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Passivation Effect of CsPbI3 Quantum Dots on the Performance and Stability of Perovskite Solar Cells

Passivation Effect of CsPbI3 Quantum Dots on the Performance and Stability of Perovskite Solar Cells hv photonics Article Passivation Effect of CsPbI Quantum Dots on the Performance and Stability of Perovskite Solar Cells Genjie Yang, Dianli Zhou, Jiawen Li and Junsheng Yu * State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China; genjieyang@std.uestc.edu.cn (G.Y.); dlzhou@uestc.edu.cn (D.Z.); 202011050805@std.uestc.edu.cn (J.L.) * Correspondence: jsyu@uestc.edu.cn Abstract: The quality of active layer film is the key factor affecting the performance of perovskite solar cells. In this work, we incorporated CsPbI quantum dots (QDs) materials into the MAPbI 3 3 perovskite precursor to form photoactive layer. On one hand, CsPbI QDs can be used as nucleation center to enhance the compactness of the perovskite film, and on the other hand, partially CsPbI QDs can be dissociated as anions and cations to passivate vacancy defects in the perovskite active layer. As a result, the film quality of the active layer was improved remarkably, thus exciton recombination was reduced, and carrier transfer increased accordingly. The devices based on doped-CsPbI QDs film had higher short circuit current, open circuit voltage and filling factor. Finally, the power conversion efficiency (PCE) was greatly enhanced from 14.85% to 17.04%. Furthermore, optimized devices also exhibited better stability. This work provides an effective strategy for the processing of high-quality perovskite films, which is of great value for the preparation and research of perovskite photoelectronic devices. Keywords: perovskite solar cells; CsPbI quantum dots; passivation; active layer 1. Introduction Citation: Yang, G.; Zhou, D.; Li, J.; Solar energy has been attracting attention as an important representative of renewable Yu, J. Passivation Effect of CsPbI and clean energy, and the development of all kinds of low-cost and high-performance Quantum Dots on the Performance solar cells is of great research significance [1–3]. Among them, the new generation of and Stability of Perovskite Solar Cells. perovskite solar cells (PSCs) have many advantages, such as low cost, long exciton diffusion Photonics 2022, 9, 3. https://doi.org/ length, adjustable band gap, etc., which have attracted much attention [4–7]. In recent 10.3390/photonics9010003 years, perovskite solar cells have achieved a breakthrough in power conversion efficiency Received: 30 November 2021 (PCE) from 3.8% to 25.5%, and are considered as the most promising new solar cells [8,9]. Accepted: 20 December 2021 However, it is well known that a large number of defects in polycrystalline perovskite films Published: 22 December 2021 have an important influence on carrier recombination and ion migration in perovskite solar cells, in which nonradiative recombination is the main means of charge loss, and it largely Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in determines the power conversion efficiency and stability of perovskite solar cells [10,11]. published maps and institutional affil- Therefore, optimizing the film forming quality of the perovskite active layer and selecting iations. the appropriate interface passivation strategy to passivate the defects of the perovskite film can improve charge extraction and transport to enhance the performance of the perovskite solar cells significantly [12–14]. In this context, Li et al. [15] used 4-ammonium chloride butyl phosphonic acid as the additives in the perovskite precursor solution. Through the Copyright: © 2021 by the authors. strong hydrogen bonds of organic additives and perovskite, the crystallization rate of Licensee MDPI, Basel, Switzerland. perovskite film was adjusted. Eventually the PCE increased from 8.8% to 16.7%. However, This article is an open access article the passivating material used in the method reported above is based on organic molecules, distributed under the terms and and for the perovskite host material, the organic molecules may introduce impurity ions, conditions of the Creative Commons potentially causing impurity defects in the perovskite film. In addition, different chain Attribution (CC BY) license (https:// lengths of organic molecules may lead to spectral shift caused by polarity changes, which creativecommons.org/licenses/by/ will also affect the intrinsic optical properties of perovskites. Therefore, it is of great 4.0/). Photonics 2022, 9, 3. https://doi.org/10.3390/photonics9010003 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 3 2 of 10 significance to select suitable passivators to reduce the introduction of irrelevant ions, and optimize the proportion of elements to prepare efficient and stable PSCs. In recent years, perovskite quantum dots (QDs) materials have attracted much at- tention due to their excellent photoelectric properties [16–19]. Perovskite quantum dots and three-dimensional perovskite materials have similar elemental composition, physical and chemical properties and lattice parameters. Therefore, the combination of three- dimensional perovskite materials and perovskite quantum dots has become a research hotspot. Therefore, it is of important research value to use inorganic quantum dot materials to modify the functional layer of perovskite solar cells to improve the performance and stability of perovskite solar cells. In this work, we introduced CsPbI QDs into the precursor solution of PSCs. On the one hand, part of CsPbI QDs were used as nucleation center to improve the morphology of the active layer film. On the other hand, the partially disintegrated CsPbI QDs were used as the anions and cations to fill the ion vacancy defects in the active layer to improve the film quality in terms of the element proportion and crystallinity of the active layer. Finally, inorganic perovskite QDs were used to regulate the growth of perovskite films to achieve high crystallinity, large grain size and fewer defects. As a result, reduced exciton recombination and improved carrier transfer help to enhance the device efficiency finally. The PCE of the optimized devices reached 17.04%, which is much higher than that of the control devices (14.85%). In addition, optimized devices also exhibit good stability, and the performance decay is much slower than that of conventional standard devices in the same environment. 2. Materials and Methods Chlorobenzene (CB), anhydrous N, N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. Other materials such as lead iodide (PbI ), methyl ammonium iodide (MAI), poly (3, 4-ethylenedioxythiophene): polybenzenesulfonate (PEDOT: PSS), phenyl- C61-methyl butyrate (PCBM) were purchased from Xi’an Polymer Light Technology Corp. All these materials were used as received. A cesium source was prepared with 0.163 g Cs CO (Sigma-Aldrich, St. Louis, MO, 2 3 USA, 99.9%), 8 mL octadecenes (Ode, Tokyo, Japan, 90%, Alfa AESAR) and 0.5 mL oleic acid (OA, 90%, Alfa AESAR) heated in a 100 mL flask at vacuum temperature for two hours. Specifically, the solution was degassed by heating in vacuum for 1 h (90 C), followed by further heating to 150 C under N until the Cs CO was completely dissolved, and finally 2 2 3 cooled under N for use. To synthesize CsPbI QDs, 0.138 g PbI (Sigma-Aldrich), 10 mL 2 3 2 ODE (Alfa Aesar, Haverhill, MA, USA) and a volume ratio of 1 were prepared under N condition: A mixture of 1 OA and oleic acid amine (Ola, Aladdin) was injected into a 50 mL three-necked flask, degassed in a vacuum at 120 C to ensure complete dissolution of PbI , and then heated to 170 C under N . Immediately, 1 mL of cesium source solution was injected. After five seconds, the colloid solution was placed in in an ice-water bath to be cooled. At the end of the reaction, the accumulated QDs was separated by centrifugation (10,000 g, 10 min). Finally, CsPbI QDs were dispersed in n-octane for later use. The device structure of perovskite solar cell is shown in Figure 1a, and the cross- sectional image SEM of the device is shown in Figure 1b. PEDOT: PSS, MAPbI , PCBM and Bphen are selected as the hole transport layer, perovskite photoactive layer, electron transport layer and hole barrier layer of the device. ITO and Ag were used as anode and cathode, respectively. Before the preparation of PEDOT: PSS layer, the 15 W/SQ ITO coated glass substrate was continuously washed in an ultrasonic bath with water-detergent solution, acetone solvent, deionized water and isopropanol (IPA) solvent, respectively. The cleaned ITO substrate was placed in an oven and dried at 80 C, followed by ozone-UV treatment for 20 min. Then, PEDOT: PSS solution was drip-coated on ITO substrate at 3000 rpm for 60 s. After 20 min of hot annealing at 150 C, the substrate was transferred into the glove box (O , H O < 1 ppm). Perovskite precursor solution was prepared by 2 2 mixing 744 mg MAI and 254.3 mg PbI in 1 mL dimethylformamide (DMF). For different 2 Photonics 2022, 9, x FOR PEER REVIEW 3 of 11 ozone-UV treatment for 20 min. Then, PEDOT: PSS solution was drip-coated on ITO sub- strate at 3000 rpm for 60 s. After 20 min of hot annealing at 150 °C, the substrate was Photonics 2022, 9, 3 transferred into the glove box (O2, H2O < 1 ppm). Perovskite precursor solution was pre- 3 of 10 pared by mixing 744 mg MAI and 254.3 mg PbI2 in 1 mL dimethylformamide (DMF). For different control groups, 1 wt%, 2 wt% and 3 wt% CsPbI3 QDs dispersions were added into the precursor solution, respectively. Then, precursor solution was stirred in the glove control groups, 1 wt%, 2 wt% and 3 wt% CsPbI QDs dispersions were added into the box for more than 4 h (500 rpm, 40 °C). Drops of 45 µL mixed perovskite precursor solu- precursor solution, respectively. Then, precursor solution was stirred in the glove box for tion were spin-coated at 4000 rpm on the ITO/PEDOT:PSS substrate for 25 s. After a delay more than 4 h (500 rpm, 40 C). Drops of 45 L mixed perovskite precursor solution were of 7 s, 200 µL of the chlorobenzene (CB) anti-solvent was dropped and spin-cast onto the spin-coated at 4000 rpm on the ITO/PEDOT:PSS substrate for 25 s. After a delay of 7 s, 200 precursor film. Afterward, the prepared films were dried at 110 °C for 20 min. The con- L of the chlorobenzene (CB) anti-solvent was dropped and spin-cast onto the precursor centration of PCBM solution was 20 mg/mL, the PCBM solution was spin-coated at 3000 film. Afterward, the prepared films were dried at 110 C for 20 min. The concentration of rpm on the perovskite film for 40 s, and then annealed at 120 °C for 10 min. The PCBM PCBM solution was 20 mg/mL, the PCBM solution was spin-coated at 3000 rpm on the layer in our experiment is about 60 nm. Subsequently, Bphen was deposited at a rate of 1 perovskite film for 40 s, and then annealed at 120 C for 10 min. The PCBM layer in our −1 1 Å s under high vacuum conditions. Followed by the deposition of Ag as anode at a dep- experiment is about 60 nm. Subsequently, Bphen was deposited at a rate of 1 Å s under −1 2 osition speed of 5 high vacuum conditions. Å s . TheFollowed active area of these P by the deposition SCs wof as Ag 0.02 cm as anode . at a deposition speed 1 2 of 5 Å s . The active area of these PSCs was 0.02 cm . Figure 1. (a) The device structure of PSCs; (b) cross-sectional image SEM of the device. (c) UV-vis Figure 1. absorption (a) T spectr he dev um iceand stru PL ctuspectr re of PSC um s of ; (CsPbI b) cross- QDs. sectional image SEM of the device. (c) UV-vis absorption spectrum and PL spectrum of CsPbI3 QDs. We used Shimazu UV1700 UV-visible absorption system to measure the UV-visible absorption spectra of CsPbI QDs and perovskite films. Scanning electron microscopy We used Shimazu UV1700 3 UV-visible absorption system to measure the UV-visible (SEM) (FEI Inspect F50) was used to measure the surface morphology of perovskite films. absorption spectra of CsPbI3 QDs and perovskite films. Scanning electron microscopy The distribution of the elements was detected by X-ray dispersive X-ray analysis (EDS). (SEM) (FEI Inspect F50) was used to measure the surface morphology of perovskite films. The crystal structure was characterized by X-ray diffraction (XRD) (D2 PHASER). Using The distribution of the elements was detected by X-ray dispersive X-ray analysis (EDS). a time-dependent single photometer system (FL-TCSPC, Horiba Jobin Yvon), stimulated The crystal structure was characterized by X-ray diffraction (XRD) (D2 PHASER). Using by a 550 nm picosecond pulse laser, photoluminescence spectra (PL) and time-resolved a time-dependent single photometer system (FL-TCSPC, Horiba Jobin Yvon), stimulated photoluminescence spectra (TRPL) were measured. An AM1.5G solar simulator was used by a 550 nm picosecond pulse laser, photoluminescence spectra (PL) and time-resolved as the light source, and the lighting power is 100 mW/cm . The current density voltage photoluminescence spectra (TRPL) were measured. An AM1.5G solar simulator was used (J-V) curves of the device under illumination were measured with a Keithley4200 semicon- as the light source, and the lighting power is 100 mW/cm . The current density voltage (J- ductor analyzer. External quantum efficiency (EQE) curves were obtained using xenon V) curves of the device under illumination were measured with a Keithley4200 semicon- lamps calibrated through a monochromator calibrated by a standard silicon solar cell. All ductor analyzer. External quantum efficiency (EQE) curves were obtained using xenon measurements were made at room temperature. Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 lamps calibrated through a monochromator calibrated by a standard silicon solar cell. All Photonics 2022, 9, 3 4 of 10 measurements were made at room temperature. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization and Test 3.1. Characterization and Test In order to characterize the optical properties of CsPbI3 QDs, we measured the UV- In order to characterize the optical properties of CsPbI QDs, we measured the UV-Vis Vis spectrum and steady-state photoluminescence spectrum. As 3 shown in Figure 1a,c, a spectrum and steady-state photoluminescence spectrum. As shown in Figure 1a,c, a strong strong PL emission peak can be observed at 687 nm for CsPbI3 QDs, which is consistent PL emission peak can be observed at 687 nm for CsPbI QDs, which is consistent with with the absorption and emission wavelength range of CsPbI3 material system reported the absorption and emission wavelength range of CsPbI material system reported in in literature [20]. It is well known that the photoelectric properties 3 of quantum dot mate- literature [20]. It is well known that the photoelectric properties of quantum dot materials rials are related to their size and purity. We observed that the PL emission peak intensity are related to their size and purity. We observed that the PL emission peak intensity of of CsPbI3 QDs is symmetric, and the width of the half-wave peak is narrow (35 nm), which CsPbI QDs is symmetric, and the width of the half-wave peak is narrow (35 nm), which indicates that the CsPbI3 QDs are high in purity. CsPbI3 QDs with good uniformity are indicates that the CsPbI QDs are high in purity. CsPbI QDs with good uniformity are more 3 3 more easily dispersed in the process of mixing the active layer, which would play a ho- easily dispersed in the process of mixing the active layer, which would play a homogenized mogenized passivation effect on the defects in different positions to ensure the uniformity passivation effect on the defects in different positions to ensure the uniformity and flatness and flatness of active layer film. of active layer film. In order to prove that CsPbI3 QDs are uniformly mixed into the perovskite film, we In order to prove that CsPbI QDs are uniformly mixed into the perovskite film, we obtained the element distribution image by EDS Mapping, and the test results are shown obtained the element distribution image by EDS Mapping, and the test results are shown in Figure 2. As we can see, Cs elements representing the characteristic elements of CsPbI3 in Figure 2. As we can see, Cs elements representing the characteristic elements of CsPbI QDs were distributed in the whole test area, indicating that CsPbI3 QDs were successfully QDs were distributed in the whole test area, indicating that CsPbI QDs were successfully incorporated into the film of MAPbI3 active layer. In Figure 2b, the intensity distribution incorporated into the film of MAPbI active layer. In Figure 2b, the intensity distribution of Cs elements in the whole test area has good evenness, indicating that CsPbI3 QDs are of Cs elements in the whole test area has good evenness, indicating that CsPbI QDs are evenly distributed in the perovskite active layer, which is conducive to the formation of evenly distributed in the perovskite active layer, which is conducive to the formation of uniform and stable perovskite film. In addition, the incorporation of Cs element intro- uniform and stable perovskite film. In addition, the incorporation of Cs element introduces duces metallic inorganic cations into the active layer of perovskite to achieve the effect of metallic inorganic cations into the active layer of perovskite to achieve the effect of cation cation passivation, and further improve the morphology of the film. passivation, and further improve the morphology of the film. Figure 2. EDS elemental mapping images of (a) all elements; (b) Cs; (c) Pb; (d) I in the MAPbI film Figure 2. EDS elemental mapping images of (a) all elements; (b) Cs; (c) Pb; (d) I in the MAPbI3 film doped with CsPbI QDs. doped with CsPbI3 QDs In order to more directly study the influence of CsPbI QDs on the morphology of In order to more directly study the influence of CsPbI3 QDs on the morphology of perovskite films, we tested the surface morphology of perovskite active layer films doped perovskite films, we tested the surface morphology of perovskite active layer films doped with different proportions of CsPbI QDs by SEM. As shown in Figure 3, the surface morphologies of perovskite films doped with different proportions of CsPbI QDs are obviously different. The perovskite film without CsPbI QDs has small grains and poor crystallinity. With the moderate addition of CsPbI QDs, the grain size of perovskite film 3 Photonics 2022, 9, x FOR PEER REVIEW 5 of 11 with different proportions of CsPbI3 QDs by SEM. As shown in Figure 3, the surface mor- phologies of perovskite films doped with different proportions of CsPbI3 QDs are obvi- ously different. The perovskite film without CsPbI3 QDs has small grains and poor crys- Photonics 2022, 9, 3 5 of 10 tallinity. With the moderate addition of CsPbI3 QDs, the grain size of perovskite film grad- ually becomes larger. We have made a quantitative size distribution in Figure 3. For per- ovskite films doped with 0, 1 wt%, 2 wt%, 3 wt%, their average grain diameter is 247.25 nm, 28 gradually 1.22 nm, 35 becomes 2.87 nm lar, 27 ger. 4.90 We nm have respecti madeve a quantitative ly. This is beca size usedistribution as the nuclein atiFigur on cent e 3 er, . For CsPbI per 3ovskite QDs can promote t films doped he crystal with 0, 1 gro wt%, wth during 2 wt%, t 3he perovski wt%, theirte crysta average lliza grain tion process diameter is and i 247.25 nducnm, e the 281.22 forma nm, tion 352.87 of thre nm, e-di274.90 mensio nm nal rperov espectively skite gra . This ins iswi because th large assize. On the the nucleation other hand, t center, CsPbI he doping of QDs can CsPbI promote 3 QDs introduces the crystal growth additi during onal metal the per caovskite tionic halogen a crystallization ni- ons, whi process ch not onl and induce y regula the formation tes the elem of ent propor three-dimensional tion of the per acovskit tive lae yer, but grains with also passi largevsize. - On the other hand, the doping of CsPbI QDs introduces additional metal cationic halogen ates some ion vacancy defects and surface grain boundaries, thus improving the film qual- anions, which not only regulates the element proportion of the active layer, but also ity of the active layer. However, with excessive incorporation of CsPbI3 QDs, as shown in passivates some ion vacancy defects and surface grain boundaries, thus improving the film Figure 3d, the film morphology of the active layer doped with 3% CsPbI3 QDs is not as quality of the active layer. However, with excessive incorporation of CsPbI QDs, as shown good as that of the active layer doped with 2% CsPbI3 QDs in Figure 3c. This is because 3 in Figure 3d, the film morphology of the active layer doped with 3% CsPbI QDs is not as excessive doping will lead to the imbalance of element proportion and affect the crystal- 3 good as that of the active layer doped with 2% CsPbI QDs in Figure 3c. This is because linity of perovskite and the film morphology. In genera3l, the active layer of perovskite excessive doping will lead to the imbalance of element proportion and affect the crystallinity doped with 2% CsPbI3 QDs has the largest grain size, which can reach nearly micron level. of perovskite and the film morphology. In general, the active layer of perovskite doped with The smooth and dense perovskite film can form a closer interface contact with the 2% CsPbI QDs has the largest grain size, which can reach nearly micron level. The smooth transport layer, thus reducing the interface recombination of excitons and improving the and dense perovskite film can form a closer interface contact with the transport layer, thus extraction efficiency of photogenerated carriers [21]. In addition, the active perovskite reducing the interface recombination of excitons and improving the extraction efficiency layer with large grains is generally characterized by a low grain boundary density, which of photogenerated carriers [21]. In addition, the active perovskite layer with large grains reduces the density of defect states in the perovskite film and effectively reduces the is generally characterized by a low grain boundary density, which reduces the density of charge recombination in defects. The experimental results prove that the incorporation of defect states in the perovskite film and effectively reduces the charge recombination in CsPbI3 QDs can improve the morphology of the perovskite active layer, so as to prepare defects. The experimental results prove that the incorporation of CsPbI QDs can improve high-quality perovskite films and improve the photovoltaic performance of the devices the morphology of the perovskite active layer, so as to prepare high-quality perovskite potentially. films and improve the photovoltaic performance of the devices potentially. Figure 3. SEM images of the MAPbI3 films with different CsPbI3 concentrations: (a) 0, (b) 1 wt%, Figure 3. SEM images of the MAPbI3 films with different CsPbI3 concentrations: (a) 0, (b) 1 wt%, (c) (c) 2 wt%, (d) 3 wt%. 2 wt%, (d) 3 wt%. In order to further study the effect of CsPbI QDs on the photoelectric properties of per In order to further study ovskite films, the active the effect of C layer was characterized sPbI3 QDs on bythe photoelectric properties of spectra and crystallization tests. Figure 4a shows the UV-Vis absorption spectrum of the perovskite film with and without perovskite films, the active layer was characterized by spectra and crystallization tests. FigCsPbI ure 4a sho QDs. ws the It can UV be -Vis seen absorption that the s active pectrum layer of the perovski film doped with te filCsPbI m with QDs and wi has thout almost 3 3 the same absorption spectrum and absorption intensity compared with the film without CsPbI QDs, both of which only exhibit MAPbI characteristic absorption. This indicates 3 3 that the incorporation of CsPbI QDs does not change the light absorption capacity of the active layer. Then, we tested the XRD peak patterns of two kinds of perovskite films. As shown in Figure 4b, the XRD of two films both showed three main intense diffraction peaks at 2q = 14.4 , 28.8 and 31.8 , which represent the (110), (220), (310) planes, respectively, of Photonics 2022, 9, x FOR PEER REVIEW 6 of 11 CsPbI3 QDs. It can be seen that the active layer film doped with CsPbI3 QDs has almost the same absorption spectrum and absorption intensity compared with the film without CsPbI3 QDs, both of which only exhibit MAPbI3 characteristic absorption. This indicates that the incorporation of CsPbI3 QDs does not change the light absorption capacity of the active layer. Then, we tested the XRD peak patterns of two kinds of perovskite films. As shown in Figure 4b, the XRD of two films both showed three main intense diffraction Photonics 2022, 9, 3 6 of 10 peaks at 2θ = 14.4°, 28.8° and 31.8°, which represent the (110), (220), (310) planes, respec- tively, of the MAPbI3 perovskite crystalline structure [22,23]. This suggests that there is no alloy state of MAxCs1-xPbI3 in the active layer. However, compared with control film, the the MAPbI perovskite crystalline structure [22,23]. This suggests that there is no alloy state perovskite film doped with CsPbI3 QDs shows higher and sharper characteristic peaks in of MA Cs PbI in the active layer. However, compared with control film, the perovskite 1-x 3 all three diffraction angles. The intensity of the diffraction peak corresponds to the crys- film doped with CsPbI QDs shows higher and sharper characteristic peaks in all three tallinity of the film. In general, the strong XRD diffraction intensity indicates that the crys- diffraction angles. The intensity of the diffraction peak corresponds to the crystallinity of tallinity of this orientation is good, which is conducive to the formation of large and uni- the film. In general, the strong XRD diffraction intensity indicates that the crystallinity form grains [24]. The results of XRD show that the doping of CsPbI3 QDs can enhance the of this orientation is good, which is conducive to the formation of large and uniform crystallinity of perovskite films and obtain higher quality perovskite films to reduce exci- grains [24]. The results of XRD show that the doping of CsPbI QDs can enhance the ton recombination and enhancing carrier extraction, which is consistent with the results crystallinity of perovskite films and obtain higher quality perovskite films to reduce exciton of SEM. recombination and enhancing carrier extraction, which is consistent with the results of SEM. Figure 4. (a) UV-VIS absorption spectra; (b) XRD pattern; (c) steady-state photoluminescence spec- Figure 4. (a) UV-VIS absorption spectra; (b) XRD pattern; (c) steady-state photoluminescence spectra; tra; (d) time-resolved photoluminescence of perovskite films spectra without and with CsPbI3 QDs. (d) time-resolved photoluminescence of perovskite films spectra without and with CsPbI QDs. In order to investigate the effect of the introduction of CsPbI3 QDs on the energy In order to investigate the effect of the introduction of CsPbI QDs on the energy transfer and carrier recombination of PSCs, we measured the PL and TRPL spectra of two transfer and carrier recombination of PSCs, we measured the PL and TRPL spectra of perovskite films with the test structure of ITO/PEDOT: PSS/perovskite active layers. Fig- two perovskite films with the test structure of ITO/PEDOT: PSS/perovskite active layers. ure 4c shows the PL spectra of the two films. Both of them show the characteristic PL Figure 4c shows the PL spectra of the two films. Both of them show the characteristic PL spectrum of MAPbI3, which indicates that the introduction of CsPbI3 will not form alloy spectrum of MAPbI , which indicates that the introduction of CsPbI will not form alloy 3 3 state of MAxCs1−x PbI3 in the active layer. However, compared with the control device, the state of MA Cs PbI in the active layer. However, compared with the control device, the x 1x 3 PL peak of the CsPbI3 QDs-doped film shows a blue shift from 766 nm to 760 nm, indicat- PL peak of the CsPbI QDs-doped film shows a blue shift from 766 nm to 760 nm, indicating ing that the surface trap of the active layer is effectively passivated [25]. In addition, com- that the surface trap of the active layer is effectively passivated [25]. In addition, compared pared with the undoped film, the PL peak strength of the doped film shows an obvious with the undoped film, the PL peak strength of the doped film shows an obvious quenching quenching phenomenon, which indicates that the doped CsPbI3 QDs is conducive to the phenomenon, which indicates that the doped CsPbI QDs is conducive to the extraction of carriers from perovskite films. Figure 4d shows the time-resolved photoluminescence (TRPL) spectrum, which can be used to further study the charge transfer kinetics. According to the TRPL results, the rapid decay lifetime of the perovskite films doped with CsPbI QDs is significantly lower than that of the undoped films. Since reduced fast decay lifetimes indicate faster and efficient charge-carrier transfer at the interface, it can be concluded that perovskite doped CsPbI QDs has better interfacial properties and charge transfer capacity. 3 Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 extraction of carriers from perovskite films. Figure 4d shows the time-resolved photolu- minescence (TRPL) spectrum, which can be used to further study the charge transfer ki- netics. According to the TRPL results, the rapid decay lifetime of the perovskite films doped with CsPbI3 QDs is significantly lower than that of the undoped films. Since re- Photonics 2022, 9, 3 7 of 10 duced fast decay lifetimes indicate faster and efficient charge-carrier transfer at the inter- face, it can be concluded that perovskite doped CsPbI3 QDs has better interfacial proper- ties and charge transfer capacity. In order to further investigate the effect of CsPbI QDs on passivating defects, we In order to further investigate the effect of CsPbI3 Q 3Ds on passivating defects, we estimated the trap-state density of perovskite film by equation [26]: estimated the trap-state density of perovskite film by equation [26]: 2εε V 0 rTEL 2# # V n = 0 r TEL t 2 (1) n = (1) t qL qL where nt is the trap state density, VTFL is the trap-filled limit voltage, L is the thickness of where n is the trap state density, V is the trap-filled limit voltage, L is the thickness of t TFL the perovskite films (400 nm), q is the elementary charge, ε0 is the vacuum permittivity, εr the perovskite films (400 nm), q is the elementary charge, # is the vacuum permittivity, # is relative dielectric constant of MAPbI3 (6.5). As showed in Figure 5, the trap-filled limit is relative dielectric constant of MAPbI (6.5). As showed in Figure 5, the trap-filled limit voltages of 0.46 V and 0.24 V were measured in the electron-only devices. According to voltages of 0.46 V and 0.24 V were measured in the electron-only devices. According to the the equation, the trap state density of two perovskite films can be calculated to be 2.73 × equation, the trap state density of two perovskite films can be calculated to be 2.73  10 15 15 −3 10 and 1.42 × 10 cm , respectively, indicating that some defects have been passivated 15 3 and 1.42  10 cm , respectively, indicating that some defects have been passivated by by introducing CsPbI3 QDs. The reduced trap density is related to the improved quality introducing CsPbI QDs. The reduced trap density is related to the improved quality of of the perovskite film, which is beneficial for device stability and carrier dynamics. It can the perovskite film, which is beneficial for device stability and carrier dynamics. It can be be mutually confirmed with XRD, PL, and TRPL results. mutually confirmed with XRD, PL, and TRPL results. Figure 5. J-V characteristics under dark conditions of electron-only devices containing the perovskite film (a) without and (b) with CsPbI QDs. Figure 5. J-V characteristics under dark conditions of electron-only devices containing the perov- skite film (a) without and (b) with CsPbI3 QDs. 3.2. Performance of PSCs In order to further study the effect of CsPbI QDs on the performance of perovskite 3.2. Performance of PSCs devices, we prepared a series of perovskite solar cells using perovskite thin films with In order to further study the effect of CsPbI3 QDs on the performance of perovskite different doping ratios of CsPbI QDs as the active layer, and tested their photovoltaic devices, we prepared a series of perovskite solar cells using perovskite thin films with performance. Besides, in order to ensure the repeatability of our devices, 20 devices different doping ratios of CsPbI3 QDs as the active layer, and tested their photovoltaic were fabricated and characterized at each ratio. The distribution of their performance performance. Besides, in order to ensure the repeatability of our devices, 20 devices were parameters is showed in Figure 6. And Table 1 is the average performance statistics of fabricated and characterized at each ratio. The distribution of their performance parame- these devices. The results show that CsPbI QDs doped in the active layer has a great ters is showed in Figure 6. And Table 1 is the average performance statistics of these de- influence on the performance of PSCs, changing the short-circuit current, open-circuit vices. The results show that CsPbI3 QDs doped in the active layer has a great influence on voltage and filling factor of the devices. The PSCs doped with 2% CsPbI QDs showed the the performance of PSCs, changing the short-circuit current, open-circuit voltage and fill- best performance, with the short-circuit current, open-circuit voltage, fill factor and energy ing factor of the devices. The PSCs doped with 2% 2 CsPbI3 QDs showed the best perfor- conversion efficiency reaching 22.27 mA cm , 0.961 V, 79.63% and 17.04%, respectively. mance, with the short-circuit current, open-circuit voltage, fill factor and energy conver- The short-circuit current, open-circuit voltage and fill factor of PSCs without CsPbI QDs are −2 sion efficiency re 2aching 22.27 mA cm , 0.961 V, 79.63% and 17.04%, respectively. The 21.32 mA cm , 0.934 V, 74.59% and 14.85%, respectively. In contrast, the doping of CsPbI short-circuit current, open-circuit voltage and fill factor of PSCs without CsPbI3 QDs are QDs significantly increased short-circuit current, open-circuit voltage, fill factor, resulting in −2 21.32 mA cm , 0.934 V, 74.59% and 14.85%, respectively. In contrast, the doping of CsPbI3 a 15% increase in PCE. The higher performance of the device with CsPbI QDs is due to the QDs significantly increased short-circuit current, open-circuit voltage, fill factor, resulting better quality of the perovskite active layer film, which reduces exciton recombination and improves carrier transfer efficiency, which is consistent with the previous characterization conclusion of the films. The passivated perovskite film also formed the better interface contact with both hole transport layer and electron transport layer, which is conducive to the improvement of open-circuit voltage to improve device performance [27,28]. Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 in a 15% increase in PCE. The higher performance of the device with CsPbI3 QDs is due to the better quality of the perovskite active layer film, which reduces exciton recombination and improves carrier transfer efficiency, which is consistent with the previous characteri- zation conclusion of the films. The passivated perovskite film also formed the better in- Photonics 2022, 9, 3 8 of 10 terface contact with both hole transport layer and electron transport layer, which is con- ducive to the improvement of open-circuit voltage to improve device performance [27,28]. Figure 6. Distribution of the (a) J , (b) V , (c) FF, and (d) PCE of the devices based on perovskite SC OC Figure 6. Distribution of the (a) JSC, (b) VOC, (c) FF, and (d) PCE of the devices based on perovskite films with different CsPbI3 QDs concentrations. films with different CsPbI3 QDs concentrations. Table 1. Photovoltaic parameters of PSCs. Table 1. Photovoltaic parameters of PSCs. Devices Jsc (mA/cm ) Voc (V) FF (%) PCE (%) Devices Jsc (mA/cm ) Voc (V) FF (%) PCE (%) Control 21.32  0.78 0.934  0.005 74.59  2.01 14.85  0.53 Control 21.32 ± 0.78 0.934 ± 0.005 74.59 ± 2.01 14.85 ± 0.53 1 wt% 21.46  0.58 0.942  0.003 76.17  2.07 15.39  0.19 1 wt% 21.46 ± 0.58 0.942 ± 0.003 76.17 ± 2.07 15.39 ± 0.19 2 wt% 22.27  0.63 0.961  0.003 79.63  1.29 17.04  0.33 2 wt 3 wt% % 19.73 22.27 ± 0.  0.46 63 0.953 0.961 ± 0. 0.002003 74.11 79.63 ± 1.  2.52 29 17 13.93 .04 ± 0.  0.29 33 3 wt% 19.73 ± 0.46 0.953 ± 0.002 74.11 ± 2.52 13.93 ± 0.29 At the same time, we also showed J-V characteristics and EQE curves of PSCs with At the same time, we also showed J-V characteristics and EQE curves of PSCs with different doping ratios of CsPbI QDs, and the test results are shown in Figure 7a,b. It can different doping ratios of CsPbI3 QDs, and the test results are shown in Figure 7a,b. It can be seen that, within the wavelength range of the response of the device, the EQE curves be seen that, within the wavelength range of the response of the device, the EQE curves of the PSCs doped with 2 wt% CsPbI QDs is higher than those of other devices, which is of the PSCs doped with 2 wt% CsPbI3 QDs is higher than those of other devices, which is consistent with the J variation trend of the J-V curve. The value of the integrated current SC consistent with the JSC variation trend of the J-V curve. The value of the integrated current obtained from the EQE curves is approximately the same as the measured current. obtained from the EQE curves is approximately the same as the measured current. Next, we tested the PCE output stability of optimized and control devices. As shown in Figure 7d, we applied a working voltage of 0.85 V to both devices under the simulated sunlight of AM 1.5 g with a light intensity of 100 mW cm , and recorded the time function curve of the photocurrent density of PSCs. As can be seen from the Figure 7c, the output peak of current density of both devices reached rapidly after the beginning of illumination. After 60 s continuous illumination, the output value of current density and steady-state 2 2 PCE of two devices reached 22.15 mA cm , 16.91% and 21.12 mA cm , 14.68%. It is basically consistent with the photocurrent density and PCE value in J-V test. We also compare the hysteresis of the two devices in Figure 7d,e. It can be seen that the hysteresis of the optimized device under forward scanning and reverse scanning is much weaker than that of the control device, which is due to the passivated defects and fewer trap states weaken the hysteresis effectively [29,30]. Finally, we studied the environmental stability of the device. The stability test of the unencapsulated device was carried out for 7 days at room temperature and atmospheric conditions. As can be seen from the Figure 7f, after 7 days of testing, the efficiency of the device without CsPbI QDs has decreased to 17% of the original efficiency, but the efficiency of the device with doping CsPbI QDs still remains more than 50% of the origin, which shows more excellent environmental stability. Photonics 2022, 9, x FOR PEER REVIEW 9 of 11 Photonics 2022, 9, 3 9 of 10 Figure 7. (a) J-V characteristics of PSCs; (b) EQE and corresponding integrated J of PSCs; SC Figure 7. (a) J-V characteristics of PSCs; (b) EQE and corresponding integrated JSC of PSCs; (c) The (c) The steady-state power output of PSCs; (d) J-V curve hysteresis of the control device; (e) J-V curve steady-state power output of PSCs; (d) J-V curve hysteresis of the control device; (e) J-V curve hys- hysteresis of the optimized device; (f) Long time stability of PSCs. teresis of the optimized device; (f) Long time stability of PSCs. 4. Conclusions Next, we tested the PCE output stability of optimized and control devices. As shown In summary, we demonstrate a defect passivation strategy by introducing CsPbI QDs in Figure 7d, we applied a working voltage of 0.85 V to both devices under the simulated into the perovskite active layer. Through a series of film characterization and device tests, −2 sunlight of AM 1.5 g with a light intensity of 100 mW cm , and recorded the time function we prove that the introduction of CsPbI QDs can effectively improve the crystallization of curve of the photocurrent density of PSCs. As can be seen from the Figure 7c, the output the perovskite active layer, passivate the defects, and thus reduce the exciton recombination peak of current density of both devices reached rapidly after the beginning of illumina- and improve the carrier transfer ability. The device doped with CsPbI QDs has higher short tion. After 60 s continuous illumination, the output value of current density and steady- circuit current, open circuit voltage and fill factor, and achieved a significant improvement −2 −2 state PCE of two devices reached 22.15 mA cm , 16.91% and 21.12 mA cm , 14.68%. It is in power conversion efficiency from 14.85% to 17.04%. In addition, the optimized devices basically consistent with the photocurrent density and PCE value in J-V test. We also com- also have better environmental stability. This work provides a novel way to prepare high- pare the hysteresis of the two devices in Figure 7d,e. It can be seen that the hysteresis of quality perovskite films, which may be of great value for the construction and preparation the optimized device under forward scanning and reverse scanning is much weaker than of efficient perovskite photoelectronic devices. that of the control device, which is due to the passivated defects and fewer trap states Author weaken Contributions: the hysteresis Conceptualization, effectively [29,30G.Y ]. . and D.Z.; methodology, G.Y.; software, J.L.; vali- dation, Fina G.Yll . and y, we J.Y st .; udie formal d tanalysis, he environment G.Y.; investigation, al stabilityG.Y of the .; reso device. The urces, G.Y.; st data ability curation, test oG.Y f the .; writing—original draft preparation, G.Y.; writing—review and editing, D.Z. and J.Y.; visualization, unencapsulated device was carried out for 7 days at room temperature and atmospheric G.Y.; supervision, J.Y.; project administration, J.Y.; funding acquisition, J.Y. All authors have read and conditions. As can be seen from the Figure 7f, after 7 days of testing, the efficiency of the agreed to the published version of the manuscript. device without CsPbI3 QDs has decreased to 17% of the original efficiency, but the effi- ciency of the device with doping CsPbI3 QDs still remains more than 50% of the origin, Funding: This work was financially supported by the Foundation of National Natural Science which shows more excellent environmental stability. Foundation of China (NSFC) (Grant Nos.61421002, 61675041, and 51703019) and the Sichuan Science and Technology Program (2019YFG0121, 2019YJ0178, 2020YFG0279, and 2020YFG0281), Sichuan Youth Software Innovation Project Funding Project Contract (Grant Nos. 2021073, 2021107). This 4. Conclusions work is also sponsored by the Sichuan Province Key Laboratory of Display Science and Technology. In summary, we demonstrate a defect passivation strategy by introducing CsPbI3 Institutional Review Board Statement: Not applicable. QDs into the perovskite active layer. Through a series of film characterization and device tests, we prove that the introduction of CsPbI3 QDs can effectively improve the crystalli- Informed Consent Statement: Not applicable. zation of the perovskite active layer, passivate the defects, and thus reduce the exciton Data Availability Statement: The data presented in this study are available on request from the recombination and improve the carrier transfer ability. The device doped with CsPbI3 QDs corresponding author. The data are not publicly available due to privacy. has higher short circuit current, open circuit voltage and fill factor, and achieved a signif- Conflicts of Interest: The authors declare no conflict of interest. icant improvement in power conversion efficiency from 14.85% to 17.04%. In addition, the optimized devices also have better environmental stability. This work provides a novel References way to prepare high-quality perovskite films, which may be of great value for the con- 1. Chu, S.; Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294–303. [CrossRef] struction and preparation of efficient perovskite photoelectronic devices. 2. Green, M.A.; Ho-Baillie, A.; Snaith, H.J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506–514. [CrossRef] Photonics 2022, 9, 3 10 of 10 3. Gao, P.; Gratzel, M.; Nazeeruddin, M.K. 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Passivation Effect of CsPbI3 Quantum Dots on the Performance and Stability of Perovskite Solar Cells

Photonics , Volume 9 (1) – Dec 22, 2021

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Abstract

hv photonics Article Passivation Effect of CsPbI Quantum Dots on the Performance and Stability of Perovskite Solar Cells Genjie Yang, Dianli Zhou, Jiawen Li and Junsheng Yu * State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China; genjieyang@std.uestc.edu.cn (G.Y.); dlzhou@uestc.edu.cn (D.Z.); 202011050805@std.uestc.edu.cn (J.L.) * Correspondence: jsyu@uestc.edu.cn Abstract: The quality of active layer film is the key factor affecting the performance of perovskite solar cells. In this work, we incorporated CsPbI quantum dots (QDs) materials into the MAPbI 3 3 perovskite precursor to form photoactive layer. On one hand, CsPbI QDs can be used as nucleation center to enhance the compactness of the perovskite film, and on the other hand, partially CsPbI QDs can be dissociated as anions and cations to passivate vacancy defects in the perovskite active layer. As a result, the film quality of the active layer was improved remarkably, thus exciton recombination was reduced, and carrier transfer increased accordingly. The devices based on doped-CsPbI QDs film had higher short circuit current, open circuit voltage and filling factor. Finally, the power conversion efficiency (PCE) was greatly enhanced from 14.85% to 17.04%. Furthermore, optimized devices also exhibited better stability. This work provides an effective strategy for the processing of high-quality perovskite films, which is of great value for the preparation and research of perovskite photoelectronic devices. Keywords: perovskite solar cells; CsPbI quantum dots; passivation; active layer 1. Introduction Citation: Yang, G.; Zhou, D.; Li, J.; Solar energy has been attracting attention as an important representative of renewable Yu, J. Passivation Effect of CsPbI and clean energy, and the development of all kinds of low-cost and high-performance Quantum Dots on the Performance solar cells is of great research significance [1–3]. Among them, the new generation of and Stability of Perovskite Solar Cells. perovskite solar cells (PSCs) have many advantages, such as low cost, long exciton diffusion Photonics 2022, 9, 3. https://doi.org/ length, adjustable band gap, etc., which have attracted much attention [4–7]. In recent 10.3390/photonics9010003 years, perovskite solar cells have achieved a breakthrough in power conversion efficiency Received: 30 November 2021 (PCE) from 3.8% to 25.5%, and are considered as the most promising new solar cells [8,9]. Accepted: 20 December 2021 However, it is well known that a large number of defects in polycrystalline perovskite films Published: 22 December 2021 have an important influence on carrier recombination and ion migration in perovskite solar cells, in which nonradiative recombination is the main means of charge loss, and it largely Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in determines the power conversion efficiency and stability of perovskite solar cells [10,11]. published maps and institutional affil- Therefore, optimizing the film forming quality of the perovskite active layer and selecting iations. the appropriate interface passivation strategy to passivate the defects of the perovskite film can improve charge extraction and transport to enhance the performance of the perovskite solar cells significantly [12–14]. In this context, Li et al. [15] used 4-ammonium chloride butyl phosphonic acid as the additives in the perovskite precursor solution. Through the Copyright: © 2021 by the authors. strong hydrogen bonds of organic additives and perovskite, the crystallization rate of Licensee MDPI, Basel, Switzerland. perovskite film was adjusted. Eventually the PCE increased from 8.8% to 16.7%. However, This article is an open access article the passivating material used in the method reported above is based on organic molecules, distributed under the terms and and for the perovskite host material, the organic molecules may introduce impurity ions, conditions of the Creative Commons potentially causing impurity defects in the perovskite film. In addition, different chain Attribution (CC BY) license (https:// lengths of organic molecules may lead to spectral shift caused by polarity changes, which creativecommons.org/licenses/by/ will also affect the intrinsic optical properties of perovskites. Therefore, it is of great 4.0/). Photonics 2022, 9, 3. https://doi.org/10.3390/photonics9010003 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 3 2 of 10 significance to select suitable passivators to reduce the introduction of irrelevant ions, and optimize the proportion of elements to prepare efficient and stable PSCs. In recent years, perovskite quantum dots (QDs) materials have attracted much at- tention due to their excellent photoelectric properties [16–19]. Perovskite quantum dots and three-dimensional perovskite materials have similar elemental composition, physical and chemical properties and lattice parameters. Therefore, the combination of three- dimensional perovskite materials and perovskite quantum dots has become a research hotspot. Therefore, it is of important research value to use inorganic quantum dot materials to modify the functional layer of perovskite solar cells to improve the performance and stability of perovskite solar cells. In this work, we introduced CsPbI QDs into the precursor solution of PSCs. On the one hand, part of CsPbI QDs were used as nucleation center to improve the morphology of the active layer film. On the other hand, the partially disintegrated CsPbI QDs were used as the anions and cations to fill the ion vacancy defects in the active layer to improve the film quality in terms of the element proportion and crystallinity of the active layer. Finally, inorganic perovskite QDs were used to regulate the growth of perovskite films to achieve high crystallinity, large grain size and fewer defects. As a result, reduced exciton recombination and improved carrier transfer help to enhance the device efficiency finally. The PCE of the optimized devices reached 17.04%, which is much higher than that of the control devices (14.85%). In addition, optimized devices also exhibit good stability, and the performance decay is much slower than that of conventional standard devices in the same environment. 2. Materials and Methods Chlorobenzene (CB), anhydrous N, N-dimethylformamide (DMF) were purchased from Sigma-Aldrich. Other materials such as lead iodide (PbI ), methyl ammonium iodide (MAI), poly (3, 4-ethylenedioxythiophene): polybenzenesulfonate (PEDOT: PSS), phenyl- C61-methyl butyrate (PCBM) were purchased from Xi’an Polymer Light Technology Corp. All these materials were used as received. A cesium source was prepared with 0.163 g Cs CO (Sigma-Aldrich, St. Louis, MO, 2 3 USA, 99.9%), 8 mL octadecenes (Ode, Tokyo, Japan, 90%, Alfa AESAR) and 0.5 mL oleic acid (OA, 90%, Alfa AESAR) heated in a 100 mL flask at vacuum temperature for two hours. Specifically, the solution was degassed by heating in vacuum for 1 h (90 C), followed by further heating to 150 C under N until the Cs CO was completely dissolved, and finally 2 2 3 cooled under N for use. To synthesize CsPbI QDs, 0.138 g PbI (Sigma-Aldrich), 10 mL 2 3 2 ODE (Alfa Aesar, Haverhill, MA, USA) and a volume ratio of 1 were prepared under N condition: A mixture of 1 OA and oleic acid amine (Ola, Aladdin) was injected into a 50 mL three-necked flask, degassed in a vacuum at 120 C to ensure complete dissolution of PbI , and then heated to 170 C under N . Immediately, 1 mL of cesium source solution was injected. After five seconds, the colloid solution was placed in in an ice-water bath to be cooled. At the end of the reaction, the accumulated QDs was separated by centrifugation (10,000 g, 10 min). Finally, CsPbI QDs were dispersed in n-octane for later use. The device structure of perovskite solar cell is shown in Figure 1a, and the cross- sectional image SEM of the device is shown in Figure 1b. PEDOT: PSS, MAPbI , PCBM and Bphen are selected as the hole transport layer, perovskite photoactive layer, electron transport layer and hole barrier layer of the device. ITO and Ag were used as anode and cathode, respectively. Before the preparation of PEDOT: PSS layer, the 15 W/SQ ITO coated glass substrate was continuously washed in an ultrasonic bath with water-detergent solution, acetone solvent, deionized water and isopropanol (IPA) solvent, respectively. The cleaned ITO substrate was placed in an oven and dried at 80 C, followed by ozone-UV treatment for 20 min. Then, PEDOT: PSS solution was drip-coated on ITO substrate at 3000 rpm for 60 s. After 20 min of hot annealing at 150 C, the substrate was transferred into the glove box (O , H O < 1 ppm). Perovskite precursor solution was prepared by 2 2 mixing 744 mg MAI and 254.3 mg PbI in 1 mL dimethylformamide (DMF). For different 2 Photonics 2022, 9, x FOR PEER REVIEW 3 of 11 ozone-UV treatment for 20 min. Then, PEDOT: PSS solution was drip-coated on ITO sub- strate at 3000 rpm for 60 s. After 20 min of hot annealing at 150 °C, the substrate was Photonics 2022, 9, 3 transferred into the glove box (O2, H2O < 1 ppm). Perovskite precursor solution was pre- 3 of 10 pared by mixing 744 mg MAI and 254.3 mg PbI2 in 1 mL dimethylformamide (DMF). For different control groups, 1 wt%, 2 wt% and 3 wt% CsPbI3 QDs dispersions were added into the precursor solution, respectively. Then, precursor solution was stirred in the glove control groups, 1 wt%, 2 wt% and 3 wt% CsPbI QDs dispersions were added into the box for more than 4 h (500 rpm, 40 °C). Drops of 45 µL mixed perovskite precursor solu- precursor solution, respectively. Then, precursor solution was stirred in the glove box for tion were spin-coated at 4000 rpm on the ITO/PEDOT:PSS substrate for 25 s. After a delay more than 4 h (500 rpm, 40 C). Drops of 45 L mixed perovskite precursor solution were of 7 s, 200 µL of the chlorobenzene (CB) anti-solvent was dropped and spin-cast onto the spin-coated at 4000 rpm on the ITO/PEDOT:PSS substrate for 25 s. After a delay of 7 s, 200 precursor film. Afterward, the prepared films were dried at 110 °C for 20 min. The con- L of the chlorobenzene (CB) anti-solvent was dropped and spin-cast onto the precursor centration of PCBM solution was 20 mg/mL, the PCBM solution was spin-coated at 3000 film. Afterward, the prepared films were dried at 110 C for 20 min. The concentration of rpm on the perovskite film for 40 s, and then annealed at 120 °C for 10 min. The PCBM PCBM solution was 20 mg/mL, the PCBM solution was spin-coated at 3000 rpm on the layer in our experiment is about 60 nm. Subsequently, Bphen was deposited at a rate of 1 perovskite film for 40 s, and then annealed at 120 C for 10 min. The PCBM layer in our −1 1 Å s under high vacuum conditions. Followed by the deposition of Ag as anode at a dep- experiment is about 60 nm. Subsequently, Bphen was deposited at a rate of 1 Å s under −1 2 osition speed of 5 high vacuum conditions. Å s . TheFollowed active area of these P by the deposition SCs wof as Ag 0.02 cm as anode . at a deposition speed 1 2 of 5 Å s . The active area of these PSCs was 0.02 cm . Figure 1. (a) The device structure of PSCs; (b) cross-sectional image SEM of the device. (c) UV-vis Figure 1. absorption (a) T spectr he dev um iceand stru PL ctuspectr re of PSC um s of ; (CsPbI b) cross- QDs. sectional image SEM of the device. (c) UV-vis absorption spectrum and PL spectrum of CsPbI3 QDs. We used Shimazu UV1700 UV-visible absorption system to measure the UV-visible absorption spectra of CsPbI QDs and perovskite films. Scanning electron microscopy We used Shimazu UV1700 3 UV-visible absorption system to measure the UV-visible (SEM) (FEI Inspect F50) was used to measure the surface morphology of perovskite films. absorption spectra of CsPbI3 QDs and perovskite films. Scanning electron microscopy The distribution of the elements was detected by X-ray dispersive X-ray analysis (EDS). (SEM) (FEI Inspect F50) was used to measure the surface morphology of perovskite films. The crystal structure was characterized by X-ray diffraction (XRD) (D2 PHASER). Using The distribution of the elements was detected by X-ray dispersive X-ray analysis (EDS). a time-dependent single photometer system (FL-TCSPC, Horiba Jobin Yvon), stimulated The crystal structure was characterized by X-ray diffraction (XRD) (D2 PHASER). Using by a 550 nm picosecond pulse laser, photoluminescence spectra (PL) and time-resolved a time-dependent single photometer system (FL-TCSPC, Horiba Jobin Yvon), stimulated photoluminescence spectra (TRPL) were measured. An AM1.5G solar simulator was used by a 550 nm picosecond pulse laser, photoluminescence spectra (PL) and time-resolved as the light source, and the lighting power is 100 mW/cm . The current density voltage photoluminescence spectra (TRPL) were measured. An AM1.5G solar simulator was used (J-V) curves of the device under illumination were measured with a Keithley4200 semicon- as the light source, and the lighting power is 100 mW/cm . The current density voltage (J- ductor analyzer. External quantum efficiency (EQE) curves were obtained using xenon V) curves of the device under illumination were measured with a Keithley4200 semicon- lamps calibrated through a monochromator calibrated by a standard silicon solar cell. All ductor analyzer. External quantum efficiency (EQE) curves were obtained using xenon measurements were made at room temperature. Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 lamps calibrated through a monochromator calibrated by a standard silicon solar cell. All Photonics 2022, 9, 3 4 of 10 measurements were made at room temperature. 3. Results and Discussion 3. Results and Discussion 3.1. Characterization and Test 3.1. Characterization and Test In order to characterize the optical properties of CsPbI3 QDs, we measured the UV- In order to characterize the optical properties of CsPbI QDs, we measured the UV-Vis Vis spectrum and steady-state photoluminescence spectrum. As 3 shown in Figure 1a,c, a spectrum and steady-state photoluminescence spectrum. As shown in Figure 1a,c, a strong strong PL emission peak can be observed at 687 nm for CsPbI3 QDs, which is consistent PL emission peak can be observed at 687 nm for CsPbI QDs, which is consistent with with the absorption and emission wavelength range of CsPbI3 material system reported the absorption and emission wavelength range of CsPbI material system reported in in literature [20]. It is well known that the photoelectric properties 3 of quantum dot mate- literature [20]. It is well known that the photoelectric properties of quantum dot materials rials are related to their size and purity. We observed that the PL emission peak intensity are related to their size and purity. We observed that the PL emission peak intensity of of CsPbI3 QDs is symmetric, and the width of the half-wave peak is narrow (35 nm), which CsPbI QDs is symmetric, and the width of the half-wave peak is narrow (35 nm), which indicates that the CsPbI3 QDs are high in purity. CsPbI3 QDs with good uniformity are indicates that the CsPbI QDs are high in purity. CsPbI QDs with good uniformity are more 3 3 more easily dispersed in the process of mixing the active layer, which would play a ho- easily dispersed in the process of mixing the active layer, which would play a homogenized mogenized passivation effect on the defects in different positions to ensure the uniformity passivation effect on the defects in different positions to ensure the uniformity and flatness and flatness of active layer film. of active layer film. In order to prove that CsPbI3 QDs are uniformly mixed into the perovskite film, we In order to prove that CsPbI QDs are uniformly mixed into the perovskite film, we obtained the element distribution image by EDS Mapping, and the test results are shown obtained the element distribution image by EDS Mapping, and the test results are shown in Figure 2. As we can see, Cs elements representing the characteristic elements of CsPbI3 in Figure 2. As we can see, Cs elements representing the characteristic elements of CsPbI QDs were distributed in the whole test area, indicating that CsPbI3 QDs were successfully QDs were distributed in the whole test area, indicating that CsPbI QDs were successfully incorporated into the film of MAPbI3 active layer. In Figure 2b, the intensity distribution incorporated into the film of MAPbI active layer. In Figure 2b, the intensity distribution of Cs elements in the whole test area has good evenness, indicating that CsPbI3 QDs are of Cs elements in the whole test area has good evenness, indicating that CsPbI QDs are evenly distributed in the perovskite active layer, which is conducive to the formation of evenly distributed in the perovskite active layer, which is conducive to the formation of uniform and stable perovskite film. In addition, the incorporation of Cs element intro- uniform and stable perovskite film. In addition, the incorporation of Cs element introduces duces metallic inorganic cations into the active layer of perovskite to achieve the effect of metallic inorganic cations into the active layer of perovskite to achieve the effect of cation cation passivation, and further improve the morphology of the film. passivation, and further improve the morphology of the film. Figure 2. EDS elemental mapping images of (a) all elements; (b) Cs; (c) Pb; (d) I in the MAPbI film Figure 2. EDS elemental mapping images of (a) all elements; (b) Cs; (c) Pb; (d) I in the MAPbI3 film doped with CsPbI QDs. doped with CsPbI3 QDs In order to more directly study the influence of CsPbI QDs on the morphology of In order to more directly study the influence of CsPbI3 QDs on the morphology of perovskite films, we tested the surface morphology of perovskite active layer films doped perovskite films, we tested the surface morphology of perovskite active layer films doped with different proportions of CsPbI QDs by SEM. As shown in Figure 3, the surface morphologies of perovskite films doped with different proportions of CsPbI QDs are obviously different. The perovskite film without CsPbI QDs has small grains and poor crystallinity. With the moderate addition of CsPbI QDs, the grain size of perovskite film 3 Photonics 2022, 9, x FOR PEER REVIEW 5 of 11 with different proportions of CsPbI3 QDs by SEM. As shown in Figure 3, the surface mor- phologies of perovskite films doped with different proportions of CsPbI3 QDs are obvi- ously different. The perovskite film without CsPbI3 QDs has small grains and poor crys- Photonics 2022, 9, 3 5 of 10 tallinity. With the moderate addition of CsPbI3 QDs, the grain size of perovskite film grad- ually becomes larger. We have made a quantitative size distribution in Figure 3. For per- ovskite films doped with 0, 1 wt%, 2 wt%, 3 wt%, their average grain diameter is 247.25 nm, 28 gradually 1.22 nm, 35 becomes 2.87 nm lar, 27 ger. 4.90 We nm have respecti madeve a quantitative ly. This is beca size usedistribution as the nuclein atiFigur on cent e 3 er, . For CsPbI per 3ovskite QDs can promote t films doped he crystal with 0, 1 gro wt%, wth during 2 wt%, t 3he perovski wt%, theirte crysta average lliza grain tion process diameter is and i 247.25 nducnm, e the 281.22 forma nm, tion 352.87 of thre nm, e-di274.90 mensio nm nal rperov espectively skite gra . This ins iswi because th large assize. On the the nucleation other hand, t center, CsPbI he doping of QDs can CsPbI promote 3 QDs introduces the crystal growth additi during onal metal the per caovskite tionic halogen a crystallization ni- ons, whi process ch not onl and induce y regula the formation tes the elem of ent propor three-dimensional tion of the per acovskit tive lae yer, but grains with also passi largevsize. - On the other hand, the doping of CsPbI QDs introduces additional metal cationic halogen ates some ion vacancy defects and surface grain boundaries, thus improving the film qual- anions, which not only regulates the element proportion of the active layer, but also ity of the active layer. However, with excessive incorporation of CsPbI3 QDs, as shown in passivates some ion vacancy defects and surface grain boundaries, thus improving the film Figure 3d, the film morphology of the active layer doped with 3% CsPbI3 QDs is not as quality of the active layer. However, with excessive incorporation of CsPbI QDs, as shown good as that of the active layer doped with 2% CsPbI3 QDs in Figure 3c. This is because 3 in Figure 3d, the film morphology of the active layer doped with 3% CsPbI QDs is not as excessive doping will lead to the imbalance of element proportion and affect the crystal- 3 good as that of the active layer doped with 2% CsPbI QDs in Figure 3c. This is because linity of perovskite and the film morphology. In genera3l, the active layer of perovskite excessive doping will lead to the imbalance of element proportion and affect the crystallinity doped with 2% CsPbI3 QDs has the largest grain size, which can reach nearly micron level. of perovskite and the film morphology. In general, the active layer of perovskite doped with The smooth and dense perovskite film can form a closer interface contact with the 2% CsPbI QDs has the largest grain size, which can reach nearly micron level. The smooth transport layer, thus reducing the interface recombination of excitons and improving the and dense perovskite film can form a closer interface contact with the transport layer, thus extraction efficiency of photogenerated carriers [21]. In addition, the active perovskite reducing the interface recombination of excitons and improving the extraction efficiency layer with large grains is generally characterized by a low grain boundary density, which of photogenerated carriers [21]. In addition, the active perovskite layer with large grains reduces the density of defect states in the perovskite film and effectively reduces the is generally characterized by a low grain boundary density, which reduces the density of charge recombination in defects. The experimental results prove that the incorporation of defect states in the perovskite film and effectively reduces the charge recombination in CsPbI3 QDs can improve the morphology of the perovskite active layer, so as to prepare defects. The experimental results prove that the incorporation of CsPbI QDs can improve high-quality perovskite films and improve the photovoltaic performance of the devices the morphology of the perovskite active layer, so as to prepare high-quality perovskite potentially. films and improve the photovoltaic performance of the devices potentially. Figure 3. SEM images of the MAPbI3 films with different CsPbI3 concentrations: (a) 0, (b) 1 wt%, Figure 3. SEM images of the MAPbI3 films with different CsPbI3 concentrations: (a) 0, (b) 1 wt%, (c) (c) 2 wt%, (d) 3 wt%. 2 wt%, (d) 3 wt%. In order to further study the effect of CsPbI QDs on the photoelectric properties of per In order to further study ovskite films, the active the effect of C layer was characterized sPbI3 QDs on bythe photoelectric properties of spectra and crystallization tests. Figure 4a shows the UV-Vis absorption spectrum of the perovskite film with and without perovskite films, the active layer was characterized by spectra and crystallization tests. FigCsPbI ure 4a sho QDs. ws the It can UV be -Vis seen absorption that the s active pectrum layer of the perovski film doped with te filCsPbI m with QDs and wi has thout almost 3 3 the same absorption spectrum and absorption intensity compared with the film without CsPbI QDs, both of which only exhibit MAPbI characteristic absorption. This indicates 3 3 that the incorporation of CsPbI QDs does not change the light absorption capacity of the active layer. Then, we tested the XRD peak patterns of two kinds of perovskite films. As shown in Figure 4b, the XRD of two films both showed three main intense diffraction peaks at 2q = 14.4 , 28.8 and 31.8 , which represent the (110), (220), (310) planes, respectively, of Photonics 2022, 9, x FOR PEER REVIEW 6 of 11 CsPbI3 QDs. It can be seen that the active layer film doped with CsPbI3 QDs has almost the same absorption spectrum and absorption intensity compared with the film without CsPbI3 QDs, both of which only exhibit MAPbI3 characteristic absorption. This indicates that the incorporation of CsPbI3 QDs does not change the light absorption capacity of the active layer. Then, we tested the XRD peak patterns of two kinds of perovskite films. As shown in Figure 4b, the XRD of two films both showed three main intense diffraction Photonics 2022, 9, 3 6 of 10 peaks at 2θ = 14.4°, 28.8° and 31.8°, which represent the (110), (220), (310) planes, respec- tively, of the MAPbI3 perovskite crystalline structure [22,23]. This suggests that there is no alloy state of MAxCs1-xPbI3 in the active layer. However, compared with control film, the the MAPbI perovskite crystalline structure [22,23]. This suggests that there is no alloy state perovskite film doped with CsPbI3 QDs shows higher and sharper characteristic peaks in of MA Cs PbI in the active layer. However, compared with control film, the perovskite 1-x 3 all three diffraction angles. The intensity of the diffraction peak corresponds to the crys- film doped with CsPbI QDs shows higher and sharper characteristic peaks in all three tallinity of the film. In general, the strong XRD diffraction intensity indicates that the crys- diffraction angles. The intensity of the diffraction peak corresponds to the crystallinity of tallinity of this orientation is good, which is conducive to the formation of large and uni- the film. In general, the strong XRD diffraction intensity indicates that the crystallinity form grains [24]. The results of XRD show that the doping of CsPbI3 QDs can enhance the of this orientation is good, which is conducive to the formation of large and uniform crystallinity of perovskite films and obtain higher quality perovskite films to reduce exci- grains [24]. The results of XRD show that the doping of CsPbI QDs can enhance the ton recombination and enhancing carrier extraction, which is consistent with the results crystallinity of perovskite films and obtain higher quality perovskite films to reduce exciton of SEM. recombination and enhancing carrier extraction, which is consistent with the results of SEM. Figure 4. (a) UV-VIS absorption spectra; (b) XRD pattern; (c) steady-state photoluminescence spec- Figure 4. (a) UV-VIS absorption spectra; (b) XRD pattern; (c) steady-state photoluminescence spectra; tra; (d) time-resolved photoluminescence of perovskite films spectra without and with CsPbI3 QDs. (d) time-resolved photoluminescence of perovskite films spectra without and with CsPbI QDs. In order to investigate the effect of the introduction of CsPbI3 QDs on the energy In order to investigate the effect of the introduction of CsPbI QDs on the energy transfer and carrier recombination of PSCs, we measured the PL and TRPL spectra of two transfer and carrier recombination of PSCs, we measured the PL and TRPL spectra of perovskite films with the test structure of ITO/PEDOT: PSS/perovskite active layers. Fig- two perovskite films with the test structure of ITO/PEDOT: PSS/perovskite active layers. ure 4c shows the PL spectra of the two films. Both of them show the characteristic PL Figure 4c shows the PL spectra of the two films. Both of them show the characteristic PL spectrum of MAPbI3, which indicates that the introduction of CsPbI3 will not form alloy spectrum of MAPbI , which indicates that the introduction of CsPbI will not form alloy 3 3 state of MAxCs1−x PbI3 in the active layer. However, compared with the control device, the state of MA Cs PbI in the active layer. However, compared with the control device, the x 1x 3 PL peak of the CsPbI3 QDs-doped film shows a blue shift from 766 nm to 760 nm, indicat- PL peak of the CsPbI QDs-doped film shows a blue shift from 766 nm to 760 nm, indicating ing that the surface trap of the active layer is effectively passivated [25]. In addition, com- that the surface trap of the active layer is effectively passivated [25]. In addition, compared pared with the undoped film, the PL peak strength of the doped film shows an obvious with the undoped film, the PL peak strength of the doped film shows an obvious quenching quenching phenomenon, which indicates that the doped CsPbI3 QDs is conducive to the phenomenon, which indicates that the doped CsPbI QDs is conducive to the extraction of carriers from perovskite films. Figure 4d shows the time-resolved photoluminescence (TRPL) spectrum, which can be used to further study the charge transfer kinetics. According to the TRPL results, the rapid decay lifetime of the perovskite films doped with CsPbI QDs is significantly lower than that of the undoped films. Since reduced fast decay lifetimes indicate faster and efficient charge-carrier transfer at the interface, it can be concluded that perovskite doped CsPbI QDs has better interfacial properties and charge transfer capacity. 3 Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 extraction of carriers from perovskite films. Figure 4d shows the time-resolved photolu- minescence (TRPL) spectrum, which can be used to further study the charge transfer ki- netics. According to the TRPL results, the rapid decay lifetime of the perovskite films doped with CsPbI3 QDs is significantly lower than that of the undoped films. Since re- Photonics 2022, 9, 3 7 of 10 duced fast decay lifetimes indicate faster and efficient charge-carrier transfer at the inter- face, it can be concluded that perovskite doped CsPbI3 QDs has better interfacial proper- ties and charge transfer capacity. In order to further investigate the effect of CsPbI QDs on passivating defects, we In order to further investigate the effect of CsPbI3 Q 3Ds on passivating defects, we estimated the trap-state density of perovskite film by equation [26]: estimated the trap-state density of perovskite film by equation [26]: 2εε V 0 rTEL 2# # V n = 0 r TEL t 2 (1) n = (1) t qL qL where nt is the trap state density, VTFL is the trap-filled limit voltage, L is the thickness of where n is the trap state density, V is the trap-filled limit voltage, L is the thickness of t TFL the perovskite films (400 nm), q is the elementary charge, ε0 is the vacuum permittivity, εr the perovskite films (400 nm), q is the elementary charge, # is the vacuum permittivity, # is relative dielectric constant of MAPbI3 (6.5). As showed in Figure 5, the trap-filled limit is relative dielectric constant of MAPbI (6.5). As showed in Figure 5, the trap-filled limit voltages of 0.46 V and 0.24 V were measured in the electron-only devices. According to voltages of 0.46 V and 0.24 V were measured in the electron-only devices. According to the the equation, the trap state density of two perovskite films can be calculated to be 2.73 × equation, the trap state density of two perovskite films can be calculated to be 2.73  10 15 15 −3 10 and 1.42 × 10 cm , respectively, indicating that some defects have been passivated 15 3 and 1.42  10 cm , respectively, indicating that some defects have been passivated by by introducing CsPbI3 QDs. The reduced trap density is related to the improved quality introducing CsPbI QDs. The reduced trap density is related to the improved quality of of the perovskite film, which is beneficial for device stability and carrier dynamics. It can the perovskite film, which is beneficial for device stability and carrier dynamics. It can be be mutually confirmed with XRD, PL, and TRPL results. mutually confirmed with XRD, PL, and TRPL results. Figure 5. J-V characteristics under dark conditions of electron-only devices containing the perovskite film (a) without and (b) with CsPbI QDs. Figure 5. J-V characteristics under dark conditions of electron-only devices containing the perov- skite film (a) without and (b) with CsPbI3 QDs. 3.2. Performance of PSCs In order to further study the effect of CsPbI QDs on the performance of perovskite 3.2. Performance of PSCs devices, we prepared a series of perovskite solar cells using perovskite thin films with In order to further study the effect of CsPbI3 QDs on the performance of perovskite different doping ratios of CsPbI QDs as the active layer, and tested their photovoltaic devices, we prepared a series of perovskite solar cells using perovskite thin films with performance. Besides, in order to ensure the repeatability of our devices, 20 devices different doping ratios of CsPbI3 QDs as the active layer, and tested their photovoltaic were fabricated and characterized at each ratio. The distribution of their performance performance. Besides, in order to ensure the repeatability of our devices, 20 devices were parameters is showed in Figure 6. And Table 1 is the average performance statistics of fabricated and characterized at each ratio. The distribution of their performance parame- these devices. The results show that CsPbI QDs doped in the active layer has a great ters is showed in Figure 6. And Table 1 is the average performance statistics of these de- influence on the performance of PSCs, changing the short-circuit current, open-circuit vices. The results show that CsPbI3 QDs doped in the active layer has a great influence on voltage and filling factor of the devices. The PSCs doped with 2% CsPbI QDs showed the the performance of PSCs, changing the short-circuit current, open-circuit voltage and fill- best performance, with the short-circuit current, open-circuit voltage, fill factor and energy ing factor of the devices. The PSCs doped with 2% 2 CsPbI3 QDs showed the best perfor- conversion efficiency reaching 22.27 mA cm , 0.961 V, 79.63% and 17.04%, respectively. mance, with the short-circuit current, open-circuit voltage, fill factor and energy conver- The short-circuit current, open-circuit voltage and fill factor of PSCs without CsPbI QDs are −2 sion efficiency re 2aching 22.27 mA cm , 0.961 V, 79.63% and 17.04%, respectively. The 21.32 mA cm , 0.934 V, 74.59% and 14.85%, respectively. In contrast, the doping of CsPbI short-circuit current, open-circuit voltage and fill factor of PSCs without CsPbI3 QDs are QDs significantly increased short-circuit current, open-circuit voltage, fill factor, resulting in −2 21.32 mA cm , 0.934 V, 74.59% and 14.85%, respectively. In contrast, the doping of CsPbI3 a 15% increase in PCE. The higher performance of the device with CsPbI QDs is due to the QDs significantly increased short-circuit current, open-circuit voltage, fill factor, resulting better quality of the perovskite active layer film, which reduces exciton recombination and improves carrier transfer efficiency, which is consistent with the previous characterization conclusion of the films. The passivated perovskite film also formed the better interface contact with both hole transport layer and electron transport layer, which is conducive to the improvement of open-circuit voltage to improve device performance [27,28]. Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 in a 15% increase in PCE. The higher performance of the device with CsPbI3 QDs is due to the better quality of the perovskite active layer film, which reduces exciton recombination and improves carrier transfer efficiency, which is consistent with the previous characteri- zation conclusion of the films. The passivated perovskite film also formed the better in- Photonics 2022, 9, 3 8 of 10 terface contact with both hole transport layer and electron transport layer, which is con- ducive to the improvement of open-circuit voltage to improve device performance [27,28]. Figure 6. Distribution of the (a) J , (b) V , (c) FF, and (d) PCE of the devices based on perovskite SC OC Figure 6. Distribution of the (a) JSC, (b) VOC, (c) FF, and (d) PCE of the devices based on perovskite films with different CsPbI3 QDs concentrations. films with different CsPbI3 QDs concentrations. Table 1. Photovoltaic parameters of PSCs. Table 1. Photovoltaic parameters of PSCs. Devices Jsc (mA/cm ) Voc (V) FF (%) PCE (%) Devices Jsc (mA/cm ) Voc (V) FF (%) PCE (%) Control 21.32  0.78 0.934  0.005 74.59  2.01 14.85  0.53 Control 21.32 ± 0.78 0.934 ± 0.005 74.59 ± 2.01 14.85 ± 0.53 1 wt% 21.46  0.58 0.942  0.003 76.17  2.07 15.39  0.19 1 wt% 21.46 ± 0.58 0.942 ± 0.003 76.17 ± 2.07 15.39 ± 0.19 2 wt% 22.27  0.63 0.961  0.003 79.63  1.29 17.04  0.33 2 wt 3 wt% % 19.73 22.27 ± 0.  0.46 63 0.953 0.961 ± 0. 0.002003 74.11 79.63 ± 1.  2.52 29 17 13.93 .04 ± 0.  0.29 33 3 wt% 19.73 ± 0.46 0.953 ± 0.002 74.11 ± 2.52 13.93 ± 0.29 At the same time, we also showed J-V characteristics and EQE curves of PSCs with At the same time, we also showed J-V characteristics and EQE curves of PSCs with different doping ratios of CsPbI QDs, and the test results are shown in Figure 7a,b. It can different doping ratios of CsPbI3 QDs, and the test results are shown in Figure 7a,b. It can be seen that, within the wavelength range of the response of the device, the EQE curves be seen that, within the wavelength range of the response of the device, the EQE curves of the PSCs doped with 2 wt% CsPbI QDs is higher than those of other devices, which is of the PSCs doped with 2 wt% CsPbI3 QDs is higher than those of other devices, which is consistent with the J variation trend of the J-V curve. The value of the integrated current SC consistent with the JSC variation trend of the J-V curve. The value of the integrated current obtained from the EQE curves is approximately the same as the measured current. obtained from the EQE curves is approximately the same as the measured current. Next, we tested the PCE output stability of optimized and control devices. As shown in Figure 7d, we applied a working voltage of 0.85 V to both devices under the simulated sunlight of AM 1.5 g with a light intensity of 100 mW cm , and recorded the time function curve of the photocurrent density of PSCs. As can be seen from the Figure 7c, the output peak of current density of both devices reached rapidly after the beginning of illumination. After 60 s continuous illumination, the output value of current density and steady-state 2 2 PCE of two devices reached 22.15 mA cm , 16.91% and 21.12 mA cm , 14.68%. It is basically consistent with the photocurrent density and PCE value in J-V test. We also compare the hysteresis of the two devices in Figure 7d,e. It can be seen that the hysteresis of the optimized device under forward scanning and reverse scanning is much weaker than that of the control device, which is due to the passivated defects and fewer trap states weaken the hysteresis effectively [29,30]. Finally, we studied the environmental stability of the device. The stability test of the unencapsulated device was carried out for 7 days at room temperature and atmospheric conditions. As can be seen from the Figure 7f, after 7 days of testing, the efficiency of the device without CsPbI QDs has decreased to 17% of the original efficiency, but the efficiency of the device with doping CsPbI QDs still remains more than 50% of the origin, which shows more excellent environmental stability. Photonics 2022, 9, x FOR PEER REVIEW 9 of 11 Photonics 2022, 9, 3 9 of 10 Figure 7. (a) J-V characteristics of PSCs; (b) EQE and corresponding integrated J of PSCs; SC Figure 7. (a) J-V characteristics of PSCs; (b) EQE and corresponding integrated JSC of PSCs; (c) The (c) The steady-state power output of PSCs; (d) J-V curve hysteresis of the control device; (e) J-V curve steady-state power output of PSCs; (d) J-V curve hysteresis of the control device; (e) J-V curve hys- hysteresis of the optimized device; (f) Long time stability of PSCs. teresis of the optimized device; (f) Long time stability of PSCs. 4. Conclusions Next, we tested the PCE output stability of optimized and control devices. As shown In summary, we demonstrate a defect passivation strategy by introducing CsPbI QDs in Figure 7d, we applied a working voltage of 0.85 V to both devices under the simulated into the perovskite active layer. Through a series of film characterization and device tests, −2 sunlight of AM 1.5 g with a light intensity of 100 mW cm , and recorded the time function we prove that the introduction of CsPbI QDs can effectively improve the crystallization of curve of the photocurrent density of PSCs. As can be seen from the Figure 7c, the output the perovskite active layer, passivate the defects, and thus reduce the exciton recombination peak of current density of both devices reached rapidly after the beginning of illumina- and improve the carrier transfer ability. The device doped with CsPbI QDs has higher short tion. After 60 s continuous illumination, the output value of current density and steady- circuit current, open circuit voltage and fill factor, and achieved a significant improvement −2 −2 state PCE of two devices reached 22.15 mA cm , 16.91% and 21.12 mA cm , 14.68%. It is in power conversion efficiency from 14.85% to 17.04%. In addition, the optimized devices basically consistent with the photocurrent density and PCE value in J-V test. We also com- also have better environmental stability. This work provides a novel way to prepare high- pare the hysteresis of the two devices in Figure 7d,e. It can be seen that the hysteresis of quality perovskite films, which may be of great value for the construction and preparation the optimized device under forward scanning and reverse scanning is much weaker than of efficient perovskite photoelectronic devices. that of the control device, which is due to the passivated defects and fewer trap states Author weaken Contributions: the hysteresis Conceptualization, effectively [29,30G.Y ]. . and D.Z.; methodology, G.Y.; software, J.L.; vali- dation, Fina G.Yll . and y, we J.Y st .; udie formal d tanalysis, he environment G.Y.; investigation, al stabilityG.Y of the .; reso device. The urces, G.Y.; st data ability curation, test oG.Y f the .; writing—original draft preparation, G.Y.; writing—review and editing, D.Z. and J.Y.; visualization, unencapsulated device was carried out for 7 days at room temperature and atmospheric G.Y.; supervision, J.Y.; project administration, J.Y.; funding acquisition, J.Y. All authors have read and conditions. As can be seen from the Figure 7f, after 7 days of testing, the efficiency of the agreed to the published version of the manuscript. device without CsPbI3 QDs has decreased to 17% of the original efficiency, but the effi- ciency of the device with doping CsPbI3 QDs still remains more than 50% of the origin, Funding: This work was financially supported by the Foundation of National Natural Science which shows more excellent environmental stability. Foundation of China (NSFC) (Grant Nos.61421002, 61675041, and 51703019) and the Sichuan Science and Technology Program (2019YFG0121, 2019YJ0178, 2020YFG0279, and 2020YFG0281), Sichuan Youth Software Innovation Project Funding Project Contract (Grant Nos. 2021073, 2021107). This 4. Conclusions work is also sponsored by the Sichuan Province Key Laboratory of Display Science and Technology. In summary, we demonstrate a defect passivation strategy by introducing CsPbI3 Institutional Review Board Statement: Not applicable. QDs into the perovskite active layer. Through a series of film characterization and device tests, we prove that the introduction of CsPbI3 QDs can effectively improve the crystalli- Informed Consent Statement: Not applicable. zation of the perovskite active layer, passivate the defects, and thus reduce the exciton Data Availability Statement: The data presented in this study are available on request from the recombination and improve the carrier transfer ability. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Dec 22, 2021

Keywords: perovskite solar cells; CsPbI3 quantum dots; passivation; active layer

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