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Ambient fabrication of perovskite solar cells through delay-deposition technique

Ambient fabrication of perovskite solar cells through delay-deposition technique The establishment of perovskite solar cells (PSCs) in terms of their power-conversion efficiency (PCE) over silicon-based solar cells is undeniable. The state-of-art of easy device fabrications of PSCs has enabled them to rapidly gain a place in third-generation photovoltaic technology. Numerous obstacles remain to be addressed in device efficiency and stability. Low performance owing to easily degraded surface and deterioration of perovskite film quality resulting from humidity are issues that often arise. This work explored a new approach to producing high-quality perovskite films prepared under high relative humidity (RH = 40%–50%). In particular, the ubiquitous 4-tert-butylpyridine (tBp) was introduced into lead iodide (PbI ) precursor as an additive, and the films were fabricated using a two-step deposition method followed by a delay-deposition technique of methylammonium iodide (MAI). High crystallinity and controlled nucleation of MAI were needed, and this approach revealed the significance of time control to ensure high-quality films with large grain size, high crystallography, wide coverage on substrate, and precise and evenly coupled MAI molecules to PbI films. Compared with the two-step method without time delay, a noticeable improvement in PCE from 3.2 to 8.3% was achieved for the sample prepared with 15 s time delay. This finding was primarily due to the significant enhancement in the open-circuit voltage, short-circuit cur - rent, and fill factor of the device. This strategy can effectively improve the morphology and crystallinity of perovskite films, as well as reduce the recombination of photogenerated carriers and increase of current density of devices, thereby achieving improved photovoltaic performance. Keywords 4-tert-Butylpyridine · High relative humidity · Two-step · Delay deposition · Perovskite solar cells Introduction particular, the certified power-conversion efficiency (PCE) reached in the year 2020 (25.2%) outperformed that of multi- The development of perovskite solar cells (PSCs) based crystalline silicon solar cells (23.3%) [6]. To date, solution on hybrid organic–inorganic halide as absorber material processing is the most adopted approach for PSC fabrication yields remarkable properties, including low-temperature because of the facile procedures, low-temperature require- solution processability, high light absorption coefficient, ment, can be produced in the laboratory, and low-cost fab- tunability, and ambipolar carrier-diffusion properties [1 –5]. rications [7–11]. Owing to these characteristics, the PSC fabrication methods Two methods that are frequently used by researchers used to achieve desired outcomes have been diversified. In across the region are one- and two-step (TS) deposition method. For the one-step method, lead iodide (PbI ) and methylammonium iodide (MAI) solution are mixed to form * Norasikin Ahmad Ludin perovskite films. Both organic and inorganic precursors are sheekeen@ukm.edu.my required to be dissolved before deposition onto the substrate. Solar Energy Research Institute, University Kebangsaan However, the capability of controlling the film properties Malaysia, Selangor, Malaysia including thickness, surface roughness, and uniformity of School of Engineering, UOW Malaysia KDU, Selangor, grain size are reduced owing to uncontrolled precipitation in Malaysia one-step deposition, thereby leading to inconsistent photo- Fuel Cell Institute, University Kebangsaan Malaysia, voltaic performance [12, 13]. Owing to these restraints, the Selangor, Malaysia Vol.:(0123456789) 1 3 11 Page 2 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 TS method has been introduced to overcome the drawbacks under ambient relative humidity (RH = 40%–50%). We of the one-step method. In the TS method, PbI solution investigated the effect of MAI deposition with a time delay in dimethylformamide (DMF) was spin coated onto TiO from 0 to 15 s. The samples were denoted as TS-0s, DDTS- films followed by the dipping of MAI solution in 2-propanol 5s, DDTS-10s, and DDTS-15s, which indicated the method to form high-quality perovskite films for PSCs [14]. Many name (i.e. delay deposition (DDTS) or TS) and the time studies have implemented the TS method to increase the delay. By altering the deposition technique of MAI precur- PCE of PSCs [12, 15–17]. Apart from deposition, several sor, perovskite film crystallisation seemed to significantly strategies have been incorporated for controlling morpho- improve as the interfacial reaction with PbI proceeded. The logical properties, such as vacuum flash-assisted solution smooth and pinhole-free perovskite film resulted in 8.3% processes [18], moisture mechanism control [15, 16], and PCE for the DDTS technique. mixed precursors [19]. Despite all these brilliant innovative technologies, the performance of PSCs is relatively vulner- able to humidity, thereby restricting the fabrication process Experimental section of such solar cells [20]. To address this issue, feasible meth- ods of fabricating PSCs in ambient atmosphere are highly Materials desirable to develop. −1 In achieving very dense and smooth perovskite morphol- Fluorine-doped tin oxide (FTO) glass (15  Ω sq ) was ogy, the crucial part is to regulate the nucleation and crys- purchased from Solaronix. TiO blocking layer and paste tallisation kinetics during film formation to obtain perovs- (18NR-T) were purchased from Dyesol. PbI (99%), kite films with high efficiency and stability [21]. To control MAI, lithium bis(trifluoromethanesulfonyl)imide (Li- perovskite crystallisation, previous studies have developed TFSI; 99%), tBp (96%), and 2,2′,7,7′-tetrakis-(N,N-di- various methods, such as solvent engineering [22], composi- p-methoxyphenylamine)-9,9′-spirobifluorene (spiro- tion re-engineering [3], introducing additional additive, and OMeTAD; 99%) were purchased from Sigma–Aldrich. others [7, 15, 23, 24]. Amongst these methods, the easiest Absolute ethanol, N,N-dimethylformamide anhydrous and most effective way to achieve the specific target is by (DMF; 99.8%), and chlorobenzene (99%) were purchased adding additives to the precursor perovskite solution [25]. from R&M Chemicals. Before using dimethyl sulfoxide (DMSO) as an additive, H O was added to perovskite precursor to form an inter- PSC fabrication 2+ mediate stabiliser with Pb ion to grow high-quality per- ovskite films with high photovoltaic performance [22, 23]. The FTO substrates were etched with zinc powder and HCl Meanwhile, 1,8-diiodooctane additives have been proven and then cleaned using ethanol, acetone, and isopropanol to successfully expedite the nucleation of crystals and to in an ultrasonic bath for 15 min each. The substrate was modulate the kinetics of crystal growth during crystallisa- rinsed with deionised water and dried under nitrogen flow. tion, leading to more uniform perovskite morphology [5]. A compact TiO blocking layer (bl-TiO ) was deposited onto 2 2 The use of 4-tert-butylpyridine (tBp) as an additive is not the precleared FTO by spin coating (3000 rpm, 30 s) 1 mL new, and numerous studies have shown that this additive of titanium isopropoxide in 1 mL of ethanol. A mesoporous can improve the PCE and photovoltaic performance of PSCs TiO layer (mp-TiO ) was diluted with absolute ethanol at 2 2 device. The tBp is frequently used in the hole-transport layer 1:9 ratio and coated onto FTO/bl-TiO substrate by spin (HTL) of PSC owing to its ability to control the uniformity coating (4000 rpm for 20 s) [29]. The substrates were sin- of HTL by avoiding the aggregation of lithium salt [26]. tered on a hotplate at 450 °C and 30 min). For the precursor The chemical bond formed between tBp and perovskite crys- solution preparation, 553 mg of PbI was mixed in 1 mL of tals promotes the ability of the interface to become more DMF and 100 μL of tBp followed by 30 mg of C H NH I 3 3 selective for holes, thereby boosting diode rectification [27]. in 1 mL of IPA. Both solutions were heated at 60 °C for Similarly, the use of tBp in the perovskite layer improves the 3 h under moderate stirring. Then, 70 μL of PbI precur- final product’s performance. Besides, tBp also plays a sig- sor solution was spin coated on the mp-TiO /bl-TiO /FTO 2 2 nificant role in perovskite crystallisation as high grain size substrate at 3000 rpm for 60 s before heating at 70 °C for and pinhole-free perovskite surface are reportedly achieved 30 min. About 200 μL of MAI precursor solution was depos- after tBp treatment [28]. ited onto the substrate then spin coated at 3000 rpm for 20 s In the current work, tBp was added into PbI precursor, before heating at 95 °C for 30 min. For the TS method, the replacing DMSO to enhance the photovoltaic performance MAI precursor solution was dropped before spin program of PSCs. Through this route, the addition of tBp into DMF meanwhile 5, 10, and 15 s before the spin program ended as an additive enabled the formation of a porous layer packed for the DDTS technique. The HTL was prepared by mix- PbI nanocrystal, and the fabrication process was conducted ing 1 mL of spiro-OMeTAD solution (72.3 mg in 1 mL of 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 3 of 8 11 chlorobenzene), 17.5 µL of Li-TFSI solution (520 mg of to give 100 mW/cm using a standard Si photovoltaic cell Li-TFSI in 1 mL of acetonitrile), and 28.8 µL of tBp. After (Daystar Meter). J–V curves were recorded with a Keithley cooling, 50 µL of spiro-OMETAD was dropped onto perovs- 2400 source metre at 0.1  V/s scan rate. Steady-state PL kite/mp-TiO /bl-TiO /FTO substrate followed by spin coat- spectra were obtained using a fluorescence spectrometer 2 2 ing at 4000 rpm for 20 s. Finally, the device was completed (FLS920, Edinburgh Instruments) at an excitation wave- with silver (Ag) top electrode deposited by thermal evapora- length of 515 nm. All procedures were performed in ambi- tion. The active area of the cell was 0.07 cm . ent air without any humidity control. Characterisation Result and discussion The surface morphology and cross-sectional images of the samples were observed by field-emission scanning electron Figure 1a shows the device schematic of a PSC, and Fig. 1b microscopy (FESEM; ZEISS, Merlin Compact) and Nano- shows a cross-sectional FESEM image of the as-prepared surf Easyscan2 atomic force microscopy (AFM). XRD PSCs with layer-by-layer intermixing. Two different proce- spectra were obtained using an X-ray diffractometer model dures were applied to deposit MAI and fabricate perovskite Bruker D8 advance operated at 2θ angle. Optical absorp- thin films, as shown in Fig.  2a. Solar cells were fabricated tion spectra were recorded using a Lambda 35 Perkin Elmer under RH = 40%–50%. The effects of the delay-deposition UV–visible (UV–Vis) spectrophotometer. Solar simulated procedure of MAI on film morphology, crystal growth, and AM 1.5G sunlight was used with a solar simulator calibrated photovoltaic performance were systematically observed. Fig. 1 a Device schematic of perovskite solar cell, and b cross-section FESEM of perovskite solar cells Fig. 2 a Schematic of conven- tional two-step method and delay-deposition procedure of MAI for perovskite film fabrica- tion, and b photographs of PbI , TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s films 1 3 11 Page 4 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 Figure 2b shows the photographs of PbI , TS-0s, DDTS-5s, was shiny. This finding was ascribed to the smoother film DDTS-10s, and DDTS-15s films. The as-prepared PbI film owing to the fast transformation to perovskite crystal [31]. was pale yellow and became shiny dark yellow after 30 min Many reports have proven that the TS deposition method of thermal annealing. The surface was smooth and well crys- results in high quality and uniform surface morphology, tallised [30]. After TS and DDTS procedures, the perovskite but it critically depends on the handling of PbI film and films became dark brown. However, TS-0s film seemed to conversion process to MAPbI (during MAI intercalation) be dimmer, whereas DDTS-5s, DDTS-10s, and DDTS-15s [17]. Figure  3a–d shows the surface topography and 3D films were mirror-like and dense. The TS-0s film used stand- view of the samples through AFM. The root-mean-square ard procedure to deposit MAI where the MAI precursor was (RMS) roughness values of TS-0s, DDTS-5s, DDTS-10s, deposited onto the PbI layer substrate and spin coated [14]. and DDTS-15s were 29.92, 14.75, 13.21, and 10.42 nm, We observed the film turn into a dim dark brown, indicat- respectively. These values eventually decreased, as shown ing the transformation of PbI to perovskite. This dimmer in Fig. 3(i). The surface roughness of TS-0s showed a higher or blurred surface was caused by the light-scattering effect, RMS value, which was correlated with the aforementioned which stemmed from the rough surface of the perovskite dim and blurred surface of the sample. A smoother surface layer. The surface of DDTS-5s and DDTS-10s films looked was observed when MAI precursor was deposited onto the brighter brown, and DDTS-15s showed a lighter colour and PbI layer substrate 5 s before the spinning program ended Fig. 3 a–d AFM images of TS-0s, DDTS-5s, DDTS-10s, and DDTS- ovskite films prepared with regulated time delay (from 0 to 15 s). (j) 15s, respectively. e–h FESEM surface images of TS-0s, DDTS-5s, XRD patterns of perovskite films with regulated time delay (from 0 DDTS-10s, and DDTS-15s, respectively. (i) RMS roughness of per- to 15 s) 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 5 of 8 11 (i.e. DDTS-15s). The topographical information obtained from FESEM images confirmed the morphology of the samples in Fig. 3(e–h). The compact and full-coverage per- ovskite film comprised packed and small grain sizes using TS methods (i.e. TS-0s), and a few pinholes appear which anticipated increasing leakage current and recombination losses in the device. Meanwhile, the grain boundaries were much clearer for DDTS samples. Pinholes and uneven grains can be observed for DDTS-5s and DDTS-10s film surfaces which can be attributed to the poorly crystallised perovskite structure. Moreover, the DDTS-15s film surface was dense and smooth. To investigate the crystal size further, Fig. 3(j) displays the X-ray diffraction (XRD) patterns of perovskite films grown with TS and DDTS techniques. The DDTS-10s and DDTS-15s samples had a consistent peak for PbI at Fig. 4 Absorbance spectra of perovskite films with a regulated time 2θ = 12.6°, which corresponded to the 001 facet. However, delay (from 0 to 15 s). The Tauc plot of each sample is plotted in the inset the intensity of the diffraction peak (001) for DDTS-MAPbI was quenched, suggesting that more amount of PbI was suc- cessfully transformed into perovskite upon using the delay deposition of MAI. The XRD pattern of TS-0s and DDTS- 5s did not show the same peak, revealing that PbI residues intensity for wavelengths over 800 nm, which was attrib- cannot be converted into perovskite after MAI deposition uted to the light-scattering effect caused by the poor sur - through the standard procedure and short time delay. Thus, face morphology as described earlier [14]. This finding we found that the 15 s delay deposition of MAI precursor was supported by the transmittance spectra, where lower was effective and sufficient to obtain high-quality perovs- transmittance was recorded for this sample. In other words, kite. Furthermore, tBp treatment played a significant role in most photons with longer wavelengths were not transmit- the crystallisation [28, 32]. Most characteristic peaks cor- ted properly but were scattered and reflected by the film. responded well to MAPbI (at 2θ = 14.45° and 28.68°) for all However, the observed light absorption in this region can samples. The crystallinity of the samples is very important be expected not to contribute to the formation of elec- to evaluate using the full width at half-maximum (FWHM) tron–hole pairs owing to the bandgap [35]. DDTS-15s of the characteristic diffraction peaks, and smaller FWHM film exhibited the lowest light absorption compared with of diffraction peak is known to correspond to higher crys- DDTS-5s and DDTS-10s films. Using the Tauc analysis tallinity. The crystal size varied from 53.6, 48.2, 45.1, and shown in the inset of Fig. 4, the estimated energy bandgaps 31.9 nm for TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s of TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s were 1.51, films, respectively, in agreement with previous findings [33]. 1.57, 1.58, and 1.59 eV, respectively. The bandgap was Some miscellaneous peaks were also detected, indicating quenched proportionally with different deposition times. that the residual CH NH I decomposed and the product was The steady-state photoluminescence (PL) spectra were 3 3 I , as shown in the following reactions [34]: obtained to further validate this phenomenon, and results are shown in Fig.  5. High-quality perovskite films are CH NH I (aq) ⇋ CH NH + HI(aq) 3 3 3 2(aq) (1) commonly related to high PL efficiency, which reduces non-radiative recombination. From the PL peak exhib- 4HI(aq) + O (aq) ⇋ 2H O + 2I (2) ited centred at 770 nm, the peak intensity was strongly 2 2 suppressed for DDTS-15s film. This finding indicated hv that photo-excited electrons were collected efficiently at (3) 2HI(aq) ⟺ H + I 2 2 the interface. The slower charge extraction rate of TS-0s was expected and can be defined as non-radiative charge To investigate the mechanism for the better photoelec- recombination by defects or trapping holes in the film. Per - tric properties of perovskite film with different times and ovskite films were deposited onto m-TiO . Furthermore, deposition procedures of MAI, optical properties were for both cases, shifted peaks were observed owing to the examined. Figure 4 displays the absorption spectra of the usage of tBp in PbI precursor. PL intensity also decreased perovskite films with time delays obtained using UV–Vis with prolonged time delay from 0 to 15 s. These results spectroscopy. TS-0s film exhibited increased absorption confirmed the analyses of AFM and FESEM images. 1 3 11 Page 6 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 (RH = 70%–80%). Figure  6b depicts the statistics of the photovoltaic parameters of the respective devices. The external quantum efficiency (EQE) spectra of the samples were subsequently obtained, and results are shown in Fig.  7. The current produced was evaluated when the quantum efficiency was integrated over the entire electro - magnetic spectrum. IPCE intensity increased from 300 to 800 nm with prolonged time delay from 0 to 15 s. The cur- rent density calculated from EQE spectra was 9.37, 12.38, 13.9, and 16.96 mA/cm for TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s films, respectively, which were close to the photovoltaic performance obtained from LIV testing (Table  1). EQE can be measured based on the following formula: electron∕sec EQE = photon∕sec Fig. 5 Steady-state photoluminescence (PL) spectra of perovskite current∕(charge of 1 electron) films with a regulated time delay (from 0 to 15 s) (total power of photons)∕(energy of one photon) (4) In addition, the statistical photovoltaic parameters of all The photovoltaic performance of the cell device was samples are summarised in Table  1. The average current determined by the crystallinity and morphology of the density, J increased from 9.4 to 17.2 mA/cm with pro- sc resulting perovskite film. Incompatibility, defects, and pin- longed time delay from 0 to 15 s. The average open-circuit hole surface generally lead to increased trapping holes and voltage, V and Fill-factor, FF slightly increased from 0.87 oc high recombination losses. Figure 6a shows the photocur- to 0.98 V and 39–50%, respectively, upon changing the time rent density–voltage (J–V) curves of PSC device fabricated delay. Eventually, the enhancement in efficiency was primar - using different time delays for MAI deposition in the same ily attributed to the increase in J owing to the perfect trans- sc batch under 1 sun illumination. Five samples comprising formation of PbI into MAPbI , enlargement of MAPbI 2 3 3 ten devices were used to validate the reliability of this study. crystallinity, light absorption of perovskite, and improved They were prepared without any encapsulation in ambi- film morphologies of perovskite. The low V and FF val- oc ent air and under a RH of approximately RH = 40%–50% ues of TS-0s film were due to the non-uniformity of the and most likely the situation even worst during testing Fig. 6 a Photocurrent density–voltage (J–V) curves of PSCs using perovskite regulated time delay (from 0 to 15  s). b Average and standard deviation of photovoltaic performance parameters of at least 5 samples (10 devices) 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 7 of 8 11 when the RH was partially controlled (RH = 40%–50%). We suggest that further investigation is needed to increase the effectiveness of this new technique with proper device encapsulation. Acknowledgements This work was performed with the support of the Universiti Kebangsaan Malaysia Research Grant, specifically, the Dana Impak Perdana (DIP-2019-025) and Internal Grant UOW Malaysia KDU (KDURG-2017-1-004). Declarations Conflict of interest No conflicts of interest are declared. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, Fig. 7 External quantum efficiency (EQE) spectra using perovskite provide a link to the Creative Commons licence, and indicate if changes regulated time delay (from 0 to 15 s) were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in Table 1 Photovoltaic-performance parameters (short-circuit photo- the article’s Creative Commons licence and your intended use is not current, open-circuit photovoltage, fill factor, and power-conversion permitted by statutory regulation or exceeds the permitted use, you will efficiency) extracted from J–V measurements for devices under 1 sun need to obtain permission directly from the copyright holder. To view a −2 illumination (AM 1.5G, 100 mV  cm ) for 10 devices copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . Sample V (V) J (mA/cm ) FF (%) PCE (%) oc sc TS-0s 0.870 9.4 39.00 3.2 References DDTS-5s 0.956 12.7 44.50 5.4 DDTS-10s 0.978 14.2 44.76 6.2 1. Tanaka, K., Takahashi, T., Ban, T., Kondo, T., Uchida, K., Miura, DDTS-15s 0.980 17.2 50.00 8.3 N.: Comparative study on the excitons in lead-halide-based perovskite-type crystals CH3NH3PbBr 3 CH3NH3PbI3. Solid State Commun. 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Motion-dispensing as an effective strategy for preparing efficient Energy Mater. (2015). https:// doi. org/ 10. 1002/ aenm. 20150 0328 high-humidity processed perovskite solar cells. J. Alloys Compd. 23. Huang, F., Dkhissi, Y., Huang, W., Xiao, M., Benesperi, I., (2021). https:// doi. org/ 10. 1016/j. jallc om. 2020. 157320 Rubanov, S., Zhu, Y., Lin, X., Jiang, L., Zhou, Y., Gray-Weale, A., Etheridge, J., McNeill, C.R., Caruso, R.A., Bach, U., Spiccia, Publisher’s Note Springer Nature remains neutral with regard to L., Cheng, Y.B.: Gas-assisted preparation of lead iodide perovs- jurisdictional claims in published maps and institutional affiliations. kite films consisting of a monolayer of single crystalline grains 1 3 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials for Renewable and Sustainable Energy Springer Journals

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Abstract

The establishment of perovskite solar cells (PSCs) in terms of their power-conversion efficiency (PCE) over silicon-based solar cells is undeniable. The state-of-art of easy device fabrications of PSCs has enabled them to rapidly gain a place in third-generation photovoltaic technology. Numerous obstacles remain to be addressed in device efficiency and stability. Low performance owing to easily degraded surface and deterioration of perovskite film quality resulting from humidity are issues that often arise. This work explored a new approach to producing high-quality perovskite films prepared under high relative humidity (RH = 40%–50%). In particular, the ubiquitous 4-tert-butylpyridine (tBp) was introduced into lead iodide (PbI ) precursor as an additive, and the films were fabricated using a two-step deposition method followed by a delay-deposition technique of methylammonium iodide (MAI). High crystallinity and controlled nucleation of MAI were needed, and this approach revealed the significance of time control to ensure high-quality films with large grain size, high crystallography, wide coverage on substrate, and precise and evenly coupled MAI molecules to PbI films. Compared with the two-step method without time delay, a noticeable improvement in PCE from 3.2 to 8.3% was achieved for the sample prepared with 15 s time delay. This finding was primarily due to the significant enhancement in the open-circuit voltage, short-circuit cur - rent, and fill factor of the device. This strategy can effectively improve the morphology and crystallinity of perovskite films, as well as reduce the recombination of photogenerated carriers and increase of current density of devices, thereby achieving improved photovoltaic performance. Keywords 4-tert-Butylpyridine · High relative humidity · Two-step · Delay deposition · Perovskite solar cells Introduction particular, the certified power-conversion efficiency (PCE) reached in the year 2020 (25.2%) outperformed that of multi- The development of perovskite solar cells (PSCs) based crystalline silicon solar cells (23.3%) [6]. To date, solution on hybrid organic–inorganic halide as absorber material processing is the most adopted approach for PSC fabrication yields remarkable properties, including low-temperature because of the facile procedures, low-temperature require- solution processability, high light absorption coefficient, ment, can be produced in the laboratory, and low-cost fab- tunability, and ambipolar carrier-diffusion properties [1 –5]. rications [7–11]. Owing to these characteristics, the PSC fabrication methods Two methods that are frequently used by researchers used to achieve desired outcomes have been diversified. In across the region are one- and two-step (TS) deposition method. For the one-step method, lead iodide (PbI ) and methylammonium iodide (MAI) solution are mixed to form * Norasikin Ahmad Ludin perovskite films. Both organic and inorganic precursors are sheekeen@ukm.edu.my required to be dissolved before deposition onto the substrate. Solar Energy Research Institute, University Kebangsaan However, the capability of controlling the film properties Malaysia, Selangor, Malaysia including thickness, surface roughness, and uniformity of School of Engineering, UOW Malaysia KDU, Selangor, grain size are reduced owing to uncontrolled precipitation in Malaysia one-step deposition, thereby leading to inconsistent photo- Fuel Cell Institute, University Kebangsaan Malaysia, voltaic performance [12, 13]. Owing to these restraints, the Selangor, Malaysia Vol.:(0123456789) 1 3 11 Page 2 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 TS method has been introduced to overcome the drawbacks under ambient relative humidity (RH = 40%–50%). We of the one-step method. In the TS method, PbI solution investigated the effect of MAI deposition with a time delay in dimethylformamide (DMF) was spin coated onto TiO from 0 to 15 s. The samples were denoted as TS-0s, DDTS- films followed by the dipping of MAI solution in 2-propanol 5s, DDTS-10s, and DDTS-15s, which indicated the method to form high-quality perovskite films for PSCs [14]. Many name (i.e. delay deposition (DDTS) or TS) and the time studies have implemented the TS method to increase the delay. By altering the deposition technique of MAI precur- PCE of PSCs [12, 15–17]. Apart from deposition, several sor, perovskite film crystallisation seemed to significantly strategies have been incorporated for controlling morpho- improve as the interfacial reaction with PbI proceeded. The logical properties, such as vacuum flash-assisted solution smooth and pinhole-free perovskite film resulted in 8.3% processes [18], moisture mechanism control [15, 16], and PCE for the DDTS technique. mixed precursors [19]. Despite all these brilliant innovative technologies, the performance of PSCs is relatively vulner- able to humidity, thereby restricting the fabrication process Experimental section of such solar cells [20]. To address this issue, feasible meth- ods of fabricating PSCs in ambient atmosphere are highly Materials desirable to develop. −1 In achieving very dense and smooth perovskite morphol- Fluorine-doped tin oxide (FTO) glass (15  Ω sq ) was ogy, the crucial part is to regulate the nucleation and crys- purchased from Solaronix. TiO blocking layer and paste tallisation kinetics during film formation to obtain perovs- (18NR-T) were purchased from Dyesol. PbI (99%), kite films with high efficiency and stability [21]. To control MAI, lithium bis(trifluoromethanesulfonyl)imide (Li- perovskite crystallisation, previous studies have developed TFSI; 99%), tBp (96%), and 2,2′,7,7′-tetrakis-(N,N-di- various methods, such as solvent engineering [22], composi- p-methoxyphenylamine)-9,9′-spirobifluorene (spiro- tion re-engineering [3], introducing additional additive, and OMeTAD; 99%) were purchased from Sigma–Aldrich. others [7, 15, 23, 24]. Amongst these methods, the easiest Absolute ethanol, N,N-dimethylformamide anhydrous and most effective way to achieve the specific target is by (DMF; 99.8%), and chlorobenzene (99%) were purchased adding additives to the precursor perovskite solution [25]. from R&M Chemicals. Before using dimethyl sulfoxide (DMSO) as an additive, H O was added to perovskite precursor to form an inter- PSC fabrication 2+ mediate stabiliser with Pb ion to grow high-quality per- ovskite films with high photovoltaic performance [22, 23]. The FTO substrates were etched with zinc powder and HCl Meanwhile, 1,8-diiodooctane additives have been proven and then cleaned using ethanol, acetone, and isopropanol to successfully expedite the nucleation of crystals and to in an ultrasonic bath for 15 min each. The substrate was modulate the kinetics of crystal growth during crystallisa- rinsed with deionised water and dried under nitrogen flow. tion, leading to more uniform perovskite morphology [5]. A compact TiO blocking layer (bl-TiO ) was deposited onto 2 2 The use of 4-tert-butylpyridine (tBp) as an additive is not the precleared FTO by spin coating (3000 rpm, 30 s) 1 mL new, and numerous studies have shown that this additive of titanium isopropoxide in 1 mL of ethanol. A mesoporous can improve the PCE and photovoltaic performance of PSCs TiO layer (mp-TiO ) was diluted with absolute ethanol at 2 2 device. The tBp is frequently used in the hole-transport layer 1:9 ratio and coated onto FTO/bl-TiO substrate by spin (HTL) of PSC owing to its ability to control the uniformity coating (4000 rpm for 20 s) [29]. The substrates were sin- of HTL by avoiding the aggregation of lithium salt [26]. tered on a hotplate at 450 °C and 30 min). For the precursor The chemical bond formed between tBp and perovskite crys- solution preparation, 553 mg of PbI was mixed in 1 mL of tals promotes the ability of the interface to become more DMF and 100 μL of tBp followed by 30 mg of C H NH I 3 3 selective for holes, thereby boosting diode rectification [27]. in 1 mL of IPA. Both solutions were heated at 60 °C for Similarly, the use of tBp in the perovskite layer improves the 3 h under moderate stirring. Then, 70 μL of PbI precur- final product’s performance. Besides, tBp also plays a sig- sor solution was spin coated on the mp-TiO /bl-TiO /FTO 2 2 nificant role in perovskite crystallisation as high grain size substrate at 3000 rpm for 60 s before heating at 70 °C for and pinhole-free perovskite surface are reportedly achieved 30 min. About 200 μL of MAI precursor solution was depos- after tBp treatment [28]. ited onto the substrate then spin coated at 3000 rpm for 20 s In the current work, tBp was added into PbI precursor, before heating at 95 °C for 30 min. For the TS method, the replacing DMSO to enhance the photovoltaic performance MAI precursor solution was dropped before spin program of PSCs. Through this route, the addition of tBp into DMF meanwhile 5, 10, and 15 s before the spin program ended as an additive enabled the formation of a porous layer packed for the DDTS technique. The HTL was prepared by mix- PbI nanocrystal, and the fabrication process was conducted ing 1 mL of spiro-OMeTAD solution (72.3 mg in 1 mL of 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 3 of 8 11 chlorobenzene), 17.5 µL of Li-TFSI solution (520 mg of to give 100 mW/cm using a standard Si photovoltaic cell Li-TFSI in 1 mL of acetonitrile), and 28.8 µL of tBp. After (Daystar Meter). J–V curves were recorded with a Keithley cooling, 50 µL of spiro-OMETAD was dropped onto perovs- 2400 source metre at 0.1  V/s scan rate. Steady-state PL kite/mp-TiO /bl-TiO /FTO substrate followed by spin coat- spectra were obtained using a fluorescence spectrometer 2 2 ing at 4000 rpm for 20 s. Finally, the device was completed (FLS920, Edinburgh Instruments) at an excitation wave- with silver (Ag) top electrode deposited by thermal evapora- length of 515 nm. All procedures were performed in ambi- tion. The active area of the cell was 0.07 cm . ent air without any humidity control. Characterisation Result and discussion The surface morphology and cross-sectional images of the samples were observed by field-emission scanning electron Figure 1a shows the device schematic of a PSC, and Fig. 1b microscopy (FESEM; ZEISS, Merlin Compact) and Nano- shows a cross-sectional FESEM image of the as-prepared surf Easyscan2 atomic force microscopy (AFM). XRD PSCs with layer-by-layer intermixing. Two different proce- spectra were obtained using an X-ray diffractometer model dures were applied to deposit MAI and fabricate perovskite Bruker D8 advance operated at 2θ angle. Optical absorp- thin films, as shown in Fig.  2a. Solar cells were fabricated tion spectra were recorded using a Lambda 35 Perkin Elmer under RH = 40%–50%. The effects of the delay-deposition UV–visible (UV–Vis) spectrophotometer. Solar simulated procedure of MAI on film morphology, crystal growth, and AM 1.5G sunlight was used with a solar simulator calibrated photovoltaic performance were systematically observed. Fig. 1 a Device schematic of perovskite solar cell, and b cross-section FESEM of perovskite solar cells Fig. 2 a Schematic of conven- tional two-step method and delay-deposition procedure of MAI for perovskite film fabrica- tion, and b photographs of PbI , TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s films 1 3 11 Page 4 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 Figure 2b shows the photographs of PbI , TS-0s, DDTS-5s, was shiny. This finding was ascribed to the smoother film DDTS-10s, and DDTS-15s films. The as-prepared PbI film owing to the fast transformation to perovskite crystal [31]. was pale yellow and became shiny dark yellow after 30 min Many reports have proven that the TS deposition method of thermal annealing. The surface was smooth and well crys- results in high quality and uniform surface morphology, tallised [30]. After TS and DDTS procedures, the perovskite but it critically depends on the handling of PbI film and films became dark brown. However, TS-0s film seemed to conversion process to MAPbI (during MAI intercalation) be dimmer, whereas DDTS-5s, DDTS-10s, and DDTS-15s [17]. Figure  3a–d shows the surface topography and 3D films were mirror-like and dense. The TS-0s film used stand- view of the samples through AFM. The root-mean-square ard procedure to deposit MAI where the MAI precursor was (RMS) roughness values of TS-0s, DDTS-5s, DDTS-10s, deposited onto the PbI layer substrate and spin coated [14]. and DDTS-15s were 29.92, 14.75, 13.21, and 10.42 nm, We observed the film turn into a dim dark brown, indicat- respectively. These values eventually decreased, as shown ing the transformation of PbI to perovskite. This dimmer in Fig. 3(i). The surface roughness of TS-0s showed a higher or blurred surface was caused by the light-scattering effect, RMS value, which was correlated with the aforementioned which stemmed from the rough surface of the perovskite dim and blurred surface of the sample. A smoother surface layer. The surface of DDTS-5s and DDTS-10s films looked was observed when MAI precursor was deposited onto the brighter brown, and DDTS-15s showed a lighter colour and PbI layer substrate 5 s before the spinning program ended Fig. 3 a–d AFM images of TS-0s, DDTS-5s, DDTS-10s, and DDTS- ovskite films prepared with regulated time delay (from 0 to 15 s). (j) 15s, respectively. e–h FESEM surface images of TS-0s, DDTS-5s, XRD patterns of perovskite films with regulated time delay (from 0 DDTS-10s, and DDTS-15s, respectively. (i) RMS roughness of per- to 15 s) 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 5 of 8 11 (i.e. DDTS-15s). The topographical information obtained from FESEM images confirmed the morphology of the samples in Fig. 3(e–h). The compact and full-coverage per- ovskite film comprised packed and small grain sizes using TS methods (i.e. TS-0s), and a few pinholes appear which anticipated increasing leakage current and recombination losses in the device. Meanwhile, the grain boundaries were much clearer for DDTS samples. Pinholes and uneven grains can be observed for DDTS-5s and DDTS-10s film surfaces which can be attributed to the poorly crystallised perovskite structure. Moreover, the DDTS-15s film surface was dense and smooth. To investigate the crystal size further, Fig. 3(j) displays the X-ray diffraction (XRD) patterns of perovskite films grown with TS and DDTS techniques. The DDTS-10s and DDTS-15s samples had a consistent peak for PbI at Fig. 4 Absorbance spectra of perovskite films with a regulated time 2θ = 12.6°, which corresponded to the 001 facet. However, delay (from 0 to 15 s). The Tauc plot of each sample is plotted in the inset the intensity of the diffraction peak (001) for DDTS-MAPbI was quenched, suggesting that more amount of PbI was suc- cessfully transformed into perovskite upon using the delay deposition of MAI. The XRD pattern of TS-0s and DDTS- 5s did not show the same peak, revealing that PbI residues intensity for wavelengths over 800 nm, which was attrib- cannot be converted into perovskite after MAI deposition uted to the light-scattering effect caused by the poor sur - through the standard procedure and short time delay. Thus, face morphology as described earlier [14]. This finding we found that the 15 s delay deposition of MAI precursor was supported by the transmittance spectra, where lower was effective and sufficient to obtain high-quality perovs- transmittance was recorded for this sample. In other words, kite. Furthermore, tBp treatment played a significant role in most photons with longer wavelengths were not transmit- the crystallisation [28, 32]. Most characteristic peaks cor- ted properly but were scattered and reflected by the film. responded well to MAPbI (at 2θ = 14.45° and 28.68°) for all However, the observed light absorption in this region can samples. The crystallinity of the samples is very important be expected not to contribute to the formation of elec- to evaluate using the full width at half-maximum (FWHM) tron–hole pairs owing to the bandgap [35]. DDTS-15s of the characteristic diffraction peaks, and smaller FWHM film exhibited the lowest light absorption compared with of diffraction peak is known to correspond to higher crys- DDTS-5s and DDTS-10s films. Using the Tauc analysis tallinity. The crystal size varied from 53.6, 48.2, 45.1, and shown in the inset of Fig. 4, the estimated energy bandgaps 31.9 nm for TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s of TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s were 1.51, films, respectively, in agreement with previous findings [33]. 1.57, 1.58, and 1.59 eV, respectively. The bandgap was Some miscellaneous peaks were also detected, indicating quenched proportionally with different deposition times. that the residual CH NH I decomposed and the product was The steady-state photoluminescence (PL) spectra were 3 3 I , as shown in the following reactions [34]: obtained to further validate this phenomenon, and results are shown in Fig.  5. High-quality perovskite films are CH NH I (aq) ⇋ CH NH + HI(aq) 3 3 3 2(aq) (1) commonly related to high PL efficiency, which reduces non-radiative recombination. From the PL peak exhib- 4HI(aq) + O (aq) ⇋ 2H O + 2I (2) ited centred at 770 nm, the peak intensity was strongly 2 2 suppressed for DDTS-15s film. This finding indicated hv that photo-excited electrons were collected efficiently at (3) 2HI(aq) ⟺ H + I 2 2 the interface. The slower charge extraction rate of TS-0s was expected and can be defined as non-radiative charge To investigate the mechanism for the better photoelec- recombination by defects or trapping holes in the film. Per - tric properties of perovskite film with different times and ovskite films were deposited onto m-TiO . Furthermore, deposition procedures of MAI, optical properties were for both cases, shifted peaks were observed owing to the examined. Figure 4 displays the absorption spectra of the usage of tBp in PbI precursor. PL intensity also decreased perovskite films with time delays obtained using UV–Vis with prolonged time delay from 0 to 15 s. These results spectroscopy. TS-0s film exhibited increased absorption confirmed the analyses of AFM and FESEM images. 1 3 11 Page 6 of 8 Materials for Renewable and Sustainable Energy (2021) 10:11 (RH = 70%–80%). Figure  6b depicts the statistics of the photovoltaic parameters of the respective devices. The external quantum efficiency (EQE) spectra of the samples were subsequently obtained, and results are shown in Fig.  7. The current produced was evaluated when the quantum efficiency was integrated over the entire electro - magnetic spectrum. IPCE intensity increased from 300 to 800 nm with prolonged time delay from 0 to 15 s. The cur- rent density calculated from EQE spectra was 9.37, 12.38, 13.9, and 16.96 mA/cm for TS-0s, DDTS-5s, DDTS-10s, and DDTS-15s films, respectively, which were close to the photovoltaic performance obtained from LIV testing (Table  1). EQE can be measured based on the following formula: electron∕sec EQE = photon∕sec Fig. 5 Steady-state photoluminescence (PL) spectra of perovskite current∕(charge of 1 electron) films with a regulated time delay (from 0 to 15 s) (total power of photons)∕(energy of one photon) (4) In addition, the statistical photovoltaic parameters of all The photovoltaic performance of the cell device was samples are summarised in Table  1. The average current determined by the crystallinity and morphology of the density, J increased from 9.4 to 17.2 mA/cm with pro- sc resulting perovskite film. Incompatibility, defects, and pin- longed time delay from 0 to 15 s. The average open-circuit hole surface generally lead to increased trapping holes and voltage, V and Fill-factor, FF slightly increased from 0.87 oc high recombination losses. Figure 6a shows the photocur- to 0.98 V and 39–50%, respectively, upon changing the time rent density–voltage (J–V) curves of PSC device fabricated delay. Eventually, the enhancement in efficiency was primar - using different time delays for MAI deposition in the same ily attributed to the increase in J owing to the perfect trans- sc batch under 1 sun illumination. Five samples comprising formation of PbI into MAPbI , enlargement of MAPbI 2 3 3 ten devices were used to validate the reliability of this study. crystallinity, light absorption of perovskite, and improved They were prepared without any encapsulation in ambi- film morphologies of perovskite. The low V and FF val- oc ent air and under a RH of approximately RH = 40%–50% ues of TS-0s film were due to the non-uniformity of the and most likely the situation even worst during testing Fig. 6 a Photocurrent density–voltage (J–V) curves of PSCs using perovskite regulated time delay (from 0 to 15  s). b Average and standard deviation of photovoltaic performance parameters of at least 5 samples (10 devices) 1 3 Materials for Renewable and Sustainable Energy (2021) 10:11 Page 7 of 8 11 when the RH was partially controlled (RH = 40%–50%). We suggest that further investigation is needed to increase the effectiveness of this new technique with proper device encapsulation. Acknowledgements This work was performed with the support of the Universiti Kebangsaan Malaysia Research Grant, specifically, the Dana Impak Perdana (DIP-2019-025) and Internal Grant UOW Malaysia KDU (KDURG-2017-1-004). Declarations Conflict of interest No conflicts of interest are declared. Open Access This article is licensed under a Creative Commons Attri- bution 4.0 International License, which permits use, sharing, adapta- tion, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, Fig. 7 External quantum efficiency (EQE) spectra using perovskite provide a link to the Creative Commons licence, and indicate if changes regulated time delay (from 0 to 15 s) were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in Table 1 Photovoltaic-performance parameters (short-circuit photo- the article’s Creative Commons licence and your intended use is not current, open-circuit photovoltage, fill factor, and power-conversion permitted by statutory regulation or exceeds the permitted use, you will efficiency) extracted from J–V measurements for devices under 1 sun need to obtain permission directly from the copyright holder. To view a −2 illumination (AM 1.5G, 100 mV  cm ) for 10 devices copy of this licence, visit http://cr eativ ecommons. or g/licen ses/ b y/4.0/ . Sample V (V) J (mA/cm ) FF (%) PCE (%) oc sc TS-0s 0.870 9.4 39.00 3.2 References DDTS-5s 0.956 12.7 44.50 5.4 DDTS-10s 0.978 14.2 44.76 6.2 1. Tanaka, K., Takahashi, T., Ban, T., Kondo, T., Uchida, K., Miura, DDTS-15s 0.980 17.2 50.00 8.3 N.: Comparative study on the excitons in lead-halide-based perovskite-type crystals CH3NH3PbBr 3 CH3NH3PbI3. Solid State Commun. 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