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Synthesis of Si and CdTe quantum dots and their combined use as down-shifting photoluminescent centers in Si solar cells

Synthesis of Si and CdTe quantum dots and their combined use as down-shifting photoluminescent... This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and their use, either on their own or combined, as photoluminescent (PL) down-shifting nanostructured coatings aimed to enhance the photovoltaic efficiency of polycrystalline silicon solar cells. To this end, the front face of a set of silicon cells was coated with different volume ratios of the above-mentioned QDs, or some of its mixtures, dispersed in PMMA layers. Previously, the absorption and the PL (exc = 380 nm) response of the dispersions of the QDs were measured. It was observed that the PL response of the mixtures was strongly affected in location, spread, and intensity of the emission peak according to the volume ratio involved. As compared to the unmixed CdTe samples, a notorious red-shift of the main peak location was obtained for a couple of mixed QDs’ dispersions, which was one of the project objectives given that Si solar cells respond better to photons with wavelengths in the 650–700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells covered with and without nanostructured PMMA/QDs layers tested under AM 1.5G solar simulator conditions. It was found that the use of the proposed mixtures of QDs gave an increase of 1.53% in solar cell power conversion efficiency. Keywords Silicon solar cells · Cadmium telluride · Quantum dots · Down-shifting Introduction [1]. Intrinsic and extrinsic losses account for the energy unaccounted for. Two of these intrinsic losses, related to The total solar energy density outside the atmosphere, the Si band-gap energy E , are depicted in Fig. 1 for two known as the irradiance solar constant at AM0 conditions, segments of the solar spectrum, namely, for the wavelength is I = 1367 Watt/m ; and approximately, 98% of the power ranges of λ ≤ 1107  nm and λ > 1107  nm, or equivalently, SC density in the solar spectrum lays in the 200–2500  nm for photon energy values E ≥ E and E < E eV, know- ph G ph G wavelength range, that is, from the near UV to the medium ing that E = 1.12 eV. For the latter range, photons are not near IR and has a modal peak at about 500 nm. It has been absorbed in Si and, therefore, their energy is not usable. reported that, due to fundamental losses, the maximum effi- This is indicated by the parameter I (λ) in Fig. 1 which NA ciency theoretically achievable in a Si solar cell is of 30.1% corresponds to an E (λ) loss of approximately 20% of the ph incident I , represented by the shaded area under E in SC G the same Fig.  1. Regarding the E ≥ E range, absorbed ph G * J. E. Pelayo-Ceja photons are sufficiently energetic to generate electron–hole pcje@hotmail.es (e–h) pairs; however, the energy in excess of E is lost by carrier collisions with the semiconductor lattice (phonon MEMS Research Lab, Department of Physics creation) and, consequently, generate heat. Assuming that and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA all the energy in excess of E goes to phonon creation, the energy loss term can then be defined as follows: Centro de Ciencias Exactas e Ingenierías, Universidad de Guadalajara, Blvd. Gral. Marcelino García Barragán 1421, Olímpica, 44430 Guadalajara, Jal., Mexico E ()− E ph G I = 100 (%) for  ≤ 1107 nm, 3 (1) Departamento de Física, Universidad de Sonora, Blvd. Luis E () ph Encinas y Rosales, Col. Centro, 83000 Hermosillo, Sonora, Mexico Vol.:(0123456789) 1 3 14 Page 2 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 Fig. 1 Distribution of the spectral fundamental losses in Si which describes how the ratio of the areas between E and (i.e., UV range) in Si solar cells is far from efficient. Specifi- ph E varies along the E ≥ E wavelength range. The loss cally, in the 200–400 nm wavelength range, 1–EQE ≈ 2/3, G ph G fraction corresponding to the above expression, indicated indicating that an average of around three absorbed photons by the shaded area over E in Fig. 1, integrates to ~ 35% of are needed to produce an effective current of one electron, I , indicating that the usable energy reduces to approxi- and thus, the recombination of two e–h pairs takes place. SC mately 45% of I in the aforementioned wavelength range. Several technical approaches, mostly related to emitter SC Ostensibly, the larger the energy of the photon absorbed, diffusion or front surface passivation optimization, have the larger the portion of energy lost. This is indicated in been disclosed aimed at mitigating the above-mentioned Si Fig. 1 by the I parameter associated with the segmented solar cells’ UV-response limitations [3–5]. The method dis- means for die ff rent photon energies, parameter that increases cussed in this paper is based on employing the down-shifting from a relatively small loss of I = 13.4% for wavelengths photoluminescent (DSPL) properties observed in QDs [6, of ~ 950 nm, for example, to I = 76.8% for a photon wave- 7], which are capable of absorbing UV photons and sub- length of ~ 257 nm. Ostensibly, a Si cell response for high- sequently emitting lower energy photons that have a better energy photons (smaller wavelengths) is rather modest. match for Si solar cells [8]. Si and CdTe QDs were selected A concomitant effect of the heat loss is an increase of the for achieving this goal, incorporating them into a PMMA [9] solar cell temperature and of its leakage current, resulting matrix applied as a DSPL coating layer on polycrystalline in degradation of the device fill factor I–V response and of Si solar cells. its power conversion efficiency. Specifically, the window surface of a set of silicon cells Si solar cells are also affected by other losses mainly was coated with different volume ratios of the above-men- related to carrier low lifetime and/or short diffusion length tioned QDs dispersed in PMMA layers. Previously, the which imply increased e–h pair recombination loss rate [2]. absorption and the PL response of several dispersions with This loss is monitored in solar cells by measuring their exter- varying quantities and proportions of QDs were determined nal quantum efficiency (EQE), which provides the spectral using UV excitation with a wavelength of 380 nm. It was information regarding how close a single absorbed photon observed that the PL response of the mixtures was strongly is of producing an effective circuit current of one electron. affected in location, spread, and intensity of the emission Furthermore, the 1–EQE parameter defines the e –h pair peak according to the volume ratio involved. Specifically, recombination per absorbed photon. Figure 2 provides an compared to the unmixed CdTe QD samples, a blue-shift of example for a typical EQE plot for a Si solar cell; the 1–EQE the main CdTe peak location was observed in SiQD-rich dis- trace is also added along with a segmented distribution of persions and a red-shift in CdTe-rich dispersions that could it. Ostensibly, the absorption of short-wavelength photons help increase the efficiency of Si solar cells considering 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 3 of 8 14 Fig. 2 EQE and carrier recombination loss distribution (1–EQE) that they respond better to photons with wavelengths in the Synthesis of CdTe quantum dots 650–700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells coated with nanostructured CdTe quantum dots were obtained using a chemical syn- PMMA layers, tested under AM 1.5G solar simulator con- thesis approach as described in [12]. The used process is ditions, and compared to the previous test results of the bare as follows: (1) a solution of 2 g of NaOH in 50 ml of DI solar cells. is prepared and set aside; (2) in a 100 ml flask, 0.053 g of cadmium acetate dihydrate (Cd(CH COO) ·2H O, 99.5%) 3 2 2 is diluted in 50 ml of DI and stirred for 5 min; (3) 18 μl of Experimental details thioglycolic acid (TGA, 90%) is added to the flask; (4) the solution pH is adjusted by dropwise adding the NaOH dis- Synthesis of Si quantum dots persion while agitating until a pH value between 10.5 and 11 is reached; (5) additional stirring is done during 5 min; (6) The silicon QDs’ solutions were synthesized at room temper- in a separate flask, 0.0101 g of potassium tellurite (K TeO , 2 3 ature [10] in a water-based dispersion, following the method 95%) is diluted in 50 ml of water; (7) stirring is kept during reported by Wang et al. [11]. The process was observed to 5 min; (8) the potassium tellurite solution is added to the be relatively straightforward and produced consistent results. TGA solution; (9) stirring is done during 5 min; (10) 0.08 g The synthesis procedure can be briefly described as follows: of sodium borohydride (NaBH , 99%) is added; (11) the mix (1) initiate with 4 ml of deionized water (DI) in a vial; (2) is transferred to a single-neck, round-bottom flask attached dropwise addition of 1.5  ml of APTES (3-aminopropyl) to a Liebig condenser; (12) the flask is placed in an oil bath triethoxyline 99% with a syringe; (3) stir during 10 min; kept at 100 °C and the solution is refluxed during the desired (4) pour 1.25 ml of DI in a separate vial to which 19.81 g synthesis time; based on a previous work [13], refluxing (0.1 M) of (+)-Sodium l -ascorbate ≥ 98% (SA) are added; times of 8 and 12 h were chosen for this project. At the end (5) stir during 10 min; (6) the second vial is poured into of the refluxing time, the QDs’ solutions were left at rest the first one, while agitation is applied; (7) agitation con- enough time, approximately 3 h, to reach room temperature. tinues during 30 min; (8) samples are settled during 24 h; Then, a volumetric relation of 1:1 of CdTe QDs solution (9) uncapped 5 ml vials containing 1 ml of each solution are and acetone (acetone HPLC, ≥ 99.9% Sigma-Aldrich) was placed in a desiccator during 24 h; (10) sediments are recol- poured in a plastic tube that was centrifuged for 10 min at lected and incorporated, or mixed and then incorporated, to 10,000 rpm. Once the residual liquid part was removed, the the PMMA (495 PMMA A2 from Microchem) to form the solids were dispersed again but this time with 1 ml of DI and DSPL solution. the solution was put in an ultrasonic bath (Branson 5510) for 1 3 14 Page 4 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 bulk 5 min. Next, 1 ml of acetone was added and the solution was where E is the band-gap energy of the quantum dot, E QD centrifuged for 10 min at 10,000 rpm. This procedure was is the band-gap energy of the bulk semiconductor,  is the repeated for three times after removing the remaining liquid ∗ ∗ Planck constant, m and m are the effective masses of elec- e h part at the end of the ultrasonic bath and centrifugation. trons and holes in a particular material, m is the electron mass at rest, and r is the radius of the quantum dot. Si and CdTe QDs’ blending Mixtures of Silicon and Cadmium telluride quantum dots were realized using four proportions, that is, xSi:yCdTe-8 h Results and discussion and xSi:yCdTe-12 h where the x:y proportions corresponded to the volume ratios of 2:1, 1:1, 1:2, and 1:3 for both mix- Size and band‑gap results ture types. Given that the 1:2 proportions showed better PL response than the one observed for the 2:1 case, it was The estimated energy gaps and diameter values for the Si decided to incorporate the 1:3 ratio and drop the 3:1 case. and CdTe QDs, along with their respective m exponent, All blends were homogenized by a 15 min sonication step. are given in Table 1. Note that an allowed direct interband The QDs of Si, CdTe, or their mixtures, one at a time, optical transition type was detected for the Si QDs case in were individually dispersed in 1 ml of PMMA. In each case, contrast to the well-established indirect nature for the bulk four dispersions were realized in PMMA corresponding to material. individual QDs volumes of 0.5 ml, 1.0 ml, 1.5 ml, or 2.0 ml. Therefore, a total of 44 different coating samples were fabri- cated, 12 for the individual QDs and 16 for each one of the QD optical characterization two mixtures according to all the possible combinations of the volume ratios x:y and the proportions QDs:PMMA. Six The UV–Vis absorption and the photoluminescence of the previously characterized solar cells (bare cells) were used synthetized quantum dots solutions were recorded using with each of these dispersions where, in each case, the dis- an Ocean Optics Flame-S-UV–Vis spectrometer; for the persion was applied by spin-coating according to the param- emission spectra, an excitation wavelength of 380 nm was eters: 300 μl dose, 4000 rpm, 50 s, and approximately 66 nm employed. The corresponding absorption and photolumines- of layer thickness. Commercially available polycrystalline cent spectra for the individual and mixed QDs are shown solar cells (Eco-worthy Company) were used (ARC finish, in Fig. 3. Figure 3a and b shows the registered absorption 52 mm × 38 mm, 200 μm thick). After the spin-coating pro- and emission spectra, whereas Fig. 3c and d shows the cor- cess, the substrate was placed on a plate at 180 °C to evapo- responding normalized graphs of the emission spectra of the rate the solvents and to fix the thin films. mixtures; the normalized spectra are to better evaluate the influence of the mixtures on the shift of the emission spec- Size and band‑gap calculations tra. In fact, the intensity of the respective QDs peaks varied when modifying the volumetric content of each constituent Material bandgaps were determined using Tauc’s graphical in the dispersions (as shown in Fig. 3c, d). In addition, in procedure [14] based on the following expression: both cases, the characteristic photoluminescent peaks of the Si and CdTe QDs were observed to exhibit a relatively small, = (E − E ) , ph g (2) but not negligible, shift as the mixture evolved from a Si-rich ph condition to a CdTe-rich dispersion. Specifically, in the case of the QDS dispersions involving Si and CdTe QDs with a where α is the QDs absorption coefficient, E is the incident ph synthesis time of 8 h (see Fig. 3c and Table 2), the Si peak photon energy, E is the bandgap, A is a constant called the was generally observed to be blue-shifted, whereas the CdTe band tailing parameter, and m is a parameter related to the nature of the exciton optical transition, which can only take the values of 0.5, 2, 3/2, and 3 corresponding to allowed Table 1 Energy bandgap, particle diameter, and optical transition direct, allowed indirect, forbidden direct, and forbidden indi- parameter value for QDs rect transitions, respectively. The values of the diameters QD case were calculated using the Brus equation [15]: Si CdTe-8 h CdTe-12 h 2 2 2 1 1 1.8e bulk E = E + + − , QD Parameter E (eV) 2.47 2.28 2.15 2r m ∗ m m ∗ m 4 r g e 0 h 0 0 QDs D (nm) 2.79 3.21 3.46 (3) m 0.5 0.5 0.5 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 5 of 8 14 Fig. 3 Absorption and photoluminescent emission spectra of Si and CdTe QDs dispersions Table 2 Shift observed in the characteristic PL peaks in Si and Table 3 Shift observed in the characteristic PL peaks in Si and CdTe- CdTe-8 h QD heterogeneous dispersions 12 h QD heterogeneous dispersions Si:CdTe QD ratio Si peak shift (nm) CdTe-8 h Si:CdTe QD ratio Si peak shift (nm) CdTe-12 h peak shift peak shift (nm) (nm) 2:1 − 4.3 − 8.1 2:1 − 0.3 − 5.8 1:1 − 3.8 − 4.1 1:1 − 1.3 − 3.8 1:2 − 4.3 + 4.9 1:2 − 1.3 + 5.2 1:3 − 5.3 + 5.9 1:3 − 5.3 + 10.2 peak migrated from a blue-shift of ~ 8.1 nm in a Si-rich dis- persion to a red-shift of ~ 5.9 nm in a CdTe-rich condition. Si QDs exhibited a similar behavior as that observed with In view of the wavelength-dependent absorption prob- smaller CdTe QDs (see Fig. 3d and Table 3) where the Si ability displayed in Fig. 1, the prospective control of the peak was also observed to be blue-shifted; however, the red-shift of CdTe QD PL peak is of interest because of CdTe-12 h peak migrated from a blue-shift of ~ 5.8 nm in its anticipated influence on solar cell performance. The a Si-rich dispersion to a red-shift of ~ 10.2 nm in a CdTe- quantum dot interaction previously described also has a rich condition. dependence on the size of the nanoparticles involved. Spe- The observed effects are thought to be due to quan- cifically, dispersions comprised of CdTe QDs with a syn- tum dot physical adsorption leading to surface energy thesis time of 12 h in combination with the aforementioned variations. However, the computational modeling of the observed effects has not been performed to date. 1 3 14 Page 6 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 Fig. 4b. Nevertheless, the benefit for the Si QDs is within the Silicon solar cell coating and characterization with QDs uncertainty, so it practically remained unchanged, as can be seen in Table 4 for the rounded values and their correspond- Based on the previous observations, the 1Si:3CdTe-12 h ing uncertainty. The solar cells with the PMMA/CdTe-8 h, on the other hand, gave the best increase in efficiency of the mixture was selected to incorporate on solar cells, since it exhibited the largest red-shift, and the CdTe-12 h has the individual QDs of this study (0.83% according to results in Table  4), though the mixture of PMMA/Si:CdTe-12 h lowest band-gap energy, or the highest size, as can be seen in Table 1. Besides, its emission spectrum is located at longer was the best of the mixtures (1.53% as given in Table 4). Specifically, these results correspond to the volume ratios wavelength regarding the Si and CdTe-8 h QDs. Sonication was employed to disperse the blend in polymethylmeth- of 1 ml/2 ml for PMMA/(individual QDs: Si, CdTe-8 h, or CdTe-12 h) and 1 ml/1.5 ml for PMMA/(Si:CdTe-12 h). acrylate (495 PMMA Microchem) prior to coating the win- dow side of a set of polycrystalline Si solar cells using the The smoothed EQEs for bare and coated cells are shown in Fig. 4. In all cases, there is a better use of high-energy spin-coating method [16]. There were six solar cells tested for each different condition considered in this study; that is, photons by the use of the QDs, as can be appreciated on the left side of the EQEs and corresponding to the down- test of the bare solar cells and then test of the coated solar cells, considering the individual volume ratios with each shifting effect of the QDs used in this work. In addition, the transition between both types of EQEs, with and without individual QDs and each mixture. Each set of polycrystalline solar cells was measured with- QDs, for the high-energy photons is realized according to the photoluminescence response of the QDs. As shown by out and with the incorporation of the aforementioned down- shifting QDs using an AM1.5 solar simulator (Oriel Sol2A). the EQEs of the Si:CdTe-12 h QDs mixture in Fig. 4d, there is a greater contribution of the more energetic photons and The collected values of open-circuit voltage (V ), short- oc circuit current (J ), and fill factor (FF) were recorded (see also the transition of both EQEs occurs at a longer wave- sc length, this as a combined result of the down-shifting and Table 4) and employed to calculate the power conversion efficiency (PCE) of the devices tested, which increased from the red-shift effects (see Fig.  3 and Table 3). Therefore, it is reasonably to get a greater efficiency, calculated in this case 14.6 to 14.78% in the best case when the down-converting film was incorporated to the solar cells. as an increment of 1.53%. For a comparative analysis, the external quantum effi- ciency (EQE) was registered for the bare and after coated Conclusions solar cells; the Δs reported in Table 4 were determined with the calculated values, not with the rounded ones. Accord- Our results indicate that it is possible to obtain a PCE ing to both results, the Si QDs have presented the smallest difference (0.39% as shown in Table  4) between prior and increase of 1.53% in Si solar cells using a QDs mixture of 1Si:3CdTe12 h; individually, none of the QDs considered after the application of the thin film of PMMA/Si QDs, see Fig. 4a. On the other hand, the EQE spectrum for the solar in our study showed a better efficiency boost. This result is mostly due to the observed red-shift of the DSPL of mixtures cells without and with the thin film of PMMA/CdTe-12 h QDs shows a better benefit than the cells incorporating the which produce photons with increased EQE in Si. Therefore, the ability to modify the down-shifting characteristics of PMMA/Si QDs layer (0.48% as reported in Table 4), see Table 4 Solar cell 2 Samples V (mV) J (mA/cm ) FF (%) PCE (%) oc sc characterization Si QDs Bare cells 612.1 34.90 69.2 14.8 ± 0.3 Si QDs 612.2 34.88 69.5 14.8 ± 0.4 % Δ of Means 0.016 − 0.06 0.43 0.39 CdTe8 QDs Bare cells 604.0 34.39 70.0 14.54 ± 0.36 CdTe8 QDs 604.6 34.56 70.2 14.66 ± 0.45 % Δ of Means 0.09 0.51 0.23 0.83 CdTe12 QDs Bare cells 613.0 34.6 68.9 14.62 ± 0.06 CdTe12 QDs 615.4 34.27 69.3 14.68 ± 0.11 % Δ of Means 0.39 − 0.95 0.58 0.48 1Si:3CdTe12 QDs Bare cells 615.5 34.38 68.8 14.6 ± 0.1 1Si:3CdTe12 QDs 617.9 34.72 68.9 14.78 ± 0.06 % Δ of Means 0.39 0.99 0.15 1.53 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 7 of 8 14 Fig. 4 EQE spectra of solar cells with and without Si and/or CdTe-12 h QDs quantum dots by employing heterogeneous mixtures could References prove a valuable tool to provide cost reduction of photovol- 1. Hirst, L.C., Ekins-Daukenes, N.J.: Fundamental losses in solar taic structures for power generation. cells. Prog. Photovolt. 19(3), 286–293 (2011) 2. Nilofar, A., Kamaruzzaman, S., Shideh, A., Kasra, S., Alghoul, Acknowledgements The authors would like to acknowledge the M., Omidreza, S., Saleem, H.Z.: A review on the role of materials National Science Foundation Grant No. ECCS 1650571 U.S. Army science in solar cells. Renew. Sustain. Energy Rev. 16, 5834–5847 Research office (Grant W911NF-13-1-0110), CONACYT, the Depart- (2012) ment of Physics and Astronomy, University of Texas at San Antonio, 3. Lopez-Delgado, R., Higareda-valenzuela, H., Zazueta-Raydnaud, and the Department of Projects Engineering-CUCEI, University of A., Pelayo, J.E., Ayon, A.: Solar cell efficiency improvement Guadalajara, México, for the support provided for this project. employing down-shifting silicon quantum dots. Microsyst. 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Synthesis of Si and CdTe quantum dots and their combined use as down-shifting photoluminescent centers in Si solar cells

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Springer Journals
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Copyright © 2019 by The Author(s)
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Materials Science; Materials Science, general; Renewable and Green Energy
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2194-1459
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Abstract

This paper describes the synthesis and characterization of Si and CdTe quantum dots (QDs) and their use, either on their own or combined, as photoluminescent (PL) down-shifting nanostructured coatings aimed to enhance the photovoltaic efficiency of polycrystalline silicon solar cells. To this end, the front face of a set of silicon cells was coated with different volume ratios of the above-mentioned QDs, or some of its mixtures, dispersed in PMMA layers. Previously, the absorption and the PL (exc = 380 nm) response of the dispersions of the QDs were measured. It was observed that the PL response of the mixtures was strongly affected in location, spread, and intensity of the emission peak according to the volume ratio involved. As compared to the unmixed CdTe samples, a notorious red-shift of the main peak location was obtained for a couple of mixed QDs’ dispersions, which was one of the project objectives given that Si solar cells respond better to photons with wavelengths in the 650–700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells covered with and without nanostructured PMMA/QDs layers tested under AM 1.5G solar simulator conditions. It was found that the use of the proposed mixtures of QDs gave an increase of 1.53% in solar cell power conversion efficiency. Keywords Silicon solar cells · Cadmium telluride · Quantum dots · Down-shifting Introduction [1]. Intrinsic and extrinsic losses account for the energy unaccounted for. Two of these intrinsic losses, related to The total solar energy density outside the atmosphere, the Si band-gap energy E , are depicted in Fig. 1 for two known as the irradiance solar constant at AM0 conditions, segments of the solar spectrum, namely, for the wavelength is I = 1367 Watt/m ; and approximately, 98% of the power ranges of λ ≤ 1107  nm and λ > 1107  nm, or equivalently, SC density in the solar spectrum lays in the 200–2500  nm for photon energy values E ≥ E and E < E eV, know- ph G ph G wavelength range, that is, from the near UV to the medium ing that E = 1.12 eV. For the latter range, photons are not near IR and has a modal peak at about 500 nm. It has been absorbed in Si and, therefore, their energy is not usable. reported that, due to fundamental losses, the maximum effi- This is indicated by the parameter I (λ) in Fig. 1 which NA ciency theoretically achievable in a Si solar cell is of 30.1% corresponds to an E (λ) loss of approximately 20% of the ph incident I , represented by the shaded area under E in SC G the same Fig.  1. Regarding the E ≥ E range, absorbed ph G * J. E. Pelayo-Ceja photons are sufficiently energetic to generate electron–hole pcje@hotmail.es (e–h) pairs; however, the energy in excess of E is lost by carrier collisions with the semiconductor lattice (phonon MEMS Research Lab, Department of Physics creation) and, consequently, generate heat. Assuming that and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA all the energy in excess of E goes to phonon creation, the energy loss term can then be defined as follows: Centro de Ciencias Exactas e Ingenierías, Universidad de Guadalajara, Blvd. Gral. Marcelino García Barragán 1421, Olímpica, 44430 Guadalajara, Jal., Mexico E ()− E ph G I = 100 (%) for  ≤ 1107 nm, 3 (1) Departamento de Física, Universidad de Sonora, Blvd. Luis E () ph Encinas y Rosales, Col. Centro, 83000 Hermosillo, Sonora, Mexico Vol.:(0123456789) 1 3 14 Page 2 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 Fig. 1 Distribution of the spectral fundamental losses in Si which describes how the ratio of the areas between E and (i.e., UV range) in Si solar cells is far from efficient. Specifi- ph E varies along the E ≥ E wavelength range. The loss cally, in the 200–400 nm wavelength range, 1–EQE ≈ 2/3, G ph G fraction corresponding to the above expression, indicated indicating that an average of around three absorbed photons by the shaded area over E in Fig. 1, integrates to ~ 35% of are needed to produce an effective current of one electron, I , indicating that the usable energy reduces to approxi- and thus, the recombination of two e–h pairs takes place. SC mately 45% of I in the aforementioned wavelength range. Several technical approaches, mostly related to emitter SC Ostensibly, the larger the energy of the photon absorbed, diffusion or front surface passivation optimization, have the larger the portion of energy lost. This is indicated in been disclosed aimed at mitigating the above-mentioned Si Fig. 1 by the I parameter associated with the segmented solar cells’ UV-response limitations [3–5]. The method dis- means for die ff rent photon energies, parameter that increases cussed in this paper is based on employing the down-shifting from a relatively small loss of I = 13.4% for wavelengths photoluminescent (DSPL) properties observed in QDs [6, of ~ 950 nm, for example, to I = 76.8% for a photon wave- 7], which are capable of absorbing UV photons and sub- length of ~ 257 nm. Ostensibly, a Si cell response for high- sequently emitting lower energy photons that have a better energy photons (smaller wavelengths) is rather modest. match for Si solar cells [8]. Si and CdTe QDs were selected A concomitant effect of the heat loss is an increase of the for achieving this goal, incorporating them into a PMMA [9] solar cell temperature and of its leakage current, resulting matrix applied as a DSPL coating layer on polycrystalline in degradation of the device fill factor I–V response and of Si solar cells. its power conversion efficiency. Specifically, the window surface of a set of silicon cells Si solar cells are also affected by other losses mainly was coated with different volume ratios of the above-men- related to carrier low lifetime and/or short diffusion length tioned QDs dispersed in PMMA layers. Previously, the which imply increased e–h pair recombination loss rate [2]. absorption and the PL response of several dispersions with This loss is monitored in solar cells by measuring their exter- varying quantities and proportions of QDs were determined nal quantum efficiency (EQE), which provides the spectral using UV excitation with a wavelength of 380 nm. It was information regarding how close a single absorbed photon observed that the PL response of the mixtures was strongly is of producing an effective circuit current of one electron. affected in location, spread, and intensity of the emission Furthermore, the 1–EQE parameter defines the e –h pair peak according to the volume ratio involved. Specifically, recombination per absorbed photon. Figure 2 provides an compared to the unmixed CdTe QD samples, a blue-shift of example for a typical EQE plot for a Si solar cell; the 1–EQE the main CdTe peak location was observed in SiQD-rich dis- trace is also added along with a segmented distribution of persions and a red-shift in CdTe-rich dispersions that could it. Ostensibly, the absorption of short-wavelength photons help increase the efficiency of Si solar cells considering 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 3 of 8 14 Fig. 2 EQE and carrier recombination loss distribution (1–EQE) that they respond better to photons with wavelengths in the Synthesis of CdTe quantum dots 650–700 nm range. This effect was confirmed in a set of polycrystalline Si solar cells coated with nanostructured CdTe quantum dots were obtained using a chemical syn- PMMA layers, tested under AM 1.5G solar simulator con- thesis approach as described in [12]. The used process is ditions, and compared to the previous test results of the bare as follows: (1) a solution of 2 g of NaOH in 50 ml of DI solar cells. is prepared and set aside; (2) in a 100 ml flask, 0.053 g of cadmium acetate dihydrate (Cd(CH COO) ·2H O, 99.5%) 3 2 2 is diluted in 50 ml of DI and stirred for 5 min; (3) 18 μl of Experimental details thioglycolic acid (TGA, 90%) is added to the flask; (4) the solution pH is adjusted by dropwise adding the NaOH dis- Synthesis of Si quantum dots persion while agitating until a pH value between 10.5 and 11 is reached; (5) additional stirring is done during 5 min; (6) The silicon QDs’ solutions were synthesized at room temper- in a separate flask, 0.0101 g of potassium tellurite (K TeO , 2 3 ature [10] in a water-based dispersion, following the method 95%) is diluted in 50 ml of water; (7) stirring is kept during reported by Wang et al. [11]. The process was observed to 5 min; (8) the potassium tellurite solution is added to the be relatively straightforward and produced consistent results. TGA solution; (9) stirring is done during 5 min; (10) 0.08 g The synthesis procedure can be briefly described as follows: of sodium borohydride (NaBH , 99%) is added; (11) the mix (1) initiate with 4 ml of deionized water (DI) in a vial; (2) is transferred to a single-neck, round-bottom flask attached dropwise addition of 1.5  ml of APTES (3-aminopropyl) to a Liebig condenser; (12) the flask is placed in an oil bath triethoxyline 99% with a syringe; (3) stir during 10 min; kept at 100 °C and the solution is refluxed during the desired (4) pour 1.25 ml of DI in a separate vial to which 19.81 g synthesis time; based on a previous work [13], refluxing (0.1 M) of (+)-Sodium l -ascorbate ≥ 98% (SA) are added; times of 8 and 12 h were chosen for this project. At the end (5) stir during 10 min; (6) the second vial is poured into of the refluxing time, the QDs’ solutions were left at rest the first one, while agitation is applied; (7) agitation con- enough time, approximately 3 h, to reach room temperature. tinues during 30 min; (8) samples are settled during 24 h; Then, a volumetric relation of 1:1 of CdTe QDs solution (9) uncapped 5 ml vials containing 1 ml of each solution are and acetone (acetone HPLC, ≥ 99.9% Sigma-Aldrich) was placed in a desiccator during 24 h; (10) sediments are recol- poured in a plastic tube that was centrifuged for 10 min at lected and incorporated, or mixed and then incorporated, to 10,000 rpm. Once the residual liquid part was removed, the the PMMA (495 PMMA A2 from Microchem) to form the solids were dispersed again but this time with 1 ml of DI and DSPL solution. the solution was put in an ultrasonic bath (Branson 5510) for 1 3 14 Page 4 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 bulk 5 min. Next, 1 ml of acetone was added and the solution was where E is the band-gap energy of the quantum dot, E QD centrifuged for 10 min at 10,000 rpm. This procedure was is the band-gap energy of the bulk semiconductor,  is the repeated for three times after removing the remaining liquid ∗ ∗ Planck constant, m and m are the effective masses of elec- e h part at the end of the ultrasonic bath and centrifugation. trons and holes in a particular material, m is the electron mass at rest, and r is the radius of the quantum dot. Si and CdTe QDs’ blending Mixtures of Silicon and Cadmium telluride quantum dots were realized using four proportions, that is, xSi:yCdTe-8 h Results and discussion and xSi:yCdTe-12 h where the x:y proportions corresponded to the volume ratios of 2:1, 1:1, 1:2, and 1:3 for both mix- Size and band‑gap results ture types. Given that the 1:2 proportions showed better PL response than the one observed for the 2:1 case, it was The estimated energy gaps and diameter values for the Si decided to incorporate the 1:3 ratio and drop the 3:1 case. and CdTe QDs, along with their respective m exponent, All blends were homogenized by a 15 min sonication step. are given in Table 1. Note that an allowed direct interband The QDs of Si, CdTe, or their mixtures, one at a time, optical transition type was detected for the Si QDs case in were individually dispersed in 1 ml of PMMA. In each case, contrast to the well-established indirect nature for the bulk four dispersions were realized in PMMA corresponding to material. individual QDs volumes of 0.5 ml, 1.0 ml, 1.5 ml, or 2.0 ml. Therefore, a total of 44 different coating samples were fabri- cated, 12 for the individual QDs and 16 for each one of the QD optical characterization two mixtures according to all the possible combinations of the volume ratios x:y and the proportions QDs:PMMA. Six The UV–Vis absorption and the photoluminescence of the previously characterized solar cells (bare cells) were used synthetized quantum dots solutions were recorded using with each of these dispersions where, in each case, the dis- an Ocean Optics Flame-S-UV–Vis spectrometer; for the persion was applied by spin-coating according to the param- emission spectra, an excitation wavelength of 380 nm was eters: 300 μl dose, 4000 rpm, 50 s, and approximately 66 nm employed. The corresponding absorption and photolumines- of layer thickness. Commercially available polycrystalline cent spectra for the individual and mixed QDs are shown solar cells (Eco-worthy Company) were used (ARC finish, in Fig. 3. Figure 3a and b shows the registered absorption 52 mm × 38 mm, 200 μm thick). After the spin-coating pro- and emission spectra, whereas Fig. 3c and d shows the cor- cess, the substrate was placed on a plate at 180 °C to evapo- responding normalized graphs of the emission spectra of the rate the solvents and to fix the thin films. mixtures; the normalized spectra are to better evaluate the influence of the mixtures on the shift of the emission spec- Size and band‑gap calculations tra. In fact, the intensity of the respective QDs peaks varied when modifying the volumetric content of each constituent Material bandgaps were determined using Tauc’s graphical in the dispersions (as shown in Fig. 3c, d). In addition, in procedure [14] based on the following expression: both cases, the characteristic photoluminescent peaks of the Si and CdTe QDs were observed to exhibit a relatively small, = (E − E ) , ph g (2) but not negligible, shift as the mixture evolved from a Si-rich ph condition to a CdTe-rich dispersion. Specifically, in the case of the QDS dispersions involving Si and CdTe QDs with a where α is the QDs absorption coefficient, E is the incident ph synthesis time of 8 h (see Fig. 3c and Table 2), the Si peak photon energy, E is the bandgap, A is a constant called the was generally observed to be blue-shifted, whereas the CdTe band tailing parameter, and m is a parameter related to the nature of the exciton optical transition, which can only take the values of 0.5, 2, 3/2, and 3 corresponding to allowed Table 1 Energy bandgap, particle diameter, and optical transition direct, allowed indirect, forbidden direct, and forbidden indi- parameter value for QDs rect transitions, respectively. The values of the diameters QD case were calculated using the Brus equation [15]: Si CdTe-8 h CdTe-12 h 2 2 2 1 1 1.8e bulk E = E + + − , QD Parameter E (eV) 2.47 2.28 2.15 2r m ∗ m m ∗ m 4 r g e 0 h 0 0 QDs D (nm) 2.79 3.21 3.46 (3) m 0.5 0.5 0.5 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 5 of 8 14 Fig. 3 Absorption and photoluminescent emission spectra of Si and CdTe QDs dispersions Table 2 Shift observed in the characteristic PL peaks in Si and Table 3 Shift observed in the characteristic PL peaks in Si and CdTe- CdTe-8 h QD heterogeneous dispersions 12 h QD heterogeneous dispersions Si:CdTe QD ratio Si peak shift (nm) CdTe-8 h Si:CdTe QD ratio Si peak shift (nm) CdTe-12 h peak shift peak shift (nm) (nm) 2:1 − 4.3 − 8.1 2:1 − 0.3 − 5.8 1:1 − 3.8 − 4.1 1:1 − 1.3 − 3.8 1:2 − 4.3 + 4.9 1:2 − 1.3 + 5.2 1:3 − 5.3 + 5.9 1:3 − 5.3 + 10.2 peak migrated from a blue-shift of ~ 8.1 nm in a Si-rich dis- persion to a red-shift of ~ 5.9 nm in a CdTe-rich condition. Si QDs exhibited a similar behavior as that observed with In view of the wavelength-dependent absorption prob- smaller CdTe QDs (see Fig. 3d and Table 3) where the Si ability displayed in Fig. 1, the prospective control of the peak was also observed to be blue-shifted; however, the red-shift of CdTe QD PL peak is of interest because of CdTe-12 h peak migrated from a blue-shift of ~ 5.8 nm in its anticipated influence on solar cell performance. The a Si-rich dispersion to a red-shift of ~ 10.2 nm in a CdTe- quantum dot interaction previously described also has a rich condition. dependence on the size of the nanoparticles involved. Spe- The observed effects are thought to be due to quan- cifically, dispersions comprised of CdTe QDs with a syn- tum dot physical adsorption leading to surface energy thesis time of 12 h in combination with the aforementioned variations. However, the computational modeling of the observed effects has not been performed to date. 1 3 14 Page 6 of 8 Materials for Renewable and Sustainable Energy (2019) 8:14 Fig. 4b. Nevertheless, the benefit for the Si QDs is within the Silicon solar cell coating and characterization with QDs uncertainty, so it practically remained unchanged, as can be seen in Table 4 for the rounded values and their correspond- Based on the previous observations, the 1Si:3CdTe-12 h ing uncertainty. The solar cells with the PMMA/CdTe-8 h, on the other hand, gave the best increase in efficiency of the mixture was selected to incorporate on solar cells, since it exhibited the largest red-shift, and the CdTe-12 h has the individual QDs of this study (0.83% according to results in Table  4), though the mixture of PMMA/Si:CdTe-12 h lowest band-gap energy, or the highest size, as can be seen in Table 1. Besides, its emission spectrum is located at longer was the best of the mixtures (1.53% as given in Table 4). Specifically, these results correspond to the volume ratios wavelength regarding the Si and CdTe-8 h QDs. Sonication was employed to disperse the blend in polymethylmeth- of 1 ml/2 ml for PMMA/(individual QDs: Si, CdTe-8 h, or CdTe-12 h) and 1 ml/1.5 ml for PMMA/(Si:CdTe-12 h). acrylate (495 PMMA Microchem) prior to coating the win- dow side of a set of polycrystalline Si solar cells using the The smoothed EQEs for bare and coated cells are shown in Fig. 4. In all cases, there is a better use of high-energy spin-coating method [16]. There were six solar cells tested for each different condition considered in this study; that is, photons by the use of the QDs, as can be appreciated on the left side of the EQEs and corresponding to the down- test of the bare solar cells and then test of the coated solar cells, considering the individual volume ratios with each shifting effect of the QDs used in this work. In addition, the transition between both types of EQEs, with and without individual QDs and each mixture. Each set of polycrystalline solar cells was measured with- QDs, for the high-energy photons is realized according to the photoluminescence response of the QDs. As shown by out and with the incorporation of the aforementioned down- shifting QDs using an AM1.5 solar simulator (Oriel Sol2A). the EQEs of the Si:CdTe-12 h QDs mixture in Fig. 4d, there is a greater contribution of the more energetic photons and The collected values of open-circuit voltage (V ), short- oc circuit current (J ), and fill factor (FF) were recorded (see also the transition of both EQEs occurs at a longer wave- sc length, this as a combined result of the down-shifting and Table 4) and employed to calculate the power conversion efficiency (PCE) of the devices tested, which increased from the red-shift effects (see Fig.  3 and Table 3). Therefore, it is reasonably to get a greater efficiency, calculated in this case 14.6 to 14.78% in the best case when the down-converting film was incorporated to the solar cells. as an increment of 1.53%. For a comparative analysis, the external quantum effi- ciency (EQE) was registered for the bare and after coated Conclusions solar cells; the Δs reported in Table 4 were determined with the calculated values, not with the rounded ones. Accord- Our results indicate that it is possible to obtain a PCE ing to both results, the Si QDs have presented the smallest difference (0.39% as shown in Table  4) between prior and increase of 1.53% in Si solar cells using a QDs mixture of 1Si:3CdTe12 h; individually, none of the QDs considered after the application of the thin film of PMMA/Si QDs, see Fig. 4a. On the other hand, the EQE spectrum for the solar in our study showed a better efficiency boost. This result is mostly due to the observed red-shift of the DSPL of mixtures cells without and with the thin film of PMMA/CdTe-12 h QDs shows a better benefit than the cells incorporating the which produce photons with increased EQE in Si. Therefore, the ability to modify the down-shifting characteristics of PMMA/Si QDs layer (0.48% as reported in Table 4), see Table 4 Solar cell 2 Samples V (mV) J (mA/cm ) FF (%) PCE (%) oc sc characterization Si QDs Bare cells 612.1 34.90 69.2 14.8 ± 0.3 Si QDs 612.2 34.88 69.5 14.8 ± 0.4 % Δ of Means 0.016 − 0.06 0.43 0.39 CdTe8 QDs Bare cells 604.0 34.39 70.0 14.54 ± 0.36 CdTe8 QDs 604.6 34.56 70.2 14.66 ± 0.45 % Δ of Means 0.09 0.51 0.23 0.83 CdTe12 QDs Bare cells 613.0 34.6 68.9 14.62 ± 0.06 CdTe12 QDs 615.4 34.27 69.3 14.68 ± 0.11 % Δ of Means 0.39 − 0.95 0.58 0.48 1Si:3CdTe12 QDs Bare cells 615.5 34.38 68.8 14.6 ± 0.1 1Si:3CdTe12 QDs 617.9 34.72 68.9 14.78 ± 0.06 % Δ of Means 0.39 0.99 0.15 1.53 1 3 Materials for Renewable and Sustainable Energy (2019) 8:14 Page 7 of 8 14 Fig. 4 EQE spectra of solar cells with and without Si and/or CdTe-12 h QDs quantum dots by employing heterogeneous mixtures could References prove a valuable tool to provide cost reduction of photovol- 1. 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