Enhancing the photo-currents of CdTe thin-film solar cells in both short and long wavelength regions

Transcription

1 Enhancing the photo-currents of CdTe thin-film solar cells in both short and long wavelength regions Naba R. Paudel * and Yanfa Yan + Department of Physics & Astronomy, and Wright Center for Photovoltaics Innovation and Commercialization, The University of Toledo, Toledo, OH 43606, USA ABSTRACT The recent increases in the record efficiency of CdTe thin-film solar cell technology largely benefited from enhancements in short circuit current densities (J SC ) in the short-wavelength regions by reducing the thicknesses of CdS window layers. Here we report that the J SC can be enhanced in both short and long wavelength regions by using CdSe as the window layer. Comparing to CdS, CdSe has a higher solubility in CdTe, resulting in stronger interdiffusion at the CdSe/CdTe interface and the formation of CdTe 1-x Se x alloys with high x values. Due to bowing effects, the CdTe 1-x Se x alloys exhibit narrower band gaps than CdTe, enhancing the J SC in the CdTe-based solar cells for long-wavelengths. We further report that the use of combined CdS/CdSe window layers can realize high open circuit voltages and maintain the J SC enhancements. Our results suggest a viable approach to improve the performance of CdTe thin-film solar cells. KEYWORDS: CdSe, CdTe, solar cell, sputtering, CSS * naba.paudel@utoledo.edu; + yanfa.yan@utoledo.edu

2 CdTe has proven to be one of the most promising absorber materials for producing highefficiency and low-cost thin-film solar cells. The record efficiency of laboratory size CdTe thin-film solar cells has dramatically improved in the last three years, from 17.3% in 2011 [1] to 21% in 2014 [2]. The record efficiency has now tied to the best laboratory size Cu(In, Ga)Se 2 (CIGS) thin-film solar cells [3]. In addition to being able to achieve high efficiency cells, CdTe exhibits advantages for fabricating thin-film solar cells, such as near optimal band gap and easy for manufacturing [4]. The recent rapid increase in record cell efficiency is largely due to the enhancement of the short circuit current (J SC ) in the short-wavelength regions (the so called improvement in blue response). The improvement in open circuit voltage (V OC ) has not been significant [3, 5]. High efficiency CdTe solar cells typically use n-type CdS window layers to form hetero junctions [5 8]. During CdTe depositions and CdCl 2 heat treatments, interdiffusion occurs at CdS/CdTe interfaces, forming CdS 1- yte y window layers and CdTe 1-x S x in CdTe absorbers. It has been shown that CdS 1-y Te y is highly defective and photo inactive [9]. Photons absorbed by the CdS 1-y Te y window layer will not generate any photocurrents. Therefore, reducing the thickness of the CdS window layer has been the most used approach to enhance the blue response and therefore the J SC for CdTe thin-film solar cells. Due to bowing effects, CdTe 1-x S x alloys exhibit reduced band gaps as compared to CdTe [10 12]. Because CdTe 1-x S x is photoactive, the diffusion of S into CdTe results in enhanced photocurrent collection in the long wavelength regions [ nm]. To realize a significant band gap reduction (photocurrent enhancement), a significant amount of S must be incorporated into CdTe [13]. However, the solubility of CdS in CdTe has found to be very low, partially due to the size mismatch between S and Te [14 16]. Therefore, J SC gain via S diffusion has been found not effective. Se has less size mismatch with Te than S and therefore has a much higher solubility in CdTe than S [17, 18]. Experimental results indicate that the band gap of CdTe 1-x Se x can be as low as 1.32 ev with x = ~0.38 [19]. Therefore, using CdSe as the window layer may significantly enhance the interdiffusion and reduce the band gap of CdTe and therefore increase the current enhancement in both short and long wavelength regions. In the past, thin-film solar cells based on Cd (Se, Te) alloys have been fabricated, but the cells have shown rather low efficiencies (lower than 5% under AM1.0 illumination) [20 25]. In this study, we report on the realization of ~15% efficiency CdTe thin-film solar cells using CdSe as the window layers. We find that J SC enhancements in both short and long wavelength regions can be realized by using CdSe window layers. Though CdSe has a narrower band gap (~1.7 ev) [20] than CdS (2.4 ev), the enhancement in current collection in short wavelength regions can still be achieved by optimizing the thickness of the CdSe layer. The J SC enhancements in the long wavelength regions are due to the strong interdiffusion at the CdSe/CdTe interface, which leads to the formation of CdTe 1-x Se x alloys with high x values. Due to bowing effects, the CdTe 1-x Se x alloys exhibit narrower band gaps than CdTe, enhancing the J SC in the CdTe-based solar cells for longwavelengths. Our best small area cell with a CdSe window layer has shown a cell efficiency of 14.7% under an AM1.5 illumination, comparable to that of the best reference cell with a CdS window layer (14.8%). Due to the band gap reduction, the V OC is about 40 mv lower than that of the best reference

3 cell using a CdS window layer. We show that using combined CdS/CdSe bi-layers can reduce the V OC loss and maintain the J SC enhancements. Our results suggest a viable approach to improve the performance of CdTe thin-film solar cells. CdTe solar cells were fabricated on commercial SnO 2 :F/SnO 2 coated soda lime glass substrates supplied by NSG North America. CdSe thin films were magnetron sputtered with a 50 watt RF power at 250 C substrate temperature and in an ambient of 10 mtorr argon pressure [26]. CdTe absorber layers were grown at 607 C substrate temperature by close-space sublimation (CSS) using 5N (99.999%) purity source materials supplied from Materion [27]. The typical thickness of CSS-grown CdTe layers was about 4 m. Post-deposition CdCl 2 treatment was carried out at 390 C for 30 minutes in dry air ambient [28]. After cooling to the room temperature, the thin-film samples were rinsed with methanol and transferred into a thermal evaporator for back contact deposition, without pursuing any chemical treatment step [29]. Cu (3 nm)/au (35 nm) bilayer metal contacts were thermally evaporated with a shadow mask containing 20 dots with an active area of 0.08 cm 2 per dot. The devices were annealed at 200 C for 20 minutes in N 2 ambient to facilitate Cu diffusion. For comparative studies, sputter deposited CdS [28] and combined CdS/CdSe window layers were also used to complete the CdTe solar cells. Finished solar cells underwent J-V characteristics measurements under an AM1.5 illumination and external quantum efficiency (EQE) measurements at room temperature. The best cells in the series have applied 110 nm thick thermally evaporated MgF 2 anti-reflection coating (ARC) layers. In our cell fabrications, deposition parameters such as substrate temperature and deposition time for CdTe deposition and CdCl 2 heat treatment temperature and time are fixed. The amount of Se diffused into CdTe in CdTe cells will mostly depend on the thicknesses of the CdSe window layers. We therefore have examined how the thickness of the CdSe widow layer may affect the performance of CdTe/CdSe cells, by varying the thickness of CdSe layer from 0 nm to 750 nm. About 20 dot cells from each device were measured and the averaged results including V OC, J SC, fill factor (FF), and efficiency (Eff) are shown in Table 1. The results from the best dot cell of each CdSe thickness are shown in the parentheses. The cells with 60 nm, 100 nm, and 150 nm thick CdSe layers showed high average J SC (>25.5 ma/cm 2 without ARC). Such high J SC s indicate that the majority of the CdSe layers had been consumed due to the interdiffusion at the CdSe/CdTe interfaces, which occurred during the high temperature CdTe deposition and CdCl 2 heat treatment. When the thickness of CdSe layer was increased to ~350 nm, the average J SC decreased to 15.4 ma/cm 2. Such a large reduction in J SC indicates that a significant portion of CdSe window layer remained after the solar cell was fabricated. Further increase of the CdSe layer thickness to 750 nm led to a very low average J SC (5.2 ma/cm 2 ), due to significant photon absorption by the CdSe window layer. The V OC s are generally low as compared to the V OC s for CdTe cells using CdS window layers [27, 28]. The lower V OC s are mainly due to the reduced band gaps of CdTe 1-x Se x alloys. However, most of the V OC s are higher than that of our reference devices without any window layers [see table 3]. The highest average V OC (766 mv) was observed for cells using about the 100 nm CdSe window layers. All the cells showed poorer FFs as compared to the cells using CdS window layers.

4 To better understand the J SC variations, EQE of the best cells for each CdSe window layer thickness was measured and the results are shown in Figure 1. For the cells with 60 nm, 100 nm, and 150 nm thick window layers, the EQE curves showed high collection in short wavelength region, giving superior blue response. The EQE curves indicated nearly zero loss in the wavelength regions above the band gap of CdSe, suggesting very little residual CdSe window layers. During high temperature deposition of CdTe and post deposition CdCl 2 heat treatment, almost the entire CdSe layers had diffused into CdTe, forming CdTe 1-x Se x alloys. The blue responses are above 85% at 400 nm, which are nearly as good as the blue responses of the record cells [30]. Considering that our cells were fabricated on commercial FTO-coated SLG substrates without any ARCs, such blue responses are very good. When the CdSe thicknesses increased to 350 nm and 750 nm, the EQE spectra showed greatly reduced blue response, dropping to ~34% and ~0% at 400 nm, respectively. These J SC reductions were due to the photon absorptions from the residual CdSe window layers. It is noted that the blue responses started to decrease at wavelengths around 750 nm-780 nm. These indicate that significant Te have diffused in the CdSe window layers, forming CdSe 1-x Te x layers which have smaller band gaps than the CdSe layers. The enhanced current collections in the long wavelength regions, 825 nm 930 nm, in the EQE spectra (Fig. 1) indicate the variations of Se contents in CdTe 1-x Se x alloys. When the thicknesses of the CdSe window layer increased, the observed redshift for the absorption edge increased. Because the CdSe window layers were much thinner than the CdTe absorbers (~4 m), the diffusion of CdSe into CdTe will only form CdTe 1-x Se x with x values smaller enough to exhibit bowing effects. Therefore, the improved redshift is anticipated due to the band gap reduction of the absorbers [12, 31]. The band gaps of the CdTe 1-x Se x alloys can be estimated from the absorption edges in the EQE spectra using the approach reported by Helmers et al. [32]. As shown in Table 2, the measured band gaps decreased from ev to ev as the thickness of the initial CdSe film increased from 0 nm to 750 nm. For the cell showing the highest cell efficiency (with a 100 nm thick CdSe window layer), the band gap was measured to be ev, about ev lower than that of the standard CdS/CdTe cell (1.43 ev). The cell with 60 nm CdSe window layer showed the highest band gap, but not the highest V OC. A possible explanation is that the 60 nm thick CdSe had more complete consumption than other window layers during CdTe depositions and CdCl 2 heat treatments. Therefore, there could be some direct contact between FTO and the absorber layer, a leading source for poor V OC for CdTe thin-film solar cells. To compare how window layers affect the performance of CdTe cells, we have fabricated cells with four types of window layers: window-free (0 nm), standard CdS (~ 130 nm), typical CdSe (~ 100 nm), and combined window layer (CdS (15 nm)/cdse (100nm)). Figures 2(a) 2(d) show the measured V OC, J SC, FF, and Eff distribution of 20 dot cells for each of the window layers. The cells without window layers showed much poorer efficiencies than the cells with window layers, despite good current collections. For these cells, the p-n junctions are at the CdTe/TCO interfaces, suffering from non-radiative recombination loss at the interfaces. When a ~130 nm thick CdS layer was incorporated, the V OC s were improved significantly. However, the CdS window layer absorbs

5 photons in the short wavelength region and result in current loss. Therefore the J SC s for cells with window layers were lower than that of the cells without window layers. When a ~100 nm thick CdSe layer was incorporated, both improved V OC s and J SC s were observed compared to the window-free device. The average V OC was not as high as that of the cell with standard CdS window layer, but the average J SC is higher [27, 28]. Because all high V OC CdTe-based solar cells use CdS window layers, a very thin CdS layer (~15 nm) was added to the CdSe window layer, forming the CdS (15 nm)/cdse (100nm) combined window layer. As shown in Fig. 2, the cells with combined window layers exhibited both higher V OC s and J SC s than the cell with 130 nm CdS window layer. However, these cells showed much poorer FF than the cells using CdSe or CdS window layers, resulting in lower conversion efficiencies. If the FF can be improved to be comparable to the cells with CdS window layers, higher efficiencies should be expected for cells. The performance parameters of the best cells for each of different window layers are presented in Table 3. These best cells were coated with about 110 nm thick MgF 2 ARCs. The best CdSe/CdTe cell showed a 14.7 % cell efficiency under AM1.5 illumination, which is nearly as good as the efficiency of the standard CdS/CdTe cell, 14.8%. Even with a poor FF of 64.1%, the best cell with a combined CdS/CdSe window layer showed an efficiency of 14.1%, which is only slightly lower than the standard cell with CdS window layer. Figures 3(a) and 3(b) show the light current voltage (JV) curves and EQE spectra of the CdTe cells presented in Table 3. The light JV curves show that the cell with a 130 nm CdS window layer has a slightly lower J SC than the window-free cell. The observed J SC difference is lower than expected. However, the light JV curve does not provide much insight for understanding why the difference is lower than expected. The cell with a 100 nm thick CdSe window layer and the cell with a CdS (15 nm)/cdse (100nm) combined window layer exhibited similar J SC. Both are much higher than that of the cell with a 130 nm CdS window layer. The observed EQE spectra shown in Figure 3(b) provide insights for understanding the J SC variations shown in Table 3 or Fig. 3(a). The spectrum for the CdTe cell without window layer showed no current loss for the blue photons. Because there is no window layer, the absorption edge in the long wavelength region corresponds to the band gap of pure CdTe. However, the cell showed current loss in the long wavelength regions. This explains why the J SC for the cell with 130 nm CdS window layer is not significantly lower than that of the windowfree cell. As a 130 nm CdS window layer was added, the absorption edge showed a ev redshift. When a 100 nm thick CdSe window layer was added, a much larger redshift, 0.1 ev, was observed. Furthermore, the cell showed almost no current loss for the blue photons. With the enhanced current collections in both the short and long wavelength regions, the cell with a ~100 nm CdSe window layer collected about 3.2 ma/cm 2 more current than the standard cell with a ~130 nm CdS window layer. It is noted that the cell with combined CdS (15 nm)/cdse (100 nm) window layer showed an almost identical EQE spectrum with the cell using a 100 nm CdSe window layer. This strongly suggests that further improvement on the cell with combined window layer is possible.

6 In conclusion, we have shown that the J SC s of CdTe cells can be enhanced in both the short and long wavelength regions by using CdSe window layers. The J SC enhancements were found to be due to no or very thin window layer and the absence of a miscibility gap for CdSe-CdTe system. The best small area cell with a CdSe window layer has shown a cell efficiency of 14.7% under an AM1.5 illumination, comparable to that of the best reference cell with a CdS window layer (14.8%). We have further shown that the use of combined CdS/CdSe window layers led to high V OC and preservation of the J SC enhancements gained from using the CdSe window layer. Our results suggest a possible approach for further improving the performance of CdTe thin-film solar cells. ACKNOWLEDGEMENT The authors would like to thank Prof. Alvin D. Compaan and Dr. Dohyoung Kwon for their valuable suggestions and Dr. David Strickler from NSG (Pilkington NA) Toledo, OH for supplying SnO 2 :F/SnO 2 coated soda-lime glass substrates. This work was supported by the U.S. Department of Energy, Foundational Program to Advance Cell Efficiency (F-PACE) program under contract No and the Ohio Research Scholar Program (ORSP). REFERENCES 1. M.A. Green, K. Emery, Y. Hishikawa, W. Warta, and E.D. Dunlop, Prog. Photovoltaics 20, 606 (2012) M.A. Green, K. Emery, Y. Hishikawa, W. Warta, and E.D. Dunlop, Prog. Photovoltaics 22, 701 (2014). 4. M.D. Wild-Scholten, M. Sturm, M.A. Butturi, M. Noack, K. Heasman, and G. Timo, in Proceedings of 25 th EUPVSEC, 2010, pp J. Britt and C. Ferekides, Appl. Phys. Lett. 62, 2851 (1993). 6. X. Wu, Solar Energy 77, 803 (2004). 7. L. Kranz, C. Gretener, J. Perrenoud, R. Schmitt, F. Pianezzi, F. La Mattina, P. Blosch, E. Cheah, A. Chirila, C.M. Fella, H. Hagendorfer, T. Jager, S. Nishiwaki, A.R. Uhl, S. Buecheler and A.N. Tiwari, Nat. Commun. 4, 2306 (2013). 8. B.A. Korevaar, J.R. Cournoyer, O. Sulima, A. Yakimov, and J.N. Johnson, Prog. Photovoltaics (2014) DOI: /pip N.W. Duffy, L.M. Peter, and R.L. Wang, J. Electroanal. Chem. 532, 207 (2002). 10. K. Wei, F.H. Pollak, J.L. Freeouf, D. Shvydka, and A.D. Compaan, J. Appl. Phys. 85, 7418 (1999) 11. D. W. Lane, Sol. Energy Mater. Sol. Cells 90, 1169 (2006).

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8 Table Captions: Table 1. Average performance parameters of 20 dot cells for CdTe cells with different thickness of sputtered CdSe window layers. Data shown in parentheses are from the best cells. Table 2. Effective band gap of the CdSe x Te 1-x absorber as a function of CdSe window layer thickness. The data shown in the last row were from the cells completed with ~30 nm thick sputtered CdS window layers. Table 3. Performance parameters of best CdS/CdTe, CdSe/CdTe and window free CdTe cells. These solar cells have ~110 nm thick thermally evaporated MgF 2 ARCs. The diode factor (A) and reverse saturation current (J O ) were estimated using dark J-Vs (not shown). Figure Captions: Figure 1: Measured EQE spectra of CdTe cells with CdSe window layers for five different CdSe thicknesses. Figure 2: Distribution of a) V OC b) J SC, c) FF and d) Eff of CdTe solar cells with four types of window layers. The cell performance parameters were from 20 dot cells. These cells do not have ARCs. [Note: The confidence limit of the box is 75% - 25% and that for whisker is 95% - 5%]. Figure 3: a) J-V and b) EQE curves of CdTe cells with four different window layers: window free, 130 nm thick CdS layer, 100 nm thick CdSe layer and combined 15 nm CdS/100 nm CdSe window layer. The solar cells received 110 nm thick thermally evaporated MgF 2 ARCs.

9 List of Tables: Table 1 CdSe thickness V OC (mv) J SC (ma/cm 2 ) FF (%) Eff (%) 0 nm 687 (709) 24.7 (24.8) 55.1 (55.8) 9.3 (9.8) 60 nm 716 (731) 26.2 (26.5) 59.3 (60.4) 11.1 (11.8) 100 nm 766 (779) 25.7 (26.4) 67.4 (68.3) 13.3 (14.0) 150 nm 757 (768) 25.5 (26.6) 61.9 (66.0) 12.4 (13.5) 350 nm 695 (713) 15.4 (17.7) 62.9 (66.9) 6.7 (8.5) 750 nm 709 (724) 5.2 (6.4) 69.3 (69.8) 2.5 (3.2) Table 2 Sputtered CdSe thickness (nm) Effective band gap of CdSe x Te 1-x absorber (ev) (CdS) 1.43 (CdS x Te 1-x )

10 Table 3 Window layer V OC (mv) J SC (ma/cm 2 ) FF (%) Eff (%) R S ( -cm 2 ) R SH ( -cm 2 ) A J O (ma/cm 2 ) None CdS CdSe CdS/CdSe List of Figures:

11 Figure 1

12 Figure 2

13 Figure 3