Fabrication of the Amorphous Silicon Thin Layers in HIT Solar Cells

Transcription

1 Fabrication of the Amorphous Silicon Thin Layers in HIT Solar Cells Abstract The intrinsic and n-type amorphous silicon (a-si) thin layers of the p-type substrate HIT solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD). Transmission spectra, conductivity and deposition rate of the a-si films were measured and the PECVD parameters were optimized through optical and electrical analysis. The substrate temperature and the RF power in the fabrication of the intrinsic a-si layer were optimized as 2 and 1W respectively to obtain the wide optical band gap and the small deposition rate, and the doping concentration (PH 3 /SiH 4 ) in fabrication of the n-type a-si layer was set as 1.5% due to the high conductivity and the relatively wide optical band gap. The p-type substrate HIT solar cells with different doping concentrations in n-type a-si layers were fabricated and the best open circuit voltage was 391mV when the doping concentration was 1.5%, which supported the optimization of the doping concentration mentioned before. The pretreatment of the c-si wafer by hydrofluoric acid before a-si deposition was proved to be useful to passivate the (i)a-si/(p)c-si interface, which has greatly improved both the open circuit voltage and the short circuit current. But the treating time should be optimized; otherwise the performance will deteriorate by over-etching. Keywords: HIT, a-si, PECVD, HF acid pretreatment 1 Introduction HIT (Heterojunction with Intrinsic Thin-layer) solar cells are attracting more and more attention since it was born in 1992 [1-4]. Compared to the a-si solar cells, the efficiency of HIT solar cells is much higher and S.W effect has not been found in this solar cell [5].On the other hand, the low temperature technology of HIT solar cells makes it possible to reduce the cost by the application of low-quality Si materials [6] and the high temperature performance is improved a lot compared to the c-si solar cells [7]. Recently, M. Taguchi et al. has reported the n-type substrate HIT solar cell with the highest efficiency of 21.5% fabricated by PECVD [8]. The p-type substrate nc-si /c-si HIT solar cells with high conversion efficiency were also developed by Zhang et al. with Hot-wire CVD (HWCVD) [9]. At the same time, the industrialization of the HIT solar cell has already been achieved by Sanyo [1]. It seems that the HIT solar cells are becoming the cost-effective alternatives for present c-si and a-si solar cells. The p/n junction of the p-type substrate HIT solar cell is formed by the n-type amorphous silicon (a-si) layer and the p-type crystalline silicon substrate. To passivate the interface of the heterojunction, an intrinsic amorphous silicon thin layer is inserted between the highly doped layer and the substrate, which greatly improves the performance of the solar cells and forms the so-called HIT structure. As the p/n junction is the main part of the solar cell, and the intrinsic buffer layer is the feature of such heterojunction structure, the fabrication of the amorphous silicon layers is very important for the HIT solar cells. In this research, the amorphous silicon layers of the p-type substrate HIT solar cell were fabricated by PECVD. The effect of the PECVD parameters on the properties of the amorphous silicon films and the performance of the HIT solar cells were investigated by the optical and electrical characterization. The substrate temperature, RF power and the doping concentration were optimized to meet the requirements of the HIT solar cells. Besides, the pretreatment of the c-si wafer before a-si deposition by hydrofluoric acid was also investigated and the treating time was optimized by comparing the performances of the HIT solar cells -1-

2 2 Experimental The n-type and intrinsic amorphous silicon thin layers were fabricated in separate chambers of the six-chamber PECVD system excited by standard radio frequency (13.56MHz). 1% SiH 4 and.2% PH 3 (both diluted by H 2 ) were used and the gas pressure of the chamber was kept at 1 Torr. For optimization of the PECVD parameters, we deposited the intrinsic and n-type amorphous silicon materials on Corning 759 glass respectively to study the optical and electrical characteristics. For intrinsic a-si films, the SiH 4 was supported as 4sccm. As the RF power was kept at 15W, the substrate temperature of the samples was changed from 18 to 24.When the substrate temperature was set as 2, the RF power varied from 8W to 15W. For the n-type a-si films, the substrate temperature and RF power were set as 2 and 1W respectively, and the PH 3 /SiH 4 was in the range of.5 and.5. The SHIMADZU UV255 UV-VIS Spectrophotometer was used to measure the transmission spectra and the optical band gap was calculated by Tauc Method [11]. To obtain the conductivity of the films, several Al electrodes were evaporated on the amorphous silicon films to get the resistance by ZC43 High Resistance Meter, and thicknesses of the films were measured by the SLOAN DEKTAK ⅡA Surface Profilometer after part of the film was etched by the SPD4 Plasma Etching Machine. At the same time, deposition rates could also be obtained by the thickness and deposition time. HIT solar cell with a simple configuration of Front Electrode/n-a-Si/i-a-Si/p-c-Si/Al was fabricated, as is shown in Figure 1. Because we mainly investigated the amorphous silicon layers in our research, other structures such as the anti-reflection coating (AR) or the back surface field (BSF) [12-13] was not included. Commercial Czochralski c-si (1) wafers of 1~2.5Ω cm were used. Silicon wafers were cleaned by RCA process. Then the Al back electrodes were thermally evaporated on the rear side of the wafers and thermal annealed in N 2 gas. Prior to the growth of a-si film, the wafer was pretreated by 1% hydrofluoric (HF) acid. The heterojunction were then fabricated by successive deposition of intrinsic a-si layer and n-type a-si layer. The substrate temperature was 2, the RF power was 1W, the gas pressure was 1Torr, and the doping concentration of the n-type a-si layers was between.5% and 2.7%. The thicknesses of the intrinsic and the n-type layers were 5nm and 3nm respectively. Finally, front electrodes were evaporated on the top. The performance of the HIT solar cells was measured under a sun simulator of AM1.5 and 1 mw/cm 2 power density at room temperature. Front Electrode (n) a-si (i) a-si (p) c-si Al Figure 1: The structure of the HIT solar cell in our research 3 Results and discussion 3.1 Fabrication of the intrinsic a-si layer The measured transmission spectra of the intrinsic a-si films were shown in Figure 2, from which we calculated the absorbing coefficient and obtained the optical band gap according to the Tauc Method in Figure

3 Ts=18 Ts= Ts=22 Ts=24 Figure 2: Transmission spectra of the intrinsic a-si layers fabricated in different substrate temperatures (Ts) Ts=18 Ts=2-3-

4 Ts=22 Ts=24 Figure 3: Optical band gap of the intrinsic a-si layers fabricated in different substrate temperatures (Ts) Optical band gap(e g )/ev Deposition rate/(nm/s) Substrate Temperature/ Substrate Temperature/ (a) (b) Figure 4: Effect of the substrate temperature on both optical band gap (a) and deposition rate (b) of the intrinsic a-si layer The calculated optical band gaps of all the samples are plotted in Figure 4 (a). In Figure 4 (a), the optical band gap of the intrinsic a-si film decreases while the substrate temperature is increasing. It is probably caused by the overflow of the hydrogen in higher temperature. As one important difference between the HIT solar cells and the a-si thin film solar cells is that the main active layer is changed from the intrinsic layer to the crystalline silicon base because the minority carrier diffusion length is longer in c-si than in a-si, the optical band gap of the intrinsic layer should be wider to improve the transmittance of this layer. Thus, a smaller substrate temperature should be selected. Figure 4 (b) shows that when the substrate temperature goes up, the deposition rate of the intrinsic a-si layer increases, too. It seems that the dominant reaction in the chemical vapor deposition is not the gas phase polymerization reaction, but the surface reaction process of the radicals absorbed on the substrate. As the surface reaction rate slows down in a lower substrate temperature, the deposition rate also decreases. As the function of the intrinsic thin layer is to passivate the interface between the heavily doped amorphous layer and the crystalline substrate, the defect density of the layer itself should be minimized to increase the solar cell efficiency by enhancing the open-circuit voltage and the fill factor. Therefore, -4-

5 the deposition rate needs to be smaller to obtain a high quality intrinsic layer. On the other hand, as the conductivity of the intrinsic layer is low, the open-circuit voltage will be reduced a lot if the intrinsic layer is too thick. Thus, a smaller deposition rate is also necessary to obtain an ultra thin film. However, as our experiments shows, the quality of the film decreases when the substrate temperature is too low, we would like to select 2 as the substrate temperature in our research. The relation between the deposition rate of the intrinsic a-si and the RF power is shown Figure 5. The deposition rate keeps increasing until the RF power comes up to 12W and gradually decreases. It can be explained as follows. At the beginning, the increasing RF power accelerates the dissociation of SiH 4, and increases the deposition rate. However, when the RF power is even higher, the influence of hydrogen increases because of the increased hydrogen flux to the surface with respect to the SiH 3 flux [14]. The hydrogen can produce etching of the deposited film by breaking the Si-Si bond and hence decrease the deposition rate Deposition rate/(nm/s) RF Power/W Figure 5: The relationship between the intrinsic a-si deposition rate and the RF power For the intrinsic layer in HIT solar cells, the deposition rate should be smaller in order to obtain an ultra thin film with high quality. On the other hand, the RF power should be high enough to make a firm contact between the film and the substrate. As the film made in 8W is found to drop easily from the substrate, we would like to choose 1W as the RF power to fabricate the intrinsic a-si layer. 3.2 Fabrication of the n-type a-si layer PH 3 /SiH 4 =.5 PH 3 /SiH 4 =.15-5-

6 PH 3 /SiH 4 =.27 PH 3 /SiH 4 =.5 Figure 6: Transmission spectra of the n-type a-si layers with different PH 3 /SiH 4 values PH 3 /SiH 4 =.5 PH 3 /SiH 4 = PH 3 /SiH 4 =.27 PH 3 /SiH 4 =.5 Figure 7: The optical band gap (E g ) of the n-type a-si layers with different PH 3 /SiH 4 values. -6-

7 Figure 6 gives the measured transmission spectra of the n-type a-si films, and the optical band gap in each doping concentrations obtained by the Tauc Method is shown in Figure 7 respectively. The calculated optical band gaps of all the samples are plotted in Figure 8 (a) E g /ev 1.95 σ d /(s/m) PH 3 /SiH 4 PH 3 /SiH 4 (a) (b) Figure 8: The variation of the optical band gap (a) and the dark conductivity (σ d ) (b) of the n-type a-si layer caused by different gas phase doping concentration Figure 8 (a) shows that the optical band gap of the n-type a-si is decreasing with the increasing of the PH 3 gas phase doping concentration. It is probably because more phosphorus atoms are going to replace hydrogen atoms in the Si-H bonds with the increasing doping concentration. As the hydrogen flows over, the optical band gap decreased as a result. The variation of the dark conductivity of the n-type a-si versus the PH 3 doping concentration is shown in Figure 8 (b). When PH 3 /SiH 4 increases from.5% to 1.5%, the dark conductivity increases due to the carriers brought by PH 3. However, in the range of 1.5% to 2.7%, the dark conductivity decreases sharply by more than 2 orders of magnitude and reaches its saturation gradually. It can be explained by the saturation of the effective doping and the scattering of the carriers made by excess phosphorus atoms [15]. The HIT solar cells require the window layer to have a wide optical band gap (E g ) to maximize the light absorption in the crystalline silicon base and a high conductivity for reducing cell s series resistance. Considering the effect of the doping ratio (PH 3 /SiH 4 ) on both aspects, we argue that the most suited n-type a-si layer for HIT solar cells can be obtained when the gas phase doping concentration is between.5% and 1.5%. 3.3 Fabrication of the p-type HIT solar cells To find out the best doping concentration of the n-type a-si layer, we fabricated HIT solar cells with the doping ratio from.5% to 2.7%. The performances of those solar cells were listed in Table 1. It showed from Table 1 that the best open circuit voltage (Voc) 391mV was achieved when the doping concentration was 1.5%. It is probably because of the large conductivity and the relatively wide optical band gap at that point, as mentioned before. Although the conductivity is fairly high when the doping ratio is 1.5%, the short circuit current (Isc) is smaller than the sample in.5%. The result can be explained by the large optical band gap of the sample with the doping concentration of.5%. When the doping concentration goes even higher, both the Voc and the Isc decreased obviously, seen from the performance of the sample in 2.7%. The performances of the HIT solar cells are in accordance to the -7-

8 electronic and optical analysis in 3.2. Comparing the performances of the HIT solar cells, we would like to select 1.5% as the doping concentration in the fabrication of the n-type a-si layers. Table 1: Performances of the HIT solar cells with different doping concentration (PH 3 /SiH 4 ) PH 3 /SiH 4 (%) Voc (mv) Isc (ma) Pretreatment by hydrofluoric acid Table 2: Performance of the HIT solar cells pretreated by hydrofluoric acid for different time Treating time (s) Voc (mv) Isc (ma) Previous researches showed that the interface between the intrinsic a-si layer and the c-si substrate is very important for the performance of the heterojunction solar cells [16]. Thus, the surface of the c-si wafer should be pretreated before the deposition of the amorphous silicon layers. In our research, we use hydrofluoric acid to remove the native SiO 2 and passivate the interface. To investigate the effect of hyfrofluoric acid on passivation, we dipped the c-si wafers into the 1% hydrofluoric acid for different time and made p-type substrate HIT solar cells with those wafers. The performances of the HIT solar cells have been listed in Table 2. It can be seen from Table 2 that the open circuit voltage (Voc) and the short circuit current (Isc) have been largely improved after 6 seconds pretreatment. However, when the time goes too long, the performance deteriorated. The deterioration may be caused by the increased defects in the interface brought by the over-etching of the hydrofluoric acid to the silicon wafer. Therefore, the treating time should be optimized and seriously controlled in the pretreatment. 4 Conclusions The intrinsic and n-type amorphous silicon (a-si) thin layers of the p-type substrate HIT solar cells were fabricated by PECVD. Transmission spectra, conductivity and deposition rate of the a-si films were measured and the PECVD parameters were optimized through optical and electrical analysis. The substrate temperature and the RF power in the fabrication of the intrinsic a-si layer were optimized as 2 and 1W respectively to obtain the wide optical band gap and small deposition rate. The doping concentration (PH 3 /SiH 4 ) in fabrication of the n-type a-si layer was set as 1.5% due to the high conductivity and the relatively wide optical band gap. The p-type substrate HIT solar cells with different doping concentrations in n-type a-si layers were fabricated and the best open circuit voltage was 391mV when the doping concentration was 1.5%, which supported the optimization of the doping concentration mentioned before. The pretreatment of the c-si wafer by hydrofluoric acid before a-si deposition was proved to be useful to passivate the (i)a-si/(p)c-si interface, which has greatly -8-

9 improved both the open circuit voltage and the short circuit current. But the treating time should be optimized; otherwise the performance will deteriorate by over-etching. References [1] M. Taguchi, M. Tanaka, T. Matsuyama, et al. Improvement of the conversion efficiency of polycrystalline silicon thin film solar cell, Int'l PVSEC-5, 199, [2] Wakisaka K, Taguchi M, Sawada T et al. More than 16% solar cells with a new HIT (doped-si/non-doped a-si/crystalline Si) structure, Conference Record, the 11nd IEEE PVSC, Las Vegas, 1991, [3] M. Tanaka, M. Taguchi, T. Matsuyama, et al. Development of new a-si/c-si heterojunction solar cells: ACJ-HIT (Artificially Constructed Junction Heterojunction with Trinsic Thin-Layer), Jpn. J. Appl. Phys., 1992, [4] T. Takahama, M. Taguchi, S. Kuroda, et al., High efficiency single- and polycrystalline silicon solar cells using ACJ-HIT structure, 11th E. C. PVSEC, 1992, [5] Y. Hishikawa, M. Sasaki, S. Tsuge, et al. Material control for high-efficiency amorphous silicon solar cells, Mat. Res. Soc. Symp.Proc., 1993, [6] T. Sawada, N. Terada, S. Tsuge, et al. High-efficiency a-si/c-si heterojunction solar cell, First WCPEC, Hawaii, 1994, [7] M. Taguchi, K. Kawamoto, S. Tsuge, et al. HIT TM cells-high-effciency crystalline Si cells with novel structure. Progress in Photovoltaics: Research and Application, 2, [8] M. Taguchi, A. Terakawa, E. Maruyama, et al. Obtaining a higher Voc in HIT cells. Progress in Photovoltaics: Research and Application, 25, [9] Zhang, Zhu, Liu, et al. High-Efficiency n-nc-si:h/p-c-si Heterojunction Solar Cells. Chinese Journal of Semiconductor, 27, [1] M. Tanaka, S. Okamoto, S. Tsuge, et al. Development of HIT solar cells with more than 21% conversion efficiency and commercialization of highest performance HIT modules. 3rd World Conference on Photovoltaic Energy Conversion, Osaka. Japan, 23, [11] Bakry A M, El-Naggar A H. Doping effects on the optical properties of evaporated a-si:h films. Thin Solid Films, 2, [12] Damon-lacoste, P.R.I. Cabarrocas, P. Chatterjee, et al. About the efficiency limits of heterojunction solar cells. Journal of Non-Crystalline Solids, 26, [13] H.D. Goldbach, A. Bink, R.E.I. Schropp. Thin p ++ uc-si layers for use as back surface field in p-type silicon heterojunction solar cells. Journal of Non-Crystalline Solids, 26, [14] V. I. Kuznetsov, R. C. van Oort, and J. W. Metselaar, Plasma deposition of hydrogenated amorphous silicon: Effect of rf power, J. Appl. Phs., 1989, 575 [15] Chen, Gao, Yang et al. Microstructure and electric characteristics of phosphorus-doped hydrogenated silicon films, Piezoel Ectectrics & Acoustooptics, 26, [16] Y. Xu, Z. Hu, H. Diao, et al. Heterojunction solar cells with n-type nanocrystalline silicon emitters on p-type c-si wafers. Journal of Non-Crystalline Solids, 26,