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The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells
Optical Materials xxx (2016) 1e6
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The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells Jaran Sritharathikhun*, Sorapong Inthisang, Taweewat Krajangsang, Patipan Krudtad, Suttinan Jaroensathainchok, Aswin Hongsingtong, Amornrat Limmanee, Kobsak Sriprapha Solar Energy Technology Laboratory (STL), NECTEC, National Science and Technology Development Agency (NSTDA), 112 Thailand Science Park, Thanon Phahonyothin Tambon Khlong Nueng, Amphoe Khong Luang, Pathum Thani 12120, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2016 Received in revised form 18 October 2016 Accepted 1 November 2016 Available online xxx
Hydrogenated amorphous silicon oxide (a-Si1xOx:H) film was used as a buffer layer at the p-layer (mcSi1xOx:H)/i-layer (a-Si1xGex:H) interface for a narrow band gap hydrogenated amorphous silicon germanium (a-Si1xGex:H) single-junction solar cell. The a-Si1xOx:H film was deposited by plasma enhanced chemical vapor deposition (PECVD) at 40 MHz in a same processing chamber as depositing the p-type layer. An optimization of the thickness of the a-Si1xOx:H buffer layer and the CO2/SiH4 ratio was performed in the fabrication of the a-Si1xGex:H single junction solar cells. By using the wide band gap aSi1xOx:H buffer layer with optimum thickness and CO2/SiH4 ratio, the solar cells showed an improvement in the open-circuit voltage (Voc), fill factor (FF), and short circuit current density (Jsc), compared with the solar cells fabricated using the conventional a-Si:H buffer layer. The experimental results indicated the excellent potential of the wide-gap a-Si1xOx:H buffer layers for narrow band gap aSi1xGex:H single junction solar cells. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydrogenated amorphous silicon oxide layer Buffer layer Amorphous silicon germanium single junction solar cell
1. Introduction Recently, wide band gap hydrogenated amorphous silicon oxide (a-Si1xOx:H) and hydrogenated microcrystalline silicon oxide (mcSi1xOx:H) materials have attracted much attention as promising materials for the fabrication of high performance Si-based thin film solar cells, wherein they are applied as p-, n- and i-layers instead of the conventional hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (mc-Si:H) layers. Wide band gap p-type microcrystalline silicon oxide (p-mc-Si1xOx:H) has been used as a window layer material because of its low light absorption and high conductivity [1]. On the other hand, intrinsic hydrogenated amorphous silicon oxide (i-a-Si1xOx:H) has been used as the top cell of multi-junction solar cells for enhancing the spectral response in the short wavelength region [2e6]. In the case of silicon-based thin film solar cells, it is widely known that buffer
* Corresponding author. E-mail address: [email protected] (J. Sritharathikhun).
layers play the role of controlling the quality of the interface between the p- and i- layers in order to increase both the open circuit voltage (Voc) and the fill factor (FF). For this purpose, conventional intrinsic hydrogenated amorphous silicon (i-a-Si:H) has been used [7e11]. Some research groups have used i-a-Si1xOx:H as buffer layers in a-Si:H solar cells in order to obtain a good compromise between a relatively high effective built-in potential (Vbi) and a relatively low recombination rate at the p/i interface [12e14]. Our group has previously reported a high efficiency of 9.4% for a hydrogenated amorphous silicon germanium (a-Si1xGex:H) single junction solar cell by using the a-Si1xGex:H VU-shape band gap profiling technique [15] and 11% for a-Si:H/a-Si1xGex:H tandem solar cells using the conventional a-Si:H as a buffer layer for the interface between wide gap p-mc-Si1xOx:H and narrow gap i-aSi1xGex:H [16]. However, there is still room for investigation of the a-Si1xOx:H buffer layer in narrow band gap a-Si1xGex:H solar cells as well as wide band gap a-Si:H and a-Si1xOx:H solar cells. Moreover, the use of wide band gap a-Si1xOx:H buffer layer between wide band gap p-mc-Si1xOx:H and narrow band gap i- aSi1xGex:H, (resulting in a new device structure) needs to be
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Please cite this article in press as: J. Sritharathikhun, et al., The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells, Optical Materials (2016), http://dx.doi.org/10.1016/ j.optmat.2016.11.002
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J. Sritharathikhun et al. / Optical Materials xxx (2016) 1e6 Table 1 Deposition condition for a-Si1xOx:H films. Parameters
Values
SiH4/H2/CO2 Plasma frequency Deposition temperature Deposition pressure Power density Thickness
1/10/0e0.6 40 MHz 180 C 500 mTorr 42 mW/cm2 0.4 mm
investigated. In this paper, we have applied a-Si1xOx:H as a buffer layer in aSi1xGex:H single junction solar cells. The electrical, optical, and structural properties of the prepared a-Si1xOx:H films were characterized. The study focused on the effects of variation in thickness of the a-Si1xOx:H buffer layer and the carbon dioxide (CO2) to the silane (SiH4) ratio (during the thin film preparation) on the performance of the a-Si1xGex:H single-junction solar cells. A comparison between solar cells containing the conventional a-Si:H buffer layer and those containing a-Si1xOx:H buffer layer, developed in this study, was also carried out. 2. Experimental details 2.1. Deposition of the a-Si1xOx:H films The a-Si1xOx:H films were prepared on soda lime glass and silicon substrate by high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 40-MHz and substrate temperatures of approximately 180 C in the chamber used for the deposition of p-layer. A gas mixture of silane (SiH4), hydrogen (H2), and CO2 was used as the precursor. The base pressure in the deposition chamber was kept at about 2 106 Torr by using a turbo molecular pump backed up by a rotary pump. The deposition conditions for a-Si1xOx:H films are summarized in Table 1. The CO2/SiH4 ratio, during the preparation of a-Si1xOx:H films, was varied between 0.0 and 0.6 to obtain films of varying thickness
(maximum thickness ¼ 0.4 mm). The Eopt of samples was measured by spectroscopic ellipsometry (SE) (J.A. Woollam, V-VASE series). The data was fitted and analyzed using the Tauc-Lorentz model [17]. 2.2. Fabrication of a-Si1xGex:H single junction solar cells a-Si1xGex:H single-junction solar cells were fabricated using the cluster type plasma enhanced chemical vapor deposition (PECVD) technique to obtain the following device structure: TCO coated glass (U-type, Asahi)/aluminum-doped zinc oxide (ZnO:Al)/ p-mc-Si1xOx:H (30 nm)/a-Si1xOx:H buffer layer (0e9.8 nm)/aSi1xGex:H (300 nm)/n-type microcrystalline silicon (n-mc-Si:H) (30 nm)/ZnO:Al/Ag/Al, as shown in Fig. 1(a); the band diagram and the optical band gap of the p-i-n layer of the solar cells are shown in Fig. 1(b). For the deposition of p-mc-Si1xOx:H layer, a-Si1xOx:H buffer layer, and n-mc-Si:H layer, 40 MHz generator was used, while 27 MHz generator was employed for the i-a-Si1xGex:H layer deposition. The a-Si1xOx:H buffer layer was deposited in a same processing chamber as depositing the p-layer after p-layer deposition. In order to investigate the role of the buffer layer on the performance of the solar cells, a-Si1xOx:H buffer layer of varying thickness (0e9.8 nm) and varying the CO2/SiH4 ratio in the range of 0.0e0.8, were used. The solar cells were isolated by laser scribing to an active area of 1.0 1.0 cm2. The photovoltaic (PV) parameters of single junction solar cells were investigated under standard test conditions (STC) at AM 1.5, 100 mW/cm2, 25 C using the light source and filter of a solar simulator (Wacom, model WXS-155S-L2). The external quantum efficiency (EQE) of the solar cells was characterized using a quantum efficiency measurement system (PV Measurement, QEW7). 3. Results and discussions 3.1. Effect of the CO2/SiH4 ratio on the electrical and optical properties of the a-Si1xOx:H films Fig. 2 shows the dark (sd) and photo conductivity (sph) of the aSi1xOx:H films deposited using various CO2/SiH4 ratios in chambers
Fig. 1. (a). Schematic structure of the a-Si1xGex:H single junction solar cell. (b). Schematic band diagram of the p-i-n layer of the a-Si1xGex:H single junction solar cell.
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0.0) to 2.01 107 S/cm (CO2/SiH4 ratio of 0.6). This result suggests that the increase of oxygen (O) concentration via a high CO2 flow rate has a deleterious effect on the electrical properties of the a-Si1xOx:H films. Since these films were deposited in the chamber used for the deposition of p-layer, contaminated by diborane gas, the sd of these samples was found to be relatively higher than that of the i-aSi1xOx:H films (normally 109e1010 S/cm) that were deposited in the chamber used for the deposition of i-layer. Therefore, the aSi1xOx:H films deposited in the chamber used for the deposition of p-layer tended to be weakly p-type. Table 2 presents the fitting parameters of the Tauc-Lorentz model of the a-Si1xOx:H films deposited on glass substrate with various CO2/SiH4 ratios, fitted using the Tauc-Lorentz model using the spectroscopic ellipsometry technique. We utilized Eopt from Tauc gap (Eg in Table 2) in order to prevent the error from Tauc's plot estimation. Eopt increased from 1.76 (a-Si:H) to 1.95 (aSi1xOx:H) eV with an increasing CO2/SiH4 ratio from 0.0 to 0.6. Fig. 2. The dark (sd) and photo conductivity (sph) of the a-Si1xOx:H films deposited using various CO2/SiH4 ratios in chambers used for the deposition of p- and i-layers. Table 2 Fitting parameters from SE measurement of a-Si1xOx:H films as a function of CO2/ SiH4 ratio. CO2/SiH4 ratio Mean square error Roughness (nm) Thickness (nm) Eg (eV) (MSE) [18] 0.0 0.2 0.4 0.6
5.34 11.50 13.93 12.46
3.34 4.35 3.96 5.44
370.29 390.57 405.33 387.42
1.76 1.79 1.85 1.95
* The Mean Squared Error (MSE) is the value used to quantify the difference between the generated and measured data.
used for the deposition of p- and i-layers. sd of the a-Si1xOx:H films deposited in the chamber used for the deposition of p-layer decreased from 5.12 108 to 1.40 108 S/cm as the CO2/SiH4 ratio increased from 0.0 to 0.6. The sph decreased from 2.30 105 (CO2/SiH4 ratio of
3.2. Effects of a-Si1xOx:H buffer layer thickness on the performance of a-Si1xGex:H single junction solar cells In our previous work, we successfully achieved a high efficiency of 9.4% with a-Si1xGex:H single solar cells [15]. For this, the aSi1xGex:H bottom cell was fabricated by using a combination of the V- and U-shape band gap profiling techniques. Therefore, in this experiment we used the same band gap profiling technique to deposit the i-a-Si1xGex:H absorber layer. The CO2/SiH4 ratio was kept constant at 0.2. We have optimized the a-Si1xOx:H buffer layer thickness by varying the deposition time by 0, 75, 110, 150, 190 and 225 s. The estimated thickness values of 0, 3.3, 4.9, 6.8, 8.5 and 9.8 nm of the a-Si1xOx:H buffer layers were measured and were fitted using the spectroscopic ellipsometry technique. Please note that the thickness of a-Si1xOx:H films was the sum of the surface roughness and the bulk layer thickness. Fig. 3(a) and (b) show the PV parameter and illuminated current-voltage (JV) curve of the a-Si1xGex:H single-junction solar
Fig. 3. (a). PV parameters of the a-Si1xGex:H single junction solar cells with a-Si1xOx:H buffer layers of varying thickness. (b). Illuminated JV curve of the a-Si1xGex:H single junction solar cells with a-Si1xOx:H buffer layers of varying thickness.
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Fig. 4. Spectral response of the a-Si1xGex:H single junction solar cells with aSi1xOx:H buffer layers of varying thickness.
cells with various a-Si1xOx:H buffer layer thickness. The cell without the buffer layer shows a Voc of only 0.77 V, with an FF of 0.68. By inserting the a-Si1xOx:H buffer layer of 3.3 nm, Voc increased to 0.78 V, however, FF dropped to 0.61 because very thin buffer layer probably leads to poor film quality and island growth. With thicker buffer layers (4.9, 6.7, 8.5 nm), both Voc and FF increased, and the conversion efficiency (Eff) reached the maximum value of 9.73%, with Voc ¼ 0.80 V, Jsc ¼ 17.7 mA/cm2 and FF ¼ 0.69, with 8.5 nm thick buffer layer. When 9.8 nm thick buffer layer was used, Voc seemed stable but the FF significantly dropped from 0.69 to 0.63. Increasing of Voc with increasing buffer-layer thickness
indicated that wide band gap a-Si1xOx:H buffer layer played a crucial role to compensate the band gap discontinuity between pmc-Si1xOx:H and i-a-Si1xGex:H layer. In this experimental result, it was found that a thickness of buffer layer at around 6.0e8.5 nm is required for band gap discontinuity between the p and i-layers [19,20]. However, with too thick a buffer layer, it was found that the FF of solar cell tended to decrease due to the decreasing internal electric field [20,21]. Fig. 4 shows the external quantum efficiencies of solar cells using a-Si1xOx:H buffer layer with various thickness. The spectral response was slightly improved by inserting thin a-Si1xOx:H buffer layer compared to the solar cell fabricated without buffer layer. The optimum wide band gap buffer layer is effective in reducing the recombination defects and trap centers at the p/i interface. As a result, a better carrier collection and internal electric field distribution of the cell with the optimum buffer thickness showed a better solar cell performance compared to a buffer-less solar cell. This effect not only improved the Voc an FF but it also slightly improved the Jsc [22]. However, when the thickness of the aSi1xOx:H buffer layer became higher than 8.5 nm, the solar cell showed lower spectral response due to higher light absorption by thick a-Si1xOx:H buffer layer, which agreed with the decrease in Jsc. 3.3. Effects of CO2/SiH4 ratio of a-Si1xOx:H buffer layer on the performance of a-Si1xGex:H single-junction solar cells In the previous part, we discussed the optimization of the aSi1xOx:H buffer layer thickness. In this part, we have attempted to discuss the effects of CO2/SiH4 ratio of a-Si1xOx:H buffer layer between p/i interface on the performance of a-Si1xGex:H singlejunction solar cells by keeping constant the other layer deposition
Fig. 5. (a). PV parameters of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer. (b). Illuminated JV curve of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer.
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help to improve optical and p/i interface properties for multijunction Si-based thin film solar cells, allowing more efficient solar cell performance. Acknowledgement This work was supported by National Electronics and Computer Technology Center (NECTEC) THAILAND under Grant No. P1350323. References
Fig. 6. Spectral response of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer.
conditions. The CO2/SiH4 ratio was varied from 0.0 to 0.8 while the thickness of the a-Si1xOx:H buffer-layers was maintained at about 8.5 nm (optimum thickness from previous part). Fig. 5(a) and (b) present the PV parameters and illuminated JV curve of cells fabricated with various CO2/SiH4 ratios for the buffer layer between p/i interface. The results demonstrated that PV parameters were significantly improved by inserting a tiny amount of CO2/SiH4 ratio (from 0.0 to 0.2). At the CO2/SiH4 ratio of higher than 0.2, the Voc still increased to 0.80 V at the CO2/SiH4 ratio of 0.8. According to these results, it was observed that the CO2/SiH4 ratio played a role in improving the Voc of solar cells because of the wider band gap of the a-Si1xOx:H buffer layer. The Jsc improved abruptly from 17.0 to 18.7 mA/cm2 when the CO2/SiH4 ratio increased from 0.0 to 0.4 and then it seemed to remain constant at the higher CO2/SiH4 ratio. As shown in Fig. 6, the EQE of the solar cells gradually increased with increasing buffer layer CO2/SiH4 ratio. However, the FF reached its maximum value at the CO2/SiH4 ratio of 0.2 and then gradually decreased due to the effect of band gap mismatch between the buffer and the i-layer, and the increase of the defect density [23,24] in the buffer layer as well as increase of a-Si1xOx:H buffer layer resistance. As a result, the conversion efficiency (Eff) tended to decrease with increase of the CO2/SiH4 ratio due to the decrease of the FF. 4. Conclusions We have applied the a-Si1xOx:H buffer layer in the aSi1xGex:H single junction solar cell. Emphasis has been placed on the buffer thickness and the CO2/SiH4 ratio for layer deposition. The a-Si1xOx:H buffer layer in the study showed weakly p-type doped property due to boron contamination in p-type process chamber. The optical band gap of a-Si1xOx:H can be adjusted to be wider than that of the conventional a-Si:H by increasing the CO2/SiH4 ratio, however, a tradeoff between optical and electrical properties should be considered. Significant improvements of PV parameters such as Voc, Jsc, FF and efficiency were achieved by inserting the thin a-Si1xOx:H buffer layer between p-mc-SiO:H/i-a-SiGe:H interface with low CO2/SiH4 ratios. According to our results, the optimum thickness of a-Si1xOx:H buffer layer was about 8.5 nm. The highest efficiency of 9.73% has been achieved from the cell using a CO2/SiH4 ratio of 0.2 for the buffer layer deposition (Voc ¼ 0.80 V, Jsc ¼ 17.7 mA/cm2 and FF of 0.69). Additional optimization and better understanding of this a-Si1xOx:H buffer layer possibly will
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The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells Jaran Sritharathikhun*, Sorapong Inthisang, Taweewat Krajangsang, Patipan Krudtad, Suttinan Jaroensathainchok, Aswin Hongsingtong, Amornrat Limmanee, Kobsak Sriprapha Solar Energy Technology Laboratory (STL), NECTEC, National Science and Technology Development Agency (NSTDA), 112 Thailand Science Park, Thanon Phahonyothin Tambon Khlong Nueng, Amphoe Khong Luang, Pathum Thani 12120, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 July 2016 Received in revised form 18 October 2016 Accepted 1 November 2016 Available online xxx
Hydrogenated amorphous silicon oxide (a-Si1xOx:H) film was used as a buffer layer at the p-layer (mcSi1xOx:H)/i-layer (a-Si1xGex:H) interface for a narrow band gap hydrogenated amorphous silicon germanium (a-Si1xGex:H) single-junction solar cell. The a-Si1xOx:H film was deposited by plasma enhanced chemical vapor deposition (PECVD) at 40 MHz in a same processing chamber as depositing the p-type layer. An optimization of the thickness of the a-Si1xOx:H buffer layer and the CO2/SiH4 ratio was performed in the fabrication of the a-Si1xGex:H single junction solar cells. By using the wide band gap aSi1xOx:H buffer layer with optimum thickness and CO2/SiH4 ratio, the solar cells showed an improvement in the open-circuit voltage (Voc), fill factor (FF), and short circuit current density (Jsc), compared with the solar cells fabricated using the conventional a-Si:H buffer layer. The experimental results indicated the excellent potential of the wide-gap a-Si1xOx:H buffer layers for narrow band gap aSi1xGex:H single junction solar cells. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydrogenated amorphous silicon oxide layer Buffer layer Amorphous silicon germanium single junction solar cell
1. Introduction Recently, wide band gap hydrogenated amorphous silicon oxide (a-Si1xOx:H) and hydrogenated microcrystalline silicon oxide (mcSi1xOx:H) materials have attracted much attention as promising materials for the fabrication of high performance Si-based thin film solar cells, wherein they are applied as p-, n- and i-layers instead of the conventional hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (mc-Si:H) layers. Wide band gap p-type microcrystalline silicon oxide (p-mc-Si1xOx:H) has been used as a window layer material because of its low light absorption and high conductivity [1]. On the other hand, intrinsic hydrogenated amorphous silicon oxide (i-a-Si1xOx:H) has been used as the top cell of multi-junction solar cells for enhancing the spectral response in the short wavelength region [2e6]. In the case of silicon-based thin film solar cells, it is widely known that buffer
* Corresponding author. E-mail address: [email protected] (J. Sritharathikhun).
layers play the role of controlling the quality of the interface between the p- and i- layers in order to increase both the open circuit voltage (Voc) and the fill factor (FF). For this purpose, conventional intrinsic hydrogenated amorphous silicon (i-a-Si:H) has been used [7e11]. Some research groups have used i-a-Si1xOx:H as buffer layers in a-Si:H solar cells in order to obtain a good compromise between a relatively high effective built-in potential (Vbi) and a relatively low recombination rate at the p/i interface [12e14]. Our group has previously reported a high efficiency of 9.4% for a hydrogenated amorphous silicon germanium (a-Si1xGex:H) single junction solar cell by using the a-Si1xGex:H VU-shape band gap profiling technique [15] and 11% for a-Si:H/a-Si1xGex:H tandem solar cells using the conventional a-Si:H as a buffer layer for the interface between wide gap p-mc-Si1xOx:H and narrow gap i-aSi1xGex:H [16]. However, there is still room for investigation of the a-Si1xOx:H buffer layer in narrow band gap a-Si1xGex:H solar cells as well as wide band gap a-Si:H and a-Si1xOx:H solar cells. Moreover, the use of wide band gap a-Si1xOx:H buffer layer between wide band gap p-mc-Si1xOx:H and narrow band gap i- aSi1xGex:H, (resulting in a new device structure) needs to be
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J. Sritharathikhun et al. / Optical Materials xxx (2016) 1e6 Table 1 Deposition condition for a-Si1xOx:H films. Parameters
Values
SiH4/H2/CO2 Plasma frequency Deposition temperature Deposition pressure Power density Thickness
1/10/0e0.6 40 MHz 180 C 500 mTorr 42 mW/cm2 0.4 mm
investigated. In this paper, we have applied a-Si1xOx:H as a buffer layer in aSi1xGex:H single junction solar cells. The electrical, optical, and structural properties of the prepared a-Si1xOx:H films were characterized. The study focused on the effects of variation in thickness of the a-Si1xOx:H buffer layer and the carbon dioxide (CO2) to the silane (SiH4) ratio (during the thin film preparation) on the performance of the a-Si1xGex:H single-junction solar cells. A comparison between solar cells containing the conventional a-Si:H buffer layer and those containing a-Si1xOx:H buffer layer, developed in this study, was also carried out. 2. Experimental details 2.1. Deposition of the a-Si1xOx:H films The a-Si1xOx:H films were prepared on soda lime glass and silicon substrate by high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at 40-MHz and substrate temperatures of approximately 180 C in the chamber used for the deposition of p-layer. A gas mixture of silane (SiH4), hydrogen (H2), and CO2 was used as the precursor. The base pressure in the deposition chamber was kept at about 2 106 Torr by using a turbo molecular pump backed up by a rotary pump. The deposition conditions for a-Si1xOx:H films are summarized in Table 1. The CO2/SiH4 ratio, during the preparation of a-Si1xOx:H films, was varied between 0.0 and 0.6 to obtain films of varying thickness
(maximum thickness ¼ 0.4 mm). The Eopt of samples was measured by spectroscopic ellipsometry (SE) (J.A. Woollam, V-VASE series). The data was fitted and analyzed using the Tauc-Lorentz model [17]. 2.2. Fabrication of a-Si1xGex:H single junction solar cells a-Si1xGex:H single-junction solar cells were fabricated using the cluster type plasma enhanced chemical vapor deposition (PECVD) technique to obtain the following device structure: TCO coated glass (U-type, Asahi)/aluminum-doped zinc oxide (ZnO:Al)/ p-mc-Si1xOx:H (30 nm)/a-Si1xOx:H buffer layer (0e9.8 nm)/aSi1xGex:H (300 nm)/n-type microcrystalline silicon (n-mc-Si:H) (30 nm)/ZnO:Al/Ag/Al, as shown in Fig. 1(a); the band diagram and the optical band gap of the p-i-n layer of the solar cells are shown in Fig. 1(b). For the deposition of p-mc-Si1xOx:H layer, a-Si1xOx:H buffer layer, and n-mc-Si:H layer, 40 MHz generator was used, while 27 MHz generator was employed for the i-a-Si1xGex:H layer deposition. The a-Si1xOx:H buffer layer was deposited in a same processing chamber as depositing the p-layer after p-layer deposition. In order to investigate the role of the buffer layer on the performance of the solar cells, a-Si1xOx:H buffer layer of varying thickness (0e9.8 nm) and varying the CO2/SiH4 ratio in the range of 0.0e0.8, were used. The solar cells were isolated by laser scribing to an active area of 1.0 1.0 cm2. The photovoltaic (PV) parameters of single junction solar cells were investigated under standard test conditions (STC) at AM 1.5, 100 mW/cm2, 25 C using the light source and filter of a solar simulator (Wacom, model WXS-155S-L2). The external quantum efficiency (EQE) of the solar cells was characterized using a quantum efficiency measurement system (PV Measurement, QEW7). 3. Results and discussions 3.1. Effect of the CO2/SiH4 ratio on the electrical and optical properties of the a-Si1xOx:H films Fig. 2 shows the dark (sd) and photo conductivity (sph) of the aSi1xOx:H films deposited using various CO2/SiH4 ratios in chambers
Fig. 1. (a). Schematic structure of the a-Si1xGex:H single junction solar cell. (b). Schematic band diagram of the p-i-n layer of the a-Si1xGex:H single junction solar cell.
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0.0) to 2.01 107 S/cm (CO2/SiH4 ratio of 0.6). This result suggests that the increase of oxygen (O) concentration via a high CO2 flow rate has a deleterious effect on the electrical properties of the a-Si1xOx:H films. Since these films were deposited in the chamber used for the deposition of p-layer, contaminated by diborane gas, the sd of these samples was found to be relatively higher than that of the i-aSi1xOx:H films (normally 109e1010 S/cm) that were deposited in the chamber used for the deposition of i-layer. Therefore, the aSi1xOx:H films deposited in the chamber used for the deposition of p-layer tended to be weakly p-type. Table 2 presents the fitting parameters of the Tauc-Lorentz model of the a-Si1xOx:H films deposited on glass substrate with various CO2/SiH4 ratios, fitted using the Tauc-Lorentz model using the spectroscopic ellipsometry technique. We utilized Eopt from Tauc gap (Eg in Table 2) in order to prevent the error from Tauc's plot estimation. Eopt increased from 1.76 (a-Si:H) to 1.95 (aSi1xOx:H) eV with an increasing CO2/SiH4 ratio from 0.0 to 0.6. Fig. 2. The dark (sd) and photo conductivity (sph) of the a-Si1xOx:H films deposited using various CO2/SiH4 ratios in chambers used for the deposition of p- and i-layers. Table 2 Fitting parameters from SE measurement of a-Si1xOx:H films as a function of CO2/ SiH4 ratio. CO2/SiH4 ratio Mean square error Roughness (nm) Thickness (nm) Eg (eV) (MSE) [18] 0.0 0.2 0.4 0.6
5.34 11.50 13.93 12.46
3.34 4.35 3.96 5.44
370.29 390.57 405.33 387.42
1.76 1.79 1.85 1.95
* The Mean Squared Error (MSE) is the value used to quantify the difference between the generated and measured data.
used for the deposition of p- and i-layers. sd of the a-Si1xOx:H films deposited in the chamber used for the deposition of p-layer decreased from 5.12 108 to 1.40 108 S/cm as the CO2/SiH4 ratio increased from 0.0 to 0.6. The sph decreased from 2.30 105 (CO2/SiH4 ratio of
3.2. Effects of a-Si1xOx:H buffer layer thickness on the performance of a-Si1xGex:H single junction solar cells In our previous work, we successfully achieved a high efficiency of 9.4% with a-Si1xGex:H single solar cells [15]. For this, the aSi1xGex:H bottom cell was fabricated by using a combination of the V- and U-shape band gap profiling techniques. Therefore, in this experiment we used the same band gap profiling technique to deposit the i-a-Si1xGex:H absorber layer. The CO2/SiH4 ratio was kept constant at 0.2. We have optimized the a-Si1xOx:H buffer layer thickness by varying the deposition time by 0, 75, 110, 150, 190 and 225 s. The estimated thickness values of 0, 3.3, 4.9, 6.8, 8.5 and 9.8 nm of the a-Si1xOx:H buffer layers were measured and were fitted using the spectroscopic ellipsometry technique. Please note that the thickness of a-Si1xOx:H films was the sum of the surface roughness and the bulk layer thickness. Fig. 3(a) and (b) show the PV parameter and illuminated current-voltage (JV) curve of the a-Si1xGex:H single-junction solar
Fig. 3. (a). PV parameters of the a-Si1xGex:H single junction solar cells with a-Si1xOx:H buffer layers of varying thickness. (b). Illuminated JV curve of the a-Si1xGex:H single junction solar cells with a-Si1xOx:H buffer layers of varying thickness.
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Fig. 4. Spectral response of the a-Si1xGex:H single junction solar cells with aSi1xOx:H buffer layers of varying thickness.
cells with various a-Si1xOx:H buffer layer thickness. The cell without the buffer layer shows a Voc of only 0.77 V, with an FF of 0.68. By inserting the a-Si1xOx:H buffer layer of 3.3 nm, Voc increased to 0.78 V, however, FF dropped to 0.61 because very thin buffer layer probably leads to poor film quality and island growth. With thicker buffer layers (4.9, 6.7, 8.5 nm), both Voc and FF increased, and the conversion efficiency (Eff) reached the maximum value of 9.73%, with Voc ¼ 0.80 V, Jsc ¼ 17.7 mA/cm2 and FF ¼ 0.69, with 8.5 nm thick buffer layer. When 9.8 nm thick buffer layer was used, Voc seemed stable but the FF significantly dropped from 0.69 to 0.63. Increasing of Voc with increasing buffer-layer thickness
indicated that wide band gap a-Si1xOx:H buffer layer played a crucial role to compensate the band gap discontinuity between pmc-Si1xOx:H and i-a-Si1xGex:H layer. In this experimental result, it was found that a thickness of buffer layer at around 6.0e8.5 nm is required for band gap discontinuity between the p and i-layers [19,20]. However, with too thick a buffer layer, it was found that the FF of solar cell tended to decrease due to the decreasing internal electric field [20,21]. Fig. 4 shows the external quantum efficiencies of solar cells using a-Si1xOx:H buffer layer with various thickness. The spectral response was slightly improved by inserting thin a-Si1xOx:H buffer layer compared to the solar cell fabricated without buffer layer. The optimum wide band gap buffer layer is effective in reducing the recombination defects and trap centers at the p/i interface. As a result, a better carrier collection and internal electric field distribution of the cell with the optimum buffer thickness showed a better solar cell performance compared to a buffer-less solar cell. This effect not only improved the Voc an FF but it also slightly improved the Jsc [22]. However, when the thickness of the aSi1xOx:H buffer layer became higher than 8.5 nm, the solar cell showed lower spectral response due to higher light absorption by thick a-Si1xOx:H buffer layer, which agreed with the decrease in Jsc. 3.3. Effects of CO2/SiH4 ratio of a-Si1xOx:H buffer layer on the performance of a-Si1xGex:H single-junction solar cells In the previous part, we discussed the optimization of the aSi1xOx:H buffer layer thickness. In this part, we have attempted to discuss the effects of CO2/SiH4 ratio of a-Si1xOx:H buffer layer between p/i interface on the performance of a-Si1xGex:H singlejunction solar cells by keeping constant the other layer deposition
Fig. 5. (a). PV parameters of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer. (b). Illuminated JV curve of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer.
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help to improve optical and p/i interface properties for multijunction Si-based thin film solar cells, allowing more efficient solar cell performance. Acknowledgement This work was supported by National Electronics and Computer Technology Center (NECTEC) THAILAND under Grant No. P1350323. References
Fig. 6. Spectral response of the a-Si1xGex:H single junction solar cells with various CO2/SiH4 ratios for the a-Si1xOx:H buffer layer.
conditions. The CO2/SiH4 ratio was varied from 0.0 to 0.8 while the thickness of the a-Si1xOx:H buffer-layers was maintained at about 8.5 nm (optimum thickness from previous part). Fig. 5(a) and (b) present the PV parameters and illuminated JV curve of cells fabricated with various CO2/SiH4 ratios for the buffer layer between p/i interface. The results demonstrated that PV parameters were significantly improved by inserting a tiny amount of CO2/SiH4 ratio (from 0.0 to 0.2). At the CO2/SiH4 ratio of higher than 0.2, the Voc still increased to 0.80 V at the CO2/SiH4 ratio of 0.8. According to these results, it was observed that the CO2/SiH4 ratio played a role in improving the Voc of solar cells because of the wider band gap of the a-Si1xOx:H buffer layer. The Jsc improved abruptly from 17.0 to 18.7 mA/cm2 when the CO2/SiH4 ratio increased from 0.0 to 0.4 and then it seemed to remain constant at the higher CO2/SiH4 ratio. As shown in Fig. 6, the EQE of the solar cells gradually increased with increasing buffer layer CO2/SiH4 ratio. However, the FF reached its maximum value at the CO2/SiH4 ratio of 0.2 and then gradually decreased due to the effect of band gap mismatch between the buffer and the i-layer, and the increase of the defect density [23,24] in the buffer layer as well as increase of a-Si1xOx:H buffer layer resistance. As a result, the conversion efficiency (Eff) tended to decrease with increase of the CO2/SiH4 ratio due to the decrease of the FF. 4. Conclusions We have applied the a-Si1xOx:H buffer layer in the aSi1xGex:H single junction solar cell. Emphasis has been placed on the buffer thickness and the CO2/SiH4 ratio for layer deposition. The a-Si1xOx:H buffer layer in the study showed weakly p-type doped property due to boron contamination in p-type process chamber. The optical band gap of a-Si1xOx:H can be adjusted to be wider than that of the conventional a-Si:H by increasing the CO2/SiH4 ratio, however, a tradeoff between optical and electrical properties should be considered. Significant improvements of PV parameters such as Voc, Jsc, FF and efficiency were achieved by inserting the thin a-Si1xOx:H buffer layer between p-mc-SiO:H/i-a-SiGe:H interface with low CO2/SiH4 ratios. According to our results, the optimum thickness of a-Si1xOx:H buffer layer was about 8.5 nm. The highest efficiency of 9.73% has been achieved from the cell using a CO2/SiH4 ratio of 0.2 for the buffer layer deposition (Voc ¼ 0.80 V, Jsc ¼ 17.7 mA/cm2 and FF of 0.69). Additional optimization and better understanding of this a-Si1xOx:H buffer layer possibly will
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