During the fabrication of our cells, the robot arm took the sample away from the PECVD chamber after the deposition of the p-layer. An in-situ cleaning was carried out to prevent the boron contamination when depositing the next layer. We applied H2
plasma treatment between the cleaning process and the next layer deposition. To verify the etching effect and chemical annealing of H2 plasma on the performance of a-Si:H thin solar cells, H2 plasma treatment at the interface of each layer of solar cells
47
was employed. The H2 flow rate was 270 sccm, the processing pressure was controlled at 200 Pa and the input power was 200 W. The treatment time at each interface was 5 sec. The solar cells were prepared at the optimum condition concluded in previous sections. The thickness of main absorber layer was controlled at around 300nm.
Figure 4.22 illustrates the I-V curves of a-Si:H single junction solar cells with H2
plasma treatment at different interface. The square symbol represents the baseline cell without any treatment which shows the lowest Jsc. The first H2 plasma treatment was applied at p/b interface, The Jsc significantly improved from 13.30 mA/cm2 to 14.02 mA/cm2 and the conversion efficiency from 8.87％ to 9.33％. The impurity such as water vapor、oxygen、organic gas may incorporate onto the surface of p-type layer.
The p/i interface is one of the most critical spots that influence the performance of p-i-n solar cell greatly. The etching from H2 plasma can remove those impurities, providing a new and clean surface for the next layer deposition. Furthermore, the H2
plasma can passivate dangling bonds, and reduce the p/i interface induced defects.
48 treatment @ p/b interface treatment @ p/b b/i interface treatment @ p/b b/i & i/n interface
Figure 4.22 The I-V curves of a-Si:H single junction solar cells with H2 plasma treatment at different interfaces.
The values of Voc and F.F. in Table 4.5 seem to be insensitive to this treatment.
The short H2 plasma treatment time is likely to have no influence on the electric property of p-type layer. Compared to the cell with H2 plasma treatment only at p/i interface, the H2 plasma treatment at b/i interface contributes to a further improvement in Jsc. It can be seen from Table 4.4 that the Jsc increased from 14.02 mA/cm2 to 14.39 mA/cm2 and the conversion efficiency improved from 9.33 ％ to 9.45％. Because the buffer layer we used is a-SiC, the heterojunction still exists due to the diversity of bandgap of a-Si and a-SiC. Besides, the removal of impurities on the surface, the treatment at the interface between buffer layer and intrinsic layer seems to suppress the formation of defect state in this region. However, it is difficult to measure the defect density at this ultra thin b/i interface layer. Some experiments should be conducted to certificate the assumption.
49
Tab le 4.5 The performance of single junction solar cells with H2 plasma treatment at different interface.
The H2 plasma treatment at i/n interface was applied after p/i、b/i interface treatment. As can be seen in Table 5.3, the decrease in Jsc can be observed. The Jsc decreased from 14.39 mA/cm2 to 13.78 mA/cm2 and the conversion efficiency degraded from 9.45 ％to 9.26％ compared to the cell with p/i、b/i H2 plasma treatment. The n-layer was deposited immediately after the deposition of intrinsic layer. Namely, the H2 plasma treatment at i/n interface was taken without exposure in the transfer chamber. The impurities contamination from transfer chamber can be ignored. Similar to the treatment at p/i interface, the treatment can reduce the defect which was induced from doped layer. However, due to the superstrate structure of our cells, the treatment was carried out at the surface of the absorber layer. The damage of H2 plasma on the surface of intrinsic layer severely deteriorates the performance of solar cells. However, it was surprising that the F.F. increased from 73.33％ to 75.68
％ compared to the cell with p/i、b/i H2 plasma treatment. The H2 plasma seems able to suppress the leakage current which increases the shunt resistance of the device.
50
4.8. The Best Solar Cell in this Study
Figure 4.23 shows the I-V curves and external quantum efficiency (EQE) of the best a-Si:H single junction solar cells in this study. The efficiency of the best cell in this study is 9.46%.
300 400 500 600 700 800
0
Figure 4.23 The I-V curves and EQE of the best a-Si:H single junction solar cells in this study.
51
Conclusion
The properties of p-type a-SiC with different CH4 to SiH4 flow rate ratio were presented. The addition of carbon enlarges the optical bandgap of p-a-SiC:H.
However, the σd decreases with the increasing concentration of carbon in p-a-SiC:H.
films. This result can be attributed the existence of increasing carbon concentration in films that weaken the ability of B doping.
The device quality a-Si:H thin film was prepared through the source gas of H2 diluted SiH4 by PECVD. The solar cell prepared at shorter electrode spacing showed better performance due to less gas phase reaction as depositing the absorber layer. No significant difference observed in the conductivity of n-type a-Si:H as varying the PH3
to SiH4 gas ratio.
The intrinsic a-SiC prepared with the source gases of SiH4、CH4、H2 by PECVD.
The optical band gap enlarged but the photo conductivity decreased as increasing CH4 to SiH4 flow rate ratio. The incorpuration of carbon in a-Si may introduce defects in the film which act as the recombination center of photo-generated carriers. That is the reason for the decrease in the photo conductivity as increasing CH4 to SiH4 flow rate ratio.
The optimal thickness of each layer in a-Si:H solar cells has been discussed.
The performance of solar cells was drastically enhanced by the introduction of bandgap profiling at the p/i interface. The etching effect and passivation mechanism of H2 plasma treatment played an important role at the junction of each layer. The application of hydrogen plasma treatment improved the performance of a-Si:H solar cells. By applying all measures mentioned in this report the conversion efficiency of solar cells was enhanced up to 9.46%
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Future Work
The optical bandgap of our p-type a-SiC is still lower than 2.0 eV. Wide optical bandgap of the window layer can avoid optical losses in this layer. A further fine-tune of p-layer should be taken to enlarge the optical bandgap of p-layer. The suitable condition of H2 plasma can be investigated to achieve higher conversion efficiency of our solar cells. The method of measuring the defect density at p/b、b/i interface layer should be verified to discuss the performance of solar cells as applying H2 plasma treatment. The substrate we used is commercial SnO2 coated glass. The ability of light trapping in our solar cells was limited. The development of new substrate should be conducted.
Additionally, in this study we considered only single-junction solar cell. This work can be extended for tandem solar cells, with a-Si:H cell as the bottom cell and the a-SiGe or μc-Si thin film solar cell as the top cell for higher efficiency.
53
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