Improved efficiency with In2O3 as window layer for CIGSe-solar cells

Intrinsic Undoped.

In solar cells it is desirable that as much as possible of the sunlight should be absorbed in the absorber layer. However, there is usually some absorption already in the front contact, the so called window layer, which leads to that fewer electrons are excited in the absorber layer and less current is produced by the solar cell. This decreases the solar cell’s efficiency. In this study, In2O3 was investigated as a new alternative for the window layer for CIGSe-solar cells and compared to the conventional layer of intrinsic ZnO (i-ZnO) and aluminum doped ZnO (AZO).

CIGSe-solar cells were processed on a glass substrate as a stack of Mo/CIGSe/buffer layer/window layer/contact, with ZnSnO (ZTO) or CdS as buffer layer and In2O3 or i-ZnO/AZO as window layer. The solar cells’ optical and electronic properties, efficiency and current were investigated.

In2O3 was not able to crystallize on CdS as buffer layer, and therefore only the other three combinations were characterized. For electronic properties, the window layers’ thickness was adjusted for a certain resistance. Here, In2O3 was seen to have a lower resistivity than AZO, which probably was due to a crystal structure with large grains where the electrons have high mobility. In2O3 also had a shifted absorption curve compared to i-ZnO/AZO, which leads to that more light is transmitted to the absorber layer. For the three combinations the efficiency was investigated for different wavelengths. Similarly to the optical results, the efficiency was higher for several spectral ranges for In2O3. ZTO as a buffer layer instead of CdS led to an improved efficiency for shorter wavelengths than 550 nm, although the efficiency increased even more below 450 nm with In2O3 as window layer. From the differences in efficiencies between the configurations, the potential increase in current was calculated to +1.7 mA/cm2 compared to zinc oxide as window layer on top of ZTO.

Other buffer layers than CdS has earlier been seen to increase the current, but simultaneously also decrease in voltage which counteracts the improvement and leads to the same overall efficiency. Also for solar cells with In2O3 a small voltage drop was seen. The voltage drop was however smaller than for i-ZnO/AZO on ZTO and due to the higher current, these solar cells still had the highest efficiency of the characterized solar cells. For solar cells with thicker window layers, which could be relevant for example for larger solar cell modules where the front contact has to lead the current further, the current increased even more for In2O3 compared to the types with i-ZnO/AZO. Since the voltage also decreased, the total efficiency was somewhat higher than the ones with ZTO but similar to the solar cells with CdS. In conclusion, it can be said that In2O3 is promising as a window layer due to the decreased optical absorption and therefore higher current. However, more research is needed on the stability of In2O3, as well as methods to be able to deposit In2O3 on CdS.

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