Development of high-efficiency Cu (InGa) Se_2 solar cells with highly resistive ZnO buffer layer

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In this study, we have been applied a high resistivity ZnO as a buffer layer of Cu (InGa) Se_2 thin-film solar cells. To investigate an optimum processing parameters for a high resistivity ZnO, undoped ZnO films were deposited by ALD process on glass substrates. DEZn and H_2O were used as reactant gases. These source gases were alternately introduced into the chamber with Ar as a carrier gas. By using this technique, a high resistivity ZnO (1kΩcm) was obtained.We prepared solar cells with an undoped ALD-ZnO buffer layer with different resistivity and with different thickness in order to determine an optimum buffer layer for maximum cell performance. First of all, we checked the dependence of the cell performances on the resistivity of buffer layer. The cell efficiency is significantly dependent on the buffer layer resistivity. Low values of Voc and FF were obtained for low resistive ZnO, and large Voc and FF were obtained when the resistivity of ZnO is greater than 1kΩcm. This result s … More upports the conclusion that the significantly higher resistive buffer layer can improve the performance of CIGS solar cells. Next, we investigated the cell performance on the buffer layer thickness. The buffer layerresistivity is greater than 1kΩcm. The device performance was improved with increasing the buffer layer thickness. However it deteriorated when the buffer layer thickness increased above 100nm. The deterioration in efficiency can be explained by an intcrease in the series resistance of the cells. Thus, we found that a 70nm-thick ALD-ZnO was suitable for the application of solar cells. In the study, Cd-free direct ZnO/CIGS solar cells exhibited active area efficiency of 13.1% without an antireflection coating.We also developed the new technique to improve the cell performance, that is, Zn-doping technique. The relatively high efficiencies of over 10% were obtained by the Zn irradiation onto the CIGS surface with beam intensities between 2.0x1O^<-8> Torr to 1.0x10^<-7> Torr, while the efficiency was as low as 5% without the Zn doping. The EBIC signal celarly showed that the pn homojunction of solar cells located at/near heterointerface with the proper Zn irradiation. Based on these results, the new buffer layer (In (OH, S) : Zn^<2+>) was also proposed. By using this new bufffer layer, a cell efficiency of 13.7% was obtained. Less