Flexible CdTe solar cells with high photovoltaic conversion efficiency

CdTe/CdS solar cells have been grown with a low temperature deposition process on flexible polyimide film and glass substrates in a superstrate configuration. Tin doped indium oxide (ITO) was used as transparent front electrical contact and a highly transparent and a resistive oxide layer was introduced between the ITO and CdS to enable the reduction of the CdS layer thickness resulting in increased current density, processing yield and reproducibility. The device structure consists of ITO, intrinsic ZnO, CdS, CdTe, Cu/Au back contact. The ITO and ZnO layers were deposited by rf sputtering while the other layers were grown by high vacuum thermal evaporation. During the whole process the substrate temperature did not exceed 420°C. The photovoltaic parameters of the flexible CdTe cells are comparable to CdTe/CdS cells on glass substrates except of a difference in the current density, which is due to the lower transmission of the polyimide substrate. Flexible CdTe/CdS cells on polyimide exceeding 10% efficiency were reproducibly produced. Nearly 9% efficiency was achieved with an in-line compatible pentacene gold electrical back contact without bromine methanol etching on glass substrate. No adhesion problem of the solar-cell structure on polyimide occurred even when rolled to small radius of curvatures. Simple processing and high performance of these high efficiency solar cells offer several advantages for manufacturing and applications.