Journey towards CdTe-based graded band gap electrodeposited solar cells on P-type window

Photovoltaic (PV) technology being the world’s most utilised renewable source of energy copes with many challenges technically and socially. Cadmium Telluride (CdTe)-based thin-film PVs are the most commercially successful solar cells that hold potential to develop further. This work proposes a novel device structure for CdTe-based solar cells which is implementable using electrodeposition, an easy and affordable fabrication technique for the developing world. Inclusion of a wide band gap p-type semiconductor as a window layer of a graded band gap device and use of suitable back diffusion barrier layers to mitigate thermionic back diffusion, have been proposed to potentially improve the open circuit voltage (Voc) of the cells. This concept has been tested and verified with GaAs and Perovskite based solar cells, but this work aims to investigate the concept for CdTe-based devices grown with inexpensive electrodeposition technique for the first time. In the material selection phase of this work, firstly, CdTe:Mg, CdMnTe and ZnTe layers are electrodeposited on glass/FTO surface, characterised, optimised and compared to be the p-type wide band gap window layer. ZnTe layers have been grown both in n-type and p-type with reasonably good crystallinity and selected as the window material for the device. Secondly, CdTe layers have been electrodeposited from cheap, low purity cadmium precursors, characterised and growth conditions were optimised to be used as the absorber layer of the cell. Thirdly, CdS thin films have been electrochemically grown, characterised, optimised and compared with n-ZnTe layers to be used as the hole back-diffusion barrier (hbdb) layer at the back of the device. Considering the ease of production, n-ZnTe has been selected as the n-type hbdb layer for the device. Following the selection of materials and finding their optimal growth parameters, firstly, ZnTe/CdTe based devices are fabricated employing the conventional n-window approach having an architecture of glass/FTO/n-ZnTe/n-CdTe/i-CdTe/p-CdTe/p-ZnTe/Au. Growth parameters of the materials used in individual layers have been optimised in the material selection phase. These devices exhibited 3.63% of highest efficiency with Voc, Jsc and Fill-factor of 430 mV, 30.2 mAcm-2 and 0.28 respectively. Finally, according to the conceptualised novel device design, p-window based glass/FTO/p-ZnTe/p-CdTe/i-CdTe/n-CdTe/n-ZnTe/In architecture has been fabricated producing highest efficiency of 5.41% with Voc, Jsc and fill-factor of 560 mV, 33.3 mAcm-2 and 0.28 respectively. Despite having concerns about the material quality of the layers, experiments have shown that p-window based graded band gap CdTe solar cells exhibit greater Voc, hence better performance compared to their n-window counterpart.