A comparison of organic back contact materials for CdTe solar cells

Whilst significant research effort has been focused on inorganic back surface materials to establish a quasi-ohmic contact to CdTe solar cells, the use of organic layers is relatively unexplored, yet offers a highly controllable route to tailoring band offsets between CdTe and metal contacts. This work assesses the impact on CdTe device performance of three such organic materials; spiro-OMeTAD, poly(3-hexythiophene-2,5-diyl) (P3HT) and poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO). Whilst PFO appears to increase the contact barrier with large rollover in JV curves, and severely limits performance, the inclusion of either P3HT or spiro-OMeTAD at the back contact was shown to effectively eliminate rollover in JV curves by reducing the contact barrier height, as confirmed by temperature dependent JV measurements. However, a reduced barrier height was not found to lead to a significant increase in cell efficiency, as inclusion of the organic layer results in an accompanying increase in series resistance.