Polymer Back Contacts and the Efficiency of Cadmium Telluride (CdTe) Solar Cells*

First Solar has announced that the highest efficiency of CdTe/CdS solar cells is still lower than the theoretical limit. This comes from how hard it is to form a good ohmic back contact on p-type CdTe. CdTe has a high electron affinity (~4.5 eV), so a metal with a high work function is necessary to form a good ohmic contact with CdTe. Conductive polymers are candidates for the back contact because they have high work functions, high conductivities, are easy to process, and cost less. Our previous studies show that poly(3,4-ethylenedioxythiophene) polystyrene sulfonate can be used as the back contact of CdTe solar cells with promising results.

In this work, we show our studies on the effect of the work function and conductivity of the polymer back contact on the performance of CdTe/PEDOT:PSS solar cells. CdTe solar cells were fabricated with PEDOT:PSS back contact with different work functions and conductivities. We show correlations between the polymer work function and the barrier height at the back contact as well as the open-circuit voltage of the solar cell. The results provide us criteria in order to choose suitable polymer back contact for the fabrication of efficient CdTe solar cells.