Cadmium Telluride Thin Film Solar Cells with Copper-Free, Low Barrier Back Contacts*

Formation of a good back contact to CdTe thin film solar cell is a prime challenge due to the high work function of CdTe. Traditionally, Cu doping has been widely used to reduce the barrier between CdTe and back contact. However, Cu can fast diffuse into the main junction CdS/CdTe which is shunting the device over time. To overcome this, copper-free inorganic and organic buffer layers have been used as a back contact for CdTe devices such as Sb₂Te₃, As₂Te₃, MoO_x, single-walled carbon nanotubes (SWCNTs), poly (3-hexylthiophene) (P3HT), and poly (3,4-ethyelenlenedioyyiohene): poly (styrene sulfonate) (PEDOT: PSS). Here, we demonstrate that by using layers of Te/ZnTe /Te at the back contact of CdTe without any intentional use of Cu. We found that the barrier height at the back of these Cu-free CdTe devices is lower than that of the CdTe devices with Cu/Au back contact, due to reduce band banding and the reduced carrier recombination at the back of the device, which leads to higher the open circuit voltage (V_OC).