Combinatorial tuning of work function and optical properties in CuZnSe thin films for efficient bifacial CdTe solar cells

Novel hole-transporting materials with high work function (WF) and transmittance are crucial for high-efficiency bifacial CdTe solar cells. Herein, the CuZnSe alloy films prepared by combinatorial sputtering exhibit tunable optical band gap (2.25-2.57 eV), WF (5.05-5.45 eV) and band structure with variable Zn content. A large WF of 5.45 eV for Cu0.46Zn0.74Se and small valence band offset of 0.13 eV at Cu0.46Zn0.74Se/CdTe interface have been determined by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), respectively, which can promote the hole transport from CdTe absorber to back electrode. A maximum power conversion efficiency (PCE) of 15.3% was achieved for CdTe solar cells when using Cu0.46Zn0.74Se/Au as back contact. The device characterizations indicate that Cu0.46Zn0.74Se has the excellent effects on reducing back-contact barrier and suppressing carrier recombination. Moreover, Cu0.46Zn0.74Se/ITO back contact yielded high back-illuminated efficiencies of 4.2% and 5.3% for the 3.2 mu m and 1.7 mu m CdTe devices, respectively, due to increased transmittance and effective carrier collection. This work provides a promising strategy to obtain optically transparent Cu-containing selenides with high work function by alloying, which have attractive application prospect for bifacial CdTe solar cells.

Automated summary

CuZnSe alloy films prepared by combinatorial sputtering exhibit tunable optical properties and band structure, which promote hole transport in CdTe solar cells and improve the efficiency.