High-Efficiency Cd-Free CZTSSe Solar Cells with Organic Semiconductor PCBM as a Buffer Layer

Replacing the traditional CdS buffer layer with more environmentally friendly materials is an urgent issue for environment-friendly CZTSSe solar cells. Herein, a nontoxic, flexible, and conductive organic molecular semiconductor PCBM to replace CdS is used and a Cd-free CZTSSe solar cell is obtained. By introducing mild spin-coated ZnO window layers, the damage of PCBM in conventional sputtering ZnO process is effectively avoided, and a higher efficiency than previously reported organic semiconductor buffer layer systems and comparable to CdS buffer layer devices are achieved. The flexible PCBM covers the surface of the CZTSSe absorber layer densely and forms a flatter film surface, which is beneficial for the following ZnO deposition. The CZTSSe/PCBM interface exhibits a typical “spike-like” energy band alignment with a conduction band offset value of only 0.1 eV. Compared with CZTSSe/CdS interface, the CZTSSe/PCBM heterojunction possesses larger interfacial recombination resistance and a longer carrier lifetime. The ideal coating ability, suitable energy band alignment, as well as the better interface carriers transfer behavior suggest that the organic molecular semiconductors can be used instead of the CdS buffer layer to pave the way for truly environment-friendly kesterite photovoltaic devices.