Reduced recombination through CZTS/CdS interface engineering in monograin layer solar cells

Abstract

The power conversion efficiency of Cu2ZnSnS4 (CZTS) solar cells is still limited by deep defects, low minority carrier lifetime and high recombination rates at the CZTS/CdS interface. The objective of this study was to find an effective method to reduce interface recombination of CZTS monograin layer solar cells. A two-step heterojunction formation process was applied by controlling the intermixing of Cd and Cu in the CZTS/CdS interface, which resulted in improved device efficiency of up to 11.7%. Surface analysis by x-ray photoelectron spectroscopy confirmed Cd diffusion into the surface of CZTS after CdS air-annealing by forming an ultra-thin Cu2Zn x Cd1−x SnS4 layer. Moreover, external quantum efficiency measurements showed that the absorption edge shifts to longer wavelengths with the addition of Cd into the CZTS surface layer. This surface modification and replacement of a CdS:Cu buffer layer by fresh CdS greatly reduced the interface recombination and improved the junction quality, contributing to an enhancement of J SC ∼3 mA cm−2 (from 20.5 to 23.6 mA cm−2) and fill factor ∼14% (from 59.4% to 67.7%). The serial resistance of the CZTS monograin layer solar cells was significantly reduced from 2.4 Ω cm2 to 0.67 Ω cm2. To understand the electrical behavior of the highest-efficiency CZTS monograin layer solar cell in more detail, the temperature-dependent current–voltage characteristics were analyzed.