Admittance spectroscopy on single-crystal Cu_2ZnSnSe_4 solar cells: Back-contact effects and metastabilities

摘要

Admittance spectroscopy has become a commonly used device-level technique to probe the defect structure of kesterite materials. While this technique holds promise, phenomena such as current barriers and metastabilities cause difficulty in the interpretation of results. In this work, devices fabricated on single-crystalline CZTSe absorber layers are used to explore these effects in a more idealized system than thin films. Differences in behavior between admittance signatures of absorbers with Cu/Zn + Sn ratios of 0.86 and 0.77 are observed and mainly attributed to the formation of a low-temperature current barrier at the CZTSe/carbon-based back-contact interface in the more Cu-depleted devices. The effect of this barrier is significantly reduced by the implementation of higher-work function Au or MoO_(3-x) back-contact material. Furthermore, distinct metastable doping densities are observed in each device type with the implementation of 3-h external bias conditioning of white, blue, or red light as well as 1 V applied forward bias. The origin of this behavior is attributed to a V_(Se)-V_(Cu) defect complex that is predicted to exist in related chalcogenide materials.