Effects of cation composition on carrier dynamics and photovoltaic performance in Cu2ZnSnSe4 monocrystal solar cells

Abstract

Understanding the relationship of doping density, carrier lifetime, and interface recombination to device performance is critical to designing solar cells with high power conversion efficiency (PCE). In turn, it is necessary to understand how bulk material composition determines doping density and carrier lifetime. The most efficient kesterite Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells have had Cu-poor, Zn-rich compositions, while more stoichiometric compositions have lower PCEs. However, thin films are grown under highly non-equilibrium conditions, complicating fundamental studies. Here we report on a set of CZTSe monocrystals with varied cation stoichiometry, enabling correlation of bulk composition to material and device properties without the complication of grain boundaries or secondary phases. Copper-poor, zinc-rich compositions (Cu/(Zn + Sn) = 0.77–0.90 and Zn/Sn = 1.17–1.25) yield bulk carrier lifetimes longer than 200 ps and PCE >5%. In contrast, near-stoichiometric compositions, with Cu/(Zn + Sn) > 0.90 and Zn/Sn < 1.15, have carrier lifetimes shorter than 20 ps and PCE <2%. CZTSe/CdS interface recombination velocity has a similar value to the CZTSe surface recombination velocity, with values of 104–105 cm/s determined by time-resolved terahertz spectroscopy and transport-recombination modeling. Device modeling reveals the dependence of open circuit voltage (VOC) on doping density, carrier lifetime and interface recombination. For a crystal with low doping density of 1015 cm−3, the maximum VOC is limited by the bulk lifetime. Higher VOC can be attained with higher doping density, but interface recombination becomes more significant with increased lifetime and doping density. These simulations indicate limitations and potential pathways to high performance.