RESUMO
Kesterite-based Cu2 ZnSn(S,Se)4 semiconductors are emerging as promising materials for low-cost, environment-benign, and high-efficiency thin-film photovoltaics. However, the current state-of-the-art Cu2 ZnSn(S,Se)4 devices suffer from cation-disordering defects and defect clusters, which generally result in severe potential fluctuation, low minority carrier lifetime, and ultimately unsatisfactory performance. Herein, critical growth conditions are reported for obtaining high-quality Cu2 ZnSnSe4 absorber layers with the formation of detrimental intrinsic defects largely suppressed. By controlling the oxidation states of cations and modifying the local chemical composition, the local chemical environment is essentially modified during the synthesis of kesterite phase, thereby effectively suppressing detrimental intrinsic defects and activating desirable shallow acceptor Cu vacancies. Consequently, a confirmed 12.5% efficiency is demonstrated with a high VOC of 491 mV, which is the new record efficiency of pure-selenide Cu2 ZnSnSe4 cells with lowest VOC deficit in the kesterite family by Eg /q-Voc. These encouraging results demonstrate an essential route to overcome the long-standing challenge of defect control in kesterite semiconductors, which may also be generally applicable to other multinary compound semiconductors.
RESUMO
Copper indium gallium diselenide-based technology provides the most efficient solar energy conversion among all thin-film photovoltaic devices. This is possible due to engineered gallium depth gradients and alkali extrinsic doping. Sodium is well known to impede interdiffusion of indium and gallium in polycrystalline Cu(In,Ga)Se2 films, thus influencing the gallium depth distribution. Here, however, sodium is shown to have the opposite effect in monocrystalline gallium-free CuInSe2 grown on GaAs substrates. Gallium in-diffusion from the substrates is enhanced when sodium is incorporated into the film, leading to Cu(In,Ga)Se2 and Cu(In,Ga)3Se5 phase formation. These results show that sodium does not decrease per se indium and gallium interdiffusion. Instead, it is suggested that sodium promotes indium and gallium intragrain diffusion, while it hinders intergrain diffusion by segregating at grain boundaries. The deeper understanding of dopant-mediated atomic diffusion mechanisms should lead to more effective chemical and electrical passivation strategies, and more efficient solar cells.
RESUMO
We apply spectroscopic ellipsometry (SE) to identify secondary phases in Cu2ZnSnSe4 (CZTSe) absorbers and to investigate the optical properties of CZTSe. A detailed optical model is used to extract the optical parameters, such as refractive index and extinction coefficient in order to extrapolate the band gap values of CZTSe samples, and to obtain information about the presence of secondary phases at the front and back sides of the samples. We show that SE can be used as a non-destructive method for detection of the secondary phases ZnSe and MoSe2 and to extrapolate the band gap values of CZTSe phase.
