Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se-2 absorber materials

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se-2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se-2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium-copper divacancy complex (V-Se-V-Cu), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at E-v+0.98eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V-Se-V-Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V-Se-V-Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.Peer reviewe

Impact of deep level defects induced by high energy neutron radiation in β-Ga2O3

The effects of high energy neutron irradiation on the deep level defect concentration profile throughout the bandgap of β-Ga2O3 were investigated by a combination of deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS). For the unintentionally doped edge-defined film-fed growth-grown (010) β-Ga2O3 substrates investigated here, it was found that the dominant effects of neutron irradiation were to produce defects detected by DLOS having energy levels of EC −1.29 eV and EC −2.00 eV, with no discernable impact on traps within ∼1 eV of the conduction band edge. Commensurate with the introduction of these states was a significant amount of net doping reduction, for which lighted capacitance-voltage studies revealed that both of these irradiation-induced deep states are responsible, likely through a compensation mechanism. The sensitivity of the EC −1.29 eV and EC −2.00 eV states on irradiation suggests an intrinsic source, and whereas the EC −2.00 eV state was already present in the as-grown material, the EC −1.29 eV state was not detected prior to irradiation. DLOS and DLTS revealed other defect states at EC −0.63 eV, EC −0.81 eV, and EC −4.48 eV, but none of these responded to neutron irradiation for two different 1 MeV equivalent fluences 8.5 × 1014 cm−2 and 1.7 × 1015 cm−2, which is consistent with the behavior expected for defect states having an extrinsic source