Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se2 absorber materials

Abstract

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 + 0.98 eV, 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.

A Comparison of Charge Carrier Dynamics in Organic and Perovskite Solar Cells

“…Such thermally activated detrapping has, for example, been reported in transient emission studies of copper indium gallium selenide (CIGS). [186] In any case, for all devices employed for Figure 4, V OC reconstructions using the measured TPV decay times were found to be in excellent agreement with directly measured device V OC , confirming that the TPV assay of carrier lifetime is a direct determinant of device function.…”

“…As such, the requirements for selective HTL/ETL contacts may be more severe for PSCs than OSCs, with a range of strategies being employed to achieve more selective extraction and minimize surface recombination in PSC devices. [ 176–224 ]…”

Showcasing the optical properties of monocrystalline zinc phosphide thin films as an earth-abundant photovoltaic absorber

“…12,13 Doping engineering is critical for the formation of homo-and heterojunctions with properties tailored for photovoltaic applications and reaching the expectations of this promising PV material. Defects can also influence other electronic properties, such as carrier lifetimes, 14 and can lead to localized bandgap fluctuations, 15 all of which contribute to the efficiency of electronic devices. 16 Therefore, a thorough understanding of the defects, their electronic properties and their interplay with the growth conditions is crucial for the development of photovoltaic cells with this earth-abundant absorber.…”

Large‐Area (Ag,Cu)(In,Ga)Se2Thin‐Film Solar Cells with Increased Bandgap and Reduced Voltage Losses Realized with Bulk Defect Reduction and Front‐Grading of the Absorber Bandgap

“…Therefore, we use the EQE ‐determined bandgap moving forward. The 1.24 eV bandgap is wider than earlier‐reported MiaSolé ACIGS absorbers ( E g = 1.21 eV, V OC = 742 mV, voltage deficit, E g /q — V OC = 0.468 V), [ 20 ] and the voltage deficit of 0.463 V is only 57 mV higher than for the record efficiency small area E g = 1.22 eV device. [ 17 ]…”

“…[ 19 ] Earlier MiaSolé devices demonstrated successful Ag‐alloying for 17.7% efficient ACIGS solar cells, but defect‐associated metastabilities still persisted. [ 20 ]…”