Composition‐Dependent Passivation Efficiency at the CdS/CuIn_1-xGa_xSe₂ Interface

Abstract

The bandgap of CuIn1-xGaxSe2 (CIGS) chalcopyrite semiconductors can be tuned between ≈1.0 and ≈1.7 eV for Ga contents ranging between x = 0 and x = 1. While an optimum bandgap of 1.34 eV is desirable for achieving maximum solar energy conversion in solar cells, state‐of‐the‐art CIGS‐based devices experience a drop in efficiency for Ga contents x > 0.3 (i.e., for bandgaps >1.2 eV), an aspect that is limiting the full potential of these devices. The mechanism underlying the limited performance as a function of CIGS composition has remained elusive: both surface and bulk recombination effects are proposed. Here, the disentanglement between surface and bulk effects in CIGS absorbers as a function of Ga content is achieved by comparing photogenerated charge carrier dynamics in air/CIGS and surface‐passivated ZnO/CdS/CIGS samples. While surface passivation prevents surface recombination of charge carriers for low Ga content (x < 0.3; up to 1.2 eV bandgap), surface recombination dominates for higher‐bandgap materials. The results thus demonstrate that surface, rather than bulk effects, is responsible for the drop in efficiency for Ga contents larger than x ≈ 0.3.

Toward Understanding Chalcopyrite Solar Cells via Advanced Characterization Techniques

“…[101,102] Ballabio et al recently demonstrated differences in surface passivation according to the Ga composition at the CdS/CIGS interface using an optical pump−TPS. [103] Figure 9c-e presents the normalized differential transmittance of the terahertz probe for CIGS films with different levels of Ga as a function of the delay time. All of the graphs exhibit a biexponential decay curve, with decay time constants obtained from exponential curve fitting.…”

“…Adapted with permission. [103] Copyright 2019, Wiley-VCH. and carrier lifetime using high-frequency and long-wavelength beams.…”

“…Other distinctive techniques for monitoring carrier dynamics are terahertz probe spectroscopy (TPS) [ 100–103 ] and transient absorption spectroscopy (TAS). [ 104 ] Both techniques detect changes in the optical properties due to photoexcited carriers.…”

“…Therefore, the change in the transmittance of the terahertz probe beam can be used to indicate the photoconductivity and excess carrier density. [ 103 ] The carrier dynamics of the absorber layer are examined by tracing the difference in the transmittance compared to the ground state (−ΔT/T) according to the time delay between excitation via the optical pump and the transferred terahertz pulse (i.e., the pump−probe delay). The carrier lifetime is evaluated by observing the change in the transmittance of the terahertz beam, reflecting the decay curve of photogenerated carrier relaxation over time.…”

“…Normalized optical pump−TPS dynamics with a pump−probe delay time for CuIn1-xGaxSe2 samples with a relative Ga content of c) x = 0.3, d) x = 0.5, and e) x = 0.7. Adapted with permission [103]. Copyright 2019, Wiley-VCH.…”