Charge Carrier Lifetime Fluctuations and Performance Evaluation of Cu(In,Ga)Se₂ Absorbers via Time‐Resolved‐Photoluminescence Microscopy

Abstract

The open‐circuit voltage (VOC) is the main limitation to higher efficiencies of Cu(In,Ga)Se2 solar cells. One of the most critical parameters directly affecting VOC is the charge carrier lifetime. Therefore, it is essential to evaluate the extent to which inhomogeneities in material properties limit the carrier lifetime and how postdeposition treatments (PDTs) and growth conditions affect material properties. Time‐resolved photoluminescence (TRPL) microscopy is employed at conditions similar to one sun to study carrier lifetime fluctuations in Cu(In,Ga)Se2 with light (Na) and heavy (Rb) alkalis, different substrates, and grown at different temperatures. PDT lowers the amplitude of minority carrier lifetime fluctuations, especially for Rb‐treated samples. Upon PDT, the grains’ carrier lifetime increases, and the analysis suggests a reduction in grain boundary recombination. Furthermore, lifetime fluctuations have a small impact on device performance, whereas VOC calculated from TRPL (and continuous‐wave PL) agrees with device values within the limits of investigated PDT samples. Finally, up to about half a per cent external radiative efficiencies are experimentally determined from TRPL metrics, and internal radiative efficiencies are approximated. The findings demonstrate that the highest absorber material quality investigated is still limited by nonradiative recombination (grain or grain boundary) and is comparable to state‐of‐the‐art absorbers.

Band gap engineering by cationic substitution in Sn(Zr1−xTix)Se3 alloy for bottom sub-cell application in solar cells

“…Nevertheless, much higher than 100 ns photo-generated carrier lifetime is essential to achieve high-performance solar cells. 51,52 In summary, we demonstrate a proof-of-concept that Sn(Zr,Ti) Se 3 with Ti-rich composition is a photo active material in the SWIR region which is imperative for the bottom sub-cell absorber.…”

Alkali element (Li, Na, K, and Rb) doping of Cu2BaGe1−xSnxSe4 films

“…The few ns bulk minority carrier lifetimes for both non-doped and alkali-doped CBGTSe films are also, however, significantly shorter than lifetimes of state-of-the-art chalcopyrite or perovskite materials, which can reach several hundred nanoseconds to microseconds. 71–73 Furthermore, all alkali-doped samples exhibit even shorter decay times ( τ 1 = 2.9–5.0 ps and τ 2 = 0.9–1.3 ns) than non-doped samples ( τ 1 = 5.8 ps and τ 2 = 1.5 ns). Shorter decay times, τ 1 and τ 2 , point to higher surface recombination velocity (∝1/ τ 1 ) and bulk recombination rates (∝1/ τ 2 ), indicating that alkali-doping likely creates additional defects that may serve as recombination centers.…”

Ag Alloying in Cu2−yAgyBa(Ge,Sn)Se4 Films and Photovoltaic Devices

“…In addition, the longer ≈ns lifetimes for ACBGTSe films are still limited compared to lifetimes in state‐of‐the‐art chalcopyrite or perovskite materials, which can reach several hundred nanoseconds to microseconds. [ 35–37 ] Both the fast and slow decay components do not noticeably change regardless of Ag content, indicating that substitution of Cu by Ag does not significantly impact film surface and bulk recombination properties. Such negligible changes in decay components implies that the dominant recombination center that determines charge carrier recombination in the ACBGTSe films may not be influenced by Cu/Ag cations.…”

“…The inset table indicates sum mobility, , determined from OPTP. b) Sum mobility, , as a function of terahertz frequencies measured by OPTP and modeled by the Drude model of free charge carrier transport (dashed line, assuming scattering time, T scat , of 3.8 fs and reduced mass, m r , of 0.11 [ 9 ] ) for AAC Exp = 5%.…”