Lateral Charge Carrier Transport in Cu(In,Ga)Se₂ Studied by Time‐Resolved Photoluminescence Mapping

Abstract

Electronic transport in a semiconductor is key for the development of more efficient devices. In particular, the electronic transport parameters carrier lifetime and mobility are of paramount importance for the modeling, characterization, and development of new designs for solar cells and optoelectronic devices. Herein, time‐resolved photoluminescence mapping under low injection and wide‐field illumination conditions is used to measure the carrier lifetime and analyze the lateral charge carrier transport in Cu(In,Ga)Se2 absorbers grown at different temperatures, on different substrates, and subject to different postdeposition treatments (PDT) with light or heavy alkalis. To estimate the carrier mobility, numerical simulations of carrier diffusion transport to areas of increased recombination (defects) are used, similarly as observed experimentally. Mobilities are found in the range of 10–50 cm2 V−1 s−1, and effective minority carrier lifetime between 100 and 800 ns resulting in carrier diffusion lengths of 2–9 μm depending on the sample. Finally, the factors limiting carrier mobility and the implications of carrier diffusion on the measured carrier lifetimes are discussed.

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

“…Different L D is accomplished by varying the effective carrier lifetimes (τ eff , shown as tags on the curves in ns) and fixed carrier mobility (40 cm 2 V −1 s −1 ). [ 21 ] PL GB, ratio represents the fraction of carriers recombined at the GB within the optical resolution (2 µm, solid lines) and for an illustrative case in which the optical resolution of the system is hypothetically enhanced four times (500 nm, dashed lines).…”

“…[ 20 ] We have previously characterized the lateral diffusion length ( L D ) of PDT samples ranging from 2 to 9 µm. [ 11,21 ] The value for No PDT absorbers could not be characterized due to pronounced inhomogeneities. Overall, it is reasonable to expect that nonuniformities in carrier lifetime of No PDT absorber are more visible because its diffusion length is significantly lower than the PDT counterparts.…”

“…However, this is not the case. By comparing, for example, the Steel and No PDT samples, Steel sample exhibits L D in the range of the optical resolution of the system (2 to 3 µm), [ 21 ] and it is reasonable to expect lower L D for No PDT sample. Hence, the impact of carrier diffusion on the lifetime inhomogeneities is minimum for both samples.…”

Exploiting the Optical Limits of Thin‐Film Solar Cells: A Review on Light Management Strategies in Cu(In,Ga)Se2

“…Furthermore, the relatively low carriers’ diffusion length for CIGS might raise fabrication and architectural issues, limiting the IBC approach. [ 265 ] Hence, for the current substrate‐type architecture, light management schemes at the front surface should provide a broadband and omnidirectional AR effect, without significant light scattering. Nanoscale features, with critical dimensions significantly smaller than the incident wavelength values, do not promote light scattering, and a significant AR performance can be achieved.…”

“…Furthermore, the CIGS carrier diffusion length lower than 10 μm demands a micro-/nanofabrication process step in the solar cell fabrication and may increase the probability of electrical shunt formation, as the gap between the contacts needs to be smaller or comparable with this value. [82,265]…”