Benchmarking photoactive thin‐film materials using a laser‐induced steady‐state photocarrier grating

Abstract

In this study, we assess the charge carrier diffusive transport quality of traditional and emerging thin-film photoactive absorber materials used for photovoltaic applications. We use a steady-state photocarrier grating technique, which has so far been predominantly used for amorphous silicon-based materials, to obtain ambipolar diffusion lengths as well as minority and majority carrier mobility-lifetime products. The measurements are performed at volume-averaged generation rates of G = 10 20 -10 21 cm À3 s À1 and low electric field strengths of E = 20-200 V cm À1 . The absorbing capability of the materials is analysed by calculating an effective optical absorption depth, and we compare its value with the obtained electronic ambipolar diffusion length. The effective absorption depths are independent of the band-gap values so that our assessment is also relevant for multijunction solar cells. We observe that for silicon-based thin-film materials, the ambipolar diffusion length (with a value lower than 150 nm) is more than twice as short as their effective absorption depth, while for copper indium gallium selenide chalcopyrite and halide perovskite materials, the diffusion length (with a value up to 367 nm) is similar or larger than the effective absorption depth. The presented method can be used as a rapid assessment of the optoelectronic quality of photoactive thin-film materials.

Increasing the scope and precision of the steady-state photocarrier grating technique by measuring the photocurrents at several voltages

“…Similar results were obtained using the phenomenological model that does not distinguish between trapped and free charges [2][3][4]. The first term of expression (15) coincides with the results obtained with the phenomenological model, whereas the other two present differences. The reason comes from the fact that one of the assumptions of the phenomenological model is not sound within the MT model.…”

“…which presents just two fitting parameters, and . The first term of [see equation (15)] is the square of the ambipolar diffusion length, [22]. As was pointed out before [6], the RZW formula could only be used for the estimation of when the two last terms of equation ( 15) are negligible (i.e., in the "lifetime regime").…”

“…Over the years, the SSPG technique has become a standard method for material characterization in several laboratories [11], where it has been effectively applied to a-Si:H and its alloys, hydrogenated microcrystalline silicon and germanium, chalcopyrite and III-V semiconductors [12,13], and also to hybrid perovskites [14][15][16]. We focus the analysis on a-Si:H-like materials because they are one of the most thoroughly studied thin-film materials in the bibliography, and there is a broad consensus on their basic optoelectronic modeling [17][18][19].…”

Influence of Environment and Light-Stress on the Optoelectronic Properties of Triple-Cation Perovskite Thin Films

“…In contrast, the SSPG method proposed first by Ritter et al for studying L d in photoconductive insulators, provides a fast, direct, and accurate experiment to study the carrier ambipolar diffusion length. Moreover, it has been proven to be suitable for many materials (for a review see ref ), and it was also shown that it could be applied to hybrid perovskites . This technique was recently upgraded into an automated experiment including a cryostat allowing the study of materials under different conditions, that is, in air and under vacuum at different temperatures .…”

Understanding Detrimental and Beneficial Grain Boundary Effects in Halide Perovskites

“…To understand the role of grain boundaries in halide perovskites on electronic transport, we used the steady‐state photocarrier grating (SSPG) technique to measure the ambipolar charge‐carrier diffusion length within the films . This technique relies on measuring lateral carrier transport in the film in the presence of sinusoidal laser interference (an optical grating) created by two monochromatic lasers (450 nm excitation wavelength) where one laser is weaker than the other.…”