Bandgap of thin film solar cell absorbers: A comparison of various determination methods

Abstract

Knowledge of the absorber bandgap is often needed for assessing the junction quality of a thin film solar cell, for example when computing the open-circuit voltage deficit. The bandgap is typically estimated from the routine measurement of the external quantum efficiency (EQE) of finished devices. However the extraction becomes ambiguous in the case of very thin absorbers, or in presence of bandgap gradients or collection issues of charge carriers. This work reviews several methods for the determination of the bandgap from EQE measurements and discusses their validity conditions. The numerical results are compared based on experimental EQE of several different thin film solar cells such as CuInGaSe 2 , CuInSe 2 , CdTe, CuZnSnSe and perovskite, and systematic trends are identified. Numerical simulations are also performed to illustrate the behavior of the different bandgap extraction methods in presence of a bandgap gradient and different magnitudes of the exponential tail states.

Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells

“…25 . Moreover, band-gap energies of CIGSe absorbers (i.e., the minimum band-gap energy at the notch position) can be determined reliably also from the EQE data, as already reported by Carron et al 35 , in contrast to from PL spectra.…”

Advanced Alkali Treatments for High‐Efficiency Cu(In,Ga)Se2 Solar Cells on Flexible Substrates

“…The increase in the PV performance essentially results from increases in V OC and J SC , while most other PV parameters remain almost unchanged (FF, ideality factor, J 0 or series resistance as shown in Figure S2, Supporting Information). To understand the clear statistical increase in efficiency and lift the ambiguity caused by sample‐to‐sample bandgap variations, the optical bandgap of a few individual cells was extracted from EQE measurements using the inflection point method . In the following, for each of these cells the V OC and J SC are compared to the Shockley–Queisser limits V OC ,SQ and J SC,SQ obtained from the optical bandgap (tabulated values available in ref.…”

“…To understand the observed evolution, we performed optical TMM simulations assuming perfect collection as detailed in refs. and . Simulation series were conducted by varying the CGI composition on four different GGI grading profiles with various notch widths.…”

Progress and Perspectives of Thin Film Kesterite Photovoltaic Technology: A Critical Review

“…Then, the V OC deficit is estimated as (Equation )where the V OC(Shockley–Queisser) is the maximum thermodynamic V OC achievable for a given absorber bandgap, and the V OC(experimental) is the V OC obtained from the J – V analysis under AM1.5G conditions. Most commonly, the bandgap value used for estimating the V OC(Shockley–Queisser) is extracted from the external quantum efficiency (EQE) or internal quantum efficiency (IQE) (a detailed description and comparison of the methods for extracting the bandgap has been published recently by Carron et al).…”