Effects of material properties of band‐gap‐graded Cu(In,Ga)Se₂ thin films on the onset of the quantum efficiency spectra of corresponding solar cells

Abstract

Polycrystalline Cu(In,Ga)Se 2 (CIGSe) thin-film solar cells exhibit gradual onset in their external quantum efficiency (EQE) spectra whose shape can be affected by various CIGSe material properties. Apart from influences on the charge-carrier collection, a broadening of the EQE onset leads to enhanced radiative losses in open-circuit voltage (V oc ). In the present work, Gaussian broadening of parameters describing the EQE onset of thin-film solar cells, represented by the standard deviation, σ total , was evaluated to study the impacts of the effective band-gap energy, the electron diffusion length, and the Ga/In gradient in the CIGSe absorber. It is shown that σ total can be disentangled into contributions of these material properties, in addition to a residual component σ residual . Effectively, σ total depends only on a contribution related to the Ga/In gradient as well as on σ residual . The present work highlights the connection of this compositional gradient, the microstructure in the polycrystalline CIGSe absorber, and the luminescence emission with the residual component σ residual . It is demonstrated that a flat band-gap with no compositional gradient in the bulk of the CIGSe absorber is essential to obtain the lowest σ total values and thus result in lower recombination losses and gains in V oc .

VOC‐losses across the band gap: Insights from a high‐throughput inline process for CIGS solar cells

“…One feature that is extracted during this analysis is the width of the band gap distribution, which is achieved by fitting a Gaussian to the derivative of the EQE around the band gap. 30 This width, taken here as the full width at half maximum of the Gaussian, is a measure for the deviation from the SQ model: The wider the band gap distribution, the further the EQE deviates from the ideal step function. Furthermore, since this Gaussian samples a part of the EQE not too from where commonly the Urbach energy is extracted, there exists a strong correlation between the two (see Figure S4c for details).…”

“…The losses discussed here are deviations from the optimum found in the SQ model. One feature that is extracted during this analysis is the width of the band gap distribution, which is achieved by fitting a Gaussian to the derivative of the EQE around the band gap 30 . This width, taken here as the full width at half maximum of the Gaussian, is a measure for the deviation from the SQ model: The wider the band gap distribution, the further the EQE deviates from the ideal step function.…”

“…The second analysis fits a Gauss function to the first derivative of the EQE centered around the band gap. 30 The full width at half maximum of the fitted Gaussian is a measure for the band gap distribution in the absorber and its mean gives the band gap. 24,31 The mean of the Gaussian fit yields the inflection point of the EQE onset, which in turn gives a good definition of the band gap (minimum band gap energy at the GGI's notch).…”

“…The EQE is analyzed in the part red‐shifted with respect to the absorber's band gap by fitting an exponential function in order to extract the Urbach energy, which samples the subgap absorption of the absorber. The second analysis fits a Gauss function to the first derivative of the EQE centered around the band gap 30 . The full width at half maximum of the fitted Gaussian is a measure for the band gap distribution in the absorber and its mean gives the band gap 24,31 .…”

Advantage of Zn(O,S) Over CdS Buffer for Low-Gap (Ag,Cu)(In,Ga)Se2 in Tandem Applications

“…We focus our analysis here on V OC and its dependence on buffer choice. Achieving insight into V OC loss mechanisms is possible by studying the EQE according to a method proposed by Rau et al and often used to assess voltage losses. − EQE values for CIGS cells with absorbers of different GGI between 0 and 0.4 are shown in Figure a with the CdS and Zn­(O,S) buffer. The band gap E g shifts with decreasing GGI toward longer wavelengths and is independent of the choice of buffer.…”