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 .
The present work comprises a practical tutorial on the topic of correlative microscopy and its application to optoelectronic semiconductor materials and devices. For the assessment of microscopic structure–property relationships, correlative electron microscopy, combined also with scanning-probe and light microscopy, exhibits a collection of indispensable tools to analyze various material and device properties. This Tutorial describes not only the various microscopy methods but also the specimen preparation in detail. Moreover, it is shown that electron microscopy can serve to monitor phase segregation processes on various length scales in semiconductor nanoparticles and thin films. Algorithms used to extract phase information from high-resolution transmission electron micrographs are explained.
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