Roland Mainz

Polycrystalline Cu(In,Ga)Se₂ (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, ????_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 ????_total can be disentangled into contributions of these material properties, in addition to a residual component ????_residual. Effectively, ????_total depends only on a contribution related to the Ga/In gradient as well as on ????_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 ????_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 ????_total values and thus result in lower recombination losses and gains in V_oc.

High‐efficiency Cu(In,Ga)Se₂ (CIGS) thin‐film solar cells are based on polycrystalline CIGS absorber layers, which contain grain boundaries, stacking faults, and dislocations. While planar defects in CIGS layers have been investigated extensively, little is still known about the impact of dislocations on optoelectronic properties of CIGS absorbers. Herein, evidence for an optoelectronic inactivity of dislocations in these thin films is given, in contrast to the situation at grain boundaries. This unique behavior is explained by the extensive elemental redistribution detected around dislocation cores, which is connected with the dislocation strain field, probably leading to a shift of defect states toward the band edges.

The heating and cooling phase diagram of CsPb(Br_xI_1−x)₃ is determined by combinatorial synthesis and high throughput X-ray characterization.

The decomposition mechanism of Cu₂BaSnS₄ is studied by in situ diffraction and fluorescence analysis revealing “Sn loss” and Cu₂Ba₃Sn₂S₈ at high temperatures.

The bandgap of CuInSe₂ thin film photovoltaic absorbers depends on the Cu content, although the nature of this dependence is still a matter of debate. While theoretical results predicted a widening or stable bandgap with decreasing Cu content, the few experimental data available point to a narrowing of the bandgap. Here, we apply photoreflectance spectroscopy at room temperature to near-stoichiometric polycrystalline CuInSe₂/CdS heterojunctions with a lateral Cu gradient to analyze the electronic transitions in the vicinity of the fundamental absorption edge of CuInSe₂ absorber as a function of Cu deficiency. The results indicate that the lowest bandgap transition at 1.02 eV notably decreases by 20–30 meV for slightly Cu deficient samples, strengthening the case for an association of a lower Cu content with a narrower bandgap. In contrast, the higher energy transition at 1.25 eV does not show a redshift, which requires further theoretical explanation.

The selenization of stacked elemental metallic layers (CuSn–Zn) is a commonly reported approach in kesterite Cu₂ZnSnSe₄ (CZTSe) processing. CZTSe formation via this approach usually involves a reaction route containing binary selenides, such as SnSe_2−x. The high volatility of these phases at the necessary annealing temperatures (500–550 °C) makes this reaction pathway prone to Sn loss, which makes it challenging to control the composition and quality of the grown material. Herein, an approach based on stacked elemental and alloyed precursors is reported, and the benefits of using a Zn/CuSn/Zn configuration are discussed. The absence of nonalloyed elemental Sn helps in suppressing the formation and subsequent evaporation of SnSe_2−x phases, preventing Sn loss from the film during selenization. This reaction pathway involves a process scheme which 1) starts with the growth of CZTSe in a “Cu‐rich” environment, 2) includes a shift of the composition by supply of SnSe_2−x vapor, and 3) terminates in the “Cu‐poor” regime, leading to device efficiencies above 10%. This composition shift in the presented process appears similar to the final stage of the commonly known CIGSe three‐stage coevaporation.