Grazing Incidence Cross-Sectioning of Thin-Film Solar Cells via Cryogenic Focused Ion Beam: A Case Study on CIGSe

Cryogenic focused ion beam (Cryo-FIB) milling at near-grazing angles is employed to fabricate cross-sections on thin Cu­(In,Ga)­Se₂ with >8x expansion in thickness. Kelvin probe force microscopy (KPFM) on sloped cross sections showed reduction in grain boundaries potential deeper into the film. Cryo Fib-KPFM enabled the first determination of the electronic structure of the Mo/CIGSe back contact, where a sub 100 nm thick MoSe_<i>y</i> assists hole extraction due to 45 meV higher work function. This demonstrates that CryoFIB-KPFM combination can reveal new targets of opportunity for improvement in thin-films photovoltaics such as high-work-function contacts to facilitate hole extraction through the back interface of CIGS

Nanoscale Characterization of Back Surfaces and Interfaces in Thin-Film Kesterite Solar Cells

Combinations of sub 1 μm absorber films with high-work-function back surface contact layers are expected to induce large enough internal fields to overcome adverse effects of bulk defects on thin-film photovoltaic performance, particularly in earth-abundant kesterites. However, there are numerous experimental challenges involving back surface engineering, which includes exfoliation, thinning, and contact layer optimization. In the present study, a unique combination of nanocharacterization tools, including nano-Auger, Kelvin probe force microscopy (KPFM), and cryogenic focused ion beam measurements, are employed to gauge the possibility of surface potential modification in the absorber back surface via direct deposition of high-work-function metal oxides on exfoliated surfaces. Nano-Auger measurements showed large compositional nonuniformities on the exfoliated surfaces, which can be minimized by a brief bromine–methanol etching step. Cross-sectional nano-Auger and KPFM measurements on Au/MoO₃/Cu₂­ZnSn­(S,Se)₄ (CZTSSe) showed an upward band bending as large as 400 meV within the CZTSSe layer, consistent with the high work function of MoO₃, despite Au incorporation into the oxide layer. Density functional theory simulations of the atomic structure for bulk amorphous MoO₃ demonstrated the presence of large voids within MoO₃ enabling Au in-diffusion. With a less diffusive metal electrode such as Pt or Pd, upward band bending beyond this level is expected to be achieved

Tailoring Photoelectrochemical Performance and Stability of Cu(In,Ga)Se₂ Photocathode via TiO₂‑Coupled Buffer Layers

We report on the photoelectrochemical (PEC) performance and stability of Cu­(In,Ga)­Se₂ (CIGS)-based photocathodes for photocatalytic hydrogen evolution from water. Various functional overlayers, such as CdS, TiO₂, Zn_<i>x</i>Sn_<i>y</i>O_<i>z</i>, and a combination of the aforementioned, were applied on the CIGS to improve the performance and stability. We identified that the insertion of TiO₂ overlayer on p-CIGS/n-buffer layers significantly improves the PEC performance. A multilayered photocathode consisting of CIGS/CdS/TiO₂/Pt exhibited the best current–potential characteristics among the tested photocathodes, which demonstrates a power-saved efficiency of 2.63%. However, repeated linear sweep voltammetry resulted in degradation of performance. In this regard, we focused on the PEC durability issues through in-depth chemical characterization that revealed the degradation was attributed to atomic redistribution of elements constituting the photocathode, namely, in-diffusion of Pt catalysts, out-diffusion of elements from the CIGS, and removal of the metal-oxide layers; the best-performing CIGS/CdS/TiO₂/Pt photocathode retained its initial performance until the TiO₂ overlayer was removed. It was also found that the durability of CIGS photocathodes with a TiO₂-coated metal-oxide buffer layer such as Zn_<i>x</i>Sn_<i>y</i>O_<i>z</i> was better than those with a TiO₂-coated CdS, and the degradation mechanism was different, suggesting that the stability of a CIGS-based photocathode can be improved by careful design of the structure