Grain engineering: How nanoscale inhomogeneities can control charge collection in solar cells

Abstract

Statistical and correlative analysis are increasingly important in the design and study of new materials, from semiconductors to metals. Non-destructive measurement techniques, with high spatial resolution, capable of correlating composition and/or structure with device properties, are few and far between. For the case of polycrystalline and inhomogeneous materials, the added challenge is that nanoscale resolution is in general not compatible with the large sampling areas necessary to have a statistical representation of the specimen under study. For the study of grain cores and grain boundaries in polycrystalline solar absorbers this is of particular importance since their dissimilar behavior and variability throughout the samples makes it difficult to draw conclusions and ultimately optimize the material. In this study, we present a nanoscale in-operando approach based on the multimodal utilization of synchrotron nano x-ray fluorescence and x-ray beam induced current collected for grain core and grain boundary areas and correlated pixel-by-pixel in fully operational Cu(In (1−x) Ga x)Se 2 solar cells. We observe that low gallium cells have grain boundaries that over perform compared to the grain cores and high gallium cells have boundaries that under perform. These results demonstrate how nanoscale correlative X-ray microscopy can guide research pathways towards grain engineering low cost, high efficiency solar cells.

Sodium enhances indium-gallium interdiffusion in copper indium gallium diselenide photovoltaic absorbers

“…The APT analyses show Na and In but no Ga enrichment at the planar defects observed in the film. Grain boundaries of polycrystalline films tend to show In 50 or Ga 57 enrichment, along with substantial amounts of Na.…”

“…refs. 50 and 57 , respectively), on the other hand, could suggest a different solubility of In and Ga supercomplexes in CIGS that may depend on CGI ratio. If either of the two supercomplexes segregates preferentially at the grain boundaries, it could hinder In/Ga interdiffusion across the boundary due to Zener pinning.…”

The fox and the hound: in-depth and in-grain Na doping and Ga grading in Cu(In,Ga)Se2 solar cells

“…(ii) Less frequent Ga enrichment of grain boundaries, i.e. their detrimental nature for device efficiency; 43,86 (iii) Na-induced hindered grain growth. 26,41,43 A second hypothesis relates to the asymmetry between In and Ga diffusivity, as already suggested by Witte et al in Na-free CIGS.…”

“…Furthermore, the premise offers a convenient explanation for the detrimental nature of the grain boundaries in Ga-rich CIGS, compared to conventional CIGS. 43,86 As vacancy antisite complexes are stable, by lowering the In migration barrier, Na allows the In excess to migrate to its equilibrium location (the grain boundaries). This has long been regarded as a benecial passivating effect, due to band gap widening at grain boundaries and consequent reduction of charge carrier recombination.…”

Resonant x-ray ptychographic nanotomography of kesterite solar cells

“…The most important feature of this technique is the possibility for in situ or operando studies. The impact of these defects and features can be quantified with measurements of x-ray beam induced current or voltage (XBIC/XBIV [50][51][52]), which is the natural extension of this study. The missing requirements for such studies are the ability to electrically contact the prepared samples and a different scheme for the ptychographic scan.…”