Cadmium Selective Etching in CdTe Solar Cells Produces Detrimental Narrow-Gap Te in Grain Boundaries

Abstract

Recent advances in design and processing technology have made possible commercialization of polycrystalline (px)-CdTe as a photovoltaic absorber. Grain boundaries (GBs) are the most prominent structural defects in these devices and undergo significant changes during device fabrication. However, the effects of device fabrication processes on these GBs are not entirely understood. Prevailing models of GBs in thin-film photovoltaics consider individual GBs to have homogeneous properties in their area. Here, using an aberration-corrected scanning transmission electron microscope (STEM)-based low-loss and core-loss electron energy-loss spectroscopy (EELS), we show that back-surface etching of CdTe leads to inhomogeneity within individual grain boundaries. We observe that etching the back surface leads to the conversion of a region of GBs from CdTe to an elemental Te, which has an only 0.33 eV band gap, as deep as 1 μm from the back surface. The presence of elemental Te in GBs this deep into the absorber layer will increase recombination in the absorber layer and limit the extractable open-circuit voltage, thus reducing device efficiency. However, additive methods for back contact formation such as deposition of Te, ZnTe, or other materials preserve the CdTe stoichiometry of the GBs. Thus, especially for the next generations of CdTe-based cells having longer minority carrier diffusion length and/or thinner absorber layers, additive back contacting methods are superior.

Identification the Role of Grain Boundaries in Polycrystalline Photovoltaics via Advanced Atomic Force Microscope

“…C‐AFM and KPFM represent efficient methodologies for probing the localized characteristics of polycrystalline PV materials. Generally, adsorption in GBs is prone to occur, [ 73 ] and the energy at GBs surpasses that of GIs, thereby promoting chemical reactions. [ 74 ] Additionally, the presence of GBs limits atomic dislocations, thereby enhancing crystal strength.…”

First‐Principles Study of the 30° and 90° Partial Dislocations in HgTe, CdTe, and Hg0.7Cd0.3Te

“…Several studies of dislocations in CdTe using DFT are available in the open literature. Some examples include the study of dislocations formed in grain boundaries, [12][13][14] the glide 60°p erfect, [15] 30°partial, [15,16] and 90°partial dislocations. [15] There has been some discrepancy regarding the most stable core configuration for the 30°partial dislocation.…”

Экситонная люминесценция двойных монослоев CdTe в матрице ZnTe

“…Гетероструктуры на основе халькогенидов кадмия и цинка применяются в преобразователях солнечной энергии в электрическую [1][2][3][4][5] и в лазерных устройствах, работающих в зеленой и синей областях спектра [6]. Гетероструктуры на основе полупроводников II−VI, как правило, являются напряженными, влияние напряжений на высоту барьеров в дырочных квантовых ямах (КЯ) оказывается на порядок более сильным по сравнению с химическим сдвигом [7].…”