Atomistic insights of multiple stacking faults in CdTe thin-film photovoltaics: A DFT study

Stacking faults in CdTe were studied using DFT simulations. Twin and tetrahedral stacking fault energies are significantly lower than previously suggested, strongly correlating with their high density observed experimentally. No long range ordering was found for tetrahedral stacking faults while a resistance for polytype clustering was calculated. All experimentally observed faults were shown to be electronically benign when considered in isolation but increased density may produce shallow electron trap states

Atomistic modelling of defect passivation in polycrystalline CdTe photovoltaics

Cadmium Telluride is a promising material for solar power generation, with a record photovoltaic efficiency of 22.1%. However as-deposited cells are Cd-s/Te-s in CdTe to Te-p > Cd-s/Se-s in CST which affects the spatial extent of the orbitals involved and may in turn affect recombination. Although some evidence for this is found from optical absorption spectra further work at higher levels of theory is needed to confirm this hypothesis. Nevertheless this work provides a valuable baseline for further exploration of this material.</div

Enhancement of photovoltaic efficiency in CdSexTe1−x (where 0≤x≤1): Insights from density functional theory

Recent advancements in CdTe photovoltaic eciency have come from selenium grading, which reduces the band gap and signicantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic eects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease lin- early as Se concentration increases in line with Vegard's Law. The simulated band gap bowing shows strong agreement with experimental values. Selenium, by itself does not introduce any defect states in the band gap and no signicant changes to band structure around the Γ point are found. Band oset values suggest a reduction of recombination across the CdSeTe/MgZnO interface at x 0:1875, which corresponds with the Se concentration used experimentally. Band structure analysis of two cases x=0.03125 and x=0.4375, shows a change from dominant Te/Se contributions in the conduction band minimum to Cd/Se contributions as Se concentration is increased, hinting at a change in optical transition characteristics. Further calculations of optical absorption spectra suggest a reduced transition probability particularly at higher energies, which conrms experimental predictions that Se passivates the non-radiative recombination centres

Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

The conversion efficiency of as-deposited, CdTe solar cells is poor and typically less than 5%. A CdCl2 activation treatment increases this to up to 22%. Studies have shown that stacking faults (SFs) are removed and the grain boundaries (GBs) are decorated with chlorine. Thus, SF removal and device efficiency are strongly correlated but whether this is direct or indirect has not been established. Here we explain the passivation responsible for the increase in efficiency but also crucially elucidate the associated SF removal mechanism. The effect of chlorine on a model system containing a SF and two GBs is investigated using density functional theory. The proposed SF removal mechanisms are feasible at the 400 ∘C treatment temperature. It is concluded that the efficiency increase is due to electronic effects in the GBs while SF removal is a by-product of the saturation of the GB with chlorine but is a key signal that sufficient chlorine is present for passivation to occur

Data for the article: Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the missing piece

Input VASP files for all the data reported in Nature Communications. INCAR and POSCAR files for:1) 0 Cl GB structures with and without stacking faults and input files required for NEB.2) Structures and input files which generate table 1 showing SF removal energy change with increasing Cl concentration.3) 14 Cl GB structures from figure 7 and input files required for NEB.4) Structures and input files required to generate DOS and ParCHG structures from figure 8 and 9.5) DOS plotting programAdded prefix to title on 06/09/2021.</div

Chlorine passivation of grain boundaries in cadmium telluride solar cells

Cadmium Telluride is the most commercially important second generation thin film photovoltaic, with a record solar cell conversion efficiency of 22.1%. However as-deposited cells are <5% efficient and require a cell activation treatment with CdCl2 at about 400 ◦C to reach commercially viable efficiencies. Such a treatment is a routine process during CdTe module manufacturing. However, the precise mechanisms at work for this remarkable efficiency enhancement are not well understood. In this paper, atomistic modelling techniques are used to improve the fundamental understanding of the structural and electronic properties of CdTe by modelling the effects of chlorine and other elements with their interaction with extended defects and grain boundaries. Studies at high spatial resolution with NanoSIMS, TEM and Energy Dispersive X-ray analysis shows that chlorine atoms are concentrated at grain boundaries in CdTe after the CdCl2 treatment. DFT calculations show that both ClTe and for the first time Cli are stabilised at the grain boundaries compared to bulk CdTe. Similar defect formation energies of these defects suggests both will be present at the grain boundaries. As expected, four single particle levels are present in the Σ3 (112) GB band gap which explains the low efficiencies prior to treatment. ClTe substitutions passivate one of these levels and partially passivate another two. Remarkably further addition of Cli fully passivates the remaining single particle levels. This passivation of single particle levels is most likely to be the primary cause of the efficiency enhancement on chlorine treatment. Further to this, alternative halogens were then trialled as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends

Inert gas bubble formation in magnetron sputtered thin-film CdTe solar cells

© 2020 The Authors. Cadmium telluride (CdTe) solar cells are deposited in current production using evaporation-based tech- niques. Fabricating CdTe solar cells using magnetron sputtering would have the advantage of being more cost-efficient. Here, we show that such deposition results in the incorporation of the magnetron working gas Ar, within the films. Post deposition processing with CdCl 2 improves cell efficiency and during which stacking faults are removed. The Ar then accumulates into clusters leading to the creation of voids and blisters on the surface. Using molecular dynamics, the penetration threshold energies are determined for both Ar and Xe, with CdTe in both zinc-blende and wurtzite phases. These calculations show that more Ar than Xe can penetrate into the growing film with most penetration across the (111) surface. The mechanisms and energy barriers for interstitial Ar and Xe diffusion in zinc-blende are determined. Barriers are reduced near existing clusters, increasing the probability of capture-based cluster growth. Barriers in wurtzite are higher with non-Arrhenius behaviour observed. This provides an explanation for the increase in the size of voids observed after stacking fault removal. Blister exfoliation was also modelled, showing the formation of shallow craters with a raised rim

Inert gas bubble formation in magnetron sputtered thin-film CdTe solar cells

Cadmium telluride (CdTe) solar cells are deposited in current production using evaporation-based tech- niques. Fabricating CdTe solar cells using magnetron sputtering would have the advantage of being more cost-efficient. Here, we show that such deposition results in the incorporation of the magnetron working gas Ar, within the films. Post deposition processing with CdCl 2 improves cell efficiency and during which stacking faults are removed. The Ar then accumulates into clusters leading to the creation of voids and blisters on the surface. Using molecular dynamics, the penetration threshold energies are determined for both Ar and Xe, with CdTe in both zinc-blende and wurtzite phases. These calculations show that more Ar than Xe can penetrate into the growing film with most penetration across the (111) surface. The mechanisms and energy barriers for interstitial Ar and Xe diffusion in zinc-blende are determined. Barriers are reduced near existing clusters, increasing the probability of capture-based cluster growth. Barriers in wurtzite are higher with non-Arrhenius behaviour observed. This provides an explanation for the increase in the size of voids observed after stacking fault removal. Blister exfoliation was also modelled, showing the formation of shallow craters with a raised rim