Atomistic modelling of defect passivation in polycrystalline CdTe photovoltaics
thesisposted on 25.11.2020, 15:25 by Michael Watts
Cadmium Telluride is a promising material for solar power generation, with a record photovoltaic efficiency of 22.1%. However as-deposited cells are <1% efficient and require an activation treatment with CdCl2 to reach commercially viable efficiencies. Such treatments have been performed universally during CdTe solar cell manufacturing for decades yet the mechanism for this remarkable efficiency enhancement is not well understood. In this thesis, 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 together with extended defects and grain boundaries. After chlorine treatment the high concentration of stacking faults (SFs) present in CdTe grains is significantly reduced. Density functional theory (DFT) calculations of SFs in bulk show that tetrahedral type SFs have very low stacking fault energies and are electronically benign while polytype SFs reduce the bulk band gap but are much higher energy defects and are therefore unlikely to form. Simulations with multiple tetrahedral SFs show that although small defect states are present close to the conduction band minimum they are not sufficient to cause the low efficiency observed in untreated CdTe solar cells. The removal of SFs is therefore not the reason for the efficiency enhancement of the CdCl2 treatment. Chlorine atoms are concentrated at grain boundaries (GBs) in CdTe after the CdCl2 treatment. DFT calculations show that both ClTe and Cli are stabilised at GBs compared to bulk CdTe and the similar defect formation energies of these defects suggests both will be present at GBs. Four single particle levels are present in the _3 (112) GB band gap prior to treatment. ClTe substitutions passivate one of these levels and partially passivate another two. Further addition of Cli fully passivates the remaining levels. This passivation is likely to be the primary cause of the chlorine treatment's efficiency enhancement. Alternative elements are then trialled as activation treatments. All halogens show similar electronic effects and their defect formation energies follow ionic radii trends. Arsenic and phosphorus also show partial passivation as tellurium substitutions but sulphur and selenium produce more single particle levels in the band gap. The current models of passivation at GBs in CdTe do not cause the removal of stacking faults which means they are missing an important aspect of the CdCl2 treatment. A novel SF removal process, where the action of Cli moving to ClTe in the GBs causes a Tei cascade along the SF, is proposed and investigated. While molecular mechanics simulations give a successful proof of concept test for this mechanism the extreme sensitivity of the SF/GB structure's energy to the presence of multiple Cl atoms means DFT simulations are inconclusive. The latest high efficiency CdTe cells include selenium grading from the pn junction into the absorber layer. The created CdSexTe1-x (CST) alloy shows less recombination in grain interiors than CdTe but the reason for this is not well understood. Using DFT simulations, the lattice constant of CST is shown to follow Vegard's Law and the band gap shows bowing in agreement with existing experimental and theoretical works. The Se concentration which produces the optimal band offset for recombination reduction is in agreement with values used in high performance cells. However no defects are found within the band gap which disagrees with experimental observations. This suggests these defect states are due to external factors such as existing native defects which are not included in this work. The nature of the electron transition changes from Te-p > 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.