Cadmium telluride (CdTe) solar cells are the leading thin film alternative to conventional silicon technology, currently accounting for around 5% of the global photovoltaic market. Alloying selenium (Se) into the CdTe absorber produces the highest efficiency devices (at 22%) because it reduces the bandgap and improves performance by passivating the defects that reduce luminescence. But exactly how Se diffuses through CdTe has remained unclear until now.

Researchers from IriG at the University of Grenoble Alpes and CEA used density functional theory (DFT) calculations to identify all the possible diffusion paths of Se in CdTe [Selvaraj et al., Applied Physics Letters 119 (2021) 062105,]. The DFT calculations demonstrate that Se has a dual role in boosting the efficiency of CdTe solar devices.

“Recently experimental efforts have indicated a reduction in the concentration of non-radiative recombination centers in Se-alloyed CdTe,” explains Pascal Pochet, who led the research. “[We wanted to] provide an atomistic-level explanation.”

Pochet and colleagues’ calculations reveal that chalcogenide interstitials diffuse through bulk CdTe via a unique two-step mechanism. Se creates a diffusion channel for Te interstitials, enabling the diffusion of both chalcogenide atoms through the material and reach Cd vacancies and Te antisites. The diffusing chalcogenide interstitials also interact with non-radiative centers, passivating broken bonds at defects.  The researchers also identify nine possible complexes that can passivate the Cd vacancy and the Te antisite, which make up the two main non-radiative recombination centers.

“Understanding of the Se incorporation diffusion path in CdTe matrix and its interaction with native non-radiative recombination centers was a bottleneck [in the development of] CdTe-based devices, which have seen the impressive improvements in efficiencies from 16.7% to 22.1 % in the last decade,” explains Pochet. “Our findings on the role of Se in the passivation mechanism at the atomic scale might allow [this approach to be] combined with other passivation strategies and improved cell architecture to achieve the target of 25% solar efficiency.”

CdTe solar cells have a relatively short payback time, given their low cost per watt, and show marginal power efficiency losses in hot climates compared with silicon photovoltaics. Despites these attractive attributes, CdTe solar technologies would be more attractive still if solar efficiencies could be boosted. This new insight into the atomistic mechanisms underlying Se passivation could enable further improvements in CdTe solar cells.

“Our study creates room for further development of the CdTe research community’s understanding and knowledge of the various doping and design strategies at play in CdTe photovoltaics,” says Pochet. “This new insight into the atomistic mechanisms underlying Se passivation could enable further improvements in CdTe solar cells,” adds Sameer Gupta, co-first author and PhD student.

Role of Se in passivation of CdTe solar cells.
Role of Se in passivation of CdTe solar cells.