In-situ Sb-doped CdSeTe solar cell fabrication and optimization

Abstract

Cadmium telluride (CdTe) thin film solar cells are historically limited by a lower than expected open circuit voltage (VOC), typically less than 0.9 V, due to difficulty achieving high p-type doping. To overcome this barrier, this work investigates in situ antimony (Sb) doping of polycrystalline CdSeTe absorbers during vapor transport deposition (VTD) to boost acceptor concentration and VOC. A custom pyrolyzer-assisted Sb-doping system was designed to introduce Sb vapor into the growing CdTe film in a controlled manner. This approach achieved significantly enhanced p-type carrier densities on the order of 10^16 cm^–3, which is 1–2 orders of magnitude higher than in conventional Cu doped CdTe, addressing the longstanding doping limitations in CdTe, where carrier density is typically below 10^15 cm^–3. Capacitance voltage (CV) profiling and secondary ion mass spectroscopy (SIMS) techniques confirmed that approximately 20 percent of the introduced Sb atoms were electrically active acceptors, demonstrating one of the highest dopant activations in the literature. Structural characterization through scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirmed that Sb doping did not negatively impact the absorber’s crystalline quality or grain morphology. ☐ The influence of high-temperature anneal (HTA) treatments on the performance of Sb-doped CdSeTe solar cells has been investigated. The HTA treatment of the thermally evaporated CdSe0.25Te0.75/CdTe front stack converts the mixed-phase film into a single zinc blende structure by recrystallizing the photo-inactive wurtzite phase. Subsequently, HTA treatment of the full device stack after vapor transport deposition of Sb-doped CdTe (CdTe: Sb) absorbers promotes Se-Te intermixing and enhances CdTe: Sb grain growth. Comprehensive characterization using XRD, SEM, cathodoluminescence spectrum imaging, and time-resolved terahertz spectroscopy revealed that the combined HTA treatments significantly improved film crystallinity, reduced defect densities, and enhanced carrier dynamics. Under optimized conditions, HTA treatments increased the VOC of CdTe: Sb devices from 419 mV to 849 mV. The findings demonstrate the potential of HTA treatments to improve the structural and electrical properties of CdSexTe1-x solar cells. ☐ Compared to conventional Cu doping, the in situ Sb process yielded comparable or improved device performance alongside greater stability, attributed to Sb’s incorporation as a substitutional dopant with minimal diffusion. These findings indicate that Sb can serve as an effective alternative to Cu as a stable p-type dopant, resulting in CdSeTe solar cells with improved VOC and paving the way for next-generation CdTe photovoltaics striving for efficiencies closer to the theoretical limit.