Large area survey grain size and texture optimization for thin film CdTe solar sells using xenon-plasma focused ion beam (PFIB)

Microstructural analysis of high efficiency thin film CdTe solar cells has been obtained over large areas. Analysis regions are device cross-sections approximately 0.325 mm in length. The samples have been prepared using a xenon-plasma focused ion beam (Xe-PFIB). The detailed images of the microstructure were obtained using backscattered electron imaging and electron backscatter diffraction (EBSD). As deposited devices and those with a low level of cadmium chloride treatment both show strong (111) growth texture. A high density of twins is seen in the columnar grains. Three As doped FTO/CdZnS/CdTe with varying process conditions we devices with 13.1 %, 16.3% and 17% conversion efficiency were investigated. Lowest efficiency device was CdCl2 treated at 420°C for 10 minutes while the 16.3 and 17% devices were both treated at 440°C for 10 minutes. The large area analysis revealed a partial recrystallisation state in the 16.3% efficient device which was induced by an incomplete chloride activation process. The analysis confirms that the efficiency of the devices tends to correlate with grain size. It also showed that a strong correlation exists between device efficiency and the randomization of the texture away from the (111) grain orientation. EBSD can be used to survey large areas and to mark out features for more detailed analysis using transmission electron microscopy (TEM). As an example, we show how using an EBSD scanned cross-sectional area can identify a partially recrystallized region which is then extracted and analyzed in detail using TEM

Exploring laser annealing as an alternative approach to improve CZTS thin film quality for photovoltaic cells

With the ever-growing global demand for renewable energy sources, Cu2ZnSnS4 (CZTS) thin films can complement the existing Silicon-based photovoltaic (PV) cell market. However, low carrier mobility and lifetime, non-uniformity, and defects in the film structure have reduced the widespread application of CZTS. Post-process annealing is a critical step in improving cell quality. Nevertheless, conventional furnace annealing methods have failed to enable this technology's commercialisation, and the heating of the complete PV cell limits the selection of available substrates. This research explored alternative approaches using diode laser annealing for CZTS films deposited on flexible Mo foils in different gas atmospheres. SEM and XRD spectroscopy techniques were used to analyse the laser-annealed samples. Processing parameters were established by which grain enlargement was observable with the pure CZTS phase retained. The results demonstrated that laser annealing has the potential to transform nanocrystalline as deposited films into a crystal structure with more intense and sharper diffraction peaks of kesterite structure. The key findings build confidence in laser annealing as an alternative method to enhance the crystallinity, surface morphology, chemical composition of CZTS thin films for nextgeneration PV cells. Laser annealing can potentially reduce the reliance on vacuum furnace heating of PV cells and enable annealing on more thermally sensitive substrates.</p

Impact of in-situ Cd saturation MOCVD grown CdTe solar cells on As doping and V_OC

In-situ Cd-saturated growth of polycrystalline CdTe:As thin film was performed by metal organic chemical vapour deposition at a low temperature of 350 °C, to investigate the impact on As doping and device VOC. Device characterization showed conversion efficiency of ∼14%, and VOC of 772 mV, which is an improvement to the baseline device with CdTe:As absorber layer grown at 390 °C under non-saturated conditions. When the low temperature Cd-saturated growth was combined with chlorine heat treatment at a higher temperature of 440 °C (in contrast with the standard 420 °C) for 10 min, device efficiency improved to ∼17% with a high VOC of 877 mV. As a result, ∼100 mV boost in VOC from baseline is demonstrated with Cd-saturated CdTe:As device. Micro-photoluminescence and time-resolved photoluminescence measurements performed on these Cd-saturated CdTe:As devices confirmed that minority carrier lifetime significantly improved.</p