Impact of In-situ Cd saturation MOCVD grown CdTe solar cells on As doping and VOC

Polycrystalline CdTe thin film solar cells grown under Cd-rich conditions has been shown to be a promising strategy for further device improvement [1]. A combination of the above with group-V element doping are of interest for maximising the p-type doping concentration, without compromising minority carrier lifetime [2]. For a long time now, increasing hole concentration of >1x1016 cm-3 in thin polycrystalline CdTe films has been limiting and challenging [3]. In-situ Cd saturation growth of polycrystalline CdTe:As thin films was performed by metal organic chemical vapour deposition (MOCVD) at a low temperature of 350°C, to investigate the impact on As doping and device VOC. SIMS measurements on the Cd-rich CdTe:As layers revealed high As concentration of 1. ${15 -1.20\times 1018}$ As cm-3. Device characterisation showed PCE of ~14%, a VOC of 772 mV, with a corresponding C-V derived acceptor concentration (NA) of ${2.0\times 1016}$ cm-3. This is a small improvement to the baseline device with CdTe:As absorber layer grown at 390 oC and not under Cd saturation conditions [4]. A much larger improvement in PCE and Voc was achieved when the low temperature Cd saturation growth was combined with higher temperature chlorine heat treatment (CHT). This was achieved for these Cd-saturated CdTe:As devices by performing a post-growth CdCl2 activation process at an elevated temperature of ${440}^{\circ}\mathrm{C}$ for 10 mins compared with the standard 420 oC. The higher CHT temperature, however, made no significant improvement with the non-saturated baseline growth. An efficiency of ~17% was measured with a high VOC of 835 mV. Compared to the baseline device, the VOC has been enhanced by ~13%. Micro - photoluminescence (micro-PL) spectra measurements performed on these Cd-rich CdTe:As samples indicated improvement in minority carrier lifetimes, which was consistent with time resolved photoluminescence (TRPL) measurements confirming that carrier lifetime almost doubled. Optimisation of As doping efficiency as a function of Cd/Te precursor partial pressure ratio and its impact on device PV characteristics, especially VOC is also investigated and will be reported. The results that will be presented in this report further lend strength to the merit of Cd-saturated growth ambient, for higher efficiency polycrystalline CdTe:As thin film solar cells.