Abstract
Recent developments in CdTe solar cell technology have included the incorporation of ternary alloy Cd(Se,Te) in the devices. CdTe absorber band gap grading due to Se alloying contributes to current density enhancement and can result in device performance improvement. Here we report Cd(Se,Te) polycrystalline thin films grown by a chamberless inline atmospheric pressure metal organic chemical vapour deposition technique, with subsequent incorporation in CdTe solar cells. The compositional dependence of the crystal structure and optical properties of Cd(Se,Te) are examined. Selenium graded Cd(Se,Te)/CdTe absorber structure in devices are demonstrated using either a single CdSe layer or CdSe/Cd(Se,Te) bilayer (with or without As doping in the Cd(Se,Te) layer). Cross-sectional TEM/EDS, photoluminescence spectra and secondary ion mass spectroscopy analysis confirmed the formation of a graded Se profile toward the back contact with a diffusion length of ~1.5 μm and revealed back-diffusion of Group V (As) dopants from the CdTe layer into Cd(Se,Te) grains. Due to the strong Se/Te interdiffusion, CdSe in the Se bilayer configuration was unable to form an n-type emitter layer in processed devices. In situ As doping of the Cd(Se,Te) layer benefited the device junction quality with current density reaching 28.3 mA/cm 2 . The results provide useful insights for the optimisation of Cd(Se,Te)/CdTe solar cells. Compositional control of optical band gap in Cd(Se,Te) thin films (0 ≤ x ≤ 1) (left) and the Se profile in finished Cd(Se,Te) devices (right) fabricated by MOCVD. • Cd(Se,Te) thin films (0 ≤ x ≤ 1) deposited with linear compositional control using a chamber-less MOCVD process. • CdSe-only and CdSe/CdSe 0.2 Te 0.8 bilayers utilised for Cd(Se,Te) grading in devices. • In-situ and ex-situ group V (arsenic) doping of the graded Cd(Se,Te) obtained. • Device performance improved for thinner CdSe layers and in-situ doped bilayers. • Good control of Se profile achieved via CdSe and CdSe/CdSe 0.2 Te 0.8 properties.
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