2, Arizona State University, Tempe, Arizona, United States
3, Colorado State University, Fort Collins, Colorado, United States
For a top cell absorber in a two junction tandem solar cell, the numerical simulations indicate that the band gap of 1.72 eV is required for the current matching with the bottom cell. The band gap of Cd-Zn-Te alloy can be altered to 1.72 eV based on the concentration of different elements and is a good candidate for the fabrication of low cost tandem solar cells. The devices fabricated from as-deposited polycrystalline films of the Cd-Zn-Te alloy require a post deposition chloride treatment to improve the absorber quality. From the literature, it is known that CdCl2 treatment is an essential post deposition process to improve the performance of devices fabricated with polycrystalline CdTe and the treatment can be implemented on devices with Cd-Zn-Te. It is also known that a buffer layer of Cd-Se-Te in between the emitter and CdTe absorber improves the carrier lifetimes. During the post deposition treatment on Cd-Zn-Te, CdCl2 reacts with the bulk and forms volatile ZnCl2 compound. Due to the loss, the alloy reduces to a lower band gap material which is not suitable for the top cell in a two junction tandem solar cell.
In this study, a polycrystalline Cd-Se-Te layer was incorporated in between the emitter and polycrystalline Cd-Zn-Te absorber. To prevent Zn loss, an Al2O3 layer was deposited on the back surface of Cd-Zn-Te before the CdCl2 treatment. The composition of CdSe0.2Te0.8 and Cd0.6Zn0.4Te was kept constant. The thickness of Cd-Se-Te layer was varied from 0 nm to 300 nm to investigate the effect on the device performance. The thickness of Cd-Zn-Te with a band gap of 1.72 eV and the CdCl2 treatment were the same for all the samples. After the CdCl2 treatment, Al2O3 was etched from the back surface of Cd-Zn-Te. The devices were fabricated after copper doping of Cd-Zn-Te and application of electrodes.
In the external quantum efficiency (EQE) graphs, Cd-Zn-Te without Cd-Se-Te layer retained a sharp band edge at 1.72 eV at zero voltage bias conditions. The retention of the band edge at 1.72 eV indicated that Al2O3 acts as a barrier to prevent zinc loss during the CdCl2 treatment. The band edge shift towards the longer wavelength regions with increasing thickness of Cd-Se-Te layer in between the emitter and Cd-Zn-Te. In all the samples, the band edge remained unchanged at the forward and reverse voltage bias conditions in the EQE measurements. The optical transmissions measurements conducted before fabricating the devices were in good agreement with the EQE graphs. The time resolved photoluminescence measurement on 300 nm thick Cd-Se-Te in between the emitter and Cd-Zn-Te showed Tau1 and Tau2 values of 1.04 ns and 3.46 ns. To understand the structural and compositional changes in Cd-Se-Te/Cd-Zn-Te, X-ray diffraction and cross-section viewed under transmission electron microscope were used.