Effect of selenium and chlorine co-passivation in polycrystalline CdSeTe devices

Abstract

CdTe-based solar cell efficiency has rapidly improved over the last few years. Some of the reasons have been a change to the absorber composition including the incorporation of selenium, and better front contact and emitter materials in CdTe photovoltaic devices. In addition to the increase in short-circuit current by reducing the bandgap, Se plays other important roles in passivation of defects thus improving the conversion efficiency of CdSeTe/CdTe graded absorber photovoltaic devices. Here, we combine structural and optical characterizations with first principles calculations to investigate the role of Se and Cl segregation in CdSeTe devices. We find that in the presence of Se and Cl, the minority carrier lifetime improves due to a reduction of midgap defect states. We also correlate this effect with defect passivation in CdSeTe devices and suggest innovative ways to further improve CdTe-based photovoltaic efficiency.

Exceeding 200 ns Lifetimes in Polycrystalline CdTe Solar Cells

“…[16,17] Carrier lifetime measured by time-resolved photoluminescence through glass increased substantively to %5-30 ns in typical devices when CdS was replaced with CdSeTe bandgrading; this was a key component of the increase in efficiency from 16.7% to 22.1%. [3][4][5][18][19][20][21][22] Eliminating CdS substantively reduced absorption of sunlight with wavelengths less than 520 nm, thereby increasing the photocurrent at the blue end of the solar spectrum. At the same time, the CdSeTe alloy formed by Se diffusion during the postdeposition CdCl 2 anneal decreased the bandgap to %1.4 eV, thereby increasing photocurrent in the red region of the solar spectrum.…”

Understanding the Role of CdTe in Polycrystalline CdSexTe1–x/CdTe‐Graded Bilayer Photovoltaic Devices

“…Similar effects have been observed with the CdSe x Te 1– x /CdTe bilayers by Fiducia et al and Guo et al where the Se diffusion lowers the bandgap of the CdSe x Te 1– x /CdTe bilayer absorber. [ 22,23 ] The comparison of TEM images shows the grain size near the front junction in the CdSeTe layer to be relatively smaller than the CdTe‐only device. The CdSeTe film was deposited at lower substrate temperature of 420 °C, whereas the CdTe film was deposited at the higher substrate temperature of 500 °C (see Experimental Section).…”

Characterizing the Back-Contact Interface for CdTe PV through HRSTEM, EELS, and XEDS

“…CdTe PV devices show promise for higher energy and cost efficiency in the near future. Carrier recombination in the absorber has been improved, which now highlights the importance of back contact engineering to transport the photo-electrons to the electrode [1] [2]. Atomic-resolution characterization of the back contact is a necessary to quantify defects at the interface that limit efficiency.…”