NREL awards $2M for CdTe PV research

Six new projects will support the Cadmium Telluride Accelerator Consortium (CTAC)

The US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) has awarded $2 million to fund six projects to support the Cadmium Telluride Accelerator Consortium (CTAC).

Announced in August 2022, CTAC is a three-year consortium intended to accelerate the development of CdTe technologies by lowering the cost and increasing the efficiency of these thin-film solar cells.

There are three topic areas: high efficiency devices; characterisation, modelling, and simulation; and tellurium supply.

High efficiency devices

The University of Utah will develop sputtered, doped widegap materials and bilayer stacks for back contacts to state-of-the-art CdSeTe/CdTe absorbers. The aim is to focus on p-type materials that have energy level alignment predicting hole selectivity, that are also amenable to passivation, and that have a wide gap to provide transparency for enhanced bifaciality or back mirror cell optics.

The team will obtain state-of-the-art absorber stacks from CTAC partners and fabricate sputtered back contacts. It will also continue to develop surface photovoltage and SPV spectroscopy techniques to characterise back contact band structure, traps, and recombination activity.

The University of Delaware will develop new approaches for processing Cd_1-xZn_xTe solar cells that overcome previously reported difficulties, such as ineffective chloride activation and passivation, which prevented the realisation of high performance with increased open-circuit voltage (VOC) relative to CdTe.

The approach will be based on two hypotheses. First is that modification of film growth, including in situ antimony incorporation, can form more equilibrated films with reduced defects and enhanced grain sizes, reducing the need for high-temperature activation. The second is that alternative halide activation chemistries during post-deposition treatments can minimise the deleterious effects of CdCl₂ activation. A final goal of the project will be to confirm the viability of Cd_1-xZnxTe by demonstration of a thin-film solar cell with VOC ≥ 1.0 V.

The University of South Florida will develop alternative device architectures based on n-type CdTe/CdSe_xTe_1-x(CST) thin-film absorbers to create opportunities to overcome the efficiency limitations associated with the current state-of-the-art p-type CdTe/CST solar cells. The project aims to build upon advances in n-CdTe/CST films that demonstrated group III and VII n-type doping for CdTe films. We will focus on the development of p-type heterojunction partners for n-CdTe/CST absorbers.

Characterisation, modelling, and simulation

Arizona State University will combine the power of hard X-ray microscopy (XRM) and soft X-ray and electron spectroscopies to probe arsenic doped CdSeTe absorbers and devices.

XRM will probe the chemical distribution, atomic environment, and current collection at the nanoscale for the arsenide and selenium absorption edges. Electron and soft X-ray spectroscopies will enable an area-integrating determination of the electronic structure at surfaces (band edges, surface bandgap) and interfaces (band alignment), in addition to the chemical bonding environment of the sulphur, chlorine, and oxygen in the device.

The team aims to tackle two main questions: How do the chemical states of arsenide (and neighbouring atoms) evolve between initial deposition and post-activation? What stressors and processes enhance or prevent activation of As dopants?

The University of Utah will assess the role of microstructures in advanced CdTe devices. The goal is to improve the limiting open-circuit voltage while retaining the maximum values of short-circuit current and fill factor of CdTe solar cells by developing a novel architecture built on a comprehensive understanding of local carrier dynamics. The aim is to investigate the interfacial and microstructural characteristics of advanced CdTe (CdSe(_1-x)Te_x) Passivated Emitter and Rear Contact (PERC) solar cells.

A microcontact array platform with tunable pattern geometry will enable measurements of global (patterned CdTe PERC) and local carrier transport, delineating the contribution of grain bulk and grain boundaries to overall photovoltaic performance. Using complementary electron/optical microscopy, we will correlate the transport characteristics to the microstructural properties of each sample set (e.g., GrV-doped vs. copper (Cu)-doped CdTe PERCs).

Tellurium supply

The Missouri University of Science and Technology will enhance Te recovery from copper processing (CP) by optimising the current operations to capture the Te, gold, and silver that are presently lost to tails.

The work covers three areas. First is mineralogical analysis of different processing streams of the flotation circuit of CP ores to identify tellurium carriers and modes of occurrence (i.e., Te in the crystal lattice versus tellerium-rich inclusions in larger minerals). Second is the evaluation of different approaches and flow sheet options for enhanced separation of tellurium, silver, and gold minerals from processing streams of CP ores. Finally, the team will conduct a techno-economic assessment of the capital and operating costs of the developed flow sheets for successful implementation, which could increase the domestic production of tellurium from CP ores by at least 50 percent.

CTAC is funded by the US DOE’s Solar Energy Technologies Office (SETO).