HOCOM · A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices

A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices

Optoelectronic devices such as solar cells and light emitting diodes (LEDs) are key to a sustainable energy future. They have two fundamental building blocks. First, a photoactive material to transform light into electrical current or vice versa. Second, contact materials to collect (or inject) this current. Because light has to reach the photo-active material from outside (as in solar cells) or it has to exit the device and reach the environment (as in LEDs) one of the two contacts has to be transparent. There is a long-standing problem in the science and technology of transparent contacts. Specifically, we are only able to produce transparent contacts where electrons carry the current. However, there is a second type of current flowing in the opposite direction in the photo-active material: a hole current. This current is carried by electron vacancies (“holes”), which are in some way analogous to bubbles in water. The performance level of transparent hole conductors is, however, much lower than transparent electron conductors. Using a simple performance figure of merit, this means in practice that the product of their electrical conductivity and their optical transmission is not as high as it should be for successful application in real optoelectronic devices. This issue severely limits our design options for solar cells and LEDs, it prevents the realization of transparent electronics, and it is a symptom of a gap in our scientific understanding of materials. Advances in hole transparent conductors would likely lead to improvement across all areas of optoelectronics (a key field for renewable energy and energy efficiency). Answering the scientific question “what makes a good transparent hole conductor?” would also be likely to trigger new fields and opportunities in materials science. The main goal of HOCOM is to experimentally evaluate certain phosphide materials as potential transparent hole conductors. Synthesis of these candidate materials in thin-film form (as relevant for optoelectronic devices) is made possible by a unique deposition setup at the National Renewable Energy Laboratory (NREL, USA) dedicated to phosphides. Detailed characterization of the most interesting phosphides, as well as collaboration with computational scientists to further study these materials, took place at the Helmholtz Zentrum Berlin (HZB, Germany). After thorough evaluation of BP and CaCuP as candidate materials, we confirmed their hole-conducting character and concluded that their electrical properties can be suitable for our envisioned application. Obtaining high optical transparency seems more challenging, probably due to the less-than-perfect crystalline quality obtainable with the film deposition technique used in this project, which enhances light absorption. Nevertheless, we concluded that BP and CaCuP could be useful materials in other applications where full transparency is not required.

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