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

Periodic Reporting for period 2 - HOCOM (A Transparent Hole Conductor by Combinatorial Techniques for Next-Generation Energy Conversion Devices)

Periodo di rendicontazione: 2021-04-01 al 2022-03-31

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.
In the initial 3-month secondment at HZB, we investigated the moisture-dependent electrical properties of CuI, which had emerged as a particularly promising transparent hole conductor shortly before the beginning of HOCOM. Surprisingly, we found that the conductivity of CuI increases upon water adsorption. We published detailed humidity-dependent electrical characterization in a journal article and proposed possible mechanisms for the conductivity improvement.

In the 16-month outgoing phase at NREL, we applied combinatorial research techniques to rapidly evaluate the potential of selected phosphide thin films as transparent hole conductors. The growth chamber was a unique sputter system equipped with diluted phosphine, which allowed us to grow phosphide films by reactive sputtering of metallic targets. The two main materials we investigated were BP (boron phosphide) and CaCuP. Under specific growth and annealing conditions, the BP films exhibited hole conduction as desired and were partially transparent. However, the performance figure of merit was rather low. We published a journal article with these results.

As a second track, we investigated reactive sputtering of CaCuP. This compound could indeed be synthesized in one step at an appropriate temperature. We found very high hole conductivity under specific process conditions. The optical transparency was less than optimal, in disagreement with theoretical expectations.

In the 12-month return phase at HZB, we worked with computational collaborators to understand this discrepancy by calculating the light absorption coefficient of CaCuP with the inclusion of electron-phonon coupling (i.e. the so-called indirect optical transitions). We found that indirect optical absorption in experimental samples was an order of magnitude above the computational prediction. We believe this to be at least partially due to crystalline imperfections within the films, increasing the strength of indirect transitions. Growth of single-crystalline or large-grained CaCuP by epitaxial growth techniques might result in weaker absorption of light and thus better performance as a transparent conductor.

Even at the current stage of development, CaCuP still exhibits a reasonably high figure of merit and a very remarkably high conductivity for a p-type semiconductor without external doping. Thus, our results warrant further investigation of this exotic compound. We published our results on CaCuP in a journal article.

I presented the results of this project at various conferences. I had oral presentations at four Materials Research Society (MRS) conferences, one European Materials Research Society conference, and I gave invited talks on this project at the MCARE conference (American Ceramic Society) and at the yearly meeting of the German CuI research network. This dissemination work allowed me to find new collaborators on this project. Most importantly, we started collaborating on CaCuP with the computational groups of David Scanlon (UCL, UK) and Bartomeu Monserrat (Cambridge, UK), which resulted in a joint article. Synchrotron-based characterization of CaCuP films is also planned as part of this collaboration. We also collaborated with the National Institute of Standards and Technology (NIST) on calorimetry measurements for thin films. Finally, we collaborated with the group of Klaus Habicht at HZB for temperature-dependent electrical measurements and thermoelectric characterization of various films. This collaboration resulted in a joint article.
This project has been the first explicit experimental investigation of BP and CaCuP as transparent hole conductors. CaCuP had never been reported in thin-film form, and BP had never been synthesized by reactive sputtering. We were able to grow both compounds as single phases and demonstrate encouraging performance towards our targeted application. We expect that this early-stage demonstration will fuel further research in reactive sputtering of phosphides, which could be a versatile method to rapidly explore the highly uncharted field of phosphide thin films. Our results also confirm the need to go beyond oxide materials when searching for new potential transparent hole conductors. In general, HOCOM is convincing us even more of the need to rapidly explore new materials which could exceed the performance of existing materials. This is particularly important in a society which is more and more dependent on high-performance materials to achieve its urgently needed renewable energy goals.