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.
