Zn3P2 as an earth-abundant photovoltaic material: from growth to device

The development of cost-effective and earth-abundant semiconducting materials is imperative for the sustainable deployment of photovoltaic technology. Zinc phosphide (Zn3P2) is a promising candidate for terawatt-scale electricity generation. It has a near-optimal bandgap of 1.5 eV, a high visible-light absorption coefficient (10^4-10^5 cm-1) near the band edge, a long minority-carrier diffusion length (5-10 µm), and both zinc and phosphorous are earth-abundant elements. However, the highest recorded efficiency stands at 6% for multi-crystalline Zn3P2 wafer solar cells. In the last 40 years, there has not been any significant improvement in the efficiencies of Zn3P2-based solar cells. Material challenges include a lack of compatible substrate for growth and control over the doping of the material. One of the key issues associated with Zn3P2-based solar cells is the limited understanding of the correlation between the functional properties and the crystalline structure and defect density of the material. In this thesis, we explore an alternative approach for the growth of Zn3P2 that circumvents the need for a lattice-matched substrate. In addition, we investigate the electrical properties of Zn3P2 thin films and highlight the role of composition and growth conditions on the material property. Finally, we demonstrate a minimally processed solar cell device with an efficiency value of 4.4% and propose a working principle. In the first part of this thesis, we demonstrate the growth of Zn3P2 on graphene using molecular beam epitaxy. The growth proceeds via van der Waals epitaxy and shows a preferentially crystallographic orientation. We delve into the growth process by identifying the key growth parameters and determining the limiting factors. The material structure and optical functionality were also studied using Raman and photoluminescence spectroscopy. The second part of this thesis presents the progress on the understanding of the electrical properties of Zn3P2 thin films. We used several different characterization techniques to demonstrate the role of composition and microstructure on the electrical transport mechanism and carrier concentration. We showcase a hole mobility of 125 cm2/Vs for high-quality single crystalline Zn3P2 thin films and the ability to modify the carrier density by the composition (Zn-to-P ratio). In the final part of the thesis, we showcase a Zn3P2 thin film-based solar cell with a relatively high open-circuit voltage (0.528 V) and short-circuit current (13.7 mA//cm2). We investigate the dominant recombination mechanism in the material using the ideality factor obtained from dark and light measurements. We highlight the possible loss mechanisms of the solar cell and provide a perspective on this new generation of Zn3P2-based solar cells.