A new generation of abundant and low-toxicity photovoltaic materials for Intermediate Band Solar Cells

The ever increasing concerns regarding CO2 emissions and international agree-ments on CO2 targets, has lead to increased efforts to realize low carbon renewa-ble energy alternatives. Present solar cells (SC) often have the drawbacks of being low efficiency or being composed of rare or toxic materials. Thus there is a huge need to find better alternatives. This has lead to a novel “Third generation” of solar cells: so-called intermediate band solar cells (IBSC) offer a theoretical effi-ciency of 63%, around 20% higher than single gap SC [1]. Furthermore, it is pre-dicted that IBSCs can be fabricated at low cost, and from abundant and nontoxic materials, such as transition metal oxides (TMO) [2,3]. In this work, we study the fundamental properties of two TMOs which have been identified as promising candidates for IBSCs: MoO3 and Cu2O. In both cases we demonstrate precise control of the electronic properties, and characterise the materials using X-ray and ultraviolet (UV) photoemission spectroscopies. In the case of MoO3 we demonstrate that oxygen vacancies are formed both by heat and exposure to UV light (see figure). We conducted an “accelerated ageing” test, and conclude that the material is insufficiently robust for use as a real IBSC. Con-versely, our study of Cu2O reveals that N+ doping is very robust, surviving heat, high fluxes of UV and X-ray exposure and air exposure. In summary, we find that MoO3 is less than ideal for IBSCs, and Cu2O is a very promising alternative. REFERENCES [1] A. LUQUE AND A. MARTI, PHYS. REV. LETTS. 78, 5014 (1997). [2] T. Lin and D. Lichtman, J. Vac. Sci. &Tech. 15, 1689 (1978). [3] C. Malerba, et al., Solar Energy Materials & Solar Cells 105, 192 (2012).