Abstract
During the last decade, transition metal oxides have been actively investigated as hole- and electron-selective materials in organic electronics due to their low-cost processing. In this study, four transition metal oxides (V2O5, MoO3, WO3, and ReO3) with high work functions (>5 eV) were thermally evaporated as front p-type contacts in planar n-type crystalline silicon heterojunction solar cells. The concentration of oxygen vacancies in MoO3−x was found to be dependent on film thickness and redox conditions, as determined by X-ray Photoelectron Spectroscopy. Transfer length method measurements of oxide films deposited on glass yielded high sheet resistances (~109 Ω/sq), although lower values (~104 Ω/sq) were measured for oxides deposited on silicon, indicating the presence of an inversion (hole rich) layer. Of the four oxide/silicon solar cells, ReO3 was found to be unstable upon air exposure, while V2O5 achieved the highest open-circuit voltage (593 mV) and conversion efficiency (12.7%), followed by MoO3 (581 mV, 12.6%) and WO3 (570 mV, 11.8%). A short-circuit current gain of ~0.5 mA/cm2 was obtained when compared to a reference amorphous silicon contact, as expected from a wider energy bandgap. Overall, these results support the viability of a simplified solar cell design, processed at low temperature and without dopants.
Highlights
Crystalline silicon (c-Si) solar cells are a mature technology with competitive energy prices in several markets, efforts toward higher efficiencies, lower costs and lesser environmental impacts continue to lead the photovoltaic community
This paper explores the use of four transition metal oxides (V2O5, MoO3, WO3, and ReO3) as front p-type contacts in planar n-type crystalline silicon (n-Si) solar cells
Oxygen deficiency has been reported for thermally evaporated V2O5 [18], WO3 [7] and ReO3 [13], which allows for fine-tuning of the desired work function by means of different process conditions and post-deposition treatments [19,20]
Summary
Crystalline silicon (c-Si) solar cells are a mature technology with competitive energy prices in several markets, efforts toward higher efficiencies, lower costs and lesser environmental impacts continue to lead the photovoltaic community. The development of thin-film and dye-sensitized/organic photovoltaics has introduced novel materials with excellent optoelectronic properties that could substitute standard silicon dopants Such materials have recently been reported in conjunction with p- and n-type c-Si, including organic polymers (PEDOT:PSS [2], P3HT [3]), transparent conductive oxides (ZnO [4]), transition metal oxides (TiO2 [5], MoO3 [6], WO3 [7]) or their combination [8,9], reaching power conversion efficiencies as high as 18.8%[10] for MoO3/n-type c-Si heterojunctions. A distinctive attribute of these materials is their preferential conductivity for one kind of charge carrier (i.e., holes) while blocking the other kind (electrons), aiding in the separation of photogenerated carriers [11] They are usually wide bandgap semiconductors (highly transparent) with low contact resistivities, the primary benefit is their processability by low-temperature techniques (vacuum thermal evaporation, atomic layer deposition) or by thin-film coating methods. The compositional, optical and electrical properties of these oxides will be discussed in terms of the solar cell design, making emphasis on their advantages (optical gains) and disadvantages (post-annealing degradation) when compared to conventional a-Si:H layers
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