Sub-gap defect density characterization of molybdenum oxide: An annealing study for solar cell applications
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epartment of Engineering, University of Palermo, Palermo 90128, Italy Photovoltaic Materials and Devices group, Delft University of Technology, Delft 2628 CD, The Netherlands 3 Department of Chemical Sciences, University of Catania, Catania 95125, Italy 2
© The Author(s) 2020 Received: 17 June 2020 / Revised: 2 August 2020 / Accepted: 3 August 2020
ABSTRACT The application of molybdenum oxide in the photovoltaic field is gaining traction as this material can be deployed in doping-free heterojunction solar cells in the role of hole selective contact. For modeling-based optimization of such contact, knowledge of the molybdenum oxide defect density of states (DOS) is crucial. In this paper, we report a method to extract the defect density through nondestructive optical measures, including the contribution given by small polaron optical transitions. The presence of defects related to oxygen-vacancy and of polaron is supported by the results of our opto-electrical characterizations along with the evaluation of previous observations. As part of the study, molybdenum oxide samples have been evaluated after post-deposition thermal treatments. Quantitative results are in agreement with the result of density functional theory showing the presence of a defect band fixed at 1.1 eV below the conduction band edge of the oxide. Moreover, the distribution of defects is affected by post-deposition treatment.
KEYWORDS molybdenum oxide, density of states, polaron theory, silicon heterojunction solar cell
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Introduction
The photovoltaic market is rapidly growing [1] and the crystalline silicon (c-Si) technology dominates the market [2]. As large-area industrial efficiencies have recently overcome the 24% mark [3] with passivated emitter rear cell (PERC) architecture and the 25% mark [4, 5] with silicon heterojunction (SHJ) architecture, there is a demand for novel concepts and materials aimed at increasing further the power conversion efficiency. The SHJ architecture combines the advantages of thin-film silicon technology with a c-Si absorber to selectively collect the generated carriers. In other words, it employs the so-called passivating contacts technology, since the thin-film silicon layers allow collecting the charge carriers efficiently while concurrently limiting the recombination losses [6, 7]. Typically, SHJ structures use a stack of hydrogenated amorphous silicon (a-Si:H) [8–11] for surface passivation and doped layers that allow high open-circuit voltage in solar cells leading to record efficiencies above 26% [11, 12]. However, a-Si:H layers also exhibit parasitic absorption of photons [13], and a limited thermal budget [7]. In this respect, the use of transition metal oxides (TMO) as an alternative to the doped amorphous silicon layers [14] is gaining particular interest owing to their remarkable optoelectronic properties including low parasitic absorption. Moreover, TMOs deposition can be carried out with simple and low-cost processes [15]. In fact, several groups devoted research works on TMOs-based solar cells [16–
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