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Band gap tuning of solution processed ferroelectric perovskite BiFe CoO thin films 1-x
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Pamela Machado, Mateusz Scigaj, Jaume Gàzquez, Estel Rueda, Antonio Sánchez-Díaz, Ignasi Fina, Martí Gibert-Roca, Teresa Puig, Xavier Obradors, Mariano Campoy-Quiles, and Mariona Coll Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04380 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019
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Chemistry of Materials
Band gap tuning of solution processed ferroelectric perovskite BiFe1xCoxO3 thin films Pamela Machado, Mateusz Scigaj, Jaume Gàzquez, Estel Rueda, Antonio Sánchez-Díaz, Ignasi Fina, Martí Gibert-Roca, Teresa Puig, Xavier Obradors, Mariano Campoy-Quiles and Mariona Coll* Institut de Ciència de Materials de Barcelona ICMAB-CSIC, Campus UAB Bellaterra, 08193, Spain
ABSTRACT: Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications due to their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap towards the visible while simultaneously retaining ferroelectricity. To address these two issues, herein it is performed an elemental composition engineering of BiFeO3 by substituting Fe by Co cations, as means to tune the characteristics of the transition metal-oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase and stable BiFe1-xCoxO3 thin films for x0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 eV to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, non-optimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible of the Co-substituted phase is improved with respect to the unsubstituted oxide.
1. Introduction All-oxide photovoltaics (PV) is an appealing and versatile approach to help overcome the most persistent challenges in the current PV technologies: to increase the power conversion efficiencies (PCE) and to lower cost processing with stable and eco-friendly elements. Indeed, oxides are progressively being integrated in solar cells as passivation components, selective layers, transparent conducting electrodes and in a less extent as light harvesters.1–4 Recently, semiconductor ferroelectric oxides with perovskite structure have received a great deal of attention because they can be used as photoactive layers,5–8 inspired by the rapid increase in PCE (up to 23.7%) of the analogous lead halide perovskites.9–13 An exceptional property of ferroelectric perovskite oxides is that its non-centrosymmetric structure provides a unique route to spontaneously separate charge carriers, known as bulk photovoltaic effect, achieving extremely large above band gap open-circuit voltages on a single-phase material.14,15 The aforementioned aspect should allow to speculate that PCE beyond the Shockley-Queisser limit might be obtained.16–18 Additionally, they possess a spontaneous electric polarization which allows controlling the direction of the PV current.19 It is worth to note the high stability at ambient conditions of such metal oxides, as well as their capability to be manufactured by low-cost methodologies.20 Nonetheless, most of the conventional ferroelectric oxides with perovskite structure typically absorb in the ultraviolet energy range (band gap Eg 3-4 eV, far from the ideal value of ~1.4 eV for the maximum PCE) and exhibit very low photocurrents at air mass 1.5 global conditions (1
