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Emergent Models for Artificial Light-Harvesting

INTRODUCTION AND BACKGROUND Photovoltaic (PV) solar energy conversion is one of the most promising emerging renewable energy technologies and is likely to play an ever-increasing role in the future of electricity generation. The potential of photovoltaics is considered to be much larger than the current energy demand, which is equivalent to ≈13,000 millions tons of oil (BP, 2014). Together with other renewable energy sources, photovoltaics could thus considerably aid the decarbonization of the energy sector in the near future, an objective, which is (should be) seriously sought after to meet the IPCC 2°C target1. Indeed, PV energy conversion is an ever-increasing research field having an impact on other sectors: technological (e.g., dealing with energy storage and distribution issues) and economical (e.g., evaluating the costs of PV devices compared to conventional energy technologies). The typical working principle of a photovoltaic cell is quite simple, and well understood in terms of doped inorganic semiconductors (Wurfel, 2009). Photovoltaic technology has significantly moved forward since the first silicon (Si) solar cell having an efficiency of ≈4% was invented in 1950s in Bell Laboratories, following the breakthrough of the p–n junction developed by Shockley, Brattain, and Bardeen (Chapin et al., 1954). Since then, research on PV has led to singlejunction and multi-junction solar cells with record efficiencies in controlled (laboratory) environments of ≈29% (Green et al., 2014) and 46% (Fraunhofer-ISE, 2014), respectively. However, the efficiency of the most popular technologies in the commercial market – which belong to the so called first generation of solar cells, mainly based on crystalline Si – is in the 10–18% range and around 25% in laboratory. A second generation of cells, mainly aimed at reducing the fabrication costs, is based on thin film architectures, where light is absorbed and charge generated in a solid thin layer of semiconductor (e.g., CdTe, CIGS), with efficiencies restrained to around 22%. Beyond these technologies, a variety of other PV designs has emerged, mainly driven by the progress on materials research and the need of low-cost manufacture devices, organic photovoltaic (OPV) being one of the most interesting solutions. The latest scientific developments in this field have taken various directions (using different concepts, materials, and geometries), all having the potential to result in high-efficiency PV devices. Most current approaches, which have been termed third generation PV [see e.g., Green (2003)], explore schemes beyond the guidelines given by Shockley and Queisser (1961), which are based on the assumption that each photon absorbed above the band gap of the semiconductor photovoltaic material generates a single electron-hole pair. These methods include solar cells exploiting multiple-exciton generation in both inorganic and organic materials, hot carriers collection, upand down-conversions. Reviewing the extensive literature about the past and present PV schemes is beyond the scope of this Opinion, many excellent reviews having already succeeded in this (challenging) task. Here, we will illustrate and highlight our point of view on an emergent strategy for PV and artificial light-harvesting in general. This strategy holds onto an interdisciplinary framework encompassing biology and quantum mechanics. Ideas, which have been developed within this nascent trend, are inspired by the recent discovery of quantum effects in biological systems (Mohseni et al., 2014), and aim to model and reproduce the highly efficient reactions often exhibited by these systems using the tools of quantum mechanics and nanotechnology platforms. Novel theoretical proposals, for instance (Scully, 2010; Blankenship et al., 2011; Scully et al., 2011; Creatore et al., 2013; Zhang et al., 2015), have underlined the relevance of effects of quantum coherence to enhance the performance of photocells whose working principle is inspired by the architecture of biological light-harvesting complexes (LHCs).