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While multi-junction geometries have the potential to boost the efficiency of organic solar cells, the experimental gains yet obtained are still very modest. This work proposes an alternative spectral splitting device concept in which various individual semiconducting junctions with cascading band gaps are laid side by side, thus the name RAINBOW. Each lateral sub-cell receives a fraction of the spectrum that closely matches the main absorption band of the given semiconductor. Here, simulations are used to identify the important material and device properties of each RAINBOW sub-cell. Using the resulting design rules, three systems are selected, namely PBDB-T-2F:IO-4Cl, PBDB-T-2F:Y6 and PTB7-Th:COTIC-4F, and their potential as sub-cells in this geometry is experimentally investigated. With the aid of a custom built setup that generates spectrally spread sunlight on demand, the simulations are experimentally validated, showing that this geometry can lead to a reduction in thermalization losses and an improvement in light harvesting, which results in a relative improvement in efficiency of 46.6% with respect to the best sub-cell. Finally, a working proof of concept monolithic device consisting of two sub-cells deposited from solution on the same substrate is fabricated, thus demonstrating the feasibility and the potential of the RAINBOW solar cell concept. This article is protected by copyright. All rights reserved., The Spanish "Ministerio de Ciencia e Innovación (MICINN)" is gratefully acknowledged for its supportthrough grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo OchoaCentre of Excellence program and the AEI/FEDER(UE) grants PGC2018-095411-B-I00(RAINBOW),TED2021-131911B-I00and PID2021-128924OB-I00(ISOSCELLES). The authors also thank the Cata-lan agency AGAUR for grant2021-SGR-00444. MCV acknowledges a FPI fellowship (PRE2019-089855)from MICINN co-financed by the European Social Fund and MGR acknowledges the scholarship FPU16/02631 from the Spanish "Ministerio de Educación". MCV and MGR also thank the PhD programme in Materi-als Science from Universitat Autònoma de Barcelona in which both were enrolled. KV and QL acknowl-edge funding by the European Research Council (ERC, grant agreement864625)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Near infrared photodetectors are a widespread and fundamental technology in many disciplines, from astronomy and telecommunications to medical sciences. Current technologies are now striving to include new aspects in this technology such as wearability, flexibility and tunability. Organic photodetectors easily offer many of those advantages but their relatively high bandgaps hinder NIR operation. In this work, we demonstrate solution processed organic photodetectors with improved NIR response thanks to a nanostructured active layer in the shape of a photonic crystal. The latter strongly increases the charge transfer state absorption, which is normally weak but broadband, increasing the optical path of light and resulting in remarkable photoresponse significantly below the band gap of the blend. We show responsivities up to 50 mA W−1 at 900 nm for PBTTT:PC71BM based photodetectors. On top of that, by varying the lattice parameter of the photonic crystal structure, the spectral response of the photodetectors can be tuned beyond 1000 nm. Furthermore, our photonic structure can be easily implemented in the device in a single nanoimprinting step, with minimal disruption on the fabrication process, which makes this approach very promising for upscaling., We greatly acknowledge financial support from the Ministerio de Ciencia, Innovación y Universidades MICINN with projects PGC2018-095411-B-I00, MAT2016-79053-P and MAT2015-70850-P and the “Severo Ochoa” excellence program SEV-2015-0496; Generalitat de Catalunya program AGAUR 2017-SGR-00488; and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grants no. CoG648901 and StG637116). P. M. acknowledges financial support from an FPI contract (2017) of the MICINN (Spain) cofounded by the ESF. M. G. R. acknowledges financial support from an FPU grant (no. 16/02631) (2017) of the MICINN (Spain). M. G. R. and P. M. B. acknowledge the departments of Physics, Chemistry and Geology of the Autonomous University of Barcelona (UAB) as coordinators of the PhD programme in Materials Science. We acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).

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 extraordi-narily high efficiencies. One of the biggest challenges so far is to reduce their band gap towards the visible while simultane-ously 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 x=0.3 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 unsubsti-tuted oxide., We would like to acknowledge Prof. Josep Fontcuberta for fruitful discussions and comments on this study and for providing access to his experimental facilities. This research was supported by the Spanish MINECO (“Severo Ochoa” Programme for Centres of Excellence in R&D SEV-2015-0496 and FIPNANOSOLE- 2016, MAT2017-83169-R, MAT2014-51778-C2- 1-R, MAT2015-70850-P, MAT2017-85232-R (AEI/FEDER, EU)), the Generalitat de Catalunya (2017-SGR-1519, 2017 SGR- 1377), the European Research Council-ERC (FOREMAT under grant agreement no. 648901) and “L’Oreal UNESCO For Women in Science”. M.C., J.G. and I.F. acknowledge RyC contracts (2013-12448, 2012-11709 and RYC-2017-22531). I.F. is grateful to MINECO for MAT2015-73839-JIN project. The HRTEM microscopy work was conducted at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, which is a DOE Office of Science User Facility.