Science and Technology of Advanced Materials | 2019
Focus issue on organic and hybrid photovoltaics
Abstract
The ongoing interest in organic and perovskite solar cells comes from the forecasted excellent potential for lowcost solar electricity generation that these technologies offer. Organic and perovskite solar cells can be fabricated from abundant materials with high-throughput compatible processes on flexible substrates and promise near future large-scale electricity production as well as novel and alternative applications in buildings, portable electronics or in tandem thin film devices. Research activities on perovskite solar cells have been exploding over the last years and a simple title search including ‘perovskite solar cell’ in the web of science delivers well over 1500 hits for the time January–October 2018 alone! Likewise, organic solar cells are still in the spotlight and recent advances with ternary blends and non-fullerene acceptors allow now for power conversion efficiencies exceeding 10% almost as a matter of routine. In this Focus Issue of Science and Technology of Advanced Materials, we present a collection of excellent research and review articles on recent milestones in materials development, device architectures and advanced characterization techniques that highlight the great progress that is being made in both photovoltaic technologies. The efficiency of ternary organic solar cells depends on the developing nanostructured network of donor and acceptor phases during film formation. In a typical ternary system for photovoltaic applications, a third sensitizing component is added to an underlying high-performing binary system. However, finding the ideal three-component composition by trial and error is a cumbersome process. The research paper of Makha and Heier et al. [1] demonstrates a general approach to understanding the behaviour of organic ternary blend solar cells from simple thermodynamic principles. The authors show that phase diagrams are a fundamental tool to better understand and control the morphology of ternary blend, thereby optimizing the solar cell performance in a rational way. Newman and Tsoi et al. [2] carried out a detailed study on the burn-in period of a high-performing solution-processed organic small molecule solar cell under illumination. Device performance was followed as a function of prior solvent vapour annealing time, controlling the crystallization of the donor phase. Results from grazing incident X-ray diffraction, UVvis absorbance, Raman and photoluminescence experiments indicate a correlation between the increase in cell stability and the degree of crystallinity of the donor; however, the degradation originates not directly from the crystallinity changes but correlates with changes in molecular conformation. Two articles are dealing with ‘photon management’ in organic and perovskite solar cells. Zhang and Toudert [3] review recent works reporting on optical management approaches for maximizing the light absorption and reducing transmission and reflection losses in perovskite solar cells. These include the design of optical cavities, the incorporation of plasmonic or dielectric nanostructures, the use of antireflection coatings and the structuration of the internal layers. This review points out that the performance of a solar cell is not only determined by the proper materials selection and fabrication method, but that optical optimization can substantially contribute to efficiency enhancement. The article of Pascual-San-José and Campoy-Quiles et al. [4] considers that organic materials and perovskites are interesting for building-integrated applications, where the colour and aesthetic appeal of a solar panel plays an important role. The authors implemented a theoretical methodology to assess colour tuning in polymer-based solar cells. They compare quantitatively interference effects, binaries with different donors and acceptors, and ternary systems where the third component is either active or a simple dye. The concept can be applied to perovskite solar cells as well but the colour tuning capability of perovskites is smaller than for organic materials. The methodology allows also addressing the inverse problem, in which the materials and geometry that produce a targeted colour are obtained by a fitting routine.