From Photosynthesis to Photovoltaics: Finding ... - ACS Publications

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From Photosynthesis to Photovoltaics: Finding Right Structures for High Photoconversion Efficiency

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synthetic systems with the quantum yield of nearly 100% for charge separation across a 10 nm thick membrane, OPV devices may be projected to have much higher yield of charge carrier collection at the electrode if the energetic landscape of the active layer can be optimized as it has been in natural photosynthetic systems through evolution. The lesson learned here is that OPV devices also need to have optimization at different structural levels, from local structures of polymers and small molecules, domain size and shape, to film morphology with potential gradient across the active materials between the two electrodes. The OPV studies have increased our understanding of fundamental electronic processes in organic materials, which can be branched out into other applications, such as photocatalysis for solar fuel generation. The Journal welcomes novel studies with structural and morphological optimization for each step of electronic processes in OPV materials and their potential applications in energy-related topics beyond solar electricity generation. Emerging perovskite solar cells have attracted extensive studies around the world, as I observed recently in three major conferences this summer: the International Conference of Materials in Singapore, the American Chemical Society Annual meetings, and the SPIE conference. In an estimate from the Web of Science, there have been >6000 publications since 2013 on perovskite solar cells and >9700 publications on organic photovoltics during the same period. Recently, a record of 22.1% device PCE in high-performance perovskite solar cells (PSCs) containing formamidinium with multiple cations and mixed halide anions has been published.3 This study is representative of many investigations in tuning compositions of the materials, aiming at higher PCE and stability and less toxicity. Meanwhile, the research front in electronic processes in perovskite grows exponentially within and beyond solar cell applications. Both theoretical and experimental works have been carried out on fundamental electronic processes in these materials, including properties of “excitons” and effects of structural dynamics of atoms of lattices accompanying charge carrier generation on ultrafast time scales. A key obstacle for perovskite materials to be used broadly in solar cells is the stability of these materials and devices under ambient conditions. Extensive investigations are dedicated to determining the origins of materials’ deterioration and methods for prevention of such processes. Significant progress has been made toward potential long-term and large-scale applications of these materials in solar cells and photocatalysis. We expect to see rapid advances in these areas, and the Journal welcomes novel studies on these materials and their applications with potential impact in the future energy landscape.

lthough natural photosynthetic systems and organic photovoltaic (OPV) devices appear to have nothing in common, they actually share many fundamental electronic processes involving light energy conversion and utilization, such as exciton generation/splitting and partition/ migration. While the separated holes (present as protons) and electrons across the membrane in natural photosynthetic systems establish chemical potentials for driving chemical reactions, the charges collected by two opposite electrodes in a photovoltaic device provide electricity for other devices. What is uniquely shown in natural photosynthetic systems is the high quantum yield of photon to electron/charge conversion efficiency largely facilitated by the fine-tuned local potential energy gradient of nearly identical chromophores imbedded in protein matrices, chlorophylls and their derivatives. The finetuned local environment for each chlorophyll ensures unidirectionality of electron transfer and efficient final charge separation across the membrane. In comparison, organic photovoltaic materials and devices have been struggling with power conversion efficiency (PCE) that suffers from the loss of charge recombination and trapping originating from intrinsic low dielectric constants, defects in organic materials, and lowmobility charge carriers compared to inorganic counterparts for photovoltaic applications. Such losses are very likely due to uncontrollable morphology of organic materials (donor− acceptor distance, orientation, electronic coupling, donor− acceptor domain size/shape/component, etc.), which makes it nearly impossible to build an energy gradient for unidirectional charge movements favoring photovoltaic functions as in natural photosynthetic systems. Therefore, it is a great challenge in the OPV field to achieve ideal local and long-range structure and morphology for optimized PV functions, and frequently it is unclear what that is for a specific OPV active layer. The challenge can be shown from data compiled from 150 OPV devices from the literature with PCE versus different device properties (such as optical energy gap, ΔEg,opt; energy offset of LUMO levels between the electron donor and acceptor materials, or perceived charge transfer driving force, ΔEcs; open circuit voltage, Voc; fill factor, FF; and short-circuit current, Jsc).1 To our surprise, ΔEg,opt, ΔEcs, and Voc have poor correlations with PCE, while Jsc and FF correlate reasonably well with PCE (Figure 1). Such results suggest that the bottleneck of the PCE in OPV materials is the extraction of carriers from the charge-separated states after exciton splitting, which manifest many energy loss mechanisms, such as geminate and diffuse charge recombination, trapped states, and energy dissipation as heat. The current record PCEs of OPV devices surpasses 13%,2 an impressive value, while large-area devices demand an even higher PCE to overcome the losses due to device assembly architectures. Compared to natural photo© 2017 American Chemical Society

Published: October 13, 2017 2516

DOI: 10.1021/acsenergylett.7b00885 ACS Energy Lett. 2017, 2, 2516−2517

Editorial

http://pubs.acs.org/journal/aelccp

ACS Energy Letters

Editorial

Figure 1. (a) OPV basic definitions: optical gap, Eg,opt; the transport gap, Eg,T; and the free-energy loss associated with exciton dissociation, ΔECS. (b) Device figures-of-merit including short-circuit current, JSC; open-circuit voltage, VOC; fill factor, FF; and the maximum efficiency, PCE. (c−g) PCE vs above-defined materials and device properties. Adapted from ref 1. Copyright 2015 American Chemical Society.

Lin X. Chen, Senior Editor, ACS Energy Letters

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Northwestern University, Evanston, Illinois 60208, United States

AUTHOR INFORMATION

ORCID

Lin X. Chen: 0000-0002-8450-6687 Notes

Views expressed in this editorial are those of the author and not necessarily the views of the ACS.

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REFERENCES

(1) Jackson, N. E.; Savoie, B. M.; Marks, T. J.; Chen, L. X.; Ratner, M. A. The Next Breakthrough for Organic Photovoltaics? J. Phys. Chem. Lett. 2015, 6, 77−84. (2) Heliatek Press Release: Heliatek sets new Organic Photovoltaic world record efficiency of 13.2%. 2016. (3) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide-based Perovskite Layers for Efficient Solar Cells. Science 2017, 356, 1376−1379.

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DOI: 10.1021/acsenergylett.7b00885 ACS Energy Lett. 2017, 2, 2516−2517