Date Published: October 09, 2018
Publisher: John Wiley and Sons Inc.
Author(s): Jean Roncali, Ion Grosu.
Single material organic solar cells (SMOSCs) are based on ambivalent materials containing electron donor (D) and acceptor (A) units capable to ensure the basic functions of light absorption, exciton dissociation, and charge transport. Compared to bicomponent bulk heterojunctions, SMOSCs present several major advantages such as considerable simplification of cell fabrication and a strong stabilization of the morphology of the D/A interface, and thus of the cell lifetime. In addition to these technical issues, SMOSCs pose fundamental questions regarding the possible formation, and dissociation of excitons on the same molecular D–A architecture. SMOSCs are developed with various approaches, namely “double‐cable” polymers, block copolymers, oligomers, and molecules that differ by the donor platform: polymer or molecule, the nature of A, the D–A connection, and the intra‐ and intermolecular interactions of D and A. Although for several years the maximum efficiency of SMOSCs has remained limited to 1.0–1.5%, impressive progress has been recently accomplished leading to SMOSCs with 4.0–5.0% efficiency. Here, recent advances in the synthesis of D–A materials for SMOSCs are presented in the broader context of the chemistry of organic photovoltaic materials in order to discuss possible directions for future research.
Solar energy is expected to provide a major contribution to an increasing energy demand in a context of reduction of the carbon emission associated with the use of fossil energy sources. In this context, the photovoltaic (PV) conversion of solar light into electricity is an area of intense research activity. Although the industry of solar PV modules based on silicon solar cells is well established, research on alternative PV technologies has been pursued for several decades.1 In this regard, organic photovoltaic (OPV) cells are potentially attractive because of a unique combination of properties: lightweight, flexibility, plasticity, low environmental impact, and low cost. Research on OPV was initiated in the mid‐seventies, in the wake of earlier basic research on the photovoltaic properties of organic conjugated systems.2, 3, 4 These first OPV cells basedon dyes or pigments which had not been specifically designed for PV conversion were poorly efficient with power conversion efficiencies (PCEs) of ≈0.01–0.10%.5 The fabrication of the first organic heterojunction by contacting an electron donor material (D) with an electron acceptor (A) by Tang in 1986 represents a milestone in OPV research.6 Due to the high electric field generated at the D/A interface, excitons diffusing to the interfacial zone are efficiently dissociated, allowing for the first time, the PCE to reach values close to 1.0%.6 Today, planar heterojunction (PHJ) cells essentially fabricated by vacuum deposition are still investigated for both technological and basic research and remain an invaluable tool for the screening of new active materials.7, 8, 9, 10, 11
In the past few years, research on polymer‐based SMOSCs has been focused on two types of materials, namely polymers or copolymers with pendant acceptor groups and “in‐chain” systems in which the acceptor is directly inserted in the polymer backbone.
The advantages of molecular donors versus polymers as active OPV materials have been extensively discussed in particular regarding the reproducibility of synthesis and purification and the analysis of structure–properties relationships.30, 31, 32, 33, 34, 35, 36, 37, 38, 40, 96 In the specific case of SMOSCs, it seems clear that at such an early stage of research, molecular systems can significantly contribute to better understand the relevant structural factors and thus to progress toward the definition of synthetic principles for the design of active materials. A huge number of dyads combining fullerenes and molecular π‐conjugated systems have been synthesized.16, 17, 18, 19, 60, 61, 62, 63 In most cases, these compounds were synthesized as models of molecular heterojunctions for the analysis of the elemental mechanisms of photoinduced electron and/or energy transfer and only a few of them have been evaluated in SMOSCs. As for polymer‐based materials, molecular materials for SMOSCs can be also divided into “side‐chain” and “in‐chain” systems.
Recent advances in the synthesis of SMOCS materials and the PCE of 4.0–5.0% recently reported definitely demonstrate that SMOSCs are not doomed to low efficiency and that further progress can be expected in a near future. Furthermore, the stabilization of the morphology of the active layer, which remains one of the major problems of two‐component BHJs has been demonstrated in several works. For historical reasons, “double‐cable” polymers with fullerene acceptors have been the most widely investigated model of active material for SMOSC and the PCE of 5.58% recently reported for a cell based on P3HT and C60 equals the best results obtained with two‐component cells. Considerable effort has been invested in the synthesis of D–A block copolymers. These materials generally require complex syntheses and thorough purifications. However, except for a reported PCE of 3.0%, which remains an isolated result,84 the efficiency of SMOSCs based on these materials remain modest. On the other hand, the high PCE (≈4.0%) recently obtained with a fully conjugated D–A polymer based on advanced building blocks92 can be expected to stimulate further work in this direction.
The authors declare no conflict of interest.