Although the field of polymer solar cell has seen much progress in device performance in the past few years, several limitations are holding back its further development. For instance, current high-efficiency (>9.0%) cells are restricted to material combinations that are based on limited donor polymers and only one specific fullerene acceptor. Here we report the achievement of high-performance (efficiencies up to 10.8%, fill factors up to 77%) thick-film polymer solar cells for multiple polymer:fullerene combinations via the formation of a near-ideal polymer:fullerene morphology that contains highly crystalline yet reasonably small polymer domains. This morphology is controlled by the temperature-dependent aggregation behaviour of the donor polymers and is insensitive to the choice of fullerenes. The uncovered aggregation and design rules yield three high-efficiency (>10%) donor polymers and will allow further synthetic advances and matching of both the polymer and fullerene materials, potentially leading to significantly improved performance and increased design flexibility.

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The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a solution-processed organic photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene derivative have been the most commonly used electron donor and electron acceptor, respectively. While most advances of the device performance come from the design of new polymer donors, fullerene derivatives have almost been exclusively used as electron acceptors in organic photovoltaics. Recently, nonfullerene acceptor materials, particularly small molecules and oligomers, have emerged as a promising alternative to replace fullerene derivatives. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chemical, thermal, and photostability. The facile functionalization of these molecules affords excellent tunability to their optoelectronic and electrochemical properties. Within the past five years, there have been over 100 nonfullerene acceptor molecules synthesized, and the power conversion efficiency of nonfullerene organic solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the molecular design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphology, and device physics are discussed.

We report a series of difluorobenzothiadizole (ffBT) and oligothiophene-based polymers with the oligothiophene unit being quaterthiophene (T4), terthiophene (T3), and bithiophene (T2). We demonstrate that a polymer based on ffBT and T3 with an asymmetric arrangement of alkyl chains enables the fabrication of 10.7% efficiency thick-film polymer solar cells (PSCs) without using any processing additives. By decreasing the number of thiophene rings per repeating unit and thus increasing the effective density of the ffBT unit in the polymer backbone, the HOMO and LUMO levels of the T3 polymers are significantly deeper than those of the T4 polymers, and the absorption onset of the T3 polymers is also slightly red-shifted. For the three T3 polymers obtained, the positions and size of the alkyl chains play a critical role in achieving the best PSC performances. The T3 polymer with a commonly known arrangement of alkyl chains (alkyl chains sitting on the first and third thiophenes in a mirror symmetric manner) yields poor morphology and PSC efficiencies. Surprisingly, a T3 polymer with an asymmetric arrangement of alkyl chains (which is later described as having an "asymmetric bi-repeating unit") enables the best-performing PSCs. Morphological studies show that the optimized ffBT-T3 polymer forms a polymer:fullerene morphology that differs significantly from that obtained with T4-based polymers. The morphological changes include a reduced domain size and a reduced extent of polymer crystallinity. The change from T4 to T3 comonomer units and the novel arrangement of alkyl chains in our study provide an important tool to tune the energy levels and morphological properties of donor polymers, which has an overall beneficial effect and leads to enhanced PSC performance.

Although it is known that molecular interactions govern morphology formation and purity of mixed domains of conjugated polymer donors and small-molecule acceptors, and thus largely control the achievable performance of organic solar cells, quantifying interaction-function relations has remained elusive. Here, we first determine the temperature-dependent effective amorphous-amorphous interaction parameter, χ(T), by mapping out the phase diagram of a model amorphous polymer:fullerene material system. We then establish a quantitative 'constant-kink-saturation' relation between χ and the fill factor in organic solar cells that is verified in detail in a model system and delineated across numerous high- and low-performing materials systems, including fullerene and non-fullerene acceptors. Our experimental and computational data reveal that a high fill factor is obtained only when χ is large enough to lead to strong phase separation. Our work outlines a basis for using various miscibility tests and future simulation methods that will significantly reduce or eliminate trial-and-error approaches to material synthesis and device fabrication of functional semiconducting blends and organic blends in general.

Non-fullerene organic solar cells with power conversion efficiencies of up to 6.3% are reported using properly matched donor and acceptor.

donor:acceptor ratio, molecular weight, and processing parameters (annealing/ coating temperature, choices of co-solvent or solvent additive, solvent annealing, etc.) come into play in determining the final morphology of a PSC and thus key performance parameters, such as open-circuit voltage (V oc ), short-circuit current density (J sc ), and fill factor (FF). For a new photovoltaic polymer or a new donor:acceptor combination, new solvents and other processing conditions are usually empirically explored and optimized within a limited range of variables. Furthermore, there is currently no widely recognized conceptual framework that allows determining the best-matched acceptor material for a given donor polymer and only by carefully considering the influences of each parameter individually can these factors be exploited. All of these variables make it extremely challenging to predict the optimal BHJ morphology and performance of PSCs when designing and using new materials. [2,7,8,[11][12][13][14] In the BHJ of a PSC device, the polymeric electron donor (D) and organic electron acceptor (A) materials form a complex structure and morphology with the goal of optimizing simultaneously photon absorption, exciton separation, and charge transport. Generally, the disorder and semi-crystalline (sometimes nearly amorphous) qualities of the organic/polymeric photovoltaic materials make their blends difficult to form a simple two-phase morphology with pure aggregated donor and acceptor phases. [15] A three-phase morphology (see Figure 1a) including pure donor aggregates, pure acceptor aggregates, and amorphous intermixed portions (consisting of dispersed fullerene in the mostly amorphous polymer) has been previously observed or inferred by us and others in a wide range of PSC material systems. [16][17][18][19][20] Recent kinetic Monte Carlo simulations [21] have suggested that this three-phase morphology may be favorable as the electronic structure of both the donor and acceptor depends on the level of aggregation, thus providing an electronic landscape that can help to sweep out charges created in the mixed phases. Molecular dynamics simulations of a matrix of polymer:acene blends pointed to the significance of thermodynamic molecular interactions in determining the formation of BHJ morphologies. [14] Practically, these molecular interactions determine an upper limit of purity of the mixed phases in the blend films, [22] which will likely impact charge creation and recombination processes in devices. [13,17,23,24] Polymer solar cells (PSCs) continue to be a promising low-cost and lead-free photovoltaic technology. Of critical importance to PSCs is understanding and manipulating the composition of the amorphous mixed phase, which is governed by the thermodynamic molecular interactions of the polymer donor and acceptor molecules and the kinetics of the casting process. This progress report clarifies and defines nomenclature relating to miscibility and its relevance and implications to PSC devices in light of new developments. U...