Small reorganization energy acceptors enable low energy losses in non-fullerene organic solar cells.

Minimizing energy loss is of critical importance in the pursuit of attaining high-performance organic solar cells. Interestingly, reorganization energy plays a crucial role in photoelectric conversion processes. However, the understanding of the relationship between reorganization energy and energy losses has rarely been studied. Here, two acceptors, Qx-1 and Qx-2, were developed. The reorganization energies of these two acceptors during photoelectric conversion processes are substantially smaller than the conventional Y6 acceptor, which is beneficial for improving the exciton lifetime and diffusion length, promoting charge transport, and reducing the energy loss originating from exciton dissociation and non-radiative recombination. So, a high efficiency of 18.2% with high open circuit voltage above 0.93 V in the PM6:Qx-2 blend, accompanies a significantly reduced energy loss of 0.48 eV. This work underlines the importance of the reorganization energy in achieving small energy losses and paves a way to obtain high-performance organic solar cells.

The Crystallinity Control of Polymer Donor Materials for High-Performance Organic Solar Cells.

Bulk heterojunction (BHJ) organic solar cells (OSCs) can be regarded as one of the most promising energy generation technologies for large-scale applications. Despite their several well-known drawbacks, the devices where polymers are employed as the donor are still leading the OSC universe in terms of performance. Such performance generally depends upon various critical factors such as the crystallinity of the material, the crystallization process during the film formation, and also the final film morphology. Despite a few reviews on the structure of the polymer donor materials and device performance, not enough attention has been paid toward the crystallinity problem. Herein, the structure and crystallinity of the representative polymer donor materials and the corresponding device properties have been briefly reviewed. Furthermore, several typical methods for controlling the crystallinity of materials have been summarized and illustrated as well. Moreover, the obstacles lying in the way of successful commercialization of such polymer solar cells have been systematically discussed. The in-depth interpretation of the crystallinity of the polymer donors in this article may stimulate novel ideas in material design and device fabrication.

Synergistic Optimization Enables Large-Area Flexible Organic Solar Cells to Maintain over 98% PCE of the Small-Area Rigid Devices.

Slot-die coating is generally regarded as the most effective large-scale methodology for the fabrication of organic solar cells (OSCs). However, the corresponding device performance significantly lags behind spin-coated devices. Herein, the active layer morphology, flexible substrate properties, and the processing temperature are optimized synergistically to obtain high power conversion efficiency (PCE) for both the flexible single cells and the modules. As a result, the 1 cm2 flexible devices produce an excellent PCE of 12.16% as compared to 12.37% for the spin-coated small-area (0.04 cm2 ) rigid devices. Likewise, for modules with an area of 25 cm2 , an extraordinary PCE of 10.09% is observed. Hence, efficiency losses associated with the upscaling are significantly reduced by the synergistic optimization. Moreover, after 1000 bending cycles at a bending radius of 10 mm, the flexible devices still produce over 99% of their initial PCE, whereas after being stored for over 6000 h in a glove box, the PCE reaches 103% of its initial value, indicating excellent device flexibility as well as superior shelf stability. These results, thus, are a promising confirmation the great potential for upscaling of large-area OSCs in the near future.

All-small-molecule organic solar cells with over 14% efficiency by optimizing hierarchical morphologies.

The high efficiency all-small-molecule organic solar cells (OSCs) normally require optimized morphology in their bulk heterojunction active layers. Herein, a small-molecule donor is designed and synthesized, and single-crystal structural analyses reveal its explicit molecular planarity and compact intermolecular packing. A promising narrow bandgap small-molecule with absorption edge of more than 930 nm along with our home-designed small molecule is selected as electron acceptors. To the best of our knowledge, the binary all-small-molecule OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-molecule photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.

Large-Area Organic Solar Cells: Material Requirements, Modular Designs, and Printing Methods.

The printing of large-area organic solar cells (OSCs) has become a frontier for organic electronics and is also regarded as a critical step in their industrial applications. With the rapid progress in the field of OSCs, the highest power conversion efficiency (PCE) for small-area devices is approaching 15%, whereas the PCE for large-area devices has also surpassed 10% in a single cell with an area of ≈1 cm2 . Here, the progress of this fast developing area is reviewed, mainly focusing on: 1) material requirements (materials that are able to form efficient thick active layer films for large-area printing); 2) modular designs (effective designs that can suppress electrical, geometric, optical, and additional losses, leading to a reduction in the PCE of the devices, as a consequence of substrate area expansion); and 3) printing methods (various scalable fabrication techniques that are employed for large-area fabrication, including knife coating, slot-die coating, screen printing, inkjet printing, gravure printing, flexographic printing, pad printing, and brush coating). By combining thick-film material systems with efficient modular designs exhibiting low-efficiency losses and employing the right printing methods, the fabrication of large-area OSCs will be successfully realized in the near future.

Modulation of the Molecular Orientation at the Bulk Heterojunction Interface via Tuning the Small Molecular Donor-Nonfullerene Acceptor Interactions.

The interfacial molecular packing orientation of the nonfullerene systems at the donor-acceptor interface is considered as one of the key parameters in fabricating high-performance devices because of the anisotropic molecular characteristics of conjugated donors (D) and nonfullerene acceptors (A). However, regulating the interfacial orientation for the nonfullerene systems is still scarcely studied. Herein, modulation of the interfacial molecular packing orientation of bulk heterojunction layer is successfully realized via tuning the D-A interactions. The results indicate that the molecule with relatively shorter alkyl side chain (2F-C4C6) because of weak D-A interactions is unable to influence the molecular orientation of the active layer, as compared to their longer alkyl side-chain counterpart (2F-C6C8), which demonstrates strong D-A interactions and thus efficiently modulates the overall packing orientation. The power conversion efficiencies of 6.41 and 8.23% are obtained for the relatively short and long alkyl side-chain donors with IDIC acceptor, respectively. Hence strong D-A interactions because of long enough alkyl side chain on a donor small molecule can modify the interfacial molecular packing orientation of the system, leading to a better performing device.

Critical Role of Vertical Phase Separation in Small-Molecule Organic Solar Cells.

An inverted device structure is a more stable configuration than a regular device structure for solution-processed organic solar cells (OSCs). However, most of the solution-processed small-molecule OSCs (SM-OSCs) reported in the literature used the regular device structure, and a regular device normally exhibits a higher efficiency than an inverted device. Herein, a representative small-molecule DR3TBDTT was selected to figure out the reason for photovoltaic performance differences between regular and inverted devices. The mechanisms for a reduced open-circuit voltage ( Voc) and fill factor (FF) in the inverted device were studied. The reduced Voc and FF is due to the vertical phase separation with excess [6,6]-phenyl-C71-butyric acid methyl ester near the air/blend surface, which leads to a reduction in build-in voltage and unbalanced charge transport in the inverted device. Another reason for the reduced FF is the unfavorable DR3TBDTT crystallite orientation distribution along the film thickness, which is preferential edge-on crystallites in the top layer of the blend film and the increased population of face-on crystallites in the bottom layer of the blend film. This study illustrates that the morphology plays a key role in photovoltaic performance difference between regular and inverted devices and provides useful guidelines for further optimization of the morphology of solution-processed SM-OSCs.