No abstract

This research provides a structure-property relation that sheds light on morphological stability of NF-OSCs by using the thermodynamic and the kinetic perspectives. We show that NF-OSCs can suffer from excessive amorphousamorphous phase separation in the blends and crystallization of NF-SMA. The former instability channel can be eliminated in systems with an optimal miscibility, whereas the excessive phase separation in low miscibility systems and NF-SMA crystallization need to be suppressed through the utilization of polymers or NF-SMAs with low flexibility.

Organic solar cells that have all-polymer active layers may have several advantages compared with polymer−small molecule systems including improved mechanical and thermodynamic stability; however, an all-polymer active layer does not guarantee robust mechanical behavior. Here, we consider key parameters that may influence the mechanical behavior and power conversion efficiency of all-polymer solar cells (all-PSCs). Considerations include the thermal transition temperature of the polymers, the molecular weight (MW) of the polymers, and film morphology. The impact these features have on mechanical behavior is probed by measuring the cohesive fracture energy (G c ), crack onset strain, and elastic modulus. We find that the selection of ductile polymers with high MW enhances interchain interactions that improve the mechanical resilience of the films. High-MW polymers are also found to maximize the power conversion efficiency (PCE). Using this strategy, BHJ films with the best reported combination of G c (7.96 J m −2 ) and PCE (6.94%) are demonstrated. Finally, it is found that increasing the film thickness increases the fracture energy of the films but at the cost of PCE. These findings provide a fundamental perspective on the design strategy to achieve high performance and mechanically robust organic solar cells.

We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, morphological, and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, upshift of the energy levels, and enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (V OC ), short-circuit current density (J SC ), and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6 to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3 to 12.6% as the acceptor core size extends.

Organic solar cells (OSCs) are one of the most promising cost-effective technologies for utilizing solar energy with a short energy payback time and in semitransparent applications. [1-3] Bulk heterojunction OSCs comprising a donor and acceptor blend as the photoactive layer have achieved impressive improvements in power conversion efficiencies (PCEs) over the past 25 years. [4-7] Owing to the rapid development of high-performance non-fullerene small molecular acceptors (SMAs), PCE of over 15-17% has been achieved in various systems. [8-12] These promising results make non-fullerene OSCs competitive with other types of next-generation solar technology. Among start-of-the-art non-fullerene SMAs, the fused-ring electron acceptor named ITIC, comprising a indacenodithieno[3,2-b]-thiophene core and 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile end-groups, represents the most extensively studied SMA that achieved high performance with various donor polymers. [13-15] A family of high-performance ITIC derivatives was then designed and synthesized to modify the energy levels, light absorption, and molecular packing of the materials, as well as the morphology of the devices, which significantly boosted the performance of non-fullerene OSCs. [16-18] Generally, design rules for modifying energy levels and optical properties exist, but design rules for miscibility and stability are largely missing, [19-22] yet they are of vital importance to guarantee a long operational lifetime. While some conceptual understanding exists that stability must be related to the glass transition of the SMA and the polymer, [19,23-25] the relation of molecular design to the glass transition and crystallization, in general, is unknown and complex, due in part to the complex amphiphilic nature of the materials. One of the most widely used chemical modification approaches to modulate non-fullerene SMA characteristics is the introduction of electron-withdrawing halogen atoms, which has generated a number of high-performance acceptors. [26,27] Given that fluorine has the highest electronegativity, fluorination can effectively modify the highest occupied molecular orbital of organic semiconductors without introducing undesirable steric hindrance like other, more bulky electron-deficient groups. [28-30]