No abstract

The highly efficient single-junction bulk-heterojunction (BHJ) PM6:Y6 system can achieve high open circuit voltages (V OC) while maintaining exceptional fill-factor (FF) and short-circuit current (J SC) values. With a low energetic offset, the blend system was found to exhibit radiative and nonradiative recombination losses that are among the lower reported values in the literature. Recombination and extraction dynamic studies revealed that the device shows moderate nongeminate recombination coupled with exceptional extraction throughout the relevant operating conditions. Several surface and bulk characterization techniques were employed to understand the phase separation, long-range ordering, as well as donor:acceptor (D:A) inter-and intramolecular interactions at an atomic-level resolution. This was achieved using photo-conductive atomic force microscopy (pc-AFM), grazing incidence wide angle x-ray scattering (GIWAXS), and solid-state 19 F Magic-Angle Spinning (MAS) NMR spectroscopy. The synergy of multifaceted characterization and device physics was used to uncover key insights, for the first time, on the structure-property relationships of this high performing BHJ blend. Detailed information about atomically resolved D:A interactions and packing revealed that the high performance of over 15% efficiency in this blend can be correlated to a beneficial morphology that allows high J SC and FF to be retained despite the low energetic offset.

Narrow bandgap n-type molecular semiconductors are relevant as key materials components for the fabrication near-infrared organic solar cells (OSCs) and organic photodetectors (OPDs). We thus designed nearly isostructural nonfullerene electron acceptors, except for the choice of solubilizing units, which absorb from 600 to 1100 nm. Specific molecules include CTIC-4F, CO1-4F, and COTIC-4F, whose optical bandgaps are 1.3, 1.2, and 1.1 eV, respectively. Modulation of intramolecular charge transfer characteristics was achieved by replacing alkoxy groups with alkyl groups on thiophene spacers that connect an electron-rich cyclopentadithiophene core to peripheral electron-poor fragments. OSCs incorporating CTIC-4F and CO1-4F with PTB7-Th achieve power conversion efficiencies of over 10% with short-circuit current densities as high as ∼25 mA·cm–2. The same blends achieve OPD responsivities of 0.52 A·W–1 at ∼920 nm. These findings highlight outstanding opportunities to tune further molecular design so that OPDs may ultimately compete with their silicon counterparts.

Some fundamental questions in the organic solar cell (OSC) community are related to the role of bulk and interfacial morphology on key processes such as charge generation, recombination, and extraction...

Figure 2. Chemical structures of the D polymers and small molecules, NFAs, and FAs discussed in this review (R 1 : -CH(C 4 H 9 )(C 2 H 5 ); R 2 , -C 6 H 13 ; R 3 , -C 8 H 17 ).