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International audience; In this work, we demonstrate several organic amorphous donor−acceptor systems that exhibit sub-bandgap features over a more than 2 eV spectral range. An in-depth study of one of these systems, NPB:HAT-CN (NPB is N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine and HAT-CN is 1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile), reveals that the broad sub-bandgap features are attributed to multiple electronic charge transfer (CT) state transitions, broadened by energetic disorder sourcing from the fluctuations of intramolecular conformations and by the disordered intermolecular environment. These unique CT features originate from an unconventional donor and acceptor selection that reveals new insight about photocurrent generation and nonradiative recombination. Unlike materials employed in high performing organic solar cells, the materials studied here feature large optical energy gaps with very large frontier orbital energy level offsets, creating high bandgap devices with low open-circuit voltage. In addition to multiple electronic CT levels, we reveal that the internal quantum efficiency of these multiple CT transitions is not constant but photon energy dependent and with photoluminescence that originates primarily from the second lowest electronic CT state implying slow (relative to radiative and nonradiative rates) internal conversion within the CT manifold. Overall, this class of donor−acceptor pairs provides an opportunity to probe CT states in unique ways to potentially unravel their role in carrier generation−recombination and energy loss mechanisms. ■ INTRODUCTION With more than 15% power conversion efficiency recently achieved 1 and coupled with the ability to be grown on flexible substrates on a large scale with low-cost fabrication techniques, organic solar cells are making a strong case for energy conversion applications. Most advances in power conversion efficiency are the result of impressive material and interface engineering and innovation. However, due to the lack of a complete understanding and consensus of carrier generation and recombination processes as well as loss mechanisms at charge generating donor−acceptor (D-A) interfaces, 2 the actual limits of organic solar cell performance remain unknown. In this context, intermolecular charge transfer (CT) excitations at D−A interfaces play an important role in the photophysical processes and energy losses of organic solar cells. 3−6 In order to understand how carrier generation and recombination are affected by CT states, a better comprehension of their origin and energetic distribution is crucial. More specifically, some key issues that are currently under inquiry include whether the carrier dissociation efficiency Special Issue: Jean-Luc Bredas Festschrift

In contemporary organic solar cell (OSC) research, small A-D-A molecules comprising electron donor (D) and acceptor (A) units are increasingly used as a means to control the optoelectronic properties of photovoltaic blends. Slight structural variations to these A-D-A molecules can result in profound changes to the performance of the OSCs. Herein, we study two A-D-A molecules, BTCN-O and BTCN-M, which are identical in structure apart from a subtle difference in the position of alkyl chains, which force the molecules to adopt different equilibrium conformations. These steric effects cause the respective molecules to work better as an electron donor and acceptor when blended with benchmark acceptor and donor materials (PC71BM and PBDB-T). We study the photophysics of these “D:A” blends and devices using a combination of steady-state and time-resolved spectroscopic techniques. Time-resolved photoluminescence reveals the impact of the molecular conformation on the quenching of the A-D-A emission when BTCN-O and BTCN-M are blended with PBDB-T or PC71BM. Ultrafast broadband transient absorption spectroscopy demonstrates that the dynamics of charge separation are essentially identical when comparing BTCN-M and BTCN-O based blends, but the recombination dynamics are quite dissimilar. This suggests that the device performance is ultimately determined by the morphology of the blends imposed by the A-D-A conformation. This notion is supported by X-ray scattering data from the “D:A” films, and electroluminescence data and pump-push-photocurrent spectroscopy on the “D:A” devices. Our findings provide insight into the remarkable structure-function relationship in A-D-A molecules, and emphasize the need for careful morphological and energetic considerations when designing high-performance OSCs.

An organic solar cell based on a bulk heterojunction (BHJ) of a polymer and a methanofullerene ([60]PCBM or [70]PCBM) exhibits a complex morphology that controls both its photovoltaic and mechanical compliance (robustness, flexibility, and stretchability). Methanofullerenes are excellent electron acceptors; however, they have relatively high cost and production energy (in the purest samples) compared to other small-molecule semiconductors. Moreover, [60]PCBM and [70]PCBM-typical of van der Waals solids-can be stiff and brittle. Stiffness and brittleness may lower the yield of working modules in roll-to-roll manufacturing, shorten the lifetime against mechanical failure in outdoor conditions, and jeopardize wearable and portable applications that demand stretchability or extreme flexibility. This paper tests the hypothesis that technical grade PCBM (incompletely separated but otherwise pure blends containing >= 90% [60]PCBM or [70]PCBM) could lower the cost of manufacturing organic solar cells while simultaneously increasing their mechanical stability. Measurements of the tensile modulus of five methanofullerene samples, technical grades and 99% grades of both [60]PCBM and [70]PCBM, and a 1:1 mixture [60]PCBM and [70]PCBM, along with their blends with regioregular poly(3-hexylthiophene) (P3HT), lead to two important conclusions: (1) films of pure [70]PCBM are approximately five times more compliant than films of pure [60]PCBM; BHJ films with [70]PCBM are also more compliant than those with [60]PCBM. (2) BHJ films comprising technical grades of [60]PCBM and [70]PCBM are approximately two to four times more compliant than are films made using 99% grades. Tensile modulus is found to be an excellent predictor of brittleness: BHJs produced with technical grade methanofullerene accommodate strains 1.4-2.2 times greater than those produced with 99% grades. The smallest range of stretchability was found for BHJs with 99% [60]PCBM (fracture at 3.5% strain), while the greatest was found for technical grade [70]PCBM (11.5% strain). Mechanical properties are correlated to the microstructures of the blended films informed from analyses of UV-vis spectra using the weakly interacting H-aggregate model. Photovoltaic measurements show that solar cells made with technical grade [70]PCBM have similar efficiencies to those made with higher-grade material but with decreased cost and increased mechanical robustness.