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Efficiencies of organic solar cells have practically doubled since the development of non‐fullerene acceptors (NFAs). However, generic chemical design rules for donor‐NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor–acceptor interface. It is shown that an acceptor–donor–acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ≈75 Debye Å balances the losses in the open circuit voltage and gains in charge generation efficiency. [ABSTRACT FROM AUTHOR]

Abstract: Reaching device efficiencies that can rival those of polymer‐fullerene Bulk Heterojunction (BHJ) solar cells (>10%) remains challenging with the “All‐Small‐Molecule” (All‐SM) approach, in part because of (i) the morphological limitations that prevail in the absence of polymer and (ii) the difficulty to raise and balance out carrier mobilities across the active layer. In this report, the authors show that blends of the SM donor DR3TBDTT (DR3) and the nonfullerene SM acceptor O‐IDTBR are conducive to “All‐SM” BHJ solar cells with high open‐circuit voltages (VOC) >1.1 V and PCEs as high as 6.4% (avg. 6.1%) when the active layers are subjected to a post‐processing solvent vapor‐annealing (SVA) step with dimethyl disulfide (DMDS). Combining electron energy loss spectroscopy (EELS) analyses and systematic carrier recombination examinations, the authors show that SVA treatments with DMDS play a determining role in improving charge transport and reducing non‐geminate recombination for the DR3:O‐IDTBR system. Correlating the experimental results and device simulations, it is found that substantially higher BHJ solar cell efficiencies of >12% can be achieved if the IQE and carrier mobilities of the active layer are increased to >85% and >10−4 cm2 V−1 s−1, respectively, while suppressing the recombination rate constant k to <10−12 cm3 s−1. [ABSTRACT FROM AUTHOR]

Abstract: Fullerenes and their derivatives are widely used as electron acceptors in bulk‐heterojunction organic solar cells as they combine high electron mobility with good solubility and miscibility with relevant semiconducting polymers. However, studies on the use of fullerenes as the sole photogeneration and charge‐carrier material are scarce. Here, a new type of solution‐processed small‐molecule solar cell based on the two most commonly used methanofullerenes, namely [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC60BM) and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC70BM), as the light absorbing materials, is reported. First, it is shown that both fullerene derivatives exhibit excellent ambipolar charge transport with balanced hole and electron mobilities. When the two derivatives are spin‐coated over the wide bandgap p‐type semiconductor copper (I) thiocyanate (CuSCN), cells with power conversion efficiency (PCE) of ≈1%, are obtained. Blending the CuSCN with PC70BM is shown to increase the performance further yielding cells with an open‐circuit voltage of ≈0.93 V and a PCE of 5.4%. Microstructural analysis reveals that the key to this success is the spontaneous formation of a unique mesostructured p–n‐like heterointerface between CuSCN and PC70BM. The findings pave the way to an exciting new class of single photoactive material based solar cells. [ABSTRACT FROM AUTHOR]

Abstract: Achieving efficient bulk‐heterojunction (BHJ) solar cells from blends of solution‐processable small‐molecule (SM) donors and acceptors is proved particularly challenging due to the complexity in obtaining a favorable donor–acceptor morphology. In this report, the BHJ device performance pattern of a set of analogous, well‐defined SM donors—DR3TBDTT (DR3), SMPV1, and BTR—used in conjunction with the SM acceptor IDTTBM is examined. Examinations show that the nonfullerene “All‐SM” BHJ solar cells made with DR3 and IDTTBM can achieve power conversion efficiencies (PCEs) of up to ≈4.5% (avg. 4.0%) when the solution‐processing additive 1,8‐diiodooctane (DIO, 0.8% v/v) is used in the blend solutions. The figures of merit of optimized DR3:IDTTBM solar cells contrast with those of “as‐cast” BHJ devices from which only modest PCEs <1% can be achieved. Combining electron energy loss spectrum analyses in scanning transmission electron microscopy mode, carrier transport measurements via “metal‐insulator‐semiconductor carrier extraction” methods, and systematic recombination examinations by light‐dependence and transient photocurrent analyses, it is shown that DIO plays a determining role—establishing a favorable lengthscale for the phase‐separated SM donor–acceptor network and, in turn, improving the balance in hole/electron mobilities and the carrier collection efficiencies overall. [ABSTRACT FROM AUTHOR]

The short exciton diffusion length associated with most classical organic semiconductors used in organic photovoltaics (5-20 nm) imposes severe limits on the maximum size of the donor and acceptor domains within the photoactive layer of the cell. Identifying materials that are able to transport excitons over longer distances can help advancing our understanding and lead to solar cells with higher efficiency. Here, we measure the exciton diffusion length in a wide range of nonfullerene acceptor molecules using two different experimental techniques based on photocurrent and ultrafast spectroscopy measurements. The acceptors exhibit balanced ambipolar charge transport and surprisingly long exciton diffusion lengths in the range of 20 to 47 nm. With the aid of quantum-chemical calculations, we are able to rationalize the exciton dynamics and draw basic chemical design rules, particularly on the importance of the end-group substituent on the crystal packing of nonfullerene acceptors. The short-range diffusion length of organic semiconductors severely limits exciton harvesting and charge generation in organic bulk heterojunction solar cells. Here, the authors report exciton diffusion length in the range of 20 to 47 nm for a wide range of non-fullerene acceptors molecules. [ABSTRACT FROM AUTHOR]

Ternary organic solar cells (OSCs) are among the best‐performing organic photovoltaic devices to date, largely due to the recent development of nonfullerene acceptors. However, fullerene molecules still play an important role in ternary OSC systems, since, for reasons not well understood, they often improve the device performance, despite their lack of absorption. Here, the photophysics of a prototypical ternary small‐molecule OSC blend composed of the donor DR3, the nonfullerene acceptor ICC6, and the fullerene derivative PC71BM is studied by ultrafast spectroscopy. Surprisingly, it is found that after excitation of PC71BM, ultrafast singlet energy transfer to ICC6 competes efficiently with charge transfer. Subsequently, singlets on ICC6 undergo hole transfer to DR3, resulting in free charge generation. Interestingly, PC71BM improves indirectly the electron mobility of the ternary blend, while electrons reside predominantly in ICC6 domains as indicated by fast spectroscopy. The improved mobility facilitates charge carrier extraction, in turn leading to higher device efficiencies of the ternary compared to binary solar cells. Using the (photo)physical parameters obtained from (transient) spectroscopy and charge transport measurements, the device's current–voltage characteristics are simulated and it is demonstrated that the parameters accurately reproduce the experimentally measured device performance. [ABSTRACT FROM AUTHOR]