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Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss ( E g /q - V OC ) in polymer:non-fullerene acceptor (NFA) organic solar cells (OSCs). In this work, transient absorption spectroscopy is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. The CT state lifetime is observed to show an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing/hybridization between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset Δ E CT-S1 is reduced. This study highlights how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

Wavelength-selective harvesting by organic solar cells (OSCs) has attracted significant research attention due to the unique potential of these materials for smart photovoltaic window applications. Here, a visibly transparent OSC is demonstrated by utilizing both near-infrared (NIR)-absorbing polymer donor and nonfullerene acceptor (NFA) materials with narrow optical band gaps of less than 1.4 eV. Despite the substantial overlap in absorption spectra between the donor and acceptor, sufficient lowest unoccupied molecular orbital (LUMO) and highest occupied molecule orbital (HOMO) energy offsets for efficient charge separation with concurrent very low voltage losses yield a power conversion efficiency (PCE) of 9.13%. Moreover, with the introduction of an ultrathin Ag film (8 nm) as a transparent top electrode, semitransparent OSCs exhibit an excellent dual-side photovoltaic performance of 5.7 and 3.9% under bottom and top illumination, respectively, with high transmittance reaching 60% at wavelengths from 400 to 600 nm. This approach is expected to provide a new perspective in developing the highly efficient and transparent OSCs.

With the emergence of nonfullerene electron acceptors resulting in further breakthroughs in the performance of organic solar cells, there is now an urgent need to understand their degradation mechanisms in order to improve their intrinsic stability through better material design. In this study, we present quantitative evidence for a common root cause of light-induced degradation of polymer:nonfullerene and polymer:fullerene organic solar cells in air, namely, a fast photo-oxidation process of the photoactive materials mediated by the formation of superoxide radical ions, whose yield is found to be strongly controlled by the lowest unoccupied molecular orbital (LUMO) levels of the electron acceptors used. Our results elucidate the general relevance of this degradation mechanism to both polymer:fullerene and polymer:nonfullerene blends and highlight the necessity of designing electron acceptor materials with sufficient electron affinities to overcome this challenge, thereby paving the way toward achieving long-term solar cell stability with minimal device encapsulation., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

To investigate the influence of donor molecule crystallinity on photovoltaic performance in all-small-molecule solar cells, two dithieno[2,3- d:2',3'- d']-benzo[1,2- b:4,5- b']dithiophene (DTBDT)-based small molecules, denoted as DTBDT-Rho and DTBDT-S-Rho and incorporating different side chains, are synthesized and characterized. The photovoltaic properties of solar cells made of these DTBDT-based donor molecules are systemically studied with the [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) fullerene acceptor and the O-IDTBR nonfullerene acceptor to study the aggregation behavior and crystallinity of the donor molecules in both blends. Morphological analyses and a charge carrier dynamics study are carried out simultaneously to derive structure-property relationships and address the requirements of all-small-molecule solar cells. This study reveals exciton decay loss driven by large-scale phase separation of the DTBDT molecules to be a crucial factor limiting photocurrent generation in the all-small-molecule solar cells incorporating O-IDTBR. In the all-small-molecule blends, DTBDT domains with dimensions greater than 100 nm limit the exciton migration to the donor-acceptor interface, whereas blends with PC 71 BM exhibit homogeneous phase separation with smaller domains than in the O-IDTBR blends. The significant energy losses in nonfullerene-based devices lead to decreased J sc and fill factor values and unusual decrease in V oc values. These results indicate the modulation of phase separation to be important for improving the photovoltaic performances of all-small-molecule blends. In addition, the enhanced molecular aggregation of DTBDT-S-Rho with the alkylthio side chain leads to higher degrees of phase separation and unfavorable charge transfer, which are mainly responsible for the relatively low photocurrent when using DTBDT-S-Rho compared with that when using DTBDT-Rho. On the other hand, this enhanced molecular aggregation improves the crystallinity of DTBDT-S-Rho and results in its increased hole mobility.

A series of naphthalene diimide (NDI)-based small molecules were synthesized as nonfullerene acceptors and incorporated in all-small-molecule solar cells. Three NDI-based small molecules, NDICN-T, NDICN-BT, and NDICN-TVT, were designed with different linkers between two NDI units to induce the different conjugation length and modulate the geometric structures of the NDI dimers. The small NDI-based dimer electron acceptors with slip-stacked structures that facilitate π-π stacking interactions and/or hinder excessive aggregation exhibited different morphological behaviors, such as miscibility or crystallinity in bulk heterojunction blends with 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (DTS-F) electron donors. The photovoltaic devices prepared with NDICN-TVT gave the highest power conversion efficiency (PCE) of 3.01%, with an open-circuit voltage (V oc ) of 0.75 V, a short-circuit current density (J sc ) of 7.10 mA cm -2 , and a fill factor of 56.2%, whereas the DTS-F:NDICN-T and DTS-F:NDICN-BT devices provided PCEs of 1.81 and 0.13%, respectively. Studies of the charge-generation properties, charge-transfer dynamics, and charge-transport properties for understanding the structure-property relations revealed that DTS-F:NDICN-TVT blend films with well-developed domains and well-ordered crystalline structures performed well, whereas an excessive miscibility between DTS-F and NDICN-BT disrupted the crystallinity of the material and yielded a poor device performance.

We report six asymmetric alkylated anthracene-based molecules with different alkyl side chain lengths for use in organic field-effect transistors (OFETs). Alkyl side chains can potentially improve the solubility and processability of anthracene derivatives. The crystallinity and charge mobility of the anthracene derivatives may be improved by optimizing the side chain length. The highest field-effect mobility of the devices prepared here was 0.55 cm(2)/(V s), for 2-(p-pentylphenylethynyl)anthracene (PPEA). The moderate side chain length appeared to be optimal for promoting self-organization among asymmetric anthracene derivatives in OFETs, and was certainly better than the short or long alkyl side chain lengths, as confirmed by X-ray diffraction measurements.

Three types of amorphous thienothiophene (TT)-benzothiadiazole (BT) based copolymers (PFTTBT) were synthesized by incorporating alkyl-substituted fluorene moieties as a third component in the polymer backbone. Their optical, electrochemical, morphological, and photovoltaic properties were examined by a comparison with those of a crystalline TT-BT derivative (PTTBT14). PTTBT14 was reported to have a high hole mobility (0.26 cm(2)/(V s)) due to the pronounced interchain ordering but poor photovoltaic power conversion efficiency (PCE) of 2.4-2.6% was reported due to excessively strong self-interactions with poor miscibility with fullerene structures. By incorporating fluorene units, the UV-vis spectra showed an increased bandgap (∼1.9 eV) with the disappearance of the packing-originated shoulder peak, and the valence band decreased compared to crystalline PTTBT14. The amorphous PFTTBT polymers showed substantially improved photovoltaic properties compared to PTTBT14, even though they showed poor hole mobility (∼10(-6) cm2/(V s)) and fill factor. The optimal devices were achieved by blending with excess PC71BM (polymer:PC71BM=1:4 by weight), showing little improvement in the thermal and additive treatments. Under simulated solar illumination of AM 1.5 G, the best PCE of 6.6% was achieved for a PFehTTBT:PC71BM device with an open-circuit voltage of 0.92 V, a short-circuit current of 15.1 mA/cm2, and a fill factor of 0.48. These results suggest that it is useful to disrupt partially the interchain organizations of excessively crystalline polymers, enabling fine-control of intermolecular ordering and the morphological properties (i.e., miscibility with fullerene derivatives, etc.) to utilize the advantages of both crystalline and amorphous materials for further improving PCE of polymer solar cells.

The molecular packing structures of two conjugated polymers based on alkoxy naphthalene, one with cyano-substituents and one without, have been investigated to determine the effects of electron-withdrawing cyano-groups on the performance of bulk-heterojunction solar cells. The substituted cyano-groups facilitate the self-assembly of the polymer chains, and the cyano-substituted polymer:PC71BM blend exhibits enhanced exciton dissociation to PC71BM. Moreover, the electron-withdrawing cyano-groups lower the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the conjugated polymer, which leads to a higher open circuit voltage (V(OC)) and a lower energy loss during electron transfer from the donor to the acceptor. A bulk-heterojunction device fabricated with the cyano-substituted polymer:PC71BM blend has a higher V(OC) (0.89 V), a higher fill factor (FF) (51.4%), and a lower short circuit current (J(SC)) (7.4 mA/cm(2)) than that of the noncyano-substituted polymer:PC71BM blend under AM 1.5G illumination with an intensity of 100 mW cm(-2). Thus, the cyano-substitution of conjugated polymers may be an effective strategy for optimizing the domain size and crystallinity of the polymer:PC71BM blend, and for increasing V(OC) by tuning the HOMO and LUMO energy levels of the conjugated polymer.