New semi-crystalline photovoltaic polymers were synthesized and the optimized device exhibited 9.39% efficiency in a ∼300 nm thick single-cell device.

The molecular weight of a conjugated polymer is one of the key factors determining the electrical, morphological, and mechanical properties as well as its solubility in organic solvents and miscibility with other polymers. In this study, a series of semicrystalline poly[(2,5-bis(2-hexyldecyloxy)phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole)] (PPDT2FBT) polymers with different number-average molecular weights (M(n)'s) (PPDT2FBT(L), M(n) = 12 kg/mol; PPDT2FBT(M), M(n) = 24 kg/mol; PPDT2FBT(H), M(n) = 40 kg/mol) were synthesized, and their photovoltaic properties as electron donors for all-polymer solar cells (all-PSCs) with poly[[N,N'-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)] (P(NDI2OD-T2)) acceptor were studied. The M(n) effect of PPDT2FBT on the structural, morphological, electrical, and photovoltaic properties was systematically investigated. In particular, tuning the M(n) induced dramatic effects on the aggregation behaviors of the polymers and their bulk heterojunction morphology of all-PSCs, which was thoroughly examined by grazing incident X-ray scattering, resonant soft X-ray scattering, and other microscopy measurements. High M(n) PPDT2FBT(H) promoted a strong "face-on" geometry in the blend film, suppressed the formation of an excessively large crystalline domain, and facilitated molecularly intermixed phases with P(NDI2OD-T2). Therefore, the optimized all-PSCs based on PPDT2FBT(H)/P(NDI2OD-T2) showed substantially higher hole and electron mobilities than those of PPDT2FBT(L)/P(NDI2OD-T2), leading to a power conversion efficiency exceeding 5%, which is one of the highest values for all-PSCs reported thus far.

A highly efficient fullerene-free polymer solar cell (PSC) based on PDCBT, a polythiophene derivative substituted with alkoxycarbonyl, achieves an impressive power conversion efficiency of 10.16%, which is the best result in PSCs based on polythiophene derivatives to date. In comparison with a poly(3-hexylthiophene):ITIC-based device, the photovoltaic and morphological properties of the PDCBT:ITIC-based device are carefully investigated and interpreted.

Development of high-performance unipolar n-type organic semiconductors still remains as a great challenge. In this work, all-acceptor bithiophene imide-based ladder-type small molecules BTI n and semiladder-type homopolymers PBTI n ( n = 1-5) were synthesized, and their structure-property correlations were studied in depth. It was found that Pd-catalyzed Stille coupling is superior to Ni-mediated Yamamoto coupling to produce polymers with higher molecular weight and improved polymer quality, thus leading to greatly increased electron mobility (μ). Due to their all-acceptor backbone, these polymers all exhibit unipolar n-type transport in organic thin-film transistors, accompanied by low off-currents (10-10 A), large on/off current ratios (10), and small threshold voltages (∼15-25 V). The highest μ, up to 3.71 cm V s, is attained from PBTI1 with the shortest monomer unit. As the monomer size is extended, the μ drops by 2 orders to 0.014 cm V s for PBTI5. This monotonic decrease of μ was also observed in their homologous BTI n small molecules. This trend of mobility decrease is in good agreement with the evolvement of disordered phases within the film, as revealed by Raman spectroscopy and X-ray diffraction measurements. The extension of the ladder-type building blocks appears to have a large impact on the motion freedom of the building blocks and the polymer chains during film formation, thus negatively affecting film morphology and charge carrier mobility. The result indicates that synthesizing building blocks with more extended ladder-type backbone does not necessarily lead to improved mobilities. This study marks a significant advance in the performance of all-acceptor-type polymers as unipolar electron transporting materials and provides useful guidelines for further development of (semi)ladder-type molecular and polymeric semiconductors for applications in organic electronics.

Ternary organic solar cells are promising alternatives to the binary counterpart due to their potential in achieving high performance. Although a growing number of ternary organic solar cells are recently reported, less effort is devoted to morphology control. Here, ternary organic solar cells are fabricated using a wide-bandgap polymer PBT1-C as the donor, a crystalline fused-ring electron acceptor ITIC-2Cl, and an amorphous fullerene derivative indene-C bisadduct (ICBA) as the acceptor. It is found that ICBA can disturb π-π interactions of the crystalline ITIC-2Cl molecules in ternary blends and then help to form more uniform morphology. As a result, incorporation of 20% ICBA in the PBT1-C:ITIC-2Cl blend enables efficient charge dissociation, negligible bimolecular recombination, and balanced charge carrier mobilities. An impressive power conversion efficiency (PCE) of 13.4%, with a high fill factor (FF) of 76.8%, is eventually achieved, which represents one of the highest PCEs reported so far for organic solar cells. The results manifest that the adoption of amorphous fullerene acceptor is an effective approach to optimizing the ternary blend morphology and thereby increases the solar cell performance.

We report a series of benzotriazole-based semicrystalline π-conjugated polymers with noncovalent conformational locks for applications in polymer field-effect transistors. The benzotriazole moiety is a versatile electron-deficient building block that offers two chemically functionalizable sites, 2(N) and 5, 6(C) positions, allowing easy modulation of the solution processability and electronic structures of the resulting polymers. Fluorine or alkoxy substituents were introduced to the benzotriazole unit to enhance the molecular ordering through intra- and intermolecular F···S, F···H–C, C–F···πF, or S···O attractive interactions. The fluorinated polymer (PTBTz-F) showed remarkably enhanced hole mobility (μh = 1.9 cm2/(V·s), on/off ratio = 8 × 107) upon thermal annealing at 305 °C, compared to the unsubstituted one (PTBTz) (μh = 7.0 × 10–3 cm2/(V·s), on/off ratio = 3 × 106). Alkoxy unit substitution (PTBTz-OR) also improved the carrier mobility up to 0.019 cm2/(V·s) with an on/off ratio of 4 × 105. Fluorine or alkoxy substitution induced tight interchain ordering with edge-on orientation, as confirmed by X-ray diffraction measurements. In particular, fluorinated PTBTz-F showed high thermal stability (T d 453 °C) and the remarkable device characteristics with deep frontier orbital levels.

A new impedance-photocurrent device analysis technique is developed to elucidate the complex factors that dictate the fill factor in operating devices.

A well-organized donor-acceptor crystalline structure is examined for high -performance nonfullerene solar cells. By thermal annealing, nanoscale structures of both donor and acceptor domains are successfully modulated, followed by -significant changes in the resulting -photovoltaic characteristics. When annealed at 90 °C, a maximum power conversion efficiency of 7.64% with a -remarkable open-circuit voltage of 1.03 V is obtained.