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.

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 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.

A new donor (D)-acceptor (A) conjugate, benzodithiophene-rhodanine-[6,6]-phenyl-C 61 butyric acid methyl ester (BDTRh-PCBM) comprising three covalently linked blocks, one of p-type oligothiophene containing BDTRh moieties and two of n-type PCBM, is designed and synthesized. A single component organic solar cell (SCOSC) fabricated from BDTRh-PCBM exhibits the power conversion efficiency (PCE) of 2.44% and maximum external quantum efficiency of 46%, which are the highest among the reported efficiencies so far. The SCOSC device shows efficient charge transfer (CT, ≈300 fs) and smaller CT energy loss, resulting in the higher open-circuit voltage of 0.97 V, compared to the binary blend (BDTRh:PCBM). Because of the integration of the donor and acceptor in a single molecule, BDTRh-PCBM has a specific D-A arrangement with less energetic disorder and reorganization energy than blend systems. In addition, the SCOSC device shows excellent device and morphological stabilities, showing no degradation of PCE at 80 °C for 100 h. The SCOSC approach may suggest a great way to suppress the large phase segregation of donor and acceptor domains with better morphological stability compared to the blend device.

Power conversion efficiency up to 8.6% is achieved for a solution-processed tandem solar cell based on a diketopyrrolopyrrole-containing polymer as the low-bandgap material after using a thin polyelectrolyte layer to modify the electron-transport ZnO layers, indicating that interfacial engineering is a useful approach to further enhancing the efficiency of tandem organic solar cells.

Four different kinds of photovoltaic polymers were synthesized by controlling the intrachain noncovalent coulomb interactions through the incorporation of alkoxy-or alkylthio substituted phenylene, 4,7-di(furan-2-yl)benzothiadiazole and 4,7-di(thiophen-2-yl)benzothiadiazole as a building block. Fine-modulation of the interplay of dipole-dipole, H-bond and chalcogen-chalcogen interactions (O•••S, O•••H, S•••S, S•••F, etc.) along the polymeric backbone influenced the chain planarity, interchain organization, film morphology, electrical, and photovoltaic properties significantly. By replacing the alkoxy substituents with alkylthio groups, the torsional angle increased (136~168°) due to the absence of an O•••S attractive coulomb interaction (and/or increased S•••S steric hindrance), enhancing the amorphous nature with hindered interchain packing. The alkoxy-substituted polymers exhibited nanofibrillar structures, showing strong interlamellar scattering peaks up to (300) with tight face-on π-π stacking in grazing incidence X-ray scattering. The measured carrier mobility of the alkoxy-containing polymers was 1~2 orders of magnitude higher than that of the alkylthio-containing polymers. The incident-light-intensity-dependent photovoltaic characteristics clearly suggested efficient charge generation/extraction with less charge recombination for the alkoxy-containing semi-crystalline polymers. The resulting photovoltaic energy conversion efficiency of the PPDT2FBT, PPDF2FBT, PPsDF2FBT and PPsDT2FBT blended devices with PC70BM was measured to be 8.28, 5.63, 5.12, and 0.55%, respectively. This study suggests an important molecular design guideline for the further optimization of photovoltaic polymers and devices by fine-controlling the interplay of the weak noncovalent coulomb interactions.

We report eco- and human-friendly fabrication of organic field-effect transistors (OFETs) and polymer solar cells (PSCs) using only ethanol as a processing solvent at ambient condition, in stark contrast to that involving the use of halogenated and/or aromatic solvents. New ethanol-processable electroactive materials, p-type polymer (PPDT2FBT-A) and n-type bis-adduct fullerene acceptor (Bis-C60-A) are designed rationally by incorporation of oligoethylene glycol (OEG) side-chains. By ethanol processing, PPDT2FBT-A shows a broad light absorption in the range of 300–700 nm and highly crystalline interchain ordering with out-of-plane interlamellar scattering up to (400) with strong π–π stacking. As a result, the ethanol-processed PPDT2FBT-A OFETs yield high charge-carrier mobilities up to 1.0 × 10–2 cm2 V–1 s–1, which is the highest value reported to date from alcohol-processed devices. Importantly, the ethanol-processed PPDT2FBT-A OFET outperformed that processed using typical processing solvent, chlorobenzene (CB), with ∼10-fold enhancement in hole mobility, because the highly edge-on oriented packing of PPDT2FBT-A was produced by ethanol-process. Also, for the first time, significant photovoltaic performance was achieved for the ethanol-processed device of PPDT2FBT-A and Bis-C60-A due to the formation of an interpenetrating nanofibrillar morphology of highly crystalline PPDT2FBT-A polymers. The relationships between molecular structure, nanoscale morphology and electronic properties within ethanol-processed OFETs and PSCs were elucidated by comparing to typical CB-processed devices. These comparisons provide important guidelines for the design of new ethanol/water-soluble active layer materials and their use in the development of green solvent-processed efficient OFETs and PSCs.