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To develop wide bandgap donor materials for high performance nonfullerene polymer solar cells (NF PSCs), bis(carboxylate) substituted benzo[1,2-b:4,5-b]dithiophene (BDT) weak electron acceptor (A) unit was used to polymerize with 5-alkylthiophene-2-yl-substituted BDT or 5-alkylthiothiophene-2-yl-substituted BDT donor unit to construct donor-acceptor (D-A) alternative polymers PBDT-BDTC and PBDTS-BDTC, respectively. PBDT-BDTC and PBDTS-BDTC exhibit wide bandgaps over 1.90 eV and low lying highest occupied molecular orbital (HOMO) levels below −5.50 eV. The absorption and energy levels of these two polymers match well with those of the typical low bandgap acceptor ITIC-Th, resulting in a good complementary absorption from 300 to 900 nm and a low HOMO level offset of about 0.15 eV. NF PSCs with an inverted device structure were fabricated and power conversion efficiencies (PCEs) higher than 7% were achieve. Furthermore, PBDT-BDTC based PSCs exhibit much better photovoltaic performance than PBDTS-BDTC based ones after the treatment with 0.5% 1,8-diiodooctane (DIO). The optimized devices gave a PCE of 8.32% with an open-circuit voltage (Voc) of 0.92 V, a short-circuit current density (Jsc) of 13.91 mA/cm2, and a fill factor (FF) of 65.21%, indicating that PBDT-BDTC is a promising polymer donor for NF PSCs. To the best of our knowledge, a PCE of 8.32% is the highest record for bis(carboxylate) substituted BDT based polymers.

To extend the absorption and enhance the electron mobility of fused-ring electron acceptors (FREAs), we designed and synthesized new FREAs (TTIC and TTIC-M) based on 4,9-dihydrothieno[3′,2':4,5]cyclopenta[1,2-b]thieno[2″,3'':3′,4'] cyclopenta[1′,2':4,5]thieno[2,3-d]thiophene (DTCTT) and 1,1-dicyanomethylene-3-indanone (IC). DTCTT unit possesses strong electron donating ability, which can broaden the absorption of the resulting acceptor molecule by enhanced internal charge transfer (ICT) effect. The S atoms on the fused ring structure can enhance the electron transport by intermolecular noncovalent S-S interaction. Furthermore, the introduction of a methyl group at the terminal IC unit can elevate the LUMO energy level of FREAs and afford higher open circuit voltage (Voc). As expected, TTIC and TTIC-M showed a broad absorption with the edge extending to the near-infrared (NIR) region (900 nm) and the optical band gaps of TTIC and TTIC-M is 1.40 and 1.44 eV, respectively. Devices based on PBDB-T:TTIC gave a PCE of 10.61% with an outstanding Jsc of 22.26 mA cm−2, a Voc of 0.77 V and an FF of 0.62. More noticeably, devices based on PBDB-T:TTIC-M provided a higher Voc of 0.80 V and a PCE of 11.48% with a Jsc of 20.51 mA cm−2 and an FF of 0.70. 11.48% is the best performance for polymer solar cells based on small band gap nonfullerene acceptors (

Vinylene ( V )- and ethynylene ( E )-bridged perylene diimide dimers ( PDI-V and PDI-E ) were designed, synthesized and used as nonfullerene acceptors for polymer solar cells. Our researches revealed that the linkage between two PDI units has a great impact on the molecular geometry, the optical properties, the blend film morphology, the molecular packing orientation, and the photovoltaic performance. Computational calculations via density functional theory (DFT) showed that PDI-E and PDI-V possessed planar and twisted geometric structures, respectively. TEM investigations showed that PTB7-Th : PDI-V based blend film exhibited a uniform morphology with small domain size and PTB7-Th : PDI-E based one showed apparent phase separation with large domain size. GIWAXS results revealed that the PDI-V can influence PTB7-Th to take on a face-on orientation, which is beneficial for vertical charge transport to increase J sc . A PCE of 4.51% with a V oc of 0.76 V, a J sc of 10.03 mA cm −2 , and an FF of 0.59 was obtained for PSCs based on PTB7-Th : PDI-V , which is almost two times higher than that of PTB7-Th : PDI-E based devices, which showed a PCE of 2.66%, a V oc of 0.66 V, a J sc of 7.33 mA cm −2 , and an FF of 0.55. These results help to gain deeper insight into the design of new nonfullerene small molecular acceptors for high efficiency PSCs.

A novel medium band gap copolymer ( PBO ) based on benzodithiophene (BDT) and benzoxadiazole (BOz) was synthesized and applied in polymer solar cells (PSCs). The introduction of two alkylthio side chains onto the BDT unit can endow the resulting polymer ( PBO ) with good solubility in solutions and good crystallinity in the solid state. The low HOMO level of −5.52 eV and the broad absorption ranging from 350 to 700 nm make PBO a promising donor material for PSCs. Power conversion efficiency higher than 7% has been achieved for devices using PBO as the donor material and PC 71 BM or ITIC as the acceptor material.

Two simple dithienosilole unit (DTS)-based nonfused small molecular acceptors (SiOC2C6–4F and SiOC2C6–4Cl) using two DTS moieties linked alkoxyl substituted benzene ring as the central core (D) and bis(5,6-difluro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)dimalononitrile or bis(5,6-dichloro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)dimalononitrile as the end groups have been synthesized. The optical, electrochemical properties and device photovoltaic performance of these two molecules were investigated in detail. DFT results indicates S⋯O interactions involving alkoxyl groups of benzyl rings and thienyl S atoms of DTS can act as conformational “locks” to enhance molecular planarity. when the polymer PBDB-T was chosen as the donor, the PBDB-T:SiOC2C6–4F-based device provided a high PCE of 9.68% with a Voc of 0.90 V, a Jsc of 17.02 mA/cm2 and an FF of 63.00%, while the PBDB-T:SiOC2C6–4Cl-based device exhibited a PCE of 11.29% with a Voc of 0.87V, a Jsc of 20.35 mA/cm2 and an FF of 63.73%, which is so far the best performance reported for organic photovoltaic using silole-based small-molecule acceptors.

A series of PDI-based small molecular acceptors (FPDI-I, FPDI-II, F2PDI-I and F2PDI-II) using 9H-fluorene as the central core and two PDI units or two PDI dimer units (PDI2) as the terminal groups have been synthesized. The optical, electrochemical and photovoltaic properties of these four acceptors have been systematically investigated. These acceptors exhibit twisted structures mainly due to the dihedral angles between the central fluorene core and the two terminal PDI units. The twisted molecular structures can effectively prevent the acceptor molecules from forming oversized aggregates. The two terminal fused PDI dimer units in F2PDI-I and F2PDI-II can enhance their molar absorption coefficients. Using PBDB-T as the donor polymer, F2PDI-I and F2PDI-II based devices exhibit higher and more balanced charge mobilities. As a result, organic solar cells (OSCs) based on PBDB-T:F2PDI-I give the best device performance with a power conversion efficiency (PCE) of 6.82%, a Voc of 0.90 V, a Jsc of 11.32 mA cm−2 and an FF of 66.65%.

A series of novel star-shaped small molecules with triphenylamine as the core, oligothiophene as the spacer and 1,8-naphthalimide as the electron withdrawing end-group were synthesized and used as donor materials for solution-processed organic solar cells. Direct arylation and Suzuki cross-coupling were used to synthesize the key intermediates and star-shaped molecules. The star-shaped molecules showed good thermal stability, intense absorption in the range of 300–700 nm, and deep HOMO energy level. A bulk heterojunction organic solar cell with one of the star shaped molecules in combination with PC71BM as the active layer gave a power conversion efficiency of 2.32% with a high open-circuit voltage up to 0.94 V. The influence of the spacer between the triphenylamine core and the peripheral 1,8-naphthalimide function group on the optical and photovoltaic properties was also investigated. Our studies indicated that 1,8-naphthalimide can be used as a promising building block for the synthesis of novel donor materials for organic solar cells.