Regio-random (P1) and -regular (P2) difluorobenzene-naphthalene-containing polymer acceptors were developed for bulk-heterojunction all-polymer solar cells (all-PSCs). P2 exhibited significantly higher crystallinity in thin films, providing high spectral absorptivity and electron mobility than P1. When used in all-PSC devices, P2 afforded a respectably higher power conversion efficiency of over 5%.

AbstractA novel, star-shaped electron acceptor, DMTPA-PDI3, derived from a planar dimethylmethylene-bridged triphenylamine core with three acetylene-linked perylene diimide (PDI) units is developed as a nonfullerene acceptor for organic solar cells (OSCs). DMTPA-PDI3 manifests significantly reduced intramolecular twisting, enabling sufficient system-wide π-electron delocalization leading to broadened spectral absorption and raised lowest unoccupied molecular orbital level. As a result, higher and more balanced hole and electron transport properties are observed. Active layers for OSCs comprising DMTPA-PDI3 acceptor and PBT7-Th donor exhibit suppressed intermolecular aggregation, giving rise to uniform nanophase network formation. These OSC devices have afforded respectably high power-conversion efficiency of about 5%.

Fullerene-based bulk heterojunction organic solar cells (BHJ-OSCs) represent one of the current state-of-the-art organic solar cells. Nonetheless, most of these devices still suffer from adverse performance degradation due to thermally induced morphology changes of active layers. We herein demonstrate that the photovoltaic performance stability of BHJ-OSCs can be profoundly enhanced with an appositely functionalized 9-fluorenylidene malononitrile. The latter, through charge transfer (CT) interactions with a donor polymer, enables the formation of a “frozen” 3-dimensional mesh-like donor polymer matrix, which effectively restrains free movement of embedded fullerene molecules and suppresses their otherwise uncontrolled aggregation. 9-Fluorenylidene malononitrile derivatives with multiple CT interaction sites are particularly effective as preservation of a power conversion efficiency of over 90% under severe thermal stress has been accomplished. The generality of this novel strategy has been affirmed with several common donor polymers, manifesting it to be hitherto the most efficient approach to stabilized fullerene-based BHJ-OSCs.

•A new, structurally random perylene diimide copolymer acceptor, PPDI, has been readily synthesized.•PPDI possesses a greatly twisted conformation and exhibits weak intermolecular interactions.•The all-polymer solar cells based on PPDI/PTB7-Th active layer provide the PCE of 5.35%.A new, structurally random perylene diimide (PDI) copolymer acceptor, PPDI, has been readily synthesized from Stille cross-coupling polycondensation of two pairs of regioisomeric dibromo-PDIs with 2,5-bis(trimethylstannyl)thiophene. PPDI possesses a complex structural configuration with a greatly twisted conformation, and thus exhibits weak intermolecular interactions. A solution of PPDI and a polymer donor such as PBT7-Th affords a smooth homogenous film without large crystalline domain formation. This composite film absorbs strongly throughout the visible spectrum, and when used as an active layer in all-polymer solar cells, provides a power conversion efficiency of over 5%.Download high-res image (212KB)Download full-size image

Two novel small molecular non-fullerene acceptors, BF-PDI2 and BF-PDI4, respectively, carrying two and four peripheral perylenediimide moieties, are obtained from appositely functionalized 9,9′-bifluorenylidene and perylenediimide via Suzuki cross-coupling reaction. Owing to their twisted molecular structures, these acceptors possess good solution processability and exhibit relatively weak intermolecular interactions in both solution and the solid state as evidenced from their spectral characteristics. Both display an optical bandgap of 1.94 eV, and their LUMOs as estimated from electrochemical data are −3.83 and −3.92 eV for BF-PDI2 and BF-PDI4, respectively. Both can be utilized as a fullerene acceptor replacement in bulk heterojunction organic solar cells with a polymer donor, PBT7-Th, affording reasonable power conversion efficiencies of 2.46 and 4.43%, respectively.

•D-A copolymers containing thiophene-fused benzochalcogenodiazole are synthesized.•Optoelectronic properties are tunable by changing the chalcogen atoms.•The chemical structures and phase separation determine device performances.Appositely functionalized dithienobenzo-thiadiazole and dithienobenzo-oxadiazole-monomers were prepared and used in the synthesis of conjugated electron donor-acceptor (D-A) polymers. Detailed systematic investigations were carried out to study the effects of chalcogen atoms and three donor units on the optical and electrochemical properties as well as photovoltaic and field-effect transistor performance of the D-A polymers. All polymers displayed good thermal properties. Polymers containing benzooxadiazole moiety showed deeper LUMO levels as compared to their benzothiadiazole-containing analogues, whereas those derived from weak donor unit exhibited deeper HOMO levels than those with stronger donors. Photovoltaic power conversion efficiency of over 2% and hole field-effect mobility of 2.6 × 10−2 cm2V−1s−1 and on/off ratio of over 105 were obtained. These results demonstrate that dithienobenzo-chalcogenodiazole structures are potentially useful electron acceptor building blocks for the construction of D-A polymers for organic electronics applications.

•Two random donor-acceptor polymers with different donor moieties synthesized.•The polymer with a stronger donor moiety exhibited higher photovoltaic efficiency.•Decisive influence of donor moiety over the polymer performance demonstrated.Two random electron donor-acceptor (D-A) polymers comprised of two acceptor moieties, difluorobenzothiadiazole and isoindigo, and one donor moiety (P1 and P2) have been synthesized in good yields to study the effects of donor structures on organic thin-film transistor (OTFT) and bulk-heterojunction organic solar cell (BHJ-OSC) performance. Both polymers possess reasonably good solubility in common processing solvents for organic electronics. P1, which contains a benzodithiophene donor structure, shows poor charge transport ability in OTFTs and low solar power conversion efficiency (PCE) in BHJ-OSC devices with PC71BM acceptor. On the other hand, P2, with a dithienylthieno[3,2-b]thiophene donor moiety, exhibits distinctly higher hole field-effect mobility and significantly much better PCE under similar conditions. These results have demonstrated the decisive influence of donor moiety over the optoelectronic properties of D-A polymers.

•A dye based new building block, 2,1,3-benzothiadiazole-bridged bis-isoindigo.•A new regioregular conjugated polymer with a D–A2–A1–A2 structure.•A hole mobility of 0.02 cm2 (V s)−1 was achieved with the new material in ambient conditions.Two isoindigos were symmetrically connected by 2,1,3-benzothiadiazole core to form an acceptor2–acceptor1–acceptor2 monomer. The new monomer was used as electron acceptor unit in construction of a unique donor/acceptor polymer with donor–acceptor2–acceptor1–acceptor2 repeating units. The optical and electrochemical properties of the monomer and polymer were investigated. The optical band gap of monomer and polymer were about 1.86 and 1.65 eV, respectively. Both monomer and polymer showed relatively low-lying highest occupied molecular orbital energy levels and the corresponding values were −5.90 and −5.56 eV for monomer and polymer, respectively. The semiconducting properties of new materials were also studied. Organic field effect transistors based on solution processed thin films of the polymer displayed typical p-channel characteristics with a hole mobility of 0.02 cm2 (V s)−1 in ambient conditions.

Random and regioregular electron donor–acceptor (D–A) polymers comprising of 3,6-bis(3-octyltridecyl)thieno[3,2-b]thiophene donor and 5,6-difluorobenzo[c][1,2,5]thiadiazole acceptor moieties were synthesized to study the effects of both structural regioregularity and molecular weight on their performance in organic thin-film transistors (OTFTs) and bulk-heterojunction (BHJ) polymer solar cells (PSCs). All the polymers displayed similar molecular orbital energy levels regardless of their structures and molecular weights (MWs). The regioregular D–A polymers exhibited higher molecular orders than their random counterparts, and the degrees in molecular ordering improved with increased MW, leading to higher charge transport capability. However, undesirably high MW led to limited polymer solubility and thus fabrication difficulties resulting in poorer device performance. A reasonably high power conversion efficiency of about 7.6% was provided by the new regioregular D–A polymer semiconductor of mid-MW properties with excellent solution processability and molecular self-assembly efficiency.

The modification of silicon dioxide (SiO2) gate dielectrics with silane self-assembled monolayers (SAMs) via silylation was conducted to study their impacts on polymer field-effect transistor (FET) performance. SAMs formed from silylating agents with long alkyl chains such as octadecyl gave high field-effect mobility but a lower on/off ratio because of lower SAM coverage of the gate dielectric surface. In contrast, SAMs from silylating agents with phenyl or medium alkyl chains (octyl) provided a high on/off ratio from high SAM surface coverage but lower mobility due to their inefficiency in promoting molecular ordering of the channel semiconductor. By treating the SiO2 dielectric surface with two silylating agents, one with an octadecyl chain and one with an octyl or phenyl chain, in a proper sequence, a high-performance “hybrid” dual-silane SAM could be created, enabling attainment of both a high mobility and on/off ratio, together with other desirable FET properties.

5,6-Difluorobenzo[c][1,2,5]thiadiazole (FBT)-based conjugated donor–acceptor (D-A) polymers with straight and branched side chains were synthesized via Stille-coupling copolymerization to study their physical, optoelectronic and photovoltaic properties. The results show that both the nature of pendant side chains and the electron acceptor strength of the acceptor moiety of D–A polymers have critical impacts on material and photovoltaic properties. Better π–π stacking of polymer backbones enabled by appropriate substituents such as fluorine atoms and branched alkyl chains leads to a reasonably high power conversion efficiency of over 6% when the polymer is utilized as a donor material with PC71BM as an active layer in bulk heterojunction solar cells.

Regio-random (P1) and -regular (P2) difluorobenzene-naphthalene-containing polymer acceptors were developed for bulk-heterojunction all-polymer solar cells (all-PSCs). P2 exhibited significantly higher crystallinity in thin films, providing high spectral absorptivity and electron mobility than P1. When used in all-PSC devices, P2 afforded a respectably higher power conversion efficiency of over 5%.

The modification of silicon dioxide (SiO2) gate dielectrics with silane self-assembled monolayers (SAMs) via silylation was conducted to study their impacts on polymer field-effect transistor (FET) performance. SAMs formed from silylating agents with long alkyl chains such as octadecyl gave high field-effect mobility but a lower on/off ratio because of lower SAM coverage of the gate dielectric surface. In contrast, SAMs from silylating agents with phenyl or medium alkyl chains (octyl) provided a high on/off ratio from high SAM surface coverage but lower mobility due to their inefficiency in promoting molecular ordering of the channel semiconductor. By treating the SiO2 dielectric surface with two silylating agents, one with an octadecyl chain and one with an octyl or phenyl chain, in a proper sequence, a high-performance “hybrid” dual-silane SAM could be created, enabling attainment of both a high mobility and on/off ratio, together with other desirable FET properties.