The device efficiency of polymer:fullerene bulk heterojunction solar cells has recently surpassed 11%, as a result of synergistic efforts among chemists, physicists, and engineers. Since polymers are unequivocally the “heart” of this emerging technology, their design and synthesis have consistently played the key role in the device efficiency enhancement. In this article, the first focus is a discussion on molecular engineering (e.g., backbone, side chains, and substituents), then the discussion moves on to polymer engineering (e.g., molecular weight). Examples are primarily selected from the authors contributions; yet other significant discoveries/developments are also included to put the discussion in a broader context. Given that the synthesis, morphology, and device physics are inherently related in explaining the measured device output parameters (Jsc, Voc and FF), we will attempt to apply an integrated and comprehensive approach (synthesis, morphology, and device physics) to elucidate the fundamental, underlying principles that govern the device characteristics, in particular, in the context of disclosing structure-property correlations. Such correlations are crucial to the design and synthesis of next generation materials to further improve the device efficiency.

Herein, a successful application of V2O5·nH2O film as hole transporting layer (HTL) instead of PEDOT:PSS in polymer solar cells is demonstrated. The V2O5·nH2O layer was spin-coated from V2O5·nH2O sol made from melting-quenching sol–gel method by directly using vanadium oxide powder, which is readily accessible and cost-effective. V2O5·nH2O (n ≈ 1) HTL is found to have comparable work function and smooth surface to that of PEDOT:PSS. For the solar cell containing V2O5·nH2O HTL and the active layer of the blend of a novel polymer donor (PBDSe-DT2PyT) and the acceptor of PC71BM, the PCE was significantly improved to 5.87% with a 30% increase over 4.55% attained with PEDOT:PSS HTL. Incorporation of V2O5·nH2O as HTL in the polymer solar cell was found to enhance the crystallinity of the active layer, electron-blocking at the anode and the light-harvest in the wavelength range of 400–550 nm in the cell. V2O5·nH2O HTL improves the charge generation and collection and suppress the charge recombination within the PBDSe-DT2PyT:PC71BM solar cell, leading to a simultaneous enhancement in Voc, Jsc, and FF. The V2O5·nH2O HTL proposed in this work is envisioned to be of great potential to fabricate highly efficient PSCs with low-cost and massive production.Keywords: hole transporting layer; melting-quenching sol−gel method; PEDOT:PSS; polymer solar cell; vanadium(V) oxide hydrate

A novel angular shaped monomeric donor of 4,5-bis(2-ethylhexyloxy)benzo[2,1-b:3,4-b′]diselenophene (BDSe) was synthesized and exploited as the “donor” moiety to copolymerize with 4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazole (DTBT) for constructing the “donor–acceptor” (D–A) polymer PBDSe-DTBT. The selenium-substituted polymer displayed both a low optical band gap of 1.71 eV and a deep HOMO level of −5.4 eV as well as excellent solubility and thermal stability. With 1% DIO as the film processing additive, bulk heterojunction solar cell based on the PBDSe-DTBT:[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) blend provided a promising average power efficiency (PCE) of 5.6%, with an open circuit voltage (Voc) of 0.80 V, a short circuit current (Jsc) of 12.30 mA cm−2 and a fill factor (FF) of 0.57. Compared to devices processed without additives, 1% DIO additive processed BHJ devices showed enhanced absorption coefficient, better molecular packing, suppressed bimolecular recombination as well as a more balanced hole and electron transport, leading to significant improvement of Jsc and FF.

To effectively enhance the electrochemical energy storage performance of graphene-based materials, oxygen-rich in-plane pores were incorporated into graphene backbones through sonochemical etching of graphene oxide (GO) and sequential chemical reduction treatment. The etching of GO produced a large amount of in-plane nanoscale pores with a high content of oxygen atoms. The hydrazine reduction treatment of etched GO (EGO) resulted in a porous basal plane with good electrical conductivity and preserved electrochemically active oxygen atoms. It is found that good conductivity and in-plane nanoscale pores of reduced EGO (REGO) with enriched electrochemically active oxygen groups synergistically reinforce the electric double-layer capacitance (EDLC) and the pseudocapacitance performance over a wide power density range. Electrodes using REGO demonstrated a maximum energy density of 47 W h kg−1, which is almost three times that of reduced graphene oxide (RGO) without etching treatment, while simultaneously displaying a high power density of up to 100 kW kg−1 with considerable energy density.

A facile procedure for the synthesis of dithieno[5,6-b:11,12-b′]coronene-2,3,8,9-tetracarboxylic tetra(2-ethylhexyl)ester (DTCTTE-EH) from readily available perylene-3,4,9,10-tetracaroboxylic dianhydride is described. The electronic properties of DTCTTE-EH were elucidated on the basis of UV–vis spectra, emission spectrum and electrochemical measurement, which demonstrate that DTCTTE is a new class of components for promising semiconducting materials.Dithieno[5,6-b:11,12-b′]coronene-2,3,8,9-tetracarboxylic tetraester (DTCTTE), a novel class of semiconducting polycyclic aromatic molecules, was synthesized and characterized.

Three structurally related conjugated molecules (BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2) in star shape have been designed and synthesized as donor materials for small molecule based bulk heterojunction (BHJ) solar cells. The structural features of these molecules include a planarized benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) with a C3h symmetry as the central core and three conjugated arms incorporating electron deficient benzo[2,1,3]thiadiazole (BTD) units, with arms being linked to the core via different number of thiophene connecting units (e.g., 0, 1, 2 corresponding to BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2, respectively). Comparative analyses of optical and electronic properties indicate that the molecules bearing more thiophene units between the BTT core and the BTD arms possess higher-lying HOMO levels while their LUMO levels remain almost unchanged. The improvement of BHJ device performance, with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor, is observed with increasing number of thiophene units between the BTT core and BTD arms, from BTT-BTD-0 to BTT-BTD-1 and BTT-BTD-2. The BTT-BTD-2:PC61BM based BHJ devices show the highest power conversion efficiency (PCE) of 0.74%, with an open-circuit voltage (Voc) of 0.69 V, a short-circuit current density (Jsc) of 2.93 mA cm−2, and a fill factor (FF) of 0.37 under 1 sun (100 mW cm−2) AM 1.5G simulated solar illumination. The PV performance of BTT-BTD-2 is further improved when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is used as the electron acceptor, yielding the best device performance with Jsc of 4.13 mA cm−2, Voc of 0.72 V, FF at 0.46 and PCE of 1.36%. The effect of the different number of thiophenes linking the BTT core and the conjugated BTD arms has been clearly demonstrated on regulating optical and electrochemical properties of the three molecules and their BHJ device performances.

As strong electron acceptors, perylene diimides (PDI) have been widely utilized to prepare polymeric acceptors with an alternating donor–acceptor (D–A) structure for all-polymer solar cell applications, and which are used to provide low open circuit voltage (Voc). Cyclic voltammetry measurements reveal that both the HOMO and LUMO levels of a structurally related analogue to PDI, perylene tetracarboxylic tetraester (PTTE), are 0.3 eV higher than those of the corresponding PDI. It was envisioned that replacing PDI by the weaker electron acceptor of PTTE in making polymeric acceptors with D–A structure could help to raise the LUMO level of the resultant polymers, which would lead to a higher Voc when these PTTE based polymers are used as acceptors in all-polymer solar cell devices. In this work, a new conjugated alternating copolymer, poly([perylene tetracarboxylic tetra(2-hexyldecyl)ester-1,7-diyl]-alt-5,5′′-(2,2′:5′,2′′-terthiophene)) (PPTTE-TerT), was synthesized via Stille coupling reaction and characterized as an electron acceptor. The copolymer shows good solubility in common aliphatic organic solvents and good thermal stability. Differential scanning calorimetry (DSC) and grazing incident X-ray diffraction (GIXRD) measurements indicate that the copolymer at solid state is amorphous. PPTTE-TerT has a relatively broad absorption in the visible region from 400–650 nm with an optical band gap of 1.91 eV. The energy levels of HOMO and LUMO derived from the onset of the first oxidation and reduction potential of the cyclic voltammograms are at −5.6 and −3.54 eV, respectively. All-polymer solar cells of regioregular poly(3-hexylthiophene) (RR-P3HT) as the donor and PPTTE-TerT as the acceptor at an optimized donor–acceptor weight ratio of 1:0.7 achieved the best power conversion efficiency of 0.76%. The Voc is 0.83 V, the short-circuit current (Jsc) is 2.38 mA cm−2 and the fill factor (FF) is 0.34 under 1 sun (100 mW cm−2) AM1.5G solar illumination. The Voc value is about 0.3 V higher than that of the all-polymer solar cell based on blending RR-P3HT with the acceptor copolymer comprised of alternating PDI-terthiophene units, which is consistent with the fact that the LUMO level of PTTE is 0.3 eV higher than that of PDI. These results indicate that PTTE is a promising electron-accepting unit to construct polymeric acceptors to be used in all-polymer BHJ solar cells.

Four new [6,6]-phenyl-C61 and C71 butylsaure n-dibutyl amides (PCBDBA) with mono- and bis-adduction on C60 and C70 cages, respectively, have been synthesized as models to study the effect of the mono- and bis-adduction on fullerene cages on device performance when used as electron acceptors with the donor of regioregular P3HT in bulkheterojunction organic photovoltaics (BHJ-OPV). The optoelectronic, electrochemistry, and photovoltaic properties of these mono- and bis-products were fully investigated. The best device performance of these fullerene derivatives were obtained from the two monoadducts with power conversion efficiency (PCE) of 1.77% for C60 derivative and 1.90% for C70 derivative, respectively, which are close to PCBM’s 2.43%. The results revealed the structure–function relationship among the monoadduct and bisadduct derivatives of C60 and C70 with the BHJ-OPV performance.Keywords: C60; C70; fullerene; photovoltaic; solar cell;

Three structurally related conjugated molecules (BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2) in star shape have been designed and synthesized as donor materials for small molecule based bulk heterojunction (BHJ) solar cells. The structural features of these molecules include a planarized benzo[1,2-b:3,4-b′:5,6-b″]trithiophene (BTT) with a C3h symmetry as the central core and three conjugated arms incorporating electron deficient benzo[2,1,3]thiadiazole (BTD) units, with arms being linked to the core via different number of thiophene connecting units (e.g., 0, 1, 2 corresponding to BTT-BTD-0, BTT-BTD-1 and BTT-BTD-2, respectively). Comparative analyses of optical and electronic properties indicate that the molecules bearing more thiophene units between the BTT core and the BTD arms possess higher-lying HOMO levels while their LUMO levels remain almost unchanged. The improvement of BHJ device performance, with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as the acceptor, is observed with increasing number of thiophene units between the BTT core and BTD arms, from BTT-BTD-0 to BTT-BTD-1 and BTT-BTD-2. The BTT-BTD-2:PC61BM based BHJ devices show the highest power conversion efficiency (PCE) of 0.74%, with an open-circuit voltage (Voc) of 0.69 V, a short-circuit current density (Jsc) of 2.93 mA cm−2, and a fill factor (FF) of 0.37 under 1 sun (100 mW cm−2) AM 1.5G simulated solar illumination. The PV performance of BTT-BTD-2 is further improved when [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) is used as the electron acceptor, yielding the best device performance with Jsc of 4.13 mA cm−2, Voc of 0.72 V, FF at 0.46 and PCE of 1.36%. The effect of the different number of thiophenes linking the BTT core and the conjugated BTD arms has been clearly demonstrated on regulating optical and electrochemical properties of the three molecules and their BHJ device performances.

To effectively enhance the electrochemical energy storage performance of graphene-based materials, oxygen-rich in-plane pores were incorporated into graphene backbones through sonochemical etching of graphene oxide (GO) and sequential chemical reduction treatment. The etching of GO produced a large amount of in-plane nanoscale pores with a high content of oxygen atoms. The hydrazine reduction treatment of etched GO (EGO) resulted in a porous basal plane with good electrical conductivity and preserved electrochemically active oxygen atoms. It is found that good conductivity and in-plane nanoscale pores of reduced EGO (REGO) with enriched electrochemically active oxygen groups synergistically reinforce the electric double-layer capacitance (EDLC) and the pseudocapacitance performance over a wide power density range. Electrodes using REGO demonstrated a maximum energy density of 47 W h kg−1, which is almost three times that of reduced graphene oxide (RGO) without etching treatment, while simultaneously displaying a high power density of up to 100 kW kg−1 with considerable energy density.