•Three isopropylphenyl substituted asymmetric PDI derivatives were reported.•Effects of isopropyl chain position of PDIs on property and PV performance were studied.•These PDIs showed similar LUMO energy levels to PC61BM.•The meta-substituted 3-iPP-PDIs showed the best performance in polymer solar cells.Perylenediimide derivatives (PDI) are among the most promising non-fullerene electron acceptor materials for use in organic solar cells. However, owing to the intensive intermolecular interactions, the non-functionalized PDI molecules showed high tendency of aggregation in solid film, which leads to poor device performance. In this paper molecular geometry of PDI derivatives was finely tuned by introducing a bulky isopropyl group on the bay-phenyl unit, and influences of such a bulky alkyl group on the optical and electrochemical properties were systematically studied. Results indicated that the bulky isopropyl group on the para- and meta-position of the bay-phenyl group has negligible influence on the twist angle between the PDI core and the bay-phenyl unit, and these two compounds (4-iPP-PDI and 3-iPP-PDI) have similar molecular properties. However, large steric hindrance of the ortho-isopropyl group causes a large twist between the PDI core and the bay-phenyl unit, which leads to conjugation break, and consequently to a blue-shifted absorption spectrum and an increased optical band gap for the final PDI compound (2-iPP-PDI). Polymer solar cells using these bay-phenyl functionalized PDIs as the electron acceptor were fabricated and tested. And the meta-substituted PDI compound 3-iPP-PDI show the better device performance than the para- and ortho-substituted compounds (4-iPP-PDI and 2-iPP-PDI), which was ascribed to the proper nano-scale phase separation and high electron mobility of the blended film. The current results proved that molecular geometry of PDI derivatives can be finely regulated through introducing bulky alkyl side chain on the bay-substitution group to achieve a balanced property of crystallinity and electron mobility.

•Two branched oligothiophene cored PDI derivatives were synthesis.•These PDIs showed similar LUMO energy levels to PC61BM.•The bigger molecule 6T-PDI4 showed better device performance over 3T-PDI2.•Higher charge carrier mobility and less SRH recombination was found for 6T-PDI4.Two perylenediimide derivatives (3T-PDI2 and 6T-PDI4) based on dendritic terthiophene (3T) and sexithiophene (6T) cores were designed and synthesized. The rigid and dendritic oligothiophene core ensures a non-planar structure of these two compounds, which was confirmed by theoretical molecular structure simulation. The optical and electrochemical investigation results showed that both compounds have similar LUMO energy level of −4.1 eV, demonstrating that the LUMO orbital is mainly localized on the PDI moiety. Polymer solar cells using 3T-PDI2 and 6T-PDI4 as the electron acceptor, and PTB7-Th as the donor were fabricated and tested. A power conversion efficiency (PCE) of 4.12% was obtained for the 6T-PDI4 based device, which is higher than that of 3T-PDI2 based device (3.41%). Higher charge carrier mobility and less Shockley-Read-Hall recombination were also measured for the 6T-PDI4 based device. The current work demonstrates that dendritic PDI derivatives could serve as non-fullerene acceptor for use in polymer solar cells.Download high-res image (195KB)Download full-size image

•A polymerizable fullerene acrylate, PC61BA,, was designed and synthesized.•PC61BA has a similar molecular shape and size as PC61BM.•PC61BA displays similar basic optoelectronic properties as PC61BM.•PC61BA PSCs exhibited similar efficiency as PC61BM cells, but had better thermostability.The continuous microstructure evolution occurring in active layers of polymer-fullerene solar cells is one of the main causes for their device instability. With aim to tackle it, this work developed a new polymerizable fullerene acceptor, [6,6]-phenyl-C61-butyl acrylate (PC61BA). It was found that PC61BA has similar light-absorption properties and HOMO and LUMO energy levels as [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM), and can be converted into insoluble oligomers upon heating at 150 °C. Polymer-fullerene solar cells using poly(3-hexylthiophene) (P3HT) as donor and PC61BA as acceptor exhibited an optimized efficiency of 3.54%, the performance comparable to P3HT/PC61BM cells (optimized efficiency: 3.70%). But, the former possess much better thermal stability than the latter owing to aggregation suppression by the polymerized PC61BA. These results indicate that PC61BA, unlike most previous reported, is a unique polymerizable fullerene derivative that can be used alone as acceptor to achieve both efficient and thermally stable polymer solar cells.Download high-res image (93KB)Download full-size image

Two small molecules named PI-DPP and NI-DPP with a DPP core as the central strong acceptor unit and phthalimide/naphthalimide as the terminal weak acceptor were designed and synthesized. The effects of terminal phthalimide/naphthalimide units on the thermal behavior, optical and electrochemical properties, as well as the photovoltaic performance of these two materials were systematically studied. Cyclic voltammetry revealed that the lowest unoccupied molecular orbitals (LUMO) (~ -3.6 eV) of both molecules were intermediate to common electron donor (P3HT) and acceptor (PCBM). This indicated that PI-DPP and NI-DPP may uniquely serve as electron donor when blended with PCBM, and as electron acceptor when blended with P3HT, where sufficient driving forces between DPPs and PCBM, as well as between P3HT and DPPs should be created for exciton dissociation. Using as electron donor materials, PI-DPP and NI-DPP devices exhibited low power conversion efficiencies (PCEs) of 0.90% and 0.76% by blending with PCBM, respectively. And a preliminary evaluation of the potential of the NI-DPP as electron acceptor material was carried out using P3HT as a donor material, and P3HT:NI-DPP device showed a PCE of 0.6%, with an open circuit voltage (VOC) of 0.7 V, a short circuit current density (JSC) of 1.91 mA•cm-2, and a fill factor (FF) of 45%.

By varying the fluorine atom number and position, four novel fluorinated diphenyl-diketopyrrolopyrrole compounds, o-fDPP, o-ffDPP, m-fDPP, and m-ffDPP, were designed and synthesized. These key building blocks were then used to construct polymers with the benzo[1,2-b:4,5-b′]dithiophene (BDT) donor moiety. Using this new system a systematic study to compare the numbers and positions of fluorine decoration was performed to understand their effect on the electronic properties, morphology and solar cell device performance. In comparison with the parent, non-fluorinated polymer, meta-fluorinated polymers showed a red-shift in the absorption, whereas such a shift was minimal for the ortho-fluorinated polymers. On introducing fluorine atoms into diphenyl-diketopyrrolopyrrole only a very small influence on both the HOMO and LUMO energy levels was observed, which was verified by theoretical calculations. However, fluorination can influence the planarity of the backbone, by introducing F⋯S interactions. Grazing incidence X-ray diffraction (GIXD) showed that meta-fluorinated polymers had an enhanced crystal size, while ortho-fluorinated polymers were less crystalline, in comparison with the non-fluorinated polymer. Resonant soft X-ray scattering (RSoXS) showed that fluorinated polymer:PC71BM blends tended to form larger phase separated domains, leading to a smaller Jsc than that of the non-fluorinated analogue. The photovoltaic performance results were corroborated with the electrochemical and morphological characterization of these polymers. Consequently, the Pm-fDPP-based device exhibited the highest PCE of 1.40% among the fluorinated polymers. Although the Pm-fDPP-based device had slightly larger Voc and FF values than that of non-fluorinated PDPP, the overall device performance was lower than that of PDPP (1.7%) because of the smaller Jsc.

•Three p-alkylphenyl substituted asymmetric PDI derivatives were synthesized.•These PDIs were used as electron acceptor in solution processed organic solar cells.•Effects of p-alkyl chain length on property and PV performance of PDIs were studied.•PDI molecule with more intensive steric hindrance gave better device performance.In this report, three asymmetrical perylenediimide derivatives (PDI) substituted on the bay-position with para-alkylphenyl groups were synthesized, on which the substituted alkyl side chain was n-propyl (4-PP-PDI), n-hexyl (4-HP-PDI), or n-nonyl (4-NP-PDI) group. The effect of alkyl chain length on the optical and electrochemical properties, thermal behavior, as well as the photovoltaic performance of these materials in solution processed polymer solar cells were systematically studied. Results indicated that the para-alkyl side chain length showed negligible influence on the spectroscopy and redox behaviors of the materials, but significant influence on the photovoltaic performance. The propyl substituted compound 4-PP-PDI showed the best photovoltaic performance of VOC = 0.63 V, JSC = 1.93 mA cm−2, FF = 0.63 and a power conversion efficiency of 0.77%, which was attributed to the balanced intermolecular interaction of PDI molecules and the donor-acceptor phase separation of the blend films.

An ethynylene-linked benzo[1,2-b:4,5-b′]dithiophene-alt-diketopyrrolopyrrole alternating copolymer, EDPP, was designed and synthesized to improve the open-circuit voltage of organic solar cells. The influence of the ethynylene on optoelectronic properties, energy levels, crystallinity, film morphology, and photovoltaic performance was investigated. Optical and electrochemical tests showed that introduction of ethynylene into the polymer backbone resulted in a larger bandgap, deeper HOMO energy level, and enhanced crystallinity due to the planar conformation and electron-withdrawing properties. Grazing incidence wide-angle X-ray scattering (GIWAXS) showed that the pure EDPP film preferentially adopted a face-on orientation with a π–π stacking distance of 3.65 Å. After thermal annealing the face-on and edge-on orientations coexisted and the overall degree of crystallinity increased. Blending with PC71BM did not disrupt the crystallinity of the EDPP. Resonant soft X-ray scattering (RSoXS) showed that the EDPP:PC71BM blend films contained large domains, a few hundred nanometers in size. As a result, EDPP:PC71BM photovoltaic devices exhibited a high open-circuit voltage of 0.88 V, but a low short-circuit current, with a moderate power conversion efficiency of 1.98%.

We present a method for the modification of the structure of thieno[3,4-c]pyrrole-4,6-dione (TPD) to gain two novel acceptors TPN and TIN. And a series of donor–acceptor–donor-type oligomers 3TTPD3T, 3TTPN3T, and 3TTIN3T, based on TPD, TPN and TIN, have been designed and synthesized for solution-processed small-molecule bulk-heterojunction (BHJ) solar cells. The impact of these different central acceptor moieties on their optical, electrochemical properties, morphology, and solar cell performance were studied. Optical and electrochemical data showed that structural modification of TPD exhibited a slight impact on the band gap and HOMO/LUMO energy levels. Atomic force microscopy and grazing incidence wide-angle X-ray scattering studies revealed that the central acceptors of these oligomers played an important role in determine the morphology of solid-state packing. Oligomer 3TTPD3T showed weak crystallinity in a thin film. The larger conjugation plane counterpart 3TTPN3T showed an improved structure order. Surprisingly, the structurally twisted compound 3TTIN3T showed a strong crystalline nature with large crystalline domains of several hundred nanometers. The photovoltaic properties of these oligomers were evaluated by fabricating BHJ devices with PC71BM. 3TTPD3T/PC71BM blends with a weight ratio of 2:1 showed a best power conversion efficiency of 1.87%, with a Jsc of 4.93 mA cm−2, a Voc of 0.87 V, and a fill factor of 44%.

An ethynylene-linked benzo[1,2-b:4,5-b′]dithiophene-alt-diketopyrrolopyrrole alternating copolymer, EDPP, was designed and synthesized to improve the open-circuit voltage of organic solar cells. The influence of the ethynylene on optoelectronic properties, energy levels, crystallinity, film morphology, and photovoltaic performance was investigated. Optical and electrochemical tests showed that introduction of ethynylene into the polymer backbone resulted in a larger bandgap, deeper HOMO energy level, and enhanced crystallinity due to the planar conformation and electron-withdrawing properties. Grazing incidence wide-angle X-ray scattering (GIWAXS) showed that the pure EDPP film preferentially adopted a face-on orientation with a π–π stacking distance of 3.65 Å. After thermal annealing the face-on and edge-on orientations coexisted and the overall degree of crystallinity increased. Blending with PC71BM did not disrupt the crystallinity of the EDPP. Resonant soft X-ray scattering (RSoXS) showed that the EDPP:PC71BM blend films contained large domains, a few hundred nanometers in size. As a result, EDPP:PC71BM photovoltaic devices exhibited a high open-circuit voltage of 0.88 V, but a low short-circuit current, with a moderate power conversion efficiency of 1.98%.

We present a method for the modification of the structure of thieno[3,4-c]pyrrole-4,6-dione (TPD) to gain two novel acceptors TPN and TIN. And a series of donor–acceptor–donor-type oligomers 3TTPD3T, 3TTPN3T, and 3TTIN3T, based on TPD, TPN and TIN, have been designed and synthesized for solution-processed small-molecule bulk-heterojunction (BHJ) solar cells. The impact of these different central acceptor moieties on their optical, electrochemical properties, morphology, and solar cell performance were studied. Optical and electrochemical data showed that structural modification of TPD exhibited a slight impact on the band gap and HOMO/LUMO energy levels. Atomic force microscopy and grazing incidence wide-angle X-ray scattering studies revealed that the central acceptors of these oligomers played an important role in determine the morphology of solid-state packing. Oligomer 3TTPD3T showed weak crystallinity in a thin film. The larger conjugation plane counterpart 3TTPN3T showed an improved structure order. Surprisingly, the structurally twisted compound 3TTIN3T showed a strong crystalline nature with large crystalline domains of several hundred nanometers. The photovoltaic properties of these oligomers were evaluated by fabricating BHJ devices with PC71BM. 3TTPD3T/PC71BM blends with a weight ratio of 2:1 showed a best power conversion efficiency of 1.87%, with a Jsc of 4.93 mA cm−2, a Voc of 0.87 V, and a fill factor of 44%.