Shuguang Wen

Hyperconjugated side chained benzodithiophene and 4,7-di-2-thienyl-2,1,3- benzothiadiazole based polymer for solar cells

A novel donor–acceptor (D–A) copolymer (P3TBDTDTBT), including hyperconjugated side chained benzodithiophene as a donor and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) as an acceptor, was designed and synthesized. Due to the introduction of the hyperconjugated side chain, the resultant polymer exhibited good thermal stability with a high decomposition temperature of 437 °C, a low band-gap of 1.67 eV with an absorption onset of 742 nm in the solid film, and a deep highest occupied molecular orbital (HOMO) energy level of −5.26 eV. Finally, the polymer solar cell (PSC) device based on this polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) showed the best power conversion efficiency (PCE) of 3.57% with an open-circuit voltage (Voc) of 0.78 V, a short-circuit current density (Jsc) of 8.83 mA cm−2 and a fill factor (FF) of 53%.

High efficiency solution-processed two-dimensional small molecule organic solar cells obtained via low-temperature thermal annealing

A new two-dimensional (2D) organic small molecule, DCA3T(T-BDT), was designed and synthesized for solution-processed organic solar cells (OSCs). DCA3T(T-BDT) exhibited a deep HOMO energy level (−5.37 eV) and good thermal stability. The morphologies of the DCA3T(T-BDT):[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) blends were investigated by atomic force microscopy and the crystallinity was explored by X-ray diffraction (XRD) and 2D grazing incidence wide-angle X-ray scattering (GIWAXS), respectively. The morphologies of the blends were strongly influenced by the blend ratio of DCA3T(T-BDT):PC61BM and annealing temperature. The effect of thermal annealing on the photovoltaic performance of DCA3T(T-BDT)-based small molecule organic solar cells (SMOSCs) was studied in detail. When DCA3T(T-BDT) was used as a donor with PC61BM as an acceptor, high efficiency SMOSCs with a power conversion efficiency of 7.93%, a high Voc of 0.95 V, Jsc of 11.86 mA cm−2 and FF of 0.70 were obtained by a thermal annealing process at only 60 °C, which offers obvious advantages for large scale production compared with solvent additive or interfacial modification treatment.

Thienothiophene-based copolymers for high-performance solar cells, employing different orientations of the thiazole group as a π bridge

In this work, a thiazole moiety was employed as a π bridge incorporated into the backbone of quinoid polymers. The new strategy combined the characteristics of a thiazole unit with a deep HOMO energy level and a thieno[3,4-b]thiophene moiety (TT) with broad absorption. Two isomeric D–A copolymers, PTBTz-2 and PTBTz-5, were synthesized, with different orientations of the thiazole to the TT moiety. Interestingly, in comparison with PTBTz-5, PTBTz-2 exhibited an even lower HOMO energy level, a higher dipole moment, and a more planar molecular configuration, together with preferable phase domains and good intermixing with PC71BM. Thus, a superior PCE of 9.72% for the photovoltaic device was obtained, with a remarkable JSC of 16.84 mA cm−2, which is among the highest values for a single-junction solar cell. This is an increase of ∼40% in PCE in comparison with PTBTz-5 (PCE = 6.91%) and twice as much as for PBT-0F with thiophene as the π-bridge (PCE = 4.5%). This work not only provides a promising high-performance thiazole-containing system, but also reveals that the orientation of the asymmetric unit along the polymer backbone plays a crucial role and should be taken into account in future molecule design.

New small molecules with thiazolothiazole and benzothiadiazole acceptors for solution-processed organic solar cells

A new thiazolothiazole based small molecule (DTTz-DTBTT) has been designed and synthesized. The small molecule exhibited good thermal stability and excellent solubility. The optical gap of DTTz-DTBTT was estimated to be 1.65 eV. The solution-processed photovoltaic device based on DTTz-DTBTT and PC61BM exhibited a power conversion efficiency of 1.64%.

Design, synthesis and photovoltaic properties of two π-bridged cyclopentadithiophene-based polymers

Two fluorinated D–A type conjugated polymers, PCPDT-DTFBT (P1) and PCPDT-DTDFBT (P2) with an extended π-bridge, were synthesized through the palladium-catalyzed Stille coupling reaction. Both P1 and P2 exhibit a narrow band gap (1.63 eV for P1 and 1.60 eV for P2) and low lying energy level with the highest-occupied molecular orbital (HOMO) of −5.16 and −5.19 eV, respectively. Because of the insertion of the 4-hexylthiophene π-bridge between the donor and acceptor units, P1 and P2 exhibit excellent solubility in common organic solvents. Particularly for P2, the improved solubility was conducive to the film forming ability with a root-mean-square roughness (RMS) value of 3.60 nm and a nanoscale bicontinuous interpenetrating network in the active layer. As a result, a short-circuit current (JSC) of 13.58 mA cm−2, an open circuit voltage (VOC) of 0.70 V, and a fill factor (FF) of 61.6% were obtained, giving a high energy conversion efficiency (PCE) of 5.85% after device optimization.

Dithieno[3,2-b:2′,3′-d]silole-based low band gap polymers: the effect of fluorine and side chain substituents on photovoltaic performance

Three alkyl-thiophene π-bridged polymers, PDTS-hDTFBT (P-hF), PDTS-hDTDFBT (P-hDF) and PDTS-ehDTDFBT (P-ehDF), with different number of F atoms and side chain substituents are synthesized through a palladium catalyzed Stille coupling reaction. P-hF, P-hDF and P-ehDF show a narrow band gap of 1.56, 1.56 and 1.60 eV with deep lying highest-occupied molecular orbital (HOMO) energy levels of −5.17, −5.21 and −5.35 eV, respectively. The optimized P-hDF-based photovoltaic device exhibits an open circuit voltage of 0.593 V, a short-circuit current density of 15.98 mA cm−2, a fill factor of 64.8% and a high energy conversion efficiency of 6.14%, which is partially ascribed to the deep HOMO energy level and good coplanarity. The performance is among the highest reported ones in devices based on polymers with dithieno[3,2-b:2′,3′-d]silole (DTS) as the electron-rich unit and 2,1,3-benzothiadiazole (BT) derivatives as the electron-deficient unit.

Aromatic Heterocycle 1,3,4-Oxadiazole-Substituted Thieno[3,4-b]thiophene to Build Low-Bandgap Polymer for Photovoltaic Application

Electron-deficient heterocycle 1,3,4-oxadiazole is first introduced to the 2-position of thieno[3,4-b]thiophene (TT) to construct a new building block 2-(thieno[3,4-b]thiophen-2-yl)-5-(alkylthio)-1,3,4-oxadiazole (TTSO) with alkylthio chain. The polymer PBDT-TTSO based on TTSO and benzodithiophene (BDT) exhibits a deep lying highest occupied molecular orbital (HOMO) energy level of -5.32 eV and low-bandgap of 1.62 eV. The power conversion efficiency (PCE) of 5.86% is obtained with a relatively high V OC of 0.74 V, a J SC of 13.1 mA cm(-2), and FF of 60.5%. Furthermore, as S atom in thioether can be oxidized easily, the optoelectronic properties of PBDT-TTSO treated with different oxidants are preliminary investigated. Interestingly, the oxidation products still maintain high PCE with reduction less than 30%. This work demonstrates a new method to regulate HOMO energy levels by introducing electron-deficient aromatic heterocyclic moiety.

Benzothiadiazole – an excellent acceptor for indacenodithiophene based polymer solar cells

Two tetradodeoxyphenyl-substituted indacenodithiophene (IDT) based polymers, PIDT3T and PIDTDTBT, were achieved by copolymerizing IDT with terthiophene (3T) or di-2-thienyl-2′,1′,3′-benzothiadiazole (DTBT). Although these two polymers show significantly different UV-vis absorption spectra and band gaps (2.08 eV and 1.75 eV), the HOMO levels (−5.35 eV and −5.30 eV) of these polymers are almost the same. Polymer solar cells (PSCs) based on polymers with the benzothiadiazole (BT) unit show relatively high short-circuit current density (Jsc) due to the relatively wide and high photo-electronic response and high hole mobility. Thanks to the four long aryl side chains on IDT, the polymer thin film shows an amorphous nature, and the AFM root-mean-square roughness (RMS) value of the polymer/PCBM blend film is only around 0.3 nm which can contribute to the homogenous bulk heterojunction structures without significant phase separation. Finally, decent power conversion efficiency (PCE) of 4.52% is achieved by the benzothiadiazole based polymer and PC71BM composite. By comparison study, we demonstrate why BT is an excellent acceptor unit for indacenodithiophene-based PSCs.

A diketopyrrolopyrrole-based low bandgap polymer with enhanced photovoltaic performances through backbone twisting

A diketopyrrolopyrrole (DPP)-based π-conjugated polymer is a very promising low band gap electron donor material for polymer solar cells. We have incorporated alkyl groups into the 4-position of the thiophene rings connected to the DPP fragment, which is proven to be beneficial for improving the open-circuit voltage (VOC) and short-circuit current density (JSC). Two DPP-based polymers are synthesized with benzo[1,2-b:4,5-b′]dithiophene (BDT) as the electron-donating segment. Both the polymers show good solubility, slightly wide optical band gaps and deep HOMO energy levels when incorporating the alkyl groups. Heterojunction solar cells are fabricated with polymer:PC71BM as the active layer. VOC and JSC are simultaneously enhanced compared to the device performance of traditional DPP-BDT alternating polymers. A power conversion efficiency (PCE) of 8.11% was obtained, which indicates that rational utilization of backbone torsion is a promising strategy to improve the photovoltaic performance.

Low band-gap polymers based on easily synthesized thioester-substituted thieno(3,4-b) thiophene for polymer solar cells

A new acceptor S-alkyl thieno[3,4-b]thiophene-2-carbothioate-based acceptor (TTS) was firstly developed via easy of synthesis and applied in the construction of donor-acceptor (D-A) type conjugated polymers, by replacing the alkyl chain of ketone-substituted thieno[3,4-b] thiophene with an alkylthio side chain. Then, two new TTS-based polymers PBDTT-TTSO and PBDTT-TTSE were synthesized by Stille coupling reaction. The TTS acceptor moieties made the polymers exhibit a lower band gap (similar to 1.5 eV) and appropriate HOMO and LUMO energy levels relative to the fullerene acceptors, which could make the polymers perform well in photovoltaic devices. Solar cells were fabricated with the structure ITO/PEDOT: PSS/polymer: PC71BM/Ca/Al. The polymer PBDTT-TTSO device exhibits a power conversion efficiency (PCE) of 4.7% with an open circuit voltage (V-OC) of 0.68 V, a short circuit current density (J(SC)) of 12.7 mA cm(-2), and a fill factor (FF) of 54.6% while the PBDTT-TTSE device yields a higher PCE of 5.8% with a V-OC of 0.70 V, a J(SC) of 14.6 mA cm(-2), and a FF of 56.7%. The results indicate that thioester-substituted thieno[3,4-b]thiophene (TTS) is a promising building block for further design of high performance photovoltaic polymers.