Chuantao Gu

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.

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.

Efficiency enhancement in an indacenodithiophene and thieno[3,4-c]pyrrole-4,6-dione backbone photovoltaic polymer with an extended thieno[3,2-b]thiophene π-bridge

Two novel donor–π–acceptor (D–π–A) type polymers based on indacenodithiophene (IDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) with thiophene (PIDT-t-TPD) or thieno[3,2-b]thiophene (PIDT-tt-TPD) as π-bridges were designed and synthesized. The π-conjugated backbone length in the repeating unit of the polymer chain was varied by different π-bridges. Both polymers exhibit planar molecular structures, similar broad optical band gaps (Eoptg) and low-lying highest occupied molecular orbital (HOMO) energy levels. However, PIDT-tt-TPD with the more extended π-conjugated backbone shows higher hole mobility and better miscibility with (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM). Polymer solar cells (PSCs) based on PIDT-tt-TPD:PC71BM show the highest power conversion efficiency (PCE) of 5.23% with a short-circuit current density (Jsc) of 11.22 mA cm−2. Considering the limited absorption of PIDT-tt-TPD (400–650 nm), the Jsc value is very impressive. In contrast, PIDT-t-TPD: PC71BM only shows a PCE of 2.47% with a low Jsc of 5.52 mA cm−2. The lower Jsc value of PIDT-t-TPD is consistent with its limited external quantum efficiency (EQE) response (lower than 40%). The present results demonstrate that extending the length of a π-conjugated backbone by an appropriate π-bridge in the D–A polymer improves the photovoltaic performance of PSCs.

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.

Effect of two facile synthetic strategies with alterable polymerization sequence on the performance of N-vinyl carbazole-based conjugated porous materials

Four (N-vinyl carbazole)-based polymers were prepared through two facile synthetic strategies including both free radical polymerization and oxidative polymerization in different sequences. N-vinyl carbazole containing an ethylenic bond and a carbazole group is a superior candidate to study the effects of polymerization methods and sequences on the performance of conjugated microporous materials (CMPs). P2 was obtained with a BET of 878.46 m2 g−1, 12.8 times that of P1 (68.65 m2 g−1) in path 1, whereas in path 2, P3 (621.18 m2 g−1) and P4 (660.62 m2 g−1) were obtained without obvious difference in surface areas and pore structure. The dominant pore of linear polymer P1 centered at 3.94 nm, which is consistent with the polymers of intrinsic microporous (PIMs) analogues, and the counterparts P2 (ultramicropores, 0.54 nm), P3 (1.13 nm), and P4 (1.17 nm) are typical CMPs. P2 shows the best gas uptake abilities and absorbancies for organic solvents among the polymers. The results demonstrate that polymerization methods and sequences can have a great influence on the performance of materials, and only by careful choice of polymerization method and fine adjustment of the polymerization sequence can one obtain conjugated porous materials with optimum performance. Path 1, which is cost effective, high yielding and pollution free, should be regarded as the optimum selection process to prepare the N-vinyl carbazole-based porous materials. The reusability of P2 and P4 shows their impressive stability after 5 cycles of use or acid/base treatment. It is worth noting that P2 and P4 with a high porosity and pore volume are promising materials in carbon dioxide uptake, and in methane and hydrogen storage.

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.

Incorporating a vertical BDT unit in conjugated polymers for drastically improving the open-circuit voltage of polymer solar cells

In order to search for new wide band gap materials, vertical benzodithiophene (BDT) is designed as an electron donating unit to construct D–A type conjugated polymers. Two polymers PVB1 and PVB2 were synthesized by the Stille coupling reaction with benzothiadiazole (BT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) as the accepting moieties, respectively. These polymers show wide optical band gaps of over 2.0 eV and low HOMO energy levels of below −5.5 eV. Polymer solar cell (PSC) devices were fabricated using the above polymers as donors and fullerene derivatives as acceptors. PSCs based on PVB1 and PC71BM showed a power conversion efficiency (PCE) of 2.84% with a Voc of 1.00 V, a Jsc of 6.80 mA cm−2 and a FF of 0.42. And PSCs based on PVB2 and PC71BM showed a PCE of 2.05% with a Voc of 1.09 V, a Jsc of 5.33 mA cm−2 and a FF of 0.35. Both polymers showed a high Voc of over 1.0 V, which should be attributed to the deep-lying HOMO levels of the polymers. They would be potential candidates as wide band gap components to construct tandem solar cells.

Nanoscale phase separation control in rationally designed conjugated polymer solar cells processed using co-additives

A conjugated polymer based on benzo[1,2-b:4,5-b′]dithiophene with a thiophene-conjugated side chain and N-alkylthieno[3,4-c]pyrrole-4,6-dione was synthesized. When shortening the alkyl to linear hexyl on thiophene, the polymer (PBDT2T6–TPD) still has good solubility in common solvents and the hole mobility is improved. By introducing two additives (1,8-diiodooctane and 1,6-dibromohexane) as co-additives in chlorobenzene solution, a nanoscale phase separation with excellent bicontinuous interpenetrating network and balanced hole mobility and electron mobility was obtained in the blend film of PBDT2T6–TPD and [6,6]-phenyl C61 butyric acid methyl ester. Combined with an appropriate side chain of polymer and post-processing of the device, the solar cells exhibit a power conversion efficiency of 6.73% with a remarkable short circuit current density of 12.91 mA cm−2.

Design, synthesis and optical properties of small molecules based on dithieno[3,2-b:2′,3′-d]stannole and stannafluorene

In this paper, a series of stable Sn-containing heteroaromatic conjugated oligomers, dialkyl dithieno[3,2-b:2′,3′-d]stannole (DTSn) and stannafluorene (SnF) derivatives, were designed and synthesized employing a new synthetic route. The distinctive feature of this route is that terminal groups are linked to the backbone before the stannole cycle closed, which can avoid the cleavage of stannole in the Stille reaction. Three Sn-heteroaromatic small molecules, DTSn-1, SnF-1 and SnF-3, have been obtained and characterized. The density functional theory (DFT) calculations show that the alkyl groups are further displaced from the conjugated backbone due to the larger atomic radius of Sn, which allows a stronger π-stacking interaction to occur. Interestingly, the fluorescence intensity of DTSn-1 is increased in the presence of Al3+ due to interaction between Al3+ and Sn. However, the emissions of the three Sn-containing oligomers can be quenched by Ru3+, which renders these compounds potential candidates as fluorescence detectors for Ru3+.