Dangqiang Zhu

Simple planar perovskite solar cells with a dopant-free benzodithiophene conjugated polymer as hole transporting material

Dopant-free poly[(4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b;4,5-b′]dithiophene)-2,6-diyl-alt-(4-(2-ethylhexanoyl)-thieno[3,4-b] thiophene)-2,6-diyl] (PBDTTT-C) polymer is used as hole transporting material (HTM) in electron transporting material (ETM) free planar perovskite solar cells (PSCs). The devices with a PBDTTT-C HTM show higher power conversion efficiency (PCE = 9.95%) than the devices with a P3HT HTM (PCE = 6.17%) with enhanced short circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF) in a simple device configuration (ITO/CH3NH3PbI3/PBDTTT-C/MoO3/Ag), due to the suitable energy level, better carrier mobility and lower interfacial charge recombination.

Compact Layer Free Perovskite Solar Cells with a High-Mobility Hole-Transporting Layer.

A high-mobility diketopyrrolopyrrole-based copolymer (P) was employed in compact layer free CH3NH3PbI3 perovskite solar cells as a hole-transporting layer (HTL). By using the P-HTL, the 6.62% device efficiency with conventional poly-3-hexylthiophene was increased to 10.80% in the simple device configuration (ITO/CH3NH3PbI3/HTL/MoO3/Ag). With improved short circuit current density, open circuit voltage, and fill factor, the higher power conversion efficiency of P-HTL device is ascribed to the higher carrier mobility, more suitable energy level, and lower interfacial charge recombination. Advantages of applying P-HTL to perovskite solar cells, such as low cost, low-temperature processing, and excellent performance with simple cell structure, exhibit a possibility for commercial applications.

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%.

WO3 with surface oxygen vacancies as an anode buffer layer for high performance polymer solar cells

The exploration of inexpensive and efficient anode buffer layers is essential in large scale commercial applications of polymer solar cells (PSCs). Here, we report a simple way that can significantly enhance the power conversion efficiency (PCE) and extend the lifetime of PSCs. A solution-based tungsten oxide (WO3) layer with surface oxygen vacancies (VOs) is introduced as an efficient anode buffer layer between the active layer and indium tin oxide (ITO) glass. The PCEs of PSCs based on P3HT:PC61BM and PBDTTT–C:PC71BM active layers are improved by 24% (from 3.84% to 4.76%) and 27% (from 5.91% to 7.50%) with the introduction of the WO3 (VO) anode buffer layer, respectively, compared to that of the conventional PEDOT:PSS layer. The excellent performance is ascribed to the greatly improved fill factor and enhanced short circuit current density of the devices, which are benefited from the surface with lots of VOs for better interfacial contact and excellent charge transport properties of the WO3 (VO) layer. The impressive PCE, good stability, easy fabrication and compatibility with solution processed organic photovoltaic devices support this materials potential applications in PSCs for both wide bandgap and narrow bandgap polymers.

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.

High-performance inverted planar perovskite solar cells without a hole transport layer via a solution process under ambient conditions

High-quality CH3NH3PbI3 perovskite films were directly prepared on simple treated ITO glass in air under a relative humidity of lower than 30%. Due to efficient charge transport at the ITO (or PCBM)/CH3NH3PbI3 interfaces, the champion and average PCEs of 12.78% and 10.85% are obtained in the inverted perovskite solar cells without any hole transport layer, which provides a promising method for low-cost and easy processing industrialization of perovskite solar cells.

Improve the photovoltaic performance of new quinoxaline-based conjugated polymers from the view of conjugated length and steric hindrance

In this work, we have synthesized two new quinoxaline derivatives: 2,3-bis(n-octylthiomethyl)-5,8-dibromoquinoxaline (QS) and 2,3-bis[(5-octylthio)thiophen-2-yl]-5,8-dibromoquinoxaline (QTS); in addition, three new donor–acceptor (D–A) copolymers: poly{4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]-dithiophene-alt-2,3-bis(n-octylthiomethyl)quinoxaline} (PBDTQS), poly{4,8-bis(2-ethylhexyloxy)-benzo[1,2-b:4,5-b′]-dithiophene-alt-2,3-bis[(5-octylthio)thiophen-2-yl]quinoxaline} (PBDTQTS), and poly{2,3-bis[(5-octylthio)thiophen-2-yl]quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl} (PTQTS) were designed from the view of extending the length of conjugated side chain and reducing the steric hindrance of building blocks. Replacing the carbon atom in the side chain of polymer PBDTQS with a thiophene ring could increase the conjugation length and improve the absorption in the visible region of the copolymer PBDTQTS and PTQTS. Furthermore, polymer PTQTS exhibited a more planar backbone and increased intermolecular π-stacking compared to PBDTQTS, which was because the thiophene unit in PTQTS had smaller size and less steric hindrance than those of benzo[1,2-b:4,5-b′]-dithiophene unit in PBDTQTS. For the optimized polymer solar cell of PTQTS : PC61BM, PCE of 3.73% with Voc of 0.76 V, Jsc of 9.41 mA cm−2 and FF of 52.33% under an AM 1.5 G solar simulator with an intensity of 100 mW cm−2 was achieved, which was the best performance among the three copolymers. The results implied that the PTQTS with thiophene as the donor unit and QTS as the acceptor unit in the main chain would be a promising donor candidate in the application of polymer solar cells.

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.

High open-circuit voltage solution-processed organic solar cells based on a star-shaped small molecule end-capped with a new rhodanine derivative

A new star-shaped small molecule named TCNR3TTPA, with a triphenylamine (TPA) unit as the central building block and 2-(1,1-dicyanomethylene)-3-octyl rhodanine (CNR) as the end-capped group, has been designed and synthesized. TCNR3TTPA showed a deep highest occupied molecular orbital (HOMO) energy level (−5.60 eV) and broad absorption. The solution-processed bulk heterojunction (BHJ) solar cells based on TCNR3TTPA:PC61BM (1:1, w/w) exhibited a high open-circuit voltage (Voc) of 0.99 V, a short-circuit current density (Jsc) of 5.76 mA/cm2, and a power conversion efficiency (PCE) of 2.50% under the illumination of AM 1.5 G, 100 mW/cm2. The high Voc is ascribed to the strong electron-with- drawing ability of the end-capped 2-(1,1-dicyanomethylene)-3-octyl rhodanine group. These results demonstrated that the Voc of small-molecule organic solar cells could be increased by introducing a strong electron-withdrawing end-capped block, and that this is an effective strategy to design high-performance small molecules for organic solar cells.

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.