Xiaonan Xue

Ternary Organic Solar Cells Based on Two Compatible Nonfullerene Acceptors with Power Conversion Efficiency >10%

Two different nonfullerene acceptors and one copolymer are used to fabricate ternary organic solar cells (OSCs). The two acceptors show unique interactions that reduce crystallinity and form a homogeneous mixed phase in the blend film, leading to a high efficiency of ≈10.3%, the highest performance reported for nonfullerene ternary blends. This work provides a new approach to fabricate high-performance OSCs.

High-performance conjugated terpolymer-based organic bulk heterojunction solar cells

Recently, conjugated terpolymers comprising three components have attracted tremendous attention. However, quite a few examples of high-performance terpolymers have been reported. We present here two novel terpolymers named PtDDA and PtDAA, in which bithiophene (BT) and benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) were chosen as the donor and acceptor units, respectively. Thieno[3,2-b]thiophene (TT) and thiazolo[5,4-d]thiazole (TTz) were used as the third component. It is interesting to find that the PtDDA terpolymer shows a typical D1–D2–D1–A1 structure while PtDAA shows a D1–A1–D1–A2 structure. Without using additives or post-annealing processes, PtDAA-based solar cells show a high PCE of 8.1%, with an unprecedented fill factor (FF) of 0.74, which is much higher than those of PtDDA-based devices (PCE = 3.4%, FF = 0.55). The high efficiency of 8.1% is one of the highest values reported so far for organic solar cells based on conjugated terpolymers. The high performance is mainly ascribed to the efficient carrier transport in the PtDAA:PC71BM active layer, high crystallinity of PtDAA, and high domain purity. The results suggest that constructing conjugated terpolymers with one donor and two acceptor units is an effective strategy for designing high-performance solar cell materials.

Influence of aromatic heterocycle of conjugated side chains on photovoltaic performance of benzodithiophene-based wide-bandgap polymers

Extensive efforts have been focused on the study of wide-band gap (WBG) polymers due to their important applications in multiple junction and ternary blend organic solar cells. Herein, three WBG copolymers named PBDT(X)-T1 (X = O, S, Se) were synthesized based on the benzodithiophene (BDT) donor unit and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (T1) acceptor unit. Different aromatic heterocycle groups (furan, thiophene and selenophene) were introduced to modify the BDT unit to investigate the influence of conjugated side chains on the photovoltaic properties of conjugated polymers. Photophysical properties, electrochemistry, charge transport and crystalline properties of the polymers were studied to discuss the role of chalcogen atoms on the performance of conjugated polymers. Solar cells based on these three WBG copolymers were fabricated. Among them, the PBDT(Se)-T1-based solar cell shows the best photovoltaic performance with the highest power conversion efficiency (PCE) of 8.52%, an open-circuit voltage (Voc) of 0.91 V, and a high fill factor (FF) of 72%. The high crystallinity and preferential face-on orientation in the blend film partially explain the superior photovoltaic performance achieved in PBDT(Se)-T1-based solar cells. The results indicate the important role of chalcogen atoms in conjugated side chains and that high photovoltaic performance can be realized through side chain engineering of BDT-based WBG conjugated polymers.

Enhanced open-circuit voltage in methoxyl substituted benzodithiophene-based polymer solar cells

The open-circuit voltage (Voc) is one of the important parameters that influence the power conversion efficiency (PCE) of polymer solar cells. Its value is mainly determined by the energy level offset between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor. Therefore, decreasing the HOMO value of the polymer could lead to a high Voc and thus increasing the cell efficiency. Here we report a facile way to lower the polymer HOMO energy level by using methoxyl substituted-benzodithiophene (BDT) unit. The polymer with the methoxyl functionl group (POBDT(S)-T1) exhibited a HOMO value of–5.65 eV, which is deeper than that (–5.52 eV) of polymer without methoxyl unit (PBDT(S)-T1). As a result, POBDT(S)-T1-based solar cells show a high Voc of 0.98 V and PCE of 9.2%. In contrast, PBDT(S)-T1-based devices show a relatively lower Voc of 0.89 V and a moderate PCE of 7.4%. The results suggest that the involvement of methoxyl group into conjugated copolymers can efficiencly lower their HOMO energy levels.

High-performance wide-bandgap copolymers based on indacenodithiophene and indacenodithieno[3,2-b]thiophene units

In this work, we demonstrated the synthesis and characterization of two novel donor–acceptor type conjugated polymers, IDT-T1 and IDTT-T1, which consist of indacenodithiophene (IDT) and indacenodithieno[3,2-b]thiophene (IDTT) donor and 1,3-bis(5-bromothiophen-2-yl)-5,7-bis(2-ethylhexyl)-4H,8H-benzo[1,2-c:4,5-c]dithiophene-4,8-dione (T1) acceptor building blocks. The resulting copolymers exhibited a wide bandgap of about 1.90 eV with a comparatively low-lying highest occupied molecular orbital (HOMO) energy level. However, IDTT-T1 with an extended fused-ring unit was found to have better coplanarity and higher carrier mobility (0.11 cm2 V−1 s−1). As a consequence, organic solar cells employing IDTT-T1 as the donor material afforded a power conversion efficiency (PCE) of 6.58%, while IDT-T1-based devices yielded a relatively lower PCE of 6.26%.

A novel bifunctional A–D–A type small molecule for efficient organic solar cells

A novel A–D–A type small molecule named DTFBR was designed and synthesized, in which the fused rings of fluorene, benzothiadiazole and rhodamine were employed as the core donor, bridge acceptor and terminal acceptor unit respectively. DTFBR exhibited a broad absorption band covering the wavelength range from 350 nm to 700 nm and the HOMO and LUMO energy levels are −5.54 eV and −3.68 eV, respectively. Moreover, DTFBR showed bipolar carrier transport behavior, evidenced by the SCLC measurement, where the electron and hole mobilities are calculated to be 2.21 × 10−4 cm2 V−1 s−1 and 8.95 × 10−5 cm2 V−1 s−1, respectively. The appropriate energy levels and bipolar carrier transport properties make DTFBR a promising bifunctional photovoltaic material. It can function as the acceptor when it is paired with a poly(3-hexylthiophene) (P3HT) donor, yielding a power conversion efficiency (PCE) of 3.7%. When [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) was used as the acceptor, it can function well as a donor and the corresponding organic solar cells (OSCs) showed a PCE of 2.5% with a high open-circuit voltage (Voc) of ∼1.1 V. Our results shed new light on multifunctional photovoltaic material design and OSC application.