Guangwu Li

Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

Triindole-cored star-shaped molecules for organic solar cells

Two new triindole-cored star-shaped molecules SM-1 and SM-2 have been designed and synthesized, and their optical, electrochemical, thermal, transport and photovoltaic properties have been investigated in detail. SM-1 and SM-2 exhibited good thermal stability, intensive absorption in a broad region, and relatively high hole mobility. Photovoltaic performances of these two molecules were investigated by fabricating bulk heterojunction solar cell devices with a blend film of SM-1:PC71BM or SM-2:PC71BM as the active layer. Organic solar cells (OSCs) based on SM-1:PC71BM and SM-2:PC71BM gave power conversion efficiencies (PCEs) of 2.05% and 2.29%, respectively. A PCE of 2.29% is the best result for all the reported triindole-based photovoltaic materials, indicating that triindole-based small molecules could become promising donor materials for solution-processed OSCs.

4-Alkyl-3,5-difluorophenyl-Substituted Benzodithiophene-Based Wide Band Gap Polymers for High-Efficiency Polymer Solar Cells.

Two novel polymers PTFBDT-BZS and PTFBDT-BZO with 4-alkyl-3,5-difluorophenyl substituted benzodithiophene as the donor unit, benzothiadiazole or benzooxadiazole as the acceptor unit, and thiophene as the spacer have been synthesized and used as donor materials for polymer solar cells (PSCs). These two polymers exhibited wide optical band gaps of about 1.8 eV. PSCs with the blend of PTFBDT-BZS:PC71BM (1:2, by weight) as the active layer fabricated without using any processing additive and any postannealing treatment showed power conversion efficiency (PCE) of 8.24% with an open circuit voltage (Voc) of 0.89 V, a short circuit current (Jsc) of 12.67 mA/cm(2), and a fill factor (FF) of 0.73 under AM 1.5G illumination, indicating that PTFBDT-BZS is a very promising donor polymer for PSCs. The blend of PTFBDT-BZO:PC71BM showed a lower PCE of 5.67% with a Voc of 0.96 V, a Jsc of 9.24 mA/cm(2), and an FF of 0.64. One reason for the lower PCE is probably due to that PTFBDT-BZO has a smaller LUMO offset with PC71BM, which cannot provide enough driving force for charge separation. And another reason is probably due to that PTFBDT-BZO has a lower hole mobility in comparison with PTFBDT-BZS.

1,8-Naphthalimide-Based Planar Small Molecular Acceptor for Organic Solar Cells

Four small molecular acceptors (SM1-4) comprising a central benzene core, two thiophene bridges and two 1,8-naphthalimide (NI) terminal groups were designed and synthesized by direct C-H activation. SM1 has a planar chemical structure and forms H-aggregation as films. By attachment of different substituents on the central benzene ring, the dihedral angles between the two NI end groups of SM1-4 gradually increased, leading to a gradual decrease of planarity. SM1-4 all possess a high-lying LUMO level, matching with wide band gap (WBG) polymer donors which usually have a high-lying LUMO level. When used in OSCs, devices based on SM1 and WBG donor PCDTBT-C12 gave higher electron mobility, superior film morphology and better photovoltaic performance. After optimization, a PCE of 2.78% with a V(oc) of 1.04 V was achieved for SM1 based devices, which is among the highest PCEs with a V(oc) higher than 1 V. Our results have demonstrated that NI based planar small molecules are potential acceptors for WBG polymer based OSCs.

A nonfullerene acceptor for wide band gap polymer based organic solar cells

A new 1,8-naphthalimide based planar small molecular acceptor and two benzothiadiazole based wide band gap (WBG) polymer donors P1 and P2 were synthesized for nonfullerene organic photovoltaic cells (OPVs). Devices based on fluorinated polymer P2 achieved a highly improved PCE of 3.71% with an open circuit voltage (V(oc)) of 1.07 V, which is beyond the currently known levels for nonfullerene OPVs with the V(oc) higher than 1 V.

A 1,8-naphthalimide based small molecular acceptor for polymer solar cells with high open circuit voltage

A novel small molecule NI-T-NI with a thiophene core and two 1,8-naphthalimide terminal groups was synthesized via direct C–H activation and used as the acceptor for polymer solar cells. NI-T-NI exhibits a good crystallinity and can form H-aggregates in the solid state. NI-T-NI has a rather high-lying LUMO level, which is beneficial for achieving a high Voc. In cooperation with a high-lying LUMO level polymer PCDTBT-C12, a PCE of 2.01% with a high Voc of 1.30 V has been achieved. As far as we know, a Voc of 1.30 V is the highest value reported for single junction organic solar cells. Our results have demonstrated that 1,8-naphthalimide could be a useful building block for the synthesis of promising acceptor materials for polymer solar cells.

An effective way to reduce energy loss and enhance open-circuit voltage in polymer solar cells based on a diketopyrrolopyrrole polymer containing three regular alternating units

A novel diketopyrrolopyrrole (DPP)-based conjugated polymer (PCDPP) was designed, synthesized and used as a donor material for polymer solar cells (PSCs). By increasing the planarity of polymer chains and reducing the energy loss in devices, we have simultaneously acquired a high short-circuit current (Jsc) and a large open-circuit voltage (Voc) in PSCs based on PCDPP, which is a regular alternating ternary conjugated polymer. This polymer has a medium optical band gap (1.55 eV) with low-lying HOMO and LUMO energy levels. In addition, PCDPP exhibits a very good planarity from density functional theory (DFT) calculations and forms a fibrillar network in the active layer of solar cells. Because of these integrated favourable effects, PCDPP-based photovoltaic devices exhibit a high power conversion efficiency (PCE) of 9.02% which is among the highest values reported so far for devices based on DPP-containing polymers. More importantly, the Voc of our PCDPP-based devices can reach as high as 0.86 V, which is much higher than that (<0.7 V) of high-efficiency solar cells based on other DPP polymers. These results provide a promising way to minimize the energy loss and to realize high Voc and Jsc values at the same time in devices to obtain high power conversion efficiencies.

Side Chain Influence on the Morphology and Photovoltaic Performance of 5-Fluoro-6-alkyloxybenzothiadiazole and Benzodithiophene Based Conjugated Polymers

Three conjugated polymers (P1-P3) with benzodithiophene derivatives as the donor unit, 5-fluoro-6-(2-hexyldecyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5] thiadiazole as the acceptor unit and thiophene as the spacer were designed, synthesized, and used as donor materials for polymer solar cells (PSCs). The influence of side chains at the benzodithiophene unit on the performance of PSCs was investigated. PSCs with the blend of P2:PC71BM (1:2, by weight) as the active layer show the highest power conversion efficiency (PCE) of 6.88%, with an open circuit voltage (Voc) of 0.76 V, a short circuit current (Jsc) of 14.67 mA/cm(2), and a fill factor (FF) of 0.62. Our research revealed that the variation of side chains had a great influence on the morphology of blend films, which is crucial to the performance of PSCs. As indicated by transmission electron microscopy, the blends of P1:PC71BM (1:2) and P2:PC71BM (1:2) formed nanofibers, whereas the blends of P3:PC71BM (1:2) formed spherical domains. Therefore, we concluded that formation of a more interpenetrating phase-separated donor-acceptor network with a larger interfacial area and proper percolation in the blends from P1 to P2 is mainly responsible for better performance in the corresponding devices.

1,8-Naphthalimide-based nonfullerene acceptors for wide optical band gap polymer solar cells with an ultrathin active layer thickness of 35 nm

Three novel 1,8-naphthalimide-based small molecular acceptors (NI-A-C4, NI-A-C6 and NI-A-C8) were designed and synthesized. The LUMO levels of these three small molecules were high-lying, which significantly reduce the energy loss between the wide optical band gap polymer (WBGP) PBDTBT-C12 and the acceptors and result in a high open circuit voltage (Voc) in solar cells. In addition, these three acceptors are planar, crystalline and H-aggregated in the solid state, which can facilitate the electron transport in blend films and lead to superior electron mobility and short circuit current (Jsc). A PCE of 4.05% with a Voc of 1.08 V was obtained for PBDTBT-C12:NI-A-C6-based polymer solar cells (PSCs) in an active layer thickness of 35 nm. Such a PCE is comparable to that of PBDTBT-C12:PC71BM-based optimized devices (4.07%) and better than devices with an active layer thickness of approximately 30 nm (2.72%). Besides, 35 nm is the thinnest active layer thickness for PSCs with a PCE above 4% and the absorption onset of PBDTBT-C12:NI-A-C6-blend films was as low as 630 nm, leading to a significantly high average visible transmittance up to 76.1%. Ultimately, a relatively high PCE and ultrahigh transmittance were achieved simultaneously, demonstrating that 1,8-naphthalimide-based small molecules are promising acceptors for tandem or semi-transparent PSCs.

Benzothiadiazole based conjugated polymers for high performance polymer solar cells

Three novel copolymers P1–3 with alkylthiophenyl substituted benzodithiophene as the donor unit, thiophene as the spacer, and benzothiadiazole as the acceptor unit have been designed, synthesized, and used as donor materials for polymer solar cells. Polymer solar cells with P3:PC71BM blends as the active layer exhibited a high power conversion efficiency (PCE) of 7.7% and a good tolerance to the change of film thickness. PCE higher than 7.3% can be obtained with the thickness of the active layer ranging from 90 to 380 nm, indicating that P3 is a very promising donor material for practical application.