Jicheng Zhang

Ternary-Blend Polymer Solar Cells Combining Fullerene and Nonfullerene Acceptors to Synergistically Boost the Photovoltaic Performance

A ternary-blend strategy is presented to surmount the shortcomings of both fullerene derivatives and nonfullerene small molecules as acceptors for the first time. The optimal ternary device shows a high power conversion efficiency (PCE) of 10.4%. Moreover, a significant enhancement in PCE (≈35%) relative to both of the binary reference devices, which has never been achieved before in high-efficiency ternary devices, is demonstrated.

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.

Simultaneous enhancement of the molecular planarity and the solubility of non-fullerene acceptors: effect of aliphatic side-chain substitution on the photovoltaic performance

Three planar nonfullerene acceptors (FTIC-C8C6, FTIC-C6C6 and FTIC-C6C8) comprising a central fluorenedicyclopentathiophene (FT) core and two 2-methylene-(3-(1,1-dicyanomethylene)-indanone) terminal groups are designed and synthesized. The coplanarity of the molecular backbone can be maintained through a locked conformation via intramolecular noncovalent interactions. The solubility of these nonfullerene acceptors is very good because the FT core can bear enough flexible aliphatic side-chain substitutions. Thus, the dilemma of the planarity–solubility tradeoff can be minimized. Through changing the length of the six flexible aliphatic side chains at the central FT core, we can easily adjust the π–π interactions of nonfullerene acceptors and optimize the nanoscale morphology of the photoactive layers. Among these three small molecular acceptors, FTIC-C6C8 based active layers show the best morphology together with the highest electron and hole mobility. These inherent advantages of FTIC-C6C8 guarantee it a high power conversion efficiency of 11.12% when used in non-fullerene polymer solar cells with a wide-bandgap polymer donor PBDB-T. Our results provide an appropriate molecular design strategy for building high-performance nonfullerene acceptors and show that optimizing alkyl-side chains is a very effective way to further improve the photovoltaic performance of devices.

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.

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.

5,6-Difluorobenzothiadiazole and silafluorene based conjugated polymers for organic photovoltaic cells

To achieve 5,6-difluorobenzothiadiazole and 2,7-linked silafluorene based soluble conjugated polymers, flexible side chains were attached at different positions of the conjugated polymers. Three soluble polymers PSiF-D(OT)DFBT, PSiF-TTDFBT, and PDOSiF-DTDFBT were prepared and used as donor materials for polymer solar cells. PSiF-D(OT)DFBT exhibits a band gap of 2.06 eV with a deep HOMO of −5.64 eV. PSiF-TTDFBT shows a band gap of 1.75 eV with the HOMO of −5.23 eV. PDOSiF-DTDFBT is of a band gap of 1.86 eV with the HOMO level of −5.37 eV. Among these three polymers, PDOSiF-DTDFBT shows the highest field effect transistor (FET) hole mobility up to 3.31 × 10−2 cm2 V−1 s−1, PDOSiF-DTDFBT:PC71BM blend films show the highest SCLC mobility up to 5.10 × 10−4 cm2 V−1 s−1, and polymer solar cells (PSCs) with the blend of PDOSiF-DTDFBT:PC71BM (1 : 1, by weight) as the active layer gave a power conversion efficiency (PCE) of 4.03% with an open circuit voltage (Voc) of 0.73 V, a short circuit current (Jsc) of 8.55 mA cm−2, and a fill factor (FF) of 0.65. Our studies also reveal the structure–property relationship of 2,7-linked silafluorene and 5,6-difluorobenzothiadiazole based conjugated polymers.

Efficient polymer solar cells processed by environmentally friendly halogen-free solvents

The use of environmentally friendly halogen-free organic solvents for the fabrication of polymer solar cells will be of great importance for future practical applications. In this work, a new alternative conjugated polymer with 3,4-bis(octyloxy)-phenyl substituted benzo[1,2-b:4,5-b]dithiophene as the donor unit and benzo[c][1,2,5]thiadiazole as the acceptor unit was synthesized and used as the donor material for polymer solar cells. This polymer showed good solubility in halogen-free solvents such as toluene, o-xylene and so on. The blend film morphology, charge mobility and photovoltaic performance were investigated in halogen-free solvents. The photovoltaic devices fabricated from o-xylene with N-methyl-2-pyrrolidone as additive provided the best power conversion efficiency of 4.57%, comparable to that fabricated from halogenated solvents such as 1,2-dichlorobenzene/1,8-diiodooctane with a power conversion efficiency of 4.33%. Our results demonstrate that halogen-free solvents are promising for the fabrication of high efficiency polymer solar cells.