Heng Lu

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.

Enhancing the performance of polymer photovoltaic cells by using an alcohol soluble fullerene derivative as the interfacial layer.

Alcohol soluble fullerene derivative (FN-C60) has been synthesized and used as a cathode interfacial layer for high-efficiency polymer solar cells (PSCs). To examine the function of the FN-C60 interfacial layer, polymer solar cells were fabricated with blends of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM as the active layer. In comparison to the bare Al electrode, power conversion efficiencies (PCEs) of P3:PC71BM, HXS-1:PC71BM, PDFCDTBT:PC71BM, and PDPQTBT:PC71BM based PSCs were increased from 3.50 to 4.64%, 4.69 to 5.25%, 2.70 to 4.60%, and 1.52 to 2.29%, respectively, when FN-C60/Al was used as the electrode. Moreover, the overall photovoltaic performances of PSCs with the FN-C60/Al electrode were better than those of cells with LiF/Al electrode, indicating that FN-C60 is a potential interfacial layer material to replace LiF.

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.

High-Efficiency Large-Bandgap Material for Polymer Solar Cells

High-molecular-weight conjugated polymer HD-PDFC-DTBT with N-(2-hexyldecyl)-3,6-difluorocarbazole as the donor unit, 5,6-bis(octyloxy)benzothiadiazole as the acceptor unit, and thiophene as the spacer is synthesized by Suzuki polycondensation. HD-PDFC-DTBT shows a large bandgap of 1.96 eV and a high hole mobility of 0.16 cm(2) V(-1) s(-1) . HD-PDFC-DTBT:PC71 BM-based inverted polymer solar cells (PSCs) give a power conversion efficiency (PCE) of 7.39% with a Voc of 0.93 V, a Jsc of 14.11 mA cm(-2) , and an FF of 0.56.

Evaluating the photovoltaic properties of two conjugated polymers synthesized by Suzuki polycondensation and direct C-H activation

Two conjugated polymers HXS-1 and PDFCDTBT were prepared by direct C-H activation and Suzuki polycondensation and their chemical structures were characterized by 1H NMR spectroscopy. The molecular weight of conjugated polymer synthesized by direct C-H activation is lower than the corresponding polymers prepared by Suzuki polycondensation. Conjugated polymers synthesized by direct C-H activation have considerable solubility in common organic solvents and form amorphous film. The photovoltaic property of conjugated polymers synthesized by direct C-H activation is inferior to the corresponding polymers synthesized by Suzuki polycondensation.

Performance Enhancement of Polymer Solar Cells by Using Two Polymer Donors with Complementary Absorption Spectra

Performance enhancement of polymer solar cells (PSCs) is achieved by expanding the absorption of the active layer of devices. To better match the spectrum of solar radiation, two polymers with different band gaps are used as the donor material to fabricate ternary polymer cells. Ternary blend PSCs exhibit an enhanced short-circuit current density and open-circuit voltage in comparison with the corresponding HD-PDFC-DTBT (HD)- and DT-PDPPTPT (DPP)-based binary polymer solar cells, respectively. Ternary PSCs show a power conversion efficiency (PCE) of 6.71%, surpassing the corresponding binary PSCs. This work demonstrates that the fabrication of ternary PSCs by using two polymers with complementary absorption is an effective way to improve the device performance.

Engineering the band gap and energy level of conjugated polymers using a second acceptor unit

Three novel isoindigo based donor–acceptor (D–A) conjugated polymers P1–3 have been synthesized by Suzuki polycondensation and utilized as donor materials for polymer solar cells (PSCs). These three polymers are of the same backbone, but have different substituents. All these polymers exhibit high thermal stability and broad absorption in the range of 300 to 770 nm. Hole mobilities of polymer films spin coated from 1,2-dichlorobenzene (DCB) solutions are 7.00 × 10−4, 2.37 × 10−3 and 2.90 × 10−4 cm2 V−1 s−1 for P1, P2 and P3, respectively. PSCs based on P2:PC71BM (1 : 2 by weight) with a 2% DIO additive displayed a power conversion efficiency (PCE) of 3.41% with a short-circuit current density (Jsc) of 7.57 mA cm−2, an open-circuit voltage (Voc) of 0.85 V, and a fill factor (FF) of 53%, under the illumination of AM 1.5G (100 mW cm−2). XRD diffraction measurements have shown that these polymers have a short π–π stacking distance in the solid state. The results demonstrate that these conjugated polymers could be promising donor materials in the application of polymer solar cells.

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.

Spin-coated Ag nanoparticles onto ITO substrates for efficient improvement of polymer solar cell performance

We investigated photovoltaic performances of polymer solar cells depending on three materials (P1/PC71BM, P2/PC71BM, P3/PC71BM) that embedded with a certain sized (35 nm) Ag nanoparticles (NPs) between indium tin oxide (ITO) and PEDOT : PSS anode buffer layer. The power conversion efficiency of the three solar cells enhanced by 13.6%, 21.1%, and 16.0%, respectively, and the increase mainly originated from Jsc. The other measurements and investigations demonstrated that both the optical and electrical properties made contributions to the development of device performances. The introduction of Ag NPs between ITO substrates and PEDOT : PSS layers is one of the most effective approaches to improve the polymer solar cell performance.

Enhancing the performance of polymer solar cells by tuning the drying process of blend films via changing side chains and using solvent additives

A series of new conjugated polymers (P1–P3) with 3,6-difluorocarbazole as a donor unit and benzoxadiazole as an acceptor unit were synthesized and used as donor materials for polymer solar cells (PSCs). The morphology of blend films was regulated by controlling the drying process via tuning the solubility of polymers and using solvent additives. Enhancing the solubility of polymers via increasing the volume of side chains can decrease the domain size of polymers and using 1,8-diiodooctane (DIO) as a solvent additive can give an even better vertical phase separation, leading to a significant enhancement of the power conversion efficiency (PCE) of up to 5.71% for P3 based PSCs. The improving of the interface between polymers and PC71BM phases as well as the formation of vertical phase separation after using DIO as an additive are probably responsible for the high open circuit voltage (Voc) of devices.