Zicheng Ding

Polymer Acceptor Based on Double B←N Bridged Bipyridine (BNBP) Unit for High‐Efficiency All‐Polymer Solar Cells

A novel polymer acceptor based on the double B←N bridged bipyridine building block is reported. All-polymer solar cells based on the new polymer acceptor show a power conversion efficiency of as high as 6.26% at a photon energy loss of only 0.51 eV.

An Electron-Deficient Building Block Based on the B←N Unit: An Electron Acceptor for All-Polymer Solar Cells.

A double B←N bridged bipyridyl (BNBP) is a novel electron-deficient building block for polymer electron acceptors in all-polymer solar cells. The B←N bridging units endow BNBP with fixed planar configuration and low-lying LUMO/HOMO energy levels. As a result, the polymer based on BNBP units (P-BNBP-T) exhibits high electron mobility, low-lying LUMO/HOMO energy levels, and strong absorbance in the visible region, which is desirable for polymer electron acceptors. Preliminary all-polymer solar cell (all-PSC) devices with P-BNBP-T as the electron acceptor and PTB7 as the electron donor exhibit a power conversion efficiency (PCE) of 3.38%, which is among the highest values of all-PSCs with PTB7 as the electron donor.

Developing Conjugated Polymers with High Electron Affinity by Replacing a CC Unit with a B←N Unit

The key parameters of conjugated polymers are lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Few approaches can simultaneously lower LUMO and HOMO energy levels of conjugated polymers to a large extent (>0.5 eV). Disclosed herein is a novel strategy to decrease both LUMO and HOMO energy levels of conjugated polymers by about 0.6 eV through replacement of a C-C unit by a B←N unit. The replacement makes the resulting polymer transform from an electron donor into an electron acceptor, and is proven by fluorescence quenching experiments and the photovoltaic response. This work not only provides an effective approach to tune the LUMO/HOMO energy levels of conjugated polymers, but also uses organic boron chemistry as a new toolbox to develop conjugated polymers with high electron affinity for polymer optoelectronic devices.

Functionalized graphene quantum dots as a novel cathode interlayer of polymer solar cells

The cathode interlayer (CIL) plays an important role in maximizing the photovoltaic efficiency of polymer solar cells (PSCs). The principle to design organic/polymeric CIL materials is to functionalize π-conjugated skeletons with specific polar/ionic groups. Here, using the principle of organic/polymer CIL materials, we developed graphene quantum dots functionalized with tetramethylammonium at the edge (GQDs-TMA) to be used as a CIL for PSCs with good device performance. The peripheral tetramethylammonium groups can form an interfacial dipole with the cathode to decrease the work function. Graphene quantum dots are used as the π-conjugated skeleton because of their facile synthesis, high conductivity and good film-forming capability. As a result, using an active layer of PCDTBT:PC71BM, a power conversion efficiency (PCE) of 7.01% is achieved with GQDs-TMA as the CIL, much higher than that (<6.5%) with Ca, LiF, or ZnO as the CIL. PSCs with a conventional configuration using GQDs-TMA as the CIL and PTB7-Th:PC71BM as the active layer show a PCE of 8.80%, which is the highest reported so far for PSCs containing graphene materials. Moreover, GQDs-TMA can also work well when high work function metals (e.g. Ag, Au) are used as the cathode. To the best of our knowledge, this is the first report on solution-processed graphene derivatives as a CIL with excellent PSC device performance.

Diketopyrrolopyrrole-based Conjugated Polymers Bearing Branched Oligo(Ethylene Glycol) Side Chains for Photovoltaic Devices

Conjugated polymers are essential for solution-processable organic opto-electronic devices. In contrast to the great efforts on developing new conjugated polymer backbones, research on developing side chains is rare. Herein, we report branched oligo(ethylene glycol) (OEG) as side chains of conjugated polymers. Compared with typical alkyl side chains, branched OEG side chains endowed the resulting conjugated polymers with a smaller π-π stacking distance, higher hole mobility, smaller optical band gap, higher dielectric constant, and larger surface energy. Moreover, the conjugated polymers with branched OEG side chains exhibited outstanding photovoltaic performance in polymer solar cells. A power conversion efficiency of 5.37 % with near-infrared photoresponse was demonstrated and the device performance could be insensitive to the active layer thickness.

Development of a donor polymer using a B ← N unit for suitable LUMO/HOMO energy levels and improved photovoltaic performance

The LUMO/HOMO energy levels of conjugated polymers are key parameters for their applications as polymer electron donors for polymer solar cells (PSCs). The widely-used strategy to tune the LUMO/HOMO levels of polymer donors is to develop D–A type polymers based on an alternating electron-donating unit (D) and an electron-accepting unit (A). In this paper, we report a novel approach to tune the LUMO/HOMO levels of polymer donors via replacing a C–C unit by a B ← N unit for enhanced PSC device performance. The control polymer PCPDT shows the LUMO/HOMO levels of −2.71 eV/−4.98 eV, which are both much higher than those required for an ideal polymer donor. By replacing a C–C unit with a B ← N unit, the resulting polymer PBNCPDT exhibits much lower LUMO/HOMO levels of −3.23 eV/−5.20 eV. PBNCPDT also shows a narrower optical bandgap (Eg = 1.73 eV) than that (Eg = 1.85 eV) of PCPDT, which is helpful for harvesting of sunlight. Moreover, PBNCPDT with the B ← N unit is not a typical D–A type conjugated polymer because its LUMO and HOMO are both delocalized over the whole conjugated framework. As the control PSC device based on PCPDT exhibits an open-circuit voltage (Voc) of 0.45 V and power conversion efficiency (PCE) of 0.63%, the device of PBNCPDT shows much improved Voc of 0.82 V and PCE of 3.74%. These results indicate that a B ← N unit can be used to develop polymer donors for high-performance PSC devices.

Low-bandgap polymer electron acceptors based on double B ← N bridged bipyridine (BNBP) and diketopyrrolopyrrole (DPP) units for all-polymer solar cells

Broad absorption spectra and small optical bandgaps of polymer electron acceptors are very important for the sunlight harvesting of all-polymer solar cells (all-PSCs). Conjugated polymers based on the double B ← N bridged bipyridine (BNBP) unit are a new class of polymer electron acceptors, which suffer from narrow absorption spectra and large bandgaps. In this manuscript, we report a new polymer electron acceptor (P-BNBP-DPP) based on the BNBP unit and the dithienyl-diketopyrrolopyrrole (DPP) unit with a small bandgap and improved sunlight-harvesting capability. P-BNBP-DPP exhibits a broad absorption band with the onset absorbance at 796 nm and a small optical bandgap of 1.56 eV. Moreover, P-BNBP-DPP shows the low LUMO/HOMO energy levels of −3.87 eV/−5.45 eV and a high electron mobility of 2.1 × 10−4 cm2 V−1 s−1. An all-PSC device with P-BNBP-DPP as the acceptor and poly[(ethylhexyl-oxy)-benzodithiophene-(ethylhexyl)-thienothiophene] (PTB7) as the donor produces a power conversion efficiency of 2.69% with a broad external quantum efficiency response in the range of 300–800 nm. These results suggest an effective approach to tune the absorption spectra of BNBP-based polymer electron acceptors.

Organic solar cells based on a polymer acceptor and a small molecule donor with a high open-circuit voltage

Organic solar cells (OSCs) based on a small molecule donor (SD) and a polymer acceptor (PA) exhibit low power conversion efficiency (PCE) due to the limited number of small molecule donor–polymer acceptor combinations. In this work, we employ a polymer acceptor based on the double B ← N bridged bipyridyl (BNBP) unit to develop SD/PA-type OSCs. With poly[(N,N′-bis(2-hexyldecyl)-diamine-bis(difluoro-borane)-2,2-bipyridine)-alt-(2,5-thiophene)] (P-BNBP-T) as the acceptor and 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) as the donor, the OSC device shows a high open-circuit voltage (VOC) of 1.08 V and a PCE of 3.50%. The VOC is ca. 0.3 V greater than that of other OSCs based on p-DTS(FBTTh2)2 due to the larger offset between the HOMO energy level of p-DTS(FBTTh2)2 and the higher-lying LUMO energy level of P-BNBP-T. The PCE of p-DTS(FBTTh2)2/P-BNBP-T is higher than that of any other OSCs based on the p-DTS(FBTTh2)2/polymer acceptor blend reported so far. These results indicate that the BNBP-based polymer acceptors are promising for high-performance SD/PA-type OSCs. While the as-cast p-DTS(FBTTh2)2/P-BNBP-T blend film exhibits low molecular packing order and large-size phase separation, processing with solvent additive 1,8-diiodoctane (DIO) leads to continuous networks with small crystalline grains of p-DTS(FBTTh2)2 in the blend film. The resulting OSC device exhibits the best photovoltaic performance because of the improved exciton dissociation efficiency and charge transport ability.

A double B←N bridged bipyridine (BNBP)-based polymer electron acceptor: all-polymer solar cells with a high donor : acceptor blend ratio

A new polymer electron acceptor (P-BNBP-CDT) composed of an alternating double B←N bridged bipyridine (BNBP) unit and a cyclopenta-[2,1-b:3,4-b′]-dithiophene (CDT) unit has been developed. P-BNBP-CDT exhibits strong light absorption in the visible range of 500–650 nm and suitable LUMO/HOMO energy levels (ELUMO/HOMO) of −3.45 eV/−5.64 eV, which are very complementary to that (ELUMO/HOMO = −3.2 eV/−5.2 eV) of the widely-used polymer donor, poly(3-hexylthiophene) (P3HT). All-polymer solar cells (all-PSCs) with P3HT as an electron donor and P-BNBP-CDT as an electron acceptor exhibit power conversion efficiencies (PCEs) exceeding 1.0% with high donor : acceptor blend ratios (w : w, from 0.5 : 1 to 9 : 1). The highest PCE of these devices is 1.76% with a high donor : acceptor blend ratio of 5 : 1. These results not only indicate that BNBP-based polymers are promising for P3HT : polymer acceptor devices, but also suggest the potential for low cost and facile device processing of all-PSCs.

Manipulating active layer morphology of molecular donor/polymer acceptor based organic solar cells through ternary blends

The development of molecular donor/polymer acceptor blend (MD/PA)-type organic solar cells (OSCs) lags far behind other type OSCs. It is due to the large-size phase separation morphology of MD/PA blend, which results from the high crystallinity of molecular donors. In this article, to suppress the crystallinity of molecular donors, we use ternary blends to develop OSCs based on one polymer acceptor (P-BNBP-fBT) and two molecular donors (DR3TBDTT and BTR) with similar chemical structures. The ternary OSC exhibits a power conversion efficiency (PCE) of 4.85%, which is higher than those of the binary OSCs (PCE=3.60% or 3.86%). To our best knowledge, it is the first report of ternary MD/PA-type OSCs and this PCE is among the highest for MD/PA-type OSCs reported so far. Compared with the binary blends, the ternary blend exhibits decreased crystalline size and improved face-on orientation of the donors. As a result, the ternary blend exhibits improved and balanced charge mobilities, suppressed charge recombination and increased donor/acceptor interfacial areas, which leads to the higher short-circuit current density. These results suggest that using ternary blend is an effective strategy to manipulate active layer morphology and enhance photovoltaic performance of MD/PA-type OSCs.