Chuandong Dou

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.

Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All-Polymer Solar Cells

We demonstrate that polymer electron acceptors with excellent all-polymer solar-cell (all-PSC) device performance can be developed from polymer electron donors by using B←N units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain B←N units, the π-π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all-PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all-PSCs with state-of-the-art polymer acceptors.

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.

An alternating polymer of two building blocks based on B←N unit: Non-fullerene acceptor for organic photovoltaics

B←N coordination bond can be used to develop polymer electron acceptors for efficient all-polymer solar cells (all-PSCs). Here, we report a new alternating conjugated polymer containing two building blocks based on B←N unit. The polymer exhibits strong light absorption in the visible range, low-lying LUMO/HOMO energy levels and moderate electron mobility. The resulting all-PSC devices exhibit power conversion efficiencies of 1.50%–2.47%.

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.

n‐Type Azaacenes Containing B←N Units

We disclose a novel strategy to design n-type acenes through the introduction of boron-nitrogen coordination bonds (B←N). We synthesized two azaacenes composed of two B←N units and six/eight linearly annelated rings. The B←N unit significantly perturbed the electronic structures of the azaacenes: Unique LUMOs delocalized over the entire acene skeletons and decreased aromaticity of the B←N-adjacent rings. Most importantly, these B←N-containing azaacenes exhibited low-lying LUMO energy levels and high electron affinities, thus leading to n-type character. The solution-processed organic field-effect transistor based on one such azaacene exhibited unipolar n-type characteristics with an electron mobility of 0.21 cm2  V-1  s-1 .

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.