Abstract
Two two-dimensional (2D) donor–acceptor (D-A) type conjugated polymers (CPs), namely, PBDT-TVT-BT and PBDT-TVT-FBT, in which two ((E)-(4,5-didecylthien-2-yl)vinyl)- 5-thien-2-yl (TVT) side chains were introduced into 4,8-position of benzo[1,2-b:4,5-bʹ]dithiophene (BDT) to synthesize the highly conjugated electron-donating building block BDT-TVT, and benzothiadiazole (BT) and/or 5,6-difluoro-BT as electron-accepting unit, were designed to systematically ascertain the impact of fluorination on thermal stability, optoelectronic property, and photovoltaic performance. Both resultant copolymers exhibited the lower bandgap (1.60 ~ 1.69 eV) and deeper highest occupied molecular orbital energy level (EHOMO, –5.17 ~ –5.37 eV). It was found that the narrowed absorption, deepened EHOMO and weakened aggregation in solid film but had insignificant influence on thermal stability after fluorination in PBDT-TVT-FBT. Accordingly, a PBDT-TVT-FBT-based device yielded 16% increased power conversion efficiency (PCE) from 4.50% to 5.22%, benefited from synergistically elevated VOC, JSC, and FF, which was mainly originated from deepened EHOMO, increased μh, μe, and more balanced μh/μe ratio, higher exciton dissociation probability and improved microstructural morphology of the photoactive layer as a result of incorporating fluorine into the polymer backbone.
Highlights
It has proved that constructing donor–acceptor (D-A) type CPs and/or small molecules (SMs) appears to be one of the most simplistic, promising, attractive, and successful strategies, since the alternating D-A conjugated blocks in the polymer backbone led to the reduction of the band gap by hybridizing the highest occupied molecular orbital (HOMO) with the lowest unoccupied molecular orbital (LUMO), and enhanced the inter- and intra-molecular interactions directly governing the molecular ordering and π–π stacking for conjugated materials in the solid state
In order to maximize the power conversion efficiency (PCE) of polymer solar cells (PSCs), it was of critical importance that, on one hand, enlarging the difference as much as possible between the EHOMO of donor and ELUMO of acceptor could obtain a larger open-circuit voltage (V OC ), the suitable ELUMO paired with the electron acceptor was juggled for charge separation and transport, while on the other hand, reducing the optical band gap toward harvesting more photons by up-shifting the EHOMO or down-shifting the ELUMO was anticipated to yield higher short-circuit current density (JSC ) [7,8]
PBDT-TVT-FBT were synthesized via typical Stille polymerization strategy [93], and the further post-processing and purification steps were accomplished in accordance with the previously reported method [89]
Summary
Of all the third generation solar cell technologies (i.e., organic solar cells, perovskite solar cells, and dye-sensitized solar cells, and so on), bulk-heterojunction (BHJ) polymer solar cells (PSCs) have attracted broad academic and industrial interests, because of attractive features including flexibility, lightweight, large area, low-cost production, and environmental friendliness [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. The alkyloxy side chain onto BDT in PTB7 was replaced with alkylthiothienylenevinylene thiophene (RS-TVT) to develop a 2D CP PBT-TVT, and a greatly improved absorption ranged from 300 to 550 nm, reduced optical band gap from 1.63 to 1.53 eV, 0.06 down-shifted EHOMO , more ordered solid film stacking, and 2.22-times raised μh were observed, leading to synergistically increased V OC and JSC and corresponding 9.7% enhanced PCE from 7.41%.
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