Abstract
Two random conjugated polymers (CPs), namely, PIDTT-TBT and PIDTT-TFBT, in which indacenodithieno[3,2-b]thiophene (IDTT), 3-octylthiophene, and benzothiadiazole (BT) were in turn utilized as electron-donor (D), π-bridge, and electron-acceptor (A) units, were synthesized to comprehensively analyze the impact of reducing thiophene π-bridge and further fluorination on photostability and photovoltaic performance. Meanwhile, the control polymer PIDTT-DTBT with alternating structure was also prepared for comparison. The broadened and enhanced absorption, down-shifted highest occupied molecular orbital energy level (EHOMO), more planar molecular geometry thus enhanced the aggregation in the film state, but insignificant impact on aggregation in solution and photostability were found after both reducing thiophene π-bridge in PIDTT-TBT and further fluorination in PIDTT-TFBT. Consequently, PIDTT-TBT-based device showed 185% increased PCE of 5.84% profited by synergistically elevated VOC, JSC, and FF than those of its counterpart PIDTT-DTBT, and this improvement was chiefly ascribed to the improved absorption, deepened EHOMO, raised μh and more balanced μh/μe, and optimized morphology of photoactive layer. However, the dropped PCE was observed after further fluorination in PIDTT-TFBT, which was mainly restricted by undesired morphology for photoactive layer as a result of strong aggregation even if in the condition of the upshifted VOC. Our preliminary results can demonstrate that modulating the π-bridge in polymer backbone was an effective method with the aim to enhance the performance for solar cell.
Highlights
By virtue of the intriguing features of environmentally friendly, inexhaustibility, and widespread distribution, solar energy as one kind of green renewable energy has yielded more and more attention [1,2]
Power conversion efficiencies (PCEs) of polymer solar cells (PSCs) have surpassed over 11% [8] in fullerene-accepter system and 15% in non-fullerene system [9], respectively, profited by the design of novel materials, optimization of morphology for photoactive layer and the deepened understanding of the structure-performance relationship [4,10,11,12,13,14,15,16,17,18]
Tremendous efforts have been devoted to design and explore donor-acceptor (D-A) type conjugated polymers (CPs) since their absorption, energy levels, molecular planarity, and charge mobility and morphology could be finely regulated by means of judiciously selecting the D and A subunits and alternating them into the polymer backbone so as to tune the photo-induced intramolecular charge transfer (ICT)
Summary
By virtue of the intriguing features of environmentally friendly, inexhaustibility, and widespread distribution, solar energy as one kind of green renewable energy has yielded more and more attention [1,2]. MBG and WBG copolymers were still of importance in the PSCs, because the relatively large band gap cannot only render the low-lying highest occupied molecular orbital energy level (EHOMO ) in order to achieve the higher open-circuit voltage (V OC ), and couple with the electron-acceptor so as to broaden the absorption range and to garner the high short-circuit current density (JSC ) [32,33,34]. It should be noted that the fluorination can improve the molecular planarity and enhance the charge mobility but would inevitably reduce CPs’ solubility, resulting in the difficulty in the process of solution-processed fabrication [74] These abovementioned results suggested that tuning the conjugated π-bridges and introducing fluorine into the polymer backbone were the facile and wise strategies that could effectively modulate the absorption, energy level, coplanarity, aggregation and charge transport, morphology of the photoactive layer, and the photovoltaic performance. The worsen PCE was observed after further introducing fluorine in PIDTT-TFBT
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