Xiaoxi Wu

Achieving High-Performance Ternary Organic Solar Cells through Tuning Acceptor Alloy

Acceptor alloys based on n-type small molecular and fullerene derivatives are used to fabricate the ternary solar cell. The highest performance of optimized ternary device is 10.4%, which is the highest efficiency for one donor/two acceptors-based ternary systems. Three important parameters, JSC , VOC , and FF, of the optimized ternary device are all higher than the binary reference devices.

Tuning Voc for high performance organic ternary solar cells with non-fullerene acceptor alloys

Open circuit voltage (Voc) is a critical parameter for ternary organic solar cells, while its mechanism is obscure. Here we employed two non-fullerene molecules TPE-4PDI and FT-2PDI (perylenediimide, a PDI-based small molecule) to form acceptor alloys with ITIC-Th (indacenodithieno, an IDDT-based small molecule) for ternary systems. The results demonstrated that the experimental Voc values fit the simulation data accurately based on the equation not only in our new ternary systems but also in other reported small molecular alloy based ternary systems. More importantly, TPE-4PDI is more efficient to enhance the Voc of ternary solar cells as the third component than FT-2PDI, since TPE-4PDI possesses a larger quasi frontier orbital density (Ne) value. Thus, upon tuning the weight ratio of the TPE-4PDI:ITIC-Th acceptor alloy, high performance ternary solar cells with an efficiency over 11% were achieved. This contribution has shed light on understanding the mechanisms of ternary solar cells and demonstrated a method for enhancing Voc efficiently to achieve high performance solar cells.

Wide bandgap small molecular acceptors for low energy loss organic solar cells

Non-fullerene organic solar cells (OSCs) have attracted great attention due to their advantages including tunable light absorption and low cost fabrication. Many important strategies have been used to achieve high performing OSCs including increasing the charge transport mobility and reducing the energy loss (Eloss). In this contribution, two wide bandgap small molecular acceptors (IDTzCR and IDTCR) were designed and synthesized for OSCs. Through replacing the thiophene moieties with thiazole ones, charge transport mobility was increased due to introducing S⋯N noncovalent conformational locks, resulting in a significant enhancement of photovoltaic performances. Furthermore, IDTCR based OSCs afforded a record low Eloss value for “narrow bandgap donor:wide bandgap acceptor” systems due to the small LUMO/LUMO energy offset. This contribution showed a novel method to achieve excellent wide bandgap acceptors for OSCs and sheds lights on understanding the relationship between the materials properties and device performances.

Fine-tuning solid state packing and significantly improving photovoltaic performance of conjugated polymers through side chain engineering via random polymerization

Alkyl chain engineering has been employed to tune the physicochemical properties of conjugated polymers. Usually, several building blocks with different alkyl chains need to be synthesized in multiple steps, which is synthetically costly. Also, it is challenging to systematically tune the alkyl chains to enhance device performances due to the complex morphological characteristics of the conjugated polymers and the effects of molecular weight and purity. Here we designed and synthesized a series of conjugated polymers with similar molecular weights through random polymerization of BDT building blocks with two TT building blocks possessing different alkyl chains. Upon simply altering the molecular ratio of two TT units, the physicochemical characteristics, solid state packing, and photovoltaic properties of the conjugated polymers were systematically tuned. As a result, a conjugated polymer that can pair with both fullerene (PC71BM) and non-fullerene (ITIC-Th) acceptors to generate high power conversion efficiencies (10.3% and 9.1%, respectively) was achieved. This contribution provides a novel strategy for designing high performance conjugated polymers.

Significantly improving the efficiency of polymer solar cells through incorporating noncovalent conformational locks

Noncovalent conformational locks have been widely employed to construct highly planar and rigid conjugated systems for organic electronics. In this paper, two conjugated polymers (PDTffBT–TVT and PDTffBT–TVTOEt) were synthesized through the Stille coupling of 4,7-di(thien-2-yl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) with (E)-2-(2-(thiophen-2-yl)vinyl)thiophene (TVT) and (E)-1,2-diethoxy-1,2-di(thiophen-2-yl)ethane (TVTOEt), respectively, to investigate the effect of incorporation of the S⋯O noncovalent conformational locks on the performance of the polymer based bulk heterojunction solar cells. The physicochemical properties and photovoltaic characteristics of the conjugated polymers were fully investigated with different characterization techniques, which demonstrated that incorporation of the noncovalent conformational locks improved the rigidity of the backbone, leading to enhanced charge transport mobilities, and thus higher Jsc and FF. As a result, the efficiencies of the solar cells were significantly improved from 2.59% (PDTffBT–TVT) to 6.16% (PDTffBT–TVTOEt).

Combination of Noncovalent Conformational Locks and Side Chain Engineering to Tune Crystallinity of Nonfullerene Acceptors for High-Performance P3HT based Organic Solar Cells

P3HT-based organic solar cells (OSCs) have great advantages for commercialization including straightforward and scalable synthesis and well-developed roll-to-roll manufacturing technology. However, it is difficult to control the morphology of P3HT:acceptor blend films due to their highly crystalline characteristics. In this work, we designed and synthesized two thiazole (Tz) containing small molecular acceptors with an A–π–D–π–A type structure for the P3HT donor material. Both small molecules exhibit a good planar configuration due to incorporation of S⋯N noncovalent conformational locks. Upon changing the side chains, the interchain π–π stacking and the crystallinity of the small molecules were fine-tuned. Interestingly, P-IDTzR with bulky side chains exhibits suitable crystallinity, which matches well with P3HT. As a result, the P3HT:P-IDTzR blend films demonstrate optimal morphology, leading to a larger short circuit current (JSC), an enhanced fill factor (FF), and thus a larger power conversion efficiency (5.01%). This contribution provides important guidance in designing nonfullerene acceptors for high-performance P3HT based OSCs.