One of the key issues limiting the efficiency of non-fullerene organic solar cells (NF-SCs) is the low electron mobility and strong recombination loss. In this paper, we report an approach of fine-tuning the parameters relative to the film-forming kinetics to increase the power conversion efficiency, which significantly improved from 1.4 up to 6.1%. The film-forming process was judiciously optimized by carefully manipulating the following four parameters: the additive content during film processing, the volume of the host solvent for solvent vapor annealing (SVA), the volume ratio of the additive versus the host solvent for SVA, and the time for SVA. Through such controls, the photocurrent dramatically increased from 5.40 to 12.83 mA/cm 2 and the fill factor from 32.61 to 56.43% as a result of the reduction of the monomolecular and bimolecular loss and the improvement of the electron mobility. These improvements in the electric properties are associated with the reconstruction of the film morphology, i.e., solvent annealing of the ascast active film leads to the improvement of the phase segregation and the consequent enhancement of the self-aggregation of the blend donor and acceptor molecules in the solar cell active film.
Volatile solid additives (SADs) are considered as a simple
yet
effective approach to tune the film morphology for high-performance
organic solar cells (OSCs). However, the structural effects of the
SADs on the photovoltaic performance are still elusive. Herein, two
volatilizable SADs were designed and synthesized. One is SAD1 with
twisted conformation, while the other one is planar SAD2 with the
S···O noncovalent intramolecular interactions (NIIs).
The theoretical and experimental results revealed that the planar
SAD2 with smaller space occupation can more easily insert between
the Y6 molecules, which is beneficial to form a tighter intermolecular
packing mode of Y6 after thermal treatment. As a result, the SAD2-treated
OSCs exhibited less recombination loss, more balanced charge mobility,
higher hole transfer rate, and more favorable morphology, resulting
in a record power conversion efficiency (PCE) of 18.85% (certified
PCE: 18.7%) for single-junction binary OSCs. The universality of this
study shed light on understanding the conformation effects of SADs
on photovoltaic performances of OSCs.
A non-fullerene, all-small-molecule solar cell (NF-SMSC) device uses the blend of a small molecule donor and a small molecule acceptor as the active layer. Aggregation ability is a key factor for this type of solar cell. Herein, we used the alkylthienyl unit to tune the aggregation ability of the diketopyrrolopyrrole (DPP)-based small molecule donors. Replacing two alkoxyl units in BDT-O-DPP with two alkylthienyl units yields BDT-T-DPP, and further introducing another two alkylthienyl units into the backbone produces BDT-T-2T-DPP. With the introduction of alkylthienyl, the backbone becomes twisted. As a result, the ππ-stacking strength, aggregation ability, and crystallite size all obey the sequence of BDT-O-DPP > BDT-T-DPP > BDT-T-2T-DPP. When selected a reported perylene diimide dimer of bis-PDI-T-EG as acceptor, the best NF-SMSC device exhibits a power conversion efficiency of 1.34, 2.01, and 1.62%, respectively, for the BDT-O-DPP, BDT-T-DPP, and BDT-T-2T-DPP based system. The BDT-T-DPP/bis-PDI-T-EG system yields the best efficiency of 2.01% among the three combinations. This is due to the moderate aggregation ability of BDT-T-DPP yields moderate phase size of 30-50 nm, whereas the strong aggregation ability of BDT-O-DPP gives a bigger size of 50-80 nm, and the weak aggregation ability of BDT-T-2T-DPP produces a smaller size of 10-30 nm. The BDT-T-DPP/bis-PDI-T-EG combination exhibits balanced hole/electron mobility of 0.022/0.016 cm(2)/(V s), whereas the BDT-O-DPP/bis-PDI-T-EG and the BDT-T-2T-DPP/bis-PDI-T-EG blend show a hole/electron mobility of 0.0011/0.0057 cm(2)/(V s) and 0.0016/0.11 cm(2)/(V s), respectively.
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