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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In this paper, we report an efficient nonfullerene solar cell based on small molecules of p-DTS(FBTTh2)2 and bis-PDI-T. Characterization data indicate that the nature of the acceptor aggregate is a key factor that affects the photocurrent. There is a good relationship between the short-circuit current density (J(SC)) and the phase size of the acceptor-rich domains. The phase size of the acceptor-rich domains is tuned by both the additive types and additive content. As the kind of additive goes from 1-chloronaphthalene (CN) to 1,8-octanedithiol (ODT) and 1,8-diiodooctane (DIO), by this order the solubility of the acceptor in the additive is down, the phase size significantly decreases from over 400 nm down to 30 nm. Also, the acceptor's domain size decreases from 80 to 30 nm as the DIO content ([DIO]) is down from 1% to 0.15%. Following this trend, less DIO remains in the wet film as residue after the host chloroform evaporates, and thus less acceptor can be dissolved in the residue DIO. This decreasing of DIO content acts on the film-morphology similarly as the additive changes down to the one having a lower solubility. Accordingly, our results indicate that it is the dissolved amount of the organic component in the residue additive solvent of the wet film that plays a role in turning the phase size. The efficiency from this small molecule system is significantly raised from 0.02% up to 3.7% by selecting the additive type and fine-tuning the additive content.

We report herein a new solution-processable small molecule acceptor, a selenophenyl bridged perylene diimide dimer, that gives 4.0% efficiency when employing PBDTTT-C-T as the polymer donor and a conventional cell structure.

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The experimental current is scaled by the generation, transportation and recombination loss of the mobile carriers. In this paper, we show that a change of small molecule/perylene diimide (PDI) weight ratio from 1:1 to 3:1 does not lead to different transportation or different recombination loss in the solar cell, while it results in different generation of the mobile carriers. With the increase of the donor/acceptor (D/A) ratio, the blend film has an enhanced absorption because the donor has a stronger light‐harvesting ability than the PDI acceptor. With respect to the contribution from the enhancement of the absorption of the solar photons, our data demonstrate that the phase size of the acceptor (donor) domains is the key factor that determines the generation of the mobile carriers. We observe a good relationship between the average short‐circuit current density (JSC) and phase size of the acceptor domains. Through fine‐tuning the D/A ratio, the best compatibility between the small molecule donor and the PDI acceptor is obtained at a mediate D/A ratio of 1.3:1, at which the absorbed solar photons are exploited efficiently, yielding an average power conversion efficiency of over 5.07 ± 0.10%. Compatibility and charge separation are modulated by varying donor weight ration in small molecule donor:perylene diimide based non‐fullerene solar cells, which affords improved exploitation of the solar photons absorbed by the photoactive layer. The acceptor phase size is a key factor that scales the photocurrent. An efficiency of 5.1% is obtained from this non‐fullerene small molecule system as a result of modulation of compatibility.