Critical factors governing vertical phase separation in polymer–PCBM blend films for organic solar cells

Kim, Min; Lee, Jaewon; Jo, Sae Byeok; Sin, Dong Hun; Ko, Hyomin; Lee, Hansol; Lee, Seung Koo; Cho, Kilwon

doi:10.1039/c6ta06508b

Cited by 67 publications

(56 citation statements)

References 63 publications

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“…The relatively large difference in surface energies of the PffBT4T-2OD ( γ = 29.1 mN m −1 ) and the bis-PDI ( γ = 41.1 mN m −1 ) is likely to result in a poor miscibility in the blend film, producing a slightly larger phase-separated film morphology. Whereas in the PffBT4T-2OD:PC 70 BM blend film, the spherical shape fullerene molecule and small difference in surface energies of the components enabled the PC 70 BM ( γ = 31.6 mN m −1 ) molecule to easily penetrate and diffuse into the polymer region, which may have hindered strong aggregation of polymer 34,35 . However, the bis-PDI molecule could not penetrate into the polymer region because of large surface energy difference, and allow polymer to crystallize largely, as can be seen in TEM images.…”

Section: Resultsmentioning

confidence: 99%

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Singh

Suranagi

Lee

et al. 2018

Sci Rep

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Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.

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Section: Resultsmentioning

confidence: 99%

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Singh

Suranagi

Lee

et al. 2018

Sci Rep

Self Cite

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“…One method for predicting the solidification pathway from a homogenous solution models the phase separation process using the homogeneous Flory–Huggins free energies of the multicomponent blend . So far, such models have only been rigorously applied to three component systems—solvent:donor:acceptor (S:D:A) mixtures—but this methodology can be extended to include more components .…”

Section: Mechanistic Understanding Of Solvent Additive Effects On Bhjmentioning

confidence: 99%

“…Equation can used to calculate a ternary phase diagram of S:D:A mixture that can predict the phase behavior of the ternary blend. An example is provided in Figure a and construction of such ternary phase diagrams are explained elsewhere in the literature …”

Section: Mechanistic Understanding Of Solvent Additive Effects On Bhjmentioning

confidence: 99%

“…Starting from an initially homogenous single phase, L–L demixing begins upon crossing from the one‐phase to the two‐phase regions. The onset of L–L demixing strongly correlates with the interaction parameter between the donor and the acceptor, χ DA . A donor and acceptor pair with low compatibility (high χ DA ) exhibit a large two‐phase region.…”

Section: Mechanistic Understanding Of Solvent Additive Effects On Bhjmentioning

confidence: 99%

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Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

et al. 2018

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Organic photovoltaics (OPV) have the advantage of possible fabrication by energy-efficient and cost-effective deposition methods, such as solution processing. Solvent additives can provide fine control of the active layer morphology of OPVs by influencing film formation during solution processing. As such, solvent additives form a versatile method of experimental control for improving organic solar cell device performance. This review provides a brief history of solution-processed bulk heterojunction OPVs and the advent of solvent additives, putting them into context with other methods available for morphology control. It presents the current understanding of how solvent additives impact various mechanisms of phase separation, enabled by recent advances in in situ morphology characterization. Indeed, understanding solvent additives' effects on film formation has allowed them to be applied and combined effectively and synergistically to boost OPV performance. Their success as a morphology control strategy has also prompted the use of solvent additives in related organic semiconductor technologies. Finally, the role of solvent additives in the development of next-generation OPV active layers is discussed. Despite concerns over their environmental toxicity and role in device instability, solvent additives remain relevant morphological directing agents as research interests evolve toward nonfullerene acceptors, ternary blends, and environmentally sustainable solvents.

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“…Due to the observed miscibility (melting point depression) and increased repulsive interaction While the Flory-Huggins theory is widely utilised, there has been limited determination of χFH via direct methods for application within the OPV field. 100,258,385,[405][406][407][408][409] Furthermore, the majority of these reports have been released throughout the duration of this project, highlighting the relevance and increasing interest in such an analysis. In particular, only a handful of these reports 258,406,407 correlate the experimentally derived χFH to device output.…”

Section: Conclusion Of Thermal Behaviourmentioning

confidence: 99%

Development of a Library of Novel Compounds for Use in Organic Solar Cells

McGregor¹

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Currently, the most common method of optimising organic solar cells is to synthesise new materials before fabricating and testing devices under a wide range of processing conditions to empirically determine the optimum conditions, with other measurements supporting the explanation. This is both a laborious and material intensive process. Simplification of device optimisation processes would likely enable devices to be more easily manufactured at a lower cost. In turn, it would be expected for organic solar cells to become more accessible to the consumer, decreasing the need for fossil fuels. Two approaches to simplifying organic solar cell manufacture were investigated.First, a series of triphenylamine (TPA) centred materials were synthesised to act as nonfullerene acceptors. However, as all four synthesised TPA materials showed comparable optoelectronic properties which were complimentary to that of common donor P3HT, no distinction towards the relative device performance could be made based on these parameters alone. As bulk heterojunction (BHJ) morphology is critical to device performance, differential scanning calorimetry (DSC) in conjunction with X-ray diffraction (XRD) provided improved screening capabilities. When blended with P3HT at varying concentrations and deposited from solution, the morphological differences of the resulting blended films were probed by thermal methods to successfully predict the optimum donor:acceptor pair. Application of Flory-Huggins theory and Kyu plots to determine the interaction parameter ( 0 ) reveal that the materials must be miscible to enable low-energy interfaces, while phase separation into pure domains enables charges to be extracted from the device through continuous percolation pathways. The donor:acceptor pair with the highest positive value of 0 demonstrated a PCE of 2.0% and JSC above 4 mA/cm 2 , while the pair with the lowest 0 value lead to poor device performance (<0.4% PCE) due to excessive mixing. DSC methods were also able to provide a calculated critical solvation concentration (wcrit); below which the donor and acceptor were expected to become immiscible and perform poorly due to limited phase separation and non-ordered regimes. Additional insights provided by DSC analysis include the observation of a coldcrystallisation process; where liquidation upon heating nucleates crystallisation. Typically due to the entrapment of disordered phases, the DSC experiment was able to provide insight towards highly variable device performance, which otherwise may have been attributed to manufacturing errors.Secondly, further simplification of organic solar cells to homojunction (i.e., single component) devices was completed. Materials with low exciton binding energies are able to efficiently separate into free charge carriers and limit recombination pathways. The binding energy of an exciton is dependent on a number of factors, such as that described by the Wannier-Mott construct, which states that the exciton binding energy is proportional to the inverse square of the d...

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Critical factors governing vertical phase separation in polymer–PCBM blend films for organic solar cells

Abstract: This study presents an effective guide to vertical phase separation of polymer–fullerene blends based on systematic comparison of compatibility, crystallization, and processing conditions in observing the vertical morphology.

Cited by 67 publications

References 63 publications

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells

Solvent Additives: Key Morphology‐Directing Agents for Solution‐Processed Organic Solar Cells

Development of a Library of Novel Compounds for Use in Organic Solar Cells

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