Nucleation-limited fullerene crystallisation in a polymer–fullerene bulk-heterojunction blend

Lindqvist, Camilla; Sanz‐Velasco, Anke; Wang, Ergang; Bäcke, Olof; Gustafsson, Stefan; Olsson, Eva; Andersson, Mats R.; Müller, Christian

doi:10.1039/c3ta11018d

Cited by 63 publications

(108 citation statements)

References 27 publications

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“…Fullerenes have the strong tendency to vitrify, i.e., they form a molecular glass. [ 4,5 ] Typically, this resistance of fullerenes towards crystallization is further aggravated in bulk-heterojunction blends, in particular if the donor material is a high molecular weight polymer. [ 6 ] The most widely used fullerene derivative [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) has a high glass transition temperature T g PCBM ≈ 110-140 °C.…”

Section: Introductionmentioning

confidence: 99%

“…[7][8][9] As a result, polymer:fullerene blends tend to have a T g blend above ambient temperature. [ 5,7,9,10 ] Thus, it is possible to freeze in a certain degree of phase separation between PCBM and the donor polymer after evaporation of the processing solvent. However, upon heating above T g blend fullerene molecules start to diffuse in the polymer matrix, slowly aggregate and eventually form micrometer-sized crystals.…”

Section: Introductionmentioning

confidence: 99%

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Fullerene Nucleating Agents: A Route Towards Thermally Stable Photovoltaic Blends

Lindqvist

Bergqvist

Feng

et al. 2014

Advanced Energy Materials

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light-harvesting active layer is composed of bulk-heterojunctions, i.e., blends of a polymeric electron-donor (hole-conductor) and an electron-acceptor (electron-conductor), with fullerene derivatives yielding particularly promising results. Thanks to recent advances in the synthesis of donor materials, power-conversion effi ciencies of 8-10% can now be achieved with lab-scale devices. [ 1 ] The precise bulk-heterojunction nanostructure, i.e., the distribution of donor and acceptor molecules, is crucial for maximizing the photovoltaic performance for a given blend composition. This is because a compromise has to be made between two critical aspects: i) a large contact area between donor and acceptor molecules aids charge generation and thus a fi nely intermixed blend is favored, and ii) percolation of separated, relatively phase-pure donor and acceptor domains to improve charge transport to the electrodes. Therefore, the ideal nanostructure features an intermediate degree of phase separation, which has to be carefully optimized through processing parameters such as the choice of solvent or solvent mixture, the blend stoichiometry and polymer molecular weight as well as post-deposition thermal or vapor annealing.The bulk-heterojunction nanostructure of non-crystalline polymer:fullerene blends has the tendency to rapidly coarsen when heated above its glass transition temperature, which represents an important degradation mechanism. We demonstrate that fullerene nucleating agents can be used to thermally arrest the nanostructure of photovoltaic blends that comprise a non-crystalline thiophene-quinoxaline copolymer and the widely used fullerene derivative [6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM). To this end, C 60 fullerene is employed to effi ciently nucleate PCBM crystallization. Sub-micrometersized fullerene crystals are formed when as little as 2 wt% C 60 with respect to PCBM is added to the blend. These reach an average size of only 200 nanometers upon introduction of more than 8 wt% C 60 . Solar cells based on C 60 -nucleated blends indicate signifi cantly improved thermal stability of the bulk-heterojunction nanostructure even after annealing at an elevated temperature of 130 °C, which lies above the glass transition temperature of the blend. Moreover, we fi nd that various other compounds, including C 70 fullerene, single-walled carbon nanotubes, and sodium benzoate, as well as a number of commercial nucleating agents-commonly used to clarify isotactic polypropylene-permit to control crystallization of the fullerene phase.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Fullerene Nucleating Agents: A Route Towards Thermally Stable Photovoltaic Blends

Lindqvist

Bergqvist

Feng

et al. 2014

Advanced Energy Materials

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“…[26,27] Furthermore, it was shown that mixtures of different fullerenes can improve the thermal stability of BHJs. [28][29][30][31][32] Despite an intense research, known techniques to stabilize BHJs either reduce only partially thermal degradation or produce poor device efficiencies. So it is a primary challenge for processing of thermally stable fullerenebased high-efficiency PSCs to identify novel strategies for inhibiting fullerene diffusion inside BHJs.…”

Section: Introductionmentioning

confidence: 99%

Toward High‐Temperature Stability of PTB7‐Based Bulk Heterojunction Solar Cells: Impact of Fullerene Size and Solvent Additive

Dkhil

Pfannmöller

Saba

et al. 2016

Advanced Energy Materials

105

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The use of fullerene as acceptor limits the thermal stability of organic solar cells at high temperatures as their diffusion inside the donor leads to phase separation via Ostwald ripening. Here it is reported that fullerene diffusion is fully suppressed at temperatures up to 140 °C in bulk heterojunctions based on the benzodithiophene‐based polymer (the poly[[4,8‐bis[(2‐ethylhexyl)oxy]‐benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]‐thieno[3,4‐b]thiophenediyl]], (PTB7) in combination with the fullerene derivative [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC70BM). The blend stability is found independently of the presence of diiodooctane (DIO) used to optimize nanostructuration and in contrast to PTB7 blends using the smaller fullerene derivative PC70BM. The unprecedented thermal stability of PTB7:PC70BM layers is addressed to local minima in the mixing enthalpy of the blend forming stable phases that inhibit fullerene diffusion. Importantly, although the nanoscale morphology of DIO processed blends is thermally stable, corresponding devices show strong performance losses under thermal stress. Only by the use of a high temperature annealing step removing residual DIO from the device, remarkably stable high efficiency solar cells with performance losses less than 10% after a continuous annealing at 140 °C over 3 days are obtained. These results pave the way toward high temperature stable polymer solar cells using fullerene acceptors.

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“…In accordance with our previous report TQ1:PC 61 BM films feature micrometer-sized PC 61 BM crystals, as confirmed by single-crystal like electron diffraction patterns that agree with previously published work. 9,10 In contrast, for TQ1:PCBM 8:2 , we observe a coarser nanostructure with $50 nanometer-large domains that remain disordered as evidenced by the conspicuous absence of sharp diffraction spots in corresponding electron diffraction patterns.…”

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confidence: 77%

“…This is corroborated by UV-vis spectra of annealed films, which show a clear decrease in light transmission below the bandgap of the polymer for TQ1:PC 61 BM but not TQ1:PCBM 8:2 after annealing for 10 min at 140 C. 25 We explain the observed invariance in light transmission for samples containing PCBM mixtures with the absence of light scattering from microscopic fullerene crystals. 6,10,26 Evidently, the use of PCBM mixtures that contain 20 wt. % PC 71 BM strongly hinders fullerene crystallization.…”

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confidence: 99%

Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend

Lindqvist

Bergqvist

Bäcke

et al. 2014

Applied Physics Letters

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Printing of polymer:fullerene solar cells at high speed requires annealing at temperatures up to 140 °C. However, bulk-heterojunction blends that comprise a non-crystalline donor polymer often suffer from insufficient thermal stability and hence rapidly coarsen upon annealing above the glass transition temperature of the blend. In addition, micrometer-sized fullerene crystals grow, which are detrimental for the solar cell performance. In this manuscript, we present a strategy to limit fullerene crystallization, which is based on the use of fullerene mixtures of the two most common derivatives, PC61BM and PC71BM, as the acceptor material. Blends of this fullerene mixture and a non-crystalline thiophene-quinoxaline copolymer display considerably enhanced thermal stability and largely retain their photovoltaic performance upon annealing at elevated temperatures as high as 170 °C.

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Nucleation-limited fullerene crystallisation in a polymer–fullerene bulk-heterojunction blend

Cited by 63 publications

References 27 publications

Fullerene Nucleating Agents: A Route Towards Thermally Stable Photovoltaic Blends

Fullerene Nucleating Agents: A Route Towards Thermally Stable Photovoltaic Blends

Toward High‐Temperature Stability of PTB7‐Based Bulk Heterojunction Solar Cells: Impact of Fullerene Size and Solvent Additive

Fullerene mixtures enhance the thermal stability of a non-crystalline polymer solar cell blend

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