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

Lindqvist, Camilla; Bergqvist, Jonas; Bäcke, Olof; Gustafsson, Stefan; Wang, Ergang; Olsson, Eva; Inganäs, Olle; Andersson, Mats R.; Müller, Christian

doi:10.1063/1.4870997

Cited by 50 publications

(53 citation statements)

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confidence: 99%

“…This observation may be explained by the reduced tendency of fullerenes to crystallize upon mixing, which is well documented for substituted fullerenes (PCBMs) and leads to the formation of disordered thin films. [15][16][17] We note that as a result of improved film formationFETs based on fullerene mixtures allow for greater reproducibility. The FET device characteristics are largely independent of the fullerene ratio with a threshold voltage ℎ ~ 6-7…”

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High‐Entropy Mixtures of Pristine Fullerenes for Solution‐Processed Transistors and Solar Cells

et al. 2015

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cells 2 / 18Fullerenes are an intriguing class of organic semiconductors that feature a high electron affinity and exceptional charge transport properties, desirable for a number of thin-film optoelectronic applications such as field-effect transistors (FETs) and organic solar cells. Despite their desirable solid-state properties a major disadvantage of pristine C60 and C70 is the seemingly poor solubility in organic solvents, [1,2] which however can be substantially improved by attaching exohedral moieties to the fullerene cage, albeit at the expense of a high electron mobility. [3][4][5] In addition, pristine fullerenes suffer from the strong tendency to crystallize during solvent removal, which complicates formation of continuous thin films. As a result, substituted fullerenes, such as in particular phenyl-Cx-butyric acid methyl esters (PCBMs; x = 61 or 71), are by far the most widely studied electron acceptor material for organic photovoltaic applications. The trade-off between increased solubility and reduced electron mobility has so far been acceptable for thin-film solar cells because the latter is still sufficiently high to not limit solar cell performance, which more strongly depends on the precise active layer nanostructure. However, as future improvements in device efficiency rely on thick active layers and high-mobility materials, [6] the use of pristine fullerenes may prove to be advantageous provided that solubility issues can be addressed and a favorable nanostructure can be achieved.The most efficient active layer nanostructure of organic solar cells is the bulkheterojunction, which is typically prepared by co-processing a fullerene acceptor and a suitable donor from a common organic solvent. Although devices based on the pristine fullerene C70 have been reported for both polymer and small-molecule donors with a powerconversion efficiency PCE of 5.1 % and 5.9 %, respectively, [7,8] the most competitive results are currently achieved with the more soluble PCBMs, which can yield a PCE of more than 10 %. [9] As low cost is the major selling point of organic photovoltaics, it is desirable to choose the most cost-effective materials. In this regard, the use of PCBMs as the electron 3 / 18 acceptor is not optimal. The additional synthesis steps required to prepare these derivatives from pristine fullerenes considerably increase the energy footprint, which implies a higher environmental impact as well as an overall higher materials cost. For instance, Anctil et al.calculated a life cycle embodied energy as low as about 8 GJ kg -1 for the as-synthesized C60:C70 mixture, which increases to 25 GJ kg -1 for C60 and 38 GJ kg -1 for C70 after separation and electronic-grade purification. [10] In contrast, preparation of substituted fullerenes consumes significantly more energy, i.e. 65 GJ kg -1 and 90 GJ kg -1 for electronic-grade PC61BM and PC71BM, respectively. Therefore, the use of pristine fullerenes and in particular C60:C70 mixtures may result in significant energy savings provided that the ability to f...

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High‐Entropy Mixtures of Pristine Fullerenes for Solution‐Processed Transistors and Solar Cells

et al. 2015

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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.…”

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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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“…12 Another promising concept to reduce degradation in fullerene-based solar cells is the use of third components which can improve the mechanical, photo, air and thermal stability of organic solar cells. 13 Such third components can be crosslinkers, [14][15][16] conventional fullerenes, [17][18][19] modified fullerene derivatives, [20][21][22] compatibilizers 23,24 or insulating polymers. 25 In this work, we overcome the aggregation of the fullerene acceptor PC 61 BM during thermal annealing by employing ternary D-D-A blends comprising two commercially available donor polymers.…”

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confidence: 99%

Enhanced thermal stability of organic solar cells comprising ternary D-D-A bulk-heterojunctions

et al. 2017

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Ternary absorber blends have recently been identified as promising concepts to spectrally broaden the absorption of organic bulkheterojunction solar cells and hence to improve their power conversion efficiencies. In this work, we demonstrate that D-D-A ternary blends comprising two donor polymers and the acceptor PC 61 BM can also significantly enhance the thermal stability of the solar cell. Upon harsh thermal stress at 120°C for 2 h, the ternary solar cells show only a minor relative deterioration of 10%. Whereas the polymer/fullerene blend PTB7-Th:PC 61 BM is rather unstable under these conditions, its degradation was efficiently suppressed by incorporating the near infrared-absorbing polymer PDTP-DFBT. Spectroscopic ellipsometry investigations and an effective medium analysis of the ternary absorber blend revealed that the domain conformation in presence of PDTP-DFBT remains stable whereas the domain conformation changes in its absence. The ternary PTB7-Th:PDTP-DFBT:PC 61 BM solar cells yield thermally stable power conversion efficiencies of up to 6%.npj Flexible Electronics (2017) 1:11 ; doi:10.1038/s41528-017-0011-z INTRODUCTIONThe continuous improvement of materials and device architectures for organic bulk-heterojunction solar cells nowadays constantly enables power conversion efficiencies (PCEs) beyond 10%.1, 2 While the PCEs of state-of-the-art organic solar cells are sufficient to meet the demands of some first applications and hence to foster market entry, the device stability often drags behind. The stability of organic solar cells is often limited by a metastable bulk-heterojunction morphology, the diffusion of electrode or buffer layer components into adjacent layers, material decomposition through oxygen and water ingress, irradiation, mechanical stress or heat.3-5 Yet, excellent thermal stability of the organic solar cells is a prerequisite for most future applications. Besides thermal stress when the solar cell is exposed to the sun, the devices may have to endure harsh conditions during fabrication, e.g., when manufacturing car roofs, tiles or façade elements, typically requiring lamination temperatures of 120°C for several hours. 6,7 Upon thermal stress, one of the major degradation mechanisms is the aggregation of fullerene acceptors, commonly [6,6]-phenyl C 71 -butyric acid methyl ester (PC 71 BM) or [6,6]-phenyl C 61 -butyric acid methyl ester (PC 61 BM). 8,9 In the recent literature, several methods were discussed to hamper fullerene aggregation-for example sample irradiation during annealing, 10 structural modification of fullerenes 11 or replacing fullerenes with non-fullerene acceptors.12 Another promising concept to reduce degradation in fullerene-based solar cells is the use of third components which can improve the mechanical, photo, air and thermal stability of organic solar cells.13 Such third components can be crosslinkers, [14][15][16] In this work, we overcome the aggregation of the fullerene acceptor PC 61 BM during thermal annealing by employing ternary D-D-A blends comprisi...

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

Cited by 50 publications

References 25 publications

High‐Entropy Mixtures of Pristine Fullerenes for Solution‐Processed Transistors and Solar Cells

High‐Entropy Mixtures of Pristine Fullerenes for Solution‐Processed Transistors and Solar Cells

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

Enhanced thermal stability of organic solar cells comprising ternary D-D-A bulk-heterojunctions

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