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

Abstract

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...

Radio Frequency Coplanar ZnO Schottky Nanodiodes Processed from Solution on Plastic Substrates

“…The electron mobility in these fi lms was evaluated using fi eld-effect measurements yielding values up to 2 cm 2 V −1 s −1 . [ 21,24 ] The latter value is among the highest reported to date for any solution-processed molecular semiconductor, hence making C 60 highly attractive for application in high-performance RF diodes. temperature in nitrogen atmosphere.…”

“…Although electron‐transporting methano‐fullerenes are particularly attractive due to their solubility, unsubstituted C 60 offers significantly higher electron mobility and a significant cost advantage . Unfortunately, solution deposition of pure C 60 layers is troublesome due to the hydrophobicity of C 60 . To circumvent this problem, we blended C 60 (20 mg mL −1 ) with high molecular weight polystyrene (5 wt%) acting as the binder.…”

“…Using this formulation we were able to grow uniform C 60 :polystyrene blend films up to 700 nm in thickness. The electron mobility in these films was evaluated using field‐effect measurements yielding values up to 2 cm 2 V −1 s −1 . The latter value is among the highest reported to date for any solution‐processed molecular semiconductor, hence making C 60 highly attractive for application in high‐performance RF diodes.…”

“…[ 23 ] Unfortunately, solution deposition of pure C 60 layers is troublesome due to the hydrophobicity of C 60 . [ 21,24 ] To circumvent this problem, we blended C 60 (20 mg mL −1 ) with high molecular weight polystyrene (5 wt%) acting as the binder. Using this formulation we were able to grow uniform C 60 :polystyrene blend fi lms up to 700 nm in thickness.…”

Tailorable PC71BM Isomers: Using the Most Prevalent Electron Acceptor to Obtain High‐Performance Polymer Solar Cells

“…Efforts have primarily been directeda t the pairs C 60 -C 70 , [7] C 60 -PC 61 BM, [8] PC 71 BM-IC 60 BA, [9] PC 61 BM-PC 71 BM. Efforts have primarily been directeda t the pairs C 60 -C 70 , [7] C 60 -PC 61 BM, [8] PC 71 BM-IC 60 BA, [9] PC 61 BM-PC 71 BM.…”