Iodine‐Stabilized Organic Nanoparticle Dispersions for the Fabrication of 10% Efficient Non‐Fullerene Solar Cells

Abstract

An entirely different route to the ecofriendly processing of organic semiconductor thin films is their deposition from aqueous or alcoholic dispersions, [4] reinforcing the unique environmental sustainability of organic solar cells. Recent reports also featured acetonitrile as a promising dispersion agent due to its high permittivity and hence its excellent screening of stabilizing charges. [5] Besides the omission of toxic solvents during coating, this deposition route i) disentangles solution processing from the need of solubility, ii) advances multi-layer deposition without the need for orthogonal solvents and hence iii) is less dependent on molecular engineering, eventually giving access to a broader choice of semiconductors for optoelectronic applications. [6] The prevailing challenge of this approach is the colloidal stabilization of the dispersion. [7,8] Only very few organic semiconductors exhibit sufficient intrinsic colloidal stability, with its most prominent example being poly(3-hexylthiophene-2,5-diyl) (P3HT). Its high intrinsic colloidal stability allows the synthesis of P3HT nanoparticle dispersions by rapid solvent exchange, where a P3HT solute is nanoprecipitated upon injection into miscible ethanol. [9,10] The high colloidal stability of P3HT also enables the formation of nanoparticle dispersions of blends of P3HT and indene-C 60 bisadduct (ICBA). Such P3HT:ICBA nano particle dispersions can be synthesized with reasonably high concentrations and can be used as inks to fabricate solar cells with maximum power conversion efficiencies (PCEs) of 4.5%. [11] Other organic semiconductors such as the latest highperformance combinations of polymers and non-fullerene acceptors do not exhibit this intrinsic colloidal stability. To disperse organic semiconductors that would otherwise coagulate rapidly, surfactants have been introduced as stabilizing agents. Best results on the stabilization of organic nanoparticle dispersions were achieved by employing poloxamers, but extensive subsequent purification steps were needed to reduce the final surfactant content which otherwise would have remained in the light-harvesting layer where it would have hampered the solar cell performance. [12,13] Still, this concept produced nanoparticulate solar cells with PCEs of 7.5%. [8] Recently, we found that the intrinsic colloidal stability of P3HT stems from its unusual but well-known tendency to electrically charge itself which in turn fosters electrostatic repulsion of the nanoparticles. [7] Accordingly, extrinsic oxidation of the light-harvesting polymers, e.g. by electrical p-doping with F 4 TCNQ or temporarily even by photodoping with visible light, can enhance the colloidal stability of nanoparticle dispersions. [5,7] Yet, the large ionization potentials of most high-performance light-harvesting polymers render High-performance organic solar cells are deposited from eco-friendly semiconductor dispersions by applying reversible electrostatic stabilization while omitting the need for stabilizing surfactants. The addition ...

Sub-4 nm mapping of donor–acceptor organic semiconductor nanoparticle composition

“…55 High efficiency OPV devices have been achieved with nanoparticles presenting intermixed morphology, generally obtained from the nanopre- cipitation method, but also the miniemulsion method in several cases. 8,13,56,57 One can highlight the 7.5% PCE achieved for the PBQ-QF:ITIC system 8 from water-based dispersions and 4.3% for P3HT:ICBA 58 from alcohol-based dispersions. It has been shown in several studies by the Venkataraman group 59,60 that, if the surfactant is efficiently removed, the charge transport in OPV devices is not drastically affected and it is comparable to thin films deposited from organic solvents.…”

Green‐Processed Non‐Fullerene Organic Solar Cells Based on Y‐Series Acceptors

“…Achieving high performance requires careful consideration of all these factors. [82][83][84][85][86][87][88][89][90][91][92][93][94][95] Numerous studies have been conducted on the BHJ systems of P3HT:PCBM or P3HT:ICBA, demonstrating that surfactant-free nanoparticles can lead to PCEs of around 4% in OSCs [84] However, it has been observed that other "donor-acceptor" conjugated polymers and non-fullerene acceptors cannot remain stable without surfactants, as they tend to become electrically charged and subsequently repel the nanoparticles. [96] While surfactants play a crucial role in the formation of stable nanoparticles, they also possess an insulating nature that can have negative effects on electronic properties.…”

Water-based solar cells over 10% efficiency: designing soft nanoparticles for improved processability

“…15 It is worth mentioning that a few research teams have explored the use of media other than water, such as alcohols or acetonitrile. 16,17…”

“…15 It is worth mentioning that a few research teams have explored the use of media other than water, such as alcohols or acetonitrile. 16,17 Two preparation techniques are mainly used to produce aqueous dispersions: miniemulsion and nanoprecipitation. There are fundamental differences between the mechanisms of each technique, influencing nanoparticle formation.…”