Relating Structure to Efficiency in Surfactant-Free Polymer/Fullerene Nanoparticle-Based Organic Solar Cells

Abstract

Nanoparticle dispersions open up an ecofriendly route toward printable organic solar cells. They can be formed from a variety of organic semiconductors by using miniemulsions that employ surfactants to stabilize the nanoparticles in dispersion and to prevent aggregation. However, whenever surfactant-based nanoparticle dispersions have been used to fabricate solar cells, the reported performances remain moderate. In contrast, solar cells from nanoparticle dispersions formed by precipitation (without surfactants) can exhibit power conversion efficiencies close to those of state-of-the-art solar cells processed from blend solutions using chlorinated solvents. In this work, we use small-angle neutron scattering measurements and transient absorption spectroscopy to investigate why surfactant-free nanoparticles give rise to efficient organic solar cells. We show that surfactant-free nanoparticles comprise a uniform distribution of small semiconductor domains, similar to that of bulk-heterojunction films formed using traditional solvent processing. This observation differs from surfactant-based miniemulsion nanoparticles that typically exhibit core-shell structures. Hence, the surfactant-free nanoparticles already possess the optimum morphology for efficient energy conversion before they are assembled into the photoactive layer of a solar cell. This structural property underpins the superior performance of the solar cells containing surfactant-free nanoparticles and is an important design criterion for future nanoparticle inks.

Recent advances in the green processing of organic photovoltaic devices from nanoparticle dispersions

“…The phase separation into NPs has been studied by SANS with contrast variation for composite P3HT:ICBA NPs prepared by nanoprecipitation from chloroform solution into methanol. 46 It was confirmed that the two materials were homogeneously distributed in the NPs having a diameter of around 100 nm. By SANS, it was not possible to observe phase separation down to the 10 nm size observable by this technique.…”

Nanomorphology of eco-friendly colloidal inks, relating non-fullerene acceptor surface energy to structure formation

“…We note that the nanoprecipitation method, compared to the miniemulsion method, has been previously reported to produce nanoparticles with a predominantly uniform phase separation. 42,[68][69][70] Our study constitutes the first report of the nanoscale morphology of P3HT:eh-IDTBR nanoparticles prepared from both the nanoprecipitation and the miniemulsion method, both being core-shell in nature. It is possible that for the nanoprecipitated P3HT:eh-IDTBR nanoparticles a uniform phase separation formed inititally, but then self-assembly to a core-shell structure occurred during the 3 hour 60 1C THF solvent removal step (from the continuous aqueous phase), a temperature above the reported T g of P3HT.…”

Can Polymer Solar Cells Open the Path to Sustainable and Efficient Photovoltaic Windows Fabrication?

“…and Gärtner et al . both argued that better mixing of P3HT and PC 61 BM or ICBA can be achieved in NPs prepared using reprecipitation compared to miniemulsion . The as cast P3HT : ICBA NP PSCs have low PCEs of 0.1 % which can be gradually increased to 1.2 %, 3.0 %, 3.7 % and 3.9 % by applying thermal annealing at 100 °C, 150 °C, 180 °C and 200 °C, respectively.…”

“…Schwarz et al and Gärtner et al both argued that better mixing of P3HT and PC 61 BM or ICBA can be achieved in NPs prepared using reprecipitation compared to miniemulsion. [67,68] The as cast P3HT: ICBA NP PSCs have low PCEs of 0.1 % which can be gradually increased to Adapted with permission from, [49] Copyright 2017, Elsevier B.V. (b) Schematic representation of the miniemulsion process. Adapted with permission from, [55] Copyright 2017, American Chemical Society .…”