Impact of Fullerene Structure on Nanoscale Morphology and Miscibility and Correlation of Performance on Small Molecules: Fullerene Solar Cell

Abstract

This manuscript reports the impact of fullerene structure on the morphology and miscibility of small molecules via a fullerene bulk heterojunction solar cell. The small angle neutron scattering and neutron reflectometry measurements were analyzed to provide quantifiable measures of the morphology of the resultant mixtures, offering miscibility, domain sizes, interfacial area between the small molecule and fullerene, and depth profiles in the mixtures. These results indicate that the bis-adduct fullerenes exhibit lower miscibility in small molecules. Correlation of miscibility and morphology to photovoltaic properties indicates that small molecule/fullerene miscibility is crucial to rationally optimize the design of fullerenes for use in small molecule organic photovoltaics. A higher open circuit voltage was obtained for bis-adduct fullerene devices which, however, does not translate to an increased power conversion efficiency. This decrease in performance is associated with the lower miscibility of bis-fullerene, which decreases the probability of the dissociation of excitons and enhances charge recombination rate in the miscible region. A quantitative analysis shows that an increase in the average separation of fullerenes in the miscible region is detrimental to electron transport in the miscible region, especially for a distance greater than ∼11 Å.

Scattering techniques for mixed donor–acceptor characterization in organic photovoltaics

“…Most successful, however, have been X-ray and neutron scattering techniques including grazing incidence wide angle X-ray scattering (GIWAXS), 6,7,10,17,18 resonant soft X-ray scattering (RSoXS), 5,6,[8][9][10][19][20][21] and small angle neutron scattering (SANS). [22][23][24] In this focus article we will provide background on the working principles of GIWAXS, RSoXS, and SANS and how they can be leveraged to characterize pure and mixed phases in BHJ films. Then, we will review the current literature characterizing mixed phase in fullerene systems and the potential challenges and outlook for characterizing mixed phases in emerging non-fullerene acceptor systems.…”

“…Small angle neutron scattering (SANS) has also been used to characterize the extent of mixing in BHJs. [22][23][24][40][41][42] In SANS, neutrons instead of X-rays are used to probe samples. Therefore, contrast in neutron scattering arises from nuclear interactions instead of electronic interactions as for X-ray scattering.…”

Impact of Fullerene Intercalation on Structural and Thermal Properties of Organic Photovoltaic Blends

“…The acceptor diameter varies from 15 Å (PC70BM) to 20 Å (bis-PC60BM). 15 The neat polymer predominantly absorbs between 400 nm and 650 nm, with evidence of a shoulder at low energy ( Table 1). The main fullerene absorption band lies at higher energy (wavelengths less than 350 nm).…”

Key Tradeoffs Limiting the Performance of Organic Photovoltaics

“…A significant issue is that it is still not fully clear what an ideal morphology should look like at various length scales of the BHJ, and whether there is a unique good morphology. Vertical segregation, (either obtained from different solubilities and interactions with substrates, or by changing the relative deposition rates during vacuum deposition), domain purity, connectivity, and cluster size have all been argued to be important. Furthermore, it is now clear that in binary polymer:fullerene and small molecule:C 60 blends, a mixed phase (can) coexist with the pure D an A domains .…”