Compact Bis-Adduct Fullerenes and Additive-Assisted Morphological Optimization for Efficient Organic Photovoltaics

Abstract

Bis-adduct fullerenes surrounded by two insulating addends sterically attenuate intermolecular interaction and cause inferior electron transportation. In this research, we have designed and synthesized a new class of bis-adduct fullerene materials, methylphenylmethano-C60 bis-adduct (MPC60BA), methylthienylmethano-C60 bis-adduct (MTC60BA), methylphenylmethano-C70 bis-adduct (MPC70BA), and methylthienylmethano-C70 bis-adduct (MTC70BA), functionalized with two compact phenylmethylmethano and thienylmethylmethano addends via cyclopropyl linkages. These materials with much higher-lying lowest unoccupied molecular orbital (LUMO) energy levels successfully enhanced the Voc values of the P3HT-based solar cell devices. The compact phenylmethylmethano and thienylmethylmethano addends to promote fullerene intermolecular interactions result in aggregation-induced phase separation as observed by the atomic force microscopy (AFM) and transmission electron microscopy (TEM) images of the poly(3-hexylthiophene-2,5-diyl) (P3HT)/bis-adduct fullerene thin films. The device based on the P3HT/MTC60BA blend yielded a Voc of 0.72 V, a Jsc of 5.87 mA/cm(2), and a fill factor (FF) of 65.3%, resulting in a power conversion efficiency (PCE) of 2.76%. The unfavorable morphologies can be optimized by introducing a solvent additive to fine-tune the intermolecular interactions. 1-Chloronaphthalene (CN) having better ability to dissolve the bis-adduct fullerenes can homogeneously disperse the fullerene materials into the P3HT matrix. Consequently, the aggregated fullerene domains can be alleviated to reach a favorable morphology. With the assistance of CN additive, the P3HT/MTC60BA-based device exhibited enhanced characteristics (a Voc of 0.78 V, a Jsc of 9.04 mA/cm(2), and an FF of 69.8%), yielding a much higher PCE of 4.92%. More importantly, the additive-assisted morphological optimization is consistently effective to all four compact bis-adduct fullerenes regardless of the methylphenylmethano or methylthienylmethano scaffolds as well as C60 or C70 core structures. Through the extrinsic additive treatment, these bis-adduct fullerene materials with compact architectures show promise for high-performance polymer solar cells.

Morphological Control of Donor/Acceptor Interfaces in All-Polymer Solar Cells Using a Pentafluorobenzene-Based Additive

“…Various approaches have been attempted to improve the blend morphology of the interconnected network of polymer donors and acceptors, such as adjusting the polymer molecular weight, − D/A blending ratios, ,, thermal annealing, − and solvent additive processing. − Among these processing methods, solvent additive processing is considered as an effective and simple way to control the nanoscale structure of phase separation and polymer ordering in D/A blend systems. − Neher et al demonstrated that 1-chloronaphthalene (CN) (1:1 p -xylene:CN) can suppress the preaggregation of the acceptor polymer, poly­[[ N , N ′-bis­(2-octyldodecyl)­napthalene-1,4,5,8-bis­(dicarboximide)-2,6-diyl]- alt -5,5′-(2,2′-bithiophene)] (P­(NDI2OD-T2)) in a poly­(3-hexylthiophene-2,5-diyl) (P3HT)/P­(NDI2OD-T2) blend film and showed that this phenomenon exhibited a large increase in J SC . Hou et al also reported an increased FF in a blend system consisting of benzo­[1,2- b :4,5- b ′]­dithiophene (BDT)–thiophene donor polymer (PBDTBDD-T) and BDT–naphthalene diimide (NDI) acceptor polymer (PBDTNDI-T) after processing chlorobenzene (CB) with 3 vol % CN and showed that this process induced high domain purity and improved the molecular ordering of the blend film.…”

High LUMO energy pyrrolidinofullerenes as promising electron-acceptor materials for organic solar cells

“…6 It is notable that biscycloadducts work well only in combination with crystalline polymers such as P3HT and PBDTBDD. [3][4][5][6][7][8][9][10][11][12][13] The crystallization of the polymer during film formation also induces ordering of the fullerene phase. Furthermore, different isomers of biscycloadducts have rather different LUMO energies which induce energetic disorder in the material represented by a complex isomer mixture.…”

The C70 Fullerene Adducts with Low Anisotropy of Polarizability are More Efficient Electron Acceptors for Organic Solar Cells. The Minimum Anisotropy Hypothesis for Efficient Isomer-Free Fullerene-Adduct Photovoltaics

“…Identification, chromatographic separation, and purification of such isomers are very hard, nontrivial, and expensive tasks. Therefore, in the first works, fullerene bis- and trisadducts were introduced into photovoltaic devices as mixtures of regioisomers. ,− Later, it has been recognized that using the purified regioisomers substantially enhances key output parameters of OSCs. This has been first demonstrated with the solar cells based on the substituted bis­(dihyrdonaphtyl)­C 60 adducts in the foremost work .…”