Polymer-Fullerene Miscibility: A Metric for Screening New Materials for High-Performance Organic Solar Cells

Abstract

The improvement of the power conversion efficiency (PCE) of polymer bulk heterojunction (BHJ) solar cells has generally been achieved through synthetic design to control frontier molecular orbital energies and molecular ordering of the electron-donating polymer. An alternate approach to control the PCE of a BHJ is to tune the miscibility of the fullerene and a semiconducting polymer by varying the structure of the fullerene. The miscibility of a series of 1,4-fullerene adducts in the semiconducting polymer, poly(3-hexylselenophene), P3HS, was measured by dynamic secondary ion mass spectrometry using a model bilayer structure. The microstructure of the bilayer was investigated using high-angle annular dark-field scanning transmission microscopy and linked to the polymer-fullerene miscibility. Finally, P3HS:fullerene BHJ solar cells were fabricated from each fullerene derivative, enabling the correlation of the active layer microstructure to the charge collection efficiency and resulting PCE of each system. The volume fraction of polymer-rich, fullerene-rich, and polymer-fullerene mixed domains can be tuned using the miscibility leading to improvement in the charge collection efficiency and PCE in P3HS:fullerene BHJ solar cells. These results suggest a rational approach to the design of fullerenes for improved BHJ solar cells.

Domain Compositions and Fullerene Aggregation Govern Charge Photogeneration in Polymer/Fullerene Solar Cells

“…Previous efforts have identifi ed that the miscibility between conjugated polymers and fullerene can affect the morphological evolution, [ 10,20,59,67,68 ] charge transport, [ 15,69 ] and bimolecular charge recombination. [ 16,70 ] Here, the combination of device testing, morphological characterization and spectroscopy suggests that the miscibility between conjugated polymers and fullerene can also affect polaron yields at ultrafast time scales.…”

“…[ 13,14 ] Variations in the fi lm compositions by 10-20% can affect charge mobilities by orders of magnitude, [ 15 ] and consequently, modulate bimolecular charge recombination. [ 16 ] Furthermore, the amorphous materials within mixed phases are expected to have a larger band gap than pure (crystalline) phases, creating an energetic driving force to push carriers away from mixed regions and thereby potentially suppress charge recombination. [17][18][19] Collins et al have hypothesized that exciton dissociation could be enhanced within molecularly mixed phases due to close contact between donor and acceptor molecules, but likewise, charge recombination should also increase.…”

Quantitative Analysis of the Molecular Dynamics of P3HT:PCBM Bulk Heterojunction

“…This has already been evidenced for the bis and tris adduct of PCBM by differential scanning calorimetry, 36 and replacing PCBM side chain has been proven an effective method to reduce the fullerene miscibility with poly(3-hexylselenophene), ,P3HS. 34 Recently, advances have been made in the synthesis of non-fullerene acceptors. 37 A trade-off between planarity and miscibility has to be reached in order to create a suitable blend microstructure for efficient OPV devices.…”

Quantification of Nano‐ and Mesoscale Phase Separation and Relation to Donor and Acceptor Quantum Efficiency, Jsc, and FF in Polymer:Fullerene Solar Cells

“…[ 5,8,[32][33][34][35] This has caused a re-evaluation of the actual morphologies, initiated by the observation of miscibility of fullerene in many donor polymers. [ 8,23,[36][37][38] Given the apparent universality of miscibility and thus the likely presence of a mixed phase, the presence of at least one, if not two pure phases was thought to be benefi cial for good fi ll factor the morphology of the promising low bandgap polymer poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo [3,4-c] pyrrole-1,4-diyl}-alt -{[2,2′:5′,2′′-terthiophene]-5,5′′-diyl}] and [6,6]phenyl C71 butyric acid methyl ester, PDPP3T:PC 71 BM-based device. This complements a recent report on use of ternary solvents to optimize, for the time being, the same DPP based system.…”