Axisymmetric and Asymmetric Naphthalene-Bisthienothiophene Based Nonfullerene Acceptors: On Constitutional Isomerization and Photovoltaic Performance

Abstract

Two pairs of constitutional isomers of fused-octacyclic nonfullerene acceptors (NFAs) based on a naphthalene-bisthienothiophene core with or without fluorination at the ending groups have been developed. Compared with the axisymmetric NFAs N66-IC and N66-2FIC with two six-member-ring bridges, their asymmetric constitutional isomers N65-IC and N65-2FIC both with one six-member-ring bridge and one five-member-ring bridge exhibit remarkable red-shifted absorption, higher crystallinity, and slightly down-shifted LUMO energy levels. Organic solar cells based on PBDB-T-2F:N65-2FIC achieved a promising power conversion efficiency of 10.19%, which is three times higher than that of its counterpart PBDB-T-2F:N66-2FIC cell (3.46%). While being blended with PBDB-T as the donor material, the asymmetric acceptor analogue N65-IC based solar cell pronounces a PCE of 9.03%, being significantly improved from that of 5.45% for the PBDB-T:N66-IC based cell, which is in consistency with the results from those cells from their both fluorinated donor and acceptor counterparts. Design rules on either both fluorinated, both nonfluorinated, or cross-combined donor/acceptors for device fabrication has been explored. In addition, PBDB-T-2F:N65-2FIC possesses very promising device stability with 85% of its initial PCE after an exposure time of 1500 h under one sun illumination, which is meaningful for their future commercial devices.

Recent Developments of Polymer Solar Cells with Photovoltaic Performance over 17%

“…[57] Moreover, asymmetric end groups are also the research direction of ADA'DA-type acceptors. [58][59][60] Compared with asymmetric side-chain and central core modification, end group is more efficient in shortening synthesis time. [61,62] Yan et al [63] studied the effect of asymmetry halogen substitution on the properties of materials.…”

Computational Evolution of High-Performing Unfused Non-Fullerene Acceptors for Organic Solar Cells

“…While A-DA D-A is the most common sequence, attributed to the high success of this sequence in Y-series NFAs, other sequences are emerging as strong contenders, with asymmetry showing much promise 19 . Asymmetric NFAs may have larger dipole moments, broader absorption, higher electron mobility, and lower crystallinity than their symmetric counterparts [31][32][33][34][35] .…”

p-Tetrafluorophenylene Divinylene-Bridged Nonfullerene Acceptors as Binary Components or Additives for High-Efficiency Organic Solar Cells

“…Organic photovoltaic (OPV) devices possess more advantageous features and potential than do inorganic solar cells in terms of their lighter weight, semitransparency, and ability to be fabricated through printing on flexible carriers. − The evolution of nonfullerene acceptors as n-type materials has progressively advanced the development and applicability of OPV devices. − Furthermore, the modification of nonfullerene acceptors with fused-ring electron acceptors through electronic effects can improve the power conversion efficiency (PCE) of devices; , specifically, the PCE of OPV devices has been increased to 17–18% using modification techniques involving the displacement of the core structure of fused-ring aromatics, modification of soluble side chains, and incorporation of electron-poor end groups. − The modification engineering of such fused-ring aromatics aims at creating near-infrared absorption, , which relies on the extension of the number of fused rings, − unidirectional extension of fused rings, varying constitutional isomers of the core structure, , and linearity of the core . The number of fused-ring units could alter the optical and electrochemical properties of the molecule acceptors; accordingly, the PCE can be improved through the engineering of fused-ring units. − …”