Intermolecular Arrangement of Fullerene Acceptors Proximal to Semiconducting Polymers in Mixed Bulk Heterojunctions

Abstract

Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging,d espite the correlation between structural characteristics and efficiency in organic photovoltaics (OPVs). This study reveals that the substitution patterns of linear and branched alkyls ide chains on electron-donating/-accepting alternating copolymers can control the positions of an acceptor molecule (C 60 )around the p-conjugated main chains in mixed BHJs.T wo-dimensional solid-state NMR demonstrates am arked difference in the location of C 60 in the blend films.Acopolymer with an electron-accepting unit positioned in close proximityt oC 60 demonstrated higher OPV performance in combination with various fullerene derivatives.T his molecular design offers precise control over the interfacial molecular structure,thereby paving the way for overcoming the current limitations of OPVs comprising mixed BHJs.This decade has witnessed the remarkable development of organic photovoltaics (OPVs) as ap romising renewable energy source owing to their potential use in lightweight, flexible,a nd large-area devices. [1][2][3] This progress has been driven by the synthesis of novel semiconducting polymers with optical and electronic properties that are carefully tuned by the design of p-conjugated structures.D onor-acceptor (D-A) alternating copolymers consisting of electron-donating and -accepting units have been widely used as ak ey method for tuning electronic properties. [4][5][6][7][8][9] Thep hotoactive layers of highly efficient OPVs are mixed bulk heterojunction (BHJ) structures typically composed of ab lend of as emiconducting copolymer as the electron donor and fullerene derivatives as the electron acceptor. [10,11] Recently,the power conversion efficiency (PCE) of OPVs comprising BHJs of polymers and fullerene derivatives reached over 11 %PCE. [12] There have been substantial advances in understanding the mechanism of OPVs.I th as been shown that the properties of the organic interfaces strongly affect the charge separation and recombination processes. [13] Thec hargetransfer (CT) state at the interface has as trong correlation with the OPV performance. [14][15][16][17][18][19] Therefore,t he spatial arrangement of the donor and acceptor can strongly affect the photophysical and electronic processes in OPVs,a s suggested by theoretical calculations [20][21][22] and experimental studies using planar heterojunctions as model systems. [23][24][25][26] Taken together,t hese studies suggest that precise control of the BHJ structure at the molecular level could be crucial to improving the performance of OPVs further.T he interfacial structures of mixed BHJs,however,are difficult to analyze at the molecular scale because of the limitations of analytical methods. [27] Controlling the interfacial structures is even more challenging in BHJs.Systematic studies of polymer synthesis suggest that the structures of the p-conjugated main chains are not the only factor that af...

Side-Chain Engineering of Donor–Acceptor Conjugated Small Molecules As Dopant-Free Hole-Transport Materials for Efficient Normal Planar Perovskite Solar Cells

“…Specifically, the RMS value was gradually reduced from ∼9.6 nm (BTTI-C12) to ∼7.2 nm (BTTI-C8) and then to ∼6.5 nm (BTTI-C6) (Figure S4), indicating the formation of a smoother film after employing a shorter side chain BTTI HTM, which may lead to a good interfacial contact between the perovskite/HTM and/or the HTM/anode electrode, leading to an improved FF of PSCs. 36,37 Furthermore, we measured the two-dimensional grazing-incidence wide-angle Xray scattering (2D-GIWAXS, Figures S5 and S6) of the perovskite/HTMs bilayer films, with pure perovskite film as the control. The intensity of the diffraction peak weakens apparently from the perovskite/BTTI-C12 film to the perovskite/BTTI-C6 film, indicating the formation of a denser pinhole-free film of BTTI-C6.…”