Delocalization of exciton and electron wavefunction in non-fullerene acceptor molecules enables efficient organic solar cells

Abstract

A major challenge for organic solar cell (OSC) research is how to minimize the tradeoff between voltage loss and charge generation. In early 2019, we reported a non-fullerene acceptor (named Y6) that can simultaneously achieve high external quantum efficiency and low voltage loss for OSC. Here, we use a combination of experimental and theoretical modeling to reveal the structure-property-performance relationships of this state-of-the-art OSC system. We find that the distinctive π–π molecular packing of Y6 not only exists in molecular single crystals but also in thin films. Importantly, such molecular packing leads to (i) the formation of delocalized and emissive excitons that enable small non-radiative voltage loss, and (ii) delocalization of electron wavefunctions at donor/acceptor interfaces that significantly reduces the Coulomb attraction between interfacial electron-hole pairs. These properties are critical in enabling highly efficient charge generation in OSC systems with negligible donor-acceptor energy offset.

17.1 %‐Efficient Eco‐Compatible Organic Solar Cells from a Dissymmetric 3D Network Acceptor

“…Figure S4 shows the powder x ‐ray diffraction pattern generated from single crystal data from BTIC‐2Cl‐γCF 3 , it was found strong signals of 2 θ =6.08°, 7.16° and 26.48°, the corresponding distances are 14.52, 12.34, and 3.36 Å according to the Bragg equation 2 d sin θ =λ in XRD, respectively, and the q value are 0.29, 0.51, and 1.87 Å −1 according to d =2π/ q , respectively. From the GIWAXS data, we found a set of values q =0.29 and 0.53 Å −1 in the IP direction and q =1.87 Å −1 in the OOP direction by the binary film, which is matching well with the data from single crystal, indicating that the packing modes of BTIC‐2Cl‐γCF 3 should be partially reserved in the blend film [29, 34] …”

“…The X‐ray single crystal structure of BTIC‐2Cl‐γCF 3 was determined to reveal its packing information in the solid state [26, 31–35] . Figure 1 b shows the monomolecular state of BTIC‐2Cl‐γCF 3 from a top view.…”

Charge Photogeneration in Non‐Fullerene Organic Solar Cells: Influence of Excess Energy and Electrostatic Interactions

“…The latter agrees well with a recent report of electron–hole separation distance of 5.1 nm in another NFA blend PM6:Y6. [ 49 ] Due to the larger separation of electron–hole pairs in PTB7‐Th:ITIC at higher photon energies, their Coulomb binding energy decreases from 141 to 75 meV. Thus, the activation energy for free charge carrier formation is reduced for the CT states with excess kinetic energy.…”

Asymmetric Acceptors Enabling Organic Solar Cells to Achieve an over 17% Efficiency: Conformation Effects on Regulating Molecular Properties and Suppressing Nonradiative Energy Loss

“…[ 45–47 ] Most importantly, Y6's banana shape allows it to stack closely to form a 3D framework with effective charge transfer/transport channels (bipolar transport property). [ 45–51 ] Consequently, the blend of Y6 with PM6 achieved an unprecedented PCE (15.7%) with a 0.50 eV E loss in binary devices. [ 6 ]…”

“…[45][46][47] Most importantly, Y6's banana shape allows it to stack closely to form a 3D framework with effective charge transfer/transport channels (bipolar transport property). [45][46][47][48][49][50][51] Consequently, the blend of Y6 with PM6 achieved an unprecedented PCE (15.7%) with a 0.50 eV E loss in binary devices. [6] Limited by the unique packing features and synthetic difficulty of Cadogan reaction, the main optimizations performed currently for Y6 were focused on its side chains and terminal groups hoping to fine-tune the NFA's crystalline properties.…”