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.

Fig. 1. Molecular and single-crystal structures, and wavefunction distribution of Y6.

a–e Molecular structure of Y6 (a). b Molecular pairs in the Y6 single crystal. c Top and d side views of the extended-crystal structure (the blue column is the stack of end groups in the b direction, the pink column is the stack of end groups in the c direction, and the green one is a molecular packing pair of D–A′ fragment). e Calculated valence band maximum (VBM) for hole (left) and conduction band minimum (CBM) for electron (right) wavefunctions at the Γ point (center of the Brillouin zone) in the Y6 single crystal.

Fig. 2. GIWAXS characterization of Y6 based systems.

a–f Two-dimensional GIWAXS patterns (top) and profiles (bottom) of a, d the pristine Y6, b, e pristine PBDB-T-2F, and c, f PBDB-T-2F:Y6 films, respectively.

Fig. 3. Molecular-dynamics (MD) simulations of Y6 based systems.

a–c Radial distribution function (g) data for Y6 (a). The labels for the acceptor moieties are shown in Fig. 1a. The blue lines represent the data from the pristine acceptor films and the orange lines, the data from the donor/acceptor blends. Illustration of the molecular-dynamics simulations results for the packing in b the pristine Y6 and c PBDB-T-2F:Y6 films. A significant amount of Y6 dimers in both the pristine and blended films show similar packing compared to the Y6 crystal structures.

Fig. 4. Charge recombination and separation properties.

a Photoluminescence spectra of the Y6, IT-4F, and ITIC films excited at 660 nm together with their quantum efficiencies. b Semi-logarithmic plots of EQE evaluated by FTPS (EQE_FTPS) (black spheres) and normalized EL (dark yellow spheres) as a function of energy for devices based on PBDB-T-2F:Y6. The ratio of ϕ_EL/ϕ_bb was used to plot the EQE in the low energy regime (red line), where ϕ_EL and ϕ_bb represent the emitted photon flux and the room-temperature blackbody photon flux, respectively. The normalized PL spectra (orange lines) were measured based on the binary blend films. c Natural transition orbitals of the interfacial CT states by using the TD-ωB97XD/6-31G(d,p) method coupled with the PCM model for molecular clusters (left: one PBDB-T-2F donor fragment with one Y6 molecule; right: one PBDB-T-2F donor fragment with three Y6 molecules). Due to the delocalization of the electron wavefunction, the estimated distance (d_e–h) between the hole and electron at the donor/acceptor interface increases from 22 Å for one Y6 molecule to 51 Å for clusters of three Y6 molecules.