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Charge transport is one of the key factors in the operation of organic solar cells. Here, we investigate the electron and hole transport in the non-fullerene acceptor (NFA) IT-4F, by a combination of space-charge-limited current measurements and multiscale molecular simulations. The electron and hole mobilities are fairly balanced, amounting to 2.9 × 10−4 cm2 V−1 s−1 for electrons and 2.0 × 10−5 cm2 V−1 s−1 for holes. Orientational ordering and electronic couplings facilitate a better charge-percolating network for electrons than for holes, while ambipolarity itself is due to sufficiently high electron affinity and low ionization energy typical for narrow-gap NFAs. Our findings provide a molecular-level understanding of the balanced hole and electron transport in an archetypical NFA, which may play a key role in exciton diffusion and photogenerated hole transfer in organic solar cells.

Charge transport in the amorphous organic small molecules α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and Spiro-TAD (2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene) is investigated in solution-processed films and compared to charge transport in vacuum-deposited films of the same molecule. By optimizing the solution-deposition conditions, such as solvent and concentration, equal charge-transport parameters for solution-processed and vacuum-deposited films are demonstrated. Modeling of the charge carrier transport characteristics was performed by drift-diffusion simulations. The dependence of the charge carrier mobility on temperature, carrier density, and electric field was found to be the same for vacuum deposition and solution processing. In both material processing cases, hole mobilities of 4 × 10−8 m2 V−1 s−1 for spiro-TAD and 0.9 × 10−8 m2 V−1 s−1 for α-NPD are obtained, demonstrating that solution processing can be a viable alternative to vacuum deposition in terms of charge transport.

Ferroelectricity has been proposed as one of the potential origins of the observed hysteresis in photocurrent-voltage characteristics of perovskite based solar cells. Measurement of ferroelectric properties on perovskite solar cells is hindered by the presence of (in)organic charge transport layers. Here we fabricate metal-perovskite-metal capacitors and unambiguously show that methylammonium lead iodide is not ferroelectric at room temperature. We propose that the hysteresis originates from the movement of positive ions rather than ferroelectric switching.