Perovskite–fullerene hybrid materials suppress hysteresis in planar diodes

Solution-processed planar perovskite devices are highly desirable in a wide variety of optoelectronic applications; however, they are prone to hysteresis and current instabilities. Here we report the first perovskite–PCBM hybrid solid with significantly reduced hysteresis and recombination loss achieved in a single step. This new material displays an efficient electrically coupled microstructure: PCBM is homogeneously distributed throughout the film at perovskite grain boundaries. The PCBM passivates the key PbI3− antisite defects during the perovskite self-assembly, as revealed by theory and experiment. Photoluminescence transient spectroscopy proves that the PCBM phase promotes electron extraction. We showcase this mixed material in planar solar cells that feature low hysteresis and enhanced photovoltage. Using conductive AFM studies, we reveal the memristive properties of perovskite films. We close by positing that PCBM, by tying up both halide-rich antisites and unincorporated halides, reduces electric field-induced anion migration that may give rise to hysteresis and unstable diode behaviour.

and c), respectively. a) and d) show conductivity at grain boundary areas is higher than that at grain center areas in both samples. The perovskite sample treated with PCBM has much higher conductivity near grain boundary areas at positive sample bias voltages, which points to improved electron extraction from PCBM at grain boundaries. Significant hysteresis is consistently observed in perovskite control sample (e).

Supplementary Note 1 | Density functional theory study of PCBM in-situ passivation effect on perovskite in hybrid solid
Calculations were performed within the Density Functional (DFT) formalism using the Perdew-Burke-Ernzerhof (PBE) [1] GGA exchange correlation functional. All calculations were performed utilizing the CP2K [2] package within Gaussian-augmented plane waves (GAPW) dual basis set using the molecularly optimized MOLOPT [3] double  -valence polarized (mDZVP) basis set implemented in CP2K code which has very small Basis Set Superposition errors (BSSE) in gas and condensed phases [4][5][6][7] . The grid cutoff was 300 Ry, which is suitable for the Goedecker-Teter-Hutter pseudopotentials [8] . Spin polarized (LSDA) and spin-unpolarized caculations (LDA) were performed in the case of the odd and even number of electrons, respectively. The structural minimization was performed with the help of the Broyden-Fletcher-Goldfarb-Shanno algorithm [9] (BFGS).
We have modelled the passivation of the perovskite surface with PCBM ligand. Surface slabs were modelled as (001) terminated slabs of tetragonal structure with 14 and 15 interchanging monolayers i.e. stoichiometric asymmetric slab (MAI-PbI 2 ) and off-stoichiometric symmetric slabs (PbI 2 -PbI 2 or MAI-MAI termination). In light of recently proposed ferroelectricity of the perovskite [10] we have studied polar stoichiometric slabs i.e. even -numbered slabs (in our case 14 monolayers). It was found previously [11] that unreconstructed (001) tetragonal termination possesses low surface energy and as a result represents the most probable surface termination. 100 Å of vacuum was added on top of the slab surface (with and without PCBM attached to the surface for consistency). Dipole-slab correction was used to remove artificial dipole-dipole interaction across periodic images in vacuum as implemented in the CP2K code of version 2.5. A 3x3 (26.88 Å x 26.88 Å) periodicity was used in the xy-plane. The basis set superposition error (BSSE) [4,12] in PCBM binding energy was estimated using the counterpoise correction method [13] to be ~3 meV and was subsequently neglected. To create dipole-free slab in case of symmetric terminations we have performed the Born-Openheimer molecular dynamics in NVT ensemble with 0.5 fs time step over the 10 ps to allow methylammonium rotational degrees of freedom to smoothen out and chose the configurations with net-zero dipole along normal to the perovskite surface (001), subsequently relaxed (Supplementary Figure  3). Similarly we have chosen the structures with mean dipole moment in the case of the even-number asymmetric slabs. Such approach provides configurations that are ensemble representative at 300 K. No band bending was observed in the case of 14M case (however at larger thickness band bending becomes apparent), which could be explained by the topmost mobile MAI layer partially compensating the built-in electric field due to non-zero dipole moment as can be seen from Supplementary Figure 3. Supplementary Figure 4 shows the density of states (DOS) of symmetric off-stoichiometric slabs (15 monolayers) where one can see no in-gap states.
To demonstrate the passivation effect caused by PCBM, we have concentrated on PbI 2 -terminated surface as it was shown to posses small surface energy [11] and therefore be very stable. PCBM was attached in two configuration as depicted in Supplementary Figure  5.
Adsorption energies were calculated using the Supplementary Equation 1: relaxed relaxed relaxed PCBM ads The binding energies are given in the Supplementary Table 1 and Supplementary  Table 3. Negative values indicate binding.
One can see that in the case of symmetrically off-stoichiometric slab (PbI 2 -PbI 2 ) PCBM easily desorbs whereas in the case of stoichiometric polar slab, binding energies correspond to weak physisorption.
To understand electronic properties further, we took the most anticipated defect [14] in bulk, namely Pb I antisite (Pb -atom is being substituted by I atom) and performed defect formation energies calculations (H f ) on the surface (Supplementary Equation 2) to verify that it maintains the low formation energy and forms a trap state in the gap as observed in the bulk.