Heterogeneous Photon Recycling and Charge Diffusion Enhance Charge Transport in Quasi-2D Lead-Halide Perovskite Films

Abstract

The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. Here we show that such effects are not detrimental to the charge transport for carefully processed films comprising a selfassembled thin layer of quasi-two-dimensional (2D) perovskite interfaced with a 3D MAPbI 3 perovskite layer. We apply a combination of time-resolved photoluminescence and photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited. Through modeling of the recorded dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective "heterogeneous photon recycling". We show that this combination fully compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskites with highly efficient charge transporting properties.

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

“…Discerning FA (CH 5 N 2 , MW = 172) and EA (C 2 H 8 N, MW = 173) cations is difficult since they have similar molecular weights. However, it has been reported by previous works that bulky cations in 2D/3D perovskites are likely to accumulate at the buried interface . X‐ray photoelectron spectroscopy measurement shows an identical surface chemical composition of EA‐FACs and FACs film (Table S1, Supporting Information), which may indicate that EA falls to the buried interface.…”

Nonradiative Energy Transfer through Distributed Bands in Piezochemically Synthesized Cesium and Formamidinium Lead Halide Perovskites

“…Recently, photon recycling in perovskites was proposed to have a positive impact to solar cells . This recycling phenomenon occurs in bulk films and large crystals of perovskites and is the result of repeated absorption and emission of photons . Thus, the effective photoluminescence (PL) lifetime increases in such samples.…”

“…For example, Motti et al. detected an increase in PL lifetime from 4 ns to 1 μs or longer in a quasi‐two‐2D/3D lead iodide perovskite film; this is due to the reabsorption of photons emitted from the higher‐bandgap quasi‐2D layer by the lower‐bandgap 3D layer.…”

“…Apart from the multiple radiative absorption–emission events in photon recycling, the distribution of energy states or bandgaps should facilitate nonradiative energy transfer. Energy cascading is known for compositionally homogenous perovskite aggregates and films; it takes place across different crystallites with closely spaced energy states . In samples with varied perovskite compositions, energy transfer takes place from the larger‐bandgap (donor) to the smaller‐bandgap (acceptor) parts, which is assigned to Förster resonance energy transfer (FRET) .…”

“…The long PL lifetime associated with the red‐shifted emission helps us to consider nonradiative energy transfer in perovskite pellets. As with thick films and single crystals of perovskites, photon recycling is often correlated with the reabsorption–emission processes,– and the contribution of nonradiative energy transfer in this process is largely neglected. In addition to the above distribution of energy states, we find the PL lifetime values of the red‐ and blue‐shifted spectra are largely different.…”