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The incorporation of Ruddlesden–Popper (RP)/3D perovskite heterostructures into photovoltaic cells has been shown to increase both the efficiency and stability of the devices. Here, a series of methylammonium lead triiodide (MAPbI3) thin films treated with varying 2-phenylethylammonium (PEA) concentrations are investigated with static and ultrafast spectroscopic techniques to reveal the mechanisms of the observed performance benefits. Transient absorption spectroscopy is employed to elucidate the effect of a surface RP layer on the excited state of the MAPbI3 films and reveal that several different RP structures are formed and participate in the charge-carrier dynamics. The passivation effects of PEA are investigated with optical pump–terahertz probe (OPTP) experiments using a variety of excitation conditions to simultaneously probe the surface and bulk recombination dynamics. Fitting models to the OPTP data for each excitation scheme allows the material parameters that govern the ultrafast dynamics to be quantified. It is found that as the PEA concentration increases, the surface recombination velocity exhibits a monotonic decrease, suggesting the RP layer is effective at passivating surface traps. Furthermore, the bulk monomolecular recombination rate is also found to decrease with the addition of PEA, indicating that the benefits of this passivation approach are not limited to the upper surface of the MAPbI3 films.

In perovskite solar cells, photovoltaic action is created by charge transport layers (CTLs) either side of the light-absorbing metal halide perovskite semiconductor. Hence, the rates for desirable charge extraction and unwanted interfacial recombination at the perovskite-CTL interfaces play a critical role for device efficiency. Here, the electrical properties of perovskite-CTL bilayer heterostructures are obtained using ultrafast terahertz and optical studies of the charge carrier dynamics after pulsed photoexcitation, combined with a physical model of charge carrier transport that includes the prominent Coulombic forces that arise after selective charge extraction into a CTL, and cross-interfacial recombination. The charge extraction velocity at the interface and the ambipolar diffusion coefficient within the perovskite are determined from the experimental decay profiles for heterostructures with three of the highest-performing CTLs, namely C60, PCBM and Spiro-OMeTAD. Definitive targets for the further improvement of devices are deduced: fullerenes deliver fast electron extraction, but suffer from a large rate constant for cross-interface recombination or hole extraction. Conversely, Spiro-OMeTAD exhibits slow hole extraction but does not increase the perovskite’s surface recombination rate, likely contributing to its success in solar cell devices.

Improved knowledge of the influence of temperature upon layered perovskites is essential to enable perovskite-based devices to operate over a broad temperature range and to elucidate the impact of structural changes upon the optoelectronic properties. We examined the Ruddlesden–Popper layered perovskite 2-thiophenemethylammonium lead iodide (ThMA2PbI4) and observed a structural phase transition between a high- and a low-temperature phase at 220 K using temperature-dependent X-ray diffraction, UV–visible absorption, and photoluminescence (PL) spectroscopy. The structural phase transition altered the tilt pattern of the inorganic octahedra layer, modifying the absorption and PL spectra. Further, we found a narrow and intense additional PL peak in the low-temperature phase, which we assigned to radiative emission from a defect-bound exciton state. In both phases we determined the thermal expansion coefficient and found values similar to those of cubic 3D perovskites, i.e., larger than those of typical substrates such as glass. These results demonstrate that the organic spacer plays a critical role in controlling the temperature-dependent structural and optoelectronic properties of layered perovskites and suggests more widely that strain management strategies may be needed to fully utilize layered perovskites in device applications.

Layered hybrid perovskites (LPKs) are promising as alternatives or additives to 3D metal halide perovskites for optoelectronic applications including photovoltaic cells, LEDs and lasers due to their increased stability. However, high exciton binding energies in these materials mean that excitons are the majority species under the operating conditions of many devices. Although the efficiency of devices that incorporate LPKs has been increasing, much is still unknown about the interplay of excitons and free charge-carriers in these materials, which is vital information for understanding how optoelectronic properties dictate device efficiency. In this work, we employ optical pump/THz probe spectroscopy (OPTP) and visible transient absorption spectroscopy (TAS) to analyse the optoelectronic properties and charge-carrier dynamics of phenylethylammonium lead iodide (PEA)2PbI4. By combining these techniques, we are able to disentangle the contributions from excitons and free charge-carriers. We observe fast cooling of free charge-carriers and exciton formation on a timescale of ∼400 fs followed by slower bimolecular recombination of residual free charge-carriers with a rate constant k2 ∼ 109 cm3s−1. Excitons recombine via two monomolecular processes with lifetimes t1 ∼ 11 ps and t2 ∼ 83 ps. Furthermore, we detect signatures of exciton–phonon coupling in the transient absorption kinetic traces. These findings provide new insight into the interplay between free charge-carriers and excitons as well as a possible mechanism to further understand the charge-carrier dynamics in LPKs.