Hideyuki Kunugita

Metal halide perovskite materials (MHPs) are promising for several applications due to their exceptional properties. Understanding excitonic properties is essential for exploiting these materials. For this purpose, we focus on CsPbBr3 single crystals, which have higher crystal quality, are more stable, and have no Rashba effect at low temperatures compared to other 3D MHPs. We have estimated exciton energy positions, longitudinal-transverse splitting energy, and damping energy using low-temperature reflection spectra. Under high excitation intensity, two biexciton emissions (M-emission) and exciton–exciton scattering emission (P-emission) were observed. We assign the two M-emissions to the emission to the states of longitudinal and transverse excitons, i.e., ML and MT emissions. From the energy position of the MT emission, the biexciton binding energy has been estimated to be ∼2 meV. By analyzing P-emission obtained from the back side of the sample, we have estimated the exciton binding energy to be 17.8–23.7 meV. This estimation minimizes the influence of the wavenumber distribution in the scattering process. In addition, time-resolved transmittance measurements using pulsed white light have revealed the group velocity dispersion. Comparing experimental results with theoretical calculations using the Lorentz model clarifies that exciton dynamics in CsPbBr3 can be described with a simple Lorentz model. These insights enhance the understanding of exciton behavior and support the development of exciton-based devices using MHPs.

It is well known that the surface trap states and electronic disorders in the solution-processed CH3NH3PbI3 perovskite film affect the solar cell performance significantly and moisture sensitivity of photoactive perovskite material limits its practical applications. Herein, we show the surface modification of a perovskite film with a solution-processable hydrophobic polymer (poly(4-vinylpyridine), PVP), which passivates the undercoordinated lead (Pb) atoms (on the surface of perovskite) by its pyridine Lewis base side chains and thereby eliminates surfacetrap states and non-radiative recombination. Moreover, it acts as an electron barrier between the perovskite and hole-transport layer (HTL) to reduce interfacial charge recombination, which led to improvement in open-circuit voltage (V-oc) by 120 to 160 mV whereas the standard cell fabricated in same conditions showed Voc as low as 0.9 V owing to dominating interfacial recombination processes. Consequently, the power conversion efficiency (PCE) increased by 3 to 5% in the polymer-modified devices (PCE=15%) with V-oc more than 1.05 V and hysteresis-less J-V curves. Advantageously, hydrophobicity of the polymer chain was found to protect the perovskite surface from moisture and improved stability of the non-encapsulated cells, which retained their device performance up to 30 days of exposure to open atmosphere (50% humidity).

We developed anew and simple solvent vapor-assisted thermal annealing (VA) procedure which can reduce grain boundaries in a perovskite film :for fabricating highly efficient perovskite solar cell (PSCs). By recycling of solvent molecules evaporated from an as-prepared perovskite film as a VA vapor source, named the pot-root VA (PR-VA) method, finely controlled and reproducible device fabrication was achieved for formamidiniurn (FA) and methylammonium (MA), mixed cation-halide perovskite (FAPbI(3))(0.85)(MAPbBr(3))(0.15). The mixed perovskite was crystallized on a low-temperature prepared brookite TiO2 mesoporous scaffold When exposed to very dilute solvent vapor, small grains in the perovskite film gradually unified into large grains, resulting in grain boundaries which were highly reduced and improvement of photovoltaic performance in PSC. PR-VA-treated large grain perovskite absorbers exhibited stable photocurrent-Voltage performance with high fill factor and suppressed hysteresis, achieving the, best conversion efficiency of 18.5% for a 5 X 5 mm(2) device and 15.2% for a 1.0 X 1.0 cm(2) device.

Formamidine (FA) and cesium (Cs) cations were introduced into quasi-two-dimensional (2D) perovskites as B site cations. The unique crystalline growth of the resulting (n-C6H13NH3)(2)FAPb(2)I(7), which promotes charge transport in photovoltaic solar cells, was confirmed, as was the stability of this material. The photovoltaic properties of (n-C6H13NH3)(2)FAPb(2)I(7) were found to be superior to those of other homologous quasi-2D perovskite compounds.