Large Band Gap Narrowing and Prolonged Carrier Lifetime of (C4H9NH3)2PbI4 under High Pressure

Abstract Due to their superior optical and electronic properties and good stability, 2D organic–inorganic halide perovskites (OIHPs) exhibit strong potential for optoelectronic applications. However, the large band gap, short carrier lifetime, and high resistance hinder their practical performance. In this work, the band gap is successfully tuned, the carrier lifetime is prolonged, and the resistance of (C4H9NH3)2PbI4 (BA2PbI4) is reduced directly using high pressure. The band gap is decreased to less than 1 eV at 35.0 GPa, and the highest pressure is studied. The carrier lifetime at 9.9 GPa is 20 times longer than that at ambient conditions. Moreover, the resistance is reduced by four orders of magnitude at 34.0 GPa accompanying band gap narrowing. This work indicates that pressure plays an effective role in tuning the optical and electronic structures of BA2PbI4, and also provides a strategy to synthesize high‐performance OIHP materials.

High pressure impedance measurement Impedance spectra were measured via using a Solartron1260 impedance analyzer and 1296 dielectric interface. The powder sample was loaded in a Mao-type symmetric DAC with a pair of 300-μm culets and placed in cBN gasket hole with a diameter on the order of 150 μm. 2 m thick Pt foil was used as electrodes. A two-electrode configuration was used for measuring the impedance spectra. Two electrodes were attached to the top and bottom culet, respectively. Cubic boron nitride powder mixed with epoxy was used for the insulation between the platinum electrode and metal gasket. Impedance spectra were collected from 1 × 10 -2 Hz to 1 × 10 6 Hz.

High pressure absorption measurement
In situ high-pressure UV-vis absorption spectroscopy measurements were performed on an UV-vis absorption spectrophotometer with a response time of 1 s. Absorption spectra (250−1000 nm) were measured using a deuterium−halogen light source. The sample was loaded in a Mao-type symmetric DAC with a pair of 300-μm culets and placed in T301 steel gasket hole with a diameter on the order of 150 μm. Silicon oil was used as pressure-transmitting medium. The silicone oil and diamond signal was recorded as background.
High pressure PL measurement PL spectra were collected by using a Horiba LabRAM HR Evolution Raman spectrometer with a 473 nm laser as the excitation. The sample was loaded in a Mao-type symmetric DAC with a pair of 400-μm culets and placed in T301 steel gasket hole with a diameter on the order of 200 μm. Silicon oil was used as pressure-transmitting medium.

High pressure PL lifetime measurement
In situ high-pressure time-resolved photoluminescence measurement was conducted at the Center for Nanoscale Materials (CNM), ANL. 400 nm laser was used as excitation. The sample was loaded in a Mao-type symmetric DAC with a pair of 400-μm culets and placed in T301 steel gasket hole with a diameter on the order of 200 μm. Silicon oil was used as pressure-transmitting medium. The measured PL decay curves were fitted using triple exponential functions. Due to the short lifetimes, the instrument response function (IRF) was reconvoluted during the fitting process. The mean PL time was calculated via <> = (A 1 * 1 2 +A 2 * 2 2 +A 3 * 3 2 )/( A 1 * 1 +A 2 * 2 +A 3 * 3 ). In Situ Synchrotron High Pressure Powder XRD In situ synchrotron high pressure powder XRD experiments were carried out at 4W2 beamline of Beijing Synchrotron Radiation Facility (BSRF). Monochromatic X-ray with wavelength of 0.6199 Å was employed. The powder sample was loaded in a Mao-type symmetric DAC with a pair of 300-μm culets and placed in T301 steel gasket hole with a diameter on the order of 150 μm.

GSAS Refinement
We firstly used LeBail mode to make sure the simulated peak position well match with the that of obseved peak. Then, we used Rietveld mode to better refine the data.

Theoretical simulation
The underlying ab initio structural relaxations and electronic properties calculations are performed in the frame work of density functional theory within generalized gradient approximation Perdew-Burke-Ernzerhof (GGA-PBE), as implemented in the VASP code. The projector augmented wave (PAW) pseudopotentials are adopted with the PAW potentials taken from the VASP library. The cutoff energy (600 eV) for the expansion of the wave function into plane waves and Monkhorst-Pack k-meshes (kpoints density 0.02 Å -1 ) are chosen to ensure that all the energy calculations are well converged to better than 1 meV/atom.