Direct Observation of Charge Injection From CH₃NH₃PbI_3−xCl_x to Organic Semiconductors Monitored With sub‐ps Transient Absorption Spectroscopy

Abstract

Thin films of the hybrid perovskite CH3NH3PbI3−xClx are prepared in contact to poly(3,4‐ethylenedioxythiophene)polystyrene sulfonate (PEDOT:PSS) or phenyl‐C61‐butyric acid methyl ester (PCBM) as model interfaces relevant in inverted perovskite solar cells (IPSCs) which provide some significant advantages over the established mesoporous cell concepts. Pump–probe absorption spectroscopy with femtosecond resolution is used to analyze injection of photo‐generated charge carriers from the perovskite to the PEDOT:PSS and PCBM layers by directly monitoring characteristic spectroscopic signatures of charge carriers in these contact phases. Such signal is identified for holes injected into the PEDOT:PSS layer, which appeared within the resolution of the measurement setup (<200 fs) proving ultrafast hole injection. Changes in the transient signals characteristic for the perovskite layer as reflected in the time constants obtained by global analysis confirmed this observation. Ultrafast injection of electrons into PCBM is also found based on the decay of signals characteristic for the excited state of the perovskite as well as by newly arising signals assigned to injected electrons into PCBM. Efficient injection of charge carriers following excitation of the perovskite film to both organic electron and hole conductors is thereby directly shown.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

“…Meanwhile, they found that the electron and hole injection at the interface proceeded within the first 3 ps after photoexcitation. These ultrafast time scales of interfacial electron and hole transfer are consistent with later studies on charge extraction at the HP-extractor interface using TA spectroscopy. − A complementary study by Ponseca, Jr. et al using time-resolved terahertz spectroscopy provided further evidence of this ultrafast charge carrier extraction in the MAPbI 3 /TiO 2 system …”

Minimizing Voltage Losses in Perovskite Solar Cells

“…However, there is still lack of solid evidence to support the proposed passivation mechanisms, which are usually attributed to ionic interaction, coordination bonding, and hydrogen bonding. In recent years, various advanced transient characterization techniques with high time resolutions (from picosecond to nanosecond), such as time‐resolved photoluminescence (TRPL), [ 81 ] transient absorption spectroscopy, [ 82 ] transient voltage measurement (TPV), [ 83 ] hyperspectral photoluminescence imaging, [ 81 ] thermal admittance spectroscopy, and drive‐level capacitance profiling (DLCP), [ 84,85 ] have been implemented to reveal the correlation between additives and charged defects for pinpointing the exact passivation mechanisms. From an in situ characterization perspective, these powerful diagnostic tools could offer guidelines to tailor the functional groups and design new passivation molecules to heal the trap states at the GBs and on the surfaces and therefore further minimize the trap‐mediated voltage losses of PSCs.…”

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se2 Solar Cells

“…State-ofthe-art CIGSe solar cells make extensive use of a bandgap engineering approach, varying the [Ga]/([Ga]+[In]) ratio (GGI ratio) across the absorber thickness. The introduction of Ga into the perovskite photovoltaic materials to measure a wide range of charge carrier dynamics including recombination, charge carrier transfer and charge trapping, [24][25][26][27][28][29] only few TAS studies have been conducted in the CIGSe field. For example, Okano et al elucidated the intraband charge carrier relaxation and localization processes in an ultra-thin film CIGSe absorber layer (film thickness around 200 nm) on femtosecond to picosecond timescales.…”

“…Though transient absorption spectroscopy (TAS) has been utilized in the fields of organic and hybrid organic–inorganic perovskite photovoltaic materials to measure a wide range of charge carrier dynamics including recombination, charge carrier transfer and charge trapping, [ 24–29 ] only few TAS studies have been conducted in the CIGSe field. For example, Okano et al.…”