Charge transfer and recombination at the metal oxide/CH₃NH₃PbClI₂/spiro-OMeTAD interfaces: uncovering the detailed mechanism behind high efficiency solar cells

Abstract

In recent years, organometal halide perovskite-based solid-state hybrid solar cells have attracted unexpected increasing interest because of their high efficiency (the record power conversion efficiency has been reported to be over 15%) and low fabrication cost. It has been accepted that the high efficiency was mainly attributed to the strong optical absorption (absorption coefficient: 15,000 cm(-1) at 550 nm) over a broader range (up to 800 nm) and the long lifetimes of photoexcited charge carriers (in the order of 10 ns - a few 100 ns) of the perovskite absorbers. However, much of the fundamental photophysical properties of perovskite relating to the high photovoltaic performance are remained to be investigated. The charge separation and recombination processes at the material interfaces are particularly important for solar cell performances. To better understand the high efficiency of perovskite solar cells, we systematically investigated the charge separation (electron and hole injection) and charge recombination dynamics of CH3NH3PbClI2 hybrid solar cells employing TiO2 nanostructures as the electron transfer material (ETM) and spiro-OMeTAD as the hole transfer material (HTM). The measurements were carried out using transient absorption (TA) techniques on a time scale from sub-picoseconds to milliseconds. We clarified the timescales of electron injection, hole injection, and recombination processes in TiO2/CH3NH3PbClI2/spiro-OMeTAD solar cells. Charge separation and collection efficiency of the perovskite-based solar cells were discussed. In addition, the effect of TiO2 size on the charge separation and recombination dynamics was also investigated. It was found that all TiO2-based perovskite solar cells possessed similar charge separation processes, but quite different recombination dynamics. Our results indicate that charge recombination was crucial to the performance of the perovskite solar cells, which could be effectively suppressed through optimising nanostructured TiO2 films and surface passivation, thus pushing these cells to even higher efficiency.

Interface structure between titania and perovskite materials observed by quartz crystal microbalance system

“…[10][11][12] Figure 1 shows the QCM system (Meywafosus Co. LTD) employed in this experiment. The titania (anatase) compact layer (500 nm) was prepared on a Au QCM sensor by atomic layer deposition (ALD) (Sunable ALD R200 Basic, Picosun) using TiCl 4 and water as the precursor.…”

“…5,6,9 The surface of the porous titania affects the electron collection as well as the crystal growth of the perovskite crystals. [10][11][12] For example, we have reported that the electron trap density of the perovskite/titania layer is decreased by passivating the surface of the titania. 10,12 In addition, the crystal size of the perovskite becomes larger when the titania surface is passivated with amino acid hydroiodic acid (HI) salt.…”

“…[10][11][12] For example, we have reported that the electron trap density of the perovskite/titania layer is decreased by passivating the surface of the titania. 10,12 In addition, the crystal size of the perovskite becomes larger when the titania surface is passivated with amino acid hydroiodic acid (HI) salt. 11 The interface between titania and the perovskite crystal is crucial for understanding the photoconversion; however, the interface has not been discussed in detail.…”

Theoretical limit of power conversion efficiency for organic and hybrid halide perovskite photovoltaics

“…of µs. [30][31][32][33][34] The electron transfer from CH 3 NH 3 PbI 3 to TiO 2 occurs on a time scale of picoseconds, which is much faster than the time scale of charge recombination inside…”

Stability Study of Flexible 6,13-Bis(triisopropylsilylethynyl)pentacene Thin-Film Transistors with a Cross-Linked Poly(4-vinylphenol)/Yttrium Oxide Nanocomposite Gate Insulator

“…Here, yttrium oxide (Y 2 O 3 ) nanoparticles were used to form the nanocomposite gate insulator, which exhibit a relatively wide band gap energy of 6.0 eV compared to that (3.2 eV) of the commonly used TiO 2 nanoparticles for nanocomposite gate insulators; the wide band gap feature of Y 2 O 3 is very advantageous on aspects of illumination stability of TFTs [13]. The first type of device used cross-linked poly(40vinylphenol)(c-PVP)/Y 2 O 3 nanocomposite gate insulators, and the second type used normal c-PVP gate insulators.…”