Management of transition dipoles in organic hole-transporting materials under solar irradiation for perovskite solar cells

Abstract

In organic hole-transporting material (HTM)-based p−i−n planar perovskite solar cells, which have simple and low-temperature processibility feasible to flexible devices, the incident light has to pass through the HTM before reaching the perovskite layer. Therefore, photo-excited state of organic HTM could become important during the solar cell operation, but this feature has not usually been considered for the HTM design. Here, we prove that enhancing their property at their photo-excited states, especially their transition dipole moments, can be a methodology to develop high efficiency p−i−n perovskite solar cells. The organic HTMs are designed to have high transition dipole moments at the excited states and simultaneously to preserve those property during the solar cell operation by their extended lifetimes through the excited-state intramolecular proton transfer process, consequently reducing the charge recombination and improving extraction properties of devices. Their UV-filtering ability is also beneficial to enhance the photostability of devices.

Interfacial Dipole in Organic and Perovskite Solar Cells

“…In addition to direct characterization with UPS and SKPM methods, the direction and strength of interfacial dipole can be indirectly inferred from knowledge of the molecular structure and arrangement. Conventional structural and spectral methods such as X-ray diffraction (XRD), , optical absorption, and fluorescence spectroscopy have been exploited to understand molecular ordering. Sum frequency generation (SFG) spectroscopy, a second-order nonlinear optical technique, is particularly noteworthy.…”

“…For example, small-molecule SAMs with the positive charge end pointing to NiO and the negative charge end pointing to the active layer have been explored . Interestingly, Ok et al demonstrated a light-dependent molecular dipole interlayer formed by inserting phenoxazine- and imidazole-containing small molecules between NiO and the perovskite active layer . The dipole moment of the small-molecule layer had its positive charge end pointing to NiO and the negative charge end pointing to perovskite, resulting in a greater effective work function of NiO.…”

<span>Progress in&nbsp;</span><span>D−A−D-type Small&nbsp;</span><span>Molecule</span><span>&nbsp;Hole-Transport Materials&nbsp;</span><span>for Perovskite Solar Cells</span>

“…Donor (D)−acceptor (A)−donor (D) molecules have been widely demonstrated as efficient HTMs due to their intramolecular charge transfer (ICT) character and high dipole moment, which could induce self-doping and built-in potential to boost charge extraction. [37][38][39][40] Besides, the HOMO and lowest unoccupied molecular orbital (LUMO) of the D−A−D molecules can be readily tuned by adopting appropriate D and A building blocks. Usually, strong intramolecular charge transfer characteristic is presented in D−A−D-type molecules, which not only affects their band-gap and optical properties but also may enhance the intermolecular packing in the thin film as well as the charge carrier mobility.…”

“…Usually, strong intramolecular charge transfer characteristic is presented in D−A−D-type molecules, which not only affects their band-gap and optical properties but also may enhance the intermolecular packing in the thin film as well as the charge carrier mobility. [37][38][39][40][41]…”

A perylene diimide dimer-based electron transporting material with an A–D–A structure for efficient inverted perovskite solar cells

“…Recently, the combination of electron donor (D) and electron acceptor (A) moieties into the conjugated systems has been proved as an efficient strategy due to their high charge mobility with the self-doping effect, which is mainly related to the strong intramolecular charge transfer (ICT) and strong dipole–dipole interactions. 58–62…”