Effects of Cyclic Tetrapyrrole Rings of Aggregate-Forming Chlorophyll Derivatives as Hole-Transporting Materials on Performance of Perovskite Solar Cells

Abstract

Organic hole-transporting materials (HTMs) are essential components of high-performance perovskite solar cells (PSCs). Three zinc-coordinated chlorophyll derivatives with bacteriochlorin, chlorin, and porphyrin macrocycles, namely, ZnBChl, ZnChl, and ZnPor, respectively, were newly synthesized and employed as HTMs in PSCs. The difference in the π backbones of these HTMs causes differences in their photophysical properties, and thus different hole-extraction abilities, as revealed by steady-state photoluminescence spectra. The power conversion efficiencies (PCEs) of PSCs with a typical mesoporous structure, fluorine-doped tin oxide/compact TiO 2 /mesoporous TiO 2 / CH 3 NH 3 PbI 3 /HTM/Ag, are 8.26%, 11.88%, and 0.68% for ZnBChl, ZnChl, and ZnPor, respectively. The small PCE of the ZnPor-based PSC is partially attributed to the small energy gap of the highest occupied molecular orbital (HOMO) levels between ZnPor and CH 3 NH 3 PbI 3 perovskite. Therefore, we increased this energy gap slightly by shifting the HOMO level of CH 3 NH 3 PbI 3 perovskite downward by incorporating formamidinium and bromide ions into the crystal lattice of CH 3 NH 3 PbI 3 . As a result, the PCE of the ZnPor-based PSC improved to 4.04%, and it exhibited a clearly normal current−voltage curve, indicating better energy alignment between ZnPor and the modified perovskite. In addition, the barriers both in the perovskite/ ZnPor interface and in the ZnPor layer originated from the delocalization of π-electrons on the symmetric aggregates determine the low PCE of ZnPor-based PSCs; this was deduced from measurements of atomic force microscope, ultraviolet photoelectron spectroscopy, and the electric impedance spectroscopy.

Semisynthetic Chlorophyll Derivatives Toward Solar Energy Applications

“…To compare the photovoltaic performance of Chl and its analogues (BChl and Por), three types of cyclic tetrapyrroles with the same peripheral substitutes were investigated as an HTM for PSC devices (Figure ) . The different degrees of the π‐conjugation influenced their photophysical properties, leading to a different hole extraction ability.…”

“…Effects of cyclic tetrapyrrole rings of aggregate‐forming semisynthetic chlorophylls as HTMs on performance of PSCs. Reproduced with permission . Copyright 2018, American Chemical Society.…”

Chlorophylls in thin-film photovoltaic cells, a critical review

“…In addition to the fully natural compounds that can be isolated from the bio-raw materials, a large number of "semisynthetic" chlorophyll dyes are available. 4,[8][9][10][11][12] They can be produced by a mere chemical modication of natural precursors, for instance by replacement of the peripheral groups, de-or remetallation. [8][9][10][11] Such protocols serve to adapt the target product to the practical applications in various elds of medicine, biochemistry or optoelectronics.…”

Metallated Macrocyclic Derivatives as a Hole – Transporting Materials for Perovskite Solar Cells

“…Recently, Wang et.al reported three different Zn centered macrocyclic derivatives namely, bacteriochlorin (ZnBChl), chlorin (ZnChl), porphyrin (ZnPor) for introduction as HTMs in PSCs as shown in Figure (a) and presented a comparative study. These molecules have similar structures with different degrees of π conjugation on the cyclic tetrapyrrole ring which may affect the photophysical properties and performance of solar cells . Here, ZnChl based solar cells show higher performance compared with ZnBChl and ZnPor, as ZnChl's HOMO levels are well suited to the perovskite layer and it has higher charge carrier mobility with the J‐aggregates.…”

“…High aggregate ZnChl molecules have the lower HOMO energy level and more intermolecular π‐π interaction leads to achieving a high performance of 11.88 % (all other parameters are listed in Table ). The zinc chlorophyll J‐aggregates HTM works efficiently without any additives and could be very beneficial to produce low‐cost PSCs for large‐scale production in the future …”