Molecularly Designed Zinc (II) Phthalocyanine Derivative as Dopant‐Free Hole‐Transporting Material of Planar Perovskite Solar Cell with Preferential Face‐on Orientation

Abstract

Efficient and stable hole‐transporting materials (HTMs) are necessary for perovskite solar cells (PSCs) with excellent efficiency and long‐term stability. Here, two A3B‐type metal phthalocyanine (MPc) compounds are prepared as dopant‐free HTMs for conventional n‐i‐p structured PSCs. Mono‐n‐butyl‐substituted zinc phthalocyanine and hexamethyl‐mono‐n‐butyl‐substituted zinc phthalocyanine (Me6Bu‐ZnPc) are synthesized through ring‐expansion method, and their exact structures are characterized using nuclear magnetic resonance and mass spectroscopy. The molecular orientation of the developed HTM thin films against the underlying surface is studied using X‐ray diffraction. Different substituents in MPcs can strongly affect their molecular orientation, resulting in different hole mobilities. The favored face‐on molecular alignment is only observed for Me6Bu‐ZnPc on the perovskite layer, proving the crucial role of methyl substituents in controlling the molecular alignment through the special interactions between the MPc molecule and different sites of perovskite material on the surface. PSCs using Me6Bu‐ZnPc as a dopant‐free HTM yields the highest reported power‐conversion efficiency (PCE) of 17.41%. With its high hydrophobicity and good coverage, Me6Bu‐ZnPc HTM thin film acts as an encapsulation layer, which leads to significantly increased long‐term stability. The Me6Bu‐ZnPc‐based devices retain over 90% of their initial PCE after 1400 h storage at 25 °C and with a relative humidity of 75%.

Novel Electron Transport Layer Material for Perovskite Solar Cells with Over 22% Efficiency and Long‐Term Stability

“…[5−10] Up to now, most of the efficient PSCs have the planar heterojunction architectures (i.e., regular n-i-p and inverted p-i-n planar architectures), in which the perovskite absorber layer is sandwiched between the electron transport layer (ETL) and the hole transport layer (HTL). Among them, optimization of the electron energy level and charge mobility of the transport layer plays a key role in improving the PCE of the planar heterojunction PSCs, [11,12] which is a key determinant of the charge extraction and charge recombination at the transport layer/perovskite (i.e., ETL/perovskite and HTL/perovskite) interfaces. [13,14] In traditional n-i-p planar PSCs, compact TiO 2 thin film is a commonly used ETL layer, mainly due to its appropriate conduction band (CB) energy level matching with the perovskite materials and its high transmittance over the whole visible spectra.…”

A Review on Solution‐Processable Dopant‐Free Small Molecules as Hole‐Transporting Materials for Efficient Perovskite Solar Cells

“…Besides the above example that demonstrates the importance of side chains attached on the arms, sidechains directly on the Pc cores were also reported, such as butyl, [140] methyl, [143] and hexyl sulfanyl groups. [144] Xu et al [143] developed a six-methyl and one-butyl-modified ZnPc molecule termed Me 6 Bu-ZnPc with a control molecule H 6 Bu-ZnPc which bore only one butyl sidechain. The six methyl groups featured Me 6 Bu-ZnPc with a face-on molecular arrangement atop perovskite layer, and the butyl group provided an enough solubility.…”

“…Although dopant-free SM-HTMs based on Pc cores were successfully applied in PSCs previously, [160][161][162][163] the solutionprocessed molecules began to reach a PCE of over 15% just in 2019. [137][138][139][140][142][143][144] In order to benefit describing the evolution of all the Pc-based HTMs, the molecules with TPA or DPA arms are also included in this part. Subsequently, we divide the SM-HTMs based on Pc cores into three classes, one with TPA or DPA arms, [139,142] one with aromatic arms other than TPA or DPA, [137,138] and one with alkyl, alkoxyl, or alkyl sulfanyl sidechains.…”

“…Subsequently, we divide the SM-HTMs based on Pc cores into three classes, one with TPA or DPA arms, [139,142] one with aromatic arms other than TPA or DPA, [137,138] and one with alkyl, alkoxyl, or alkyl sulfanyl sidechains. [140,143,144] Due to the widely reported success of using TPA or DPA to construct SM-HTMs, TPA arms were also picked to modify the periphery of Pc cores, though very few highly efficient SM-HTMs were reported. Muccini et al [139] conducted a research on such a molecule named OMe-TPA-CuPc with a control…”

Chlorophylls in thin-film photovoltaic cells, a critical review

“…Looking to the future, we fear that the (photo-)thermal stability issues will dominate the other when developing the PVC devices based on chlorophylls, as it happened with the hybrid perovskites in 2010s. 4,12,50,65 The intramolecular structures however can be relatively stable: the thermal destruction of methyl Pheo was observed only at t 0 > 351 C. 26 Any kind of derivatization reduces stability, for example destruction of chlorin e 6 begins at t 0 > 307 C.…”