It has been ten years since the birth of perovskite solar cells (PSCs). Remarkable progress has been made, with their efficiency increased from its initial 3.8% to 24.2%, and the introduction of organic hole-transport layer materials has improved the stability, efficiency and lifetime of perovskite solar cells. The commercialization of PSCs has been inhibited by its long synthesis cycle, low yield, high cost and low stability due to doping, and the efficiency of PSCs appear to have reached their upper limit. Research is currently focused on how to improve the efficiency and lifetime of the devices, while reducing the costs and working out how to manufacture them at an industrial scale. There are also scientifically significant questions that need answering around solving energy and environmental problems.

A research group led by Associate Professor Xu Zongxiang from the Department of Chemistry at Southern University of Science and Technology has focused on metal phthalocyanine-based organic semiconductor molecular design and optoelectronic device applications. Metal phthalocyanine is an excellent semiconductor material with simple synthesis and purification, strong molecular regulation and high stability. It is widely used in many kinds of organic optoelectronic devices. The research group recently had three papers published in top energy-related journals, Advanced Energy Materials (IF=24.884) and Solar RRL (Expected IF of ~7). This is the first time undergraduate students at SUSTech have published papers in journals with an impact factor of more than 20.

The paper published in Advanced Energy Materials was titled “High‐Performance and Stable Perovskite Solar Cells Based on Dopant‐Free Arylamine‐Substituted Copper(II) Phthalocyanine Hole‐Transporting Materials,” and was selected as the back cover of the journal. SUSTech was the first communication unit. Research assistant Feng Yaomiao and junior student Hu Qikun were the co-first authors. Associate Professor Xu Zongxiang and Michele Muccini were the co-correspondent authors. Other co-authors were Dr Andrea Lorenzoni and Francesco Mercuri of the Italian National Research Council.

The traditional hole-transport material Spiro-OMeTAD is represented by arylamine compounds. Arylamine compounds have been a focus due to their structural diversity and easily tunable frontier orbital energy levels, with improved film forming capacity and high thermal & morphological stability. The non-planar conformation of the arylamine compound, combined with the distortion between the core nitrogen and aryl groups, makes the arylamine compound-based films mostly amorphous. This reduces the charge carrier mobility of the arylamine compound and results in the need for p-type doping of the arylamine compound. It further reduces the device stability. The pseudo 3D conjugated structure of the arylamine and its propeller conformation in the hole-transporting material prevents close contact between the perovskite and the hole-transporting material, thereby reducing charge extraction rate of the perovskite.

Associate Professor Xu Zongxiang’s research group originally introduced a triphenylamine group into the phthalocyanine molecule. The addition of the triphenylamine group combined its advantages with the rigid large π conjugate plane and high carrier mobility of the copper phthalocyanine. The material was applied to the perovskite solar cell without doping, achieving a conversion efficiency of 19.7% (the highest record of the currently reported doping-free phthalocyanine hole-transporting material), and still maintained 92% of its initial PCE in 960 hours. These results open a new way toward the fabrication of inexpensive, highly efficient and stable PSCs.

Link to the paper: https://onlinelibrary.wiley.com/doi/10.1002/aenm.201901019

The first paper published in “Solar RRL” is titled “Molecularly Designed Zinc (II) Phthalocyanine Derivative as Dopant‐Free Hole‐Transporting Material of Planar Perovskite Solar Cell with Preferential Face‐on Orientation.” Research assistant Dong Lei, junior student Hu Qikun and postdoctoral researcher Ehsan Rezaee were the co-first authors. Associate Professor Xu Zongxiang was the sole corresponding author.

The research team had previously produced and synthesized a series of tetraalkane-substituted phthalocyanines, achieving conversion efficiencies of up to 17.8%. However, there are four isomers in these phthalocyanines, and the existence of isomers reduces the crystallization quality of phthalocyanine thin films. This results in uneven device performance which makes it difficult to further improve device efficiency, limiting development and application in perovskite solar cells.

Following their previous research, Associate Professor Xu Zongxiang’s research group obtained a tetrabutyl-substituted zinc phthalocyanine without isomers through a simple ring expansion process. The purity of zinc phthalocyanine was verified by nuclear magnetic resonance spectroscopy. Further application of phthalocyanine materials in perovskite battery devices found a more uniform performance with a 21% improvement in average efficiency comparing with zinc phthalocyaine with isomer. The perovskite solar cell could maintain an initial efficiency of more than 90% for 750 hours. This presents a new idea for the design and synthesis of high performance phthalocyanine-based hole transport materials.

Link to the paper: https://onlinelibrary.wiley.com/doi/pdf/10.1002/solr.201900119

The other “Solar RRL” paper was titled “Dopant‐Free Hole‐Transporting Layer Based on Isomer‐Pure Tetra‐Butyl‐Substituted Zinc(II) Phthalocyanine for Planar Perovskite Solar Cells.” Junior student Hu Qikun was the first author with co-first authors research assistant Dong Lei and postdoctoral researcher Ehsan Rezaee. Associate Professor Xu Zongxiang was the sole corresponding author.

Early research from the team had indicated that octamethyl modified metal phthalocyanines that form face-on molecular configurations on the perovskite layer would greatly increase the carrier mobility. The power conversion efficiency of phthalocyanines is increased by more than 50% compared with pristine metal phthalocyanine , but their organic solubility is poor. The devices can only be prepared by a vacuum deposition process. The problem of high energy consumption affects the further commercial application of these materials.

Based on their previous research, the research group synthesized a modified asymmetric zinc phthalocyanine named hexal-methyl-mono-n-butyl substituted ZnPc. Their approach ensured that phthalocyanines could be deposited on the perovskite layer with full face-on orientation, which realized simple solution process of preparation for phthalocyanine films.

The “face-on” arrangement of phthalocyanine induces power conversion efficiency increased by 70% compared with edge-on Pc and was also very stable in an unpackaged test of 1400 hours. This is the first report of soluble phthalocyanines with complete “face-on” arrangement on perovskite.

The study provides new ideas for the molecular design of phthalocyanine perovskite hole transport materials. The further replacement of phthalocyanine central metals is expected to achieve higher power conversion efficiency.

The related research work was supported by the Shenzhen Municipal Science and Technology Innovation Committee’s basic research discipline layout project and the Shenzhen Development and Reform Commission’s Flexible Solar Cell R&D Engineering Research Center.

Link to the paper: https://onlinelibrary.wiley.com/doi/abs/10.1002/solr.201900182