Design, synthesis and applications of novel hole transporting materials for Perovskite solar cells
Creators: name: REDDY, Govind
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)
classificationCode: 400909
subject: Photonic and electro-optical devices, sensors and systems (excl. communications)
classificationCode: 401807
subject: Nanomaterials

description: Perovskite solar cells are one of the rapidly advancing technologies in photovoltaic research to convert solar energy into electrical energy as we move towards a carbon free society. This technology started its journey in 2009 with cells having modest efficiencies of ~3.8% by replacing the organic dye in a solid-state dye sensitized solar cell with a perovskite light harvester. Within 10 years, the power conversion efficiency has jumped to 25.2 % (2019), with the added benefit of simple fabrication methods, making commercialization of this technology a possibility. The key layers present in a perovskite solar cell are the light harvesting perovskite, the hole transporting layer and the back contact (Au), with commercialization hampered because of the instability of the perovskite and the cost associated with the other two layers. Of particular interest is the hole transport layer, which plays a key role for the protection and performance of the perovskite solar cell. Up until now, benchmark hole transport materials (HTMs) such as spiro-OMeTAD and PTAA have displayed the highest efficiency, but the cost of these materials hinders the manufacture of low-cost perovskite solar cells, contributing up to 25-30% of the overall cost of the cell. In addition, chemical additives are necessary to match the HTM band alignments to perovskite, and to improve the hole mobility. Use of such additives causes problems, as doping leads to highly hydrophilic hole transporting layers, causing rapid degradation of the complete device, leading to a drop in performance.

In order to address these issues, this thesis has focussed on the design, synthesis, and characterization of a range of novel small molecules to be used as hole transporting layers. Although a variety of organic small molecules have been developed before, there is still scope for new organic materials with improved properties, and greater understanding of the material classes currently in use, that may lead higher efficiency in perovskite cells, improved chemical stability and lower cost. Careful molecular engineering has resulted in two kinds of new hole transporting materials (HTM) being developed and described in this thesis: organic and organo-macrocyclic (porphyrin) derivatives. 

The first results presented in this thesis concern the synthesis of two new organic materials with the configurations of donor-acceptor (D-A) and donor-spacer-acceptor (D–π–A), namely OHM-1 and OHM-2 respectively. Here, Phenothiazine acts as a donor (D), Phenonthro-imidazole acts as an acceptor (A) and Phenyl acts as spacer. These materials were developed through simple synthetic schemes with the minimized number of steps, and simple purification procedures allowing cost-effective upscale. As these hole carriers have long alkyl chains, they exhibit some hydrophobic nature, which is an important characteristic to protect solar cells from moisture. Both theoretical and experimental analysis was carried out for these two molecules. Theoretically, OHM-1 and OHM-2 displayed low hole reorganization energy than electron one and both materials act as p-type or hole extracting materials. Insertion of the phenyl spacer decreases the band gap, and improves the conductivity of OHM-2 relative to OHM-1. Theoretical calculations of these two molecules initially confirmed their promise as hole carriers, leading to their synthesis using low-cost starting materials. Experimental characterisation elucidated the significant effect of the phenyl spacer on the optical, electrical, and electrochemical properties towards hole transporting phenomena. Ultimately, this strategic molecular engineering of HTMs based on modelling of new materials followed by synthesis and characterisation will lead to new paths for the development of materials for optoelectronic devices such as perovskite solar cells.
 
Next, the synthesis of two novel organo-macrocyclic materials with the configuration of donor-spacer-donor (D–π–D) namely HPPHT (metal-free) and ZPPHT (having Zn metal in its macrocycle) respectively, was undertaken. Here, Phenothiazine acts as a donor (D), and Bis ethynyl porphyrin acts as the spacer. These hole carriers were designed with long alkyl chains to improve solubility in organic solvents and to give them a hydrophobic nature. Synthesis of these molecules was optimised via condensation and Pd-coupling reactions with low-cost precursors. The cost of these HTMs was estimated to be ~$85/g for ZPPHT and ~$112/g for HPPHT. The introduction of Zn into the D–π–D porphyrin molecular system not only improved the planarity and hydrophobicity of the molecule, but also allowed a pinhole free surface to be screen printed, which led to a higher water contact angle 114.2°, and stability of more than 120 hours in water and under ambient conditions. Dopant free perovskite solar cells were fabricated by screen printing and employing HPPHT and ZPPHT as the HTL. ZPPHT achieved over 11% efficiency under 100 mW/cm2, which is the highest reported efficiency so far for a D–π–D porphyrin hole-extraction layer using a carbon-graphene composite as a cathode. A larger mini-module was also fabricated by adopting a similar procedure to the test cell device, and the corresponding ZPPHT device had an efficiency of 3.54 %. Overall, the efficiency was ascribed to the strong phenothiazine donor linked to the porphyrin, with intra molecular charge transfer via Zn metal leading to an increase its conductivity relative to HPPHT.

To reduce the cost of above macrocyclic HTMs, HPPHT and ZPPHT were re-synthesized, by minimizing the number of steps, and Cu, which exhibits high conductivity and is cheaper was used, leading to the Cu based D–π–D porphyrin (CPPHT), which was synthesized and compared to HPPHT and ZPPHT. These D–π–D porphyrin molecular systems exhibited appropriate energy levels for perovskite, improved electrical conductivity for the Cu derivative, and also acceptable thermal stability for solar cell applications. The higher performance of selective HTM free- Carbon nanoparticle Graphene Composite CGC layered PSC devices were converted to HTM (HPPHT, CPPHT and ZPPHT) based CGC layered PSC devices by a front layer infiltration process. The power conversion efficiency (PCE) of these three HTMs was in the range of 10-11.0% without dopant. To further increase the performance of the D–π–D porphyrin-based perovskite devices, Li-TFSI/t-BP was introduced via drop casting. The device performance improved by 13% compared to that of the control PSCs. Furthermore, CPPHT based solar cells showed superior short-term and operational stability against moisture than the Li-TFSI/t-BP CPPHT solar cells; due to the Li-TFSI/t-BP engrossing moisture and damaging the interface. 

This thesis thus presents a series of results and thorough discussions on the fundamental relationship between the structure and performance of the molecules we have designed, synthesised and tested, extending our understanding of the factors that lead to effective hole transport materials in perovskite solar cells.