Development of Functionalized Tetravalent Phthalocyanines for Low-cost Organic Photovoltaics

Abstract

Organic photovoltaic (OPV) devices possess several advantages over their silicon-based counter parts, namely mechanical flexibility, inexpensive high throughput processing and lightweight modules. These characteristics have prompted the scientific community to intensify efforts to make such technology economically viable. In the past two decades, due to clever molecular engineering, the power conversion efficiency (PCE) of OPVs has increased from around 2% to almost 20%. Unfortunately, such accomplishments have come from the design and development of organic semiconducting compounds whose synthesis and purification are increasingly complex, hindering commercial deployment. Alternatively, axially substituted silicon phthalocyanines (R2-SiPc) are promising candidates for low-cost OPVs. SiPc dyes have been employed in organic electronics for almost 50 years, owning to their high molar extinction coefficient, high chemical and thermal stability, and ease of manufacturing. In this thesis, we aimed to explore how the physical, chemical, and electrochemical properties of R2-SiPc derivatives interplay with their performance in OPVs. We have demonstrated that the solubility of the derivatives plays a significant role on their performance as ternary additives in Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric acid methyl ester (P3HT and PC61BM, respectively) bulk heterojunctions. We demonstrated that when employed as stand-alone acceptors, the final morphology of P3HT/R2-SiPc films, and consequently their performance, correlates to basic thermodynamic properties such as critical radius, miscibility, and crystallization enthalpy. Such properties have also been correlated with the length and branching of axial silane groups, serving as a guide for molecular design, in particular, with respect to the solubilizing alkyl groups present in many organic semiconductors. We have demonstrated that R2-SiPc derivatives can be paired with polymers other than P3HT, to achieve respectable metrics, namely Voc, but the overall efficiency is limited due to their shallow highest occupied and lower unoccupied molecular orbitals (HOMO and LUMO) energy levels. As such, we have also explored peripherally fluorinated R2-FXSiPcs derivatives and characterized how the degree of fluorination affects the electrochemical and physical properties of the R2-SiPcs. Through control of the degree of fluorination, it was possible to pair, in OPVs, R2-FXSiPcs with polymers that unfluorinated R2-SiPcs was not originally compatible with, such as poly[[6,7-difluoro[(2-hexyldecyl)oxy]-5,8-quinoxalinediyl]-2,5-thiophenediyl]] (PTQ10) and even achieve air-stable electron conduction. Ultimately, we found that the degree of fluorination controls the charge carrier (electron or holes) that is preferably conducted in the R2- FXSiPcs phase. The results stablish novel property-performance relationships that can be used to move away from trial-and-error approaches and designing organic electronics and further stablishes silicon phthalocyanines as promising cores for organic electronics.