Studies of Conjugated Small Molecules, Polymers and Organic-Inorganic Hybrid Materials for Photovoltaic Applications
This dissertation focuses on the photophysical properties and device studies of three different types of materials, namely polyoxometalate (POM)-containing organic-inorganic hybrids, near-IR absorbing conjugated polymers and discotic polycyclic aromatic hydrocarbons. POM-containing organic-inorganic hybrids are interesting because of the potential roles, such as electron acceptors and electron transporters, that POM clusters may be able to play. A molecular hybrid (Mo6-Fe) containing a hexamolybdate cluster on one end, a ferrocenyl unit on the other and a conjugated bridge has been explored for photovoltaic applications. This hybrid shows direct charge transfer absorption in its UV-vis absorption spectrum. While the hybrid itself show very poor photovoltaic properties, solar cells fabricated from its PCBM blends show attractive device performance with power conversion efficiencies up to 1.8%. To develop a POM-containing hybrid system that exhibits the desired morphologies for solar cells, three POM-containing rod-coil hybrid diblock copolymers (PS-Mo6-PT1-3) with different rod block lengths have been carefully studies on their optical, electrochemical, morphological and photovoltaic properties. Solar cells fabricated from these hybrid diblock copolymers (without added PCBM) did show photovoltaic effects with around 0.01% power conversion efficiency. While good open circuit voltage (1.25 V) is observed for the pristine film, the short circuit photocurrent is dismally low. Annealing improves the photocurrent by one order of magnitude and also the fill factor, presumably due to the formation of desired phase-separated domains. The overall photocurrent is still very low, likely due to the poor photoinduced charge transfer from the PT backbone to the POM cluster, which is corroborated by femtosecond time-resolved fluorescence studies. Two new POM-containing main-chain conjugated polymers (P10 and P11) have been synthesized and characterized in detail. Simple single layer solar cells based on P10 show respectable power conversion efficiencies up to 0.31%. Incident photo-to-current conversion efficiency measurements have confirmed the photocurrent contribution of both the organic π-segments and the POM clusters. Near infrared (NIR) absorbing conjugated polymers combining alkoxy-substituted bithiophene units with different comonomers have been synthesized. Due to the electron-donating properties of the alkoxy substituents and the near planar structure of the bithiophene unit, conjugated polymers containing 3,3’-dialkoxy bithiophene units show strong propensity towards electrophiles such as H⁺. The protonated polymers show strong NIR absorptions due to intra-chain charge transfer. The strength and the wavelengths of the NIR absorption depend on the comonomers as well. Comonomers with stronger electron donating properties (such as NDT over INDT) lead to more intense NIR absorptions and longer NIR absorption wavelengths. The protonation process is found to follow the first order reaction kinetics. While the NIR-absorbing polymers, when blended with PC₇₁BM, show photovoltaic properties, only the absorptions in the UV-visible range are able to produce photocurrent. Three PAH molecules, all based on the TBP core but with different peripheral substituents, have been studied as hole transporting materials. The SCLC hole mobility of compounds 1a-1c pristine films deposited by spin-coating were measured to be 8.58×10⁻⁴ cm² V⁻¹ s⁻¹, 5.42×10⁻³ cm² V⁻¹ s⁻¹, and 2.30×10⁻⁴ cm² V⁻¹ s⁻¹, respectively, which are typical for solution-processed organic thin films. Thermal annealing improved the mobility of all three compounds with compound 1b showing the largest increase and an appealing SCLC hole mobility of 3.63×10⁻² cm² V⁻¹ s⁻¹ was obtained. The XRD studies of thin films before and after annealing suggest better molecule orders for both 1a and 1b after thermal annealing, which is likely responsible for their improved hole mobility.
Table of Contents
Introduction of organic photovoltaic devices -- Polyoxometalate containing organic-inorganic hybrid materials for photovoltaic applications -- Near infrared absorption polymers with 3, 3'-diakoxy substitute bithiophene units -- Hole mobility studies of selected polycyclic aromatic hydrocarbons