Porphyrin-based materials for organic solar cells

  • Hongda Wang

Student thesis: Doctoral Thesis


A series of novel porphyrin materials with pushpull framework were designed and synthesized for organic solar cells (OSCs). To start with, a brief overview on the background of OSC, including dye-sensitized solar cells (DSSCs) and bulk heterojunction (BHJ) solar cells, and the porphyrin based materials for OSC applications was presented in Chapter 1. In Chapter 2, an efficient panchromatic light harvesting was demonstrated by the co-adsorption of a porphyrin molecule HD18 or HD19 and N719 in dye-sensitized solar cells. It is apparent that the porphyrin sensitizers show strong absorption in the Soret (400500 nm) and Q bands (600700 nm), while N719 shows efficient spectral response in the 500600 nm (between the Soret and Q bands), and the combination of these two kinds of dye molecules might display strong spectral response in the full-colour region. Mechanistic investigations were carried out by various spectral and electrochemical characterizations. The best co-sensitized device based on HD18 + N719 shows considerably enhanced power conversion efficiency of 8.27%, while those individually sensitized by HD18 and N719 display efficiencies of 6.74% and 6.90%, respectively. Subsequently, an optimized co-sensitized device based on the porphyrin HD18 and organic dye PT-C6 was fabricated by a stepwise adsorption of HD18 and PT-C6. The best performance of JSC/mA cm-2 =19.61, VOC/V = 0.74, FF = 0.69 and η = 10.1%, is superior to that of the individual device made from either HD18 (η = 7.4%) or PT-C6 (η = 8.2%) under the same conditions of fabrication. The post-adsorption of PT-C6 on the porphyrin-sensitized TiO2 anode surface not only enhances the spectral response of solar cells, but also greatly retards the back reaction between conduction-band electrons in TiO2 and the oxidized species ( I_3^-) in the electrolyte. In Chapter 3, a series of new donor-π-acceptor (DπA) porphyrin sensitizers with extended π-conjugation units were designed and synthesized for DSSC applications. Appending a phenothiazine (PTZ) donor moiety to the well-investigated porphyrin core and a variety of acceptors with electron deficient property at the opposite side can significantly red shift the absorption spectra to 700 nm in dyes (24). These different acceptor groups exert a significant influence on the electrochemical and photovoltaic properties of these sensitizers. These dyes have been evaluated in dye-sensitized solar cells, showing efficiencies of 0.90~7.29% with I^-/I_3^- based electrolytes. A detailed investigation on their physical, photophysical and electrochemical properties provided some important information on the factors affecting the main photovoltaic parameters. In Chapter 4, we designed and synthesized another series of dyes based on the rigid 2-aryl-1H-imidazo[4,5-b]porphyrin donors, in which an electron-accepting group was incorporated at the position 2 of imidazo unit via an aromatic spacer. Their photophysical and electrochemical properties, theoretical calculations and dye-sensitized solar cell performances have been investigated. The spectroelectrochemical data suggests the 1H imidazo unit can extend the conjugation length and lower the optical gap. As expected, the π conjugated substituents in all these dyes produced panchromatic absorption spectra over a wide range of wavelengths and IPCE spectra featuring a broad plateau in the region 430650 nm. In addition, both DFT computational and electrochemical data indicate a smaller HOMOLUMO energy gap for HD31Zn than that for dye 1, suggesting that a slightly more facile conjugation between the porphyrin core and the diketopyrrolopyrrole (DPP) unit through the 1H imidazo unit in HD31. Both Dye 1 and HD31Zn exhibited strong solvation effect in different solvents. The effects of solvents and their structures on the photophysical and photochemical properties and device performance have been studied in detail. The results indicate that porphyrin fused heterocycle as an effective electron donor and a suitable spacer between the donor and the acceptor can reduce the molecular aggregation through solvation effects. In Chapter 5, a series of conjugated DπA small molecules (YJ1YJ6, YJ13YJ15 and YJ16YJ19) for bulk heterojunction solar cells (BHJSCs) were prepared by the Sonogashira cross coupling of the electron rich porphyrin units with electron deficient benzothiadiazole (BT), DPP, or 3-ethylrhodanine moieties. The peripheral side chains on the porphyrin units like alkoxyl phenyl, alkyl, and (triisopropylsilyl)ethynyl (TIPS) can alter the solubility, conformation, and electronic properties of the obtained DπA small molecules, allowing the tuning of their photovoltaic properties when blended with fullerene derivatives. The presence of these side chains groups on porphyrin donor units affects the torsion angles between the side chains and the conjugated main chain, but resulting in only slightly different energy levels for the highest occupied molecular orbital (HOMO) for these molecules. Their performance in solution-processed solar cells is under studying. In Chapter 6, we reported the synthesis, electrochemical properties, and optical properties of seven novel BODIPY based π-conjugated materials. These dyes were synthesized via the Stille coupling reactions between the BODIPY units and electron donating groups (EDGs), such as 4,8-bis(5-(2-ethylhexyl)thiophen-2- yl)benzo[1,2-b:4,5-b′]dithiophene (BDT), 9,9-dioctyl-9H-fluorene (FL) or thieno[3,2-b]thiophene (TH). These donors were rationally chosen based on their gas phase ionization potential (IP) values estimated by density functional theory (DFT) calculations. Cyclic voltammetry of these dyes in dichloromethane solutions reveals that HOMOs of the resulting dyes correlated well with the ionization potentials (donor strength) of the donors. On the contrary, the lowest unoccupied molecular orbital (LUMO) energy levels of all dyes are fairly invariant, independent of the donors used. This suggests that the BODIPY moiety provides the primary influence on the LUMO levels of the materials. Two series of YJ9YJ11 and YJ21YJ23 show strong visible absorption in the red region. In addition, we presented the first example of a donor-acceptor BODIPY- containing conjugated copolymers, HDP6 and HDP7, with absorption over the entire spectrum of visible light and part of near infrared region (300900nm) making them suitable as additive for light-harvesting antenna. These dyes provide us with a toolset to tune the frontier molecular orbital energy levels, while retaining the low band gap and broad absorption of these dyes. Overall, these BODIPY molecules exhibited appropriate lower lying LUMO levels (3.70 ~ 3.86 eV) when compared with that of the P3HT, indicating their potential as acceptors for many donor materials in BHJSCs.
Date of Award21 Apr 2015
Original languageEnglish
SupervisorRick W K WONG (Supervisor)

User-Defined Keywords

  • Porphyrins
  • Solar cells.

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