Photochemical and photophysical properties of diketopyrrolopyrrole organic pigments
Various aspects of the pigment class based on 1,4-diketo-3,6-diphenyl-pyrrolo(3,4-c)pyrrole (DPP) are investigated. The thesis consists of two parts: the first part, motivated by the degradation problems of DPP, focuses on the molecular photophysics of the DPP class of compounds. The second part, motivated by photovoltaic applications, focuses on the electrical and photovoltaic properties of solid state DPP. Investigation of the photoluminescence of the main commercial derivative, p-Cl DPP (PCD) showed, that the emission of the pigment in polymer matrices may be attributed to aggregates at the surface and that it is sensitized by monomers of DPP. From the distinct excitation spectrum of the monomers it is thus concluded that these play a major role in the photoactivity and photodegradation of the pigments. In solution the dominant role of singlet oxygen in the degradation of the molecule has been shown. The quenching of the singlet excited state of DPP by oxygen efficiently produces singlet oxygen and triplet States of DPP. The latter is again an efficient sensitizer of singlet oxygen. The triplet state energy has been measured for DPP (28.5±2 kcal/mol) and the p-dimethylamino derivative DMADPP (29.5±2 kcal/mol). The triplet energy for DMADPP is slightly higher than for DPP, although the respective singlet energies have the opposite trend (DPP: 56 kcal/mol and DMADPP: 51 kcal/mol). Based on molecular orbital calculations this observation is attributed to the structure of the HOMO and LUMO orbitals and it is concluded that the large singlet to triplet energy splitting is a property of the core of the pyrrolopyrrole chromophore. DPP and DTPP (dithioketo-DPP) are used in electrochemical and heterojunction solid state photovoltaic cells based on porous TiO2 electrodes. MCDPP, a derivative of DPP using a carboxylic anchoring group yielded an incident photon to current conversion efficiency of 47% in a photoelectrochemical cell. The solid state photovoltaic cells yielded poor photocurrents. The use of MCDPP in the TiO2/DPP heterojunction increased the photocurrent. Surface photovoltage spectroscopy and Kelvin probe measurements were used to determine the workfunction of DPP (5.38 eV), DTPP (4.95 eV) and DMADPP (5.08 eV) and to investigate surface and interface band bendings of DTPP on SnO2. A band diagram is proposed for this system. DTPP and DPP showed both p-type band bending.