|Title||Artificial photosynthesis : towards the development of molecular photodiodes|
|Source||Agricultural University. Promotor(en): T.J. Schaafsma. - S.l. : Savenije - ISBN 9789054856467 - 113|
|Publication type||Dissertation, internally prepared|
|Keyword(s)||fotosynthese - photosynthesis|
Since quite some time similarities have been noted between the photo-generation of charge carriers (electrons and holes) in a molecular semiconductor and the photosynthetic reaction centre of bacteria. For example, the mechanism for charge carrier generation by light in phthalocyanine films is largely dependent on the presence of ionised impurities. These impurities have been proposed to play a role similar to that of the special pair in bacterial reaction centra. In addition, the redox chain cascade in the reaction centra has a comparable function for charge transport as the internal field of a Schottky barrier. Although the present work applies to entirely different molecular system, this analogy is confirmed by the results of our experiments.
Chapter 1 contains a general introduction and the theoretical framework to understand the basics of charge separation processes in organic donorlacceptor combinations and in p/n heterojunctions. In this study the expertise at the Department of Molecular Physics with water-soluble porphyrin heterodimers has been used as a starting point to examine the photophysical and charge-transfer properties of molecular assemblies upon organising them as two thin layers on top of each other. A complication arises from the requirement to produce pinhole-free films for (photo) electrical measurements.
Experimental procedures to perform photo-electric measurements, optical characterisation and other techniques are described in Chapter 2.
In Chapter 3 we describe the adsorption of a fourfold positively charged tetrakis(4 - methyl pyridinium) porphyrin and its metal derivatives on glass surfaces from basic aqueous solutions. The thickness and the depth profile of the chemical composition of these films has been determined using optical and ion beam techniques. Upon exposure to a buffered porphyrin solution, the glass surface is initially almost completely coated with a monomolecular porphyrin layer (5*10 13molecules*cm -2). Subsequently, the area of the glass surface available for deposition increases due to leaching of the glass by the buffer solution, resulting in an almost linear increase of the amount of adsorbed organic material with the period of exposure.
In Chapter 4 a different technique to produce thin films on transparent, electrically conducting substrates (ITO) is introduced, i.e. the electro-polymerisation of hydroxy-phenyl substituted metallo-porphyrins. During electro-polymerisation an ether bond is formed between the phenyl rings of two porphyrin units, leaving the porphyrin macrocycles undisturbed. Using this deposition technique the thickness as well as the redoxpotentials of the layer can be accurately controlled. Using spectro-electrochemical techniques we can conclude that the oxidised film effectively consists of dimeric porphyrin mono-cations as the building blocks. To characterise charge carrier conduction in the polymerised films, diffusion coefficients were determined and found to be (1-2).10 -10cm 2s -1for films of porphyrins with varying number of hydroxyphenyl substituents, indicating that no correlation exists between the value of the diffusion coefficient and the number of cross links per porphyrin unit. This suggests that charge transport is mainly from ring to ring.
Chapter 5 demonstrates a photovoltaic effect in organic heterojunctions, made by the consecutive deposition of two different types of porphyrins onto an ITO substrate equipped with a mercury top contact, using a home-built measuring cell. From photoelectric measurements of the separate and composite electron donor- and acceptor porphyrin layers with varying order of the layers it can be concluded that the photoactive region is located at or near the interface of both porphyrin layers. Changing the redoxpotentials for the porphyrin donor/ acceptor combinations results in different values of the observed short circuit current, demonstrating the dependence of the photo-induced electron transfer on the exothermicity of the electron transfer. Depending on the layer thickness and type of porphyrins, the open circuit voltage is typically ≤0.4 Volt and the short circuit current ≤130 μA.cm -2using illumination of 13 mWatt.cm -2at 440 nm.
To clarify the photovoltaic mechanism of these porphyrin heterojunctions cells in greater detail Chapter 6 reports the results of impedance spectroscopy providing evidence for the formation of a depletion layer upon contacting the two porphyrin films. This depletion layer can be described in the same way as in a conventional p/n heterojunction of inorganic semiconductors. From Mott-Schottky plots the doping concentration is found to be ≈10 17cm -3for electro-polymerised zinc tetra-(hydroxy phenyl) porphyrin films and two orders of magnitude higher for spin-coated films of free base tetra (methyl pyridinium) porphyrin. Excitation of these cells with a nanosecond laser pulse reveals a lower limit for the photocurrent response time of < 10.
From the results of Chapters 5 and 6 we conclude that charge separation and thus the photo- active part of the cell is confined to the interface of both layers. For charge collection, consecutive dissociation of the electron/hole pair competes with charge recombination, in particular in organic heterojunctions with presumably low values of the dielectric constant. The internal field over the heterojunction space charge layer causes the photo-induced charge carriers which are generated at the interface to drift through the bulk layer to the electrodes. This explanation for the photovoltaic effect in organic heterojunctions implies that for a device to be efficient, the rate for electron transfer at the interface must be optimised, next to the presence of an internal field. This also implies that for a sandwich-type cell, such as described in this work (but also many other cells reported in literature) the efficiency (η ≤2%) is restricted by the absorbance of only a few dye monolayers. For application of this kind of charge generation layers in organic solar cells an antenna layer must be added to the cell.