Hybrid copper–indium disulfide/polypyrrole photovoltaic structures prepared by electrodeposition
Introduction
Conjugated polymers (CP) are a novel class of conductive materials which combine the optical and electrical properties of semiconductors with the processing advantages, good mechanical properties and low cost of polymers. CP offer new opportunities for producing coatings that function as an integral part of electronic devices, e.g. solar cells, to produce electricity from sunlight [1], [2]. Although a number of CP was synthesized, polypyrrole (PPy) was one of the most intensively studied polymers during the last decade [3], [4]. PPy shows several application advantages, such as easy synthesis in aqueous media with a wide range of possible dopants, relatively high stability of electrical conductivity and good mechanical properties [4].
The combination of CP with inorganic semiconductors, e.g. CuInS2 (CIS), is attractive for use in thin-film photovoltaic cell structures [2]. For example, a Schottky barrier-type PV junctions were formed between electrodeposited CISe and heavily doped poly(3-methylthiophene) prepared by electropolymerization [5]. Also, Schottky barriers were observed in CdS/poly(3-methylthiophene) and CdSe/poly(3-methylthiophene) junctions. The value of energy conversion efficiency 1.3%, uncorrected for light losses, was obtained for this CdSe/poly(3-methylthiophene) junction [6]. The multilayer p-i-n structures consisting of CIS and poly(3,4-ethylenedioxythiophene) thin films were prepared and investigated for PV applications by the authors [7].
CIS polycrystals show a very high optical absorption coefficient of more than 104/cm. The bandgap of CIS can be adjusted between about 1.1 and 1.8 eV. It is a very stable material well adapted to the solar spectrum, presently intensively studied for photovoltaic applications. In this work, the so-called CISCuT deposition method was applied to form the polycrystalline CIS absorber layer on a copper tape substrate [8].
Among various techniques for the preparation of these hybrid inorganic–organic structures with PPy, the electrodeposition of the polymer onto the surface of CIS deserves special attention because it can be easily controlled by regulating the polymerization current or potential and the time for producing high-quality PPy films with predictable properties [9]. Moreover, electrodeposition is an inexpensive, low-temperature and relatively non-polluting method. However, the process of PPy electrosynthesis at the surface of semiconductors with high electrical resistance is far from being easily controllable due to non-linear electrical processes in the semiconductors. PPy films of good quality are most often polymerized, using certain sulfonic acids and salts (naphthalene sulfonates, toluene sulfonates, etc.) as dopants with planar aromatic groups. These dopants induce structural regularity in the polymer film. Also, they can absorb more easily at the electrode surface and thus facilitate the first steps of the polymerization, leading to an improved adhesion of PPy with a high conductivity of the film [3], [9].
PPy has always p-type conductivity when doped with sulfonates, but the type of conductivity in CIS strongly depends on the composition and defects and may be p-or n-type. The polymer layer of p-type is considered as an alternative for the traditional buffer layer and window layer in the conventional cell structure. The purpose of our work is to determine appropriate conditions of electrochemical preparation of thin PPy films with a good adherence to the top CIS layer and to investigate the obtained structures. In order to prepare Cu/CIS/PPy photovoltaic structures, appropriate concentrations of reagents, current densities, potentials of electrodeposition and illumination conditions were selected experimentally.
Section snippets
Experimental
For our investigations, the Cu/CIS structures were received from Institut für Solartechnologien (Frankfurt (Oder), Germany). The so-called CISCuT deposition method [8] was used to form a polycrystalline CIS absorber layer on a copper tape substrate. The idea of the method is fast roll-to-roll sulfurization in Sx+N2 gas atmosphere at 500–600 °C of 0.8 μm indium precursor on the copper tape substrate. Quick chemical reaction of indium and copper from the substrate with sulfur from the gas phase
Results and discussion
The morphology of CIS and PPy films was studied using the SEM technique. Fig. 1 shows the SEM micrographs of the surface of CIS in the Cu/CIS structure (a) and from the PPy surface in the Cu/CIS/PPy structure (b). The thickness of the layers was about 1.5 μm for the CIS layer and 2 μm for the PPy layer. The top CIS layer has a relatively homogeneous polycrystalline grain structure and grain boundaries with an average grain size of 500 nm. As noted [10], these crystallites grow from the CIS
Conclusions
The appropriate conditions for electrodeposition of high-quality PPy films with a good adherence to the surface of polycrystalline CIS were determined in the dark and under illumination. Microscopical investigations confirm a deep adhesive penetration of electrodeposited amorphous PPy into the CIS structure. An original method of selective electrodeposition of PPy under illumination helps to passivate the shunting areas of the cell absorber. It was shown that cell improvements occur due to the
Acknowledgements
Financial support from Estonian Science Foundation (Grant 5190), the European Polymer Solar Battery–EURO-PSB project (Contract ENK5-CT-2002-00687) and the INTAS Project Ref. Nr. 03-51-4561 is acknowledged.
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