Polymer-based Hybrid Cells
The solar power conversion offers outstanding potential due to its required minor investment and variety of applications as nicely illustrated by conventional silicon-based photovoltaic systems found everywhere on this planet and beyond. However, conventional inorganic photovoltaic systems are associated with high fabrication costs and limited adaptation to novel requirements. By now, organic materials are an inherent part of the research on tomorrow's photovoltaic energy conversion systems, because they promise reduced fabrication costs, simple processing and the potential employment of flexible substrates. A potential approach includes the so called hybrid photovoltaic cells (see figure 1), which are a combination of two independent solar cell technologies: the dye-sensitized solar cell (DSSC) and the polymer solar cell consisting merely of p- and n-type conjugated polymers. The DSSC consists of dye-sensitized nanostructured titanium dioxide on a compact transparent semi-conducting oxide (titania) and the multi-component redox-electrolyte filled in between the two electrodes. Although they are still some of the most efficient organic solar cells, the presence of the liquid electrolyte creates sealing issues which also initiated the research for an alternative hole- conducting material. For the hybrid photovoltaic cells the electrolyte is replaced by a p-type conducting polymer acting as hole-conducting and electron-blocking layer of the device. This combination of the inorganic semiconductor and a semi-conducting polymer provides the practical advantages of the organic material, e.g. synthetically tailored properties, simple processibility, mechanical flexibility, as well as the high electron mobility of the inorganic material. The performance of these hybrid devices significantly depends on the morphology of the nanostructured inorganic material, i.e. titania, because the morphology determines the volume-to-surface ratio and hence the surface available for interfacial reactions. Moreover, the morphology influences charge carrier transfer routes and thus electron-hole recombination probabilities. Therefore, the tailoring of the desired morphology yielding an efficiently performing hybrid solar cell is of great importance and considered to be the crucial step of the layer stack build-up. As a consequence, the preservation of the tailored morphology is the main requirement for the successive stages of preparation towards the device.
The performance significantly depends on the morphology of the nanostructured tiania, because the morphology determines the volume-to-surface ratio and hence the surface available for interfacial reactions. Moreover, in photovoltaics the morphology influences charge carrier transfer routes and thus electron-hole recombination probabilities. In order to accomplish the requirements arising from these versatile applications it is important to prepare the desired morphology with high reproducibility and homogeneously spread out over areas of squarecentimeters. There are numerous reports about the synthesis of nanoscale titania materials of different morphologies such as nanoparticles, nanorods, nanowires and nanotubes, nanovesicles, structures of mesoscale networks as well as lamellae. Many synthesis approaches combine organic and inorganic materials whereas the organic component controls the mesoscale structure, e.g. block copolymers, and the inorganic component, namely titania, provides a special functionality resulting in the formation of a functional and structured hybrid system. The main structure control is based on the formation of separated microphases and thus the self-organization mechanism of block copolymers.
To achieve titania thin films with variable morphologies and in a reproducible way, we apply a rather simple recipe using an amphilic diblock copolymer P(S-b-EO) as the structure-directing agent and combined with a so called good-poor solvent pair induced phase separation coupled with a sol-gel chemistry (see figure 2). The diblock copolymer is dissolved in 1.4-dioxane, which is a good solvent for both blocks, the major PS and minor PEO constitution parts. The successive addition of HCl as a poor solvent for PS results in the formation of cylindrical micelles in the solution. Furthermore, the added sol-gel precursor of titanium tetraisopropoxide (TTIP) is coupled to the surface of the PEO domain but can also be incorporated into the PEO domain. At last the self-organization mechanism leads to the formation of nanostructures by thin film preparation via spin-coating. A detailed investigation of the structure templating process was reported recently summarized by a phase diagram of titania morphologies representing the rich capabilities of the preparation process of tailor-made morphologies.
The investigated stages of the preparation towards the hybrid solar cell are schematically presented in Figure 3. In the typical hybrid photovoltaic device multiple functional layers are sandwiched between two electrodes. In our system the electrodes are represented on one side by the anode of fluorine-doped tin oxide (FTO) on glass substrate and on the other side by the cathode of a thermally evaporated gold layer. In the progression of this investigation, the FTO-anode will be considered as the bottom electrode, since FTO on glass is the substrate from where the stack build-up starts. Actually this is the top-electrode when operating the device under the incident sunlight. The stack build-up starts with the nanostructured titania nestling directly on top of the FTO. With the absence of the corresponding compact titania layer underneath the nanostructured titania, a novel approach is employed. This approach comprises the integrated self-encapsulation of the titania nanostructures hence an encapsulation mechanism supplied by its own preparation components. Therefore, a solution containing the triblock copolymer poly(ethyleneoxidemethacrylate-block-dimethylsiloxane-block-ethyleneoxide-methacrylate) ((PEO)MA-b-PDMS-b-MA(PEO)) in combination with a titania-based sol-gel chemistry is spin-coated resulting in a thin polymer nanocomposite film. The thin film is formed by the triblock copolymer acting as structure-directing agent, originating from the formation of separated microphases and thus the self-organization mechanism of block copolymers. In addition, the phases of (PEO)MA contain the titanium supplied by the sol-gel chemistry, which is the initiator of the incorporation into this particular phase. Because of the block ratio of (PEO)MA to PDMS, the PDMS-phase will form the matrix with embedded (PEO)MA-phases, hence embedded titania. Afterwards the PDMS-matrix is successively removed by plasma etching resulting in exposed titania nanostructures. In the final step the remaining polymer nanocomposite film with titania sticking out is calcined at elevated temperatures. As a result the titania is converted to crystalline anatase phase and the remaining PDMS turns into silicon-oxi-carbide (SiOC) type ceramic. In contrast to the conventional approach employing the compact titania layer, the SiOC acts as the blocking layer between the conducting polymer and the transparent anode across the titania nanostructures. In this stage of the preparation, a ruthenium dye is anchored to the titania and forms a monolayer of dye molecules on the surface of the exposed nanostructures as depicted in the cross section scheme in Figure 3. In the next step, the actual organic photoactive layer of poly(3-hexylthiophene) (P3HT) is spin-coated on the prepared inorganic nanostructures. As the uppermost sandwiched layer in direct contact to the cathode, a layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) is spin-coated on P3HT. The role of the PEDOT:PSS polymer mixture is to block the free electrons from reaching the cathode, but enabling the conduction of the holes towards the cathode.
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last change: June 1, 2012