CHESS · Block Copolymers for High Efficient Solar Cells with novel Structures

Block Copolymers for High Efficient Solar Cells with novel Structures

Organic Electronics (OE) is a science and technology field that relies on carbon-based semiconductors to deliver devices with unique characteristics. The technological profits that OE will bring to the Society are expected to be significant and for this it occupies a high-rank position in the list of priorities defined by the European Union. Consequently, it represents one of the most flourishing research fields the last decades. Among the organic electronic devices, Organic Photovoltaics (OPVs) constitute one of the most rapidly emerging directions in the field, due to the increasing global demand on renewable energy resources. The “soft” nature of organics offers better mechanical compatibility with mechanically flexible substrates which suits the non-planar formats often required for the fabrication of such devices. Moreover organics profit from their low production costs, which can be combined with environmentally friendly and sustainable production processes. Thus, OPVs are preferable with respect to their inorganic counterparts however, the low efficiencies achieved so far and the limited lifetimes they exhibit remain the bottleneck to their prospective commercialization. Poor morphology has been blamed for the failure of many innovative materials to achieve high performance and the role of thermodynamics in controlling nanomorphology to overcome these problems has been stressed. This IEF project aims to address this hurdle through a wise incorporation of block copolymers in the blend that forms the active layer of OPVs, i.e. the layer where photons are absorbed and electric charges are generated. The self-assembly properties of block copolymers as well as their ability to form well controlled nanostructures and to act as compatibilizers in the blends of the respective homopolymers are exploited to form stable nanomorphologies with optimum domain size, according to the specifications required for OPV applications. Our target is to fabricate highly efficient solar cells with enhanced morphological stability and prolonged lifetimes, applying process techniques that can be easily adopted by industry. During the 17 months that this project lasted, an integrated study has been performed, starting from the design and synthesis of the copolymers till their incorporation in photovoltaic devices and the evaluation of their performance. The copolymer that we opted to study is the poly(3-hexyl thiophene)-b-polyisoprene, P3HT-b-PI, rod-b-coil block copolymer, with P3HT being the rod-like block and PI the coil-like one. P3HT is used in order to exploit its semiconducting nature as the electron-donor in photovoltaic devices. PI was chosen due to its low glass transition temperature (~ -50oC – -70oC), which suggests that the PI chains are not frozen at room temperature. P3HT-b-PI copolymers of various total molecular weights and volume fractions of the rod-like block in the copolymer, f(P3HT), have been synthesized and the ternary blends formed upon blending the P3HT-b-PI copolymer with the respective P3HT and PI homopolymers have been studied. Initial characterization of the blends by means of differential scanning calorimetry, DSC, has been performed and it will be complemented by small and wide angle x-ray scattering experiments, SAXS/WAXS, in order to derive the complete phase diagrams of these blends. Next, we utilized the synthesized P3HT-b-PI copolymers to fabricate photovoltaic devices. These solar cells comprise PCBM : P3HT : P3HT-b-PI ternary blends as the active layer, where PCBM stands for [6,6]-phenyl-C61-butyric acid methyl ester and it is a fullerene-based electron-acceptor small molecule. Thus, we were able to study the effect of the incorporation of the rod-b-coil copolymer P3HT-b-PI in the structure of the archetypical P3HT:PCBM active layers, and relate it to the device performance of the corresponding solar cells. An extensive study of the morphology of the resulting active layers have been performed by means of optical microscopy, scanning force microscopy, UV-vis absorption spectroscopy, neutron reflectometry, and grazing incidence X-ray diffraction, GIXD. This detailed characterization revealed that the P3HT-b-PI copolymer acts as a nucleation agent, promoting the crystallization of P3HT. We concluded that the presence of the copolymer drives the formation of an optimized bulk heterojunction network that stimulates photon absorption, efficient exciton dissociation and improved charge transport. Subsequently, a maximum power conversion efficiency of 4.5 + 0.1% was achieved. Our study demonstrates that adding a wisely-designed block copolymer into the archetypical P3HT:PCBM BHJ is a valuable and efficient method to optimize the active layer morphology and to improve device performance in the corresponding organic photovoltaic cells. Our approach can be implemented in any pair of donor/acceptor materials, as long as the block copolymer that will be used is wisely chosen. Thus, efficiencies above the 4.5% one that we achieved can be reached. A subsequent large-scale integration of our method to devices could result in the commercialization of our products, which would benefit the EU in a community level and contribute positively in the competitiveness of Europe with other countries.

Data: CORDIS, © European Union