Solar photovoltaic technology is one of the fastest growing renewable energy technologies that can help meet increasing global energy demands. OPVs represent a transformative technology with great potential for high-throughput manufacturing at very low cost, whereas use of earth-abundant, non-toxic materials renders the technology eco-friendly. OPVs can also benefit from a larger selection of functional materials given that material properties such as charge mobility, band gap, photovoltage or intrinsic stability can be tuned by careful design and synthesis. Therefore, selection of optimal materials to optimise light absorption and photovoltage remains challenging and ongoing improvements in material design are urgently needed. Comprising a broad collection of universities, research institutes and industrial partners, the EU-funded project MUJULIMA (Innovative materials for multiple junction OPVs and for improved light management) was formed to address some of the concerns facing OPVs. Key to achieving project goals was the selection of high-performance photoactive materials for application in solution-processed multiple junction solar cell stacks. Use of donor-acceptor conjugated polymers with sufficient variation in optical band gaps helped achieve high efficiencies in single, double and triple junction devices. In addition, use of up- and down-converter materials provided better light management capabilities and an increase in energy conversion efficiency. Beyond the synthesis of new materials, development of proper ink formulations, optimisation of the cell architecture and development of the inkjet process were required to reach high OPV energy conversion efficiencies. Researchers also developed accelerated ageing tests to identify and remediate degradation mechanisms so as to further improve lifetime and stability of the modules. The project consortium demonstrated their achievements in three applications: small, indoor OPVs for smart home devices; medium-sized panels for urban furniture; and a large, flexible solar roof for commercial buses. Given the significant advantages in terms of cost and flexibility of processing, if device performance is improved and stability issues resolved, the significant savings offered by OPVs will render them a larger share in the solar energy market.
Solar cells, organic photovoltaics, light management, band gap, photovoltage, MUJULIMA