Charge-selective transparent conductors for solution-processed organic solar cells (2017)
Organic solar cells (OSCs) become increasingly popular for harvesting solar energy, due to their potential for low-cost manufacturing and mechanical flexibility. As the efficiency of laboratory-scale devices increases, developing materials and processes that would enable low-cost roll-to-roll fabrication of such devices gains increasing research interest. In order to promote OSCs as a viable substitute for silicon-based solar cells, it is necessary to synthesize materials that can offer high performances, roll-to-roll processability, and potential for flexibility, via processes that are scalable, and do not rely heavily on costly fabrication conditions, such as high temperature, vacuum processing, or inert atmospheres.This research is focused on two related aspects within this goal. The first part of the research concerns the fabrication of highly stretchable transparent conductive electrodes (TCEs) as replacement for conventional indium tin oxide (ITO) TCEs. Sparse meshes of metallized polyacrylonitrile nanofibers (NFs) fabricated via the scalable electrospinnig method are used to realize TCEs with performances comparable to ITO electrodes (sheet resistance, Rs, of 155 Ω/□, with transparency, T, of 95%) and with unprecedented electromechanical stretchability (only 56% increase in resistance at 100% strain). Furthermore, by incorporating the metallized NFs into matrices of solution-processed, charge-selective layers, composite charge-selective TCEs are fabricated. Annealed at the appropriate temperature, these charge-selective TCEs achieve performances superior to ITO and on a par with the uncoated NF TCEs. Using ZnO as the matrix, electron-selective composite TCEs with Rs = 23 Ω/□ at T = 95% are fabricated. Using MoO₃, hole-selective TCEs with Rs = 35.5 Ω/□ at T = 94% are obtained. The second part of the research, involves the fabrication of OSCs using a scalable, low-temperature spray-coating process in air. By employing an accelerated air drying post-deposition stage, we were able to achieve large-area pinhole-free coatings of P3HT:PCBM layers with sub-nanometer surface roughness, through single-pass spray-coating at 25 °C substrate temperature. OSCs fabricated in air through this process, achieve power conversion efficiencies up to 2.57%, comparable to the reference devices fabricated via spin-coating in nitrogen atmosphere. The introduced process is successfully used to fabricate fully-sprayed, as well as large-area OSCs.