Spray Deposition Technique for Utilizing Förster Energy Transfer in Bulk Heterojunction Organic Solar Cells: Role of Applied Voltage
TAUHEED MOHAMMAD a, Viresh Dutta a, Mahesh Kumar b, Suresh Chand b
a Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi New Delhi-110016, India
b Ultrafast Optoelectronics and Terahertz Photonics Lab, CSIR-National Physical Laboratory New Delhi 110012, India
Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV19)
Roma, Italy, 2019 May 12th - 15th
Organizers: Prashant Kamat, Filippo De Angelis and Aldo Di Carlo
Publication date: 11th February 2019

In   recent   years,   organic   solar   cells   (OSCs)   have   attracted   much attention due to their potentially low-cost of fabrication, flexibility, solution processability and roll to roll production. The use of ternary structure-active layer, which is fabricated by blending a second donor or acceptor into a binary blend, is emerging as a promising strategy to broaden the absorption bandwidth of the photoactive layer for increasing the power conversion efficiency (PCE) since they contain the major merits of both single bulk heterojunction (BHJ) solar cells and tandem solar cells. In the ternary system the low-bandgap polymer serves as a sensitizer, energy transfers from a wide-bandgap polymer to a low bandgap polymer and the polymers work separately. To enhance the PCE of OSCs, absorption spectral range should be increased while preserving efficient exciton harvesting. Forster resonance energy transfer (FRET) has been identified as an important process in exciton harvesting, transport and dissociation. The spectral absorption range can be broadened by utilizing suitable sensitizers that efficiently transfer absorbed energy to the photoactive materials. Here, we prepared thin films from ternary blend of PCE-10, PCDTBT with the acceptor PC71BM in chlorobenzene by using novel one-step spray coating method. Spray deposition technique is a versatile coating strategy that can be used to deposit utilitarian films of different materials over large areas at speed. The coupling of an electric field with fluid motion plays significant role in aerosol generation for thin film coating. Net charge present on the droplet surface causes Coulombic repulsion reduces the binding force due to surface tension that holds the droplet together. The unstable condition, when the surface charge density is sufficiently high, the electrostatic force overcomes surface tension and the droplet disrupts, termed Coulombic fission. Droplets charge densities increases as the applied voltage at the nozzle is increased which decreases the droplet size since droplets rupture by electrical repulsion forces and hence rate of atomization increases. The application of applied voltage provides a means for generating ultrafine droplets of uniform size without the need for a very small orifice and high pressure which is indispensable for suitable spray deposited thin film fabrication. Absorption, photoluminescence, femtosecond transient absorption spectra with time resolved decay traces and Kinetics traces were recorded to elucidate the role of applied DC voltages during the deposition of these films.

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