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The role of solvent selection and post processing treatment in controling morphology of bulk heterojunction squaraine organic solar cells

Squaraine dyes are a class of electron-donating (p- type) organic molecules, which when blended with appropriate electron acceptors, form bulk heterojunction active layers in small molecule donor organic solar cells (SMD-OSCs). Their thermal and potochemical stability, high utilization of the solar spectrum that extends into the NIR region and ease of synthesis make them excellent candidates for the fabrication of cost efficient, flexible and large area solar panels. In this work, we present squaraine (SQ) donor and fullerene (PC71BM) acceptor solar cells composed from 2, 4-Bis[4-(N, N-diisobutylamino)-2, 6- dihydroxyphenyl] squaraine dye (DIB- SQ). The DIB-SQ wassynthesized using a novel, improved and eco-friendly procedure. Model solar cells of DIB-SQ:PCBM in a ratio of 1:3 were subjected to an extensive optimization through solvent selection (chlorobenzene, chloroform and various mixtures of the two solvents) and spin coating parameters in an ITO/ZnO/SQ:PC71BM/MoO3/Ag inverted structure. Through the course of optimization, the magnitude of influence of certain parameters was elucidated. UV-Vis absorption, recorded on both solutions and thin films, confirmed major differences in absorption behavior for compounds dissolved in different solvents and was further utilized to indicate the degree of SQ dispersion throughout the PC71BM matrix. Solar cells were electrically characterized by means of light intensity dependence measurements, which in combination with extended J/V and EQE measurements gave an insight as to which charge recombination mechanism is dominant in the devices. . Furthermore when this was put in context with morphological studies, i.e. AFM (atomic force microscopy) and TEM (transmission electron microscopy) the influence of processing solvent, spin coating parameters and post process treatment on the morphology and size domains of SQ in BHJ configuration was further clarified and correlated very well to JSC, VOC, FF and PCE. The results gave significant insight on the overall methodology of optimizing new squaraine derivatives for photovoltaic application.

squaraines, bulk heterojunction, electron transport

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