engleski

On the importance of structural analysis for understanding the morphology of squaraine based bulk heterojunction organic solar cells

Squaraine dyes are a class of electron-donating 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 photochemical stability, high utilization of the solar spectrum that extends into the NIR region and ease of synthesis make them excellent candidates for fabrication of cost efficient, flexible and large area solar cells (SCs). However, the properties of SQ-SCs highly depend on the degree of crystallinity and superstructural order, where the importance lies in achieving the highest crystalline organisation, and controlled and continuous growth in direction perpendicular to constituent layer plane. Namely the loss of control over continuous one-dimensional crystallization would yield large crystal domains and eventually donor-acceptor phase separation. An optimized morphology can greatly increase the short circuit current as well as improve both series and shunt resistance which contribute to the fill factor and overall efficiency. The key parameters that influence the final morphology are the donor-to-acceptor ratio and the type of deposition technique. The scope of this work is to highlight the importance of structural analysis for correlating squaraine crystallization and packing inside the acceptor matrix to performance parameters of the assembled SCs. To display this, the morphology and structure of drop cast and spin cast thin films from various solvents and post processing conditions were comparatively studied by grazing incidence X-ray diffraction (GIXRD) and grazing incidence X-ray scattering (GISAXS) and then correlated to electrical (J/V measurements) and absorption (UV-Vis and photoluminescence) data. As expected, the drop cast films showed much higher orders of crystallinity and were used as reference in investigation of the spin cast blends. For SCs with optimised morphology, an increase in short circuit current up to an order of magnitude could be observed in comparison to those without any order of crystallinity, which is due to the increased hole mobility of the squaraine when crystalline.

squaraines, organic solar cells, thin films

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