Optical properties and stability of Coumarin 7 luminescent down shifting layers in different host polymers

Abstract

In this work, pristine Coumarin 7 (C7) layers and mixtures of C7 with Poly(methyl methacrylate) (PMMA), Polycarbonate (PC) and Polyurethane (PU) were produced by drop casting and characterized by UV-vis absorption and Photoluminescence spectroscopy, aiming at investigating the effect of the different host polymers on the optical properties of the luminescent material. The optical properties of the luminescent down shifting LDS layers are adjusted by using the different polymers, different solvents and by changing the relative concentration of the host, resulting in Photoluminescence Quantum Yield (PLQY) values of 98, 92 and 96% for C7:PMMA, C7:PC and C7:PU layers, respectively. The overall enhancement in emission is about 5 times when compared to pristine C7 layers (PLQY = 19% in DMSO solvent). The effect of the different LDS layers in a simulated Perovskite Solar Cell (PSC) was assessed using Rothemundś model. The External Quantum Efficiency (EQE) curve of the simulated device was obtained using SCAPS-1D software and calculations with Rothemund's model show enhancements of about 11, 15 and 17% in the short-circuit current density when C7:PC, C7:PU and C7:PMMA LDS layers are used, respectively. The stability of the layers under different stressing conditions was examined with Differential Scanning Calorimetry DSC measurements and optical characterizations. There is no dramatic effect of the temperature on the optical properties of the layers, however, upon interaction with light, PLQY values are significantly reduced during the first hours of light soaking. C7:PMMA layers were the more stable combination after the light-soaking stability tests, indicating that PMMA is the most suitable option as host matrix for organic LDS dyes, with potential applications in photovoltaic devices. Our work highlights the importance of choosing the host polymer and evaluate its stability for the successful application of organic LDS layers in solar cells.