Effect of blend layer thickness on the dielectric response of PCDTBT:PC71BM-based bulk heterojunction solar cells

Bulk heterojunction solar cells based on a blend of polyN-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and 6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM), were studied. The organic photoactive layers were spin coated onto a poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT-PSS) interfacial layer at speeds of 600 and 2000 rpm. The molecular structure of PCDTBT, PC₇₁BM, and the PCDTBT:PC₇₁BM blend was investigated using Fourier-transform IR (FTIR) spectroscopy, which confirmed the absence of interactions between the individual components of the composite. The dielectric properties of PCDTBT:PC₇₁BM-based solar cells were studied under illumination by means of impedance analysis. The dielectric constant, impedance, and ac conductance were analyzed as a function of frequency at different bias voltages close to the open circuit voltage (Voc). We found that the dielectric constant, dielectric loss, and conductance increased with increasing PCDTBT:PC₇₁BM thickness. Impedance spectroscopy analysis revealed decreases in charge recombination and the resistance of the whole device with increasing spin coating speed for the active layer. Moreover, an increase in recombination resistance for the solar cells was observed close to V_OC.