Investigation of the electrical, optical and photophysical properties of PTB7:PCBM-thin films

Conjugated polymers offer flexible and lightweight solar cells devices which are fabricated via cost-effective solution processing techniques; however, these devices suffer from lower efficiency and short-term lifespan. To improve the efficiency and stability of device, the knowledge of the mechanism of the sun light absorption, dissociation of excitons into the free electron-hole pair and their transport to the collecting electrodes are essential. Herein, the impact of PCBM concentrations on photophysical and optical and properties of PTB7 were investigated by spectroscopic ellipsometer and fluorescence spectroscopy. It was noticed that with increase of PCBM concentrations, the dielectric constant (ε r ) and refractive index ( n ) of PTB7 thin films decreases which ascribed to the reduction of electric polarization and increase of the surface roughness. The emission intensity was noted to be dropped and charge carrier lifetime decreases for PTB7:PCBM bulkhetrojucntion as compared to pristine PTB7, confirms the charge transfer between donor and acceptor. The charge transport mechanism in PTB7 thin film was investigated in hole only device architecture by recording the temperature-dependent current-density vs. voltage ( J–V ) curves. The charge transport for the temperatures above 220 K was governed by the trap-free space charge limited (SCL) conduction described by Mott-Gurney square law and switched to the trap-filling SCL conduction for T ≤ 220 K. For the temperature 220 K, the trap density H b , hole mobility µ and characteristics trap depth E t were evaluated to be 2.98 × 10 17 cm −3 , µ = 6.08 × 10 −5 cm 2 V −1 s −1 and 25 meV, respectively.