Quantifying interface states and bulk defects in high‐efficiency solution‐processed small‐molecule solar cells by impedance and capacitance characteristics

The AC properties of high‐efficiency ( η = 8.01% under standard 100 mW/cm 2 AM1.5 illumination) small‐molecule bulk heterojunction (SM BHJ) solar cells (p‐DTS(FBTTh 2 ) 2 /PC 70 BM) at different DC biases and frequencies of small amplitude (±10 mV) AC signal in the dark at room temperature were investigated in details. We showed the presence of interface states at the heterojunction interface and determined their parameters from the analysis of spectral distributions of real and imaginary components of the measured impedance. The dielectric constant of BHJ ε BHJ = 2.9 was determined from the geometrical capacitance of totally depleted BHJ layer. We explained quantitatively the effect of interface states and series resistance on the measured C‐V characteristics of the SM BHJ solar cells at both low and high frequencies. The quantitative value of the density of defect states in the bulk N = 1.05 × 10 16 cm −3 was determined from the high frequency C‐V characteristic corrected by the effect of the series resistance.