Dye-sensitized solar cell from a new organic n-type semiconductor/polyaniline composite: insight from impedance spectroscopy

Abstract

Aniline was in situ polymerized in the presence of N,N-di((S)-1-carboxylethyl)-3,4 : 9,10 perylenetetracarboxyldiimide (PTCDA, n-type semiconductor) in dilute acetic acid medium at different concentrations of PTCDA to produce PANI–PTCDA composites (P10, P15 and P20) with PTCDA concentrations 9.8, 13.1 and 21.3% (w/w), respectively. Both FESEM and TEM images indicated that the nanotubular morphology of PANI was retained, with formation of new hairy surfaces in the composites. The FTIR and UV-Vis spectra suggested the presence of π-stacking interaction between PTCDA and PANI produced in the doped state. Blue shifts in the π to π* and π to polaron band transitions indicated H-aggregate formation and secondary de-doping, respectively. Among the composites, the DC-conductivity of P15 sample exhibited the highest value (1.1 × 10⁻² S cm⁻¹) and fully reversible nature of the photocurrent. The HOMO and LUMO levels for PTCDA (−5.85 eV and −3.90 eV), determined from cyclic voltammetry, were lower than those of PANI, which transfers its photo-excitons to LUMO of PTCDA generating the photocurrent. Dye-sensitized solar cells (DSSCs) fabricated with N719 dye exhibited the highest power conversion efficiency (PCE) of 2.88% for the P15 sample. The IPCE results indicated a maximum 52% conversion of incident photon in the range 360–660 nm and the longevity of the cell was found to be satisfactory. Nyquist plots of the impedance spectra of the DSSCs indicated the presence of three semicircles in each sample, representing a complex equivalent circuit diagram consisting of three resistance and capacitance circuits. The middle semicircle yields the resistance (R_rc) and chemical capacitance (C_μ) between PANI–PTCDA and electrolyte interface and these are highest for the P15 sample yielding higher lifetime and hence lesser back-reaction of the injected photoelectrons at the PANI–PTCDA/electrolyte interface, contributing to the highest PCE value in the composites.