Tuning of nonlinear optical bistability tuned by fullerene concentration in polyaniline-fullerene (PANI-C60) hybrid organic films

Third-order nonlinear optical (NLO) parameters, NLO absorption coefficient (β), imaginary part of susceptibility ( χ i ( 3 ) ) , saturation intensity and figure of merit of polyaniline–fullerene (PANI-C 60 ) films were investigated using open-aperture z-scan measurement with a 532 nm Nd:YAG laser. Switching of saturable absorption (SA) to reverse saturable absorption (RSA) behaviour (NLO bistability) was observed in these organic hybrid films by changing the concentration of C 60 in the PANI matrix. It is attributed to the charge transfer complex (CTC) formation between PANI and C 60 in the film. PANI and low C₆₀-doped films exhibited SA, indicative of ground-state bleaching, while higher C₆₀ loadings triggered a switch to RSA due to enhanced excited-state absorption facilitated by efficient intersystem crossing induced by CTC formation. A transition from SA to RSA was confirmed from the observed sign reversal in β and χ i (3) . Values of β and χ i ( 3 ) , were shifted from –59.80 cm/GW to 99.3 cm/GW and –0.52 × 10 −10 esu to 0.96 × 10 −10 esu, respectively, in PANI and PANI-C₆₀. Films were prepared via the drop-casting technique with varying C₆₀ concentrations. SEM analysis showed morphological changes from uniform domains of PANI to C 60 -rich agglomerates. The CTC formation is confirmed by the FTIR and UV–visible absorption spectroscopy. The bandgap reduction from 2.62 eV in the PANI film to 2.20 eV at the highest C₆₀ concentration, with an increase in Urbach energy from 0.40 eV to 0.95 eV, indicated enhanced disorder and localised states. To the best of our knowledge, this is the first report of a concentration-driven SA-to-RSA transition in PANI–C₆₀ solid films, establishing their potential in photonic applications. • SA to RSA switching in PANI-C 60 films observed with increasing C 60 concentration. • CTC formation enhances ESA and ISC, at higher C 60 concentrations leads RSA. • SA attributed to ground-state bleaching in PANI at lower C 60 doping levels. • NLO coefficients β and χᵢ⁽³ ⁾ switch sign, confirming SA–RSA crossover. • First solid-state study of dopant-controlled bistability in PANI–C 60 hybrid films.