Temperature tunable flexible photo absorbers based on near-infrared 1D photonic crystal hybridized W-doped VO2 nanostructures

Vanadium dioxide is a potential candidate for energy efficient smart windows and have crystalline phase transition temperature (Tc) at 68 °C. So far, literatures mainly emphasis on different synthetic strategies of tungsten doped VO2 which is a most effective dopant to reduce Tc of VO2 to near room temperatures. Until now, there is no report shows the incorporation of flexible 1D photonic crystals as spectrally selective, temperature tunable device to control the changes in optical transmission modulations of W-VO2, especially in the near IR region for smart window application. W- doped VO2 with various tungsten contents were synthesized with a facile hydrothermal route. We found that, with 1.1 at % of tungsten doping in intrinsic VO2, the metal to insulator (MIT) transition temperature is brought down to 37°C from 68°C. IR transmission of VO2 thin film can be reduced from 70 % to 40 % around room temperature, after doping. Significant absorption enhancement has been observed for both VO2 and W-doped VO2 films, deposited over tunable SiO2/Ta2O5 based distributed Bragg reflector (DBR) fabricated over flexible PET (poly-ethylene terephthalate) substrates. On depositing VO2 over ~ 70% reflecting DBR, optical transmission is reduced to ~ 15 % from 35 % while the temperature varies to 380 K from 300 K in IR regime. Number of stacks plays a crucial role for effective IR extinctions. A high quality DBR is fabricated by increasing no. of stacks from 4 to 7, with optical transmission of DBR reduced to nearly 5 % in stop band. However, with 1.1 at % of W-VO2 over such 95 % reflecting flexible DBR, optical transmission vanishes nearly, around room temperature itself in the stop bands of that DBR, which clearly indicates the significant absorption enhancement. W-VO2/DBR hybrid can substantially modulate the solar heat flux and also imbuing DBR over flexible PET substrates offers retrofitting of the existing windows for energy economy. Thus these structures have promising potential applications for optical devices and practical design for smart windows.