Executive Summary : | Perovskites, specially all-inorganic perovskites (CsPbX3), a class of materials of topical importance, exhibit very narrow colour pure and very bright photoluminescence (PL) emission, exotic photo-physics and exciton dynamics and are thus considered for several deep-tech optoelectronic applications e.g. single photon source/detector, solar cells and LEDs. However, these perovskites contain toxic metal Pb, thereby hinders the sustainable application potential. Moreover these perovskite materials suffer from serious stability issue. In order to circumvent these problems there are mainly two approaches adopted by the researchers : (a) single perovskites, replacing Pb with Sn or Zn ; and (b) double perovskites, replacing Pb with (i) In and water or (ii) Na/K and In. Out of these possibilities, perovskites containing Sn (CsSnX3) has serious stability issues and with Zn, the extent of Pb replacement is very small (only 10%). Through the other alternative, i.e. double perovskite approach, 100% Pb replacement is possible (as in Cs2InX5.H2O and Cs2NaInX6) and these perovskites are quite stable under ambient condition, however, these pristine perovskites are nearly non-photoluminescent. In order to circumvent this problem, doping with Sb3+ or Bi3+ and thus, doped Pb free perovskites have been adopted. These doped double perovskites are quite stable under ambient condition. However, there are still several drawbacks and unanswered questions in these perovskites, as mentioned below, which deserve very serious and through investigations : 1. How to enhance the PLQY of Pb free double perovskites to near unity? 2. PL emission spectrum is quite broad (FWHM of PL emission ~ 100 nm / 600meV). How to reduce the FWHM of PL emission to ~40 nm/ 300 meV)? 3. What is the rationale behind very large Stokes shift (150-300 nm) ? 4. Are there any intermediate excited states in between initially excited state and the state from which PL occurs? 5. Is it possible to modulate the band engineering so that PL emission wavelength can be altered ? 6. What is the origin of large Stokes shifted PL emission - is it self-trapped-emission? 7. How the extent of electron-phonon interaction can be modulated? 8. Why the PL emission lifetime is in s domain? Is it possible to alter the excited state lifetime? 9. How fast are the hot electron trapping dynamics, hot electron cooling dynamics, hot hole trapping dynamics? Ultrafast transient absorption based investigations will be able to answer these questions. 10. Ultra-sensitive single particle spectroscopic investigations will be carried out to understand how fast is the electron and hole trapping and detrapping dynamics? 11. How does the above-mentioned properties get altered because of different extent of Sb3+ or Bi3+ doping, different types of surface passivating ligands etc.? Thus, toxic metal free double perovskites with outstanding optical properties necessary for sustainable applications can be achieved. |