Colloidal quantum dots based hybrid LEDs and photovoltaics

Abstract

Colloidal quantum dots (QDs) have proven themselves as excellent light emitters and strong absorbers. This work aims to use QDs to enhance the performance of light emitting diodes (LEDs) and photovoltaics. Photonic quasi crystals (PQC) are used to bring QDs close to quantum wells (QWs) of InGaN LED. This thesis demonstrates the colour conversion effective quantum yields of 123% and 110% for single colour QDs and white LEDs respectively. High colour conversion quantum yield was made possible through efficient coupling of QDs and QWs by both radiative and non-radiative resonant energy transfer (RET). Existence of RET between QWs and QDs is demonstrated using the time resolved photoluminescence spectroscopy. The PQC LED module with current tunable submodules was inkjet printed with different colours of QDs. Reproducibility and correlated colour temperature tunability of colour tunable module using inkjet printing is also demonstrated. Lead sulfide (PbS) QDs as a superficial layer on Si solar cell has shown the absolute and relative photon conversion efficiencies of 1.37% and 20% respectively. This improvement in photon conversion was achieved through luminescent down shifting and RET from QDs to underlying silicon. The PbS QD at the surface also serves as a refractive index matching layer thus the light scatters, increasing the coupling of photon into the Si solar cell. CdSe/ZnS (core/shell) QDs were also hybridised on the planar Si solar cell and optical enhancements were investigated. It was demonstrated that the QD layer can also serve as an efficient refractive index layer and optimum thickness was found through dispersing QDs at different spin speeds. Finally, PQC was used to bring CdSe/ZnS QDs in proximity to metallurgical junction of the Si solar cell. For optimised QD layer, the relative enhancement of Jsc was found to be 17.5%. It was shown that by increasing the air fill fraction of the PQC solar cell, efficient light trapping can be achieved. On hybridisation with CdSe/ZnS QDs, short circuit current of 31.67mA/cm2 was demonstrated. When the comparison was drawn between the champion hybrid PQC solar cell and planar device, the hybrid PQC showed an absolute and relative Jsc enhancement of 9mA/cm2 and 41% respectively