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Abstract
Solar power has been focused on as an alternative for exhaustible and environment-unfriendly energy resources. Crystalline silicon solar cells are market-dominating solar energy devices based on a mature silicon technology. However, the limiting efficiency of crystalline silicon solar cells is 29.8% due to intrinsic loss mechanisms. One of the major loss mechanisms is transmission of sub-band-gap photons. In the case of crystalline silicon solar cells, approximately 20% of the solar energy is wasted by the transmission losses. These transmitted photons could contribute to the power conversion of solar cells through optical conversion of two or more sub-band-gap photons to generate one above-band-gap photon.
Trivalent erbium doped hexagonal sodium yttrium fluoride has been selected as an upconverter due to the ideal energy states of erbium and low phonon energies of hexagonal sodium yttrium fluoride that reduce non-radiative quenching mechanisms during upconversion processes. However, this upconverter has also suffered from low absorption, non-linear power dependence and the narrow ground state absorption bandwidth of erbium.
This thesis aims to fabricate efficient upconversion silicon solar cells using efficient upconverting nanocrystal, direct deposition techniques of upconverting nanocrystals on solid state solar cells and broad-band sensitisation of lead sulphide (PbS) quantum dot that extends the available solar spectrum for the narrow bandwidth.
The meso-structured erbium doped hexagonal sodium yttrium fluoride nanocrystals were deposited on a bifacial crystalline silicon solar cell by spin-casting, resulting in higher absorption, brighter upconversion luminescence and high photoconversion efficiency. The highest external quantum efficiency measured was 0.125% under a laser excitation at 1525 nm and 0.071 W/cm2. With the upconverting nanocrystals, the addition of PbS quantum dot layers to the upconverting layers exhibited an extended spectral response to shorter wavelengths. The results of the optical converter made using erbium doped hexagonal sodium yttrium fluoride nanocrystals and PbS quantum dots demonstrate the potential of reduced transmission losses and enhanced photocurrent generation of sub-band-gap photons.