PP3, optics and characterisation, targets experimental demonstration that previously accepted theoretical conversion limits can be increased by use of structures that have a high local density of optical states, with particular emphasis on thin-film organic and inorganic solar cells. Of special interest are devices thinner than the wavelength of light where there are opportunities for much stronger absorption of light than would normally be expected from the device thickness involved.
There are two main themes in this program package. The aim of the first theme, PP3.1, is to develop methods and theories to better understand the structure-property relationships in thin film organic and earth-abundant solar cells. This activity will contribute to programs PP2.1 and PP2.2 and provide a valuable resource for the ACAP program overall. In 2015 this work has built on the results reported in 2014 with the establishment of a systematic relationship between electrode work functions and maximum open-circuit voltage in planar homojunction organohalide perovskite solar cells and the development of new, combined methodologies to study recombination and transport physics and their relationship to structure in organic solar cells. The application of related techniques to produce the first truly narrowband photodetectors with no input optical filtering resulted in a high profile 2015 paper in Nature Photonics.
The second theme studies plasmonic and nanophotonic light trapping for a range of cell structures. Work at ANU and UNSW (PP3.2a) applies a photoluminescent spectroscopy method to compare a range of surface textures for silicon wafers and to quantify their light-trapping performance against standard texturing methods. In 2015 the team demonstrated the use of photoluminescence spectroscopy to quantitatively measure light trapping without the need to make a finished solar cell. High light-trapping efficiency was measured for random pyramids and plasmonic structures in combination with very low front reflectance resulting from reactive ion etch texturing. This work
was also extended to new materials this year, with its application to accurate determination of absorption coefficient of perovskite materials. Complementary work at UNSW (PP3.3c) uses polystyrene nanospheres to pattern plasmonic light-trapping arrays for either transparent conducting layers on the front or light-trapping layers on the rear of thin-film photovoltaics. Simulation and preliminary experimental results show that some nanosphere configurations provide significant enhancement relative to the use of a mirror as a rear reflector. Small errors in the optical constants of silver can be magnified several-fold if it is used in plasmonic structures. Use of accepted values can lead to significant inaccuracies in the modelling and interpretation of results. The team at UNSW, working on the PP3.2c task, has conducted a systematic study on the optical constants of this important metal, which is of importance for simulations of plasmonic enhancements and for extracting important parameters. Moreover, the relationship between grain sizes and relaxation times of silver was elucidated both experimentally and theoretically. Task PP3.2f has developed and applied cutting-edge characterization techniques, especially time-resolved photoluminescence (TRPL), for materials for photovoltaics. This work has, in 2015, developed new analytical and numerical calculation methods for analysis of silicon bricks and wafers, allowing the effects of bulk and surface defects to be reliably separated, and a new TRPL tool was built and shown to have exceptional performance.