Characterisation of non-silicon devices
Perovskite solar cells – Luminescence imaging
Despite an astonishing performance improvement in such a short time, perovskite-based solar cells suffer from some major problems. One key challenge for this technology appears to be the stability of the devices; cells tend to undergo degradation, especially upon exposure to high temperature and humidity. Key electronic material and device properties, such as bulk doping and series resistance, tend to change once exposed to light. Another challenge associated with perovskite-based solar cells, which is common to other solution-based thin film solar cells, is the uniformity of the different constituting layers. To date, perovskite solar cells have been mainly fabricated on relatively small substrates; however, commercial applications require scaling-up the process to a much larger substrate area. This upscaling requires the ability to monitor the uniformity of the fabrication process. Lateral variations in the opto-electronic properties can be expected particularly for solution spreading techniques that are commonly used for the fabrication of perovskite solar cells.
Luminescence images of a perovskite solar cell before (a) and after (b-d) a sequence of three current–voltage measurements using a slow voltage sweep rate (13.4 mV/s): (a) EL image before this sequence; (b) EL image after this sequence; (c) PL image after this sequence; and (d) EL image of the same solar cell after 24 hours of storage and after this measurement sequence was done at different location. The EL images were taken under a forward bias voltage of 1.45 V; the PL image was taken at open–circuit condition.
Perovskite thin films and solar cells – Injection dependent measurements
Lead-halide based perovskite semiconductors exhibit long effective minority carrier lifetimes approaching the radiative limit. However, further improvements remain bottlenecked by non-radiative interfacial recombination which varies with the excess carrier densities. It is therefore crucial to establish methods which allow quantitative identification of the contribution of surfaces and interfaces to the perovskite-based devices’ recombination losses.
Owing to the large radiative recombination coefficient of perovskites, photoluminescence (PL)-based techniques such as time-resolved PL (TR-PL) and absolute steady-state spectral PL (SS-PL) have been used to identify the impact of different interfaces (passivation layer or selective contacts) on the device recombination. These techniques measure the total device recombination of the bulk and all interfaces, therefore, requiring assumptions and/or analysis methods to decouple the surface recombination contribution. In this project, we develop a novel method to quantify the surface/interfaces recombination based on analysing the injection-dependent effective lifetime determined from the absolute SS-PL.
Panel A illustrates the PL spectra at absorbed photon flux of 1-Sun equivalent. Panel B provides the corresponding τeff(∆n) curves. The black symbol on each curve is the 1-Sun equivalent excess carrier density. The radiative lifetime, corrected for photon recycling, is shown for comparison. The filled-in areas represent the experimental uncertainties of τeff and ∆n (Δµ). Panel C presents the Kane-Swanson method applied to extract J0s. The filled-in areas represent the experimental uncertainties of τeff and τrad. Please note that: 1 yA = 10-24 A.
Tandem solar cells
While currently, passivated emitter rear contact (PERC) silicon solar cells dominate the solar photovoltaic market, passivating contact and heterojunction solar cells are expected to be the next major players in the near future with their certified laboratory power conversion efficiency above 27.5%. Looking farther into the future, most PV technology projections consider tandem solar cells, with efficiencies noticeably exceeding that of single-junction silicon solar cells, as the most probable ultimate PV technology. It is well recognised that silicon cells with their continuously reducing cost and unquestionable long-term stability, will constitute the bottom cell of most types of terrestrial tandem solar cells (at least in the initial stages). However, it is not clear, yet, which technology will be preferred as the top cell.
Although the performance of series-connected multi-junction solar cells has been remarkably improved in recent years, further improvements require rapid and reliable access to the spatial electronic properties of each of the sub-layers in the device. In this project, we aim to develop a novel luminescence imaging system to investigate any type of monolithic tandem solar cells. The required theoretical modelling to reveal quantitative information regarding the spatial distributions of the electrical and optical properties of each of the individual sub-cells and layers is developed as well. These spatial distribution maps include, but not limited to, the implied open-circuit voltage, series resistance, surface recombination associated current densities, and the local ideality factor.