Development of novel characterisation methods

Temperature-dependent Suns-Voc

Suns-Voc is a technique whereby an open-circuit voltage (Voc) is measured along with the incident illumination on a sample. Its main advantage is that the cell's performance is measured rapidly at a large range of illumination values. By incorporating a temperature stage into the setup, we allow for the acquisition of cell performance data in a 2-D landscape, varying with both temperature and illumination. Using this setup, we investigate the temperature and illumination response of a wide variety of silicon and non-silicon solar cells, in order to better assess their real-world performance. 

Temperature-dependent Suns-Voc data for six different solar cell structures in the temperature range of 140 – 30°C. The coloured dashed lines indicate the lines for an ideality factor of 1 or 2, plotted for comparison to the data. The purple points indicate the pseudo maximum power point.

Outdoor photoluminescence imaging

To operate photovoltaic power plants at maximum capacity, it is desirable to identify cell or module failures in the field at the earliest possible stage. Currently used field inspection methods cannot detect many of the electronic defects that can be revealed with luminescence-based techniques.
In this project, PL images are acquired using the sun as the sole illumination source by separating the weak luminescence signal from the much stronger ambient sunlight signal. This is done by using an appropriate choice of optical filtering and modulation of the cells’ bias between the normal operating point and open circuit condition. The switching is achieved by periodically changing the optical generation rate of at least one cell within the module. This changes the biasing condition of all other cells that are connected to the same bypass diode. This method has the advantage that it can deliver high quality images revealing electrical defects in individual cells and entire modules, without requiring any changes to the electrical connections of the photovoltaic system.

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(a) Our outdoor PL image acquired using contactless modulation, and (b) indoor electroluminescence image.

Two-photon absorption

In this project, we are developing new characterisation methods using two-photon absorption (2PA) time-resolved PL (TR-PL). We first investigated the fundamental limitations of 2PA to determine the low-injection bulk lifetime of different semiconductor materials. We then developed a novel method to measure the quality of silicon bricks and ingots using 2PA.
In the 2PA process, two sub-band-gap photons with total energy greater than the band-gap energy are simultaneously absorbed and their total energy is transferred to a single electron in the valence band. The electron is excited across the optical band-gap into the conduction band. The process requires high light intensities, usually by combining ultra-fast pulsed excitation with strong spatial focusing. The 2PA process produces excess free-carriers, with most excess carriers generated at the focal region where the local light intensity is highest. Due to the localised excitation, the PL decay rates are biased towards the carrier kinetics near the focal point. By positioning the focal point within the bulk of the sample, a depth profile can be obtained.

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(a) Colour-map of the excess carrier density, normalised to the peak density for bulk excitation. (b) Diagram of solid immersion lens used in conjunction with conventional lens to achieve a 2PA measurement.

Temperature-dependent photoluminescence imaging with non-uniform illumination

Photoluminescence (PL) imaging is a powerful inspection technique for research laboratories and production lines. It is used for a wide range of applications across the entire manufacturing chain from bricks and ingots to modules. However, common PL imaging systems have three main limitations: (a) due to the uniform illumination, the acquired images are affected by lateral carrier flow, resulting in image blurring; (b) sample’s non-uniformity is measured at different injection levels; and (c) images are commonly taken at room temperature, although, there is valuable information in temperature-dependent measurements.
In this project, we are developing a novel temperature-dependent PL imaging system that is not affected by lateral balancing currents. By adaptively adjusting the light intensity at each pixel, we set a uniform excess carrier density across the sample. Hence, the lateral currents are eliminated. The non-uniformity of the material’s electrical properties and temperature characteristics can then be extracted from the excitation image. This novel approach presents a significant improvement in accuracy and resolution compared to conventional PL imaging techniques and is therefore, expected to be beneficial for any PL-based quantitative analysis

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Schematic of the temperature dependent PL imaging setup with non-uniform illumination.