Spatially and Spectrally Resolved Characterisation of Solar Cells

Vincent Tsai, Dr. Martin Bliss, Dr. Tom Betts, Prof. Ralph Gottschalg

Centre for Renewable Energy Systems Technology

School of Electronic, Electrical and Systems Engineering, Loughborough University

Keywords - photoluminescence; electroluminescence; characterisation; spectral; spatial; mapping

I.  Introduction

Photoluminescence (PL) and electroluminescence (EL) are powerful and advanced techniques for the characterisation of solar modules and cells. They can be used to acquire both spatially and spectrally resolved information from solar cells. In PL, the solar cell is illuminated using an external source, which causes excitation of excess carriers from the valence band to the conduction band. In EL, excess carriers are excited by forward biasing a solar cell using a power supply [1]. Essentially, the solar cell is operating as a light emitting diode (LED). The excited carriers then relax back down from the conduction band to the valence band where they recombine. This results in the emission of photons with energies around the bandgap of the solar cell if the recombination is radiative. These emitted photons are not in the visible range, but can be detected by a cooled charge coupled device (CCD) camera. A grayscale image of the solar cell is produced from the detected photons. In addition, spectral information of the emitted photons can be obtained through the use of a monochromator [2]. Since PL is contactless, it can be applied throughout the different stages of fabrication of a solar cell while EL can only be conducted on a finished device as it requires metal contacts [1].

II.  discussions

Currently, a combined EL/PL measurement system is being developed at CREST [2]. This system aims to integrate the advantages of the speed of camera based EL/PL with the additional information from spectral EL/PL. Previous work with this system [3] used spatial EL imaging to map the series resistance of a mono-crystalline silicon (c-Si) solar cell, which followed the work of Hinken et al [4]. Figure 1 shows an EL image taken in the previous work.

From the acquired EL image in Figure 1, it is clear that spatial EL imaging can reveal extrinsic defects such as micro-cracks and broken finger lines, which are not visible to the naked eye. These are shown in the EL image by a decrease in the pixel intensity i.e. darker areas.

In addition to the intensity of each pixel, in an EL image, each pixel contains spectrally integrated information [5]. The EL spectrum can reveal further information on material defects, properties and recombination mechanisms that simple spatial EL imaging cannot. Therefore, it would be advantageous if the EL spectrum of the entire solar cell could be mapped, which also applies to PL.

III.  Conclusions

EL and PL can be used to obtain useful spatial and spectral characteristics of solar cells. However, the EL/PL system at CREST is still under development and further time is required before the spectral and spatial characterisation of solar cell samples can be properly conducted. Once this system has been fully implemented, future work could also include introducing time resolved photoluminescence (TRPL) to the system which would allow for the measurement of minority carrier lifetime.

References

[1]  T. Trupke, J. Nyhus, J. Haunschild, “Luminescence imaging for inline charactersiation in silion photovoltaics,” Phys. Status Solidi RRL 5, No. 4, 131–137, 2011, / DOI 10.1002/pssr.201084028

[2]  M. Bliss, X. Wu, K. Bedrich, T.R. Betts, R. Gottschalg, “Spatially and Spectrally Resolved Electroluminescence Measurement System for PV Characterisation,” unpublished.

[3]  V. Tsai, 2014 “Analysis of Electroluminescence Images of Solar Modules”. Unpublished (MEng), Loughborough University.

[4]  D. Hinken, K. Ramspeck, K. Bothe, B. Fischer, and R. Brendel, “Series resistance imaging of solar cells by voltage dependent electroluminescence”. Applied Physics Letters, vol. 91, 182104, 2007.

[5]  D. Abou-Ras, T. Kirchartz and U Rau: “Advanced Characterization Techniques for Thin Film Solar Cells”, Wiley-VCH, 2011, Chapter 3, pp61-77

Figure 1 Captured EL image of a 2×2 cm2 c-Si solar cell