Mattias K. Juhl

Evaluating Crystalline Silicon Solar Cells at Low Light Intensities Using Intensity-Dependent Analysis of I–V Parameters

This paper discusses the influence of different solar cell loss mechanisms at low light intensities and presents a simple method for the analysis of solar cell performance under various illumination intensities below 1 sun. Suns-PL and Suns - Voc are used to measure the intensity-dependent pseudo I-V curves of symmetric test structures and of finished silicon solar cells in an intensity range between 1 sun and 10-3 suns. The solar cell parameters from the pseudo I-V curves are compared with the parameters evaluated by intensity-dependent measurements of the whole I-V curve. The pseudo efficiency and pseudo fill factor are found to be in good agreement with the real values at low intensities as the influence of the series resistance vanishes. Based on this finding, we compare the passivation quality of silicon dioxide and silicon nitride in combination with emitter windows on test structures. Above 0.1 suns, both passivation layers show similar performance. Below 0.1 suns, the pseudo fill factors and pseudo efficiencies of the silicon nitride passivated sample are strongly reduced compared with the sample with silicon dioxide. The open-circuit voltage starts differing below 0.01 suns.

Photoluminescence Imaging of Silicon Bricks

Photoluminescence imaging techniques have recently been extended to silicon bricks for early production quality control and electronic characterisation in photovoltaics and microelectronics. This contribution reviews the state of the art of this new method which is fundamentally based on spectral luminescence analyses. We present highly resolved bulk lifetime images that can be rapidly extracted from the side faces of directionally solidified or Czochralski grown silicon bricks. It is discussed how detailed physical modelling and experimental verification give good confidence of the best practice measurement errors. It is also demonstrated that bulk lifetime imaging can further be used for doping and interstitial iron concentration imaging. Additionally, we show that full spectrum measurements allow verification of the luminescence modelling and are, when fitted to the theory, another accurate method of extracting the absolute bulk lifetime.

Full Spectrum Photoluminescence Lifetime Analyses on Silicon Bricks

Bulk lifetime and doping images on silicon bricks can be obtained by spectral luminescence intensity ratio analysis as established recently. Here, we report on calibrated full spectrum band-to-band luminescence measurements taken on the flat side faces of mono- and multicrystalline silicon bricks at room temperature. Our results verify the physical modeling used for the spectral intensity ratio imaging. We further investigate three fitting methods employing spectrally resolved photoluminescence data to obtain bulk lifetime information.

Correcting the inherent distortion in luminescence images of silicon solar cells

Luminescence imaging of Silicon solar cells is typically performed with a silicon CCD, which is a poor absorber of silicon luminescence (900-1300 nm). This leads to a phenomenon referred to as photon smearing in the CCD, where a photon incident on one pixel may be absorbed in another. This makes the image blurry and quantitative analysis of this data inaccurate. Also resolution, contrast, and sharpness of the image are reduced at features such as grain boundaries, and sample edges. An already established method of recovering the original luminescence signal incident on the CCD is to deconvolve a Point Spread Function (PSF) with the resultant image. This paper focuses on a novel method for determining the PSF from a measurement of the Edge Spread Function, which greatly increases the Signal to Noise ratio over methods where the PSF is measured directly. The determined PSF and its application to luminescence images, is compared and contrasted with previously published PSF determination methods.

Assessing the Performance of Surface Passivation Using Low-Intensity Photoluminescence Characterization Techniques

This paper applies quasi-steady-state photoluminescence (QSS-PL) and photoluminescence imaging to characterize the recombination properties of various surface passivation techniques. Particular interest is given to the performance at low excess carrier densities where many types of surface passivation show a strong increase in surface recombination velocity. These techniques are then used to further understand the ability of parasitic effects such as nonuniform illumination, edge recombination and areas of high recombination to affect these measurements. Furthermore, a new technique for edge isolation using laser doping is shown to be effective against the effect of edge recombination. This technique is useful to implement when using QSS-PL to analyze small samples as carriers conducted to the edge regions can dramatically alter the effective lifetime in low injection.

The impact of voltage independent carriers on implied voltage measurements on silicon devices

The electrical performance a solar cell is determined from direct measurements of the current voltage relationship, while the so-called implied current-voltage measurements are often performed to estimate the performance of partially processed samples. Implied current voltage measurements are commonly obtained from quasi steady state photoconductance and quasi steady state photoluminescence measurements, where the implied voltage is inferred from the average excess carrier density. As will be shown here, this approach can be problematic due to the presence of excess carriers that do not contribute to the terminal voltage. These carriers are referred to as voltage independent carriers, a concept that is not widely known or generally accepted. This paper provides the theoretical background for the distinction of voltage dependent and voltage independent carriers. It is shown that the relative impact of voltage independent carriers on implied voltage data depends strongly on device parameters and on the illu...

Anomalously high lifetimes measured by quasi-steady-state photoconductance in advanced solar cell structures

Quasi-Steady-State Photoconductance is widely used in photovoltaics industry to measure the effective minority carrier lifetime of silicon wafers, a key material parameter affecting final solar cell efficiency. When interpreting photoconductance based lifetime measurements, it is important to account for various artefacts that can cause an over-estimation of the carrier lifetime, such as minority carrier trapping. This paper provides experimental evidence for another artefact in photoconductance lifetime measurements, affecting samples that have a conductive layer that is interrupted by lines of the opposite polarity doping, forming laterally alternating regions of p/n doping. This structure often appears in the emitter region of samples used to monitor the lifetime of interdigitated back contact cells. The cause of this artefact is linked to a reduction in the measured dark conductance. Experimental data are presented that suggest this is due to the formation of a phototransistor type structure on the sa...

Photoluminescence Imaging of Silicon Wafers and Solar Cells With Spatially Inhomogeneous Illumination

Photoluminescence imaging is a fast and powerful spatially resolved characterization technique, commonly used for silicon wafers and solar cells. In conventional measurements, homogeneous illumination is used across the sample. In this paper, we present a photoluminescence imaging setup that enables inhomogeneous illumination with arbitrary illumination patterns to determine various parameters of solar cells and solar cell precursors. To demonstrate the strength of the proposed inhomogeneous illumination imaging, a set of proof-of-concept measurements have been conducted; these measurements include contactless series resistance imaging, emitter sheet resistance, and diffusion length measurements. The results indicate that the use of inhomogeneous illumination significantly extends the range of photoluminescence imaging applications for the characterization of silicon wafers and solar cells.

Impact of Edge Recombination in Small-Area Solar Cells with Emitter Windows

This paper investigates the use of emitter windows with varying passivation layers in an intensity range between 1 and 10-3 suns. The results are compared with a cleaved sample without emitter windows. It is found that the passivation of the nondiffused region outside the emitter windows is very important to reduce recombination. The surface passivation schemes investigated are the three most commonly used for solar cells: aluminum oxide, silicon dioxide, and silicon nitride. The aluminum oxide and silicon dioxide resulted in a reduction in edge recombination of 8 and 4.56 times, respectively. The silicon nitride passivation resulted in worse performance than the unpassivated sample, as a result of increased recombination. The impact of the thickness of the region outside of the emitter was investigated by reducing the outside area from a 2-mm border to a 200-μm border. The aluminum oxide sample was hardly influenced, while the silicon dioxide passivated sample suffered as the carrier was now able to travel to the edge and recombine. The performance of the silicon nitride passivated sample was improved with a reduction of the outside region. However, the performance is still reduced compared with the control sample with unpassivated emitter edges.

Evaluating the accuracy of point spread function deconvolutions applied to luminescence images

Luminescence imaging is a widely used characterization technique for silicon photovoltaics. However, the tools used to acquire images typically utilize a silicon CCD array for detection, which is a poor absorber at silicon luminescence wavelengths. This leads to a smearing effect in the measured image which can be characterized by a point spread function (PSF). If the true PSF is known then the measured image can be restored through deconvolution. Several methods exist for determining a PSF for a particular imaging system and different extraction techniques can lead to variations in the PSF result, yet no studies have provided comprehensive analysis of PSF deconvolution accuracy when applied to luminescence imaging. In this work, several new techniques have been designed and investigated in order to test PSF deconvolution results, with a view to quantifying improvement or errors generated and potentially leading towards improved image restoration.