Photothermal beam deflection technique for non-destructive evaluation of thin film photovoltaic cells

Abstract

Listen

Translate article

Photothermal beam deflection technique is used to measure the electronic transport properties of the photovoltaic cell layers at different fabrication stages. The open circuit voltage and short circuit current of the cell is shown to be depend on the transport properties of the window layer material. The photothermal technique is suitable adapted to measure the series resistance, optimum load resistance and photovoltaic efficiency of a three layer (transparent conducting oxide /absorber layer/window layer) compound semiconductor thin film photovoltaic cell. The cell parameter values measured using photothermal technique is in agreement with the electrically measured values. The possibility of efficient and continuous evaluation of the photovoltaic cell during different fabrication stages using photothermal technique is demonstrated. Real time characterization and continuous non-contact non-destructive evaluation of a device during its fabrication stage is important for improving the performance, quality and reliability of products in photovoltaic industry. A technique to measure the parameters that foretells any possible degradation and failure during the initial stages of fabrication is essential for lowering the production costs and to assuring the certified quality. Thermal wave techniques have gained increasing attention in process control mainly due to their non-invasive nature whereby any possibility of contamination can be avoided. Photothermal beam deflection technique (PTBD) is an efficient non-destructive method for evaluating semiconductor thin films and photovoltaic devices. Since demonstration for first time in 1880, these techniques have evolved in different stages and now do multi-purpose analyses of a variety of materials like polymers, ceramics, semiconductors and various biological samples [1]. Photothermal techniques are highly sensitive and have become a tool for absorption studies in both semiconductor films and devices [2]. This technique is shown to be effective for monitoring the device at processing stages and for rapidly identifying the changes during the processing itself [3]. The ability to monitor each stage of device processing can lead to the identification of parameters, which critically affect the device quality. This includes monitoring the quality of substrate, the subsequent device layers and determination of their electronic parameters. Photothermal approach was adopted by Daniele Fournier et al [4] to measure transport processes based on the principle of optical beam deflection. This measurement technique, which is an extension of photothermal deflection, has significant advantages as it is contactless and directly yields both thermal and electronic transport parameters within the bulk / at surface / interface of a semiconductor. Another important advantage of the technique is that it can be used to measure these transport properties in a spatially resolved (µ3 ) manner. This model allows for the explicit measurement of important parameters such as thermal and electronic diffusivity, electronic mobility, and carrier recombination kinetics-both in bulk and surface or interface. Later, in 1995, photoacoustic (PA) heat-transmission measurements were used to study transport in a nearly intrinsic Ge single crystal by M D Dramicanin et al [5]. D M Todorovic et al [6] investigated photoacoustic effect as a function of the modulation frequency in a ‘transmission-detection configuration’ for semiconductor samples. Dependence of photoacoustic effect on thermal diffusion, thermoelastic, and electronic-transport parameters was identified. Infrared photothermal radiometry was found to be extremely useful in measuring thermal and several electronic properties of semiconductors. Photothermal radiometry (PTR) allows measurement of optically induced black body radiation from semiconductor surface. It has been shown that the frequency-domain PTR signal is extremely sensitive to the photoexcited carrier plasma-wave in semiconductors. Hence this technique has been attractive for ion implantation and process monitoring with carrier plasma wave technology. Applications of technique to ion implanted Si wafer for qualitative analysis of the influence of thermal annealing and sensitivity of the carrier plasma wave to implantation induced damage in Si technique have been reported [7]. This technique was successfully demonstrated to be capable of characterization of ion implantation effects on the electronic properties, as well as monitoring the contamination of Si wafers [8] and Si electronic devices [9]. But all these works were done under the assumption that semiconductors were homogeneous or discontinuously homogeneous; however modern device processing requires improvement of the spatial resolution of PTR, allowing for measurements within the ‘scribe lines’. Three dimensional measurement techniques taking into