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In this paper we measure, spatially resolved, the efficiency potential of multicrystalline material. The excess carrier lifetime in the bulk affects the open-circuit voltage, the short-circuit current and the fill factor of a solar cell differently as different injection conditions apply. Therefore, we measure the bulk lifetime injection-dependent and analyze each IV -parameter separately. Furthermore, multicrystalline wafers with laterally varying bulk recombination need a spatially resolved analysis to explore the impact of large grains or dislocation clusters on the efficiency. The crucial information we need is the spatially resolved injection-dependent bulk lifetime. For this purpose, we use lifetime calibrated photoluminescence imaging with a varying generation rate. Measuring a surface passivated wafer after the important high temperature steps of a solar cell process ensures that we attain the bulk recombination that is relevant for the solar cell. Combining this information with a cell simulation, we deduce the maximum efficiency for every image point. We also discuss how to obtain global values from these images and investigate the effect of charge currents flowing laterally through the emitter using Sentaurus Device and Spice network simulations. A comparison of these simulated global values to measured IV -parameters on finished solar cells shows good agreement between the two. A detailed analysis of two mc-wafers from different feedstock shows limiting IV -parameters in different regions, efficiency reductions due to dislocations and explains differences in the fill factor of two equally processed solar cells. The analysis shows that evaluating the material related efficiency limitation of silicon wafers has to be done at the efficiency level, as the lifetime at one generation level alone is not sufficient.

Reliable process control or predictions of solar cell efficiencies from minority carrier lifetimes on silicon wafers require precise lifetime measurements. For inhomogeneous material quality this implies the necessity of adequately averaged spatially resolved lifetime measurements. Materials such as, e.g. multicrystalline upgraded metallurgical grade silicon frequently feature relatively low lifetimes, high trap densities, and several material parameters (charge carrier mobilities and net dopant concentration) that are not straightforwardly predictable or measurable. As this may substantially compromise conventional lifetime measurements, we present a solely luminescence based lifetime imaging technique, which requires virtually no a priori information about material parameters. Our approach is based on a calibration of a wafer's photoluminescence image through a precise lifetime determination of a part of this wafer via quasi-steady-state photoluminescence. Carrier mobilities, net dopant concentration, and surface morphology leave the determination of lifetime virtually unaffected, the injection dependence of lifetime is properly taken into account, and lifetimes down to the timescale of a microsecond can be reliably measured.

Spontaneous photoemission of crystalline silicon provides information on excess charge carrier density and thereby on electronic properties such as charge carrier recombination lifetime and series resistance. This paper is dedicated to separating bulk recombination from surface recombination in silicon solar cells and wafers by exploiting reabsorption of spontaneously emitted photons. The approach is based on a comparison between luminescence images acquired with different optical short pass filters and a comprehensive mathematical model. An algorithm to separate both front and back surface recombination velocities and minority carrier diffusion length from photoluminescence (PL) images on silicon wafers is introduced. This algorithm can likewise be used to simultaneously determine back surface recombination velocity and minority carrier diffusion length in the base of a standard crystalline silicon solar cell from electroluminescence (EL) images. The proposed method is successfully tested experimentally. Copyright © 2009 John Wiley & Sons, Ltd.

Broad range injection-dependent carrier lifetime measurements in crystalline silicon are most relevant for both defect level spectroscopy, and for the investigation of recombination properties of novel solar cell technologies. The approach presented in this paper combines a determination of the effective carrier lifetime via time-dependent (quasi-steady-state) photoluminescence with a steady-state photoluminescence lifetime scan over a broad range of excess carrier densities. Thereby, the power of a virtually artifact-free time-domain approach is combined with the high sensitivity of steady-state photoluminescence at extremely low injection conditions. Time-dependent fluctuations of dark photocurrent measurements are identified as the essential source of uncertainty under such low injection. A measurement design which eliminates this source of uncertainty is presented and tested. Lifetime measurements at excess carrier densities as low as 10 8 cm −3 are shown in detail.

Minority carrier lifetime is the most crucial material parameter for the performance of a silicon solar cell. While numerous methods exist to determine carrier lifetime on solar cell precursors prior to metallization, only very few techniques are capable of implicitly extracting effective minority carrier lifetimes from metallized silicon samples. In this paper, a measurement technique for effective minority carrier lifetime on silicon solar cells and metallized cell precursors via quasi-steady-state photoluminescence is presented. The setup requirements for this measurement technique are elaborated, experimental evidence of the reliability of such measurements down to carrier lifetimes in the range of a microsecond is provided, and a lifetime calibration of spatially resolved photoluminescence images of solar cells via the presented measurement technique is sketched. Finally, the very good agreement between the obtained effective carrier lifetime and the corresponding open circuit voltage of a solar cell is demonstrated.

A simultaneous determination of injection dependent minority carrier lifetime and net dopant concentration in crystalline silicon wafers from quasi-steady-state photoluminescence (QSSPL) is theoretically derived and experimentally implemented. The time shift between maxima of a time modulated irradiation intensity and the respective photoluminescence intensity is linked to effective minority carrier lifetime. In addition, the ratio of peak curvatures of irradiation intensity and photoluminescence intensity reveals the net dopant concentration of the respective material. Thus, we found a luminescence based technique to determine injection dependent minority carrier lifetime in silicon wafers, which requires a priori information neither about carrier mobilities nor about net dopant concentration.

In 2007 Wuumlrfel [J. Appl. Phys. 101, 123110 (2007)] introduced a method to determine spatially resolved minority carrier diffusion lengths in silicon solar cells from electroluminescence intensity ratios. The key feature of this method was the exploitation of reabsorption of luminescence within a solar cell through optical short pass filters. The first part of this work deals with some experimental challenges which we encountered with the practical application of this method. A procedure of removing an artifact due to typical lateral filter inhomogeneities is introduced. Moreover, temperature dependence of luminescence is discussed and incorporated into the underlying model. The second part of this work aims at a determination of spatially resolved carrier diffusion lengths from photoluminescence (PL) on silicon wafers. The shortcomings of a diffusion length determination from PL intensity ratios are discussed. A straightforward method to determine spatially resolved integral excess charge carrier density from single PL images is introduced, thereby rendering a calibration with another measuring technique unnecessary. We show that quantitative information about recombination properties, such as minority carrier diffusion length, can be directly extracted from single PL images of silicon wafers.

A model for the combined effect of dislocations and grain boundaries on minority carrier lifetime has been developed. Lifetime varies with dislocation density, grain boundary misorientation, and the coincidence site lattice (CSL) nature of the boundaries. Minority carrier lifetime was measured with high spatial resolution (50 mu m) using the carrier density imaging (CDI) technique on a silicon nitride passivated multicrystalline sample. Dislocation density was measured on the same sample by image recognition of optical microscope pictures of a Secco etched surface. Grain boundaries were then mapped and characterized by electron backscatter diffraction (EBSD). Lifetime was simulated based on the dislocation and grain boundary measurements. Parameters were chosen to match closely the simulated and measured maps. Very good two-dimensional (2D) correlation was obtained by assigning roughly equal importance to recombination at dislocations and grain boundaries. The value for the capture cross section, which gives the best correlation, is 4x10(-14) cm(-3). This is in the range of values reported for interstitial transition metals like, for instance, iron. It appears necessary to include also the effect of grain boundaries to explain recombination in low lifetime areas. Sub grain boundaries were particularly recombination active and are dominating the number of active grain boundaries.

An overview is given on recent developments at Fraunhofer ISE in the field of diagnostic techniques based on carrier lifetime measurements. The status of the different lifetime spectroscopy methods and the diagnostic capabilities of infrared carrier lifetime imaging are outlined. A combination of short acquisition times with high spatial resolution and high dynamic range under (quasi-) steady-state conditions gives IR lifetime imaging a high potential for the application in material and process characterization. Measuring in the emission mode provides access to the temperature dependence of lifetime and thus spatially resolved temperature dependent lifetime spectroscopy. Images of injection dependent lifetimes may be measured. For the anomalous increase at low injection a modeling description was developed. Assuming carrier trapping as origin of the increase, spatially resolved trap parameter analysis is possible. A new approach to suppress the anomalous increase by the application of sub-bandgap light was demonstrated.

Lower PV systems cost can be achieved if less silicon material is used in modules and if higher solar cell efficiencies can be achieved cost effectively. In this study the efficiency limits of mass production high efficiency laser grooved buried grid solar cells have been modelled for thinner and thinner wafers. PC1D modelling has been coupled with a 3D ray tracing simulation RAYN to predict cells performance. Given suitable surface passivation, light trapping and minority carrier lifetime, solar cell efficiency can actually increase with decreasing wafer thickness. Cells were made by the RP-PERC process, using thinned industrial grade Czochralski silicon wafers. Cells (4cm/sup 2/) of over 20% efficiency were fabricated in wafers with a final thickness of 115 /spl mu/m. Standard production LGBG cells had poor BSFs but on process optimisation nearly 17% efficiency were made in 140 /spl mu/m micron wafers on an industrial production line.

Depending on the specific impurities and defect spectrum of a silicon material, the minority carrier lifetime can react diversely to rapid thermal processing (RTP). We have measured the lifetime of silicon materials before and after RTP and have diffused solar cells either by RTP or by conventional quartz tube furnace processing (CFP). Our investigations show that the lifetime of Fz-Si can be preserved during RTP resulting in up to 18.7 % efficient solar cells. For 1.4 /spl Omega/cm PV-grade Cz-Si, the stable lifetime after light degradation could be improved by up to 60 % by RTP. In the case of EFG-Si, the same average cell efficiency was obtained with RTP diffusion as with the industrial reference diffusion process. However, in the case of block-cast mc-Si, the lifetime decreases with increasing diffusion temperature indicating that P-diffusion in the second range might provide insufficient gettering.

Lifetime mappings are common tools for assessing the quality of mc-silicon for solar cell production. Nevertheless it is a difficult problem to directly relate lifetime mappings to overall solar cell parameters. This paper intends to show that this correlation is possible quantitatively. We have correlated actual low-level injection lifetimes obtained by carrier density imaging (CDI) measurements with overall cell parameters of solar cells processed on adjacent wafers. The 2D lifetime-structure is taken account for by appropriate weighing functions that include the whole information given in the frequency distribution of bulk lifetimes. Thus a one dimensional cell model (PC1D) can be applied. Good general agreement between predicted and measured cell parameters has been achieved, deviations are discussed. Further insight into the gettering behavior of block-east and Bridgman-grown mc-silicon was attained.

SR-LBIC measurements are an important tool for mapping effective diffusion lengths of processed solar cells. This paper investigates the extraction of bulk diffusion length and back surface recombination velocities from SR-LBIC data. Two models for fitting data are studied, one of which takes account for a physical model proposed by Basore (1993), the other one, Spiegel et al. (2000), is a simplification thereof. Simulations with varying noise levels show, that noise is a more severe problem for L/sub bulk/ and S/sub back/ mappings as compared to L/sub eff/ mappings, though it should be possible to generate L/sub bulk/ and S/sub back/ mappings, if the noise level is below approximately 3%. First results of SR-LBIC data analysed with this tool are presented showing the necessity of having at least one measurement at a wavelength that corresponds to a penetration depth around half of the cell thickness.

Carrier lifetime is very sensitive to electrically active defects. Apart from detecting the presence of recombination active defects, lifetime measurements allow for a direct identification of defects if the temperature and injection dependence of carrier lifetime is analyzed. In the present work, the Shockley–Read–Hall (SRH) equation is analyzed from a theoretical point of view in order to demonstrate the possibilities and basic restrictions of i_njection- and t_emperature-d_ependent l_ifetime s_pectroscopy (IDLS and TDLS), respectively. To show the variety of attainable curve shapes and to evaluate their suitability for a spectroscopic determination of defect parameters, the general impact of the defect and material parameters on the injection and temperature dependence of SRH lifetime is investigated systematically. From this the basic requirements for an unambiguous defect characterization are derived. The experimental applicability of lifetime spectroscopy is demonstrated by TDLS and IDLS measurements performed on an intentionally molybdenum contaminated Si sample using the microwave-detected photoconductance decay technique and the quasi-steady-state photoconductance technique, respectively. The energy level of 0.335 eV below the conduction band determined from TDLS corresponds well with deep level transient spectroscopy-values reported in literature and could also be determined from the corresponding IDLS curve though not unambiguously.

Effective lifetimes as measured via dynamic vs. steady-state techniques may drastically diverge in the presence of a highly nonuniform spatial distribution of excess carrier recombination. Based on a Green's function approach to the continuity equation, simple mode weighting rules were derived for both steady-state effective lifetime and quasi-steady-state harmonically modulated lifetime. Their validity was experimentally confirmed. Such mode weighting rules provide a full quantitative understanding of the mismatch between these two types of effective lifetime. Also, they allow the determination of individual recombination and transport properties from effective lifetimes measured at highly nonuniform configurations of recombination rate.

The authors compare the limiting effects of high-efficiency rear contacted solar cells, screen-printed rear contacted solar cells and emitter-wrap-through solar cells for different cell thickness by means of numerical device simulation. For high diffusion lengths, the high-efficiency rear contacted cell and the emitter-wrap-through solar cell show a decreasing short circuit current density with decreasing cell thickness, whereas the screen-printed rear contact cell shows the reverse behavior due to a "pumping" effect of the floating emitter. The impact of the emitter sheet resistance in the via hole of the emitter-wrap-through cell on the illuminated IV-curve is investigated for illumination from the front and from the rear side. The presented analysis allows an optimization of rear contacted cells for application as high-efficiency and as low-cost structures.

Recent studies have revealed that the metastable defect causing the lifetime degradation in standard boron-doped Czochralski silicon (B-Cz-Si) is correlated with boron and oxygen. This is confirmed by the authors' results from a comprehensive investigation of carrier lifetime in p-type Cz-Si. While no degradation and excellent lifetimes close to the theoretical limit are observed for all gallium-doped samples, the stable degraded lifetime of the B-Cz-Si samples is strongly reduced by the Cz-specific defect. In order to allow realistic simulations of cells from B-Cz-Si, the measured doping dependence of the bulk lifetime in B-Cz-Si is modeled by a simple empirical expression. It is demonstrated that the optimal doping concentration leading to maximum efficiency is strongly shifted with the used solar cell structure and the actual degradation state. While the maximum stable efficiency for a high efficiency RP-PERC solar cell is predicted for a base doping of 5.10/sup 15/ cm/sup -3/ using B-Cz-Si, the optimal value for an industrial cell with screen printing emitter varies between 10/sup 15/ cm/sup -3/ for a cell with boron BSF and 10/sup 16/ cm/sup -3/ for a cell without BSF. The physical background leading to these optima is discussed in detail. Finally, the authors verified the theoretical predictions for the high efficiency cell structure fabricating RP-PERC cells on B-Cz-Si.

We studied the behavior of iron in multicrystalline silicon during phosphorus diffusion by spatially resolved measurements and physical modeling. We present improvements to the previously used models for internal gettering in multicrystalline silicon and phosphorus diffusion gettering. 2-dimensional simulations are used for optimization of the phosphorus diffusion processes for intentionally contaminated wafers regarding the iron distribution, without changing the emitter characteristics. Simulations and experimental results show a reduced interstitial iron concentration after an additional low temperature step at the end of the phosphorus diffusion. The concentration of iron precipitates was reduced by a short annealing at 900°C before the phosphorus diffusion, leading to a carrier lifetime three times higher than compared to the standard process.

Based on quasi-steady-state photoluminescence, we present an approach to extract minority carrier lifetime from silicon wafers without a priori information about any material parameter (e.g., dopant concentration or mobility). A sinusoidal oscillation of irradiation of a silicon sample in time stimulates a likewise oscillating excess carrier density. Our approach is based on the fact that—in the quasi-steady-state regime—the time shift between the maxima of irradiation intensity and the intensity of radiative recombination is linked to effective minority carrier lifetime. Exploiting the continuity equation, it is possible to determine injection dependent minority carrier lifetime from there.

Carrier density imaging (CDI) is introduced as a new, spatially resolved carrier lifetime measurement technique in solar cell production. CDI provides the actual local lifetimes as compared to standard lifetime mapping techniques (e.g. MW-PCD) which yield the differential lifetime only. Most important, CDI is an extremely fast measurement technique: A measurement on a 100/spl times/100 mm/sup 2/ wafer under low-level injection conditions can be performed on a timescale of seconds whereas a standard MW-PCD map needs about 2 hours for a measurement with identical resolution even if high injection conditions are chosen. The combination of a spatially resolved and fast measurement predestines CDI for in-line process control. It is possible to achieve actual lifetime maps with overall measurement times on the order of seconds on as cut samples as well as on emitter-diffused wafers without any surface passivation. Improvements for further reduction of measurement time are discussed.