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We present an approach for examining and understanding the impact of material and process variations on solar cell efficiencies using the example of an industrial feasible multicrystalline silicon (mc-Si) passivated emitter and rear cell (PERC) process. We fabricate and characterize more than 800 mc-Si PERC cells with a broad material variation and model the experimentally achieved solar cell efficiencies based on numerical 3-D device simulations, metamodeling, and Monte Carlo runs. We subject the simulated distribution of cell efficiencies to a variance-based sensitivity analysis, extracting and ranking the process- and material-related input parameters according to their share of the total variance of cell efficiencies and highlighting the parameters that need to be tuned and controlled most accurately. We are able to explain 90% of the measured total variance which divides into 68%abs. material- and 22%abs. process-related influences. Experimental indication of fill factor (FF) losses due to laterally inhomogeneous bulk lifetimes is found. The presented methodology and its findings provide a fundamental tool for a better understanding of the dependencies in a mc-Si PERC process and lays the groundwork for optimizing the quality and the yield of production lines. Furthermore, the approach is transferrable to other solar cell concepts and production lines.

Light-induced degradation (LID) of multicrystalline silicon (mc-Si) solar cell performance has been reported to be surprisingly strong for conditions relevant under field operation. In paticular, solar cells with dielectrically passivated rear sides such as passivated emitter and rear cells are affected by this LID effect that can cause a loss of more than 10%rel in cell efficiency. With the root cause for the observed degradation being unknown to date, the underlying defect, however, has also been reported to be permanently deactivated under the same conditions at even longer time scales. However, a severe power loss due to the mc-Si specific LID is observed before the cells recover due to the long time scales of the regeneration under these conditions. Hence, regeneration on short time scales similar to the fast regeneration processes being reported for the boron–oxygen defect within p-type Czochralski-grown silicon is highly desirable also for p-type mc-Si, especially since mc-Si currently dominates industrial solar cell production. Within this work, partial regeneration of the defect causing the mc-Si specific LID is shown to be possible within less than 30 s, reducing the impact of LID by up to 60%, which leads to a significantly increased performance of the regenerated mc-Si solar cells.

Novel material classes for solar cell production e.g. high performance multicrystalline silicon or epitaxially grown wafers have a huge impact on solar cell performance. A speedup of these developments calls for a rapid assessment of the material quality in the as-cut stage already.This work introduces a generic architecture for the material rating of wafers in terms of solar cell quality. Our approach allows for a simultaneous prediction of the open-circuit voltage of the solar cell and the image of the dark-saturation current density (j 0 ) from photoluminescence images of as-cut wafers. In the sense of theory-guided data-analysis, we combine a data-driven machine learning approach with known physical constraints, here given by the one-diode equation.Due to this statistical optimization, our method derives the j 0 values for the occluded regions beneath the busbar. From the derived j 0 values, we also evaluate the impact of material-related defects and grid metallization structure on the dark-saturation current density.

Bifacial solar cells and modules are a promising approach to increase the energy output of photovoltaic systems, and therefore decrease levelized cost of electricity (LCOE). This work discusses the bifacial silicon solar cell concepts PERT (passivated emitter, rear totally diffused) and BOSCO (both sides collecting and contacted) in terms of expected module cost and LCOE based on in-depth numerical device simulation and advanced cost modelling. As references, Al-BSF (aluminium back-surface field) and PERC (passivated emitter and rear) cells with local rear-side contacts are considered. In order to exploit their bifacial potential, PERT structures (representing cells with single-sided emitter) are shown to require bulk diffusion lengths of more than three times the cell thickness. For the BOSCO concept (representing cells with double-sided emitter), diffusion lengths of half the cell thickness are sufficient to leverage its bifacial potential. In terms of nominal LCOE, BOSCO cells are shown to be cost-competitive under monofacial operation compared with an 18% efficient (≙ pMPP = 18 mW/cm2) multicrystalline silicon (mc-Si) Al-BSF cell and a 19% mc-Si PERC cell for maximum output power densities of pMPP ≥ 17.3 mW/cm2 and pMPP ≥ 18.1 mW/cm2, respectively. These values assume the use of $10/kg silicon feedstock for the BOSCO and $20/kg for the Al-BSF and PERC cells. For the PERT cell, corresponding values are pMPP ≥ 21.7 mW/cm2 and pMPP ≥ 22.7 mW/cm2, respectively, assuming the current price offset (≈50%, at the time of October 2014) of n-type Czochralski-grown silicon (Cz-Si) compared with mc-Si wafers. The material price offset of n-type to p-type Cz-Si wafers (≈15%, October 2014) currently accounts for approximately 1 mW/cm2, which correlates to a conversion efficiency difference of 1%abs for monofacial illumination with 1 sun. From p-type mc-Si to p-type Cz-Si (≈30% wafer price offset, October 2014), this offset is approximately 2.5 mW/cm2 for a PERT cell. When utilizing bifacial operation, these required maximum output power densities can be transformed into required minimum rear-side illumination intensities for arbitrary front-side efficiencies ηfront by means of the performed numerical simulations. For a BOSCO cell with ηfront = 18%, minimum rear-side illumination intensities of ≤ 0.02 suns are required to match a 19% PERC cell in terms of nominal LCOE. For an n-type Cz-Si PERT cell with ηfront = 21%, corresponding values are ≤ 0.11 suns with 0.05 suns being the n-type to p-type material price offset. This work strongly motivates the use of bifacial concepts to generate lowest LCOE. Copyright © 2016 John Wiley & Sons, Ltd.

Bifacial applications are a promising way to increase the performance of photovoltaic systems. Two silicon solar cell concepts suitable for bifacial operation are the passivated emitter, rear totally diffused (PERT) and the both sides collecting and contacted (BOSCO) cell concepts. This work investigates the bifacial potential of these concepts by means of in-depth numerical device simulation and experiment with a focus on the impact of varying material quality. It is shown that the PERT cell concept (representing a structure with front-side emitter only) requires high-minority-carrier-diffusion-length substrates with Lbulk > 3 × W (with cell thickness W) to exploit its bifacial potential, while the BOSCO cell (representing a structure with double-sided emitter) can already utilise its bifacial potential on substrates with significantly lower diffusion lengths down to Lbulk ≈ 0.5 × W. Experimentally, BOSCO cells with and without activated rear-side emitter are compared. For rear-side illumination, the activated rear-side emitter is measured to increase internal quantum efficiency at wavelengths λ

Microcracks in silicon wafers reduce the strength of the wafers and can lead to critical failure within the solar-cell production. Both detection of the microcracks and their impact on fracture strength of the wafers are addressed within this study. To improve the accuracy of the crack detection in photoluminescence (PL) and infrared transmission (IR) images of as-cut wafers, we introduce a pattern recognition approach based on local descriptors and support-vector classification. The learning model requires a set of labeled data generated by an artificial insertion of cracks. Within this evaluation, the algorithm detects 81% of the cracks for PL-images and 98% for IR-images at precision rates above 98% in each case, which outperforms the quality of pure IR-intensity-based crack-detection systems with a hit-rate of 65% at a precision of 59%. The proposed algorithm may be combined with the images of the grain structure to avoid the confusion of cracks and grain boundaries. Moreover, the comprehensive set of wafers allows the impact of crack morphology on wafer strength to be investigated. Despite complex crack morphologies, the theoretically expected dependence between crack length and fracture strength is confirmed. Therefore, sorting criteria are derived to rate the cracks with respect to the expected fracture strength of the wafer based on the measured crack length only.

Recombination-active defects e.g. dislocations in multicrystalline silicon (mc-Si) wafers impact the quality of solar cells. These defects can be quantified during the incoming control of silicon wafers with Photoluminescence (PL) imaging and used to rate the solar cell quality. In this work, we analyze the relevance of defect patterns in PL images and grain-boundary (GB) data for current-voltage (IV) prediction by means of image processing algorithms. Based on a large set of empirical data of passivated emitter and rear cells (PERC), a sparse prediction model is trained for each IV-parameter. Our results include both, the prediction of different quality parameters and the relevance of the features extracted from PL images and GB images. We achieve mean absolute prediction errors as low as 2.72 mV and 0.18 mA/cm2 for open circuit voltage (Voc) and short circuit current density (Jsc) respectively, and 0.18% for efficiency as combined parameter. In this evaluation, the wafer data set is split into training group and test group. Therefore the results show the prediction of unknown material. This makes the prediction more challenging but represents a realistic use case for production. The comparative overview of the relevant feature set shows a difference between the prediction of short-circuit current and open-circuit voltage prediction.

Bifacial solar modules are expected to increase their market share in the coming years. To date, most bifacial cells in production and under research are realized on n-type silicon substrates. However, p-type silicon has been dominating photovoltaic industry, partly due to its lower cost. In this work, we present an approach to combine bifacial applications with the industrial p-type passivated emitter and rear (PERC) structure: a bifacial PERC cell (biPERC). Simulation results are presented that showcase a high potential for bifacial gains of the biPERC cell. First experimental results of the biPERC concept on large-area multicrystalline silicon substrates yield a monofacial efficiency of 17.8%. The biPERC cell, therefore, is an easy-to-apply modification to a PERC route and enables higher output powers due to bifacial applicability.

We present new experimental data on light-induced degradation due to the boron oxygen defect in compensatedn-type silicon. We are the first to show that both defect components known fromp-type silicon are formed in compensatedn-type silicon. A parameterization of the injection dependent recombination activity of the slower formed defect component is established. The formation kinetics of both defect components are studied and modeled under different conditions. It is found that the same rate factors as inp-type can describe the degradation, if the actual hole concentration under illumination is taken into account. The regeneration process known to permanently deactivate boron oxygen defects inp-type is successfully applied ton-type material and the illumination stability of the regenerated state is tested and proven.

Light-induced degradation (LID) has been identified to be a critical issue for solar cells processed on boron-doped silicon substrates. Typically, Czochralski-grown silicon (Cz-Si) has been reported to suffer from stronger LID than block-cast multicrystalline silicon (mc-Si) due to higher oxygen concentrations. This work investigates LID under conditions practically relevant under module operation on different cell types. It is shown that aluminium oxide (AlOx) passivated mc-Si solar cells degrade more than a reference aluminium back surface field mc-Si cell and, remarkably, an AlOx passivated Cz-Si solar cell. The defect which is activated by illumination is shown to be doubtful a sole bulk effect while the AlOx passivation might play a certain role. This work may contribute to a re-evaluation of the suitability of boron-doped Cz- and mc-Si for solar cells with very high efficiencies. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

To investigate the rear side of bifacial p-type Czochraslki-grown silicon PERC solar cells, the present work combines Sentaurus Device simulation – calibrated with extensively characterized samples – and the subsequent fabrication of solar cells according to the simulation findings. The authors investigate the physical alteration of rear-side characteristics in the context of an additional rear-side illumination. The additional injection represents an further factor for the balance of carrier generation, recombination and series resistances which in turn influences the design rules for the rear side layout. Our detailed bifacial simulations include these physical aspects and we derive design solutions for different bifacial illumination scenarios for a bifacial p-doped PERC solar cell. Using an industrial PERC process, solar cells with laser contact openings (LCO) and a rear aluminum grid were produced according to the simulation results with a wide variation in rear side layout parameters. The PERC batches showed a rather constant medium (front side) efficiency of η = 20.8±0.2% and a bifaciality of 66 to 77% depending on the rear layout, allowing us to investigate the rear-side characteristics in detail and to compare them with the effects predicted by the simulations. We processed an aluminum rear contact grid with finger widths as small as 100 µm and successfully aligned it onto the LCO with 30 µm contact openings on full-area 156x156 mm 2 wafers. We reached good accordance between the monofacial measurements from front and rear side and our simulation model and could thus predict bifacial illumination results by modeling for two issues: 1. Planar rear sides have an advantage over pyramid textured rear sides for 1000 W/m² front illumination unless additional rear illumination exceeds 250 W/m². 2. As soon as any rear illumination is added to the front-illuminated PERC solar cell, 100 µm thin fingers at the rear side have an output power advantage compared to 150 µm and 200 µm wide fingers.

For bifacial applications, double-sided collecting solar cell structures can be beneficial. The recently introduced "BOth Sides COllecting and COntacted" (BOSCO) solar cell is such a structure and allows the use of standard module interconnection technology. The structure features emitter areas on both sides, which are interconnected by diffused vias. It favors the use of silicon substrates with low-to-medium diffusion length and low resistivity for the maximum benefit compared with other structures, such as Al-BSF and PERC. Within this study, we discuss the potential of the BOSCO cell structure and its applicability for certain silicon material types. Experimental results on different multicrystalline silicon (mc-Si) materials yield monofacial efficiencies (independently confirmed on a non-reflecting chuck) of 17.4% on large-area wafers from block-cast electronic-grade mc-Si and 16.9% for low-quality upgraded metallurgical-grade mc-Si. These values represent a gain of 0.6-0.7% (abs) compared with Al-BSF cells processed in parallel. The bifacial properties are investigated under outdoor testing conditions, yielding a gain in output power of 13% compared with monofacial operation.

The BOSCO solar cell represents a bifacial structure with double-sided collection. The structure allows the use of standard module interconnection technology and favours the use of material with low to medium diffusion length and low resistivity for maximum benefit towards other structures, such as Al-BSF and PERC. Within this work, we present first results on different multicrystalline silicon materials yielding a monofacial efficiency of 17.4 % on large-area wafers from block-cast mc-Si. This value represents a gain of ~0.7 %abs compared to Al-BSF cells processed in parallel. The applicability for bifacial operation is demonstrated by a significantly increased quantum efficiency for rear side illumination. These results make the BOSCO solar cell concept a promising candidate to further boost the output of utility-scale PV plants even when using low-cost wafers of low to medium diffusion length material. Sketch and photographs of the BOSCO solar cell concept.

The BOSCO (“ BO th S ides CO llecting and CO ntacted”) solar cell features a double-sided emitter and contact grids on both surfaces. The emitter region on the rear is connected to the front side by diffused vias. The structure allows the use of standard module interconnection technology and favours the use of silicon substrates with low to medium diffusion length and low resistivity for maximum benefit towards other structures, such as Al-BSF and PERC. Within this work, we summarize the latest results on multi-crystalline silicon (mc-Si). Monofacial efficiencies of 17.4% on large-area wafers from 9N (99.9999999% pure) block-cast mc-Si and 16.9% for low-quality 5N upgraded metallurgical-grade mc-Si have been achieved. These values represent a gain of 0.6 to 0.7% abs compared to Al-BSF cells processed in parallel. First tests of bifacial operation under outdoor conditions yield a gain in output power of 13% compared to monofacial operation. This effect makes the BOSCO solar cell concept a promising candidate to enable bifacial operation, even for low-cost wafers of low to medium diffusion length material.

Light-induced degradation (LID) in boron-doped p-type Czochralski (Cz) silicon is caused by a boron-oxygen (BO) complex, which may be permanently deactivated by simultaneous illumination and heating leading to a permanent regeneration of carrier lifetime and solar cell performance. Up to now, regeneration has only been investigated on wafer and solar cell level. In this work, we investigate whether the regeneration gain on solar cell level can be transferred to module level. For this purpose, we fabricated solar cells with passivated emitter and rear on boron-doped p-type Cz and float zone wafers. The cells are extensively characterised regarding losses in cell performance due to LID by preparing different defect states with certain temperature and illumination treatments. The effect of the injection-dependent carrier lifetime caused by the BO complex in its active state on fill factor is investigated in detail theoretically and experimentally, developing a descriptive explanation. Finally, it is shown by integrating solar cells in the degraded and regenerated state into solar modules, that the regeneration effect can be transferred to module level and is stable upon subsequent illumination of the module.

The recombination induced by soldering pads, screen printed with a commercially available paste containing primarily silver and an aluminium amount of 1-5% wt , is investigated concerning the open-circuit voltage for aluminium back surface field (Al-BSF) solar cells. The saturation current density J o,pad due to soldering pads is found to be in the range of 26.7 10 3 to 85.4 10 3 fA/cm 2 for different conditions of the rear silicon surface. The recombination under soldering pads leads to a voltage drop of Δ V oc = 6.7 mV for Al-BSF solar cells with 6.2% of the rear side being covered with soldering pads compared to a cell with full-area Al-BSF. Within a microstructural analysis of the interface between the silicon surface and the soldering pads it is found that Al-alloying from the Al share in the soldering pad paste only occurs insularly. Furthermore, Al-BSF cells with and without residual rear side emitter and varying area fraction of soldering pads, using a soldering pad paste with and without an Al-share, are investigated. It is found that the non-overcompensated emitter regions under the soldering pads of Al-BSF solar cells affect cell performance dramatically in case of full area soldering pad, especially for an Al-free pad paste.

In this paper, we introduce a predictive, physics-based model, i.e., the so-called tilted-mirror model (tm-model), for optical modeling of rough rear surfaces on silicon solar cells. An enhanced method of using transfer matrices at the rear-side interface of solar cells is developed and combined with Monte Carlo ray tracing. As a result, a physically consistent and precise simulation of the spectral reflectance is achieved, thus leading to a predictive quality of the simulations that could previously not be reached for solar cells with a remaining irregular rear-surface roughness. This advance in optical simulation enables the researcher to directly analyze the effects of varying rear-side passivation materials and thicknesses, as well as the impact of different surface morphologies on the gained charge-carrier generation rate of a solar cell. A comparison with the Phong model shows that the tm-model is able to simulate the generated photocurrent Jph more accurately, as it is shown that the Phong model tends to overestimate this value due to imprecise calculation of charge-carrier generation. In an application of the tm-model to passivated emitter and rear cells, it is shown that a strong planarization of the rear surface leads to an improvement in photogenerated current up to 0.13 mA/cm2 compared with a weak planarization.

Photoluminescence (PL) imaging is a promising characterization technique for rating and sorting of multicrystalline silicon (mc-Si) as-cut wafers concerning to their material quality. It is inline applicable and yields high resolution images showing recombination active defects from crystallization which influence solar cell performance. In this contribution the basic concepts in ongoing work concerning relevant defects, rating results, statistics, algorithms and general approaches for the rating are summarized. Since 2009 wafers are sorted into five quality classes at Fraunhofer ISE. Details on the rating criteria are given and three examples for application are presented: (i) the impact of edge contaminations on the solar cell results, (ii) a comparison of wafer suppliers based on random samples from a statistical basis of 10,000 wafers and (iii) the results of an advanced rating and sorting of wafers for the manufacturing of highly efficient MWT-PERC solar cells. The results confirm that PL imaging can be used for a very precise rating of material quality.

Carrier mobility in silicon plays a crucial role for photovoltaic applications. While the influence of doping on mobility in standard monocrystalline silicon is well understood, recent research has been focused on the effects of crystal defects in multicrystalline (mc) silicon and of the presence of both acceptors and donors in compensated silicon, both introducing additional scattering centers influencing carrier mobility. In this work measurements of the majority carrier mobility have been carried out in two blocks of compensated multicrystalline silicon. Confirming existing results we come to the conclusion that with increasing compensation level mobilities may be significantly lower than predicted by Klaassen's mobility model, which is basically suited for the description of mobilities in compensated silicon as it accounts for scattering on both acceptors and donors. However, as this model is based on mobility data from uncompensated silicon, a compensation-related reduction of screening is not taken into account sufficiently. To describe mobilities in compensated silicon, a modification of Klaassen's mobility model based on published mobility data in compensated silicon is suggested.

A fast and thorough characterization of grain structure in multicrystalline silicon (mc-Si) is crucial to improve crystal growth and thus bulk lifetime in solar cells. The presented characterization techniques are based on simple optical measurements on as-cut mc-Si wafers. An insight into the entire brick is gained by connecting 2D-information, computed via advanced pattern recognition techniques, over brick height. We identify robust statistical key parameters. Their development within typical bricks of different cast-Si techniques is compared and it is found that the distinct behavior of different materials in the lower part of the brick subsides towards the brick top where grain size distribution is similar.

Microcracks that are induced in early processing stages, especially before emitter diffusion, strongly influence the current–voltage $(I-V)$ characteristics of the solar cell. We focus on the impact of crack morphology measured by photoluminescence imaging in the as-cut stage on the electrical solar cell parameters. To provide a sufficient statistical base, microcracks are intentionally induced in a well-defined way in multi- (mc-Si) and mono- (Cz-Si) crystalline silicon wafers in the as-cut stage, the damaged wafers being processed to solar cells afterwards. From the dataset, a sorting criterion for microcracks concerning their electrical impact is derived, which depends on wafer thickness and material type. It is shown that cracks above 4 mm2 lead with high probability to severe shunts and, thus, need to be sorted out. Investigations by means of scanning electron microscopy (SEM) and electron-beam induced current (EBIC) measurements reveal that shunts with very low parallel resistance in Cz-Si solar cells can be attributed to metal-to-metal contacts between front and rear sides of the solar cell. Moreover, it is shown that the reduced robustness of Cz-Si compared with mc-Si concerning the formation of shunts at microcracks originates from a widening of the crack channels above 10 μm in alkaline texturing, which facilitates the formation of metal-to-metal contacts.

Grain structure and grain competition have a strong impact on bulk lifetime in multicrystalline (mc) silicon. A fast and thorough characterization of grain structure is crucial in order to improve industrial crystal growth. This work introduces key parameters of grain structure, extracted with a newly developed image processing tool. Four bricks grown with different concepts were chosen to investigate the value of the identified key parameters and to identify characteristic developments along the brick. Optical measurements on as-cut wafers from these bricks serve as a basis to extract grain structure properties, i.e., size, shape, homogeneity and distribution of grain size. By connecting the 2D-information over brick height, a statistical insight into the entire brick is gained. Weighted percentiles of grain area offer a robust measure to characterize grain size distribution. As twinning has a large impact on grain competition, twinned grains are detected via grain shape. Additionally, regions with strong grain competition are highlighted for investigations on grain overgrowth. It is found that the share of twin grains increases with brick height in high-performance mc (HPM) silicon with fine-granular seeds from almost zero up to about 15% whereas it remains rather constant over the whole brick height in standard mc-Si. The results of the investigated bricks show clearly that towards the brick top, the material differences in grain size decrease. This suggests that an energetically favorable state may exist for grain structure development.

The optimization of the metallization pattern and the emitter doping profiles and geometry for selective emitter solar cells require reliable and fast simulation models. The computational effort of one-dimensional models is usually much lower than that of two-dimensional models, which in turn allow for more realistic calculations. We review the literature on one-dimensional and two-dimensional models for the simulation of selective emitter solar cells. We compare the approaches for various emitter profiles and widths of the highly doped areas. We show that the one-dimensional and the two-dimensional approaches show similar trends and only small deviations concerning the short-circuit current density and the open-circuit voltage. Concerning the fill factor and the efficiency, the agreement is still reasonable for the investigated selective emitter structures. However, the one-dimensional approach leads to a more profound understanding and a more realistic simulation of the fill factor.

Wafer quality is extremely important in determining yield and efficiency of solar cells. Ideally, this wafer quality should be determined for incoming wafers before solar cell fabrication based on the electronic quality of the wafers. Recent papers have discussed methodologies for doing this by using lifetime measurement and pattern recognition of photoluminescence (PL) images. This paper compares results from quasi-steady-state photoconductance (QSSPC) lifetime measurements with PL imaging pattern recognition of dislocations. By using a more complete analysis of the lifetime and the PL data than performed in some recent publications, a more detailed physical picture is presented here, which reconciles contradictions between previous results. In particular, the differences between PL and QSSPC lifetime measurements on as-cut wafers are discussed. The trends in voltage prediction based on measured lifetime, doping, and PL-determined dislocation densities are shown. Copyright © 2012 John Wiley & Sons, Ltd.

Double side collecting silicon solar cells such as the emitter wrap through and the both sides contacted and collecting concept are one possibility to maintain high efficiencies even on low lifetime material. To investigate the intrinsic advantages and limitations of this concept we use the solution of the continuity equation under low level injection conditions for a double side collecting (n + pn + ) and a single side collecting (n + pp + ) solar cell. In general, compared to the single side collecting structure, the double side collecting structure yields the highest advantages concerning the roduct of the short circuit current density and the open circuit voltage Jsc·Voc on highly doped material with diffusion lengths in the range of half the cell thickness. A realistic calculation of the fill factor of such devices is done using two dimensional numerical simulations. It is shown that a major reduction of the fill factor is caused by laterally varying voltage induced non-generation losses and ohmic losses, both caused by the majority carrier flow in the base. Electro- and photoluminescence images verify and illustrate the lateral voltage distribution under different illumination levels and terminal voltages.

The authors discuss industry related approaches at Fraunhofer ISE for bifacial p-type silicon solar cells, taking into account the well-known "passivated emitter and rear cell" (PERC), "passivated emitter and rear totally diffused" (PERT) and "passivated emitter and locally diffused" (PERL) architectures. In the case of PERC, challenges in terms of alignment, printability and the importance of bifaciality are addressed. In the case of PERT, a co-diffusion process is utilized to form the emitter and the back surface field simultaneously avoiding also critical shunts that can arise at the edges of such devices. For the PERL technology, the industrial feasible pPassDop approach is discussed. We report on front side energy conversion efficiencies for PERC of 21.4%, PERT of 20.5%, and PERL of 19.8%. Furthermore, bifaciality factors for PERC of 0.7, for PERT of 0.86, and for PERL of 0.89 are presented.

We present an approach to predict the equilibrium temperature of hot spots in crystalline silicon solar cells based on the analysis of their temporal evolution right after turning on a reverse bias. For this end, we derive an analytical expression for the time-dependent heat diffusion of a breakdown channel that is assumed to be cylindrical. We validate this by means of thermography imaging of hot spots right after turning on a reverse bias. The expression allows to be used to extract hot spot powers and radii from short-term measurements, targeting application in inline solar cell characterization. The extracted hot spot powers are validated at the hands of long-term dark lock-in thermography imaging. Using a look-up table of expected equilibrium temperatures determined by numerical and analytical simulations, we utilize the determined hot spot properties to predict the equilibrium temperatures of about 100 industrial aluminum back-surface field solar cells and achieve a high correlation coefficient of 0.86 and a mean absolute error of only 3.3 K.We present an approach to predict the equilibrium temperature of hot spots in crystalline silicon solar cells based on the analysis of their temporal evolution right after turning on a reverse bias. For this end, we derive an analytical expression for the time-dependent heat diffusion of a breakdown channel that is assumed to be cylindrical. We validate this by means of thermography imaging of hot spots right after turning on a reverse bias. The expression allows to be used to extract hot spot powers and radii from short-term measurements, targeting application in inline solar cell characterization. The extracted hot spot powers are validated at the hands of long-term dark lock-in thermography imaging. Using a look-up table of expected equilibrium temperatures determined by numerical and analytical simulations, we utilize the determined hot spot properties to predict the equilibrium temperatures of about 100 industrial aluminum back-surface field solar cells and achieve a high correlation coefficient of 0...

Silicon crystals made from solar-grade feedstock, in general, contain boron and phosphorus atoms. Due to the resulting compensation effects, resistivity measurements on a single wafer alone are not sufficient to describe the electrical transport characteristics of the material, and existing standard models are not applicable. In this paper, guidelines for a consistent description of material parameters in compensated silicon are presented. It is shown that the mobility along the whole ingot is described more precisely by a recently published advanced mobility model accounting for compensation than by Klaassen's mobility model, especially for high compensation. Thus, a consistent description of the material parameters along the ingot may be achieved from only Scheil's equation, as well as the advanced mobility model, if the initial dopant concentrations in the melt are known. It is demonstrated that the set of material parameters may be consistently derived from resistivity measurements, only with no complex measurements of additional parameters within an error of 11% for compensation ratios $C_{{\rm R}} , if the base resistivity is measured at several positions along the crystal and if the simulations are based on Scheil's equation and the advanced mobility model.

Solar cells made from multicrystalline silicon (mc-Si) wafers play an important role in photovoltaics. Nevertheless, tools for the optical simulation of these devices are scarce. In the present work, the reflectance and charge carrier generation of mc-Si cells and modules are for the first time simulated successfully in the complete spectral range including light trapping and escape light, as the comparison with measured reflectance of the finished cells and mini-modules shows. The “spherical caps” geometry is used to model the front surface reflection of iso-textured silicon solar cells. The characteristic angles of the spherical caps are determined from the reflectance of iso-textured wafers for three different texture strengths. Based on this calibration, the reflectance and charge carrier generation rates of cells encapsulated with EVA and glass are simulated and analysed. Iso-textured cells with full-area aluminium back surface field (Al-BSF) and with passivated emitter and rear (PERC) are quantitatively compared regarding the photo-generated current density jPh. The simulations demonstrate that the direct cell-to-module loss of iso-textured mc-Si cells with Al-BSF (0.7 mA/cm2) is smaller than for PERC cells (1.2 mA/cm2).

Current losses at surfaces play a crucial role in the optimization of high-efficiency silicon solar cells. We present a new approach to characterize the surface recombination activity of interdigitated-back-contact (IBC) silicon solar cells by comparing experimental and simulated photoluminescence images (PL). The different recombination properties of the p- and n-doped regions of IBC cells in combination with the operating condition lead to contrast profiles in the PL image that vary with bias voltage. We achieve a good matching of experimental and simulated data for the investigated cells enabling the analysis of how sensitive the simulated contrast patterns are to changes in the surface recombination at the emitter, back- and front-surface-field and if a better matching of the experimental PL images is possible. Using these PL images in combination with simulations around the open circuit voltage Voc the determination of surface recombination velocities S and emitter saturation currents J0 on finished cells is made possible for the first time. This approach also opens a new path towards loss analysis of finished PERL, PERC, MWT and other silicon solar cells.

The minority carrier lifetime in boron-doped oxygen-contaminated Czochralski (Cz) silicon is strongly reduced under illumination or carrier injection. This process can be fully reversed by a 200 degrees C anneal step. In several recent studies it was shown that boron and oxygen are the major components of the underlying metastable Cz-specific defect. The energy level of the defect in its active state A was determined to be around midgap [Schmidt et al., J. Appl. Phys. 86, 3175 (1999)] while the energy level of the defect in its passive state P is very shallow. The Cz-specific defect in its passive state can be identified with the shallow thermal donor. The kinetics of the excess carrier-induced transformation from state P to state A can be described using recombination-enhanced defect reaction theory. On the basis of these experimental facts different solutions for the reduction or elimination of the metastable defect are suggested. Two promising solutions are discussed in more detail: the use of gallium-doped Cz silicon and the introduction of high-temperature anneals into the process sequence. Gallium-doped Cz silicon shows no degradation and excellent lifetimes over a wide resistivity range, although the concentration of interstitial oxygen is in the same range as in standard Cz silicon. Stable solar cell efficiencies comparable to FZ silicon have been achieved. If standard boron-doped Cz silicon is used, the defect concentration can be reduced permanently by a high-temperature anneal using conventional tube or rapid thermal processing. This leads to an improvement of the carrier lifetime by a factor of 2-3

In p-type Czochralski-grown (Cz) silicon a light-induced degradation of the minority-carrier lifetime is well known in the literature. Reducing the extent of this degradation would significantly improve the stable effective lifetime and thus the related performance of solar cells. In this work, the reduction of the density of the metastable defect underlying the degradation is performed by rapid thermal annealing (RTA). For a proper analysis it is extremely important to avoid contamination by the RTA furnace. Both, SiNx and SiO2 were examined as a barrier layer. A 60 nm SiNx layer was proven to act as the most effective barrier layer, allowing maintenance of a very high lifetime of 700 μs on 1.25 Ω cm p-type FZ material. A design-of-experiments (DOE) study was used to analyze the effect of five process parameters on the stable effective lifetime. Especially, the plateau temperature shows a strong correlation with τd, the stable effective lifetime after light-induced degradation. The effect of plateau temperature on τd of Cz- and FZ-Si wafers is examined in the temperature range of 700–1050°C for plateau time 120 s. It was found that the stable effective lifetime of all RTA-treated Cz-wafers is increased compared with the initial stable effective lifetime before processing. The highest increase of stable effective lifetime (by a factor of around 2) is obtained at 900°C with a process time of 120 s. This increase in lifetime is reflected in a reduced concentration of the metastable defect. Copyright © 2001 John Wiley & Sons, Ltd.

We have investigated the effect of the light-induced deep-level recombination centre specific to boron-doped, oxygen-contaminated Czochralski (Cz) silicon on the current-voltage characteristic of Cz silicon solar cells by means of numerical simulation and experiment. The device simulation predicts the occurrence of a shoulder in the current-voltage curve after activating the characteristic recombination centre. The physical reason for the non-ideal diode behaviour, characterised by a local ideality factor greater unity, is the strongly injection-level-dependent bulk lifetime produced by the deep-level centre. The increased ideality factor causes a degradation in fill factor with the magnitude of degradation depending on the doping concentration of the Cz silicon base. In order to verify the theoretical predictions experimentally, we have performed measurements on high-efficiency Cz silicon solar cells. Current-voltage curves recorded before and after light degradation clearly show the theoretically predicted change in shape and the reduction in fill factor. An excellent quantitative agreement between calculation and experiment is obtained for the subtracted current-voltage curves measured after and before illumination.

We introduce a fast and easy to apply method for determining the local series resistance of standard silicon solar cells. For this method only two electroluminescence images taken at different voltages are needed. From these two images, the local voltage and the local current density through the device can be calculated. Knowing these parameters for each pixel yields the local series resistance. By calculating the cell's dark saturation current from the lower voltage image, the method also works with multicrystalline material. We show images, acquired in only 300 ms and compare them with other luminescence based series resistance images. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A set of p-type Czochralski (Cz) silicon materials grown by Shin-Etsu Handotai was used for a comprehensive investigation, including carrier lifetime measurements and fabrication of high-efficiency solar cells at Fraunhofer ISE. The set of different materials consists of gallium and boron doped wafers grown with the Cz method and boron doped wafers grown with the magnetic Czochralski (MCz) method. A clear correlation of the Cz-specific lifetime degradation and the concentration of boron and interstitial oxygen was observed. Thus, gallium-doped wafers with a high concentration of interstitial oxygen of 13·7 ppm showed no degradation. Excellent stable lifetimes of 1098 μs and 862 μs were determined for boron-doped MCz wafers and for gallium-doped Cz wafers, respectively. This high lifetime level was maintained or even improved throughout the cell process optimized for Cz silicon and record efficiencies of 22·7% and 22·5% were achieved for boron-doped MCz silicon and gallium-doped Cz silicon, respectively. Copyright © 1999 John Wiley & Sons, Ltd.

By combining data from temperature- and injection-dependent lifetime spectroscopy (TDLS and IDLS) measured by means of the microwave-detected photoconductance decay technique and the quasi-steady state photoconductance technique, respectively, the exact electronic structure of the metastable defect in standard boron-doped Czochralski (Cz) silicon has been determined. A detailed Shockley-Read-Hall analysis of the entire TDLS curve reveals that the Cz-specific defect acts as an attractive Coulomb center (/spl sigma//sub n/(T)=/spl sigma//sub n0/T/sup -2/) which is localized in the upper band gap half at E/sub C/-E/sub t/=0.41 eV and has an electron/hole capture cross section ratio k:=/spl sigma//sub n///spl sigma//sub p/ of 9.3. A new routine for data evaluation allows a transparent SRH analysis of IDLS and TDLS data and enables the accuracy and consistency of the determined defect parameters to be assessed. For the metastable defect in boron-doped Cz-Si perfect agreement between IDLS and TDLS has been found, which demonstrates the excellent performance of lifetime spectroscopy.

Progress in metallization techniques and emitter formation technologies needs fast and reliable tools for the optimization of the metallization pattern of crystalline silicon solar cells. We present and compare three models and discuss their validity and accuracy for different emitter sheet, finger and contact resistances, and finger widths. All models yield a similar optimal number of metallization fingers for a 156 mm × 156 mm large solar cell and broad plateaus concerning the dependence of the conversion efficiency on the finger spacing, but they differ concerning the absolute value of the fill factor and the dependence of the open-circuit voltage on the number of fingers.

In the last years photoluminescence (PL) imaging has become a standard characterization method for conventional silicon solar cells and wafers providing spatially resolved information for material characterization and process control. In this work, the method of the c oupled determination of d ark saturation c urrent and series r esistance (C-DCR) is evaluated on conventional silicon solar cells in terms of its accuracy, reliability and informative value based on a large number of conventional multicrystalline silicon solar cells. The statistical evaluation is based on a comparison of the series resistance mean values obtained from the C-DCR method with the global values obtained from the IV-characteristics within the standard cell testing. Furthermore, examples for its application are given and demonstrate that the minimum exposure time needed for the C-DCR method is short enough to allow an inline application of the method.

During the production of silicon solar cells crack detection systems can help to sort out damaged wafers and reduce wafer breakage before they enter the production line. In order to be cost effective, the crack detection system needs to minimize false detections as much as possible. False detections in crack detection systems occur when bad wafers are not detected or when good wafers are falsely detected as bad. The first error leads to an increase in cell breakage, the second error raises cell costs because non-damaged wafers are sorted out prior to cell processing. In this work a model has been developed to calculate the maximum allowable error rates of crack detection systems in order to achieve a cost per wafer benefit. Therefore a breakage rate dependent throughput calculation, based on manufacturing data, has been implemented. A sensitivity analysis shows that avoiding a high sorting out rate is crucial to favor the implementation of a crack detection system.

Acidic texturing of multicrystalline silicon (mc-Si) wafers often leads to rough surfaces with strong etch attacks (trench structures), especially at sites with crystal defects. The appearance of trench structures on the wafer surface has been cited in earlier publications and has been often recognised as being harmful to solar cell performance [Mathijssen et al., 2009] . In this work, an in-line measurement method for these structures is presented, using a line camera system with diffuse illumination. The presented method can be used for the in-line quality control of the acidic texturization. The number of trench structures is quantified in the images via a newly developed algorithm by an adaptive threshold method. With the help of AFM images the measurement method could be regarded in detail. It has been shown that the true area fraction of trench structures only lies between 0.3% and 1.8% instead of 2–12%, which is estimated from measurements. As expected, the number of trench structures strongly depends on the texture strength and the number of crystal defects in as-cut material. Therefore, if the texturization method remains constant, it is possible to easily measure material quality by measuring the fraction of trench structures and the presented measurement method could be used as cheap alternative to photoluminescence measurements. The reliability of the algorithm is demonstrated by the correlation between material quality, texture strength and the resulting number of trench structures.

We use photoluminescence imaging (PL) for quality control of the crystallization process at Fraunhofer ISE in order to find defects which will later limit the efficiencies of solar cells. Bricks of multicrystalline feedstock from electronic grade (EG) and upgraded metallurgical (UMG) silicon are subsequently wafered and solar cells are manufactured using a standard industrial solar cell process. PL is employed on bricks, as-cut wafers and finished cells and supplemented by additional measurement techniques such as microwave photo conductance decay (MW-PCD) or I–V curve measurements. In UMG material, the major problem is the presence of background dopants, which lead to compensation effects such as a pn-type changeover. As the cell efficiency of wafers within or beyond the type changeover drops significantly in the standard process, these wafers have to be detected reliably in the incoming test to be separated and introduced to an adapted solar cell process or discarded. To do such a separation, the position of the wafer from the brick needs to be known.

For effectively textured silicon surfaces for solar cell applications two sometimes contradicting preconditions have to be met. On the one hand a roughening of the surface which reduces the amount of incident light reflected on the surface and leads to higher short circuit currents and on the other hand a minimization of the total surface area which results in higher open circuit voltages [1]. Acidic texturing of multi-crystalline silicon (mc-Si) wafers often leads to rough surfaces with strong etch attacks (rift structures) especially at crystal defects. However, in this work we come to the conclusion that, besides the rift structures slightly increase surface area and might also decrease parallel resistance, they do not have a significant influence on solar cell efficiency as well as on open circuit voltage, short circuit current and fill factor. Moreover, the rift structures are an indicator for material quality. Furthermore, in this work is has been found that for these acidic created textures surface roughness correlates with weighted reflection and hence only one of these parameters has to be measured.

Luminescence images of silicon solar cells contain information about local recombination properties and local series resistance. It is difficult to separate the information and interpret single images correctly and quantitatively though, which greatly limits the use of single luminescence images, in particular for the application as an in-production characterization tool. We therefore developed a fast method based on photoluminescence imaging for a spatially resolved coupled determination of the dark saturation current and series resistance (C-DCR). (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

At Fraunhofer ISE the fabrication of high-efficiency solar cells was extended from a laboratory scale to a small pilot-line production. Primarily, the fabricated cells are used in small high-efficiency modules integrated in prototypes of solar-powered portable electronic devices such as cellular phones, handheld computers etc. Compared to other applications of high-efficiency cells such as solar cars and planes, the illumination densities found in these mainly indoor applications are significantly below 1 sun. Thus, special care was taken to keep the cell efficiency level high even at very low illumination levels. For this reason, particularly the cell border was analyzed and optimized carefully. The excellent cell characteristics achieved at low illumination densities increase the benefit of a solar power supply for such devices by an order of magnitude if compared to standard solar cells.

If high efficiency solar cells made from Czochralski material are exposed to sunlight or forward bias, their performance degrades due to a rearrangement of complex which most probably contains oxygen and boron. This degradation, which can be reversed by annealing, is reflected not only in the solar cell parameters but also in the dark I-V characteristics. We have imaged the dark forward current across a 4 cm/sup 2/ cell and a 100 cm/sup 2/ cell containing striations by lock-in thermography after annealing the cell at 200/spl deg/C and after degrading it for 24 hrs. The difference between these two images corresponds to the local action of the B-O complex. We have found that this complex is distributed homogeneously across both cells, and that some observed weak local shunts do not show any recombination-induced degradation. Especially, the striation pattern in the 100 cm/sup 2/ cell did not react on degradation and annealing.

We present a spatially resolved method to determine the short-circuit current density of crystalline silicon solar cells by means of lock-in thermography. The method utilizes the property of crystalline silicon solar cells that the short-circuit current does not differ significantly from the illuminated current under moderate reverse bias. Since lock-in thermography images locally dissipated power density, this information is exploited to extract values of spatially resolved current density under short-circuit conditions. In order to obtain an accurate result, one or two illuminated lock-in thermography images and one dark lock-in thermography image need to be recorded. The method can be simplified in a way that only one image is required to generate a meaningful short-circuit current density map. The proposed method is theoretically motivated, and experimentally validated for monochromatic illumination in comparison to the reference method of light-beam induced current.

The lifetime degradation induced by light illumination or carrier injection which is observed in Czochralski-grown silicon (Cz-Si) leads to a significant decrease of solar cell efficiency. Thus, the reduction of this effect has a high potential for the improvement of Cz-Si solar cells. In the present work both, the analysis of the underlying defect and its technological reduction are discussed. A clear correlation of the Cz-specific metastable defect with the oxygen and boron concentration in Cz-Si has been observed. Especially, recently performed lifetime measurements on oxygen-free boron-doped p-type MCz silicon and gallium-doped oxygen-contaminated Cz-silicon, both of which show no degradation, confirm this hypothesis. While the quantitative correlation between the defect concentration and boron is linear, the increase of the defect concentration induced by the interstitial oxygen concentration is superlinear, i.e. it follows a potential law of power approximately 5. Beyond the defect analysis, two different ways to reduce the metastable defect concentration are discussed. A proper material choice by substituting or reducing one of the major components of the metastable defect can completely avoid the degradation effect. The excellent performance of oxygen-free MCz-Si and gallium-doped Cz-Si is reflected in the achieved record efficiencies of 22.7% and 22.5%, respectively. In standard boron-doped oxygen-contaminated p-type Cz-Si a strong reduction of the metastable defect concentration can be achieved by a high-temperature process step resulting in an improvement of the stable bulk lifetime by a factor of 2–4.

Recent investigations have firmly established that the metastable defect which is responsible for the light-induced degradation in Czochralski silicon (Cz-Si) is correlated with oxygen and boron. Thus, an attractive way to improve the material quality is to substitute boron by gallium as the p-type dopant. The authors have verified that no degradation and excellent carrier lifetimes close to the theoretical limit are observed over a wide doping concentration range. Stable efficiencies higher than 20% can be achieved with an RP-PERC cell structure on gallium-doped Cz-Si with resistivities from 0.12 /spl Omega/ cm to 1.5 /spl Omega/ cm. A maximum efficiency of 22.2% was obtained at 0.3 /spl Omega/ cm despite the Ga-doped Cz-Si having a significant concentration of interstitial oxygen. Standard Cz-Si (boron-doped, oxygen-contaminated) can be improved by an optimized high-temperature step without external gettering. Choosing the optimal process parameters, it is possible to increase the stable lifetime significantly in both conventional tube furnace and rapid thermal processing (RTP) system. Using the RTP system, the stable lifetime can be improved by a factor of two within 120 s.