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This paper presents a comprehensive overview on printing technologies for metallization of solar cells. Throughout the last 30 years, flatbed screen printing has established itself as the predominant metallization process for the mass production of Silicon solar cells. For this reason, we will provide a detailed review on its history, its evolution over time and how the continuous efforts of the scientific and industrial community for further improvements revolutionized the entire PV industry. Furthermore, we will guide the reader through the physics on Silicon solar cell metallization, the fundamentals on contact formation and what type of challenges and requirements these topics create for printing technologies. The main topic of this review addresses the flatbed screen printing process mechanics, it´s different process sequences, corresponding screen technology and the very important impact of paste rheology on the printing result. Finally, we will compare alternative upcoming printing approaches to flatbed screen printing and discuss how they might solve upcoming challenges in metallization in terms of increasing the throughput rates at an ever decreasing grid line width and reduced silver consumption.

Simulation is essential for a comprehensive analysis of the performance of solar cells. The rear contact pitch of passivated emitter and rear cells (PERC cells) is a crucial device parameter that influences not only the electrical performance but also the optical performance of these cells. This article investigates the applicability of the analytical light trapping model by Basore to account for the optical influence of the rear contact pitch as a simpler alternative to ray tracing. First, we manufacture three different groups of cells with different rear contact pitches, where the metallization fraction f met varies between 0 and 54%. Second, the reflectance of the cells is measured. Subsequently, we fit the model parameters R f and R b (internal reflection on the front and back surface, respectively) to the measured reflectance. While we confirm a nonlinear relation between f met and the measurable spectra found in previous works, our results reveal linear relations between f met and R b with the adjusted coefficients of determination of R adj² > 0.97, as well as between f met and the charge carrier generation rates with R adj² > 0.94. These relations allow a simple and rapid optical simulation of f met variation for PERC cells. The same approach is likely applicable to any local contacts also in other cell concepts.

In this study, we introduce a temperature screening image analysis to investigate the temperature dependence of boron-oxygen-related defect regeneration achieved by using one sample. For that purpose, we induce a temperature gradient in a single sample over a broad temperature range in our laser-based rapid thermal processing furnace, while other influencing factors are kept constant. Spatially resolved measurements of the temperature during the regeneration process (thermographic images) and photoluminescence (PL) images at different boron-oxygen-related defect states are recorded. By a pixelwise assignment of the PL images to the temperature image, the effectiveness of the regeneration process in terms of regeneration completeness is evaluated for each pixel. In this experiment, we investigate the temperature dependence of a boron-oxygen-related defect regeneration in a temperature range of 100-500 °C for different treatment times of 2-30 s at an illumination intensity of 100 kW/m². Thereby, we determine the temperature regimes that allow for efficient regeneration for the respective regeneration parameter set with a single sample. The results can be used for industrial optimization of a boron-oxygen-related defect regeneration process. Furthermore, this technique can also be applied to other temperature-dependent process optimizations and even fundamental research.

Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since January 2020 are reviewed.

Convolutional neural networks can be trained to assess the material quality of multicrystalline silicon wafers. A successful rating model has been presented in a related work, which directly evaluates the photoluminescence (PL) image of the wafer to predict the current–voltage parameters after solar cell production. This paper presents the results of two visualization techniques to understand what has been learned in the network. First, we reveal what has been learned in the PL image by visualizing the spatial quality distribution of the wafers based on the activation maps of the network. The method is denoted as regression activation mapping. We compare regression activation maps with $j_0$ images of solar cells to show the semantically meaningful representation of the trained features. Second, we show what has been learned in the data by mapping the learned network representation of all wafers into a low-dimensional subspace. Visualizations reveal the smoothness of our representation with respect to the PL input and measured quality. This technique can be used to detect material anomalies or process faults for samples with high prediction errors.

This paper investigates deep convolutional neural networks (CNNs) for the assessment of defects in multicrystalline silicon (mc-Si) and high-performance mc-Si wafers for solar cell production based on photoluminescence (PL) images. We identify and train a CNN regression model to forecast the $I\text{--}V$ parameters of passivated emitter and rear cells from given PL images of the as-cut wafers. The presented end-to-end model directly processes the PL image and does not rely on the human-designed image feature. Domain knowledge is replaced by a model based on a huge variety of empirical data. The comprehensive dataset allows for the evaluation of the generalizability of the model with test wafers from bricks and manufacturers not presented in the training set. We achieve mean absolute prediction errors as low as $\text{0.11}\%_{\text{abs}}$ in efficiency for test wafers from “unknown” bricks, which improves handcrafted feature-based methods by $\text{35}\%_{\text{rel}}$ at simultaneously lower computational costs for prediction. Samples with high prediction errors are investigated in detail showing an increased iron point defect concentration.

Strong band bending within crystalline silicon is important for improving carrier selectivity in the contact regions of silicon heterojunction solar cells. In this work, we compare the use of capacitance-voltage as an alternative to the surface photovoltage method for probing the built-in voltage associated with crystalline silicon band bending. The surface photovoltage technique is incapable of measuring structures with non-transparent contacts, while the capacitance-voltage technique shows saturation at high built-in voltages due to the formation of an inversion layer. The shift in built-in voltage and effect on open circuit voltage are demonstrated for variation across a range of amorphous Si p-doping levels. These experimental results are supported by simulation work, confirming the underlying band structure and carrier concentrations of photovoltaic Si heterojunctions and clarifying the limitations of capacitance measurements at high built-in voltages. Errors arising in capacitance-voltage measurements of these structures are explained and linked to deviations from the Mott-Schottky model approximations of a one-sided junction in full depletion.

Within the EURAMET ENG55 “PhotoClass” project, several characteristics of photovoltaic (PV) devices beyond their performance at Standard Test Conditions were investigated, including measurements at varying irradiance and temperature. Four groups of PV devices of different size and technology were prepared and corresponding round-robins were run between partner laboratories with substantially different facilities and methods – namely based on spectral or integral measurements. This paper presents the outcome of the four inter-laboratory comparisons dealing with temperature coefficient measurements of the short-circuit current of PV devices, from reference-cell size to full-size commercial modules of mainly several c-Si technologies, but also with some examples of CIGS and GaAs devices. The measurement results are compared via En number assessment, hence including measurement uncertainties. The main outcome of this measurement exercise is a very good agreement of all the laboratories although completely different approaches were applied. In some cases, laboratory measurement uncertainties are even considered rather conservative and could therefore be revised. Furthermore, a comparison between bare cells and commercial modules of the same technology is made, which may represent useful information for PV manufacturers.

In High-Performance mc-Si [1] random grain boundaries, although being recombination active, often enhance material quality by reducing dislocations. With this work, we take a step towards statistical large-scale investigations of crystal defects via a combined analysis of different inline-measurements on as-cut wafers: photoluminescence images for the extraction of recombination-active structures and reflection and infrared transmission images for the extraction of the grain structure. The combined extraction of recombination-active structures and grain structures allows isolating dislocations from grain boundaries for all material types. To discern dislocations from other recombination-active defect structures, an image-processing-based analysis technique has been developed. By applying this separation on wafers from various bricks of our material set, typical developments of grain structure and dislocations can be identified. As a particular application, we investigate the correlation between the development of dislocations in higher parts of the brick and grain size in the lower parts. The results support theory quantitatively: Dislocation ratio in the upper brick part shows a correlation with the square root of the weighted median of the grain size in the lower brick part (R≈0.85). However, the results also show that grain size distribution, in particular grain size homogeneity, has to be considered to account for a stronger distinction between materials.

The extraction of the implied open-circuit voltage ( iV OC ) and saturation current density under the metal fingers ( j 0met ) using a single photoluminescence (PL) image is demonstrated in this article. The method employed to derive these parameters is based on an analytical modeling of the local p-n-junction voltage between the metallization fingers and on the analysis of the p-n-junction voltage variations obtained using a calibrated PL image. The image is analyzed in the Fourier space; taking advantage of the periodicity of the metal grid, the relevant information is extracted from the first two Fourier peaks. The ( iV OC ) and ( j 0met ) are calculated from the peaks intensities. This method is compared with one that uses two PL images. Both methods agree within their uncertainty ( ± 200 fA/cm² for j 0met ). The main advantage of the method developed in this article is that only one image is required.

The realistic measurement of solar cells is key for the whole PV industry, as accurate information about cell power is one of the most important aspects in solar cell purchase and PV module design. The omission of busbars introduces new challenges to the current-voltage measurement of solar cells, since contact to every single grid finger has to be established with independent current and voltage contacts. It is not just the shadow correction of the measurement unit that needs to be carried out more laboriously, but also the contacting of the front metal grid, which is more critical because of the high resistivity of the grid fingers. The position of the voltage sensing contact and the number of current contacts can thus have a noticeable impact on the measured performance of busbarless solar cells. Measured efficiencies are highly dependent on the contacting schemes used in different measurement systems, as these vary in contact number and sensing configuration. Two different main approaches for measuring busbarless solar cells have evolved, representing either realistic or idealized application of the cells in the module. The pros and cons of both approaches are discussed in detail in this paper. Realistic measurement conditions lead to efficiencies which best predict module performance, but are hard to realize and require knowledge about the subsequent module design. Although not their primary purpose, the use of idealized measurement conditions can make it easier to achieve record cell efficiencies, but with the disadvantage of limited comparability with busbar-based solar cell concepts. Idealized conditions can moreover lead to hidden losses in performance of the solar cells, related to the application in a module, which in turn causes inflated cell-to-module (CTM) losses. If solar cells are bought in terms of $/Wp and modules are sold likewise, the economic implications arising from the different measurement configurations have to be considered. Whichever approach is used for the measurement of busbarless solar cells, full disclosure of the measurement configuration is absolutely essential.

We present two approaches for high‐accuracy aligning of patterning processes with each other when fabricating solar cells. We introduce the approaches on the example of two different patterning processes of which one is adjustable (laser process) and one is not adjustable (screen‐printing process). The basic idea is to measure the coordinates of the applied structures of each involved patterning process at discrete grid points with respect to a reference coordinate system. We chose the grid points such that they completely define the final cell pattern. Then, we adjust the grid point coordinates of one of the patterning processes (the laser process) according to the pattern of the other process (the screen‐printing process). The laser then performs the patterning by connecting the corrected grid points with each other in the desired direction. We perform the associated high‐precision measurement of the patterns' coordinates by using either a high‐precision offline coordinate measuring machine or a high‐resolution inline camera system with subsequent computer‐based data processing. The latter inline method enables high throughput and is, in turn, of great interest for mass production of solar cells. In this paper, we demonstrate the alignment procedure approaches on ""pPassDop"" solar cells by adjusting a locally applied laser process to the directly following screen‐printing step. This proof of principle includes both above‐mentioned methods for coordinate determination in separate cell batches. Our innovative alignment procedures so far demonstrated the successful matching of 40‐mm‐wide screen‐printed contact fingers to 70‐mm‐wide laser‐processed lines over the entire area of 6‐inch solar cells.

Cracks in silicon solar cells reduce cell efficiency and may lead to critical failures in cell and module production because of breakage. Detecting cracks in an early production stage is, therefore, advisable. Several approaches for crack detection systems have been developed in the past years. In order to compare the system performance, the rating done by the system has to be compared with a reference, i.e., a ground truth defining the crack's presence, position, and/or area. This information can also be used for training of detection algorithms based on supervised learning, e.g., convolutional neural networks. In the present article, we show that finding the ground truth is ambiguous and depends strongly on the operator and the applied metrology. For this end, we apply six crack detection tools to a set of 120 multicrystalline silicon passivated emitter and rear cells that were sorted out in industrial production because of cracks, and compare the human rating of six operators for every tool. The operators’ precision, i.e., the fraction of the contrasts detected as cracks that actually are cracks, ranges from 73% to 100% and their recall, i.e., the fraction of all actual cracks that have been found, from 30% to 92% for all tools and operators. To achieve the highest precision and recall, we suggest rating the images of an optical near-infrared system combined with luminescence imaging by three operators conjointly for the definition of ground truth for the present as well as for larger sample sets.

A round‐robin proficiency test (RR PT) on thin‐film multi‐junction (MJ) photovoltaic (PV) cells was run between 13 laboratories within the European project CHEETAH. Five encapsulated PV cells were circulated to participants for being tested at Standard Test Conditions (STC). Three cells were a‐Si/mc‐Si tandem PV devices, each of which had a different short‐circuit current ratio between the top junction and the bottom one; the remaining two cells were single‐junction PV devices made with material representative of the individual junctions in the MJ cells. The RR PT's main purpose was to assess the capability of the participating laboratories, in terms of employed facilities and procedures, to test MJ PV devices. Therefore, participants were requested to perform STC measurements of all cells according to their own procedure, which might not include external quantum efficiency measurements. The European Solar Test Installation (ESTI) of the Joint Research Centre (JRC) provided the reference calibrations against which the participants' results are compared. ESTI made also a verification of the cells performance at STC at the end of the RR PT, in order to allow a comparison between the initial stable state at which the cells were calibrated (just before circulation) and the one they had reached at the end of the RR PT. The overall results of the RR PT are here presented and discussed together with some aspects of MJ PV testing that emerged as not adequately applied or largely missing. Their full implementation is expected to improve the consistency of future results.

Today’s PV production chain heavily relies on screen printing as the predominant metallization process because of its robust production capabilities. However, the continuous demand to further increase throughput rates while at the same time decreasing printed structure sizes is going to create fundamental challenges in the upcoming years. In order to address this problem, robust fine-line screens at competitive production costs need to be developed. Throughout this study, we present an optimization of screen architectures by utilizing an evolutionary algorithm. Individual screen architectures are defined by genes that represent industrial manufacturing parameters. Our algorithm then solves for the optimal trade-off between printability of a screen, its mechanical stability and optimal mesh tension to ensure sufficient snap-off mechanics. Further, a computational fluid dynamics simulation is performed to find the optimal thickness of the corresponding emulsion layer in respect to the underlying optimized mesh architecture. Our results predict mesh configurations with mesh counts up to MC = 1268 1/inch and wire diameters down to d =6.7 μm in order to enable fine-line printing below 10 μm.

The angle-dependent spectral response is measured for six differently textured silicon solar cells before and after encapsulation. Deviations from Lambert's cosine law and differences due to the textures can be clearly determined by means of an uncertainty analysis. This effort in highly accurate angle-dependent measurement is needed for reliable prediction of annual performance. Energy rating using common simulation models for three different climate zones shows differences of more than 1% in yearly energy yield solely due to the different angle-dependent behavior of the textures. Modules featuring solar cells with random upright pyramids exhibit the largest angular losses of the studied textures.

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.

In this paper we present a new update to PC1Dmod, which extends the original PC1D program by implementing Fermi-Dirac statistics and a range of state-of-the-art models in order to improve the accuracy of c-Si device simulation. In PC1Dmod 6.2 the list of models is further expanded to include a parameterization of incomplete ionization of dopants (Altermatt et al., J. Appl. Phys.100, 113715, 2006), with parameters for phosphorus, boron, arsenic, gallium and aluminum. The results have been verified against a previous implementation of the model. We show that the inclusion of incomplete ionization is of particular importance for moderately doped surface regions with doping densities in the range ∼5 × 10 17 to ∼5×10 19 cm -3 , resulting in a deviation of up to 15% in the simulated recombination current density and sheet resistance. The effect of incomplete ionization is also shown to be more pronounced in devices made from compensated Si material. Furthermore, the default solar spectrum has also been updated in PC1Dmod 6.2, taking advantage of the increased maximum file size to include a more realistic representation. The generation profiles calculated using PC1Dmod 6.2 together with input from OPAL 2 agree well with results from Sentaurus TCAD for both planar and pyramidally textured surfaces, thus facilitating a more direct comparison between PC1Dmod 6.2 and other simulation programs utilizing standard spectra. The simulation comparison is also extended to the electrical performance of the modeled devices, showing good agreement in the calculated short-circuit current density for a range of planar and textured solar cells with varying emitter doping. Finally, several new output options, including emitter saturation current and injection-dependent lifetime curves have been added, enabling a simpler and more direct way for users to access and plot important device properties.

Simulating the fluid flow during a wet chemical process bath, allows for a deeper understanding of the flow conditions in the reaction basin. To validate the simulation, dye visualization as well as laser doppler velocimetry (LDV) and particle image velocimetry (PIV) were carried out for the first time in a transparent replica. Dye experiments led to a qualitative validation of the simulation model, as vortices and direction of rotation were detected in the basin. In summary, the repeated experiments show a good agreement with the simulation that there are areas in the basin where high relative standard deviations occur, mostly in areas with a very low velocity. The LDV velocity is higher compared to the theoretical velocity, while the simulation and PIV measurements match the theoretical value. The PIV measurements are in better agreement with the simulation, since the mean velocity in the ${z}$ -direction is 0.0039 m/s and 0.0033 m/s for the PIV and the simulation, respectively. The PIV results show a change in the velocity direction with time, which can also be seen in the simulation results. The PIV measurements therefore confirm the simulation results of a time-dependent change in the flow.

We determine the uncertainties on simulated efficiencies of silicon solar cells due to uncertainties of the fundamental physical models. To determine these, we refit their parameters to the underlying measurement data. Using a metamodeling and Monte Carlo simulation approach, we then deduce how these propagate to the simulated solar cell efficiency. This is evaluated for 150- $\mu \text{m}$ -thick 1 $\Omega $ cm p-type standard and advanced silicon passivated emitter and rear cells (PERC), as well as for an ideal silicon solar cell. When sticking to the best-known set of physical models, we determine resulting efficiency uncertainties as low as 0.06%abs for usual PERC cells and 0.1%abs for an ideal cell. In a variance-based sensitivity analysis, we find the uncertainties of the bandgap and the hole density-of-states effective mass as well as the Auger recombination in the limiting case to dominate the efficiency uncertainty. In addition to these relatively low uncertainties, larger discrepancies may arise when applying different physical models for otherwise fixed device properties. We determine comparably large efficiency discrepancies of up to 0.6%abs for the two most prominent bandgap narrowing (BGN) models, and up to 0.15%abs for the two most recent parameterizations of free carrier absorption. We also show evidence that for the specific case of a strong space charge region being present at a recombining surface, Schenk’s BGN model fails to replicate experiments. Those discrepancies highlight the necessity of further research on those models. Finally, an error of up to 0.08%abs is observed by simplifying diffused regions on textured surfaces to a planar geometry.

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.

We investigate the origin of the apparently reduced recombination parameter J 0 of highly doped regions obtained from e.g. QSSPC lifetime measurements at high injection densities as well as the influence of recombination at bulk defects on the recently updated J 0 -analysis of QSSPC measurements in the Sinton lifetime tester software. Using the example of crystalline silicon, we show that the frequently observed reduction and underestimation of J 0 , which is strongly pronounced at high injection levels, originate from neglecting the finite carrier diffusion coefficient. This work presents an analysis taking into account the finite carrier diffusion coefficient and thus enabling to evaluate charge carrier lifetime data at high injection levels leading to strongly increased consistency of the determined J 0 . Furthermore, we give a simple correction term for the recently published analysis leading to a total independence of a reasonably injection-independent Shockley–Read–Hall (SRH)-lifetime in the substrate. Combining the proposed methods, the determination of J 0 from numerically simulated lifetime-data is accurate within 3% of the input J 0 =100 fA/cm 2 over an injection range of 10 14 –3·10 16 cm −3 . The application of the proposed method to measured data of crystalline silicon devices is in excellent agreement with the simulations and shows a strongly improved linearity and thus constant extracted J 0 compared to the former and recently published methods, which underestimate J 0 at high injection levels, nearly independently from doping type and -level of the substrate. The new and simple method is implemented in a spreadsheet calculator enabling fast and robust data evaluation similar to the Sinton lifetime tester software.

The upgrade of state of the art p-type silicon solar cell production lines to passivated rear side technology (PERC) will be one of the major trends in the next years and new production processes for further cost reduction will continuously gain relevance. In 2007, we have introduced the laser based foil metallization technology “FolMet”: the rear electrode of p-type PERC devices as well as the local contact is fabricated by attaching conventional aluminum foil during the so-called laser fired contact process to the silicon wafer. This process features improved internal optical properties, a huge cost saving potential and a simplified cell production process. In this publication we focus on the acceleration of the laser process, which is together with module assembly issues a remaining challenge towards industrialization. We carried out comprehensive simulations, to better understand the correlation between different laser parameters on melting- and evaporation depth of the 8 μm thin aluminum foil. We determined lower limits for crucial laser pulse parameters to successfully attach the foil onto the substrate and validated these parameters experimentally. According to these results, we set up a system based on a pulsed high power laser featuring repetition rates Frep ≤ 2 MHz with an unique ultrafast polygon scanning system, allowing for scan-speeds vscan ≤ 1000 m/s. Thereby, we demonstrate processing times tpro < 0.8 s for industrial wafer, which corresponds to a reduction in laser process time by the factor of 20 compared to state of the art laser scanning technology.

Shingled interconnection of solar cells allows increased output power density pout by (i) increasing the active cell area within the module, (ii) decreasing shading losses, and (iii) reducing both series resistance per cell as well as interconnection losses. An additional increase in pout is possible by shingled interconnection using bifacial solar cells, allowing the light capture from the rear side as well. The ""p‐type silicon shingled passivated edge, emitter, and rear (pSPEER)"" solar cell concept introduced and examined in this work as an approach for fabrication of bifacial shingle solar cells is based on the passivated emitter and rear cell (PERC) concept. This article portrays bifacial pSPEER solar cells that are fabricated using industrial 6‐inch p‐type Czochralski‐grown silicon PERC precursors. After contact firing, the 6‐inch host wafers are separated by means of conventional laser scribing and manual mechanical cleaving into six pSPEER solar cells each with an area of 23 × 148 mm2. Current‐volt­age measure­ments of these bifacial pSPEER solar cells with front side illumination yield peak pout,f = 20.4 mW cm−2 on a black background (total cell area, busbar included). For an irradiance of 1000 W m−2, this pout,f is equivalent to an energy conversion efficiency of 20.4%. By additional rear side illumination with an irradiance of 100 W m−2, peak pout = 21.5 mW cm−2 is obtained for these pSPEER solar cells.

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.

We present a paper about vignetting in the context of luminescence imaging of solar cells and wafers. The physical phenomenon vignetting and its contributions are reviewed. A simple model to simulate vignetting and a method of measuring the vignette by the variation of aperture are introduced. The simulated and the measured results are compared showing good agreement with less than 2.1% deviation. For the investigated luminescence setup, it is shown that vignetting can cause an intensity decrease of up to 20% toward the image corners for wide opened apertures. To quantify the correction quality, a correction quality factor is adopted from a recent publication and applied to the suggested method. Additionally, the correction quality is quantified by investigating the linear correlation between luminescence intensity and charge carrier product measured via photoconductance on a suitably prepared sample. The correlation coefficient increases from R2 = 0.92 to R2 = 0.95 after the correction.

For the extraction of spatially resolved solar cell parameters, a variety of methods has been introduced in the past years. Nearly all methods use the fact that the local luminescence intensity can be calibrated to local junction voltage. The different methods however use different approaches for the calibration. In this work we discuss the different approaches used throughout literature. We investigate the key assumption of two approaches which assumes that there are no lateral junction voltage gradients at sufficiently low excitation conditions. Our investigation is based on circuit simulations and experimental results on an example cell. We give an explanation for the remaining contrasts in luminescence images taken at these low excitation conditions. A third approach which does not rely on the key assumption is discussed with respect to measurement time in comparison to the other approaches. Finally the role of the short circuit current is discussed and a modification to the approach as known from literature is proposed.

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.

In this paper we present a one-dimensional numerical simulation study concerning the electrical properties of the TCO / a-Si:H(p) / a-Si:H(i) / c-Si(n) hole contact in amorphous / crystalline silicon heterojunction (SHJ) solar cells. Simulations where performed with Sentaurus TCAD from Synopsys considering the implemented barrier tunneling model. The modification of the a-Si:H(p) / a-Si:H(i) / c-Si(n) p/n junction by the opposing TCO / a-Si:H(p) junction and its influence on the device recombination for the relevant working conditions are investigated. We demonstrate the relevance of the effective screening length in the amorphous silicon, which depends on the effective space-charge in the a-Si:H(p) and the work function mismatch at the TCO / a-Si:H(p) interface. We highlight the importance of an improved work function matching as this presents an additional degree of freedom for the design of the SHJ and is an important parameter to reach both a very high Voc and FF.

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.

Thermal silicon oxides are known to very effectively passivate silicon surfaces. Choosing a water vapor ambient instead of a dry oxygen atmosphere increases the oxidation rate by about one order of magnitude and considerably reduces process time and costs. State of the art pyrox systems produce steam by pyrolysis of hydrogen and oxygen gas. A new approach is the purification of vaporized deionized (DI) water. In this work we present a direct comparison of both steam generation systems, which are connected to the same quartz tube of an industrial high quality tube furnace. The higher steam saturation of the direct steam process enhances the growth rate by about 20% compared to a pyrolytic steam based process. On low-resistivity p-type substrates, excellent surface recombination velocities of around 25 cm/s are found for both systems after a forming gas anneal. Detailed characterization shows similar physical properties of the oxide layers grown by either steam from pyrol ysis or purified steam. Moreover, thermal oxide rear surface passivated silicon solar cells show similar high efficiencies exceeding 18.0% irrespective of the applied steam generation technology.

Simple analytical and statistical models for the evaluation of the temperature of PV-modules from climatic data (ambient temperature, global solar irradiation, and wind speed) are investigated. The parameters which describe the effects of cell technology and module design were evaluated from outdoor exposure data at different climatic regions. The models were validated by comparison of the simulated module temperatures with measured module temperatures at different test sites. A simplified way to determine a Realistic Nominal Module Temperature (ROMT) instead of the Nominal Operating Cell Temperature (NOCT) is proposed.

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.

A model is introduced which divides a solar cell locally into a 'baseline' region, showing a homogeneous exponential characteristic with a low ideality factor, and local 'shunt' regions which additionally show an inhomogeneous depletion region recombination current with a high, locally varying ideality factor. By measuring only three lock-in thermograms and the corresponding currents at one reverse and two different forward biases, the local ohmic contributions, the positions of the shunt regions, the diode parameters of the baseline region, and the local diode parameters of the shunt regions are obtained. Based on these data a good fit of dark current-voltage characteristics of real solar cells over the whole bias range is obtained. These data may be used in a local network model of the cell. [GRAPHICS] Measured dark I-V characteristic (squares) and characteristic simulated from evaluating three lock-in thermograms (line).

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...

The determination of the emitter saturation current density J0e of symmetrical test structures, analyzed by injection-dependent lifetime spectroscopy (IDLS), in high as well as in low injection regimes is compared. A detailed investigation of the influence of different models for the Auger recombination on the evaluation of the measured minority carrier lifetime is performed. It can be concluded that a good agreement for the extraction of J0e under high and low injection conditions for lightly and highly doped emitters on textured and untextured surfaces was obtained if the Auger parameterization of Kerr et al. is applied. For deep-diffused emitter profiles on textured surfaces a significant increase of J0e well above the geometrical surface area ratio of untextured and textured surfaces of 1.7 was observed. A verification of the experimentally determined values for J0e is performed by numerical device simulations. The simulated surface recombination velocity for the investigated emitter doping profiles is in good agreement to analytical calculations. Copyright © 2010 John Wiley & Sons, Ltd.

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.

With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver-aluminum (Ag-Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j0,met and give detailed insight to which degree a solar cell's performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j0,met until their tips reach depths where boron doping concentration is lower than approximately 1018 cm−3. Deeper spikes then lead to an exponential increase in j0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j0-results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.

This paper introduces and explains a simulation-assisted approach for determining and ranking the most influential causes of variations in experimentally obtained solar cell efficiencies, using the example of an industrially feasible multicrystalline silicon (mc-Si) passivated emitter and rear cell (PERC) process. The approach presented is especially helpful for ramping up PERC production; however, since it is basically transferable to any solar cell concept, it can also be applied to optimize established production lines.

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.

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 ifront by means of the performed numerical simulations. For a BOSCO cell with ifront = 18%, minimum rear-side illumination intensities of

This work introduces a novel industrial feasible back-contact back-junction crystalline silicon solar cell technology with exclusively highly doped surfaces. The near-surface phosphorus doping is realized in one single tube furnace diffusion step, while the alloying of screen-printed aluminum forms the p(+)-doping. A first experimental verification leads to a conversion efficiency of 20.1%. Further possible improvements are proposed and verified with numerical simulation. We perform comprehensive step-by-step optimization led by insights from free energy loss analysis, resulting in a conversion efficiency potential of 22.4% under realistic assumptions for design optimization.

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 L-bulk > 3 x 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 L-bulk approximate to 0.5 x 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 lambda < 850 nm by up to 45%(abs) (factor of 9) and 30%(abs) (factor of 2) for cells processed on p-type multicrystalline silicon substrates with L-bulk approximate to 0.3 x W and L-bulk approximate to 2.6 x W, respectively. For PERT cells processed on n-type Czochralski-grown silicon substrates, an according increase in internal quantum efficiency for rear-side illumination of more than 20%(abs) (factor of 1.3) is measured when changing from a substrate with L-bulk approximate to 3.0 to 10.0 x W. The performed simulations and experiments demonstrate that the BOSCO cell concept is a promising candidate to successfully exploit bifacial gain also on low- to medium-diffusion-length substrates such as p-type multicrystalline silicon, while PERT cells require a high-diffusion- length substrate to utilise their bifacial potential. Furthermore, the BOSCO cell concept is shown to be a promising option to achieve highest output power densities, even when using lower quality and therefore possibly more cost-effective silicon substrates.

The most straightforward way to map the photo-induced short circuit current density ( J sc ) in solar cells is light beam-induced current (LBIC) mapping. Recently several methods for J sc imaging based on camera-based photoluminescence and illuminated lock-in thermography imaging were proposed. This letter reports an alternative method for J sc imaging, which is solely based on the evaluation of dark lock-in thermography images. This method is particularly advantageous to improve the accuracy of dark lock-in thermography based local efficiency analysis of solar cells.

During cooling of Czochralski grown silicon ingots, thermal donors can be formed. These oxygen clusters influence the measurement of the base resistivity in the as-cut state. Thermal donors distort the calculation of the emitter sheet resistance during inline control because two measurements-one before and one after emitter diffusion-are needed. In this paper, we quantify the negative influence of the thermal donors on the base resistivity measurements, as well as on the calculation of the emitter sheet resistance, and present a new photoluminescence-based method for the determination of the dopant-related base resistivity, despite the presence of thermal donors in the as-cut state of Czochralski grown silicon wafers. With the photoluminescence base resistivity method, the dopant-related base resistivity and, therefore, the emitter sheet resistivity can be calculated with significantly higher accuracy than standard base resistivity measurements.

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.

Within the silicon photovoltaics (PV) community, there are many approaches, tools, and input parameters for simulating solar cells, making it difficult for newcomers to establish a complete and representative starting point and imposing high requirements on experts to tediously state all assumptions and inputs for replication. In this review, we address these problems by providing complete and representative input parameter sets to simulate six major types of crystalline silicon solar cells. Where possible, the inputs are justified and up-to-date for the respective cell types, and they produce representative measurable cell characteristics. Details of the modeling approaches that can replicate the simulations are presented as well. The input parameters listed here provide a sensible and consistent reference point for researchers on which to base their refinements and extensions.

In this paper, we present a new, updated version of the commonly used semiconductor device simulator PC1D named PC1Dmod 6.1. The new program is based on the previously published command line version cmd-PC1D 6.0, which has implemented several new options related to the device physics, but now uses an updated version of the original PC1D graphical user interface. The program thus provides the possibility for using Fermi–Dirac statistics and a selection of state-of-the-art models for crystalline silicon, including injection-dependent band gap narrowing, carrier mobility and Auger recombination, in a familiar setting. Version 6.1 also has implemented the recently published band gap narrowing model by Yan and Cuevas, which is based on empirical studies of a large selection of both n + and p + emitters, in addition to Schenk's model. It has also implemented the mobility model for compensated material by Schindler et al. Finally, the maximum number of nodes, time steps and wavelengths have been increased in order to reduce unnecessary constraints on simulations and external files. The results from the PC1Dmod 6.1 simulations have been compared with those of other simulation tools and with previously published data to verify the correct implementation of the new models. Emitter saturation currents calculated using PC1Dmod 6.1 showed an excellent agreement with those obtained using the emitter recombination calculator EDNA 2, and the new program was able to successfully reproduce previously published experimental data and previous implementations of the models. Both PC1Dmod 6.1 and the command line version cmd-PC1D 6.1 are open source software, and are freely available for download.

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).