In this work, we applied internal quantum efficiency mapping to study the recombination activity of grain boundaries in High Performance Multicrystalline Silicon under different processing conditions. Wafers were divided into groups and underwent different thermal processing, consisting of phosphorus diffusion gettering and surface passivation with hydrogen rich layers. After these thermal treatments, wafers were processed into heterojunction with intrinsic thin layer solar cells. Light Beam Induced Current and Electron Backscatter Diffraction were applied to analyse the influence of thermal treatment during standard solar cell processing on different types of grain boundaries. The results show that after cell processing, most random-angle grain boundaries in the material are well passivated, but small-angle grain boundaries are not well passivated. Special cases of coincidence site lattice grain boundaries with high recombination activity are also found. Based on micro-X-ray fluorescence measurements, a change in the contamination level is suggested as the reason behind their increased activity.

Wafers from a hybrid silicon ingot seeded in part for High Performance Multicrystalline, in part for a quasi-mono structure, are studied in terms of the effect of gettering and hydrogenation on their final Internal Quantum Efficiency.The wafers are thermally processed in different groups – gettered and hydrogenated. Afterwards, a low temperature heterojunction with intrinsic thin layer cell process is applied to minimize the impact of temperature. Such procedure made it possible to study the effect of different processing steps on dislocation clusters in the material using the Light Beam Induced Current technique with a high spatial resolution. The dislocation densities are measuredusing automatic image recognition on polished and etched samples. The dislocation recombination strengths are obtained by a correlation of the IQE with the dislocation density according to the Donolato model. Different clusters are compared after different process steps. The results show that for the middle of the ingot, the gettering step can increase the recombination strength of dislocations by one order of magnitude. A subsequent passivation with layers containing hydrogen can lead to a decrease in the recombination strength to levels lower than in ungettered samples.

The performance of centimeter-sized energy devices is regulated by inhomogeneously distributednanoscale defects. To improve device efficiency and reduce cost, accurate characterization of thesenanoscale defects is necessary. However, the multiscale nature of this problem presentsa characterization challenge, as non-destructive techniques often specialize in a single decade of lengthscales, and have difficulty probing non-destructively beneath the surface of materials with sub-micronspatial resolution. Herein, we push the resolution limits of synchrotron-based nanoprobe X-rayfluorescence mapping to 80 nm, to investigate a recombination-active intragranular defect in industrialsolar cells. Our nano-XRF measurements distinguish fundamental differences between benign anddeleterious dislocations in solar cell devices: we observe recombination-active dislocations to containa high degree of nanoscale iron and copper decoration, while recombination-inactive dislocationsappear clean. Statistically meaningful high-resolution measurements establish a connection betweencommercially relevant materials and previous fundamental studies on intentionally contaminatedmodel defect structures, pointing the way towards optimization of the industrial solar cell process.Moreover, this study presents a hierarchical characterization approach that can be broadly extended toother nanodefect-limited energy systems with the advent of high-resolution X-ray imaging beamlines

We push the resolution limits of synchrotron-based nano-X-ray fluorescence mapping below 100 nm to investigate the fundamental differences between benign and deleterious dislocations in multicystalline silicon solar cells. We observe that after processing recombination-active dislocations contain a high degree of nanoscale iron and copper decoration, while recombination-inactive dislocations appear clean. To study the origins of the distinct metal decorations around different dislocations we analyze as-grown samples as well as specimens at different stages of processing. We complement our X-ray studies with micro-Raman mapping to understand the relationship between metallic decoration and stress fields around dislocations.

Multicrystalline silicon is the most used materialfor the production of silicon solar cells. The quality of the asgrown material depends on the quality of the feedstock andthe crystallization process. Bulk impurities, crystal defectslike dislocations and of course the grain boundaries determinethe material quality and thus the solar cell conversionefficiency. Therefore minority carrier lifetime measurementsare often done to characterize the material quality. Butthe measured values are from limited use because it is knownthat the solar cell process itself can dramatically change theminority carrier lifetime and the solar cell efficiency. In orderto obtain more detailed information of the behaviour ofdifferent defect types additionally high-resolution LBIC(light beam induced current)-measurements have been done.Since LBIC needs a pn-junction for photocurrent generationthe LBIC technique has been combined with the a-Si/c-Siheterojunction cell process, which makes it possible tomanufacture solar cells even from as cut wafers withoutchanging the material quality. With this combination ofmeasurement and preparation techniques it was possible toanalyze the influence of the diffusion process and the firingprocess on the behaviour of the three different defect types: grain boundaries, dislocation networks and bulk impurities.

Dislocations limit solar cell performance by decreasing minority carrier diffusion length, leading to inefficient charge collection at the device contacts [1]. However, studies have shown that the recombination strength of dislocation clusters within millimeters away from each other can vary by orders of magnitude [2]. In this contribution, we present correlations between dislocation microstructure and recombination activity levels which span close to two orders of magnitude. We discuss a general trend observed: higher dislocation recombination activity appears to be correlated with a higher degree of impurity decoration, and a higher degree of disorder in the spatial distribution of etch pits. We present an approach to quantify the degree of disorder of dislocation clusters. Based on our observations, we hypothesize that the recombination activity of different dislocation clusters can be predicted by visual inspection of the etch pit distribution and geometry.

Dislocation clusters in multicrystalline silicon limit solar cell performance by decreasing minoritycarrier diffusion length. Studies have shown that the recombination strength of dislocation clusterscan vary by up to two orders of magnitude, even within the same wafer. In this contribution, wecombine a surface-analysis approach with bulk characterization techniques to explore theunderlying root cause of variations in recombination strength among different clusters. We observethat dislocation clusters with higher recombination strength consist of dislocations with a largervariation of line vector, correlated with a higher degree of variation in dislocation etch-pit shapes(ellipticities). Conversely, dislocation clusters exhibiting the lowest recombination strength containmostly dislocations with identical line vectors, resulting in very similar etch-pit shapes. Thedisorder of dislocation line vector in high-recombination clusters appears to be correlated withimpurity decoration, possibly the cause of the enhanced recombination activity. Based on ourobservations, we conclude that the relative recombination activity of different dislocation clustersin the device may be predicted via an optical inspection of the distribution and shape variation ofdislocation etch pits in the as-grown wafer.

In this work, we investigated the Si pre-amorphization implantation (PAI) assisted low temperatureannealing process to activate boron implantation in n-Si in a hydride vapor phase epitaxy (HVPE) reactor, which canbe used for the Si subcell fabrication in the III-V/Si tandem solar cells enabled by the corrugated epitaxial lateralovergrowth (CELOG). A uniform boron activation in Si and a low emitter sheet resistance of 77 /sq was obtained atannealing temperatures of 600-700°C. High-resolution x-ray diffraction was used to study the recrystallization ofamorphous silicon and the incorporation of boron dopants in Si. Hall measurements revealed p-type carrierconcentrations in the order of 1020 cm-3. The n-Si wafers with B implantation activated at 700°C by HVPE wereprocessed to solar cells and characterized by the standard light-current-voltage measurement under AM1.5 spectrumand external quantum efficiency measurements. The developed B implantation and low temperature activationprocesses are applied to the InP/Si seed template preparation for CELOG, on which CELOG GaInP over a Si subcellwith a direct heterojunction was demonstrated.

The evolution during silicon solar cell processing of performance-limiting iron impurities isinvestigated with synchrotron-based x-ray fluorescence microscopy. We find that during industrialphosphorus diffusion, bulk precipitate dissolution is incomplete in wafers with high metal content,specifically ingot border material. Postdiffusion low-temperature annealing is not found to alterappreciably the size or spatial distribution of FeSi2precipitates, although cell efficiency improvesdue to a decrease in iron interstitial concentration. Gettering simulations successfully modelexperiment results and suggest the efficacy of high- and low-temperature processing to reduce bothprecipitated and interstitial iron concentrations, respectively.

The diffusion length of minority charge carriers in the silicon bulk Ldiffis an important characteristicof optoelectronic devices fabricated from low cost silicon wafers. In this study computer simulationshave been carried out to calculate the beneficial effects of a macroscopic surface texturization on thecharge carrier generation and the collection probability. Textured solar cells should be able tocollect charge carriers more effectively resulting in an increased current due to the special emittergeometry resulting from the texture, decreased reflection losses, and the inclined penetration of thelight. In order to prove this expected behavior, deeply V-textured solar cells have been processedand characterized on low cost silicon reaching an Ldiffof about 25 mm. Spatially resolved highresolution measurements of the internal quantum efficiency exhibit a strongly increased signal in thetexture tips which is the first experimental proof of the increased charge carrier collectionprobability of deeply textured solar cells. This effect can further be seen in cross sectional electronbeam induced current measurements and the mechanical texture results in an overall gain in shortcircuit current density of about 11% and in efficiency of about 8% relatively.

A scheme for passivating thin multi-crystalline silicon solar cells compatible to massproduction is presented. Wafers with a thickness of 180 mm were processed into solarcells. The otherwise severe bowing has been avoided by reduced aluminium coverageon the rear surface. The process scheme includes a silicon nitride firing through stepfor conventional screen printed contacts, where a silicon nitride layer on the rearsurface acts as surface passivation layer and enables a gain in efficiency of 0.6%[abs.].

Sponge-LID decreases the Al-BSF cell efficiency by up to 10 %rel. and is only partially recoverable at 200°C. This contributionshows that Sponge-LID occurs at and near most grain boundaries, but only in the centre of the affected cell.  Furthermore,Sponge-LID is not the only type of LID in the silicon bulk. High-resolution Light Beam Induced Current mapping reveals localinternal quantum efficiency losses of up to 8 %rel. at dislocation clusters and small angle grain boundaries, which recover(nearly) fully at 200°C. Nevertheless, this dislocation-related LID appears to reduce the Al-BSF efficiency by less than 1 %rel.

In this study, we have utilized characterization methods to identify the nature of metal impurityprecipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we haveutilized synchrotron-based x-ray fluorescence and x-ray absorption spectromicroscopy to study theelemental and chemical nature of these impurity precipitates, respectively. We have detectednanometer-scale precipitates of Fe, Cr, Ni, Cu, and Au in multicrystalline silicon materials from avariety of solar cell manufacturers. Additionally, we have obtained a direct correlation between theimpurity precipitates and regions of low light-induced current, providing direct proof that metalimpurities play a significant role in the performance of multicrystalline silicon solar cells.Furthermore, we have identified the chemical state of iron precipitates in the low-performanceregions. These results indicate that the iron precipitates are in the form of oxide or silicatecompound. These compounds are highly stable and cannot be removed with standard siliconprocessing, indicating remediation efforts via impurity removal need to be improved. Futureimprovements to multicrystalline silicon solar cell performance can be best obtained by inhibitingoxygen and metal impurity introduction as well as modifying thermal treatments during crystalgrowth to avoid oxide or silicate formation.

The influence of phosphorus gettering is studied in thiswork applied to an acidic textured multicrystalline siliconsubstrate. The texturization was achieved with an HF/HNO3 solution leading to nanostructures on the silicon surface. It has been demonstrated in previous works that this textured surface decreases the reflectance on the solar cell and increases the surface area improving the photon collection and enhancing the short circuit current.

The present study investigates the effect on the minoritycarrier lifetime of the phosphorous diffusion when it is carried out on this textured surface. The lifetime is measured by means microwave photoconductance decay and quasi steady state phototoconductance devices. The diffused textured wafers are used to fabricate solar cells and their electrical parameters are analyzed.

Crystal growth processes of multicrystalline silicon and their potential for further development are reviewed. Important parameters for theassessment of the final efficiency of the solar cells and the production yield are the bulk lifetime and the mechanical stability. The distributionand morphology of lattice defects can be related to the electrical properties. In particular oxygen in multicrystalline ingots and carbon in EFGribbons and their interaction with extended defects such as dislocations and grain boundaries will be discussed. The fracture strength dependson microcracks that are introduced through wafer slicing. The current understanding will be reviewed here.

Crystal defects such as grain boundaries affect the overall performance of a solar cell. The light beam induced current method allows for the localized quantification of the impact on the internal quantum efficiency of such defects. This work presents a method to estimate the separate impact of grain boundaries on the internal quantum efficiency (IQE) of multicrystalline silicon solar cells by correlating LBIC topographs with optical images of etched samples. Segmenting the impact of the grain boundaries on the IQE against those of other defects in our samples showed that the grain boundaries remain the most detrimental. The average IQE at 826 nm was reduced by up to 1.29 % (vs 0.25 % for other defects) absolute for standard multicrystalline and up to 1.15 % (vs 0.28 % for other defects) absolute for high performance multicrystalline silicon.

Herein, a method to quantify the amount of reduction in the internal quantumefficiency (IQE) of multicrystalline silicon solar cells that can be attributed to grainboundaries is presented. By correlating the IQE maps obtained via light beaminduced current (LBIC) topography with optical images of Secco-etched samples,the distribution of IQE values at the positions of grain boundaries can be comparedwith the distribution of IQE values for all other positions where other defects mayexist. The segmentation of IQE at 826 nm maps of the samples shows grainboundaries to be more detrimental compared with the combined effects of all otherdefects that may exist in other regions of the cell. The grain boundaries reduce theaverage IQE by up to 4.07% absolute for the cell from the bottom of the ingot.

Research on the structural defects of silicon such as grain boundaries and dislocations, their spatial distributionand how they impact the resulting solar cell  performance often proceed by polishing the sample, etching toreveal the dislocations and grain boundaries, and then scanning the surface to image the defects and recordtheir corresponding positions. While a lot of work has been devoted to developing appropriate etches and howto correlate the etch pits to cell performance, materials pertaining to preparation of samples for defect etching, which is a crucial step to ensure successful imaging and analysis, are limited. This work describes a methodof polishing multicrystalline silicon solar cell samples in preparation for defect etching. The method described herein:

- Utilizes both mechanical and chemical mechanical polishing.

- Can be applied to both fabricated silicon solar cells and as-cut wafers.

Having a photovoltaic (PV) system raises the question of whether it runs as expected.Measuring its energy yield takes a long time and the result still contains uncertainties from varyingweather conditions and possible shading of the modules. Here, a free software PVcheck to measurethe peak power of the system is announced, using the power data of a single sunny day. The softwareloads a data file of the generated power as a function of time from this day. This data file is providedby typical inverters. The software then simulates this power curve using known parameters like angleand location of the PV system. The assumed peak power of the simulation can then be adjusted sothat the simulated curve matches the measured one. The software runs under Microsoft Windows™and makes use of the free library pvlib python. The simulation can be refined by importing weatherdata like temperature, wind speed, and insolation. Furthermore, curves describing the nominalmodule efficiency as a function of the illumination intensity as well as the power-dependent inverterefficiency can be included in the simulation. First results reveal a good agreement of the simulationwith experimental data. The software can be used to detect strong problems in PV systems afterinstallation and to monitor their long-time operation.

Die Dissertation demonstriert ein Verfahren zur Messung der normalisierten Rekombinationsstärke (Gamma) von Versetzungen in blockerstarrten Siliziumsolarzellen. Es basiert auf der topografischen Messung der internen Quanteneffizienz IQE und der Versetzungsdichte. Unter Verwendung des theoretischen Modells von Donolato und Simulationen mit dem Programm PC1D werden für einzelne Versetzungscluster aus dem gemessenen Zusammenhang zwischen der IQE und der Versetzungsdichte die Gamma-Werte bestimmt. Zur Messung der Versetzungsdichte wurden neue Methoden der automatischen Bildanalyse zur Erkennung von angeätzten Kristalldefekten mit dem Computer entwickelt. Die in den Solarzellen gemessenen Gamma-Werte variieren über ca. eine Größenordnung innerhalb weniger Millimeter. Regelmäßig angeordnete Versetzungen haben kleinere Gamma-Werte gegenüber ungeordneteren Verteilungen. Der Gamma-Wert korreliert signifikant mit den Prozesstemperaturen bei der Solarzellherstellung. Es wird gezeigt, in welcher Weise eine Remote-Plasma-Wasserstoffpassivierung die lokale IQE und die Gamma-Werte beeinflusst.

After installation of a PV system, the question arises if it works correctly. Comparing the yield with some prediction gives a hint if the system is OK, but takes a long time and depends on the weather during that period. Also the effect of shading by trees or buildings is hard to estimate and often not included in the prediction. We present a solution based on the evaluation of the power curve of the PV system from a sunny day. The peak power of the PV system is obtained by simulating this power curve using the peak power as a fit parameter. The simulation uses the public library pvlib python including the clear sky model of Ineichen and Perez, and a database for the air turbidity. The temperature of the PV modules is calculated from air temperature and wind speed. The software with its graphical user interface for Microsoft Windows is freely available. The simulation of irradiance and module temperature is compared to experimental data. As an example, the peak power of some newly installed PV systems was evaluated. Furthermore, a degradation analysis of the peak power over a 10-year period of a test system is performed.

A calibrated measurement of the short circuit current and the surface reflection coefficient can be directly converted into the internal quantum efficiency(IQE) of a solar cell. The IQE at a wavelength of 833 nm were measured on ingot, EFG and RGS silicon solar cells with a spatial resolution of 6 μm. Ingot solar cells were found to be predominantly influenced by a homogeneous distribution of recombination centers. However, if the dislocation densities exceeded a certain limit the IQE was reduced by recombination at dislocations. This limit varied in different parts of the wafer. EFG solar cells only showed a lifetime reduction by dislocations whereas the investigated solar cells made of RGS silicon were dominated by recombination at grain boundaries. The RGS silicon was further investigated by TEM- measurements, which showed that the extended defects were highly decorated with SiO2- and SiC precipitates.

An improved technique is presented to measure the normalized recombination strength Gat dislocations in silicon solar cells that were fabricated of cast grown silicon. G is the number ofrecombinations per unit time, length, and excess carrier density divided by the minority carrierdiffusion coefficient D. The measurement is based on fitting the theoretical correlation betweeninternal quantum efficiency IQE at a single wavelength and dislocation density r to the measureddata. The IQE is measured topographically by the light beam induced current (LBIC) method. Foreach point of the LBIC map a dislocation density is determined by analysing the etched samplesurface with an image recognition programme. The theory for IQE(r) combines Donolato'sprediction for L(r) with a calculation of IQE(L) made by the computer programme PC1D. L is thediffusion length of the minority carriers. The programme PC1D takes special properties of the solarcell process into account. The method was applied to solar cells made by a conventional furnaceprocess as well as a rapid thermal process (RTP). In the latter case a correlation between G and theemitter diffusion temperature was found. Finally TEM measurements were made to investigatedislocations with different values of G.

Minority carrier recombination is studied in multicrystalline ingot cast silicon solar cells. The normalized recombination strength G of dislocations is obtained by correlating topogramsof the internal quantum efficiency (IQE) with those of the dislocation densityr.G is obtained by fitting an extended theory of Donolato to the experimental data. The measured G-values vary significantly between adjacent dislocation clusters and correlate with the spatial pattern of the dislocations. All G-values are strongly dependent on the parameters of the solar cell process. The influence of phosphorus diffusion and hydrogenation is shown. After solidification of the silicon, impurities from the crucible enter the ingot and deteriorate its border regions during cooling to room temperature. These deteriorated border regions can be significantly improved byan additional low temperature anneal that is applied after phosphorus diffusion. The experiments indicate that the mechanism of the anneal is external phosphorus gettering into the emitter.