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Laser diffusion from PECVD layers can result in the incorporation of impurities like nitrogen into crystalline silicon. It is shown that the nitrogen can have a significant influence on the laser doping process. The incorporated nitrogen can affect the material properties resulting in misinterpretation of measurement results and accumulation of nitrogen at the surface can lead to negative effects such as improper contact formation. An approach reducing the amount of nitrogen content in the PECVD layer and the LBSF is presented. This new approach allows for a partial decoupling of the passivation and doping properties of the passivation layers. The doping efficiency of the laser doping process was significantly improved while keeping the recombination properties low. The higher doping efficiency was found to be of major importance for a reproducible level of LBSF/metal contact resistivity on the rear side. Using the adapted process with reduced nitrogen content, it is shown that the doping concentration is high enough to be contacted by screen printed silver pastes. Solar cells using the new approach are presented reaching efficiencies up to 20.9% on a cell area of 149 cm 2 . The influence of the higher doping efficiency reflected into the new solar cells allowing fill factors of up to 80.1%.

Rear side hexagonal sphere gratings are demonstrated as diffractive structures that enhance the light path length in the near infrared, where crystalline silicon solar cells suffer from weak absorption. Moreover, the rear side sphere grating can be added behind a solar cell with flat rear surface, giving an “electrically flat but optically rough” device with high efficiency potential. Here, a thin passivating tunnel-contact layer electrically separates the sphere grating from the cell's base. Solar cells with the rear side sphere grating have obtained a VOC of up to 710 mV and a FF of up to 81.9%. External quantum efficiency measurements show a current density gain of 1.4 mA/cm2 due to the sphere grating. This leads to an overall efficiency of up to 22.1% for the solar cells with planar front side and rear side sphere grating. Estimates for perovskite–silicon and III/V-silicon tandem devices show that the efficiency of tandems can be enhanced by up to 2.4% absolute with a sphere grating on the rear side. Thus, sphere gratings could improve Si-based tandem devices that are limited due to a low current in the Si bottom cell.

The performance of PERC solar cells benefits from a smooth rear surface due to a reduced surface recombination and increased light trapping. State of the art surface roughness is defined solely by the polishing silicon removal. However the initial texture height can vary, therefore there is no universal definition of roughness. In this work roughness parameters from an earlier study will be used to determine the rear surface roughness of mono-crystalline industrial p-type solar cells [1]. The effects of rear side roughness on the passivation quality and thus minority carrier lifetime will be discussed. Additionally the influence of rear side roughness on reflection and therefore light trapping will be investigated. Furthermore solar cells with PERC concept are presented. Up to now used roughness parameters describe vertical structures and thus represent polishing results but they are not sensitive to horizontal structure expansion. Effective lifetime of polished samples first increases with decreasing roughness. For further polishing values stagnate or decrease. Implied VOC of saw damage etched surface is higher compared to polished surface although they show similar vertical roughness, hence there are more parameters influencing passivation quality than vertical roughness. According to roughness measurements nano-roughness, valleys and horizontal structure expansion seem to affect passivation quality in addition to vertical roughness. Decreasing roughness increases reflection at 1200 nm and thus increases light trapping. As the highest benefit in light trapping appears with decreasing structure height, vertical structure expansion seems to affect back reflection less than horizontal structures. Solar cell results cannot confirm the results of implied VOC and reflection measurements but prove the benefit of smooth rear sides. For polished polished rear sides the gain in efficiency is decreasing with increasing silicon removal.

Continuous wave laser processes used to create solar cells with selective emitter and plated Ni–Cu front contacts are a widely discussed topic. Particularly low adhesion of the front metal contact has been identified as a serious challenge. In this work a detailed surface characterization of laser doped and patterned front sides of solar cells shows that formation of silicon oxynitride hinders nickel silicide formation and reduces contact adhesion of Ni–Cu plated contacts. In order to overcome the observed tradeoff between metal contact adhesion and penetration of the pn-junction, this paper presents a novel process sequence based on the formation of a deep selective emitter using a green continuous waver laser and subsequent patterning of the dielectric using a nanosecond-pulsed laser.

The selective transport of electrons and holes to the two terminals of a solar cell is often attributed to an electric field, although well-known physics states that they are driven by gradients of quasi-Fermi energies. However, in an illuminated semiconductor, these forces are not selective, and they drive both charge carriers toward both contacts. This paper shows that the necessary selectivity is achieved by differences in the conductivities of electrons and holes in two distinct regions of the device, which, for one charge carrier, allows transport to one contact and block transport to the other contact.

We have studied surface passivation layers for the application on n -type p+ nn+ bifacial silicon solar cells. Thereby, we have examined their optimal composition and thickness with regards to passivation quality, optical properties, and especially the contact formation during a co-firing step. These parameters were addressed in separate investigations: 1) simulation of the optical properties of a bifacial silicon solar cell, 2) measurement of the passivation quality on lifetime samples, 3) measurement of contact resistance (of aerosol printed fingers) to analyze the contact formation during the co-firing process, and 4) differential scanning calorimetry measurement were conducted to fundamentally understand reactions during contact formation in a fast firing furnace. The passivation layers tested were silicon nitride (SiNx), titanium oxide (TiO 2), and silicon oxide (SiO2) on lowly phosphorus-doped silicon n+-layers, whereas aluminum oxide (Al2O3) stacks, capped with SiNx and TiO2, were studied on lowly boron-doped silicon p+-layers. The results show that a dielectric stack, consisting of 10-nm-thick Al2O3 and 60-nm-thick SiNx layers on the boron-diffused silicon front side and a single 50-nm SiNx layer on the phosphorus-diffused silicon rear side, provides low emitter saturation current density (J0e), high optical absorption current density, and low contact resistance for printed and co-fired contacts.

Within this work we investigate the lifetime of soft PDMS stamps as a critical parameter for the up-scaled fabrication of microstructures via nanoimprint lithography. We propose the characterization of the PDMS hardness as a suitable parameter for the stamp degradation. The increasing hardness for a larger number of imprints is attributed to resist diffusion into the PDMS volume and possibly subsequent cross-linking. Force-distance measurements with an atomic force microscope were applied to demonstrate for two acrylic materials that a lower average molecular weight leads to a faster degradation. Using a temperature-assisted UV-imprint process with an epoxy material (SU-8 2002) enabled more than 100 imprints without measurable degradation due to the complex molecular structure which we assume to reduce the resist diffusion. This can be a key development towards a higher stamp lifetime and finally an industrial realization of NIL.

Well suited and reliable values of the optical and electrical properties of thin indium tin oxide (ITO) films are needed in order to choose the optimal deposition parameters and to perform reliable modeling for solar cells design. In this work, a new method will be presented to evaluate the ITO transparency directly on silicon substrates. The effects of each deposition parameter that influences the ITO transparency and conductivity force a trade-off in the frame of values useful for SHJ solar cells. The deposition of our optimized ITO film on a textured wafer yields a weighted average reflectance as low as 4.4±0.2%. The deposition of an MgF2/ITO double-layer anti-reflection coating (DL-ARC) on textured cells increases the efficiency from 17.9%, measured immediately after contacts have been added to the ITO, to 18.4% after the MgF2 deposition. An annealing step at 200 °C for 10 min proved to further increase the efficiency up to 18.9%, for a total gain of 1%.

We demonstrate diffractive rear side gratings to enhance the near infrared light trapping and thus the quantum efficiency of wafer based crystalline silicon solar cells. Binary crossed gratings with a period of 1 µm, produced via nanoimprint lithography and plasma etching, are electrically decoupled from the solar cell by a thin dielectric passivation layer, creating an electrically flat, but optically rough rear side. We fabricated solar cells with thicknesses of 250, 150 and 100 µm and demonstrate a short circuit current density gain due to the grating of 1.2, 1.6 and 1.8 mA/cm 2 for solar cells with planar front surface. For solar cells with pyramidally textured front surface the grating also leads to a small current density gain in the near infrared of approximately 0.3 mA/cm 2 according to EQE measurements, leading to the best cell's efficiency of 21.1%. By optical simulations we show the potential of the grating structure and identify losses in the fabricated solar cells.

We investigate the properties of silver thick film contacts on phosphorus doped alkaline textured silicon surfaces with varying surface doping levels. Using scanning electron microscopy, we find that the surface near doping concentration influences the formation of silver crystallites especially those that are in direct contact with both the silicon surface and the contact finger. The observed correlation between direct silver crystallite imprint coverage and specific contact resistance allows for extracting the specific contact resistance of individual crystallites to be approximately 3.3 µΩ cm 2 , which agrees well with theoretical predictions. Furthermore we present a new approach that allows for a quantitative separation of direct and indirect current conduction channels. For our experimental conditions, the assumption of current conduction exclusively by transport through silver crystallites adequately describes the experimental data. Finally we fabricate bifacial n-type solar cells with peak efficiencies of 19.9%.

We demonstrate a low surface recombination velocity of 14 cm/s with only 1.5 nm thin titanium dioxide (TiO2) layers on undiffused 10 Omega cm p-type crystalline silicon. The TiO2 nanolayers were deposited by thermal atomic layer deposition at 150 degrees C and 200 degrees C substrate temperatures using tetrakis-dimethyl-amido titanium as the Ti precursor and water as the oxidant. The influence of a post-deposition anneal in forming gas at different temperatures was investigated. We have observed that a subsequent anneal in forming gas at 350 degrees C enhances the surface passivation quality of the TiO2 layers tremendously. Increasing the thickness of the TiO2 layers leads to a reduction of the surface passivation quality. Introducing a thin interfacial layer of silicon oxide (1.6 nm) grown by rapid thermal oxidation underneath the TiO2 layer improves the surface passivation of thicker TiO2 layers (5.5 and 15 nm). These results show that ultrathin TiO2 layers with a thickness of only 1.5 nm can be used to effectively passivate the c-Si surface.

An approach for n -type passivated emitter and rear locally diffused (PERL) solar cells is presented that combines the PassDop technology with Ni- and Cu-plated front contacts. As a step toward an eased industrial implementation, a new PassDop layer-based on a- ${\text {SiN}}_{x}$ :P is presented and compared with the original layer based on a- ${\text {SiC}}_{x}$ :P. We show that the PassDop concept can be used as a rear-side approach for these solar cells that reach energy-conversion efficiencies up to 23.5% on a small-area (2 × 2 cm2) when using the layer based on a- ${\text {SiC}}_{x}$ :P, and 22.8% when using the layer based on a- ${\text {SiN}}_{x}$ :P. By applying the same technology on a larger scale, we achieved efficiencies that exceed 22% on 143.2 cm2. We show that Ni-plating on boron emitters allows for excellent contact properties. With laser contact opening in conjunction with Ni- and Cu-plating, similar results for photolithographic opening (TiPdAg seed layer and Ag-plating) can be reached. Combining these technologies, cells are presented using PassDop as the rear-side approach and Ni-plating to contact the boron emitter with first results to achieve up to 21% cell efficiency on 148.6 cm2.

The OPTOS formalism is a matrix-based approach to determine the optical properties of textured optical sheets. It is extended within this work to enable the modelling of systems with an arbitrary number of textured, plane-parallel interfaces. A matrix-based system description is derived that accounts for the optical reflection and transmission interaction between all textured interfaces. Using OPTOS, we calculate reflectance and absorptance of complete photovoltaic module stacks, which consist of encapsulated silicon solar cells featuring textures that operate in different optical regimes. As exemplary systems, solar cells with and without module encapsulation are shown to exhibit a considerable absorptance gain if the random pyramid front side texture is combined with a diffractive rear side grating. A variation of the sunlight’s angle of incidence reveals that the grating gain is almost not affected for incoming polar angles up to 60°. Considering as well the good agreement with alternative simulation techniques, OPTOS is demonstrated to be a versatile and efficient method for the optical analysis of photovoltaic modules.

A matrix-based approach efficiently simulates ray- and wave-optical phenomena together within multi-scale structured sheets, for increased efficiency in textured silicon solar cells.

Currently, in the photovoltaic industry, wet chemical etching technologies are used for saw damage removal and surface texturing. Alternative to wet chemical etching is plasma etching. However, as for example, the linear microwave plasma technique, developed by Roth&Rau, has not been implemented in the photovoltaic industry for etching, because of the very low etch rate (

Carrier-selective contacts are one of the key enabling technologies to approach very high conversion efficiencies close to the theoretical limit of silicon solar cells. The tunnel oxide passivated contact (TOPCon) approach is an alternative to classical heterojunction solar cells enabling efficiencies up to 24.4%. The tunnel oxide is a core element of this contact as it has to reduce the minority carrier recombination but simultaneously must not hamper the majority carrier flow. This paper focuses on ozone-based oxidation techniques, which can potentially be cost effective and industrially feasible methods for the realization of ultra-thin tunnel oxide layers as an alternative to the oxidation in nitric acid (HNO3) reference process. All investigated oxides were applied to the electron-selective contact (n-TOPCon) on planar and textured surfaces. It will be shown that both ozone based oxidation techniques (UV/O3 photo-oxidation and wet-chemical oxidation in ozonized DI-H2O) enable high implied open circuit voltage (iVOC) values exceeding 720 mV on planar and 710 mV on textured surfaces, respectively. Further oxide properties as stoichiometry and layer thickness were analyzed by means of X-ray photoelectron spectroscopy (XPS), spectral ellipsometry (SE) and transmission electron microscopy (TEM). In compliance with earlier results it was found that a minimum oxide layer thickness (approximately 1.3 nm) and a high amount of oxygen-rich sub oxide species are required to obtain a good surface passivation. Using such oxides, a wider range of temperatures can be used during the TOPCon annealing. Applying the ozone-based oxide layers to n-TOPCon solar cells resulted in a high VOC of up to 719 mV and a peak efficiency of 24.9%. Similar results were obtained with the HNO3 reference process (VOC=716 mV, i=24.8%).

In this paper, large-area honeycomb (HC) texturing of multi-crystalline Si substrates with a newly developed process chain based on roller nanoimprint lithography and inline-capable plasma etching is demonstrated. The realized HC-textures show optical properties superior to random pyramids on c-Si, while the surface passivation quality is comparable. First, results on 156 × 156 mm2 multi-crystalline Si solar cells with an aluminum back-surface field show an efficiency of i = 17.8% compared with i = 17.3% on isotextured reference samples. This efficiency gain of 0.5% results from an increase in Jsc of up to 1.3 mA/cm2, which underlines the high optical and electrical quality of the texture. On a smaller area (50 × 50 mm2), the gain in Jsc of HC-textured cells compared with isotextured references was even higher with up to 2.5 mA/cm2.

Laser-induced selective Si doping and simultaneous ablation of a dielectric passivation layer is a promising technology for the creation of efficient and cost-effective solar cells. In this paper, the electrical quality of emitters produced with a 532-nm continuous-wave laser will be discussed using elaborate analysis of quasi-steady-state photoconductance (QSSPC) measurements. It will be shown that these emitters cause good charge carrier shielding, which leads to emitter saturation current densities as low as 240 fA/cm2 for unpassivated surfaces. If an SiNx layer is present during laser doping, the emitter recombination increases by a factor of three. This detrimental effect is put down to the formation of microcavities within the recrystallized Si. A model of the ablation mechanism and cavity formation for long laser pulses is proposed, with the experimental data in this study serving as a limiting case for long irradiation lengths.

A novel atmospheric pressure dry texture process is investigated in order to create nanostructures at the c-Si surface. The texture process uses diluted molecular fluorine (F2) as the process gas. F2 is partially dissociated at an elevated temperature before it is delivered to the c-Si wafer. Thermal activation of fluorine occurs on Si wafer surface in a dissociative chemisorption process leading to the removal of Si in the form of volatile SiFx species. The etching process can be controlled to form nanostructures with different aspect ratios and surface reflection values. In this work, we dry textured multicrystalline (mc) Si wafers to reach weighted surface reflection ∼12% in the wavelength range of 250-1200 nm. Nanotextured mc Si wafers were used to prepare p-type Al-BSF solar cells. The fabricated nanostructured cells show a gain in short circuit current (Jsc) of ∼0.5 mA/cm2 and reached a conversion efficiency of 17.3%.

We report recent achievements in adapting industrially used solar cell processes on nanotextured surfaces. Nanostructures were etched into c-Si surfaces by dry exothermic plasma-less reaction of F species with Si in atmospheric pressure conditions and then modified using a short post-etching process. Nanotextured multicrystalline wafers are used to prepare Al-BSF solar cells using industrially feasible solar cell processing steps. In comparison to the reference acidic textured solar cells, the nanostructured cells showed gain in short circuit current (Jsc) of up to 0.8 mA/cm2 and absolute gain in conversion efficiency of up to 0.3%. The best nanotextured solar cell was independently certified to reach the conversion efficiency of 18.0%

Reducing wafer thickness while increasing power conversion efficiency is the most effective way to reduce cost per Watt of a silicon photovoltaic module. Within the European project 20 percent efficiency on less than 100-mu m-thick, industrially feasible crystalline silicon solar cells ("20pl mu s"), we study the whole process chain for thin wafers, from wafering to module integration and life-cycle analysis. We investigate three different solar cell fabrication routes, categorized according to the temperature of the junction formation process and the wafer doping type: p-type silicon high temperature, n-type silicon high temperature and n-type silicon low temperature. For each route, an efficiency of 19.5% or greater is achieved on wafers less than 100 mu m thick, with a maximum efficiency of 21.1% on an 80-mu m-thick wafer. The n-type high temperature route is then transferred to a pilot production line, and a median solar cell efficiency of 20.0% is demonstrated on 100-mu m-thick wafers.

The world's ever increasing demand for energy necessitates technologies that generate electricity from inexhaustible and easily accessible energy sources. Silicon photovoltaics is a technology that can harvest the energy of sunlight. Its great characteristics have fueled research and development activities in this exciting field for many years now. One of the most important activities in the solar cell community is the investigation of alternative fabrication and structuring technologies, ideally serving both of the two main goals: device optimization and reduction of fabrication costs. Inkjet technology is practically evaluated along the whole process chain. Research activities cover many processes, such as surface texturing, emitter formation, or metallization. Furthermore, the inkjet technology itself is manifold as well. It can be used to apply inks that serve as a functional structure, present in the final device, as mask for subsequent structuring steps, or even serve as a reactant source to activate chemical etch reactions. This article reviews investigations of inkjet-printing in the field of silicon photovoltaics. The focus is on the different inkjet processes for individual fabrication steps of a solar cell. A technological overview and suggestions about where future work will be focused on are also provided. The great variety of the investigated processes highlights the ability of the inkjet technology to find its way into many other areas of functional printing and printed electronics.

Dielectric double layers of thermal silicon dioxide-chemical vapour deposition (CVD) silicon nitride are found to produce excellent passivation of silicon surfaces by combining a chemical reduction of surface defect states, with a field effect reduction of carriers at the surface due to charge in the dielectrics. The charge present in such double-layers has previously been attributed to be characteristic of the interface between the two. However, experimental evidence shows this is indirect and inconclusive. This manuscript reports direct measurements that show the charge lies within 10 nm of the interface between passivating double layers of thermal silicon dioxide-plasma CVD silicon nitride. In addition, the passivation efficiency of oxide-nitride layers, deposited using optimised conditions, was found to be largely unaffected by extra charge subsequently added to the film. The passivation efficiency of textured surfaces or those produced using non-optimised deposition conditions is found to be highly dependent on the field effect component provided by extra deposited charge. Using such extra field effect component, surface recombination velocities

Almost every monocrystalline silicon solar cell design includes a wet chemical process step for the alkaline texturing of the wafer surface in order to reduce the reflection of the front side. The alkaline texturing solution contains hydroxide, an organic additive usually 2-propanol and as a reaction product silicate. The hydroxide is consumed due to the reaction whereas 2-propanol evaporates during the process. Therefore, the correct replenishment for both components is required in order to achieve constant processing conditions. This may be simplified by using analytical methods for controlling the main components of the alkaline bath. This study gives an overview for a successful analytical method of the main components of an alkaline texturing bath by titration, HPLC, surface tension and NIR spectrometry.

In this paper, the properties of hydrogenated aluminum nitride layers (AlN:H) as an excellent passivation layer of silicon surfaces are examined. The structural and chemical properties of the AlN:H bulk are analyzed using a great variety of measurements techniques such as GI-XRD, FTIR, SEM, HR-TEM, corona charging and a thickness dependent measurements of the surface passivation. A model for the formation of negative fixed charges at the interface to silicon is presented based on the charge state of ionized point defects in AlN. Additionally, stacks of AlN:H and SiNx:H are introduced into silicon photovoltaics and their combination with two different types of thin low temperature silicon oxide layers is studied. Excellent passivation results are presented for highly doped p-type silicon and highly doped n-type silicon using the previous described stacks. Emitter saturation current densities of textured samples show, that 75 Ω/sq. phosphorous emitter can be passivated as efficient as with the typically used SiNx:H allowing maximum open circuit voltages of 657 mV. This is supported by measurement of cell parameters of p-type solar cells using these stacks as a combined anti-reflective coating and passivation layer. Furthermore, the passivation level reached for high efficiency 70 Ω/sq. boron emitters is nearly as high as with excellent passivating PECVD Al2O3/SiNx stacks. Emitter saturation current densities down to 61 fA/cm2 are presented corresponding to a maximum open circuit voltage of 696 mV. This indicates that the invented stacks containing AlN:H can also be applied as combined anti-reflective coating and passivation layers in n-type cells.

We demonstrate an innovative process to fabricate uniformly shaped plasmonic nanoparticles. Laser interference lithography, nano-imprint lithography and a lift-off process are employed for the controlled production of periodically arranged nanoparticles on large areas. Round and elliptic silver particles with diameters of about 200 nm on an area of 5×5cm2 are investigated. Measurements of resonant absorption by the metal particles are in agreement with data computer-simulated by rigorous coupled wave analysis. We observe that the plasmonic resonance of elliptic particles depends on the polarization of incident light and that porosity of the metal influences the plasmonic band.

It is investigated how potential drop sheet resistance measurements of areas formed by laser-assisted doping in crystalline Si wafers are affected by typically occurring experimental factors like sample size, inhomogeneities, surface roughness, or coatings. Measurements are obtained with a collinear four point probe setup and a modified transfer length measurement setup to measure sheet resistances of laser-doped lines. Inhomogeneities in doping depth are observed from scanning electron microscope images and electron beam induced current measurements. It is observed that influences from sample size, inhomogeneities, surface roughness, and coatings can be neglected if certain preconditions are met. Guidelines are given on how to obtain accurate potential drop sheet resistance measurements on laser-doped regions.

Rear side passivation and local back surface field formation are two of the main technological challenges for n-type PERL silicon solar cells. A promising approach is the PassDop process. This process combines a phosphorous doped passivation layer deposited on the rear side with a subsequent laser process to create both a local contact opening as well as a local back surface field. In this paper we introduce a new layer system based on doped amorphous silicon nitride (the fPassDop process) which is able to passivate the n-type surface after a firing step as typically used for screen printed contacts. After the firing step, an effective recombination velocity

The nanostructured surface – also called black silicon (b-Si) – is a promising texture for solar cells because of its extremely low reflectance combined with low surface recombination obtained with atomic layer deposited (ALD) thin films. However, the challenges in keeping the excellent optical properties and passivation in further processing have not been addressed before. Here we study especially the applicability of the ALD passivation on highly boron doped emitters that is present in crystalline silicon solar cells. The results show that the nanostructured boron emitters can be passivated efficiently using ALD Al2O3 reaching emitter saturation current densities as low as 51 fA/cm2. Furthermore, reflectance values less than 0.5% after processing show that the different process steps are not detrimental for the low reflectance of b-Si.

Silicon solar cells typically feature textured surfaces on the front side to increase light absorption. An unwanted side effect of the texture is an increase in surface recombination compared with smoother surfaces. On the rear side of the solar cell, light absorption is not an issue; therefore, planar surfaces are used to decrease surface recombination. In processing, a planar surface can be achieved by wet chemical single-side etching of previously textured surfaces, resulting in a smoothed rear surface. This study investigates surface passivation of these chemically polished surfaces in dependence on the degree of smoothness. Surface passivation is achieved by thin thermally grown silicon oxides. Special focus is set on injection dependence and the influence of postmetallization annealing. Measured optical properties of different surfaces are compared with different optical simulation models. Finally, recombination and optical properties are connected to solar cell performance of fabricated passivated emitter and rear cells.

In this study, the feasibility of creating one dielectric layer system acting simultaneously as antireflection coating, phosphorous doping source, masking against metal plating, and surface passivation is presented. Moreover, a similar layer stack is described, which behaves as rear-side surface passivation, boron dopant source, and internal reflection mirror. The optical characteristics of these layers are especially investigated and optimized for a solar cell's front- and rear-side coating. By consequent use of the multifunctional layers, a totally diffused solar cell with rear-side passivation and local rear contacts can be produced using only one wet chemical bath sequence, one multilayer vacuum step, and one high-temperature process. We present the first proof of concept for such a solar cell production using multifunctional plasma-enhanced chemical vapor deposition layers resulting already in a conversion efficiency of 18.3%.

This work investigates a double layer stack system that can be used for surface passivation of crystalline silicon. The stack consists of amorphous silicon-rich silicon oxynitride and amorphous silicon nitride on top. Both layers are fabricated by means of plasma-enhanced chemical vapour deposition. We investigate the stack in terms of changes in the hydrogen content and distribution within the different stack layers due to a high temperature treatment. For that purpose the stack is studied by Fourier-transformed infrared spectroscopy and nuclear reaction analysis before and after fast firing at 850 degrees C. Our results determine the bottom silicon oxynitride layer as very hydrogen-rich. Furthermore, we identify the silicon nitride capping layer as diffusion barrier to atomic hydrogen but still allowing an effusion of molecular hydrogen. We present a qualitative model that explains our findings and distinguishes between atomic and molecular hydrogen.

In this study, back-contacted back-junction n-type silicon solar cells featuring a large emitter coverage (point-like base contacts), a small emitter coverage (point-like base and emitter contacts), and interdigitated metal fingers have been fabricated and analyzed. For both solar cell designs, a significant reduction of electrical shading losses caused by an increased recombination in the non-collecting base area on the rear side was obtained. Because the solar cell designs are characterized by an overlap of the B-doped emitter and the P-doped base with metal fingers of the other polarity, insulating thin films with excellent electrical insulation properties are required to prevent shunting in these overlapping regions. Thus, with insulating thin films, the geometry of the minority charge carrier collecting emitter diffusion and the geometry of the interdigitated metal fingers can be decoupled. In this regard, plasma-enhanced chemical vapor deposited SiO 2 insulating thin films with various thicknesses and deposited at different temperatures have been investigated in more detail by metal-insulator-semiconductor structures. Furthermore, the influence of different metal layers on the insulation properties of the films has been analyzed. It has been found that by applying a SiO 2 insulating thin film with a thickness of more than 1000nm and deposited at 350°C to solar cells fabricated on 1cm and 10cm n-type float-zone grown silicon substrates, electrical shading losses could be reduced considerably, resulting in excellent short-circuit current densities of more than 41mA/cm 2 and conversion efficiencies of up to 23.0%.

An analysis of the nucleation mechanism of pyramids formed in (100) silicon in Na2CO3/NaHCO3 solution has been carried out. This texturization process of silicon by means of Na2CO3/NaHCO3 solutions is of special interest because it can be applied to the silicon solar cell industry to texture solar cell surfaces to decrease the front reflection and enhance light trapping in the cells. For this purpose, two microscopy techniques-scanning electron microscopy and atomic force microscopy-have been used to study the different stages of pyramidal nucleation and formation. The different aspects and factors involved in the texturization process require different analysis conditions and microscopy resolution. Tracing the transformation of determined surface areas and structures has been achieved, contributing clarification of the mechanism of pyramid nucleation in Na2CO3/NaHCO3 solutions.

Thin layers of Al2O3 are known to feature excellent passivation properties on highly boron-doped silicon surfaces. In this paper, we present a detailed study of the passivation quality of Al2O3 single layers and stacks of Al2O3 and antireflection SiNx on boron-doped emitters, where the Al2O3 was deposited by plasma-assisted atomic layer deposition and the SiNx by plasma-enhanced chemical vapor deposition. The passivation quality was studied for different atomic layer deposition temperatures, as a function of the Al2O3 layer thickness, as well as on samples with planar and random pyramids textured surfaces. These investigations were performed for different boron emitter diffusions, such as shallow, industrial emitters with high surface concentrations, as well as driven-in emitters with low surface concentrations. For all these variations, we compared systematically different thermal post-deposition treatments to activate the Al2O3 passivation, i.e., annealing processes at moderate temperatures and short high-temperature processes, as required for firing printed metal contacts. Therefore, symmetrically processed p+np + samples were fabricated, which were characterized with the photoconductance decay technique to determine emitter saturation current densities. Finally, the longtime stability of the Al2O3/SiNx stacks with planar and textured surfaces was investigated with an accelerated ultraviolet (UV) exposure experiment, miming about 34 month of outdoor performance.

Stacks of aluminum oxide and silicon nitride are frequently used in silicon photovoltaics. In this Letter, we demonstrate that hydrogenated aluminum nitride can be an alternative to this dual-layer stack. Deposited on 1 ohm cm p-type FZ silicon, very low effective surface recombination velocities of 8 cm/s could be reached after firing at 820 °C. This excellent passivation is traced back to a high density of fixed charges at the interface of approximately -1 × 1012 cm-2 and a very low interface defect density below 5 × 1010 eV-1 cm-2. Furthermore, spectral ellipsometry measurements reveal that these aluminum nitride layers have ideal optical properties for use as anti-reflective coatings.

In this work, we present a novel fire-through contact (FTC) approach for the formation of local rear contacts for rear surface passivated silicon solar cells. The FTC approach aims at an easy integration of rear surface passivation into typical production lines. Besides rear surface passivation, no new technology is required as FTC only relies on common printing equipment. In this work, different FTC contact geometries and printing approaches are investigated using lifetime and resistance measurements. A PC1D model is implemented to allow for a comparison of different FTC configurations on cell level. Finally, the integration of the FTC approach into metal wrap through solar cells is presented.

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.

In this paper, we present guidelines for the design of backside gratings for crystalline silicon solar cells. We use a specially developed method based on a combination of rigorous 3D wave optical simulations and detailed semiconductor device modeling. We also present experimental results of fabricated structures. Simulation-based optimizations of grating period and depth d of a binary grating and calculations of the optical and electrical characteristics of solar cells with optimized gratings are shown. The investigated solar cell setup features a thickness of dbulk=40m and a flat front surface. For this setup, we show a maximum increase in short-circuit current density of jSC=1.8mA/cm2 corresponding to an efficiency enhancement of 1% absolute. Furthermore, we investigate different loss mechanisms: (i) an increased rear surface recombination velocity S0,b because of an altered surface caused by the introduction of the grating and (ii) absorption in the aluminum backsi de reflector. We analyze the trade-off point between gain due to improved optical properties and loss due to corrupted electrical properties. We find that, increasing the efficiency by 1% absolute due to improved light trapping, the maximum tolerable recombination velocity is S0,b(max)=5.2×103cm/s. From simulations and measurements, we conclude that structuring of the aluminum backside reflector should be avoided because of parasitic absorption. Adding a dielectric buffer layer between silicon and the structured aluminum, absorption losses can be tuned. We find that for a planar reflector, the thickness of a SiO2 buffer layer should exceed dSiO2=120nm.

In this paper we address the fundamental challenge of forming an efficient contact between the p-type a-Si:H layer and n-type TCO's as contact layers in amorphous/crystalline silicon heterojunction (SHJ) solar cells. We point out the capability of Suns- V oc measurements to give valuable insights into the formation of Schottky barriers and its influence on the solar cell fill factor FF. The influence of the a-Si:H(p) doping on the Schottky characteristic is shown for test structures and on device level. Test structures are used to probe the influence of various contact layers on the effective work function at the a-Si:H/contact layer interface. A very good correlation between the vacuum work function of different contact layers and the open-circuit voltage is observed for test structures. Therefore, we could demonstrate the work function mismatch between a-Si:H and ITO and a-Si:H and various metals as contact layers. For small area n-type silicon solar cells featuring an a-Si:H(p) rear emitter and a diffused front surface field (FSF), it is shown that by improving the carrier transport between the a-Si:H(p) and the contacting layer, ITO(n) or metal, FF above 80% can be obtained. Furthermore, we demonstrate that a TCO is not mandatory for the rear SHJ emitter, which simplifies the cell structure and allows for proper junction engineering. We obtained high internal rear side reflection with a single metal layer and an efficiency of 22.8% for these TCO-less SHJ emitter solar cells. As these solar cells feature FF of up to 81.5%, they clearly demonstrate the high FF potential of the silicon heterojunction which can be achieved by proper junction engineering.

Random pyramid texturization of Si(1 0 0) substrates in alkaline solutions with addition of surface active compounds is considered. Technological difficulties connected with the application of alcohol additives are analyzed and a new solution for the wet-chemical texturization is proposed. The commonly applied alcohols in the texturization process, such as isopropanol and tertbutanol have been replaced in this work by diols (the alcohols with multiple hydroxyl groups). The results of texturization in KOH solution with 1,2-pentanediol are presented. Optimization of the solution composition and process conditions is performed. Measurements of the coefficient of light reflection carried out in the spectral range from 300 nm to 1200 nm, for the textured substrates with and without an additional anti-reflection layer are presented. The reflection coefficient for the substrates obtained in the optimal etching conditions (1 M KOH + 2% 1,2-pentanediol, 90 °C, 20 min) is close t o 10%, which is comparable with the coefficient of light reflection obtained in KOH solutions with isopropyl alcohol addition. Unquestioned advantages of the solutions with diol addition (higher process temperature, shorter etching time, technological improvements) show that they can successfully replace the currently applied solutions.

We present a numerical simulation study on the optimization of locally contacted rear surface passivated p-Si solar cells considering float-zone and Czochralski-grown bulk material, latter by taking different amounts of oxygen concentrations into account and, thus, varying the quality of the bulk material. The conversion efficiency potential is figured out by a broad variation of the nominal base resistivity, thickness and rear contact distance of the solar cell. To focus on the bulk and rear side recombination properties, the front side contact and emitter properties have been idealized to avoid recombination losses in this region. It turns out that high level injection effects playing a major role in the description of high resistivity bulk materials.

The passivation mechanisms and qualities of Al2O3, SiNx, SiO2 and a-Si:H( i ) on p - and n -type silicon are investigated by quasi-steady-state photoluminescence measurements. This technique allows effective lifetime measurements in an extremely large injection range between 1010 cm-3 and 1017 cm-3. The measurements are discussed focusing on injections below 1012 cm-3 in order to determine the most effective passivation layer for solar cells arranged for indoor applications. Fixed negative charges in the passivation layer cause field-effect passivation due to band bending leading to either accumulation or inversion at the passivation layer/silicon interface. Accumulation causes a stable passivation quality at low level injection. Inversion leads to effective lifetime losses similar to the losses in the space charge region. On p -type silicon the most effective surface passivation at low injections is provided by Al2O3 or a-Si:H( i ). The n -type silicon samples passivated with a-Si:H( i ) show the best effective lifetimes. SiNx and SiO2 show lifetimes one order of magnitude below a-Si:H( i ). Al2O3 on n -type is the most effective passivation at high injections around 1015 cm-3. Due to inversion losses at low level injections the passivation quality decreases more than two orders of magnitude for injections around 1010 cm-3.

To considerably improve the conversion efficiency of industrial solar cells, the effective passivation of the rear surface is a prerequisite. Thermal grown silicon oxides provide an excellent level of surface passivation on lowly doped p-type surfaces. However, dry thermal oxidation processes require relatively high temperatures (?1000°C) and, due to the low growth rate, long process times. To decrease both oxidation temperature and process time, the dry oxidation process can be replaced by a wet oxidation. The most common way to introduce high purity water vapor into the oxidation tube is the pyrolytic generation from high purity gases (H2 and O2). A more easy and cost effective option for the supply of water vapor that was applied within this work is the direct purification of steam. The passivation quality of dry and wet oxides, the latter grown from pyrolytic generated water vapor and purified steam was compared within this work. The passivation quality obtained for the wet oxides grown from purified steam was found to be comparable to those oxides grown from pyrolytic generated water vapor. On laser fired contacts (LFC) solar cells conversion efficiencies well above 20% could be achieved independent of the oxide that was applied for the rear side passivation.

The performance of silicon heterojunction (SHJ) solar cells is discussed in this paper in regard to their dependence on the applied amorphous silicon layers, their thicknesses and surface morphology. The emitter system investigated in this work consists of an n-doped, hydrogenized, amorphous silicon carbide a-SiC:H(n) layer with or without a pure, hydrogenized, intrinsic, amorphous silicon a-Si:H(i) intermediate layer. All solar cells were fabricated on p-type FZ-silicon and feature a high-efficiency backside consisting of a SiO2 passivation layer and a diffused local boron back surface field, allowing us to focus only on the effects of the front side emitter system. The highest solar cell efficiency achieved within this work is 18.5%, which is one of the highest values for SHJ-solar cells using p-type substrates. A dependence of the passivation quality on the surface morphology was only observed for solar cells including an a-Si:H(i) layer. It could be shown that the fill factor suffers from a reduction due to a reduced pseudo fill factor for emitter thicknesses below 11 nm due to a lower passivation quality and/or a higher potential for shunting thorough the a-Si emitter to the crystalline wafer with the conductive indium tin oxide layer. Furthermore, the influence of a variation of the doping gas flow (PH3) during the plasma enhanced chemical vapor deposition of the doped amorphous silicon carbide a-SiC:H(n) on the solar cell current–voltage characteristic-parameter has been investigated. We could demonstrate that a-SiC:H(n) shows in principle the same dependence on PH3-flow as pure a-Si:H(n).

Thin layers of Al2O3 always require a thermal post-deposition treatment to activate the passivation on crystalline silicon surfaces. In this work, we studied the influence of different thermal post-deposition treatments for the activation of passivating ALD Al2O3 single layers and Al2O3/SiNx stacks. For the stacks, especially with less than 5 nm Al2O3, a short high temperature process at 800 °C results in a remarkably lower surface recombination compared to a commonly applied annealing at 425 °C. We observed that four ALD cycles of Al2O3 are sufficient to reach the full potential of surface passivation, and even with one atomic layer of Al2O3 (one ALD cycle) emitter saturation current densities as low as 45 fA/cm2 can be reached on boron-diffused emitters.

Comparison of the open-circuit voltage (external voltage V(oc,ext)) determined by Suns-V(oc) measurements with the implied voltage V(oc, impl) determined by transient photoconductance decay lifetime measurements can yield a quick and easy analysis of silicon heterojunction (SHJ) solar cells, especially in regard to finding the optimum doping concentration of the emitter layer [or back surface field (BSF)]. A sufficiently high doping concentration of the emitter and BSF is mandatory to extract the internal Fermi-level splitting and thus the internal voltage, at the solar cell contacts. However increasing the concentration of doping gases during the deposition of doped amorphous silicon layers results in a reduction of the interface passivation quality and Voc, impl. The best trade off is realized when the ratio of V(oc,ext) to V(oc,impl) (external/internal V(oc)-ratio zeta) reaches a saturation value near 1 upon increasing the doping concentration. AFORS-HET (Automat FOR Simulation of HETerostructures) simulations resulted in the conclusion that the characteristics of the external/internal Voc-ratio are mainly determined by the active doping concentration (doping minus defect concentration).

A back-contact back-junction solar cell on n-type silicon with an aluminum-alloyed emitter is introduced, where both structuring and metallization are realized by screen-printing. The process sequence for realizing the cell with a pitch of 2 mm is displayed and described. The cell parameters are shown for three different emitter coverages on the rear side (45%, 58%, and 72%). An analysis of the saturation current density Jο of the different areas was carried out on lifetime samples, and the calculated Voc is compared to the measured one. The potentially critical edge of the Al finger is analyzed in a scanning electron microscopy cross section. No damaging of the aluminum paste to the passivation layer and a homogenous p+-layer over the whole finger width is observed. A conversion efficiency of 19.7% is presented for a cell with an aperture area of 16.65 cm2 that was exclusively processed in Fraunhofer ISE PV-TEC, which is an industrial-like fabrication environment.

n-Type solar cells with passivated rear surface and point contacts have been proven to have an enormous efficiency potential. However, an industrially feasible process for the realization of the passivated locally contacted rear side of this solar cell type is still missing. Therefore, a rear passivation scheme based on doped amorphous silicon carbide was investigated. The newly developed PassDop layer results in excellent surface passivation and, at the same time, acts as a doping source. After the PECVD of the PassDop layer, contact points are locally opened by a laser pulse, and simultaneously, a local back surface field is formed using the phosphorus contained in the layer. In the last step, the rear side is contacted by the evaporation of aluminum. Due to the very effective passivation of the rear side by the doped passivation layer as well as the excellent contact formation by the laser process, the best cell (aperture area of 4 cm(2)) exhibits an open-circuit voltage of 701 mV and a fill factor of 80.1%, resulting in a confirmed solar cell efficiency of 22.4%.