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This article describes the successful integration of a passivated, highly reflecting Ge rear side into a III-V multijunction solar cell. The use of lowly doped Ge and the new rear side leads to the aimed increase in Ge cell current up to 1.6 mA.cm (exp -2), demonstrated by external quantum efficiency and I-V measurements. For the contact formation, two different types of laser processes were conducted and evaluated-the laser fired contact route and the PassDop route. In both cases, the formation of a local back surface field preserves the passivation of the contact points. A laser pitch and laser power variation leads to a good performing back contact. The passivation effect is proven experimentally and is qualitatively accessed with cell simulations.

By adjusting the process flow of a newly developed transparent conductive adhesive (TCA), the first mechanically stable interconnection of a glued III-V on Si tandem solar cell was established utilizing a ZnO-based adhesive. The measured $V_{OC}$ of 1691 mV is a first proof of concept for the developed TCA. The functionality of the two sub-cells in the tandem device was proven by external quantum efficiency (EQE) measurements. The reflectance at the bond interface of such a glued tandem solar cell remains the limiting factor. In order to reduce the optical losses, the effect of a textured silicon surface was investigated in test structures, which indeed showed reduced reflectance. Furthermore, this textured test structure showed an improved connecting resistivity as low as $83\ \mathrm{m}\Omega\text{cm}^{2}$ .

Gallium phosphide (GaP) is, in theory, a near-ideal heteroemitter for silicon solar cells due to its electronic and crystal properties. In this paper, we present n-type gallium phosphide on p-type silicon heterojunction solar cells which have been prepared by direct growth via metal–organic vapor phase epitaxy (MOVPE). The devices show very promising results in quantum efficiency and current density. However, the open-circuit voltage of 560 mV is far from ideal. The investigation of two different nucleation processes reveals a significant influence of the antiphase domain density at the GaP/Si interface on the open-circuit voltage.

International audience; We report on the effect of electron and proton irradiation on effective minority carrier lifetimes (tau(eff)) in p-type Ge wafers. Minority carrier lifetimes are assessed using the microwave-detected photoconductance decay (mu W-PCD) method. We examine the dependence of tau(eff) on the p-type doping level and on electron and proton radiation fluences at 1 MeV. The measured tau(eff) before and after irradiation are used to estimate the minority carriers' diffusion lengths, which is an important parameter for solar cell operation. We observe tau(eff) ranging from approximate to 50 to 230 mu s for Ge doping levels between 1 x 10(17) and 1 x 10(16) at.cm(-3), corresponding to diffusion lengths of approximate to 500-1400 mu m. A separation of tau(eff) in Ge bulk lifetime and surface recombination velocity is conducted by irradiating Ge lifetime samples of different thicknesses. The possible radiation-induced defects are discussed on the basis of literature

III–V multijunction solar cells grown on a Si solar cell are an attractive approach to reduce the cost of high-efficiency solar cells. When using the Si wafer as an active solar cell, it is crucial to avoid degradation of the minority carrier lifetime in the Si during the metal–organic vapor phase epitaxy (MOVPE) process. After heating a Si wafer in a MOVPE reactor under a H2 atmosphere, we observed a strong degradation of its lifetime. By analyzing the annealed samples with photoluminescence and quasi-steady-state photoconductance, we found an iron contamination of up to ${2} \times {{10}}^{{12}}\text{cm}^{-{3}}$ . The measured iron concentration could be identified as the major source for the decrease of the minority carrier lifetimes of the Si wafers. By using diffusion barriers, we identified the graphite susceptor as the main source of the iron contamination. This knowledge offers pathways to preserve the minority carrier lifetime in the Si wafers during the MOVPE process.

We present n-type epitaxially grown wafers deposited in a reactor that allows a process transfer to inline high-throughput reactors. Those wafers exhibit an effective lifetime of up to 1720 μs locally for a phosphorous concentration of 2·1015 cm-3 and a wafer thickness of about 100 μm. In these wafers the most detrimental defects are stacking faults with polycrystalline silicon inclusions. Comparing two samples with stacking faults densities differing by one order of magnitude revealed a difference in average effective minority carrier lifetime of also one order of magnitude (reduction from more than 1500 μs down to 111 μs for the defective sample). A solar cell fabricated from a 200 μm thick epitaxial wafer of low stacking fault density and a phosphorous concentration of 3·1016 cm-3 reaches an independently confirmed efficiency of 20%, an open circuit voltage of up to 658 mV, a short circuit current density of up to 39.6 mA/cm2 and a fill factor of up to 76.9%. Differences between this cell and FZ references can be attributed to a reduced bulk lifetime caused by the high doping concentration and most probably additional recombination due to polycrystalline silicon inclusions in stacking faults, although their amount is comparably low. A second solar cell made of an epitaxial wafer with a high stacking fault density exhibits an efficiency reduction of 0.5% absolute compared to the cell made of the high quality epitaxial wafer. This result underlines the importance of minimizing the stacking fault density in epitaxial wafers, in particular the density of those with polysilicon inclusions.

Monolithic tandem cells involving a top cell with Si nanocrystals embedded in SiC (Si NC/SiC) and a c-Si bottom cell have been prepared. Scanning electron microscopy shows that the intended cell architecture is achieved and that it survives the 1100 degrees C anneal required to form Si NCs. The cells exhibit mean open-circuit voltages V-oc of 900-950mV, demonstrating tandem cell functionality, with 580mV arising from the c-Si bottom cell and 320mV arising from the Si NC/SiC top cell. The cells are successfully connected using a SiC/Si tunnelling recombination junction that results in very little voltage loss. The short-circuit current densities j(sc) are, at 0.8-0.9 mAcm(-2), rather low and found to be limited by current collection in the top cell. However, equivalent circuit simulations demonstrate that in current-mismatched tandem cells such as the ones studied here, higher j(sc), when accompanied by decreased V-oc, can arise from shunts or breakdown in the limiting cell rather than improved current collection from the limiting cell. This indicates that V-oc is a better optimisation parameter than j(sc) for tandem cells where the limiting cell exhibits poor junction characteristics. The high-temperature-stable cell architecture developed in this work, coupled with simulations highlighting potential pitfalls in tandem cell analysis, provides a suitable route for optimisation of Si NC layers for photovoltaics on a tandem cell device level. Copyright (c) 2016 John Wiley & Sons, Ltd.

Sub-bandgap sunlight provides a source of heat generation in solar cells that is detrimental to performance, especially in space applications where heat dissipation is limited. In this work we assess the impact that an advanced rear-side contact scheme for multi-junction solar cells has on the cell temperature. Our results show that this scheme reduces the optical power absorption below the bandgap of germanium by 81% compared to a standard, full metallization design. Measurements of the electrical and thermal power fluxes performed in vacuum demonstrate that this lower near-infrared light absorption results in 8% less heat dissipated in the cell with the novel rear-side contact scheme when operating at 25 °C. Modelling of the operating temperature for both cells when fully encapsulated with glass indicates that this effect will also result in a reduction of the operating temperature of 9 °C for the novel design.

Multijunction solar cells with four junctions are expected to be the next-generation technology for both space and concentrator photovoltaic applications. Most commercial triple junction solar cells are today grown on germanium, which also forms the bottom subcell. Extending this concept to four junctions with an additional∼1-eV subcell was proven to be challenging. We investigate a new cell concept, which uses direct wafer bonding to combine a metamorphic GaInAs/Ge bottom tandem solar cell with a GaInP/AlGaAs top tandem on GaAs resulting in a monolithic four-junction cell on germanium. This article summarizes results of the cell developments, which have been resulting in a four-junction concentrator cell with 42% efficiency. We implemented a new passivated Ge backside technology to enhance the current generation in the Ge junction, and we propose realistic steps to realize solar cells with 45% efficiency using this cell architecture.

In this work a quantitative approach to assess the specific material related efficiency limits of epitaxially grown silicon wafers is demonstrated. Based on experimental results of injection dependent carrier lifetime images on these wafers the absolute losses of identified defects, namely decorated stacking faults, defects from inhomogeneous processing and underlying homogeneously distributed recombination centers, have been quantified and compared. The losses from decorated stacking faults have been determined as a function of their lateral density. The obtained loss diagrams allow for systematic material optimization.

For future 4-junction Ge based multi-junction solar cells, the current generated in the Ge sub-cell gets very important. We developed an efficient rear-side passivation stack, which results in minority carrier lifetimes $(\tau _{eff}) \approx 200\mu s$ and investigated its performance in an accelerated aging experiment (1MeV electron irradiation). The aging caused a strong lifetime decrease down to $\tau _{eff}=4\mu s$, whereas the carrier mobility stayed constant. These experimental values provide the basis for Beginning-of-Life and End-of-Life solar cell simulations, which show that the potential of the rear-side passivation for 3-junction solar-cell performance is limited, but very promising for 4-junction solar cells.

The integration of III-V compound semiconductors on a silicon bottom cell offers the opportunity to form two- and three-junction solar cells with a conversion efficiency exceeding 30%. This paper reports on the progress in the heteroepitaxial nucleation of gallium phosphide (GaP) on silicon, which allows the fabrication of a silicon bottom cell with front-surface passivation by a thin single-crystalline GaP window layer. GaP has a low lattice-mismatch to Si and an indirect bandgap energy of 2.26 eV, which leads to low absorption. At the same time, GaP can be doped with silicon to form an n-type contact layer. In this publication, we investigate n-Si/p-Si homojunction solar cells with a GaP window and contact layer. Metal-organic vapor phase epitaxy was used to deposit the 60-nm GaP window layer with a low density of antiphase boundaries at the heterointerface and without misfit dislocations. Open-circuit voltages of up to 634 mV have been obtained under 1-sun AM1.5g conditions for devices without antireflective coatings.

The outstanding demonstration of quantum confinement in Si nanocrystals (Si NC) in a SiC matrix requires the fabrication of Si NC with a narrow size distribution. It is understood without controversy that this fabrication is a difficult exercise and that a multilayer (ML) structure is suitable for such fabrication only in a narrow parameter range. This parameter range is sought by varying both the stoichiometric SiC barrier thickness and the Si-rich SiC well thickness between 3 and 9 nm and comparing them to single layers (SL). The samples processed for this investigation were deposited by plasma-enhanced chemical vapor deposition (PECVD) and subsequently subjected to thermal annealing at 1000–1100 °C for crystal formation. Bulk information about the entire sample area and depth were obtained by structural and optical characterization methods: information about the mean Si NC size was determined from grazing incidence X-ray diffraction (GIXRD) measurements. Fourier-transform infrared spectroscopy (FTIR) was applied to gain insight into the structure of the Si–C network, and spectrophotometry measurements were performed to investigate the absorption coefficient and to estimate the bandgap E04. All measurements showed that the influence of the ML structure on the Si NC size, on the Si–C network and on the absorption properties is subordinate to the influence of the overall Si content in the samples, which we identified as the key parameter for the structural and optical properties. We attribute this behavior to interdiffusion of the barrier and well layers. Because the produced Si NC are within the target size range of 2–4 nm for all layer thickness variations, we propose to use the Si content to adjust the Si NC size in future experiments.

Reduction of solar cell fabrication costs is still of importance and can be achieved by going towards thinner silicon wafers. The aim is to achieve thicknesses in the range of 100 mu m and below while avoiding further kerf loss. The approach using porous silicon as a sacrificial detachment layer and as a seed layer for epitaxial growth can accomplish that goal [1]. Several research groups have already shown the high potential of this technique [2]. Most processes have been conducted in microelectronic-grade batch or single-wafer reactors, questioning the industrial feasibility of this concept. In this work n-type silicon wafers, epitaxially grown in a quasi-inline Atmospheric Pressure Chemical Vapour Deposition (APCVD) reactor, will be presented. The wafers were characterized using confocal white light microscopy, quasi steady state photoconductance decay (QSSPC), microwave detected photoconductance decay (MWPCD) and spreading resistance profiling (SRP) measurements. The first batch of n-type wafers showed mean effective carrier lifetimes exceeding 100 mu s on 45 x 45 mm(2) and locally more than 300 mu s. A detailed analysis on the thickness distribution, bulk lifetime and surface recombination velocity, affecting the effective carrier lifetime was conducted. For the second batch of n-type wafers mean effective carrier lifetimes of over 260 mu s on 25 x25 mm(2) and locally over 500 mu s were determined. This corresponds to a local bulk lifetime of over 800 mu s and shows the high material quality of epitaxial layers grown in our developed APCVD reactors.

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of Si tandem solar cells since their bandgap exceeds that of bulk silicon and can be tuned by adjusting nanocrystal size. Due to this effect, size control is required to maintain a uniform bandgap throughout a Si NC film. This can be achieved by annealing superlattices of Si-rich and stoichiometric dielectrics. This paper reviews the progress that has been made using host matrices SiO2, Si3N4, and SiC. Si NCs in SiO2 show excellent NC size and shape control and strong quantum-confinement-related photoluminescence, however electrical conductivity is poor. Ordering and size control is also obtained in Si3N4, but conclusive evidence of quantum confinement is lacking. Preparing ordered but separated Si NCs in SiC is difficult, but the narrow parameter space in which this is possible has been elucidated, good electrical conductivity was obtained, and functioning single-junction and tandem cells have been produced. Si NC formation can now be well-controlled in all three materials, and the key weaknesses for photovoltaics have been identified to be the electrical conductivity of SiO2, and defect density for Si3N4 and SiC. Addressing these is expected to lead to competitive Si tandem solar cells.

An alternative boron emitter diffusion process called rapid vapor-phase direct doping (RVD) is studied and applied to n -type silicon solar cells with a tunnel oxide passivated electron contact (TOPCon). The RVD emitter diffusion process occurs under an atmosphere containing only the dopant gas and hydrogen. Thus, compared with standard tribromide diffusion processes, no oxygen is present. Hence, no boron glasses form during the RVD process. Consequently, a faster diffusion process with fewer chemical treatments after the diffusion process compared with standard tribromide processes is possible. In this paper, three different RVD emitter surface dopant concentrations and dopant depths were achieved by process parameter variations. These RVD emitters were applied to TOPCon cells, and their cell characteristics were compared with profiles of TOPCon reference cells with standard boron-diffused emitters. Up to 24.0% cell efficiency, 697.6 mV open-circuit voltage, 41.8 mA/cm ${^2}$ short-circuit current density, and 82.1% fill factor were reached by the best TOPCon cell with an RVD emitter. Nevertheless, compared with the reference, all cells with RVD emitters exhibited efficiency losses. Hence, to further optimize cells with RVD emitters, in-depth characterizations were conducted. The cell efficiency of cells with an RVD emitter is mainly limited by two main reasons: First, effective carrier lifetime degradation was observed, resulting in voltage losses, and second, for RVD diffusion temperatures above 980 $\mathrm{^\circ }$ C, a flattening of textured cell surfaces was detected leading to current losses. In order to overcome these issues, an adapted two-step RVD emitter diffusion process is suggested for future experiments.

The material quality degradation of silicon wafers by metal impurities, various crystal defects as well as light and thermally induced mechanisms is very important for the solar cell performance and has been investigated by various groups. In this paper, the material degradation during epitaxial deposition at high temperatures above 1100°C will be discussed. Annealing experiments in hydrogen atmosphere are done with the laboratory rapid thermal chemical vapor deposition reactor to mimic the thermal process conditions for epitaxial growth of silicon from the gas phase. A general investigation of crystallographic and electronic properties of n- and p-type silicon wafers has been done between 950°C and 1150°C. A detailed sensitivity analysis of process parameters like cooling ramp, peak temperature, duration, and ambient gasses has been conducted. The degradation mechanism by metal impurities has been investigated by using silicon wafers with different diffusion barriers. Besides effective minority carrier lifetime, measurements by quasi steady state photo conductance, etch pit density, Raman spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy measurements have been done. The presented results have been used to improve the deposition process of epitaxial thin-film solar cells, the production of silicon foils with a thickness

New developments for space solar cells mainly address efficiency improvements and weight reduction. In this paper we developed amorphous SiC based layer stacks for passivation and enhanced reflection on thin and lowly doped (2×1016 − 1×1017 at/cm3) Ge wafers. Passivated Ge samples with minority carrier lifetimes of more than $300\ \mu \mathrm{s}$ and surface recombination velocities of just 17 cm/s are presented. Thermal annealing at 400°C and additional “mirror” layer deposition do not harm the minority carrier lifetimes or lead to an even slight increase. Electron irradiation with fluences of 1×1015 e/cm2and more lead to strong material degradation and lifetimes of just $5\ \mu \mathrm{s}$.

Multi-wire sawing has emerged as the leading technology in wafer production for a variety of semiconductor materials. This study investigates the process stability and efficiency of conventional semiconductor multi-wire slurry saws in routinely machining translucent, high-density alumina ceramics. The brittle and fine-grained translucent alumina ceramics with extreme hardness and wear resistance represents a major challenge for the process. The alumina ceramic substrates are used for sensor applications, energy storage technology and applications in power electronics. An ideal adaptation of the sawing process parameters to the workpiece properties guarantees the efficiency of the slurry sawing process and the quality of the ceramic wafers. An indicator of the efficiency and cutting ability of the sawing process is the size of the bow of the wire web. The first time was shown that the wire bow can be used for the characterization of the sawing processes for hard and brittle technical ceramics. It was found that a longer workpiece length, a higher number of wafers and stronger abrasive wear lead to an increased size of the bow. The rocking frequency has no measurable influence on the size of the bow. Knowledge of these relations is an extremely valuable tool in the sawing process development for hard and brittle technical ceramics.

In this work the processing sequence for an integrated interconnection concept for crystalline silicon thin-films (c-Si TF) is introduced. This concept is designed to reduce cost in the production of large-area crystalline silicon thin-film modules. Combining the advantages of classical thin-film and wafer technologies, a soldering-free interconnection of cells in the module is realized. In order to investigate the interconnection concept, a small scale laboratory process was developed and is discussed here. Mini-modules were fabricated using this processing sequence on silicon on insulator (SOI) wafers with epitaxially grown back surface field (BSF) and base layer. Despite a non-optimized metallization sequence an open circuit voltage of over 3 V could be achieved for a mini-module consisting of five cells. Mini-modules were also encapsulated without loss in efficiency after encapsulation.

The quality and cost of a wafer are important prerequisites for a high-efficiency, low-cost solar cell. Standard wafers are fabricated using a dicing process, which leads to the loss of a substantial amount of material (up to 40%), the kerf loss. The kerf loss is costly, but in standard technologies, it is unavoidable. As the wafer thickness decreases, these losses become more significant. Using an alternative approach, by applying a sacrificial detachment layer on top of a thin seed layer made of porous silicon for a follow-on thick epitaxial growth, so-called EpiWafers can be produced, and kerf losses can be avoided. An advantage of this EpiWafer production process is that the thickness of the wafers can be deliberately chosen according to requirements, e.g. between 50 µm and 250 µm. The wafer production costs are lower than standard wafering techniques if high-throughput porosification tools and chemical vapour deposition reactors are used. In this work, solar cells with n-type wafers epitaxially grown in an atmospheric pressure chemical vapour deposition reactor are presented. The processed wafers feature an n-type doping density of 3×1016 cm−3 and a mean thickness of 150 µm. The best EpiWafer cell reached an open circuit voltage of 657 mV, a short circuit current of 39.6 mA/cm2 and a fill factor of 77%. A detailed analysis of the losses caused by stacking faults and cell process issues is presented. The resulting energy conversion efficiency of 20% proves the high efficiency potential of this material.

Silicon nanocrystals (Si NCs) are a promising candidate for the top cell of an all-Si tandem solar cell with a band gap from 1.3-1.7 eV, tuneable by adjusting NC size. They are readily produced within a Si-based dielectric matrix by precipitation from the Si excess in multilayers of alternating stoichiometric and silicon-rich layers. Here we examined the luminescence and transport of Si NCs embedded in SiC. We observed luminescence that redshifts from 2.0 to 1.5 eV with increasing nominal NC size. Upon further investigation, we found that this redshift is to a large extent due to Fabry-Pérot interference. Correction for this effect allows an analysis of the spectrum emitted from within the sample. We also produced p-i-n solar cells and found that the observed I-V curves under illumination could be well-fitted by typical thin-film solar cell models including finite series and parallel resistances, and a voltage-dependent current collection function. A minority carrier mobility-lifetime product on the order of 10-10 cm2/V was deduced, and a maximum open-circuit voltage of 370 mV achieved.

Silicon-carbon (Si-C) thin films play a key role in many technological applications such as hard coatings, high-power electronics, and photovoltaics. In photovoltaics in particular annealed Si-C thin films containing Si quantum dots are used to develop solar cells with improved efficiency. The oxidation of these films during the annealing step, which is unavoidable in the high-throughput processes required for photovoltaics, was explored using scanning electron microscopy, Fourier-transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. SiO2 surface layers 5 to 14 nm thick were observed even in nominally inert furnace atmospheres, while annealing with graphitic carriers leads to the formation of SiOxCy films a few nm thick. To avoid the formation of either compound and thereby reduce the impact of the particular furnace used on the Si-C film an encapsulation layer made of a-Si:H was developed. It is shown that 40 nm of this layer can protect an Si-C film from oxidation. The SiO2 and residual Si formed are removed using standard etchants with only minimal impact on the Si-C film. It was found that this process depends critically on the thickness of a-Si:H deposited but is fairly insensitive to the parameters of the etching process. The encapsulation process presented herein is a key step towards the fast large-scale annealing of Si-C films required for photovoltaic applications, and has the potential to greatly simplify the thermal treatment of a wide range of thin films. © 2012 Elsevier B.V.

Silicon wafers have still a significant contribution to the total cost of production for silicon solar cells. One cost driver when using classical wafering techniques is kerf loss. With the approach using porous silicon as a detachment layer and as a seed layer for epitaxy kerf losses can be avoided. In this work, solar cells with epitaxially grown n-type wafers are presented. The best EpiWafer-cell reaches an open circuit voltage of 657.5 mV, a short circuit current of 39.6 mA/cm2 and a fill factor of 77%. The resulting energy conversion efficiency of 20% proves the high quality of this material.

III-V materials are very attractive top absorbers for highly efficient Si based multi-junction solar cells. In case of direct growth on Si wafers, a high quality hetero interface for the passivation of the silicon bottom cell should be achieved and the degradation of the Si minority carrier bulk lifetime has to be avoided. In this paper we show that diffusion barriers can reduce the contamination of the Si absorber and lead to minority carrier lifetimes of 1120 μs after thermal treatment at 1050 °C in the metal-organic vapor phase epitaxy reactor. With an adapted growth and solar cell process we could fabricate Si solar cells with an open circuit voltage of 634 mV.

Improvements in emitter passivation as well as back contact passivation lead to several high efficiency solar cell concepts. PERC (Passivated Emitter Rear Contact) solar cells on monocrystalline wafers above 21 % [1] have been achieved, but the diffusion processes with POCL3 and BBr3 have emerged as one of the limiting factors. The formation of high efficiency emitters by APCVD (Atmospheric Pressure Chemical Vapour Deposition) has several advantages compared to the standard diffusion. Epitaxial emitters can be optimised to match the passivation and metallisation of high efficiency concepts as well as industrial approaches like nickel plating. Simulations by PC1D show the potential of epitaxial emitters by featuring low contact resistance in combination with high shunt resistance, a good blue response and low emitter saturation current (J(0e)) [2,3]. The emitter profiles can be designed in depth and in doping in the range from 1x10(17) cm(-3) up to 1x10(20) cm(-3) for p-type and n-type emitters. Simple reference solar cells with J(0e)=46 fA/cm(2) and efficiencies of eta = 17.5 % on float zone (FZ) and eta = 16.1 % on multicrystalline (mc) wafers have been processed. Furthermore an epitaxial selective emitter has been developed. The deposition process itself has been enhanced to improve the emitter profiles as well as material quality.

Crystalline silicon thin film solar cells, based on the epitaxial wafer equivalent, require a reflecting interlayer between substrate and active layers to increase the generated current and reach similar efficiencies as wafer based solar cells. With the epitaxial lateral overgrowth technique, a reflecting dielectric layer can be implemented. In this paper results of solar cells with overgrown, patterned SiO 2 films are shown. A beneficial optical effect due to the interlayer and also a reduced effective diffusion length within the epitaxially grown silicon layer are observed. Cells with reflecting interlayer and non-optimized absorber layer thicknesses therefore exhibit lower efficiencies than cells without SiO 2 . Slightly higher currents are observed with textured front sides. Significantly increased effective diffusion lengths and cell performances can be reached with non-merged active silicon layers. Reasons might be the avoidance of defects generated at merging points and of unpassivated surface areas in voids remaining where two growth fronts merge.

The influence of temperature on the open-circuit voltage (VOC) of crystalline silicon solar cells is analysed using different semiconductor temperature models with different levels of accuracy. The strongest influence besides the direct dependence of the intrinsic carrier concentration on temperature results from the temperature dependence of the band gap and the effective density of states, while the incomplete ionization plays a minor role for the implied voltage. However incomplete ionization can play an important role for the external voltage at temperatures below 50K due to imperfect selectivity of the emitter and back surface field. The observed saturation of VOC towards low temperatures is caused by the effective density of states. The temperature dependence from 80 K to 300 K and the intensity dependence as a function of temperature and illumination density were measured on a silicon wafer solar cell resulting in a maximum voltage of 1012 mV at T=85.8K. The measured values could be well described by theory.

A new cell concept has been developed that enables epitaxial c-Si thin-film solar cells to be made using isolating substrates. The Recrystallised Wafer-Equivalent on an Isolating Substrate (RexWISe) cell concept relies on an array of mini-silicon rods through the substrate to enable standard contacting. Processing techniques have been developed to produce the rods by drilling holes through the substrate, coating the holes with an intermediate layer, filling the holes with a seeding layer deposition and then recrystallising the seeding layer. Subsequently, the active layers of the cell are epitaxially grown onto the recrystallised layer and then this “Wafer-Equivalent” structure is metallised like a standard wafer solar cell. The first solar cells have been produced to test the RexWISe process and a “proof-of-concept” efficiency of almost 8% was achieved.

Amorphous Si x C 1− x :H single layers with varying stoichiometry were prepared using plasma enhanced chemical vapour deposition (PECVD). The as-deposited layers were annealed at different temperatures ( T a ) from 500 to 1100 °C. The influence of annealing on layers with varying composition x was investigated by a variety of analytical techniques including photoluminescence (PL) spectroscopy, reflectance and transmittance measurements, grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectrometry (FTIR). Before annealing, PL measurements show a shift of the spectra to higher energies with increase in carbon content. Calculations of the optical bandgap by other groups confirm this trend. The FTIR investigations show in addition to the expected Si–C bonds also the formation of oxygen and hydrogen related bonds within the layers. After annealing, solely a broad PL signal appears at energies around 2.0 eV. This might be due to luminescence of defects, created by the incorporated amount of carbon and the evolution of crystalline phases in the layers. GIXRD measurements confirmed the formation of both Si NCs after 700 °C and SiC NCs after annealing at 1000 °C. The FTIR spectra exhibit a shift of the SiC absorption band and an increase in the Si–C bond density due to the crystallization of SiC. After annealing a hydrogen passivation was performed, which leads to a strong decrease in the PL intensity. However, annealed Si-rich films show a peak around 1.2 eV after H-passivation, which might originate from quantum confinement effects in Si NCs.

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 cm2) 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%.

Fraunhofer ISE has a long experience in the field of surface passivation for crystalline silicon wafers. Novel rear-surface passivation layer systems have led to excellent results. Using a low-temperature passivation stack of hydrogenated amorphous silicon and plasma-enhanced chemical vapor deposition (PECVD) silicon oxide an efficiency of up to 21.7% has been achieved. Thermally stable passivation can be proven with all-PECVD stacks of silicon oxide, silicon nitride, and silicon oxide (PECVD-ONO), i.e. after contact firing. Solar cell efficiencies of up to 20.0% have been reached with PECVD-ONO. In parallel, Fraunhofer ISE is working on silicon carbide (SiC x ) layers, which provide excellent and thermally stable passivation, as well deposited by PECVD. Solar cells with SiC x layers as rear passivation led to efficiencies of up to 20.2%.

The optical and photovoltaic properties of Si NCs/SiC multilayers (MLs) are investigated using a membrane-based solar cell structure. By removing the Si substrate in the active cell area, the MLs are studied without any bulk Si substrate contribution. The occurrence is confirmed by scanning electron microscopy and light-beam induced current mapping. Optical characterization combined with simulations allows us to determine the absorption within the ML absorber layer, isolated from the other cell stack layers. The results indicate that the absorption at wavelengths longer than 800 nm is only due to the SiC matrix. The measured short-circuit current is significantly lower than that theoretically obtained from absorption within the ML absorber, which is ascribed to losses that limit carrier extraction. The origin of these losses is discussed in terms of the material regions where recombination takes place. Our results indicate that carrier extraction is most efficient from the Si NCs themselves, whereas recombination is strongest in SiC and residual a-Si domains. Together with the observed onset of the external quantum efficiency at 700–800 nm, this fact is an evidence of quantum confinement in Si NCs embedded in SiC on device level.

In this publication we present p- and n-type Si foils with thicknesses between 40 and 150 µm produced with epitaxy on porous Si followed by mechanical lift-off. We discuss the influence of thickness and surface passivation quality on the relation between τeff and τbulk. Appearing inhomogeneity of minority carrier lifetimes over the sample area is found to be due to porous Si removal and surface cleaning issues. However, on n-type Si foils we could achieve τbulk values of up to 800 µs. As such Si foils are sufficient for conversion efficiencies well above 20 % [1] we are confident that solar cells can be processed with our material exceeding this benchmark.

Crystalline silicon thin-film solar cells (CSITF; high temperature approach) on low-cost substrates will only get their chance on the market when they can be introduced into an industrial solar cell process. Therefore, a conductive diffusion barrier intermediate layer (IL) is necessary. The most promising candidate is, in our opinion, SiC. Not just because doping with boron or phosphorous can change the electrical behavior effectively. It also matches well in thermal expansion coefficients with RBSiC (reaction-bonded) ceramics which is a very promising low-cost substrate. The passivation performance and the possibility to use the thin layer as a dopant source for an in situ back surface field are additional benefits. In this paper, we will show that the electrical conductivity of SiC can become sufficiently high with temperature treatments. We will furthermore present FTIR and SIMS measurements which would give information about the changing of bonding conditions and the distribution of the dopant boron with different deposition temperatures and annealing.

Cost reduction is still the driving force in photovoltaic industry. One promising way to tackle this issue in the crystalline silicon technology is to reduce the consumption of highly purified Si raw materials. To avoid kerf losses and to manufacture still high quality Si foils with thicknesses in the range of 50 µm the porous Si approach [1, 2] is one potential solution. Several investigations could show that although Si is an indirect semiconductor such thin layers are sufficient for conversion efficiencies well above 20 % [3] given good material quality and optical confinement. At Fraunhofer ISE we have been working on high-throughput solutions for all necessary technologies such as porosification, reorganization plus Si epitaxy and lift-off. In this paper we will present the first reliable process to produce Si foils with reorganization and epitaxial thickening done in an industrially relevant inline tool. The paper will furthermore give a status report on the different technology steps.

The electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO 2 superlattices are studied for different silicon excess and layer thicknesses. The precipitation and crystallization of the Si excess present within the SRON layers is induced by a post-deposition annealing treatment, in order to form Si nanocrystals (Si-NCs). The electrical characterization performed in dark conditions allowed for deducing the charge transport mechanism through the superlattice structure, found to follow the Poole-Frenkel law. In addition, the EL investigation revealed the correlation between EL excitation and transport mechanisms, suggesting that impact ionization of high-energy conduction electrons dominates the whole frame. The reduction of the SiO 2 barrier thickness and the increase in the Si excess were found to enhance the carrier transport through the superlattices due to the reduction of the electrons mean free path, which, in turn, modifies the EL properties.

This paper demonstrates the usefulness of optical microcharacterization for process control and defect analysis for epitaxial grown crystalline silicon thin films (cSiTF). Using a confocal laser scanning microscope, we gradually analyze material and electrical properties of epitaxial grown cSiTFs and interface quality between cSiTF and substrate on the microscopic scale. With micro-Raman spectroscopy, the interface quality and the doping density within the epitaxial layers are evaluated. Microphotoluminescence spectroscopy investigations reveal the highly recombinative character of stacking faults decorated with strongly deformed silicon fractions that are frequently formed during epitaxial growth at low temperatures. The microlight beam-induced current delivers detailed information about local inhomogeneities of the charge collection. Finally, these methods are applied to fully processed solar cells with an epitaxial absorber. A globally observed cell voltage drop is linked to parasitic metal deposition on highly recombination active pyramidal stacking faults in the epitaxially grown cSiTF layers. By means of microreverse-biased electroluminescence, it is shown that this parasitic metal deposition on stacking faults causes local prebreakdowns.

Silicon nanocrystals (Si NCs) embedded in Si-based dielectrics provide a Si-based high-bandgap material (1.7 eV) and enable the construction of crystalline Si tandem solar cells. This chapter focusses on Si NC embedded in silicon carbide, because silicon carbide offers electrical conduction through the matrix material. The material development is reviewed, and optical modeling is introduced as a powerful method to monitor the four material components, amorphous and crystalline silicon as well as amorphous and crystalline silicon carbide. In the second part of this chapter, recent device developments for the photovoltaic characterization of Si NCs are examined. The controlled growth of Si NCs involves high-temperature annealing which deteriorates the properties of any previously established selective contacts. A membrane-based device is presented to overcome these limitations. In this approach, the formation of both selective contacts is carried out after high-temperature annealing and is therefore not affected by the latter. We examine p-i-n solar cells with an intrinsic region made of Si NCs embedded in silicon carbide. Device failure due to damaged insulation layers is analyzed by light beam-induced current measurements. An optical model of the device is presented for improving the cell current. A characterization scheme for Si NC p-i-n solar cells is presented which aims at determining the fundamental transport and recombination properties, i.e., the effective mobility lifetime product, of the nanocrystal layer at device level. For this means, an illumination-dependent analysis of Si NC p-i-n solar cells is carried out within the framework of the constant field approximation. The analysis builds on an optical device model, which is used to assess the photogenerated current in each of the device layers. Illumination-dependent current-voltage curves are modelled with a voltagedependent current collection function with only two free parameters, and excellent agreement is found between theory and experiment. An effective mobility lifetime product of 10 10 cm2/V is derived and confirmed independently from an alternative method. The procedure discussed in this chapter is proposed as a characterization scheme for further material development, providing an optimization parameter (the effective mobility lifetime product) relevant for the photovoltaic performance of Si NC films.

An optoelectronic analysis of SiC with and without embedded silicon nanocrystals is presented. The layers are implemented as the absorber to membrane-based p-i-n solar cells and characterized by illumination-dependent IV measurements and electroluminescence. The silicon nanocrystals in silicon carbide were prepared by solid-phase crystallization and contacted with doped a-SixC1-x:H. The fundamental transport and recombination properties, i.e. the effective mobility lifetime product, of the nanocrystal layer is determined at device level. For this means, illumination-dependent current-voltage curves are modeled with a voltage-dependent collection function with only two free parameters, and excellent agreement is found between theory and experiment. Electroluminescence measurements are presented and conclusions are drawn regarding the electronic structure of the SiC host matrix.

In this work we present a structural, optical and electrical characterization of Si x C 1− x /SiC multilayer systems with different silicon content. After the deposition process, an annealing treatment was carried out in order to induce the silicon nanocrystals formation. By means of energy-filtered transmission electron microscopy (EFTEM) we observed the structural morphology of the multilayers and the presence of crystallized silicon nanoprecipitates for samples annealed up to 1100 °C. We discuss the suitability of optical techniques such as Raman scattering and reflectance and transmittance ( R&T ) for the evaluation of the crystalline fraction of our samples at different silicon excess ranges. In addition, the combination of R&T measurements with simulation has proved to be a useful instrument to confirm the structural properties observed by EFTEM. Finally, we explore the origin of the extremely high current density revealed by electrical measurements, probably due to the presence of an undesired defective SiC y O z ternary compound layer, already supported by the structural and optical results. Nevertheless, the variation of the electrical measurements with the silicon amount indicates a small but significant contribution from the multilayers.

Silicon nanocrystal quantum dots in a dielectric matrix form a material with higher band gap than silicon, but still compatible with silicon technology. So far, devices using silicon nanocrystals have been realized either on silicon wafers, or using in-situ doping in the superlattice deposition which may hinder the nanocrystal formation. In this paper, a vertical PIN device is presented which allows to investigated the electrical and photovoltaic properties of nanocrystal quantum dot layers. The device structure circumvents any influence of a substrate wafer or dopants and provides full flexibility in the material choice of both, i.e. electron and hole, contacts. Furthermore, not-high-temperature stable contact materials can be applied. Devices have been realized using SiC/Si nanocrystal multilayers as the i-region and doped a-SixC1-x:H layers as electron and hole contacts. First devices show open-circuit voltage of up to 400mV.

A membrane based silicon nanocrystal p-i-n diode is presented that enables the photovoltaic characterization of silicon quantum dots produced by high-temperature routes. The membrane p-i-n diode decouples silicon nanocrystal formation from the formation of selective contacts and therefore enables extraction of the full photovoltaic potential. Open-circuit voltages of up to 370 mV are shown, which is encouraging for future tandem solar cells.

We report on the development of a laser doping process for the formation of a local back surface field (LBSF) for n-type silicon solar cells. Local point contacts are formed by applying a laser process to a doped, passivating layer of amorphous silicon carbide (PassDop layer). The exposure to laser radiation results in local doping and opening of the passivation layer at the same time. By variation of the laser parameters and the dopant content in the layer, strength and depth of the doping can be controlled. Both parameters are varied and the formation of a LBSF structure on lifetime samples is investigated. High dopant content in the passivation layer and comparably low laser fluencies yield the best electrical results, evidencing the formation of an effective LBSF in combination with a restricted laser induced damage in the silicon crystal.

In this paper the applicability and efficiency of different intermediate layer (IL) stacks for the implementation in recrystallized wafer equivalent (RexWE) solar cells are investigated. The requirements for the IL in the RexWE concept are short term stability at temperatures above 1400 °C, high reflectivity for wavelengths exceeding 600 nm, electrical conductivity and acting as a diffusion barrier against metallic impurities. Various combinations of stoichiometric SiC layers, silicon rich SiC layers and SiO 2 layers were tested as IL stacks regarding their performance after a zone melting recrystallization (ZMR) process. For the first time, samples with an IL consisting of a SiC multilayer were recrystallized and successfully processed to solar cells.

The subject of this work is the development of an electrically conductive Rugate filter for photovoltaic applications. We believe that the optical as well as the electrical performance of the filter can be adapted especially to the requirements of crystalline Si thin-film and amorphous/crystalline silicon tandem solar cells. We have deposited amorphous hydrogenated silicon carbide layers (a-Si x C 1-x :H) with the precursor gases methane (CH 4 ), silane (SiH 4 ) and diborane (B 2 H 6 ) by applying Plasma Enhanced Chemical Vapour Deposition (PECVD). By variation of the precursor flows, a floating refractive index n from 1.9 to 3.5 (at 633 nm) could be adjusted quite accurately. Different complex layer stacks (up to 200 layers) with a sinusoidal refractive index variation normal to the incident light were deposited in just 80 min on an 100x100 mm 2 area. Transmission measurements show good agreement between simulation and experiment, which proves our ability to control the deposition process, the good knowledge of the optical behaviour of the different SiC single layers and the advanced stage of our simulation model. The doped single layers show lateral conductivities that were extremely dependent on the Si/C ratio.

An effective light trapping system is required in silicon solar cells in order to collect a large amount of photons. That is why we focus our investigation on the fabrication and evaluation of two types of optical systems introduced on the back side of solar cells. The aim of these structures is to enhance the light trapping of the long wavelength photons (above 1000 nm). On the one hand, we evaluated a Si/SiO2 linear nanograting; on the other hand, hexagonal nanostructures fabricated with SiO2 nanoparticles and a filling matrix are under investigation. In this paper, we describe the fabrication processes developed for both approaches and we present the solar cell results and characterisation. For the first approach, we show a reflectance reduction on test structures, which occurs at the same wavelength as the increase of absorption induced by the simulated gratings. Moreover, we demonstrate the feasibility of the fabrication of silicon solar cells with the hexagonal nanostructures as a diffractive back reflector. Although no short circuit current increase has been observed due to a poor rear side passivation, a current gain up to 0.3 mA/cm2 is possible in the wavelength range of 1050-1150 nm due to these nanostructures. Finally, we also comment on the advantages and drawbacks of each approach and on the feasibility to introduce these systems in the solar cell process flow.

In this paper the potential of amorphous silicon carbide used as an emitter for silicon heterojunction solar cells is presented. Especially the annealing behaviour of the open-circuit voltage V oc of n-doped amorphous silicon carbide heterojunction emitter solar cells is investigated in detail. We present our results of a significant open-circuit voltage improvement of more than 100 mV on both a flat and a textured front surface triggered by thermal annealing on a hot plate. The observed open-circuit voltage behaviour can be described best by a stretched exponential function, which in general describes relaxation rates in complex systems. Further we investigated the optimum conditions of a post deposition annealing step in order to reach the highest efficiency. During this analysis we also observed deterioration in solar cell performance when the structure is annealed for a very long time. In conclusion, we suppose that a diffusion of weakly bonded or free hydrogen, activated by the annealing which saturates dangling bonds in the amorphous layer itself and most likely more important at the heterojunction interface, is responsible for the strong improvement in V oc and efficiency.

This paper is addressed to the surface passivation of c-Si and c-Ge wafers by plasma enhanced chemical vapor deposition (PECVD) of Si-rich a-SiC x . We report upon excellent effective lifetimes on 1 Ωcm CZ p-Ge substrates exceeding 400 µs after an annealing step of 450°C. The comparison of the thermal stability of the passivation quality on the respective substrates reveals experimentally, that contrary to the system c-Si/a-SiC x , hydrogen plays a minor role in the electrical passivation of c-Ge surfaces by a-SiC x . This phenomenon is discussed considering the different work functions and band gaps of the respective materials and different possibilities for the role of carbon in the passivating matrix are outlined.

In this paper we present an advanced upconverter system concept to reduce the sub-bandgap losses of silicon solar cells. We address the issue of the narrow absorption range of common upconverter materials. This problem can be overcome by the combination of the upconverter with a broadly absorbing fluorescent material, which emits in the absorption range of the upconverter. However, possible fluorescent materials also absorb in the emission range of the upconverter. We therefore propose an advanced system setup, which avoids unwanted absorption by separating upconverter and fluorescent material with a selectively reflective photonic structure. The incorporation of the fluorescent material in a fluorescent concentrator allows for additional geometric concentration increasing the efficiency of the upconversion process. We estimate that a total system efficiency of up to 25% could be possible with an Er based upconverting system and a silicon solar cell.