Martin C. Schubert

• Dr. rer. nat.
• Head of Department at Fraunhofer Institute for Solar Energy Systems

Epitaxially grown silicon wafers (EpiWafers) have a lower carbon footprint than conventional wafers produced by ingot crystallization but have also a lower initial material quality which can be significantly improved by gettering. We show that in situ gettering during the application of asymmetric n‐type and p‐type tunnel oxide passivating contacts...

Following the impressive efficiencies achieved for two‐terminal perovskite/silicon dual–junction solar cells, perovskite/perovskite/silicon triple‐junction cells have now gained attention and are rapidly developing. In a two‐terminal triple‐junction cell, maximizing the open‐circuit voltage ( V OC ) is not straightforward as it requires understandi...

A model for hydrogen in silicon is presented, which accounts for both in‐diffusion and out‐diffusion from a passivation layer (e.g., SiN x ), as well as the known hydrogen reactions within the silicon matrix. The model is used to simulate hydrogen diffusion and reactions during contact firing in a solar cell process, with a particular focus on vari...

Understanding the behavior of mobile ions in perovskite‐based solar cells (PSCs) is crucial for improving their performance and stability, which belong to the key hurdles in advancing this technology toward commercialization. This study explores the role of mobile ions in PSCs using the comprehensive technology computer‐aided design model which is...

With the surge of UV‐transparent module encapsulants in the photovoltaic industry aiming to boost quantum efficiency, modern silicon solar cells must now inherently withstand UV exposure. UV‐induced degradation (UVID) of nonencapsulated laboratory and industrial solar cells from several manufacturers is investigated. Passivated emitter rear contact...

In monolithic perovskite silicon dual‐junction solar cells, it is crucial that the subcells are current‐matched to maximize performance. The most precise method to determine the current (mis)match of a monolithic dual‐junction solar cell is a spectrometric measurement with, e.g., a light‐emitting diode (LED)‐based solar simulator. However, recoding...

In this contribution, we investigate the formation and dissociation of boron–hydrogen (BH) pairs in crystalline silicon under thermal equilibrium conditions. Our samples span doping concentrations of nearly two orders of magnitude and are passivated with a layer stack consisting of thin aluminum oxide and hydrogen-rich silicon nitride (Al 2O 3/SiN...

Luminescent coupling (LC) is a key phenomenon in monolithic tandem solar cells. This study presents a nondestructive technique to quantitatively evaluate the LC effect, addressing a gap in the existing predictions made by optical modeling. The method involves measuring the ratio of photons emitted from the high bandgap top cell that escape through...

Perovskite‐based triple‐junction solar cells have recently gained significant attention and are rapidly developing, thanks to the insights gained from the advancement in its dual‐junction counterparts. However, employing perovskite materials in multijunction solar cells with more than two junctions brings new challenges that have not yet been addre...

Aiming for highly efficient solar cells based on wafers with a low carbon footprint, silicon (Si) EpiWafers are grown epitaxially on reusable, highly doped Si substrates with a stack of porous Si layers (PorSi) for detachment. A state‐of‐the‐art p‐type Si EpiWafer exhibiting a minority charge carrier lifetime of up to 2.2 ms detected at an excess c...

Perovskite-silicon (Pero-Si) tandem solar cells have made remarkable progress in recent years, achieving certified cell efficiencies of up to 33.9%. However, accurately measuring the efficiency and current density-voltage (JV) curves of these devices poses various challenges including the presence of mobile ions within the perovskite absorber that...

Perovskite‐based triple‐junction solar cells offer the potential for highly efficient and cost‐effective photovoltaic energy conversion. This paper aims to provide a roadmap for the optical properties of perovskite/perovskite/silicon triple‐junction cells. A comprehensive opto‐electrical model for the perovskite/perovskite/silicon structure is deve...

Conventional silicon (Si) wafers are produced by energy‐intensive ingot crystallization which is responsible for a major share of a solar cell’s carbon footprint. Our work explores Si EpiWafers that are produced by direct epitaxial deposition of trichlorosilane on a re‐usable substrate. This approach requires less energy and material and hence offe...

Precise solar cell measurements become more and more challenging due to the increasing complexity of metallization patterns and the sensitivity to rear side illumination for bifacial cell concepts. In this context, the measurement conditions under which conversion efficiencies are determined need to be closely examined: Different efficiency values...

Spectrometric characterization allows for accurate determination of the current matching point and investigation of sub‐cell properties of multi‐junction solar cells. It has been widely used for dual‐junction solar cells. Although the concept has been suggested for triple‐junction solar cells, it has only been applied for the variation of two sub‐c...

As the efficiency of perovskite silicon tandem solar cells is increasing, the upscaling for industrial production is coming into focus. Spatially resolved, quantitative, fast, and reliable contactless measurement techniques are demanded for quality assurance and to pinpoint the cause of performance losses in perovskite silicon tandem solar cells. I...

Optimally enhancing the performance of perovskite silicon tandem solar cells comes with accurate identification of loss origins in the device in combination with optoelectrical device simulations assessing the respective efficiency gains to prioritize optimization pathways. Herein, various characterization methods, namely, spectrally resolved photo...

The efficiency of perovskite/silicon tandem solar cells has exceeded the previous record for III–V-based dual-junction solar cells. This shows the high potential of perovskite solar cells in multi-junction applications. Perovskite/perovskite/silicon triple-junction solar cells are now the next step to achieve efficient and low-cost multi-junction s...

As the efficiency of perovskite silicon tandem solar cells is increasing, the upscaling for industrial production is coming into focus. Spatially resolved, quantitative, fast, and reliable contactless measurement techniques are demanded for quality assurance and to pinpoint the cause of performance losses in perovskite silicon tandem solar cells. I...

The top cell of a perovskite silicon tandem solar cell requires several material layers on each side of the perovskite absorber to efficiently extract electrons and holes, respectively. These layers must meet multiple requirements simultaneously, namely, low interface recombination, good charge carrier selectivity, low contact resistivity, and high...

In current silicon solar cell technologies, hydrogen is incorporated into the solar cell during the fast-firing process. It passivates defects at the surface and in the bulk, but also leads to light- and elevated-temperatureinduced degradation (LeTID). Although it is known that the hydrogen content and the LeTID extent can be reduced by employing a...

The recombination parameter J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> _s provides an important metric to characterize surface recombination. For its calculation, numerous methods and models have to be applied. Since the models for the Auger and radiative recombination in cr...

With the remarkable advances in semiconductor processing, devices such as solar cells have fewer and fewer defects that impact their performance. Determination of the defects that currently limit the device performance, predominantly by increasing the charge carrier recombination rate, has become more challenging with standard methods like deep lev...

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measur...

Improving electrical and optical properties is important in manufacturing high‐efficiency solar cells. Previous studies focused on individual gettering and texturing methods to improve solar cell material quality and reduce reflection loss, respectively. This study presents a novel method called saw damage gettering with texturing that effectively...

Perovskite silicon tandem solar cells can overcome the efficiency limit of silicon single‐junction solar cells. In 2‐terminal perovskite silicon tandem solar cells current matching of the sub‐cells is an important requirement. In this work, we report a current‐matched tandem solar cell using a planar front/ rear side textured silicon hetero‐junctio...

Laterally inhomogeneous cell parameters in large scale perovskite/silicon (PSC/Si) tandem solar cells may significantly influence the device performance. The lateral quality of the absorber can be analysed with electroluminescence (EL), photoluminescence (PL) and thermographic methods. In addition to a global impact on cell performance, such latera...

For measuring the current-voltage (I–V) characteristics of busbarless solar cells, there is a certain degree of freedom in the choice of the contacting configuration as none has been defined as standard yet. This leads to the question of how the energy conversion efficiency of a busbarless solar cell should be specified when there are different mea...

In monolithic perovskite/silicon tandem solar cells, it is important to know which sub‐cells is limiting the overall current to adapt the perovskite absorber thickness and bandgap accordingly. The current matching situation is usually analyzed by integrating measured external quantum efficiencies. However, this method can lead to significant errors...

For upscaling silicon based tandem solar cells from small laboratory sizes to full size formats compatible with industrial production, two-dimensional (2-D) and 3-D effects like metal grid layout, perimeter design, and lateral inhomogeneities gain importance for tandem cell development. For understanding and quantifying such effects, 3-D tandem mod...

Passivated contact cell architectures have the potential for higher efficiencies than the currently dominant PERC technology. Further development requires greater understanding of the passivation mechanism and potential surface related degradation, especially at polysilicon-oxide-crystalline silicon contacts. In particular, the hydrogenation provid...

Combining the advantages of a high‐efficiency solar cell concept and a low carbon footprint base material is a promising approach for highly efficient, sustainable, and cost‐effective solar cells. In this work, we investigate the suitability of epitaxially grown p‐type silicon wafers for solar cells with tunnel oxide passivating contact rear emitte...

We investigate possible correlations between the temporal evolution of Light and elevated Temperature Induced Degradation (LeTID) and of boron-hydrogen pairs. For this, samples were treated under dark annealing conditions at 160°C and the formation and annihilation of the LeTID defect and the formation of BH-pairs were studied simultaneously. Simul...

One drawback of passivating contacts in crystalline silicon solar cells is the current loss due to parasitic absorption within the involved material layers. When employed on an illuminated side of the cell, the full spectrum of the incident light will be partly absorbed before reaching the silicon bulk. Additionally, near-infrared (NIR) absorption...

Gallium-doped silicon material has been rapidly gaining importance in the photovoltaic industry as a boron-oxygen defect-free material with promising minority carrier lifetime. We investigate the influence of different cell process flows [passivated emitter and rear cell tunneling-oxide-passivating contact, and a “hot oxidation” process] on the bul...

In order to push silicon solar cell efficiencies further towards their limit, as well as to ensure accuracy of luminescence based characterization techniques, an accurate modeling of radiative recombination is important. It is well-known that the radiative recombination coefficient Brad of silicon shows a substantial charge carrier density dependen...

Recent progress in surface passivation technology and wafer pretreatment already resulted in significant improvements in the achievable minority charge carrier lifetime of crystalline silicon. This work further exemplifies this by studying the lifetime on lowly doped crystalline silicon wafers passivated by poly-Si. To ensure credible lifetime meas...

By investigating the formation of the light‐ and elevated temperature‐induced degradation (LeTID) defect under dark annealing conditions alongside the formation of the boron‐hydrogen‐complex, we have found both formations being limited by presumably the same reaction. Starting with this observation, two possible mechanisms of LeTID defect formation...

Czochralski‐grown gallium doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear (PERC) solar cells and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski‐grown wafers in dependence of resistivity, finding effect...

Commercial silicon is prone to form silicon oxide precipitates during high-temperature treatments typical for solar cell production. Oxide precipitates can cause severe efficiency degradation in solar cells. We have developed a model describing the nucleation and growth of oxide precipitates that considers silicon self-interstitial defects and surf...

The interest in cast mono silicon is increasing due to its lower energy consumption and resulting smaller carbon footprint, lower oxygen content and resulting less oxygen-related defects as well as easy scalability to large wafer formats like 210 × 210 mm² full square. As a cast silicon alternative to high performance multicrystalline (hpm) silicon...

Czochralski-grown gallium-doped silicon wafers are now a mainstream substrate for commercial passivated emitter and rear cell (PERC) devices and allow retention of established processes while offering enhanced cell stability. We have assessed the carrier lifetime potential of such Czochralski-grown wafers in dependence of resistivity, finding effec...

The effect of light- and elevated temperature-induced degradation (LeTID) can be nonpermanently reversed by charge carrier injection below the degradation temperature (commonly used degradation temperatures are above ~70 °C). In this study, we show that the rate of temporary recovery depends strongly on the excess carrier density. We observe that t...

As the electrical characteristics of silicon solar cells depend significantly on their temperature T of operation, it is vital to analyze and understand the contributions of the various cell properties in detail in order to optimize silicon solar cells for improved energy output in realistic operation conditions. Within a detailed electro-optical s...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

This review explores the current state of the art in spatially resolved characterization of mixed‐halide perovskite solar cells. As the size of perovskite cells and modules continues to grow, quantification of the spatial distribution of key cell parameters will become increasingly valuable in predicting ultimate cell‐level performance and tracking...

The silicon surface texture significantly affects the current density and efficiency of perovskite/silicon tandem solar cells. However, only a few studies have explored fabricating perovskite on textured silicon and the effect of texture on perovskite films because of the limitations of solution processes. Here we produce conformal perovskite on te...

In this paper we present a contactless transient carrier spectroscopy and imaging technique for traps in silicon. At each pixel, we fit the transient decay of the trap emission which allows us to obtain both the trap time constant and trap concentration. Here we show that this technique allows for high-resolution images. Furthermore, this technique...

The poster shows investigations of the lifetime at the highest level. We reached a lifetime of 135 ms on p-type material and a lifetime of 1/3 s on n-type material. We determine the J0 for the surface as well as the limitation of the lifetime, possibly by SRH recombination.

Hybrid metal halide perovskite‐based thin‐film photovoltaics have the potential to become the next generation of commercialized PV technology with certified power conversion efficiencies reaching 24 % on 0.1 cm2 area devices. Recent efforts in upscaling of this technology resulted in an efficiency of 12.6 % for 354 cm2 modules. Still, upscaling los...

In this work the existing SRH parametrizations for the FeGa defect are re-evaluated by a deliberately iron contaminated sample set of varied doping densities. The evolution of the cross-over point is analyzed for this aim, due to its characteristic dependency on the defect parameters of the metastable iron states. It can give insight into the defec...

In this work we present results of a series of experiments to investigate the origin of the defects causing light and elevated temperature induced degradation (LeTID). It has been demonstrated that LeTID effects can be observed even in high purity monocrystalline silicon. The experiments are therefore performed on float-zone silicon and feature a v...

Edge losses in silicon solar cells are becoming more important in current photovoltaic research, especially in shingled cell modules with high perimeter to area ratios. Hence, in this study a new approach is presented to quantify edge recombination losses by using photoluminescence (PL) measurements combined with device modelling. The main focus of...

To understand and maximize the annual energy output of silicon based photovoltaic modules beyond standard testing conditions (STC) it is of utmost importance to get insight into the temperature dependent device properties of silicon solar cells. In this study we demonstrate an approach which allows for both a global and a local characterization of...

To maximize the annual energy output of silicon based photovoltaic modules beyond standard testing conditions (STC), it is important to assess temperature dependent device properties of silicon solar cells. In this study we characterize the temperature dependence of the dark saturation current density J 0 of diffused regions (e.g. emitter layers)....

Multicrystalline solar cells made from n‐type silicon feedstock have shown record efficiencies of 22.3% in a TOPCon cell structure. Still, material related carrier recombination limits the attainable efficiency. In this article, we summarize findings of metallic impurity and structural defect concentration present in n‐type mc silicon and elaborate...

In this study, we present a method to predict the local temperature-dependent performance of silicon solar cells from wafer lifetime images, which enables local investigation of silicon solar cell parameters under realistic operation conditions. The multicrystalline silicon wafers investigated underwent high-temperature steps equivalent to emitter...

Understanding the origin of thermomechanical stress in solar cells is a key factor to extend the lifetime of photovoltaic modules. However, the methods to determine the stress are very limited. With the confocal micro-Raman spectroscopy, we present a contactless method, which is able to measure through the front glass and is well-known in the field...

In this work, the established method of iron imaging is transferred from B‐doped silicon to Ga‐doped material. For this purpose, the pairing and splitting conditions are investigated and a preparation procedure suggested that ensures a sufficient fraction of iron–gallium pairing and splitting, respectively. Furthermore the defect parameters availab...

In order to systematically improve perovskite-based solar cell technologies, it is crucial to identify performance limits and determine both global and local loss mechanisms quantitatively. One of the most important steps toward competitiveness is the upscaling of perovskite solar cells, which can be achieved, e.g., via solution-based blade coating...

We present a novel method to analyze the local current-voltage I-V characteristics of both solar cells and cell precursors. The method, which we call “Suns-ILIT”, is based on illuminated lock-in thermography (ILIT) measurements and photoluminescence imaging (PLI) in open-circuit conditions with varying illumination intensities. Compared to conventi...

In this study, we present a method to predict the local temperature dependent performance of silicon solar cells from wafer lifetime images which enables local investigation of silicon solar cell parameters under realistic operation conditions. The multicrystalline silicon wafers investigated underwent high temperature steps equivalent to emitter d...

Correlations between defect-related luminescence (DRL) and recombination mechanisms of multicrystalline silicon wafers are investigated by hyperspectral photoluminescence (PL) imaging at cryogenic temperatures (∼80 K) and by PL-based techniques for charge carrier lifetime at room temperature. This unique combination of measurement techniques is use...

The main goal of a solar cell's contact is to simultaneously minimize the contact resistivity ρ $_\text{cont}$ and the effective recombination, the latter often expressed via $J_\text{0, cont}$ . To model the resulting solar cell characteristics, the simplest approach is to use those two measurable quantities as an effective boundary condition. W...

Photoluminescence imaging is a fast and powerful technique to extract information about the nonuniformity of the recombination properties of silicon wafers and solar cells. In conventional photoluminescence imaging, where a uniform illumination is used, lateral carrier flows within the sample lead to smeared and quantitatively inaccurate results. I...

N-type multicrystalline silicon (mc-Si) is a promising alternative to the dominant p-type mc-Si for solar cells because it combines the cost advantages of mc-Si while benefiting from higher tolerance to transition metal contamination. A detailed understanding of the relative roles of point defect and precipitated transition metals has enabled advan...

We demonstrate a novel method to measure the lateral distribution of the recombination current without an external load, hence without the need to establish an electrical contact. In combination with photoluminescence imaging, which can be used to calculate the lateral implied open-circuit voltage iVOC distribution, accurate local current-voltage I...

GLOBAL cell efficiencies are measured at standardized testing conditions (STC) to be able to compare technology improvements worldwide in a systematic manner. However, these STC are rarely met in real operation conditions of solar cells as especially temperature and irradiation intensity differ a lot to laboratory standards. In this study we presen...

Silicon is the most studied semiconductor, having almost every aspect of it being investigated. All this information is spread over a large set of publications, review articles and textbooks and cannot be found in a single location. Furthermore, the available data is not always consistent and depends on the techniques and samples used. This problem...

We report on the formation of wrinkle-patterned surface morphologies in cesium formamidinium based CsxFA1-xPb(I1-yBry)3 perovskite compositions with x = 0–0.3 and y = 0–0.3 under various spin-coating conditions. By varying the Cs and Br content, perovskite precursor solution concentration, and spin-coating procedure, the occurrence and characterist...

A central quantity to assess the high quality of monocrystalline silicon (on scales beyond mere purity) is the minority charge carrier lifetime. We demonstrate that the lifetime in high purity float zone material can be improved beyond existing observations, thanks to a deeper understanding of grown-in defects and how they can be permanently annihi...

Models for the calculation of losses in PV systems are widely applied but typically focus on single components (i.e. the solar cell). We discuss relevant models and combinations thereof to analyze losses from wafer to system. We propose a holistic approach to analyze losses from laboratory to environmental conditions. The proposed approach focusses...

In this work, we show the latest progress in the confocal micro-Raman spectroscopy for determination of stress within embedded solar cells. We present measurements proving that the module front glass has no influence on the stress measurement. We also present the first large area stress mapping on a quarter solar cell within a single-cell laminate....

In this contribution, we present our recent results for high efficiency multicrystalline silicon solar cells. Based on n-type high-performance multicrystalline silicon substrates in combination with the TOPCon solar cell concept featuring a full area passivating back contact and a boron-diffused emitter as well as a plasma-etched black-silicon text...

An approach that determines the charge carrier lifetime from photoluminescence (PL) imaging that is virtually not affected by lateral charge carrier drift and diffusion and image smearing due to photon scattering is proposed. The approach attempts to create a laterally uniform charge carrier density within a sample with non‐homogeneous recombinatio...

A new approach to model edge recombination in silicon solar cells is presented. The model accounts for recombination both at the edge of the quasi-neutral bulk as well as at an exposed space-charge-region (SCR), the latter via an edge-length-specific diode property with an ideality factor of 2: a localized J $_{02}$ , $_{edge}$ . The model is imple...

In the following sections some practical aspects of using lock-in thermography in the functional diagnostics of electronic components will be discussed and illustrated by measurement examples of a typical, thermally thin sample (solar cell) and a thermally thick one (integrated circuit). All these discussions are based on the theoretical findings p...

In Sect. 3.1, a number of thermography and lock-in thermography approaches from literature are described and discussed, both steady-state and non-steady-state, showing the large variety of thermography measurement possibilities. Many of these systems are not called thermography but rather thermo-AFM, or photothermal or thermo-elastic investigations...

Some applications of Lock-in Thermography for the investigation of a thermally thin sample (solar cell) and a thermally thick one (IC) were already presented in the examples given in Chap. 5. In the following section we will present some more applications, showing the universal applicability of this technique to different fields of functional diagn...

In this book, the technique of lock-in thermography is being reviewed with special emphasis on its application to the characterization and functional testing of electronic components. The investigation of shunting phenomena in solar cells, which our lock-in thermography originally was developed for, among a lot of other applications is presented to...

First, in Sect. 2.1 the general basics of infrared (IR) thermography are briefly reviewed, which are also applicable to IR camera based lock-in thermography. In Sect. 2.2, the principles of the lock-in technique itself and of its digital realization are described.