Martin Hermle

• PhD
• Head of Department at Fraunhofer Institute for Solar Energy Systems

Self‐assemble monolayers (SAMs) have become state‐of‐the‐art hole‐selective contacts for high‐efficiency perovskite‐based solar cells due to their easy processing, passivation capability, and low parasitic absorption. Nevertheless, for the deposition of SAMs with a monolayer thickness and a high packing density on metal oxide substrates, critical c...

Within this work, we present key results of the transnational European research project “Bussard”. The aim of this project is the development and evaluation of various innovative approaches for highly efficient cell concepts such as tunnel oxide passivating contact (TOPCon) solar cells considering the whole process chain including front-end, back-e...

One challenge in thin‐film based solar cells, including perovskite‐silicon tandem cells, is the defect‐free deposition of the thin‐film layers. Such defects can result in high local parasitic current losses, that is, local shunt spots. Depending on the nature of the defects, their geometrical distribution can either be microscopic, for example, ind...

Optical losses of perovskite/silicon tandem solar cells can be effectively reduced by optimizing the thin‐film layer thicknesses. Herein, the thicknesses of DC sputtered indium tin oxide (ITO) films, which serve as the front electrode and the recombination layer connecting the subcells, are optimized to reach high transparency and good lateral char...

Silicon solar cells with both-side full-area passivating hole and electron contacts are viable candidates for application as bottom cells in tandem architectures. In this contribution, cells with poly-Si based contacts at both sides are investigated as a potential upgrade to the emerging i-TOPCon single junction devices featuring a passivating cont...

Plasma-Enhanced Chemical Vapor Deposition (PECVD) is an attractive tool for TOPCon production, as it enables uniformly in situ doped amorphous silicon (a-Si) and dielectric layer depositions with high throughput. However, a lean process requires in situ interfacial oxide growth in the same tool. In this work, we use Plasma-Assisted N2O Oxidation (P...

Both-sides TOPCon solar cells are an interesting candidate for a highly efficient and thermally robust Silicon (Si) bottom cell for tandem devices, such as Perovskite-Si solar cells. However, preparation of p-type TOPCon on a textured surface is necessary which is particularly challenging. This work aims to gain a deeper understanding of the limiti...

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...

Multi-junction solar cells are the most famous approach to overcome the power conversion efficiency (PCE) limit of single-junction solar cells. Metal halide perovskite absorber materials offer low-cost fabrication and tunable...

Capitalizing on the existing silicon industry, fully textured perovskite‐silicon tandem solar cells have a great potential to penetrate the electricity market. While the use of textured silicon with large pyramid size (> 1 μm) enhances the power conversion efficiency (PCE), it also presents process complications. To achieve high performance, meticu...

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...

This article reports on the reduction of indium consumption in bifacial rear emitter n-type silicon heterojunction (SHJ) solar cells by substituting the transparent conducting oxide (TCO) indium tin oxide (ITO) with aluminum doped zinc oxide (AZO). AZO, ITO, and stacks of both TCOs are sputtered at room temperature and 170 °C on both sides of SHJ s...

Monolithic perovskite silicon tandem solar cells promise high efficiency and cost advantage. In such tandem devices a conductive front electrode of high transparency is necessary for lateral transportation of the charges. This study focuses on optimizing the electro-optical properties of DC sputtered ITO films as front electrode in monolithic two-t...

Transparent conducting oxides with high infrared transparency and high mobility are crucial for further enhancing the efficiency of crystalline silicon solar cells. Numerous hydrogen-doped or cation-doped indium oxide films have been intensively studied, while anion doping in indium oxide films is still under development. Here we investigate the fl...

Perovskite silicon tandem solar cells can overcome the efficiency limit of silicon single‐junction solar cells. In two‐terminal perovskite silicon tandem solar cells, current matching of subcells is an important requirement. Herein, a current‐matched tandem solar cell using a planar front/ rear side‐textured silicon heterojunction bottom solar cell...

This paper focusses onto the first successful realization of an industry-relevant in situ B-doped direct-plasma PECVD process for p-type TOPCon as well as the fundamental understanding of the junction’s working principle. A key element in this approach is controlling the diffusion of B across the poly-Si/SiOx/c-Si junction upon high-temperature ann...

This work investigates in detail plating of Ni/Cu/Ag contacts as an alternative metallization approach for industrial bifacial tunneling oxide and passivating contacts (i-TOPCon) silicon solar cells. We have achieved a 23.3 % champion cell efficiency on a front and rear plated bifacial TOPCon silicon solar cell on industrial precursors on a 9 busba...

This work aims for the development of an industrially feasible Si bottom cell for two-terminal perovskite/Si tandem solar cells. A tunnel oxide and poly-Si based passivating front contact (TOPCon) as a replacement of the phosphorus diffused emitter is investigated to upgrade the industrial PERC technology to tandem bottom solar cells (TOPerc). It i...

This work aims for providing a low-resistive passivating polycrystalline Si tunnel junction (poly-Si TJ) for interconnecting the sub cells in Perovskite/Si tandem solar cells. To account for the parasitic interdiffusion of dopant atoms during high-temperature processing, the incorporation of an additional diffusion blocking interlayer between the n...

This work addresses general aspects of the front side TCO and grid electrode for application in a Perovskite- Silicon tandem device by optical and electrical modelling. We highlight the need to optimize the front electrode for the tandem case, where in contrast to silicon single junction devices, there is (i) about half the current, which significa...

In this work, the intrinsic hydrogenated amorphous silicon (a-Si:H) layer of silicon heterojunction (SHJ) solar cells was modified to improve carrier transport while maintaining excellent passivation of the c-Si absorber surface. The microstructure of different multilayer intrinsic a-Si:H films was measured and its influence on contact resistance a...

Low‐cost and high‐efficiency tandem solar cells are promising candidates for a future industrial mass production. Nowadays, the passivated emitter and rear cell (PERC) technology makes up the major market share; therefore, it is an attractive option to use the PERC technology as bottom cell concept for a perovskite–silicon tandem device. Long‐term...

Multijunction solar cells (MJSCs) have attracted attention as next-generation solar cells. In particular, GaAs//Si-based MJSCs are highly efficient with low cost and are expected to gain new applications, such as on-vehicle integrations. In this article, we examined a highly efficient In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xli...

Passivating contacts based on poly‐Si/SiOx structures also known as TOPCon (tunnel oxide passivated contacts) have a great potential to improve the efficiency of crystalline silicon solar cells, resulting in more than 26% and 24% for laboratory and industrial cells, respectively. This publication gives an overview of the historical development of s...

Polysilicon contact structures with ultra‐thin atomic layer deposited (ALD) oxide and nitride interlayers based on SiOx, SiNx, AlOx, AlNx, and TiOx either as part of an interlayer stack when applied on top of a conventional thermally‐grown SiOx or as a single interlayer were investigated. ALD SiOx single interlayers provided a very good passivation...

III–V//Si multijunction solar cells offer a pathway to increase the power conversion efficiency beyond the fundamental Auger limit of silicon single‐junctions. In this work, we demonstrate how the efficiency of a two‐terminal wafer‐bonded III–V//Si triple‐junction solar cell is increased from 34.1 % to 35.9 % under an AM1.5g spectrum, by optimising...

To unlock the full potential of perovskite–silicon tandem solar cells with >30% efficiency at presumably low cost, the transparent conductive oxides (TCOs) and metal grid at the front side need to be adapted compared to classical silicon heterojunction (SHJ) solar cells. By means of optical and electrical modelling, we consider the main aspects to...

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passiva...

The performance of a low-resistive p+/n+ poly-Si tunneling junction (SiTJ) based on a tunnel oxide passivating contact in dependence on the thermal budget of the applied post-deposition treatment is studied. We present two approaches to reduce the performance limiting parasitic dopant interdiffusion and, thus, the contact resistivity, without impai...

Interfacial layers consisting of organic molecules with a permanent dipole moment exhibit enhanced charge carrier selectivity when applied as electron‐selective contacts in crystalline silicon (c‐Si) heterojunction solar cells. It is found that thermal annealing has a detrimental effect on the charge carrier selectivity of dipole materials based on...

Multijunction (MJ) solar cells achieve high efficiencies by effectively utilizing the solar spectrum. Previously, we have developed III‐V MJ solar cells using smart stack technology, a mechanical stacking technology that uses a Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//CuxIn1−yGaySe2 three‐junction solar cell by applying...

In typical industrial processing of tunnel oxide passivated contact (TOPCon) solar cells, poly‐Si is deposited on the entire back of the cells. During the deposition process, a wrap‐around of poly‐Si onto the edges and the front side of the cells is virtually unavoidable if chemical vapor deposition processes are used. Plasma‐enhanced chemical vapo...

In this work application of inkjet printing of phosphorus inks as dopant source for fabrication of silicon solar cells with tunnel oxide passivating contacts (TOPCon) was demonstrated. Inkjet printing is of great importance for the formation of selectively doped TOPCon layers of industrial TOPCon cells to provide a low contact resistance, while cre...

The photovoltaic industry is dominated by crystalline silicon solar cells. Although interdigitated back-contact cells have yielded the highest efficiency, both-sides-contacted cells are the preferred choice in industrial production due to their lower complexity. Here we show that omitting the layers at the front side that provide lateral charge car...

III–V/Si multi‐junction solar cells are potential successors to the silicon single‐junction cell due to their efficiency potential of up to 40% in the radiative limit.[1] Herein, latest results of epitaxially integrated GaInP/GaAs/Si triple‐junction cells are presented. To reduce parasitic absorption losses, which have limited the current density i...

The implied voltage iV $_{OC}$ is a popular parameter for the electronic quality of solar cell test samples. It is used to characterize properties like the passivation quality of surface coatings. While iV $_{OC}$ fundamentally depends on the sample properties besides the electronic quality of the interface, such influences have not been systemat...

Indium oxide doped with tin (ITO) is the most commonly used material for lateral transport window layers in silicon heterojunction (SHJ) solar cells, as it currently offers the best combination of physical properties, industrial deposition capability, and module reliability. However, typically applied ITO layers by far do not exploit the full elect...

Resistive losses in silicon heterojunction (SHJ) solar cells are partly linked to transport barriers at the amorphous silicon/crystalline silicon (a-Si:H/c-Si) and transparent conductive oxide (TCO)/a-Si:H interfaces. A key parameter is the position of the Fermi-level on either side of the junction which we modify by a systematic doping variation o...

Hydrogenation of poly-Si based passivating contacts (TOPCon) is an essential process to achieve a very high level of surface passivation, especially on textured surface. This contribution is dedicated to improve the understanding of the hydrogenation mechanism. We compare different hydrogen sources with regard to their ability to chemically passiva...

The aim of this work is to demonstrate the maturity of the TOPCon technology by conducting a round-robin on symmetrically processed lifetime samples in the leading European PV institutes EPFL, ISC, CEA-INES, ISFH, IMEC and Fraunhofer ISE within the H2020 funded project called HighLite. For all layers, dark saturation current-densities ranging betwe...

Perovskite–silicon tandem solar cells have shown a rapid progress within the past 5 years in terms of their research cell efficiency and are currently being investigated as candidates for the next generation of industrial PV devices. This raises the question of which silicon bottom cell will be most suitable for tandem application. Currently, the s...

Titanium oxide (TiO x ) has recently emerged as an electron-selective passivating contact for solar cell and semiconductor device applications. The mechanism behind this function has been attributed to the lower energy barrier for electrons than holes at the TiO x /semiconductor interface. Here we report an antithetic function of TiO x nanolayers (...

The present invention relates to a process for producing one or more electrical contacts on a component, comprising (a) applying one or more coatings on the component, where at least one of the coatings is a coating of an electrically conductive material, (b) applying a self-passivating metal or semiconductor and/or a dielectric material on the coa...

Perovskite-silicon tandem solar cells have shown a rapid progress within the past five years in terms of their cell efficiency in research and are currently being investigated as a candidate for next generation of industrial PV devices. This gives rise to the question which silicon bottom cell will be most suitable for tandem application. Currently...

These days low-pressure chemical vapor deposition (LPCVD) is commonly used by the photovoltaic industry to deposit Si layers for tunnel oxide passivated contact (TOPCon). This work summarizes the development of an alternative TOPCon deposition process using a tube plasma-enhanced chemical vapor deposition (PECVD) tool. In the first part, the main r...

A vital part of poly-Si passivating contacts is the ultrathin interfacial oxide layer sandwiched between heavily-doped poly-Si and c-Si absorber that will herein be investigated in detail. The stoichiometry of differently prepared interfacial oxides in the as-grown state was investigated by XPS. The findings are correlated with the thermal stabilit...

Industrial TOPCon cells commonly feature a TOPCon/SiNx stack for the passivation at the rear. In this layer stack system, the SiNx needs to function as a hydrogen source during the firing step to ensure a good passivation after firing. Herein, the composition of this SiNx layer is varied, and it is shown that a refractive index of n > 2 is desirabl...

In recent years, the incorporation of hydrogen into indium and zinc oxide based TCOs has been recognized as an effective technique to improve the charge carrier mobility and hereby to relax the transparency-conductivity tradeoff within the thin films. On the other hand, the process sequence of poly-Si/SiO $_{text{x}}$ based contacts typically requ...

The terrestrial photovoltaic market is dominated by single‐junction silicon solar cell technology. However, there is a fundamental efficiency limit at 29.4%. This is overcome by multijunction devices. Recently, a GaInP/GaAs//Si wafer‐bonded triple‐junction two‐terminal device is presented with a 33.3% (AM1.5g) efficiency. Herein, it is analyzed how...

The efficiency of photovoltaic energy conversion is a decisive factor for low-cost electricity from renewable energies. In recent years, the efficiency of crystalline silicon solar cells in mass production has increased annually by about 0.5–0.6%abs per year. In order to maintain this development speed, new technologies must be developed and transf...

Titanium oxide (TiOx) nanolayers grown by atomic layer deposition are investigated with respect to their application as carrier selective contacts for crystalline silicon (c-Si) solar cells. Although TiOx is known to act as an electron contact, in this work the selectivity of TiOx layers is found to be widely tunable from electron to hole selective...

Monolithic perovskite silicon tandem solar cells can overcome the theoretical efficiency limit of silicon solar cells. This requires an optimum bandgap, high quantum efficiency, and high stability of the perovskite. Herein, a silicon heterojunction bottom cell is combined with a perovskite top cell, with an optimum bandgap of 1.68 eV in planar p–i–...

To improve silicon heterojunction solar cells even further, minimizing transport losses within the charge carrier selective junctions and layers is mandatory. With this in mind, we present a systematic quantification of the transport losses of the electron (contact resistivity, ρ $_{\rm c}$ $\approx$ 30 mΩ·cm²) and hole (ρ $_{\rm c}$ $\approx$...

Most efficient perovskite solar cells are synthesized by the one-step spin coating method. However, when applied on µm-sized textures, e.g. for efficient monolithic perovskite silicon tandems, no conformal film formation is achieved. In this respect, we investigate hybrid evaporation/ spin coating for mixed cation mixed halide perovskite absorbers...

____________________________________________________________________________________ Postprints available at: http://publica.fraunhofer.de/authors/messmer,%20c* ____________________________________________________________________________________ Minimizing transport losses in novel solar cell concepts is often linked to improvements at the transpa...

____________________________________________________________________________________ Postprints available at: http://publica.fraunhofer.de/authors/messmer,%20c* ____________________________________________________________________________________ Passivating contacts created via a thin interfacial oxide and a highly doped polysilicon layer, e.g., th...

Perovskite silicon tandem solar cells have the potential to overcome the efficiency limit of single junction solar cells. For both, monolithic and mechanically stacked tandem devices a semi-transparent perovskite top solar cell including a transparent contact is required. Usually this contact consists of a metal oxide buffer layer and a sputtered t...

Perovskite silicon tandem solar cells are a promising technology to overcome the efficiency limit of silicon solar cells. Although highest tandem efficiencies have been reported for the inverted p‐i‐n structure, high‐efficiency single junction perovskite solar cells are mostly fabricated in the regular n‐i‐p architecture. In this work, regular n‐i‐...

Multijunction (MJ) solar cells achieve very high efficiencies by effectively utilizing the entire solar spectrum. Previously, we constructed a III‐V//Si MJ solar cell using the smart stack technology, a unique mechanical stacking technology with Pd nanoparticle array. In this study, we fabricated an InGaP/AlGaAs//Si three‐junction solar cell with a...

We present a p⁺/n⁺ poly-Si tunneling junction (TJ) based on a tunnel oxide passivated contact (TOPCon) that enables both low contact resistivity ρc and high implied open-circuit voltages iVoc. It will be shown that the charge carrier profile of the TJ and the junction resistance are strongly affected by the applied thermal budget, consisting of a f...

__________________________________________________________________________________ For more details see: www.doi.org/10.1109/JPHOTOV.2019.2957642 __________________________________________________________________________________ Passivating contacts like the tunnel-oxide passivating contact (TOPCon) technology are promising candidates for a next ge...

TOPCon is an appealing choice for next-generation solar cells as it minimizes surface recombination, enables low contact resistivities, and provides high thermal stability thereby rendering it compatible with screen-printed metallization. While TOPCon is commonly realized by low-pressure chemical vapor deposition (LPCVD), this paper discusses the u...

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.

Recently, we demonstrated an efficiency of 25.8% for a both sides contacted silicon solar cell. These cells were realized on n-type Si featuring a boron-doped p+ emitter at the front surface and a full-area tunnel oxide passivating contact (TOPCon) at the rear surface. In this work, we present a detailed electrical and optical loss analysis of this...

This work addresses the development of transparent conductive oxides (TCOs) for Si solar cells featuring SiOx/poly-Si based passivating contacts. Minimizing the thickness of the parasitically absorbing poly-Si layer is essential to reduce losses in the generation of current. However, as the poly-Si shields the critical absorber surface region durin...

The diffusion of dopants from the poly-Si layer through the interfacial oxide into the crystalline silicon wafer is studied experimentally and via numerical simulation. It is found that phosphorus piles up at the poly-Si/SiOx interface but boron readily diffuses through the oxide into the silicon and thereby forms a less shallow junction which adve...

In this work indium tin oxide (ITO) was deposited at low temperature using DC magnetron sputtering under the addition of hydrogen gas. Optical and electrical film properties were studied as a function of deposition and subsequent annealing parameters. A strong correlation between the bulk properties and the ITO film morphology could be observed. Po...

This paper discusses how differently grown ultra-thin interfacial oxide of poly-Si based passivating contacts correspond to high-temperature annealing as well as its sensitivity to hydrogenation. It will be shown on symmetrical lifetime samples that tunnel oxide passivating contacts (TOPCon) featuring thermally grown interfacial oxide layer allow a...

III–V on Si multijunction solar cells exceede the efficiency limit of Si single‐junction devices but are often challenged by expensive layer transfer techniques. Here, progress in the development of direct epitaxial growth for GaInP/GaAs/Si triple‐junction solar cells is reported. III–V absorbers with a total thickness of 4.9 μm are grown onto a Si...

The main advantages of the organic–inorganic halide perovskite solar cell technology are high efficiencies achieved after short development time in combination with rather simple solution-based processing. In this paper, we address remaining challenges by presenting a low-temperature two-step hybrid evaporation-spincoating method that combines high...

Back‐contacted back‐junction n‐type Si solar cells with locally overcompensated diffusion regions are investigated in two different designs. In the “buried emitter” design, boron‐doped (B‐doped) emitter diffusions are partially diffused and locally overcompensated by phosphor‐doped (P‐doped) back surface field (BSF) diffusions, leading to n‐type re...

Perovskite solar cells became a game changer in the field of photovoltaics by reaching power conversion efficiencies (PCE) beyond 23%. To achieve even higher efficiencies it is necessary to increase the understanding of crystallization, grain formation and layer ripening. In this study, by a systematic variation of the MAI concentrations we changed...