The EU crystalline silicon (c-Si) PV manufacturing industry has faced strong foreign competition in the last decade. To strive in this competitive environment and differentiate itself from the competition, the EU c-Si PV manufacturing industry needs to (1) focus on highly performing c-Si PV technologies, (2) include sustainability by design, and (3) develop differentiated PV module designs for a broad range of PV applications to tap into rapidly growing existing and new markets. This is precisely the aim of the 3.5 years long H2020 funded HighLite project, which started in October 2019 under the work program LC-SC3-RES-15-2019: Increase the competitiveness of the EU PV manufacturing industry. To achieve this goal, the HighLite project focuses on bringing two advanced PV module designs and the related manufacturing solutions to higher technology readiness levels (TRL). The first module design aims to combine the benefits of n-type silicon heterojunction (SHJ) cells (high efficiency and bifaciality potential, improved sustainability, rapidly growing supply chain in the EU) with the ones of shingle assembly (higher packing density, improved modularity, and excellent aesthetics). The second module design is based on the assembly of low-cost industrial interdigitated back-contact (IBC) cells cut in half or smaller, which is interesting to improve module efficiencies and increase modularity (key for application in buildings, vehicles, etc.). This contribution provides an overview of the key results achieved so far by the HighLite project partners and discusses their relevance to help raise the EU PV industries' competitiveness. We report on promising high-efficiency industrial cell results (24.1% SHJ cell with a shingle layout and 23.9% IBC cell with passivated contacts), novel approaches for high-throughput laser cutting and edge re-passivation, module designs for BAPV, BIPV, and VIPV applications passing extended testing, and first 1-year outdoor monitoring results compared with benchmark products.

Passivating contacts consisting of heavily doped polycrystalline silicon (poly-Si) and ultrathin interfacial silicon oxide (SiOx) films enable the fabrication of high-efficiency Si solar cells. The electrical properties and working mechanism of such poly-Si passivating contacts depend on the distribution of dopants at their interface with the underlying Si substrate of solar cells. Therefore, this distribution, particularly in the vicinity of pinholes in the SiOx film, is investigated in this work. Technology computer-aided design (TCAD) simulations were performed to study the diffusion of dopants, both phosphorus (P) and boron (B), from the poly-Si film into the Si substrate during the annealing process typically applied to poly-Si passivating contacts. The simulated 2D doping profiles indicate enhanced diffusion under pinholes, yielding deeper semicircular regions of increased doping compared to regions far removed from the pinholes. Such regions with locally enhanced doping were also experimentally demonstrated using high-resolution (5-10 nm/pixel) scanning spreading resistance microscopy (SSRM) for the first time. The SSRM measurements were performed on a variety of poly-Si passivating contacts, fabricated using different approaches by multiple research institutes, and the regions of doping enhancement were detected on samples where the presence of pinholes had been reported in the related literature. These findings can contribute to a better understanding, more accurate modeling, and optimization of poly-Si passivating contacts, which are increasingly being introduced in the mass production of Si solar cells.

We investigate hole-selective passivating contacts that consist of an interfacial layer of silicon oxide (SiOx) and a layer of boron-doped SiCx(p). The fabrication process of these contacts involves an annealing step at temperatures above 750 °C which crystallizes the initially amorphous layer and diffuses dopants across the interfacial oxide into the wafer to facilitate charge transport, but it can also disrupt the SiOx layer necessary for wafer-surface passivation. To investigate the transport mechanism of the charge carriers through the selective contact and its changes during the annealing process, we utilize various characterization methods, such as transmission electron microscopy, micro Raman spectroscopy, and conductive atomic force microscopy. Combining the latter with a sequential removal of material, we assemble a tomographic reconstruction of the crystallized layer that reveals the presence of preferential vertical transport channels.

We present a p-type passivating rear contact that complies with integration into standard solar cell manufacturing with phosphorus-diffused front side. Our contact structure consists of a thin SiOx tunneling layer grown by wet chemistry and a stack of layers deposited in one single run by plasma-enhanced chemical vapor deposition. The layers of the stack were tailored to protect the interfacial oxide layer, to act as a source for boron diffusion into the wafer and to connect to the external metallisation with low contact resistivity. We found that this stack tolerated annealing at 900 °C over a wide range of dwell times: for 15 min anneals we obtained dark saturation current densities (J o ) as low as 10 fA·cm -2 (after hydrogenation) and after 12-fold increase of the annealing time to 180 min, J 0 was only increased to 12 fA·cm -2 . These values corresponded to implied open circuit voltages (iV oc ) of 718 and 715 mV, respectively. To test passivating rear contacts under realistic operation conditions, we combined them with an n-type heterojunction into hybrid solar cells. With conversion efficiencies abovementioned 22% and V oc > 705 mV, these devices demonstrated high level of rear surface passivation. Finally, we demonstrated the integration of the hole selective rear contact with a POCl 3 diffusion process. To this end, we added a phosphorus diffusion barrier to our layer stack by depositing one additional layer of amorphous SiO x on top of the stack. For symmetric samples with this layer structure on both sides, we observed iVoc values of 714 and 712 mV on n- and p-type silicon wafers after hydrogenation, respectively. Co-diffused cells with POCl 3 front diffused emitter and rear passivating contact resulted so far in efficiencies of 20.4% and 20.1% for n- and p-type wafers, respectively.

A monolithic two-terminal perovskite/silicon tandem solar cell based on an industrial, high-temperature tolerant p-type crystalline silicon bottom cell with a steady-state power conversion efficiency of 25.1% is demonstrated.

A method for rapid quantitative imaging of dopant distribution using secondary ion mass spectrometry (SIMS) is described. The method is based on SIMS imaging of the cross-section of a reference sample with a known concentration profile. It is demonstrated for the case of boron quantification in silicon in a SIMS imaging mode. A nonlinear relationship between the secondary ion intensity and the concentration is observed. A detection limit of 3 (±2) × 1017 at./cm3 (~6 ppm) is determined with 39 nm pixel-size for the used experimental conditions. As an application example, a boron concentration profile in a passivating contact deposited on a textured Si surface is analyzed.

In this contribution, we present developments aiming at overcoming remaining challenges for an industrial process integration of passivating contact technologies. In particular, SiC x -based hole and electron passivating contacts activated respectively during a short and a long annealing step are developed. Implied Voc's up to 730mV have been obtained on symmetrical structures featuring SiC x (p) layers having undergone short annealing, and above 750mV for those featuring SiC x (n) submitted to long annealing. Finally, these contacts are implemented as easy add-ons in existing industrial PERC/PERT like cell architectures. First integration tests of SiC x (p) in PERC like cell featuring front phosphorous emitter have led to Voc of 686mV and cell efficiency up to 21.7% on 6inch p-type Cz. Integration of SiC x (n) in PERT like solar cells featuring a front boron emitter enabled Voc of 691mV and efficiency of 22.5% on 6 inch n-type Cz.

We present an electron selective passivating contact based on a tunneling SiOx capped with a phosphorous doped silicon carbide and prepared with a high-temperature thermal anneal. We investigate in detail the effects of the preparation conditions of the SiCx (n) (i.e., gas flow precursor and annealing temperature) on the interface recombination rate, dopant in-diffusion, and optical properties using test structures and solar cells. On test structures, our investigation reveals that the samples annealed at temperatures of 800-850 degrees C exhibit an increased surface passivation toward higher gas flow ratio (r = CH4/(SiH4 + CH4)). On textured and planar samples, we obtained best implied open-circuit voltages (i-V-OC) of 737 and 746 mV, respectively, with corresponding dark saturation current densities (J(0)) of similar to 8 and similar to 4 fA/cm(2). The SiCx (n) layers with different r values were applied on the textured front side of p-type c-Si solar cells in combination with a boron-doped SiCx(p) as rear hole selective passivating contact. Our cell results show a tradeoff between V-OC and short-circuit current density (J(SC)) dictated by the C-content in the front-side SiCx (n). On p-type wafers, best V-OC = 706 mV, FF = 80.2%, and J(SC) = 38.0 mA/cm(2) with a final conversion efficiency of 21.5% are demonstrated for 2 x 2 cm(2) screen-printed cells, with a simple and patterning-free process based on plasma depositions and one annealing step 800 degrees C < T < 850 degrees C for the formation of both passivating contacts.

Passivating contacts are indispensable for achieving high conversion efficiency in crystalline-silicon solar cells. Their realization and integration into a convenient process flow have become crucial research objectives. Here, we report an alternative passivating contact that is formed in a single post-deposition annealing step called ‘firing’, an essential step for current solar cell manufacturing. As firing is a fast (<10 s) and high-temperature (>750 °C) anneal, the required microstructural and electrical properties of the passivating contact are stringent. We demonstrate that tuning the carbon content of boron-doped silicon-based thin films inhibits firing-induced layer delamination without preventing a partial crystallization. The latter promotes charge-carrier selectivity, even in the absence of a diffused doped region beyond the oxide, by inducing hole accumulation near the wafer surface. We fabricated proof-of-concept solar cells employing the developed technology, demonstrating an open circuit voltage of 698 mV and an efficiency of 21.9%, and show how it could be a drop-in replacement for today’s rear contacts based on locally opened dielectric passivation stacks. To minimize recombination losses and therefore increase the conversion efficiency of crystalline silicon solar cells, researchers have relied on passivating contacts. Here, the authors demonstrate a hole-selective passivating contact that exploits the firing step currently employed in industrial manufacturing.

Recently, the charge carrier transport mechanism of passivating contacts, which feature an ultra-thin oxide layer, has been investigated by studying temperature-dependent current-voltage (I-V) characteristics. The measurement revealed that tunneling is the dominant transport path for tunnel oxide passivated contact (TOPCon) with wet chemically grown oxide layer. Furthermore, higher annealing temperatures led to the deterioration of the surface passivation most likely because of excessive pinhole formation. In this contribution, we are going to extend the previous study by analyzing other interfacial oxides as well. We will show that extremely low recombination current densities and low contact resistivity values can be achieved by differently processed TOPCon structures, which are characterized by a predominant tunnel transport as well as one where current flow via pinholes likely predominates. Furthermore, an I-V(T) study on solar cells with passivating rear contact reveals that fill factor transitions from a nonlinear to linear behavior when the Si layer turns partially crystalline.

We investigate the properties of an electron selective front contact based on a phosphorous doped mixed-phase SiOx/Si layer stack at device level. The addition of the SiOx phase to the Si layer targets reduced optical absorption, pursuing the goal of a broad-band transparent full-area passivating contact for front-side application. To demonstrate the validity of our approach we realised a planar hybrid solar cell with the mixed-phase SiOx/Sibased passivating contact on the front side and a hydrogenated amorphous (i/p) silicon heterojunction as rear hole-selective contact. With this structure, we obtained a V-oc of 691 mV, a Jsc of 33.9 mA/cm(2), a fill factor of 79.4% and an efficiency of 18.6% on a planar n-type FZ Si-wafer. Temperature-dependent IV-measurements at solar cell level were performed in order to understand the physical mechanisms behind charge carrier transport and surface passivation of the mixed-phase SiOx/Si layer stack. The results were compared to those of a standard silicon heterojunction (SHJ) cell on a similar planar substrate. The temperature dependence of the IV-curves in the range from-100 degrees C to +75 degrees C reveals that the hybrid cell is less temperature sensitive with respect to the SHJ cell. Furthermore, at low temperatures, the analysis reveals a reduction of the voc temperature coefficient of the hybrid cell, whereas for the SHJ cell a saturation occurs. This behaviour hints that the barrier imposed by the SiOx/Si-based contact is less pronounced than the barrier imposed by a standard SHJ contact.

We analyze the recombination properties of passivating electron selective contacts based on nanostructured silicon oxide. Our contact design is based on an interfacial buffer oxide capped with a bilayer structure of phosphorus-doped silicon oxide and silicon which is annealed at 900 degrees C. We investigate in detail the effects of the initial dopant concentration in the bilayer and of the anneal dwell time on dopant in-diffusion, contact formation, and interface recombination. Our investigation addresses also the hydrogenation of interface defects and the effect of indium-tinoxide (ITO) sputtering, allowing us to separate the interplay between enhanced field-effect passivation, Auger recombination, and interface recombination. After thermal annealing, the passivating electron selective contact presented here attains a saturation current density (J(0)) of 12.4 fA cm(-2) for medium doping, which improves further upon hydrogenation to J(0) = 8.1 fA cm(-2). For specific contact resistances <500 m Omega cm(2), however, higher doping concentrations are required. For those doping concentrations, the saturation current density is 13.9 fA cm(-2) and increases by 10% upon sputter-deposition of an ITO layer on top of the electron selective stack.

In devices with intrinsic amorphous silicon layer on a crystalline silicon substrate, the light absorbed in the amorphous layer can be weakly electronically coupled into the silicon base. Such carrier injection has previously been reported from measurements on finished devices containing stacks of intrinsic and doped amorphous silicon layers. Here, we use spectral response of photoluminescence, a contactless approach, to investigate this carrier injection on significantly simpler structures. In such devices, the effect of absorption in the front layer can be measured by the internal quantum efficiency. A highly absorbing front layer is expected to cause a drop in the quantum efficiency at short wavelengths. However, if electron-hole pairs that are generated in the front layer are subsequently injected into the base, the optical losses will be reduced, resulting in a partial recovery of the quantum efficiency at short wavelengths. Here, we quantify the efficiency of carrier injection from the intrinsic amorphous silicon front layer to the crystalline silicon base, by measuring the spectral response of photoluminescence heterojunction test structures. For devices with just an intrinsic amorphous silicon layer, the carrier injection from the layer was found to be close to unity.

In this study we present a boron-doped silicon carbide layers as a hole-selective contact which is compatible with short annealing time (typically <; 1 minute) as the one used for firing of metal pastes. The application of such layers on symmetrically processed test structures lead to implied open circuit voltages up to 715 mV and contact resistances below 75 mΩ.cm2. Proof-of-concept p-type solar cells employing such passivating contact stack over the full-rear side and a POCl3 diffused emitter metallized with firing-through of Ag-paste were processed, leading to a first conversion efficiency of 21.4%.

We present electron- and hole-selective passivating contacts based on wet-chemically grown interfacial SiOx and overlying in-situ doped silicon carbide (SiCx) deposited by plasma-enhanced chemical vapor deposition. After annealing at 850 degrees C, excellent surface passivation on the p-type planar crystalline silicon wafer is obtained for both electron- and hole-selective contacts. Their potential is demonstrated at the device level by employing a simple process flow, in which the junction formation of the two polarities is achieved with a single coannealing step. Both-side-contacted patterning-free planar p-type cells with an area of 4 cm(2) and screen-printed metallization reach a fill factor of 83.4% and a open-circuit voltage of 726 mV. Zirconium-doped indium oxide with excellent optoelectrical properties is used as a front electrode. The decrease in the parasitic absorption in the front electrode results in higher photogenerated current. By realizing front-side-textured and rear-side-planar p-type cells, an efficiency of up to 22.6% is achieved.

Intrinsic amorphous silicon provides excellent surface passivation on crystalline silicon. It has previously been shown, that carriers that are photo generated in the amorphous silicon can be efficiently electronically injected into the crystalline silicon. A method to quantify the efficiency of such carrier injection using the spectral response of photoluminescence has recently been demonstrated. This is a contactless method and it can be applied to incomplete device structures. Here we use this technique to measure partially processed heterojunction devices with different capping layers to quantify their impact on the carrier injection efficiency. Silicon nitride capping on amorphous silicon is shown to have minimum impact on the high carrier injection efficiency of the amorphous layer but doped amorphous capping layer on the other hand were seen to have a strong effect on the carrier injection efficiency. A model was developed to understand the material properties of the amorphous layer. The reduction in carrier injection efficiency with doped amorphous silicon capping layers were attributed to the large defects in the doped layer.

Tandem solar cells that feature a high-bandgap perovskite cell on top of a lower bandgap silicon cell have the potential to reach efficiencies > 30%. Here, we present a versatile hybrid deposition method that yields conformal perovskite cells directly on textured silicon bottom cells, a prerequisite to achieve highest photocurrents and hence efficiencies. Furthermore, this low-temperature evaporation/spin-coating 2-step method produces high-quality perovskite materials with different bandgaps, here varied in the range of 1.5 eV to 1.8 eV. This flexibility enables the fabrication of monolithic 2-terminal perovskite/textured Si tandems that feature high photocurrents of about 19.5 mA/cm(2).