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We apply phosphorus-doped silicon nitride (SiNX:P) layers on the front surface of p-type Czochralski-grown silicon (Cz-Si) passivated emitter and rear cells (PERC). The layers are formed using industrial-type plasma-enhanced chemical vapor deposition. They provide excellent surface passivation with implied open-circuit voltages text{i}V{text{OC}} up to 695 mV and similar optical properties as conventional undoped SiNX anti-reflection layers with text{i}V{text{OC}} peaking at 685 mV. The emitter dark saturation current density at the passivated textured surface is j{rm 0e} â 40 fA/cm² for a lowly-doped emitter with sheet resistance R{text{sh}} â 160 Ω/sq. For realizing laser-doped selective emitters (LDSE), local laser processing is applied to introduce additional dopants from the SiNX:P layer into the silicon. Thereby, highly doped areas are formed in which R{text{sh}} is decreased down to 55 Ω/sq. We refer to this as the nPassDop approach, which provides both lo cal high doping and local structuring of the anti-reflection coating in a single process step. Low contact resistivity of around 1.5 mΩcm2 is measured for screen-printed and fired silver contacts on the locally laser-doped areas. A first proof of concept of large-area p-type Cz-Si PERC solar cells with front side text{SiN}{text{X}}text{:P} passivation and LDSE structure yield energy conversion efficiencies up to 20.9%.

8th World Conference on Photovoltaic Energy Conversion; 1-6, In 1997, a visionary study addressed limitations of the cost reduction potential for high efficiency solar cell devices. The results of this study triggered increased efforts for the development of production technology at Fraunhofer ISE. Many new approaches with substantial impact have been developed to the demonstration level. Economic analysis and a technology readiness terminology have been established to aid in assessing technological approaches, while the set-up of the PV technology evaluation center (PV-TEC) allowed for testing equipment, processes and materials on a higher maturity level. Reference processes building the backbone of the technology have been developed consistently further and device efficiencies follow a roughly 0.5%/a progress gradient since 2005, which is in good agreement with reports for an industrial pilot line. Today, we can rely on reference processes for PERC, TOPCon and SHJ in the PV-TEC with total and large area efficiencies of up to 22.7%, 24.0% and 24.1%, respectively. Further learning includes that for production technology R&D for PV devices high average power conversion efficiencies must be targeted and that scaling should be combined with thorough testing of products along the value chain. All cost and value determining parameters like production efficiency distribution, yield, capital and operational expenditure as well as degradation and performance under field conditions have to be monitored.

Herein, boron-doped cast-monocrystalline silicon wafers that have been fabricated using the Seed Manipulation for ARtificially controlled defect Technique (SMART mono-Si) are examined. Their suitability for passivated emitter and rear cell (PERC) fabrication is investigated. Applying a zero busbar layout energy conversion efficiencies of i = 21.9% for SMART mono-Si, i = 22.2% for gallium-doped Cz-Si (Cz-Si:Ga), and i = 22.3% for boron-doped Cz-Si (Cz-Si:B) are achieved at similar doping levels between 0.7 O cm

Current studies reveal the expectation that photovoltaic (PV) energy conversion will become the front-runner technology to stem against the extent of global warming by the middle of this century. In 2019, the passivated emitter and rear cell (PERC) design has taken over the majority of global photovoltaic solar cell production. The objective of this paper is to review the fundamental physics of the underlying cell architecture, its development over the past few decades to an industry main stream product, as well as an in-depth characterization of current cells and the future potential of the device structure. The early development of PERCs was set by an intriguing series of improvements starting in 1989 and resulting in a long-standing energy conversion efficiency record of 25.0% set up in 1999. It took a decade of intense technological development to implement this structure as an upgrade to existing production lines and another decade to increase the efficiency of industrially manufactured cells to over 22%. Our analysis of state-of-the-art large-area screen-printed PERCs is based on the pilot-line technology in the Photovoltaic Technology Evaluation Center at the Fraunhofer ISE, which is assumed to be representative of current state-of-the art cell processing. The main recent cell efficiency improvements have been achieved thanks to fine line metallization taking advantage of the high quality emitter formation and passivation and to improvements in material quality. In order to enhance the energy yield of the PV modules, innovations in interconnection technology like multibusbar and shingling technology as well as bifaciality are supported by PERC developments. Over the years, ongoing improvements have been made in the understanding of PERCs by analytical and numerical modeling of these devices. We show a study based on 3D numerical modeling and an extrapolation of the PERC device structure and technology to achieve an efficiency of 26%. This result surpasses earlier investigations due to the combination of technology components, as further improved front contact and emitter design as well as rear passivation and mirrors. We expect that PERCs can also play a strong role at the bottom of multijunction solar cells and will defend a strong position in global PV production beyond the end of the now starting decade.

37th European Photovoltaic Solar Energy Conference and Exhibition; 388-393, In this study, we investigate and demonstrate the general feasibility of a new approach for a self-aligned selective emitter process technology for passivated emitter and rear cell (PERC) technology based on inkjet printing of an UV-polymer ink in combination with plating for metallization. First, we present the major findings about the single process development of inkjet printing (minimum printed structures of 36 μm), emitter etch-back (from Rsh = 80 Ω/sq to 144 Ω/sq with an etch time of 10 s), low temperature passivation (depositions temperatures of up to 250°C), a lift-off process (at elevated temperatures of 600°C) and demonstrate that the UV-polymer ink shows sufficient chemical and thermal stability to the processes involved. Finally, the successful integration into the PERC process sequence with upscaling to an industrial wafer size M2 and a first working cell device with an efficiency = 19.68% is demonstrated. In comparison, a conventional mask&etch selective emitter approach using inkjet printing of an hotmelt ink in combination with a semi-automated enhanced alignment algorithm between inkjet and screen printing is processed in parallel showing = 21.54% and a gain of = 0.16%abs compared to PERC cell with homogeneous emitter.

We present two approaches for high‐accuracy aligning of patterning processes with each other when fabricating solar cells. We introduce the approaches on the example of two different patterning processes of which one is adjustable (laser process) and one is not adjustable (screen‐printing process). The basic idea is to measure the coordinates of the applied structures of each involved patterning process at discrete grid points with respect to a reference coordinate system. We chose the grid points such that they completely define the final cell pattern. Then, we adjust the grid point coordinates of one of the patterning processes (the laser process) according to the pattern of the other process (the screen‐printing process). The laser then performs the patterning by connecting the corrected grid points with each other in the desired direction. We perform the associated high‐precision measurement of the patterns' coordinates by using either a high‐precision offline coordinate measuring machine or a high‐resolution inline camera system with subsequent computer‐based data processing. The latter inline method enables high throughput and is, in turn, of great interest for mass production of solar cells. In this paper, we demonstrate the alignment procedure approaches on ""pPassDop"" solar cells by adjusting a locally applied laser process to the directly following screen‐printing step. This proof of principle includes both above‐mentioned methods for coordinate determination in separate cell batches. Our innovative alignment procedures so far demonstrated the successful matching of 40‐mm‐wide screen‐printed contact fingers to 70‐mm‐wide laser‐processed lines over the entire area of 6‐inch solar cells.

37th European Photovoltaic Solar Energy Conference and Exhibition; 516-520, This work reviews on our industrial-oriented passivated emitter and rear cell (PERC) baseline process for Czochralski-grown silicon (Cz-Si) wafers at the Fraunhofer ISE PV-TEC pilot-line. We perform several front side optimizations based on homogeneous emitter doping: finger width reduction of the screen-printed silver fingers, improved silver paste, and implementation of low-temperature thermal oxidation. This yields peak energy conversion efficiencies of 22.1% for boron-doped Cz-Si from LONGi and 22.2% for gallium-doped Cz-Si from Fraunhofer CSP. We show that gallium-doped Cz-Si wafers offer an industrially feasible option to further improve PERC-type but also other solar cell concepts on p-type Cz-Si. We also demonstrate the possibility to omit regeneration procedures that are needed to suppress the boron-oxygen-related light-induced degradation effects as known for conventional borondoped Cz-Si.

37th European Photovoltaic Solar Energy Conference and Exhibition; 364-369, We demonstrate tube furnace BBr3 diffusion processes for the formation of high-quality homogeneous boron emitters with industrial cycle times of around 2 hours. They feature emitter dark saturation current densities as low as 17 fA/cm² for textured surfaces at a sheet resistance of about 150 Ω/sq. In order to achieve the respective doping profiles with a maximum charge carrier concentration slightly above 1019 cm-3 and profile depths of about 800 nm, we optimize the atmospheric pressure BBr3 diffusion such that we make use of an increased maximum temperature (below 1000°C) that yields accelerated diffusion of boron atoms. In addition, careful parameter adjustment assures that the total boron doping dose in the silicon is maintained, despite the temperature increase. This optimization shows a great potential in reducing cycle times without compromising the quality of the formed boron emitters and their respective doping profiles.

37th European Photovoltaic Solar Energy Conference and Exhibition; 164-169, 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 results of the German federal project “Upgrade Si-PV” are summarized. The goal of this project was the transfer of the TOPCon process to an industrial-proven tube PECVD system. In the second part, recent progress on the implementation of this process into a TOPCon cell is presented. It is demonstrated that the PECVD system is capable of producing screen-printing compatible TOPCon layers yielding recombination currents of ≈ 3 fA/cm² and ≈ 200 fA/cm² in the passivated and metallized region, respectively. The best TOPCon cell which was realized with the PECVD process yielded the following current-voltage parameters: Voc = 700 mV, FF = 79.6%, Jsc = 41.2 mA/cm², and = 22.95%.

Achieving high output power densities pout of silicon-based PV modules requires an increase of cell efficiency as well as a reduction of cell-to-module (CTM) losses. Solar cell shingling, an approach first introduced in the 1950s, targets the reduction of CTM losses mainly by: 1) eliminating the cell spacing through the overlapping of neighbouring cells; 2) decreasing the shading losses by covering the busbar with a neighbouring cell's active area; and 3) reducing the series resistance losses at the interconnection level. This paper reports on the latest advances in passivated emitter and rear cell (PERC)-based shingled solar cell activities at Fraunhofer ISE. The approach taken is to fabricate 6" host wafers from Czochralski-grown silicon and separate them after metallization and contact firing into bifacial p-type shingled passivated edge, emitter and rear (pSPEER) solar cells. The separation is performed by laser-assisted processes: 1) laser scribing and mechanical cleaving, or 2) thermal laser separation. Since the separation process leaves the edges without the intended passivation, high edge recombination rates are expected. For that reason, a photoluminescence-based method to characterize edge recombination has been developed and verified by Quokka3 simulations. In order to further increase the pSPEER output power density pout for a cell without the intended edge passivation, a post-metallization/separation edge passivation method, i.e. Passivated Edge Technology (PET), has been developed. The implementation of PET in pSPEERPET solar cells leads to an enhanced designated area pout = 23.5mW/cm2 (considering an additional rear-side irradiance G r = 100W/m2). In the transition to shingled-module assembly, the study follows up with the cure kinetics of electrically conductive adhesives (ECAs) and mechanical-model-based methods to gain a better understanding of the joint between pSPEER cells within strings. A CTM analysis using the SmartCalc.CTM software shows a comparison of a parallel-stringing topology with a matrix topology of the cell interconnection. The reduced form factor of shingled solar cells makes them very appealing and effective for use in integrated module products, which is demonstrated by a successful automotive application, additionally profiting from the high pout attained. Drawing from the authors' expertise in customized module and surface design, a vehicle integrated PV solution with a highly aesthetic appearance is presented.

36th European Photovoltaic Solar Energy Conference and Exhibition; 304-308, 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 use of a plasma-enhanced chemical vapor deposition (PECVD) tool, which are commonly used for deposition of SiNx or AlOx. It will be shown that thick screen-printing compatible TOPCon layers providing excellent surface passivation can be realized with such tool. Additionally, the firing stability of TOPCon/SiNx stack will be discussed and first solar cell results will be presented. The IV parameters of the best solar cell were: Voc = 691.2 mV, FF = 80.7%, Jsc = 40.4 mA/cm², and = 22.5%.

35th European Photovoltaic Solar Energy Conference and Exhibition; 371-374, Using atmospheric pressure chemical vapour deposition (APCVD) of phosphosilicate glass (PSG) allows the separation of the PSG deposition and the thermal drive-in, adding another degree of freedom and control to the emitter formation. A large survey is carried out varying the process parameters such as phosphorus concentration and layer thickness of the PSG layer and the temperature and duration of the thermal drive-in in a tube furnace. We show that the data of the survey can train a metamodel, which can predict the emitter sheet resistances Rsh based on the process parameters. An emitter with Rsh = 120 /sq and an emitter dark saturation current density j0e = 40 fA/cm² after firing (textured surface, SiNX passivation) is demonstrated using this approach. For the formation of a selective emitter, laser diffusion is carried out. The laser process takes place between the PSG layer deposition and the drive-in process. A 1.4 μm deep profile with a peak concentration of 7×1019 cm-3 at the surface is fabricated using a longpulse infrared laser. This process leads to a specific contact resistivity C = 4.2 mcm² using a commercially available screen-printed and fired silver paste. Specific contact resistivity down to C = 1 mcm² is reached using a short-pulse green laser process.

35th European Photovoltaic Solar Energy Conference and Exhibition; 379-384, This study pursues the development of a laser-doped selective emitter (LDSE) for p-type silicon passivated emitter and rear solar cells with screen-printed and fired silver contacts on the front. The LDSE is formed via local laser doping from the two-layer stack system of phosphosilicate glass and silicon dioxide that is located on the wafer surface after tube furnace diffusion using phosphorus oxychloride (POCl3) as liquid dopant precursor. We aim for minimum emitter dark saturation current density at the LDSE-metal interface j0e,met and minimum specific contact resistance c. We use both an atmospheric pressure POCl3 diffusion process and a high throughput low pressure POCl3 diffusion process. Both POCl3 processes are combined with a green nanosecond laser process at wavelength = 532 nm and pulse repetition rates 60 kHz ≤ frep ≤ 100 kHz. Furthermore, we investigate high-speed infrared laser processes at = 1064 nm and frep = 2 MHz for which heat accumulation is expected to become relevant during LDSE formation. For some of the tested laser processes within this work, c ≈ 1 mΩcm2 is achieved. At the same time, simulations with the Quokka3 skin solver that are based on the LDSE doping profiles show that the LDSEs have the potential to lead to a j0e,met reduction by more than 50% compared to the not laser-doped emitter.

35th European Photovoltaic Solar Energy Conference and Exhibition; 410-413, Shingling bifacial solar cells leads to higher output power densities pout of silicon-based photovoltaic modules. This paper provides a short update on the current developments at Fraunhofer ISE in Czochralski-grown silicon bifacial “p-type shingled passivated edge, emitter, and rear (pSPEER)” solar cells. In the batch presented in this work, conventionally separated pSPEER cells are fabricated. Illuminated current-voltage measurements of the most efficient pSPEER cell results in a designated front side energy conversion efficiency of f = 21.4% for an irradiance Gf = 1000 W/m2. The pSPEER cells show similar front side efficiencies independent of the pre-separation position within the 6-inch host wafer. Considering an additional rear side irradiance of Gr = 100 W/m2, the most efficient pSPEER cell attains a total designated output power density pout = 22.8 mW/cm2.

The concept of attaching a second deposition step at the end of boron tribromide (BBr3) diffusion is introduced, where second deposition describes an active ni­tro­gen flow through the BBr3 bubbler. This approach pro­vides a high­er boron dose in the borosilicate glass (BSG) which facilitates the formation of laser‐doped se­lec­tive emitters. It is found that the second deposition hardly impacts the as‐diffused charge carrier con­cen­tra­tion profile in comparison to BBr3 dif­fu­sion without second deposition. The emitter sheet re­sis­tance Rsh ≈ 110 O sq−1 and emitter dark saturation cur­rent den­si­ty j0e ≈ 25 fA cm−2 (alkaline textured, Al2O3/SiNX passi­va­tion) are similar for both processes. The BBr3 diffusion process forms a BSG/silicon dioxide (SiO2) stack layer on the silicon. The BBr3 diffusion with second deposition step results in an 8 nm thicker BSG/SiO2 stack layer (total thickness: 42 nm) with fac­tor two higher boron dose compared to the BBr3 diffusion without second deposition. After laser doping, the charge carrier concentration is higher for the BBr3 pro­cess with second deposition resulting in stronger local doping with about 10 O sq−1 lower Rsh. For laser‐doped and Al2O3/SiNX‐passivated areas, a promising process combination results in j0e = (250 ± 30) fA cm−2 at Rsh = (65 ± 1) O sq−1.

35th European Photovoltaic Solar Energy Conference and Exhibition; 564-568, We present the investigation of metallization fraction of bifacial p-type silicon shingled passivated edge, emitter and rear (pSPEER) solar cells intended for shingled module integration. For the first group G1 (cell dimension: 23 mm x 148 mm), ten fabricated pSPEER cells, five each with a different busbar layout (single and double busbar), are utilized to measure the contact widths and calculate the metal coverage. Taking into account the shingling interconnected cell is overlapped, the reduced and therefore designated area metallization fraction is represented by the metal coverage of fingers and redundant line. The rear side metallization fraction for G1 for single and double busbar layouts based on designated area remain for both busbar types almost the same with around 25.9%. The same applies to the front side resulting in around 3.0%. For the second group G2 (cell dimension: 22 mm x 148 mm), five fabricated pSPEER cells are used. Metallization fraction for this group is lower than G1. The designated area metal coverage fraction on the rear side is 19.8%. The front side features metallization fraction of 2.8%. A complete overlap should be ensured precisely to avoid any variations in metallization fraction affecting the expected short-circuit current density values.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 631-636, Laser-doped selective emitters feature the advantage of more effective shielding of minority charge carriers from the metal contacts while allowing for low emitter saturation current density j0e in the photoactive area. The formation of emitters by diffusion processes using phosphorus oxychloride (POCl3) with incorporated in-situ oxidation gains more and more attention as it allows for low j0e below 90 fA/cm² on textured surface with silicon nitride passivation in industrial cycle times. Hence, the combination of both—POCl3 diffusion with in-situ oxidation and laser doping—is very interesting. We examine four different POCl3 diffusions with in-situ oxidation and one reference POCl3 diffusion without in-situ oxidation in terms of their suitability for selective emitter laser doping. Detailed characterizations of the grown layers on the silicon surface are performed after diffusion with respect to the individual layer thicknesses of the phosphosilicate glass (PSG) and the intermediate silicon dioxide (SiO2) layer as well as the stacks’ total phosphorus doses. The as-diffused depth-dependent charge carrier concentration profiles show that a second PSG deposition step attached after drive-in to the diffusion processes hardly impacts their course. We find that POCl3 diffusions with in-situ oxidation—especially those with second deposition step—allow for effective laser doping. Thereby, the intermediate SiO2 layer thickness plays a key role: the thicker the layer is the less phosphorus can be incorporated additionally from the PSG layer into the silicon.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 844-848, Increasing the output power density of a photovoltaic module is a reliable way of lowering electricity production costs. Besides increasing the solar cells’ conversion efficiency, a further option is lowering electrical and optical cell to module losses. The method of shingling singulated monofacial solar cell stripes is known since Dickson Jr.’s patent in 1956. First, it increases the packing density of active cell area in the module. Second, the active cell area is busbar-less reducing shading losses. Third, due to the reduced area of the solar cell stripes, the generated current per cell is less which results in a reduction of the overall series resistance of the cell interconnection within the module. We call our cell concept for this approach – which is based on the p-type silicon bifacial passivated emitter and rear cell (PERC) concept – “shingled passivated edge, emitter, and rear” – or “SPEER”. These cells are then to be interconnected by shingling the p-busbar of the first cell onto the n-busbar of the second cell, constituting the first bifacial shingled module of its kind. Each adjacent shingle covers the busbar with active cell area and minimizes spacing losses in the module. This work covers the optimization of the SPEER concept on cell level with the simulation tool Quokka3. The optimized cell provides the basis for full usage of the cell-to-module gains compared to standard modules. Key issues for optimizing the SPEER cells, which will form the module, concern the characteristics and amount of recombination at their cut edges. This directly affects the question of ideal contour-to-area ratio and thus, the width of the SPEER cells. Using latest experience from bifacial PERC cells and literature values for edge recombination as simulation input, we are able to define a region of interest between 15 mm and 25 mm stripe width for building the first SPEER prototypes. We identify the need for edge treatment, be it an emitter window or edge passivation, as crucial for the success of stripe cells against more conventional cell layouts.

The loss analysis of state-of-the-art p-type Czochralski-grown silicon passivated emitter and rear solar cells (PERC) fabricated in a manner close to industrial production is presented in this paper. The 6-inch solar cells are featuring a homogeneous emitter on the front side, an Al2O3 passivation layer and local contacts on the rear side. The peak energy conversion efficiencies obtained are 21.1% for a standard antireflection coating (ARC) and 21.4% for a double-layer ARC. The loss analysis is based on an extended characterization of the solar cells and of special samples, which allow the separation of the contributions of each region of the solar cell including metallization. Their impact is determined experimentally on the open-circuit voltage, the short-circuit current density, and the fill factor. Based on the measurements, the devices are numerically simulated. The free energy losses are analyzed using the simulation model. Based on the results from the loss analysis, it is found that the integration of a selective emitter structure and the further improvement of the rear surface would allow for efficiencies above 22% in the short term.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 418-423, We present 6-inch bifacial p-type Czochralski-grown silicon passivated emitter and rear locally-diffused (PERL) solar cells with “pPassDop” layer stack on the cell’s rear side. The “pPassDop” layer stack consists of an aluminum oxide and a boron-doped silicon nitride layer serving as both surface passivation and doping source. Local laser processing introduces boron and aluminum atoms from the “pPassDop” layer stack into the silicon. The electrical contacting of the formed line-shaped p-doped back surface field is realized by screen-printed and fired pure silver contacts (i.e. without aluminum). The fabricated PERL solar cells reach high bifaciality of up to 89%. The monofacial peak front side energy conversion efficiency, measured with contact bars on both sides on a black non-conducting chuck, is given by 19.8%. Fill factors of more than 79% and specific contact resistances in the single-digit mΩcm²- range prove the successful low-resistance contacting on both sides. It is important to stress that the same commercial state-of-the-art firing-through pure silver screen-printing paste is used for the cell’s front and rear side metallization. A special alignment procedure ensures that the rear silver grid with finger widths of about 65 μm is placed over the whole wafer on top of the about 37 μm-wide laser-doped and opened structures.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 928-931, We discuss a detailed analysis of the potential efficiency improvements of state-of-the-art industrial p-type multicrystalline silicon solar cells with passivated emitter and rear with an average energy conversion efficiency of 19.0%. The aim is to identify the limitations of cells currently fabricated in industry, not of record laboratory cells, and to demonstrate approaches for research institutes and industry to further optimise the devices using industrially applicable processes. According to the analysis, front surface reflection of the active cell area bears the largest potential for increasing the efficiency by replacing the standard acidic texture by a honeycomb texture, which is estimated to improve the efficiency from 19% to 20%. Reducing shading and recombination at the front metal is the second largest potential efficiency increase. We expect that reducing the metal finger width from 50 μm to 40 μm or below is feasible and increases the efficiency further to 20.2%.

33rd European Photovoltaic Solar Energy Conference and Exhibition; 406-412, This paper gives a close-up insight into recent and future developments that are performed with industrial focus at Fraunhofer ISE’s PV-TEC pilot-line to increase the energy conversion efficiency of 6-inch p-type Czochralski- grown silicon (Cz-Si) passivated emitter and rear cells (PERC) to 22% and above. First, the current status of PERC solar cell fabrication allowing for conversion efficiencies up to 21.5% is discussed. Then, we examine four key aspects in detail that need to be considered for optimizing the cells’ front side to boost the cell efficiency to the 22% regime. We demonstrate selective emitter laser doping out of the phosphosilicate glass layer, which is formed by a gas phase phosphorus oxychloride diffusion process. After diffusion and wet-chemical emitter etch back, the field emitter features a very low saturation current density of only 31 fA/cm² (textured, SiNx-passivated). Specific contact resistances of 1 mΩcm2 confirm the low-resistance contacting of the laser-doped surfaces using a commercially available silver screen printing paste. Apart from developing an accurate alignment procedure to match laserstructured and screen-printed layouts, we have also optimized our single-step screen-printing process for finger widths of 38 μm at 16 μm height. Based on simulations we find that efficiencies up to 22.5% are possible when the optimized process routes are integrated into PERC solar cells.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 317-322, We investigate the impact of high-temperature processes on phosphorus-doped n-type Czochralski-grown silicon (Cz-Si) wafers. Wafers from five ingots are subjected to high-temperature process sequences with different temperatures, times, and gas atmospheres. The process sequences are specifically chosen to represent different routes for the fabrication of high-efficiency n-type Cz-Si solar cells. As POCl3 diffusion is known for its effective gettering of metal impurities, it is preferably used as the last high-temperature step. However, our results show that other process sequences, e.g. with BBr3 diffusion as last high-temperature step, enable high charge carrier lifetimes in the range of a few milliseconds as well. For some materials and process sequences, ring-shaped defect structures are observed while for others increased charge carrier lifetimes by a factor up to 2.6 are found compared to their initial values prior to high temperature processing.

With this work, we introduce numeric three-dimensional simulation of metal spiking into highly boron-doped surfaces of n-type silicon solar cells, which is moreover performed with a simulation of the quasi-steady-state photoconductance technique. This setup serves as a virtual experiment to simulate the dark saturation current density j0,met of metallized boron-doped emitters with respect to metal spikes originating from the silver-aluminum (Ag-Al) contact. With the results obtained from this simulation model we approach quality and quantity of increased j0,met and give detailed insight to which degree a solar cell's performance is possibly harmed by this effect. We show that metal spikes penetrating into boron-doped emitters are of harmless nature concerning j0,met until their tips reach depths where boron doping concentration is lower than approximately 1018 cm−3. Deeper spikes then lead to an exponential increase in j0,met as more and more carriers from emitter and also the base are able to diffuse to its tip and recombine there. With the help of j0-results obtained experimentally in combination with the simulation results, we discuss the influence of spikes on emitter recombination, the benefits that can be achieved with deeper emitter doping profiles, and suggestions for the further development of pastes to contact boron-doped surfaces.

32nd European Photovoltaic Solar Energy Conference and Exhibition; 1038-1043, In this paper, seven Czochralski (Cz) and one Float Zone p-type silicon materials are processed as PERC solar cells in the Fraunhofer ISE PVTEC pilot line. In order to compare materials with different bulk resistivities, the rear contact distance is varied in order to keep the spreading resistance in the same range. Asprocessed and regenerated, energy conversion efficiencies between 20.5% and 21.0% are achieved. Within processing and measurement accuracy, all materials performed on the same level when comparing the solar cell conversion efficiencies. Implied open-circuit voltage samples revealed to be more sensitive to the material quality than the solar cells and give some more details. Due to the small differences in the results, we will not provide a ranking of the material tested in this paper. However, all the materials tested here are capable of achieving at least an efficiency of 20.7%, when applied in a PERC solar cell process.

We present 6-inch n-type Cz-Si metal wrap through (MWT) solar cells with screen-printed and fired contacts achieving energy conversion efficiencies up to 20.4%. These so-called n-type high-performance MWT (n-HIP-MWT) structures feature different rear side configurations at the external p-type contacts with respect to the phosphorus-doped back surface field (BSF). It is shown that the alteration of the phosphorus dopant concentration at the silicon surface below the external rear p-type contacts allows for both decreasing reverse-bias-induced losses and adjusting the current flowing in reverse mode.

Bifacial applications are a promising way to increase the performance of photovoltaic systems. Two silicon solar cell concepts suitable for bifacial operation are the passivated emitter, rear totally diffused (PERT) and the both sides collecting and contacted (BOSCO) cell concepts. This work investigates the bifacial potential of these concepts by means of in-depth numerical device simulation and experiment with a focus on the impact of varying material quality. It is shown that the PERT cell concept (representing a structure with front-side emitter only) requires high-minority-carrier-diffusion-length substrates with L-bulk > 3 x W (with cell thickness W) to exploit its bifacial potential, while the BOSCO cell (representing a structure with double-sided emitter) can already utilise its bifacial potential on substrates with significantly lower diffusion lengths down to L-bulk approximate to 0.5 x W. Experimentally, BOSCO cells with and without activated rear-side emitter are compared. For rear-side illumination, the activated rear-side emitter is measured to increase internal quantum efficiency at wavelengths lambda < 850 nm by up to 45%(abs) (factor of 9) and 30%(abs) (factor of 2) for cells processed on p-type multicrystalline silicon substrates with L-bulk approximate to 0.3 x W and L-bulk approximate to 2.6 x W, respectively. For PERT cells processed on n-type Czochralski-grown silicon substrates, an according increase in internal quantum efficiency for rear-side illumination of more than 20%(abs) (factor of 1.3) is measured when changing from a substrate with L-bulk approximate to 3.0 to 10.0 x W. The performed simulations and experiments demonstrate that the BOSCO cell concept is a promising candidate to successfully exploit bifacial gain also on low- to medium-diffusion-length substrates such as p-type multicrystalline silicon, while PERT cells require a high-diffusion- length substrate to utilise their bifacial potential. Furthermore, the BOSCO cell concept is shown to be a promising option to achieve highest output power densities, even when using lower quality and therefore possibly more cost-effective silicon substrates.

31st European Photovoltaic Solar Energy Conference and Exhibition; 495-501, Opposed to phosphorus n+-emitters on p-type silicon solar cells, p+-emitters on n-type silicon solar cells show altered properties with respect to e.g. dopant diffusion, electrical contacting, and metal induced recombination. This work tries to paint a consistent picture whose results base on deep-penetrating metal spikes (often also referred to as crystallites) formed below Ag-Al screen printing pastes during the contact firing process. By optimizing BBr3 diffusion processes, emitter dark saturation current densities of as low as 30 fA/cm² on alkaline textured and passivated surface are obtained. For considering the interplay between boron doping profile and spike depth regarding the specific contact resistance C, BBr3 diffusion processes are adapted yielding profiles with similar surface concentration but different junction depth. Apart from the maximum boron concentration, the junction depth of the doping profiles impacts C obtained for two examined Ag-Al pastes: deeper junctions lead to lower C. This finding is confirmed by an analytical model taking metal spikes with penetration depths of several 100 nm into account. The known issue of increased recombination below the Ag-Al metallization is addressed by a new simulation approach, which models the actual geometric shape of metal spikes in a three-dimensional (3D) simulation utilizing the quasisteady- state photoconductance (QSSPC) technique. With these 3D QSSPC simulations, we discuss the impact of metal spiking in boron emitters in relation to depth and surface coverage of the spikes, and discuss the scenario which leads to dark saturation current densities of metallized boron emitters in the range of several 1000 fA/cm².

31st European Photovoltaic Solar Energy Conference and Exhibition; 637-641, In this work, we investigate boron diffusion processes for emitter formation on the front side of n-type Cz-Si solar cells with an edge length of 156 mm. The processes are performed in an industrial tube furnace from Tempress Systems using boron tribromide (BBr3) as liquid dopant source. An initial optimization of BBr3 diffusion processes yields a significant improvement in the homogeneity in sheet resistance Rsh across the wafers and from wafer to wafer for full load runs. A standard deviation of the Rsh across the wafer in the range of 3% is achieved for a mean Rsh ≈ 70 Ω/sq. Dark saturation current density j0e = 60 fA/cm² is extracted from lifetime samples with alkaline textured surface and PECVD Al2O3/SiNx passivation after firing. A second optimization aimed at reducing emitter recombination by decreasing the maximum boron doping concentration Nmax near the surface. By adapting the post-oxidation incorporated within the BBr3 diffusion process, Nmax is decreased to 1.8·1019 cm-3 while the junction depth increases to slightly above 800 nm (Rsh ≈ 115 Ω/sq, below 4%). This results in a reduction in j0e to a value of j0e = 30 fA/cm², which corresponds to an open-circuit voltage limit of 717 mV. Despite the fairly low Nmax, low spe-cific contact resistance below 4 mΩcm² is found for screen-printed and fired contacts using commercially available silver-aluminum paste. Hence, post-oxidation is found to be a promising method for manipulating boron doping profiles while maintaining high homogeneity in Rsh.

29th European Photovoltaic Solar Energy Conference and Exhibition; 635-641, We present high-performance metal wrap through+ (n-HIP-MWT+) solar cells (239 cm² area) on n-type Czochralski-grown silicon (Cz-Si) wafers with a boron-doped front-side emitter. Peak conversion efficiencies of = 19.5 % are measured using a black chuck. Short-circuit current density jSC, open-circuit voltage VOC, and fill factor FF are 39.6 mA/cm², 650 mV, and 75.7 %, respectively. The loss in VOC, caused by front and rear side metallization, is found to be 16 mV for the n-HIP-MWT+ as well as for H-pattern cells with electrically non-contacting busbars fabricated in parallel, whereas the use of electrically contacting busbars in case of the H-pattern cells leads to a significantly higher VOC loss of 24 mV. The saturation current density underneath the front silver-aluminum (Ag-Al) contacts is determined to be j0,met,Ag-Al ≈ 3500 fA/cm², which is three times larger than j0,met,Ag found for the silver contacts on the rear side. The high j0,met,Ag-Al values are assumed to originate from deep spikes formed during Ag-Al paste contact firing. This assumption is supported by numerical simulations performed with Sentaurus Device.

29th European Photovoltaic Solar Energy Conference and Exhibition; 1342-1347, In this work, we optimize the front side of p-type Czochralski-grown silicon (Cz-Si) solar cells by investigating industrial-type phosphorus diffusion processes, and by adapting the front-side metallization. The utilization of a metallization grid with less coverage, combined with double printing, results in an increase in conversion efficiency of 0.5 %abs for p-type Cz-Si H-pattern cells with aluminium back surface field. By incorporating in-situ oxidation into the diffusion process, we realize a surface doping concentration of ≈ 2·1020 cm-3 for our improved emitter. With this diffusion process, a low emitter dark saturation current density of 85 fA/cm2 is achieved while maintaining low specific contact resistance ≤ 4 mΩcm2. The application of this emitter results in a gain in of 0.4 %abs for p-type Cz-Si high-performance metal wrap trough (HIP-MWT) solar cells in comparison to a diffusion process without in-situ oxidation. For the best performing HIP-MWT cell, reaches 20.5 %. Furthermore, we test the stability and reproducibility of our improved diffusion process in ten runs with 200 wafers each. The mean sheet resistance is found to be (85 ± 2) Ω/sq, revealing high homogeneity over full-load runs.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1043-1047, Different diffusion barrier layers applied on the boron-doped surfaces of silicon wafers with sawdamage etched surfaces are investigated with respect to their applicability in the fabrication process of n-type silicon solar cells using sequential tube furnace diffusions with a first BBr3- and a subsequent POCl3-diffusion. The barrier layers consist of silicon oxide and/or silicon nitride deposited either by plasma-enhanced chemical vapor deposition or sputter technology. The layer itself must act as barrier against phosphorus diffusion into the silicon wafer during POCl3-diffusion. Furthermore, it has to ensure that no substantial depletion of boron occurs at the wafer surface. The boron doping surface concentrations and profile depths measured after POCl3-diffusion depend on the applied diffusion barrier. If solely silicon oxide barrier layers are used, depletion of boron at the wafer surface and deeper profiles are observed, which we attribute to oxygen diffusion through the barrier and growth of a thin thermal oxide film at the silicon. With a thin silicon nitride layer incorporated into the diffusion barrier system, no significant change in the boron doping profile is detected. Numerical simulations of the boron diffusion during the POCl3- process agree well with the measurements and support these findings.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1126-1130, The improvement of the contact resistance with the modification of the dopant profile is investigated for n-type solar cells. Metallization was performed by screen printing. The dopant profile is controlled by laser doping. Different powers of a laser with a wavelength of = 355 nm and a pulse length of Laser ≈ 25 ns were tested. Laser doping was carried out on two emitters (Rsheet ≈ 70 and 90 Ω/sq.) and their corresponding borosilicate glass (BSG) precursor layers, which were grown with the emitter during tube furnace diffusion. Only a small part of the boron is diffused from the BSG layer into the silicon. This is explained by the low heat diffusion in the BSG layer, since it was found that the surface dopant concentration diffused into silicon increases after laser doping with higher laser powers. The boron profiles obtained by laser doping presents lower surface concentration but deeper dopant profiles than the emitters without laser doping. The emitter saturation current density results indicate that the recombination increases strongly for laser doping samples. Specific contact resistances were reduced half for laser doping samples in comparison to the homogeneous tube furnace diffused emitters, reaching a minimum value of 5.2±1.2 mΩ·cm2.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1099-1104, This work gives a review on the present status of p-type metal wrap through (MWT) silicon solar cell development and presents future expectations. With novel materials and approaches, many critical issues that have been discussed as potential show stoppers in the past are now overcome. The MWT technology, especially when combined with rear surface passivation, has the potential to significantly decrease cost of ownership. According to our calculations, MWT-based modules show a cost advantage of 2 % against H-pattern approaches; cell conversion efficiency is expected to reach 21 to 22 % in the medium term.

28th European Photovoltaic Solar Energy Conference and Exhibition; 1080-1084, This work investigates the integration of a wet chemical single-side emitter etching process into the process sequence of screen printed p-type Czochralski-grown silicon MWT-BSF (metal wrap through aluminum back surface field) solar cells. The MWT-BSF solar cells fabricated with single-side emitter etch back achieve a median conversion efficiency of 18.4 % (as processed), while MWT-BSF solar cells isolated by laser grooves show a slightly lower median conversion efficiency of 18.2 % (as processed). Repeated current-voltage (I-V) measurements under forward and reverse bias using stabilized MWT-BSF solar cells with single-side emitter etch back confirm that a novel via paste enables stable results under forward bias (standard operation mode), although the via paste is in direct contact with the p-type base. MWT-BSF solar cells with single-side emitter etch back show increasing current densities under reverse bias without affecting the cell performance after repeated I-V measurements, this behaviour could be used for an integrated bypass approach. The results confirm the successful integration of single-side emitter etch back into the MWT-BSF process sequence allowing for the reduction of the process sequence by one process step compared to MWT-BSF solar cells isolated by laser grooves.

28th European Photovoltaic Solar Energy Conference and Exhibition; 761-765, This paper summarizes the results of the project “MASSE”, which focused on the research and development of technologies and process sequences for mass production of 20 % efficient monocrystalline p-type silicon solar cells. This goal is addressed by the use of PERC (passivated emitter and rear cell) and MWT-PERC (metal wrap through PERC) structures. Within the frame of the project, several technologies have been investigated in detail and optimised carefully. The various technologies are modular, which allows for an application both in PERC and MWT-PERC structures. We propose the use of thermal oxidation for simultaneous high quality front and rear surface passivation. On high-quality float-zone silicon, which we use for determining the efficiency potential of these structures, the best PERC solar cells achieve a conversion efficiency of 20.0 %; reduced shading in MWTPERC type devices increases this value to 20.3 %. In both cases, screen printing forms the front and rear contacts. The implementation of dispensed silver front contacts in MWT-PERC structures increases the maximum cell efficiency even further to 20.6 %. Solder pads enable module assembly with existing equipment. By using rectangular masks for aperture area definition, conversion efficiencies of 17.7% for PERC and 18.2% for MWT-PERC modules are measured.

27th European Photovoltaic Solar Energy Conference and Exhibition; 1335-1338, A newly developed characterisation method at Fraunhofer ISE allows a precise evaluation of via contact formation by determining the via resistance for metal wrap through (MWT) solar cells. Based on a previously presented via characterisation method for test structures, the new measurement setup is able to automatically characterise via-contacts directly on a solar cell. Due to its flexible design, the setup is adjustable to any cell size and any contact arrangement. The modified four point probe measurement setup with flat electrodes on the front and spiky electrodes on the rear of the cell facilitates the measurement and precise the alignment on a each MWT-via contact. The new device is an important characterisation instrument for the evaluation and optimization of via-contacts directly on cell level.

27th European Photovoltaic Solar Energy Conference and Exhibition; 2001-2006, MWT (metal wrap through) solar cells allow higher efficiencies, while only requiring two additional process steps. One of these process steps – the contact isolation between the external n-contact on the rear and the rear p-contact is the focus of this work. The possibility of realizing this isolation by the etching paste isishape® SolarEtch® SiD instead of a laser groove, which was presented at this conference two years ago, is investigated further. Moreover, this rear contact isolation is combined with the paste driven edge isolation simultaneously which was relocated to the rear of the solar cell. In addition to the experimental approach, the two isolation technologies as well as an isolation implemented by a thermal oxide, which works as a diffusion barrier and passivates the surface at the same time, were simulated to validate the experimental results. A proof of concept is given by two MWT cell runs, which show high parallel resistances and indicated higher efficiencies for the isolation by etching paste compared to the isolation by laser grooves. These results are reinforced by the conducted simulation.

27th European Photovoltaic Solar Energy Conference and Exhibition; 590-595, Via metallization is a main challenge in fabrication of metal wrap through (MWT) solar cells. Special attention has to be given to the right choice of both metallization paste and suction process, in order to ensure a low via-related series resistance. Five out of six via pastes investigated show average via resistances of less than 4 mΩ by using an appropriate suction process. The contact formation of the via paste to the base is also of particular interest since in more advanced MWT structures no rear emitter is present, as for example the high-performance MWT (HIPMWT) approach on p-type silicon. The contact separation and minimum leakage current to the base has to be ensured by the non-contacting via paste itself. Dark current-voltage characteristics in forward and reverse bias condition show quite different behavior for each of the six via pastes tested on p- and n-type test structures. The obtained results are very promising for a transfer of the HIP-MWT concept to n-type silicon. With completely screen-printed contacts, a large-area p-type float-zone silicon HIP-MWT solar cell reaches a peak conversion efficiency of 20.3% and confirms the high potential of the HIP-MWT structure for industrial fabrication of passivated MWT solar cells.

We present metal wrap through (MWT) silicon solar cells with passivated surfaces based on a simplified device structure. This so-called HIP-MWT structure (high-performance metal wrap through) does not exhibit an emitter on the rear side and therefore simplifies processing. The confirmed peak efficiency of the fabricated solar cells with an edge length of 125 mm, screen printed contacts and solder pads is 20.2%. To our knowledge, this is the highest value reported for large-area p-type silicon solar cells to date. Schematic structure of the fabricated HIP-MWT solar cells

This paper focuses on the latest developments from research on MWT (metal wrap-through) solar cells at Fraunhofer ISE. An overview of the current cell results for mc-Si and Cz-Si material with both Al-BSF and passivated rear side is presented. Recent progress in cell technology and challenges to reaching efficiencies of 20% for industrially processed large-area MWT solar cells are also discussed. Up to recently, MWT cell efficiencies of up to 19% for Cz-Si and up to 17.5% for mc-Si have been reached with industrially feasible processing. Improvements to the design of the MWT cell to increase cell efficiency and to allow an easy module assembly are also presented in this paper, as are first calibrated IV measurements of MWT solar cells.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 1097-1101, This paper focuses on the latest developments from research on MWT (metal wrap through) solar cells at Fraunhofer ISE. An overview of the current cell results for mc-Si and Cz-Si material with both Al-BSF and passivated rear side is presented. Recent progress in cell technology and the challenges in order to reach efficiencies for pilot-line processed large area MWT solar cells towards 20% are discussed. Up to now MWT cell efficiencies over 18.5% for Cz-Si and over 17% for mc-Si are reached with pilot-line processing. Improvements of the MWT cell design in order to increase cell efficiency further and to allow an easy module assembly are shown. Furthermore first calibrated IV measurements of MWT solar cells are presented.