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Passivating contacts based on SiOx/poly-Si exhibit excellent contact and surface passivation properties enabling very high solar cell conversion efficiencies. In this paper, we investigate and opti...

We present a customizable and potentially cost-efficient technique of coloring photovoltaic modules by laminating colored textiles onto photovoltaic cover glass (CoTex). A white nonwoven fabric is imprinted with any color, design, or graphic and is then laminated onto an arbitrary, frameless photovoltaic module. The short-circuit current losses of these modules range from 12% to 32% and depend on the type and coverage of the color, i.e., how much of the solar cell structure remains visible. Uncolored fabrics generate a grayish photovoltaic module with a short-circuit current density loss of 11% relative to a standard module. We fabricate CoTex building-integrated photovoltaic test modules and experimentally analyze their durability, UV stability, energy yield, hiding of the solar cells and appearance.

Degradation and regeneration of recombination parameters can occur in the bulk and at the surfaces of silicon solar cells. This article focuses on the time-resolved analysis of the recombination properties of textured 1.7 Ω cm boron-doped p -type Cz-Si and 5 Ω cm phosphorus-doped n -type Cz-Si wafers, where the surfaces are passivated by n + poly-Si on interfacial oxide layers exposed to a rapid thermal annealing (RTA) step in a conventional firing furnace. We observe a thermally activated instability in the lifetime over the entire examined injection range. Our experiments show that minority carrier injection (e.g., by illumination) is not required. Degradation in the surface passivation quality of the poly-Si on oxide layer—corresponding to an increase of the saturation current density J 0 by up to a factor of five—causes the degradation of the effective lifetime. Interestingly, the surface passivation fully regenerates under prolonged annealing and finally improves even beyond the initial state. Both the extent of the lifetime degradation and the change in J 0 depend on the postprocessing treatment temperature which we varied between 80 and 400 °C. Our results indicate that two different processes are responsible for the degradation and the regeneration. Reference samples which did not receive an RTA treatment show no degradation of the surface passivation quality. The RTA treatment applied therefore triggers the degradation effect. A large improvement of the surface passivation quality under prolonged annealing (e.g., at 400 °C) is observed for all samples examined in this study.

The setting up of a practical electrically driven light commercial demonstration vehicle with integrated photovoltaics PV is reported. The demonstrator vehicle is equipped with 15 modules based on the crystalline Si amorphous Si heterojunction technology. The nominal total peak power under standard testing conditions is 2180 amp; 8201;Wp. Specifically, the PV converted energy is fed into the high voltage HV; 400 amp; 8201;V board net for a utilization of the large capacity of the HV battery and thus for direct range extension. The demonstrator vehicle is equipped with irradiation, wind, temperature, magnetic, and global positioning system sensors. Irradiation and temperature as well as the energy flows from modules, maximum power point trackers MPPTs , low voltage buffer battery to HV battery via DC DC, and from the HV battery to the loads during an exemplarily test drive day May 31, 2021 are monitored. The range extension obtained at this day on our test route 51 59 amp; 8242; N, 9 31 amp; 8242; E was 36 amp; 8201;km, the corresponding CO2 savings account for amp; 8776;2.3 amp; 8201;kg. The chain efficiency of the electronic components from the input side of the MPPTs to the HV output side of the DC DC was 68.6 , whereas the DC DC itself has an average efficiency of 90

Advanced mathematical methods, like machine learning or genetic algorithms, have the potential to further accelerate the computer-aided optimization of processes. In this paper we combine the power of sophisticated numerical simulations with these modern concepts. The goal is to combine the strength of both approaches, high predictive quality from numerical models and fast prediction power of machine learning and genetic algorithms. We demonstrate this on a POCl 3 diffusion process and optimize an industry relevant PERC solar cell up to 23.4%. This approach is not limited to POCl 3 or PECR cells and can be applied to other cell architectures or processes.

The accurate calculation of annual insolation for a large amount of building surfaces in urban environments is a key prerequisite for a targeted increase of the installed PV capacity. In this study, we build a digital city model upon publicly available geographic data. We use this model in combination with machine learning for performing PV potential analyses on large scales and at low computational costs. We train the machine learning algorithm using ground truth values for insolation, which we determine from forward ray tracing calculations for the city of Hanover, Germany. We find that our machine learning approach is able to predict the annual insolation for all types of building surfaces independent from their tilt and orientation. The RMSE relative to the mean ground truth value is 3.2% for rooftops and 8.8% for facades. The calculation time is reduced by a factor of 20 compared to the forward ray tracing approach.

We present a novel cell concept that combines the tandem cell approach with the passivated emitter and rear cells (PERC) mainstream technology. As an interface between Si bottom and top cell, we utilize passivating n+-type polysilicon on oxide (POLO) contacts and a p+ poly-Si/n+ poly-Si tunneling junction. Our full area PERC+ Si bottom cells are fabricated within a typical industrial process sequence where the POCl3 diffusion and SiNx deposition are replaced by the POLO junction formation processes. The implied open-circuit voltage i V oc that is measured on these devices reaches up to 708 mV (684 mV) under 1 sun (under filtered spectrum to simulated top cell absorption). On sister cells with planar front side, the respective i V oc values are 718 mV (696 mV). In order to understand the device physics of our ultra-abrupt p+ poly-Si/n+ poly-Si tunneling junction, we determined the carrier lifetime in the poly-Si by time-resolved photoluminescence. The extracted lifetimes of 42–54 ps enter as input parameter for numerical Sentaurus Device simulations. These simulations reveal the importance of band-to-band and trap-assisted tunneling for a low tunneling junction resistivity of 2.95 mΩ·cm2. Experimentally, an upper limit for the combined junction resistance of the p+ poly-Si/n+ poly-Si/SiOx stack of 100 mΩ·cm2 is determined.

In the first part of this paper, we review the developments which led to the present state-of-the-art in the surface passivation of today's industrially predominant dopant-diffused crystalline silicon (c-Si) solar cells, based on dielectric layers such as silicon oxide, silicon nitride, aluminum oxide and stacks thereof. In the second part of this review, we focus on the future developments in the field of c-Si solar cells based on carrier-selective passivation layers. Whereas the dielectric layers are insulating and are hence applied only for passivating the non-contacted areas of the silicon surface, the carrier-selective passivation layers are intended to provide an effective passivation of non-contacted as well as contacted areas of a c-Si solar cell, thereby increasing the efficiency potential of c-Si solar cells significantly. Due to the fact that the carrier-selective layers are implemented in a contact, besides the good passivation properties for minorities, these layers must also provide a good majority carrier transport, i.e. they have to provide a low contact resistance. Both properties, i.e. suppression of minority-carrier recombination as well as good majority-carrier transport, define the selectivity of the carrier-selective contact, which is an important figure of merit for the assessment and comparison of different types of carrier-selective contacts. One very promising type of carrier-selective passivation layer is based on heavily doped polycrystalline silicon layers deposited on a thin silicon oxide layer, the latter providing the excellent passivation while enabling efficient majority-carrier transport via pin-holes and/or tunneling. Moreover, we discuss metal oxides and conductive polymers, which have only recently been applied to c-Si photovoltaics, but seem to have a promising potential as low-cost selective contact materials. We finally compare combinations of the various options of carrier-selective layers concerning their combined selectivities and efficiency potentials.

We present a simulation-based study for identifying promising cell structures, which integrate poly-Si on oxide junctions into industrial crystalline silicon solar cells. The simulations use best-case measured input parameters to determine efficiency potentials. We also discuss the main challenges of industrially processing these structures. We find that structures based on p-type wafers in which the phosphorus diffusion is replaced by an n-type poly-Si on oxide junction (POLO) in combination with the conventional screen-printed and fired Al contacts show a high efficiency potential. The efficiency gains in comparsion to the 23.7% efficiency simulated for the PERC reference case are 1.0% for the POLO BJ (back junction) structure and 1.8% for the POLO IBC (interdigitated back contact) structure. The POLO BJ and the POLO IBC cells can be processed with lean process flows, which are built on major steps of the PERC process such as the screen-printed Al contacts and the $$\text{Al}_\text{2 }\text{O}_\text{3 }/\text{SiN }$$ Al 2 O 3 / SiN passivation. Cell concepts with contacts using poly-Si for both polarities ($$\text{POLO}^2$$ POLO 2 -concepts) show an even higher efficiency gain potential of 1.3% for a $$\text{POLO}^2$$ POLO 2 BJ cell and 2.2% for a $$\text{POLO}^2$$ POLO 2 IBC cell in comparison to PERC. For these structures further research on poly-Si structuring and screen-printing on p-type poly-Si is necessary.

38th European Photovoltaic Solar Energy Conference and Exhibition; 154-157, Polysilicon-on-oxide (POLO) passivating contacts and interdigitated back-contact (IBC) cell technologies have recently attracted a lot of interest as candidates for the implementation in the next generation of solar cells. An IBC cell with POLO junctions for both polarities – a POLO²-IBC cell – has to electrically isolate the highly defective p+ and n+ poly-Si regions on the rear side of the cell to avoid parasitic recombination. Inserting an initially undoped, intrinsic (i) region between the p+ and n+ poly-Si regions was demonstrated to successfully prevent the parasitic recombination in the transition region of ISFH’s 26.1%- efficient POLO²-IBC cell. In order to further improve the conversion efficiency towards 27%, we apply hydrogen-donating dielectric layer stacks to the p+-(i)-n+ POLO interdigitating rear side to enhance the passivation quality of the POLO junctions. We indeed show a significant improvement of POLO junctions on symmetrical full-area homogenously doped reference samples, but when we apply a hydrogen-donating layer stack on the p+-(i)-n+ POLO interdigitating rear side, we observe a strong degradation in the performance of the POLO²-IBC cell. We attribute this to the formation of a conductive channel between the p+ and n+ poly-Si regions due to the strong negative charge density of the hydrogen-donating layer stack.

This article investigates the impact of the back-surface-field (BSF) thickness variation within a local aluminum contact on the performance of passivated emitter and rear contact solar cells. A significant difference of BSF thickness between contact endings and the center of dash-shaped contacts is verified experimentally by a comprehensive statistical analysis using scanning electron microscopy. The impact of local BSF thickness differences on the cell performance is studied with 3-D technology computer-aided design (TCAD) device simulations. Several device parameters such as BSF thicknesses, the doping concentration in the BSF profile at rear contacts, or the metallized area fraction at the cell rear side are varied. Our simulation study shows that the open-circuit voltage is mainly affected by locally reduced BSF thicknesses, resulting in an efficiency loss up to 0.14% abs or 0.84% abs , respectively, if an area fraction of 1% or 20% within a local contact has reduced BSF thicknesses. This effect can be minimized either by reducing the metallized area fraction at the cell rear side or by increasing the doping concentration in the BSF profile at aluminum rear contacts. In addition, we demonstrate that the 3-D simulations can be approximated with 2-D simulations by applying a single doping profile with an average BSF thickness, calculated with the harmonic mean.

Passivating contacts formed by poly-Si on oxide (POLO) junctions yield high passivation qualities after an appropriate annealing process at temperatures between 800°C and 1050°C. In today's typical cell process, firing is applied in the end of cell production mainly to form screen printed contacts. Thus, a high stability of the passivation quality against this firing process is required - and also expected since a previous high-temperature process for POLO junction formation implies a much higher thermal budget. However, in this work we found a significant decrease in effective lifetimes of up to 75% for n-type POLO samples with ~ 1.5 nm interfacial oxide at firing temperatures of 620°C to 900°C. This holds without a supply of hydrogen (no capping layers). Experiments with hydrogen-rich dielectric capping layers show, however, that a coating with AlO x :H as opposed to SiN y :H (n = 2.05), can significantly increase the stability of the passivation upon firing. Capacitance-voltage measurements show that the saturation current density correlates to the density of defect states at the SiO x /c-Si interface when varying the firing temperature. Although firing with hydrogen supplying layers such as AlO x :H seems to be viable, our results may indicate that the chemical configuration of the SiO x /Si interface changes from Si-O to Si-H bonds upon firing. If this hypothesis holds true, possible implications on the long-term stability of the passivation quality should be evaluated.

Unusually high carrier lifetimes are measured by photoconductance decay on n-type Czochralski-grown silicon wafers of different doping concentrations, passivated using plasma-assisted atomic-layer-deposited aluminum oxide (Al2O3) on both wafer surfaces. The measured effective lifetimes significantly exceed the intrinsic lifetime limit previously reported in the literature. Several prerequisites have to be fulfilled to allow the measurement of such high lifetimes on Al2O3-passivated n-type silicon wafers: (i) large-area wafers are required to minimize the impact of edge recombination via the Al2O3-charge-induced inversion layer, (ii) an exceptionally homogeneous Al2O3 surface passivation is required, and (iii) very thick silicon wafers are needed. Based on our lifetime measurements on n-type silicon wafers of different doping concentrations, we introduce a new parameterization of the intrinsic lifetime for n-type crystalline silicon. This new parameterization has implications concerning the maximum reachable efficiency of n-type silicon solar cells, which is larger than assumed before.

We demonstrate damage-free laser contact openings in silicon oxide layers on polycrystalline silicon on oxide (POLO) passivating contacts. A pulsed UV-laser evaporates the upper part of the polycrystalline silicon layer, lifting off the silicon oxide layer on top. On n-type POLO (and p-type POLO, respectively) samples a saturation current density of 2 fA cm−2 (6 fA cm−2) and an implied open-circuit voltage of 733 mV (727 mV) are achieved with a laser contact opening area fraction of 12.3% (8.7%). The application of this ablation process in an interdigitated back contact solar cell leads to an independently confirmed power conversion efficiency of 26.1%. The excellent contact quality of the laser contact openings is proven by the low series resistance of 0.1 Ω cm2 on the solar cell with a contact area of only 3%.

Carrier selective junctions using a poly-silicon/ silicon oxide stack on crystalline silicon feature low recombination currents J0 whilst allowing for low contact resistivity ρ C . We describe the limiting current transport mechanism as a combination of homogeneous tunneling through the interfacial silicon oxide layer and transport through pinholes where the interfacial silicon oxide layer is locally disrupted. We present an experimental method and its theoretical basis to discriminate between homogenous tunneling and local pinhole transport mechanisms on n + /n or p + /p junctions by measuring the temperature-dependent contact resistance. Theory predicts opposing trends for the temperature dependencies of tunneling and pinhole transport. This allows identifying the dominant transport path. For the contact resistance of two differently prepared poly-Si/ silicon oxide/ c-Si junctions we either find clear pinhole-type or clear tunneling-type temperature dependence. Pinhole transport contributes more than 94 % to the total current for the sample with a 2.1 nm-thick interfacial silicon oxide that we anneal at a temperature of 1050 °C to achieve highest selectivity. In contrast pinhole transport contributes less than 35 % to the total current for the sample with a 1.7 nm-thick silicon oxide that we annealed at only 700 °C in order to avoid pinholes.

We demonstrate the application of a liquid-processed doped silicon precursor as a doping source for the fabrication of interdigitated back contact solar cells. We integrate phosphorus- as well as boron-doped liquid silicon in our n-type interdigitated back contact cell process based on laser-structuring. The cell with the phosphorus back surface field from liquid silicon has an efficiency of 20.9% and the cell with the boron emitter from liquid silicon has an efficiency of 21.9%. We measure saturation current densities of 34 fA cm−2 on phosphorus-doped layers with a sheet resistance of 108 Ω/sq and 18 fA cm−2 on boron-doped layers with a sheet resistance of 140 Ω/sq using passivated test samples.

We report on an industrial large area, screen-printed, double-side contacted cell with polysilicon on oxide (POLO) junctions on both sides and an energy conversion efficiency of 22.3% ( A = 244.15 cm 2, V oc = 714 mV, FF = 81.1%, J sc = 38.5 mA/cm2, measured in-house). This cell shows an extraordinarily low series resistance below 0.05 Ω cm2. This confirms the low specific junction resistance observed recently for POLO junctions. The present cell suffers from 1) low short-circuit current due to parasitic absorption in the rather thick poly-Si (30 nm), as well as in the indium tin oxide, 2) deterioration of the recombination behavior upon sputter deposition of a transparent conductive oxide (TCO), and 3) shunts near the edge due to nonadapted TCO edge exclusion. We address all of these limitations experimentally. In particular, we developed a plasma-enhanced chemical vapor deposition process for ZnO:Al, which does not compromise the passivation of the POLO junctions underneath. An estimation of the efficiency potential (based on the two-diode model and the assumption that all these building blocks can be successfully combined on a cell level) shows that 25.3% can be achieved with this cell concept. We also look into potential cost advantages of the POLO junction scheme for this cell structure, such as the usage of p-type Cz-Si material and the omission of Ag fingers.

We introduce a method for the quantification of perimeter recombination in solar cells based on infrared lifetime measurements. We apply this method at a 25.0%-efficient interdigitated back contact (IBC) silicon solar cell with passivating contacts. The implied pseudo-efficiency determined by infrared lifetime mapping is 26.2% at an intermediate process step. The 1.2%abs loss is attributed to a process-related reduction in surface passivation quality, recombination in the perimeter area, and series resistance. The 2 × 2 cm2 -sized cell is processed on a 100 mm wafer. We determine the implied pseudo-efficiency with illuminated and with shaded perimeter area during infrared lifetime mapping. The difference between both implied pseudo-efficiencies yields the efficiency loss by perimeter recombination, which is determined to be 0.4%abs for a wafer resistivity of 1.3 Ω cm and even 0.9%abs for a wafer resistivity of 80 Ω cm.

The optimization of a solar cell requires a detailed knowledge of the efficiency limiting power losses. The free energy loss analysis (FELA) and the synergistic efficiency gain analysis (SEGA) both allow for studying these power losses. We compare both approaches for industrial passivated emitter and rear solar cells. The resistive losses calculated with both approaches are in good agreement with each other. In contrast, we find deviations of more than 20% for losses because of charge carrier recombination. From an analytical description of both methods, we conclude that these deviations are caused by a shift of the maximum power point voltage in an improved cell which is not considered in the FELA. The deviation of the losses critically depends on the specific loss channel investigated. In contrast with implied front-side power gains which are usually underestimated, rear-side or absorber gains can be over or underestimated by the FELA depending on the magnitude of the power loss. These differences follow from the Fermi level splitting which is affected differently by recombination currents at different locations. For an accurate determination of the efficiency improvement potential of individual recombination loss channels, we therefore recommend to perform a SEGA.

The maximum achievable silicon single junction solar cell efficiency is limited by intrinsic recombination and by its limited capability of absorbing sun light. For Lambertian light trapping the maximum theoretical solar cell efficiency is around 29.5%. Recently a new approach for light trapping has been proposed for silicon photovoltaics. Highly regular structures with a size in the range of the wavelengths of the incident light act as so-called photonic crystals. Such structures allow wave-interference light trapping beyond the Lambertian limit. Applying these photonic crystals to silicon solar cells can help to reduce the absorber thickness and thus to minimizing the unavoidable intrinsic recombination. From a simulation study, we can conclude that 31.6% is the maximum possible single junction solar cell efficiency for a 15 μm-thin substrate. Furthermore, we present a process flow for the preparation of regular inverted pyramid structure, that acts as photonic crystal. Finally, regular inverted pyramid structures are prepared on polished and shiny-etched, i. e. on surfaces with a certain roughness, substrates. Surface passivation of these structured surfaces shows as good lifetimes as on conventional randomly pyramid textured surface. Excellent total saturation current densities on asymmetric samples of 4 ± 2 fA/cm2 for n-type and of 4.5 ± 2.2 fA/cm2 on p-type substrates are obtained.

Polycrystalline silicon (poly-Si) layers on thin silicon oxide films have received strong research interest as they form excellent carrier selective junctions on crystalline silicon substrates after appropriate thermal processing. Recently, we presented a new method to determine the pinhole density in interfacial oxide films of poly-Si on oxide (POLO)-junctions with excellent electrical properties. The concept of magnification of nanometer-size pinholes in the interfacial oxide by selective etching of the underlying crystalline silicon is used to investigate the influence of annealing temperature on pinhole densities. Eventually, the pinholes are detected by optical microscopy and scanning electron microscopy. We present results on the pinhole density in POLO-junctions with J 0 values as low as 1.4 fA/cm 2 . The stability of this method is demonstrated by proving that no new holes are introduced to the oxide during the etching procedure for a wide range of etching times. Finally, we show the applicability to multiple oxide types and thickness values, differently doped poly-Si layers as well as several types of wafer surface morphologies. For wet chemically grown oxides, we verified the existence of pinholes with an areal density of 2×10 7 cm −2 even already after annealing at a temperature of 750 °C (lower than the optimum annealing temperature for these junctions).

Carrier-selective contact schemes, like polysilicon on oxide (POLO), provide low contact resistivities while preserving an excellent passivation quality. These junctions offer an important additional feature compared to a-Si/c-Si heterojunctions. We find that the formation of n -type POLO junctions lead to a huge increase of the Shockley-Read-Hall (SRH) lifetime of the substrate, a prerequisite for highly efficient solar cells. The SRH lifetime improvement can be observed for both bulk polarities and for a variety of bulk resistivities. Thus we suggest that the highly doped POLO junction getters impurities that have more or less symmetric SRH capture cross sections. We are able to achieve SRH lifetimes of > 50 ms. By applying POLO junctions to interdigitated back contact cells, we achieve cells with an efficiency of 25%.

We review the famous Cox & Strack equation that is commonly applied in contact resistance measurements of samples with a negligible contact resistance at the sample backside. We apply geometric interpretations to extend the Cox & Strack model a) to samples that have a non-negligible contact resistance not only on the front- but also on the back-side and b) to junctions with a buried contact resistance underneath a conductive layer. Case a) is for example a symmetric sample with a single material junction on both sample surfaces. Case b) could be a poly-Si/SiOx/c-Si or a-Si/c-Si hetero-junction. We compare our analytic treatment with rigorous finite-element simulations and find a relative agreement between 2.5 % and 36 % depending on the sample geometry and resistance values. We apply the method to analyze the contact resistance of lifetime samples with both-sided n+/n-type poly-Si junctions.