Search

This work mainly focuses on modeling the impact of bulk resistivity $ \rho_b $ on the bifacial n -type passivated emitter rear and totally diffused rear-junction ( n -PERT-RJ) solar cells using Quokka3. Through simulation, under the front illumination, the light-generated excess carrier flow behavior in the bulk is studied. The regulatory function of the rear-junction solar cells’ bulk on the electron flows distributary is discussed. Finally, how the different $ \rho_b $ values affect the hole-current flow behavior and the FSF performance is revealed. This explains very well the reasons for decreasing short-circuit current J SC because of the low $ \rho_b $ wafers in n -PERT-RJ solar cells. The studies also provide insights on the lowest value of $ \rho_b $ that can be applied to n -PERT-RJ solar cells without the efficiency deterioration. We also simulated the n -PERT-RJ solar cell performance and bifaciality when applying different $ \rho_b $ values under rear illumination. Last but not the least, investigation on how $ \rho_b $ affects the output performance will be extended to when n- PERT-RJ solar cells are served as the bottom subcell in a four-terminal tandem configuration.

We present laser-doped interdigitated back contact (IBC) solar cells with efficiencies of 23% on an area of 244 cm2 metallized by a screen-printed silver paste. Local laser doping is especially suited for processing IBC cells where a multitude of pn-junctions and base contacts lay side by side. The one-sided deposition of boron-doped precursor layers by atmospheric pressure chemical vapor deposition (APCVD) is a cost-effective method for the production of IBC cells without masking processes. The properties of the laser-doped silicon strongly depend on the precursor’s purity, thickness, and the total amount of boron dopants. Variations of the precursor in terms of thickness and boron content, and of the laser pulse energy density, can help to tailor the doping and sheet resistance. With saturation-current densities of 70 fA/cm2 at sheet resistances of 60 Ohm/sq, we reached maximum efficiencies of 23% with a relatively simple, industrial process for bifacial IBC-cells, with 70% bifaciality measured on the module level. The APCVD-layers were deposited with an inline lab-type system and a metal transport belt and, therefore, may have been slightly contaminated, limiting the efficiencies when compared to thermal-diffused boron doping. The use of an industrial APCVD system with a quartz glass transport system would achieve even higher efficiencies.

As compared with the traditional back surface field (BSF) solar cells, the n -type passivated emitter rear and totally diffused rear-junction ( n -PERT-RJ) solar cells have much lower carrier recombination losses on their rear sides. In order to obtain more benefits (higher open-circuit voltages) from the currently passivated rear sides, the implementation of the point contact concept is an intuitive solution. In this article, we present industrial screen-printed point contacts that are metallized with aluminum (Al) pastes containing silicon additives (Si-add). To understand the impacts of the Si-add on the performance our n -PERT-RJ solar cells, the detailed loss analysis on the open-circuit voltage ( $V_{OC}$ ), short-circuit current density ( $J_{SC}$ ), and fill factor are performed separately. The effect of Si-add and pitch of laser contact openings (LCO) on the point contact characteristics are also investigated. In the last part, we introduce an innovative “point-line” contact concept on the rear side, which is composed of dot-shaped LCO and Al metal finger design, resulting in an n -PERT-RJ solar cell to reach $V_{OC}$ of 692 mV and peak efficiency of 22%. The 60% bifaciality of the device opens the way for an additional power output on module level, which further reduces the levelized cost of electricity. These features make the rear-side point contact design more flexible to be applied in industrial mass production.

Stable high voltages in solar cells can be transferred directly to modules with no losses and are becoming increasingly important. Solar cells with high voltages have lower temperature coefficients and therefore higher energy yield in PV systems, particularly for those exposed to high temperatures (e.g., in desert regions). This paper presents the optimization of a large area n -type passivated emitter rear and totally diffused rear-junction ( n -PERT-RJ) solar cell to reach open-circuit voltages exceeding 690 mV and 22% stable solar cell efficiencies by using industrial diffusions and screen-printed metallization on both sides. With a free energy loss analysis and Quokka3 simulations, the recombination and resistive loss mechanisms in the n -PERT-RJ solar cells are identified and a guideline for future development is given, showing a practical roadmap toward 700 mV open-circuit voltages and solar cell efficiencies approaching 23%.

The ZEBRA technology - since several years under development at ISC Konstanz - is a bifacial IBC solar cell concept based on a standard industrial low cost manufacturing process still without passivating contacts. In this work, we present results of the industrial implementation of the ZEBRA cell technology in a 200 MW p /year production line in Xining (China) in a collaboration between HHSD/SPIC Xi'an Solar Power and ISC Konstanz. In particular, we report on processes for this advanced bifacial IBC technology and on first results achieved in industrial production at SPIC. At the current stage of ramp-up of the industrial cell line, average energy conversion efficiencies of 23.6% and open circuit voltages above 700 mV have been achieved. These values are reached using a simple cell process, based on standard LP-BBr 3 - and POCl 3 -tube furnace diffusion steps and on a screen printing metallization process that is compatible with any commercial format (e.g. also M6, M8 and M12) of n-type Cz-Si wafers. Modules have been fabricated in SPIC's industrial manufacturing line by soldering of ribbons on half-cut ZEBRA-cells. Thereby a P mpp of around 360 W p has been achieved using a module design based on 120 G1 half-cells corresponding to an efficiency of around 21.2%.

Bifacial photovoltaic (PV) technology has received considerable attention in recent years due to the potential to achieve higher annual energy yield compared to its monofacial counterpart. Higher annual energy yield is a crucial factor (even more than to further reduce the module costs) because, with the use of higher power PV modules, the high BOS (Balance of System) costs can be reduced, resulting in the lowest LCOE (levelised cost of energy). The International Technology Roadmap for Photovoltaic (ITRPV) predicts an upward trend for the shares of crystalline silicon (c-Si) bifacial PV cells and modules in the global PV market in the next decade, i.e., more than 35% in 2028. Two key enabling factors have been identified to promote the widespread use of c-Si bifacial PV devices, namely the bifacial PV performance measurement method/standard for indoor characterisation and comprehensive simulation models for outdoor performance characterisation. Both will increase the bankability of bifacial PV technology. In this paper, a comprehensive review of the state-of-the-art of the c-Si bifacial PV performance characterisation and simulation is presented. First, an overview of the indoor characterisation of c-Si bifacial PV cells and modules is presented, followed by an overview of the outdoor characterisation of c-Si bifacial PV modules and the draft technical specification, IEC TS 60904-1-2. The second part of this paper reviews the current status of bifacial PV performance modelling, which includes the three primary sub-models: optical, electrical, and thermal models. This paper also provides an overview of the required future research to address the challenges associated with the characterisation and simulation of c-Si bifacial PV devices.

We present strong arguments and experimental findings for the definition of a photovoltaic module accelerated ageing procedure for high irradiance and desert conditions to create a Desert Label for the particular Atacama Desert climatic conditions. A special focus on high ultraviolet solar radiation dosage in combination with either damp heat, salt mist or aggressive thermal cycling are presented. Indoor experimental results, strengthened by outdoor monitoring of sister mini-modules at the Atacama Desert, allowed selecting the appropriate ageing tests while exploring the reliability and durability of a wide range of module configurations, including various typical encapsulants, high efficiency cell technologies and module designs, for industrial applications.

Low photovoltaic module costs imply that increasing the energy yield per module area is now a priority. We argue that modules harvesting sunlight from both sides will strongly penetrate the market but that more field data, better simulation tools and international measurement standards are needed to overcome perceived investment risks.

An in situ oxidation during the boron tribromide diffusion process to form $\text{p}^{+}$ -doped junctions on crystalline Si solar cells leads to the formation of a layer stack system consisting of a borosilicate glass (BSG) (a binary B $_{2}$ O $_{3}$ –SiO $_{2}$ system) and a SiO $_{2}$ layer at the Si interface. We present a method to passivate the $\text{p}^{+}$ -doped regions by using this in situ grown SiO $_{2}$ in combination with a plasma-enhanced-chemical-vapor-deposited SiN $_{x}$ layer. We show that the etching rate of the BSG layer, in a hydrofluoric acid solution, varies over the wafer. The etching rate depends on its local B $_{2}$ O $_{3}$ content in the BSG and is markedly higher than that of the SiO $_{2}$ layer. This difference in the etching rates can be used to controllably etch back the BSG layer in order to obtain a thin and uniform passivating oxide layer for solar cell application. Using this oxide/SiN $_{x}$ stack, we obtained implied $V_{\text{oc}}$ of 705 mV and $J_{0e}$ as low as 14 fA/cm $^{2}$ on symmetrically diffused boron emitters on $\text{n}$ -type Czochralski wafers. These passivation results are comparable, on similar structure and boron emitters, with state-of-the-art Al $_{2}$ O $_{3}$ -based passivation methods. Moreover, we have successfully implemented this passivation method in the fabrication of n-type passivated emitter and rear totally diffused and interdigitated back contact solar cells, avoiding the need of adopting additional process steps and costs for boron emitter passivation.

Large-area (251.96 cm2) bifacial interdigitated-back-contact (IBC) solar cells are presented in this work. We employ front floating emitter (FFE) to replace the front surface field (FSF) to simplify the process sequences. A simplified process flow is exploited to fabricate the IBC solar cells through industrial equipment and compatible processes. Double side boron diffusion followed by etch-back to form the emitter and lightly doped front surface, reducing high-temperature influence on the bulk lifetime and fabrication complexity. Ion implantation and anneal process is applied for base doping. The cell conversion efficiency reaches 22.92%, independently certificated by Fraunhofer Institute for Solar Energy System CalLab (Fraunhofer ISE). The IBC solar cells feature an open metallization grid, which offers a bifacial response with the bifaciality reaches to 72%. Only one-step mask and opening procedure was used, which greatly simplified the process. These results demonstrate the feasibility of this simplified process for manufacturing low-cost high-efficiency of IBC cells. Key parameters such as surface recombination J0, metal contact recombination J0-metal, contact resistivity of the cells are extracted by specially designed structures. Loss analysis shows that further efficiency improvement can be attained though reducing the contact resistance and metal contact recombination. The results in this work indicates the potential of this novel process for producing low-cost high-efficient IBC solar cells.

Surface recombination of minority charge carriers is a significant loss mechanism in crystalline-Si (c-Si) solar cells. For n + (phosphorous) doped regions a sufficient surface passivation is achieved by using a silicon nitride (SiN X ) film deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). However, on p + boron doped regions, SiN X does not passivate the surface. Instead, a stack of layers comprising of a thin silicon dioxide (SiO 2 ), or aluminium oxide (Al 2 O 3 ), and a SiN X film is commonly used to passivate boron doped regions. In this study we investigated the passivation quality of boron doped emitters by varying the composition of SiO 2 /SiN X stack layers. For this purpose, n-PERT (passivated emitter, rear totally-diffused) solar cells with boron doped front side emitter and phosphorous doped back-surface-field (BSF), as well as symmetrical boron doped structures, were fabricated on 6-inch n-type wafers. The results show that the optimum passivation is achieved by a stack layer of a thermal SiO 2 , with a thickness of at least 10 nm, and a SiN X layer with a low refractive index. The chemical composition of SiN X capping layer plays an important role for surface passivation. A more Si-rich SiN X layer show significant degradation in surface passivation of the stack, due to the increase in the density of interface states (D it ) and fixed positive charges (Q tot ) at the interface.

The pitch of the rear side laser contact opening (LCO) strongly influences the performance of n-type Passivated Emitter and Rear Totally diffused (n-PERT) solar cells. The rear emitter of a back junction (BJ) cell was formed by boron doping [1,2]. The considered n-PERT, BJ cell structure is based on a p-type passivated emitter and rear cell (p-PERC) process sequence [3] upgraded by a boron diffusion on the rear side. Due to the application on n-type substrates, the cells have no light-induced degradation (LID) [4]. In this work we focus on fine tuning the LCO pitch of the device. We found that the LCO pitch directly affected the quality of the contact formation, leading to a variation on the solar cell performance investigated. By combined electroluminescence (EL) and photoluminescence (PL) measurements. Voids were identified (It’s about 60 μm width, 25 μm depth.) by scanning electron microscopy (SEM) and confirmed the different nature of the defects contributing to EL and PL contrast. Void formation is attributed to the diffusion of Si and Al at the Si/Al metallization interface. We found that void formation can be controlled by different LCO pitches. Based on this result we achieved a 0.35% absolute efficiency gain in average.

Ensuring high-quality front contacts in terms of low contact and line resistance while keeping the recombination losses low in p-type silicon solar cells has been challenging for the development of silver pastes. In this work, three silver pastes were used to create a contact on a lightly doped emitter of 90 Ω/sq, produced in a tube furnace by POCl3 diffusion, with an electrically active phosphorus surface concentration of 8 × 1019 cm–3. The peak firing temperature ( T peak) and belt speed ( v belt) of the firing step were varied to study how the silver pastes performed. No shunting was produced after firing as pseudo fill factor exceeded 83%. Efficiencies up to 19% were measured. When firing at higher v belt, a higher T peak permitted higher efficiencies due to better sintering and contact formation as well as a better passivation. A selective etching procedure was applied in order to investigate the contact interface. It was found that contact imprints were preferably located at pyramid tips for all groups. The paste producing larger and deeper contact imprints led to the lowest contact resistance (4 mΩcm2) but higher line resistance (0.46 Ω/cm) and higher saturation current density (30 fA/cm2 above) compared with the other two pastes.

In order to evaluate module materials, the maximum theoretical value of the photo-generated current density, was calculated. The calculation was performed for four different solar cells, a standard p-type, passivated emitter and rear contact (PERC), bifacial cell and interdigitated back contact (IBC) considering a solar spectrum of Atacama Desert, the transmittance of several glass-encapsulant-glass structures and quantum efficiency. Regarding the solar spectrum in Atacama, an average air mass (AM) at noon for this location averaged 1.17 and the photovoltaic (PV) modules tilt angle was 20°. When studying the impact of using glass and encapsulants combined with a solar cell under the same solar spectrum, ethylene vinyl acetate (EVA) with low ultraviolet (UV) cutoff led to the higher current density values, up to 2% higher with the IBC solar cell compared to the other solar cells. The highest current gain, when studying the impact of the two spectra in the 300–1200 nm wavelength range, was 7.4% for the IBC solar cell, obtained with a standard 3.2 mm glass, a thermoplastic material (TM) as encapsulant. Considering the UV part of the spectrum, the current gain was maximized with a glass with an anti reflection coating (ARC) combined with the TM encapsulant for the IBC solar cell (25%). A quantification of losses due to reflection in the glass and absorption in the encapsulant revealed that the glass with ARC and the TM encapsulant different than EVA led to the lowest reflection and absorption losses. In this case, the reflection and absorption came down to 4.8% and 0.9%, respectively, contrasting with the 7% and 2.8% loss produced with the standard glass and EVA encapsulants.

Screen printed front side contacts were investigated in n-type, high efficiency, single-crystalline Si solar cells with B doped emitters. Cells were processed identically and two different front side pastes were used: paste Ag–Al yielded efficiencies of 20.0%, and paste Ag, without Al, yielded 16.1% efficiency only. Cells contacted with paste Ag–Al yielded a continuous glass layer, a significantly higher density of Ag colloids, and an Al mole fraction of ∼1 at%. In contrast, cells contacted with paste Ag revealed a thinner but continuous glass layer as compared to cells processed with paste Ag–Al. It contained a lower density of Ag colloids and Al was below the detection limit of the EDX microanalysis. In n-type cells pyramidal shaped Ag nanocrystals did not appear, whereas for p-type cells they are formed at intersections of differently {111} oriented surface planes at the Si emitter. We did not find Ag/Al spikes at the Si emitter for cells contacted with paste Ag–Al. Cells contacted with paste Ag–Al had a smaller series resistance as well as a significantly lower specific contact resistance (4 mΩ cm2). A higher series resistance and specific contact resistance >100 mΩ cm2 was found for cells contacted with paste Ag. It was concluded that the specific contact resistance decreases with increased wetting and increased density of Ag colloids in the glass layer. The series resistance of the Ag–Al alloy contact depended only weakly on temperature, whereas the series resistance linearly increased with temperature for contacts processed with paste Ag. p- and n-type cells with maximum efficiency yield a similar microstructure of the front side contact, similar series and contact resistance, respectively. A high density of Ag colloids in the glass layer was found for cells with low specific contact resistance. This suggests that also for n-type cells a three dimensional percolative charge transport through the glass layer is assisted by the presence of metallic Ag colloids.

Bifacial solar cells enable the absorption of light also by the cell's rear side, hence increasing the energy yield of a bifacial module, Y b compared to the energy yield of a monofacial module installed under the same conditions, Y m , by BG = ( Y b - Y m ) / Y m , the bifacial gain. This contribution presents a simulation model for the prediction of the BG of bifacial PV modules (stand-alone and integrated in a PV field). The model has been implemented as a software tool and the results obtained by applying the tool to various relevant system configurations (ground albedo, geographical locations, module height and tilt, diffuse irradiation fraction) are shown. These results allow to determine the optimum installation parameters for highest BG for a given installation site. Finally, the tool is validated by comparing the simulated results with the actual BG monitored on bifacial modules during several months on an outdoor testing site.

In this study we designed and fabricated n -PERT solar cells and we investigated the effect of different boron emitter profile (emitter surface concentration and emitter junction depth) on the cell performance. The emitter depth was varying in the range 0.4μm and 0.75 μm and the resulting R Sh between 74Ω/sq and 140Ω/sq. From QSSPC measurements we observed that a better passivation is achieved in case of low R Sh . almost independently on the emitter depth. However, to achieve the best efficiency at cell level it is necessary to use deeper doping profiles, since for shallow doping profiles the metal recombination losses increase considerably.

We investigate the shade performance of modules assembled with interdigitated-back-contact (IBC) cells that consist of abutted p+n+ regions on the rear side. Because of the soft breakdown characteristic of these cells, the shade performance of such modules is different from the standard front-contact modules that exhibit high breakdown voltage. We fabricated IBC cells that feature early and relatively homogeneous breakdown characteristic, and then these cells were assembled into 60-cell modules. The module power was measured under illuminated and several shaded conditions, and the expected enhancement of module power under partial shading was verified. Further, a SPICE model was built to simulate the shade performance of cells with different breakdown voltages. We found that when the shaded area in the module exceeds a certain value, the dissipated power from the shaded cells is higher than the power generated by the non-shaded cells and hence the activation of bypass diodes is beneficial. Due to this reason bypass diodes are still necessary for such modules.

Compared with standard monofacial photovoltaic (PV) systems, the simulation of the energy yield of bifacial PV systems is more challenging since the impact of factors such as the installation height, the ground albedo, shadowing of neighbored rows, the diffuse irradiance fraction, and the PV module design is more pronounced. Therefore, many academic institutions as well as companies are currently working on the development of suitable modeling tools that allow an accurate energy yield prediction of bifacial systems. In this article, we present the results of energy-yield simulations of bifacial PV systems with fixed tilt and horizontal single-axis tracking (HSAT) in comparison to their monofacial counterparts using a tool that has been developed at ISC Konstanz. In addition, the simulated data are compared with measured results. The energy yield of fixed tilt bifacial systems is simulated as a function of the number of rows and number of modules in a row as well as a function of the installation height. The simulated data have been compared with measured data obtained using a PV system with continuously changing tilt angels. The accuracy of the simulated data is shown to be from +/ $-$ 0.1% to +/ $-$ 4% depending on the tilt angle of the bifacial modules. In addition, the energy yield of bifacial HSAT PV systems have been simulated and compared with measured data for a bifacial HSAT system in Chile. In this comparison, the use of ray tracing instead of the view factor (VF) concept for modeling of the rear irradiance reduced the deviation between simulated and measured gain significantly. Therefore, the approach of using VF-based calculations for the front irradiance and ray tracing for the rear irradiance was then used to evaluate different bifacial system configurations in comparison to monofacial ones. Especially, the influence of a variation of the ground coverage ratio on the energy yield for various monofacial, bifacial fixed tilt, and HSAT systems was studied.

In this work, a high voltage, both sides screen-printed n-type passivated emitter and rear totally diffused rear junction (n-PERT-RJ) solar cell concept with Al point contacts on its rear side is proposed. A dash pattern for the firing through Ag contact is applied instead of performing the selective doping on the front surface field (FSF), that effectively reduced the area-weighted metal-induced recombination current J0, Met (Ag). The impact of different n-bulk properties on the n-PERT-RJ solar cell performance is revealed by Quokka3 simulation. An innovative “point-line” concept is then introduced which is composed of dot-shaped laser contact opening (LCO) and Al metal grid, aims to possess bifacial gain in addition to the preservation of high VOC in order to further reduce the levelized cost of energy (LCOE). This industrial type n-PERT-RJ solar cell demonstrates the same performance level as the advanced passivated contact solar cell concepts, the cost of ownership (COO) is close to the p-PERC solar cells, last but not least, shows the performance stability after light and elevated temperature induced degradation (LeTID) tests.

This paper focuses on the technology mix proposed for the Diego de Almagro “Solar Technology District” (DTS), intended to be built at the south of the Atacama Desert in Chile, which combines Concentrated Solar Power (CSP) with Photovoltaic (PV) plants to take advantage of both systems attributes: easy installation and low levelized cost of electricity (LCOE) of PV and dispatchability and low cost for storing energy of CSP. Thermal energy storage systems as well as batteries are considered. Local conditions such as solar irradiation, water cost and availability and energy demand profile in Chile are noteworthy and important to be discussed. The methodology followed to optimize the power plant configurations focused on minimizing the LCOE and also considered the available land and the water consumption. Additionally, a rather new PV technology was investigated and selected for the solar park: Bifacial PV modules. The results obtained are useful to project developers seeking for combinations of PV and CSP to satisfy specific electricity demand in an economic, clean and reliable way

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

A major motivation for bifacial photovoltaics (PV) is an expected additional energy yield, compared to monofacial panels, due to the two-sided light sensitivity. The potential for an improved module power output and energy yield was repeatedly demonstrated by simulations [1-8], measurements on stand-alone modules [9-14] or installations [15-19] in various orientations. However, uncertainties concerning the actual output of projected systems still deter possible investors. Even in the PV community, the real quantitative benefit due to bifaciality and the best-suited technical concepts are still under discussion [20-22], as reflected by numerous publications dealing with these topics.

We developed a low-cost and high-efficiency IBC solar cells process ready to be transferred to industrial mass production. This so-called ZEBRA technology is lito-process free and it requires only one mask for the gas-phase diffusions. In this paper, we review the latest optimizations of the ZEBRA process in the attempt to achieve high open-circuit voltage (Voc) reducing the metallization losses and improving the passivation of phosphorous and boron doped regions. By a significant reduction of the contact area of the devices, we demonstrate a way to make screen printed Ag point-contacts feasible and to improve the efficiency of the ZEBRA cells of more than 22.3% and a Voc of 668 mV. Moreover we introduce a potential industry scenario in which we forecast ZEBRA cells efficiencies up to 22.8 % in mass production. Additionally, update CoO calculations are presented to demonstrate the economic feasibility of the ZEBRA process. At last, we introduce the new generation of ZEBRA cell (Gen2) as the latest process development. The J 0,e and J 0,met of the p+ and n+ regions are reduced by lowering the surface concentration of the doping profiles, meanwhile, the fine tuning of the contacts formation allow for a RC < 2.5 mΩ.cm2. Excellent surface passivation obtained with oxide/SiNx stack layers with implied Voc of 710 mV and Joe=14 fA/cm2 on symmetrically diffused boron emitter. Finally, solar cells with energy conversion efficiencies up to 23.2% were fabricated.

Despite of recent improvements in silver paste technology, the open-circuit voltages $( V_{oc})$ of silicon solar cells with screen printed and firing-through silver contacts continue to be limited by the high recombination currents at the metal contacts $( J_{{0} {,met}})$. To maximize solar cell efficiencies, the contact resistance $( \rho_{c})$ and $J_{{0} {,met}}$ must be simultaneously minimised. In this study we investigated the origin of $J_{{0} {,met}}$ for screen printed silver pastes on $p^{+}$ and $n^{+}$ doped regions and correlate this with contact formation phases during the firing process. We show that, during the contact firing process, the $J_{{0} {,met}}$ significantly increases and even saturates to its final value at temperatures well below 700°C, which is a temperature range below that is needed for contact formation. The same is observed on both $p^{+}$ and $n^{+}$ diffused junctions with planar or random pyramids textured Si surfaces passivated by a SiO 2 /SiN x layer stack. This show that most of the metal induced recombination loses originates from the etching of the dielectric layers by the glass frit and less during the contact formation process where $\rho_{c}$ is minimised. Furthermore, we demonstrate that increasing the SiN x passivating layer thickness leads to a significant reduction in $J_{{0} {,met}}$, possibly due to an incomplete etching of dielectric layers under the contact, whereas the $\rho_{c}$ remain low and constant under optimum firing conditions. These findings could help design metallization pastes optimised to reduce dielectric etching, and hence $J_{{0} {,met}}$, without affecting $\rho_{c}$.

Soiling is currently one of the major issues for decreasing power production of PV plants in Chile. In order to prevent significant power losses cleaning frequencies of more than one time per month are scheduled. In the present study, we investigate the impact of soiling on energy yield (YA) of bifacial solar modules with different material combinations. We fabricated half-size and full-size one cell modules employing different glass thicknesses, encapsulants, transparent backsheets and glass-glass module configurations. Based on indoor current-voltage (IV) measurements which were carried out on front and rear side of each bifacial module type at different irradiance levels, a mathematical YA model was applied. Front and rear measurements were taken separately and simultaneously assuming the same bifaciality factor at a given irradiation. YA estimations show that module groups with half-cell design achieve the highest YA values compared to full-cell design. In addition, vertically mounted bifacial modules (VMBM) produce higher YA than tilted mounted bifacial modules (TMBM) if the power loss rate due to soiling is higher than 0.10% per day. To validate the YA model, real outdoor measurements were carried out in the Atacama Desert in Chile for TMBM and for VMBM.

Studying encapsulated n -type interdigitated back contact (IBC) solar cells with front floating emitter (FFE) we observe a power degradation mechanism which behaves similar to the PID-p degradation for n -typeIBC solar cells with front surface field described in or for bifacial n -typesolar cells in . This power drop is not shunt-related and arises as loss of short circuit current and open circuit voltage in analogy to the findings in . The power degradation occurs when biasing FFE-IBC minimodules with negative potential against the ground and can be fully regenerated by reversing the polarity of the bias potential. The susceptibility to PID for our IBC minimodules depends on the encapsulation material to higher or lesser extent; until now no material was identified which can totally prevent PID. Bifacial IV measurements and EL recordings let us conclude that the PID we observe is non-local and causes a homogenous degradation of the front surface of the solar cell. Silvaco Atlas simulations show that a total loss of surface passivation is not sufficient to explain the effect.

In this work we conducted a comparison study of our n -type PERT (BiSoN) and IBC (ZEBRA) cell technologies. We investigated the effect of material quality and base resistivity on the performance and efficiency potential of the two device architectures. We observed only a slight efficiency variation (less than 3% relative ) for wafer bulk lifetimes ranging from 1.2-6.5 ms and base resistivities from 2-9 Ωcm, with no significant difference between the n-PERT andthe IBC concept. We achieved cell efficiencies, as high as 20.4% for BiSoN concept and 21.5% for ZEBRA on 15.6×15.6 cm 2 n-type Cz Si wafers. Additionally, we demonstrated that both cell concepts have a high bifacial performance, both at cell and module level.

The soft reverse IV characteristic of interdigitated-back-contact (IBC) silicon solar cells consisting of contiguous p + and n + regions on the rear side (Figure 1) was investigated in this study. Our IBC cell concept, which is a 6-inch IBC cell with a diffused phosphorous BSF and a boron front floating emitter, features a relatively low breakdown voltage of about - 3.7 V. We measured a negative temperature coefficient at reverse bias in the dark. The low breakdown voltage and the negative temperature coefficient both support the hypothesis that the high reverse current is caused by the tunneling effect [1] . We demonstrated that by changing the doping profiles, the breakdown voltage of the IBC cells can be modified. Our simulations show that the reverse bias current is mainly caused by the band-to-band tunneling across the borders of the p + and n + regions, and that the breakdown voltage changes with doping profiles. Furthermore, a qualitative agreement is found between the experimental and simulated reverse currents. In principle, the same theory may apply for studying the early breakdown behavior of MWT, EWT or any other cells that feature p + n + junctions.

Crystalline silicon solar cells are currently the leading technology in the photovoltaic market with no great expectable change in the shares. The scientific community works on the further development and improvements of state-of-the-art as well as new solar cell materials. This paper reports on a chemical methodology for selective etching to study the metallization step in monocrystalline silicon solar cells. The object of study is a complete processed silicon solar cell which was cleaved via laser beam on the back side and broken per hand to obtain stripes of the size 15.6×1 cm². In the following a sequence of etching chemical solutions to selectively remove the components of the front side silver contact was applied. Scanning electron microscopy was used to investigate contact interface after each etching step. The silver finger, the glass and the silver crystallites grown in silicon could be removed. It came out that the silver crystallites preferably grow at the pyramid tips and edges of the textured wafer. A characterization with Energy Dispersive X-Ray Spectrometry was performed to quantify the components of the silver contact after each chemical etching step. While the weight percentage of silver reduced by more than 90% after an aqua regia treatment, it increased by 13% after hydrofluoric acid. Silver was practically eliminated after a second aqua regia bath. Similarly, the content of glass was also determined. The approach serves for interface investigations in semiconductor technology where screen printing approaches are used for the metallization.

In this work, we present an n-type Passivated Emitter and Rear Totally diffused Rear Junction (n-PERT-RJ) solar cell structure with a dot-shaped Laser Contact Opening (LCO) pattern on the rear side. The fully screen printed devices are investigated for rear side Al paste metallization with various Si-additive content. A hypothesis of the screen printed point contact formation and the role of the Si-additives played in the alloying process are proposed based on the analysis of microstructure and electrical properties of the solar cells. Adapted process conditions led to an n-PERT-RJ solar cell with 21.78% efficiency and 686mV open-circuit voltage. The process fits to simple industrial implementation and has the potential to exceed the 22% efficiency level in mass production.

We present an approach of implementing the experimental microscopic observations at metal-Si emitter interface into a 2-D simulation model to investigate the metallization-induced recombination losses of crystalline Si solar cells with firing through metallization. This metal-Si interface is typically characterized by few Ag crystallites grown into Si, whereas the main part of the interface area is covered by a glass layer. By using this simulation model, we were able to disentangle the effect that each of these microscopic features has on solar cell performance. The model assumes the metal-Si interface to be either Ohmic or Schottky, with current extraction occurring only through the direct Ag crystallites-Si interface. The simulation results show negligible degradation in open-circuit voltage (V OC ) and pseudo-fill factor (pF F) if the metal-Si interface comprises the sole presence of Ag crystallites, as long as these crystallites grow only superficially and are not penetrating the p-n junction. A more detrimental effect is observed if the emitter area beneath the glass layer is affected by etching. For more aggressive metallization pastes or for overfiring scenarios where larger in-grown crystallites are observed that are protruding through the p-n junction, the model assuming a Schottky contact qualitatively explains the shunting behavior observed experimentally.

Bifacial modules have the advantage of capturing sunlight from front as well as from rear surfaces, and therefore, they are able to produce larger amounts of energy, compared with standard (monofacial) modules. However, their performance depends on the spatial distribution of the irradiance incident on the rear module surface, which is strongly affected by several site-specific conditions, such as albedo, reflective surface size, module elevation, and tilt angle. In this study, we elaborate upon the individual and combined effects of these factors on the annual energy yield of stand-alone south-facing bifacial modules through simulations at two site locations with contrary climatic conditions. Following the optimization of the tilt angle of bifacial modules dependent on the site, albedo, and module elevation, we demonstrate that the annual energy yield of a bifacial module increases linearly with albedo, which shows a monotonically increasing but, in addition, saturating behavior versus reflective surface size, and increases up to a certain module elevation. Through the simultaneous consideration of these dependences, we suggest an optimal positioning of bifacial modules. Finally, we show that under these optimal conditions, bifacial modules can supply up to 25% more energy compared with standard modules.

In this study, we investigate the metallization‒induced recombination losses of high efficiency bifacial n‒type and p‒type crystalline Si solar cells. From the experimental data, we found that the most efficiency limiting parameter by the screen‒printed metallization is the open‒circuit voltage (VOC) of the cells. We investigated the mechanism responsible for this loss by varying the metallization fraction on either side of the cell and determined the local enhancement in the dark saturation current density beneath the metal contacts (J0(met)). Under optimum fabrication conditions, the J0(met) at metal‒p+ (boron) emitter interfaces was found to be significantly higher compared with the values obtained for metal‒n+ emitters. A two‒dimensional simulation model was used to get further insight into the recombination mechanism leading to these VOC losses. The model assumes that metal contacts penetrate (or etch) into the diffused region following the firing process and depassivate the interface. Applying this model to our n‒type solar cells with a boron p+ emitter, we demonstrated that the simple loss of passivated area beneath the metal contact cannot explain the degradation observed in the VOC of the cell without considering a significant etching or metal penetration into the emitter region. Copyright © 2014 John Wiley & Sons, Ltd.

Bifacial solar modules are becoming increasingly more attractive stimulated by the development of new solar cell structures enabling to capture solar insolation on both front and rear surfaces. The possibilities to be installed in the conventional south facing orientation or even vertically are further advantages. Although the potential of bifacial modules have been already shown in specific time intervals and for various ground albedos there is a lack in simulation studies up to now. In this study, we simulated the annual energy yield (AEY) of south facing bifacial modules using a rigorous calculation method of the ground reflected radiation reaching the rear module surface. The necessary tilt angle optimization is done incorporating the influence of module elevation and considering the inherent albedo coefficient. These simulations are able to reproduce measurement observations and show that at optimum tilt angles produced annual energy can be increased by 30% compared to a standard module simply by positioning modules two meters above ground instead of a close to ground installation. Furthermore, a linear relationship between albedo coefficient and AEY is demonstrated.

Good quality contacts between metal and silicon emitter are crucial for high crystalline solar cell efficiencies. We investigate the impact of defects originating from electrically inactive phosphorus on contact formation within silver thick film metallized silicon solar cells. For this purpose, emitters with varying sheet resistance, depth, and dead layer were metallized with silver pastes from different generations. Macroscopic contact resistivity measurements were compared with the microscopic contact configurations studied by scanning electron microscopy. The density of direct contacts between Ag crystallites grown into Si and the Ag finger bulk is essential for low contact resistivity. The presence of glass-free regions needed for such direct contacts depends on the paste composition and on the surface texture, and does not vary with the Si emitter properties. Indeed, the decrease in contact resistivity correlates with increasing density of Ag crystallites embedded in the Si surface. Furthermore, the density of Si surface-embedded Ag crystallites scales proportional to the electrically inactive P and is independent of the sheet resistance. Using the newest silver paste, the Ag crystallite density is independent of the emitter doping, but the Ag crystallite size increases as a function of the thickness of the dead layer. Transmission electron microscopy characterization of the excess P-doped Si crystal lattice shows that significant strain and Si bond weakening may play a major role for both Ag crystallite nucleation and growth. Finally, we studied Si crystal defects by metallizing nanocracks, dislocations, and grain boundaries and found that Ag crystallite nucleation is defect-property dependent. Copyright © 2013 John Wiley & Sons, Ltd.

Massive savings in silicon solar cell production could be achieved by replacing the costly Ag in front side metallization pastes against cheaper metals like Ni or Zn. Regarding the attempt to split the metallization process up into a two step process, first printing a paste containing Ni or Zn instead of Ag for contact formation and second printing a conducting paste containing e.g. Cu, this work is restricted to the investigation of the contacting paste. Simple pastes containing only Ni or Zn respectively, rudimentary glass frit, organic binder and solvent were tested on wafers with a low resistivity emitter using various fast firing profiles and annealing. Our first attempts using Ag compatible frits have shown that it is possible to create a contact to Si yet it is significantly worse than with comparable silver pastes. SEM measurements of the contact area after selective metal/glass etch have shown a strong Si etching by the investigated pastes and only insufficient crystallite formation. This explains the reduced open circuit voltage, pseudo fill factor and the high contact resistivity measured on the cells. Our investigations show that it is very hard to exchange Ag by another metal without fundamentally changing the paste formulation and/or the firing conditions.

Up to date most spin-on diffusion sources based on wet chemical created boron-silicate glass (BSG) lead to a strongly degraded carrier lifetime after diffusion. In this study the implication of a spin-on boron diffusion source is applied to the fabrication of large area n-type CZ bifacial solar cell and the performance of the spin-on source is compared to a BBr3 boron diffusion concerning the resulting lifetime and emitter saturation current J0e. Comparable J0e values and a comparable lifetime have been achieved resulting in a top efficiency of 19.38% in comparison to a top efficiency of 19.60% achieved by BBr3 diffusion.