PERT Solar Cells Research Articles

TiO2 Antireflection Coating Deposited by Electro-Beam Evaporation: Thin Film Thickness Effect on Weighted Reflectance and Surface Passivation of Silicon Solar Cells

Titanium dioxide was extensively used in solar cell industry and currently has been studied to produce passivated contacts in PERC/PERT and TOPCon solar cells. The aim of this paper was to analyze the impact of the thickness of TiO2 thin films deposited by electro-beam evaporation on the weighted reflectance and the surface passivation on silicon solar cells. Thin films with different thicknesses were deposited to produce PERT solar cells, varying from 50 to 90 nm. The surface passivation was enhanced as the thickness was increased. For instance, at 400 nm, the internal quantum efficiency increased from 71% to 76% when the thickness of the TiO2 was augmented from 50 nm to 90 nm. The lowest weighted reflectance was obtained in samples with 80 nm thick TiO2 films. Considering the compromise between antireflection properties and surface passivation, the highest efficiency solar cells were produced with 80 nm thick TiO2.

Mathematical Modelling of a Novel Hetero-junction Dual SIS ZnO-Si-SnO Solar Cell

For the last few decades scientists across the world have achieved significant improvement in performance of conventional silicon p-n junction solar cell. Sophisticated high temperature doping technology is unavoidable in the fabrication of these conventional solar cells. Back in 1970s scientists proposed an alternative solar cell technology with Schottky barrier which can cut down the burden on thermal budget of manufacturing process. Later the metal-semiconductor Schottky barrier further modified with hetero junction semiconductor-semiconductor solar cells. A thin intrinsic layer sandwiched between semiconductor-semiconductor junctions can repair the junction defect efficiently. These SIS solar cells became popular for its low thermal budget and considerable efficiency. In this paper we have tried to propose a mathematical model of a novel dual side SIS solar cell which is basically a multi junction solar cell. We have introduced a third semiconductor layer (SnO) at the back side of the cell which can provide an inversion layer much similar to PERT solar cell. This structure is first of its kind and thus a theoretical analysis is required before implementation. We have studied the effect of this back field on the performance of the cell and propose a mathematical model based on reciprocity theorem of charge collection. The efficiency of conventional ZnO-pSi SIS solar cell was computed ~5.2% while the back SnO p+ layer is expected to enhance the efficiency up to ~7.9% according to our mathematical model. We have concluded with significant mathematical justification to implement this structure with proposed electro-chemical experiments.

Optimization of TOPCon Structured Solar Cell Using AFORS-HET

The tunnel oxide passivated contact (TOPCon) structured solar cell, improved version of first-generation PERT solar cell, delivers the highest power conversion efficiency, and is the most commercially available. The first-generation solar cell still suffers from recombination losses, which strongly impacts the conversion efficiency of the cell. To reduce the losses, efforts need to be made to suppress the recombination by passivating the rear junction using a dielectric layer, tunneling layer, along with an excellent field-effect passivation. The tunneling current is strongly dependent on the thickness of the dielectric layer, i.e., an increase in SiO2 thickness results in the decrease of the tunneling current by more than one magnitude. Although raising the doping in the n+-Si layer enhances the tunneling probability of an electron, which allows for a thicker layer for better passivation. The paper discusses the results of a numerical simulation tool to study the impact of surface recombination on the performance of TOPCon solar cells. The solar cell is simulated at different surface recombination velocity observing typical solar cell parameters with varying tunnel oxide (SiO2) thickness and doping of field-effect passivation (n+-Si) to obtain the optimized value. Optimization of SiO2/n+-Si backside structure is essential to obtain a structure reaching a high theoretical efficiency.

Interface analysis of Ag/n‐type Si contacts in n‐type PERT solar cells

AbstractTo increase efficiencies of bifacial solar cells, emitter, back surface field (BSF), and metal patterns must be optimized. We study the influence of paste volume, through multiple prints, of two silver pastes on the contact formation at the rear side of n‐type passivated emitter and rear totally diffused (n‐PERT) solar cells with two BSF doping profiles. Differences in fingers' electrical properties were found between pastes. Contact resistivity shows a relative difference of 27.6%, partially explained by changes in the silver crystallites formation at the Ag/Si interface and in the crystallites' penetration depth. Variations in crystallites formation and penetration between pastes can reach 38.4% and 48.8%, respectively. Line resistance shows a difference between pastes, appearing as the main cause of an absolute efficiency difference of 2.9%. Fingers' structural and electrical properties are modified by increasing the paste volume. Microstructure analysis reveals that additional metallic printing does not only increase line cross sectional area but also increases the formation of silver crystallites, which can reach a relative increment of 23.9% between first and second prints. Further printing does not necessarily decrease contact resistivity, but reduces line resistance in up to 94.9%, which results in an absolute efficiency increase of 2.2%. In addition, the higher presence of silver oxide in the finger is related to a higher efficiency in the formation of silver crystallites. Finally, BSF doping has an influence in the open circuit voltage, short circuit current density, and contact resistivity, with differences that can reach 8.7 mV, 0.2 mA/cm2, and 6.1 mΩcm2, respectively, depending on paste and number of prints.

Synergistic Tandem Solar Electricity-Water Generators

Summary Energy and clean water are two intertwined fundamental elements for human civilization and sustainable development. Here, we devise a monolithic tandem solar electricity-water generator that synergistically produces electricity and clean water by utilizing the full spectrum of solar irradiance. The tandem generator has two components: a top infrared-transparent photovoltaic device using above-band-gap photons, and a bottom solar water purifier using the below-band-gap photons. More strikingly, a well-designed water-proof thermal interconnecting layer (WTIL) makes these two components work synergistically: the bottom purifier serves as an evaporative cooler of the top solar cell to increase its efficiency, whereas the thermalization energy of the top cell is reutilized by the bottom purifier for producing more clean water. We experimentally demonstrate that a prototype hybrid tandem solar device with WTIL can generate electricity with a power output of 204 W m−2 and purify water at a rate of 0.80 kg m−2 h−1 under 1-sun illumination.

Analysis of different conductive pastes to form the contact with the boron back surface field in PERT silicon solar cells

The industrial production of solar cells of the PERC family is growing, because the potential to increase the efficiency due to the passivation of the rear face. The PERT solar cell is a cost-effective structure of the PERC configurations. The goal of this paper is to analyze the influence in the electrical parameters of different metal pastes used to form the contact with the boron back surface field of PERT solar cells passivated with silicon dioxide on both sides and developed with a cost-effective process. The boron doped BSF and phosphorus emitter were carried out with reduction of steps. Solar cells were processed with three different conductive pastes: (1) an aluminum paste (PV381), (2) a silver/aluminum paste (PV3N1) and (3) a silver paste (PV51G), with different viscosity and solids content. The pastes were produced by DuPont. The PV381 and PV3N1 pastes produced solar cells with the efficiency of 16.2% and 15.9%, respectively. The higher open circuit voltage was achieved with the aluminum paste, indicating that this paste is more effective to produce the selective back surface field. The PV51G paste is not suitable to form the rear contact.

Optical and electrical performance of rear side epitaxial emitters for bifacial silicon solar cell application

In this work we investigated the optical and electrical performance of p-type epitaxial layers as the rear emitter of bifacial n-type PERT solar cells. In the first part of this paper, the surface morphology of epitaxial layers grown on textured surfaces is studied. Because of the epitaxial growth, a pyramids-rounding effect is observed as a result of {311} and {911} facet propagation. The growth pattern was quantified and modelled. In the second part of this paper, the optical performance of semi-device test structures is evaluated. The trend of the optical results in bifacial solar cell structures indicates that a final pyramid angle at the rear side around 20° gives the maximum light absorption in the wafer substrate. In this work we demonstrate that the epitaxial growth of the emitter on the textured rear side of these devices can already give a pyramid angle of 25° without having to introduce any additional polishing steps to modify the morphology of the textured surface. In the last part of this paper, we present the electrical results for semi-device structures created to quantify the recombination losses in the passivated and metallized regions of those p-type epitaxial emitters. These results indicate that by introducing a rear epitaxial emitter in the bifacial n-type PERT cell structure, we can increase the implied Voc up to 17 mV compared to a diffused emitter with the same sheet resistance.

Investigation of phosphorus diffused back surface field (BSF) in bifacial nFAB solar cells

Recently bifacial n-type passivated emitter and rear totally diffused (PERT) monocrystalline silicon solar cells have become one hot spot in photovoltaic (PV) industries, due to good bifaciality, high and stabilised conversion efficiency. Unlike passivated emitter and rear contacts (PERC) structure, n-PERT solar cells require the use of a thin and uniform back surface field (BSF) layer, usually achieved by phosphorus doping. In this study, we optimised the phosphorus diffusion process in terms of surface concentration, junction depth and carrier lifetime. The effects of different phosphorus BSF were investigated by fabricating n-type front and back contact (nFAB) PERT solar cells using industrial feasible approaches and M2 size Czochralski (Cz) monocrystalline Si wafers (6 in., 244.32 cm2). Good phosphorus profiles were developed, which gives low parasitic absorption, low contact resistance and low J0 value when passivated with silicon nitride (SiNx) layer. The optimised champion cell shows a high Voc of 666.5 mV, Jsc of 40.2 mA/cm2, fill factor of 79.9%, efficiency of 21.43% from front side-illumination, together with a good bifaciality factor of 93.0%.

Effect of Oxygen Precipitation in Silicon Wafer on Electrical Characteristics of Fully Ion-Implanted n-Type PERT Solar Cells

Fully ion-implanted n-type PERT solar cells with boron-implanted emitter and phosphorus-implanted back surface field were fabricated on the n-type silicon wafers obtained from the top part of the magnetic field-applied Czochralski ingot. It was observed that the electrical parameters are widely dispersed; among 11 solar cells, the best performing achieved 20.4% front side conversion efficiency, whereas the worst achieved 19.0%. Although the silicon wafers had low oxygen concentrations of 3–4 × 1017 atoms/cm3, the density of oxygen precipitates in the silicon wafers was on the order of 109 /cm3 as a consequence of the fully ion-implanted n-type PERT silicon solar cell processes. In addition, it was observed that the front side conversion efficiencies of the solar cells depended on the density of oxygen precipitates. Furthermore, the behavior of the oxygen precipitation during the fabrication processes of the solar cell is discussed.

Internal resistance of rear totally diffused solar cells with line shaped contacts

We present an analytical model for the internal resistance of passivated emitter and rear totally diffused (PERT) solar cells. First, we apply the model of Saint-Cast for the spreading resistance of a passivated emitter and rear cell (PERC) structure with line-shaped contacts. To account for the additional vertical current flow through the silicon wafer and the lateral current flow through the back surface field of a PERT structure, we add a parallel current path using common analytical expressions. We compare the analytical models with two-dimensional numerical simulations based on Quokka 3 and find deviations of less than 6% for the internal resistance. In addition, we compare the analytical model of the internal resistance of PERC and PERT solar cells with experimental data of the series resistance of PERC and PERT solar cells.

Influence of nitride and nitridation on the doping properties of PECVD-deposited BSG layers

High efficiency p-type passivated emitter and rear (PERC) silicon (Si) solar cells are becoming industrial standard with demonstrated efficiencies above 22.6 %. Alternatively to the PERC design there is the possibility to apply a full-area boron back-surface-field (B-BSF) to the solar cell rear side for implementation of passivated emitter and rear totally diffused solar cells (PERT). In this work we investigate plasma enhanced chemical vapor deposited (PECVD) boron silicate glass (BSG) layers employing the precursor gases nitrous oxide (N2O), silane (SiH4) and diborane (B2H6). We experimentally demonstrate the impact of nitride (N) content in the BSG on the boron (B) doping efficiency within a thermal drive-in step, i.e. a subsequent high temperature diffusion step post BSG-deposition. It is found that the N content of the BSG which is mainly controlled by the N2O:SiH4 precursor gas flow ratio has to be kept as low as possible to achieve optimal B doping results of the Si bulk material. In addition an in-situ nitridation process has been developed to prevent build-up of a boron rich layer (BRL) on the highly doped Si surface. PERT and PERC solar cells were processed from material with different base resistivities and equal solar cell efficiencies were achieved.

Co-diffused bi-facial PERT solar cells

The authors present a fully solderable, co-diffused and bi-facial PERT solar cell where the rear BSG/SiN stack acting both as dopant and passivation source reaching a mean efficiency of 20.4 %. The conductivity induced by the presence of the p+ layer at the rear reduces series resistance losses significantly by simultaneously enabling the use of lowly doped Si wafers that reduceB-O degradation while maintaining a high fill factor. Parallel processed, mono-facial pendants reach the same efficiency level. A higher FF for the mono-facials is compensated by a higher Voc for the bi-facial PERT devices. SEM analysis on test structures show an overall superior, local Al-Si contact in terms of voiding and BSF thickness with reducing printed finger width. At reverse bias, the “conventional process” shows a typical behaviour for such architecture of elevated current flow around the edges. For the “adapted process” up to -12 V, the authors demonstrate a virtually shunt-free device exhibiting reverse currents far below 1 A with no additional process step.

Boron Emitter Formation by Plasma Immersion Ion Implantation in n-type PERT Silicon Solar Cells

The use of plasma immersion ion implantation (PIII) is a relevant approach for the development of advanced solar cells technologies at lower cost (€/Wp). In this paper, we report on the development of homogeneous boron (B) doping of n-type crystalline silicon (cSi) substrate by the PIII technique. Using diborane (B2H6) as gas precursor, various doping profiles were identified fitting the requirements for boron-doped emitters in n-type PERT solar cells. Particularly, saturation current density (J0e) of 50 fA/cm2 were achieved on symmetrical samples for a 94 Ω/sg textured B-emitter passivated with SiO2/SiN stack. Bifacial n-type Passivated Emitter Rear Totally-diffused (n-PERT) solar cells were fabricated using the PIII technology and conversion efficiencies up to 19.8% on 239 cm2 Cz-Si wafers were obtained. As a consequence, these results indicate that PIII can compete with beamline technology but will have lower running cost. It is therefore a promising technology to create high efficiency cSi solar cells.

Effective Surface Recombination of p+ Layers Doped Using Ion Implantation or Surface Deposited B Sources

Two key factors need to be fulfilled in order to enable high efficiency bifacial p-type PERT cell. High bulk lifetime values retention during production processes, and minimized effective surface recombination, Seff in the p+ layer. The influence of boron doped layer parameters on the effective surface recombination in the p+ layer of an n+-p-p+ bifacial PERT solar cell is evaluated. Back IQE data of n+-p-p+ cells with over-doped p+ layer were used for evaluation of Seff values in the p+ layer. Simulation was made for several doping profiles with the surface doping concentration, Bs, in the range 1018 - 1020 cm-3. According to simulation, the dominating parameter controlling the effect of the built-in charge in the passivation layer on the effective surface recombination is the Bs level. For Bs values above ∼1019 cm-3 this charge has no significant influence. p+ layer doping was made by B ion implantation as well as by thermal diffusion from deposited B source. Measurements were performed on symmetrically B doped n-Si wafers with different passivation. Good agreement between simulation and measurements was found.

Bifacial, fully screen-printed n-PERT solar cells with BF2 and B implanted emitters

For boron implants made with ion implanters designed for PV applications, a variety of ion species BFx (x=0.2) is implanted, while BF2 accounts for the lions share. We investigate the impact of the BF2 implantation dose and the annealing conditions on the resulting electrical characteristics of industrial bifacial n-type PERT solar cells. For an annealing temperature of 950°C, we observe a steep increase in the emitter saturation current density for BF2 implant doses above 1.75×1015cm−2.We compare co-annealed BF2- and B-implanted solar cells and achieve maximum energy conversion efficiencies of 20.6% with BF2 and 21.0% with B, respectively. Separate annealing processes for the p+ and n+ doped regions result in even higher energy conversion efficiencies of 20.8% with BF2 and 21.5% with B emitter implant. An optimized double-layer ARC applied on the separately annealed cells further increases the efficiency up to 20.9% for BF2 and up to 21.8% for B emitter implant. All efficiency values are independently confirmed.The solar cells have bifacial factors between 97.3% and 99.4%. The BF2-implanted solar cells with highest efficiency feature an emitter with a sheet resistance of 180 Ω/sq. contacted by a commercially available Ag/Al paste with specific contact resistance of less than 10mΩcm2.

Development of a large area n-type PERT cell with high efficiency of 22% using industrially feasible technology

A high-efficiency front junction n-type passivated emitter and a rear total diffusion (n-type PERT) solar cell with the front boron diffusion passivated by a Al2O3/SiNx stack layer deposited by plasma enhanced chemical vapor method and the rear with phosphorus total diffusion and Al evaporated local contact are presented in this paper. The main purpose of this study was to develop industrially feasible front junction n-type PERT solar cells with high-efficiency; these were realized on a large area of n-type industrial 5- and 6-in. wafers. An average of 21.85% cell efficiency was achieved on 5-in. wafers, and the highest cell convert efficiency of 21.98% was achieved with Voc of 683.8mV, Jsc of 40.13mA/cm2, and FF of 80.11%. For 6-in. cells, we get Fraunhofer independently confirmed efficiency of 21.49% with a Voc of 674.1mV, a Jsc of 39.77mA/cm2, a FF of 80.18%. Details of cell fabrication and results are presented, followed by a best cell power loss analysis on Voc, Isc and FF.

Large-Area n-Type PERT Solar Cells Featuring Rear p+ Emitter Passivated by ALD Al2 O3

We present large-area n-type PERT solar cells featuring a rear boron emitter passivated by a stack of ALD Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and PECVD SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> . After illustrating the technological and fundamental advantages of such a device architecture, we show that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> stack employed to passivate the boron emitter is unaffected by the rear metallization processes and can suppress the Shockley-Read-Hall surface recombination current to values below 2 fA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , provided that the Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> thickness is larger than 7 nm. Efficiencies of 21.5% on 156-mm commercial-grade Cz-Si substrates are demonstrated in this study, when the rear Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> passivation is applied in combination with a homogeneous front-surface field (FSF). The passivation stack developed herein can sustain cell efficiencies in excess of 22% and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> above 685 mV when a selective FSF is implemented, despite the absence of passivated contacts. Finally, we demonstrate that such cells do not suffer from light-induced degradation.

Effect of the Back Surface Reflector and Passivated Rear Contacts With PERT Solar Cells

We use Silvaco software (atlas tcad) simulation to investigate the effect of dielectric layer deposed on rear surface of solar cells passivated emitter and rear totally diffused (PERT). For an improved performance for this solar cell, several physical factors must be considered, such as the light trapping behavior, and the resulting passivation performance and rear surface recombination currents were investigated. Particular consideration will be given to the back surface reflector (BSR) impact on reflection surface, interface passivation, and on the I–V characteristics. Numerical simulations show that using a layer of two dielectrics (SiNx/SiO2) with optical indices and thickness optimized in combination with contacts located (optimized metallization fraction f) at the rear surface allow for energy conversion efficiencies of 21.26% compared to a single layer of dielectric SiN 21.01%.

Hydrogen Passivation of Laser-Induced Defects for Laser-Doped Silicon Solar Cells

Hydrogen passivation of laser-induced defects (LasID) is shown to be essential for the fabrication of laser-doped solar cells. On first-generation laser-doped selective emitter solar cells where open-circuit voltages were predominately limited by the full-area back surface field, a 10-mV increase and 0.4% increase in the pseudo-fill factor were observed through hydrogen passivation of defects generated during the laser doping process, resulting in an efficiency gain of 0.35% absolute. The passivation of such defects becomes of increasing importance when developing higher voltage devices and can result in improvements in implied open-circuit voltage on test structures up to 50 mV. On n-type PERT solar cells, an efficiency gain of 0.7% absolute was demonstrated with increases in open-circuit voltage and pseudo-fill factor by applying a short low-temperature hydrogenation process using only hydrogen within the device. This process was also shown to improve the rear surface passivation, increasing the short-circuit current of approximately 0.2 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> of wavelengths from 950 to 1200 nm compared with that achieved using an Alneal process. Subsequently, an average efficiency of 20.54% was achieved.

Influence of the boron emitter profile on V OC and J SC losses in fully ion implanted n-type PERT solar cells

In this work, we report on fully ion implanted 156 × 156 mm2 n-type PERT solar cells fabricated in an industry-capable process. The implant damage of phosphorous and boron was co-annealed in a single furnace annealing step. The cells feature a screen-printed front side metallization and an evaporated rear side metallization. The influence of boron emitter profile on the open-circuit voltage VOC and the short-circuit current density JSC was studied by comparing two boron doses (2.5 × 1015 cm−2, 3 × 1015 cm−2) and two annealing durations (20 min, 80 min). The solar cells reach tightly distributed efficiencies above 20% (20.3% maximum). Cells featuring an emitter implanted with 2.5 × 1015 cm−2 boron dose annealed for 80 min show the highest open-circuit voltages up to 668 mV. As compared to implied open-circuit voltages measured on non-metallized cell precursors, this corresponds to a metallization-induced voltage loss of 14 mV. For the shorter annealing time (corresponding to a shallower profile) but same boron dose, the VOC loss is 18 mV. The fact that the cells with the higher boron dose of 3 × 1015 cm−2 showed lower VOC values indicates that the recombination at the Ag/Al-p+ Si contacts is not totally dominating. We determine the recombination current densities of the metallized emitter regions to 900–1900 fA cm−2. Contrary to the dependence of VOC on the emitter profile, JSC is lower for deeper emitters. The loss in JSC is visible in the internal quantum efficiencies IQE at short wavelengths. Strategies for an optimization of both quantities, JSC and VOC, are discussed.