Local Back Surface Field Research Articles

Surface Recombination Velocity of Local Al-contacts of PERC Solar Cells Determined from LBIC Measurements and 2D Simulation

Industrial production volumes of passivated emitter and rear contact (PERC) solar cells increase due to significantly higher cell efficiencies compared to full area back surface field (BSF) solar cells at similar costs. The main features of PERC cells are dielectric surface passivation of the rear and local contact formation with Al leading to a p+p junction beneath the Al/eutectic. For non-optimized process conditions, the eutectic in the local Al contact area does not form and so-called voids result. Since it is known that there are voids causing high or low recombination activity, a determination of the surface recombination velocity (SRV) is necessary for identification of the potential for process optimization. The passivation quality of the BSF, locally formed in the rear side contacts, is studied in detail via local internal quantum efficiency (IQE) measured by high resolution light beam induced current (LBIC). The significant spreading of the IQE values is attributed to a variation in local BSF layer thickness at different areas. The SRV of the local contact is determined by fitting the LBIC measurements of voids by 2D simulations. These simulations are based on a detailed modeling of SRV in local contact areas involving a non-uniform SRV in the void's vicinity. The non-uniform SRV in voids is traced back to laser induced damage nearby the local contact opening in the dielectric layer. Additionally the existence of laser damaged areas close to filled contacts is demonstrated in this work.

Si paste technology for high-efficiency solar cells

This study demonstrates the successful implementation of industrially feasible local B doping as a local back surface field via the Si paste technology to fabricate high-efficiency solar cells. Si paste formed by 25wt% p-type Si nanoparticles and 75wt% organic solvent is used as the B source. Si paste is fully screen-printed on the preprocessed p-type Si rear surface, and then a homogenous Si cladding layer is formed by picosecond laser cladding. During this process, B diffuses into the Si substrate. The Si cladding layer itself is a heavily doped layer and forms a metallurgical bond between Si substrate. After co-firing with Al paste, cells obviously avoid Kirkendall void formation at contact regions. The heavily doped Si cladding layer and localized B doping can decrease the contact resistance and reduce the minority carrier recombination at the local metal contacts. This optimized rear surface design finally contributes to push the overall average cell efficiency over 20.3% on p-type CZ wafers.

P+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy

Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μm thick p+-doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p+-doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p+-doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitance-voltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 1019 cm−3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process.

Void Formation on PERC Solar Cells and Their Impact on the Electrical Cell Parameters Verified by Luminescence and Scanning Acoustic Microscope Measurements

Ideally formed local Al-contacts of passivated emitter and rear contact solar (PERC) cells feature an eutectic and an uniform local back surface field (LBSF). Under certain conditions the eutectic is missing after the co-firing process, referring to the well-known voids. So far light beam induced current (LBIC) measurements are used to obtain information concerning the passivation quality of the LBSF in local contacts in general. In addition, the destructive technique of scanning electron microscopy (SEM) is established for distinguishing whether a void features a sufficiently thick BSF-layer or a very thin/no BSF-layer. However, both methods are very time consuming.This paper shows a non-destructive and fast characterization of solar cells by applying electroluminescence (EL) and photoluminescence (PL) measurements to investigate the effect on the electrical parameters after locating the voids by scanning acoustic microscopy (SAM). For filled contacts EL and PL measurements correlate well with the resulting values for series resistance (RS) and dark saturation current density (j0): the formed LBSF leads to a good surface passivation (high PL signal intensity, low value for j0) and the eutectic layer ensures a good electrical contact (high EL signal intensity, low value for RS). Voids with a sufficiently thick LBSF show a high PL signal intensity whereas the intensity is significantly reduced for a very thin or completely missing LBSF. Increased values for RS can be explained by the missing eutectic layer. In addition, the electrical connection of the LBSF to the paste can be derived from the value of RS.

Laser Treatment to form An Effective Base Contact in a - Si:H/c-Si Heterojunction Solar Cells

In this paper we investigate the p-type a-Si:H/ia-Si:H/p-type c-Si structure, commonly used as base contact in amorphous/crystalline silicon heterojunction solar cell when fabricated on p-type c-Si wafer. Even though the most effective amorphous silicon/crystalline silicon heterostructure is based on n-type c-Si due to higher bulk lifetime, the p-type c-Si still remains the most common and cheaper substrate for silicon based solar cell. In particular we study the effect of localized 532nm pulsed laser treatment at different laser conditions in order to reduce the cell series resistance due to the base contact. In this approach the p-type a-Si:H layer is used as a source of boron dopant. Depending on the thickness of the p-type a-Si:H film, when the laser beam is focused on p-type a-Si:H layer the boron can be transferred into the c-Si base to form an overdoped region and then an effective local Back Surface Field, able to enhance the hole collection at the metal of the base electrode in the p-type c-Si based heterojunction solar cell. The application of a thin Aluminum layer on top of the amorphous silicon to be treated by laser is also concerned. Series resistance of a transverse structure composed by the laser treated p-type a-Si:H/c-Si/opposite surface contacted by InGa is considered to optimize the laser procedure. Values as low as 0.5Ωcm2 are obtained when the aluminum layer is adopted.

Laser doping of boron-doped Si paste for high-efficiency silicon solar cells

Boron laser doping (LD) is a promising technology for high-efficiency solar cells such as p-type passivated locally diffused solar cells and n-type Si-wafer-based solar cells. We produced a printable phosphorus- or boron-doped Si paste (NanoGram® Si paste/ink) for use as a diffuser in the LD process. We used the boron LD process to fabricate high-efficiency passivated emitter and rear locally diffused (PERL) solar cells. PERL solar cells on Czochralski Si (Cz-Si) wafers yielded a maximum efficiency of 19.7%, whereas the efficiency of a reference cell was 18.5%. Fill factors above 79% and open circuit voltages above 655 mV were measured. We found that the boron-doped area effectively performs as a local boron back surface field (BSF). The characteristics of the solar cell formed using NanoGram® Si paste/ink were better than those of the reference cell.

Dielectric Charge Tailoring in PECVD SiO<inline-formula> <tex-math>${}_x$</tex-math> </inline-formula>/SiN <inline-formula> <tex-math>${}_x$</tex-math> </inline-formula> Stacks and Application at the Rear of Al Local Back Surface Field Si Wafer Solar Cells

State-of-the-art surface passivation results are obtained on undiffused p-type commercial-grade Czochralski Si wafers with effective surface recombination velocity S eff values of ∼8 cm/s and implied open-circuit voltage iV oc values of up to 715 mV with an industrially fired dielectric stack of silicon oxide and silicon nitride (SiO ${}_x$ /SiN ${}_x$ ) deposited in an industrial inline plasma-enhanced chemical vapor deposition reactor. We are able to controllably vary the total positive charge density Q total in the stack by more than one order of magnitude (1011–10 12 cm−2) with no impact on midgap interface state density D it,midgap (5 × 1011 eV−1·cm−2) by altering the deposition temperature of the SiO ${}_x$ layer in the stack. We show experimentally that, for inversion conditions, S eff scales with the inverse square of the charge density $1/Q_{{\rm total}}^2$ , which is in good agreement with theory. Based on the measured injection-level–dependent minority carrier lifetimes and the total positive charge densities, it is shown that films with higher positive charge density have higher 1-sun V oc and fill factor ( FF ) potential. Large-area alloyed aluminum local back surface field solar cells confirmed this by showing higher conversion efficiency by 0.17% absolute due to improved cell V oc and FF of the solar cells featuring a SiO ${}_x$ /SiN ${}_x$ stack with a higher Q total.

High efficiency n-type silicon solar cells featuring passivated contact to laser doped regions

Minimizing carrier recombination at cell contacts becomes increasingly important for reaching high efficiency. In this work, the passivated contact concept is implemented into n-type silicon solar cells with laser-processed local back surface fields. The passivation and contact characteristics of the SiO2/amorphous silicon (a-Si:H) stack on localized laser doped n+ regions are investigated. We find that the SiO2/a-Si:H stack provides not only good passivation to laser doped n+ regions but also allows a low contact resistivity after thermal annealing. With the implementation of the SiO2/a-Si:H passivated contact, an absolute efficiency gain of up to 1.5% is achieved for n-type solar cells.

Laser Fired Local Back Contact C-Si Solar Cells Using Phosphoric Acid for Back Surface Field

We report on a laser doping process for the formation of a local back surface field (BSF) using phosphoric acid (H3PO4) for n-type passivated emitter rear totally diffused silicon solar cells. The sheet resistance of the BSF layer was varied by changing the H3PO4 concentration. The BSF layer was passivated using SiNx. With the passivated BSF, the LBC solar cell shows an improved open circuit voltage. A laser power of 44 mW with 10 kHz resulted in a 45-Ω/sq BSF layer with effective lifetime of 290 μs and a higher Voc of 623 mV. With the optimized laser parameters, devices with the best electrical results yielded a short circuit current density of 36 mA/cm2 and an efficiency of 18.26%.

Accurate extraction of the series resistance of aluminum local back surface field silicon wafer solar cells

Aluminum local back surface field (Al-LBSF) silicon wafer solar cells are currently intensively investigated in the photovoltaic community and are expected to enter mass production in the near future. In this work we show that this solar cell architecture can pose significant challenges in the determination of the series resistance at the maximum power point. We also show that some of the traditional methods for extracting the series resistance of these cells result in a severe underestimation, due to injection dependent saturation current densities. By using a combination of electro- and photo-luminescence images, we demonstrate that the series resistance of Al-LBSF solar cells can be accurately determined.

Optical Modeling of Alkaline Saw-Damage-Etched Rear Surfaces of Monocrystalline Silicon Solar Cells

In this study, we propose a geometric optical model to represent alkaline saw-damage-etched (SDE) surfaces of monocrystalline silicon wafers. An experimental study is carried out to characterize the optical properties of alkaline SDE surfaces on monocrystalline silicon wafers. Based on the surface characteristics measured by goniometry and height profiling, a geometric optical model is developed to describe the SDE surface with two parameters: characteristic angle and planar fraction. Using the path-tracing method, spectral reflectance simulations are carried out for four different types of samples. With the measured characteristic angle of 22° and planar fraction of 0.25 or 0.36, we find that this representation of SDE surface can predict the reflection and transmission with a root-mean-square error (RMSE) of the equivalent current density from 0.19 to 0.57 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The developed model is also applied to the optical loss analysis of aluminum local back surface field (Al-LBSF) solar cells with an SDE rear surface. We find that SDE rear surfaces provide better light trapping than planar surfaces. As a consequence, Al-LBSF solar cells with pyramids on the front and an SDE rear are predicted to produce 0.6 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> more photocurrent than similar cells with a planar rear surface.

Laser ablation of aluminum oxide and silicon nitride rear-side passivation for i-PERC cell

Rear-side passivated solar cells are one of the next-generation concepts for industrial mass production lines. Our production line has achieved greater than 19% for a commercial-grade with a homogeneous POCl3 emitter. In this study we investigated an industrial passivated emitter and rear cell (PERC) structure with optimized laser ablation after the deposition of rear-side passivation layers of aluminum oxide and silicon nitride. Microscopy was carried out to compare the quality of the rear-side contact, opening, and local back surface field area. The rear-side pattern design of the line width and spacing was studied to improve the cell performance. Minimization of the series resistance is influenced by the contact area and line spacing. We have processed an i-PERC cell for evaluating the cell parameters. The optimized rear line contact scheme and screen-printing conditions made it possible to realize an increase Jsc, Voc, and the conversion efficiency. Finally, we attained significant improvements of Jsc = 39.06 mA/cm2, Voc = 647 mV, FF = 78.09, and conversion efficiency = 19.73%

Analysis of thin bifacial silicon solar cells with locally diffused and selective back surface field

The aim of this work is to present the development and comparison of thin n+pp+ industrial bifacial silicon solar cells produced with the local screen-printed Al back surface field (BSF) to those with the selective BSF doped with aluminum-boron. To produce solar cells with selective BSF, the boron diffusion based on spin-on dopant was introduced in the process sequence. The thin SiO2 layer (10 nm) thermally grown did not produce good passivation on the rear face and wafers were contaminated during aluminum diffusion in the belt furnace. The implementation of selectively doped BSF improved the efficiency by reflecting minority charge carriers and the wafer contamination by belt furnace was compensated by boron diffusion. The bifacial solar cells with B-Al selective BSF achieved an efficiency of 13.7% / 8.9% (front / rear illumination) and presented lower sensitivity to the belt furnace processing and to the quality of the rear surface passivation.

PassDop rear side passivation based on Al2O3/a-SiCx:B stacks for p-type PERL solar cells

In this study we present a new approach for p-type PERL solar cells based on an Al2O3 passivation in combination with boron doped amorphous SiCx. During a laser diffusion process, the rear side passivation is locally opened and simultaneously dopants are driven from the stack into the silicon to create a local back surface field resulting in a sheet resistance in the range of 15Ω/sq. We show that the main contribution to the dopants in the local back surface field originates from the a-SiCx:B layer, while aluminum is mainly present near the surface. This p-type PassDop stack is compatible with a firing step, reaching surface recombination velocities lower than 3cm/s. With this stack, small area solar cells on p-type float-zone silicon were processed, achieving energy conversion efficiencies up to 21.4% and fill factors of up to 82.5% with PVD contacts in a proof-of-concept batch.

Impact of Auger recombination parameterisations on predicting silicon wafer solar cell performance

For high-efficiency silicon wafer solar cells, Auger recombination is becoming one of the most important efficiency limiting factors. For this purpose it is desirable to be able to use different Auger recombination parameterisations in advanced computer simulations. In this paper we present a method to implement arbitrary Auger parameterisations in the software package Sentaurus TCAD, enabling two- and three-dimensional simulation of solar cells using different Auger parameterisations. As examples, we implemented and investigated three different Auger parameterisations (proposed by Altermatt et al., by Kerr and Cuevas, and by Richter et al.) from the literature. For verification, we simulate Auger lifetimes for different doping densities and injection levels in crystalline silicon. The simulated Auger lifetimes are found to agree well with analytical solutions (differences less than 0.001 %). We then employ the three different Auger parameterisations for fitting measured effective lifetime curves of both $$n$$ n -type and $$p$$ p -type float-zone silicon lifetime samples and show which models are applicable under which conditions. We further compare the difference between the three Auger parameterisations by simulating characteristics of a screen-printed aluminium local back surface field silicon wafer solar cell. The simulation results agree well with the characterisation results. We find that the choice of Auger parameterisation can lead to significant differences in the predicted solar cell behaviour under one-Sun illumination. We demonstrate that different Auger parameterisations may result in significant differences in the blue response, by simulating a heavily doped emitter of an aluminium local back surface field silicon wafer solar cell.

Influence of atomic layer deposition Al2O3 nano-layer on the surface passivation of silicon solar cells

A stack of Al2O3/SiNx dual layer was applied for the back side surface passivation of p-type multi-crystalline silicon solar cells, with laser-opened line metal contacts, forming a local aluminum back surface field (local Al-BSF) structure. A slight amount of Al2O3, wrapping around to the front side of the wafer during the thermal atomic layer deposition process, was found to have a negative influence on cell performance. The different process flow was found to lead to a different cell performance, because of the Al2O3 wrapping around the front surface. The best cell performance, with an absolute efficiency gain of about 0.6% compared with the normal full Al-BSF structure solar cell, was achieved when the Al2O3 layer was deposited after the front surface of the wafer had been covered by a SiNx layer. We discuss the possible reasons for this phenomenon, and propose three explanations as the Ag paste, being hindered from firing through the front passivation layer, degraded the SiNx passivation effect and the Al2O3 induced an inversion effect on the front surface. Characterization methods like internal quantum efficiency and contact resistance scanning were used to assist our understanding of the underlying mechanisms.

Firing-stable PassDop passivation for screen printed n-type PERL solar cells based on a-SiNx:P

Rear side passivation and local back surface field formation are two of the main technological challenges for n-type PERL silicon solar cells. A promising approach is the PassDop process. This process combines a phosphorous doped passivation layer deposited on the rear side with a subsequent laser process to create both a local contact opening as well as a local back surface field. In this paper we introduce a new layer system based on doped amorphous silicon nitride (the fPassDop process) which is able to passivate the n-type surface after a firing step as typically used for screen printed contacts. After the firing step, an effective recombination velocity <5cm/s can be reached with this layer. The measured sheet resistance is in the range of 60Ω/sq after the laser process. In a first test the fPassDop process is applied to small area solar cells achieving a conversion efficiency of 21.3% (675mV Voc). Additionally, we fabricated large area n-type solar cells with screen printed front side contacts achieving 20.1% efficiency and Voc of 668mV.

Fabrication of c-Si solar cells using boric acid as a spin-on dopant for back surface field

Cost effective solar cells are paramount for solar power to compete with traditional power generation. Here we present results on two low cost, both side textured c-Si solar cells. One solar cell had an Al back surface field (BSF), while the other had boron BSF fabricated by using spin-on boric acid as a p-type dopant. As compared to the Al-BSF solar cell, an improvement in efficiency of 1.7% was observed for a solar cell with boron BSF. Since boron BSF is stronger than Al-BSF, an improvement in efficiency can be attributed to an increase in long wavelength response, collection efficiency and reduced back surface recombination. The boron BSF solar cell showed an efficiency of 12.9% with Voc of 0.56 V, Jsc of 32.2 mA cm−2 and FF of 0.72. These parameters are expected to significantly increase further with the addition of layers such as anti-reflection/passivation layers at the front side, and back surface passivation with local back surface field, etc.

Investigation of Rear Side Selective Laser Ablation and Damage Etching Process for Industrial PERC Solar Cells

We fabricated industrial passivated emitter and rear cell (i-PERC) solar cells by upgrading a conventional solar cell process for a commercial grade 6 inch p-type wafer. The complex fabrication process of the rear side opening to form the local back surface field (LBSF) limits mass production of PERC solar cells, therefore, we focused on the selective ablation of rear side passivation layer using laser and laser-induced damage etching. Optical microscope and electroluminescence (EL) images reveal that the damage etching after laser ablation improved its surface quality, and the shunt resistance (Rsh) was also increased by optimizing the damage etching. We obtained 19.73% conversion efficiency on i-PERC solar cells.

Optimization of Laser Processes for Local Rear Contacting of Passivated Silicon Solar Cells

Laser Firing Contact (LFC) and Laser Doping (LD) have become potential alternatives to the Al BSF thermal processing conventionally used in p-type c-Si solar cell rear contacts. Optimized LFC and LD processes allow, not only the generation of efficient micro-contacts, but also the diffusion of p-type doping impurities reducing the surface recombination velocity due to the formation of a local back surface field (BSF). In this work, three different laser strategies to create ohmic micro-contacts are studied: 1) evaporated Aluminum LFC, 2) Aluminum foil LFC and 3) Aluminum oxide (Al2O3) LD. The laser source used was a pulsed Nd-YAG 1064nm laser working in the nanosecond regime. Laser parameters were explored to optimize the electrical behavior of the contacts and their carrier recombination rate. Optimized laser parameters lead to specific contact resistance in the 1.0 - 1.3 mΩ·cm2 range for all three strategies. From the point of view of carrier recombination, better results were obtained for Al2O3 LD, probably related to the lower energy pulse needed to create the contact. Next, the three proposed laser approaches were applied to the back surface of heterojunction silicon solar cells. Contact quality was not limiting any cell performance indicating that the contact quality is good enough to be applied in high-efficiency c-Si cell concepts. On the other hand, surface recombination velocity at the rear surface on the final devices also points out to Al2O3 LD as the best alternative.