Emitter Sheet Resistance Research Articles

Q.ANTUM NEO with LECO Exceeding 25.5 % cell Efficiency

In recent years, we have developed the Q.ANTUM NEO solar cell, a rear-passivated double-sided contact solar cell that achieves power conversion efficiencies exceeding 25.5 %. Our streamlined and cost-efficient process leverages the proprietary Laser Enhanced Contact Optimization (LECO) technology. The LECO process has undergone extensive optimization and has been successfully implemented in both our pilot line and mass production facilities, resulting in significant improvements in solar cell efficiency. LECO facilitates a novel formation of the contact interface between the silicon surface and metallization, leading to remarkably low j0,met values (as low as 160 fA cm−2). The contact resistivity of 20 μm wide contacts on a high sheet resistance emitter is below 1 mΩ cm2, with minimal impact on fill factor (FF) and overall cell efficiency. Rigorous reliability tests, including TC600 and DH3000, demonstrate module degradation well below 5%rel. Our champion cell conversion efficiency achieved in the pilot line using LECO technology reaches 25.6 %, accompanied by a notable open-circuit voltage exceeding 730 mV.

TCAD based numerical exploration of industrially feasible tunnel oxide passivated contact on p-type silicon

Tunnel Oxide Passivated Contact (TOPCon) is considered as an immediate successor of Passivated Emitter and Rear Contact (PERC). Rear side full area hole selective passivating contact with front n+ diffused emitter on p-type silicon (Si) (p-TOPCon) would be an incremental upgrade from PERC. However, limited studies are present on p-type Si and most of them are on TOPCon with rear full area electron selective contact (n-TOPCon). In this work, we explore the potential of p-TOPCon by incorporating industry relevant material and process parameters in Sentaurus TCAD simulations. These include bulk lifetime (τb), wafer resistivity (ρ), polysilicon related rear parameters, front emitter sheet resistance (Rsheet) and its passivation (Sfront), and finger width (Wfinger). Simulation results are benchmarked against PERC and n-TOPCon. p-TOPCon outperforms PERC and has lower sensitivity to wider range of τb, ρ, and Sfront. Simulation suggests that an efficiency of 23.83% can be achieved for industrially feasible τb of 5 ms, ρ of 1 Ω-cm, emitter Rsheet of 132 Ω/sq, and Wfinger of 20μm for p-TOPCon. Replacement of diffused emitter with n-TOPCon would be ideal after accomplishing above anticipated advancements and in such a scenario, efficiency > 24.4% can be achieved on p-type Si.

Improving the performance of GaInP solar cells through rapid thermal annealing and delta doping

We show that the effect of rapid thermal annealing (RTA) on carrier lifetime in GaInP grown by molecular beam epitaxy depends strongly on both doping type and density, and that these disparities must be accounted for to realize high-performance GaInP solar cells. Although the photoluminescence intensity and lifetime of lightly doped p- and n-GaInP improved greatly with RTA, heavily doped n + -GaInP showed sharp degradation upon RTA, preventing the realization of GaInP front-junction solar cells with low emitter sheet resistance. Since a low series resistance is important to achieve high fill factor (FF), we designed a front-junction cell utilizing a thin, lightly doped n-type emitter with delta doping to enhance conductivity, attaining an open-circuit voltage (V OC ) of 1.40 V and FF of 86%. We then designed rear-heterojunction solar cells to further leverage the relatively long lifetime of lightly n-doped GaInP (∼19 ns). With the help of delta doping in the n-AlInP window to improve surface passivation, we attained a V OC of 1.42 V, similar to cells grown by metal-organic vapor phase epitaxy. • Rapid thermal annealing greatly improves luminescence of lightly doped n- and p-GaInP but degrades heavily doped n + -GaInP. • We demonstrate front-junction cells with V OC = 1.40 V, the highest value for cells grown by molecular beam epitaxy (MBE). • We present the first rear-junction GaInP solar cells grown by MBE with V OC of 1.42 V; delta doping the window improves surface passivation.

Industrial High Throughput Emitter Formation and Thermal Oxidation for Silicon Solar Cells by the High Temperature Stack Oxidation Approach

Herein, the high temperature stack oxidation (HiTSOx) approach for the fabrication of passivated emitter and rear cells (PERCs) is investigated. This approach features a combination of phosphorus oxychloride (POCl3) diffusion shortened to the phosphosilicate glass (PSG) deposition phase as well as high temperature thermal oxidation using stacked wafers. During the latter thermal oxidation, the incorporated phosphorus is redistributed and diffuses deeper into the silicon wafer. The simultaneously growing thermal dioxide serves as a passivation layer. Due to the use of stacked wafers, the throughput of the HiTSOx approach is three times higher in comparison to state‐of‐the‐art oxidation at moderate temperature. Applying a busbarless metallization layout, a median energy conversion efficiency of η = 22.2% is achieved for the HiTSOx approach, being similar to the performance of the reference group with state‐of‐the‐art PERC processing also with η = 22.2%. Despite stacking of the wafers during the thermal oxidation, an excellent homogeneity of the oxide layer thickness of ±1 nm over the wafer surface is found, whereas the passivation quality features dark saturation current densities j0e as low as j0e = (30 ± 3) fA cm−2 at emitter sheet resistances Rsh = (199 ± 6) Ω sq−1.

Spatially Resolved Determination of Metallization-Induced Recombination Losses Using Photoluminescence Imaging

Metallization induced recombination losses are one dominant loss mechanism for current industrial solar cells. A precise determination of these losses is important for contacting technology optimization, as well as precise solar cell modeling. Usually, for state-of-the-art approaches to determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> , it is assumed that the samples itself exhibit spatially uniform properties (e.g., carrier lifetime or sheet resistance) or that the used reference samples are identical to the metallized samples. Finally, in most cases, only one global j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> -value for the entire sample is given, neglecting possible spatial inhomogeneities. In this article, we mostly eliminate the necessity for the assumptions of perfect sample homogeneity by means of an interpolation scheme of the photoluminescence (PL) signal. Thereby, we can predict the PL signal of a virtually nonmetallized test field with a relative standard deviation of about σ ≈ 0.7%. Additionally, we determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> for specific test fields at different positions on the sample and correlate the results to the local emitter sheet resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> , the local peak firing temperature of the sample during the fast firing process T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> , and the test field finger spacing d. For our samples, a reduction of d from d = 1000 μm to d = 200 μm leads to a reduction of j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> by up to 18%. This strong effect is physically unexpected and so far not considered by the state-of-the-art approach, frequently performed in the photovoltaic community. Further, we perform a sensitivity and error analysis which reveals that we are able to determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> within an estimated accuracy between 15% and 18%.

A Computationally Efficient Multidiode Model for Optimizing the Front Grid of Multijunction Solar Cells under Concentration

We have developed a computationally efficient simulation model for the optimization of redirecting electrical front contacts for multijunction solar cells under concentration, and we present its validation by comparison with experimental literature results. The model allows for fast determination of the maximum achievable efficiency under a wide range of operating conditions and design parameters such as the contact finger redirecting capability, period and width of the fingers, the light concentration, and the metal and emitter sheet resistivity. At the example of a state-of-the-art four-junction concentrator solar cell, we apply our model to determine ideal operating conditions for front contacts with different light redirection capabilities. We find a 7% relative efficiency increase when enhancing the redirecting capabilities from 0% to 100%.

EFFECT OF PHOSPHOROUS/BORON DOPING PROFILE DIFFERENCES ON THE PERFORMANCE OF SILICON SOLAR CELLS

This research work was done under title “Effect of phosphorous/boron doping profile differences on the performance of silicon solar cells”. Emitter diffusion either phosphorous or boron is quite challenging in photovoltaic industry. It directly affects the emitter saturation current density and the emitter quantum efficiency of silicon solar cells. Our main objective was to make the comparison of both phosphorous and boron diffused emitters for different peak dopant concentrations in silicon solar cells. It was done by using EDNA 2 simulations. We used different parameters in EDNA 2 and simulated the high efficiency solar cells with boron as back ground and phosphorous as emitter. Then we simulated the solar cells with phosphorous as back ground and boron as emitter. We varied the peak dopant concentration of phosphorous as well boron from 1.6E+17 to 3.9E+20. The best internal quantum efficiency of emitter for phosphorous diffused emitters was 95.1 %, obtained at 1.6E19 (cm-3 ) with an effective emitter depth of 0.675 (µm). However, the best internal quantum efficiency of emitter for boron diffused emitters was 80.6 %, obtained at 3.9E19 (cm-3 ). It has an effective emitter depth of 0.732 (µm) that is greater than obtained from phosphorous diffused emitters. We concluded that the phosphorous diffused emitters have much better performance than boron diffused emitter in silicon solar cells. They have better internal quantum efficiency of emitters at lower peak dopant concentration. They have lower emitter sheet resistance with lower effective emitter depth, as also required during silicon solar cell fabrication.

Inverted rear-heterojunction GaInP solar cells using Te memory effect

Tellurium allows attaining heavy n-type doping levels in GaAs, which is suited to achieve very low contact resistivities in solar cells. Besides, it modifies the energy bandgap of MOVPE-grown GaInP by reducing the group-III sublattice ordering and presents a strong memory effect which induces residual n-type doping in subsequent layers, potentially detrimental to the performance of the solar cell. In this work, we present an inverted rear-heterojunction GaInP solar cell that employs a thick Te-doped GaInP layer as absorber, with a doping profile obtained exclusively by controlling the memory effect of Te coming from the preceding growth of a heavily doped GaAs contact layer. In this way, GaInP is partially disordered with the use of no additional surfactant, leading to an increase in the solar cell bandgap of around 35 meV as compared to traditional samples doped with silicon. In the proof-of-concept experimental devices developed so far, the use of a rear-heterojunction configuration and the bandgap increase results in a global open-circuit voltage enhancement of 109 mV. The photocurrent decreases by 1.32 mA/cm2, mostly due to the bandgap blue-shift, with about 0.35 mA/cm2 attributable to lower carrier collection efficiencies. These preliminary results are discussed by analyzing the I-V curve parameters and quantum efficiencies of a Te-doped rear-heterojunction, a Si-doped rear-heterojunction and a Si-doped front-junction solar cell. An additional advantage is that the emitter sheet resistance is reduced from 551 to 147 Ω/□, which offers potential for higher efficiencies through lower front grid shadowing factors, as demonstrated with the concentrator measurements presented.

A Novel TLM Analysis for Solar Cells

Despite its widespread usage, the linear regression method used to obtain contact resistivity and sheet resistance from the transmission line model (TLM) can lead to huge errors. The method assumes that contact parameters (both electric and geometric) are identical across the probed structure, such as a solar cell. In contrast, the contact parameters display various process-associated inhomogeneities. This paper analyzes the limitations of the linear fit method by using mathematical analysis and measurements on the front-side Ni–Cu plated contacts of a passivated emitter and rear contact (PERC) solar cell. It presents a new evaluation method based on the compartmentalization of contact resistance and emitter sheet resistance of a TLM network. The relative increase in the uncertainty of the extracted contact parameters with decreasing contact resistance is verified and linked to strong error coupling. A large error (20%–180%) is observed in the extracted values of the contact resistance for an assumed (5%–40%) scatter in the sheet resistance values, when a linear fit is used on simulated data with absolutely no pre-estimated error in the contact resistance. The presented method enables the detection and elimination of the influence of bad contacts along with minimal error propagation, which is imperative for contacts with small opening width and lower values of contact resistivity.

Performance enhancement techniques for the front and back of nanostructured “black silicon” solar cells

Nanostructured black silicon (bSi) exhibits a broadband antireflection (AR) response due to graded-index and scattering effects, unlike traditional quarter-wavelength dielectric AR coatings. We present various techniques to improve the front- and back-surface performance of nanostructured bSi solar cells. Ammonium dihydrogen phosphate (ADP) is used for proximity doping to reduce the physical impact on the bSi nanostructures during front-surface emitter formation. An optimum concentration of 2 wt. % of ADP is found to result in a typical solar cell emitter sheet resistivity of 50 Ω / sq. Potassium hydroxide is used to etch off the highly doped region of the bSi solar cell front emitter, which results in lower surface recombination and up to a 23% increase in short wavelength (400 to 600 nm) internal quantum efficiency of the bSi solar cell. To reduce the series resistance and enhance surface passivation, forming gas anneal is employed, improving bSi cell’s overall efficiency by over 31%. By optimizing the back-surface-field formed by sputtered aluminum (Al), the backside recombination rate is reduced, improving external quantum efficiency by up to 11% in the long wavelength (>900 nm) region.

Towards 22% efficient screen-printed bifacial n-type silicon solar cells

This paper outlines the development of fully screen-printed, uniformly diffused, bifacial, n-type silicon solar cells produced using a lean processing sequence and industry-standard equipment. An average batch efficiency of 21.4% was measured at standard test conditions for an optimised bifacial cell batch (i.e. illumination on the front surface only). The results of open circuit voltage (VOC) and fill factor (FF) loss analyses are presented for select batches. By targeting reductions in the largest recombination components, optimised batch VOC was increased by 15 mV compared to the baseline case. The two largest recombination components limiting VOC in optimised devices are front contact recombination and passivated rear surface recombination. The largest FF loss components for optimised devices are the emitter sheet resistance and front contact resistance.

The Effect of Nonuniform Emitter Sheet Resistance on PERC Solar Cell Performance

The aim of this research work is to study the effects of non-uniform emitter sheet resistance on the performance of PERC solar cells. For this purpose, we used different simulation techniques including EDNA 2, MATLAB and Griddler 2.5 Pro. We calibrated the phosphorous doping profiles with Nmax of 4E20, 3.5E20, 3E20, 2.5E20, 2E20, 1.5E20 and 1E20 by using EDNA 2. The results show that the emitter saturation current densities are decreased for corresponding increase in sheet resistance during phosphorous diffusion. In this way we find a relationship between emitter sheet resistances and recombination current densities and developed a MATLAB function from it. By assuming maximum emitter sheet resistance and the corresponding minimum recombination current density at the centre of the cell, we determined their average values from the same MATLAB function. These average values were then used as input parameters for computer simulations Griddler 2.5 Pro, to assess the effect of non-uniformities in emitter sheet resistance and recombination currents on the performance of p- PERC solar cells. The open-circuit voltage, short-circuit current density, fill factor and efficiency are calculated as a function of the difference between the maximum and minimum value of emitter sheet resistance. The results show that the open-circuit voltage and the efficiency of p-PERC solar cell are decreased if the difference between maximum and minimum value of emitter sheet resistance is increased from 0.45 ohm/sq to 19.3 ohm/sq. Whereas the fill factor is increased for the corresponding increase in the difference between maximum and minimum value of emitter sheet resistance.

Investigation of Tellurite Based Fire-Through Dielectric (FTD) Ag Paste Contacts for Si Solar Cell with High Sheet Resistance Emitter

This paper reports the importance of PbO/TeO2 ratio in the formation of Ag2Te, which is responsible for increasing the reformed glass conductivity after sintering. The presence of Ag2Te, a degenerate semiconductor, seen in the Raman Spectra, after HNO3 etching of the gridline and its absence in the HF etched counterpart, supports this assertion. Thus, the increase glass conductivity after sintering has the potential of enabling formation of high quality contacts to emitter with high sheet resistance.

High efficiency screen-printed n-type silicon solar cell using co-diffusion of APCVD boron emitter and POCl3 back surface field

We present the fabrication and analysis of Passivated Emitter and Rear Totally Diffused (PERT) solar cells on n-type silicon using a co-diffusion process. In a single high temperature step, a BSG/SiOx stack deposited by APCVD and a POCl3 back surface field diffuse into the wafer to form the boron doped emitter and phosphorus doped back surface field. The SiOx layer on top of BSG acts as a masking layer to prevent cross-doping of phosphorus as well as a blocking layer for boron out-diffusion. This resulted in an initial sheet resistance of 76 Ω/□ with good uniformity and a final p+ emitter sheet resistance of 97 Ω/□ after boron rich layer removal. Additionally, bulk lifetime was investigated before and after the high temperature step that resulted in an increase from 1.2 ms to 1.5 ms due to a POCl3 gettering effect. A peak cell efficiency of 20.3% was achieved and each recombination component in terms of saturation current density was calculated and analyzed to understand the cell for further efficiency enhancement.

Structure and composition of phosphosilicate glass systems formed by POCl3 diffusion

The phosphosilicate glass (PSG) layer system grown on the silicon surface during diffusion processes with phosphorus oxychloride (POCl3) is a two-layer stack system consisting of a PSG and a silicon dioxide (SiO2) layer. Understanding the stack layers’ structure and composition is essential for further optimizing POCl3 diffusion processes. For diffusion processes with in-situ oxidation (i.e. a high oxygen gas flow during the drive-in step), we find that the intermediate SiO2 layer thickness increases significantly from 8 nm (no in-situ oxidation) to 43 nm, while the PSG layer thickness remains constant at about 8 nm. This thick SiO2 layer seems to hinder the diffusion of phosphorus atoms from the PSG through the SiO2 layer into the silicon. Implementation of a second deposition step with active N2-POCl3 flow at the end of another diffusion process type increases the thickness of the PSG layer from 14 nm to 25 nm, while at the same time the intermediate SiO2 layer thickness decreases by about 40% to 5 nm. The total phosphorus dose within the PSG/SiO2 stack layer thereby increases by a factor of three and is determined to be about 1.7·1015 cm-2. These findings make the approach of a second deposition step very interesting for, e.g., laser doping applications to form selective emitters. For diffusion processes with different N2-POCl3 to oxygen (N2-POCl3:O2) ratios during deposition, we find only small influences on the PSG/SiO2 stack layers present after drive-in. However, the resulting emitter sheet resistances are strongly impacted by the N2-POCl3:O2 ratio; the measured sheet resistances continuously decrease from 343 Ω/sq for a N2-POCl3:O2 ratio of 0.8 to 72 Ω/sq for a ratio of 4.0.

Understanding the influence of tellurium oxide in front Ag paste for contacting silicon solar cells with homogeneous high sheet resistance emitter

This paper investigates TeO2, one of the front Ag paste additives, to understand its role in low contact and gridline resistances for screen-printed Si solar cell. It is concluded that TeO2 aids the reduction of molten glass frit viscosity during contact co-firing. This in turn, leads to uniform flow of molten glass frit, both in the gridline bulk and interface of gridline and SiNx. Therefore, the uniform wetting and etching of SiNx and consequently larger contact area of metal to Si compared to its counterpart without TeO2. Hence, the current transport mechanism from Si to gridline can be said to be both direct and tunneling. The Raman spectra showed a blue shift in the phase of the TeO2 after contact co-firing in the gridline bulk confirming a crystalline γ-TeO2.

Photoluminescence Imaging of Silicon Wafers and Solar Cells With Spatially Inhomogeneous Illumination

Photoluminescence imaging is a fast and powerful spatially resolved characterization technique, commonly used for silicon wafers and solar cells. In conventional measurements, homogeneous illumination is used across the sample. In this paper, we present a photoluminescence imaging setup that enables inhomogeneous illumination with arbitrary illumination patterns to determine various parameters of solar cells and solar cell precursors. To demonstrate the strength of the proposed inhomogeneous illumination imaging, a set of proof-of-concept measurements have been conducted; these measurements include contactless series resistance imaging, emitter sheet resistance, and diffusion length measurements. The results indicate that the use of inhomogeneous illumination significantly extends the range of photoluminescence imaging applications for the characterization of silicon wafers and solar cells.

Further Studies on the Effect of SiNx Refractive Index and Emitter Sheet Resistance on Potential-Induced Degradation

We present the impacts of silicon nitride (SiN x ) antireflection coating refractive index and emitter sheet resistance on potential-induced degradation of the shunting type (PID-s). Previously, it has been shown that the cell becomes more PID-s-susceptible as the refractive index decreases or the emitter sheet resistance increases. To verify the effect of refractive index on PID-s, we fabricated cells with varying SiN x refractive index (1.87, 1.94, 2.05) on typical p-type base solar cells with ∼60 Ω/sq emitters. However, none of these cells showed output power degradation, regardless of the refractive index. Further investigation of the emitter showed that the PID-s was suppressed at ∼60 Ω/sq due to the extremely high surface phosphorus concentration (6 × 1021 cm−3), as measured by secondary ion mass spectrometry. Furthermore, PID-s was observed on cells possessing a high emitter sheet resistance (∼80 Ω/sq). The emitter surface phosphorus concentration plays an important role in determining PID-s susceptibility.

Two dimensional device simulation and performance optimization of n-type silicon solar cell structure using PC2D

In this paper, we analyze the impact of various parameters on the performances of the n-type monocrystalline silicon solar cell experimented by Fraunhofer Institute for Solar Energy Systems (ISE) in Germany. We studied, especially the influence of the base parameters (lifetime, resistivity and thickness), the emitter sheet resistance and the back surface field (BSF) sheet resistance, on the solar cell performances.To optimize this cell we have used PC2D which is a solar cell device simulator that models two-dimensional effects entirely within a Microsoft Excel spreadsheet. With an Al2O3/SiNx front side boron emitter passivation, the metallization parameters were optimized by the authors getting efficiency of 19.60%. If all the parameters have ideal values our optimization provided an efficiency of 20.05% for homogeneous emitter with sheet resistance of 75Ω/□. Furthermore, the study of the emitter led to a new structure developed recently: the selective emitter of n-type solar cell achieving efficiency of 20.20% with sheet resistance of 50Ω/□ under the contacts and 100Ω/□, between contacts.

Thin Al2O3 passivated boron emitter of n‐type bifacial c‐Si solar cells with industrial process

AbstractWe have presented thin Al2O3 (~4 nm) with SiNx:H capped (~75 nm) films to effectively passivate the boron‐doped p+ emitter surfaces of the n‐type bifacial c‐Si solar cells with BBr3 diffusion emitter and phosphorus ion‐implanted back surface field. The thin Al2O3 capped with SiNx:H structure not only possesses the excellent field effect and chemical passivation, but also establishes a simple cell structure fully compatible with the existing production lines and processes for the low‐cost n‐type bifacial c‐Si solar cell industrialization. We have successfully achieved the large area (238.95 cm2) high efficiency of 20.89% (front) and 18.45% (rear) n‐type bifacial c‐Si solar cells by optimizing the peak sintering temperature and fine finger double printing technology. We have further shown that the conversion efficiency of the n‐type bifacial c‐Si solar cells can be improved to be over 21.3% by taking a reasonable high emitter sheet resistance. Copyright © 2017 John Wiley &amp; Sons, Ltd.