Rear Point Contacts Research Articles

Three-Dimensional Numerical Simulation of Rear Point Contact Crystalline Silicon Solar Cells

High efficiency monocrystalline solar cells commonly adopts rear point contacts of limited extension and passivation of the uncontacted bottom silicon surface region in order to improve performance. Modeling and analysis of advanced solar cells is strategic to optimize the device design and to minimize the losses. Several competing physical mechanisms must be accounted for in order to properly analyze rear point contact solar cells and a three-dimensional (3D) analysis of complex geometries is required. In this work we describe the issues related to the design of the mesh grid for the 3D numerical simulations, the definition of appropriate boundary conditions and the main adopted assumptions. Examples of numerical simulations are reported.

Rear point contact structures for performance enhancement of semi-transparent ultrathin Cu(In,Ga)Se2 solar cells

Semi-transparent ultrathin Cu(In,Ga)Se2 (CIGSe) solar cells offer many promising applications but their efficiencies are still limited. In this work, SiO2 point contact structures were prepared using nanosphere lithography on Sn:In2O3 (ITO) substrate and ultrathin CIGSe solar cells were fabricated on top. It was discovered that the SiO2 point contact structure at the CIGSe/ITO interface behaves significantly different from at the CIGSe/Mo interface and does not improve the rear interface reflectivity or short-circuit current density. However, the SiO2 point contact structure brings pronounced electrical benefits, arising from the joint effect of reduced back recombination and induced field effect by internal charges. Semi-transparent ultrathin CIGSe solar cells with an absorber thickness of 380 nm exhibited a significant increase in open circuit voltage of 26 mV and fill factor of 9.4% (absolute). Consequently, the efficiency is improved by 23% (from absolute 6.8%–8.4%).

Photovoltaics: Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer (Adv. Mater. Interfaces 2/2018)

On the left a schematic of an ultrathin Cu(In,Ga)Se2 solar cell with passivated rear interface and on the right a transmission electron microscopy of the real device. In article number 1701101 Pedro M. P. Salomé and co-workers show a significant reduction in the rear recombination, improving the electrical performance of solar cells.

Passivation of Interfaces in Thin Film Solar Cells: Understanding the Effects of a Nanostructured Rear Point Contact Layer

AbstractThin film solar cells based in Cu(In,Ga)Se2 (CIGS) are among the most efficient polycrystalline solar cells, surpassing CdTe and even polycrystalline silicon solar cells. For further developments, the CIGS technology has to start incorporating different solar cell architectures and strategies that allow for very low interface recombination. In this work, ultrathin 350 nm CIGS solar cells with a rear interface passivation strategy are studied and characterized. The rear passivation is achieved using an Al2O3 nanopatterned point structure. Using the cell results, photoluminescence measurements, and detailed optical simulations based on the experimental results, it is shown that by including the nanopatterned point contact structure, the interface defect concentration lowers, which ultimately leads to an increase of solar cell electrical performance mostly by increase of the open circuit voltage. Gains to the short circuit current are distributed between an increased rear optical reflection and also due to electrical effects. The approach of mixing several techniques allows us to make a discussion considering the different passivation gains, which has not been done in detail in previous works. A solar cell with a nanopatterned rear contact and a 350 nm thick CIGS absorber provides an average power conversion efficiency close to 10%.

Realization of point contact for stacks Al2O3/SiNx rear surface passivated solar cells

A simple locally-sintered process for realizing rear point contacts of the Passivated emitter and rear cells, of which rear surface is passivated by Al2O3/SiNx double layers, is demonstrated in this paper. As compared to conventional cells passivated by aluminium back surface field, the cells show higher open circuit voltage and short circuit current. The sintering effect on the final passivation quality of stacks Al2O3/SiNx during the sintering process is investigated and discussed in detail.

Understanding the impact of point-contact scheme and selective emitter in a c-Si BC-BJ solar cell by full 3D numerical simulations

Abstract This work presents a detailed analysis of the impact of a rear point-contact (PC) scheme and a selective emitter (SE) design on the performance of a crystalline silicon (c-Si) back contact-back junction (BC-BJ) solar cell featuring narrower highly-doped back surface field (BSF) areas and wider lowly-doped emitter areas. The analysis was performed both through light and dark J-V simulations by using a rigorous full three-dimensional (3D) electro-optical modeling approach to accurately include the 3D effects of several competing physical mechanisms occurring in a PC structure. Simulation results show that the adoption of a relatively thick and wide metallization contacting the silicon at the rear side only via small circular-shaped holes gives an efficiency improvement above 0.3%abs with respect to a conventional BC-BJ solar cell featuring a linear-contact (LC) metallization scheme. Moreover, the introduction of an optimized SE design, featuring a local deeper highly-doped (HDOP) profile underneath rear point contacts and a narrower lowly-doped (LDOP) profile at the non-contacted rear interfaces, can lead to a further efficiency enhancement of about 0.2%abs, which increases with the emitter width.

Process development and comparison of various boron emitter technologies for high-efficiency (~21%) n-type silicon solar cells

This paper shows for the first time a comparison of commercial-ready n-type passivated emitter , rear totally diffused solar cells with boron (B) emitters formed by spin-on coating, screen printing, ion implantation, and atmospheric pressure chemical vapor deposition. All the B emitter technologies show nearly same efficiency of ~20%. The optimum front grid design (5 busbars and 100 gridlines), calculated by an analytical modeling, raised the baseline cell efficiency up to 20.5% because of reduced series resistance. Along with the five busbars, rear point contacts formed by laser ablation of dielectric and physical vapor deposition Al metallization resulted in another 0.4% improvement in efficiency. As a result, 20.9% efficient n-type passivated emitter, rear totally diffused cell was achieved in this paper. Copyright © 2016 John Wiley & Sons, Ltd.

Theoretical study of the impact of rear interface passivation on MWT silicon solar cells

In this study we analyze the impact of a rear point contact (RPC) scheme in metal wrap through (MWT) solar cells with passivated base by means of numerical simulations. We adopt a modeling methodology which avoids cpu and memory intensive simulations of the whole true three-dimensional solar cell. The proposed approach exploits simplified three-dimensional simulation in combination to the adoption of calibrated semi-empirical models to account for the RPC scheme in an effective way. The sensitivity of the main figures of merit of the complete MWT cell to geometrical and doping parameters such as substrate thickness, hole size as well as substrate resistivity is discussed. From our simulations we observe that the adoption of a passivated base enhances significantly the conversion efficiency. In particular, we calculate an increase of efficiency up to $$1.07\, \%_{\mathrm{abs}}$$1.07%abs with respect to MWT without RPC and up to $$1.33\, \%_{\mathrm{abs}}$$1.33%abs with respect to conventional front contact solar cells, depending upon the substrate resistivity and the substrate thickness. The optimum base contact fraction results within the range 3---5 % and it is higher in the case of lower substrate doping or larger holes.

Three-dimensional numerical analysis of hybrid heterojunction silicon wafer solar cells with heterojunction rear point contacts

This paper presents a three-dimensional numerical analysis of homojunction/heterojunction hybrid silicon wafer solar cells, featuring front-side full-area diffused homojunction contacts and rear-side heterojunction point contacts. Their device performance is compared with conventional full-area heterojunction solar cells as well as conventional diffused solar cells featuring locally diffused rear point contacts, for both front-emitter and rear-emitter configurations. A consistent set of simulation input parameters is obtained by calibrating the simulation program with intensity dependent lifetime measurements of the passivated regions and the contact regions of the various types of solar cells. We show that the best efficiency is obtained when a-Si:H is used for rear-side heterojunction point-contact formation. An optimization of the rear contact area fraction is required to balance between the gains in current and voltage and the loss in fill factor with shrinking rear contact area fraction. However, the corresponding optimal range for the rear-contact area fraction is found to be quite large (e.g. 20-60 % for hybrid front-emitter cells). Hybrid rear-emitter cells show a faster drop in the fill factor with decreasing rear contact area fraction compared to front-emitter cells, stemming from a higher series resistance contribution of the rear-side a-Si:H(p+) emitter compared to the rear-side a-Si:H(n+) back surface field layer. Overall, we show that hybrid silicon solar cells in a front-emitter configuration can outperform conventional heterojunction silicon solar cells as well as diffused solar cells with rear-side locally diffused point contacts.

Performance improved by point‐contact electrodes and SiO 2 /SiN X layers at rear

The rear point-contact fabricated through the laser-opening technique for mass production was applied on the photovoltaic cells. Laser opening, different layers for passivation (SiO2) and protection (SiN X ) were employed to investigate their impact on the performances of solar cells. The SiN X layer protects the SiO2 layer from being burnt through by aluminium paste at the co-firing step. A conversion efficiency (η) of 16.91% with an open-circuit voltage of 628 mV was obtained for the optimal cell, a stack structure with SiO2 and SiN X layers, which also achieves a lower contact resistance of 6.66 mΩ·cm2 and a higher light-beam-induced current of 80.77 mA/cm2. The optimal cell also showed longer lifetime and 3–4% increased quantum efficiency in the visible wavelength range. Therefore, the developed process has simplicity and reliability, is fast and cost-effective and could be applied to industrial applications.

20.7% efficient ion‐implanted large area n‐type front junction silicon solar cells with rear point contacts formed by laser opening and physical vapor deposition

ABSTRACTIn this work, we report on ion‐implanted, high‐efficiency n‐type silicon solar cells fabricated on large area pseudosquare Czochralski wafers. The sputtering of aluminum (Al) via physical vapor deposition (PVD) in combination with a laser‐patterned dielectric stack was used on the rear side to produce front junction cells with an implanted boron emitter and a phosphorus back surface field. Front and back surface passivation was achieved by thin thermally grown oxide during the implant anneal. Both front and back oxides were capped with SiNx, followed by screen‐printed metal grid formation on the front side. An ultraviolet laser was used to selectively ablate the SiO2/SiNx passivation stack on the back to form the pattern for metal–Si contact. The laser pulse energy had to be optimized to fully open the SiO2/SiNx passivation layers, without inducing appreciable damage or defects on the surface of the n+ back surface field layer. It was also found that a low temperature annealing for less than 3 min after PVD Al provided an excellent charge collecting contact on the back. In order to obtain high fill factor of ~80%, an in situ plasma etching in an inert ambient prior to PVD was found to be essential for etching the native oxide formed in the rear vias during the front contact firing. Finally, through optimization of the size and pitch of the rear point contacts, an efficiency of 20.7% was achieved for the large area n‐type passivated emitter, rear totally diffused cell. Copyright © 2014 John Wiley & Sons, Ltd.

Fully Ion-Implanted and Screen-Printed 20.2% Efficient Front Junction Silicon Cells on 239 cm $^{\bf 2}$ n-Type CZ Substrate

In this study, we present fully ion-implanted screen-printed high-efficiency 239 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> n-type silicon solar cells that are fabricated on pseudosquare Czochralski wafers. Implanted boron emitter and phosphorous back-surface field (BSF) were optimized to produce n-type front junction cells with front and back SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> surface passivation and rear point contacts. Average efficiency of 19.8%, with the best efficiency of 20.2%, certified by Fraunhofer ISE, Freiburg, Germany, was achieved. In addition, the planarized rear side gave better surface passivation, in combination with optimized BSF profile, raised the average efficiency to ~20% for the fully implanted and screen-printed n-type passivated emitter, rear totally diffused cells.

Improved Rear Surface Passivation of Cu(In,Ga)Se$_{\bf 2}$ Solar Cells: A Combination of an Al$_{\bf 2}$O $_{\bf 3}$ Rear Surface Passivation Layer and Nanosized Local Rear Point Contacts

An innovative rear contacting structure for copper indium gallium (di) selenide (CIGS) thin-film solar cells is developed in an industrially viable way and demonstrated in tangible devices. The idea stems from the silicon (Si) industry, where rear surface passivation layers are combined with micron-sized local point contacts to boost the open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ) and, hence, cell efficiency. However, compared with Si solar cells, CIGS solar cell minority carrier diffusion lengths are several orders lower in magnitude. Therefore, the proposed CIGS cell design reduces rear surface recombination by combining a rear surface passivation layer and nanosized local point contacts. Atomic layer deposition of 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> is used to passivate the CIGS surface and the formation of nanosphere-shaped precipitates in chemical bath deposition of CdS to generate nanosized point contact openings. The manufactured 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> rear surface passivated CIGS solar cells with nanosized local rear point contacts show a significant improvement in V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> compared with unpassivated reference cells.

Efficiency and stability enhancement of laser-crystallized polycrystalline silicon thin-film solar cells by laser firing of the absorber contacts

Polycrystalline silicon thin-film solar cells produced by continuous-wave diode-laser crystallization at the University of New South Wales were recently reported to have reached a conversion efficiency above 10%. One drawback of these cells, however, was that they exhibited efficiency degradation within several hours after the cell fabrication was completed. In this work we show that by applying laser firing to the rear point contacts of the solar cells, it is possible to stabilize and even to enhance the performance of these devices. Our investigation indicates that it is the poor quality of the contact between the aluminum and the silicon absorber that causes the cell degradation and offers an elegant and industrial-compatible process to improve the cell performance. This is the first time that the laser firing process, initially developed for alloying an aluminum layer through a dielectric layer on crystalline silicon wafer solar cells, is being applied to polycrystalline silicon thin-film solar cells.

Development of rear surface passivated Cu(In,Ga)Se2 thin film solar cells with nano-sized local rear point contacts

For the first time, a novel rear contacting structure for copper indium gallium (di)selenide (CIGS) thin film solar cells is discussed theoretically, developed in an industrially viable way, and demonstrated in tangible devices. The proposed cell design reduces back contacting area by combining a rear surface passivation layer and nano-sized local point contacts. Atomic layer deposition (ALD) of Al2O3 is used to passivate the CIGS surface and the formation of nano-sphere shaped precipitates in chemical bath deposition (CBD) of CdS to generate point contact openings. The Al2O3 rear surface passivated CIGS solar cells with nano-sized local rear point contacts show a significant improvement in open circuit voltage (VOC) compared to unpassivated reference cells. Comparing the passivated devices to solar cell capacitance simulator (SCAPS) modeling indicates that this increase is attributed to a decrease in rear surface recombination of a few orders.

Performance Analysis of Rear Point Contact Solar Cells by Three-Dimensional Numerical Simulation

The adoption of local point contacts at the back surface of high-efficiency monocrystalline silicon solar cells is strategic in order to reduce the recombination losses at the rear side of the device. However, the reduction of the rear-contact surface leads to an increase of series resistance losses. In this paper, we present an extensive analysis based on 3-D optoelectronic numerical device simulations in order to highlight the dependence of the conversion efficiency on the main geometrical and technological parameters of the cell, such as the pitch and the size of the rear point contacts and the substrate resistivity. A state-of-the-art device simulator has been successfully adopted in order to accurately solve the transport equations in the semiconductor by taking into account all the loss mechanisms that are crucial in order to address the design of the cell.

Optimization of Rear Point Contact Geometry by Means of 3-D Numerical Simulation

In this work three-dimensional (3-D) numerical simulations, validated by the experimental measurements of a reference cell, have been performed to optimize the rear contact geometry of a PERC-type solar cell, featuring a high sheet resistance (140Ω/sq) phosphorus-doped emitter and a front-side metallization with narrow and highly-conductive electro-plated copper lines (40μm wide) on lowly resistive Ti contacts. The simulation results show that an optimization of the rear point contact design potentially leads to an efficiency improvement of 0.68%abs compared to the reference cell.

Mono-Crystalline Silicon Solar Cell Optimization and Modeling

The optimization of a mono-crystalline silicon solar cell is complicated due to various competing physical mechanisms that determine its performance. In this paper a three-dimensional (3D) electrical simulation template for mono-crystalline silicon solar cell optimization is developed using Synopsys Sentaurus tool suite, and is used to demonstrate an optimization space of over 1 percent in terms of cell power conversion efficiency. The key design parameters include rear point contact area coverage, substrate doping concentration, cell thickness, and the junction. Comparison between three-dimensional analysis and the simplified two-dimensional and one-dimensional cases is also provided.

Industrially Feasible Rear Passivation and Contacting Scheme for High-Efficiency n-Type Solar Cells Yielding a $V_{\rm oc}$ of 700 mV

n-Type solar cells with passivated rear surface and point contacts have been proven to have an enormous efficiency potential. However, an industrially feasible process for the realization of the passivated locally contacted rear side of this solar cell type is still missing. Therefore, a rear passivation scheme based on doped amorphous silicon carbide was investigated. The newly developed PassDop layer results in excellent surface passivation and, at the same time, acts as a doping source. After the PECVD of the PassDop layer, contact points are locally opened by a laser pulse, and simultaneously, a local back surface field is formed using the phosphorus contained in the layer. In the last step, the rear side is contacted by the evaporation of aluminum. Due to the very effective passivation of the rear side by the doped passivation layer as well as the excellent contact formation by the laser process, the best cell (aperture area of 4 cm2) exhibits an open-circuit voltage of 701 mV and a fill factor of 80.1%, resulting in a confirmed solar cell efficiency of 22.4%.

Effect of electroless nickel on the series resistance of high-efficiency inkjet printed passivated emitter rear contacted solar cells

Many existing and emerging solar cell technologies rely on plated metal to form the front surface contacts, and aluminium to form the rear contact. Interactions between the metal plating solutions and the aluminium rear can have a significant impact on cell performance. This paper describes non-uniform nickel deposition on the sintered aluminium rear surface of passivated emitter and rear contacted (PERC) cells patterned using an inkjet printing technique. Rather than being plated homogeneously over the entire rear surface as is observed on an alloyed aluminium rear, the nickel is plated only in the vicinity of the point openings in the rear surface silicon dioxide dielectric layer. Furthermore, this non-uniform nickel deposition was shown to increase the contact resistance of the rear point contacts by an order of magnitude, resulting in higher series resistance values for these fabricated PERC cells.