L. Serenelli

Back Enhanced Heterostructure with INterDigitated contact - BEHIND - solar cell

In this paper we investigate in detail how the heterostructure concept can be implemented in an interdigitated back contact solar cell, in which both the emitters are formed on the back side of the c-Si wafer by amorphous/crystalline silicon heterostructure, and at the same time the grid-less front surface is passivated by a double layer of amorphous silicon and silicon nitride, which also provides an anti-reflection coating. The entire process, held at temperature below 300degC, is photolithography-free, using a metallic self-aligned mask to create the interdigitated pattern. An open-circuit voltage of 695 mV has been measured on this device fabricated. The mask-assisted deposition process does not influence the uniformity of the deposited amorphous silicon layers. Several technological aspects that limit the fill factor are considered and discussed.

Advances in screen printing metallization for a-Si:H/c-Si heterojunction solar cells

Amorphous / crystalline silicon heterojunction is the most attractive technique to obtain high efficiency solar cells. Usually such a kind of cells is produced starting from n-type silicon wafers, because of several advantages, like the high bulk lifetime and the possibility to easily contact the n-type base with i-n amorphous layers. The emitter is usually covered by Transparent Conductive Oxides (TCO) which works as high conductive layer and Anti Reflection Coating (ARC). The device is completed by a metal grid, made by screen printed silver, sintered at low temperature. Both the TCO and the grid strongly influence the final cell series resistance, and consequently the cell efficiency. When p-type wafer is adopted as substrate for heterojunction cell, the base contact is more difficult to obtain because of the energy bands alignment between the c-Si and the p-type a-Si:H layer. Recently n-type doped SiOx layer has attracted interest as emitter layer in heterojunction device, therefore in this work we show the results obtained on the metallization of n-type SiOx/ptype c-Si heterojunction solar cells by means of low temperature screen printing technique. In particular a new kind of low temperature sintering (<; 200°C) screen printable silver paste has been developed able to ensure high linear conductivity, low specific contact resistivity and strong adhesion to TCOs. We present electrical characterization using Transfer Length Method (TLM) technique. Since the base contact of SiOx/c-Si heterojunction is ensured by laser doping technique starting from p-type a-Si:H layer, we also show how the screen printed Ag paste can enhance the base contact of this solar cell.

Contact Formation on a-Si:H/c-Si Heterostructure Solar Cells

In this chapter a description of the contact formation in a-Si:H/c-Si heterojunction solar cell is detailed. Firstly the doping of amorphous films is reported together with the possibility to enhance the amorphous film conductivity by using Chromium Silicide formation on top of the doped films. Then a finite difference numerical model is used to describe the a-Si:H/c-Si heterojunction solar cell in which both contacts are made by amorphous films. In particular to evaluate the effect of the bandgap mismatch between amorphous and crystalline silicon at the base contact a detailed investigation is presented comparing experimental current voltage characteristics of heterojunction contacts with the results of a simulation based on numerical model. Subsequently, details about formation and properties of a transparent conductive oxide and a screen printing procedure to form metallic grids are presented as a common way to form the heterojunction solar cell electrodes. Finally three examples of heterojunction solar cells are proposed using different approaches to form the contacts. In particular a double side heterojunction cell fabricated on multicrystalline silicon is presented, a laser fired local contact for the rear side of the cell is shown and finally an interdigitated back contact is described. All the investigations are based on our experience on heterostructure solar cells developed in the past years.

Electroplated Nickel/Tin Solder Pads for Rear Metallization of Solar Cells

In this study, we report on the feasibility of formation of nickel/tin solder pads and bus bars directly electroplated onto the aluminum screen-printed rear metallization layer of silicon-based solar cells. A localized wet processing technique via dynamic liquid drop/meniscus is used to perform the electrodeposition procedure. Excellent mechanical and electrical parameters of electroplated contacts are measured, thus proving the reliability of the proposed approach suitable for industrial application. Adhesion of electroplated nickel/tin solder pads is ensured through a two-step electrochemical pretreatment procedure, resulting in mean peel force values ranging from 2.5 to 3.8 N/mm. Electroplating of solder pads directly onto the screen-printed aluminum layer allows us to obtain a full homogeneous back surface field on the solar cell, resulting in an efficiency gain in 0.31-0.48% abs range. Furthermore, the proposed method completely removes the need for silver in the rear-side metallization layer of silicon-based solar cells.

Surface photovoltage as a tool to monitor the effect of hydrogen treatment on a-Si: H/c-Si heterojunction

The amorphous/crystalline silicon technology has demonstrated its potentiality leading to high efficiency solar cells. We propose the use of surface photovoltage technique as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface. We investigate the effect hydrogen plasma treatments performed on thin amorphous silicon buffer layer deposited over crystalline silicon surface and we compare its effect with that of thermal annealing on the interface. The surface photovoltage technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality.

Localized metal plating on aluminum back side PV cells

In this work we demonstrate a new selective metallization technique to perform localized plating on the screen-printed Al contact using the innovative approach based on Dynamic Liquid Drop/Meniscus that is able to touch the cell back contact in specific defined positions and show that it is possible to produce suitable electrical and mechanical contact with Al-Si and thus to replace the silver from the back contact in the cell manufacturing process reducing the solar cell cost. A fast pre-treatment process was developed to clean and prepare the surface of the aluminum on the back side of PV cells allowing direct plating with good electrical contact. Several commercial aluminum screen printable pastes have been experimented also having different distribution of sphere particles dimensions. We have used high resolution Scanning Electron Microscopy (SEM) and compositional microanalysis with Energy Dispersive X-Ray microanalysis (EDX) to evaluate the metal dispersion within aluminum-silicon inter-diffused region and Transfer Length Method and current-voltage measurements to estimate the specific contact resistivity of the metal contact and series resistance of the overall solar cell device. We have found that the interconnection ribbon soldered on tin contacts plated on screen printed aluminum back contact shows adhesion higher (> 1N/mm) than that verified on screen printed silver over silicon. The main difference between a tin pad and a nickel-tin pad will be shown. Efficiency increase and fill factor are compared respect standard Al-Ag back contact PV cell.

Silicon Based Photovoltaic Cells For Concentration–Research And Development Progress In Laser Grooved Buried Contact Cell Technology

The Laser grooved buried contact silicon solar cell (LGBC) process employed by Narec currently produces LGBC cells designed to operate at concentrations ranging from 1–100 suns and has demonstrated efficiencies at 50X of over 19% and at 100X of over 18.2% using 300 μm CZ silicon[1] wafers. As part of the LAB2LINE[1], APOLLON[2] and ASPIS[3] projects funded under the European Commission Framework Programs (FP6 and FP7) we have made improvements to the LGBC process to improve efficiency or make the cell technology more suitable for industrial CPV receiver manufacturing processes. We describe a process which hybridizes LGBC and more standard screen printing technologies which yields at least a 6% relative improvement at concentration when using more readily available 200 μm thick CZ wafers. We describe a pioneering front dicing technique (FDT). The FDT process is important in small cells where edge recombination effects are detrimental to the performance. We show that by using this new technique we can produ...

Plasma dry etching for selective emitter formation in crystalline silicon based solar cell

Selective emitter formation in silicon based solar cell is recently becoming a common technique to enhance the blue response of solar cell. In this work, we suggest a novel procedure based on a self alignment thought to overcome the realignment problems that still limit its industrial request. The idea is based on a plasma dry etching procedure of the emitter region using the metal grid of the cell as a mask. In particular the plasma etching can reduce the thickness of a homogeneous heavily doped emitter reducing both doping concentration and sheet resistance. Since there is no necessity of realignment step in the proposed selective emitter fabrication process it can be useful and appealing for industrial PV manufacturing.

Porous silicon solar cells

We developed a new process for the fabrication of crystalline solar cell, based on an ultrathin silicon membrane, taking advantage of porous silicon technology. The suggested architecture allows the costs reduction of silicon based solar cell reusing the same wafer to produce a great number of membranes. The architectures combines the efficiency of crystalline silicon solar cell, with the great absorption of porous silicon, and with a more efficient way to use the material. The new process faces the main challenge to achieve an effective and not expensive passivation of the porous silicon surface, in order to achieve an efficient photovoltaic device. At the same time the process suggests a smart way to selective doping of the macroporous silicon layers despite the through-going pores.

Evaluation of Hydrogen plasma effect in a-Si:H/c-Si interface by means of Surface Photovoltage measurement and FTIR spectroscopy

The amorphous/crystalline silicon technology has demonstrated its potentiality leading to high efficiency solar cells. To enhance the interface quality we investigate the effect of hydrogen plasma and thermal annealing treatments performed on thin amorphous silicon layer deposited over crystalline silicon surface. To this aim we use surface photovoltage technique, as a contact-less tool for the evaluation of the energetic distribution of the state density at amorphous/crystalline silicon interface, and FTIR spectroscopy of the same samples to evaluate the evolution of Si-H and Si-H2 bonds. The surface photovoltage technique results to be very sensitive to the different experimental treatments, and therefore it can be considered a precious tool to monitor and improve the interface electronic quality. We found that thermal annealing produces a metastable state which goes back to the initial state after just 48 hours, while the effect of hydrogen plasma post-treatment results more stable. In particular the latter reduces the defect density of one order of magnitude and keeps constant also after one month. The hydrogen plasma is able to reduce the defect density but at the same time increases the surface charge within the a-Si:H film due to the H+ ions accumulated during the plasma exposure, leading to a more stable configuration.