ES2638068B1 - High efficiency photoelectric cell - Google Patents

Abstract

High-efficiency, low-cost, non-toxic photovoltaic cell where the metal grid used in the upper part of conventional silicon cells is replaced by a conductive metal layer invisible to solar radiation.

Description

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DESCRIPTION

High efficiency photoelectric cell.

Object of the invention

Sector of the "Materials Science" applied to "Nanotechnology" manipulating matter at the atomic scale and creating a material that experiences a remarkable improvement in the energy efficiency of solar and photovoltaic cells.

Background of the invention

A solar panel is a module that harnesses the energy of solar radiation. The term includes both the solar collectors used to produce hot water (usually domestic) and the photovoltaic panels used to generate electricity.

In the case of photovoltaic panels, these are formed by numerous cells that convert light into electricity. These cells depend on the photovoltaic effect by which light energy produces positive and negative charges in two nearby semiconductors of different types, thus producing an electric field capable of generating a current.

The structure for silicon solar cells typical of the modern solar cell industry is illustrated in Figure 1. These cells have collector fingers (Figure 1 (1)) and subsequent contact obtained by screen printing, in addition to presenting a surface texturing by chemical attack among other things. In general, commercial cells do not include optimized selective emitter, nor the passivation of surface states, particularly in the issuer, in addition to the serigraphic technology used imposes limits for the reduction of losses, both due to series resistance and shading, due to the collector fingers on the lattice of the solar cells. Therefore, maximum efficiency in these "industrial" cells is only 6 to 15%, depending on the technology used.

The most efficient silicon solar cell (24.7%) so far has been carried out using a sophisticated structure called "Passivated Emmiter and Rear Locally diffused (PERL)", illustrated in Figure 2. This structure, developed by Martin Green and his group in Australia [Progress and Outlook for High-Efficiency Crystalline Silicon Solar Cells, MA Green, J. Zhao, A. Wang, S.R. Wenham, Solar Energy Materials & Solar Cells 65 9-16 (2001) 1, is of great importance because it has shown that it is possible to achieve conversion efficiencies close to the theoretical maximum efficiency (32%, Shockley-Queisser limit) of silicon cells under sunlight (global standard AM1.5 spectrum at 1 kW m-2), but require manufacturing processes such as multi-level photolithography, oxidation and diffusion of impurities in ultra-clean laboratories (microelectronic class for manufacturing integrated circuits) and the use of silicon wafers of extreme quality, so that they cannot be industrialized at production costs appropriate for their terrestrial application.

In addition to the typical mechanisms of energy loss (thermalization of carriers by the network; loss of voltage at the junction and contacts; recombination of holes and electrons) we have others that exist in practice and that can be reduced by design and application of the appropriate manufacturing processes. Among these we can mention: surface recombination due to the presence of defects in the surface of any material; and the reflection of solar radiation on the surface of the solar cell. The first can be reduced by what is called surface passivation. If

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Of silicon, it is usually done by depositing thin layers of an oxide or nitride that eliminate loose bonds on the surface. [G. Santana and A. Morales-Acevedo, Solar Energy Materials and Solar Cells 60 (2000) 135]. In the second case, the reflectance is usually reduced by depositing one or two layers of materials with adequate thickness and refractive index on the face on which the light falls [A. Morales-Acevedo, E. Luna- Arredondo and G. Santana, 29th IEEE Photovoltaic Specialists Conference, New Orleans, LA (IEEE, New York, 2002), p. 293.] In this way there would be a greater optical coupling between the air and the silicon. Another way to achieve the latter is by texturing the surface with pyramidal arrangements (Figure 2 (2)), so that the light is "trapped" inducing multiple reflection towards the photocell. Therefore, the most advanced silicon solar cell has an emitter structure type n +, the base is type p and the posterior region is type p + (ie a silicon solar cell n + - p - p +, Figure 2 (3)). It should be noted that under the metal lattice finger (Figure 2 (1)) there will be an n ++ region to form a good ohmic contact. It should also be noted that a silicon oxide or nitride layer is normally included to achieve passivation of the emitter surface (Figure 2 (4)). This is the type of structure known as "solar cell with selective emitter". However, a solution like the proposal is not known. In this case, it is a question of replacing the metal grid used in the upper part of the commercial cells with a conductive metallic layer that is invisible to solar radiation. The elimination of the metal grid (Figure 1 and 2 (1)) that partially reflects the light (shading), allows an increase in the efficiency of the solar cell as a result of the use of the entire semiconductor area to perform the photoelectric effect. In addition, removing the metal grid eliminates all the toxic components necessary for its attachment to the cell surface. In this way, they are able to manufacture solar cells by a procedure that results in more efficient solar cells, of lower toxicity and of lower cost.

Description of the invention

The proposed solution consists in supplanting the metal grid used in the upper part of the commercial cells, by a conductive metallic layer and invisible to solar radiation. In this way there will be no loss of energy by reflection of solar radiation on the surface of the solar cell (shading), increasing the efficiency of energy conversion.

The method used for the formation of micelles containing a metallic core was that of microemulsion, which basically consists of preparing organic micelles containing around 500 atoms of Pt [O. Guillén-Villafuerte, G. García, B. Anula, E. Pastor, N.C. Blanco, M.A. López-Quintela, A. Hernández-Creus and G.A. Plans, Angew. Chem. Int. Ed. 118 (2006) 4372].

In the present work, several micelles with different metals in their nucleus have been successfully synthesized, such as Pt, Sn, Ni, Rh, Ir, Cu, Fe, Pd, Cr and Ag. These micelles with metallic nucleus were deposited and distributed uniformly on the surface of a solar cell in the absence of the metal lattice. For this, an aliquot of the micelle solution was deposited on the solar plate (in the absence of the conductive grid). Subsequently, the sample was subjected to heat treatment (300 ° C for 30 minutes) under a reducing atmosphere (H2 / N2: 1/10). It was observed that the metallic deposit is uniform, has good electrical conductivity and even more important is that the photoelectric effect is produced effectively, that is to say the metallic deposit results in a conductive layer and invisible to solar radiation.

Description of the drawings

Figure 1: Main characteristics of a typical silicon solar cell, where the use of collecting fingers (1) is indicated.

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Figure 2: Main characteristics of the silicon solar cell that has the highest efficiency so far. It highlights the use of collecting fingers (1), surface texturing with pyramid arrangements (2) and the structure of the "barrier zone" (3).

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Figure 3: Power curve under light irradiation of a commercial solar panel in the absence of the metal lattice (A) and of a monolayer of micelles containing Pt inside it on a commercial solar plate in the absence of the metallic lattice (B).

15 Figure 4: Power curve under light irradiation of a commercial solar plate in the absence of the metal lattice (A) and of multilayers of micelles containing Pt inside it on a commercial solar plate in the absence of the metallic lattice (C).

Preferred Embodiment of the Invention

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Figures 3 and 4 show the effect of depositing a monolayer (Figure 3 (B)) and multilayers (Figure 4 (C)) of micelles containing Pt inside on a commercial solar panel in the absence of the metal lattice (Figures 3 ( A) and 4 (A)). It is observed how the maximum power as well as the potential in short circuit developed by the solar cell 25 increases slightly when depositing the micelles that contain platinum in its nucleus.

Claims (2)

ES 2 638 068 A2 1. Low-cost, non-toxic high efficiency photovoltaic cell characterized by the replacement of the metal grid used in the upper part of conventional cells 5 by a conductive metallic layer and invisible to solar radiation. 2. Method for the deposit of a metal monolayer or multilayers used in photovoltaic cells lacking metal grids characterized according to claim (1) comprising the following steps: 10 l. Deposit an aliquot of micelles containing a metal core (eg Pt, Sn, Ni, Rh, Ir, Cu, Fe, Pd, Cr and Ag, etc.) on a commercial solar cell (mono and / or polycrystalline silicon wafers) in the absence of the metal lattice (collecting fingers). 15 II. Drying under a flow of inert gas. III. Drained wash with acetone. IV. Heating at a temperature of approximately 300 ° C during 20 approximately half an hour under a flow of reducing gas.