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Browse Prior Art Database

Method for Si and GaAs Solar Cell Diffusion

IP.com Disclosure Number: IPCOM000080350D
Original Publication Date: 1973-Dec-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 3 page(s) / 58K

Publishing Venue

IBM

Related People

Hovel, HJ: AUTHOR [+2]

Abstract

One of the most important and difficult steps in fabricating solar cells is the diffusion step. In order to obtain efficient devices, it is necessary to make the depth of penetration of the diffusion equal to about 0.6 micron or less, because of the problems associated with surface recombination. At the same time, the doping must be made high enough to prevent significant lateral series resistance, without degrading the lifetime and diffusion length to an appreciable degree. In current state-of-the-art Si and GaAs cells, this delicate compromise has never totally been accomplished.

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Method for Si and GaAs Solar Cell Diffusion

One of the most important and difficult steps in fabricating solar cells is the diffusion step. In order to obtain efficient devices, it is necessary to make the depth of penetration of the diffusion equal to about 0.6 micron or less, because of the problems associated with surface recombination. At the same time, the doping must be made high enough to prevent significant lateral series resistance, without degrading the lifetime and diffusion length to an appreciable degree. In current state-of-the-art Si and GaAs cells, this delicate compromise has never totally been accomplished.

In Si cells for example, it is found that the diifusion degrades the lifetime in the diffused region to the order of nanoseconds. In GaAs, it has proven difficult to obtain shallow diffusions with surface concentrations high enough to minimize series resistance. Si and GaAs solar cells currently available are limited in their efficiency by both series resistance and poor "blue" spectral response, both of which are determined by the properties of this diffused region.

The present approach consists of the diffusion of suitable dopants through a thin, partially diffusion-masking layer with a pattern of grid lines formed in it by photolithographic techniques. The layer thickness and type are chosen to influence surface concentration and junction depth to the desired values.

One embodiment of the device is shown in Fig. 1. An SiO(2) or other film 1 is applied to a semiconductor substrate 2 by any well known techniques. Film 1 is of the order of 1000 angstroms in thickness, but the thickness to be used is determined by the diffusion-masking properties of the film. A pattern of grid lines 3 is opened in the film by photolithographic methods. The pattern results in islands of film surrounded on four sides, as shown in Fig. 1. Substrate 2 is placed in a diffusion furnace with a suitable dopant source and at a suitable temperature. Since film 1 acts as a partial mask, the diffusion of dopant is considerably shallower beneath film 1 than in grid lines 3. The surface concentration b...