Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Screen-printed selective emitter for industrial multicrystalline silicon solar cells

In workpackage two WP2 the most promising, low cost and innovative solutions were studied, to enhance the carrier collection in the blue spectral region, and to minimise surface recombination. Selective emitters formed by screen printing and low temperature passivation schemes were setup, and an increase of one absolute point in efficiency respect to homogeneous emitter, titanium dioxide layer cells were demonstrated.

A selective emitter formation process was developed, based on the selective printing of dopant pastes.

The purpose was to establish the P-incorporation conditions by using a screen printed deposition of a doped paste, especially elaborated by one of the partner of this project, with a grid design similar to the grid used for the contacts in order to form the low resistive part of the emitter under the metallic contacts. For the low doped region of this selective emitter we used both a dopant paste and a spray-on deposition followed by firing step that will yield to form the two high and low doped regions that are suitable for selective emitter industrial silicon solar cells.

The selective emitter was optimised and tested based on the following parameters: measurement of the differential of doping by scanning 4-point probe, reflectance etc, control of the lateral diffusion of the doped paste, measurement of the gradient of doping, characterisation of the junctions properties and effects on the base material.
The selective emitter formation was achieved by screen printing; for our experiments we have employed the Soltech P101 thick film dopant paste.

The first part of the work consisted in verify the effectiveness of selective emitter formation on single crystal, Cz wafers.

First, the homogeneous diffusion from Soltech P101 was optimized, to find the best processing conditions.

Firing was carried out in a standard conveyor belt furnace, in air atmosphere, at temperatures of about 920-950 °C, for 15-20 min, giving, on homogeneous devices, 20 - 40 ohm/square sheet resistance value. The set-up of a full screen printing selective emitter process is impossible without a suitable system of re-alignment of the different patterns, metallisation and selective diffusion. This was achieved on our high resolution screen printing system, equipped with a special optically resolved recognizing system, which allows the realignment of the different patterns within 10 microns.

Selective emitters (20/100 ohm/square) have been obtained by screen printing the dopant paste through a diffusion mask with the same grid pattern of the metallization mask, followed by an optimized firing step in the belt furnace.

Due to out-diffusion induced by the screen printed doped paste, a high-low emitter is formed, allowing a significant increase in the blue spectrum region carrier collection.
To complete the selective emitter structure, different kinds of front surface treatments have been studied: lightly doped screen printed emitters, sprayed-on, thermal oxidation, etc.

Contacts have been made by screen printing (for back contact, an Al based paste was used). Firing of contacts has been carried out in air atmosphere, in an IR belt furnace (peak Temp 860 C, v=50-60 ipm), and temperature profiles have been adjusted to get the best Fill Factor.

A short circuit current gain of up to 3 mA/sqcm has been demonstrated, respect to homogeneous emitters, and open circuit increase of 15 mV.

Several batches of cells have been fabricated with efficiency of about 15%.

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ENEA CR Casaccia
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