Skip to main content

Very high efficiency multibandgap solar cells based on III-V compounds including growth on alternative substrates

Objective

Demonstrate the feasibility of very high efficiency solar energy photovoltaic conversion (above 30%) by means of tandem cells epitaxially grown in III-V materials. Study the possibilities of cost reduction by means of heteroepitaxy on cheap substrates.
Several methods of fabricating 3-terminal, gallium arsenide and silicon tandem solar cells were explored. Firstly, epitaxial life off (ELO) was employed to transplant high efficiency solar cells to alternative substrates. Although this technique was successful with layers as thin as 500 nm, problems arose with cells larger than 1 cm{2} due to stresses induced when the wax carrier was removed.

A new lift off technique was developed, mesa release and deposition (MRO) which combines the heteroepitaxial growth process for gallium arsenide on silicon with the possibility of removing the epitaxial layer from the substrate, relieving the thermally induced stress from the heteropitaxial layer. This method also proved unsuitable for areas larger than 5 mm{2}. It was concluded that neither ECO nor MRD was better than gallium arsenide cells grown directly on silicon.

In the third approach, the stacked cell process (SCP), the gallium arsenide cell was grown on a gallium arsenide substrate and glued face down to a silicon cell using transparent, space grade silicone rubber, after which the substrate was etched away. Spinning the silicone layer proved unsatisfactory, since air bubbles remained. Better results were obtained by using a droplet of silicone and damping the materials to be glued. An intermediate terminal was provided either by including a metal foil between the 2 cells during the gluing process, or by using a spot of conductive glue.

Transparent top cell contacts on the gallium arsenide layer were tested. A low temperature screen printing technique was devised to protect the glue, which can tolerate a maximum of 200 C.

Very high efficiency solar cells were developed. Gallium arsenide doped with tellurium was used to make a high energy tandem cell. A new annealing procedure was developed. The best cell produced had an efficiency of 26%. A wide band gap cell was made using (aluminium, gallium) arsenide. Minority carrier lifetime proved to be very sensitive to growth temperature; epitaxy must therefore be performed at high temperature.

A mechanically stacked tandem of gallium arsenide and (gallium, indium) arsenide was constructed. Stacking was carried out in 2 steps. First, each cell was mounted on an independent support. Secondly, the supports were stacked. Each support was made of 2 squared copper slabs. The cell was glued onto the bottom slab with a conduction epoxyde resin (back contact). The upper slab was glued onto the bottom one with an epoepoxyde resin with electrical insulation and thermal conduction properties. The stacking of both supports was realized with an electrical isolating resin and optical alignment was optimized by observing the bottom cell photocurrent while moving one support, before the resin reticulation phase.

Silicon was used as the substrate for gallium arsenide cells by means of the aluminium arsenide nucleation technique. Various methods were employed to reduce the residual stress between the lattices. Use of an intermediate buffer layer was best. The buffer was gallium (arsenide, phosphide) (Ga(As(1-x)Px)), where x can be calculated from the thermal expansion coefficients of the cell and substrate.

A material study of the growth of gallium arsenide solar cells on germanium substrates by metal organic vapour phase epitaxy has been made. The influence of the substrate misorientation, substrate cleaning process, growth procedure and initial growth conditions on the properties of the grown material have been investigated. To grow material with a good morphology and a long minority carrier lifetime, growth must be performed at a rate of 30 nm/minute with a ratio V/III of 45. Incorporation of indium into the gallium arsenide, forming indium gallium arsenide (in (0.016) Ga (0.984) As) permitted a lattice match, and hence no misfit dislocations.

Optimization for solar cell application was investigated. The gallium arsenide was protected from autodoping by coating the germanium substrate with silicon oxide (SiO2). Control of undesirable gallium arsenide to germanium interface properties was achieved by using a high doping level of substrate, a long annealing time before growth, and a low mital growth temperature. One cell showed a very good short circuit current, and a reasonable open circuit voltage (1.08 V), corresponding to an efficiency of 9.28%, which was low due to the poor quality of the window layer. With an improved window layer such a cell would give an efficiency of nearly 20%.

A 2-junction monolithic tandem solar cell in a 3-terminal configuration based on the aluminium gallium arsenide and gallium arsenide system has been developed. The cell is made of a liquid phase epitaxy (LPE) grown p-n-p stack of layers. The upper cell, bandgap 1.89 eV, presents an aluminium gallium arsenide (Al(0.32)Ga(0.68)As) p-n junction obtained by diffusion of beryllium in a tin doped layer during an isothermal process. The cell was specifically designed to work in linear concentrators. The device had a trial efficiency of 26.6% at 40 AM1.5D. It exhibited an increased blue response due to the thin front surface window. The high value of the diffusion length in LPE grown materials made feasible the suppression of the collection grid, but since it was found that conversion efficiency was limited by the series resistance, it was concluded that a collection grid was necessary to improve the efficiency.

An attempt was made to grow aluminium gallium arsenide layers by LPE on aluminium gallium arsenide on silicon substrates. The major difficulty encountered was the partial dissolution of the silicon substrate into the growth liquid bath.
This programme is the continuation of the 1986-1989 Alterna action. The present research is essentially based on Metallorganic Vapour Phase Epitaxy (MOVPE) The central photovoltaic material of the project is GaAs.

I. Very high efficiency.

Two aspects of achieving high efficiency will be investigated:

A) Monolithic integration
The GaAs solar cell efficiency can be improved by growing an additional large gap cell on top of GaAs (Al,Ga)As and specially (Al,Ga,In)P alloys will investigated for two- and three-terminal monolithic tandems.

B) Mechanically stacking
This technique will be developed for the coupling of cells obtained in the past Alterna action and the present JOULE Programme, involving the III-V (Al,Ga)As, (Al,Ga,In)P, (Ga,In)As, (Ga,In)(As,P).

II. Cost reduction of III-V compound-based cells.

Silicon and germanium will be used as alternative substrates for the grown of:

A) GaAs/Si(Al,Ga)As/Si (growth techniques MOVPE and Liquid phase Epitaxy).

B) GaAs on Ge and also GaAs on Ge on Si.
The GaAs/Si heteroepitaxial growth raises difficult problems lattice accommodation between 2 materials, whereas GaAs and Ge are lattice-matched. on both substrate materials will be investigated.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Centre National de la Recherche Scientifique (CNRS)
Address
Rue Bernard Grégory Parc De Valbonne Sophia Antipolis
06560 Valbonne
France

Participants (3)

INTERUNIVERSITAIR MIKRO-ELEKTRONICA CENTRUM VZW
Belgium
Address
75,Kapeldreef 75
3001 Heverlee
Institut National des Sciences Appliquées de Lyon (INSA)
France
Address
20 Avenue Albert Einstein
69621 Villeurbanne
STICHTING KATHOLIEKE UNIVERSITEIT
Netherlands
Address

6500 GL Nijmegen