In order to raise conversion efficiency of solar energy into electric or chemical energy, we have designed composite semiconductor photoelectrodes. The ZnSe/GaAs/Ge is a hopeful group of matching materials on rare occasion for well covering the solar spectrum

Table 1 Properties of several semiconductors.

Semiconductor	Bandgap (eV)	Lattice constant(A)	Gap type	Crystal structure
Si	1.11	5.431	Indirect	Diamond
Ge	0.67	5.657	Indirect	Diamond
YP	1.00	5.652	Direct	Zincblende
GaAs	1.43	5.653	Direct	Zincblende
Ga0.69Al0.31As	1.66	5.653	Direct	Zincblende
In0.496Ga0.504P	1.93	5.653	Direct	Zincblende
In0.5Al0.5P	2.30	5.653	Direct	Zincblende
ZnSe	2.58	5.668	Direct	Zincblende

1. The design of one possible structure: As shown in Fig.1. It is a hetero-single-crystal structure with three function layers doped gradiently. Their lattice constants are near the same, the mismatch is smaller than 0.05A, which can be considered as an judgement standard according to the literature, so a fine hetro-single crystal composition can be formed. The conducting types are N+,N,P, forming two tandem gradient hetrojunction fields to fully separate and collect the photo-generated carriers in any depth. depth h N+ -ZnSe 2N GaAs 2P Ge 3 1015 16 17 18 19 cm-3 doped concentration Fig.1. Schematic structure of the hetero-single-crystal thin-film composite semiconductor photoelectrode with three function layers doped gradiently. Their bandgaps equal to 2.58, 1.43, 0.67 eV, respectively,are well distributed in the range of solar spectrum. They can cover 94% of the total solar energy according to the data of solar spectral irradiance under AM 1.5.
The theoretical effciency is 56%.

Table 2. The data for solar spectral direct irradiance under AM 1.5. E E0-E0-(m) (W·m-2·m-1) (W·m-2 ) E0- 0.3050 3.40.02 0.0000 0.4000556.0 22.00 0.0286 0.5000 1026.7 107.75 0.1402 0.6100 1088.8 225.00 0.2928 0.7100 1002.4 326.49 0.4249 0.8000873.4 405.77 0.5281 0.9050630.4 485.65 0.6321 1.0400582.9 554.26 0.7214 1.1000366.2 584.69 0.7610 1.2000375.2 611.28 0.7965 1.3200223.4 655.80 0.8536 1.4425 51.6 661.57 0.8611 1.5200239.3 671.78 0.8744 1.6100210.5 692.95 0.9091 1.7400158.2 718.72 0.9355 1.8000 28.6 724.33 0.9428

The thickness of the three layers are decided by the ir absorptioncoefficients. The calculating formula is I=I0exp(-bX), in which the I0 is the entrance light intensity, the b is the absorption coefficients, the X is the thickness, the I is the transmission light intensity. Two microns are enough for each layer. This thin film structure makes the consumption of semiconductor materials be less and the inner resistance be lower.
The surface layer absorbs photons with higher energy, the middle layer absorbs photons with medium energy, and the base layer absorbs photons with lower energy. The ZnSe layer is transparent for visible light, it benefits photons entering the next layers.
The suitable technique for composing is MOCVD (Metal Organic Chemical Vapor Deposition). When this kind of composite photoelectrode is used in photovoltaic (PV) solar cells, the working mechanism is analyzed as following.

2. Working mechanism in photovoltaic (PV) solar cells:
The‘+’&‘-’two poles all are of ohmic connection with the surface of the Ge and ZnSe layers respectively. In the inner circuit, the photo-generated carriers are separated and collected by the built-in electric fields, the photo-generated electrons flow from type P to type N+. The quantum efficiency is high, whereas the energy difference between the electrons and holes is nearly equal to the smallest Eg of the composite material (0.67eV of the Ge layer), that is the output voltage is drawn down by the smallest Eg of the composite material.
The experiment results on spectrum-response curves (SRC) of samples have demonstrated the tendency that the SRC of the three-layer Composite photoelectrode has been widened compared with that of the two-layer and one-layer photoelectrodes. This means that the photons have penetrated into inner layers, produced photo-generated electron-hole pairs, and are separated & collected by hetro-junction fields. Every function layer contributes to the photoelectric conversion.
However, There is a valley between two peaks of SRC of ZnSe and GaAs. This is caused by the too big Eg difference between ZnSe and GaAs materials, which makes the overlap of the SRC of the two materials not good. When adding a middle layer (Ga0.3Al0.7As layer) between them, the valley is filled. This means that it is possible to raise the covering rate of photoelectrode for solar spectrum and the conversion efficiency of solar energy by adding the middle layer.

3. Comparing with the other similar design:
According to the proceedings of 25th IEEE Photovoltaic Specialists Conference, Washington, DC, USA, May 13-17, 1996, a high-efficiency photovoltaic project involving many of the national laboratories and several universities has been initiated under the umbrella of the U.S. Department of Energy Center of Excellence for the Synthesis and Processing of Advanced Materials.
The project is focused on two areas: (1) Silicon-Based Thin films, (2) Next-Generation Thin-film Photovoltaics, which will be concerned with the possibilities of new advances and breakthroughs in the materials and physics of photovoltaics using non-silicon-based materials.
Some papers concerning the area (2) are:

1. EXPERIMENTAL RESULTS OF GaInP2/GaAs/Ge TRIPLE JUNCTION CELL DEVELOPMENT FOR SPACE POWER SYSTEMS.
2. MANUFACTURING EXPERIENCE WITH GaInP2/GaAs/Ge SOLAR PANELS FOR SPACE DEMONSTRATION.
3. etc..

In this design the used semiconductor materials, GaInP/GaAs/Ge, are similar to prior design, ZnSe/GaAs/Ge; and the output voltage is higher than the prior design. It is composed of top cell, middle cell, and bottom cell.
Between the three cells there are two tunnel junctions. The number of layers is 15 layers, the thickness of each layer is 0.1 to 0.4 µm. Its practical efficiency has reached to 25.67%. However still there are some problems worth to be discussed: The selection of semiconductor materials, GaInP/GaAs/Ge, is not optimum, especially for high frequency band, because the Eg of the GaInP (1.93eV) is some small for solar spectrum when its lattice constant is matching with that of GaAs, as shown in Table 1. Outstanding Professor Martin A.Green (Australia) indicates the Egs of multi-junction cells should be from 0.6eV to 2.6eV, as shown in Table b.

Table b.
The optimum Egs of multi-junction solar cells and efficiency (1000 AM1).

Number of gaps	Efficiency (%)	Energy bandgaps (eV)
1	32.4	1.4
2	44.3	1.0 1.8
3	50.3	1.0 1.6 2.2
4	53.9	0.8 1.4 1.8 2.2
5	56.3	0.6 1.0 1.4 1.8 2.2
6	58.5	0.6 1.0 1.4 1.8 2.0 2.2
7	59.6	0.6 1.0 1.4 1.8 2.0 2.2 2.6
8	60.6	0.6 1.0 1.4 1.6 1.8 2.0 2.2 2.6
9	61.3	0.6 0.8 1.0 1.4 1.6 1.8 2.0 2.2 2.6

The number of epitaxial layers of the design is too many (15 layers), so the interfece loss of energy will be high. As well as the thickness of each function layer is too thin for absorbing photons (0.1 to 0.4 µm).

4. Conclusion:
The composition of four kinds of materials, i.e. ZnSe/Ga0.69Al0.31As/GaAs/Ge and ZnSe/In0.496Ga0.504P/GaAs/Ge, combined with multi-junction cell and tunnel junctions structure, can be considered as optimum design for solar energy conversion.

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emails: gcli@hebust.edu.cn and ghli@sjzri.edu.cn and bicelli@ipmchx.chfi.polimi.it

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Last updated April 20, 1999