New approaches for high efficiency cascade solar cells

Solar Cells, 21 (1987) 413 - 418

413

NEW APPROACHES FOR HIGH EFFICIENCY CASCADE SOLAR CELLS* T. KATSUYAMA, M. A. TISCHLER, D. MOORE, N. HAMAGUCHI, N. A. ELMASRY and S. M. BEDAIR North Carolina State University, Department o f Electrical and Computer Engineering, Raleigh, NC 27695-7911 (U.S.A,)

(Recieved May 1986; accepted July 3, 1986)

S u mmar y New approaches for current problems facing cascade solar cells in I I I - V c o m p o u n d s are addressed. Spinodal decomposition of metastable alloys such as A1GaAsSb resulting in the separation of low band gap phases is proposed as an in terconnect between the two cells. A new superlattice structure based on the ternary alloys GaAsP and InGaAs has been used to reduce defects originating f r om a GaAs substrate or a lattice-mismatched interface. This superlattice structure with Eg = 1.25 eV is used to ext end p h o t o n absorption b e y o n d the band gap of GaAs w i t h o u t the i nt roduct i on of lattice defects.

1. I n t r o d u c t i o n Cascade solar cells may be the only promising approach to achieving conversion efficiencies of greater than 30% [ 1 ]. The first monolithic cascade solar cell, which was built in 1979 [2], raised expectations that such a high conversion efficiency could be achieved. However, to the best of our knowledge, cascade cells built of I I I - V c o m p o u n d s still have efficiencies that are lower than are those obtained in single-junction GaAs cells. There are several problems [3] t h a t have to be addressed before efficiencies greater than 30% can be achieved. In this paper we will outline the main problems and discuss new approaches to improve device performance. Initial experimental results will also be presented.

*Paper presented at the 7th Photovoltaic Advanced Research and Development Project Review Meeting, Denver, CO, U.S.A., May 13, 1986. 0379-6787/87/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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2. Various approaches 2.1. L o w resistance interconnect Individual cells (low and high band gap) grown alone usually have high efficiencies. However, when the whole structure is built by connecting the individual cell through an interconnect, such as a tunnel junction, the performance of the high band gap cell deteriorates. This is because the interco n n ect itself typically has a fairly high density of defects that propagate to the to p cell, thus limiting its performance. Also, during the process of growing the t op cell the i nt e r connect is exposed to high temperatures that result in the deterioration of its electrical properties. One potential solution is to make the i nt erconnect from a thin film of a metastable alloy such as GaAsSb or A1GaAsSb. Annealing this metastable film results in spinodal decomposition and separation into two phases with high and low band gaps [4]. The low band gap phase, e.g. Ga-Sb, will act as an in ter co nnect between the two cells. Annealing will take place after all the cascade structures have been built. Thus a top cell with o p t i m u m quality can be obtained in the monolithic structure. 2.2. Lattice mismatch between different c o m p o n e n t s Op timu m band gap combinations are 1.1 - 1.6 eV for cells operating at r o o m tem pe r at ur e and 1.2 - 1.8 eV for ceils operating at 475 °C. One o f the problems is the lack of available substrates with a lattice constant that matches that of the ternary and quaternary c o m p o u n d s which have the o p t i m u m band gap combinations. Also, in some cases, considerations o f material quality may impose the presence of lattice mismatch between the top and b o t t o m cells. Efforts to reduce defects by step and continuous compositional grading techniques have resulted in partial success. We propose the use of strained-layer superlattices that can be grown lattice matched to GaAs or A1GaAs compounds. These new superlattice structures are based on the GaAsP/AIInGaAs material systems. We have recently reported [5] that GaAsP/InGaAs strained-layer superlattices can be grown lattice matched to a GaAs substrate. In this superlattice structure, the GaAsP layers will be under tension, whereas the InGaAs layers will be u n d e r compression. Dislocations propagating up from the substrate, or originating from any lattice-mismatched interface, e n c o u n t e r strain fields and bend over, and are thus forced to move laterally toward the edge of the substrate. We have recently reported [6] the use of GaAsP/InGaAs strainedlayer superlattices as buffer layers to produce subsequently grown GaAs epilayers which are almost dislocation free on substrates that have 104 dislocation cm-5. This superlattice can thus be used as an intermediate buffer layer between the several c o m p o n e n t s of the solar cell structure to prevent propagation of dislocations from one cell to the other. 2.3. Use o f GaAsP/InGaAs as a part o f the b o t t o m junction The A1GaAs/GaAs (top c e l l / bot t o m cell) combination is the most attractive cascade structure from the point of view of lattice matching and good electronic properties.

415 The main problem with this structure is that the band gap of the GaAs (E~ = 1.43 eV) is too high for optimum collection of solar radiation. For such a structure with open-circuit voltage Voc ~ 2 V, an increase of 1% in the short-circuit current will result in an increase of 2% in the collection efficiency. Thus any slight increase in the ability of this structure to absorb long wavelength photons results in a significant improvement in efficiency. The absorption can be increased by using the GaAsP/InGaAs strained-layer superlattice (SLS) as a part of the b o t t o m cell. It is expected that about 70% of the photons in the energy range from 1.25 to 1.43 eV may be collected by the SLS which has an effective band gap of 1.25 eV and a total thickness of about 1 pm. The efficiency of minority carrier diffusion through the superlattice layers can be improved by reducing the width and the height of the GaAsP barrier layers.

3. Growth and characterization of GaAsP/InGaAs

superlattices

The SLS structure has been grown by metal-organic chemical vapor deposition using triethylindium, trimethylgallium, AsH 3 and PH 3 as sources for the indium, gallium, arsenic and phosphorus respectively. The latticematched condition is achieved, for equal layer thicknesses, with y ~ 2x, for the InxGa l_xAs and GaAsl yPy layers. A superlattice consisting of 45 bartiers of GaAs0.sP0.2 alternating with 45 wells of In0.1Ga0.gAs was grown on a GaAs substrate. These ternary layers have equal thicknesses of about 170 A. The period of the SLS was obtained from the X-ray diffraction pattern [4] shown in Fig. 1. The zero-order diffraction peak n = 0 gives the lattice parameter of the SLS, with a lattice mismatch to the GaAs substrate of less than 0.1%. The extra satellite peaks arise from the periodicity of the structure [4], giving a period of about 350 A, which is consistent with the value obtained from the total thickness of the SLS structures obtained on a cleaved sample. Transmission electron microscopy was also performed to characterize the superlattice structure. The samples for transmission electron microscopy were prepared by lapping and ion milling two pieces bonded together face to face. They are viewed in cross-section with the electron beam parallel to the (100) zone axis. Figure 2 shows a transmission electron micrograph of a tenperiod GaAsP/InGaAs superlattice indicating the uniform thickness of the ternary layers.

4. Superlattice p - n

junctions

A superlattice p - n junction test structure, shown in Fig. 3, was fabricated. Selenium (from H2Se ) and zinc (from dimethylzinc) were used as the n- and p-type dopants respectively. Doping levels for both n and p type were in the 1017 cm -3 range. The superlattice was made of alternating layers of

416

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Fig. 1. X-ray diffraction pattern (400) of a GaAs0.sP0.2/In0.1Ga0.vAs SLS. The numbers above the peaks show the order of the satellite peaks originating from the periodicity of the SLS.

Fig. 2. Cross-sectional transmission electron micrograph of a GaAsP/InGaAs SLS. GaAs0.72P0.28 and In0.14Ga0.86As with equal thicknesses o f a b o u t 150 A each. The total thicknesses o f the n - t y p e and p - t y p e superlattice are a b o u t 0 . 3 / a m each. The slope o f the log(current) v s . voltage characteristics o f these superlattice structures gave a d i o d e f a c t o r of a b o u t 2, indicating t h a t the c u r r e n t t r a n s p o r t is c o n t r o l l e d by g e n e r a t i o n and r e c o m b i n a t i o n processes in the

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Fig. 3. S c h e m a t i c diagram of t h e G a A s P / I n G a A s SLS p - n j u n c t i o n test cell. Fig. 4. C u r r e n t - v o l t a g e c h a r a c t e r i s t i c s of t h e test cell: curve a, w i t h o u t i l l u m i n a t i o n ; curve b, w i t h i l l u m i n a t i o n .

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Fig. 5. P h o t o e u r r e n t spectral r e s p o n s e of t h e test cell. T h e peak located at a b o u t 9 2 0 0 • c o r r e s p o n d s to t h e c u r r e n t g e n e r a t e d in t h e I n G a A s wells.

depletion region• Current-voltage characteristics of typical test device structures are shown in Fig. 4, with and without illumination• The open-circuit voltage for illumination of about 1 Sun is close to 0.85 V. The spectral response for a GaAs cell with part of the active region made of a GaAsP/ InGaAs superlattice (Fig. 3) is shown in Fig. 5. As shown in this figure, the b o t t o m cell spectral response can be extended to wavelengths longer than those achieved in GaAs cells without the superlattice structure• It should be emphasized that this is achieved w i t h o u t introducing any defects that are present in a lattice-mismatched system• This test structure was not optimized for maximum absorption; however, the great flexibility of this ternary alloy

418 strained-layer s t r u c t u r e p r o v i d e s the ability to design a solar cell o p t i m i z e d f o r high efficiency.

5. C o n c l u s i o n N e w a p p r o a c h e s to i m p r o v e the e f f i c i e n c y o f cascade solar cells in I I I - V c o m p o u n d s have b e e n discussed. As a low resistance i n t e r c o n n e c t b e t w e e n the t w o cells, the a p p l i c a t i o n o f spinodal d e c o m p o s i t i o n o f m e t a stable alloys was p r o p o s e d . A G a A s P / I n G a A s SLS g r o w n b y m e t a l - o r g a n i c c h e m i c a l v a p o r d e p o s i t i o n was i n t r o d u c e d to e x t e n d p h o t o n a b s o r p t i o n b e y o n d the G a A s b a n d gap w i t h o u t generating misfit dislocations. T h e initial e x p e r i m e n t a l results o f t h e SLS p n j u n c t i o n t e s t cells s h o w e d an increased range f o r p h o t o n a b s o r p t i o n and an o p e n - c i r c u i t voltage o f a b o u t 0.85 V (with illumination). This indicates t h a t the t e r n a r y alloy SLS can be used as a b o t t o m cell o f a cascade solar cell to achieve higher efficiency.

Acknowledgment This w o r k is s u p p o r t e d b y the Solar E n e r g y R e s e a r c h I n s t i t u t e .

References 1 M. Lamorte and D. Abbot, Solid State Electron., 26 (1979) 467. 2 S. M. Bedair, J. R. Hauser and M. Lamorte, Appl. Phys. Lett., 34 (1979) 38. 3 S. M. Bedair, S. B. Phatak and J. R. Hauser, IEEE Trans. Electron Devices, 27 (1980) 822. 4 G. C. Fonstand, M. Quillec and S. Garone, J. Appl. Phys., 49 (1978) 5920. 5 S. M. Bedair, T. Katsuyama, M. Timmons and M. A. Tischler, J. Cryst. Growth, 68 (1984) 477. 6 M. A. Tischler, T. Katsuyama, N. Elmasry and S. M. Bedair, Appl. Phys. Lett., 46 (1985) 294.