(n+-p) silicon solar cell

Solar Cells, 14 (1985) 133 - 138

133

INDIUM TIN OXIDE/(n+-p) SILICON SOLAR CELL A. CHAOUI, R. ARDEBILI and J. C. MANIFACIER Centre d 'Electronique de MontpeUier, Universitd des Sciences et Techniques du Languedoc, place Eugdne Bataillon, 34060 Montpellier Cddex (France)

(Received May 9, 1984; accepted September 14, 1984)

Summary The solar cell structure consists of a shallow n+-p junction prepared on a p-type silicon substrate using phosphorus diffusion at a temperature of 800 °C. A conductive layer of indium tin oxide is then deposited by spray pyrolysis. Both polycrystalline and monocrystalline silicon have been used. The air mass 1 efficiencies for the monocrystalline and polycrystalline substrates are ~? = 14% and ~ = 12% respectively. The cells have a total area of 4 cm 2 and their stability and reliability is very good.

1. Introduction In this paper an account is given of experimental work pursued over the last five years in the processing of SnO2/Si and indium tin oxide (ITO)/Si solar cells by a spray method. We used both polycrystalline and monocrystalline p-type silicon substrates. Similar studies had been performed earlier using n-type substrates [1]. However, in that case the efficiency was usually smaller (in the range 8%- 12%). Moreover, because of the lower fabrication temperature the stability was n o t as high as for p-type substrates. Numerous papers have been published recently in the field of SnO2/Si and ITO/Si solar cells either on p-type [2 - 6] or on n-type [7 - 12] substrates. These experimental results show that the conversion efficiency depends strongly on the m e t h o d of preparation of the SnO2 or ITO film and on the t y p e (n or p) of the silicon substrate. An explanation has been given recently by Ashok e t al. [13] for these contradictory results.

2. Fabrication t e c h n o l o g y We first recall the fabrication technology used for an n-type silicon substrate [1]. An alcoholic solution of InCl 3 and SnC14 is sprayed through a heating furnace onto the substrate which is kept at a temperature in the 0379-6787/85/$3.30

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134

CHEMICAL ETCHING AND HF RINSING OF P-Si SUBSTRATES

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Fig. 1. Main fabrication phases of ITO/p-Si solar cells.

range 380 - 400 °C. For a p-type substrate the same procedure leads to very low barrier height, as would be expected from an abrupt S c h o t t k y barrier free o f surface states [14, 15]. In order to obtain a high open-circuit photovoltage it was f ound necessary t o diffuse an n + layer o n t o the p-type substrate before spraying on the ITO layer. The structure then corresponds to a buried h o m o j u n c t i o n . Figure 1 gives the various steps o f the fabrication processes. The silicon substrate has a resistivity p ~ 2 - 4 ~ cm. First it is polished, chemically etched and rinsed in a 10% H F solution. Subsequently a drop o f orthophosphoric acid (H3PO4) is spread o n t o one face of the 2 cm × 2 cm silicon substrate. (A very similar m e t h o d of preparation was first report ed by Takakura and Hamakawa [3] .) The depth o f the n ÷ layer was evaluated using secondary ion mass s p e c t r o m e t r y and was f o u n d to be of the order of 70 80 nm. The substrate is t hen annealed in an argon ambient (gas flow rate, 0.5 1 min -1) at a t e m p e r a t u r e of 800 °C for 30 min. The back cont act (p÷-p) is made by aluminium evaporation and annealing at the A1-Si eutectic temperature (T ~ 577 °C). Before the aluminium evaporation (thickness o f aluminium, approximately 500 nm) the substrate is rinsed in a 10% H F solution; it is rinsed again after the f o r m a t i o n o f the eutectic. The ITO layer is prepared using a spray m e t h o d [16]. The solution is of composition InC13:SnC14(5H20 ) :H20 :CH3CH2OH in the ratio 3 4 : 1 : 1 7 5 : 1 7 5

135 IOO

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~,(urn) Fig. 2. Reflection spectra R (%): curve 1, polished silicon substrate; curve 2, ITO/p-Si structure. b y weight. The substrate t e m p e r a t u r e is fixed at 470 °C and the deposition time is optimized in order t o obtain an efficient antireflection coating. For these deposition parameters a thickness o f 75 nm is obtained in a time o f 4 rain 15 s. The sheet resistance RD is approxi m at el y 40 ~2, which corresponds to a deep blue col our f or the cell. Figure 2 gives total reflection spectra o f polycrystalline silicon substrate with and w i t h o u t t he ITO layer. Since the resistivity of the ITO layer is n o t low enough (p ~ 2 - 4 × 10 -4 ~ cm), it is necessary to add a collecting grid. Various processes were tried: metallic evaporation gave very good results but is inconvenient; e p o x y paste (screen-printing process) is very simple to apply b u t an i m p o r t a n t degradation o f the fill factor occurred after a few days (this was attr ib ut ed to a tearing of the ITO layer); silver paste annealed at 200 °C gave th e best results t hough there were some stability problems under severe endurance tests.

3. Cell characteristics Figure 3 shows the cur r ent dens i t y- vol t age (J-V)characteristics in the dark and u n d er air mass (AM) 1 simulated illumination. Figure 3(a) corresponds to a single-crystal silicon p-type substrate o f {111) orientation and with a resistivity p ~ 3.7 ~ cm. Figure 3(b) corresponds t o a polycrystalline SILSO p-type substrate with a resistivity p ~ 3 ~ cm. The cells have an area o f 4 cm :. Th e conversion efficiencies o f t he active area (approximately 3.5 cm 2) are 14.1% (for the single-crystal substrate) and 12.2% (for t he polycrystalline substrate). T he difference is due to a decrease in the short-circuit

136

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(b) Fig. 3. J - V characteristics of ITO/p-Si solar cells in the dark and under simulated AM 1 illumination: (a) for a single-crystal p-Si substrate 07 = 14.1%; F F = 0.74; Vo¢ = 0.565 V; Js¢ -- 33.7 mA cm-2); (b) for a polycrystalline p-Si (SILSO) substrate (v/= 12.2%; F F = 0.75; Voc = 0.555 V;Jsc = 29.3 mA cm-2).

current from 34 mA cm -2 for the monocrystalline substrate to 29 mA cm -2 for the polycrystalline substrate. Hundreds of cells have been prepared and these results, which correspond to an optimization of the fabrication parameters, are highly reproducible for monocrystalline substrates. F o r t h e p o l y c r y s t a l l i n e S I L S O s u b s t r a t e t h e r e is a s l i g h t v a r i a t i o n . F o r example, ten 2 cm × 2 cm cells made from the same substrate gave an openc i r c u i t p h o t o v o l t a g e in t h e r a n g e 5 5 0 - 5 6 0 m V , a s h o r t - c i r c u i t c u r r e n t d e n s i t y Jsc ~ 2 7 - 2 9 m A c m - 2 a n d a fill f a c t o r v a r y i n g b e t w e e n 0 . 7 2 a n d

137 0.75. This variation in the fill factor is attributed to lateral variations in the lifetime of the minority carriers.

4. Discussion The stability of these cells (with either monocrystalline substrates or polycrystalline SILSO substrates) is excellent. They worked at a temperature of 75 °C for a m o n t h while exposed to AM 1 illumination and delivering their m a x i m u m power to a load. No degradation at all was observed in the J - V characteristics. At higher temperatures above 100 °C the cells were stable under dry ambient conditions. The cells were not encapsulated and under wet ambient conditions (water vapour at a temperature of 100 °C) the metallic contact (silver paste) pulled away from the back aluminium contact. This degradation does not affect the internal structure, since when new contacts are made the initial cell performances are recovered. (These endurance tests were performed at Photowatt International, Caen.) It should be noted that under similar weathering conditions some slight irreversible degradation is observed for ITO/Si or SnO~/Si cells made on an n-type substrate [8, 17]. ITO and SnO2 doped with fluorine or a n t i m o n y have very similar electrical and optical properties. SnO2 is less expensive than ITO and so would seem to be more suitable. Unfortunately, the SnO2/p-Si structures have a much higher series resistance (a lower fill factor) and so the efficiencies are only in the region of 10%. We have no clear explanation for this. It is believed to result from a higher contact resistance (between the SnO2 and the silver paste) and/or from the presence of an insulating layer (metal/insulator/semiconductor (MIS) structure) which develops between the silicon and the SnO2. We have also made cells with a double ITO/SnO2 layer. First an ITO layer of thickness 15 - 20 nm was deposited, followed by a layer of SnO2 doped with fluorine. The total thickness (approximately 75 nm) corresponds to an optimized antireflection coating (the indexes of refraction of SnO2 and ITO are very similar, in the range 1.9 - 2). The J - V characteristics of these cells were identical with and presented the same stability as the cells with a single ITO layer. Other steps were tentatively tried in order to diminish the costs and to simplify the fabrication process. For example, we used as a back ohmic contact a layer of ITO prepared by spraying directly onto the silicon substrate at a temperature of about 450 °C. Such a contact is o'f very low resistance (ohmic contact) and does not require any steps under vacuum. Cells prepared in this way showed an efficiency of the order of 10%. The front (barrier) and back (ohmic) contacts can be made simultaneously, simplifying greatly the fabrication process. To conclude, ITO/p-Si solar cells were prepared by a very simple m e t h o d of diffusion and spraying. Apart from the very high efficiencies (in excess of 18%) reported for some silicon solar cells such as the minority car-

138 rier MIS (min-MIS) s t r u c t u r e [ 1 8 ] , c o n v e r s i o n efficiencies are in t h e same range as f o r h o m o j u n c t i o n silicon cells. The p r o b l e m s o f d e g r a d a t i o n o f the metallic c o n t a c t w h e n tested in water v a p o u r at 100 °C are at p r e s e n t u n d e r s t u d y . The s t a n d a r d screen-printing p r o c e d u r e [ 1 9 ] used for these c o n t a c t s does n o t w o r k with I T O layers. We are c u r r e n t l y t r y i n g to a d a p t the electroless nickel plating m e t h o d developed by Sullivan a n d Eigler [ 2 0 ] to these layers. The first results s h o w t h a t nickel c o n t a c t s o n I T O are strongly adherent, easily soldered and stable in tests u n d e r w e a t h e r i n g c o n d i t i o n s .

Acknowledgments The assistance o f J. D o n o n with the e n d u r a n c e tests p e r f o r m e d at P h o t o w a t t I n t e r n a t i o n a l , Caen, is gratefully a c k n o w l e d g e d .

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

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