Photovoltaic effect of Ag-containing chalcogenide glasses

Journal of Non-Crystalline Solids 77 & 78 (1985) 1157-1160 North-Holland, Amsterdam

1157

PHOTOVOLTAIC EFFECT OF Ag-CONTAINING CHALCOGENIDE GLASSES Takeshi KAWAGUCHI and Shigeo MARUNO Nagoya Institute of Technology, Showa-ku, Nagoya 466, Japan Photoelectric cells of chalcogenide glasses containing a large quantity of Ag exhibit a photovoltage of several tens of m i l l i v o l t s . The photovoltage increased at the early stage of illumination and was gradually followed by decrease. The polarity of the photovoltage for illuminated electrode changes from positive to negative and f i n a l l y the photovoltage reached to the saturation level. The polarity change occurs e a r l i e r with increasing the intensity and the photon energy of illuminated l i g h t and further with decreasing the temperature of cell. The open-circuit photovoltage of ITO/Ge-S-Ag/Au cell reached about 0.5 V in magnitude and the energy conversion efficiency was about 1.5xi0-2%. I. INTRODUCTION Photovoltaic a c t i v i t i e s of silver bromideI and silver surfide 2 show photovoltage of some ten m i l l i v o l t range. Recently, i t has been found that photoelectric cells of chalcogenide glasses containing a large quantity of Ag exhibit a marked photovoltage.

The photovoltage increases with increasing Ag-concentra-

tion being containing within the glass.

In the present study, the photovoltaic

e f f e c t of Asz5S4oAg45 and Ge2zS49Ag3o bulk glasses and vacuum-deposited amorphous Gez2S28Ag6o f i l m , sandwiched between conductive electrodes, is reported. 2. EXPERIMENTAL Size of the bulk glass was 2mm in thickness and I0 mm in diameter.

Elec-

trodes f o r bulk samples were prepared by evaporation of metal a f t e r the glass had been polished mechanically with alumina powder. as the electrodes.

Gold f i l m was commonly used

The electrode about 150 ~ thick toward i l l u m i n a t i o n side was

semitransparent over the range of 350-700 nm in wavelength.

The Ge-S-Ag films

were prepared by evaporating Ge3oS7o glass fragments and m e t a l l i c Ag simultaneously onto gold or ITO f i l m substrates using two W-baskets in 1.5xlO-6Torr vacuum at about 5 X/sec deposition rate.

The p h o t o e l e c t r i c c e l l s were exposed

to each monochromatic l i g h t : 578,547,435 and 363 nm in wavelength. 3. RESULTS AND DISCUSSION When the c e l l was subjected to the exposure to l i g h t with higher energy than the o p t i c a l band gap of glasses, the c e l l e x h i b i t e d the photovoltage and then the i l l u m i n a t e d side became always p o s i t i v e regardless of m e t a l l i c electrodes of 0022-3093/85/$03.30 © Else~er Science Pub~she~ B.V. (No~h-HoHandPhysicsPub~shmg Dwision)

T. KawaguchL ~ Maruno / Ag-containing chalcogenide glasses

1158

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Time (rain)

FIGURE ] (a) Spectra] response of photovo]tage of structurally different type of bulk g]asses with a large quantity of Ag. Illuminated electrode is positive. (b) Time dependenceof open-circuit photovo]tage for different intensities of l i g h t and relaxation curve after turning o f f of illumination. different kinds (Au,A],Ag,Cr,Pt).

The spectra] response of the photovo]tage has

a peak at around 2.2 eV for As15S40Ag45 glass and 2.6 eV for Ge21S49Ag30one, as shown in f i g . ] ( a ) . Through the experiment for several As-S-Ag and Ge-S-Ag bu]k glasses with high Ag content from 25 to 45 at%, sandwiched in between Au electrodes, i t was found that the photovo]tage peaks appear at the ]ight energy higher than the optical band gap of the glasses by about 0.5 eV and the peaks s h i f t toward lower energy l i n e a r l y with increasing Ag content in glasses. The photovo]tage increases at the i n i t i a l stage of illumination and reaches gradua]ly to the saturation ]eve] ( f i g . ] ( b ) ) .

With increasing ]ight intensity,

the relaxation time of which the photovo]tage reaches the highest value becomes short, and a]so the highest value of photovo]tage increases.

When the intensity

of illuminated l i g h t is r e l a t i v e l y low (<0.1 mN/cm2), the saturated photovo]tage (maximum value: Ms) increases in proportion to logarithm of the l i g h t intensity (Cl) 10C

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FIGURE 2 (a) Relation between saturated photovo]tage (Vs) and ]ight intensity. (b) Time dependence of open-circuit photovo]tage for r e l a t i v e l y high intensity of light.

T Kawaguchi, S Maruno / Ag-containing chalcogenide glasses

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FIGURE 3 Relation between o p e n - c i r c u i t photovo]tage and i l l u m i n a t i o n time f o r d i f f e r e n t temperatures of c e i l (a) and f o r d i f f e r e n t wavelengths of i l l u m i n a t e d l i g h t (b). (fig.2(a)).

The value of the saturated photovo]tage in the case of A] or Ag

electrodes was low in comparison with other metal electrodes.

According to the

scheme f o r the p h o t o e l e c t r i c phenomena, the p h o t o v o ] t a i c e f f e c t of samples cont a i n i n g a large q u a n t i t y of Ag is considered to be due to the Dember (Photod i f f u s i o n ) p h o t o v o ] t a i c one as well as the case of s l i v e r bromide ] and s l i v e r su] f i d e 2 . When the i n t e n s i t y of l i g h t exceeded a

c e r t a i n value (about 0.4 mW/cm2 f o r

As1554oAg45), the photovoltage increased at the earlE stage of i l l u m i n a t i o n and was g r a d u a l l y followed by the decrease.

The p o l a r i t y of photovo]tage changes

from p o s i t i v e to negative w i t h i n several minutes f o r r e l a t i v e l y high i n t e n s i t y of i l l u m i n a t i o n and f i n a l l y shown in f i g . 2 ( b ) .

the photovo]tage reaches to the s a t u r a t i o n ] e v e ] , as

The time, from the making a s t a r t of i l l u m i n a t i o n to change

of the photovo]tage p o l a r i t y , decreases with increasing the i n t e n s i t y of l i g h t and f u r t h e r decreases with decreasing the temperature of c e l l (

~'~ J I~ 20F / I

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FIGURE 4 (a) Change in o p e n - c i r c u i t photovo]tage with turning o f f and on of i l l u m i n a t i o n . Time i n t e r v a l of i l l u m i n a t i o n o f f + o n is 2 min. (b) Recovering photovo]tage (Vp) as a function of the i ] ] u m i n a t i o n time. The i n s e r t shows the d e f i n i t i o n of Vp.

T. KawaguchL S. Maruno / Ag-containing chaleogenide glasses

1160

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Photovoltage

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FIGURE 5 (a) Time dependenceof open-circuit photovoltage of Au/Gez2S28Ag6o film/Au cell for different intensities of light. (b) Relation between photovoltage and photocurrent of ITO/GeI2S2BAg6ofilm/Au cell. 3(a)).

In the case of the exposure to light with shorter wavelength, the polar-

ity changes at the earlier stage (fig.3(b)). I t is considered through the present experiment that the photovoltage of chalcogenide glasses containing Ag consists of positive compornent and negative one and the relaxation time of the l a t t e r is greater than that of the former. The change of the polarity takes place when the negative compornent is prior to the positive one. Moreover, the results of figs.4(a) and 4(b) indicate that the positive compornent decreases with increasing the illumination time.

Whenthe

illumination had been turned off after the prolonged illumination, the residual voltage (negative) remained for several days at room temperature. This fact gives evidence that Ag+ ions in the glasses accumulate near the electrode of illumination side. The photoelectric phenomenaof Au/Ge-S-Ag film/Au cell were the same as those of the cell of bulk glass. The cell of Au/GeI2S28Ag60/Auexhibited the photovoltage of several hundreds m i l l i v o l t range (fig.5(a)), Such high photovoltage might be thought to be caused by high Ag-concentration of the film.

On the

other hand, the photoelectric phenomenaof ITO/Ge-S-Ag film/Au cell were widely different from those of the cell with Au electrodes. In this cell, the PN junction is formed at the interface between ITO and Ge-S-Ag film. The open-circuit photovoltage of ITO/Gez2S28Ag60/Aucell reached 0.5 V in magnitude and the conversion efficiency obtained from the relation of photovoltage and photocurrent (fig.5(b)) was 1.5xi0-2%. REFERENCES I) J.I. Masters, J.Electrochem. Soc. If7 (1970) 1378. 2) H. Ivanova and R. Andreychin, Compt. rend. Acad. bulg. Sci. 20 (1967) 1251.