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Photo-injection of electrons into antracene from electrolytic electrodes
Solid State Communications, Vol. 4. PP. 615-617, 1966. Pergamon Press Ltd. ,Printed in Great Britain
PHOTO-INJECTION OF ELECTRONS INTO ANTRACENE FROM ELECTROLYTIC ELECTRODES B. J. Mulder Philips Research Laboratories, N. V. Philips’ Gloeilampenfabrieken, Eindhoven, Netherlands (Received 22 August 1966 by G. W. Rathenau)
When anthracene crystals with electrolytic electrodes are illuminated with strongly absorbed light through the negative electrode an efficient injection of electrons takes place when the solution in the negative electrode compartment is both alkaline and strongly reductive (with potassium stannite).
PHOTOCURRENTS in anthracene crystals with electrolytic electrodes (aqueous solutions of neutral salts, bases, acids) have their origin exclusively at the surface of the crystals in contact with the positive solution, where holes are injected in a reaction between crystal excitons and electron acceptors (e. g. adsorbed oxygen or oxidative impurities). 1 2 Illumination through the positive electrode-solution with light that is strongly absorbed by the specimen gives a photocurrent (i+) 50 100 times larger than the current (L) measured with the same light intensity under illumination through the negative electrode solution. The small current i. also has its origin at the positive electrode; it is a result of re absorption of fluorescent light generated at the negative electrode and reabsorbed near the pos3 The photocurrent i~,and consequentitive one. ly also i_, can be diminished by adding powerful reductants like sulphite ions (in alkaline solutions) and Cr2~or V2~ ions (in acidic solutions) to the positive electrode solution. 2 ~ -
-
-
The present communication reports about photocurrents in anthracene under illumination through the negative electrolytic electrode which have their origin in the injection of electrons. Such true “negative” photocurrents were observed upon addition of powerful reductants to the solution in the negative-electrode compartment.
in the negative compartment. The crystal was rinsed with a solvent (benzene), followed by water, before the solution was applied. It can be seen in the figure that the “negative” photocurrent can be generated with a relatively high quantum efficiency (maximum observed efficiency 0. 12 in the peak at 25,300 cm 1 did not saturate with the voltage even when the latter was increased to 1000 V across a crystal 30 ~ thick. The solution in the positive compartment was also an alkaline solution of stannite, to inhibit the injection of holes at the positive electrode (via reabsorption of fluorescent light). Under these conditions the photocurrent i_ was up to 16 times larger than i+ (using mercury light ofa 365 m~-i with theofdensity j~of the dark current small fraction the photocurrent density (~d being 10 -12 10 ~1A/ 0. 1 cm,2 depending on the voltage). When stannite was omitted from the alkaline solution in the negative compartment the photocurrent i_ was about 500 times smaller. Clearly, the photocurrent i_ measured with stannite in the negative electrode compartment is a true electron current injected into the crystal in a reaction between excitons and stannite ions. -
Although the injections of electrons is, of course, determined by the extremely reductive properties of stannite (we measured a potential of 1080 mV vs. a saturated calomel electrode), another essential condition for the
The figure shows the excitation spectrum of the photo-current i_ measured with a iN solution of potassium stannite (K 3SnO2)in 5N KOH
-
photo-injection of electrons seems to be the 615
616
PHOTO-INJECTION OF ELECTRONS
alkalinity of the solution. An acidic solution of chromo-ions (0. 1 N in 0. iN sulphuric acid) with a potential of -500 mV gave hardly any electron injection or none at all. * On the other hand, a saturated, alkaline solution of sodium sulphite with ~ potential of only (-220)mV (_250)mVgave a pronounced electron áurrent, though with considerably lower efficiency than with the solution of stannite, and even with an alkaline solution of sodium thiosulphate (-140 mV) some electron injection was observable. The negative results
Vol. 4,No. 11 I
I
1
b polarized
0.:
-
with reaction ed electrons hydrogen-ions solutions on anthracene. can be with ascribed Its counterpart photogeneratto aconductrapid seems to ion partment, be inacidic the theof observed presence through inhibition a ofreaction alkali in of ofthe the the positive hole positive comholes with OH-ions at the surface. 2 The response of the stannite-determined “negative” photocurrent was slower than the almost instantaneous response of theholecurrent measured under illumination of the same crystal through the positive electrode. The response of L to ultraviolet light could be made more rapid by illuminating simultaneously with red light. This indicates that in our solution-grown crystals electrons are trapped more readily than holes. Although the response of the negative photocurrent was relatively slow its stationary level was proportional to the intensity of the light. Accordingly the shape of the photoconduction spectrum was independent of the intensity, The shape of the photoconduction spectrum of i_ (see Fig. 1) closely resembles that of the absorption spectrum of anthracene. It can be analysed in terms of the excitons diffusion model of the photoconduction. ~ In accordance with this model a plot of the inverse quantum efficiency of the photoconduction vs.the inverse absorption co efficient of anthracene gave a straight line. From the plot the mean diffusion path of the excitons perpendicular to the surface was calculated to be 550-600 This value is rather high compared with the value of the mean diffusion path determined for the same crystals from the excitation * A criterion for injection of electrons was the shape of the excitation spectrum of the photocurrent. A spectrum symbatic with the absorption spectrum of anthracene, as in Fig. 1. indicates injection of electrons. When injection of holes via reabsorption of fluorescent light is the dominant process, the spectrum of i. is antibatic with the absorption spectrum. -
~.
0.05 ~lariz~
024
o000
2J000
___________________ —~
J
~u00
000 ________
Wovenumber (cm’)
FIG. 1
The excitation spectrum of the electron photo-injected into anthracene from an alkaline solution of stannite. The photocurrents are expressed as quantum efficiencies (number of electrons in the outer circuit per absorbed photon). The spectra given were measured under illumination with light polarized parallel with respectively the a and the b axis of the crystal. -
spectrum of the hole current measured with alkaline electrodes and from that of the sensitized fluorescence emitted by dyes adsorbed at th surface of anthracene crystals 2 (400-500 1) The high values found from the spectrum of the “negative” photocurrent can be understood on the assumption that the stannite ions operate not only at the very surface of the crystals but also at small depths below the surface. This would weaken the structure in the excitation spectrum and increase the calculated values of the exciton diffusion path. The penetration of small ions, notably hydroxonium ions, into anthracene crystals has also been invoked to account for high apparent mean-diffusion-paths determined from the hole current measured with non-alkaline solutions in the positive compartment. The assistance of Mr. G. Vermeulen in this work is acknowledged.
Vol.4, No.11
PHOTO-INJECTION OF ELECTRONS
617
References 1.
KALLMANN H. and POPE M., J. Chem. Phys. 32, 300 (1960).
2.
MULDER B. J., (to be published) Philips Res. Repts.
3.
MULDER B. J., de JONGE J. and VERMEULEN, Rec. Tray. chim. Pays-Bas 85, 31(1966).
4.
STEKETEE J.W. and de JONGE J., Philips Res. Re~s. 17, 363 (1962).
5.
MULDER B.J. and de JONGE J., Philips Res. Repts. 21, 188 (1966).
—
a
On mesure la photoconductivitd de cristaux d’anthrace’ne l’aide d’dlectrodes e~1ectro1ytiques. La solution sltude dans le con~artlnient e~1ectroden~gativeest la fois alcaline et r~ductrice(stannite de pctasslum). Si on irradie avec de Ia lumiêre fortement absorbée par l’anthracêne ii se produit tine injection d’e~lectronsefficace.
a
a
PHOTO-INJECTION OF ELECTRONS INTO ANTRACENE FROM ELECTROLYTIC ELECTRODES B. J. Mulder Philips Research Laboratories, N. V. Philips’ Gloeilampenfabrieken, Eindhoven, Netherlands (Received 22 August 1966 by G. W. Rathenau)
When anthracene crystals with electrolytic electrodes are illuminated with strongly absorbed light through the negative electrode an efficient injection of electrons takes place when the solution in the negative electrode compartment is both alkaline and strongly reductive (with potassium stannite).
PHOTOCURRENTS in anthracene crystals with electrolytic electrodes (aqueous solutions of neutral salts, bases, acids) have their origin exclusively at the surface of the crystals in contact with the positive solution, where holes are injected in a reaction between crystal excitons and electron acceptors (e. g. adsorbed oxygen or oxidative impurities). 1 2 Illumination through the positive electrode-solution with light that is strongly absorbed by the specimen gives a photocurrent (i+) 50 100 times larger than the current (L) measured with the same light intensity under illumination through the negative electrode solution. The small current i. also has its origin at the positive electrode; it is a result of re absorption of fluorescent light generated at the negative electrode and reabsorbed near the pos3 The photocurrent i~,and consequentitive one. ly also i_, can be diminished by adding powerful reductants like sulphite ions (in alkaline solutions) and Cr2~or V2~ ions (in acidic solutions) to the positive electrode solution. 2 ~ -
-
-
The present communication reports about photocurrents in anthracene under illumination through the negative electrolytic electrode which have their origin in the injection of electrons. Such true “negative” photocurrents were observed upon addition of powerful reductants to the solution in the negative-electrode compartment.
in the negative compartment. The crystal was rinsed with a solvent (benzene), followed by water, before the solution was applied. It can be seen in the figure that the “negative” photocurrent can be generated with a relatively high quantum efficiency (maximum observed efficiency 0. 12 in the peak at 25,300 cm 1 did not saturate with the voltage even when the latter was increased to 1000 V across a crystal 30 ~ thick. The solution in the positive compartment was also an alkaline solution of stannite, to inhibit the injection of holes at the positive electrode (via reabsorption of fluorescent light). Under these conditions the photocurrent i_ was up to 16 times larger than i+ (using mercury light ofa 365 m~-i with theofdensity j~of the dark current small fraction the photocurrent density (~d being 10 -12 10 ~1A/ 0. 1 cm,2 depending on the voltage). When stannite was omitted from the alkaline solution in the negative compartment the photocurrent i_ was about 500 times smaller. Clearly, the photocurrent i_ measured with stannite in the negative electrode compartment is a true electron current injected into the crystal in a reaction between excitons and stannite ions. -
Although the injections of electrons is, of course, determined by the extremely reductive properties of stannite (we measured a potential of 1080 mV vs. a saturated calomel electrode), another essential condition for the
The figure shows the excitation spectrum of the photo-current i_ measured with a iN solution of potassium stannite (K 3SnO2)in 5N KOH
-
photo-injection of electrons seems to be the 615
616
PHOTO-INJECTION OF ELECTRONS
alkalinity of the solution. An acidic solution of chromo-ions (0. 1 N in 0. iN sulphuric acid) with a potential of -500 mV gave hardly any electron injection or none at all. * On the other hand, a saturated, alkaline solution of sodium sulphite with ~ potential of only (-220)mV (_250)mVgave a pronounced electron áurrent, though with considerably lower efficiency than with the solution of stannite, and even with an alkaline solution of sodium thiosulphate (-140 mV) some electron injection was observable. The negative results
Vol. 4,No. 11 I
I
1
b polarized
0.:
-
with reaction ed electrons hydrogen-ions solutions on anthracene. can be with ascribed Its counterpart photogeneratto aconductrapid seems to ion partment, be inacidic the theof observed presence through inhibition a ofreaction alkali in of ofthe the the positive hole positive comholes with OH-ions at the surface. 2 The response of the stannite-determined “negative” photocurrent was slower than the almost instantaneous response of theholecurrent measured under illumination of the same crystal through the positive electrode. The response of L to ultraviolet light could be made more rapid by illuminating simultaneously with red light. This indicates that in our solution-grown crystals electrons are trapped more readily than holes. Although the response of the negative photocurrent was relatively slow its stationary level was proportional to the intensity of the light. Accordingly the shape of the photoconduction spectrum was independent of the intensity, The shape of the photoconduction spectrum of i_ (see Fig. 1) closely resembles that of the absorption spectrum of anthracene. It can be analysed in terms of the excitons diffusion model of the photoconduction. ~ In accordance with this model a plot of the inverse quantum efficiency of the photoconduction vs.the inverse absorption co efficient of anthracene gave a straight line. From the plot the mean diffusion path of the excitons perpendicular to the surface was calculated to be 550-600 This value is rather high compared with the value of the mean diffusion path determined for the same crystals from the excitation * A criterion for injection of electrons was the shape of the excitation spectrum of the photocurrent. A spectrum symbatic with the absorption spectrum of anthracene, as in Fig. 1. indicates injection of electrons. When injection of holes via reabsorption of fluorescent light is the dominant process, the spectrum of i. is antibatic with the absorption spectrum. -
~.
0.05 ~lariz~
024
o000
2J000
___________________ —~
J
~u00
000 ________
Wovenumber (cm’)
FIG. 1
The excitation spectrum of the electron photo-injected into anthracene from an alkaline solution of stannite. The photocurrents are expressed as quantum efficiencies (number of electrons in the outer circuit per absorbed photon). The spectra given were measured under illumination with light polarized parallel with respectively the a and the b axis of the crystal. -
spectrum of the hole current measured with alkaline electrodes and from that of the sensitized fluorescence emitted by dyes adsorbed at th surface of anthracene crystals 2 (400-500 1) The high values found from the spectrum of the “negative” photocurrent can be understood on the assumption that the stannite ions operate not only at the very surface of the crystals but also at small depths below the surface. This would weaken the structure in the excitation spectrum and increase the calculated values of the exciton diffusion path. The penetration of small ions, notably hydroxonium ions, into anthracene crystals has also been invoked to account for high apparent mean-diffusion-paths determined from the hole current measured with non-alkaline solutions in the positive compartment. The assistance of Mr. G. Vermeulen in this work is acknowledged.
Vol.4, No.11
PHOTO-INJECTION OF ELECTRONS
617
References 1.
KALLMANN H. and POPE M., J. Chem. Phys. 32, 300 (1960).
2.
MULDER B. J., (to be published) Philips Res. Repts.
3.
MULDER B. J., de JONGE J. and VERMEULEN, Rec. Tray. chim. Pays-Bas 85, 31(1966).
4.
STEKETEE J.W. and de JONGE J., Philips Res. Re~s. 17, 363 (1962).
5.
MULDER B.J. and de JONGE J., Philips Res. Repts. 21, 188 (1966).
—
a
On mesure la photoconductivitd de cristaux d’anthrace’ne l’aide d’dlectrodes e~1ectro1ytiques. La solution sltude dans le con~artlnient e~1ectroden~gativeest la fois alcaline et r~ductrice(stannite de pctasslum). Si on irradie avec de Ia lumiêre fortement absorbée par l’anthracêne ii se produit tine injection d’e~lectronsefficace.
a
a