Dye-Sensitized Photoconductivity and Photovoltaic Effect in Silicon

Mikhail A. Goryaev and Rene A. Castro

Herzen State Pedagogical University of Russia, 48 Moika Emb., St. Petersburg, Russia

Keywords: Internal Photo Effect, Powdered and Monocrystalline Silicon, Dye Sensitization.

Abstract: In this paper we investigate the influence of organic dyes applied to the semiconductor surface on the

effectiveness of the direct current photoconductivity of powdered silicon and photovoltaic effect in the

monocrystalline silicon. Dyes on the semiconductor surface effectively increase the photoconductivity of

powdered samples in the dye absorption band. The effectiveness of the monocrystal sensitization is smaller

because the specific surface area of finely divided powders is several orders of magnitude larger than it is in

monocrystals. The optimum concentrations of dye molecules on the semiconductor surface for the internal

photo effect sensitization are determined. We also discuss the mechanism of the sensitization based on the

theory of non-radiative resonant inductive energy transfer.

1 INTRODUCTION

Organic dyes effectively sensitize the

photoconductivity and photolysis in broad-band

semiconductors (ZnO, AgHal, TiO

, etc) in the dye

absorption range (Akimov, et al, 1980, Goryaev,

2013). In the silver halide photography dye

sensitization is the most effective method of varying

the sensitivity level and spectrum of photographic

materials (James, 1977). Photoelectrochemical cells

based on the dye-sensitized titanium dioxide provide

an alternative to silicon solar batteries (Gratzel,

2003). The thickness of solar cells based on

crystalline silicon must be hundreds of microns,

because the extinction coefficient of the

semiconductor is relatively low in the range of

indirect electron transitions (Alferov, et al, 2004,

Afanasyev, et al, 2011, Duffie and Beckman, 2013).

Modern electron photography systems have the

silicon CCD matrices as image sensors in (Boyle,

2010), and using light filters for color separation

system leads to the loss of energy and valuable

information (Goryaev and Dudnikov, 1990). In the

present study we investigated the internal photo effect

in silicon and the influence of the application of

organic dyes to the semiconductor surface on its

effectiveness.

2 EXPERIMENTAL

Silicon powdered samples with micron-sized

microcrystals and flat monocrystals of 1-2 cm

with a

thickness of 0.5 mm were used for the investigation.

The semiconductor surface was covered with dyes

from the ethanol solution with a definite

concentration by means of a natural evaporation of

the solvent at room temperature. The direct current

(DС) photoconductivity measurements were

performed in special surface type cells for

investigating the electro-physical properties of

powder semiconductors (Povkhan, et al, 1974), in

which the sample in the pellet form was pressed to a

quartz plate under constant pressure (approx. 10

kg/cm

). The quartz plate was covered with a raster

of platinum electrodes; the distance between the

electrodes was 0.1 mm, and the effective length of

electrodes was 80 mm. The photocurrent



measured under the conditions of a constant sweeping

electric field was normalized to number of light E

(Goryaev, 2015). The method used for the

investigation of the monocrystalline samples was the

measurement of the photo-EMF at the modulated

irradiation (Akimov, 1966). To estimate the

effectiveness of the photovoltaic effect, the measured

signals



were normalized to number of light

quanta E.

130

Goryaev, M. and Castro, R.

Dye-Sensitized Photoconductivity and Photovoltaic Effect in Silicon.

DOI: 10.5220/0006525701300135

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 130-135

ISBN: 978-989-758-286-8

3 RESULTS AND DISCUSSION

3.1 DC Photoconductivity

The photoconductivity spectra of powdered silicon

are shown on figures 1 and 2: the undyed sample

(curve 1) and the samples with the dyes applied to the

semiconductor surface (curve 2). The results

demonstrate that the photoconductivity of dyed

samples in the absorption range of the dyes (curve 2,

Fig. 1 and Fig. 2) is higher than the photoconductivity

of the original silicon sample (curve 1, Fig. 1 and 2)

by more than an order of magnitude.

Figure 1: Photoconductivity spectra of undyed silicon (1)

and samples dyed with Rhodamine 6G (a) and Erythrosine

(b) with concentrations of 10

-4

Mole/g of Si (2) and 2×10

-4

Mole/g of Si (3) with concentrations of 10

-4

Mole/g of Si

(2) and 2×10

-4

Mole/g of Si (3).

The increase in photoconductivity in the

absorption range of the dye cannot be related to the

carrier generation in the dye layer because the dark

conductivity of both the powdery dye layers and the

thin dye films deposited from a solution on a quartz

plate with electrodes is several orders of magnitude

smaller than the conductivity of undyed silicon

samples, while the photoconductivity is absent

altogether. This circumstance explains the decrease in

the photocurrent for the dye deposited in a large

amount on the semiconductor surface (Goryaev,

2015).

The magnitude of the observed photocurrent

sensitization effect depends on the amount of dye

applied to the semiconductor surface non-

monotonely: when the dye concentration is increased,

the photoconductivity at first increases and then

decreases (curves 2 and 3, Fig. 2). The decrease in the

photocurrent when the large amount of dye is

deposited on the semiconductor surface is explained

by the fact that a solid insulating dye film is formed

on silicon, and it hinders the transition of charge

carriers among the microcrystals. The optimum

concentration of dye molecules on the semiconductor

surface for the photocurrent sensitization of

powdered samples is about of 10

-4

Mole/g of Si.

Figure 2: Photoconductivity spectra of undyed silicon (1)

and samples dyed with Crystal Violet with concentrations

of 10

-4

Mole/g of Si (2) and 2×10

-4

Mole/g of Si (3).

The investigation of photoconductivity spectra

shows the interesting results for the samples with the

dye crystal violet applied to the semiconductor

surface (Fig. 2). On increasing dye concentration not

only a decreasing photoconductivity in the dye

absorption band due to the insulating dye film

formation is observed, but an additional absorption

band appears in the long-wave region (curves 2 and

3, Fig. 2). This fact may be explained by the J-

aggregates formation at the large amount of dye on

the semiconductor surface. The absorption band of

dye J-aggregates lies in the longer wave region and

their sensitization effectiveness is larger than that for

the dye in a non-aggregated state (James, 1977).

3.2 Photovoltaic Effect

For the monocrystalline silicon the spectral

sensitization of condenser photo-EMF is also

Dye-Sensitized Photoconductivity and Photovoltaic Effect in Silicon

131

observed in the absorption band of the dye (curves 2

and 1, Fig. 3). In comparison to powdered silicon the

magnitude of the sensitized photo effect in silicon

monocrystals is significantly smaller than in

polycrystalline samples (Goryaev, 2017). This is a

consequence of the fact that the specific surface area

of monocrystals is several orders of magnitude

smaller than in microcrystals of finely divided

powders. It should also be noted that during a

characteristic absorption the absorption and carrier

generation happens rather deep in the crystal because

of a small extinction coefficient of silicon. The role

of near-surface photoelectron generation increases

during the light absorption by dyes because the

efficient energy transfer radius during the dye

sensitization is several nanometers large (Akimov, et

al, 1980). The thickness of monocrystals is very

significant, therefore the number of such near-surface

photoelectrons is small in comparison with the

carriers generated due to the characteristic absorption

in the volume of the semiconductor.

Figure 3: Photo-EMF spectra of undyed silicon

monocrystals (1) and samples dyed with Rhodamine 6G

with dye molecule concentrations of 30 nm

-2

(2) and 60 nm

(3).

The magnitude of the observed photo effect

sensitization depends on the amount of dye applied to

the semiconductor surface non-monotonely as well:

when the dye concentration is increased on

monocrystals the photo-EMF at first increases and

then decreases in an absorption band of the dye

(curves 2 and 3, Fig. 3). Dye sensitization is observed

at the optimum concentration of dye molecules on the

monocrystal surface about of 30 nm

-2

. Taking into

account the fact that the surface density of molecules

in the dye monolayer is 0.7-1.4 nm

-2

(James, 1977),

the optimum thickness of the dye film formed on the

monocrystal surface is around 30 monolayers or 10-

15 nanometers. As the length of the efficient radius of

energy transfer at the spectral dye sensitization is 5-7

nanometers (Akimov, et al, 1980), part of the energy

absorbed by the dye does not reach the semiconductor

for the larger film thickness. Besides, increasing the

film thickness increases the role of the intramolecular

energy conversion rate on the vibrational and

rotational degrees of freedom during light absorption

by the dye. Those processes lead to the filter effect in

an absorption band of the dye applied on the solid

surface, as it is observed for the photophysical and

photochemical processes in the dye – aluminum

hydride system (Goryaev and Pimenov, 1975,

Goryaev, 1980; 2000).

3.3 Mechanism of Sensitization

The efficiency of spectral sensitization for the

photoelectron formation in a solid after the light

absorption by the dye applied on the surface is

determined by the following ratio:

= P

, (1)

where P

– the efficiency of absorption determined

by the dye extinction coefficient and its

concentration; P

– transfer efficiency of excitation

energy from dye to semiconductor; P

– the

efficiency of photoelectron formation in a solid as a

result of the excitation energy transfer from dye.

Some processes occur in the dye-semiconductor

system during light absorption by the dye. After the

dye photoexcitation non-radiative and radiative

electron transitions can realize in molecules. The

possibility of electron excitation energy exchange

between the vibrational and rotational degrees of

freedom and the rate of the internal energy

degradation are determined by the rigidity of the dye

molecule skeleton. For dyes applied on the solid

surface the molecular structure becomes more rigid

and intramolecular conversion probability becomes

insignificant. As result the luminescent quantum yield

of dyes is measured in tens of percent on the

dielectrics surfaces (Goryaev, 1980; 1981; 1997,

Goryaev and Smirnov, 2015). But the luminescence

is completely absent on metals, narrow-band

semiconductors and well-sensitizable photosensitive

broad-band semiconductors (for example, silver

halides and zinc oxide) because of the effective

photoexcitation energy transfer from dyes to the

solids (Akimov and Goryaev, 1984).

After the light absorption the photoexcitation

energy received by the dye can be transferred to the

semiconductor, be observed as luminescence and

degrade within the molecule with the corresponding

transition constants k

, k

and k

. Transfer efficiency

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

132

of the excitation energy is determined by the

following ratio:

























(2)

The intramolecular energy conversion rate k

becomes insignificant when compared to k

and k

for

the majority of dyes applied on the solid surface

because of the hardening of the molecule structure.

Therefore the luminescence becomes the main

competing process with the effective electronic

excitation transfer to the semiconductor. The

fluorescent state lifetimes of dyes applied on the solid

surface are 10

-11

– 10

-10

s (Akimov and Goryaev,

1984, Goryaev, 2013), and photoexcitation transfer

rate constant k

in effectively sensitized

semiconductors exceeds 10

-1

(Akimov, et al, 1980,

Goryaev, 2013), so the quantum effectiveness of the

photophysical and photochemical processes

sensitization in the semiconductor



approaches unity

under such circumstances.

According to the theory of non-radiative resonant

inductive energy transfer, the transfer probability

depends on the overlap integral of the donor

luminescence spectrum and the acceptor absorption

spectrum (Ermolaev, et al, 1996):

 













































(3)

In equation (3): q

and



– are, accordingly, the

quantum yield and duration of luminescence decay of

the donor in the absence of quenching, I

– quantum

radiation spectral density of luminescence,





–

molar decimal absorption coefficient, R – distance

between the donor and the acceptor, n – the

environment refractive index,



– wave number, C –

constant.

Local electronic states of the forbidden band play

the role of the energy acceptors in broad-band

semiconductor sensitization (Akimov, et al, 1980,

Akimov and Goryaev, 1984, Goryaev, 2013). The

existence of such local states in the semiconductors

capable of accepting energy from the dye is the main

reason for the luminescence quenching in these

systems. The integrated density 10

-10

-2

of the

surface states occupied by electrons is sufficient for

the complete luminescence quenching (Goryaev,

2013). This is the necessary requirement for the

sensitization of broad-band semiconductors. Such

surface states are responsible for the weak absorption

and photoconductivity in the impurity region of the

semiconductors. The dyes applied on the solid surface

provide the spectral sensitization of the photophysical

and photochemical processes in the semiconductors

because of increasing the absorption in this region.

In silicon this spectral region corresponds to

electronic transitions from the valence band to the

conduction band. The densities of states in these

bands are quite sufficient for the complete dye

luminescence quenching because of accepting the

photoexcitation energy. The intrinsic light absorption

by the semiconductor is formed by indirect electronic

transitions, and free carriers obtained by the intrinsic

absorption are generated in the volume of the

semiconductor. The near-surface free carriers

generation during the light absorption by dyes

produces a significant increase in the efficiency of the

photoelectric effect (Fig. 4).

Figure 4: The generation of free carriers in the volume of

the semiconductor due the intrinsic absorption in silicon (1)

and in the subsurface region due the energy transfer from

the dye (2).

The sensitization process efficiency after the

transfer of excitation from a dye to the semiconductor

is determined by the fate of an electron appearing in

the semiconductor conductivity band. This electron

may remain free for a sufficient period of time,

enabling the sensitized photoeffect, or may quickly

recombine. If the free carriers are formed with a

constant k

and recombination of carriers in a

semiconductor occurs with constant k

the

effectiveness of the photoelectron formation in a solid

after electronic excitation transfer is determined by

the formula:





(4)

The additional reason for the large differences of

sensitized photoelectric effect in monocrystalline and

powder silicon is that photo-EMF has both drift and

diffusion components (Akimov, 1966). If the

spectrum of the drift component repeats the

photoconductivity spectrum, the diffusion photo-

EMF is determined not by the concentration of

carriers but by their gradient. Depending on the

Dye-Sensitized Photoconductivity and Photovoltaic Effect in Silicon

133

subsurface band bending, drift and diffusion currents

have different signs. At the dye – silicon

heterojunction the change of photopotential character

during illumination in different spectral regions was

discovered (Komolov, et al, 2006). The subsurface

band bending in semiconductors can be controlled by

an external electric field or an additional illumination

(Akimov, 1966), as well as the adsorption of the

electron-acceptor and electron-donor molecules on

the semiconductor surface (Pimenov and Goryaev,

1976).

4 CONCLUSIONS

Observed effective dye sensitization of the internal

photoelectric effect in silicon can be used in such

semiconductor devices for converting the light energy

into electric energy as solar panels or image sensors

in modern electronic photography systems. In the

case of silicon solar batteries increasing the near-

surface free carriers generation may decrease the

thickness of cells. The application of dye sensitization

in CCD matrices will enable the improvement of the

color separation system during the recording of

colored images as in traditional silver halide

photography. Selecting appropriate dyes to provide

an effective increase in sensitivity in the required

spectral range instead of using light filters can

decrease losses of energy and of valuable information

in electronic photography.

ACKNOWLEDGEMENTS

This research was supported by the Ministry of

Education and Science of the Russian Federation

(Project No. 3.5005.2017/BY).

REFERENCES

Afanasyev, V.P., Terukov, E.I., Sherchenkov, A.A., 2011.

Thin Film Solar Cells Based on Silicon. State

Electrotech. Univ. St. Petersburg.

Akimov, I.A., 1966. The internal photoeffect investigation

in semiconductors by the condenser method. In Soviet

J. Optical Technology. V. 33. No 5. P. 248-257.

Akimov, I.A., Cherkasov, Yu.A., Cherkashin, M.I., 1980.

Sensitized Photoeffect. Nauka. Moscow.

Akimov, I.A., Goryaev, М.А., 1984. Photoprocesses in

semiconductors with adsorbed dye. In Zh. fizicheskoi

khimii, V. 58, No 5. P.1104-1107.

Alferov, Zh.I., Andreev, V.M., Rumyantsev, V.D., 2004.

Solar Photovoltaics: Trends and Prospects. In

Semiconductors. V. 38. No 8. P. 899-908.

Boyle, W.S., 2010. Nobel Lecture: CCD—An extension of

man’s view. In Rev. Modern Physics. V. 82, fasc. 3. P.

2305—2306.

Duffie, J.A., Beckman, W.A., 2013. Solar Engineering of

Thermal Processes, John Wiley & Sons, Inc. New

York.

Ermolaev, V.L., Bodunov, E.N., Sveshnikova, E.B. 1996.

Inductive-resonant mechanism of nonradiative transi-

tions in ions and molecules in condensed phase. In

Physics-Uspehi, V. 39. No 3. P. 261-282.

Goryaev, М., 2013. Physical fundamentals of solid state

photochemistry. Photolysis of inorganic solids.

Lambert Acad. Publ., Saarbrucken.

Goryaev, М.А., Dudnikov, Yu.A., 1990. Electronic and

hybrid imaging systems. In Zh. nauchnoi i prikladnoi

fotografii i kinematografii. V. 35. No 5. P. 386-394.

Goryaev, М.А., 2015. Spectral Sensitization of Internal

Photoeffect in Silicon. In Izvestiya Rossiiskogo

gosudarstvennogo pedagogicheskogo universiteta. V.

176. P. 71-75.

Goryaev, М.А., 2015. Dye sensitization of

photoconductivity of polycrystalline silicon. In Russian

J. Physical Chemistry A. V. 89. No. 12. P. 2320-2321.

Goryaev, М.А., 2017. Dye Sensitization of Silicon. In

International J. Modern Engineering Research. V. 7.

No 5. P. 39-43.

Goryaev, М.А., 1980. Method of determining the

luminescence spectra of powdered systems. In Soviet

Technical Physics Letters, V. 6. No 9. P. 484-485.

Goryaev, М.А., 1981. Quantum yield and luminescence

spectra of dyes in the adsorbed state. In Optics and

Spectroscopy. V. 51. No 6. P. 562-564.

Goryaev, М.А., 1997. Spectral dependence of the

luminescence quantum yield of adsorbed dyes. In

Optics and Spectroscopy. V. 82. No 5. P. 723-725.

Goryaev, М.А., Smirnov, A.P., 2015. Luminescence of dye

adsorbed on silver stearate and sensitization of

photothermographic materials. In Izvestiya Rossiiskogo

gosudarstvennogo pedagogicheskogo universiteta. V.

173. P. 50-54.

Goryaev, М.А., Pimenov, Yu.D., 1975. The action of an

electric field on photochemical processes in aluminum

hydride. In Soviet J. Optical Technology. V. 42. No 9.

P. 538-541.

Goryaev, М.А., 1980. Photoprocesses in aluminum hydride

with an adsorbed dye. In Optics and Spectroscopy. V.

49. No 6. P. 625-627.

Goryaev, М.А., 2000. Vacancy mechanism of the

photolysis of aluminum hydride. In Optics and

Spectroscopy. V. 88. No 1. P. 42-45.

Gratzel, M., 2003. Dye-sensitized Solar Cells. In J.

Photochem. Photobiol. C: Photochem. Rev. V. 4. No 2.

P. 145-153.

James, T.H., 1977. The Theory of the Photographic

Process, Мacmillan Publ. Company. New York.

Komolov, S.A., Gerasimova, N.B., Aliev Yu.G., et al,

2006. Photoelectronic properties of organic films on the

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

134

silicon surface. In Zh. tekhnicheskoi fiziki,. V. 76. No 7.

P. 76-80.

Pimenov, Yu.D., Goryaev, М.А., 1976. Scales of

perturbation undergone by an adsorbent-semiconductor

with adsorption of molecules of donor-acceptor type. In

Kinetics and Catalysis. V. 17. No 4. P. 899-903.

Povkhan, T.I., Demidov, K.B., Akimov, I.A., 1974. Cells

for research of electrophysical properties of powdered

semiconductors. In Pribory and Tekhnika

Eksperimenta. No 3. P. 217-218.

Dye-Sensitized Photoconductivity and Photovoltaic Effect in Silicon

135