The necessary requirements for the photoeffect sensitization are the existence of electron states in the semiconductors capable of accepting energy from the dye and subsurface band bending. Methods for control of the sensitization efficiency are discussed.
Keywords: Internal Photo Effect, Semiconductors, Dye Sensitization

Goryaev, M A

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M A Goryaev

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Conditions for the Dye Sensitization of Photoprocesses in Semiconductors

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 KnE Energy & PhysicsPhIO-2018VII International Conference on Photonics and Information OpticsVolume 2018Conference PaperConditions for the Dye Sensitization ofPhotoprocesses in SemiconductorsM. A. GoryaevHerzen Russian State Pedagogical University, Saint Petersburg, RussiaAbstractThe necessary requirements for the photoeffect sensitization are the existence ofelectron states in the semiconductors capable of accepting energy from the dyeand subsurface band bending. Methods for control of the sensitization efficiency arediscussed.Keywords: Internal Photo Effect, Semiconductors, Dye Sensitization.1. INTRODUCTIONSpectral dye sensitization of photoelectric and photochemical sensitivity in visible andinfrared spectral regions is well known for broad-band semiconductors (ZnO, AgHal,TiO2, etc). In the conventional photography dye sensitization is the most effectivemethod of varying the sensitivity level and spectrum of photographic materials basedon silver halides [1-4]. Photoelectrochemical cells based on the dye-sensitized titaniumdioxide provide an alternative to silicon solar batteries [5]. The main disadvantage ofsolar cells based on crystalline silicon is relatively low absorption coefficient in the bandof indirect electron transitions, which necessitates the thickness of photoelectric trans-ducers produced on the basis of this semiconductor to be hundreds of micrometers[6-8]. Silicon CCD matrices are used as image sensors in modern electron photographysystems, and light filters used for the color separation system lead to a loss of energyand valuable information [9, 10]. The dye sensitization of the photoconductivity inpowdered silicon and photovoltaic effect in the monocrystallic silicon were found inworks [11, 12]. In the present study we consider the photoelectric sensitivity formationprocesses in semiconductors with dyes applied on the surface and the possibilities tocontrol this processes.How to cite this article: M. A. Goryaev, (2018), “Conditions for the Dye Sensitization of Photoprocesses in Semiconductors” in VII InternationalConference on Photonics and Information Optics, KnE Energy & Physics, pages 122–129. DOI 10.18502/ken.v3i3.2022 Page 122Corresponding Author:M. A. Goryaevmgoryaev@mail.ruReceived: 28 January 2018Accepted: 15 March 2018Published: 25 April 2018Publishing services provided byKnowledge EM. A. Goryaev. This article isdistributed under the terms ofthe Creative CommonsAttribution License, whichpermits unrestricted use andredistribution provided that theoriginal author and source arecredited.Selection and Peer-reviewunder the responsibility of thePhIO Conference Committee. KnE Energy & Physics PhIO-20182. MateriaLs and methodsSemiconductor powder samples with micrometer-sized microcrystals and flatmonocrystals of 1-2 cm2 with a thickness of 0.5 mm were used for the investigation.The semiconductor surface was covered with dyes from the ethanol solution witha definite concentration by means of a natural evaporation of the solvent at roomtemperature. The photoconductivity measurements were performed in special surfacetype cells for investigating the electro-physical properties of powder semiconductors[13], in which the sample in the pellet form was pressed to a quartz plate underconstant pressure (approx. 10 kg/cm2). The quartz plate was covered with platinumelectrodes in the form of a solution; the distance between the electrodes was 0.1 mm,and the effective length of electrodes was 80 mm. The measurements were carriedout in conditions of a constant sweeping electric field [14]. The method used for theinvestigation of the monocrystal samples was the measurement of the condenserphoto-EMF at the modulated irradiation [15]. To estimate the effectiveness of thephotoeffect, the measured signals ΔI𝑝ℎand ΔU𝑝ℎ were normalized to the same numberof incident light quanta E.3. RESULTS AND DISCUSSIONLet us consider the processes occurring in the dye-semiconductor system during lightabsorption by the dye. During the dye photoexcitation both non-radiative and radiativeelectron transitions can occur in the molecule. The rate of the internal energy degra-dation depends on the rigidity of the dye molecule skeleton, which determines thepossibility of electron excitation energy exchange between the vibrational and rota-tional degrees of freedom. For the majority of dyes in the adsorbed state the molec-ular structure becomes more rigid and intramolecular conversion probability becomesinsignificant. Therefore, the luminescent quantum yield of adsorbed dyes is measuredin tens of percent on dielectrics, while luminescence is completely absent on metals,narrow-band semiconductors and well-sensitizable photosensitive broad-band semi-conductors (for example, silver halides and zinc oxide) because of the effective pho-toexcitation energy transfer to the adsorbent [2-4, 16].One of the main reasons for the luminescence quenching in these systems is theexistence of local electron states in the forbidden gap of the semiconductor capableof accepting energy from the dye. For the complete luminescence quenching a certainconcentration of the surface state occupied by electrons, and an integrated density 109DOI 10.18502/ken.v3i3.2022 Page 123 KnE Energy & Physics PhIO-2018- 1010 cм−2 is sufficient [2-4]. This is an essential requirement for the sensitization ofbroad-band semiconductors, in which surface states occupied by electrons are respon-sible for the weak absorption and photoconductivity in the impurity region. The dyessignificantly increase the absorption in this region, thus providing spectral sensitizationof the internal photoelectric effect.In silicon this energy interval corresponds to electron transitions from the valenceband to the conduction band; therefore, the density of states capable of acceptingthe photoexcitation energy of the dye is quite sufficient for the complete lumines-cence quenching. Figure 1 shows the photoconductivity spectra of the starting siliconsample and the samples with the dyes applied to the semiconductor surface. Theresults show that the photoconductivity of dyed samples in the absorption range of thedyes (curves 2 and 3, Fig. 1) is higher than the photoconductivity of undyed silicon bymore than an order of magnitude (curves 1, Fig. 1). The increase in photoconductivityin the absorption range of the dye cannot be attributed to the carrier generation inthe dye layer because the dark conductivity of both the powdery dye layers and thethin dye films deposited from a solution on a quartz plate with electrodes is severalorders of magnitude smaller than the conductivity of undyed silicon samples, wherethe photoconductivity is absent altogether.Figure 1: Photoconductivity spectra of undyed silicon (1) and samples dyedwith erythrosine (2) and crystalviolet (3).The significant increase in the efficiency of generating free carriers during theenergy transfer from the dye comparedwith the intrinsic absorption of a light quantumby the semiconductor may be explained by the fact that this absorption is formedby indirect electronic transitions. It should also be noted that during a characteristicabsorption the absorption and carrier generation happens rather deep in the crystalDOI 10.18502/ken.v3i3.2022 Page 124 KnE Energy & Physics PhIO-2018because of a small extinction coefficient of silicon. The role of near-surface photoelec-tron generation increases during the light absorption by dyes because the efficientenergy transfer radius during the dye sensitization is several nanometers large [2].In a spectrum of condenser photo-EMF of monocrystal silicon the sensitization ofphotoeffect of the dyed samples in the absorption band of the dye (curve 2, Fig. 2) incomparison with undyed silicon (curve 1, Fig. 2) is also observed.When the dye concentration is increased on monocrystals the photoeffect at firstincreases and then decreases in an absorption band of the dye as well (curves 2 and 3,Fig. 2). The optimum concentration of dye molecules on the monocrystal surface for asensitization is 3×1015 cm−2. Taking into account that the area of dye molecules is 0.7-1.5 nm2[1], it corresponds to the dye film thickness of about 30 monolayers or 10-15nanometers. As the efficient radius of energy transfer at the dye sensitization is 5-7nanometers in length [2], part of the energy absorbed by the dye does not reach thesemiconductor. Besides, with the increase in the film thickness during light absorptionby the dye, the role of the intramolecular energy conversion rate on the vibrationaland rotational degrees of freedom increases. All this results in a manifestation of thefilter effect in an absorption band of the dye applied on the crystal surface, as it isobserved in dye sensitization of aluminum hydride [4, 17].Figure 2: Photo-EMF spectra of undyed silicon monocrystals (1) and samples dyed with rhodamine 6Gwith dye molecule concentrations of 3×1015 cm−2 (2) and 6×1015cm−2 (3).The adsorbed dye has a filtering effect on the photoconductivity spectrum of AlH3in the doped region (curves 1’ and 1, Fig. 3), i.e. the energy absorbed by dye convertsinside the molecule, part of the incident light does not reach semiconductor and thefailure in the photoconductivity spectrum is observed in the absorption band of thedye.DOI 10.18502/ken.v3i3.2022 Page 125 KnE Energy & Physics PhIO-2018 Figure 3: Photoconductivity spectra of undyed (1,2) and dyed with rhodamine 6G (1’,2’) AlH3. 1,1’ - initial;2,2’ – exposed by UF light sample; 3 - absorption spectrum of adsorbed dye.During aluminium hydride photolysis the considerable luminescence quenching ofthe adsorbed dye is observed due to the emergence of new defects, accepting theenergy [4, 18] and photoconductivity strongly increases in the doped region (Fig. 3).However, luminescence quenching is not accompanied by the emergence of a sen-sitized photocurrent in the semiconductor and a filtering effect of adsorbed dye isobserved for the exposed aluminium hydride as well (curves 2’ and 2, Fig. 3). Thus,the energy from the dye transferred to the semiconductor centers that do not providesensibility of the photoelectric effect.The magnitude of the sensitized photoeffect in silicon monocrystals is significantlysmaller than in powder samples. This is not only a consequence of the fact that thespecific surface area of finely divided powders is several orders of magnitude largerthan in monocrystals. The additional reason for the large differences of sensitizedphotoelectric effect in monocrystalline and powder silicon is that photo-EMF has bothdrift and diffusion components [15]. If the spectrum of the drift component repeats thephotoconductivity spectrum, the diffusion photo-EMF is determined not by the con-centration of carriers but by their gradient. Depending on the subsurface band bending,drift and diffusion currents have different signs. At the dye – silicon heterojunction thechange of photopotential character during illumination in different spectral regionswasdiscovered [19]. The subsurface band bending in semiconductors can be controlled byan external electric field or an additional illumination [15], as well as the adsorptionof the electron-acceptor and electron-donor molecules on the semiconductor surface[20].DOI 10.18502/ken.v3i3.2022 Page 126 KnE Energy & Physics PhIO-2018Shallow local centres take part in the formation of band bending. Full luminescencequenching of dyes adsorbed on zinc and cadmium sulphides also does not alwaysguarantee the sensitized photoelectric effect in these semiconductors and the weakfilter effect caused by partial internal energy degradation in the dye is only observed inthe impurity photoconductivity spectrum [3]. The absence of a sensitized photoelectriceffect in these cases is due to the fact that energy from the dye transferred to thecenters, which are ineffective for photoconductivity due to small life time of generatedfree carriers (fast recombination).The introduction of shallow local centres (adhesion levels) can change the tempo ofthe carriers recombination in semiconductors. In the original aluminumhydride and zincsulfide integrated surface concentration of levels is 109-1010 cm−2 [2-4] that is clearlyinsufficient to create subsurface electric fields, significantly changing the surface carri-ers recombination. In good sensitized AgI and ZnO the concentration of adhesion levelsis more than 1012 cm−2 [2, 4]. After the introduction of chlorine ions in the ZnS lattice,concentration of shallow donor levels in the semiconductor reaches 1014 cm−2 [21] andsuch processing of sulphides of zinc and cadmium can turn them from nonsensitized togood sensitized photoconductors [2]. Aluminium quazimetallic particles appearing asa result of photolysis can be the centers accepting the photoexcitation energy from andye in aluminum hydride, and the energy transferred from the dye is quickly dissipatedon these centres – the products of photolysis. At the same time the secondary pho-tochemical processes develop in the semiconductor and ultimately an effective dyesensitization of the integral photochemical sensitivity of aluminium hydride has beenobserved [17, 22].4. ConclusionsMethods of creating conditions for the dye sensitization of photoprocesses in semi-conductors considered here enable us to control the efficiency of spectral sensitization.The sensitization of the internal photoelectric effect in silicon after creating appropriateconditions discovered here can be used in such semiconductor devices for convertingthe light energy into electric energy, as solar panels or CCD matrices used as imagesensors in modern electronic photography systems. In the first case, the conversionefficiency can be increased and the thickness of silicon cells can be decreased. Inthe second case, the use of dye sensitization will allow an improvement in the colorseparation system during the recording of colored images by choosing appropriateDOI 10.18502/ken.v3i3.2022 Page 127 KnE Energy & Physics PhIO-2018dyes instead of light filters to provide an effective increase in sensitivity in the requiredspectral range, similar to the traditional silver halide photography.References[1] James T.H., The Theory of the Photographic Process. Мacmillan Publ. Company. NewYork. 1977.[2] Akimov I.A., Cherkasov Yu.A., Cherkashin M.I., Sensitized Photoeffect. Nauka.Moscow. 1980.[3] Akimov I.A., Goryaev М.A., Photoprocesses in semiconductors with adsorbed dye.Zh. fizicheskoi khimii, 1984. V. 58, No 5. P.1104-1107.[4] Goryaev, М., Physical fundamentals of solid state photochemistry. Photolysis ofinorganic solids. Lambert Acad. 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P. 2320-2321.[12] Goryaev М.A., Dye Sensitization of Silicon. International J. Modern EngineeringResearch. 2017. V. 7. No 5. P. 39-43.[13] Povkhan T.I., Demidov K.B., Akimov I.A., Cells for research of electrophysicalproperties of powdered semiconductors. Pribory and Tekhnika Eksperimenta. 1974.No 3. P. 217-218.[14] Goryaev М.A., Spectral Sensitization of Internal Photoeffect in Silicon. IzvestiyaRossiiskogo gosudarstvennogo pedagogicheskogo universiteta. 2015. V. 176. P. 71-75.DOI 10.18502/ken.v3i3.2022 Page 128 KnE Energy & Physics PhIO-2018[15] Akimov I.A., The internal photoeffect investigation in semiconductors by thecondenser method. Soviet J. Optical Technology. 1966. V. 33. No 5. P. 248-257.[16] Goryaev М.A., Spectral dependence of the luminescence quantum yield ofadsorbed dyes. In Optics and Spectroscopy. 1997. V. 82. No 5. P. 723-725.[17] Goryaev М.A., Photoprocesses in aluminum hydride with an adsorbed dye. Opticsand Spectroscopy. 1980. V. 49. No 6. P. 625-627.[18] Goryaev М.A., Akimov I.A., 1979. Optics and Spectroscopy. V. 47. No 2. P. 409-411.[19] Komolov S.A., Gerasimova N.B., Aliev Yu.G., et al, Photoelectronic properties oforganic films on the silicon surface. Zh. tekhnicheskoi fiziki., 2006. V. 76. No 7. P.76-80.[20] Pimenov Yu.D., Goryaev М.A., Scales of perturbation undergone by an adsorbent-semiconductor with adsorption of molecules of donor-acceptor type. Kinetics andCatalysis. 1976. V. 17. No 4. P. 899-903.[21] Demidov К.B., Titov A.V., Akimov I.A., Total spectrum of localized electronic levelsin nonsensitized and sensitized by dye photoconductor. Zh. nauchnoi i prikladnoifotografii i kinematografii. 1981. V. 26. No 5. P. 375-377.[22] Goryaev М.A., Energetics of intrinsic defects and the mechanism of secondaryphotochemical properties in AlH3. Optics and Spectroscopy. 1998. V. 84. № 6. P.870-872.DOI 10.18502/ken.v3i3.2022 Page 129

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