Spray Pyrolysis in Solar Cells and Gas Sensors

8/2/2019 Spray Pyrolysis in Solar Cells and Gas Sensors

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Prog. Crystal Growth Charact. 1981,Vol. 4, pp. 221 - 248.OPergamon Press Ltd. Printed in Great Britain

0146- 3535/B1/0701 -0221$05.00/0

SPRAY PYROLYSIS IN SOLAR CELLS ANDM. S. Tomar and F. J. Garcia

Dpto. de Fisica, Universidad Simon Bdivar,Apartado 80858 Caracas, 1 OBOA Venezuela

&&rJ&ed 17th Februwy 1981)

GAS SENSORS

ABSTRACTThe present article offers a review of the technique of spray pyrolysisdeposition of thin films. General aspects of the method, such as its inherent

simplicity and low energy consumption, are briefly considered. The basicprocess and apparatus utilized in spray pyrolysis are described in general andalso for particular applications. Specific techniques used to deposit thinfilms of metal oxides, oxide semiconductors, sulfide and selenide semiconductorsand related compounds, and the properties of the resulting deposited films arediscussed. The most cornnon pplications of these sprayed films are presented,with special reference to recent results in gas detection and photovoltaic solarenergy conversion, including thin film solar cells, solar cell antireflectioncoatings, and thin film gas sensors. A general review of the published work inthese areas is also included, together with a number of pertinent references.

1, INTRODUCTIONBigh quality single crystals of semiconductor materials are generally pre-

pared by several established crystal growth techniques (1, 2). &I a parallelline thin films of these materials are usually produced by vacuum evaporation,sputtering, electron beam evaporation and CVD etc. (3) to mention a few. Allthese techniques have their advantages and disadvantages, depending on a par-ticular application. In the present review we want to focus attention on asimple though useful technique termed "SPRAY PYROLYSIS" which has been used toproduce several compound semiconductors whose utility in various devices e.g.solar cells, gas sensors, antireflection coatings etc., have been realized.

Chamberlin et al (4, 5) used the spray pyrolysis (SP) to produce severalII-VI compounds in search of low cost photoconductors. With this techniquethese authors also prepared for the first time a heterojunction solar cell ofCu sleds (6). A reactive spray technique has long been used to prepare trans-pafent conducting oxide layers (7-14), but a serious attention (15-20) to producehigh quality oxide films such as Sn02,CdSn04, In203, Ti02, etc., has been givenaround mid seventies onwards due to their applications in conductive transparentelectrodes,particularly in solar cells and gas sensing elements. It perhaps isthe demand of time to search for low cost techniques and their utility in this

221


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222 M. S. Tomar and F. J. Garciaperiod of energy crisis. It is therefore assumed that the discussion in thisarticle is concerned with the materials and their devices produced by spraypyrolysis technique if not otherwise mentioned.

2 , S P RAYP Y ROL YS I S E C HNI QUE S P >In principle, spray pyrolysis is a simple technique where an ionic solution(prepared by starting materials in appropriate stoichiometric proportions) con-

taining the constituent elements of the compound, is sprayed over a heated sub-strate (T around 300C to 5OO'C).their chl%%ies Generally the metals are in solution as, nitrates or acetates. Sulphur and selenium are present in theforms of N,N dimethylthiourea or thiourea and N,N dimethyl selenourea. Severalsulphides and selenides: binary, ternary and even complex sphalerite solid so-lutions (4, 21-24) have been prepared using this technique. The ionic solutionproduces complex ions (4) presumbly (Metal chloride + Ureas) i.e. CdC12(SCN H4)or CdC12(SeCN2H4) in the case of CdS or CdSe products (4, 25), these sulphi e3and selenide ions do not precipitate in the solution. When the solution issprayed over a heated substrate, the complex ions then decompose and the inor-ganic sulphide/selenide deposits on the substrate in the form of very adherentfilms. The organic products of the chemical reaction do not hold the tempera-ture encountered on the substrate and subsequently evaporate along with othergaseous species. Nitrogen is used as carrier (flushing) gas but compressed dryair was used frequently for oxides. Sometimes a rotating substrate was alsoused to acquire uniform films. The typical schematic diagrams of SP setup areshown in Fig. 1 which are generally used (4,27,28). The approximate chemicalreactions during the deposition process are given in Fig. 2A. Table 1 showssome of the important starting materials used in various spray deposited films.The quality of the deposited film seems to depend strongly on substrate tempera-ture, spray rate, degree of atomization, anion to cation ratio and the sub-strate environment (4).

A reactive spray technique as mentioned above to produce oxide films uti-lizes similar chemical solutions but allows the spray constituents to reactwith the ambient (say oxygen) (15-18) or helps in hydrolysis and sometimesa preheater is used (15) which helps in fast oxidation process. The problemsencountered presently in sprayed films are due to non-uniform droplet sixesformed at the outlet of the spray nozzle which contribute to the poor grain sizeand hence lower transparency. This effect has been treated by Lampkin (29)through an investigation of the aerodynamics of the spray nozzle and by Som andMukharjee (30) with a study on the coefficient of discharge and spray cone angleof the swirl spray atomizing nozzle.

3 , P R E P ARAT I ONND P R OP E R T I E S S UP H I DE S , E LE NI DE S , T C, >As mentioned above, several sulphide, selenides and oxides were prepared

by SP. In this section we will give an account of the preparation and charac-terization of these films.3.1. II-VI. Inorganic Compounds

An extensively studied material among all the II-VI compounds is CdS due toits utility in heterojunction solar cells as a large bandgap window material.3.1.1. Cadmium Sulphide (CdS). The starting materials generally used to pre-pare CdS were an equimolar ionic solution of CdCl, N,N dimethyl thiourea (orthiourea) (4). This solution was sprayed over a fieated substrate around 300Cto 5oo"c. The spray rate was generally maintained in the range of 2mlfmin to


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Spray Pyrolysis in Solar Cells and Gas Sensors 223

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224 M. S. Tomar and F. .I. arcia

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cu Cuprous ChlorideCopper Acetate

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T ABL E - ISTART TNG MATER lALS FOR SPRAY PYROLYSISStarting Materials Substrate

TemperatureSpray Rate

Cadmium ChlorideCadmium NitrateCadmium AcetateCadmium Formate

CdS-350"to45O'C with N2or air ascarrier gas

In the rangeof 2 mllmin. to30 mllmin. forsulphides andselenides

Lead ChlorideLead AcetateLead Nitrate+ Alcohol

Gallium ChlorideGallium Nitrate+ AlcoholTin Chloride(SnC14.5H20)TetramethylinThioureaN,N dimethyl-ThioureaAmmonium ThiocyanateN,N dimethyl-selenoureaTitanium IsoproxideTetraisopropyl titaanate + H20

ZnCdS 400-45O'Cwith N or airas car?ier gasTernaries likeCuInSe2,CuInS2300 to 400Cin excess ofselenourea andN2 as carriergas,/ excess ofThiourea

Oxides like -SnO ,In 0 :Sn4502600zC3

:zfao&TiO- 46ooc

SnO2- 400-5oo"c

In the range of20 mllmin. to150 mlimin.for oxides


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170C24SCN2H4 6SCN2 II4 18NH4SCN

180C6SCN2 I-I4 18NH4 SCN 2 6CH5N3HSCN 3 ( N2H8CS3) + 9 (NH4SCN)180-270C6CH5N3HSCN 2 C6HgNllHSCN 2 (NH31 + 5 (NH4ScY)

I


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226 M. S. Tomar and F. J. Garcia3Oml/min depending on the nature of the study one is interested in (31-33). Theformation of CdS over the heated substrate was assumed (4) to occur by the decom-position of the intermediate complex ion (formed in the solution) CdCl (SCN H4).In most of the work reported, this assumption was taken fcr granted (4:31,333.In recent reports (25,26) however the authors demonstrated that an intermediatereaction path was involved to form the final product. The composition of thefinal m;.terial depended on the temperature of preparation. The decompositionpaths proposed are shown in Fig. 2A and the thermogram of the film (during prep-aration) is shown in Fig. 2.B. The proportion of cholorine to cadmium in the filmas a function of the temperature is shown in Fig. 3 (25,26,34). It can be seenfrom these results that the chlorine content decreases as fast as the preparationtemperature of the substrate increases.3.1.1.A Electron Transport. The spray deposited films generally have high car-rier concentration and high resistivities. Micheletti and Mark (31) showed thatthe absorption of oxygen on CdS films reduced both the Hall mobility (u) and elec-tron density (n), the former by a larger factor than the latter. The films werephotosensitive. An activation energy of 0.03eV for electron density was not af-fected by chemisorption, whereas the values of activation energy for Hall mobili-ty increased by chemisorption essentially from a zero value to values between 0.08and 0.21 eV, due to the additional scattering introduced by chemisorption (31,34).Bube and co-workers in their extensive studies (32,33) on these films observedthat the dark electron mobility was thermally activated. The photoconductivitywas caused primarily by an increase in electron mobility. During photoexitationthe holes are trapped at grain boundaries and the electrons thus contribute ton-type photoconductivity. When the average grain size in the film is less thanthe mean free path in the grain, the scattering seems to dominate. The grainsize of these films are small (" l-Z-urn) s observed by several workers (28,35,36). The holes trapped at the grain boundaries decrease the barrier height andbarrier width. The decrease in the barrier height was indicated by a decreaseof mobility activation energy with photoexitation (31,33) and the decrease inbarrier width increases the tunneling probability of electrons through the bar-rier. Therefore the tunneling process seems to dominate the electrical trans-

I . C

0.6400 To(C)

Fig. Z-B, Thermogram of complex CdCl2(SCN2H4), (from Ref. 25,26).


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b

C

300 400 WC)Fig. 3. Quantity of Chlorine in the film versus temperature f preparation ) atthe end of spray, b) post-treatment: successive nnealing for one hourfor a film prepared at 260C, c) after annealing at 500C for one hour.

(from Ref. 34,261.


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228 M. S. Tomar and F. J. Garciaport at high photoexcitation and at low photoexcitation both tunneling and thermalexcitations are parallel processes as observed by Wu and Bube (33). The indepen-dence of dark electron density on temperature indicates that the donor level (as-sumed to be due to the un-compensated chlorine donors) was shallow (- 0.03eV) inthese films (31,33). However, it was observed that there seems to exist insulat-ing grain boundaries which played an important role in the measured variations ofthe mobilities (37). These insulating grain boundaries were formed due to chemi-sorption of oxygen at the grain boundaries. Under photoexcitation this chemi-sorbed oxygen at the grain boundaries and on the film surface acts as additionalrecombination centers and reduces photoexcited electron lifetime. The insulatinggrain boundaries produced by oxygen absorption thus decreases the tunneling prob-ability for electron transport through the intergrain barriers. Therefore, whenthe film was exposed to air (absorption of oxygen) after deposition, both theelectron mobility and density were observed to be reduced (31,35,38) with the mo-bility decrease being larger (both under dark and light). As a result the ob-served increase in resistivity was justified (33).

Fig. (4) shows the measured average crystal grain size versus film depositiontime (37). Both dark and light electron mobilities vary with the thickness andit was estimated to be due to the grain size variations. Kwok (37) predicted thatthe conventional thermoelectric power equation (38) can only be applicable as longas intergranular separations in the film are small.3.1.lB Temperature Effects. In the films deposited at a substrate temperaturelower than 350C (T 80%) have been obtainedwith CdSnO3 films (153). An advantage of sprayed cadmium stannate films oversputtered ones is the elimination of the post-deposition annealing step requiredin sputtered films.

6, APPLICATIOMSF OXIDE ILMS6.1. Solar Cells Using Oxide Semiconductors.

The benefits of using a wide band gap semiconductor as the top material inheterojunction solar cells have been recognized for some time. Such a semicon-ductor acts as an optical window for most of the solar spectrum thus allowing theradiation to reach the bottom absorber semiconductor with little attenuation.Because of this transparency the electron-hole pairs produced by the incoming ra-diation are created in a large proportion inside the depletion region of the junc-tion, resulting in a noticeable improvement of the spectral response of the cellat short wavelengths relative to the spectral response of most diffused junctioncells. Similarly, metal-insulator-semiconductor (MIS) cells also exhibit thistype of behavior, and with a suitable thickness of the interfacial insulatinglayer they are capable of producing open circuit voltages comparable or higher(141) than those obtained with diffused junctions. On the,other hand, the highsheet resistance associated with the small thickness (- 5OA) required for goodtransparency of the metal in the usual MIS configuration has led to the use ofwide band gap transparent oxide semiconductors with degenerate doping as substi-tutes of the metal thin film, Cells incorporating such an oxide semiconductorand an absorber (smaller band gap) semiconductor generally contain also an inter-facial insulating layer (SIS structure) which improves the performance of the cellas in the case of MIS cells. The behaviour of this type of SIS cell, which isroughly similar to that of the MIS structure, has been widely studied and a con-siderable amount of related work has been done and reported in different publica-tions (100,108,135,139,141). Wide gap (> 3eV) oxide semiconductor films presentseveral properties which make them attractive for use in solar cell applications:a> They are highly conductive and transparent for most of the solar spectrum.b) The indices of refraction of these materials have values (1.2-2.1) which arein the appropriate range for antireflection coating applications. c) The fabri-cation of these films can be carried out by simple deposition processes, at lowtemperature and low cost as in the case of spray pyrolysis deposition. The oxidesemiconductor, to form a solar cell, can at the same time complete the junctioncreating the necessary potential barrier, act as a transparent collector of car-riers, serve as an antireflection layer and provide environmental protection ofthe cell. The usual methods of fabrication of this kind of cell allow the utili-zation ofpolycrystalline absorber semiconductors, instead of monocrystalline,without many of the problems encountered when a diffused junction is formed inpolycrystalline material. All these characteristics represent significant advan-


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Spray Pyrolysis in Solar Cells and Gas Sensors 239tages of oxide semiconductor-absorber emiconductor olar cells over the more com-mon diffused junction cells. Using this type of structure it appears feasibleto produce high efficiency olar cells at lower costs than are presently possiblewith diffused junction technology.One of the most important onsiderations n the fabrication f oxide semi-conductor-absorber emiconductor olar cells is the choice of appropriate ombina-tion of materials. According to theoretical and diagram considerations, oachieve significant hotovoltaic ehaviour with p-type absorber semiconductors,the work function of the oxide semiconductor ust be less than or equal to theelectron affinity of the absorber semiconductor. Conversely ith n-type semi-conductors he work function of the oxide semiconductor s to be greater than orequal to the electron affinity plus the band-gap of the absorber semiconductor(107). Following this idea indium-tin xide (ITO) and ZnO should be adequate forp-type silicon and Sn02 for n-type. In theory these two combinations roduceminority carrier diodes which are ideal for photovoltaic onversion ince theypresent Schockley type behaviour. This fact is generally confirmed in the litera-ture (108-135) pecially in the case of sputtered ells, but not in all spray-deposited cells. Work in this field has been mainly confined to ITO, In 03, andSn02, usually with silicon as the absorber, and a few experiments sing E 2Sn04and ZnO. InP (136), GaAs (129), CdTe (71), etc., have also been utilized as absor-bers but they have shown up until now very modest performance xcept in the casesof InP (132,136,137) here an efficiency f 14.4% (AM2) has been reported in acell using ITO, and ZnO (sprayed)/CdTe single rystal) cell with efficiency f8.8% has been reported (71,72) in spite of a large lattice mismatch (Table II).About 3% efficiency as also been demonstrated ith an ITO/GaAs cell (129). Shew-chun et al (138) present in Table III of that reference properly chosen combina-tions of oxide semiconductors nd the above mentioned absorber semiconductorsincluding heoretical ossible efficiencies. As noted earlier some cells fabri-cated by spray pyrolysis of the oxide semiconductor xhibit a behaviour oppositeto the theory and to the experimental esults obtained with other methods of de-position. In this respect, there have been reports of ITO/n-Si and SnO /psicells made by pyrolysis. Silicon cells with sprayed IT0 show photovolt .c behav-iour only when n-type Si is used, since p-type Si produces an ohmic contactwith sprayed IT0 (139). Ashok et al (139) propose that contrary to what happensin sputtered ITOfp-Si cells, where minority carriers generated in the absorber(electrons n the conduction and of silicon) simply slide down to the conductionband of ITO, in sprayed ITOfn-Si cells the photogenerated arriers (holes in thevalence band of silicon) reach the IT0 by recombination ith electrons in the con-duction band of IT0 via interface tates which are necessarily equired for thisprocess. At the same time, the existence of an interfacial nsulating xidelayer is fundamental or the proper operation of the oxide semiconductor-absorber semiconductor (SIS) cell for the same reason as in the MIS cell. Thisinterfacial nsulating xide allows the creation of a strong inversion layer inthe surface of the absorber semiconductor hich diminishes the effect of the lat-tice mismatch that may exist between the oxide semiconductor nd the absorbersemiconductor. This inversion layer behaves as a pseudo p-n junction if properattention is given to the choice of the electron affinity of the insulating nter-facial oxide layer. One advantage of the SIS cell over the MIS cell is that thepresence of an energy band gap in the oxide semiconductor locks majority carrierflow if there are no interfacial tates in the insulating xide to absorber semi-conductor interface. To avoid Tunnel-limited low of the photogenerated arriersthe thickness,of he interfacial ayer must be carefully controlled to valuesless than 10 A. In practice, only a small range of lo-15 1 seems to be usefulfor good photovoltaic ehaviour (138). Generally this interfacial xide is grownfrom the absorber semiconductor y oxidation, roducing a native oxide, which hasthe advantage over deposited oxides of inducing less trap-producing islocationsof the crystal structure together ith simpler fabrication nd easy control of theoxide thickness. Using this SIS structure a number of oxide semiconductor-absor-ber semiconductor ells have been constructed, any of them with efficiencies


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240 M. S. Tomar and F. .I. arciaaround 10%. Ion-beam sputtered IT0 on SiO -p-Si cells have shown an efficiencyof 16.5% (140), already approaching near t e limit efficiency of silicon and in-dicating a good perspective for sprayed type cells. Similarly spray depositedcells with SIS structure have produced above 10% efficiences. Several recentresults with this type of cells using n-type silicon as the absorber semiconductorare presented in Table TI.

The values of efficiency reported are comparable to sputter-deposited SISsilicon cells and in the same range as n-p diffused silicon cells. As can be seenfrom the table the work on In 0 and IT0 has been in n-type silicon, structurethat when fabricated by sputter ng does not produce significant efficiency.a How-ever, a recent report (134) has shown an efficient electron-beam deposited ITO/n-Si solar cell similar to the sprayed ITOfn-Si SIS cell. This not yet completelyunderstood behavior of ET0 on both n and p-type silicon and the influence of thedeposition technique still requfre further investigation. Several factors arelimiting the efficiency of these cells. Shewchun et al (138) estimate that 10%to 20% reflection loss takes place at the surface of sputtered ITO. Non-degener-acy and work function alteration due to the fabrication method of the oxide semi-conductor together with interfacial layer losses due to interfacial layer defectstates are the other mechanisms that tend to lower the efficiency of these SIScells. In conclusion it can be said that the spray pyrolysis method of deposi-tion has demonstrated its usefulness in the production of high efficiency solarcells in a relatively simple, fast, and inexpensive way without the need of vacuumor the use of high temperature processes. In general the use of IT0 presents sev-eral advantages over Sn02 since it has better chemical stability and affords thepossibility of modifying the alloy composition thus permitting a certain ampli-tude in the electrical properties of the material. For large scaleimplementationhowever, Sn02 might be a better candidate since there have been some questions asto the availability of sufficient indium to satisfy the possible demand. Otheroxide semiconductors still require further research to fully ascertain their po-tential.6.2. Gas Sensors.

Gas sensors based on semiconducting metal oxides can experiment a change intheir electrical conductance when gases like carbon monoxide, alcohols, and hy-drocarbons are present at high temperatures. The sensor consists typically of afilm of an oxide semiconductor on an insulating substrate with two metal elec-trodes attached. This type of detection was first used to detect gases in air in1962 (142). One of the most appropriate oxide semiconductors for this applica-tion seems to be Sn02 (143-145). There are two models that can explain thechange in conductance of the oxide semiconductor: Transfer of electrons from theabsorbed gas molecule to the oxide semiconductor, and release into the conductionband of the oxide semrconductor of electrons captured by the previously chemi-sorbed oxygen which reacts with the absorbed gas (146). The second model seemsto represent better observed experimental results (147). Since the detectionmechanism relies on the oxygen absorption ability of the oxide semiconductor sur-face, a high area to volume ratio is desirable. This requirement clearly favorsthe use of thin film oxide semiconductors in this type of detectors. Long lifeoperability of the sensor depends on maintaining a disordered surface during itsworking life at the elevated temperatures encountered in the environment. Sev-eral gas sensors utilizing films of SnO have been reported in the literature(142-145,148,149). Sn02 sensors exhibi $ a dependence of their conductance on thesquare root of carbon monoxide partial pressure and require the presence of oxy-gen and also operation within a certain temperature range to properly absorb anddesorb the gas (147). The times rec@ad by the sensor to detect the gas and toregenerate are of special importance since the measurement of gas concentrationsusually need small values of these times.

pink et al (149) have produced fast spray deposited sensors with SnO2 as the


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Spray Pyrolysis in Solar Cells and Gas Sensors