J. Phys. D: Appl. Phys., Vol. 11, 1978. Printed in Great Britain. 0 1978

Cu,S /CdS thin-film solar cells using chemically sprayed CdS films

W C Siu and HL Kwok The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

Received 28 July 1977, in final form 9 November 1977

Abstract. This paper presents the results of a study of the photovoltaic effect of Cuss/ CdS thin-film solar cells formed on chemically sprayed CdS films using a dipping method. The main advantages of using CdS chemically sprayed thin films include low cost and the possibility of production in a large scale. Our investigation centres on the study of the effects of the properties of the chemically sprayed CdS films on the photovoltaic response of the resulting cells. This includes the study of the parameters of film deposition, film thickness, grain-size and structure, spectral response and I-Vcharacteristics. In general, we found that good photovoltaic cells can be made using the chemically sprayed films. The photovoltaic response in the cells has been limited by the series resistance losses as well as a grain-size effect which is responsible for the lowering of the shunting resist- ance observed in cells made on relatively thicker films. The cell performance is believed to be dependent on the copper content and the distribution within the cell. This has been observed in the changes of the cell's response with heat treatment. Normally, a brief heat treatment at 300 "C can bring the cells to their peak performance. No particular benefits can be derived from the addition of dopants. The maximum short-circuit current and open-circuit voltage obtained at 1 A M 0 were 6.2 mA cm-2 and 0.30 V, respectively, for a cell doped with iodine. The fill-factor was about 05-0.6 at 50 % sunlight intensity.

1. Introduction

This paper presents the results of a detailed study on Cu,S/CdS thin film solar cells formed on chemically sprayed CdS films. Although much work has already been done on cells fabricated on evaporated films, there is little or no work reported on cells made with chemically sprayed films. The chemical spraying method of thin-film deposition was first studied by Chamberlin and Skarman (1966) and later by Shallcross (1967), Micheletti and Mark (1967) and Wu et aZ(1972). Most of the work has been on isolated properties of the films, and no systematic study of the film properties in relation to device applica- tions have been made. It is found that the properties of the chemically sprayed films are quite similar to evaporated films and a lot of the effects observed in evaporated films (photosensitivity, grain-size effects and oxygen absorption) are also present. There are many reasons why the chemically sprayed films are attractive in device applications. The most outstanding features are the low cost, the requirement of a relatively simple set-up and the possibility of large-scale production. The main purpose of this work is to assess the suitability of using the chemically sprayed films in the making of solar cells. In relation to this objective, we have made a large number of chemically sprayed CdS films prepared under different physical conditions and various impurity dopings. Solar cells are made

669

670 W C Siu and H L Kwok

on these films using a dipping method and the photovoltaic processes are examined. Our main concern is the effect of the film properties on the resulting solar cells and no direct attempt has been made to study the properties of the Cu,S layer.

Our work can be divided into two parts. The first part is a study of the photovoltaic response of the cells when the physical properties of the chemically sprayed CdS films are changed. This includes changes caused by varying the parameters in film deposition and cell formation as well as the impurities intentionally added to the films. The second part is a study of the photovoltaic conversion mechanism. The effects of shunting and series resistance, heat treatment, spectral response and I-V characteristics are examined.

2. Experimental procedure

2.1. Film deposition and cell formation

The deposition of thin polycrystalline films using the chemical spraying method is well- established in the literature (Chamberlain and Skarman 1966, Cusano 1967). A schematic diagram of our set-up is shown in figure 1. The chemicals used in the spraying-solution were 'Chemical Grade' cadmium chloride and thiourea at concentration of 0.025 Molar.

Figure 1. A schematic of the chemical spraying set-up.

In the film-deposition, the solution was sprayed in a fine mist on clean glass substrates (made by Chance Proper) heated to 300-400 "C. The cleaning process for the substrates followed essentially those recommended for integrated circuits work. Some of the glass substrates were coated with indium oxide or tin oxide for use as electrodes. A preliminary investigation showed that good and reproducible films could be obtained using a spray-rate of 2.8 ml min-1 and a substrate temperature of around 340 "C. This would yield a deposition rate of about 400 A min-1. For the purpose of studying the effects of film thickness, films had been deposited with thicknesses from 0.5-7-0 pm. Impurities were incorporated in some of the films. Doping was done in the spraying solution. Iodine was added to the spraying solution in the form of soluble CdIz and indium in the form of soluble he l a . An excess of chlorine or sulphur could be achieved by either increasing the acidity of the solution or increasing the proportion of thiourea in the solution.

CusSICdS thin-jilm solar cells using chemically sprayed CdS films 671

The cells were formed using a dipping method. This method involved dipping the cadmium sulphide films in a dilute aqueous solution of cuprous chloride at 78 "C. Small amounts of hydroxylamine hydrochloride and potassium chloride were also added to ensure that the concentration of the cuprous ions remained high. Hydrochloric acid was used to adjust the pH of the resulting solution. A careful study of the variation of the photovoltaic response of a set of trial cells with the pH of the dipping solution and the dipping time showed that optimal results could be obtained when the cells were dipped for 2.5 s in a pH of 2.5. The corresponding Cuss layer was estimated to be between 0.2 and 0.3 pm.

Evaporated indium electrodes were used throughout this work. Comb-shaped electrodes were used in most of the cells to reduce the series resistance effect. Some of the cells were made without conducting substrates, and in those cases, Cuss layers were first formed in a comb-shaped pattern using standard photolithographic techniques so that a pair of comb-shaped electrodes could be placed on both the Cuss and the cadmium sulphide. The cells formed in this configuration had higher series resistance compared with those that had conducting substrates. No anti-reflection coatings were used for any of the cells. The as-prepared cells (immediately after cell-formation) had rather inferior photovoltaic response and a brief annealing at 250-300 "C in a nitrogen atmosphere was necessary to give the best results. Such an annealing process before the cell formation was found to be ineffective.

2.2 . Electrical and optical measurements

Thermoelectric and photothermoelectric measurements were used to estimate the carrier concentration in the films. A thermoelectric constant of 2.0 was assumed. The measure- ments, together with the conductivity data, gave the mobility. Light response measure- ments were taken using a tungsten light source filtered through a water bath. The maximum light intensity achieved was about 50 mW cm-2 (calibrated against direct sun- light intensity). Neutral density filters (Oriel Optics Corp.) were used to reduce the light intensity whenever necessary. Spectral response measurements were taken using a calibrated Bausch and Lomb grating monochromator covering a spectral range between 0.4 and 1.6 pm. Heat treatment was done in a carbolite furnace flushed with high-purity nitrogen. The electrical measurements were done in a vacuum dewar in vacuum or flushed with nitrogen.

.

3. Results

Figure 2 shows optical micrographs of a typical 'thick' film ( N 5 pm) and a typical 'thin' film (< 1 pm). It is observed that their structures are quite different, even though the only difference between them has been in the time of deposition. The 'thick' films have large grains (with diameters of 1-4 pm) which are easily identified under the microscope. The overall surface appearance is rather rough. The 'thin' films, however, have fine grains (submicron) and have no identifiable boundaries between the grains. The surface is quite smooth. The large black dots in the 'thin' film appear to be precipitations of iodine added to these particular films. As it appears, a certain amount of crystal- growth must have occurred for the 'thick' films. Figure 3 shows the light and dark data of the carrier concentration and mobility for films of different thicknesses. It is found that the thicker films are much more conducting and this has been the result of an increase

672 W C Siu and H L Kwok

I 0 1 0 ? O 3 0 , o - 4 0 L L d -- 7 0 3 0 L --

1 0 10 6L---- J--- - _L _I --i

T hiLAnc>is I p m ) Thickne.5 [pm)

Figure 3. (U) Light and dark electron concentration, (b) light and dark mobilities as a function of the film thickness measured at 60 "C.

in the mobility with increasing film thickness. Figure 4 shows the photovoltaic output of a number of iodine-doped cells heat-treated at temperatures between 150 and 400 "C. It is observed that the best response occurred when the cells were heat-treated for about 15 min at around 300 "C. Further heat treatment beyond this temperature tends to degrade the cells substantially. Figure 5 shows the photovoltaic output for some iodine- doped cells for different film thicknesses. The two sets of data (A, B) correspond to cells made with different electrode spacings. The larger value for the short-circuit current in A has been a consequence of the smaller series resistance when the electrode density has been increased. The results shown in figures 4 and 5 have all been obtained from cells without conducting substrate electrodes. Figure 6 shows the spectral current response of an iodine-doped cell subjected to different degrees of heat treatment. The cell was first baked in air at 160 "C for 12 min and then at 250 "C for 15 min. (This process, by our experience, would roughly bring the cell to the state of optimal spectral response.) It was then successively heat-treated at 300 "C for 5, 30, 120 and 240 min. Two response peaks can be identified, and they correspond to the near-intrinsic absorption of CdS at 0.49 pm and an extrinsic absorption at about 0.6-0.7 pm. It is observed that the spectral current response improved significantly with the initial heat treatment but deteriorated rapidly for heat treatment of 30 min or more. The relative importance of the two absorp- tion peaks also shifted with successive heat treatment. Figure 7 shows the spectral response of the short-circuit current for cells that are 'undoped', and doped with InC13, CdI2 and excess chlorine respectively. It is noted that there are striking similarities between the 'undoped' cells and the one doped with iodine. All the cells have received similar heat treatment at 250 "C for 15 min and at 300 "C for 5 min. It is observed that both the shape of the peaks as well as their position are dependent on the type of impnri- ties present. Figure 8 shows some typical current-voltage (I-V) characteristics of some 100 cells before and after heat-treatment at 300°C for 15 min. The cells are classified

Cu,S/CdS thin-Jilm solar cells using chemically sprayed CdS films 673

Time l h )

I I 0 1 2 3 L 5 6 7 8 9 10 11

Time ( h l

Figure 4. (a) Open-circuit voltage, (b) relative short-circuit current as a function of the period of heat treatment at temperatures between 150"-400 "C.

according to the photovoltaic response. Figure 9 shows the temperature dependence of the 1-V characteristics of some of our better cells. The voltage drop due to the series resistance has been corrected. It is noted that the slopes are roughly independent of temperature for small current while the intercepts vary with change in temperature. Table 1 summarises the photovoltaic output of the different cells we measured. For the best cells, we have obtained an open-circuit voltage of 0.3 V and a short-circuit current of 6.2 mA cm-2 calculated for a light intensity of 100 mW cm-2. (The actual light intensity during the measurements was about 50 mW cm-2.)

4. Discussions

4.1. Effects of film thickness

It is observed that the thick films (see figure 5 ) actually have inferior photovoltaic response compared with the thinner films. This is contrary to the idea that the cell's output current is mainly limited by the series resistance. This also implies that the photovoltaic conver- sion mechanisms in these films are more complicated than a simple heterojunction as

674 W C Siu and H L Kwok

I I 1 I I

-__L,---- 0 2 L 6 0 10

Thickness (pml Figure 5. (U) Open-circuit voltage, (b) short-circuit current as a function of the film thick- ness. A, 10 electrode slots per cm; B, 5 electrode slots per cm.

Cu,S/CdS thin-film solar cells using chemically sprayed CdS films 675

3 Wavelength I pml

Figure 7. Spectral response of the short-circuit current for cells doped with different impurities: 0 Inch, x excess HCI, A CdI2, 0 undoped.

( i i ) ( i i i l

i i i l ( i i i l

Figure 8. Light and dark I-Vcharacteristics: (U) before heat treatment; (b) after heat treatment; and (c) over-heat-treated, The labels (i), (ii) and (iii) are for cells with good photovoltaic response, fair photovoltaic response and poor photovoltaic response.

676 W C Siu and H L Kwok

v - R' I 1 V )

Figwe 9. I-V characteristics measured in the dark at 203,248 and 295 K. The voltage drops due to the series resistance effect have been corrected.

Table 1. Photovoltaic outputs

Substrate Dopants voc 0 Is, (mA cm-2)

Glass I2 0.36 1.8 (max.)

Glass Undoped 0.35 1.7 (max.)

0.43 1 .2

0.42 1 .2

Glass Excess HCI, pH=4 0.40 0.5 pH=3 0.38 0.6

Glass Excess S 0.20 0 . 2 - ___--

Glass I n c h 0.26 0.1

SnOs I2 0.31 5 . 5 Undoped 0.30 5.9 (max.)

I n 0 I2 0.30 6.2 (max.) _____.

Undoped 0.31 5.6

The cells were similarly prepared and were heat-treated for 15 min at 300°C after fabrication. The light intensity was normalised to 1 AMO.

suggested for single crystals. In these films, it is known that the structure and the electrical properties are sensitive to the grain size and the method of film preparation. The observed decrease in the open-circuit voltage and the short-circuit current with increasing film thickness certainly suggest that the junction properties must have changed. Also, it is found in the study of the I-I/ characteristics that the decrease in VOc is directly related to the decrease in the shunting resistance. Recognising the large difference in the grain size

CuzS/CdS thin-jilm solar cells using chemically sprayed CdS jilms 677

of the thick films and the thin films, we propose that the poor response in the thick films must have been associated with the grain size difference. The dipping process used in the cell formation is known to be quite sensitive to the structure of the substrate. For example: the dipping process for a given concentration of the dipping solution that takes several minutes to complete in single crystals will take seconds in the films. This gives a good idea of the drastic differences in the reaction rates. In single crystals, it is expected

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I /

LJ-J ,.!--*-..

Figure 10. (a) A typical structure of a thick CdS film showing the effects of crystallite growth during film deposition; (b) cell-structure for films with large grains; (c) cell- structure for films with fine grains; ( d ) chemical reaction at the grain surface of a crystallite.

that the reaction will take place along the crystal planes, while in the thin films, the process depends on both the rate of solution penetration into the films (through the grain boundaries) as well as the reaction rate at the grain surface. The reaction at the grain surface depends largely on the grain size. A fine grain structure ensures that the reaction can proceed rapidly and to completion throughout the grains while a coarse structure is likely to produce partially reacted grains. These partially reacted grains may have cores that remain unreacted (in CdS form). Figure 10 shows a somewhat exaggerated picture of the two structures. The partially reacted grains can have conducting channels that extend deep into the CdS bulk and are responsible for the observed low shunting resist- ance. Also, these inhomogeneous regions can increase the series resistance and the recombination losses in the cells resulting in a decrease in the short-circuit current.

678 W C Siu and H L Kwok

4.2. Heat-treatment effects

The as-prepared cells normally have poor photovoltaic response and a small amount of heat treatment is necessary to bring the cells to their peak performance. A simple pro- cedure for our cells is to heat-treat them at 300 “C for 15 min. The spectral current response during successive heat-treatment is a good indication of what happens during cell-formation and cell-degradation. It is noted that for short-periods of heat treatment at temperatures up to 300 “Cy the overall response curve increases rapidly. This is accom- panied by the building-up of the absorption peak at 06-0.7 pm and an overall broadening of the whole response spectrum. This increase in the long wavelength response is a clear indication of the increasing important role of the Cu,S layer. It is long known that the chalcocite phase of the Cu,S (with absorption peaks at 1.2 and 1.8 eV) is responsible for the improved response in the Cu2S/CdS solar cells and a transformation to the chalcocite phase with initial heat-treatment can give rise to the observed increase.

Heat treatment at 300°C for 30 min or more degrades the cells. This could be the consequence of the diffusion of copper into the CdS bulk resulting in the widening of the junction and in the deficiency of copper in the Cu,S layer. The primary effects of a widened junction are an increase in the recombination losses and a lowering in the near- intrinsic absorption as the CdS side of the junction is now located further down the surface. The effect in the lowering of the copper concentration in the Cu,S layer is more complicatcd. Nakayama (1969) reported an energy gap of 1.5-1.8 eV for the djurleite phase (CUl.QBS) and a higher absorption edge for CuzS with less copper content. Thus, the formation of these copper-deficient phases seems to agree with the observed narrowing in the spectral response curves as well as the merging of the two peaks with heat treatment.

4 . 3 . Effects of doping

Although iodine is known to give rise to sensitising centres (Bube 1960) in the CdS films, cells made on films doped with iodine do not seem to give better photovoltaic response. It is most likely that iodine is precipitated out during the heat treatment process. Doping CdS films with other types of impurities, however, all tend to degrade the photovoltaic response of the resulting cells.

4.4 . I-V characteristics

The I-V characteristics of the solar cells are often useful in the study of the current transport mechanism within the cells especially when a well-defined model can be used, For the cells used in this study, the current transport mechanism can be quite complicated and it is doubtful if meaningful quantitative study of the I-V characteristics can be obtained. Qualitatively, however, a number of features can be identified and can be useful in understanding the cell behaviour. It is observed that, in general, the photo- voltaic response of the ‘non-heat-treated’ cells are poor and their I-V curves are insensi- tive to illumination. After some heat treatment, the I-Vcurves are more ‘diode-like’ and the ‘cross-over effect’ becomes significant. The dark resistance increases somewhat, but to a lesser extent in the better cells. The shunting resistance in the better cells remains roughly unchanged. Nearly all of the over-heat-treated cells have low shunting resistance in the light and high cut-in voltages in the dark. Thus, the low open-circuit voltages observed in some of the cells are found to be associated with the low shunting resistance under illumination. One parameter of some interest is the ‘apparent’ diffusion potential $

CuzS/CdS thin-fim solar cells using chemically sprayed CdSJilms 679

- 2 3.0- 5. - 0

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.- 4-

c

a c 2 0 - .Q m 2

73

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of the cells. This parameter can be obtained by extrapolating the forward I-V curves to the voltage values at I=O. Normally, this parameter gives an indication of the barrier- height the electrons have to surmount to cross the junction and it should correspond to an absorption edge in the spectral response measurements. The word 'apparent' has been added to distinguish it from the diffusion potential at equilibrium. Figure 11 shows how

-

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the 'apparent' diffusion potential $ varies with heat treatment at 300 "C. The magnitude increases from less than 1.0 for slightly heat-treated cells to somewhat less than 2.0 in over-heat-treated cells. This change is another indication of the possible transformation of the CuzS layer from the chalcocite phase to the other copper deficient phases.

The temperature dependence of the I-V characteristics can be used to distinguish the kind of current transport mechanism in the cells. The lack of any temperature dependence found in the I-V characteristics of the better cells suggests that tunnelling is important. This seems to agree with the observations found in the single crystals (Gill and Bube 1970, Lindquist and Bube 1972). For the cells with poor photovoltaic response, it is found that the slopes increased slowly at lower temperatures, indicating that diffusion current may also be important.

5. Conclusions

In this work, we have made a detailed study of the properties of the Cu2S/CdS thin film solar cells formed on chemically sprayed CdS films. We have examined the photovoltaic effect in these cells and their dependence on the properties of the chemically sprayed CdS films. It is observed that the photovoltaic output can be limited by a bulk effect caused

680 W C Siu and H L Kwok

by the series resistance and a junction effect when the size of the crystallites becomes too large. The junction effect, not emphasised before, is unique in polycrystalline films with large crystallites and is believed to be due to the formation of a region of inhomogeneous Cu,S and CdS at the interface of the heterojunction. Such an inhomogeneous structure can lower the shunting resistance and increase the recombination losses. This is particu- larly responsible for the lowering in the open-circuit voltage. Heat-treatment effects are also investigated and it is found that a brief annealing at a moderate temperature can improve the cell performance possibly through the enhancement of the chalcocite phase of the Cu,S layer. Excessive heat treatment will degrade the cell performance and this can be caused by the diffusion of copper into the CdS bulk. Doping the films with impurities does not seem to increase the response of the cells in any way and it is found that the better cells are often made without doping. Tunnelling appears to be important for the current transport in the better cells and this agrees with the observations found in other Cu,S/CdS systems. In general, we found that the performance of the cells is quite sensitive to the method of preparation. Although a ‘standard’ set of parameters can be proposed, it is not at all certain that one can define the best ‘recipe’ for the optimal cell. In this work, we have made a comprehensive study of these cells and have identified the structural and heat-treatment effects. The cell efficiency has been estimated to be about 1 % (with a fill factor of 0.5 at 50 % AMO). This figure is somewhat low compared with best figures reported and we believe that improvements can be made if steps are taken to optimise the cell geometry and to include a layer of anti-reflecting material. Further improvements will have to come from means of reducing the series resistance losses as well as the grain-size effect. Bearing in mind that our main purpose has been to assess the application of these films to photovoltaic conversion, we believe that we have demon- strated that these cells can compete well with cells made on evaporated films, especially when cost is also considered.

Acknowledgment

The authors wish to express their gratitude to the referees for their helpful comments and recommendations.

References

Bube R H 1960 Photoconductivity of Solids (New York: Wiley) Chamberlin R R and Skarman JS 1966 J. Electrochem. Soc. 113 86-9 Cusano D A 1967 Physics and Chemistry of 11-IV Compounds (New York: Wiley) Gill W D and Bube R H 1970 J. Appl. Phys. 41 3731-8 Lindquist P F and Bube R H 1972 J. Appl. Phys. 43 2839-50 Micheletti F B and Mark P 1967 Appl. Phys. Lett. 10 136-8 Nakayama N 1969 Japan. J. Appl. Phys. 8 450-62 Shallcross F V 1967 RCA Rev. 569-81 Wu CH, Feigelson R S and Bube R H 1972 J Appl. Phys. 43 756-8

J. Phys. D: Appl. Phys., Vol. 1 I , 1978-W C Sirr and H L Kwok (see pp 669-80)

Figure 2. Light microscopic pictures of (a) a typical thick film (approx. 5 pm) and (b) a typical thin film (< 1 ym). Magnification x 600.

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