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Thin Solid Films 480–4

Materials issues in very thin film CdTe for photovoltaics

S.J.C. Irvinea,*, V. Barrioza, A. Stafforda, K. Duroseb

aDepartment of Chemistry, University of Wales, Bangor Gwynedd LL57 2UW, UKbDepartment of Physics, University of Durham, UK

Available online 28 December 2004

Abstract

A study is made of extremely thin absorber layers of CdTe deposited onto planar substrates in order to characterise their properties. Metal

organic chemical vapour deposition (MOCVD) was used to deposit the CdS and CdTe films under controlled conditions of substrate

temperature and VI/II organometallic ratio. In situ laser reflectance was used to monitor the thickness of the films and also the roughening

from loss of reflected intensity. The roughness of the final structure was measured with atomic force microscopy (AFM) and compared with

that of both indium tin oxide (ITO)/glass substrates and CdS deposited on them. Absorber layer thicknesses of between 50 and 500 nm were

deposited, the thicker layers being grown in two stages, with a highly As doped cap layer to act as a contact. A degradation of the current–

voltage characteristics was observed from good rectification at 500 nm thick absorbers to an ohmic characteristic at 100 nm thickness. This

could not be simply explained by the non-coalescence of grains, based on the AFM results, but attributed to defects on the ITO/glass

substrate. Improvements in the cleaning procedure resulted in good rectification for 100 nm absorber layer. It was demonstrated that CdTe

covers non-planar CdS conformally within 50 nm.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Deposition; CdTe; Photovoltaics

1. Introduction

The absorber layer thickness for CdTe is normally

between 2 and 10 Am, but most of the band gap radiation

is absorbed in less than 2 Am in a planar structure. Thicker

films are often used to avoid pinholes through the absorber

layer that can lead to shorting from the back contact. The

pinholes can arise from incomplete coalescence of the CdTe

grains during deposition, or due to defects in the underlying

surface, and are a continuing concern in thin film

polycrystalline devices. A further problem is encountered

in very thin CdTe where the minority carrier diffusion length

is considerably less than the absorber thickness as this will

reduce the photocurrent and photoconversion efficiency of

the device. Successful PV devices rely on a fortuitous

passivation of the grain boundaries that repel the minority

carriers and increase the probability of diffusion to the

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.11.130

* Corresponding author. Tel.: +44 1248 382383; fax: +44 1248 370528.

E-mail address: sjc.irvine@bangor.ac.uk (S.J.C. Irvine).

junction [1–3]. Other factors that can affect the collection

efficiency are built-in fields, junction location and carrier

diffusion barriers.

The extremely thin absorber (ETA) cell has been

proposed to improve the electrical collection efficiency of

polycrystalline thin film photovoltaics [4]. The loss in

optical absorption efficiency can be compensated by using a

porous or nano-porous structure. For example, Ernst et al.

[4] used a micro-porous TiO2 substrate for the CdTe ETA

layer and coated with a total thickness of 500–2000 nm of

CdTe. The estimated coating thickness on the TiO2 micro-

pores was 100–200 nm. More recent work by Ernst et al. [5]

has reduced this to 150 nm, using electrodeposition of CdTe

onto nano-porous TiO2. The coverage was very good with

local variations in thickness from 100 to 200 nm. However,

the quantum efficiency was low in this structure, falling to

below 40% at 800 nm. Mercury was alloyed with the CdTe

in an attempt to improve the absorption of the film. The

device characteristics of these ETA cells have been to have a

low short circuit current and very low fill factor. This was

attributed to the large band offset between TiO2 and CdTe.

81 (2005) 76–81

S.J.C. Irvine et al. / Thin Solid Films 480–481 (2005) 76–81 77

However, these authors were able to show an enhancement

over the planar equivalent structure of a factor of 50 in the

quantum efficiency.

Konenkamp et al. [6] have investigated different nano-

structured substrates, including TiO2 and ZnO. The ZnO

produced a very columnar structure and it was concluded

that this might be a good morphology for ETA cells. The

same approach was taken by Levy-Clement et al. [7] who

produced a columnar ZnO film by electrodeposition. These

columns were subsequently coated with CdTe by vapour

phase epitaxy under a dynamic vacuum. The coating

uniformity appeared to be excellent with an average

thickness of just 50 nm.

This paper investigates some of the materials issues in

depositing ETA layers of CdTe by depositing in a conven-

tional planar geometry onto a CdS window layer. The

chosen method for depositing the CdS and CdTe layers was

metal organic chemical vapour deposition (MOCVD) as this

has shown to be a useful method for controlling grain size,

VI/II stoichiometric ratio and intentional introduction of

dopants [8–11]. The latter two advantages offer the

opportunity for experimenting with the characteristics of

nucleation, development of surface morphology and elec-

trical characteristics of the films. In this study, a series of

layers were grown where the absorber layer thickness was

changed in each experimental structure. It was also possible

to investigate the CdS window layer characteristics and

produce films with different degrees of roughening. The

film deposition was monitored in situ using laser interfer-

ometry (635 nm diode laser) and ex situ measurements of

the films included atomic force microscopy (AFM) and

measurement of I–V characteristics. A particular aspect to

this work was to investigate the effects of very thin absorber

layers without the complication of deposition onto a nano-

structured surface. Issues such as shunting due to pinholes

and non-coalescence of grains were studied.

Table 1

Deposition conditions for the CdTe absorber layer deposited onto the CdS

window layer

RUN no. CdTe thickness/nm,

undoped+As

doped cap

Organometallic

VI/II ratio

Growth

temperature of

CdTe films/8C

1 500+250 2.5 320/350

2 300+250 2.4 320/350

3 100+250 2.5 320/350

4 500+250 2.5 320/320

5 250+250 2.5 320/320

6 100+250 2.5 320/320

7 100+250 2.5 320/320

8 100+250 2.5 320/320

ETA1 53 1.0 350

2. Experimental

Details of the experimental procedure for the MOCVD

growth of CdS/CdTe have been given elsewhere [8]. The

substrates were indium tin oxide (ITO) coated glass

substrates supplied by Merck and Delta Technology. The

organometallics used in these experiments were dimethyl-

cadmium (DMCd), ditertiarybutylsulphide (DTBS) and

diisopropyltelluride (DIPTe), all supplied by Epichem. The

dopant precursors were n-hexylchloride (n-hexCl) for the

CdS layer and tris-dimethylaminoarsenic (DMAAs) for the

CdTe layer. The layer structure was CdS:Cl/CdTe/CdTe:As.

For the initial series of layers, the As doped cap layer (250

nm thick) was grown as a contact layer to the undoped CdTe

absorber layer, which varied in thickness from 100 to 500

nm. This cap layer was grown with an organometallic VI/II

ratio of 0.6 and a flow of DMAAs of 9 sccm at 20 8C.Experiments on thicker layers had shown that the photo-

current for undoped layers was less than 1 mA cm�2,

without subsequent annealing and therefore As doping was

included for these growths where CdCl2 annealing was not

used. The deposition temperatures for the cap were 350 8Cfor layers 1–3 and 320 8C for layers 4–8. The significance of

growth temperature in this paper was largely associated with

the change in the morphology where the film became

rougher at higher temperatures.

The in situ laser reflectance monitor was a single wave-

length instrument operating at 635 nm, normal incidence, and

supplied by ORS. The output of the monitor gives reflected

intensity versus time and real time analysis of growth rate,

thickness and film roughening. The column showing the VI/

II ratio in Table 1 is the ratio of the organometallic precursors

entering the reactor cell. This was calibrated at the start of

each run using an Epison ultrasonic monitor [12].

The CdS/CdTe structures were made into test devices for

characterisation of their photovoltaic parameters in order to

correlate with the growth conditions and the in situ

monitoring. The substrates were cleaved into 1�2 cm strips

and etched at one end to remove the CdTe and CdS film.

Gold contacts were evaporated through a mask to leave a

1�1 cm contact pad on the CdTe top surface I–V

characteristics were measured using an HP I–V meter and

illumination using a quartz halogen lamp with 100 mW/cm2

intensity on the photovoltaic device. No annealing treatment

was given prior to making these measurements.

The deposition conditions for the different absorber

layers are shown in Table 1. The absorber layers were

grown at 320 8C in order to encourage a smoother growth

and encourage complete coverage of the CdS. Layers 7 and

8 were deposited under the same conditions as 6 but the

ITO/glass substrates were prepared differently in order to

change the nucleation conditions. The substrate for run 7

was ultrasonically cleaned in deionised water, and 8 was

ultrasonically cleaned in methanol.

Characterisation of the surface morphology was carried

out with a Digital Instruments Nanoscope 3Awith a new tip

fitted prior to the measurements. The measurements were

made in tapping mode (target volts 1.0 V) at 1.064 Hz scan

rate.

S.J.C. Irvine et al. / Thin Solid Films 480–481 (2005) 76–8178

3. Results and discussion

3.1. Characteristics of thin absorber layers

The thicknesses of the MOCVD grown layers were

measured in situ using laser reflectometry and a typical

interferometer trace is shown in Fig. 1 for layer 2. The first

broad peak is approximately one interference oscillation of

CdS which was grown for 20 min. The relatively slow

growth rate relates to the low temperature of 290 8C that is

needed to obtain high quality CdS layers. The growth

process was then stopped while the substrate temperature

was reset to 320 8C under a flow of pure hydrogen. The

subsequent series of three oscillations corresponds to the

growth of 300 nm of CdTe absorber layer followed by a

further growth pause. During this period, the substrate was

heated to 350 8C and a further three oscillations can be

observed corresponding to the doped CdTe cap layer. It can

be seen in Fig. 1 that there is an overall decrease in the

reflectance signal during the cap layer growth which is

indicative of surface roughening, thus losing intensity from

the reflected beam due to scattering. Some roughening is

also occurring during the absorber layer growth but is less

pronounced. The laser reflectance plots for the first series 1–

3, where the absorber layer was reduced from 500 to 100

nm, all show the same characteristic of more roughening

occurring during the cap layer growth.

I–V measurements were made on this series of layers and

a number of them were not rectifying, indicating that the

gold contact was shorting through to the CdS layer. This is

not normally observed for the more usual, thicker absorber

layer of 2 Am. Examples are shown in Fig. 2 from the first

series where the cap layer was grown at the higher

temperature of 350 8C. I–V curves from the first two layers

in the series, are shown where the thicker absorber layer

shows good rectification and the second is showing very

poor characteristics but still some photoresponse. The third

Fig. 1. Laser reflectance monitoring of layer 2 showing time-resolve

layer in the series with just a 100 nm thick absorber layer

was purely ohmic with no photoresponse.

Although the films are sufficiently coherent to give Fabry

Perot interference oscillations, small regions of pinholes

could still exist and would dominate the I–V characteristics

seen in Fig. 2(b). A peak height analysis was performed using

an AFM where a 10�10 ı̀m area of the surface was scanned

on a number of these layers. The distribution did not change

significantly with absorber layer thickness and the centre of

the Gaussian distribution was less than the cap layer

thickness. An example is of the AFM height analysis, for

layer 1, where the total CdTe layer thickness of 750 nm gave a

maximum recorded height on the histogram of 264 nm.

The in situ laser reflectance monitoring in Fig. 1 had

indicated that the higher growth temperature of the cap layer

was forming a much rougher layer and could be responsible

for most of the height distribution shown in the AFM

images. The series of growth runs 3–5 were a repeat of the

series 1–3 but with the growth temperature of the cap layer

reduced to 320 8C, the same as the absorber layer growth

temperature. This resulted in less roughening during the cap

layer growth, with relatively little decrease in the reflected

intensity. The AFM height analysis for layer 4, which has

the same absorber layer thickness as layer 1, did not show

any significant changes. This would indicate that most of

the observed roughening has occurred before the CdTe

layers have been grown and the CdTe is replicating the

underlying surface.

The I–V characteristics for the second series of layers did

not show a trend as in the first series, with layers 4 and 5

showing ohmic characteristics and very poor I–V response

for layer 6. The latter was the thinnest absorber layer in this

series but the AFM image showed a similar height

distribution to that of the 500 nm thick absorber layers.

The inconsistency of these results would indicate that

shorting of these layers cannot be explained simply by the

morphology and poor CdTe grain coalescence. However,

d interference oscillations for the three layers in this structure.

Fig. 2. I–V curves (under 100 mW/cm2 halogen lamp illumination) for thin absorber layers (a) layer 1, 500 nm thick and (b) layer 2, 300 nm thick.

S.J.C. Irvine et al. / Thin Solid Films 480–481 (2005) 76–81 79

pinholes could be formed around foreign particulates left on

the ITO surface after cleaning, and this is investigated

further in the next section.

Fig. 3. I–V plot for layer 7 where the substrate had

The following two layers in this series (runs 7 and 8)

were grown with a different surface preparation for the ITO

surface in replace of the normal methanol rinse. Both

been cleaned ultrasonically in deionised water.

S.J.C. Irvine et al. / Thin Solid Films 480–481 (2005) 76–8180

substrates were cleaned in an ultrasonic bath to ensure that

all particulates were removed with layer 7 being cleaned in

deionised water and layer 8 being cleaned in methanol. Both

layers were grown with absorber layer thickness of 100 nm,

which was the thickness most likely to show ohmic

characteristics due to pinholes. Layer 7 (with the deionised

water clean) had the best I–V characteristic with rectifying

behaviour and a reasonable photoresponse, for a thin

absorber layer, and is shown in Fig. 3. Layer 8 displayed

similar characteristics to layer 6 but with a very poor

rectification. The rectification in Fig. 3 shows an improve-

ment over the ETA cells reported by Ernst et al. [4], but in

their work, the absorber was deposited onto nano-porous

TiO2 and the photocurrent was considerably higher than

with the planar structure in the present work.

3.2. Morphological characterisation of ITO substrate and

CdS window layers

Possible causes for pinhole formation and surface rough-

ening were investigated by examining the surface morphol-

ogy of ITO layers with optical microscopy. Fig. 4 (a) shows

the surface of an as received ITO layer. A high density of

Fig. 4. Optical microscope images of (a) ITO surface with normal rinse and

(b) ITO surface after ultrasonic cleaning procedure.

Fig. 5. AFM images, 5�5 Am scan area and 2 Am height scale, of CdS

surfaces for different film thicknesses of (a) 120 nm, (b) 240 nm and (c)

500 nm.

particulates can be seen on the surface, probably due to

debris from cleaving the substrates and is not removed by

normal rinsing procedures. A new cleaning procedure was

adopted which involved cleaning in an ultrasonic bath filled

with deionised water and refluxing in toluene. This

procedure produced much cleaner surfaces (shown in

Fig. 4(b)) and was adopted for subsequent growths of CdS

window layers.

S.J.C. Irvine et al. / Thin Solid Films 480–481 (2005) 76–81 81

A series of CdS layers were deposited at a substrate

temperature of 300 8C where for each layer the thickness

was increased from 120 to 240 and 500 nm, as measured

using the in situ interferometer. The AFM images for these

layers are shown in Fig. 5. The RMS roughness measure-

ments for these three films were 8, 26, and 49 nm,

respectively, showing progressive roughening for thicker

CdS layers but a uniform grain structure. From these results,

it is possible to choose different CdS templates for the

subsequent growth of CdTe ETA layers and investigate the

grain growth and extent of surface coverage as the surface

progressively roughens.

3.3. Characterisation of ETA CdTe onto non-planar CdS

The intermediate thickness for a CdS layer of 240 nm

was selected to study the effect of CdTe coverage. The

interferogram from the in situ monitor gave two oscillations

from the growth of the CdS layer (corresponding to 240 nm)

followed by a pause of 1200 s where the reactor temperature

was reset to 350 8C. The CdTe layer was grown for 69 s and

had an estimated thickness of 53 nm. The in situ monitor

showed approximately half of an interference oscillation,

consistent with the estimated thickness. The AFM character-

isation of the surface, taken under the same conditions as for

the CdS layers, gave an RMS roughness of 10 nm. This

makes an interesting contrast with the RMS roughness for a

CdS layer of 26 nm. The AFM images indicate that the film

has become smoother as a result of growing the CdTe layer.

Some evidence for this effect was also observed by Ernst et

al. [5] who showed that CdTe was filling the nano-structured

pores in the TiO2 substrate. This further supports the idea

that CdTe can provide excellent surface coverage for

extremely thin films and problems with shorting of the

devices have to be attributed to other effects such as

particulates on the substrate surface.

4. Conclusions

Planar CdTe solar cell structures with extremely thin

absorbers have been studied to gain more insights into the

relationship of device to materials characteristics.

Using MOCVD, it was possible to grow a two-level

CdTe structure with an undoped layer followed by a highly

As doped cap layer.

Comparison of CdTe/CdS/TCO devices showed an

increasing tendency for shorting in the structures with

thinner CdTe. Structures were grown under conditions to

give a smoother CdTe layer but the same tendency was not

observed. The AFM characterisation of these layers

indicated that all showed complete grain coalescence, and

that the surface coverage of the CdS with CdTe was good.

Examination of the surface of a typical ITO/glass substrate

using optical microscopy showed particulates consistent

with surface contamination that could cause pinholes in the

growth. Ultrasonic cleaning in deionised water resulted in

good rectification for the thinner structure and demonstrated

that ETA cells without pinhole shorting were possible.

Characterisation of CdS layer varying in thickness from

120 to 500 nm showed good coverage of the TCO, but an

increase in the roughening of the surface.

Preliminary experiments at sub 100 nm thick growth of

CdTe onto a similarly grown CdS film showed that not only

was the coverage excellent but that some improvement in

the morphology had taken place with a 53 nm thick film of

CdTe reducing the RMS roughness from 26 to 10 nm.

Deposition of extremely thin CdTe will now be inves-

tigated on more structured surfaces to increase the solar

absorption and to test whether the good rectification

observed in these experiments can be maintained.

Acknowledgements

The Authors gratefully acknowledge the support of

EPSRC through GR/R20816/01 for the support of Dr Anne

Stafford. This research was supported by Epichem who is

gratefully acknowledged for their generosity and encourage-

ment. The authors also wish to thank John Cambridge for the

AFM measurements and Steve Jones for technical support.

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