Highly structured TiO2/In(OH)xSy/PbS/PEDOT:PSS for photovoltaic applications

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Solar Energy Materials & Solar Cells 89 (2005) 13–25

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Highly structured TiO2/In(OH)xSy/PbS/PEDOT:PSS for photovoltaic applications

R. Bayon1, R. Musembi, A. Belaidi, M. Bar, T. Guminskaya,M.-Ch. Lux-Steiner, Th. Dittrich�

Hahn-Meitner-Institut, Abteilung SE2, Glienicker Str. 100, D-14109 Berlin, Germany

Received 9 July 2004; accepted 20 November 2004

Available online 18 January 2005

Abstract

A system of highly structured TiO2/In(OH)xSy/PbS/PEDOT:PSS has been developed and

investigated by photovoltage spectroscopy, X-ray photo- and Auger electron spectroscopies,

electron microscopy, and photovoltaic response. TiO2, In(OH)xSy, PbS, and PEDOT:PSS

serve as electron conductor, buffer layer, absorber, and hole conductor, respectively. Both

buffer and absorber layers were prepared by chemical bath deposition. The band gap of as-

prepared In(OH)xSy varied between 2.4 and 3.5 eV depending on the pH-value of the solution.

In addition, the band gap of the PbS could be widened to about 0.85 eV making the

application as absorber for solar cells feasible. At present, corresponding solar cell devices

reach short-circuit current densities of about 8mA/cm2 and open-circuit voltages of about

0.3V.

r 2004 Elsevier B.V. All rights reserved.

Keywords: Highly structured TiO2; PbS; Photovoltage spectroscopy

1. Introduction

The development of alternative photovoltaic systems and solar cell materials isimportant from both the fundamental and technological viewpoints. An example of

see front matter r 2004 Elsevier B.V. All rights reserved.

.solmat.2004.11.011

nding author. Tel.: +4930 8062 2090; fax: +4930 8062 3199.

dress: [email protected] (T. Dittrich).

dress: IMRA-EUROPE, 220 rue Albert Caquot, Sophia-Antipolis Cedex, 06904 France.

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R. Bayon et al. / Solar Energy Materials & Solar Cells 89 (2005) 13–2514

such devices are the so-called eta cells (eta: extremely thin absorber), which make useof highly structured surfaces [1]. In these solar cells, the absorber layer is embeddedbetween transparent electron and hole conductor [1]. In this way, the processes oflight absorption followed by charge separation and carrier transport are separated inspace, similarly to dye-sensitized solar cells [2]. The practical challenge of the etaconcept is to find suitable materials and the appropriate technologies for theirpreparation. Candidate materials as transparent electron conductors are TiO2 [1]and ZnO [3]. As transparent hole conductors, both inorganic (e.g., CuI [4], CuSCN[5] and CuAlO2 [6]) and organic (e.g., OMeTAD [7]) materials are underinvestigation for use in this kind of solar cells. For the absorber material, a largevariety of compounds such as semiconductor nanoparticles [8,9], CuInS2 [10,11] orCdTe [1] could be applied to eta cells. One of the crucial points in the development ofeta cells is the engineering of the electronic properties of the large electronconductor–absorber interface. It has been shown that buffer layers such as metaloxides [12], In2S3 [11,13] or In(OH)xSy [14] between electron conductor andabsorber improve the eta cell performance by decreasing recombination losses at thisinterface.In this work, a highly structured eta cell such as TiO2/In(OH)xSy/PbS/

PEDOT:PSS has been developed. In this device, highly structured (microporous)TiO2, deposited onto glass/SnO2:F substrates by spray pyrolysis was used as theelectron conductor. Poly(3,4-ethylenedioxythiophene) doped with polystyrenesulfonic acid (PEDOT:PSS) acted as the hole conductor and was deposited by spincoating. A thin film of In(OH)xSy prepared by chemical bath deposition (CBD,[15,16]) was used as buffer layer. For the absorber, a PbS layer prepared also byCBD was used. In order to do so, it was necessary to widen the direct band gap ofthe PbS in comparison to its bulk value of 0.42 eV [17] for better adaptation to thesolar spectrum. This was achieved by adjusting the deposition parameters of the PbS.X-ray photoelectron spectroscopy (XPS), X-ray excited Auger electron spectro-

scopy (XAES), photovoltage (PV) spectroscopy, electron microscopy and photo-voltaic response (current–voltage and quantum efficiency (QE)) measurements wereapplied for the characterization of the layers and the corresponding solar cells.

2. Experimental

Highly structured TiO2 was deposited onto glass/SnO2:F by spray pyrolysis usinga solution of Ti-isopropoxide in isopropanol [18]. In(OH)xSy was prepared by CBDfrom a solution containing 0.025M InCl3, 0.1M thioacetamide (TA) and differentconcentrations of hydrochloric acid (HCl) at 70 1C for 25–35min following a similarprocedure as described elsewhere [15,16]. The concentration of HCl was varied from0.02 to 0.001M resulting in pH-values between 1.7 and 3.0. For solar cellpreparation, the In(OH)xSy deposition was performed up to three times in 0.005MHCl at 70 1C for 30min each time. Subsequently, the In(OH)xSy layers were annealedin Ar atmosphere at 300 1C for 30min prior to PbS deposition. PbS was prepared byCBD following the conditions described by Garcıa et al. [16] from a solution

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R. Bayon et al. / Solar Energy Materials & Solar Cells 89 (2005) 13–25 15

containing Pb(CH3OOH)2, NaOH, triethaolamine, thiourea and H2O at 40 1C for10min. Undiluted PEDOT:PSS (BAYTRON-P) was deposited on top of theIn(OH)xSy/PbS layers by spin coating. To avoid clustering, the PEDOT:PSS waskept under ultrasonic agitation and injected through a membrane onto the sample justbefore spin coating (final velocity of spinning 2000 rpm) was performed. The area ofthe solar cells was determined by the evaporated gold contacts (area 0.031 cm2).The morphology and distribution of elements were studied by scanning electron

microscopy (S4100 HITACHI) and energy dispersive X-ray spectroscopy (EDX).The PbS layer was characterized by XPS and XAES measurements using a Mg Ka

excitation source and a VG CLAM4 electron spectrometer. The spectrometer wascalibrated such that the Cu 3p line, the Au 4f7/2 line, the Cu L3MM line and the Cu2p3/2 line appeared at 75.13, 84.00, 334.95 and 932.67 eV [19], respectively. Chargereferencing was made to the adventitious C 1s line at 285.00 eV [20]. For thedetermination of the exact binding energy of the emission lines, the spectra werefitted with Voigt functions.The layers were further characterized by PV spectroscopy in the same arrangement

as described recently [18]. A halogen lamp (for characterization of PbS andIn(OH)xSy) or a Xe lamp (for characterization of TiO2) was used for excitation of thePV signal. The PV spectra were not corrected to the photon flux of the lamp. Theoptical band gap (Eg) and the energy value of the exponential states below the bandgap (Et) were determined by PV spectroscopy. Current–voltage measurements(Keithley 237) were performed on the solar cells at room temperature in the dark andunder illumination (halogen lamp, illumination intensity 100mW/cm2). The QE wasmeasured in a set-up with a halogen lamp, monochromator and a calibrated Siphotodiode.

3. Results and discussion

3.1. Band gap of the In(OH)xSy buffer layer

In order to gain information about the optical band gap energy of the In(OH)xSy

buffer, respective layers were deposited onto plane glass/SnO2:F substrates. By thevariation of the HCl concentration in the chemical bath (0.02–0.001M), whichresults in different pH-values (1.7–3.0), different In(OH)xSy layers were prepared.The optical band gap energy of these In(OH)xSy ‘buffers’ was obtained by measuringtheir PV spectra. Fig. 1 shows the dependence of the determined optical band gapenergies (Eg) of as-deposited CBD-In(OH)xSy thin films on the pH-value in thechemical bath. Reference values of Eg are also indicated for In2S3 (Eg ¼ 2:0 eV [21])and In2O3 (Eg ¼ 3:7 eV) [22,23]. Eg is about 2.4 eV for pHo2.6. For higherpH-values, the value of Eg increases up to about 3.4 eV (pH 3.0). The strongdependence of Eg on the pH-value of the solution is related to changes in thestoichiometry of In(OH)xSy. As reported in a previous work, a higher content ofsulphide is expected for lower pH-values while a higher content of hydroxide and/oroxide is expected for higher pH-values [15]. The dependence of Eg on the

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2.0

2.5

3.0

3.5

4.0

2.0 2.5 3.0

In2O3

In2

S3

In(OH)XS

Y

pH of the solution

Eg

(eV

)

Fig. 1. Dependence of the optical band gap as determined by PV spectroscopy of as-prepared In(OH)xSy

on the pH-value of the CBD. Reference values are also shown for In2S3 [21] and In2O3 [22].


stoichiometry of a similar compound InOxSy was studied by Barreau et al. [24] bycombining optical measurements and XPS. With respect to Barreau, the valence aswell as the conduction band shift downwards and upwards, respectively, due to thepresence even of small amounts of oxygen resulting in a widening of the optical bandgap. Similar investigations were carried out for CBD-In(OH)xSy thin films, where thevalence band shifts downwards by 0.9 eV while the conduction band shifts upwardsby 0.3 eV [25]. Interestingly, the argument that the increase of Eg might be induced,not only by the composition, but also by the quantum size effect due to In2S3 nano-particles in the deposited In2S3 layers [26] can be rejected for the CBD-In(OH)xSy

thin films since there is a strong dependence of Eg on the pH-value, but not on thedeposition time or on repetitive deposition procedures.For further studies and preparation of eta cells, CBD-In(OH)xSy buffer layers

were made in solutions containing 0.005M HCl (pH ¼ 2:3). Subsequently, theIn(OH)xSy buffer was annealed in Ar-atmosphere at 300 1C prior the absorberdeposition. Consequently, Eg decreased for these CBD-In(OH)xSy layers from 2.4 toabout 2.2 eV. This is an indication for the dehydration of the hydroxide content inthe buffer.

3.2. Chemical bonding in the PbS absorber layer

Information about the chemical bonding in the PbS prepared by CBD (CBD-PbS)was obtained by XPS and XAES. For this purpose, the CBD-PbS layer was

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deposited on a plane glass/SnO2:F substrate covered with CBD-In(OH)xSy. Fig. 2(a)shows the respective Pb NOO Auger signal. Its unusual shape (for comparison see[26]) points to a contribution of Auger electrons from at least two different Pbcompounds to this line. The Pb 4f7/2 emission in Fig. 2(b) confirms this suggestion. Aclear shoulder can be observed at higher binding energies (component [I]:138.370.1 eV) compared to the energetic position of the main contribution [II] at137.370.1 eV. Component [II] can be identified as PbS since the determined bindingenergy is in good agreement with the respective literature data (137.3–137.8 eV) [27].Contribution [I] to the Pb 4f7/2 XPS can be ascribed to PbO (137.4–139.3 eV) [27].This is also confirmed by the O 1s emission (Fig. 2(c)). The comparison withliterature data reveals that the main component [Ia] at 530.870.1 eV can also beattributed to PbO (527.5–531.6 eV) [28,29]. However, a small shoulder at higherbinding energies (component [Ib]: 532.870.1 eV) can be observed. In the literature,one can also find reports about this doublet O 1s structure of PbO with a separationof approximately 1.7–2.0 eV where the high-energy component is ascribed to oxygen-containing surface contaminants such as adsorbed water or OH� [20]. In order toestimate the composition of the CBD-PbS absorber, the respective areas of thecorresponding contributions to the Pb 4f7/2 XPS line are set in relation. Thus, theratio of the areas under the fitted curves, which correspond to the contributions [I]and [II] to the Pb 4f7/2 emission (see Fig. 2(b)) reveals a PbO/PbS-ratio of around1.0/3.8. Hence, the CBD-PbS is more a PbOZS1�Z with Z � 0:21: Nevertheless, wewill refer to this material still as PbS.On the first glance, no XPS or Auger signals related to the underlying glass/

SnO2:F/In(OH)xSy structure can be observed in the respective survey spectrum of thePbS absorber (not shown), i.e. the CBD-PbS covers it completely. However, aftervery detailed and prolonged measurements, a tiny signal appears for the most intenseXPS line of the In(OH)xSy buffer, the In 3d emission. But, this signal is shifted tolower binding energies compared to the In 3d line of the uncovered In(OH)xSy,which indicates that the chemical environment of these In 3d photoelectrons is

80 85 90 95 100(a)

Inte

nsity

less

Bac

kgro

und

(arb

.un.

) Mg Kα

Pb NOO

hydr

oxid

e

oxid

esu

lfide

Kinetic Energy (eV)140 139 138 137 136 135

(b)

II

I

Inte

nsity

(ar

b.un

.)

Binding Energy (eV)

Pb 4f7/2

Mg Kα

534 532 530 528 526

I a

I b

O 1s

Mg K

(c)

I a

I b

Binding Energy (eV)

Inte

nsity

(ar

b. u

n.)

Mg Kα

I oxide

II sulfide

Ia oxide

Ib hydroxide

Fig. 2. Pb NOO (a), Pb 4f7/2 (b) and O 1s (c) detail spectra of the PbS/In(OH)xSy/SnO2:F/glass test

structure. The reference positions [27] for the Pb N6O45O45 Auger line of Pb(OH)2, PbO and PbS are also

shown as well as the fits of the XPS spectra.

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different. In our opinion, this might be a hint for diffusion of In into the CBD-PbS.An additional explanation of the observed energy shift would be a band bendingcaused by the deposition of the p-conducting absorber on the n-conducting bufferlayer. The clarification of this subject remains for future experiments as, e.g., adetailed XPS and XAES characterization of the considered test structure with PbSlayers of different thickness.

3.3. Analysis by photovoltage spectroscopy

PV spectroscopy was performed in order to obtain information about both, Egand Et, the energy values of the exponential absorption tails. The onset of the PVsignal indicates the separation in space of photogenerated electron–hole pairs whenlight absorption sets on. Fig. 3 compares PV spectra of highly structured TiO2 afterdeposition of In(OH)xSy (a), PbS (b), and In(OH)xSy/PbS (c). In all cases, theIn(OH)xSy layers were annealed in Ar-atmosphere for 30min. In Fig. 3, both the in-phase (filled circles) and the phase-shifted by 901 (open circles) components withrespect to the chopped light are presented. A positive signal of the in-phasecomponent means that excess electrons are moving into the bulk of the thin film with

2 3

0

1

-1

0

0

1

(a)

(b)

(c)

41

Pho

tovo

ltage

(m

V)

Photon energy (eV)

halogen lamp

fmod = 8 HzTiO2 / In(OH)XSY

TiO2 / PbS

in phase

phase shifted by 90°

TiO2 / In(OH)XSY / PbS

Fig. 3. PV spectra of highly structured TiO2 after deposition of In(OH)xSy (a), PbS (b) and In(OH)xSy (c).

The spectra are shown for the components in phase (filled circles) and phase shifted by 901 (open circles) to

the chopped light intensity.

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respect to the holes. This case is shown by the In(OH)xSy layer deposited on TiO2, incontrast to the PbS (Fig. 3(b)), which displays the opposite situation. This result wasquite surprising since an injection of electrons into the TiO2 would be expected forboth materials [14] and hence a positive sign of the in-phase PV signal also for PbS.To our opinion, traps might play a dominating role for the formation of the PVsignal of the PbS layer deposited on TiO2. Nevertheless, detailed experiments will beneeded for getting better understanding of this phenomenon.For the case of PbS deposited on In(OH)xSy (Fig. 3(c)), the sign of the in-phase PV

signal is positive at photon energies below 2.4 eV. This means, in contrast to PbSdeposited on TiO2, that electrons are separated towards the In(OH)xSy deposited onTiO2. The sign changes to negative at higher photon energies at which the PV signalis rather low. Therefore, the charge separation changes in sign under the conditionsof weak and strong absorption. The situation is quite similar for the PV signalshifted in phase by 901. For this signal, the sign may also change depending on theabsorption. A detailed description of the responsible processes is impossible atpresent, since the involved traps for electrons and holes are not well known. FromFig. 3, the values of Eg can be estimated for both In(OH)xSy (a) and PbS (b). For thefirst one, a value of 2.2 eV is obtained and for the second one Eg amounts to about0.85 eV.In Fig. 4, the amplitude of the PV spectra (determined as the square root of the

sum of the squares of the in-phase and phase-shifted by 901 signals) recorded inFig. 3 are shown in a logarithmic scale. For comparison, the spectra of a highlystructured TiO2 substrate and of as-prepared In(OH)xSy are included. For the TiO2,the value of Et amounts to about 45meV, which points to the high quality of theanatase prepared by spray pyrolysis. Further, the annealing process of In(OH)xSy

leads not only to a shift of Eg to lower energies (around 2.2 eV instead of 2.4 eV) butalso to an increase of Et from 80meV for the as-prepared sample to 190meV for theannealed one. In addition, charge separation starts already at much lower photon

0. 01

0. 1

1

PbS

on

Photon energy (eV)1.0 1.5 2.0 2.5 3.0 3.50.5

PV

am

plitu

de (

mV

)

In(OH)xSy

TiO2

after anneling

In(OH)xSy

as preparedon TiO

2

Fig. 4. Spectra of the PV amplitude for as-prepared In(OH)xSy (open circles, pH ¼ 2.4), annealed

In(OH)xSy (filled circles), PbS prepared on TiO2 (open triangles), PbS prepared on In(OH)xSy (filled

triangles) and TiO2 (squares).

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energies for the annealed sample which indicates the generation of deep defect statesdue to annealing of In(OH)xSy. As remark, the value of Eg decreases even furtherafter prolonged annealing at 300 1C in high vacuum. This demonstrates that theannealing procedure has a great influence on the photoelectric properties of theIn(OH)xSy layer. For the case of PbS, the values of Eg ¼ 0:85 eV and of Et ¼ 40meVare identical for both the layers deposited onto TiO2 and onto In(OH)xSy. Therefore,the formation of a mixed homogeneous InPbx(OH)ySz phase due to an ion exchangemechanism seems unlikely for the given preparation procedure.The band gap of the CBD-PbS is widened in comparison to bulk PbS

(Eg ¼ 0:42 eVÞ [17]. We suppose that the widening of Eg results dominantly fromthe composition of the CBD-PbS, since the PbS is rather a PbOZS1�Z as revealed byXPS and XAES measurements (see discussion above). For the tetragonal andorthorhombic phases of PbO, the values of Eg are 1.9 and 2.8 eV, respectively [30,31].According to Wei and Zunger [32], it is possible to determine the optical band gapenergy of a ternary compound by a sort of a simple linear combination of the Eg’s ofthe single components. Assuming an optical bowing parameter equal to 1, thisresults for Z ¼ 0:21 in an optical band gap energy for our PbOZS1�Z absorber of0.69 eV if the oxidic phase in the absorber is tetragonal or of 0.88 eV and if the oxidicabsorber content is of orthorhombic structure. This is in good agreement with ourmeasurements. However, since the formation of tetragonal PbO is favoured at lowertemperatures [31], an additional influence of a quantum-size effect due to nano-particles resulting also in a widening of the optical band gap cannot be ruled outcompletely. In fact, optical band gap energies of up to 2.32 eV have been reported forCBD-grown PbS nanoparticle films [33].

3.4. Morphology and composition of solar cell structures

Fig. 5 shows a cross-section of a typical glass/SnO2:F/TiO2/In(OH)xSy/PbS/PEDOT:PSS/Au structure (a) and the respective distributions of the elements Au (b,back-contact), C (c, present in the organic hole conductor), Ti (d, in highlystructured TiO2), Pb (e, in the PbS layer) and In (f, in the In(OH)xSy layer) measuredby EDX. The SEM picture shows that the TiO2 prepared by spray pyrolysis is highlystructured although the relative increase of the surface area is much less than for thenanoporous TiO2, which is used for dye-sensitized solar cells. Nevertheless, thestructures and pores of the TiO2 being in the order of microns have advantage forgood light scattering. It has to be remarked that pores close to the SnO2:F front-contact are nearly empty, i.e. neither In(OH)xSy, PbS nor PEDOT:PSS havecompletely penetrated inside the pores.From EDX mapping, it can be seen that the distribution of the evaporated Au is

not very homogeneous at the surface, probably due to the cracking during the cross-section preparation. The PEDOT:PSS did not penetrate far into the pores whichshows that the deposition by spin coating has to be further optimized. Ti isdistributed relatively homogenously over the cross-section of the sample. In and Pbare also detected practically over the whole cross-section while their concentration ismuch higher at the surface region of the TiO2. This shows, despite the open porous

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Fig. 5. Cross-section of a typical glass/SnO2:F/TiO2/In(OH)xSy/PbS/PEDOT:PSS/Au structure (a) and

the respective distributions of the elements Au, C, Ti, Pb and In (b–f, respectively).


structure of the TiO2 prepared by spray-pyrolysis that precipitation of In(OH)xSy

and PbS is decreased at pore-walls close to the SnO2:F front contact. It seems thatprecipitation starting at the surface region hinders the penetration of In and Pbdeeper into the pores.

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3.5. Solar cell performance

The performance of solar cells (area 3mm2) based on glass/SnO2:F/TiO2/In(OH)xSy/PbS/PEDOT:PSS/Au structures was well reproducible. Fig. 6 showscurrent–voltage characteristics of a solar cell in the dark and under illumination. Itcan be seen that the rectifying behaviour of the cell is good. Under illumination, theopen-circuit voltage and the short-circuit current density were 0.28V and 7.4mA/cm2, respectively. The efficiency of the given cell is 0.83% and the fill factor amountsto 0.4, which is relatively good in comparison to previously reported results [34]. Thehigh series resistance seems to be the main problem for getting high efficiencies at thepresent stage. In our opinion, there is a big potential for increasing the solar cellperformance if the structural properties of the cell are improved.Fig. 7 shows the spectral dependence of the external QE for the sample shown in

Fig. 5. QE was measured between 1.37 and 3.55 eV. The value of QE increases from5.5% at 1.37 eV to 13% at 1.9 eV. The value of QE reaches the maximum of about16% at the photon energy of about 2.3 eV. In the region between 2.0 and 2.8 eV, thevalue of QE is above 14%. It is rather interesting that the external QE reaches thehighest values in the region where the PV signal is not so high and even changes sign(see Fig. 3(c)). This shows that the injection of both electrons and holes into TiO2and PEDOT:PSS is important and that the PEDOT:PSS layer has a strong influenceon the behaviour of the charge separation and injection processes in the solar cell.The efficiency of solar cells based on glass/SnO2:F/TiO2/In(OH)xSy/PbS/

PEDOT:PSS/Au structures is plotted as a function of the open-circuit voltage andshort-circuit current density in Fig. 8. The efficiencies of the analysed cells rangedbetween 0.2% and 0.8% and did not depend on the open-circuit voltage (values

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-10

-5

0

Cur

rent

den

sity

(m

A/c

m2 )

Voltage (V)

VOC = 0.28 V

ISC = 7.4 mA/cm2

FF = 0.4

η = 0.83 %

Fig. 6. Current–voltage characteristics in the dark and under illumination for a TiO2/In(OH)xSy/PbS/

PEDOT:PSS/Au solar cell.

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1 30

10

20VOC = 0.28 V

ISC = 7.4 mA/cm2

FF = 0.4

η = 0.83 %E

xter

nal q

uant

um e

ffici

ency

(%

)

Photon energy (eV)2

Fig. 7. Spectral dependence of the external QE for the cell shown in Fig. 6.

0.0 0.3 0.60.0

0.5

1.0

0 6 9

VOC (V) ISC (mA/cm2)

Effi

cien

cy (

%)

3

Fig. 8. Dependence of the energy conversion efficiency on the open-circuit voltage and on the short-circuit

current density.


around 0.3V). A linear correlation was found for the dependence of the efficiency onthe short-circuit current density.

4. Conclusions

An organic–inorganic hybrid solar cell based on TiO2/In(OH)xSy/PbS/PED-OT:PSS has been developed on highly structured TiO2. Both In(OH)xSy buffer andPbS absorber layer are important for this kind of eta cell. The main limitations of

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these devices are not clear yet, but are reflected mainly in the low VOC obtained inthis work. For improving the performance of the given eta cell, the highly structuredTiO2 has to be optimized in such a way that the material would become denser in thecontact region with the SnO2:F and more porous in the surface region. On the otherhand, CBD-In(OH)xSy does not form a compact layer [35]. This probably enhancesthe precipitation of PbS but hinders (i) a good transport of photogenerated chargecarriers to the injecting contact and (ii) a good penetration of PEDOT:PSS into thepores. Therefore, techniques like ILGAR [36], which lead to a better conformaldeposition on the highly structured TiO2, might be more appropriate. In addition, abetter penetration of the spin-coated PEDOT:PSS into the pores has to be achievedin general. It may be that the alternative techniques such as dilution or in situpolymerization will help to solve this problem. Specific questions such as theinfluence of the layer structure and interfaces on the charge separation remainunclear and should be considered in future investigations. The further optimizationof the whole cell assembly in terms of deposition techniques of all the componentsand the significant increase of the conversion efficiency seem promising with respectto low thermal budget and low cost processing.

Acknowledgements

The authors are grateful to the European Comission (HPRN-CT-2000-00141,A. B.), to the German BMBF (01SF0007) and BMWA (0329889) and to theDeutscher Akademischer Austauschdienst (DAAD, R. M.) for financial support andto H.-J. Muffler, I. Sieber for discussions and K. Fostiropoulos for some electronmicroscopy micrographs.

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