237

ISSN 1995-0780, Nanotechnologies in Russia, 2009, Vol. 4, Nos. 3–4, pp. 237–243. © Pleiades Publishing, Ltd., 2009.Original Russian Text © S.A. Gavrilov, A.A. Dronov, V.I. Shevyakov, A.N. Belov, E.A. Poltoratskii, 2009, published in Rossiiskie nanotekhnologii, 2009, Vol. 4, Nos. 3–4.

INTRODUCTION

Energy storage, production, and conversion aresome of the main directions in the development of sci-ence and technology; in these fields, considerable eco-nomic growth is expected thanks to the introduction ofnanotechnologies. This is realistic as long as conditionsof mass production at low prime cost of production andextended availability of the developed technologies areprovided. The creation of photoelectric convertersbased on nanocrystalline titanium oxide is a strikingexample of incorporating nanotechnological designs inmass production [1]. Thanks to the low prime cost oftheir production and their unique mass–dimensionalcharacteristics, such solar cells built on flexible sub-strates have become the foundation for creating porta-ble charging devices.

The increase in the cost of conventional energysources and the outlined deficit of silicon wafers, whichare conventionally the basis for solar cells, are a prob-lem for the developers of photoelectric converters try-ing to produce technologies that can be alternatives tosilicon technology. The record efficiencies of solar cellsbased on III–V heterostructures do not allow for theirmass production because of high production expensesand the necessity of large initial investments needed toform the production infrastructure. In both cases thereason for the high prime cost of electrical energy,which is produced by the direct transformation of theoptical energy to electric, is the high technologicalexpense for materials of single-crystal substrates [2].One way to solve this problem is by developing thin-film elements on lower-cost carriers such as metal andpolymer ribbons [3]. Among the thin-film elements, the

structures designed based on nanocrystalline oxidesoccupy a special place. In them, the effective spatialseparation of charge carriers is realized in the nanome-ter-scale absorption layers, while the high values of thephotocurrent are achieved due to the developed surface.This concept is shown in industrial samples by theexample of the Grätzel batteries [4]. However, the pres-ence of liquid between the electrodes considerablydecreases the range of the working temperatures ofdevices. Replacing liquid with gel electrolytes [5] stilldid not allow one to attain profitable levels of effi-ciency, which should exceed the current prime cost ofbuilding by 5–6%.

This work is devoted to developing a design for a solarcell with an extremely thin absorption layer (SCETAL) inwhich the electrolyte is absent and all functional layersare solid. The concept of solid-state SCETALs was sug-gested in [6] and was called an eta cell (extremely thinabsorber cell). In these structures materials character-ized by a high defectiveness are used, which consider-ably increases the recombination rate in the carrierbulk. The efficiency of eta-cells increases as the thick-ness of the layer that absorbs the light decreases to sizescomparable with the diffusion length of charge carriers;i.e., for highly defective films, they decrease to severaltens of nanometers. However, this leads to a decrease inthe number of absorbed photons per time unit in theactive layer. This problem is solved thanks to the use oftransparent conducting porous matrices based on tita-nium dioxide or zinc oxide nanocrystals with electron-type conduction. These matrices increase the effectivearea of heterojunctions by two orders of magnitude andsubstantially decrease the optical losses thanks to a

Ways to Increase the Efficiency of Solar Cells with Extremely Thin Absorption Layers

S. A. Gavrilov

a

, A. A. Dronov

a

, V. I. Shevyakov

a

, A. N. Belov

a

, and E. A. Poltoratskii

b

a

Moscow Institute of Electronic Engineering (Technological University), proezd 4806 5, Zelenograd, Moscow, 124498 Russia

b

Lukin Scientific Research Institute of Physical Problems, Zelenograd, Moscow, Russiae-mail: pcfme@miee.ru

Abstract

—The effect of design parameters, electrical properties, and technological modes of formation on theefficiency of the photoelectrical conversion of solar cells with extremely thin absorbing layers based on theSnO

2

:F/TiO

2

/In

2

S

3

/In

x

Pb

1 –

x

S/CuSCN is investigated. It is shown that both a decrease in resistance due to anincrease in roughness and a decrease in the average grain size of the TiO

2

films is attained with deposition usingthe sol–gel method under conditions of increased humidity. The use of Ni as the contact metal to a planarizingCuSCN layer provides the reduced value of the transient resistance. The optimization of the sequential resis-tance of the TiO

2

film and contact resistance to the CuSCN layer provided an increase in the efficiency of pho-toelectric converters by a factor of more than four. The structures of solar cells formed in optimal technologicalmodes showed the following characteristics:

J

sc

= 9 mA/cm

2

,

U

oc

= 720 mV, and they had an efficiency of 2.9%.

DOI:

10.1134/S1995078009030112

EXPERIMENT

238

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GAVRILOV

et al.

decrease in the reflecting ability and the multiplerefraction in the porous structure.

The eta-cell concept was developed by several Euro-pean research groups in France and Germany [7–9].The authors of these studies used TiO

2

deposited by thesol–gel method or electrochemically deposited ZnO fil-amentous nanocrystals as the

n

-type transparent elec-trode, which is called an electron acceptor. To form theabsorbing layer, electrochemical or chemical absorp-tion from aqueous solutions was used. The maximumefficiency attained in these works was no higher than1.5%. The lower-than-excepted efficiency was a conse-quence of the fact that the chemical and electrochemi-cal deposition methods did not provide a conform coat-ing of the nanostructured carrier by the material of theabsorbing layer.

To solve this problem, we used the method ofmolecular layering (ML), the principles of which weredeveloped in the mid-1960s under the direction ofAleskovskii [10]. The main idea of the ML method con-sists of providing conditions for irreversible chemicalreactions between the functional groups on the surfaceof solid and externally supplied reagent molecules.Growing the layers of the specified thickness is done bythe multiple alternate treatment of the surface of thedeposited material, which provides the formation offunctional groups of active atoms on the surface whichare able to react with new portions of reagents. There isa method of achieving chemical assembly from solu-tions called the method of ion layering (IL) [11, 12]. InEnglish-speaking literature, this method is called suc-cessive ion layer adsorption and reaction (SILAR). Theformation of semiconductor compounds by the methodof molecular layering is a cyclic process which involvesfour stages, namely, the adsorption of cations from thecation precursor solution; the removal of physicallyadsorbed complexes by washing in water; the chemi-sorption of anions from the anionic precursor solution,which is accompanied by the reaction of the formationof the chemical compound; and the removal of the reac-tion products by washing in water. The thickness of the

film formed during such deposition is controlled by theamount of layering cycles.

The eta cell involves a transparent substrate with aconductive coating, a nanostructured electron acceptor,an ultrathin absorbing layer, and a hole acceptor. Theselection of materials for such a structure is performedbased on the band energy diagram of the multilayeredheterostructure, which contains a built-in electric fieldproviding the separation of photogenerated electronsand holes. Our previous investigations showed that inthe SnO

2

:F/TiO

2

/In

2

S

3

/In

x

Pb

1 –

x

S/CuSCN system, inwhich the electron acceptor is TiO

2

, the role of theabsorbing layer is played by the In

2

S

3

/In

x

Pb

1 –

x

S het-erostructure; the hole acceptor is CuSCN, providing thehighly efficient separation of charge carriers [13, 14].

In this work we present the results of investigationswhich substantiate the necessity of further optimizingthe constructive parameters, the electrical properties,and the technological modes of formation of functionalSCETAL (eta cell) layers based on theSnO

2

:F/TiO

2

/In

2

S

3

/In

x

Pb

1 –

x

S/CuSCN heterostructure.

EXPERIMENTAL

Figure 1 represents the structure of the transversesection of eta cells studied in the present work.

We used energy-saving architecture glass (layer 1)with a film of conducting SnO

2

:F deposited (layer 2) onthe surface as substrates for the formation of eta cells.Prior to the deposition of the first TiO

2

layer, the sub-strates were subjected to special liquid–chemical treat-ment, which provided the removal of organic and inor-ganic pollutions from the surface.

The deposition of TiO

2

(layer 3) was performed bythe method of controllable immersion of the verticallyarranged substrate in a 10% (vol. fraction) solution oftitanium(IV) isopropoxide Ti(OC

3

H

7

)

4

in isopropanolCH

3

CH(OH)CH

3

. Vibrationless immersion with a rateof 120 mm/min was performed using a KSV LMX2automated complex (KSV Instruments Ltd., Finland)for the layer-by-layer deposition of the films by the

1

2345

6

7

hv

Fig. 1.

Transverse section of the thin-film structure of the photoelectric converter based on the SnO

2

:F/FiO

2

/In

2

S

3

/In

x

Pb

1 –

x

S/CuSCNstructure.

NANOTECHNOLOGIES IN RUSSIA

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WAYS TO INCREASE THE EFFICIENCY OF SOLAR CELLS 239

immersion method. Films with different microstruc-tures were obtained by varying the relative humidity ofthe surrounding atmosphere in the deposition chamber.

Dust particles can ingress on the deposited surfaceduring the deposition. Therefore, deposition was per-formed in two stages with obligatory intermediateannealing in air at 450

°

C for 20 min. The intermediateannealing provided the decomposition of the dust mate-rial under the effect of high temperature. The repeateddeposition provided the healing of transparent poresformed in the TiO

2

film in the course of the precedingannealing. In the course of the subsequent annealing at450

°

C for 1 h, oxide with an anatase crystal structurewas formed. Of all the polymorphs of TiO

2

, it has thehighest electron diffusivity [4].

The In

2

S

3

buffer layer (layer 4), which provides lowrecombination losses, was deposited by the IL methodfrom solutions of 10 mM InCl

3

and 10 mM Na

2

S (20 ILcycles). The absorbing In

x

Pb

1 –

x

S layer (layer 5) wasobtained similarly from the solution of the cation pre-cursor containing 9 mM InCl

3

and 1 mMPb(CH

3

COO)

2

(30 IL cycles). Then the annealing wasperformed at 150

°

C for 1800 s in air. The

p

-type contact(acceptor of holes) was formed on a specially devel-oped installation that fills in the space between thenanocrystals with the degenerate CuSCN semiconduc-tor (layer 6). For this purpose, the substrate was placedon a mobile stage heated to 80

°

C and 0.07 M of CuSCNsolution in propylsulfide was supplied using a specialdosing unit. At the final stage, metal contacts to theSnO

2

:F and CuSCN layer were formed by thermalevaporation in a vacuum (layer 7).

To reveal the correlation of conductivity of the basicTiO

2

layer with the technological modes of its deposi-tion, local metal contacts were formed on the oxide sur-face and the reduced electrical resistance (

Ω

/cm

2

)between the local metal contact and SnO

2

:F film wasdetermined.

Using atomic force microscopy (an SZM-Solver 4Rmicroscope was used), the surface morphology of theTiO

2

films was investigated in a semicontact mode andthe average surface roughness was determined on area1

×

1

µ

m in size.

To investigate the

I–V

characteristics, the variableload resistance rated in a range of 0–1 M

Ω

was con-nected immediately to the contacts of photoelectricconverters. During the measurements, the samples wereilluminated from the substrate side by an optical sourcewith an emission power of 100 mW/cm

2

.Based on the measured

I–V

characteristics, wedetermined the open-circuit voltage

U

oc

, the density ofthe short-circuit current

J

sc

, and the maximum value ofthe specific output power

P

m

as the multiplication of thecurrent by the voltage. The established

P

m

determinesthe efficiency

η

of the solar cell according to the knownrelation

where

P

s

is the specific power of the incident solar radi-ation equal to approximately 100 mW/cm

2

.In determining the characteristics of solar cells, the

filling factor FF of the current–voltage characteristic isimportant. It is determined by the following formula:

The values of

U

oc

,

J

sc

, and FF allow one to com-pletely characterize the parameters of solar cells.

To determine the parameters of ohmic contacts ofmetal to CuSCN, we used the method of the Kelvinresistor [15]. This method allows one to determine thetransient resistance of the contact with one measure-ment; it is independent of the geometric sizes of othercontacts, other than the measuring one; and it providesincreased accuracy in determining the specific transientresistance of contacts.

RESULTS AND DISCUSSION

Based on the known data on the electronic proper-ties of the materials used in this work (Table 1), we con-structed a schematic of the mutual arrangement of theedges of the bands of the allowed states and the Fermilevels (Fig. 2). In constructing the diagram, we used the

ηPm

Ps

------,=

FFPm

JscUoc---------------.=

Table 1.

Parameters of the electron structure of materials entering the composition of the eta cells under study

Semiconductor Conduction type Electron affinity, eV Work function, eV Band gap, eV Reference

SnO

2

: F

n

+

4.7 4.7 3.9 [15]

TiO

2

n

4.3 5.2 3.2 [4]

InO

x

S

1 − x n [15]

x = 0 4.65 4.7 2.1

x = 0.07 4.25 4.7 2.5

x = 0.14 3.85 4.7 2.9

InOxS1 − x:Pb p 3.3 4.7 1.8 [16]

CuSCN p+ 1.9 5.3 3.4 [6]

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GAVRILOV et al.

data on the location of energy levels of InOxS1 – x at x =0.07 based on the data on the oxygen content in thefilms obtained in [13]. The presented band diagramindicates that the electrons and holes, which are gener-ated during the irradiation, will move with a high prob-ability preferentially towards SnO2:F and CuSCN,respectively.

It is known that the efficiency of actual solar cellsdepends on the recombination at the boundaries ofjunctions and the sequential and shunt resistances of theelement [16, 17]. The current–voltage characteristic ofthe solar cell is described by the equation

(1)

where J0 is the saturation current, Rs is the series resis-tance, and Rp is the shunt resistance.

It follows from Eq. (1) that the efficiency decreaseswith an increase in recombination losses (which can bejudged by the value of J0). Up to now, no exact data areavailable on the distribution of the electric field at theboundaries of heterojunctions under consideration.Therefore, from this viewpoint, it is rather difficult tojudge the main channels of recombination losses in thestructure. However, we can evaluate the power losses inthe eta cell based on evaluations of the magnitude of theseries resistance of the layers.

According to Eq. (1), Rs most strongly affects theshape of the I–V characteristic at large currents andhigh bias voltages, while the effect of Rp manifestsitself at low voltages, where the current flowing acrossthe junction is small compared with the shunt current.Power losses exist in any actual solar cell, because Rs > 0and Rp < ∞. To provide little power loss, it is necessaryto decrease Rs and increase Rp. In this case, series resis-tance plays the decisive role in the operation of the solarcell. The value of Rs for the eta cell is formed by theresistances of the layers entering the composition of the

J J0q U JRs–( )

kT---------------------------exp 1–

U JRs–Rp

------------------- JL,–+=

solar cell and contact resistances at the boundaries ofheterojunctions.

Taking into account that SnO2:F and CuSCN are thedegenerate semiconductors and the thicknesses of thelayers of In2S3 and InxPb1 – xS are smaller than the dif-fusion length of charge carriers [13], we can expect thatTiO2 mostly contributes to the series resistance.According to existing notions, the charge transport inTiO2 is performed via diffusion [18, 19]. The electrontransport is performed through the defect states, whichare conditioned by the presence of atomic vacancies ofoxygen in the near-surface layers [20, 21]. Conse-quently, nanocrystalline films with large specific sur-face areas possess the best conductivity [20].

The way TiO2 with the sol–gel deposition is formedis described by the following sequential reactions

≡Ti–OR + H2O ⇔ ≡Ti–OH + ROH,

≡Ti–OR + Ti–OH ⇔ ≡Ti–O–Ti≡ + ROH,

≡Ti–OH + ≡Ti–OH ⇔ ≡Ti–O–Ti≡ + H2O,

where R are the C3H7 groups with the use of titaniumtetraisopropoxyde as the precursor.

From the general theory of kinetics of chemicalreactions, the rate of such process increases as the con-centration of titanium tetraisopropoxide and watervapor pressure in the surrounding medium increaseduring the synthesis of the films by the method of pull-ing the substrate from the melt. The hydrolysis ratedetermines the microstructure of deposited films. Toobtain highly dispersed systems, it is necessary toachieve high supersaturation with a limited growth rateof forming particles [22].

In the course of pulling the substrate from the solu-tion, a meniscus is formed in the region of the solution–air interface. The thickness of the meniscus and, conse-quently, the amount of titanium isopropoxyde on thesurface unit are determined by the kinetics of wettingand decrease as the pulling rate decreases. The concen-tration supersaturation in such a process is provideddue to the evaporation of the solvent (isopropanol) andadsorption of water vapors. It is evident that high super-saturation by water molecules can be provided thanksto an increase in the humidity of the atmosphere in thedeposition installation. The decrease in the growth rateof the already-formed nuclei is achieved due to thedepletion of the liquid layer during precipitation.

Table 2 represents the values of structural parame-ters and the reduced electrical resistance of the TiO2layer obtained at different humidity values of the atmo-sphere under conditions of an identical deposition rateof 120 mm/min. The values of efficiency of photoelec-tric converters created with the use of the correspond-ing TiO2 layers are also presented here. We used Ni asthe contact metal to CuSCN.

Using the AFM (Fig. 3) it was established that, ashumidity increases, the roughness of the films increases

–1

–8

–2

–3

–4

–5

–6

–7

E, eV

SnO2:F

3.9 eV

TiO2

3.2 eV

InO0.07S0.93

2.5 eV

In9PbOxSy

1.8 eV

CuSCN

3.4 eV

Fig. 2. Energy characteristics of the layers entering thecomposition of the SnO2:F/TiO2/In2S3/InxPb1 – xS/CuSCNstructure.

NANOTECHNOLOGIES IN RUSSIA Vol. 4 Nos. 3–4 2009

WAYS TO INCREASE THE EFFICIENCY OF SOLAR CELLS 241

and the average size of the crystallites constituting thefilm decreases.

As expected, the resistance of the TiO2 filmsdecreases as the humidity increases. The decrease in theresistance of the TiO2 films increases the efficiency ofthe eta cell.

As noted above, the contribution to the series resis-tance of actual solar cells is also introduced by theohmic contacts to the planarizing layer of the holeacceptor. This quantity also affects the output power ofthe solar cells and, finally, the efficiency value.

In the literature there are almost no data on selectingmaterials for building ohmic contacts to the planarizinglayer of the hole acceptor based on CuSCN and on thevalues of the transient resistance of ohmic contacts ofvarious metals to it. The qualitative characteristics ofthe ohmic contacts are the transient resistance Rc andspecific transient resistance ρc.

We built test structures of solar cells in which In, Cu,Au, and Ni were used as contact materials to the pla-narizing CuSCN layer. All samples were formed basedon titanium oxide deposited at the highest atmospherichumidity. Table 3 represents the measured average val-ues of resistivity of ohmic contacts, open-circuit volt-ages, densities of the short-circuit current, filling factor,and efficiency of the corresponding eta cells.

Table 2. Values of the reduced resistance of the TiO2 layer

Formation mode of TiO2 1 2 3

Relative humidity in a chamber, % 30 60 80

Roughness, nm 10 25 110

Average grain size, nm 250 150 80

Reduced resistance of the TiO2 layer, Ω/cm2

12–13 4–6 2.3–2.4

η, % 0.95 1.2 2.9

500 nm

500 nm

100

80

60

40

20

0 1 2 3 µm

nm

12

10

8

6

4

2

0 1 2 3 µm

nm

(a)

(b)

Fig. 3. Two-dimensional AFM images of the surface and transverse section of the TiO2 layer deposited at various humidity valuesof the surrounding medium: (a) 30%, (b) 80%.

242

NANOTECHNOLOGIES IN RUSSIA Vol. 4 Nos. 3–4 2009

GAVRILOV et al.

It follows from the obtained data that the value ofthe transient resistance of ohmic contacts to theCuSCN-based planarizing layer of hole acceptordecreases as the work function of the used metalsincreases. This can be explained by the fact that CuSCNis a degenerate ion p-type semiconductor, and the clas-sic Schottky formation theory of the potential barrier inthe metal–semiconductor compound is valid for it. Theheight of the energy barrier qϕb is determined by thefollowing expression in this case [23]:

(2)

where ϕm is the work function of the metal, and Eg andχ are the band gap and electron affinity of the semicon-ductor.

It is known that when metal contacts the semicon-ductor, the transient resistance increases exponentiallyas the values of qϕb increase [23]. According to thedata presented in Table 1, the band gap of CuSCNequals 3.4 eV and the electron affinity is 1.9 eV. Thevalues of the work function for In, Cu, Au, and Ni are4.2, 4.3, 5.2, and 5.3 eV, respectively; according toexpression (2), the potential barrier height for them is1.1, 1.0, 0.1, and 0 eV, respectively. The decrease incontact resistance alongside the increasing potentialbarrier height between the metal and CuSCN explainsthe increase in the filling factor of the I–V characteristicof the eta cell.

Along with this, a simultaneous increase in the val-ues of Uoc and Jsc is observed. These quantities are asso-ciated with each other by the following equation at JL J0 [18]:

(3)

Because all the studied eta cell structures were iden-tically built, except for the selection of the material forthe ohmic contact, the photocurrent density for themshould remain almost invariable. Consequently, theincrease in Uoc and Jsc is associated with a decrease in J0.This decrease is easily explained by the considerationof the generation–recombination processes in thedepleted CuSCN region. According to the Shockleytheory, at first approximation, J0 ~ W, where W is the widthof the space-charge region [23]. In turn, W ~ (qϕb)1/2. Con-sequently, the width of the space-charge region and

qϕb Eg q ϕm χ–( ),–=

UockTq

------JL

J0-----⎝ ⎠

⎛ ⎞ 1+lnkTq

------Jsc

J0------⎝ ⎠

⎛ ⎞ .ln≈=

recombination losses decrease when there is a decrease inthe barrier height at the metal–semiconductor interface.

A simultaneous increase in the values of FF, Uoc,and Jsc leads to a substantial increase in efficiency η,which is determined by the expression

CONCLUSIONS

The performed investigations showed that the func-tioning of eta cells follows the main regularities estab-lished for conventional photoelectric converters.

The performed optimization of only two parame-ters, like the series resistance of the TiO2 film and thecontact resistance to the CuSCN layer, providedincreasing efficiency by a factor of more than four. Theplanned directions of the further developments, specif-ically, an increase in the region of the surface of hetero-junctions due to the use of methods of directed controlover the morphology of TiO2 and the optimization ofthe electronic and optical properties of chalcogenidesemiconductors, should allow it to attain a level of η >5%, which, according to experts, is the profitabilitythreshold for this type of solar cell.

ACKNOWLEDGMENTS

This work was performed in the context of the Fed-eral Target Program “Investigating and Developing Pri-ority Directions of the Development of Russian Scien-tific and Technological Complex for 2007–2012,”SC no. 02.513.11.3151.

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η FFUoc Jsc

Ps------------------.=

Table 3. Results of the measurements of the test-structure characteristics of solar cells

Material of the ohmic contact with the pla-

narizing layer

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