Solar Energy Materials & Solar Cells 94 (2010) 1753–1758

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

Experimental and simulation analysis of the dye sensitizedsolar cell/Cu(In,Ga)Se2 solar cell tandem structure

W.L. Wang a,b, H. Lin a,n, J. Zhang a, X. Li a, A. Yamada b,c, M. Konagai b,c, J.B. Li a

a State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of Chinab Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1 O-Okayama, Meguro-Ku, Tokyo 152-8552, Japanc Photovoltaics Research Center, Tokyo Institute of Technology, 2-12-1 O-Okayama, Meguro-Ku, Tokyo 152-8552, Japan

a r t i c l e i n f o

Article history:

Received 19 July 2009

Received in revised form

16 May 2010

Accepted 18 May 2010Available online 9 June 2010

Keywords:

Dye sensitized solar cell

CIGS solar cell

Tandem cell

Transmittance

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.05.041

esponding author. Tel./fax: +86 10 627 7267

ail address: hong-lin@tsinghua.edu.cn (H. Lin

a b s t r a c t

A novel tandem structure composed of dye sensitized solar cell (DSC) and CuInGaSe2 (CIGS) solar cell

shows the potential to increase conversion efficiency by fully utilizing the sunlight. In our research, a

tandem solar cell with a high open-circuit voltage of 1.15 V and a conversion efficiency of 10.46%, which

is both larger than that of DSC and CIGS solar cell has been prepared. To further increase the tandem

efficiency, equivalent circuit analysis is carried out to find out the corresponding factors. It is also found

that there is limitation for the tandem structure to increase the efficiency. Transmittance and

performance of DSC top cell are found to be the essential factors which determine the tandem effect.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Tandem solar cell structure has attracted the attention ofresearchers around the world as it provides another path towardshigher efficiencies. Theoretically multi-junction tandem solar cellcan fully take advantage of the sunlight so that the efficiency can beremarkably increased. A world record efficiency of 32% has beenachieved by the GaInP/GaAs/Ge tandem structure [1]. However, asthe number of junction increases, the optical loss and match for eachjunction part become complicated problems to solve. Thus, as thebasis of multi-junction tandem cell, two-junction tandem solar cellsare widely being studied nowadays. It has been reported that theoptimum band gaps for series-connected two-junction tandem cellwere 1.72 eV for the top cell and 1.14 eV for the bottom cell [2].Considering this, two candidates, dye sensitized solar cell (DSC) andCuInGaSe2 (CIGS) solar cell, appear to have the potential to achievehigher efficiency when stacked together.

Dye sensitized solar cell is a novel solar cell of low cost and easyto produce. The highest efficiency is 11.3% [3]. DSC uses the dye witha high band gap of 1.7 eV (N719) to absorb the visible light.Cu(In,Ga)Se2 solar cell is one kind of thin film solar cell, which isstable and of high conversion efficiency and nowadays the efficiencyreaches 19.9% [4]. However, although the band gap of CIGS can beadjusted from 1.04–1.68 eV, the highest efficiency is obtained at aband gap of 1.15 eV. Considering the characteristics of these twosolar cells, the enhancement of the conversion efficiency from the

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2.

).

DSC/CIGS tandem structure can be expected. However, both DSC andCIGS are widely studied in their own types of tandem structures.A dye sensitized tandem cell using dye sensitized photocathode wasreported [5], which provides an idea of optimizing the DSC structure.Murayama and Mori [6] have parallelly connected two DSCs withdyes that absorb light of different wavelengths. Compared with10.8 mA/cm2 of a single cell, the short-circuit current was improvedto 13.3 mA/cm2 by the tandem structure. The expansion of spectralresponse for such a tandem structure was also studied by Kubo et al.[7]. As to CIGS, the band gap of CuGaSe2 (1.68 eV) ideally satisfiesthe requirements for the top cell. Series-connected CuGaSe2/CuInGaSe2 tandem cell was reported to achieve a high open-circuitvoltage of 1.18 V with the conversion efficiency of 7.4% [8]. Thisrecord for open-circuit voltage was refreshed to 1.5 V usingAgInGaSe2/CuInGaSe2 tandem cell by Nakada et al. [9].

The tandem cell of DSC and CIGS was first realized by Gratzel withconversion efficiency greater than 15% [10]. However, to what extentthis tandem structure can improve the conversion efficiency and thekey factors to improve the tandem structure have not yet beenstudied. In our research, the mechanically stacked two-terminaltandem cell has been prepared. To further find the limitation for thetandem structure and to find the factors to improve the tandemeffect, equivalent circuit analysis was carried out.

2. Experimental

DSC was made using a screen printing method [11] based onthe techniques founded by Graztel. TiO2 nanocrystalline particles

Fig. 1. Schematic representation of the DSC/CIGS structure; DSC and CIGS are

series connected.

Table 1Solar cell performances of tandem, top and bottom cells.

Cell Voc (V) Jsc (mA/cm2) FF (%) Z (%)

Tandem 1.147 13.58 67 10.46

Top (DSC-I) 0.679 13.40 76 6.99

Bottom (CIGS-I) 0.477 13.57 57 3.71

Bottom (without top) 0.544 29.94 63 10.27

Table 2Comparison between original CIGS-II cell and tandem cell.

Cell Voc (V) Jsc (mA/cm2) FF (%) Z (%)

CIGS-II 0.610 36.95 74 16.76

Bottom (CIGS-II) 0.548 14.87 71 5.78

Tandem 1.240 14.21 73 12.45

W. Wang et al. / Solar Energy Materials & Solar Cells 94 (2010) 1753–17581754

with the particle size of 20 nm were prepared using hydrothermalmethod. The TiO2 paste was then screen printed onto the FTOglass (10O/&, Nippon Sheet Glass, Japan). After TiCl4 treatmentand post annealing, a 5 mm TiO2 semi-transparent thin film layerwas obtained. The TiO2 film was then soaked in 0.5 mMsynthesized N-719 dye (Bu4N)2 [Ru(dcbpyH)2(NCS)2] solutionfor 24 h [12]. After being washed in ethanol and dried, asemi-transparent photoanode was obtained. Counter electrodewas synthesized using thermal decomposition of H2PtCl6 solutionyielding a semi-transparent platinum layer with thickness of3 nm. The electrolyte was composed of 0.6 M BMI I (methylben-zimidazolium iodide), 0.03 M I2, 0.10 M guanidinium thiocyanateand 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile andvaleronitrile [11].

The CIGS solar cell was fabricated by depositing ZnO:B/i-ZnO/CdS/CIGS layers on Mo coated soda lime glass substrate [13]. TheCIGS absorption layer was deposited using 3-stage process byMolecular Beam Epitaxy (Eiko Co. Ltd., Japan). The fluxes of Cu, In,Ga and Se elements were precisely controlled in order to controlthe composition of the CIGS layer. After depositing the CdS bufferlayer by chemical bath and ZnO:B/i-ZnO window layer by MetalOrganic Chemical Vapor Deposition (G30001, Yoshida Kogyo KK,Japan), the CIGS solar cell was completed.

The DSC/CIGS tandem structure was realized by overlaying theDSC on the CIGS solar cell using two-terminal connection asshown in Fig. 1.

The photovoltaic performances of the solar cells weremeasured by an Impedance Analyzer (4294A, Agilent, USA) underan AM1.5 solar condition using a xenon lamp (UXL-S310MY,Ushio, Japan) with power density of 100 mW/cm2. The opticalproperties of the cells and electrodes were measuredusing UV–vis–NIR spectrophotometer (Solidspec-3700, Shimadzu,Japan). The quantum efficiency (QE) was measured by QEmeasuring systems (YQ-250, JASCO, Japan) as a part of data forsimulation. The carrier densities and mobility for TCO weremeasured using a Hall Effect measurement system (HMS-3000,Ecopia, Korea).

Table 3Tandem efficiency for four-terminal and two-terminal connections.

Cell Tandem efficiency Z (%)

Four-terminal Two-terminal

Bottom (CIGS-I) 10.70 10.46

Bottom (CIGS-II) 12.77 12.45

3. Results and discussion

3.1. Performance of the DSC/CIGS tandem solar cell

A semi-transparent DSC sample (DSC-I) with high efficiency ofnearly 7% (all the areas were 0.08 cm2) was obtained by

controlling the thickness of TiO2 layer. CIGS solar cells with theefficiency varying from 10.26% (CIGS-I) to 16.77% (CIGS-II)were fabricated by changing the composition of the CIGSabsorption layer. When DSC-I and CIGS-I were series connected,a tandem cell was obtained yielding a high open-circuit voltageVoc¼1.147 V, a short-circuit current Jsc¼13.58 mA/cm2, a fillfactor FF¼0.67 and a high efficiency Z¼10.46% (as shown inTable 1). Compared with the original DSC cell and CIGS cell, thetandem cell produces a higher efficiency. The voltage wasremarkably increased, thanks to the two-terminal connectionstructure. Also, as the photocurrents of top cell and bottom cellwere nearly the same, the current loss due to the series-connectedcircuit was minimized. Therefore, the novel tandem cell isdemonstrably effective to improve cell performance because itfully takes the utilization of the sunlight. However, if theefficiency of the bottom cell was further increased, there waslimitation for the tandem cell to increase the efficiency. As it wasshown in Table 2, when CIGS-II was used as the bottom cell theefficiency of tandem cell was lower than the CIGS-II single cell.This phenomenon leaves the issues to be studied that to whatextent the tandem cell can increase the efficiency and how it canbe improved. It will be discussed later.

Four-terminal connection of the tandem cell was also studiedwhen setting DSC-I as the top cell (as shown in Table 3). It can befound that the difference of cell performance between two-terminal and four-terminal connections is rarely small. It isbecause of the transmittance of the top cell and the QE of bottomcell that a photocurrent near the top cell is developed. It showsthat the DSC/CIGS tandem structure can minimize the loss fromthe series connection.

3.2. Equivalent circuit analysis for tandem cell output

Equivalent circuit analysis has been widely used to study theelectric mechanism for silicon, CIGS and DSC solar cells [14–16].

W. Wang et al. / Solar Energy Materials & Solar Cells 94 (2010) 1753–1758 1755

For our DSC/CIGS tandem cell, as there is big optical loss from thetop cell, the equivalent circuit analysis for the bottom cell isimportant. One-diode model was used to analyze the performanceof the bottom cell (as shown in Fig. 2). The equation of theequivalent circuit is as follows:

J¼ Jph�Js exp qVþ JRs

nkT

� ��1

� ��

Vþ JRs

Rshð1Þ

Jph is the photocurrent density, Js the saturation current density, Rs

the series resistance per unit area, Rsh the parallel (shunt)resistance per unit area, n the diode factor, q the elementaryelectric charge, k the Boltzmann constant and T the workingtemperature.

From the equation it can be found that if Jph, Js, Rs, Rsh and n areobtained, the J–V characteristics of the bottom cell can beobtained by simulation. The photocurrent Jph can be calculatedby combining the transmittance of top cell and the QE of bottomcell. Js, Rs, Rsh and n are obtained by curve fitting of J–V curve oforiginal CIGS solar cell. After getting the J–V characteristics of thebottom cell from simulation, the tandem cell performance can bepredicted by combining the J–V characteristics of the bottom cellwith that of the DSC top cell.

Fig. 3 shows simulated and real J–V characteristics of twotandem samples using DSC-I as a top cell and CIGS-I or CIGS- II asa bottom cell. The simulation result and the real curve match well,

Fig. 3. Comparison of simulated and real J–V curves of tandem cells.

Fig. 2. Equivalent circuit for a CIGS solar cell.

which shows the equivalent circuit analysis is reliable. Thus, theequivalent circuit analysis can be further used to study thelimitation for the tandem structure to increase the efficiency.

3.3. Limitation for DSC/CIGS tandem cell to increase the efficiency

It has been shown earlier that the tandem efficiency does notkeep increasing with increase in the efficiency of CIGS while fixingDSC-I as the top cell. To find such a turning point where thetandem structure stops improving the performance, simulationmethod based on equivalent circuit analysis can be used. It helpsto avoid the heavy job in making a number of solar cells withdifferent conversion efficiencies. For the DSC-I sample, the electriccharacteristics of the CIGS bottom cell were changed to simulateimaginary samples with different efficiencies. Then the tandemperformances were calculated using equivalent circuit analysis.Thus, the relationship between original CIGS solar cells andcorresponding tandem solar cells could be obtained. Fig. 4 showswhen the efficiency of a CIGS solar cell is greater than 10.82%, thetandem efficiency will be lower than the original CIGS cell. Thisresult is in accordance with the experimental data, which shows itis a reliable and efficient way of analyzing the tandem issues.

Transmittance from a top cell, performance of a top cell andmatching of two cells, etc. were considered to be very importantfactors in deciding the performance of a tandem structure. In thisstudy, the matching of DSC and CIGS proves to be good. Hence, weconsider transmittance and performance of a top cell as the mostimportant factors.

As shown in Fig. 5(a), assume the transmittance of DSC couldbe enhanced (DSC-II) while keeping the cell performance byimproving the utilization of the incident sunlight. The transmittedsunlight led to a higher photocurrent, which in turn increased theperformance of the bottom cell. As a result, the limitationefficiency increases from 10.82% (with DSC-I) to 12.46% (withDSC-II), as shown in Fig. 5.

Fig. 6 shows the influence of performance of the top cell on thelimitation efficiency. Transmittance of the top cell were assumedto be the same while increasing the short-circuit current or theopen-circuit voltage. It is obvious that either the short-circuitcurrent or the open-circuit voltage increases, the limitationefficiency increases remarkably. This is due to the driving forcecoming from the improvement of the top cell.

Fig. 4. Limitation for DSC/CIGS tandem cell to increase the efficiency (DSC-I was

fixed as a top cell).

Fig. 5. (a) Transmittance of different DSC samples (J–V characteristics were assumed to be constant). (b) The influence of the transmittance of the top cell on limitation

efficiency.

Fig. 6. Performance of the top cell affects the efficiency of the tandem cell. The

transmittance of the DSC sample was fixed (DSC-I). When the short-circuit current

density or the open-circuit voltage of DSC-I increases by 1 and 2 mA/cm2 or 0.1 V,

respectively, the limitation efficiency of tandem cell increases.

W. Wang et al. / Solar Energy Materials & Solar Cells 94 (2010) 1753–17581756

3.4. Factors affecting the transmittance in near infrared region

To further increase the efficiency of the tandem solar cell,transmittance and performance of the top cell both need to beimproved. To improve the performance of the top cell, increasingthe photocurrent is an efficient way. There are several ways to geta higher photocurrent such as increasing the TiO2 layer thickness,using reflection layer to expand the spectral response region andso on. However, these methods are all accompanied by loss intransmittance. Thus, further increasing the performance in thepresent condition seems to be a difficult way. On the other hand,taking the transmittance spectrum (as shown in Fig. 5(a)) intoaccount, it can be found that the optical loss especially in longwavelength region (900–1200 nm) is quite high. Considering thatCIGS bottom cell also absorbs the photons efficiently in this nearinfrared region, improving the near infrared transmittance can befeasible.

To understand which part of the DSC is the key source decidingthe near infrared loss, the components of the cell should be

studied respectively. As the band gap of the N719 dye is 1.7 eV,which decides it only absorbs the visible light, the dye is not whatwe are looking for. This explanation can also be applied to that ofthe TiO2 whose band gap is 3.2 eV. Fig. 7(a)–(c) show thetransmittance of electrolyte, Pt layer and transparent conductingoxide (TCO) substrates, respectively. After 700 nm, there wasnearly no loss from the electrolyte (0.001 M I2) which means theelectrolyte can be excluded. For Pt layer, it indeed decreased thetransmittance in the whole spectral region, but the decrease wasnot so serious in the near infrared region. Finally, the TCOsubstrates, namely FTO substrates (14 O/&, China buildingmaterials academy, China) and ITO substrates (12 O/&, Chinabuilding materials academy, China), were studied. Fig. 7(c) showsthat in the near infrared region, the TCO substrates especially ITOare weakly transmitted which shows they are the main causes forthe big optical loss in the long wavelength region.

To find the causes for the TCO which decreases the nearinfrared transmittance, the electronic properties of ITO and FTOwere studied by Hall Effect measurement. Table 4 shows thecarrier densities and the mobility of the ITO and FTO from whichit can be found that the carrier density of ITO is larger than that ofFTO. Then it can be concluded that the optical loss is due to thefree carrier absorption of the TCO [9]. Thus, the present TCOs forDSC should be improved. The free carrier density should bedecreased to reduce the free carrier absorption while increasingthe mobility in order to keep high conductivity. In2O3:Mo andIn2O3:Ti seem to be good replacements according to their lowconcentrations and high mobility [17]. Therefore, furtherimprovement in the Jsc of the filtered bottom cell is expected byreplacing ITO or FTO with these high-mobility TCOs.

4. Conclusion

A DSC/CIGS tandem structure using two-terminal junctionwas studied. A tandem cell with a high open-circuit voltage of1.15 V and a high efficiency of 10.46%, which is greater thanany of the single cell, has been prepared. Equivalent circuitanalysis was carried out to find the factors enhancing theefficiency of the tandem cell, transmittance and performance ofthe top cell were found to be the key factors affecting thepotential of the tandem structure to increase the efficiency.Improving the TCO substrates, which are being widely used

Fig. 7. Transmittance of (a) electrolyte (0.001 M), (b) Pt-deposited FTO (Pt layer thickness: 3 nm) and (c) ITO and FTO.

Table 4Electronic properties of ITO and FTO.

ITO FTO

Thickness (nm) 260 360

Carrier density (1020/cm3) 8.385 7.682

Mobility (cm2/V s) 20.077 17.35

W. Wang et al. / Solar Energy Materials & Solar Cells 94 (2010) 1753–1758 1757

nowadays, and which have a strong absorption in near infraredregion, is an efficient way of improving the tandem structure. Amuch higher efficiency can be expected by appropriate designand materials.

Acknowledgements

This work was financially supported by the National HighTechnology Research and Development Program of China (863Program, 2006AA03Z218), National Basic Research Program ofChina (973 Program, 2007CB607504) and NSF of China(50672041).

Appendix A. Supplementary material

Supplementary data associated with this article can be foundin the online version at doi:10.1016/j.solmat.2010.05.041.

References

[1] M.A. Green, K. Emery, D.L. King, Y. Hisikawa, W. Warta, Solar cell efficiencytables (version 27), Prog. Photovolt.: Res. Appl. 14 (2006) 455–461.

[2] T.J. Coutts, K.A. Emery, J.S. Ward, Modeled performance of polycrystallinethin-film tandem solar cells, Prog. Photovolt.: Res. Appl. 10 (2002) 195–203.

[3] M. Gratzel, Mesoscopic solar cells for electricity and hydrogen productionfrom sunlight, Chem. Lett. 34 (2005) 8–13.

[4] I. Repins, M.A. Contreras, B. Egaas, C. DeHart, J. Scharf, C.L. Perkins, B. To, R. Noufi,Short communication: accelerated publication 19.9%-efficient ZnO/CdS/CuInGaSe2

solar cell with 81.2% fill factor, Prog. Photovolt.: Res. Appl. 16 (2008) 235–239.[5] J.J. He, H. Lindstrom, A. Hagfeldt, S.-E. Lindquist, Dye-sensitized nanostruc-

tured tandem cell-first demonstrated cell with a dye-sensitized photo-cathode, Sol. Energy Mater. Sol. Cells 62 (2000) 265–273.

[6] M. Murayama, T. Mori, Novel tandem cell structure of dye-sensitized solar cellfor improvement in photocurrent, Thin Solid Films 516 (2008) 2716–2722.

[7] W. Kubo, A. Sakamoto, T. Kitamura, Y. Wada, S. Yanagida, Dye-sensitized solarcells: improvement of spectral response by tandem structure, J. Photochem.Photobiol. A 164 (2004) 33–39.

W. Wang et al. / Solar Energy Materials & Solar Cells 94 (2010) 1753–17581758

[8] S. Nishiwaki, S. Siebentritt, P. Walk, M.C. Lux-Steiner, A stacked chalcopyritethin-film tandem solar cell with 1.2 V open-circuit voltage, Prog. Photovolt.:Res. Appl. 11 (2003) 243–248.

[9] T. Nakada, S. Kijima, Y. Kuromiya, R. Arai, Y. Ishii, N. Kawamura, H. Ishizaki, N.Yamada, Chalcopyrite thin-film tandem solar cells with 1.5 V open-circuit-voltage, in: Conference Record of the 2006 IEEE 4th World Conference onPhotovoltaic Energy Conversion, 2006, pp. 400–403.

[10] P. Liska, K.R. Thampi, M. Gratzel, D. Bremaud, D. Rudmann, H.M. Upadhyaya,A.N. Tiwari, Nanocrystalline dye-sensitized solar cell/copper indium galliumselenide thin-film tandem showing greater than 15% conversion efficiency,Appl. Phys. Lett. 88 (2006) 201303.

[11] X. Li, H. Lin, S.M. Zakeeruddin, M. Gratzel, J. Li, Interface modification of dye-sensitized solar cells with pivalic acid to enhance the open-circuit voltage,Chem. Lett. 38 (2009) 322–323.

[12] M.K. Nazeeruddin, S.M. Zakeeruddin, R. Humphry-Baker, M. Jirousek, P. Liska,N. Vlachopoulos, V. Shklover, C.H. Fischer, M. Gratzel, Acid�base equilibria of(2,20-bipyridyl-4,40-dicarboxylic acid)ruthenium(II) complexes and the effect

of protonation on charge-transfer sensitization of nanocrystalline titania,Inorg. Chem. 38 (1999) 6298–6305.

[13] F.Y. Meng, Y. Chiba, A. Yamada, M. Konagi, Growth of Zn1�xMgxO films withsingle wurtzite structure by MOCVD process and their application toCu(InGa)(SSe)2 solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 1887–1891.

[14] Z. Ouennoughi, M. Chegaar, A simpler method for extracting solar cell parametersusing the conductance method, Solid-State Electron. 43 (1999) 1985–1988.

[15] A.E. Delahoy, A.M. Payne, Determination of the internal series resistance ofCIS and CIGS photovoltaic cell structures, in: Conference Record of the 25thIEEE Photovoltaic Specialists Conference, 1996, pp. 841–844.

[16] S. Sodergren, A. Hagfeldt, J. Olsson, S.-E. Lindquist, Theoretical-models for theaction spectrum and the current–voltage characteristics of microporoussemiconductor-films in photoelectrochemical cells, J. Phys. Chem. 98 (1994)5552–5556.

[17] A.E. Delahoy, S.Y. Guo, Transparent and semitransparent conducting filmdeposition by reactive-environment, hollow cathode sputtering, J. Vac. Sci.Technol. A 23 (2005) 1215–1220.