journal jjap dye-sensitized solar cells with conversion efficiency of 11.1%

8/4/2019 Journal JJAP Dye-Sensitized Solar Cells With Conversion Efficiency of 11.1%

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Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%

Yasuo C HIBA , Ashraful I SLAM , Yuki W ATANABE , Ryoichi K OMIYA , Naoki K OIDE and Liyuan H AN

Solar Systems Development Center, Sharp Corporation, 282-1 Hajikami, Katsuragi, Nara 639-2198, Japan

(Received June 7, 2006; accepted June 9, 2006; published online June 23, 2006)

Dye-sensitized solar cells (DSCs) using titanium dioxide (TiO 2 ) electrodes with different haze were investigated. It was foundthat the incident photon to current efficiency ( IPCE ) of DSCs increases with increase in the haze of the TiO 2 electrodes,especially in the near infrared wavelength region. Conversion efficiency of 11.1%, measured by a public test center, wasachieved using high haze TiO 2 electrodes. This indicates that raising the haze of TiO 2 electrodes is an effective technique forimprovement of conversion efficiency. [DOI: 10.1143/JJAP.45.L638 ]

KEYWORDS: dye-sensitized solar cells, haze, internal resistance, TiO 2 electrode, IPCE

Dye-sensitized solar cells (DSCs) have been widelyinvestigated as a next-generation solar cell because of theirsimple structure and low manufacturing cost. 1,2) In general,a DSC comprises a nanocrystalline titanium dioxide (TiO 2 )

electrode modied with a dye fabricated on a transparentconducting oxide (TCO), a platinum (Pt) counter electrode,and an electrolyte solution with a dissolved iodide ion/tri-iodide ion redox couple between the electrodes. Althoughcertied conversion efficiency using black dye has beenreported to be 10.4% by the Swiss Federal Institute of Technology in Lausanne (EPFL), 3) efficiency of over 10%has rarely been achieved due to insufficient understanding of the mechanism of DSCs, which is different from that of conventional solar cells.

It is well known that the conversion efficiency ( ) of solarcells can be represented as follows: 4)

FF I SC V OC =P in ; 1 where FF , I SC , V OC , and P in are ll factor, short circuitcurrent, open circuit voltage, and incident power, respec-tively. Equation (1) suggests that it should be necessary toimprove these three parameters in order to raise conversionefficiency. In our previous studies, we investigated theinternal resistance of DSCs using electrochemical impe-dance spectroscopy (EIS) measurement as a means of investigating DSC mechanisms and proposed an equivalentcircuit for modeling DSCs based on the results of EISanalysis. 5) We also found that the series resistance of DSCsconsists of three resistance elements, namely, the sheet

resistance of the TCO, the resistance of ionic diffusion in theelectrolyte, and the resistance at the interface between thecounter electrode and the electrolyte. We discovered that FF increases with decrease in the internal resistance elementsand reported a resulting rise in certied conversion effi-ciency to 10.2%. 6) In order to further improve the efficiency,we then considered ways of elevating the other factors suchas short circuit current density ( J SC ) and V OC .

There are two approaches to improving J SC . One is todevelop a new dye which can absorb incident light of longerwavelengths, and the other is to increase the extent of lighttrapping within the TiO 2 electrodes. Many panchromaticdyes have been developed such as -diketonato Ru(II)complex. 7,8) Unfortunately, the certied efficiency has notbeen improved as there is no dye superior to black dye. As

for the latter approach, Usami made the theoretical ndingthat light scattering magnitude in DSCs could be controlledby adding submicron particles to TiO 2 electrodes composedof nanocrystalline particles. 9) Most related experimental

work involves making a rough estimate of the light-scattering magnitude from the light-harvesting efficiency( LHE ) of the dye-treated TiO 2 electrodes. 10) However, it isvery difficult to obtain the exact magnitude and to elevate itbecause LHE is affected by the absorption characteristics of the dye. An effective index of the light-trapping effect of TiO 2 electrodes is therefore needed.

In addition, it is very important to conrm that the J SCobtained from currentvoltage ( I V ) measurement is con-sistent with the value estimated from incident photon tocurrent efficiency ( IPCE ) spectra, because J SC can bedescribed by integrating the product of the incident photonux density [ F ] and IPCE of the cell over thewavelength ( ) of the incident light, expressed as

J SC Z qF 1 r IPCE d ; 2 where q is the electron charge and r the incident light lossin light absorption and reection by the conducting glass. Itis difficult to obtain accurate IPCE spectra of DSCs because IPCE strongly depends on measurement conditions such aschopping frequency and bias light. Recently, Hishikawa et al. reported that IPCE spectra increase with decrease of chopping frequency under AC mode. They suggested thatthe IPCE of DSCs should be measured under white bias light

and in AC mode with low chopping frequency of 1 2 Hz orin DC mode and that J SC measured from the I V curveshould be veried with the correct IPCE measurement. 11)

In the present letter, we attempt to introduce the conceptof haze to estimate the optical path length in TiO 2 electrodesand discuss the relationship between haze in the infraredregion of TiO 2 electrodes and IPCE spectra. We veried J SCnot only by I V measurement but also IPCE measurement.We also report the achievement of high performance inDSCs.

TiO 2 electrodes with different haze were prepared usinga screen-printing method. DSCs were fabricated using apublished procedure. 6) The TiO 2 electrodes on TCO sub-strates were placed in a mixture solution of tert -butyl alcoholand acetonitrile ( 1 : 1 ) at a concentration of 2 10 4 M of black dye. Deoxycholic acid was added to the dye solutionas a coadsorbent at a concentration of 20 mM. Pt-coated E-mail address: [email protected]

Japanese Journal of Applied PhysicsVol. 45, No. 25, 2006, pp. L638 L640

# 2006 The Japan Society of Applied Physics

L638

JJAP Express Letter
http://dx.doi.org/10.1143/JJAP.45.L638http://dx.doi.org/10.1143/JJAP.45.L638http://dx.doi.org/10.1143/JJAP.45.L638http://dx.doi.org/10.1143/JJAP.45.L638


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glass was used for the counter electrodes. The compositionof the electrolyte solution in acetonitrile was: dimethyl

propyl imidazolium iodide (0.6 M), lithium iodide (0.1 M),iodine (0.05 M), and tert -butylpryidine (0.5 M). IPCE spectra were measured with monochromatic incident lightof 1 10 16 photon/cm 2 under 100 mW/cm 2 white bias lightin DC mode (CEP-2000, Bunkoh-Keiki). 11) Currentvoltage( I V ) characteristics of DSCs were measured using a digitalsource meter (2400, Keithley) under a standard air mass(AM 1.5) with simulated solar illumination at 100 mW/cm 2

(WXS-155S-10, Wacom). 12) Haze was controlled by theaddition of submicron particles (400 nm diameter) to theTiO 2 electrodes. Haze, dened as the ratio of diffusedtransmittance to total optical transmittance, was measured atthe wavelength of 800 nm using an integral sphere.

Figure 1 shows the dependence of IPCE spectra on haze,which varied in the range from 3 to 76%. Here, the haze at800nm was used as an index as this is the wavelength inwhich we want to improve IPCE . Figure 1 indicates that IPCE is greatly increased by increase in haze, especially inthe infrared region, where it increased from 10 to 50% withincrease in haze from 3 to 76%. On the other hand, IPCE spectra in the visible region, for example around 600 nm,increased only gradually, from 65 to ca. 80%, with increasein haze from 3 to 53% and reached saturation with furtherincrease in haze. These results suggest that IPCE of 80%in the visible wavelength region is easily obtained using

electrodes of medium haze TiO 2 because of the largeextinction coefficient of the dye, while high IPCE in theinfrared region demands high haze TiO 2 because of thesmall extinction coefficient of the dye. For example, black dye in ethanol has an extinction coefficient of about 7 10 3 mol 1 cm 1 at around 600nm, while extinction coef-cient is about 200 mol 1 cm 1 at around 800 nm. 3) Exper-imental evidence was thus found that the IPCE of DSCs iseffectively improved by increase in the haze of TiO 2electrodes.

Figure 2 shows the relationship between J SC calculatedfrom eq. (2) and J SC measured from I V characteristics atvarying haze at 800 nm. In the calculation, F is assumedto be AM 1.5 standard spectrum. A linear relationshipbetween measured J SC and calculated J SC is observed. Theslope of the line is very close to 1, suggesting that IPCE could be accurately measured under DC mode. The reason

for using DC mode can be explained as follows: in theequivalent circuit model proposed in our previous study, 5)

there are two large capacitance elements (1 F and 2 mF)parallel to the series resistance. It is these capacitanceelements that delay the response of the DSCs. Since it allowsthe effect of the capacitance elements to be excluded, the DCmode as used here is preferable for IPCE measurement.

As a result, the highest J SC of 21 mA/cm 2 was achievedwhen using TiO 2 electrodes with haze of 76%. This resultindicates that haze is a useful index of the improvement of J SC and energy conversion efficiency in DSCs. When usingTiO 2 electrodes with haze of 76%, we almost alwaysobtained high J SC of 20.8 21.1 mA/cm 2 . The cell gap wasalso investigated. It was found that the resistance of the ionicdiffusion in the electrolyte decreases when the counterelectrode is placed directly on the TiO 2 electrode, so thatthe total series resistance was successfully decreased to1.6 cm 2 .

For the sake of fairness, I V characteristics wereindependently measured by the Research Center for Photo-voltaics, the National Institute of Advanced IndustrialScience and Technology (AIST, Japan), using a black metal

mask with an aperture area of 0.219 cm2

under standard AM1.5 sunlight (100.0 mW/cm 2 ), as shown in Fig. 3. An overallconversion efficiency of 11.1% was achieved, which is thehighest efficiency conrmed by a public test center.

In conclusion, we investigated the improvement of DSCperformance using TiO 2 electrodes with high haze. It wasfound that the increase of haze in TiO 2 electrodes at 800 nmeffectively improves the IPCE of DSCs. It is thereforeproposed that haze is a useful index of improvement inconversion efficiency. Moreover, high J SC was veried notonly by I V measurement but also by precise IPCE measurement in DC mode. The highest DSC efficiencyrecorded so far, 11.1% (0.219 cm 2 ), was obtained andconrmed by AIST. These ndings appear to emphasizethe important role of the haze of TiO 2 electrodes in deviceperformance and should aid the further development of DSCs for photovoltaic applications.

0

20

40

60

80

100

400 600 800 1000

Wavelength (nm)

I P C E ( % )

Haze 76%Haze 60%

Haze 53%Haze 36%Haze 10%Haze 3%

Fig. 1. Dependence of IPCE spectra on haze of TiO 2 electrodes. Haze inthe gure was measured at 800 nm.

10

15

20

25

10 15 20 25

3%

10%

36%

53%

60%

76%

Measured Jsc (mA/cm 2 )

C a

l c u

l a t e d J s c

( m A / c m

2 )

Fig. 2. Relationship between J SC calculated from eq. (2) and J SCmeasured from I V characteristics. The solid line shows a good t tothe data, y 0 :99 x 0 :008 . Percentage values indicate the haze of eachTiO 2 electrode at 800 nm.

Jpn. J. Appl. Phys., Vol. 45, No. 25 (2006) Express Letter Y. C HIBA et al.

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1) B. ORegan and M. Gra tzel: Nature 353 (1991) 737.2) M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mu ller,

P. Liska, N. Vlachopoulos and M. Gra tzel: J. Am. Chem. Soc. 115(1993) 6382.

3) M. K. Nazeeruddin, P. Pe chy, T. Renouard, S. M. Zakeeruddin, R.Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover,L. Spiccia, G. B. Deacon, C. A. Bignozzi and M. Gra tzel: J. Am.Chem. Soc. 123 (2001) 1613.

4) See for example, S. M. Sze: Physics of Semiconductor Devices (Wiley,New York, 1981) 2nd ed., p. 807.

5) L. Han, N. Koide, Y. Chiba and T. Mitate: Appl. Phys. Lett. 84 (2004)2433.

6) L. Han, N. Koide, Y. Chiba, A. Islam, R. Komiya, N. Fuke, A. Fukui

and R. Yamanaka: Appl. Phys. Lett. 86 (2005) 213501.7) A. Islam, H. Sugihara, M. Yanagida, K. Hara, G. Fujihashi, Y.

Tachibana, R. Katoh, S. Murata and H. Arakawa: New J. Chem. 26(2002) 966.

8) A. Islam, F. A. Chowdhury, Y. Chiba, R. Komiya, N. Fuke, N. Ikedaand L. Han: Chem. Lett. 34 (2005) 344.

9) A. Usami: Sol. Energy Mater. Sol. Cells 64 (2000) 73.10) Y. Tachibana, K. Hara, K. Sayama and H. Arakawa: Chem. Mater. 14

(2002) 2527.11) Y. Hishikawa, M. Yanagida and N. Koide: Proc. 31st IEEE Photo-

voltaic Specialists Conf., Florida, 2005, p. 67.12) N. Koide and L. Han: Rev. Sci. Instrum. 75 (2004) 2828.

Fig. 3. Currentvoltage characteristics of DSC sensitized with black dye. Results were measured at 25 C with an aperture area of 0.219 cm 2 using a black metal mask and irradiance of 100.0 mW/cm 2 . Short circuit current I SC 4 :57 mA; short circuit currentdensity J SC 20 :9 mA/cm 2 ; open circuit voltage V OC 736 mV; ll factor FF 72 :2 %; maximum power voltage V PMAX 583 mV; maximum power current I PMAX 4 :16 mA; maximum power P MAX 2 :429 mW; efficiency = 11.1%. The solid line anddashed line indicate current and power, respectively. This measurement was independently carried out by the Research Center forPhotovoltaics, AIST.

Jpn. J. Appl. Phys., Vol. 45, No. 25 (2006) Express Letter Y. C HIBA et al.

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journal jjap dye-sensitized solar cells with conversion efficiency of 11.1%

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