Highly Efficient Dye-sensitized Solar Cells

Invited Paper

Highly Efficient Dye-sensitized Solar Cells Liyuan Han*, Ashraful Islam Advanced Photovoltaics Center, National Institute for Materials Science 1-2-1 Sengen Tsukuba, Ibaraki, 305-0047, Japan ABSTRACT The present paper discusses the strategy for improvement of efficiency of dye-sensitized solar cells based the equivalent circuit model. The influence of these elements upon cell efficiency in areas such as short circuit current density (JSC), open circuit voltage (VOC), and fill factor (FF) was examined. It was demonstrated that the haze factor of TiO2 electrodes is a useful index when fabricating light-confined TiO2 electrodes to improve JSC, and that blocking the TiO2 surface with molecules is an effective way of reducing interfacial charge recombination at the TiO2 surface and of improving shunt resistance and VOC. FF was also improved by reduction of the internal series resistance, which is composed of the redox reaction resistance at the platinum counter electrode, the resistance of carrier transport by ions in the electrolyte, and resistance due to the sheet resistance of the transparent conducting oxide. Finally, the highest efficiency scores of 11.1% and 10.4% (aperture illumination area of 0.219cm2 and 1.004cm2, respectively) were confirmed by a public test center. Keywords: Dye sensitized solar cell, haze factor, internal resistance, equivalent circuit.

1. INTRODUCTION Much attention has been paid to the development of dye-sensitized solar cells (DSCs), because of their low cost of production and high energy-conversion efficiency [1-3]. In general, a DSC comprises a nanocrystalline titanium dioxide (TiO2) electrode modified with a dye, a platinum (Pt) counter electrode, and an electrolyte solution with a dissolved iodide ion/tri-iodide ion (I−/I3−) redox couple between the electrodes as shown in Fig. 1(a). Differing from conventional solar cells, the principle of DSCs is considered similar to that of photosynthesis in a leaf, the light absorption and carrier transportation in DSCs occur separately. The process is thought to be as follows: electrons are injected from the photoexcited dye into the conduction band of TiO2, from where they pass through the nanoparticles to the transparent conducting oxide (TCO) current collector and into the external circuit. The dye is regenerated by electron transfer from a donor, typically iodide ions, which are dissolved in the electrolyte that is present in the porous semiconductor. The triiodide ions formed during the reaction diffuse to the counter electrode, where they are reduced back to iodide by the conduction band electrons that have passed through the external circuit that performs the electrical work.

C3

R3

Iph Z2

C1

R1

I R

Rsh

(b)

(a)

Fig. 1 a) Sketch of dye-sensitized solar cell. b) Equivalent circuit model for DSCs. Z2 functions to rectify the current and represented as a diode. The sum of R1, R3 and Rh largely corresponds to the series resistance of DSCs. A constant-current source Iph and shunt resistance Rsh are in parallel with Z2. C1 and C3 are capacitance elements. *[email protected]; phone 81 29 859-2305; fax 81 29 859-2301

Organic Photovoltaics X, edited by Zakya H. Kafafi, Paul A. Lane, Proc. of SPIE Vol. 7416 74160X · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.829181

Proc. of SPIE Vol. 7416 74160X-1

In a previous paper, we investigated the internal resistance of DSCs through electrochemical impedance spectroscopy measurement as a means of researching DSC mechanisms. Four resistance elements were observed in the impedance spectra. According to our analysis, these internal resistance elements are related to the charge transfer processes at the Pt counter electrode (R1), the charge transportation at the TiO2/dye/electrolyte interface (R2), ionic diffusion in the electrolyte (R3), and the sheet resistance of TCO (Rh). R2 was found to act like the resistance of a diode as it was dependent on the applied bias voltage, while R1, R3, and Rh showed the behavior of series-internal resistance. These results were used to construct the equivalent circuit of DSCs shown in Fig. 1(b). The present paper deals with the way how to improve the conversion efficiency based on the analysis of the equivalent circuit. The current-voltage (I-V) characteristics with the confirmed highest efficiency for DSCs were also reported.

2. STRATEGY FOR RAISING EFFICIENCY It is well known that the conversion efficiency (η) of solar cells can be represented as follows:

η = (JSC x VOC x FF) / Pin

(1)

where JSC, VOC, FF and Pin are short circuit current density, open circuit voltage, fill factor, and incident solar radiation, respectively. Equation (1) suggests that it should be necessary to improve these three parameters in order to raise conversion efficiency. The methods how to improve JSC, VOC, FF is investigated in this section. 2.1 Improvement of JSC JSC, which is described by a constant-current source in the equivalent circuit, can be calculated by integrating the product of an incident photon flux density (F(λ)) and the incident photon to the current efficiency (IPCE(λ)) of the cell over the wavelength (λ) of the incident light as in eq. (2).

J SC = ∫ qF (λ )(1 − r (λ )) IPCE (λ )dλ

(2)

where q is the electron charge, F(λ) is the incident photon flux density at wavelength λ, r(λ) is the incident light loss due to the light absorption and reflection by the conducting glass, and IPCE(λ) is defined as the incident monochromatic photon-to-electron conversion efficiency. In the case of DSCs, light absorption occurs mainly at the photo-sensitive dye, as it is necessary to increase the IPCE of the photo-sensitive electrodes with dye. Therefore, IPCE is expressed as: IPCE (λ) = LHE (λ) φe-inj(λ) ηCC(λ) = LHE (λ) Φ(λ)ET

(3)

where LHE(λ) is light-harvesting efficiency, φe-inj(λ) is electron injection yield from the dye excited state into TiO2, ηCC(λ) is charge collection efficiency at the front TCO electrodes, and Φ(λ)ET is defined as electron-transfer yield, that is,

the product of electron injection yield and charge collection efficiency.

The values of φe-inj(λ) and ηCC(λ) in DSCs have been examined by studies of electron transfer mechanism, for example transient absorption spectroscopy [4], and Φ(λ)ET has been found to be close to 1 in high-efficiency DSCs [5, 6]. It is therefore very important to increase LHE to improve short circuit current. The LHE of the cell depends heavily on 1) the properties of the dye, such as extinction coefficient and dye uptake on TiO2 electrodes, and 2) the optical path length within the electrodes. Here we discuss the properties of the dye and optical path length within the electrode for the purpose of improvement of JSC. 2.1.1 Development of sensitizer Many dyes, including organic dyes [7-9] and transition metal complexes [1-3, 10-17], have been employed in DSCs. One of the advantages of organic dyes is their high molar extinction coefficient. According to Hara et al., over 7% efficiency was achieved, which is nevertheless still lower than with the transition metal complexes based on Ru(II), Os(II), Pt(II), Fe(II), Re(I), and Cu(I) that have been developed for DSCs. [14] Of these, Ru(II)-based charge-transfer polypyridyl


complexes show the highest energy conversion efficiency. This is because of their intense charge-transfer (CT) absorption in the whole visible range, moderately intense emission with fairly long lifetime in fluid solutions at ambient temperatures, high quantum yield for the formation of the lowest CT excited state, and redox reactivity and ease of tunability of redox properties. Photoexcitation of the metal-to-ligand charge-transfer (MLCT)-excited states of the adsorbed dye leads to an efficient injection of electrons into the conduction band of the TiO2. IPCE values exceeding 70% have been reported in a number of cases. The most efficient transition metal complexes employed so far in these solar cells are the Ru(II) bipyridyl complex [Ru(dcbpy)2(NCS)2{(C4H9)4N}2 referred to as N719] [2] and the Ru(II) terpyridyl complex [Ru(tctpy)(NCS)3{(C4H9)4N}3 referred to as Black Dye] [3]. Of these two complexes, black dye has the higher JSC value, with 21 mA/cm2 against 17 mA/cm2, because it absorbs light of longer wavelength. This suggests that JSC could be improved by extending the dye absorbance to cover longer wavelengths. The molecular design of new dyes that can absorb light up to the infrared wavelength region presents a challenging task in the further improvement of JSC. Sugihara et al. reported an efficient Ru(II) 4,4'-dicarboxy-2,2'-bipyridine sensitizer containing one chelating β-diketonate ligand instead of two thiocyanate ligands [15]. The strong σ-donating nature of the negatively charged oxygen donor atom of the β- diketonato ligand destabilizes the ground state energy level of the dye, leading to lower energy shift of MLCT transitions compared to the thiocyanato-ruthenium(II) complex. In addition, the β-diketonato-ruthenium(II) sensitizer of Ru(4, 4', 4''- tricarboxy- 2, 2': 6', 2''- terpyridine) (β-diketonate)(NCS) with one NCS ligand shows efficient panchromatic sensitization of nanocrystalline TiO2 solar cells [16]. Recently, we have developed a new β-diketonato Ru(II) complex, Ru (4, 4', 4''- tricarboxy- 2, 2': 6', 2''- terpyridine) (4, 4, 4- trifluoro- 1- (4fluorophenyl) butane- 1, 3- dione)(NCS) {(C4H9)4N}2, which shows very efficient panchromatic sensitization over the whole visible range extending into the near infrared region (ca. 950nm) [17]. However, because of its lower VOC compared to N719 and black dye, a conversion efficiency of 8.9% was attained under standard AM 1.5 irradiation (100 mW/cm2) with JSC of 20.4 mA/cm2. As mentioned above, although many dyes have been synthesized for improvement of performance, dyes with higher efficiency than black dye have not yet been developed. 2.1.2 Improvement of optical path length within TiO2 electrode Another approach to improvement of JSC is to increase the optical path length in TiO2 electrodes. The optical path length can be increased by the addition of relatively large particles to small particles to induce light scattering of the electrodes, which is also called the light-trapping effect. Usami theoretically points out that light-scattering magnitude in DSCs could be controlled by adding submicron particles to TiO2 electrodes composed of nanocrystalline particles [18]. Most of the experimental work was concerned with rough estimation of the light-scattering magnitude from the LHE of the dyeadsorbed TiO2 electrodes [5]. Grätzel et al. report that over 10% efficiency with N719 dye was achieved by using a TiO2 electrode with double layers, one a transparent layer with particle size of about 20 nm and the other a scattering layer with particle size of 400 nm [19]. However, this high efficiency has not yet been reproduced by other groups, as the light-scattering effect is affected by certain subtle properties of the TiO2 electrodes, such as the configuration of the TiO2 particles, the distribution of particle size, the method of preparation of the electrode, and the sintering conditions. Meanwhile, the concept of haze factor is well known as an index of TCO substrates for light scattering in the field of thin-film solar cells, such as amorphous silicon solar cells [20] and microcrystalline silicon solar cells [21, 22]. In the relevant studies, an effective way of trapping light within the cell was found to be to increase the haze factor of the TCO substrates while controlling the surface morphology of TCO. Hence conversion efficiency can be improved by using a TCO with large haze factor due to the increase in the optical path length in the cells. In the same manner, we have recently investigated the optical path length in DSCs using the haze factor of TiO2 electrodes [23]. We also found that haze factor could be used as a suitable index for estimating the light scattering of TiO2 electrodes in DSCs. Here, the haze factor of TiO2 electrodes was defined as the ratio of diffused light to the total light transmitted through the electrodes. In the case of DSCs, IPCE spectrum is primarily governed by the absorption spectrum of the dye. In general, IPCE in the long wavelength region is lower than in the short wavelength region because the extinction coefficient of the dye at longer wavelength is much lower. For example, black dye in ethanol has an extinction coefficient of about 7x103 mol-1 cm-1 at 620 nm, while extinction coefficient is about 200 mol-1 cm-1 at 800 nm. Thus, although it is easy to make a DSC with black dye with IPCE of over 70% at 600 nm, IPCE at 800 nm is usually limited to 20%. For this reason, in order to


raise cell efficiency to over 10%, it is necessary to increase the haze factor of the TiO2 electrodes in the infrared wavelength region to increase the optical path length. Figure 2 shows the dependence of IPCE spectra on haze factors of 3% to 76% in TiO2 electrodes at 800 nm. IPCE spectra at 800 nm increase enormously from 10% to 50% with increase in haze factor from 3% to 76%. On the other hand, IPCE spectra at around 600 nm increase gradually from 65% to ca. 80% with increase in haze factor from 3% to 53% and become saturated with further increase in haze factor. These results tell us that IPCE of 80% in the visible wavelength region is easily obtained using electrodes with medium haze factor because of the large extinction coefficient of the dye, while high IPCE in the infrared region demands high haze factor because of the small extinction coefficient of the dye. 100 90 80

IPCE (%)

70 60 50 Haze 76% Haze 60% Haze 53% Haze 36% Haze 3%

40 30 20 10 0 400

500

600

700

800

900

1000

Wavelength (nm)

Fig. 2 The incident photon to current conversion efficiency (IPCE) of the DSCs using modified TiO2 electrodes with different haze factor.

Because JSC is integral to IPCE over the sensitized wavelength region, the JSC of DSCs will be improved with increase in haze factor especially in the infrared region using modified TiO2 electrodes. We have demonstrated that JSC increases in direct proportion to haze factor at 800 nm. The highest JSC of 21 mA/cm2 was achieved with haze factor of 76%. This result also indicates that the haze factor of the TiO2 electrodes is a useful index for improving short circuit current in DSCs. 2.2 Improvement of Voc The total current through the cell is described in eq. (4) from the analysis of the equivalent circuit in Fig. 1. [24]

⎧ ⎛ q(V + IRS ) ⎞ ⎫ V + IRS I = I ph − I 0 ⎨exp⎜ ⎟ − 1⎬ − nkT RSh ⎠ ⎭ ⎩ ⎝

(4)

Based on eq. (4), VOC is determined under conditions where the current source Iph is equal to the sum of the current through the diode (Id) (the second term on the right-hand side of the equation) and the current through the shunt resistance (Ish) (the third term on the right-hand side of the equation). If we assume Iph to be constant, I0 in Id and/or Ish should be reduced for increase in VOC. The value of I0 in the diode current of the equivalent circuit is strongly related to charge transfer at the TiO2/dye/electrolyte interface in the reverse bias region, but since the mechanism of charge transfer in this region is still unclear, it is at present difficult to control I0. Meanwhile, there have been many attempts to reduce Ish by blocking the surface of TiO2 with various molecules such as tert-butylpyridine (TBP) [2, 3, 25], alkylaminopyridine additives [26], combinations of acetic acid and methylpyrimidine or methylbenzimidazole [27], and pirimidine additives [28].


DSCs with TBP-treated dye-sensitized TiO2 electrodes have high open circuit voltage because of the increased shunt resistance caused by blocking the TiO2 surface with TBP molecules. For example, Rsh increases from 1 kΩcm2 to 2 kΩcm2 with TBP treatment [25]. However, JSC is unfortunately decreased by adding TBP to the electrolyte. It is well known that the conduction band of nanocrystalline TiO2 electrodes depends on the pH of the electrolyte and that pyridine functions as a base within the electrolyte. Haque et al. [29] therefore concluded that the shift of the conduction band to more negative potentials results primarily from the interaction of TBP with dye-sensitized TiO2 electrodes. This negative shift reduces the density of acceptor states available for electron injection and thereby retards the kinetics of electron injection. This is the main reason for the trade-off between VOC and JSC in DSCs with TBP. Recently, we have found an answer to this issue in THF molecules, which are effective in enhancing VOC without loss of JSC. The results are shown in Fig. 3. [30]

20

C urrent D ensity (m A /cm 2)

18 16 14 12 10 8 6 4 2 0 0

0.2

0.4

0.6

0.8

V oltage (V )

Fig. 3 Current-voltage characteristics of the DSCs with (closed line) and without (dashed line) THF in electrolyte.

2.3 Improvement of FF It is well known that FF increases with decreasing series resistance. In the case of DSCs, the series resistance (Rs) is composed of the three resistance elements of R1, R3 and Rh as shown in the equivalent circuit in Fig. 1(b). Here, reduction of R1, R3 and Rh will be discussed in that order. 8

4

(a)

(b)

6 2

R3 (Ωcm )

R1 (Ωcm2)

3 2 1 0 0.85

4 2 0

0.9

0.95 1/RF

1

1.05

0

20 40 60 80 Thickness of electrolyte layer (mm)

Fig. 4 (a) Dependence of R1 on roughness factor (RF) of the counter electrodes and (b) Dependence of R3 on thickness of electrolyte layer.


Since R1 is resistance related to charge-transfer processes occurring at the Pt counter electrodes, it should increase with increase in the surface area of the counter electrodes. In order to decrease R1, we introduced a roughness factor (RF) as an index of the surface area of the counter electrodes. Here, RF is defined as the ratio of the total surface area to the projected area of the counter electrodes. The total surface area was measured by atomic force microscope (AFM) and the projected area was by optical microscope. Counter electrodes with different roughness factors were prepared by depositing Pt thin film on glass substrates with different roughness factors. It was found that R1 is inversely proportional to RF (Fig. 4a). This result suggests that increase in the RF of the counter electrode leads to an accelerated rate of I3reduction through the increased surface area of the counter electrode. In brief, R1 can be decreased by 2 Ωcm2 by increasing the RF of the counter electrodes from 1.0 to 1.1. R3 is related to carrier transport by ions within the electrolyte. In order to decrease R3, it is necessary to decrease the friction between two ions or between ions and solvent molecules in the electrolyte. We therefore used acetonitrile with low viscosity as the electrolyte solvent. In addition, to shorten the length of the electrolyte layer, the relationship between R3 and the thickness of the electrolyte layer (d) was investigated (Fig. 4b). This figure shows that cell performance can be improved by setting the counter electrode close to the TiO2 film. The extrapolation of R3 vs. d data toward d = 0 gives a value of R3 close to 0.7 Ωcm2. We consider this resistance to be related to the resistance of the electrolyte in the porous TiO2 film, because the thickness is defined as the distance between the TiO2 film and the counter electrode. Consequently, we can decrease R3 to 0.7 Ωcm2 by decreasing d to zero. Finally, Rh, which is mainly attributed to the sheet resistance of the TCO substrates, should be discussed. It was also found that Rh increases in proportion to the increase in the sheet resistance of the TCO [31]. Although Rh could be theoretically reduced to 0 by using TCO with much lower sheet resistance, the efficiency of the cell would decrease due to decrease in the transmittance of the TCO. For example, while sheet resistance of 10 Ω/sq in the TCO, which is the norm in DSCs, gives Rh of about 1.0 Ωcm2, sheet resistance of 5 Ω/sq reduces Rh to 0.5 Ωcm2, but also reduces transmittance at wavelength of 600 nm from 83% to 78%. The optimum value for the sheet resistance of TCO is thought to be around 10 Ω/sq with transmittance of over 80% in the visible spectral region. In order to improve conversion efficiency, TCO, the Pt counter electrode, and the thickness of the electrolyte layer were optimized simultaneously. After optimization, a cell with R1 of 0.4 Ωcm2, R3 of 0.7 Ωcm2, and Rh of 0.7 Ωcm2 was fabricated, so that the total series resistance of the cell was successfully decreased from 2.7 Ωcm2 to 1.8 Ωcm2.

Fig. 5 Current-voltage characteristics of the DSCs.

2.4 Highly efficient dye-sensitized solar cells The cell performance of DSCs is optimized by control of JSC, VOC and FF as discussed above. In order to correctly evaluate the energy conversion efficiency [32, 33], the I-V characteristics of the cells were also measured at the AIST public test center in Japan. Energy conversion efficiency of 10.4% at 1.004 cm2 and 11.1% at cell size of 0.219 cm2 were finally achieved as shown in Fig. 5.


In order to achieve a further large improvement in energy conversion efficiency, an understanding of the mechanism of DSCs must be fostered by analyzing charge transfer mechanisms and making every effort to improve JSC, VOC and FF as discussed before. The successful completion of future tasks — which include the development of a new sensitizer with an expanded wavelength region, suppression of recombination current in cells, the development of new redox species with more positive redox potential than I-/I3-, the development of counter electrodes with very high RF, the development of a new electrolyte solvent with low viscosity, and the development of new TCO substrates with extremely low sheet resistance and high transmittance — will bring the energy conversion efficiency of DSCs up to the level of silicon solar cells.

3. CONCLUSIONS We successfully improved the energy conversion efficiency of DSCs through a strategy of improving the three parameters of JSC, VOC, and FF. The haze factor of TiO2 electrodes was found to be a useful index of increase in JSC. It was also found that THF molecules could improve VOC without loss of JSC, and that FF was optimized by minimization of internal series resistance, that is, enlargement of the surface area of the counter electrodes, increase in the thinness of the electrolyte layer, and optimization of the sheet resistance of TCO. As a result, the highest efficiency values of 11.1% and 10.4% (at aperture illumination areas of 0.219cm2 and1.004cm2, respectively) were achieved.

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