Increasing the performance of cis-dithiocyanato(4,4′-dicarboxy-2,2′-bipyridine)(1,10-phenanthroline) ruthenium (II) based DSC using citric acid as co-adsorbant

Abstract

We describe the use of citric acid as co-adsorbent in Dye Solar Cells (DSC). As we demonstrate herein, it is possible to increase the photocurrent of DSC without loosing photovoltage when we employed cis-dithiocyanato(4,4′-dicarboxy-2,2′-bipyridine)(1,10-phenanthroline) ruthenium (II) (AR20). Moreover, we show that the dye molecular structure also plays a major role due to its interaction with the sensitizer.

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Broader context

The non-covalent interactions between the different chemical species present in a dye solar cell are not yet well understood. In this communication, we show how the change in one of the ligands coordinated to the ruthenium metal centre can induce, in the presence of citric acid as co-adsorbent, an important increase in the light-to-energy power conversion efficiency.

The basic research to improve DSC stability power conversion efficiency stems from the need to reach commercial applications in the next five to six years. Although there is not yet a clear niche application for DSC it is widely accepted that DSC represents a low-cost alternative to traditional single gap semiconductor crystalline devices.^1–5

The research strategies used to increase DSC efficiency have traditionally focused on the composition,^6,7 size^8–10 and shape^11–13 of the semiconductor nanoparticles, the adsorption of dyes with broad absorption band and high molar extinction coefficient^14–17 in the Visible–Near IR of the solar spectrum, the composition of the electrolyte,^18–22 and the use of coadsorbents.^23–25 Regarding the latter strategy, several kinds of additives such as deoxycholic acid,²⁶hexadecylmalonic acid,²⁷guanidinium thiocyanante,²⁸tert-butylpyridine²⁹ have been added to dye sensitized solar cells in order to improve the short-circuit photocurrent (J_SC) or the open circuit photovoltage (V_OC) of the solar cells. In this communication we have focused on the study of a new co-adsorbent and its effect on the DSC using two different ruthenium dyes (cis-dithiocyanato(4,4′-dicarboxy-2,2′-bipyridine)(1,10-phenanthroline) ruthenium (II) (AR20) and cis-bis(isothiocyanato) bis-(4,4′-dicarboxylato-2,2′-bipyridine) bis-(tetrabutylammonium) ruthenium (II), also known as N719 (Fig. 1).†The AR20dye was chosen due to the structural differences from the dye paradigm in DSC, the N719 sensitizer. The more conjugated ligand in AR20, a phenanthroline, gives AR20 different molecular electronic properties compared to N719 and it also interacts with the electrolyte differently, as we have recently demonstrated.³⁰

Fig. 1 Molecular structure of the molecules AR20, N719 and citric acid.

Citric acid is a tri-protic weak organic acid with a pK_a of 6.4. The presence of three carboxylate groups on its molecular structure ensures anchoring of this acid onto the TiO₂ surface, and it can act as a buffer on the surface of the semiconductor nanoparticles.

The UV–Visible absorption spectra of ruthenium-sensitized 4 µm TiO₂ films were measured. The films were immersed into a 300 µM solution of either AR20 or N719 sensitizers containing different concentrations of citric acid in acetonitrile/tert-butanol (1 : 1) solution. Firstly, it is important to note that a decrease of 20% and 30% of the total dye adsorption is observed for AR20 and of 40% and 60% in the case of N719, when we use citric acid concentrations of 150 and 300 µM, respectively. Secondly, in the N719dye, the maximum absorbance wavelength when adsorbed onto a TiO₂ film was red-shifted when citric acid was present.

This batochromic shift of 9 nm may be due to the interaction between the dye and the citric acid due to a change on the TiO₂ surface pH. Interestingly, the absorption spectrum of the AR20 sensitizer is not shifted. Different groups have already reported that the treatment of the TiO₂ surface with different organic molecules can cause shifts in the conduction band edge, by positively or negatively charging the TiO₂ surface and, thus, it is possible to increase or decrease the device photocurrent, respectively.^31,32 Hence, to further prove this hypothesis, we carried out photo-induced charge extraction measurements to register the shift of the semiconductor band edge.

The DSCs were prepared using 8 µm thick TiO₂ films (20 nm particle diameter) plus 4 µm TiO₂ scatter layer (400 nm particle diameter). The electrolyte composition used was, 1 M BMII (1-butyl-3-methylimidazolium iodide), 0.03 M I₂, 0.05 M LiI, 0.1 M guanidinium thiocyanate (GuNCS) and 0.5 M tert-butylpyridine in acetonitrile (MeCN) : valeronitrile (VCN) 85 : 15. Fig. 3 illustrates the measured experimental points at different light bias for the two different devices.

Fig. 2 The absorption spectra of N719/TiO₂2 and AR20/TiO₂2 (film thickness 4 µm) at different concentrations of citric acid as co-adsorbent.

Fig. 3 Charge extraction measurements for AR20 and N719dye solar cells at different citric acid concentrations.

As we see, at equal photo-induced charge density, the photovoltage of the cell with the dye and the co-adsorbed citric acid (300 µM) is about 60 mV and 70 mV lower for AR20 and N719, respectively. These results indicate that the addition of citric acid (300 µM) shifts the conduction band edge downwards in both cases. Thus, an increase in short circuit photocurrent (I_SC) or a decrease in open circuit voltage (V_OC) must be observed.

As listed in Table 1, DSC based on the sensitization of TiO₂ using AR20 shows a high increase in power conversion efficiency but, this is not only due to the increase in photocurrent but also in photovoltage and fill factor. This was unexpected since as mentioned above, and based on Fig. 3, we expected an increase only in photocurrent. Moreover, taking the N719 as a blank (standard DSC using N719 gave us above 7.5% power conversion efficiencies under standard measurement conditions) we observed an important decrease in device efficiency due to the decrease in photocurrent, photovoltage and fill factor. Therefore, the presence of citric acid not only affects/interacts with the TiO₂ and shifts the conduction band edge position but also interacts with the anchored dye.

Table 1 Photovoltaic characteristics of DSC (cell area 0.196 cm²) sensitized with N719 and AR20 with different amounts of citric acid

Dye	Citric acid concentration (10^−6M)	I SC/mA	V OC/V	ff (%)	η ^a (%)
a Power conversion efficiency at 1 sun (100 mW cm^−2).
AR20	0	1.83	0.61	48.5	2.8
AR20	150	2.88	0.65	63.4	6.1
AR20	300	2.36	0.64	63.2	4.8
N719	0	3.65	0.72	57.6	7.7
N719	150	3.27	0.69	49.8	5.8
N719	300	2.54	0.64	30.3	2.5

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To shed some light on these results we carried out light-induced photovoltage transient spectroscopy, a technique that is used to measure the recombination rate between the photo-injected electrons and the oxidised electrolyte.^33–35Fig. 4 shows the measured electron lifetimes at each light bias. As can be seen, for AR20DSC the presence of citric acid induces slow recombination dynamics at each bias, including light bias equal to V_OC. On the contrary for N719DSC, although at low light intensities (low-induced V_OC) it shows similar recombination lifetimes, at light bias equal to V_OC the recombination is much faster when in the presence of citric acid.

Fig. 4 Recombination lifetime vscellV_OC for AR20 and N719DSC with different concentrations of citric acid.

The results in Fig. 4 clearly show that one of the major reasons for the increase in power conversion efficiency for AR20 based DSC is the slower recombination dynamics due to the presence of citric acid as an additive in combination with the AR20 sensitizer.

In conclusion, the results confirm that the use of citric acid in DSC sensitised with AR20 results in slower recombination dynamics and an important increase in power conversion efficiency from 2.76% to 6.08%. However, while in the case of AR20, with phenanthroline, citric acid slows the interfacial charge transfer recombination dynamics; for N719, with bipyridine bearing carboxylic acids as the ligand, it accelerates the process. Further experiments to uncover the molecular mechanism of this unprecedented interaction between citric acid and the dye are being carried out to increase the device efficiency further.

Acknowledgements

AR and EP would like to acknowledge the MICIIN for financial suport. Project CONSOLIDER-HOPE CSD-0007-2007 and CTQ-2007-60746-BQU and to the ICIQ Foundation.

Notes and references

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Footnote

† The synthesis of AR20 was carried out as previously reported in Ref. 22N719 was purchased from Solaronix.

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