Reduced charge recombination by the formation of an interlayer using a novel dendron coadsorbent in solid-state dye-sensitized solar cells

Abstract

3,4,5-Tris(dodecyloxy)benzoic acid (DOBA) and the Z907 dye were coadsorbed to form a light-sensitizing monolayer in a solid-state dye-sensitized solar cell (sDSC). Coadsorption of DOBA which has three hydrocarbon chains permitted preparation of a denser monolayer of dyes and DOBA. This dense monolayer formed interlayer between TiO₂ and Spiro-OMeTAD (hole conductor), effectively preventing charge recombination, while half of the photocurrent was dissipated via recombination reaction when Z907 solely anchored on the surface of TiO₂. Moreover, the DOBA induced a lower population of density-of-state (DOS) in the surface of TiO₂, shifting the position of the conduction band (CB) toward negative values. This resulted in higher open-circuit voltage (V_OC) for the device made with Z907 and DOBA than that of the Z907-sensitized device. These surface properties were investigated using electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent/photovoltage spectroscopy (IMPS and IMVS).

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Introduction

Dye-sensitized solar cells (DSC) may present alternatives to conventional silicon solar cells for the production of electricity from solar light due to their reliable efficiencies and low production costs. The conversion efficiency is enabled through the use of a mesoporous nanocrystalline oxide, which provides an extremely high surface area for the adsorption of light-absorbing sensitizers.¹ Liquid electrolyte-based DSCs have achieved efficiencies exceeding 11%.^2,3 Typical DSC structures are composed of a mesoporous wide band gap oxide sensitized by light-absorbing dyes, along with a liquid electrolyte and a redox species. In principle, photo-generated electrons are injected from the excited dye into the conduction band of the TiO₂ after light absorption. The injected electron then diffuses to the transparent conducting anode where the oxidized dye is regenerated by donation of electrons from I⁻ in the electrolyte. The resulting oxidized form of I⁻, I₃⁻, is then regenerated at the counter electrode.

The potential for leakage of the volatile electrolyte or corrosion by the toxic iodine have hindered the commercial applications of DSCs.⁴ Solid-state organic hole transport materials (HTMs), rather than liquid electrolytes, have been used to create fully electronic solar cells.^5–7 Spiro-OMeTAD (2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spiro-bifluorene) is one of the most widely-used HTMs due to its respectable charge carrier mobility,⁸ amorphous nature, and high solubility, which enable penetration of the material into mesoporous titania films a few micrometres thick.⁹ Iodine-free solid-state dye-sensitized solar cells (sDSCs) that incorporate spiro-OMeTAD as the HTM exhibit power conversion efficiencies (PCEs) in the range of 2%–6%, depending on the optimal fabrication conditions in a given laboratory.^10–17 This PCE is far below that of liquid electrolyte-based DSCs because the optimal thickness of a nanocrystalline TiO₂ layer, less than 2 μm¹⁸ (cf. ∼16 μm for liquid electrolyte DSCs)¹⁹ prevents most of the light incident on the absorbing region of the composite from being absorbed by the active medium.

These thin films incur performance limitations, such as fast recombination between the photo-injected electrons and the holes in the spiro-OMeTAD. Studies of charge recombination have revealed that recombination in sDSCs is two orders of magnitude faster than in liquid DSCs²⁰ because injected electrons recombine with the cationic HTM⁺, unlike anionic I₃⁻ in a liquid electrolyte. Several strategies have been proposed for retarding the recombination rate at the TiO₂/dye/HTM heterojunction. One successful approach, based on molecular engineering, is to capture Li cations near the TiO₂ surface or polymer matrix in the HTM by introducing ion-coordinating moieties into the framework of the dye^17,21 or HTM.^22,23 Because the addition of lithium salts to an HTM successfully retards recombination between electrons in the TiO₂ and holes in the HTM due to Coulombic screening of both electrons and holes,²⁴ this approach enhances the photocurrent and the solar cell performance. In addition, modifications to the dye structure upon introduction of hydrophobic alkyl chains can impose potential barriers (K68) to recombination and enhance sDSC performance by reducing charge recombination.¹⁷ Another approach to retarding recombination is to passivate the TiO₂ surface by coating with an inorganic oxide^25,26 or co-grafting with organic small molecules.²⁷ Li et al. reported that atomic layer deposition (ALD) of ZrO₂ onto the TiO₂ surface effectively passivated the surface trap states, which significantly enhanced the PCEs.²⁵ A more convenient approach to passivation has been to co-graft dyes with amphiphilic organic compounds that contain carboxylic or phosphonic acid end groups. Several molecules, such as chenodeoxycholic acid (CDCA)²⁸ or dineohexyl phosphinic acid (DINHOP),²⁹ have been used as coadsorbents in liquid electrolyte-based DSCs. 4-Guanidinobutyric acid (4-GBA) dramatically improves the properties of a sDSC by increasing both J_SC and V_OC.²⁷ The mechanism underlying the reduced charge recombination upon coadsorption of the organic compounds and dyes stems from the highly compact monolayer that forms during the coadsorption process.²⁷ The insulating molecular layer effectively shields recombination processes by blocking holes from accessing recombination centers on the TiO₂ surface. In addition, amphiphilic compounds can modulate V_OC by influencing the position of the conduction band in TiO₂ relative to the oxidation potential of the HTM, either positively or negatively, depending on the dipole moment of the coadsorbent.

Given this background, it is apparent that control over the interfacial properties of a TiO₂/dye/HTM heterojunction is crucial for realizing efficient solar cells. We report herein one method for reducing charge recombination. We designed and synthesized dendron-based coadsorbents having three hydrocarbon chains (DOBA) to take advantage of both the dye functionalization and coadsorbent approaches. Three dodecyloxy chains were introduced at positions 3, 4, and 5 with respect to the carboxylic groups on the aromatic ring. The three hydrocarbon chains per molecule were sufficient to maximize the passivation effects without incurring significant loss in the light harvesting properties by occupying sensitizer adsorption sites: Ru(4,4′-dicarboxylic acid-2,2′-bypiridine)(4,4′-dinonyl-2,2′-bipyridine) (NCS)₂ (coded as Z907). The chains were expected to form an interlayer between the surface of TiO₂ and HTM layer. Z907 includes two alkyl chains that help reduce the recombination reactions, further enlarging the potential barrier of the compound.

The interfacial properties and charge recombination were characterized by electrochemical impedance spectroscopy (EIS) and intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements. The position of the TiO₂ conduction band was estimated by cyclic voltammetry, revealing that DOBA effectively passivated the TiO₂ surface and shifted the TiO₂ conduction band toward negative values, thereby enhancing J_SC and V_OC.

Experimental section

Synthesis of coadsorbent

All solvents and reagents, unless otherwise stated, were of analytical quality and used as received. The synthesis of coadsorbent 3,4,5-tris-dodecyloxy-benzoic acid (DOBA) is illustrated in Fig. 1.

Fig. 1 Synthesis of the DOBA. Reagents and conditions: a) 1-bromododecane/compound 1, potassium carbonate (K₂CO₃), potassium iodide (KI), butanone, reflux, b) DOBA; tetrahydrofuran (THF), MeOH, potassium hydroxide (KOH) in water, refluxed for 3 h.

Synthesis of methyl 3,4,5-tris(dodecyloxy)benzoate (2)³⁰. 1-Bromo-dodecane (5.0 g, 20 mmol) was added to a suspension of methyl 3,4,5-tris(hydroxy)benzoate (1.0 g, 5.5 mmol), K₂CO₃ (14 g, 80 mmol), KI (0.4 g, 2.4 mmol) and 4 Å molecular sieves in dry butanone (100 mL), and the mixture was heated under reflux for 3 days. The reaction mixture was filtered and the butanone distilled off. The crude product was purified by column chromatography (SiO₂, CH₂Cl₂/hexane = 8/2) yielded white powder (3.7 g, 5.3 mmol, 97%). ¹H–NMR (400 MHz, CDCl₃): δ (ppm) = 0.88 (t, 9 H, CH₂–CH₃), 1.29 (m, 54 H, –(CH₂)₉–CH₃), 1.79 (m, 6 H, CH₂–CH₂–CH₂–O), 3.88 (s, 3 H, COO–CH₃), 4.00 (m, 6 H, CH₂–O), 7.24 (s, 2 H, CH_ar).

Synthesis of 3,4,5-tris(dodecyloxy)benzoic acid (DOBA)³⁰. Compound 2 (2.6 g, 3.8 mmol) was dissolved in THF (8 mL) and methanol (50 mL). A solution of KOH (1.8 g, 30 mmol) in water (10 mL) was then added dropwise to the mixture with stirring. The mixture was stirred at room temperature. overnight, followed by reflux for 4 h. The solvents were evaporated and ice/water (30 mL) was added to the residue. The mixture was acidified with HCl conc. until pH = 3 and the product was filtered off, washed with water and then with hexane (2.5 g, 3.7 mmol, 98%). ¹H–NMR (400 MHz, CDCl₃): δ (ppm) = 0.87 (t, 9 H, CH₂–CH₃), 1.29 (m, 54 H, –(CH₂)₉–CH₃), 1.75 (m, 6 H, CH₂–CH₂–CH₂–O), 3.98 (m, 6 H, CH₂–O), 7.27 (s, 2 H, CH_ar).

Device preparation

Two kinds of sDSCs made from the TiO₂, which stained by Z907 (device-A) and by Z907 together with DOBA (device-B) were prepared. The TiO₂ films were deposited on FTO substrate (15 Ωcm⁻¹, Pilkington) patterned through etching with zinc powder and HCl (4 N) to give the required electrode. The FTO was pre-treated with a 20 mM aqueous TiCl₄ at 60 °C for 6 h. Samples used for efficiency measurements were coated with a compact layer of TiO₂ (100 nm) by aerosol spray pyrolysis deposition at 450 °C using oxygen as the carrier gas.³¹ Mesoporous TiO₂ films were fabricated by doctor-blading TiO₂ paste from Solaronix (T20/SP series). These sheets were then slowly heated to 500 °C (ramped over 30 min) and baked at this temperature for 30 min under an oxygen flow. The resulting thickness of TiO₂ film, measured by surface profiler (α-step), was ca. 2 μm. Acidic TiCl₄ treatment was repeated before slowly reheating (ramped over 30 min) to 450 °C and baking at this temperature for 30 min with subsequent cooling to 80 °C. The mesoporous TiO₂ electrodes were placed for 18 h in dye solutions consisted of 3 mM Z907 (denoted to electrode-A) and a mixture of 3 mM Z907 and various concentrations (3 mM, 9 mM, and 15 mM) of DOBA (denoted to a series of electrode-B) in acetonitrile and tert-butyl alcohol (1 : 1 v/v). The dye coated mesoporous films were briefly rinsed in acetonitrile and dried in air for one minute. Spiro-OMeTAD (home-made according to the literature procedure)³² was completely dissolved in chlorobenzene by warming and stirring the solution at 70 °C for 30 min. Following the literature,²⁷tert-butylpyridine (tBP) (0.11 M) and lithiumbis-(trifluoromethyl-sulfonyl)imide (Li-TFSI) (21 mM) were added to the solution as additives, while Li-TFSI was pre-dissolved in acetonitrile at 0.65 M as stock solution, then diluted to the HTM solution to give final concentration. A small quantity (10–70 μL) of the spiro-OMeTAD solution was deposited onto each TiO₂ film at room temperature and left for 1 min before spin coating at 2000 RPM for 30 s. After spin coating, the film was dried overnight at room temperature in air. Finally, a 100 nm gold layer was evaporated on the top of the electrode-A and -B coated with the spiro-OMeTAD under ultrahigh vacuum (−10⁻⁶ torr), giving device-A and -B, respectively. Illustrative models of the two kinds of electrodes are shown in Fig. 2.

Fig. 2 Illustration of TiO₂ surface sensitized with Z907 (device-A), and Z907 + DOBA (device-B).

Results and discussion

The extent of dye loading was examined by UV-Vis spectroscopy. The absorbances of electrode-A and electrode-B (1 : 1 to 1 : 5 ratio of Z907 : DOBA) are shown in Fig. 3. The intensity was ca. 89% at a 1 : 1 molar ratio of DOBA to Z907, indicating that the reactivity of the carboxylic acid in Z907, which anchored the compound onto the TiO₂ surface, was higher than the reactivity of DOBA. The relative quantity of absorbed Z907 gradually decreased as the ratio of DOBA increased. This intensity reached 50% at a five-fold higher concentration (a molar ratio of 1 : 5). These results suggested that DOBA effectively retarded the rate of dye adsorption due to the highly competitive anchoring process between the dye and the coadsorbent, as proposed previously.³⁴ Therefore, Z907 was expected to be well-dispersed on the TiO₂ surface, and the remaining empty sites were expected to be occupied by DOBA molecules.

Fig. 3 (a) Absorbance spectra of electrode-A (Z907-coated TiO₂, black line) and three electrodes (Z907+DOBA-coated TiO₂) at various concentrations (1 : 1 Z907 : DOBA (—), 1 : 3 ([dash dash, graph caption]), and 1 : 5 ([dash dot, graph caption])).

The relationship between the Z907 (or DOBA) content and the photovoltaic performances of device-A and devices-B prepared from the corresponding electrodes were investigated. A representative J–V curve is shown in Fig. 4. Device-A, employing only Z907, showed a J_SC of 4.5 mA cm⁻², a V_OC of 684 mV, and a FF of 0.59, corresponding to an overall efficiency of 1.81%. Although we obtained a slightly lower value for the control device (device-A), these results were consistent with those of others, who reported a range of 1–3%^27,33 under identical conditions. A low light harvesting efficiency (LHE) presents a major limitation to device performance. The LHE for sDSCs employing a thin TiO₂ electrode is related to the light absorbance according to LHE = 1 − 10^−A. Thus, it is possible that the low amounts of Z907 in electrodes-B reduced the performance of the electrode compared to electrode-A.

Fig. 4 J–V Characteristics of the devices used in this study: device A, sensitized with Z907 (black line); device B, co-sensitized with Z907/DOBA (red line) at various ratios (1 : 1 (red line), 1 : 3 (green line), and 1 : 5 (blue line)). Masked active area of 0.16 cm². Measured at the full-intensity sunlight irradiation using a 450 W Xenon lamp light source filtered to AM 1.5 conditions.

The PCEs of device-B, however, were higher than those of device-A at all ratios, despite the lower degree of dye loading. The highest efficiency was achieved in device-B prepared from a solution containing a 1 : 1 molar ratio of Z907 : DOBA. J_SC and V_OC both increased, resulting in a 2.13% PCE, which was 18% higher than that of device-A. The efficiency gradually decreased as the ratio increased, as seen in device-B employing a 1 : 5 ratio of Z907 : DOBA. The J_SC of device-B (1 : 5) was 0.1 mA cm⁻² higher than in device-A, even though the amount of dye was half that of device-A, indicating that light harvesting by the sensitizers may not be the limiting factor in device performance. As shown in Fig. 5, the harvesting ability of the dyes, which could be obtained by dividing the PCE (or J_SC) by the dye loading, greatly increased with increasing DOBA content. The enhanced performance of device-B was due to several effects. In principle, the photocurrent is governed by light harvesting efficiency, electron injection yield, and charge collection efficiency as the following equation: J_SC = η_LHE × η_inj × η_coll. The light harvesting efficiency, as explained previously, is not a limiting factor for these experiments. In terms of electron injection, we reported recently that photocurrent could be enhanced by co-grafting organic molecules along with the N719 dye, which reduced dye aggregation and increased the electron injection efficiency.³⁴ However, an increase in V_OC could not be rationalized from a lack of dye aggregation in this case. Furthermore, the increase in V_OC which is not only an indirect indication of the position of conduction band of TiO₂ in a negative way but also the reduction of driving forces for electron injection (ΔG_inj), does not follow the trend in J_SC. A more plausible explanation is that charge recombination was retarded more effectively in device-B than in device-A. The organic molecules probably acted as an efficient passivation layer, thus formed an interlayer between the surface of TiO₂ and HTM layer, by imposing a potential barrier to recombination through the increased hydrophobicity of the TiO₂ surface. In other words, 51% of photoinduced electrons are dissipated by charge recombination when only Z907 is anchored on the surface of TiO₂ compared to Z907 and DOBA (1 : 5) anchored together. The loss of photocurrent by charge recombination gradually decreased from 41% and 33% by increasing the molar ratio of Z907 : DOBA from 1 : 1 to 1 : 3, respectively. This result suggested that the dense monolayer formed by DOBA having three hydrophobic alkyl chains and Z907 induced an interlayer between the surface of TiO₂ and Spiro-OMeTAD (HTM) layer, effectively preventing charge recombination between photoinduced electrons and cationic HTMs.

Fig. 5 PCE/[dye] (black column) and J_SC/dye (red column) for varying [Z907] : [DOBA] ratios.

The effects of DOBA on the interfacial properties were investigated by measuring the surface properties of electrode-A and electrode-B. Water contact angle (Θ_water) measurements revealed the surface properties of TiO₂. Θ_water for the Z907-sensitized TiO₂ (electrode-A) was 120° at 25 °C (Fig. 6(b)), which was about 108° larger than Θ_water for bare TiO₂ (Fig. 6(a)). This result indicated that the two alkyl chains in Z907 conveyed hydrophobic properties to the surface. In contrast, electrode-B yielded a Θ_water of 132° (Fig. 6(c)), even though the Z907 content was 11% lower than on electrode-A, as shown in Fig. 3. These results indicated that coadsorption of Z907 with DOBA formed a monolayer that was denser than that formed by Z907 alone, thereby enhancing the hydrophobicity of the electrode-B surface. This result was ascribed to the number of alkyl groups and chain lengths (e.g., three dodecyl groups in DOBAvs. two octyl groups in Z907). The enhanced hydrophobicity of the TiO₂ surface provided fewer opportunities for recombination between the photo-injected electrons and the holes. Furthermore, a spin-coating chlorobenzene solution spread easily and flowed through the three-dimensional porous network area of the electrode-B, thereby facilitating pore-filling.³⁵ The wetting characteristics of the sensitized TiO₂ film surfaces were useful indicators of device performance because they reflected the dye-sensitized electrode/HTM interfacial properties.

Fig. 6 Water contact angles on a screen-printed TiO₂ surface: (a) on a bare TiO₂ surface (12°), (b) on a TiO₂ surface sensitized with Z907 (120°), and (c) on a TiO₂ surface sensitized with Z907 and DOBA (132°).

EIS measurements were conducted to improve our understanding of the effects of the hydrophobic DOBA coadsorbent on the device performance. The charge transfer rates at the interfaces of the dye-sensitized TiO₂/HTM or the HTM/counter electrode (CE) were obtained from the EIS measurements.^16,25 Recently, Bisquert et al. proposed a physical model of the complex charge transfer and transport processes occurring in DSCs.³⁶Fig. 7(a), which shows the Nyquist plots of each device under a forward bias of −0.65 V, serves as an illustrative example of this conceptual model. Two distinct semicircles were observed in the Nyquist plot based on EIS measurements. The first semicircle, in the high-frequency range of the Nyquist plot, was related to the charge exchange processes at the HTM (spiro-MeOTAD)/CE (Au) interface. The low-frequency semicircle was related to recombination between electrons in the TiO₂ conduction band and holes in the HTM at the TiO₂/spiro-OMeTAD interface. The electron transport resistance through the TiO₂ network (R_trans) was not measured in this measurement. A transmission line model was fit to the impedance data to obtain the recombination resistance (R_ct) and the electron lifetime (τ_e). As shown in Fig. 7(b) and 7(c), the recombination resistance of device-B (1 : 1) is one order of magnitude larger than that of device-A over all ranges under a −0.4 V to −0.7 V forward bias. The electron lifetimes were longer in device-B (1 : 1) than in device-A, indicating the extent of DOBA surface passivation due to the interlayer formed by DOBA.

Fig. 7 (a) Nyquist plots for the solid state DSCs device-A (black line) and device-B (1 : 1) (red line), under a forward bias of −0.65 V under dark conditions. The solid line corresponds to the values derived using the fitting model. (b) The recombination resistance (R_ct) as a function of the applied bias voltage obtained from impedance measurements in the dark for device-A (black line) and device-B (1 : 1) (red line). (c) Estimated electron lifetimes (τ_e) for device-A (black line) and device-B (1 : 1) (red line) using the equation τ_e = 1/2πf, where the f is the frequency at the maximum in the Nyquist plot.

The interfacial characteristics of DSCs were conveniently assessed using optical impedance techniques, such as IMPS/IMVS.³⁷ The IMPS provided information about the electron transit (transport) time (τ_trans) through the nanocrystalline TiO₂ to the photoanode under short-circuit conditions, and the IMVS measured the recombination lifetime (τ_e) of the photo-injected electrons, which recombined with the holes in the HTM or oxidized dyes under open-circuit conditions. Fig. 8 shows the results of the IMPS and IMVS measurements at various light intensities. The transport times (τ_trans) in the two devices were nearly identical, which was not surprising because the electrodes were fabricated under identical conditions, including TiO₂ deposition and film thickness. The electron lifetime (τ_e) of device-B (1 : 1), however, was longer than that of device-A, which was consistent with the results of the EIS measurements. A longer τ_e suggests a lower chance of successful charge recombination, thereby reducing loss and enhancing the photocurrent and photovoltage.

Fig. 8 Electron transport times (τ_trans, solid line) and electron lifetimes (τ_e, dashed line) for device-A (black) and device-B (1 : 1) (red), from IMPS/IMVS measurements.

Apart from the higher photocurrents generated by device-B, V_OC was 60 mV larger than that of device-A. The changes in V_OC and the trapping states induced by the presence of DOBA were investigated by cyclic voltammetry. Experiments were performed on each TiO₂electrode-A and electrode-B (1 : 1) in the ionic liquid 1-butyl-3-methylimidazolium bromide (BMIB). The capacitive currents of the electrodes revealed gradual onsets under a forward bias. As shown in Fig. 9, the onset was around −0.38 V (vs. Ag/AgCl) for electrode-A, whereas electrode-B (1 : 1) displayed an onset at −0.49 V, indicating a negative shift in the conduction band of the TiO₂. Furthermore, the total injected charge (Q) of electrode-B (1 : 1) increased more slowly than in electrode-A due to better surface passivation by DOBA.

Fig. 9 (a) Cyclic voltammograms for electrode-A (black line) and electrode-B (1 : 1) (red line), and (b) energy levels in the mesoscopic TiO₂.

Conclusion

In the present study, we successful introduced an interlayer between HTM layer and the surface of TiO₂ using DOBA having three hydrophobic alkyl chains. This interlayer effectively prevented charge recombination between photoinduced electrons and cationic HTMs, resulting in efficient solid-state dye-sensitized solar cells. The hydrophobic dendron-based organic molecules, DOBA, expedited not only the formation of compact monolayer on the surface of TiO₂ but also reduced the chances for electrons in TiO₂ recombining with holes in spiro-OMeTAD. As a result, 51% of losses in photocurrent by charge recombination for Z907-sensitized solar cells were prevented by the interlayer. Furthermore, DOBA induced the negative shift of the conduction band of TiO₂, resulting in higher open circuit voltage.

Acknowledgements

This work was supported by a grant (M2009010025) from the Fundamental R&D Program for Core Technology of Materials, funded by the MKE, by the Mid-career Researcher Program and Future-based Technology Development Program (Nano Fields) through NRF, funded by the MEST (No. 2010-0000406, No. 2010-0019119), and by the second stage of a BK21 (Brain Korea 21) grant.

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Footnote

† Present address: Department of Inorganic Chemistry, School of Chemistry, Madurai Kamaraj University, Palkalai Nagar, Madurai 625 021, Tamilnadu, India.

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