Improving energy relay dyes for dye-sensitized solar cells by use of a group of uniform materials based on organic salts (GUMBOS)

Abstract

In this study, GUMBOS (a group of uniform materials based on organic salts) derived from rhodamine B chloride, 1,1′-diethyl-2,2′-carbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, and meso-tetra(4-carboxyphenyl)porphine were synthesized and characterized for application as energy relay dyes (ERDs) in dye-sensitized solar cells (DSSCs). A facile ion exchange reaction was employed for synthesis of GUMBOS. These GUMBOS exhibited improved characteristics in comparison to their respective parent dyes, including increased solubility, thermal stability, molar extinction coefficient, and fluorescence quantum yield. In addition, excellent spectral overlap integral and Förster resonance energy transfer efficiency were observed between various GUMBOS based-ERDs (donors) and the photosensitizing dye, N719 [di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)] (acceptor). DSSC devices were fabricated and solar efficiency was evaluated in the absence and presence of ERDs using N719 as the photosensitizing dye. DSSCs in the presence of GUMBOS-based ERDs exhibited increased solar efficiencies in comparison to DSSCs in the absence of ERDs. Moreover, increases in solar efficiencies were found to be dependent on the counterions used in GUMBOS synthesis.

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Introduction

With an ever increasing world population, a reliable, low-cost, and green energy source is urgently needed. Among the various types of renewable energy sources, the use of solar cells to harness energy from the sun to produce electricity has attracted tremendous interest due to high electricity production potential.¹ Dye-sensitized solar cells (DSSCs), as third generation solar cells, are known for low cost and facile manufacturing processes.^2–6 In addition, DSSCs can possess the useful characteristics of flexibility and/or transparency.^2–6 However, solar conversion efficiencies acquired with DSSCs are relatively low in comparison to other generation solar cells.

Several unique approaches have been explored to improve solar energy conversion efficiency of DSSCs to make such systems more competitive with current solar technologies. Some examples include the use of mirror-like cathodes,⁷ light scattering electrolytes,⁸ luminescent external coatings,⁹ and coating of photoanodes with polymers¹⁰ to improve the short circuit current (J_sc) which consequently enhances solar efficiency. Furthermore the use of multiple dyes has also been studied for improvement of J_sc. For example, the application of co-adsorbed photosensitizing dyes has demonstrated improved solar efficiency for DSSCs.^11–13 In this approach, two photosensitizing dyes are employed to extend absorption of light in order to cover a greater portion of the electromagnetic spectrum, thereby increasing light harvesting and consequently solar efficiency. However, in the design of photosensitizing dyes, many criteria must be considered. Some of these criteria include incorporation of a linking group, broad absorption of the electromagnetic spectrum, efficient electron injection into the semiconductor, and subsequent regeneration by a redox couple.^3–5,14

In some studies, use of a second dye that is dissolved in the electrolyte, known as an energy relay dye (ERD), has been examined.^15–23 In such an arrangement, the ERD absorbs light in a wavelength range where absorption of the photosensitizing dye is minimal. The ERD acts as a donor to the photosensitizing dye (acceptor) via use of Förster resonance energy transfer (FRET). This results in enhanced incident photon to current efficiency (IPCE) of the photosensitizer and ultimately, enhanced solar conversion efficiency.¹⁵ In addition, ERDs should possess high molar extinction coefficients and excellent FRET efficiencies with the photosensitizing dye.²⁴ Thus, significant overlap of the fluorescence emission spectrum of the ERD with the absorbance spectrum of the photosensitizing dye is one requirement for high FRET efficiency. To meet this requirement, the distance between the donor and acceptor must be very small for efficient energy transfer.²⁵ Furthermore, ERDs should not aggregate in solution as this can lead to fluorescence quenching, thus impeding FRET efficiency between the ERD and photosensitizing dye.²⁴ Hardin and coworkers were first to develop ERDs for application in DSSCs.¹⁵ In their work, a perylene derivative (PTCDI) and a zinc-phthalocyanine dye (TT1) were investigated as the ERD and photosensitizing dye, respectively. However, use and development of new ERDs have been hindered due to low solubility of ERDs in the electrolyte.^19,25,26

Our research group has previously reported on a group of uniform materials based on organic salts (GUMBOS) for tuning properties of organic salt compounds. Properties that can be tuned include, but are not limited to stability, solubility, melting point, and fluorescence quantum yield.^27–30 Since GUMBOS are composed of two oppositely charged ions, properties of these materials can be easily tailored by varying either the cation or anion. Benefits of the tunable properties of GUMBOS have been exploited in numerous studies.^28–30 In one example, the hydrophobicity of aminopyrene was tailored by varying the counteranion to develop improved matrices for hydrophobic and hydrophilic protein analysis for matrix assisted laser desorption ionization mass spectrometry (MALDI-MS).²⁸ In another example, the hydrophobicity of rhodamine 6G was tuned to develop nanoparticles which were selectively toxic to cancer cells and non-toxic to normal cells.²⁹ More recently, Siraj and coworkers have demonstrated the ability to tune the molar extinction coefficient and fluorescence quantum yield of a carbazole-based dye by employing various anions to form GUMBOS.³⁰ In particular, tuning of solubility and spectral properties of dyes through ion exchange in GUMBOS represents a unique approach to acquire more promising and extremely efficient ERDs.

In the present study, GUMBOS and their respective parent dyes are evaluated for application as ERDs in DSSCs. More specifically, rhodamine-, cyanine-, and porphyrin-based GUMBOS are examined for this application. All compounds are characterized by use of thermal gravimetric analysis, and ultraviolet-visible absorbance and fluorescence spectroscopies to evaluate their applications as possible ERDs for DSSCs. Furthermore, spectral overlap integral and energy transfer efficiency are investigated, in solution, between various GUMBOS-based ERDs and the photosensitizing dye N719 [di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II)]. Finally, DSSCs are fabricated for assessment of GUMBOS-based ERDs in DSSCs and solar conversion efficiency results are evaluated in comparison to the respective parent dyes.

Materials and methods

Materials

meso-Tetra(4-carboxyphenyl)porphine was obtained from Frontier Scientific, Inc. (Logan, UT). Rhodamine B, 1,1′-diethyl-2,2′-carbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, sodium tetraphenylborate, lithium bis(trifluoromethanesulfonyl)imide, trihexyltetradecylphosphonium chloride, butylmethylimidazole iodide, iodine, guanidinium thiocyanate, tert-butylpyridine, chloroplatinic acid, and fluorine-doped tin oxide coated glass slides were purchased from Sigma Aldrich (St. Louis, MO). Di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (N719) was obtained from Solaronix (Switzerland). Dichloromethane, sodium hydroxide, and acetonitrile were purchased from Fisher Scientific (Waltham, MA), and ethanol (200 proof) from EMD (Billerica, MA). Triply deionized ultrapure water (18.2 MΩ cm) was obtained from an Aries high purity water system (West Berlin, NJ).

Synthesis of GUMBOS

GUMBOS were synthesized by use of an ion exchange reaction in a biphasic solvent system as described in previous studies.^29,31–34 Molecular structures of all compounds used for GUMBOS syntheses, are provided in Fig. 1. Rhodamine- and cyanine-based GUMBOS were synthesized by anion exchange of rhodamine B chloride ([RhB][Cl]), 1,1′-diethyl-2,2′-carbocyanine iodide ([PC][I]), or 3,3′-diethylthiacarbocyanine iodide ([TC1][I]) with lithium bis(trifluoromethanesulfonyl)imide ([Li][NTf2]), lithium bis(pentafluoroethane)sulfonamide ([Li][BETI]), or sodium tetraphenyl borate ([Na][TPB]). In this procedure, [RhB][Cl], [TC1][I], or [PC][I] was dissolved in dichloromethane and [Li][NTf2], [Li][BETI], or [Na][TPB] was dissolved in water. The organic and aqueous solutions were combined in a 1 to 1.1 mole ratio of reactants, respectively, where the ratio of water to dichloromethane was 1 to 5 (v/v). After stirring for 24 h, the organic layer was washed several times with distilled water to remove any excess lithium iodide or sodium iodide byproduct. The organic layer was then separated and solvent was removed under reduced pressure. Lastly, GUMBOS were freeze-dried to remove any traces of water. Two rhodamine-based GUMBOS were synthesized: [RhB][NTf2] and [RhB][BETI], and five cyanine-based GUMBOS were also prepared: [TC1][NTf2], [TC1][BETI], [TC1][TPB], [PC][NTf2], and [PC][BETI].

Fig. 1 Molecular structures of compounds employed in GUMBOS syntheses. Rhodamine B [RhB][Cl], 3,3′-diethylthiacarbocyanine iodide [TC1][I], 1,1′-diethyl-2,2′-carbocyanine iodide [PC][I], meso-tetra(4-carboxyphenyl)porphine [H]₄[TCPP], lithium bis(trifluoromethanesulfonyl)imide [Li][NTf2], lithium bis(pentafluoroethane)sulfonamide [Li][BETI], sodium tetraphenyl borate [Na][TPB], and trihexyltetradecyl phosphonium chloride [P66614][Cl].

Porphyrin-based GUMBOS were formed by cationic exchange of meso-tetra(4-carboxyphenyl)porphine ([H]₄[TCPP]) with trihexyltetradecyl phosphonium chloride ([P66614][Cl]).^32,34,35 The porphyrin dye was dissolved in water, with the aid of sodium hydroxide to form the sodium salt ([Na]₄[TCPP]), and [P66614][Cl] was dissolved in dichloromethane. A mole ratio of 4 to 1.1 between [P66614][Cl] and [Na]₄[TCPP] was used in a biphasic solvent system that was composed of water and dichloromethane (1 : 5, v/v). This mixture was allowed to stir for 48 h. A similar procedure was used to wash and isolate porphyrin GUMBOS from the dichloromethane layer as described for rhodamine and cyanine GUMBOS. Using this cation exchange method, GUMBOS were obtained with high percentage yields (>90%), and formation of products was confirmed by use of electrospray ionization mass spectrometry (Table 1). Melting points of GUMBOS and reaction schemes used to form GUMBOS are provided in the ESI.†

Table 1 Electrospray ionization mass spectral peaks of GUMBOS in positive and negative ion modes

	Positive		Negative
	Actual	Expected	Actual	Expected
[RhB][NTf2]	443.2468	443.23	279.9179	279.92
[RhB][BETI]	443.2449	443.23	379.9116	379.91
[TC1][NTf2]	365.1144	365.11	279.9178	279.92
[TC1][BETI]	365.1146	365.11	379.9115	379.91
[TC1][TPB]	365.1140	365.11	319.1655	319.17
[PC][NTf2]	355.2061	355.22	279.9162	279.92
[PC][BETI]	355.2069	355.22	379.9104	379.91
[P66614]4[TCPP]	483.5046	483.51	788.1980	788.19

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Characterization of GUMBOS

Thermal stabilities of the parent dyes and GUMBOS were evaluated using thermogravimetric analysis (TGA) measurements, which were collected using a TA instruments 2950 TGA HR V6.1A (New Castle, DE). In this measurement, the change in mass was examined from room temperature to 600 °C using a ramp rate of 10 °C per min.

Spectroscopic properties of GUMBOS were studied by use of a Shimadzu UV-3101PC UV-Vis scanning spectrometer (Shimadzu, Columbia, MD). Fluorescence emission spectra were obtained using a Spex Fluorolog-3 spectrofluorimeter (model FL3-22TAU3; Jobin Yvon, Edison, NJ). Photostability was measured by recording the fluorescence intensities that were acquired every 0.1 s over a time period of 3000 s using fluorescence kinetics measurements. The initial fluorescence intensity is given by I₀ and the fluorescence intensity at different times is represented by I. Excitation and emission slit widths of 14 nm (maximum) were used for photostability experiments. A ratio of fluorescence intensities (I/I₀) is plotted to examine the photodegradation rate upon exposure of light. All spectroscopic measurements were collected using quartz cuvets (Starna Cells, Atascadero, CA) at room temperature (20 °C).

Fabrication and evaluation of DSSCs

DSSCs were assembled using fluorine-doped tin oxide (FTO) coated glass slides as the supporting substrate for the counter electrodes. These electrodes were prepared by drop casting a solution of 5 mM chloroplatinic acid in isopropanol onto FTO glass slides.³⁶ These electrodes were then sintered at 400 °C for 20 min. Working electrodes were prepared by soaking a titanium dioxide nanoparticle coated FTO glass (MS001610, Dyesol, Queanbeyan, Australia) in a dye solution of 0.2 M N719 in ethanol for sensitization. After 48 h of immersion, working electrodes were removed and rinsed with ethanol to minimize excess dye. For assembly of DSSCs, the working electrode was sandwiched together with the counter electrode using a 100 μm thick hot-melt sealing film as a spacer (Meltonix 1170-100, Solaronix). DSSCs were sealed by applying heat and pressure with a hot press at 110 °C. An electrolyte solution consisting of 0.60 M butylmethylimidazolium iodide, 0.03 M iodine, 0.10 M guanidinium thiocyanate, 0.50 M tert-butylpyridine was prepared in acetonitrile.³⁷ In experiments with ERDs, the ERD was dissolved in a separate solution of electrolyte to achieve a final concentration of 1 mM. DSSCs were filled with electrolyte solution through two holes that were drilled into each counter electrode. Current–voltage characteristics of solar cells were measured with a Keithly 2400 source meter (Cleveland, OH). A solar light simulator (Model: 67005, Oriel, Stratford, CT) was used to simulate sunlight under one sun AM 1.5 G (100 mW cm⁻²) illumination provided by a 150 W xenon arc lamp (Model: 6256, Oriel, Stratford, CT). Devices were fabricated and tested in triplicate using an active area of 0.25 cm².

Results and discussion

Thermal stability studies

Thermal stabilities of parent dyes and GUMBOS were evaluated using thermogravimetric analysis. In this method, thermal stability was determined by scanning the temperature from 25 to 600 °C. An onset temperature of decomposition (T_onset) was determined for each parent dye and GUMBOS, and values of T_onset for all compounds are reported in Table 2. Values of T_onset for all parent dyes were found to be greater than or equal to 270 °C, with the lowest degradation temperature reported for TC1 and the highest for TCPP. Upon conversion to GUMBOS, there was a significant increase in thermal stability for all RhB- and PC-based GUMBOS (i.e. [RhB][NTf2], [RhB][BETI], [PC][NTf2], and [PC][BETI]). Escalation in thermal decomposition temperature of 40 °C and greater was observed for all RhB- and PC-based GUMBOS. This observation is consistent with previous studies reported by our research group where significant improvement in thermal stability was noted upon conversion of a dye into GUMBOS.³⁰ Anion dependent thermal stability of ionic liquids has also been reported by Lee and coworkers.³⁸ Additionally, T_onset values for PC-based GUMBOS determined in this study are in good agreement with our previous study.³¹ For TC1-based compounds, T_onset increased upon conversion of [TC1][I] to [TC1][NTf2] and [TC1][BETI]. However, thermal stability remained relatively constant when converting to [TC1][TPB]. For TCPP-based compounds, decreased thermal stability was observed upon conversion of [H]₄[TCPP] into [P66614]₄[TCPP]. This decrease in thermal stability was attributed to introduction of the P66614 cation since [P66614][Cl] also has a similar T_onset as [P66614]₄[TCPP]. The ability to tune thermal stability of compounds through use of GUMBOS is interesting not only for application in DSSCs, but also in other fields where thermal stability is important, e.g. in organic light emitting diodes (OLEDs).³⁰

Table 2 Onset temperature (T_onset) of degradation of GUMBOS and respective parent dyes. Values were determined using thermogravimetric analysis

Compound	Tonset (°C)
[RhB][Cl]	312
[RhB][NTf2]	356
[RhB][BETI]	360
[TC1][I]	270
[TC1][NTf2]	364
[TC1][BETI]	364
[TC1][TPB]	265
[PC][I]	300
[PC][NTf2]	374
[PC][BETI]	370
[H]4[TCPP]	423
[P66614]4[TCPP]	340
[Li][NTf2]	390
[Li][BETI]	363
[Na][TPB]	393
[P66614][Cl]	360

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One beneficial property of GUMBOS that can be exploited to improve performance of ERDs is the ability to tune the solubility of these compounds. As discussed previously, solubility of a compound can be easily tuned by simply varying the counterion in the desired GUMBOS.^28,29,39 In this regard, solubilities of all synthesized GUMBOS were tested in acetonitrile, which is also used as the primary solvent in the electrolyte of DSSCs. TC1-, PC-, and TCPP-based GUMBOS exhibited increased solubility in comparison to the respective parent dyes. In particular, NTf2-and BETI-containing counteranions of TC1 and PC displayed higher solubility in acetonitrile than the respective parent dyes. However, [TC1][TPB] demonstrated similar solubility as the parent dye. Likewise, no significant change in solubility was observed with RhB-based GUMBOS with respect to the parent compound. Similar to cyanine-based GUMBOS, [P66614]₄[TCPP] also exhibited increased solubility in comparison to the parent dye in acetonitrile. Thus, nearly all GUMBOS can be dissolved in the electrolyte at higher concentrations than their respective parent dyes. This demonstrates that GUMBOS based-ERDs are superior when compared to their respective parent compounds, since low solubility is a common problem that is encountered with ERDs.^19,25,26 Furthermore, the ion exchange method that was used to form GUMBOS represents a facile approach to tuning the solubility of dyes in order to produce more efficient ERDs.

Spectroscopic studies

Spectral properties of the parent dyes and GUMBOS were evaluated using ultraviolet-visible absorbance (UV-Vis) and fluorescence spectroscopies. Normalized UV-Vis spectra of the four parent dyes are given in Fig. 2. Maximum absorbance wavelengths were observed at 542, 559, 607, and 416 nm for RhB, TC1, PC, and TCPP parent compounds, respectively. RhB, TC1, and PC absorbed over the wavelength range of 450–570 nm, 460–590 nm, and 500–650 nm, respectively. Absorbance spectra of TCPP consisted of a Soret band (360–410 nm) and four Q bands (475–650 nm).

Fig. 2 Normalized ultraviolet-visible absorbance spectra of the parent dyes [RhB][Cl], [TC1][I], [PC][I], and [H]₄[TCPP] that were used to form GUMBOS.

Upon ion exchange to form GUMBOS, shapes of the absorbance spectra of GUMBOS were found to be similar to the parent dyes from which the GUMBOS were derived. This can be observed from the nearly complete overlap of the normalized absorbance spectra of the parent dyes and respective GUMBOS. This was true for all RhB, TC1, PC, and TCPP GUMBOS, including the various anion variations. Normalized absorbance spectra of the parent dyes and GUMBOS are given in Fig. 3.

Fig. 3 Normalized ultraviolet-visible absorbance spectra of RhB-, TC1-, PC-, and TCPP-based GUMBOS and respective parent dyes.

Although spectral shape did not change substantially when dyes were converted into GUMBOS, significant changes in molar extinction coefficient were observed. High molar extinction coefficients were also observed, which is another critical characteristic for ERDs since such dyes must efficiently absorb sunlight to enhance solar efficiency of DSSCs.²⁴ The molar extinction coefficients for RhB-, PC-, and TCPP-based compounds were greater than 1 × 10⁵ L mol⁻¹ cm⁻¹. However, the molar extinction coefficients of TC1 compounds were slightly lower (>1 × 10⁴ L mol⁻¹ cm⁻¹), in comparison to RhB, PC, and TCPP dyes. Ion exchange tended to produce a slight increase in molar absorptivity of RhB and TCPP GUMBOS, in comparison to the parent dye. This is attributed to introduction of bulky counterions, which helps to prevent aggregation of dye molecules.³⁵ Most importantly, the molar extinction coefficient was high for all compounds used in this study, indicating potential utility as ERDs in DSSCs (Table 3).²⁴

Table 3 Molar extinction coefficient (ε) and fluorescence quantum yield (ϕ_fl) of parent dyes and GUMBOS

Compound	ε (L mol^−1 cm^−1)	ϕfl (%)
[RhB][Cl]	8.30 × 10^4 ± 2.38 × 10^3	51.86 ± 0.20
[RhB][NTf2]	1.43 × 10^5 ± 0.29 × 10^3	49.92 ± 0.09
[RhB][BETI]	1.26 × 10^5 ± 3.33 × 10^3	52.64 ± 0.14
[TC1][I]	7.05 × 10^4 ± 0.52 × 10^3	3.13 ± 0.01
[TC1][NTf2]	4.48 × 10^4 ± 1.26 × 10^3	4.11 ± 0.02
[TC1][BETI]	1.17 × 10^5 ± 3.55 × 10^3	3.09 ± 0.01
[TC1][TPB]	5.98 × 10^4 ± 2.47 × 10^3	4.63 ± 0.04
[PC][I]	2.82 × 10^5 ± 1.14 × 10^4	0.11 ± 0.01
[PC][NTf2]	1.47 × 10^5 ± 7.69 × 10^3	0.74 ± 0.01
[PC][BETI]	1.33 × 10^5 ± 1.80 × 10^3	0.21 ± 0.01
[H]4[TCPP]	4.28 × 10^5 ± 8.98 × 10^3	2.69 ± 0.21
[P66614]4[TCPP]	5.11 × 10^5 ± 1.43 × 10^4	3.45 ± 0.22

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Spectral characteristics of dyes were further examined using fluorescence spectroscopy. Normalized fluorescence emission spectra of all parent dyes and GUMBOS are given in Fig. 4. To obtain fluorescence emission spectra, excitation wavelengths of 543, 553, 604, and 515 nm were used for RhB, TC1, PC, and TCPP dyes, respectively. Fluorescence emission spectra for dyes used in this study were increasingly red-shifted in the order of RhB, TC1, PC, to TCPP. For RhB, TC1, and PC dyes, slight shifting of the wavelength of maximum fluorescence intensity was observed upon conversion of parent dyes into GUMBOS. However, no clear trend in direction or magnitude of wavelength shift (i.e. bathochromatic or hypsochromic) was observed. For the TCPP parent dye, fluorescence emission spectra was nearly identical to that of [P66614]₄[TCPP].

Fig. 4 Normalized fluorescence emission spectra of RhB-, TC1-, PC-, and TCPP-based GUMBOS and the respective parent dyes.

Fluorescence quantum yields of parent dyes and GUMBOS were also evaluated by use of an absolute method. Upon conversion of parent dyes into GUMBOS, fluorescence quantum yield tended to increase with exception of [RhB][NTf2] and [TC1][BETI] GUMBOS. We attribute this to the introduction of bulky counteranions, which minimizes aggregation, thus leading to increases in fluorescence quantum yield.⁴⁰ Values of fluorescence quantum yield are reported in Table 3.

Förster Resonance Energy Transfer (FRET)

FRET efficiencies were studied between the photosensitizing dye and parent dyes or GUMBOS to evaluate the applicability of these compounds as potential ERDs. Transfer of energy from the ERD to the photosensitizing dye must be efficient in order to utilize the beneficial properties of ERDs.¹⁵ To evaluate the potential of these GUMBOS as ERDs in DSSCs, fluorescence spectral characteristics were examined in relation to the absorbance of the photosensitizing dye (N719). The normalized absorbance spectrum of the acceptor (N719) and normalized fluorescence spectra of the donors (parent dyes) are provided in Fig. 5. The photosensitizing dye (N719) absorbs visible light up to 730 nm; whereas, fluorescence emission of the ERDs occurs in the region of 550–750 nm. Fluorescence emission of ERDs exists in a region where absorbance of N719 is low, thus suggesting that use of these ERDs will enhance photocurrent in that region of the visible spectrum.^12,20

Fig. 5 Normalized absorbance spectrum of N719 (acceptor) and normalized fluorescence spectra of the parent dyes [RhB][Cl], [TC1][I], [PC][I], and [H]₄[TCPP] (donors).

Spectral overlap integral and energy transfer efficiency were evaluated between the donor (parent dyes or GUMBOS) and acceptor (N719). Spectral overlap integral (J(λ)) was calculated using the following equation:

where ε(λ) is the molar extinction coefficient (M⁻¹ cm⁻¹) of the acceptor (N719) at wavelength λ, and f(λ) is the normalized fluorescence wavelength of the donor (GUMBOS).³¹ Additionally, energy transfer efficiency (E) was determined using the following equation:where F_d is the fluorescence of the donor and F_da is the fluorescence of the donor in the presence of the acceptor.³¹

Values of J(λ) and E are summarized in Table 4 using parent dyes and GUMBOS as donors and N719 as the acceptor. Values of J(λ) increased in the order of TCPP < PC < TC1 < RhB. The highest values of J(λ) were expected for RhB-based compounds since these compounds have the most blue-shifted fluorescence emission and significant overlap with the absorption spectra of N719. Likewise, TCPP-based compounds provided the lowest J(λ) values since fluorescence emission was the most red-shifted and decreased spectral overlap with the acceptor. Use of [PC][NTf2] provided a significant increase in J(λ) value as compared to the parent dye [PC][I]. This increase was attributed to the slight blue-shifted fluorescence emission of [PC][NTf2] (Fig. 6). However, in general anion variations (i.e. NTf2⁻, BETI⁻, TPB⁻) of TC1 and RhB GUMBOS did not result in any significant changes in J(λ) values.

Table 4 Spectral overlap (J(λ)) and energy transfer efficiency (E) of parent dyes or GUMBOS (donors) with N719 (acceptor)

	J(λ) (M^−1 cm^−1 nm^−4)	E (%)
[RhB][Cl]	4.46 × 10^14 ± 2.21 × 10^12	8.86 ± 0.26
[RhB][NTf2]	4.33 × 10^14 ± 1.53 × 10^13	12.67 ± 1.08
[RhB][BETI]	4.39 × 10^14 ± 1.35 × 10^12	11.35 ± 1.58
[TC1][I]	3.89 × 10^14 ± 5.89 × 10^12	40.71 ± 1.42
[TC1][NTf2]	3.97 × 10^14 ± 4.03 × 10^12	39.40 ± 2.09
[TC1][BETI]	3.83 × 10^14 ± 6.93 × 10^10	35.71 ± 1.01
[TC1][TPB]	3.95 × 10^14 ± 6.31 × 10^11	41.69 ± 0.72
[PC][I]	1.92 × 10^14 ± 4.21 × 10^12	33.35 ± 3.41
[PC][NTf2]	2.15 × 10^14 ± 5.87 × 10^12	42.43 ± 2.51
[PC][BETI]	1.85 × 10^14 ± 1.07 × 10^13	33.30 ± 3.66
[H]4[TCPP]	1.36 × 10^14 ± 2.40 × 10^12	11.23 ± 0.38
[P66614]4[TCPP]	1.36 × 10^14 ± 2.44 × 10^11	8.03 ± 0.71

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Fig. 6 Normalized fluorescence intensity of parent dyes in the absence and presence of N719 (acceptor). Spectra for each compound were normalized by setting the highest intensity in absence of N719 to a value of 1.

Energy transfer efficiency was determined using equimolar concentrations of both the donor (parent dye or GUMBOS) and the acceptor (N719). Fluorescence spectra of parent dyes in the absence and presence of N719 are presented in Fig. 6. All fluorescence spectra of GUMBOS in the absence and presence of N719 are given in the ESI.† Additionally, fluorescence emission of [P66614]₄[TCPP] was studied in the presence of increasing concentration of N719. The corresponding Stern–Volmer plot is included in the ESI.† Using data presented in Table 4, it was determined that the highest values of energy transfer efficiency were obtained when utilizing cyanine-based compounds (TC1 and PC) and similar values of energy transfer efficiency were observed with RhB and TCPP-based compounds. These studies suggest that FRET efficiency can be improved by applying GUMBOS as ERDs. However, the most significant property of GUMBOS is the increase of solubility in the electrolyte of DSSCs, which was not possible using the parent compounds.

Photostability analysis

Photostability is another important property of ERDs since DSSCs have extended exposure to sunlight.³ Thus, these compounds should not be prone to decay by photobleaching. Photostabilities of parent dyes and GUMBOS were monitored over a time period of 3000 s using fluorescence kinetics measurements. Photostability curves of the parent dyes [RhB][Cl], [TC1][I], [PC][I], and [H]₄[TCPP] are displayed in Fig. 7. All parent dyes as well as GUMBOS exhibited excellent photostability for the duration of these experiments.

Fig. 7 Photostability of [RhB][Cl], [TC1][I], [PC][I], and [H]₄[TCPP] over a 3000 s time period. I represents fluorescence intensity at different times and I₀ is the initial fluorescence intensity.

Solar efficiency

DSSCs were fabricated with N719 employed as the photosensitizing dye due to the high solar conversion efficiency that has been achieved with this dye.⁴¹ Rhodamine, cyanine, and porphyrin GUMBOS and the respective parent dyes were applied as ERDs, and solar cell efficiencies were evaluated in the presence of each ERD. These ERDs were dissolved in the electrolyte before injection into the DSSC, and detailed fabrication of DSSCs can be found in the experimental section. Photovoltaic characteristics of these DSSCs such as short circuit current (J_sc), open current voltage (V_oc), fill factor (FF), and solar efficiency (η) are summarized in Table 5.

Table 5 Photovoltaic characteristics of DSSCs prepared using N719 as a photosensitizing dye in the absence and presence of energy relay dyes

	Jsc (mA cm^−2)	Voc (V)	FF^a	η^b (%)
a Fill factor (FF) = (Pmax)/(Jsc × Voc).b Solar conversion efficiency (η) = [Jsc (mA cm^−2) × Voc (V) × FF]/[100 (mW cm^−2)] × 100 (%).
N719-No ERD	13.08 ± 0.17	0.70 ± 0.01	0.57 ± 0.01	5.19 ± 0.07
N719-[RhB][Cl]	14.82 ± 0.08	0.73 ± 0.01	0.52 ± 0.01	5.63 ± 0.02
N719-[RhB][NTf2]	15.46 ± 0.03	0.73 ± 0.01	0.53 ± 0.01	5.95 ± 0.03
N719-[RhB][BETI]	14.17 ± 0.20	0.72 ± 0.01	0.56 ± 0.01	5.76 ± 0.01
N719-[TC1][I]	13.75 ± 0.05	0.72 ± 0.01	0.54 ± 0.01	5.38 ± 0.03
N719-[TC1][NTf2]	15.58 ± 0.07	0.72 ± 0.01	0.49 ± 0.01	5.45 ± 0.06
N719-[TC1][BETI]	15.46 ± 0.11	0.73 ± 0.01	0.48 ± 0.01	5.31 ± 0.04
N719-[TC1][TPB]	14.33 ± 0.10	0.73 ± 0.01	0.52 ± 0.01	5.51 ± 0.05
N719-[PC][I]	12.51 ± 0.04	0.71 ± 0.01	0.58 ± 0.01	5.21 ± 0.01
N719-[PC][NTf2]	12.96 ± 0.01	0.69 ± 0.01	0.62 ± 0.01	5.52 ± 0.01
N719-[PC][BETI]	11.60 ± 0.04	0.70 ± 0.02	0.64 ± 0.01	5.25 ± 0.01
N719-[H]4[TCPP]	11.59 ± 0.01	0.73 ± 0.01	0.66 ± 0.01	5.65 ± 0.01
N719-[P66614]4[TCPP]	13.18 ± 0.11	0.70 ± 0.01	0.64 ± 0.01	5.93 ± 0.01

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For all DSSC devices that were tested, minor changes to the open current voltage and fill factor were acquired. However, changes in short circuit current and solar efficiency, in the absence and presence of ERDs, were observed. Upon addition of ERDs, the efficiency of DSSCs tended to be higher compared to the control (absence of ERD). The highest solar efficiencies were obtained using rhodamine and porphyrin GUMBOS. This likely results from the high fluorescence quantum yield and molar absorptivity, respectively, displayed by these compounds. The higher values of short circuit current that were acquired using ERDs indicated increased light harvesting capability, leading to enhanced solar efficiency.¹⁵ Although, cyanine-based ERDs displayed high energy transfer efficiency with N719 in solution, lower solar efficiencies were obtained in comparison to rhodamine- and porphyrin-based ERDs. This was attributed to low fluorescence quantum yield of PC compounds and low molar absorptivity of TC1 compounds, thus indicating a correlation between these factors and solar efficiency enhancement.

In Fig. 8, the current–voltage curves for DSSCs without ERDs and containing rhodamine-based ERDs are presented. It is interesting to note that solar efficiency can be influenced by the identity of the counterion of the ERD. For example, the NTf2 counterion tended to achieve higher solar efficiencies as compared to BETI and Cl or I when using RhB-, TC1-, and PC-based compounds as ERDs. The TPB anion variation of TC1 exhibited the highest solar efficiency of all TC1 compounds. This was attributed to the higher fluorescence quantum yield of that particular anion variation. However, one distinct disadvantage of the TPB anion variation is limited solubility in acetonitrile.

Fig. 8 Current–voltage curves for DSSCs using N719 as a photosensitizing dye in the absence and presence of rhodamine B-based energy relay dyes.

Conclusions

In this study, rhodamine-, cyanine-, and porphyrin-based GUMBOS were synthesized, characterized, and investigated for application as ERDs in DSSCs using N719 as a photosensitizing dye. GUMBOS exhibited beneficial properties compared to the respective parent dyes such as increased molar extinction coefficient, fluorescence quantum yield, and increased solubility in the electrolyte. GUMBOS and the respective parent dyes were evaluated as FRET donors to the photosensitizing dye, N719, using solution-based UV-Vis and fluorescence measurements. Good spectral overlap and energy transfer efficiency were observed between ERDs and the photosensitizing dye. Upon application of GUMBOS as ERDs, the solar efficiency tended to be higher in comparison to the parent dye due to the beneficial properties of GUMBOS. Furthermore, it was observed that counteranion variation also plays a role in enhancement of solar efficiency. Thus, the ion exchange approach used to form GUMBOS represents a simplistic method for tuning the properties of dyes, e.g. stability, solubility, molar absorptivity, and fluorescence quantum yield to produce more efficient ERDs.

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. CHE-1307611, CHE-1508726, and the Philip W. West Endowment. This research is supported in part by a fellowship to PEK from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21980b

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