Probing Recombination Mechanism and Realization of Marcus

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C 2018, 122, 14113−14127

Probing Recombination Mechanism and Realization of Marcus Normal Region Behavior in DSSCs Employing Cobalt Electrolytes and Triphenylamine Dyes Suraj Soman,*,†,‡ Sourava C. Pradhan,† Muhammed Yoosuf,† Manikkedath V. Vinayak,† Sivasankaran Lingamoorthy,†,‡ and Karical R. Gopidas*,†,‡

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†

Photosciences and Photonics Section, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, 695019 Kerala, India ‡ Academy of Scientific and Innovative Research (AcSIR), New Delhi 110001, India S Supporting Information *

ABSTRACT: Cobalt based, outer-sphere, one-electron redox shuttles represents an exciting class of alternative electrolyte to be used in dye-sensitized solar cells. The flexibility of redox potential tuning by varying the substituents on peripheral organic ligands renders them the advantage of achieving higher photovoltage. However, higher recombination experienced in these systems by employing diffusion-limited cobalt species serves as a bottleneck which significantly limits attaining higher performance. The focus of the present contribution is to systematically investigate in detail the effect of structural variations and steric hindrance of organic triphenylamine dyes (TPAA4 and TPAA5) which differs in the number and nature of binding groups and peripheral hole accepting units on the recombination reactions and mass transport variations employing two different cobalt electrolytes, [Co3]3+/2+ and [Co(phen)3]3+/2+, having variable driving force for recombination. The detailed photovoltaic analysis provides us the information that modification of the architecture of organic dyes plays a decisive role in determining the performance, in particular, employing alternate one-electron outer-sphere redox systems. From our analysis, for both the dyes the charge recombination with the oxidized cobalt species was found to happen in the Marcus normal region which is attributed to the shift in conduction band (CB) that influenced the driving force for recombination. The current observation was quite exciting since the redox systems employed in the present study were previously documented to exhibit Marcus inverted recombination behavior. The impact of structural variations of dyes, change in conduction band, effect of nature of electrolyte species, and its interaction with the semiconductor on the recombination reactions was explored in detail using a range of small and large perturbation techniques.

1. INTRODUCTION Dye-sensitized solar cells (DSSC) have gained considerable attention as a potential third-generation photovoltaic technology due to their ease of fabrication, low production cost, flexibility of integration into various substrates, color tunability, comparatively less environmental issues, and exceptional performance under indoor/diffused light-harvesting conditions.1−8 In DSSCs each component has its specific role to play which contributes to the overall performance of the device, out of which the molecular structure of dyes and redox electrolytes plays an important role. Redox electrolytes directly influence the kinetic parameters such as regeneration, recombination, and diffusion which need to be carefully controlled in a way to achieve higher efficiencies.9−13 For more than two decades since the discovery of DSSCs, iodide/ triiodide electrolyte dominated the research field as the universal electrolyte of choice. Iodide electrolyte is endowed with properties such as excellent regeneration, slower © 2018 American Chemical Society

recombination, high solubility, good stability under operating conditions, and compatibility with commonly used Ru-based thiocyanate dyes.14−17 Even though iodide/triiodide electrolyte possesses several of these unique properties, still they are limited by the voltage tunability, larger driving force for regeneration, formation of iodide−dye complexes which complicates the evaluation of the actual mechanism happening in these systems, and inability to showcase good performance employing novel organic dyes due to higher recombination and quenching of the dye excited state.18−21 The first one-electron, outer-sphere alternate redox shuttle to be used in DSSCs was a ferrocene/ferrocenium (Fc/Fc+) redox Special Issue: Prashant V. Kamat Festschrift Received: February 7, 2018 Revised: March 27, 2018 Published: March 27, 2018 14113

DOI: 10.1021/acs.jpcc.8b01325 J. Phys. Chem. C 2018, 122, 14113−14127

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The Journal of Physical Chemistry C couple. Even though Fc/Fc+ proved to regenerate the dye quite efficiently, the recombination takes place at a very fast rate which retards the overall performance of the device. In addition to this, the fast self-exchange kinetics prevents using this as a model system to study recombination.22−25 This was followed by the successful employment of a one-electron, outer-sphere cobalt metal complex based redox electrolyte which proved to be the best to achieve higher photovoltaic performance employing organic dyes. The control that was gained in the redox potential by engaging these redox shuttles helps in managing the driving force for regeneration and recombination.26,27 Outer-sphere cobalt-based redox electrolytes have a more straightforward mechanism of operation which provides an excellent platform to evaluate the charge transfer properties of DSSCs while also employing organic dyes with variable architectures.28,29 Even though the first report on cobalt-based electrolytes appeared in 2001, since then very few papers were published in this area until 2010, where Feldt et al. successfully demonstrated efficiencies of 6.7% employing cobalt electrolytes with triphenylamine dyes.30 This was followed by contributions from the Grätzel group with efficiencies reaching up to 13%.31−33 Still comparatively less research has been focused on understanding the fundamentals of the interfacial electron transfer process employing alternative redox shuttles. The recombination reaction involving electron transfer from TiO2 to oxidized species in the electrolyte is the primary limiting reaction in DSSCs employing cobalt electrolytes which limits the widespread application of these redox systems.13,25,34 This necessitates the need to study recombination in more detail, employing cobalt redox shuttles. The interception of electrons by the CoIII species can also be selectively modulated by redesigning the dye architecture. Due to large inner sphere reorganizational energies induced by spin change [high spin (d7) to low spin (d6) going from Co2+ to Co3+], cobalt complexes exhibit very slow electron self-exchange kinetics. This gives us enough room to study the recombination dynamics in more detail, employing cobalt electrolytes in comparison to iodide/ferrocene redox systems.35,36 However, unlike I−/I3−, due to more positive redox potential the recombination rate is found to be much higher compared to iodine.37−39 Recombination is pitched in terms of electron lifetime which is evaluated using a variety of techniques such as OCVD, IMVS, and EIS.40−48 A general method to prevent this recombination is by applying pre/post blocking layers by the TiCl4 method. Even though this prevents recombination these layers create blockage toward the electron injection process.49−52 Hamann et al. have successfully demonstrated how to retard this recombination by carefully employing Al2O3 passivation layers of nanometer size thickness deposited by ALD which physically blocks the oxidized species getting closer to TiO2, thereby improving the charge collection.51,53 However, this method has limitations, as the thickness of these blocking layers needs to be carefully tuned in such a way that it will not create a barrier to injection. In this regard, modifying dye architectures in a way leading to building a blockage for the oxidized species in getting closer to TiO2 can be considered as a classical approach that forbids recombination but at the same time improves the lifetime. Recently our group has also contributed to this by investigating the charge transfer dynamics in detail for the two triphenylamine dyes (TPAA4 and TPAA5) that have variable steric and binding properties

and its influence on the recombination reaction with an iodide redox mediator.82,83 In the present contribution, we are employing two different propeller-shaped triphenylamine dyes which have a different number of hole acceptors on the periphery and varied number and nature of binding groups along with two different cobalt electrolytes having variable redox potentials in a way to probe how the variation in the structure of dyes and nature of electrolytes influence the recombination pathways in dyesensitized solar cells. Cobalt-based electrolytes being oneelectron, outer-sphere molecules, the recombination dynamics can be explained using Marcus theory.54,55 Unfortunately there are very few reports correlating Marcus theory to recombination in DSSCs employing outer-sphere cobalt-based electrolytes.30,50,52,56−58 In the present contribution Marcus theory was applied in a way to understand/differentiate the effect of conduction band shift toward the recombination driving force, employing two different outer-sphere cobalt electrolytes used along with triphenylamine dyes (TPAA4 and TPAA5) (Figure 1). This provides a clear indication that in addition to the steric

Figure 1. Chemical structure of dyes and electrolytes employed in the present study.

effect of the dye the nature of electrolyte plays a crucial role in determining the recombination driving force from the semiconductor conduction band toward the oxidized species in the electrolyte. This observation points out the fact that it is hard to have a universal best cobalt electrolyte that is compatible/best performing with organic dyes; instead from dye to dye based on the orientation on the surface, ability to prevent recombination, influence on conduction band shift, etc., we need to judiciously choose the redox system in a way to attain superior performance.

2. EXPERIMENTAL SECTION 2.1. General Methods. All chemicals were purchased from Sigma-Aldrich unless otherwise noted. Dyes TPAA4 and TPAA5 were prepared according to the published procedures.82,83 Cobalt redox mediators cobalt(II)-trisbipyridine hexafluorophosphate [Co(bpy)3](PF6)2 and cobalt(III)-trisbipyridine hexafluorophosphate [Co(bpy)3](PF6)3 and cobalt(II)-trisphenanthroline hexafluorophosphate [Co(phen)3]14114


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modulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) measurements were carried out using the electrochemical workstation (PGSTAT 302N) equipped with an FRA and LED driver to drive the white LED. Photovoltage responses of the cells were analyzed in the frequency range of 1 Hz to 1 kHz. The amplitude of the sinusoidal modulation for IMVS and IMPS measurements was 10% of the DC light. Open-circuit voltage decay (OCVD) measurements were carried out using a white LED, with the output set to match the photocurrent obtained under 100 mW cm−2. Measurement involves turning off the illumination in a steady state and then monitoring the subsequent decay of photovoltage (VOC). Charge extraction (CE) measurements were carried out using a white LED light coupled with the Autolab (PGSTAT 302N) and LED driver. CE involves switching on the illumination under different applied bias followed by switching off the light and then extracting the charge by short-circuiting the device. Photocurrent transients were recorded using a white LED which was switched off and switched on for 5 s each for a couple of times, and the corresponding photocurrent was monitored.

(PF6)2 and cobalt(III)-trisphenanthroline hexafluorophosphate [Co(phen)3](PF6)3 were synthesized according to published procedures.26−28,84 Identification of both redox electrolytes by NMR is documented in the Supporting Information. 2.2. Solar Cell Fabrication. Initially cleaning of working electrodes involves sonication of 1.5 cm2 FTO (Tec15, Dyesol) glass substrates in soap solution followed by gentle rubbing with kimwipes succeeded by sonication in Millipore water for 30 min and sonication in IPA and acetone for 15 min with each of them followed by UV−O3 treatment. The substrates were dipped in 40 mM aqueous TiCl4 solution for 30 min at 80 °C and annealed at 500 °C for 30 min. Doctor blading technique was used to deposit a transparent layer of mesoporous TiO2 (18NRT, Dyesol) layer by making a 0.3 cm2 gasket using scotch tape. The TiO2-coated glass plates were then kept for programmed heating at 325 °C for 15 min, 450 °C for 15 min, and 500 °C for 30 min. This was followed by post blocking layer deposition by heating the electrodes in 40 mM TiCl4 for 30 min at 80 °C followed by annealing at 500 °C for 30 min. The sintered photoanodes were then slowly cooled, and once the temperature reached 70 °C they were immersed in triphenylamine dye solutions in acetonitrile (0.3 mM) and kept at room temperature overnight. No CDCA was employed in any of the devices. Solar cells were assembled, using a 25 μm thick thermoplastic Surlyn frame, with an electrochemically deposited poly(3,4-ethylene dioxythiophene) (PEDOT) counter electrode (TEC8). Counter electrodes were initially cleaned by sonication of the predrilled 1.5 cm2 FTO (TEC8, Dyesol) glass substrates in soap solution, Millipore water, and ethanol, respectively, for 30 min each, followed by UV−O3 treatment. PEDOT electrodes were prepared by electropolymerization of 3,4-ethylene dioxythiophene (EDOT) from a micellar aqueous solution of 0.1 M sodium dodecyl sulfate (SDS) and 0.01 M EDOT. The electrolyte was introduced into the predrilled holes, and the holes were sealed using a glass coverslip. Cobalt electrolytes were prepared using the following composition: 0.22 M [Co(bpy)3(PF6)2], 0.05 M [Co(bpy)3(PF6)3], 0.1 M LiTFSI, and 0.2 M TBP in acetonitrile and 0.22 M [Co(phen)3(PF6)2], 0.05 M [Co(phen)3(PF6)3], 0.1 M LiTFSI, and 0.2 M TBP in acetonitrile. 2.3. Solar Cell Characterization. The photocurrent− voltage (J−V) measurements of the devices were carried out using an Oriel 3A (Model PVIV- 94043A) Class-AAA solar simulator accompanied by a Keithley E 2400 source meter. The light intensities were calibrated using a certified reference Si solar cell, to an intensity of 100 mW cm−2 (1 sun at AM 1.5G). A circular black mask of area 0.1256 cm2 was always used during the J−V measurements to avoid significant additional contribution from light impinging on the device outside the active area (0.3 cm2). The incident photon-to-current conversion efficiency (IPCE) measurement of devices was performed under DC mode using a 250 W xenon lamp coupled with a Newport monochromator, calibrated using a certified reference solar cell. The electrochemical impedance spectroscopy (EIS) measurements of devices were carried out using an Autolab (PGSTAT 302N) workstation equipped with FRA under forward bias in the dark. The impedance spectra were recorded at an applied bias around VOC, with a 10 mV alternating potential superimposed on the applied bias. The entire equivalent circuit fitting was carried out using NOVA 1.11 software. The measurements were performed in a frequency range of 100 mHz to 100 kHz in equally spaced logarithmic steps. Intensity-

3. RESULTS AND DISCUSSION The two major recombination pathways in dye-sensitized solar cells involves the back electron transfer from TiO2 to the oxidized dye molecule and to the oxidized species present in the electrolyte. In devices employing redox shuttles that have fast regeneration kinetics, the major recombination pathway involves the back electron transfer to the oxidized species present in the electrolyte. In the current contribution, we are employing two different propeller-shaped triphenylamine dyes which differ in the number of peripheral hole acceptor moieties along with two different cobalt electrolytes with variable redox potentials, in a way to probe how these structural variations of dyes and redox couples influence the recombination reaction in dye-sensitized solar cells. TPAA4 and TPAA5 dyes were selected for this study since both of them have quite similar ground and excited state potentials [1.01 V TPAA4 and 1.00 V for TPAA5 vs NHE measured by cyclic voltammetry] but differ in the steric properties due to structural variations.82 Thereby, employing electrolytes with the same kind of additives, the variation in injection and regeneration will be minimal which provided us the opportunity to track the effect of structural modifications of the dyes on the recombination phenomenon employing alternate cobalt-based electrolytes [Co(bpy)3 placed at a potential of 0.56 V and Co(phen)3 at 0.62 V vs NHE] which triggered a shift in CB. The concentration of TPAA4 on TiO2 was found to be 4.58 × 10−7 mol cm−2, and for TPAA5 it was found to be 8.61 × 10−7 mol cm−2. In addition to this, the spatial separations of the dye cation in both the dyes are different. Clifford and co-workers already demonstrated that the recombination mechanism in DSSCs is influenced by the charge separation between the dye cation and TiO2 surface.85 According to the quasi-static approximation, the measured electron lifetime depends on the steady-state relationship between free electrons in the conduction band and trapped electrons.86−88 In the present case, all devices were fabricated employing pre- and post-blocking layers which passivate the surface states; therefore, wherever recombination is mentioned directly relates to the electron transfer from the conduction band to the oxidized species in the electrolyte. 14115


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thickness was kept at 6 μm to reduce the mass transport issues to minimal. Initially to understand the influence of structural effect of dyes and nature of cobalt electrolyte on conduction band position we derived the chemical capacitance (Cμ) of mesoporous titania film employing electrochemical impedance spectroscopic measurement of the fabricated devices. The variation of chemical capacitance as a function of Fermi voltage (VF) is given in Figure 3(a). It is quite clear from the capacitance plot that when [Co(bpy)3]3+/2+ was used as the electrolyte the conduction band of TPAA5 shifted to more negative potentials compared to TPAA4. This shift in the conduction band is attributed to (a) variation in the number of binding groups and nature of binding in between the two dyes and (b) structural repercussions that restrict/allow the additives in the electrolyte to get in close contact with the semiconductor, resulting in a shift in the conduction band. If the shift is due to the variation in number and nature of the binding groups, it should hold even while changing the electrolyte. In the present case on changing the electrolyte from [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+, there is decidedly less shift in the conduction band. This indicates that the structural variations of dyes affect the possible cause that led to the shift in the conduction band in between both the dyes employing [Co(bpy)3]3+/2+. TPAA5 with less steric effect contributed to more infiltration channels compared to TPAA4, resulting in more t-butylpyridine to get closer to TiO2, leading to a more negative shift in the conduction band. The recombination kinetics were investigated in detail measuring lifetime using three different techniques, viz., electrochemical impedance spectroscopy (EIS), intensitymodulated photovoltage spectroscopy (IMVS), and opencircuit voltage decay measurements (OCVD). Lifetime derived by fitting the obtained Nyquist plot by the transmission line model is given in Figure 3(b) for both TPAA4 and TPAA5. From the figure, it is quite clear that for both the electrolytes TPAA4 showed better lifetime in comparison to TPAA5. The same trend is reflected in IMVS (Figure 3c) and OCVD (Figure 3d) results which exhibit a better lifetime trend for TPAA4 compared to TPAA5. As shown in Scheme 1 the orientation of TPAA4 on the TiO2 surface bordered by two TPA groups at the periphery efficiently prevented the approach of CoIII species, leading to better lifetime. In addition to this TPAA5 resulted in a more negative shift in conduction band which has led to a higher driving force for electron recombination employing TPAA5, resulting in a lower lifetime. However, the extent of variation in lifetime between TPAA4 and TPAA5 employing both the electrolytes does not show much difference [Figure 3(b), (c), and (d)], which clearly shows that more than the driving force for recombination between the conduction band of TiO2 and redox potential of the dye, the steric effect of sensitizers played a more significant role in determining recombination. Transport time (τd) for TPAA4 and TPAA5 is given in Figure 4(a). Employed along with both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+, TPAA5 showcased a lower transport time than TPAA4. TPAA5 having two binding cyanoacetic acid groups helped in achieving a better electron transfer from the dye excited state to the TiO2 conduction band which might have resulted in an improved injection. This helped in attaining better electron transport in TiO2 leading to a shorter transport time resulting in better diffusion coefficient (Dn) for TPAA5 in comparison to TPAA4 for both the electrolytes. Unfortunately,

3.1. Variation in Charge Recombination Reaction between TPAA4 and TPAA5 Employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. In dye-sensitized solar cells, one of the primary limiting electron transfer reactions toward achieving higher charge collection efficiency and higher performance involves the back electron transfer from the semiconductor metal oxides to the oxidized species in the electrolyte.59−61 Many groups have carried out systematic studies in a way to understand the dynamics of this interception reaction to tune the performance of both dyes and electrolytes to achieve better photovoltaic performance.47,62−65,82 Even though due to spin change it was anticipated to have slow recombination employing cobalt redox mediators, in reality, the recombination of conduction band electrons to oxidized CoIII species in electrolyte under device operating conditions takes place at a very fast rate.66−68 In addition to this the mass transport limitation and slow regeneration kinetics employing more positive cobalt redox shuttles again worsen the situation.30,69,81 Many research groups including Hupp et al. and Hamann et al. have successfully employed a range of insulating blocking layers including passivating layers deposited employing the ALD technique using materials like TiO2 and Al2O3 to reduce the rate of recombination.51,52,49,70−73 However, very few reports are available which involves the detailed study of this recombination mechanism as a function of dye architecture and nature of redox mediator. This needs to be explored more as it offers a simple method to control the recombinations while employing outer-sphere redox mediators in a way to achieve better performance without compromising the electron injection which is a significant drawback of using blocking layers. In this regard here we are analyzing the effect of recombination of two triphenylamine dyes that have almost similar ground- and excited-state potentials (Figure 2) but differ

Figure 2. Schematic representation of the energetics of dyes and electrolytes employed in the present study. The recombination reaction that is probed in detail is marked with red arrows.

in the number of binding groups and nature of binding on the TiO2 surface.82 This variation has a profound influence on determining the performance employing two different outersphere, cobalt-based electrolytes, [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. These unsubstituted cobalt electrolytes are engaged in the present study in a way to avoid high diffusion issues which will allow us to probe the recombination mechanism in more detail. In addition to this, the TiO2 film 14116


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Figure 3. Variation of (a) chemical capacitance (Cμ) of TiO2 as a function of corrected potential measured by EIS technique. (b) Electron lifetime (τn) as a function of corrected potential measured by EIS. (c) Electron lifetime (τn) as a function of LED current measured by the IMVS technique. (d) Lifetime as a function of applied potential obtained from OCVD measurements for devices fabricated using TPAA4 and TPAA5 employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.

Scheme 1. Representation of a Possible Recombination Mechanism in TPAA4 and TPAA5 Employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+

collection efficiency which was reflected in the short-circuit current where TPAA5 gave better Jsc (8.22 mA cm−2) compared to TPAA4 (7.23 mA cm−2), but due to higher recombination prevailing in TPAA5 both Voc and FF were decreased which resulted in a net lower performance. 3.2. Nonlinearity in Recombination Kinetics and Mass Transport Effects. The light intensity dependence of opencircuit potential employing TPAA4 and TPAA5 with both cobalt redox mediators ([Co(bpy) 3 ] 3+/2+ and [Co(phen)3]3+/2+) is given in Figure 5a. Voc is related to light

the shorter transport time and higher diffusion coefficient attained did not translate into better diffusion length and charge collection efficiency for TPAA5 employing [Co(bpy)3]3+/2+ due to a more significant shift in the conduction band and structural effects of dye which led to more recombinations for TPAA5 in comparison to TPAA4. Lower collection leads to a lower current, and more recombination leads to a lower voltage for TPAA5 in comparison to TPAA4 employing [Co(bpy)3]3+/2+. However, in the case of [Co(phen)3]3+/2+ going from TPAA4 to TPAA5 resulted in better diffusion length and charge 14117


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Figure 4. (a) Transport time (τd) measured as a function of LED current by IMPS technique. (b) Diffusion coefficient (Dn) measured as a function of LED current. (c) Diffusion length (Ln) measured as a function of LED current and (d) charge collection efficiency (ηcc) measured as a function of LED current for devices fabricated using TPAA4 and TPAA5 employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.

Figure 5. Light intensity dependence of (a) the open-circuit voltage (Voc) and (b) the current density (Jsc) under short-circuit conditions for DSSCs sensitized with TPAA4 and TPAA5 with [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.

intensity by the relation

dVoc d log I

=

2.30kBT βq

intensity of light there should be a linear increase in the conduction band electrons which leads to more recombination rate with higher light intensity.30 For the ideal case, the slope should be 59 mV/decade which leads to a β value of 1. Any deviation from this ideality is due to the nonlinear recombination kinetics as a result of either a shift in the conduction band or due to enhanced recombination

where I is input light

intensity; kB is Boltzmann’s constant; T is the temperature; and β = (1/m), where m is the diode ideality factor. β is calculated from the slope of the plot of log of light intensity vs Voc.74,75 It is a known fact that if the recombination involves back electron transfer solely from the conduction band, with the increase in 14118


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Table 1. Tabulated J−V Characteristics of TPAA4 and TPAA5 along with the Recombination Parameter β for Devices Fabricated with [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ Electrolytes dye electrolyte TPAA4-Co(bpy)3 TPAA5-Co(bpy)3 TPAA4-Co(phen)3 TPAA5-Co(phen)3

Voc (mV) 856 643 834 735

± ± ± ±

9.0 42.0 8.0 9.0

Jsc (mA cm−2) 8.62 8.47 7.23 8.22

± ± ± ±

0.3 0.1 0.5 0.2

FF 0.65 0.36 0.61 0.45

± ± ± ±

0.1 0.2 0.1 0.2

efficiency (%)

β

± ± ± ±

0.694 0.208 0.687 0.171

4.78 1.96 3.7 2.72

0.1 0.2 0.2 0.2

Figure 6. Plots of current transients measured at an illumination intensity of 100 mW cm−2 for DSSCs sensitized with TPAA4 and TPAA5 with [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.

sensitized solar cells.79 Here, as explained before for TPAA4, employed along with [Co(bpy)3]3+/2+ there is a considerable shift in conduction band; therefore, the variation in slope from ideality is expected to be contributed from this displacement, whereas for [Co(phen)3]3+/2+ the difference in slope between both dyes is more influenced by surface state recombinations as in this case the shift in conduction band is minimal. A decrease in slope obtained from the intensity-dependent voltage measurement for TPAA4 along with both electrolytes corresponds to an increase in steric bulk of the dyes which leads toward a better lifetime. The increase in slope for TPAA5 can be correlated to the availability of more penetration channels leading towards more recombination and a lower lifetime as explained in the previous sections. In addition to this, the electrostatic repulsion induced by the two flanked TPA groups in TPAA4 compared to one in TPAA5 might have created extended blockage for the approach of oxidized CoIII species in the electrolyte to getting closer to the semiconductor contributing toward a better lifetime. The remarkable difference

from surface states/trap states. There are a few reports which explored this deviation employing both iodine and cobalt electrolytes and also by varying the concentration of additives such as Li+ in the electrolyte.30,74,76−78 We had recently shown that there is little contribution from trap states/surface states toward recombination employing TPAA4 and TPAA5 dyes along with iodide redox shuttle.82 However, it is not necessary that this should hold the same while using cobalt electrolytes, as it is already known that employing cobalt electrolytes with more positive redox potentials triggers recombination from surface states.13 In the present case for TPAA4 and TPAA5 employed with [Co(bpy)3]3+/2+ the slope was 28.3 mV/decade and 85.5 mV/ decade, and the corresponding β values were 0.694 and 0.208, respectively (Table 1). However, employing TPAA4 and TPAA5 with [Co(phen)3]3+/2+ the slope was found to be 34.5 mV/decade and 85.8 mV/decade, respectively, and corresponding β values of 0.687 and 0.171 were obtained. There are many hypotheses regarding nonlinearity in dye14119


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Figure 7. (a) Current density versus potential (J−V) characteristic curves and (b) IPCE of TPAA4- and TPAA5-based dye-sensitized solar cells along with [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ measured under 100 mW cm−2, AM 1.5G illumination. (c) Dark J−V measurements for TPAA4 and TPAA5 along with [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ electrolytes.

in β value between TPAA4 and TPAA5 where TPAA4 exhibited a much decreased slope suggests lower recombination in TPAA4 with respect to TPAA5 regardless of the electrolyte. The variation of short-circuit current density with light intensity is shown in Figure 5b. Current density is related to input light intensity by the equation Jsc ∝ Iα where α is a constant.80 If the system is not limited by mass transport, then the increase in short-circuit current with respect to light intensity should be linear, and α will be close to 1. A deviation from linearity at higher intensities is a clear indication of mass transport issues. In the present case employing [Co(bpy)3]3+/2+ with both TPAA4 and TPAA5, the increase in Jsc was found to be more or less linear, whereas on changing the electrolyte to [Co(phen)3]3+/2+ a slight deviation from linearity is observed at higher light intensities which are in accordance with the current transient results (Figure 6). The calculated α value for all the four sets was more or less the same with values close to 1. The mass transport limitations in these systems were evaluated in more detail by employing transient photocurrent measurements as given in Figure 6. A large modulation of incident light was generated using a white LED light source whose output intensity was set to match the photocurrent obtained under 1 sun illumination (100 mW/cm2). It has been well documented by Nelson et al. and Hagfeldt et al. that the diffusion limitation of the oxidized CoIII species determines the performance of the devices employing alternate cobalt redox

electrolytes.30,81 The ratio of initial peak current to steady state current is much less while employing [Co(bpy)3]3+/2+ in comparison to [Co(phen)3]3+/2+, clearly indicating that the mass transport issues are lower while employing small molecules like [Co(bpy)3]3+/2+ in comparison to [Co(phen)3]3+/2+. This trend was also reflected in the Jsc vs light intensity measurements (Figure 5b), where [Co(phen)3]3+/2+ employed devices showed more deviation from linearity at higher intensities. Since we used a thin 6 μm TiO2 layer, the ratio of peak current to steady current and diffusion limitation was found to be comparatively less with respect to thicker electrodes. 3.3. Photovoltaic Performance Analysis (I−V, IPCE, and Dark Current Measurements). The current density− voltage (J−V) characteristics were measured for TPAA4 and TPAA5 sensitized solar cells using both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ redox electrolytes which are shown in Figure 7a, and the tabulated results are summarized in Table 1. DSSCs fabricated using TPAA4 with [Co(bpy)3]3+/2+ gave values of Jsc = 8.62 ± 0.3 mAcm−2, Voc = 856 ± 9.0 mV, and FF = 0.65 ± 0.1, corresponding to an overall power conversion efficiency of 4.78 ± 0.1%. At the same time TPAA5-based devices employing [Co(bpy)3]3+/2+ gave values of Jsc = 8.47 ± 0.1 mAcm−2, Voc = 643 ± 42.0 mV, and FF = 0.36 ± 0.2, leading to an overall power conversion efficiency of 1.96 ± 0. 2% only. Due to the severe back electron transfer at the semiconductor− 14120


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counterparts.29,82 In addition to this, the recombination parameter (β) can also be corelated to FF. A higher β value will result in lower recombination and a better lifetime, leading to improved FF. A higher β value showcased by TPAA4 employed along with both [Co(bpy)3 ]3+/2+ and [Co(phen)3]3+/2+ is attributed to the better retardation of recombination which ultimately helped in achieving higher FF. In addition to recombination, conduction band shift induced by either modification of dyes, electrolytes, additives in electrolyte, or other factors also produces changes to β parameter.78,79 We came across another striking observation in photovoltaic performance when the electrolyte was switched over using the same dye. For TPAA4 better performance was observed by employing [Co(bpy)3]3+/2+, whereas TPAA5 gave better performance using [Co(phen)3]3+/2+. On changing the electrolyte from [Co(bpy)3]3+/2+ to [Co(phen)3]3+/2+ for TPAA4 there is a 23% decrease in photovoltaic performance, whereas for TPAA5 there is a 28% increase in photovoltaic performance. This is quite interesting, and we employed Marcus theory in a way to correlate this behavior with the CB shift which resulted in the variation in recombination driving force and is addressed in the following section. 3.4. Evaluation of Recombination Using Marcus Theory Differentiating the Effect of [Co(bpy)3]3+/2+ to [Co(phen)3]3+/2+ on TPPA4 and TPAA5. To understand the back electron transfer and to predict the charge transfer dynamics of injected electrons in the semiconductor electrode to the oxidized species in the electrolyte, Marcus theory was used as a successful method.54,55 Very few systematic studies have been done so far in this regard where one of the seminal papers was from Hamann et al. where they had experimentally observed the Marcus inverted behavior for recombination of CB electrons employing an outer-sphere, one-electron redox shuttle and a single-crystal ZnO electrode. The same group did study the effect of CB shift, leading to a variation in recombination driving force by varying the pH at the ZnO interface.36,90 Recently, Hagfeldt and co-workers have shown both Marcus normal and inverted behavior by changing the driving force, employing a series of cobalt electrolytes with D35 dye. They had intentionally fixed the quasi-Fermi level in a way to have a qualitative comparison of rate constants. They observed Marcus inverted recombination behavior, employing cobalt electrolytes with potentials between 0.5 and 0.85 V vs NHE for devices fabricated using D35 dye.58 Gao et al. have also shown recently both Marcus normal and inverted behavior, employing cobalt electrolytes by changing the donor groups in triphenylamine dyes and also by varying the number of dyes on the semiconductor surface by changing the dye concentration.89 Recently, T. J. Meyer and co-workers have relooked into this back electron transfer process in more detail, employing two Ru-based dyes and a series of four triphenylamines which are having their redox potentials spanned at around 0.5 eV range in a way to study the correlation between reaction kinetics and driving force. They successfully showed a Marcus normal region behavior where the rate of electron transfer increased with increase in the recombination driving force.91 This points out the need for a more detailed evaluation in a way to enhance the fundamental knowledge on electron interception reactions in DSSCs. In the present contribution, we have successfully showcased a Marcus normal region behavior for two cobalt electrolytes which are endowed with oxidation potentials placed in between the potential window of 0.5−0.85 V. The variation

electrolyte interface, in TPAA5, a significant decrement in voltage, current, and FF and hence overall reduction in power conversion efficiency was observed going from TPAA4 to TPAA5 employing [Co(bpy)3]3+/2+. Devices fabricated using TPAA4 with [Co(phen)3]3+/2+ gave values of Jsc = 7.23 ± 0.5 mA cm−2, Voc = 834 ± 8.0 mV, and FF = 0.61 ± 0.1, corresponding to an overall power conversion efficiency of 3.7 ± 0.2%. At the same time TPAA5-based devices employing [Co(phen)3]3+/2+ gave values of Jsc = 8.22 ± 0.2 mA cm−2, Voc = 735 ± 9.0 mV, and FF = 0.45 ± 0.2, leading to an overall power conversion efficiency of 2.72 ± 0.2%. For both the electrolytes on going from TPAA4 to TPAA5, a decrease in overall performance was observed. TPAA5 with less steric effect provided more penetration channels for the access of oxidized CoIII species to get closer to the semiconductor surface (Scheme 1), resulting in more recombination and lower lifetime leading to a decrease in Voc and net overall photovoltaic performance as explained before. However, interestingly the extent of decrease is different employing different redox shuttles. By using [Co(bpy)3]3+/2+ there is an increment of 59% in efficiency going from TPAA5 to TPAA4, whereas when [Co(phen)3]3+/2+ was employed the extent of increase got reduced to 26%. The variation of shift in conduction band between both the dyes while changing the cobalt electrolyte (Figure 3a) resulted in this riveting behavior. By using [Co(bpy)3]3+/2+ there is a substantial negative shift in conduction band for TPAA5 in comparison to TPAA4 which resulted in more driving force for recombination for TPAA5 compared to TPAA4. This coupled with low steric effect lead to more penetration channels for TPAA5, resulting in more recombination, lowering the lifetime resulting in reduced performance. When [Co(phen)3]3+/2+ was employed there is considerably little shift in the conduction band; therefore, the driving force for recombination is minimal here, and the reduced performance for TPAA5 resulted from higher recombination initiated by the lesser steric effect which attracted more Co III closer to the TiO2 surface. The improvement in performance for TPAA4 is also quite visible from the IPCE spectra where TPAA4-based devices showed better IPCE response in comparison to TPAA5. The dark current measurements given in Figure 7c also reflect this variation in lifetime and voltage. The superposition principle holds well for DSSCs also. Therefore, the measured dark current gives a proper indication towards the relevant recombination process taking place in DSSCs that limits the open-circuit voltage and net photovoltaic performance. From Figure 7c it is quite evident that TPAA4 which prevented more recombination has a better dark current which resulted in achieving better voltage compared to TPAA5. The variation in fill factor (FF) can also be used in a way to evaluate the variation in performance between both the dyes and its influence with the electrolyte. The recombination taking place from the semiconductor electrode to the oxidized species in the electrolyte influences the FF to a greater extent. In the present contribution as shown in Table 1, it is quite clear that on moving from TPAA4 to TPAA5 employing both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ there is a considerable decrease in FF. Improved FF obtained for TPAA4 using both the electrolytes clearly supports superior lifetime showcased by TPAA4 in comparison to TPAA5 as explained in detail in the previous sections. In general it is observed that, due to the higher mass-transport resistance, employing outer-sphere cobalt redox mediators leads to lower FF compared its iodide 14121


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which resulted in a Marcus normal region recombination behavior employing cobalt electrolytes ([Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+) which were previously shown to have Marcus-inverted recombination kinetics with organic dyes, in particular, triphenylamine dyes. The shift in conduction band for TPAA4 employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ are shown in Figure 8a, and the shift in conduction band for TPAA5 for both the electrolytes are given in Figure 8b. It is quite apparent from both the graphs that employing [Co(bpy)3]3+/2+ electrolyte resulted in a more negative shift in the conduction band for both the dyes. However,the extent of variation is much greater for TPAA5 compared to TPAA4. This could be rationalized by considering the less steric effect prevailing in TPAA5 which creates more infiltration channels for the electrolyte to access the semiconductor. The lifetime results are given in Figures 8b and 8c for TPAA4 and TPAA5 dyes using both the electrolytes. The lifetime results were also in tune with the shift in conduction band pointing out the role of driving force for recombination. In both the cases by employing [Co(bpy)3]3+/2+ there is a negative shift in conduction band which increased the driving force for recombination, resulting in lower lifetime trend for TPAA4 and TPAA5 employing [Co(bpy)3]3+/2+ electrolyte [Figure 8(c) and (d)]. Since this driving force for recombination is much higher for TPAA5, the extent of reduction in lifetime engaging [Co(bpy)3]3+/2+ is much higher for TPAA5 in comparison to [Co(phen)3]3+/2+. The same trend was observed in the results obtained by carrying out IMVS measurements which are given in the Supporting Information (Figure S5). Now when we compare this with the photovoltaic performance, we can observe that for TPAA4 changing electrolyte from [Co(phen)3] to [Co(bpy)3] resulted in an increase in voltage from 834 mV to 856 mV which can be attributed to the negative shift in CB. Here, since the shift in CB is not that high the effect of a slight increase in recombination driving force [as a result of the CB shift] has a minimal impact on Voc, whereas for TPAA5 on changing the electrolyte from [Co(phen)3] to [Co(bpy)3] there is a considerable decrease in voltage from 735 mV to 643 mV. This clearly details the importance of recombination driving force. In the case of TPAA5, the shift in CB by employing [Co(bpy)3]3+/2+ is much higher with respect to the shift obtained for TPAA4 employing [Co(bpy)3]. Pushing the CB above a specific potential will result in a situation where driving force for recombination overpowers the increase in voltage developed as a result of the negative shift in the conduction band. Thus, for TPAA5 even with a negative shift in CB employing [Co(bpy)3]3+/2+, the device ended up having lower voltage. We also evaluated the variation in transport time by changing the electrolyte. Surprisingly there was negligible change in transport time by changing the electrolyte for both the dyes (Figure S6, Supporting Information). Therefore, the variation in conduction band by changing the electrolyte did not contribute to any variation in transport. For TPAA4 both current and voltage increased on going from [Co(phen)3]3+/2+ to [Co(bpy)3]3+/2+, whereas for TPAA5 voltage decreased and current increased moving from [Co(phen)3]3+/2+ to [Co(bpy)3]3+/2+ electrolyte. The variation in voltage has been explained in detail before. To understand the increase in current we evaluated the variation in diffusion length (Ln) (Figure 9) and charge collection efficiency (ηcc) (Figure 10) for TPAA4 and TPAA5 employing both [Co(bpy)3]3+/2+ and

in recombination behavior is attributed to the shift in CB which modified the recombination driving force. To the best of our knowledge, such a kind of detailed study in this regard has not been reported so far. On analyzing the PV results obtained (Table 1), the variation in voltage and efficiency values realized in between both the cobalt electrolytes provided valuable information on the recombination reaction of conduction band electrons with the oxidized species in the electrolyte. It is quite impressive to note that for TPAA4 on moving from [Co(bpy)3]3+/2+ to [Co(phen)3]3+/2+ which is placed 60 mV more positive to that of former there is a 22 mV decrease in voltage from 856 mV to 834 mV, whereas for TPAA5 there is a 92 mV increase in voltage on going from [Co(bpy)3]3+/2+ to [Co(phen)3]3+/2+ from 643 mV to 735 mV. In the case of both the dyes, [Co(phen)3]3+/2+ gave a better lifetime in comparison to [Co(bpy)3]3+/2+. Witnessing the higher voltage along with better lifetime showcased by more positive [Co(phen)3]3+/2+ redox shuttle, we initially thought of having a Marcus-inverted recombination behavior for [Co(phen)3]3+/2+ as has been suggested previously by Hagfeldt and co-workers.30,58 They observed Marcus inverted recombination behavior for cobalt electrolytes with potentials positive to 0.5 V vs NHE and Marcus normal region behavior employing cobalt redox systems with potentials negative to 0.5 V vs NHE fabricated along with D35 dye. In the present case after analyzing the shift in conduction band in detail, we figured out that the driving force for recombination got higher in the case of [Co(bpy)3]3+/2+ due to a negative shift in conduction band which resulted in lower lifetime pushing the recombination to happen in the Marcus normal region (as shown in Scheme 2). In the present contribution, recombination from the surface states can be considered to be minimal as has been explained in the intensity-dependent recombination kinetics section. We used a range of small and large perturbation techniques to investigate in detail the origin and effect of the shift in conduction band Scheme 2. Representation of Shift in Conduction Band Attained by Employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ along with TPAA4 and TPAA5a

a

CBbpy corresponds to the position of CB when [Co(bpy)3]3+/2+ was employed as the electrolyte, and CBphen corresponds to the position of CB when [Co(phen)3]3+/2+ was employed as the electrolyte. Positions are approximate and not scaled up to the actual values. 14122


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Figure 8. (a) Variation in chemical capacitance (Cμ) for TPAA4 as a function of corrected potential measured by EIS technique employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. (b) Variation in chemical capacitance (Cμ) for TPAA5 as a function of corrected potential measured by EIS technique employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. (c) Electron lifetime as a function of corrected potential measured by EIS technique for TPAA4 employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. (d) Electron lifetime (τn) as a function of corrected potential measured by EIS technique for TPAA5 employing [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.

Figure 9. (a) Diffusion length (Ln) measured as a function of LED current for TPAA4 and (b) diffusion length (Ln) measured as a function of LED current for TPAA5 employing both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ electrolytes.

[Co(phen)3]3+/2+. The results are quite exciting as for TPAA4 there is minimal variation in both Ln and ηcc (Figures 9a and 10a), whereas for TPAA5 there is a considerable increase in both diffusion length and charge collection efficiency employing [Co(phen)3]3+/2+ which helped in achieving higher current.

On account of the complexity involved in understanding the detailed charge transfer recombination dynamics in systems engaging cobalt electrolytes, we believe that the present contribution will help the scientific community to rationalize the design of new dyes to be used with alternate cobalt-based 14123


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Figure 10. (a) Charge collection efficiency (ηcc) measured as a function of LED current for TPAA4 and (b) charge collection efficiency (ηcc) measured as a function of LED current for TPAA5 employing both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ electrolytes.

and electrolytes needs to be used in a way to achieve higher performance in devices employing outer-sphere, one-electron redox mediators.

electrolytes. Further detailed experimentation to quantify the reorganizational energy along with the kinetic parameters is required to gain more fundamental insight and understanding of the predicted mechanism which is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

4. CONCLUSION In the present study, the recombination we observed is influenced directly by changing the dye architecture/redox shuttle. The dye having two peripheral hole acceptors and a branched network prevented the approach of the oxidized species in the electrolyte coming closer to the semiconductor surface for TPAA4, thereby exhibiting better lifetime employing both [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+. Using [Co(bpy)3]3+/2+ even though TPAA5 showed better τd and Dn, the variation in CB and structural effects that triggered more recombination lowered the lifetime leading to a lower ηcc for TPAA5 in comparison to TPAA4. At the same time by employing [Co(phen)3]3+/2+ the extent of CB shift was minimal, and the reduced lifetime obtained was judged mainly by the steric effect which helped in achieving better Ln and ηcc for TPAA5. We observed a Marcus normal region behavior in the case of both triphenylamine dyes, TPAA4 and TPAA5, employed along with two different cobalt electrolytes. It is a general strategy for researchers to make use of cobalt redox mediators which are placed at a more positive potential with respect to the conventional iodide/triiodide system along with organic dyes in a way to gain more voltage. With the advantage of low diffusion issues, less bulky nature and lowest charge transfer resistance at the counter electrode [Co(bpy)3]3+/2+ seemed to be the most common electrolyte of choice. We also observed a similar kind of behavior, where [Co(bpy)3]3+/2+ performed better than [Co(phen)3]3+/2+ with TPAA4 dye which is more capable of preventing recombination. However, when these electrolytes were used along with a recombination limited dye (TPAA5) having more infiltration channels, [Co(phen)3]3+/2+ even with less regeneration efficiency and lower driving force for regeneration compared to [Co(bpy)3]3+/2+ showcased better performance than its bipyridine counterpart. This variation is attributed to the shift in conduction band induced by the nature of dye and electrolyte which changed the driving force for recombination. This is quite an interesting observation which dictates the need to have the utmost care while deciding on which combination of dyes

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01325. Synthesis and characterization details of cobalt electrolytes, lifetime measurements using IMVS technique and transport time results from IMPS measurements (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; [email protected]. *E-mail: [email protected]. ORCID

Suraj Soman: 0000-0003-3535-8179 Karical R. Gopidas: 0000-0003-2897-7633 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. gratefully acknowledges financial support from DSTINSPIRE Faculty Award (IFA 13-CH-115). We also thank DST for the DST-SERI Project. S. C. P thank DST-SERI, M. Y. thank CSIR and S. L thank DST-IFA for research fellowships.

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REFERENCES

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