Couple in Quantum Dot Solar Cells

Subscriber access provided by UNIV OF DURHAM 2+/3+

Improving the Efficiency of the Mn Couple in Quantum Dot Solar Cells: The Role of Spin Crossover Matthew C. Kessinger, Rachael Langlois, Jonathan Roof, Shaunak Shaikh, James M. Tanko, and Amanda J Morris J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01361 • Publication Date (Web): 21 Apr 2018 Downloaded from http://pubs.acs.org on April 21, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Improving the Efficiency of the Mn2+/3+ Couple in Quantum Dot Solar Cells: The Role of Spin Crossover Matthew C. Kessinger; Rachael Langlois; Jonathan Roof; Shaunak M. Shaikh.; James M. Tanko.; Amanda J. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 48

Abstract

In this study, we present the synthesis of a family of first-row transition metal redox mediators based on the bis[hydrotris(pyrazolyl)]borate manganese(II/III) (MnTp2) redox couple. Using cyclic voltammetry, the electrochemical properties and characteristic spin crossover inherent in this class of metal complexes was analyzed. From the electrochemical analysis the standard heterogeneous rate constant (ks,h) was estimated. These constants were 2-3 orders of magnitude lower than other outer sphere redox couples, such as Co(bpy)32+/3+ with ks,h values decreasing from 1.52 × 10-2 cm/s in Co(bpy)3 to 1.43 × 10-5 cm/s in bis[hydrotris(4-ethylpyrazolyl)]borate manganese(II/III). It was theorized that the drastic reduction in the rate of electron transfer could be used to increase the lifetimes of the injected electrons in quantum dot sensitized solar cells (QDSSCs). Indeed, this was found to be the case with the slope of open-circuit voltage decay measurements being an order of magnitude lower in the Mn-based redox couples, compared to Co(bpy)3 when using cells prepared under the same conditions. This increase led us to then focus on optimizing the electrolyte solvent to assess the current-voltage characteristics of cells prepared using the MnTp2 family of redox mediators. These cells displayed enhanced power conversion efficiencies when compared to Co(bpy)3 despite poor diffusion throughout the nanostructured TiO2 film. Analysis of the quenching rate constant via Stern-Volmer quenching analysis suggested that the MnTp2 family of redox mediators possesses an adequate ability to regenerate the quantum dot sensitizer, with values of kq being on similar orders of magnitude as other Co- and Cu-based redox couples employed in dye-sensitized solar cells. Ultimately, it was concluded that the increase in the lifetime of the injected electron, working in concert with increased open circuit voltage potentials, was the source of the significantly improved power conversion efficiencies.


2

Page 3 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Since the first efficient sensitization of wide bandgap semiconductors with Ru-based dyes was reported,1 the scientific community has devoted considerable attention toward utilizing such photoanodes (or cathodes) for the direct conversion of sunlight into energy. Termed dyesensitized solar cells (DSSCs), this technology presents itself as a cost-efficient alternative to traditional single-crystal silicon photovoltaics.2-5 The performance of DSSCs is limited to the thermodynamic maximum set by the Shockley-Queisser limit for a single-junction, planar device of approximately 32% power conversion efficiency.6-7 In an attempt to surpass this limit, inorganic sensitizers based on type II-VI semiconductor nanocrystals, known as quantum dots (QDs), were introduced as Generation III photovoltaics. Generation III photovoltaics are so termed because they fundamentally break one of the limiting factors considered in Shockley and Queisser’s calculation: (1) absorption of all photons higher than 1.1 eV in energy from unconcentrated sunlight; (2) one electron in the external circuit per photon absorbed; (3) rapid thermalization of electrons to the lowest vibrational level in the light absorbing molecule or material. QDs have been observed to generate multiple excited charge carriers upon absorption of high energy photons.8-11 Thus, they break the second limiting factor noted above. With the inclusion of multiple exciton generation, Hanna and Nozik calculated a new theoretical maximum of 44.4% for QDSSCs.12 Given the potential of QDSSCs to reach record efficiencies, it is surprising that the highest efficiencies achieved by QDSSCs remain lower than that of DSSCs,13 with efficiencies only recently exceeding 11 %.14-15 To this point, a fair amount of research has been conducted on the optimization of QDSSC efficiency from the perspective of the photoanode and cathode. A significant amount of work has been focused on the optimization of the sensitizing quantum dot. Specifically, a wide variety of


3


Page 4 of 48

quantum dot semiconductor absorbers have been explored and CdS, CdSe, and CdSeTe have emerged as the standard materials.14,

16-17

The sensitization method has also been extensively

studied, including chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR), and the connection of pre-formed monodisperse quantum dots to the wide bandgap semiconductor via molecular linkers.18 More recently, optimization efforts have shifted to alterative counter electrodes to promote the rapid regeneration of the electron acceptor and stability toward the redox electrolyte. 14, 19-21 Interestingly, there have been relatively few studies dealing with the optimization of the liquid electrolyte and, more specifically, the redox mediator responsible for completion of the electrical circuit.16,

22-23

In this case, the sulfide/polysulfide (S2-/Sx2-) electrolyte has received the most

attention.15, 24-25 The S2-/Sx2- electrolyte shares many similar characteristics with the DSSC goldstandard, the I-/I3- electrolyte, in that it is heavily influenced by the choice of electrolyte solvent, supporting additives, and counter electrode material.14, 19, 26-28 However, the S2-/Sx2- electrolyte has often been limited by its complex and poorly understood electrochemistry. Pioneering work by Kamat, et al. informed the community that the addition of a CuxS passivation layer to the surface of CdSe vastly enhances the rate of charge transfer (kct) of photogenerated holes to the polysulfide electrolyte.29 This breakthrough provided crucial insight into the role that CuxS plays in hole mediation at the electrode-electrolyte interface, and helped facilitate the development of crucial counter electrode materials for use with S2-/Sx2-. That said, the high overpotential for QD regeneration, exhibited by S2-/Sx2-, has limited the ability to reach satisfactory open circuit voltages (Voc) and thus, power conversion efficiencies. As such, the development of electrolytes capable of overcoming the limitations of S2-/Sx2- is paramount.


4

Page 5 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In alternative redox mediator research, there are two prevailing approaches to maximize the power conversion efficiency of a solar cell, η. To introduce the fundamental basis for each route, a discussion of the mathematical expression η and the terms therein is necessary, eq. 1-5. Power conversion efficiency can be simply thought of as the ratio of the input power, Pin, to the maximum output power observed by the cell, Pmax. The Pmax would be determined at the point where the product of the cell voltage and resultant current is greatest. In practice, Pmax is determined by the product of the short circuit photocurrent (JSC), the open circuit voltage (VOC), and the fill factor (FF). If broken down into component fundamental process efficiencies, it is clear that three major processes define sensitized solar cell efficiency – injection efficiency (ηinj), regeneration efficiency (ηreg), and charge collection efficiency (ηcc), eq. 2-3. The third process, ηcc, reflects the comparison of the rates of electron transport in the wide bandgap semiconductor to that of deleterious recombination of the electron to the oxidized mediator or sensitizer. Thus, the electron lifetime, τo, is critical, eq. 3.

=

= () " # $%&

(1)

(2)

!

=

'(

,ln + 0 ./-

(3)


5


Page 6 of 48

The two approaches to maximized efficiency focus on one of these critical processes, specifically, ηreg or ηcc. The ηinj is defined by the ratio of the rate of injection (kinj) to that of radiative (kr) and non-radiative relaxation (knr) of the sensitizer.

# =

' # '$ + ' $

(4)

Since the rate of electron injection typically occurs on the femtosecond to picosecond timescale and relaxation on the nanosecond time scale30, the large difference in observed rate constants necessitates that ηinj be near unity for most standard sensitizers.31 Therefore, appropriate emphasis is placed on the other two processes. Similar to ηinj, ηreg is defined by the ratio of the rate of regeneration (kreg) to that of radiative (kr) and non-radiative relaxation (knr) of the sensitizer. To increase ηreg, steps should be taken to maximize kreg. When exploring one electron outer-sphere redox shuttles, like the manganese compounds explored herein, the rate of regeneration can be expressed by the Marcus Cross relation.

'23 = 4'(56/5) '(86/8) 985 :23 ;23

(5)

The Marcus Cross relation states that the overall rate constant for regeneration will be related to the self-exchange rates for the dye (kD+/D) and the mediator (kR+/R), as well as the equilibrium constant for the reaction (KRD). Therefore, the strategy employed is to use highly reversible electrochemical couples that exhibit rapid self-exchange. Ferrocene is a classic example of a redox mediator in this category and indeed, exhibits near quantitative regeneration efficiency.32 Additionally, Hamann has pioneered work in this area with the development of Co(ttcn)23+/2+ and [Co(ptpy)3]+/0.32-33 The typical downfall to this approach is that recombination to the oxidized


6

Page 7 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


form of these mediators from the TiO2 conduction band is rapid, which results in a decreased ηcc. It should be noted that the use of blocking layers has been shown to attenuate these effects.34 The second strategy focuses on increasing τo and ηcc through slowed recombination. In opposition to the use of compounds with rapid self-exchange rates, complexes that undergo a structural rearrangement upon a charge transfer event and therefore, quasi-reversible to irreversible electron transfer properties, are used. The quintessential redox mediator that exhibits such properties is Co(bpy)3.35 Indeed, DSSC efficiencies reported utilizing this mediator have seen rapid advances in recent years with current efficiencies exceeding those of I-/I3-.36 The success of Co(bpy)3-based electrolytes has been attributed to charge transfer induced spin crossover (CTISC) from the high spin (HS) d7 electron configuration to the low spin (LS) d6 electron configuration upon regeneration. The spin change is accompanied by geometric constriction of the ligand environment, which imparts a kinetic barrier to recombination between the injected electron and the Co3+ form. Ultimately, the kinetic barrier can also affect regeneration of the mediator at the counter electrode and/or by self-exchange through solution. Thus, in both strategies there are trade-offs that must be considered. Outside of cobalt based redox mediators, few CTISC complexes have been reported. In work by Schultz, it was introduced that metal complexes based on manganese(II) atoms surrounded by an N6 coordination system such as that of bis-metal-pyrazolylborate complexes undergo CTISC during the stepwise oxidation from Mn2+ (HS d5) > Mn3+ (LS d4) > Mn4+ (HS d3).37 This spin exchange event, observed across a series of MnTp2 analogues, where Tp stands for the hydrotris(pyrazolyl)borate ligand, was attributed to the wide Mn-N-N bite angle (c.a. at 119.93738

), affording a ligand field strength that lies close to the spin exchange barrier. The identity of

substituents on the pyrazole ring itself plays a key role in directing both the electronic and


7


Page 8 of 48

magnetic effects of these metal complexes, with steric effects playing a larger role than inductive effects within the MnTp2 family of metal complexes.39 From this reasoning, it was believed that a series of bis[hydrotrispyrazolyl])borate]manganese(II) complexes, with systematic substitution at the 4-position of pyrazole with linear alkyl groups, could be prepared. These metal complexes would exhibit the characteristic CTISC associated with the MnTp2 redox couple. They would also present themselves as prime candidates for incorporation into QDSSCs as redox mediators, affording reduced recombination kinetics and enhanced open circuit potentials. Previously, our lab presented a new class of alternative redox mediators based on bis-chelated Mn2+/3+ complexes utilizing the poly(pyrazolyl)borate ligand (MnTp2, MnTp2*, and Mn(pzTp)2). These complexes displayed reduction potentials more positive than other redox electrolytes, which translated directly to increases in the open circuit potential of similar magnitude.40-41 In addition, the prepared cells produced agreeable fill factors between 55 % and 66 %, a two-fold increase over S2-/Sx2- measured under similar conditions.42 Despite these advantages, however, the MnTp2, MnTp2*, and Mn(pzTp)2 redox mediators displayed inferior solubility in a wide range of common solvents used in QDSSC electrolytes, with a maximum concentration of 50 mM being achieved with MnTp2 in γ-butyrolactone.40 To this end, there is significant interest in synthesizing analogues of MnTp2 which would display enhanced solubility while also retaining the superior electronic properties of MnTp2 in relation to other transition metal redox mediators. In this study, a series of pyrazoles substituted in the R4 position were synthesized and used to prepare

a

series

of

coordination

complexes

based

on

the

manganese(II)-

bis[hydrotris(pyrazolyl)borate] (MnTp2) redox mediator (Figure 1).


8

Page 9 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Structures of the MnTp2 series of redox mediators, the vertical N-N bonds indicate the presence of a third pyrazole ring present perpendicular to the plane of the paper. This

series

of

alkylated

ligands

afforded

increased

solubility

to

the

manganese

hydrotris(pyrazolyl)borates in nitrile based solvents such as butyronitrile and valeronitrile, with the maximum solubility for all of the prepared metal complexes obtained in a binary mixture of 80:20 THF:valeronitrile (v:v). The prepared redox mediators exhibit charge transfer-induced spin crossover (CTISC) as evidenced by their effective magnetic moments (µeff) determined via NMR. A cathodic shift in the reduction potential for complexes prepared using the alkylated ligands compared to that of MnTp2 was observed and was attributed to the electron-donating nature of the alkylated pyrazole rings relative to that of unsubstituted pyrazole. The lifetime of the injected electron was determined via the transient voltage decay experiments under open circuit conditions, confirming the reduced recombination kinetics exhibited by this class of redox mediators. The injected electron lifetimes were correlated to apparent rates of electron transfer


9


Page 10 of 48

determined via electrochemical measurements and photoluminescence quenching to discern design rules for the development of alternative redox mediators. Experimental Chemicals. Phosphorus (V) oxychloride, calcium (II) chloride, manganese (II) chloride tetrahydrate, cadmium (II) nitrate tetrahydrate, butanal, hexanal, and octanal were all purchased from Alfa Aesar. Hydrazine monohydrochloride was purchased from TCI Chemicals. Thallium (I) acetate and 2,2’-bipyridine were purchased from Acros Organics. Cobalt (II) chloride tetrahydrate was purchased from Amresco Chemicals. Pyrazole, potassium borohydride, ferrocene, zinc (II) nitrate hexahydrate, titanium (IV) chloride, sodium sulfide, thioglycolic acid (TGA), and bromine were purchased from Sigma Aldrich. Sodium sulfide nonahydrate was purchased from Oakwood Chemical. Nitrosonium hexafluoroantimonate was purchased from Strem Chemicals. Commercial titania paste was purchased from Solaronix. All solvents and chemicals were used as received without further purification unless indicated otherwise. Synthesis

of

1,1-diethoxyalkanes.

1,1-diethoxybutane,

1,1-diethoxyhexane,

and

1,1-

diethoxyoctane were prepared using a modified literature procedure.43 The same general procedure was used for the preparation of each of the aforementioned 1,1-diethoxyalkanes. As such, a detailed procedure will be listed for 1,1-diethoxybutane only. 1,1-diethoxybutane. 21 g (194 mmol) of CaCl2 was added to a 100 mL round bottom flask. To the solution was added 20 mL (342 mmol) of 200 proof EtOH, and allowed to stir for 5 minutes. 20 mL of butyraldehyde (222 mmol) was dissolved in 10 mL of EtOH (171 mmol) and added to the CaCl2/EtOH slurry dropwise. After complete addition of the butyraldehyde, the flask was equipped with a reflux condenser and stirred at room temperature for two hours. Afterwards, the reaction mixture was allowed to stand overnight. The organic layer was decanted off and the


10

Page 11 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


remaining solid washed thoroughly with CH2Cl2. The combined organic portions were dried with MgSO4 and filtered through celite. The solvent was then reduced under rotary evaporation to yield 1,1-diethoxybutane in acceptable purity. Yield: 89.3%. 1H NMR (400 MHz, CDCl3) 4.48 (t, 1H, CH), 3.62 (q, 2H, O-CH2), 3.47 (q, 2H, O-CH2), 1.56 (m, 4H, CH2CH2 ), 1.19 (t, 6H, CH3), 0.91 (t, 3H, CH3). 1,1-diethoxyhexane. Yield: 94.0%. 1H NMR (400 MHz, CDCl3) 4.42 (t, 1H, C-H), 3.57 (m, 2H, O-CH2), 3.42 (m, 2H, O-CH2), 1.53 (m, 2H, CH2CH2CH2CH2), 1.24 (m, 2H, CH2CH2CH2CH2), 1.14 (t, 6H, CH3), 0.83 (t, 3H, CH3). 1,1-diethoxyoctane. Yield: 95%. 1H NMR (400 MHz, CDCl3) 4.3 (t, 1H, CH), 3.45 (m, 2H, OCH2), 3.3 (m, 2H, O-CH2), 1.42 (m, 2H, (CH2)6), 1.11 (m, 10H, (CH2)6), 1.02 (t, 6H, CH3), 0.71 (t, 3H, CH3). Synthesis of 4-alkylpyrazoles. 4-ethyl-1H-pyrazole, 4-butyl-1H-pyrazole, and 4-hexyl-1Hpyrazole were synthesized using literature procedures.44 The same general procedure was used for the synthesis of each pyrazole, and a detailed procedure for the preparation of 4-ethyl-1Hpyrazole is given below. 4-ethyl-1H-pyrazole. A 300 mL, 3-neck round bottom flask equipped with a rubber septa, a gas inlet tap, a magnetic stirrer, and a pressure equalizing addition funnel was connected to a schlenk line. The apparatus was placed under inert atmosphere via evacuation and nitrogen backfill (3x). 17.4 mL (186 mmol) of POCl3 was added to the apparatus via injection, and chilled to 0 °C. Following this, 18 mL (247 mmol) of anhydrous DMF was added dropwise. The vilsmeier reagent precipitated from solution in pale yellow crystals. Then, 15 mL (85 mmol) of 1,1diethoxybutane was added to the reaction flask, dropwise. The apparatus was then placed into a preheated oil bath set to 80 °C and heated for two hours while stirring. A gradual color change


11


Page 12 of 48

from pale yellow, to orange, to a dark brown/red occurred. Afterwards, the reaction mixture was cooled to room temperature and poured onto 200 g of crushed ice and stirred overnight. The solution was neutralized using anhydrous K2CO3 added in 2 g portions to prevent overflowing the beaker due to effervescence. Once the pH of the solution reached pH 7.0, it was poured into a separatory funnel and additional water was added to dissolve the precipitated salts. This solution was then extracted with DCM (5x75 mL), CHCl3 (5x75 mL), Ether (3x75 mL), and THF (2x75 mL). The combined organic extracts were dried with MgSO4 and the solvent removed under reduced pressure to provide a dark red/brown oil. This oil was then dissolved in methanol and reacted with 8.62 g (126 mmol) of hydrazine monohydrochloride dissolved in deionized (DI) H2O. The reaction mixture was allowed to reflux for 2 h, after which it was cooled to room temperature, and neutralized with NaHCO3 until a neutral pH was obtained. The reaction mixture was then poured into a separatory funnel and extracted with DCM (5x75 mL). The combined extracts were dried with MgSO4, and the solvent removed under reduced pressure. The resulting brown oil was transferred to a distillation apparatus and distilled under vacuum (130 °C/13 mbar) to isolate the desired product. Yield: 52% (based on starting acetal.) 1H NMR (400 MHz, CDCl3) 12.81 (s, br, 1H, N-H), 7.43 (s, 2H, CH-3,5-Pz), 2.55 (m, 2H, CH2), 1.23 (t, 2H, CH3). 4-butyl-1H-pyrazole. The resulting red/brown oil from the cyclization reaction was distilled under vacuum (145 °C/13 mbar) to isolate the desired product as a slightly yellow oil. Yield: 51.9% (based on starting acetal.) 1H NMR (400MHz, CDCl3) 12.08 (s, br, 1H, N-H), 7.40 (s, 2H, CH-3,5-Pz), 2.49 (m, 2H, CH2-CH2CH2CH3), 1.55 (m, 2H, CH2-CH2CH3), 1.35, (m, 2H, CH2CH2CH3), 0.93 (t, 3H, CH3).


12

Page 13 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4-hexyl-1H-pyrazole. The resulting red/brown oil from the cyclization reaction was distilled under vacuum (165 °C/13 mbar) to isolate the desired product as a waxy white solid. Yield: 83% (based on starting acetal.) 1H NMR (400 MHz, CDCl3) 7.40 (s, 2H, CH-3,5-Pz), 2.49 (t, 2H, CH2(CH2)4CH3), 1.56 (m, 2H, CH2(CH2)3CH3), 1.30 (m, 6H, CH2CH2CH2), 0.88 (t, 3H, CH3). Synthesis of Mn2+ and Mn3+ Complexes. MnTp2, MnTp2(4-Et), MnTp2(4-Bu), and MnTp2(4-Hex) where Tp represents the hydrotris(pyrazolylborate) ligand and Tp(4-x) represents hydrotris(4alkylpyrazolyl)borate ligand, along with their oxidized salts, MnTp2SbF6, MnTp2(4-Et)SbF6, MnTp2(4-Bu)SbF6, and MnTp2(4-Hex)SbF6, were synthesized using modified literature procedures.37, 40, 45-46

Bis[hydrotris(1-[4-ethylpyrazolyl])borate]manganese(II) (MnTp2(4-Et)). 5.25 g of 4-ethyl-1Hpyrazole (54 mmol) and 0.8396 g of KBH4 (16 mmol) was added to a 25 mL round bottom flask. The reaction mixture was heated under nitrogen while stirring first to 80 °C, then 120 °C, then to 150 °C, and finally to 180 °C, until H2 evolution had ceased. The reaction progress was monitored via 11B NMR until all of the starting KBH4 (400 MHz, -35 ppm), had been converted to KTp4-Et (400 MHz, 0.54 ppm). The reaction was then cooled to room temperature and dissolved in 30 mL of Acetonitrile. The suspension was centrifuged (10,000 rpm, 3x), the supernatant collected, and the solvent removed under reduced pressure. The resulting oil was dissolved in hot deionized water, and 2.2192 g (8.4 mmol) of thallous acetate was added to precipitate the Tp4-Et ligand as its thallium(I) salt. This salt was then dissolved in DCM and reacted in the correct stoichiometric ratio with a THF solution of MnCl2·4H2O to produce MnTp24-Bu. The resulting reaction mixture was then filtered through an alumina plug, and the solvent removed under evaporation. The resulting oil was then dissolved again in hexanes and filtered through a silica plug. The solvent was again removed under evaporation to yield a


13


Page 14 of 48

crystalline white powder which was characterized via mass spectroscopy and cyclic voltammetry. (m/z calc.: 649.34, found: 649.3396 [M]+). Bis[hydrotris(1-[4-butylpyrazolyl])borate]manganese(II) (MnTp2(4-Bu)). The resulting reaction mixture from the reaction of 4-butyl-1H-pyrazole with KBH4 was dissolved in 30 mL of acetonitrile and centrifuged (10,000 rpm, 2x). The supernatant was collected, then the precipitate was again suspended in 30 mL of hexanes and centrifuged (10,000 rpm, 2x). The two supernatant fractions were combined and the solvent removed under reduced pressure. The resulting oil was dissolved in hot deionized water and thallous acetate was again added to form the thallium(I) salt of the Tp4-Bu ligand. The mixture was stirred overnight, before being extracted with DCM (50mL, 4x). The combined extracts were dried using MgSO4 and the solvent removed under reduced pressure. The resulting oil was kept overnight at -10 °C to solidify. The resulting solid was dissolved in DCM and a stoichiometric THF solution of MnCl2·4H2O was added to form the MnTp24-Bu complex. The reaction solution was filtered through alumina, and the solvent removed under reduced pressure. The resulting oil was then dissolved in hexanes and filtered through silica, and the solvent removed under reduced pressure to produce a fluffy white powder, which was characterized via mass spectroscopy and cyclic voltammetry (m/z calc.: 817.53, found: 818.5350 [M+H]+). Bis[hydrotris(1-[4-hexylpyrazolyl])borate]manganese(II) (MnTp2(4-Hex)). The resulting reaction mixture from the reaction of 4-hexyl-1H-pyrazole with KBH4 was dissolved in 30 mL of hexanes and centrifuged (10,000 rpm, 3x). The supernatant was collected and the fractions combined. The solvent was removed under reduced pressure, and the resulting oil was dissolved in a hot methanol/water solution (40:60; v:v). Thallous acetate was then added to form the thallium(I) salt of the Tp(4-Hex) ligand. The aqueous solution was stirred overnight before being extracted


14

Page 15 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with DCM (50 mL, 4x). The combined extracts were dried using MgSO4, filtered, and the solvent removed under reduced pressure. The resulting oil was then dissolved in DCM, and a stoichiometric amount of MnCl2·4H2O dissolved in THF was added to form the MnTp2(4-Hex) complex. The reaction was filtered through alumina, and the solvent removed under reduced pressure. The oil was then dissolved in hexanes and filtered through silica, and the solvent removed under reduced pressure. The resulting oil was allowed to solidify at room temperature overnight into a fluffy white powder which was characterized by mass spectroscopy and cyclic voltammetry. (m/z calc.: 986.72, found: 986.7290 [M]+). MnTp2SbF6, MnTp2(4-Et)SbF6, MnTp2(4-Bu)SbF6, and MnTp2(4-Hex)SbF6. The oxidized form of each metal complex was prepared through the reaction of the requisite MnTp(4-x) metal complex with an equimolar amount of NOSbF6 dissolved in MeCN. The reaction mixture went from clear to yellow almost immediately with the evolution of NO(g), indicating oxidation of the metal complex. The reaction mixtures were stirred for 1 h. They were then filtered through a fine frit and the solvent removed under reduced pressure to yield the desired product. Synthesis

of

Co2+

bis(hexafluorophosphate)

and

Co3+

Complexes.

[Co(bpy)3][PF6]2

and

Cobalt(II) cobalt(III)

tris(2,2’-bipyridine) tris(2,2’-bipyridine)

tris(hexafluorophosphate) [Co(bpy)3][PF6]3 were synthesized using previously reported methods.41 To synthesize [Co(bpy)3][PF6]2, 1.000g (7.7 mmol) of CoCl2·6H2O was dissolved in H2O, and placed in an addition funnel. Following this procedure, 4.000g (26 mmol) of 2,2’bipyridine was dissolved in a 150 mL RBF in MeOH. The aqueous solution of CoCl2·6H2O was then added dropwise to the methanolic ligand solution while stirring. The color quickly changed from clear to dark yellow. The solution was stirred for 1 h before the dropwise addition of an aqueous solution of 3.544 g (19 mmol) of KPF6 was used to precipitate the desired metal


15


Page 16 of 48

complex. The tan powder was collected via filtration and washed with excess H2O, then dried under vacuum overnight. [Co(bpy)3][PF6]3 was synthesized in a similar manner beginning with the dropwise addition of an aqueous solution of 0.5000 g (3.8 mmol) of CoCl2·6H2O to a stirred methanolic solution of 1.8648 g (11.9 mmol) of 2,2’-bipyridine. The solution was stirred for 5 minutes before the slow addition of 0.2 mL of bromine. The reaction mixture was then filtered to remove the precipitate, and the desired metal complex was obtained by the addition of an aqueous solution of 2.4 g (13 mmol) of KPF6. The solid was collected via filtration, washed with ample H2O, and then dried overnight under vacuum. Electrochemical Characterization. All electrochemical analysis was performed on a Princeton Applied Research (EG&G/PAR) Model 283 potentiosat/galvanostat. Instrumental parameters were controlled using Electrochemistry PowerSuite v. 2.58 from Princeton Applied Research (Advanced Measurement Technology Inc.). Cyclic voltammograms were performed using a three electrode arrangement with a glassy carbon working electrode polished using diminishing sizes of alumina slurry, ranging from 1 µm to 0.05 µm between each scan. The reference electrode was 0.01 M Ag(NO3) dissolved in distilled acetonitrile with 0.1 M TBAPF6. The counter electrode for the cell was platinum mesh. Distilled butyronitrile was used as the solvent with 0.3 M TBAPF6 as the supporting electrolyte. Electrochemical measurements were carried out under anaerobic conditions with 95% IR compensation. Potentials were referenced to the NHE electrode using the Fc/Fc+ couple. Standard heterogeneous rate constants were calculated using two different methods. For the Co2+/3+ couple, which displayed peak splitting under 250 mV, a method developed by Nicholson was used. This method relates the peak splitting between the cathodic and anodic peaks (∆EP) at


16

Page 17 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


a given sweeprate (ν) to the kinetic parameter ψ.47 In the case of the MnTp2 family of redox mediators, whose peak splitting lies outside the useable range provided in the work of Nicholson, the apparent heterogeneous rate constant was calculated directly from the peak splitting by eq 1 in ref. 39.48 Photoanode Fabrication.

Photoanodes

were

fabricated

using previously published

procedures.40, 49 Fluorine-doped Tin Oxide (FTO, 10 Ω/sq Hartford glass) was first cleaned by sonication for 15 min each in a solution of alconox, 200 proof ethanol, and finally acetone. Afterwards, a transparent blocking layer of compact TiO2 (cp-TiO2) was applied via thermal decomposition of a 60mM aqueous solution of TiCl4 at 70 °C (2x, 30 min). The electrodes were then rinsed with deionized H2O and dried. The TiO2 active layer was deposited using the doctor blade method, and using a commercial transparent TiO2 paste (T/SP, 100% anatase, 15-20nm from Solaronix). The deposited TiO2 films were dried in an oven at 80 °C for 30 min. and then at 130 °C overnight before being calcinated at 500 °C for 1 h. After cooling to room temperature, a post treatment of aqueous TiCl4 (20 mM) was applied for 1h. Afterwards, the electrodes were rinsed with DI water and dried. The electrodes were then sensitized using a successive ionic layer adsorption and reaction (SILAR) process to deposit a CdS active layer on the prepared FTO/cp-TiO2/mp-TiO2 electrodes. Briefly, the electrodes were submerged, first in a precursor solution of 0.2 M Cd(NO3)2·4H2O dissolved in 50 mL of 1:1, H2O:EtOH (v:v) for 1 min. The electrodes were then removed from the solution and rinsed for 1 min using the neat solvent mixture. Next, they were submerged in 0.2 M Na2S·9H2O dissolved in 50 mL of 1:1, H2O:EtOH (v:v) for one min. After removal from the second precursor solution, the electrodes were again rinsed for one min. This process is considered one SILAR cycle of CdS. The electrodes were sensitized with 8 cycles of CdS followed by 2 cycles of ZnS using 0.1 M solutions of


17


Page 18 of 48

Zn(NO3)2·4H2O and 0.1 M Na2S·9H2O. The electrodes in this state are hereafter referred to as CdS-8-ZnS-2. Device Fabrication. Photovoltaic devices were prepared using a modified 3-electrode cell. A Teflon block (1.6 cm × 2.7 cm × 3.9 cm), was drilled to allow for the use of a Pt-mesh counter electrode and an interior chamber to be filled with electrolyte (Figure S1). The prepared manganese complexes were compared to Co(bpy)3 as a redox mediator in a two electrode configuration, with the CdS-8-ZnS-2 electrode as the working electrode, and a Pt-mesh counter electrode. The cobalt and manganese electrolytes consisted of a 10:1 molar ratio of the reduced and oxidized species for each manganese and cobalt redox mediator. The named concentration refers to the concentration of the reduced (majority) species with 0.5 M LiClO4 as supporting electrolyte dissolved in either 80:20, THF:Valeronitrile (v:v) for the manganese electrolyte, or acetonitrile for the cobalt electrolyte, unless specified otherwise. Device Characterization. Three photovoltaic cells were prepared in series for each measurement. The cells were measured by first masking the cell to an active area of 0.1256 cm2. The cells were then illuminated at 20%, 40%, 60%, 80%, and 100% sun intensities using a Newport LCS-100 solar simulator (AM 1.5G spectrum). The current-voltage response of the cells was recorded at a scan rate of 20 mV/s using linear sweep voltammetry. Open-circuit voltage decay (OCVD) measurements were collected by disrupting the cell from steady state conditions and recording the voltage decay at 50 ms intervals. Unless stated otherwise, the results and standard deviations presented in this work indicate the average of three cells prepared in parallel. Synthesis of Colloidal Quantum Dots. Colloidal CdS quantum dots capped with thioglycolic acid (TGA) were prepared using previously published procedures,50 wherein, 25 mL of DMSO


18

Page 19 of 48 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was purged with Ar(g) for 30 min. Following the purge, 1.25 mmol of Cd(NO3)2·4H2O was added to the solution, the pH adjusted to 9.0 via the addition of aqueous 0.1 M NaOH, and the mixture heated to 70 °C while stirring under nitrogen. This temperature was maintained for 30 min, after which, 180 µL of thioglycolic acid (TGA) was added to the solution, and the mixture was stirred again for 30 more minutes. During this time, 0.625 mmol of Na2S was dissolved in degassed DMSO. After 30 min, the Na2S solution was slowly added to the reaction mixture, over the course of 5 min, via an addition funnel. The reaction mixture was stirred for 30 min at 70 °C, after which it was removed from the heat, cooled to room temperature, and stored in a vial under Ar(g), without access to light until further use. Photoluminescence Measurements. Photoluminescence (PL) and PL quenching measurements were made on a Photon Technology International Inc. (PTI Inc.) Quanta-Master 400 series fluorometer, using a 75 W xenon lamp as the source and a model 914 photo multiplier tube detector (PMT). A suspension of colloidal CdS QDs formed by mixing 2.0 mL of the QD stock solution with 1.0 mL of toluene. The solution was purged under Ar(g) for 30 min prior to the measurements to avoid quenching from oxygen. The temperature was held constant for all measurements. For steady state emission measurements, the sample was excited at 380 nm and the emission measured between 400 nm and 650 nm. The excitation and emission slit widths, as well as the step size and integration time, were held consistent across each sample and measurement. Lifetime measurements were conducted by exciting the sample at 340 nm from an LED and monitoring the fluorescence decay as a function of time. Results and Discussion Analysis and Characterization of Charge Transfer-Induced Spin Crossover. To assess the spin crossover character of the MnTp2 family of redox mediators, magnetic measurements were


19


Page 20 of 48

carried out using the Evans Method. This method relates the chemical shift of a pure solvent signal to the displacement in the chemical shift observed in the signal for solvent, with a dissolved paramagnetic component, through the following relationship (eq. 6).51-52

- − +

Couple in Quantum Dot Solar Cells

Recommend Documents