Udo Bach,bcd Richard A. Evans,b C. Andre Ohlina and Leone Spiccia*a
The abundance and low toxicity of manganese have led us to explore the application of manganese
complexes as redox mediators for dye sensitized solar cells (DSCs), a promising solar energy conversion
technology which mimics some of the key processes in photosynthesis during its operation. In this paper,
we report the development of a DSC electrolyte based on the tris(acetylacetonato)manganese(III)/(IV),
[Mn(acac)3]0/1+, redox couple. PEDOT-coated FTO glass was used as a counter electrode instead of the
conventionally used platinum. The influence of a number of device parameters on the DSC performance
was studied, including the concentration of the reduced and oxidized mediator species, the concentration
of specific additives (4-tert-butylpyridine, lithium tetrafluoroborate, and chenodeoxycholic acid) and the
thickness of the TiO2 working electrode. These studies were carried out with a new donor–p–acceptor
sensitizer K4. Maximum energy conversion efficiencies of 3.8% at simulated one Sun irradiation (AM 1.5 G;
1000 W m�2) with an open circuit voltage (VOC) of 765 mV, a short-circuit current (JSC) of 7.8 mA cm�2
and a fill factor (FF) of 0.72 were obtained. Application of the commercially available MK2 and N719 sensiti-
zers resulted in an energy conversion efficiency of 4.4% with a VOC of 733 mV and a JSC of 8.6 mA cm�2
for MK2 and a VOC of 771 mV and a JSC of 7.9 mA cm�2 for N719. Both dyes exhibit higher incident
photon to current conversion efficiencies (IPCEs) than K4.
Introduction
With the possibility of the earth’s oil reserves potentiallydepleting before the end of this century, it is essential todevelop sustainable and renewable energy sources that canfulfil the world’s ever increasing energy demand while at thesame time minimizing the detrimental effects of the presentday energy utilization on climate, the environment and health.1
Among the energy sources available, solar energy is expected toplay a crucial role as a future sustainable energy source.1
The dye sensitized solar cell (DSC) is one promising tech-nology that mimics the natural photosynthetic process wheresunlight is directly converted to usable energy. The sensitizer inthe DSC absorbs the incident sunlight and induces the electrontransfer within the cell, imitating the role of chlorophyll in agreen leaf.2–4 DSCs can also harvest solar energy at low cost, lowpayback times and with energy conversion efficiencies of>12%.1,5,6 As shown in Fig. 1, DSCs are typically composed ofa dye-sensitized mesoporous semiconductor electrode, a coun-ter electrode and an electrolyte containing a redox couple. Theelectrolyte facilitates charge transfer between the two electrodesand dye regeneration. The latter is accomplished throughelectron transfer from the reduced redox species to the photo-oxidized dye.7–9
The redox mediator should possess the following key proper-ties in order to be suitable for DSC applications: (i) lowabsorption in the visible region (400–800 nm) so that it doesnot compete with the dye for the incident photons; (ii) fast andselective electron transfer to the oxidized dye (dye regeneration)and from the counter electrode to the oxidized form of themediator; and (iii) a high diffusion coefficient to ensure quickionic charge transport across a cell gap of typically Z10 mm.10,11
Iodide/triiodide (I�/I3�) has been the most widely investi-
gated redox mediator in DSCs. However, in recent years thecorrosiveness and complex two-electron redox chemistry have
a School of Chemistry, Monash University, Victoria 3800, Australia.
Bag 10, Clayton South, 3169 Victoria, Australiac Department of Materials Engineering, Faculty of Engineering, Monash University,
Wellington Road, Clayton, 3800 Victoria, Australiad Tech Fellow, The Melbourne Centre for Nanofabrication, 151 Wellington Road,
Clayton, 3168 Victoria, Australia
† Electronic supplementary information (ESI) available: Synthesis of a K4 sensi-tizer, UV-visible spectra of [Mn(acac)3], [Mn(acac)3]BF4, [Mn(acac)3]0/1+ and[Co(bpy)3]2+/3+ based electrolytes, Photovoltaic parameters for the dependenceof the efficiency of the DSC on the [Mn(acac)3]/[Mn(acac)3]+ ratio, LiBF4 concen-tration, tBP concentration, TiO2 layer thickness, chenodeoxycholic acid concen-tration and different counter electrodes. EIS data for the symmetric cells preparedwith different counter electrodes, Photovoltaic parameters and IPCE data andIMVS/IMPS data for I�/I3
�, [Co(bpy)3]2+/3+ and [Mn(acac)3]0/1+ based devices. SeeDOI: 10.1039/c3cp54894e
Received 25th November 2013,Accepted 6th December 2013
fuelled the search for alternative electrolytes.12 Excellent deviceperformances have been achieved with substitutes such as ferro-cene (Fc/Fc+),13 a variety of cobalt complexes6,14,15 and organiccompounds.12,16 A large number of cobalt compounds, such as[Co(phen)3]2+/3+, [Co(dtb)3]3+/2+, [Co(dmb)3]3+/2+, [Co(terpy)2]2+/3+,[Co(Cl-terpy)2]2+/3+, have been investigated with a variety of sensi-tizers11,12,14 (phen = 1,10-phenanthroline, dtb = 4,40-ditertiary-butyl-2,20-bipyridine, dtm = 4,40-dimethyl-2,20-bipyridine andterpy = 2,20,60,200-terpyridine). In a recent remarkable achievement,the [Co(bpy)3]3+/2+ redox couple was applied in DSCs in combi-nation with an organic donor–acceptor dye and a porphyrin dye toachieve a photo-voltage of almost 1.0 V and a power conversionefficiency of 12.3%.6
Our group has very recently investigated the application ofthe cobalt complexes of higher denticity (pentadentate andhexadentate) polypyridyl ligands as redox shuttles in DSCsand has found that the stability of the devices is substantiallyimproved.17,18 In addition, the Fc/Fc+ redox couple was shownto be a suitable replacement for the traditional I�/I3
� redoxcouple with a light to electricity conversion efficiency of 7.5%,arising from the more favourable redox potential of the Fc/Fc+
electrolyte compared to that of the I�/I3� electrolyte.9,13,19
Other examples of redox mediators recently used in DSCsinclude copper complexes,20 nickel complexes,21 Br�/Br2,21,22
SCN�/SCN2,22 SeCN�/SeCN222–24 and organic redox couples,
such as 2,2,6,6-tetramethyl-1-piperidinyloxy, TEMPO.16 Due totheir instability, energy incompatibility with sensitizer dyes ortheir intrinsic low diffusion coefficients in the electrolytes,these redox couples have shown lower efficiencies than theI�/I3
� redox couple.20–26
In searching for less toxic metal complexes to apply in DSCs,we have investigated a new electrolyte based on thetris(acetylacetonato) manganese(III)/(IV), [Mn(acac)3]0/1+, com-plexes (Fig. 2(a)). To the best of our knowledge, this is the first
time that the application of a manganese compound as a DSCredox mediator has been reported. It is noteworthy that due tothe natural abundance, low toxicity and appropriate redoxchemistry of manganese, a Mn cluster has evolved in natureto play the crucial role of regenerating the dye following lightinduced oxidation during the process of photosynthesis.27,28
The acac ligand offers the possibility of fine-tuning the redoxpotential of these complexes through the modification of theacac ligand at either the central methylene or the terminalmethyl groups.
In this paper, we have optimized the performance of[Mn(acac)3]0/1+ based DSCs in conjunction with a new organicsensitizer, K4 (Fig. 2(c)). The influence of the composition ofthe electrolyte, the titanium dioxide working electrode (WE)thickness and the counter electrode (CE) was studied. Fourdifferent types of CE were tested, viz. thermally decomposedplatinum/fluorine-doped tin oxide (FTO), sputter-coated Pt/FTO,sputter-coated gold/FTO, and poly(3,4-ethylenedioxythiophene)(PEDOT)/FTO. The concentrations of the redox couple, 4-tertiary-butylpyridine (tBP), lithium tetrafluoroborate (LiBF4) and cheno-deoxycholic acid (cheno), were all considered in the optimizationof the electrolyte composition. Testing of the optimized[Mn(acac)3]0/1+ redox couple with a commercial organic sensiti-zer, MK2, and a ruthenium sensitizer, N719 (Fig. 2(b)), led to anefficiency of 4.4%.
ExperimentalMaterials and reagents
Reagents and chemicals were, unless otherwise specified, pur-chased from Sigma Aldrich and Merck Specialty Chemicals(Sydney, Australia) and were used as received. The TiO2 pastewas purchased from JGC Catalysts and Chemicals Ltd (Kanagawa,Japan). MK2, 2-cyano-3-[50 0 0-(9-ethyl-9H-carbazol-3-yl)-30,300,30 0 0,4-tetra-n-hexyl-[2,20,50,200,500,20 0 0]-quater thiophen-5-yl] acrylic acidand N719, di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato)ruthenium(II) were purchased fromSigma-Aldrich.
Fig. 1 Structure of a typical DSC comprising two conducting electrodesand an electrolyte containing a redox couple. The electrodes are typicallyconstructed on glass substrates, coated with a transparent conductingoxide (TCO) layer. The working electrode consists of titanium dioxide(TiO2) that is sensitized with a dye whereas the counter electrode istypically fabricated with a catalyst which promotes the reduction of theoxidized form of the redox couple in the electrolyte. The two conductingglass substrates are connected via the external circuit.
Fig. 2 Chemical structure of (a) [Mn(acac)3]0/1+ and the sensitizers (b)N719 (c) MK2 and (d) K4.
The dye, 3-6-(4-(bis(4-((2-ethylhexyl)oxy)phenyl)amino)phenyl)-4-(2-ethylhexyl))-4H-dithieno[3,2-b:20,30-d]pyrrol-2-yl)-2-cyanoacrylicacid (K4), was synthesized by reacting the corresponding aldehydeprecursor, 6-(4-(bis(4-((2-ethylhexyl)oxy)phenyl)amino)phenyl)-4-(2-ethylhexyl))-4H-dithieno[3,2-b:20,3 0-d]pyrrole-2-carbaldehyde,under reflux with cyanoacetic acid in a 1 : 1 chloroform/acetonitrile mixture in the presence of piperidine as base(Fig. S1†). 4-(2-Ethylhexyl)-4H-dithieno[3,2-b:20,30-d]pyrrole wassynthesized via literature procedures.29 All synthetic and char-acterization details are provided in the ESI.†
Electrolyte composition
A typical [Mn(acac)3]0/1+ electrolyte was prepared by the addi-tion of 0.30 M [Mn(acac)3], 0.10 M nitrosyl tetrafluoroborate(NOBF4), 0.50 M tBP, 0.10 M LiBF4 and 0.010 M chenodeoxy-cholic acid in acetonitrile. The concentrations of these compo-nents were changed in some experiments as described in theparticular sections. For comparison, DSCs were also con-structed with a [Co(bpy)3]2+/3+ electrolyte, containing 0.165 M[Co(bpy)3](B(CN)4)2, 0.045 M [Co(bpy)3](B(CN)4)3, 0.80 M tBPand 0.10 M LiClO4 in acetonitrile, and an I�/I3
� electrolyte,containing 0.03 M I2, 0.50 M tBP, 0.6 M 1-butyl-3-methylimidazolium iodide (BMII) and 0.10 M guanidium thio-cyanate (GuSCN) in acetonitrile/valeronitrile (85 : 15 vol%).
Working electrode
The mesoporous TiO2 working electrode was fabricated with a2 mm transparent TiO2 (anatase) layer (30 nm particle size) anda 2 mm scattering TiO2 (anatase) layer (400 nm particle size) byscreen-printing. The film was then sintered at 500 1C andtreated with a 20 mM aqueous TiCl4 solution.30 Prior to deviceassembly, the TiO2 films were re-sintered at 500 1C for 30 minutesand immersed hot (ca. 60 1C) in a dye solution of either MK2(0.30 mM MK2) in a 1 : 1 (v/v) mixture of acetonitrile and toluene,K4 (0.30 mM K4 and 5.0 mM cheno) in a 1 : 1 (v/v) mixture ofethanol and chlorobenzene or N719 (0.30 mM N719) in a 1 : 1 (v/v)mixture of tertiary-butanol and acetonitrile for 16 hours at roomtemperature.
Counter electrode
Pre-drilled FTO films were used to prepare the Pt/FTO counterelectrodes. One drop of 10 mM of hexachloroplatinic acid inisopropanol was added to the conductive side of each film,smeared with a Pasteur pipette, and allowed to dry. The filmwas then heated at ca. 400 1C for 15 minutes using a heat gun.For the poly(3,4-ethylenedioxythiophene) (PEDOT)/FTO counterelectrodes, a solution consisting of 0.10 M of 3,4-ethylene-dioxythiophene (EDOT) and 0.10 M of lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI) in acetonitrile was used to deposit PEDOTusing a three-electrode system. A pre-drilled FTO film was used asthe working electrode, with a platinum wire mesh as the counterelectrode and a platinum wire as the reference electrode. Aconstant potential (1.1 V) was applied for 30 seconds toelectro-polymerize EDOT into PEDOT on the glass surface.31–34
Sputter-coated metal/FTO counter electrodes were also usedand were prepared by direct current sputtering the metal (goldor platinum were used) on cleaned FTO glass under argon at4 � 10�12 Torr for 10 minutes.
Device fabrication
The CE and the WE were sealed using a 25 mm thick 6 � 6 mmSurlyn gasket (Solaronix). Vacuum back filling was used toinsert the electrolyte into the cell. The back filling hole wasthen sealed with a piece of aluminium-backed Surlyn preparedby melting 25 mm Surlyn onto aluminium foil at 120 1C.
Current–voltage and spectral response measurements
Current–voltage (I–V) characteristics of DSCs under dark andilluminated conditions were recorded using a Keithley 2400source meter. A sun simulator by Oriel (1000 W Xe lamp)was used to produce simulated solar irradiation (AM 1.5,1000 W m�2). The output of the light source was calibratedusing a silicon photodiode covered with a KG3 filter (PeccellTechnologies, Japan). Light intensities were adjusted by using afilter wheel equipped to place an appropriate mesh filter in thelight path.9,35
A Xe lamp (Oriel, 150 W) and a monochromator (Corner-stone 260) were used to generate monochromatic light tocharacterize the spectral response, or IPCE (Incident Photonto Current Conversion Efficiency), of the DSCs. A Keithley 2400source meter was used to measure the photo-currents undershort-circuit conditions and monochromatic illumination. Theoutput of the light source was calibrated by a silicon photo-diode (Peccell Technologies, Japan). An area slightly smallerthan the active area of the test cell was used as the illuminationspot.9,35
IMVS/IMPS experiments were performed in an earthed Faradaydark-box where the noise was eliminated at low light intensi-ties. A circular LED array (Luxeonstar), which consists of sevenred (610 nm) LEDs (LXML-PM01-010) with a concentratingcluster lens, Polymer 263 – a Fibre Optic Concentrator, wasused. The centre LED was modulated to a depth of ca. 2% usinga purpose built LED driver with an adjustable DC offset and aStanford lock-in-amplifier (SR810). The illumination intensitywas varied by a set of neutral density filters and further lightintensities were accessed by switching the outer ring LEDs offand on. A purpose built battery powered high impedancevoltage follower (input impedance 1012 O) was used to measurethe photo-voltage and a battery powered current pre-amplifier(Stanford SR570) was used to measure the photo-current. Thephase and amplitude of the resultant alternating photo-current(or photo-voltage) were captured using the lock-in amplifier(SR810) under computer control (Labview).36
Charge extraction measurements
The setup for the charge extraction measurements was iden-tical to that in previous literature37 and the same light source
was used as described in the IMVS/IMPS measurements. A NiUSB 6251 BNC data acquisition board was used to record thedata. The results were reported after correction for the porosityof the TiO2 film (0.64, EC/TiO2 0.8).
Redox potential measurements
The redox potential of the [Mn(acac)3]0/1+ redox couple in theelectrolyte was measured using the rest potential method.38 Atwo-electrode setup with Ag/AgNO3 (0.59 V versus the normalhydrogen electrode, NHE) as the reference electrode and aplatinum wire as the working electrode was employed.38 Theelectrolyte consisted of 0.50 M [Mn(acac)3], 0.10 M NOBF4,1.20 M tBP, 0.050 M LiBF4 and 0.010 M cheno in acetonitrile.The potential at the Pt working electrode was recorded as theredox potential of the electrolyte.
Results and discussion
In this study, manganese complexes have been introduced asDSC redox mediators. Our interest was drawn towards manga-nese due to number of potential advantages. Unlike conven-tional I�/I3
�, the manganese redox couple is a one-electron,outer-sphere redox mediator, which could promote fast dyeregeneration. The low toxicity of manganese is a further advan-tage that could lead to more environmentally friendly devices.We have selected the [Mn(acac)3]0/1+ redox couple specifically asits redox potential is appropriately positioned to be applied inconjunction with organic sensitizers with good visible lightabsorption properties.
The UV-Visible spectra of [Mn(acac)3] and [Mn(acac)3]BF4
show absorption maxima at 550 nm and 543 nm (Fig. S2†) withmolar extinction coefficients of 74.9 M�1 cm�1 and 143 M�1
cm�1, respectively. A comparison of the UV-visible spectra ofthe [Mn(acac)3]0/1+ electrolyte and a typical [Co(bpy)3]2+/3+ elec-trolyte (Fig. S3†) shows that the former has a much lowerabsorption of visible light. Thus, [Mn(acac)3]0/1+ based electro-lytes have the potential to improve DSC performance by redu-cing the competitive light absorption with the sensitizer.
The performance of [Mn(acac)3]0/1+ as a DSC redox mediatorwas tested with two donor–p–acceptor dyes with compatibleenergy levels and high molar extinction coefficients. Thisensured excellent light harvesting, even when adsorbed ontorelatively thin (B4 mm) TiO2 films. Use of this thickness isreported to minimize charge recombination at the semicon-ductor–electrolyte interface.17 The first sensitizer was a newpyrrole dye, K4, which has a half-wave potential of 0.82 V vs.NHE and a molar extinction coefficient of 50 700 M�1 cm�1 at516 nm in acetonitrile. Secondly, an optimized redox mediatorwas tested with a commercially available carbazole dye (MK2)which has a redox potential of 0.92 V vs. NHE17 and a molarextinction coefficient of 38 400 M�1 cm�1 at 441 nm in aceto-nitrile.39 Adsorption of K4 onto TiO2 was routinely performedin the presence of chenodeoxycholic acid as a co-adsorbent, asthis was reported to improve DSC performance by reducing dyeaggregation and improving surface passivation.13 The compatibility
of [Mn(acac)3]0/1+ with metal based sensitizers was also testedusing N719, a dye with a redox potential of 0.95 V vs. NHE40 andmolar extinction coefficient of 14 700 M�1 cm�1 at 535 nm inethanol.41
A comparison of the redox potential of a [Mn(acac)3]0/1+
based electrolyte and the HOMO and LUMO levels of K4,MK2 and N719 is shown in Fig. 3. The redox potential of the[Mn(acac)3]0/1+ based electrolyte is +0.49 V vs. NHE which is140 mV higher than that of the I�/I3
� redox couple (+0.35 V vs.NHE), indicating the suitability of this redox couple for use inDSCs.12 The dye regeneration driving force was about 0.50 V forall three, K4, MK2 and N719. Optimization of device perfor-mance was initially carried out using K4 as a sensitizer.
In the first instance, four different counter electrodes –thermally decomposed Pt/FTO, sputter-coated Pt/FTO, sputter-coated Au/FTO and PEDOT/FTO – were tested to determinewhich one would lead to the best performing devices. A commonelectrolyte composition was used (0.20 M [Mn(acac)3], 0.10 M[Mn(acac)3]BF4, 0.10 M LiBF4 and 0.50 M tBP in acetonitrile) andthe working electrode consisted of a 4 mm transparent TiO2 layerand a 6 mm scattering TiO2 layer sensitized with K4. PEDOT wasfound to be the most suitable catalyst for the manganeseelectrolyte; yielding a higher JSC (6.6 mA cm�2) and FF (0.75)than the others (Table S6†). To further confirm the data observedfor the characteristic I–V measurements, symmetric cells (counterelectrode–electrolyte–counter electrode) were constructed. Asshown in Fig. 4, electrochemical impedance spectroscopy (EIS)was carried out on symmetric cells comprising identicalcounter-electrodes filled with the electrolyte. The PEDOT-based cells showed the lowest charge transport resistance(Table S7†) towards the manganese based electrolyte at thecounter electrode/electrolyte interface which contributes to animproved performance of devices assembled with PEDOT overother counter electrodes.
Fig. 3 Energy level diagram of DSC components and approximate redoxpotentials of the electrolytes.
In the subsequent device optimization, the concentrationsof the redox mediators and additives were varied to determinethe optimal electrolyte composition. K4 sensitized TiO2 films(4 mm transparent layer and 6 mm scattering layer) were used asthe working electrode throughout this optimization process. Tooptimize the [Mn(acac)3]/[Mn(acac)3]+ ratio, a series of cellswere constructed with electrolytes having different ratios ofreduced to oxidized complexes, generated by the addition ofNOBF4 to [Mn(acac)3], as well as constant tBP (0.50 M) andLiBF4 (0.10 M). As shown in Fig. S4 and Table S1,† there was aslight increase in efficiency of up to 2.9% upon increasing[Mn(acac)3] from 0.20 M to 0.50 M which was followed by adecrease from 2.9% to 1.9% upon a further increase to 1.0 M. At1.0 M [Mn(acac)3], a significant drop in JSC was observed, fromB5.5 mA cm�2 to 3.1 mA cm�2. For the intermediate 4 : 1 ratiowith 0.40 M [Mn(acac)3] and 0.10 M [Mn(acac)3]BF4 (againformed using NOBF4), the DSC performance parameters wereZ = 2.8%, VOC = 712 mV, JSC = 5.6 mA cm�2 and FF = 0.74.
Lithium salts are commonly used in DSCs as additives toimprove the charge diffusion inside the titania film and tocontrol the conduction band edge of TiO2.42 Moreover, thisshifts the conduction band of the TiO2 to a more positivepotential (vs. NHE) and thus improves the injection of electronsfrom the sensitizer into the titania film. Optimization ofthe lithium salt concentration at fixed levels of [Mn(acac)3](0.50 M), NOBF4 (0.10 M) and tBP (0.50 M) improved theefficiency to 3.1% when using 0.050 M LiBF4 (Fig. S5, TableS2†), which arose from minor increases in VOC (723 mV) and JSC
(5.8 mA cm�2). Improved charge diffusion inside the titaniumdioxide film may have led to the higher JSC observed at thisconcentration. Further increases in the LiBF4 concentrationresulted in a slight decrease in efficiency due to a decrease inJSC (4.6 mA cm�2).
The addition of tBP shifts the position of the conductionband edge of the titanium dioxide negatively to yield a higherVOC, and also passivates the surface of TiO2.43 Therefore, theeffect of varying the tBP concentration was studied at fixed
levels of [Mn(acac)3] (0.50 M), NOBF4 (0.10 M) and LiBF4
(0.050 M). A DSC efficiency of 3.6% was observed upon increas-ing the tBP concentration to 1.20 M, which was accompanied bysignificant increases in VOC to 772 mV and JSC to 6.9 mA cm�2
(Fig. S6 and Table S3†). One origin of the improved perfor-mance could be a reduction in charge recombination arisingfrom passivation of TiO2 surface states through interactionwith tBP.43
An examination of the effect of the thickness of the trans-parent and scattering TiO2 layers on device performance(Fig. S7 and Table S4†) indicated that the device efficienciesdecrease when the thickness of the transparent layer wasincreased from 2 to 4 mm. In addition, devices made with a4 mm scattering layer showed higher efficiencies than thosemade with 2 and 6 mm scattering layers. Consequently, TiO2
films with a 2 mm transparent layer and a 4 mm scattering layerwere used in subsequent studies. Thinner films have beenfound to result in devices with higher electron lifetime due toa reduction of interfacial recombination enhancing the chargecollection efficiency.44
Chenodeoxycholic acid (cheno) is commonly used as aco-adsorbent in dye solutions to minimize dye aggregationonto TiO2 surfaces.45 Recent reports show that the addition ofcheno in the electrolyte improves the device performance as itcan block the access of the oxidized species in the electrolyte tothe TiO2 surface and thereby reducing the degree of recombi-nation.13,46 Furthermore, the presence of cheno in the electro-lyte has been reported to provide a ‘repair mechanism’ for anypart of the TiO2 that becomes exposed over time.13 The additionof cheno beyond 5 mM improved JSC (>7.5 mA cm�2) comparedto the devices without cheno (JSC o 6.9 mA cm�2). There was novariation in device efficiency (3.8% to 3.9%) when 5.0 mM to10.0 mM cheno was added, indicating that the dye layerabsorbed onto the TiO2 is dense and benefits only slightly fromcheno as an additive (Fig. S8 and Table S5†). At higher con-centrations a decrease in efficiency arising from a decrease inVOC and JSC was observed. A maximum VOC of 765 mV and a JSC
of 7.8 mA cm�2 were observed at 10.0 mM cheno.The results of these studies led to an optimized [Mn(acac)3]0/1+
based electrolyte composition which was used in assemblingDSCs with MK2 and N719 as sensitizers. Characteristic I–V curvesof the optimized K4, MK2 and N719 devices are presented inFig. 5 and JSC, VOC, FF and efficiency are summarized in Table 1.
A slightly higher VOC (765 mV vs. 733 mV) and a fill factor(0.72 vs. 0.69) was observed for the K4 and N719 in comparisonto the MK2 devices. In contrast, the current density generatedby the MK2 devices (8.6 mA cm�2) was higher than that of thedevices with K4 (7.8 mA cm�2) and N719 (7.9 mA cm�2). Theseresults are further supported by the IPCE curves (Fig. 5). Over-all, the [Mn(acac)3]0/1+ electrolyte shows a maximum conversionefficiency of 3.9% with K4, 4.4% with MK2 and N719 atsimulated one Sun (1000 W m�2) irradiation. The results ofthe TiO2 layer thickness optimization for N719 are provided inTable S9 (ESI†). The compatibility of [Mn(acac)3]0/1+ with bothmetal-free (K4, MK2) and metal based (N719) sensitizers high-lighted another advantage of this DSC redox mediator.
Fig. 4 Nyquist plot of EIS spectra measured between 105 and 10�1 Hz oncounter–counter electrode symmetric cells based on the [Mn(acac)3]0/1+
The IPCE values reflect the ratio of photo-current densityproduced in the external circuit under monochromatic illumi-nation to the photon flux that strikes the cell.47 With K4sensitized DSCs, a maximum IPCE of 48% was obtained at495 nm with an average integrated current density of 5.7 mAcm�2 and N719 based devices achieved an IPCE of 50% at
525 nm with an average integrated current density of 6.93 mA cm�2
(Fig. 5). In contrast, the MK2 sensitized cells showed betterphoton to current conversion efficiencies over the 400–600 nmwavelength range (54% at 475 nm) than other two dyes, whichyielded a higher integrated current density (7.5 mA cm�2). Theobserved current densities for K4 and MK2 devices reflect thenarrow IPCE curves, which indicate the low light harvestingefficiency below B400 nm and above B600 nm. The IPCE curvefor N719 is broader than for the other dyes but the maximumIPCE is still lower than that of MK2. The lower IPCE observedfor the [Mn(acac)3]0/1+ based devices is explained by the IMVS/IMPS data (Fig. 6(b)) for [Mn(acac)3]0/1+ based DSCs where thelifetime of the electrons injected into TiO2 is an order ofmagnitude shorter than for the [Co(bpy)3]2+/3+ and I�/I3
� baseddevices. A lower current density results from higher chargerecombination within [Mn(acac)3]0/1+ based devices.
Finally, the [Mn(acac)3]0/1+ based electrolyte was comparedwith conventional I�/I3
� and [Co(bpy)3]2+/3+ based electrolytesin conjunction with MK2 under identical conditions. The I–V,IPCE and IMVS/IMPS studies were carried out for comparison.The I–V data (Fig. S9, Table S8†) showed that the devices basedon the [Co(bpy)3]2+/3+ couple perform better than devices with
Fig. 5 (a) Characteristic I–V curves at simulated one Sun (1000 W m�2) and0.1 Sun; (b) IPCE measurements under low light conditions (o2% Sun) ofDSCs based on [Mn(acac)3]0/1+ electrolytes sensitized with K4, MK2 and N719.
Table 1 Detailed photovoltaic parameters of the devices prepared withthe K4, MK2 and N719 dyes and the [Mn(acac)3]0/1+ electrolyte measured attwo different light intensities
a Film thickness = 2 mm transparent +2 mm scattering titania layer.b Film thickness = 4 mm transparent +2 mm scattering titania layer. The[Mn(acac)3]0/1+ electrolyte consists of 0.50 M [Mn(acac)3], 0.10 M NOBF4,1.20 M tBP, 0.050 M LiBF4 and 0.010 M cheno in acetonitrile.
Fig. 6 (a) Corrected VOC vs. electron density; (b) electron lifetime vs.charge density, measured IMVS and charge extraction.
I�/I3� and [Mn(acac)3]0/1+ with an energy conversion efficiency
of 6.1% (VOC = 820 mV, JSC = 10.6 mA cm2, FF = 0.70). The lowerefficiencies of both I�/I3
� and [Mn(acac)3]0/1+ based devices aredue to the lower current density and the open circuit voltagegenerated. The IPCE data (Fig. S10†) are in good agreementwith I–V data, where [Co(bpy)3]2+/3+ based devices reached thehighest IPCE (ca. 70%) followed by I�/I3
� (ca. 60%) and[Mn(acac)3]0/1+ (ca. 54%) based devices.
IMVS/IMPS along with charge extraction measurementswere carried out to further analyse the electron lifetime withinthese devices. The mean charge transport time (td) of electronswithin the TiO2 film was determined by the IMPS measure-ments (Fig. S11†) which was found to be similar in all threedevices based on [Mn(acac)3]0/1+, I�/I3
� and [Co(bpy)3]2+/3+ inconjunction with MK2. The electron lifetime within the TiO2
film was determined using IMVS (Fig. 6). According to theseresults, the [Mn(acac)3]0/1+ based devices have a shorter elec-tron lifetime compared to [Co(bpy)3]2+/3+ and I�/I3
� baseddevices which results in the lower current density (Table S8†),as explained above. As anticipated, the more positive redoxpotential of [Co(bpy)3]2+/3+ (0.56 V vs. NHE) compared with thatof [Mn(acac)3]0/1+ (0.49 V vs. NHE), and the longer electronlifetime lead to a higher VOC for the [Co(bpy)3]2+/3+ baseddevices. A favourable shift in the conduction band of TiO2 inI�/I3
� based devices did not result in a higher VOC due to thelower redox potential of the electrolyte (ca. 0.36 V vs. NHE).Overall, recombination losses are limiting the device perfor-mance of the [Mn(acac)3]0/1+ based DSCs.
The application of more strongly absorbing dyes could beone way to improve device performance. An alternativeapproach could include strategies to limit recombination lossesby adding insulating coatings to the TiO2 film,48,49 modifyingthe dye and/or adding bulky alkyl chains to the acetylacetonatoligand.50
Conclusions
In this study, we have applied a low toxicity metal-based redoxcouple, [Mn(acac)3]0/1+, and a novel donor–p–acceptor dye, K4 inthe dye sensitized solar cell, a promising technology that mimicskey natural photosynthetic processes. The optimization of thecomposition of the electrolyte, the counter electrode and theworking electrode led to a 3.8% conversion efficiency for devicessensitized with K4 under a simulated one Sun (1000 W m�2)irradiation. Use of the optimized electrolyte in conjunction withthe MK2 and N719 dyes resulted in a higher energy conversionefficiency (4.4%) than for K4. This highlights the compatibility ofthe [Mn(acac)3]0/1+ based redox mediator with both metal-free andmetal based sensitizers. The electron lifetime of [Mn(acac)3]0/1+
based DSCs is shorter than that of reported I�/I3� and
[Co(bpy)3]2+/3+ based DSCs, which indicates faster electron recom-bination at the photoelectrode and results in lower efficiencies ofthe [Mn(acac)3]0/1+ DSCs. Modification of the dye and the acetyl-acetonate ligand with the bulky alkyl chain could reduce chargerecombination reactions and lead to a longer electron lifetime
within the TiO2 film. Substitution of electron withdrawing groups,such as halogens in the acetylacetonate ligand, would pave theway to developing manganese redox couples with more positiveredox potentials that may lead to DSCs with higher open circuitvoltages.
Acknowledgements
We acknowledge financial support from the Australian SolarInstitute, Victorian State Government Department of PrimaryIndustry, Bluescope Steel, Innovia Films, Innovia Securityand Bosch (Victorian Organic Solar Cells consortium), andMonash University (for providing I. R. P. with an InternationalPostgraduate Research Scholarship and an Australian Post-graduate Award). Support of the Australian Centre forAdvanced Photovoltaics by the Australian Governmentthrough the Australian Renewable Energy Agency (ARENA) isalso gratefully acknowledged. CAO thanks the AustralianResearch Council for Discovery project grants DP110105530and DP130100483, and a QEII fellowship. We also thank MrJiangjing He (Monash University) for providing the recipe ofthe EDOT solution.
Notes and references
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