Toward high-efficiency dye-sensitized solar cells with aphotoanode fabricated via a simple water-basedformulationRobert Lupitskyy1†, Venkat Kalyan Vendra2†, Jacek Jasinski1, Delaina A. Amos1,2,Mahendra K. Sunkara1,2 and Thad Druffel1*1 Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY, USA2 Department of Chemical Engineering, University of Louisville, Louisville, KY, USA
dye-sensitized solar cell; aqueous; photoanode; Ti-organic precursor; sintering
*Correspondence
Thad Druffel, Conn Center for Renewable Energy Research, University of Louisville, Louisville, KY, USA.E-mail: [email protected]†These authors contributed equally.
Received 22 January 2013; Revised 4 March 2014; Accepted 10 March 2014
1. INTRODUCTION
Dye-sensitized solar cells (DSCs), pioneered by O’Reganand Grätzel in 1991 [1] have emerged as a promisinglow-cost photovoltaic technology in the recent years[2,3]. The current state-of-the-art DSC has achieved singlelab cell efficiencies of 12.3% [4], which is comparable withcompeting amorphous silicon and organic thin film tech-nology [5]. Compared with other types of photovoltaics,DSCs have the unique advantage of using non-vacuum,near atmospheric processes, and low-cost chemicals fortheir fabrication. These near atmospheric processes aresimply incorporated into continuous manufacturing plat-forms including roll-to-roll techniques that are widely usedin the printing industry [3]. Another advantage of DSCs isthat they are particularly suitable for indoor applicationsdue to their excellent sensitivity to low-intensity light.
These two attributes along with the ability to produceconformal coatings define a major contribution for DSCin the building integrated photovoltaic market.
The DSC is unique among the PV technologies as thecharge generation and transfer are separated into twodistinct processes. The technology incorporates three maincomponents: (i) a dye that absorbs the incident photonscreating an electron/hole pair; (ii) a semiconducting thinfilm anode that is designed to transport the electrons to acharge collecting conductor; and (iii) a redox couple thatshuttles electrons from the counter electrode to thedepleted dye through an oxidation and reduction process.A key component of the DSC is the mesoporous semicon-ducting film that anchors the dye molecules and acts as aconduit to transport photogenerated electrons to theconductive substrate. The performance of the DSC stronglydepends on the dye adsorption, electron transport, and
PROGRESS IN PHOTOVOLTAICS: RESEARCH AND APPLICATIONSProg. Photovolt: Res. Appl. (2014)
Published online in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/pip.2502
electron recombination characteristics of this mesoporoussemiconducting film. Current state-of-the-art DSCs use anenergy intensive process and flammable organic solventsfor manufacturing a several microns thick mesoporoustitanium dioxide (TiO2) film. Reduction in materials andprocessing costs of DSCs is very important for commercialproduction and to achieve a competitive levelized costof energy.
Anatase TiO2 nanoparticles used for the making high-efficiency DSCs are usually prepared through a protractedprocess by a hydrothermal method [6]. These are thendispersed into organic solvents by extensive processes notlimited to flocculation, triple centrifugation, ultrasonication,addition of organic binders, solvent exchange, and homoge-nization using a three-roll mill [7]. The hydrothermal methodis a batch process resulting in low yields of the final productoften contained in a solvent at low concentrations making itchallenging to scale economically. Template-based methodsoffer an attractive way of making porous TiO2 particles forinclusion in mesoporous films but are not economical toscale up [8].
Achieving a low-cost TiO2 thin film through traditionalprinting technology needs to be based on inexpensive,available, non-hazardous, and environmentally friendlymaterials. A commercially available nanosized TiO2 powder,Aeroxide® P25 (Evonik Industries, Essen Germany),manufactured via a high temperature flame pyrolysis pro-cess, is a suitable source of inexpensive titanium oxide.P25 powder consists of anatase and rutile phases at a 3:1 ratiowith average diameters approximately 25 and 85 nm, respec-tively, and they exist separately by forming their agglomer-ates that range between 50 and 200 nm [9]. A recent studydetermined that films of anatase and rutile phases mixed atthis ratio resulted in a DSC of higher performance than apure anatase thin film [10]. P25 has been extensively usedin the research of DSCs with several compositions that canbe classified as (i) pastes with organic binders [11,12]; (ii)formulations with metal-organic binders [13,14]; and (iii)binder-free formulations [15,16].
Ito and co-workers reported the fabrication of high-effi-ciency DSC using P25 nanoparticles [11]. Although theuse of P25 nanoparticles lowers the preparation time, theirprocedure still has several processing steps similar to thoseused for dispersing the TiO2 colloids made by hydrother-mal methods. The interconnections between the TiO2
nanoparticles play a central role in determining the electrondynamics in the mesoporous semiconducting film such asthe hydrothermal treatment of titanium precursors such astitanium alkoxides and titanium tetrachloride (TiCl4) toimprove necking that has been investigated by severalgroups [17]. However, the major drawbacks of thesemethods are the poor chemical stability of the titanium pre-cursors, which have high sensitivity toward temperatureand humidity and long autoclaving times (~12 h) requiredto crystallize TiO2 [18]. In addition, these methods donot offer a control of the porosity of the film and haveresulted in low efficiencies when compared with championcells made with the anatase TiO2.
The use of water as a solvent for dispersing the TiO2
nanoparticles is very desirable from a safety and costperspective. It is important that a water-based formulationbe stable and yield high porosity after deposition. Ethylcellulose is typically used in pastes for superior porosity;however, it has poor water solubility. Aqueous TiO2
nanoparticle dispersions are accomplished using surfacefunctionalization and pH control and will cross the stabilityratio as solvent is removed leading to agglomeration. Hence,it becomes extremely challenging to fabricate highly porousnanoparticles films using water-based formulations.
Titanium (IV) bis(ammonium lactato)dihydroxide (TALH)is a water-soluble, non-hazardous compound stable at neutralpH and ambient conditions [19]. Several reports haveinvestigated the hydrolysis of TALH to form anatase or rutilephases by varying the solution pH and temperature [20]. Theformation of TiO2 by the hydrolysis of TALH has beenreported for making photocatalysts. For example, thehydrolysis of TALH at 90 °C in the presence of urea wasused for formation of anatase nanoparticles coating oncarbon nanotubes [21], on barium ferrite [22], and silica-modified cobalt ferrite [23]. Gutiérrez-Tauste et al. reportedthe application of TALH in a TiO2 paste using UV irradia-tion to remove the organics from the film [13]. The authorsdid not discuss the effects of TALH concentration on the filmmicrostructure and properties such as changes in porosity,crystallinity, and electron dynamics that significantly alterthe photovoltaic performance. The state-of-the-art DSCfabrication utilizes a TiCl4 post-treatment to improve perfor-mance, and a recent comprehensive study showed that thinfilm morphology and electron dynamics drastically changedwith TiCl4 concentration [24].
In this work, we focus on an alternative TiO2 formula-tion with simple preparation routine using water as asolvent. This aqueous formulation consists of P25 TiO2
particles and TiO2 precursor TALH that are simply mixedusing very mild agitation. The use of TALH results in atwo-fold advantage (i) increased porosity resulting in highdye adsorption and (ii) thermal decomposition of TALHduring sintering leading to the formation of anatasebridging between the P25 nanoparticles and thus improvesthe interparticle connectivity. The mechanism of howTALH affects the performance of the DSCs is investigatedin detail in this work.
2. RESULTS AND DISCUSSION
2.1. Performance and optimization
Current–voltage characteristics of single layer DSCsfabricated from TiO2 formulations with varying TALHconcentrations are shown in Figure 1 and summarized inTable I. The DSCs assembled without TALH exhibit ashort-circuit current density (Jsc) of 10.8mA/cm2, anopen-circuit voltage (Voc) of 0.71V, a fill factor of 0.63,and an overall conversion efficiency (η) of 4.8%. These re-sults are consistent with the previously reported performance
Toward simple high-efficiency dye sensitized solar cells R. Lupitskyy et al.
of a DSCmade with a binder-free H2O/t-butanol formulation[16]. As the TALH concentration in the aqueous P25 formu-lation was increased, the Jsc, Voc, and η increased until a max-imum was reached at 0.43molar (M). At this optimumconcentration, a Jsc of 16.64mA/cm
2, Voc of 0.74V, fill factorof 0.66, and η of 8.1% were observed. Further increase inTALH concentrations resulted in lower performance reducingboth the Voc and Jsc. A complete DSC prepared with blockingand scattering layers sandwiching the active layer using theoptimized formulation resulted in an efficiency of 9.2%. Thisis similar to the efficiency obtained by Ito et al.who preparedP25 nanoparticle films through complex processing methodusing organic solvents and a polymeric binder.
Both the Voc and Jsc are maximized at a TALH concen-tration of 0.43M that would suggest (i) increased porosityresulting in the higher dye loading hence higher chargeinjection and (ii) improved electron transport due to betterconnectivity between the TiO2 particles. To verify thishypothesis, the dye loading characteristics, changes in thefilm morphology and electron transport and recombinationwere investigated further and discussed in the next section.
2.2. Mechanism and structure
Substantive bridging between neighboring nanoparticlesby thermal decomposition of the TALH precursor andcrystallization of the formed TiO2 will improve electron
transport and lower recombination in the film. The electronrecombination time constant and transport time constantfor DSCs made with different TALH concentration areshown in Figure 2 and compared well with our own studiesusing ethyl cellulose-based pastes [25]. The maximumelectron lifetime and the minimum transport time constantsoccur at 0.43M TALH and these closely correlate withobserved maximum DSC performance. Faster electrontransport and suppressed electron recombination suggestthat addition of TALH improves interconnections betweenthe particles.
The porosity of the nanoparticle films also has animpact on solar cell performance and could explain thediscrepancy in recombination and transport at higherTALH concentrations. At very low porosities seen for highTALH concentrations, the transport of electrolyte into thepores is slow and the dye cannot be regenerated efficiently[26]. The slow transport of electrolyte results in increasedrecombination losses with oxidized species. It was alsoshown that the adsorption of the dye passivates the surfacestates on the TiO2 particles, reducing the recombinationlosses. At very high porosity, there are elevated densitiesof surface trap states that increase the electron transit time,and electron recombination and fewer coordination sitesamong TiO2 NPs would imply a longer path for the elec-tron to travel. Consequently, the slower electron transportalso leads to increased recombination losses in the TiO2
film. At the optimum TALH concentration, the fast elec-tron transport could lower the recombination losses andthus improves the electron lifetimes. At high TALH con-centrations (>1M), the porosity of the P25 nanoparticlefilm is low and the electron transit time would be expectedto be faster when compared with transit times in the filmsmade from the optimized formulation. However, theslower electron transport can be explained through fissurescreated during the thermal decomposition of TALH afore-mentioned concentration of 1M. The presence of fissuresin the nanoparticle film would mean a longer path thatthe electron has to travel to reach the conducting substrate,thus resulting in a slower transport time scale.
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Active layer + scattering layer Jsc= 20 mA/cm2
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= 9.2 %
(a) (b)
Figure 1. (a) Current–voltage characteristics for varying TALH concentrations and (b) Current–voltage characteristics for an optimizedformulation with both active layer and scattering layer.
Table I. Performance of DSC with various TALH concentrations.
Concentration ofTALH, M (P25:TALH, molar ratio) Jsc, mA/cm2 Voc, V FF η, %
The changes in the film morphology with varyingTALH concentrations were analyzed by scanning electronmicroscopy (SEM). SEM micrographs of the nanocry-stalline mesoporous films prepared using formulationcontaining 0, 0.14, 0.43, and 1.30M TALH are shown in
Figure 3. Brunauer-Emmett-Teller (BET) measurement ofthe surface area resulted in 53m2/g for pure P25 film,supporting the manufacturer’s specification and is 60m2/gat the optimal TALH concentration. This trend is also con-firmed from the UV–vis measurements on the dye-sensitized
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Figure 3. Scanning electron images of titanium dioxide films fabricated with formulations containing varying amount of TALH (a) 0MTALH, (b) 0.14M TALH, (c) 0.43M TALH, and (d) 1.30M TALH. (e) Absorbance of N-719 dye on the titanium dioxide films fabricated
with varying TALH concentrations.
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Figure 2. (a) Electron transport time constant and (b) electron lifetimes for DSCs fabricated with varying amounts of TALH (c) XPSspectrum for different TALH concentrations (d) chronoampereometry measurements at �0.8 V Ag/AgCl in a 0.5M LiClO4 electrolyte,
inset shows the surface density of trap states, at varying concentrations of TALH.
Toward simple high-efficiency dye sensitized solar cells R. Lupitskyy et al.
films made with varying TALH concentrations showingincreased absorbance as TALH increases (Figure 3(e)).
It has been shown that Ti3+ trap states limit electrontransport in TiO2 films and the passivation of these withincreasing TALH concentration could be a possible justifi-cation for improved transport [27]. X-ray photoelectronspectroscopy (XPS) was used to determine the surface pas-sivation of the films. Figure 2(b) shows the Ti 2p XPSspectrum of the titania films prepared with differentconcentration of TALH. The Ti 2p3/2 and Ti 2p1/2 peaksobserved at 458.3 and 464 eV indicate titanium in its +4state [28]. There is no evidence for any Ti+3 (457.6 eV)in any of the samples. The XPS results show that improve-ment in the transport and recombination properties withaddition of TALH is not due to passivation of the Ti+3 traps.An electrochemical method based on chronoameperometry,developed by Wang et al. was also adapted to measure thesurface trap state concentration [29]. The current decay isrelated to the filling of the surface traps states in TiO2. Theconcentration of the traps can be estimated from thecharge accumulated in these traps obtained by integratingthe area under the current transients. The surface trap stateconcentration remained constant at 1013 cm�2 indicatingthat no new trap states are created with the addition ofTALH (Figure 2(d)).
Prior studies on TALH have been focused on the hydro-lysis of TALH at low temperatures. Hydrolysis of TALH atabove 100 °C yields uniform anatase nanoparticles [19]where the crystal size was mainly dependent on the reac-tion temperature, increasing from around 2 nm at 120 °Cto about 20 nm at 300 °C with no dependence of size onprecursor concentration. Kinsinger et al. [30] reported thesynthesis of rutile and anatase nanoparticles from TALHusing the hydrothermal method at 150 °C for differentdurations (1–72h). At neutral pH, rutile phase was formedand increasing the alkalinity favored the formation of anatasephase. There have been no reports studying the directthermal decomposition of TALH at 500 °C.
The overall process of thermal decomposition of TALHoccurring in the presence of oxygen can be summarized inthe following equation:
CH O�ð ÞCO2NH4½ �2Ti OHð Þ2þ502→500oC
TiO2
þ2NH3þ6CO2þ6H2O
(1)
XPS (Supporting Information) did not show thepresence of nitrogen in the P25 film after sintering,suggesting that all nitrogen was indeed released duringthe thermal treatment. To validate the formation of
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JCPDS of Anatase TiO2JCPDS of Rutile TiO2
P25 NPs
Anatase NPs from TALH
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Figure 4. (a) XRD diffraction pattern of anatase nanoparticles (NP) formed from thermal decomposition of TALH at 500 ºC (b) schematicshowing the formation of anatase nanoparticle bridging between P25 nanoparticles.
Toward simple high-efficiency dye sensitized solar cellsR. Lupitskyy et al.
TiO2 during the thermal decomposition of TALH at 500 °C, the powder obtained by the thermal decomposition wasanalyzed by X-ray diffraction (XRD). The XRD pattern ofpowder obtained by thermal treatment of TALH is shownin Figure 4(a). The main characteristic peak at 25.4° wasassigned to the (101) Miller index of anatase. Thediffraction pattern showed other characteristic peaks ofthe anatase structure, corresponding to (004), (200),(211), and (213) crystalline planes (JCPDS schedule:21–1272). No other TiO2 crystalline phase was found inthis sample. The XRD pattern of anatase TiO2 fromdecomposed TALH exhibits broader peaks than P25TiO2, suggesting smaller crystalline size in accordancewith the Scherrer equation. Thus, the anatase TiO2 formedby the decomposition of TALH should result in thebridging between the P25 nanoparticles (Figure 4(b)).
The nanoparticle films were also scrutinized using aTEM at a much higher magnification to visualize the inter-action between the TiO2 formed from the decomposedTALH and the P25 particles (Figure 5). High resolutiontransmission electron microscope (HRTEM) micrographsof anatase TiO2 nanoparticles obtained by thermal decom-position of TALH and anatase TiO2 nanoparticles creatingbridges between P25 nanocrystals during thermal sinteringof the P25-TALH formulation are shown in Figure 5(b,c).This observed bridging accounts for the improved electrontransport between P25 nanocrystals. The enhancement inelectron transport and lowering of recombination was alsoconfirmed from the transient current and voltage decaymeasurements.
The aforementioned analysis conclusively shows thatTALH affects the performance of DSC via changes in themorphology of TiO2 film. First, it increases porosity ofthe film, which results in the increased dye adsorption.Second, anatase bridges between P25 particles are formedupon thermal decomposition of TALH, which accountsfor improved electron transport. The existence of anoptimal concentration of TALH can be explained in termsof reaching optimal porosity of the nanocrystalline filmupon thermal decomposition of TALH at that concentra-tion. Theoretical evaluation (refer to the SupplementaryInformation) showed that formulation with optimal TALHconcentration (0.43M) upon thermal sintering of the TiO2
film will create porosity of 58%, which is in the range that
is considered optimal for DSC performance (50–60%) [31].Formulations with lower (0.14M) and higher (1.30M)TALH concentrations yield porosities that are out of thisrange (37 and 73%, respectively), which correlates withlower performance of DSC made with the aforementionedformulations. This result is experimentally supported by thedye absorbance measurements as a function of TALHconcentration (Figure 3).
3. CONCLUSION
In this work, we developed and tested an aqueous formula-tion for preparation of thermally sintered photoanodes forDSCs that is an environmentally friendly, easy-to-makealternative to conventional non-aqueous TiO2 pastes madewith terpineol and ethyl cellulose. The formulation is com-posed of inexpensive, commercially available materials:P25-TiO2 particles and water-soluble, non-hazardous,stable TiO2 precursor TALH. We demonstrated that highlyefficient DSCs with overall power conversion efficiency ashigh as 9.2% can be manufactured using this formulation,which is comparable with the performance of DSCs madewith P25-TiO2, ethyl cellulose, and terpineol. Furthermore,the three components of the formulation were simplymixed using mild ultrasonication and a stir bar for only15min.
The overall efficiency of single TiO2 layer DSCsincreased from 4.8% to 8.1% upon addition of TALH withan optimal concentration at 0.43M. TALH affects twoparameters of the semiconducting TiO2 film: surface areaand electron transport. Thermal decomposition of lactateligands of TALH increases porosity of the film, which inturn increases the amount of adsorbed dye molecules.The analysis revealed the formation of bridging betweenP25 particles by ~3 nm anatase nanoparticles formed uponTALH decomposition, which accounts for improvedelectron transport.
The developed formulation has a great potential forindustrial application in DSC manufacturing; it is inexpen-sive, environmentally friendly, easy to make, and does notrequire special handling. It can be easily modified andtailored for a particular TiO2 source, deposition technique,or sintering process.
cba
Figure 5. Transmission electron microscopy image of titanium dioxide nanoparticles after sintering—(a) P25 nanoparticles, (b) anatasetitanium dioxide NPs from TALH sintered at 500 ºC, and (c) P25 nanoparticles with anatase nanoparticle bridging formed from TALH.
Toward simple high-efficiency dye sensitized solar cells R. Lupitskyy et al.
Titanium dioxide nanoparticles Aeroxide® TiO2 P25(specific surface area 50m2/g) were obtained from EvonikIndustries (CAS# 1309-63-3). TALH (CAS# 65104-06-5),50wt.% solution in water. TiCl4 (CAS# 7550-45-0) and200 proof ethanol (CAS# 64-17-5) were purchased fromSigma Aldrich (St. Louis, MO, USA). Fluorinated tinoxide (FTO)-coated glass slides (19mm×38mm×2.3mm, TEC 8, Sheet resistance = 8 ohms/□) were purchasedfrom Hartford Glass Inc. (Hartford, IN, USA), N-719 Dye(CAS# 207347-46-4), platinum counter electrode solution(CELS), scattering layer TiO2 paste (WER2-O), Surlyn-30thermoplastic sealant, iodide/tri-iodide electrolyte solution(EL-HPE) were purchased from Dyesol (Queanbeyan,NSW, Australia). Hot melt adhesive (Crystal Bond™) waspurchased from Ted Pella, Inc. (Redding, CA).
4.2. TiO2 formulation
The TiO2 nanoparticle formulation was prepared by slowlyadding water and TALH to P25 powder. The mixture washomogenized using ultrasonication for 15min with simul-taneous mechanical stirring (Scheme 1).
Ti02 þ TALH þ H20→ultrasonication
Formulation
4.3. Solar cell fabrication
Fluorinated tin oxide glass slides were cleaned with ethanolfor 15min in an ultrasound bath and then subjected to oxy-gen plasma for 10min to remove remaining organics. For se-lect experiments, the TiO2 formulation was deposited ontothe clean FTO slides using ‘doctor-blade’ method. A scotchtape (Scotch Magic™ Tape, thickness = 50μm, 3M St. Paul,MN, USA) was used as a mask to form 5×5mm square area.The electrodes were subsequently sintered at 500 °C for 1 h.The average film thickness was measured to be 11μm asmeasured by a profilometer.
One optimized cell was made with blocking and scatter-ing layers. Prior to the deposition of the TiO2 formulation,the FTO slides were pretreated with 40mM aqueous TiCl4solution. The slides were kept in the solution for 30min at70 °C and subsequently rinsed with ethanol. After the TiO2
films were deposited as described earlier, post-treatment ofthe electrodes with 40mM aqueous TiCl4 solution wasperformed. The electrodes were kept in the solution for30min at 70 °C, rinsed with ethanol, and annealed at500 °C for 15min. A scattering layer was further appliedusing WER2-O paste from Dyesol. The electrodes wereannealed again at 500 °C for 15min. For all samples, aftercooling down, the electrodes were immersed in a 0.5-mMsolution of N-719 dye in ethanol for ~20 h. The counterelectrodes were prepared by a brush deposition of platinumCELS onto the FTO glass slide and heating the electrode to
420 °C for 20min and subsequently cooling it down atroom temperature. Holes were drilled into the counterelectrode prior to applying the platinum CELS. The cellswere fabricated by sandwiching the photoanode and thecounter electrode with a Surlyn film. The sandwiched cellwas heated at 100 °C for 5min and allowed to cool at roomtemperature. The iodide/tri-iodide electrolyte was intro-duced into cells using a backfilling method and holes weresealed using the hot melt adhesive.
4.4. Solar cell characterization
The DSCs were illuminated using a 150W Xenon lampsolar simulator (Model #96000, Newport Corporation,Redding, CA) equipped with an Air Mass 1.5 filter. Allelectrochemical measurements were performed using apotentiostat (EG&G-PAR 273A, Princeton AppliedResearch). The light intensity incident on the cells wasadjusted to 100mW/cm2 using a silicon power meter(Model# S120UV, Thor labs). The electrical measure-ments on all the DSCs in the study have been performedwithout a shadowing mask. For the current decay measure-ments, the light from the solar simulator (100mW/cm2)was used as the bias light and a light pulse from a diodelaser (~550nm) was used as the perturbation light. The timeconstant was determined by an exponential fit to the photo-current decay [32]. The electron lifetimes were determinedusing the open-circuit voltage decay technique describedby Bisquert and co-workers [33]. For the electrochemicaltrap state measurements, the procedure described by Wangand co-workers was followed [29]. TiO2 working electrodeswere prepared on FTO glass and immersed in a 0.5MLiClO4 electrolyte solution (pH ~7). Ag/AgCl wasemployed as a reference electrode and a Pt mesh was usedas a counter electrode. The working electrode was equili-brated at 0V for 5min and the potential was shifted to�0.8V. The transient current decay was monitored.
4.5. Thin film characterization
Scanning electron micrographs were taken using NOVANanoSEM 600 SEM (FEI Company, Hillsboro, OR,USA). TEM studies were performed using Tecnai F20field emission gun transmission electron microscope oper-ating at 200 kV (FEI Company, Hillsboro, OR, USA).XRD spectra were recorded using Discovery D-8 X-raydiffractometer (Bruker, Billerica, MA, USA). A smallvolume of TALH was drop casted onto a glass slides andheated to 500 °C for 4 h and the resulted powder wasanalyzed by XRD. The dye adsorption on films containingvarying amounts of TALH was analyzed using a Lambda950 UV–vis spectrophotometer. TiO2 layer thickness wasmeasured using Alpha-Step 500 Surface Profiler (TencorInstruments, Milpitas, CA, USA). N2 adsorption BET sur-face areas were determined in a Micromeritics Tristar-3000porosimeter. Before the surface area measurements, thesamples were degassed at 300 °C for 3 h.
Toward simple high-efficiency dye sensitized solar cellsR. Lupitskyy et al.
Chemical structure of TALH (Figure S1). Comparisonbetween conventional ethyl cellulose-based TiO2 pastepreparation routine and aqueous TiO2 formulationdiscussed in this paper (Figure S2). Low magnificationSEM micrographs showing the effect of TALH concentra-tion on the porosity of the sintered TiO2 film (Figure S3).Ti2p XPS spectra of the mesoporous films made from for-mulations with varying TALH concentration (Figure S4).Theoretical evaluation of the effect of TALH concentrationof the porosity of the sintered TiO2 films.
ACKNOWLEDGEMENTS
The authors would like to acknowledge the support of theConn Center for Renewable Energy Research during thiswork. The personnel and the work were supported by USDepartment of Energy through DE-EE0003206 and DE-FG02-07ER46375. The authors would also like to ac-knowledge the technical discussion on trap state measure-ments with Dr. Heli Wang from NREL.
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Toward simple high-efficiency dye sensitized solar cellsR. Lupitskyy et al.
