Solar Energy Materials & Solar Cells 95 (2011) 1123–1130

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Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/solmat

A study of stabilization of P3HT/PCBM organic solar cells by photochemicalactive TiOx layer

Jiguang Li a, Sunhee Kim b, Sean Edington b, Joseph Nedy a, Shinuk Cho c,d, Kwanghee Lee c,d,Alan J Heeger c,d, Mool C. Gupta a,n, John T. Yates Jrb

a Department of Electrical & Computer Engineering, University of Virginia, Charlottesville, VA 22904, USAb Department of Chemistry, University of Virginia, Charlottesville, VA 22904, USAc Heeger Center for Advanced Materials, University of California at Santa Barbara, Santa Barbara, CA 93106, USAd Gwangju Institute of Science and Technology, Gwangju, Republic of Korea

a r t i c l e i n f o

Article history:

Received 18 September 2010

Received in revised form

8 December 2010

Accepted 9 December 2010Available online 24 December 2010

Keywords:

Organic solar cells

Stability

TiOx

P3HT/PCBM

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.12.030

esponding author. Tel.: +1 434 924 6167.

ail address: mgupta@virginia.edu (M.C. Gupta

a b s t r a c t

We describe a study of the stabilization behavior of P3HT/PCBM organic solar cells under air and UV

irradiation using a 20 nm thin TiOx protection layer made by partial hydrolysis of a Ti-alkoxide and spin

coating in air. Data on the degradation of solar cell performance under air and under UV exposure are

presented indicating that significant improvements are observed with TiOx layer protection. The

protection mechanism has been investigated by transmission IR and UV spectroscopy and by ESR

spectroscopy. The results of this study suggest how sol–gel derived TiOx films containing organic

functionalities serve as effective passivation films for protection from oxygen when excited by photons,

where the photooxidation of the bound organic moieties causes oxygen gas scavenging.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

Currently, organic solar cells have two major limitations, namelylower conversion efficiency and poor long-term environmental stabi-lity. This paper addresses the long-term stability of regioregular poly(3-hexylthiophene) (P3HT)/phenyl-C61-butyric acid methyl ester(PCBM) based organic solar cells under ultraviolet (UV) and environ-mental degradation conditions. Degradation of organic solar cells ingeneral has been investigated by several groups and is considered acomplex phenomenon as it may involve several mechanisms, such aswater absorption by a PEDOT:PSS layer, reaction of the metal/organicinterface and oxidation of the organic layer [1–4]. Kawano et al. [1]showed that water absorption by a PEDOT:PSS layer (hole blocking filmused in organic solar cell device structure) accounted for some of theobserved degradation. Reese et al. [2] studied the degradation oforganic photovoltaic P3HT/PCBM devices and concluded that themetal/organic interface can be major source for cell degradation.Gevorgyan et al. [3] concluded from their study that the degradationmechanism in ambient atmosphere was related to chemical reactionsof the active layer and metal electrodes with O2 and H2O. It was alsoconcluded that the decay of organic solar cells is complex andmay involve several mechanisms. Jørgensen et al. [4] pointed out

ll rights reserved.

).

that oxygen can oxidize the organic layer, in particular under lightirradiation.

Norrman et al. [5] showed that the oxygen diffuses throughpinholes in the aluminum electrode and illumination acceleratesoxidation/degradation. Lira-cantu et al. [6] showed incorporationand transport of oxygen using a combination of isotopic labelingand time of flight secondary ion mass spectrometry. Norrman et al.[7] studied water-induced degradation of polymer solar cells by H218O labeling. The water effect was found to be similar to the effect ofmolecular oxygen. The results on the stability for the roll-to-rollprocess for flexible polymer solar cells has also been reported [8]and ascribed the moisture effect due to interaction with aluminum.Hauch et al. [9] have produced flexible organic P3HT:PCBM bulkheterojunction modules with more than 1 year outdoor lifetime bypackaging with transparent barrier films. However, further studiesare needed to address the important issue of long-term stability oforganic solar cells.

Different approaches of achieving higher stability have beenconsidered, such as the use of improved active materials and enca-psulation schemes. Recently, Lee et al. [10] have shown enhancedenvironmental stability for P3HT/PCBM-based organic solar cellsusing a TiOx layer on top of the organic active layer. Cho et al. [11]demonstrated improvements in field effect transistor device life-time by the application of a thin TiOx layer on top of the device. TheTiOx layer allows the flow of electrons and blocks transport of holes.TiOx is postulated to block the passage of oxygen and humidity into

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–11301124

the active layer. The mechanism proposed is based on oxygen defi-ciencies in the TiOx film providing adsorption sites for O2 [11,12].The role of the photoexcitation of TiOx layers was not postulated inthese works [10–12].

There is another possible mechanism based on the TiOx layeracting as a photochemically activated oxygen scavenger. The TiOx

layer contains both Ti-OR (OR¼alkoxide) functionalities and Ti-OHgroups. The Ti-OR functionalities are photooxidized, consuming O2

and generating CO2, H2O gas products and [HCOO-] and Ti-OHmoieties. This photo-chemical reaction forms the basis for TiOx

films to remove oxygen when exposed to ultraviolet light.In this paper we have examined the individual effect of oxygen

and UV radiation as well as their combined effect on the degrada-tion behavior of P3HT/PCBM organic solar cell devices and the impro-vements provided by placing a TiOx layer between the active layer anda metal electrode. Using IR and ESR spectroscopy measurements weshow that the improvements are due to photooxidation of the boundorganic moieties causing oxygen gas scavenging.

Fig. 2. Organic solar cell fabrication process: color coded for process in air (purple)

and process in vacuum (orange). (For interpretation of the references to color in this

figure legend, the reader is referred to the web version of this article.)

2. Experimental

The organic solar cell device structure was based on the P3HTand PCBM material system. The organic solar cell device structureused in this study is shown in Fig. 1 and it has been described pre-viously [13]. The indium tin oxide (ITO) coated glass was used as asubstrate and cleaned with acetone and isopropyl alcohol and sub-sequently dried in nitrogen. Highly conducting poly (3,4-ethyle-nedioxylenethiophene):polystyrene sulfonic acid (PEDOT:PSS) waspurchased from H.C. Stark with a conductivity of 1�10�3 S/cm.PEDOT:PSS was spin coated at 4000 rpm onto an ITO coated glasssubstrate to achieve a thickness of 40 nm. The substrate was bakedfor 15 min at 110 1C in air.

The P3HT material was purchased from Sigma Aldrich Corp andPCBM from NanoC Inc; they were dissolved in dichlorobenzenesolution to 0.45 and 0.55 wt%, respectively. The solution was spincoated on an ITO coated glass substrate to produce a film of 120 nmthickness. The films were prebaked at 60 1C for 30 min in vacuumand then the top aluminum electrode was deposited by electronbeam evaporation in vacuum on the device without TiOx. The sampleswere post baked at 130 1C in air for 2 min before measurements ofsolar cell efficiency.

The TiOx material was based on titanium isopropoxide[Ti(OCH(CH3)2)4], 2-methoxyethanol [CH3OCH2CH2OH] and etha-nolamine [H2NCH2CH2OH], as described previously [13], and wasprovided by Heeger’s group at UC Santa Barbara. The sol–gelprecursor was dissolved in methanol to obtain a 2% by volumeconcentration solution. Under these conditions it is likely that theisopropoxide groups were exchanged with methoxide groups. Thesolution was spin coated on the P3HT/PCBM active layer at3000 rpm to produce a 20 nm thin film. The coated sample wasbaked at 90 1C in air for 10 min, where the precursor converted to

Fig. 1. Organic solar cell device structure.

TiOx through hydrolysis. The device fabrication method andprocess parameters used are shown in Fig. 2.

For UV light exposure studies of the devices, a 500 W Oriel66,033 Hg arc lamp was used. The radiation from the lamp wasfiltered with a Thorlab FGUV11S square colored glass UV filter toremove IR radiation. The lamp spectrum, compared with the solarspectrum and the transmission property of the filter are shown inFig. 3. The intensity of light incident on the sample was variedthrough changing the distance from light source to sample. Thelamp was held at 50 cm from the sample mounted inside thechamber and an aperture was used to limit the beam size. The lampsupplied continuous radiation except during the few minutes ofmeasurement time. No heating of the samples was observed. TheUV power density was measured using a Thorlab D3MM bolometerand was kept at 10 mW cm�2, which corresponds to 1.6�1016

photons cm�2 s�1 above 3.9 eV photon energy.The photovoltaic cell performance was measured under AM1.5

illumination from a calibrated solar simulator with irradiationintensity of 100 mW cm–2. Efficiencies are 2.8% and 2% for thedevice with TiOx and without TiOx, respectively, in air. The solar cellsize is around 0.12 cm2. The initial photovoltaic characterizationparameters are listed in Table 1. Three types of ambient conditionswere used for this study that included air, oxygen and dry N2 atmo-spheres. All experiments were carried out at room temperature.

For mechanistic studies of the behavior of TiOx films, the mate-rial was dissolved to 10% by volume in methanol, and used at 300 Kto spin coat�200 nm films onto a CaF2 single crystal disk, 25 mm indiameter and 2 mm in thickness. The disk was coated and stored inthe dark in air prior to use in these investigations. The thickness ofthe spin coated films was measured using a Veeco Dektak 8 stylusprofilometer apparatus to determine the spacing between the top

Fig. 3. 500 W Hg UV lamp spectrum, AM 1.5 solar spectrum and transmission of

FGUV11S band passing filter.

Table 1Initial photovoltaic characterization parameters.

Initial

efficiency (%)

Initial

Voc (V)

Initial Jsc

(mA/cm2)

Initial fill

factor

Device without TiOx 2 0.67 5.1 54

Device with TiOx 2.8 0.64 7.2 57

Fig. 4. Variation of normalized organic solar cell efficiency under combined effect of

UV and air exposure (UV power density¼10 mW/cm2).

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–1130 1125

of the film and the CaF2 support plate. The spin-coated disk wastransferred to a stainless steel ultrahigh vacuum IR cell [14,15]containing windows for both the IR measurements as well as forirradiation of the film at a 451 incidence angle with UV radiationfrom an Oriel 66,033 Hg arc lamp. The radiation from the lamp wasfiltered with a 10 cm column of water to remove IR radiation. Thebase pressure in the cell was �5�10�8 Torr. Simultaneous UVirradiation and observation of changes in the transmission IRspectrum were carried out in vacuum and under an O2 atmosphereat a pressure of 20 Torr. Separate IR spectra of the thin film and ofthe gas phase could be measured during irradiation by periodicallytranslating the TiOx film out of the IR beam. Thus, by differencespectroscopy where the gas phase spectra are subtracted from thespectra of the film plus gas phase, we measured the spectraldevelopments in the film alone. To check the assignment of infraredbands produced when TiOx-bound formate species were formedduring photooxidation, 18O2 [99% isotopic purity] was employed insome experiments, and bands assigned to oxygen motions in theproduced HCOO– species were observed to shift to lower wavenumbers compared to similar experiments with 16O2.

Transmission IR measurements were made with a BrukerTensor 27-FTIR spectrometer with a liquid nitrogen-cooled MCTdetector. The spectrometer optical path was continuously purgedwith dry air, scrubbed of CO2. The instrumental resolution was2 cm�1 and typical spectra involved acquisition of 128 scans. Duringsome measurements (Fig. 10), a small amount of ice condensedinternally on the MCT detector giving a broad ice absorption bandcentered near 3300 cm�1, and this band is to be disregarded. Mostmeasurements did not involve this effect.

ESR measurements were carried out in a 4 mm diameter quartzESR tube containing flakes of the fresh TiOx film. The flakes arescraped from a glass slide, where TiOx solution was spread anddried overnight in darkness at room temperature in a N2 environ-ment. A Bruker ESP 3000 spectrometer was employed. It was automa-tically calibrated internally. The ESR measurements were made underN2 (gas) or O2 (gas) at 1 atmosphere pressure. Exposure of the TiOx tothe UV lamp was carried out in the ESR tube.

UV absorption spectroscopy on the TiOx film, deposited on aCaF2 plate, was carried out using a Perkin-Elmer Lambda 25 UV–visspectrometer. The film was sequentially heated in air on a hotplateto temperatures up to �573 K as judged by a thermocouple con-tacted with the CaF2 plate. Heating was followed immediately byspectral measurements.

Fig. 5. Variation of normalized organic solar cell efficiency in air and nitrogen in

dark with time.

3. Results and discussion

3.1. Study of solar cell degradation and improvements by TiOx

Using the fabricated solar cell devices, performance degradationwas measured. All the devices without TiOx protection have beenfound to degrade to lower than 80% of initial efficiency during 12 h.Therefore 12 h time is long enough to study the degradation accor-ding to the recommended procedures for reporting organic solarcell stability [16]. We can clearly see the degradation trend andimprovement that could be achieved by TiOx. Some of the degra-dation occurs during our fabrication process because most of the

fabrication has been done in air. But the major degradation isgoverned by the UV and gas exposures.

Fig. 4 shows the change in normalized solar cell efficiency as afunction of ultraviolet light/air exposure time for the P3HT/PCBMorganic solar cell and for a cell containing the TiOx protection layer.Each set of data is derived from the average of 8 devices on a singlesubstrate. These data clearly indicate enhanced solar cell efficiencystabilization under UV irradiation in air caused by the presence ofthe TiOx layer. These data indicate the combined effect of oxygenand UV light on the change in solar cell efficiency. In order to assessthe individual effects of UV and air exposure, further studies wereconducted.

Fig. 5 shows the change in normalized efficiency under air expo-sure. Significant degradation in solar cell efficiency was noted underair exposure. When the experiment was repeated under nitrogen nodegradation was noted. The data also shows that significant protec-tion is provided from air degradation by the presence of a TiOx layer ontop of the active layer. Next, we examined the effect of UV irradiationonly on solar cell performance. Fig. 6 compares the protected andunprotected solar cells performance when exposed to UV radiation ina N2 atmosphere. Fig. 7 compares solar cell degradation in 1 atm ofoxygen under weak (10 mW/cm2) and strong (300 mW/cm2) UVirradiation. For both cases the presence of the TiOx layer enhances theperformance over time. Solar cells degrade more rapidly when the UVflux is increased 30 times.

The TiOx protection is more effective in the presence of both UVand O2. The TiOx layer when exposed to UV irradiation scavengesoxygen and consumes UV photons providing protection from UV andair exposure to organic solar cell devices. This is in accordance withspectroscopic measurements of the photochemically activated TiOx

film to be shown later.The degradation of P3HT/PCBM organic solar cell devices can be

represented by an exponential term that describes the fast initial

Fig. 7. Variation of normalized organic solar cell efficiency with UV exposure time

while the sample was surrounded by the 1 atm oxygen environment (weak UV

power density¼10 mW/cm2 and strong UV power density¼300 mW/cm2).

Fig. 9. Current voltage characteristics data indicating the significant change in I–V

curve shape due to air degradation.

Fig. 6. Variation of normalized organic solar cell efficiency with UV exposure time

while the sample was in a nitrogen atmosphere (UV power density¼10 mW/cm2).

Fig. 8. Variation of (a) open circuit voltage(Voc), (b) short circuit current (Isc) and

(c) fill factor with UV exposure time when sample was surrounded by nitrogen

atmosphere (UV power density¼10 mW/cm2).

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–11301126

decay in combination with a second exponential term describingthe long-term degradation.

Z¼ Z0½ae�gtþbe�dt� ð1Þ

where Z0 is the initial solar cell efficiency and a, b, g and d are curvefitting parameters [17].

The existence of a double exponential expression for the degra-dation of solar cells upon exposure to UV irradiation and air(or oxygen) atmosphere suggests that two degradation processes actin parallel with time constant g (h�1) and d (h�1). For Fig. 4, wheredegradation is due to combined effect of UV and air, the followingvalues for the various constants were obtained: a¼0.189, b¼0.804,g¼2.275 and d¼0.0701. While the photocell decay process may beempirically expressed as the sum of two exponential degradationprocesses, we do not believe that these two processes can be directlyattributed to separable factors such as UV degradation of the polymerand oxygen-derived degradation of the polymer and hence of thephotocell. The time constant g indicates the fast decay process and d isrelated to slow decay. For UV degradation high values of g wereobtained, indicating faster decay due to UV exposure compared to air.

We also examined the effect of the surrounding environmentand UV irradiation on the various photovoltaic performanceparameters such as open circuit voltage (Voc), short circuit current(Isc), fill factor (FF) and efficiency (Z). Fig. 8 shows the variation ofVoc, Isc, and FF under UV irradiation in a nitrogen environment.These data clearly show that major loss in solar cell efficiency under

UV irradiation occurs due to lowering of the short circuit currentand the fill factor. The open circuit voltage remains essentially cons-tant. Similar results were obtained under air exposure conditions.

A significant change in current vs. voltage curve shape wasobserved for the uncoated solar cell under air degradation and nochange was noted when the TiOx layer was used as a protectionlayer as indicated in Fig. 9. Several authors [4,18–20] have reportedthe observation of the kink in the fourth quadrant of Fig. 9 and haveprovided some explanation [21,22]. This is considered to be due tothe formation of an interface layer between active organic layer andmetal electrode. The TiOx layer scavenges oxygen and it works as abarrier layer to prevent such reaction. Therefore there is no suchkink in the device with TiOx.

3.2. TiOx improvement mechanism study

We report here the study of the mechanism of the photoche-mical activity of a TiOx film, which contains both Ti-OR functionalities

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–1130 1127

and Ti-OH groups throughout. It is found by IR spectroscopy that theresidual Ti-OR functionalities bound into the sol–gel film are photo-oxidized, consuming O2(gas) and producing CO2(gas) and H2O(gas), aswell as bound formate [HCOO–], H2O and Ti-OH moieties. The photo-activation of these films leads to O2 scavenging and forms the basis forthin films, which remove oxygen when exposed to light, therebyprotecting an underlying surface from oxygen gas. An extensiveliterature search has shown that such materials have not previouslybeen used for protecting surfaces from oxygen scavenging induced byphotochemistry [23].

Fig. 11. Surface species consumption and formation during photooxidation of TiOx

film containing alkoxide functionalities. The consumption of the alkoxide function-

ality is accompanied by the formation of carboxylate and hydroxyl surface-bound

species, as well as nitrile moieties from the oxidation of ethanolamine.

3.2.1. Infrared measurements of photooxidation of alkoxide groups in

TiOx

Fig. 10 shows the measurements of the production of CO2 (gas)and H2O (gas) during UV photoactivation of the TiOx film at 295 Kunder 16O2 (gas) at 20 Torr pressure. The spectra indicate that CO2

is monotonically generated over the 32 h irradiation period, withbroadened CO2 vibration-rotation bands being observed (asym-metric stretching mode), centered at 2349 cm�1. In addition, twospectral regions for H2O (gas) are also seen to monotonically developabsorbance, one centered near 3700 cm�1 (stretching mode) andone near 1600 cm�1 (bending mode). The infrared peak positionswere identified in accordance with the reported results as described inRef. [24].

Fig. 11 shows the infrared spectral developments which occur inthe TiOx film during an 8 h UV irradiation period at 295 K, duringwhich the 16O2 pressure was 20 Torr. In the nst(C–H) region,characteristic of the alkoxide functional groups remaining in theTiOx film, it may be seen that considerable loss of absorbance occursat 2926 and 2853 cm�1 as photooxidation of the alkoxide takesplace. The modes at the frequencies above the (C–H) stretchingregion are due to surface hydroxyl (Ti-OH) groups and adsorbedwater in the TiOx film and their absorbances are observed to growduring the photooxidation process. In addition, a very weak C�Nstretching mode [n(C�N)] at 2201 cm�1 is observed due to thephotooxidation of residual ethanolamine from the preparation.Shifting to a congested spectral region below 1750 cm�1, whereasymmetric and symmetric OCO stretching modes develop for

Fig. 10. Gas phase photooxidation products from TiOx film containing alkoxide

functionalities. CO2 (gas) and H2O(gas) are observed. The slow formation of ice on

the IR detector is an artifact to be disregarded. The spectrum through the gas phase

at each time was ratioed by the spectrum taken before UV irradiation.

Fig. 12. Comparison of the IR difference spectra (before and after UV irradiation) of

COO– species produced by photooxidation with 16O2 and 18O2.

HCOO– species, subtle increases occur in absorbance at 1567 and1359 cm�1, as marked. The vibrational frequencies observed areclose to those found for formate species [HCOO�] on TiO2 inpublished reports [25–28]. These developments below 1750 cm�1

are more easily seen when 18O2 is used during the photooxidation,as shown in Fig. 12. Here, by comparison of 16OC16O and 18OC16Omodes, shifts of �14 and �20 cm�1 are seen for the high and lowwavenumber modes, respectively. Shifts of this magnitude for 18Osubstitution for one 16O in HCOO– species are reported in theliterature [29,30].

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–11301128

3.2.2. ESR measurements of electron–hole pair formation by

irradiation of TiOx

Fig. 13 shows ESR spectra recorded in 1 atm of O2 (gas) followingUV activation at 295 K. After exposure to UV irradiation for 15 min,three strong ESR features are observed at g¼2.030, g¼2.006 andg¼2.001. The three ESR transitions are assigned to adsorbed O�2[31,32]. The trapping of the electron by O2 stabilizes the holeproduced in the TiOx film but the hole was not observed by ESR.

Fig. 14 clearly shows the lack of the production of O�2 by UVirradiation for 60 min with the TiOx sample is irradiated under1 atm of N2. This shows, in comparison to the results found for UV

Fig. 13. ESR spectrum showing the effect of UV light on TiOx under O2 at 1 atm.

(a) After UV irradiation, 15 min and (b) before irradiation.

Fig. 14. ESR spectrum of the effect of UV light on TiOx under N2 at 1 atm. (a) after UV

irradiation, 60 min; (b) before irradiation. The results, compared to Fig. 4, show that

O�2 species require O2 (gas) for photochemical formation on TiOx.

irradiation in O2 (Fig. 13), that the photo-formation of trappedelectrons is directly associated with the formation of adsorbed O�2 .

3.2.3. UV absorption study of lattice organization in TiOx films upon

annealing

Fig. 15 shows the absorption edge that is found in TiOx filmsafter annealing in air. For the spin-coated film produced at 295 K,characteristic of the films investigated here, the band edge is�3.9 eV. As the film is sequentially annealed to higher tempera-tures, we estimate that the band edge shifts smoothly downward toabout 3.6 eV after heating to 573 K, achieving a maximum rate ofchange near 400 K. During this heating, IR spectral measurementsindicate that Ti-OH groups are disappearing but alkoxide groupsremain in the film at least to 573 K.

3.2.4. Mechanistic considerations

The ability of TiOx films, containing internally chemically boundalkoxide groups, to photochemically consume oxygen is notsurprising based on the wide utility of TiO2 for the photooxidationof ex situ organic and inorganic matter [32–37]. The TiOx film carriesits own sacrificial organic reactant while also containing ultravioletlight absorbing � � �Ti–O–Ti � � � structures throughout the film,capable of making electron–hole pairs, which then promote redoxprocesses. It is found that the presence of adsorbed O2 facilitateselectron–hole pair separation by acting as an electron acceptor.

The O�2 species produced from molecular oxygen can cause theoxidation of the alkoxide moieties bound within the film. The O�2species must be able to move from its point of formation to find theorganic group and to achieve the first oxidation step. The O�2 speciesis called a superoxide species. The formation of CO2 and H2O as finalproducts of the photooxidation of alkoxide groups must involve alarge number of sequential one-electron oxidation steps; however,only HCOO- has been observed spectroscopically in this work as anintermediate chemically bound organic oxidation product alongwith Ti-OH species resulting from adsorption of some of the H2Oproduced in the photooxidation process [32–37].

The 3.9 eV absorption threshold for the spin-coated TiOx filmsprepared near room temperature is indicative of the presence ofshort � � � Ti–O–Ti � � � structures within the film, leading to elec-tron confinement and an electronic gap, which is broader than forthe infinite � � � Ti–O–Ti � � � entities within bulk TiO2. TiO2 in bulk

Fig. 15. UV absorption spectrum-TiOx upon annealing in air. The absorbance at

250 nm decreases during annealing and the spectra are normalized at this point.

J. Li et al. / Solar Energy Materials & Solar Cells 95 (2011) 1123–1130 1129

anatase or rutile crystal phases exhibits a bandgap of �3.1 eV [34].Annealing of the TiOx film to 573 K produces a 0.3 eV decrease inthe threshold for photon absorption, which is probably indicative ofthe development of � � �Ti–O–Ti � � � structures into longer, moreorganized structures as organic matter and water are removedfrom the film by heating. In these annealed structures, electronconfinement effects are reduced. Such an electronic model has beenproposed for other sol–gel oxide materials [38,39]. The develop-ment of absorption tails in the 3.7 eV region could also indicate theproduction of localized states near the band edges. Heating TiOx

films to high temperatures to decrease the threshold energy forlight absorption could be beneficial for the more efficient capture ofsolar energy in TiOx films utilized for protection of underlyingsurfaces from oxygen.

It is of interest to estimate the capacity of the bound –OR groupswithin the TiOx film for reaction with oxygen gas. A 20 nm film ofTiOx will contain about 3�1016 TiOx units per cm2. Assuming thatabout 10–50 mole % of –OCH3 groups exist in the film, then thecapacity for photo-oxidative reaction with O2 is of order of 3�1015

–1.5�1016 O2 molecules cm�2. The underlying 120 nm P3HT/PCBM active layer (1 cm2 surface area) has a volume of about1�10�5 cm3. The critical doping level for electron traps generatedby O2 is reported for P3HT/PCBM to be between 1016 cm�3 [40] and1017 cm�3 [41] or about 1011–1012 per cm2 of active area. Thus, theTiOx protective film used here has a capacity of the order of 104–105

times the quantity of O2 needed to seriously influence the activeP3HT/PCBM film performance. Also, in addition to its oxygenreactivity, the TiOx film will act as a diffusion barrier for oxygenthus further slowing down the oxygen interaction with the activelayer.

4. Conclusions

From this study on stabilization of P3HT/PCBM based organic solarcells with a TiOx layer, we can draw following conclusions: (1) TheP3HT/PCBM organic solar cell efficiency decreases with increase in airexposure time. No change was noted under a nitrogen atmosphere.A thin film of TiOx on top of the active layer provided significantprotection from air degradation. (2) The cell performance decrea-ses with increasing UV exposure under nitrogen atmosphere and athin film of TiOx layer on top of the active layer provided significantprotection from UV exposure. (3) A higher rate of loss in cell efficiencyfor the unprotected cells was noted when exposed to the combinedeffect of UV and oxygen, as compared to the individual effects.

From the TiOx mechanism study, we have discovered thefollowing properties of spin-cast TiOx sol–gel films made fromtitanium alkoxide and acting as photochemically activated oxygenscavengers:

(1)

UV irradiation of TiOx films containing alkoxide groups remain-ing from the sol–gel synthesis leads to the photochemicaloxidation of these organic groups, removing O2 (gas). H2O (gas),CO2 (gas), chemically bound formate [HCOO–], H2O(ad) andTi-OH oxidation products are also observed to form at 295 K, dueto photooxidation of the organic groups. It is the photochemicalremoval of O2 by TiOx films that is postulated to lead to passivationof organic photocells from oxygen induced degradation.

(2)

O�2 species are observed to form where TiOx films are irradiatedin O2 (gas) at 295 K. In the absence of O2 (gas), these species arenot detected by ESR upon UV irradiation. These species (super-oxides) are well-known oxidizers.

(3)

The spectral threshold energy for TiOx films made at 295 K is�3.9 eV. As the films are thermally processed up to 573 K, toremove some alkoxide and Ti-OH ligands, the threshold photonenergy becomes less distinct (distribution of localized states

near band edges) and shifts downward to �3.6 eV, indicative ofincreasing order amongst the � � �Ti–O–Ti � � � assemblies inthe films, causing decreasing electron confinement, but thelattice order is still very incomplete compared to bulk TiO2.

Acknowledgements

We thank nanoSTAR, the Institute for Nanoscale and QuantumScientific and Technological Advanced Research at the University ofVirginia for their financial support. We thank Professor David Cafisoand Wei-wei Kuo, Department of Chemistry, for use of the ESRspectrometer for portions of this work, and Professor James Demas,Department of Chemistry, for use of the UV–Vis spectrometer forportions of this work. We thank Professor Lin Pu for helpful com-ments. Research at UC Santa Barbara was supported by a grant fromthe Department of Energy (DOE, Dr. A. Kini, Program Officer) and bygrants from the US Army: CERDC (Mary Hendrickson, Chief Engineer)through General Technical Services LLC/GTS-S-09-1-196).

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