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Enhanced diode characteristics of organic solar cells using titanium suboxide electron transport layerJae Hyun Lee,1,2Shinuk Cho,1Anshuman Roy,1Hee-Tae Jung,2and Alan J.Heeger1,a͒1Center for Polymers and Organic Solids,University of California at Santa Barbara,Santa Barbara,California93106-5090,USA2Department of Chemical and Biomolecular Engineering(BK-21),Korea Advanced Institute of Scienceand Technology,Daejeon305-701,Republic of Korea͑Received26February2010;accepted24March2010;published online23April2010͒The͑dark͒diode characteristics of the organic bulk heterojunction solar cell based on the phaseseparated blend of poly͓N-9Љ-heptadecanyl-2,7-carbazole-alt-5,5-͑4Ј,7Ј-di-2-thenyl-2Ј,1Ј,3Ј-benzothiadiazole͔͒with͓6,6͔-phenyl C70-butyric acid methyl ester have been analyzed with a focuson the effect of the titanium suboxide͑TiO x͒electron transport layer.The addition of the TiO x layerinto the device structure causes the saturation current density to decrease by a factor of26and theshunt resistance to increase by a factor of12.The diode ideality factor and series resistance are,respectively,almost the same for diodes made with and without the TiO x layer.The results indicatethat the TiO x layer increases the energy barrier for hole transport and reduces the minority carrierdensity.©2010American Institute of Physics.͓doi:10.1063/1.3409116͔Polymer bulk heterojunction͑BHJ͒solar cells based on composites of semiconducting conjugated polymers and fullerene derivatives have the potential for application in low-cost,printable,portable,andflexible renewable energy sources.1–3Significant effort has been devoted to developing materials and device structures to facilitate enhanced device performance.Encouraging progress has been reported dem-onstrating power-conversion efficiencies͑PCEs͒greater than 7%.3–5One promising approach to improve device perfor-mance was the introduction of a thin layer of titanium sub-oxide͑TiO x͒as an optical spacer and an electron transport layer.6,7Fundamental and systematic understanding of the device physics is essential in order to further optimize the perfor-mance.To describe the behavior of BHJ solar cells,the single diode model and more complicated models have been proposed.Although good correlation was obtained between data and the diode model,8–10systematic studies of the effect on the diode characteristics of each component and of differ-ent processing conditions are lacking.In particular,a system-atic study of the effect of TiO x on the diode characteristics of BHJ solar cells has not yet been carried out.In this work,we investigated the effect of including a TiO x layer on the diode characteristics of the BHJ solar cell. The use of a TiO x layer between the BHJ thinfilm and the metal contact decreases the saturation current density and increases the shunt resistance of the diode.These changes result from the electron transport and hole blocking proper-ties of the TiO x layer.The classic model of the metal-semiconductor-metal de-vice configuration has often been used to describe BHJ solar cells,wherein the current density versus voltage relationship is given by the following expression as obtained from a simple equivalent circuit model including series and shunt resistance͑see inset in Fig.1͒:11J=J0ͭexpͫq nkT͑V−JAR s͒ͬ−1ͮ+V−JAR s AR sh−J ph͑V͒.͑1͒In Eq.͑1͒,J is the current density,V is the applied voltage,J0 is the reverse saturation current density,q is elementary charge,n is the diode ideality factor,k is the Boltzmann constant,T is the absolute temperature,A is the area of the solar cell,R s is the series resistance,R sh is the shunt resis-tance,and J ph͑V͒is the voltage-dependent photogenerated current density.By measuring the dark current density while sweeping the bias voltage,the diode characteristics including saturation current density,diode ideality factor,series resis-tance,and shunt resistance can be evaluated.In order to determine the role of the TiO x layer,solar cells are fabricated with and without TiO x.The fabricated solar cell structure comprises indium tin oxide͑ITO͒coated glass substrate,poly͑3,4-ethylenedioxythiphene͒: poly͑styrenesulfonate͒͑PEDOT:PSS͒,BHJ photoactive layer,TiO x͑if applicable͒,and an aluminum cathode.Thea͒Author to whom correspondence should be addressed.Electronic mail: ajhe@.FIG.1.͑Color online͒Dark currentfitting with Eq.͑1͒.The equivalent circuit is shown in the inset.The curve shows thefit to the measured dark current͑circle͒.The line below the circles shows the current density vs voltage that is obtained from Eq.͑1͒when the series resistance is zero and the shunt resistance is infinite.APPLIED PHYSICS LETTERS96,163303͑2010͒0003-6951/2010/96͑16͒/163303/3/$30.00©2010American Institute of Physics96,163303-1ITO coated glass substrate was sequentially cleaned with de-tergent,distilled water,acetone,and isopropyl alcohol fol-lowed by drying in an oven.A 40nm thick PEDOT:PSS ͑Baytron PH ͒was spin cast on the UV-ozone treated ITO coated glass.The photoactive layer is a phase separated blend ͑BHJ layer ͒of poly ͓N-9Љ-heptadecanyl-2,7-carbazole-alt-5,5-͑4Ј,7Ј-di -2-thenyl -2Ј,1Ј,3Ј-benzothiadiazole ͔͒͑PCDTBT ͒and ͓6,6͔-phenyl C 70-butyric acid methyl ester ͑PC 70BM ͒with weight ratio of 1:4.The BHJ layer was spin cast from solution in chlorobenzene and o-dichlorobenzene cosolvent ͑1:3volume ratio ͒.For the solar cell devices “with TiO x ,”the TiO x layer was introduced by spin casting under air ͑at 5000rpm for 40s ͒using a presynthesized TiO x precursor solution.12A 100nm thick aluminum electrode was deposited by thermal evaporation under vacuum of 3ϫ10−6mbar.Current density-voltage ͑J-V ͒characteristics of the devices were measured using a Keithley 236source measure unit.To characterize the photogenerated current density,a solar simulator was used ͑irradiation intensity of 100mW /cm 2with Air Mass 1.5Global filter ͒.An aperture with area of 15.7mm 2was employed during J-V character-istics measurement.The dark current density curve shown in Fig.1can be divided into three distinct regimes.13When the cell is under negative or slightly positive bias,the current is limited by the shunt resistance.The second regime,at higher voltages,shows exponential behavior where the current density is con-trolled by the diode characteristics.The reverse saturation current density ͑J 0͒can be evaluated from the y -axis inter-cept by extrapolation ͑see the dotted line in Fig.1͒.The third regime occurs at still higher positive bias,where the current is limited by the series resistance.The parameters in the model given by Eq.͑1͒͑J 0,n ,R s ,and R sh ͒can be evaluated by fitting curves for the positive bias region.With no illumination ͑J ph =0͒,the current density can be expressed as a sum of diode current density and a leakage current density from shunt resistance.In the regime with moderate bias where diode characteristics are dominant,i.e.,exp ͓q ͑V −JAR s ͒/nkT ͔ӷ1,the current density-voltage equation can be expressed aslnͩV −JAR s AR sh ͪ=ln J 0+qnkT͑V −JAR s ͒.͑2͒When the linear fitting is executed with ͑V −JAR s ͒versus ͕ln ͓J −͑V −JAR s ͒/AR sh ͔͖,the saturation current density can be calculated from the y -axis intercept,and the diode ideality factor can be calculated from the slope of the line.To arrive at a more accurate fit,the root mean square ͑RMS ͒error value was calculated with the following equation,RMS=sqrt ͕⌺͓͑J −J fit ͒/J ͔2/n data ͖,where J fit is fitted current den-sity and n data is the number of the data point.The next step for the fitting involves adjusting the series resistance to obtain the smallest RMS value with fixed satu-ration current density,diode ideality factor,and the shunt resistance.Then,the saturation current density and diode ide-ality factor were recalculated from the linear fitting by using the new assumed series resistance value.The values con-verge after several iterations.The dark current density ͑no illumination ͒versus bias voltage curves obtained from solar cells fabricated with and without TiO x are shown in Fig.2.The parameters evaluated from fitting the curve with basic diode equation are summa-rized in Table I .With the TiO x layer between the photoactive BHJ layer and the aluminum cathode electrode,the satura-tion current density decreased by 26times,and the shunt resistance increased by 12times.The diode ideality factor and the series resistance are nearly the same with and with-out the TiO x layer ͑10%increase in series resistance ͒.Figure 3shows some examples of the fitting.The TiO x layer causes a lower intercept when the line is extrapolated from the exponential regime,indicative of a lower saturation current density.Note that the slopes are the same with and without TiO x the layer,which means the cells fabricated with and without the TiO x layer have the same diode ideality fac-tor.The diode ideality factor reflects the properties of the interface;i.e.,the internal BHJ morphology.13Because the BHJ layers were deposited from the same solution with the same method,the absence of any change in the diode ideality factor when the TiO x layer was added is reasonable.The magnitude of highest occupied molecular orbital ͑HOMO ͒level of TiO x is at about Ϫ8.0eV ͑with respect to the vacuum ͒;i.e.,much lower in energy than the HOMO energies of PCDTBT or PC 70BM ͑Ϫ5.5eV or Ϫ6.0eV ,respectively ͒.5For this reason,the insertion of the TiO x layer is expected to reduce the hole carrier contribution to the cur-rent density and thereby to reduce the saturation current ͑see the energy level diagram at the inset of Fig.3͒.WeconsiderFIG.2.͑Color online ͒Current density vs bias voltage curves obtained from solar cells with two different struc-tures:without TiO x layer ͑circles ͒and with TiO x layer ͑triangles ͒,eight cells each.͑a ͒Semilog plot and ͑b ͒linear plot.TABLE I.Fitted parameters obtained from the two different solar cell struc-tures ͑with and without the TiO x layer ͒.Each entry in the table is an average of the values obtained from eight cells.Without TiO xWith TiO x Saturation current density ͑A /cm 2͒ 6.53ϫ10−122.48ϫ10−13Diode ideality factor1.46 1.47Series resistance ͑⍀/cm 2͒45.650.2Shunt resistance ͑⍀/cm 2͒7.06ϫ1048.24ϫ105the reduced saturation current to be indirect evidence that TiO x indeed serves as a hole blocking layer in a BHJ solar cell.The saturation current density is the part of the reverse current in a diode caused by diffusion of minority carriers ͑holes in these devices ͒from the neutral regions to the deple-tion region.14The saturation current is almost independent of the reverse voltage.It is determined by the number of charges able to overcome the energy barrier in the reverse bias direction.13As expected from the energy band diagrams in Fig.3,the TiO x layer functions as a hole-blocker causing the saturation current to decrease.Note that while the presence of a TiO x layer increases the shunt resistance by more than an order of magnitude,the accompanying increase in the series resistance is only 10%.This also explains why the photocurrent density ͑total cur-rent density under illumination ͒is not affected in a negative way due to the introduction of the TiO x .The higher shunt resistance also implies that the leakage current is suppressed by the TiO x layer.15When illuminated by a solar simulator,the solar cell performance is affected by these differences in the dark cur-rent density.On average,solar cells without a TiO x layer yield short circuit current density ͑J sc ͒=10.74mA /cm 2,open circuit voltage ͑V oc ͒=743mV,fill factor ͑FF ͒=0.59,leading to a PCE=4.70%.By introducing the TiO x layer,J sc =10.86mA /cm 2,V oc =882mV,and FF=0.63,and PCE =6.08%͑average values over the eight cells measured with and without the TiO x layer ͒.The difference in short circuit current density is expected from the optical spacer effect of TiO x modeled and predicted in Ref.7.To explain the differ-ence in open circuit voltage,we consider the equivalent circuit model for an “ideal”solar cell by assuming infinite shunt resistance and zero series resistance.Under open cir-cuit conditions,the measured current is ing Eq.͑1͒,we obtain the following expression for the open circuit volt-age:V oc =nkT q ln ͫJ ph ͑V oc ͒J 0+1ͬϷnkT q ln ͫJ ph ͑V oc ͒J 0ͬ,͑3͒where J ph ͑V oc ͒is the photogenerated current density at V =V oc ,which is equal to the dark current density at V oc .The J ph ͑V oc ͒is 1.32and 1.57mA /cm 2with TiO x and without TiO x ,respectively.With Eq.͑3͒,we can readily calculate the expected ratio of the open circuit voltages for devices with and without TiO x .Since the ideality factors are almost equal,the estimated ratio of the open circuit voltages for these devices is ln ͑1.32/2.48ϫ10−10͒/ln ͑1.57/6.53ϫ10−9͒=1.16.The ratio of the measured open circuit voltages is 882/743=1.18.Hence,the higher open circuit voltage in devices with TiO x can be explained by the smaller saturation current den-sity.Thus,the increase in open circuit voltage is a direct outcome of the hole blocking effect of TiO x .The increase in fill factor implies a higher maximum power point current as a result of delayed turn-on of current injection due to the hole blocking as a result of the higher energy barrier ͑see Fig.3͒.In conclusion,we have analyzed the parameters of the basic diode equation for organic solar cells using PCDTBT and PC 70BM.The TiO x layer decreases the saturation current density by 26times due to the higher energy barrier for hole transport ͑the TiO x layer blocks holes ͒.The TiO x layer de-creases the leakage current by increasing the shunt resistance by a factor of 12,yet it does not significantly increase the series resistance.The research was supported by a grant from the U.S.Army CERDEC ͑Mary Hendrickson,Chief Engineer ͒,by the Heeger Center for Advanced materials at the Gwangju Insti-tute for Advanced Materials and by the GRL Program of the Korean Government ͑Grant No.M60605000005-06A0500-00510͒.We thank Dr.G.Dennler,Dr.R.Gaudiana and col-leagues at Konarka for advice and encouragement,and for supplying the PCDTBT and the PC 70BM for our use.1G.Yu,J.Gao,J.C.Hummelen,F.Wudl,and A.J.Heeger,Science 270,1789͑1995͒.2P.Schilinsky,C.Waldauf,and C.J.Brabec,Adv.Funct.Mater.16,1669͑2006͒.3H.-Y .Chen,J.Hou,S.Zhang,Y .Liang,G.Yang,Y .Yang,L.Yu,Y .Wu,and G.Li,Nat.Photonics 3,649͑2009͒.4S.Sista,M.-H.Park,Z.Hong,Y .Wu,J.Hou,W.L.Kwan,G.Li,and Y .Yang,Adv.Mater.22,380͑2010͒.5S.H.Park,A.Roy,S.Beaupre,S.Cho,N.Coates,J.S.Moon,D.Moses,M.Leclerc,K.Lee,and A.J.Heeger,Nat.Photonics 3,297͑2009͒.6J.Y .Kim,S.H.Kim,H.H.Lee,K.Lee,W.Ma,X.Gong,and A.J.Heeger,Adv.Mater.18,572͑2006͒.7A.Roy,S.H.Park,S.Cowan,M.H.Tong,S.Cho,K.Lee,and A.J.Heeger,Appl.Phys.Lett.95,013302͑2009͒.8K.-I.Ishibashi,Y .Kimura,and M.Niwano,J.Appl.Phys.103,094507͑2008͒.9A.Cheknane,H.S.Hilal,F.Djeffal,B.Benyoucef,and J.-P.Charles,Microelectron.J.39,1173͑2008͒.10P.Kumar,S.C.Jain,V .Kumar,S.Chand,and R.P.Tandon,J.Appl.Phys.105,104507͑2009͒.11S.M.Sze,Physics of Semiconductor Devices ͑Wiley,New York,1981͒.12S.Cho,K.Lee,and A.J.Heeger,Adv.Mater.21,1941͑2009͒.13C.Waldauf,M.C.Scharber,P.Schilinsky,J.A.Hauch,and C.J.Brabec,J.Appl.Phys.99,104503͑2006͒.14R.C.Dorf,The Electrical Engineering Handbook ͑CRC,Boca Raton,1993͒.15R.Steim,S.A.Choulis,P.Schilinsky,and C.J.Brabec,Appl.Phys.Lett.92,093303͑2008͒.PCDTBT -5.5eV Energy PEDOT/PC 70BM TiO x -4.7eV-5.0eV-3.6eV -4.3eV-6.0eV-8.0eV-4.3eV-4.3eVbarrierFIG.3.͑Color online ͒Examples of dark current fitting results.Circles rep-resent measured values of dark current density for PCDTBT:PC70BM solar cell without TiO x .Solid line shows the fitted result,and the dashed lines show the extrapolated line for determining saturation current density from intercept and diode ideality factor from the slope.The triangles,line,and dashed line represent the same for solar cell with TiO x .Inset:energy level diagram of the materials in the devices.。