Vinamrita Singh & Swati Arora & P. K. Bhatnagar &

Manoj Arora & R. P. Tandon

Received: 28 November 2011 /Accepted: 21 May 2012 /Published online: 1 June 2012# Springer Science+Business Media B.V. 2012

Abstract We have investigated the degradation of P3HT:PCBM (poly(3-hexylthiophene):6,6-phenylC61 butyric acidmethyl ester) solar cell beyond 150 h of fabrication incontinuation to our earlier reported work up to 150 h offabrication. The current- voltage characteristics of degradedIndium tin oxide/poly(3,4-ethylenedioxythiopene):poly(styrenesulfonate)/poly(3-hexylthiophene):6,6-phenylC61butyric acid methyl ester/Aluminum (ITO/PEDOT:PSS/P3HT:PCBM/Al) solar cell can be explained by consideringthe tunneling current through electrode interfaces, increasein both the interface states density and the thickness ofinterface with time for150-200 h. Beyond 200 h of fabrica-tion, a significant reduction in the occupation probability atthe electrode interfaces explains the experimental results upto 300 h fairly well. Calculations based on realistic param-eters and activity at both the electrode interfaces (ITO/PEDOT:PSS and P3HT:PCBM/Al) confirm that degrada-tion at P3HT:PCBM/Al interface is more prominent thanthat at ITO/PEDOT:PSS interface.

Keywords Organic solar cell . Degradation . Occupationprobability . Tunneling current . Density of interface states

Introduction

The main problem and matter of concern with bulk hetero-junction (BHJ) organic solar cell is not the efficiency but thefast degradation [1-4]. To make the cells commerciallyviable there is a need to address the factors responsible forthis. Many attempts have been made to understand theworking principle of organic solar cells. The different mod-els such as the diode model, SCLC model, etc. have beensuccessful to some extent, but require extensive assumptionsand therefore these models do not always reproduce manyof the experimental results of organic based devices. Anumber of models have been suggested to explain P3HT:PCBM blend solar cell [5-14]. Many authors have reviewedand suggested that interfaces play an important role indeciding the device behaviour [8, 9]. Guerrero et al. [15]have mentioned that electrostatic mechanisms occurring atthe nanometer scale, both in the active layer bulk and atinterfaces, have a great influence on the overall deviceoperations. Toyoshima et al. [16] showed that the electronicstructure at the interface between the active material and theelectrode plays an important role in electron transfer. One ofthe main causes of aging is creation of interface states at theinterface layer with passage of time. This interface layercannot be regarded as a simple contact between the elec-trode and the organic layer [12]. Instead, it should be con-sidered as a thin layer (8–12 Å) resulting from reaction/diffusion of ions or growth of oxide due to atmosphericeffect with time. All these work emphasize role of interfacestates and thickness of interfaces. The absorption or adsorp-tion of atmospheric moisture and oxygen may occur at the

V. Singh : R. P. TandonDepartment of Physics & Astrophysics, University of Delhi,Delhi 110007, India

S. Arora (*)Department of Physics, Zakir Husain College, University of Delhi,Delhi 110002, Indiae-mail: drswatia@yahoo.comURL: http://people.du.ac.in/~rptandon9/Page2.html

P. K. BhatnagarDepartment of Electronic Science,University of Delhi South Campus,Delhi 110021, India

M. AroraDepartment of Physics, Ramjas College, University of Delhi,Delhi 110007, India

J Polym Res (2012) 19:9899DOI 10.1007/s10965-012-9899-0

ORIGINAL PAPER

Degradation in bulk heterojunction organic solar cells:changes in electrode interface and reduction in the occupationprobability of the interface states

polymer-electrode interface. The wet film preparation tech-niques such as spin coating and its detrimental effects at thesurface of polymers [12] with the possibility of formingspecial form of charge carriers such as solitons, polaronsand bipolarons, makes the polymer interface problem rathercomplex. Therefore, the interfacial layer influences the J-Vcharacteristics. We believe that the tunneling through theinterface states starts dominating with time after 150 h offabrication. These interface states are created with the pas-sage of time because of degradation of Aluminum (Al) andIndium Tin Oxide (ITO) electrodes [17], degradation due tochange in active polymer layer [18], degradation due toexposure of poly(3,4-ethylenedioxythiopene):poly(styrene-sulfonate) (PEDOT:PSS) to moisture [17] and degradationdue to changes at ITO/PEDOT:PSS, PEDOT:PSS/P3HT:PCBM, and P3HT:PCBM/Al interfaces [19, 20]. It has alsobeen reported that the activity at P3HT:PCBM/Al is muchmore than that of ITO/PEDOT:PSS interface [19]. There-fore, there is a need to look into the mechanisms and thechanges that occur at organic/electrode interfaces. This hasbeen taken care of in our model by suitably incorporatingthe changes in the occupation probability at thermal equi-librium (fSO), along with changes in surface state density(DS) and thickness (δ) of the interface as a function of time.Importance of occupation probability at metal/organic inter-faces has been reported by other workers also [15, 21] but tothe best of our knowledge, no quantitative analysis inregards to degradation is available in the literature. Theexperimental J-V characteristics for our typical cell (ITO/PEDOT:PSS/P3HT:PCBM/Al) are studied up to 300 h afterfabrication and are in agreement with our calculations whenthe effect of the above mentioned parameters is included inthe calculations (details of parameters are given in Resultsand Discussion section). Decrease in occupation probabilityis plausible because as the cell degrades the density of statesincreases [22] but the number of injected carriers remainsthe same and there is a detrapping of the charge carriers alsoat the interface.

Theoretical model

In bulk heterojunction organic solar cells, absorbed photonsgenerate excitons which dissociate to form charge carriersand these charge carriers are transported towards the electro-des. The holes move towards ITO and electrons movetowards Al electrode in a typical cell ITO/PEDOT:PSS/P3HT:PCBM/Al. The charge carriers face a tunneling bar-rier at both the electrode interfaces (cathode and anode).Dark and illuminated characteristics are not explained bydiode current JD only and one needs to consider the currentdue to the charge carriers which are transported through theinterfacial layer via interface states. In the present model

also, explicit calculations have been made for J-V character-istics using diode equation incorporating the tunnelingcurrent.

The continuity equation for electron current in dark isgiven by

JD ¼ qμnðxÞF þ qDn@nðxÞ@x

ð1Þ

where μ is the charge carrier mobility, n(x) is electrondensity, F is the electric field and Dn is the diffusion coef-ficient. On further simplification one arrives at the followingdiode equation for dark characteristics [23]

JD ¼q2DnNC Vbi � V þ JARSð Þ exp �ϕ

kT

� �exp q V�JARSð Þ

nkT

� �� 1

� �

dnkT 1� exp �q Vbi�VþJARSð ÞnkT

� �� �

þ V � JARS

RPA

ð2Þ

In Eq. (2) k is Boltzmann constant, T is temperature inKelvin, n is the diode ideality factor, Vbi is the built-involtage, V is the applied voltage, A is the diode cross-section area, d is sample thickness, NC is the effectivedensity of states [23], φ is the injection barrier at the elec-trode, Rs is the series resistance and Rp is the parallelresistance. The tunneling current through the interface statesat one junction is given by [24]

Jt ¼ qDsσStVmg Esð ÞΔEs exp �bk1 2=s d

� �

� fs � fso exp � BqV

kT

� �� ð3Þ

Here, Ds is the density of interface states, σSt is thecapture cross-section for interface states [5], Vm is thermalvelocity of the electrons, g(ES) is density of states at energyE0Es around a small energy range ΔEs, b is tunnelingconstant for interface states, κs is the barrier height forinterface states, δ is the thickness of the interface layer, fsois the occupational probability of interface states at thermalequilibrium and fs is final occupational probability [24, 25].In Eq. (3), B is a constant which depends on the oxidethickness, depletion width and dielectric constants of theinterface materials. Values of fs and fso have been calculatedby method used by us [24]. In our previous work [20] weconsidered all the three interfaces ITO/PEDOT:PSS,PEDOT:PSS/P3HT:PCBM, and P3HT:PCBM/Al and foundthat only ITO/PEDOT:PSS and P3HT:PCBM/Al interfaceshave more significant contribution. This is in agreementwith earlier reported work also [19]. Therefore, we areconsidering two interfaces (ITO/PEDOT:PSS and P3HT:PCBM/Al), and the tunneling current through ITO/PEDOT:PSS and P3HT:PCBM/Al interfaces are taken as

Page 2 of 6 J Polym Res (2012) 19:9899

Jt1 and Jt2 respectively. Hence the net tunneling current, JTthrough the interfaces can be obtained by

JT ¼ Jt1 þ Jt2 ð4ÞJt1 and Jt2 can be calculated using Eq. (3) using B1 for Jt1and B2 for Jt2.

For ITO/PEDOT:PSS interface

B1 ¼d1d

� � "p1"i1

� �

1þ d1d

� � "p1"i1

� � ð5Þ

and for P3HT:PCBM/Al interface

B2 ¼d2d

� � "p2"i2

� �

1þ d2d

� � "p2"i2

� � ð6Þ

Here, dielectric constant for PEDOT:PSS εP1039, forP3HT:PCBM εP203.6, at ITO/PEDOT:PSS interface εi1010 and that at P3HT:PCBM/Al interface εi203, are takenfrom reported literature [26-29].

Now, the net dark current (JD,total) is obtained by addingEqs. (2) and (4).

JD;total ¼ JD þ JT

JD;total ¼ q2DnNC Vbi�VþJARSð Þ exp �φkTð Þ exp

q V�JARSð ÞnkT

� ��1

� �

dnkT 1�exp�q Vbi�VþJARSð Þ

nkT

� �� �

þ V � JARS

RPAþ JT ð7Þ

Under ideal conditions, Rs→0and Rp→∞.

Total dark current–voltage characteristics

JD;total ¼q2DnNC Vbi � Vð Þ exp �φ

kT

� �exp qV

nkT

� �� 1� �

dnkT 1� exp �q Vbi�Vð ÞnkT

� �� � þ JT

ð8Þ

Illuminated current–voltage characteristics

Under illumination the current–voltage characteristics forBHJ solar cells without tunneling is given by

JI ¼q2DnNC Vbi � Vð Þ exp �φ

kT

� �exp qV

nkT

� �� 1� �

dnkT 1� exp �q Vbi�Vð ÞnkT

� �� � þ JLðV Þ

ð9Þ

With tunneling, the total illuminated current will be

JI ;total ¼q2DnNC Vbi � Vð Þ exp �φ

kT

� �exp qV

nkT

� �� 1� �

dnkT 1� exp �q Vbi�Vð ÞnkT

� �� �

þ JT � JLðV Þ ð10Þ

where JL(V) is the photo-generated current and has beencalculated under ideal conditions by

JLðV Þ ¼ JSCj jμt Vbi � Vð Þd2

τ is the lifetime of the charge carrier [23], JSC is the shortcircuit current density.

Calculations have been made taking realistic initial val-ues of the parameters. The thickness of the active layer, d0150 nm; charge carrier mobility μ010−4 cm2V−1 s−1 [23];life time of the charge carrier τ02×10−6 s [5]; thermalvelocity of the electrons Vm010

7 cms−1 [24]; capturecross-section for surface states σSt010

−14 m2 [5]; tunnelingconstant for surface states b00.6 [24]

Experimental details

Organic solar cells were prepared on pre-cleaned, pre-patterned indium tin oxide (ITO) coated glass (18Ω/□) sub-strate. The substrates were cleaned in ultrasonic bath usingsoap solution, followed by distilled water. The substrates werethen boiled in acetone, trichloroethylene and isopropanol, anddried in vacuum oven at a temperature of 120 °C. Finally, thesubstrates were exposed to oxygen plasma for 5 min prior todeposition of organic layers. Conducting polymer (PEDOT:PSS) of thickness 50 nmwas spin coated on ITO substrate. Toprepare the active layer, a solution of P3HT:PCBM (1:1 ratioby weight) was dissolved in chlorobenzene (as solvent) andspin coated at 1,000 rpm for 2 min to form ~150 nm thicklayer. The Al electrodes were thermally evaporated and astructure shown in Fig. 1 was obtained. The current voltagemeasurements were carried out for freshly prepared samplesand up to 300 h of fabrication at regular time intervals, usingKeithley 2400 source meter unit, interfaced with a computer.Solar simulator (Photoemission Tech. SS50AAA, USA) wasused and its intensity was set at 100 mWcm−2. The chemicalsused in fabrication of devices were procured from AldrichChemicals USA.

Results and discussions

The interface states at both the electrode interfaces andhence the tunneling current through these states starts

J Polym Res (2012) 19:9899 Page 3 of 6

dominating when the cell degrades. It is not only the inter-face state density and interface layer thickness that changeswith time (as was there up to 200 h), but there is significantchange in the occupation probability with time at variouselectrode interfaces. These effects taken together significant-ly influence the J-V characteristics of the cell (beyond 200 h.of fabrication). As reported earlier also for degradationbetween 50 and 150 h of fabrication, tunneling current startsplaying a role [20]. Our calculations show that beyond thisrange (150–300 h), the tunneling current through the elec-trode interfaces becomes quite significant. The results forthree typical values (160, 210 and 300 h of fabrication) havebeen presented here. It may be mentioned that only after~150 h of fabrication, the tunneling current starts becomingimportant and becomes significant after ~200 h. In the range(150–200 h), the experimental results are explained consid-ering both the thickness of interface and density of interfacestates which increase with time. Beyond 200 h of fabrica-tion, there is remarkable change in occupation probabilitytoo. The importance of occupation probability at metal/organic interface for organic solar cells has also beenreported by others [15, 21]. In our case, it is found that thereis a significant percentage decrease in the occupation prob-ability with time.

Figure 2(a, b) shows that the experimental results are bestexplained when (i) interface state density increases from 9×1011 cm−2 eV−1 to 2×1013 cm−2 eV−1 for both the interfacesand (ii) interface thickness increases from 8 Å to 11 Å (atITO interface) and from 8 Å to 12 Å (at Al interface) due tomore activity at Al interface [19]. Here at 160 h of fabrica-tion, the tunneling current alone explains J-V characteristicswhen change in DS and δ are considered.

However, for cells degraded beyond 200 h (Fig. 3 (a, b)),the present calculations (with increase in DS and δ) do notexplain fully the experimental results. This may be due toprobable reduction in the occupation probability of theinterface states with time because the trapped interface statestend to give up the charge carrier making these neutral withpassage of time for donor type traps [30]. The presentcalculations give the best fit for 14 % decrease in theoccupation probability of the interface states along withincrease in interface state density from 9×1011 cm−2 eV−1

to 2×1013 cm−2 eV−1 (at ITO interface) and from 9×1011 cm−2 eV−1 to 3×1013 cm−2 eV−1 (at Al interface),interface thickness increases from 8 Å to 11 Å (at ITOinterface) and from 8 Å to 12 Å (at Al interface).

Figure 4 (a, b) shows both experimental as well as cal-culated dark and illuminated characteristics of the cellaround 300 h of fabrication. The best fit of the model isobtained when surface state density further increases from

Fig 1 a Schematic diagram oforganic bulk heterojunctionsolar cell based on P3HT:PCBM blend. b Schematicdiagram of the device showingITO and Al interfaces alongwith their thicknesses anddensities of states

Fig 2 a Dark J-V characteristics and b Illuminated J-V characteristicsof degraded solar cell (~160 h); □ represents experimental curve, -o-represents JD total and ─ represents JT

Page 4 of 6 J Polym Res (2012) 19:9899

9×1011 cm−2 eV−1 to 6×1013 cm−2 eV−1 (at ITO interface)and from 9×1011 cm−2 eV−1 to 8×1013 cm−2 eV−1 (at Alinterface), while interface thickness increases from 8 Å to12 Å (at ITO interface) and from 8 Å to 13 Å (at Alinterface) and the occupation probability is further reducedfrom 14 % to 20 %.

Conclusion

To sum up the interface states start playing a prominent role indegradation around 200 h of fabrication of the solar cell whichcan be explained in terms of growth of interface state density,increase in the interface thickness and reduction in their oc-cupation probability with time. Therefore, for putting a checkon degradation there is need to control the interfaces. This canbe done (i) by using highly conducting buffer layer ofPEDOT:PSS at ITO interface and (ii) use of noble metal like

gold (Au) by adsorbing an air-stable n-type dopant acridineorange (AOB) thin films using AOB/Au as cathode. In addi-tion to this a soft deposition process which leaves a negligiblenumber of dangling bonds in organic material or denselypacked small molecules at the interface (which do not permitdiffusion of impurity from outside) might prevent increase inthe interface state density, increasing thickness of interfacelayer, and checking the reduction in occupation probability.Since the effect of interface states is very important and cannotbe ignored one must include the adverse effect on mobilityand lifetime of the carriers also.

Acknowledgement Authors wish to thank Department of Scienceand Technology Purse Grant, INDIA for financial support. We want tofurther thank Prof P.C. Mathur, Department of Electronic Science,South Campus, University of Delhi, Dr. M. Aslam Parvaiz, Principal,Zakir Husain College, University of Delhi and Dr. Pankaj Kumar,Scientist B, National Physical Laboratory for their valuable sugges-tions and support.

Fig 3 a Dark J-V characteristics and b Illuminated J-V characteristicsof degraded solar cell (~210 h); □ represents experimental curve, -o-represents JT with initial fSO value and ─ represents tunneling currentwith fSO reduced by 14 %

Fig 4 a Dark J-V characteristics and b Illuminated J-V characteristicsof degraded solar cell (~ 300 h); □ represents experimental curve, -o-represents tunneling current with initial fSO value and ─ representstunneling current with fSO reduced by 20 %

J Polym Res (2012) 19:9899 Page 5 of 6

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