Organic Electronics 24 (2015) 73–79

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Organic Electronics

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Metal salt modified PEDOT:PSS as anode buffer layer and its effect onpower conversion efficiency of organic solar cells

http://dx.doi.org/10.1016/j.orgel.2015.05.0191566-1199/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.

Burak Kadem, Wayne Cranton, Aseel Hassan ⇑Materials and Engineering Research Institute, Sheffield Hallam University, City Campus, Sheffield S1 1WB, UK

a r t i c l e i n f o

Article history:Received 20 March 2015Received in revised form 11 May 2015Accepted 14 May 2015Available online 14 May 2015

Keywords:Organic solar cellP3HT:PCBMPEDOT:PSSMetal salt treatmentElectrode modification

a b s t r a c t

The effects of metal chlorides such as LiCl, NaCl, CdCl2 and CuCl2 on optical transmittance, electrical con-ductivity as well as morphology of PEDOT:PSS films have been investigated. Transmittance spectra ofspun PEDOT:PSS layers were improved by more than 6% to a maximum of 94% in LiCl dopedPEDOT:PSS film. The surface of the PEDOT:PSS films has exhibited higher roughness associated with anincrease in the electrical conductivity after doping with metal salts. The improvement in the physicalproperties of PEDOT:PSS as the hole transport layer proved to be key factors towards enhancing theP3HT:PCBM bulk heterojunction (BHJ) solar cells. These improvements include significantly improvedpower conversion efficiency with values as high as 6.82% associated with high fill factor (61%) and largershort circuit current density (�18 mA cm�2).

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Organic solar cells (OSCs) have received significant attention dur-ing the last few decades due to their low cost processing which iscompatible with flexible substrates, high throughput and thusrequires less capital investment [1]. OSCs are mainly basedon bulk heterojunctions (BHJ) of conjugated polymers such aspoly (3-hexylthiophene) (P3HT) and fullerenes such as(6,6)-phenyl-C61 butyric acid methyl ester (PCBM). Power conver-sion efficiencies (PCE) in the region of 5% have been achieved [2]and PCE of 10% or larger [3] is projected for the foreseeable future,which makes OSCs a viable replacement of the well-established sil-icon SCs. OSCs are diverse and versatile, and can offer wide range ofproperties including tailored band gap and thus varied range of lightharvesting efficiency covering a broad range of the solar spectrum. Itcould be argued that OSCs based on BHJ comprising P3HT and PCBMare considered as the most investigated devices [4]. The nature ofcontacts on both sides of the active layer is considered as an essentialfeature to determine the device performance, and therefore theirPCE. Poly (3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate), abbreviated as PEDOT:PSS has been widely known asone of the most promising conductive polymers employed as a holetransporting layer, due to its simple processing, high transparencyand high stability. It is considered as a favorable material to replaceinorganic materials to deposit on flexible substrates for plastic elec-tronics applications such as memories, sensors, OLEDs and flexible

PV devices [5]. PEDOT:PSS dispersed in water can easily form atransparent film with high conductivity by spin coating, thus pro-ducing a low sheet resistance film [6]. The optimized combinationof conductivity and surface morphology is essential for numerousapplications of PEDOT:PSS thin films; these could be influenced bya variety of factors during the drying process including the fractionof solid content, particle size, proportion of PSS and the solution vis-cosity which could lead to different morphological and electricalproperties [7]. The photovoltaic (PV) device performance is mainlydefined by the PCE which depends on several parameters such aslight harvesting properties of the active layer, generation of excitons,exciton diffusion and separation, transportation and collection bythe cell electrodes [8]. However, the modification of both contactshas been demonstrated to play significant role in the enhancementof charge collection (electrons and holes), which could largelyimprove the device PCE [9]. To date, several treatments were carriedout to modify the properties of the PEDOT:PSS layer, which includedthe use of different organic solvents. Among those solvents are etha-nol, dimethylsulfoxide, acetonitrile, and tetra-hydrofuran whichhave resulted in a considerable improvement of PEDOT:PSS layerconductivity [10]. Through an alternative approach, Ouyang hasstudied the effect of various salts on PEDOT:PSS conductivity andit was shown that the conductivity enhanced by employing metalsalts as dopants within the aqueous PEDOT:PSS solution. This wasascribed to the binding of the metal ions to the PSS anions ofPEDOT:PSS layer [11]. Furthermore, PEDOT:PSS treatment with hex-afluoroacetone (HFA) were shown to reduce the sheet resistancefrom 7 MX/h to 40 kX/h [12]. For the work presented here, the

74 B. Kadem et al. / Organic Electronics 24 (2015) 73–79

effect of metal salts, both in powder form and as aqueous solutionson the PEDOT:PSS films’ properties such as conductivity, transmit-tance and morphology were investigated. The modified PEDOT:PSSlayers were adopted as anode buffer layer to study their effects onP3HT:PCBM OSCs performance and thus on device PCE. To the bestof the authors’ knowledge this is the first attempt to applysalt-modified PEDOT:PSS layers to P3HT:PCBM-based OSCs.

2. Experimental methods

2.1. Preparation of PEDOT:PSS solution with metal salts

PEDOT:PSS (conductive grade-483095 Aldrich) were mixedwith different metal salts (Sigma Aldrich); these are CuCl2

(99.99%), CdCl2 (99.99%), NaCl (99.99%) and LiCl (99.99%) used aspowders and as aqueous solutions. 1 mg of the metal salts wasadded separately as powders to 1 ml of PEDOT:PSS solution; thefinal solutions were sonicated for 30 min to ensure good dispersionand complete solubility of the salts. In a separate method, aqueoussolutions of each metal salt with the concentration of 1 mg ml�1

were prepared in deionized (DI) water. The solutions were soni-cated for 30 min before mixing with PEDOT:PSS solution with avolume ratio of 0.1:1 ml. Further sonication for 30 min was carriedout for efficient dispersion after mixing the PEDOT:PSS with theaqueous salts solutions. Additional samples of LiCl and CdCl2 inDI water were used in the concentrations of 5 and 10 mg ml�1 inorder to examine the role of dopant concentration on the electricaland optical properties of PEDOT:PSS.

2.2. Organic solar cells preparations

ITO-coated glass slides (sigma Aldrich; sheet resistance8–10 X/h) were cleaned using DI water, acetone and 2-propanolfor 10 min each in ultrasonic bath, respectively, and then blowndry in N2 gas. PEDOT:PSS (pure and doped) were spin coated onITO slides at a spin speed of 2000 rpm for 30 s and annealed on ahot plate at 150 �C for 10 min in ambient air. P3HT:PCBM (in 1:1ratio) active layer (14 mg ml�1) were dissolved in co-solvents ofchloroform and chlorobenzene (CB:CF) with the ratio 1:1. Theactive layers were spin coated inside a nitrogen-filled glove boxfollowed by annealing inside the glove box at 120 �C for 10 min.Film thickness of 95 nm was obtained as determined by spectro-scopic ellipsometry. A top contact of aluminum (Al) was evapo-rated with a thickness of about 100 nm, at the deposition rate of0.1–0.2 nm/sec as was monitored by a quartz crystal thicknessmonitor. All the devices were subjected to further heat treatmentinside the glove box at 120 �C for 10 min and left to cool downfor 30 min before measurements were carried out.

2.3. Measurements

A UV–visible spectrophotometer (Varian 50-scan UV–visible) inthe range of 190–1100 nm was used to study the transmittancespectra of PEDOT:PSS layers which were spin coated onto cleanedglass substrates. The active layer and PEDOT:PSS layer thicknesseswere determined using M2000 (J.A. Woollam Co., Inc.) spectroscopicellipsometry operating in the wavelength range 370–1000 nm.Morphology of PEDOT:PSS layers were examined using aNanoscope IIIa multimode atomic force microscope (AFM). DC elec-trical characteristics in the form of current density–voltage (J–V)dependence and electrical conductivity of PEDOT:PSS layers as wellas completed OSC devices were studied using a 4200 Keithley semi-conductor characterization system and the photo current was gen-erated under AM 1.5 solar simulator source of 100 mW cm�2. Thefill factor (FF) and the overall light to-electrical energy conversion

efficiency (PCE) of the solar cell were calculated according to the fol-lowing equations [13]:

PCE ð%Þ ¼ JmaxVmax

Pinð1Þ

FF ¼ JmaxVmax

JscVocð2Þ

where Jsc is the short-circuit current density (mA cm�2), Voc is theopen-circuit voltage (V), Pin is the incident light power and Jmax

(mA cm�2) and Vmax (V) are the current density and voltage at thepoint of maximum output power in the J–V curves, respectively.Furthermore, the electrical conductivity was determined usinginterdigitated platinum electrodes (IDEs) purchased fromDropSens (Spain). The IDE can be used to measure the surface con-ductivity (r) of the samples from the following relationship [14]:

r ¼ ½I=V �½n=WtL� ð3Þ

where t is the thickness of the film, W is the distance between thefingers (6.67 mm), n is the number of fingers (500), and L is the dis-tance between electrodes (5 lm).

3. Results

3.1. Transmittance spectra

Fig. 1 shows transmittance spectra of metal salts treatedPEDOT:PSS films as well as those of films prepared from as pur-chased PEDOT:PSS. Modified PEDOT:PSS films doped with metalsalts have exhibited higher transmittance as compared to purePEDOT:PSS films. Conversely, aqueous solution-treatedPEDOT:PSS films showed a higher transmittance than those treatedwith metal salt powders. In both cases, metal chloride treatedPEDOT:PSS layers are found to be useful for solar cells specificallyfor materials employed as a windows layer. The maximum trans-mittance of pure PEDOT:PSS film was in the range 88–91% withinthe wavelength range (400–650 nm), while transmittance of pow-der salts treated-PEDOT:PSS films has increased up to 93%.Aqueous solution (1 mg ml�1) of metal salts treated PEDOT:PSSfilms have however exhibited further increase in transmittance,reaching 94%. The PEDOT:PSS films treated with LiCl and CdCl2

were shown to demonstrate maximum transmittance in both casesof treatment, in powder and in aqueous solutions. Moreover, anincrease in the aqueous solution concentration of LiCl and CdCl2

has led to further improvement in transmittance.

3.2. Surface morphology

Fig. 2 shows AFM images obtained for treated and untreatedPEDOT:PSS films. The pure film has exhibited smooth surface withrms = 1.25 nm while rougher morphologies were markedly clear inthe metal salts-treated films; LiCl (10 mg ml�1 aqueous solution)doped PEDOT:PSS has exhibited more rough surface withrms = 3.38 nm. Generally speaking the brighter regions observedin the AFM images could be associated with the PEDOT whereasthe darker regions are related to the less conductive PSS regions[15]. This increase in the surface roughness may be ascribed tothe increase in grain size after metal chloride treatment. It hasrecently been shown that surface roughness and grain size of inor-ganic CdS layer has increased as a result of CdCl2 treatment [16]. Atthe same time, the group VII element (Cl) has also acted as a p-typedopant when it produces complexes with un-known native defectswithin the inorganic layer [16]. It can be assumed here that Cl�

ions play a similar role when organic layers of PEDOT:PSS are trea-ted with metal chlorides. The PSS could be removed by the metal

Fig. 1. The transmittance spectra of PEDOT:PSS films: (a) on treatment with powder salts of NaCl, CuCl2, CdCl2 and LiCl; (b) on treatment with aqueous solution of NaCl, CuCl2,CdCl2 and LiCl in DI water with the concentration 1 mg ml�1; (c) on treatment with aqueous solutions of LiCl and CdCl2 with the concentrations 5 and 10 mg ml�1.

Fig. 2. AFM images for pure (a) and LiCl aqueous solution treated PEDOT:PSS layers (b, c and d) of concentrations 1, 5 and 10 mg ml�1 respectively.

Fig. 3. (a) The cross-section of the P3HT:PCBM solar cell after modifying PEDOT:PSS layer and the scattering of the incident light within the active layer due to PEDOT:PSSsurface roughness. (b) The PEDOT:PSS after chlorine salts treatment and the dissociation of the metal ions and the Cl� within the PEDOT:PSS.

B. Kadem et al. / Organic Electronics 24 (2015) 73–79 75

ions (cations) leaving the Cl (anion) to act as a dopant impurity toimprove the PEDOT conductivity and enhance its surface

roughness as shown in Fig. 3a and b. It therefore appears that thistreatment improves the light harvesting by the active layer as well

Pure NaCl CuCl2 LiCl CdCl20.32

0.34

0.36

0.38

0.40

0.42

0.44

0.46 (a)

Co

nd

uct

ivity

(S

.cm

-1)

Metal salts

solution powder

0 5 10 15 200.41

0.42

0.43

0.44

0.45

0.46

0.47

0.48

0.49 (b)

Co

nd

ucti

vity

(S

.cm

-1)

Salt concentration (gm.ml-1)

LiClCdCl

2

Fig. 4. (a) The conductivity of PEDOT:PSS variation with salts treatment. (b) Dependence of conductivity on LiCl and CdCl2 solution concentration.

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60

70

80

90

J Dar

k (m

A.c

m-2)

Voltage (V)

Pure CdCl

2(1mg/ml)

CdCl2(5mg/ml)

CdCl2(10mg/ml)

LiCl(1mg/ml) LiCl(5mg/ml) LiCl(10mg/ml)

Fig. 5. Dark J–V characteristics for P3HT:PCBM solar cell based on PEDOT:PSStreatment with LiCl and CdCl2 aqueous solutions of different concentrations (1, 5,10 mg ml�1).

0.000 0.001 0.002 0.0030.0

0.2

0.4

0.6

0.8

1.0

1.2

Pure PEDOT:PSS I(

dV

/dI)

I (A)

Fig. 6. I(dV/dI) vs. I characteristics of P3HT:PCBM solar cell based on purePEDOT:PSS.

Table 1OSC parameters of devices having PEDOT:PSS layer treated with powder salts (NaCl,CuCl2, CdCl2 and LiCl).

76 B. Kadem et al. / Organic Electronics 24 (2015) 73–79

as decreasing the contact resistance of the interfacial layer, henceincreasing the short circuit current density (Jsc).

POWDER Pure CuCl2 NaCl CdCl2 LiCl

Voc (V) 0.62 0.62 0.6 0.62 0.59Jsc (mA cm�2) 11.9 13.37 12.92 14.8 16.1FF% 53 59 59 59 59PCE% 3.92 5 4.6 5.4 5.6

Table 2Photovoltaic parameters of devices with PEDOT:PSS layer treated with aqueoussolutions of NaCl, CuCl2, CdCl2 and LiCl with the concentration of 1 mg/ml.

Solution Pure CuCl2 Nacl CdCl2 LiCl

Voc (V) 0.62 0.63 0.61 0.62 0.6Jsc (mA cm�2) 11.9 15.4 13 16.77 17.37FF% 53 57 59 59 58PCE% 3.92 5.52 4.7 6.1 6

3.3. Electrical conductivity

Fig. 4 illustrates the variation in electrical conductivity of thePEDOT:PSS layer as a result of treatment with different metal chlo-rides (Fig. 4a), as well as the effect of concentration of the chlorideaqueous solutions (Fig. 4b). A maximum conductivity of0.41 S cm�1 was obtained for the LiCl treated film when the metalsalt was added in powder form while a conductivity of 0.43 S cm�1

was obtained using aqueous solution. Further treatment was car-ried out to examine the effect of CdCl2 and LiCl concentrationson PEDOT:PSS conductivity, using concentrations of 5 and10 mg ml�1 metal salt solutions. Fig. 4b shows the increase infilms’ conductivity with increased concentration of the aqueoussolution added to the PEDOT:PSS. The LiCl treated film(10 mg ml�1) shows higher conductivity of 0.485 S cm�1 whilethe CdCl2 treated (10 mg ml�1) films exhibited slightly lower con-ductivity of 0.48 S cm�1. Once again, this could be ascribed to theremoval of PSS chains from the PEDPT:PSS films by the cations,as illustrated in Fig. 3b. This could be further understood as theresult of dissociation of the metal salts where the higher dissocia-tion contributes higher ions concentration which significantlyreduces the coulombic attraction between the PEDOT and PSS

chains, thus more loss of PSS chains from the PEDOT:PSS film viathe salt treatment [17]. It has been established that conductionin PEDPOT:PSS films is due to charge transport across the PEDOTchains via hopping, and that it remains unchanged after impuritiesaddition [11].

Table 3Photovoltaic parameters of devices with PEDOT:PSS layer treated with different concentrations of aqueous solution of CdCl2 and LiC.

Solution Pure CdCl2 (1 mg/ml) CdCl2 (5 mg/ml) CdCl2 (10 mg/ml) LiCl (1 mg/ml) LiCl (5 mg/ml) LiCl (10 mg/ml)

Voc (V) 0.62 0.62 0.62 0.62 0.6 0.62 0.62Jsc (mA cm�2) 11.9 16.77 17.23 17.8 17.37 17.3 17.97FF% 53 59 62 61 58 63 61PCE% 3.92 6.07 6.65 6.72 6 6.77 6.82Rs (X) 111.2 86.86 62.5 53.2 59.5 56.2 52.3

B. Kadem et al. / Organic Electronics 24 (2015) 73–79 77

3.4. Series resistance determination

The series resistance (Rs) is an effective parameter which influ-ences the solar cell behavior. Fig. 5 shows the dark J–V curves forthe pure PEDOT:PSS based device and devices with CdCl2 andLiCl treated PEDOT:PSS based devices with different concentra-tions. Shen et al. have argued that dark I–V characteristics is an effi-cient method to determine the Rs using the following equation[18]:

I ¼ IL � Io eqðv�IRsÞ=nkT � 1� �

� V � IRs

Rshð4Þ

where IL is the light current, Io is the saturation current, nis the ideality factor, kT/q is the thermal energy, Rs is the seriesresistance, and Rsh is the shunt resistance. Typically, under darkconditions the photocurrent is zero and therefore it could bepresumed that Rsh is large enough to ignore the last term inEq. (4). Therefore, the dark I–V characteristics can be described asfollows:

I ¼ Io eqðV�IRsÞ=nkT � 1� �

ð5Þ

Re-arranging Eq. (5) the following relation is obtained:

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-20

-15

-10

-5

0

5

10 (a)

powder salts

J SC

(mA

.cm

-2)

Voltage (V)

PureCuCl

2

NaClCdCl

2

LiCl

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-20

-15

-10

-5

0

5

10

15 (c)

Vmax

Pmax

Jmax

J SC (

mA

.cm

-2)

Voltage (V)

Pure

CdCl2(1mg.ml-1)

CdCl2(5mg.ml-1)

CdCl2(10mg.ml

-1)

LiCl(1mg.ml-1)

LiCl(5mg.ml-1)

LiCl(10mg.ml-1)

Fig. 7. J–V characteristics for P3HT:PCBM solar cell based on PEDOT:PSS treatment withwith different CdCl2 and LiCl concentrations (c) and output power plotted as a functioPEDOT:PSS based devices (d).

V ¼ nkTq

lnIIoþ 1

� �þ IRs ð6Þ

Therefore, at higher applied voltage when I� Io, Eq. (6)becomes:

IdVdI¼ IRs þ

nkTq

ð7Þ

Using Eq. (7), Rs could be extracted from the slope at the highcurrent region of I(dV=dIÞ vs. I plot as shown in Fig. 6 for the purePEDO:PSS based device as an example. The lowest Rs was exhibitedby OSC devices with LiCl treated PEDOT:PSS with the value ofRs = 52.3 X compared to devices employing pure PEDOT:PSS layerwhich revealed Rs = 111.2 X (results are summarized in Table 3).This improvement in Rs could be a key factor among otherstowards achieving efficient electrode properties and therefore bet-ter device performance.

3.5. Photovoltaic measurements

Using measured J(V) characteristics under illumination, the PVproperties of PEDOT:PSS/P3HT:PCBM/Al OSCs were determinedwith PEDOT:PSS layers treated with different metal chlorides.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

-20

-15

-10

-5

0

5

10 (b)

Solution salts

J SC (m

A.c

m-2)

Voltage (V)

PureCuCl

2

NaClCdCl

2

LiCl

0.0 0.1 0.2 0.3 0.4 0.5 0.60.00

0.05

0.10

0.15

0.20

0.25

0.30 (Vmax

, Pmax

)(d)

Pow

er (m

W)

Applied voltage (V)

pureLiCl (10mg/ml)

several salts as powder (a), as aqueous solution (b), subjected to further treatmentn of voltage for pure PEDOT:PSS and LiCl (10 mg ml�1) aqueous solution-treated

78 B. Kadem et al. / Organic Electronics 24 (2015) 73–79

Fig. 7 shows J–V curves of the investigated OSCs with different saltstreated-PEDOT:PSS. Devices with untreated PEDOT:PSS layerhave exhibited lower PV performance with Voc = 0.62 V,Jsc = 11.9 mA cm�1, FF = 53% and PCE of 3.92%. These are in line withvalues frequently reported in the literature [19]. However, deviceswith powder metal salt treated PEDOT:PSS layers have demon-strated significant improvement in all PV parameters as revealedin Fig. 7a, and the results are summarized in Table 1. The highestPCE of 5.6% and short circuit current density Jsc = 16.1 mA cm�1

were achieved when LiCl2 powder was added to PEDOT:PSS; FFand Voc however, remained unchanged for all metal salts. Whenthe same metal salt was added to PEDOT:PSS in aqueous form asin Fig. 7b, the device PCE and short circuit current density havereached values of about 6% and 17.4 mA cm�1, respectively, in thecase of LiCl added with the concentration of 1 mg ml�1. Once again,Voc and FF remained more or less unchanged for all metal salts stud-ied in this work. Table 2 provides a summary of the electrical prop-erties of ITO/PEDOT:PSS/P3HT:PCBM/Al OSCs when metal chlorideswere used in aqueous form. The substantial increase in Jsc could beascribed to the improved electrical conductivity of PEDOT:PSS lay-ers as a result of metal salt treatment while enhanced PCE couldbe associated with the increased films’ transmittance and surfaceroughness. Values of Voc remained unchanged since its value ismainly determined by the difference between the donor’s higheroccupied molecular orbital (HOMOd) and the lower unoccupiedmolecular orbital of the acceptor (LUMOa) [20]. Furthermore, themetal salt treatment of PEDOT:PSS layers seems to solve the wettingproblem between the active layer, which is hydrophobic and thehydrophilic PEDOT:PSS [21]. Such film processing method wouldbe more favored for the fabrication of inverted OSC structure.Electrical properties of the OSCs were further improved when theconcentration of CdCl2 and LiCl in aqueous solution increased to 5and 10 mg ml�1 as demonstrated in Fig. 7c. Moreover, the estima-tion of Jmax and Vmax values is illustrated in Fig. 7c. Fig. 7d showsthe variation of output power as a function of applied voltage.

A summary of the electrical parameters of theITO/PEDOT:PSS/P3HT:PCBM/Al OSCs treated with these solutionsis given in Table 3. PCE as high as 6.82% associated with FF = 61%,Jsc � 18 mA cm�2 and Voc = 0.62 V have been recorded for LiCl trea-ted devices using a higher concentration of 10 mg ml�1. Treatmentof PEDOT:PSS with metal salts of concentrations higher than10 mg ml�1 has not shown any further improvement in device per-formance. The above reported efficiency of 6.82% is found to berepeatable and on comparing this value with PCE values obtainedfor other metal salts treatment (see Table 3 and data summarizedin Fig. 7) it can be confirmed that these results are very reliable.The optimization of electrode treatment [22–25] as well as activelayer thickness [26,27], are found to be key processing conditionsto achieve good device performance. In a recent review [11] con-ductivity enhancement of PEDOT:PSS thin films for use in polymerlight-emitting diodes and polymer solar cells has been discussedand conductivity increase of up to 1000 S cm�1 was reporteddepending on the type of additives. Recently modification of elec-tron transport layer in P3HT:PCBM-based OSC has revealed 3 timesimprovement in the cells’ PCE as compared to untreated electrodes[28]. In the present work improvement of the hole transport layervia doping of PEDOT:PSS with metal salts, preferably in solutionform, has been shown to play crucial role in improving OSCs per-formance as well as device stability.

4. Conclusion

PEDOT:PSS layers doped with different metal salts (LiCl, NaCl,CdCl2 and CuCl2) were employed as hole transport layers inP3HT:PCBM based OSCs. The effects of metal salts added as powders

or as solutions on the electrical, optical as well as morphologicalproperties of PEDOT:PSS layers were investigated. PEDOT:PSS filmsdoped with LiCl salt in solution form with the concentration10 mg ml�1 have exhibited the highest electrical conductivity aswell as highest transmittance intensities. The metal salt dopinghas resulted in a more rough surface morphology of PEDOT:PSS lay-ers compared to pristine PEDOT:PSS films. Furthermore, in dopedfilms the series resistance has exhibited much lower values of about52.3 O. These improvements in PEDOT:PSS layer structural, opticaland electrical properties are thought to be the main reasons forthe enhancement of P3HT:PCBM based solar cells performance, withPCE as high as 6.82%, FF = 61% and Jsc � 18 mA cm�2 in the case ofPEDOT:PSS doped with LiCl (10 mg ml�1 aqueous solution) device.

Acknowledgements

Burak Kadem wishes to acknowledge the PhD scholarshipawarded by the Ministry of Higher Education and ScientificResearch-Babylon University in Iraq. The support and fruitful dis-cussion with Dr. Heming Wang, Yaqub Rahaq and Vikas Kumarare gratefully acknowledged.

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