Effect of boric acid doped PEDOT:PSS layer on the performance of ...

Synthetic Metals 212 (2016) 12–18

Effect of boric acid doped PEDOT:PSS layer on the performance of P3HT:PCBM based organic solar cells

Özlem Yagcia, Serco Serkis Yesilkayaa, Süreyya Aydin Yüksela, Fatih Ongüla,Nurhan Mehmet Varalb, Mahmut Kusb, Serap Günesa, Orhan Icellia,*aYıldız Technical University, Department of Physics, Istanbul, TurkeybAdvanced Technology Research & Application Center, Konya, Turkey

A R T I C L E I N F O

Article history:Received 12 August 2015Received in revised form 7 November 2015Accepted 10 November 2015Available online xxx

Keywords:PEDOT:PSSBulk heterojunction solar cellsBoric acid

A B S T R A C T

In this study, we fabricated PEDOT:PSS film with different concentrations 0–5 mg/ml of boric acid(H3BO3) doped PEDOT:PSS film as hole collector layer. Undoped and boric acid doped PEDOT:PSS films areprepared with spin coating technique and characterized by XRD, UV, AFM, FTIR and electricalconductivity measurements. We fabricated polymer solar cells in the form of ITO/PEDOT:PSS:H3BO3/P3HT:PCBM/Al. Results show that the open-circuit voltage (Voc) and the fill factor (FF) increased byintroducing H3BO3 as dopant into the PEDOT:PSS layer due to an increase in the work function of PEDOT:PSS layer and improvement of surface roughness, respectively. The solar cells without H3BO3 showed apower conversion efficiency PCE of 1.79% whereas the cells with H3BO3 concentration of 1.25 mg/ml inPEDOT:PSS, showed a higher performance resulting in a PCE of 2.14% under AM 1.5G illumination.

ã 2015 Elsevier B.V. All rights reserved.

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1. Introduction

In recent years, there is an increasing level of interest inorganic-based solar cells as an alternative energy source. Organicsolar cell/photovoltaic (OPV) devices fabricated from the blends ofpoly(3-hexylthiopene) (P3HT) and [6,6]-phenyl C61-butyric acidmethyl ester (PCBM) are the most widely studied bulk hetero-junction systems because of their relatively good photovoltaic (PV)properties. Although the P3HT:PCBM devices exhibit excellent PVproperties compared to other bulk heterojunction OPV devices,their power conversion efficiency is still too low compared to thatof the conventional silicon PV cells [1]. Annealing by a laser [2],light harvesting by a use of a low band gap molecule [3],incorporation of nanocrystals [4] or the use of crystallizablesolvent [5] have been foreseen as methods to improve the PVefficiency of P3HT:PCBM based organic solar cells. The conven-tional polymer:fullerene solar cells include poly(3,4-ethylenediox-ythiophene): poly(styrenesulfonate) (PEDOT:PSS) as hole collectorbuffer layer because of its high work function, transparency in thevisible range, high conductivity, mechanical flexibility, and easyprocessing ability which is inserted between transparent elec-trode, typically ITO and light-absorbing organic layer [6–11].PEDOT:PSS is thought to be beneficial for device performance in a

* Corresponding author. Fax: +90 212 383 41 06.E-mail address: [email protected] (O. Icelli).

http://dx.doi.org/10.1016/j.synthmet.2015.11.0100379-6779/ã 2015 Elsevier B.V. All rights reserved.

number of ways. It not only decreases the surface roughness ofindium-tin oxide (ITO) transparent electrodes, reduces the shortcuts in the device, but also improves the selectivity of the anodedue to the higher work function relative to ITO, enhancing electronblocking and therefore maintaining higher open-circuit voltage.There are many approaches which demonstrated that propertiessuch as the electrical conductivity and the work function of PEDOT:PSS can be changed by the addition of additives (e.g, WOx, MoOx,V2O5, NiOx, Fe2O3 NPs, ethylene glycol (EG) and multi-walledcarbon nanotubes (MWCNT)) which in turn affects the deviceperformance significantly [7–13]. In these studies, the efficiency ofthe organic solar cells increased with the addition ethylen glycol(10%) [6], NiOx (5:1 ratio) [7], Fe2O3 NPs (0.7 wt%) [14], andMWCNT (4%) [15] in PEDOT:PSS which used as hole collector layer,29%,10%, 32%, 23%, respectively, also observed that increases in FFs.However, these additives are more expensive and requires difficultand long production processes [16]. The use of boric acid is analternative way for boron doping. Boric acid is one of the mostimportant boron compounds and has strategic and industrialimportance. It is a low-cost material and can be easily fabricated[17]. It is mostly used in industrial applications, such as glass,ceramics, textiles, detergents, and nuclear power, as well as in theagricultural, medical, pharmaceutical and electronics relatedsectors [18]. However, boric acid has not been widely used insolar cell studies to best of our knowledge. In one of the studies onboric acid, Ruan et al., examined the photoelectrochemicalproperties of highly ordered titanium dioxide nanotube-array

Fig. 1. Schematic of the device architecture with PEDOT:PSS:H3BO3 buffer layer.

0

400

800

1200

1600

2000

10 15 20 25 30 35 40 45 50 55 60

Inte

nsity

(a.

u.)

2Theta (o)

(a)

(b)

(c)

(d)

Fig. 2. XRD diffraction pattern of (a) PEDOT:PSS, (b) 1.25 mg/ml boric acid doped

O. Yagci et al. / Synthetic Metals 212 (2016) 12–18 13

photoanodes, fabricated by anodization of titanium in a nitric acid/hydrofluoric acid electrolyte, with and without the addition ofboric acid. They reported the presence of boric acid in theelectrolyte resulted in a TiO2 nanotube-array possessing signifi-cantly greater photoelectrochemical properties, under both UV–visspectrum illumination [17]. Rahman et al., investigated the effectof boric acid composition on the morphology, thickness, elementalcomposition, optical absorption, structure, photoluminescence ofZnO nanotubes and the performance of the DSSC utilizing the ZnOsamples. They reported that the diameter and thickness of ZnOnanotubes decreased with the increasing composition of boricacid. The DSSC utilizing ZnO nanotubes prepared at 2 wt% boricacid demonstrated the highest Jsc and h of 2.67 mA/cm2 and 0.29%,respectively [19]. The use of boric acid as a dopant in titaniumdioxide nanotubes based dye sensitized solar cells have beeninvestigated by Subramanian et al., They introduced boron into theinterstitial sites of TiO2 lattice and contributed to the shift ofconduction band. The boron-doped TiO2 nanotube arrays showedan enhanced performance compare to those of undoped TiO2

nanotubes [20].In this work, we introduced a new boron doped PEDOT:PSS

layer to improve the PV performance of P3HT:PCBM based organicsolar cells. We have shown that boron can be used as a dopant tothe PEDOT:PSS films for organic solar cells as hole collector layer. Ithas been shown that the devices employing boron showed a betterFF and efficiency.

2. Experimental

As substrates, 1.5 cm � 1.5 cm ITO covered glass sheets with asheet resistance <15 V cm, from KINTEC company, were used assubstrates. The ITO was patterned by etching with an acid mixtureof HCl:HNO3:H2O (4.5 ml:1 ml:3 ml) for 30 min. The sheet resis-tance of the ITO substrate was 13 V cm. The part of the substratewhich forms the contact is covered with a scotch tape to preventetching. The tape was removed after etching and the substrate wasthen cleaned using acetone and isopropanol in an ultrasonic bath.

The blends of P3HT: PCBM were prepared by dissolving 12 mg ofP3HT and 6.5 mg PCBM in 1 ml chlorobenzene. For the preparationof the PEDOT:PSS:H3BO3 buffer layer, various ratios of H3BO3 (0–5 mg/ml) was added in PEDOT:PSS solution. All the preparedsolutions were stirred in dark for 24 h.

For the preparation of the solar cells, PEDOT:PSS:H3BO3 wasspin coated on ITO substrates at 2000 rpm for 60 s and annealed at150 �C for 4 min under ambient conditions. The film thickness wasdetermined using Veeco Dektak profilometer. The film thicknessdid not change very much with boric acid doping. The active layerof P3HT:PCBM was spin cast onto PEDOT:PSS:H3BO3 film at800 rpm for 60 s in glove box N2 atmosphere. Finally aluminum (Al)contact metal was deposited by thermal evaporation on activelayer about 100 nm. The final device structure was in the form ofITO/PEDOT:PSS:H3BO3/P3HT:PCBM/Al (see Fig. 1). A referencedevice was fabricated without H3BO3 addition to check whetherH3BO3 had an effect on the PV performance of P3HT:PCBM based

Table 1Photovoltaic parameters of organic solar cell prepared with different concentrations of

Boric acid concentration in PEDOT:PSS (mg/ml) Film thickness (nm) V

0 56.85 00.625 56.33 01.25 56.08 02.50 57.19 03.75 55.67 05.00 56.23 0

devices. Reference device structure was in the form of ITO/PEDOT:PSS/P3HT:PCBM/Al.

The current–voltage (I–V) characteristics were measured with aKeithley 2400 source meter under simulated 100 mW/cm2 (AM.1.5G) irradiation. Optical characterizations of P3HT:PCBM activelayer, undoped and boric acid doped PEDOT:PSS films weredetermined in the wavelength range of 300–1100 nm usingPerkinElmer Lambda2 UV–vis spectrophotometer. AFM measure-ments were performed using ezAFM Atomic Force Microscopesystem for characterization of the film morphology. X-raydiffraction studies were carried out using the PanalyticalDiffractometer with CuKa radiation of 1.5418 Å XRD system. Theresistivity of undoped and boric acid doped PEDOT:PSS filmscoated on glass were measured by two probe method forperformed to compare the electrical properties of the undopedand doped films. Electrical characterization of the resistance of thefilm were obtained using Keithley 2400 sourcemeter. For fouriertransfer infrared spectrum (FTIR) of undoped and boric acid dopedPEDOT:PSS solution were recorded by a PerkinElmer spectrometer-ATR mode.

boric acid in PEDOT:PSS buffer layer.

oc (V) Jsc (mA/cm2) FF% PCE% Rs (kV) Rsh (kV)

.535 8.75 38.3 1.79 0.486 3.017

.525 8.00 36.5 1.53 0.680 2.981

.602 7.57 46.9 2.14 0.625 11.47

.612 6.37 45.8 1.78 0.600 9.153

.620 7.00 42.2 1.83 0.600 8.000

.640 5.10 39.0 1.27 1.400 9.803

PEDOT:PSS, (c) 5.00 mg/ml boric acid doped PEDOT:PSS, (d) ITO substrate.

14 O. Yagci et al. / Synthetic Metals 212 (2016) 12–18

3. Results and discussion

Fig. 2 shows the XRD pattern of undoped, boric acid dopedPEDOT:PSS films and bare ITO substrate. A weak and broad peakwas observed between 2u = 25–26� which belongs to (0 2 0) planeof PEDOT:PSS polymer [15,21,22].

The broadness of the peaks reflects the semi-crystalline natureof PEDOT:PSS. This 2u value corresponds to the d-spacing ofapproximately 3.56 Å, which can be attributed to the inter-chaindistance of PEDOT [23]. When closely examined, the broad peaks ofthe undoped and doped PEDOT:PSS are slightly shifted withrespect to one another. The peak positions of PEDOT:PSS, PEDOT:PSS (1.25 mg/ml) and PEDOT:PSS (5 mg/ml) have been extractedusing a Gaussian peak fitting after a linear background subtraction.This yields the 2u values of the peaks as 25.04, 25.28, 25.47,corresponding to the d-spacing values of 3.56, 3.53 and 3.50 Å,respectively. The lowest D-spacing is obtained from the films ofPEDOT:PSS (5 mg/ml), which indicates the closest packing of thePEDOT chains.

As also can be seen from Fig. 2, the ITO peak at 2u = 30�

disappears for the boric acid doped PEDOT:PSS films that isattributed to the homogeneous film formation on the ITO substratewhich is in accordance with the AFM studies.

Fig. 3. The AFM images of PEDOT:PSS, PEDOT:PSS (1.25 mg/ml boric a

Atomic force microscopy images of undoped and boric aciddoped PEDOT:PSS films are shown in Fig. 3. The peak to valleyheight values were 13.40, 11.61 and 8.68 for undoped PEDOT:PSS,PEDOT:PSS (1.25 mg/ml boric acid), PEDOT:PSS (5 mg/ml boricacid)/PEDOT:PSS films, respectively whereas the RMS values were1.37 nm, 1.36 nm, 1.15 nm, respectively. By the help of the AFMimages, it has been observed that the additional H3BO3 into thePEDOT:PSS layer led to a smoother surface and therefore a bettermorphology. A better film forming property led to an improved thefill factor of the solar cells.

The results of resistivity of undoped and doped PEDOT:PSS filmsare shown in Fig. 4. They demonstrate the reduction of sheetresistance by 5–20% compared to undoped PEDOT:PSS film withincreased boric acid concentration up to 1.75 mg/ml. The amountof doping concentration is increased above 1.75 mg/ml increasingresistance with the B��O peak observed in FTIR spectra shown inFig. 5.

Fig. 5 shows the FTIR spectra of undoped and boric acid dopedPEDOT:PSS at room temperature. On the PEDOT:PSS films FTIRspectra can clearly see one peak at about 1400 cm�1 caused byasymmetric B��O stretching [24] with boric acid added after1.25 mg/ml boric acid doping concentration. We observed thatdecreased resistivity with boric acid doping up to 1.75 mg/mlconcentration than increased resistance with increasing doping

cid), PEDOT:PSS (1.25 mg/ml boric acid)/PEDOT:PSS on ITO glass.

Fig. 4. Variation of resistivity of undoped PEDOT:PSS and PEDOT:PSS films withdifferent concentrations of boric acid.

0

10

20

30

40

50

60

70

80

90

100

600 1100 1600 2100 2600 3100 3600

Wave Number (cm-1)

T%

83.9

84.4

84.9

1350 140 0 145 0

Undoped0.625 mg/ml Boric acid doped1.25 mg/ml Boric acid doped2.50 mg/ml Boric acid doped3.75 mg/ml Boric acid doped5.00 mg/ml Boric acid doped

Fig. 5. FTIR spectra of undoped PEDOT:PSS and PEDOT:PSS with differentconcentrations of boric acid.

Fig. 6. I–V curves of the investigated devices in linear scale.

Fig. 7. Absorption of PEDOT:PSS/P3HT:PCBM with (a) undoped, (b) 1.25 mg/ml

O. Yagci et al. / Synthetic Metals 212 (2016) 12–18 15

amount with the B��O stretching peak has appeared as shown inFTIR spectra.

Fig. 6 shows the current–voltage (I–V) characteristics of solarcells with different H3BO3 concentration (0–5 mg/ml) in PEDOT:PSS layer measured in dark and under illumination. A referencesolar cell in the form of ITO/PEDOT:PSS/P3HT:PCBM/Al wasfabricated without any H3BO3 to better analyze the effect of

additional H3BO3 on the performance of the organic solar cells. Thecalculated cell parameters in order to determine the effect of boricacid on the characteristics of photovoltaic cells are given in Table 1.

The reference device where no H3BO3 is employed in the deviceexhibited a short-circuit current density (Jsc) of 8.75 mA/cm2 andan open-circuit voltage (Voc) of 535 mV. A fill factor of 0.39 wascalculated which led to a power conversion efficiency of 1.79%. Forthe solar cells employing 1.25 mg/ml of H3BO3 in PEDOT:PSS layerexhibited a Jsc of 7.57 mA/cm2 and a Voc of 602 mV. A fill factor of0.46 was calculated which led to a PCE of 2.14%. It has beenobserved that at a specific concentration of H3BO3 (1.25 mg/ml) thesolar cells showed the best PV performance. From photovoltaicparameters point of view, the main effect of the additional H3BO3

was to increase the Voc and the fill factor. The increase in the Voc isexplained in terms of the work function difference (Fig. 11)whereas the increase in the fill factor is attributed to the bettermorphology (Fig. 3) achieved by an additional H3BO3 into thePEDOT:PSS layer.

The power conversion efficiency of an organic solar cell iscalculated using the following formula:

h ¼ IscVocFFPin

ð1Þ

where Isc is the short-circuit current, Voc is the open-circuit voltageand FF is the fill factor. The open-circuit voltage is a sensitivefunction of energy levels of the used materials as well as theirinterfaces [25]. It has been previously demonstrated that interfa-cial effects at the metal/organic semiconductor interface (such asoxide formation) change the work function of the electrodes andinfluence the open-circuit voltage [26,27] which has been alsosupported by KPFM results in this study.

In the ideal, loss free contacts, the short-circuit current, Isc, isdetermined by the product of the photoinduced charge carrierdensity and the charge carrier mobility within the organicsemiconductors:

Isc ¼ nemE ð2Þwhere n is the density of charge carriers, e is the elementarycharge, m is the mobility, and E is the electric field. Assuming the100% efficiency for the photoinduced charge generation in a bulkheterojunction mixture, n is the number of absorbed photons perunit volume. For a given absorption profile of a given material, thebottleneck is the mobility of charge carriers. Mobility is not amaterial parameter but a device parameter. It is sensitive to thenanoscale morphology of the organic semiconductor thin film[28–33]. In a van der Waals crystal, the final nanomorphologydepends on film preparation. Parameters such as solvent type, thesolvent evaporation (crystallization) time, the temperature of the

doped PEDOT:PSS, (c)5 mg/ml doped PEDOT:PSS.

P

16 O. Yagci et al. / Synthetic Metals 212 (2016) 12–18

substrate, and/or the deposition method can change the nano-morphology [34,35].

In this study it has been observed that the blend film of PEDOT:PSS and boric acid improves the interfacial effects between the ITOand PEDOT:PSS as compared to bare PEDOT:PSS films and thereforeimproves the open-circuit voltage of the herein investigateddevices however, the prize paid is the complicated nanomorphol-ogy of the blend that is difficult to optimize and control whichresults in a lower short-circuit current density. Since the efficiencyis directly proportional to the product of Voc and Isc, although the Iscof the devices involving boric acid decreased non-dramatically, theoverall efficiency increased upon additional boric acid. Whatmatters for the performance of an organic solar cell is the overallpower conversion efficiency.

Fig. 8. The external quantum efficiency of organic solar cells with (1) undoped

Fig. 9. The relationship between the photovoltaic param

Also, as it is seen in Fig. 4 the reduction in the resistivity of thePEDOT:PSS films with boric acid doping confirms the improve-mentin serial-shunt resistance of the cells.

An increase of Rsh and a decrease of Rs or both can cause anincrease in FF for photo voltaic devices. The Rsh values for cellswhich contained boric acid in PEDOT:PSS increased from 3 kV to11.47 kV. The increase in the Rsh led to a higher FF and PCE% uponadditional boric acid.

Fig. 7 shows the absorption graph of PEDOT:PSS/P3HT:PCBMlayers with different concentration of H3BO3 in PEDOT:PSS. It hasbeen seen that the additional H3BO3 in PEDOT:PSS leads to a smallred shift in the absorption. It is well known that, the PEDOT:PSSlayer is almost transparent in the visible region of the solarspectrum. The major role of the conducting PEDOT:PSS layer inorganic solar cells is to modify and increase the work function of

EDOT:PSS, (2) 1.25 mg/ml doped PEDOT:PSS, (3) 5 mg/ml doped PEDOT:PSS.

eters of the solar cell and boric acid concentration.

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500 600

Undoped

1.25 mg/ml bori c acid dop ed

Cur

rent

Den

sity

(m

A/c

m2 )

Time (Hou r)

Fig. 10. Life time of OPV cell prepared with undoped and 1.25 mg/ml boric aciddoped.

O. Yagci et al. / Synthetic Metals 212 (2016) 12–18 17

ITO electrode, for facilitating charge transfer between ITO andorganic active layer. In this study, the transparency of the PEDOT:PSS layer in the visible portion of the solar spectrum is diminishedupon addition of H3BO3 which in turn leads to a decrease in theshort-circuit current density.

Fig. 8 shows the external quantum efficiency (EQE) of OSCsmeasurement. We compared the curve of the external quantumefficiency (EQE) between PEDOT:PSS and PEDOT:PSS:H3BO3

(different concentration) device. The spectrum span between300 and 750 nm and is in accordance with the photovoltaic data.

Fig. 9 shows the H3BO3 concentration dependence on theprincipal cell parameters of the device such as the open-circuitvoltage (Voc), the short-circuit current density (Jsc), FF and powerconversion efficiency. It is clear that the doping H3BO3 has aninfluence on all the device parameters. With increasing H3BO3

concentration the device fill factor and open-circuit voltageincreased despite the decreased the short-circuit current density.The maximum PCE and FF were obtained at the 1.25 mg/mlconcentration ratio of H3BO3.

Fig. 10 shows lifetime of OPV cell prepared with undoped and1.25 mg/ml boric acid doped PEDOT:PSS film was investigated in aglove box. At the end of 600 h it has been observed that boric aciddoped solar cells still have a good stability.

Energy level alignment is crucial for the performance of organicsolar cells and depends on the work function of the individual

-2 -1 0 1 2

0

2

4

6

8

10

12

Mag

nitu

te (n

A)

ITO PEDOT undope d 1.25 mg/ml boric a cid doped 5.00 mg/ml boric a cid doped

Surface Poten tial ( V)

(a)

(c)

(b)

(d)

Fig. 11. Magnitude–surface potential curves of (a) ITO (b) undoped PEDOT:PSS film,(c) 1.25 mg/ml boric acid doped PEDOT:PSS, and (d) 5 mg/ml boric acid dopedPEDOT:PSS films.

electrodes [36,37]. In order to understand the influence of H3BO3

blending in PEDOT:PSS on the contact potential difference, we usedscanning Kelvin probe microscopy (SKPM). Local contact potentialdifference (CPD) between doped and undoped PEDOT:PSS filmsformed on ITO substrates and AFM conductive tip (TiN) wasinvestigated. The local contact potential difference between TiN tipand bare-ITO was measured as �0.281 V and it was obtained as�0.0375 V for undoped PEDOT:PSS whereas 0.12 V and 0.124 Vwere obtained for 1.25 mg/ml and 5 mg/ml doped PEDOT:PSS,respectively. The values obtained from KPFM measurementsclarified that the work function of the ITO surface increased as aresult of the doping of PEDOT:PSS with boric acid. One of the majorroles of the PEDOT:PSS layer is to increase the work function of theITO to facilitate the charge transfer from the organic layer to theITO (see Fig. 11). The increase in the work function leads to anincrease in the Voc. The increase in the Voc is attributed to thehigher work function of 1.25 mg/ml boric acid doped PEDOT:PSS.

4. Conclusion

We have investigated the influence of addition of H3BO3 in thePEDOT:PSS solution and its consequence on the performance ofP3HT:PCBM based photovoltaic devices. Results show that thepower conversion efficiency was significantly improved byinserting H3BO3 in the PEDOT:PSS layer using as hole collectorlayer for OPV. An increase in power conversion efficiency and FFapproximately 20% and 23% were obtained between the baredevice and optimized device due to the addition of H3BO3. Themain effect of H3BO3 in the device structure was the increase in Voc

and FF. With the help of the Kelvin probe microscopy studies theincrease in the Voc is attributed to an increase in the work functionof ITO upon additional boric acid. The increase in the fill factor isattributed to the better morphology of the herein investigatedfilms achieved by an additional boric acid in PEDOT:PSS layer.

Acknowledgement

This work was supported by TUBITAK (The Scientific andTechnological Research Council of Turkey) with project no 113F391.

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