Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-<i>co</i>-Pyrrolenevinylene (AVPV) Based Solar Cell Devices

doi:10.4236/msa.2011.212227

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Materials Sciences and Applicatio ns, 2011, 2, 1702-1707

doi:10.4236/msa.2011.212227 Published Online December 2011 (http://www.SciRP.org/journal/msa)

Effect of PEDOT:PSS Layer and ITO Ozonization

in Arylenevinylene-co-Pyrrolenevinylene (AVPV)

Based Solar Cell Devices

Ankur Solanki1,2, S. Sundar Kumar Iyer2,3, Ashish Garg1,2

1Department of Materials Science & Engineering, Indian Institute of Technology, Kanpur, India; 2Samtel Center for Display Tech-

nologies, Indian Institute of Technology, Kanpur, India; 3Departmental of Electrical Engineering, Indian Institute of Technology,

Kanpur, India.

E-mail: [email protected]

Received September 26th, 2011; revised November 1st, 2011; accepted November 17th, 2011.

ABSTRACT

Arylenevinylene-co-pyrrolenevinylene (AVPV) is polymer oligomer system derived from arylbridged bispyrroles which

has been explored for photovoltaic devices. In this paper, we show the dependence of the photovoltaic device parame-

ters on the anode surface treatment in an organic single layer photovoltaic device based on AVPV as an electron donor .

Since the total quantum efficien cy includes the charge collection efficien cy at the electrodes, experiments were carried

out to vary the anode (ITO) characteristics, achieved by using ITO with or without ozonization and with or without

PEDOT:PSS (Polyethylene dioxythiophene:Polystyrene sulphonic acid) layer. Devices fabricated on ITO anode (with-

out ozonization and without PEDOT:PSS) exhibited the maximum current density (Jsc = 1.3 µA·cm–2) as compared to

those devices where ITO was ozonized as well as had a PEDOT:PSS layer (Jsc = 0.1 µA·cm–2) measured under 1 sun

illumination of AM 1.5 through a calibrated solar simulator.

Keywords: AVPV, Organic Poly mers, Ozonization, PEDOT:PSS, Organic Solar Cells

1. Introduction

Photovoltaic solar cells based on organic compounds are

promising candidates for solar energy conversion. They

have the potential for cost effectiveness, mechanical

flexibility and easy processing [1-3]. However, in order

to compete with the inorganic thin film solar cells, power

conversion efficiencies of more than 10% must be

achieved. While current research efforts have led to the

efficiencies above 8% [4] There is a lack of certainty on

the maximum achievable efficiencies and concerns on

the device reliability and environmental stability of the

organic solar cells (OSC) [5,6]. Moreover, for large area

solar cell devices, development of materials which are

environment friendly and manufacturing processes which

are ecologically favourable is required.

As reviewed by Gunes et al., [7] a variety of polymers

have been used to fabricate the organic solar cells and the

results are dependent on the energy band alignment of

the polymers with respect to other layers in the device.

The internal power conversion efficiency of organic solar

cell is mainly a product of five factors viz., 1) absorption

efficiency, 2) exciton diffusion efficiency, 3) exciton

dissociation efficiency, 4) charge transport efficiency and

5) charge collection efficiency [8,9]. It can be easily es-

timated that the polymers having low band gap (<2 eV)

absorb the light in the visible range (300 nm - 700 nm) of

solar spectrum. Since the first report of low band gap

conjugated polymer, polyisothianaphthene (PITN) [10],

much work has been carried out to prepare polymers with

low band gap energy [11,12]. Low band gap conjugated

polymers may have higher conductivity (carrier mobility)

and therefore offer potential applications in transistors,

transparent conductors, non-linear optical devices and

smart windows.

In addition, stability of the active layer and rapid deg-

radation of properties has been a major concern for OSC

devices especially under light illumination and simulta-

neous exposure to oxygen or water vapor and an under-

standing of these aspects is crucial for fabricating long

lasting devices. Degradation of organic solar cells device

can occur due to various mechanisms such as diffusion of

oxygen and moisture resulting in active layer and elec-

trode degradation as reviewed by Jorgensen et al. [13].

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene 1703

(AVPV) Based Solar Cell Devices

During last few years, researchers have designed and

synthesized a large number of π-conjugated arylenevi-

nylene-co-pyrrolenevinylene (AVPV) oligomer system

derived from arylbridged bispyrroles [14]. Bispyrroles

are fluorescent dyes and have been used as the building

blocks for fabricating the low band gap polymeric sys-

tems. In our previous work, we introduced a derivative of

AVPV and investigated its candidature for the solar cells

in single layer device structure [15]. The structure of this

molecule is shown in Figure 1. The presence of alternate

single and double bond in structure facilitate the conduc-

tion of electrons in this polymer. The device performance

of photovoltaic cell is characterized by the short-circuit

current density (Jsc), open-circuit voltage (Voc) and fill

factor (FF). For single layer device structure, maximum

ideal Voc is limited by the work function difference of

electrodes used as cathode and anode [5,16,17]. In this

work, we have a systematic study on the effect of varia-

tions in anode surface characteristics on charge collection

efficiency. The study shows that total internal efficiency

in single layer organic solar cell is also limited by the

work function of used electrodes and HOMO (highest

occupied molecular orbital)—LUMO (lowest occupied

molecular orbital) levels of polymer [18,19].

2. Experimental Details

Device Fabrication

The OSC devices were fabricated using AVPV as an

active layer. A schematic of the device structure before

encapsulation is shown in Figure 2. Transparent indium

tin oxide (ITO)-coated glass substrates were used as an-

ode for this study. Sheet resistance of the ITO layers was

16 Ω·sq-1. ITO typically has a transitivity of 80% - 90%

in the UV range of light [1]. The ITO was patterned and

then subjected to ozonization for selective experiments.

The workfunction of ITO was measured as 4.8 eV prior

to ozonization and 5.1 eV after ozonization. These values

are in good agreement with the reported data [20,21].

For a few devices, thin film of PEDOT:PSS was spin

coated on top of an ITO-coated substrates at 3000 rpm

for 90 s. This layer was dried in the nitrogen ambient at

110˚C for 1 h.

To study the effect of variation in the anode character-

istics, four different device structures were fabricated i.e.

substrates with and without ozonization and with and

without PEDOT:PSS. Active layers were spin coated at

1000 rpm from a solution containing AVPV polymer.

Concentration of AVPV solution was 10 mg·cc-1 in

chlorobenzene. The active layer was dried in a vacuum

drying chamber at 120˚C for 1 h. For optical absorption

measurements, a single layer of AVPV was deposited on

a bare glass substrate whose results are shown in Figure 3.

Figure 1. Molecular structure of Arylenevinylene-co-pyr-

rolenevinyle ne (AVPV).

Figure 2. Schematic of the organic solar cell device struc-

ture.

300 400 500 600 700

0.5

0.6

0.7

0.8

0.9

1.0

Absorption (normalized)

Wavelength (nm)

Figure 3. Normalized absorption spectrum of AVPV film in

Chlorobenzene.

Top contacts of calcium-aluminium were evaporated

on the top of the active layer in high vacuum of 10-6 mbar

to form the cathode. The final active device area for each

cell was 0.2 cm2. Finally, the devices were encapsulated

with epoxy in a nitrogen environment.

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene

1704

(AVPV) Based Solar Cell Devices

The encapsulated devices were electrically character-

ized with a Keithley 4200 parameter analyser. Light charac-

teristics were measured under 1 sun illumination of AM

1.5 through a calibrated solar simulator.

3. Results and Discussion

3.1. Light Absorption

To make a good quality organic solar cell, absorption

spectrum of the organic material should match the Solar

spectrum, as closely as possible. In addition, thickness of

the active layer should be sufficient to achieve higher

absorption. Generally, organic materials have higher ab-

sorption coefficients than inorganic semiconductors such

as silicon [22]. Low band gap materials always tends

absorb sunlight towards lower wavelength.

Figure 3 shows the normalized absorption spectrum

taken on thin films of single layer AVPV, spun coated on

a clean glass substrate. The figure shows that AVPV film

has two broad peaks at ~423 nm and ~540 nm respect-

tively. The broad nature of these peaks suggests that

AVPV absorb mainly in the visible region (between 400 -

700 nm of the solar spectrum). This compares fairly well

with the absorption spectra of well established polymer

materials such as P3HT [23] and justifies the use of this

polymer for the OSC devices as reported by us previ-

ously [15].

3.2. Effect of Variations in the Anode Surface

As a rule, the internal quantum efficiency (



IQE) of OSCs

depends on the collection efficiency of carriers at elec-

trode. Collection of the charge carriers at the electrodes

depends on the alignment of HOMO of the donor poly-

mer and work function of the anode as well as alignment

of LUMO of the polymer and work function of the cath-

ode. As the difference between metal work function and

LUMO or HOMO level reduces, it enhances the number

of collected charge carriers at electrode and hence the

internal quantum efficiency.

To the determine the effect of surface characteristics

of anode, four different device structures of single layer

AVPV devices, (A) ITO with ozonization and with PE-

DOT:PSS, (B) ITO with ionization and without PE-

DOT:PSS, (C) ITO without ionization and with PE-

DOT:PSS and (D) without ionization without PE-

DOT:PSS were fabricated. The energy band diagram of

each of these device structures are shown in Figure 4.

These device structures are fabricated to understand the

change in the device performance after changes in the an-

ode surface by means of ozonization, leading to change in

the work function of ITO, as well as use of PEDOT:PSS.

Figure 4. Energy band diagrams of single layer AVPV de-

vices: (a) ITO with ionization and with PEDOT:PSS; (b)

ITO with ionization and without PEDOT:PSS; (c) ITO

without ionization and with PEDOT:PSS, and (d) ITO

without ionization and without PEDOT:PSS.

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene 1705

(AVPV) Based Solar Cell Devices

As our cyclic voltammetry measurements showed, the

Highest Occupied Molecular Orbital (HOMO) level of

AVPV lies at 4.77 eV while PEDOT:PSS has a work

function of 5.2 eV. Hence, as shown in the schematic

band diagram in Figure 4(a), at the time of collection at

anode, holes face a barrier of 0.43 eV. This indicated

towards a reduction in the number of collected holes and

thus may have an effect on the solar cell efficiency. In

this context, ozonization of ITO also affects the collec-

tion because after ozonization as we found that ITO work

functions shift from 4.8 eV to 5.1 eV.

J-V characteristics for all these devices measured un-

der 1 sun illumination of AM 1.5 through a calibrated

solar simulator and the results are shown in Figure 5. We

find that the devices without ozonization and without

PEDOT:PSS exhibit the highest short circuit current

density Jsc, and devices with ozonization and with PE-

DOT:PSS exhibit the lowest Jsc among these structures.

To perform a comparative study, the efficiency deter-

mining parameters are mentioned as a function of anode

in Table 1. The table shows that the photovoltaic devices

made with ionization with PEDOT:PSS are inferior in

comparison to the devices from all other three type of

devices. As shown in Figure 4(a), for the device struc-

ture (A), after the exciton dissociation, holes face a bar-

rier of 0.43 eV for the collection at anode as the accumu-

lation should take place in the part of the device near

anode.

This barrier reduces the hole collection at anode ham-

pering the collection efficiency and hence total internal

efficiency. Likewise, as the anode work function changes,

barrier for collection of holes reduces from Figures 4(a)-

(d) and as a result the current density increases. The ex-

tracted device parameters for each of the devices (short

circuit current density, Jsc, open circuit voltage, Voc, Fill

Factor, FF, series and shunt resistances, Rs and Rsh re-

spectively) are shown in Table 1. In the following sec-

tions, we discuss these effects separately on each of the

device parameter.

3.2.1. Effect on Jsc

Jsc is mainly controlled by efficiency of absorption of

light, exciton dissociation, charge transport and charge

collection. For all the four devices, absorption, dissocia-

tion and transportation are likely to be similar because of

similar device thicknesses. The only variation is in the

anode characteristics and band positions at the anode

interface (Figure 4). The figure shows that as for the ITO

anode without PEDOT:PSS and without ozonization

(Device D), the ITO work function lies at 4.8 eV which

helps in aligning the HOMO level of AVPV with the

ITO work function facilitating an easy transport of holes

to the anode and hence increasing the Jsc. In contrast the

devices which have a PEDOT:PSS layer, the holes face a

larger barrier of 0.43 eV.

3.2.2. Effect on Voc

For a single layer OSC device, Voc is determined by the

difference in the work function of both the electrodes

[16]. This implies that the devices where ITO is ozonized

and is further coated with PEDOT:PSS layer should

show highest Voc (because of voltage drop at anode and

active layer interface). But in these devices, charges also

accumulate near the electrode and thus hampering the Voc.

In the device “D”, no accumulation is likely so Voc was

found to be maximum. Less Voc for device “B” having

ITO with ozonization without PEDOT:PSS and device

“C” without ozonization and with PEDOT:PSS. This

appears to suggest that anode roughness may also have

an effect on the Voc as PEDOT:PSS has a smoothening

effect on ITO.

Figure 5. J-V characteristics of single layer AVPV devices

with anode variations under 1 Sun illumination.

Table 1. A comparison of device parameters for all four

types of OSC devices with variations in anode characteris-

tics.

Devices Jsc (A·cm-2)

 10-7 Voc (V)FF (%) Rs

(106 ·cm-2)

Rsh

(106 ·cm-2)

A 1.1 1.2 24 1.7 5.0

B 2.1 1.0519 1.8 2.8

C 4.3 1.0518 1.1 1.4

D 13 1.4 9.1 0.6 0.2

Effect of PEDOT:PSS Layer and ITO Ozonization in Arylenevinylene-co-Pyrrolenevinylene

1706

(AVPV) Based Solar Cell Devices

3.2.3. Effect on FF

Fill Factor (FF) is actually the filling factor (ratio of

maximum power to the theoretical maximum power)

determined by the fraction of the photo-generated charge

carriers that actually reach the electrodes, when the built-

in field is lowered toward the open circuit voltage. In fact,

there is a competition between charge carrier recombina-

tion and charge transport. Highest shunt resistance for

device “D” results in highest recombination and hence

least FF [24].

4. Conclusions

In conclusion, we showed the effect of surface character-

istics of anode in the single layer devices on the photo-

voltaic properties of the devices. Since the total quantum

efficiency also includes the charge collection efficiency

at the electrodes the energy level diagram of the device

suggested that the alignment of ITO anode work function

and HOMO level of donor is an important condition for

better collection of holes under illumination. We found

that the devices fabricated on ITO anode (without

ozonization and without PEDOT:PSS) exhibited the

maximum current density (Jsc = 1.3 µA·cm-2) as compared

to those devices where ITO was ozonized as well as had a

PEDOT:PSS layer (Jsc = 0.1 µA·cm-2 ). This is due to re-

duced barrier to the hole transport to the anode from the

active layer when ITO is not ozonized and PEDOT:PSS

layer is absent.

5. Acknowledgements

This work was financially supported by the Department

of Science and Technology, Government of India, New

Delhi.

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