Interfacial polymer ferroelectric dipole induced electric field effect on the photovoltaicperformance of organic solar cellsAlok C. Rastogi Citation: Journal of Vacuum Science & Technology B 31, 04D112 (2013); doi: 10.1116/1.4813752 View online: http://dx.doi.org/10.1116/1.4813752 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/31/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Polymer-based parallel tandem solar cells with a transparent ferroelectric interconnecting layer Appl. Phys. Lett. 104, 083302 (2014); 10.1063/1.4866398 Polymer photovoltaic cells with a graded active region achieved using double stamp transfer printing Appl. Phys. Lett. 103, 193301 (2013); 10.1063/1.4829040 Analyzing photovoltaic effect of double-layer organic solar cells as a Maxwell-Wagner effect system by opticalelectric-field-induced second-harmonic generation measurement J. Appl. Phys. 110, 103717 (2011); 10.1063/1.3662914 Surface relief gratings on poly(3-hexylthiophene) and fullerene blends for efficient organic solar cells Appl. Phys. Lett. 91, 173509 (2007); 10.1063/1.2802561 Efficient dye-sensitized solar cells based on a 2-thiophen-2-yl-vinyl-conjugated ruthenium photosensitizer and aconjugated polymer hole conductor Appl. Phys. Lett. 89, 043509 (2006); 10.1063/1.2240296

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Interfacial polymer ferroelectric dipole induced electric field effecton the photovoltaic performance of organic solar cells

Alok C. Rastogia)

Electrical and Computer Engineering Department and Center for Autonomous Solar Power, BinghamtonUniversity, State University of New York, Binghamton, New York 13902

(Received 12 March 2013; accepted 28 June 2013; published 17 July 2013)

Effect of a dipolar electrical field on the photovoltaic properties of polymer solar cell devices in the

bulk heterojunction structure based on 1:1 blend of donor-regioregular poly(3-hexylthiophene),

P3HT, and acceptor-phenyl-C61-butyric acid methyl ester, PCBM, polymer semiconductor

material is studied. With a thin layer of poled ferroelectric P(VDFþTrFE) copolymer film

having oriented dipoles inserted at the interface between poly(3,4-ethylenedioxythiophene):

poly(styrenesulfonate) (PEDOT:PSS) and the P3HT:PCBM composite layer, significant

improvement in the short-circuit photocurrent and open circuit voltage was observed consistent

with the increase in the poling voltage. Remnant polarization charge due to dipoles produces

localized electric field of 300–700 V � lm�1. The presence of the electric field helps increase the

charge transfer exciton dissociation rates at the P3HT (donor)–PCBM (acceptor) interfaces,

reduces the exciton recombination probability and increases the carrier extraction yield resulting in

the enhancement in the photocurrent. Solar cell device in a structure having a poled

P(VDFþTrFE) copolymer film sandwiched between the two P3HT:PCBM composite layers

shows only a marginal improvement in the photovoltaic properties. This is mainly attributed to

shielding of dipolar field from free charge carries and the inferior nanoscale morphology of the

P3HT:PCBM composite layer. VC 2013 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4813752]

I. INTRODUCTION

Polymeric organic solar cells (OSCs) due to low cost

and easy processing ability are most promising for solar

electricity generation. These solar cells, based on the hole-

conducting polymeric semiconductor (electron donor) and

the electron-conducting fullerene (electron acceptor) in the

blended bulk heterojunction (BHJ) device structures, have

shown photovoltaic conversion efficiencies of 6–8%.1,2 For

further increasing the conversion efficiencies of the BHJ so-

lar cells, recent efforts have focused on the morphology of

the heterojunction interfaces at the nanoscale by utilizing

solvents atmosphere annealing,3,4 solvent additives,5 and

variations in the annealing methods.6 Other approaches such

as new polymer donor–acceptor combinations with opti-

mized band-gap energies1,2 and the solar cell designs based

on tandem structures7 have also been investigated. In the

operation of organic solar cells, donor–acceptor (D-A) inter-

face plays an important role in efficient dissociation of the

photogenerated charge transfer excitons (CTE), the elec-

tron–hole pairs bound by Coulomb attraction. The photocur-

rent in organic solar cell devices is produced when free

electrons and holes generated at the interface by CTE disso-

ciation are collected at the contacts via transport through the

acceptor and donor semiconductors, respectively.8 Various

factors that contribute to the low efficiency of organic solar

cells are generally ascribed to the short diffusion lengths that

cause the singlet excitons to recombine before arriving at the

D-A interface, CTE recombination at the interface before

dissociation,8,9 and the low free carrier mobility. Most

relevant loss processes are geminate recombination at the

interface during the polaron pair dissociation,10,11 nongemi-

nate recombination of separated polarons in transport through

the donor and acceptor semiconductors,12 and the extraction

of carriers at the device contacts.13 Recombination processes

add up to�44% loss in conversion efficiency,14 and the key to

achieve higher conversion efficiencies in solar cell is to find

ways to nullify the effects of the recombination pathways.15

The object of the present investigation is to improve the

photovoltaic performance of organic solar cells by inducing

a local electric field in the vicinity of the bulk D-A hetero-

junction interface and thus (i) affect efficient CTE dissocia-

tion in generating the free carriers, electrons, and holes, and

(ii) extract and drift free carriers apart under the field to min-

imize loss by recombination. We have created local field by

introducing the polarized ferroelectric dipoles of copolymer

of vinyildene fluoride (80%) with trifluoroethylene (20%)

P(VDFþTrFE), which has a large dipole moment (2.1 D)

and can produce large spontaneous polarization by coopera-

tive alignment of the molecular dipoles through poling. The

effect of the electric field introduced through the ferroelec-

tric P(VDFþTrFE) copolymer buffer layer at the Al contact

was demonstrated recently by Yaun et al.16,17 who showed

increase in the short-circuit photocurrent and overall

improvement in the photovoltaic conversion efficiency of

the OSC device. It was also suggested that the increase in

the open circuit voltage in the solar cells with ferroelectric

P(VDFþTrFE) buffer layer at the Al interface results from

improved contact property rather than the electric field since

the improved photovoltaic properties were found insensitive

to the field direction.18 The organic solar cells in which the

active polymer semiconductor donor–acceptor system hasa)Electronic mail: arastogi@binghamton.edu

04D112-1 J. Vac. Sci. Technol. B 31(4), Jul/Aug 2013 2166-2746/2013/31(4)/04D112/9/$30.00 VC 2013 American Vacuum Society 04D112-1

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been doped with the ferroelectric P(VDFþTrFE) copolymer

in different concentrations, the effect of the ferroelectric

field on charge separation was recently shown by Nalwa

et al.19 These authors reported quantum efficiencies

approaching 100%, indicating complete CTE dissociation

and �50% improvement in the power conversion efficiency.

In this work, the effect of dipolar field on the photocurrent

was evaluated by fabricating organic solar cell devices using

the polymer semiconductor material system based on 1:1 blend

of donor-regioregular poly(3-hexylthiophene), P3HT, and

acceptor-phenyl-C61-butyric acid methyl ester, PCBM. Two

different solar cell structures were investigated. In one struc-

ture, an interfacial dipolar electric field is introduced by incor-

porating a nanometer thin layer of polarized ferroelectric

P(VDF-TrFE) copolymer at the PEDOT:PSS anode contact

instead of at the Al cathode contact studied previously.16,17 The

other structure comprised of a trilayer interface with a thin fer-

roelectric P(VDF-TrFE) copolymer film sandwiched between

the two separate polymer semiconductor P3HT:PCBM donor–

acceptor (D-A) composite layers. Both these structures ensure

that the electric field dipoles in the ferroelectric polymer layer

are in the close proximity of the P3HT:PCBM D-A interface.

Improvements in the photocurrent density of these solar cells

are observed when subjected to positive poling at different vol-

tages. This paper reports on the results of the photovoltaic pa-

rameters under various poling and polarity conditions of the

interface dipolar electric field. These results show improved

photocurrent in organic solar cells with a ferroelectric buffer

layer is due to ferroelectric field assisted efficient exciton disso-

ciation at the D-A interface.

II. EXPERIMENT

The BHJ photovoltaic solar cell devices were based on

donor-P3HT and acceptor-PCBM materials system and fab-

ricated by the conventional spin casting technique. All devi-

ces were fabricated over indium tin oxide (ITO) coated

transparent conducting glass substrates of nominal 15–20X/sq.

sheet resistivity. The bottom electrode comprises of a

�50 nm thick PEDOT:PSS layer deposited by spin coating

over the ITO coated glass substrate at 3000 rpm for 30 s

from an aqueous dispersion of PEDOT:PSS in deionized

water in 2.5:1 volume ratio and followed by a 10 min anneal-

ing at 120 �C. This layer facilitates transport of photogener-

ated holes for collection across the ITO contact. The

photoactive component of the solar cell device consisted of a

�160 nm thick layer of a composite of donor P3HT and

acceptor PCBM polymers blended in a 1:1 weight ratio. This

layer was spin coated over the PEDOT:PSS layer from a so-

lution in dichlorobenzene having a solid content of both

polymers in the concentration of 12 mg/ml. Typically, the

spin coating is done at 800 rpm for 20 s, which is followed

by a crystallization annealing at 145 �C for 25 min under dry

N2 flow. The solar cell device is completed by thermal evap-

oration of a 100 nm thick aluminum metal top contact layer

through mask directly over the P3HT: PCBM composite

layer in a cryopumped system under �10�7 Torr pressure.

The reference solar cell device (OSC-A) made this way is

schematically shown in Fig. 1(a). In order to investigate the

effect of the interface ferroelectric field in the close vicinity

of the P3HT:PCBM composite layer, a thin film of P(VDF-

TrFE) copolymer was introduced in the solar cell device

structure. The ferroelectric film is deposited by spin coating

a solution of P(VDFþTrFE) copolymer (Solvay) with a

concentration of 2.5–5 mg�ml�1 dissolved in 80 �C heated

diethylcarbamide by stirring under moisture free ambient.

Typically, a spin rate of 2000 rpm for 60 s was used while

assiduously maintaining the isothermal conditions under a

N2 ambient to form a �5–10 nm thick film. This layer was

crystallized by annealing at �145 �C for 25–45 min. We

investigated two types of solar cell structures: one which has

a 10 nm thin P(VDFþTrFE) copolymer layer inserted at the

interfaces of the P3HT:PCBM composite layer and the

PEDOT:PSS hole conductor layer (OSC-B), and the other, a

trilayer structure, which has a 5 nm thin P(VDFþTrFE) co-

polymer ferroelectric layer sandwiched between the two

P3HT:PCBM (D-A) composite layers (OSC-C). In the first

structure, the ferroelectric dipolar field exerts an unidirec-

tional electric field at the PEDOT:PSS and P3HT:PCBM

(D-A) composite interface and the trilayer stack ensures that

the bidirectional electric field due to dipoles is in the close

proximity of the P3HT:PCBM composite interfaces on either

side. Both device structures are shown in Figs. 1(b) and 1(c).

In order to establish the interface dipolar electric field, it is

essential to orient the PVDF molecular chain such as most of

the H-F bonds are uniformly aligned in the same direction.

FIG. 1. (Color online) Schematic of three organic solar cell device structures:

(a) OSC-A, control device without a ferroelectric P(VDFþTrFE) copolymer

layer (b) OSC-B, with a 10 nm thin ferroelectric P(VDFþTrFE) copolymer

film at the PEDOT:PSS interface with P3HT:PCBM (donor–acceptor) compos-

ite layer and (c) OSC-C, with a 5 nm thin ferroelectric P(VDFþTrFE) copoly-

mer film sandwiched between two 80 nm thin P3HT:PCBM (donor–acceptor)

composite layer.

04D112-2 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-2

J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013

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This was done by electric field poling of the solar cell in

dark using a step by step enhancement of the poling voltage

varied in the range of 68–27 V using a computer controlled

ferroelectric tester Precision Pro with vision software by

Radiant Technology. The voltage pulse applied for polariza-

tion and hysteresis measurements was triangular in shape

with a total loop time of 100 ms. Smaller loop times gave an

unsaturated loop and while higher times showed an appreci-

able amount of DC conductivity in the hysteresis loop. The

current–voltage (I-V) characteristic of the solar cells was

measured using a Keithley 2602 source meter in dark and

under AM1.5 illumination.

III. RESULTS AND DISCUSSION

A. Ferroelectric P(VDF 1 TrFE) copolymer filmcrystalline structure

The dipole moment in P(VDFþTrFE) copolymer is asso-

ciated with the H-F bonds, and it is directed toward electro-

positive hydrogen from electronegative fluorine. For the

P(VDF-TrFE) film to show a net dipole moment, the molec-

ular units in the polymer chains should be aligned along a

two-fold crystalline axis. This structural form is associated

with the orthorhombic crystal structure in the b-phase in

which molecular chains are in all trans(-TTT-) zigzag config-

uration. Crystalline structure and the morphology of the

P(VDFþTrFE) copolymer film spin deposited over PEDOT

layer as in solar cell OSC-B and over P3HT: PCBM compos-

ite layer as in solar cell OSC-C device structures was stud-

ied. Figure 2 shows the x-ray diffraction patterns of the

as-spin coated and postannealed P(VDFþTrFE) copolymer

films. The diffraction patterns in the two interface structures

belonging to the solar cell devices OSC-B and OSC-C are

marked in Fig. 2. The P(VDFþTrFE) copolymer film

deposited over a PEDOT:PSS layer shows a broad diffrac-

tion peak at 2h¼ 20.2� curve (a) that belongs to the P(VDF

þTrFE) copolymer in a weakly crystalline low temperature

phase along with a cooled (CL) phase, which is crystallo-

graphically a disordered phase.20 After annealing in air at

100 and 145 �C for 45 min, intensity of this peak increases

substantially, and it turns much sharper as shown by curves

(b) and (c), respectively, in Fig. 1. This peak is assigned to

(110) (200) diffraction from a ferroelectric b-phase of

P(VDFþTrFE).21 The annealing temperature of 145 �C was

chosen because it is between the Curie temperature (Tc)

�125 �C of the copolymer, and the crystal melting tempera-

ture of 160 �C. The PVDF with simple molecular –CH2-CF2-

units has high flexibility of the skeletal chain, in spite of ster-

eochemical constraint imposed by the CF2 units. As a result,

PVDF can take various molecular and crystal structures of

which only the b-phase exhibits ferroelectricity. The as-

deposited P(VDFþTrFE) copolymer film spin coated over a

P3HT:PCBM composite layer as in the solar cell device

structure OSC-C was found amorphous (not shown here). Its

crystalline features emerge after annealing at 145 �C for

45 min a shown by curve d in Fig. 2. This peak, interpreted

as that of the P(VDFþTrFE) in b-phase combined with a

disordered CL phase is broad and of much smaller intensity

compared to the crystallized P(VDFþTrFE) copolymer

layer over PEDOT:PSS base layer. A broad shoulder at the

low 2h side arises from a thermodynamically stable but non-

ferroelectric a-phase with molecular chains in Trans-Gauche

(-TGTG0-) structure. It is apparent that a highly crystalline

form of P(VDF-TrFE) film could not be realized when spin

coated over the P3HT:PCBM base layer.

B. Ferroelectric P(VDF 1 TrFE) copolymer filmmorphology at interface

Figure 3 shows tapping mode AFM images of the P(VDF

þTrFE) copolymer films deposited in the two interface

structures. The surface morphology of a 145 �C-45 min

annealed P(VDFþTrFE) copolymer film over PEDOT:PSS

layer reveals interlocking rodlike crystalline lamellae, with

an average length of 0.4 lm and an average diameter of

0.15 lm. It is well known that the P(VDF-TrFE) in the ferro-

electric b-phase crystallizes with a rodlike morphology.

AFM image, Fig. 3(b), of a P(VDFþTrFE) copolymer film

deposited over P3HT:PCBM composite layer after crystalli-

zation anneal shows randomly distributed granular morphol-

ogy with an average size �0.2 lm. It appears that final

surface morphology of the P(VDFþTrFE) copolymer film

is mediated by simultaneous crystallization or phase-

separation of the underlying P3HT:PCBM blended layer dur-

ing the annealing process. As shown by the XRD studies, the

P(VDF-TrFE) crystallization did not result in a completely

ordered b-phase. The final morphology is interpreted as that

of the P(VDF-TrFE) crystallites embedded in underlying

segregated P3HT: PCBM domains.

C. Polarization field due to interfacial ferroelectricdipoles

A unidirectional alignment of dipoles is essential for

attaining a macroscopic polarization induced ferroelectric

field at the interface. Such a field is required for observing

FIG. 2. (Color online) X-ray diffraction pattern of ferroelectric P(VDFþTrFE)

copolymer film. Deposited over PEDOT:PSS coated substrate; curve (a) as-

deposited (b) annealed at 100 �C for 45 min (c) annealed at 145 �C for 45 min.

Deposited over P3HT:PCBM composite layer; curve (d) annealed at 145 �C for

45 min. As-deposited film is amorphous in structure.

04D112-3 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-3

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the possible effect on the singlet and charge transfer exciton

dissociation in the operation of solar cells. Dipolar orienta-

tion is achieved by step-by-step application of positive and

negative voltage pulses of increasing electric field across the

bottom ITO and top Al contacts. The polarization hysteresis

and the remnant polarization characteristics of the P

(VDFþTrFE) copolymer film was initially studied in a con-

trolled structure. Figure 4 shows polarization hysteresis at

different poling voltages of a 50 nm thick P(VDFþTrFE)

copolymer film sandwiched between the Ti film as top and

bottom electrodes in the Si/Ti/P(VDFþTrFE)/Ti controlled

structure. With the increasing poling voltage, enhancement

in dipolar switching, coercive field and remnant polarization

charge density is observed. Nucleation of ferroelectric

domains takes place at a field higher than the coercive field.

When an electric field is applied, polarization reversal starts

with the generation of nuclei in the reversed direction fol-

lowed by nuclei growth by domain wall movement.

Structurally, the polarization reversal in P(VDFþTrFE) co-

polymer occurs by chain rotation about its axis. Therefore,

the speed of polarization switching depends on orientation of

the dipoles in the film and the presence of defects that could

act as domain nucleation sites. Initial characteristics, curves

a and b show rounded hysteresis possibly by depolarization

due to the presence of free carriers at the molecular chain

sites. By application of repetitive switched voltages of

increasing magnitude, the polarization saturation corre-

sponding to the maximum possible polarization of the film,

curves c and d, is realized when these charges are swept

away by the poling electric field. The polarization poling

study of the crystallized P(VDFþTrFE) copolymer film in

the solar cell device structure OSC-B sandwiched between

the PEDOT:PSS and P3HT:PCBM composite layers is

shown in Fig. 5(a). The polarization switching is asymmet-

ric, which arises from the difference in the work function of

the contacts on both sides of the P(VDFþTrFE) copolymer

film. The saturation polarization is obtained when negative

polarization charges build-up in the P (VDFþTrFE) copoly-

mer is neutralized by accumulated charges due to free car-

riers at the P3HT-PCBM composite interface.22 Figure 5(b)

shows polarization hysteresis from P(VDFþTrFE) copoly-

mer layer in the solar cell structure OSC-C. In this structure

with a single layer P(VDFþTrFE) copolymer, a stable

poling of dipoles is found difficult perhaps arising from

depolarization effects due to insufficient space charge to

compensate for the ferroelectric polarization charge.23 In the

case of the double-layer P(VDFþTrFE) copolymer solar

FIG. 3. (Color online) AFM images of the crystallized P(VDFþTrFE) copolymer films deposited (a) over a PEDOT:PSS anode contact layer showing the

interlocking lamellae like structures, with average 0.4 lm length and 0.15 lm diameter (b) over a semiconducting P3HT:PCBM composite layer showing a

randomly distributed granular morphology with average size 0.2 lm embedded in an amorphous matrix.

FIG. 4. (Color online) Ferroelectric polarization hysteresis characteristics of

a control P(VDFþTrFE) film in the Si/Ti/P(VDFþTrFE)/Ti capacitor

structure shown in inset at different poling voltages (a) 11 V, (b) 15 V, (c)

21 V, and (d) 27 V.

04D112-4 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-4

J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013

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cell device structure as shown in Fig. 5(b), only a weak

polarization field could be established even after a high field

poling. Based on the dipole-field model described in earlier

studies,16,19 the magnitude of the electric field established at

the interface by the oriented ferroelectric dipoles is related to

the remnant polarization by the equation

E ¼ 4pePVDF � e0

� P�r � Vf ; PVDF; (1)

where P�r is the remnant polarization lC�cm�2 and Vf ;PVDF is

the volume fraction of the P(VDFþTrFE) copolymer in the

organic solar cell structures OSC-B and OSC-C. The polar-

ization direction is normal to the film plane and polarization

charge is distributed uniformly across the surface of the fer-

roelectric layer. The electric field permeates in the polymer

semiconductor layer as its surface potential is raised in

response to the polarization charge at the ferroelectric

P(VDFþTrFE) polymer surface.16 Table I shows the polar-

ization induced interface electric field at the interface in both

solar cell structures as well as in the case of controlled

P(VDFþTrFE) copolymer film. The morphology of the fer-

roelectric P(VDFþTrFE) copolymer layers in both solar

cell interface structures is conducive enough and the voltage

poling in principle should be able to produce a reasonable

polarization charge density. In reality, the magnitude of the

electric field shown in Table I determined purely on the rem-

nant polarization charge density values is estimated at a

higher side. One also expects that the charge screening

effects in the solar cell device structure will also diminish

the net electric field at the interface. However, a large frac-

tion of the calculated field might still be introduced in the

interface region.

D. Effect of ferroelectric interface field effecton photovoltaic response

Current–voltage characteristics of the organic solar cell

device in the OSC-B structure under AM1.5 radiation is

shown in Fig. 6 at different ferroelectric poling voltages

used for dipolar orientation in the P(VDFþTrFE) copoly-

mer film. The photocurrent behavior of this solar cell device

having a poled ferroelectric P(VDFþTrFE) copolymer

interface layer was observed to improve significantly relative

FIG. 5. (Color online) Ferroelectric polarization hysteresis characteristics at

different voltages of a crystallized P(VDFþTrFE) copolymer film in the so-

lar cell device structure (a) OSC-B sandwiched between PEDOT:PSS and

P3HT:PCBM composite layers and (b) OSC-C sandwiched between two

P3HT:PCBM composite layers in structures shown in the inset.

TABLE I. Ferroelectric dipolar field due to P(VDFþTrFE) copolymer layer

at the interface.

P(VDFþTrFE) film

interface in OSC

Maximum interface field (V/lm)

Poling voltage

27 V 21 V 17 V 15 V

PEDOT:PSS/P3HT:PCBM:

OSC structure-A

— — — —

PEDOT:PSS/P(VDFþTrFE)/P3HT:

PCBM:OSC structure-B

— 902 710 588

P3HT:PCBM/P(VDFþTrFE)/P3HT:

PCBM:OSC structure-C

— 498 295 —

Controlled P(VDFþTrFE) film

Si/Ti/P(VDF:TrFE)/Ti

1700 1400 1000 —

FIG. 6. (Color online) Photovoltaic performance under AM1.5 radiation of a

solar cell device in structure OSC-B with the introduction of a ferroelectric

P(VDFþTrFE) copolymer under various poling voltages. Increasing poling

voltage implies enhanced dipolar electric field at interface through increased

remnant polarization. Curves (a) control solar cell in structure OSC-A; (b), (c),

(d) and (e) are at poling fields 15, 17, 21, and 27 V, respectively; (f) and (g) are

at poling fields 21 and 27 V, respectively, poled in the reverse direction.

04D112-5 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-5

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to an identically fabricated conventional solar cell without a

ferroelectric P(VDFþTrFE) copolymer interface layer as in

structure OSC-A. The increase in the poling voltage basi-

cally results in a corresponding increase in the remnant

polarization charge (Fig. 5) and hence an increase in the

magnitude of the electric field at the interface. It is seen that

the electric field at the PEDOT:PSS and P3HT:PCBM com-

posite layer interface significantly effects the short circuit

photocurrent of the solar cell device. Basically the short cir-

cuit photocurrent density JSC is observed to scale with the

increase in the ferroelectric field. The open circuit voltage

VOC of the device also increases, but marginally. Changes in

both JSC and VOC values as a function of the polarization

voltage are summarized in Fig. 7. It may be noted that the

control solar cell device OSC-A without the ferroelectric

interface layer in this study was not optimized and the low

conversion efficiency is mainly due to a poor morphology of

the P3HT:PCBM composite layer. The main objective of

this study is to study the effect of a ferroelectric field at the

P3HT:PCBM composite layer interface over the photocur-

rent behavior of the solar cells. The increase in JSC with fer-

roelectric P(VDFþTrFE) copolymer interface layer at the

PEDOT:PSS back electrode and P3HT:PCBM composite

layer is attributed to efficient dissociation of single and

charge transfer excitons supported by the increased dipolar

electric field at the interface. In a solar cell device made with

P(VDFþTrFE) copolymer doped within the P3HT:PCBM

composite layer, a similar increase in the JSC values was

reported earlier with increasing content of P(VDFþTrFE)

copolymer. This was attributed to field associated exitonic

dissociation.19 A similar effect of the ferroelectric field in

increasing the photovoltaic efficiency of solar cells by intro-

ducing ultrathin �3 monolayer Langmuir–Blodgett films of

the P(VDFþTrFE) copolymer interface layer over the Al

cathode was reported by Yuan et al.16 Based on the require-

ment for a space charge for the stabilization of the ferroelec-

tric dipoles, however, it was argued that the increased

photovoltaic efficiency of the solar cells is primarily due to

the improvement in the performance of the Al cathode rather

than the effect of the magnitude or the direction of dipolar

field.18 In this study, the ferroelectric electric field is estab-

lished at the PEDOT:PSS side of the P3HT:PCBM compos-

ite layer interface. Since increase in the photocurrent is still

observed in this solar cell structure and it scales with rem-

nant polarization through increasing polarization field, it can

be inferred that the field assisted enhancement in SE or CTE

dissociation rate is the primary mechanism for increased

photocurrent generation. The photovoltaic conversion effi-

ciency of the solar cell devices in this study is somewhat

low, which is due to the nonoptimized P3HT:PCBM com-

posite layer morphology. It is well known that the bulk het-

erojunction properties sensitive the morphology are

unfavorably affected by poor intercalation of the P3HT

donor-PCBM acceptor domains. The underlying crystallized

P(VDFþTrFE) copolymer layer having directionally ori-

ented large lamellae-like structural features (Fig. 3) to a

large extent determine the final morphology of the over

coated P3HT:PCBM composite layer in the solar cells de-

vice being studied. It is notable that in spite of the nonopti-

mized P3HT:PCBM layer morphology, the photocurrent in

these solar cells significantly increases by ferroelectric

poling, which are ascribed to the effect of the dipolar electric

field of the ferroelectric P(VDFþTrFE) copolymer layer.

In operation of the solar cell device, photogenerated sin-

glet excitons diffuse to the P3HT donor and PCBM acceptor

interface within the composite layer. At the interface, due to

the energy difference between the lower unoccupied molecu-

lar orbital (LUMO) of P3HT and PCBM, these singlet exci-

tons decay into CTE and contribute to photocurrent

generation on dissociation into free carriers.24,25 For obtain-

ing the photocurrent, these free carriers electrons and holes

are required to be collected at the Al (cathode) and

PEDOT:PSS/ITO (anode) contacts, respectively. A compet-

ing process, which results in the reduction of photocurrent, is

the loss of charge transfer excitons before dissociation either

by a nonradiative recombination at the interface or by sur-

face diffusion as a bound polaron pair.8 Loss of CTE can be

minimized by achieving an enhanced rate of CTE

dissociation-carrier generation, kDG compared to the com-

bined rates of various CTE loss processes; interface recom-

bination kIR and surface diffusion-recombination kSR. A

higher yield YE of extraction of free carriers at the contacts

will require a built-in field at the contacts. The net short cir-

cuit photocurrent JSC is given by the equation8

JSC / kDG � YE=ðkIR þ kSR þ kDGÞ: (2)

Based on the model by Onsage-Braun26,27 the net

dissociation-carrier generation rate balancing the combined

CTE recombination rates is given by

kDR ¼qhliepe0

� 3

4pa3expð�EB=kTÞ � 1þ bþ b2

3þ b3

18þ � � �

� �:

(3)

FIG. 7. (Color online) Increase in the (a) open circuit voltage (VOC) and (b)

short-circuit photo current density (JSC) as a function of poling voltage of a

solar cell device in structure OSC-B with ferroelectric buffer layer at the

PEDOT:PSS–P3HT:PCBM interface relative to solar cell in structure OSC-

A without a ferroelectric interface field.

04D112-6 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-6

J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013

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The mobility hli term accounts for the free carrier recombi-

nation. The exponent term basically relates to the free and

bound polarons through their binding energy EB and the ther-

malization radius a. In the polymer semiconductor system

P3HT:PCBM based on the average dielectric constant ep

considerations, EB is considerably large, 0.3–0.5 eV, com-

pared to kT ¼ 0:0259 eV at 300 K.8,19 Large EB makes the

exponent term extremely small, thus intrinsically diminish-

ing the CTE dissociation rates as demonstrated by earlier

studies carried out with polymer semiconductors with

varied dielectric constant values.28,29 Effect of the electric

field E on CTE dissociation manifests through the term,

b ¼ q3E=8pepe0k2T2. Accordingly, the effect of CTE recom-

bination can be minimized by increasing the dissociation

rates through introduction of an electric field. It is estimated

that an internal field of 50� 70 V � lm�1 is required for

obtaining reasonable CTE dissociation rates.11 In the con-

ventional organic polymer solar cell structures, the only in-

ternal electric field available is due to the difference in the

work function of the two contact electrodes. This accounts

for a field of typically30 �10 V � lm�1, which is insufficient

for achieving any significant increase in CTE dissociation

rates. In the present case, with the introduction of polarized

ferroelectric P(VDFþTrFE) copolymer layer which after

poling establishes an electric field within the polymer semi-

conductor P3HT:PCBM composite layer that far exceeds the

required field (Table I) required to assist in CTE dissociation

even with taking into account some reduction due to the

charge screening effects. Increasing electric field is estab-

lished at the interface by ferroelectric dipoles aligned

through increasing poling voltage. Thus, the enhancement in

the photocurrent density observed with the increasing poling

voltage shown in Figs. 6 and 7 can be interpreted as resulting

from the ferroelectric field assisted increase in the CTE dis-

sociation rate kDR. Similarly, this field can also enhance the

free carrier extraction yield, YE by assisting the injection free

hole carriers at the PEDOT:PSS/ITO contact. To deduce this

aspect further, we investigated the photovoltaic characteris-

tic of the solar cell OSC-B in which the polarization field

direction is reversed by an opposite direction poling of the

ferroelectric interface layer. The direction of the electric

field is now directed toward P3HT:PCBM composite layer

as depicted in the polarization hysteresis in Fig. 5(a) and

photocurrent–voltage characteristic in Fig. 6. Curves f and gin Fig. 6 show the short-circuit photocurrent density JSC with

polarization induced electric field directed toward the

P3HT:PCBM side of the interface. The poling voltage in this

case is same as that of the curves d and e but with electric

field directed toward the PEDOT:PSS/ITO side of the inter-

face. Even though, the photocurrent density values are

higher compared to the solar cell without the ferroelectric

interface layer, compared to the curves d and e photocurrent

values are lower. The CTE dissociation, which is assisted by

the electric field in P3HT:PCBM composite layer, is not de-

pendent on the orientation of the electric field. However, the

electric field of correct direction will support carrier collec-

tion by drift toward the electrodes and thus minimize carrier

loss by recombination and increase the carrier extraction

yield, YE. With the electric field direction reversed, impeded

free carrier transport rather than the SE or the CTE dissocia-

tion will result in lowering of JSC values as observed. It is

worth noting that under conditions of high remnant polariza-

tion with most dipoles in ferroelectric interface layer

aligned, the electric field still extends through the semicon-

ductor polymer P3HT:PCBM composite layer because of the

incomplete charge compensation in the semiconductor due

to low free carrier concentration. This electric field can act at

the P3HT (donor) and PCBM (acceptor) domain interface

and contribute to enhanced dissociation rates of the CTE

generated by light absorption. On the other hand, at the con-

ducting PEDOT:PSS/ITO electrode interface, free carries to

a large extent compensate the polarization charge and as

such the penetration of the electric field in the PEDOT:PSS/

ITO interface region is minimized. The data of Fig. 6 show

that this field is still large enough for effective free hole car-

rier injection at the PEDOT:PSS/ITO contacts.

The observed increase in the open circuit voltage, VOC

values relative to controlled solar cell device OSC-A which

does not have a ferroelectric P(VDFþTrFE) copolymer

layer at the interface can be attributed to the interface ferro-

electric field. The VOC values relate to the energy difference

between the highest occupied molecular orbitals of P3HT

(donor) and LUMO energy of PCBM (acceptor) in the com-

posite layer. Experimentally observed VOC values are, how-

ever, considerably lower due to various charge loss

mechanisms.31,32 A range of processes, polaron and CTE

recombination,33 free carrier surface recombination34 and

inefficient singlet exciton dissociation contribute to charge

loss.8 In the case of solar cell in OSC-B structure, an

increased rate of CTE dissociation and of free carrier genera-

tion can be regarded as suppression of CTE recombination

rate which explains the increase in the VOC values. Action of

the ferroelectric field at the contacts in increasing the free

carrier collection has been discussed above for explaining

the increase in JSC. This drift field also contributes to the

reduction in bimolecular recombination lifetime. As pro-

posed by Shuttle et al.,12 nongeminate bimolecular recombi-

nation can limit the VOC values. Ferroelectric field assisted

solar cell device OSC-B shows somewhat lower fill factor

which is due to increased series resistance. Addition of a

resistive P(VDFþTrFE) copolymer thin film increases con-

tact resistance at the PEDOT:PSS electrode. Application of a

lower thickness of P(VDFþTrFE) copolymer is not an use-

ful option since a higher remnant polarization charge which

scales with the number of dipoles is also required for a rea-

sonable electric field within the P3HT:PCBM semiconductor

polymer bulk in order to observe the field effect.

We investigated the solar cell device OSC-C in the inter-

face configuration with a thin ferroelectric P(VDFþTrFE)

copolymer layer interspaced between the two thin semicon-

ductor polymer P3HT:PCBM composite layers, Fig. 1(c).

Such a structure reduces the need to have a higher P

(VDFþTrFE) copolymer thickness and at the same time ena-

bles better infiltration of the electric field across the bulk of

P3HT: PCBM composite layers on either side. Figure 8 shows

the photovoltaic characteristics under AM1.5 radiation. The

04D112-7 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-7

JVST B - Microelectronics and Nanometer Structures

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short circuit photocurrent gain and the open circuit voltage

enhancement over the base value for a control solar cell de-

vice are shown in Fig. 9. The increase in the photocurrent is

not as significant as expected; in fact it is inferior to the values

observed for solar cells with the ferroelectric P(VDFþTrFE)

copolymer layer at the interface with PEDOT:PSS. A substan-

tial photocurrent gain is only seen with higher poling voltages

at which the ferroelectric field is estimated to be

�300–500 V � lm�1. We attribute this to (i) non optimum

P3HT: PCBM composite layer morphology and (ii) charge

carrier transport limitation. In bilayer interface device struc-

ture, crystallite morphology of the P(VDFþTrFE) copolymer

layer deposited over the P3HT:PCBM composite layer is infe-

rior compared to the one formed over PEDOT:PSS. In a

�5 nm thin P(VDFþTrFE) copolymer interface layer, the

crystallites are smaller, randomly oriented and interspaced by

amorphous regions. Due to sensitivity to the substrate surface,

the nanoscale morphology of the top P3HT: PCBM composite

layer formed over underlying P(VDFþTrFE) copolymer

layer has segregated clusters (not shown here) much different

from the bottom layer in the device. Due to lack of intermix-

ing, large separation of P3HT and PCBM domains at the mi-

croscopic level are possible. This possibly nullifies the gains

due to the dipolar field in the top P3HT: PCBM composite

layer and hence produces only a smaller increase in the short

circuit photocurrent. The electric field due to dipoles in the

ferroelectric P(VDFþTrFE) copolymer film pervades on ei-

ther side into the P3HT:PCBM composite layers and, as men-

tioned above, increases CTE dissociation and free carriers

generation rates. However, the carrier transport of the free

electrons in the bottom P3HT:PCBM composite layer to the

top Al contact and that of the holes in the top P3HT:PCBM

composite layer to the bottom PEDOT:PSS/ITO contact is

impeded across the P(VDFþTrFE) copolymer layer. Thus,

lack of coupling between the bilayers inhibits the gain in pho-

tocurrent generation from translating into short-circuit photo-

current collection. We also observed a slight increase in the

fill factor accompanied by increase in the VOC values with fer-

roelectric layer poled at 21 V (Fig. 8). This increase is appa-

rently due to a combined effect of the increase in the shunt

resistance and decrease in the saturation dark current of the

trilayer solar cell device.33,35

We studied the trilayer device structure with two

P(VDFþTrFE) copolymer layers sandwiched between three

polymer semiconductor P3HT:PCBM composite layers each

of much smaller thickness �50 nm compared to 160 nm used

in solar cell devices OSC as shown in the inset of Fig. 5(b).

Instead of the photovoltaic parametric enhancement, we

find, both short-circuit photocurrent and open circuit voltage

of solar cell have degraded. A primary reason is the ineffec-

tive dipolar poling and as a result a ferroelectric field could

not be established as show in Fig. 5(b). Furthermore, the

P(VDFþTrFE) copolymer interface layers also restricted

carrier transport across the P3HT:PCBM composite layers to

contacts.

IV. CONCLUSIONS

Organic polymer BHJ solar cells show improved photo-

voltaic performance when a thin layer of poled ferroelectric

P(VDFþTrFE) copolymer film with oriented dipoles is

inserted at the interface between the PEDOT:PSS and

P3HT:PCBM composite layer. Remnant polarization due to

dipoles produces localized electric field of 300–700 V � lm�1

which enables increases the charge transfer exciton dissocia-

tion at the P3HT donor–PCBM acceptor bulk heterojunction

interfaces. This field also increases the free carrier extraction

yield by supporting the internal field due to work function

difference of the anode and cathodes. Both mechanisms

result in the increase in the short circuit photocurrent and

open circuit voltage of the interface ferroelectric solar cell

devices. The tri-layer P(VDFþTrFE) copolymer interface

device shows only a nominal improvement which is mainly

attributed to (i) the presence of free carrier which shield

FIG. 8. (Color online) Photovoltaic performance under AM1.5 radiation of a

solar cell device in structure OSC-C with the introduction of a ferroelectric

P(VDFþTrFE) copolymer film under various poling voltages. Increasing

poling voltage implies enhanced dipolar electric field at interface through

increased remnant polarization. Inset shows schematic of device structure.

FIG. 9. (Color online) Increase in the (a) open circuit voltage (VOC) and (b)

short-circuit photocurrent density (JSC) as a function of poling voltage of a

solar cell device in structure OSC-C with ferroelectric buffer layer sand-

wiched between the two P3HT:PCBM composite layers relative to solar cell

in structure OSC-A without a ferroelectric interface field.

04D112-8 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-8

J. Vac. Sci. Technol. B, Vol. 31, No. 4, Jul/Aug 2013

Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 140.254.87.149 On: Sat, 20 Dec 2014 07:00:51

dipolar field, reduces persistent polarization and effective

electric field at the interface, and (ii) nonoptimum nanoscale

morphology of P3HT:PCBM composite layer formed over

ferroelectric P(VDFþTrFE) copolymer film and acts as a

barrier for free carrier transport.

ACKNOWLEDGMENT

This work was supported by the Office of Naval Research

(ONR) under contract N00014-11-1-0658, which is grate-

fully acknowledged.

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04D112-9 Alok C. Rastogi: Interfacial polymer ferroelectric dipole induced electric field effect 04D112-9

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