Advancement in P3HT PCBM solar cells, the most efficient Polymer Photovoltaic cell

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10th

Annual Session of Students’ Chemical Engineering Congress (SCHEMCON – 2014)

Advancement in P3HT PCBM solar cells, the most

efficient Polymer Photovoltaic cell

P. Vijay*, 1

and Vedant Sumaria2.

1Department of Chemical Engineering, University of Petroleum and Energy Studies, Dehradun 248007 India

2Department of Power System Engineering, University of Petroleum and Energy Studies, Dehradun 248007 India

[email protected]

[email protected]

Abstract: Concern about global warming and

diminishing fossil fuel reserves have accelerated

searches for low cost sources of renewable energy.

Organic photovoltaic (OPV) cells are one such source.

They have couple of advantages over the conventional

semiconductors. Organic solar cells have the potential

to be low cost and efficient solar energy convertors, with

a promising energy balance. The applications of

thermoelectric polymers at low temperatures, especially

conducting polymers, have shown various advantages

such as easy and low cost of fabrication, light weight,

and flexibility. However efficiency remains quite low.

Thus efforts have been made to increase the efficiency

by varying the fabrication parameters. Poly (3-

hexythiophene) (P3HT) and 1-(3-methoxycarbonyl)

propyl-1-phenyl [6, 6] C61 (PCBM) are the most studied

polymer blend materials around the world for bulk

heterojunction structure of an organic solar cells

(OSCs). This research article is a survey on tremendous

literature published that exhibit solar cells based on

blends of P3HT and PCBM.

The basic structure of a P3HT: PCBM heterojunction

solar cell and accurate methods for measurement of the

power conversion efficiency (PCE) were also discussed.

Standard method using Air Mass 1.5 Global (1000Wm-

2, AM1.5G) solar spectrum is advised after finding

abnormalities in the PCE reported. It is noticed that

optimum thickness and area of every layer in the cell

structure is important. A detailed discussion on thermal

annealing and solvent annealing approaches to improve

device performance is presented. The effects of these

two approaches on improving polymer crystallinity,

light absorption in the polymer, carrier transport, and

blend film nano-morphology, etc. are summarized.

Polymer morphology has proven to be extremely

important in determining the optoelectronic properties

in polymer-based devices. We also investigate the effects

of polymer morphology too on the PCE of the cell.

Another important parameter affecting the efficiency

discussed is the Molecular weight ratio of P3HT and

PCBM blend active layer. Future directions and

challenges on polymer solar cell development are also

discussed

Keywords— P3HT: PCBM organic blend layer,

Polymer Photovoltaic, Organic solar cells, bulk

heterojunctions.

INTRODUCTION

With increasing energy demands, global warming

and diminishing fossil fuel reserves, it is important to

accelerate our research for a low cost source of

renewable energy. The sun deposits 173000 TW of

power onto the earth’s surface [1]

. The current power

consumption is about 13 TW and will probably

increase to 30 TW by 2050 [2]

. Conventional

photovoltaic have good power conversion efficiency

but it only contributes about 0.1% of the total

electricity [3]

. This is because of high costs of

fabrication, processing and installation of PV

systems. Thus it is important in the PV research to

produce economically viable for a better acceptance.

Today the goal in front of us is to produce PV system

with cost effective materials producing high

efficiency. After the traditional Silicon based solar

cells, there were many other alternate materials and

technologies used such as amorphous silicon (a-Si),

copper indium gallium arsenide (CIGS), Cadmium

Tellurium (CdTe) and many more [4]

.

Discovery in semiconducting (conjugated) polymers

gave a new path for research in organic electronics

and opened areas for using organic polymers in light

emitting diodes [5,6]

, thin film transistors [7-9]

, memory

devices [10,11]

, and solar cells [12,13]

. Photovoltaic cells

on organic materials are strong candidates because of

several important properties they have such as large

area, excellent mechanical flexibility, low production

costs, low specific weight, simple fabrication process

and easy tenability of chemical properties of the

organic material [12-15]

. There have been several

papers on Organic solar cells (OSCs) since their

discovery with . Among these OSCs the most

important and prominent materials in BHJs is poly(3-

hexylthiophene) and phenyl-C61-butyric acid methyl

Ester (P3HT:PCBM). It is important to know that the

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efficiency varies at a wide range from a min of 0.1%

to a max of 8.4% recently by Heliatek (a company).

Thus there is a broad scope to work on. Most of the

PCEs ranged between 3-4%. The complexity in the

organic structure is the fact that studies on P3HT:

PCBM will continue in the future till it is replaced by

another better technology at this low cost.

Fig. 1: P3HT and PCBM Structure

The Power Conversion Efficiency (PCE) of an OPV

cell can be determined by the following factors: light

absorption by the acceptor/donor (A/D) interface,

transport of excitons to the interface layer, efficiency

of the hole/exciton breaking up into electron-hole

pairs at the interface, transport of holes across the

donating layer to the anodic electrode, transport of

electrons across the acceptor layer to the cathodic

electrode, surface roughness between the donor layer

and the anodic electrode, and surface roughness

between the contact between the acceptor layer and

the cathodic electrode. Thus PCE of OPV cells can

be modified by change in the donor, acceptor,

blocking material, PV cell structure, annealing, regio-

regularity, thickness of films, and area of the films [16-

23].

Fabrication parameters such as solvent

selection, annealing time, annealing temperature and

regioregularity of the active layer are most critical

factors in the film morphology. In this paper we have

focused on the recent advances in morphology

control which is the key parameter for a high PCE.

We also discuss the accurate methodology for finding

the power conversion efficiency of an OSC.

I. STRUCTURE OF A DONOR/ ACCEPTOR (P3HT:

PCBM) HETEROJUNCTION SOLAR CELLS

Fig. 2: Small-molecule donor–acceptor heterojunction solar cell

(PEDOT: poly (3, 4-ethylenedioxythiophene); PSS: poly (styrene

sulfonic acid); ITO: indium tin oxide).

Donor Acceptor layer is the P3HT: PCBM layer with

a weight ratio of 1:1. It is usually made in a solvent

such as 1,2-dichlorobenzene (DCB) and

chlorobenzene (CB). Most devices are built on

Indium Tin oxide (ITO) on glass sheets. ITO is

chosen as it is transparent and has a good electrical

conductivity. ITO resistivity is between 10-20 Ωm-2.

Interface layer uses usually PEDOT: PSS on the ITO

glass. This acts the hole ejection layer. 2, 9-

dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is

also sometimes deposited by thermal evaporation as

a hole/exciton blocking layer on the P3HT:PCBM

blended layer under a fixed pressure for a fixed

thickness. After every deposition the cell is backed in

a glove box at different temperature and for different

time interval. Thus Pressure, temperature and

material, all are important to be précised in

fabrication.

II. ACCURATE MEASUREMENTS

Recording accurate PCE is very important and care

should be taken of that the efficiencies reported are

close to the theoretical limits of solar cells dictated by

physics [24, 25]

and in some cases even the

thermodynamic limits. There are standards which

have to be accepted. As a result of continuing

research efforts, the efficiencies of organic solar cells

are now fast approaching the levels where they could

be put into commercial applications. Significant

efforts have been made in the past to accurately

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determine the efficiency of solar cells, and a standard

test method has been established [26-28]

. One of the

best works on standardizing the procedure was put in

by Prof. Y. Yang and coworkers. Some of the

important issues with respect to OSC devices were

discussed, such as spectral responsivity and its

behavior with light-bias intensity, dependence of the

device parameters on the incident-light intensity, and

calculation and application of spectral-mismatch

factor for efficiency correction [29]

. The note

“Reporting Solar-Cell Efficiency” published in 2008

in “Solar Energy Materials and Solar Cells” was

important which stated one of the standard method

using Air Mass 1.5 Global (1000Wm-2

, AM1.5G)

solar spectrum [30-32]

. It is advised that if the power

conversion efficiencies exceed 5%, journals usually

requires the measurement be traceable to an ISO/IEC

17025 certified laboratory specializing in solar cell

characterization (ISO is the International

Organization for Standardization) [33]

. Such labs

include, but are not limited to, the National

Renewable Energy Laboratory (NREL), Japanese

National Institute of Advanced Industrial Science and

Technology (AIST), Fraunhofer Institute für Solare

Energie Systeme (Fraunhofer- ISE), or Energy

Research Center of the Netherlands (ECN). Solar

power conversion efficiency is given by the product

of current density (J), open circuit voltage (Voc), and

fill factor (FF) divided by the incoming light’s power

density. The thermodynamic limit for a single

bandgap device under one sun at AM1.5 is

approximately 33%. We would also like the

researchers to keep in mind that the most common

error in efficiency measurements stems from

improper calibration of the solar simulator.

Researchers should consult the accepted standards

defined for solar simulators [34, 35]

.

III. EFFECT OF INTERFACE AND ACTIVE LAYER

THICKNESS AND AREA

The effects of interface and active layer have not

studied been sufficiently but it is important factor for

a higher PCE of the organic photovoltaic cells. Su-

Huwan Lee and coworkers studied the effect of the

interface layer, PEDOT: PSS and LIF, to improve the

PCE of the OPV cell. OPV devices were fabricated

with a sandwiched structure of varying E SS

and iF layer thic ness CE pea ed at a E

SS thic ness of 2000 rpm improving the CE by

about 41.5% with a maximum PCE of 6.648% (Refer

Fig.3). In addition to that the PCE peaked at a LiF

layer thic ness of 0 nm improving the CE by

209.2% (Refer Fig.4)[36]

.

Fig. 2: Variation of PCE and Jsc with interface layer thickness

(here, PEDOT: PSS) [36].

Fig. 3: Variation of PCE and Jsc with interface layer thickness

(here, LiF) [36].

Further R.H.Friend and coworkers investigated by

variation of PEDOT: PSS layer thickness between 25

and 150 nm, finding the optimum device

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performance at 70nm [37]

. It is also important to note

that the surface topology became smoother at this

thic ness When researched we don’t find much

papers on this matter, thus encourage authors to work

on this field. Vishal. S et al [38]

described that that

there is no effect of thickness on the open circuit

voltage unlike Brabec et al [39]

which instead had

shown a variation. They found a variation of the

work function of the anode linearly influences the

VOC, with a scaling factor of 0.8. The authors

suggest that the work function of PEDOT is not

pinned to any of the energy levels in the active layer,

hence, the variation in VOC. It was previously

reported in [46, 47]

that the work function of the

cathode metal slightly affects the Voc of the PV cell.

Now we will discuss about the

effects of active layer on the PCE. Gang Li and

coworkers in 2005 demonstrated this by changing

active layer thickness and measuring the I-V curves.

It is found that the open circuit voltage remain

constant on varying the thickness of active layer

however the short circuit current and the power

conversion efficiency vary significantly. Highest

PCE of 4% was found for a thickness of 63nm [40]

. It

is uncommon for such small thickness of the active

layer as it goes up to 350 nm for better absorption

and thus short circuit current Jsc of OPV cells without

losing much in terms of bulk recombination. But

even thin film active layer leads to efficient cells due

good absorption coefficient and leads to favoring

percolation paths and enhanced charge transport [41,

42]. MD. S. Islam et al also demonstrated that there

was a significant rise in the absorption of light as the

thickness of the active layer increases [43]

. Similar

results were report by Moule [44]

and Kim [45]

.

Fig. 4: Short circuit current density and power conversion

efficiency as a function of active layer thickness [40].

Fig. 5: P L Quenching showing how absorption increases with

increase in the layer thickness of P3HT: PCBM [43].

Not only thickness of the films but are also affects

the efficiency of an organic cell. Mainly the area is

ranges from 5 to 15 mm2. However we find that

negligible work has been carried out in optimizing

the geometry, pixel dimensions and distribution. In

2007 D L Carroll and coworkers fabricated a small

active area (< 10 mm2), defined by the overlap of the

bottom ITO anode and top cathode, has been reported

to give rise a high Jsc values [48]

. Dhritman Gupta in

2008 published a paper especially on area dependent

efficiency of organic solar cells [49]

. The active area

ranged from 0.01 – 1 cm2. Measurements were done

on varying active layer area and varying illumination

values. In all the cases there was a drop in the Jsc for

flooded illumination. Under the restricted

illumination, the J-V characteristics for different area

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devices are identical. Further Hisashi Kitami and his

coworkers show Jsc was decreased depending on the

active area, whereas VOC and FF are not affected [50]

.

S.Y. Park also demonstrated effects of cell size in

OSCs by introduction of metal subelectrodes [51]

. It is

demonstrated that the series resistance could be

reduced significantly by inserting metal subelectrodes

onto the ITO anode, yielding a PCE of 2.60.3% up to

the cell area of 4.08 cm2. Dang Gao too worked on

area dependency and used device sizes of 0.07, 0.20

and 0.50 cm2 [52]

. Table 1 shows us the results found

from which we conclude that there is no major

change in the open circuit voltage and fill factor on

variation of active area but PCE decreased with

increase in area.

IV. TABLE 1: SUMMARY OF FULLY ILLUMINATED

DEVICE CHARACTERISTICS. ALL DEVICES WERE

FABRICATED ON THE SAME SUBSTRATE [52].

Device

Area

(cm2)

Rs

(Ωcm2)

JSC

(mA/

cm2)

VOC

(V)

FF

(%)

PCE

(%)

Illuminati

on

Intensity

(mA/cm2)

0.07 2.86 8.20 0.58 65.92 3.14 100

0.20 6.27 8.45 0.58 62.00 3.04

0.50 10.76 7.42 0.58 59.31 2.55 90

Fig. 6: Variation in Jsc and Voc with different active areas for a

P3HTPCBM solar cell under flooded white light illumination of intensity80 mW/cm2. Devices were fabricated on two different

substrates with pixel sizes of 0.01, 0.04, and 0.09 cm2 (substrate

1). The schematic diagram of the different active area devices is depicted in the inset [49].

Fig. 7: The J-V characteristics of 0.25 cm2 device under flooded

and restricted collimated illumination using a calibrated 50

mW/cm2 Xe source. The device layout has been depicted in the inset [49].

V. EFFECT OF ANNEALING

Thermal annealing can play an important role in

improving the optoelectronic properties of a polymer

blend. Annealing processes in polymer solar cells can

be divided into two categories: thermal annealing

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[53, 54] and solvent annealing

[55- 57]. Thermal annealing

can be applied either on the final device (post-

annealing) or on the polymer film only (pre-

annealing). Annealing temperature and time are the

most critical parameters in this approach. Berggren et

al. showed that thermal annealing can enhance

polymer crystallinity [58]

. Thermal annealing enables

the P3HT and PCBM nano domains sizes to be

increased up to optimized values corresponding to the

exciton diffusion lengths in each material, which

have recently been evaluated to be as low as 3 nm

and 30 nm in P3HT and PCBM, respectively [59]

.

Franz Padinger in 2003 treated P3HT: PCBM solar

cells post production with an applied external

potential higher than the open circuit voltage and

annealing temperature higher than the glass transition

temperature leading to an improved efficiency. Using

a temperature of 75 °C the optimum time for

annealing was found to be 5-6 minutes [60]

. Li et al in

2005 used AFM to show the active layer roughness

after annealing at different temperatures for 10

minutes. The surface becomes rougher as the

annealing temperature increases from 70°C to 110°C,

but on further annealing at 150°C the roughness

decreases. The broad hill like structures in the figure

9c shows that it is most rough [54]

. On further

investigation they find that the optimum condition of

annealing for best power conversion efficiency (here

4%) was 110°C for 10 minutes post production. This

is due to the fact that higher roughness at the

interface results in the presence of more defect sites

that can increase the exciton decomposition at the

interface, thereby increasing the charge generation [54]

.

Fig. 8: AFM height images of the surface of the active layer

consisting of P3HT:PCBM in (1:1 weight ratio) a) before annealing, and after annealing at b)70 °C, c)110 °C, and d)150 °C.

The annealing time for all the films was 10 min. The P3HT:

PCBM films were spin coated on top of PEDOT: PSS-coated ITO glass [54].

W. Ma et al used transmission electron microscopy

(TEM) images to study the interpenetrating networks.

It was observed that there is no distinct visibility of

the networks without annealing whereas on annealing

for 30 mins and 2hrs the morphology of the

interpenetrating Donor/Acceptor networks became

clearer and easily visible with typical feature of size

of 10nm. XRD results were also taken which told us

that annealing improved the crystallinity within the

face separated networks and thereby facilitates charge

transport to the electrode [53]

. The importance of

aluminum interface in annealing process that leads to

high efficiency was also shown.

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Figure 9: TEM images of P3HT: PCBM film bulk morphology

before annealing a), after annealing at 150°C for 30 minutes

(b), and annealing at 150°C for 2 hrs. (c) [53].

Fig. 11: a) XRD spectra of P3HT: PCBM film drop-cast onto PEDOT/ ITO substrate with and without thermal annealing at

150°C for 30 minutes. The inset shows P3HT crystal structure. b)

XRD spectra obtained from complete solar cells (with Al electrode) before and after annealing [54].

In 2007 G. Li studied the effects of solvent annealing

and it was found that solvent annealing approach is

more pronounced at higher PCBM loadings. The

variation in VOC was partially explained by the longer

conjugation length and formation of a band structure

in well-organized RR-P3HT domains in solvent

annealed solar cells [56]

. Increase in crystallinity was

also supported by reports in literature [61-63]

. The

effect of annealing on the absorption profile is

characterized with UV–vis spectroscopy. Two

phenomena are generally observed as a result of

thermal annealing. Both a red-shift in peak

absorption and an increase in magnitude of

absorption are observed for a P3HT: PCBM blend [64]

. An absorption profile of an as-cast and annealed

P3HT: PCBM is displayed in Fig. 12.

Ma et al [53]

have undertaken a comprehensive study

of the process parameters of annealing temperature

and time for a P3HT: PCBM system. A comparative

study of J–V curves of devices with treated with

different annealing temperature is displayed in Fig.

13. From the curve we understand that on moderate

temperature annealing at 70°C leads to an

improvement in both the fill factor and JSC and it is

believed that the crystallinity of the active layer blend

improves. Thermal annealing at 150°C brought about

more improvement in the two device characteristics.

The Jsc increases to a value of 9.5mA/cm2, whilst the

fill factor increases from 30% in the as-cast sample to

68% in the sample annealed at 150°C. These

parameter changes concluded the reduction in series

resistance of the device [53]

. Yang et al in 2005

investigated the effect of annealing on the external

quantum efficiency and it was found that EQE

response for the annealed sample is better for a wide

spectral range [65]

. Kim et al [66]

investigated the effect

of the annealing sequence on both the film and

electronic characteristics of P3HT: PCBM solar cells.

Post annealing step was deemed to increase the

Fig. 10: UV–vis spectra for both pure P3HT and P3HT: PCBM blend films. Data for as cast (red) and thermally annealed at 1501C for 15 min (blue) [64].

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interfacial area between the aluminum cathode and

the active layer. This improved contact lead to more

efficient charge collection. Additionally, a slight

increase in absorption was measured for the post

annealed device, which was due to an increase in

interfacial roughness between the active layer and

aluminum. The performance of the post-annealed was

better than both the untreated and pre-annealed

device, which suggests that the sequence of the

processing step is important, and that device post

annealing is crucial in obtaining efficient charge

carrier collection.

Fig. 12: Current–voltage characteristic curves of P3HT: PCBM solar cells with different treatments. Data is shown for three varying heat

treatments of (i) no thermal annealing (open squares), (ii) thermal annealing at 701C (open triangles) and (iii) thermal annealing at 150°C (filled

squares). The illumination intensity used was 80 mW/cm2 [53].

Fig. 13: EQE profile for pristine (blue) and thermally annealed at 120C for 60 min. (red) P3HT: PCBM devices [65].

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Fig. 14: Current–voltage characteristic curves for (i) untreated (squares), (ii) pre-annealed (circles), (iii) pre- and post-annealed (triangles up) and

(iv) post-annealed [66].

J Jong and his coworkers later in 2009 worked on time

dependent morphology evolution by annealing process on

polymer: fullerene blend solar cells [67]

. Morphological

changes in P3HT: PCBM were evaluated under different

annealing conditions over a time scale relevant for PCBM

diffusion and aggregation. High-temperature thermal

annealing (150°C) was a strong condition, and room-

temperature solvent annealing was a relatively mild

condition. Here solvent annealing resulted in a more

favorable BHJ morphology than thermal annealing.

However, additional thermal annealing of the blend films

after the formation of the well-ordered morphology by an

optimized solvent annealing showed a relatively stable

morphology, an observation confirmed by TEM images,

selected-area electron diffraction (SAED) patterns, and

device performances. C-W Chu et al. also used annealing

for increasing the efficiency of the solar cells [68]

. There

was an enhancement of the JSC and FF of the solvent

annealed device with thermal treatment compared to the

fast-growth device without thermal treatment which is

attributed to increase absorption in red region and

improved charge transfer properties. In this

paper we see the use of grazing-incidence

x-ray diffraction (GIXRD) which

highlights that controlled film processing

can tune polymer nanostructure in RR-

P3HT/ fullerene blend films. In 2007 we

had an interesting change with the

annealing process. C-J Ko used microwave

annealing for Polymer photovoltaic [69]

. It was inferred

microwave irradiation could penetrate through the

glass/ITO/PEDOT: PSS layers and be absorbed directly by

the active layer during microwave annealing. No heating of

the microwave occurs at the anode and only heats the

active layer and cathode selectively. Thus microwave

irradiation is a selective heating (annealing) method for

improving the efficiency of OPV devices.

Solvent vapor annealing (SVA) can be

effectively used as a post processing technique to

manipulate the morphology of polymer- fullerene [69-74]

.

Thermal annealing can lead to degradation of conjugate

polymers and flexible substrates that are not stable at high

temperatures, and it can result in coarsening of the nano

scale phase segregation, which is unfavorable for exciton

diffusion to the BHJ interface, leading to exciton

recombination and lower power conversion efficiency [75,

76]. Eric Verploegen et al in 2012 used SVA for fabrication

of or high PCE polymer- fullerene solar cells [77]

. Using

grazing incidence X- ray scattering (GIXS) for morphology

Fig 15 GISAXS of 1 1 3H − CBM blend thin films spin-cast from 10 mg/mL in chloroform

after different annealing conditions [77].

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investigation. BHJ thin films before, during, and after

exposure to the solvent vapor were checked. Grazing

incidence small-angle X-ray scattering (GISAXS) images

shows us that SVA results in smaller phase segregation

domains (approx. 52 Å) than thermal annealing at

temperatures above 140°C (>130 Å), which is more

favorable for exciton dissociation. Using some solvents

SVA induces crystallinity in the PCBM. Vapor annealing

both results in a swelling of the P3HT layers, a decrease in

the π−π stac ing distance an increase in the crystalline

order within the P3HT component, and phase segregation

of P3HT and PCBM domains. Grazing incidence wide-

angle X- ray scattering (GIWAXS) measurements were

also done.

Fig. 16: 2-D GIWAXS images of 1:1 blend thin films spin-cast from 10

mg/mL in chloroform [77].

VI. EFFECT OF REGIOREGULARITY AND MOLECULAR

WEIGHT

We know that the highest efficiencies have been achieved

using blends of P3HT and fullerene derivative such as

PCBM, but it depends critically on the material properties

and processing condition. This can be done by the self-

organizing properties of P3HT, which means that both

optical [78, 79]

and electronic [80, 81]

properties are sensitive to

the molecular packing. It is seen that there is a strong

influence of polymer regioregularity (RR) on the molecular

nanostructure, and hence on the resulting material

properties and device performance. Intraplane stacking is

found to be unaffected by this variation in RR, the

interchain spacing perpendicular to the ring plane (d b-b) is

found to increase in the lower RR case, indicating that

higher RR should indeed increase interchain interactions [82]

. n increasing RR the absorption coefficient (α) also

increases. RR tends to increase photoluminescence

intensity, suggesting a reduction in non-radiative quenching

pathways in the more ordered material. Annealing of the

highest RR films invokes a slight reduction in

photoluminescence intensity. In [82] GIXRD, J-V curve,

and TEM tests were performed to anticipate that the highest

device efficiencies will be achieved with the highest RR

P3HT. They achieved PCE of 4.4% with a EQE of 68- 73%

using P3HT polymer with the highest available RR (95.4%)

after optimizing film thickness of 175nm and device

annealing conditions. R. Mauer et al. [83]

also worked on

effects of regioregularity effects. As similar to previous

results P3HT with very high regioregularity exhibited a

well ordered solid state structure as indicated by the

pronounced vibronic features of the absorption spectrum

and the further red- shifted absorption compared to P3HT

with medium regularity. Apparently it was found that solar

cells prepared from both regioregular polymers in

combination with PCBM showed comparable PCE of about

2.5% without a full empirical optimization of the

processing parameters, thus proving that increasing order

and charge carrier mobility in P3HT films by using very

high regioregular polymers does not significantly help to

improve the device efficiency.

Another important parameter which strongly

affects the device performance is the molecular weight

(MW) of the P3HT polymer. High and medium

regioregular P3HT not only varies in regularity but also

molecular weight, with values for the weight averaged

molecular weight Mw of approximately 60000 gmol-1

for

medium regioregular P3HT and 25000 gmol-1

for high

regioregular P3HT. Ballantyne et al. [84]

studied the

influence of molecular weight on charge transport in P3HT

by the TOF (Time of Flight) technique and found that upon

changing the molecular weight from 18000 g mol-1

to

56000 g mol-1

the hole mobility decreased by roughly one

order of magnitude from 2 x 10-4

to 3 x 10-5

cm2V

-1s

-1 at

room temperature. While Ballantyne et al., [84]

Baumann et

al., [85]

and Yang et al. [86]

found almost no difference in

hole nobilities of pristine P3HT and blends with PCBM

over a large range of molecular weights and P3HT: PCBM

blending ratios, Mihailetchi et al. [87]

observed a reduction

in hole mobility by more than three orders of magnitude

when they blended a P3HT of similar high regioregularity

with PCBM. In their study Mihailetchi et al. [87]

reported an

increase in mobility along with a red-shift and decreasing

vibronic line-width of the absorption spectra of the blends,

resulting in an absorption spectrum that, except for the

PCBM contribution, is almost identical to that of pristine

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P3HT. Still due to the discrepancy in PCE, the effect of the

molecular weight on performance is not clearly displayed.

Weight ratio between the active layer

components, P3HT and PCBM, is also an important

parameter affecting the PCE of a cell. The P3HT: PCBM

power-conversion efficiency is governed by phase

separation between the two materials in the solid state. The

phase diagram clearly shows how the morphology is

affected by the composition ratio [88, 89]

. Anyhow, according

to numerous studies on ratio effects, it is generally advised

that P3HT: PCBM be used in a weight ratio between 1:0.8

and 1:1 (i.e., with a small excess of P3HT).

VII. RECENT PROGRESS IN HETEROJUNCTION P3HT:

PCBM SOLAR CELLS

When we look at the last two years, there has been a lot of

work on polymer blends of P3HT and PCBM solar cells.

There are many publications aimed at getting a better PCE

value with different various parameter changes. J. Krantz

investigated silver nanowire (Ag NW) as top electrodes in

semitransparent inverted organic solar cells. Performance

of P3HT: PCBM solar cells with Ag NW top electrode

layers is found to match very closely the performance of

reference devices based thermally evaporated, highly

reflective metal silver top electrodes. Overall, Ag NW top

electrodes are established as a serious alternate to TCO

base electrodes. Semitransparent devise with efficiencies of

over 2% are realized [90]

. PCE of P3HT: PCBM bulk

heterojunction organic solar cells were increased through

the addition of small amounts (1-10%) of metalloporphrin-

oligothiophene conjugates. Efficiency was enhanced up to

17% for the thiophene substituted Cu(II)- porphyrin,

pointing to an important effect of the metal center.

Effectiveness of porphyrins increased in the order free-

base Co< Zn< Ni< Cu [91]

eter Kohn’s work show that

crystallization which is intrinsic to most semi crystalline

polymers, followed by the enrichment, segregation, and

crystallization of PCBM provides an interdigitated

structure, which is conceptually ideal for excitonic solar

cells [92]

. D. Spoltore had shown us in his work, the mass

fraction ‘f’ of highly crystalline nanofibrillar 3H to the

total P3HT content in P3HT: PCBM solar cells are

consistently varied, controlling the temperature of Nano

fibers- P3HT casting dispersion. A systematic study of the

apparent recombination order, measured with a transient

photo voltage technique as a function of ‘f’ was presented

and a correlation order of the P3HT crystalline, and the trap

concentration in the band gap measured with an admittance

spectroscopy technique [93]

. C. Koidis et al. worked on to

study the effect of R2R printing and drying process on the

nanomorphology and nanostructure of the photoactive

P3HT: PCBM thin films printed on PEDOT: PSS

electrodes towards the fabrication of ITO free flexible

OPVs. Spectroscopic ellipsometry, atomic force

microscopy analysis, x- ray diffraction tests were done [94]

.

S. Ulum, Natalie Holmes and coworkers proved directly

the structural motif of P3HT: PCBM nano particulate

organic photovoltaic (NO- OPV) devices and had shown

how NP morphology determines device function. PCE of

1.3% was obtained. Nanoparticle structure proved with

synchrotron x- ray microscopy revealed a core- shell

structure with a PCBM rich core [95]

. Control over the

structure of donor/ acceptor blends is essential for the

development of solution processable OSCs. K H Lee and

coworkers used time-resolved Neutron Reflectometry (NR)

and in-situ annealing to investigate the nanoscale structure

and inter-diffusion of sequential spin-coated thin films of

P3HT: PCBM and correlated the evolving structure with

the device performance. Despite the complete intermixing

of PCBM and P3HT to form a structure resembling a bulk

heterojunction, essentially there was no improvement on

annealing at a temperature of 110°C [96]

.

S Khelifi et al. worked on the effect of light induced

degradation on electrical transport and charge extraction in

P3HT: PCBM OSCs. They gained a detailed insight into

the gradual degradation of P3HT: PCBM aged under

different illumination conditions. They applied numerous

advanced electrical characterization techniques and a

modeling software tool (SCAPS) to explain the failure

mechanism. TEM revealed a change in blend morphology

upon long illumination times manifested by phase

segregation in the blend [97]

. Kurniawan Foe and coworkers

fabricated P3HT and PCBM OPVs using TiOX interfacial

layer. It was found that TiOX layer coated at a spin speed of

3000 rpm improved the photon absorption of the active

layer. After 70 days of storage the JSC dropped by 16.2%,

FF dropped by 10.6% and PC by 25% whereas VOC

remained relatively stable [98]

. High precision production

process of a flexible P3HT: PCBM OPV modules with a

PCE above 3% and FF of 95% were fabricated by P. Kubis

et al. They combined the slot-die coating and high

resolution laser patterning. They even demonstrated

flexible modules showing identical performance to

optimized small scale lab devices. Devices withstood 1000s

of bending cycles [99]

. We also find an extensive review on

annealing effect on PCE of bulk heterojunction organic

S C H E M C O N - 2 0 1 4 12 | P a g e

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solar cells. It focused on changes in the physical film

properties, the electronic performance of devices, as well as

the link between film structure and electronic performance

during a thermal annealing process [100]

.

VIII. CONCLUSION AND SCOPE OF STUDY

In summary, bulk heterojunction organic solar cells with

P3HT and PCBM polymer blend as active layer are still

quite promising. A major advantage is that their

morphology are relatively easy to optimize. Meanwhile,

flexibility, large area, light-weight, and the ability to

self-repair are all attractions for research. Changes in the

important fabrication parameters have brought about a

large difference since the invention. Thus there is a large

field to work on. The largest challenge remains is the low

power conversion efficiency mainly due to poor charge

mobility. Utilization of whole spectrum and a more

efficient use of UV energy are also possible points for

breakthrough.

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