chapter 4 effects of active layer thickness and thermal annealing on p3ht-pcbm photovoltaic cells

17 Pages·2010·0.29 MB·English

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CHAPTER 4
EFFECTS OF ACTIVE LAYER THICKNESS AND
THERMAL ANNEALING ON P3HT-PCBM
PHOTOVOLTAIC CELLS
4.1 INTRODUCTION
With a steady improvement in energy conversion efficiency during the past
decades, organic photovoltaics (OPV) has evolved into a promising technology for
renewable energy made possible by the first report of planar donor-acceptor
heterojunction [1] where efficient exciton dissociation takes place. The device
efficiency got another boost with the introduction of bulk heterojunction OPV (BHJ-
OPV) [2] comprising blends of donor and acceptor materials that phase-separate into
nanoscale interpenetrating networks to dramatically increase the donor-acceptor
interface per unit volume. The most promising BHJ-OPV devices to date consist of
conjugated polymers, such as poly(3-hexylthiophene) (P3HT), blended with soluble
fullerene derivatives, such as [6,6]-phenyl C butyric acid methyl ester (PCBM).
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Both thermal annealing [3, 4] and solvent-vapor annealing [5, 6] have been performed to
optimize phase separation between P3HT and PCBM and to induce crystallization in
the P3HT domain with an objective of improving device efficiency. In addition,
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thermal annealing of completed devices has been reported to improve charge
collection efficiency by promoting active layer/electrode interaction [4]. The active
layer thickness has been limited largely to 100-200 nm, although a thicker active
layer should absorb more incident light to be more efficient in power conversion.
Thick-film polymer BHJ-OPV devices, however, have been found to be less efficient
than thin-film devices because of inadequate mobility for supporting charge transport
to electrodes for collection.[7, 8] There have been no systematic studies of how
chemical purity of conjugated polymers affects device efficiency. Using a copper
phthalocyanine:C active layer, Hiramoto et al. have reported the effects of chemical
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purity via repeated sublimations and of active layer thickness up to microns on BHJ-
OPV device efficiency.[9]
In this study we have examined the thickness-dependent device characteristics
of BHJ-OPV with a P3HT:PCBM layer thickness from 130 to 1200 nm. In particular,
it will be demonstrated that thick-film devices can be made as efficient as their thin-
film counterparts through thermal annealing. Furthermore, the observed OPV device
characteristics can be accounted for by vertical phase separation in the active layer
and Li+ diffusion from the Al/LiF cathode into the neighboring active layer.
4.2 EXPERIMENTAL
Device Fabrication
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Pre-patterned indium tin oxide (ITO) substrates (1.5 × 1.5 inch) with surface
resistance of 15 Ω / square were cleaned and treated with oxygen plasma prior to the
deposition of a thin MoO layer by thermal sublimation at 0.03 nm/s. Regioregular
3
P3HT (Rieke Metal) and PCBM (Nano-C) at a 1:1 mass ratio were dissolved in o-
dichlorobenzene by stirring at 50 oC under nitrogen atmosphere overnight. The active
layers were spin-coated from 0.2 g of the solution onto the MoO -coated ITO
3
substrates at 300 rpm for 10 s plus 600 rpm for 60 s, and kept in a covered petri dish
for solvent vapor annealing for 3 h to create bulk heterojunctions [5]. The solution
concentration was varied to adjust the film thickness measured with an optical
interferometer (Zygo NewView 5000). The devices were completed by successive
deposition of LiF and Al at 0.01 and 1 nm/s, respectively, through a shadow mask to
define an active area of 0.1 cm2. The finished device has the structure: ITO/MoO (3
3
nm)/P3HT:PCBM(x nm)/LiF(0.8 nm)/Al(100 nm), where x is varied from 130 to
1200 nm. Incremental thermal annealing was conducted by placing the tested device
on a hotplate set at 110oC for a pre-determined period of time before quenching to
room temperature on an aluminum block for the next round of measurement. Spin
coating and thermal annealing were performed in a nitrogen-filled glove box with a
oxygen level less than 1ppm, and the thermal evaporation was performed at a base
pressure less than 4 × 10−6 torr.
Device Characterization
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Devices were characterized in ambient atmosphere without encapsulation. The
current density (J) – voltage (V) characteristics were recorded with a Keithley 2400
under the illumination of white light at 100 mW/cm2. The white light was generated
using an Oriel xenon lamp solar simulator equipped with an AM 1.5 G color filter,
and the intensity is calibrated with a silicon diode traceable to National Renewable
Energy Laboratory. Defined as the ratio of maximum attainable electrical power to
the incident light power (P ), the power conversion efficiency (η) was calculated as
in
J V FF/P , where J , V and FF are short circuit current density, open circuit
SC OC in SC OC
voltage, and fill factor, respectively. The reported η values are accompanied by an
error of ±5%. For the measurement of spectral response, a beam of white light is
passed through a monochromator before being focused onto the devices with
photocurrent recorded by the Keithley 2400. The monochromatic photocurrent was
recorded at a step size of 10 nm. The external quantum efficiency (η ) is calculated
EQE
as the ratio of collected electrons to incident photon with a typical error of ±5%. For
the measurement of spectral response under applied field, the dark current under the
same applied field was subtracted from the recorded photocurrent.
4.3 RESULTS AND DISCUSSION
Depicted in Figure 4.1 is the schematic diagram of device architecture together
with the chemical structures of P3HT and PCBM. Recently, there has been a
growing interest in adding a metal oxide layer on the ITO anode, such as MoO , [10]
3
to improve hole-collection efficiency. The bulk heterojunction between P3HT and
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PCBM were created by solvent vapor annealing. [5] To facilitate electron collection, a
thin LiF layer was inserted between the active layer and the Al cathode. [11]
ee−−
AAll ((110000 nnmm))
LLiiFF((00..88 nnmm))
PP33HHTT::PPCCBBMM ((xx nnmm))
MMooOO ((33 nnmm)) hh++
33
IITTOO
GGllaassss PP33HHTT PPCCBBMM

Figure 4.1. Schematic diagram of the BHJ-OPV device architecture and chemical
structures of P3HT and PCBM.

2
15 J, mA/cm
Unannealed
10
Annealed
5
−1.0 −0.5 0 0.5 1.0
V, V
−5
−10

Figure 4.2. J-V characteristics under 100 mW/cm2 light illumination before and after
thermal annealing at 110oC for 20 min of 130-nm BHJ-OPV devices without the LiF
layer to improve electron collection-efficiency over Al cathode alone.
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As the baseline experiment, a BHJ-OPV device with a 130-nm active layer but
without a LiF layer was fabricated and characterized to yield the J-V relationship as
displayed in Figure 4.2. It was found that η = 1.3 and 0.9% before and after thermal
annealing at 110oC for 20 min, respectively, both inferior to η = 2.9 and 1.1% from
devices with a LiF layer processed under the same conditions as to be shown below.
Therefore, a LiF layer was consistently adopted in all the BHJ-OPV devices in the
present study.

20 J, mA/cm2
130 nm
220 nm 15
370 nm
540 nm 10
830 nm
5
1200 nm
−1.0 −0.5 0 0.5 1.0
V, V
−5
−10

Figure 4.3: J-V characteristics under 100 mW/cm2 light illumination of BHJ-OPV
devices with varying active layer thicknesses before thermal annealing.
A series of OPV devices with an active layer thickness varied from 130 to 1200
nm were fabricated and characterized. Their J-V curves before thermal annealing are
plotted in Figure 4.3, and the relevant performance parameters are compiled in Table
4.1. Note that all the devices exhibit a weak dependence of photocurrent on the
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applied field with virtually the same FF values around 0.62. As the film thickness
increases from 130 to 1200 nm, J decreases sharply from 8.4 to 1.3 mA/cm2
SC
accompanied by a V decreasing from 0.58 to 0.50 V. The weak dependence of
OC
photocurrent on applied field for thick-film device precludes recombination as a
possible cause for the low J . [12]
SC

55 00..88
((aa)) ((bb))
44 113300 nnmm 113300 nnmm
00..66 222200 nnmm
222200 nnmm
ee
cc 337700 nnmm
nn33 337700 nnmm
aa EE 554400 nnmm
sorbsorb22 585843430000 nnnnmmmm ηηEQEQ00..44 1 1 228800330000 nnnnmmmm
AbAb 11220000 nnmm 11220000 nnmm
00..22
11
00 00
440000 550000 660000 770000 880000 440000 550000 660000 770000 880000
WWaavveelleennggtthh,, nnmm WWaavveelleennggtthh,, nnmm

Figure 4.4: (a) UV-vis absorption spectra of P3HT:PCBM blend films with varying
active layer thicknesses before thermal annealing, and (b) Spectral responses from
BHJ-OPV devices with varying active layer thicknesses before thermal annealing; the
dotted curve represents the spectral response from a 1200-nm device under
illumination through the semitransparent cathode.
The absorption spectra of P3HT:PCBM films with varied thicknesses are presented
in Figure 4.4a. All the films show a main peak at 515 nm with two shoulders at 560
and 610 nm, which originates from the highly crystalline P3HT domains with
improved stacking. [4, 5] As the film thickness reaches 830 nm, essentially all incident
light between 350 and 610 nm is absorbed in one optical path. The spectral responses
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of the devices under short circuit conditions are presented in Figure 4.4b. The η
EQE
spectrum of the 130-nm device tracks the P3HT:PCBM absorption spectrum quite
well as expected.

Table 4.1: Performance parameters of BHJ-OPV devices with varying active layer
thicknesses before and after thermal annealing at 110oC for 20 min.
UUnnaannnneeaalleedd AAnnnneeaalleedd
TThhiicckknneessss
JJ VV FFFF ηη JJ VV FFFF ηη
((nnmm)) SSCC OOCC SSCC OOCC
((mmAA//ccmm22)) ((VV)) ((%%)) ((mmAA//ccmm22)) ((VV)) ((%%))
113300 88..22 00..5588 00..6611 22..99 44..44 00..5577 00..4422 11..11
222200 77..66 00..5588 00..6611 22..77 55..33 00..6600 00..5544 11..77
337700 66..44 00..5588 00..6622 22..33 66..77 00..6611 00..5577 22..44
554400 33..99 00..5555 00..6611 11..33 88..77 00..6611 00..5533 22..88
883300 11..66 00..5522 00..6655 00..55 1111..33 00..6611 00..4455 33..11
11220000 11..33 00..5500 00..6644 00..44 99..22 00..6611 00..4400 22..33

As the film thickness increases from 130 nm, the relative η around
EQE
absorption peak decreases and eventually dwindles to a valley for film thickness
beyond 540 nm with the maximum η shifted to the absorption edge. This inverse
EQE
spectral response indicates that photons that are more strongly absorbed by P3HT are
less likely to be converted into collected charge carriers, as observed in planar
heterojunction devices with a layer thickness much greater than exciton diffusion
length. [13] The apparent filtering effect arises from excitons created in the
neighborhood of ITO electrode have limited donor-acceptor interface within their
diffusion length to dissociate efficiently. We propose that vertical phase separation in
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a thick film yield a separated P3HT layer on ITO as inspired by numerous recent
reports. [14-16] This P3HT layer absorbs most of incident photons to produce excitons
that fail to diffuse sufficiently deep into the region rich in P3HT:PCBM interfaces
for dissociation into charge carriers, resulting in a low photocurrent density. On the
other hand, photons with wavelength in the weakly absorbing region can penetrate
deeper into the active layer where more abundant P3HT:PCBM interfaces are
available for efficient exciton dissociation. Meanwhile, the resultant holes and
electrons are transported in spatially separated P3HT and PCBM domains,
respectively, without encountering significant recombination, thereby achieving FF
values in thick-film devices comparable to those in thin-film devices. Furthermore,
the reduced exciton dissociation efficiency in vertically separated films is also
responsible for the inferior V in thick-film devices.[17] Overall, η decreases steadily
OC
from 2.9% for the 130-nm device to 0.4% for the 1200-nm device before thermal
annealing. To confirm the presence of a separated P3HT layer on ITO as an
explanation for the inverse spectral response in thick-film devices, Al(100 nm) was
replaced with a semitransparent Al(2 nm)/Ag(10 nm) having a transmittance of about
70% in the 350 to 650 nm spectral region. As shown in Figure 4.4b, illumination
through the semitransparent metal electrode of a 1200-nm device yielded a spectral
response consistent with its absorption spectrum with a much improved J = 3.8
SC
mA/cm2.
The finished devices with a 1200-nm active layer were further subjected to
thermal annealing at 110oC over incremental durations. The J-V characteristics as a
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function of annealing time are plotted in Figure 4.5, and their performance
parameters are summarized in Table 4.2. The J increases steadily upon thermal
SC
annealing, reaching the maximum value of 10.1 mA/cm2 after annealing for 40 min,
accompanied by V rising from 0.50 to 0.62 V and FF decreasing from 0.64 to 0.40,
OC
corresponding to an improvement of η from 0.4 to 2.5%. The maximum η of 2.6%
was achieved with thermal annealing for 90 min, beyond which the η value decreases
slightly to 2.2% because of the deteriorated J .
SC

15 J, mA/cm2
0 min 40 min
10
4 min 60 min
10 min 90 min
5
20 min 120 min
−1.0 −0.5 0 0.5 1.0
V, V
−5
−10

Figure 4.5: J-V characteristics under 100 mW/cm2 white light illumination of a 1200-
nm BHJ-OPV device thermally annealed at 110oC up to 120 min.
Similar to what has been reported previously, [5] thermal annealing of solvent-vapor-
annealed films did not significantly alter the absorption spectra, as shown in Figure
4.6a. However, the η around the absorption maximum is substantially improved
EQE
upon thermal annealing, thus reversing the inverse spectral response (Figure 4.6b)
through morphological modification of the separated P3HT layer on ITO. It has been
reported that thermal annealing of polymer/fullerene bilayer films induces the