Bulk heterojunction PCPDTBT:PC71BM organic solar cells deposited by emulsion-based, resonant infrared matrix-assisted pulsed laser evaporationWangyao Ge, Ryan D. McCormick, Gift Nyikayaramba, and Adrienne D. Stiff-Roberts Citation: Applied Physics Letters 104, 223901 (2014); doi: 10.1063/1.4881058 View online: http://dx.doi.org/10.1063/1.4881058 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Transient photocurrent measurements of PCDTBT:PC70BM and PCPDTBT:PC70BM Solar Cells: Evidence forcharge trapping in efficient polymer/fullerene blends J. Appl. Phys. 109, 074513 (2011); 10.1063/1.3573394 Design of organic tandem solar cells using PCPDTBT : PC 61 BM and P 3 HT : PC 71 BM J. Appl. Phys. 107, 124515 (2010); 10.1063/1.3448271 Thin films of polymer blends for controlled drug delivery deposited by matrix-assisted pulsed laser evaporation Appl. Phys. Lett. 96, 243702 (2010); 10.1063/1.3453756 The use of thermal initiator to make organic bulk heterojunction solar cells with a good percolation path Appl. Phys. Lett. 93, 043304 (2008); 10.1063/1.2965468 Efficient polymer:polymer bulk heterojunction solar cells Appl. Phys. Lett. 88, 083504 (2006); 10.1063/1.2176863

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Bulk heterojunction PCPDTBT:PC71BM organic solar cells deposited byemulsion-based, resonant infrared matrix-assisted pulsed laser evaporation

Wangyao Ge (葛王尧), Ryan D. McCormick, Gift Nyikayaramba,and Adrienne D. Stiff-Robertsa)

Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina 27708, USA

(Received 18 April 2014; accepted 18 May 2014; published online 2 June 2014)

Organic solar cells based on poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b0]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and [6,6]-phenyl C71 butyric acid

methyl ester (PC71BM) were fabricated by emulsion-based, resonant infrared matrix-assisted pulsed

laser evaporation (RIR-MAPLE). Two different deposition modes, namely simultaneous deposition

and sequential deposition, were investigated for fabricating bulk-heterojunction organic solar cells.

This work demonstrates that the RIR-MAPLE sequential deposition mode provides precise ratio

control for the fabrication of bulk-heterojunction organic solar cells. VC 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4881058]

Harvesting solar energy in an efficient and cost-effective

manner is considered as one of the most significant ways to

address today’s energy crisis.1 Over the last few decades, a

growing interest has focused on organic solar cells (OSCs)

for the purpose of harvesting solar energy using inexpensive

and earth abundant materials.2,3 During this time, many

solution-processed thin film casting methods have been

developed to fabricate OSCs, especially for the blended

active region known as bulk hetero-junctions (BHJs), com-

prising conjugated polymers as the donor phase and small or-

ganic molecules, typically fullerene derivatives, as the

acceptor phase.4 Solution-processed methods include drop-

casting, spin-coating, doctor blading, screen printing, and

ink jet printing. Although these solution-processed methods

are simple and straightforward, they have not yet demon-

strated control of film morphology and device yields across

the large areas required for solar cell modules. In addition,

solution-processed deposition methods are limited in the

ability to fabricate multilayer structures of materials with the

same solubility characteristics.5 Yet, such multilayer struc-

tures are necessary and essential for tandem solar cells that

offer absorption over a broader range of the solar spectrum

in order to maximize photocurrent generation. If the influ-

ence of the solvent on film and device properties, as well as

the limitations of material solubility, could be removed from

the fabrication of OSCs, higher yield OSC modules and

greater flexibility in tandem OSC design could be achieved.

Among different deposition techniques, pulsed laser

deposition (PLD) is one potential technique to deposit or-

ganic materials with little to no solvent influence.6 In 1982,

Srinivasan and Kawamura first successfully demonstrated

polymer deposition using PLD.7 In PLD, pure solid polymer

is used as the target for UV laser ablation under vacuum.

Typically, the high peak energy in the pulsed laser results in

rapid heating, promoting the sublimation of the material by

thermal mechanisms.8 The drawback of this technique is that

not all polymers can be deposited due to the potential photo-

chemical and structural degradation that result from directly

absorbing high energy from the UV laser. To minimize the

degradation of the polymer, a variation of the PLD technique

called matrix-assisted pulsed laser evaporation (MAPLE)

was developed.9,10 In MAPLE deposition, the target polymer

is dissolved into a volatile solvent (the host matrix) and then

frozen into a solid target by liquid nitrogen. Ideally, only the

host solvent absorbs the energy from the UV laser and evap-

orates. The remaining solute polymer is ejected from the tar-

get and deposited onto the substrate. Because the solvent

evaporates as the gas phase due to the absorption of laser

energy, direct solvent exposure on the substrate is limited.

Although being advantageous over the traditional PLD, this

UV-MAPLE deposition still results in absorption of the

high-energy laser by the guest polymer and can yield degra-

dation of the polymer, which is especially problematic for

optoelectronic applications.11 An alternative approach is to

minimize polymer degradation by tuning an infrared laser to

a specific bond that exists in the solvent but not in the target

polymer, known as resonant infrared, matrix-assisted pulsed

laser evaporation (RIR-MAPLE). This can be realized using

a free electron laser to tune the wavelength according to the

choice of solvent.12 In 2008, Pate et al. demonstrated a varia-

tion of the RIR-MAPLE technique that uses an emulsion-

based target matrix.13 A schematic diagram of RIR-MAPLE

is shown in Fig. 1. In emulsion-based RIR-MAPLE, instead

of using pure volatile solvent as the frozen matrix, a frozen

emulsion containing uniformly mixed solvent and water is

used as the matrix. An Er:YAG laser (2.94 lm), which is res-

onant only with hydroxyl bonds in the emulsion (primarily

from water), serves as the ablating laser. Considering the

high absorbance of water ice at the Er:YAG laser energy, as

well as the low energy of the IR source, the degradation of

chemical bonds and molecular weight in polymers is practi-

cally eliminated.14,15 Thus, emulsion-based RIR-MAPLE

essentially decouples the absorption of laser energy from the

guest material system, which provides wide flexibility in the

choice of organic solvents and polymer materials.

While MAPLE deposition has demonstrated great poten-

tial for polymer-based optical coatings16,17 and functional

films.18,19 to date, very few studies of polymer-based optoe-

lectronic devices fabricated using any variation of MAPLE

a)Author to whom correspondence should be addressed. Electronic mail:

adrienne.stiffroberts@duke.edu

0003-6951/2014/104(22)/223901/5/$30.00 VC 2014 AIP Publishing LLC104, 223901-1

APPLIED PHYSICS LETTERS 104, 223901 (2014)

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have been reported.20 In 2002, a P3HT:PCBM planar hetero-

junction, or bi-layer, OSC fabricated by UV-MAPLE was

reported by Caricato et al. (gPCE¼ 0.03%).21 As an advance-

ment of this sole report of OSC fabrication by any MAPLE

variation, in this paper, we report the device performance of

BHJ PCPDTBT:PC71BM OSCs deposited using the emul-

sion based, RIR-MAPLE technique. Two different deposi-

tion modes, namely, simultaneous deposition (single target

emulsion) and sequential deposition (partitioned target emul-

sions),22,23 were investigated for the BHJ active region

structure.

The detailed description of the emulsion-based RIR-

MAPLE process can be found elsewhere.24 The emulsion

target for RIR-MAPLE deposition in this study comprised

primary solvent, secondary solvent and deionized (DI) water

containing sodium lauryl sulfate (SLS). The SLS is essential

and used as the surfactant to stabilize the emulsion.

Chlorobenzene was used as the primary solvent to dissolve

either PCPDTBT (Solaris Chem Inc, Mw¼ 15-25 KDa) or

PC71BM (American Dye Source). Phenol was used as the

secondary solvent to not only enrich the target with hydroxyl

bonds, but also prevent sublimation of the frozen target

under vacuum over time. DI water was primarily used to

enrich the hydroxyl bond concentration in the emulsion tar-

get for the laser ablation. The emulsion was frozen by being

injected into the cooled target holder (77K, cooled by liquid

nitrogen). It was observed that the emulsion was frozen into

a solid ice instantly (less than 5–10 s) without any notable

phase separation between organic solvent and water.

For simultaneous deposition, PCPDTBT and PC71BM

were co-dissolved in equal parts in chlorobenzene. The

emulsion target comprised the chlorobenzene solution, phe-

nol and DI water at the ratio of 1:0.25:3 by volume. For the

simultaneous deposition frozen target, the single emulsion

was directly injected into the target holder with a BHJ ratio

of PCPDTBT to PC71BM of (1:1).

For sequential deposition, instead of preparing a single

emulsion containing a mixture of PCPDTBT and PC71BM,

a separate emulsion was prepared for each material.

PCPDTBT or PC71BM were dissolved in chlorobenzene.

The emulsion comprised the chlorobenzene solution, phe-

nol and DI water at a ratio of 1:0.25:3 by volume for

PCPDTBT and a ratio of 1:0.5:3 by volume for PC71BM.

For the sequential deposition frozen target, the target holder

was partitioned and one section was injected with the

PCPDTBT emulsion while the remaining section was

injected with the PC71BM emulsion. The BHJ ratio can

then be determined by the deposition growth rate for each

material and the target partition ratio, as opposed to the

weight ratio in solution typically used for solution-based

methods. The deposition rate for the PCPDTBT emulsion

and the PC71BM emulsion were calibrated to be roughly

�20 nm/h for a laser fluence of 2 J/cm2 per pulse, 2 Hz repe-

tition rate, and 7 cm target-to-substrate distance. All deposi-

tion times were 4 h, which resulted in OSC active regions

with thicknesses around 80 nm.

OSCs were fabricated on pre-patterned ITO-coated glass

slides that were ultrasonically cleaned, sequentially, with ac-

etone, methanol and isopropyl alcohol. Oxygen plasma treat-

ment was used for 5 min as the final step to improve the

contact between PEDOT:PSS water solution and ITO sub-

strates. Then, PEDOT:PSS (CleviosTM

PVP AI 4083) solution

was spin-coated at 4000 rpm for 30 s and baked at 150 �C for

10 min. The active layer (80 nm) was deposited by emulsion-

based RIR-MAPLE using the two different deposition

modes, as described. Finally, an aluminum cathode (150 nm)

was thermally evaporated (5 � 10�6 millibars pressure) on

top of the active layer, defining a 9 mm2 active area for each

solar cell. All of the OSCs were encapsulated using encapsu-

lation epoxy (Ossila, Ltd) before measurement in ambient

atmosphere under simulated AM 1.5G illumination

(100mW/cm2, ABET Technology solar simulator). All

FIG. 1. Idealized schematic diagram

of RIR-MAPLE system: an incident

laser pulse enters a vacuum chamber

through the optical systems. It ablates

a solid frozen target consisting of a

guest material in the hydroxyl-rich

host matrix. The guest material has a

high sticking coefficient and is depos-

ited onto the substrate, whereas the fro-

zen host matrix is evaporated by the

laser energy and pumped away.

223901-2 Ge et al. Appl. Phys. Lett. 104, 223901 (2014)

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devices were stored in a vacuum box before encapsulation

and characterization.

Before OSC fabrication and characterization, we used

the separate emulsions from the sequential deposition pro-

cess to deposit PCPDTBT-only and PC71BM-only films for

basic materials characterization. Atomic force microscopy

(AFM) and scanning electron microscopy (SEM) images

demonstrated that our deposition technique yielded nano-

sized cluster domains with minimal presence of micron-

sized clusters across the film surface. As observed from the

AFM images of PCPDTBT and PC71BM deposited by

emulsion-based RIR-MAPLE (Figs. 2(a) and 2(c), respec-

tively), most of the cluster domains were around 50 nm.

The corresponding RMS surface roughness was 9.57 nm

(PCPDTBT) and 6.57 nm (PC71BM) using a 1lm� 1 lmscan size. In order to observe possible micron-sized cluster

domains, the SEM images of the emulsion-based RIR-

MAPLE-deposited PCPDTBT and PC71BM films were also

studied (Figs. 2(b) and 2(d), respectively). These SEM

images demonstrate that clusters were observed on the film

surface as expected; however, their sizes were typically less

than 1 lm (100 nm–300 nm), which are smaller than the

typical cluster size of films deposited by UV-MAPLE techni-

ques.25,26 For low band gap polymer based OSCs, the exci-

ton diffusion length is typically 10 nm;27 thus, micron-sized

cluster domains would greatly limit the exciton dissociation

efficiency and the resulting power conversion efficiency, but

this is not a significant limiting factor in OSCs deposited by

emulsion-based RIR-MAPLE.

The absorption spectra of PCPDTBT-only and PC71BM-

only films deposited by emulsion-based RIR-MAPLE were

also examined by ultraviolet-visible-near infrared (UV–Vis-

NIR) absorbance spectroscopy (Fig. 2(e)). The measured

PCPDTBT absorbance spectrum showed two characteristic

peaks at 400 nm and 750 nm. The measured PC71BM absorb-

ance spectrum showed complementary strong absorption at

shorter wavelengths (<600 nm). These absorbance spectra

are consistent with previous measurements for films depos-

ited by spin-coating,28 indicating the preservation of optical

properties by emulsion-based RIR-MAPLE.

To test the performance of BHJ OSCs fabricated by

emulsion-based RIR-MAPLE using simultaneous or se-

quential deposition modes, current density-voltage (J-V)

measurements were conducted for the BHJ ratio

PCPDTBT:PC71BM¼ 1:1 and active region thickness of

80 nm in each case. Typical J-V curves for the BHJ OSCs

deposited by the two deposition modes are shown in Fig. 3

(a), and the calculated performance parameters are listed in

the table inset. As can be seen, the BHJ OSC fabricated by

simultaneous deposition showed much lower efficiency

(gPCE¼ 0.067%) than the BHJ OSC fabricated by sequen-

tial deposition (gPCE¼ 0.74%). In order to exclude the pos-

sibility that the different efficiencies resulted from

significantly different BHJ ratios in sequential and simulta-

neous modes, the UV-Vis-NIR absorbance spectrum of

each BHJ active region was measured (Fig. 2(b)). The

PCPDTBT and PC71BM peak intensities in each spectrum

were similar, demonstrating comparable absorption from

both components. As a result, the large difference in power

conversion efficiency for the two deposition modes was not

due to different PCPDTBT:PC71BM BHJ ratios, but rather

how the donor and acceptor materials were blended to-

gether in the film. For the simultaneous mode, PCPDTBT

and PC71BM were blended together in the emulsion, yet

PCPDTBT is relatively hydrophobic, while PC71BM is rela-

tively hydrophilic. Due to the different polarity, these mate-

rials tend to separate into different domains (more PC71BM

in water and more PCPDTBT in chlorobenzene). In addi-

tion, the emulsion freezing process may also facilitate phase

separation because different components in the emulsion

may freeze at different rates. These separated phase

domains in the frozen target for the simultaneous mode

could be transferred to the substrate during laser ablation.

In contrast, for the sequential deposition mode, PCPDTBT

and PC71BM were prepared in separate emulsions. The

combination of the laser raster pattern and the target rota-

tion result in smaller material domains on the substrate

because the influence of the solvent on material blending is

significantly reduced.

Based on this growth comparison, a systematic study

of the effect of PCPDTBT:PC71BM BHJ ratio on the solar

FIG. 2. Tapping-mode topographical AFM images (1 lm � 1lm) of (a)

PCPDTBT and (c) PC71BM deposited by emulsion-based RIR-MAPLE; and

SEM images of (b) PCPDTBT and (d) PC71BM deposited by emulsion-

based RIR-MAPLE. (e) Normalized UV-Vis-NIR absorbance spectra of a

PCPDTBT film (red) and a PC71BM film (green) deposited by emulsion-

based RIR-MAPLE.

223901-3 Ge et al. Appl. Phys. Lett. 104, 223901 (2014)

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cell performance using the emulsion-based RIR-MAPLE

sequential deposition mode was conducted. The UV-Vis-

NIR absorbance spectra of PCPDTBT:PC71BM active

regions with different BHJ ratios are shown in Fig. 4(a). As

the ratio of PCPDTBT:PC71BM increases, that is, as the

content of the polymer component increases in the BHJ,

the PCPDTBT relative peak intensities increase, while

the PC71BM relative peak intensities decrease. These trends

demonstrate the ability to precisely control the BHJ ratio by

emulsion-based RIR-MAPLE using sequential deposition.

The J-V curves for the PCPDTBT:PC71BM BHJ OSCs

with different ratios are shown in Fig. 4(b), and the corre-

sponding device parameters are summarized in Table I.

The PCPDTBT:PC71BM BHJ ratio of 1:1.5 yielded the best

device performance. From literature, the best ratio for

the spin-coated PCPDTBT:PC71BM BHJ system is between

1:2 and 1:3.29 The difference in BHJ ratio for these

RIR-MAPLE devices is reasonable in this study because

the BHJ ratio is determined by growth rate and target parti-

tion ratio rather than weight percent in the prepared

solution.

In conclusion, the emulsion-based RIR-MAPLE tech-

nique was introduced to fabricate PCPDTBT:PC71BM BHJ

OSCs. The sequential deposition mode was demonstrated to

enable precise control of the BHJ ratio for fabrication of

OSCs. The best device performance yielded a power conver-

sion efficiency of 0.86%, which constitutes an improvement

by an order of magnitude from the first MAPLE-deposited

FIG. 4. (a) Stacked UV-Vis-NIR absorbance spectra of PCPDTBT:PC71BM

blended films with different ratios deposited by sequential deposition mode.

The attribution of the peaks are highlighted by arrow marks. (b) The J-V

curves of PCPDTBT:PC71BM BHJ OSCs deposited by sequential deposition

mode.

FIG. 3. (a) J-V characteristics of PCPDTBT:PC71BM (1:1) BHJ OSCs fab-

ricated by simultaneous (red) and sequential (black) deposition modes. The

inset table is calculated performance parameters. (b) Stacked UV-Vis-NIR

absorbance spectra of blended PCPDTBT:PC71BM (1:1) films deposited by

simultaneous (red) and sequential (black) deposition modes.

TABLE I. Performance of PCPDTBT:PC71BM BHJ OSCs with different

ratios fabricated by emulsion-based RIR-MAPLE using sequential deposi-

tion mode. The ratio of the two materials is indicated in parentheses (poly-

mer: small molecule). All results were determined by averaging six devices.

BHJ ratio Jsc (mA/cm2) Voc (V) FF g (%)

(3:1) 1.76 0.61 0.36 0.39

(2:1) 2.05 0.62 0.36 0.46

(1:1) 3.12 0.58 0.41 0.74

(1:1.5) 3.43 0.56 0.45 0.86

(1:2) 2.99 0.57 0.42 0.72

(1:3) 2.40 0.58 0.43 0.60

223901-4 Ge et al. Appl. Phys. Lett. 104, 223901 (2014)

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OSC. Although, at this stage, the efficiency of solar cells fab-

ricated by RIR-MAPLE is lower than those fabricated by

spin coating, RIR-MAPLE provides a wide parameter space

(e.g., chamber pressure, substrate temperature, target-to-sub-

strate distance, laser fluence, and target chemistry) to opti-

mize thin-film deposition and device performance. In

particular, the domain sizes of deposited BHJ films are

strongly related to the target emulsion chemistry (e.g., type

of surfactant, emulsion droplet size, emulsion composition),

which is tunable and requires further study. Post-deposition

annealing and other treatments could also improve the device

performance. The capabilities of RIR-MAPLE to fabricate

multi-layer films using materials with similar solubility and

to control blended film morphology motivate further investi-

gation of RIR-MAPLE deposition as a possible commercial

technology for OSC fabrication.

This work was supported by the Office of Naval

Research under Grant No. N00014-10-1-0481.

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