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title: Fabrication of Polymer Solar Cells (PSCs): Can a ternary device perform better? A

preliminary lab report.

authors: Peter Dimoulas and Dr. Su Huang.

last updated : August 1, 2014

Introduction

We fabricated polymer solar cells (PSCs) on planar substrates. Our goal was to characterize the

performance of PSCs with 2 donors - 1 acceptor systems (ternary PSC) in relation to that of PSCs

with 1 active donor - 1 acceptor system.

A ternary PSC involves two distinct active polymer, donor-acceptor interfaces, as well as a donor-

donor interface, in contrast to a single donor-acceptor interface for standard PSCs. Ternary PSCs

have the potential to widen the effective wavelengths and increase the percent absorption thereby

enhancing the power conversion efficiency (PCE) and peak, or open-circuit voltage (Voc).

Moreover, an additional donor-donor interface may yield finer phase separation, which is ideal for

charge separation at the donor-acceptor interface (L. Gang et al., 2012). On the other hand, the

resultant energy cascades may be negatively impacted by competing donor-acceptor systems (Y.-

C. Chen et al., 2013). Furthermore, the stability and processability of the polymers requires

careful consideration in order to optimize the performance of the two, polymer donor-acceptor

interfaces. For the purposes of our study we used the following polymers: PTB7, Poly({4,8-bis[(2-

ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-

ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}); P3HT, Poly(3-hexylthiophene-2,5-diyl); PCBM,

Phenyl-C61-butyric acid methyl ester.

Regarding stability and processability of PTB7, several studies have fabricated and encapsulated

PBT7-based devices within an N2 environment, within a glove box (Y. Liang et al. 2009 and 2010;

and J. You et al. 2012). Presumably, this was to account for PTB7’s air sensitivity. Unfortunately,

this was not possible given our facilities. Additionally, recent studies have annealed PTB7-based

devices at room temperature (A. Guerrero et al. 2013; H.-Y. Park et al. 2013; S. Cho et al. 2014;

and T. Yanagidate et al. 2014). Interestingly, Dr. Su Huang previously annealed a PTB7:PCBM

device at 120°C for 10 minutes and found significant charge leakage. On the other hand, P3HT is

not known to be particularly air-sensitive. In terms of annealing, Dr. Su Huang has found that,

under conditions within our facilities, P3HT-based devices yield their best results when then

annealed at 160°C for 10 minutes (Voc = 0.63; PCE = 4.3, fill factor (FF) = 0.4; photocurrent

density (A/m2) = 141).

We examined the performance of several batches of PSCs with a variety of PTB7: P3HT: PCBM

ratios that were annealed at temperatures ranging from 22°C to 165°C. Our aim was to realize

conditions suitable for fabricating a PTB7: P3HT: PCBM based ternary PSC with enhanced

performance relative to either single, polymer interface system.

methods

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Planar substrates that include ITO on glass obtained from Candice Veligra and Noga Kornblum.

Substrates subsequently underwent oxygen, plasma cleaning within a vacuum for 10 minutes.

Filtered polymer solutions, which were passed through a filter (0.2um PTFE with 13mm diameter),

were spin-coated onto substrates. Prior to the filtering and throughout the spin-coating processes

polymer solutions were placed on a hot plate at 65°C to maintain lower solution viscosity. Coated

substrates were transferred into a glove box (with a nitrogen environment) and annealed.

Following annealing of spin-coated polymer solutions for 10 minutes, layers of MoO3 and Ag were

deposited via thermal evaporation within a vacuum. Finished devices were tested in ambient air

using PV-measurement software, from which we obtained the following measurements: open-

circuit voltage (Voc), fill factor (FF), photocurrent density, and power conversion efficiency (PCE).

For more specific information on how specific samples were treated please see table 1. Light

used to test devices is passed through a KG5 filter (300-800 nm) that is applied to better simulate

sunlight.

Results

Among the devices with single active-donor interfaces PTB7:PCBM outperformed P3HT:PCBM for

each parameter tested, including Voc, FF, photocurrent density, and PCE (tables 2 and 3).

Regardless, of annealing conditions, neither ternary device outperformed PTB7:PCBM; however,

most of ternary devices did realize a similar or improved Voc relative to P3HT:PCBM (figs. 1-4).

On average, for Voc, photocurrent density, and PCE, our ternary devices realized improved

performance at higher annealing temperatures; although, we did not find a consistent relationship

between annealing temperature and FF among polymer systems and conditions that were

considered (figs. 1-4). We also obtained similar results for 2 devices that were tested following

annealing at 2 distinct temperatures (table 4).

We also found that PTB7 devices annealed at 22°C realized an asymptotic rise in Voc and PCE

as a function of time spent exposed to our light source (figs. 5 & 8); though, we did observe an

initial rise rise in FF and photocurrent, it was more modest (figs. 6 & 7). For sample 5, PTB7: 2.5,

P3HT: 7, PCBM (from July 21), we periodically turned off the light source and subsequently found

decreases in Voc, FF, photocurrent density, and PCE.

Discussion

We fabricated several PSCs with varying ratios of PTB7, P3HT, and PCBM. Our goal was to

elucidate conditions via which a ternary device, with 2 donor-acceptor systems, PTB7: PCBM and

P3HT: PCBM, would out-perform either single donor-acceptor system. We found that some of our

ternary devices did realize an elevated Voc relative to a P3HT: PCBM donor systems; however,

this was not the case for our ternary devices in terms of FF, photocurrent density, or PCE

compared with either PTB7: PCBM or P3HT: PCBM donor systems. Among our ternary devices,

we found a positive relationship for Voc, photocurrent density, and PCE with increasing annealing

temperature; although, we did not find such a relationship for FF. Additionally, individual ternary

devices tested following annealing at room temperature and retested following annealing at a

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higher temperature also realized an increase in Voc, photocurrent density, and PCE but not for

FF.

Our P3HT:PCBM devices performed on par when compared to those completed by previous

studies (Gang et al., 2005; Reyes-Reyes et al.; 2005); however, our PTB7:PCBM devices did not

perform as well (A. Guerrero et al., 2013.; H.-Y. Park et al., 2013.; Yanagidate et al., 2014). We

believe that this difference can be attributed to our fabrication conditions. In fact, those studies

fabricated and tested their devices entirely within a glove box (with a nitrogen environment), in

addition to annealing at room temperature, and only exposed their device to air after it was

encapsulated. We propose that PTB7 is air (or oxygen) sensitive, exposure to which may limit its

ability to generate excitons.

Regarding device performance and changes in annealing temperatures, previous work revealed

that P3HT:PCBM devices generally realized enhanced performance when annealed at higher

temperatures, although this relationship is time-dependent and device performance can

deteriorate, eventually (Gang et al., 2005; Reyes-Reyes et al.; 2005). The authors suggested that

annealing at higher temperatures tends to tighten packing of P3HT polymer chains, which better

enable the generation and separation of excitons. Among our findings, we examined Voc and

PCE as a function of annealing temperature and percent composition (of P3HT and PTB7) and

found no noticeable or consistent relationship (figs. 9 and 10, and table 5).

Ternary PSCs have the potential to realized enhanced performance (L. Gang et al., 2012).

Indeed, we found that under specific conditions PTB7:P3HT:PCBM devices realized elevated Voc

relative to P3HT:PCBM alone, but not compared to PTB7:PCBM; additionally, the FF,

photocurrent density, and PCE of our ternary PSCs were less than that of both P3HT:PCBM and

PTB7:PCBM devices. Unfortunately, PTB7-based devices realized optimal performance when

fabricated within an inert environment and annealed at room temperature and P3HT-devices

perform better when annealed at higher temperatures. Therefore, we believe that our ternary

devices did not perform as well as due to the disparate processing conditions under which PTB7-

and P3HT- based PSCs perform best.

Although we found that our ternary PSCs tended not to perform as well as single-interface PSCs,

provided that multiple polymers can be processed under similar conditions; and realize phase

separation, based on their inherent polar or non-polar properties, on the order of 10-20 nms,

consistent with the working distance of excitons; ternary PSCs should perform better. Going

forward, future studies should carefully consider selection of appropriate polymers with similar,

optimal processing conditions.

Acknowledgements

This research was primarily supported by NSF Grant MRSEC DMR 111-9826 (CRISP Research

Experiences for Teachers Program). Additionally, thank you to CRISP, Professor Christine

Broadbridge and Carol Jenkins for administering this program and making this work possible. A

special thanks to Dr. Su Huang, my mentor and device making guru. I would also like to thank

Candice Pelligra and Noga Kornblum, for fabricating substrates (including ZnO nanorod forests).

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Thank you to Professor Chinedum Osuji for welcoming me into his lab and providing additional

guidance and support for this work. Finally, thank you to all the members of the Osuji lab for

being warm and friendly as well as keeping a high-energy and motivating environment.

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References

Chen, Y.-C.; C.-Y. Hsu; R. Y.-Y. Lin; K.-C. Ho; and J.T. Lin. (2013). Materials for Active Layer of

Organic Photovoltaics Ternary Solar Cell Approach. Chem. SUS Chem Reviews (6): 20-35.

Gang, L.; V. Shrotriya; Y. Yao; and Y. Yang. (2005). Investigation of annealing effects and film

thickness dependence of polymer solar cells based on poly(3-hexylthiophene). Joumal of

Applied Physics; 98, 043704.

Guerrero, A.; N.F. Montcada; J. Ajuria; I. Etxebarria; R. Pacios; G. Garcia-Belmonte; and E.

Palomares. (2013). Charge carrier transport and contact selectivity limit the operation of

PTB7-based organic solar cells of varying active layer thickness. Journal of Material

Chemistry, A (1): 12345-54.

Huang, S. (2014, July). Personal Communication.

Liang, Y.; Z. Xu; J. Xia; S.-T. Tsai; Y. Wu; G. Li; C. Ray; and L. Yu. (2010). For the Bright Future -

Bulk Heterojunction Polymer Solar cells with Power Conversion Efficiency of 7.4%.

Advanced Materials (22): E135-8.

Park, H.-Y.; D. Lim; K.-D. Kim; and S.-Y. Jang. (2013). Performance of low-temperature-annealed

solution-processible ZnO buffer layers for inverted solar cells. Journal of Material

Chemistry, A (1): 6327-34.

Reyes-Reyes, M.; K Kim; and D.L. Carrolla. (2005). High-efficiency photovoltaic devices based on

annealed poly„3-hexylthiophene… and 1-„3-methoxycarbonyl… -propyl-1-phenyl-„6,6…C61

blends. Applied Physics Letters (87), 083506.

Yanagidate, T.; S. Fujii; M. Ohzeki; Y. Yanagi; Y. Arai; T. Okukawa; A. Yoshida; H. Kataura; and

Y. Nishioka. (2014). Flexible PTB7:PC71BM bulk heterojunction solar cells with LiF buffer

layer. Japanese Journal of Applied Physics (53): 2BE05.

You, J.; C.-C. Chen; L. Dou; S. Murase; H.-S. Duan; S.A. Hawks; T. Xu; H.-J. Son; L. Yu; G. Li;

and Y. Yang. (2012). Metal Oxide Nanoparticles as an Electron-Transport Layer in High-

Performance and Stable Inverted Polymer Solar Cells. Advanced Materials (24): 5267-72.

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Figures

Figure 1

Figure 2

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Figure 3

Figure 4

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Figure 5

Lights were turned off for sample 3b between 9:20-14:05, 15:11-27:49, and 45:18-1:16:59; for

sample 5, PTB7: 2.5, P3HT: 7, PCBM between 1:02:53-1:23:23.

Figure 6

Lights were turned off for sample 3b between 9:20-14:05, 15:11-27:49, and 45:18-1:16:59; for

sample 5, PTB7: 2.5, P3HT: 7, PCBM between 1:02:53-1:23:23.

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Figure 7

Lights were turned off for sample 3b between 9:20-14:05, 15:11-27:49, and 45:18-1:16:59; for

sample 5, PTB7: 2.5, P3HT: 7, PCBM between 1:02:53-1:23:23.

Figure 8

Lights were turned off for sample 3b between 9:20-14:05, 15:11-27:49, and 45:18-1:16:59; for

sample 5, PTB7: 2.5, P3HT: 7, PCBM between 1:02:53-1:23:23.

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figure 9

figure 10

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Tables

fabrication

date

test

date

sample

IDs

polymer

solvent

polymer ratio spin-coating

(rpms)

annealing

temperature (°C)

7/9/14 7/9/14 A 3% DIO,in

Cl-benzene

5, PTB7: 20,

P3HT: 19,

PCBM

2000 22

7/9/14 7/9/14 C 3% DIO,in

Cl-benzene

5, PTB7: 20,

P3HT: 19,

PCBM

2000 90

7/9/14 7/9/14 F 3% DIO,in

Cl-benzene

5, PTB7: 20,

P3HT: 19,

PCBM

2000 120

7/11/14 7/11/14 A Cl-benzene 5, PTB7: 20,

P3HT: 12,

PCBM

2000 120

7/11/14 7/11/14 C Cl-benzene 5, PTB7: 20,

P3HT: 12,

PCBM

2000 140

7/15/14 7/15/14 1a Cl-benzene 20, P3HT: 12,

PCBM

3500 165

7/17/14 7/17/14 2b Cl-benzene 5, PTB7: 7,

PCBM

1000 22

7/18/14 7/21/14 4a Cl-benzene 5, PTB7: 2.5,

P3HT: 7, PCBM

1000 22

7/18/14 7/21/14 4a1 Cl-benzene 5, PTB7: 2.5,

P3HT: 7, PCBM

1000 95

7/18/14 7/21/14 3a Cl-benzene 5, PTB7: 2.5,

P3HT: 7, PCBM

1000 22

7/18/14 7/21/14 3a1 Cl-benzene 5, PTB7: 2.5,

P3HT: 7, PCBM

1000 121

Table 1

1 samples were initially tested and then retested after annealing in ambient air.

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The above table includes more detailed information regarding how specific samples were treated

prior to testing, obtaining I-V curves.

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Table 2

The above table includes data obtained from all tested PSCs. Abbreviations: Voc = open-circuit

voltage; FF = fill factor; PCE = power conversion efficiency.

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date sample ID

ratio of PTB7-P3HT-PCBM

annealing temp. (°C)

Voc FF Photocurrent density

PCE

7/9/14 A 5, PTB7: 20, P3HT: 19, PCBM

22 0.544 0.31 54.239 0.913

7/9/14 C 5, PTB7: 20, P3HT: 19, PCBM

90 0.615 0.29 79.444 1.423

7/9/14 F 5, PTB7: 20, P3HT: 19, PCBM

120 0.629 0.27 67.303 1.160

7/11/14 A 5, PTB7: 20, P3HT: 12, PCBM

120 0.586 0.39 69.827 1.590

7/11/14 C 5, PTB7: 20, P3HT: 12, PCBM

140 0.645 0.40 78.195 2.010

7/15/14 1a 20, P3HT: 12, PCBM

165 0.595 0.39 103.473 2.387

7/17/14 2b 5, PTB7: 7, PCBM 22 0.680 0.51 137.403 4.810

7/21/14 4a 5, PTB7: 2.5, P3HT: 7, PCBM

22 0.593 0.31 36.3148 0.672

7/21/14 4a 5, PTB7: 2.5, P3HT: 7, PCBM

95 0.638 0.38 47.1267 1.134

7/21/14 3a 5, PTB7: 2.5, P3HT: 7, PCBM

22 0.468 0.31 29.034 0.422

7/21/14 3a 5, PTB7: 2.5, P3HT: 7, PCBM

121 0.592 0.33 57.8935 1.143

Table 3

The above table includes average findings, calculated from the raw data (table 2).

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sample ID annealing regime (°C)

Voc FF Photocurrent density

PCE

4a 22-95 0.076 0.209 0.298 0.687

3a 22-121 0.265 0.073 0.994 1.706

Table 4

The above table indicates the fractional improvement in Voc, FF, photocurrent density, and PCE

for PSCs fabricated on July 18, 2014 and tested on July 21, 2014. Baseline data was obtained

following annealing at 22°C. Samples were subsequently annealed, for a second time in ambient

air, and retested.

date sample ID

ratio of PTB7-P3HT-PCBM

Percent P3HT (%)

annealing temp. (deg. C)

Voc PCE

7/17/14 2b 5, PTB7: 7, PCBM 0 22 0.680 4.810

7/21/14 4a 5, PTB7: 2.5, P3HT: 7, PCBM

33 22 0.594 0.672

7/9/14 A 5, PTB7: 20, P3HT: 19, PCBM

80 22 0.544 0.913

7/9/14 C 5, PTB7: 20, P3HT: 19, PCBM

80 90 0.615 1.423

7/21/14 4a 5, PTB7: 2.5, P3HT: 7, PCBM

33 95 0.639 1.134

7/21/14 3a 5, PTB7: 2.5, P3HT: 7, PCBM

33 121 0.592 1.143

7/11/14 A 5, PTB7: 20, P3HT: 12, PCBM

80 120 0.586 1.590

7/11/14 C 5, PTB7: 20, P3HT: 12, PCBM

80 140 0.645 2.010

7/15/14 1a 20, P3HT: 12, PCBM 100 165 0.595 2.387

Table 5

The above table includes average findings including with figures 9 and 10. Percent P3HT was

obtained by dividing the mass of P3HT (within solution) by the total mass, or sum of PTB7 and

P3HT.