Improvement in photovoltaic performance of anthracene ... - JKU

J Nanopart Res (2014) 16:2298 DOI 10.1007/s11051-014-2298-1

RESEARCH PAPER

Improvement in photovoltaic performance of anthracenecontaining PPE–PPV polymer-based bulk heterojunction solar cells with silver nanoparticles Nesrin Tore • Elif Alturk Parlak • Tu¨lay Aslı Tumay • Pelin Kavak • S¸ erife Sarıog˘lan • Sinem Bozar • Serap Gu¨nes • Christoph Ulbricht • Daniel Ayuk Mbi Egbe

Received: 13 October 2013 / Accepted: 24 January 2014 Ó Springer Science+Business Media Dordrecht 2014

Abstract In this study, we investigated the effect of silver nanoparticles (Ag NPs) in the active layer of anthracene-containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene):phenyl-C 61 butyric acid methyl ester (AnE-PVstat:PCBM)-based bulk heterojunction solar cells. By incorporating Ag NPs of 6 nm in diameter, the power conversion efficiency of AnE-PVstat:PCBM solar cells was improved to 3.10 % from a value of 2.46 % for the solar cells fabricated without nanoparticles. Moreover, ISOS-L-1 stability test showed that the lifetime of the

N. Tore E. A. Parlak T. A. Tumay (&) S¸ . Sarıog˘lan ¨ BI˙TAK Marmara Research Center, Chemistry TU Institute, 41470 Gebze, Kocaeli, Turkey e-mail: [email protected] N. Tore Department of Physics, Gebze Institute of Technology, 41400 Gebze, Kocaeli, Turkey P. Kavak S. Bozar S. Gu¨nes Department of Physics, Yıldız Technical University, 34220 Esenler, ˙Istanbul, Turkey C. Ulbricht Battery Research Centre (MEET), Institute of Physical Chemistry, University of Muenster, Corrensstr. 46, 48149 Muenster, Germany C. Ulbricht D. A. M. Egbe Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry, Johannes Kepler University, Altenbergerstr. 69, 4040 Linz, Austria

cells was also significantly improved by doping Ag NPs into the active layer. Keywords Organic solar cells Silver nanoparticles PPE–PPVs Stability test Energy conversion

Introduction The bulk heterojunction (BHJ) solar cell concept, based on interpenetrating phases of donor polymers and acceptor fullerene derivatives, is highly appealing as it is a promising candidate to realize effective harvesting of solar energy at low costs (Yu et al. 1995; Brabec et al. 2001). Poly(phenylene-vinylenes) (PPVs) were one of the first semiconducting polymer species investigated for BHJ application. Introducing acetylene units into the PPV structure opened way to new types of conjugated systems (PPE–PPVs) that show outstanding optoelectronic properties (Egbe et al. 2005a; Tekin et al. 2006). This class of compounds has successfully been used either as donor or acceptor components in solar cells. Open-circuit voltages (Voc) as high as 950 mV and 1.50 V have been achieved in BHJ cells (Hoppe et al. 2004; Al-Ibrahim et al. 2005; Egbe et al. 2005b) and from polymer:polymer bilayer cells (Egbe et al. 2004; Kietzke et al. 2006), respectively. The short-circuit current densities (Jsc) and the fill factors (FF) were

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found to be greatly dependent on the triple bond/ double bond ratio as well as the nature and size of the solubility-promoting side chains (Egbe et al. 2007). Hybrid polymer photovoltaic cells attract much attention and versatile systems with various inorganic compounds such as CdSe, PbS, TiO2, PbSe, ZnO, CuInS2, etc., in the form of nanoparticles, nanorods, nanodots, and tetrapods were reported (Huynh et al. 2002; Sun et al. 2005; Kwong et al. 2004; McDonald et al. 2005; Qi et al. 2005; Arici et al. 2003). Rand et al. (2004) presented an efficiency enhancement in tandem ultrathin-film organic solar cells by using silver nanoparticles (Ag NPs, *5 nm in diameter). Also, novel metal nanocrystallites from silver and gold are of high interest due to their close lying conduction and valence band in which electrons move freely. Furthermore, there are a number of demonstrations of plasmon-enhanced absorption and charge generation in organic solar cells (Stratakis and Kymakis 2013; Lu et al. 2012; Wu et al. 2011, 2013; Gan et al. 2013; Wang et al. 2011). Using small amounts of metal nanostructures can lead to small variations in the absorbance, changes in the film morphologies, and increase in short-circuit current values (Parlak et al. 2013). Recently, we reported on the effect of Ag NPs on poly[N-900 -hepta-decanyl-2,7-carbazole-alt-5,5-(40 ,70 di-2-thienyl-20 ,10 ,30 -benzothiadiazole)] (PCDTBT)based photovoltaic cells (Parlak et al. 2013). By embedding the NPs into the active polymer layer, a considerable improvement in the efficiency of the devices could be achieved. Plasmonic effects of Ag NPs on PPE–PPV solar cells have not been reported yet in the literature. In this contribution, we investigated the effect of Ag NPs on the performance of BHJ solar cells employing the PPE–PPV polymer AnE-PVstat. Furthermore, the performance stability of the cell assemblies (with and without Ag NPs) under standard AM1.5 G illumination was also investigated.

J Nanopart Res (2014) 16:2298

by transmission electron microscopy (TEM) and Malvern Zetasizer Nano-ZS instrument was described elsewhere (Mucur et al. 2012). The AnE-PVstat polymer (Fig. 1), equally equipped with linear octyl and branched 2-ethylhexyl side chains at the PPE and PPV parts, was synthesized according to a well-established procedure (Egbe et al. 2010). The hole transport layer material, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS, Clevios P), was purchased from Heraeus, and phenyl-C61-butyric acid methyl ester (PCBM) was received from Sigma-Aldrich. AnEPVstat and PCBM solutions with a concentration of 25 mg/mL were prepared using 1,2-dichlorobenzene which was purchased from Alfa Aesar. The active layer formulations with 0.02 and 0.01 % of Ag NPs (in the final active blend) and the one without Ag NPs were prepared by mixing AnE-PVstat polymer and PCBM solution in a 1:3 blend ratio. Device fabrication The cells were fabricated on indium tin oxide (ITO)coated glass substrates with a sheet resistance of 25 X/ cm. The substrates were cleaned in an ultrasonic bath with acetone, isopropyl alcohol, and deionized water, successively for 5 min; and dried under a flow of nitrogen. Afterward, the hole transport layer (PEDOT:PSS) was spin-coated onto the substrates and annealed at 100 °C on a hotplate for 10 min. Then, the active layer solutions were spin-coated onto PEDOT:PSS layer at 800 rpm for 1 min. Structured aluminum (Al) cathode layer (100 nm) was deposited by vacuum evaporation through a shadow mask. Finally, the devices were encapsulated using glass and UV-curable epoxy resin (Ossila). Device architecture is shown in Fig. 2. Device characterization

Experimental Materials and solutions Silver nanoparticles (Ag NPs, *6 nm in diameter) were synthesized by reducing silver acetate with phenylhydrazine according to a literature procedure (Li et al. 2005). The characterization of the particles

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The current density–voltage (J–V) characteristics of the devices were recorded under standard solar irradiation (AM1.5, 100 mW/cm2) using a xenon lamp as a light source and computer-controlled voltage–current Keithley 2600 source meter at 25 °C. The incident photon-to-current conversion efficiency (IPCE) spectra were obtained using Newport Quantum Efficiency Measurement System. The


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Fig. 1 Chemical structures of AnE-PVstat polymer and PCBM

10

J (mA/cm2)

5 a b c

0

-5

-10 0.0

0.2

Fig. 2 Device architecture of AnE-PVstat:PCBM solar cell with Ag NPs

morphology of the blend films were investigated by atomic force microscopy (AFM, Park Systems). UV– Vis spectroscopic measurements were performed with a Varian spectrophotometer. Electrochemical impedance spectroscopy (EIS) measurements were conducted at room temperature with perturbation amplitude of 10 mV over a frequency range of 1 Hz–1 MHz using Gamry Instrument Reference 600. During ISOS-L-1 test, the devices were kept under the solar simulator for 400 h.

0.4

0.6

0.8

1.0

Voltage (V)

Fig. 3 Current density–voltage (J–V) characteristics of AnEPVstat:PCBM solar cells (a without Ag NPs; b with 0.02 % of Ag NPs; c with 0.01 % of Ag NPs)

Table 1 Photovoltaic parameters of AnE-PVstat:PCBM solar cells with and without Ag NPs Solar cell

Voc (V)

Jsc (mA/cm2)

FF

g (%)

AnE-PVstat:PCBM

0.825

5.13

0.58

2.46

AnE-PVstat:PCBM with 0.02 % of Ag NPs

0.832

5.66

0.62

2.92

AnE-PVstat:PCBM with 0.01 % of Ag NPs

0.764

8.46

0.48

3.10

Results and discussion Photovoltaic performance of AnE-PVstat:PCBM solar cells J–V characteristics of ‘‘standard’’ polymer solar cells and the silver nanoparticle-embedded solar cells are shown in Fig. 3. The intermixing of different amounts of Ag NPs in the active layer led to a change in shortcircuit current (Isc) and fill factor (FF). The increased Jsc for the BHJ cells with nanoparticles shown in

Fig. 3 implies that more light is harvested in the active layer because of light scattering effect of the nanoparticles. Apparently, the incorporation of Ag NPs caused an increase in the power conversion efficiency (PCE) of AnE-PVstat:PCBM solar cells. When the amount of Ag NPs in the active layer was decreased, the performance of the device was improved in comparison to the sample with more concentrated nanoparticles (Table 1).

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polymer polymer + Ag NPs

50

-Zim, kohm

40 30 20 10 0 0

20

40

60

80

100

120

140

Zre, kohm

Fig. 4 UV–Vis absorption spectra of AnE-PVstat:PCBM film (a without Ag NPs; b with 0.02 % of Ag NPs)

Figure 5 shows the incident photon-to-current efficiency (IPCE) spectra of the reference BHJ cell and the BHJ cells fabricated with 0.02 and 0.01 % of Ag NPs. Both IPCE values resulted from the cells with nanoparticles are greater than the IPCE of the reference cell. Those improvements in the IPCE result from the Ag NPs, particularly from efficient light scattering. Moreover, decrease in the amount of nanoparticles used in the active layer increased the IPCE value of the device. This result can be attributed to the increased photocurrent generation, which is also compatible with J–V curves shown in Fig. 3.

80 72 64

IPCE (%)

56 48 40 32 24

a b c

16 8 0 -8 300

400

500

600

700

800

Wavelength (nm)

Fig. 5 IPCE spectra of AnE-PVstat:PCBM solar cells (a without Ag NPs; b with 0.02 % of Ag NPs; c with 0.01 % of Ag NPs)

The UV–Vis spectra of AnE-PVstat:PCBM blend and the blend with 0.02 % of Ag NPs are shown in Fig. 4. There are two absorbance maxima which are 544 and 580 nm for the AnE-PVstat:PCBM blend, 548 and 580 nm for the Ag NPs-doped AnE-PVstat:PCBM blend. While for the pure blend the maximum at higher energy is slightly more intense than the maximum at longer wavelengths, for the Ag-doped blend this appears inverted. Furthermore, it seems that there is a considerable increase in the absorbance at low energies between 650 and 800 nm range for the AnE-PVstat:PCBM blend with Ag NPs which may be attributed to a strong light scattering upon light illumination (Kalfagiannis et al. 2012). We assume that this might be a major cause for the increase in the efficiency of AnE-PVstat:PCBM solar cells with embedded Ag NPs.

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Fig. 6 Nyquist plot of AnE-PVstat:PCBM solar cells without Ag NPs and with 0.02 % of Ag NPs

Characterization by alternating current impedance spectroscopy Characteristic frequency of Nyquist plot at low frequency side of the curve allows the determination of the effective lifetime (sn) related to electron–hole recombination (Parlak et al. 2013). sn is determined by the relation (Garcia-Belmonte et al. 2008): sn ¼ 1=2pf ;

ð1Þ

where f is frequency of Nyquist plot at the midpoint. In the dark, hole and electron recombination time (sn) values of 0.4 ms (solar cell assemblies with 0.02 % of Ag NPs) and 0.5 ms (solar cell assemblies without Ag NPs) were observed (Fig. 6). From the other part, the characteristic frequency of Nyquist plot at high frequency side allows the determination of the diffusion time (sn) of electrons (Garcia-Belmonte et al. 2008; Perrier et al. 2012) and global mobilities through the relation:


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Fig. 7 AFM height images of AnE-PVstat:PCBM blends a AnE-PVstat:PCBM blend (section: 2 lm 9 2 lm), b AnEPVstat:PCBM blend with 0.02 % of Ag NPs (section:

l ¼ eL2 kb Tsd ;

ð2Þ

where e is the electronic charge, L is the thickness of the active layer, kb is the Boltzmann constant, and T is the actual temperature (21 °C in the dark) of the measurement. Smaller effective lifetimes mean higher mobilities leading to higher efficiencies (GarciaBelmonte et al. 2008). While the charge-transfer resistance (Rct) value is 81 kX for the Ag NPs-doped system, it is 127 kX for the Ag NPs-free system. As

2 lm 9 2 lm), c AnE-PVstat:PCBM blend (section: 5 lm 9 5 lm), d AnE-PVstat:PCBM blend with 0.02 % of Ag NPs (section: 5 lm 9 5 lm)

compared, both Rct and sn values are smaller for solar cells with Ag NPs, which exhibit the higher efficiency. Morphology study of AnE-PVstat:PCBM blends To obtain a deeper insight into the relation between morphology and performance of the fabricated BHJ solar cells, devices were analyzed by AFM. The AFM topographic images of an AnE-PVstat:PCBM blend and an AnE-PVstat:PCBM blend with 0.02 % of Ag

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(b)

1.1

1.0

1.0

0.9

0.9

0.8

0.8

Normalized Jsc

Normalized Voc

(a) 1.1

0.7 0.6 0.5 0.4 0.3

without Ag NPs with Ag NPs

0.2

0.7 0.6 0.5 0.4 0.3 without Ag NPs with Ag NPs

0.2

0.1

0.1

0.0 0

50

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0

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(d)

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0.8 0.7 0.6 0.5 0.4 0.3 0.2

300

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450

0.8 0.7 0.6 0.5 0.4 0.3 0.2


0.1

250

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Normalized efficiency

Normalized FF

(c)

200

Time (h)


0.1 0.0

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Time (h)

0

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Time (h)

Fig. 8 a Normalized Voc versus time, b normalized Jsc versus time, c normalized FF versus time, d normalized efficiency versus time plots of both AnE-PVstat:PCBM solar cells and AnE-PVstat:PCBM solar cells with 0.02 % of Ag NPs

NPs are depicted in Fig. 7. The roughness values were determined as 0.8 nm for basic blend and 2.0 nm for Ag-doped blend. Although the roughness values for films from the Ag NPs-doped blends are higher, they appear slightly more homogenous in their grain distribution. Furthermore, the grain shapes seem to be slightly more ordered for the blends with embedded Ag NPs. The increased contact area affects the amount of charge collected at metal–polymer interphase (Yu et al. 1995). Laboratory weathering test The consensus stability testing protocols for organic photovoltaic materials and devices (ISOS) were established based on interlaboratory studies and gives detailed descriptions of how to perform lifetime

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measurements of organic photovoltaics (OPVs) under both indoor and outdoor conditions (Reese et al. 2011). For indoor testing, ISOS-L-1 was carried out to obtain insights into the performance of AnEPVstat:PCBM solar cells under long-term permanent stress. The ISOS-L-1 test indicated that the AnEPVstat:PCBM solar cells with 0.02 % of Ag NPs possessed a good durability maintaining about 60 % PCE after 400-h test run. In contrast, AnEPVstat:PCBM solar cells without NPs showed only about 45 % PCE at the end (Fig. 8). Normalized Voc versus time graphs of both solar cells with and without Ag NPs indicated that open-circuit voltages were very stable (100 %) for a long time. Furthermore, normalized FF versus time graphs showed that FF of the solar cells doped with Ag NPs is more durable (90 %) than that of the Ag NPs-free solar cells (70 %). The


incorporated Ag NPs may hinder segmental motion of the polymer chains, leading to an increase in the glass transition temperature; which may increase the stability of solar cells.

Conclusion We have investigated the effect of Ag NPs on the photovoltaic performance of AnE-PVstat:PCBM solar cells. Moreover, ISOS stability tests of those cells were performed and evaluated. It was observed that the PCEs of AnE-PVstat:PCBM solar cells were improved from 2.46 to 3.10 % by the incorporation of Ag NPs. The ISOS-L-1 stability test indicated that AnE-PVstat:PCBM solar cells with Ag NPs exhibit an increased durability. Both morphology and impedance results seem to support the benefit of the addition of Ag NPs for the performance of the solar cells. Acknowledgments DAM Egbe and C Ulbricht are grateful to the Deutsche Forschungsgemeinschaft (DFG) for financial support in the framework of the priority program SPP 1355.

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