All organic-based solar cell and thermoelectric generator hybrid device system using highly conductive PEDOT:PSS film as organic thermoelectric generator

Solar Energy 134 (2016) 479–483

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Solar Energy journal homepage: www.elsevier.com/locate/solener

All organic-based solar cell and thermoelectric generator hybrid device system using highly conductive PEDOT:PSS film as organic thermoelectric generator Jung Joon Lee a,1, Dohyuk Yoo a,1, Chanil Park a, Hyang Hee Choi b,⇑, Jung Hyun Kim a,⇑ a b

Department of Chemical and Biomolecular Engineering, Yonsei University, 262 Seongsanno, Seodaemun-gu, Seoul 120-749, Republic of Korea Institute of Nanoscience and Nanotechnology, Yonsei University, Seoul 120-749, Republic of Korea

a r t i c l e

i n f o

Article history: Received 5 December 2015 Received in revised form 3 May 2016 Accepted 6 May 2016 Available online 26 May 2016 Keywords: Organic solar cell Thermoelectric generator Hybrid device PEDOT:PSS

a b s t r a c t We report the fabrication of an all organic-based solar cell and thermoelectric generator hybrid device using an organic solar cell (OSC) and a single highly conductive PEDOT:PSS film as the solar cell and thermoelectric generator (TEG), respectively. The Seebeck coefficient of the single PEDOT:PSS film was measured as 19.8 lV/K. When the two devices were hybridized, hybridization loss (i.e., reduction in FF) caused by the internal resistance of the PEDOT:PSS film used as the organic thermoelectric generator (OTEG) decreased, because of the decreased resistance of the PEDOT:PSS film. The hybridization loss of FF was neglected when the resistance of the PEDOT:PSS film was below 1.36 O. The PEDOT:PSS films as OTEG were fabricated by drop-casting of the PEDOT:PSS solution on glass. The temperature difference in the PEDOT:PSS film was 5 °C in the hybrid device. As the resistance of the PEDOT:PSS film was 1.36 O, after hybridization, the PCE of the hybrid device was enhanced by the increased open circuit voltage (Voc) generated from the PEDOT:PSS film. The result shows that the PEDOT:PSS film can be applied as an OTEG material for all organic-based solar cell and thermoelectric generator hybrid devices. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Over the past decades, significant development has been made to photovoltaic (PV) devices as next-generation renewable energy resources for solar energy conversion. The key to improve the power conversion efficiency (PCE) of a PV device is the utilization of irradiated solar light with a broad wavelength range. To this end, effort has been devoted toward the development of new active materials and device structure design (Chen et al., 2014a, b; Dou et al., 2012; Li, 2012; Scharber et al., 2006). However, most PV devices absorb only a part of the visible and infrared light in the full sunlight wavelength. Thus, long-wavelength infrared light cannot be efficiently used. In most PV devices, unabsorbed longwavelength infrared light with low energy photons, as well as

Abbreviations: OSC, organic solar cell; TEG, thermoelectric generator; OTEG, organic thermoelectric generator; PV, photovoltaic; PCE, power conversion efficiency; DSSC, dye-sensitized solar cell; PSC, polymer solar cell; PEDOT:PSS, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate; DMSO, dimethyl sulfoxide. ⇑ Corresponding authors. E-mail addresses: [email protected] (H.H. Choi), [email protected] (J.H. Kim). 1 The first two authors made equal contributions to this work. http://dx.doi.org/10.1016/j.solener.2016.05.006 0038-092X/Ó 2016 Elsevier Ltd. All rights reserved.

some UV light with photons having energy higher than the bandgap energy of the active layer, are spontaneously transformed into heat. This waste heat energy corresponds to
40% of the solar spectral irradiance (Nelson, 2003). Therefore, it is necessary to recycle the waste heat generated from the solar-facing PV device. A thermoelectric generator (TEG) can convert heat directly into electric energy via the Seebeck effect; hence, a PV–TE hybrid device not only allows for the utilization of a broader spectrum of the irradiated solar light, but also prevents overheating that decreases the efficiency and stability of the PV device during operation (Royne et al., 2005). Several groups have reported PV–TE hybrid device systems based on various PV devices such as Si solar cells, dye-sensitized solar cells (DSSCs), and polymer solar cells (PSCs) Chang and Yu, 2012; Deng et al., 2013; Park et al., 2013; Wang et al., 2011; Zhang et al., 2013. However, there is hardly any report on all organic-based PV–TE hybrid device systems using an organicbased TEG, because PV devices have mostly been hybridized with a commercialized TE module based on inorganic materials. A part of TE as well as PV device should be also needed to be organized for PV–TE hybrid device to have advantages of organic electronic device with low-cost, lightness, flexibility and mass and easy

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fabrication. Thus, all organic-based PV–TE hybrid systems can minimize manufacturing cost and complexity in production. A PV–TE hybrid device based on organic materials, with a doped P3HT film as the organic TEG (OTEG), has been reported, but its PCE is very low, 2.1  102% (Suzuki et al., 2010). In a PV–TE hybrid device system, the internal resistance of the TEG is a very important factor affecting hybridization loss because the PV and TE components are electrically connected in series. The hybridization loss due to the high internal resistance of the TEG decreases the FF of the hybrid device, which in turn results in a reduction in the PCE. Therefore, it is essential to select an organic thermoelectric material with low resistance as well as good thermoelectric characteristics as the OTEG. Among the various organic thermoelectric materials based on conductive polymers, poly(3,4-ethylenedioxy thiophene) polystyrene sulfonate (PEDOT:PSS) exhibits the highest conductivity with promising thermoelectric properties (Lee et al., 2014; Park et al., 2014; Yue and Xu, 2012). Hence, PEDOT:PSS was selected as the most suitable OTEG candidate for the all organic-based PV–TE hybrid device system in our research. In this study, we focus on the effect of the resistance of a single PEDOT:PSS film on hybridization loss of the hybrid device, at various film thicknesses, to verify the applicability of PEDOT:PSS films as OTEG material for all organic-based PV–TE hybrid systems. The resistance of the PEDOT:PSS films decreased from 58.73 to 1.36 O with an increase in the film thickness from 0.19 to 12.30 lm. The single PEDOT:PSS film with a resistance of 1.36 O did not induce any hybridization loss that would lead to FF reduction. The results show that the PEDOT:PSS film is applicable as an OTEG material for all organic-based PV–TE hybrid devices. 2. Experimental 2.1. Materials PEDOT:PSS solution (Clevios PH 1000) was purchased from Heraeus Ltd. Dimethyl sulfoxide (DMSO, 99%) was purchased from Samchun Pure Chemicals (S. Korea). All materials were used without further purification. 2.2. Fabrication of the hybrid device First, the OSCs were fabricated by a previously reported procedure (Lee et al., 2015). Glass coated with ITO (150 nm thick, 10 X/ h) was cleaned by sequential ultrasonic treatment in acetone and isopropanol. The cleaned ITO glass was dried at 100 °C under vacuum for 1 h and then treated using an ultraviolet–ozone cleaner for 15 min. PEDOT/PSS solutions (Clevios PH) mixed with PEGME in a PSS/PEGME weight ratio of 1:0.5 were spin-coated onto the cleaned ITO glass at 2000 rpm for 40 s. The PEDOT:PSS-coated ITO glass samples were then thermally annealed at 150 °C for 30 min in air and transferred to a nitrogen-filled glove box. P3HT (Rieke Metals, 4002-EE) and C61-butyric acid methyl ester (PCBM) (Nano-C, 99%) in a weight ratio of 1:0.6 were dissolved in chlorobenzene. The solution containing P3HT and PCBM was stirred for 1 d at 50 °C, filtered through a 0.2 lm PTFE syringe filter, and spin-coated onto the PEDOT:PSS-coated ITO glass at 600 rpm for 40 s. The thin films were then thermally annealed on a hot plate at 120 °C for 10 min. To complete the structure of the device, top contact was formed through sequential thermal evaporation of lithium fluoride (0.6 nm) and aluminum (100 nm) through a shadow mask under vacuum (pressure: 2  106 Torr). Finally, the fabricated OSCs were encapsulated in a nitrogen-filled glove box using UV epoxy resin and covered glass. Second, highly conductive PEDOT:PSS films were prepared to be used as the organic TEG. PEDOT:PSS solution (Clevios PH 1000)

were filtered through a 5.0 lm nylon syringe filter, spin-coated twice at 1500 rpm for 60 s onto glass, and thermally annealed on a hot plate at 150 °C for 5 min in air (thickness = 190 nm). To prepare PEDOT:PSS films with thickness of a few to several tens of micrometers, 50, 100, and 200 lL PEDOT:PSS solutions were drop-casted onto glass and annealed on a hot plate at 80 °C for 30 min in air. All the PEDOT:PSS films had dimensions of 2 cm  1 cm. Finally, the OSC–OTEG hybrid device was fabricated by attaching one-half of the PEDOT:PSS film, which acts as the OTEG, onto the backside of the OSC using scotch tape, so that the other half of the films protruded from the OSC. 2.3. Characterization and measurements The resistance of the PEDOT:PSS films was measured by using a two-point probe meter with a Fluke 289 RMS multimeter after deposition of silver paste on the PEDOT:PSS films to enable Ohmic contact. The distance between two points of silver paste deposition was 6 mm. The thicknesses of the PEDOT:PSS films were measured by a surface profiler (Alpha step IQ, KLA-Tencor). Current/voltage curves were recorded by a electrochemical workstation (Keithley Model 2400) and a solar simulator (1000 W xenon lamp, Oriel, 91193), which provided a simulated AM 1.5 spectrum (100 mW/ cm2). The simulated light was calibrated with a Si solar cell (Fraunhofer Institute for Solar Energy Systems, Mono-Si + KG filter, Certificate No. C-ISE269) to a sunlight intensity of 1 (100 mW/cm2). The temperature at the important points of the OSC–OTEG hybrid device was measured using a Fluke 289 RMS multimeter. The Seebeck coefficient of the PEDOT:PSS films with silver paste was measured by using a home-built setup consisting of two Peltier devices to maintain a controlled temperature gradient. The Peltier devices were controlled by using a Keithley 2400 source-measure unit and a Keithley 2200-30-5 power supply. Two thermocouples were used to measure the temperature gradient across the PEDOT:PSS films. 3. Results and discussion 3.1. OSC–OTEG hybrid device system Fig. 1 illustrates the OSC–OTEG hybrid device system using highly conductive PEDOT:PSS films as an OTEG. The temperature gradient of the commercialized TE module based on inorganic materials used as the TEG has been formed in the upper and lower direction vertically in most previously reported studies of the PV– TE hybrid device (Chang and Yu, 2012; Deng et al., 2013; Park et al., 2013; Wang et al., 2011; Zhang et al., 2013). One of the many advantages of organic materials compared to inorganic materials is easy and large-area film coating by using a solution process. Hence, as shown in Fig. 1(a), the OSC was hybridized with the prepared PEDOT:PSS film in mainly the horizontal direction as the OTEG. This is because using the common coating method in the horizontal direction with spin, bar, and spray coating is much easier to fabricate large-area films than the coating method in the vertical direction. The range of the thickness of the prepared PEDOT:PSS film is from a few hundred nanometers to several tens of micrometers. Thus, as shown in Fig. 1(a), it is almost impossible for the temperature gradient inside the PEDOT:PSS film to be formed in the vertical direction film because the thickness of the prepared PEDOT:PSS film is not thick enough. However, as shown in Fig. 1(b), covering a part of the PEDOT:PSS films out of area of OSC by sunshade can generate a temperature gradient in the PEDOT:PSS film. Thus, the output voltage of the PEDOT:PSS film (VTE) as the OTEG can be obtained using the following equation

V TE ¼ SðT TEG;hot side  T TEG;cold side Þ ¼ SDT

ð1Þ

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(a)

(b)

h+ Fig. 1. (a) Schematic illustration of the OSC–OTEG hybrid device using a highly conductive PEDOT:PSS film as the OTEG and (b) with sunshade for the temperature gradient of the OTEG.

where S in the Seebeck coefficient of the PEDOT:PSS film. In Fig. 1 (b), the TTEG,hot side and TTEG,cold side of the PEDOT:PSS film as the OTEG were measured using a fluke 289 RMS multimeter. The measured temperatures are shown in Table 1. The TTEG,hot side and TTEG,cold side of the PEDOT:PSS film were 35.4 and 30.3 °C, respectively, and the ambient temperature during the experiment was 29.2 °C. Thus, the temperature different of 5.1 °C was naturally generated by only covering a part of the PEDOT:PSS films out of area of OSC with sunshade without any special energy supply and equipment to form the temperature different in PEDOT:PSS film. The equivalent circuit of the OSC–OTEG hybrid device system could be represented as shown in Fig. 2. The output voltage of the hybrid device (Vhybrid) is the same as the sum of the output voltage of the OSC (VPV) and the output voltage of the PEDOT:PSS film as the OTEG (VTE) because the PV and TE are connected in series. However, the fill factor (FF) of the hybrid device can be decreased by adding the internal resistance from the TEG compared to that of only the OSC (Park et al., 2013; Wang et al., 2011; Zhang et al., 2013). It is expected that the short-circuit current density (Jsc) of the hybrid device will be determined using the Jsc of the OSC only because the PEDOT:PSS film is very highly conductive as a conductor. In previous studies of the PV–TE hybrid device, the efficiency of the PV–TE hybrid devices is higher than that of only the PV devices, because the effects of voltage added by the TE device are higher than the effects of the FF reduced using the internal resistance of the TE device on the increase in efficiency of the PV–TE hybrid devices (Park et al., 2013; Wang et al., 2011; Zhang et al., 2013). Thus, in order to maximize the efficiency increase in the hybrid device, in addition to a higher Seebeck coefficient and temperature difference in the TE device for higher VTE, a lower internal resistance of the TE device for minimization of FF loss or lossless FF is also very important. More precisely, matching the internal resistance of the TE device with the circuit of the PV device is essential for the minimization of the FF loss or lossless FF in the hybridization between the TE and PV device (Park et al., 2013). Thus, the resistance of the PEDOT:PSS film was measured using a two-point probe meter depending on the thickness to sufficiently lower its resistance. The distance measured between the two points was 6 mm. The thickness of the PEDOT:PSS films was controlled using an amount of drop-casted PEDOT:PSS solution. As shown in Fig. 3, as the thickness of the PEDOT:PSS films

Table 1 Temperature measurement results of the important points of the OSC–OTEG hybrid device. TTEG,hot Temperature (°C)

side

35.4

TTEG,cold

side

30.3

DTTEG

Tambient

5.1

29.2

PV

TE

+ Rsh, PV

VPV+VTE

− Ri, TE

Rs, PV

VTE = S(Thot –Tcold ) Fig. 2. Equivalent circuit of the OSC–OTEG hybrid device connected in series.

increases from 0.19 to 12.30 lm, the resistance of the PEDOT:PSS films decreases from 58.73 to 1.36 O. 3.2. Performance of OSC–OTEG hybrid device The focus was to confirm the applicability of the PEDOT:PSS film as a material of the OTEG for all organic based PV–TE hybrid devices. Thus, in this study, we first investigate the performance of the hybrid device depending on the resistance of each single PEDOT:PSS film as the OTEG. Fig. 4 shows the photocurrent density–voltage curves of the OSC only and OSC–OTEG hybrid devices under AM 1.5 (100 mW/cm2) illumination. The photovoltaic performance of these devices was summarized in Table 2 from the photocurrent density–voltage curves of Fig. 4. The OSC

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60

Resistance (Ω)

50

40

30

20

10

0

0

2

4

6

8

10

12

Thickness ( µ m) Fig. 3. Graph of the resistance of the PEDOT:PSS films depending on the thickness.

2

2

Current density (mA/cm )

0 OSC only OSC-OTEG hybrid (ROTEG = 58.73 Ω)

-2

OSC-OTEG hybrid (ROTEG = 4.30 Ω) OSC-OTEG hybrid (ROTEG = 2.32 Ω)

-4

OSC-OTEG hybrid (ROTEG = 1.36 Ω)

-6 -8

-10 -12

0.0

0.2

0.4

0.6

Voltage (V) Fig. 4. Photocurrent density–voltage (JV) curves of the OSC–OTEG hybrid device depending on the resistance of each PEDOT:PSS film as the OTEG.

only performs with a Jsc of 10.35 (±0.02) mA/cm2, Voc of 587.70 (±0.30) mV, FF of 48.12 (±0.07)%, and PCE of 2.93 (±0.01)%. In the hybrid devices, the hybridization loss of the FF by the internal resistance of the PEDOT:PSS film as the OTEG was decreased as the resistance of the PEDOT:PSS film was decreased. The hybridization loss of the FF was neglected when the resistance of the PEDOT: PSS film was below 1.36 O. As the resistance of the PEDOT:PSS film as the OTEG was 1.36 O, the hybrid device with which the single PEDOT:PSS film was connected in series with the OSC performs with a Jsc of 10.37 (±0.03) mA/cm2, Voc of 587.80 (±0.30) mV, FF of

48.13 (±0.08), and PCE of 2.94 (±0.01)%. The Seebeck coefficient and temperature different in the PEDOT:PSS film were 19.8 lV/K and 5.1 °C, respectively. The PCE of the hybrid device was enhanced by the increased Voc as the voltage generated from the single PEDOT:PSS film as the OTEG. Here, note that only a single PEDOT:PSS film was used as the OTEG in the hybrid device. Although there is a 0.01% increase in the PCE by hybridizing the OSC with a single PEDOT:PSS film with 1.36 O at 12.30 lm, the PCE of the hybrid device could be further increased visibly through modulation of the PEDOT:PSS film. Wei et al reported PEDOT:PSS thermoelectric modules (Wei et al., 2014). Basically, to achieve a high-output power, the p-and n-type organic thermoelectric material, such as PEDOT:PSS, are necessary to make a pair in organicbased module fabrication. However, the resistance of the current n-type organic thermoelectric materials is not as lower as that of the p-type organic thermoelectric material of the PEDOT:PSS for the lossless FF in the PV–TE hybrid device (Dubey and Leclerc, 2011; Zhang et al., 2014). In addition, the n-type organic thermoelectric material is very unstable in air (Russ et al., 2014; Schlitz et al., 2014). Hence, the n-type organic thermoelectric material is limited to be used as a pair with PEDOT:PSS in organic-based module fabrication. However, as mentioned above, the output power of the PEDOT:PSS film can be increased through its modulation (Wei et al., 2014). It is essential that the module is fabricated while lowering the internal resistance of the module as much as possible to avoid or minimize the FF loss. Thus, the lossless FF in the OSC– OTEG hybrid device with a single PEDOT:PSS film of 1.36 O at 12.30 lm as the OTEG shows that the PEDOT:PSS film is sufficiently applicable as a material of the OTEG for all organic based PV–TE hybrid devices. The series connections among the PEDOT:PSS film could produce higher voltage because of the additional increase in the voltage of the hybrid device, which could result in increased efficiency of the hybrid device. Thus, the Seebeck coefficient of the PEDOT: PSS films was measured depending on the number of PEDOT:PSS films connected in series. Fig. 5(a)–(c) shows the schematic illustration, optical image, and Seebeck coefficient of the PEDOT:PSS films connected in series, respectively. The resistance and thickness of each PEDOT:PSS film were 1.36 O and 12.30 lm, respectively. As shown in Table 3, the Seebeck coefficient of the PEDOT: PSS film was increased depending on the number of PEDOT:PSS films connected in series almost without loss of the Seebeck coefficient, which could increase the voltage of the hybrid device. However, the increase in the efficiency of the hybrid device could be limited because when current is constant, increase in the voltage could result in increase in the resistance also using the following equation:

V ¼ IR

ð2Þ

However, the parallel connections could provide higher current density (Wei et al., 2014). Thus, the series and parallel connections in the PEDOT:PSS thermoelectric module fabrication are necessary

Table 2 Photovoltaic performance of the OSC–OTEG hybrid device depending on the resistance of each single PEDOT:PSS film as the OTEG. Device type

Thickness of PEDOT:PSS film (lm)

Resistance of PEDOT:PSS film (O)

Jsc (mA/cm2)

Voc (mV)

FF (%)

PCE (%)

OSC only Hybrid

– 0.19 (±0.02) 3.40 (±0.02) 7.76 (±0.36) 12.30 (±1.78)

– 58.73 (±3.00) 4.30 (±0.23) 2.32 (±0.08) 1.36 (±0.02)

10.35 (±0.02) 10.36 (±0.03) 10.37 (±0.03) 10.36 (±0.02) 10.37 (±0.03)

587.70 (±0.30) 589.14 (±0.52) 588.11 (±0.20) 587.90 (±0.25) 587.80 (±0.30)

48.12 (±0.07) 43.15 (±0.11) 47.47 (±0.04) 48.10 (±0.05) 48.13 (±0.08)

2.93 (±0.01) 2.63 (±0.02) 2.90 (±0.01) 2.93 (±0.01) 2.94 (±0.01)

00

00

00

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(a)

(b)

(c)

Hot Side

Seebeck coefficient (μV/ K)

120

Cold Side

100 80 60 40 20 0

6 PEDOT:PSS films connected in series

1

2

3

4

5

6

Number of PEDOT:PSS films connected in series

Fig. 5. (a) Schematic illustration of the PEDOT:PSS films connected in series. (b) Optical image of six PEDOT:PSS films connected in series. (c) Graph of the Seebeck coefficient depending on the number of PEDOT:PSS films connected in series.

Table 3 Seebeck coefficient depending on the number of PEDOT:PSS films connected in series. Number of PEDOT:PSS films connected in series Seebeck coefficient (lV/K)

1 19.8 (±0.2)

2 39.7 (±0.1)

3 59.2 (±0.1)

4 78.98 (±0.2)

5 98.59 (±0.2)

6 118.28 (±0.2)

to lower the internal resistance of the module as much as possible to avoid or minimize the FF loss. Additional studies are being conducted in this regard to further increase the efficiency of the hybrid device. 4. Conclusions In conclusion, we have demonstrated the applicability of a highly conductive PEDOT:PSS film as a material of the OTEG for all organic based PV–TE hybrid device system. The hybridization loss of the FF was neglected when the resistance of a single PEDOT:PSS film was below 1.36 O with 12.30 lm as the OTEG in the OSC–OTEG hybrid device. However, to further increase the efficiency of the hybrid device using the PEDOT:PSS films as OTEG, PEDOT:PSS thermoelectric modulation with the lower internal resistance is necessary. Acknowledgments This research was supported by the Nano Material Technology Development Program through the National Research Foundation of South Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP, South Korea) (NRF-2014M3A7B4050960, 2014M3A7B4051745) and the Priority Research Center Program through the NRF funded by the Ministry of Education, Science and Technology (No. 2009-0093823). References Chang, H., Yu, Z.-R., 2012. Integration of dye-sensitized solar cells, thermoelectric modules and electrical storage loop system to constitute a novel photothermoelectric generator. J. Nanosci. Nanotechnol. 12, 6811–6816. Chen, C.C., Chang, W.H., Yoshimura, K., Ohya, K., You, J., Gao, J., Hong, Z., Yang, Y., 2014a. An efficient triple-junction polymer solar cell having a power conversion efficiency exceeding 11%. Adv. Mater. 26, 5670–5677. Chen, Z., Cai, P., Chen, J., Liu, X., Zhang, L., Lan, L., Peng, J., Ma, Y., Cao, Y., 2014b. Low band-gap conjugated polymers with strong interchain aggregation and very

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