Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system

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Solar Energy Materials & Solar Cells 91 (2007) 1187–1193 www.elsevier.com/locate/solmat

Optimization of process parameters for high-efficiency polymer photovoltaic devices based on P3HT:PCBM system Sung-Ho Jina,, B. Vijaya Kumar Naidua, Han-Soo Jeona, Sung-Min Parka, Jin-Soo Parka, Sung Chul Kima, Jae Wook Leeb,, Yeong-Soon Galc a

Department of Chemistry Education and Interdisciplinary Program of Advanced Information and Display Materials, Pusan National University, Busan 609-735, Republic of Korea b Department of Chemistry, Dong-A University, Busan 604-714, Republic of Korea c Polymer Chemistry Lab, College of General Education, Kyungil University, Hayang 712-701, Republic of Korea Received 18 March 2007; received in revised form 31 March 2007; accepted 2 April 2007 Available online 8 May 2007

Abstract Here, we report the fabrication of high-efficiency poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) blend photovoltaic device. Process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio and cathodes structures are optimized in making the devices. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, made by mixing 6 wt% glycerol to PEDOT:PSS, is used as a buffer layer. Highest efficiency of 4.64% was obtained for the device made with 1:0.7 ratio of P3HT to PCBM, o-dichlorobenzene:chloroform cosolvent, newly developed slow process and G-PEDOT:PSS. Film morphology is evaluated by atomic force microscopy (AFM). Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements are also performed for the best device. r 2007 Elsevier B.V. All rights reserved. Keywords: Photovoltaic devices; P3HT; Cosolvent; Drying time; G-PEDOT:PSS

1. Introduction Converting solar energy into electrical energy is becoming important due to the crisis in conventional energy sources nowadays. There are various natural resources available to generate energy. Converting solar energy into electrical energy is one of such exploitation of the natural sources. Silicon-based inorganic photovoltaics are the best utilized for the last few decades in this direction. But, the drawbacks such as manufacturing costs and cumbersome fabrication process made researchers to look into easily processable nature and low-cost polymer materials. Much work has been done for almost last one decade on polymer photovoltaics, but the lower power conversion efficiency (PCE) limits their commercial usage [1–10]. After introCorresponding author. Tel.: +82 51 510 2727; fax: +82 51 581 2348.

E-mail addresses: [email protected] (S.-H. Jin), [email protected] (J. Wook Lee). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.04.001

duction of bulk-heterojunction concept, the PCE of polymer photovoltaics is nearing to 5% [7–10]. But, these values are not sufficient to meet realistic specifications for commercialization. Conceptually, the formation of bulk-heterojunction phase allows for bulk separation of photoinduced excitons and high-mobility removal of electron through the nanophase. Poly(3-hexylthiophene) (P3HT) has been the mostly used p-type material [7–10] in polymer photovoltaics along with a fullerene derivative, [6,6]-phenyl C61butyric acid methyl ester (PCBM) as an electron acceptor. Since hole is typically the high-mobility carrier in regioregular P3HT [11], the enhanced electron mobility was achieved by addition of electron acceptor. However, the difficulty in these systems arises when we account for the effects of morphological modifications in P3HT phase due to the introduction of nanophase [1,12]. There are significant number of studies that investigate the effects of processing parameters on blended photoactive nanophase

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[12–16]. Even, the molecular weight [17] and regioregularity [18] of P3HT also affect the performance of P3HT:PCBM devices. Generally, the electric power extracted from a photovoltaic device depends on both the photocurrent and photovoltage of the diode under illumination of a given intensity. In order to increase the PCE of a photovoltaic device, the practicable approach is to increase the photocurrent as much as possible, since the photovoltage is limited by the built-in potential and it is the difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the electron donor and acceptor materials [19]. Different device geometries and interface morphologies are evaluated for the purposes of trapping more light, dissociating excitons more efficiently, transporting charges with fewer impediments in order to extract more photocurrent [20]. Indeed, the solvents used for the preparation of active layer have shown a strong impact on its morphology, which influences the generation of photocurrent in the devices [15,16]. Unfortunately, till to date no conclusive result was made for optimal processing of the nanophase. In this paper, we tried to optimize the process parameters such as solvent, solvent drying conditions, cathode structures and modified buffer layer in the fabrication P3HT:PCBM devices for achieving high PCE. For the first time, we used cosolvent systems to make active layer of P3HT:PCBM composite and G-PEDOT:PSS, which is made by mixing 6 wt% glycerol to PEDOT:PSS (Baytron ‘‘P’’ AI 4083) is used as a buffer layer. The solvent drying conditions and the ratio of donor to acceptor are controlled in making the active layer. Aluminum (Al) and lithium fluoride (LiF)/Al cathodes are used. The device characteristics were correlated with morphology of the active layer characterized by atomic

force microscopy (AFM) measurements. Time-of-flight (TOF) and incident photon-to-current conversion efficiency (IPCE) measurements were studied for the best photovoltaic device.

2. Experimental 2.1. Photovoltaic device fabrication The photovoltaic devices were fabricated using blend solutions of P3HT and PCBM with the device structure displayed in Fig. 1. Indium–tin-oxide (ITO) coated glass, cleaned ultrasonically was used as transparent electrode. ITO surface was modified by spin coating 35 nm PEDOT:PSS or G-PEDOT:PSS layer after exposing it to ozone for 10 min. This layer was dried on hotplate at 120 1C for 10 min in air. P3HT purchased from Aldrich was used as supplied. PCBM was prepared according to the published procedure [21]. Cosolvents were prepared by mixing the respective solvents in 1:1 wt ratio. About 110 nm thickness of active layer was spin coated from 1.5 wt% blend solutions on the dried PEDOT:PSS or G-PEDOT:PSS layer after filtering through 0.45 mm PP syringe filters. Morphology of the active layer was modified by controlling the solvent drying time and conditions. Active layer dried on hotplate for 10 min at 120 1C is referred as ‘‘normal’’ process where as quick dried on hotplate for 1 min at 120 1C, then dried in covered Petri dish for 30–40 min at an ambient atmosphere is named as ‘‘slow’’ process. Photovoltaic device structure was completed by depositing either Al or LiF/Al as cathode under vacuum less than 3  10 6 Torr. Thermal annealing of the completed photovoltaic devices was carried out at 150 1C for 30 min inside the glove box.

Cathode

Photoactive Layer (G)-PEDOT:PSS ITO Glass Fig. 1. Photovoltaic device structure used for the fabrication of the devices.

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2.2. Sample preparation for AFM and TOF measurements Single layer device fabricated with active layer thickness of 3.5 mm without buffer PEDOT:PSS layer was used TOF measurements. In order to know the under laying morphology of active layer in the final devices, samples for AFM measurements were made by removing the Al cathode from the final devices using sticky tape. 2.3. Measurements

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Table 1 Photovoltaic properties of the devices made with different solvents and processes using 1:0.5 ratios of P3HT to PCBM, PEDOT:PSS and Al cathode Solvent

Process

VOCa (V)

JSCb (mA/cm2)

FFc (%)

PCEd (%)

DCB DCB:CB DCB:CHL DCB:CHL

Normal Normal Normal Slow

0.625 0.610 0.630 0.637

8.48 7.82 9.28 9.78

58.0 53.0 59.3 63.3

3.07 2.53 3.46 3.94

a

VOC (V): open-circuit voltage. JSC (mA/cm2): short-circuit current density. c FF: fill factor. d PCE: power conversion efficiency. b

Performance of photovoltaic devices were measured using a calibrated AM 1.5G solar simulator (Oriel 300 W) at 100 mW/cm2 light intensity adjusted with standard PV reference cell (2 cm  2 cm monocrystalline silicon solar cell, calibrated at NREL, Colorado, USA). Current density–voltage (J–V) curves were recorded using Keithley 236. All fabrication steps and characterization measurements were performed at an ambient atmosphere. Thickness of the thin films was measured using a KLA Tencor Alpha-step IQ surface profiler with an accuracy of 71 nm. The data given in this paper were verified by making each device more than five times. 3. Results and discussion The interpenetrating network (IPN) structure formed between electron donor and acceptor within the active layer of bulk-heterojunction photovoltaic devices is an important parameter to be studied for obtaining high PCE. It is a challenge to organize electron donor and acceptor materials such that their interface area is maximized, while typical dimensions of phase separation are within the exciton diffusion range and continuous, preferably short pathways for transport of charge carriers to the electrodes are ensured [1,22,23]. But, still this nanometer morphology was not completely estimated. Many parameters such as the processing conditions of blended active layer [23,24] and thermal treatment [6,25] affect this nanoscale morphology. For the first time, we used cosolvent systems in making the active layer of P3HT:PCBM composite because the phase separation is strongly influenced by the solubility of the blend components. The choice of solvent not only affects the thickness of the film at a given spin speed, but also the morphology of the film. A mixture of solvents may give better solubility of the blend components than single solvent. Different solvents have different solubilities and different mixing of the components in the blend. This results in both a larger internal donor–acceptor interface area and/or increased charge mobility in one of the components. The photovoltaic properties of the devices fabricated with o-dichlorobenzene:chlorobenzene (DCB:CB) and odichlorobenzene:chloroform (DCB:CHL) cosolvents along with DCB solvent using 1:0.5 ratio of P3HT to PCBM, PEDOT:PSS and Al cathode are given in Table 1. These

devices are made in normal process (see the Experimental Section for details). We have chosen these cosolvent systems based on physical properties of the solvents mainly the vapor pressure and also considering the earlier work [7–9]. A slight variation in open-circuit voltage (VOC) and large variation in short-circuit current density (JSC) and fill factor (FF) was observed for these devices. The overall photovoltaic performance of DCB:CHL cosolvent device was improved whereas DCB:CB device was decreased compared to the DCB device. The solubility of P3HT:PCBM blend system might be good in DCB:CHL cosolvent. The different evaporation rates of these solvents due to large difference in vapor pressures might be given good film morphology, that could be responsible for this improved performance compared to DCB:CB cosolvent. Higher JSC and FF of the DCB:CHL cosolvent device increases the PCE to 3.46% compared to other solvent devices. The solvent drying time and conditions have shown strong influence on the morphology of the active layer [8]. For a given spin speed and concentration, the time it takes for the film to dry is different for two different solvents and it depends on solvent’s vapor pressure. Hence, devices were fabricated with slow and normal processes with varied drying conditions (see the Experimental Section for details) using DCB:CHL cosolvent, PEDOT:PSS and Al cathode. The J–V curves of these devices are presented in Fig. 2 and data is included in Table 1. The slow process increases the JSC and FF of the device compared to the normal process. The improved JSC and FF increases the PCE of the slow process device up to 3.94%. In order to establish whether the different processes of composite films could be explained by difference in morphology, AFM images of the active layer films were taken by removing Al cathode using sticky tape from these devices and images are presented in Fig. 3. Both the films revealed smooth surfaces with r.m.s roughness of 3.05 and 2.31 nm for normal and slow processes, respectively. However, smoother texture was observed for slow process film than normal process. This suggests the more orderly arrangement of P3HT chains in slow process film than normal process film.

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1190

0 Normal

Current density [mA/cm2]

Slow -3

-6

-9

0.0

0.2

0.4

0.6

Bias [V] Fig. 2. The J–V curves of the devices fabricated with normal and slow processes using 1:0.5 ratio of P3HT to PCBM and DCB:CHL cosolvent measured at 100 mW/cm2 light illumination.

Fig. 3. AFM images of the films of the devices made with (a) normal and (b) slow processes using DCB:CHL cosolvent. Note that the color scale is 0–50 nm.

The slow process has some advantages than normal process. The quick drying on hotplate at 120 1C for 1 min and later allowing more time in covered Petri dishes at an ambient atmosphere will give different evaporation rates at two different conditions for the same film. This causes the formation of self-organized ordered structure in the P3HT:PCBM system. This might be improved the JSC and PCE of the slow process device compared to normal process device. Different research groups obtained their best results with different ratio of P3HT to PCBM [7–9]. Dyakonov and coworkers [13] studied the effect of donor to acceptor ratio and estimated that the best ratio may lie in between 1:1 and 1:0.9 ratio of P3HT to PCBM. The charge balance depends on overall film thickness and most of the researchers do not

disclose under which conditions their optimal concentrations are relevant. Hence, it is important to use appropriate amount of PCBM in the blended active layer. Here, we study the effect of P3HT and PCBM composition on photovoltaic performance using DCB:CHL cosolvent system, PEDOT:PSS, slow process and Al cathode. The photovoltaic characteristics of the devices fabricated with 1:1, 1:0.7 and 1:0.5 ratio of P3HT to PCBM are presented in Table 2. VOC of the devices decreased slightly as the amount of PCBM increases in the blended active layer. The device made with 1:0.7 ratios has shown highest JSC, FF, and increases the PCE up to 4.15%. AFM images of the active layers were investigated with 1:1, 1:0.7, and 1:0.5 ratio of P3HT to PCBM and the images are shown in Fig. 4. The r.m.s roughness of 1:1 film is 8.67 nm whereas

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the r.m.s roughness of 1:0.7 and 1:0.5 films are 2.45 and 2.31 nm, respectively. The texture of 1:0.7 and 1:0.5 films are much smoother compared to 1:1 film due to the more ordered structures. Once, obtaining the better performance with slow process using DCB:CHL cosolvent, PEDOT:PSS and 1:0.7 ratio of P3HT to PCBM, photovoltaic devices were made using Al and LiF/Al cathodes and the data is given in Table 3. Slightly improved VOC, JSC, and FF were observed for LiF/Al cathode device when compared to Al cathode device. These improved properties increase the PCE of LiF/Al cathode device up to 4.47%. After studying the effect of solvents, its drying conditions, electron donor to acceptor ratio and cathode materials on the performance, photovoltaic devices were made with G-PEDOT:PSS buffer layer using 1:0.7 ratio of P3HT to PCBM, DCB:CHL cosolvent and slow process. It has been shown that the addition of high-molecular weight Table 2 Photovoltaic properties of the devices made with different ratio of P3HT to PCBM using DCB:CHL solvent, PEDOT:PSS, slow process and Al cathode P3HT:PCBM

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

1:0.5 1:0.7 1:1

0.637 0.629 0.618

9.78 10.16 9.64

63.3 64.9 63.9

3.94 4.15 3.81

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alcohols such as glycerol to PEDOT:PSS will increase its conductivity [26]. The conductivity of PEDOT:PSS increased from 1.26  10 2 to 1.51 S/cm for G-PEDOT:PSS by adding 6 wt% glycerol. The J–V curves of these devices are presented in Fig. 5 and the data are summarized in Table 3. An improved JSC was observed for G-PEDOT:PSS devices due to the improved charge collection compared to PEDOT:PSS devices. This improvement may be due to the swelling and aggregation of colloidal PEDOT-rich particles, forming a highly conductive network [27,28]. Apart from this, the surface potential of the film will decrease due to the presence of glycerol in G-PEDOT:PSS. The increased JSC increases the PCE of devices made with GPEDOT:PSS to 4.64% and 4.62% for Al and LiF/Al devices, respectively.

Table 3 Photovoltaic properties of the devices made with different cathodes and G-PEDOT:PSS using 1:0.7 ratios of P3HT to PCBM, DCB:CHL cosolvent and slow process Cathode VOC (V) JSC (mA/cm2) FF (%) PCE (%) PEDOT:PSS PEDOT:PSS G-PEDOT:PSS G-PEDOT:PSS

Al LiF/Al Al LiF/Al

0.629 0.655 0.653 0.641

10.16 10.26 12.51 12.34

64.9 66.5 56.8 58.3

4.15 4.47 4.64 4.62

Fig. 4. AFM images of the films of the devices fabricated with (a) 1:1, (b) 1:0.7, and (c) 1:0.5 ratio of P3HT to PCBM, respectively using DCB:CHL cosolvent and slow process. Note that the color scale for the films is 0–100 nm.

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1192

0 G-PEDOT (LiF/Al) PEDOT (LiF/Al)

Current density [mA/cm2]

-3

G-PEDOT (Al) PEDOT (Al) -6

-9

-12

0.0

0.2

0.4

0.6

Bias [V]

Fig. 5. The J–V curves of the devices fabricated with G-PEDOT:PSS and different cathodes using DCB:CHL cosolvent and slow process measured at 100 mW/cm2 light illumination.

100 P3HT:PCBM (1:1) P3HT:PCBM (1:0.7)

IPCE [%]

75

50

25

0 400

500 600 Wavelength [nm]

700

Fig. 6. Incident photon-to-current conversion efficiency (IPCE) curves for the devices fabricated with different ratio of P3HT to PCBM using DCB:CHL cosolvent and slow process.

Fig. 6 displays the IPCE results of photovoltaic devices fabricated with different ratio of P3HT to PCBM using DCB:CHL cosolvent and slow process. The IPCE maximum of 69% at 490 nm was observed for 1:1 device. The IPCE maximum increases to 83% at 490 nm for 1:0.7 device, which is one of the highest IPCE values reported for polymer photovoltaics. TOF measurements were investigated for the device fabricated with 3.5 mm thicker active layer without PEDOT:PSS buffer layer using 1:0.7 ratio of P3HT to

PCBM, DCB:CHL cosolvent, slow process and Al cathode. The hole mobility of 5.0  10 6 cm2/V/s 1 and electron mobility of 6.5  10 6 cm2/V/s 1 were observed. Eventhough the values are little lower than literature values [8], good balance between the hole and electron mobility was observed. The most important point for commercialization of organic/polymer photovoltaic devices is the stability of the devices. A few studies on the lifetime of photovoltaic devices have been reported [29–33]. Here, we used a simple

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and crude method to study the stability of our best device under two different conditions. First, the devices were fabricated using our best conditions and the PCE of the devices was measured immediately after completing the fabrication. After measuring the efficiency, one device is immediately taken into glove box and kept in dark condition and another one is kept at the ambient atmosphere under dark. The PCE of these devices were periodically measured and again kept back under the same conditions. The whole measurement step was completed within 2–3 min. We continued the measurements up to 7 days. After 7 days, the Voc of the device is not varied, but the JSC and PCE of the device are reduced. We observed only 10% loss in PCE for the device stored in glove box, whereas 63% loss was observed for the device stored in room atmosphere compared to the PCE of the devices measured immediately after completing the fabrication. 4. Conclusions In conclusion, we optimized different process parameters like solvent, solvent drying conditions, electron donor to acceptor ratio, cathode structures and buffer layer using GPEDOT:PSS to achieve high PCE for P3HT:PCBM devices. DCB:CHL cosolvent system, 1:0.7 ratio of P3HT to PCBM, slow process, Al cathode and G-PEDOT:PSS are found to be the best conditions for making P3HT:PCBM photovoltaic devices, which has shown highest PCE of 4.64%. We believe that these results will help in understanding the important roles played by these process variables on the performance of polymer photovoltaics. Acknowledgments This research was supported by the University IT Research Center (ITRC) Project of the Ministry of Information and Communication (J. W. Lee) and by the Korea Science and Engineering Foundation (KOSEF) (S.H. Jin) Grant funded by the Korea government (MOST) (no. M10600000157-06J0000-15710). References [1] S.E. Shaheen, C.J. Brabec, N.S. Sariciftci, F. Padinger, T. Fromherz, J.C. Hummelen, Appl. Phys. Lett. 78 (2001) 841. [2] C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Adv. Func. Mater. 11 (2001) 15. [3] H. Spanggaard, F.C. Krebs, Sol. Energy Mater. Sol. Cells 83 (2004) 125. [4] K.M. Coakley, M.D. McGehee, Chem. Mater. 16 (2004) 4533.

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