Solar Energy Materials & Solar Cells 95 (2011) 852–855

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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Spray-coated organic solar cells with large-area of 12.25 cm2 Sun-Young Park a,b, Yong-Jin Kang a, Seunghun Lee a, Do-Geun Kim a, Jong-Kuk Kim a, Joo Hyun Kim b, Jae-Wook Kang a,n a b

Department of Material Processing, Korea Institute of Materials Science, Changwon 641-831, Republic of Korea Division of Applied Chemical Engineering, Department of Polymer Engineering, Pukyong National University, Busan 608-739, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 August 2010 Received in revised form 26 October 2010 Accepted 31 October 2010 Available online 20 November 2010

This study evaluated the possibility of utilizing a spray-coating process for large-area organic solar cells (OSCs) combined with a metal electrode geometry. The effects of the cell area in spray-coated OSCs were investigated systematically by introducing a metal sub-electrode and grid-electrode to realize large-area cells of up to 12.25 cm2. The series resistance could be reduced significantly by inserting a metal gridelectrode into the indium tin oxide (ITO) anode, yielding a power conversion efficiency of 2.11% at a cell area of 12.25 cm2 and 2.49% at an effective photocurrent generated area of 11.23 cm2 under AM.1.5 simulated illumination. This is comparable to the 3.13% obtained in the cell produced by spray-coating at a cell area of 0.38 cm2. & 2010 Elsevier B.V. All rights reserved.

Keywords: Spray-coating process Organic solar cell Large-area Metal grid-electrode

1. Introduction Solar energy has promising potential as an alternative to fossil fuels. Polymer-based organic solar cells comprising p-conjugated polymers and fullerene derivatives have attracted considerable interest owing to their lightness, simple solution processing and flexibility [1–4]. Over the last few years, many research groups have focused on increasing the efficiency of organic solar cells. Recently, organic solar cells (OSCs) using thieno[3,4-b]thiophene and benzodithiophene polymers (PTBs) and the fullerene derivative PC71BM from a dichlorobenzene/1,8-diiodoctane mixed solvent showed an increase in power conversion efficiency (PCE) of up to 7.4% in a cell area of 0.1 cm2 [5]. However, a scale-up of the cell area leads to a decrease in PCE caused by an increase in series resistance, owing to the high sheet resistance of the transparent electrode (e.g. indium tin oxide, ITO) and the difficult optimization of large-area thin film deposition [6–8]. The fabrication of largescale, efficient and solution processable OSCs requires scalable methods of device processing showing large-area deposition with little efficiency loss. The spin-coating method, which is currently the laboratory standard, is not compatible with a large-area process or roll-to-roll; thus alternatives are needed. The spray-coating process is well established in graphic art, industrial coatings and painting [9]. This method employs small amounts of material in a dilute solution and is well suited to the largearea deposition of a controlled device thickness. It also minimizes the

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Corresponding author. Tel.: + 82 55 280 3572; fax: + 82 55 280 3570. E-mail address: [email protected] (J.-W. Kang).

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.10.033

amount of material discarded during deposition. The spin-coating method results in large amounts of material being cast off during spinning. Several groups have reported the fabrication of OSCs using a spray-coating method with results similar to those of spin-coating [9–14]. These efforts have focused mainly on process development with a small cell size (o1 cm2) and not the key advantage of that process, i.e. large-area OSC application [9–13]. Previously, the effects of the cell area in OSCs fabricated by spincoating were examined by introducing a metal sub-electrode to reduce the resistive loss of ITO [6]. The series resistance could be reduced significantly using the sub-electrode, yielding a PCE of 2.670.3% up to a cell area of 4.08 cm2. However, the devices in previous studies were based on a spin-coated active layer, which is not as ideal in terms of simplicity for large-area, roll-to-roll processing. This study examined the possibility of utilizing a spray-coating process for large-area OSCs combined with an electrode geometry. Spray-coated OSCs with metal grid-electrode geometries to reduce the efficiency loss were examined with a large cell area of up to 12.25 cm2.

2. Experimental details The OSCs fabricated on the ITO-coated glass (sheet resistance 10 O/sq.) substrate with a 5  5 cm2 size were first cleaned in an ultrasonic bath containing acetone and then in boiling isopropyl alcohol. The substrates were then dried in an oven and treated with UV–ozone for 5 min. The substrates were transferred to a nitrogenfilled glove box, and poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS; Baytron P) solutions were then spin coated

S.-Y. Park et al. / Solar Energy Materials & Solar Cells 95 (2011) 852–855

after passing the solution through a 0.45 mm filter at 5000 rpm with a thickness of approximately 40 nm, followed by annealing at 150 1C for 1 min on a hot plate. The poly(3-hexylthiophene) (P3HT):[6,6]-phenylC61 butyric acid methyl ester (PCBM) blend solution was prepared in a 1:1 mass ratio in 1,2-dichlorobenzene (20 mg/ml P3HT and 20 mg/ml PCBM). The photoactive materials were coated by spin-coating (with a spin speed of 600 rpm) in a glove box or spray-coated in air with a thickness of 250 nm. For spray-coating, the P3HT:PCBM blend solution was diluted to 10 mg/ml P3HT and 10 mg/ml PCBM. The spray-coating conditions were optimized to minimize the surface roughness by varying the solution injection rate, carrier gas flow, nozzle-to-substrate distance and printing speed. Slow evaporation and pre-annealing (150 1C for 20 min on a hot plate) were carried out in a glove box. The cathode electrode with LiF (1.2 nm) and aluminum (120 nm) were evaporated on the photoactive layer through a shadow mask at o10  6 Torr. The current density–voltage (J–V) characteristics of the OSCs were measured under AM 1.5 simulated illumination with an intensity of 100 mW/cm2 (Pecell Technologies Inc., PEC-L11 model) [15–18]. The intensity of sunlight illumination was calibrated using a standard Si photodiode detector with a KG-5 filter. The J–V curves were recorded automatically with a Keithley SMU 2410 source meter by illumination of the OSCs.

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Fig. 2. Spray-coated P3HT:PCBM layer thickness as a function of injection rate with a constant air pressure of 50 psi, nozzle-to-substrate distance of 3.5 cm and printing speed of 6 cm/min along x-axis and 1800 cm/min along y-axis.

3. Results and discussion Fig. 1 shows a schematic diagram of the spray-coating process apparatus. The deposited P3HT:PCBM layer was optimized at a solution injection rate of 200 ml/min, compressed air pressure of 50 psi, nozzle-to-substrate distance of 3.5 cm, and printing speed of 6 cm/min along x-axis and 1800 cm/min along y-axis. The spraycoating system comprises two nozzles as the core and clad. The core nozzle was connected to the injection pump for the P3HT:PCBM solution and the clad nozzle was linked to an air compressor for the carrier gas. A computer controlled xy-stage and injection pump allow a reproducible thickness of the deposited film. The spraycoating conditions were optimized to minimize the surface roughness. Fig. 2 shows spray-coated P3HT:PCBM thickness as a function of the injection rate at a constant gas pressure, nozzle-to-substrate distance and printing speed. With a single pass spray process, it was possible to produce P3HT:PCBM thin films reproducibly with thicknesses ranging from 250 to 450 nm. The 250-nm-thick spray- and spin-coated films were characterized by atomic force microscopy (AFM) to better understand the morphological properties, as shown in the inset of Fig. 3. The optimized spray-coated P3HT:PCBM film showed a similar surface roughness to the spin-coated layer with a root mean square (rms) roughness of 16 and 18 nm, respectively. Significantly lower rms

Fig. 1. Schematic diagram of spray-coating apparatus.

Fig. 3. Current density versus voltage (J–V) characteristics of organic solar cells fabricated by spin- and spray-coating methods. All devices were measured under AM 1.5 illumination conditions (100 mW/cm2). Inset—AFM images of spin- and spray-coated P3HT:PCBM layer with a thickness of 250 nm.

roughness was observed using the optimized spray-coating process. For comparison, the poorly spray-coated films showed a large rms roughness of 450 nm (data not shown). Fig. 3 shows the J–V characteristics of the spin- and spray-coated devices. The spraycoated device with a cell area of 0.38 cm2 showed a short circuit current density (Jsc) of 8.42 mA/cm2, an open circuit voltage (Voc) of 0.58 V, a fill factor (FF) of 64.2% and a PCE of 3.13%. These results are comparable to those of the spin-coated devices (Jsc ¼ 8.51 mA/cm2, Voc ¼0.58 V, FF¼ 65.0% and PCE ¼3.21%), which indicated that the spray-coating method can be used to evaluate large-area OSCs. Fig. 4 shows the OSC layout with a metal sub-electrode and gridelectrode type geometry. In the case of the OSCs with the subelectrode, a 200-nm-thick aluminum (Al) layer was deposited on the ITO anode through a shadow mask, which defined the device cell area, as shown in Fig. 4(c) [6]. The sub-electrode defines the active area and works as a conducting electrode at the same time with very low resistance. The devices contained square shaped cells, 0.38, 4.08, 9.08 and 12.25 cm2 in size. A simple way of decreasing the overall resistance of the ITO anode was the fabrication of metal grids on top of the anode, which can provide an alternative, low-resistance pathway for the current. For OSCs with a metal grid-electrode, the Al

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Table 1 Summary of spray-coated OSCs’ performance with sub-electrode and grid-electrode type geometries with different cell areas. Cell type

Cell area Jsc (mA/cm2) Voc (V) (cm2)

FF (%)

PCE (%)

Rs (O cm2)

Sub-electrode

0.38 4.08 9.08 12.25

0.58 0.58 0.58 0.58

64.2 53.5 38.5 38.2

3.13 2.64 1.79 1.68

5 20 37 42

Grid-electrode

12.25 7.85 (11.23)a (8.56)a

0.59 (0.59)a

45.5 (49.3)a

2.11 (2.49)a

32 (29)a

8.42 8.50 7.99 7.57

a Calculated values were based on the effective photocurrent generating area due to shadowing from the metal grid (Agrid,eff ¼12.25 cm2  91.7% ¼11.23 cm2).

Fig. 4. OSC layouts with metal (a) sub-electrode and (b) grid-electrode type geometries. Cross sectional layouts for (c) sub-electrode and (d) grid-electrode type. The organic layers are the PEDOT and P3HT:PCBM layers.

significantly lower and higher Rs and FF, respectively [7]. A decrease in Rs of 25% was achieved, thereby improving FF by 20%. Furthermore, Jsc increased from 7.57 to 7.85 mA/cm2, even though less area contributed to photocurrent generation due to shadowing from the metal grid-electrodes. In other words, effective Jsc of the metal grid integrated device was 8.56 mA/cm2 when only the effective photocurrent generating area (Agrid,eff ¼12.25 cm2  91.7%¼11.23 cm2) was considered, which is similar to the small-area device with Asub ¼0.38 cm2. With improved Jsc and FF, the PCE is increased significantly to 2.11% at Agrid ¼12.25 cm2 and 2.49% at Agrid,eff ¼11.23 cm2 compared to 1.68% for the large-area device with the sub-electrode at Asub ¼12.25 cm2, as summarized in Table 1. Improvement in the PCE of 25% and 48% were achieved using a grid-electrode type geometry at Agrid ¼12.25 cm2 and Agrid,eff ¼11.23 cm2, respectively. The grid-electrode based devices showed improvement in Rs, FF and PCE, due to a decrease in the resistive loss of ITO.

4. Conclusions

Fig. 5. (a) Current density versus voltage (J–V) characteristics of spray-coated OSCs for the sub-electrode type with cell areas Asub ¼0.38, 4.08 and 12.25 cm2, and gridelectrode type with a cell area of Agrid ¼ 12.25 cm2. (b) Images of spray-coated OSCs with sub-electrode and grid-electrode geometries.

metal was deposited through two shadow masks for the subelectrode and grid-electrode (Fig. 4(d)). Fig. 5 shows the effect of the cell area on the OSC performance indicating the J–V characteristics of the sub-electrode and gridelectrode geometries. There were no changes in Voc with cell area and type of geometry, confirming the independence of Voc on the series resistance (Rs). With an increase in cell area (A) from Asub ¼0.38 to 12.25 cm2, the PCE of the sub-electrode geometry cells was reduced dramatically from 3.13% to 1.68%. Rs from the inverse slope of the J–V curve at J¼0 increased significantly from 5 to 42 O cm2 with increasing cell area, resulting in a decrease in FF from 64.2% to 38.2%. The device with a grid-electrode showed better overall performance than the large-area device without a metal grid, as well as

Spray-coating is an excellent alternative to spin-coating for the fabrication of OSCs, showing PCEs with similar values. This study examined the effects of the cell area in spray-coated OSCs by introducing a metal sub-electrode and grid-electrode so as to produce large-area cells up to 12.25 cm2. The series resistance could be reduced significantly by inserting a metal grid-electrode in the ITO anode, yielding a PCE of 2.11% at Agrid ¼12.25 cm2 and 2.49% at Agrid,eff ¼11.23 cm2 compared to 1.68% at Asub ¼ 12.25 cm2 and 3.13% at Asub ¼0.38 cm2. With further optimization, all-spray deposited OSCs may become a reality in the near future for low-cost roll-to-roll manufacturing.

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