Al bilayer cells

ARTICLE IN PRESS Solar Energy Materials & Solar Cells 94 (2010) 924–929

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

Letter

The investigation of the stability and optimal encapsulation time for ITO/CuPc/C60/Al bilayer cells Xi Xi a, Fangxin Li b, Qinglei Meng b, Yuqiang Ding c, Jingjia Ji d,e, Zhengrong Shi d,e, Guohua Li b,e,n a

School of Communication and Control Engineering, Jiangnan University, Wuxi, 214122, China School of Science, Jiangnan University, Wuxi, 214122, China c School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, China d Suntech Power Co., Ltd., Wuxi, 214028, China e Jiangsu (Suntech) Institute for Photovoltaic Technology, Wuxi, 214028, China b

a r t i c l e in f o

a b s t r a c t

Article history: Received 26 September 2009 Received in revised form 30 December 2009 Accepted 2 January 2010 Available online 22 January 2010

The stability of the ITO/CuPc/C60/Al cells exposed in air without encapsulation is studied. It is found that the performances of the cells will first enhance a little during the first hour after fabrication, and then begin to decline. The reason can be attributed to the appearance of Al2O3 layer at the interface of C60/Al. The function and mechanism of this Al2O3 layer are similar to a LiF cathode buffer layer. The verification experiments were established and the optimal encapsulation time for ITO/CuPc/C60/Al cells was demonstrated. The optimal encapsulation time in this work is an hour after removing from the vacuum. & 2010 Elsevier B.V. All rights reserved.

Keywords: Organic solar cells Stability Al2O3 Encapsulation time

1. Introduction Organic solar cells (OSC) are attracting a great deal of attention because of flexibility, low cost, light weight and large-area applications [1–9]. From the first double-layer structured solar cell introduced by Tang in 1986 [10], different methods have been proposed in order to improve the power conversion efficiency and the stability of organic solar cells. The highest reported efficiency has recently risen to 5% for small molecule heterojunction devices [11–13], and to above 6% for polymer bulk-heterojunction cells [14,15]. Stability is another important factor of organic solar cells for practical applications. Lifetimes of up to 2000 h continuous operation have been reported for the devices protected from oxygen and water ingress [16]; and thousands of hours are possible with evaporated C60 (in vacuum) [17,18]. Frederik C. Krebs states that a polymer solar cell based on a process free from vacuum steps and fullerenes, which can be stored under ambient conditions (25 1C and 3575% relative humidity) in the dark for 6 months, is presented [19]. Most studies of degradation of organic solar cells have focused on the oxidative damage to the active layers [20]. Sometimes it

n Corresponding author at: School of Science, Jiangnan University, 1800 LiHu Ave. Wuxi, Jiangsu 214122, China. E-mail address: [email protected] (G. Li).

0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.01.001

can be seen from figures of some papers studying the stability of the cells that the performances enhance a little during the first few hours after removing from the vacuum [19–22]. Usually, this phenomenon is thought to be caused by anneal effect in polymer solar cells [19,22,23]. However, most of these cells are of bulkheterojunction structure. In this work, the enhancement has also been found in small molecule organic solar cells with a planeheterojunction. Hundreds of cells with the structure of ITO/CuPc/ C60/Al are fabricated in this work and more than 75% of the cells show the phenomenon that the performances enhance a little during the first hour after removing from the vacuum and then start to decline. The highest open-circuit voltage, short-circuit current density and efficiency all appeared an hour after fabrication. Since these cells are all small molecule cells with plane-heterojunctions, the anneal effect may not be a major factor to this enhancement phenomenon. There should be other reasons and the probable mechanism is also discussed in this work.

2. Experiment The cells are fabricated in a typical sandwich structure. The ITO glass substrates (with a sheet resistance of 7 O/&) is sequentially cleaned by ultrasonic treatment in acetone, isopropyl alcohol and deionized water; blown by N2 gas; and treated by UV-Ozone for 15 min. The UV wavelength used here is 185 nm. The power of the UV lamp is 20 W. It is placed in an airtight box with a capacity of

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about 40 L. The ozone gas is generated using the UV light to excite the oxygen in air inside the box. The temperature and humidity inside the box are, respectively, kept at 20 1C and 20%. CuPc and C60 were purchased from Tokyo Chemical Industry Co., Ltd. The purities of CuPc and C60 are 498.5% and 499.5%, respectively, and they are not further purified. All the layers are fabricated by vacuum evaporation at a pressure of 2.5  10 3 Pa. CuPc and C60 powders are evaporated from quartz crucibles and the crucibles are placed in an evaporation tungsten boat. All the other materials, like Al, LiF and Al2O3, are directly put in the tungsten boat. The thicknesses are 20, 40 and 100 nm for CuPc, C60 and Al, respectively, which are monitored by a quartz oscillator thickness monitor and are also checked by an ellipsometer (produced by Gaertner Scientific Corporation). The active area of the device is about 0.06 cm2. The current– voltage (J–V) characteristics were measured every 15 min for each cell with a Keithley 2400 sourcemeter under an illumination of 100 mW/cm2 with an AM1.5G sun simulator. Since light intensity and temperature can strongly influence the stability [20,22,23], the storage of the cells is very important. Between two consecutive measurements, the cells were stored under ambient conditions and without illumination, while the temperature was kept at 25 1C and the humidity was kept below 20%. The illumination was sporadically used for the measurements of the efficiency, and the irradiated time is so short that the light intensity influence can be neglected.

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3. Results and discussion The stability of the cells exposed in air without encapsulation is studied. Fig. 1 shows a typical cell’s stability in the experiments. All the photovoltaic characteristics are normalized to their initial values. The initial performances of this cell are shown as follows, Jsc = 1.41 mA/cm2, Voc = 478 mV, FF= 0.296, Z = 0.1995% (under an illumination of 100 mW/cm2 with an AM1.5G sun simulator). As the cell’s structure and the fabrication processes have not been optimized, as well as due to the limitation of the equipments, the performances of the cells are a little poor. However, they still can exhibit the results we need, the performances of the cell (except FF) do not decrease during the first hour, but they enhance a little. The Jsc and the Voc reach the maximums at about an hour after removing from the vacuum. Because of the greater enhancements of the Jsc and Voc, the efficiency also increases during the first hour, and reaches the highest value at about an hour after fabrication, although FF declines monotonously. In this work, the emphasis is focused on the reason of the gradual enhancements of performances during the first hour after fabrication. But how long the cells will stably operate is not our concern. So the stabilities shown in Fig. 1 are only during the first 2 h after fabrication. Inside the cells, since factors such as the infiltration of In into active layers [23,24] and the reaction between Al and C60

Fig. 1. A typical stability of the cells without encapsulation in the air-exposure, with the structure of ITO/CuPc/C60/Al. The initial performances: Jsc = 1.41 mA/cm2, Voc = 478 mV, FF = 0.296, Z = 0.1995%. All the data in the figures have been normalized to their initial values. The temperature was kept at 25 1C, and the humidity was kept below 20% during the two measurements. Figures (a)–(d) are the stabilities of Jsc, Voc, FF and Z, respectively.

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[17,18,23] will all cause degradation of the cells, the enhancement during the first hour should be from the circumstance. H2O vapor and oxygen are two of the most important influencing factors for the stability of organic solar cells [17–23,25–28]. It must

Fig. 2. The change of the mass of the cell with the structure of ITO/CuPc/C60/Al. The area of the substrate is 3 cm  1 cm. The cell was stored under an ambient condition (humidity: o 20%; temperature: 25 1C).

be identified which factor, water or oxygen, is the major one causing the enhancement. Some researchers found that pure oxygen does not seem to speed up degradation while humidity profoundly influences the stability leading to rapid degradation [19,22,27,28]. However, the humidity in the lab is kept so low that H2O vapor may have less influence than that of oxygen. The mass of the cell exposed in air has been measured by electronic balance to observe the absorption of H2O vapor. Fig. 2 shows the weight change of the cell with the structure of glass (1 mm)/ITO(200 nm)/ CuPc(20 nm)/C60(40 nm)/Al(100 nm) in the clean air. The mass of the cell is almost unchanged. It means that the cells have very weak ability to absorb H2O vapor from such a low humidity environment. There is another good way to prove that oxygen is the major factor in the experiments. The cells were stored under a nitrogenatmosphere condition (N2: 499%; humidity: o20%; temperature: 25 1C) without illumination during the two measurement. And the stability of one of these cells is shown in Fig. 3. It is clear that the enhancement of the performances of those cells stored in nitrogen atmosphere is not evident. The performances are much more stable. The only difference between the N2 storage condition and the ambient storage condition is the concentration of oxygen. Thus, under an ambient condition, oxygen may react with the cells, and enhance the performances.

Fig. 3. A typical stability of the cells without encapsulation under a nitrogen-atmosphere storage condition (N2: 499%; humidity: o 20%; temperature: 25 1C), with the structure of ITO/CuPc/C60/Al. The initial performances: Jsc = 1.38 mA/cm2, Voc = 482 mV, FF =0.294, Z = 0.1956%. All the data in the figures have been normalized to their initial values. Figures (a)–(d) are the stabilities of Jsc, Voc, FF and Z, respectively.

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From Figs. 1–3, it can be concluded that humidity may not be a major factor here, while the enhancements of the performances are caused by oxygen in this work. It is found the path in which oxygen can diffuse into the devices is through the outer electrode, while the diffusion through the sides of the devices is not a major channel [23,27,29–32]. At the interface of C60/Al, an Al2O3 layer may appear since the cell is exposed in air and without encapsulation [25,26]. The Al2O3 layer will block the excitons and reduce the recombination probability of excitons at this interface, while the electrons can pass through this insulating layer by tunneling. Since this Al2O3 layer is so thin, the tunneling efficiency of electrons is extremely high and the Jsc and Voc of the cells are enhanced. The function and mechanism are similar to that of LiF buffer layer. Additionally, because of the existence of Al2O3 layer at the interface of C60/Al, the reaction between the acceptor and metallic cathode [17,18,23] can be tremendously slowed down. The deceleration of the formation of ‘‘C–Al’’ [23] can also promote an improvement of the stability. However, with the continuous increase in the thickness of the Al2O3 layer, more and more electrons will be blocked by a thicker insulating layer, and the transmission efficiency of electrons will decrease. Then the performances start to decline later. Due to the gradual oxidation of Al and C60, the series resistance of the cell increases, which also results in a continuous degradation of the fill factor. In order to verify the analysis, an Al2O3 layer (1 nm) was deposited between the C60 layer and Al cathode during the fabrication. A cell with LiF cathode buffer layer (1.5 nm) was also

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fabricated for purposes of comparison. The performances of the cells are shown in Fig. 4. The short-circuit current densities, opencircuit voltages, calculated fill factors and power conversion efficiencies are summarized in Table 1. It is very clear that the Jsc and Voc of Device B and Device C are very similar. So it can be concluded that the reason of the gradual enhancements of performances during the first hour after fabrication is contributed by the growth of Al2O3 layer at the interface of C60/Al. However, the fill factor of Device B is a little lower than that of Device C. This is because during the time that the cell is exposed in air, C60 can also be oxidized [33]. Since the oxygen molecules act as electron traps in the lattice of fullerene molecules [33], the conductivity of C60 will gradually decrease, and the series resistance of the cell will increase. This situation causes a lower FF of Device B than that of Device C. Additionally, the initial performances of Device C are much better than that of Device A. This phenomenon can be attributed to the function of Al2O3 cathode buffer layer. This demonstrates that the Al2O3 cathode buffer layer does have the ability to enhance the performances. Furthermore, the performances of Device C and Device D are nearly the same. On the other hand, LiF and Al2O3 are two typical insulators and the function of Al2O3 ought to be same as that of LiF. Another set of experiments has been done in order to verify the analysis. In these experiments three pieces of the cells with the structure of ITO/CuPc/C60/Al were simultaneously fabricated so that the three cells can be considered with the same performances. One of the cells was without encapsulation, and measured immediately after fabrication (Device E). The second cell was immediately encapsulated by MgF2 (4500 nm) after fabrication, which was deposited by vacuum evaporation (Device F). The third one was placed in dark and exposed in air for an hour. The temperature was kept at 25 1C, and the humidity was kept below 20%. Then the device was encapsulated by MgF2 (Device G). The initial performances of Device F and Device G were the results

Fig. 4. The J–V curves of the cells in the verification experiments under an illumination of 100 mW/cm2 with an AM1.5G sun simulator.

Table 1 The performances of the cells in the verification experiments (under an illumination of 100 mW/cm2 with an AM1.5G sun simulator). No.

Structure

Jsc (mA/cm2)

Voc (mV)

FF

Z (%)

Device A Device B

ITO/CuPc/C60/Al initial performances ITO/CuPc/C60/Al performances an hour later ITO/CuPc/C60/ Al2O3(1 nm)/Al initial performances ITO/CuPc/C60/ LiF(1.5 nm)/Al initial performances

1.41

472

0.297

0.1977

1.74

521

0.283

0.2566

1.69

525

0.311

0.2759

1.75

518

0.321

0.2910

Device C Device D

Fig. 5. The initial J–V curves of the three cells in the encapsulation experiments under an illumination of 100 mW/cm2 with an AM1.5G sun simulator.

Table 2 The initial performances of the three cells in the encapsulation experiments (under an illumination of 100 mW/cm2 with an AM1.5G sun simulator). CELLS

Jsc (mA/cm2) Voc (mV) FF

Device E (without encapsulation) 1.36 Device F (encapsulated immediately) 1.39 Device G (encapsulated after exposing 1.78 in air for an hour)

485 488 525

Z (%)

0.293 0.1926 0.288 0.1954 0.270 0.2523

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Fig. 6. The stabilities of the cells with and without encapsulation in air-exposure, with the structures of ITO/CuPc/C60/Al. The temperature was kept at 25 1C, and the humidity was kept below 20% during the two measurements. Figures (a)–(d) are the stabilities of Jsc, Voc, FF and Z, respectively.

measured after encapsulation. Fig. 5 and Table 2 show the initial performances of the three cells. It is obvious that the initial performances of Device G are the best. This phenomenon can be attributed to the growth of Al2O3 cathode buffer layer during the storage. Fig. 6 shows the stabilities of the cells in air-exposure with different encapsulation times, as well as the stabilities of the cells without encapsulation. All the photovoltaic characteristics of Device E are normalized to their initial values, while all the data of Device F and Device G are normalized to the relevant initial values of Device E. From Fig. 6, it can be seen that the stabilities of the cells with MgF2 encapsulation are improved a lot. Although the photovoltaic characteristics of Device F are very stable, they are still a little lower than the relevant highest values of Device E. That is because the cell is separated from the circumstance so early that the Al2O3 cathode buffer layer cannot be formed. Furthermore, from Fig. 6, at any time, almost all the data of Device G, except the fill factors, are the best among the three devices. Since Device G had been exposed in air for an hour before encapsulation, there was enough time for the growth of the Al2O3 layer. The cell was encapsulated while the thickness of the insulating buffer layer reached the appropriate thickness. Under this condition, the growth of the Al2O3 layer will be stopped, and the performances of the cell will be kept stably at the highest values. Due to the oxidation of both Al and C60 during the storage, the series resistance increases and the fill factors of Device G are the lowest.

It can be concluded that the performances of the cells with the structure of ITO/CuPc/C60/Al can be improved a little after being exposed in dry air for an appropriate time. An appropriate encapsulation time is needed, if with a much better encapsulation method, the cells can be steadily kept at their best status.

4. Conclusion The stability of the ITO/CuPc/C60/Al cells exposed in air without encapsulation is studied. It is found that the performances of the cells will first enhance a little during the first hour after fabrication, and then begin to decline. It is found that a very low humidity may not be a major factor here, while the enhancements of the performances are caused by oxygen in this work. The enhancements can be attributed to the appearance of Al2O3 layer at the interface of C60/Al. The function of Al2O3 layer is similar to an insulating cathode buffer layer. Although FF declines a little, the efficiency still shows the highest value at about an hour after fabrication, because of the large enhancements of Jsc and Voc. With the unceasing increase in the thickness of Al2O3 layer, the performances then begin to decline. It can be concluded that it is good for the cells with the structure of ITO/CuPc/C60/Al to be exposed in dry air for an appropriate time. And the optimal encapsulation time is about an hour after being removed from the vacuum.

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