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Chemical Physics Letters 416 (2005) 42–46

Small-molecule organic solar cells with improved stability

Q.L. Song a,*, F.Y. Li b, H. Yang b, H.R. Wu a, X.Z. Wang a, W. Zhou a, J.M. Zhao a,X.M. Ding a, C.H. Huang b, X.Y. Hou a

a Surface Physics Laboratory (National Key Laboratory), Fudan University, Handan Road 220, Shanghai 200433, Chinab Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China

Received 13 July 2005; in final form 12 September 2005Available online 6 October 2005

Abstract

A stable small-molecule organic photovoltaic device with structure of ITOndonornacceptornbufferncathode is presented. A thin layer(�60 A) of tris-8-hydroxy-quinolinato aluminum (Alq3) instead of bathocuproine (BCP) is adopted as the buffer of the device, resultingin 150 times longer lifetime. The power conversion efficiency of the device is 2.11% under 75 mW/cm2 AM1.5G simulated illumination,and no perceptible efficiency degradation is observed for long-term storage of the device in vacuum or nitrogen-filled glove box. Moreeffective blocking of Alq3 than BCP against diffusion of cathode atoms and permeation of oxygen and/or water molecules is consideredas the main reason for the improved performance of the new device.� 2005 Elsevier B.V. All rights reserved.

1. Introduction

Organic photovoltaic (OPV) cells have the potentialadvantages of light weight and low-cost [1]. Unfortunately,the power conversion efficiency (gp) [2–10] and the lifetime[11–16] of OPV cells are far from satisfactory. gp higherthan 5% was reported recently by Forrest�s group [17],which is considered as a level suitable to commercialize thistype of OPV cells. In addition, the lifetime [11–16] of suchdevices is also too short to meet the requirement for prac-tical application. Most efforts [2–10] made in the past dec-ade focused on improvement of gp. By introducingbathocuproine (BCP) between fullerence (C60) and alumi-num cathode, organic thin film OPV cells based on copperphthalocynine (CuPc) and C60 have been demonstratedwith gp higher than 4% [10,17] recently.

Obviously, BCP used in these cells plays a very impor-tant role of blocking exciton transport and therefore isnamed exciton blocking layer (EBL) [6]. However, the life-time of these solar cells with BCP is only about a few hours

0009-2614/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2005.09.052

* Corresponding author.E-mail address: qunliang@gmail.com (Q.L. Song).

or even less without encapsulation [11,14]. This is probablydue to the instability of BCP, as it is readily crystallized inmoist environment [12]. BCP was already used to improvethe electroluminescence efficiency in organic light-emittingdiodes (OLEDs) [18], the lifetime of such OLEDs was alsoshort because of the instability of BCP [18]. Doping BCPwith PTCBI (3,4,9,10-perylenetetracarboxy-licbis-benz-imidazole) might be beneficial to improve the stability ofOPV cells [8,14], but few such stable OPV cells have beenrealized so far.

In this Letter, a small-molecule OPV cell with structureof ITOndonornacceptornbuffer layernAl, in which Alq3 isadopted to replace BCP as the buffer layer, is presented,with gp of 2.11% under 75 mW/cm2 AM1.5G simulated so-lar illumination and much improved stability. The lifetimeof the present OPV cells without encapsulation is 150 timeslonger than that of the OPV cells with a BCP buffer layer.Such a lifetime improvement results probably from the rea-son that Alq3 effectively block the oxygennwater to perme-ate through the acceptor layer. Another role of the Alq3buffer is the efficiency enhancement effect by blocking thediffusion of cathode atoms into active layer during thedeposition.

Q.L. Song et al. / Chemical Physics Letters 416 (2005) 42–46 43

2. Experimental

The present CuPcnC60 OPV cells are fabricated on thepre-cleaned glass substrates coated with transparent con-ducting ITO anode. The sheet resistance is about 30 X/square. The substrates are heated to about 150 �C in air aftersolvent cleaning and then loaded into a high vacuum cham-ber (�5 · 10�6 Pa). Before organic deposition, the sub-strates are in situ heated to about 120 �C for at least 1 h inthe high vacuum chamber. The organic films are depositedafter the substrates cooled down to room temperature. Thecathode is aluminum (Al), in situ deposited with shadowmask. The OPV cells are of 7.5 mm2 area, with 3-mm-wideITO overlapped by 2.5-mm-wide Al bar. The structure ofthree investigated devices is shown in the left inset ofFig. 1. In device A, BCP is used as the buffer layer; in deviceB, BCP mixed with Alq3 is used as the buffer layer, while indevice C, pure Alq3 thin layer is used as the buffer layer.Power conversion efficiencies are measured under illumina-tion of Oriel solar simulator producing an AM1.5G spec-trum. The intensity of AM1.5G irradiation is measuredwith a calibrated broadband optical power meter (verifiedby a standard crystalline silicon solar cell). The current–volt-age (I–V) characteristics are measured by Keithley 2400. Allthe measurements are carried out in air at room temperaturewithout encapsulation, except specially mentioned.

3. Results and discussion

Fig. 1 shows the I–V characteristics of three devices: A,B, and C fabricated for the present experiments. Short

Fig. 1. Dark and illuminated I–V characteristics of devices. The illumi-nated I–V curves are measured under 75 mW/cm2 simulated AM1.5spectrum. The left inset is the schematic structure of the organic solar cellsinvestigated, namely substratendonornacceptornbuffer layernAl, it isITOnCuPc (310 A)nC60(310 A)nBCP (100 A)nAl for device A; ITOnCuPc(300 A)nC60 (400 A)n BCP: Alq3 (80 A 1:1)nAl for device B; andITOnCuPc (300 A)nC60 (400 A)nAlq3 (60 A)nAl for device C. The rightinset shows the Fermi levels of electrodes, the highest occupied molecularorbital (HOMO) and lowest unoccupied molecular orbital (LUMO) ofCuPc, C60 and Alq3.

circuit current (Isc), open circuit voltage (Voc), and fill fac-tor (FF) of device A, in which BCP is used as buffer layer,are 5.09 mA/cm2, 0.42 V, and 0.49 under 75 mW/cm2 ofAM1.5 simulated solar spectrum irradiation, respectively.The initial gp (gp0) of device A is 1.39%, measured shortly(about 8 min) after removing the device from the vacuumsystem. But it shows rapid decease in subsequent (5 min la-ter) measurement. The effect of the new buffer is revealedby device B, in which a buffer of 50% Alq3 mixed with50% BCP is sandwiched between C60 and aluminum cath-ode. The Isc, Voc, and FF of device B are 5.8 mA/cm2,0.49 V and 0.464, respectively, under the same irradiationintensity of 75 mW/cm2. Meanwhile, these measured re-sults do not change obviously in a time interval of severalhours. From the value of Isc, Voc, and FF, the gp0 of deviceB is calculated to be 1.75%. As for device C, in which BCPis entirely substituted by pure Alq3, Isc, Voc, and FF are6.03 mA/cm2, 0.506 V and 0.52, respectively, under thesame irradiation intensity of 75 mW/cm2. Therefore, itsgp0 is 2.11%.

The degradation of gp of these three devices is shown inFig. 2. All the measurements are carried out with each cellkept in air without any encapsulation. It can be seen in theinset that the decrease of power conversion efficiency of de-vice A with BCP buffer is about 50% in 23 min. This is con-sistent with [11], in which the apparent discrepancybetween photocurrent derived from the I–V curves and thatfrom external quantum efficiency was ascribed to the deg-radation of devices when exposed to air. Recent work byHeutz et al. [14] also showed that the gp of devices withBCP buffer layer decrease by more than 70% in 2 h. By con-trast the devices with Alq3 as the buffer (device B and C)are quite stable over the same time range (inset ofFig. 2). If lifetime is defined as the degradation time of gpfrom gp0 to half of it, the lifetime for device A, B and Ccan be estimated from Fig. 2, to be 23 min, 27 and 61 h,respectively. It is remarkable that by substituting BCP withthin Alq3, the lifetime of OPV cells based on CuPcnC60 can

Fig. 2. Normalized power conversion efficiency of devices A, B and Cversus time. The inset shows the variation in the first 50 min, the abscissais enlarged.

44 Q.L. Song et al. / Chemical Physics Letters 416 (2005) 42–46

be improved over a factor of 150, and it looks like thatAlq3 mixed in the device facilitates the lifetime improve-ment, and the more Alq3 mixed in, the longer the lifetimewould be.

Since the power conversion efficiency of a cell is the prod-uct of Isc, Voc and FF, in order to clarify which one is themain factor for the gp degradation of these cells, Isc, Voc

and FF of device A and device C are plotted in Fig. 3.For device A, Isc, FF and Voc decreases by about 12%,16% and 42% in about half an hour, respectively, indicatingthe degradation of gp is mainly due to the degradation ofVoc and partly due to the degradation of Isc and FF. Sincethe structure of the three devices is the same except differentbuffer layers used, the drop of Voc of device A can be attrib-uted to the thin BCP layer, while the stability of Voc of de-vices B and C can be attributed to the presence of Alq3.Because of the instability of BCP in moist environment,the interfaces between BCP and aluminum cathode and be-tween C60 and BCP of the devices kept in air might be dam-aged. This assumption is confirmed by the fact that, if thedevices with BCP buffer are transferred into vacuum imme-diately after the measurements are carried out, their perfor-mance remains almost unchanged for several days. As fordevices B and C, whose degradation of gp is substantially

Fig. 3. (a) Degradation of device A in air. (b) Degradation of device C inair. The inset of Fig. 3b shows the variation of Voc, Isc and FF of device Cin the first 40 min.

suppressed, as shown in Fig. 2, contrast to device A, thedegradation of gp is mainly due to the decrease of Isc. In halfan hour, Isc decreases by about 2.7% and 2.5% for device Band device C, respectively, while Voc increase slightly, asshown in Fig. 3b. If these two types of devices are kept innitrogen environment, it is also found that their gp wouldnot decrease for several days, sometimes even slightly in-creased (not shown in figure) because of the slight increaseof Voc. It is expected that encapsulation to shield water andoxygen can make the lifetime of such OPV cells even longer.

The conducting levels below the LUMO of Alq3 alsomight be the reason why the transport of electron is notweakened. For another device we fabricated, with structureof ITOnCuPc (300 A)nC60 (400 A)nCuPc (60 A)nAl, themeasured Isc, Voc, FF and gp values are 5.8 mA/cm2,0.49 V, 0.53 and 2%, respectively, and its lifetime is about40 h without encapsulation. Though the LUMO level ofAlq3 or CuPc are higher than that of C60, the HOMO levelof Alq3 [24] or CuPc are also higher than that of C60, incontrast to that of BCP. It is also possible that Alq3 actsas exciton blocking layer. If this thin Alq3 or CuPc bufferlayer between C60 and cathode can be also considered toact as exciton blocking layer, the HOMO level of the bufferis not necessary to be lower than that of acceptor (C60), asshown in the right inset of Fig. 1.

Though damage caused by cathode deposition was pro-posed [6], it is not clear that how the damage affects theperformance of the cells. We suppose that the function ofthe thin buffer layer for efficiency improvement is to blockthe diffusion of aluminum atoms into the C60 films duringdeposition. The negative effect of diffused cathode atoms inC60 layer is revealed by intentionally mixing 1.2% (weight)aluminum with 275 A C60 film (herein called device D)while keeping all other parameters unchanged. In such acase, dramatic degradation of the performance can be ob-served from the I–V curve shown in Fig. 4. I–V curve of

Fig. 4. Illuminated I–V characteristics of device D and device C (alsoshown in Fig. 1), the structure of device D is ITOnCuPc (260 A)nC60(125 A)nC60 (275 A) mixed with 1.2% (weight) AlnAlq3 (60 A)nAl. Theilluminated I–V curves are measured under 75 mW/cm2 simulatedAM1.5G spectrum.

Q.L. Song et al. / Chemical Physics Letters 416 (2005) 42–46 45

device C is also shown for comparison. The Isc, Voc, andFF of device D are 1.32 mA/cm2, 0.46 V and 0.37, respec-tively, under the irradiation intensity of 75 mW/cm2.Meanwhile, the gp of device D is calculated to be 0.224%.Therefore, it can be concluded from Fig. 4 that, by blockingthe diffusion of metal atoms into organic layer, the short cir-cuit current, thus the gp can be improved significantly.

A thin buffer layer of either Alq3 or BCP, betweenacceptor and cathode, can obviously improve the efficiency.But Alq3 behaves very different from BCP if the devices arekept in air. The devices with BCP buffer degrade muchmore rapidly than that with Alq3 buffer, as shown inFig. 2. Whereas, when kept in vacuum or nitrogen-filledglove box to rule out the effect of oxygen and/or water,the devices, with either Alq3 or BCP buffer, show very longlifetime (over 3 months, which is much longer than thosekept in air). Though few studies have been reported aboutthe effect of oxygen and/or water on the lifetime of OPVcells, the effect of oxygen on the conductivity of single layerof C60 has been widely studied [21–23]. Upon exposure tooxygen, several orders of magnitude decrease of conductiv-ity of C60 has been observed by Hamed et al. [22], whileexposure to gases N2, Ar and He has no effect on the con-ductivity of C60 films. Therefore, it is assumed that the con-ductivity decrease of C60 due to the oxygen permeatingeasily through thermally deposited aluminum cathode[19,20] might be one of the reasons for the degradationof performance and lifetime of OPV cells. As shown inFig. 3, the lifetime degradation of devices with Alq3 buffercomes mainly from the decrease of Isc, this is assumed dueto the conductivity decrease of C60 film, caused by the oxy-gen permeating through the cathode and Alq3 buffer intothe C60 film. Contrarily, the degradation of devices withBCP buffer comes mainly from the degradation of Voc. Be-cause of the presence of water, BCP easily crystallizes [12]and forms numerous inner voids in those devices with BCPbuffer to facilitate more oxygen to diffuse into C60, leadingto the more rapid decrease of Isc. As seen in the inset ofFig. 3a,c, the decrease of Isc of device A in half an houris 12%, in contrast to only 2.5% decrease of device C. Inaddition, crystallization of BCP might cause the damageof the interface between BCP and cathode andnor betweenBCP and C60, resulting in rapid decrease of Voc (42% de-crease in half an hour) of device A. For devices withAlq3 buffer, besides its better oxygen blocking ability, itsbetter water withstanding ability depresses the formationof such inner voids, thus less oxygen will diffuse into C60,resulting in the slower degradation of Isc. The efficiencyof Alq3 to block the oxygen permeation, compared to thatof BCP, is further revealed by in situ current measurementsof the device ITOnC60nAlq3nAl and ITOnC60nBCPnAl un-der a fixed voltage, while filling gas into high vacuum. Thedevices of ITOnC60nBCPnAl shows much faster currentdrop (the current of which almost decreases to zero inabout one hour) than that of ITOnC60nAlq3nAl device,experimentally showing the better blocking ability ofAlq3 than that of BCP. Details of these results will be

published elsewhere. Briefly, it is the more effective oxygenblocking property and better water withstanding ability ofAlq3 that improve the lifetime of this type of OPV cells. Itis expected that the lifetime of the OPV cells can be furtherimproved by encapsulation to effectively insulate the de-vices from oxygen and/or water or using materials havingeven better oxygen blocking and water withstanding prop-erties than that of Alq3.

4. Conclusions

In summary, over 150 times increase of lifetime has beenobserved for the OPV devices with a thin Alq3 buffer layersandwiched between C60 and aluminum cathode, which canbe attributed to the more effective waternoxygen blockingeffect of Alq3 and better environment withstanding thanthat of BCP. The devices can be stored in vacuum or nitro-gen-filled glove box without obvious degradation for a longterm, revealing that the performance degradation is mainlydue to waternoxygen in the environment. In addition, sub-stitution of Alq3 for BCP as the buffer does not reducepower conversion efficiency, suggesting the role of thinAlq3 and BCP layer might be the same for efficiencyimprovement. The role of this thin layer between acceptorand cathode is likely to prevent cathode atoms from diffus-ing and oxygen molecules from permeating into the activeorganic films. Further investigation is expected to clarifythe mechanism of this thin layer.

Acknowledgments

This work is supported by CNKBRSF, and the Na-tional Natural Science Foundation of China under GrantNos. 10321003, 90401017, and 10404005.

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