Inverted small molecule organic photovoltaic cells on reflective substratesXiaoran Tong, Rhonda F. Bailey-Salzman, Guodan Wei, and Stephen R. Forrest Citation: Applied Physics Letters 93, 173304 (2008); doi: 10.1063/1.3005173 View online: http://dx.doi.org/10.1063/1.3005173 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/93/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Structural influences on charge carrier dynamics for small-molecule organic photovoltaics J. Appl. Phys. 116, 013105 (2014); 10.1063/1.4887076 Power losses in bilayer inverted small molecule organic solar cells Appl. Phys. Lett. 101, 233903 (2012); 10.1063/1.4769440 Efficient semitransparent small-molecule organic solar cells Appl. Phys. Lett. 95, 213306 (2009); 10.1063/1.3268784 Open circuit voltage enhancement due to reduced dark current in small molecule photovoltaic cells Appl. Phys. Lett. 94, 023307 (2009); 10.1063/1.3072807 Organic tandem solar cells comprising polymer and small-molecule subcells Appl. Phys. Lett. 89, 203506 (2006); 10.1063/1.2388938

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.82.252.58 On: Sat, 20 Dec 2014 21:39:48

Inverted small molecule organic photovoltaic cells on reflective substratesXiaoran Tong,1 Rhonda F. Bailey-Salzman,2 Guodan Wei,1 and Stephen R. Forrest1,a�

1Departments of Electrical Engineering and Computer Science, Physics, and Materials Scienceand Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA2Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA

�Received 9 September 2008; accepted 30 September 2008; published online 29 October 2008�

We demonstrate top-illuminated, inverted, small molecule photovoltaic cells grown on reflectivesubstrates employing copper phthalocyanine as the donor and 3,4,9,10-perylenetetracarboxylicbis-benzimidazole as the acceptor, with a sputter-deposited transparent indium tin oxide top cathodeand a metal anode, thereby reversing the conventional charge extraction properties of these contacts.The best device achieved a peak power conversion efficiency of 0.74�0.03%, reasonably consistentwith the optical simulations under 1 sun AM1.5G illumination giving 0.83�0.02%. This worksuggests that inverted organic solar cells grown on reflective substrates have potential uses such asfor power-generating coatings on opaque surfaces. © 2008 American Institute of Physics.�DOI: 10.1063/1.3005173�

Organic photovoltaic cells are gaining recognition fortheir possible use as inexpensive power sources. Conven-tional organic solar cells are fabricated on transparent sub-strates such as glass or plastic, and employ a transparentanode consisting of indium tin oxide �ITO�.1 In this work wedemonstrate inverted small molecule organic photovoltaiccells starting with a reflective substrate, and using a sputter-deposited transparent conducting oxide top cathode contact.This architecture eliminates the need for comparatively high-cost transparent substrates, allowing for use in applicationssuch as semitransparent power-generating coatings,2 or forgrowth on flexible and inexpensive opaque substrates.

In previous work, the term “inverted” has been used todescribe polymer cells grown on conventional transparentsubstrates, utilizing a Au grid as the reflective top contact,3

as well as to describe structures in which the donor is placedadjacent to the transparent cathode.4–7 In our work, we uti-lize a metal anode adjacent to the donor, and sputter-deposited ITO as the top transparent cathode, with light en-tering from the top vs. the substrate. The use of an ITOcathode eliminates the need for a transparent ITO/glass sub-strates or a conductive metal grid,3 thus enabling more flex-ible structure design without sacrificing device active area.

The inverted structure uses the donor, copper phthalo-cyanine �CuPc�, and the acceptor, 3,4,9,10-perylene-tetracarboxylic bis-benzimidazole �PTCBI�. Quartz sub-strates were solvent cleaned, then loaded into an electronbeam evaporator where a 1000 Å thick layer of Ni was de-posited at a rate of 5 Å /s. The Ni anodes were exposed toultraviolet ozone treatment for 30 min, then loaded into ahigh vacuum thermal deposition chamber with a base pres-sure of 5�10−7 Torr. Purified organic sources were grown ata pressure of 1�10−6 Torr, and a rate of 2 Å /s. The planardouble heterojunction solar cell consisted of a 400 Å thickCuPc layer, a 100 Å thick PTCBI layer, and a 100 Å thickbathocuproine �BCP� exciton blocking layer.8 A vacuumbreak occurred before attaching a shadow mask to the depos-ited layers and substrate in a high purity ��1 ppm H2O andO2� N2 ambient. The top contact consisted of a 400 Å thickITO layer sputter deposited at 20 W with the rate of 0.1 Å /s

through the shadow mask defining an array of 1 mm diam-eter circular cathodes. Performance of the cells was charac-terized in the dark and under simulated AM1.5G solar illu-mination �uncorrected for solar spectral mismatch� using a150 W Xe arc lamp.

Optical simulations of the device structure employed atransfer-matrix formalism9,10 to predict the short-circuit cur-rent density �JSC� under an AM1.5G solar spectrum at 1 sun

a�Electronic mail: stevefor@umich.edu.

FIG. 1. �a� Current-voltage characteristics for a conventional CuPc/PTCBIphotovoltaic cell structure in the dark �squares� and under simulated 1 sun,AM1.5G illumination �dashed lines�, and for the optimized inverted struc-ture in the dark �triangles�, and under illumination �dashed-dotted line�. Thesolid lines are fits to the measurement using theory in text. �b� Power con-version efficiency �circles�, open-circuit voltage �triangles� and fill factor�squares� for the inverted device structure: quartz /Ni /CuPc �350 � /PTCBI�100 � /BCP �100 � / ITO �400 � as a function of incident powerdensity.

APPLIED PHYSICS LETTERS 93, 173304 �2008�

0003-6951/2008/93�17�/173304/3/$23.00 © 2008 American Institute of Physics93, 173304-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.82.252.58 On: Sat, 20 Dec 2014 21:39:48

�100 mW /cm2� illumination. Optical constants of organicmaterials, sputtered ITO, and Ni films grown on Si substrateswere measured using ellipsometry.

Simulations suggest that the photocurrent for the in-verted device using a Ag cathode with structure:quartz/Ag �1000 � /BCP �100 � /PTCBI �300 � /CuPc�150 � / ITO �400 � should be 94% of that obtained for aconventional device with similar layer thicknesses. The 6%difference is due to Fresnel reflections at the transparentcathode surface, since light is incident through the top, highrefractive index ITO �nITO=2.05�, as opposed to through theglass substrate �nglass=1.42�. However, when this devicestructure was fabricated, it was nearly ohmic and showednegligible photoresponse, most likely due to damage in-curred during ITO sputter deposition directly onto the activeCuPc surface.11

The performance of a conventional device �glass/ITO/CuPc �200 � /PTCBI �250 � /BCP �100 � /Ag �1000 �showed an open-circuit voltage, VOC=0.47 V, a fill factor,FF=0.62, and a responsivity JSC / P0=0.041 A /W. Here, P0

is the incident optical power density under simulatedAM1.5G, 1 sun illumination. These values result in a powerconversion efficiency of �P=1.2�0.1%, which is consistentwith previous work.8 The dark current-voltage �JD-V� char-acteristic was fit to the modified ideal diode equation: JD

=JS�exp��q�V−JDRSA� /nkT��−1�, giving an ideality factorn=1.54, a series resistance RSA=1.54 � cm2, and a reversesaturation current JS=4.9�10−8 A /cm2. Figure 1�a� showsthe J-V characteristics in the dark �squares�, and under simu-lated AM1.5G, 1 sun illumination �dashed line�. The solidline represents the fit of this expression to the dark currentdata.

Performance data for the inverted structure ofquartz/Ni �1000 Å� /CuPc �350 Å� /PTCBI �100 Å� /BCP�100 Å� / ITO �400 Å� are shown in Fig. 1. The J-V charac-teristics in the dark �triangles� and under 1 sun illumination�dashed-dotted line� are plotted in Fig. 1�a�, along with thetheoretical fit to JD-V �line�. For this cell, JSC / P0 is relativelyindependent of P0, indicating that the responsivity is linearfor 0.22 mW /cm2� P0�166 mW /cm2. As shown in Fig.1�b�, the power conversion efficiency peaks at 0.74�0.03%with an incident power slightly lower than 1 sun, with VOC=0.44 V and FF=0.53 at 1 sun. The VOC increases withoutsaturating12 as P0 increases, and the peak FF=0.56 at P0=20 mW /cm2. Fits to JD-V characteristics yield a seriesresistance RSA=4.55�0.012 � cm2, an ideality factor n=1.78�0.004, and a reverse saturation current JS=1.46�10−7 A /cm2. The increased RSA and n of this structure in-dicates a somewhat higher contact resistance of the Ni cath-ode compared to the ITO anode in the control. Furthermore,there is considerable variation in performance from device todevice, possibly due to an observed sensitivity of the J-Vcharacteristics to the surface treatment of the metal prior toorganic layer deposition, to be discussed elsewhere.

The performance of the inverted device depends on thethicknesses of both the CuPc �tCuPc� and PTCBI �tPTCBI� lay-ers. Investigations of the thickness-dependent efficiencyshown in Fig. 2 employed a constant BCP layer thickness of100 Å. For tCuPc�350 Å, the photocurrent �at 1 sun� in-creases monotonically with tCuPc due to the relatively longexciton diffusion length �LD

CuPc� in the donor. The roll-off ofthe measured photocurrent at larger thicknesses of PTCBI ispartly due to the shift of the optical field intensity maximumaway from the CuPc/PTCBI interface, resulting in reducedexciton recombination at the donor-acceptor interface. From

FIG. 2. �a� Simulated �line� and measured photocurrent at one sun intensity �filled squares� for quartz /Ni /CuPc �x Å� /PTCBI �100 Å� /BCP �100 Å� / ITO�400 Å�, where x is varied from 100 to 400 Å. Power conversion efficiency, �P �squares�, open-circuit voltage, VOC �triangles�, and fill factor, FF �circles�, arealso shown under simulated 1 sun AM1.5G illumination. �b� Simulated �line� and measured photocurrent �filled squares� for quartz /Ni /CuPc �400 Å� /PTCBI�y Å� /BCP �100 Å� / ITO �400 Å�, where y is varied from 0 to 300 Å. Power conversion efficiency, �squares�, VOC �triangles�, and FF �circles�, are also shownunder simulated 1 sun AM1.5G illumination.

173304-2 Tong et al. Appl. Phys. Lett. 93, 173304 �2008�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.82.252.58 On: Sat, 20 Dec 2014 21:39:48

the fits to the data in Fig. 2�a� �solid line�, we obtain LDCuPc

=80�20 Å, and for PTCBI, LDPTCBI=30�5 Å, consistent

with previously obtained values for these materials.9 The di-vergence between the modeled and measured data at tCuPc=400 Å is ascribed to a somewhat decreased LD

CuPc in thethickest films, perhaps as a result of disruption of molecularstacking order as the film thickness is increased.9 The open-circuit voltage does not vary significantly with decreasingCuPc thickness, although �p decreases �for tCuPc�350 Å,see Fig. 2�a�� due to increased exciton quenching at the Ni-CuPc interface. Here, �p=0.74�0.03% is achieved at aCuPc thickness of 350 Å, declining to �p=0.60�0.04% at athickness of 400 Å.

Figure 2�b� shows device performance as a function ofPTCBI thickness, with tCuPc=400 Å. The low performanceof the device with 0 Å PTCBI verifies that the CuPc-Ni in-terface does not form a photoactive Schottky diode, and theheterojunction is active in generating free electrons andholes. A fit to the data with LD

PTCBI=30�5 Å and LDCuPc

=80�20 Å is shown by the solid line. As in the case ofvarying tCuPc, VOC and FF do not vary significantly withPTCBI thickness, and hence �P follows the same trends asJSC / P0, peaking at �P=0.60�0.04% for tPTCBI=100 Å. For

a 300 Å thick layer of PTCBI, �p=0.35�0.02%. FortPTCBI�100 Å, absorption in the PTCBI layer occurs at dis-tances considerably larger than the exciton diffusion lengthfrom the CuPc/PTCBI interface. In this case, photogeneratedexcitons cannot be efficiently dissociated, and hence result ina reduction in cell performance.

Using the experimentally determined values for LDCuPc

and LDPTCBI, in Fig. 3�a� we calculate the photocurrent at

1 sun, AM1.5G illumination as a function of the donor andacceptor layer thicknesses. This model simulates the J-Vcharacteristics based on the ideality factor, n and the reversesaturation current JS obtained from fits to the data in Fig. 2.In addition, the series resistance RSA is taken as a functionof the total thickness of photoactive layer, viz., RSA=0.05� ��tCuPc+ tPTCBI� /� � cm2, as measured from JD-V dataof the serial devices shown in Fig. 2. Finally, an excitonlifetime of 2 ns is assumed for both PTCBI and CuPc.9

The optimized photocurrent in Fig. 3�a� is JSC=3.2�0.002 mA /cm2, consistent with measurement. Figure3�b� indicates that the optimized device power efficiency forthe inverted structure is 0.83�0.02%, somewhat higher thanthe best device measured. The �11% difference betweensimulation and experiment primarily due to the high RSA �andhence lower FF�, which is more than ten times of that of thecontrol. This is further reflected in the ideality factor, n,which increases from 1.54 for the control to nearly 2 for theinverted structure. These variations are possibly due to un-controlled variations in the Ni surface that occur during pro-cessing, resulting in an increased contact resistance com-pared to that obtained using a conventional ITO anode.

In conclusion, we have demonstrated and modeled aninverted small molecule organic photovoltaic cell employinga reflective Ni anode and an ITO cathode. Device perfor-mance was found to be in good agreement with simulations,with a measured peak power conversion efficiency of0.74�0.03% compared to an optimized simulated value of0.83�0.02% These devices have potential applications aspower-generating coatings on a variety of flat or curvedopaque surfaces.

This work was supported in part by funds from the AirForce Office of Scientific Research and Global Photonic En-ergy Corp., Ewing, NJ.

1R. F. Bailey-Salzman, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 91,013508 �2007�.

2R. F. Bailey-Salzman, B. P. Rand, and S. R. Forrest, Appl. Phys. Lett. 88,233502 �2006�.

3M. Glatthaar, M. Niggemann, B. Zimmerman, P. Lewer, M. Riede, A.Hinsch, and J. Luther, Thin Solid Films 491, 298 �2005�.

4Q. L. Song, M. L. Wang, E. G. Obbard, X. Y. Sun, X. M. Ding, X. Y. Hou,and C. M. Li, Appl. Phys. Lett. 89, 251118 �2006�.

5Y. Şahin, S. Alem, R. de Bettignies, and J. Nunzi, Thin Solid Films 476,340 �2005�.

6C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley, S. A. Choulis,and S. J. Brabec, Appl. Phys. Lett. 89, 233517 �2006�.

7M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S. Ginley,Appl. Phys. Lett. 89, 143517 �2006�.

8P. Peumans, V. Bulovic, and S. R. Forrest, Appl. Phys. Lett. 76, 2650�2000�.

9P. Peumans, A. Yakimov, and S. R. Forrest, J. Appl. Phys. 93, 3693�2003�.

10L. A. A. Pettersson, L. S. Roman, and O. Inganas, J. Appl. Phys. 86, 487�1999�.

11P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 1 �2001�.12B. P. Rand, D. P. Burk, and S. R. Forrest, Phys. Rev. B 75, 115327 �2007�.

FIG. 3. �a� Simulated contour plot of the short-circuit current density �JSC�as a function of CuPc and PTCBI thicknesses. �b� Simulated contour plot ofpower conversion efficiency ��P� as a function of CuPc and PTCBI thick-nesses. For these plots, we assume the ideality factor n=1.78, reverse satu-ration current JS=1.46�10−7 A /cm2, and the series resistance as a functionof the CuPc and PTCBI thicknesses �tCuPc and tPTCBI� follows RSA=0.05� ��tCuPc+ tPTCBI� /Å�� cm2. The diffusion lengths of CuPc and PTCBI weretaken as 80�20 and 30�5 Å, with a lifetime of 2 ns. Values of Ni anode,ITO cathode, and BCP layers thicknesses were 1000, 400, and 100 Å, re-spectively, for these simulations.

173304-3 Tong et al. Appl. Phys. Lett. 93, 173304 �2008�

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.82.252.58 On: Sat, 20 Dec 2014 21:39:48