Solar Energy Materials & Solar Cells 95 (2011) 1489–1494

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

n Corrnn Cor

E-m

marin.r

journal homepage: www.elsevier.com/locate/solmat

Engineering of hybrid interfaces in organic photovoltaic devices

Konstantinos Fostiropoulos n, Marin Rusu nn

Institut fur Heterogene Materialsysteme, Helmholtz-Zentrum Berlin fur Materialien und Energie, Lise-Meitner Campus, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

a r t i c l e i n f o

Available online 20 January 2011

Keywords:

Organic solar cells

Phthalocyanine

C60

Hybrid interface

ITO

Mg/Ag cathode

48/$ - see front matter & 2010 Elsevier B.V. A

016/j.solmat.2010.12.027

esponding author. Tel.: 49 30 8062 42719; fa

responding author. Tel.: 49 30 8062 42604; fa

ail addresses: fostiropoulos@helmholtz-berlin

usu@helmholtz-berlin.de (M. Rusu).

a b s t r a c t

In this study, we engineer and investigate the interface structure and chemistry at the indium tin oxide

(ITO) anode (front-side electrode) as well as at the Mg�Ag cathode (back-side electrode) in metal

phthalocyanine (MePc)/C60 organic solar cells (OSCs).

For the front-side electrode, Zn-phthalocyaninetetraphosphonic acid (Zn-PTPA) and Sn-phthalocya-

nine axially substituted with tartaric acid (Sn-PTA) have been used for the surface termination of ITO

coated glass substrates. Both terminations yielded OSCs with higher fill factors and open circuit voltages,

thus increasing the power conversion efficiency by 33% and 67%, respectively. A possible influence of a

chemisorbed Zn-PTPA on the film growth of the adjacent ZnPc absorber in the vicinity of the hybrid

interface is discussed using X-ray reflectivity and near edge X-ray absorption fine structure data. Distinct

effects of the Zn-PTPA and Sn-PTA terminations on the electronic properties of the ITO surface were found

by X-ray photoelectron spectroscopy (XPS) measurements at the valence band edge. We demonstrate the

possibility to engineer the hybrid interface without additional buffer.

For the back-side electrode we report the formation of buffer-free charge carrier selective Mg�Ag

cathodes, which are applied for bulk heterojunction organic absorbers consisting of copper phthalocya-

nine (CuPc) donor and fullerene C60 acceptor materials. The chemical and structural properties of the

CuPc:C60/Mg�Ag interface are investigated by element depth profiling using secondary ion mass

spectrometry (SIMS), grazing incidence X-ray diffraction analysis (GI-XRD) and XPS.

We demonstrate that an optimum charge carrier selectivity is achieved with Mg:Ag/Ag cathode

structures, where the Mg:Ag alloy layer has a composition close to that of Ag3Mg. In addition, Mg diffusion

into CuPc:C60 layer is observed. As a result, an interaction between Mg and Cu2 + with a concurrent change

in oxidation state of both metals takes place. However, no formation of MgPc is observed.

The findings of this work are discussed against the background of the performance and electrical

properties of the corresponding MePc/C60-based organic solar cells.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

A crucial point in organic photovoltaic devices is the interfacebetween the organic absorber layer system and the inorganiccontacts. While organic layers consist of molecules with inert gasconfiguration there are no unpaired electrons at the layer’s surface.On the contrary, such unsaturated valences form at the surfaces ofmetals or transparent conductive oxides (TCO). When combiningthese two classes of materials in one device they form hybridinterfaces with unpredictable chemical and physical properties,which might be detrimental to the charge carrier transport.

Optimal transport properties in organic solar cells are achievedwhen hybrid interfaces exhibit low trap density, precise bandalignment, and consequently chemical and structural stability.

ll rights reserved.

x: +49 30 8062 43199.

x: +49 30 8062 43199.

.de (K. Fostiropoulos),

In order to avoid defective hybrid interfaces it is a commonpractice to introduce additional organic buffer layers, which createbetter controlled hybrid interfaces. The role of the organic bufferlayers lies, on one hand, in the passivation of unsaturated valencesby reacting with the inorganic side; on the other hand, theystimulate an optimum p–p overlap to the absorber, thus forminga well-defined Van der Waals interface. However, such bufferscome along with disadvantages. Therefore more specific engineer-ing of the individual hybrid interface is necessary.

At the device’s front side between the indium tin oxide (ITO)electrode and the adjacent organic absorber layer usually the polymerpolyethylenedioxithiophene:polystyrenesulfonate (PEDOT:PSS) [1,2]is deposited by an additional wet chemical process. However in thepresence of water impurities the chemical and electrical properties oforganic semi-conductors suffer. Therefore elaborate drying processeshave to be conducted. In the case of small molecules organic solar cellswhere only vacuum deposition techniques are applied any prepara-tion of such ‘‘thick’’ wet layers is undesirable. In addition thegenerated polymer layer is not completely transparent in the visiblesun spectrum.

K. Fostiropoulos, M. Rusu / Solar Energy Materials & Solar Cells 95 (2011) 1489–14941490

Recently an alternative procedure in order to substitute thePEDOT:PSS buffer has been proposed [3]. Engineering the electro-nic properties of the TCO by a specific termination using amolecular chemisorbed monolayer on the ITO surface band align-ment as well as trap passivation can be achieved [4]. Following asimilar procedure in this study we compare ITO substratesterminated either with the planar Zn-phthalocyaninetetrapho-sphonic acid [5] (Zn-PTPA) or the umbrella-like Sn (IV) phthalo-cyanine axially substituted with tartaric acid (Sn-PTA) molecule toan untreated ITO reference. Distinct effects of such TCO substrateson the JV characteristics of ZnPc/C60 bi-layer heterojunction organicsolar cells (OSCs) are demonstrated. Structural as well as electronicreasons were discussed with the help of X-ray reflectivity (XRR)and near edge X-ray absorption fine structure (NEXAFS) measure-ments at the hybrid interface as well as X-ray photoelectronspectroscopy (XPS) measurement on the TCO surfaces, respectively

At the device’s back side a metallic electrode is deposited. Theelectronic properties of the latter electrode must match with those ofthe organic absorber, which consists either of a single layer or of ablend donor:acceptor structure. Particularly, for an effective collectionof the charge carriers from a single organic layer, the work function ofthe metallic film is adjusted to the HOMO [LUMO] level of the donor[acceptor] material for the formation of an ohmic contact. In the caseof absorbers from blend structures effective charge-selective electro-des are required. These are achieved by the fine adjustment of theelectrode work function by alloying low and high work functionconducting materials. Recent developments of electrodes for organiclight-emitting diodes as well as for OSCs are based on Mg:Ag alloys[6–8]. The usage of Mg:Ag-based electrodes is stimulated by its stablechemical and consequently electronic properties, particularly posses-sing a constant work function (F) [9]. The latter can be adjustedbetween FMg¼3.4 eV and FAg¼4.7 eV by changing the [Mg] to [Ag]ratio. The variations in F with the Mg:Ag composition are, however,superimposed by structural/phase changes from cubic-Ag, cubic-Ag3Mg, cubic AgMg to hexagonal-AgMg3 and hexagonal-Mg, asshown by the phase diagram [10].

In this work, we report on the formation of effective selectiveMg�Ag contacts for blend organic absorbers consisting of copperphthalocyanine (CuPc) donor and fullerene C60 acceptor. Thechemical and structural properties of the CuPc:C60/Mg�Ag inter-face is investigated by elemental depth profiling using secondaryion mass spectrometry (SIMS), grazing incidence X-ray diffractionanalysis (GI-XRD) and XPS. The respective findings are discussedagainst the background of the photovoltaic (PV) and electricalproperties of the corresponding CuPc:C60-based OSCs.

2. Experimental

Commercial ITO glass substrates (310 nm ITO on 1.1 mm glass,5 O/square) were consecutively cleaned with acetone, ethanol, anddeionized water in ultrasonic bath. Subsequently the ITO surface

AlBathocuproine

C60

ZnPc

Chemical termination

ITO

Fig. 1. Sequentially deposited ZnPc-C60 architecture on chemically terminated ITO substr

was terminated by dipping the substrates either in a 5�10�5 Msolution of Zn-PTPA methanol/water (1/2) or a saturated ethanolsolution of Sn-PTA for 20 min. Thereafter the substrates werethoroughly rinsed with plenty of water, so that only the chemi-sorbed monolayer species remain on the ITO surface and all thephysisorbed rest gets removed. The applied Zn-PTPA molecule wassynthesized following a receipt recently published [5], whereas theSn-PTA synthesis will be published elsewhere.

ITO/ZnPc samples as well as complete ITO/ZnPc/C60/bathocu-proine (BCP)/Al (Fig. 1) devices have been produced by thermalevaporation under vacuum �3.10�8 mbar. Commercial low puritygrade ZnPc powder was purchased from Alfa Aesar. Because of thehigh purity demands of the application this material was addi-tionally purified twice through vacuum sublimation. Sublimationpurified C60 powder (499%) was delivered from MER. For thepreparation of the back contact with buffer layer Al slugs as well asBCP powder were obtained from Fluka with purity Z99.99% and97%, respectively. They were used without further purification.

The structure profile in z-direction (perpendicular to substrate)of a 30 nm ZnPc layer on treated ITO was determined by analyzingXRR data collected at wavelength l¼1.541 A and power P¼900 W.NEXAFS spectroscopy was performed at the linear polarizedASTRID SX-700 beam-line of ISA in Aarhus/Denmark with mono-chromator resolution DE¼0.44 eV. The molecular orientation of a3 nm ZnPc layer on ITO was characterized by collecting spectra atthe carbon K-edge at 284 eV. For each sample, Auger electronemission spectra at four incident angles y¼201, 451, 701, and 901 ofthe synchrotron beam with respect to the substrate’s plane wererecorded. The effect of the termination molecules on the TCO workfunction was analyzed by low intensity [11] X-ray photoelectronspectroscopy (XPS) using a Specs XR 50 X-ray tube with Mg-Ka

spectrum under ultrahigh vacuum conditions at o3.10�10 mbar.SIMS measurements were conducted using a Cameca-IMS-Sc-Ultra

instrument [12,13]. The depth profiles were taken with 1 keV Cs+

primary ion beam at a current of 5 nA. The elements M were recordedas MCs+ (24Mg, 63Cu, 34S, and 113In) and MCs2

+ (12C and 16O) clusters andtheir individual intensity signals were normalized with respect to theCs+ and Cs2

+ intensities. The recording of the elements in this modeallows minimizing the matrix effect [14]. During the SIMS measure-ments, it was checked that no mass interferences occurred between24Mg and 12C, for example. GI-XRD (CuKa1,2; incidence angle 1.21)analysis was performed using a Bruker D8 diffractometer. The XPSsurface sensitive measurements were carried out at a base pressureo10�9 mbar at the CISSY experimental station at the BESSY IIsynchrotron radiation source in Berlin [15]. The data were recordedusing the synchrotron radiation (1486.6 eV) and a VG Scientific CLAM4 electron analyzer. The electrical parameters of the OSC devices withan area of 6.370.1 mm2 were determined from dark current–voltage(J–V) measurements. The PV parameters (power conversion efficiency(Eff), fill factor (FF), open circuit voltage (Voc), short-circuit currentdensity (Jsc)) were determined from J–V measurements performedunder AM1.5G conditions (100 mW/cm2, 25 1C).

Ag Mg

Selective contact

ITO

ate (left). Co-deposited CuPc-C60 blend architecture with Mg/Ag back contact (right).

K. Fostiropoulos, M. Rusu / Solar Energy Materials & Solar Cells 95 (2011) 1489–1494 1491

Experiments were performed on 70 nm thick CuPc:C60 layersdeposited by organic vapor phase deposition (OVPD) [16] on 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS)-coated indium tin oxide (ITO)/glass substrates. Mg and Ag layerswere sequentially deposited by physical vapor deposition tocomplete the device structure in Fig. 1 (right). The optimizationof the Mg-layer thickness d was performed on ITO/PEDOT(70%)/70-nm-CuPc:C60(1:1)/d-nm-Mg/100-nm-Ag test devices (type A).Fully optimized type B devices have an ITO/PEDOT (30%)/80-nm-CuPc:C60(1:1)/30-nm-Mg/90-nm-Ag structure. The absorber pre-paration temperatures were 151 and 163 1C for the optimized andtest devices, respectively. XRD and SIMS measurements wereperformed from the Ag side on type A and B devices, respectively.In addition, SIMS measurements were carried out on type C stacks,90-nm-Ag/d-nm-Mg/70-nm-CuPc:C60/mica, as references for typeA structures. In addition, XPS measurements were performed on d-nm-Mg/70-nm-CuPc:C60/ITO/glass structures of type D.

3. Results and discussion

3.1. Anode–absorber interface

Exemplarily, typical JV characteristics of organic solar cells pre-pared on 3 different ITO terminations are presented in Fig. 2. Apart

Fig. 2. Comparison of the JV characteristics of organic solar cells prepared on

different chemically terminated ITO substrates applying chemisorbed Sn-PTA (red)

and Zn-PTPA (blue) monolayers with that of an untreated reference ITO. (For

interpretation of the references to color in this figure legend, the reader is referred to

the web version of this article.)

Fig. 3. Dependence of the internal ZnPc layer structure on the chemical termination o

termination (red) on the (2 0 0) reflex of thea-modification of 30 nm ZnPc layers (left). On

on such substrates exhibit the well-known angle dependentp–p* transitions below the C e

substrate is related to better ordering of the molecular species. (For interpretation of the re

article.)

from the ITO treatment, the devices were produced under identicalconditions as described above. The recorded PV parameters showedfluctuations, which resulted in statistical deviations of the devicesefficiency of about 12%.

The Sn-PTA terminated sample (red curve) shows the highestefficiency (Z¼2.0%), which originates from very high fill factor(57%), open circuit voltage (Voc¼0.54) and high short-circuitcurrent density (Jsc¼6.3 mA/cm2). Compared to the reference cellwithout specific termination the series as well as the parallelresistance is much improved. Although if the reference Jsc

ref is 10%higher it cannot compensate for the collapse of the fill factor(FFref

¼32%) resulting in a low power conversion efficiencyZref¼1.2%. The analysis of the JV curve of the sample with Zn-

PTPA termination on ITO (blue curve) yields similar results con-cerning FF and Voc. Compared to the first sample they differ just by2%, which is within the error range of these measurements. SolelyJsc¼5.2 mA/cm2 differs significantly from the corresponding valueof the Sn-PTA sample lowering the efficiency dramatically by 20%.

On contrary to the reference, in both terminated devices thecurves show no voltage dependent reverse biased photocurrent.Since this effect does not occur in the dark it points to better chargecarrier extraction rather than low shunt resistance [17]. Theforward current of the reference is significantly lower. This is aneffect of the serial resistance, which can be influenced by detri-mental states at the hybrid interface or distort absorber structure.

The influence of the ITO termination on the growth of theadjacent ZnPc absorber as the positive effect on the cell perfor-mance was studied by means of XRR analysis on a 30 nm ZnPc layeron ITO substrates. Fig. 3 shows the reflectivity data of a sample withZn-PTPA termination compared to that of an untreated ITOreference. Both data sets exhibit distinct Kiessig oscillations inthe vertical momentum transfer range qz¼0.05–0.35 A�1, which issignificant for flat layers and a Bragg peak at qz¼0.485 A�1, whichcorresponds to the (2 0 0) reflex of the monoclinic ZnPc a-mod-ification at scattering angle 2yE6.91 [18]. The experimental datacould be described by fit curves using a bi-layer multi-stack modeland applying the Parratt formalism [19].

From our calculations we derived that due to the terminationthe surface roughness of the organic layer increased from srms¼2to 3 nm. Apparently the termination also reduces the intensity ofthe Bragg peak. The latter points to a reduced total number of theordered molecules that contribute to the reflex compared to thereference. In addition the peak is slightly shifted to lower qz

corresponding to larger lattice distance.On the other hand the NEXAFS spectra of 3 nm ZnPc layers show

the opposite effect. The terminated sample exhibits a stronger

f the ITO substrate. The X-ray reflectivity data show the influence of the Zn-PTPA

the terminated sample this reflex is weaker. The NEXAFS spectra of 3 nm ZnPc layers

dge (main peak at 285.7 eV) (right). The higher intensity variation for the terminated

ferences to color in this figure legend, the reader is referred to the web version of this

40

45

F (%

)

400

450

c (m

V)

K. Fostiropoulos, M. Rusu / Solar Energy Materials & Solar Cells 95 (2011) 1489–14941492

angle dependence of the well-known p–p* transitions below the Cedge (at 285.7, 287.7, and 289–291 eV) [20]. In both samples theorientation of the molecular planes is perpendicular to the sub-strate. However the angle dependent peak intensity variation of theterminated sample is more distinct indicating a better molecularassembly enforced by the modified hybrid interface.

Therefore we believe that the Zn-PTPA termination disturbs thenatural film growth of ZnPc resulting in a lattice strain in thevicinity of the hybrid interface. The first few upright standingmonolayers are better ordered and have a higher lattice distancethan the bulk average. The distorted overall structure profile formsa higher amorphous fraction reducing the Bragg peak and increasesmost likely the surface roughness. As well-ordered ZnPc layersexhibit highly anisotropic transport properties upright standinglayers conduct only parallel to the substrate. Therefore the higheramorphous fraction on terminated substrates might support theconductivity perpendicular to the substrate yielding higher for-ward currents.

The XPS spectra of Zn-PTPA and Sn-PTA terminated ITO surfacescompared to an untreated reference are presented in Fig. 4. Thehigh binding energy cut-off (secondary edge) of the ITO with Zn-PTPA (blue curve) is shifted by DE¼0.8 eV compared to the ITOreference (black curve) without specific termination. Its workfunction increased from Wref

¼ 4.3 to WZn-PTPA¼ 5.1 eV. The edge

of the Sn-PTA sample (red curve) shows a bimodal behavior. Itexhibits a step hence two different positions with spacing DE¼0.6eV. Linear extrapolation of the high energy step yields 4.3 eV for thework function that fits to that of untreated ITO. As expected forupright standing umbrella molecule, due to its polarization thesecond step is shifted to a higher work function value as high asWSn-PTA

¼4.9 eV. The step indicates that we have collected datafrom modified as well as non-modified ITO. Apparently theintensity of the shifted component is higher. Therefore we believethat most of the ITO surface area is terminated. There is evidencethat it is even completely covered. Since the umbrella-like Sn-PTAtermination is approximately one atomic monolayer only, a smallfraction of the emitted electrons might rise from the intrinsicITO below.

The shifted work functions of both treatments fit better to thehighest occupied molecular level of the adjacent ZnPc absorber

Fig. 4. X-ray photoelectron emission of ITO substrates with Zn-PTPA (blue), Sn-PTA

(red), and without (black) surface termination. The valence band of the Sn-PTA

sample shows two different positions. One fits to the untreated ITO reference

(4.3 eV) and the other to a higher value at 4.9 eV. The Zn-PTPA sample shows one

high value of 5.1 eV. (For interpretation of the references to color in this figure

legend, the reader is referred to the web version of this article.)

layer at 5.28 eV [21] reducing the barrier formed at the hybridinterface.

3.2. Absorber–cathode interface

The influence of the Mg-layer thickness on the charge selectivityof the Mg/Ag cathode was performed by investigating the PV andelectrical parameters of the type A devices. All PV parameters inFig. 5(a) reach maximum values for a Mg-layer thickness of 30 nm.At the same time, in Fig. 5(b) the device series resistance (Rs)decreases dramatically, while the parallel resistance (Rp) increasessignificantly. A further increase in the Mg thickness up to 100 nmresults in a monotonic decrease in all the PV parameters to valuesthat, however, are higher than those recorded on samples without aMg layer. Simultaneously, Rs and Rp decrease to minimum values.Thus, an optimum Mg thickness of 30 nm is found. Furtheroptimizations provided type B OSCs with a 30-nm-Mg/90-nm-Agcathode structure, which show enhanced electrical characteristicsand efficiencies of 3.0% (see Fig. 5, full symbols).

SIMS depth profiles of type B and C OSCs are shown in Fig. 6(a–c);the depth profiles in Fig. 6(b) were recorded from a device structurederived from the type B OSCs after lifting-off the Ag/Mg cathode. The113In profile reveals the region of the transparent ITO anode, while 12Cand 63Cu profiles are indicative for the extension of CuPc:C60 layer. 34Soriginates from the PEDOT buffer layer in Fig. 6(a) and (b). Only sulfurtraces are recorded on type C OSCs since these devices were preparedwithout any PEDOT buffer layer. A clear diffusion of Mg from Mg layerinto CuPc:C60 thin film is observed in all figures. Especially cleardemonstration of the Mg diffusion into CuPc:C60 absorber film isshown in Fig. 6(b) where high Mg concentration is recorded across theentire absorber thickness. At the same time the 107Ag profile clearlycrosses the region of the Mg layer suggesting the formation of a mixedMg:Ag layer in Fig. 6(a) and (c), while only Ag traces are registered in

1.0

1.5

2.0

3.0

Eff

(%)

30

35 F

9

10

11

121415

J sc (

mA

/cm

2 )

300

350Vo

0

0.0

0.5

3.5

7.0

0.0

1.0

2.0

3.0

Rp

(kΩ

*cm

2 )

Rs (

Ω*c

m2 )

Mg-thickness (nm)15 30 45 60 75 90 105

Fig. 5. (a) PV parameters of the type A test solar cells as a function of Mg-layer

thickness d (open symbols) and of the optimized type B devices (full symbols) under

AM1.5 conditions (100 mW/cm2). (b) Series (Rs) and parallel (Rp) resistances of the

respective devices in Fig. 5(a).

100

101

102

103

104

105

ITOPED

OTCuPc:C60MgAg

Sputter time (x103 s)

107Ag24Mg63Cu12C34S113In

ITO

PED

OT

107Ag24Mg63Cu12C34S113In120Sn

CuPc:C60

0.0

MicaCuPc:C60Mg

Sputter time (x103 s)

107Ag24Mg63Cu12C

34S

Ag

2.0 4.0 6.0 8.00.0Sputter time (x103 s)

2.0 4.0 6.0 8.00.0 4.0 8.0 10.0 12.012.0

Inte

nsity

(cnt

/s)

100

101

102

103

104

105

Inte

nsity

(cnt

/s)

100

101

102

103

104

105

Inte

nsity

(cnt

/s)

Fig. 6. SIMS depth profiles of a (a) type B OSC, (b) type B device after the lifting-off the Ag/Mg cathode, and (c) type C OSC. The dashed lines indicate the approximate positions

of interfaces.

30

ITO

(431

)

Ag

(200

)

ITO

(222

)

ITO

(411

)IT

O (4

00)

dMg = 100 nm

dMg = 60 nm

dMg = 45 nm

dMg = 30 nm

100-nm-Mg // org. / ITO_Ref.

90-nm-Ag // org. / ITO_Ref.

Ag

(111

)

Mg

(002

)

Inte

nsity

(a.u

.)

2Theta (degree)35 40 45 50

Fig. 7. GI-XRD (1.21) patterns of type A devices together with references of 100-nm-

Mg and 90-nm-Ag on CuPc:C60/ITO substrates. The Mg and Ag peaks were identified

according to JCPDS 35-0821 and 04-0783, respectively. The ITO peaks were

identified according to JCPDS 21-1250 and 29-1484.

Inte

nsity

(a.u

.)

1200 1000 800 600 400 200 0Binding Energy (eV)

Synchrotronhν = 1486.6 eV

Mg1

s

OK

LL O1s

N1s

C1s

MgK

LLM

g2s

Mg2

p

Ek = 180 eVIMFP: 0.8 nm

Ek = 1180 eVIMFP: 3.2 nm

Cu2

p 3/2

Sn3d

5/2

In3dIn3p3/2CKLL

Cu3

d

Fig. 8. XPS overview spectra of (a) 30 nm Mg and (b) 1 nm Mg on CuPc:C60/ITO

stack, and of (c) the CuPc:C60/ITO reference. IMPF denotes the inelastic mean free

path of electrons with the kinetic energy Ek.

K. Fostiropoulos, M. Rusu / Solar Energy Materials & Solar Cells 95 (2011) 1489–1494 1493

the CuPc:C60 layer in Fig. 6(b). The Mg peak in the region of the PEDOTis attributed to components of this layer since the correspondingprofiles of the type C stacks on mica in Fig. 6(c) do not contain sucha peak.

The Mg/Ag cathode structure was investigated on type A devicesby GI-XRD measurements in Fig. 7. Although clearly seen in thepattern of the Mg-Ref. sample, the peaks of the Mg hexagonalstructure, e.g. Mg(0 0 2), are not present in any of the Mg/Agpatterns recorded. In addition, with the Mg thickness increase acontinuous decrease in the intensity of Ag peaks, particularly of theAg(1 1 1) and Ag(2 0 0) peaks, is observed. These facts could beexplained by the formation of a nanocrystalline Ag:Mg phase.Moreover, the Mg diffusion into CuPc:C60 layer is supported. Theabsence of the peak shifts that are expected for the case of Mg�Agphase formation could be explained by the formation of Ag3Mg,which has a similar cubic structure and almost the same latticeparameters as Ag [12].

The formation of the Mg/CuPc:C60 interface was investigated byXPS measurements on type D stacks. Fig. 8 shows the XPS overviewspectra of 1 and 30 nm Mg on CuPc:C60/ITO structures, compared tothat of the CuPc:C60/ITO reference. The main features are thephotoemission peaks of Mg 1s (1305 eV), O 1s (532 eV), N 1s(400 eV), C 1s (284 eV), Mg 2s (90 eV), and Mg 2p (58 eV). Thedoublet at 443/445 eV can be assigned to In 3d, originating from theITO substrate. In addition to the photoemission peaks, prominent

Auger electron peaks from O KLL (at 977 eV, originating fromoxidized Mg0 or adsorbed H2O) and Mg KLL (306 eV) are visible. Thefact that almost no Mg 1s emission is visible from the 1 nm Mgsample while the Mg KLL Auger line is of comparable magnitude tothat from the 30 nm Mg sample can only be explained byattenuation of the Mg 1s signal by a covering layer containingno or very little Mg. The kinetic energy, Ek, of the electrons from the1s level excited by X-rays with energy of 1486.6 eV is only 180 eV.This corresponds to an electron mean free path (MFP) of around0.8 nm, so that the signal is attenuated by 95% after passing through2.4 nm of material (the so-called information depth). On the otherhand, the Mg KLL Auger electrons possess an Ek¼1080 eV, whichincreases the information depth to almost 10 nm. Therefore,diffusion of the initial surface Mg into the bulk of the underlyingmaterial could be assumed, which is almost inaccessible foranalysis by the ‘‘slow’’ Mg 1s photoelectrons, but well visible bythe ‘‘fast’’ Auger electrons. This is further supported by the presenceof the Mg 2s and 2p electrons, both of which have even higherkinetic energies compared to Mg KLL Auger electrons.

The detailed spectra of the Cu 2p emission of pure and 1 nm Mg-coated CuPc:C60 (‘‘Mg/CuPc’’) layers (on ITO/glass substrates) havebeen analyzed in our work in Ref. [22]. The Mg/CuPc sample wasetched in dilute sulfuric acid (0.01%) to remove any superficialelemental Mg. The pure CuPc reference sample was treatedsimilarly. A shift in the Cu 2p emission from etched Mg/CuPc by

K. Fostiropoulos, M. Rusu / Solar Energy Materials & Solar Cells 95 (2011) 1489–14941494

2.2 eV toward lower binding energy as compared to the CuPc wasobserved. This shift points toward a change in the oxidation state ofCu in CuPc from Cu2 + to either Cu+ [23] or Cu1. However, thechemical shift between Cu+ and Cu1 was too small to allow anunambiguous discrimination. Nevertheless, a redox reaction wasclearly observed, which was most likely caused by the Mg metalthat diffused into the organic matrix.

The Mg KLL Auger spectra of Mg/CuPc samples in Fig. 8 were indetail analyzed in Ref. [22] against the same spectra of MgPcreference sample. The comparative study of these spectra showed areaction of Mg with CuPc. However, since the position of the Mg2 +

peak of Mg/CuPc samples was very different from that observed onthe MgPc reference sample, the formation of MgPc in the Mg/CuPcsystem was ruled out.

In the following, we discuss the results of the investigation ofthe CuPc:C60/cathode interface formation. As shown in optimiza-tion experiments of the Mg/Ag cathode structure, a good correla-tion between the behavior of the devices Voc and Rp is found. Thissuggests that the adjustment of the Mg thickness results in asimultaneous adjustment of theFcathode for an ohmic contact to then-type C60 and a blocking contact to the p-type CuPc. As shown inthe introduction, the adjustment of F can occur through theformation of a Mg:Ag alloy, which was revealed by both SIMSand XRD measurements. Its composition for an optimum cathodework function, which results from an optimum cathode structure(30-nm-Mg/90-nm-Ag) could be close to that of Ag3Mg, as sug-gested by XRD measurements. The alteration of the PV and electricparameters with the increase in Mg thickness could be attributed todeviations of F from optimum values as well as to the effect of Mgdiffusion into CuPc:C60 layer, which were revealed by SIMS and XPSmeasurements. Excessive Mg diffusion could provide shuntingpathways, thus lowering the devices’ Rp and consequently their Voc,as confirmed by the data in Fig. 5. In addition, the XPS analysisdiscovered a reaction between the diffused Mg metal and Cu2 +.This reaction might significantly influence the electronic state ofthe CuPc/cathode interface. Furthermore, Mg diffusion couldsignificantly influence the CuPc electronic conductivity.

4. Conclusion

We investigated the effect of chemical engineering of the hybridinterfaces at the ITO anode as well as Mg�Ag cathode in Me�Pc/C60 organic solar cells.

Applying molecular termination techniques onto ITO weachieved band alignment with the organic absorber and significantpassivation of traps at the hybrid interface. The termination alsoaffected the absorber growth, thus forming a structure profile thatpossibly increases the conductivity of the absorber in z-direction(perpendicular to the substrate plane).

Furthermore, we investigated the formation and the chemicalstructure of the CuPc:C60 absorber/Mg�Ag cathode interface inOSCs and found an optimum cathode structure. An optimumcharge carrier selectivity is achieved with a Mg:Ag/Ag cathodestructure of an adjusted F, where the Mg:Ag alloy has a composi-tion close to that of Ag3Mg. In addition, Mg diffuses into theCuPc:C60 layer interacting with Cu2 +. However, no MgPc is formed.

Acknowledgments

The authors thank S. Wiesner, Tayfun Mete, Ivelin Bochukov,and Wolfram Schindler for experimental support and team work.I. Lauermann for NEXAFS and photoemission investigations,

J.N. Audinot, and Y. Fleming are gratefully acknowledged forperforming the elemental depth profiling, and S. Lehmann forvaluable discussions.

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