on the role of bathocuproine in organic photovoltaic cells

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3686

DOI: 10.1002/adfm.200800815

On the Role of Bathocuproine in Organic Photovoltaic Cells

By Hans Gommans,* Bregt Verreet, Barry P. Rand, Robert Muller, Jef Poortmans,Paul Heremans, and Jan Genoe

The effect of bathocuproine (BCP) on the optical and electrical properties of organic planar heterojunction photovoltaic cells is

quantified by current–voltage characterization under 1 sun AM 1.5D simulated solar illumination and spectral response at short-

circuit conditions. By inserting a 10 nm BCP layer in an indium tin oxide (ITO)/subphthalocyanine (SubPc)/buckminsterfuller-

ene (C60)/BCP/Al thin-film structure, an increase in power-conversion efficiency from 0.05 to 3.0% is observed, mostly reflected

in the enhanced open-circuit voltage up to 920mV. Furthermore, the incorporation of a 10-nm BCP layer in an ITO/C60/BCP/Al

structure leads to an increase in built-in potential from 250 to 850mV, as demonstrated by electroabsorption. It is argued that

BCP passivates C60 such that a 10-nm layer provides a sufficient buffer layer that prohibits Al contacting the C60 where it would

otherwise create donor states.

1. Introduction

Organic photovoltaic cells (OPVCs) are considered to be

promising devices because of their mechanical flexibility, ease

of fabrication and potential for low-cost production. A typical

OPVC consists of an organic donor and acceptor material

sandwiched between two electrodes. Naturally, matching

energy levels at each interface in order to optimize charge

injection/extraction and transport is essential and much

progress has been made by incorporating additional layers.

Even though such layers do not contribute to the absorption

and carrier generation in the photovoltaic device, structures for

which the highest efficiencies have been reported generally

include an ultrathin interface layer between the acceptor and

metal top contact.

For small-molecule OPVCs the most-common material to

use between the acceptor, here C60, and the top electrode, Al,

is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, also known

as bathocuproine (BCP). Indeed, the insertion of BCP results

in an enhanced power-conversion efficiency (PCE).[1] How-

ever, it is also known that BCP crystallizes quickly,[2,3] and for

this reason it may be desired to replace BCP with another

material in the future. In order to find such a substitute, the

working mechanisms of BCP in an OPVC need to be

understood in greater detail.

The improved power-conversion efficiency has been

attributed to an increased exciton harvesting in the active

layer.[1,4,5] As BCP is a wide-band-gap material it acts as an

exciton-blocking barrier (EBL) that prohibits excitons diffus-

ing towards the Al electrode where they would otherwise be

[*] Dr. H. Gommans, B. Verreet, Dr. B. P. Rand, Dr. R. Muller, Dr. J.Poortmans, Prof. P. Heremans, Dr. J. GenoeSOLO department, Polymer and Molecular Electronics, IMECKapeldreef 75, B-3001 Leuven, (Belgium)E-mail: [email protected]

� 2008 WILEY-VCH Verlag GmbH &

quenched.[1] Additionally, BCP can act as a diffusion barrier

for the Al and as such eliminate the creation of Al-induced

charge-transfer states in C60 that are known to quench

excitons.[1] It has also been mentioned that BCP works as

an optical spacer, allowing the position of themaximum optical

field at the charge-generating donor-acceptor heterojunction

to be tuned.[1,5]

Recently, the importance of the exciton-blocking effect of

BCP on the OPVC performance has been challenged.[2,5,7] It

was suggested that BCP mainly acts as a diffusion buffer layer

that: i) establishes an ohmic contact between the C60 and Al,[7]

ii) prevents lowering of the work function of the C60/

Al interface and as a result increases the built-in field,[6] and

iii) prevents the dipole formation at the C60/Al interface, which

would inhibit electron collection at forward bias.[2]

Finally, it has been demonstrated that neither the exact

position of the lowest unoccupied orbital of the interface

layer,[5,8] nor its value for the band gap,[8]seem to relate to the

photovoltaic performance.

In this paper, we report on current density–voltage (J–V)

characterization in the dark and under illumination as well as

measurements of external quantum efficiency (hEQE) con-

ducted on OPVCs in which chloro[subphthalocyaninato]bo-

ron(III) (SubPc) was applied as the donor material.[9,10] We

calculate the optical absorption, including interference effects,

and calculate the internal quantum efficiency (hIQE). We then

quantify the effect of the incorporation of BCP on the

absorption and quantum efficiencies for both the donor and

acceptor material.

Subsequently, J–V characterization and electroabsorption

(EA) experiments on C60 films sandwiched between indium

tin oxide (ITO) and Al electrodes are discussed. Here, the

layer thickness of the C60 was varied, as well as alternating

the presence and absence of the BCP layer. These

experiments allowed us to characterize the nature of the

C60/Al and the C60/BCP/Al contacts and extrapolate how these

Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 3686–3691

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H. Gommans et al. /Bathocuproine in Organic Photovoltaic Cells

Table 1. The short circuit current, JSC, open-circuit bias, VOC, fill factor, FF,and power-conversion efficiency, hP, as determined from the J–V charac-terization in Figure 1. The first and second rows show the parameters for adevice with and without BCP, respectively.

Device Pin [a] [mW cmS2] JSC [mA cmS2] VOC [mV] FF [%] hP [%]

With BCP 100 5.42 920 60.7 3.03

Without BCP 100 3.12 70 25.1 0.05

findings influence the OPVC performance in a qualitative

manner.

Based on the experimental and modelling results, we are

able to make a fair comparison of the mechanisms suggested in

the literature and contrast each of its effects on the device

performance. In the end, we reach a conclusion about BCP as

an interface layer that resolves many facets of the discussion.

[a]Light intensity.

2. Results and Discussion

The role of the BCP layer inOPVCs was investigated by J–V

characterization in the dark and under 1 sun AM 1.5D

simulated solar illumination. The device structures are

indicated in the caption of Figure 1. The short-circuit current,

JSC, open-circuit voltage, VOC, fill factor, FF, and the power-

conversion efficiency, hP, are given in Table 1.

In the dark, the current for the layer stack including BCP

exhibits a rectification ratio at �1V of �104 and a parallel

(shunt) resistivity of 1.8� 106V cm2. Without BCP the parallel

resistivity reduces tremendously to 3.6� 101 V cm2, while the

rectification has completely disappeared. As a result, the

magnitude of the dark current becomes comparable to JSC(�3.1mA cm�2) at a bias of 90mV and the VOC is reduced to

70mV. The power-conversion efficiency drops to 0.05%. We

conclude that the positive effect of BCP on the photovoltaic

(PV) performance closely relates to an increased shunt

resistance.

Figure 1. J–V characteristics at room temperature for photovoltaic struc-tures in the dark (black curve) and under 1 sun simulated AM 1.5D solarillumination (grey curve). The structures investigated are glass/ITO (100)/SubPc (13)/C60 (32.5)/BCP (10)/Al (80) (dashed curve) and glass/ITO(100)/SubPc (13)/C60 (42.5)/Al (80) (solid curve). The numbers in bracketsindicate the thickness in nm.

Adv. Funct. Mater. 2008, 18, 3686–3691 � 2008 WILEY-VCH Verl

Figure 2 shows the modelled absorptance spectrum for the

SubPc and C60 layers in the device structures. The optical

interference as a result of multiple refractive interfaces is

calculated by commercially available simulation software

(SCOUT).[11] From these spectra it is immediately clear that

the effect of BCP as an optical spacer for the chosen SubPc/C60

structures may be neglected. Devices where the 10-nm-thick

BCP is replaced by an additional 10 nm of C60, the C60

absorption maximum at 450 nm increases from 0.67 to 0.74

(Fig. 2a and 2b). The absorptance spectrum in the SubPc layer

remains virtually unaffected in the device structures, mainly

due to our choice of a constant total device thickness, which

only minimally disturbs the optical interference effects within

the device structure. The calculated increase in the absorp-

tance spectrum could result in, at most, a 0.36mA cm�2 current

increase, assuming charge separation and collection efficien-

cies of 100%. However, devices without BCP demonstrated a

reduction in JSC under 1 sun illumination (see Table 1).

Convolution of the measured hEQE with AM 1.5 yields a JSClowered by 0.9mA cm�2 (Fig. 2).

If the C60 layer thickness is constant for both device structures

(325 A), an additional BCP layer does allow for an increase in

absorption in both donor and acceptor materials, and, as such,

acts as an optical spacer. In such case, the total absorption in

both the donor and acceptor material yields an increase from

0.618 to 0.675. If the internal quantum efficiency is assumed to

be 100%, the JSC is calculated to increase from 7.77mA cm�2 to

8.83mA cm�2. It can be observed that experimental value for

JSC is typically a factor of 2 lower than these values.

Also shown in Figure 2 is the measured hEQE at short-circuit

conditions for the twoOPVCs.Convolution of these spectrawith

AM. 1.5 yields short-circuit currents of 4.0 and 3.1mA cm�2 for

the devices with and without BCP respectively. Apart from an

overall reduction in hEQE for devices without BCP, the spectral

features coincide closely. At the SubPc absorption maximum

(590nm) the hEQE for devices with and without BCP is 46% and

39%, respectively. At the C60 absorptance maximum at 450 nm,

the equivalent values are 29% and 22%.

In Figure 2c, hIQE is determined from the measured hEQE

divided by the combined absorptance spectrum of the donor

and acceptor layer. The SubPc maximum in hIQE for devices

with and without BCP is 55% and 47%, respectively, while for

C60 it is 37% and 26% respectively. At wavelengths higher than

650 nm the hIQE is 10–30%. However, we determined that

coevaporated SubPc:C60 (1:1 vol %) films yield an extinction

ag GmbH & Co. KGaA, Weinheim www.afm-journal.de 3687

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Figure 2. Experimentally determined external quantum efficiency, hEQE,(circles), and the calculated absorptance (dashed curves) in an OPVC:a) with BCP and b) without BCP. The solid curve in (a) is the modelled hEQE

obtained for an exciton diffusion length of 95 A for SubPc and 110 A for C60.The modelled hEQE in (b) assumes ideal quenching at the C60/Al interface(solid line) and zero quenching (dashed dotted curve). c) Internal quantumefficiency, hIQE, for the structures with BCP (dark-gray curve) and without(light-gray curve).

3688

coefficient in this range that is significantly higher than that for

pristine C60 or SubPc. In fact, a similar increase in absorption in

the near-infrared was found for coevaporated donor material

(Zn-phthalocyanine) and C60, which was attributed to the

creation of a charge-transfer state at the donor/acceptor (D/A)

heterojunction.[12] For this reason it cannot be ruled out that

the SubPc-C60 interface in reality may yield higher absorption

than estimated here by the optical modelling (based on bulk

parameters of the pristine materials) and hence, the hIQE signal

cannot unambiguously be attributed to the C60 layer.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

At450nm, the difference in the hIQE ismore pronounced than

the difference in the hEQE. This can be understood as the

exciton-diffusion length in C60 is significantly smaller than the

C60 layer thickness: excitons generated near the Al contact have

to cover a longer distance to the D/A heterojunction. As a

consequence, the probability for charge separation and collec-

tion reduces for excitations generated in the C60. In addition, it is

generally assumed that the C60/Al interface has a high exciton-

quenching rate as opposed to the C60/BCP interface.

In order to quantify the additional reduction in the C60

absorption range, we modelled the hEQE, in which we included

the charge-generation rate due to exciton diffusion in the donor

and acceptor materials following the transfer-matrix-based

approach outlined in ref. [13] The results are shown in Figure 2.

The mismatch between the measured and modelled hEQE for

energies exceeding 3 eV is attributed to the crude estimation of

the absorption of the glass substrate in this range. The ITO/

SubPc and C60/BCP heterojunctions were assumed to be ideal

nonquenching interfaces with negligible recombination.

For the structure with BCP, the fitted exciton-diffusion

lengths in SubPc and C60 were estimated to be (95� 5) and

(110� 5) A, respectively. For the structure without BCP we

kept these values for the diffusion lengths constant. In order to

account for the 20% overall reduction in the measured hEQE,

the fits in Figure 2b were simply lowered by this number. This

overall reduction will be further discussed below. Two curves

are shown in Figure 2b for which the C60/Al interface is

assumed to be either an ideal quenching or an ideal

nonquenching interface. Better agreement with the experi-

mental data is shown in the case where the C60/Al interface is

modelled as a perfectly quenching interface.

The positive effect of BCP on the PCE has been explained

by its exciton blocking properties near the Al electrode.[1] We

confirm here that in the case where the BCP is absent allowing

for quenching at the C60/Al interface clearly improves the fit to

the hEQE spectra in the C60 absorption range. However, this

cannot account for the observed 20% overall increase in the

hEQE when BCP is included. If we assume that the exciton-

quenching rate, defined as the reciprocal of quenching

probability, at the D/A heterojunction is unaffected by the

BCP, the 20% increase in the hEQE can only be explained by an

improved charge-collection efficiency. This can be motivated

by an increased parallel resistance that, in turn, increases the

built-in field, or alternatively, a reduced series resistance. In the

subsequent paragraphs, J–V characterization and EA studies

on C60 diode structures allow us to characterize the effect of

BCP on these two parameters.

Figure 3 demonstrates the J–V characterization at room

temperature for C60 layers of 100- and 200-nm thickness

sandwiched between ITO and Al electrodes. The positive-bias

direction in the graph corresponds to electron injection from

the Al and hole injection from the ITO. The zero-bias

conductivities, dJ/dV, for the 100-nm and 200-nm films are

found to be 9� 10�3 S cm�2 and 2� 10�5 S cm�2, respectively.

At 1V bias, they become 3� 10�1 S cm�2 and 1� 10�4 S cm�2

respectively. Despite the difference in the bottom and top

& Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 3686–3691

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Figure 3. J–V characteristics at room temperature for a set of C60 diodestructures. Open squares: device area¼ 135 mm2; glass/ITO (100)/C60

(100)/BCP (20)/Al (80); open circles: device area¼ 135 mm2; glass/ITO(100)/C60 (200)/BCP (10)/Al (80); open diamonds: device area¼ 33 mm2;glass/ITO (100)/C60 (100)/BCP (10)/Al (80); solid diamonds: devicearea¼ 33 mm2; glass/ITO (100)/C60 (100)/Al (80); solid circles: devicearea¼ 135 mm2; glass/ITO (100)/C60 (200)/Al (80). The numbers inbrackets indicate the thickness in nm.

contacts, the curves lack rectification. A clear field-activated

behavior in the conductivity is observed for the C60 films under

investigation. Multiple bias scans between �2 and 2V for the

100-nm films and between �4 and 4V for the 200-nm films

indicate that the conductivity is unstable and converges

towards linear (shunted) J–V curves within 10 scans. For films

with a smaller layer thickness (50 nm), linear J–V curves were

obtained in the first scan. The lack of device stability is also

responsible for the noisy spectrum of the glass/ITO (100)/C60

(100)/BCP (10)/Al (80) structure. Increasing the layer thick-

ness generally improves the electrical stability.

J–V characterization was also performed on diode structures

that included a 10-nm BCP layer between the C60 and Al

layers. The reverse (shunt) current was lowered by a factor of

10 (100) for 100-nm (200-nm) C60 and the conductivity at 0V

was 4� 10�5 (4� 10�6) S cm�2. The forward conductivity

exceeded that for structures without the BCP layer and was 0.3

(0.11) S cm�2 at 1V. The rectification imposed by the ITO and

Al electrodes thus becamemore pronounced: 5� 102 (3� 103).

Increasing the BCP layer thickness from 10 nm to 20 nm

reduced the forward current by a factor of 100, demonstrating

its low conductivity.

Early studies of Al growth on C60 thin films showed that a

significant amount of diffusion and doping occurs.[14,15] This

may explain the superlinear dependence of the conductivity on

thickness in combination with the absence of rectification.

Generally, C60 with ITO and Al contacts would follow an

electron-dominated space-charge-limited behavior, where the

current follows the Mott–Gurney law, J ¼ 98 "0"rm

V2

L3, where e0is the permittivity of vacuum, er is the dielectric constant, m is

the electron mobility and L is the layer thickness. Even though

the term V2 is found to describe a limited bias range in the

forward direction for the separate structures, the thickness

dependence of the conductivity does not exhibit J� 1/L3. The


non-linear relation of conductivity with film thickness rules out

injection-limited transport equally well.

BCP is a known example of a ligand that forms stable

complexes with metal ions (typically Cuþ).[16] It has been

argued that BCP also forms stable complexes with positively

ionized Al3þ.[17] Regarding the results obtained from the J–V

characterization, we argue that BCP will prevent Al diffusion

into the C60 film, possibly by the formation of stable complexes.

This decreases the dopant concentration and, as a conse-

quence, reduces the conductivity at zero and reverse bias. In

the case of complete carrier depletion in the C60 film, the

electric field should increase towards the upper limit of VBI/L

at zero bias, where VBI is the built-in potential. In order to

quantify the modification to VBI by the presence of the BCP

layer, we conducted EA experiments. The built-in field in

OPVCs is of the utmost importance for the PCE as it facilitates

the charge-carrier transport towards the electrodes. In this

context, it may be emphasized that, in contrast to conventional

photovoltaics, VBI is generally not a measure for VOC.[18] For

example, charge separation of excitons at the D/A hetero-

junction results even in the absence of the built-in field, which

is generally not the case for conventional PV devices and this

leads to a chemical potential gradient that drives the

photovoltaic effect. Although the VOC is thus generally not

equal to, but higher than, the VBI,[18] it is beyond dispute that

optimization of the collection efficiency reflected in JSC and FF

requires a maximized built-in field.

VBI can be estimated from the external dc bias required to

null the EA signal at the first-harmonic modulation fre-

quency.[19,20] EA is fundamentally based on the Starck effect

and is a well-established characterization technique for organic

semiconductors.[21]

As we were interested in the maximum signal to noise ratio,

we measured the absolute EA signal as a function of incident

wavelength at short-circuit conditions at room temperature

(Fig. 4). Also shown is the calculated absorptance in C60, taking

into account the multiple refractive interfaces in the layered

structure, as indicated in the caption. Peaks are observed at 505

and 545 nm and correspond to the inflection points in the

absorption spectrum of the structure.

In Figure 5, the EA signal at room temperature is shown as a

function of bias for two structures. By choosing the excitation

wavelength at the peak maximum (Fig. 4) we could determine

VBI within 100mV at room temperature. This variation is

mainly given by the reproducibility and stability of the

structure. The incorporation of 10 nm BCP is immediately

clear as VBI increases from 250mV up to 850mV.

From the difference in the work function of the ITO (4.8 eV)

and Al (4.3 eV), the Fermi-level difference is estimated to be

500mV, assuming vacuum-level alignment. For the structure

without BCP, a smaller VBI was determined. This can be

attributed to band bending and Fermi level pinning at the Al

contact, which in turn is triggered by dopant states. Since the

early nineties, it has been known that vacuum deposition of Al

onto C60 results in significant diffusion and creation of donor

states in C60 films.[14,15] From Raman spectra it was concluded


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Figure 4. Absolute electroabsorption signal (black dots) on a glass/ITO(100)/C60 (200)/BCP (10)/Al (80) structure as a function of excitationwavelength at short-circuit conditions (left axis) is shown. The numbers inbrackets indicate the thickness in nm. The calculated absorptance (solidcurve) in the same structure is also shown (right axis).

3690

that aluminum can transfer as many as 6 electrons to the C60

molecule.[14] Here, we suggest that the lower VBI is likely to be

due to the creation of these donor states. Accordingly, we

suggest that the BCP acts as a passivation for the C60 layer

underneath. By depositing 10 nm of BCP, a sufficiently thick

diffusion barrier for the Al is formed so as to prevent the

creation of donor states in C60 and the inevitable band bending.

Recently, it has been observed that the C60/Al and C60/Au

contact can act as a hole collector in an inverted OPVC that

gives efficiencies of 0.78 and 0.64%, respectively.[22] The

similarity in performance was attributed to Fermi level pinning

that corresponds to our findings.

It has been suggested by Wang that, under application of an

electric field, the formation of a cation complex [AlxBCPy]3xþ

will improve electron injection into the Alq3 by an enhanced

field near the electrode contact.[17] However, such a potential

drop near the interface reduces the field over the electron-

Figure 5. Electroabsorption signal at an excitation wavelength of 5 450 Aand at room temperature on C60 diode structures as a function of appliedbias. The structures investigated were glass/ITO (100)/C60 (200)/BCP(10)/Al (80) (grey) and glass/ITO (100)/C60 (200)/Al (80) (black). Thenumbers in brackets indicate the layer thickness in nm.

www.afm-journal.de � 2008 WILEY-VCH Verlag GmbH

transporting layer which is in contrast to the increase in VBI as

determined here.

The increase in VBI reflecting the increased asymmetry in

the energy diagram of the structure is in accordance with the

enhanced rectification, as determined by J–V characterization.

The conductivity onset determined for the structures with

BCP, �500mV (Fig. 3), makes an interesting comparison with

the VBI of 850mV, as determined from EA. The explanation

for this difference is the following: EA is an optical probe that

averages over the C60 layer homogeneously. In contrast, the

conductivity onset is a measure for the conductive paths with

the least resistance, supposedly the shortest distance between

two electrodes. Lateral variations in the C60 layer thickness are

known and have been determined by the surface roughness

(rms¼ 6.6 nm, obtained from atomic force microscopy (AFM)

imaging). Hence, the difference of �350mV is inherent to the

techniques used to investigate the structures.

Electrical characterization performed on the C60 diode

structures cannot naturally be extrapolated to describe the

OPVC.[23] However, the increase in VBI determined for the

diodes is attributed to the incorporation of BCP at the C60/Al

contact. Hence, it is argued that by including BCP in the

OPVC, the improved PCE can be attributed to an increased

built-in field. The VOC even exceeds the 850mV by 70mV,

which, as explained before, is well understood for the OPVC,

as the chemical potential gradients can act as a driving force.[18]

In addition, the measured increase in VBI with BCP naturally

leads to an increase in the charge-collection efficiency at short-

circuit conditions and qualitatively explains the gain in hEQE by

the overall constant factor.

3. Conclusions

In summary, the effect of BCP in OPVCs has been

characterized by J–V characterization under AM 1.5D

simulated solar illumination and spectral response. The

PCE increases with the incorporation of BCP, mainly due to

the increase in VOC. The hEQE at short-circuit demonstrates an

increase of 20% over the whole spectral range, which is

attributed to an increased built-in field that results in an

increased collection efficiency. Changing the boundary condi-

tions from ideally quenching to ideally nonquenching in the

model calculations based on the exciton diffusion equation

could explain the hEQE measurements only to a limited extent

and is quantified as of being of secondary importance to JSCand hence the PCE. J–V characterization and electroabsorp-

tion were conducted at room temperature on C60 diode

structures for different C60 layer thicknesses and alternating

the presence and absence of BCP. Both VBI and the

rectification were observed to increase with the inclusion of

BCP. We argue that the BCP forms a passivating layer for the

C60 that prevents the creation of donor states by the Al.

Both the optical and electrical mechanisms that have been

proposed in the literature to account for the increase in PCE

have been characterized by either experiment or modelling in a

& Co. KGaA, Weinheim Adv. Funct. Mater. 2008, 18, 3686–3691

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SubPc/C60 device structures for the first time. In this regard,

this work demonstrates a unique comparison between each of

the contributions of BCP to the PCE.

ER

4. Experimental

Indium tin oxide (ITO)-coated glass substrates (Merck DisplayTechnologies, ITO thickness¼ 100 nm, sheet resistance <20V persquare) were cleaned by ultrasonic treatment in, subsequently, soapsolution, deionized water, acetone and isopropyl alcohol for 20mineach. In the last step the isopropyl alcohol was heated up to its boilingtemperature. Then an oxygen-plasma treatment for 600 s wasperformed to remove the remaining carbon residue. The sampleswere transferred to the ultrahigh vacuum (UHV) chamber (basepressure 5� 10�9 Torr) by means of a load lock. subphthalocyanine(SubPc) (�220 8C) and buckminsterfullerene (C60) (�350 8C) weredeposited at a rate of �1 A s�1 at a pressure of 2� 10�8 Torr.Bathocuproine (BCP) (�120 8C) was evaporated at a rate of �1 A s�1.The deposition rates were determined from the calibration of the filmthickness by spectroscopic ellipsometry (Sopra GESP-5). All of theorganic materials had been purified before loading into the vacuum bythermal-gradient sublimation in a home-built setup. Al deposition(�1200 8C; 2� 10�8 Torr; 2–3 A s�1) was performed in a separatevacuum chamber where samples were cooled down to<0 8C for 30minprior to deposition. Without being exposed to air, the samples wereloaded into a concealed-measurement unit under a nitrogen atmo-sphere ([O2] <1 ppm and [H2O] <1 ppm). Electrical characterizationwas performed with an Agilent 4156C parameter analyzer and aKeithley 2400. Solar illumination was done using a Lot–Oriel 1000-Wxenon arc lamp with AM 1.5D filter. Calibration was performed by aKG3 band-pass filter and a silicon photodetector.

For the measurement of hEQE, light from a Xe arc lamp (Lot–Oriel300 W ozone-free) was coupled into a monochromator (Jobin–YvonH25). Optical cut-off filters of 295 nm and 550 nm were positionedbetween the source and monochromator for the 300–600nm and 550–1100 nm spectra, respectively. The monochromatic-light intensity wascalibrated under normal incidence with a Si photodiode (Newport 818-UV). The light incident on the device under test (DUT) was choppedby a square-wave at 300Hz (Stanford Research 540) and themodulated-current-signal detection was performed by a current–voltage amplifier (Stanford Research 570) and lock-in amplifier(Stanford Research 810). In this configuration hEQE in the near-infrared range could be determined up to 0.01%. In order to ensurestability of theDUT response, the pressurewas kept at below 10�4 Torrduring the measurements.

The optical interference was calculated using commerciallyavailable optical-simulation software (SCOUT) [11]. The complexrefractive index was determined for each layer used in the stack byellipsometry and served as input material parameters. A one-dimensional model stack structure was developed for which weassumed that the interfaces are dominated by the intrinsic bulkproperties of the separate materials and that the morphology can bedepicted as (optically) abrupt junctions. The 0.7-mm glass substratewas modelled as a semi-infinite medium. hIQE, was calculated as theratio of the experimentally determined hEQE over the calculatedintegrated absorption distribution of the donor and acceptor layers.

In order to conduct electroabsorption measurements, the sand-wiched ITO/C60 (/BCP)/Al structure was loaded into a cryostat (Janis)with optically transparent quartz windows. For illumination, light from


a Xe arc lamp (Lot–Oriel 300W ozone-free) was coupled into amonochromator (Jobin–Yvon H25). An optical cut-off filter of 295 nmwas positioned between the source and monochromator. The opticalsignal was detected in reflectance geometry by a femtowatt photo-receiver (New Focus 2151) connected to a lock-in amplifier (StanfordResearch 810). Signal modulation was provided for DR by a square-wave bias modulation through a function generator (500mV; 900Hz;Agilent 33120A). TheEA response generally was phase dependent andhence we determined R¼H(X2þY2) where X and Y are the in-phaseresponse and the quadrature component.

Received: June 17, 2007Published online: November 4, 2008

[1] P. Peumans, V. Bulovic, S. R. Forrest,Appl. Phys. Lett. 2000, 76, 2650.

[2] Z. R. Hong, Z. H. Huang, X. T. Zeng,Thin Solid Films 2007, 515, 3019.

[3] Z. R. Hong, Z. H. Huang, X. T. Zeng, Chem. Phys. Lett. 2006, 425, 62.

[4] P. Peumans, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 126.

[5] B. P. Rand, J. Li, J. Xue, R. J. Holmes, M. E. Thompson, S. R. Forrest,

Adv. Mater. 2005, 17, 2714.

[6] Q. L. Song, C. M. Li, M. L. Wang, X. Y. Sun, X. Y. Hou, Appl. Phys.

Lett. 2007, 90, 071109.

[7] M. Vogel, S. Doka, C. Breyer, M. C. Lux-Steiner, K. Fostiropoulos,

Appl. Phys. Lett. 2006, 89, 163501.

[8] Z. R. Hong, Z. H. Huang, X. T. Zeng, Chem. Phys. Lett. 2006, 425, 62.

[9] K. L. Mutolo, E. I. Mayo, B. P. Rand, S. R. Forrest, M. E. Thompson,

J. Am. Chem. Soc. 2006, 128, 8108.

[10] H. Gommans, D. Cheyns, T. Aernouts, C. Girotto, J. Poortmans,

P. Heremans, Adv. Funct. Mater. 2007, 17, 2653.

[11] URL: http://mtheiss.com/

[12] G. Ruani, C. Fontanini, M. Murgia, C. Taliani, J. Chem. Phys. 2002,

116, 1713.

[13] L.A.A. Pettersson, L. S. Roman,O. Inganas, J.Appl. Phys. 1999, 86, 487.

[14] A. F. Hebard, C. B. Eom, Y. Ywasa, K. B. Lyons, G. A. Thomas, D. H.

Rapkine, R. M. Fleming, R. C. Haddon, J. M. Phillips, J. H. Marshall,

R. H. Eick, Phys. Rev. B: Condens. Matter 1994, 50, 17740.

[15] D. W. Owens, C. M. Aldao, D. M. Poirier, J. H. Weaver, Phys. Rev. B:

Condens. Matter 1995, 51, 17068.

[16] M. Prudent, C. Roussel, H. H. Girault, Electrochemistry Commun.

2007, 9, 2067.

[17] Y.-M. Wang, F. Teng, Q.-C. Zhou, Y.-S. Wang, Appl. Surf. Sci. 2006,

252, 2355.

[18] B. A. Gregg, M. C. Hanna, J. Appl. Phys. 2003, 93, 3605.

[19] I. H. Campbell, T. W. Hagler, D. L. Smith, J. P. Ferraris, Phys. Rev.

Lett. 1996, 76, 1900.

[20] T.M. Brown, J. S. Kim, R. H. Friend, F. Caciallia, R. Daik,W. J. Feast,

Appl. Phys. Lett. 1999, 75, 1679.

[21] Primary Photexcitations in Conjugated Polymers: Molecular Exciton

Versus Semiconductor Band Model, 1st Edition Ed: N. S. Sariciftci,

World Scientific Publishing Company, Singapore 1997.

[22] M. L.Wang, Q. L. Song,H. R.Wu, B. F. Ding, X. D.Gao, X. Y. Sun, X.

M. Ding, X. Y. Hou, Org. Electron. 2007, 8, 445.

[23] If EA were performed at an OPVC, charge carriers would be created

at the D/A interface, which would hinder straightforward determi-

nation of the internal field.


on the role of bathocuproine in organic photovoltaic cells

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