Surface and subsurface morphology of operating nanowire:fullerene solar cells revealed by photoconductive-AFM

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Surface and subsurface morphology of operating nanowire:fullerene solar cellsrevealed by photoconductive-AFM†

Wing C. Tsoi,ab Patrick G. Nicholson,a Jong Soo Kim,b Debdulal Roy,a Tim L. Burnett,a Craig E. Murphy,a

Jenny Nelson,b Donal D. C. Bradley,b Ji-Seon Kim*b and Fernando A. Castro*a

Received 13th June 2011, Accepted 11th July 2011

DOI: 10.1039/c1ee01944a

The 3D nanometer scale phase separated morphology of organic solar cells crucially affects

performance. We demonstrate that photoconductive atomic force microscopy can provide both surface

and subsurface information in operating organic solar cells providing direct correlation between 3D

filmmorphology, local nanoscale optoelectronic properties and device characteristics. P3HT nanowire:

PCBM bulk-heterojunction working devices were investigated. The macroscopic solar cell performance

improvements upon thermal annealing, such as an increase in the short circuit current, the open circuit

voltage and the fill factor, are consistent with observed enrichment of PCBM at the air interface and

increased nanowire crystallinity. PC-AFM is able to directly resolve the associated changes in charge

transport and collection at the local scale, with an estimated depth resolution of at least 20 nm inside the

film.

1. Introduction

Power conversion efficiency of organic solar cells (OPVs) has

recently overcome the 8% barrier.1 A major driving force in the

development of the field has been the growing understanding of

the complex but crucial effect of the 3D nanometre scale

morphology on device efficiency.2–4 The morphological require-

ments for an optimal device include the formation of a large

surface area between the electron donor and electron acceptor

components, with domain sizes in the few nanometers range to

aNational Physical Laboratory (NPL), Teddington, Middlesex, TW110LW, United Kingdom. E-mail: [email protected] of Physics & Centre for Plastic Electronics, Imperial CollegeLondon, London, SW7 2AZ, United Kingdom. E-mail: [email protected]

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1ee01944a

Broader context

Polymer/fullerene thin films exhibiting phase separation on the nano

in organic solar cells. It is now clear that the arrangement of donor a

and needs to be controlled in order to allow high efficient solar cells

processing and post processing parameters in a complex way. Ther

devices to avoid change in morphology due to differences in pr

measurement methods over the last decades, there is no technique

with nanoscale resolution at the same time. The work presented her

that photoconductive AFM can provide subsurface information at

correlation between nanoscale film morphology and device charact

3646 | Energy Environ. Sci., 2011, 4, 3646–3651

maximize charge generation, whilst having percolated paths in

the donor and the acceptor phases to ensure loss-free charge

collection at the anode and cathode electrodes, respectively.2,3

However, controlling film morphology is challenging and,

unfortunately, there are no techniques so far that can charac-

terise the 3D nanoscale domain structure and optoelectronic

properties at the same time in operating device conditions.2

In the last decade the morphology of thin films has been

investigated by several methods,2 including electron tomog-

raphy,5 cross-sectional electron microscopy,6 secondary ion mass

spectroscopy,7 near-edge X-ray absorption fine structure,8 X-ray

photoelectron spectroscopy,9 small angle neutron scattering,10

and grazing incidence X-ray difraction.11 Though these tech-

niques are ideal for revealing structural and crystallographic

information, they do not provide optoelectronic information. In

parallel, scanning probe microscopies have been used to probe

both structural and electrical information at the nanoscale via

metre scale have attracted widespread interest as the active layer

nd acceptor phases in the film strongly affects device behaviour

to be developed. The 3D film morphology depends on material,

efore characterisation should ideally be made in real operating

ocessing conditions. Despite intensive development of novel

today that allows morphological and electrical characterisation

e represents a significant step in this direction. We demonstrate

least 20 nm inside the 80 nm thick active layer, providing direct

eristics.

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/c1ee01944a






http://pubs.rsc.org/en/journals/journal/EE

http://pubs.rsc.org/en/journals/journal/EE?issueid=EE004009

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surface 2D mapping.2,12 In this work we show that photocon-

ductive atomic force microscopy (PC-AFM) can provide both

surface and subsurface information in operating organic solar

cells providing direct correlation between 3D film morphology,

local nanoscale optoelectronic properties and device

characteristics.

PC-AFM is a promising recent variant of AFM, where

a conductive probe in contact-mode is used to simultaneously

measure the topography and the photocurrent generated by

a sample under illumination. Being reported for the first time in

1999,13 it started to receive significant attention from 2007

following publications by Ginger’s group where they demon-

strated 2D mapping of nanoscale photocurrent generation and

local electrical characteristics in different polymer:fullerene

systems, including poly(3-butylthiophene) nanowire:phenyl-

C61-butyric acid methyl ester (PCBM) blends.14–16 The group of

T.-Q. Nguyen reported PC-AFM studies of small molecule:

fullerene devices17 as well as adding a tunable light source to their

setup allowing the determination of local incident photon-to-

electron conversion efficiency.18 Moreover, recently NIST has

published PC-AFM results on variations in photoresponse in

poly(3-hexylthiophene) (P3HT):PCBM films19 where they

concluded that the observed inhomogeneities are due to local

variations at the surface, not reflecting the 3D arrangement

throughout the film. In fact, in common with most scanning

probe microscopies, non-destructive approaches have restricted

this method to the investigation of local surface variations.

In contrast, we demonstrate here that PC-AFM indeed can

provide subsurface morphological and electrical information and

clearly show the distribution of donor and acceptor phases

within a blend, allowing a direct correlation of local nanoscale

properties with macroscopic device characteristics. As a sample

of interest we used bulk-heterojunction solar cells with PCBM

and P3HT nanowires as electron acceptor and electron donor,

respectively. There is a growing interest in the use of semi-

conducting polymer nanowires for a number of applications,20

including photovoltaics. These wires are partially crystalline

which improves charge transport21 and, in principle, by adjusting

the width and density of the wires it should be possible to

maximise charge generation. At the same time, the large wire

length (�hundreds nm) facilitates percolation of charges through

the device reducing series resistance.20

Fig. 1 Current density vs. voltage characteristics of P3HT nanowire:

PCBM solar cells before (blue open circles) and after thermal annealing

at 150 �C for 20 min (black line). Jsc is the short circuit current (current at

zero bias) and Voc is the open circuit voltage (voltage at zero current).

2. Materials and methods

P3HT nanowire:PCBM solutions have been prepared by using

a marginal solvent (dichloromethane (DCM)) procedure as

described elsewhere.22 P3HT and PCBM at 1 : 1 weight ratio

were blended in DCM to a concentration of 3mgmL�1 by stirring

above 50 �C. Sequentially, the hot plate was turned off and the

solution was left to cool down to room temperature. Films were

spin coated onto poly(3,4-ethylenedioxythiophene)-poly(styrene

sulfonate) (PEDOT:PSS) coated ITO substrates to form around

80 nm thick films. Finally, metal top electrodes (Ca(20nm)/Al

(100nm)) were evaporated sequentially through a shadow mask

to form 6 devices on each film. Annealed cells were heated on

a hot plate at 150 �C for 20 min, before cathode deposition. All

samples were prepared and characterized under inert

atmosphere.


Current density vs. voltage (JV) curves were measured using

a computer controlled Keithley source meter. Power conversion

efficiency was calculated as the ratio of maximum generated

electrical power (given by the product: open circuit voltage

(Voc) � short circuit current (Jsc) � fill factor (FF)) to the

incident light power (AM1.5G spectrum at 1000 W m�2). The fill

factor is given by the ratio of the maximum power point

(maximum product of voltage and current) to the product of Voc

and Jsc.

AFM, SKPM and EFM measurements were performed on

a Bruker Dimension Icon (double pass in air, 30 nm lift) using

40 Nm�1 Ir/Pt coated tips (Budget Sensors). PC-AFM (including

topography measurements) was performed on an Asylum

Research MFP-3D mounted on top of an inverted microscope,

using 0.2 N m�1 Cr/Au coated tips (Budget Sensors) under inert

nitrogen atmosphere. A 532 nm laser beam was used through

a 50x microscope objective to form a 100 micrometre diameter

spot on the active layer (power density of �6600 W m�2). This

wavelength was chosen to match the absorbance peak of P3HT.

3. Results and discussion

Fig. 1 shows the macroscopic current density vs. voltage (JV)

characteristics under 1000 W m�2 AM1.5G white light for

a typical P3HT nanowire:PCBM solar cell before (blue open

circles) and after (solid line) thermal annealing. Annealing

induces an increase in device efficiency from 2.0% to 2.8%.

However, it is not possible, from the JV characteristics alone, to

determine the mechanisms responsible for the increase in

performance, since all parameters change at the same time. Short

circuit current (Jsc) increases from 7.5 mA cm�2 to 8.4 mA cm�2,

open circuit voltage (VOC) increases from 0.53 V to 0.60 V and fill

factor (FF) changes from 50% to 55%. Thermal annealing is

widely used for post processing organic solar cells23 but its effect

on device parameters is highly material dependent.2 Often, the

main effect in bulk heterojunction devices is a change in film

morphology,24 although changes in material crystallinity and

interface mixing (or demixing) have also been reported.25,26

To shed some light on the mechanisms at play behind the

increase in performance, we employed several scanning probe

Energy Environ. Sci., 2011, 4, 3646–3651 | 3647


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microscopies to investigate the morphology and electrical prop-

erties of operating devices. Fig. 2 shows standard AFM, elec-

trostatic force microscopy (EFM) and scanning Kelvin probe

microscopy (SKPM) taken at the exact same location, and

PC-AFM images taken on a similar area of the annealed sample

where the blend is not covered by the Al top electrode. Standard

AFM (Fig. 2a) shows that the film is relatively flat (�2 nm RMS

roughness) and, in common with the EFM results (Fig. 2c), some

nanowire features are just discernable on the surface of the film.

The SKPM image (Fig. 2b) is almost featureless and the small

variation in surface potential (26 mV peak-to-trough) seems to

suggest just one material present on the surface. These results

indicate that the nanowires are embedded within the film but

don’t give us any information about how they are arranged.

The PC-AFM photocurrent image (Fig. 2d) is far clearer,

showing a remarkable level of quantitative detail about the

distribution of P3HT nanowires in the sample. Taking into

account tip convolution effects, the width (ca. 20 nm full width at

half maximum) and length (over a micrometer) of the nanowire

features revealed by the PC-AFM compare remarkably well with

previously reported TEM images.27 Note that PC-AFM was

measured under short-circuit conditions (no applied bias) and

the image represents local photocurrent under 532 nm excitation

only. From the difference in current levels (z scale ranges from

�8 pA, dark region, to +28 pA, bright regions) and the shape of

the features it is plausible to suggest that the negative (dark)

areas correspond to PCBM-rich zones and the positive, high

current (bright) regions represent the P3HT nanowires. Note

that the work functions of the AFM gold-coated tip (top elec-

trode) and PEDOT:PSS (bottom electrode) are similar

(measured difference of 100 mV, see Electronic Supplementary

Information†) and therefore there is no significant electric field

present to drive charges to either electrode. This means that the

PC-AFM current is formed mainly by charge diffusion and the

Fig. 2 a) AFM topography, b) SKPM, c) EFM, and d) PC-AFM photocu

annealing; e) magnification of cross point between two nanowires highlightin

contrast, the scales range from zero to, respectively, a) 15 nm; b) 26 mV; c) 0


AFM tip is able to collect both positive and negative charges. In

a first approximation, a change in photocurrent polarity then

indicates a change in material type on the surface if the probed

area is formed of areas rich in electrons and areas rich in holes. In

our samples these areas are formed by, PCBM and P3HT,

respectively. However, we expect this ability to discriminate

between materials with different electrical properties to be

general and therefore applicable also to, for instance, quantum

dots and inorganic semiconductor nanowires.

The PC-AFM images show that the photocurrent from the

nanowires is not uniform. Though the overall value is within

what would be expected from the macroscale JV characteristics

(tens of pA), there seems to be significant variation along single

wires (Fig. 2e). This could suggest different partial PCBM

coverage of the wires, local variation in molecular orientation

or different levels of P3HT crystallisation. Note that the junc-

tion shown in Fig. 2e) is composed of two �20 nm thick P3HT

nanowires, while the top surface of the film is essentially flat

(RMS � 2 nm); this morphology implies that the nanowires

are, for the most part, embedded in the spin coated film. The

fact that PC-AFM is able to display this junction, both as

positive (scale colour yellow-red, Fig. 2e) and slightly negative

(scale colour blue-green) photocurrent, means that subsurface

information is collected at least 20 nm deep inside the film by

this method.

Given the distinct advantage offered by PC-AFM we focused

our attention on this technique. Fig. 3 shows PC-AFM images of

annealed (3a, 3c) and non-annealed (3b, 3d) devices at an applied

forward bias of 0.5 V. A bias was applied here to allow better

visualisation by means of higher contrast. As expected, the

overall current level in the annealed image is greater although the

difference (over 50%) does not directly correlate with that seen in

the macroscopic short-circuit current (ca. 11%, Fig. 1) suggesting

additional contributing factors (see below).

rrent measurements of a P3HT nanowire:PCBM solar cell after thermal

g inhomogeneous current generation within wires. Represented in color

.31� and d) from �8 pA to 28 pA.



Fig. 3 Current map at an applied bias of +0.5V for annealed (a) and not annealed (b) P3HT nanowire:PCBM blends. c) and d) are exactly the same

images as, respectively, a) and b) but the scale is such that negative current is represented in blue.

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The most striking differences between the annealed (3a) and

the non-annealed samples (3b) are the apparent width of the

nanowires (narrower for the annealed sample) and the apparent

density of nanowires present in the image (higher for the

annealed sample). Note that these trends were consistently

observed when measuring different areas of the film, on different

days and using different tips. A decrease in nanowire width with

thermal annealing could be a signature of increased nanowire

crystallinity27 due to closer molecular packing, which would

induce better charge transport leading to higher Jsc and

better FF.

To further investigate the information contained in the images

we changed the current scale of Fig. 3a and 3b to highlight only

the positive current (Fig. 3c and 3d). Note that Fig. 3c and 3d are

exactly the same as Fig. 3a and 3b, except that all negative

current (electrons collected by the tip) is shown in blue. Such

a change in scale allows visualising only the positive current

regions (holes collected by the tip), which should be a signature

of nanowires directly exposed on the surface. Apparently, some

nanowires are completely exposed to the air interface (providing

positive current), some are buried under the PCBM-rich matrix

and others are just partially covered by PCBM-rich areas (at the

subsurface of the blend) as highlighted (white circles) in Fig. 3a

(3b) and 3c(3d). If the wire was completely exposed at the top

surface (or completely buried) we would expect the same current

polarity everywhere, as discussed above. The fact that we can

visualise nanowires that are not exposed at the surface (Fig. 3a

and 3b) again indicates that PC-AFM is providing subsurface

information.

The apparent increase in nanowire density (Fig. 3a and 3b) can

be a result of the higher current generation and better transport

in the annealed device, which increases the PC-AFM image


contrast allowing us to distinguish nanowire features deeper into

the film. However, we cannot exclude the possibility of more

nanowires being formed during annealing (presumably from

P3HT crystallites within the PCBM phase) although the

observed density difference is probably too large to be entirely

due to this.

Interestingly, by decoupling the surface information in the

PC-AFM data from that of the subsurface (Fig. 3), we observe

a lower nanowire density (48%) on the film surface after

annealing when compared to the non-annealed device (50%)

(Fig. 3c and 3d, respectively). A similar increase in PCBM

coverage was recently observed by other authors in a nanowire:

PCBM blend system.28 This is consistent with an increase of Voc

for the macroscopic device data (Fig. 1), since the presence of

P3HT bridges between top and bottom electrode is known to

result in photovoltage loss.29 It is also supported by X-ray

photoelectron spectroscopy of these devices that indicate PCBM

enrichment at the surface with annealing. A detailed device study

including XPS data will be published elsewhere.

Fig. 4 provides further evidence that PC-AFM is able to probe

and differentiate subsurface information. Fig. 4a and 4b show,

respectively, a high-resolution photocurrent map and the corre-

sponding topography. The solid line indicates where the cross

section shown in Fig. 4c was measured. The topography is again

quite smooth (�2 nm RMS), consistent with that observed in

Fig. 2, and the depressed regions correspond to locations where

the nanowire features are evident. This anticorrelation was also

observed for non-annealed devices. As expected, we observe

more positive current (up to 30 pA) where the P3HT wires are.

However, despite exhibiting similar topography, the difference in

the current image of the two nanowires included in the cross

section is striking. The one on the left shows positive current and



Fig. 4 a) Photocurrent and b) topography (z-scale ranges from 0 to 10 nm) maps of the annealed P3HT nanowire:PCBM blend. c) Cross section line of

the current (red dashed line) and height (black line) maps as shown, respectively, in a) and b). d) Schematic drawing of the proposed film morphology at

the cross section view under illumination. The grey area represents the electron-rich PCBM phase and black circles represent the hole-rich P3HT

nanowires partially exposed (left) and buried (right). Arrows represent position of local current measurements during scanning.

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has a similar shape to that observed in the topography whereas

the one on the right shows only a decrease in negative current and

the shape in the current image has blurred slightly with respect to

the topography. We believe this is due to different depths of the

nanowires relative to the film surface, as shown schematically in

Fig. 4d.

While the wire on the left is partially exposed on the top

surface, we expect the one on the right to be covered with a thin

PCBM rich layer, similar to the highlighted features observed in

Fig. 3. The decrease in negative current could be explained by an

increased charge recombination (electrons from PCBM with

holes from P3HT) due to the presence of a thin PCBM layer

covering the nanowire (Fig. 4d).

4. Conclusions

In conclusion, we demonstrated that PC-AFM reveals both thin

film morphological and opto-electronic information in operating

organic solar cells, providing direct correlation between surface

and subsurface structure, local optoelectronic properties and

device characteristics. PC-AFM has shown that thermal

annealing of P3HT nanowire:PCBM blends induces much

improved local charge generation and transport, possibly due to

increased nanowire crystallinity improving short-circuit current

and fill factor. The morphology of the devices is complex: the

nanowires exist at different depth planes and contribute differ-

ently to the overall device photocurrent, with variations present

even within a single nanowire. Our results indicate an increase of


PCBM at the top film surface with annealing, which agrees with

the observed increase in Voc of the solar cell. The apparent

reduction in nanowire width after thermal annealing could be an

indication of increased crystallisation, which would explain the

higher current level from the nanowires in the annealed blends.

A main advantage of PC-AFM when characterising organic

solar cells and phase separated films is related to the fact that

photocurrent needs to travel vertically through the film to be

collected at the surface with the AFM tip, meaning that both

surface and subsurface information is contained in the recorded

current signal. Previous work by other groups have tried to gain

insight into vertical material concentration gradients by ana-

lysing the intensity dependence of the photocurrent30 and tried to

correlate changes in conductive-AFM dark current with surface

composition.28,31 Here, we demonstrated that surface and

subsurface signals can be decoupled which allows investigation

of the internal thin film morphology. Moreover, PC-AFM only

probes material that is active in the photovoltaic process (i.e. an

isolated donor or acceptor region buried deep in the sample will

not contribute to the signal) making this technique particularly

attractive to study organic solar cells. It is expected that the depth

information that can be discerned by PC-AFM will have

a material dependent range and a detailed investigation will be

part of further studies. Our results so far indicate that we can

probe at least �20 nm below the surface of the 80 nm thick film.

The ability to discriminate between electron-rich and hole-rich

nanoscale phases in operating device structures and to directly

correlate that to optoelectronic properties makes PC-AFM



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a valuable tool for the intelligent design of thin film devices based

on nanoscale structures.

Acknowledgements

The authors thank Andy Wain (NPL), Joanna Lee (NPL) and

Roland Hany (EMPA, Switzerland) for discussions. The work

was supported by the EPSRC-NPL Post-Doctoral Research

Partnership (EP/G062056/1) and the SUPERGEN Excitonic

Solar Cell Consortium Grant (EP/G031088/1).

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Surface and subsurface morphology of operating nanowire:fullerene solar cells revealed by photoconductive-AFM