NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Overcoming Degradation in 

Organic Photovoltaics: 

Illuminating the Role of 

Fullerene Functionalization

Preprint

Matthew T. Lloyd, Andres Garcia, 

Joseph J. Berry, Matthew O. Reese, 

David S. Ginley, and Dana C. Olson

Presented at the 37th IEEE Photovoltaic Specialists Conference 

(PVSC 37)

Seattle, Washington

June 19-24, 2011

Conference Paper

NREL/CP-5200-51745

July 2011
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1

OVERCOMING DEGRADATION IN ORGANIC PHOTOVOLTAICS: ILLUMINATING THE 

ROLE OF FULLERENE FUNCTIONALIZATION

Matthew T. Lloyd1, Andres Garcia1, Joseph J. Berry1, Matthew O. Reese1, David S. Ginley1, and Dana C. Olson1

1

National Renewable Energy Laboratory, Golden, Colorado 80401, USA

ABSTRACT

Photobleaching rates are investigated for thin films of 

poly(3-hexylthiophene) (P3HT) blends employing either an 

indene-C60 bisadduct (ICBA) or [6,6]-phenyl-C61-butyric 

acid methyl ester (PCBM) as the electron acceptor. 

Relative to the bisindene, PCBM significantly enhances 

resistance to photobleaching of the P3HT donor polymer. 

We tentatively attribute a decrease in the charge transfer 

rate as the mechanism responsible for the more rapid 

photobleaching in the sample containing the bisindene 

adduct. In order to elucidate the influence of the 

photobleaching rate on the initial performance of 

unencapsulated devices, we also monitored the timedependent behavior for P3HT:fullerene inverted devices. 

Under conditions of constant illumination, we observe 

essentially identical behavior in device performance 

parameters regardless of the energy levels of the electron 

acceptor. We conclude that over the time frame measured 

for these devices, the primary degradation mechanism of 

the active layer is independent of the electron acceptor, 

despite the enhanced tolerance to photobleaching it may 

impart to the donor material.

INTRODUCTION

Recently, several groups have investigated metal oxides, 

such as TiO2 or ZnO, as an electron transport layer that 

modifies the effective work function of ITO to render its 

use as a cathode, thereby inverting the polarity of the 

device.[1-3] The anode (or hole-collecting electrode) then 

requires a high work function metal such as Ag or Au by 

itself or in conjunction with a high work function hole 

transport layer (HTL) such as poly (3,4-

ethylenedioxythiophene): poly(styrenesulfonate) 

(PEDOT:PSS). One implication of the inverted device

architecture is the low chemical reactivity of the metal 

oxide layers and environmental stability of high work 

function top contacts especially in contrast to the Ca/Al 

alternative. Because of the inherent stability of the 

constituent layers, this architecture is being used as the 

basis for scaled-up production of OPV modules. For 

unencapsulated devices tested in air, the inverted 

architecture enables lifetimes of several hundred hours 

compared to <10 hours for conventional devices.[4]

Degradation of the low work function top contact 

represents the single dominant degradation mechanism in 

OPV devices and further effort is required to understand 

higher order processes responsible for the inevitable but 

more gradual decay in performance for inverted devices. 

Of particular interest is the nature of the electron accepting 

molecule and how changes in the energy levels of the 

frontier orbitals can lead to dramatic changes in 

photostability of the accompanying electron donor 

material.

In bulk heterojunction (BHJ) OPV devices, the open-circuit 

voltage (VOC) can be manipulated by changing the 

energetic offset between the donor material, typically 

P3HT, and the electron acceptor materials, often a soluble 

fullerene derivative. By increasing energy level offset 

between the donor highest occupied molecular orbital 

(HOMO) and the acceptor lowest unoccupied molecular 

orbital (LUMO) one can extract a larger VOC. Recently, He 

et al. employed an indene-C60 bisadduct (ICBA) which, 

relative to PCBM, shifts the LUMO level 0.17 eV closer to 

the vacuum level.[5] Optimized devices utilizing ICBA 

produce a VOC of 840 mV and power conversion 

efficiencies of 6.5%.[6] Thus far, there have been no 

studies to link the shift LUMO level to the photostability or 

device lifetime of bulk heterojunction thin films or OPV 

devices.

Figure 1. Molecular structure for (a) indene-C60

bisadduct (ICBA) and (b) [6,6]-phenyl-C61-butyric acid 

methyl ester (PCBM).

This paper compares the photobleaching behavior and the 

influence of the bleaching rate on early-stage device 

lifetime for two functionalized fullerene acceptors. For the 

former, we monitored the change in the absorption spectra 

over time for P3HT:fullerene BHJ films. Additionally, we 

continuously monitored the J-V performance of 

unencapsulated inverted devices under constant 

illumination to investigate the initial performance transients 

over the course of 200 hours.
2

RESULTS AND DISCUSSION

Figure 2a illustrates the change in UV-Vis absorption for a 

bare film of P3HT:PCBM as a function of exposure to 

constant illumination (~100 mW/cm2

) for 77 hours. 

Processing steps for these films were identical to those 

used for typical inverted device architectures, i.e. 1:1 

donor acceptor blending ratio and phase separation 

formed by a slow dry procedure.[4] Absorption peaks for 

the initial films are apparent for the P3HT at 515 and 550 

nm along with a distinct low energy absorption shoulder at 

~610 nm. The latter is known to be associated with 

interchain interactions in well-ordered domains of P3HT.[7]

Successive absorption measurements performed after 

prolonged illumination indicate a decrease in the height of 

each absorption peak over time. After 77 hours, the 

absorption profile exhibits features indicating much of the 

initial order is present in the aged film, although the 

fraction of light absorbing chromophores is significantly 

reduced. It is noteworthy that the higher energy 

absorption peaks associated with absorption by PCBM 

show similar behavior in that the peak height decreases 

but does not vanish over time.

Figure 2. (a) UV-Vis absorption spectra for thin films 

of P3HT:PCBM after exposure to continuous 

illumination (halogen lamp, ca. 100 mW/cm2

) in air. (b) 

Absorption spectra for identically prepared samples 

utilizing the ICBA acceptor molecule.

Figure 2b displays the change in absorption for 

P3HT:ICBA under identical illumination conditions. The 

initial absorption profile for P3HT is essentially identical for 

both composite films. The main difference arises after the 

sample has been illuminated for more than 5.5 hours, 

marked by a large drop in the π-π* absorption peaks and 

near disappearance of the low-energy absorption 

shoulder. This change in the absorption profile indicates a 

much more rapid loss in the number of light absorbing 

chromophores in P3HT and a loss of structural order in the 

polymer not found in the sample containing PCBM. 

Likewise, the absorption peak due to ICBA (250 nm) 

becomes smaller and disappears entirely by 21.5 hours, 

rendering a broad featureless absorption spectrum for the 

ICBA component of the film after prolonged illumination.

In polymer:fullerene photovoltaic devices, the polymer 

component is responsible for the vast majority of 

absorption in the visible spectrum and thus predominately 

contributes to photocurrent generation and the power 

conversion efficiency. To focus on the degradation 

behavior of the polymer portion of the film, we monitored 

the decrease in the peak absorption of P3HT, which is 

located at 515 nm in these films. Fig. 3 shows the linear 

decrease in the fraction of light absorbed, normalized to 

the initial value, for the absorption peak as the films are 

illuminated for 77 hours. The number of light absorbing 

chromophores is reduced by about 10% in the film 

containing PCBM, while the absorption in the ICBA 

decrease approximately 80% in the same period of time 

(77 hours). Despite similar initial absorption profiles and 

identical film thickness, heterojunctions formed from 

PCBM display substantially improved resistance to 

photobleaching. We were able to reproduce this behavior 

in subsequent experiments utilizing lower light intensities 

and P3HT of a higher molecular weight (not shown). 

However, we note that in the higher molecular weight 

films, the photobleaching for both types of acceptors 

occurred at lower rate than reported in Fig. 3.

Figure 3. Normalized fraction of absorbed light at the 

515 nm absorption peak for P3HT:PCBM (triangles) 

and P3HT:ICBA (circles), as a function of light soaking 

duration. Linear fits to the data are indicated by black 

lines.

The literature points to several potential mechanisms 

responsible for the loss of pi-conjugation and subsequent 

photoinduced bleaching in P3HT; and consensus has yet 

to be reached for a single degradation pathway.[8]

However, in ambient conditions it has been shown that 
3

continuous illumination of pristine P3HT results in a linear 

decrease in the peak absorption of the polymer, and the 

eventual loss of all light absorbing chromophores. It is well 

known that an efficient quencher (such as C60 or a 

functionalized fullerene derivative) of the P3HT excited 

state can be introduced to significantly stabilize P3HT 

against photobleaching.[9, 10] Tolerance to 

photobleaching is induced by rapid charge transfer to the 

acceptor, thereby quenching the reactive photoexcited 

state residing on P3HT. ICBA clearly functions as an 

efficient electron acceptor, yielding photovoltaic devices 

with external quantum efficiency approaching 65%.[6] In 

Fig. 3, however, linear fits for the photobleaching rate 

indicate that the bleaching rate for the ICBA sample 

(1.1x10-2 %/hr) is nearly an order of magnitude higher than 

that of the PCBM stabilized sample (1.6x10-3 %/hr). The 

implication of this significant difference in photobleaching 

rate suggests differences in the nature of the charge 

transfer process between P3HT and ICBA. One possible 

explanation is slower donor/acceptor charge transfer for 

ICBA relative to PCBM. Although the donor-acceptor 

charge transfer process may be efficient overall, as 

evidenced by the high yield EQE, the charge transfer rate 

may be slower. If this is the case, the P3HT excited state 

may persist for a sufficient time to react with oxygen in the 

film, and initiate the photo-oxidation process. PCBM, 

which is known to quench P3HT on a picosecond time 

scale[11], will quench P3HT sufficiently fast to prevent 

appreciable reaction with oxygen. The slower charge 

transfer step for ICBA is also supported by smaller 

electron affinity of ICBA, which is responsible for the 

higher VOC in photovoltaic devices. The smaller electron 

affinity is responsible for a lower donor/acceptor energetic 

offset and consequently reduces the driving force for 

charge transfer. It is likely that this smaller LUMO-LUMO 

offset will result in a reduction in the rate of the charge 

transfer process from the polymer to ICBA. Further 

measurements to directly probe the excited state lifetime 

of P3HT in P3HT:ICBA heterojunctions are underway in 

our laboratory.

In order to evaluate the impact of the higher 

photobleaching rate of P3HT:ICBM on the lifetime of 

photovoltaic devices, we employed a home-built

automated combinatorial light soaking system to monitor 

performance parameters as a function of illumination. The 

samples were exposed to constant illumination provided 

by a sulfur plasma lamp (~70 mW/cm2

) in air and J-V

characteristics were recorded at 30-minute intervals. The 

change in each photovoltaic parameter over 200 hours is 

shown in Fig. 4. The VOC and fill factor for the PCBM 

sample degrades very little (2-5%) over the course of 200 

hours, while the short-circuit current density (JSC) 

demonstrates exponential decay. This inverted 

P3HT:PCBM control cell exhibits typical behavior 

observed for many devices in our laboratory.[4] As shown 

in Fig. 4a, the VOC for the ICBA sample increases by 

approximately 15% over the first 25 hours, which we are 

not able to explain at this time. Such behavior is typically 

found in samples where the bare Ag electrodes are used 

to collect holes due to the increase in work function as the 

samples age.[4] In this case, a PEDOT:PSS hole-transport 

layer is used under the Ag contact, and an initial 

performance increase is not expected. Furthermore, a 

concomitant increase in fill factor (Fig. 4b) is not observed 

for ICBA over the same time period. The JSC for the ICBA 

sample demonstrates a qualitatively similar decay profile 

compared to the PCBM device. Overall, in this single 

comparison of the two acceptor materials, the 

performance transients are remarkably similar. Additional 

experiments are needed for complete statistical relevance.

Figure 4. Performance parameters as a function of 

illumination time for unencapsulated 

ITO/ZnO/BHJ/PEDOT:PSS/Ag, where the BHJ layer 

employs P3HT:PCBM (blue) or P3HT:ICBA (red). 

Changes are shown for the normalized (a) open-circuit 

voltage, (b) fill factor, and (c) short-circuit current over 

the course of 200 hours.

Bare films (i.e. fee of metal contacts) demonstrate 

significantly faster photobleaching for BHJs containing the 

ICBA acceptor molecule. Surprisingly, the same trend is 

not readily observable in the early stage lifetime behavior 

in devices with active layers prepared in an identical 

fashion. It is likely that during the initial stages of 

operation in unencapsulated devices, oxygen and 

moisture are effectively blocked from the bulk of the active 

layer and most of the active area of the OPV device is 

protected by the PEDOT:PSS/Ag top contact. In that 

case, regardless of the type of acceptor employed, the 

BHJ is shielded from the oxidizing effects of the ambient 

environment and will not photo-degrade.[10] Moreover, we 

believe that the initial exponential loss in photocurrent 
4

stems from edge-in diffusion, while the end-of-life behavior 

is determined by ingress through defects in the top 

contact.[4, 12] Hence, it is likely that the photobleaching 

bleaching behavior found in thin films represents an 

additional decay channel that is not primarily responsible 

for the decay of JSC found in unencapsulated inverted 

devices.

CONCLUSION

In summary, the difference in photobleaching rates of 

P3HT were compared in thin films employing either a 

bisindene adduct or PCBM as the electron acceptor. 

Relative to the bisindene, PCBM significantly enhances 

resistance to photobleaching of the P3HT donor polymer. 

We tentatively attribute a decrease in the donor-acceptor 

charge transfer rate for ICBA as the mechanism 

responsible for the more rapid photobleaching of P3HT. 

Additionally we monitor the time-dependent behavior for 

P3HT:fullerene inverted devices to investigate the 

influence of the photobleaching rate on the initial 

performance parameters of unencapsulated devices. 

Under conditions of constant illumination, we observe 

essentially identical behavior in device performance 

regardless of the energy levels of the electron acceptor. 

We conclude that over the timeframe measured for these 

devices, the primary degradation mechanism in 

unencapsulated OPV devices is independent of the 

acceptor-stabilized photobleaching rate of the donor 

material.

ACKNOWLEDGMENTS

This work was funded through the US Department of 

Energy under Contract no. DOE-AC36-08GO28308 with 

the National Renewable Energy Laboratory DOE SETP 

program through the National Center for Photovoltaics.

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