Photodegradation in Encapsulated Silole-Based Polymer: PCBM Solar Cells Investigated using Transient Absorption Spectroscopy and Charge Extraction Measurements

TLDR: In this paper, the effect of photodegradation on optical and electrical features has been investigated on encapsulated photovoltaic blend devices comprised of the silole-based donor-acceptor polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM).

Abstract: Light induced degradation has been observed in the performance of organic solar cells in the absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic blend devices comprised of the silole-based donor–acceptor polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power conversion efficiency, primarily as a result of a reduction in short circuit current, JSC. The initial burn-in phase of the photodegradation has been examined using a combination of transient absorption spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction by linearly increasing voltage) and time-resolved charge extraction using a nanosecond switch. These measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities at long times. At the lower charge densities operating at short circuit conditions and under continuous white light illumination, where bimolecular recombination is much less significant, the JSC decreases after light soaking due to a reduction in the efficiency of trapped charge carrier extraction.

Figures

Figure 8. The photo-CELIV curves of the encapsulated KP115:PCBM (1:2) photovoltaic device before and after light soaking (LS), measured using a nanosecond time-resolved switch and a 532 nm 0.3 µJ.cm-2 excitation pulse followed, after a delay time of 100 µs, by a 4 V voltage pulse with a width of 5 µs. The dark CELIV traces are also shown. The inset shows the mobility values over delay times from 10 µs to 10 ms, prior to light-soaking with and without the switch, and after light soaking with the switch.

Figure 9. Photocurrent decays of the encapsulated KP115:PCBM (1:2) photovoltaic device before and after light soaking (LS), measured using a 50 Ω resistance, 0.4 V constant reverse bias and a 1 µJ.cm-2 532 nm laser pulse.

Figure 11. The differences in light soaking behaviour between devices with thick (~ 150 nm) and thin (~ 70 nm) active layers.

Figure 10. Schematic representation illustrating the effect of light soaking on the density of states of the KP115 HOMO. After light soaking, the bimodal distribution of free (grey curve) and localised (black curve) charge carriers changes, with the distribution of localised states becoming both deeper and broader. The horizontal lines represent the centre of each gaussian.

Figure 4. a) The transient absorption spectra of the non-degraded KP115:PC60BM device on the early nanosecond timescale. The microsecond-timescale transient absorption spectra of (b) KP115:PC60BM at 1 µs before and after 72 hours of light soaking (LS) and (c) KP115:PC70BM encapsulated films in transmission, all using 10 µJ.cm -2 532 nm excitation. These results were reproducible over several samples and almost identical results were measured for devices.

Figure 3. The ground state absorbance spectrum of the encapsulated KP115:PCBM (1:2) photovoltaic blend film before and after 72 hours of light soaking (LS).

Full text

University of Wollongong University of Wollongong
Research Online Research Online
Australian Institute for Innovative Materials -
Papers
Australian Institute for Innovative Materials
1-1-2013
Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encapsulated silole-based polymer: Pcbm solar cells
investigated using transient absorption spectroscopy and charge extraction investigated using transient absorption spectroscopy and charge extraction
measurements measurements
Tracey M. Clarke
University of Wollongong
, tclarke@uow.edu.au
Christoph Lungenschmied
Konarka Technologies
Jeff Peet
Konarka Technologies
Nicolas Drolet
Konarka Technologies
Kenji Sunahara
National Institute of Advanced Industrial Science and Technology
See next page for additional authors
Follow this and additional works at: https://ro.uow.edu.au/aiimpapers
Part of the Engineering Commons, and the Physical Sciences and Mathematics Commons
Recommended Citation Recommended Citation
Clarke, Tracey M.; Lungenschmied, Christoph; Peet, Jeff; Drolet, Nicolas; Sunahara, Kenji; Furube, Akihiro;
and Mozer, Attila J., "Photodegradation in encapsulated silole-based polymer: Pcbm solar cells
investigated using transient absorption spectroscopy and charge extraction measurements" (2013).
Australian Institute for Innovative Materials - Papers
. 1016.
https://ro.uow.edu.au/aiimpapers/1016
Research Online is the open access institutional repository for the University of Wollongong. For further information
contact the UOW Library: research-pubs@uow.edu.au

Photodegradation in encapsulated silole-based polymer: Pcbm solar cells Photodegradation in encapsulated silole-based polymer: Pcbm solar cells
investigated using transient absorption spectroscopy and charge extraction investigated using transient absorption spectroscopy and charge extraction
measurements measurements
Abstract Abstract
Light induced degradation has been observed in the performance of organic solar cells in the absence of
oxygen and a detailed analysis of the effect of this photodegradation on optical and electrical features
has been accomplished. This photodegradation study has been performed on encapsulated photovoltaic
blend devices comprised of the silole-based donor-acceptor polymer KP115 blended with [6,6]-phenyl
C61-butyric acid methyl ester (PCBM). Photodegradation induces an almost 20% decrease in power
conversion eHciency, primarily as a result of a reduction in short circuit current, JSC. The initial burn-in
phase of the photodegradation has been examined using a combination of transient absorption
spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction by linearly
increasing voltage) and time-resolved charge extraction using a nanosecond switch. These
measurements reveal a bimodal KP115 polaron population, comprised of both delocalised and localised/
trapped charge carriers. The photodegradation results are consistent with an alteration of this bimodal
KP115 polaron population, with the polarons becoming trapped in a broader, deeper density of localised
states. Under laser illumination and at open circuit conditions, this enhanced trapping after light soaking
inhibits charges from undergoing bimolecular recombination, leading to higher extracted charge densities
at long times. At the lower charge densities operating at short circuit conditions and under continuous
white light illumination, where bimolecular recombination is much less signiGcant, the JSC decreases
after light soaking due to a reduction in the eHciency of trapped charge carrier extraction.
Disciplines Disciplines
Engineering | Physical Sciences and Mathematics
Publication Details Publication Details
Clarke, T. M., Lungenschmied, C., Peet, J., Drolet, N., Sunahara, K., Furube, A. & Mozer, A. J. (2013).
Photodegradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient
absorption spectroscopy and charge extraction measurements. Advanced Energy Materials, 3 (11),
1473-1483.
Authors Authors
Tracey M. Clarke, Christoph Lungenschmied, Jeff Peet, Nicolas Drolet, Kenji Sunahara, Akihiro Furube, and
Attila J. Mozer
This journal article is available at Research Online: https://ro.uow.edu.au/aiimpapers/1016

11111111114111
DOI: 10.1002/aenm.201300337
Article type: Full Paper
Photodegradation in encapsulated silole-based polymer:PCBM solar cells investigated
using transient absorption spectroscopy and charge extraction measurements
Tracey M. Clarke,* Christoph Lungenschmied, Jeff Peet, Nicolas Drolet, Kenji Sunahara,
Akihiro Furube, and Attila J. Mozer*
[*] Tracey M. Clarke, Attila J. Mozer
ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research
Institute, University of Wollongong, North Wollongong, NSW 2500, Australia
E-mail: tclarke@uow.edu.au, attila@uow.edu.au
Christoph Lungenschmied, Jeff Peet, Nicolas Drolet
Konarka Technologies, 116 John St., Suite 12, Lowell, Massachusetts 01852, USA.
Kenji Sunahara, Akihiro Furube
National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5,
1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Keywords: charge transport, conducting polymer, organic solar cell, photodegradation,
transient absorption spectroscopy
Light induced degradation has been observed in the performance of organic solar cells in the
absence of oxygen and a detailed analysis of the effect of this photodegradation on optical and
electrical features has been accomplished. This photodegradation study has been performed
on encapsulated photovoltaic blend devices comprised of the silole-based donor-acceptor
polymer KP115 blended with [6,6]-phenyl C61-butyric acid methyl ester (PCBM).
Photodegradation induces an almost 20 % decrease in power conversion efficiency, primarily
as a result of a reduction in short circuit current, J
SC
. The initial burn-in phase of the
photodegradation has been examined using a combination of transient absorption
spectroscopy and charge extraction measurements, including photo-CELIV (charge extraction
by linearly increasing voltage) and time-resolved charge extraction using a nanosecond switch.
These measurements reveal a bimodal KP115 polaron population, comprised of both

22222222224222
delocalised and localised/trapped charge carriers. The photodegradation results are consistent
with an alteration of this bimodal KP115 polaron population, with the polarons becoming
trapped in a broader, deeper density of localised states. Under laser illumination and at open
circuit conditions, this enhanced trapping after light soakings inhibits charges from
undergoing bimolecular recombination, leading to higher extracted charge densities at long
times. At the lower charge densities operating at short circuit conditions and under continuous
white light illumination, where bimolecular recombination is much less significant, the J
SC
decreases after light soaking due to a reduction in the efficiency of extraction of trapped
charge carriers.
1. Introduction
Even though efficiency records of organic solar cells based on blends of conjugated polymers
and fullerene derivatives have been frequently broken over recent years, insufficient device
lifetime may threaten the widespread implementation of this versatile technology. The present
certified world record for a single junction polymer solar cell has recently surpassed 9 %.
[1]
Given the trend in efficiencies, the commercial success of this promising technology is
increasingly possible. However, with so much emphasis on increasing power conversion
efficiencies, relatively little attention has been applied to the stability of such organic
photovoltaic devices.
[2, 3]
This is a key issue and considerable research is required to
investigate mechanisms of degradation and strategies to enhance device lifetime.
A recent work addressing this point examined the effect of long-term photodegradation on
encapsulated PCDTBT:PC
70
BM solar cells.
[4]
This high-performing blend, which has
achieved efficiencies of over 7 %,
[5, 6]
exhibited an initial sharp loss in performance upon
photo-illumination over the first few hundred hours (the burn-in phase), followed by a

33333333334333
remarkably stable efficiency over several thousand hours. The degradation mechanism was
concluded to be a photochemical reaction in the active layer that creates sub-bandgap states,
thereby increasing the energetic disorder of the system. Further work done by Leclerc et al.,
[7]
albeit under ambient conditions (air), showed that the photo-oxidation mechanism in this
blend involved polymer chain scission and cross-linking reactions. The burn-in phase has
been noted in other blends, such as P3HT:PCBM,
[8]
and may be a general phenomenon of
these type of organic solar cells.
A polymer that is beginning to receive considerable attention is KP115, poly [(4,4’- bis (2-
ethylhexyl) dithieno [3,2-b:2’,3’-d] silole) -2,6-diyl-alt-(2,5-bis 3-tetradecylthiophen-2-yl
thiazolo 5,4-d thiazole)-2,5diyl].
[9-13]
This polymer, although it does not attain the high
efficiencies of PCDTBT:PCBM in thin (< 100 nm) active layer thickness devices, has
particularly interesting characteristics in thick (> 200 nm active layer) devices. KP115:PCBM
devices, which can reach efficiencies of 5 %, possess the rare but highly desirable
characteristic that the active layer can be considerably thicker without a detrimental effect on
the efficiency. However, we also observed that photodegradation effects have a greater impact
on the efficiency of photovoltaic devices with thicker active layers (vide infra). Since
commercially produced polymer solar cells will likely have thicker active layers to maximise
light absorption and to facilitate the high speed coating of consistent layer thicknesses, it is
vital to examine the degradation mechanisms of photovoltaic devices that exhibit good
performance using thicker active layers.
Here we present a detailed study of the photodegradation of KP115:PC
60
BM photovoltaic
devices. The devices studied here have an inverted geometry, with a Ag/hole-injecting layer
(HIL)/active layer (140 nm thickness)/ electron-injecting layer (EIL)/ITO structure, where the
HIL and EIL are Konarka proprietary materials. Encapsulation with glass to inhibit oxygen

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Photodegradation in encapsulated silole-based polymer: pcbm solar cells investigated using transient absorption spectroscopy and charge extraction measurements" ?

Clarke et al. this paper investigated photode degradation in encapsulated silole-based polymer: Pcbm solar cells investigated using transient absorption spectroscopy and charge extraction measurements. 

Q2. What are the future works in "Photodegradation in encapsulated silole-based polymer: pcbm solar cells investigated using transient absorption spectroscopy and charge extraction measurements" ?

Photodegradation, even in encapsulated devices, and charge carrier trapping are clearly significant issues that need further research and must be addressed for successful commercialisation. 

Q3. What is the effect of light soaking on the polaron decay?

After light soaking, the localised polaron decay dynamics slow down, suggesting the presence of energetically deeper trap states, while the delocalised polaron kinetics are less affected. 

Q4. Why is it important to examine the degradation mechanisms of photovoltaic devices that exhibit good performance?

Since commercially produced polymer solar cells will likely have thicker active layers to maximise light absorption and to facilitate the high speed coating of consistent layer thicknesses, it is vital to examine the degradation mechanisms of photovoltaic devices that exhibit good performance using thicker active layers. 

Q5. What is the effect of enhanced charge carrier trapping on the active layer?

The enhanced charge carrier trapping influences the recombination characteristics of the active layer, reducing the efficiency of charge extraction under short circuit conditions. 

Q6. What is the effect of enhanced charge carrier trapping on the solar cell?

The detrimental effect this enhanced charge carrier trapping has on JSC is also supported by the thickness dependent light soaking results, where charge carrier trapping is expected to have a greater effect in thicker devices, as confirmed by the larger photodegradation effect. 

Q7. What is the common reason why the charge carrier trapping is not observed in this experiment?

Charge carrier trapping is typically associated with a concomitant decrease in charge carrier mobility, which is not observed here.[45-47] 

Q8. What is the value of the charge carrier after light soaking?

At low charge densities, however, β is considerably lower after light soaking (by over an order ofmagnitude), indicating a greater proportion of deeply trapped charge carriers unable to participate in bimolecular recombination. 

Q9. What does light soaking do to the charge carrier?

After light soaking, therefore, the enhanced charge carrier trapping causes charge carrier extraction to be impeded more strongly and thus JSC decreases by a larger fraction. 

Q10. What is the kinetics of the trapped polarons at 1000 nm?

A distinct alterationin the decay dynamics at 1000 nm is observed upon photodegradation: the kinetics of the trapped polarons are appreciably slower, with α decreasing from 0.43 to 0.13 (Figure 5b). 

Q11. What is the kinetics of the delocalised polarons at 1000 n?

This suggests that it is partially delocalised polarons that are undergoing bimolecular recombination at this probe wavelength or, alternatively, both delocalised and localised polarons are present, thus the kinetics are a combination of both trap-free and traplimited recombination respectively. 

Q12. What does the TRCE technique allow to monitor?

This TRCE technique also allows the photovoltage decay over time to be monitored, allowing the charge density as a function of photovoltage to be examined. 

Q13. What is the effect of active layerthickness on the decrease in JSC?

Another piece of evidence for the significant role that enhanced charge carrier trapping plays in the decrease in JSC after photodegradation is a consideration of the effect of active layerthickness. 

Q14. What is the difference between the positive and negative polaron bands?

In the case of P3HT, the positive polaron has a substantially higher molar extinction coefficient compared to the negative PCBM anion, thus the latter’s absorption at 1070 nm is not visible and the polymer polaron band dominates the spectrum. 

Q15. What is the effect of light soaking on the charge carrier density?

In order to examine more closely the effects of photodegradation on the charge carrier density and decay dynamics, a time-resolved charge extraction (TRCE) technique was utilised. 

Q16. What is the main contributor to the decrease in JSC observed after light soaking?

It is likely that this loss in charge extraction efficiency is the main contributor to the decrease in JSC observed after light soaking.