Oxygen-Induced Doping as a Degradation Mechanism in Highly

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Oxygen-induced doping as a degradation mechanism in highly efficient polymer organic solar cells Andreas Weu, Joshua Kress, Fabian Paulus, David BeckerKoch, Vincent Lami, Artem A. Bakulin, and Yana Vaynzof ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02049 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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ACS Applied Energy Materials

Oxygen-induced doping as a degradation mechanism in highly efficient polymer organic solar cells

Andreas Weu,†§ Joshua A. Kress,†§ Fabian Paulus,#§ David Becker-Koch,†§ Vincent Lami,†§ Artem A. Bakulin‡ and Yana Vaynzof†§*

† Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany

§

Centre for Advanced Materials, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany

#

Institute for Physical Chemistry, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

ACS Paragon Plus Environment

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‡ Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom

Corresponding Author *Email: [email protected]

KEYWORDS Device

stability,

organic

field

effect

transistors,

photo-thermal

deflection

spectroscopy, transient absorption spectroscopy, organic photovoltaics, oxygen doping


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ABSTRACT

Despite tremendous advances in improving the efficiency of organic solar cells above 14%, the environmental stability of such devices remains an essential and widely inadequately addressed challenge. Understanding the underlying principles of device degradation is a critical step towards further development and commercialisation of organic photovoltaics. Herein, we report on the effect of exposure to oxygen on the operation and degradation of highly efficient PffBT4T-2OD:PC71BM absorption

(TA)

photovoltaic

measurements,

devices.

Ultra-fast

ultra-sensitive

pump-probe

photo-thermal

transient deflection

spectroscopy (PDS) in combination with field-effect transistors suggest that oxygen-induced doping of the active layer is responsible for the severe degradation of the photovoltaic performance. We find that light exposure further accelerates this effect without causing photo-oxidation of the materials.


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INTRODUCTION The last decades have seen organic semiconductors receive enormous attention from the scientific community. Their attractive properties, which include flexibility,

semi-transparency

and

the

ability

to

tune

their

optoelectronic

properties, make organic materials a promising alternative to their inorganic counterparts.1–3 In the field of organic photovoltaic devices, the development of new materials has resulted in significant advances in performance, with power conversion efficiencies (PCE) surpassing 10% in a number of systems.4,5 While most research efforts focus on optimizing the device efficiency and finding alternative fabrication procedures for these material systems,6–16 little is known about the operational and environmental stability of such devices. Nevertheless, a detailed understanding of the degradation mechanisms in organic electronic devices is essential for potential commercial application.17 The stability of polymers in organic solar cells in the presence of oxygen has been previously addressed by several studies.18–20 For the well-known material system

(poly(3-hexylthiophene):

phenyl-C61-butyric


acid

methyl

ester)

4

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P3HT:PC61BM, exposure to oxygen in the absence of UV light leads to reversible

p-type

doping,

which

causes

an

increase

in

charge

carrier

concentration and hence higher bimolecular recombination, lowering the device performance.21–25 Additionally, it has been reported that exposure to oxygen impedes charge carrier mobility due to the formation of deep traps.26,27 Exposure to both oxygen and light, on the other hand, commonly leads to irreversible photo-oxidation of the polymer through radical reactions in several material systems.24,25,28–30 The role of fullerenes in environmental stability has also been discussed; dimerization under the influence of UV light is reported for certain fullerene derivatives leading to the formation of large clusters and thus hindering charge generation and transport.31 In ambient atmospheres fullerenes have been shown to photo-oxidize, reducing electron mobility by deep trap formation.32–37 Another

aspect

of

device

performance

degradation

is

related

to

the

morphological changes the active layer undergoes during the device’s lifetime. A prerequisite for efficient charge generation and transport in organic solar cells


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is a finely mixed bi-continuous network of donor and acceptor material in a bulk-heterojunction (BHJ) on the order of the exciton diffusion length (~10nm).38 In

the

material

system

of

poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-

(3,3’’’-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophen-5,5’’’-diyl)]

(PffBT4T-2OD):

PC61BM, Li et al. observed a de-mixing of polymer and fullerene phases on that length scale upon degradation in nitrogen and continuous illumination. Consequently, charge generation was strongly inhibited, leading to a drastic burn-in effect in short-circuit current (JSC) and thus fast deterioration of performance.39 Furthermore, the addition of processing additives such as 1,8diiodoctane (DIO) has been shown to be detrimental to device stability as the small scale morphology becomes very sensitive to external influences.40–44 Cha et al. also investigated this polymer blended with the acceptor PC71BM under the influence of continuous 1 sun illumination in nitrogen. They found the strong reduction in fill factor (FF) to be the main contribution to the loss in PCE, which might stem from light-induced disruption or cleavage at the PC71BM side group. Exposure to thermal stress, on the other hand, leads to a rapid


6

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decrease of JSC due to thermally induced phase separation of the donor and acceptor domains.45 While these studies focused on the degradation of this material system under light in a nitrogen atmosphere, the effect of exposure to oxygen remains unexplored.

Here,

we

present

a

detailed

study

of

the

oxygen-induced

degradation mechanisms in PffBT4T-2OD:PC71BM organic solar cells. By using femtosecond-transient

absorption

(TA)

and

ultra-sensitive

photo-thermal

deflection spectroscopy (PDS) we suggest that exposure to ambient conditions leads to efficient p-type doping and trap formation in this material system which rapidly decreases the device performance. X-ray photoemission spectroscopy (XPS) measurements suggest that while exposure to light accelerates this degradation process, it does not induce additional photo-oxidation of the active layer components.

RESULTS AND DISCUSSION


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Figure 1a shows the chemical structure and the energy level diagram of the PffBT4T-2OD:PC71BM system. The energy levels were extracted from ultraviolet photoemission spectroscopy and UV-vis absorption and are in agreement with previous results.12,13,45,46 Figure 1b shows the J-V characteristics of the best working solar cell with PCE of 10.23%, which is similar to the maximum reported efficiency of this material system.12,13 The investigated solar cells showed an average VOC of (0.75 ± 0.01) V, JSC of (-18.19 ± 1.38) mA/cm², FF of (66.16 ± 4.78) % and PCE of (8.98 ± 1.01) %. The inset shows the structure of the solar cells in inverted architecture: a glass substrate coated with a thin layer of indium tin oxide (ITO) is used as the bottom electrode, while a Cesium-doped zinc oxide (ZnO:Cs) serves as the electron transporting layer47,48. The deposition of all subsequent layers takes place under inert conditions. The active layer consists of a BHJ of PffBT4T-2OD:PC71BM, onto which molybdenum trioxide (MoO3) as hole transport layer and silver (Ag) as top electrode were evaporated. The choice of this architecture was motivated by the previous observations that standard architecture devices that commonly


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employ PEDOT:PSS as a hole extracting layer and a low work function metal cathode (e.g. Ca/Al or LiF/Al) degrade significantly faster due to the oxidation of the metal electrode and the acidic nature of PEDOT:PSS. As we are interested in degradation processes within the active layer, we chose the more stable inverted architecture that employs metal oxides as charge extraction layers.

a

5

b d 0

current density (mA/cm²)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5 -10

VOC: 0.74V JSC: 19.9mA/cm² FF: 69.6% PCE: 10.23%

-15 -20

Light Dark

-25 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 voltage (V)


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Figure 1. (a) Chemical structure and energy level diagram of PffBT4T2OD:PC71BM system. (b) J-V characteristics of a pristine solar cell prior to degradation. The inset shows the structure of the inverted architecture solar cell.

To monitor the degradation of the photovoltaic devices, directly after fabrication, they were introduced into a gas-tight, home-build environmental sample holder, in which a predetermined atmosphere for degradation experiments could be created. Either nitrogen or a mixture of 20 Vol-% oxygen and 80 Vol-% nitrogen with a total humidity below 20 ppm was used as atmosphere for degradation studies and J-V curves were measured repeatedly while the samples were kept in this environment. Due the low humidity level, the contribution of water to the device degradation in our studies is considered to be negligible and is excluded from discussion. We differentiate between solar cells stored in the dark (apart from illumination during the J-V measurements) and solar cells stored under continuous 1 sun illumination (AM1.5) in between


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J-V-measurements.

Measurements

were

performed

every

20

minutes,

a

substantial heating of the samples can be excluded since the experimental setup provides a steady flow of gas around the substrates stabilizing the temperature at 23-25°C. The temporal evolution of open-circuit voltage, short-circuit current, fill factor and power conversion efficiency of several PffBT4T-2OD:PC71BM solar cells under different environmental conditions is depicted in Figure 2. All values have been normalized to the initial performance at t = 0 (see Figure S1 for unnormalized data). Panels (a) and (b) show reference measurements in nitrogen in dark and under illumination. The solar cells which were kept in the dark under inert conditions show the least amount of degradation, however, still result in a loss of ~20% of initial power conversion efficiency after 15 hours. This performance loss is possibly a result of subtle morphological changes, which might be caused by the destabilizing effects of the processing additive DIO40–42. Illuminating the OSCs constantly increases the degradation rate considerably: the efficiency drops to 40% after 15 hours, mainly caused by the


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rapid decrease of JSC. Light-induced degradation in inert atmosphere has been extensively

studied

before;

small

scale

morphological

changes

and

bond

disruption at the fullerene have been suggested.39,45 Our results are comparable to previously reported studies, although the degradation of VOC is much more pronounced in all our experiments, which may be due to differences in the fabrication procedures. Panels (c) and (d) depict the evolution of photovoltaic performance of the solar cells in simulated dry air environments. The OSCs stored in the dark degrade on a similar timescale as the ones degraded in nitrogen under light. About 6070% of the initial performance is lost after 15 hours, dominated by the severe drop in JSC. The FF remains relatively stable at > 90% of its initial value throughout the course of the experiment and after 1 hour no further decay is observed. On the other hand, the VOC is reduced by 30% during the measurement, which is an unusually strong effect for polymer solar cells 24,27,49,50

and in stark contrast to previously reported degradation experiments in

nitrogen. Indeed, Li et al. and Cha et al. found a stable VOC after storage of 5


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days in the dark under ambient conditions.

The difference may be

39,45

explained by the smaller light dose or a stabilizing effect of a higher humidity in their experiments. While we observed some batch-to-batch variations slightly affecting the overall magnitude of degradation, the strong reduction of the VOC remained a constant feature in all our experiments.

a

0.8 0.6 0.4

FF VOC JSC PCE

0.2 0.0

0

1

2

3

b

1.0

normalized values

normalized values

1.0

0.8 0.6 0.4

FF VOC JSC PCE

0.2

4

5

6

7

8

9 10 11 12 13 14 15

0.0

0

1

2

3

4

5

c

1.0

0.6 0.4

FF VOC JSC PCE

0.2 0.0

0

1

2

3

7

8

9 10 11 12 13 14 15

FF VOC JSC PCE

1.0

normalized values

0.8

6

time (hours)

time (hours)

normalized values

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8

d

0.6 0.4 0.2

4

5

6

7

8

9 10 11 12 13 14 15

0.0

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14 15

time (hours)

time (hours)


13


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Figure 2. Normalized J-V parameters for solar cells degraded in (a) nitrogen under dark conditions, (b) nitrogen with constant illumination, (c) 20% oxygen atmosphere in dark and (d) 20% oxygen with light. The data is averaged from measurement of 8 solar cells for each degradation conditions. See Figure S1 for the un-normalized data.

Figure 2d shows the solar cell characteristics in the same oxygen atmosphere while

continuously

illuminated

by

1

sun

equivalent

solar

radiation.

The

degradation proceeds in two phases: after an initial exponential decay (‘burn-in’) of about 6 hours the solar cell parameters decay linearly. The fill factor remains stable after the first 10h of degradation at approximately 80% of its initial value.

After 15h of degradation the VOC is reduced to half its original

value and the JSC drops to 30%, compared to 50-60% in just light or oxygen alone.

Considering

the

observed

behavior

under

different

environmental

conditions, we can conclude that both oxygen and light have significant implications on the device performance and the combination of the two leads to very rapid performance deterioration.


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To exclude (photo-) oxidation of the active layer upon exposure to oxygen we conducted XPS and UPS studies on the degraded films. In general, photooxidation results in the formation of oxidized sulfur, carbon and/or nitrogen species and changes the overall composition of the studied material. XPS measurements acquired both prior to and after degradation in 20 Vol-% oxygen and light for 20h reveal no changes in chemical composition and chemical environment (see Figure S2). The choice of this prolonged degradation period (at which point the devices reach

Oxygen-Induced Doping as a Degradation Mechanism in Highly

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