XPS Analysis of the Improved Operational Stability of Organic Solar

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XPS Analysis of Improved Operational Stability of Organic Solar Cells Using V2O5 and PEDOT:PSS Composite Layer: Effect of Varied Atmospheric Conditions Haya Alhummiany, Saqib Rafique, and Khaulah Sulaiman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b13016 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 9, 2017

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The Journal of Physical Chemistry

XPS Analysis of Improved Operational Stability of Organic Solar Cells Using V2O5 and PEDOT:PSS Composite Layer: Effect of Varied Atmospheric Conditions Haya Alhummiany1*, Saqib Rafique2, 3 and Khaulah Sulaiman2

1

Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia

2

Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia 3

Centre of Nanotechnology, King Abdulaziz University, Jeddah, 21589, Saudi Arabia

*Corresponding author: [email protected]

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Abstract This work investigates the enhanced stability of organic solar cells (OSCs) fabricated with hybrid hole transport layer (HTL) by incorporating vanadium pentaoxide (V2O5) nanoparticles in PEDOT:PSS.OSCs have been fabricated in controlled and ambient atmospheric conditions by employing pristine PEDOT:PSS HTL and its hybrid variant. Stability and degradation analysis were carried out by using photovoltaic and X-ray photoelectron spectroscopy (XPS) measurements, respectively. Normalised photovoltaic characteristics showed that OSCs with hybrid HTL outperformed the pristine device and retained their performance as compared to their pristine counterparts when the fabrication was carried out in nitrogen filled golvebox and devices were tested after encapsulation for seven days. However, OSCs which were fabricated and characterised in ambient air showed severe degradation in photovoltaic performance, mainly due to drastic decay in short circuit current and open circuit voltage for both device variants. Further, XPS was applied to probe the stability of HTL variants under ageing in ambient air. The device instability was mainly ascribed to the indium diffusion from the anode into the HTL and its concentration increased from 0.4 to 2.8% within 250 h of ambient exposure of pristine HTL, while insignificant increase was recorded in the indium content of the hybrid HTL. This confirms the remarkable reduction in indium diffusion brought by the presence of V2O5 nanoparticles.


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1 Introduction During the past few years, remarkable progress has been recorded in bulk heterojunction (BHJ) organic solar cells (OSCs) that have pushed the device efficiency towards the efficient 10% regime.1,2,3 Although, the lifetime of OSCs can be improved by adopting various encapsulation techniques but this approach would significantly increase the production cost.4 Most of the OSCs which are being fabricated today are tested under controlled environment without even being exposed to ambient air containing natural humidity. Hence, for the successful widespread commercialization and large scale production of OSCs, the stability of OSCs also needs to be improved along with their efficiency.5 The

BHJ

OSCs

ethylenedioxythiophene):

in Poly

the

conventional

(styrenesulfonate)

device

structure

(PEDOT:PSS)

uses as

Poly (3,4-

standard

hole

transporting layer (HTL) which also favours the device degradation due to its inherent hygroscopic and acidic nature.6 The acidic PEDOT:PSS causes indium loss from indium-tin oxide (ITO). Corroded interface between ITO and PEDOT:PSS arises reliability issues while reducing the cell life drastically.7 Such constrains have brought the use of high work function metal oxides such as NiO, WO3, MoO3 and V2O5 as good alternatives to standard PEDOT:PSS HTL.8,9 These metal oxides proved to substantially enhance the cell stability, however, PEDOT:PSS still outperforms its alternatives in terms of device efficiency.10,11 Therefore, combination of metal oxides and PEDOT:PSS is expected to compliment the drawbacks of either of the individual materials. In this context, we recently used an organicinorganic hybrid composite consisting of PEDOT:PSS and vanadium pentaoxide (V2O5) as HTL.12 The resultant device showed significantly enhanced device stability. Combination of PEDOT:PSS and V2O5 has also been studied by several other groups. For example S.J. Lee et al..13 studied the doping of PEDOT:PSS with V2O5 nanoparticles and demonstrated to have highly stable device as compared to pristine PEDOT:PSS based device. Similarly J. Pan et 3 ACS Paragon Plus Environment


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al.14 used combination of PEDOT:PSS and V2O5 nanowires to fabricate OSCs with higher stability. In another study, J. Kim et al.15 studied the effect of V2O5 doping in PEDOT:PSS HTL for polymer light-emitting diode. But most of the work is related to simply evaluating the efficiency of the fabricated device and less attention has been given to identify the key degradation factors which have been suppressed by addition of V2O5 to improve the lifetime stability. Moreover, most of the degradation factors arise from the ambient atmosphere i.e. moisture and molecular oxygen ingress from the atmosphere. OSCs are currently the ultimate in terms of complexity and they exhibit the most uncontrollable situation in terms of stability. In order to improve the device life time, we need to understand the degradation mechanism of the solar cells during operation. Device degradation is rather a complex phenomenon comprises of several factors, of which many of them are not identified yet. Herein we present the synthesis and application of solution processable V2O5 as an HTL in OSCs. V2O5 nanoparticles were mixed in PEDOT:PSS to form an organic-inorganic hybrid HTL and further compared with the OSCs employing pristine PEDOT:PSS HTL. In order to identify the degradation effects arising from ambient atmospheres, the OSCs were fabricated in controlled mode in an inert atmosphere (Nitrogen filled golvebox) and tested after encapsulation. For comparison, both variants of OSCs were fabricated in ambient atmosphere and tested in an open chamber without encapsulation, the so-called open mode. This work is focused on device life time, stability and degradation effects, in particular pertaining to HTL. X-ray photoelectron spectroscopy (XPS) has been used to characterise the chemical changes that occur on surface of HTL as a function of time. Several groups have adopted XPS to study degradation factors in OSCs, but, most of these studies are focused on the changes occurring in the photoactive blend and less attention has been given on the changes related to HTL.16,17,18

The photovoltaic performance and atmospheric induced

degradation of OSCs with pristine and hybrid HTL have been evaluated and compared. We 4 ACS Paragon Plus Environment

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experimentally confirmed that fabrication conditions play a key role in device performance where the stability of the device can be improved significantly under controlled conditions. Moreover, It was found that using the hybrid HTL yielded the better operational stability of the device as compared to that with pristine PEDOT:PSS HTL.

2 Experimental Method 2.1 Materials PEDOT:PSS solution (PH1000) has been purchased from H.C. Starck, Germany and used as received. Both poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl2',1',3' benzothiadiazole)] (PCDTBT), and (6,6)-Phenyl C71 butyric acid methyl ester (PC71BM) were purchased from Lumtec, Taiwan. Pre-patterned (ITO) glass substrates with a sheet resistance, Rs = 15 Ω and dimensions of 1.5 × 2.0 cm (Ossila, UK) were used as the working electrode. Each ITO coated substrate has six active pixels with an active area of 30 × 1.5 mm (4.5 mm2) for every pixel. NH4VO3 and nitric acid (HNO3) for V2O5 synthesis were obtained from BDH Chemicals, UK and Sigma Aldrich, respectively. All other necessary chemicals such as chloroform etc.; were purchased commercially and used as received without further purification. HTL and active layer solutions were filtered by using 0.45 and 0.25µm PTFE filters respectively, purchased from Whatman, Germany.

2.2 Synthesis of V2O5 nanoparticles V2O5 nanoparticles used in the present study were synthesised by co-precipitation method.19 Ammonium metavanadate (NH4VO3) was used as precursor to synthesize V2O5 in the presence of surfactant triton X-100. In a typical process, 20 g of NH4VO3 was primarily dissolved in 1.0 litre of deionised water, and then 0.2g of triton X-100 was added. The solution was continuously stirred and the temperature was maintained at 90 °C for 60 minutes to ensure complete emulsification. Thereafter, 35 wt. % nitric acid (HNO3) was added drop by drop to acidify the VO3 solution. After acidification, brownish black precipitate was 5 ACS Paragon Plus Environment


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obtained which was left overnight and later washed with excess of water until neutralization. Traces of triton X-100 surfactant were removed from the precipitates by rinsing with a mixture of ethanol-acetone (1:1 by volume). The precipitates were dried in vacuum oven at 100 °C for 12 h after which the dried powder was grinded and subsequently calcined at 500 °C for 5 h in a muffle furnace at a heating and cooling rate of 25 °C/min respectively to finally obtain yellowish-brown V2O5 nanoparticles.

2.3 Preparation of HTL variants Two variants of HTL were spun coated in the normal geometry device structure. In the reference device, we used pristine PEDOT:PSS HTL whereas, for the second type of device, organic-inorganic composite of PEDOT:PSS and V2O5 had been used to function as HTL. The hybrid organic-inorganic composite was prepared by mixing V2O5 nanoparticles in the PEDOT:PSS aqueous solution at the concentration of 1mg/ml. The hybrid composite suspension was homogenized by ultrasonication and stirring. Prior to the spin coating of HTL, PEDOT:PSS aqueous solution was filtered by using commercially available 0.45µm PTFE filters.

2.4 Solar cell fabrication procedure under various atmospheric conditions Pre-patterned ITO coated glass substrates were cleaned in ultrasonic bath with soap water, acetone, isopropyl alcohol and de-ionized (DI) water sequentially, for 15 minutes each. The cleaned substrates were then blown dry by nitrogen stream and were further treated with UV-ozone for 20 minutes. Two variants of HTL were spun coated on ITO substrates and the thickness for each type of HTL was optimised by using different spinning speeds to finally get the desired thickness of about 40 nm at 4000 rpm for 1 minute. The HTLs were annealed at 120°C for 30 minutes.


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The photoactive binary blend of donor and acceptor materials in optimized volumetric ratio was later spun coated on both types of HTLs. PCDTBT (donor) and PC71BM (acceptor), the two constituents of photoactive binary blend, were dissolved in chloroform at the concentration of 10mg/ml. Both solutions were left on a magnetic stir plate overnight which were further mixed in the optimized volumetric ratio of 1:4 (PCDTBT:PC71BM) to obtain the active layer blend. Active layer solution was filtered by using 0.25µm PTFE filter prior to spin coating at 2000 rpm for 20 seconds in order to get desired thickness of about 70nm. To complete the fabrication process, Aluminium (Al) electrodes with 100 nm thickness were thermally evaporated on top of photoactive layer under vacuum (10-6 Torr) for both types of devices. For the degradation analysis, OSCs devices were fabricated under two atmospheric conditions. To study the degradation in air, both types of devices were fabricated in ambient atmosphere and tested in an open chamber without encapsulation in dark as well as under illumination. All the photovoltaic parameters such as open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF) and power conversion efficiency (η) were measured after regular intervals. For the stability tests in controlled environment, the devices were fabricated in a nitrogen filled atmosphere under the controlled relative humidity (RH) and oxygen conditions. The two variants of devices were further encapsulated with glass to protect from ambient atmosphere.

2.5 Instrumentations Surface morphology was characterised by using field emission scanning electron microscope (FESEM) model JEOL JSM-7600F and atomic force microscopy (AFM) model SPM PROBE VT AFM XA 50/500 Omicron, Germany. Structural information was acquired by Ultima-IV (Rigaku, japan) multipurpose X-ray diffraction (XRD) system. Powder 7 ACS Paragon Plus Environment


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diffractometer with Cu K-α source (λ=0.154060nm) was used to scan in 2θ range of 10 to 70°. Active layer and HTL solutions were spun coated by Laurell spin coater (WS-650MZ23NPP).Thickness for each layer was measured by using surface profiler measuring system (KLA Tencor (P-6), Taiwan). Current-voltage (I–V) curves were measured

by

using

Keithley 236 source measure unit (SMU) along with an air mass 1.5 Global (AM 1.5 G) solar simulator with an irradiation intensity of 100mW/cm2. Light intensity was calibrated by a Newport power meter 1918-R with calibrated Si-detector 818-UV. For XPS compositional analysis, the HTLs were replicated on ITO coated glass and subjected to ageing until 250 h in ambient atmosphere. XPS data were obtained by using PHI 5000 Versa Probe Scanning ESCA Microprobe (PHI 5000 Versa Probe II, USA), equipped with monochromatic Al-Kα (hν = 1486.6 eV) X-ray source. Multipack Software (VERSION 9, ULVAC-PHI, Inc. Japan) was used to analyse the XPS data and for the fitting of Gaussian-Lorentzian line shapes.

3 Results and Discussion Simple spin coating technique has been used in this work for the fabrication of devices with pristine PEDOT:PSS HTL and its hybrid variant. The advantage of the present technique is its facile route and easy experimental setup. Addition of V2O5 into the PEDOT:PSS remarkably improved the performance of the device, especially the stability. One of the root causes of the device degradation is indium diffusion which was successfully suppressed by addition of V2O5 into the PEDOT:PSS aqueous suspension. In addition, V2O5 also acts as the electron blocking layer which effectively blocks the flow of electrons towards the anode and facilitates the flow of holes. The schematic of the present work has been presented in the Figure 1 below.


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Figure 1: Schematic representation of solar cells fabrication procedure with two variants of HTL under ambient and controlled atmospheric conditions.

3.1 Structural and morphological characterizations V2O5 nanoparticles synthesized by co-precipitation method were characterized for crystalline structure by using XRD. The XRD pattern shows intense reflections at 15.53°, 20.45°, 21.83°, 26.31°, 31.13°, 32.57°, 34.47°, 41.39°, 47.51°, 51.37° and 62.23°. Sharp and noise free peaks exhibit high crystallinity of the prepared material. XRD patterns presented in Figure 2 shows Shcherbinaite phase of V2O5 with orthorhombic crystal structure and well indexed with the ICDD PDF-2 card number: 01-072-0433. The cell dimensions are a=11.5100, b=4.3690, c=3.5630 and calculated grain size is 48.21 nm by using Scherer's equation.


010

10

20

30

020 120 411 600 320 002 610 012 611 312 701 711

011 301 111

110

200

310

Intensity (a.u.)

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101


40 50 2(degree)

60

70

Figure 2: XRD pattern of V2O5 nanoparticles synthesised by co-precipitation method

Device performance greatly depends on surface morphology of each layer in OSCs. In our work, the incorporation of V2O5 nanoparticles into the PEDOT:PSS solution has been carried out by dispersing these particles in PEDOT:PSS aqueous suspension. We analyzed the surface morphology of both HTL variants by using AFM and SEM surface scans. The SEM image of pristine PEDOT:PSS shows a smooth sheet of polymer with no features on the surface. However, the hybrid HTL exhibits a loosely packed surface morphology with typical granular structure. This typical granular surface increased the surface roughness of the HTL and can be observed in AFM images. Figure 3a and c shows AFM images of pristine HTL and its hybrid variant with the scan size of 2µm×2µm, respectively. As we can see from AFM images, pristine HTL shows root mean square (RMS) roughness of 1.29 nm, whereas, hybrid HTL shows RMS roughness of 2.26 nm which is approximately double of the pristine HTL. The increase in the RMS roughness was expected and agrees with the previously reported work.12,20 Although the increase in the surface roughness is undesired, however, the advantages of V2O5 based HTL outweigh its disadvantages. It is confirmed from our results that addition of V2O5 nanoparticles successfully suppressed the diffusion of indium from ITO 10 ACS Paragon Plus Environment

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substrate into the subsequent layers which is one of the root causes of device degradation. In addition, it also reduces the leakage current by blocking the negative charge carriers.12 Moreover, addition of V2O5 nanoparticles helps in enhancing the surface adhesion.

Figure 3: AFM and SEM images of (a, b) pristine PEDOT:PSS HTL and its (c, d) hybrid variant consisting of V2O5 nanoparticles dispersed in PEDOT:PSS aqueous suspension.

3.2 Photovoltaic characterisation For the stability analysis of the OSCs we recorded current density-voltage (I-V) data for both variants of devices (Pristine PEDOT:PSS and PEDOT:PSS+V2O5) in ambient and



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(a) (a)

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controlled atmospheres after every 24h for seven days (168 h). The experimental conditions for both atmospheric modes have been defined in section 2.4. Figure 4 shows normalised (a) Jsc (b) Voc (c) FF and (d) PCE of the both variants of devices. It can be observed from Figure 4a that fabrication condition significantly affected the Jsc of the devices in open air. The normalised Jsc values decreased to almost zero for both variants of devices fabricated and tested in open air without any encapsulation. However, the device fabricated in N2 atmosphere and encapsulated to test in air retained almost 100% of its Jsc in case of hybrid device and more than 80% of the initial value in case of pristine PEDOT:PSS. This shows that incorporation of V2O5 in the PEDOT:PSS significantly enhanced its Jsc. Similarly, Voc also experienced 80 % reduction in its initial values for both variants when devices were fabricated and tested in open air as shown in Figure 4b. However, it retained more than 90% of the initial Voc in case of controlled conditions for both variants of device. Figure 4c shows reduction in FF over the period of 7 days. It is observed that FF reduced to almost 70% of its value for the pristine device fabricated and tested in open mode, however, ~ 20% improvement has been observed in the hybrid device and it retained almost 90% of its initial values. In the controlled mode, the hybrid device retained 100% of the initial FF whereas, it reduced to 90% of its values for the pristine device, hence, exhibiting 10% improvement brought by the addition of V2O5 nanoparticles in PEDOT:PSS. Finally, from the efficiency measurements normalized to their initial values, V2O5 based device retained 93% of its initial value, showing 25% improvement as compared to its pristine HTL based counterpart where the efficiency dropped to 68% of the initial value, when tested under controlled mode for one week as shown in Figure 4d. However, the PCE plunged down to almost zero when both variants were fabricated and tested in open mode. The observed decay in the device PCE is attributed to a drastic decrease in the photovoltaic parameters particularly, the Jsc when the devices were fabricated and tested in ambient conditions. It was confirmed from the I-V 12 ACS Paragon Plus Environment

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characterizations that V2O5 nanoparticles had significantly enhanced the stability of hybrid OSCs as compared to their pristine counterparts. However , addition of V2O5 in PEDOT:PSS was observed to be more effective when devices were protected from the surrounding environment during fabrication and testing phase, yielding higher values for FF, Jsc and Voc.

Figure 4: Normalized photovoltaic parameters i.e. (a) Jsc (b) Voc (c) FF and (d) PCE of normal architecture BHJ OSC devices with pristine PEDOT:PSS HTL and their hybrid variants as a function of time for one week (168 h) in ambient and controlled conditions.

The observed decay in the device stability has been thoroughly investigated by using XPS surface scan of HTL variants and discussed in detail in section 3.3. The key known factors responsible for the degradation in ambient atmosphere are the presence of oxygen and humidity which causes severe degradation in the optical, chemical, mechanical and structural 13 ACS Paragon Plus Environment


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performance of the device. Krebs et al.21 in their analysis of the failure mechanism pertaining to OSCs stated that atmospheric moisture diffuses through aluminium electrodes and further penetrates through all the layers reaching to ITO electrode which leads to severe degradations at every level in layered structure of BHJ OSCs. T. S. Glen et al.22 in their work related to degradation study of OSCs due to air exposure and K. Norman al.17 in their study on degradation due to oxygen and water in OSCs stated that air induced degradations often occur at the interface between active layer and PEDOT:PSS layer. In our case owing to the hygroscopic nature of PEDOT:PSS it was annealed and properly dried before active layer fabrication. Thus it is most likely possible that after exposing to ambient conditions for longer time, it may absorb moisture which subsequently leads to its rapid degradation. Moreover, PEDOT:PSS is highly acidic which upon contact causes severe damage to the ITO electrode. As a result, the ITO reacts with the PEDOT:PSS and diffuse into it arising severe degradation issues. In OSCs, both intrinsic and extrinsic degradation factors equally affect the HTL stability. As discussed above, PEDOT:PSS as benchmark HTL material suffers from moisture and oxygen ingress due to its hygroscopic nature. Moreover, its acidic nature also corrodes underneath anode. All these factors degrade the device performance. In this work, we seek a detailed understanding of these factors and the possible improvement brought by the addition of V2O5 nanoparticles in the PEDOT:PSS aqueous suspension. Although, we presented the device stability data for 168 h, however, the XPS study was prolonged to 250 h to develop a deeper understanding of degradation factors affecting the device performance, particularly in the context of HTL.

3.3 XPS analysis of chemical changes with ageing OSCs degradation process goes through several chemical changes which can be best studied by XPS analysis. We investigated chemical changes pertaining to both HTL variants 14 ACS Paragon Plus Environment

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induced by ageing. Survey spectra for both variants of HTL were recorded as a function of exposure time to ambient atmosphere in order to ascertain the diffusion of degradation elements such as (1) indium and Sn from the ITO substrate and (2) oxygen mainly from the atmosphere, into the HTLs. Figure 5 shows the representative survey spectra of pristine PEDOT:PSS HTL and its hybrid variant recorded for fresh sample and 250 h aged sample in ambient atmosphere whereas, the survey spectra for 5 and 25 h have been shown in Figure S1 of the supporting information. The data extracted from these spectra have been presented in Table 1. The nominal XPS concentration of indium was present on the surface of polymeric (PEDOT:PSS) layer for all the samples recorded for fresh, 5 and 25 h of ageing in both variants of HTL. Upon ageing for 250 h, there is a significant indium uptake in pristine PEDOT:PSS HTL rising more than seven times of the initial concentration. Indium concentration in the fresh sample has been found to be 0.4% which increased up to 2.8% of the total concentration in 250 h of ageing. Additionally, there were no significant change recorded in the Sn content for both variants of HTL and it is found to be 0.1% or less in all samples.



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Figure 5: Survey spectra of (a, b) pristine PEDOT:PSS HTL and (c, d) PEDOT:PSS+V2O5 based hybrid HTL recorded for fresh sample and after 250 h of aging in ambient atmosphere.

Presence of indium and Sn in the HTL is due to the product of etch reaction between the PEDOT:PSS HTL solution and the ITO substrate. It is evident from our results that etching of ITO may starts to occur during the spin coating of the PEDOT:PSS solution. Our observation matches well with the previously reported data.23,24,25 PEDOT:PSS with a pH value between 1 and 2, etches the ITO surface, causing its deterioration, that leads to the incorporation of indium into the subsequent layers.26 This phenomenon accounts for one of the leading factors effecting device performance. The possible reason for detection of high concentration of indium as compared to Sn in the samples is that Indium dissolve more


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readily in PEDOT:PSS as compared to Sn during the spin coating process and later on during aging in ambient.27 Table 1: Summary of Atomic concentration (%) for each element of pristine PEDOT:PSS HTL and PEDOT:PSS+ V2O5 HTL Ageing time in ambient atmosphere Fresh 5 hours 25 hours 250 hours Fresh 5 hours 25 hours 250 hours

C 67.9 67.5 67.7 68.7

Atomic concentration (%) by XPS Pure PEDOT:PSS HTL O S In 24.0 7.6 0.4 24.7 7.3 0.5 23.8 7.1 1.3 22.5 2.8 6.0

68.5 69.3 68.1 65.2

PEDOT:PSS+ V2O5 HTL 23.4 7.5 23.3 6.8 24.3 7.0 24.6 6.3

0.4 0.4 0.4 0.6

Sn 0.1 < 0.1 0.1 < 0.1 < 0.1 < 0.1 0.1 --

V 0.1 0.2 0.1 0.8

Na ---2.5

In contrast, the PEDOT:PSS+ V2O5 sample exhibited only an insignificant change in In 3d signals and almost negligible Sn concentration, which implies that ITO diffusion has been successfully suppressed by the addition of V2O5 nanoparticles in PEDOT:PSS. S. J. Lee et al.13 in their recent work on effect of mixing V2O5 in PEDOT:PSS HTL on the OSCs performance, described that HOMO level of V2O5 is much better than that of PEDOT:PSS, therefore, one can expect better performance of hybrid HTL compared to pristine PEDOT:PSS HTL. Furthermore, in their work the improvement is mainly attributed to the superior optical properties of the hybrid HTL as compared to its pristine PEDOT:PSS counterpart. Whereas, in our investigations, it has been observed that addition of V2O5 nanoparticles in PEDOT:PSS might mitigate the diffusion of ITO into the HTLs, since atomic % for In did not have significant changes as compared to reference HTL (shown in Table 1). Additionally, it will help to block the electrons and avoid current leakage.28 While, the concentration of vanadium at the surface of HTL in hybrid sample increased from 0.1% to 0.8% in 250 h. 17 ACS Paragon Plus Environment


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Table 1 also shows the oxygen uptake for both types of HTLs as a function of exposure time to air. For both variants of HTL, the oxygen content did not change significantly which might be attributed to occurrence of more than one phenomenon that balance the incoming oxygen contents with the outgoing oxygen. As reported by Norrman et al.17 in their recent study on degradation patterns pertaining to water and oxygen of an OSC, PEDOT:PSS is commercial material which contains one or more unknown additives, therefore, it is not possible to explain this phenomenon in detail. In addition, XPS only reveals surface information with a maximum probe depth of 5 to 10nm, therefore it does not represent the bulk material properties; that is the material itself serves as a barrier, so nominal oxidation can also occur in the bulk of the film .17 We recorded 7.6% and 7.5% sulphur content in as prepared samples of pristine and hybrid HTL respectively, which reduced to 6.0% and 6.3% after ageing of the samples for 250 h. Similar to oxygen, no substantial change recorded in carbon concentration in both variants of HTL and remained 67±2% in all samples. 3.3.1 Study of core-level spectra

The exact chemical changes, hence degradation mechanisms, can be studied from the individual spectra for the respective elements. XPS profile for two variants of HTL showed distinct peaks for S 2p, C 1s and O 1s peaks along with the In 3d and Sn 3d spectra appeared as a result of indium and Sn diffusion into the HTL. The hybrid HTL also features V 2p3 peaks in all its samples due to addition of V2O5 nanoparticles into the PEDOT:PSS. Sodium (Na) signals were also detected in the survey scan of hybrid HTL at 250 h of ageing which could be attributed to an impurity or a trace element in PEDOT:PSS solution. 29 In order to investigate the chemical changes caused by degradation with time, the XPS signals corresponding to these elements were observed during the ageing of samples. 18 ACS Paragon Plus Environment

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Figure 6a, b shows high resolution S 2p spectra of freshly prepared pristine PEDOT:PSS film and after ageing for 250 h. The electrons in the core shells are slightly perturbed by the surrounding chemical changes which influence their binding energies. This causes a slight shift in peak position, a so-called chemical shift.17 For both conditions, the S 2p spectra appear in two components.16 The XPS band with lower binding energies in fresh and aged samples originate from the sulphur atoms in the thiophene ring of PEDOT and particularly attributed to sulphur atoms linked to carbon in the chemical structure of PEDOT,30,31,32 whereas the peaks with higher binding energies are attributed to sulphur atoms in the sulphonic acid PSS polymer. The PEDOT component is approximately half the intensity of PSS component. The relative intensities of these two components correspond well with their relative proportion in the PEDOT:PSS matrix. The relative intensity of the fresh sample is slightly higher than that of its aged counterpart exhibiting high concentration of S 2p in the sample. The chemical changes related to carbon were observed from C 1s spectra shown in Figure 6a (fresh) and 6b (aged). The C 1s core level spectra show a strong and defined peak near 284.8 eV in both samples which is attributed to (C-C) bonds. While, the shoulder peak at about 286 eV is corresponding to the (C-O) bond. The C 1s spectra for aged sample appear as typical of oxygen containing polymer where an additional shake-up peak from the oxygenated carbon is emerging at 298.11eV and corresponds to the (COO- or C (=O) O) bond. 17 The shake-up structure of the C 1s spectra in the aged sample indicates the break-up of aromatic rings and presence of a significant increase in the oxidised carbon in the polymer.16 This phenomenon might affect the OSCs performance leading to inability of the polymer to transfer the charge. XPS spectra for O 1s (Figure 6a) show the two components at 530.88 and 532.59 eV in fresh samples corresponding to C=O and C-O bonds, respectively. The spectra in Figure 6b show the same two components in aged samples at 530.69 and 19 ACS Paragon Plus Environment


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531.80 eV with C=O and C-O bonds, respectively, but with different intensities and a slight shift towards higher binding energies. The peaks are relatively stronger in fresh samples but they have diminished and broadened, suggesting that C=O and C-O oxidation of carbon atoms in the polymer chain has occurred due to presence of the C=O and C-O bonds in the side chains.16,33 The two components of the O 1s spectra at higher and lower binding energies are assigned to PEDOT and PSS moieties, respectively.34

Figure 6: XPS spectra of PEDOT:SS including S 2p, C 1s and O 1s core level spectra for (a) fresh and (b) 250 h-aged samples in ambient atmosphere.

We observed the presence of indium content in all samples of both HTL variants immediately after the spin coating, as discussed in several previous reports that ITO surface is etched during the spin coating of

acidic PEDOT:PSS.23,24,25 Relatively much higher


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concentration of indium in aged sample confirms that ambient atmosphere plays a significant role in the migration of indium into the HTL. In order to determine the chemical state of diffused indium and Sn into the polymer layer, we acquired the high resolution XPS spectra. Figure 7 shows the chemical states for indium and Sn as components of ITO. The In 3d spectra for indium confirm the doublet characteristic of In (3d) with the two components. The two peaks at the 444.90 and 445.98 eV correspond to In 3d5/2 in both samples whereas, the peaks at 452.57 eV in freshly prepared sample and 453.53 eV in aged sample are attributed to In 3d3/2, indicating In(III).35 The peak at 444.90 eV in freshly prepared sample is attributed to the characteristic line of In2O3 .25,36 After the ageing of samples, the peak positions of indium has been shifted (approx. 1 eV) towards higher binding energies. The peak position in the XPS spectrum of indium in the aged sample at 445.98 eV corresponds to the indium ions in a salt. The count rate of Sn in all samples is almost negligible due to the lower concentration in ITO compared to indium, no significance changes were detected in Sn concentration and spectral configuration, therefore core level spectra for fresh sample is only presentenced here in Figure 7a. The XPS spectra of Sn exhibit two components at 486.14 and 495.60 eV corresponding to Sn 3d5/2 and Sn 3d3/2, respectively.37 In2O3 readily reacts with acidic PEDOT:PSS and its dissolution is much faster than Sn.27 Water absorbed from the atmosphere is a key player in accelerating the migration of indium and Sn from the ITO surface and their concentration could be much higher depending on level of humidity and exposure time to the atmosphere.36 The implications can be in the form of its influence on charge transport through PEDOT:PSS layer. The salt ions act as a trap for the charges and reduce the charge transportation.25 They can even further migrate to active layer and affect the charge separation and transport in the polymer blend. The influence on charge transport must be investigated separately.



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Figure 7: XPS spectra of PEDOT:SS including core level spectra for In 3d, Sn 3d (a) fresh , In 3d and V 2p3 (b) aged samples in ambient atmosphere.

Moreover, there was no significance changes recorded in the core level spectra for hybrid layer. In addition, negligible level of vanadium (V 2p3) were found and hence count rate were very less in the freshly prepared sample, therefore chemical states for aged sample only has been shown in Figure 7b. It is important to mention that O 1s signals are very close to V 2p1/2 and influence the background underneath the V2p1/2 signal and hence the intensity of the measured signals for V 2p3. Several authors38,39,40 include O 1s signal along with V 2p3 signals to extract the Shirley background, however we did not include O 1s signals in our spectra. In our XPS investigations, the binding energy of V 2p3/2 and V 2p1/2 core level spectra is 517.01 and 523.70 eV, respectively, in good agreement with the


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previously reported XPS results for V2O5

38,40,41

and exhibiting V5+ oxidation state caused

by oxygen vacancy defects.39 It should be emphasized that the results from XPS are corresponding to the outer 5 to 10 nm of the HTLs; that is, the results do not represent the bulk material properties. It is well know that PEDOT:PSS reacts with water, probably attributed to its tendency to absorb water. But in our case, there were an insignificant change in the oxygen content during 250 h of aging. Instead, the major failure mechanism for the device degradation was indium migration into the HTLs. This might be due to the fact that PEDOT:PSS is relatively resilient toward molecular oxygen. PEDOT:PSS both loses and gain oxygen, and in initial hours of studies oxygen loss outweigh the oxygen gain which upon exposing to ambient for longer hours causes an increase in oxygen content.17 In addition, PCDTBT:PC71BM is protected by the PEDOT:PSS layer which acts as a poor barrier against the ITO diffusion due to its acidic nature and consequently active layer is also effected by indium diffusion causing severe stability issues for the OSCs.

3.4 Proposed mechanism of stability enhancement due to V2O5 based hybrid HTL The governing mechanism behind the successful suppression of indium diffusion due to addition of V2O5 nanoparticles in PEDOT:PSS aqueous matrix is predominately attributed to the formation of self-assembled thin compact layer of V2O5 nanoparticles at the bottom of HTL (on top of ITO) which can effectively act as a barrier to the indium diffusion caused by the acidic PEDOT:PSS. Moreover, it helps in blocking the electrons and avoids current leakage. Almost similar phenomenon has been observed by H.Q. Wang et al.28 in their recent article related to the use of V2O5 nanoparticles as an HTL. They reported that tiny V2O5 nanoparticles form a thin compact layer on top of ITO surface that present the etching



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of ITO substrate. Although, they used pure V2O5 based HTL without mixing with PEDOT:PSS and ethanol was used as a solvent for V2O5 dispersion, however, V2O5 being soluble in aqueous solvent can effectively disperse in PEDOT:PSS aqueous suspension and probably leads to the formation of similar compact thin layer of V2O5 nanoparticles during spin coating as reported by W.H. Wang et al.28,19 The XPS survey scans carried out at different stages of ageing in ambient air also support the proposed mechanism by which V2O5 nanoparticles stop the etching of ITO substrate. It can be observed from Table 1 that almost negligible concentration of “V” was found in the upper 5-10 nm of hybrid HTL and it reached to maximum of 0.8 % in 250 h which indicates that V2O5 nanoparticles were probably settled down at the bottom of the HTL. The findings of the current work suggest that fabrication atmosphere plays an important role in the device stability. Moreover, device ageing is another root cause of degradation. Although, in our investigation apparently, indium diffusion is the major cause of device degradation along with the atmospheric factors but it is difficult to hold this phenomenon responsible for overall deterioration of the solar cell performance. It is still unknown to us whether many factors contribute or one factor act as a bottleneck; most likely an interface phenomenon or it could be ascribed to the Al electrode. Previous studies confirmed that Al and active layer interface gets oxidized due to water diffusion through Al grains; this oxidation acts as a charge blocking layer which accounts significantly in total efficiency loss with time. 42 ,43 On the basis of our findings, we thus demonstrated that major failure mechanism is related to the indium diffusion. We tentatively ascribe this to some unfavourable properties of PEDOT:PSS that upon contact with ITO electrode causes its deterioration. The possible implication of this is to improve the PEDOT:PSS hazardous properties in the context of organic solar cells, probably with the doping of metal oxides.


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Conclusions In summary, we investigated the stability and degradation of PCDTBT:PC 71BM based OSCs fabricated with pristine PEDOT:PSS and its PEDOT:PSS+ V2O5 hybrid variant under varied atmospheric conditions. A 25% increase in stability of OSCs containing hybrid HTL signifies the effectiveness of V2O5 in PEDOT:PSS. XPS results suggested that the device instability was mainly due to the indium diffusion from the substrate into the subsequent layers rather than the effect from oxygen (O) content. Such diffusion occurred due to etching of ITO by acidic PEDOT:PSS. Addition of V2O5 nanoparticles in the PEDOT:PSS solution greatly improved the device stability and proved to be a good barrier to the ITO migration into the upper layers as compared to pristine PEDOT:PSS layer. Moreover, OSCs with longer life time can be fabricated in controlled fabrication and testing environment with proper encapsulation to mitigate atmospheric degradations. Our findings suggest a simple, solution processed and economical approach to improve the lifetime of the OSCs by improving the interfacial HTL with V2O5 nanoparticles.

Associated Content Supporting Information Further supporting information [XPS survey spectra of pristine PEDOT:PSS HTL and its hybrid variant consisting of V2O5 nanoparticles dispersed in PEDOT:PSS aqueous suspension after 5 and 25 h of ageing]. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgement This work is supported by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia, under grant No. (G-464-363-37). The authors, 25 ACS Paragon Plus Environment


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therefore, gratefully acknowledge the DSR technical and financial support.

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XPS Analysis of the Improved Operational Stability of Organic Solar

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