Poly(4-vinylpyridine): A New Interface Layer for Organic Solar Cells

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Poly(4-vinylpyridine), A New Interface Layer for Organic Solar Cells Anirudh Sharma, Renee Kroon, David Andrew Lewis, Gunther G Andersson, and Mats R. Andersson ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12687 • Publication Date (Web): 06 Mar 2017 Downloaded from http://pubs.acs.org on March 6, 2017

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ACS Applied Materials & Interfaces

Poly(4-vinylpyridine), A New Interface Layer for Organic Solar Cells Anirudh Sharma,ǂ Renee Kroon,ǁ David A. Lewis,¥ Gunther G. Andersson,¥ and Mats R. Anderssonǂ* ǂ

ǁ

Future Industries Institute, University of South Australia, Adelaide, SA 5095, Australia. Chemistry and Chemical Engineering, Chalmers University of Technology, SE-41296

Göteborg, Sweden. ¥

Flinders Centre for Nanoscale Science and Technology, Flinders University, Sturt Road,

Bedford Park, Adelaide, 5042, Australia. KEYWORDS: Pyridine, cathode interface layer, work function, inverted OSCs, morphology

ABSTRACT: Poly(4-vinylpyridine) (P4VP) was used as a cathode interface layer in inverted OSCs fabricated using poly[2,3-bis-(3-octyloxyphenyl)quinoxaline-5-8-diyl-alt-thiophene2,5-diyl] (TQ1) and PC71BM (phenyl C71 butyric acid methyl ester) as the donor and acceptor materials, respectively. We successfully demonstrate that the work function of underlying ITO electrode can be significantly reduced by ~ 0.7 eV, after modifying the surface with a thin film of P4VP. Photo conversion efficiency of 4.7 % was achieved from OSCs incorporating P4VP interface layer between the ITO and BHJ. Thin P4VP layer, when used to modify ZnO electron transport layer in inverted OSCs, reduced the ZnO work function from 3.7 eV to 3.4 eV, which resulted in a noteworthy increase in open circuit 1 ACS Paragon Plus Environment


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voltage from 840 mV to 890 mV. On simultaneous modification of ZnO with P4VP and optimization of the BHJ morphology by using solvent additive chloronapthalene (CN), photo-conversion efficiency of OSCs was significantly increased from 4.6 % to 6.3 %. The enhanced device parameters are also attributed to an energetically favorable material stratification, as a result of an enrichment of PC71BM towards the P4VP interface.

1. INTRODUCTION Organic solar cells (OSCs) offer an attractive alternative as a potential source of renewable energy to meet the ever increasing energy demand of the growing population. These cells are promising due to anticipated low costs of large scale fabrication and their ability to be light and flexible, making them compatible with a wide range of applications. Continued research on OSCs have led to significant improvement in device performance with power conversion efficiency (PCE) of 11.7 % being reported.1 Despite recent improvements in performance, device stability still remains a challenge for commercialization of this technology. The degradation of devices is caused largely by the interfacial instability, migration of electrode materials and the diffusion of water and oxygen into the device.2-3 In conventional OSCs, significant device degradation has been reported due to etching of ITO by acidic and hygroscopic PEDOT:PSS layer4-5 and diffusion of oxygen through the cathode interface.2 Inverted devices, in which the transparent electrode (ITO in most cases) acts as a cathode and high work function metal electrode behaves like an anode, prevents an ITO-PEDOT:PSS interface in the device stack and has also been shown to perform better than conventional devices.6 In inverted devices, n-type materials such as zinc oxide (ZnO),7-8 titanium oxide (TiO2)9 and other transition metal oxides10 are commonly used as electron transport layer. ZnO can conveniently be solution processed to form thin ZnO interlayers. However, to achieve improved crystallinity and charge mobility, metal oxide films require a post2 ACS Paragon Plus Environment

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deposition thermal treatment up to 300 ˚C,11-13 which makes this method much less feasible for large-scale manufacturing of OSCs. Though an alternative approach of depositing ZnO on flexible substrates using nanoparticle inks has been demonstrated,[15] such thin films composed of nanoparticles still need post deposition annealing,[16] not only to sinter the particles, but also promote adhesion to substrate and remove the ligands. In order to avoid such processing limitations of inorganic oxide materials and ease the process of device fabrication, various organic materials such as polyethylene oxide,14-15 ethoxylated

poly-

ethylenimine(PEIE), poly(3,3′-([(9′,9′-dioctyl-9H,9′H-[2,2′-bifluorene]-9,9-diyl)bis(4,1-phen ylene)]bis(oxy))bis(N,N- dimethylpropan-1-amine)) (PFPA-1)16 and poly [(9,9-bis(3´-( N , N - dimethylamino) propyl)-2,7-fluorene)- alt -2,7-(9,9–dioctylfluorene) (PFN) have been employed as cathode interfacial layer.17-19 Lately, poly(N-vinylpyrrolidone)20-22 and pyridine modified fullerene23 has been used as an interface layer and for work function modification of electrode in inverted OSCs, respectively. In this work, we demonstrate the use of poly(4-vinylpyridine) (P4VP) (Figure 1a) as a cathode interfacial layer in inverted organic solar cells. P4VP interface layer is fully room temperature processable, making P4VP highly compatible with devices fabricated on flexible substrates such as PET. Solubility of P4VP in IPA and insolubility in water is another advantage, as it also offers a perfect orthogonal system for sequential deposition of water based solar inks. This article provides a comprehensive understanding of the influence of P4VP interlayer on the work function of the cathode, as well as the performance of organic solar cells.

The changes induced in the open circuit voltage and the photocurrent are

attributed to the modification of the interface as well as the BHJ morphology, as a result of P4VP modification. PC71BM enrichment towards the P4VP interface resulting in an energetically favourable material stratification, is also discussed. 3 ACS Paragon Plus Environment


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Figure 1. (a) Schematic of an inverted organic solar cell depicting P4VP as an interface layer and its interaction with PC71BM (b) chemical structure of TQ1. 2. EXPERIMENTAL SECTION 2.1 Sample Preparation. Patterned ITO-coated glass substrates (10 Ω/□, Xin Yan Technology LTD) were cleaned in pyroneg (5 % solution) at 90 °C for 20 minutes. The samples were then rinsed in deionized (DI) water followed by successive sonication for 10 minutes each in DI water, acetone and isopropanol. Subsequently UV-ozone cleaning of the ITO substrates was applied for 20 minutes. 2.2 Device Fabrication. ZnO films were deposited from a Zinc oxide sol-gel prepared as reported elsewhere.24 Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), 99.9 % chemical purity, supplied by Sigma Aldrich) (1 g) and ethanolamine (NH2CH2CH2OH, Sigma Aldrich, 99.5 %, 0.28 g) was dissolved overnight in 2-methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.8 %, 10 mL). The prepared solution was then spin-coated on cleaned ITO substrates at 3000 rpm for 60 seconds followed by 10 minutes annealing in a preheated oven at 280 °C resulting in ZnO thickness of ~ 25- 30 nm. For device fabrication, all ZnO coated samples and clean ITO (where specified) substrates were transferred to the glove box prior to further fabrication of devices. P4VP interface layer 4 ACS Paragon Plus Environment

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was then deposited by spin-coating ~ 0.13 wt % of P4VP (Mw = 60,000) solution in isopropanol (IPA) (1 mg ml-1) at 5000 rpm on clean ITO or on ZnO coated ITO, as specified. Hetero-junction layer of TQ1 (synthesized in our lab, Mn = 63000 g mol-1, Mw = 154000 g mol-1, PDI 2.8) and PC71BM (Solenne) was spin coated from a 1:2.5 solution (25 mg ml-1) in oDCB. For tuning the morphology and to achieve fine grain structure, 2 vol % CN was added to the solvent. For measuring reference UP and XP spectra, pure TQ1 (7 mg ml-1) and PC71BM (18 mg ml-1) solutions in oDCB were spin-coated on P4VP modified ITO. A molybdenum oxide hole transport layer (12 nm) was then thermally evaporated on the BHJ layer (using a Covap system supplied by Angstrom Engineering) before evaporating metallic Ag electrode (80 nm) using a shadow mask, which defined the active area to be 0.1 cm-2. The devices were measured using an Oriel solar simulator fitted with a 150 W xenon lamp (supplied by Newport), filtered to give an output of 100 mW cm-2 at AM1.5. 2.3 Electron Spectroscopy. Electron spectroscopy measurements were performed in an ultrahigh vacuum chamber built by SPECS (Germany). UPS measurements were performed using a low intensity UV light (He I line) with an excitation energy of 21.21 eV. The base pressure of the UHV chamber is a few 10-10 mbar. The spectra of electrons emitted from the sample were recorded at a pass energy of 10 eV with a hemispherical analyzer, at which the analyzer has an energy resolution of 400 meV as evaluated from the Fermi edge of polycrystalline silver. The work function of the sample is determined as the difference between the excitation energy and the length of the spectrum. The latter is given as the difference of the high binding energy cut-off and the cut-off of the spectrum at the lowest binding energy, as described elsewhere.25 The apparatus was further equipped with a non-monochromatic X-ray source for Mg and Al Kα. All XPS measurements reported here were performed using a non-monochromatic Mg source operated at 200 W. The angle between the He*/UV light irradiation and analyzer and 5 ACS Paragon Plus Environment


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the X-ray irradiation and the analyzer are both 54°. All XP spectra were referenced to the C 1s peak which was set to 285 eV. For performing UPS and XPS studies on the BHJ-P4VP interface, BHJ layer was washed off by spin-coating oDCB at 2000 rpm. 2.4 Contact Angle Measurements. Static contact angle of de-iodized water against the surfaces was measured using a PAT-1 tensiometer supplied by Sinterface Technologies (Berlin, Germany). The reported contact angles are average values measured over a period of one minute keeping the volume of the droplet constant. 2.5 Thin Film Characterization. The topography of samples was examined using a multimode AFM (supplied by Bruker) in tapping mode using Si tips. P4VP film thickness was measured using a Variable Angle Spectroscopic Ellipsometer (VASE®) utilizing WVASE32® software (J.A. Woollam Co., Inc.). P4VP film thicknesses were also measured using Neutral Impact Collision Ion Scattering Spectroscopy (NICISS)4 and were found to be in agreement with that measured using ellipsometer, within experimental uncertainty.

3. RESULTS AND DISCUSSION Work function modification Work function of ITO was modified by spin-coating a thin layer of P4VP with a thickness of 5 ± 1 nm. The surface modification was first confirmed by x-ray photoelectron spectroscopy (XPS) and the changes induced in the work function were studied using Ultraviolet photoelectron spectroscopy (UPS). Figure 2a shows the comparison between the XP spectra acquired from clean ITO as well as from ITO and ZnO surfaces modified using P4VP. The presence of the N1s peak and a significant increase in the C1s peak in case of P4VP modified ITO and ZnO samples compared to that of clean ITO demonstrates the presence of P4VP on both ITO and ZnO surfaces, thus confirms successful surface modification with P4VP. Figure 6 ACS Paragon Plus Environment

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2b shows a vacuum level shift in the secondary electron (S.E.) cut-off of the UP spectra of ITO and ZnO, after P4VP modification. This vacuum level shift (Figure 2c) causes significant reduction of over 0.7 eV in work function of ITO from 4.6 eV to 3.9 eV and alters the ZnO work function from 3.7 eV to 3.4 eV. This is in agreement with a recent report of work function modification using other pyridine compounds.23 The origin of this reduction of work function is related to the presence of nitrogen with a free electron pair in the pyridine ring of P4VP. The work function of both ITO and ZnO modified by P4VP was found to only marginally increase to 4.15 eV and 3.6 eV, respectively after four weeks of ageing in ambient air atmosphere (Figure S1, supplementary information), indicating remarkably high stability of these P4VP modified electrodes.

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Figure 2. (a) XP survey spectra showing the presence of nitrogen and carbon on P4VP modified ITO and ZnO (b) UP spectra of ITO and ZnO with and without P4VP modification (c) schematic diagram showing the origin of work function change due to the presence of P4VP on the ITO surface. Contact Angle and Material Stratification For an inverted device structure as fabricated in this paper (Figure 1), high quality contact between the PC71BM from the BHJ layer to the cathode is desirable26 for efficient charge transport from the BHJ layer to the ITO electrode. Therefore we study the changes in the contact angle of the ITO and ZnO surface after P4VP modification and its influence on the adhesion of PC71BM on the substrate. The contact angle of UV cleaned ITO was found to be ~20˚ whereas the contact angle increased to ~ 45˚ (Table 1) when the ITO was modified with P4VP. This result demonstrates that, though the modification of ITO surface with P4VP alters the surface energy of the substrate, the resultant surface is still hydrophilic. Rinsing an ITO surface with oDCB resulted in a contact angle of 58˚, whereas coating PC71BM from oDCB and CHCl3 resulted in a contact angle of 52˚ and 93˚, respectively (Table 1). This shows that PC71BM film is only coated from a highly volatile CHCl3 solution. However, when ITO is first modified with P4VP and subsequently with PC71BM from oDCB, the resulting contact angle was 92˚, showing that P4VP modification of ITO facilitates adhesion of PC71BM to the substrate, resulting in an ITO surface that is coated with PC71BM. This is due to a charge transfer complexation which is known to occur between pyridine and fullerene.27-28 This demonstrates that from an oDCB solution, a PC71BM covered surface can be achieved on a P4VP modified ITO substrate as compared to that on clean ITO surface with no P4VP modification. It must also be noted that the film thickness of P4VP was found to be unchanged after spin-coating only oDCB, indicating that oDCB does not wash off the P4VP layer on the surface of the cathode. On the other hand, PC71BM was found to attach to the 8 ACS Paragon Plus Environment

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ZnO surface with and without the P4VP modification as seen from similar contact angles in both cases (Table 1). This is due to the possible interaction between the ZnO and the ester function in PC71BM. To further investigate material stratification in an actual device scenario, we utilize XPS and UPS to study the surface of TQ1:PC71BM BHJ films spin-coated on P4VP modified ITO and also the BHJ - P4VP interface, after washing off the BHJ layer. Firstly, XPS studies showed an enrichment of TQ1 on the surface of the BHJ film in agreement with previous reports (see supplementary information).29 Secondly, on washing the BHJ layer off the P4VP modified ITO, XPS confirms (a) the presence of a very thin BHJ layer remaining on the surface after washing and (b) the presence of P4VP interface layer between the ITO electrode and the BHJ layer (see supplementary info). Importantly, the presence of P4VP between the ITO and BHJ layer, after sequential deposition of BHJ followed by washing off the BHJ layer, shows good adhesion of P4VP to the ITO substrate and absence of any intermixing with the BHJ layer. However, deeper probing depths of XPS (5 - 8 nm) compared to that of UPS (2- 3 nm) limits obtaining reliable elemental compositions of a complex BHJ-P4VP interface with the presence of both PCBM and TQ1 in the remaining BHJ layer. Thus we employ UPS to further probe the material stratification. The spectroscopic features of an UP spectrum corresponds to emissions from specific orbitals in a molecule,16 thus the changes in the spectral features are directly related to the changes in the surface composition. Hence, a UP spectrum from a sample composed of more than one species is a linear combination of the spectra from each species.30 Figure 3 shows a UP spectra of BHJ layer spin-coated on P4VP modified ITO (identical to that in case of devices) before and after washing with oDCB. It must be pointed out that after washing the BHJ layer, signals corresponding to P4VP, In and Sn were found in XPS, whereas no P4VP contribution was found in the UPS spectrum (see supplementary 9 ACS Paragon Plus Environment


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information). This demonstrates that the residual BHJ layer left after washing is thin enough to see features of P4VP/ITO interface in XPS but thick enough to not probe the P4VP/ITO interface. It can be estimated that the remaining BHJ layer above the P4VP interface layer has a thickness of only about 2 – 3 nm. Interestingly, the spectrum of the washed BHJ layer had similar features to that of unwashed BHJ layer except that it had reduced intensity between 8 eV to 10 eV, which mainly corresponds to the contribution from TQ1 in UPS.30 In order to confirm the removal of TQ1 upon washing the BHJ layer, UP spectrum from a BHJ blend sample was modelled by subtracting the spectra of pure TQ1 and the resultant spectrum was found to be almost identical to that of measured spectrum from a washed sample.31-32 The fitting procedure can be described by (1)

,

where S indicates the measured UP spectra of the washed BHJ layer, the pristine BHJ layer and the TQ1 and a the weighting factors for the pristine BHJ and TQ1 UP spectra. The UP spectrum of the blend after washing has been modelled with 1.0 and 0.12. The same fitting cannot be achieved by subtracting the UP spectrum of PCBM. This proves that the P4VP-BHJ interface is enriched with PC71BM.


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Figure 3. UP spectra showing various spectroscopic features of pure TQ1, PC71BM and TQ1:PC71BM (1:2.5) BHJ film spin-coated on P4VP modified ITO. Table 1. Water contact angle measurements on ITO and ZnO surface before and after P4VP and PC71BM deposition.

Sample

Contact Angle (˚)

ITO

~20

ITO-P4VPa

~45

ITO-oDCBb

58

ITO-PC71BMb

52

ITO-PC71BMc

93

ITO-P4VPa-PC71BMb

92

ZnO

40

ZnO-PC71BMb

102

ZnO-P4VPa

55

ZnO-P4VPa-PC71BMb

98

a) P4PV was spin coated from IPA (1 mg ml-1) @ 5000 rpm for 60 sec. b) oDCB & PC71BM (from oDCB, 17.5 mg ml-1) was spin coated @ 500 rpm for 60 sec followed by 3000 rpm for 30 sec. c) PC71BM was spin coated from CHCl3 (17.5 mg ml-1) @ 2000 rpm for 60 sec.

Bulk-heterojunction Morphology Morphology of active layer plays an important role for OSCs and can influence charge separation and transport. Thus, atomic force microscopy (AFM) studies were performed to study the impact of P4VP modification of cathode on the BHJ morphology. Topographical images taken by atomic force microscopy (AFM) confirm a concomitant change33 in the 11 ACS Paragon Plus Environment


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surface morphology of the bulk hetero-junction (BHJ) layer as a result of changed surface energy after P4VP modification of both ITO and ZnO coated ITO surfaces. For TQ1:PC71BM films spin-coated on clean ITO, phase separated island shaped domains uniformly spread across the sample area with rms roughness of 0.6 nm were found (Figure 4).34 TQ1:PC71BM films spin coated on P4VP modified ITO shows a different lateral structure with cluster-like features periodically present on the surface (Figure 4b). Such features have previously been reported due to the aggregation of PC71BM35 as a result of increased phase separation between the donor and acceptor material. Since height images of AFM only measures the surface topography, phase images were also studied in order to chemically map the phase separated domain. However, for all samples, phase images were found to be similar to the height images (supplementary information), suggesting a ‘skin’ layer of material covering the surface.35

a

b

c

d


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e

f

g

h

Figure 4. AFM images (2 x 2 µm2) showing the surface topography of TQ1:PC71BM (1:2.5) films spin coated on (a) ITO (b) P4VP modified ITO (c) ZnO coated ITO (d) P4VP modified ZnO coated ITO, and films from TQ1:PC71BM blend with 2 % CN on (e) ITO (f) P4VP modified ITO (g) ZnO coated ITO (h) P4VP modified ZnO coated ITO.

The surface morphology of TQ1:PC71BM films spin-coated on ZnO coated ITO (Figure 4c) also displayed phase separated domains, however the domain size was found to increase with near spherical domains spread across the surface, when BHJ layer was spin-coated on P4VP modified ZnO (Figure 4d). Non-optimal morphology with larger domains results in decreased donor-acceptor interface, which is detrimental for efficient charge separation and extraction and leads to reduced photocurrent.29 To modify the morphology of the BHJ layer, 2 % chloronaphthalene (CN)29 was used as an additive in the TQ1:PC71BM solution, after which the resultant surface morphology of all samples spin-coated on various substrates displayed a feature less nanostructure. For BHJ layer spin-coated on both ZnO and P4VP modified ZnO, much smoother surface was observed at the analyzed scale (Figure 4g, h), indicating increased miscibility between 13 ACS Paragon Plus Environment


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TQ1:PC71BM without large phase separation, thus overcoming the influence of surface energy of substrate on the BHJ morphology.

Photovoltaic Devices Finally, the photovoltaic performance of TQ1:PC71BM (1:2.5) devices incorporating P4VP modified ITO and P4VP modified ZnO as well as reference solar cells with only ZnO electron transport layer were investigated with and without chloronapthalene (CN) additive in the active layer. Figure 5 shows the current density-voltage (J-V) curves corresponding to the best performing devices. Highest device performance was achieved when P4VP film thickness was 5 nm. For thicker P4VP films, the device performance was found to deteriorate (Figure S7, supporting information). The photovoltaic performance parameters are summarized in Table 2.

With no additive in the BHJ layer, a photo conversion efficiency

(PCE) of 2.5 % was achieved for devices incorporating P4VP modified ITO with a noteworthy Voc of 894 mV as compared to 4.3 % PCE and 840 mV of Voc for reference devices with inverted structure having only ZnO as an electron transport layer. It must be noted that although a record PCE of 6 - 7 %34,

36

has been reported for conventional

TQ1:PC71BM devices, reduced Voc and lower PCE has been found for similar devices in an inverted geometry.37 Thus, the differences between the Voc and PCE of control devices reported in this study compared to those reported earlier34 are attributed to a different choice of interface materials, device structure and batch difference of TQ1. Despite successful modification of ITO with P4VP which resulted in high Voc of over 890 mV, J-V curves of the ITO-P4VP devices showed a slight s-shape accompanied by a high series resistance (Rs) of ~ 36 Ωcm2, indicating poor charge separation and charge transport, thus limiting the PCE of the device. The reason for poor photovoltaic performance can be 14 ACS Paragon Plus Environment

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found in the AFM image of BHJ film spin coated on P4VP modified ITO (Figure 4b), showing a non-optimal morphology with large discrete domains across the surface.

Figure 5. Representative current density-voltage curves (a) light and (b) dark of devices fabricated with or without P4VP modification of the cathode. However, after incorporating 2 % CN in the BHJ layer, s-shape in the J-V curve was found to considerably reduce, accompanied by a reduction in Rs to 17 Ωcm2. Also, in agreement 15 ACS Paragon Plus Environment


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with previous reports,36 as a result of increased homogeneity and finer surface morphology of the TQ1:PC71BM layer (Figure 4f), an increase in Jsc and FF was observed resulting in power conversion efficiency of 4.7 %, which is comparable to that of a control device with ZnO interlayer (Table 2). This demonstrates that P4VP is a promising interface material for organic solar cells, which can be used to achieve low work function electrodes. Moreover, its room temperature processability and stability in ambient air atmosphere makes it compatible for applications in large scale roll-to roll fabrication of OSCs. P4VP was further explored to modify the ZnO layer in inverted OSCs. In disagreement with a recent report by Sylvianti et al. on P4VP interface layer,38 significant increase in Voc from 840 mV to 890 mV was observed in addition to an improvement in photocurrent after modifying the ZnO interface layer with P4VP. The increased Voc is attributed to (a) the reduced work function of ZnO with P4VP modification as seen from UPS (Figure 2) and (b) the enrichment of PC71BM at the P4VP-BHJ interface (as seen from contact angle and UPS measurements), which would result in selective charge extraction at the cathode, which is also known to increase the Voc.39 Also, further reduced workfunction of ZnO as a result of P4VP modification would increase the internal electric field in the device, leading to better charge extraction and could explain the improvement observed in Jsc. However, a notable reduction in FF from 68 % down to 46 % was observed after P4VP modification of ZnO (Table 2), resulting in a reduced PCE of 3.4% as compared to that of 4.3% for ITO-ZnO only devices. The reduction in the FF is reflective of the formation of even larger domains on the surface of the BHJ layer (Figure 4d), as a result of the altered surface energy of ZnO after P4VP modification. Thus, the drop in PCE is mainly attributed to the non-optimum surface morphology of BHJ. Dramatic enhancement in the photovoltaic performance of devices incorporating P4VP modified ZnO was observed on addition of 2 % CN additive in the BHJ layer, resulting in Jsc 16 ACS Paragon Plus Environment

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of 10.1 mA due to well intermixed polymer-fullerene network (Figure 4h),Voc of 887 mV, remarkable FF of 70 % and PCE of 6.3 % (Table 2). This is the best performance reported so far for a TQ1:PC71BM device fabricated with an inverted structure.36-37 We attribute the enhanced performance of these devices mainly to the (a) increased Voc as a result of P4VP modification of ZnO and (b) simultaneously achieving optimal BHJ morphology leading to enhanced Jsc and high fill factor. Figure 5b shows dark J-V curves of the best performing devices. The series resistance was extracted from the inverse slope of the I-V curve at 1 volts, whereas the currents in the low and reverse bias region is indicative of the leakage currents. As can be seen from Figure 5b, except the ITO-P4VP devices, the series resistance of devices incorporating ZnO is not significantly influenced by the BHJ morphology. In general, rectification of devices incorporating ZnO was found to be better than that of those with ITO modified using P4VP. P4VP modification of ZnO was found to reduce the leakage currents and resulted in improved rectification, which is ascribed to an enhanced hole blocking effect of ZnO after P4VP modification. Reduction of undesired charge flow at the cathode interface as a result of P4VP modification, could also explain the improved FF (Table 2).40 Upon successful demonstration of P4VP as a promising interface layer in TQ1:PC71BM devices, its applicability was also tested on other donor-acceptor system such as PTNT:PC71BM. Inverted devices fabricated with P4VP interface were found to give similar Voc as obtained using a ZnO electron transport layer and PCE obtained were comparable to that reported in the literature41 (see supplementary information). Table 2: Photovoltaic performance parameters of TQ1:PC71BM based OSCs incorporating P4VP modified ITO or ZnO. Mean values ± standard deviation from six devices.

Jsc

Voc

FF


Rs

PCE


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[mA cm-2]

[Volts]

[%]

[Ω cm2]

[%]

ITO-P4VP

6.4 ± 0.1

0.894± 0.009

44 ± 6

35.8

2.5 ± 0.1

2.6

ITO-P4VP (with CN)

9.2 ± 0.3

0.870± 0.011

58 ± 1

17.2

4.7 ± 0.1

4.8

ITO-ZnO

7.5 ± 0.2

0.840± 0.005

68 ± 2

1.2

4.3 ± 0.3

4.7

ITO-ZnO (with CN)

8.5 ± 0.3

0.868± 0.005

62 ± 5

1.1

4.6 ± 0.4

5.1

ITO-ZnO-P4VP

8.3 ± 0.4

0.890± 0.009

46 ± 2

2

3.4 ± 0.3

3.7

ITO-ZnO-P4VP (with CN)

10.1 ± 0.3

0.887± 0.004

70 ± 2

1.3

6.3 ± 0.2

6.5

Devices

[Max. Value]

4. CONCLUSIONS P4VP is demonstrated as a promising interface material which significantly reduces the work function of ITO from 4.6 eV down to 3.9 eV, whereas the work function of ZnO was reduced from 3.7 eV to 3.4 eV. The work function of modified electrodes proved remarkably stable in ambient air atmosphere with only ~ 0.2 eV increase in work function after 4 weeks of air exposure. While P4VP modification of both ITO and ZnO was found to significantly enhance the Voc, it also influenced the BHJ morphology due to altered surface energy of the ITO and ZnO. The presence of P4VP on the ITO surface was found to facilitate the attachment of PC71BM to the substrates, as confirmed by contact angle measurements. UPS measurements also show an enrichment of PC71BM from the BHJ towards the P4VP modified ITO electrode, resulting in an energetically favourable stratified structure of donor and acceptor materials. On simultaneous optimization of (a) interfacial energetics using P4VP modification and (b) BHJ morphology using solvent additive CN, PCE of 4.7 % was achieved for ITO-P4VP devices, whereas PCE was significantly increased from 4.6 % to a noteworthy 6.3 %, for ITO-ZnO devices when ZnO was modified using P4VP. This study presents P4VP as a 18 ACS Paragon Plus Environment

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promising interface material as a replacement of its inorganic and organic counterparts with added benefits of room temperature solution processability using less toxic solvent, making it compatible with large scale roll-to-roll fabrication.

ASSOCIATED CONTENT Supporting Information Work function stability plot, contact angle, elemental composition from XPS, UPS spectrum of P4VP modified ITO, PTNT:PC71BM device data, AFM phase images of BHJ on various substrates. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The authors would like to thank University of South Australia and the South Australian government (PRIF) for financial support. The facilities at Flinders University are supported by the Australian Nano Fabrication Facility (ANFF) and the Australian Microscopy and 19 ACS Paragon Plus Environment


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Microanalysis Research Facility (AMMRF), which are gratefully acknowledged. A. S. greatly acknowledge Simarpreet Kaur from FII, University of South Australia for assisting with thickness measurements on Ellipsometer.

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TOC Graphic


Poly(4-vinylpyridine): A New Interface Layer for Organic Solar Cells

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