Organic solar cells of enhanced efficiency and stability using zinc oxide:zinc tungstate nanocomposite as electron extraction layer

Organic Electronics 71 (2019) 227–237

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Organic solar cells of enhanced efficiency and stability using zinc oxide:zinc tungstate nanocomposite as electron extraction layer

T

Anastasia Soultatia,∗, Apostolis Verykiosa,b, Thanassis Speliotisa, Mihalis Fakisb, Ilias Sakellisa,c, Hajar Jaouanid,e, Dimitris Davazogloua, Panagiotis Argitisa, Maria Vasilopouloua a

Institute of Nanoscience and Nanotechnology, National Center for Scientific Research Demokritos, Agia Paraskevi, 15310, Athens, Greece Department of Physics, University of Patras, 26504, Patras, Greece c Solid State Physics Section, Physics Department, National and Kapodistrian University of Athens, Panepistimioupolis, 15784, Zografos, Athens, Greece d University Hassan II, Faculty of Science Ain Chock, Department of Renewable Energy & Dynamic Systems, Casablanca, Morocco e Higher School of Textile and Clothing Industries (ESITH), REMTEX Laboratory, Casablanca, Morocco b

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc tungstate Zinc oxide Sodium metatungstate Organic solar cells Electron extraction layer Efficiency Stability

In this work, the enhanced performance of inverted bulk heterojunction (BHJ) organic solar cells (OSCs) using zinc oxide (ZnO):polyoxometalate (POM), in particular sodium metatungstate (Na6H2W12O40), nanocomposite films as electron extraction layers (EELs) is demonstrated. The addition in the precursor solution of ZnO of sodium metatungstate results in the formation of ZnO:ZnWO4 nanocomposite as evidenced by X-ray diffraction, Fourier transform infrared and photoluminescence measurements. The formation of ZnO:ZnWO4 heterointerface reduces the work function of the nanocomposite material leading to a more favorable electron extraction/ transport at the organic blend/electron transport layer interface. Additionaly, the amount of zinc interstitial defects is suppressed having a profound positive effect on device stability. As a result, simultaneously improved open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) are obtained in the devices using the ZnO:ZnWO4 nanocomposites. Therefore, both of the inverted BHJ OSCs composed of either poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) or P3HT:indene-C60 bisadduct (IC60BA) photoactive blends show a significant performance enhancement when using the nanocomposite electron extraction layer, exhibiting a 27% and 23%, respectively, improvement in their power conversion efficiency (PCE) values compared to the reference devices based on pristine ZnO. In addition, the devices with the ZnO:ZnWO4 layer exhibit a remarkable stability enhancement retaining 95% of their initial PCE value upon storage for 500 h.

1. Introduction Our society is constructed around the central concept of energy, the demand of which is constantly increasing in order to sustain our way of living. Although fossil fuels were the primary source of energy, the researcher's interest has been focused on the renewable harmless energies, with the production of energy by harvesting the solar power to be the most challenging. In recent years, OSCs have attracted more attention due to their low-cost and requirements suitable for large-scale processing [1–5]. In addition, with the advent of non-fullerene acceptors OSCs power conversion efficiency (PCE) has reached a validated benchmark target of 15% in single-junction non-fullerene acceptors (NFAs)-based devices [6,7], and over 17% in double-junction tandem OSCs [8], but their successful commercialization depends on both high-

∗

efficiency and sufficient long-time stability. The inverted OSC architecture has been proposed as an effective strategy for the enhancement of device stability [9–11]. In this device architecture, the photoactive layer consisting of a polymer-donor and a fullerene (and more recently non-fullerene) acceptor is sandwiched between the transparent cathode (comprised of either indium tin oxide (ITO) or fluorinated tin oxide (FTO)) and the metallic anode (usually aluminum (Al) or silver (Ag)). Titanium dioxide (ΤiΟ2) or zinc oxide (ZnO) are commonly used as electron extraction layers (EELs) directly deposited on the bottom transparent electrode of the inverted architecture in order to promote electron selectivity and reduce the interfacial barriers for effective electron transport and extraction [12–20]. More specifically, ZnO is one of the most efficient and transparent metal oxide, used in many highly-efficient optoelectronic devices, such as

Corresponding author. E-mail address: [email protected] (A. Soultati).

https://doi.org/10.1016/j.orgel.2019.05.023 Received 5 November 2018; Received in revised form 10 May 2019; Accepted 11 May 2019 Available online 16 May 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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and a sodium metatungstate with the chemical structure Na6H2W12O40 (named hereafter as POM-W), as potential EEL in inverted OSCs. We make our selection of the metatungstate material to formulate the ZnWO4 component motivated by the efficient catalytic activities of 1:12 tungstates [54,55]. Both of the inverted bulk heterojunction (BHJ) OSCs composed of poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C71butyric acid methyl ester (PC71BM) and P3HT:indene-C60 bisadduct (IC60BA) show significantly enhanced performance by inserting the ZnO:ZnWO4 layer at the cathode side of the device to facilitate electron transport/extraction. The incorporation of the ZnO:ZnWO4 EEL in the device structure provides a large decrease in the work function as well as an effective passivation of surface defects (mostly Zni) of ZnO resulting in an increase of the open-circuit voltage (Voc) and an enhancement in temporal stability of the OSCs which maintain nearly 95% of their initial PCE value after 500 h storage in the dark. Also, both Jsc and FF of the ZnO:ZnWO4 modified devices are considerably improved, which is correlated to the more efficient electron transport and collection and the enhanced nanomorphology of the photoactive layer when coated on top of the composite layer.

solar cells, lasing and light emitting diodes (LEDs) [21–24]. Despite its high transparency in the visible range, environmental stability, and solution-based processing, there is a major concern with ZnO which lies in its surface defects such as oxygen vacancies (VO) and zinc interstitials (Zni) [25–28]. It has been established that, while VO may be responsible for the unintentional n-type conductivity of ZnO, Zni present active recombination centers for photoexcited electron-hole pairs resulting in the limitation of the energy conversion efficiency of the organic solar cells [29]. In addition, they also serve as oxygen chemisorption centers, thus negatively affecting the device stability [30]. Moreover, the relatively high work function (WF) of ZnO results in the formation of a significant electron extraction barrier at the photoactive blend/ZnO interface having a profound negative effect on the device performance. An excellent strategy to overcome those limitations is the surface modification by coating ZnO layer with self-assembled monolayers [31], or conjugated polyelectrolytes [32], which passivate surface defects and consequently improve the device performance. Recently, our group demonstrated that the insertion of ultra-thin atomic layer deposited (ALD) metal oxides such as hafnium, aluminum, and zirconium oxides, between of the ZnO electron transport layer and the photoactive blend film also passivates surface defects of ZnO, leading to improved electron collection at the cathode/photoactive layer interface [33,34]. The effectiveness of hydrogen-doping and fluorine (SF6) plasma surface treatment of ZnO on the surface trap passivation was also investigated in details [35,36]. Additionally, these surface modification strategies positively influence the WF and contact resistance while also improve the conductivity of cathode interfacial layer. However, these strategies could be challenging for large-area processing technologies such as rollto-roll and inkjet printing as they require additional steps after the deposition of ZnO. Therefore, in order to avoid the more complicated two-step film-formation, a single-step film-development by a simple solution-processing is required. Lampande et al. reported a solution processed n-type mixed electron transport layer based on ZnO:Cs2CO3 used in inverted polymer solar cells [37]. They demonstrated an enhanced performance of the ZnO:Cs2CO3-based devices due to easy control of the mixed oxide layer's morphology and the improvement of the interfacial contact between the electron transport layer and the photoactive film. Moreover, enhanced performance of inverted polymer solar cells using poly (ethylene oxide)-modified ZnO as electron transport/extraction layer was demonstrated by Shao et al. [38]. On the other hand, zinc tungstate (ZnWO4) has been widely used as effective photocatalyst and as anode material for lithium-ion batteries due to several advantages such as excellent chemical stability, nontoxicity and several unique optoelectronic characteristics [39–41]. In addition, the formation of ZnO/ZnWO4 heterointerfaces has been found to highly improve charge separation of the photogenerated electronhole pairs [42]. However, the application of such materials as charge transport layers in photovoltaic devices has never been reported. A few synthetic approaches have been proposed for the synthesis of ZnO/ ZnWO4 nanocomposites such as precipitation, mechanochemical reaction, sol-gel, and microemulsion [43–45]. One of the simplest approaches is the addition of an appropriate polyoxometalate (POM) compound such as phosphotungstic or silicotungstic acids into a Zn containing precursor solution [46]. In addition, ZnO/POM and TiO2/ POM composites have been used as photoanodes in dye-sensitized solar cells (DSSCs) [47–50], resulting in enhanced photovoltaic effect which may be attributed to the hindrance of charge recombination losses in the DSSCs and the increased efficiency of the exciton dissociation. Similarly, Dong et al. demonstrated an enhanced performance of perovskite solar cells (PSCs) using POM to modify the electron transport layer [51]. In particular, SiW12–TiO2-based PSC showed improved electron extraction efficiency and enhanced conductivity as compared with the pristine TiO2-based device due to high electron mobility of POM [52,53]. In this work, we investigate for the first time a single-step solutionprocessed ZnO:ZnWO4 nanocomposite layer, formed by blending ZnO

2. Experimental section 2.1. Preparation of ZnO and ZnO:ZnWO4 nanocomposite films Fluorine-doped tin oxide (FTO) coated on a glass substrate from Sigma Aldrich was cleaned with soap water, deionized water, acetone and isopropyl alcohol (IPA), each for 10 min, and finally dried by nitrogen flow. ZnO precursor solution 0.5 M was prepared by dissolving zinc acetate dihydrate purchased from Sigma Aldrich in 2-methoxyethanol and ethanolamine under vigorous stirring. ZnO:ZnWO4 nanocomposites were prepared by mixing appropriate amounts of ZnO solution with a sodium metatungstate (Na6H2W12O40) (purchased from Sigma Aldrich and used without further purification) solution with a concentration of 5% w/w in methanol under vigorous stirring for 10 min. The FTO substrates were spin-coated at 2000 rpm for 40 s with the resulting solutions and immediately placed on a hot plate at 80 °C. Then, the films were annealed at 250 °C for 20 min. 2.2. Device fabrication Inverted organic solar cells were fabricated on the 50 nm thick ZnO or ZnO:ZnWO4 layers spin-coated on FTO substrates. A solution containing a mixture of P3HT:PC71BM (5.5 mg/mL:4.5 mg/mL) in chloroform or P3HT:IC60BA (17 mg/mL:17 mg/mL) in chlorobenzene was spin-coated on top of the electron extraction layer to produce the photoactive layer. The ∼100 nm P3HT:PC71BM film was consequently annealed on a hot plate at 135 °C for 10 min, while the ∼80 nm P3HT:IC60BA films (which were prepared in nitrogen environment) were annealed at 150 °C for 10 min. Afterward, a 20 nm of sub-stoichiometric molybdenum oxide (MoOx) was deposited on top of the different photoactive layers to serve as the hole extraction layer (HEL) [56,57]. Finally, a 150 nm aluminium (Al) was thermally deposited on HEL through a shadow mask at a base pressure of 10−6 Torr. The photoactive area of the device was 12.56 mm2. 2.3. Thin film and device characterization The Fourier transform infrared (FTIR) transmittance spectra of the samples were recorded using a Bruker Tensor 27 FTIR spectrometer with a DTGS detector. Scanning electron microscopy (SEM) images of samples surfaces were taken using a JEOL field emission electron microscope. The UV–vis transmittance and absorption spectra of the samples were recorded using a PerkinElmer Lambda 40 UV–vis spectrometer. The thickness measurements were carried out using an Ambios XP-2 profilometer. The surface morphology of thin films was also analyzed using atomic force microscopy (AFM) measurements with 228

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3. Results and discussion

a NT-MDT Solver PRO scanning probe microscope operating in semicontact mode. XRD measurements of the samples were carried out with X-ray powder diffraction on a Siemens D-500 diffractometer with Cu radiation in Bragg-Brentano geometry. Photoluminescence measurements on ZnO and ZnO:ZnWO4 films were carried out using a Horiba Jobin-Yvon iHR320 Spectrometer with a He–Cd laser (325 nm) as excitation source. The PL dynamics of P3HT were studied under magic angle conditions, by using a Time-Correlated Single Photon Counting spectrometer (Fluotime, Picoquant) employing a pulsed diode laser emitting at 470 nm with 60 ps pulse duration as the excitation source (Picoquant). Detection of the fluorescence was realized by a microchannel plate photomultiplier at the peaks of the PL spectra and the Instrument's Response Function was ∼80 ps. Upon fitting the PL decays with a multiexponential function, the lifetimes τi and their normalized pre-exponential factors Ai were obtained. The PL average (mean) lifetime < τ > was calculated by means of < τ > =

∑i Ai τi ∑i Ai

Fig. 1a shows the FTIR spectra of pristine ZnO and POM-W films and of ZnO:ZnWO4 nanocomposite films. In the FTIR spectrum of ZnO the characteristic Zn–O band appears at 498 cm−1. In the spectrum of POM-W the absorption bands in the region of 700–1000 cm−1 are assigned to the W–O–W and W=O bonds of the Keggin structure Na6H2W12O40. In the case of ZnO:ZnWO4 nanocomposite film, the existence of typical peaks of ZnO and one characteristic broad peak at 870 cm−1 is evident. The latter can be assigned to ZnWO4 mixed oxide suggesting the formation of a composite ZnO:ZnWO4 film upon the addition of the sodium metatungstate into the ZnO precursor solution [58]. XRD measurements of the ZnO and ZnO:ZnWO4 films were also performed and are presented in Fig. 1b. It is demonstrated that the POM-W addition into the precursor solution of ZnO although did not affect the wurtzite structure of ZnO film (as evidenced by the characteristic peaks of ZnO diffractogram) has also promoted in part the formation of the ZnWO4 state [59]. XRD patterns show a broadening of the diffraction peaks due to POM-W addition indicating, according to Scherrer formula, a decrease in the crystal size of the ZnO:ZnWO4 sample in accordance with the literature [59]. In addition, pristine and composite films were investigated using scanning electron microscopy (SEM) to unravel their surface morphology; the corresponding topographic images are presented in Fig. 1c and d. It is observed that the ZnO particle size decreases upon the formation of ZnWO4 phase, in agreement with the decrease in the crystallite size indicated by XRD. ZnO is a hexagonal wurtzite structure whereas ZnWO4 exhibits a monoclinic wolframite structure, therefore ZnWO4 can be regarded as a distorted hexagonal structure after formation of the composite material, which can be assumed to result in a superior crystal surface and a smaller diameter [60]. The distortion of

. The contact po-

tential difference (CPD) measurements were performed with a singlepoint Kelvin probe system KP010. The current density–voltage (J–V) characteristics of the inverted OSCs were measured with a Keithley 2400 source-meter in the dark and under an illumination intensity of 100 mW cm−2 using a Xe lamp as the illumination source equipped with a AM 1.5G filter in ambient conditions. The external quantum efficiency (EQE) measurements were obtained using an Autolab PGSTAT-30 potensiostat. The wavelength was controlled with Oriel 1/8 monochromator and the source was a 300W Xe lamp. The capacitance–voltage (C–V) characteristics were measured using an Agilent 4284A LCR meter at a frequency of 10 KHz and a AC bias of 25 mV.

Fig. 1. (a) FTIR spectra of pristine ZnO, POM-W and ZnO:ZnWO4 substrates. (b) XRD patterns of ZnO and ZnO: ZnWO4 samples. SEM images of (c) ZnO and (d) ZnO:ZnWO4 films. 229

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and W-5d, and the position of the orbital levels between O-2p, W-5d, and Zn-3d is different from each other thus making the ZnO:ZnWO4 heterointerface a type II heterojunction (Fig. 3a). Presumably, the intimate contacts at ZnO:ZnWO4 interface offers an appropriate combination of the energy bands favoring a transfer vector of photo-induced charge carriers from one to another. Fig. 3b shows the schematic diagram of charge carrier transfer between ZnO and ZnWO4. When the ZnO:ZnWO4 nanocomposite is irradiated by UV light, photoinduced electrons (e-) could be transferred from the CB of ZnO to the lower lying bands of W-5d. On the other hand, holes (h+) could be transferred from the lower O-2p states in WO6 complex to the VB of ZnO. This accumulation of charge carriers at the heterojunction increases the probability of recombination luminescence of electron-hole (transition 2 in Fig. 3b) which can be responsible for the enhanced broad visible PL intensity of the composite material. The band-to-band emission of the ZnWO4 phase (transition 3) corresponds to higher energy compared to that of pristine ZnO (transition 1) which can explain the blue shift in the NBE emission of the nanocomposite. Note that the observed blue-shift in the NBE peak for the ZnO:ZnWO4 films could be also be attributed to lower concentration of Zni sites in the composite sample. Zinc interstitials are well-known localization centers for excitons in ZnO located few tens of meVs below the conduction band edge [62]. A large density of defect states derived from Zni may act as deactivation centers thus causing a pronounced red shift in the NBE peak positioning in the pristine material. By decreasing such defects upon the addition of POM, the NBE peak should exhibit a profound hypsochromic shift. In any case, this shift may be the result of one of the above reasons or, most probably, of a combination of both. Fig. 4a displays the contact potential difference (CPD) -measured with the Kelvin probe technique-of ZnO:ZnWO4 film compared to the pristine ZnO. A large reduction of the CPD of about 0.7 eV for the ZnO:ZnWO4 substrate is demonstrated, indicating a significant WF decrease in the composite film surface, which could be beneficial for electron extraction, when ZnO:ZnWO4 film is used as electron extraction layer in inverted organic solar cells. This WF reduction may be related to the stronger n-type character of ZnWO4 compared to ZnO as qualitatively indicated from the energy difference between the Fermi level and CB minima of those materials illustrated in Fig. 3b. In order to investigate the effect of inserting the composite layer on the device performance, inverted BHJ OSCs were fabricated. The device architecture comprised of a commonly used blend of P3HT and PC71BM based photoactive layer, a sub-stoichiometric MoOx film as hole extraction layer and either pristine ZnO or ZnO:ZnWO4 electron extraction layer is illustrated in Fig. 4b, while the energy level diagram based on the Kelvin probe measurements of the same devices is shown in Fig. 4c. From the energy level diagram, it is observed that upon the insertion of POM-W into the ZnO precursor solution, the WF value of the

the hexagonal structure of ZnO upon the reaction with sodium metatungstate for the formation of ZnWO4 can be explained as follows: as mentioned above, under ambient conditions the thermodynamically stable phase of ZnO is that of hexagonal (ZnO6) wurtzite symmetry. The POM anion can be described as an assembly of 12 edge and cornershared WO6 octahedra. The structure of the nanocomposite material formed upon the reaction of ZnO with POM is composed of zig-zag metal–oxygen chains made up of edge-sharing ZnO6 and WO6 octahedra. Each of the (ZnO6–ZnO6)n and (WO6–WO6)n chains is interlinked to four chains of the other type. All the metal–oxygen octahedra are distorted from perfect octahedral geometry leading to the distortion of the hexagonal wurtzite structure and the formation of the monoclinic wolframite structure. Atomic force microscopy (AFM) (Fig. S1, Supporting Information) reveals that the composite film presents a smoother surface with a root-mean-square (RMS) roughness of 2.63 nm compared to the pristine ZnO film (RMS = 2.94 nm). It is known that a smooth and uniform surface nanomorphology of the interfacial layers could be beneficial on device performance, since they improve electron extraction, form a better contact at the interface between them and the photoactive layer and decrease the leakage current of the device. Fig. 2a shows the Tauc-plots of both ZnO-based films as derived from UV–Vis absorption measurements (Fig. S2a). The energy bandgap (Eg) of each material was calculated by extrapolating the linear part of plots of (αhv)2 versus the energy of the exciting photons with an assumption that these samples are direct semiconductors. A small increase in the Eg derives for the composite material; this bandgap widening has been also previously reported upon the formation of the ZnWO4 phase [61]. No change in the transmittance spectrum of ZnO:ZnWO4 film is observed (Fig. S2b), while both films are more than 80% transparent in the visible range, and therefore they could be used as cathode interfacial layer in inverted OSCs where the sunlight passes into the device. Fig. 2b presents the steady-state photoluminescence (PL) of ZnO and ZnO:ZnWO4 films upon excitation at 325 nm. ZnO exhibits a strong UV emission peak located at 394 nm attributed to the near-band-edge (NBE) emission [62]. In the spectrum of ZnO:ZnWO4 sample, this peak is shifted to higher energies (centered at 382 nm). However, the most characteristic feature of the PL spectrum of the composite material is the strong visible broad emission band in the range of 440–750 nm. To explain the origin of the broad visible emission of the composite material, we should take into account the energy differences between the valence band (VB) and conduction band (CB) edges of both ZnO and ZnWO4. The O-2p and the W-5d orbital levels have dominantly contributed to VB and CB edges of ZnWO4 [63]. The position of Zn-4s orbitals, which define the CB minimum of ZnO, is higher than that of W-5d. The position of Zn-3d orbital levels is deeply below VB, and its energy bandwidth is assumed to be about 1.0 eV [63]. From the width of these orbital levels, Zn-3d is smaller than O-2p

Fig. 2. (a) Tauc plots as derived from absorption measurements taken in 50 nm thick films of ZnO and ZnO:ZnWO4 deposited on FTO substrates. (b) Steady-state photoluminescence measurements of thick ZnO and ZnO:ZnWO4 films (formed via drop casting) deposited on Si substrates. 230

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Fig. 3. (a) The schematic energy diagram and (b) the radiative transitions upon UV excitation of ZnO:ZnWO4 nanocomposite.

resultant ZnO:ZnWO4 composite film is significantly reduced to 3.4 eV. As a result, a large net interfacial dipole (Δ) with its positive pole pointing toward the photoactive layer and its negative pole toward the FTO is formed as illustrated in Fig. 5a. Consequently, an interfacial dipole-induced vacuum level (VL) shift occurs which brings the lowest unoccupied molecular orbital (LUMO) level of the acceptor closer to the cathode thus reducing the electron extraction barrier. Therefore, the WF reduction of ZnO:ZnWO4 is related to the enhancement of electron extraction. In addition, the direction of the dipole plays an additional role to the device performance by driving the electrons toward the cathode and simultaneously enhancing the built-in voltage of the OSC. Furthermore, the formation of type II heterojunction between ZnO and ZnWO4 in the ZnO:ZnWO4 nanocomposite material (Fig. 3a and b) may promote fast charge separation and extraction at the cathode interfaces thus benefiting the device performance. To support this argument, we next evaluated the performance characteristics of OSCs illustrated in Fig. 3b. The illuminated current density-voltage (J-V) characteristics under the AM 1.5G radiation light intensity of 100 mW cm−2 of P3HT:PC71BM based OSCs with pristine ZnO and ZnO:ZnWO4 EELs are shown in Fig. 5b. The corresponding electrical parameters of the inverted BHJ OSCs are listed in Table 1. It is

noted that, in order to investigate the dependence of the content of POM-W in the ZnO on the device performance, OSCs based on the different concentration of POM-W doping were also fabricated and tested (Fig. S3 and Table S1). The reference device using the ZnO as EEL shows a Jsc of 9.54 mA cm−2, a Voc of 0.59 V, a FF of 0.56, and a PCE of 3.15%. The device performance was significantly enhanced when ZnO:ZnWO4 was used as the electron extraction layer. The device based on the ZnO added POM-W (5% w/w) exhibits enhanced PCE as high as 4.02% with Jsc of 10.47 mA cm−2, Voc of 0.63 V and FF of 0.61 which represents a 28% enhancement compared to the reference cell. In order to find the reason for the significant improvement of the device performance, the series (Rs) and shunt (Rsh) resistance of the above devices were calculated from the J-V curves (also summarized in Table 1). It is well known that the Rs is attributed to the ohmic losses occurring in the entire solar cell, which mean the contact resistance at the interface, the bulk resistance of the different interfacial layers and also the photoactive blend film. Moreover, the Rsh is attributed to the charge carrier losses originated from the recombination of charges at the interface or in the bulk and the current leakage. The device using the ZnO:ZnWO4 electron extraction layer shows smaller Rs of 2.9 Ω cm2 and higher Rsh of 610 Ω cm2, compared to the reference device based on

Fig. 4. (a) Kelvin probe measurements of ZnO and ZnO:ZnWO4 surfaces. (b) Device structure of the inverted OSCs. (c) Energy level alignment at the various interfaces of the inverted OSCs. 231

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Fig. 5. (a) Schematic illustration of the dipole's formation at the cathode/fullerene acceptor interface. (b) Current density-voltage (J–V) characteristics of the ZnO and ZnO:ZnWO4 based devices under 1.5AM illumination. (c) J-V dark characteristics and (d) external quantum efficiency (EQE) of the same devices.

characteristics for polymer:fullerene organic solar cells have been reported [64,65]. The external quantum efficiency (EQE) spectra of the devices with ZnO and ZnO:ZnWO4 EELs are shown in Fig. 5d. A similar tendency as that of PCE of the inverted OSCs using the pristine ZnO and ZnO:ZnWO4 is observed, confirming the improved charge generation efficiency in the ZnO:ZnWO4 based device. Particularly, the device with the composite EEL exhibits higher value of EQE (maximum EQE value ≈ 79%) in comparison with the reference device with EQEmax ≈ 69%. It is also clearly observed that both the inverted OSCs show a remarkable spectral response in the range from 300 nm to 750 nm, resembling the UV–Vis absorption spectra of the P3HT:PC71BM blend film (Fig. S5) spin-coated on top of the different EELs. From the above results, it is suggested that the improvement of PCE in the device with the composite EEL may be originated by the reduced series resistance, the enhanced charge generation efficiency and/or the reduced recombination losses. Fig. 6 presents the statistical analysis of OSCs performance of a batch of 8 individual devices using the pristine and ZnO:ZnWO4 as EEL. The statistical data for Jsc, Voc, FF and PCE values measured under 1.5AM illumination along with the standard box plots, are shown in Fig. 6a–d, respectively. It is clearly seen that the average values of Jsc, Voc and FF of the ZnO:ZnWO4 based devices is higher than that of the device using the pristine ZnO substrate, leading to higher PCE values. Most importantly, the high reproducibility of device performance when using the composite layer is demonstrated, showing the influence of the ZnO doping on the efficiency of the organic solar cells. In order to support our suggestion of increased charge generation efficiency, the photocurrent density (Jph, defined as JL-JD where JL and JD is the current densities under illumination and in the dark, respectively) of the devices with different EELs were plotted (shown in

Table 1 Device characteristics of organic solar cells having the device structure glass/ FTO/ZnO or ZnO:ZnWO4/P3HT:PC71BM/MoOx/Al (mean values and standard deviations were extracted from a batch of 8 independent devices). EEL

Jsc (mA cm−2)

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

ZnO ZnO:POM-W

9.54 10.47

0.59 0.63

0.56 0.61

3.15 4.02

3.8 2.9

430 610

the pristine ZnO with Rs = 3.8 Ω cm2 and Rsh = 430 Ω cm2. Accordingly, the reduced Rs of the ZnO:ZnWO4 based OSC indicates a more efficient electron collection resulting in the improvement of Jsc and FF, while the increase of Rsh attributed to the reduced charge carrier recombination and leakage current could explain the enhancement of Voc. The reduced leakage current is also observed in Fig. 5c where the dark J-V characteristics on logarithmic scale of the reference device and the device with ZnO: ZnWO4 are shown. The leakage current of the device with the composite layer is lower than that of the reference cell, indicating that the charge recombination is suppressed inside the ZnO:ZnWO4 based device. In addition, from the slope of J-V curves at intermediate voltages, the dark ideality factor (n) of the devices was determined. As can be observed in Fig. S4, where the differentiated characteristic curves according to the equation n=(kT/q dlnJ/dV)−1 (where k is Boltzmann's constant, T the temperature and q the elementary charge) are plotted as a function of voltage, the ideality factor of the ZnO:ZnWO4 based device shows a lower value of 1.74, than that of the reference cell (n = 1.97), which can be attributed to a slower recombination rate at the cathode/photoactive layer interface. Note that ideality factors of typically 1.3–2.0 determined from dark J-V

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Fig. 6. Statistical data on electrical parameters of BHJ organic solar cells with different EELs: (a) Jsc, b) Voc, c) FF, and d) PCE. The data were recorded from a batch of 8 individual devices using the pristine ZnO and ZnO:ZnWO4 substrates as EELs.

a frequency of 10 kHz of the devices with different cathode interfacial layers are plotted and presented in Fig. 7d. The device built-in voltage (Vbi) can be determined by intersecting the 1/C2 curve with the horizontal voltage axis. It is clearly shown that the Vbi increases in the case of the ZnO:ZnWO4 based device, resembling the increase of the opencircuit voltage of the same device as derived from the J-V measurements (Table 1). This improvement suggests that efficient charge separation and extraction occurs using the composite film as cathode interfacial layer in inverted BHJ OSCs. Next, the nanomorphology of the P3HT:PC71BM blend film spincoated on top of pristine ZnO and ZnO:ZnWO4 substrates was investigated, since the morphology of the photoactive layer affects the Jsc and FF, and thus the performance of the OSCs. Fig. 8a and b demonstrate the AFM two-dimensional (2D) 5 × 5 μm2 topographies of the ZnO/P3HT:PC71BM and ZnO:ZnWO4/P3HT:PC71BM surface, respectively. AFM images reveal a small increase of the phase separation when the BHJ blend deposited on top of the POM-doped ZnO substrate, indicating the formation of a better interfacial contact between the photoactive film and electron extraction layer, which is confirmed by the SEM and AFM images of the different cathode interfacial substrates shown in Fig. 1c, d, and Figure S1, respectively. Such uniform and smooth surface morphology of the EELs facilitate the physical contact at the ZnO:ZnWO4/photoactive layer interface, leading to reduced series resistance and low leakage current of the OSCs. The improved nanomorphology of the photoactive blend film also confirmed by the crystallinity investigation of the P3HT:PC71BM blend film deposited on the pristine ZnO and ZnO:ZnWO4 substrates, using XRD measurements (Fig. S6). It is observed that the intensity of the characteristic diffraction peak of P3HT at the 2θ = 5.4° is similar in both films, while in the case of P3HT:PC71BM blend film deposited on ZnO:ZnWO4 substrate, the appearance of two other diffraction peaks at the 2θ = 10.6° and 2θ = 15.9° is clearly seen, indicating the enhanced face-on stacking of P3HT when deposited on top of the composite film. This enhancement

Fig. 7a) as a function of the effective voltage (Veff, defined as Vo-V where Vo is the voltage at which the Jph is zero) [66,67]. The calculated charge dissociation probabilities (P (E,T)) [68,69] shown in Fig. 7b for the devices using either the pristine ZnO or ZnO:ZnWO4 EELs are 89.8% and 94.4%, respectively, which is another factor that contributes to the enhanced EQE value of the composite layer based device. It is known that the higher dissociation probability is related to the weaker geminate recombination at the polymer:fullerene interface in BHJ blend film. In addition, at a large reverse voltage (Veff > 1), the saturated photocurrent density of the OSC with the composite EEL is slightly higher than that of the device using the pristine ZnO, suggesting that more photogenerated excitons are dissociated into free carriers in the case of ZnO:ZnWO4 based device. Therefore, the maximum exciton generation rate (Gmax, estimated by the Jph,sat = qGmaxL where L is the thickness of the photoactive layer) is slightly higher when using the ZnO:ZnWO4 EEL compared to the reference device. The enhancement of the photocurrent density of the composite EEL based OSC, attributed to both increased exciton dissociation rate and charge dissociation probability may also depend on the selectivity of the cathode interface. Accordingly, electron-only devices with the structure FTO/ZnO or ZnO:ZnWO4/P3HT:PC71BM/Al were fabricated. Fig. 7c displays the J-V characteristics in a semi-logarithmic scale measured in dark conditions. A significant increase in the electron current density is observed for the device with ZnO:ZnWO4 layer, which can be attributed to the improved electron transport due to the reduced WF of the composite film and the decreased electron extraction barrier. We can, therefore, conclude that the ZnO:ZnWO4 layer facilitates exciton dissociation and improves electron extraction at the cathode/photoactive film interface. The effective exciton dissociation and the enhanced electron transport were further confirmed by capacitance measurements taken in the BHJ organic solar cells. Using the Mott-Schottky equation (1/C2 = 2/qεN (Vbi – V) where ε is the permittivity of the photoactive material and N the doping level), the corresponding 1/C2 versus V characteristics taken at 233

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Fig. 7. (a) Photocurrent density and (b) exciton dissociation probability versus the effective voltage of ZnO and ZnO:ZnWO4 based OSCs. (c) J-V characteristics measured in dark of electron-only devices with the structure glass/FTO/ZnO or ZnO:ZnWO4/P3HT:PC71BM/Al. (d) Mott-Schottky plot derived from capacitance versus voltage (C–V) measurements of the same devices.

on the photoactive blend film orientation could be beneficial to charge transport, and thus the device performance. To further investigate the surface properties of the cathode interfacial layers, their wetting characteristics were examined using contact angle measurements. Figs. S7a and b show the contact angle between a drop of deionized water and/or diiodomethane and the different EELs. The corresponding surface tension values of the same films are summarized in Table S2. The ZnO:ZnWO4 surface seems to be more hydrophilic with a water contact angle of 63.5°, while the hydrophobic nature of ZnO film is evident by the 77.5° angle recorded between its surface and a water drop. The improved hydrophilicity of the composite ZnO substrate could promote the enrichment with fullerene (which is more hydrophilic than P3HT [70]) of BHJ photoactive film near the electron extraction contact. Accordingly, a large density of PC71BM clusters could appear near the enhanced hydrophilic ZnO:POM-W substrate, leading to the formation of larger phase separation, and thus efficient exciton dissociation. Furthermore, the adhesion of P3HT:PC71BM solution to EELs was estimated, as shown in Fig. S8. It is observed that the contact angle between a drop of P3HT:PC71BM and the EELs is decreased from 15.7° in the case of pristine ZnO film to 12.6° for the ZnO:ZnWO4 substrate, facilitating an enhanced coverage of blend film on the composite ZnO substrate. Additionally, a small increase in the exciton lifetime of P3HT 20 nm thick films when deposited on ZnO:ZnWO4 was estimated compared to that deposited on pristine ZnO, as observed in TRPL dynamics taken upon excitation at 470 nm, which are shown Fig. 8c and d for detection at 660 and 720 nm, respectively. The corresponding fitting parameters are summarized in Tables S3 and S4. As the P3HT films deposited on the two substrates are processed under identical conditions, the slightly larger exciton lifetime of the organic film on the composite film could be related to a reduction

of the interface trap state density that generally acts as effective recombination center for photogenerated electrons and holes. However, except of the high device efficiency, the temporal stability plays a vital role for the future commercialization of the OSCs. Therefore, the influence of the ZnO:ZnWO4 on the device stability upon storage was also investigated. Fig. 9a–d represent the variation of normalized PCE, Jsc, Voc, and FF over a period of 500 h for pristine ZnO and ZnO:ZnWO4 based OSCs. It is noted that the devices were intentionally unencapsulated and stored inside a nitrogen-filled desiccator between measurements, while the electrical measurements were performed in dry air (relative humidity ∼ 20%) at room temperature. It is observed that the ZnO:ZnWO4 based OSC preserves over 95% of the initial PCE value after 500 h, while the reference device with the ZnO EEL seems to strongly degrade with its PCE to be decreased to 60% of its initial value after 500 h. This high stability of the ZnO:ZnWO4 based device can be assigned to the restriction of the adsorption of oxygen molecules into the surface of ZnO upon the addition of POM-W to form the composite state. The high stability improvement of the composite film-modified device can be attributed to the reduction of Zni defects of ZnO upon the formation of the composite phase as evidenced by the PL spectra discussed above. It has been established that oxygen chemisorption occurs readily on to interstitial zinc sites present on the oxide's surface [30]. Upon decreasing the concentration of Zni defects, thereby passivating them, we prohibit the adsorption of corrosive agents thus boosting the device stability. To verify our approach that the ZnO:ZnWO4 is an efficient but also universal EEL, OSCs based on another well-studied blend photoactive system (P3HT: indene-C60 bisadduct (IC60BA)) were fabricated. Fig. 10a shows the illuminated J-V curves of the devices with the structure FTO/ZnO or ZnO:ZnWO4/P3HT:IC60BA/MoOx/Al, while the 234

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Fig. 8. 2-Dimensional AFM 5 × 5 μm2 topographies of P3HT:PC71BM blend film spin-coated on (a) ZnO and (b) ZnO:ZnWO4 substrates. Transient PL spectra detected at (c) 660 nm and (d) 720 nm of P3HT films deposited on pristine and composite ZnO layers.

photoactive blend layer.

device structure is shown as an inset. A similar trend in the performance of the P3HT:IC60BA based devices is observed compared to the previous study with the P3HT:PC71BM blend photoactive layer. The corresponding device parameters are presented in Table S5. The device with the ZnO:ZnWO4 EEL exhibits higher PCE value of 5.42% with Jsc = 10.29 mA cm-2, Voc = 0.85 V and FF = 0.62, than that of the reference device (PCE = 4.39%, Jsc = 9.41 mA cm−2, Voc = 0.79 V and FF = 0.59). Note that despite the significant efficiency enhancement in our OSCs upon using the composite EEL the absolute PCE values are lower compared to the state-of-the-art efficiencies for the same photoactive blend [33], probably because we processed our photoactive blends from solutions in chlorobenzene (instead of the more appropriate 1,2 dichlorobenzene). In addition, from EQE spectra of the same devices shown in Fig. 10b, the enhancement of Jsc becomes evident when using the POM-doped ZnO layer. The reduced leakage current and the enhanced charge generation efficiency are also demonstrated (the J-V curves measured in dark and the photocurrent density versus the effective voltage of P3HT:IC60BA based OSCs with ZnO and ZnO:ZnWO4 as EELs are shown in Figs. S9a and b, respectively). Moreover, the 1/C2 – V plots shown in Fig. S9c of the same devices reveal the enhanced exciton dissociation and electron collection efficiency attributing to the increased Vbi for the device with the ZnO:ZnWO4 substrate, which is further supported by the increase in current density obtained in electron-only devices with the structure FTO/ZnO or ZnO:ZnWO4/P3HT:IC60BA/Al (Fig. S9d). The above results indicate the successful use of the composite ZnO:ZnWO4 film as a universal electron extraction layer in inverted organic solar cells, resulting in high power conversion efficiency value independently the

4. Conclusions In conclusion, the influence of using a ZnO:ZnWO4 composite film, formed upon the addition of a POM derivative, in particular sodium metatungstate, into the precursor ZnO solution, as an electron extraction layer on the performance of inverted organic solar cells is investigated. The reduced recombination loss of carrier at the ZnO:ZnWO4/photoactive layer interface and the increased electron collection attributed to the reduced WF of the composite ZnO film, leading to improved device electrical parameters, are demonstrated. In addition, the enhanced nanomorphology of the photoactive layer deposited on top of ZnO:ZnWO4 substrate also improves the Jsc and FF of the device. As a result, the PCEs are significantly increased reaching the value of 4.02% and 5.42% for the P3HT:PC71BM and P3HT:IC60BA based OSCs, respectively. The devices using the ZnO:ZnWO4 layer also exhibit an outstanding long-term stability in air maintaining about 95% of their PCE initial value after 500 h in storage, which is attributed to the reduction of Zni defect density in the ZnO:ZnWO4 film.

Acknowledgements ΙΚΥ Scholarship Programs, Strengthening Post-Doctoral Research Human Resources Development Program, Education and Lifelong Learning, co-financed by the European Social Fund – ESF and the Greek government is acknowledged. 235

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Fig. 9. Stability measurements in ambient air; variation (as derived of 4 individual devices of both kinds) of normalized (a) PCE, (b) Jsc, (c) Voc and (d) FF over a period of 500 h for ZnO and ZnO:ZnWO4 based OSCs.

Fig. 10. J–V characteristics of the FTO/ZnO or ZnO:ZnWO4/P3HT:IC60BA/MoOx/Al OSCs under 1.5AM illumination and (b) external quantum efficiency (EQE) of the same devices.

Appendix A. Supplementary data

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