Surface Modification of ZnO Layers via Hydrogen Plasma Treatment

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Surface Modification of ZnO Layers via Hydrogen Plasma Treatment for Efficient Inverted Polymer Solar Cells Vasilis Papamakarios, Ermioni Polydorou, Anastasia Soultati, Nikos Droseros, Dimitris Tsikritzis, Antonios M. Douvas, Leonidas Palilis, Mihalis Fakis, Stella Kennou, Panagiotis Argitis, and Maria Vasilopoulou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09533 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 24, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33

ACS Applied Materials & Interfaces

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

Solar Cells

Vasilis Papamakarios,1 Ermioni Polydorou,1,2 Anastasia Soultati,1 Nikos Droseros,2 Dimitris Tsikritzis,3 Antonios M. Douvas,1 Leonidas Palilis,2 Mihalis Fakis,2 Stella Kennou,3 Panagiotis Argitis,1 and Maria Vasilopoulou1*

1

Institute of Nanoscience and Nanotechnology (INN), National Center for Scientific Research Demokritos, 15310 Aghia Paraskevi Attikis, Athens, Greece, 2

Department of Physics, University of Patras, 26500 Patras, Greece

3

Department of Chemical Engineering, University of Patras, 26500 Patras, Greece E-mail: * [email protected]

Abstract Modifications of the ZnO electron extraction layer with low-pressure hydrogen plasma treatment increased the efficiency of inverted polymer solar cells (PSCs) based on four different photoactive blends, namely poly(3-hexylthiophene):[6,6]-phenyl C71 butyric acid methyl ester (P3HT:PC71BM), P3HT:1′,1′′,4′,4′′tetrahydro-di[1,4]methanonaphthaleno-[5,6]ullerene-C60

(P3HT:IC60BA),

poly[(9-(1-octylnonyl)-9H-

carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl]:PC71BM (PCDTBT:PC71BM),

and

(poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-

fluoro-2-(2-ethylhexy)carbonyl]thieno[3,4-b]thiophenediyl]]):PC71BM (PTB7:PC71BM) irrespective of the donor:acceptor combination in the photoactive blend. The drastic improvement in device efficiency is dominantly attributable to the reduction in the work function of ZnO followed by a decreased energy barrier for electron extraction from fullerene acceptor. In addition, recombination losses and improved nanomorphology of the photoactive blend in the devices with the H plasma treated ZnO layer were observed while exciton dissociation also improved with hydrogen treatment. As a result, the inverted PSC consisting of the P3HT:PC71BM blend exhibited a high power conversion efficiency (PCE) of 4.4%, the one consisting

1 ACS Paragon Plus Environment


Page 2 of 33

of the P3HT:IC60BA blend exhibited a PCE of 6.6% whereas our champion devices with the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

PCDTBT:PC71BM and PTB7:PC71BM blends reached high PCEs of 7.4% and 8.0%, respectively.

Keywords: Polymer solar cells, hydrogen plasma, ZnO, reduced work function, electron extraction, charge generation efficiency.

1. Introduction The ever-increasing energy demands of the last decades together with the raise of public environmental awareness have led to the search for environmental friendly energy sources. Solar cells have become an extremely popular option for energy production since they take advantage of the planet’s most plentiful and widely distributed renewable energy source – the sun – while they are independent from large infrastructure. In the last decade, polymer solar cells (PSCs) have emerged as an alternative technology to their inorganic counterparts and have attracted a lot of research interest as they can be processed at low cost on large areas, have light weight and conform to flexible substrates.1-8 The best PSCs reported to date are based on a photoactive layer composed of a blend of an electron donating conjugated polymer and an electron accepting fullerene derivative, which form a bulk heterojunction (BHJ) interpenetrating network. The conventional device geometry of PSCs comprises a bottom indium tin oxide (ITO) anode, a poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole extraction interlayer, a BHJ active layer and a low work-function metal cathode. Although high power conversion efficiencies have been observed in these conventional structures, the oxidation of the air-sensitive, low work function metal cathode (such as Ca) and etching of ITO by the acidic PEDOT:PSS hole-transporting layer render these structures unstable.911

In addition, several other degradation mechanisms may play a role in device lifetime, such as

morphological degradation, UV-induced aging and oxygen exposure.12-15 One way to overcome the instability and improve the device lifetime is to use an inverted device architecture.16 Moreover, in order to render mass production and practical applications of PSCs feasible, apart from increasing stability, module efficiencies higher than 10% are required. One of the limiting factors in achieving these performance goals is the energetic losses caused by the recombination of the electron-hole pairs (excitons) at the interfaces.17 2 ACS Paragon Plus Environment

Page 3 of 33


To suppress the energetic losses and reach their full potential, various anode and cathode interlayers have 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

been used for efficient transport of charge carriers after the dissociation of the exciton that is generated in the active layer, acting thus as hole -or electron - selective layers, respectively. As a result, power conversion efficiencies (PCEs) improved from 2% to more than 11% in about a decade.18 Among various materials available for interfacial layers, transition metal oxides (TMOs) have great potential due to their wide range of energy level aligning capabilities, better environmental stability, higher optical transparency and easier synthesis routes than alkali metal compounds,19 aqueous conducting polymers20,21 and conjugated polyelectrolytes.22,23 Zinc oxide (ZnO) in particular, has been widely used as an electronselective material to modify the bottom cathode contact in PSCs with an inverted architecture due to its relatively low work function, which allows an Ohmic contact to be formed with fullerene derivatives used in the photoactive blends, its optical transparency, n-type conductivity, and solution processability.24-26 However, the effectiveness of ZnO layers in PSCs was found to be strongly dependent on their surface properties and on the trap sites and defects present there.27,28 Therefore, modification of surface properties of ZnO layers has been raised as major issue for the optimization of inverted PSCs. An effective approach to solve these problems is to modify the ZnO surface with a self-assembled monolayer (SAM) of organic molecules like fullerene derivatives.29-34 Using polyelectrolytes to modify ZnO is an alternative approach to renovate its surface. Heeger's group demonstrated that the power conversion efficiency (PCE) was increased over 25% for small molecule organic solar cells by incorporating polyethylenimine ethoxylated (PEIE) atop of ZnO.35 In other work, exposure of the devices to ultraviolet (UV) light, the so-called light-soaking approach,36-38 where trap-filling in the ZnO layer by UV light generated charges leading to an increased conductivity of this layer, is proposed. These fascinating results show a bright future of modifying ZnO surface with suitable methods to achieve electron extraction interlayers for highly efficient inverted PSCs. In a recent work by Gao et al. the effect of surface modification of the ZnO layer by high pressure H plasma treatment on the device performance was investigated.39 It was found that the open circuit voltage (Voc) of the solar cells was dramatically increased -the increased Voc being attributed to the reduced work function of ZnO caused by H-plasma treatment- while the short-circuit current density (Jsc) exhibited a consistent decrease with increasing the duration of H-plasma treatment which may be due to sputtering caused by high 3 ACS Paragon Plus Environment


Page 4 of 33

pressure H plasma. In this work, we demonstrate the simultaneous improvement of Voc, Jsc and fill factor 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(FF) of the devices caused by the enhanced charge extraction/transport properties and charge generation quantum yield and the reduced recombination losses achieved in the devices having the active blends deposited on ZnO layers treated with a low-pressure H plasma. By using several experimental techniques we elucidate that application of low-pressure H plasma as ZnO surface modifier creates an effective cathode interlayer which: (i) exhibits reduced work function when compared to the as-deposited ZnO, which is beneficial for device operation, (ii) decreases recombination losses due to hydrogen assisted passivation of defect states present at its surface. In addition, low-pressure H plasma treatment of the ZnO underlayer induces (iii) better nanomorphology of the photoactive blend, leading to enhanced charge generation efficiency and (iv) enhanced exciton dissociation at metal oxide/photoactive blend interface. Upon inserting the low-pressure H plasma treatment of the ZnO layer, PSC with long term stability and high PCES were fabricated. Hence, the surface modification of n-type metal oxides with low-pressure H plasma opens up alternative methods for the interface design of polymer solar cells.

2. Materials and Methods Zinc Oxide Layer Preparation. Solution-processed (sol-gel derived) ZnO films with a thickness of 40 nm were prepared using zinc acetate in 2-methoxyethanol:2-amino-ethanol as a precursor solution with a concentration of 0.50 M by spin-coating at 4000 rpm followed by thermal treatment at 200 °C for 60 min. Zinc acetate was purchased from Sigma–Aldrich and used without further purification. Device Fabrication. PSC devices were fabricated on fluorinated tin oxide (FTO) coated glass substrates which were purchased from Sigma-Aldrich and served as the cathode electrode. The FTO exhibited a mean thickness of 100 nm, a sheet resistance of 13 Ω/square and a visible transmission higher than 90%. Substrates were ultrasonically cleaned with a standard solvent regiment (15 min each in acetone and isopropanol). The ZnO layer was then

deposited followed by the active layer. The active layer was consisting of P3HT:PC71BM blend (10 mg ml-1 for P3HT, 8 mg ml-1 for PC71BM in chlorobenzene) with a thickness of 120 nm or P3HT:IC60BA (17 mg ml1

for P3HT, 17 mg ml-1 for PC71BM in chlorobenzene) with a thickness of 180 nm or PCDTBT:PC71BM (1.5

mg ml-1 for P3HT, 5.5 mg ml-1 for PC71BM in a mixed solvent (dichlorobenzene:chlorobenzene = 3:1) with a thickness of 70 nm or PTB7:PC71BM (10 mg ml-1 for PTB7, 15 mg ml-1 for PC71BM in chlorobenzene 4 ACS Paragon Plus Environment

Page 5 of 33


where 3% per volume of DIO was added) with a thickness of 80 nm. After spin coating, the photoactive 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

layer was annealed either at 150°C (P3HT:PC71BM, P3HT:IC60BA) or at 70 oC (PCDTBT:PC71BM, PTB7:PC71BM) for 15 min in air. Then, an approximately 20 nm-thick under-stoichiometric molybdenum oxide (MoOx) layer was deposited on top of the active layer, using the previously reported hot-wire deposition method,40,41 to serve as the hole extraction layer. The devices were completed with a 150 nm thick aluminium anode, deposited in a dedicated thermal evaporator at a pressure of 10-6 Torr through a shadow-mask, which defined the device active area to be equal to 12.56 mm2. The devices were then measured in air at room temperature without additional encapsulation. All chemicals were purchased from Sigma-Aldrich and used with no further purification. Measurements and Instrumentation. All plasma treatments for the ZnO films were performed in dc plasma for 20 min at room temperature. The plasma power was set at 500 W and the pressure was set at 1.3 Pa while the dc voltage was 0V. X-ray photoelectron spectra (XPS) and Ultraviolet Photoelectron Spectra (UPS) were recorded by Leybold EA-11 electron analyzer operating in constant energy mode at pass energy of 100 eV and at a constant retard ratio of 4 eV for XPS and UPS respectively. All binding energies were referred to the C 1s peak at 284.8 eV of surface adventitious carbon, respectively. The X-ray source for all measurements was an unmonochromatized Al Kα line at 1486.6 eV (12 keV with 20 mA anode current). The valence band spectra of Zn oxides were evaluated after recording the UPS spectra of about 40 nm thick films deposited on an FTO substrate, taken from the batch used for OPV cell fabrication. For the UPS measurements, the He I (21.22 eV) excitation line was used. A negative bias of 12.22 V was applied to the samples during UPS measurements in order to separate secondary electrons originating from sample and spectrometer and to estimate the absolute work function value from the high BE cut-off region of the UPS spectra. The analyzer resolution is determined from the width of the Au Fermi edge to be 0.16 eV. The steady state photoluminescence spectra of P3HT on various substrates were taken by means of a Fluoromax spectrometer (Horiba) upon excitation at 550nm. The films were placed on a specific holder for solid samples and the spectra were corrected for the sensitivity of the detector. The PL dynamics of the samples were studied under magic angle conditions, by using a Time Correlated Single Photon Counting (TCSPC) system (Fluotime, Picoquant) equipped with a Microchannel Plate photomultiplier. The excitation of the 5 ACS Paragon Plus Environment


Page 6 of 33

samples was realized at 400 or 470 nm by means of two diode pulsed lasers. The Instrument's Response 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Function (IRF) was in both cases approximately 80 ps. A three-exponential function convoluted with the IRF was used for fitting the dynamics. The thicknesses of ZnO films and active layers were estimated with ellipsometry. EQE measurements were carried out using an Autolab PGSTAT-30 potentiostat, with a 300 W Xe lamp in combination with an Oriel 1/8 monochromator for dispersing the light in an area of 0.5 cm2. A Thorlabs silicon photodiode was used for the calibration of the spectra. All measurements were performed in air. The structural characterization of all samples was carried out by wide angle X-ray diffraction (reflection mode) using a Bruker D8 Discover diffractometer with Ni-filtered Cu-Kα radiation source (λ=1.5406 Å) equipped with a LynxEye position sensitive detector. Absorption measurements were taken using a Perkin Elmer Lampda 40 UV/Vis spectrophotometer. GIWAXS was employed on the BM26B-DUBBLE beamline (λ = 1.033 Å, sample-to-detector distance of 2.1 m using a Pilatus 1M 981×1043 pixel detector with pixel size 172 µm) at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Current densityvoltage characteristics of the fabricated solar cells were measured with a Keithley 2400 source-measure unit. Cells were illuminated with a Xe lamp and an AM 1.5G filter to simulate solar light illumination conditions with an intensity of 100 mW/cm2 (1 sun), as was recorded with a calibrated silicon photodiode. To accurately define the active area of all devices we used aperture masks during the measurements with their area equal to those of the Al contacts (12.56 mm2).

3. Results and discussion Polymer solar cells characteristics. The inverted device architecture and the molecular structures of the organic semiconductors used in this study are shown in Figure 1a. First, the influence of plasma conditions on the device operation using the well-known P3HT:PC71BM system as the active layer was investigated. The current density-voltage (J-V) characteristics under simulated 1.5 AM solar irradiation taken in these devices (PSC 1) when using as-deposited ZnO or H plasma ZnO -treated under different pressures for about 20 min- as the electron extraction interlayer, are shown in Figure 1 b while Table 1 summarizes the devices operational characteristics. (The plasma power and dc voltage were set at 500 W and 0 V, respectively, after having performed preliminary experiments, which revealed that these were the appropriate conditions for 6 ACS Paragon Plus Environment

Page 7 of 33


achieving optimum device performance as shown in Figure S1, Supporting Information). From the J-V 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

curves shown in Figure 1b and the results summarized in Table 1 becomes evident that the devices showed simultaneous improvement in their Voc, Jsc and FF when using a low pressure H plasma treatment of ZnO interlayers. In particular, compared to the reference device with the as-deposited ZnO which exhibited a PCE of 3.1%, a more than 40% improvement was accomplished by the device with the ZnO layer treated with H plasma with a pressure of 1.3 Pa which gave the highest PCE value of 4.4%. The improvement in devices performance relative to the reference one was evident for treatments of ZnO layer with H plasma having pressures of up to 5.0 Pa. However, in higher pressures the performance decreases significantly as a result of the reduced current of the devices whereas in the pressure of 100.0 Pa the device operational characteristics completely deteriorate. The large enhancement of Jsc and FF in the devices with the lowpressure H plasma treated ZnO is also reflected in the considerable reduction of their series resistance (Rs) compared with the reference one and those using ZnO treated at higher plasma pressures (Table 1), as evidenced from the significantly increased slopes in the dark current measurements of the former devices presented in Figure 1 c. Reduced Rs suggests improved contact between the active layers and cathodes, which facilitates the free charge-carrier extraction that increases Jsc and FF in the devices incorporating lowpressure H plasma ZnO layers. In addition, the devices with the low-pressure hydrogen plasma treated ZnO exhibit increased shunt resistance (Rsh) which however starts to decrease when using the higher pressure H plasma. This is in accordance with the reduced leakage current exhibited by the devices using the lowpressure H plasma treated ZnO. It is well-known that shunt resistance represents any parallel high conductivity paths (shunts) across the solar cell junction. Shunt paths lead the current away from the intentional load, and their effects are detrimental to the device performance. In addition, significantly increased EQE values were observed in low-pressure H plasma ZnO layer incorporating devices when compared with their as-prepared counterparts (Figure 1 d). This enhancement in EQEs suggests that the photon-to-electron conversion processes are very efficient in the low-pressure H plasma-ZnO incorporating devices and provide clear evidence for enhanced charge generation/extraction probably attributed to a decrease in the charge extraction barrier and/or a decrease in recombination losses as well as better nanomorphology of the photoactive blend, as discussed below. 7 ACS Paragon Plus Environment


Page 8 of 33

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

modified devices results also in its superior environmental stability as compared with the reference ones. In Figures 2a, 2b, 2c and 2d the variation of PCE, Jsc, Voc and FF of the above devices using different cathode interlayers is presented as a function of storage time under ambient conditions. A faster drop in operational characteristics of the reference device was observed, while the device with the H plasma ZnO interlayer exhibits better stability, maintaining nearly unaffected its original values, even after storage in air for more than 2500 hours. The slower degradation rate of the H plasma ZnO modified solar cells is an additional benefit of their use as efficient charge transport and also protective layers. Regarding the origin of the enhanced stability of devices using the low-pressure H plasma-ZnO layers it has been shown recently that defects or dangling bonds present on the surface of the ZnO layers may reduce the stability of the solar cell due to oxygen chemisorption that occurs readily at room temperature on such ZnO surfaces, specifically to interstitial zinc (Zni) defects.42 Passivation of such defects is possible by using a H plasma treatment of ZnO which can explain, except of better performance, the enhanced stability of devices using these passivated ZnO films. Reduction of the electron injection barrier with H plasma treatment of ZnO. To further explore the reason for enhanced devices performance when using the H plasma-ZnO layers, ultraviolet photoelectron spectroscopy (UPS) was used to probe the electronic properties of ZnO and H plasma-ZnO (the plasma conditions were: 1.3 Pa, 500 W and 0 V and the treatment time was 20 min) on FTO substrates (Figure 3a). The UPS spectrum of ZnO shows two distinct peaks labeled a, and b, which arise from the Zn3d, and O2p bands at ~11 eV and ~6.6 eV binding energies, respectively.43-45 The intensity of peak b is increased after the H-plasma treatment due to the increased density of -OH groups as it is also shown in O1s XPS peak (Figure S2) and FTIR transmittance spectra (Figure S3). FTO/ZnO film possesses a WF of 4.5 eV, while FTO/H plasma-ZnO shows a reduced WF of 4.2 eV (0.3 eV smaller than ZnO). The work function of ZnO is sensitive to the preparation process and similar work functions values (4.3-4.5 eV) have been reported for sol-gel ZnO films which were annealed to temperatures higher than 150 Co.46-49 Interestingly, a shift of about 0.3 eV to higher binding energy of the Zn3d band was also observed (Figure 3a). The WF shift towards lower values of the H plasma treated ZnO can be explained as follows: During the hydrogen plasma 8 ACS Paragon Plus Environment

Page 9 of 33


treatment –OH groups are formed on the surface of ZnO. Hydroxyl is however electrically polar, with a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dipole moment of 1.67 Debye (5.57x 10-30 Cm).50 It is thus reasonable to assume that the formation of hydroxyl groups on the surface of H plasma treated ZnO would readily lead to the formation of an electrical dipole which consequently reduces the work function of ZnO, as illustrated in Figure 3b, where the energy level alignment of materials used in our devices (the anode side was omitted), is shown. The ionization energy of 7.7 eV of ZnO layer derived from the valence band edge of ~3.2 eV, as estimated from the near Fermi region of the UPS spectrum whereas the conduction band edge of -4.3 eV was calculated by subtracted the energy gap value of about 3.4 eV (Figure S4), which is slightly larger than other values reported in the literature.51 Initially (when using pristine ZnO), there is a significant energy level offset between the conduction band edge of ZnO (4.3 eV) and the LUMO level of the PCBM acceptor (4.0 eV). In the case of H plasma-ZnO however this energy offset is decreased by the formation of an electrical dipole (which also reduces the WF of ZnO) having its positive pole showing towards the PC71BM side and facilitating electron extraction via the conduction band of ZnO (Figure 3b). To this effect of electrical dipole formation, we believe that the relatively strong dipole moment of hydroxyl groups is essential. The above data indicate that the reduction of 0.3 eV in the WF and the improved interfacial energy level alignment of the H plasma-ZnO interlayer could allow the formation of an ohmic contact with PC71BM acceptor, representing a clear benefit and leading to enhanced electron injection/extraction efficiency and increased photocurrent in the complete device. In addition, the authors speculate that the enhanced Voc exhibited by the devices with the H plasma-ZnO films resulted also from a significant dipole that was nascently formed on the hydrogen treated ZnO surface, since this dipole leads to an enhanced built-in potential across the device. Charge carrier transport/extraction measurements. To shed more light on the actual nature of FTO/ ZnO/PC71BM contacts we have also fabricated electron-only devices by replacing the MoOx/Al interface with a simple Al cathode. Space-charge-limited-current (SCLC) measurements are a commonly used tool to determine the electron extraction efficiency of the devices and also to estimate the mobility of organic materials. Using the Mott–Gurney law the SCL current density (JSCL) is given by eqn (1), where ε0 is the electric permittivity of free space, εr the relative dielectric constant of the active layer (ranging between 3 9 ACS Paragon Plus Environment


Page 10 of 33

and 4), µ the charge carrier mobility, L the thickness of the device and Vin the voltage dropped across the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sample given by eqn (2).52 V2 9 J SCL = ε ο ε r µ in3 8 L

(1)

Vin = V − Vbi − Vrs

(2)

where V is the applied voltage, Vbi the built-in voltage and Vrs is the voltage drop due to the series resistance of the contacts. In the case of a field dependent mobility, as described by the Poole–Frenkel effect, the SCL current density is given by the modified Mott–Gurney equation (3): V2 9  0.89 β  J SCL = ε ο ε r µ in3 exp  V L 8  L 

(3)

where β is the electric field-activation factor of mobility, which accounts for the degree of disorder and particularly for the energetic level distribution of the carrier hopping sites in the material, i.e., a smaller b indicates a lower degree of energetic disorder.53 The field dependent SCLC expression yielded a reasonably good fit to the measured J-V curves of single-carrier devices. As it can be seen from Figure 4a, the device with the H plasma ZnO interlayer exhibits significantly enhanced electron current, indicating the reduced contact resistance at the cathode after hydrogen plasma treatment of ZnO, while the calculated electron mobility in this device was 3.7 × 10-3 cm2 V-1 s-1 compared with 1.9 × 10-3 cm2 V-1 s-1 in the reference one. In order to explain the enhanced electron mobility we speculate that by considerably reducing the electron injection/extraction barrier using the lower work function H plasma modified ZnO layer, we are able to enhance electron injection/conduction and obtain true SCL electron current in PC71BM. We then calculate an electron mobility higher than that in the reference device with the ZnO interlayer. This mobility is comparable to measured values and more accurately reflects the intrinsic value of the PC71BM (compared to a contact-limited effective value that is calculated from the reference device).54 Moreover, the threshold voltage is reduced in the modified ZnO interlayer-based device and the overall J-V curve shifts to lower voltages. Next, transient photocurrent (TPC) measurements were performed to study the photocarrier decay dynamics under an extraction field for the device with the H plasma-ZnO interlayer and for the reference one. By 10 ACS Paragon Plus Environment

Page 11 of 33


applying an external dc bias we can change the internal field as well as alter the amount of time the carriers 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

spend in the device prior to extraction, making it possible to observe carrier recombination dynamics in these systems.55 Figure 4b shows the TPC traces for the different devices while they are operating at their maximum power point (500 mV applied bias). The device is under constant illumination of the solar simulator, while a small perturbation is given by a pulsed laser to introduce a small photocurrent. The single exponential decay of the transient photocurrent for our devices is due to photo-generated carriers recombining either in the bulk or at the electrodes/photoactive layer interface. Since these devices have the same hole extraction contact and the same photoactive layer, the difference in carrier lifetime must be due to the difference in the cathode contact. The results indicate that the photogenerated carriers recombine at a faster rate in the reference device. Upon H plasma treatment of the ZnO cathode the rate of carrier recombination is reduced, resulting in a longer carrier lifetime and better device performance. The observed enhancement in carrier lifetime in the modified devices indicates that carrier density inside those devices is decreased, as photogenerated electrons are easily swept toward the cathode electrode due to the reduced electron extraction barrier and the increased internal field strength, resulting in a lower recombination rate. To explain the reduced recombination rate in our H plasma modified devices we should take into account that, except of the reduction in the electron injection barrier, passivation of defects or dangling bonds on the surface of the ZnO also promotes reduced recombination losses. Indeed, passivation of defect emission of the H plasma ZnO layer was verified using steady-state PL spectroscopy (Figure S5 and relevant discussion). The significant suppression of the visible photoluminescence of the H plasma-ZnO layer indicates that the observed change in the TPC measurements of P3HT when deposited on top of this layer is indeed due to reduced recombination losses at the ZnO/active layer interface. In order to further investigate the effect of H plasma treatment of ZnO layer on the cell built in potential we measured the capacitance-voltage (C-V) characteristics (in dark) of P3HT:PC71BM based PSCs for different ZnO layers (Figure 4 c). C-V measurements in devices based on organic semiconductors are usually exhibiting Mott-Schottky characteristics and can be used to evidence the device built-in field (Vbi) which is also related to the presence of charge extraction barriers at electrode contacts, and the doping level N. In Figure 4d the Mott-Schottky characteristics of the devices as derived from their C-V measurements are 11 ACS Paragon Plus Environment


Page 12 of 33

shown, where C-2=(2/qεN) (Vbi-V), ε≈εο is the permittivity of the blend and q is the elementary charge.56 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The built-in voltage (Vbi) is defined by the intersection of the C-2 curve and the horizontal bias axis. On the other hand, the upper-limit of the dopant (carrier) density (N) is determined from the slope of the Mott– Schottky plot. For the device with the ZnO we obtained a Vbi of 0.44 V while the use of H plasma-ZnO as the cathode interlayer delivered an increased Vbi increase of 0.57V. This increase in Vbi matches the increase in Voc and indicates that the use of the hydrogen treated ZnO cathode interlayer induces large strengthening of the device built-in field. This is due to the higher work function difference between our modified cathode and the anode (MoOx/Al) contacts.

Probing changes in nanomorphology of the active blend films on different ZnO layers. Because we found that H plasma treatment may alter the surface properties of ZnO films (e.g. wetting of the photoactive layer and/or reduced surface roughness, Figures S6 and S7), we also try to identify possible changes in the nanomorphology/crystallinity of the photoactive blend films. In Figure 5 a, the X-ray diffraction (XRD) measurements of similar P3HT:PC71BM blend films deposited on different ZnO substrates are shown. When deposited on the un-treated ZnO film, the P3HT:PC71BM layer exhibits a weak diffraction peak at 2θ=5.4o indicating that P3HT contains a low amount of crystallites with lamellae oriented perpendicularly to the substrate (along the a-axis, edge-on orientation).57,58 For the film deposited on the H plasma-ZnO film however the characteristic diffraction patterns (h00) (h=1,2,3) of P3HT crystallites become pronounced, as evidenced by the increased intensity of the peak at 2θ=5.4° and by the appearance of new peaks at 10.6° and 15.9° which are attributed to the primary (100), secondary (200) and tertiary (300) diffraction patterns, respectively, indicating that the stacking perpendicular to the substrate is significantly enhanced. Moreover, the a new diffraction peak appears at 2θ=23.2°, which corresponds to the π–π stacking direction (010) of P3HT chains, in the blend film deposited on the H plasma-ZnO which is indicative of a significant face-on packing (along the b-axis). Therefore, the main result derived from the XRD study is that when the blend P3HT:PC71BM film is deposited on top of the H plasma-ZnO surface, it exhibits a strong enhancement on its orientation, not only along the a-axis normal to the substrate (edge-on orientation), but also along the b-axis parallel to the underneath metal oxide’s surface (face-on orientation) (Figure 5d). Similar results with XRD were deduced using grazing incidence X-ray scattering (GIWAXS) of P3HT:PC71BM films deposited onto 12 ACS Paragon Plus Environment

Page 13 of 33


different ZnO layers (Figure 5 b). The region selected for integration is from q = 0.3 to 1.0 Å-1. The peaks 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

observed at q∼0.39 Å-1 and ∼0.75 Å-1 are the (100) and (200) peaks of the P3HT component, respectively. As discussed above, better edge-to-edge crystallization of P3HT chains in the blend deposited on the H plasma ZnO can be deduced from the increase in intensity of these peaks.Noting that, two distinct diffraction peaks at 2θ=12.7° and 25.8° are also appearing in the XRD pattern of the blend film deposited on the H plasma-ZnO film (Figure a). We speculate that these peaks could be attributed to the formation of PC71BM crystallites or aggregates since it has been shown that fullerene derivatives may crystallize and/or aggregate in bulk-heterojunction films particularly after thermal or solvent vapor annealing (Figure 5d).59,60 The improved nanomorphology of the blend film on top of the H plasma-ZnO suggests enhanced charge generation which was verified with absorption spectroscopy (Figure 5 c), which showed that although in all blend films there is a clear vibronic structure of the P3HT absorption component, this is more pronounced in the film deposited on the H treated ZnO surface. Taking into account that in crystalline materials there is an energetic driving force for charge carriers to leave amorphous, mixed regions of bulk heterojunctions, and to dominantly transported in pure, ordered phases which allows efficient charge generation as well as extraction and also may benefit the stability against light-induced traps we thus conclude that the enhanced crystallization exhibited by the blend films deposited on H plasma ZnO is highly beneficial for devices performance and also for their stability.

Photophysical characterization of polymer/ZnO interfaces. Thus far, we have presented experimental evidence of better active blend film nanomorphology and reduced electron extraction barrier leading to enhanced electron collection at the cathode contact in the case of devices fabricated on top of the H plasmaZnO surface. However, an additional question is whether the improved charge collection also follows from more efficient dissociation of excitons formed in the P3HT (donor), or whether the charge generation yield is unchanged and H plasma-ZnO is only more efficient at collecting electrons generated within the P3HT. Photoluminescence quenching experiments constitute a valuable tool towards evaluating an oxide’s ability to induce exciton dissociation at the heterointerface with an adjacent polymer. Steady-state PL measurements were performed on thin P3HT films, with 10 nm and 30 nm thickness, deposited on ZnO and H plasma-ZnO layers and the spectra –which were quite similar for both thicknesses- are shown in Figure 13 ACS Paragon Plus Environment


Page 14 of 33

6a. The two emission peaks of P3HT are centered at 660 nm and 720 nm in both spectra corresponding to 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the 0-0 transition and the 0-1 and 0-2 side-bands but two important observations were made: An overall reduction of the PL intensity and an enhanced reduction in the intensity of the lower energy emission peak is observed for P3HT on H plasma-ZnO versus P3HT on ZnO. These changes are even more pronounced in the PL spectra of P3HT:PC71BM thin films on top of the H plasma-ZnO (Figure 6b). Decreasing PL intensity with H modification suggests that the excitons generated in the photoactive layer have been dissociated in a more effective manner. Furthermore, this lower energy vibronic sideband emission is followed by lower intensity tail emission at even lower energies which has also been attributed to bandtail/trap states of P3HT.62,63 The presence of localized band-tail states is well documented in organic semiconductors, and current flow in disordered semiconductors is generally described in terms of a transport energy that separates mobile states and localized band-tail states.62,63 Our results of quenched low energy emission of P3HT could be attributed to either possible filling of the HOMO band-tail states of P3HT e.g. due to better film nanomorphology or enhanced dissociation of excitons (especially of the lower energy ones) in the H plasma-ZnO/photoactive blend interface via charge transfer from the P3HT to the oxide. Next, in an attempt to observe differences in charge kinetics in these systems transient PL measurements were performed on approximately 30 and 10 nm thick P3HT films on ZnO and H plasma ZnO layers. The excitation wavelength was 400 nm while the detection wavelength was either 660 nm (Figure 6c) or 720 nm (Figure 6d). Identical dynamics were observed upon 400 nm excitation. PL decay parameters as derived from the analysis of the above measurements are summarized in Table 2. In both detection wavelengths, a decrease exciton lifetime is observed when P3HT is deposited on hydrogen plasma modified metal oxide. A slightly enhanced reduction in lifetime is observed for low energy excitons.64 These results indicate that hydrogen plasma surface modification of ZnO improves the efficiency of exciton dissociation at the heterointerface with the photoactive polymer (P3HT), resulting in the generation of a greater quantity of charge. In other words, H plasma-ZnO seems to be more effective electron acceptor than its as-deposited counterpart which is an additional benefit along with its improved electron extraction capability. This is attributable to the large suppression of defect emission in the H plasma-ZnO layer (Figure S5) which results in reduced exciton recombination at the metal oxide/photoactive blend interface. While the precise 14 ACS Paragon Plus Environment

Page 15 of 33


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

previously for P3HT on N doped ZnO,62 and on silicon,65 where ultrafast charge transfer from P3HT to N:ZnO and silicon was observed. Despite the fact that it is not straightforward to quantify this effect (enhanced exciton dissociation) and compare it with charge generation efficiency in terms of their exact influence on the improved solar cell characteristics or to which one is more significant, we think that both synergistically act to improve overall solar cell performance.

Universal improvement in efficiency and stability of PSCs with H plasma-ZnO. Up to now, our results indicate that our approach of using low-pressure hydrogen plasma treatment of ZnO has the ability to improve the characteristics of P3HT:PC71BM-based solar cells. In order to explore its universal applicability in polymer photovoltaics we next fabricated devices based on the P3HT:IC60BA (PSC2), PCDTBT:PC71BM (PSC3) and PTB7:PC71BM(PSC4) systems. Remarkably, the trend observed in PSCs 1 using ZnO layers treated under different H plasma conditions is also seen for PSCs 2 (Figure S8 a and Figure 7a), PSCs 3 (Figure S8 b and Figure 7d) and PSCs 4 (Figure S8 c and Figure 7g). In particular, the devices treated with hydrogen with low-pressure (ranging from 0.7 to 5.0 Pa) exhibit higher efficiencies from the reference ones which however does not seem to apply to devices treated at higher pressures. In particular, while the reference devices showed a relatively low PCE of 4.4% for PSCs 2, of 5.4% for PSCs 3 and 5.6% for PSCs 4 the devices with the ZnO layers processed with H plasma with pressure 1.3 Pa exhibited significantly enhanced performance as revealed from their high PCE values of 6.6%, 7.4% and 8.0%, respectively (Table 1). In all cases, the efficiency enhancement is a direct result of highly improved Voc, Jsc and also FF in our H-modified ZnO devices. Reduced series and increased shunt resistances (Table 1) derived from the dark J– V curves taken in the above devices and shown as insets in Figure 7 revealed the better quality of the cathode contacts in the devices with the modified ZnO layers. In addition, EQE measurements witnessed the enhanced charge generation efficiency in the devices with the modified ZnO (Figures 7b, 7e and 7h) while once again we also verified the beneficial effect of surface modification of ZnO in the device long term stability (Figures 7c, 7f, 7i). These results unambiguously demonstrate that low-pressure hydrogen plasma treatment of ZnO films used as electron extraction/collection layers is a promising strategy for applications in various types of polymer solar cells. 15 ACS Paragon Plus Environment


Page 16 of 33

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

Surface modification of ZnO electron extraction layers by low-pressure H plasma treatment resulted in a remarkable enhancement of Voc, Jsc and FF in all inverted PSC devices, resulting in high power conversion efficiencies, irrespective of the donor:acceptor combination blend used in the photoactive layer. The improved Voc and overall performance is attributed to the reduced work function of ZnO, as derived from UPS measurements, and lowering of the electron extraction barrier and overall improved charge extraction/collection. XRD measurements revealed the improved nanomorphology/crystallinity of the photoactive blend atop the hydrogen modified ZnO substrate. PL spectra and dynamics indicated also the more efficient exciton dissociation at the heterointerface of H plasma-ZnO/P3HT versus that formed in the samples with the as deposited oxide. These finding show that low-pressure H plasma treatment can be used as a cost-effective technique to improve the surface properties of ZnO electron extraction layers, opening a novel and efficient route for the development of high performance PSC devices.

Acknowledgments The project “Implementing advanced interfacial engineering strategies for highly efficient hybrid solar cells” (Acronym: IMAGINE-HYSOL) was implemented under the "ARISTEIA II" Action of the OPERATIONAL PROGRAMME EDUCATION AND LIFELONG LEARNING" and was co-funded by the European Social Fund (ESF) and National Resources.

Supporting Information Additional Figures (Figure S1-S8) and discussion are included in the Supporting Information: J-V characteristic curves demonstrating the influence of plasma conditions on the device performance, O 1s XPS peaks and FTIR spectra of different ZnO layers, tauc plots and photoluminescence spectra of ZnO layers, contact angle measurements and atomic force microscopy images of ZnO films and J-V characteristics of polymer solar cells based on different donor:acceptor combinations using ZnO electron transport layers processed under different plasma conditions.


Page 17 of 33


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

(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer Photovoltaic Cells: Enhanced Efficiencies via a Network of Internal Donor-Acceptor Heterojunctions. Science 1995, 270, 1789– 1791.

(2) Chen, H.-Y.; Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nature Photon. 2009, 3, 649–653.

(3) Brabec, C. J.; Gowrisanker, S.; Halls, J. J. M.; Laird, D.; Jia, S.; Williams, S. P. Polymer-Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2010, 22, 3839–3856.

(4) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474–1476.

(5) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Efficient Photodiodes from Interpenetrating Polymer Networks. Nature 1995, 376, 498–500.

(6) Park, S. H.; Roy, A.; Beaupré, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction Solar Cells with Internal Quantum Efficiency Approaching 100%. Nature Photon. 2009, 3, 297–302.

(7) Krebs, F. C. All Solution Roll-to-Roll Processed Polymer Solar Cells Free from Indium-Tin-Oxide and Vacuum Coating Steps. Sol. Energy Mater. Sol. Cells 2009, 10, 761–768.

(8)

Søndergaard, R.; Hösel, M.; Angmo, D.; Larsen-Olsen, T.-T.; Krebs, F. C. Roll-to-Roll Fabrication of Polymer Solar Cells. Mater. Today 2012, 15, 36–49.

(9)

Ecker, B.; Nolasco, J. C.; Pallarés, J.; Marsal, LL. F.; Posdorfer, J.; Parisi, J.; von Hauff, E. Degradation Effects Related to the Hole Transport Layer in Organic Solar Cells. Adv. Funct. Mater.

2011, 21, 1–7. (10) Jørgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradation of Polymer Solar Cells. Sol. Energy Mater. Sol. Cells 2008, 92, 686–714.

(11) Hau, S. K.; Yip, H.-L.; Jen, A. K.-Y. A Review on the Development of the Inverted Polymer Solar 17 ACS Paragon Plus Environment


Page 18 of 33

Cell Architecture. Polym. Rev. 2010, 50, 474–510. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(12) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580–612.

(13) Tromholt, T.; Madsen, M. V.; Carlé, J. E.; Helgesen, M.; Krebs, F. C. Photochemical Stability of Conjugated Polymers, Electron Acceptors and Blends for Polymer Solar Cells Resolved in terms of Film Thickness and Absorbance. J. Mater. Chem. 2012, 22, 7592–7601.

(14) Schaffer, C. J.; Palumbiny, C. M.; Niedermeier, M. A.; Jendrzejewski, C.; Santoro, G.; Roth, S. V.; Müller-Buschbaum, P. A Direct Evidence of Morphological Degradation on a Nanometer Scale in Polymer Solar Cells. Adv Mater. 2013, 25(46), 6760–6764.

(15) Schafferhans, J.; Baumann, A.; Wagenpfahl, A.; Deibel C.; Dyakonov, V. Oxygen Doping of P3HT:PCBM Blends: Influence on Trap States, Charge Carrier Mobility and Solar Cell Performance. Org. Electron. 2010, 11, 1693–1700.

(16) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. On the Origin of the OpenCircuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904–909.

(17) Kirchartz, T.; Taretto, K.; Rau, U. Efficiency Limits of Organic Bulk Heterojunction Solar Cells. J. Phys. Chem. C 2009, 113, 17958–17966.

(18) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Solar Cell Efficiency Tables (Version 45). Prog. Photovolt. Res. Appl. 2015, 23, 1–9.

(19) Wu, C. I.; Lin, C. T.; Chen, Y. H.; Chen, M. H.; Lu, Y. J.; Wu, C. C. Electronic Structures and Electron-Injection Mechanisms of Cesium-Carbonate-Incorporated Cathode Structures for Organic Light-Emitting Devices. Appl. Phys. Lett. 2006, 88, 152104.

(20) Gustafsson, G.; Cao, Y.; Treacy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Flexible LightEmitting Diodes made from Soluble Conducting Polymers. Nature 1992, 357, 477–479.

(21) Heywang, G.; Jonas, F. Poly(alkylenedioxythiophene)s − New, Very Stable Conducting Polymers. Adv. Mater.1992, 4, 116–118.

(22) He, Z.; Zhong, C.; Huang, X.; Wong, W. Y.; Wu, H.; Chen, L.; Su, S.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer 18 ACS Paragon Plus Environment

Page 19 of 33


Solar Cells. Adv. Mater. 2011, 23, 4636–4643. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(23) Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer. J. Am. Chem. Soc.2011, 133, 8416–8419.

(24) Yip, H.-L.; Jen, A. K.-Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994–6011.

(25) Chen, S.; Small, C. E.; Amb, C. M.; Subbiah, J.; Lai, T. H.; Tsang, S. W.; Manders, J. R.; Reynolds, J. R.;So, F. Inverted Polymer Solar Cells with Reduced Interface Recombination. Adv. Energy Mater. 2012, 2, 1333–1337.

(26) Pacholski, C.; Kornowski, A.; Weller, H. Self-Assembly of ZnO: From Nanodots to Nanorods. Angew. Chem. 2002, 41, 1188–1191.

(27) Liang, Z.; Zhang, Q.; Wiranwetchayan, O.; Xi, J.; Yang, Z.; Park, K.; Li, C.; Cao, G. Effects of the Morphology of a ZnO Buffer Layer on the Photovoltaic Performance of Inverted Polymer Solar Cells. Adv. Funct. Mater. 2012, 22, 2194–2201.

(28) Ma, Z.; Tang, Z.; Wang, E.; Andersson, M. R.; Ingana¨s, O.; Zhang, F. Influences of Surface Roughness of ZnO Electron Transport Layer on the Photovoltaic Performance of Organic Inverted Solar Cells. J. Phys. Chem. C 2012, 116, 24462–24468.

(29) Cheng, Y.-J.; Hsieh, C.-H.; He, Y.; Hsu, C.-S.; Li, Y. Combination of Indene-C60 Bis-Adduct and Cross-Linked Fullerene Interlayer Leading to Highly Efficient Inverted Polymer Solar Cells. J. Am. Chem. Soc. 2010, 132, 17381–17383.

(30) Lee, B. R.; Jung, E. D.; Nam, Y. S.; Jung, M.; Park, J. S.; Lee, S.; Choi, H.; Ko, S.-J.; Shin, N. R.; Kim, Y.- K.; Kim, S. O.; Kim, J. Y.; Shin, H.- J.; Cho , S.; Song, M. H. Amine-Based Polar Solvent Treatment for Highly Efficient Inverted Polymer Solar Cells. Adv. Mater. 2014, 26, 494–500.

(31) Yang, T.; Wang, M.; Duan, C.; Hu, X.; Huang, L.; Peng, J.; Huang, F.; Gong, X. Inverted Polymer Solar Cells with 8.4% Efficiency by Conjugated Polyelectrolyte. Energy Environ. Sci. 2012, 5, 8208–8214.

(32) Hau, S. K.; Cheng, Y.- J.; Yip, H.- L.; Zhang, Y.; Ma, H.; Jen, A. K.- Y. Effect of Chemical 19 ACS Paragon Plus Environment


Page 20 of 33

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

Polymer Solar Cells. ACS Appl. Mater. Interfaces 2010, 2, 1892–1902.

(33) Yip, H.-L.; Hau, S. K.; Baek, N. S.; Ma, H.; Jen, A. K.-Y. Polymer Solar Cells that Use SelfAssembled Monolayer-modified ZnO/Metals as Cathodes. Adv. Mater. 2008, 20, 2376–2382.

(34) O'Malley, K. M.; Li, C.- Z.; Yip, H.- L.; Jen, A. K.- Y. Enhanced Open-Circuit Voltage in High Performance Polymer/Fullerene Bulk-Heterojunction Solar Cells by Cathode Modification with a C60 Surfactant. Adv. Energy Mater. 2012, 2, 82–86.

(35) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C. A. J. Heeger, Efficient Solution-Processed Small-Molecule Solar Cells with Inverted Structure. Adv. Mater. 2013, 25, 2397–2402.

(36) Lilliedal, M. R.; Medford, A. J.; Madsen, M. V.; Norrman, K.; Krebs, F. C. The Effect of PostProcessing Treatments on Inflection Points in Current-Voltage Curves of Roll-to-Roll Processed Polymer Photovoltaics. Sol. Energy Mater. Sol. Cells 2010, 94, 2018–2031.

(37) Kuwabara, T.; Kawahara, Y.; Yamaguchi, T.; Takahashi, K. Characterization of Inverted-Type Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by ac Impedance Spectroscopy. ACS Appl. Mater. Interfaces 2009, 1, 2107–2110.

(38) Cheun, H.; Fuentes-Hernandez, C.; Zhou, Y.; Potscavage, W. J.; Kim, S.-J.; Shim, J.; Dindar, A.; Kippelen, B. Electrical and Optical Properties of ZnO Processed by Atomic Layer Deposition in Inverted Polymer Solar Cells. J. Phys. Chem. C 2010, 114, 20713–20718.

(39) Gao, H. L. ; Zhang, X. W.; Meng, J. H.; Yin, Z. G.; Zhang, L. Q.; Wu, J. L.; Liu, X. Enhanced Efficiency in Polymer Solar Cells via Hydrogen Plasma Treatment of ZnO Electron Transport Layers. J. Mater. Chem. A 2015, 3, 3719–3725.

(40) Vasilopoulou, M.; Douvas, A. M.; Georgiadou, D. G.; Palilis, L. C.; Kennou, S.; Sygellou, L.; Soultati, A.; Kostis, I.; Papadimitropoulos, G.; Davazoglou, D.; Argitis, P. The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics. J. Am. Chem. Soc. 2012, 134, 16178–16187.

(41) Vasilopoulou, M.; Palilis, L. C.; Georgiadou, D. G.; Kennou, S.; Kostis, I.; Davazoglou, D.; Argitis, 20 ACS Paragon Plus Environment

Page 21 of 33


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

Diodes using Substoichiometric MoOx Anode Interfacial Layer. Appl. Phys. Lett. 2012, 100,

013311. (42) MacLeod, B. A.; Tremolet de Villers, B. J.; Schulz, P.; Ndione, P. F.; Kim, H.; Giordano, A. J.; Zhu, K.; Marder, S. R.; Graham, S.; Berry, J. J.; Kahn, A.; Olson, D. C. Stability of Inverted Organic Solar Cells with ZnO Contact Layers Deposited from Precursor Solutions. Energy Environ.

Sci. 2015, 8, 592–601. (43) Özgür, Ü.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doǧan, S.; Avrutin, V. S.; Cho, J.; Morko̧, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys.2005, 98, 041301: 1–103, doi: 10.1063/1.1992666.

(44) Sharma, A.; Franklin, J. B.; Singh, B.; Andersson, G. G.; Lewis, D. A. Electronic and Chemical Properties of ZnO in Inverted Organic Photovoltaic Devices. Org. Electron. 2015, 24, 131–136.

(45) Ozawa K.; Mase, K. Angle-Resolved Photoelectron Spectroscopy Study of Hydrogen Adsorption on ZnO (1010). Phys. Status Solidi 2010, 207, 277–281.

(46) Yu, W.; Huang, L.; Yang, D.; Fu, P.; Zhou, L.; Zhang, J.; Li, C. Efficiency Exceeding 10% for Inverted Polymer Solar Cells with a ZnO/Ionic Liquid Combined Cathode Interfacial Layer. J. Mater. Chem. A 2015, 3, 10660–10665.

(47) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679–1683.

(48) White, M. S.; Olson, D. C.; Shaheen, S. E.; Kopidakis, N.; Ginley, D. Inverted Bulk-Heterojunction Organic Photovoltaic Device Using a Solution-Derived ZnO Underlayer. Appl. Phys. Lett. 2006, 89, 143517.

(49) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed Small-Molecule Solar Cells with Inverted Structure. Adv. Mater. 2013, 25, 2397-2402.



Page 22 of 33

(50) Stuhl, B. K.; Hummon, M. T.; Yeo, M.; Quéméner, G.; Bohn, J. L.; Ye, J. Evaporative Cooling of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the Dipolar Hydroxyl Radical. Nature 2012, 492, 396–401.

(51) Sun, Y.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low- Temperature-Annealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679–1683.

(52) Mott, N.; Gurney, R. Electronic Processes in Ionic Crystals; Oxford University Press:London, 1940.

(53) Palilis, L. C.; Uchida M.; Kafafi Z. H. Electron Injection in “Electron-Only” Devices Based on a Symmetric Metal/Silole/Metal Structure. IEEE J. Sel. Top. Quantum Electron. 2004, 10, 79–88, No. 1.

(54) Do, T. T.; Hong, H. S.; Ha, Y. E.; JPark, Y.; Kang, Y.-C.; Kim, J. H. Effect of Polyelectrolyte Electron Collection Layer Counteranion on the Properties of Polymer Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 3335–3341.

(55) Tiwana, P.; Docampo, P.; Johnston, M. B.; Herz, L. M.; Snaith, H. J. The Origin of an Efficiency Improving “Light Soaking” Effect in SnO2 Based Solid-state Dye-Sensitized Solar Cells. Energy Environ. Sci. 2012, 5, 9566–9573.

(56) He, Z. C.; Zhong, C. M.; Huang, X.; Wong, W. Y.; Wu, H. B.; Chen, L. W.; Su, S. J.; Cao, Y. Simultaneous Enhancement of Open-Circuit Voltage, Short-Circuit Current Density, and Fill Factor in Polymer Solar Cells. Adv. Mater. 2011, 23, 4636–4643.

(57) Chen, L.-M.; Xu, Z.; Hong, Z.; Yang Y. Interface Investigation and Engineering - Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem., 2010, 20, 2575–2598.

(58) Vasilopoulou, M. The Effect of Surface Hydrogenation of Metal Oxides on the Nanomorphology and the Charge Generation Efficiency of Polymer Blend Solar Cells. Nanoscale, 2014, 6, 13726– 13739.

(59) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stühn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Correlation Between Structural and Optical Properties of Composite Polymer/Fullerene Films for Organic Solar Cells. Adv. Funct. Mater. 2005, 15, 1193–1196. 22 ACS Paragon Plus Environment

Page 23 of 33

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


(60) Description of the Nanostructured Morphology of [6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM) by XRD, DSC and Solid-State NMR. Magn. Reson. Chem. 2011, 49, 242–247.

(61) Chen, L. -M.; Xu, Z.; Hong, Z.; Yang Y. Interface Investigation and Engineering - Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 2575–2598.

(62) Musselman, K. P.; Albert-Seifried, S.; Hoye, R. L. Z.; Sadhanala, A; Muñoz-Rojas, D.; MacManusDriscoll, J. L.; Friend, R. H. Improved Exciton Dissociation at Semiconducting Polymer:ZnO Donor:Acceptor Interfaces via Nitrogen Doping of ZnO. Adv. Funct. Mater. 2014, 24, 3562–3570.

(63) Pingel, P.; Neher, D. Comprehensive Picture of p-type Doping of P3HT with the Molecular Acceptor F4TCNQ. Phys. Rev. B 2013, 87, 115209.

(64) Zhang, Y.; de Boer, B.; Blom, P. W. M. Controllable Molecular Doping and Charge Transport in Solution-Processed Polymer Semiconducting Layers. Adv. Funct. Mater. 2009, 19, 1901-1905.

(65) Herrmann, D.; Niesar, S.; Scharsich, C.; Köhler, A.; Stutzmann, M.; Riedle, E. Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photovoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption. J. Am. Chem. Soc. 2011, 133, 18220-18233.



Page 24 of 33

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

Figure 1

(a) The reverse device structure FTO/ZnO or H plasma-ZnO/active layer/MoOx/Al. The chemical structures of the organic semiconductors used, are also shown. (b) and (c) Current density versus voltage (J-V) characteristics under 1.5 AM illumination and in dark, respectively, of the P3HT:PC71BM based devices using ZnO layers treated with H plasma with different pressure. (d) EQE measurements of the same devices.

Figure 2

Variation of (a) PCE, (b) Jsc, (c) Voc and (d) FF over time for the P3HT:PC71BM-based devices using different ZnO cathode interlayers.

Figure 3

(a) UPS spectra taken on the ZnO and H-plasma-ZnO films deposited on an FTO substrate. (b) Schematic energy level diagram of FTO/ZnO/P3HT:PCBM interfaces illustrating the formation of an energy dipole due to the presence of hydroxyl groups on the surface of H plasma-treated ZnO.

Figure 4

(a) J−V curves in double-logarithmic scale obtained in electron-only P3HT:PC71BM-based devices with the different ZnO cathode interfaces, measured in the dark. (b) Transient photocurrent of P3HT:PC71BM-based devices with the different ZnO cathode interfaces near the maximum power point. (c) C-V measurements (in dark) and (d) Mott-Schottky plots of P3HT:PC71BM-based devices using different ZnO cathode interlayers.

Figure 5

Effect of hydrogen plasma treatment of ZnO on the P3HT:PC71BM blend film nanomorphology: (a) XRD graphs, (b) GIWAXS and (c) normalized to blend film thicknesses absorption spectra of the films deposited on as-prepared ZnO and H plasma ZnO, respectively. (d) Illustration of the formation of extended P3HT crystallites and PC71BM aggregates.

Figure 6

Steady-state PL spectra of (a) P3HT (30 nm) and (b) of P3HT:PC71BM blend (100 nm) on different ZnO layers. Transient PL dynamics of P3HT thin film (30 nm) on different ZnO layers detected at (c) 670 nm and (d) 720 nm.

Figure 7

(a), (d), (g) Current density versus voltage (J-V) characteristics under 1.5 AM illumination of PSCs based on P3HT:IC60BA, PCDTBT:PC71BM and PTB7:PC71BM


using different ZnO

Page 25 of 33


interlayers. The dark currents are shown as insets. (b), (e), (h) EQE measurements of the same 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

devices. (c), (f), (i) Variation of PCE versus time of the same devices.



Page 26 of 33

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

Table 1. Device characteristics of PSCs having the device configuration FTO/zinc oxide/active layer /MoOx/Al (mean values and standard deviations were extracted from a batch of 36 devices). PSC 1 (P3HT:PC71BM) Cathode:

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

ZnO or H plasma-ZnO 0.0 Pa

Rs (Ω cm2)

Rsh (Ω cm2)

9.0(±0.15)

0.60(±0.01)

0.58(±0.01)

3.1(±0.15)

3.5

1100

0.7 Pa

9.1(±0.15)

0.63(±0.01)

0.59(±0.01)

3.4(±0.10)

3.1

1150

1.3 Pa

10.4(±0.10)

0.67(±0.01)

0.63(±0.01)

4.4(±0.15)

1.4

2000

2.5 Pa

10.1(±0.10)

0.67(±0.01)

0.62(±0.01)

4.2(±0.15)

1.5

2000

5.0 Pa

9.7(±0.15)

0.64(±0.01)

0.61(±0.01)

3.8(±0.10)

1.9

1850

10.0 Pa

7.0(±0.10)

0.62(±0.01)

0.58(±0.01)

2.5(±0.10)

2.9

1600

50.0 Pa

5.1(±0.10)

0.60(±0.01)

0.57(±0.01)

1.7(±0.10)

3.3

1500

100.0 Pa

3.2(±0.10)

0.44(±0.01)

0.41(±0.01)

0.6(±0.15)

6.6

700

Cathode

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

ZnO

8.10 (±0.15)

0.80(±0.01)

0.60(±0.01)

4.4(±0.25)

5.1

1050

H plasma-ZnO 1.3 Pa

10.90 (±0.10)

0.84(±0.01)

0.75(±0.01)

6.6(±0.15)

1.4

1300

Cathode

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

ZnO

9.90(±0.20)

0.82(±0.10)

0.60(±0.10)

5.4(±0.25)

5.1

700

H plasma-ZnO 1.3 Pa

11.5(±0.10)

0.88(±0.10)

0.73(±0.10)

7.4(±0.15)

2.5

1100

Cathode

Jsc (mA/cm2)

Voc (V)

FF

PCE (%)

Rs (Ω cm2)

Rsh (Ω cm2)

ZnO

11.5(±0.20)

0.74(±0.10)

0.65(±0.10)

5.6(±0.20)

6.2

780

H plasma-ZnO 1.3 Pa

15.2(±0.15)

0.80(±0.10)

0.68(±0.10)

8.0(±0.15)

2.4

1060

PSC 2 ((P3HT:IC60BA)

PSC 3 (PCDTBT:PC71BM)

PSC 4 (PTB7:PC71BM)

Table 2. Fitting parameters, obtained by means of a three-exponential function, of the P3HT films on ZnO substrates with different treatment. Excitation wavelength: 470 nm. Detection wavelengths: 670 and 720 nm. Sample P3HT on ZnO P3HT on H plasmaZnO

λdet (nm) 670 720 670

Α1 0.75 0.80 0.79

τ1 (ns) 0.11 0.14 0.09

Α2 0.24 0.20 0.19

τ2 (ns) 0.35 0.43 0.3

Α3

Surface Modification of ZnO Layers via Hydrogen Plasma Treatment

Recommend Documents