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High performance planar perovskite solar cells by ZnO electron transport layer engineering
Author’s Accepted Manuscript High performance planar perovskite solar cells by ZnO electron transport layer engineering Qingzhi An, Paul Fassl, Yvonne J. Hofstetter, David Becker-Koch, Alexandra Bausch, Paul E. Hopkinson, Yana Vaynzof www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30423-8 http://dx.doi.org/10.1016/j.nanoen.2017.07.013 NANOEN2071
To appear in: Nano Energy Received date: 24 April 2017 Revised date: 3 July 2017 Accepted date: 6 July 2017 Cite this article as: Qingzhi An, Paul Fassl, Yvonne J. Hofstetter, David BeckerKoch, Alexandra Bausch, Paul E. Hopkinson and Yana Vaynzof, High performance planar perovskite solar cells by ZnO electron transport layer engineering, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.07.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High performance planar perovskite solar cells by ZnO electron transport layer engineering
Qingzhi An1,2, Paul Fassl1,2, Yvonne J. Hofstetter1,2, David Becker-Koch1,2, Alexandra Bausch,1,2 Paul E. Hopkinson1,2 and Yana Vaynzof1,2* 1 Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, Heidelberg University, Germany 2
Centre for Advanced Materials, Im Neuenheimer Feld 225, Heidelberg University, Germany *Corresponding author. [email protected] Abstract ZnO as electron extraction layer in photovoltaic devices has many advantages, including high mobility and low processing temperature. However, it has been underutilized in perovskite solar cells due to the reported instabilities of perovskite layers deposited on ZnO resulting in poor device performance. Herein, we modify the ZnO layer by incorporating Cs or Li dopants in its bulk and depositing a selfassembled monolayer on its surface. This combined approach of engineering both the bulk and surface properties of ZnO results in significant improvements in the performance of planar MAPbI3 perovskite solar cells with a maximum power conversion efficiency of 18%, accompanied by a reduction in hysteresis and a significant enhancement of the device stability. Our work makes engineered solutionprocessed ZnO layers a practical alternative to TiO2 as electron extraction layers in perovskite solar cells, while also eliminating the need for high temperature sintering steps from the device fabrication. Keywords: Perovskite solar cells, extraction layer engineering, ZnO 1. Introduction Planar perovskite solar cells can be made in two architectures commonly referred to as ‘standard’ and ‘inverted’ depending on the order of deposition of hole and electron transport layers that sandwich the perovskite active layer. In a standard architecture, an electron transport layer (ETL) - typically an n-type oxide - is deposited first,
followed by the perovskite and hole transport layer (HTL). 1 In an inverted architecture, the HTL is deposited first - most commonly a conducting polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) - followed by the perovskite layer and ETL, typically a fullerene derivative. 2 Significant scientific efforts have been devoted to developing various HTLs including polymeric hole transporters such as poly[N,N-bis(4-butylphenyl)-N,N-bis(phenyl)-benzidine] (polyTPD) and poly(triarylamine) (PTAA), 3 - 4 or inorganic hole transporters such as CuSCN5 and NiO.6-7 ETL research has focused on metal oxides such as TiO2, ZnO and SnO2. While TiO2 is by far the most commonly used ETL, it has several significant disadvantages. TiO2 layers require a sintering step at high temperature of ~450 oC, rendering them unsuitable for deposition on indium tin oxide (ITO) or flexible plastic substrates. 8 Fu et al has shown that lowering the processing temperatures for the formation of TiO2 is possible, however this results in a reduced power conversion efficiency of only 9.95%. 9 Moreover, TiO2 has a lower electron mobility than other ETLs (for example, ZnO) and many surface and bulk defects that adversely affect the performance and stability of the photovoltaic devices.10 Unlike TiO2, ZnO and SnO2 do not require a sintering step at high temperature and have also been investigated as electron extraction layers in perovskite photovoltaic devices. Recent reports have showed that devices with a SnO2 as an electron extraction layer can result in efficiencies comparable or even surpassing those of TiO2.11-15 In the case of ZnO, despite the first promising report of highly efficient solar cells,16 subsequent investigations revealed that perovskite films deposited on ZnO suffer from severe decomposition due to the presence of hydroxyl groups or acetate ligands on the metal oxide surface.17-18 Even in the absence of these groups, the basic nature of ZnO can
result in perovskite decomposition due to proton transfer reactions taking place at the perovskite/ZnO interface.19 Two types of modification of the electron extraction layer have been investigated previously: doping the metal oxide layer, or using an interlayer to modify its surface properties. In the case of the widely used TiO2 ETL, a variety of dopants has been investigated, including Al,10 Li, 20 W, 21 Mg, 22 Nb, 23 Y, 24 and other additives. 25 - 26 Various interfacial modifiers have also been studied including benzoic acid derivatives, 27 thiols, 28 glycine, 29 C60 derivatives, 30 - 31 polyoxyethylene (PEO), 32 caesium bromide (CsBr), 33 caesium Carbonate (Cs2CO3), 34 MgO 35 and ZrO2. 36 Significantly less attention has been devoted to ZnO as extraction layer due to the previously mentioned instabilities observed for perovskite films deposited on ZnO. Despite this, promising results were shown in the case of Al doped ZnO 37-39 or 3aminopropanioc acid modified ZnO, 40 where an increase in performance was reported. Interestingly, it was observed that the deposition method of the ZnO plays a critical role in determining the final performance of the device. Various methods such as deposition from sol-gel, 41 chemical bath deposition (CBD), 42 atomic layer deposition
43
and sputtering38 have all been investigated. Even with these
modifications, the performance of perovskite solar cells that utilize ZnO as ETL is significantly lower than that of comparable devices on TiO2 or SnO2. Specifically, devices made using solution processed ZnO from sol-gel show at best power conversion efficiencies of 11%,41 and those processed from ZnO nanoparticles a maximum of 15.7%.16 In this work, we report a complete engineering of both the bulk and surface properties of a sol-gel ZnO ETL, by combining doping of the ZnO layer and surface modification with a self-assembled monolayer, both of which are realised using solution processing at relatively low temperatures. This approach allows us to
achieve efficiencies up to 18% with the standard perovskite MAPbI3, the highest reported thus far for solution processed ZnO, and comparable to devices made on TiO2 or SnO2. 2. Experimental Section 2.1 Materials Methylammonium iodide (CH3NH3I) was purchased from Dyesol. Spiro-oMeTAD (99.7%) was purchased from Borun New Material Technology Co., Ltd. PCBA powders (99%) were purchased from Solenne BV. All other materials were purchased from Sigma-Aldrich and used as received. 2.2 Device Fabrication Pre-patterned indium tin oxide (ITO) coated glass substrates (PsiOTech Ltd., 15 Ohm/sqr) were cleaned sequentially with 2 % hellmanex detergent, deionized water, acetone, and isopropanol, followed by 10 min oxygen plasma treatment. A ZnO sol-gel was prepared following a previously reported recipe.44 Doping with Cs or Li was accomplished by adding either caesium carbonate or lithium acetate to the solgel solution in 2% and 3% MR, respectively. These molar ratios were chosen as they yielded the most efficient devices. Immediately after plasma cleaning, ZnO solution (with or without dopants) was spin-coated at 2000 rpm for 45 s and annealed for 30 min at 200 °C. Afterwards, the substrates were modified with a SAM of PCBA following previously reported procedure. 44 For the perovskite layer a lead acetate trihydrate recipe was used: CH3NH3I and Pb(Ac)2·3(H2O) (3:1, molar ratio) were dissolved in anhydrous N, N –dimethylformamide (DMF) with a concentration of 40 wt% with the addition of hypophosphorous acid solution (6 µl / 1 ml DMF). The perovskite solution was spin coated at 2000 rpm for 60 s in a drybox (RH < 0.5 %). After drying for 5 min, the as-spun films were annealed at 80 °C for 30 min.
Subsequently, spiro-MEOTAD solution (80 mg Spiro-OMeTAD dissolved in 1ml chlorebenze with 17.3µl Li-TFSI (520 mg/ml acetonitrile) and 28.5 µl 4-tertbutylpyridine) was spin coated on top of the perovskite layers at 2000 rpm for 45 seconds. To complete the device, a 80 nm silver electrode was deposited via thermal evaporation under high vacuum. 2.3 Device Characterisation The current density-voltage (J-V) measurements were performed under simulated AM 1.5 sunlight at 100 mW cm-2 irradiance (Abet Sun 3000 Class AAA solar simulator) with a Keithley 2450 Source Measure Unit. The light intensity was calibrated with a Si reference cell (NIST traceable, VLSI) and corrected by measuring the spectral mismatch between the solar spectrum, the spectral response of the perovskite solar cell and the reference cell. The mismatch factor was calculated to be around 10 %. The cells were scanned from forward bias to short circuit and back at a rate of 0.15 V s-1 and 0.025 V s-1 after holding under illumination at 1.2 V for 2 s. To eliminate the contribution of the area surrounding each pixel, a mask that covers these areas from illumination was employed. For transient photocurrent measurements, the light of an inorganic LED (Thorlabs TO-1 ¾, λ = 465 nm) was pulsed by a function generator (Agilent/Keysight 33510B; pulse length = 2 ms) and focused on the solar cell. The resulting photocurrent was measured with an oscilloscope (Picoscope 5443A) with a 50 Ω terminator placed across the oscilloscope input. 2.4 ZnO layer characterisation ZnO samples for XPS/UPS measurements were prepared as described above on ITO substrates and then transferred to the ultrahigh vacuum chamber of the XPS system (Thermo Scientific ESCALAB 250Xi). The XPS measurements were performed
using a XR6 monochromated Al Kα source (h = 1486.6 eV) and a pass energy of 20 eV. UPS measurements were carried out using a He discharge lamp (h = 21.2 eV) and a pass energy of 2 eV. Measurements were collected at three different spots for each sample. XPS depth profiling was performed using an Ar cluster source with cluster energy of 4000 eV. AFM (Bruker MultiMode) was performed in tapping mode in air with silicon tips (Bruker NTESPA) to study the surface morphology of the undoped and doped ZnO films. Layers for PDS characterization were prepared on spectrosil in an identical fashion to those for photovoltaic devices. In the N2 glovebox the samples were placed into a sample holder filled with Fluorinert FC-770 (IoLiTec). A 150 W Xenon short-arc lamp (Ushio) provides light for a monochromator (Cornerstone 260 Oriel, FWHM 16 nm) to achieve a chopped, tunable, monochromatic pump beam. The heat caused through absorption of the pump light in the films changes the refractive index of the Fluorinert. This change is detected by deflecting a probe He-Ne-laser (REO) whose displacement in turn is measured by a position-sensitive-detector (Thorlabs, PDP90A). The magnitude of the deflection is determined by a lock-in amplifier (Amatec SR 7230) and directly correlated to the absorption of the film. 2.5 Perovskite layer characterisation To study the effect of doping and surface modification of ZnO on the perovskite film formation, scanning electron microscopy was conducted using an Ultra FE-SEM Gemini Ultra 55 (Zeiss) microscope with a working distance of 3 mm and an acceleration voltage of 1.6 kV. Samples were mounted on standard SEM holders using conductive silver paste to avoid sample charging. Photoluminescence (PL) measurements were carried out inside an integrating sphere (LabSphere) with
excitation by a 447 nm diode laser (Dragon Lasers). The spectra were recorded using a QE65000 (Ocean Optics) spectrometer.
3. Results and discussion 3.1 Electron transport layer characterisation Figure 1a schematically shows the structure of the planar photovoltaic devices. Solution processed electron extracting ZnO layer was deposited from a sol-gel solution with zinc acetate precursor. To introduce the dopants into the ZnO layer, lithium acetate or caesium carbonate were added to the sol-gel solution. To modify the surface of the electron extraction layer, a self-assembled monolayer (SAM) of [6,6]-Phenyl C61 butyric acid (PCBA) was applied. The chemical structures of the materials are shown in Figure 1b.
Figure 1: (a) Schematic photovoltaic device structure (b) chemical structure of Zinc and Li acetate, Cs carbonate and PCBA. The changes in ZnO composition with doping were investigated by X-ray photoemission spectroscopy (XPS). Figure 2a shows the O1s, Zn2p, Li1s and Cs3d high resolution spectra obtained on ZnO, ZnLiO and ZnCsO films. As expected, ZnO films show no signal from Cs or Li. Li1s and Cs3d peaks can be seen for the ZnLiO
and ZnCsO, respectively, confirming the incorporation of the dopants into the inorganic layer. The atomic percentages of Li and Cs are in agreement with the stoichiometry of the sol-gel precursor (2% Cs and 3% Li). The surface structure of the oxide films was characterised by AFM and is shown in Figure 2b. We find that doping has nearly no effect on the film surface structure with very similar RMS roughness obtained for each layer (~1-2 nm). The films are overall smooth, uniform and pin-hole free, as is desirable for planar solar cell fabrication.
Figure 2: (a) O1s, Zn2p, Li1s, Cs3d XPS spectra measured on ZnO (red), ZnLiO (blue) and ZnCsO (green) (b) AFM micrographs of ZnO, ZnLiO and ZnCsO, the scale bar is 400 nm. The electronic structure of the undoped and doped ZnO layers was characterised by photoemission spectroscopy (UPS). We find that doping has no effect on the electronic energy levels of the metal oxide layer as both the photoemission onset (Figure 3a) and the onset of the valence band (Figure 3b) remain unchanged. As a result, the work function and the ionisation potential are found to be at 3.6 eV and 6.8 eV, respectively, in excellent agreement with previous measurements.45-46 Modification of the oxide surface with a PCBA self-assembled monolayer alters the energetics of the surface. For both undoped and doped ZnO, the photoemission onset shifts by 0.65 eV, resulting in a work function of 4.25 eV. The low energy onset of
the valence band for PCBA modified oxides is found to be 1.9 eV below the Fermi level, resulting in an ionisation potential of 6.15 eV. Previous results on Sr doped ZnO showed that surface properties of the ZnO layer may have an effect on the formation of the PCBA monolayer, with denser monolayers forming on Sr doped ZnO. 47 As the absorption of the PCBA monolayer is too weak to be probed by traditional UV-vis spectroscopy, we employed the more sensitive photothermal deflection spectroscopy (PDS). We find a nearly identical absorption of the monolayer on both undoped and doped ZnO, indicating that the coverage on all surfaces is very similar. The thickness of the PCBA monolayer can be estimated by the attenuation of the Zn2p peak in XPS measurements. We find very similar layer thicknesses (~10-11 Å) of the SAM, in agreement with what is expected for a fullerene monolayer.
Figure 3: UPS spectra of the secondary emission onset of (a) bare and (c) PCBA modified ZnO, ZnLiO and ZnCsO. Valence band regions of (a) bare and (d) PCBA modified ZnO, ZnLiO and ZnCsO. (e) Photothermal deflection spectroscopy (PDS) of ZnO, ZnLiO and ZnCsO (open symbols) and PCBA/ZnO, PCBA/ZnLiO and
PCBA/ZnCsO (filled symbols). (f) Energy diagram summary for bare and PCBA modified ZnO, ZnLiO and ZnCsO. 3.2 Perovskite film formation on engineered ZnO layers The film formation of perovskite on ZnO surfaces has been shown to be problematic due to the decomposition of the perovskite by deprotonation reactions with the intrinsically basic ZnO surface which are accelerated by surface hydroxyl groups or acetate ligands and at elevated temperatures.19 In most reports using solution processed ZnO as the ETL, a two step inter-diffusion method was employed together with a minimized heat treatment after the deposition of the MAI on top of the PbI2 film to limit the magnitude of this degradation process. However, a complete and phase pure conversion to high quality MAPbI3 is hard to attain on planar substrates without a careful control of processing parameters. 48-50 In this work, we employ a simple one step deposition method based on a lead acetate trihydrate precursor. This method has been shown to yield high efficient devices in both, the standard architecture on top of TiO2 and the inverted architecture on top of PEDOT:PSS, with ordered crystalline perovskite grains of high electronic quality.51-52 We modified the annealing conditions using a lower temperature of 80° for 30 min to achieve a complete conversion to perovskite while minimizing heat induced degradation. To characterise the perovskite film formation on the undoped and doped ZnO layers with and without PCBA modification, we imaged the cross section of the perovskite films on the different substrates using scanning electron microscopy (SEM). Figure 4a shows that on ZnO, the perovskite layer consists of small crystallites and exhibits many gaps near the interface with ZnO. Doping of ZnO with Cs or Li does not improve the perovskite film formation (Figure 4b and 4c): the perovskite layers show irregular small grains and large gaps near the oxide surface.
Metal oxide surface modification with the PCBA SAM has a pronounced effect on the perovskite layer structure. For both undoped and doped ZnO, the perovskite layer consists of large uniform grains of approximately 0.5 µm lateral size that protrude throughout the entire film with highly improved interfacial contact. This is particularly beneficial for photovoltaic devices as it ensures that charges are transported through a single grain. 53 - 55 Previous reports employed a PCBM layer (thicknesses between 10 nm to 50 nm) on top of a ZnO layer in order to minimize decomposition of the perovskite layer and improve the film formation and charge extraction.17, 56-58 However, PCBM is soluble in N,N-dimethylformamide (DMF), the most commonly used solvent for perovskite precursors, and will be partly washed away, limiting the applicability of this approach.57,59-60 In contrast, the PCBA SAM is more resistant against the DMF solvent 61 and our results demonstrate that a single monolayer of fullerenes is sufficient to achieve a positive effect on film formation and charge extraction using our one step deposition routine.
Figure 4: Cross section SEM images of perovskite films on bare and PCBA modified ZnO (a, d), ZnCsO (b, e) and ZnLiO (c, f).
3.3 Device Performance To investigate the changes in performance of photovoltaic devices, between 60 and 80 solar cells with each type of electron extraction layer were fabricated and characterised. Figure 5 shows the histograms of the photovoltaic performance parameters obtained from these measurements. Figure 5a shows that the open circuit voltage (VOC) of the devices does not change with the introduction of dopants and only very slightly increases upon modification with PCBA. This is in agreement with the UPS measurements which showed that doping does not alter the energetic structure of the metal oxide. The small increase in VOC for PCBA modified devices is consistent with results reported for PCBA modified TiO2 extraction layers and is likely to be a result of the PCBA layer functioning as a hole blocking layer.61-62 Unlike the VOC, the short circuit current (JSC) (Figure 5b) and fill factor (FF) (Figure 5c) are highly dependent on the electron extraction layer. Doping of ZnO results in a 20-30% increase in the JSC from an average of 10.4 mA/cm2 to 11.8 and 12.6 mA/cm2 for Cs and Li, respectively. This is accompanied by an increase of the average FF, resulting in an overall average power conversion efficiency improvement from 4.7% to 5.6-5.8%, comparable to previous reports of perovskite solar cells that employ ZnO as extraction layer.17 The relatively poor performance of the devices is not surprising considering the structure of the perovskite film imaged by SEM (Figure 4). The gaps at the perovskite/metal oxide interface result in very poor charge extraction lowering both the JSC and FF. We also note that the spread of the photovoltaic performance parameters is rather large, which is likely to be a result of different degree of poor interfacial contact of the perovskite on different areas of the metal oxide.40, 63
(a)
(b)
(c)
(d)
Figure 5: Photovoltaic performance parameters: (a) VOC, (b) JSC, (c) FF and (d) PCE histogram of a total of 414 bare and PCBA modified photovoltaic device. Modification of the ZnO surface with PCBA results in a drastic improvement of the photovoltaic performance. In the case of undoped ZnO, the average JSC is increased by nearly 75% (to 17.5 mA/cm2) and the average FF is significantly improved, rising from 40.3% to 56.8%. These improvements result in an enhancement of the PCE to an average of 10.2% (220% higher than unmodified ZnO). PCBA modified doped ZnO layers show an even better performance. Similarly to the improvements of the unmodified devices, an enhancement in JSC results in an average of 19.5 mA/cm2 and 20.5 mA/cm2 for Cs and Li doped layers, respectively – an increase of approximately 20%. The fill factors also increase to an average of 69-72%. Notably, the distribution
of performance is narrowed, suggesting that upon modification with PCBA the perovskite film formation becomes more reproducible. A maximum power conversion efficiency of 18.02% is achieved for PCBA modified Li doped devices - over double the maximum efficiency of unmodified ZnO. An additional advantage of modifying the properties of the electron extraction layer is a significant reduction in the hysteresis of the devices. Figure S1 shows the J-V curves of devices with and without PCBA measured in the reverse and forward directions. While doping does not seem to effect the hysteresis of the devices, interfacial modification with PCBA results in devices with a smaller degree of hysteresis, in agreement with previous observations of devices that employ a fullerene derivatives.64-65 In addition, doping of ZnO results in a reduction of series resistance in devices with and without PCBA (Table S1). The stability of the devices is also substantially improved with the use of a PCBA monolayer. Figure S3 shows the evolution in power conversion efficiency of Li doped unencapsulated devices both with and without PCBA modification stored in air in the dark at room temperature (~30 % relative humidity) between measurements. The PCE of the device without a PCBA modification gradually decreases and is reduced by ~40% after two weeks of measurements. On the other hand, the PCBA modified devices show very stable performance with only minor variations. This suggests that the improved perovskite layer structure on PCBA modified oxides results in not only enhanced performance, but also enhanced stability, even in a humid environment. Specifically, eliminating the voids at the interface of the ETL and perovskite reduced the ingress of water molecules from the ETL side and thus results in an increased stability.
3.4 Origins of improved device performance To characterise the efficiency of charge extraction by the engineered ZnO layers, we compared the photoluminescence (PL) of the perovskite layer deposited on different electron extraction layers (Figure 6a) and summarised the PL quantum efficiencies for each case in Table S2. The significantly lower PL of the films deposited on PCBA modified oxides indicates that charge extraction in these layers is improved. The improved extraction can also be seen in transient photocurrent measuremenets (Figure 6b). Transport times are dramatically reduced for the devices modified with PCBA, with the doped modified layers showing the shortest times (Table S3). These results are consistent with improved electron extraction and suggest that both the surface properties of the metal oxide layer and the bulk properties (affected by doping) influence the extraction efficiency within the device.
Figure 6: (a) Photoluminescence of perovskite on ZnO, ZnLiO and ZnCsO (empty symbols) and PCBA/ZnO, PCBA/ZnLiO and PCBA/ZnCsO (filled symbols), (b) Transient photocurrent measurements on PCBA modified and unmodified ZnO, ZnCsO and ZnLiO. Our measurements thus far showed that doping does not change the absorption, film microstructure or the energy levels of metal oxide. Modification with PCBA ensures that the surface properties of the undoped and doped metal oxide layers are very similar. This suggests that the improvements in charge extraction and overall performance with doping are related to the bulk properties of the ZnO layer. To
examine these properties we performed XPS depth profiling of the doped oxide layers and followed the evolution of the Zn2p and O1s spectra. For all metal oxide layers, the Zn2p3/2 peak was centred at 1022.4 eV, in agreement with previous measurements. The O1s spectra are shown in Figure 7a,b,c for ZnO, ZnLiO and ZnCsO, respectively. They consist of two peaks – a low binding energy peak at 531 eV, assigned to metal-oxygen bonds and a high binding energy peak at 532.5 eV, which is associated with hydroxyl (O-H) groups.66
Figure 7: O1s spectra of (a) ZnO, (b) ZnLiO and (c) ZnCsO, (d) Percentage of O-H oxygen of total oxygen (e) Oxide stoichiometry (defined as O-Zn/Zn) as a function of etching time. Following the evolution of the O-H peak (Figure 7d) shows that more OH groups are present at the surface than in the bulk of the metal oxide. This is to be expected considering the hydrophilic nature of ZnO. Within the bulk of the film the O-H percentage stabilizes and is ~20% lower for doped ZnO films. Presence of O-H
species has been previously associated with trapping of electrons, 67-68 so a lower concentration of O-H within the doped ZnO layers would suggest a decreased density of traps at the surface and within the bulk of the metal oxide film. The stoichiometry of the films can be estimated by calculating the ratio O-Zn/Zn. The stoichiometry is always smaller than 1 due to a large density of oxygen vacancies as is frequently observed for solution processed ZnO films.
69
We find that the
stoichiometry of the doped films is ~15% higher than the undoped ZnO, demonstrating the overall improved quality of the doped films. Elucidating the exact mechanism of doping is beyond the scope of this work, however we note that our results are in agreement with previous investigations of similar dopants in ZnO extraction layers of organic photovoltaic devices.70-71 These reports suggest that such doping of ZnO reduces the oxygen vacancies within the film (i.e. improves the film stoichiometry) and as a result increases its electron conductivity. Another indication for the density of traps within ZnO films can be obtained by measuring the increase in conductivity under UV-light illumination. Solution processed ZnO layers have many structural defects, resulting in a variety of subbandgap states that can trap charges.72 Illuminating the films with UV-light fills these traps and leads to an increase in the film conductivity,73-74 thus the relative increase in conductivity can be related to the total density of traps. Figure 8 shows that the increase in conductivity of ZnO is significantly larger than that of the doped films, illustrating the lower density of traps within the doped layers.
Figure 8: Enhancement of oxide conductivity upon illumination under a solar simulator. 4. Conclusions To summarise, our results demonstrate that once the bulk and surface properties of solution processed ZnO layers have been engineered, it can serve as an efficient electron extraction layer in perovskite photovoltaic devices. We show that this can be achieved by combining doping of the metal oxide with a modification by a selfassembled monolayer, both of which can be easily performed in solution and at low processing temperatures. Our approach results in a significant improvement in the device performance up to 18%, one of the highest values ever reported employing standard MAPbI3 and solution processed ZnO as the ETL. Our results demonstrate that ZnO can be engineered to be a competitive alternative to TiO2, especially for applications that require low temperatures, such as printed and flexible perovskite photovoltaics.
Acknowledgements We would like to kindly thank Prof. Uwe Bunz for providing access to the device fabrication facilities. We are also grateful to Prof. Annemarie Pucci and Prof. Rasmus
Schroeder for access to AFM and SEM, respectively. Paul Fassl thanks the HGSFP for a scholarship.
References [1] M. Habibi, F. Zabini, M. R. Ahmadian-Yazdi, M. Eslamian, Renew. Sustainable Energy Rev. 2016, 62, 1012. [2] T. Liu, K. Chen, Q. Hu, R. Zhu, Q. Gong, Adv. Energy Mater. 2016, 6, 1600457. [3] D. Zhao, M. Sexton, H.-Y. Park, G. Baure, J. C. Nino, F. So, Adv. Energy Mater. 2015, 5, 1401855. [4] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476. [5] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, Z. Liu, C. Huang, Nano Lett. 2015, 15, 3723−3728. [6] J. H. Kim, P. W. Liang, S. T. Williams, N. Cho, C. C. Chueh, M. S. Glaz, D. S. Ginger, A. K.-Y. Jen, Adv. Mater. 2015, 27, 695. [7] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Grätzel, L. Han, Science,. 2015, 350, 944. [8] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, L. Mora-Sero, J. Bisquert, H. J. Snaith, J. H. Nicholas, Nano. Lett. 2014, 14, 724. [9] N.Fu, C. Huang, Y. Liu, X. Li, W. Lu, L. Zhou, F. Peng, Y. Liu, H. Huang. ACS Appl. Mater. Interfaces 2015, 7, 19431. [10] S. K. Pathak, A. Abate, P. Ruckdeschel, B. Roose, K. C. Gödel, Y. Vaynzof, A. Santhala, S.-I. Watanabe, D. J. Hollman, N. Noel, A. Sepe, U. Wiesner, R. H. Friend, H. J. Snaith, U. Steiner, Adv. Funct. Mater. 2014, 24, 6046. [11] W. Ke, G. Fang, Q. Liu, L. Xiong, P. Qin, H, Tao, J. Wang, H. Lei, B. Li, J. Wan, G. Yang. Y. Yan, J. Am. Chem. Soc. 2015,137, 6730. [12] J. Song, E. Zheng, J. Bian, X.-F. Wang, W. Tian, Y. Sanehira, T. Miyasaka, J. Mater. Chem. A 2015, 3, 10837. [13] G. S. David, P. McMeekin, W. Rehman, G. E. Eperon, M. T. H. Michael Saliba, A. Haghighirad, N. Sakai, B. R. Lars Korte, M. B. Johnston, L. M. Herz , H. J. Snaith, Science, 2016, 351, 151.
[14] J. P. C. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, F. Giordano, T. J. Jacobsson, A. R. Srimath Kandada, S. M. Zakeeruddin, A. Petrozza, A. Abate, M. K. Nazeeruddin, M. Grätzel, A. Hagfeldt, Energy Environ. Sci. 2015, 8, 2928. [15] E. H. Anaraki, A. Kermanpur, L. Steier, K. Domanski, T. Matsui, W. Tress, M. Saliba, A. Abate, M. Grätzel, A. Hagfeldt, J.-P. Correa-Baena, Energy Environ. Sci. 2016, 9, 3128. [16] D. Y. Liu, T. L. Kelly, Nat. Photonics 2014, 8, 133. [17] Y. Cheng, Q.-D. Yang, J. Xiao, Q. Xue, H.-W. Li, Z. Guan, H.-L. Yip, S.-W. Tsang, ACS Appl. Mater. Interfaces 2015, 7, 19986. [18] Y. Dkhissi, S. Meyer, D. Chen, H. C. Weerasinghe, L. Spiccia, Y.-B. Cheng, R. A. Caruso, ChemSusChem 2016, 9 (7), 687. [19] J. Yang, B. D. Siempelkamp, E. Mosconi, F. De Angelis, T. L. Kelly, Chem. Mater. 2015, 27, 4229. [20] F. Giordano, A. Abate, J. P. Correa Baena, M. Saliba, T. Matsui, S. H. Im, S. M. Zakeeruddin, M. K. Nazeeruddin, A. Hagfeldr, M. Graetzel, Nat. Comm. 2016, 7, 10379. [21 .
, .
, .
, .
, .
, .
, Electrochim. Acta 2016, 195, 143.
[22] H. Zhang, J. Shi, X. Xu, L. Zhu, Y. Luo, D. Li, Q. Meng, J. Mater. Chem. A 2016, 4, 15383. [23] D. H. Kim, G. S. Han, W. M. Seong, J.-W. Lee, B. J. Kim, N.-G. Park, K. S. Hong, S. Lee, H. S. Jung, ChemSusChem 2015, 8 (14), 2392. [24] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [25] H.-H. Wang, Q. Chen, H. Zhou, L. Song, Z. St Louis, N. De Marco, Y. Fang, P. Sun, T.-B. Song, H. Chen, Y. Yang, J. Mater. Chem. A 2015, 3, 9108. [26] M. Lv, W. Lv, X. Fang, P. Sun, B. Lin, S. Zhang, X. Xu, J. Ding, N. Yuan, RSC Adv. 2016, 6, 35004. [27] L. F. Zhu, Y. Z. Xu, J. J. Shi, H. Y. Zhang, X. Xu, Y. H. Zhao, Y. H. Luo, Q. B. Meng, D. M. Li, RCS Advances, 2016, 6, 82282. [28] J. Cao, J. Yin, S. Yoan, Y. Zhao, J. Li, N. Zheng, Nanoscale 2015, 7, 9443. [29] Y. C. Shih, L. Y. Wang, H. C. Hsieh, K. F. Lin, J. Mater. Chem. A 2015, 3, 9133.
[30] K. Wojciechowski, S. D. Stranks, A. Abate, G. Sadoughi, A. Sadhanala, N. Kopidakis, G. Rumbles, X.-Z. Li, R. H. Friend, A. K.-Y. Jen, H. J. Snaith, ACS Nano 2014, 9 (12), 12701. [31] J. Kim, G. Kim, T.-K. Kim, S. Kwon, H. Back, J. Lee, S.-H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [32] H. P. Dong, Y. Li, S. F. Wang, W. Z. Li, N. Li, X. D. Guo, L. D. Wang, J. Mater. Chem. A 2015, 3, 9999. [33] W. Li, W. Zhang, S. Van Reenen, R. J. Sutton, J. Fan, A. A. Haghighirad, M. B. Johnston, L. Wang, H. J. Snaith, Energy Environ. Sci. 2016, 9, 490. [34] H. Dong, X. Guo, W. Li, L. Wang, RCS Adv. 2014, 4, 60131. [35] X. Guo, H. Dong, W. Li, N. Li, L. Wang, ChemPhysChem 2015, 16, 1727. [36] H.-W. Kang, J.-W. Lee, D.-Y. Son, N.-G. Park, RSC Adv. 2015, 47334. [37] X. Zhao, H. Shen, Y. Zhang, X. Li, X. Zhao, M. Tai, J. Li, J. Li, X. Li, H. Lin, ACS Appl. Mater. Interfaces 2016, 8 (12), 7826. [38] Z.-L. Tseng, C.-H. Chiang, S.-H. Chang, C.-G. Wu, Nano Energy 2016, 28, 311. [39] M. A. Mahmud, N. K. Elumalai, M. B. Upama, D. Wang, M. Wright, T. Sun, C. Xu, F. Haque, A. Uddin, RSC Adv. 2016, 6, 86108. [40] L. Zuo, Z. Gu, T. Ye, W. Fu, G. Wu, H. Li, H. Chen, J. Am. Chem. Soc. 2015, 137, 2674. [41] J. Kim, G. Kim, T. K. Kim, S. Kwon, H. Back, J. Lee, S. H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [42] J. Zhang, T. Pauporté, J. Phys. Chem. C 2015, 119, 14919. [43] X. Dong, H. Hu, B. Lin, J. Ding, N. Yuan, Chem. Commun. 2014, 50, 14405. [44] Y. Vaynzof, D. Kabra, L. Zhao, P. K. H. Ho, A. T. S. Wee, R. H. Friend, Appl. Phys. Lett. 2010, 97 (3), 156. [45] Y. Sevinchan, P. E. Hopkinson, A. A. Bakulin, J. Herz, M. Motzkus, Y. Vaynzof, Adv. Mater. Interfaces 2016, 3, 1500616. [46] C. Hinzmann, O. Magen, Y. J. Hofstetter, P. E. Hopkinson, N. Tessler, Y. Vaynzof, ACS Appl. Mater. Interfaces 2017, 9 (7), 6220.
[47] O. Pachoumi, A. A. Bakulin, A. Sadhanala, H. Sirringhaus, R. H. Friend, Y. Vaynzof, J. Phys. Chem. C 2014, 118 (33), 18945. [48] C. Bi, Y. Shao, Y. Yuan, Z. Xiao, C. Wang, J. Mater. Chem. A 2014, 2, 18508. [49] C. Bi, Q. Wang, Y. Shao, Y. Yuan, Z. Xiao, J. Huang, Nat. Commun. 2015, 6, 7747. [50] J. B. Patel, R. L. Milot, A. D. Wright, L. M. Herz, M. B. Johnston, J. Phys. Chem. Lett. 2015, 7, 96.# [51] S. Bai, N. Sakai, W. Zhang, Z. Wang, J. T.-W. Wang, F. Gao, H. J. Snaith, Chem. Mater. 2017, 29, 462. [52] W. Zhang, S. Pathak, N. Sakai, T. Stergiopoulos, P. K. Nayak, N. K. Noel, A. A. Haghighirad, V. M. Burlakov, D. W. DeQuilettes, A. Sadhanala, W. Li, L. Wang, D. S. Ginger, R. H. Friend, H. J. Snaith, Nat. Commun. 2015, 6, 10030. [53] F. Zhang, J. Song, L. Zhang, F. Niu, Y. Hao, P. Zeng, H. Niu, J. Huang, J. Lian, J. Mater. Chem. A 2016, 4, 8554. [54] J. S. Yun, A. Ho-Baillie, S. Huang, S. H. Woo, Y. Heo, J. Seidel, F. Huang, Y.-B. Cheng, M. a. Green, J. Phys. Chem. Lett. 2015, 6, 875. [55] J. Li, J.-Y. Ma, Q.-Q. Ge, J. Hu, D. Wang, L.-J. Wan, ACS Appl. Mater. Interfaces 2015, 7, 28518. [56] J. Kim, G. Kim, T. K. Kim, S. Kwon, H. Back, J. Lee, S. H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [57] F. Fu, T. Feuer, T. Jäger, E. Avancini, B. Bissig, S. Yoon, S. Buecheler and A. N. Tiwari, Nat. Comm. 2015, 6, 8932. [58] J. H. Kim, C.-C. Chueh, S. T. Williams, A. K.-Y. Jen, Nanoscale 2015, 7, 17343. [59] S. Ryu, J. Seo, S. S. Shin, Y. C. Kim, N. J. Jeon, J. H. Noh, S. Il Seok, J. Mater. Chem. A 2015, 3, 3271. [60] W. Qiu, J. P. Bastos, S. Dasgupta, T. Merckx, I. Cardinaletti, M. Jenart, C. Nielsen, R. Gehlhaar, J. Poortmans, P. Heremans, I. Mcculloch, D. Cheyns, J. Mater. Chem. A 2017, 5, 2466. [61] Y. Dong, W. Li, X. Zhang, Q. Xu, Q. Liu, C. Li, Z. Bo, Small 2016, 12, 1098. [62] T. Ma, D. Tadaki, M. Sakuraba, S. Sato, A. Hirano-Iwata, M. Niwano, J. Mater. Chem. A 2016, 4, 4392. [63] S. D. Stranks, H. J. Snaith, Nat. Nanotechnol. 2015, 10, 391.
[64] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Comm. 2014, 5, 5784. [65] J. H. Hei, H. J. Han, D. Kim, T. K. Ahn, S. H. Im, Energy Environ. Sci., 2015, 8, 1602. [66] E. Kim, Y. Vaynzof, A. Sepe, S. Guldin, M. Scherer, P. Cunha, S. V. Roth, U. Steiner, Adv. Funct. Mater. 2014, 24, 863. [67] X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang, R. Liu, Sci. Rep. 2014, 4, 4596. [68] O. Pachoumi, A. A. Bakulin, A. Sadhanala, H. Sirringhaus, R. H. Friend, Y. Vaynzof, J. Phys. Chem. C 2014, 118 (33), 18945. [69] M. Krzywiecki, L. Grzadziel, A. Sarfraz, D. Iqbal, A. Szwajce, A. Erbe, Phys. Chem. Chem. Phys. 2015, 17, 10004. [70] H. Peng, W. Xu, F. Zhou, J. Zhang, C. Li, Synth. Met. 2015. 205, 164. [71] L. Zuo, S. Zhang, S. Dai, H. Chen, RCS Adv. 2015, 5, 49369. [72] V. Ischenko, S. Polarz, D. Grote, V. Stavarache, K. Fink, M. Driess, Adv. Funct. Mater. 2005, 15, 1945. [73] O. Pachoumi, C. Li, Y. Vaynzof, K. K. Banger, H. Sirringhaus, Adv. Energy. Mater. 2013, 3, 1428. [74] C. Wang, C. Li, S. Wen, P. Ma, Y. Liu, R. C. I. MacKenzie, W. Tian, S. Ruan, J. Mater. Chem. A 2017, DOI: 10.1039/C7TA00229G
Qingzhi An is a Ph.D student in Kirchhoff Institute for Physics and the Centre for Advanced Materials of Heidelberg University under the supervision of Prof. Yana Vaynzof. She received her B.Sc. from Dalian University of Technology and her M.Sc. in Physics from Heidelberg University. She is currently working on functional interface and extraction layer modifications in hybrid solar cells.
Paul Fassl received his B.Sc. and M.Sc. degrees in Physics from Heidelberg University in 2010 and 2013, respectively. In his master thesis at the InnovationLab GmbH, his research focused on the optimization of graded mixed host organic light-emitting diodes. Since 2014, he is pursuing his Ph.D. in the group of Prof. Yana Vaynzof in Heidelberg, where his research is mainly focused on understanding environmental effects and degradation phenomena in organic-inorganic hybrid perovskite materials and devices. Paul is the recipient of Heidelberg Graduate School of Fundamental Physics (HGSFP) scholarship.
Yvonne Jasmin Hofstetter received her B.Sc. degree in Physics from Heidelberg University in 2015. She is now a master student under supervision of Prof. Yana Vaynzof. Her research focuses on modification of interfaces in hybrid photovoltaics and characterisation of polymers and metal oxides using x-ray and ultraviolet photoemission spectroscopies.
David Becker-Koch received his B.Sc. degree in Physics from the University of Zurich in 2014. He continued his studies at the University of Heidelberg where he joined the group 'Physical Principles of Organic and Hybrid Photovoltaics' lead by Prof. Dr. Yana Vaynzof working on photothermal deflection spectroscopy (PDS). David graduated in 2016 with a M.Sc. in Physics and in 2017 started his Ph.D. under Prof. Vaynzof focusing on the research of new interface modification layers for thin film solar cells.
Alexandra Bausch received her B.S. degree in Physics in 2015 from University of Constance and carried out her Bachelor thesis at the Molecular Imaging Center at National Taiwan University. Currently, she is pursuing her M.S. degree in Physics at Heidelberg University under the supervision of Prof. Yana Vaynzof. Her research interests include studying the effect of film fabrication parameters on optical properties of perovskite materials, as well as degradation of perovskite layers.
Paul Hopkinson obtained his PhD in physics from the University of Cambridge under the supervision of Professor Dame Athene Donald. He then joined the Cambridge Optoelectronics group as a research associate, focussing on degradation of organic photovoltaics. At the University of Heidelberg he was the senior research associate in the Vaynzof research group, responsible for a range of technical activities. Paul is now a science and technology consultant at the Science Group PLC (UK).
Since 2014, Yana Vaynzof is a Juniorprofessor at Heidelberg University (Germany), where she leads the ‘Physical Principles of Organic and Hybrid Photovoltaics’ group. She received a B.Sc degree (summa cum laude) in electrical engineering from the Technion - Israel Institute of Technology (Israel) in 2006, and a M.Sc. degree in electrical engineering from Princeton University (USA) in 2008. In 2011, she obtained a Ph.D. degree in physics under the supervision of Prof. Sir. Richard Friend at the Optoelectronics Group, Cavendish Laboratory, University of Cambridge (UK) and joined the Microelectronics group at the University of Cambridge as a Postdoctoral Research Associate focusing on the study of surfaces and interfaces in organic and hybrid optoelectronics. Yana Vaynzof was the recipient of a number of fellowships and awards, including the Pinzi Award for Academic Excellence, Knesset (Israeli Parliament) Award for contribution to the Israeli Society, Gordon Y. Wu Fellowship, Henry Kressel Fellowship and the ERC Starting Grant.
Research highlights:
The bulk and surface properties of ZnO extraction layers are engineered using doping and a modification with a self-assembled monolayer. This complete engineering approach results in tripling of the average device power conversion efficiency reaching up to 18%, accompanied by reduced hysteresis and improved stability. The enhancement in performance originates from improved perovskite film formation, more efficient electron charge extraction and reduced density of traps in the bulk of the oxide layer. All modifications are performed in solution at low temperature and can be applied to printed perovskite solar cells or those fabricated on flexible substrates.
Graphicala abstract
PII: DOI: Reference:
S2211-2855(17)30423-8 http://dx.doi.org/10.1016/j.nanoen.2017.07.013 NANOEN2071
To appear in: Nano Energy Received date: 24 April 2017 Revised date: 3 July 2017 Accepted date: 6 July 2017 Cite this article as: Qingzhi An, Paul Fassl, Yvonne J. Hofstetter, David BeckerKoch, Alexandra Bausch, Paul E. Hopkinson and Yana Vaynzof, High performance planar perovskite solar cells by ZnO electron transport layer engineering, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.07.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
High performance planar perovskite solar cells by ZnO electron transport layer engineering
Qingzhi An1,2, Paul Fassl1,2, Yvonne J. Hofstetter1,2, David Becker-Koch1,2, Alexandra Bausch,1,2 Paul E. Hopkinson1,2 and Yana Vaynzof1,2* 1 Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, Heidelberg University, Germany 2
Centre for Advanced Materials, Im Neuenheimer Feld 225, Heidelberg University, Germany *Corresponding author. [email protected] Abstract ZnO as electron extraction layer in photovoltaic devices has many advantages, including high mobility and low processing temperature. However, it has been underutilized in perovskite solar cells due to the reported instabilities of perovskite layers deposited on ZnO resulting in poor device performance. Herein, we modify the ZnO layer by incorporating Cs or Li dopants in its bulk and depositing a selfassembled monolayer on its surface. This combined approach of engineering both the bulk and surface properties of ZnO results in significant improvements in the performance of planar MAPbI3 perovskite solar cells with a maximum power conversion efficiency of 18%, accompanied by a reduction in hysteresis and a significant enhancement of the device stability. Our work makes engineered solutionprocessed ZnO layers a practical alternative to TiO2 as electron extraction layers in perovskite solar cells, while also eliminating the need for high temperature sintering steps from the device fabrication. Keywords: Perovskite solar cells, extraction layer engineering, ZnO 1. Introduction Planar perovskite solar cells can be made in two architectures commonly referred to as ‘standard’ and ‘inverted’ depending on the order of deposition of hole and electron transport layers that sandwich the perovskite active layer. In a standard architecture, an electron transport layer (ETL) - typically an n-type oxide - is deposited first,
followed by the perovskite and hole transport layer (HTL). 1 In an inverted architecture, the HTL is deposited first - most commonly a conducting polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) - followed by the perovskite layer and ETL, typically a fullerene derivative. 2 Significant scientific efforts have been devoted to developing various HTLs including polymeric hole transporters such as poly[N,N-bis(4-butylphenyl)-N,N-bis(phenyl)-benzidine] (polyTPD) and poly(triarylamine) (PTAA), 3 - 4 or inorganic hole transporters such as CuSCN5 and NiO.6-7 ETL research has focused on metal oxides such as TiO2, ZnO and SnO2. While TiO2 is by far the most commonly used ETL, it has several significant disadvantages. TiO2 layers require a sintering step at high temperature of ~450 oC, rendering them unsuitable for deposition on indium tin oxide (ITO) or flexible plastic substrates. 8 Fu et al has shown that lowering the processing temperatures for the formation of TiO2 is possible, however this results in a reduced power conversion efficiency of only 9.95%. 9 Moreover, TiO2 has a lower electron mobility than other ETLs (for example, ZnO) and many surface and bulk defects that adversely affect the performance and stability of the photovoltaic devices.10 Unlike TiO2, ZnO and SnO2 do not require a sintering step at high temperature and have also been investigated as electron extraction layers in perovskite photovoltaic devices. Recent reports have showed that devices with a SnO2 as an electron extraction layer can result in efficiencies comparable or even surpassing those of TiO2.11-15 In the case of ZnO, despite the first promising report of highly efficient solar cells,16 subsequent investigations revealed that perovskite films deposited on ZnO suffer from severe decomposition due to the presence of hydroxyl groups or acetate ligands on the metal oxide surface.17-18 Even in the absence of these groups, the basic nature of ZnO can
result in perovskite decomposition due to proton transfer reactions taking place at the perovskite/ZnO interface.19 Two types of modification of the electron extraction layer have been investigated previously: doping the metal oxide layer, or using an interlayer to modify its surface properties. In the case of the widely used TiO2 ETL, a variety of dopants has been investigated, including Al,10 Li, 20 W, 21 Mg, 22 Nb, 23 Y, 24 and other additives. 25 - 26 Various interfacial modifiers have also been studied including benzoic acid derivatives, 27 thiols, 28 glycine, 29 C60 derivatives, 30 - 31 polyoxyethylene (PEO), 32 caesium bromide (CsBr), 33 caesium Carbonate (Cs2CO3), 34 MgO 35 and ZrO2. 36 Significantly less attention has been devoted to ZnO as extraction layer due to the previously mentioned instabilities observed for perovskite films deposited on ZnO. Despite this, promising results were shown in the case of Al doped ZnO 37-39 or 3aminopropanioc acid modified ZnO, 40 where an increase in performance was reported. Interestingly, it was observed that the deposition method of the ZnO plays a critical role in determining the final performance of the device. Various methods such as deposition from sol-gel, 41 chemical bath deposition (CBD), 42 atomic layer deposition
43
and sputtering38 have all been investigated. Even with these
modifications, the performance of perovskite solar cells that utilize ZnO as ETL is significantly lower than that of comparable devices on TiO2 or SnO2. Specifically, devices made using solution processed ZnO from sol-gel show at best power conversion efficiencies of 11%,41 and those processed from ZnO nanoparticles a maximum of 15.7%.16 In this work, we report a complete engineering of both the bulk and surface properties of a sol-gel ZnO ETL, by combining doping of the ZnO layer and surface modification with a self-assembled monolayer, both of which are realised using solution processing at relatively low temperatures. This approach allows us to
achieve efficiencies up to 18% with the standard perovskite MAPbI3, the highest reported thus far for solution processed ZnO, and comparable to devices made on TiO2 or SnO2. 2. Experimental Section 2.1 Materials Methylammonium iodide (CH3NH3I) was purchased from Dyesol. Spiro-oMeTAD (99.7%) was purchased from Borun New Material Technology Co., Ltd. PCBA powders (99%) were purchased from Solenne BV. All other materials were purchased from Sigma-Aldrich and used as received. 2.2 Device Fabrication Pre-patterned indium tin oxide (ITO) coated glass substrates (PsiOTech Ltd., 15 Ohm/sqr) were cleaned sequentially with 2 % hellmanex detergent, deionized water, acetone, and isopropanol, followed by 10 min oxygen plasma treatment. A ZnO sol-gel was prepared following a previously reported recipe.44 Doping with Cs or Li was accomplished by adding either caesium carbonate or lithium acetate to the solgel solution in 2% and 3% MR, respectively. These molar ratios were chosen as they yielded the most efficient devices. Immediately after plasma cleaning, ZnO solution (with or without dopants) was spin-coated at 2000 rpm for 45 s and annealed for 30 min at 200 °C. Afterwards, the substrates were modified with a SAM of PCBA following previously reported procedure. 44 For the perovskite layer a lead acetate trihydrate recipe was used: CH3NH3I and Pb(Ac)2·3(H2O) (3:1, molar ratio) were dissolved in anhydrous N, N –dimethylformamide (DMF) with a concentration of 40 wt% with the addition of hypophosphorous acid solution (6 µl / 1 ml DMF). The perovskite solution was spin coated at 2000 rpm for 60 s in a drybox (RH < 0.5 %). After drying for 5 min, the as-spun films were annealed at 80 °C for 30 min.
Subsequently, spiro-MEOTAD solution (80 mg Spiro-OMeTAD dissolved in 1ml chlorebenze with 17.3µl Li-TFSI (520 mg/ml acetonitrile) and 28.5 µl 4-tertbutylpyridine) was spin coated on top of the perovskite layers at 2000 rpm for 45 seconds. To complete the device, a 80 nm silver electrode was deposited via thermal evaporation under high vacuum. 2.3 Device Characterisation The current density-voltage (J-V) measurements were performed under simulated AM 1.5 sunlight at 100 mW cm-2 irradiance (Abet Sun 3000 Class AAA solar simulator) with a Keithley 2450 Source Measure Unit. The light intensity was calibrated with a Si reference cell (NIST traceable, VLSI) and corrected by measuring the spectral mismatch between the solar spectrum, the spectral response of the perovskite solar cell and the reference cell. The mismatch factor was calculated to be around 10 %. The cells were scanned from forward bias to short circuit and back at a rate of 0.15 V s-1 and 0.025 V s-1 after holding under illumination at 1.2 V for 2 s. To eliminate the contribution of the area surrounding each pixel, a mask that covers these areas from illumination was employed. For transient photocurrent measurements, the light of an inorganic LED (Thorlabs TO-1 ¾, λ = 465 nm) was pulsed by a function generator (Agilent/Keysight 33510B; pulse length = 2 ms) and focused on the solar cell. The resulting photocurrent was measured with an oscilloscope (Picoscope 5443A) with a 50 Ω terminator placed across the oscilloscope input. 2.4 ZnO layer characterisation ZnO samples for XPS/UPS measurements were prepared as described above on ITO substrates and then transferred to the ultrahigh vacuum chamber of the XPS system (Thermo Scientific ESCALAB 250Xi). The XPS measurements were performed
using a XR6 monochromated Al Kα source (h = 1486.6 eV) and a pass energy of 20 eV. UPS measurements were carried out using a He discharge lamp (h = 21.2 eV) and a pass energy of 2 eV. Measurements were collected at three different spots for each sample. XPS depth profiling was performed using an Ar cluster source with cluster energy of 4000 eV. AFM (Bruker MultiMode) was performed in tapping mode in air with silicon tips (Bruker NTESPA) to study the surface morphology of the undoped and doped ZnO films. Layers for PDS characterization were prepared on spectrosil in an identical fashion to those for photovoltaic devices. In the N2 glovebox the samples were placed into a sample holder filled with Fluorinert FC-770 (IoLiTec). A 150 W Xenon short-arc lamp (Ushio) provides light for a monochromator (Cornerstone 260 Oriel, FWHM 16 nm) to achieve a chopped, tunable, monochromatic pump beam. The heat caused through absorption of the pump light in the films changes the refractive index of the Fluorinert. This change is detected by deflecting a probe He-Ne-laser (REO) whose displacement in turn is measured by a position-sensitive-detector (Thorlabs, PDP90A). The magnitude of the deflection is determined by a lock-in amplifier (Amatec SR 7230) and directly correlated to the absorption of the film. 2.5 Perovskite layer characterisation To study the effect of doping and surface modification of ZnO on the perovskite film formation, scanning electron microscopy was conducted using an Ultra FE-SEM Gemini Ultra 55 (Zeiss) microscope with a working distance of 3 mm and an acceleration voltage of 1.6 kV. Samples were mounted on standard SEM holders using conductive silver paste to avoid sample charging. Photoluminescence (PL) measurements were carried out inside an integrating sphere (LabSphere) with
excitation by a 447 nm diode laser (Dragon Lasers). The spectra were recorded using a QE65000 (Ocean Optics) spectrometer.
3. Results and discussion 3.1 Electron transport layer characterisation Figure 1a schematically shows the structure of the planar photovoltaic devices. Solution processed electron extracting ZnO layer was deposited from a sol-gel solution with zinc acetate precursor. To introduce the dopants into the ZnO layer, lithium acetate or caesium carbonate were added to the sol-gel solution. To modify the surface of the electron extraction layer, a self-assembled monolayer (SAM) of [6,6]-Phenyl C61 butyric acid (PCBA) was applied. The chemical structures of the materials are shown in Figure 1b.
Figure 1: (a) Schematic photovoltaic device structure (b) chemical structure of Zinc and Li acetate, Cs carbonate and PCBA. The changes in ZnO composition with doping were investigated by X-ray photoemission spectroscopy (XPS). Figure 2a shows the O1s, Zn2p, Li1s and Cs3d high resolution spectra obtained on ZnO, ZnLiO and ZnCsO films. As expected, ZnO films show no signal from Cs or Li. Li1s and Cs3d peaks can be seen for the ZnLiO
and ZnCsO, respectively, confirming the incorporation of the dopants into the inorganic layer. The atomic percentages of Li and Cs are in agreement with the stoichiometry of the sol-gel precursor (2% Cs and 3% Li). The surface structure of the oxide films was characterised by AFM and is shown in Figure 2b. We find that doping has nearly no effect on the film surface structure with very similar RMS roughness obtained for each layer (~1-2 nm). The films are overall smooth, uniform and pin-hole free, as is desirable for planar solar cell fabrication.
Figure 2: (a) O1s, Zn2p, Li1s, Cs3d XPS spectra measured on ZnO (red), ZnLiO (blue) and ZnCsO (green) (b) AFM micrographs of ZnO, ZnLiO and ZnCsO, the scale bar is 400 nm. The electronic structure of the undoped and doped ZnO layers was characterised by photoemission spectroscopy (UPS). We find that doping has no effect on the electronic energy levels of the metal oxide layer as both the photoemission onset (Figure 3a) and the onset of the valence band (Figure 3b) remain unchanged. As a result, the work function and the ionisation potential are found to be at 3.6 eV and 6.8 eV, respectively, in excellent agreement with previous measurements.45-46 Modification of the oxide surface with a PCBA self-assembled monolayer alters the energetics of the surface. For both undoped and doped ZnO, the photoemission onset shifts by 0.65 eV, resulting in a work function of 4.25 eV. The low energy onset of
the valence band for PCBA modified oxides is found to be 1.9 eV below the Fermi level, resulting in an ionisation potential of 6.15 eV. Previous results on Sr doped ZnO showed that surface properties of the ZnO layer may have an effect on the formation of the PCBA monolayer, with denser monolayers forming on Sr doped ZnO. 47 As the absorption of the PCBA monolayer is too weak to be probed by traditional UV-vis spectroscopy, we employed the more sensitive photothermal deflection spectroscopy (PDS). We find a nearly identical absorption of the monolayer on both undoped and doped ZnO, indicating that the coverage on all surfaces is very similar. The thickness of the PCBA monolayer can be estimated by the attenuation of the Zn2p peak in XPS measurements. We find very similar layer thicknesses (~10-11 Å) of the SAM, in agreement with what is expected for a fullerene monolayer.
Figure 3: UPS spectra of the secondary emission onset of (a) bare and (c) PCBA modified ZnO, ZnLiO and ZnCsO. Valence band regions of (a) bare and (d) PCBA modified ZnO, ZnLiO and ZnCsO. (e) Photothermal deflection spectroscopy (PDS) of ZnO, ZnLiO and ZnCsO (open symbols) and PCBA/ZnO, PCBA/ZnLiO and
PCBA/ZnCsO (filled symbols). (f) Energy diagram summary for bare and PCBA modified ZnO, ZnLiO and ZnCsO. 3.2 Perovskite film formation on engineered ZnO layers The film formation of perovskite on ZnO surfaces has been shown to be problematic due to the decomposition of the perovskite by deprotonation reactions with the intrinsically basic ZnO surface which are accelerated by surface hydroxyl groups or acetate ligands and at elevated temperatures.19 In most reports using solution processed ZnO as the ETL, a two step inter-diffusion method was employed together with a minimized heat treatment after the deposition of the MAI on top of the PbI2 film to limit the magnitude of this degradation process. However, a complete and phase pure conversion to high quality MAPbI3 is hard to attain on planar substrates without a careful control of processing parameters. 48-50 In this work, we employ a simple one step deposition method based on a lead acetate trihydrate precursor. This method has been shown to yield high efficient devices in both, the standard architecture on top of TiO2 and the inverted architecture on top of PEDOT:PSS, with ordered crystalline perovskite grains of high electronic quality.51-52 We modified the annealing conditions using a lower temperature of 80° for 30 min to achieve a complete conversion to perovskite while minimizing heat induced degradation. To characterise the perovskite film formation on the undoped and doped ZnO layers with and without PCBA modification, we imaged the cross section of the perovskite films on the different substrates using scanning electron microscopy (SEM). Figure 4a shows that on ZnO, the perovskite layer consists of small crystallites and exhibits many gaps near the interface with ZnO. Doping of ZnO with Cs or Li does not improve the perovskite film formation (Figure 4b and 4c): the perovskite layers show irregular small grains and large gaps near the oxide surface.
Metal oxide surface modification with the PCBA SAM has a pronounced effect on the perovskite layer structure. For both undoped and doped ZnO, the perovskite layer consists of large uniform grains of approximately 0.5 µm lateral size that protrude throughout the entire film with highly improved interfacial contact. This is particularly beneficial for photovoltaic devices as it ensures that charges are transported through a single grain. 53 - 55 Previous reports employed a PCBM layer (thicknesses between 10 nm to 50 nm) on top of a ZnO layer in order to minimize decomposition of the perovskite layer and improve the film formation and charge extraction.17, 56-58 However, PCBM is soluble in N,N-dimethylformamide (DMF), the most commonly used solvent for perovskite precursors, and will be partly washed away, limiting the applicability of this approach.57,59-60 In contrast, the PCBA SAM is more resistant against the DMF solvent 61 and our results demonstrate that a single monolayer of fullerenes is sufficient to achieve a positive effect on film formation and charge extraction using our one step deposition routine.
Figure 4: Cross section SEM images of perovskite films on bare and PCBA modified ZnO (a, d), ZnCsO (b, e) and ZnLiO (c, f).
3.3 Device Performance To investigate the changes in performance of photovoltaic devices, between 60 and 80 solar cells with each type of electron extraction layer were fabricated and characterised. Figure 5 shows the histograms of the photovoltaic performance parameters obtained from these measurements. Figure 5a shows that the open circuit voltage (VOC) of the devices does not change with the introduction of dopants and only very slightly increases upon modification with PCBA. This is in agreement with the UPS measurements which showed that doping does not alter the energetic structure of the metal oxide. The small increase in VOC for PCBA modified devices is consistent with results reported for PCBA modified TiO2 extraction layers and is likely to be a result of the PCBA layer functioning as a hole blocking layer.61-62 Unlike the VOC, the short circuit current (JSC) (Figure 5b) and fill factor (FF) (Figure 5c) are highly dependent on the electron extraction layer. Doping of ZnO results in a 20-30% increase in the JSC from an average of 10.4 mA/cm2 to 11.8 and 12.6 mA/cm2 for Cs and Li, respectively. This is accompanied by an increase of the average FF, resulting in an overall average power conversion efficiency improvement from 4.7% to 5.6-5.8%, comparable to previous reports of perovskite solar cells that employ ZnO as extraction layer.17 The relatively poor performance of the devices is not surprising considering the structure of the perovskite film imaged by SEM (Figure 4). The gaps at the perovskite/metal oxide interface result in very poor charge extraction lowering both the JSC and FF. We also note that the spread of the photovoltaic performance parameters is rather large, which is likely to be a result of different degree of poor interfacial contact of the perovskite on different areas of the metal oxide.40, 63
(a)
(b)
(c)
(d)
Figure 5: Photovoltaic performance parameters: (a) VOC, (b) JSC, (c) FF and (d) PCE histogram of a total of 414 bare and PCBA modified photovoltaic device. Modification of the ZnO surface with PCBA results in a drastic improvement of the photovoltaic performance. In the case of undoped ZnO, the average JSC is increased by nearly 75% (to 17.5 mA/cm2) and the average FF is significantly improved, rising from 40.3% to 56.8%. These improvements result in an enhancement of the PCE to an average of 10.2% (220% higher than unmodified ZnO). PCBA modified doped ZnO layers show an even better performance. Similarly to the improvements of the unmodified devices, an enhancement in JSC results in an average of 19.5 mA/cm2 and 20.5 mA/cm2 for Cs and Li doped layers, respectively – an increase of approximately 20%. The fill factors also increase to an average of 69-72%. Notably, the distribution
of performance is narrowed, suggesting that upon modification with PCBA the perovskite film formation becomes more reproducible. A maximum power conversion efficiency of 18.02% is achieved for PCBA modified Li doped devices - over double the maximum efficiency of unmodified ZnO. An additional advantage of modifying the properties of the electron extraction layer is a significant reduction in the hysteresis of the devices. Figure S1 shows the J-V curves of devices with and without PCBA measured in the reverse and forward directions. While doping does not seem to effect the hysteresis of the devices, interfacial modification with PCBA results in devices with a smaller degree of hysteresis, in agreement with previous observations of devices that employ a fullerene derivatives.64-65 In addition, doping of ZnO results in a reduction of series resistance in devices with and without PCBA (Table S1). The stability of the devices is also substantially improved with the use of a PCBA monolayer. Figure S3 shows the evolution in power conversion efficiency of Li doped unencapsulated devices both with and without PCBA modification stored in air in the dark at room temperature (~30 % relative humidity) between measurements. The PCE of the device without a PCBA modification gradually decreases and is reduced by ~40% after two weeks of measurements. On the other hand, the PCBA modified devices show very stable performance with only minor variations. This suggests that the improved perovskite layer structure on PCBA modified oxides results in not only enhanced performance, but also enhanced stability, even in a humid environment. Specifically, eliminating the voids at the interface of the ETL and perovskite reduced the ingress of water molecules from the ETL side and thus results in an increased stability.
3.4 Origins of improved device performance To characterise the efficiency of charge extraction by the engineered ZnO layers, we compared the photoluminescence (PL) of the perovskite layer deposited on different electron extraction layers (Figure 6a) and summarised the PL quantum efficiencies for each case in Table S2. The significantly lower PL of the films deposited on PCBA modified oxides indicates that charge extraction in these layers is improved. The improved extraction can also be seen in transient photocurrent measuremenets (Figure 6b). Transport times are dramatically reduced for the devices modified with PCBA, with the doped modified layers showing the shortest times (Table S3). These results are consistent with improved electron extraction and suggest that both the surface properties of the metal oxide layer and the bulk properties (affected by doping) influence the extraction efficiency within the device.
Figure 6: (a) Photoluminescence of perovskite on ZnO, ZnLiO and ZnCsO (empty symbols) and PCBA/ZnO, PCBA/ZnLiO and PCBA/ZnCsO (filled symbols), (b) Transient photocurrent measurements on PCBA modified and unmodified ZnO, ZnCsO and ZnLiO. Our measurements thus far showed that doping does not change the absorption, film microstructure or the energy levels of metal oxide. Modification with PCBA ensures that the surface properties of the undoped and doped metal oxide layers are very similar. This suggests that the improvements in charge extraction and overall performance with doping are related to the bulk properties of the ZnO layer. To
examine these properties we performed XPS depth profiling of the doped oxide layers and followed the evolution of the Zn2p and O1s spectra. For all metal oxide layers, the Zn2p3/2 peak was centred at 1022.4 eV, in agreement with previous measurements. The O1s spectra are shown in Figure 7a,b,c for ZnO, ZnLiO and ZnCsO, respectively. They consist of two peaks – a low binding energy peak at 531 eV, assigned to metal-oxygen bonds and a high binding energy peak at 532.5 eV, which is associated with hydroxyl (O-H) groups.66
Figure 7: O1s spectra of (a) ZnO, (b) ZnLiO and (c) ZnCsO, (d) Percentage of O-H oxygen of total oxygen (e) Oxide stoichiometry (defined as O-Zn/Zn) as a function of etching time. Following the evolution of the O-H peak (Figure 7d) shows that more OH groups are present at the surface than in the bulk of the metal oxide. This is to be expected considering the hydrophilic nature of ZnO. Within the bulk of the film the O-H percentage stabilizes and is ~20% lower for doped ZnO films. Presence of O-H
species has been previously associated with trapping of electrons, 67-68 so a lower concentration of O-H within the doped ZnO layers would suggest a decreased density of traps at the surface and within the bulk of the metal oxide film. The stoichiometry of the films can be estimated by calculating the ratio O-Zn/Zn. The stoichiometry is always smaller than 1 due to a large density of oxygen vacancies as is frequently observed for solution processed ZnO films.
69
We find that the
stoichiometry of the doped films is ~15% higher than the undoped ZnO, demonstrating the overall improved quality of the doped films. Elucidating the exact mechanism of doping is beyond the scope of this work, however we note that our results are in agreement with previous investigations of similar dopants in ZnO extraction layers of organic photovoltaic devices.70-71 These reports suggest that such doping of ZnO reduces the oxygen vacancies within the film (i.e. improves the film stoichiometry) and as a result increases its electron conductivity. Another indication for the density of traps within ZnO films can be obtained by measuring the increase in conductivity under UV-light illumination. Solution processed ZnO layers have many structural defects, resulting in a variety of subbandgap states that can trap charges.72 Illuminating the films with UV-light fills these traps and leads to an increase in the film conductivity,73-74 thus the relative increase in conductivity can be related to the total density of traps. Figure 8 shows that the increase in conductivity of ZnO is significantly larger than that of the doped films, illustrating the lower density of traps within the doped layers.
Figure 8: Enhancement of oxide conductivity upon illumination under a solar simulator. 4. Conclusions To summarise, our results demonstrate that once the bulk and surface properties of solution processed ZnO layers have been engineered, it can serve as an efficient electron extraction layer in perovskite photovoltaic devices. We show that this can be achieved by combining doping of the metal oxide with a modification by a selfassembled monolayer, both of which can be easily performed in solution and at low processing temperatures. Our approach results in a significant improvement in the device performance up to 18%, one of the highest values ever reported employing standard MAPbI3 and solution processed ZnO as the ETL. Our results demonstrate that ZnO can be engineered to be a competitive alternative to TiO2, especially for applications that require low temperatures, such as printed and flexible perovskite photovoltaics.
Acknowledgements We would like to kindly thank Prof. Uwe Bunz for providing access to the device fabrication facilities. We are also grateful to Prof. Annemarie Pucci and Prof. Rasmus
Schroeder for access to AFM and SEM, respectively. Paul Fassl thanks the HGSFP for a scholarship.
References [1] M. Habibi, F. Zabini, M. R. Ahmadian-Yazdi, M. Eslamian, Renew. Sustainable Energy Rev. 2016, 62, 1012. [2] T. Liu, K. Chen, Q. Hu, R. Zhu, Q. Gong, Adv. Energy Mater. 2016, 6, 1600457. [3] D. Zhao, M. Sexton, H.-Y. Park, G. Baure, J. C. Nino, F. So, Adv. Energy Mater. 2015, 5, 1401855. [4] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476. [5] S. Ye, W. Sun, Y. Li, W. Yan, H. Peng, Z. Bian, Z. Liu, C. Huang, Nano Lett. 2015, 15, 3723−3728. [6] J. H. Kim, P. W. Liang, S. T. Williams, N. Cho, C. C. Chueh, M. S. Glaz, D. S. Ginger, A. K.-Y. Jen, Adv. Mater. 2015, 27, 695. [7] W. Chen, Y. Wu, Y. Yue, J. Liu, W. Zhang, X. Yang, H. Chen, E. Bi, I. Ashraful, M. Grätzel, L. Han, Science,. 2015, 350, 944. [8] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, L. Mora-Sero, J. Bisquert, H. J. Snaith, J. H. Nicholas, Nano. Lett. 2014, 14, 724. [9] N.Fu, C. Huang, Y. Liu, X. Li, W. Lu, L. Zhou, F. Peng, Y. Liu, H. Huang. ACS Appl. Mater. Interfaces 2015, 7, 19431. [10] S. K. Pathak, A. Abate, P. Ruckdeschel, B. Roose, K. C. Gödel, Y. Vaynzof, A. Santhala, S.-I. Watanabe, D. J. Hollman, N. Noel, A. Sepe, U. Wiesner, R. H. Friend, H. J. Snaith, U. Steiner, Adv. Funct. Mater. 2014, 24, 6046. [11] W. Ke, G. Fang, Q. Liu, L. Xiong, P. Qin, H, Tao, J. Wang, H. Lei, B. Li, J. Wan, G. Yang. Y. Yan, J. Am. Chem. Soc. 2015,137, 6730. [12] J. Song, E. Zheng, J. Bian, X.-F. Wang, W. Tian, Y. Sanehira, T. Miyasaka, J. Mater. Chem. A 2015, 3, 10837. [13] G. S. David, P. McMeekin, W. Rehman, G. E. Eperon, M. T. H. Michael Saliba, A. Haghighirad, N. Sakai, B. R. Lars Korte, M. B. Johnston, L. M. Herz , H. J. Snaith, Science, 2016, 351, 151.
[14] J. P. C. Baena, L. Steier, W. Tress, M. Saliba, S. Neutzner, T. Matsui, F. Giordano, T. J. Jacobsson, A. R. Srimath Kandada, S. M. Zakeeruddin, A. Petrozza, A. Abate, M. K. Nazeeruddin, M. Grätzel, A. Hagfeldt, Energy Environ. Sci. 2015, 8, 2928. [15] E. H. Anaraki, A. Kermanpur, L. Steier, K. Domanski, T. Matsui, W. Tress, M. Saliba, A. Abate, M. Grätzel, A. Hagfeldt, J.-P. Correa-Baena, Energy Environ. Sci. 2016, 9, 3128. [16] D. Y. Liu, T. L. Kelly, Nat. Photonics 2014, 8, 133. [17] Y. Cheng, Q.-D. Yang, J. Xiao, Q. Xue, H.-W. Li, Z. Guan, H.-L. Yip, S.-W. Tsang, ACS Appl. Mater. Interfaces 2015, 7, 19986. [18] Y. Dkhissi, S. Meyer, D. Chen, H. C. Weerasinghe, L. Spiccia, Y.-B. Cheng, R. A. Caruso, ChemSusChem 2016, 9 (7), 687. [19] J. Yang, B. D. Siempelkamp, E. Mosconi, F. De Angelis, T. L. Kelly, Chem. Mater. 2015, 27, 4229. [20] F. Giordano, A. Abate, J. P. Correa Baena, M. Saliba, T. Matsui, S. H. Im, S. M. Zakeeruddin, M. K. Nazeeruddin, A. Hagfeldr, M. Graetzel, Nat. Comm. 2016, 7, 10379. [21 .
, .
, .
, .
, .
, .
, Electrochim. Acta 2016, 195, 143.
[22] H. Zhang, J. Shi, X. Xu, L. Zhu, Y. Luo, D. Li, Q. Meng, J. Mater. Chem. A 2016, 4, 15383. [23] D. H. Kim, G. S. Han, W. M. Seong, J.-W. Lee, B. J. Kim, N.-G. Park, K. S. Hong, S. Lee, H. S. Jung, ChemSusChem 2015, 8 (14), 2392. [24] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [25] H.-H. Wang, Q. Chen, H. Zhou, L. Song, Z. St Louis, N. De Marco, Y. Fang, P. Sun, T.-B. Song, H. Chen, Y. Yang, J. Mater. Chem. A 2015, 3, 9108. [26] M. Lv, W. Lv, X. Fang, P. Sun, B. Lin, S. Zhang, X. Xu, J. Ding, N. Yuan, RSC Adv. 2016, 6, 35004. [27] L. F. Zhu, Y. Z. Xu, J. J. Shi, H. Y. Zhang, X. Xu, Y. H. Zhao, Y. H. Luo, Q. B. Meng, D. M. Li, RCS Advances, 2016, 6, 82282. [28] J. Cao, J. Yin, S. Yoan, Y. Zhao, J. Li, N. Zheng, Nanoscale 2015, 7, 9443. [29] Y. C. Shih, L. Y. Wang, H. C. Hsieh, K. F. Lin, J. Mater. Chem. A 2015, 3, 9133.
[30] K. Wojciechowski, S. D. Stranks, A. Abate, G. Sadoughi, A. Sadhanala, N. Kopidakis, G. Rumbles, X.-Z. Li, R. H. Friend, A. K.-Y. Jen, H. J. Snaith, ACS Nano 2014, 9 (12), 12701. [31] J. Kim, G. Kim, T.-K. Kim, S. Kwon, H. Back, J. Lee, S.-H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [32] H. P. Dong, Y. Li, S. F. Wang, W. Z. Li, N. Li, X. D. Guo, L. D. Wang, J. Mater. Chem. A 2015, 3, 9999. [33] W. Li, W. Zhang, S. Van Reenen, R. J. Sutton, J. Fan, A. A. Haghighirad, M. B. Johnston, L. Wang, H. J. Snaith, Energy Environ. Sci. 2016, 9, 490. [34] H. Dong, X. Guo, W. Li, L. Wang, RCS Adv. 2014, 4, 60131. [35] X. Guo, H. Dong, W. Li, N. Li, L. Wang, ChemPhysChem 2015, 16, 1727. [36] H.-W. Kang, J.-W. Lee, D.-Y. Son, N.-G. Park, RSC Adv. 2015, 47334. [37] X. Zhao, H. Shen, Y. Zhang, X. Li, X. Zhao, M. Tai, J. Li, J. Li, X. Li, H. Lin, ACS Appl. Mater. Interfaces 2016, 8 (12), 7826. [38] Z.-L. Tseng, C.-H. Chiang, S.-H. Chang, C.-G. Wu, Nano Energy 2016, 28, 311. [39] M. A. Mahmud, N. K. Elumalai, M. B. Upama, D. Wang, M. Wright, T. Sun, C. Xu, F. Haque, A. Uddin, RSC Adv. 2016, 6, 86108. [40] L. Zuo, Z. Gu, T. Ye, W. Fu, G. Wu, H. Li, H. Chen, J. Am. Chem. Soc. 2015, 137, 2674. [41] J. Kim, G. Kim, T. K. Kim, S. Kwon, H. Back, J. Lee, S. H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [42] J. Zhang, T. Pauporté, J. Phys. Chem. C 2015, 119, 14919. [43] X. Dong, H. Hu, B. Lin, J. Ding, N. Yuan, Chem. Commun. 2014, 50, 14405. [44] Y. Vaynzof, D. Kabra, L. Zhao, P. K. H. Ho, A. T. S. Wee, R. H. Friend, Appl. Phys. Lett. 2010, 97 (3), 156. [45] Y. Sevinchan, P. E. Hopkinson, A. A. Bakulin, J. Herz, M. Motzkus, Y. Vaynzof, Adv. Mater. Interfaces 2016, 3, 1500616. [46] C. Hinzmann, O. Magen, Y. J. Hofstetter, P. E. Hopkinson, N. Tessler, Y. Vaynzof, ACS Appl. Mater. Interfaces 2017, 9 (7), 6220.
[47] O. Pachoumi, A. A. Bakulin, A. Sadhanala, H. Sirringhaus, R. H. Friend, Y. Vaynzof, J. Phys. Chem. C 2014, 118 (33), 18945. [48] C. Bi, Y. Shao, Y. Yuan, Z. Xiao, C. Wang, J. Mater. Chem. A 2014, 2, 18508. [49] C. Bi, Q. Wang, Y. Shao, Y. Yuan, Z. Xiao, J. Huang, Nat. Commun. 2015, 6, 7747. [50] J. B. Patel, R. L. Milot, A. D. Wright, L. M. Herz, M. B. Johnston, J. Phys. Chem. Lett. 2015, 7, 96.# [51] S. Bai, N. Sakai, W. Zhang, Z. Wang, J. T.-W. Wang, F. Gao, H. J. Snaith, Chem. Mater. 2017, 29, 462. [52] W. Zhang, S. Pathak, N. Sakai, T. Stergiopoulos, P. K. Nayak, N. K. Noel, A. A. Haghighirad, V. M. Burlakov, D. W. DeQuilettes, A. Sadhanala, W. Li, L. Wang, D. S. Ginger, R. H. Friend, H. J. Snaith, Nat. Commun. 2015, 6, 10030. [53] F. Zhang, J. Song, L. Zhang, F. Niu, Y. Hao, P. Zeng, H. Niu, J. Huang, J. Lian, J. Mater. Chem. A 2016, 4, 8554. [54] J. S. Yun, A. Ho-Baillie, S. Huang, S. H. Woo, Y. Heo, J. Seidel, F. Huang, Y.-B. Cheng, M. a. Green, J. Phys. Chem. Lett. 2015, 6, 875. [55] J. Li, J.-Y. Ma, Q.-Q. Ge, J. Hu, D. Wang, L.-J. Wan, ACS Appl. Mater. Interfaces 2015, 7, 28518. [56] J. Kim, G. Kim, T. K. Kim, S. Kwon, H. Back, J. Lee, S. H. Lee, H. Kang, K. Lee, J. Mater. Chem. A 2014, 2, 17291. [57] F. Fu, T. Feuer, T. Jäger, E. Avancini, B. Bissig, S. Yoon, S. Buecheler and A. N. Tiwari, Nat. Comm. 2015, 6, 8932. [58] J. H. Kim, C.-C. Chueh, S. T. Williams, A. K.-Y. Jen, Nanoscale 2015, 7, 17343. [59] S. Ryu, J. Seo, S. S. Shin, Y. C. Kim, N. J. Jeon, J. H. Noh, S. Il Seok, J. Mater. Chem. A 2015, 3, 3271. [60] W. Qiu, J. P. Bastos, S. Dasgupta, T. Merckx, I. Cardinaletti, M. Jenart, C. Nielsen, R. Gehlhaar, J. Poortmans, P. Heremans, I. Mcculloch, D. Cheyns, J. Mater. Chem. A 2017, 5, 2466. [61] Y. Dong, W. Li, X. Zhang, Q. Xu, Q. Liu, C. Li, Z. Bo, Small 2016, 12, 1098. [62] T. Ma, D. Tadaki, M. Sakuraba, S. Sato, A. Hirano-Iwata, M. Niwano, J. Mater. Chem. A 2016, 4, 4392. [63] S. D. Stranks, H. J. Snaith, Nat. Nanotechnol. 2015, 10, 391.
[64] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Nat. Comm. 2014, 5, 5784. [65] J. H. Hei, H. J. Han, D. Kim, T. K. Ahn, S. H. Im, Energy Environ. Sci., 2015, 8, 1602. [66] E. Kim, Y. Vaynzof, A. Sepe, S. Guldin, M. Scherer, P. Cunha, S. V. Roth, U. Steiner, Adv. Funct. Mater. 2014, 24, 863. [67] X. Zhang, J. Qin, Y. Xue, P. Yu, B. Zhang, L. Wang, R. Liu, Sci. Rep. 2014, 4, 4596. [68] O. Pachoumi, A. A. Bakulin, A. Sadhanala, H. Sirringhaus, R. H. Friend, Y. Vaynzof, J. Phys. Chem. C 2014, 118 (33), 18945. [69] M. Krzywiecki, L. Grzadziel, A. Sarfraz, D. Iqbal, A. Szwajce, A. Erbe, Phys. Chem. Chem. Phys. 2015, 17, 10004. [70] H. Peng, W. Xu, F. Zhou, J. Zhang, C. Li, Synth. Met. 2015. 205, 164. [71] L. Zuo, S. Zhang, S. Dai, H. Chen, RCS Adv. 2015, 5, 49369. [72] V. Ischenko, S. Polarz, D. Grote, V. Stavarache, K. Fink, M. Driess, Adv. Funct. Mater. 2005, 15, 1945. [73] O. Pachoumi, C. Li, Y. Vaynzof, K. K. Banger, H. Sirringhaus, Adv. Energy. Mater. 2013, 3, 1428. [74] C. Wang, C. Li, S. Wen, P. Ma, Y. Liu, R. C. I. MacKenzie, W. Tian, S. Ruan, J. Mater. Chem. A 2017, DOI: 10.1039/C7TA00229G
Qingzhi An is a Ph.D student in Kirchhoff Institute for Physics and the Centre for Advanced Materials of Heidelberg University under the supervision of Prof. Yana Vaynzof. She received her B.Sc. from Dalian University of Technology and her M.Sc. in Physics from Heidelberg University. She is currently working on functional interface and extraction layer modifications in hybrid solar cells.
Paul Fassl received his B.Sc. and M.Sc. degrees in Physics from Heidelberg University in 2010 and 2013, respectively. In his master thesis at the InnovationLab GmbH, his research focused on the optimization of graded mixed host organic light-emitting diodes. Since 2014, he is pursuing his Ph.D. in the group of Prof. Yana Vaynzof in Heidelberg, where his research is mainly focused on understanding environmental effects and degradation phenomena in organic-inorganic hybrid perovskite materials and devices. Paul is the recipient of Heidelberg Graduate School of Fundamental Physics (HGSFP) scholarship.
Yvonne Jasmin Hofstetter received her B.Sc. degree in Physics from Heidelberg University in 2015. She is now a master student under supervision of Prof. Yana Vaynzof. Her research focuses on modification of interfaces in hybrid photovoltaics and characterisation of polymers and metal oxides using x-ray and ultraviolet photoemission spectroscopies.
David Becker-Koch received his B.Sc. degree in Physics from the University of Zurich in 2014. He continued his studies at the University of Heidelberg where he joined the group 'Physical Principles of Organic and Hybrid Photovoltaics' lead by Prof. Dr. Yana Vaynzof working on photothermal deflection spectroscopy (PDS). David graduated in 2016 with a M.Sc. in Physics and in 2017 started his Ph.D. under Prof. Vaynzof focusing on the research of new interface modification layers for thin film solar cells.
Alexandra Bausch received her B.S. degree in Physics in 2015 from University of Constance and carried out her Bachelor thesis at the Molecular Imaging Center at National Taiwan University. Currently, she is pursuing her M.S. degree in Physics at Heidelberg University under the supervision of Prof. Yana Vaynzof. Her research interests include studying the effect of film fabrication parameters on optical properties of perovskite materials, as well as degradation of perovskite layers.
Paul Hopkinson obtained his PhD in physics from the University of Cambridge under the supervision of Professor Dame Athene Donald. He then joined the Cambridge Optoelectronics group as a research associate, focussing on degradation of organic photovoltaics. At the University of Heidelberg he was the senior research associate in the Vaynzof research group, responsible for a range of technical activities. Paul is now a science and technology consultant at the Science Group PLC (UK).
Since 2014, Yana Vaynzof is a Juniorprofessor at Heidelberg University (Germany), where she leads the ‘Physical Principles of Organic and Hybrid Photovoltaics’ group. She received a B.Sc degree (summa cum laude) in electrical engineering from the Technion - Israel Institute of Technology (Israel) in 2006, and a M.Sc. degree in electrical engineering from Princeton University (USA) in 2008. In 2011, she obtained a Ph.D. degree in physics under the supervision of Prof. Sir. Richard Friend at the Optoelectronics Group, Cavendish Laboratory, University of Cambridge (UK) and joined the Microelectronics group at the University of Cambridge as a Postdoctoral Research Associate focusing on the study of surfaces and interfaces in organic and hybrid optoelectronics. Yana Vaynzof was the recipient of a number of fellowships and awards, including the Pinzi Award for Academic Excellence, Knesset (Israeli Parliament) Award for contribution to the Israeli Society, Gordon Y. Wu Fellowship, Henry Kressel Fellowship and the ERC Starting Grant.
Research highlights:
The bulk and surface properties of ZnO extraction layers are engineered using doping and a modification with a self-assembled monolayer. This complete engineering approach results in tripling of the average device power conversion efficiency reaching up to 18%, accompanied by reduced hysteresis and improved stability. The enhancement in performance originates from improved perovskite film formation, more efficient electron charge extraction and reduced density of traps in the bulk of the oxide layer. All modifications are performed in solution at low temperature and can be applied to printed perovskite solar cells or those fabricated on flexible substrates.
Graphicala abstract