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Very wide-bandgap nanostructured metal oxide materials for perovskite solar cells
L. L. Larina1, O.V. Alexeeva1, O.V. Almjasheva2, V. V. Gusarov3, S. S. Kozlov1, A.B. Nikolskaia1, M.F. Vildanova1, O.I. Shevaleevskiy1
1 Department of Solar Photovoltaics, Institute of Biochemical Physics RAS, Kosygin St. 4, Moscow, 119334, Russia 2St. Petersburg Electrotechnical University "LETI", Professora Popova St. 5, Saint Petersburg, 197376, Russia 3Ioffe Physical-Technical Institute RAS, Politekhnicheskaya St. 26, Saint Petersburg, 194021, Russia shevale2006@yahoo.com, almjasheva@mail.ru, victor.v.gusarov@gmail.com
PACS 73.63.Bd DOI 10.17586/2220-8054-2019-10-1-70-75
Very wide-bandgap undoped and Y2O3-doped ZrO2 nanoparticles were synthesized and their structural, optical, morphological and energy characteristics were investigated. It was found that the bandgap value in ZrO2 decreases with Y2O3 doping. The developed materials were used for fabrication of nanostructured photoelectrodes for perovskite solar cells (PSCs) with the architecture of glass/FTO/ZrO2-Y2O3/CH3NH3PbI3/spiro-MeOTAD/Au. The power conversion efficiency in the PSCs based on ZrO2-Y2O3 photoelectrodes was significantly higher than that for undoped ZrO2 photoelectrodes. We have found that nanostructured layers, based on very wide-bandgap materials could efficiently transfer the injected electrons via a hopping transport mechanism.
Keywords: nanostructures, ZrO2 , thin films, semiconductors, solar photovoltaics, perovskite solar cells.
Received: 10 November 2018 Revised: 18 January 2019
1. Introduction
Nanostructured materials are widely used for the development of next-generation solar cells (SCs) since they enable fabrication of high efficiency and low-cost devices which are promising for mass production of photovoltaic technologies [1,2]. Recently, a considerable interest is focused on inorganic-organic metal halide perovskite solar cells (PSCs) in which the record power conversion efficiency (PCE) exceeded 22 % [3] and reached 27.3 % in perovskite-silicon tandem solar cell [4]. PSC's architecture comprises a mesoscopic layer of metal-oxide nanoparticles on a conductive substrate, which plays a role of the electron-conductive photoelectrode, a perovskite (CH3NH3PbI3) layer deposited on top of the photoelectode, a hole-conductive layer and a metallic counter electrode [5,6].
One of the key components of the PSC is an electron-conductive photoelectrode, which consists of metal oxide semiconductor nanoparticles organized in a mesoscopic architecture. Nanostructured layers of titanium dioxide (TiO2) with the band gap (Eg) of 3.0 - 3.2 eV are generally used as photoelectrodes in PSCs [7,8]. At the same time, some other wide-bandgap materials were also successfully used in photoelectrodes [9]. The application of a very wide-bandgap metal oxide, such as ZrO2 with Eg ~ 5.7 eV, is of special interest for this purpose [10]. Condensed layers of wide-bandgap materials are dielectrics with insulator type conductivity behavior and can't be used as a conductive medium. However, their analogs with nanostructured morphology demonstrate high electron-conductive abilities, due to the large concentration of the nanoparticle surface defects. A number of publications confirmed that in nanostructured systems with Eg > 5 eV, the effective transfer of the injected electrons was observed, while the density of the electrons in the conduction band was negligible [9]. Charge transport through the nanostructured layer can be realized on the basis of a hopping conduction mechanism through localized states within forbidden zone [10].
The formation of crystal phase and morphology in ZrO2 as well as optical and electrical properties of ZrO2 nanoparticles strongly depend on the synthesis conditions [11]. A significant advantage of ZrO2 material is its ability to be doped with yttrium oxide (Y2O3), which allows one to vary the optoelectronic characteristics of ZrO2 -Y2 O3-based nanostructured systems. Doping with rare-earth metals or niobium (Nb) allows to significantly improve the transport characteristics of the photoelectode and to increase the PCE of the PSCs [12,13]. Previously reports of PSCs fabricated using undoped ZrO2-based photoelectode have been made [14]. In this work, we have synthesized ZrO2 nanoparticles and yttrium oxide doped ZrO2-Y2O3 systems which were used for fabrication of the nanostructured electron-conductive photoelectrodes for PSCs. Using the developed ZrO2-Y2O3-based photo-electrodes, we have prepared a series of PSCs and provided comparative measurements of the main photovoltaic parameters.
2. Experimental
2.1. Materials and samples preparation
Nanocrystalline zirconium dioxide was prepared by hydrothermal treatment of zirconium oxyhydroxide precipitated from a solution of ZrOCl2 (chemical pure grade) with concentrated aqueous NH4OH. Hydrothermal treatment was performed at T = 250 °C and P = 70 MPa over 4 h. The Y2O3-doped ZrO2 nanoparticles were obtained by hydrothermal treatment of co-precipitated zirconium and yttrium hydroxides from solutions of the corresponding metal salts. The conditions of hydrothermal treatment were chosen according to the data in [11] and corresponded to complete dehydration of zirconium hydroxide.
To fabricate a nanostructured photoelectrode based on ZrO2-Y2O3 system, we utilized a known technique; pastes from ZrO2 and ZrO2-Y2O3 nanopowders were prepared in organic solvent [6]. The photoelectodes were formed by depositing the pastes on the glass substrates with a conductive FTO coating. The ZrO2 and ZrO2-Y2O3 layers with a thickness of about 200 nm were deposited using spin-coating method, followed by sintering at 500 °C for 30 min.
The PSC fabrication process was provided under ambient conditions with high humidity 50 - 60 %) using a one-step method described previously [15]. During the fabrication process, ZrO2-based photoelectodes were first coated with a photosensitive perovskite (CH3NH3PbI3) layer, obtained from lead iodide and methylammonium iodide precursor solutions, followed by depositing a layer of spiro-MeO-TAD as a hole-transporting material [7,14]. The PSC fabrication process was completed by thermal evaporation of conductive Au contacts with a thickness of 50 nm using vacuum system VUP-4. As a result, we have prepared PSCs with a device architecture of glass/FTO/ZrO2-Y2O3/CH3NH3Pbl3/spiro-MeOTAD/Au, in which the doping content of Y2O3 was varied from 0 % (undoped system) to 3 and 10 mol.%.
2.2. Characterization studies
The structure and composition of nanostructured ZrO2-Y2 O3 system were determined by X-ray diffraction (XRD) analysis in the 13 - 65 ° range (Cu Ka radiation) using Rigaku Corporation SmartLab 3 diffractometer. The optical properties were investigated using UV-vis double-beam spectrophotometer Shimadzu 3600 with an integrating sphere ISR-3100 (Shimadzu, Japan), followed by an analysis of diffuse reflection spectra over a wavelength range 200 - 900 nm. The morphology of the films was investigated using dual-beam scanning electron microscope (SEM) Helios NanoLab 660 (FEI, USA).
The measurements of the photovoltaic parameters for PSCs were provided under standard illumination conditions (AM1.5G) with PIN = 1000 W/m2 by recording the current-voltage characteristics (J-V) using Abet Technologies Solar Simulator (Abet, USA) as a light source and Keithley 4200-SCS Parameter Analyzer (USA) for recording the current-voltage characteristics (J-V). The PCE (n) of the PSC was calculated from the J-V data using the known formula:
n = Jse ■ V°c ■ FF ^ 100%, (1)
Pin
where JSC - short-circuit current density, VOC - open-circuit voltage, FF - fill factor and Pin - light intensity of solar radiation.
3. Results and discussion
In Fig. 1, we present comparative data of XRD patterns for the powders of undoped ZrO2 nanoparticles and for ZrO2-Y2O3 system with Y2O3 doping level of 3 and 10 mol.%. XRD results for the samples, obtained using hydrothermal processing of co-precipitated zirconium and yttrium, reveal the co-existence of tetragonal 53 %) and monoclinic 47 %) phases in ZrO2 nanoparticles. The addition of 3 mol.% Y2O3 to ZrO2 leads to the formation of a predominantly pseudo-cubic modification of ZrO2 (c-ZrO2) and a trace amount of the monoclinic modification of m-ZrO2 (up to 5 %), the addition of 10 mol.% Y2O3 leads to the complete disappearance of m-ZrO2. The crystallite size of zirconia phases, determined by the X-ray line broadening method using the Scherrer equation, was found to be 16 and 14 nm for m-ZrO2 and t-ZrO2, respectively. The obtained results shows that 3 mol.% Y2O3 additive does not affect the crystallite size. The addition of 10 mol.% Y2O3 decreases the crystallite size down to 5 nm, which can be explained by the formation of the "core-shell" structure in which the shell is enriched with yttria [16].
Figure 2 shows the dependence of the diffuse reflection spectra for the powders of undoped ZrO2 and ZrO2-Y2O3 system with Y2O3 content of 3 and 10 mol.%. XRD data have shown that yttria doping stabilizes the high-temperature tetragonal ZrO2 phase. This result revealed that ZrO2-Y2O3 samples have a monophase structure and, thus, the semiconductor properties of these materials could be characterized by a direct transition from the
Fig. 1. XRD patterns for ZrO2 nanoparticles with a varied Y2O3 content
Fig. 2. Diffuse reflectance spectra for the powders of undoped ZrO2 and ZrO2-Y2O3 system
valence to the conduction band. Following the Kubelka-Munk theory, the value of the optical energy bandgap (Eg ) for direct transitions can be determined from the Tauc plots [17]:
,(hv - Eg )V2
a(hv) = C-
hv
(2)
where a - optical absorption coefficient, C - constant, hv - photon energy.
The Eg values for ZrO2 and ZrO2-Y2O3 system were defined with linear extrapolation of (ahv)2 plots with the photon energy axis (Fig. 3). The results obtained showed that Eg value also enhances with the increase of doping concentration from 5.74 eV in ZrO2 to 5.63 eV in ZrO2-Y2O3 (3 %). However, in ZrO2-Y2O3 (10 %), the Eg value was found to be 5.45 eV, which can be explained by a significant decrease of the nanoparticle size for that particular sample, to about 5 nm.
Typical scanning electron microscopy (SEM) surface image of the undoped ZrO2 nanostructured layer deposited on a conductive glass substrate (Fig. 4) indicates the agglomeration of ZrO2 sphere-like crystallites. SEM results show that the average particle size was approximately 30 - 40 nm. Fig. 5 presents the cross-sectional SEM image of the undoped ZrO2 electron transport layer spin-coated on FTO glass substrate. It is seen that FTO conductive layer is covered with ~ 200 nm uniform ZrO2-based mesoscopic layer. Fig. 6 presents J-V characteristics, recorded for PSCs under standard illumination AM 1.5G. Photovoltaic parameters for all the investigated PSCs are summarized in Table 1. Comparative studies of the PSCs based on undoped and Y2O3-doped ZrO2 photoelectrodes showed that doping leads to the increase of the short-circuit current values and improves the fill factor of the devices, resulting in the increase of total PCE values. The best performance of 11.4 % was obtained for the PSC with ZrO2-Y2O3 (10 %) photoelectrode that significantly exceeds the corresponding value of 5.9 % for PSC based on undoped ZrO2 photoelectrode.
The performance of ZrO2-based PSCs developed in this study was higher than that in TiO2 based PSCs with much higher observed VOC. The major difference between the above mentioned configurations of PSCs concerns the different charge transport mechanisms at the perovskite/photoelectrode interface for ZrO2 and TiO2 electrodes. Fig. 7 presents schematic energy band diagrams demonstrating the energy band structure for PSCs
Fig. 3. Eg values for ZrO2-Y2O3 system extracted from (ahv)2 vs. photon energy graphics
)
. I Landing E spot mag I WD I H FW I det I tilt I ■ 400 nm '
' I 5.00 keV 3 0 200 000 x | 5 3 mm 1 49 pm TLD ! -0 |
Fig. 4. SEM image of undoped ZrO2 nanostructured layer spin-coated on a conductive glass substrate
Fig. 5. Cross-sectional SEM image of the ZrO2-based photoelectrode Table 1. Photovoltaic characteristics of ZrO2-Y2O3 based PSCs
PCE parameters Photoelectrode
ZrO2 ZrO2/3 mol.% Y2O3 ZrO2/10 mol.% Y2O3
Voc , V 0.94 1.0 1.0
Jsc , m/m2 10.9 13.6 15.4
FF, a.u. 0.58 0.69 0.74
n, % 5.9 9.4 11.4
Fig. 6. J-V characteristics of the PSCs based on ZrO2-Y2O3 photoelectrodes under simulated AM 1.5G (1000 W/m2) irradiance
based on a ZrO2 photoelectrode (Fig. 7(a)) and on traditional TiO2 photoelectrode (Fig. 7(b)). The band diagram in Fig. 7(b) demonstrates that the conduction band edge of perovskite has the energy above the conduction band edge of TiO2 [18] that enables a classic photoexcited electron transfer from the perovskite layer to the TiO2 photoelectrode. Unlike the previously described situation, the conduction band edge of ZrO2 has much higher energy (Fig. 7(a)), leaving the conduction band edge of perovskite far below, which makes it impossible to transfer the electrons from the perovskite to ZrO2 in terms of the classical charge transfer mechanism. It is also known that under ambient temperature, ZrO2 is an insulator with poor carrier transport characteristics and its practical applicability as a charge carrier transporting material is questionable. However, several publications confirmed that the mechanism of charge transport in nanostructured wide-bandgap electrodes, being of primary physical and technical significance, is different from that in the bulk materials [19]. It was also shown that rare earth oxide doping initiates the creation of core-shell structures and results in a high concentration of surface defects [16] that significantly improves the transport characteristics of the mesoscopic photoelectrodes and increases the efficiency of the solar cells [20,21]. The latter is possible due the large concentration of the nanoparticle surface defects. A number of publications confirmed that in nanostructured systems with Eg > 5 eV, the effective transfer of the injected electrons was observed, while the density of the electrons in the conduction band was negligible [19]. In our study, we observed the effective electron conduction through the nanostructured ZrO2 layer that can be explained on the basis of the hopping conduction mechanism through localized states within forbidden zone of ZrO2 [10].
Fig. 7. Schematic energy band diagrams comparing the energy band structures for PSCs based on ZrO2 (a) and TiO2 photoelectrodes (b)
4. Conclusions
As a result, we have developed the technology and provided synthesis of both undoped and Y2O3-doped ZrO2 nanoparticles for which the structural, optical and energy characteristics were investigated. It was found that the band-gap value in ZrO2 decreases with increased Y2O3 doping. The developed materials were used for fabrication of nanostructured thin film photoelectrodes for constructing and providing a comparative study of the PSCs with the architecture of glass/FTO/ZrO2-Y2O3/CH3NH3PbI3/spiro-MeOTAD/Au. The power conversion efficiency in the PSCs based on ZrO2-Y2O3 photoelectrodes was shown to be significantly higher than that for undoped ZrO2 photoelectrodes. We have found that nanostructured layer, based on very wide-bandgap ZrO2 nanoparticles, could efficiently transfer the injected electrons to the back contact through the hopping transport mechanism via trap states in the forbidden zone of ZrO2. The obtained results demonstrate the possibility of using a very wide-bandgap oxide nanostructured materials with Eg values exceeding 5 eV for fabrication electron-conductive layers, including their successful application as mesoscopic photoelectrodes for perovskite solar cells.
Acknowledgements
This work was supported by the Russian Science Foundation under grant No. 17-19-01776. References
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