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NANOSYSTEMS: Kozlov S.S., et al. Nanosystems:
PHYSICS, CHEMISTRY, MATHEMATICS Phys. Chem. Math., 2023,14 (5), 584-589.
http://nanojournal.ifmo.ru
Original article DOI 10.17586/2220-8054-2023-14-5-584-589
Planar perovskite solar cells with La2NiMnO6 buffer layer
Sergey S. Kozlov1, Anna B. Nikolskaia1, Olga K. Karyagina1, Ekaterina K. Kosareva2, Olga V. Alexeeva1, Vasilisa I. Petrova1, Oksana V. Almjasheva3'4, Oleg I. Shevaleevskiy1
1 Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Moscow, Russia
2N. N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Science, Moscow,
Russia
3St. Petersburg State Electrotechnical University "LETI", St. Petersburg, Russia 4loffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg, Russia
Corresponding author: Anna B. Nikolskaia, [email protected]
PACS 84.60.Jt
Abstract Thin films of La2NiMnO6 (LNMO) double perovskite oxide were first used as buffer layers in planar perovskite solar cells (PSCs) with the architecture of glass/FTO/LNMO/CH3NH3PbI3/Spiro-MeOTAD/Au. All PSCs were fabricated under ambient conditions and their photovoltaic parameters were measured under standard illumination (AM1.5G, 1000 W/m2). Power conversion efficiency (PCE) values (10-11 %) for the PSCs developed were comparable with those obtained for conventional PSCs with compact TiO2 (cTiO2) layer, but the stability of PSCs with LNMO buffer layer was significantly higher than for cTiO2-based PSCs. Keywords nanostructures, double perovskite oxides, perovskite solar cells, solar photovoltaics Acknowledgements This work was supported by the Russian Science Foundation under grant No. 20-6947124.
For citation Kozlov S.S., Nikolskaia A.B., Karyagina O.K., Kosareva E.K., Alexeeva O.V., Petrova V.I., Almjasheva O.V., Shevaleevskiy O.I. Planar perovskite solar cells with La2NiMnO6 buffer layer. Nanosystems: Phys. Chem. Math., 2023, 14 (5), 584-589.
1. Introduction
The search and development of new types of solar cells with high efficiency and stability is one of the most urgent tasks of the modern photovoltaics [1,2]. In the last decade, the great interest has been focused on the studies of perovskite solar cells (PSCs), in which hybrid organic-inorganic materials, such as CH3NH3PbX3 (X = Cl-, Br- or I-), are used as photosensitive layers. Improvement of the PSC fabrication technology allowed one to achieve high values of power conversion efficiency (PCE), which nowadays exceeds 25 % [3-5]. Low cost and simple manufacturing methods make PSCs a promising alternative to traditional silicon-based solar cells. However, perovskite materials used in PSCs are characterized by low stability and degrade under ambient conditions or under continuous illumination. As a result, PCE of PSCs is significantly decreased over time, which limits the application prospects [6,7].
Previously, it was shown that the highest PCE values can be achieved using PSCs with a planar structure [8,9]. The architecture of the state-of-the-art planar PSC sample is glass/FTO/cTiO2/CH3NH3PbI3/SpiroMeOTAD/Au, where FTO is a fluorine-doped tin oxide conductive coating (F:SnO2), cTiO2 is a compact layer of TiO2 nanoparticles with the thickness ~ 50 nm. The functions of cTiO2 layer include blocking hole transfer from the perovskite material to the FTO coating and preventing direct contact between CH3NH3PbI3 layer and the FTO surface [10,11]. However, PSCs with planar structure were found to be the most sensitive to external factors such as humidity, temperature, and illumination. The latter is due to both the degradation of the perovskite material itself and the instability of the TiO2 layer when exposed to ultraviolet radiation. Long-term illumination causes desorption of oxygen molecules inside the TiO2 material, which initiate the degradation of the perovskite layer [12-14]. In addition, FTO layer is characterized by relatively rough and non-uniform surface, which may lead to non-uniform deposition of cTiO2 layer on FTO substrate. The latter causes the appearance of morphological defects in the perovskite layer deposited on top of FTO/cTiO2 substrate, which also initiates rapid degradation of PSCs [12,13]. In the literature, attempts to reduce the degradation effects in PSCs by applying CH3NH3PbI3 layer directly to the FTO coating were described, but this leaded to the decrease in PCE values [15,16]. Thus, nowadays an important task for PSC improvement is to find new materials suitable for use in planar PSC as a buffer layer that do not contribute to the degradation of perovskite material.
In this work, a highly stable inorganic material such as double perovskite oxide La2NiMnO6 (LNMO) was first used as a buffer layer in planar PSCs. The LNMO thin films on glass substrates were obtained by spin coating and were
© Kozlov S.S., Nikolskaia A.B., Karyagina O.K., Kosareva E.K., Alexeeva O.V., Petrova V.I., Almjasheva O.V., Shevaleevskiy O.I., 2023
investigated using optical spectroscopy and X-ray diffraction techniques, as well as atomic force microscopy (AFM). Planar PSCs with the architecture of glass/FTO/LNMO/CH3NH3Pbl3/Spiro-MeOTAD/Au were fabricated under ambient conditions at high humidity level 50 %) and their photovoltaic (PV) parameters were measured. The data obtained demonstrate a new approach to PSCs optimization for long-term operation in outdoor conditions.
2. Experimental
2.1. Buffer layer preparation
The appropriate amounts of La(NO3)3 • 6H2O, Ni(NO3)2 • 6H2O and Mn(NO3)2 • 6H2O powders were dissolved in a mixture of 2-ethoxyethanol and acetic acid (4:1 v/v) to obtain 0.3 M stock solution. The LNMO thin films were obtained by spin coating of the stock solution on the surface of non-conductive glass substrates (3000 rpm, 30 s), followed by annealing at 550 - 650 °C during 2 hrs in the muffle furnace. The obtained LNMO thin films were used for XRD, AFM and optical measurements. LNMO-based buffer layers were deposited on the FTO-glass substrates following the same spin coating and annealing protocols using the 5-fold diluted stock solution and were used further for PSCs fabrication. FTO glass substrates coated by cTiO2 layer were obtained following the technique described in [17].
2.2. Device fabrication
Planar PSCs were fabricated under ambient conditions at relatively high humidity level 50 - 60 %) according to the procedure described in [18]. Perovskite (CH3NH3PbI3) layer was formed on the surface of LNMO layer using a conventional one-step deposition method [19]. A layer of Spiro-MeOTAD hole transport material was spin-coated onto the surface of the perovskite layer (3000 rpm, 20 s). The final stage of PSC fabrication was the deposition of ~ 50-nm thick Au contacts by vacuum thermal evaporation using the VUP-4 vacuum post. Schematic representation of the PSC architecture is illustrated in Fig. 1. The state-of-the-art planar PSCs were fabricated using FTO glass substrates coated by cTiO2 layer via the same technique as described above [18].
f^Tl hsh r^n Spiro-MeOTAD
CH3NH3PbI3 LNMO FTO
Glass
Fig. 1. Schematic representation of the planar PSC architecture
3. Characterization studies
The X-ray diffraction (XRD) measurements of LMNO thin films were provided using DRON-3M X-ray diffrac-tometer with Cu Ka radiation (A = 1.5405 A) as the X-ray source. Atomic force microscope (AFM) NTEGRA Prima (NT-MDT) with HA_NC probes (TipsNano, resonant frequency 140 kHz, stiffness constant 3.5 N/m) was used to investigate the surface morphology and to evaluate average particle size in LNMO thin films. The optoelectronic properties of the LNMO layers were studied using UV-Vis spectrophotometer (Shimadzu UV-3600, Japan) with an ISR-3100 integrating sphere in the wavelength range of 300 - 1400 nm.
Photovoltaic (PV) characteristics of the developed PSCs were measured under standard illumination conditions (AM1.5G, 1000 W/m2) by recording the current density-voltage (J-V) characteristics using Keithley 4200-SCS Semiconductor Characterization System (Keithley, USA) and Abet Technologies 10500 solar simulator with Xenon lamp (Abet, USA) as the light source. The PCE (n) values of the PSCs were calculated from the J-V data using the known formula [20]:
n = Jse • Voc • FF 100% (1)
PIN
where JSC - short-circuit current density, VOC - open-circuit voltage, FF - fill factor and PIN - incoming light intensity.
4. Results and discussion
XRD patterns of the LNMO thin films deposited on non-conductive glass substrates and annealed at different temperatures (550 - 650 °C) are shown in Fig. 2(a). LNMO films obtained at 575 - 650 °C showed XRD reflexes corresponding to monoclinic, rhombohedral and orthorhombic phases of LNMO, confirming the triple phase structure of La2NiMnO6 double perovskite oxides [21]. The absence of additional XRD peaks indicates the purity of the samples. In contrast, LNMO film obtained at 550 ° C was found to possess amorphous structure.
29, degrees Wavelength, nm
(a) (b)
Fig. 2. XRD patterns (a) and transmittance spectra (b) of LNMO thin films, obtained at different annealing temperatures
Transmittance spectra for LNMO thin films deposited on the surface of non-conductive glass substrates is shown on Fig. 2(b). Using these data and Tauc equation for direct transitions, the energy bandgap (Eg) values were obtained (Table 1) [22]. It can be seen that an increase in annealing temperature leads to a decrease in the Eg value for the studied LNMO layers from 1.75 to 1.04 eV. As a result, all LNMO samples possesses semiconductor properties.
Table 1. The energy bangap (Eg) values calculated for LNMO thin films, obtained at different annealing temperatures (T)
T, °C 550 575 600 650
Eg, eV 1.75 1.50 1.05 1.04
AFM images of LNMO thin layers deposited on FTO glass substrates show that regardless of annealing temperature, fabricated layers are characterized by homogeneous structure with uniform particle distribution (Fig. 3). The average particle size determined using AFM was found to be around 60 - 70 nm. A comparison of the particle size (Fig. 3) and the crystallite size determined from data about the broadening of X-ray diffraction lines indicates that the particles are polycrystalline, consisting of crystallites 15 ± 2 nm in size.
The LNMO thin films fabricated on the surface of FTO conductive glasses were used for fabrication of planar PSCs with the architecture of FTO/LNMO/CH3NH3PbI3/Spiro-OMeTAD/Au under ambient conditions. State-of-the-art planar PSC with cTiO2 layer was constructed for providing comparative analysis. J-V curves recorded under standard illumination (1000 W/m2, AM1.5G) for PSCs with LNMO and cTiO2 layers are shown in Fig. 4(a). The PV parameters for all PSCs are listed in Table 2. It can be seen that the PSCs with LNMO layers demonstrated PCE values (10-11 %) comparable to the efficiency obtained for conventional TiO2-based PSCs (Table 2). The highest PCE was observed for the PSC samples with LNMO thin films prepared at annealing temperature 600 ° C. The results confirm that LNMO materials can be successfully used as buffer layers in planar PSCs.
PV parameters of planar PSCs with LNMO and cTiO2 layers were investigated during long-term storage under ambient conditions. The data obtained (Fig. 4(b)) revealed that PSCs with LNMO-based buffer layer demonstrate more stable behavior than PSCs with cTiO2 layer. Thus when PSCs were exposed under ambient conditions for 35 days, PCE for PSC with LNMO thin film was almost unchanged, while PCE for cTiO2-PSC decreased more than 60 %.
Fig. 3. AFM surface image (left) and particle size distribution (right) for LNMO thin layer obtained at 600 °C
(a)
(b)
Fig. 4. J-V curves (a) and stability test (b) for PSCs under standard illumination (1000 W/m2, AM1.5G) Table 2. The PV parameters for PSCs under standard illumination (1000 W/m2, AM1.5G)
Sample Jsc, mA/cm2 Voc ,V FF, a.u. n, %
PSC with cTiO2 21.0 0.96 0.65 12.6
PSC with LNMO 575 ° C 19.3 0.86 0.58 10.0
PSC with LNMO 600 ° C 20.5 0.89 0.62 11.4
PSC with LNMO 650 ° C 20.2 0.89 0.58 10.5
Summing up, it was shown that La2NiMnO6 (LNMO) double perovskite oxide thin films demonstrate semiconductor properties and their optoelectronic characteristics including bandgap values could be easily tuned by changing the synthesis conditions, namely, the annealing temperature. La2NiMnO6-based thin films were successfully used as buffer layers in planar PSCs, showing PCE values comparable with those obtained for conventional TiO2-based PSCs. Due to the high stability of LNMO double perovskite oxides towards moisture and oxygen, incorporation of LNMO buffer layer in PSCs architecture allowed to significantly increase the stability of planar PSCs under ambient conditions as compared to TiO2-based PSCs behavior.
5. Conclusions
Thin films of La2NiMnO6 (LNMO) double perovskite oxide were synthesized on the glass substrates via spin coating and were first used as buffer layers in planar PSCs. LNMO thin films revealed higher homogeneity and stability in comparison with the state-of-the-art cTiO2 layers. The optical and photoelectrical properties of LNMO layers can be tuned by changing the annealing temperature. Planar PSCs with the architecture of FTO/LNMO/CH3NH3Pbl3/Spiro-OMeTAD/Au were fabricated under ambient conditions. The maximum PCE value of 11.4 % under standard illumination conditions (AM1.5G, 1000 W/m2) was obtained for PCE with LNMO buffer layer annealed at 600 °C and was comparable to the PCE for PSCs with cTiO2 layer. The stability of PSCs based on LNMO buffer layer was significantly higher than the stability of PSCs with cTiO2 layer. The data obtained demonstrate a novel approach for optimization of PSCs designed for long-term operation in outdoor conditions.
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Submitted 15 September 2023; revised 10 October 2023; accepted 12 October 2023
Information about the authors:
Sergey S. Kozlov - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0000-0002-8660-5646; [email protected]
Anna B. Nikolskaia - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0000-0002-7430-4133; [email protected]
Olga K. Karyagina - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0000-0002-6702-5195; [email protected]
Ekaterina K. Kosareva - N. N. Semenov Federal Research Center for Chemical Physics, Russian Academy of Science, Kosygin St., 4, Moscow, 119991, Russia; ORCID 0000-0001-8090-2052; [email protected]
Olga V. Alexeeva - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0000-0001-8982-3959; [email protected]
Vasilisa I. Petrova - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0009-0001-6357-2052; [email protected]
Oksana V. Almjasheva - St. Petersburg State Electrotechnical University "LETI", Professora Popova St., 5, St. Petersburg, 197376, Russia; Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya St., 26, St. Petersburg, 194021, Russia; ORCID 0000-0002-6132-4178; [email protected]
Oleg I. Shevaleevskiy - Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St., 4, Moscow, 119334, Russia; ORCID 0000-0002-8593-3023; [email protected]
Conflict of interest: the authors declare no conflict of interest.
