Thermal Analysis of Metal-Organic Precursors for Functional Cu-ΝiOx Metal Oxide as Hole Transporting Layer in Inverted Perovskite Solar Cells: Role of Combustion Chemistry in Thin Films versus Powders
- Introduction
Perovskite solar cells (PSCs) have witnessed significant progress related to improved efficiency within the last decade. For example, the energy of PVCs rose from 3.8% in 2009 to more than 24% in the year 2019. According to many experts, many of the most currently used electron transport layers in perovskite solar cells, such as require considerably high temperatures during synthesis. This has significantly limited their potential commercial applications. Consequently, the emerging development of inverted perovskite solar cells based on the use of p-type NiOx as the primary hole transport layer (HTL) on the electrodes has received significant attention in recent years due to its immense potential benefits.
Cu doped nickel oxide (Cu-NiOx) films are important nanoparticles which have recently demonstrated significant promise as hole transporting layer (HTL) for modern perovskite solar cells (PVSCs) due to their unique properties such as intrinsic p-type semiconducting properties and enhanced electrical conductivity[1]. Besides, Cu-NiOx also has high optical transmittance and a deep-lying valence band of (~5.4 eV). According to many experts, all these properties are critically important in improving the efficiency of perovskite solar cells, thereby enhancing the viability of their commercial applications. For example, the novel properties of Cu-NiOx such as p-type semiconducting properties not only improve the conductivity of the thin-film solar cells but also reduce their annealing temperature[2].
The hole transporting layer (HTL), such as Cu-NiOx are increasingly being used in a wide range of semiconductor applications to enhance performance through improved conductivity. However, despite the considerable potential of Cu doped nickel oxide hole-transporting layer prepared using the traditional sol-gel method, its commercial application is hindered by issues. For example, Cu:NiOx HTL is not only associated with significantly high costs of device fabrication but also restricted techniques of printing[3]This is mainly attributed to the high relatively high temperatures of >400 °C, which is required when annealing Cu:NiOx HTL for purposes of achieving higher crystallinity. Additionally, many of the current technologies for synthesis of metal technologies use not only slow but also highly equipment-intensive techniques, which further increase the overall costs of such processes[4]. Don't use plagiarised sources.Get your custom essay just from $11/page
Combustion method is widely regarded to be one of the promising alternative routes of synthesis which have been found to significantly enhance the efficiency of achieving high crystallinity for solution-processed metal oxides like Cu-NiOx. For example, unlike the traditional sol-gel process, this requires more than 400 °C; the combustion method works typically at much lower temperatures. Generally, the exothermic reaction involved in the combustion method usually provides considerably lower transition energy for enhancing the formation of metal oxides, thereby eliminating the need for high thermal energy. This is critically important as it allows for the use of relatively lower processing temperatures. The low temperatures help in achieving crystallinity as opposed to relying on endothermic sol-gel reactions. Endothermic sol-gel reactions require exceptionally high thermal energy to assist in the formation of metal lattices and removal of organic residuals.
There are many differences between the combustion synthesis of metal oxides on films versus combustion in Bulk materials. Firstly, Solution combustion synthesis (SCS) is increasingly being adopted as one of the popular methods for the synthesis of metal oxides on semiconductor thin films due to its relative cost-effectiveness, simplicity and enhanced efficiency[5] For example, combustion synthesis of metal oxides on semiconductor thin films particularly takes advantage of the exothermic nature of the involved redox reaction. This acts as an important source of energy for its localized heating. As a result, it allows it to convert the precursor solutions to the desired metal oxide at relatively lower temperatures[6]On the other hand, combustion in Bulk materials such as petroleum fuel, coal, or organic wastes involves the use of high temperatures to achieve the burning of the materials. However, this often comes at considerable costs and risks of material loss during combustions.
The laboratory project involved experimentally investigating the viability and effectiveness of the proposed combustion thin-film deposition approach in the production of high quality of Cu-NiOx films. Different chemical compositions obtained from the reaction of metal nitrates, namely [0.05 mmol Cu(NO3)2 and 0.95 mmol Ni(NO3)2] with acetylacetone fuel additive in Bulk and thin films were mainly investigated. The results revealed that combustion could only effectively work in Bulk materials rather than in films. This is likely to be attributed to the fact that acetylacetone usually is removed from the firms before the decomposition of nitrates, while glycine is left in the film during the annealing process. Besides, it was also observed that the decomposition of Cu:Ni(NO3)2 occurs at relatively high temperatures of about 270 °C without the use of fuel.
The study of Solution combustion synthesis (SCS) on film versus combustion in Bulk materials revealed many similarities and differences when it comes to the processing of Cu:ΝiOx for use as hole transporting layer (HTL). For example, the results showed significant improvement of the crystallinity of the films with the addition of Cu dopants[7]. On the other hand, the required temperature for annealing also significantly reduced when using solution combustion synthesis (SCS) on film as compared to combustion in Bulk materials. Finally, the organic solvents which are employed during the fabrication of devices with Cu:ΝiOx as HTL play a dual role of enabling the formation of the metal oxides lattices and removing organic residuals[8]. For example, the organic solvent 2-methoxy ethanol played a dual role of acting both as a solvent and also as a fuel in addition to cetylacetonate.
- Materials and Methods
- Material Synthesis
The primary materials used in the experiment included 0.05 mmol Cu(NO3)2, 0.95 mmol Ni(NO3), Kapton substrates, Cu:NiOx films, and 10 ml 2-methoxy ethanol. The other materials comprised of different concentrations of acetylacetonate and bulk material fuels, namely Urea and glycine. The first step in the synthesis process involved dissolving both the 0.95 mmol Ni(NO3)2 and 0.05 mmol Cu(NO3)2 in a solution of 10 ml 2-methoxyethanol. Next, different concentrations of acetylacetonate and glycine were added to the solution as fuels. The solution was then gently stirred at room temperature for about 3 hours.
The next important step involved examining the combustion synthesis behavior of the Cu:NiOx films on Kapton substrates using thermogravimetric analysis (TGA) in the air (200 ml min-1) based on a heating rate of 5 °C min-1. On the other hand, the corresponding combustion synthesis behavior of the bulk mixtures was also investigated using thermogravimetric analysis with similar conditions being observed. Finally, the crystallinity of the combustion synthesis of the Cu:NiOx, was examined by XRD measurements. The resultant solution was evaporated at about 100°C until the formation of a sol-gel was achieved.
- Characterization
Thermogravimetric analysis (TGA) analysis was carried out using a thermogravimetric analyzer set at air (200 ml min-1) and a heating rate of 5 °C min.-1 The thermogravimetric analyzer continuously measured the changes in the mass of the Cu:NiOx films with the change in temperature over time. A TGA curve was then plotted to characterize the Cu:NiOx decomposition patterns and determine the optimum temperature for the synthesis reaction.
- Results and Discussion
- Results
XRD results from Bulk and films of CuNiOx
The crystallinity of the combustion synthesis of the Cu:NiOx, was examined using XRD analysis. As shown in the results, only for the amount of 0.1 acetylacetonates (ratio fuel to oxidizer) and above 300 oC annealing temperature was obtained pure crystal phase of NiO. The combustion method exhibited distinct diffractions with comparable intensities at 2θ = 37.8° and 43.7°, representing the typical diffraction peaks of (111) and (200) planes for the cubic-structured NiO.
Figure1. XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (0.1-mole ratio compare to nitrates –fuel lean) acetylacetonate as fuel, respectively.
Figure 2: XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (1.5-mole ratio compare to nitrates- fuel-rich) acetylacetonate as fuel, respectively.
Similarly, the crystallinity of combustion synthesis of the Cu: xrd measurements also examined the niox in Bulk solution combustion synthesis (SCS). Different initial volumes of the fuel (w/o, 0,1, and 1.5Acac) were compared and analyzed. In the case of 0.5mmol concentration of precursors (Bulk), XRD patterns of combustion synthesized samples (Bulk), the peaks appeared at 44.0°, 52.3° and 76.5° (assigned for (111), (200) and (220) planes, respectively) are indexed for face-centered cubic (FCC) Ni phase (JCPDS No. 87-0712). The characteristic peaks of NiO appeared at 2θ= 37.20°, 43.0°, 62.87°, and 75.20° and can be indexed to the cubic crystalline structure of NiO as (111), (200), (220) and (311) planes, respectively (JCPDS No. 01-089-5881). It may be concluded that higher crystallinity of combustion-synthesized samples (Bulk) evident from the intensity and sharper peaks appeared in the XRD patterns. As shown in figure 2 below, the high crystallinity of mulk materials evidenced by the sharp XRD peaks is attributed to its release of higher energy in an exothermic reaction. This is mainly because Bulk material has a high concentration of precursors as compared to films.
Figure 3. XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors and acetylacetonate as fuel, respectively.
Figure 4: XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors without fuel, respectively.
The crystallinity of the combustion synthesis of the Cu:NiOx, was examined by xrd measurements. Only for the amount of 0.1 acetylacetonates (ratio fuel to oxidizer) and above 300 oC annealing temperature was obtained pure crystal phase of NiO. The combustion method exhibited distinct diffractions with comparable intensities at 2θ = 37.8° and 43.7°, representing the typical diffraction peaks of (111) and (200) planes for the cubic-structured NiO.
Figure 5: XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (0.1-mole ratio compare to nitrates –fuel lean) acetylacetonate as fuel, respectively.
Figure 5. XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (1.5-mole ratio compare to nitrates- fuel abundant) acetylacetonate as fuel, respectively.
Concerning Bulk SCS, the crystallinity of combustion synthesis of the Cu:NiOx, was examined by xrd measurements. Additionally, we compared different initial volumes of the fuel (w/o, 0,1, and 1.5Acac). In the case of 0.5mmol concentration of precursors (Bulk), XRD patterns of combustion synthesized samples (Bulk), the peaks appeared at 44.0°, 52.3° and 76.5° (assigned for (111), (200) and (220) planes, respectively) are indexed for face-centered cubic (FCC) Ni phase (JCPDS No. 87-0712). The characteristic peaks of NiO appeared at 2θ= 37.20°, 43.0°, 62.87°, and 75.20° and can be indexed to the cubic crystalline structure of NiO as (111), (200), (220) and (311) planes, respectively (JCPDS No. 01-089-5881). It may be concluded that higher crystallinity of combustion-synthesized samples (Bulk) evident from the intensity and sharper peaks appeared in the XRD patterns.
Figure 6. XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors and acetylacetonate as fuel, respectively.
Figure 7. XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors without fuel, respectively.
The crystallinity of the combustion synthesis of the Cu:NiOx, was examined by xrd measurements. Only for the amount of 0.1 acetylacetonates (ratio fuel to oxidizer) and above 300 oC annealing temperature was obtained pure crystal phase of NiO. The combustion method exhibited distinct diffractions with comparable intensities at 2θ = 37.8° and 43.7°, representing the typical diffraction peaks of (111) and (200) planes for the cubic-structured NiO.
Figure 8. XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (0.1-mole ratio compare to nitrates –fuel lean) acetylacetonate as fuel, respectively.
Figure 9. XRD patterns of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and (1.5-mole ratio compare to nitrates- fuel abundant) acetylacetonate as fuel, respectively.
(films for different annealing temperatures…fuel-rich)
On the other hand, about Bulk SCS, the crystallinity of combustion synthesis of the Cu:NiOx, was examined by xrd measurements. Additionally, we compared different initial volumes of the fuel (w/o, 0,1, and 1.5Acac). In the case of 0.5mmol concentration of precursors (Bulk), XRD patterns of combustion synthesized samples (Bulk), the peaks appeared at 44.0°, 52.3° and 76.5° (assigned for (111), (200) and (220) planes, respectively) are indexed for face-centered cubic (FCC) Ni phase (JCPDS No. 87-0712). The characteristic peaks of NiO appeared at 2θ= 37.20°, 43.0°, 62.87°, and 75.20° and can be indexed to the cubic crystalline structure of NiO as (111), (200), (220) and (311) planes, respectively (JCPDS No. 01-089-5881). It may be concluded that higher crystallinity of combustion-synthesized samples (Bulk) evident from the intensity and sharper peaks appeared in the XRD patterns.
Figure 10: XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors and acetylacetonate as fuel, respectively.
Figure 11: XRD patterns of combustion-synthesized samples (Bulk) prepared from metals nitrates (Cu, Ni) as precursors without fuel, respectively.
- TGA results from Bulk and films of CuNiOx
The combustion synthesis behavior of the Cu:NiOx, was examined through thermogravimetric analysis (TGA) in the air (200 ml min-1) with a heating rate of 10 °C min-1. Also, the combustion synthesis behavior of the corresponding bulk mixtures was examined by TGA analysis with the same conditions, shortly after the resulting solution was evaporated at about 80 and 100 °C until a sol-gel was formed, and the impact of the different preheating temperature was investigated. Additionally, we compared different initials volumes of the precursors. TGA results showed that full combustion could occur only in Bulk materials with large initial size of the precursor, while it was observed that if the precursor were heated at 100 for an extended period (~48 hours), the combustion for 0.1 acac couldn’t occur. Further, for a small amount of initial volume of precursors (~50 μL), the TGA profiles resample the behavior of film, which is described in more detail in the next slide.
Figure 12: Bulk- SCS- 1000nm film instead of um)
Figure 12: Absorption spectra for different final thicknesses of Cu:ΝiOx annealing at 150 oC and 300 oC respectively (please analyze the graph)
As shown in the figure above, the width of the film at 150oC is ~80 nm, while at 300oC is ~50 nm suggesting that residues have to remain into 150oC treated film. In this regard, the treatment of film at 150oC reveals no prominent absorption at the specific photon energies, while treatment given at 300oC shows a bandgap at ~3.1eV.
Figure 13: P.L. measurements for different final thicknesses of Cu:ΝiOx annealing at 150 oC and 300 oC respectively
Photoluminance measurements clearly show the transition at 400 nm (~3.1eV) ascribed to Cu:NiOx for all samples. An additional Peak at ~300 nm is also observed, which is very prominent for the films annealed at 150OC for 60 to 180 min. In contrast, the peak intensity for film annealed at 300 OC is very weak. It is hypothesized that the peak can be ascribed to NO–3 (originate from the oxidizer), which may cause an increase in the hydrophobicity.
Figure 14: (from film to Bulk instead of uL is nm thickness of film precursors – gel- metal nitrates, acetylacetonate, and 2-methoxy ethanol solvent)
Figure 15: (films instead of 50uL is 50 nm film)
Figure 16: TGA curves of combustion-synthesized samples (films) prepared from metals nitrates (Cu, Ni) as precursors and acetylacetonate as fuels, respectively.
Results from devices
The results from the devices suggest that with and without acetylacetone, we have two stages of combustion at 130 oC and 300oC for 80oC preheat, respectively. Also, suggesting that the 2-methoxy ethanol acts as solvent and fuel simultaneously. If the higher preheated temperature is applied, it seems that the fuels (Acac and 2-methoxy ethanol) are evaporated, and thus no combustion can be observed at 130oC. As shown in Figure 1, for the Cu: NiOx film with 0.1 acetylacetonates (ratio fuel to oxidizer), mass loss occurs more gradually during the first stage of combustion, at 130oC. To investigate the functionality of the combustion-based Cu:NiOx HTL films in PVSCs, devices were fabricated with the structure of ITO/Cu:NiOx/CH3NH3PbI3 /P.C. [70] B.M./Al. Different thicknesses (15 nm, 30 nm, and 60 nm) and annealing temperatures (150 oC and 300 oC for one h and 0.5 h respectively) of the combustion-based Cu:NiOx HTL films in the corresponding devices were studied….
The figure shows the J–V curve of the device using a 30 nm Cu:NiOx HTL thickness fabricated at 150 oC of annealing for 1 h. None of the Cu:NiOx combustion-based films as HTL treated at 150 oC for different reaction times (1 h to 12 h) delivered any functional device.
The optimized thickness and annealing temperature of the combustion derived Cu:NiOx HTL in the device were 15 nm and 300 oC for 0.5 h, respectively, showing enhanced charge extraction and negligible J-V hysterics, compared to thicker films, delivering a PCE of 11.66 %. Further research is expected to suggest ways in which other fuel additives can fabricate Cu:NiOx films with combustion pathway[9].
- Conclusion
In conclusion, many parameters that can potentially increase the performance of inverted perovskite solar cells based on the use of p-type NiOx as the first hole transport layer (HTL) have been widely investigated by recent empirical studies. Generally, one of the essential parameters which have been identified is the nature of the hole transporting layers. For example, several hole transporting layers (HTLs) based on the implementation of a wide range of organic and inorganic materials have been investigated to determine their potential advantages as functional Hole Transporting Layer in Inverted Perovskite Solar Cells. For example, inverted PSCs employing Cu:NiOx as a hole transport layer have are widely preferred due to their low processing temperature as well as enhanced stability as compared to other organic HTLs. Besides, Cu:ΝiOx HTLs are also suitable for the manufacture of flexible large-scale devices. The beneficial qualities of NiOx based HTLs are mainly attributed to the inconsistent band position and low conductivity of NiOx based HTLs. Similarly, inorganic HTLs such as Cu:NiOx also offer an essential advantage of providing a wide optical bandgap, which results in high transparency within the visible range as well as superior hole mobility.
References
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Yu, X., Zeng, L., Zhou, N., Guo, P., Shi, F., Buchholz, D.B., Ma, Q., Yu, J., Dravid, V.P., Chang, R.P. and Bedzyk, M. “Ultra‐Flexible, “Invisible” Thin‐Film Transistors Enabled by Amorphous Metal Oxide/Polymer Channel Layer Blends.” Advanced Materials 27, no. 14 (2015): 2390-2399.
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[1] Branquinho, R., Santa, A., Carlos, E., Salgueiro, D., Barquinha, P., Martins, R. and Fortunato, E. “Solution
Combustion Synthesis: Applications in Oxide Electronics.” Developments in Combustion Technology (2016): 397-417.
[2] J.W., Hennek, Smith, J., Yan, A., Kim, M.G., Zhao, W., Dravid, V.P., Facchetti, A. and Marks, T.J. Oxygen “
Getter” effects on microstructure and carrier transport in low-temperature combustion-processed a-InXZnO (X= Ga, Sc, Y, La) transistors. Journal of the American Chemical Society, Vol. 135, no.29, 2013, pp.10729-10741.
[3]Everaerts, K., Zeng, L., Hennek, J.W., Camacho, D.I., Jariwala, D., Bedzyk, M.J., Hersam, M.C. and Marks, T.J. ”
Printed indium gallium zinc oxide transistors. Self-assembled nanodielectric effects on low-temperature combustion growth and carrier mobility.” ACS applied materials & interfaces Vol. 5, no. 22, 2013, pp. 11884-11893.
[4] Kim MG, Kanatzidis MG, Facchetti A, Marks TJ. Low-temperature fabrication of high-performance metal oxide
thin-film electronics via combustion processing. Nature materials. 2011 May;10(5), pp.382.
[5] Liu, X., Tsai, K.W., Zhu, Z., Sun, Y., Chueh, C.C. and Jen, A.K.Y. A Low‐Temperature, Solution Processable Tin
Oxide electron-transporting Layer Prepared by the Dual‐Fuel Combustion Method for Efficient Perovskite Solar Cells. ” Advanced Materials Interfaces 3, no. 13 (2016): 1600122.
[6] Wen, W., & Wu, J. M. “Eruption combustion synthesis of NiO/Ni nanocomposites with enhanced properties for
dye-absorption and lithium storage.” ACS applied materials & interfaces Vol. 3, no. 10, 2011, p.4112-4119
[7] Yu, X., Zeng, L., Zhou, N., Guo, P., Shi, F., Buchholz, D.B., Ma, Q., Yu, J., Dravid, V.P., Chang, R.P. and
Bedzyk, M. “Ultra‐Flexible, “Invisible” Thin‐Film Transistors Enabled by Amorphous Metal Oxide/Polymer Channel Layer Blends.” Advanced Materials 27, no. 14 (2015): 2390-2399.
[8] Cochran, E.A., Park, D.H., Kast, M.G., Enman, L.J., Perkins, C.K., Mansergh, R.H., Keszler, D.A., Johnson, D.W.
and Boettcher, S.W. “Role of combustion chemistry in the low-temperature deposition of metal oxide thin films from solution.” Chemistry of Materials 29, no. 21 (2017): 9480-9488
[9] Kim, J.H., Liang, P.W., Williams, S.T., Cho, N., Chueh, C.C., Glaz, M.S., Ginger, D.S. and Jen, A.K.Y. “High‐
performance and environmentally stable planar heterojunction perovskite solar cells based on a solution‐processed copper‐doped nickel oxide hole‐transporting layer.” Advanced materials 27, no. 4 (2015): 695-701.