Effects of Co-Addition of Copper, Sodium and Ethylammonium to CH₃NH₃PbI₃ Perovskite Compound  ^†

Abstract

From the band calculation, the copper d-orbital band formed slightly above the valence band maximum would function as an acceptor level promoting the generation of carriers. In addition, the excitation processes from the p-orbital of iodine and the d-orbital of copper to the s-orbital of sodium could suppress carrier recombination. Total energy calculations showed that, the stability of the crystal structure decreases with the addition of copper and sodium, but increases with the addition of ethylammonium. Therefore, it is expected that the combination of these compounds can compensate for the disadvantage of unstable crystal structure. The calculated results could be obtained by optimizing the composition of the perovskite and the annealing conditions.

1. Introduction

There have been many investigations to improve the conversion efficiency and durability of devices by adding compounds to the perovskite precursor solution, and these are still being actively researched. In this study, copper (Cu), sodium (Na) and ethylammonium (EA) were focused on. Cu compounds are mainly used as hole transport materials, such as copper thiocyanate (CuSCN) and copper oxide (CuO or Cu₂O), which are expected to be stable inorganic hole transport materials that can replace organic hole transport materials [1,2,3,4,5]. Many studies have been reported on the addition of alkali metals to perovskite precursor solutions [6,7,8,9,10,11], but relatively few studies have been reported on the addition of copper [12,13,14,15]. A previous study reported that the addition of CuBr₂ and NaCl to perovskite precursor solutions improved the conversion efficiency and durability of the devices [16,17]. In that study, it was reported that the lattice distortion of perovskite crystals was reduced by the addition of a small quantity of Cu, and the lattice defects were suppressed by the transfer of Na to the lattice defect sites after the desorption of methylammonium (MA), which led to the enhanced conversion efficiency and durability of the devices.

In addition to the experimental investigation of the addition effect, the analysis using first-principles calculations has been actively studied. CH₃NH₃PbI₃ is known as the most general perovskite used as a photoelectric conversion material. First-principles calculations using a structural model in which organic cations, metal cations, and halogen anions are substituted by other molecules or atoms can be used to estimate the stability of the crystal structure. In a previous study reported in 2019, first-principles calculations showed that substituting EA for MA enhanced the stability of the crystal structure [18]. The calculated energy gap values were agreed well with the experimental values, and the correspondence between the calculated and experimental results indicates that the addition effect can be estimated by first-principles calculations. There have been other attempts to improve the properties of devices by substituting MA with larger organic cations [19,20,21,22,23,24]. Furthermore, numerous studies on the effects of halogen additions have been conducted, both computationally and experimentally [25,26,27,28,29]. The selectivity of the halogen is very important because the energy gap and the crystallinity of the perovskite film changes for halogen.

The purpose of this study is to investigate the addition effects of Cu, Na and EA using experimental characterization and first-principles calculations. Co-addition of Cu and Na enhanced the J_SC, V_OC, and FF, resulting in higher conversion efficiency. This result is attributed to the enhancement of carrier generation by Cu addition and the suppression of carrier loss by Na addition. The properties are expected to be further improved by using other organic cations or alkali metals, or by optimizing the experimental conditions.

2. Experimental Procedures and Calculation

Detailed fabrication processes were described in the previous works [30,31,32,33]. F-doped tin oxide (FTO) substrates were cleaned in an ultrasonic bath with acetone and methanol and dried under nitrogen gas. The 0.15 M TiO₂ precursor solution was spin-coated on the FTO substrate at 3000 rpm for 30 s, and the coated substrate was then annealed at 125 °C for 5 min. The 0.30 M TiO₂ precursor solution was spin-coated on the TiO₂ layer at 3000 rpm for 30 s, and the resulting substrate was annealed at 125 °C for 5 min. The process to form the 0.30 M precursor layer was performed twice. Then, the FTO substrate was sintered at 550 °C for 30 min to form a compact TiO₂ layer. To form the mesoporous TiO₂ layer, a TiO₂ paste was prepared from the TiO₂ powder with poly(ethylene glycol)in ultrapure water. The solution was mixed with acetylacetone and Triton X-10 for 30 min. Then, the TiO₂ paste was spin-coated on the compact TiO₂ layer at 5000 rpm for 30 s. The resulting cell was annealed at 550 °C for 30 min to form the mesoporous TiO₂ layer.

To prepare the perovskite compounds, mixed solutions of CH₃NH₃I (2.4 M, Showa Chemical, Tokyo, Japan) and PbCl₂ (0.8 M, Sigma-Aldrich, Tokyo, Japan) in N,N-dimethylformamide (DMF, Sigma-Aldrich, 0.5 mL) were prepared for the standard cell. Pb or MA in the perovskite structure was expected to be substituted by Cu, Na or EA, respectively. These perovskite solutions were then introduced into the TiO₂ mesopores by spin-coating at 2000 rpm for 60 s, which is followed by annealing in air. During the spin-coating, a hot air-blowing method was applied. Temperatures of the cells during the air-blowing were set at 90 °C. A polysilane solution was prepared by mixing chlorobenzene (Fujifilm Wako Pure Chemical Corporation, Osaka, Japan, 0.5 mL) with decaphenylcyclopentasilane (DPPS, Osaka Gas Chemicals, Osaka, Japan, OGSOL SI-30-10, 10 mg). During the last 15 s of the third spin-coating of the perovskite precursor solutions, the DPPS solution was also spin-coated on the perovskite layer [34,35,36,37]. The annealing temperature was gradually increased from 90 °C until the entire film turned black [38].

A solution of spiro-OMeTAD in chlorobenzene was mixed with a solution of lithium bis(tri-fluoromethylsulfonyl)imide in acetonitrile and tris(2-(1H-pyrazol-1-yl)-4-tert-butyl pyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] in acetonitrile (0.5 mL) for 24 h. The former solution with 4-tertbutylpyridine was mixed with the Li-TFSI solution and FK209 solution (0.004 mL) for 30 min at 70 °C. Then, the spiro-OMeTAD solution was spin-coated on the perovskite layer at 4000 rpm for 30 s. All procedures were performed in ambient air. Finally, gold (Au) electrodes were evaporated as top electrodes using a metal mask for the patterning.

First principle calculations of all structures were performed within the generalized gradient approximation in the form of the Perdew-Burke-Ernzerhof exchange-correlation functional using Quantum Espresso. The cut-off energy of the planar wavefunction was set to 80 eV for the structural optimization and to 25 eV for the other calculations. 4 × 4 × 4 k-point mesh was used for calculations of all structures.

3. Results and Discussion

The band structure, partial density of states and parameters were obtained by first-principles calculations. The total substitution structure model used in the calculations is shown in Figure 1. The energy gap increases when iodine is substituted with bromine or chlorine compared to MAPbI₃, which is the most general perovskite used as a photoelectric conversion material. In general, an increase in the energy gap may lead to an increase in the open-circuit voltage, therefore, the use of bromide or chloride as an additive may improve the open-circuit voltage.

The total energy per cell of the structural model is obtained by first-principles calculations, and a smaller value means that the crystal structure is more stable. Since the total energy decreases by substituting iodine with bromine or chlorine, the crystal structure is expected to be stabilized. The result that the addition of EA improves the stability of the crystal structure is in good agreement with previous studies.

Calculations for the partial substitutional structure model were also performed [39,40,41,42,43,44]. The partial substitution structure model used in the calculations is shown in Figure 2. Compared to the total substitutional structure model, the partial substitutional structure model contains more atoms, and thus requires a longer time for calculation. In contrast to MA_0.875EA_0.125PbI₃, the addition of copper resulted in the formation of a shallow band of Cu d-orbitals slightly above the band of iodine p-orbitals, decreasing the energy gap and the mobility of holes. Furthermore, comparing MA_0.875EA_0.125Pb_0.875Cu_0.125I₃ and MA_0.750EA_0.125Na_0.125Pb_0.875Cu_0.125I₃, by substituting Na for a part of MA, a band of Na s orbitals is formed slightly above the conduction band minimum, which results in a slight increase in electron mobility. The energy gap is increased by substituting some of the iodine with bromine, and this result is in good agreement with the calculated results of the total substitution structure model.

In the calculated results of the partial substitution structure model, it is indicated that the addition of Cu forms a shallow band of Cu d-orbitals slightly above the band of iodine p-orbitals, which reduces the energy gap and decreases the hole mobility. However, if we consider the Cu d-orbital band as an acceptor level, it may have a positive effect on the performance of the device.

The current-voltage (J-V) curve and the external quantum efficiency (EQE) obtained from the present perovskite photovoltaic devices. From the calculated results of the partial substitution structure model, it was expected that the addition of Cu would decrease the energy gap and reduce the hole mobility. However, from the measured results, the co-adding of Cu and Na slightly increased the energy gap and further enhanced the short-circuit current density, open-circuit voltage, and fill factor, resulting in higher conversion efficiency. Therefore, the Cu d-orbitals are considered to function as acceptor levels.

4. Conclusions

In this study, the effects of co-addition of Cu, Na and EA into the perovskite precursor solution were investigated using the first-principles calculations. The addition of these additives to the perovskite precursor solution increased the energy gap and improved the open circuit voltage in all devices compared to the standard system. According to first-principles calculations, replacing all of the iodine with bromine or chlorine increased the energy gap, indicating that the improvement in V_OC was due to the effect of halogen. When some of the lead was substituted with Cu, a shallow band of Cu d-orbitals was formed slightly above the valence band maximum, suggesting that the addition of Cu decreases the energy gap and reduces the hole mobility. When a small amount of Cu was added in the experiment, the conversion efficiency decreased slightly, but the energy gap increased slightly. This suggests that the Cu energy levels of formed in the forbidden band may function as acceptor levels. The excitation of electrons from the p-orbital of iodine to the d-orbital of Cu is thought to have promoted the generation of carriers, leading to the improvement of J_SC.