Photovoltaic Properties and Microstructures of Polysilane-Added Perovskite Solar Cells ^†

Abstract

CH₃NH₃PbI₃ perovskite photovoltaic devices inserted with a polysilane layer were fabricated and characterized. A Decaphenylcyclopentasilane (DPPS) in chlorobenzene solution was spin-coated between the perovskite layer and the hole transport layer of spiro-OMeTAD, and the resulting device was annealed at 190 °C. The DPPS-treated devices had higher conversion efficiencies than the standard ones, and they were stable in ambient air. Microstructural observations suggested that DPPS would work effectively as a bulk-hetero structure with perovskite layers.

1. Introduction

Perovskite solar cells are attracting attention as an alternative to silicon solar cells, which are widely studied [1,2,3,4,5,6,7]. Although silicon solar cells have high conversion efficiencies, they have demerits in terms of cost and weight. Perovskite solar cells provide high conversion efficiencies, low cost and an easy fabrication process [8,9,10]. However, these lead halide compounds are typically unstable in the air [11,12,13], and the stability of the perovskite photovoltaic devices should be improved for the actual cell modules [14,15,16,17].

The instability of perovskite photovoltaic devices is due to the migration of CH₃NH₃ (MA) and iodine (I), and reactivity with H₂O [18,19,20,21,22,23]. To improve the stability of perovskite photovoltaic devices, polymeric materials and organic semiconductors have been investigated [24,25,26]. For example, poly(ethylene oxide), poly(ethylene glycol) and poly(propylene carbonate) have been used to protect the perovskite layer from oxygen and moisture [27,28] and to enhance its stability. Both polymeric materials formed cross-linked networks of perovskite grains, which suppressed the defects. Other methods were also developed to ensure long-term stability by passivating the grain boundaries of perovskite grains, using Lewis acid bases and functional groups to increase the activation energy of decomposition [29,30,31].

Polysilane derivatives were introduced between the perovskite layer and the hole transport layer to improve long-term stability and photovoltaic properties [32,33,34,35]. The Polysilane derivatives exhibit two important advantages. The first is that Polysilanes are p-type semiconductors that facilitate hole transfer and rectification at the pn junction [36,37]. The second derives from Polysilanes having high stabilities at elevated temperatures up to ~300 °C and are therefore expected to act as a protective layer across the perovskite surface. In fact, Polysilanes have been applied to perovskite solar cells, and the photovoltaic properties were improved, especially by adding Decaphenylcyclopentasilane (DPPS) [33,34,35]. The DPPS-doped devices reported so far are based on spin-coating of DPPS on the perovskite layer using the anti-solvent method, and it is necessary to focus on the morphology of the interface, which is closely related to stability and carrier transport.

The purpose of the present work is to investigate the photovoltaic properties, stabilities and microstructures of DPPS-treated CH₃NH₃PbI₃ (MAPbI₃) perovskite solar cells. In this study, the perovskite layer was formed by annealing the film at 140 °C or 190 °C and the DPPS was treated by an anti-solvent method. The effects of DPPS treatment on the photovoltaic properties, stability and microstructures of MAPbI₃ perovskite solar cells were investigated using current density voltage (J-V) characteristics, X-ray diffraction (XRD) and transmission electron microscopy (TEM).

2. Experimental Procedure

Figure 1a shows a schematic diagram of the fabricated device. All fabrication processes were performed under atmospheric conditions in ambient air [38,39], and the temperature and humidity were ~25 °C and ~50%, respectively. F-doped tin oxide (FTO, Nippon Sheet Glass Company, Tokyo, Japan, ~10 Ω/□) substrates were cleaned with methanol and acetone in an ultrasonic bath and an ultraviolet ozone cleaner (Asumi Giken, Tokyo, Japan, ASM401N). Next, 0.15 and 0.30 M precursor solutions of TiO₂ compact layers were prepared from 1-butanol (Wako Pure Chemical Industries, Osaka, Japan) and titanium diisopropoxide bis(acetylacetonate) (Sigma Aldrich, Tokyo, Japan). These precursor solutions of compact TiO₂ were spin-coated on the FTO substrate at 3000 rpm for 30 s, and the substrates were annealed at 125 °C for 5 min. To form a uniform, compact TiO₂ layer, the 0.30 M precursor solution was spin-coated twice. Next, the FTO substrate was annealed at 550 °C for 30 min to form the compact TiO₂ layer. After that, a TiO₂ paste (precursor solution for mesoporous TiO₂) was spin-coated on the compact TiO₂ layer at 5000 rpm for 30 s. This TiO₂ paste was prepared by mixing distilled water (0.5 mL), poly (ethylene glycol) PEG-20000 (Nacalai Tesque, Kyoto, Japan, PEG #20000, 20 mg) and TiO₂ powder (Aerosil, Tokyo, Japan, P-25, 200 mg). This solution was further mixed with the surfactant Triton X-100 (Sigma Aldrich, 10 μL) and acetylacetone (Wako Pure Chemical Industries, 20 μL) for 30 min, and it was left untouched for 24 h to remove bubbles in the solution. To form the mesoporous TiO₂ layer, the TiO₂-coated substrates were annealed at 550 °C for 30 min [40,41,42,43].

To prepare the perovskite compounds, solutions of PbCl₂ (Sigma Aldrich, Tokyo, Japan, 111.2 mg) and CH₃NH₃I (Tokyo Chemical Industry, 190.8 mg) with the 1:3 molar ratio were mixed in N,N-dimethylformamide (Sigma Aldrich, 0.5 mL) at 60 °C for 24 h. These perovskite precursor solutions were normally spin-coated during the first coating. During the second and third spin-coating steps, an air-blowing method was employed, as illustrated in Figure 1b. The cells were maintained at 90 °C during the air-blowing [44]. DPPS (Osaka Gas Chemicals, OGSOL SI-30-15, Osaka, Japan, 10 mg) solutions were prepared in chlorobenzene (0.5 mL) and dropped onto the perovskite layer during the last 15 s of the third spin-coating of the perovskite precursor solutions. As shown in Figure 1b, This process was called D, which indicates perovskite and DPPS layers in an alternating sequence. The standard device was annealed at 140 °C for 20 min. The devices with DPPS layers were annealed at 190 °C for 16 min.

Then, a spiro-OMeTAD layer was formed as a hole transport layer by spin-coating, and the spiro-OMeTAD layer was formed below the gold electrodes for all the fabricated devices in the present work. Finally, gold (Au) electrodes were formed by evaporation [45,46,47,48]. All the fabricated cells in the present work were put into dark storage at a temperature of 22 °C and ~30% humidity in ambient air.

Microstructural analysis was conducted by an X-ray diffractometer (Bruker, Billerica, MA, USA, D2 PHASER) and a transmission electron microscope (Thermo Fisher Scientific Tokyo, Japan, Talos F200X). The surface morphologies of the perovskite layers were examined using an optical microscope (Nikon, Tokyo, Japan, Eclipse E600). The current density voltage characteristics of the fabricated devices were measured (Keysight, Santa Rosa, CA, USA, B2901A) under a solar simulator (San-ei Electric, Osaka, Japan, XES-301S) with irradiation at 100 mW cm⁻².

The role of all layers is described as follows: the FTO is a transparent electrode, the compact TiO₂ layer is an electron transport layer, the mesoporous TiO₂ layer is a scaffold layer for perovskite compounds, the perovskite layer is a photoactive layer, the DPPS layer is a protective and hole transport layer, the spiro-OMeTAD is a hole transport layer and the Au layer is an electrode layer.

3. Results and Discussion

Table 1. Measured photovoltaic parameters of solar cells.

Devices	JSC (mA cm−2)	VOC (V)	FF	RS (Ω cm−2)	RSh (Ω cm−2)	η (%)	ηave (%)
Standard	18.9	0.844	0.410	5.90	85.3	6.55	5.19
D × 1	17.0	0.846	0.443	5.07	110	6.38	5.65
D × 2	21.3	0.696	0.466	8.38	290	6.89	5.36
D × 3	23.2	0.771	0.620	5.59	2100	11.09	9.19
D × 4	17.3	0.877	0.493	7.21	124	7.50	5.96

shows the photovoltaic parameters of the TiO₂/perovskite/spiro-OMeTAD solar cells under illumination, which indicates the effects of the DPPS. The device without DPPS provided a short-circuit current density (J_SC) of 18.9 mA cm⁻², and a conversion efficiency (η) of 6.55%. The J_SC was increased from 18.9 to 23.2 mA cm⁻² by performing operation D 3 times, and the η increased to 11.09%. The surface morphology of the perovskite film was improved by operation D, and the DPPS blocked electrons and suppressed the recombination between holes and electrons. XRD patterns of the perovskite solar cells showed highly (100)-oriented crystals of the perovskite compounds, which were formed by the hot air-blowing method [44]. All devices presented a few peaks corresponding to PbI₂, which indicated the effectiveness of the DPPS layer against high-temperature annealing at 190 °C. Among these devices, the smallest PbI₂ peak was observed for the D × 3 device, which indicates that DPPS suppressed the decomposition of perovskite crystals most effectively in this study. Although the DPPS has a good electron-blocking effect, the DPPS has high electric resistivity. When the DPPS was deposited 4 times, the J_SC decreased and V_OC increased, which would be due to the high electric resistivity and the electron-blocking effect of the DPPS, respectively. All the devices with DPPS showed a significant increase in conversion efficiency after one week, indicating that DPPS contributes to the suppression of decomposition of perovskite grains at room temperature.

For the D × 3 device with the best characteristics, stability measurements of the photovoltaic parameters were carried out for up to 226 days.

Table 2. Changes of parameters for D × 3 device.

Time (Day)	JSC (mA cm−2)	VOC (V)	FF	RS (Ω cm−2)	RSh (Ω cm−2)	η (%)	ηave (%)
As-prepared	23.2	0.771	0.620	5.59	2100	11.09	9.19
149 days	18.6	0.862	0.707	4.54	842	11.38	9.46
191 days	21.4	0.887	0.701	5.17	1840	13.33	11.22
226 days	19.3	0.893	0.701	5.99	1410	12.10	10.40

shows the changes in parameters for the D × 3 device. The device showed an increase in η over time, with the highest η of 13.33% after 191 days.

DPPS and the PbI₂ crystal generated by the decomposition of the perovskite crystal blocked the recombination of electrons and holes, and the blocking effect saturated. Basically, PbI₂ inhibits carrier migration and degrades the photoelectric properties of the device. However, when a small amount of PbI₂ is added between the perovskite layer and the hole transport layer, the photoelectric properties are improved. R_sh decreased significantly at the 149th day but then rose again at the 191st day. This may be the effect of hysteresis of the J-V measurement, which would be caused by a carrier charge in the TiO₂ layer. Since a small number of carriers might be generated and charged in the TiO₂ layer during J-V measurements under light irradiation and current flow, the R_sh might reduce at the 149th day.

The microstructures of the D × 3 device with the best characteristics were investigated by TEM, and the TEM image is shown in Figure 2a. The TEM observation revealed several types of lattice fringes, and the enlarged high-resolution image of Figure 2b shows a lattice distance of 6.2 Å, which corresponds to (100) of the perovskite crystal. This was also confirmed by the Fourier transform, as shown in Figure 2c. In addition, an amorphous DPPS phase was observed around the perovskite layers [30], and the DPPS may form a bulk-hetero structure with perovskite layers, as illustrated in Figure 2d. In the previous work [37], the DPPS was simply added to the perovskite/γ-butyrolactone solutions. In contrast, the DPPS was dissolved in chlorobenzene in the present work. The clear difference is that chlorobenzene is a poor solvent for the perovskite solution, which leads to the formation of the thin DPPS layer on the perovskite layer. In the actual device, the scheme was repeated three times, and the advantage of this method is the formation of a clearer DPPS layer compared with the previous work, as shown in Figure 2d.

4. Conclusions

In summary, MAPbI₃ perovskite photovoltaic devices treated with a polysilane layer were fabricated and characterized. It was confirmed that the insertion of the DPPS layer between the perovskite layer and the hole transport layer improved the photovoltaic properties compared with the standard one. In particular, the device in which the operation D was performed three times showed the highest J_SC and conversion efficiency immediately after the fabrication, and higher photovoltaic properties were observed even after 191 days. The presence of DPPS have contributed to the suppression of the decomposition of the perovskite crystals and to the smooth transport of carriers. From TEM observations, it was confirmed that there was an amorphous phase covering the perovskite grains. This amorphous phase is considered to be the DPPS, and it would work effectively as a bulk-hetero structure with perovskite layers.