Effect of Electrochemically Deposited MgO Coating on Printable Perovskite Solar Cell Performance

Abstract

Herein, we studied the effect of MgO coating thickness on the performance of printable perovskite solar cells (PSCs) by varying the electrodeposition time of Mg(OH)₂ on the fluorine-doped tin oxide (FTO)/TiO₂ electrode. Electrodeposited Mg(OH)₂ in the electrode was confirmed by energy dispersive X-ray (EDX) analysis and scanning electron microscopic (SEM) images. The performance of printable PSC structures on different deposition times of Mg(OH)₂ was evaluated on the basis of their photocurrent density-voltage characteristics. The overall results confirmed that the insulating MgO coating has an adverse effect on the photovoltaic performance of the solid state printable PSCs. However, a marginal improvement in the device efficiency was obtained for the device made with the 30 s electrodeposited TiO₂ electrode. We believe that this undesirable effect on the photovoltaic performance of the printable PSCs is due to the higher coverage of TiO₂ by the insulating MgO layer attained by the electrodeposition technique.

1. Introduction

Perovskite solar cell (PSC) technology has made tremendous progress over the last few years, with a significant increase in power conversion efficiency with recent devices reaching over 22% [1]. Initially, perovskite (CH₃NH₃PbI₃) was used as a sensitizer to replace organic dye molecules in dye-sensitized solar cells by Miyasaka et al. [2]. However, corrosion of the CH₃NH₃PbI₃ by the I⁻/I₃⁻ electrolyte hindered the interest of this new sensitizer until realizing the possibility of replacing the electrolyte with a solid organic hole transport material (HTM), spiro MeOTAD [3]. Since then, both mesoscopic and planar heterojunction PSCs have been fabricated with different architectures and preparation methods [4,5,6,7]. Recently, huge interest was given to printable PSCs with carbon counter electrodes as they demonstrate enormous potential for achieving high efficiency, long lifetime and low manufacturing costs which may lead to future commercialization [7,8,9].

In PSCs, often a thin compact TiO₂ layer (~50 nm) is formed in order to prevent back electron transport from fluorine-doped tin oxide (FTO) to either perovskite or HTM. However, unevenness, surface defects and the presence of pin holes in this layer are often responsible for reducing the cell performance. Therefore, over the last years, many attempts have been made to enhance the overall cell performance of PSCs by retarding the back transfer of photo-generated electrons through the FTO/TiO₂ interface by surface modification of TiO₂ using insulating metal oxides [10,11,12] and hydroxides [13,14] or high band gap semiconductors [15,16] that form a blocking layer between the perovskite sensitizer and TiO₂ layer to block the back electron flow towards the HTM. For instance, TiO₂ modification with a monolayer of silane [17] and ZnO with 3-aminopropanioc acid self-assembled monolayer [18] have been found to enhance the device efficiency by retarding the back electron transfer processes of PSCs. Wang et al. [19] found that magnesium oxide and magnesium hydroxide, formed at the surface of TiO₂, suppress the recombination, achieving an improvement of V_oc and hence photo-conversion efficiency (PCE). Similarly, Jung et al. [10] coat an ultrathin MgO layer on TiO₂ and found an improvement in the fill factor and V_oc of the device, which they believe to be due to the retarded charge recombination at the interface between MgO and CH₃NH₃PbI₃. Conversely, Ke et al. [20] and Liu et al. [21] have shown that a higher V_oc can be obtained for planar PSCs without a compact TiO₂ layer, suggesting that the recombination pathways in PSCs are still unclear and more investigation with different device configurations is required for better understanding.

Compared to other widely used coating techniques such as spin coating and screen printing, the electrodeposition is considered to be a versatile technique for producing surface coatings, owing to its precise controllability, better adherence to substrate, rapid deposition rate with a higher uniformity, room temperature operation and relatively low cost [22,23,24]. In this study, we grew a conformal Mg(OH)₂ coating by the electrodeposition method on the surface of FTO/TiO₂ and investigated the effect of this insulating oxide on their photovoltaic device performance. Our results confirmed that there is an adverse effect on the device performance with the MgO coating obtained by electrodeposition of Mg(OH)₂ on the printable PSCs.

2. Materials and Methods

Fluorine-doped tin oxide (FTO) glass substrates were etched with a laser before being cleaned ultrasonically with detergent, deionized water and ethanol successively. Then, some of the substrates were coated with a TiO₂ compact layer by aerosol spray pyrolysis at 500 °C using a precursor containing 300 μL of titanium diisopropoxide bis (acetylacetonate). The treated film was annealed at 500 °C for 30 min inside an oven. Then, the mesoporous TiO₂ layer was deposited by screen printing using a TiO₂ paste [3 g of F-6 powder (Showatitanium, Toyama, Japan) with 0.5 mL of acetic acid, 15 mL of ethyl cellulose (45–55 mPa·s, TCI, 10 wt % in EtOH) and 50 g of α-terpineol]. After the coating, the film was dried at 125 °C for 5 min and sintered at 500 °C for 30 min using an oven. The Mg(OH)₂ coatings were electrodeposited on the FTO/TiO₂ or FTO substrates in an aqueous electrolyte solution composed of Mg(CH₃COO)₂·4H₂O having a concentration of 0.01 M. The electrodeposition was carried out in the three-electrode configuration using the TiO₂-coated FTO or FTO substrate as the working electrode with the cathode area of 1 cm², Ag/AgCl electrode, and Pt as the reference and counter electrodes, respectively. The electrodeposition was conducted at a constant current of 0.6 mA (Chronopotentiometry) using a Potentiostat/Galvanostat. After the deposition, the films were removed from the electrolyte solution, washed with distilled water and allowed to dry at room temperature. The electrodeposition time was varied for 10 s, 30 s, 1 min, 2 min, 4 min, 6 min, 10 min and 20 min and the deposited Mg(OH)₂ is converted to MgO during the post-annealing of successive layers. In devices A and B, a ZrO₂ space layer was printed on the film using a ZrO₂ paste [3 g of ZrO₂ powder (40–50 nm, Alfa Aesar, Lancashire, UK) with 0.5 mL of acetic acid, 15 mL of ethyl cellulose (45–55 mPa·s, TCI, 10 wt % in EtOH) and 50 g of α-terpineol]. The film was annealed at 400 °C for 30 min inside an oven after drying at 125 °C for 5 min on a hot plate. Then, a NiO mesoporous layer was coated on respective devices using a NiO paste consisting of 3 g of NiO powder (20 nm, Iolitec Ionic Liquids Technologies GmbH, Heilbronn, Germany) with 0.5 mL of acetic acid, 15 mL of ethyl cellulose (45–55 mPa·s, TCI, 10 wt % in EtOH) and 50 g of α-terpineol. After the pre-drying at 125 °C, the NiO layer was sintered at 500 °C for 30 min using an oven. Finally, a carbon black/graphite layer was coated on the top of the ZrO₂ or NiO layer by the screen printing method using a pot milled carbon black/graphite paste [1 g of Printex L6 carbon (Evonik Industries, Frankfurt, Germany), 1 g of graphite, 4 g of graphite flakes (~325 mesh, Alfa Aeser, Haverhill, MA, USA), 5 g of TiO₂ (P25), 20 g of α-terpineol and 30 g of ethyl cellulose (10 wt % in EtOH) and sintered at 400 °C for 30 min inside an oven. The synthesis of CH₃NH₃PbI₃ (MAPbI₃) and deposition on the devices was carried out by the two-step deposition method onto the carbon black/graphite layer (using 1.2 M PbI₂ in DMF solution and CH₃NH₃I solution (10 mg/mL)]. Upon drying at 70 °C for 30 min, the films darkened in colour, indicating the formation of MAPbI₃ in the solar cell. All of the processes were performed under ambient conditions.

Photovoltaic measurements were conducted using an AM 1.5 solar simulator equipped with a xenon lamp (Yamashita Denso, Tokyo, Japan). The power of the simulated light was calibrated to 100 mW·cm⁻² by a reference Si photodiode (Bunkou Keiki, Tokyo, Japan). J–V curves were obtained by applying an external bias to the cell and the generated photocurrent was measured with a B2901A, Agilent voltage current source (Santa Clara, CA, USA). The active area of the cells was fixed at 0.04 cm². The SEM images were obtained by JOEL-JSM-6510 scanning electron microscopes (JEOL, Tokyo, Japan) and EDX measured using a TE3030, Hitachi machine (Tokyo, Japan).

3. Results and Discussion

In order to investigate the effect of MgO on the photovoltaic performance of printable PSCs, we have deposited a MgO layer on FTO/TiO₂ (devices B and C) or FTO (device D) by varying the Mg(OH)₂ electrodeposition time on different mesoscopic structures (Figure 1). The deposited Mg(OH)₂ is converted to MgO upon post-annealing of successive layers. The devices made with a standard four-layer structure of TiO₂/ZrO₂/NiO/Carbon (MAPbI₃) are depicted as A and the devices with the same architecture with MgO coating on TiO₂ are illustrated as device B. Device C is employed with the intention of fabricating a thin insulation layer of MgO on TiO₂ and FTO (with the deposition time) before coating the hole-transporting NiO layer. Device D is designed according to the meso-super-structured solar cell structure where the MgO layer is acting as a scaffold for MAPbI₃ in the device.

During the electrodeposition of Mg(OH)₂ in the devices B, C and D, the below electrochemical reactions could take place at the cathode surface [13],

$${\mathrm{NO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{NO}}_2^{-}+2{\mathrm{OH}}^{-}$$

(1)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to 2{\mathrm{OH}}^{-}+{\mathrm{H}}_2$$

(2)

As a result of these reactions (i.e., with the formation of OH⁻), a steep increase of local pH (~9) in the electrodeposition solution near to the cathode could occur, leading to the formation of Mg(OH)₂ due to the poor solubility of Mg(OH)₂ (K_sp of Mg(OH)₂ is 1.2 × 10⁻¹¹ mol³·dm⁻⁹ at 25 °C) [25,26]. This flocculated Mg(OH)₂ in the solution can be hetero-coagulated on TiO₂ and FTO electrostatically due to their opposite surface charges. (Isoelectric points of TiO₂ ~ 6.2 FTO ~ 6 and Mg(OH)₂ ~ 12) [13]. Once the electrodeposition is started, it is more likely that the low resistive exposed FTO surface (i.e., pinholes) and the TiO₂ surface in the vicinity are coated with Mg(OH)₂. However, with the increasing deposition time, the alkaline pH boundary extends further away from the interior surface which could lead to the TiO₂ nanoparticles being covered by Mg(OH)₂.

The surface topographic FEG-SEM images of electrodeposited Mg(OH)₂ for 2 min and 10 min deposition times on FTO glass substrate are shown in Figure 2a,b respectively. The images showed that the substrates are completely covered by electrodeposited flower-like Mg(OH)₂ spheres [13,27]. Figure 2c,d shows the cross-sectional images of devices C and D respectively. As shown in Figure 2c, the thickness of the TiO₂<MgO>/ZrO₂/NiO (MAPbI₃) is around 2 µm whereas the carbon counter electrode is estimated to be around 25 µm. The MgO layer in the device cannot be seen clearly as it is too thin to be visible in the cross-sectional images. However, EDX (Figure 2e) mapping of the cross-sectional image of device D, confirms the presence of Mg, Ni and carbon in the electrode.

The influence of the MgO layer and its thickness (by varying the Mg(OH)₂ electrodeposition time) on the photovoltaic performance of these different printable PSC structures was evaluated by their I–V characteristics. Figure 3 illustrates the variations observed in the I–V characteristics of the TiO₂/ZrO₂/NiO/Carbon(MAPbI₃) structured device (device A) against the Mg(OH)₂ deposition time (device B). The device A yielded a V_oc of 0.78 V, a short-circuit current density (J_sc) of 9.30 mA·cm⁻² and a fill factor of 0.44, which correspond to a PCE of 3.19%. The I–V plots and the average photovoltaic parameters in

Table 1. Summary of the average solar cell parameters of devices A and B against the electrodeposition times of Mg(OH)₂.

Time	Jsc (mA·cm−2)	Voc (V)	Fill Factor	PCE (%)
0	9.30 ± 0.23	0.78 ± 0.01	0.44 ± 0.01	3.19 ± 0.14
30 s	8.48 ± 0.38	0.75 ± 0.01	0.52 ± 0.01	3.26 ± 0.15
1 min	6.32 ± 0.68	0.48 ± 0.04	0.36 ± 0.01	1.10 ± 0.20
2 min	2.26 ± 0.17	0.20 ± 0.02	0.26 ± 0.01	0.12 ± 0.02
4 min	1.69 ± 0.19	0.47 ± 0.03	0.34 ± 0.02	0.27 ± 0.06
6 min	1.32 ± 0.16	0.22 ± 0.03	0.26 ± 0.01	0.07 ± 0.02
10 min	0.61 ± 0.03	0.79 ± 0.01	0.53 ± 0.01	0.26 ± 0.01

of devices show that the device’s performance is clearly influenced by the systematic growth of Mg(OH)₂ on TiO₂. The cell prepared with the 30 s Mg(OH)₂ electrodeposited electrode showed a V_oc of 0.75 V, J_sc of 8.48 mA·cm⁻², fill factor of 0.52 and PCE of 3.26%. It is noticeable that the J_sc has systematically decreased as the Mg(OH)₂ coating time varied from 1 to 10 min, suggesting the blocking of porous TiO₂. It is more likely that the coverage of Mg(OH)₂ on TiO₂ prevents MAPbI₃ on TiO₂ which was evident by the small reduction of photocurrent density up to 30 s. The trend was continued as the electrodeposition time further increased. The device with 10 min of Mg(OH)₂ coating showed the lowest J_sc of 0.61 mA·cm⁻², with a trivial improvement in the V_oc (0.79 V) and fill factor (0.53) compared to device A.

In device C, Mg(OH)₂ was electrodeposited after coating the mesoporous TiO₂ layer without a compact TiO₂ layer and the MgO layer was replaced with TiO₂ and ZrO₂ layers in the device D. As shown in Figure 4a and

Table 2. Summary of the average solar cell parameters of devices C and D against the electrodeposition time of Mg(OH)₂.

Time (min)	Jsc (mA·cm−2)	Voc (V)	Fill Factor	PCE (%)
Device C
0	4.99 ± 0.56	0.10 ± 0.01	0.26 ± 0.01	0.14 ± 0.03
0.5	3.75 ± 0.51	0.06 ± 0.01	0.26 ± 0.01	0.05 ± 0.01
1	1.67 ± 0.09	0.02 ± 0.01	0.25 ± 0.01	0.009 ± 0.002
2	0.53 ± 0.03	0.021 ± 0.002	0.25 ± 0.01	0.0029 ± 0.0002
4	3.45 ± 0.22	0.019 ± 0.002	0.243 ± 0.006	0.02 ± 0.01
6	1.74 ± 0.03	0.071 ± 0.001	0.26 ± 0.01	0.032 ± 0.001
10	1.55 ± 0.04	0.058 ± 0.001	0.251 ± 0.002	0.023 ± 0.001
Device D
10	0.31 ± 0.01	0.097 ± 0.001	0.27 ± 0.01	0.0079 ± 0.0001
20	0.32 ± 0.01	0.12 ± 0.01	0.27 ± 0.01	0.0099 ± 0.0002

, device C demonstrated poor device performance compared to the device A. This is probably due to the higher recombination rate at the TiO₂/NiO interface despite the MgO layer on TiO₂. The bare device demonstrated a J_sc of 4.99 mA·cm⁻², a V_oc of 0.10 V and a FF of 0.26, leading to a PCE of 0.14%.The device made with a coating of Mg(OH)₂ for 30 s on the FTO/TiO₂ electrode showed a PCE of 0.05%, with a significant decrease in both the J_sc (from 4.99 to 3.75 mA·cm⁻²) and V_oc (from 0.10 to 0.06 V). Although an improvement in PCE was observed when the electrodeposition time increases from 2 to 4 min, the device efficiency is still inferior to the performance of the bare solar cell (Table 2).

Following the meso-super-structured solar cell proposed by Snaith et al. [3], we fabricated device D with FTO/Mg(OH)₂/NiO/Carbon(MAPbI₃) configuration. In this structure, MgO acts as a scaffold for MAPbI₃. The I–V characteristics of the devices with different electrodeposition times of Mg(OH)₂ are shown in Figure 4b. It was clear that up to 10 min electrodeposition time of Mg(OH)₂, the diode characteristic did not appear due to the shorting resulted by the low thickness of the MgO layer. However, the diode characteristic emerged above the 10 min electrodeposition time of Mg(OH)₂. The device made with the 20 min deposited MgO layer showed a J_sc of 0.32 mA·cm⁻², a V_oc of 0.12 V, a fill factor of 0.27 and a PCE of 0.01% (Table 2). However, the higher thickness of MgO does not improve the photocurrent density of the device as expected, which could be due to the higher sheet resistance that results from the thick insulating MgO layer at the higher deposition times.

Overall, the results confirmed that the MgO coating on FTO/TiO₂ (devices B and C) or FTO (device D) in printable PSCs does not show much benefit in improving the PCE compared to the bare devices, which could be due to the higher coverage of TiO₂ by the insulating MgO layer attained by the electrodeposition of Mg(OH)₂.

4. Conclusions

In summary, we studied the effect of MgO coating on the photovoltaic performance of printable PSCs. Electrodeposition of Mg(OH)₂ was conducted on the surface of mesoporous TiO₂ on FTO in devices A, B and C and FTO in device D. The effect of electrodeposition time on the performance of printable PSCs was evaluated on the basis of their key cell parameters. The overall results confirmed that the insulating MgO coating has an adverse effect on the photovoltaic performance of the solid state printable PSCs. We believe that this adverse effect on the photovoltaic performance of the printable PSCs is due to the higher coverage of TiO₂ by the insulating MgO layer attained by the electrodeposition of Mg(OH)₂.