Efficient perovskite solar cells based on low-temperature solution-processed (CH₃NH₃)PbI₃ perovskite/CuInS₂ planar heterojunctions

Abstract

In this work, the solution-processed CH₃NH₃PbI₃ perovskite/copper indium disulfide (CuInS₂) planar heterojunction solar cells with Al₂O₃ as a scaffold were fabricated at a temperature as low as 250°C for the first time, in which the indium tin oxide (ITO)-coated glass instead of the fluorine-doped tin oxide (FTO)-coated glass was used as the light-incidence electrode and the solution-processed CuInS₂ layer was prepared to replace the commonly used TiO₂ layer in previously reported perovskite-based solar cells. The influence of the thickness of the as-prepared CuInS₂ film on the performance of the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cells was investigated. The ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell showed the best performance and achieved power conversion efficiency up to 5.30%.

Background

Thin-film solar cells have attracted considerable attention because of simplified and low-cost fabrication procedures compared to conventional silicon-based solar cells. The thin-film solar cells based on inorganic photovoltaic materials processed with expensive vacuum-based techniques and/or high-temperature sintering exhibit high efficiency [1–3]. However, the use of these thin-film solar cells is still limited because the manufacturing costs are still relatively high. To lower the cost of device fabrication, the low-temperature solution-based techniques such as spin coating and chemical bath deposition are needed to prepare inorganic photovoltaic materials. The thin-film solar cells based on the solution-processed inorganic nanocrystals such as PbS [4, 5], CdTe [6, 7], CdSe [8], copper indium disulfide (CuInS₂) [9, 10], and Cu₂ZnSnS₄[11] have been demonstrated, but their maximum solar power conversion efficiency is still low. The main reason for the low efficiency is that the low-temperature solution-processed inorganic nanocrystals are typically amorphous or poorly crystalline, leading to poor charge carrier transport because of short carrier diffusion lengths (typically about 10 nm). Therefore, the new solution-based techniques to improve the crystalline structures of inorganic nanocrystals are needed. For example, to enhance the carrier transport in CuInS₂, a method of using a molecular-based precursor solution has been presented [10] to synthesize CuInS₂ nanocrystals with a polycrystalline structure at relatively lower temperatures (<250°C) for the solution-processed inorganic solar cells. Besides, the inorganic materials which can be processed with solution-based techniques and generated charge carriers with long diffusion lengths in the bulk are sought.

The recently reported semiconducting perovskite materials such as (CH₃NH₃)PbX₃ (X = Cl, Br, I) could fulfil these requirements. These perovskites have high charge carrier mobilities and long charge carrier lifetime, which means that the light-generated charges have long carrier transport lengths [12]. It has been reported that the effective diffusion lengths are about 100 nm for both electrons and holes [13, 14]. In addition, these perovskites with a direct bandgap have a broad range of light absorption and high extinction coefficient [15, 16]. Due to their super electrical properties and super light-harvesting characteristics, the perovskites have been used in a variety of nanostructured solar cells and have achieved high-power conversion efficiencies (>9%) [16–20]. In solid-state sensitized solar cells, the CH₃NH₃PbI₃ used as the sensitizer has led to a high-power conversion efficiency of 15% [17]. The other perovskite-based nanostructured solar cells that commonly incorporated the perovskite as the absorbing layer between an n-type electron-transporting layer such as TiO₂ and a p-type hole-transporting layer such as 2,2′,7,7′-tetrakis(N, V-di-p-methoxyphenylamino)-9,9′- spirobifluorene (Spiro-OMeTAD) have also demonstrated high efficiencies [15, 18, 21]. Moreover, the research results reported by Lee et al.[16], Etgar et al.[22], and Ball et al.[23] showed that the perovskites have good charge (electron or hole)-transport properties, resulting in high efficiencies of the solar cells. Nevertheless, in these perovskite-based nanostructured solar cells, the transparent TiO₂ compact layer between the conducting substrate and perovskite materials or the scaffold (Al₂O₃) generally requires high-temperature sintering at about 500°C [15, 17, 18, 23, 24], which limits substrate choice and is incompatible with the low-cost solar technology. Therefore, the low-temperature solution-processed semiconductor materials that could replace the TiO₂ for the perovskite-based nanostructured solar cells are needed. In addition to the preparation method, the electronic energy levels (EELs) of those substitute materials are needed to match the EELs of the perovskite materials for efficient charge transfer. For this purpose, ZnO compact layer and ZnO nanorods are recently prepared by electrodeposition and chemical bath deposition, respectively, to replace the TiO₂ by Kumar et al. [25]. It is worth noting that the materials used to replace the TiO₂ do not necessarily have to be the n-type semiconductors such as the ZnO because the perovskites can conduct not only positive holes [22, 26] but also electrons [16].

It is known that CuInS₂ as a p-type semiconductor is a very promising light-absorbing material for its direct bandgap of 1.5 eV, which is closely matched to the best bandgap (1.45 eV) of the solar cell materials [27]. Recently, a method of using a molecular-based precursor solution to synthesize CuInS₂ nanocrystals at relatively lower temperatures (250°C) has been presented by Li et al. [10]. More importantly, the valence band level (−5.6 eV) of the CuInS₂ is also matched to that (−5.6 or −6.5 eV) [13] of the (CH₃NH₃)PbI₃, which is very beneficial to the hole transfer from the (CH₃NH₃)PbI₃ to the CuInS₂. Therefore, replacing the TiO₂ with CuInS₂ is reasonable. In this study, for the first time, the p-type semiconductor material, CuInS₂, as both the light harvester and hole transporter is prepared by the reported method [10] to replace the commonly used n-type TiO₂ in the perovskite-based solar cells. Moreover, the indium tin oxide (ITO) glass rather than the commonly used fluorine-doped tin oxide (FTO) glass in previously reported perovskite-based solar cells is used as a light-incidence electrode because the CuInS₂ film can directly be deposited on the ITO glass at a temperature as low as 250°C. After the deposition of CuInS₂ film, the Al₂O₃ and (CH₃NH₃)PbI₃ are successively deposited on the CuInS₂ film to form the CuInS₂/(CH₃NH₃)PbI₃ planar heterojunction. The porous Al₂O₃ layer acts as a scaffold. Finally, an evaporated Ag top electrode was deposited on the (CH₃NH₃)PbI₃ at a pressure of 10⁻⁶ Torr to complete the device fabrication. The schematics and energy diagram of the prepared solar cells are shown in Figure 1a, b, respectively. The surface morphology, structure characterization, and optical property of the prepared CuInS₂/(CH₃NH₃)PbI₃ film are studied. Furthermore, the influence of the thickness of the CuInS₂ film on the power conversion efficiency of the fabricated CuInS₂/(CH₃NH₃)PbI₃ planar heterojunction solar cell is investigated.

Figure 1

Schematic diagram (a) and energy diagram (b) of the ITO/CuInS ₂ /Al ₂ O ₃ /(CH ₃ NH ₃ )PbI ₃ /Ag device.

Methods

Materials

Indium acetate (In(OAc)₃, 99.99%), copper iodode (CuI, 99.999%), thiourea (≥99.0%), 1-propionic acid (≥99.5%), γ-butyrolactone (≥99.0%), aluminum oxide (Al₂O₃, 20 wt.% in isopropanol), methylamine (40 wt.% in H₂O), hydroiodic acid (57 wt.% in water), diethyl ether, and PbI₂ (99.999%) were purchased from Sigma-Aldrich (St. Louis, MO, USA). All the reagents were used without further purification. Indium tin oxide-coated glass slides (ITO, ≤15 Ω/square, Wuhu Token Sci. Co., Ltd., China) were cleaned by successive ultrasonic treatment in deionized water, acetone, and isopropyl alcohol and then dried at 100°C for 10 min.

Synthesis of CuInS₂ nanocrystal film on ITO substrate

CuInS₂ nanocrystals were synthesized through a spin-coating method, which is similar to that reported by Li et al. [10]. Briefly, CuI (0.11 mmol), In(OAc)₃ (0.1 mmol), and thiourea (0.5 mmol) were dissolved in a mixture of 1-butylamine (0.6 mL) and 1-propionic acid (40 μL) under a nitrogen atmosphere in a glovebox (O₂ < 0.1 ppm, H₂O < 0.1 ppm). The mixture was shaken for 1 min, and after which, the obtained CuInS₂ precursor solution was then spin-cast onto the cleaned ITO substrates at 4,000 rpm for 30 s. Then, the obtained films were calcined at 150°C for 10 min and then heated to 250°C and held for 15 min at this temperature. To change the thickness of the CuInS₂ film, a spinning-drying cycle was repeated several times. The ITO/CuInS₂ sample after n cycles of CuInS₂ deposition was denoted as ITO/CuInS₂(n).

Synthesis of methylammonium iodide and (CH₃NH₃)PbI₃

Methylammonium iodide (CH₃NH₃I) was synthesized by reacting methylamine (aqueous, 40 wt.%) and hydroiodic acid (aqueous, 57 wt.%) in an ice bath for 2 h with stirring, as described elsewhere [18]. After that, the solvent was evaporated and the precipitate was washed using diethyl ether three times and dried at 60°C for 24 h in a vacuum oven. The resulting product, CH₃NH₃I, was used without further purification. To obtain a (CH₃NH₃)PbI₃ precursor, the synthesized CH₃NH₃I was mixed with PbI₂ at a 1:1 mol ratio in γ-butyrolactone (40% by weight) at 60°C.

Solar cell fabrication

First, a Al₂O₃ layer was introduced on the ITO/CuInS₂(n) films by spin coating isopropanol solution containing Al₂O₃ nanoparticles at 4,000 rpm for 60 s. After that, the films were dried at 150°C for 30 min to obtain ITO/CuInS₂(n)/Al₂O₃. Then, the prepared ITO/CuInS₂(n)/Al₂O₃ films were spin-coated with the obtained (CH₃NH₃)PbI₃/γ-butyrolactone solution at 3,000 rpm for 60 s and then dried at 100°C for 1 h to form crystalline (CH₃NH₃)PbI₃. The prepared ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films were naturally cooled to room temperature. All the experiment was finished in a nitrogen glovebox (O₂ < 0.1 ppm, H₂O < 0.1 ppm). Finally, a silver back contact layer was deposited by thermal evaporation onto the ITO/CuInS₂(n)/(CH₃NH₃)PbI₃ films from a silver wire (99.999%).

Characterization

The surface morphology and structure of the prepared ITO/CuInS₂(n) and ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films were characterized using a scanning electron microscope (SEM) (JSM-7001 F, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) and power X-ray diffractometry (XRD) (DX-2500, Dandong Fangyuan Instrument Co., Ltd., Dandong, China), respectively. It should be noted that, for XRD measurement, the CuInS₂ and (CH₃NH₃)PbI₃ films are individually deposited on the cleaned glass without ITO layer to exclude the influence of the substrate on the XRD measurement. UV-visible absorption measurements were conducted using a UV–vis spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan). Current density-voltage (J-V) characteristics of the as-prepared solar cells were measured using a Keithley 2410 SourceMeter (Keithley Instruments, Inc., Cleveland, OH, USA). A solar simulator (Newport Inc., Irvine, CA, USA) was used as the light source to provide AM 1.5 G simulated solar light (100 mW/cm²). Before each measurement, the light intensity was determined using a calibrated Si reference diode. For all measurements, the effective illumination area of the cells was 4 mm². The monochromatic incident photon-to-electron conversion efficiency (IPCE) spectra for the fabricated solar cells were measured using a commercial setup (QTest Station 2000 IPCE Measurement System, Crowntech, Macungie, PA, USA).

Results and discussion

The morphology, structures, and chemical composition of the as-prepared CuInS₂ were studied with SEM studies accompanied by energy dispersive X-ray spectrometry (EDX). Figure 2a shows a typical top-view SEM image of the ITO/CuInS₂(1) film. As shown in Figure 2a, the surface of the ITO substrate is covered with the CuInS₂ film. The CuInS₂ film is composed of CuInS₂ nanoparticles, and these CuInS₂ nanoparticles appear to be fused together after heating at 250°C. Moreover, some voids can be clearly seen in the ITO/CuInS₂ (1) film, which can be explained by the decomposition of volatile surface ligands and precurse materials [10]. For a comparison, the SEM top image of the ITO/CuInS₂(2) is displayed in Figure 2b. It can be observed that, compared to the ITO/CuInS₂(1) film, the number of the voids in the ITO/CuInS₂(2) film decreases significantly, indicating that the voids in the ITO/CuInS₂(1) have been filled by the CuInS₂ precursor after two times of spin coating. In addition, the cross-sectional SEM images of the ITO/CuInS₂(2), ITO/CuInS₂(2)/Al₂O₃, and ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag films are shown in Figure 2c. It clearly shows that the CuInS₂(2) film with an average thickness of 400 nm is formed on the ITO glass, and there are no obvious voids in the film. After the deposition of Al₂O₃, the thickness of the ITO/CuInS₂(2)/Al₂O₃ film increased. After the deposition of (CH₃NH₃) PbI₃, the thickness of the ITO/CuInS₂ (2)/Al₂O₃/(CH₃NH₃) PbI₃ film further increased to about 650 nm. The more important thing is that the (CH₃NH₃) PbI₃ precursor solution permeated into the porous Al₂O₃ layer to form the (CH₃NH₃) PbI₃. The corresponding EDX spectrum of ITO/CuInS₂ (2) is shown in Figure 2d, which shows the film is mainly composed of copper (Cu), indium (In), and sulfur (S). The chemical compositional analysis reveals that the atomic ratio of Cu, In, and S is 25.33%, 24.9%, and 49.77%, respectively, close to 1:1:2, which confirms the formation of CuInS₂.

Figure 2

The morphology, structures, and chemical composition of the as-prepared CuInS ₂ . (a) Top view of the ITO/CuInS₂(1) film; (b) the top-view SEM image of the ITO/CuInS₂(2) film; (c) the cross-sectional images of the ITO/CuInS₂(2), ITO/CuInS₂(2)/Al₂O₃, and ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag films; and (d) the EDX spectrum of the ITO/CuInS₂(2) film.

To characterize the crystal structure of the CuInS₂, a typical XRD pattern of the as-prepared CuInS₂(3) film on a clean glass substrate is shown in Figure 3. The well-defined peaks can be referred to a tetragonal CuInS₂ (112), (204), (220), (116), and (312) (JCPDS file no. 85–1575), which is in agreement with the reported results [10, 28, 29].

Figure 3

The XRD pattern of the as-prepared CuInS ₂ (3) film.

Figure 4a shows a top-view SEM image of the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃ film. By comparing this image with that (Figure 2b) of the ITO/CuInS₂(2) film, it can be clearly observed that the (CH₃NH₃)PbI₃ film was deposited on the CuInS₂. However, the solution-processed (CH₃NH₃)PbI₃ films are not very uniform and coated the CuInS₂ film only partially with micrometer-sized (CH₃NH₃)PbI₃ platelets, which is very similar to the observed phenomenon in the (CH₃NH₃)PbI₃-coved compact TiO₂ film [21]. Furthermore, to characterize the crystal structure and phase composition of the synthesized (CH₃NH₃)PbI₃ film, the XRD analysis of the prepared (CH₃NH₃)PbI₃ film was performed and shown in Figure 4b. It can be seen from Figure 4b that the diffraction peaks are in good agreement with the tetragonal phase of the (CH₃NH₃) PbI₃ perovskite [17, 30].

Figure 4

SEM image and XRD analysis. (a) Top-view SEM image of the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃ film and (b) the XRD analysis of the prepared (CH₃NH₃)PbI₃ film.

To study the light absorption properties of the prepared ITO/CuInS₂(n) and ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films for application in photovoltaic devices, light absorption studies are carried out. Figure 5 shows the UV–vis absorption spectra of the ITO/CuInS₂(1) and ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films (n = 1, 2, and 3). As shown in Figure 5, the ITO/CuInS₂(1) film has light absorption at wavelengths below 825 nm, which is similar to the reported results [10, 31, 32]. After the (CH₃NH₃)PbI₃ film was deposited on the ITO/CuInS₂(1) film, the absorbance of the spectra of the ITO/CuInS₂(1)/Al₂O₃/(CH₃NH₃)PbI₃ film increases significantly in the UV region as well as the visible region. For the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films after other spin-cast cycles (n = 2 and 3) in our experiments, similar results are also obtained, which can be attributed to the light absorption of the deposited (CH₃NH₃)PbI₃ film. Moreover, for the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃ films, Figure 5 also illustrates that the light absorbance was enhanced with an increase in spin-cast cycle number n, indicating an increased deposition amount of CuInS₂. In addition, the absorption lines of the ITO/CuInS₂/(CH₃NH₃)PbI₃ films are very similar to those of the reported FTO/TiO₂/(CH₃NH₃)PbI₃ films [33], which further confirms the formation of (CH₃NH₃)PbI₃ film and shows the potential of the ITO/CuInS₂/Al₂O₃/(CH₃NH₃)PbI₃ films in photovoltaic application.

Figure 5

UV–vis absorption spectra of the ITO/CuInS ₂ (1) and ITO/CuInS ₂ ( n )/Al ₂ O ₃ /(CH ₃ NH ₃ )PbI ₃ films ( n = 1, 2, and 3).

The J-V characteristics of the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cells under simulated AM 1.5 G solar irradiation and in the dark are shown in Figure 6. All device parameters under the light illumination, the open-circuit voltage (V_oc), the short-circuit photocurrent (J_sc), the fill factor (FF), and the solar power conversion efficiency (η), extracted from the J-V characteristics are summarized in Table 1. For the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cells, with the increase of CuInS₂ deposition number n from 1 to 2, the J_sc increased from 8.85 to 9.92 mA/cm², the V_oc increased from 0.74 to 0.76 V, the FF increased from 0.51 to 0.70, and the η increased from 3.31% to 5.30%. These results might be caused by the voids in the CuInS₂ film. As shown in Figure 2a, some voids have been found in the ITO/CuInS₂(1) film. When the (CH₃NH₃)PbI₃ precursor solution was spin-cast onto the ITO/CuInS₂(1)/Al₂O₃ film, these voids might be filled by the (CH₃NH₃)PbI₃ precursor solution. Therefore, similar to the observed phenomenon in the mesoporous-TiO₂/(CH₃NH₃)PbI₃ film, the (CH₃NH₃)PbI₃ probably infiltrated to the bottom of the CuInS₂ film and had a contact with the hole collection electrode (i.e., the ITO electrode), which will enhance the probability of recombination between electrons in the (CH₃NH₃)PbI₃ and holes in the ITO in the ITO/CuInS₂(1)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cell. In contrast, as shown in Figure 2c, there are few voids in the ITO/CuInS₂(2) film, which may effectively reduce the charge recombination at the ITO/(CH₃NH₃)PbI₃ interface in the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cell. This explanation is supported by the J-V characteristics of the ITO/CuInS₂(n)/(CH₃NH₃)PbI₃/Ag in the dark (Figure 6) since the charge recombination can be typically represented by the dark current [2, 34, 35]. It can be observed that the dark current density of the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell is lower than that of the ITO/CuInS₂(1)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell, which indicates that the charge recombination is reduced in the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell.

Figure 6

J - V characteristics. J-V characteristics of the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cells under simulated AM 1.5G solar irradiation and in the dark (n = 1, 2, and 3).

Table 1 Summary of device performance under white light illumination with an intensity of 100 mW/cm ²

However, the device performance of the ITO/CuInS₂ (n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell decreased as the CuInS₂ deposition number n increased further from 2 to 3. It can be found from Table 1 that, compared to the ITO/CuInS₂ (2)/Al₂O₃/(CH₃NH₃) PbI₃/Ag cell, all device parameters (J_sc, FF, and η) except V_oc of the ITO/CuInS₂(3)/Al₂O₃/(CH₃NH₃) PbI₃/Ag cell decreased. For the ITO/CuInS₂ (3)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell, the J_sc, FF, and η decreased to 8.98 mA/cm², 0.38, and 2.60%, respectively. The main reason for the decreased device performance may be the increased thickness of CuInS₂ film. As shown in Figure 1, in the ITO/CuInS₂/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell, the CuInS₂ mainly conducted holes. Therefore, increasing the thickness of CuInS₂ film would increase the hole-transfer resistance and lead to an increase in overall series resistance (R_s) in the cells, which would inevitably lead to the degradation of J_sc and FF. The R_s can be calculated from the inverse slope of the illuminated J-V characteristics at J = 0. The R_s values for the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag and ITO/CuInS₂(3)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cells are 6.4 and 29.6 Ω/cm², respectively, which were calculated from the illuminated J-V characteristics (shown in Figure 6). Obviously, compared to the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell, the R_s of ITO/CuInS₂(3)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell increased due to the increased thickness of the CuInS₂ film. Furthermore, a too thick CuInS₂ film may dramatically reduce the amount of light absorbed by the (CH₃NH₃)PbI₃ film, which results in a sizeable reduction in the number of the photo-generated electrons in the (CH₃NH₃)PbI₃ film and therefore reduces the J_sc and FF.

Figure 7 shows the IPCE spectra of the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cell. It can be observed that the solar cell shows a spectral response in the almost entire wavelength region from 370 to 1,000 nm. The IPCE of over 31% is observed at a wavelength range from 370 to 750 nm. Furthermore, for the IPCE value, a sharp decrease in the wavelength region from 750 to 820 nm is observed. The threshold wavelength of 820 nm is related to the bandgap of about 1.5 eV for (CH₃NH₃)PbI₃[36]. These results are in agreement with the previously reported IPCE spectra for the perovskite solar cells without a CuInS₂ layer [19, 36, 37]. It should be noted that, for these reported perovskite solar cells, the IPCE values are nearly zero in the long wavelength region (820 to 1,000 nm). However, in our case, IPCE of over 9% is observed at an entire wavelength range from 820 to 1,000 nm, resulting from the photocurrent originating from the CuInS₂ layer. Therefore, the CuInS₂ layer can improve the IPCE values of the solar cells in the long wavelength region.

Figure 7

IPCE spectra of the ITO/CuInS ₂ ( 2 )/Al ₂ O ₃ /(CH ₃ NH ₃ )PbI ₃ /Ag solar cell.

Our experimental results demonstrated that, for the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cells, the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell showed the highest solar power conversion efficiency of 5.30%. However, it should be noted that the highest power conversion efficiency presented here was just taken from the solar cells with a simplified architecture. The solar cell architecture can be further optimized. For example, a hole-selective layer can be inserted between the ITO and the CuInS₂ layers to reduce the charge recombination at the ITO/CuInS₂ interface. Similarly, inserting an electron-selective layer between the (CH₃NH₃)PbI₃ layer and the Ag electrode may also suppress the charge recombination at the (CH₃NH₃)PbI₃/Ag interface. Therefore, the solar cells with an architecture that incorporates the charge (hole or electron)-selective layer may achieve higher power conversion efficiency, which is our future study.

Conclusions

In summary, the solution-processed (CH₃NH₃)PbI₃ perovskite/CuInS₂ planar heterojunction solar cells with a Al₂O₃ scaffold have been successfully fabricated, in which the CuInS₂ films as both the light harvester and hole transporter were prepared at a relatively low temperature (250°C) via a simple solution-based chemical approach to replace the commonly used n-type TiO₂ layer. The influence of the thickness of CuInS₂ film on the performance of the fabricated ITO/CuInS₂/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cells was investigated. Our experimental results demonstrated that an optimum power conversion efficiency of up to 5.30% can be achieved by the ITO/CuInS₂(2)/Al₂O₃/(CH₃NH₃)PbI₃/Ag cell. Optimizing the device architecture may further improve the performance of the ITO/CuInS₂(n)/Al₂O₃/(CH₃NH₃)PbI₃/Ag solar cells. The present research findings offer a new approach to achieve low-cost and high-efficiency solar cells.