Efficient perovskite solar cells based on low-temperature solution-processed (CH3NH3)PbI3 perovskite/CuInS2 planar heterojunctions
© Chen et al.; licensee Springer. 2014
Received: 18 July 2014
Accepted: 27 August 2014
Published: 2 September 2014
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%.
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 , copper indium disulfide (CuInS2) [9, 10], and Cu2ZnSnS4 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  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 . 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% . 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., Etgar et al., and Ball et al. 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. . 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 .
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. . 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 . 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%).
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
Summary of device performance under white light illumination with an intensity of 100 mW/cm 2
V oc (V)
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 (Jsc, FF, and η) except Voc of the ITO/CuInS2(3)/Al2O3/(CH3NH3) PbI3/Ag cell decreased. For the ITO/CuInS2 (3)/Al2O3/(CH3NH3)PbI3/Ag cell, the Jsc, 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 (Rs) in the cells, which would inevitably lead to the degradation of Jsc and FF. The Rs can be calculated from the inverse slope of the illuminated J-V characteristics at J = 0. The Rs 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 Rs 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 Jsc and FF.
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.
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.
This work was supported by the Henan University Distinguished Professor Startup Fund, Natural Science Foundation of Henan University (2013YBZR046), National Natural Science Foundation of China-Talent Training Fund of Henan (U1404616), and Seed Fund of Young Scientific Research Talents of Henan University (0000A40540).
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