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Abstract
Herein, the low-cost and eco-friendly zinc cation (Zn2+) is used to replace part of the lead cation (Pb2+) in methylammonium lead iodide (CH3NH3PbI3). The modified perovskite material, CH3NH3PbxZn1-xI3, is then obtained and successfully applied in the construction of hole-conductor-free perovskite solar cells (PSCs) based on carbon counter electrodes. The obtained PSCs with 1 mol% Zn doping dramatically facilitate the formation of dense, high surface coverage perovskite films with large grain size and superior crystallinity. Especially, the power conversion efficiency is up to 15.37%, which is a 14.8% increase, compared to the pristine PSCs. This work finds a superior way to further research lead-reduced PSCs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic-inorganic hybrid perovskite materials have emerged as promising light absorbers due to various pivotal properties, such as low cost, strong capacity for light absorption, adjustable band gap, long carrier diffusion length, and high carrier mobility and lifetime [1–4]. Accordingly, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased rapidly from an initial 3.8% in 2009, when perovskite was first applied to solar cells, to the certified world record of 23.2% in 2018 [5–9]. Due to the essential role played by the perovskite material in light absorption performance, as well as to the photovoltaic properties of the device, research on PSCs has drawn great international attention in recent years, and has imponderable potential for development. However, there are many problems with the typical perovskite material, CH3NH3PbI3, such as poor stability, high toxicity and inability to light absorption above 800 nm, which hinders the further development of PSCs [10–12].
Generally, the basic composition of perovskite materials is ABX3, where A stands for cations like methylammonium (CH3NH3+ or MA+), formamidinium (HC(NH2)2+ or FA+) and Cs+; B stands for metal cations like Pb2+, Sn2+ and Ge2+; and X stands for halogen anions like Cl-, Br- and I-. The probable structure and crystal stability of perovskite are mainly limited by the tolerance factor t and the octahedral factor μ, which are calculated by the following Eq. (1):
where r stands for the ionic radius, and 0.81 < t <1.11, 0.44 < μ < 0.90 are the prerequisites to form the perovskite structure [13]. As the tolerance factor increases, the symmetry of the perovskite structure increases, resulting in a decrease in the band gap [14]. Therefore, the crystal structure and band structure of the perovskite material change whenever a component of ABX3 is replaced by other ions, which makes doping or substitution a good way of enhancing the photovoltaic properties of perovskite materials. Constrained by the tolerance factor, the ionic radius of A must be much larger than that of B, thus the choice of A site is limited; the choice of X site is only limited to halogen anions, as well as similar anions. By comparison, the choice of B site can be very broad, in addition to the above-listed Pb2+, Sn2+ and Ge2+, it currently extends to Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Mg2+, Ca2+, Sr2+, Ba2+ and many other divalent metal cations, and even to heterovalent metal cations [15–34]. Meanwhile, the bond angle of B-X has a significant influence on the band gap, the latter decreasing as the former increases, resulting in an extension of the absorption spectrum [35]. Considering that the use of Pb may have adverse effects on the environment, life safety and social development, which hinders the commercialization of PSCs, the development of lead-reduced or lead-free PSCs is of great importance.Hole-conductor-free PSCs based on carbon counter electrodes were first fabricated by Han’s group in 2013 with a PCE of 6.64%. Nowadays, the PCE of these PSCs can reach as high as 15.6% on account of the many advantages of carbon materials, such as abundant resources, low cost, and the similarity to the Fermi level of gold [36–42]. The elimination of expensive hole transport materials and the substitution of noble metal electrodes could greatly enhance the development and commercialization of PSCs. In summary, hole-conductor-free and lead-reduced or lead-free PSCs based on carbon counter electrodes are well worth studying and our previous work focused on the modification of electron transport layer [43–48].
In this work, low-cost and eco-friendly Zn2+ was used to replace part of the Pb2+ in MAPbI3, and the resulting perovskite material, MAPbxZn1-xI3, was then successfully applied to the construction of hole-conductor-free PSCs based on carbon counter electrodes. Obtained results proved that Zn was doped into the perovskite structure and replaced part of Pb, based on X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS), and confirmed that the incorporation of an appropriate amount of Zn could significantly boost the photovoltaic properties of PSCs. Specifically, 1 mol% Zn doping conduced to the improvement of surface morphology, crystallinity and light absorption performance of the perovskite layer, as well as markedly lessened the grain boundaries and defects, where non-radiative recombination usually occurs. Accordingly, the PCE acquired under AM 1.5G irradiation at 100 mW/cm2 intensity increased dramatically, from 13.39% to 15.37%, which amounted to a 14.8% enhancement compared to the pristine PSCs.
2. Experimental
2.1 Materials
Lead iodide (PbI2) and methylammonium iodide (MAI) were purchased from Xi’an p-OLED (China). Titanium diisopropoxide bis(acetylacetonate), anhydrous zinc iodide (ZnI2), dimethylsulfoxide (DMSO) and N, N-dimethylformamide (DMF) were bought from Sigma-Aldrich (US). TiO2 paste, ZrO2 paste and low-temperature carbon electrode paste was acquired from Shanghai MaterWin New Materials (China).
2.2 Synthesis of perovskite precursor solutions
To obtain high-quality perovskite films, the perovskite precursor solutions were prepared in a N2 glovebox by the Lewis acid-base adduct approach [49–51]. Zn2+ was introduced to replace part of the Pb2+ in MAPbI3 by dissolving a PbI2 and ZnI2 mixture in DMF with equimolar DMSO and MAI, where PbI2 and ZnI2 served as the Lewis acid, DMSO served as the Lewis base and DMF served as the solvent. The precursor solutions were stirred thoroughly for several hours at room temperature to form the 1:1:1 adduct of MAI·PbxZn1-xI2·DMSO.
2.3 Device fabrication
The as-prepared PSCs were based on the structure of FTO/compact TiO2/mesoporous TiO2/ZrO2/perovskite/C, as demonstrated in Fig. 1(a), which is similar to the structure obtained by Han’s group, while the preparation technology and the choice of carbon materials are distinct. The cross-sectional SEM image of the PSC (the scale bar is 100nm) is shown in Fig. 1(b), the thickness of FTO, compact TiO2 layer, mesoporous TiO2/perovskite layer and ZrO2/perovskite layer was roughly 500 nm, 40 nm, 150 nm and 150 nm, respectively. In addition, the carbon counter electrodes were about 30 µm, which is upon the perovskite film.
Fig. 1 (a) Schematic structure of the fabricated perovskite solar cells and (b) cross-sectional SEM image of the PSC.
Firstly, the fluorine-doped tin oxide (FTO) glass substrates (7Ω/sq.) were sequentially cleaned with detergent, acetone, isopropanol, and ethanol for 30 min each by using an ultrasonic cleaner. Before use, the FTO glass substrates were put in an ultraviolet (UV) ozone cleaner for about 20 min, for further treatment. Subsequently, the precursor solution composed of titanium diisopropoxide bis(acetylacetonate) and ethanol (1:19, volume ratio) was spin-coated on the FTO glass substrates at 4500 rpm for 20 s and annealed at 150 °C for 10 min. After cooling down, the above procedure was repeat once to form the compact TiO2 layer. Next, the mesoporous TiO2 layer was deposited by spin-coating at 4000 rpm for 30 s, using the TiO2 paste diluted in ethanol with a weight ratio of 1:4, and calcined at 500 °C for 30 min. Then, the ZrO2 layer was deposited by spin-coating at 5000 rpm for 30 s, using the ZrO2 paste diluted in ethanol with a weight ratio of 1:5, and calcined at 500 °C for 30 min. Later on, 40μl of perovskite precursor solutions, which composed of 1 mmol PbI2 and ZnI2 mixture, 159 mg MAI, 78 mg DMSO and 600 mg DMF, were spin-coated on the ZrO2 layer at 1000 rpm for 10 s and 4000 rpm for 20 s. During the second spin-coating step, 0.2 mL toluene was added dropwise to remove the solvent, and the obtained MAPbxZn1-xI3 films were annealed at 100 °C for 10 min. Finally, low-temperature carbon electrode paste was used to prepare the carbon counter electrodes by screen-printing method and annealed at 100 °C for 20 min.
2.4 Characterization
Surface morphologies of the as-prepared samples were observed by a scanning electron microscope (SEM, JSM-IT300, JEOL, Japan). The X-ray diffractometer (XRD, D8 Advance, AXS, Germany) was used to analyze the crystal structure of the as-prepared samples with Cu-Kα radiation. Elemental composition and chemical bonding in the as-prepared samples were explored by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, US). The UV-vis spectrometer (UV-3600, Shimadzu, Japan) was used to obtain the UV-visible absorption spectra of the as-prepared samples. Steady-state photoluminescence (PL) spectra of the as-prepared samples were determined by a fluorescence spectrophotometer (RF-6000, Shimadzu, Japan), using a 532 nm excitation light. A monochromatic incident photon-to-electron conversion efficiency spectrometer (IPCE, Newport Corporation, US) was used to acquire the external quantum efficiency (EQE) of the as-prepared samples. Current-voltage (J-V) characteristics of the as-prepared samples were measured by a source meter (2400, Keithley, US) together with a sunlight simulator (Oriel Sol3A, Newport Corporation, US), under AM 1.5G irradiation, at 100 mW/cm2 intensity.
3. Results and discussion
Figure 2 presents the SEM images of perovskite films with different Zn doping concentrations to explore their effect on the surface morphology of perovskite films. Evidently, 1 mol% Zn doping significantly enlarged the grain size (from ~200 nm to over 1 μm) and surface coverage of perovskite films, which contributed the observed decrease in grain boundaries and defects. Consequently, the photogenerated carriers resulting from the perovskite layer could be conducted in a single crystal grain and collected by transport layers without being hindered by grain boundaries and defects, therefore reducing the non-radiative recombination losses. Nevertheless, increasingly more pinholes and small grains were introduced as the Zn doping concentration exceeded 1 mol%, leading to a decline in the light absorption capacity of perovskite films. The observed small grains could be PbI2 or ZnI2, which were produced by the decomposition of MAPbxZn1-xI3 due to the hot and humid environment. In short, the introduction of 1 mol% Zn2+ into the MAPbI3 layer not only facilitated the formation of dense, high surface coverage perovskite films with large grain size, but also decreased the grain boundaries and defects, enhancing the photovoltaic properties of PSCs.
XRD was used to investigate the impact of Zn2+ doping on the crystal structure of MAPbI3. As shown in Fig. 3(a), the diffraction peaks of perovskite films with Zn doping presented no impure phases, being well matched with the pristine ones as a tetragonal perovskite structure. Obviously, the intensities of the (110) facet and (220) facet diffraction peaks were gradually enhanced as the Zn doping concentration increased from 0 to 3%, indicating that the incorporation of Zn, at an appropriate level, could improve the crystallinity of perovskite films. For further discussion, the detailed view and relevant information of the representative diffraction peak at the (110) facet are shown in Fig. 3(b) and Table 1, respectively. These reveal that the diffraction peak at the (110) facet moved to a larger angle gradually compared with the pristine one. According to Bragg's law, Eq. (2):
the interplanar distance (d) would hence decline, demonstrating that Zn2+ doping caused the lattice of MAPbI3 to shrink. Considering that the ionic radius of Zn2+ (~74 pm) is smaller than that of Pb2+ (~119 pm), Zn doping should be substitutional doping rather than interstitial doping, otherwise the lattice would expand. The shoulder peak at the (110) facet corresponding to the perovskite (002) facet became stronger as the Zn doping concentration increased from 0 to 3%. According to similar previously reported results [52,53], this might be caused by MAZnI3, thus excessive Zn doping could induce phase transition. Meanwhile, the average crystalline size was calculated by the Scherrer Eq. (3):where τ stands for the mean size of the ordered (crystalline) domains, K stands for a dimensionless shape factor, and β stands for the line broadening at half the maximum intensity [54]. Apparently, Zn doping contributed to the increase in average crystalline size of perovskite films, based on the results in Table 1. Evidently, Zn was doped into the perovskite structure and successfully replaced part of Pb, enhancing the crystallinity and crystalline size of perovskite films, which coincides with the results of the SEM images above.Fig. 3 (a) XRD patterns of perovskite films with different Zn doping concentrations and (b) detailed view of the (110) diffraction peak ranging from 13.75° to 14.75°.
Table 1. Relevant information of the representative diffraction peak at the (110) facet.
To further explore the elemental composition and chemical bonding of the perovskite films with and without Zn doping, XPS was used for analysis and the generated spectra are displayed in Fig. 4. The XPS spectra of perovskite films with 1 mol% Zn doping possessed a small binding energy peak in the range of 1000 to 1050 eV compared with the pristine ones in Fig. 4(a), which was assigned to Zn 2p. To acquire more details about the difference after doping with 1 mol% Zn, Figs. 4(b) to 4(d) present the XPS core level spectra of Zn 2p, I 3d, and Pb 4f, respectively. The peak at 1022.49 eV belongs to Zn 2p3/2 in Zn2+, the two peaks at ~630.5 and ~619 eV respectively represent I 3d3/2 and I 3d5/2 in I-, and the two peaks at ~143 and ~138.2 eV respectively belong to Pb 4f5/2 and Pb 4f7/2 in Pb2+. These peaks of the XPS core level spectra suggest that the above elements were in the right chemical state to form the perovskite structure. In contrast to the pristine spectra, not only did 1 mol% Zn doping reduce the intensities of the peaks for both I 3d and Pb 4f, but also caused the peaks of I 3d to move to lower binding energy, while the peaks of Pb 4f slightly moved to higher binding energy. Consequently, the binding energy difference between Pb 4f7/2 and I 3d5/2 was reduced by 0.14 eV, from 480.93 to 480.79 eV, signifying that the Pb-I bond was weakened after doping 1 mol% Zn due to the lower electronegativity of Zn than Pb. In concert with the XRD results, this shows that Zn was doped into the perovskite structure and successfully replaced part of Pb.
Fig. 4 (a) XPS spectra of perovskite films with and without Zn doping, and XPS core level spectra of (b) Zn 2p, (c) I 3d, and (d) Pb 4f.
Figure 5(a) shows the UV-vis absorption spectra of perovskite films with different Zn doping concentrations, to investigate their effect on the light absorption performance of the perovskite layer. In Fig. 5(a), it is apparent that as the Zn doping concentration increased from 0 to 3%, the absorbance of the perovskite films, ranging from 400 to 690 nm, was strengthened at first due to the enlarged grain size and high surface coverage of the perovskite films, the perovskite films with 1 mol% Zn doping had the best absorption performance on the whole. Then the absorbance of the perovskite films was weakened due to the increase in the number of pinholes and defects, especially when Zn doping concentration exceeded 2 mol%, which is in agreement with the SEM images. Besides, the absorption spectra of perovskite films with Zn doping exhibited red shifts from 690 nm and the absorption edge was smoother compared with the pristine one, thus light absorption still existed over 800 nm, demonstrating that Zn2+ doping reduced the band gaps of MAPbI3. Regarding the decrease in band gaps observed as the Zn doping concentration increased from 0 to 3%, the small change of Pb-I bond angle in MAPbI3 crystal structure after doping Zn2+ might have functioned, according to the XRD patterns and XPS spectra. Overall, the introduction of 1 mol% Zn2+ into the MAPbI3 layer markedly enhanced the light absorption performance of PSCs.
Fig. 5 (a) UV-vis absorption spectra of perovskite films with different Zn doping concentrations and (b) steady-state PL spectra of perovskite films with different Zn doping concentrations on glass substrates.
Steady-state PL spectra of the MAPbI3 layers with different Zn2+ doping concentrations on glass substrates were determined using an excitation light of 532 nm to detect the trap or defect density in perovskite films. As exhibited in Fig. 5(b), the intensities of the PL peaks increased initially and then declined as the Zn doping concentration increased from 0 to 3%. The perovskite films with 1 mol% Zn doping had the strongest PL peak intensity, owing to the superior crystallinity of the film with fewer traps or defects, which minimizes the non-radiative recombination losses and prolongs the carrier lifetime. In contrast, the PL intensity was distinctly weakened as the Zn doping concentration exceeded 2 mol%, ascribing to the incremental non-radiative recombination occurring at pinholes or defects, which was in accordance with the inferences seen on the SEM images. Moreover, red shifts of the PL peaks were observed as the Zn doping concentration increased, coinciding with the conclusions of UV-vis absorption spectra that Zn2+ doping reduced the band gaps of MAPbI3.
The J-V characteristic curves of PSCs with different Zn doping concentrations are displayed in Fig. 6(a), which were measured under AM 1.5G irradiation (100 mW/cm2), at a reverse scan rate of 0.15 V/s, whereas the corresponding photovoltaic parameters are presented in Table 2. After doping Zn into the perovskite layer, the short-circuit current density (JSC) was largely promoted, attributing to the enlarged grain size and high surface coverage of perovskite films. In contrast, the open-circuit voltage (VOC) decreased gradually as the Zn doping concentration increased, on account of the reduced band gaps and deteriorating interface properties. Specifically, the pristine PSCs had an inferior photovoltaic performance, with a JSC of 20.93 mA/cm2, VOC of 1.03 V, a fill factor (FF) of 62.1%, and PCE of 13.39%, while the PSCs based on 1 mol% Zn doping had a better photovoltaic performance, with a greatly enhanced JSC of 23.56 mA/cm2, slight reduced VOC of 1.01 V, an improved FF of 64.6%, and the resulting PCE as high as 15.37%, which was a 14.8% increase compared to the pristine PSCs. However, the photovoltaic performance became worse as the Zn doping concentration exceeded 2 mol%, possibly due to the increasing number of pinholes and defects. In Fig. 6(b), the J-V curves under forward and reverse scan are exhibited. The results uncover the pristine PSCs shown obvious hysteresis, while the PSCs based on 1 mol% Zn doping shown almost no hysteresis. Therefore, it can be concluded that Zn could effectively suppress the hysteresis in the J-V curves of the PSCs, which is attributed to the reduced trap states after doping Zn. To ensure the accuracy and reproducibility of the results, ten sets of photovoltaic parameters based on 1 mol% Zn doping and the pristine PSCs are shown in Fig. 6(c), for comparison. Noticeably, the JSC, FF and PCE of all the PSCs were improved to varying degrees after doping 1 mol% Zn into the perovskite layer, with the average JSC increasing from 20.50 mA/cm2 to 23.20 mA/cm2, the average FF from 61.8% to 63.5% and the average PCE from 12.95% to 14.90%, indicating an excellent reproducibility for the as-prepared samples.
Fig. 6 (a) J-V characteristics of PSCs with different Zn doping concentrations, (b) the J-V curves under forward and reverse scan, based on 1 mol% Zn doping and the pristine PSCs, and (c) ten sets of photovoltaic parameters based on 1 mol% Zn doping and the pristine PSCs.
Table 2. Relevant photovoltaic parameters corresponding to Fig. 6(a)a.
In Fig. 7, the EQE spectra and the corresponding integrated current densities based on 1 mol% Zn doping and the pristine PSCs are exhibited. After doping 1 mol% Zn into the perovskite layer, the EQE was significantly promoted, reaching more than 90% within a large range, which was much higher than the pristine one, demonstrating that the separation and transportation efficiency of carriers were greatly improved, as was the light absorption performance. In addition, the integrated current densities were estimated from the EQE spectra to be 20.04 mA/cm2 for the pristine PSCs and 22.36 mA/cm2 for the 1 mol% Zn-doped PSCs, which were roughly consistent with the JSC values acquired from the J-V characteristic curves.
Fig. 7 EQE spectra and the corresponding integrated currents based on 1 mol% Zn doping and the pristine PSCs.
4. Conclusions
In conclusion, low-cost and eco-friendly Zn2+ was introduced to replace part of Pb2+ in MAPbI3, and the resulting modified perovskite material, MAPbxZn1-xI3, was then successfully applied to the construction of hole-conductor-free PSCs based on carbon counter electrodes with remarkable properties. As shown, Zn was doped into the perovskite structure and replaced part of Pb. PSCs with 1 mol% Zn doping not only dramatically facilitated the formation of dense, high surface coverage perovskite films with large grain size and superior crystallinity, conducing to an improvement in light absorption performance, but also markedly lessened the grain boundaries and defects, where non-radiative recombination usually occurs. Especially, the PCE of the 1 mol% Zn-doped PSCs developed from 13.39% to as high as 15.37%, amounting to a 14.8% increase compared to the pristine PSCs. Additionally, this work highlights a superior method of enhancing the photovoltaic properties of hole-conductor-free PSCs based on carbon counter electrodes, facilitating the further progress of lead-reduced or lead-free PSCs.
Funding
National Natural Science Foundation of China (NSFC) (61505151); Excellent Dissertation Cultivation Funds of Wuhan University of Technology (2017-YS-77); Fundamental Research Funds for the Central Universities under Grant WUT (2018IB017).
Acknowledgments
The authors sincerely thank Dr. H. Lu for offering many constructive suggestions and the teachers in Material Research and Testing Center of WHUT for assisting with relevant tests.
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