Perovskite Solar Cells Based on Low-Temperature Processed Indium

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Perovskite solar cells based on low-temperature processed indium oxide electron selective layers Minchao Qin, Junjie Ma, Weijun Ke, Pingli Qin, Hongwei Lei, Hong Tao, Xiaolu Zheng, Liangbin Xiong, Qin Liu, Zhiliang Chen, Junzheng Lu, Guang Yang, and Guojia Fang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12849 • Publication Date (Web): 21 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

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ACS Applied Materials & Interfaces

Perovskite solar cells based on low-temperature processed indium oxide electron selective layers Minchao Qin,a Junjie Ma,a Weijun Ke,a Pingli Qin,a Hongwei Lei,a Hong Tao,a Xiaolu Zheng,a Liangbin Xiong,a Qin Liu,a Zhiliang Chen,a Junzheng Lu,a Guang Yanga, and Guojia Fang*a

a

Key Lab of Artificial Micro- and Nano-Structures of Ministry of Education of China, School of Physics and Technology, Wuhan University, Wuhan 430072, People’s Republic of China

*Corresponding Author. E-mail: [email protected]

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ABSTRACT: Indium oxide (In2O3) as a promising n-type semiconductor material has been widely employed in optoelectronic applications. In this work, we applied low-temperature solution-processed In2O3 nanocrystalline film as an electron selective layer (ESL) in perovskite solar cells (PSCs) for the first time. Taking advantages of good optical and electrical properties of In2O3 such as high mobility, wide band gap, and high transmittance, we obtained In2O3-based PSCs with a good efficiency exceeding 13% after optimizing the concentration of the precursor solution and the annealing temperature. Furthermore, to enhance the performance of the In2O3based PSCs, a phenyl-C61-butyric acid methyl ester (PCBM) layer was introduced to modify the surface of the In2O3 film. The PCBM film could fill up the pinholes or cracks along In2O3 grain boundaries to passivate the defects and make the ESL extremely compact and uniform, which is conducive to suppressing the charge recombination. As a result, the efficiency of the In2O3-based PSC was improved to 14.83% accompanying with VOC, JSC and FF being 1.08 V, 20.06 mA cm-2 and 0.685, respectively.

KEYWORDS: perovskite solar cells, indium oxide, electron selective layer, PCBM, low temperature, sol-gel method

1.

Introduction Recent years, inorganic-organic lead halide perovskite materials have attracted

extensive attention due to their ambipolar charge transport,1 high absorption coefficient,2 high carrier mobility,3 and long electron-hole diffusion lengths.4,5 As a result, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been quickly increased to 20.1% from the initial PCE of 3.8%.6-12

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Generally, an efficient PSC contains a transparent electrode, an electron selective layer (ESL), a perovskite absorber layer, a hole selective layer (HSL), and a back electrode. The ESL, which plays a crucial role in transporting photo-generated electrons and suppressing the electron-hole recombination, is one of the most important components in PSCs.13 An excellent ESL is mainly ascribed to its good optical and electrical properties. TiO2 is the most common ESL materials in PSCs and plays a vital role to keep pushing the performance of PSCs to an even higher level due to its good transparency, high mobility (0.1-4 cm2 V-1 s-1), and well-matched energy band with perovskite.14-17 Nevertheless, a high-quality TiO2 film usually requires high-temperature annealing at ~450˚C to form TiO2 crystals, causing the cost increase and impeding the development of flexible devices. To solve this problem, ZnO, which mobility is 205-300 cm2 V-1 s-1, has been employed in PSCs by Kumar et al.,18-21 and the ZnO-based PSCs have exhibited satisfactory performance with the highest PCE reaching 17.6%.22 However, chemically non-inertness of ZnO is detrimental to longterm stability of PSCs. Recently, SnO2 has gathered much attention owing to its high transparency and high mobility (up to 240 cm2 V-1 s-1),23-26 leading the PCE of the SnO2-based PSCs to 18%.27 Some other ESL materials with high mobility also have been explored such SrTiO3 (5-8 cm2 V-1 s-1) and Zn2SnO4 (10-30 cm2 V-1 s-1).28,29 Despite these achievements on exploiting different ESL materials in PSCs, it is still meaningful to find other potential ESL candidates to provide more options in the field and improve the performance of PSCs. Indium oxide (In2O3) is a promising n-type semiconductor material that has not only a wide band gap (~3.75 eV), but also a high mobility (~20 cm2 V-1 s-1) and a good thermal stability.30,31 In addition, its highly transparent property is also conducive to its optoelectronic applications.32 These satisfying optical, thermal and

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electrical properties imply that In2O3 should be a good potential ESL candidate in PSCs. However, there are still no reports on In2O3-based PSCs. In this work, we use low-temperature solution-processed In2O3 films as ESLs in PSCs and the In2O3-based PSC with a PCE of above 13% was obtained after optimizing the concentration of the precursor solution and the annealing temperature. The morphology, structure, and optical characteristics of In2O3 films are also deeply investigated. The In2O3 films coated on FTO substrates exhibit good antireflection property, leading to better light absorption by the perovskite layer. High mobility and wide band gap of In2O3 films also contribute to the performance improvement of PSCs. Furthermore, we improve the PCE of In2O3-based PSCs to 14.83% after introducing an optimized phenyl-C61-butyric acid methyl ester (PCBM) layer between the In2O3 ESL and the perovskite layer. Our results, therefore, suggest that In2O3 is a good alternative ESL material for efficient PSCs.

2.

Experimental

2.1. Materials In(NO3)3·4.5H2O (Sinopharm Chemical Reagent Corporation Co., Ltd, 99.5%) was dissolved in ethanol and stirred for an hour to prepare In(NO3)3 precursor solutions. 462 mg lead iodide (PbI2) (Aladdin reagent, 99.99%) was dissolved in 1 ml N,N-dimethylformamide (DMF) (Sinopharm Chemical Reagent Co., Ltd) and stirred under 70˚C overnight. Methylammonium iodide (MAI) was prepared according to a previous literature.33 32.3 ml hydroiodic acid (Sigma-Aldrich, 57 wt.% in water, 99.99%) and 30 ml methylamine (Sigma-Aldrich, 33 wt.% in absolute ethanol) were mixed and stirred in the ice bath for 2 h. The mixture was dried at 50˚C by rotary evaporation to produce synthesized chemicals MAI. Then, the precipitate was washed

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with diethyl ether for three times and recrystallized from a mixed solvent of diethyl ether and ethanol. Finally, MAI was obtained after drying at 70˚C for 24 h under vacuum. 10 mg, 15 mg, 20 mg, and 25 mg PCBM was dissolved in 1 ml chlorobenzene solution and stirred under 40˚C for hours. The hole selective material was composed of 68 mM 2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene (spiro-OMeTAD) (Shenzhen Feiming Science and Technology Co., Ltd, 99.0%), 55 mM TBP (Aladdin reagent), and 26 mg Li-TFSI (Aladdin reagent) in acetonitrile and chlorobenzene (1:10 in volume ratio).

2.2. Solar cell fabrication FTO glass substrates (Asahi Glass, 14 Ohm/square) were sequentially rinsed by sonication in detergent, DI water, acetone and ethanol, and finally dried in air. In2O3 thin films were spin-coated on the FTO substrates by sol-gel method. The spinning rate was set at 2500 rpm for 40 s. Then the In2O3 thin films were annealed for 40 min at 150˚C, 200˚C, 300˚C, 400˚C, and 500˚C, respectively. After cooling, a solution of PCBM was spin-coated on the In2O3 film and dried on a hotplate under 150˚C for 10 min. Perovskite (MAPbI3) absorber layers were deposited in the samples by sequential deposition method according to the reported procedure.34 The PbI2 films were deposited on the In2O3 films by spin-coating at 1900 rpm for 40 s in a glove box and annealed at 70˚C for 30 min. Then, the samples were dipped in 10 mg/mL MAI dissolved in isopropanol for several minutes and annealed at 70˚C for 40 min. The spiro-OMeTAD solution was spin-coated on perovskite film at 2500 rpm for 45 s. Finally, a thin gold electrode was deposited by thermal evaporation.

2.3. Characterization

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The morphologies of In2O3 film and perovskite absorber layer were observed by a high-resolution field emission scanning electron microscope (SEM) (JSM 6700F, Japan). Transmission electron microscopy (TEM) was conducted by a JEOL-2010 TEM. Compositions and kinetic energy spectra of the In2O3 nanocrystalline films were characterized by an X-ray photoelectron spectroscopy (XPS)/ Ultraviolet (UV) photoelectron spectroscopy (UPS) system (Thermo Scientific, Escalate 250Xi). The crystallinity of In2O3 films was examined by an X-ray diffraction (XRD) (D8 Advance, Bruker AXS, Germany). Photoluminescence (PL) spectra were obtained with a 532 nm laser, as the excitation source, pulsed at a frequency of 9.743 MHz. J-V characteristics of solar cells were performed on a CHI 660D electrochemical work station (Shanghai Chenhua Instruments, China) with a standard ABET Sun 2000 Solar Simulator. A standard silicon solar cell was used to calibrate the light intensity. All the cells were measured under a 100 mW cm-2 (AM 1.5 simulated irradiation) illumination with a scan rate of 0.1 V s-1. The area of the Au electrode was 0.09 cm2. The transmission spectra of the In2O3 films coated on FTO substrates were measured by an ultraviolet-visible spectrophotometer (CARY5000, Varian) in a wavelength range of 300-800 nm at room temperature. Incident photon-to-current conversion efficiency (IPCE) was measured by a QE/IPCE system (Enli Technology Co. Ltd) in the 320-800 nm wavelength range at room temperature.

3. Results and discussion Figure 1a presents the scheme of the In2O3-based PSC with regular structure in this study: a FTO coated glass as the front electrode, an In2O3 thin film as the ESL, a perovskite absorber layer (CH3NH3PbI3), a spiro-OMeTAD as the HSL, and Au as the back contact. The energy band diagram containing various ESL materials is shown in

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Figure 1b and UPS measurements of the In2O3 film is shown in Figure S1, indicating that the In2O3 ESL has a proper energy level alignment with perovskite layer.

Figure 1. (a) Schematic view of the device structure and (b) energy band diagram of the device containing various ESL materials.

The In2O3 films were fabricated on the FTO substrates by sol-gel method, following an annealing process at 200˚C. We first optimized the thickness of the In2O3 films by changing the concentration of the precursor solution, which different precursor concentrations (0.05 M, 0.10 M, 0.15 M, and 0.20 M) were obtained by adding different amount of In(NO3)3·4.5H2O into a fixed volume of ethanol. Here, the resultant In2O3 films were named after 0.05-In2O3, 0.10-In2O3, 0.15-In2O3, and 0.20In2O3. The PSCs based on different In2O3 ESLs were fabricated and photovoltaic performances of these devices were measured. Figure 2a shows the J-V curves of the PSCs without and with different In2O3 films, and the photovoltaic parameters are summarized in Table 1.

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Figure 2. (a) J-V curves of the PSCs based on the In2O3 ESLs prepared by different precursor concentrations. The corresponding (b) IPCE spectra, and (c) transmission spectra. (d) Tauc plot for In2O3 film.

It is clear that the performance of the PSCs with In2O3 ESLs is much better than that of the PSC without an In2O3 ESL. The best performance is obtained from the PSC with the 0.10-In2O3 thin film which the thickness is ca. 120 nm, achieving a PCE of 13.01%, an open-circuit voltage (VOC) of 1.07 V, a short circuit current density (JSC) of 17.90 mA cm-2, and an FF of 0.679. In contrast, the PSC fabricated in the same way, except without an In2O3 ESL, has a low PCE of 4.44% with a VOC of 0.99 V, a JSC of 10.61 mA cm-2, and an FF of 0.421. For the 0.05-In2O3 thin film, it may be too thin to form a continuous and compact layer, causing more serious recombination between electrons and holes. The In2O3 films obtained from a high concentration precursor solution, such as 0.15 M and 0.20 M, are too thick to act as an efficient ESL. Those films cannot extract electrons quickly and suppress the charge recombination effectively, which are partially responsible for the lower JSC and VOC. Hence, 0.10 M is the optimal molarity of precursor to produce high efficiency PSCs. The IPCE were 8


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conducted to confirm the trend of JSC in the J-V curves, which are consistent with the JSC of the PSCs with different In2O3 ESLs (Figure 2b). For the PSC with 0.10-In2O3 ESL, the value of JSC integrated from its IPCE spectrum is 18.11 mA cm-2, which is closely agreement with the value obtained from the J-V curve. For comparison, we also fabricated TiO2-based PSCs since TiO2 is the most common ESL materials in PSCs. It should be noted that TiO2 films required a high-temperature annealing at ~450 ˚C although the PCE of the TiO2-based PSCs is 14.41% (Figure S2). The transparency of the ESL is also an essential factor to influence the light absorption in PSCs. The transmission spectra of different In2O3 films and bare FTO are presented in Figure 2c. It is notable that the 0.05-In2O3 and 0.10-In2O3 thin films coated on FTO substrates are antireflective, and such a good optical property can increase the light transmittance and facilitate the generation of electron-hole pairs, sustaining the better performance of the PSCs based on the 0.05-In2O3 and 0.10-In2O3 thin films. For the In2O3 films formed from high concentration precursor solution such as 0.15 M and 0.20 M, however, the optical absorption edges shift to longer wavelengths and the transmittance is much lower because the In2O3 films are too thick. The poor optical property of the 0.15-In2O3 and0.20-In2O3 films is responsible for the mediocre performance of the In2O3-based PSCs. Figure 2d presents the dependence of (αhυ)2 on hυ for the In2O3 films and the optical band-gap energy (Eg) has been evaluated according to the intercept of the linear portion of the curve to the energy axis. The calculated optical Eg of the In2O3 film is about 3.75 eV, which is in agreement with the literature.35 The wide band gap of In2O3 is very beneficial for the light absorption by perovskite layer so that the performance of PSCs could be enhanced, and the absorbance spectrum of the perovskite layer deposited on In2O3 ESL is shown in Figure S3. To illustrate the charge transfer in the device, we carried

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out the steady-state PL measurements of perovskite films based on In2O3 layers and FTO (Figure S4). The perovskite film deposited on the In2O3/FTO substrate shows a much lower PL intensity than the film deposited on the FTO substrate, indicating In2O3 ESLs play an important role in transferring electrons and hindering the recombination between electrons and holes.

Table. 1 Photovoltaic parameters for the PSCs without and with different In2O3 films. Molarity (M)

VOC (V)

JSC (mA cm-2)

FF

PCE (%)

Rs (Ω cm2)

A

FTO

0.99

10.61

0.421

4.44

-

-

0.05

1.05

16.88

0.632

11.20

3.65

4.29

0.10

1.07

17.90

0.679

13.01

3.52

3.46

0.15

0.99

17.56

0.607

10.54

4.36

3.93

0.20

0.98

16.21

0.496

7.88

7.32

7.29

Moreover, Figure S5 shows the plots of -dV/dJ vs (JSC-J)-1 and linear fitting curves of the PSCs based on different In2O3 films (0.05 M, 0.10 M, 0.15 M and 0.20 M) at illumination. The series resistance (RS) and ideality factor (A) can be calculated from the intercept and slope of the linear fitting results (Table 1).36,37 A low value of RS and a proper value of A are necessary for a high-performance PSC. As can be seen in Table 1, the PSC with the 0.10-In2O3 ESL has the lowest values of both RS (3.52 Ω cm2) and A (3.46), which further demonstrates that the 0.10-In2O3 thin film is the best 10


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ESL in the In2O3-based PSCs. In contrast, the PSCs with other In2O3 ESLs have lower PCEs owing to their high values of RS and A. Figure S5 exhibits the plots of ln(JSC-J) vs. V+RSJ based on the 0.10-In2O3 film and there is a linear fitting curve on them. The value of reverse saturated current density (J0) is 8.94 × 10-8 mA cm-2 derived from the linear fitting results. In the light of the parameters of the cell shown in Figure S5, the ideal VOC of the cell is calculated to be 1.09 V, which is very close to the experimental result.

Figure 3. (a) XRD patterns of In2O3 films coated on glass substrates after annealing at 150˚C, 200˚C, 300˚C, 400˚C, and 500˚C, respectively. (b) TEM and (c) SAED images of an In2O3 nanocrystalline film.

The XRD patterns of In2O3 films as a function of annealing temperature (150˚C, 200˚C, 300˚C, 400˚C, and 500˚C) are shown in Figure 3a. The In2O3 films are amorphous and there are no distinct peaks in the XRD pattern when annealed at 150ºC. However, characteristic peaks of the In2O3 film appeared when the annealing temperature rose up to 200ºC, which means the In2O3 crystals were formed. The XRD 11



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patterns of the In2O3 films, annealed at 200 ºC or higher, all show a good match with pure In2O3 (PDF card number 01-071-2194) and reveal strong orientation along (222) direction at ~30.65º. It is seen that the crystallization of the In2O3 film is enhanced with the increment of annealing temperature. In addition, we calculated the average crystallite size of the In2O3 by using the Scherrer’s equation and the parameters of FWHM and the average crystallite size of In2O3 are summarized in Table S1. The In2O3 film whose annealing temperature is 200 ºC has the highest FWHM value, therefore, the crystallite size of In2O3 is the smallest. We measured the crystallite size of In2O3 annealing at 200 ºC is ca. 10 nm from the images of the transmission electron microscopy (TEM) (Figure 3b), which is close to the value of 11.74 nm we calculated. Furthermore, the In2O3 films, formed by the same process, are demonstrated to be nanocrystalline according to the selective area electron diffraction (SAED) images shown in Figure 3c. The small In2O3 particles are expected to form a compact and uniform blocking layer. When the annealing temperature is raised, the value of FWHM will decrease, indicating that the crystallite size of In2O3 will become larger. Large In2O3 crystalline grains may increase the roughness of In2O3 film, which is detrimental to the device fabrication. To verify the assumption, we fabricated the PSCs based on In2O3 films with different annealing temperature and the J-V curves are shown in Figure S6. The PSC based on the In2O3 film with 200 ºC annealing temperature achieved the best performance while other PSCs exhibited a lower VOC or JSC.

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Figure 4. XPS spectra of (a) survey, (b) In 3d, (c) O 1s, and (d) N 1s peaks for an In2O3 film coated on a glass substrate.

To further demonstrate the composition of the In2O3 film under 200 ºC annealing temperature, we conducted the XPS, as shown in Figure 4. By resolving the XPS spectra, we found that the composition of the films prepared by In(NO3)3·4.5H2O precursor at a low temperature is pure In2O3. Figure 4a presents the full XPS spectrum survey, revealing the presence of In and O. In Figure 4b, the binding energies of 445.1 eV and 452.5 eV are attributed to the In 3d5/2 and In 3d3/2 peak, respectively.32,38 The main binding energy of 530.1 eV corresponds to the O 1s peak, which is the O2-state in In2O3 (Figure 4c). Figure 4d shows that there is no residual N in the In2O3 films. Therefore, we conclude that the In(NO3)3·4.5H2O is converted to In2O3 completely after annealing at 200˚C for 40 min. The surface morphologies of the In2O3 film were also investigated and the top view SEM images at different magnifications are presented in Figure 5a and b. We found that some unexpected pinholes and cracks existed along In2O3 grain boundaries, and it is possibly due to the separation occurrence during the annealing process. The imperfect surface coverage could lead to direct contact of perovskite layer and FTO, 13



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which may cause the severe recombination. In order to modify the surface of the In2O3 film, we introduced a PCBM layer between the In2O3 ESL and the perovskite layer. PCBM, which has been widely used in PSCs as an electron transporting layer,4042

has an excellent ability to extract and transport charge carriers. Figure 5c and d

show the morphology of the PCBM layer formed on the In2O3 film. It is noteworthy that the PCBM/In2O3 layer is extremely compact and uniform, which is expected to promote the performance of the In2O3-based PSCs. Figure 5e and f also present the morphology of perovskite absorber layer (CH3NH3PbI3) and the cross-sectional SEM image of a completed device, respectively.

Figure 5. Top-view SEM images of In2O3 film at (a) low and (b) high magnifications, PCBM/In2O3 film at (c) low and (d) high magnifications, and (e) perovskite absorber layer. (f) Cross-sectional SEM image of a device.

To gain further insight into the effect after introducing a PCBM layer in the

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In2O3-based PSCs, we first optimized the thickness of PCBM layers and found the highest PCE was achieved when using the PCBM precursor that 20 mg PCBM dissolved in 1 mL chlorobenzene (Figure S7). A too thin PCBM layer might not be capable enough to passivate the defects efficiently while a too thick layer presents a very poor transmittance. Then we compared the performance of the In2O3-based PSCs with and without a PCBM layer, the corresponding J-V curves are shown in Figure 6a. The PSC with a PCBM layer has a higher JSC of 20.06 mA cm-2, a VOC of 1.08 V, an FF of 0.685, and a higher PCE of 14.83%, demonstrating that PCBM has a positive effect on In2O3-based PSCs. For comparison, we also fabricated PSCs with only PCBM layer which have a low PCE of 8.74% with a poor FF, demonstrating the importance of In2O3 layer in electron selection. To confirm the JSC of the PSCs, the normalized IPCE spectra of the In2O3-based PSCs with and without a PCBM layer are shown in Figure S8. Besides, we found that the J-V curves shown in Figure S9 exhibited less hysteresis after introducing a PCBM layer, showing that the PCBM layer can passivate the ESL interface and reduce hysteresis. To check the reproducibility of the devices, we fabricated 50 cells with and without a PCBM layer and the histogram of PCEs is shown in Figure 6b. It is conspicuous that the overall performance of the In2O3-based PSCs with a PCBM layer is better than that without a PCBM layer.

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Figure 6. (a) J-V curves and (b) a histogram of PCEs for 50 cells of the In2O3-based PSCs with and without a PCBM layer. Steady-state efficiency of the In2O3-based PSC (c) without and (d) with a PCBM layer at a constant bias voltages of 0.74 V and 0.78 V, respectively.

The steady-state efficiencies of the In2O3-based PSCs without and with a PCBM layer are also conducted, which are shown in Figure 6c and d, respectively. For the In2O3-based PSC without a PCBM layer, the JSC firstly decreased quickly and then stabilized when measured at a constant bias voltage of 0.74 V. In contrast, the JSC of the PSC containing a PCBM layer decreased very little for 300 s. A steady-state current density of 16.26 mA cm-2 and a steady-state efficiency of 12.68% were achieved under a constant bias voltage of 0.78 V. The constant bias voltage of 0.74 V and 0.78 V are consistent with the voltage at the maximum power points of J-V curves of the cells without and with a PCBM layer, respectively.

4. Conclusions In conclusion, In2O3 as a new ESL material was employed in PSCs and In2O3-

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based PSCs with a PCE exceeding 13% were obtained in this work. The morphology, structure, and optical characteristics of In2O3 films were also deeply investigated to further confirm the feasibility for In2O3 acting as an ESL material. To enhance the performance of the In2O3-based PSCs, we introduced an optimized PCBM layer between the In2O3 film and the perovskite layer, improving the PCE of the In2O3based PSCs up to 14.83% and the steady-state efficiency to 12.68%. Our results suggest that In2O3 is a good alternative ESL material for efficient PSCs due to its high mobility, wide band gap, and high transmittance. The performance of the In2O3-based PSCs is expected to be improved after optimizing the fabrication process of the perovskite absorber layer.

Acknowledgments This work was financially supported by the National High Technology Research and Development Program (2015AA050601), the National Natural Science Foundation

of

China

(61376013,

91433203,

J1210061),

Special Program of the Postdoctoral Science Foundation of China

the General and (2013M531737,

2014T70735), Suzhou Science & Technology Bureau (SYG201449). We thank the nano center of Wuhan University for SEM, XPS, and UPS measurements, and Jie Cao for his help in the experiment.

Supporting Information Available: Plots of -dV/dJ vs (JSC-J)-1 and ln(JSC-J) vs V+RSJ, J-V curves of the PSCs based on In2O3 films with different annealing temperature, J-V curves of the In2O3-based PSCs containing PCBM with different thickness, the parameters of FWHM and the average crystallite size of In2O3.

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References (1) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H. J.; Sarkar, A.; Nazeeruddin, M. K.; Grätzel, M.; Seok, S. I., Efficient Inorganic-organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486-491. (2) Im, J. H.; Lee, C. R.; Lee, J. W.; Park, S. W.; Park, N. G., 6.5% Efficient Perovskite Quantum-dot-sensitized Solar Cell. Nanoscale 2011, 3, 4088-4093. (3) Hodes, G., Perovskite-Based Solar Cells. Science 2013, 342, 317-318. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341-344. (5) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J., ElectronHole Diffusion Lengths > 175 mu m in Solution-grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. (6) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (7) Kim, H. S.; Lee, C. R.; Im, J.-H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Grätzel, M.; Park, N. G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (8) Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398.

18


Page 18 of 24

Page 19 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(9) Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T. B.; Duan, H. S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y., Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542-546. (10) Zhao, Y.; Zhu, K. Efficient Planar Perovskite Solar Cells Based on 1.8 eV Band Gap CH3NH3PbI2Br Nanosheets via Thermal Decomposition. J. Am. Chem. Soc. 2014, 136, 12241-12244. (11) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., Compositional Engineering of Perovskite Materials for High-performance Solar Cells. Nature 2015, 517, 476-480. (12) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., High-performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234-1237. (13) Wei Chen, Y. W., Youfeng Yue, Jian Liu, Wenjun Zhang, Xudong Yang, Han Chen, Enbing Bi, Islam Ashraful, Michael Grätzel, Liyuan Han, Efficient and Stable Large-area Perovksite Solar Cells with Inorganic Charge Extraction Layers. Science 2015, 350, 944-948. (14) Im, J. H.; Jang, I. H.; Pellet, N.; Grätzel, M.; Park, N. G., Growth of CH3NH3PbI3 Cuboids with Controlled Size for High-efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, 9, 927-932. (15) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Il Seol, S., Solvent Engineering for High-performance Inorganic-organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. (16) Ke, W.; Fang, G.; Wang, J.; Qin, P.; Tao, H.; Lei, H.; Liu, Q.; Dai, X.; Zhao, X., Perovskite Solar Cell with an Efficient TiO2 Compact Film. ACS Appl. Mater. Interfaces 2014, 6, 15959-15965.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(17) Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Grätzel, M.; Han, H., A Hole-conductor-free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295-298. (18) Kumar, M. H.; Yantara, N.; Dharani, S.; Grätzel, M.; Mhaisalkar, S.; Boix, P. P.; Mathews, N., Flexible, Low-temperature, Solution Processed ZnO-based Perovskite Solid State Solar Cells. Chem. Commun. 2013, 49, 11089-11091. (19) Liu, D.; Kelly, T. L., Perovskite Solar Cells with a Planar Heterojunction Structure Prepared Using Room-temperature Solution Processing Techniques. Nat. Photonics 2014, 8, 133-138. (20) Son, D. Y.; Im, J. H.; Kim, H. S.; Park, N. G., 11% Efficient Perovskite Solar Cell Based on ZnO Nanorods: An Effective Charge Collection System. J. Phys. Chem. C 2014, 118, 16567-16573. (21) Mahmood, K.; Swain, B. S.; Amassian, A., 16.1% Efficient Hysteresis-Free Mesostructured Perovskite Solar Cells Based on Synergistically Improved ZnO Nanorod Arrays. Adv. Energy Mater. 2015, 5, 1500568. (22) Liu, T.; Hu, Q.; Wu, J.; Chen, K.; Zhao, L.; Liu, F.; Wang, C.; Lu, H.; Jia, S.; Russell, T.; Zhu, R.; Gong, Q., Mesoporous PbI2 Scaffold for High-Performance Planar Heterojunction Perovskite Solar Cells. Adv. Energy Mater. 2015, 6, 1501890. (23) Dong, Q.; Shi, Y.; Wang, K.; Li, Y.; Wang, S.; Zhang, H.; Xing, Y.; Du, Y.; Bai, X.; Ma, T., Insight into Perovskite Solar Cells Based on SnO2 Compact ElectronSelective Layer. J. Phys. Chem. C 2015, 119, 10212-10217. (24) Ke, W.; Fang, G.; Liu, Q.; Xiong, L.; Qin, P.; Tao, H.; Wang, J.; Lei, H.; Li, B.; Wan, J.; Yang, G.; Yan, Y., Low-temperature Solution-processed Tin Oxide as an Alternative Electron Transporting Layer for Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 6730-6733.

20


Page 20 of 24

Page 21 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Li, Y.; Zhu, J.; Huang, Y.; Liu, F.; Lv, M.; Chen, S.; Hu, L.; Tang, J.; Yao, J.; Dai, S., Mesoporous SnO2 Nanoparticle Films as Electron-transporting Material in Perovskite Solar Cells. Rsc Adv. 2015, 5, 28424-28429. (26) Song, J.; Zheng, E.; Bian, J.; Wang, X.-F.; Tian, W.; Sanehira, Y.; Miyasaka, T., Low-temperature SnO2-based Electron Selective Contact for Efficient and Stable Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 10837-10844. (27) Correa Baena, J. P.; Steier, L.; Tress, W.; Saliba, M.; Neutzner, S.; Matsui, T.; Giordano, F.; Jacobsson, J.; Srimath Kandada, A. R.; Zakeeruddin, S. M.; petrozza, a.; Abate, A.; Mohammad K, N.; Grätzel, M.; Hagfeldt, A., Highly Efficient Planar Perovskite Solar Cells through Band Alignment Engineering. Energy Environ. Sci. 2015, 8, 2928-2934. (28) Bera, A. ; Wu, K. ; Sheikh, A. ; Alarousu, E. ; Mohammed, O. F. ; Wu, T., Perovskite Oxide SrTiO3 as an Efficient Electron Transporter for Hybrid Perovskite Solar Cells. J. Phys. Chem. C 2014, 118, 28494-28501. (29) Shin, S. S. ; Yang, W. S. ; Noh, J. H. ; Suk, J. H. ; Jeon, N. J. ; Park, J. H. ; Kim, J. S. ; Seong, W. M. ; Seok, S. I., High-performance Flexible Perovskite Solar Cells Exploiting Zn2SnO4 Prepared in Solution Below 100 ˚C. Nat. Commun. 2015, 6, 7401. (30) Lau, L. N.; Ibrahim, N. B.; Baqiah, H., Influence of Precursor Concentration on the Structural, Optical and Electrical Properties of Indium Oxide Thin Film Prepared by a Sol-gel Method. Appl. Surf. Sci. 2015, 345, 355-359. (31) Korotcenkov, G.; Brinzari, V.; Ivanov, M.; Cerneavschi, A.; Rodriguez, J.; Cirera, A.; Cornet, A.; Morante, J., Structural Stability of Indium Oxide Films Deposited by Spray Pyrolysis During Thermal Annealing. Thin Solid Films 2005, 479, 38-51.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32) Qin, P.; Fang, G.; Sun, N.; Fan, X.; Zheng, Q.; Cheng, F.; Wan, J.; Zhao, X., Ptype Indium Oxide Thin Film for the Hole-transporting Layer of Organic Solar Cells. Thin Solid Films 2012, 520, 3118-3124. (33) Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M., Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. J. Am. Chem Soc. 2012, 134, 17396-17399. (34) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Sequential Deposition as a Route to High-performance Perovskite-sensitized Solar Cells. Nature 2013, 499, 316-319. (35) Tahar, R. B. H.; Ban, T.; Ohya, Y.; Takahashi, Y., Optical, Structural, and Electrical Properties of Indium Oxide Thin Films Prepared by the Sol-gel Method. J. Appl. Phys. 1997, 82, 865-870. (36) Shi, J.; Dong, J.; Lv, S.; Xu, Y.; Zhu, L.; Xiao, J.; Xu, X.; Wu, H.; Li, D.; Luo, Y.; Meng, Q., Hole-conductor-free Perovskite Organic Lead Iodide Heterojunction Thin-film Solar Cells: High Efficiency and Junction Property. Appl. Phys. Lett. 2014, 104, 063901. (37) Tao, H.; Ke, W.; Wang, J.; Liu, Q.; Wan, J.; Yang, G.; Fang, G., Perovskite Solar Cell Based on Network Nanoporous Layer Consisted of TiO2 Nanowires and Its Interface Optimization. J. Power Sources 2015, 290, 144-152. (38) Gurlo, A.; Ivanovskaya, M.; Pfau, A.; Weimar, U.; Gopel, W., Sol-gel Prepared In2O3 Thin Films. Thin Solid Films 1997, 307, 288-293. (39) Liu, C.; Wang, K.; Du, P.; Meng, T.; Yu, X.; Cheng, S. Z.; Gong, X., High Performance Planar Heterojunction Perovskite Solar Cells with Fullerene Derivatives as the Electron Transport Layer. ACS Appl. Mater. Interfaces 2015, 7, 1153-1159.

22


Page 22 of 24

Page 23 of 24

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(40) Chen, Y.; Peng, J.; Su, D.; Chen, X.; Liang, Z., Efficient and Balanced Charge Transport Revealed in Planar Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 4471-4475. (41) Jeng, J. Y.; Chen, K. C.; Chiang, T. Y.; Lin, P. Y.; Tsai, T. D.; Chang, Y. C.; Guo, T. F.; Chen, P.; Wen, T. C.; Hsu, Y. J., Nickel Oxide Electrode Interlayer in CH3NH3PbI3 Perovskite/PCBM Planar-Heterojunction Hybrid Solar Cells. Adv. Mater. 2014, 26, 4107-4113. (42) Seo, J.; Park, S.; Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yoon, S. C.; Seok, S. I., Benefits of Very Thin PCBM and LiF Layers for Solution-processed P-i-n Perovskite Solar Cells. Energy Environ. Sci. 2014, 7, 2642-2646.

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Perovskite Solar Cells Based on Low-Temperature Processed Indium

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