Efficient and Stable Perovskite Solar Cell with High Open-Circuit

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Efficient and Stable Perovskite Solar Cell with High Open Circuit Voltage by Dimensional Interface Modification Wei Luo, Cuncun Wu, Duo Wang, Yuqing Zhang, Zehao Zhang, Xin Qi, Ning Zhu, Xuan Guo, Bo Qu, Lixin Xiao, and Zhijian Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22040 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Efficient and Stable Perovskite Solar Cell with High Open Circuit Voltage by Dimensional Interface Modification

Wei Luo, Cuncun Wu, Duo Wang, Yuqing Zhang, Zehao Zhang, Xin Qi, Ning Zhu, Xuan Guo, Bo Qu, Lixin Xiao *, Zhijian Chen *

State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University, Beijing,100871, PR China.

KEYWORDS: dimensional interface engineering, energy level modification, defect passivation, high open circuit voltage, ambient stability, perovskite solar cell

ABSTRACT High efficiency organic–inorganic hybrid perovskite solar cells have experienced rapid development and attracted significant attention in recent

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years. However, instability to ambient environment such as moisture is a facile challenge

for

application

of

perovskite

solar

cells.

Herein,

1,8-

octanediammonium iodide (ODAI) is employed to construct a 2D modified interface by in situ combined with residual PbI2 on the Formamidinium lead iodide (FAPbI3) perovskite surface. The ODA2+ is seem to lie horizontally on the surface of 3D perovskite due to its substitution for two FA+, which could protect the bulk perovskite more effectively. The unencapsulated perovskite solar cells showed notably improved stability, which remained 92% of its initial efficiency after storage in ambient environment for 120 days. In addition, a higher open circuit voltage of 1.13 V compared to that of control device (1.04 V) was obtained due to the interface energy level modification and defect passivation. A champion power conversion efficiency of 21.18% was therefore obtained with a stabilized power output of 20.64% at the maximum power point for planar perovskite solar cells.


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1. INTRODUCTION Since first reported by Miyasaka in 20091, organic-inorganic hybrid perovskite solar cells (PSCs) have received tremendous interest in the photovoltaic field due to their low-cost, simple fabrication and unique photovoltaic properties including outstanding light harvesting, high charge carrier mobility and long diffusion length.2-6 To improve the device performance such as efficiency and stability, plenty of researches are focused on examining the perovskite compositions7,8, deposition method9,10, device architectures11,12, interface engineering13,14 and so on. So far, remarkable performance progress has been achieved with certified power conversion efficiency (PCE) now exceeding 23%.15 Though the reported efficiency of PSCs is even higher than that of polycrystalline silicon solar cells16, the device stability in ambient environment is still the main barrier for the commercialization of PSCs in the future. The perovskite crystal could be decomposed back to the precursors by moisture, oxygen, heat and electric field due to its low formation energy and hydroscopic nature.17-19 Several strategies have been adopted to improve the environment stability of PSCs, such as precursor additive20,21 and interface passivation engineering22-27. As compared to precursor additive engineering, interface passivation engineering might be more effective because it can fix interfacial defects, protect bulk crystal and optimize energy level alignment. Several recent studies revealed that interface passivation can significantly mitigate



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nonradiative recombination in cells, yielding both improved Voc and device stability.22-24 Yang and co-workers demonstrated a graded 3D-2D perovskite interface by post-treated with henethylammonium iodide (PEAI), which modified the interface energy level and reduced nonradiative recombination, leading to an increased Voc and enhanced thermal stability.25 Liu et al. employed the methimazole to construct a surface patch by in situ converting residual PbI2 at grain boundary, thus suppressing the ion migration and prolong the device stability.26 Park and co-workers introduced Formamidinium hexafluorophosphate (FAPF6) modification on the top of the perovskite film to partial replace the iodide with PF6- ion, which effectively reduced the defect density and finally improved the photovoltaic performance.27 In this work, we introduced the 1,8-octanediammonium iodide (ODAI) for the first time to construct a 2D modified interface by simply spin-coating the ODAI/isopropyl alcohol (IPA) solution onto the top of Formamidinium lead iodide (FAPbI3) based perovskite film. Contrast to conventional 2D passivation materials in the previous studies24,25 such as PEAI, there are two amino and iodides in ODAI which could combined more residual PbI2 to form 2D perovskite. As illustrated in Figure 1, the PEA+ is like standing vertically on the bulk perovskite, while the ODA2+ is seem to lie horizontally on the surface of perovskite due to its substitution of two FA+, which could protect the bulk perovskite

more

effectively.

This

dimensionally

graded

layer

also

advantageously modified the interface energy level, thus increasing the open


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circuit voltage (Voc) from 1.04 V to 1.13 V, which is among the highest value for FAPbI3 based perovskite solar cell.28,29 A champion power conversion efficiency of 21.18% (stabilized at 20.64%) with negligible J–V hysteresis was obtained, consequently. Due to the long hydrophobic alkyl chain of ODAI at the interface as well as the grain boundaries, the device stability against moisture was tremendously enhanced as expected, which maintained 92% of its original efficiency after stored in dark ambient environment with relative humidity of 20 ~ 40% for 120 days.

Figure 1. Schematic of the surface of perovskite with a) PEAI modification wherein the PEA+ was seem to stand vertically; b) ODAI modification wherein the ODA2+ was seem to lie horizontally. (Color = atom: red = I; grey = Pb; blue = H; yellow= C; brown = N)

2. EXPERIMENTAL SECTION 2.1. Materials Indium tin oxide (ITO) glass substrates with ITO thickness of 180 nm and



sheet resistance of 8 Ω/sq were purchased from Huayulianhe Co., Ltd. SnO2 colloid precursor (tin(IV) oxide, 15% in H2O colloidal dispersion), anhydrous N, N-dimethylformamide (DMF) and chlorobenzene were obtained from Alfa Aesar. PbI2 (99.9985%), 1,8-octanediammonium iodide (ODAI, 99.5%) and spiroOMeTAD were purchased from Xi’an Polymer Light Technology Co., Ltd. Formamidinium iodide (FAI, 99.5%) and methylamine chloride (MACl, 99.5%) were acquired from Borun New Material Technology Co., Ltd. Lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI) and 4-tert-butylpyridine (tBP) were obtained from Aldrich. 1-Methyl-2-pyrrolidinone (NMP, 99.5%) was purchased from Aladdin. All these commercially available materials were used as received without any further purification.

2.2. Device fabrication The ITO coated glass substrates were cleaned ultrasonically in detergent, deionized water, acetone and isopropyl alcohol each for 30 min, sequentially with ultraviolet-ozone modification for 10 min. The SnO2 colloid precursor was ultrasonically diluted by H2O (1:6 volume ratio) for 45 min. The final solution was spin-coated onto glass/ITO substrates at 3,000 rpm for 30 s, then baked on a hot plate at 150 °C for 30 min. The fabrication method of perovskite films was according to another submitted paper of our group (see it for more details). The perovskite precursor solution (1.25 M) was prepared by mixing the PbI2 and FAI (1:1 molar ratio) in DMF with additives of MACl and NMP. After


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completely dissolved, the perovskite precursor solution was spin-coated onto the SnO2/ITO substrate at 1,000 rpm for 2 s and 4,000 rpm for 10 s. The coated film was quickly moved to a small sample chamber which was pumped to 9.0 Pa in about 90 s. A brown and transparent gas-pump dried film was obtained and then taken out to be annealed at 150 °C for 20 min. For ODAI-modified devices, 50 μL ODAI/IPA (2mg/ml) was dispersed on the pristine perovskite film hold for 2 s and then spin coated at 4000 rpm for 30 s. Subsequently, the resulted film was annealed at 150 ℃ for 10 min to remove the residual solvent. The hole transport layer was spin-coated with a Spiro-OMeTAD solution at 4,000 rpm for 40 s, where 1mL Spiro-OMeTAD/chlorobenzene (72.3 mg/mL) solution was employed with the addition of 54 μL Li-TFSI/acetonitrile (170 mg/mL) and 30 μL tBP. So far, all processes were performed in ambient atmosphere. Finally, an 80 nm Au back electrode was deposited by thermal evaporation at a pressure of < 2×10-3 Pa. The active area was 0.09 cm2.

2.3. Characterization The current density-voltage (J-V) curves were measured with a scan speed of 0.5 V/s under 100 mW/cm2 AM 1.5G simulated illumination using a Keithley 2611 Semiconductor Characterization System with a Newport solar simulator as a light source, and the current density was revised with a mask. Incident photon-to-current conversion efficiency (IPCE) spectrum was observed using a lock-in amplifier (Model SR830 DSP) coupled with a 1/4 m monochromator



(Crowntech M24-s) and 150 W tungsten lamp (Crowntech). Atomic force microscope (AFM) images were taken by an Agilent Series 5500 and scanning electron microscope (SEM) images were collected with a Hitachi S-4800. The X-ray diffraction (XRD) patterns of perovskite films on the glass/ITO/SnO2 substrates were obtained by using a D/MAX-2000 X-ray diffractometer with monochromatic Cu Kα irradiation (l ¼ 1.5418 Å) at a scan rate of 6 °/min. The absorption spectrum was recorded with a UV-visible spectrophotometer (Agilent 8453). Photoluminescence (PL) was measured with NaonLog infrared fluorescence spectrometer (Nanolog FL3-2Ihr) with excitation at 470 nm. Timeresolved photoluminescence (TRPL) were carried out by an ultrafast lifetime spectrometer (Delta flex). Highest occupied molecular orbital (HOMO) level was measured by a photoelectron spectrometer (Riken Keiki AC-2). The thickness of the perovskite film is measured with a KLA-Tencor Alpha-Step Surface Profiler. The water contact angle was measured by a contact angle meter (OCA2O Dataphysics). All of the measurements were performed in ambient atmosphere at room temperature without encapsulation.

3. RESULTS AND DISCUSSION The morphology of the pristine and ODAI-modified perovskite film was scanned by atomic force microscopy (AFM) and presented in Figure S1. Comparing with the pristine perovskite film, the ODAI-modified perovskite film has less pin-holes and more smooth surface with root mean square (RMS)


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roughness decreasing from 32.5 nm to 19.6 nm. The SEM image of the pristine perovskite film was shown in Figure 2a. As we can see, there are some bright grains which should be ascribed to PbI2 according to the previous study30, the XRD patterns (Figure 2c) also shows an obvious peek of PbI2 at 12.64o correspondingly. While with ODAI modification, the bright grains disappeared, replaced with lots of small platelets passivated at the grain boundary (Figure 2b). Moreover, the perovskite grain size was increased as shown as the black contact areas. The XRD pattern of ODAI-modified perovskite film shows almost no peak of PbI2 but enhanced intensity at peaks around 13.93o and 28.06o, which correspond to the (111) and (222) crystal planes of trigonal perovskite phase (α-phase), respectively31. We assume that ODAI modification converts the residual PbI2 into an ODAI-based 2D perovskite and therefore enhances the crystallinity of bulk perovskite. It did not change the bulk crystal structure since no observation of additional peaks. Moreover, the XRD patterns of the pristine and ODAI-modified perovskite film stored under 40 ~ 60% humidity environment for one month was shown in Figure 2d. The pristine perovskite film was decomposed badly as a higher intensity at PbI2 peak shown in XRD pattern, while the ODAI-modified perovskite film remained the same XRD pattern as the fresh one. It demonstrated that the ODAI modification significantly improved the stability of perovskite film at ambient environment due to the passivation of hydrophobic 2D perovskite on the surface and grain boundary.



Figure 2. a, b) SEM images of perovskite films a) without and b) with ODAI modification, respectively. c, d) XRD patterns of c) fresh and d) aged perovskite films stored under 40 ~ 60% relative humidity environment for one month based on SnO2/ITO/glass substrate.

In order to investigate the effectiveness of ODAI modification on device performance, we prepared devices with the structure of ITO/SnO2/perovskite with or without ODAI modification/spiro-OMeTAD/Au, as seen in Figure S2. The average photovoltaic parameters of Jsc, Voc, FF and PCE from 30 devices for each group were shown in Figure S3 and summarized in Table S1. The control devices showed an average PCE of 19.27%, along with a Voc of 1.029 V, Jsc of 24.70 mA/cm2 and FF of 0.756. While modified with ODAI, the average Voc was significantly increased to 1.117 V, leading to an increased PCE of 20.33%


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in spite of a slight decrease of FF. A champion device with a Voc of 1.13 V, Jsc of 24.90 mA/cm2, FF of 0.75 and PCE of 21.18% was obtained (Figure 3a), which had almost no hysteresis with a scan speed of 0.5 V/s (Figure 3b) as the same to the control device (Figure S4). This is attributed to both the excellent transport ability of SnO232 and the outstanding quality of FAPbI3 perovskite film. Integration of IPCE spectrum in the range of solar emission yields AM 1.5 photocurrents of 22.73 mA/cm2 and 22.49 mA/cm2 for the ODAI-modified and control device (Figure 3c), with an acceptable mismatch compared to the measured Jsc. Stabilized power output measurement is also carried out by holding the bias voltage at the maximum power point (Vmax = 0.88 V for the control device and Vmax = 0.92 V for ODAI-modified device), as presented in Figure 3d. Stabilized current density of ODAI-modified and control device was almost the same with the value of 22.21 mA/cm2 and 22.43 mA/cm2, respectively, but the steady-state PCE of ODAI-modified device was increased to 20.64% compared with that of control device (19.55%), which was very close to the PCE value of J-V measurement, further demonstrating the actual and stable performance of both the control and ODAI-modified device.



Figure 3. a) J-V curves of the best control and ODAI-modified device with reverse scan. b) J-V curves of the best ODAI-modified device with different scan direction at 0.5 V/s. c) IPCE spectrum along with the corresponding integrated Jsc of the control and ODAI-modified device. d) stabilized current density and PCE of the control and ODAI-modified device measured at the maximum power point for 240 s with a bias voltage of 0.92 V and 0.88 V, respectively.

To understand the inherent mechanism of the improved performance of devices with ODAI modification, the highest occupied molecular orbital (HOMO) level of two kinds of perovskite film were measured, as presented in Figure 4a, b. The HOMO level of ODAI-modified perovskite film was changed to -5.46 eV from -5.36 eV of pristine perovskite. It’s indicates that the ODAI self-assembled


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layer acts like a dipole layer, which optimizes the interface energy band alignment, thus leading to a higher Voc of devices. Figure 4c shows the steadystate photoluminescence (PL) of the pristine and ODAI-modified perovskite film fabricated on glass substrate. The PL peak at 811 nm of the pristine perovskite is corresponding with the absorption edge (812 nm) as seen in Figure 4d. When with ODAI modification, the absorption edge of perovskite film was slightly blue shift to 805 nm due to the formation of 2D perovskite on the surface. Accordingly, the PL peak was also blue shift to 804 nm with higher intensity, which indicates that the nonradiative recombination was inhibited due to the defect passivation by ODAI modification. There are three more PL peaks at 573 nm, 636 nm and 675 nm, which is supposed to belong to the ODAI-based 2D perovskite. The time-resolved PL measurements of two kinds of perovskite films fabricated on glass substrate were conducted and shown in Figure S5. We fitted the decay curves by the biexponential decays with a fast component τ1 and a slow component τ2, and the detailed fit data are listed in Table S2. The fast decay component could be assigned to the interfacial charge separation property, while the slow component corresponded to a nonradiative recombination caused by the trap state of perovskite film.33,34. For the ODAImodified film, the decrease of τ1 indicates that the 2D perovskite on the surface act as a role of carrier extraction layer, while the increase of τ2 indicates that the ODAI modification effectively eliminates the trap states at the 3D perovskite surface and suppressed the nonradiative recombination, which accords with



the steady-state PL measurement result.

Figure 4. a, b) the HOMO level of a) control and b) ODAI-modified perovskite film. c) steady-state photoluminescence (PL) and d) absorption spectra of perovskite film with or without ODAI modification on glass substrate. Inset, (αhυ)2 as a function of wavelength, through which the absorption edges were calculated to 805 nm and 812 nm for the control and ODAI-modified perovskite film, respectively. e, f) Dark I-V curves of the electron-only devices (ITO/SnO2/perovskite/PCBM/Ag) for space charge limited current analysis: e)


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control; f) ODAI-modified.

To further demonstrate the origin of the Voc rise, the space charge limited current (SCLC) measurement was used to calculate the trap density of the perovskite film35. The electron-only devices with the configuration of ITO/SnO2/perovskite with or without ODAI modification/PCBM/Ag were prepared. As shown in the dark I–V characteristic curves (Figure 4e, f), the trapfilled limit voltage (VTFL) values of the control and ODAI-modified devices were estimated to 0.290 V and 0.166 V, respectively. The trap density (N) is determined by the VTFL according the following equation:36

N=

2εεε0VTFL eL2

(1)

where ε = 46.9 and ε0 = 8.85 × 10 ―12 𝐹/𝑚 are the relative dielectric constants of FAPbI331 and the vacuum permittivity, respectively; e is the elementary charge of the electron; L ~ 380 nm is the thickness of the obtained perovskite film measured by a surface profiler, which could be also verified by the crosssectional SEM images as shown in Figure S6. Compared with the control device with the trap density of 1.04 × 1016 cm-3, the ODAI-modified device delivered a lower trap density of 5.87 × 1015 cm-3. It demonstrated that the defect of perovskite layer was effectively passivated by ODAI modification, and the unwanted carrier recombination was thus suppressed, which contributed to an increased Voc. The stability of ODAI-modified devices was improved significantly as shown



in Figure 5a, wherein devices without encapsulation were stored at ambient environment with humidity about 20 ~ 40% under dark condition. The ODAImodified devices still maintained 92% of its initial PCE after storage for 120 days, which shows greatly improved stability compared to the control devices. As seen in the inset, The ODAI-modified perovskite film remained the black phase, while the control film degraded badly to yellow PbI2, as confirmed by the XRD measurement above. We suppose that the stability improvement of ODAI modification device is due to the great hydrophobicity of the 2D perovskite on the surface, wherein the long alkyl group of ODAI could prevent the 3D perovskite from the ambient moisture effectively. The contact angle of the water droplet on the control and ODAI-modified perovskite films was measured as shown in Figure 5b. The ODAI-modified perovskite film displays a clearly more hydrophobic surface with a contact angle of 85.9° than the pristine perovskite film with a contact angle of 54.2°. We also took a video of dropping water on two kinds of perovskite films as seen in supporting information. The pristine film turned to yellow color as soon as the water dropped onto the surface, while the ODAI-modified film still remained black perovskite phase.


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Figure 5. a) The device performance as a function of storage time at ambient environment with humidity about 20 ~ 40% under dark, and the inset was the final photographs of the control (left) and ODAI-modified (right) device. b) Contact angle measurement of the water droplet on the pristine and ODAImodified perovskite film.

4. CONCLUSIONS In summary, we demonstrate a dimensional interface engineering to improve the device performance by in situ modified the perovskite film with ODAI. The dimensionally graded layer formed by ODAI and residual PbI2 advantageously passivated the grain boundary and modified the interface energy level, thereby significantly increased the Voc from 1.04 V to 1.13 V. The ODAI-modified devices also exhibited an improved ambient stability due to the hydrophobic alkyl group of ODAI, which still retained 92% of its initial after storage in dark ambient environment with relative humidity of 20 ~ 40%. Our work provides a simple and valid strategy to fabricate the efficient and stable



perovskite solar cells.

ASSOCIATED CONTENT Supporting Information Further information relating to the best performance device and statistic photovoltaic parameters of devices without or with ODAI modification, the biexponential decay fitted carrier lifetime and TRPL spectra of two kinds of perovskite films fabricated on glass substrate, AFM images of the pristine and ODAI-modified perovskite film, schematic of the device configuration, the statistic photovoltaic parameters of Jsc, Voc, FF and PCE of the control and ODAI-modified devices from 30 devices for each group, J-V curves of the typical control device with different scan direction at 0.5 V/s, cross-sectional SEM images of devices.

AUTHOR INFORMATION Corresponding Author *Email: [email protected], [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Key Basic Research and Development Program of China (Grant No.2016YFB041003) and the National Natural Science Foundation of China (11574009, U1605244, 61575005, 11574013, 61775004). ABBREVIATIONS CCR2, CC chemokine receptor 2; CCL2, CC chemokine ligand 2; CCR5, CC chemokine receptor 5; TLC, thin layer chromatography.

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Doping. Energy Environ. Sci., 2017, 10, 2095-2102.



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Efficient and Stable Perovskite Solar Cell with High Open-Circuit

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