Performance Planar Perovskite Solar Cells

Province, Soochow University, Suzhou 215006, China. ABSTRACT. 4,7-diphenyl-1,10-phenanthroline (Bphen) is an efficient electron transport and hole ...
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Organic−Inorganic Hybrid Interfacial Layer for High-Performance Planar Perovskite Solar Cells Hao Yang,† Shan Cong,† Yanhui Lou,*,†,‡ Liang Han,† Jie Zhao,† Yinghui Sun,† and Guifu Zou*,†,‡ †

Soochow Institute for Energy and Materials InnovationS, College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, and ‡Key Laboratory of Advanced Carbon Materials and Wearable Energy Technologies of Jiangsu Province, Soochow University, Suzhou 215006, China S Supporting Information *

ABSTRACT: 4,7-Diphenyl-1,10-phenanthroline (Bphen) is an efficient electron transport and hole blocking material in organic photoelectric devices. Here, we report cesium carbonate (Cs2CO3) doped Bphen as cathode interfacial layer in CH3NH3PbI3−xClx based planar perovskite solar cells (PSCs). Investigation finds that introducing Cs2CO3 suppresses the crystallization of Bphen and benefits a smooth interface contact between the perovskite and electrode, resulting in the decrease in carrier recombination and the perovskite degradation. In addition, the matching energy level of Bphen film in the PSCs effectively blocks the holes diffusion to cathode. The resultant power conversion efficiency (PCE) achieves as high as 17.03% in comparison with 12.67% of reference device without doping. Besides, experiments also demonstrate the stability of PSCs have large improvement because the suppressed crystallization of Bphen by doping Cs2CO3 as a superior barrier layer blocks the Ag atom and surrounding moisture access to the vulnerable perovskite layer. KEYWORDS: planar perovskite solar cells, Bphen, Cs2CO3, stability, interface engineering nation.18,19 To reduce the recombination in the interface between PCBM layer and cathode, researchers have proposed some efficient interface engineering methods to improve the device efficiency.20−24 Seok and co-workers insert a very thin LiF layer between the Al electrode and the PCBM layer.25 Compared to the device without a LiF layer, both of the short circuit current density (JSC) and the fill factor (FF) can be improved a lot. It is also reported that bathocuproine (BCP) has been applied to modify the PCBM and metal electrode interface.19 Noticeably, all of the above devices have shown remarkable performance after the interfacial layers were modified. As is well-known, 4,7-diphenyl-1,10-phenanthroline (Bphen), an organic small material, is a hole blocking and efficient electron transport material in organic electronic devices (OLEDs) owing to its outstanding electron transporting ability.26−28 As for perovskite solar cells, Bphen has been inserted between the electron transport layer PCBM and the cathode for effective interface modification.21,29,30 In addition, a thin Cs2CO3 coated on Bphen to tune the function was reported.31 Nevertheless, readily crystallization of Bphen causes unevenness of film surfaces leading to decrease of the conduct area with electrode and hindering the electron collection. In

1. INTRODUCTION Organic−inorganic halide perovskite solar cells (PSCs) appear explosive growth as a promising technology for the photovoltaic industry due to some unique advantages, such as extremely long carrier diffusion length and high power conversion efficiency (PCE).1,2 The PCE of PSCs has reached more than 20% since the initial 3.8% in the past several years.3−5 Usually, two types of device architectures, planar heterojunction (PHJ) and mesostructure, have been widely reported.6,7 Compared with the mesostructure requiring a high temperature treatment to form the scaffold layer, PHJ structure has a simple fabrication with lower temperature processing.8−10 Especially, the devices with a poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS)/perovskite/fullerene (PCBM) planar junction have attracted much attention because of the simple solution process.11−13 In a typical PHJ perovskite solar cell, the perovskite layer is sandwiched between an electron transport layer (ETL) and a hole transport layer (HTL).6 When sunlight shines on a solar cell, the free carriers are generated in the perovskite layer, and then transport across ETL or HTL and the corresponding interfaces into electrodes. Therefore, besides perovskite layer, carrier transport layers and interfaces also play significant roles for the device performance.14−17 Interface engineering can not only passive the perovskite layer surface, but also align the energy level of electrode, which is beneficial for enhancing the open-circuit voltage (VOC) and reducing the charge recombi© 2017 American Chemical Society

Received: May 12, 2017 Accepted: August 25, 2017 Published: August 25, 2017 31746

DOI: 10.1021/acsami.7b06681 ACS Appl. Mater. Interfaces 2017, 9, 31746−31751

Research Article

ACS Applied Materials & Interfaces addition, the rough surface might not block effectively the water or oxygen entering the perovskites. Herein, cesium carbonate (Cs2CO3) is doped into solution-processed Bphen as an interface layer between the perovskite/PCBM and the Ag electrode in CH3NH3PbI3−xClx-based planar perovskite solar cells. By introducing Cs2CO3, the Bphen film energy level is got modification for effectively blocking holes transport to the electrode interface and reducing recombination. In addition, the Cs2CO3 dopant plays a role in suppressing the crystallization of Bphen to form smooth interface layer for close contact. The PCE of the device achieves as high as 17.03% after Cs2CO3 doped Bphen introduced as interfacial layer. Moreover, the unencapsulated device stored in ambient condition with 40− 60% relative humidity exhibits excellent stability compared with the control device.

Figure 1. (a) Chemical structures of Bphen and Cs2CO3; (b) energy level diagram of the device (Per represents perovskite); (c) schematic of the device structure; (d) cross-sectional SEM image of the optimized device structure.

2. EXPERIMENTAL SECTION Materials. PbCl2 (99.999%) and Cs2CO3 (99.995%) were purchased from Sigma-Aldrich. Bphen (99%) was received from Xi’an polymer light technology Co. Methylammonium iodide (MAI) was synthesized using methylamine and hydroiodic. The MAI and PbCl 2 with a molar ratio of 3:1 were dissolved in N,Ndimethylformamide (DMF, 99.8%) to prepare the perovskite precursor solution, which was stirred at 70 °C overnight in a glovebox. Solar Cell Fabrications. The perovskite solar cells were fabricated as described in our previous report.22 Bphen and Cs2CO3 are dissolved in ethanol. The Bphen:Cs2CO3 interfacial layer was deposited onto the PCBM layer from a 0.5 mg mL−1 solution by spin-coating at 4000 rpm for 40 s without additional treatment. More details are described in the Supporting Information. Characterization. The surface morphologies were estimated using atomic force microscope (Bruker) and field emission scanning electron microscope (Hitachi SU8010). Photoluminescence (PL) measurements were carried out with a Horiba spectrofluorometer (Fluoromax4) in air. Electrochemical impedance spectroscopy (EIS) was performed with an IM6e Electrochemical Workstation (ZAHNER, Germany). All the above measurements were performed for pristine Bphen and Bphen doped with 20 wt % Cs2CO3. The current density− voltage (J−V) characteristics of different devices were obtained from a programmable Keithley 2400 source meter under AM 1.5G solar irradiation at 100 mW cm−2 in air condition. The J−V curve in Figure 5b is obtained by backward scanning.

without Cs 2CO 3 doping, thereby reducing the charge recombination at cathode to improve the VOC and the FF. Photoluminescence (PL) spectra were used to determine the charge transfer ability of the Cs2CO3 doped Bphen interfacial layer. As shown in Figure 2b, a strong peak at 770 nm is exhibited in the pristine perovskite film with an excitation light of 525 nm applied in air, indicating a strong carrier recombination. After being coated with PCBM layer, an obvious quenching effect is observed due to effective extraction of PCBM electron transport layer. Particularly, the peak intensity is dramatically reduced compared with that of pristine perovskite film, whereas the Bphen: Cs2CO3 layer is coated on top of the PCBM layer. The distinct quenching effect demonstrates that the carrier recombination is suppressed to some extent at the interfaces. Simultaneously, there are more efficient electrons transporting from perovskite layer to cathode. To further investigate the interface charge transport in the device, the electrical impedance spectroscopy (EIS) is performed. Figure 2c shows the Nyquist plots of three PSCs with different interface layers. And the equivalent circuit model fitted by the impedance data is shown in the inset. According to the fitted equivalent circuit, the internal series resistance consists of the sheet resistance (RS) of the cell, the charge transfer resistance (RCT) at interfaces between the perovskite and the carrier selective layer and between the carrier selective layer and the electrode.32,33 Because all samples have the same device structure, the radius of the semicircle for device with interface layer is lower than that for the reference device, which is attributed to the difference of the interfaces between the PCBM layer and the Ag electrode. As shown in Figure 2c, when introducing Bphen: Cs2CO3, the RCT is decreased, which suggests that more efficient charge extraction happens at the interface. And it keeps pace with the results from PL spectra. Generally, the interface of each layer plays crucial role in the stacked device. A smooth interface contact between the top electrode and the perovskite layer can effectively suppress the charge recombination and prevent the leakage current at the electrode interface to obtain high-performance for PSCs. Surface topography of different interfacial films was examined by AFM in tapping mode with a 5 μm square scan area. The AFM images of the perovskite layer coated with different interfacial layer are shown in Figure 3. Some dark shadows representing voids come insight in the pristine perovskite films,

3. RESULTS AND DISCUSSION The chemical structures of Bphen and Cs2CO3, schematics of energy level diagram and the device structure, and crosssectional scanning electron microscopy (SEM) image of the PSC are shown in Figure 1. The device architecture is ITO/ PEDOT:PSS/perovskite/PCBM/Bphen:Cs2 CO 3 /Ag (100 nm). From SEM image, every layer can be observed clearly. Bphen film doped with Cs2CO3 act as interface layer at the cathode side. It can effectively block holes transport to cathode and electron transport layer. To find out the influence of Cs2CO3 on the energy level of Bphen layer in the device, we carried out ultraviolet photoemission spectroscopies (UPS) measurements to examine the electronic structures of pristine Bphen and Bphen: Cs2CO3. Figure 2a displays the magnified plots of UPS spectra for Bphen and Bphen:Cs2CO3. According to the UPS spectra obtained from a helium lamp emitting at 21.2 eV, the highest occupied molecular (HOMO) energy level of Bphen is calculated to be 6.2 eV. After doping with Cs2CO3, it is effectively increased to 6.4 eV as shown in Figure 1b. The deeper HOMO level of Bphen doped with Cs2CO3 can block holes transport to Ag cathode more effectively than the one 31747

DOI: 10.1021/acsami.7b06681 ACS Appl. Mater. Interfaces 2017, 9, 31746−31751

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) Magnified UPS spectra of Bphen and Bphen: Cs2CO3; (b) PL spectra of perovskite film on glass with different interface layers; (c) Nyquist plots of PCBM, PCBM/Bphen, and PCBM/Bphen:Cs2CO3-based PSCs measured at an applied voltage close to open voltage in the dark.

contact between the perovskite layer and the silver electrode can reduce the contact resistance and carrier recombination, which is beneficial for the VOC and FF. To further understand the influence of Cs2CO3 doping on morphology, polarized optical microscope images were captured for the films of Bphen and Bphen:Cs2CO3 coated on glass (Figure 4). The obvious large-scale crystallization is observed for Bphen film without Cs2CO3 in Figure 4a. After doping with Cs2CO3, it is interesting to find that the aggregation of the Bphen molecules becomes weak with uniform distribution, indicating the crystallization extent of Bphen is efficiently suppressed (Figure 4b). The less-crystalline Bphen:Cs2CO3 layer contributes to a smoother surface, which keeps pace with the AFM analysis from Figure 3. Combined with the AFM images (Figure 3) and the polarized optical images (Figure 4), it verifies that doping Cs2CO3 in Bphen can effectively suppress the crystallization of Bphen film. CH3NH3PbI3−xClx-based inverted planar perovskite solar cells were fabricated to investigate the effects of Cs2CO3 doped Bphen layer on the cell performance. Figure 5a shows the J−V characteristics of the PSCs with Cs2CO3 (varied ratio) doped Bphen interfacial layers. The key parameters of these devices are summarized in Table 1 and Table S1. The device with bare PCBM presents a VOC of 0.88 V, JSC of 19.17 mA cm−2, FF of 0.66, and PCE of 11.03%. After introducing Bphen, the PCE increase to 12.67%. As for the device employing a Cs2CO3 doped Bphen interfacial layer, the photovoltaic performance is obviously improved. It originated from the suitable energy level (Figure 2a) and smooth interface contact (Figure 3d).We carried out J−V measurement for the different ratio of Bphen: Cs2CO3 in the PSCs. Figure 5b shows the device with a Cs2CO3 (20 wt %) layer has a champion performance with a PCE of 17.03%, VOC of 1.01 V, FF of 0.76 and JSC of 22.14 mA cm−2. Figure S1 shows a histogram of the performance for the Bphen and Bphen: Cs2CO3 devices, indicating a good

Figure 3. AFM images of (a) perovskite, (b) PCBM on perovskite, (c) Bphen on perovskite/PCBM, and (d) Bphen:Cs2CO3 on perovskite/ PCBM.

which results in a relative nonuniform surface with a rootmean-square roughness (RMS) of 22.86 nm. After deposition of PCBM, the surface roughness immediately decreases to 8.04 nm as shown in Figure 3b. Surfaces roughness further decrease to be 6.58 and 5.66 nm after PCBM is modified with Bphen and Bphen: Cs2CO3 in Figure 3c and Figure 3d, respectively. There is an obvious difference between the morphology of pristine perovskite and perovskite with PCBM layers. Finally, the adoption of Bphen: Cs2CO3 layer results in the most flat and smooth surface for the PCBM layer. Accordingly, the inserted moification layer can effectively fill the voids in the perovskite/PCBM surface. A smooth and compact interface

Figure 4. Polarized optical microscope images of (a) pristine Bphen and (b) Bphen:Cs2CO3. 31748

DOI: 10.1021/acsami.7b06681 ACS Appl. Mater. Interfaces 2017, 9, 31746−31751

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ACS Applied Materials & Interfaces

Figure 5. (a) J−V curves of all the devices. (b) J−V curve of the device with best performance. IPCE spectra of the device based on Bphen:Cs2CO3 is inset.

Table 1. Parameters of PSCs with Different Interface Layers device PCBM PCBM/Bphen PCBM/Bphen:Cs2CO3 (10%) PCBM/Bphen:Cs2CO3 (20%) PCBM/Bphen:Cs2CO3 (30%)

JSC (mA cm−2)

VOC (V)

FF (%)

PCE (%)

avg (%)

19.17 20.14 20.92

0.88 0.91 0.92

66 69 73

11.03 12.67 14.05

10.65 12.06 13.36

22.14

1.01

76

17.03

16.12

19.28

0.90

71

12.32

11.80

reproducibility. The incident photon-to-current efficiency (IPCE) spectra of Bphen: Cs2CO3 (20 wt %) based device is shown in the inset of Figure 5b. It shows a high spectral response from 400 to 750 nm constant with UV−vis absorption spectrum in Figure S2. To investigate the hysteresis of the device, the forward and backward scan is performed and shown in Figure S3. No apparent hysteresis is observed with different scan directions. The stability of the PSCs in ambient is another key parameter for the practical devices. It is noting that our unencapsulated devices are stored under ambient (room temperature, humidity 40−60%) in dark conditions and tested under 1 sun illumination every 25 h in air. As shown in Figure 6, it is clearly seen that there was no larger change on the degradation of VOC and FF in the Bphen: Cs2CO3 (20 wt %) based device in comparison with those of the device with Bphen. As a result, the optimized device with about 80% of initial PCE reserved for 175 h, whereas the device without doping maintains only 20% in Figure 6d. The experiment demonstrates that Cs2CO3 doping benefits the improvement of the device stability. And it might be caused from doping Cs2CO3 in Bphen efficiently suppressing the crystallization of Bphen to form a smooth surface blocking the Ag atom diffusion and moisture permeating to the sensitive perovskite layer.

Figure 6. Stability of (a) VOC, (b) JSC, (c) FF, and (d) PCE for different unencapsulated devices with pristine Bphen and Bphen:Cs2CO3 (20 wt %) interface layer under ambient condition.

PCE was improved significantly compared with reference device. In addition, suppressed crystallization of Bphen by doping Cs2CO3 makes this layer a superior barrier to block the Ag atom and surrounding moisture access to the vulnerable perovskite layer, resulting in an improved stability of the device. The desirable interfacial layer could contribute to efficiency as well as stability for planar-perovskite solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06681. Device fabrications and figures showing histogram of performance, UV−vis absorbance, and reverse and forward J−V curves (PDF)

4. CONCLUSION In conclusion, a solution-processed Cs2CO3-doped Bphen has been employed as an interface layer to modify the contact between perovskite/PCBM and the cathode in planar perovskite solar cells. After introducing Cs2CO3 into Bphen film, the deeper HOMO can block hole transport to the cathode and decrease recombination of carriers. The suppressed crystallization of Bphen by doping Cs2CO3 improved the contact with Ag electrode to enhance electron extraction. As a result, the

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guifu Zou: 0000-0002-8342-7768 31749

DOI: 10.1021/acsami.7b06681 ACS Appl. Mater. Interfaces 2017, 9, 31746−31751

Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Scientific Research Foundation, the “973 Programthe National Basic Research Program of China” Special Funds for the Chief Young Scientist (2015CB358600), the Excellent Young Scholar Fund from National Natural Science Foundation of China (21422103), Jiangsu Fund for Distinguished Young Scientist (BK20140010), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars (2014−2017).

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DOI: 10.1021/acsami.7b06681 ACS Appl. Mater. Interfaces 2017, 9, 31746−31751