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Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlayer Hong Zhang, Hamed Azimi, Yi Hou, Tayebeh Ameri, Thomas Przybilla, Erdmann Spiecker, Mario Kraft, Ullrich Scherf, and Christoph J Brabec Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502864s • Publication Date (Web): 27 Aug 2014 Downloaded from http://pubs.acs.org on September 3, 2014
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Chemistry of Materials
Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlayer Hong Zhang*,†,∫, Hamed Azimi†, Yi Hou†, ∫, Tayebeh Ameri†, Thomas Przybilla‡, Erdmann Spiecker‡, Mario Kraft|, Ullrich Scherf|, Christoph J. Brabec†,§ †
Institute of Materials for Electronics and Energy Technology (i-MEET), Friedrich- Alexander-University Erlangen-Nuremberg, Martensstraße 7, 91058 Erlangen, Germany §
Bavarian Center for Applied Energy Research (ZAE Bayern), Haberstraße 2a, 91058 Erlangen, Germany
∫
Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany
|
Macromolecular Chemistry Group (buwmakro) and Institute for Polymer Technology (IfP), Bergische Universität Wuppertal, Gaußstraße 20, 42119 Wuppertal, Germany
‡
Center for Nanoanalysis and Electron Microscopy (CENEM), University of Erlangen-Nuremberg, Cauerstrasse 6, 91058 Germany
ABSTRACT: Highly efficient hybrid organic/inorganic perovskite planar heterojunction (PHJ) solar cells (ITO / PEDOT:PSS / CH3NH3PbI3-XClX / PCBM / polyelectrolyte interlayer /Ag) are fabricated based on the interface layers all solution-processed at low temperature. Relative to the control device, the power conversion efficiency (PCE) increased significantly from 8.53% for the control device to 12.01% (PEIE) and 11.28% (P3TMAHT) via incorporation of a polyelectrolyte interlayer. The improvement in PCE for devices is chiefly assigned to the effective influence of polyelectrolyte interlayers on reducing the work function of the subsequently deposited metal electrode, thereby lowering the electroninjection barriers.
Light weights, mechanically flexibility, solutionprocessibility, low cost and high efficiency photovoltaic devices have been studied intensively in the last decade.1-5 In 2009, Miyasaka et al.6 reported a first attempt using perovskite as sensitizers incorporated into dye-sensitized solar cells, which represents a revolutionary step in perovskite photovoltaic technology. Organolead halide perovskite materials, ABX3 (A = CH3NH3 or NHCHNH3, B = Pb, and X = Br, Cl, or I) are the subject of extensive investigations. This kind of perovskite materials offer a broad range of attractive features such as a direct optical bandgap, a low exciton bonding energy and long diffusion length, a broad range of light absorption, excellent carrier transport and crystallinity.4,5,7-13 Recently, significant progress has been realized in organolead halide perovskite photovoltaic devices with efficiencies over 15% 10,14-18, attracting tremendous attention in the photovoltaic industry. Yang et al. reported a record-breaking efficiency of 19.3% in lead-based perovskite solar cells.19 However, the fabrication of most efficient perovskite solar cells is typically performed based on employing a high-quality condensed TiO2 layer as electron transporting layer, which requires high-temperature processing (450°C) for a long time (1 hour).17,18 Efficient perovskite solar cells can be also fabricated using a hybrid planar heterojunction (PHJ), in which the perovskite layer is sandwiched between a holetransport layer, poly(3,4-ethylenedioxythiophene) poly(styrenesulphonate) (PEDOT:PSS), and an electrontransport layer, [6,6]-phenyl-C61-butyricacid methyl (PCBM). The advantages of this structure are the simplici-
ty as well as low-temperature solution-processability.5,10,20However, the barrier at the contact interface between a Fermi level of various electrode metals (e.g. Ag, Au) and the lowest unoccupied molecular orbital (LUMO) of the organic material (PCBM) still exists in organic optoelectronic devices, leading to poor electron injection and extraction.23,24 The charge injection and extraction at the metal-organic semiconductor interface has a significant impact on the electrical properties of the semiconductor devices. To minimize the contact barrier, the interface between the metal electrode and the PCBM layer should be a quasi-ohmic contact. The suitability of the interface opens the opportunity to apply the interface design rules for the organic solar cell to the perovskite solar cell technology. That strategy has led to efforts in interfacial engineering of hybrid organic/inorganic perovskite PHJ solar cells. High fill factors were demonstrated with e.g. the use of thermal vapour deposited LiF25, bathocuproine (BCP)20,22 or fullerene (C60)20 on top of the PCBM layer. All these systems improved the contact properties and enhanced the device efficiency. However, the thermal vapour deposition of an interface layer is the contradiction to the concept of large scale fabrication and cost-effective solution processing.
22
In this work, we demonstrate that high efficient hybrid organic/inorganic perovskite PHJ solar cell can be fabricated by inserting an ultrathin polyelectrolyte layer, either ethoxylated polyethyleneimine (PEIE) or poly[3-(6trimethylammoniumhexyl)thiophene] (P3TMAHT). Both
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layers are processed from methanol on top of PCBM and beneath the Ag electrode. The power conversion efficiency (PCE) increases from 8.53% to 12.01% (PEIE) and 11.28% (P3TMAHT) for the solution-processed polyelectrolytemodified interfaces. The creation of a surface dipole between PCBM and Ag electrode opens up an alternative pathway to engineer the interface of hybrid organic/inorganic perovskite PHJ solar cells. It is noteworthy to mention that the identical strategy was successfully demonstrated for organic solar cells.22,23 (b) (a)
(c) (d)
Figure 1. a) Schematic of the hybrid organic/inorganic perovskite PHJ solar cell structure and chemical structures of the PEIE and P3TMAHT. b) Cross-sectional SEM image showing the device structure of the hybrid organic/inorganic perovskite PHJ solar cell without Ag electrode. The thickness of PEDOT:PSS, CH3NH3PbI3-XClX, and PCBM layers are 50, 180, and 65 nm, respectively. c) Schematic energy level diagrams of devices with and without the polyelectrolyte interlayer under flat band conditions. d) A photograph of the smooth perovskite film.
Figure 1a. shows the molecular structures of the interlayer materials, PEIE and P3TMAHT, and the device configuration of the hybrid organic/inorganic perovskite PHJ solar cell. The polyelectrolyte interlayers were subsequently deposited by spin-casting from PEIE (0.2% w/v) and P3TMAHT (0.01% w/v) solutions in methanol. A cross-sectional scanning electron microscopy (SEM) image of the device without the Ag electrode is shown in Figure 1b. The cross-section was prepared using a focused ion beam (FEI Helios NanoLab 660) operating at 30 kV and subsequently imaged with the electron beam of the same instrument using an accelerating voltage of 2 kV. The perovskite layer has an average film thickness of approximately 180 nm. The CH3NH3PbI3-XClX film appeared to be rather smooth at the surface, providing a full coverage of the PEDOT:PSS layer (see Figure 1d). Furthermore, it can be seen that a 65 nm thick PCBM layer does perfectly cover the entire CH3NH3PbI3-XClX surface. To investigate the influence of PEIE and P3TMAHT on the work function of top electrode, the work function of Ag electrode was measured before and after modification with polyelectrolyte layers. The work function of Ag was found to decrease from 4.70 eV to 3.97 eV (PEIE) and 4.13
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eV (P3TMAHT). This significant decrease in work function can be attributed to the formation of a surface dipole26, which is attributed to the “push-back” or “cushion” effect. The interfacial modification of PEIE or P3TMAHT/Ag interfaces was previously reported for organic photoelectric devices.23,24,26,27 The energy level diagrams of devices with and without the polyelectrolyte interlayer under flat band conditions are illustrated in Figure 1c. The interface dipoles (PEIE or P3TMAHT) with a negative charge toward the metals and the corresponding positive charge toward the PCBM layer result in the lowering of the vacuum level of the metal.23,24,26 Under irradiation, free charge carriers are generated in the CH3NH3PbI3-XClX layer. The oppositely charged holes and electrons can be extracted and transferred by the PEDOT:PSS and the PCBM, respectively. The ITO anode collects holes and the Ag cathode collects the electrons. The modified work function of Ag - PCBM interface minimizes electrical losses upon injection or extraction of electrons. The current density-voltage (j-V) characteristics for typical perovskite PHJ solar cells based on different polyelectrolyte interlayers are shown in Figure 2, and a summary of the device performance is tabulated in Table 1. The control device without a polyelectrolyte interlayer exhibits an open circuit voltage (Voc) of 0.849 V, a short circuit current density (Jsc) of 16.00 mA cm-2, a fill factor (FF) of 61.29% and a power conversion efficiency (PCE) of 8.53%. The series resistance (RS) is 7.96 Ω cm2 and the shunt resistance (RShunt) is 0.90 kΩ cm2. As shown in Figure 2(a), the control device exhibits a strong s-shaped j-V curve under illumination, which means that a contact barrier prevents an efficient electron injection at the PCBM/Ag interface. Such an injection barrier primarily implies an increased series resistance. The PCE increases significantly from 8.53% for the control device to 12.01% (PEIE) and 11.28% (P3TMAHT) when the polyelectrolyte interlayer is inserted between PCBM layer and Ag electrode. The improvement is mainly due to an increased FF. Relative to the control device, the incorporation of the polyelectrolyte interlayer gave rise to an increase in Jsc from 16.00 mA/cm2 to 17.32 mA/cm2 (PEIE) and 17.10 mA/cm2 (P3TMAHT). Similarly, Voc enhanced from 0.849 V to 0.899 V (PEIE) and 0.899 V (P3TMAHT) and FF improved from 61.29% to 77.10% (PEIE) and 74.10% (P3TMAHT). The series resistance decreased from 7.96 Ω cm2 to 1.00 Ω cm2 (PEIE) and 0.99 Ω cm2 (P3TMAHT), obviously due to the reduction of the injection barrier between the PCBM layer and the Ag electrode, and is in line with an improved FF. To study the impact of methanol processing on the CH3NH3PbI3-XClX/PCBM stack, we fabricated another type of reference device by spin-coating of a pure methanol solution on the top of PCBM layer using a sequence of steps similar to those for the polyelectrolyte interlayer devices described above. As shown by the green line in Figure 2 and the data of the device performance in Table 1, methanol-treated devices show negligible impact with Voc and Jsc values remaining unchanged and FF values slightly increased. This clearly indicated that the im-
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Chemistry of Materials
provement in performance after PEIE or P3TMAHT insertion is due to the presence of the thin polyelectrolyte interlayers, and methanol processing has no effect on the performance of devices. Figure 2b shows that the dark current densities with the thin polyelectrolyte interlayers were significantly suppressed in the regime from -1 to 0.8 V, consistent with a reduced leakage current density and an increased shunt resistance (Rsh) (0.90, 8.18, and 3.14 kΩ cm2 for the control, PEIE, and P3TMAHT, respectively). In the regime from 0.8 to 2 V, the injected current density is higher with than without the polyelectrolyte interlayers. This is in agreement with a reduced injection barrier upon inserting a polyelectrolyte interlayer. (a)
(c)
(b)
generation of a surface dipole. Both effects, the surface dipole layers as well as the better protected interface (reduced shunt) are in excellent agreement with the experimental findings. Table 1. Key values of the j-V characteristics of hybrid organic/inorganic perovskite PHJ solar cell with and without polyelectrolyte interlayers. Each data represents the average from 5 cells. Voc
Jsc
FF
PCE (%)
Devices cathode configuration
(mV)
(mA cm2 )
(%)
PCBM/Ag
849±1
16.00±0.48
60.29±2.30
8.53 (8.82)
PCBM/methanol/Ag
849±1
16.10±0.67
61.75±6.25
7.97 (8.92)
PCBM/PEIE/Ag
899±1
17.32±0.31
77.10±1.27
12.01 (12.36)
PCBM/P3TMAHT/Ag
899±1
17.10±0.42
74.10±1.34
11.28 (11.88)
avg (best)
(d)
Figure 2. a) j-V characteristics of the hybrid organic/inorganic perovskite PHJ solar cells without interlayer (black) and with thin layers of PEIE (red) and P3TMAHT (blue) under illumination of an AM 1.5G solar simulator (100 2 mW/cm ). Methanol (green) was spin-cast on top of the PCBM layer. b) Corresponding logarithmic plot of dark j-V characteristics. c) EQE spectra of perovskite solar cells corresponding to (a). d) Current density versus voltage characteristics of ITO / ZnO / PCBM (65nm) / with and without polyelectrolyte interlayers /Ag electron-only devices. Inset: corresponding logarithmic plot of current density versus voltage characteristics of electron-only devices.
The external quantum efficiency (EQE) curves of devices with and without different polyelectrolyte interlayers are shown in Figure 2c. The improved PCEs of the devices with PEIE or P3TMAHT layers are also consistent with the higher EQE values. The photocurrent integrated from the EQE data are 15.2, 15.1, 16.7 and 16.5 mA/cm2 for the control, methanol treated, PEIE, and P3TMAHT, respectively. The hybrid organic/inorganic perovskite PHJ solar cells exhibit a spectral response from the visible to nearinfrared wavelength regime (300 to 800 nm) with a broad and flat peak around 70∼80% at approximately 400∼750 nm. The higher EQE values of the device with PEIE or P3TMAHT layer in the visible to near-infrared wavelength regime suggest that the polyelectrolyte interlayers more efficiently collect electrons of the PCBM/Ag electrode, on account of successfully reducing energy barrier at the interface between the PCBM and Ag resulting from the
To study the effects of electron injection efficiency at PCBM/Ag interface, we fabricated electron-only devices with the following device configuration: ITO/ZnO/PCBM/polyelectrolyte interlayers/Ag. As shown in Figure 2d, the electron current density significantly increased (up to 3 times) for PEIE or P3TMAHT based devices when injected from the polyelectrolyte interlayer/Ag electrode, but remained the same when injected from the ITO/ZnO electrode. This asymmetric improvement confirms that the increase in electron current density is due to improved injection efficiency of the Ag electrode. It is also clear that the polyelectrolyte interlayers improved the electron injection current for Ag electrode, compared to the control devices. This is also in agreement with a reduced injection barrier at PCBM/Ag interface. In parallel, in a methanol-treated device, the j-V characteristics remain nearly symmetric, with the current injected from the Ag electrode only being slightly higher than that injected from ZnO/ITO. Figure 3 shows the surface morphologies obtained by atomic force microscopy (AFM). The surface of the pristine CH3NH3PbI3-XClX layer is relatively smooth, with a root mean square (RMS) roughness of about 16.7 nm in an area of 10 µm × 10 µm (see Figure 3a). After deposition of PCBM, the surface was much smoother (rms roughness = 6.1 nm) and remained homogeneous, as shown in Figure 3b. In this work, the perovskite film formed has a relatively low roughness, which allows full surface coverage with a very thin PCBM layer (approximately 65 nm thick). Notably, full surface coverage of the perovskite film by the PCBM layer is crucial to prevent leakage due to direct contact between the metal and the perovskite film.20,25 Further important point is that the full surface coverage by the PCBM layer can prevent migration of the polar solvent such as methanol to perovskite layer. We found this important to ensure good reproducibility of the de-
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vice performance. To study the possible effect of the methanol solvent on perovskite film, we fabricated devices with the following device structure: ITO/PEDOT:PSS/CH3NH3PbI3-XClX/methanol/PCBM/Ag. This structure device did not work at all, and the corresponding j-V characteristic was presented in Figure S1. We also found that the fabricated perovskite film turned from dark brown to light brown after treatment with methanol, indicating a severe damage to the perovskite layer. (see Figure S2). Since CH3NH3I is re-dissolved in methanol. The RMS roughness of the PCBM/CH3NH3PbI3XClX layer after treatment with methanol is 16.4 nm. The RMS roughness of a PEIE layer and a P3TMAHT layer deposited on the PCBM / CH3NH3PbI3-XClX planar heterojunction films are 8.4 and 10.3 nm, respectively. (a)
(b)
RMS = 16.7 nm
RMS = 6.1 nm
(d)
(e)
RMS = 8.4 nm
RMS = 10.3 nm
(c)
RMS = 16.4 nm
Figure 3. Surface topographic AFM images images (size: 10 × 2 10 μm ) of a) the pristine CH3NH3PbI3-XClX perovskite film, b) after deposition of PCBM on the CH3NH3PbI3-XClX perovskite film, c) the PCBM/CH3NH3PbI3-XClX layer after treatment with methanol, d) after deposition of PEIE on the PCBM/CH3NH3PbI3-XClX layer and e) after deposition of P3TMAHT on the PCBM/CH3NH3PbI3-XClX layer.
In conclusion, we fabricated hybrid organic/inorganic perovskite PHJ solar cells via incorporation of fully lowtemperature solution processed polyelectrolyte interlayers. The insertion of a polyelectrolyte interlayer improved the PCE to reach 12.01% for the PEIE device and 11.28% for the P3TMAHT device. We attribute the improvement in PCE for devices as compared to devices without PEIE or P3TMAHT, to the formation of surface dipoles. Both PEIE and P3TMAHT, effectively reduce the work function of the subsequently deposited metal, thereby lowering the electron-injection barrier to PCBM. Thus, this study provides a practical route for the fabrication of high efficiency perovskite thin film solar cells based on solution processing at low temperatures.
ASSOCIATED CONTENT Supporting Information. Detailed information about experimental methods. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *
Email:
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ACKNOWLEDGMENT The authors gratefully acknowledge the support of the Cluster of Excellence “Engineering of Advanced Materials (EAM)”, Energy Campus Nuremberg (EnCN, Solarfactory), “Synthetic Carbon Allotropes” (SFB 953) project, DFG research training group GRK 1896 and the Erlangen Graduate School in Advanced Optical Technologies (SAOT) at the University of Erlangen-Nuremberg, which is funded by the German Research Foundation (DFG) within the framework of its “Excellence Initiative”. This work has been partially funded by the China Scholarship Council (CSC). The authors also thank the support of Solar Technologies go Hybrid (SolTech) project from Bavarian Ministry of Science.
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Title Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlayer Author Hong Zhang, Hamed Azimi, Yi Hou, Tayebeh Ameri, Thomas Przybilla, Erdmann Spiecker, Mario Kraft, Ullrich Scherf and Christoph J. Brabec
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