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We report perovskite solar cells with a new device structure that employ highly conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the top electrode replacing commonly used metal electrodes. The PEDOT:PSS top electrode is prepared from its aqueous solution through a transfer-lamination technique rather than direct spin-coating, which converts the CH3NH3PbI3 into PbI2. Perovskite solar cells with the structure of glass/FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD/PEDOT:PSS yield a maximum open-circuit voltage (VOC) of 1.02 V, and a maximum power conversion efficiency (PCE) of 11.29% under AM1.5 100 mW/cm2 illumination. The whole device was fabricated in air without high-vacuum deposition which simplifies the processing and lowers the threshold of both scientific research and industrial production of perovskite solar cells.
© 2015 Optical Society of America
1. Introduction
Organic-inorganic lead halide perovskite (CH3NH3PbX3, X = Cl, Br, I)–based thin film photovoltaic devices have been attracting great attention in the past a few years owing to their high efficiency with easy processing [1–10]. In these perovskite solar cells, metals (typically Au, Ag or Al) are generally used as the top electrodes. For example, in fluorine-doped tin oxide (FTO)/TiO2-based perovskite solar cells [with a typical structure of FTO/c-TiO2/(m-TiO2 or Al2O3/)perovskite/HTL/Au or Ag where HTL denotes hole-transporting layer, c-TiO2 denotes compact TiO2 and m-TiO2 denotes mesoporous TiO2], Au or Ag is used as the top electrodes for hole collection [3–5]. In indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-based perovskite solar cells derived from organic photovoltaics [with a typical structure of ITO/PEDOT:PSS/perovskite/fullerene/Al], Al is used as the top electrode for electron collection [11–15]. These metal electrodes are deposited from high vacuum, which is highly energy-consuming and requires expensive high-vacuum deposition facility. It raises the threshold of both scientific research and large-area industrial production of perovskite solar cells. Recently, thick carbon layers, carbon nanotubes (CNT) and metal meshes have been reported as the top electrodes in perovskite solar cells [16–19]. The incorporation of these electrodes eliminates high-vacuum processing, but the preparation of the electrodes is relatively complicated.
Conducting polymer PEDOT:PSS [Fig. 1(a)] has high conductivity (up to several thousand S/cm) and is widely used as an electrode in organic electronic devices such as organic light-emitting diodes, organic photovoltaics and organic field-effect transistors [20–26]. PEDOT:PSS is dispersed in water and has excellent film-forming ability. The PEDOT:PSS films can be easily prepared via spin-coating, spraying or printing. However, due to the moisture-sensitive nature of CH3NH3PbI3 perovskite film, direct coating of PEDOT:PSS aqueous solution would destroy the CH3NH3PbI3 perovskite film. The CH3NH3PbI3 perovskite is changed to PbI2 as indicated by x-ray diffraction (XRD) patterns shown in the following section. Transfer-lamination technique that the target film layer is firstly deposited onto a transfer medium substrate and then transferred onto the receiving substrate has been successfully employed to transfer organic layers (such as PEDOT:PSS and organic semiconducting layers) for organic electronic devices [27–34]. With this technique, the aqueous solution of PEDOT:PSS will not directly contact the layers containing CH3NH3PbI3 perovskite film. Instead, the dried PEDOT:PSS film layer will be laminated onto the top and thus the perovskite film will be not destroyed. This will potentially solve the incompatibility between aqueous PEDOT:PSS solution and moisture-sensitive CH3NH3PbI3 perovskite.
Fig. 1 (a) Schematic of the perovskite solar cell employing PEDOT:PSS as the top electrode and the chemical structure of PEDOT:PSS; (c) Energy diagram (versus vacuum energy level) of each layer in the device. The unit of energy level is electron volts (eV).
Here, by using the transfer-lamination technique we demonstrate a new type of device structure of perovskite solar cell that employs high-conductivity PEDOT:PSS as the top electrode and the solar cell fabrication is free from vacuum processing. Unlike direct spin-coating of PEDOT:PSS solution which turns CH3NH3PbI3 into PbI2 immediately, the use of the transfer-lamination technique keeps the structure of CH3NH3PbI3 perovskite film unchanged during the deposition of PEDOT:PSS. As a result, the new type of perovskite solar cells with PEDOT:PSS top electrodes exhibit outstanding photovoltaic performance with an averaged open-circuit voltage (VOC) of 0.97 V, an averaged power conversion efficiency (PCE) of 9.05% and a maximum VOC of 1.02 V, a maximum PCE up to 11.29%, over 20 cells under AM1.5 100 mW/cm2 illumination.
2. Experimental section
2.1 Sample preparation
FTO-coated glasses (Pilkington, TEC-7) with a sheet resistance of 7 ohm/sq were patterned by wet etching with zinc powder and concentrated hydrochloric acid (12 M). The substrates were then cleaned in ultrasonic baths of detergent diluted in deionized water, deionized water, acetone and isopropanol in sequence. Finally, a nitrogen flow was used to dry the etched FTO glass substrates.
The pattered substrates were coated with a roughly 40 nm compact TiO2 layer by aerosol spray pyrolysis at 450 °C. Upon cooling down to room temperature in air, a 200-250 nm thick TiO2 nanocrystalline layer (PASOL HPW-18NR) was deposited on top of the c-TiO2 by screen printing and then sintered at 500°C for 30 min. PbI2 (99%, Sigma Aldrich) in DMF (≥ 99%, Aldrich) with a concentration of 461 mg mL−1(~1 M) was spin-coated onto the substrates at 6500 rpm for 60 s with an acceleration of 2000 rpm/s and kept at 70 °C for 30 min afterwards. After cooling down to room temperature, the PbI2 films were dipped into a solution of CH3NH3I in 2-propanol (10 mg mL−1) for 60 s and spin-coated at 1000 rpm for 10 s with an acceleration of 1000 rpm/s to remove the remaining solution and smooth the resulting CH3NH3PbI3 films. The obtained CH3NH3PbI3 films were then annealed at 70 °C for 30 min for further crystallization. The spiro-OMeTAD-based hole-transporting layer was spin-coated onto the CH3NH3PbI3 film at 4000 rpm for 30s with an acceleration of 1000 rpm/s from a solution mixture [80 mg spiro-OMeTAD (99%, Nichem), 28.5 μl 4-tert-butylpyridine (TBP, 96%, Aldrich) and 17.5 μl lithium-bis(trifluoromethanesulfonyl)imide) (Li-TFSI, 99.95%, Aldrich) solution (520 mg Li-TFSI in 1 ml acetonitrile (≥99.9%, Aldrich) dissolved in 1mL chlorobenzene (99.8%, Aldrich)].
2.2 PEDOT:PSS film transfer lamination
To achieve PEDOT:PSS film-transfer lamination, a piece of PDMS was firstly cut and attached onto a clean glass, then treated under air plasma for 50 s to tune the surface hydrophilic. PEDOT:PSS PH1000 (Heraeus) was spin-coated onto the PDMS substrates with a two-step procedure: 500 rpm for 5s and then 1000 rpm for 1 min. To improve the wetting property and conductivity, 5 wt.% ethylene glycol (98%, Aldrich) and 0.1 wt.% polyethylene glycol 2,5,8,11-tetramethyl-6-dodecyne-5,8-diol ether (Superwet-340, TOYNOL) were added into the PH1000 before the spin coating. Prior to PEDOT:PSS film transfer, substrates of FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD were exposed to air plasma for 5 s. Drying for about 10 min in air, the PEDOT:PSS on top of PDMS was cut into 4 mm-wide and 6 mm-long rectangle shape and transferred onto the receiving surface facing down with PEDOT:PSS contacting the spiro-OMeTAD film intimately. Finally, the PDMS was slowly peeled off and PEDOT:PSS film left on the sample to finish the device fabrication. The thickness of PEDOT:PSS film was about 120 nm. Finally, Ag paint (Leitsilber 200, Ted Pella Inc.) was put onto the edge of PEDOT:PSS film and FTO for electrical contact for measuring the performance. The effective device area ranging from 2 to 10 mm2 was determined precisely under an optical microscope (DM4000 M, Leica). Note that the whole device fabrication process was conducted under ambient condition (Temperature: 25 °C, Moisture: 45%).
2.3 Film and device characterization
The current density-voltage (J-V) characteristics of the devices were measured without encapsulation using a Keithley 2400 source-measure unit. The cells were illuminated by a 450 W Newport solar simulator (model 91192-1000) equipped with an AM 1.5G filter at a calibrated intensity of 100 mW cm−2. Unlike Au or Ag counter electrode, semitransparent PEDOT:PSS can also transmit light, thus a piece of white paper was placed below the device as a reflector with light illuminating from the FTO side. XRD measurement (X’pert PRO, PANalytical B.V.) was conducted to analyze the crystalline structure of perovskite before and after PEDOT:PSS deposition. The SEM measurement of the device was performed by a field-emission scanning microscope (Quanta 3D FEG, FEI).The thickness of the transfer-laminated PEDOT:PSS film was measured through a step profiler (Veeco Dektak 150) and the conductivity of the transfer-laminated PEDOT:PSS film was measured by four-point probe (RTS-8 (504))
3. Results and discussions
Figure 1(a) shows the schematic of the CH3NH3PbI3 perovkite solar cell with the new device structure (glass/FTO/c-TiO2/m-TiO2/CH3NH3PbI3/spiro-OMeTAD/PEDOT:PSS), where conducting polymer PEDOT:PSS is used as the top electrode instead of metal. PEDOT:PSS has a work function of about 5.0 eV which is close to that of Au and is energetically favorable for hole collection from the spiro-OMeTAD layer as shown in Fig. 1(b). The PEDOT:PSS layer has been demonstrated as an efficient top electrode for hole collection in organic solar cells. Therefore, here the PEDOT:PSS can be expected to work effectively as a top electrode in the perovskite solar cell.
PEDOT:PSS is a polymer mixture of PEDOT and PSS [Fig. 1(a)]. The mixture is dispersed in water solution for film processing. It is known that the CH3NH3PbI3 perovkite is very sensitive to moisture. Direct coating of PEDOT:PSS aqueous solution on top of the CH3NH3PbI3 perovkite film/spiro-OMeTAD would damage the CH3NH3PbI3 perovskite film. The color of the CH3NH3PbI3 perovkite film changes from dark brown to yellow after direct spin coating of aqueous PEDOT:PSS solution, as shown in the inset of Fig. 3(c). The CH3NH3PbI3 is found to be converted into PbI2 during the coating by XRD measurement (as shown in Fig. 3 and will be discussed in the following).
With the transfer-lamination method, whereby the PEDOT:PSS film is firstly deposited from its aqueous solution onto a PDMS substrate and then the film is transferred onto the receiving surface (CH3NH3PbI3 perovskite/spiro-OMeTAD film), the incompatibility between aqueous PEDOT:PSS solution and moisture-sensitive CH3NH3PbI3 perovskite can be solved. Figure 2(a) illustrates the transfer-lamination procedure of PEDOT:PSS film that can be briefly summarized as follows (more details are provided in the Experimental Section): (1) aqueous PEDOT:PSS solution (with 5% EG and 0.1% additive surfactant) is dropped onto plasma-treated PDMS substrate and spin-coated to form a film; (2) the PEDOT:PSS film is then dried in air without thermal annealing; (3) PDMS with the PEDOT:PSS film is peeled off from the glass substrate and put onto the surface of plasma-treated spiro-OMeTAD film facing down with the PEDOT:PSS contacting the spiro-OMeTAD layer; (4) PDMS substrate is peeled off and the PEDOT:PSS film is left on the spiro-OMeTAD as the top electrode. With the transfer-lamination technique, the PEDOT:PSS film is successfully deposited on the CH3NH3PbI3 perovskite/spiro-OMeTAD layers while keeping the CH3NH3PbI3 perovskite safe, as confirmed in Fig. 3. The color of the CH3NH3PbI3 film doesn’t change after the deposition of PEDOT:PSS film, as shown in the inset of Fig. 3(d). Conductivity of the transfer-laminated PEDOT:PSS film was measured to be 308 S/cm on a clean glass substrate.
Fig. 2 (a) Preparation procedure of PEDOT:PSS top electrode using the film-transfer lamination method:(1)aqueous PEDOT:PSS solution is dropped onto air plasma-treated PDMS substrate (~1 mm) and spin-coated to form a film; (2) the PEDOT:PSS film is dried in air for about 10 min without thermal annealing; (3) PDMS with PEDOT:PSS films are peeled off from the glass substrate and put down onto the surface of air plasma-treated spiro-OMeTAD film; (4) PDMS substrate is peeled off to separate the PEDOT:PSS film from the PDMS substrate; (5) the PEDOT:PSS film is left on the spiro-OMeTAD as the top electrode; (b) SEM image of the surface of PEDOT:PSS top electrode of a fabricated device. The edge in the middle of the image is formed by the PEDOT:PSS top electrode, the left side with PEDOT:PSS and the right side without PEDOT:PSS film. The inset is a image of another sample of PEDOT:PSS film on top of spiro-OMeTAD in larger area. (c) cross-section image of the whole device obtained from the white square in (b) with Au and Pt nanoparticles on top, the left side is with PEDOT:PSS top electrode and the right side without PEDOT:PSS.
Fig. 3 X-ray diffraction patterns of different samples: (a) CH3NH3PbI3; (b) CH3NH3PbI3/spiro-OMeTAD; (c) CH3NH3PbI3/spiro-OMeTAD with spin-coated PEDOT:PSS on top; (d) CH3NH3PbI3/spiro-OMeTAD with transfer-laminated PEDOT:PSS film on top. All the samples for XRD measurement are prepared on glass substrate.
To obtain more information of the new perovskite solar cells, surface morphology of PEDOT:PSS top electrode and the cross-section image of the whole device are measured using scanning electron microscopy (SEM) as shown in Fig. 2(b) and 2(c). In the set of Fig. 2(b), the transfer-laminated PEDOT:PSS film on top of spiro-OMeTAD layer can be clearly distinguished by the edge. In Fig. 2(b), the border can also be clearly seen showing that the left side is with PEDOT:PSS film while the right side is without. As for the cross-section image of the cell, because the PEDOT:PSS top electrode is thin and soft, it is hard to obtain a sharp and clean cross section by simple cutting. Therefore, we applied a focused ion beam (FIB) to drill a hole right on the border of the PEDOT:PSS layer to obtain a clean cross section. Prior to the drill, a thick platinum layer is deposited within the sample area denoted by a red rectangle in Fig. 2 to protect the soft PEDOT:PSS film from being damaged during the FIB drill. The cross-section image of the device at the drill point is shown in Fig. 2(c), the left side is with PEDOT:PSS and the right side is without. Layers of FTO/c-TiO2/m-TiO2/CH3NH3PbI3 perovskite can be relatively easy to recognize and distinguish from the SEM image. However, layers of PEDOT:PSS and spiro-OMeTAD are difficult to distinguish as there is not enough contrast between them. It also means the two layers have good contact. It should be noted that Au particles and Pt layers [in Fig. 2(c)] on top of the PEDOT:PSS or spiro-OMeTAD are deposited for helping the SEM measurement (doesn’t exist in the real devices).
To confirm the feasibility of film transfer-lamination technique in preparing PEDOT:PSS top electrode for perovskite solar cells, we carried out XRD measurement (shown in Fig. 3). Pure CH3NH3PbI3 film shows strong diffraction peaks at 14.05, 24.35, 28.34 and 31.88°, which correspond to the (100), (111), (200) and (210) lattice planes of cubic CH3NH3PbI3 crystals, respectively [35]. A small diffraction peak at 12.6° can be attributed to the unreacted PbI2 [4, 36]. Figure 3(b) presents XRD pattern for the CH3NH3PbI3 with a layer of spiro-OMeTAD film on top. It exhibits almost identical diffraction peaks and intensities as shown in Fig. 3(a) except that the peak at 12.6° is slightly stronger, which should be ascribed to the preparation and measurement variation sample by sample as it is known that the coating of spiro-OMeTAD solution does not harm the perovskite film. Figure 3(c) shows the XRD pattern of the sample prepared by spin-coating aqueous PEDOT:PSS solution onto the CH3NH3PbI3/spiro-OMeTAD substrate. The direct spin-coating of aqueous PEDOT:PSS converts CH3NH3PbI3 into PbI2 as evidenced by the dominating diffraction peak at 12.6°. If the PEDOT:PSS film is prepared by transfer-lamination technique, CH3NH3PbI3 retains its original crystalline structure as shown in Fig. 3(d) where XRD pattern is the same as that of pure CH3NH3PbI3 in Fig. 3(a). The transfer-lamination dry process does not degrade crystallinity of CH3NH3PbI3 perovskite film.
Figure 4(a) shows the current density-voltage (J-V) characteristics of a representative solar cell under 100 mW/cm2 AM1.5 illumination. Figure 4(b) is the scattergram of photovoltaic performance of 20 devices. Averaged VOC, short-circuit current density (JSC), fill factor (FF) and PCE are 0.97 V, 16.40 mA/cm2, 0.57 and 9.05%, respectively, with a maximum PCE of 11.29% and a maximum VOC of 1.02 V. Furthermore, because hysteresis effect often appears in the perovskite solar cells during the J-V measurement [10, 37], which may be associated with large defect density within or near the surface of the perovskite absorber or its ferroelectric properties [38, 39], the influence of the scanning direction on the J-V characteristics of a typical fabricated cell is also included in Fig. 4(a). We do see the hysteresis effect in our device, but it is not very significant. At a scan rate of 300 mV/s, the cell displays VOC, JSC, FF and PCE of 993 mV, 17.41 mA/cm2, 0.63 and 10.91% when measured from forward bias to short circuit. Under the opposite scanning direction, the cell displays VOC, JSC, FF and PCE of 979 mV, 16.67 mA/cm2, 0.60 and 9.82%, respectively.
Fig. 4 (a) J-V characteristics of a fabricated device under 100 mW/cm2 AM 1.5 illumination measured from forward bias to short circuit (filled square) and the opposite direction (open circle) at a scan rate of 300 mV s−1. (b) Scattergram of the device photovoltaic performance measured from forward bias to short circuit over 20 cells.
4. Conclusion
Conducting polymer PEDOT:PSS is employed as the top electrode in perovskite solar cell. With the film transfer lamination technique, cost-effective PEDOT:PSS has been proven as a substitute for Au or Ag metal electrode which neither requires costly material nor expensive high-vacuum thermal evaporation system. Perovskite solar cells with PEDOT:PSS top electrodes have achieved a maximum PCE of 11.29% and an averaged PCE up to 9.05% over 20 cells. The performance of this type of cell may be further enhanced by optimizing the PEDOT:PSS layer and the perovskite films. The whole device fabrication was fully conducted in air, significantly simplifying the fabrication of perovskite solar cells and lowering the threshold of both scientific research and large-area industrial production. Moreover, the high transparency of PEDOT:PSS electrode makes it promising for potential applications in tandem perovskite solar cells.
Acknowledgments
The work is supported by the Recruitment Program of Global Youth Experts, the National Natural Science Foundation of China (Grant No. 21474035), and the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2014YQ013). The authors thank Prof. Hongwei Han for the help of preparing the TiO2 layers and Mr. Yuanzhi Cao and Prof. Jun Zhou for the help of FIB SEM measurement. The authors also acknowledge the Analytical and Testing Center of HUST for the help of the XRD measurement.
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