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Solar Cells
Efficient Perovskite Hybrid Solar Cells Through a Homogeneous High-Quality Organolead Iodide Layer Kai Wang, Chang Liu, Pengcheng Du, Hao-Li Zhang,* and Xiong Gong*
Fabricating homogeneous and high-quality perovskite thin films via low-temperature solution processing is a challenge to realizing high-efficiency perovskite hybrid solar cells (pero-HSCs). Here, an approach is reported to realize smooth surface morphology of methylammonium lead iodide (CH3NH3PbI3) perovskite thin films via using strong-polar ethanol solution rather than less-polar isopropanol solution, which was previously used as the solvent for preparing perovskite thin films. In comparison with the pero-HSCs processed from isopropanol solution, more than 40% enhanced efficiency is observed from pero-HSCs processed from ethanol solution. The enhanced efficiency is attributed to a homogeneous high-quality perovskite thin film with dramatically low root-mean-square roughness and completely conversion of lead (II) iodide (PbI2) to CH3NH3PbI3. The findings provide a simple way to realize high-efficiency high-reproducible pero-HSCs.
1. Introduction Efficiently and economically harnessing the solar energy has always been an interesting topic for human beings to generate renewable energy.[1–7] Very recently, the high-efficient and low-cost perovskite hybrid solar cells (pero-HSCs) have shown great potentials to be the fourth-generation photovoltaics, breaking the deadlock previously dominated by organic and inorganic photovoltaics.[8–12] Because of their ideal physical properties for photovoltaics such as broad absorption coverage with high extinction coefficients,[13] long charge carrier diffusion length,[14] and good electrical transport properties,[15] the methylammonium lead halide
K. Wang, C. Liu, P. Du, Prof. X. Gong Department of Polymer Engineering The University of Akron Akron, OH 44325, USA E-mail: [email protected] P. Du, Prof. H.-L. Zhang State Key Laboratory of Applied Organic Chemistry College of Chemistry & Chemical Engineering Lanzhou University Lanzhou 730000, P. R. China E-mail: [email protected] DOI: 10.1002/smll.201403399 small 2015, DOI: 10.1002/smll.201403399
(CH3NH3PbI3)-based pero-HSCs showed remarkable device efficiencies with the highest value over 19%.[16] Moreover, the advanced feature of low-temperature solution processability allows pero-HSCs possessing tremendous promise in propelling solar power into the marketplace. However, one profound limitation is to form a homogeneous pinhole-free perovskite thin film, which is crucial to reproduce high-efficient pero-HSCs.[17] It has been reported that the quality of solution-processed perovskite thin films was poor with incomplete surface coverage, incomplete chemical conversion of lead (II) iodide (PbI2) to CH3NH3PbI3, large amount of defects and pinholes, and dramatically rough surface morphologies.[18–20] All of these drawbacks certainly limit further boosting efficiency of pero-HSCs.[17,19,21,22] Hence, to form a high-quality perovskite thin film is one of the preoccupations to realize reproducible efficient peroHSCs. Tremendous effects have been paid to make highquality perovskite thin films.[23–27] However, it is still a huge challenge to realize smooth perovskite thin films without pinholes or defects.[28,29] In this study, we report an approach to realize smooth surface morphology of CH3NH3PbI3 via using stronger polar ethanol solution rather than isopropanol solution, which was previously used as the solution for preparing perovskite thin films. As a result, the pero-HSCs processed from ethanol solution possesses both enhanced a short-circuit current density (JSC) of 17.31 mA cm−2 and a fill factor (FF)
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Scheme 1. Procedures for fabrication of perovskite hybrid solar cells from isopropanol or ethanol solutions and device structure of perovskite hybrid solar cells.
of 77.2%, with a corresponding power conversion efficiency (PCE) of 11.45%. In contrast, the pero-HSCs processed from isopropanol solution exhibits a JSC of 14.38 mA cm−2, a FF of 64.8%, and a PCE of 8.21%.
2. Results and Discussion The procedure of pero-HSCs fabrication and device structure of pero-HSCs are shown in Scheme 1. Compared with previously reported fabrication of pero-HSCs with a mesoporouse structure,[13] planar heterojunction pero-HSCs processed from corresponding solution followed with low-temperature thermal annealing is considerably a simple way to fabricate efficient pero-HSCs. The perovskite layer is inserted into two charge carrier transferring layers, poly(3,4-ethylenedioxythi ophene):polystyrene sulfonate) (PEDOT:PSS) and phenylC61-butyric acid methyl ester (PC61BM), where PEDOT:PSS and PC61BM layers act as the hole transfer layer (HTL) and the electron extraction layer (ETL), respectively. The current densities versus voltage (J–V) characteristics of pero-HSCs were carried out to investigate the device performance dependent on the precursor solutions. Figure 1a presents the J–V characteristics of pero-HSCs under AM 1.5-simulated illumination. The pero-HSCs processed from isopropanol solution (represented as the iso-pero-HSCs) show an average PCE of 8.21% with an open-circuit voltage (VOC) of 0.88 V, a JSC of 14.38 mA cm−2, and a FF of 64.80%. The PCE from the iso-pero-HSCs is comparable to the reported value (8.1%) from the pero-HSCs with a similar device structure.[30] However, the pero-HSCs processed from ethanol solution (represented as the eth-pero-HSCs) exhibit an average PCE of 11.45% with a VOC of 0.86 V, a JSC of 17.31 mA cm−2, and a FF of 77.2%. In order to understand underlying mechanisms for high PCE from the eth-pero-HSCs, the series resistance (RS) and shunt resistance (RSH) were estimated from the slope of J–V curves at open-circuit and short-circuit conditions, respectively. The RS and RSH values from pero-HSCs are summarized in Table 1. For solar cells, keeping the RS as low as possible is of paramount importance because large RS will decrease JSC, VOC, FF, and consequently PCEs.[31,32] The RS from the eth-pero-HSCs is 0.92 kΩ cm2, which is smaller than 2.03 kΩ cm2 from the iso-pero-HSCs. The lower RS from the eth-pero-HSCs is due to smaller contact resistance and lower bulk resistance of the high-quality perovskite layer, indicating that high currents can flow through the cell at
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low applied voltages.[33] Contrarily to the RS, the RSH must be higher to avoid current loss at the junction[34] diminishing the photocurrent and hence the solar cell performance. The RSH from the eth-pero-HSCs is 246.7 kΩ cm2, which is larger than 222.3 kΩ cm2 from the iso-pero-HSCs. The larger RSH indicates that shorts or leakages of the photocurrent are minimal in the eth-pero-HSCs. Moreover, in comparison with the iso-pero-HSCs, the eth-pero-HSCs exhibit a dramatically increased FF of 77.2%, resulting from an enlarged RSH, particularly a reduced parasitic resistance at the CH3NH3PbI3/ PC61BM interface.[35] To gain further insight into the Jsc limiting mechanisms, the incident photon to current efficiency (IPCE) spectra of pero-HSCs were measured and displayed in Figure 1b. The IPCE specifies the ratio of extracted electrons to incident photons at a given wavelength. The IPCE spectra of peroHSCs are in good agreement with the absorption spectrum of CH3NH3PbI3 (Figure 1c).[7,15,36] The energy gap (Eg) estimated from IPCE spectrum is 1.55 eV, which is consistent with that from absorption spectrum (Figure 1c).[7,15,36] Moreover, it was found that the eth-pero-HSCs possess enhanced photoresponse in comparison with the iso-pero-HSCs. The iso-pero-HSCs show a wide photoresponse from visible to the near-infrared, with over ≈60% IPCE ranging from 400 to 600 nm, while the eth-pero-HSCs give a noticeable improvement of the photocurrent from 500 to 800 nm, with a peak IPCE of ≈80% at ≈730 nm. In order to investigate the origin of high IPCE peaked at 730 nm, we measure the absorption spectra of CH3NH3PbI3/PC61BM thin films, where CH3NH3PbI3 layers are processed from either isopropanol or ethanol solutions. As shown in Figure 1c, the CH3NH3PbI3 films processed from isopropanol exhibit higher absorption coefficient than that processed from ethanol at the wavelength below 500 nm, where the PC61BM has relatively strong absorption. While in the region from 500 to 800 nm, both bilayer thin films exhibit similar absorption coefficient, which is different from the IPCE spectra. Therefore, the high IPCE at ≈730 nm from the eth-pero-HSCs is probably due to the reflection effect at the back contact.[36] Table 1. Photovoltaic performance of perovskite hybrid solar cells (pero-HSCs) processed from isopropanol or ethanol solutions. Devices
Precursor solution
VOC JSC FF [V] [mA cm−2] [%]
RS RSH [kΩ cm2] [kΩ cm2]
PCE [%]
iso- pero-HSCs Isopropanol 0.88
14.38
64.8
2.03
222.3
8.21
eth- pero-HSCs
17.31
77.2
0.92
246.7
11.45
Ethanol
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0.86
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(a) iso-pero-HSCs eth-pero-HSCs
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0
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-8
-12
-16
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0.4
0.6
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100
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80
60
40
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450
500
550
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650
700
750
800
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(c)
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Absorption (a.u.)
1
61
eth-perovskite/PC BM 61
PC BM 61
0.8
0.6
0.4
and eth-pero-HSCs, respectively. These values are in good agreement with those obtained from the J–V characteristics (Figure 1a). The J–V characteristics of eth-pero-HSCs and iso-peroHSCs under different scan rates and directions are shown Figure 2.The device performance parameters of eth-pero-HSCs and iso-pero-HSCs under different scan rates and directions are summarized in Table 2. The iso-pero-HSCs exhibit different PCEs under different scan rates or scan directions. When the scan rate is 30 mV s−1, the iso-pero-HSCs exhibits a PCE of 8.83% under reverse scan (from 1 to −1 V) and a PCE of 7.52% under forward scan (from −1 to 1 V); when the scan rate is 0.60 V s−1, the iso-pero-HSCs show a PCE of 8.92% under reverse scan and a PCE of 8.41% under forward scan. However, the eth-pero-HSCs possess almost identical value of PCEs (≈11.49%) for different scan rates either at 30 mV s−1 or 0.60 V s−1 and different scan directions, either in the reverse scan (from 1 to −1 V) and in the forward scan (from −1 to 1 V). Therefore, there is no photocurrent hysteresis observed from the eth-pero-HSCs at different scan rates and scan directions. The statistical comparison of histograms from 100 peroHSCs is presented in Figure 3. In general, the reproducibility of pero-HSCs can be obtained by utilizing Gaussian fitting. The deviation of real data from the standard Gaussian distribution is probably due to the insufficiently small base. However, it still can be noted that all the photovoltaic parameters of the eth-pero-HSCs have a smaller standard deviation in comparison to those of the iso-pero-HSCs. The average VOC, JSC, FF, PCEs of the eth-pero-HSCs are 0.86 ± 0.07 V, 17 ± 3 mA cm−2, 77 ± 4%, and 11.5 ± 1.5%, respectively. In contrast, the iso-pero-HSCs exhibits the average VOC of 0.88 ± 0.07 V, JSC of 14 ± 4 mA cm−2, FF of 65 ± 8%, and corresponding PCEs of 8.2 ± 2.3%. These results demonstrate that the reproducibility of the eth-pero-HSCs is higher than that of the iso-pero-HSCs. To further understand the difference in the device performance of pero-HSCs, X-ray diffraction (XRD) was carried out to study the film morphology of perovskite layers. Figure 4 presents the XRD patterns of the perovskite films processed from isopropanol and ethanol solutions, respectively. The main diffraction peaks at 13.9°, 28.5°, 32.0°, 40.6°, and 43.6° are assigned to the (110), (220), (310), (224), and (314) planes,[37,38] respectively. These diffraction peaks Table 2. The device performance parameters of pero-HSCs measured under different scan rates and scan directions.
0.2
Scan rate Scan [V s−1] direction
0 400
500
600
700
800
0.03
Wavelength (nm) Figure 1. a) The J–V characteristics of and b) incident photon to current efficiency (IPCE) spectra of pero-HSCs, and c) UV–vis absorption spectra of CH3NH3PbI3/PC61BM bilayer and pristine PC61BM, where the CH3NH3PbI3 is prepared from either isopropanol or ethanol solutions.
Forward
Devices
VOC [V]
JSC [mA cm−2]
FF [%]
PCE [%]
iso-pero-HSCs
0.82
14.31
63.70
7.52
0.87
14.98
67.80
8.83
0.87
17.43
75.80
11.48
0.87
17.50
75.60
11.51
Reverse Forward
eth-pero-HSCs
Reverse 0.60
Forward
iso-pero-HSCs
Reverse
The integrated photocurrent densities from the IPCE spectra are 14.14 and 17.21 mA cm−2 for iso-pero-HSCs small 2015, DOI: 10.1002/smll.201403399
Forward Reverse
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eth-pero-HSCs
0.87
14.27
68.00
8.41
0.89
14.79
68.10
8.92
0.87
17.47
75.90
11.49
0.87
17.51
75.60
11.46
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Voltage (V)
Voltage (V)
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1
Voltage (V)
Forward scan
-5
Reverse scan -10
-15
-20 -1
-0.5
0
0.5
1
Voltage (V)
Figure 2. The J–V characteristic of eth-pero-HSCs and iso-pero-HSCs measured under different scan directions: forward scan, from −1 to 1 V and reverse scan, from 1 to −1 V and different scan rates: 30 mV s−1 and 0.6 V s−1.
indicate that CH3NH3PbI3 possesses an orthorhombic crystal structure. However, the CH3NH3PbI3 thin film processed from isopropanol solution exhibits two representative peaks for PbI2, indicating that PbI2 was not completely converted
into CH3NH3PbI3. In comparison, no PbI2 peak was detected in the CH3NH3PbI3 thin films processed from ethanol solution, indicating that PbI2 was completely converted into CH3NH3PbI3. Due to poor optical property and low
Figure 3. Comparison of histograms of photovoltaic parameters for the perovskite hybrid solar cells processed from isopropanol (red) and ethanol (blue) solutions.
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(134)
*
(224)
*
(114)
(220)
Intensity (a.u.)
(110)
iso-perovskite eth-perovskite
* PbI2 5
10
15
20
25
30
35
40
45
2 Theta (deg) Figure 4. X-ray diffraction (XRD) patterns of the CH3NH3PbI3 layer prepared from either isopropanol or ethanol solutions.
electrical conductivity of PbI2 over those of CH3NH3PbI3, the unreacted PbI2 within CH3NH3PbI3 would deteriorate final device performance of pero-HSCs. Moreover, a dramatically rough surface morphology of perovskite would definitely affect the interfacial contact between CH3NH3PbI3 layer and the PC61BM ETL. Thus, the inferior device performance is therefore expected from the iso-pero-HSCs. Atomic force microscopy (AFM) and top view scanning electron microscopy (SEM) were carried out to further inspect the morphological difference in CH3NH3PbI3 films. Figure 5 illustrates the AFM topographic images of CH3NH3PbI3 thin films. The CH3NH3PbI3 thin film processed from isopropanol solution exhibits a large root-meansquare (RMS) roughness (Rq) of 66.56 nm with crystals of
Figure 5. Tapping-mode atomic force microscopy (AFM) height images of CH3NH3PbI3 perovskite films prepared from a) isopropanol and b) ethanol solutions; and the corresponding AFM phase images of CH3NH3PbI3 perovskite films prepared from c) isopropanol and d) ethanol solutions. small 2015, DOI: 10.1002/smll.201403399
larger dimensions to micrometer scale (Figure 5a). Noticing that the top PC61BM layer has a thickness of ≈100 nm, the PC61BM film with such thickness is too thin to completely cover the surface of CH3NH3PbI3 film. Thus, a direct contact between CH3NH3PbI3 film and top alumina (Al) electrode is expected, which induces a non-geminate charge carrier recombination between the CH3NH3PbI3 layer and top Al electrode,[39] decreasing the efficiency of pero-HSCs. In contrast, CH3NH3PbI3 film processed from ethanol solution shows a much smoother surface with a dramatically decreased Rq of 10.58 nm (Figure 5b). Such flat surface suggests a complete coverage of PC61BM layer afterwards, avoiding direct contact between CH3NH3PbI3 layer and Al cathode. The small Rq indicates a smoother surface with less sharp traps or pinholes, thus leading to a better contact between CH3NH3PbI3 layer and the PC61BM ETL. In this way, the electron extraction efficiency is elevated, resulting in increased JSC and FF (Figure 1a). Furthermore, such a homogeneous film allows top Al electrode to be smoothly back contacted on the top of the PC61BM ETL, ensuring good reflection from top Al electrode. This is probably the origin of high IPCE (80%) at ≈730 nm for the eth-pero-HSCs (Figure 1b). In addition, the size of each crystal in the CH3NH3PbI3 film processed from ethanol solution is almost identical with a uniform, small scale of ≈200 nm, implying a more compact and denser film with less voids and traps. Such unique characteristics indicate a better electrical properties and lower bulk resistance of CH3NH3PbI3 film processed from ethanol solution as compared with the CH3NH3PbI3 film processed from isopropanol solution. Figure 6 compares the top-view SEM images of CH3NH3PbI3 thin films. As shown in Figure 6a, the CH3NH3PbI3 film processed from isopropanol solution shows a rough surface with long wrinkle domains, which is consistent with the large Rq from AFM measurement. As shown in Figure 6b, the CH3NH3PbI3 film processed from ethanol solution shows a smooth surface without any inhomogeneous domains. The inserted Figure 6a,b present high-resolution images with a magnification of ×50 000. It is clear that the crystal sizes of CH3NH3PbI3 film prepared from either ethanol or isopropanol solutions are almost the same, but the crystals of CH3NH3PbI3 processed from isopropanol solution are easily to be aggregated to large domains, whereas the crystals of CH3NH3PbI3 prepared from ethanol solution are dispersed homogeneously. Thus, these results further demonstrate
Figure 6. Top view scanning electron microscope (SEM) images of CH3NH3PbI3 films prepared from a) isopropanol and b) ethanol solutions.
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that the surface roughness of CH3NH3PbI3 film processed from ethanol solution is lower than that from isopropanol solution. The film morphological difference induced by CH3NH3PbI3 layers processed from different solutions is probably due to the different solvent polarity. The polarity of ethanol is 5.2, which is larger than 3.9 for isopropanol. In the processing of PbI2 reacted with methylammonium (MA) iodide to form CH3NH3PbI3, the polar solvent probably can offer an additional van der Waals interaction (typically 0.4–4 kJ mol−1)[40] with either PbI2 or MAI. The CH3NH3PbI3 with orthorhombic crystal structure is metastable with weak hydrogen bonds between amino group and halide ions, and van der Waals bonds between each MA group.[41] Thus the polar solvent-induced weak interaction will eventually affect film morphology of resultant CH3NH3PbI3 layer in different scales.[42] The further studies on the correlation between the film morphology of CH3NH3PbI3 layer and the polarity of solvents, which are used for processing CH3NH3PbI, are under the investigation, and the observation will be reported elsewhere. In order to further investigate the electrical properties of pero-HSCs dependent on the film morphology of CH3NH3PbI3 layer, the impedance spectroscopy (IS) is carried out to investigate the internal series resistances (RS) of pero-HSCs. The IS analysis enables us to monitor the detailed electrical properties of the interfaces that cannot be determined by direct current measurements. The RS consists of the sheet resistance (RSheet) of the electrodes, the charge-transfer resistance (RCT) at the interfaces between the electrode and charge carrier selective layer, as well as charge carrier selective layer and CH3NH3PbI3 layer, inside of CH3NH3PbI3 layer.[43] For both pero-HSCs, the main difference is the RCT at the interfaces between the CH3NH3PbI3 layer and the PC61BM ETL. Figure 7 shows the Nyquist plots under the light intensity of 100 mW cm−2 for pero-HSCs at an applied
1200
iso-pero-HSCs
R
eth-pero-HSCs
CT
R
Reactance -Z" (Ohm)
S
1000 C 800
600
400
200
0
0
200
400
600
800
1000
1200
Resistance Z' (Ohm) Figure 7. Nyquist plots at V ≈ VOC for perovskite hybrid solar cells processed from either isopropanol or ethanol solutions.
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voltage close to VOC of pero-HSCs. From the plots, RCT of 1.17 and 0.85 kΩ is obtained for the iso-pero-HSCs and the eth-pero-HSCs, respectively. A lower RCT in the eth-peroHSCs indicates a better contact at the interface between CH3NH3PbI3 layer and the PC61BM ETL, and better charge carrier transferring properties, thus enhanced PCEs for the eth-pero-HSCs.
3. Conclusion We reported efficient pero-HSCs processed from ethanol solution instead of the previously reported isopropanol solution. The CH3NH3PbI3 layer processed from ethanol solution possesses a much smooth surface, therefore a better contact between the CH3NH3PbI3 layer and the PC61BM ETL, and a better light reflection from the back top Al electrode, resulting in enhanced electron extraction efficiency, decreased interfacial charge carrier recombination, and increased photoabsorption of CH3NH3PbI3 layer. As a result, the pero-HSCs processed from ethanol solution exhibits JSC of 17.31 mA cm−2, VOC of 0.86 V, FF of 77.2%, and corresponding PCE of 11.45%. In contrast, the pero-HSCs processed from isopropanol solution exhibits JSC of 14.38 mA cm−2, VOC of 0.88 V, FF of 64.8%, and corresponding PCE of 8.21%. By changing the solvent used for processing the CH3NH3PbI3 layer, ≈40% enhanced PCEs is observed from pero-HSCs. Therefore, our findings provide a simple method to approach high efficiency of pero-HSCs with high reproducibility.
4. Experimental Section Materials: Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were purchased from Clevious and 1-Material Inc., respectively, and used as received without further treatment. Lead (II) iodide, anhydrous N,N-dimethylformamide (DMF), ethanol (99.5%), isopropyl alcohol (99.7%), hydroiodic acid (99.99%), methylamine were purchased from Sigma–Aldrich and used as received without further purification. Peroskite Precursor Preparation: PbI2 was firstly dissolved in DMF to give a light yellow solution with a concentration of 400 mg mL−1. After siring at 70 °C for 12 h, let PbI2 sit overnight to give a clear supernatant. Methylammonium iodide (MAI) was dissolved in isopropanol and ethanol, respectively, to give a colorless solution with a concentration of 35 mg mL−1. Perovskite Film Preparation: The CH3NH3PbI3 films were prepared following two-step solution deposition. The PbI2 supernatant was preheated at 70 °C for 5 min. Then, the PbI2 supernatant solution was pin-coated on the top of the substrates and then followed thermal annealing at 70 °C for 10 min. Subsequently, after the PbI2 layer was cooled down to room temperature, MAI is casted from either isopropanol solution or ethanol solution on the top of PbI2 followed with thermal annealing at 100 °C for 2 h. Pero-HSCs Fabrication: All pero-HSCs were fabricated on precleaned indium tin oxide (ITO)-coated glasses. After ITO substrates were treated with UV-ozone for 20 min under an ambient atmosphere, a ≈40-nm-thick film of PEDOT:PSS was cast on top of ITO,
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followed with thermal annealing at 150 °C for 10 min. After that, ≈ 300-nm-thick CH3NH3PbI3 thin films were prepared via two-step deposition as described. Afterward, ≈100-nm-thick PC61BM was deposited from chlorobenzene solution. Finally, a 100-nm-thick Al was thermally deposited on the top of PC61BM layer in the vacuum with a base pressure of 6 × 10−6 mbar through a shadow mask. The device area was measured to be 0.045 cm2. Perovskite Film Characterization: The SEM images were obtained by using a field-emission scanning electron microscope (JEOL-7401). The AFM images were obtained by using a NanoScope NS3A system (Digital Instrument) to observe the surface morphologies and thicknesses of various thin films. XRD patterns of perovskite films coated on poly(ethylene terephthalate) (PET) substrate were obtained by using a Bruker AXS Dimension D8 X-ray system. The absorption spectra of perovskite films coated on ITO/glass substrate were measured by HP 8453 UV–vis spectrophotometer. Pero-HSCs Characterization: The J–V characteristics of peroHSCs were obtained by using a Keithley model 2400 source measure unit. A Newport Air Mass 1.5 Global (AM1.5G) full-spectrum solar simulator was applied as the light source. The light intensity was 100 mW cm−2, which was calibrated by utilizing a monosilicon detector (with a KG-5 visible color filter) of National Renewable Energy Laboratory to reduce the spectral mismatch. The IPCE was measured through the IPCE measurement setup in use at ESTI for cells and mini-modules. A 300 W steady-state xenon lamp provides the source light. Up to 64 filters (8 to 20 nm width, range from 300 to 1200 nm) are available on fourfilter wheels to produce the monochromatic input, which is chopped at 75 Hz, superimposed on the bias light and measured via the usual lock-in technique. Bias light is necessary to put the device under examination close the operating irradiance condition. After collecting the IPCE data, the software also integrates the data with the AM1.5G spectrum and gives the calculated JSC value, which is helpful for checking the accuracy of the measurement. The impedance spectra (IS) were obtained using an HP 4194A impedance/gain-phase analyzer, all under illumination, with an oscillating voltage of 10 mV and frequency of 1 Hz to 1 MHz. Pero-HSCs were held at their respective open circuit potentials obtained from the J–V measurements, while the IS spectrum was being recorded.
Acknowledgements The authors at the University of Akron thank NSF (1351785) for financial support.
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Received: November 14, 2014 Revised: December 10, 2014 Published online:
small 2015, DOI: 10.1002/smll.201403399
Solar Cells
Efficient Perovskite Hybrid Solar Cells Through a Homogeneous High-Quality Organolead Iodide Layer Kai Wang, Chang Liu, Pengcheng Du, Hao-Li Zhang,* and Xiong Gong*
Fabricating homogeneous and high-quality perovskite thin films via low-temperature solution processing is a challenge to realizing high-efficiency perovskite hybrid solar cells (pero-HSCs). Here, an approach is reported to realize smooth surface morphology of methylammonium lead iodide (CH3NH3PbI3) perovskite thin films via using strong-polar ethanol solution rather than less-polar isopropanol solution, which was previously used as the solvent for preparing perovskite thin films. In comparison with the pero-HSCs processed from isopropanol solution, more than 40% enhanced efficiency is observed from pero-HSCs processed from ethanol solution. The enhanced efficiency is attributed to a homogeneous high-quality perovskite thin film with dramatically low root-mean-square roughness and completely conversion of lead (II) iodide (PbI2) to CH3NH3PbI3. The findings provide a simple way to realize high-efficiency high-reproducible pero-HSCs.
1. Introduction Efficiently and economically harnessing the solar energy has always been an interesting topic for human beings to generate renewable energy.[1–7] Very recently, the high-efficient and low-cost perovskite hybrid solar cells (pero-HSCs) have shown great potentials to be the fourth-generation photovoltaics, breaking the deadlock previously dominated by organic and inorganic photovoltaics.[8–12] Because of their ideal physical properties for photovoltaics such as broad absorption coverage with high extinction coefficients,[13] long charge carrier diffusion length,[14] and good electrical transport properties,[15] the methylammonium lead halide
K. Wang, C. Liu, P. Du, Prof. X. Gong Department of Polymer Engineering The University of Akron Akron, OH 44325, USA E-mail: [email protected] P. Du, Prof. H.-L. Zhang State Key Laboratory of Applied Organic Chemistry College of Chemistry & Chemical Engineering Lanzhou University Lanzhou 730000, P. R. China E-mail: [email protected] DOI: 10.1002/smll.201403399 small 2015, DOI: 10.1002/smll.201403399
(CH3NH3PbI3)-based pero-HSCs showed remarkable device efficiencies with the highest value over 19%.[16] Moreover, the advanced feature of low-temperature solution processability allows pero-HSCs possessing tremendous promise in propelling solar power into the marketplace. However, one profound limitation is to form a homogeneous pinhole-free perovskite thin film, which is crucial to reproduce high-efficient pero-HSCs.[17] It has been reported that the quality of solution-processed perovskite thin films was poor with incomplete surface coverage, incomplete chemical conversion of lead (II) iodide (PbI2) to CH3NH3PbI3, large amount of defects and pinholes, and dramatically rough surface morphologies.[18–20] All of these drawbacks certainly limit further boosting efficiency of pero-HSCs.[17,19,21,22] Hence, to form a high-quality perovskite thin film is one of the preoccupations to realize reproducible efficient peroHSCs. Tremendous effects have been paid to make highquality perovskite thin films.[23–27] However, it is still a huge challenge to realize smooth perovskite thin films without pinholes or defects.[28,29] In this study, we report an approach to realize smooth surface morphology of CH3NH3PbI3 via using stronger polar ethanol solution rather than isopropanol solution, which was previously used as the solution for preparing perovskite thin films. As a result, the pero-HSCs processed from ethanol solution possesses both enhanced a short-circuit current density (JSC) of 17.31 mA cm−2 and a fill factor (FF)
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Scheme 1. Procedures for fabrication of perovskite hybrid solar cells from isopropanol or ethanol solutions and device structure of perovskite hybrid solar cells.
of 77.2%, with a corresponding power conversion efficiency (PCE) of 11.45%. In contrast, the pero-HSCs processed from isopropanol solution exhibits a JSC of 14.38 mA cm−2, a FF of 64.8%, and a PCE of 8.21%.
2. Results and Discussion The procedure of pero-HSCs fabrication and device structure of pero-HSCs are shown in Scheme 1. Compared with previously reported fabrication of pero-HSCs with a mesoporouse structure,[13] planar heterojunction pero-HSCs processed from corresponding solution followed with low-temperature thermal annealing is considerably a simple way to fabricate efficient pero-HSCs. The perovskite layer is inserted into two charge carrier transferring layers, poly(3,4-ethylenedioxythi ophene):polystyrene sulfonate) (PEDOT:PSS) and phenylC61-butyric acid methyl ester (PC61BM), where PEDOT:PSS and PC61BM layers act as the hole transfer layer (HTL) and the electron extraction layer (ETL), respectively. The current densities versus voltage (J–V) characteristics of pero-HSCs were carried out to investigate the device performance dependent on the precursor solutions. Figure 1a presents the J–V characteristics of pero-HSCs under AM 1.5-simulated illumination. The pero-HSCs processed from isopropanol solution (represented as the iso-pero-HSCs) show an average PCE of 8.21% with an open-circuit voltage (VOC) of 0.88 V, a JSC of 14.38 mA cm−2, and a FF of 64.80%. The PCE from the iso-pero-HSCs is comparable to the reported value (8.1%) from the pero-HSCs with a similar device structure.[30] However, the pero-HSCs processed from ethanol solution (represented as the eth-pero-HSCs) exhibit an average PCE of 11.45% with a VOC of 0.86 V, a JSC of 17.31 mA cm−2, and a FF of 77.2%. In order to understand underlying mechanisms for high PCE from the eth-pero-HSCs, the series resistance (RS) and shunt resistance (RSH) were estimated from the slope of J–V curves at open-circuit and short-circuit conditions, respectively. The RS and RSH values from pero-HSCs are summarized in Table 1. For solar cells, keeping the RS as low as possible is of paramount importance because large RS will decrease JSC, VOC, FF, and consequently PCEs.[31,32] The RS from the eth-pero-HSCs is 0.92 kΩ cm2, which is smaller than 2.03 kΩ cm2 from the iso-pero-HSCs. The lower RS from the eth-pero-HSCs is due to smaller contact resistance and lower bulk resistance of the high-quality perovskite layer, indicating that high currents can flow through the cell at
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low applied voltages.[33] Contrarily to the RS, the RSH must be higher to avoid current loss at the junction[34] diminishing the photocurrent and hence the solar cell performance. The RSH from the eth-pero-HSCs is 246.7 kΩ cm2, which is larger than 222.3 kΩ cm2 from the iso-pero-HSCs. The larger RSH indicates that shorts or leakages of the photocurrent are minimal in the eth-pero-HSCs. Moreover, in comparison with the iso-pero-HSCs, the eth-pero-HSCs exhibit a dramatically increased FF of 77.2%, resulting from an enlarged RSH, particularly a reduced parasitic resistance at the CH3NH3PbI3/ PC61BM interface.[35] To gain further insight into the Jsc limiting mechanisms, the incident photon to current efficiency (IPCE) spectra of pero-HSCs were measured and displayed in Figure 1b. The IPCE specifies the ratio of extracted electrons to incident photons at a given wavelength. The IPCE spectra of peroHSCs are in good agreement with the absorption spectrum of CH3NH3PbI3 (Figure 1c).[7,15,36] The energy gap (Eg) estimated from IPCE spectrum is 1.55 eV, which is consistent with that from absorption spectrum (Figure 1c).[7,15,36] Moreover, it was found that the eth-pero-HSCs possess enhanced photoresponse in comparison with the iso-pero-HSCs. The iso-pero-HSCs show a wide photoresponse from visible to the near-infrared, with over ≈60% IPCE ranging from 400 to 600 nm, while the eth-pero-HSCs give a noticeable improvement of the photocurrent from 500 to 800 nm, with a peak IPCE of ≈80% at ≈730 nm. In order to investigate the origin of high IPCE peaked at 730 nm, we measure the absorption spectra of CH3NH3PbI3/PC61BM thin films, where CH3NH3PbI3 layers are processed from either isopropanol or ethanol solutions. As shown in Figure 1c, the CH3NH3PbI3 films processed from isopropanol exhibit higher absorption coefficient than that processed from ethanol at the wavelength below 500 nm, where the PC61BM has relatively strong absorption. While in the region from 500 to 800 nm, both bilayer thin films exhibit similar absorption coefficient, which is different from the IPCE spectra. Therefore, the high IPCE at ≈730 nm from the eth-pero-HSCs is probably due to the reflection effect at the back contact.[36] Table 1. Photovoltaic performance of perovskite hybrid solar cells (pero-HSCs) processed from isopropanol or ethanol solutions. Devices
Precursor solution
VOC JSC FF [V] [mA cm−2] [%]
RS RSH [kΩ cm2] [kΩ cm2]
PCE [%]
iso- pero-HSCs Isopropanol 0.88
14.38
64.8
2.03
222.3
8.21
eth- pero-HSCs
17.31
77.2
0.92
246.7
11.45
Ethanol
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PC BM 61
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0.6
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and eth-pero-HSCs, respectively. These values are in good agreement with those obtained from the J–V characteristics (Figure 1a). The J–V characteristics of eth-pero-HSCs and iso-peroHSCs under different scan rates and directions are shown Figure 2.The device performance parameters of eth-pero-HSCs and iso-pero-HSCs under different scan rates and directions are summarized in Table 2. The iso-pero-HSCs exhibit different PCEs under different scan rates or scan directions. When the scan rate is 30 mV s−1, the iso-pero-HSCs exhibits a PCE of 8.83% under reverse scan (from 1 to −1 V) and a PCE of 7.52% under forward scan (from −1 to 1 V); when the scan rate is 0.60 V s−1, the iso-pero-HSCs show a PCE of 8.92% under reverse scan and a PCE of 8.41% under forward scan. However, the eth-pero-HSCs possess almost identical value of PCEs (≈11.49%) for different scan rates either at 30 mV s−1 or 0.60 V s−1 and different scan directions, either in the reverse scan (from 1 to −1 V) and in the forward scan (from −1 to 1 V). Therefore, there is no photocurrent hysteresis observed from the eth-pero-HSCs at different scan rates and scan directions. The statistical comparison of histograms from 100 peroHSCs is presented in Figure 3. In general, the reproducibility of pero-HSCs can be obtained by utilizing Gaussian fitting. The deviation of real data from the standard Gaussian distribution is probably due to the insufficiently small base. However, it still can be noted that all the photovoltaic parameters of the eth-pero-HSCs have a smaller standard deviation in comparison to those of the iso-pero-HSCs. The average VOC, JSC, FF, PCEs of the eth-pero-HSCs are 0.86 ± 0.07 V, 17 ± 3 mA cm−2, 77 ± 4%, and 11.5 ± 1.5%, respectively. In contrast, the iso-pero-HSCs exhibits the average VOC of 0.88 ± 0.07 V, JSC of 14 ± 4 mA cm−2, FF of 65 ± 8%, and corresponding PCEs of 8.2 ± 2.3%. These results demonstrate that the reproducibility of the eth-pero-HSCs is higher than that of the iso-pero-HSCs. To further understand the difference in the device performance of pero-HSCs, X-ray diffraction (XRD) was carried out to study the film morphology of perovskite layers. Figure 4 presents the XRD patterns of the perovskite films processed from isopropanol and ethanol solutions, respectively. The main diffraction peaks at 13.9°, 28.5°, 32.0°, 40.6°, and 43.6° are assigned to the (110), (220), (310), (224), and (314) planes,[37,38] respectively. These diffraction peaks Table 2. The device performance parameters of pero-HSCs measured under different scan rates and scan directions.
0.2
Scan rate Scan [V s−1] direction
0 400
500
600
700
800
0.03
Wavelength (nm) Figure 1. a) The J–V characteristics of and b) incident photon to current efficiency (IPCE) spectra of pero-HSCs, and c) UV–vis absorption spectra of CH3NH3PbI3/PC61BM bilayer and pristine PC61BM, where the CH3NH3PbI3 is prepared from either isopropanol or ethanol solutions.
Forward
Devices
VOC [V]
JSC [mA cm−2]
FF [%]
PCE [%]
iso-pero-HSCs
0.82
14.31
63.70
7.52
0.87
14.98
67.80
8.83
0.87
17.43
75.80
11.48
0.87
17.50
75.60
11.51
Reverse Forward
eth-pero-HSCs
Reverse 0.60
Forward
iso-pero-HSCs
Reverse
The integrated photocurrent densities from the IPCE spectra are 14.14 and 17.21 mA cm−2 for iso-pero-HSCs small 2015, DOI: 10.1002/smll.201403399
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0.87
14.27
68.00
8.41
0.89
14.79
68.10
8.92
0.87
17.47
75.90
11.49
0.87
17.51
75.60
11.46
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Voltage (V)
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-5
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-0.5
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Figure 2. The J–V characteristic of eth-pero-HSCs and iso-pero-HSCs measured under different scan directions: forward scan, from −1 to 1 V and reverse scan, from 1 to −1 V and different scan rates: 30 mV s−1 and 0.6 V s−1.
indicate that CH3NH3PbI3 possesses an orthorhombic crystal structure. However, the CH3NH3PbI3 thin film processed from isopropanol solution exhibits two representative peaks for PbI2, indicating that PbI2 was not completely converted
into CH3NH3PbI3. In comparison, no PbI2 peak was detected in the CH3NH3PbI3 thin films processed from ethanol solution, indicating that PbI2 was completely converted into CH3NH3PbI3. Due to poor optical property and low
Figure 3. Comparison of histograms of photovoltaic parameters for the perovskite hybrid solar cells processed from isopropanol (red) and ethanol (blue) solutions.
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(134)
*
(224)
*
(114)
(220)
Intensity (a.u.)
(110)
iso-perovskite eth-perovskite
* PbI2 5
10
15
20
25
30
35
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2 Theta (deg) Figure 4. X-ray diffraction (XRD) patterns of the CH3NH3PbI3 layer prepared from either isopropanol or ethanol solutions.
electrical conductivity of PbI2 over those of CH3NH3PbI3, the unreacted PbI2 within CH3NH3PbI3 would deteriorate final device performance of pero-HSCs. Moreover, a dramatically rough surface morphology of perovskite would definitely affect the interfacial contact between CH3NH3PbI3 layer and the PC61BM ETL. Thus, the inferior device performance is therefore expected from the iso-pero-HSCs. Atomic force microscopy (AFM) and top view scanning electron microscopy (SEM) were carried out to further inspect the morphological difference in CH3NH3PbI3 films. Figure 5 illustrates the AFM topographic images of CH3NH3PbI3 thin films. The CH3NH3PbI3 thin film processed from isopropanol solution exhibits a large root-meansquare (RMS) roughness (Rq) of 66.56 nm with crystals of
Figure 5. Tapping-mode atomic force microscopy (AFM) height images of CH3NH3PbI3 perovskite films prepared from a) isopropanol and b) ethanol solutions; and the corresponding AFM phase images of CH3NH3PbI3 perovskite films prepared from c) isopropanol and d) ethanol solutions. small 2015, DOI: 10.1002/smll.201403399
larger dimensions to micrometer scale (Figure 5a). Noticing that the top PC61BM layer has a thickness of ≈100 nm, the PC61BM film with such thickness is too thin to completely cover the surface of CH3NH3PbI3 film. Thus, a direct contact between CH3NH3PbI3 film and top alumina (Al) electrode is expected, which induces a non-geminate charge carrier recombination between the CH3NH3PbI3 layer and top Al electrode,[39] decreasing the efficiency of pero-HSCs. In contrast, CH3NH3PbI3 film processed from ethanol solution shows a much smoother surface with a dramatically decreased Rq of 10.58 nm (Figure 5b). Such flat surface suggests a complete coverage of PC61BM layer afterwards, avoiding direct contact between CH3NH3PbI3 layer and Al cathode. The small Rq indicates a smoother surface with less sharp traps or pinholes, thus leading to a better contact between CH3NH3PbI3 layer and the PC61BM ETL. In this way, the electron extraction efficiency is elevated, resulting in increased JSC and FF (Figure 1a). Furthermore, such a homogeneous film allows top Al electrode to be smoothly back contacted on the top of the PC61BM ETL, ensuring good reflection from top Al electrode. This is probably the origin of high IPCE (80%) at ≈730 nm for the eth-pero-HSCs (Figure 1b). In addition, the size of each crystal in the CH3NH3PbI3 film processed from ethanol solution is almost identical with a uniform, small scale of ≈200 nm, implying a more compact and denser film with less voids and traps. Such unique characteristics indicate a better electrical properties and lower bulk resistance of CH3NH3PbI3 film processed from ethanol solution as compared with the CH3NH3PbI3 film processed from isopropanol solution. Figure 6 compares the top-view SEM images of CH3NH3PbI3 thin films. As shown in Figure 6a, the CH3NH3PbI3 film processed from isopropanol solution shows a rough surface with long wrinkle domains, which is consistent with the large Rq from AFM measurement. As shown in Figure 6b, the CH3NH3PbI3 film processed from ethanol solution shows a smooth surface without any inhomogeneous domains. The inserted Figure 6a,b present high-resolution images with a magnification of ×50 000. It is clear that the crystal sizes of CH3NH3PbI3 film prepared from either ethanol or isopropanol solutions are almost the same, but the crystals of CH3NH3PbI3 processed from isopropanol solution are easily to be aggregated to large domains, whereas the crystals of CH3NH3PbI3 prepared from ethanol solution are dispersed homogeneously. Thus, these results further demonstrate
Figure 6. Top view scanning electron microscope (SEM) images of CH3NH3PbI3 films prepared from a) isopropanol and b) ethanol solutions.
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that the surface roughness of CH3NH3PbI3 film processed from ethanol solution is lower than that from isopropanol solution. The film morphological difference induced by CH3NH3PbI3 layers processed from different solutions is probably due to the different solvent polarity. The polarity of ethanol is 5.2, which is larger than 3.9 for isopropanol. In the processing of PbI2 reacted with methylammonium (MA) iodide to form CH3NH3PbI3, the polar solvent probably can offer an additional van der Waals interaction (typically 0.4–4 kJ mol−1)[40] with either PbI2 or MAI. The CH3NH3PbI3 with orthorhombic crystal structure is metastable with weak hydrogen bonds between amino group and halide ions, and van der Waals bonds between each MA group.[41] Thus the polar solvent-induced weak interaction will eventually affect film morphology of resultant CH3NH3PbI3 layer in different scales.[42] The further studies on the correlation between the film morphology of CH3NH3PbI3 layer and the polarity of solvents, which are used for processing CH3NH3PbI, are under the investigation, and the observation will be reported elsewhere. In order to further investigate the electrical properties of pero-HSCs dependent on the film morphology of CH3NH3PbI3 layer, the impedance spectroscopy (IS) is carried out to investigate the internal series resistances (RS) of pero-HSCs. The IS analysis enables us to monitor the detailed electrical properties of the interfaces that cannot be determined by direct current measurements. The RS consists of the sheet resistance (RSheet) of the electrodes, the charge-transfer resistance (RCT) at the interfaces between the electrode and charge carrier selective layer, as well as charge carrier selective layer and CH3NH3PbI3 layer, inside of CH3NH3PbI3 layer.[43] For both pero-HSCs, the main difference is the RCT at the interfaces between the CH3NH3PbI3 layer and the PC61BM ETL. Figure 7 shows the Nyquist plots under the light intensity of 100 mW cm−2 for pero-HSCs at an applied
1200
iso-pero-HSCs
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Resistance Z' (Ohm) Figure 7. Nyquist plots at V ≈ VOC for perovskite hybrid solar cells processed from either isopropanol or ethanol solutions.
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voltage close to VOC of pero-HSCs. From the plots, RCT of 1.17 and 0.85 kΩ is obtained for the iso-pero-HSCs and the eth-pero-HSCs, respectively. A lower RCT in the eth-peroHSCs indicates a better contact at the interface between CH3NH3PbI3 layer and the PC61BM ETL, and better charge carrier transferring properties, thus enhanced PCEs for the eth-pero-HSCs.
3. Conclusion We reported efficient pero-HSCs processed from ethanol solution instead of the previously reported isopropanol solution. The CH3NH3PbI3 layer processed from ethanol solution possesses a much smooth surface, therefore a better contact between the CH3NH3PbI3 layer and the PC61BM ETL, and a better light reflection from the back top Al electrode, resulting in enhanced electron extraction efficiency, decreased interfacial charge carrier recombination, and increased photoabsorption of CH3NH3PbI3 layer. As a result, the pero-HSCs processed from ethanol solution exhibits JSC of 17.31 mA cm−2, VOC of 0.86 V, FF of 77.2%, and corresponding PCE of 11.45%. In contrast, the pero-HSCs processed from isopropanol solution exhibits JSC of 14.38 mA cm−2, VOC of 0.88 V, FF of 64.8%, and corresponding PCE of 8.21%. By changing the solvent used for processing the CH3NH3PbI3 layer, ≈40% enhanced PCEs is observed from pero-HSCs. Therefore, our findings provide a simple method to approach high efficiency of pero-HSCs with high reproducibility.
4. Experimental Section Materials: Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) were purchased from Clevious and 1-Material Inc., respectively, and used as received without further treatment. Lead (II) iodide, anhydrous N,N-dimethylformamide (DMF), ethanol (99.5%), isopropyl alcohol (99.7%), hydroiodic acid (99.99%), methylamine were purchased from Sigma–Aldrich and used as received without further purification. Peroskite Precursor Preparation: PbI2 was firstly dissolved in DMF to give a light yellow solution with a concentration of 400 mg mL−1. After siring at 70 °C for 12 h, let PbI2 sit overnight to give a clear supernatant. Methylammonium iodide (MAI) was dissolved in isopropanol and ethanol, respectively, to give a colorless solution with a concentration of 35 mg mL−1. Perovskite Film Preparation: The CH3NH3PbI3 films were prepared following two-step solution deposition. The PbI2 supernatant was preheated at 70 °C for 5 min. Then, the PbI2 supernatant solution was pin-coated on the top of the substrates and then followed thermal annealing at 70 °C for 10 min. Subsequently, after the PbI2 layer was cooled down to room temperature, MAI is casted from either isopropanol solution or ethanol solution on the top of PbI2 followed with thermal annealing at 100 °C for 2 h. Pero-HSCs Fabrication: All pero-HSCs were fabricated on precleaned indium tin oxide (ITO)-coated glasses. After ITO substrates were treated with UV-ozone for 20 min under an ambient atmosphere, a ≈40-nm-thick film of PEDOT:PSS was cast on top of ITO,
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followed with thermal annealing at 150 °C for 10 min. After that, ≈ 300-nm-thick CH3NH3PbI3 thin films were prepared via two-step deposition as described. Afterward, ≈100-nm-thick PC61BM was deposited from chlorobenzene solution. Finally, a 100-nm-thick Al was thermally deposited on the top of PC61BM layer in the vacuum with a base pressure of 6 × 10−6 mbar through a shadow mask. The device area was measured to be 0.045 cm2. Perovskite Film Characterization: The SEM images were obtained by using a field-emission scanning electron microscope (JEOL-7401). The AFM images were obtained by using a NanoScope NS3A system (Digital Instrument) to observe the surface morphologies and thicknesses of various thin films. XRD patterns of perovskite films coated on poly(ethylene terephthalate) (PET) substrate were obtained by using a Bruker AXS Dimension D8 X-ray system. The absorption spectra of perovskite films coated on ITO/glass substrate were measured by HP 8453 UV–vis spectrophotometer. Pero-HSCs Characterization: The J–V characteristics of peroHSCs were obtained by using a Keithley model 2400 source measure unit. A Newport Air Mass 1.5 Global (AM1.5G) full-spectrum solar simulator was applied as the light source. The light intensity was 100 mW cm−2, which was calibrated by utilizing a monosilicon detector (with a KG-5 visible color filter) of National Renewable Energy Laboratory to reduce the spectral mismatch. The IPCE was measured through the IPCE measurement setup in use at ESTI for cells and mini-modules. A 300 W steady-state xenon lamp provides the source light. Up to 64 filters (8 to 20 nm width, range from 300 to 1200 nm) are available on fourfilter wheels to produce the monochromatic input, which is chopped at 75 Hz, superimposed on the bias light and measured via the usual lock-in technique. Bias light is necessary to put the device under examination close the operating irradiance condition. After collecting the IPCE data, the software also integrates the data with the AM1.5G spectrum and gives the calculated JSC value, which is helpful for checking the accuracy of the measurement. The impedance spectra (IS) were obtained using an HP 4194A impedance/gain-phase analyzer, all under illumination, with an oscillating voltage of 10 mV and frequency of 1 Hz to 1 MHz. Pero-HSCs were held at their respective open circuit potentials obtained from the J–V measurements, while the IS spectrum was being recorded.
Acknowledgements The authors at the University of Akron thank NSF (1351785) for financial support.
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Received: November 14, 2014 Revised: December 10, 2014 Published online:
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