2PbI3 for Planar Perovskite Solar Cell - ACS Publications - American

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Nucleation and Growth Control of HC(NH2)2PbI3 for Planar Perovskite Solar Cell Jincheol Kim,† Jae S. Yun,† Xiaoming Wen,† Arman Mahboubi Soufiani,† Cho Fai Jonathan Lau,† Benjamin Wilkinson,† Jan Seidel,‡ Martin A. Green,† Shujuan Huang,† and Anita W. Y. Ho-Baillie*,† †

The Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia ‡ School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia S Supporting Information *

ABSTRACT: HC(NH2)2PbI3 perovskite solar cells have emerged as a promising alternative to CH3NH3PbI3 perovskite solar cells due to their better thermal stability and lower bandgap. In this work, we have demonstrated a reliable fabrication technique for HC(NH2)2PbI3 planar perovskite solar cells by controlling nucleation and crystallization processes of the perovskite layer through a combination of gas-assisted spin coating and the addition of HI additive in the perovskite precursor. A narrow distribution of power conversion efficiencies (PCEs) can be achieved with an average of 13% with negligible hysteresis when measured at a scanning rate of 0.1 V/s. The best performance device has a PCE of 16.0%. It is shown that by using optimized conditions we can consistently form dense, uniform, pinhole-free good crystalline, lead-iodide-impurities-free HC(NH2)2PbI3 film that has been comprehensively characterized by scanning electron microscopy, X-ray diffraction, Kelvin probe force microscopy, photoluminescence, and electroluminescence in this work. FA-based perovskites,3,10−12,15 which counteracts the advantages of FA-only devices. In addition, most of the FA-based perovskite cells demonstrated so far including the two record cells3,5 have a mesoscopic architecture requiring the use of mesoporous (mp-) TiO2, which requires a higher temperature process. Few demonstrations on the simpler planar structures have been reported. A notable one by Eperon et al. has the highest power conversion efficiency (PCE) of 14.2% while the average is 9.8%8 after adding a small amount of aqueous hydriodic acid (HI) to a precursor solution mixture containing PbI2, FAI, and dimethylformamide (DMF) for obtaining continuous perovskite films. We further investigate how varied amounts of HI additive affect the nucleation and crystallization process of FAPbI3 analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) measurement, Kelvin probe force microscopy (KPFM), photoluminescence (PL), and electroluminescence (EL) measurements. Together with the use of nitrogen gas purge during the spin coating (the so-called gas-assisted method)16 for the first time on FAPbI3 perovskite solar cells, we successfully fabricate FAPbI3 planar cells with narrow power conversion efficiency distribution, achieving an average of 13% without hysteresis at a scanning rate of 0.1 V s−1 and the highest efficiency of 16.0% measured under reverse (VOC to JSC) scan.

1. INTRODUCTION In recent years, organic−inorganic halide perovskite solar cells have generated tremendous research interests as one of the promising alternatives to the existing photovoltaic technologies because of their unique features such as excellent diffusion length, strong light absorption, high open-circuit voltage, ambipolar charge transport, and solution processability.1 Since the 9.7% efficient solid-state cell demonstration in 2012, progress in improving their power conversion efficiencies has been rapid, leading to the most recent independently certified record at 22.1% in 2016.2 Although details are yet to be known on the latest record cell, formamidinium HC(NH2)2+ (FA) has been used as the organic cation in the world record cells reported by Seok’s group in Korean Research Institute of Chemical Technology (KRICT). FA-based perovskite solar cells also have higher thermal stability and absorb slightly larger proportion of the sun’s spectrum due to lower bandgap compared with the methylammonium CH3NH3+ (MA)containing counterparts.3−12 Nevertheless, demonstrations of FA-based perovskite solar cells have been limited with fewer than 30 to-date since its first demonstration in ref 13, despite hundreds of demonstrations reported for MA-based perovskite solar cells. One of the major challenges with FA-based perovskites is associated with the transition to the undesirable nonperovskite δ-phase (yellow in color). The desired perovskite α-phase (black in color) can be achieved via a high-temperature process (e.g., 120−170 °C) and remains stable if in solid form.14 Others have attempted to stabilize the α-phase by incorporating MAI or MABr into the © 2016 American Chemical Society

Received: March 8, 2016 Revised: May 6, 2016 Published: May 9, 2016 11262

DOI: 10.1021/acs.jpcc.6b02443 J. Phys. Chem. C 2016, 120, 11262−11267

Article

The Journal of Physical Chemistry C

(≤0.05 g mL−1 = 0.05 g of HI in 1 mL of FAPbI3/DMF solution) is used, suggesting insufficient nucleation and supersaturation, resulting in lower number of nuclei before crystallization of the perovskite layer.16,17 Similar observation of sparse perovskite film with isolated crystals can be found in ref 16 when no or insufficient additive is used. The XRD patterns in Figure 1c indicate that FAPbI3 films formed by lower HI concentration (≤0.05 g mL−1) have impurities of lead iodide (12.64°). The Bragg peaks at 13.92, 19.76, 24.35, 28.09, and 31.42° are indexed to (110), (200), (202), (220), and (222) diffraction of FAPbI3, respectively, corresponding to the black cubic perovskite phase.14 The full width at half-maximum (fwhm) of the (110) peaks for various samples is also shown in Figure S1 in Supporting Information. Samples with lower HI additive concentration have broader fwhm than those from samples with higher HI concentration (≥0.10 g mL−1). With increasing HI additive, the solubility of perovskite solution increases, which leads to higher supersaturation point promoting nucleation, which has been demonstrated in ref 17. The fast drying process assisted by the N2 flow during spin coating also promotes a large number of nuclei. Therefore, the combined effects of HI additive and gas-assisted approach cause fast nucleation before crystallization.16 Therefore, the FAPbI3 layer is denser and more uniform when HI concentration is ≥0.10 g mL−1 (see Figure 1b) and without the presence of PbI2 impurities; see XRD in Figure 1c. When 0.2 g mL−1 of HI additive is used, although fwhm indicates reasonable crystallinity, pin holes are apparent in the film; see SEM images in Figure 1b. We believe that the pin holes are caused by higher water concentration present in the HI additive, which could lead to degradation and dissolution of the perovskite layer as the amount of HI additive increases.17,18 To gain further insight into the perovskite film formed using different concentrations of HI additive, we performed Kelvin probe force microscopy (KPFM) measurements in the dark to characterize local electrical properties of the film surface. FAPbI3 without a hole transport layer or electrode was fabricated on bl-TiO2/FTO glass such that KPFM directly probes the perovskite film surface. The FTO layer is grounded during the measurement. The measurement is carried out under open-circuit condition, as illustrated in Figure S2 using the same methodology as previously reported,19 and the results are repeatable. Figure 2a−d shows the topography of the perovskite films on bl-TiO2/FTO glass prepared using different concentrations of HI additive. Figure 2e−h show the contact potential difference (CPD) measured by KPFM on the same film. When HI additive ≥0.10 g mL−1, the grains (200−400 nm in size) of films are distinguishable in both the topography maps (Figure 2c,d) and CPD maps (Figure 2g,h). When HI additive ≤0.05 g mL−1, the contrast between the grain boundaries and the grain interiors is very small in the CPD maps (Figure 2e,f). This shows that signals measured by the AFM and KPFM are due to topographical differences rather than a measure of electrical property of the films. The small CPD contrast in the multicrystalline film may be due to the poor crystallization, as also shown in Figure S1 in the Supporting Information. Some localized regions (highlighted by white arrows) in Figure 2e,f with particularly high CPDs (74 and 120 mV higher) indicate the presence of different phase, which is most likely due to the presence of lead iodide impurities, as also shown in Figure 1c. As the concentration of HI additive increases, grains become more distinguishable in the CPD maps, with the grain boundaries having different CPD

2. RESULTS AND DISCUSSION To investigate the effects of varied HI additives on the quality of FAPbI3 film, we prepared FAPbI3 on blocking layer (bl)TiO2 on FTO substrate using the method described in the Supporting Information using the setup illustrated in Figure 1a.

Figure 1. (a) Scheme of FAPbI3 device fabrication: gas-assisted spincoating deposition without (left)/with (right) HI additive, (b) topview SEM images, and (c) XRD patterns of FAPbI3/bl-TiO2/FTO formed using varied HI contents in the precursor under the same annealing temperature of 160 °C (scale bar: 1 μm).

Although the gas-assisted deposition method is effective on MAPbI3,16 the technique alone without further treatment is less effective on FAPbI3, yielding poor coverage, nonuniform film with pin holes in the absence of HI additive; see the top-view SEM image labeled “HI 0.00 g/mL” in Figure 1b due to insufficient nucleation and crystallization of the layer. As varied amounts of aqueous HI (57 wt % in water) are added to the perovskite precursor solution, denser and more uniform film starts to form. Irregular size grains are still observed (see Figure 1b) when a lower HI concentration 11263

DOI: 10.1021/acs.jpcc.6b02443 J. Phys. Chem. C 2016, 120, 11262−11267

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Figure 2. Atomic force microscopy (AFM) topography images of FAPbI3 on bl-TiO2/FTO glass deposited by gas-assisted spin coating and with HI additive at various concentrations (a) 0, (b) 0.05, (c) 0.10, and (d) 0.20 g mL−1. The corresponding KPFM images of the same film are shown in panels e−h.

Figure 3. (a) PL decay traces at 800 nm measured by time-correlated single-photon counting (TCSPC) technique and (b) average electroluminescence intensities as a function of applied voltage bias of FAPbI3 on bl-TiO2/FTO glass formed using varied HI additive concentrations.

values as expected.19−22 There is no abnormally high CPD region in Figure 2g. The CPD at grain boundaries is typically 5−20 mV lower than the CPD within the grain interiors. This agrees with what was commonly observed in MAPbI 3 perovskite materials previously reported.19,23−25 When HI additive concentration ≥0.20 g mL−1, although the grain size appears larger, pin holes are evident in the film; see Figure 2d,h for regions highlighted by white arrows. These holes have much (up to 155 mV) lower CPD, resulting in nonuniform electrical characteristics of the film and hence final device. To investigate the charge carrier dynamics, we measured the steady-state and time-resolved photoluminescences (PLs) of the same samples at 200 mW/cm2 excitation intensity. Figure S3 shows the PL spectra of the samples exhibiting a strong peak at ∼808 nm, consistent with the reported band gap and absorption spectrum of FAPbI3,8 although a slight red shift is observed when FAPbI3 is formed using HI additive with concentration ≥0.20 g mL−1. This could be due to a bandgap narrowing effect when impurity concentration is too high as the HI additive increases. Using time correlated single photon counting (TCSPC) technique we measured the PL decay of the samples near the perovskite bandgap (Figure 3a). Biexponential fitting (see Supporting Information) is used to extract the fast component of the PL decay traces,26−28 which shows obvious

reduction in the time constants with HI additive concentration. In particular, the time constant is lowest when HI concentration ≥0.10g mL−1 due to improved carrier extraction, thereby improving conversion efficiency of final devices to be presented in a later section. Electroluminescence (EL)29−31 is also carried out on the same devices without the added effect of light soaking on the perovskite material. Figure 3b shows the average EL intensity as a function of voltage bias with varied amounts of HI additive. The lowest voltage bias at 0.85 V was selected due to the limitation of the CCD camera resolution for detecting signals below 0.85 V, given that the EL efficiency of the perovskite devices is low at low voltages.32 The EL response of the FAPbI3 perovskite film without any HI additive is below the detection limit of the system for the voltage range measured, reflecting the poor quality of the film, and is therefore not shown in Figure 3b. With increasing HI content, the EL efficiency improves (and exponentially with increasing voltage bias) and is at its maximum when HI = 0.10 g mL−1. The similarly poor EL performance of the samples at low bias voltage for perovskite film prepared using HI ≥ 0.10 g mL−1 is due to high dark saturation current from high defect density or possible shunting. At higher HI concentration of 0.20 g mL−1, the EL intensity suffers and increases at a slower rate with voltage due 11264

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Table 1. Average Solar Cell Performance of Devices with FAPbI3 Formed Using Different Concentrations of HI Additive and Annealed at 160 °C

a

HI concentration (g mL−1)a

Jsc (mA cm−2)

Voc (mV)

FF

Rs (Ω cm)

Rsh (Ω cm)

PCE (%)

standard deviation of PCE

0.00 0.05 0.10 0.20

12 20 23 22

651 755 905 806

42 47 63 43

27 24 18 27

421 1836 2204 1543

3 7 13 8

1.04 2.44 0.97 1.55

HI contents in 1 mL of FAPbI3/DMF solution

Figure 4. (a) Current density−voltage (J−V) curves of the best performing FAPbI3 devices within each group of cells that use different concentrations of HI additive annealed at 160 °C. Distributions of energy conversion efficiencies of devices with FAPbI3 formed (b) using different concentrations of HI additive and annealed at 160 °C. Error bars are standard deviations of device efficiencies. (c) Distribution of device performance of FAPbI3 planar solar cells fabricated using 0.1 g of HI in 1 mL of FAPbI3/DMF solution and annealed at 160 °C. (d) J−V curve of the best performing HC(NH2)2PbI3 planar device in this work.

given in Figure S5 in the Supporting Information. Figure 4a shows the effect of different HI additive concentration used on the current density−voltage (J−V) curves of the final device. The averaged electrical characteristics are summarized in Table 1. The trend agrees well with the results from SEM, XRD, PL, and EL, whereby FAPbI3 devices fabricated without or with insufficient HI have defective absorber with impurities affecting photogenerated currents, which have a detrimental effect on both VOC and FF. An overdose of HI additive affects the VOC and FF of the device, as predicted by SEM (the presence of pinholes), KPFM (the nonuniform electrical characteristics of the film), and EL (increase in series resistance). We have also fabricated cells with FAPbI3 annealed at different temperatures; see Table S2 for their electrical characteristics. The presence of lead iodide impurities has a detrimental effect on FF and therefore cell efficiency, while JSC and VOC remain respectable. The spread of the device performance is larger when HI additive concentration other than 0.10 g mL−1 or annealing temperature other than 160 °C is used; see Figure 4b and Figure S6. Using the optimized conditions, 0.10 g of HI in 1 mL of FAPbI3/DMF solution in conjunction with the gas-

to an increase in nonradiative recombination and parasitic resistances (see Table 1) due to the presence of pin holes as observed in Figures 1 and 2. All of these characterizations confirm that an HI additive of 0.10 g mL−1 (0.10 g of HI in 1 mL of FAPbI3/DMF solution) is the optimized concentration suitable for fabricating high-quality FAPbI3 film that is dense and pinhole free with good crystallinity and forms a good interface with the carrier transport layer. We have also tested the effect of different annealing temperatures on the quality of the perovskite layer. Good crystallinity with identical XRD spectrum can be achieved for annealing temperature ranging from 140 to 180 °C; see Figure S4 in the Supporting Information. Lead iodide impurities (12.64°) start to appear when annealing temperature is ≥180 °C in the XRD pattern (black). We believe the grains with white patches observed in the SEM image correspond to the lead iodide impurities (also observed in ref 23), which contributed to the degradation of device performance. Finally, full devices consisting of Spiro-OMeTAD/FAPbI3/ bl-TiO2/FTO/Glass are fabricated using the optimized conditions. A cross-sectional SEM image of such device is 11265

DOI: 10.1021/acs.jpcc.6b02443 J. Phys. Chem. C 2016, 120, 11262−11267

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The Journal of Physical Chemistry C

consistent with each other. Preliminary thermal cycling test shows our devices are stable between −40 and 80 °C. The combined use of gas assistance and the HI additive developed in this work are not be limited to formamidinium perovskite solar cells but can be applied to perovskite cells of other materials. Insights into perovskite layer formation and solar cell operation will be useful for further development of highefficiency perovskite solar cells.

assisted spin coating method and annealing temperature of 160 °C, we are able to fabricate cells reliably with an average power conversion efficiency of 13%. Figure 4c shows the distribution of 53 devices using the optimized condition. The average efficiency achieved in our work is higher than the average efficiency of 9.7% for FAPbI3 planar cell reported in ref 8, where only HI additive (without gas assistance during spin coating) is used to improve the quality of the perovskite film. The typical J−V curves measured under different scanning conditions of a 13% cell are also shown in Figure S7a in the Supporting Information. Hysteresis of J−V is negligible at a scanning rate of 0.1 V s−1. This is in agreement with the reported result.33 The current density of the cell at maximum power point (∼0.73 V) stabilizes within seconds at ∼18 mA cm−2, resulting in a stabilized PCE of 13%; see Figure S7b in the Supporting Information. PCE measured under the VOC to JSC scan is close to the stabilized PCE and is therefore a reasonable representation of the photovoltaic performance. The PCE of the best performing device in this work is 16.0% measured under the VOC to JSC scan; see Figure 4d. The external quantum efficiency (EQE) spectrum of the cell is also shown in Figure S8. The broader spectral response to up to 840 nm shows the advantage of the lower bandgap FAPbI3 solar cells over the MAPbI3 counterparts (spectral response to ∼800 nm).8 The slightly lower JSC (20.2 mA cm−2) estimated from EQE is due to the lack of light soaking that stabilizes the current density of the cell, which takes a few seconds.9,34 The thermal stability of the cells from −40 to 80 °C was investigated in a preliminary thermal cycling test. Cells were placed in hermetically sealed enclosure that was then subjected to ∼30 h (10 cycles) of thermal cycling; see Figure S9a in the Supporting Information for the temperature and relative humidity recorded during the thermal cycling test. Details of a typical thermal cycle (IEC 61646 standard) are given in the Supporting Information. The solar cell performance essentially remained stable after 10 thermal cycles; see Figure S9 for the J−V curves. Further investigations into the thermal stability test will be carried out in the future and full 200 cycles from the IEC 61646 standard will be conducted.

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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/acs.jpcc.6b02443. Materials preparation, sample fabrication, characterization method, and additional data. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 2 9385 4257. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australian-based activities of the Australia U.S. Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). This work is also supported by ARENA through 2014 RND075 project. Seidel also acknowledges funding from the Australian Research Council through DP140102849 and DP140100463. We thank the Analytical Centre at UNSW for their technical support.

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REFERENCES

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3. CONCLUSIONS We have successfully demonstrated a reliable fabrication method for FAPbI3 perovskite solar cell with narrow spread in device efficiency achieving an average of 13% without hysteresis at a scanning rate of 0.1 V s−1 through controlling the nucleation and crystallization process using nitrogen purge during spin coating and HI additive solution. The highest cell efficiency has been 16.0%. Through SEM, XRD, KPFM, PL/ TRPL, EL measurements, and characterization of the solar devices, it is revealed that moderate amount of HI additive (0.10 g in 1 mL of FAPbI3/DMF solution) in conjunction with nitrogen purging during spin coating and an anneal temperature of 160 °C results in a dense, uniform, pinhole-free good crystalline film without the presence of lead iodide impurities. Insufficient HI content (0.1 g mL−1) will introduce excess water content, resulting in poor perovskite layer with pin holes and nonuniform electrical properties reflected in the increased series resistance in the solar device. The conclusions from the SEM, XRD, KPFM, PL/ TRPL, EL measurements, and device electrical results are 11266

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.6b02443 J. Phys. Chem. C 2016, 120, 11262−11267