High-Efficiency Rubidium-Incorporated Perovskite Solar Cells by Gas

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High Efficiency Rubidium Incorporated Perovskite Solar Cells by Gas Quenching Meng Zhang, Jae Sung Yun, Qingshan Ma, Jianghui Zheng, Cho-Fai Jonathan Lau, Xiaofan Deng, Jincheol Kim, Dohyung Kim, Jan Seidel, Martin A. Green, Shujuan Huang, and Anita Wing-Yi Ho-Baillie ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00697 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017

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High Efficiency Rubidium Incorporated Perovskite Solar Cells by Gas Quenching Meng Zhang*1, Jae S. Yun1, Qingshan Ma1, Jianghui Zheng1, Cho Fai Jonathan Lau1, Xiaofan Deng1, Jincheol Kim1, Dohyung Kim2, Jan Seidel2, Martin A. Green1, Shujuan Huang1 and Anita W. Y. Ho-Baillie*1 1

Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, University of

New South Wales, Sydney 2052, Australia 2

School of Materials Science and Engineering, University of New South Wales, Sydney 2052, Australia

*Corresponding Authors’ e-mail: [email protected]; [email protected]

TOC

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Abstract: We apply gas-quenching to fabricate Rubidium (Rb) incorporated perovskite films for high efficiency perovskite solar cells achieving 20% power conversion efficiency on a 65 mm2 device. Both double-cation and triple-cation perovskites containing a combination of methylammonium (MA), formamidinium (FA), caesium (Cs) and Rb have been investigated. It is found that Rb is not fully embedded in the perovskite lattice. However, a small incorporation of Rb leads to an improvement in the photovoltaic performance of the corresponding devices for both double cation and triple cation perovskite systems.


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As an emerging photovoltaic (PV) technology, organic-inorganic perovskite solar cells (PSCs) have attracted enormous research attention due to their ease of fabrication and high power conversion efficiencies (PCE)1-7. A typical formula for the organic-inorganic perovskite is ABX3, where A stands for an organic cation such as methylammonium (MA), formamidinium (FA), etc. B is a metal (typically Pb) and X represents a halogen anion (I, Br, Cl, or a mixture of these). The stability and distortion of the perovskite structure is determined by Goldschmidt tolerance factor (t), which can be calculated by = + /√2 + , where r is represents the ionic radii of the corresponding ions. The perovskite phase cannot be stabilized with t values larger than 1.0 or smaller than 0.8. With a t value of 0.91, MAPbI3 is the mostly studied perovskite material for PV. However, MAPbI3 has its limitations such as thermal stability, moisture-related degradation, and hysteretic IV behaviour8-10. In comparison, perovskite FAPbI3 with a lower band gap has an extended light absorption range, better stability and reduced IV hysteresis10-12. However, apart from the desired black phase (α-FAPbI3), FAPbI3 tends to form a non-perovskite yellow phase (δ-FAPbI3) at room temperature, which is due to a relatively large radius of the FA cation that makes t of FAPbI3 larger than 1.0. In order to reduce t, incorporation of smaller cations has been demonstrated to be successful13-17. The incorporation of MA and caesium (Cs) stabilizes the perovskite structure, resulting in better PV performance. Saliba et al. have demonstrated that the device made of FA-MA-Cs triple cation perovskite can achieve a PCE over 21% with an active area of 0.16 cm217. The incorporation of Cs also effectively reduces the crystallization temperature during the annealing process. FA-Cs and FA-MA-Cs perovskite can be fully crystallized under 100 ○C, while pristine FA perovskite requires an annealing temperature over 150 ○C15, 17

. It is also worth noting that compared with hybrid perovskites, inorganic perovskites are


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usually less sensitive to moisture. As a result, the FA1-xCsxPbI3 perovskites exhibit better photo- and moisture-stability to the pristine FAPbI3 perovskites15, 17. Building on the work mentioned above, we started to explore new inorganic cations for mixed cation perovskites. Rubidium (Rb) is a neighbour alkali metal element of Cs and has a relatively smaller ionic radius. There has been no successful application of Rb during our experiment work until recently, Rb3Sb2I9 has been reported as a lead-free perovskite light absorber18 and another two reports demonstrated embedding Rb in lead-based perovskites19-20 achieving respectable PV performance using the anti-solvent preparation. However, the antisolvent method requires the dropping of a solvent with lower solubility for the perovskite precursor seconds after the application of perovskite precursor on a spinning substrate. This method requires extra chemicals and stringent process control. The failure of the latter leads to poor reproducibility. Gas-quenching method which involves a gas stream being applied to a precursor–coated spinning or precursor-printed substrate has the advantage of good reproducibility21-22. As it does not require extra chemicals for the fabrication of good quality perovskite film and efficient devices, it has a better prospect for up-scaling21-24. Herein, we demonstrate our 65 mm2 (aperture area) Rb-incorporated perovskite devices using the gas-quenching method and precursors were prepared by dissolving RbI, FAI, MABr and PbI2 stoichiometrically in a mixed solvent of N,N-dimethylformamide (DMF) and dimethyl sulphoxide (DMSO). For comparisons, FAPbI3, FAxRb1-xPbI3, (FAPbI3)0.85(MAPbBr3)0.15, (FAxCs1-xPbI3)0.85(MAPbBr3)0.15, (FAxRb1-xPbI3)0.85(MAPbBr3)0.15 perovskite films and devices are characterised and demonstrated in this work. The FA-Rb perovskite was investigated first. Although it has been reported that the incorporation of Cs cations can lower the phase transition temperature (transition to black phase) to below 150 ○C15-16, we found that, the addition of Rb cations cation did not have the


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same effect. After annealing at 110 ○C, the Rb-added perovskite still shows a yellow colour while the Cs-added perovskite has completely turned black (Figure S1). Therefore in our work, the annealing temperature for FA-Rb films is kept at 150 ○C for full conversion to perovskite phase. The XRD patterns of the FA-Rb film with different concentrations of Rb are shown in Figure 1a. Pristine FAPbI3 show typical perovskite peaks with noticeable impurity peaks at 11.5○ and 12.6○, which can be attributed to the non-perovskite δ-phase and PbI2, respectively. It is worth noting that, even the precursor was prepared with an exact stoichiometric ratio, PbI2 can still be present in the perovskite film which does not necessarily have an adverse effect on the final device performance25-26. For the Rb-added perovskites, some other impurity peaks are present around at 10○ and 27○, which increase with Rb concentration indicating that they are Rb-rich phases which are present in pristine RbPbI3, see the corresponding XRD pattern in Figure S2. Figure 1b shows the scanning electron microscopy (SEM) images of the gas-quenched perovskite films with different concentrations of Rb. All the films show a smooth pinhole-free surface with respectable grain size. Bright impurity phases can be observed under SEM for the pristine FAPbI3. By incorporating 5% of Rb, these impurity phases can be significantly reduced. However, a higher concentration of Rb results in particles near the grain boundaries appearing bright under the SEM. This is likely to be precipitated RbPbI3 considering the XRD results. This can be verified by the backscattered electron image (Figure S3) showing the impurity particles being brighter than the normal grains indicating a higher average atomic number of these bright particles. Therefore our results show that Rb is not fully embedded in the perovskite lattice although the impact is minimal if the concentration is 5% or below. Optical characterisation shows that the addition of Rb does not lead to a significant shift in absorption onset (Figure S4) and therefore does not result in band gap change. 5 ACS Paragon Plus Environment

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The light current density vs voltage (J-V) curves of fabricated devices with the structure of FTO/compact-TiO2 (c-TiO2)/mesoporous-TiO2 (mp-TiO2)/FA1-xRbxPbI3/spiro-MeOTAD/Au are shown in Figure 1c. The device with FAPbI3 exhibits a PCE of 14.9% with an opencircuit voltage (Voc) approaching 1090 mV. The advantage of adding small amount of Rb is shown in the improved PCE at 16.2% for device with 5% of Rb. The improvement is mainly due to an enhancement in short-circuit current density (Jsc), with slight improvement in fill factor (FF). The addition of Rb deteriorates the Voc and further increase in Rb (10% or 15%) also worsens the Jsc producing devices with PCE’s below that of FAPbI3 device. This is due to the presence of large amount Rb-rich impurity phases. In addition to device performance enhancement, FA0.95Rb0.5PbI3 films and devices also show significant improvement in moisture-stability. Figure 1d shows the efficiency trend over time of un-encapsulated devices. During the storage in ambient air for 4 weeks (in dark), the FAPbI3 device presented an obvious decrease in PCE while the FA0.95Rb0.05PbI3 device kept over 90% of its initial efficiency. It is also clearly shown in the inset figure that upon exposure to ambient air for 4 weeks, the FAPbI3 films and devices exhibited severe degradation. In comparison, the FA0.95Rb0.05PbI3 films and devices have retained dark black colour, demonstrating a better stability than the pristine FAPbI3. The improved stability and tolerance to moisture are considered to be associate with the tightened lattice upon Rbincorporation and the stabilized black phase of FAPbI315.


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Figure 1. a) XRD patterns and b) SEM images of the FA-Rb perovskite films with different concentrations of Rb; c) Light J-V curves and electrical parameters of FA1-xRbxPbI3 65mm2 (aperture area) devices with different concentrations of Rb; d) Efficiency decay of unencapsulated devices stored in ambient air for 4 weeks (inset: photos of the aged films and devices after 4 weeks of exposure to ambient air). Apart from FA-Rb perovskites, we have also demonstrated Rb incorporated MA perovskite devices which show no significant improvement after the 5% Rb incorporation (Figure S5) although the MA-Rb devices perform better than the FA-Rb devices. We therefore proceed to investigate further the mixed FAMA system given previously reported better photovoltaic performance13-14 than the MA-only or FA-only systems. In particular, we use the mixed halide FA0.85MA0.15PbI2.55Br0.45 as the baseline due to their demonstrated high photovoltaic 7 ACS Paragon Plus Environment

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performance14. The connotation FAMA is used here for convenience. The FAMA-Rb (FA0.80MA0.15Rb0.05PbI2.55Br0.45) films and devices are fabricated by incorporating 5% Rb. For comparison, Cs-incorporated FAMA-Cs (FA0.80MA0.15Cs0.05PbI2.55Br0.45) films and devices were also fabricated. A lower annealing temperature (110 ○C) was used for these samples due to the presence of MA. The UV-Vis absorption and photoluminescence (PL) data (Figure S6) show that the incorporation of Cs or Rb improves the absorption of the perovskite film. The FAMA-Cs perovskite exhibits a blue shift which is consistent with the results reported in the literature17. The blue shift from the FAMA-Rb perovskite is less significant. The shape (presence of a shoulder) of the FAMA-Rb PL peak suggests possible phase segregation. Figure 2a shows the surface morphology of these perovskite films. Some bright impurity grains can be found in the FAMA sample (Fig. 3a labeled “FAMA”), but are reduced when inorganic cation Cs is added (Fig. 3b labeled “FAMA-Cs”). However, impurity grains are present in the FAMA-Rb samples. To identify these bright particles, XRD scan of these samples are performed. The results shown in Figure 2b are in a good agreement with the SEM images. Peaks of non-perovskite are clearly seen in the FAMA sample. After the incorporation of Cs and Rb, the non-perovskite phases are significantly suppressed. However, extra peaks are present in the XRD pattern of FAMA-Rb compared to that of the FAMA-Cs film. These peaks correspond to Rb-rich impurity phase similar to those in FA-Rb samples indicating Rb-rich impurity phase are also formed in FAMA-Rb triple cation perovskite film. Interestingly, although FAMA-Rb film does not appear as neat as FAMA-Cs film, the FAMA-Rb device achieved a higher PCE of 19.6% than the FAMA and FAMA-Cs devices with PCEs of 17.1% and 19.4%, respectively. As can been seen from the inset table in Figure 2c, incorporating of Cs and Rb boost the Jsc and fill factor (FF). The boost in Jsc agrees with the improved absorption measured; see Fig. S6. The improved FF can be due to better film 8 ACS Paragon Plus Environment

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quality from the elimination of non-perovskite phase in the FAMA perovskite which can possibly act as shunting paths resulting in low shunt resistance (RSH) in FAMA samples compared to FAMA-Rb and FAMA-Cs samples. The presence of Rb-rich impurity phase does not appear to affect the VOC of the film device. This may be similar to the case when excess impurity can have a positive effect on device performance such as the case of incorporating excess PbI226-29. To understand this further, Kelvin probe force microscopy (KPFM) was performed on test structures perovskite/mp-TiO2/c-TiO2/FTO glass using the set up illustrated in Figure S7. The perovskite surface is directly accessible by the KPFM probe while the FTO layer is grounded. Open-circuit conditions are applied similar to those used in previous report25. Measurement details are described in the experimental section and the measurement has shown consistent results from repetitive measurements.


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Figure 2. a) SEM image and b) XRD patterns of the FAMA, FAMA-Cs and FAMA-Rb perovskite films. c) J-V curves and performance parameters of the solar cells fabricated with the FAMA, FAMA-Cs and FAMA-Rb perovskite films. KPFM provides spatial work function distribution of the surface of a sample. Such a measurement set up has shown to be a powerful method for providing morphological information and for quantifying local electrical properties of the film surface30. Figure 3 shows the contact potential difference (CPD) maps of the FAMA, FAMA-Cs, and FAMA-Rb test structures in the dark and under illumination. For all samples, darker grains (indicated by red arrows) can be observed for all films in the CPD maps taken in the dark, see Figures 3a to (c). These darker grains have CPD values 12±10 mV lower than those of their surroundings. Such darker grains may be due to segregation of specific compositions or point defects (such as mobile ion interstitial or vacancies)31 Under light illumination, some of the grain boundaries appear brighter (higher CPD) than the grain interiors, which agree with previous reports30, 32. Most importantly, all of these darker grains remain dark (lower CPD) in FAMA sample as indicated by the red arrows (Figure 3d) but not all dark grains in the FAMA-Cs and FAMA-Rb samples remain dark under illumination. Some of the dark grains became brighter, i.e., have higher CPD values than their surroundings after illumination in the FAMA-Cs and FAMA-Rb samples as indicated by the green arrows in Figures 3e and 3f. In these regions there is an enhancement in photo-voltage as it is equivalent to CPD under light subtracted by CPD under dark33 (CPDlight-CPDdark). This indicates that the photogenerated carriers are more effectively separated at these grains. We calculate CPDlight of bright regions in Figure 3d, 3e and 3f minus CPDdark of dark regions in Figure 3a, 3b, and 3c. It is found that CPDlight of the bright regions-CPDdark of the dark regions

are 38 mV on average for the FAMA-Cs sample while

CPDlight of the bright regions-CPDdark of the dark regions are 42 mV on average for the FAMA-Rb sample. Therefore the incorporation of Rb further improves photo-voltage enhancement. Although we 10 ACS Paragon Plus Environment

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cannot identify the compositional characteristics of these grains under KPFM, it is likely that these grains are Rb-rich phase as observed in the XRD results (Figure 2b) which contribute to enhancement in photo-voltage and therefore improvement in Voc (Figure 2c) in complete devices that have Rb incorporated.

Figure 3. KPFM measurements performed on perovskites/m-TiO2/c-TiO2/FTO/glass test structures over an area of 2 x 2 µm2 in the dark and under white light illumination equivalent to 0.6-Sun. CPD maps of (a) FAMA, (b) FAMA-Cs, and (c) FAMA-Rb in the dark. CPD maps of (d) FAMA, (e) FAMA-Cs, and (f) FAMA-Rb under illumination. Finally, in order to reduce front reflection, a MgF2 antireflection layer was coated on the glass side of the FAMA-Rb devices, see inset photo of Figure 4a. External quantum efficiency (EQE) spectra of the devices with and without MgF2 coating have been obtained to verify the effectiveness of the antireflection layer. After coating with the MgF2 antireflection layer, the external efficiency of the device has been boosted by a factor of 2~4 % as shown in Figure 4a. As a result, the current density of the device has increased by 3% approximately. The corresponding J-V curves and electrical characteristics are shown in Figure 4b. The 11 ACS Paragon Plus Environment

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enhanced Jsc led to an improvement in PCE from 19.6 % to 20.0 % or 20.4 % under reverse scans (from Voc to Jsc). These FAMA-Rb devices exhibit negligible J-V hysteresis under scan rate of 0.1 V/s. Although the J-V hysteresis becomes noticeable at high scan rate, the averaged efficiency from the J-V hysteresis remains high at 19.8%. The steady-state power output measured near the maximum power point also demonstrated a stabilized PCE at 19.8% (see Figure S8). The very low hysteresis in these devices is probably due to the presence of mp-TiO2 layer (see Figure 4c for the cross-sectional SEM image). It has been previously reported that TiO2 nanoparticles (see dots in SEM in Figure 4d viewed under back scattered mode) in the mp-TiO2 layer provide effective electron injection and therefore results in a more efficient and balanced charge separation reducing J-V hysteresis

34-35

. We have also

successfully demonstrated the use of gas-quenching to fabricate perovskite films on mesoporous cell structures extending the use of gas-quenching beyond planar devices21, 24.


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Figure 4. a) External quantum efficiency spectra of the FAMA-Rb devices with and without MgF2 antireflection layer. Inset: photo of a typical device with MgF2 antireflection layer. b) J-V curves and electrical parameters under dfferent scan conditions of the MgF2 coated

FAMA-Rb perovskite devices. c) Cross-sectional SEM image of a typical device and d) viewed under backscattered mode. In conclusion, Rb-incorporated meso-porous perovskite solar cells (aperture area of 65 mm2) have been successfully demonstrated using the gas-quenching method for the first time. Various characterisations have confirmed that the amount of Rb is to be limited to 5% for best film quality and device performance. Larger amount of Rb incorporation results in more Rb-rich impurity phases due to the incomplete incorporation of Rb into the perovskite lattice. The incorporation of 5% Rb improves the efficiency from 14.9% for a FAPbI3 cell to 16.2% for a FA0.95Rb0.05PbI3 cell. The moisture-stability of the FA0.95Rb0.05PbI3 is also more superior to the FAPbI3. We have been successful in extending the shell-life of un-encapsulated FA0.95Rb0.05PbI3 device to 1 month stored in ambient. The incorporation of Rb into FAMA mixed halide perovskite devices lead to a remarkable improvement in PCE from 17.1% for a FA0.85MA0.15PbI2.55Br0.45 cell to 19.6% for a FA0.80MA0.15Rb0.05PbI2.55Br0.45 cell. KPFM measurement of perovskite test structures under dark and illuminated conditions shows photo-voltage enhancement, particularly, in FAMA-Rb sample which explains the improvement of Voc in complete solar devices. The PCE of the champion FAMA-Rb device reaches 20.0% with negligible hysteresis. This is the highest for this kind with aperture area lager than 0.5 cm2. We have successfully extended the use gas-quenching method to fabricate perovskite films beyond planar cells. This work contributes to the development of highly efficient perovskite solar cells.


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Acknowledgements: The Australian Centre for Advanced Photovoltaics (ACAP) encompasses the Australianbased activities of the Australia-US Institute for Advanced Photovoltaics (AUSIAPV) and is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Financial support from Australian Research Council through DP160102955 program is also acknowledged. Supporting Information Available: (Experimental details, supplementary characterizations of materials and devices)

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High-Efficiency Rubidium-Incorporated Perovskite Solar Cells by Gas

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