Perovskite Solar Cells Employing Copper ... - ACS Publications

Subscriber access provided by University of South Dakota

Letter

Perovskite Solar Cells Employing Copper Phthalocyanine Hole Transport Material with an Efficiency over 20% and Excellent Thermal Stability The Duong, Jun Peng, Daniel Walter, Jin Xiang, Heping Shen, Dipankar Chugh, Mark N. Lockrey, Dingyong Zhong, Juntao Li, Klaus J. Weber, Thomas P. White, and Kylie R. Catchpole ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01483 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Perovskite Solar Cells Employing Copper Phthalocyanine Hole Transport Material with an Efficiency over 20% and Excellent Thermal Stability The Duong1,*, Jun Peng1,2, Daniel Walter1, Jin Xiang2, Heping Shen1, Dipankar Chugh3, Mark Lockrey4, Dingyong Zhong2, Juntao Li2, Klaus Weber1, Thomas P. White1,2, Kylie R. Catchpole1,* 1

Research School of Engineering, Australian National University, 32 North Road, Acton, Canberra 2601, Australia

2

State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics, Sun Yat-sen University, Guangzhou 510275, China 3

Department of Electronic Materials Engineering, Research School of Physics & Engineering, Australian National University, 56 Mills Road, Acton, Canberra 2601, Australia

4

Australian National Fabrication Facility, Research School of Physics and Engineering, Australian National University, 56 Mills Road, Acton, Canberra 2601, Australia

*

Corresponding authors: [email protected]; [email protected]

ABSTRACT: We investigate the properties of an inexpensive hole transporting material (HTM), copper phthalocyanine (CuPc), deposited by a solution-process method in perovskite solar cells (PSCs). Cracks are found to be abundant on the as-deposited CuPc films, which lead to serious shunts and interface recombination. Surprisingly, shunts and interface recombination are significantly reduced and cell performance is greatly improved after heat treatment at 85oC. We find that the enhancement is due to heat-induced migration of Au particles away from the cracks. Furthermore, Au is found to dope the CuPc film and the doping effect is greatly enhanced by the heat treatment. Using CuPc and quadruple-cation perovskite, an efficiency of over 20% and negligible hysteresis is achieved after the heat treatment, which is the highest value reported for this structure. Additionally, PSCs employing CuPc and dual-cation perovskite show excellent thermal stability after >2000 hours at 85oC and good light stability at 25oC.

1 ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 19

TOC Graphic

PSCs have achieved a certified efficiency of over 23% for single junction solar cells in normal mesoporous structure, which is a six-fold increase since they were first reported in 20091-3. PSCs with high efficiencies can also be achieved with other cell structures such as the normal planar structure4-5 and inverted structure6-8. PSCs are also promising for tandem application when combined with commercial silicon bottom cells as demonstrated by several reports9-11. Although PSCs can be fabricated using solution process methods, the cost is still high due to the presence of several costly layers in the devices. Commonly used hole transporting

materials

such

as

small

methoxyphenylamine)-9,9′-spirobifluorene

molecules

2,2′,7,-7′-tetrakis(N,N-di-p-

(Spiro-MeOTAD)

and

polymer

poly(triarylamine) (PTAA) are excessively expensive and therefore unsuitable for commercialization. There is a critical need to replace these expensive materials by cheaper alternatives. Moreover, problems with stability remain the biggest challenge to PSCs since the devices are well known to be unstable under many conditions such as heat, moisture, light and electrical bias12. Several groups have reported that the instability of PSCs is greatly affected by the hole transporting material. As an example, Spiro-MeOTAD is not thermally stable even at a temperature as low as 80 oC 13 and it is a poor barrier to metal migration from the contact into the perovskite active layer14-15. Inorganic HTMs such as NiOx have shown some promise to improve cell stability, however so far high performance of up to 20% has 2 ACS Paragon Plus Environment

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

only been obtained for inverted cell structures16-18. For normal structure PSCs using purely NiOx HTL, efficiencies of less than 11% are obtained due to the low conductivity of the NiOx layer because of insufficient sintering of the NiOx nanoparticles19 and inefficient charge extraction coming from the unremoved organic ligand in the NiOx film20. A recent result using an inorganic HTM CuSCN to achieve more than 20% efficiency in normal structure PSCs is encouraging, however the study has identified that the electrical potential induced reaction of gold with the CuSCN can severely degrade the devices under light and electrical bias in less than 24 hours21. Phthalocyanine metal complexes, especially copper phthalocyanine (CuPc), have been used as a p-type semiconductor in light-emitting diodes 22 and organic solar cells

23

due to their high mobility and excellent stability (both thermal and

chemical). CuPc in the unsubstituted form was first used in PSCs by vacuum deposition 24-25 because it is not soluble in common organic solvents. The solubility of CuPc can be enhanced by the introduction of substituents on the periphery of the macrocycle, and subsequently solution-processed CuPc films have been incorporated in PSCs with reasonable success26-29. Most recently, Kim et al. achieved a promising efficiency of 18.8% (obtained from the reverse scan) for PSCs employing a CuPc HTM by achieving strong interfacial and conformal coating of the CuPc layer on the surface of the perovskite active layer30. This is however still well below the efficiency of state-of-the-art PSCs employing Spiro-MeOTAD. In this work, we use a soluble commercially available derivative of CuPc, copper(II) 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (Sigma Aldrich 423165), as the HTM in PSCs. We discover that this solution-processed CuPc layer has abundant cracks on the surface which enable direct contact between the top metal layer and the perovskite active layer underneath, leading to serious shunts and interface recombination in the devices. Surprisingly, the shunts and interface recombination are removed to a negligible level after the devices are heat-treated at 85 oC. We find that the Au particles on the contact rearrange



after the heat treatment and migrate away from the cracks. In addition, through photoemission spectroscopy studies, Au is found to dope the CuPc film, and the doping is further enhanced by the heat treatment. An efficiency of more than 20% and negligible hysteresis for PSCs with CuPc HTM and quadruple-cation perovskite active layer is achieved after the heat treatment, which is comparable with the efficiency obtained with SpiroMeOTAD. Importantly, PSCs with CuPc HTM and dual-cation perovskite active layer are stable at 85 oC for more than 2000 hours in the dark and N2 environment. The cell is also stable under light and N2 environment at 25 oC for 100 hours. We fabricate PSCs with the cell structure glass/ indium-doped tin oxide/ compact-TiO2 / mesoporous-TiO2/ passivation layer/ perovskite/ CuPc / Au. The passivation layer here refers to the blend of 6,6-Phenyl C61 butyric acid methyl ester (PCMB) and poly(methyl methacrylate) (PMMA) as in our previous report31. The quadruple-cation perovskite composition Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45 is used as the active layer, where FA stands for formamidinium and MA stands for methylammonium. The thickness of the CuPc layer is 40 nm as revealed from the high magnification SEM cross-sectional image (Figure S1).We find that all of the as-prepared cells have inferior photovoltaic performance. As presented in Figure 1a-d and Table S1, the average efficiency is 7.5% with a wide distribution of 2.47%. This efficiency is much lower than as-prepared PSCs with the same structure but employing Spiro-MeOTAD HTM instead. However, the cell performance is improved dramatically to an average value of 18.68% with a tight distribution of 0.68% after the cells are heat-treated at 85 oC for 20 hours inside a N2 environment. After the heat treatment, we achieve an efficiency of over 20% with negligible hysteresis for the champion device (Figure 1e, f), which is the highest value reported to date for this cell structure. The External Quantum Efficiency (EQE) of the device is shown in Figure S2 together with the integrated short-circuit current density (JSC). In comparison, by applying the same heat 4 ACS Paragon Plus Environment

Page 4 of 19

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

treatment to PSCs employing Spiro-MeOTAD HTM, the average efficiency drops from 19.73% to 11.46% with a drastic reduction in all photovoltaic parameters as shown in Figure S3. This is in agreement with previous reports showing the thermal instability of PSCs using Spiro-MeOTAD HTM32-33. To investigate the possible causes of the performance enhancement in PSCs employing a CuPc HTM, we look more closely into the evolution of the J-V characteristics of the devices before and after the heat treatment (Figure 1g). The J-V curve of the as-prepared device shows a clear shunting problem as evidenced by a steep slope near to the short circuit point. After the heat treatment, all the photovoltaic parameters are improved dramatically and the J-

V curve becomes flat near to the short circuit point. We use our physical cell model with the semiconductor drift-diffusion equations extended for two additional charge carriers (anions and cations) 34 and run simulations with the Quokka3 software program 35 to investigate which factors can cause such dramatic changes in the response of the perovskite solar cells. Details of the model can be found in the supporting document (Note 1). Our simulations showed that changes in shunt resistance (RSH) and/or interface recombination were most consistent with the changes observed after the heat treatment. Changes in both parameters result in a softening in fill-factor (FF), loss of short-circuit current density (JSC) and opencircuit voltage (VOC), reflecting the changes in the cell performance following the heat treatment. In Figure 1h we plot the simulated J-V curves under changing shunt resistance, while the influence of interface recombination is plotted in Figure S4. Conversely, changes in ohmic series resistance RS alone (Figure S5) or work function (WF) of the HTM interface (Figure S6) cannot fully replicate the enhancement in cell performance. In practice, it is highly probably that all the aforementioned parameters are simultaneously affected by the heat treatment, but the simulations indicate that shunt resistance and interface recombination are likely to be the dominant factors. 5 ACS Paragon Plus Environment


Figure 1. (a, b, c, d) Comparison of as-prepared and heat-treated perovskite solar cells in terms of photovoltaic parameters including VOC, JSC, FF and power conversion efficiency (PCE). The photovoltaic parameters are extracted from the J-V scans from open circuit voltage to short circuit current with the scan rate of 50 mV/s. There are 16 cells in two batches of the study. (e) J-V scans of the champion perovskite cell in both reverse and forward direction with a scan rate of 50 mV/s. The inset shows the photovoltaic parameters 6 ACS Paragon Plus Environment

Page 6 of 19

Page 7 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

extracted from the J-V curves. (f) Steady state power conversion efficiency monitoring at the VMPP of 0.946 V for 1000 s. (g) Comparison of J-V curves of as-prepared and heat-treated perovskite solar cell. (h) Simulated results showing the effect of RSH on the J-V curve of perovskite solar cells. Using Scanning Electron Microscopy (SEM), we examine the morphology of different layers in the device before and after the heat treatment. For the perovskite layer, the perovskite crystal size increases from 100-300 nm to 300-500 nm after annealing due to the Ostwald ripening effect (Figure 2a, b). On the as-prepared CuPc film deposited on top of a perovskite film, the perovskite crystals can still be observed which indicates that the thin CuPc layer conformally covers the perovskite layer. Interestingly, we find many cracks evenly distributed on the film surface. After the heat treatment, these cracks persist on the CuPC film (Figure 2c, d). To examine the effect of the heat treatment on the gold layer, we deposited a very thin Au layer (10 nm) by thermal evaporation onto the CuPc film, which partially covers the underlying layer especially the areas around the cracks on the CuPc film. After heat treating this film, surprisingly we find that the Au particles on the contact rearrange and pull away from the cracks, and form much bigger particles (Figure 2e, f). The crystallite growth during sintering at high temperatures over 200 oC of Au and other metals such as Ag, Al and Pt has been observed36-37 , however such drastic movement and rearrangement of the metal particles on the surface even at a temperature as low as 85 oC is surprising. We suspect that this is partly due to the high surface energy of the film near to the cracks. This phenomenon can also be observed on Au layers with 20 nm thickness (Figure S7). On the as-prepared 100 nm Au film deposited on a CuPc film, the gold particles are in the range of a few nanometres and the cracks on the underlying CuPc film are still visible on the Au layer. Under the SEM cross-sectional image of a complete cell, we observe direct contact between the Au and perovskite active layer through the cracks in the CuPc (Figure S8). The direct contact 7 ACS Paragon Plus Environment


between Au and perovskite active layer can be further confirmed using Secondary Electrons (SE)/ Back Scattered Electrons (BSE) images as shown in Figure S9a, b. Contact between the Au and the perovskite through the cracks of the CuPc film can cause shunts with associated J-V performance losses as reported previously38. In addition, direct contact to the metal may increase recombination at the metal-perovskite interface by introducing a large concentration of recombination-active interface states. After the heat treatment, the Au particles become much bigger (tens to hundreds nanometres) and the cracks on the CuPc film underneath are no longer visible (Figure 2g, h). Under SE/ BSE images of the heat-treated device, we find no direct contact between Au and perovskite active layer (Figure S9c, d). This suggests that the enhancement in the cell performance after the heat treatment is due to the reduction in the direct contact between the Au and the perovskite through the cracks on the CuPc layer. This reduction in the metal-perovskite surface area may increase shunt resistance and reduce interface recombination in the devices. The shunting and recombination pathways can also be blocked by using an interlayer of MoOx (10 nm) between the CuPc film and an indium doped zinc oxide contact. The as-prepared device with MoOx/IZO contact does not show any obvious sign of shunt and no enhancement in the cell performance is observed after the heat treatment (Figure S10).


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 2. SEM images of different layers in the device before and after the heat treatment. (a) as-prepared perovskite film, (b) heat-treated perovskite film, (c) as-prepared CuPc film, (d) heat-treated CuPc film, (e) as-prepared 10 nm Au on CuPc film, (f) heat-treated 10 nm Au on



CuPc film, (g) as-prepared 100 nm Au on CuPc film, (h) heat-treated 100 nm Au on CuPc film. The scale bar is 1 µm. Next, we investigate whether the enhancement in the cell performance after the heat treatment has contributions from other factors such as the change in the properties of the CuPc layer or the perovskite/CuPc interface. For example, Yang et al. reported that the molecular alignment of the thermally-evaporated CuPc films can be altered from edge-on to face-on by substituting with the octamethyl functional groups, resulting in higher mobility, denser thin film structure and better cell performance39. Using Grazing Incident X-Ray Diffraction (GIXRD), we show no change in the diffraction pattern of the solution-processed CuPc before and after the heat treatment as the CuPc film adopts the edge-on orientation in both cases (Figure S11). To confirm that the performance enhancement does not come from the change in the CuPc or the perovskite/CuPc interface upon annealing, we perform the heat treatment on the devices prior to Au contact deposition. The Au contact is then deposited to complete the cells. As shown in Figure S12, the cell performance is inferior in this case with similar average efficiency compared to as-prepared devices. The results indicate that the heat treatment is only effective when performed on complete cells with gold contact. In other words, the enhancement in the performance is most likely linked to the change in the Au contact (as demonstrated above) or the CuPc/Au interface after the heat treatment. We use Ultraviolet Photoelectron Spectroscopy (UPS) to investigate the change in the CuPc/Au interface upon annealing. As depicted in Figure 3a, the obtained work function (WF) of the as-prepared ITO/Au (100 nm) is 5.18 eV. After the heat treatment, the WF of the ITO/Au (100 nm) shows a similar value of 5.20 eV. A slight discrepancy in WF between the as-prepared and the heat-treated ITO/Au samples can be assigned to the experimental errors from sample to sample. For the ITO/CuPc, both the as-prepared and the heat-treated samples produce an equivalent value of 4.55 eV and 4.54 eV respectively in terms of WF (Figure 3b). 10 ACS Paragon Plus Environment

Page 10 of 19

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

The XPS results also reveal no change in the Cu2p spectra of CuPc film after the heat treatment (Figure S13). This further confirms that the heat treatment does not change the property of the CuPc film itself. When depositing 5 nm of Au on top of the CuPc film, the work function increases to 4.87 eV (Figure 3c). This can be explained by the fact that the measured WF is contributed by both the CuPc WF and the Au WF. In addition, the Au particles might penetrate into the CuPc layer during the thermal evaporation and cause a surface doping effect. After annealing the CuPc sample with a thin layer of Au on top, we observe a large increase (110 meV) in the WF to 4.98 eV. The same phenomenon is also found for the ITO/CuPc/Au (10 nm) samples (Figure 3d), where the WF increases from 4.98 eV to 5.07 eV after the heat treatment. Since the coverage of the Au layer on the CuPc film decreases after the heat treatment as confirmed by the SEM results above, the change in the WF cannot be due to the contribution of the Au WF during the measurement. A plausible explanation for the change in the WF of ITO/CuPc/Au (5 nm or 10 nm) is that during the heat treatment, some Au may diffuse into the CuPc bulk, inducing the relocation of the π-π stacking of the CuPc molecules. In other words, Au itself can induce a doping effect in the CuPc layer, which changes the WF of the material. Au-doping of CuPC is thus greatly enhanced by thermally enhanced diffusion. For reference, the sample with much thicker (40 nm) gold layer ITO/CuPc/Au (40 nm) shows a constant WF of 5.20 eV with and without the heat treatment, which is the same as the WF of the ITO/Au sample (Figure S14). This indicates that a 40 nm-thick gold layer is sufficient to fully cover the CuPc surface since UPS is only sensitive to the sample surface. The UPS full spectra of these samples are provided in Figure S15. The improved WF due to the Au doping of the heat-treated CuPc/Au would provide a better energy level alignment for the perovskite device as compared to the asprepared CuPc/Au, which may contribute to improved VOC and FF. To test this hypothesis, we first deposit 10 nm of Au on the PSCs (without the Au electrode) and heat-treat the



devices. We then further deposit 90 nm of Au on the annealed devices to complete the devices. As shown in Figure S16, the modified devices have slightly higher VOC (0.909 ± 0.051) compared to the as-prepared devices (0.883 ± 0.127). However, all the devices still show serious shunting and interface recombination issue resulting in low FF and inferior PCE. This indicates that the doping effect of Au on the CuPc film only contributes slightly to the cell performance improvement upon annealing. It further confirms that the great improvement in the cell performance is mainly due to the rearrangement of the Au particles migrating away from the cracks on the CuPc surface and therefore removing the shunting and reducing interface recombination in the devices.


Page 12 of 19

Page 13 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 3. UPS measurements for the samples with/without thermal annealing post-treatment. UPS spectra of (a) Glass/ITO/Au (100 nm). (b) Glass/ITO/CuPc. (c) Glass/ITO/CuPc/Au (5 nm). (d) Glass/ITO/CuPc/Au (10 nm). Cracking on thin films deposited by spin coating has been reported previously and proposed mechanisms have been identified40. As solvent evaporates and leaves the dispersion during film spinning, particles consolidate and get packed. Cracks develop during the film formation due to the capillary pressure induced by the solvent when interfacial tension exceeds the bonding strength of the closed-packed particles41-42. Several techniques can be applied to avoid the cracks such as reducing the film thickness and manipulating the drying mechanism. As the film thickness is already around 40 nm, which is required to fully cover the perovskite film underneath, it might not be possible to reduce the film thickness. Therefore, we try to manipulate film drying mechanism by using multiple organic solvents with different boiling points and evaporation rates for the CuPc precursor solution. When using Dipropyl Sulfide with a higher boiling point (143 oC) compared to Chlorobenzene (132 oC), the CuPc film is not continuous and only covers a small portion of the perovskite film underneath. As expected, the as-prepared cell is seriously shunted with a VOC of 0.29 V, a JSC of 3.8 mA/cm2 and a FF of 0.3. After the heat treatment, the shunts can only be partly removed and the performance is improved to a VOC of 0.922 V, a JSC of 15.9 mA/cm2 and a FF of 0.43 (Figure S17a). When using Toluene solvent with a slightly lower boiling point (110 oC), the cracks on the CuPc film persist at a similar density. The as-prepared cells show inferior performance and the performance is greatly improved after the heat treatment (Figure S17b). Moving to Diethyl Sulfide with considerably lower boiling point (90 oC), the cracks persist on the CuPc films however with lower density. Unfortunately, as-prepared devices still show pronounced shunts and interface recombination and the performance is again recovered after the heat treatment (Figure S17c). We also change from static spinning to dynamic spinning of the 13 ACS Paragon Plus Environment


CuPc precursor, however it is unable to solve the cracking problem (Figure S18). We suspect that the cracking on spin-coated films of this CuPc derivate is inherent to its particle packing configuration and bonding strength, and modification of the material chemistry might be required to resolve the cracking problem. This is however out of the scope of the current study. Finally, we assess the thermal stability (85 oC in the dark and N2 environment) of PSCs employing CuPc HTM. As shown in Figure 4, we find that the performance of perovskite cells with the quadruple perovskite composition gradually decreases to less than 80% of the original performance after 2000 hours. This is in fact due to the decomposition of the perovskite active layer with the presence of the MA cation in the quadruple-cation perovskite composition. MA has been shown to be intrinsically thermally unstable evidenced by the decomposition of perovskite to lead iodide43. We also observe that the colour of the cells turns to yellow after the aging test, which further confirms the hypothesis. By removing the MA component in the perovskite composition, we make PSCs with dual-cation (Cs/FA) perovskite composition. The efficiency of dual-cation PSCs is comparable to the quadruplecation devices as the average efficiency extracted from the reverse J-V scans is 18.6% (Figure S19a-d). The champion efficiency has an efficiency of 20% in the reverse scan, 18.2% in the forward scan and a steady state efficiency of 19.3% (Figure S19e, f). The dual-cation PSCs clearly show more pronounced hysteresis behaviour compared to the quadruple-cation devices. Nevertheless, the dual-cation PSCs show superior thermal stability compared to the quadruple-cation devices. All four dual-cation cells retain more than 90% of their initial performance after more than 2000 hours at 85 oC. In our preliminary light stability study, PSCs with dual-cation perovskite and CuPc also show some promise as the device retains more than 92% of the initial performance after operating at the maximum power point under continuous illumination for 100 hours (Figure S20). 14 ACS Paragon Plus Environment

Page 14 of 19

Page 15 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Figure 4. Thermal stability of perovskite solar cells using CuPc HTM and different perovskite compositions. The stability of four cells is studied in each condition. In conclusion, using a soluble commercially available CuPc derivative we achieve more than 20% efficiency for PSCs, which is comparable with state-of-the-art devices using high performing HTMs. We discover that cracks in the CuPc film lead to direct contact between the Au and the perovskite, which causes serious shunts and interface recombination. These problems can be resolved to negligible influence by heat treatment at 85 oC due to the rearrangement of Au particles on the metal contact migrating away from the cracks on the CuPc film during the heating. In addition, Au is found to dope the CuPc film and the doping effect is greatly enhanced by the heat treatment. In term of stability, PSCs with CuPc HTM show excellent thermal stability at 85 oC for more than 2000 hours and good light stability at 25 oC for 100 hours. The work has revealed the potential of using CuPc material to achieve low cost and stable perovskite solar cells.



Acknowledgements This work has been supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). Responsibility for the views, information or advice expressed herein is not accepted by the Australian Government. Part of the experiment was performed at Australian National Fabrication Facility (ANFF) ACT Node. The authors acknowledge the Department of Electronic Materials Engineering – ANU for providing GIXRD equipment access. T. D acknowledges the support of a Postdoc Fellowship from the Australian Centre for Advanced Photovoltaics (ACAP). J. X., D. Z. and J. L. acknowledge funding from MSTC (Grant No. 2016YFA0301300), NNSFC (Grant No. 11674402) and GSTP (Grant No. 201607010044, 201607020023).

Supporting Information Available: Experimental methods, Note on simulation protocols, Table S1, S2 and Figures (S1-S20) to support the arguments in the main paper. References

(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131 (17), 60506051. (2) Jeon, N. J.; Na, H.; Jung, E. H.; Yang, T.-Y.; Lee, Y. G.; Kim, G.; Shin, H.-W.; Il Seok, S.; Lee, J.; Seo, J. A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells. Nat. Energy 2018, 3 (8), 682-689. (3) NREL Efficiency Chart. https://www.nrel.gov/pv/assets/images/efficiency-chart20180716.jpg (accessed 10/09/2018). (4) Yang, D.; Yang, R.; Wang, K.; Wu, C.; Zhu, X.; Feng, J.; Ren, X.; Fang, G.; Priya, S.; Liu, S. High Efficiency Planar-Type Perovskite Solar Cells with Negligible Hysteresis Using Edta-Complexed SnO2. Nat. Commun. 2018, 9 (1), 3239. (5) Zhu, X.; Yang, D.; Yang, R.; Yang, B.; Yang, Z.; Ren, X.; Zhang, J.; Niu, J.; Feng, J.; Liu, S. Superior Stability for Perovskite Solar Cells with 20% Efficiency Using Vacuum CoEvaporation. Nanoscale 2017, 9 (34), 12316-12323. (6) Niu, J.; Yang, D.; Ren, X.; Yang, Z.; Liu, Y.; Zhu, X.; Zhao, W.; Liu, S. GrapheneOxide Doped PEDOT:PSS as a Superior Hole Transport Material for High-Efficiency Perovskite Solar Cell. Org. Electron. 2017, 48, 165-171. (7) Zheng, X.; Chen, B.; Dai, J.; Fang, Y.; Bai, Y.; Lin, Y.; Wei, H.; Zeng, Xiao C.; Huang, J. Defect Passivation in Hybrid Perovskite Solar Cells Using Quaternary Ammonium Halide Anions And cations. Nat. Energy 2017, 2, 17102. (8) Luo, D.; Yang, W.; Wang, Z.; Sadhanala, A.; Hu, Q.; Su, R.; Shivanna, R.; Trindade, G. F.; Watts, J. F.; Xu, Z., et al. Enhanced Photovoltage for Inverted Planar Heterojunction Perovskite Solar Cells. Science 2018, 360 (6396), 1442-1446. (9) Duong, T.; Wu, Y.; Shen, H.; Peng, J.; Fu, X.; Jacobs, D.; Wang, E. C.; Kho, T. C.; Fong, K. C.; Stocks, M., et al. Rubidium Multication Perovskite with Optimized Bandgap for Perovskite‐Silicon Tandem with over 26% Efficiency. Adv. Energy Mater. 2017, 7 (14), 1700228. (10) Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T., et al. 23.6%-Efficient 16 ACS Paragon Plus Environment

Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009. (11) Sahli, F.; Werner, J.; Kamino, B. A.; Bräuninger, M.; Monnard, R.; Paviet-Salomon, B.; Barraud, L.; Ding, L.; Diaz Leon, J. J.; Sacchetto, D., et al. Fully Textured Monolithic Perovskite/Silicon Tandem Solar Cells with 25.2% Power Conversion Efficiency. Nat. Mater. 2018, 17 (9), 820-826. (12) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5 (20), 1500963. (13) Habisreutinger, S. N.; Leijtens, T.; Eperon, G. E.; Stranks, S. D.; Nicholas, R. J.; Snaith, H. J. Carbon Nanotube/Polymer Composites as a Highly Stable Hole Collection Layer in Perovskite Solar Cells. Nano Lett. 2014, 14 (10), 5561-5568. (14) Domanski, K.; Correa-Baena, J.-P.; Mine, N.; Nazeeruddin, M. K.; Abate, A.; Saliba, M.; Tress, W.; Hagfeldt, A.; Grätzel, M. Not All That Glitters Is Gold: Metal-MigrationInduced Degradation in Perovskite Solar Cells. ACS Nano 2016, 10 (6), 6306-6314. (15) Cacovich, S.; Cina, L.; Matteocci, F.; Divitini, G.; Midgley, P. A.; Di Carlo, A.; Ducati, C. Gold and Iodine Diffusion in Large Area Perovskite Solar Cells under Illumination. Nanoscale 2017, 9 (14), 4700-4706. (16) Kim, J. H.; Liang, P.-W.; Williams, S. T.; Cho, N.; Chueh, C.-C.; Glaz, M. S.; Ginger, D. S.; Jen, A. K.-Y. High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide HoleTransporting Layer. Adv. Mater. 2015, 27 (4), 695-701. (17) Yue, S.; Liu, K.; Xu, R.; Li, M.; Azam, M.; Ren, K.; Liu, J.; Sun, Y.; Wang, Z.; Cao, D., et al. Efficacious Engineering on Charge Extraction for Realizing Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10 (12), 2570-2578. (18) Chen, W.; Liu, F.-Z.; Feng, X.-Y.; Djurišić, A. B.; Chan, W. K.; He, Z.-B. Cesium Doped Niox as an Efficient Hole Extraction Layer for Inverted Planar Perovskite Solar Cells. Adv. Energy Mater. 2017, 7 (19), 1700722. (19) Cao, J.; Yu, H.; Zhou, S.; Qin, M.; Lau, T.-K.; Lu, X.; Zhao, N.; Wong, C.-P. LowTemperature Solution-Processed Niox Films for Air-Stable Perovskite Solar Cells. J. Mater. Chem. A 2017, 5 (22), 11071-11077. (20) Liu, Z.; Zhu, A.; Cai, F.; Tao, L.; Zhou, Y.; Zhao, Z.; Chen, Q.; Cheng, Y.-B.; Zhou, H. Nickel Oxide Nanoparticles for Efficient Hole Transport in P-I-N and N-I-P Perovskite Solar Cells. J. Mater. Chem. A 2017, 5 (14), 6597-6605. (21) Arora, N.; Dar, M. I.; Hinderhofer, A.; Pellet, N.; Schreiber, F.; Zakeeruddin, S. M.; Grätzel, M. Perovskite Solar Cells with Cuscn Hole Extraction Layers Yield Stabilized Efficiencies Greater Than 20%. Science 2017, 358 (6364), 768-771. (22) Yu, W.-L.; Pei, J.; Cao, Y.; Huang, W. Hole-Injection Enhancement by Copper Phthalocyanine (Cupc) in Blue Polymer Light-Emitting Diodes. J. Appl. Phys. 2001, 89 (4), 2343-2350. (23) Xue, J.; Uchida, S.; Rand, B. P.; Forrest, S. R. 4.2% Efficient Organic Photovoltaic Cells with Low Series Resistances. Appl. Phys. Lett. 2004, 84 (16), 3013-3015. (24) Kumar, C. V.; Sfyri, G.; Raptis, D.; Stathatos, E.; Lianos, P. Perovskite Solar Cell with Low Cost Cu-Phthalocyanine as Hole Transporting Material. RSC Adv. 2015, 5 (5), 3786-3791. (25) Ke, W.; Zhao, D.; Grice, C. R.; Cimaroli, A. J.; Fang, G.; Yan, Y. Efficient FullyVacuum-Processed Perovskite Solar Cells Using Copper Phthalocyanine as Hole Selective Layers. J. Mater. Chem. A 2015, 3 (47), 23888-23894. (26) Xiaoqing, J.; Ze, Y.; Jianbo, L.; Yuchen, Z.; Maowei, H.; Ning, L.; Dongping, W.; Xichuan, Y.; Licheng, S. Interfacial Engineering of Perovskite Solar Cells by Employing a 17 ACS Paragon Plus Environment


Hydrophobic Copper Phthalocyanine Derivative as Hole-Transporting Material with Improved Performance and Stability. ChemSusChem 2017, 10 (8), 1838-1845. (27) Liu, X.; Wang, Y.; Rezaee, E.; Chen, Q.; Feng, Y.; Sun, X.; Dong, L.; Hu, Q.; Li, C.; Xu, Z.-X. Tetra-Propyl-Substituted Copper (Ii) Phthalocyanine as Dopant-Free Hole Transporting Material for Planar Perovskite Solar Cells. Solar RRL 2018, 2 (7), 1800050. (28) Jiang, X.; Yu, Z.; Li, H.-B.; Zhao, Y.; Qu, J.; Lai, J.; Ma, W.; Wang, D.; Yang, X.; Sun, L. A Solution-Processable Copper(Ii) Phthalocyanine Derivative as a Dopant-Free HoleTransporting Material for Efficient and Stable Carbon Counter Electrode-Based Perovskite Solar Cells. J. Mater. Chem. A 2017, 5 (34), 17862-17866. (29) Jiang, X.; Yu, Z.; Lai, J.; Zhang, Y.; Lei, N.; Wang, D.; Sun, L. Efficient Perovskite Solar Cells Employing a Solution-Processable Copper Phthalocyanine as a Hole-Transporting Material. Sci. China: Chem. 2017, 60 (3), 423-430. (30) Kim, Y.; Yang, T.-Y.; Jeon, N.; Im, J.; Jang, S.; Shin, T.; Shin, H.-W.; Kim, S.; Lee, E.; Noh, J. Engineering Interface Structures between Lead Halide Perovskite and Copper Phthalocyanine for Efficient and Stable Perovskite Solar Cells. Energy Environ. Sci. 2017, 10 (10), 2109-2116. (31) Peng, J.; Wu, Y.; Ye, W.; Jacobs, D. A.; Shen, H.; Fu, X.; Wan, Y.; Duong, T.; Wu, N.; Barugkin, C., et al. Interface Passivation Using Ultrathin Polymer-Fullerene Films for High-Efficiency Perovskite Solar Cells with Negligible Hysteresis. Energy Environ. Sci. 2017, 10 (8), 1792-1800. (32) Zhao, X.; Kim, H.-S.; Seo, J.-Y.; Park, N.-G. Effect of Selective Contacts on the Thermal Stability of Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2017, 9 (8), 71487153. (33) Domanski, K.; Correa-Baena, J.-P.; Mine, N.; Nazeeruddin, M. K.; Abate, A.; Saliba, M.; Tress, W.; Hagfeldt, A.; Grätzel, M. Not All That Glitters Is Gold: Metal-MigrationInduced Degradation in Perovskite Solar Cells. ACS Nano 2016, 10 (6), 6306-6314. (34) Walter, D.; Fell, A.; Wu, Y.; Duong, T.; Barugkin, C.; Wu, N.; White, T.; Weber, K. Transient Photovoltage in Perovskite Solar Cells: Interaction of Trap-Mediated Recombination and Migration of Multiple Ionic Species. J. Phys. Chem. C 2018, 122 (21), 11270-11281. (35) Fell, A. A Free and Fast Three-Dimensional/Two-Dimensional Solar Cell Simulator Featuring Conductive Boundary and Quasi-Neutrality Approximations. IEEE Trans. Electron Devices 2013, 60 (2), 733-738. (36) Yu, R.; Song, H.; Zhang, X.-F.; Yang, P. Thermal Wetting of Platinum Nanocrystals on Silica Surface. J. Phys. Chem. B 2005, 109 (15), 6940-6943. (37) Cao, A.; Lu, R.; Veser, G. Stabilizing Metal Nanoparticles for Heterogeneous Catalysis. Phys. Chem. Chem. Phys. 2010, 12 (41), 13499-13510. (38) Laban, W. A.; Etgar, L. Depleted Hole Conductor-Free Lead Halide Iodide Heterojunction Solar Cells. Energy Environ. Sci. 2013, 6 (11), 3249-3253. (39) Yang, G.; Wang, Y.-L.; Xu, J.-J.; Lei, H.-W.; Chen, C.; Shan, H.-Q.; Liu, X.-Y.; Xu, Z.-X.; Fang, G.-J. A Facile Molecularly Engineered Copper (Ii) Phthalocyanine as Hole Transport Material for Planar Perovskite Solar Cells with Enhanced Performance and Stability. Nano Energy 2017, 31, 322-330. (40) Lee, W. P.; Routh, A. F. Why Do Drying Films Crack? Langmuir 2004, 20 (23), 9885-9888. (41) Prosser, J. H.; Brugarolas, T.; Lee, S.; Nolte, A. J.; Lee, D. Avoiding Cracks in Nanoparticle Films. Nano Lett. 2012, 12 (10), 5287-91. (42) Petersen, C.; Heldmann, C.; Johannsmann, D. Internal Stresses During Film Formation of Polymer Latices. Langmuir 1999, 15 (22), 7745-7751.


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(43) Bert, C.; Jeroen, D.; Nicolas, G.; Aslihan, B.; Jan, D. H.; Lien, D. O.; Anitha, E.; Jo, V.; Jean, M.; Edoardo, M., et al. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5 (15), 1500477.


Perovskite Solar Cells Employing Copper ... - ACS Publications

Recommend Documents