Efficient Carbon-Based CsPbBr3 Inorganic Perovskite Solar Cells by Using Cu-Phthalocyanine as Hole Transport Material
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KeywordsPerovskite solar cells (PSCs) Metal halide CsPbBr3 Cu-phthalocyanine (CuPc) Carbon electrode
Cu-phthalocyanine was employed as hole transport material for CsPbBr3 inorganic perovskite solar cells.
The optimal device acquires a decent power conversion efficiency of 6.21%, over 60% higher than those of the hole transport material-free devices.
The device exhibits an outstanding durability and a promising thermal stability.
Organic–inorganic perovskite solar cells (PSCs) are appearing as a hopeful new generation of photovoltaic technology and have revolutionized the prospects of emerging photovoltaic industry, because of the tremendous increase in device performance [1, 2, 3, 4, 5, 6]. The outstanding photoelectric properties, such as high absorption coefficient, suitable and adjustable band gap [7, 8, 9], ambipolar charge transport [10, 11, 12, 13], and long carrier diffusion length [14, 15], make perovskite materials very appropriate for light harvesting in photovoltaics. Since the breaking report from Miyasaka , power conversion efficiency (PCE) of such PSCs has reached a remarkable value (over 22%) in a short span [17, 18, 19], approaching the efficiency of commercialized c-Si solar cells and thin-film photovoltaic solar cells such as CdTe and Cu2ZnSn(Se,S)4 . Despite the rapid increment in PCE associated with the evolution of new perovskite materials and novel fabrication techniques, the instability of PSCs remains unresolved. The mostly studied hybrid perovskite materials, for example methylammonium lead triiodide (MAPbI3) and formamidinium lead triiodide (FAPbI3), are forceless against moisture and heat. Some organic additives in commonly used HTMs, such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and tert-butylpyridine (tBP), are also hygroscopic and deliquescent, accelerating performance degradation [21, 22, 23, 24]. Thus, precise environmental controls (gloveboxes or dryrooms) are often necessary during the fabrication of organic–inorganic hybrid PSCs. On the other side, efficient PSCs generally employ a p-type organic small-molecule or polymeric hole conductor, such as 2,2′,7,7′-tetrakis (N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) , poly(3-hexylthiophene) (P3HT) , and poly(triarylamine) (PTAA)  as hole-extraction materials to boost device efficiencies. Discouragingly, these conventional HTMs suffer from disadvantages of high synthetic cost, thermal and chemical instability, and low hole mobility or low conductivity in their pristine form [28, 29, 30], seriously hindering the viable commercialization of the emerging PSC technology. The necessary doping techniques involved in improving their carrier density and conductivity further increase the cost in production. In addition, the high-energy-consuming coating process together with the consumption of noble metals as counter electrode (such as Au and Ag, widely used in efficient state-of-the-art PSCs) gives another problem for the commercialization of PSCs. To sum up, there are mainly three cruxes for the future up-scaling of PSCs: (1) exploring novel perovskite materials and HTMs with high stability against humidity and heat; (2) developing efficient, low-cost, durable, and scalable alternative HTMs that can replace currently used organic ones; (3) searching for low-cost and scalable substitutions for noble counter electrodes.
It has been proposed that inorganic perovskites (e.g., CsPbI3 and CsPbBr3) are more stable than organic ones, due to smaller ionic radius of Cs+ than those of FA+ and MA+ cations. Many works on PSCs with inorganic perovskites as light absorber have been reported. Tan et al.  incorporated Cs+ into MA/FA hybrid perovskite to improve the photostability of solar cells. Luo et al.  prepared a CsPbI3 HTM-based PSC under fully open-air conditions with a PCE of 4.13%. Kulbak et al.  reported CsPbBr3 PSCs with different HTMs and achieved a highest PCE of 6.2%. Sutton et al.  demonstrated a CsPbI2Br-based inorganic mixed halide PSC with an efficiency up to 9.8% and high ambient stability. Both Chen’s group and Liu’s group proposed a kind of carbon-based CsPbBr3 all-inorganic PSCs and achieved optimal efficiencies of 5.0%  and 6.7% , respectively. All these PSCs using inorganic perovskite have demonstrated a relatively enhanced stability. On the other hand, p-type semiconductor CuPc, small molecular HTMs with planar configuration, is preferable in fabricating stable and efficient traditional organic PSCs [37, 38, 39]. It owns properties of low cost, ease of synthesis, low band gap, high hole mobility of 10−3–10−2 cm2 V−1 S−1 (as compared with 4 × 10−5 cm2 V−1 S−1 for spiro-OMeTAD) , good stability (starting degradation above 500 °C in air), and long exciton diffusion length (Lex ranging from 8 to 68 nm) [41, 42, 43]. Nonetheless, CuPc is never reported as HTM in inorganic perovskite photovoltaic devices. Besides, novel counter electrodes including Al , Ni , and carbon [46, 47, 48] have been explored in PSCs recently. Among them, carbon is thought to be the most promising for the electrode material because carbon is cheap, stable, inert to ion migration originating from perovskite and metal electrodes, inherently water-resistant, and therefore advantageous for good stability. The emergence of carbon counter electrode-based PSCs greatly lowers the cost and simplifies the procedures, rolling forward the development and commercialization of PSCs .
In this work, CuPc were introduced as HTM in carbon counter electrode-based CsPbBr3 inorganic PSCs. For comparison, HTM-free PSCs were also made as the control devices. The optimal CuPc-based device performance with an efficiency of 6.21% has been achieved, 63% higher than the HTM-free device. Systematic characterization and analysis were performed to reveal the underlying mechanism of the improvement originated from the CuPc HTM layer. Our results suggest that introducing CuPc between the perovskite layer and carbon electrode provides a simple and effective route to facilitate charge transfer and suppress charge recombination in PSCs. More importantly, our devices exhibit an outstanding durability and a promising thermal stability, compared with the HTM-free CsPbBr3 devices and traditional MAPbI3 devices.
3 Experimental Section
3.1 Synthesis of Carbon Paste
One gram polyvinyl acetate (PVAc) and 0.5 g hydroxypropyl cellulose were dissolved in 60 mL ethyl acetate. PVAc acted as the binder in the carbon film, and hydroxypropyl cellulose was used to adjust the viscosity of the carbon paste. 20 mL of the mixed ethyl acetate solution was blended with 2 g 40-nm graphite powder, 1 g 10-μm flake graphite, 1 g 40-nm carbon black, and 0.5 g 50-nm ZrO2 powder. The ZrO2 particles were introduced to enhance the scratch resistance performance of the carbon film [50, 51]. After vigorously milling for 2 h in an electro-mill (QM-QX0.4, Instrument Factory of Nanjing University), the printable carbon paste was ready.
3.2 Device Fabrication
Perovskite thin film and solar cells were fabricated on fluorine-doped tin oxide (FTO)-glass substrate with the sheet resistance of 14 Ω sq−1. Diluted hydrochloric acid (2 mol L−1) and zinc powder were used to pattern the fluorine-doped tin oxide substrates. After ultrasonically cleaned by acetone, ethanol, and deionized (DI) water, the FTO substrates were treated under oxygen plasma for 30 min to remove the last traces of organic residues. A thin layer of compact anatase TiO2 with 50 nm in thickness was deposited by spin-coating a mildly acidic solution of titanium isopropoxide in ethanol at 5000 rpm for 60 s and consequently annealed at 500 °C for 30 min. After cooling down to room temperature, the mesoporous TiO2 scaffold (particle size 20 nm) was formed by spin-coating TiO2 paste (DSL. 18NR-T, 20 nm, Dyesol, Australia) diluted in ethanol (2:7 weight ratio) at 5000 rpm for 60 s and consequently heating at 500 °C for 30 min. The CsPbBr3 perovskite layer was prepared by a sequential method. 1.47 g PbBr2 was dissolved in 4 mL N,N-dimethylformamide (DMF) and heated at 80 °C for 12 h under magnetic stirring. The prepared mesoporous TiO2 films were preheated to ~ 80 °C and then infiltrated with the PbBr2 precursor solution by spin-coating at 2000 rpm for 45 s and dried at 80 °C for 30 min immediately. Sequentially, the PbBr2 films were immersed in a methanol solution of 0.07 M CsBr for 15 min. After rinsed by 2-propanol and dried in air, the samples were heated to 250 °C for 5 min on a hotplate to form a uniform layer of CsPbBr3. CuPc was deposited on the perovskite film by vacuum evaporation (< 1 × 10−3 Pa) using quartz crystal monitor to determine the thickness and deposition rate. The deposition of carbon CE was conducted by doctor blade method and dried at 80 °C for 15 min. All these procedures were carried out on naturally ambient atmosphere.
The morphology of the perovskite surface and cross-sectional structure of the solar cells was observed by the field emission scanning electron microscopy (FESEM, JSM-7600F, JEOL). The formation of CsPbBr3 perovskite absorber layer has been further confirmed by X-ray diffraction (XRD) analysis (PANalytical PW3040/60) with Cu Kα radiation (λ = 1.5406 Å) from 10° to 50°. An X-ray photoelectron spectrometer (XPS, Axis Ultra DLD, Shimadzu) equipped with a monochromatic Al Kα source (1486.6 eV) was employed to determine the surface chemical composition of CsPbBr3 perovskite film. The Raman spectra of the CuPc film on glass substrate were performed by a Raman spectrometer (LabRAM HR800, Horiba JobinYvon) with a 532 nm laser source. All the XPS spectra were obtained in the constant pass energy mode, where the pass energy of the analyzer was set at 20 eV. Here the binding energy of the C 1s peak (285 eV) arising from adventitious carbon was used for the energy calibration. UV–Vis spectrophotometer (UV 2600, Shimadzu) was utilized to obtain the absorption spectra of CsPbBr3 and CsPbBr3/CuPc films. The steady-state photoluminescence measurements were taken using a spectrometer (LabRAM HR800, Horiba JobinYvon) under an excitation laser with a wavelength of 325 nm. The time-resolved photoluminescence decay transients were measured at 525 nm using excitation with a 478-nm light pulse from a HORIBA Scientific DeltaPro fluorimeter. Current density–voltage (J–V) curves were recorded under AM 1.5, 100 mW cm−2 simulated sunlight (Oriel 94043A, Newport Corporation, Irvine, CA, USA) with an electrochemical station (Autolab PGSTAT302 N, Metrohm Autolab, Utrecht, The Netherlands), previously calibrated with an NREL-calibrated Si solar cell. The measurements were taken with a black metal mask with a circular aperture (0.071 cm2) smaller than the active area of the square solar cell (1.5 × 1.5 cm2). The incident photon to current conversion efficiency (IPCE) was performed employing a xenon lamp coupled with a monochromator (TLS1509, Zolix) controlled by a computer.
4 Results and Discussion
The excess PbBr2 or the poor solubility of CsBr in methyl alcohol could facilitate the transformation at a low temperature . CsPb2Br5 crystal is reported to exhibit an inactive photoluminescence behavior and a large indirect band gap of approximately 3.1 eV , which are unfavorable in the application of photovoltaic device and need to be eliminated by future process optimization. After coated by CuPc, the XRD patterns of the film show negligible changes, mainly due to the amorphous state of the CuPc . In the Raman spectra (Fig. S4), the peak at 680.2 cm−1 is ascribed to the breathing vibration band of phthalocyanine ring, the peak at 1140.5 cm−1 is ascribed to the breathing vibration band of benzene ring, and the peaks at 1137.5, 1452.0, and 1526.5 cm−1 are attributed to the stretching vibration band of C–C, C–N, and C=C bond, respectively . UV–Vis absorbance spectra of the CuPc, FTO/TiO2/CsPbBr3, and FTO/TiO2/CsPbBr3/CuPc are also demonstrated in Fig. 2f. The CsPbBr3 film strongly absorbs light with the wavelength between 300 and 540 nm, owning to the relatively wide band gap (2.3 eV) as shown in Fig. S5. Pristine CuPc demonstrates a wide spectral ranging from 500 to 800 nm, and peaks at 625 and 696 nm, which are ascribed to the Q-band of CuPc. The peak at 625 nm is the absorbance peak of the CuPc dimer, and the peak at 696 nm comes from the CuPc monomer [63, 64]. In the presence of CuPc, an enhancement in absorption is observed, especially in the region of 537–800 nm. Correspondingly, the color of the films changes from golden yellow to light green, as shown in the inset of Fig. 2f.
Photovoltaic performance of the TiO2/CsPbBr3/CuPc/carbon devices and TiO2/CsPbBr3/carbon devices measured under simulated AM 1.5G (100 mW cm−2) condition
JSC (mA cm−2)
Long-term stability is a critical concern for practical applications of PSCs. Figure 6c presents the room-temperature stability test of the CuPc-based CsPbBr3 PSCs in comparison with the HTM-free CsPbBr3 devices and the classical CH3NH3PbI3/carbon devices. The devices without encapsulation were stored in dark with a humidity of 30–40% RH. Both the CuPc-based CsPbBr3 PSCs and HTM-free CsPbBr3 PSCs exhibit excellent stability beyond 2000 h, while the organic ones start degrading at 800 h. Thermal stability of the devices was further evaluated in a harsh environment (the humidity of 70–80% RH and the temperature of 100 °C), as shown in Fig. 6d. Obviously, the performance of CH3NH3PbI3/carbon devices decays rapidly, since the high humidity and high storing temperature accelerate the degradation of CH3NH3PbI3 perovskite light absorber. The HTM-free CsPbBr3 devices also show a PCE loss of 37% after 944 h, similar to the previous research . However, the CuPc-based CsPbBr3 devices show an outstanding thermal stability (without evident decay) during the whole testing period. The organic CH3NH3+ cation is more vulnerable to moisture and has higher volatility than the inorganic Cs+ cation, leading to rapid degradation of the CH3NH3PbI3 devices under relatively high RH and temperature environment . The introduction of the CuPc film and carbon film, which can act as shields to prevent the deliquescing of the underlying perovskite layer, obtains the best hydrophobicity and thus results in the best stability of CuPc-based CsPbBr3 PSCs.
In summary, cost-effective p-type material CuPc was introduced as HTM layer in the carbon-based CsPbBr3 inorganic PSCs. The deposited CuPc layer exhibits a nanorods morphology and an intimate contact with the perovskite layer, preventing direct contact between the perovskite layer and carbon electrode. The CuPc layer can effectively extract the photon-generated carriers and accelerate the hole-diffusion process, obtaining a decent PCE (6.21%) with high reproducibility. Compared with HTM-free CsPbBr3/carbon devices, the enhanced PCE may be ascribed to a more efficient charge transfer and a more suppressed charge recombination. Moreover, the newly developed devices demonstrate a dramatically enhanced durability under ambient atmosphere and a promising thermal stability in relatively harsh condition. The enhanced PCE and excellent stability of our devices offer a new device designing strategy and promise a reality of commercial application for PSCs with cost-effective, mass manufacturing solar technology that is compatible with current large-scale printing infrastructure.
The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 51675210 and 51675209) and the China Postdoctoral Science Foundation (Grant No. 2016M602283). We thank the Analytical and Testing Center of Huazhong University of Science and Technology for the field emission scanning electron microscopy (FESEM). We also thank the assistance from Prof. Hongwei Han and Dr. Miao Duan at Michael Grätzel Center for Mesoscopic Solar Cells of Huazhong University of Science and Technology for the time-resolved photoluminescence measurements.
- 14.S.D. Stranks, G.E. Eperon, G. Grancini, C. Menelaou, M.J. Alcocer, T. Leijtens, L.M. Herz, A. Petrozza, H.J. Snaith, Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 342(6156), 341–344 (2013). https://doi.org/10.1126/science.1243982 CrossRefGoogle Scholar
- 18.D. Bi, C. Yi, J. Luo, J.-D. Décoppet, F. Zhang, S.M. Zakeeruddin, X. Li, A. Hagfeldt, M. Grätzel, Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%. Nat. Energy 1, 16142 (2016). https://doi.org/10.1038/nenergy.2016.142 CrossRefGoogle Scholar
- 19.Best research-cell efficiencies NREL (2016). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg
- 23.W.H. Nguyen, C.D. Bailie, E.L. Unger, M.D. McGehee, Enhancing the hole-conductivity of spiro-OMeTAD without oxygen or lithium salts by using spiro(TFSI)2 in perovskite and dye-sensitized solar cells. J. Am. Chem. Soc. 136(31), 10996–11001 (2014). https://doi.org/10.1021/ja504539w CrossRefGoogle Scholar
- 29.J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N.-L. Cevey-Ha, C. Yi, M.K. Nazeeruddin, M. Grätzel, Tris (2-(1 H-pyrazol-1-yl) pyridine) cobalt (III) as p-type dopant for organic semiconductors and its application in highly efficient solid-state dye-sensitized solar cells. J. Am. Chem. Soc. 133(45), 18042–18045 (2011). https://doi.org/10.1021/ja207367t CrossRefGoogle Scholar
- 30.T. Leijtens, J. Lim, J. Teuscher, T. Park, H.J. Snaith, Charge density dependent mobility of organic hole-transporters and mesoporous TiO2 determined by transient mobility spectroscopy: implications to dye-sensitized and organic solar cells. Adv. Mater. 25(23), 3227–3233 (2013). https://doi.org/10.1002/adma.201300947 CrossRefGoogle Scholar
- 40.X. Zhang, M. Yang, W. Cheng, L. Wang, Sun, Boosting the efficiency and the stability of low cost perovskite solar cells by using CuPc nanorods as hole transport material and carbon as counter electrode. Nano Energy 20, 108–116 (2016). https://doi.org/10.1016/j.nanoen.2015.11.034 CrossRefGoogle Scholar
- 43.E. Nouri, Y. Wang, Q. Chen, J. Xu, G. Paterakis et al., Introduction of graphene oxide as buffer layer in perovskite solar cells and the promotion of soluble n-butyl-substituted copper phthalocyanine as efficient hole transporting material. Electrochim. Acta 233, 36–43 (2017). https://doi.org/10.1016/j.electacta.2017.03.027 CrossRefGoogle Scholar
- 62.K. Ishii, S. Mitsumura, Y. Hibino, R. Hagiwara, H. Nakayama, Preparation of phthalocyanine and octacyanophthalocyanine films by CVD on metal surfaces, and in SITU observation of the molecular processes by Raman spectroscopy. Appl. Surf. Sci. 33, 1324–1331 (1988). https://doi.org/10.1016/0169-4332(88)90451-5 CrossRefGoogle Scholar
- 78.X. Huang, Z. Hu, J. Xu, P. Wang, L. Wang, J. Zhang, Y. Zhu, Low-temperature processed SnO2 compact layer by incorporating TiO2 layer toward efficient planar heterojunction perovskite solar cells. Sol. Energy Mater. Sol. Cells 164, 87–92 (2017). https://doi.org/10.1016/j.solmat.2017.02.010 CrossRefGoogle Scholar
- 85.D. Song, P. Cui, T. Wang, D. Wei, M. Li et al., Managing carrier lifetime and doping property of lead halide perovskite by postannealing processes for highly efficient perovskite solar cells. J. Phys. Chem. C 119(40), 22812–22819 (2015). https://doi.org/10.1021/acs.jpcc.5b06859 CrossRefGoogle Scholar
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