Y. ZUO et al.: HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS IMPROVED WITH LOW-COST ORTHORHOMBIC ... 483–486 HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS IMPROVED WITH LOW-COST ORTHORHOMBIC Cu 2 ZnSnS 4 AS THE HOLE-TRANSPORTING LAYER VISOKO U^INKOVITE PEROVSKITNE SON^NE CELICE, IZBOLJ[ANE Z NIZKOCENOVNIM ORTOROMBSKIM Cu 2 ZnSnS 4 , DELUJO^IM KOT TRANSPORTNA PLAST ZA PRAZNINE Yinze Zuo, Liang Chen, Wenlong Jiang**, Biao Liu, Chao Zeng, Meng Li, Xingling Shi* School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003 sxl_just@163.com * ,957661528@qq.com ** Prejem rokopisa – received: 2017-10-17; sprejem za objavo – accepted for publication: 2018-02-15 doi:10.17222/mit.2017.178 Quaternary copper chalcogenide Cu2ZnSnS4 (CZTS) nanoparticles (NPs) were synthesized with the sol-gel technique and used as the hole-transporting layer (HTL), achieving a power-conversion efficiency (PCE) of 13.41 %, which is the highest PCE for Cu-based inorganic HTLs. The effects of the CZTS HTL on the optical absorption, crystallinity, crystal plane orientation and hysteresis of the J-V curve for the devices with a perovskite film were investigated and compared with spiro-OMeTAD, revealing the role of CZTS in efficient hole transporting in perovskite solar cells. In addition to the traditional use of the light absorber, this study reveals a new role of CZTS in photovoltaics, acting as an HTL, providing a completely different insight into the CZTS development. Keywords: Cu 2 ZnSnS 4 , perovskite, solar cells, hole-transporting layer Avtorji prispevka so sintetizirali nanodelce (NPs; angl.: Nano-Particles) kvaternarnega bakrovega halkogenida Cu2ZnSnS4 (CZTS) s sol–gel tehniko in jih uporabili kot transportno plast za praznine (HTL; angl.: hole transporting layer). S tem so dosegli 13,41 % u~inkovitost pretvorbe mo~i (PCE; angl.: Power Conversion Efficiency), kar je do sedaj najvi{ja dose`ena PCE z anorganskimi HTL-ji na osnovi Cu. Avtoriji so raziskali vpliv CZTS HTL na opti~no absorpcijo, kristalini~nost, orientacijo kristalne ravnine in histerezo J-V krivulje za naprave s perovskitnim filmom in ga primerjali s spiro-OMeTAD ter odkrili vlogo CZTS pri u~inkovitosti transporta praznin v perovskitnih son~nih celicah. Ta {tudija, dodatno k tradicionalni uporabi svetlobnih absorberjev, odkriva nov pomen CZTS v fotovoltaiki kot HTL, kar omogo~a popolnoma nov pogled v razvoju CZTS. Klju~ne besede: Cu2ZnSnS4, perovskit, son~ne celice, transportna plast praznin 1 INTRODUCTION In recent years, new energy materials have attracted more and more attention of the researchers. 1–4 Solar energy is seen as one of the most promising renewable energy forms in the 21 st century. The trend towards a widespread affordable use of solar energy has aroused the researchers’ attention to thin-film-based solar cells. 5 Especially metal halide perovskites (CH3NH3PbX3, X=Cl-,Br-,I-) perovskite-based hybrid solar cells are currently one of the most competitive candidates for the fabrication of solar cells as they were reported to achieve a remarkably high PCE of 26 %. 6 Different types of hole-transport materials (HTMs) such as poly(triarylamine)(PTAA), poly (3-hexylthio- phee-2,5-diyl(P3HT), poly[2,6-(4,4-bis(2-ethylhexyl)- 4Hcyclopenta[2,1-b;3,4-b’]dithiophene)-alt-4,7-(2,1,3-be nzothiadiazole)(PCPDTBT), spiro-OMeTAD, poly (3-hexylthiophene-2,5-diyl) (P3HT), 4-(diethylamino)- benzaldehyde diphenylhydrazone (DEH), MoO 3 and CuSCN were developed to fabricate perovskite-solar-cell devices. 7–10 Organic HTMs called spiro-OMeTAD and PTAA have been widely used and achieved a PCE reaching 20.54 %. However, these materials are easy to degrade and expensive for fabricating solar cells because of their complicated synthetic procedures and high- purity requirement. 11,12 Inorganic materials are advant- ageous in terms of a facile synthesis, high stability and low cost. Up to now, a few inorganic HTMs were incor- porated into PSCs, including MoO 3 , CuI and CuSCN. For instance, Seigo Ito et al. used CuSCN as the HTM for solar cells and achieved a PCE of 4.86 %, which was lower than that obtained for the spiro-OMeTAD HTM-based device. 10 After that, the CuI HTM was also applied by Qin et al. to fabricate high-efficiency and stable perovskite solar cells using a low-temperature solution-process deposition method, leading to a high PCE of 6.6 %. 11 These limited reports demonstrate the great potential of Cu-based inorganic HTMs for fabri- cating perovskite solar cells. But, the PCE of the devices using inorganic HTMs are lower than in the case of organic HTMs. Therefore, large-scale applications of perovskite solar cells are impractical, and it is preferable to develop other HTMs with lower costs. The CZTS belonging to the colloidal semiconductor, which is similar to CuSCN and CuI, exhibits a combi- Materiali in tehnologije / Materials and technology 52 (2018) 4, 483–486 483 UDK 620.3:549.641:621.383.51 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(4)483(2018) nation of attractive electronic and optical properties, such as low toxicities, the band gaps ( 1.5 eV) suitable for the solar-energy conversion, earth-abundant elemen- tal constituents and low costs. Now, the best PCE of thin-film solar-cell devices using CZTS as the light absorber was over 9 %. 13 Few papers reported on perovskite solar cells based on CZTS HTL. In this work, we synthesized smooth CZTS NPs with different phase structures and employed them on a perovskite layer as an HTL in a perovskite solar cell. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectroscopy and PCE experiments suggest that CZTS could be a good choice for the HTL to replace expensive spiro-OMeTAD in large-scale applications of perovskite solar cells. 2 EXPERIMENTAL PART The TiO 2 compact-film precursor solution in ethanol consists of 0.3 M titanium isopropoxide (Sigma–Aldrich, 99.999 %) and 0.01 M HCl. A dense TiO 2 film was applied onto an F-doped SnO 2 (FTO) substrate by spinning titanium precursor at 5000 min –1 , followed by annealing at 500 °C for 1 h. The precursor solution was deposited onto the TiO 2 /FTO substrate by spin-coating at 3000 min –1 for 18 s. During this period, 40 μL chloro- benzene was dropped on the perovskite film to recrys- tallize it in 13 s. After annealing the film on a hotplate at 100 °C for 0.5 h, the color of the film turned from yellow to black, indicating the formation of perovskite. Sub- sequently, CZTS was deposited on the perovskite film to function as the HTL. For comparison, a reference PSC device based on a spiro-OMeTAD HTL was also fabri- cated following the procedure reported in the literature. 10 Finally, an Ag electrode was thermally evaporated as an electrode. All of the layers were fabricated inside an N 2 -filled glove box. A schematic diagram of the fabri- cated FTO/ TiO 2 /perovskite/HTL/Au cell is presented in Figure 1. 3 RESULTS AND DISCUSSION The structure and optical properties were first studied to probe the effect of the CZTS NP film as a HTL. Figure 2 shows the XRD pattern and EDS image of CZTS NP thin films. The positions of diffraction peaks indicate that CZTS NPs are polycrystalline with the preferred in-plane orientations of the (210), (002), (121), (400) and (402) planes, with an orthorhombic structure matching the JCPDS file (#26-0575). The EDS image shows a ratio of different atoms in the CZTS to be 2.05:1:1:4.05, which reveals that the CZTS is syn- thesized successfully. Ultraviolent-visible (UV-vis) spectroscopy results for the TiO 2 /CH 3 NH 3 PbI 3 films before and after the CZTS HTL incorporation are compared in Figure 3, which also includes the TiO 2 /CZTS and TiO 2 /CH 3 NH 3 PbI 3 /spiro- OMeTAD films for comparison. A huge improvement in the light absorption of TiO 2 /CH 3 NH 3 PbI 3 /CZTS can be seen in the visible spectrum after the incorporation of the CZTS HTL. This can be partly attributed to the absorp- tion of the CZTS, having a relatively small bandgap of 1.49 eV as seen from its absorption spectrum. In addition, after the incorporation of spiro-OMeTAD HTL, Y. ZUO et al.: HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS IMPROVED WITH LOW-COST ORTHORHOMBIC ... 484 Materiali in tehnologije / Materials and technology 52 (2018) 4, 483–486 Figure 2: XRD pattern and EDS image of CTTS NPs Figure 1: Schematic representation of a perovskite solar cell Figure 3: UV-vis absorption spectra for: a) TiO2/perovskite/CZTS, b) TiO2/perovskite, c) TiO2/perovskite/spiro-OMeTAD, d) TiO2/ CZTS films and band gap of CZTS the TiO 2 /CH 3 NH 3 PbI 3 /spiro-OMeTAD film exhibits an absorption decrease, indicating a negative effect on the light absorption of the solar cell. Therefore, the dramatic absorption increase upon the CZTS incorporation shows a path for solving the high cost of the HTL used in pe- rovskite solar cells. Recently, it was found that the crystallization and crystal-plane orientation, especially a more preferential in-plane orientation of the (110) plane, of a perovskite film could improve the corresponding device perfor- mance. 14 GIXRD (grazing incidence X-ray diffraction) experiments were carried out to study the HTM effect on the perovskite crystallization and crystal-plane orienta- tion. Snapshots of the in-situ GIXRD experiments on the perovskite film on spiro-OMeTAD and CZTS are shown in Figures 4a and 4b. All the GIXRD patterns are at the same incidence angle and exhibit a relatively strong scattering background on the amorphous glass substrate in a range of q = 10–25 nm –1 , where q is the scattering vector (q=4 sin( )/ , is half of the diffraction angle, and is the X-ray wavelength. After the calibration, the key features of the perovskite diffraction pattern could be observed at q 10.04 nm –1 , 19.74 nm –1 and 22.21 nm –1 . These are consistent with the reflections from the (110), (220) and (310) lattice planes, respectively, 15 demon- strating that the textured perovskite crystals with a well orthorhombic crystal structure 16 were found in the perov- skite film on both spiro-OMeTAD and CZTS. Figure 4c reports on the azimuthally integrated intensity profiles for the two films derived from Fig- ures 4a and 4b, respectively. The strong and sharp (110) peaks in Figure 4c imply that the films exhibit good crystallization and a large crystal size. Compared with the perovskite film on spiro-OMeTAD, the perovskite film on CZTS has a higher and sharper perovskite (110) diffraction peak, indicating its higher quality crystalli- zation. Thus, the improved crystallization of the present film obviously contributes to the performances of the present champion perovskite solar cells. 14,16 The current density-voltage (J-V) curves of CZTS and spiro-OMe- TADHTL-based device are shown in Figure 4d. The device based on spiro-OMeTAD HTL shows a Vo c of 0.974 V, a Jsc of 23.01 mA cm –2 ,anFF of 0.56, and a PCE of 12.72 %. On the basis of the CZTS HTL, the PCE of the device reaches 13.41 %, calculated from the Vo c of 0.962 V, Jsc of 22.43 mA cm –2 , and FF of 0.62, representing the highest PCE of Cu-based inorganic HTLs reported up to now. These results indicate that CZTS acts as a low-cost and efficient HTL, being an appropriate candidate to be used as a photoelectric material for future perovskite solar-cell applications. Table 1 summarizes the photovoltaic parameters of the devices with different HTLs in different scan directions measured under AM 1.5G solar illumination at 100 mW cm –2 . Regarding the device employing spiro- OMeTAD as the HTL, the hystersis of the PCE of 29.0 % was obtained. When the CZTS NP layer was introduced into the solar cell, the hystersis of the PCE of 16.3 % was obtained, indicating a better effect on the hole transporting, a dramatic improvement in the charge extraction and reduced recombination. 13,17 Therefore, the CZTS NP layer is proved to be a suitable HTM for the use in perovskite solar cells. Table 1: Photovoltaic parameters of FTO/TiO2/CH 3 NH 3 PbI 3 perov- skite/HTM/Ag devices with different HTLs in different scan directions HTL Scan direction J SC / (mA/cm 2 ) Voc/V FF (%) Spiro Reverse 23.01 0.974 0.56 12.72 Forward 13.47 0.921 0.72 9.08 CZTS Reverse 22.43 0.962 0.62 13.41 Forward 21.02 0.925 0.57 11.22 Y. ZUO et al.: HIGH-EFFICIENCY PEROVSKITE SOLAR CELLS IMPROVED WITH LOW-COST ORTHORHOMBIC ... Materiali in tehnologije / Materials and technology 52 (2018) 4, 483–486 485 Figure 4: GIXRD pattern of CH 3 NH 3 PbI 3 deposited on: a) CZTS, b) spiro-OMeTAD, c) the corresponding radially integrated intensity plots along the ring at q 10 nm –1 for the two films derived from a) and b), respectively, d) J-V characteristics of the champion perovskite solar cells using spiro-OMeTAD and CZTS as HTLs 4 CONCLUSIONS In summary, in order to tackle the problem of high-cost organic HTMs in perovskite solar cells, we successfully demonstrated an orthorhombic structure of CZTS NPs, using the XRD and EDS results, and employed it as the HTL in the perovskite solar cells. Its champion efficiency of up to 13.41 % was demonstrated. The CZTS NP layer can efficiently extract holes out of perovskite and help improve the perovskite crystal structure in the preferred in-plane orientation of the (110) plane, as indicated by the GIXRD measurement. 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