Effect of TiO₂ Surface Treatment on the Current–Voltage Hysteresis of Planar‐Structure Perovskite Solar Cells Prepared on Rough and Flat Fluorine‐Doped Tin Oxide Substrates

Abstract

Herein,w es tudiedt he effect of surface treatment of compact TiO 2 layer by vacuum ultraviolet (VUV) light and by TiCl 4 on the I-V hysteresis of perovskite solar cells prepared on fluorine-dopedt in oxide (FTO) substrates with two different surface roughnesses.I nitially,t he cells prepared on flat FTO substrates show better results than cells prepared on rough FTO,w ith slightly higher power conversione fficiency (PCE) and less hysteresis in the I-V curves.T reatment of TiO 2 preparedo nf lat FTO has no effect on the I-V hysteresis because of better contact between TiO 2 /CH 3 NH 3 PbI 3 layers.T reatment of TiO 2 layer on rough FTO has ap ositive effect on the PCE and I-V hysteresis and stability.Despite the rapid progress in power conversion efficiency (PCE;2 2.1 %) of perovskite solar cells,t he fundamental problems pertaining to stability and origin of the currentvoltage (I-V)h ysteresis are still unsolved. [1][2][3] The I-V hysteresis creates an ambiguity about the actual power of the devices and persists as ac hallenge in this field. [4][5] To elucidate its origin,s everalh ypothesesi ncluding ion migration/displacement, ferroelectric polarization, trappingo fe lectronsa t the interfaces,a nd capacitive effects [6][7][8] were proposed. Migrationo fi ons such as CH 3 NH 3 + ,I À ,P b 2 + ,a nd H + [9] was also suggested as origin of hysteresis.D iffusion of intrinsic ionic defects in CH 3 NH 3 PbI 3 layer implies ah igh long-term stability and performance of the solar cells.I on migration can create defects in the crystal, and these defects can affect the physical and charge-transfer properties at interfaces, eventually leading to charge accumulation. Thef erroelectric polarization of the perovskite and its contributioni ss till questionable because there is no direct evidence for ferroelectricity in perovskites under operating conditions. [10] Va riationo ft emperature alters the lattice parameters,a nd the difference in thermal expansion coefficient combined with the lattice mismatch between TiO 2 and perovskite could result in interfacial defects.[8] Weak contacts and defects between two layers may hinder charge transfer at this interface, resultingi nI-V hysteresis.T op rovide further clarification, many reports have also shownc apacitive effects in I-V hysteresis phenomena. [8,[11][12][13] In one of our previous reports,w e also found that the origin of hysteresis in I-V curvesc an be explained by an equivalent-circuit modeli ncluding ac apacitance as ac omponent.[8] Experimental I-V curves were successfully reproduced by simulating the I-V behavior of the equivalent circuit. In some cases,d epending on the sample's fabrication conditions,t he normal trend of hysteresis became opposite, that is,t he reverse scan showing ah igher performance than the forward scan.[14] An inductance element representing the kinetics of the charges at the perovskite/ contacti nterfaces in the equivalent circuit can be one reason for this opposite trend. Both, presence of capacitancea nd inductance are related to the in...

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“…For the bare FTO, the average surface roughness was ca. 27.50 nm, which is consistent with the reported value [32]. The particle size of the FTO was in the range from ten to several hundred nanometers, which is advantageous to obtain a high density of CYS-SAM onto the FTO electrode.…”

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“…Recently, research direction has been focused on the slow processes occurring in the PSCs to study possible cause of hysteresis. The charge accumulation at the TiO 2 /perovskite interface coupled with ionic migration may cause this phenomenon [ 23 , 30 , 38 ].…”

“…The results showed an increment in PCE, mostly due to better charge extraction after modifying the TiO 2 layer. Following a similar strategy, Cojocaru et al [ 30 ] carried out a post-treatment to the blocking TiO 2 layer with TiCl 4 and demonstrated an efficiency enhancement of 3% when comparing reverse- versus forward-measurement directions. In a similar fashion, Grätzel et al [ 31 ] and Friend et al [ 32 ] demonstrated beneficial effects when mesoporous TiO 2 is post-treated with Li 2+ .…”

Powder Pressed Cuprous Iodide (CuI) as A Hole Transporting Material for Perovskite Solar Cells

“…The perovskite precursor solution was prepared by dissolving methylammonium iodide and lead (II) chloride in 3:1 molar ratio in 700 μL of DMF and stirring for 1 hr at 70 °C. This suspension was then spin coated onto the TiO 2 porous layer at 2000 rpm for 30 s [35]. It was then sintered at 120 °C for 45 min.…”

“…1 mL of 97 mg/mL spiro-OMeTAD in chlorobenzene mixed with 30.2 µL of 175 mg/mL Bis(trifluoromethane) sulfonamide lithium salt in acetonitrile and 9.7 µL of solution consists of 46.6% of 4-tert-butylpyridine in acetonitrile. This mixed solution was spin coated on the perovskite film at 2000 rpm for 30 s [35]. Finally, 80 nm Au electrode was deposited on spiro-OMeTAD coated devices by thermal evaporation at a constant evaporation rate of 0.2 Å/s under 1 × 10 −6 torr vacuum.…”