Using ZnCo2O4 nanoparticles as the hole transport layer to improve long term stability of perovskite solar cells

Inorganic metal oxides with the merits of high carrier transport capability, low cost and superior chemical stability have largely served as the hole transport layer (HTL) in perovskite solar cells (PSCs) in recent years. Among them, ternary metal oxides have gradually attracted attention because of the wide tenability of the two inequivalent cations in the lattice sites that offer interesting physicochemical properties. In this work, ZnCo2O4 nanoparticles (NPs) were prepared by a chemical precipitation method and served as the HTL in inverted PSCs. The device based on the ZnCo2O4 NPs HTL showed better efficiency of 12.31% and negligible hysteresis compared with the one using PEDOT:PSS film as the HTL. Moreover, the device sustained 85% of its initial efficiency after 240 h storage under a halogen lamps matrix exposure with an illumination intensity of 1000 W/m2, providing a powerful strategy to design long term stable PSCs for future production.

Perovskite solar cells (PSCs) have attracted a great deal of attention from academic and industrial researchers because of their rapid development in power conversion efficiency (PCE) from 3.8% to 25.5% within a decade 1,2 .
Perovskites are considered as ideal photovoltaic materials in solar cells due to their high absorption in the visible spectrum 3 , long carrier diffusion length 4 , high carrier mobility 5 , low exciton binding energy 6 , tunable bandgaps by exchanging atomic composition 7,8 , large area production and low cost owing to solution processability. In recent years, PSCs using multiple-cation lead halide as the absorbing layer dominate mainly because of their high stability and high reproducibility compared to single-cation perovskites like MAPbI 3 , FAPbI 3 3 as the absorbing layer 10 . The optimized device achieved a high PCE of 20.56%, a V OC of 1132 mV, a J SC of 22.95 mA/ cm 2 , and a FF of 0.79. Besides, the device exhibited stable conversion efficiency over 1000 h stored under ambient air (10 ± 5 RH%) without encapsulation. Hence, the utilization of multiple-cation perovskite material was adopted as the light absorber instead of single-or double-cation perovskites.
In recent years, inverted PSC (p-i-n) has been extensively investigated because of its simple device architecture, ease of fabrication, improved stability, and reduced hysteresis effect 11 . Besides, tandem cells with augmented efficiency can be accomplished by combing inverted PSCs with traditional solar cells such as silicon or copper indium gallium selenide solar cells 12,13 . To fabricate inverted PSCs, organic polymers such as poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PPS), poly(4,4'-bis(N-carbazolyl)-1,10-biphenyl) (PPN), poly (p-phenylene) (PPP), and polythiophene (PT) have been used as the hole transport layer (HTL) [14][15][16] . Among them, PEDOT:PSS is the most widely used organic HTL with good conductivity and availability in the area of PSCs. Moreover, PEDOT:PSS is dissolved in water or alcohols, i.e., less toxic and environmentally friendly than other polymers that require organic solvents like dichloromethane or chlorobenzene. The acidic and hygroscopic nature of PEDOT:PSS induces corrosion of transparent conducting oxides such as fluorine-doped tin oxide (FTO), which restricts the long term stability and commercialization of inverted PSCs 17,18 . These polymers cause challenges due to a susceptibility to environmental factors such as moisture and ultraviolet light exposure. Furthermore, the complicated synthesis and purification process of these materials make them very expensive and difficult for mass production. In contrast to organic polymers, inorganic hole transport materials have the advantages of high carrier mobility, superior stability, low cost, and facile preparation, such as vanadium oxide 19 , copper oxide 20 , nickel oxide 21 , and cobalt oxide (Co 3 O 4 ) 22 . Bashir et al. utilized spinel Co 3 O 4 NPs as the HTL for the fabrication of PSCs with a large-area of 70 cm 2 to achieve a PCE of 11.06% and extensive stability up to 2500 h under standard one sun illumination. In addition to those common metal oxides, spinel ternary metal oxides prepared by solution process have been gradually investigated as promising hole conductors in optoelectronics and lithium-CO 2 batteries due to their tunable optical and electrical properties [23][24][25] . Spinel ternary metal oxides with a chemical formula of AB 2 O 4 contain two inequivalent cations in the lattice sites 26 . The tetrahedral and octahedral sites are occupied by divalent (A) and trivalent (B) cations, respectively, leading to the formation of antisite defects that is energetically favored and is the source of the p-type conductivity 27 . The advantages of such ternary metal oxides include wide optical gap, better energy level alignment, and superior electrical property for serving as the HTL in optoelectronic devices. Choy and co-workers firstly proposed a controllable deamination strategy to synthesize nickel cobaltite (NiCo 2 O 4 ) NPs as the HTL in inverted PSCs 28  has also been reported to possess several features of hole transport ability, wide optical bandgap, and solution processability 29,30 , which can serve as the photocathode for the applications in photoelectrochemical water splitting and lithium-ion batteries 31,32 . Despite being a good candidate for alternative HTLs, surprisingly, no study about the use of ZnCo 2 O 4 as the HTL in PSCs has been reported so far. Therefore, for the first time, we attempted to prepare ZnCo 2 O 4 NPs as an efficient HTL in PSCs, which may bring important contribution to long term stability and enhanced photovoltaic performance of PSCs due to its inorganic and hole transport nature.
In   The scan rate for J-V measurements was 20 mV/s. The external quantum efficiency (EQE) measurements were conducted using a PV Measurement QE-R instrument which was assembled by Enli Technology Co., Ltd. from Taiwan. To exploit the stability of devices, the encapsulated PSCs were constantly exposed to a halogen lamps matrix with an illumination intensity of 1000 W/m 2 at room temperature with 40-60% relative humidity and their J-V characteristics were measured in each 24-h period.

Results and discussion
Characterization of ZnCo 2 O 4 NPs. Crystallographic information of the prepared ZnCo 2 O 4 NPs was acquired and the corresponding pattern is shown in Fig. 1a 33,34 . According to the XRD pattern, the prepared ZnCo 2 O 4 is well consistent with the spinel phase. Figure 1b displays the TEM image of ZnCo 2 O 4 NPs. These particles tend to aggregate with an average diameter of 20 nm. The residual NH 3 molecules on the surface of ZnCo 2 O 4 may deteriorate its electrical properties and thus should be removed. The FT-IR experiment was adopted to detect the removal of NH 3 , and the corresponding infrared spectra before and after calcination are depicted in Fig. 2. Before calcination, the characteristic stretching bands of NH 3 molecules were observed at 3655-2597, 1753, and 826 cm −1 , which are assigned to the N-H stretching mode, H-N-H bending vibration, and H-N-H rocking mode, respectively 35 . A significant absorption band was found at 1317 cm -1 , which was attributed to NO 3 groups from starting materials 36 . In addition,      Fig. 3c. The main signal due to lattice oxygen (O 2-) is observed at 529.4 eV that is in agreement with the previous report 40 . Besides, shoulder signals at a higher binding energy of 530.9 eV and 532.3 eV come from surface hydroxyl groups and chemisorbed oxygen 41 . The energy levels of ZnCo 2 O 4 NPs were calculated from their UPS spectra, as shown in Fig. 4. The work function (φ w ) is derived by subtracting the binding energy cutoff in the high binding energy region (around 16.7 eV) from He I photon energy (21.22 eV). Since the φ w is defined as the energy difference between the Fermi level (E F ) and the vacuum level (0 eV), the E F value of ZnCo 2 O 4 NPs is determined to be −4.52 eV from Fig. 4a. Furthermore, the binding energy cutoff in the low binding energy region reveals the energy difference between the E F and the valence band (VB) level 42  .65 nm, respectively. Apart from AFM investigation, contact angle experiment was also carried out to realize surface properties of the two HTLs. Figure S1a,b in the Supplementary Information represent the contact angles of a water droplet on the surfaces of PEDOT:PSS and ZnCo 2 O 4 NPs, revealing that ZnCo 2 O 4 NPs has a smaller contact angle of 23.6° than PEDOT:PSS film (36.8°). It is reported that the smaller contact angle facilitates the nucleation process of perovskite crystals to form a uniform layer with larger grain sizes and little pinholes 44,45 . The results from AFM and contact angle measurements reveal that ZnCo 2 O 4 NPs can serve as a better surface modifier for FTO substrates than PEDOT:PSS, which is beneficial for improving interfacial contact and hole extraction between ZnCo 2 O 4 NPs and the perovskite 46 . The crosssectional SEM images of ZnCo 2 O 4 NPs layer and PEDOT:PSS film can be seen in Figure S2a Figure S4a in the Supplementary   Information shows the transmission spectra of the ZnCo 2 O 4 NPs layer and PEDOT:PSS film from 315 to 750 nm. The transmittance was measured to be 55-90% in the range of 375-650 nm and even higher over 90% in the rage of 650-750 nm for both samples with similar spectral shapes. Therefore, we speculate that the amount of incident photons entering into devices is close. The absorption spectrum of the ZnCo 2 O 4 NPs layer is shown in Fig. S4b Fig. 7a. It is clearly seen that the perovskite deposited on the FTO substrate has the highest PL intensity, while the one on the ZnCo 2 O 4 NPs layer owns the lowest PL emission. The reduced PL emission implies hindrance of electron − hole pair recombination and improvement of J SC and FF of PSCs 26,52 . Furthermore, the TR-PL decay experiment was performed and the obtained PL decay curves of the perovskite on FTO, PEDOT:PSS film, and ZnCo 2 O 4 NPs layer are shown in Fig. 7b. The PL decay curves agree well with a biexponential decay fitting and corresponding lifetimes of τ 1 , τ 2 , and τ avg are listed in Table S1 in the Supplementary Information. It is reported that fast decay (τ 1 ) originates from nonradiative capture of free carriers and the slow decay (τ 2 ) comes from radiative recombination of remaining excitons 26 Fig. 8e for comparison. The PSC based on ZnCo 2 O 4 HTL retained 85% of its initial efficiency after 240 h storage under a halogen lamps matrix exposure at room temperature, whereas the PCE of the device based on PEDOT:PSS HTL dropped to only 0.5% of its initial efficiency after 144 h storage. Such fast deterioration can be attributed to the acidic nature of PEDOT:PSS causing corrosion to the perovskite and FTO substrate. Therefore, the use of inorganic ZnCo 2 O 4 HTL is highly beneficial for the device stability. As mentioned in the Introduction, the device using CuCo 2 O 4 as the HTL retained 71% of initial PCE after 96 h storage under a continuous yellow light irradiation 26 . Our result reveals that ZnCo 2 O 4 is a better candidate for the fabrication of stable PSCs. Figure 8f shows the EQE spectra and integrated current density of devices as a function of wavelength using ZnCo 2 O 4 NPs and PEDOT:PSS as the HTL. The results demonstrate that the device based on ZnCo 2 O 4 NPs has a higher photon-to-electron conversion capability from 300 to 750 nm compared to that based on PEDOT:PSS. The integrated current density for the devices based on ZnCo 2 O 4 NPs and PEDOT:PSS was calculated to be 18.4 and 15.45 mA/cm 2 , respectively, which are similar to the J SC values in Table 1.

Conclusions
In this study, we successfully synthesized ZnCo 2 O 4 NPs by a facile chemical precipitation method, which were employed as the HTL in inverted PSCs. The obtained ZnCo 2 O 4 NPs showed a spinel phase and an average particle size of 20 nm. The introduced NH 3  www.nature.com/scientificreports/ provides a simple and effective approach to achieve PSCs with high efficiency and long term stability that show promising use in photovoltaic application.

Data availability
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