Author Affiliations
1State Key Laboratory of Reliability and Intelligence of Electrical Equipment, School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300401, China2Center for Excellence in Nanoscience (CAS), Key Laboratory of Nanosystem and Hierarchical Fabrication (CAS), National Center for Nanoscience and Technology, Beijing 100190, China3Key Laboratory of Optoelectronic Technology & Systems (MoE), College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, China4Department of Chemical Engineering, Kansas State University, Manhattan, KS 66506, USA5Macao Institute of Materials Science and Engineering (MIMSE), Macau University of Science and Technology, Macau 999078, Chinashow less
Inverted perovskite solar cells (PSCs) have attracted interest due to their simple fabrication, long-term stability, and small hysteresis[1-3]. It is noteworthy that the quality of the hole-transport layer (HTL) largely determines the device performance. Nickel oxide (NiOx) has been paid great attention as a hole-transport material in PSCs because of its natural p-type property, low cost, good stability, and high transmittance[4, 5]. Further, NiOx has a suitable bandgap (Eg > 3.50 eV) and a well-matched valence band with perovskites, which is conducive to hole collection and electron blocking[6, 7]. NiOx-based inverted PSCs are promising for flexible and tandem solar cells due to their negligible hysteresis and low processing temperatures compatible with flexible substrates. Thus, more investigations into surface/interface modification, bandgap alignment, and the physical principles are worth paying effort to enhance device performance further.
Sol-gel method is mainly used to prepare Ni(OH)2 by reacting Ni(NO3)2 with NaOH and then calcination to obtain NiOx nanoparticles (Fig. 1(a))[8]. However, NO3− as residue cannot be removed by subsequent process. They will be embedded in Ni(OH)2, thus reducing the efficiency and long-term stability of the device. These ions can be removed at high temperatures, but this will lead to poor dispersion of the generated NiOx in solution, seriously affecting the quality of the subsequent NiOx HTL. In addition, NiOx has a relatively low inherent conductivity, which limits the efficiency enhancement of PSCs[9]. At the interface, organoiodide in perovskite can react with Ni3+ in NiOx layer, and this reaction can affect the device stability. As shown in Fig. 1(b), there are three reactions at the interface[10]: (1) Oxidation and deprotonation reactions generate iodine vapor and free protons; (2) the formation of volatile products at high temperatures, including hydrogen cyanide (HCN), methyliodide (CH3I) and ammonia (NH3); (3) with the increase of vapor pressure of free FA and MA molecules, the condensation product N-methyl formamidine can be formed. Therefore, to enhance the intrinsic stability and mobility is necessary for improving the device performance.
![(Color online) (a) Sol-gel preparation of NiOx nanoparticles. Reproduced with permission[8], Copyright 2023, Wiley. (b) Possible degradation mechanism of NiOx-perovskite heterojunction. Reproduced with permission[10], Copyright 2022, the Royal Society of Chemistry.](/Images/icon/loading.gif)
Figure 1.(Color online) (a) Sol-gel preparation of NiOx nanoparticles. Reproduced with permission[8], Copyright 2023, Wiley. (b) Possible degradation mechanism of NiOx-perovskite heterojunction. Reproduced with permission[10], Copyright 2022, the Royal Society of Chemistry.
Recently, doping ions have become an important strategy for improving NiOx conductivity. To increase hole concentration, it is preferred to dope metal cations into NiOx as acceptors. Doping metal ions like Li+[11], Cs+[12], Ag+[13], Cu2+[14], Sr2+[15], Co2+[6], Zn2+[16], rare earth ions[17] or Pb2+/Li+[18] can effectively improve NiOx conductivity. Chen et al. doped Cs+ into NiOx to increase conductivity and workfunction, resulting in a significant improvement in efficiency and stability[12]. Doping bivalent metal cations with the same valence state as Ni can improve the mobility of NiOx and device efficiency. Fig. 2(a) shows that the ionic radii of common bivalent metal cations mismatch with Ni ion radius within 10%, which can effectively promote the occurrence of substitution[19]. Dong et al. used KBr as a buffer layer between the perovskite and NiOx to improve the valence band maximum of NiOx to −5.37 eV, which matches better with perovskite and facilitates charge separation[20]. Chen et al. made facile NiOx modification by KCl to synchronously suppress interfacial recombination and ion migration[21].
Figure 2.(Color online) (a) The oxidation state and ionic radius for several metals. Reproduced with permission[19], Copyright 2020, the Royal Society of Chemistry. (b) Synthesis of NiOx nanoparticles. Reproduced with permission[22], Copyright 2022, Wiley. (c) NiOx modified by TTTS. Reproduced with permission[23], Copyright 2022, Wiley. (d) The preparation of NiOx/carbon heterostructure. Reproduced with permission[24], Copyright 2021, Wiley. (e) SAM-modified NiOx at the interface. Reproduced with permission[27], Copyright 2021, Wiley. (f) Flexible PSCs with bridging molecules. Reproduced with permission[28], Copyright 2022, Nature. (g) The energy level diagram for device with PTAA. Reproduced with permission[30], Copyright 2021, Elsevier. (h) TMSBr buffer layer inhibiting perovskite degradation. Reproduced with permission[10], Copyright 2022, the Royal Society of Chemistry.
The interfacial recombination loss and mismatched band alignment limit the performance enhancement of inverted PSCs. Though doping metal cations could improve the conductivity of NiOx layer, the impurity ions cannot be avoided. In response, Wang et al. used [BMIm]BF4 ionic liquid (IL) assisted synthesis to prepare high-quality NiOx nanoparticles[22]. [BMIm]BF4 is added before the reaction of Ni(NO3)2 with NaOH (Fig. 2(b)). The multifunctional substitution of imidazole rings produces more chemical bonds. In addition, cations can inhibit the adsorption of impurity ions on Ni(OH)2, thus obtaining NiOx-IL HTL with high conductivity. Yang et al. used TTTS as a chelating agent of Ni2+ in NiOx layer to improve its conductivity[23]. TTTS and Ni2+ are combined by strong Ni2+−N coordination bonds in NiOx, increasing the ratio of Ni3+ : Ni2+ (Fig. 2(c)). The increase of Ni3+ content adjusted the band structure of NiOx, thereby increasing the hole density and mobility, resulting in enhanced PCE over 22%. Carbon materials with good conductivity are also suitable modifiers for NiOx. Yin et al. developed a NiOx/carbon heterostructure (Fig. 2(d)) as an interlayer for fabricating efficient PSCs with good interfacial energy level alignment and more efficient charge transport[24].
The interfacial lattice mismatch and adverse reactions in NiOx-based PSCs cannot be ignored. Self-assembled molecular layers (SAMLs) are effective carrier-transport layers in PSCs due to their unique ability to manipulate interface properties as well as their simple processing and scalable manufacturing[25]. However, the defects and pinholes caused by its adsorption process can seriously degrade device performance. Therefore, SAMLs are often combined with hole-transport materials such as NiOx to improve hole transport. The phosphoric acid (PA) group in 2PACz has a strong coordination with NiOx[26], and it is easier to spin-coat 2PACz onto NiOx layer. The presence of 2PACz can promote the crystallization of perovskite and regulate the bandgap of perovskite. Sun et al. found that tridentate binding between MeO-2PACz and NiOx is superior to the double-toothed binding between MeO-2PACz and ITO (Fig. 2(e))[27]. This close contact reduces defects and pinholes at the interface, thereby improving device performance. In 2022, Li et al. used a mixture of 2PACz and MeO-2PACz as a molecular bridge at the interface to reduce interface recombination and they can act as a stress buffer layer at the interface to improve the bending durability of the flexible device (Fig. 2(f))[28]. Recently, Zhang et al. used p-chlorobenzenesulfonic acid (CBSA) self-assembly to anchor NiOx and perovskite crystals, where the chlorine end can provide a growth site for perovskite and also release the interfacial strain[29]. The sulfonic acid group in CBSA can passivate the surface defects of NiOx, which is conducive to carrier extraction.
Besides SAMLs, introducing long-chain organic molecules can also improve surface properties for bandgap alignment and charge transfer. PTAA can act as a molecular bridge. Fig. 2(g) shows that the valence band of PTAA (−5.2 eV) is very close to the valence band of NiOx (−5.1 eV) and perovskite (−5.3 eV), which can quickly transfer holes from perovskite to NiOx[30]. Recently, Li et al. obtained a PCE of 25.12% (certified 24.6%) for inverted NiOx-based PSCs by using NiOx/PTAA/Al2O3 as hole-transport layer[31]. To eliminate multistep photochemical reactions at the interface, Wu et al. constructed an aprotic trimethyl bromosulfonic acid (TMSBr) buffer layer at NiOx/perovskite interface (Fig. 2(h))[10]. TMSBr has excellent photothermal stability, and strong trap-passivation capability. The T80 lifetime for the device under AM1.5G light is 2310 h.
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