2.2.1.

Organic compounds

A hydrophobic ETM can be introduced to improve the moisture resistance of perovskite devices without loss of photovoltaic performance. The most commonly used organic ETM is fullerene-derived phenyl-${\mathrm{C}}_{61}$-butyric acid methyl ester (${\mathrm{PC}}_{61}\mathrm{BM}$). Fullerene ${\mathrm{C}}_{60}$ and ${\mathrm{PC}}_{61}\mathrm{BM}$ were first used by Jeng et al.¹² as ETMs in inverted-structured PSCs with a PCE of 3.9%. As the electron conductivity of ${\mathrm{PC}}_{61}\mathrm{BM}$ is higher than that of ${\mathrm{TiO}}_2$, the former can improve the electron extraction from ${\mathrm{MAPbI}}_3$ to the ETL. The corrosion-resistant architecture and hydrophobicity of ${\mathrm{PC}}_{61}\mathrm{BM}$ also improve air and humidity stability. The poor PCE observed is attributed to the thin perovskite absorbing layer of $<100\mathrm{nm}$, which limits the light harvesting and short-circuit current of devices. You et al.¹³ and Docampo et al.¹⁴ used a 300-nm-thick mixed halide–perovskite film and improved PCE to 11.5%. Heo et al.¹⁵ used ${\mathrm{PC}}_{61}\mathrm{BM}$ as an ETL. They found that ${\mathrm{PC}}_{61}\mathrm{BM}$ could passivate the defects at the grain boundaries.

Li et al.¹⁶ designed triphenylamine fullerene (TPA-PCBM) with a stronger charge transfer capability and higher carrier density compared with those of PCBM. A cross-linked silane functional fullerene ETL was designed by Bai et al.¹⁷ to isolate moisture from PSCs. The unsealed ${\mathrm{C}}_{60}$-SAM-based device maintained nearly 90% of its original efficiency after 30 days of exposure to the environment.

Metal oxides, such as ZnO, combined with fullerene as an ETL, improve the ohmic contact and device stability.¹⁸ This improvement is described in detail in the following sections.

2.2.2.

Inorganic compounds

Intrinsic oxygen vacancies on the ${\mathrm{TiO}}_2$ surface are suspected to be among the main reasons behind the degradation of PSCs. Liu and Kelly¹⁹ first used ZnO as a PSC ETL. ZnO has a similar energy level position and higher electron transport rate than ${\mathrm{TiO}}_2$. The PCE of a cell fabricated with this material reached 15.7% under 1.5 AM sunlight irradiation. However, ZnO is very sensitive to weak bases or acids and has a high defect density. These characteristics result in serious electron hole recombination on the material surface, which is a factor of instability in PSCs.

Brinkmann et al.¹⁸ introduced a bilayered electron extraction interlayer consisting of aluminum-doped zinc oxide (AZO) and ${\mathrm{SnO}}_x$ in inverted-structured PSC. As shown in Fig. 3, a PCBM layer is deposited on top of the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ perovskite layer. An AZO layer on ${\mathrm{PC}}_{61}\mathrm{BM}$ layer is derived from a nanoparticle dispersion, which is excellently suited to extract electrons from ${\mathrm{PC}}_{61}\mathrm{BM}$. An ${\mathrm{SnO}}_x$ layer with high electrical conductivity is deposited on top of the AZO layer by atomic layer deposition to form an outstandingly dense gas permeation barrier that effectively hinders the ingress of moisture toward the perovskite. At the same time, ${\mathrm{SnO}}_x$ can prevent the egress of decomposition products of the perovskite, which then corrode the metal electrodes and shorten the lifespan of cells. Consequently, the overall decomposition of perovskite is considerably suppressed, and the cells based on $\mathrm{AZO}/{\mathrm{SnO}}_x$ do not degrade even after 300 h in ambient air (23°C and 50% RH). Yang et al.²⁰ complexed ethylenediaminetetraacetic acid (EDTA) with ${\mathrm{SnO}}_2$ in planar-type PSCs to realize an EDTA-complexed ${\mathrm{SnO}}_2$ ($\mathrm{E}\text{-}{\mathrm{SnO}}_2$) ETL. Its electron mobility is approximately three times larger than that of ${\mathrm{SnO}}_2$. The PCE of the device with $\mathrm{E}\text{-}{\mathrm{SnO}}_2$ ETL increases to 21.60%. Moreover, the unencapsulated device based on $\mathrm{E}\text{-}{\mathrm{SnO}}_2$ maintains 92% of its initial efficiency exposed to an ambient atmosphere (35% RH) after 2880 h in the dark, whereas the device using only ${\mathrm{SnO}}_2$ provides 74% of its initial efficiency under the same storage condition. In the continuous irradiation test, the device using the $\mathrm{E}\text{-}{\mathrm{SnO}}_2$ maintains 86% of its initial efficiency, whereas the device using only ${\mathrm{SnO}}_2$ remains 38% relative to its initial efficiency after 120 h of illumination at $100\mathrm{mW}{\mathrm{cm}}^{-2}$.

Fig. 3

Scanning electron microscopy image of the device cross-section along with the assignment of the respective layers.¹⁸

Hwang and Yong²¹ applied CdS as a new inorganic ETM to replace the traditional dense ${\mathrm{TiO}}_2$ layer. CdS has a suitable conduction band energy level and no oxygen vacancy defects. Test results showed that the CdS-based PSC possesses significant light stability, and the resulting PCE retained over 90% of its initial value after 12 h of continuous standard 1.5 AM sunlight irradiation. By comparison, a ${\mathrm{TiO}}_2$-based PSC retained only 18% of its initial PCE under the same conditions.

2.3.1.

Two-layered structures

A two-layered structure can be designed to synergize different ETMs, modify the contact between ETL/perovskite interfaces, and improve device stability. Qiu et al.²² reported a ${\mathrm{PC}}_{61}\mathrm{BM}/\mathrm{ZnO}$ double-layer ETL to improve the interface between this layer and the perovskite layer because the direct contact of ${\mathrm{PC}}_{61}\mathrm{BM}$ with a metal electrode is not conducive to electron extraction due to energy level mismatch. This issue can be addressed by inserting an additional layer of ZnO into the material. This ZnO layer not only improves the energy level matching of the device but also blocks the reaction between the metal electrode and the perovskite component that breaks down the perovskite structures. Moreover, the two-layered structure can protect the perovskite layer from air because of its barrier capability.

Singh and Nalwa^23–25 demonstrated that graphene-based materials can improve heterojunctions and enhance the stability of dye-sensitized photovoltaic devices. On the basis of Singh and Nalwa’s work, Agresti et al.²⁶ used ${\mathrm{TiO}}_2/\mathrm{Li}$-modified graphene oxide (GO-Li) as a double-layer ETL. When the device was exposed to moisture, the GO-Li layer acted as a reaction center to prevent moisture infiltration. The GO-Li layer could also passivate the oxygen vacancies and defects of the mesoporous ${\mathrm{TiO}}_2$ layer. Moreover, GO-Li and ${\mathrm{TiO}}_2$ showed good energy matching, which improved the electron injection efficiency from the perovskite layer to the transport layer. The tested devices with double-layer ETL showed an improved stability when compared to the device with pure ${\mathrm{TiO}}_2$ ETL in illumination aging test, but the physical mechanism of the improvement was not sure.

Song et al.²⁷ designed a two-layered ETL of ${\mathrm{SnO}}_2/{\mathrm{TiO}}_2$. The double-layered ETL had a larger contact area with the perovskite layer, a larger free energy difference, and suitable electron mobility. In addition, it allowed smooth contact with the FTO. The efficiency of a device fabricated with this layer was stable at 20.2% with almost no hysteresis behavior.

2.3.2.

Core/shell structures

Surface trap sites could be improved by designing the ETL as a core/shell structure to increase stability. Hwang et al.²⁸ used core/shell-structured ${\mathrm{TiO}}_2/\mathrm{CdS}$ as an ETL; here, the CdS coated on the ${\mathrm{TiO}}_2$ layer surface inhibits the activation of oxygen vacancies in ${\mathrm{TiO}}_2$, as shown in Fig. 4, and the CdS shell is effectively passivated by ${\mathrm{TiO}}_2$ surface defects, thus preventing charges from recombining at the interface and oxygen from entering the perovskite. The resulting device maintained nearly 80% of its initial PCE after 12 h of continuous standard 1.5 AM sunlight exposure.

Fig. 4

Schematic of the mechanism of the CdS shell of the ${\mathrm{TiO}}_2$ layer.²⁷

3.1.1.

Organic perovskites

The first perovskite material used in the photovoltaic field is ${\mathrm{MAPbI}}_3$. Methylamine (MA) is thermally unstable, whereas formamidine (FA) is relatively stable. The decomposition temperatures of ${\mathrm{MAPbI}}_3$ and ${\mathrm{FAPbI}}_3$ are 230°C and 290°C, respectively. Mei et al.⁴⁰ used 5-aninovaleric (5-AVA) to partially replace MA in ${\mathrm{MAPbI}}_3$ in hole-conductor–free PSC. ${(5\text{-}\mathrm{AVA})}_x{\mathrm{MA}}_{1-x}{\mathrm{PbI}}_3$ exhibited better stability under standard 1.5 AM sunlight in air. Compared with ${\mathrm{MAPbI}}_3$, the ${(5\text{-}\mathrm{AVA})}_x{\mathrm{MA}}_{1-x}{\mathrm{PbI}}_3$ perovskite layer showed better interfacial contact with ${\mathrm{TiO}}_2$, which indicated a much lower defect concentration. Many studies have attempted the partial substitution of Br to form a MAPb ${({\mathrm{I}}_{1-x}{\mathrm{Br}}_x)}_3$ material.^41,42 Because the substitution of larger I atoms with smaller Br atoms in $\mathrm{MAPb}{({\mathrm{I}}_{1-x}{\mathrm{Br}}_x)}_3$ leads to the reduction of the lattice constant and a transition to a cubic phase from a tetragonal phase, the cubic phase has more compact structure than tetragonal phase. The resistance of the resulting device to humidity was significantly enhanced as the Br content increased, as shown in Fig. 5. When $x=0.20$ or 0.29, exposure to 55% relative humidity (RH) hardly affected the PCE of the cell. In addition, Br not only broadened the conduction band but also improved the electron extension in the crystal lattice, resulting in increased PCE.⁴¹

Fig. 5

Line graph of the PCE of heterojunction solar cells based on $\mathrm{MAPb}{({\mathrm{I}}_{1-x}{\mathrm{Br}}_x)}_3$ ($x=0$, 0.06, 0.20, 0.29) versus time. Cells were exposed to 55% humidity on the fourth day and maintained at 35% RH for the rest of the experiment.⁴¹

FA and Br intervention forms a perovskite material with a stable structure. Yang et al. and Jeon et al.^43,44 partially used FA instead of MA at A sites and Br instead of I at X sites, thereby improving the stability of the cell while increasing its PCE to over 18%.

3.1.2.

Inorganic perovskites

Inorganic ions generally have better thermal stability than organic ions, and many researchers have experimented with doping inorganic components at the A site to form stable perovskite compounds. McMeekin et al.⁴⁵ partially replaced FA with the inorganic ion Cs and prepared a ${\mathrm{FA}}_{0.83}{\mathrm{Cs}}_{0.17}\mathrm{Pb}{({\mathrm{I}}_{1-x}{\mathrm{Br}}_x)}_3$ perovskite cell with a band gap of 1.75 eV on the basis of doping with Br. Under standard 1.5 AM sunlight irradiation, the short-circuit current density of the cells reached $19.4\mathrm{mA}/{\mathrm{cm}}^2$, and its PCE exceeded 17%. The ${\mathrm{FA}}_{0.83}{\mathrm{Cs}}_{0.17}\mathrm{Pb}{({\mathrm{I}}_{0.6}{\mathrm{Br}}_{0.4})}_3$ lattice was observed to be most stable under a thermal stress of 130°C. Similarly, Yi et al.⁴⁶ designed a ${\mathrm{Cs}}_{0.2}{\mathrm{FA}}_{0.8}{\mathrm{PbI}}_{2.84}{\mathrm{Br}}_{0.16}$ PSC. Figure 6 shows that this cell has excellent air stability in addition to thermal stability.

Fig. 6

Comparison of the stability of ${\mathrm{Cs}}_{0.2}{\mathrm{FA}}_{0.8}{\mathrm{PbI}}_{2.84}{\mathrm{Br}}_{0.16}$ and ${\mathrm{FAPbI}}_3$ PSCs. Both devices are not packaged and stored in a dark environment.⁴⁶

Nam et al.⁴⁷ reported a K-doped all-inorganic Cs-Pb halide perovskite ${\mathrm{Cs}}_{1-x}{\mathrm{K}}_x{\mathrm{PbI}}_2\mathrm{Br}$. Doping of K ions not only promotes the formation and transport of photoexcited carriers but also shrinks the ${\mathrm{PbX}}_6$ octahedron volume. Thus, the phase stability is improved, and the life of the cell in air is prolonged. However, poor coverage and low PCE are among the disadvantages of this technique. Dong et al.⁴⁸ proposed an antisolvent-assisted multistep deposition method to prepare high-quality all-inorganic perovskite ${\mathrm{CsPbI}}_2\mathrm{Br}$. Figure 7 presents the ${\mathrm{PbI}}_2$ precursor solution [solvent = dimethyl sulfoxide (DMSO) and dimethylformamide] that is first spin-coated on the substrate. The antisolvent chlorobenzene or ethanol is then added dropwise during spin-coating to form a porous ${\mathrm{PbI}}_2$ (with DMSO) intermediate phase. The CsBr solution is filled into the pores and fully reacts to deposit a dense, full-coverage ${\mathrm{CsPbI}}_2\mathrm{Br}$ film. This method solves the shortcomings of poor shape, low crystallinity, and low coverage of ${\mathrm{CsPbI}}_2\mathrm{Br}$ prepared via the traditional one-step spin-coating process and improves the conversion efficiency of the resulting battery to 10.21%. This process also creates a new record for the ${\mathrm{CsPbI}}_2\mathrm{Br}$ cells without an HTL. Moreover, the devices displayed good long-term stability in stability tests, no significant efficiency degradation was observed in unpackaged cells after 44 days of exposure to the environment.

Fig. 7

Schematic of the preparation of ${\mathrm{CsPbI}}_2\mathrm{Br}$ by antisolvent-assisted step deposition.⁴⁸

3.1.3.

Pb-free perovskites

Most mixed perovskite materials contain the toxic heavy metal Pb. If used improperly, Pb could pollute the environment and endanger human health. Therefore, a number of researchers have attempted to replace Pb ions with other inorganic cations and produce environment-friendly Sn-based perovskites. Yang et al.⁴⁹ developed a high-performance low-Eg ($<1.4\mathrm{eV}$) ${\mathrm{MA}}_{0.5}{\mathrm{FA}}_{0.5}{\mathrm{Pb}}_{0.75}{\mathrm{Sn}}_{0.25}{\mathrm{I}}_3$ perovskite cell. The resulting cells were stable in ${\mathrm{N}}_2$ but not ideal in air because ${\mathrm{Sn}}^{2+}$ in the hybrid Sn-Pb perovskite is easily oxidized to ${\mathrm{Sn}}^{4+}$. Lee et al.⁵⁰ added ${\mathrm{SnF}}_2$ and pyrazine as inhibitors to Sn-based perovskite to reduce ${\mathrm{Sn}}^{4+}$ formation. This method greatly improved the stability of the Sn-based perovskite cells with high reproducibility. Under ambient condition, the packaged device stored in the dark maintained 98% of its initial conversion efficiency after 100 days.

Gao et al.⁵¹ used a structural adjustment strategy to inhibit the oxidation of ${\mathrm{Sn}}^{2+}$ ions in ${\mathrm{FASnI}}_3$ perovskite cells. Inserting Cs into the ${\mathrm{FASnI}}_3$ crystal structure caused crystal shrinkage, improved crystal symmetry and stability, and reduced the free energy and trap state density of the resulting crystals. More importantly, the fully Pb-free ${\mathrm{Cs}}_x{\mathrm{FA}}_{1-x}{\mathrm{SnI}}_3$ perovskite cells exhibited better long-term stability, including thermal stability, and air stability, in contrast to Cs-free perovskites.

4.2.1.

Undoped organic HTMs

When the HTL is prepared by using spiro-MeOTAD, the HTL is usually doped with $p$-type dopants, such as Li-TFSI or tributyl phosphate. Spiro-MeOTAD has poor conductivity and must be doped to increase its conductivity. However, the use of dopants requires complex preparation conditions and increases the preparation cost. Moreover, the dopant is easily deliquescent, which could exert a negative effect on the stability of the HTL and perovskite layer. Thus, developing a high-performance nondoped HTM is a viable method to improve the durability of PSC devices.

Liu et al.⁷³ used the undoped organic small-molecule tetrathiafulvalene derivative (TTF-1) as an HTM to reduce the cost of fabricating PSCs and obtained a PCE OF 11.03%. This value is equivalent to that of a $p$-type doped spiro-OMeTAD cell. However, the cells based on dopant-free TTF-1 are more stable than the cells based on common $p$-type doping spiro-OMeTAD. The hydrophobic alkyl chains of TTF-1 can increase the resistance of the HTL to humidity, and avoiding the use of deliquescent additives also improves the stability. Figure 10 shows the degradation time associated with a 20% decrease in efficiency, the lifespan of the cells without encapsulation based on dopant-free TTF-1 ($\sim 360\mathrm{h}$) was three times higher than that of cells based on $p$-type doping spiro-OMeTAD ($\sim 120\mathrm{h}$) under ambient conditions at 40% RH. Liao et al.⁷⁴ reported undoped 4,8-dithien-2-yl-benzo[1,2-d;4,5-d′]bistriazole-alt-benzo[1,2-b:4,5-b′]dithiophenes (pBBTa-BDTs) as a polymer HTM. pBBTa-BDT is $\pi$-downward oriented on perovskites and arranged in a highly ordered manner. The smooth and dense surface of this HTM can effectively prevent atmospheric moisture from affecting the perovskite layer and inhibit perovskite degradation while effectively extracting and collecting holes.

Fig. 10

Efficiency variation of optimized cells based on dopant-free TTF-1 (pristine) and p-type doping spiro-OMeTAD (doped). Unencapsulated cells were stored in air at room temperature with a humidity of about 40% and were measured under illumination at AM 1.5 G.⁷³

As an important hole-transporting material, metal phthalocyanines (MPcs) have been widely used in organic solar cells. However, most of these materials are highly insoluble and require preparation via a rigorous thermal evaporation procedure, which may destroy the underlying perovskite layer. Wu et al.⁷⁵ synthesized a soluble tert-butyl-substituted zinc phthalocyanine ($\mathrm{ZnPc}{(\mathrm{tBu})}_4$) and applied it as an undoped HTL to ${\mathrm{MAPbI}}_3$ PSCs. $\mathrm{ZnPc}{(\mathrm{tBu})}_4$ has high hole mobility and matching HOMO and LUMO energy levels and is tolerant to humidity. It can be spin-coated under ambient conditions (humidity: 30% to 70%). The unpackaged device fabricated from this material retained 85% of its initial PCE after 120 h in the dark at a humidity of 70% to 80%. The disadvantage of this material, however, is that its conversion efficiency is low (approximately only 8%) because $\mathrm{ZnPc}{(\mathrm{tBu})}_4$ has poor coverage and a large number of interfacial defects.

Caliò et al.⁷⁶ reported the synthesis of a Cu(II)-based phthalocyanine [${({}^{\mathrm{t}}\mathrm{O}\mathrm{ctPhO})}_8\mathrm{CuPc}$ 1] with 4-tert-octylphenoxy-substituted functional groups that possesses excellent solubility in a wide range of organic solvents. The ${({}^{\mathrm{t}}\mathrm{O}\mathrm{ctPhO})}_8\mathrm{CuPc}$ 1 and its analogous Zn(II) phthalocyanine [represented as ${({}^{\mathrm{t}}\mathrm{O}\mathrm{ctPhO})}_8\mathrm{ZnPc}$ 2] were applied as HTL in ${({\mathrm{FAPbBr}}_3)}_{0.85}{({\mathrm{MAPbI}}_3)}_{0.15}$ PSCs. The MPcs exhibit high thermal and chemical stability, which can help to improve the preservation of the underlying perovskite layer. The stability of unencapsulated devices with the configurations of Cu(II) and Zn(II) Pc-based and Spiro-OMeTAD-based PSCs was examined for over 200 h under ambient environment at 25°C and an RH of 50% to 60%. The PCE of PSCs with ${({}^{\mathrm{t}}\mathrm{O}\mathrm{ctPhO})}_8\mathrm{CuPc}$ 1 as the HTM only lost less than 10% of the initial value after 7 days. The ${({}^{\mathrm{t}}\mathrm{O}\mathrm{ctPhO})}_8\mathrm{ZnPc}$ 2-based devices dropped 15% of their initial value, whereas the reference devices with Spiro-OMeTAD lost over 20% of their initial efficiency value.

Liu et al.⁷⁷ used CuPc as an HTM in ${\mathrm{CsPbBr}}_3$ perovskite cells, increasing their efficiency by 60%. ${\mathrm{CsPbBr}}_3$ itself is an all-inorganic perovskite with good thermal stability and durability. The high coverage of CuPc can also exert hydrophobic effects. The overall device can exhibit excellent stability at high humidity and temperature. A schematic of the cell structure and normalized PCE over time is shown in Fig. 11. Feng et al.⁷⁸ have recently used high-solubility arylamine groups [triphenylamine (TPA) and diphenylamine (DPA)] as substituents. They designed two dopant-free HTMs based on CuPc derivatives: the methoxydiphenylamine-substituted copper phthalocyanine (OMe-DPA-CuPc) and the methoxytriphenylamine-substituted copper phthalocyanine (OMe-TPA-CuPc). The HTM/perovskite interaction occurs at the methoxy (OMe) group interface with the perovskite methylammonium sites, which improves the performance of PSCs due to the strong adhesion and enhanced interfacial coupling between HTMs and perovskites. Researchers used ${\mathrm{Cs}}_{0.05}{({\mathrm{MA}}_{0.13}{\mathrm{FA}}_{0.87})}_{0.95}\mathrm{Pb}{({\mathrm{I}}_{0.87}{\mathrm{Br}}_{0.13})}_3$ as absorption layer and OMe-TPA-CuPc as undoped HTM layer, achieving a PCE of 19.7% for the fabricated PSCs under 1 sun illumination. Devices based on OMe-DPA-CuPc HTM showed lower PCEs (16.7%) than PSCs using OMe-TPA-CuPc HTM. The fabricated PSCs based on substituted CuPc exhibited considerably better stabilities than the doped Spiro-OMeTAD-based ones. When exposed to atmosphere with a humidity of 75% and at room temperature for more than 960 h, the devices based on OMe-DPA-CuPc HTM or OMe-TPA-CuPc HTM only dropped to 92% and 89% of their initial PCEs, respectively. By contrast, the PCE of doped Spiro-OMeTAD-based PSCs dropped to 50% of the initial value after 480 h.

Fig. 11

Use of CuPc as an HTL in CsPbBr3 cells. (a) Functional diagram of CuPc. (b) Normalized images of the PCEs of PSCs based on ${\mathrm{CsPbBr}}_3/\mathrm{CuPc}/\text{carbon}$, ${\mathrm{CsPbBr}}_3/\text{carbon}$, and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3/\text{carbon}$ versus storage time in ambient air (30% to 40% RH, 25°C) without encapsulation. (c) Normalized images of the PCEs of PSCs based on ${\mathrm{CsPbBr}}_3/\mathrm{CuPc}/\text{carbon}$, ${\mathrm{CsPbBr}}_3/\text{carbon}$, and ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3/\text{carbon}$ versus storage time heated at high temperature (100°C) in a high-humidity ambient environment (70% to 80% RH, 100°C) without encapsulation.⁷⁷

4.2.2.

Inorganic HTMs

Some organic HTMs require complex synthesis processes, are sensitive to the environment, and have poor chemical stability. The use of a stable inorganic HTM, i.e., a metal oxide $p$-type HTM, such as ${\mathrm{NiO}}_x$ and ${\mathrm{CrO}}_x$. Islam et al.⁷⁹ found that ${\mathrm{NiO}}_x$ exhibits better properties than commonly used PEDOT:PSS as an HTM in inverted-structured PSCs. PEDOT:PSS is acidic and hygroscopic, easily leading to perovskite degradation. ${\mathrm{NiO}}_x$ itself has outstanding thermal and chemical stability with a large band gap and good transmittance. Researchers had sputtered a 70-nm-thick ${\mathrm{NiO}}_x$ film onto ITO glass and found that the ${\mathrm{NiO}}_x$ conductivity is higher when the ${\mathrm{Ni}}^{3+}/{\mathrm{Ni}}^{2+}$ ratio is high. However, the optical transmittance of the material was reduced at the same time. The PCE of PSCs prepared after optimization reached 15.2%. The encapsulated cells using ${\mathrm{NiO}}_x$ as an HTM could maintain more than 73% of the initial PCE at 30°C and under 50% RH for more than 1000 h. In addition, the packaged device did not degrade after 1000 h of storage in the dark at 85°C ($\sim 5\%$ RH). Degradation under illumination occurs because ${\mathrm{NiO}}_x$ is unstable under UV light. Qin et al.⁸⁰ introduced ${\mathrm{CrO}}_x$ to a planar inverted-structure PSC as an HTL. Similar to ${\mathrm{NiO}}_x$, ${\mathrm{CrO}}_x$ exhibits better stability than PEDOT:PSS, and the stability of final device with ${\mathrm{CrO}}_x$ HTM was also better. Cu was used as a dopant for ${\mathrm{CrO}}_x$, Cu could increase the conductivity of ${\mathrm{CrO}}_x$ and inhibit its reaction with the degraded perovskite product. Compared ${\mathrm{CrO}}_x$ with ${\mathrm{NiO}}_x$, the PCE of the Cu-doped ${\mathrm{CrO}}_x$ device is $\sim 11\%$ while that of the Cu-doped ${\mathrm{NiO}}_x$ device is 15.4%.⁸¹

Besides thermal and humidity stability, light stability is another direction of cell stability. To prevent the decomposition of perovskite materials caused by UV light, Zhang et al.⁸² proposed the use of ${\mathrm{CuCrO}}_2$ as an HTL. ${\mathrm{CuCrO}}_2$ is capable of achieving broad absorption in the UV region and high transmittance at wavelengths above 400 nm, thereby preventing the perovskite layer from being decomposed by UV light without affecting its light absorption properties.

4.2.3.

Graphene oxide

Yeo et al.⁸³ developed a reduced graphene oxide (RGO) film as an HTL. RGO exhibits high conductivity and can promote charge collection and delay recombination. Given that RGO is nearly neutral with few surface oxygen functional groups, it is stable in nature, does not degrade perovskite similar to acidic PEDOT:PSS, and does not promote decomposition under UV light like ${\mathrm{NiO}}_x$. Whereas ${\mathrm{NiO}}_x$ requires high-temperature and high-vacuum conditions to prepare, RGO can be prepared by the solution method at low cost. More importantly, the passivation effect of RGO can effectively reduce the decomposition of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ perovskite films caused by oxygen and water, enhance their stability, and prolong cell life. GO has a high oxygen content, which results in low electrical conductivity and is disadvantageous for HTMs. To reduce the oxygen content of this material, Yang et al.⁸⁴ reported a method to balance the functionality and conductivity of GO films by limiting its thickness. The thinner GO film resulted in higher conductivity, and the group grew a large-sized perovskite crystal of 500 to 1000 nm on the 2-nm GO film. Larger grains can effectively suppress the charge recombination due to the reduced concentration of grain boundaries. This method achieved a PCE of 16.5% without hysteresis. In addition, the encapsulated GO perovskite cells stored in the dark could maintain over 80% of its initial PCE value after 2000 h under ambient conditions at 80% RH. By contrast, the PCE of PEDOT:PSS-based device rapidly decreased to 40% of the initial PCE after just 1400 h under the same condition.