A review on the classification of organic/inorganic/carbonaceous hole transporting materials for perovskite solar cell application

2.1 Role of HTMs in PSC device structure

The stability and performance depends on the transporting layers used in the device architecture as it serves the various aspects in PSCs such as; (i) it acts as a physical/energetic barrier between ETL and perovskite layer by blocking the electron transfer; (ii) by improving the hole transporting efficiency due to its high hole mobility and its matching energy level with those of ETMs/HTMs and electrode, (iii) avoids the degradation and corrosion which can take place in the absence of an HTM at the metal-perovskite interface (Salim et al., 2015). (iv) suppresses charge recombination by fully separating the top contact from the bottom transport or contact layer leading to improved performance. Though we have a wide variety of device architectures available for achieving both stability and performance, besides this HTMs play the most vital in swapping environmental and economic drawbacks of the perovskite solar cells which are under scrutiny. The light-to-electricity conversion in PSCs can be further improved by using suitable HTMs with well-established device architecture.

2.2 Remarkable properties of HTM in PSCs

HTMs play an important role in determining the device performance and the researchers must consider the following aspects for the introduction of new HTMs; It should have suitable energy levels, providing the driving force for charge transfer (i.e. the highest occupied molecular orbital, HOMO, energy levels of the selected HTMs must be slightly higher to that of the perovskite materials). It should act as a barrier between anode and perovskite layer to block electrons without entering the anode (i.e. its lowest unoccupied molecular orbital, LUMO, energy level should be significantly lower than that of the perovskite absorber). It should have a high hole transfer efficiency (possibly > 10⁻³ cm² V⁻¹ s⁻¹) to facilitate hole conduction and prevent charge recombination. It should favor the open circuit voltage by determining the splitting of the quasi fermi-energy levels of the perovskite (Bakr et al., 2017). It should also avoid the diffusion between back electrode metal and the perovskite layer. It should be cost-effective and should have high thermal stability and resistance to external degradation factors such as moisture and oxygen for a long-term device performance and must be environment friendly.

2.3 Stability perspectives of HTMs in PSCs

The high yielding perovskite devices suffer from extreme instability towards air, water, moisture, light and heat which may be due to the instability in both the perovskite and the transporting layers. The researchers have demonstrated that replacing the lead based perovskite material with any other component may not be suitable in achieving higher performance. So, the focus is on choosing suitable HTMs. The potential to use organic material as HTMs for solar cell technology was first observed in 1975 (Tang, 1986), but that resulted in very low efficiency. After enormous effort, an efficiency of 3.8% was achieved. In 1998, Bach et al. (1998), reported about HTMs based on spiro-OMeTAD, which displayed an open-circuit voltage exceeding 0.9 V but yielded only 2.56% PCE. Then the same group in 2012, achieved an efficiency of about 9.1% after various optimizations made in transporting layers paved way to introduce long-chain polymer based HTMs such as spiro-OMeTAD, PEDOT: PSS, PTAA, and P3HT. But the problems related to instability in using these organic long chain polymers based HTMs remains a challenge. Also, this spiro-OMeTAD as HTM, requires costly additive (4-tert-butylpyridine) and dopants (typically hygroscopic Li-salts) as it suffers from poor instability caused by the spiro structure. To overcome such drawbacks the spiro-skeleton could be replaced with other ensemble to improve the stability of the interface between perovskite/HTL. Li et al. (2014), incorporated montmorillonite (MMT) as a buffer layer between spiro-OMeTAD and perovskite to improve the stability towards ambient conditions. To further improve the stability, one of the most striking approach was incorporation of spiro (TFSI)₂ as HTM to replace pristine spiro-OMeTAD. Researchers have also focused on PEDOT: PSS, its acidic nature is a concern for the long-term stability of solar cells. It also produces a lower PCE due to low hole mobility, it is sensitive to humidity and is expensive (Nguyen et al., 2014). Also, the cost of these organic materials, the dopants and the additives are very high. These drawbacks have forced researchers to find low cost materials with long-term stability. This has brought inorganic p-type materials such as copper, nickel, transition metal oxide based HTMs (CuI, NiO, Cu₂O, CuO, MoO_x, VO_x, WO_x) into PSCs. These inorganic materials have wider band gap, high conductivity and also prevents the device from exposure to the ambient atmosphere. So employing such inorganic materials in PSCs could help to achieve higher PCE, yet their sensitivity towards oxygen and moisture seems to be a hindrance. To address these issues of sensitivity towards ambient condition, carbon based materials have been used photovoltaics as an electrode in PSC. With its low-cost and abundant availability on earth, it becomes an ideal candidate for hole-conductor. Though carbon emerged as an electrode, its favorable photovoltaic properties makes it to play dual role in PSCs (both as an electrode and as a HTM) which has invoked researchers to focus on carbon and its derivatives based materials which can be used as replacement to other high cost, unstable and sensitive HTMs. Hence, in this review article, a complete detail about the different available organic/inorganic/carbonaceous based HTMs is provided.

4.1 Classification of organic long chain polymer HTMs

(Prisitne & Doped spiro-OMeTAD/PEDOT: PSS/P3HT/PTAA & Cross-linked polymers)

In this topic, we will be focusing on the latest development of the different organic conducting polymers (both long chain and small molecules), small molecules and crosslinked polymers based hole transporting material to analyze the transport properties of polymeric PV materials and have correlated and compared the results on device performance and stability.

4.1.4

4.1.4 PTTA polymer based HTMs

Lin et al. (2017) have reported that even a simple modification with poly tri-arylamine (PTAA) based HTM could avoid the need to use standard HTM and delivers a relatively high open-circuit voltage of 1.08 V and a PCE of 16.5% in a simple planar architecture. Matsui et al. (2016) have synthesized three new polymeric triarylamine derivatives with methyl phenyl ethenyl and diphenyl ethenyl functional groups and has used as HTMs. These materials work without any additives, which also showed to be a detrimental for PSC stability. Polymer V873, containing poly [bis(4-phenyl)-(3,5-dimethylphenyl)amine] main chain with methyl phenyl ethenyl fragments attached to it, when used in PSC showed a PCE of 12.3% without any additives, which is better than the usually used PTAA. Zhou et al. (2017) have reported about a low temperature processed reduced graphene oxide (r-GO)/PTAA bilayer introduced into the inverted PSCs. This bilayer HTL improved the stability of the device, as the r-GO present in the HTL strongly absorbs near-UV light and the present PTAA material acts as a defect complement for r-GO. The bilayer had the benefit of resulting in larger perovskite grains, thus decreasing grain boundary density and enhancing light harvesting. In addition, the energy level gradient in between the layers at the HTM/perovskite interface effectively inhibited charge recombination and facilitates charge extraction and transport within the device.

The energy level alignment diagram of the device components of PSCs in which (n-i-p and p-i-n) long polymer based HTM used is shown in Fig. 7.

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Fig. 7 Energy-level alignment diagram of the device component of perovskite solar cells with (relative to vacuum level) long polymer based HTM in (n-i-p) PSCs.

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4.2 Organic small molecule based HTMs (pyrene/thiophene/porphyrins and carbazoles, etc.)

Some of the organic small molecules also have the ability to act as a HTM and in this part of the study, the behaviour of the PSCs having small molecule based layer as a HTM is discussed in terms of their photovoltaic properties, device performance and stability (Wang et al., 2017). This small molecules based HTMs can be easily synthesized with high purity and high yield and can be reproduced for industrial production. Its distinct molecular structure with the suitable HOMO energy level value −5.1 to −5.5 eV helps it to be an efficient hole extraction material from the perovskite absorber in PSCs (Saliba et al., 2016). Organic small molecule-based HTMs for solar cell technology was first observed in 1975 by Tang and Albrecht, but the PCE was very low (∼1%) (Abdulrazzaq et al., 2013). Thereafter much attention has been paid to develop this cost-effective small molecule-based materials suitable to be used as HTM in PSC device. The available small-molecule based HTMs has been mainly classified as pyrene, thiophene, porphyrins, and carbazoles, etc. (Bakr et al., 2017).

Jeon et al. (2013) have introduced pyrene-core derivative (1-(N,N-di-p-methoxyphenylamine) pyrene) as small molecule HTM. Its efficient charge-transport capability makes it to be a good candidate. They have reported on three different HTMs based on pyrene core and their HOMO value (5.25 eV) is found to be close to that of spiro-OMeTAD (5.22 eV) and using that they have achieved an efficiency of ∼12.4% almost equal to that of the efficiency achieved by spiro-OMeTAD (12.7%) and they have also reported that it was more stable than spiro-OMeTAD. Yang et al. (2017) introduced a π-conjugated organic small molecule (4,4′-cyclohexylidenebis[N,N-bis(4-methyl phenyl) benzenamine] (TAPC) which is found to be an efficient HTM to replace PEDOT:PSS in p-i-n-type PSCs with structure, ITO/TAPC-HTM/MAPbI₃/PCBM/Ag. This TAPC layer seems to be smooth, uniform, hydrophobic and can be easily deposited through solution casting. The π-conjugation and crystallinity of TAPC makes it to exhibit high hole conductivity without the need for doping. Further, perovskite material deposited onto TAPC also showed smooth and uniform surface morphology, with an average grain size of ≈310 nm. Their champion device exhibited a PCE of 18.80%, much higher than PEDOT: PSS HTM based devices (12.90%). Also the PSC device exhibited superior stability under ambient conditions than the standard PEDOT: PSS based devices. Their results also indicated that the performance of the devices showed very negligible dependence on the thickness of the TAPC layer.

Thiophene-based small molecule HTMs (2,5-bis(4,4′-bis (methoxyphenyl) aminophen-4′′-yl)-3,4-ethylene dioxythiophene) (H101) for PSCs was introduced by Li et al. (2014). The PSC device with thiophene-based molecules as HTM have produced a promising PCE of 10.6% in which when the HTM is doped with cobalt salt the efficiency improved to over 13%, with the best devices matching the value of 13.7% obtained using spiro-OMeTAD doped with the same salt as a reference. The same group have developed films with superior air stability and higher solubility in common solvents compared to H101 which had deeper HOMO levels (H111 and H112 are 5.31 and 5.29 eV, respectively) than H101 (5.16 eV) and they were able to develop them by making some modifications in the thiophene core based molecules and named it as, (2,3,4,5-tetra[4,4′-bis(methoxyphenyl) aminophen-4′′-yl]-thiophene (H111) and 4,4′,5,5′-tetra[4,4′-bis(methoxyphenyl) aminophen-4′′-yl]-2,2′-bithiophene (H112). The voltage values improved upon doping H101 from 0.97 to 1.05 V matching with the value observed with spiro-OMeTAD (as the HOMO of H101 is –5.16 eV compared to –5.22 eV for spiro-OMeTAD, one expects it to produce a lower V_OC) and the FF rose from 0.57 to 0.65 (0.69 for spiro-OMeTAD). This resulted in improved PV performance, from ∼13.8% for H101 to ∼14.7% and 14.9% for H112 and H111, respectively.

Chou et al. (2016) are the first group to develop porphyrin-based HTMs (Y2 and Y2A2) in which two ethynylaniline moieties are bonded to porphyrin core horizontally and the other two alkoxy-phenyl groups are bonded vertically which resulted in very low PCE value. Chen et al. (2017) have reported on using tetra alkoxy-triphenylamine substituted porphyrin-based HTMs and studied the effect of metal ions in the central cavity of the porphyrin ring and they have achieved a PCE of >17% and this device performance seems to be lower than that of the state-of-the-art spiro-OMeTAD existing during that time. Similarly, Lee et al. (2017) have recently employed a series of novel porphyrin derivatives as HTMs namely; PZn-TPA-O (5,10,15,20-tetrakis(4-bromophenyl) porphyrin), PZn-TPA (5,10,15,20-tetrakis(5-bromopyridine-2-yl) porphyrin), PZn-DPPA-O (5,10,15,20-tetrakis(4-bromophenyl) porphyrin zinc(II)) and PZn-DPPA (5,10,15,20-tetrakis(5-bromopyridine-2-yl) porphyrin zinc(II)). These PZn-DPPA based devices showed a higher PCE value of 18% compared to PZn-TPA based devices and showed much improved air-stability in both doped and un-doped forms compared to spiro-OMeTAD. This study on the structure and properties of porphyrin-based HTMs provides a valuable information for the further development of HTMs in PSCs. The hydrophobic property of PZn-DPPA contributed in enhancing air stability, while the face-on orientation offered relatively high hole-transport. The good hole transporting property of PZn-DPPA even without dopant will be beneficial for future design for dopant-free porphyrin based HTMs. Due to the ease of synthesis, good thermal, optical and photophysical properties, this type of molecules hold great promise for practical application in commercial perovskite solar cells.

Carbazole derivatives have attracted much attention due to their interesting photovoltaic properties such as low cost of its starting material, good chemical and environmental stability, it‘s reactive sites can react with a wide variety of functional groups to fine-tune its electronic and optical properties (Prachumrak et al., 2013). Magomedov et al. (2018) have developed a series of new type of HTMs containing two diphenylamine-substituted carbazole fragments linked by a nonconjugated methylenebenzene unit in PSCs. Their results on the performance of two newly introduced HTMs (V885 and V911) were on a par with spiro-OMeTAD. Chen et al. (2017) designed and synthesized a low-cost carbazole based small molecule material, 1,3,6,8-tetra (N, N-p-dimethoxyphenylamino)-9-ethylcarbazole through a facile synthetic route. The material was characterized and applied as an HTM for low-temperature processed planar PSCs. Devices based on this new HTM showed PCE of 17.8% which is comparable to that of the available spiro-OMeTAD based devices (18.6%). A simple carbazole-based conjugated enamine V950 was synthesized, fully characterized and incorporated into a PSC by Daskeviciene et al. (2017) which displayed improved efficiency of 18%. They synthesized carbazoles using an extremely simple route from commercially available and relatively inexpensive starting reagents, resulting in lower cost of the final product compared to the commercial spiro-OMeTAD. They have stated that this types of carbazole based HTMs promise to be a commercial viable p-type organic charge conductor to be employed in perovskite solar modules. Kang et al. (2015) reported the synthesis and characterization of novel carbazole-based HTMs with two or three-arm chemical structures, which were linked through phenylene-derived central core units and the PSC fabricated using them achieved a PCE of 14.79%, which is comparable to that of the device with a commercial spiro-OMeTAD (15.23%). Malinauskas et al. (2016), Gratia et al. (2015) have reported about a new type of branched hole transporting material consisting of molecules based on methoxy diphenylamine substituted fluorene and carbazoles, with PSC performance (19.96%) equal to that of spiro-OMeTAD and PTAA. Lu et al. (2017) have used a simple synthesis method and molecular engineering to develop low-cost and star-shaped carbazole based HTMs for highly efficient PSCs. They have designed and synthesized three derivatives of SGT-405(2,7), 18 SGT-410(2,7) and SGT-411(2,7) via tuning the substitution position from the 2,7 to the 3,6 position of carbazole and used them as HTM in PSC which showed a remarkable PCE of 18.87%, which was better than spiro-OMeTAD (17.71%). Thus all the above results indicate that the carbazole-based transporting materials are a promising class of hole-transporters for PSCs.

5.1 Nickel and its derivatives based HTMs

Nickel oxide (NiO) is one of the most promising HTM with expected stability as it has good optical transparency, prevents electron leakage and has appropriate energy levels (Irwin et al., 2008). NiO layer has a wide band gap and acts as a p-type transporting layer (Jeng et al., 2014) and acts as an electron blocking layer suppressing charge recombination and enhances the hole extraction in solar cells, since the performance of the solid-state PSC device mainly depends on the charge transport (Cui et al., 2014). Among several inorganic HTM used in PSCs, NiO as HTM also has the favorable deep-lying HOMO (valence band edge value) with work function from 5 eV to 5.6 eV (Garcia et al., 2012). NiO acts as a potential HTL with methylammonium lead iodide perovskite and exhibits good hole transfer characteristics and this leads to improved device performance. The inertness of NiO makes it non-corrosive towards ITO substrates and its work function makes NiO a good HTM for PSCs. Initially, Snaith’s group used NiO layer as HTL, but they obtained a photoconversion efficiency of <0.1%, due to poor adhesion between perovskite and the NiO layer.

Hu et al. (2014) built an efficient inverted planar PSC using NiO as the hole-collecting layer by using a sequential deposition method. Since NiO has an affinity for carbonaceous species and other contaminants during storage it was treated with UV radiation and this was found to increase the work function of NiO. NiO film thickness optimization and UV-ozone (UVO) treatment improved the efficiency and a PCE of 7.6% with a V_OC of 1.05 V was obtained. Li et al. (2015) have deposited CH₃NH₃PbI₃ onto planar NiO layer through a spin-coating (8000 rpm) method and built a planar ITO/NiO/CH₃NH₃PbI₃/PCBM/BCP/Al (BCP-bathocuproine) device which exhibited an efficiency of 7.9%; they then incorporated a mesoporous NiO layer into the planar device and obtained 9.51% efficiency. They also found that the UVO treatment was beneficial for the device, and the highest V_OC they achieved was 1.04 V. These reports clearly showed that NiO is indeed a very promising functional layer in PSCs. Generally, NiO nanocrystals (NiO_nc) acts as efficient light–scattering center, which increases the effective optical path length of the device and exhibits a higher transparency than conventional NiO thin films. Zhu et al. (2014) have used sol–gel process to obtain a thin NiO nanocrystalline film and used it in NiO/CH₃NH₃PbI₃ solar-cell device. The thin NiO_nc film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH₃NH₃PbI₃. The rough NiO film surface formed by the aggregation of faceted NiO nano scale mono crystals permits the formation of an intimate, large interfacial-area junction with the CH₃NH₃PbI₃ film, and this dramatically improved the cell efficiency.

Pitchaiya et al. (2018) have recently reported about the low-cost inorganic nickel sulphide-carbon (NiS-C) composite synthesized using the simple chemical method and has used it as hybrid hole extraction and as a counter electrode material for perovskite (CH₃NH₃PbI₃)-based solar cells. They have found that the optimal bandgap of the prepared NiS-C composite material is favorable in achieving the hole extraction property. The surface morphology of the nickel sulphide carbon materials is found to be highly dependent on the NiS-carbon composition. The perovskite solar cell fabricated using NiS with 20% carbon composition as HTL exhibited a power conversion efficiency of 5.20%. Wang et al. (2014), obtained an efficient charge separation at HTM/perovskite junction by replacing the p-contact with inorganic metal oxide material NiO_x as a mesoscopic layer and NiO_nc as a thin capping layer for perovskite. When a mesoscopic electrode is used as charge collector, the sequential deposition method used for depositing the different layers of a PSCs provides better filling of perovskite into the nano-porous pores and more conformal surface coverage on the NiO_nc surface. This prevents the direct contact between the n-contact layer (PCBM) with the p-contact material (NiO_nc or NiO_x). This CH₃NH₃PbI₃ perovskite/NiO heterojunction solar cell exhibited a power conversion efficiency of 9.51% with a higher V_OC of 1.04 V and J_SC 13.24 mA cm⁻². NiO/perovskite films have a very high yield (95%) of PL quenching which shows that the NiO particles not only act as a scaffold layer but have proven to be a very efficient p-type counterpart. Li et al. (2014) developed a hybrid interfacial layer of “compact NiO/meso-Al₂O₃” for an inverted PSC, which led to marked improvement in photovoltaic performance. The hybrid interfacial layer made of an ultrathin NiO compact layer (10–20 nm) and a thin meso-Al₂O₃ layer (90 nm) exhibits “dual blocking effect”. The interfacial recombination loss could be sufficiently minimized. Moreover, ultrathin NiO and transparent meso-Al₂O₃ as a hybrid interfacial layer guarantees an excellent optical transmittance, which can minimize the optical loss in NiO based perovskite sensitized solar cells. This is substantial progress in NiO based PSCs, in view of the great challenge on a critical balance between high optical transmittance and efficient electrical blocking at the interface.

A mesoscopic PSCs device using a quadruple-layer architecture consisting of inorganic metal oxide in combination with carbon counter electrode was constructed by Cao et al. (2015). Here both the NiO and Al₂O₃ layers of the fabricated cell act as a spacer layer to separate TiO₂ and the carbon counter electrode. A reduced interfacial charge recombination was observed in quadruple-layer mesoscopic PSCs containing a NiO layer when compared to the device structure that uses only Al₂O₃ as a spacer layer. As a result, a higher PCE of 15.03%, J_SC of 21.62 mA cm⁻² and FF of 76% were achieved. Moreover, quadruple-layer mesoscopic PSC devices containing a NiO layer exhibit good stability which contributes to highly efficient PSCs. When compared to the performance of PEDOT: PSS/spiro-OMeTAD and other organic HTMs, the performance of NiO in PSCs is still not satisfactory due to a lower fill factor and reduced short-circuit current density even though there is a significant improvement in the V_OC. Although ionic radius of copper is slightly larger than that of nickel, Kim et al. (2015) observed that upon incorporating 5 at% of Cu in Ni there is a reduction in the NiO_x lattice parameter. They fabricated 5 at% Cu doped NiO_x based PSCs which exhibited a PCE of 15.4% with V_OC of 1.11 V, J_SC of 19.01 mA cm⁻² and FF of 73% which is substantially better than the performance of PEDOT: PSS based devices. Similarly, Tvingstedt et al. (2014) have stated that, for a good photovoltaic material combination the absorber should display strong PL quenching due to beneficial induced charge transfer. Their work on Cu: NiO_x MAPbI₃ films suggest that there is a great degree of PL quenching indicating enhanced hole collection and transport efficiency, and exhibit good electrical conductivity. Thus, the good performance of Cu: NiO_x MAPbI₃ film based solar cells shows the promising potential of Cu: NiO_x which can be used in perovskite tandem solar cells. Park et al. (2015) prepared NiO film with good optical transparency through a pulsed laser deposition method and used it as a p-type transport layer. The PSCs constructed using the developed nanostructured NiO film exhibited a high PCE of 17.3%. To gain more insight into how the hole extraction property depends on the morphology of NiO layer, the dark current and recombination lifetime of devices were studied. Compared to the “nanostructured” film, the “disordered” film showed a higher dark current due to its loosely packed porous microstructure, which caused an insufficient electron blocking/hole extraction.

The device structure, thickness of the HTMs used, the IPCE and photovoltaic parameters of the PSCs fabricated using Nickel based HTMs are given in Table 4. The energy level alignment diagram of the PSCs device elements in the n-i-p and p-i-n device configuration is shown in Figs. 8 and 9 respectively.

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Fig. 8 Energy-level alignment diagram of the device components of perovskite solar cells in which (relative to vacuum level) NiO as HTMs in (n-i-p).

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Fig. 9 Energy-level alignment diagram of the device components of perovskite solar cells in which (relative vacuum level) NiO is used as HTM in (p-i-n) PSCs.

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5.2 Copper and its derivatives based HTMs (CuI/Cu₂O/CuO/CuCrO₂/CuGaO₂)

Inorganic copper based p-type semiconductors such as CuI/Cu₂O/CuO/CuCrO₂/CuGaO₂ are considered to be potential hole conductors to be used in PSCs. They have been already used in dye-sensitized and quantum dot solar cells. Also, their interesting photovoltaic properties such as wide band gap high conductivity, and simple synthesizing methods (solution-based) at relatively low cost have made them worthy HTMs in PSC technology.

Christians et al. (2014) have fabricated PSCs using CuI as an HTL deposited by drop casting method. The cells exhibited an efficiency of 6%. They concluded that though CuI provides a higher fill factor than spiro-OMeTAD, the efficiency is still lower which can be attributed to the higher recombination rate. As the perovskite layer used for planar device architecture is relatively thin, CuI can easily form short-circuits between the electrode and counter electrode through pinholes in the perovskite layer. Huangfu et al. (2015) have stated that a very thin mesoporous layer cannot absorb enough perovskite, and so produces a narrow depletion region which cannot separate electron-hole pairs efficiently. But a thicker mesoporous layer leads to large recombination of the injected electrons in the TiO₂ conduction band with the holes in the perovskite. The fabricated solar cells showed a very low V_OC due to (i) low charge carrier diffusion length and (ii) the cavitation in CuI layer, which creates paths that decrease the shunt resistance and increase the series resistance. Sepalage et al. (2015) have used copper iodide deposited by doctor blade technique and the surface was found to be rough and showed gaps between particles. There was no variation in the device with respect to change in the CuI film thickness, but a comparatively high efficiency was obtained with changes in the texture of the perovskite film. They also observed that the magnitude of the charge separation is smaller at the perovskite/CuI interface when compared to the perovskite/spiro-OMeTAD interface. Further, Chen et al. (2015) have synthesized CuI transport layer using the low-cost solution process method to replace the commonly used PEDOT: PSS in inverted PHJ PSCs and observed higher performance and improved stability. The hydrophobic nature and ambient stability of CuI dramatically improved the stability of inverted PHJ PSCs. The CuI film exhibits PL quenching more efficiently than that of PEDOT: PSS films which demonstrate that CuI can be used as an HTL replacing PEDOT: PSS. The root means square (RMS) roughness was found to be increased to 8.6 nm after being covered with CuI layer. The results showed higher V_OC due to the lower HOMO energy level and higher J_SC due to more efficient charge transport process and/or light harvesting. Liu et al. (2016) have used Cu₂O and CuO as HTMs in PSCs which showed significantly enhanced PCE of 13.35% and 12.16% respectively. The PL results showed that the higher PL quenching for the Cu₂O and CuO layers, which can make them to play the role of HTMs effectively and can be a suitable alternative for the replacement of unstable PEDOT: PSS.

Recently, Qin et al. (2017) introduced a novel copper-chromium (CuCrO₂) binary metal oxide as HTM synthesized using low-temperature solution-process method for PSC. The energy levels and phase composition of this binary metal oxide CuCrO₂ can be modulated by using different copper acetylacetonate/chromium acetylacetonate. Unlike copper oxides, the copper-chromium binary metal oxide films have wide band gap of around 3.1 eV, as well as suitable energy levels for hole-transporting. As a result, PSCs with this low-temperature solution processed copper-chromium binary metal oxide HTL have achieved high PCE of 17.19% on the glass and 15.53%, which is a significant enhancement when compared to mono-metal oxide-based devices. Zhang et al. (2017) have fabricated an efficient PSC using solution-processed CuGaO₂ as HTL and have acheived an impressive PCE of 18.51% and have found that the stability of these devices to be much better compared to other inorganic HTMs. Galatopoulos et al. (2017) have fabricated PSCs by replacing the most commonly used organic PEDOT: PSS with inorganic based Cu: NiOx and CuO HTLs and the PCE values increased from 8.44% to 11.45% and 15.7% respectively. Thus, using copper and its derivatives as HTMs have shown an efficient PCE and improved stability crisis in PSCs could be an alternative to the high cost and unstable organic HTMs. The device structures, method and thickness of the HTMs used, the IPCE and photovoltaic parameters of the PSCs fabricated using copper-based HTMs are given in Table 5. The energy level alignment diagram of the PSCs device elements in the p-i-n device configuration is shown in Fig. 10.

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Fig. 10 Energy-level alignment diagram of the device component of perovskite solar cells in which (relative to vacuum level) Copper based (CuI/Cu₂O/CuO) HTMs in (p-i-n) PSCs.

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5.3 p-type semiconductor HTMs (copper phthalocyanine (CuPc) & cuprous thiocyanate (CuSCN))

Copper phthalocyanine (CuPc) – being a derivative of copper is found to be one of the alternative HTM in PSCs due to it's low-cost, high hole mobility (∼10⁻³ – 10⁻² cm² V⁻¹ s⁻¹), stability and long exciton diffusion length. Considering all the superior photovoltaic properties of CuPc, Kumar et al. (2015) have fabricated solid-state PSCs by employing copper phthalocyanine as HTM. The best cell demonstrated a J_SC of 16.3 mA cm⁻², V_OC of 0.75 V and PCE of 5%. The fabricated cells show relatively large differences between the two charge acceptor levels (conduction band of titania and the LUMO level of CuPc). The fill factor of the cell was relatively small and this limits the cell efficiency when compared to the cells made with spiro-OMeTAD as HTMs. Low-temperature printable carbon cathode based PSCs was introduced for the first time with interfacial engineered dopant-free, nanorod like copper phthalocyanine to facilitate charge transportation (Zhang et al., 2016). By incorporating CuPc nanorods as hole–selective contact material, together with the printable low temperature processed carbon as a cathode material, a high-power conversion efficiency of 16.1% was obtained successfully. CH₃NH₃PbI₃/CuPc/C solar cell exhibited a high FF of 74%, which may be due to the better contact of the cathode with CuPc and low charge transport resistance. There was no significant increase in photocurrent due to the back reflection from gold (Au) cathode. In CuPc based PSCs, light transmitted through the perovskite layer could be absorbed by the thin CuPc nanorods layer and the relatively rough surface of CuPc nanorods might also act as a scattering layer blocking the transmitted light to a large extent. Thereby, the light transmitted through the layers of CH₃NH₃PbI₃ and CuPc which get reflected from the back electrode is found to be much reduced. Nanorod–like shape CuPc exhibits stronger quenching effects (∼95%), which indicate that CuPc nanorods are more effective in hole extraction owing to the high interfacial film quality with intimate contact with the perovskite. Chavhan et al. (2014) have demonstrated the viability of using solution processed CuSCN films as hole selective contact in PSCs using a planar heterojunction and have acheived PCE of 6.4%.

Wang et al. (2018) have developed an efficient and stable doped Copper Phthalocyanine based HTM, (F4-TCNQ-doped TS-CuPc) to fabricate high-performance PSCs. p-type conductivity was realized in TS-CuPc and F4-TCNQ with obviously improved film conductivity and hole mobility. The doping also resulted in a band bending of TS-CuPc with a high HOMO level of 5.3 eV, which is beneficial for hole extraction in corresponding PSCs. The TSCuPc:F4-TCNQ composite film presented good stability because of the almost neutral precursor solution, which avoided the electrode erosion. More importantly, F4-TCNQ-doped TS-CuPc could be employed as an HTM not only in the p–i–n structure but also in the n–i–p structure PSCs. Particularly, a champion n–i–p structured device with a PCE as high as 20.16% was obtained. The developed organic small molecule semiconductor of TS-CuPc provides the diversification of HTMs in planar PSCs. It will stimulate the researchers to develop other organic-based HTMs for fabricating high-performance PSCs. Ito et al. (2014) studied the behavior of HTL against humidity in PSCs. The PSCs with spiro-OMeTAD had strong humidity effects whereas the PSCs with cuprous thiocyanate as an HTM exhibited good coverage against humidity. Hence, it was understood that the humidity effect in PSCs should be associated to the HTMs rather than perovskite materials. Solution-processed inorganic p-type semiconductors have been proposed as alternatives to PEDOT:PSS owing to their relative low cost, greater stability and potential for implementation in a hybrid tandem structure. Low temperature solution processed, thin and smooth CuSCN films can be used as effective bottom HTL in planar heterojunction p-i-n solar cells (Zhao et al., 2015). CuSCN based planar heterojunction p-i-n solar cells consistently show a PCE of 10.8% with negligible hysteresis. CuSCN based PSCs show strong PL quenching implying that charge transfer occurs efficiently at the CuSCN-perovskite interface. Favorable energy level combination at CuSCN/perovskite interface increases the V_OC substantially by 0.23 V when compared to PEDOT:PSS HTL. The solar cells with CuSCN based HTL exhibit 1% higher PCE than the solar cells with PEDOT:PSS and this is used as HTL, due to significant increase in V_OC despite slight decrease in FF and J_SC. Furthermore, Ito et al. (2014) for the first-time fabricated planar PSCs with CuSCN as HTL by sequential deposition method and the cell exhibited a PCE of 11.96%. They have reported that cell surface should be smooth with less pinholes in the perovskite layer and such a smooth perovskite layer may block the short circuit between ETL and HTL layers (i.e. TiO₂ and CuSCN). Due to partial short circuits in the planar solar cells fill factors reduces. Similarly, Qin et al. (2014) have demonstrated the use of an effective inorganic p-type CuSCN-HTM synthesized using solution-process deposition method. The presence of CuSCN leads to an improvement in the overall efficiency to 12.4% through a 65% increase in J_SC and 9% increase in V_OC. The high short circuit current and IPCE values indicate efficient charge extraction and collection from the excited perovskites by TiO₂ and CuSCN, respectively. CuSCN can extract hole carriers efficiently while blocking electrons at the CuSCN/perovskite interface. The high transparency of CH₃NH₃PbI₃ improves light absorption and helps in generating higher photocurrent in the device. The optimized CuSCN derived PSCs exhibited a PCE of 16% increase in V_OC to 1.07 V. This can be attributed to better energy alignment or reduced resistance at the HTL/perovskite interface, which minimizes the potential loss across the HTL/perovskite interface and increases the built-in potential of the device to a value more than PEDOT:PSS. High transmittance is another important reason for improved J_SC in the device (Jung et al., 2015). It is noteworthy to mention that the morphology and crystallinity of perovskite layers in both CuSCN and PEDOT:PSS HTLs are quite similar and so the higher V_OC and J_SC of the CuSCN based device is mainly attributed to a better energy level matching at the CuSCN/perovskite interface which leads to better charge extraction and transport. CuSCN film as HTL was used in inverted PSCs (Ye et al., 2015). They have discussed the feasibility of various deposition methods in obtaining high–quality perovskite films on top of a rough CuSCN layer by a one-step, fast-deposition crystallization method and have obtained an efficiency of 16.6% with CuSCN as HTM. Compared to the perovskite film present on top of a rough CuSCN layer prepared by a conventional two-step sequential deposition process, the film prepared by one-step process had lower surface roughness and the interface contact resistance between the perovskite layer and the selective contacts was small, which resulted in higher PCE (16.6%). Jung et al. (2016) have fabricated PSCs using low temperature solution-processed CuSCN as the inorganic HTL, which possesses a highly stable crystalline structure and is robust even at high temperatures. The fabricated cell delivers a PCE of 18.0%. Thus, the hydrophobic copper based derivatives CuPc and CuSCN materials act as an HTM as well as an interlayer which prevents the perovskite devices under ambient conditions.

The device structure, thickness of the HTMs used, the IPCE and photovoltaic parameters of the PSCs fabricated using CuPc and CuSCN as HTMs are given in Table 6. The energy level alignment diagram of the PSCs device elements in the n-i-p device configuration for p-Type semiconductor (CuPc/CuSCN) based HTMs is shown in Fig. 11.

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Fig. 11 Energy-level alignment diagram of the device component of PSCs with (relative to vacuum level) p-type Semiconductor (CuPc/CuSCN) based HTMs in (n-i-p) PSCs.

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5.4 Transition metal oxide based interfacial/HTMs (MoO_x/VO_x/V₂O_x/WO_x)

The acidic (pH ∼ 1) and hygroscopic properties of an organic material always result in a dramatic degradation of the PSCs in ambient conditions and this has made way for the researchers to focus on transition metal oxides such as MoO₃, V₂O₅, WO₃, etc. which have been successfully demonstrated as, as a dopant and as a material for the HTL due to their enhanced stability in PSCs and also due to their high transparency, high work function, smooth surface, high conductivity, and easy solution based synthesis process (Qian et al., 2015).

5.4.3

5.4.3 Tungsten oxide (WO₃) as HTM

Tungsten oxide (WO₃) has been employed as hole injection layer in organic light-emitting diodes to reduce the injection barrier at the polymer/electrode interface and lead to higher device performance. In the case of PSCs, WO₃ film can also significantly reduce the series resistance and improve the fill factor in conventional, tandem, and inverted PSCs. Xu et al. (2013) have prepared the (WO₃) buffer layer on an ITO electrode by solution processing from tungsten (VI) isopropoxide solution and then annealed at 150 °C. The solution-processed WO₃ layer is highly transparent in the visible range and shows an effective hole collection property. Thus, this s-WO₃ as HTM layer shows high hole mobility and high light transmittance and showed enhanced performance (6.36%) in comparison with the PEDOT: PSS modified devices. This work gives a new option for the selection of the solution-processed anode buffer layer in designing higher efficiency and more stable PSCs. The energy level alignment diagram of the device components of PSCs in which MoO_x/VO_x/V₂O_x/WO_x are used as interlayer/HTMs (p-i-n) and are shown in Fig. 12. By using transition metal based HTMs as the interfacial layers, an enhanced efficiency and improved stability can be achieved, which cannot be readily achieved with organic interfacial materials. All the above-discussed results and the fundamental insights could be useful in the development of achieving high efficiency and long-term stable PSCs devices at low cost.

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Fig. 12 Energy-level alignment diagram of the device components of PSCs in which (relative to vacuum level) transition metal oxide based interlayer/HTMs is used in (p-i-n) PSCs.

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From this review on HTM in PSCs, it has been clearly recognized that interfacial engineering plays a critical role in cell performance. It has also been stated that the open circuit voltage is dependent on the ionization potential of the HTMs.

The device structures, thickness of the HTMs used, the IPCE and photovoltaic parameters of the PSCs fabricated using transition metal oxide as HTMs is given in Table 7. Energy-level alignment diagram of the device components of PSCs in which (relative to vacuum level) transition Metal Oxide (MoO_x/VO_x/WO_x) based interlayer/HTMs are used in HTL (p-i-n) are shown in Fig. 12.

Table 7 Photovoltaic characteristics of perovskite solar cells using MoO_x, GeO_2, & V₂O₅ and CrO₃ (as an interlayer) as HTMs.

S. no.	Device gestalt	Structure type	MoOx & V2O5 (as an interlayer) based HTLs		IPCE (response range) (nm)	Photovoltaic parameters				Ref.
			Method	Thickness		VOC (V)	JSC (mA/cm2)	FF (%)	η (%)
1.	TiO2/PSK/spiro/MoOx/Al	Planar n-i-p	Thermal evaporation	10 nm	300–800	0.993	20.14	60	12.04	Yixin Zhao and Zhu (2014)
2.	ITO/MoO3/PEDOT:PSS/PSK/C60 B-phen/Ag	Planar p-i-n	Spin coating	–		0.96	20.06	67	12.78	Hou et al. (2015)
3.	ITO/MoO3/NPB/PSK/C60/BCP/Al	Planar p-i-n	Thermal evaporation	5 nm		1.12	18.1	68	13.7	Kim et al. (2015)
4.	FTO/TiO2/PSK/spiro/MoOx/Ag	Planar n-i-p	Spin coating	50 nm		0.938	18.5	0.67	11.6	Sanehira et al. (2016)
5.	ITO/PEDOT: PSS/CH3NH3PbIxCl3-x/PC61BM/-Bphen/Ag	Planar p-i-n	Spin coating	5 nm	350–800	0.71	16.93	0.61	7.46	Lou and Wang (2017)
6.	ITO/PEDOT: PSS- MoO3/CH3NH3PbIxCl3-x/PC61BM/B-phen/Ag					0.70	17.01	0.63	7.58
7.	ITO/PEDOT: PSS-GeO2/CH3NH3PbIxCl3-x/PC61BM/Bphen/Ag					0.72	17.26	0.63	7.83
8.	ITO/PEDOT: PSS-V2O5/CH3NH3PbIxCl3-x/PC61BM/Bphen/Ag					0.73	17.35	0.66	8.36
9.	ITO/PEDOT: PSS-CrO3/CH3NH3PbIxCl3-x/PC61BM/Bphen/Ag					0.68	15.63	0.60	6.38

6.1 Hole-transport-free based carbon HTMs

Zhou et al. (2014) have discussed the effect of thickness of carbon film on the electrical conductivity of carbon layer. As the thickness of the carbon layer is increased from 2 to 20 μm, the device showed a significant increase in efficiency which increased from 2 to 6.9%, and this has been attributed to reduced square and series resistance of the device. They also modified the thin film structure by sandwiching active perovskite (CH₃NH₃PbI₃) layer between n-type ZnO which acts as an ETL and carbon electrode which acts as hole selective layer (ZnO/CH₃NH₃PbI₃/C PHJ-PSCs) and have achieved PCE up to 8%. This carbon-based hole selective layers are of low cost and are of simple architecture. The facile fabrication process at low temperature without using high vacuum is a promising criterion for future production of all flexible PSCs. The use of a low-cost carbon material prepared at low temperature on top of CH₃NH₃PbI₃ without destroying the structure of perovskites offers numerous possibilities to choose materials and optimize the structure of the device and achieve good efficiency.

Zhang et al. (2016) used low-temperature-processed (100 °C) carbon paste in PSCs and reported an impressive PCE value of 8.31% and here carbon acts as both counter electrode and as hole transporter. Impedance spectroscopy results show that good charge transport characteristics are exhibited by a low-temperature processed carbon material. Moreover, long-term stability studies show that the carbon-based PSCs exhibit good stability even after 800 h. Further, the crystallite size and morphology of carbon materials play a key role in determining the performance of the cell as they influence the electrical contact at the CH₃NH₃PbI₃/Carbon interface (Yang et al., 2014). They have reported that PSCs with carbon made of smallest graphite flakes and carbon black particles exhibit the highest efficiency of 10.2%. The performance of carbon PSCs shows that it is possible soon to develop low-cost and highly efficient flexible PSCs. Moreover, they have suggested that the material contact between carbon and perovskite layer is the key to achieve good performance in PSCs. Wei et al. (2014) have reported that candle soot is an effective hole extractor and is a cost-effective, environmentally stable and abundantly available material. They have developed PSCs by interfacing the candle soot with CH₃NH₃PbI₃ films. The fabricated solar cells exhibited a remarkable efficiency of 11.02% and good long-term stability. Femtosecond time-resolved PL and distance-dependent PL measurements have confirmed that the improved PCE is largely due to the enhanced directional hole extraction. They have also fabricated planar carbon-based PSCs that have a nano-carbon hole-extraction layer prepared by inkjet printing technique with a precisely controlled pattern and low interface. This inkjet printing technique could not only be used to precisely control the pattern of the carbon electrode, but also improve the interface between the CH₃NH₃PbI₃ active layer and the carbon hole-extraction electrode. As a result, considerably higher power conversion efficiency of 11.60% has been achieved. Liu et al. (2015) have fabricated carbon-based PSCs devices by effectively depositing uniform and continuous MAPbI₃ onto ZrO₂/TiO₂ films by a sequential deposition method and obtained a high efficiency of 13.14%. They have concluded that some of the parameters, like the thickness of spacer layer, pore filling and morphology of CH₃NH₃PbI₃ perovskite, the temperature of substrate and precursor greatly influence the performance of these types of solar cells.

Zhang et al. (2015) have reported about the effect of the size of the graphite layer on the performance of the cell and they have stated that an overly thick layer of carbon counter electrode (CCE) will restrict the diffusion of the PbI₂ precursor from the CCE layer to the TiO₂ layer, resulting in incomplete pore filling. Therefore, the thickness of the CCEs should be optimized. They have fabricated devices using graphite-based carbon materials of different thicknesses (40 nm, 500 nm, 3 μm, 6 μm, 8 μm), among these, the device with 8 μm graphite thickness had a larger average pore size, smaller square resistance and hence a higher PCE exceeding 11%. It confirms that graphite thickness has a huge influence on the porosity and conductivity of carbon. Wei et al. (2015) have synthesized free-standing thermoplastic carbon film with good flexibility and conductivity and introduced it in PSCs. Due to its unique nature of thermo-plasticity, this free-standing carbon film can be directly hot pressed onto the perovskite layer as a counter electrode. By optimizing the weight ratio between graphite and carbon black to 3:1 an increase in J_SC from 18.77 to 21.30 mA/cm² was achieved and the FF improved from 0.566 to 0.634, yielding the highest PCE of 13.53%, which is one of the highest efficiencies reported for PSCs fabricated without an HTL.

The device structures, thickness of the HTMs used, the IPCE and photovoltaic parameters of the PSCs fabricated using carbon are given in the Tables 8 and 9 (as a hole-transport-free layer in Table 8 & as HTL in Table 9). Fig. 13 provides an energy-level alignment diagram of the device components of PSCs in which (relative to vacuum level) a carbon-based hole-transport-free layer (p-i-n) is shown.

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Fig. 13 Energy-level alignment diagram of the device components of perovskite solar cells with (relative to vacuum level) carbon as hole-transport-free material (n-i-p).

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6.2 Graphene and doped graphene oxide based HTMs

Compared to carbon materials, graphene oxide (GO) and reduced graphene oxides (r-GO), which have a two-dimensional sheet structure consisting of carbon atom monolayer, are considered to have a great potential to be used in PSCs as they exhibit good electrical conductivity and high specific surface area (Cho et al., 2014). The high electrical conductivity is due to the extremely high electron mobility and fast heterogeneous electron transfer (Jaison et al., 2015). Therefore, due to the afore-mentioned properties, graphene derivatives GO and r-GO have been used as a substitute for spiro-OMeTAD as HTM in PSCs. This could be a viable strategy both economically and technically, i.e., due to mobility values and stability of the device (Liu et al., 2015; Yeo et al., 2015). Replacing the organic HTL by GO may result in better efficiency and then for improving the cell stability, r-GO could be a good choice.

During the initial stages when GO was introduced as HTM in PSCs, it was employed just as a bilayer along with PEDOT: PSS. This GO bilayer modification strategy takes the advantages of the high conductivity of PEDOT: PSS and good electron-blocking capability of GO, and produces a marked increase in PCE from 10% to 13.1%.

Li et al. (2014) have studied the interface properties of PSCs and reported that the interface wettability of the HTL solution on a perovskite surface could be improved. To address this problem, GO with amphiphilic function is used to form a buffer layer between the perovskite and the HTL. After the GO modification, the contact angle of the HTL solution with the perovskite film decreased to zero degrees. The GO layer improved the contact between the perovskite and HTL, resulting in an enhancement of the short-circuit current density. Moreover, using GO as an insulating buffer layer can retard charge recombination in solar cells, as revealed by EIS measured in dark conditions, leading to a significant increase in the open-circuit voltage and the fill factor. Therefore, application of GO as a dual-function buffer layer on the perovskite layer is a useful strategy for preparing highly efficient hybrid PSCs. Yeo et al. (2015) are the first research group to introduce r-GO, as a novel HTM and fabricate and highly efficient and stable CH₃NH₃PbI₃ PSCs. The fabricated device exhibited excellent and reproducible device efficiency up to 10.8%, which was superior to the PEDOT: PSS and GO-based devices. A systematic investigation on the mechanisms responsible for the efficiency enhancement of r-GO devices revealed that the facilitated charge collection with a retarded recombination, high r-GO conductivity, better-aligned energy levels, and better growth of the perovskites on r-GO HTMs are responsible for the high efficiency. More importantly, in the ambient stability test, they found that the use of r-GO HTMs with the inherent passivation-ability greatly extended the cell lifetime compared to PSCs employing conventional PEDOT: PSS. These successful demonstrations showed r-GO to be an advanced interfacial material which has the potential to replace the conventional PEDOT: PSS HTMs because of the observed photovoltaic performance and device-stability.

Palma et al. (2016) have demonstrated the application of reduced graphene oxide as effective HTM for the long-term stability of PSCs. They have reported about the stability study that a 1987 h’ shelf life test showed that r-GO-based PSCs shows 36% increase of efficiency (PCE = 6.6%), with respect to the value recorded by r-GO-based PSCs (PCE = 4.87%). The results of endurance tests showed that the reduced graphene oxide is not affected by the issues of additives as tBP which corrodes the perovskite material when used as an additive in spiro-OMeTAD. The increase of the PCE in the r-GO-based PSCs can be related to the reduction in the density of charged bulk defects within the perovskite layer due to the light illumination during measurements.

The device structure, and thickness of the HTMs used, IPCE and photovoltaic parameters of the PSCs fabricated using carbonaceous materials GO and r-GO as HTMs are given in Table 9. Fig. 14 shows the energy-level alignment diagram of the device components of PSCs in which (relative to vacuum level) carbonaceous materials GO and r-GO are used as HTMs (p-i-n).

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Fig. 14 Energy-level alignment diagram of the device components of perovskite solar cells with (relative to vacuum level) graphene oxide/reduced graphene oxide as HTMs in (p-i-n).

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