Materials Today Energy xxx (2017) 1e13
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Organic hole-transporting materials for efficient perovskite solar cells Xiaojuan Zhao, Mingkui Wang* Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan 430074, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2017 Received in revised form 12 September 2017 Accepted 14 September 2017 Available online xxx
Nowadays hole-transporting materials based on conductive small organic molecules and polymers have become the hottest topic in high-performance perovskite solar cells. Currently, the perovskite solar cells have reached certified power conversion efficiency of 22.1%. 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)-9,90 -spirobifluorene and poly-triarylamine are the two widely used hole-transporting materials in highly efficient perovskite solar cells. It is highly desirable to seek other low-cost and efficient holetransporting material for highly efficient and stable perovskite solar cells. This review summarizes recent progress of typical small molecules, conductive polymer hole-transporting materials used for efficient perovskite solar cells. Particularly, we focus on those hole transporting material based on triphenylamine units. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Perovskite solar cells Hole-transporting material Small organic molecule Conductive polymer
1. Introduction Perovskite solar cells nowadays (PSCs) have become effective, attention grabbing topics in renewable and sustainable energy due to its impressive capability of converting solar energy into electricity with high power conversion efficiency. Over the past few years, great progresses have been made in perovskite solar cells though the investigation is still in progress [1e3]. Organicinorganic hybrid perovskite, has a specific crystal structure with the ABX3 formula (Fig. 1), where A represents the organic cations þ 2þ CH3NHþ and X repre3 (MA) or HC(NH2)2 (FA), B represents Pb sents halogen ions Cl, Br or I. The relatively large-volume A (MA or FA) cation forms a cube unit cell. B (Pb2þ) cation locates in the center of the cube unit cell and X (Cl, Br or I) anion resides surface center of the cube unit cell. Under irradiation of electromagnetic wave, perovskite can be excited with converted photocurrent. Perovskite materials possess high extinction coefficient, suitable band gap, long charge diffusion range, broad spectrum absorption range and excellent bipolar carrier transport properties to guarantee corresponding devices with high photoelectric conversion efficiency [4e6]. PSCs can be divided into the conventional neiep and the inverted peien architectures according to the electron transporting layer or the hole-transporting layer being in contact with
* Corresponding author. E-mail address:
[email protected] (M. Wang).
the transparent conductive substrates (Fig. 2), where n- and prespectively refer to n-type and p-type charge carrier transporting materials, and i refers to the perovskite optical absorption layer. For example, in the conventional planar PSCs devices [7e11], a flat electron transporting layer (ETL) is deposited on the transparentconductive oxide (TCO) substrate such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO) as cathode, then perovskite materials deposited by spin-coating or vacuum evaporation on top of ETL, followed by the deposition of flat hole transporting layer (HTL) and metal electrode, such as Au or Ag. While in the inverted PSCs [12e16], the position of ETL and HTL is simply exchanged. Compared to planar structural PSCs, the mesoporous neiep PSCs [17e21] utilize the compact TiO2 layer formed on FTO/ITO as ETL, then depositing the mesoporous TiO2 layer acting as ETL and structural scaffold. Afterwards, the perovskite precursor solution is deposited to form perovskite film followed by deposition of the HTL and metal electrode. The main function of the hole transporting materials (HTMs) in devices is to collect and transport after holes injection from light harvester, realizing effective separation of electrons and holes, which is an important and essential part of the PSCs. In order to achieve high-performance PSCs, HTMs ought to possess: (1) suitable highest occupied molecular orbital (HOMO) energy levels for matching with the valence band energy (VBE) of perovskite materials, together with ensuring holes injection and transporting at each interfaces; (2) high hole mobility and photochemical stability; (3) suitable solubility in organic solvents and good film-forming ability for processing and device fabrication; (4) suitable light
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Fig. 1. Crystalline structure of perovskite ABX3, A: MA or FA cation, B: Pb2þ cation, X: Cl, Br or I anion.
on. The organic HTMs usually contains small molecular compounds [49e52], such as 2,20 ,7,70 -tetrakis(N,N-di-p-methoxyphenylamine)9,90 -spirobifluorene (spiro-OMeTAD), and conducting polymers [53e56], including poly(3-hexylthiophene-2,5-diyl) (P3HT). Insertion of suitable HTMs between perovskite layer and metal electrode can promote the separation of electrons and holes in the functional layer interface, and thus reduce charge recombination and improve the performance of solar cells. The HTMs with stable thermodynamic and optical properties would help to improve the stability of PSCs as well. Perovskite crystal can be easily decomposed in humidity environments. Therefore, the quality of perovskite films can be mostly decided by the hydrophobicity of the hole-transporting layer. Those hydrophobic hole-transporting layers can protect perovskite materials from moisture and thus ensure perovskite materials to sustain crystals with larger grain size and fewer grain boundaries. In this review, we focus on the unique properties of HTMs for PSC with excellent photovoltaic performance. 2. The progress of HTMs for PSCs 2.1. Inorganic HTMs
Fig. 2. The illustration of typical perovskite solar cell (a) the conventional (neiep) and inverted (peien) structure and (b) energy level scheme for transporting direction of electron and hole carriers in neiep perovskite.
absorption in visible and near-IR region of the solar spectrum for high photocurrent [22e25]. Furthermore the design rules of HTMs for PSC applications are shown here. Firstly, the fill factor (FF) of PSC is mainly affected by the series resistance and conductivity of HTMs. In order to improve the FF, many efforts have been focused on improving the hole mobility and conductivity of HTMs. Secondly, the photocurrent is mainly determined by the hole extracting rate of HTMs, so the p-doped HTM is the research priority. Thirdly, the Voc of PSCs has following expression.
Egap AkT ð1 pÞgNc2 ln Voc ∝ q q PG
!
where g is the Langevin recombination constant, P being the dissociation probability of a bound electronehole pair into free charge carriers, G being the generation rate of bound electronehole pairs, Nc being the effective density of states, A being the exponential factor [26e28]. Energy band structure and carrier transport direction in an neiep PSC are illustrated in Fig. 2b. Upon sunlight irradiation, the electronehole pairs generate in light-harvester perovskite layer [29e31]. After that, electrons transport to the conduction band of electron transporting materials (ETM) and holes transport to the HOMO energy level of HTM, which are collected by cathode and anode, respectively. At last, the photocurrent comes into being in a complete cycle [32e35]. The HTMs in efficient PSCs can be categorized into inorganic and organic materials. At present, the most widely used inorganic HTMs are CuI [36e38], Cu2O [39e41], NiO [42e46], CuSCN [47,48] and so
NiO with wide optical band gap is the most investigated inorganic HTMs in inverted planar PSCs. The device using un-doped NiO film with ultrathin film thickness of 6 nm achieved a high PCE of 16.40% with high Voc and FF values of 1.04 V and 0.72, respectively [57]. Cu-doped NiOx HTM termed as Cu:NiOx further increased the device PCE to 17.74% with high FF value (0.76). This can be caused by its high conductivity (1.25 103 S cm1), resulting in a low series resistance (Rs ~ 1.8 U cm2) and high parallel resistance (Rsh ~ 3900 U cm2) [58]. The CuSCN with proper energy level has been demonstrated as an efficient HTM, exhibiting smooth and continuous surface in planar hetero-junction PSCs [59]. The PSC device using CuSCN thin film at an optimal film thickness of 40 nm gave an impressive PCE of 16.0%. Although inorganic HTMs have achieved good photovoltaic performance, most of devices fabrication needs high-temperature processes. In order to realize lowtemperature solution-processed PSCs with high efficiency, it is highly necessary to develop effective organic HTMs. 2.2. Organic small molecule HTMs 2.2.1. Triphenylamine (TPA)-based organic small molecule HTMs Triphenylamine (TPA)-based small molecule HTMs accounted for a large proportion and displayed satisfactory photovoltaic performance together with excellent thermal stability and photochemical properties [20,60e64]. Some of them hold big potential for achieving highly efficient PSCs. In the TPA units, the nitrogen atom attracts electrons from the aromatic rings through inductive effect, meanwhile those unshared electrons in the nitrogen atoms supply to the aromatic rings, consequently making them electron rich owing to p-p conjugated effect. Due to the conjugate effect being greater than induction effect, the electron cloud of TPA units is widely distributed along triphenylamine moiety. Therefore, electrons are easily lost to form positively charged vacancy, which finally facilitate the holes transmission. The compounds based on TPA unit possess low ionic potentials, solubility in organic solvents and light stability, which can meet the requirements from PSCs. However, the non-planar TPA-based compounds could induce long intermolecular distance and thus decrease the hole mobility (106 ~ 105 cm2 V1 s1). Thus, additives such as 4-tert-butylpyridine (TBP), lithium and cobalt salt additives are needed to enhance hole mobility and regulate their energy levels. A number of novel-structure cores have been reported and widely employed in TPA-based small molecule HTMs (Fig. 3). For example, after
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Fig. 3. Development of novel-structure cores used in TPA-based small molecule HTMs.
introduction of spirofluorene [65] in 2012, there have emerged others such as SAF, SFX, bifluorene, TPA and carbazole units. Their unique properties and potential application in perovskite solar cells are discussed as follows. Spiro-OMeTAD and its derivatives have been proven to be most promising HTMs for PSCs devices. In 2014, Seok et al. [66] synthesized three compounds coded as pm-, pp- and po-spiroOMeTAD, which possess suitable optical properties, good solubility and wide band gap (Fig. 4). The ortho-methoxy substituted derivative show high LUMO energy level. This could well play the role of blocking electrons, which would bring about low Rs and high
Fig. 4. Chemical structures of spiro-fluorene core TPA-based small molecule HTMs.
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Rsh as a return, giving a better FF as well as a high PCE. Their electrochemical properties including the HOMO and LUMO energy levels can be tuned via changing substitution site of the methoxy groups. The fabricated PSC devices showed PCEs of 13.9%, 14.9% and 16.7% for pm-, pp- and po-spiro-OMeTAD, respectively. The amorphous organic spiro-MeOTAD has been the most popular HTM for PSCs due to its good solubility, weak absorption within the scope of visible light, good film-forming properties, and high hole mobility [5,67,68]. Spiro-MeOTAD was introduced into solid-state sensitized mesoporous heterojunction solar cells [69]. Though the charge transfer from the sensitizers to spiro-MeOTAD in the heterojunction is very efficient, the overall PCE of these devices is relatively low due to a fast charge recombination at the dye-sensitized TiO2/spiro-MeOTAD interface as discovered latterly. To date, the PCE of mesoporous PSC using Spiro-OMeTAD as HTM has achieved 21.3% [70]. However, it is noted the synthetic conditions for spiro-OMeTAD are complicated, especially the difficult purification. Meng et al. [71] reported novel HTMs termed Spiro-E and Spiro-S (Fig. 4) via three-step synthetic routine by replacing para-methoxy substituent in spiro-OMeTAD with ethyl and methylsulfanyl groups. By heteroatom substitution in spiro-bifluorene, Spiro-E and Spiro-S lower the HOMO energy levels, which potentially can enhance the device Voc. When employed in the inverted PSCs, the high hydrophobicity of Spiro-E and Spiro-S layers is benefited to form highquality perovskite films on top of them as discussed above. Sun et al. found that charge transporting rate could be improved through breaking the carbon bond of fluorene in spiro-OMeTAD [72]. The synthesized materials show good solubility in organic solvents and high hole mobility (HT1, 1.12 104 cm2 V1 s1, HT2, 1.04 104 cm2 V1 s1) (Fig. 4), which is an order of magnitude higher that of spiro-OMeTAD (8.25 105 cm2 V1 s1). The broad three-dimensional structure ensures the HT2 forming high-quality film and efficient photovoltaic performance. Due to the lack of pep intermolecular force, it is necessary to doping spiro-OMeTAD with lithium bis(trifluoromethanesulfonyl) imide (Li-TFSI) and TBP. Introduction fluorine-dithiophene core forms a new HTM FDT (Fig. 4), which stacks in a slipped fashion through CH/p hydrogen bonds and pep interaction. And the additional thiophene-iodine interaction may improve hole transfer at the FDT/perovskite interfaces. The PSC based on FDT HTM showed impressive PCE of 20.3% with high Voc value (1.15 V), being higher than that of spiroOMeTAD under the same conditions [73]. Change the spiral cores consequently results in a series of new HTMs. For example, Chen et al. [74] synthesized three small molecule HTMs based on SAF core named as CW3, CW4 and CW5 (Fig. 5). Their results revealed that the steric interaction of HTMs might have an effect on morphological coverage. With proper alkyl chains, CW4 possesses preferable coverage on top of perovskite layer, bringing in high photocurrent as well as a better PCE of
Fig. 5. Chemical structures of SAF core TPA-based small molecule HTMs.
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16.56%. Lately Liao et al. [75] replaced the two alkyl chains with triphenylamine groups in SAF core to form SAF-OMe (Fig. 5). The SAF-OMe owns proper HOMO energy level (5.07 eV) and well lines with that of perovskite material (5.43 eV). The LUMO energy level of SAF-OMe (2.11 eV) is above that of perovskite (3.93 eV), thus blocking the reverse transporting of electrons and enhancing the Voc of the final devices. When applied in PSCs, dopant is unnecessarily used to the SAF-OMe and thus simplifies devices preparation process, which provides a new way to design novel HTMs. Fig. 5 shows new TPA-based HTMs employing a novel spiro [fluorene-9,90 -xanthene] (SFX) as core, including X59 linked by two bis(4-methoxyphenyl) amine groups in the same side [46]. The PSC devices with the X59 HTM possess minimized hysteresis and reasonable long-term stability (only 2.5% efficiency loss under dry and dark conditions). Later investigation found that by adding substitutes in SFX core, newly synthesized X60 (Fig. 6) [76] shows fast hole mobility (1.9 104 cm2 V1 s1) and high conductivity (1.9 104 S cm1). When employed in PSCs, an impressive PCE of 19.84% was achieved, which could compete with the record of spiro-OMeTAD (21.1%). Furthermore, Zhan et al. [77] changed the positions of bis(4-methoxy phenyl)aniline or bis(4methoxyphenyl) amine groups in SFX core, and synthesized three small molecule compounds for PSCs, including mp-SFX-3PA, mmSFX-2PA and mp-SFX-2PA (Fig. 6). The mp-SFX-2PA possesses larger water contact angle (111.4 ) than that of spiro-OMeTAD (80.6 ), indicating better hydrophobicity for the former, which could protect perovskite layer from moisture. Bifluorene (BF) core refers to insertion of carbonecarbon bonds or other functional groups in two fluorene units [78e81]. It is interesting to note that the torsion angle between two fluorine units plays an important role in determining device performance. For example, Nazeeruddin et al. investigated a straightforward synthetic KR216 (Fig. 7) [82]. The dihedral angle of KR216 (42 ) is smaller than spiro-OMeTAD (90 ), which might lead to an extended intermolecular pep stacking. Further investigation has shown that the hole-transporting rate of KR216 is not enough to
Fig. 6. Chemical structures of SFX core TPA-based small molecule HTMs.
Fig. 7. Chemical structures of BF core TPA-based small molecule HTMs.
sustain a high photocurrent and thus reducing the device PCE [83]. Therefore, a new BF core-based HTM named as H11 is designed (Fig. 7), whose two fluorenes are connected with carbonecarbon single bond. The H11 possesses a dihedral angel of 81 close to that of spiro-OMeTAD. H11 exhibits high hole mobility (7.0 104 cm2 V1 s1) and high conductivity (3.4 105 S cm1), ensuring efficient charge separation and hole extraction. An impressive PCE of 19.8% was obtained with H11, showing a high short current density (Jsc) value of 24.2 mA cm2. These results demonstrated that the BF core in TPA-based HTMs with high torsion angel has the potential for achieving highperformance PSCs. On the other hand, Getautis et al. [84] suggested inserting phenyl or thiophene groups between two fluorenes as demonstrated by V859 and V862 molecules (Fig. 7). Compared with spiro-OMeTAD, the synthesis of these HTMs was straightforward without redundant and costly purification. They both displayed high charge mobility (~103 cm2 V1 s1), lateral thin-film conductivity (~105 S cm1) and excellent devices stability (in the dark for 50 days) under dry conditions. Impressive PCEs of 19.47% and 19.96% similar to spiro-OMeTAD were achieved with V859 and V862, respectively. Triphenylamine core linked with terminal TPA units may form long-stability materials. Now we have realized, dopants and additives added in HTMs often lead to low stability of perovskite devices. The improved device stability can be ascribed to the avoidance of using the deliquescent additives. We emphasize that hydroscopic additives might accelerate the degradation of the PSC devices and the development of HTMs with high hole mobility in their pristine forms are of great importance from long-term stability point of view [85]. A novel dopant-free “butterfly” Z1011 (Fig. 8) was reported by Gr€ atzel et al., [86] which possesses better encapsulated device stability (PCE dropped 2.2% after 312 h light soaking) than that of spiro-OMeTAD. The Z1011 based device FF kept similar values with varying light intensity due to its high hole mobility (8.49 104 cm2 V1 s1). Adding a branch substituent and a star-shape results in the Z1013 (Fig. 8) [87], which shows good planarity and long conjugation length. The aging test at 80 C for 3 weeks confirms the Z1013 based devices stability. Although the above-mentioned dopant-free HTMs perform well in PSCs, their synthetic routines are complicated. In this regard, Jin and coworkers [88] reported low-cost CzPAF-SBF and CzPAF-SBFN synthesized within four-step (Fig. 8). The resulted PSCs devices
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Fig. 8. Chemical structures of triphenylamine as core TPA-based small molecule HTMs.
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SGT-405 displays well crystalline structure, thus can form smooth film with few traps, resulting in efficient hole transporting, extending electron transmitting life and even reducing charge recombination. The devices using these HTM showed higher FF (~0.71) Increasing length of alkyl terminal substituents resulted in a derivative SGT-411 (Fig. 9) [92], showing good solubility. The SGT411 possesses extended p-conjugated structure, thus better film formation and increased the FF. Yet the synthesis of SGT type HTMs is a little complicated. Taking this into account, a two-step synthetic V886 (Fig. 9) was presented by Nazeeruddin et al. [93] The V886 displays high hole mobility (6.4 104 cm2 V1 s1) and good filmforming capability on top of perovskite layer, resulting in high FF and Jsc values of 0.73 and 21.38 mA cm2, respectively. When applied to neiep meso-PSCs, an excellent PCE of 16.92% was obtained. Furthermore, Hagfeldt et al. [94] simplified the molecular structure of X25 (Fig. 9), characterized with X2 from prior literature [95]. From a structural point of view, slight adjustment on chemical structure of molecules would bring up a big influence onto photovoltaic performance. Compared with X2 (5.09 eV), the X25 possesses deeper HOMO energy level (5.16 eV), favoring to enhance Voc (1.07 V). Eventually, the X25 based PSCs reached an impressive PCE of 17.4%.
without encapsulation showed good stability under identical storage conditions (storage time z 500 h). Compared with CzPAF-SBF, CzPAF-SBFN shows red-shifted absorption due to its high intermolecular charge transferring, which attributes to the terminal CN groups. Hagfeldt et al. [89] investigated the effects of substituent length of X21 and X22 on photovoltaic performance (Fig. 8). According to their research, long alkyl substituents might be good for final solubility, but would hinder intermolecular stack of solid-state films. The X21 with suitable alkyl chains in fluorine moiety gave an impressive PCE of 17.33%. Among organic small molecule HTMs, carbazole ones also exhibit adorable performance. Snaith et al. reported a hydrophobic and simple-structured carbazole-based HTM EH44 (Fig. 9) [90]. The doped EH44 shows a water contact angle of 96 , indicating its high hydrophobicity and ability of protecting perovskite from moisture, thus enhance the stability of corresponding solar cells. However, an inferior FF (0.60) was usually observed for the devices due to the EH44 films with copious pinholes. In order to improve the film quality, Lee et al. [91] introduced carbazole groups and synthesized SGT-404, SGT-405 and SGT-407 (Fig. 9). Among three HTMs, the
2.2.2. Biphenyldiphenylamine-based organic small molecule HTMs Biphenyldiphenylamine-based materials (Fig. 10) refer those with different cores inserted into two triphenylamine terminal units. Xiao et al. [96] synthesized TPBC (Fig. 10), which possesses deep but suitable HOMO energy level (5.33 eV) and thus gave high Voc. Devices based on TPBC displayed good reproducibility without any dopants. Grimsdale et al. [97] reported H101 using an electron-rich EDOT as the core (Fig. 10). Given the 20 nm redshifted absorption, the H101 possesses a higher photocurrent (20.5 mA cm2) than that of spiro-OMeTAD (18.9 mA cm2). A further structural optimization of H101 produces SP-01 using longalkyl substituted EDOT as central core [98]. The SP-01 shows high water contact angle (96.1 ), indicating its hydrophobic property. Afterwards, Grimsdale et al. [99] synthesized another HTM F101 (Fig. 10) by using electron-rich furan as core. The intramolecular distance in F101 is short. However, further investigation showed that H101 and F101 gave inferior FF values and thus inferior PCEs due to the poor film-forming property, low hole transporting rate and high Rs. In order to avoid these shortcomings, Nazeeruddin et al. [100] suggested PEH-2 (Fig. 10) based on silothiophene core. As expected, the PEH-2 showed a higher 0.72 FF with lower Rs (87 U cm2) and good long-term stability under working
Fig. 9. Chemical structures of carbazole linked TPA-based small molecule HTMs.
Fig. 10. Chemical structures of biphenyldiphenylamine-based small molecule HTMs.
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environment. Despite the relatively superior FF value, the devise using PEH-2 showed low PCE. Therefore, an efficient thienothiophene core was introduced into PEH-9 (Fig. 10) [101], which possessed extended ð-conjugated structure and thus short intermolecular distance, resulting in good film-forming on top of perovskite layer. As a result, a remarkable PCE of 16.9% was achieved. In addition, the device with PEH-9 retained 93% of its original PCE after 400 h aging, demonstrating good thermal stability. This research revealed that an expanded bridge moiety linked to TPA units have positive influence on the thermal and photovoltaic properties. Following this strategy, Xu et al. [102] synthesized novel ð-conjugated TPASB and TPASBP (Fig. 10). The resulted device gave high FF (0.80) for both materials having high charge transporting mobility and low value of Rs. According to their findings, both TPASBP and TPASB could induce perovskite materials to better crystallize and form large grains on top of their layers through reducing trap states at the HTL/perovskite interfaces. When employed in the peien structured PSCs, impressive PCEs of 17.4% and 17.6% were afforded by TPASBP and TPASB respectively. It is concluded that extending ð-conjugated molecular structure has positive effects on devices' photovoltaic performance, which should be utilized in the design of novel materials. 2.2.3. Fused triphenylamine-based organic small molecule HTMs A rigid and novel fused triphenylamine was found to be an efficient core employed in hole-transporting molecules. Ko et al. [103] synthesized a novel OMeTPA-FA (Fig. 11) with bis(4methoxyphenyl)aniline linking units. OMeTPA-FA shows planar molecule configuration, which would bring in high hole mobility (3.67 104 cm2 V1 s1) and excellent device stability (a 12% reduction of the initial efficiency after 500 h aging time without any encapsulation). The compact intermolecular packing of OMeTPA-FA leads to good local crystallinity, which would result in the incomplete coverage on perovskite layer. Therefore, the terminal substituents of OMeTPA-FA were further tuned to alkyl-fluorene linkers to produce molecularly bulky nonplanar DMFA-FA (Fig. 11) [104]. Such chemical structure could impede aggregation on top of perovskite layer, hence, reducing the corresponding Rs and improving device performance. After aging under one sun illumination for 250 h, the DMFA-FA solar cell gave a 7.04% reduction of original PCE, indicating its enhanced long-term stability. These investigation above provides a new way (not being coplanar) to design small molecule HTMs for reducing partial crystallinity of HTM solid-state films.
Fig. 12. Chemical structures of paracyclophane-based small molecule HTMs.
intermolecular interactions, thus enhancing charge carrier mobility (6.32 104 cm2 V1 s1), which would bring out a high photocurrent of 22.0 mA cm2 and an admirable PCE of 17.8%. With different numbers of TPA peripheral substituents, di-TPA, tri-TPA and tetra-TPA (Fig. 12), exhibit suitable HOMO energy levels (5.28, 5.29 and 5.28 eV, respectively) [106]. It is interesting that Tetra-TPA, with four terminal TPA groups, shows increased hole-transporting mobility. Therefore, the resulted device achieved high Jsc of 22 mA cm2 and FF of 0.78 than that of di-TPA and triTPA, as a result, affording a better PCE of 18.0% [87]. It turns out that materials with cylindrical and three-dimensional molecular structure show huge potential in application of perovskite solar cells as HTMs. 2.2.5. Aza-based organic small molecule HTMs Aza-based HTMs exhibited good photovoltaic performance and compatible electrochemical properties when employed in perovskite solar cells. Triazatruxene, in which three indole units are combined by one benzene ring, can be considered as an extended delocalized p-system. Yang et al. [107] developed TPDI as HTM (Fig. 13), exhibiting good solubility in common organic solvents and suitable HOMO energy levels. Owing to the high hole mobility (3.5 103 cm2 V1 s1) and low Rs (7.5 U cm2), the TPDI-based
2.2.4. Paracyclophane-based organic small molecule HTMs To the best of our knowledge, [2,2]paracyclophane-based materials usually tend to be three-dimensional materials. Son et al. [105] introduced a novel PCP-TPA (Fig. 12) with simple synthetic routine. The cylindrical structure of PCP-TPA favored
Fig. 11. Chemical structures of fused triphenylamine-based small molecule HTMs.
Fig. 13. Chemical structures of triazatruxene-based small molecule HTMs.
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device achieved a PCE of 15.5% with high 0.749 FF. Afterwards, a two-dimensional material KR131 [108] was synthesized by replacing phenyl with n-hexyl chains in triazatruxene core of TPDI. The enlarged alkyl chains are able to ensure good solubility, making it easy for solution processable device fabrication procedure. The deep HOMO energy level (5.22 eV) of KR131 results in high device Voc (1.146 V) and thus an excellent PCE of 18.3%. In order to further simplify the synthesize process and reduce cost, Getautis et al. [109] proposed new HT3 (Fig. 13). This material performs good thermal property and amorphous crystalline capability. To be concluded, triazatruxene core based HTMs are especially promising candidates because of the excellent thermal stability, high charge mobility and no requirement of annealing steps at high temperature. The above mentioned triazatruxene-based devices usually delivered small photocurrent density, thus limited their corresponding PCEs. Chen et al. [110] reported Trux-OMeTAD (Fig. 14) by optimizing triazatruxene core. It was discovered that the TruxOMeTAD owns planar structure, hence offering high hole mobility (3.5 103 cm2 V1 s1). Furthermore, the Trux-OMeTAD could be a promising candidate because of the appropriate HOMO energy level (5.28 eV) and high LUMO energy level (2.3 eV) which could block electrons to reduce charge recombination and enhance the Voc. As expected, a remarkable PCE of 18.6% and a high Jsc of 23.2 mA cm2 were afforded. On the other hand, Kim and coworkers [111] presented a simple-structure NPB (Fig. 14) with excellent hole extraction rate and a corresponding high Voc of 1.12 V. The high LUMO energy level (2.2 eV) prevents electron from recombination at the anode side. Although this device with HTM showed a high Voc, the Jsc (18.1 mA cm2) and FF (0.67) were a little inferior as well as its PCE. Murata et al. [112] reported an azulene core based HT4 (Fig. 14) synthesized with a facile synthetic procedure. In the architecture, the five-membered ring shows electron donating property and the seven-membered ring performs electron accepting capability. With extended ð-conjugated system, the HT4 possesses high hole mobility (2.1 104 cm2 V1 s1). When employed in PSCs, a superior PCE of 16.5% with high FF value (0.71) was achieved. 2.2.6. Thiophene-based organic small molecule HTMs The non-planar structure of triphenylamine leads to long intermolecular interaction distance and thus relatively low charge transporting rate. Introduction of thiophene can enhance the molecular conjugate property and as a result to improve the final charge carrier mobility. The donoreacceptor (DeA) or
Fig. 14. Chemical structures of other aza-based small molecule HTMs.
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acceptoredonoreacceptor (AeDeA) type materials was synthesized using thiophene as structural bridge, introducing different electron donating and electron withdrawing groups. This thiophene-based small molecule has been paid attention and a series of HTMs (Fig. 15) were reported. On one hand, benzo[1,2b:4,5b0 ]-dithiophene unit shows excellent electron-donating property and has been used in HTMs. Sun et al. [113] presented an AeDeA type M1 using conjugated phenoxazine [114] as couplers. The rigid and p-conjugated system brought out enhanced hole mobility (2.71 104 cm2 V1 s1) and satisfactory conductivity (1.16 103 S cm1). However, the M1 based devices showed poor FF value (0.68) due to high Rs. Kloo et al. [115] optimized this material using benzo-[c][1,2,5]thiadiazole as linkers. The water contact angle of BDT-C1 was 107.4 , larger than that of spiro-OMeTAD, which favors good hydrophobic property and good moisture protector. As expected, the BDT-C1 based device possesses an improved FF value of 0.76 due to the relatively low Rs and high charge mobility. As mentioned above, dopants are the primary reason for damaging stability of perovskite. In this regard, Yang et al. [11] introduced thiophenesubstituted benzo[1,2b:4,5b0 ]-dithiophene as core to develop DOR3T-TBDT for extended conjugation. Without adding dopants, the DOR3T-TBDT based planar neiep devices showed better stability and efficient charge collection rate. As a result, a PCE of 14.9% was obtained. In this case, DERDTS-TBDT [116] was further reported by using electron-donating dithienosilole as structural linker. The HOMO energy level of DERDTS-TBDT was calculated to be 5.2 eV, leading to a high Voc of 1.05 V. The DERDTS-TBDT based device exhibited high stability and displayed an impressive 16.2% PCE with no dopants. These investigations above demonstrate that benzo [1,2b:4,5b0 ]-dithiophene can be a potential core for HTMs of achieving high-performance PSCs. On the other hand, Nazeeruddin et al. [117] have made new attempt to synthesize squaraine-based JK-216D and JK-217D, which have the difference at the end peripheral substituents. With low band gap of JK-216D and JK-217D (1.65 and 1.66 eV) both showed wide IPCE spectrum, corresponding to the wide absorption spectrum with high Jsc (JK-217D, 21.23 mA cm2; JK-216D, 21.75 mA cm2). However, the achieved PCEs were a little inferior due to poor FF values. As a consequence, three novel benzotrithiophene core based BTT-1, BTT-2 and BTT-3 [118] were obtained from facial synthetic procedures. The BTT-3 possesses the highest lateral conductivity (2.79 105 S cm1). Both BTT-1 and BTT-2 show similar HOMO energy levels (5.2 eV), while BTT-3 gives a slightly deep one (5.4 eV), which would facilitate the hole transportation. The BTT-1, BTT-2 and BTT-3 based devices exhibited impressive PCEs of 16.0%, 17.0% and 18.2% with superior FF values (0.72, 0.767 and 0.77), respectively. All results reveal that the introduction of thiophene in molecular structure can enhance the molecular conjugate property, reducing intermolecular interaction distance and thus increasing hole mobility. 2.2.7. Other organic small molecule HTMs As shown in Fig. 16, a large number of novel cores with TPA units as terminal substituents have been reported with adorable photovoltaic performance. For example, a spiral PST-1 was presented [119] with deeper HOMO energy level of 5.15 eV. The fabricated device showed a high Voc (1.02 V). However, a plenty of OeC, pep and CH/p intermolecular contacts in the PST-1 ultimately result in severe local crystallization and thus decrease device performance. Furthermore, Gr€ atzel et al. [120] suggested using weak intermolecular contacts like HT5. It was interesting that both in solution and solid-state films without any dopants, a red-shifted absorption for the HT5 was observed in comparison with that of spiroOMeTAD. Further studies indicates that hole mobility
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Fig. 15. Chemical structures of thiophene-based small molecule HTMs.
Additionally, Triazine-InT was developed on the base of 1,3,5triazine core [123], possessing that of CH3NH3PbI3 perovskite (5.43 eV). With high hole mobility (4.41 104 cm2 V1 s1), the Triazine-InT based devices displayed high photocurrent density of 21.56 mA cm2. On the other hand, dibenzofuran, as a newly discovered unit, was proved to be an efficient core. In this regard, Shi et al. [124] synthesized BF-002 and BF-003, with slight adjustment on chemical structure of molecules as mentioned before. The BF-002 exhibited 17-nm red-shifted absorption than BF-003, which would benefit for device photocurrent, which was reflected by an obtained high Jsc of 21.56 mA cm2. Eventually, an adorable PCE of 14.20% was afforded for the BF-002 based solar cells. All these investigations above have proven that TPA-based materials can meet the requirements (suitable energy levels, better solubility, good light stability, high hole mobility and so on) of application to the hole-transporting materials in high-quality PSCs. 2.3. Organic polymer HTMs
Fig. 16. Chemical structures of other novel core TPA-based small molecule HTMs.
(6.0 106 cm2 V1 s1) of HT5 is not enough to maintain a satisfactory Jsc. Therefore, a novel steric bulky EtheneTTPA, incorporating ethene unit as core, was brought forth [121]. As expected, the EtheneTTPA based devices exhibited broad light absorption and relatively high charge mobility (4.45 105 cm2 V1 s1), leading to a relatively higher Jsc of 21.24 mA cm2. However, a low FF of 0.67 was found in the devices. In order to improve device performance, Ko et al. [122] further optimized molecular structure by employing 2,1,3-benzothiadiazole and 3,4-ethylenedioxythiophene. With an extended structure, the DPBTDB[BMPDP]2 and DPEDOT-B[BMPDP]2 formed perfect coverage on top of perovskite layer, resulting in relatively superior FF values of 0.70 and 0.71, respectively.
Small molecule HTMs show good film forming ability and interface contact and more processing solvent compared to polymer HTMs, while the polymer HTMs possess suitable energy levels, appropriate solubility and high conductivity. Polymer HTMs are mainly divided into four types containing triphenylamine, EDOT, thiophene and benzo[1,2b:4,5b0 ]-dithiophene according to their chemical structures. The triphenylamine-based HTMs usually possess high hole mobility and are currently employed in highly efficient solar cells. The last three kinds of HTMs are enabled with light absorption capacity through introduction of thiophene and diazosulfide in order to make complementary absorption with perovskite materials, or introduction of carbon nanotubes and graphene to improve the hole mobility. 2.3.1. Triphenylamine-based organic polymer HTMs As shown in Fig. 17, triphenylamine-based polymer HTMs usually refer to materials with constitutional units of aniline [56,125]
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Fig. 18. Chemical structures of EDOT-based polymer HTMs.
Fig. 17. Chemical structures of TPA-based polymer HTMs.
and fluorene [126], which exhibit good film-forming property and high charge mobility as compared to small molecule HTMs. And thus the potential applied to PSCs is great. Taking this into consideration, Seok et al. [127] reported PTAA, PF8-TAA and PIF8TAA. It is found that the Voc is determined by the VBE of perovskite and HOMO energy level of HTMs, through studying the effect of VBE difference of MAPbI3 and MAPbBr3 and the valence band of these polymers. A further improvement of Jsc for PSC device is limited by the weak light absorption in long wavelength range of MAPbBr3. Recently, an impressive 16.2% PCE was achieved by PTAA in MAPbI3-devices. Afterwards, Huang et al. [128] found a new way to improve photovoltaic performance of PTAA-based PSCs by doping with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4-TCNQ). A best PCE of 17.5% was obtained with doping ratio of 1 wt%. Also, an optimization of perovskite to hybrid (FAPbI3)0.85 (MAPbBr3)0.15 and achieved an excellent PCE of 20.3% using PTAA as HTM [129]. These research above demonstrate that PTAA is a promising candidate polymer HTM for high-quantity PSCs. So et al. [130] reported poly-TPD with tert-butyl alkyl chain in lateral phenolic group, exhibiting >70% EQE at visible 400e750 nm, corresponding to the high Jsc of 22.0 mA cm2. It is interesting to find that negligible recombination of poly-TPD based PSCs as light intensity increased from 0.5 mW cm2 to 100 mW cm2 when surveyed light intensity-dependence. Last but not the least, Yip et al. [131] developed HSL1 and HSL2 with ionic sodium sulfonate groups into polymer backbone side alkyl chains to improve the wetting of corresponding perovskite films. Wide band-gap CH3NH3Pb(I0.3Br0.7)xCl3 x was employed, HSL2 showed the highest Voc of 1.34 V so far, which provides new thoughts of adding ionic groups to design novel polymer HTMs for achieving high electrochemical properties. 2.3.2. EDOT-based organic polymer HTMs EDOT-based polymer HTMs (Fig. 18) were divided into two kinds including PEDOT and PEDOT:PSS (poly(styrenesulfonic acid)). It is depressed to find that though the conductivity of PEDOT (2.69 101 S cm1) is high enough to reduce the device Rs, low FF value (0.68) are usually observed due to charge recombination [132]. In order to overcome this issue, Li et al. [133] introduced a ptype dopant grafted sulfonated-acetoneeformaldehyde lignin (GSL) into PEDOT. The conductivity of PEDOT:GSL (2.64 102 S cm1) was properly reduced, indicating that even slightly lower hole mobility might bring in high photovoltaic performance. When fabricated in inverted PSCs, an improved PCE of 14.94% with high FF value of 0.72 was achieved. This research
provides new information to improve performance of HTMs applied to PSCs. Shen et al. [134] reported graphene oxide (GO) modified PEDOT:PSS as HTM for PSC devices, showing the PCE of 13.1%. Ouyang et al. [135] further optimized by treating the solution of PEDOT:PSS with probe ultra-sonication. The PEDOT:PSS films could become smoother with decreasing the root-mean-square roughness from 1.55 to 1.07 nm. Consequently, an improved PCE of 15.12% was achieved. 2.3.3. Thiophene-based organic polymer HTMs Thiophene-based polymer HTMs (Fig. 19) mostly contain benzothiadiazole, fluorene, thiazolo[5,4-d]thiazole, silolo[3,2-b:4, 5-b0 ] dithiophene as constitutional units. The conjugation and hole mobility can be enhanced through the insertion of thiophene units. P3HT was the frequently used one in PSCs. Snaith et al. [136] found that the PSC devices using P3HT-functionalized single-walled carbon nanotubes (SWNTs) performed good thermal stability and high photovoltaic performance when deposited PMMA as the protective substrate on HTM layer. Li-doped P3HT nanofibrils (LN-P3HT) were further used to improve device performance [137]. The steady-state PL spectra and time-resolved PL decay reveal that LN-P3HT based PSCs possess better charge separation and hole extraction capability. The LN-P3HT based device exhibited a relatively high 15.18%
Fig. 19. Chemical structures of thiophene-based polymer HTMs.
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PCE. Bian et al. [138] reported a series of PT, PBT and PCT, with HOMO energy levels 5.2, 5.4 and 5.4 eV. It was found that HTMs owning deep HOMO energy levels give the Voc of perovskite devices [139] for the same perovskite material. Correspondingly, PBT and PCT show relatively high Voc (1.01 and 1.02 V). With an optimal layer thickness, the PBT and PCT based devices obtained the best PCEs of 16.3% and 16.5%, respectively. Moreover, Park et al. [140] synthesized an amorphous TTB-TTQ, exhibiting good filmforming property and showing good coverage on top of perovskite layer. Though the devices could obtain high Jsc (23.9 mA cm2), the high dominated recombination mechanism (bimolecular recombination) has been found to limit the PCE. Therefore, Burn et al. [141] presented the PCDTBT to minimalism bimolecular recombination via efficient charge separation and transporting. It was found that either thinner or thicker junction thickness would play a negative effect on the device photocurrent because of the insufficient device Rs. With optimal 370 nm, PCDTBT based devices gave the best 16.5% PCE. Song et al. [142] reported the PDTSTTz and PDTSTTz-4 based on dithienosilole (DTS) and thiazolothiazole (TTz), which possess high hole mobility (3.6 103 cm2 V1 s1 and 7.8 102 cm2 V1 s1) ascribing to the coplanar molecular structure and an enhanced p-conjugated property. In order to lessen the LUMO offsets of 0.2 and 0.3 eV, a pristine C60 interlayer at the cathode side was introduced, leading to efficient charge extraction and fast charge transfer ability. Several new ideas about adding ionic groups to polymer HTMs have been demonstrated by such as HSL1 and HSL2 [134] (Fig. 19). Firstly, Fang et al. [143] reported P3CT-Na introducing sodium ion in side alkyl chain. The HOMO energy level of P3CT-Na was tested to be 5.26 eV, decreasing the energy loss while holes transporting from perovskite to HTM. As mentioned above, the optimal photovoltaic performance was given with 4-nm thick hole-transporting layer. Inserting sulfonatobutoxy resulted in PhNa-1T [144], which could endure long-time stability under conditions of 25 C and 40% relative humility. The PhNa-1T film on ITO/PEN possessed smoother surface, favoring the ohmic contact of perovskite and HTM layers, resulting in high 0.774 FF. These results demonstrate that adding ionic groups to side alkyl chains is a good way to design efficient HTMs for high-performance PSCs. 2.3.4. Benzo[1,2b:4,5b0 ]-dithiophene-based organic polymer HTMs BDT-based polymers (Fig. 20) are promising hole-transporting materials used in PSCs. Zhang et al. [145] demonstrated well-
known PTB7 could sustain great grains of perovskite film and the whole device would absorb sunlight over the entire visible range. Marks et al. [146] synthesized two dopant-free DeA copolymers termed as pBBTa-BDT1 and pBBTa-BDT2, with HOMO energy levels of 5.1 and 5.2 eV, favoring hole collection from perovskite materials. The PBBTa-BDT2 possesses enhanced pep stacking along the vertical direction, enlarging the corresponding hole mobility (2.0 103 cm2 V1 s1). A best PCE of 14.5% was achieved based on these HTMs. Another dopant-free HTM comprising a random copolymer (RCP) [147], displays high charge carrier mobility (3.09 103 cm2 V1 s1). The RCP based devices showed longtime stability under 25% or 75% humidity due to its excellent hydrophobicity, in which RCP acts as effective moisture barrier for perovskite layer. The PSCs devices based on RCP achieved an impressive PCE of 17.3%. The results above demonstrate that BDT is an efficient unit for hole-transporting materials. 3. The stability perovskite soar cell Moisture, temperature and UV light have been seriously susceptible to cause the degradation of organic-inorganic perovskites. For example, we found the degradation of MAPbI3 starts with removal of organic components. H2O acts as catalyst rather than reactant in this process. However, the degradation of FAPbI3 is related to a chemical hydration reaction with H2O [148]. Under UVlight illumination, the oxygen vacancies in the common used TiO2 electron transport layer was proposed to trap photo-induced electrons in perovskite, thus deconstruct the perovskites [149]. Therefore, effective routes to prevent UV degradation include isolating the TiO2 from UV light, and even replacing the TiO2 with other electron transport materials, such as organic materials [150e155]. Several additives have been used in organic HTMs for PSCs, such as TBP and bis(trifluoromethane) sulfonimide lithium salt (Li-TFSI) for spiro-OMeTAD or PTAA, in order to control HTM morphology or provide the necessary electrical conductivity [156]. However, the utilization of those additives is problematic to long-term stability of PSC devices. Detailed investigations have identified that the hygroscopic nature of lithium salt makes the HTM highly hydrophilic and tends to chemical degradation, play negative influence on the stability of the entire device [157]. Therefore, the development of dopant-free HTMs with enhanced moisture resistance is highly desirable to probe structure-stability correlations towards the realization of stable PSCs. For the first time, the tetrathiafulvalene derivative (TTF-1, Fig. 21) without using p-type dopants was introduced into PSCs in 2014 [158]. Moreover, the resulted device showed greatly improved stability in air at a relative humidity of ~40%. This finding uncovers the prelude on the study of efficient dopant-free HTMs for PSCs for the last several years. Afterwards, even more dopant-free HTMs such as HA1, HA2 and TIPS-pentacene [3,159], and DHPT-SC, DOPTSC [6], and FA-CN, TPA-CN [160], have emerged for PSCs to improve the device stability. 4. Conclusion and outlook
Fig. 20. Chemical structures of BDT-based polymer HTMs.
In this review, we have thoroughly discussed recent development of HTMs based on small molecule and conducting polymer for high efficient PSCs (PCE > 13%). Particularly, we focus on the central-core development of triphenylamine-based small molecule materials. Especially we illustrated how HTMs properties (electrochemical properties, optimal properties and thermal properties) played on the performance of perovskite solar cells. It turns out that these HTMs perform high hole mobility of ~104 cm2 V1 s1 (SCLC), low series resistance of 1e10 U cm2, good solubility in
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Acknowledgements Financial support from the Natural Science Foundation of China (No.: 21673091), the Natural Science Foundation of Hubei Province (No.: ZRZ2015000203), Technology Creative Project of Excellent Middle & Young Team of Hubei Province (No.: T201511), and the Wuhan National High Magnetic Field Center (2015KF18) is acknowledged.
References
Fig. 21. Chemical structures of dopant-free HTMs.
organic solvents and deep but suitable HOMO energy level of 5.1 ~ 5.3 eV matching with that of perovskite materials and thus give good photovoltaic performance and high PCEs. Though 21.3% PCE has been achieved by spiro-OMeTAD based PSCs, limitations such as the complicated synthetic and subsequent purification procedures as well as the high cost still exist. Therefore, we suggest to focus on the following points in the future development of HTMs for high-quality perovskite solar cells. 1. At present most of hole transporting materials contain triphenylamine and thiophene in chemical structure. It is urgent to explore novel materials for perovskite solar cells or with complementary light absorption (strong absorbance > 800 nm) with perovskite materials, which would be significant to improve the current density of corresponding solar cells. 2. Introducing hydrophobic groups such as amide and ester to side chains can largely enhance the hydrophobicity of corresponding hole transporting materials, and thus prevent water entering into the perovskite layer, as a result enhance the stability of corresponding perovskite devices. And increasing the water contact angle would even make it, then spiro-type or largetwist-angel (~90 ) materials should be taken into account. 3. The introduction of inorganic materials (such as single-walled carbon nanotubes, graphene oxide and so on) could improve the hole mobility of corresponding hole transporting materials, and in order to increase their conductivity, BDT-type materials would be good. To be concluded, as a key part of perovskite solar cells, holetransporting material plays an indispensable role in charge extraction and transportation. Due attention should be put in electrical contact on HTM/perovskite interface.
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