3.1.1. Small-Molecule-Based HTMs
This category comprises the largest number of newly-designed HTMs for PSC research. Small molecules are suitable when thinking of PSC technology upscaling, because they have a distinct structure and molecular weight and, thus, can be easily reproduced for industrial production with high purity and high yield [
39]. Moreover, small organic molecules are superior to conjugated polymers from the viewpoints of the easier synthesis process, better reproducibility, relatively easy tuning of the optical and electrochemical properties by changing different functionalities, low molecular weight and solution processability. Most small-molecule-based HTMs contain nitrogen and sulfur, which are electron-rich atoms and, thus, particularly suitable for HTMs. They are usually based on a triphenylamine (TPA) moiety, due to the presence of the electron-rich nitrogen atom, which minimizes the intermolecular distance, leading to non-planarity of the TPA system. This ultimately results in the formation of amorphous materials, a beneficial feature for HTMs because of their capability to form smooth and pinhole-free films. This ensures uniform contact at the interface with the metal electrodes [
54]. However, as mentioned earlier, the amorphous nature of these materials imparts poor hole-mobility. Such a drawback can be overcome by using different organic and hybrid dopants (
Figure 4). Nevertheless, one issue regarding the use of dopants is their hygroscopic nature, which reduces their stability and ultimately affects the degradation of the PSCs. The discovery of new stable dopants is an open issue for molecular designers, the discussion of which goes beyond the scope of this article.
Herein, we classify the different small-molecule-based HTMs based on their chemical composition and give a summary of the photovoltaic performance of the presented HTMs (
Table 1) as compared to a reference cell containing spiro-OMeTAD.
(a) Pyrene-based HTMs
One of the first works on alternative HTMs appeared in 2013, when Seok et al. proposed a set of pyrene-core arylamine derivative HTMs, with a performance comparable to that of spiro-OMeTAD [
55]. In these molecules, the spirobifluorene core of spiro-OMeTAD is replaced by a pyrene core (PY-1, PY-2 and PY-3;
Figure 5). Methoxy (-OCH
3) groups are present also in these pyrene-based HTMs, though their position is changed from
para (as in each of the quadrants of spiro-OMeTAD) to
meta or
ortho. In fact, the -OCH
3 groups play an important role not only in controlling the electronic properties of spiro-OMeTAD by adjusting the HOMO levels of the materials, but they are also responsible for anchoring the material onto the underlying perovskite layer [
4,
5]. The electron-donating effect of methoxy groups in the
N,
N-di-p-methoxy phenyl amine (X in
Figure 5), which is directly bonded to the pyrene moiety, increases the electron density. By enhancing the electron density, both the HOMO and the lowest unoccupied molecular orbital (LUMO) energy levels (and thus, the band gap) are modified. As a result, a high PCE of 12.4% (PY-3) is achieved. PY-1 exhibited lower efficiency (3.3%), which is due to the insufficient driving force for hole injection. The well-known spiro-OMeTAD showed PCE of 12.7% under similar fabrication conditions. The slightly higher PCE of spiro-OMeTAD cells as compared to PY-3 ones originates from the higher short-circuit current (J
SC), i.e., 21 mA cm
−2 vs. 20.2 mA cm
−2, achieved for PY-3 cells, and the higher open-circuit voltage (V
OC), i.e., 1.01 V (spiro-OMeTAD) vs. 0.93 V (PY-3). This indicates more efficient charge collection via spiro-OMeTAD HTM and better matching between the quasi-Fermi level of the electrons in TiO
2 and the HOMO of spiro-OMeTAD. Conversely, a higher fill factor is obtained for PY-3 devices with respect to the reference, i.e., 69.5% (PY-3) vs. 59.5% (spiro-OMeTAD). The fill factor is related to the series resistance and shunt resistance, and the enhanced hole-transporting and electron-blocking abilities of PY-3 HTM are responsible for decreased recombination for the photogenerated charges and, thus, higher fill factor. In fact, the introduction of the pyrene core in arylamine derivatives results in HTMs with an electron-blocking ability superior to that of spiro-OMeTAD, while at the same time keeping the synthesis costs lower [
55].
(b) Truxene-core HTMs
While most of the papers dealing with alternative HTMs to spiro-OMeTAD assume the use of dopants (as for spiro-OMeTAD itself) to achieve competitive efficiencies, Chen et al. have designed a
C3h truxene-core (Trux-I) with OMeTAD terminals and hexyl side-chains (
Figure 6) [
57]. Its planar, rigid and fully-conjugated structure results in an excellent hole-mobility of the pristine material of roughly 10
−3 cm
2V
−1s
−1, nearly two orders of magnitude higher than that of spiro-OMeTAD and polytriarylamine (between 10
−5 and 10
−4 cm
2V
−1s
−1 [
98]). Indeed, its higher mobility allows one to fabricate devices with a superb PCE of 18.6%, without introducing any external dopants [
57].
Recently, Grisorio et al. have synthesized Trux-I and a new molecule named Trux-II (
Figure 6) [
56]. These star-shaped HTMs were designed by binding the bis(p-methoxyphenyl)amine groups to a truxene-based core (Trux-I) and by interspacing these electron-donating functionalities from the core with 1,4-phenylene π-bridges (Trux-II). The authors have subsequently employed them as HTMs in two different perovskite device architectures (direct and inverted). As for the inverted configuration (n-i-p), both HTMs showed a poor performance (PCE = 4.9% and 5% for Trux-I and Trux-II, respectively) with respect to spiro-OMeTAD (19.2%). However, in the case of direct device configuration (p-i-n), the trend was dramatically different: both Trux-I- and Trux-II-containing cells outperformed the spiro-OMeTAD reference (PCE = 10.2%, 13.4%, 9.5% for Trux-I, Trux-II and spiro-OMeTAD, respectively) [
56]. The huge difference in the photovoltaic behavior achieved in the two configurations depends on the intramolecular charge distributions in radical cations and on the thickness of the HTMs (5–20 nm and 150–200 nm in inverted and direct configuration, respectively). This study indicates that the performance of PSCs can be effectively tuned by ad hoc device architecture modifications.
Rakstys et al. have designed and synthesized a series of four two-dimensional triazatruxene-based derivatives (Triazatrux-I, Triazatrux-II, Triazatrux-III and Triazatrux-IV;
Figure 6) using inexpensive starting materials and simple synthetic procedures for low production costs [
58]. These centrosymmetric star-shaped HTMs, which comprise a planar triazatruxene core and electron-rich methoxy-engineered side arms, interact efficiently with the perovskite surface (a mixed perovskite composition, (FAPbI
3)
0.85(MAPbBr
3)
0.15, was chosen), thus providing better hole-injection from perovskite to HOMO levels of the HTMs, as revealed by the time-resolved photoluminescence studies. The Triazatrux-II-based solar cell exhibited power conversion efficiency of 17.7%, which is slightly higher than that of spiro-OMeTAD device (17.1%) [
58].
The triazatruxene-based design guidelines open new paths for constructing low-cost and high-performance hole-transporting materials for PSCs. Rakstys et al. recently designed for the first time a series of star-shaped triazatruxene-based donor-π-acceptor HTMs (Triazatrux-V, Triazatrux-VI and Triazatrux-VII;
Figure 6) [
59]. When studying their application in PSCs, they observed that Triazatrux-VII led to high PCEs (19%), on par with those of spiro-OMeTAD cells. This exceptionally good performance is attributed to a particular face-on stacking organization of Triazatrux-VII on perovskite (a mixed composition, (FAPbI
3)
0.85(MAPbBr
3)
0.15, was chosen) films, which favors vertical charge carrier transport through an ordered structure. These results are particularly interesting because they represent a unique example of highly-efficient PSCs based on a pristine HTM without any chemical additives or doping. This work paves the way toward the molecular design of next-generation HTMs with high mobility based on a planar donor core, p-spacer and periphery acceptor [
59].
(c) Phenothiazine-based HTMs
The phenothiazine heterocycle plays an important role in the design of high-mobility organic semiconducting materials [
99]. Because of their excellent optical, electrochemical and thermal properties, phenothiazine-based sensitizers have been widely used in DSSCs with great performance [
100]. Recently, Grisorio and co-workers designed and synthesized two phenothiazine-based molecules, which differ in the aromatic linker (PH-I and PH-II;
Figure 7) [
60]. PH-I and PH-II were synthesized through straightforward Buchwald−Hartwig and Suzuki−Miyaura cross-couplings, by binding diarylamine or triarylamine groups to a phenothiazine core, respectively. When used as HTM in PSCs, PH-I led to a poor power conversion efficiency of 2.1%, while on the other hand, PH-II exhibited an exceptional PCE of 17.6%, which is close to that obtained with spiro-OMeTAD HTM (17.7%) under the same conditions [
60]. The oxidation potential of PH-II (−5.15 eV, which is close to that of spiro-OMeTAD of −5.02 eV) results in high open-circuit voltage (1.11 V for PH-II cells vs. 0.82 V for PH-I devices). Nevertheless, the lower oxidation of PH-I (−4.77 eV) with respect to perovskite (−5.4 eV) is responsible for the more efficient hole-transfer from perovskite to the HOMO level of PH-I. The significantly different photovoltaic behavior of PH-I and PH-II is attributed to the modulation of the electron density distribution, which affects the stability of the molecules during the charge-transfer dynamics at the perovskite|HTM interface. This study demonstrates that, upon minor modifications to the phenothiazine unit, one can achieve significant changes in the PSC performances by low-cost alternatives to spiro-OMeTAD HTM.
(d) Acridine-, thiophene-, biphenyl-, bithiophene-, tetrathiophene- and phenyl-based HTMs
Chao et al. have reported an acridine-based hole-transporting material (AC-I;
Figure 8) with a 9,9-dimethyl-9,10-dihydroacridine core, prepared by an easy synthetic procedure consisting of two steps and with good reaction yields [
61]. In fact, AC-I does not contain the spirobifluorene motif typical of spiro-OMeTAD, whose preparation requires highly intricate synthetic strategies. Its hole-mobility (in the order of 10
−3 cm
2V
−1s
−1, upon doping of additives such as Li-TFSI and tertiary butyl pyridine) is comparable to that of spiro-OMeTAD, and its HOMO level (−5.03 eV) is slightly lower than that of spiro-OMeTAD (4.97 eV). When AC-I was employed as HTM for a perovskite device, a power conversion efficiency of 16.42%, comparable to that of spiro-OMeTAD under the same conditions (16.26%), was achieved after HTM thickness optimization (~250 nm), due to enhanced charge separation kinetics and recombination resistance. Hence, acridine-based derivatives can be useful low-cost alternatives to spiro-OMeTAD. The synthetic costs of AC-I are estimated to be approximately half of the costs of spiro-OMeTAD. Furthermore, AC-I can be synthesized in larger quantities with a high yield against spiro-OMeTAD [
61].
Liu and co-workers have designed two thiophene-substituted HTMs (Thio-I and Thio-II;
Figure 8), by a simple one-step synthesis of dibromo thiophene with arylamine [
62]. The substitution position of the arylamine moieties on the thiophene π-linker in the two HTMs was connected to the PSC performance via computational and experimental studies. Thio-II showed better hole-mobility than Thio-I, due to its favorable conjugation in the 2,5 positions as compared to that in the 3,4 positions of Thio-I. As a result, a good overall solar cell performance of 15.13% in a Thio-II-based PSC was achieved, which is 40% higher than that obtained with Thio-I-containing HTM. This indicates that favorable geometry of HTMs results in enhanced PSC performance. In the same work, when spiro-OMeTAD was adopted as HTM with a concentration of 20 mg mL
−1 under similar conditions, a surprisingly poor performance (PCE = 8.83%) was achieved. However, when the spiro-OMeTAD concentration was increased up to 73 mg mL
−1, PCE was enhanced up to 15.63%.
Pham et al. prepared two easily-attainable, biphenyl-based, low-cost and high-performance HTMs for PSCs (BPH-I, BPH-II;
Figure 8) via conventional Suzuki coupling reactions [
63]. In particular, BPH-II-based cells exhibited a PCE of 16.42% (spiro-OMeTAD employed under similar conditions led to PCE of 16.81%), suggesting that BPH-II could be a good low-cost replacement for spiro-OMeTAD. Regarding the device stability, PSCs based on BPH-I and BPH-II retain almost 87% of the initial performance after 10 days, similar to spiro-OMeTAD devices.
Rakstys et al. developed a bithiophene-based derivative (2,2’,7,7’-tetrakis-(
N,
N’-di-4-methoxyphenylamine)dispiro-[fluorene-9,40-dithieno[3,2-c:20,30-e]oxepine-6’,9’’-fluorene], BTHIO;
Figure 8) and studied its performance, stability and crystallography [
64]. BTHIO, a novel dispiro-oxepine derivative, was prepared by using a simple three-step synthetic procedure and low-cost precursors. When adopted as HTM, the corresponding PSC exhibited one of the best reported power conversion efficiencies of 19.4%, slightly higher than that of the spiro-OMeTAD reference cell (18.8%) under similar conditions. Furthermore, BTHIO shows significantly improved stability when compared to spiro-OMeTAD-based cells.
In a very recent report, Zimmermann et al. designed highly electron-rich tetrathiophene-fused HTMs (TETRATH-I–IV;
Figure 8), differing from each other with respect to the alkoxy groups (methyl, butyl, hexyl and decyl, respectively) [
65]. All of these derivatives are easy to synthesize and to purify. Moreover, TETRATH-I showed higher thermal stability and performance (PCE = 18.1%), comparable to the conventional spiro-derivative. Upon introduction of different alkyl groups, the solubility of the tetrathiophene core increased, but at the same time, the efficiency decreased dramatically up to 9.7% (TETRATH-IV). In case of TETRATH-I, the solubility increased by heating to 100 °C prior to spin coating. The PSC performance remained at a high level after heating, yet when a similar experiment was conducted for the traditional spiro-derivative, the PCE decreased dramatically already upon heating to 70 °C. This indicates that tetrathiophene can enable the design of thermally-stable and low-cost PSCs with high performance [
65].
Chen et al. have reported a simple HTM (3,6-difluoro-N1,N1,N2,N2,N4,N4,N5,N5-octakis(4-methoxyphenyl)benzene-1,2,4,5-tetraamine, DFTAB;
Figure 8), obtained via one-step synthesis using commercially available precursors [
66]. When utilized in a PSC, it gave rise to a PCE of 10.4% with low hysteresis. When DFTAB was used without additional ionic dopants, the corresponding device achieved a stabilized PCE of 6%. The low cost and easy synthesis render this HTM promising for future large-scale applications, especially considering that avoiding additional dopants will significantly enhance the long-term stability of the PSCs.
(e) Triazine-based HTMs
Ko and co-workers synthesized electron-deficient triazine core donor-acceptor HTMs (TRIAZ-I and TRIAZ-II;
Figure 9), differing by the spacer group (thiophene or phenyl group) and with dimethoxytriphenylamine as the donor moiety [
67]. Both TRIAZ-I and TRIAZ-II showed hole-mobility similar to that of spiro-OMeTAD. When employed as HTMs in PSCs, TRIAZ-I exhibited better performance than TRIAZ-II (12.5% and 10.90% efficiencies, respectively) due to a higher photocurrent and fill factor. Under similar conditions, spiro-OMeTAD-based PSC showed a PCE of 13.45%. Lim et al. presented two triazine core star-shaped HTMs (TRIAZ-III and TRIAZ-IV;
Figure 10) [
68]. They found that TRIAZ-IV exhibited a red-shift in the absorption band, as well as better hole-mobility as compared to TRIAZ-III, due to the presence of the electron-rich indeno[1,2-b]thio moiety. These two materials led to excellent PCEs (13.2% and 12.6% for TRIAZ-III and TRIAZ-IV, respectively), comparable to spiro-OMeTAD (13.8%).
(f) Benzotrithiophene- and squaraine-based HTMs
Ontoria et al. have obtained three benzotrithiophene-based HTMs (BZTR-I, BZTR-II and BZTR-III;
Figure 10) by straightforward cross-coupling reactions between different triphenylamine derivatives and benzotrithiophene [
69]. These materials, when further applied to PSCs, showed PCEs of 16%, 17, and 18.2% for BZTR-I, BZTR-II and BZTR-III, respectively, comparable to the reference spiro-OMeTAD (PCE = 18.1%) under similar conditions. The higher performance of BZTR-III is due to its better conductivity and good alignment of the HOMO level to the perovskite valence band. Along the same line, Benito et al. have prepared tri-arm and tetra-arm isomers (BZTR-IV and BZTR-V;
Figure 10) and studied their optical, electrochemical, photophysical properties and PSC performance [
70]. These materials are highly stable up to 430 °C, and the corresponding photovoltaic devices show superior performance with PCE of 19% for BZTR-IV and 18.2% for BZTR-V. The higher efficiency achieved with the derivative BZTR-IV may be related to its
cis-sulfur arrangement, leading to favorable interactions with the perovskite structure and better hole extraction. Paek et al. have recently designed squaraine-based (SQ-H, SQ-OC
6H
13;
Figure 10) HTMs, identifying them as excellent light harvesters in PSCs [
71]. These HTMs exhibited excellent PCEs of 14.74% (SQ-H) and 14.73% (SQ-OC
6H
13), comparable to the spiro-OMeTAD reference (PCE 15.33%). The air stability of these materials was also investigated: for SQ-H, the PCE dropped only by 12% upon 300 h of ambient exposure, while for SQ-OC
6H
13, there was no change in PCE.
(g) Fluorene- and spiro-fluorene-based HTMs
Rakstys et al. designed a novel bifluorenylidene-based HTM (FL-I;
Figure 11) with a lower band-gap (2.41 eV) compared to spiro-OMeTAD (3.00 eV), yet with a similar HOMO level (−5.09 eV vs. −5.04 eV) [
72]. When FL-I was employed as an HTM in PSC, it exhibited a power conversion efficiency of 17.8%, comparable to spiro-OMeTAD (18.4%). However, being almost 50-times less expensive than spiro-OMeTAD, FL-I is an intriguing candidate for future commercial applications of PSCs.
As already mentioned, various dopants have been used in HTMs in order to enhance their electrical conductivity, while at the same time decreasing the stability and increasing the cost of the resulting device. Based on these considerations, Wang et al. designed a dopant-free HTM (FL-II;
Figure 11), which constituted both the polytriarylamine unit (i.e., N-benzene) and spiro-OMeTAD [
73]. FL-II showed excellent PCE of 16.73% with dopants and 12.39% without dopants. The corresponding values for a spiro-OMeTAD-based device are 14.84% and 5.91%, respectively.
Focusing on device stability, Reddy and co-workers developed two fluorene-based HTMs (FL-III and FL-IV;
Figure 11) using the Suzuki coupling reaction. In their molecular design, spiro-fluorene was end-capped to a terminal fluorine group of two different sides, while carbazole was linked to the third arm of the triphenylamine core [
74]. The two materials differ through the cyano-group end-capping. For both, the HOMO levels were well aligned with spiro-OMeTAD and PEDOT:PSS. In addition, both exhibited high hole-mobility, long-term stability and good solubility. Due to such attractive characteristics, both have been employed in PSCs (replacing spiro-OMeTAD), as well as in the organic bulk-heterojunction (BHJ; replacing PEDOT:PSS). The reported PCE for FL-III was 17.25%, higher than for the spiro-OMeTAD reference (16.67%). In a BHJ cell, a PCE of 7.93% was achieved, the reference PEDOT:PSS cell yielding an efficiency of 7.74%.
Tiazkis et al. have systematically studied the structure-property relationship of a series of fluorene-based HTMs (FL-V, FL-VI, FL-VII, FL-VIII and FL-IX;
Figure 11) [
75]. They found that hole extraction, molecular planarity and charge-transport properties can be tuned by substitution of an aliphatic group to the
meta- and
para-positions of triphenylamine fragments. The poor performance observed in the case of
meta-substitution may be due to the non-favorable geometry, but in the case of
para-substitution, the overall PCE fell in the 9%–16.8% range, close to the spiro-OMeTAD reference value (17.8%).
Since spiro-OMeTAD-based derivatives are found in many of the best-performing HTMs, Bi and co-workers recently reported about a newly-designed and easily-synthesized spiro-based HTM (spiro-FL-I;
Figure 12) with excellent performance (PCE = 19.8%; 20.8% for the spiro-OMeTAD reference) [
76]. In addition, devices based on spiro-FL-I showed less hysteresis, excellent reproducibility in device fabrication and better stability under dark and dry conditions. By following a similar design principle, Xu et al. have prepared two 3D-spiro-fluorene-based HTMs (spiro-FL-II, spiro-FL-III;
Figure 12) [
77]. Spiro-FL-III exhibited better hole-mobility, better film-forming properties, higher solubility and a deeper HOMO level as compared to spiro-OMeTAD, also yielding higher PCE (20.8% vs. 18.8%; the PCE for spiro-FL-II was 13.6%). Spiro-FL-III showed also excellent stability after long-term aging for six months. Malinauskas and co-workers have reported a set of HTMs with di-substituted and tri-substituted benzene, or a di-substituted thiophene core (SPI-BI, SPI-TH and SPI-TRI, respectively;
Figure 12) with fluorine as peripheral unit [
79]. The hole-mobility of these materials is in the same order as for spiro-OMeTAD, but their advantage is that they are easy to prepare via two steps, using commercially available precursors, with an overall material cost equal to about one fifth that of spiro-OMeTAD. When employed in PSCs, these HTMs exhibited PCE values up to 20%. Liu et al. have recently reported a series of four spiro-fluorene derivatives differing from each other by the
para- and
meta-substitution of diphenylamine or triphenylamine moieties (SPI-FL-MM-3PA, SPI-FL-MP-3PA, SPI-FL-MM-2PA and SPI-FL-MP-2PA;
Figure 12) [
80]. Furthermore, these compounds showed high hole-mobility, good solubility, suitable energy levels and efficient hole-extraction combined with electron-blocking capability, which are all positive characteristics of HTMs. In the devices, SPI-FL-MP-2PA exhibited the best photovoltaic performance, with a PCE of 16.8%, slightly higher than that of the reference spiro-OMeTAD (15.5%) under similar conditions [
80]. When SPI-FL-MP-2PA was further adopted in mixed FAPbI
3/MAPbBr
3 PSCs, an enhanced PCE of 17.7%, comparable to the spiro-OMeTAD reference (17.6%), was achieved. Furthermore, SPI-FL-MP-2PA-based devices possess drastically improved stability compared to spiro-OMeTAD-containing devices, with 90% of initial PCE retained after 2000 h in ambient temperature, while spiro-based devices retained only 20% of initial PCEs under the same time exposure.
To overcome the stability drawbacks of HTM additives, Xu and co-workers have designed dopant-free HTMs based on spiro-fluorene derivatives (one-3′,6′-bis(benzyloxy)-N2,N2,N7,N7-tetrakis(4-methoxyphenyl)spiro [fluorene-9,9′-xanthene]-2,7-diamine (XDB), N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-2-ylmethoxy)spiro[fluorene-9,9′-xanthene]-2,7-diamine (XOP), N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-3-ylmethoxy)spiro[fluorene-9,9′-xanthene]-2,7-diamine (XMP), and N2,N2,N7,N7-tetrakis(4-methoxyphenyl)-3′,6′-bis(pyridin-4-ylmethoxy)spiro[fluorene-9,9′-xanthene]-2,7-diamine (XPP);
Figure 12), in which the pyridine group is the pendant to respective HTMs with different positions of nitrogen atoms (
para,
ortho and
meta substitution) [
78]. Among these, XPP exhibited a PCE of 17.2% without the use of external dopants, which is much higher than corresponding PCE for the spiro-OMeTAD cell (5.5%) under similar conditions. When planar PSCs were considered, XPP-based HTMs even yielded a PCE of 19.5% without any external dopant. Further studies of steady-state PL and transient photocurrent decays for these materials revealed much better hole-extracting and hole-transporting capabilities than for spiro-OMeTAD, thus explaining the better performance and long-term stability as compared to the conventional HTM.
One of the most interesting examples of organic HTMs is the one recently proposed by Saliba et al. [
39]. They present a novel compound, 2’,7’-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b’]dithiophene-4,9’-fluorene] (FDT;
Figure 13), where an asymmetric fluorine-dithiophene core is substituted by
N,N-di-p-methoxyphenylamine donor groups. FDT was designed on the basis of the interesting optoelectronic properties of spiro-cyclopentadithiophene derivatives. When used in a mesoscopic configuration instead of spiro-OMeTAD, it leads to one of the highest PCEs reported for small-molecule HTMs, i.e., 20.2%. The advantages of FDT are the low costs of the material (~60
$/g), and the possibility of using toluene for dissolving it, instead of the more hazardous chlorobenzene used for spiro-OMeTAD [
39]. These results are particularly interesting because they point towards an entire class of new HTMs with high performance by molecular engineering of the FDT core.
(h) Carbazole-based HTMs
Carbazole-based derivatives have attracted much attention as charge-transporting materials for organic light-emitting diodes (OLEDs), DSSCs and PSCs [
101,
102,
103]. Their interesting photophysical properties such as intense luminescence and reversible oxidation processes, together with the reasonable synthetic costs, the versatility of the carbazole reactive sites and the excellent charge transport properties justify the efforts to find novel solutions for low-cost HTMs for PSCs in this class of compounds [
10,
40,
82,
83,
84,
86,
87,
88,
89,
90,
91,
92].
A study from Wu et al. reports a carbazole-based compound (CA-I;
Figure 14) synthesized with a simple two-step reaction from inexpensive and commercially available materials [
81]. CA-I has higher hole-mobility and conductivity than spiro-OMeTAD, and it leads to 12.3% efficiency for PSCs, which is comparable to what the authors achieved with spiro-OMeTAD [
104].
Leijtens et al. have synthesized two simple carbazole-based, lithium salt-free HTMs, differing by the alkyl group (linear or branched; CA-II and CA-III, respectively;
Figure 14) [
82]. The oxidized form of these materials, when applied in PSCs, produced similar performance to that of spiro-OMeTAD-based doped devices, thus implying that simple carbazole-based design can be useful for making dopant-free PSCs. Another example is a carbazole-based HTMs with two-arm and three-arm structures, connected through TPA, phenylene or diphenylene core units (CA-IV, CA-V, CA-VI;
Figure 14) [
83]. With these systems, a PCE as high as 14.79% was obtained. Recently, Wang et al. synthesized novel carbazole-based derivatives (CA-VII, CA-VIII;
Figure 15) with a biphenyl core. Undoped CA-VII resulted in a PCE of 4.53%, close to that of spiro-OMeTAD (5.10%). CA-VIII, in turn, showed much poorer efficiency of 0.19%, due to the mismatched HOMO level with respect to the perovskite and increased electron recombination at the interface [
84].
Xu et al. have reported two HTMs (CA-IX, CA-X;
Figure 15) [
92] based on the carbazole-core. These materials had previously been used in solid-state DSSCs using (E)-3-(6-(4-(Bis(2’,4’-dibutoxy-[1,1’-biphenyl]-4-yl)amino)phenyl)-4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-b’]dithiophen-2-yl)-2-cyanoacrylic Acid (LEG4) as a sensitizer [
85]. CA-X cells exhibited higher PCE (6%) than those employing CA-IX (4.5%), while under the same conditions, the performance of the spiro-OMeTAD-based device was 5%. The good performance of CA-X in solid-state DSSCs gives strong evidence of its superiority over spiro-OMeTAD, pointing towards its further use as HTM in PSCs. The CA-X-based PSC device exhibited a power conversion efficiency of 9.8% (spiro-OMeTAD reference: 10.2%). The superiority of CA-X as compared to CA-IX might be due to lower reorganization energy, yielding higher hole-mobility than in CA-IX. Kang et al. have designed a family of dendritic carbazole-based star-shaped HTMs (CA-XI, CA-XII, CA-XIII;
Figure 16) and investigated their performance in PSCs [
86]. The trimeric structures exhibited higher conductivity than the dimeric one, due to crystallization during the fabrication process. As HTM, CA-XIII resulted in a PSC with 13% efficiency, which is comparable to that of spiro-OMeTAD PSCs (13.76%). Recently, Zhang et al. synthesized two new carbazole-based materials (CA-XIV and CA-XV;
Figure 16) using commercially available di-substituted and tri-substituted phenyl derivatives [
87]. The PSCs based on these HTMs had PCEs of 11.4% and 13.1% for CA-XIV and CA-XV, respectively (spiro-OMeTAD reference: 12.0%).
Daskeviciene et al. have also developed a simple one-step synthesis for obtaining a low-cost HTM (CA-XVI;
Figure 16) [
88]. Such an HTM led to PSCs with 17.8% efficiency, comparable to the spiro-OMeTAD reference device. Simple enamine condensation chemistry for synthesis, low-cost materials and higher efficiency suggest the applicability of CA-XVI designs as HTMs for future high-performance and low-cost organic electronic devices. Chen et al. reported a tetra-substituted carbazole-based HTM (CA-XVII;
Figure 17) via a three-step synthesis, using low-cost starting materials, with a PCE of 17.81% [
89]. The authors also investigated the role of the OMeTAD group in the system, by synthesizing a molecule similar to CA-XVII, but with a different substituent than OMeTAD. The corresponding photovoltaic device had extremely low performance (PCE close to zero), which demonstrates the importance of the OMeTAD moiety in the HTM design. Zhu and co-workers have prepared a carbazole-based molecular design with a tetra-phenylene core (CA-XVIII;
Figure 16) [
90]. CA-XVIII imparts good thermal stability and well-aligned energy levels. When used as HTM, the resulting PSCs exhibited a PCE of 12.4% without dopants, which is close to 14.3% obtained with doped spiro-OMeTAD. Recently, Zhu et al. synthesized a series of carbazole derivatives differing in the 2,7 and 3,6 positions (CA-XIX, CA-XX, CA-XXI and CA-XXII;
Figure 17) [
91]. Out of these materials, the 2,7-substituted derivatives (CA-XX and CA-XIX) showed not only good solubility due to the highly twisted structure, but also high PCEs of 16.74% and 14.92%, respectively [
91], the former being even higher than for the spiro-OMeTAD reference. On the other hand, the 3,6-substituted carbazole-based HTM CA-XXII exhibited a non-promising PCE of 13.3%.
Wu et al. have recently introduced a carbazole-based HTM including the electron-deficient benzothiadiazole (BT) core (CA-XXIII;
Figure 17) [
105], which was compared to the previously reported CA-X by Xu et al. [
92]. The introduction of a BT unit between biphenyl structures in CA-XXIII effectively enhanced the intermolecular interactions, increasing the charge transport, the hole-mobility and the thermal stability of the material. Furthermore, CA-XXIII exhibited better charge collection and transportation properties than the CA-X derivative. When used as HTM in a PSC device, PCE = 16.87% was achieved for CA-XXIII, higher than for spiro-OMeTAD (15.53%).
(i) Other small-molecules HTMs
Zong and co-workers have proposed two novel binaphthol-based designs (NPH-I, NPH-II;
Figure 18), which differ in the aromatic or aliphatic linkage to binaphthol unit [
93]. The electrochemical measurements revealed that the HOMO energy levels of both materials (−5.41 eV and −5.39 eV for NPH-I and NPH-II, respectively) are well-aligned with that of perovskite, and PCE values similar to the spiro-OMeTAD reference were reported. Li et al. have made two different HTMs by changing the π-linker unit (biphenyl vs. carbazole; OMe-I, OMe-II;
Figure 18) [
94]. PCEs of 18.34% and 16.14% were obtained for OMe-II and OMe-I, respectively. The higher efficiency of OMe-II could be due to its higher hole-mobility (2.26 × 10
−4 cm
2V
−1s
−1 vs. 7.83 × 10
−5 cm
2V
−1s
−1 for OMe-I), indicating clearly the promise of carbazole as the core unit of HTMs. Based on the work by Li et al. [
94], Nazim and co-workers have recently synthesized three low-cost thiazolo[5,4-d]thiazole-based HTMs (Thiazo-I, Thiazo-II and Thiazo-III;
Figure 19) [
95].
Their appropriate energy levels, tuned to match those of CH
3NH
3PbI
3, render them suitable to be employed in PSCs, which exhibited a PCE of 10.60%, 4.37% and 8.63% for Thiazo-II, Thiazo-I and Thiazo-III, respectively. In particular, Thiazo-II, with a furan unit in the thiazolo[5,4-d]thiazole-core, provides a good interface with the perovskite film, allowing for ultrafast and complete intermolecular hole transfer from the photoexcited perovskite layer. Three novel tetraphenylmethane(TPM)-arylamine-based hole-transporting materials (anisole, Ph-TPM, and bulky arylamine side groups DPA or TPMA, DPA-TPM and TPA-TPM;
Figure 19) were presented by Liu et al. [
96]. PSCs based on these three novel HTMs achieved good PCE values of 4.62%, 9.33% and 15.06% respectively, whereas under similar conditions, spiro-OMeTAD exhibited a PCE of 15.49%.
Finally, we want to highlight the work by Petrus et al., who have addressed the issue of reducing HTM synthetic costs by proposing a material with 3,4-ethylenedioxy thiophene (EDOT) as the central core (EDOT-AZO;
Figure 19) [
97]. EDOT-AZO was synthesized by a simple one-step Schiff base condensation chemistry of amine and aldehyde of EDOT under ambient conditions using inexpensive precursors. Indeed, currently EDOT-AZO is the least expensive HTM ever reported in the context of PSCs (the cost of EDOT-AZO is only 10
$/g). This work demonstrates how, by adopting simple chemical procedures and cost-effective raw materials, it has been possible to synthesize a high-performance material with only water as the byproduct. When EDOT-AZO was employed as HTM in planar CH
3NH
3PbI
3 PSCs, it led to a performance (PCE = 11%) comparable to that of spiro-OMeTAD-based PSCs (11.9%).
3.1.2. Polymer-Based Hole-Transporters
Figure 20 summarizes the polymeric hole-transporting materials that will be surveyed in the rest of this section. An overview of their photovoltaic performance is also presented in
Table 2.
Among the polymer-based HTMs, PTAA was the first one tested in PSCs and so far the most efficient, as well. Starting from an earlier work by Seok et al., where PTAA partly infiltrates into a mesoscopic scaffold forming a zig-zag-like structure (the achieved efficiency of the PSC was 12%), an extensive optimization process has been carried out with the utilization of mixed perovskites (MAPbBr
3/FAPbI
3), leading to a PCE of over 20% [
10,
12,
47]. The superior performance of PTAA-based PSCs arises from the exceptional hole-mobility of PTAA as compared to other polymers (10
−2–10
−3 cm
2V
−1s
−1), as well as from its strong chemical interaction with perovskite. However, the high molecular weight of PTAA does not allow easy infiltration into the pores of the TiO
2 scaffold. In addition, PTAA is an extremely expensive material (~2000
$/g [
106]). Poly(3-hexylthiophene) (P3HT) and PEDOT:PSS are very well-known conducting polymers for organic photovoltaic applications and have been adopted as HTMs in polymer solar cells [
107]. When P3HT was used as HTM in a mixed perovskite solar cell (MAPbI
3/MAPbI
3-xCl
x), an increase in the efficiency from 6.7% (for the traditional MAPbI
3 perovskite-based cells) [
104] up to 13% (for mixed-ion perovskite cells) [
108] was obtained. More advanced structures based on P3HT have been recently developed, such as bamboo-structured carbon nanotubes [
109] or P3HT-modified carbon nanotube cathodes [
110], displaying high efficiency and stability, while keeping the fabrication costs low. PEDOT:PSS has been mostly adopted in inverted planar PSCs with successful outcomes, thanks to the good matching of its work-function with the valence band of the perovskite, good film properties and low-temperature processability. You et al. have reported a 17.1% efficiency in such a PSC [
111], while the highest efficiency with PEDOT:PSS (18.1%) so far has been achieved with perovskite and hydrogen iodide (HI) additive [
30], which ensured very high quality of the perovskite film. The main well-known drawback of PEDOT:PSS is its hygroscopicity, which limits the chemical stability of PEDOT:PSS-based cells in ambient conditions.
Several other polymers have been tested as HTMs in mesoporous PSCs. Recently, Dubey et al. presented a diketopyrrolopyrrole-based polymer (PDPP3T, poly[{2,5-bis(2-hexyldecyl)-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl}-alt-{[2,2’:5’,2’’-terthiophene]-5,5’’-diyl}]) that did not decrease the PSC efficiency when used to replace the doped spiro-OMeTAD (12.32% for PDPP3T vs. 12.34% for spiro-OMeTAD) [
112]. As an additional attractive feature, the PDPP3T-based device yielded slower device degradation. PCE decreased up to 60.6% of its initial value in 172 h, while for spiro-OMeTAD cells, the loss in PCE was almost 83% of its initial value in the same time.
Stringer et al. have designed a carbazole-based co-polymer, Poly[
N-9′-heptadecanyl-2,7-carbazole-
alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) [
113], which is a well-known donor material in organic BHJ cells. Its applicability as an HTM in PSCs after doping by LiTFSI and TBP was investigated, and a PCE of 15.9%, close to that of spiro-OMeTAD reference, was reported. Its good air stability is due to the low-lying HOMO level (−5.45 eV), lower than in spiro-OMeTAD (in the range of −5.0–−5.22 eV). Thus, PCDTBT-doped HTMs have been used in standard (n-i-p) PSC device architectures, but unfortunately, very low power conversion efficiencies (4.2%) have been obtained [
114]. In [
115], the authors optimized the thickness and the doping level of PCDTBT in the FTO/c-TiO
2/mesoporous TiO
2/(FAPbI
3)
0.85 (MAPbBr
3)
0.15/PCDTBT/Au structure, reaching an excellent performance (PCE = 15.9%). These results were comparable with those with conventional spiro-OMeTAD-based PSCs (PCE = 7.4%). In one recent report by Yu et al., a co-polymer based on Poly[2,6-(4,4-bis-(2-ethylhexyl)-4
H-cyclopenta [2,1-
b;3,4-
b′]dithiophene)-
alt-4,7(2,1,3-benzothiadiazole)] (PCPDTBT) has been investigated. Upon doping of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) with PCPDTBT, a PCE of 15.1% was reached [
115], the highest ever reported for PCPDTBT-based polymers. Zhou and co-workers have recently prepared a hyper-branched carbazole-based polymer (HB-CZ) in one step via the Suzuki coupling reaction and successfully utilized it as a hole-transporting material for a perovskite device [
116]. The polymer absorbs in the UV region (which makes it a screen against PSCs’ degradation), has high hole-mobility, a deep HOMO energy level (−5.32 eV) and high LUMO energy (−2.42 eV). HB-CZ-based cells exhibited a PCE of 14.07%, which is higher than reference devices made up of commercially available HTMs such as P3HT (PCE = 9.05%) and polycarbazole (PCz) (6.60%). Liu et al. have in turn designed a highly π-extended copolymer HTM Poly{3,6-dithiophen-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-thienylenevinylene-2,5-yl} (PDVT-10), with an impressive hole-mobility of 8.2 cm
2V
−1·s
−1 and good performance in PSCs (PCE = 13.4%) without doping [
117], representing one of the highest PCEs reported for dopant-free polymer-based HTMs.
Xu and co-workers have reported a novel carbazole-based non-conjugated polymer (PVCz-OMeDAD), which bears a non-conjugated polyvinyl chain and a hole-transporting OMeDAD unit [
118]. The material is obtained by free radical polymerization of vinyl monomer using low-cost raw materials with high reaction yields. The OMeDAD moiety enhances the molecule’s hole-transporting capabilities, and a PCE of 16.09% has been reported, which is significantly better than that of the spiro-OMeTAD-based reference cells (9.62%). Gaml et al. have reported the use of benzodithiophene-based polymer poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b’]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)], PBDTT-FTTE, as an HTM in PSCs [
119]. After doping PBDTT-FTTE with 3% of diiodooctane, it led to PSCs with similar performance as conventional spiro-OMeTAD-based devices in an inert atmosphere, but with an enhanced fill factor and open-circuit voltage. Diiodooctane-doped PBDTT-FTTE cells exhibited an efficiency of 11.6%, while the PCE of the non-doped PBDTT-FTTE devices was slightly lower (PCE = 10.3%). This highlights very well the importance of diiodooctane doping for PBDTT-FTTE polymers as efficient HTMs.
Further research is mandatory to explore novel polymeric HTMs whose absorption lies beyond that of perovskite, to contribute to the external quantum efficiency of the device. In fact, the so-far proposed HTMs mostly function as charge carriers rather than light absorbers. However, polymeric HTMs offer several drawbacks when thinking of commercialization, such as their polydispersity, which results in more difficult characterization with undefined molecular weight, lower purity, batch-to-batch variation and last, but not least, poor infiltration into the nanostructured material [
4,
40].