Featuring with the advantages such as efficiently broadening the absorption area of the porphyrin, fast regeneration reaction with the reductant in redox mediator and so on, diarylamino group is one of the most popular electron-donating group in building D-A porphyrin. To clarify the function of the diarylamino in porphyrin sensitizer, we can take a close look at YD0 and YD1 porphyrins, which have similar molecular structure except that YD1 contains a DPA donor group while YD does not (Figure 5) (Lu et al., 2009). This simple difference led to an obvious variation in the absorption spectra and DSSC performance, being PCE of 2.4% for the YD0 and 6.5% for the YD1. Later impedance spectroscopy (IS) study demonstrated the diarylamino group plays a key role in retarding the I3- ions from the TiO₂ surface, which resulted of a significantly reduced charge recombination of YD1 than YD0. Besides, excited-state relaxation dynamics study by femto-second fluorescence decays measurement showed a higher electron injection rate of YD1 sensitized-TiO₂ than that of YD0. Comparing to YD1, YD2 (Figure 5) contains two more hexyl chains on diphenylamine, which has been proved to be beneficial for dye solubility, suppression molecular aggregation and reducing the recombination reaction between I3- and TiO₂. The performance of YD2 devices was improved to 8.8% by Grätzel in the same year as it created, and a higher PCE of 11.0% was achieved by using an organic co-sensitizer to supplement the porphyrin's absorption gap between the B and Q band (around 500 nm) (Bessho et al., 2010). This result is quite impressive because it was the first report on ruthenium-free sensitizer based DSSC achieving efficiency over 11%.

Compared to the ruthenium dye, the narrow B and Q band of typical porphyrins limits the light harvesting properties in DSSC. Especially considered the absorption gap between B and Q bands locates the most intensive region of the solar spectrum (around 500–550 nm), this weakness limits the current density of porphyrin sensitized solar cells. Fortunately, as Imahori et al. pointed out in 2009, elongation of the π conjugation and break the molecular symmetry can broaden and red shift the absorption bands of porphyrins (Imahori et al., 2009). However, this strategy is not always efficient if merely increase the conjugation area or decrease the symmetry of the molecule. Several related reports have shown disappoint results, in which damaged ICT properties could be one of the major reasons. For example, Yeh and Diau designed the YD14, YD15, and YD16 porphyrins (Figure 5) in 2010 (Wu et al., 2010). The YD14 porphyrin has a feature of one more triphenylamine attached onto the 20-position (position accords to Figure 2) than YD15. For YD16 porphyrin, two triphenylamine groups are located at the 5 and 15 trans-position and an additional diphenylamine is attached at the 20-position. For the sake of this design, the YD14 shows a split B band at 436 and 476 nm, and the maximum Q band at 660 nm, while they are 440, 488, and 707 nm for the YD15, 451, 600, and 660 nm for the YD16. However, the DSSCs based on these highly asymmetrical and panchromatic porphyrins only exhibited PCE of 4.2% for YD15 and 5.5% for YD16, along with a nearly 20% J_SC drop compared with the YD2 device.

In another case, by locating the two diphenylamines to the 5- and 10- positions (iso-position according to Figure 2A), Imahori and co-workers successfully broke the molecular symmetry while remained an efficient ICT properties in ZnPBAT (shown in Figure 5), which successfully obtained a remarkable PCE of 10.1% (Kurotobi et al., 2013). This rational design concept based on highly asymmetric, push-pull substitution suggests new possibilities for further improving porphyrin performance. The ZnPBAT porphyrin shows a red-shifted Q band maximum at 661 nm, leading to a broader IPCE spectra and about 2 mA/cm² higher current density at similar condition. It is interesting to note that all the cells reach their highest efficiencies after several days aging under dark. More importantly, the ZnPBAT devices retained 90% of initial performance assessed under continuous AM 1.5G illumination at 25°C.

Porphyrins constructed with dialkylaminophenyl groups always show broader light-harvesting area than the diarylamino ones, even the latter possess larger conjugation area. LD14 porphyrin (Figure S1) features with 4-ethynyl-N,N-dimethylaniline as donor group while the acceptor is similar to YD2 (Chou et al., 2016). This porphyrin showed a wide IPCE onset till to 780 nm because of a Q band maximum at 667 nm, impressive high PCE of 10.2% was obtained. By adjusting the alkyl chains in amino group from methyl to octyl, LD16 porphyrin showed even higher performance than the LD14 at same condition, which was supposed to benefit from a successfully retarded charge recombination accomplished by these optimized alkoxyl chains(Wang C.-L. et al., 2012).

Inserting an anthracene between the aniline and the triple bond is also another choice to elongate the conjugation area but remain an efficient ICT, which is demonstrated to be efficient in broadening the IPCE thus increasing the current density of porphyrin based DSSC. Lin and Diau synthesized LD31 porphyrin (Figure S1) with a Q band maximum at 691 nm (Wang et al., 2014a). The significant red-shifted absorption spectrum of LD31 makes a DSSC capable of light-harvesting ability over 800 nm, which showed a PCE of 10% as well.

As proposed in D-A metal-free organic dyes, the function of donor is pushing the electron to the acceptor group, finally to the semi-conductor. However, it's still delusive how important of the electron-donating ability of this group in porphyrin dyes. In most highly efficient dyes, electron-density of the HOMO is mainly distributed at the donor site as from the DFT calculations. In 2011, a series of phenylethynyl-substituted porphyrin sensitizers bearing with nitro, cyano, methoxy, or dimethylamino phenylethynyl substituent were prepared by Lin and Diau, aiming to examine the electron-donating or -withdrawing effects of dyes on their photovoltaic performance (Lo et al., 2010). Along with an increased electron-donating ability, the HOMO and LUMO levels for these four porphyrins negatively shift with a trend of cyano<nitro<methoxy<dimethylamino. The DFT calculation indicates that electrons are effectively injected from the dye to the TiO₂ for the Me2N-PE1 (Figure S2) and MeO-PE1 upon excitation. As a result, the DSSC performance of these four porphyrins showed a similar trend of the energy levels, in which Me2N-PE1 porphyrin featured with electron-donating group achieved a PCE of 6.1%. These results have provided us a glimpse of the relationship between the donor structure and the energy levels of the porphyrin molecule, and finally the relation to the DSSC photovoltaic properties.

introduction of an additional π-chromophore is another efficient strategy to achieve excellent light-harvesting ability in the visible and Near-IR regions for porphyrin. In 2011, Lin and Diau investigated the phenylethyne, naphthalenylethyne, anthracenylethyne, phenanthrenylethyne, and pyrenylethyne substituted porphyrins (LD1-LD4) and their application in DSSC (Wang et al., 2011). The pyrenylethyne conjugated porphyrin LD4 (Figure S2) showed much improved photovoltaic performance compared with the other porphyrins, giving an J_SC of 19.2 mA/cm² and overall efficiency of 10.1%. LD4 was further modified to LWP4 porphyrin by introducing an ethynylanthracene group between the pyrene and porphyrin. However, the absorption spectrum of LWP4 is more like the LAC3 (with only anthracene) instead of LD4 (with only pyrene), showing split B band absorption instead of red-shifted Q band (Wu et al., 2014). The significant performance difference between LD4 and LWP4 based DSSC suggested that there should be a certain length of the donor group.

Carbazole groups possess large similarity to the DPA, but with more rigid configuration (Zhao et al., 2016). Considering the “ring fusion” effect, one can expect a lower HOMO energy level of carbazole conjugated porphyrin compared to the DPA porphyrin (Qi et al., 2015). Xie et al. demonstrated that an ethynylene bridge between carbazole and porphyrin causes a decrease in the energy gaps, resulting in a red shifting of the absorption band (Sun et al., 2014; Wang et al., 2014c; Tang et al., 2015). They demonstrated that attachment of long alkyl chains at the ortho positions of the meso-substituents in porphyrin is an effective strategy to prevent the charge recombination and improve the V_OC and J_SC values, as well as the cell efficiencies. Thus, a 10.4% of PCE was achieved by XW4 co-sensitized with an organic dye. Not only with high cell efficiencies, they found that the XW4 based devices show satisfactory photostability under simulated solar light.

Attaching multiple electron-donating groups on diphenylamino also has been investigated. For example, Kim and co-workers reported a series of porphyrin sensitizers employed bulky alkoxy group substituted fluorene derivatives as electron donor moieties (Kang et al., 2012, 2013). Benefit from an efficient retardation of charge recombination, the DSSC based on 2,4-ZnP-CN-COOH displayed a PCE of 8.5% with V_OC of 739 mV. In 2014, Zhao et al. reported a series of push-pull porphyrin dyes featuring with different numbers of di-fluorene-amine at the donor moiety (Li et al., 2014a,b,c). With a C-N linkage of bis(4-hexylphenyl)amine to 9,9-dihexyl-9H-fluorene to construct the donor group, the ZZX-N4 exhibited better DSSC performance than the other analogs. They found that extension of π-conjugation area of donor group can hardly achieve a positive effect on the absorption spectra, as well as the IPCE of the DSSC devices.

To generate a large photocurrent response in DSSC, sensitizers should own broad and intense absorption. Porphyrin arrays exhibit strong electronic coupling between porphyrin rings, resulting in splitting of the B band and broadening of the Q band (Campbell et al., 2004). Indeed, the linkage of the porphyrins is a criterion factor determines the array's ICT therefore their spectroscopy properties. For example, in 2010, Yeh and Diau reported the triple bond linked porphyrin (YDD0) exhibited lower performance than single bond linkage porphyrin (YDD1) (4.1 vs. 5.2%), despite of a broader light-harvesting area of former (Mai et al., 2010). Calculation results suggested that the nearly planar structure of YDD0 facilitates π-conjugation between two porphyrin macrocycles, explaining the broader absorption area than YDD1. Whereas, the down-shift LUMO of YDD0 decreases the electron injection driving force. The planar geometry also induces serious dye aggregates, leading to a more non-radiation recombination of the excited dye. These factors eventually decrease the performance of the final device. In 2011, Segawa et al. designed a porphyrin dimer DTBC (DTBC in Figure S3) (Liu et al., 2011, 2012). The IPCE of DTBC device shows an onset to 900 nm and steeply reaches almost a plateau value close to 60% from around 400 nm until 800 nm. By employing high concentration of tBP to down-shift the conduction band of TiO₂, they succeeded in enhancing the electron-injection efficiency.

The JY07 porphyrin (Figure S3) features a double D-A branched structure, which was expect to increase the paths for efficient electron extraction (Zhang et al., 2015). JY07 with orthogonal conformation was also supposed to minimize the molecular aggregation on TiO₂ and suppress charge recombination in DSSC device. The V_OC of JY07 porphyrin was 650 mV, which was impressive for porphyrin dimer DSSC. However, inefficient conjugation of the triphenylamines with the porphyrin dimers limited the IPCE of JY07 to 725 nm.

Meanwhile, porphyrin dimers were expected to be ideal co-sensitizer to capture the solar energy in the longer wavelength area (Wu et al., 2012; Shiu et al., 2015). In 2012, Diau et al. reported a judiciously co-sensitization engineering of TiO₂ films with an organic dye, YD2-oC8 and dimer YDD6. The devices showed panchromatic spectral features in the IPCE spectrum up to 850 nm, while the PCE was 10.4%. These porphyrin-based co-sensitization systems showed only 10–15% degradation in 300 h at 25–30°C. They also reported that the porphyrins intend to desorption from the nanocrystalline particles when device temperature got higher than 60°C. Therefore, modification on the anchor group between the porphyrin and nanocrystalline become vital for improving the thermal stability of porphyrin sensitizers. We will discuss this part in the next section.

In the early years, the research of electron-donating groups in porphyrin dyes was major focused on arylamino or the derivatives. It's indeed useful in designing useful porphyrins for DSSC, such as YD2 (bis(4-hexylphenyl)amine), YD2-o-C8 (bis(4-hexylphenyl)amine), ZnPBAT (di-p-tolylamine), GY50 (bis(4-hexylphenyl)amine) (Yella et al., 2014), SM315 (bulky bis(2′,4′-bis(hexyloxy)-[1′-biphenyl]-4-yl)amine donor) etc. Recently, researchers started to put some eggs to the other bucket (Yang et al., 2014; Qi et al., 2018). For example, some electron-donating groups in organic sensitizer were successfully applied in porphyrin, such as N-annulated perylene (26 in Figure 4) (Jiao et al., 2011; Luo et al., 2013, 2016), 2-diphenylaminothiophene (23 in Figure 4) (Wang et al., 2014d), indoline (24 in Figure 4) (Pellej et al., 2014), phenothiazine (31 in Figure 4) (Xie et al., 2015), and even ferrocene (van der Salm et al., 2015). Some concepts from low-band-gap polymer were successfully applied in adjusting the absorption area and energy alignment of the porphyrins, such as weak donor-strong acceptor (Zhou et al., 2012; Bai et al., 2018; Lin et al., 2018; Xie et al., 2018), which will be discussed in the following part.

Insertion of secondary chromophores into the donor framework was reported to be efficient in broadening the IPCE while maintain the V_OC. Following this idea, Lin and Diau systematically tuned the absorption spectra by inserting acenes into the acceptor (Figure 6) (Lin et al., 2009). The best performance was achieved with LAC-3 porphyrin (5.4%), which bears with an anthracene as part of the linker. And the photovoltaic performance was decreased when it was tetracene or pentacene, which can be attributed to the inappropriate LUMO levels.

Ideally, enhance the donor's electron-donating ability and/or the acceptor's electron-withdrawing ability is beneficial to the ICT of the sensitizer, thus increases the injection efficiency and reduce the charge recombination (Wang et al., 2018). Yeh and Diau used the acrylonitrile group to build the acceptor (Lee et al., 2009). However, this porphyrin presented serious charge recombination and low injection efficiency. The authors suggested that the superior electron- withdrawing ability of cyano group first pull the electrons and then transfer them back into the porphyrin core. Another case is the ZnPF and ZnPH porphyrins (shown in Figure S4), which were reported by Imahori et al. in 2011 (Mathew et al., 2011; Sakurada et al., 2014). ZnPF featured with 4 fluorine atoms substituted on the phenyl linker, supposed to enhance the electron-withdrawing ability of the acceptor. Unfortunately, the ZnPF based DSSC showed lower performance than ZnPH, which was attributed to the more serious recombination reaction caused by the fluorine atoms.

The linker between the porphyrin and anchoring group plays a key role in modulating D-A electronic coupling as well as the magnitude of the change in dipole moment (Liu et al., 2009; Arja et al., 2018). Thiophene and its derivatives have been widely studied as linkers in organic dyes or low band-gap polymer (Zhou et al., 2015). In 2013, an efficient LW4 porphyrin employed 5-ethynylthiophene-2-carboxylic acid as acceptor was reported by Wang et al. (Lu et al., 2013a). LW4 shows similar spectral properties in comparison with its analog LD14, while the solar cells showed superior a performance (9.5 vs. 9.0%) at similar conditions. Transient photovoltage decay and charge extraction measurements revealed a prolonged recombination lifetime of LW4 devices (shown in Figure 7), indicating a less interfacial charge recombination process in the devices.

Within a rigid structure between porphyrin and carboxyl acid, sensitizers LW17, LW18, and LW19 (Figure S5) successfully indicated information on how the hetero-aromatic linker affect the DSSC performance, in terms of conjugation length, adsorption angle on TiO₂ (Lu et al., 2014a). They all have similar donor, porphyrin core and anchoring group structure, while only differentiate the linker group with phenyl, thiophene, and elongated 2-phenylthiophene. The absorption spectra (Q band maximum presented a stepwise red-shift as 674, 678, 683 nm, but obviously differentiate in DSSC performance. The resulted LW18 devices showed 8.7% PCE. Comparing to the LW17 or LW19 devices, LW18 devices presented longer electron recombination lifetime in LW18 device as revealed by transient decay measurements. This result also demonstrates that the recombination reaction on the TiO₂ interface is highly related to the linker groups.

One of the attractions in DSSC research is the controllable sensitizers' molecular structure through synthesis approach. When investigate how the linkers influence a porphyrin's spectral and photovoltaic properties, one can make the porphyrins bearing with exactly a same donor, porphyrin ring, and anchor, but only varied the linker group, such as benzene (LW1), thiophene (LW2), 3,4-ethylenedioxythiophene (EDOT, LW3), 2-phenylthiophene (LW6), 1,1′-biphenyl (LW7), 2,2′-bithiophene (LW8), and cyclopenta-[1,2-b:5,4-b′]dithiophene (CPDT, LW9) (Figure 8) (Lu et al., 2013a, 2014b,c). The Q band maximum of LW2 porphyrin can be red-shifted to 692 nm by 18 nm in comparison with LW1, whereas the LW6 porphyrin blue-shifts 14 nm compared with LW2 despite extended conjugation length. These results strongly suggested that the light-harvesting ability of porphyrin is not simply determined by the molecule's conjugation area, but also other factors, such as dipole moments, ICT process. Devices based on porphyrin LW2 and LW3 presented PCE of 7.4% and 7.6%, higher than that of LW1 (7.1%) but lower than LD14 (9.0%). Impedance spectra and transient decay measurements suggested this difference was ascribed to the variation of recombination reaction at the TiO₂/porphyrin/electrolyte interface. Bisquert et al. suggested that the floppy structure of cyanoacetic acid of the anchoring group tilted the molecules on TiO₂ film, which increased the aggregation of the molecules thus decreased its IPCE values and V_OC (Ripolles-Sanchis et al., 2012).

The linker's electron negativity was also reported to capable of influencing the porphyrin's properties. For example, when the linker switched from 1,1′-biphenyl to 2,2′-bithiophene and cyclopenta-[1,2-b:5,4-b′]dithiophene (CPDT), the absorption area and DSSC device performance for LW7, LW8, LW9 broadens and increases stepwise (Figure S6). The LUMO of these porphyrins down shift when the linker changed from bi-phenyl to CPDT. Considering an increase electron-donating ability of these linkers, it was supposed that a D-π-D-A structure might be benefit to red-shift and broaden the porphyrin sensitizer and improve the device performance (Lu et al., 2014b).

After the first report by Wang etc., a variety of thiophene conjugated rigid acceptors have been created (Sreenivasu et al., 2014; Zhang M.-D. et al., 2014). Interesting to note that furan and bithiophene linked acceptor functionated porphyrins exhibit broader absorption area than that of thiophene but dramatically dropped performance (2.8 and 1.4%). In 2015, Zhao reported ZZX-N8 porphyrin (Figure S7) utilized thieno[3,2-b]thiophene to substituted the thiophene (ZZX-N7) to expand the conjugation area in the linker place, the conjugated porphyrin achieved a slight higher PCE than the porphyrin conjugated with thiophene (7.8 vs. 7.5%) (Li et al., 2015). Choi et al. reported a porphyrin (ZnP-2 in Figure S7) featured with 4-((5-ethynylthiophen-2-yl)ethynyl)benzoic acid acceptor, in which a triple bond was inserted between the thiophene and phenyl (Chae et al., 2014). These two triple bonds between DPA and carboxylic acid group was reported to suppress the interface charge recombination reaction, delivering a 6.7% PCE of DSSC.

The strategy of introducing electron-deficient units as the auxiliary electron acceptor for push-pull porphyrins was the most successful progress in recent years. Benzothiadiazole (BTD) (Zhou et al., 2012; Xu et al., 2014; Lu F. et al., 2017), 2,3-diphenylquinoxaline (DPQ) (Fan et al., 2015), diketopyrrolopyrrole (DPP) (Warnan et al., 2012), and squaraine (SQ) (Jradi et al., 2015) etc. have been extensively exploited to re-construct D-A porphyrin. Studies demonstrated that integration of those proquinoidal units into the porphyrin structure causes strong perturbations to the electronic structure of the porphyrin ring, which lead to a broadening and red shifting of porphyrin's both B and Q-bands absorbance. For example, we synthesized analog porphyrins LW5 and LW24 (Figure 9) to analysis the benefit of BTD unit between the macrocycle and benzoic acid (Lu et al., 2015a, 2016). The LW5 and LW24 dyes feature with similar structure, except that the linker is an electron-rich unit thiophene for the former, whilst it is electron-deficient BTD unit for the latter. The LW24 porphyrin exhibits nearly 50 nm absorption onset red-shifts from 750 to 800 nm, along with more than 30% Q band absorption intensity compared with LW5. Finally, the LW24 devices presented 9.21% PCE, while it was 8.16% for the LW5 porphyrin.

It is worth to note that the BTD-conjugated porphyrins not always work efficient for DSSC application. For example, if the BTD linked between the porphyrin ring and benzoic acid, the obtained GY50 porphyrin based DSSC reached 12.8% PCE (Figure 10). Whereas it is 2.5% for the analog porphyrin GY21 (BTD connects the porphyrin and carboxyl acid directly) based DSSC despite of a similar absorption property (Yella et al., 2014). This performance difference suggests that locate the electron-deficient directly to the anchor is not a good idea. It is also criteria to choose the aromatic unit between the BTD and anchor. Take the LCVC02 and LCVC03 porphyrins (Figure 10) as examples, only a single atom (O to S) in the linker change switches-on the efficiency from 3% to over 10% (Cabau et al., 2015). Wang and co-workers investigated the suitable position for an auxiliary electron-deficient unit in the porphyrin sensitizers (Fan et al., 2015). A 50% improved PCE was reported when places the DPQ in the acceptor instead of the donor part (6.0 vs. 4.0%). In Table 1, we summarized the absorption peaks and DSSC performance of some porphyrins featured with electron-deficient linker.

By introducing benzoic acid or corresponded ester before constructing porphine macrocyle, porphyrin dyes without triple bond in the acceptor can be obtained (acceptor 18 in Figure 6) (Hayashi et al., 2015). Kim and co-workers reported series of push-pull porphyrins with benzoic acid or 2-cyano-3-phenylacrylic acid (acceptor 19 in Figure 6) (Seo et al., 2012). HKK-Por1 incorporated with 4-benzoic acid obtained a PCE of 5.9%, while 2,4-ZnP-CNCOOH porphyrin with 2-cyano-3-phenylacrylic acid acceptor achieved remarkable 8.5% with the HC-A1 co-adsorbent (Kang et al., 2013).

Through one-pot with two-step condensation reactions, followed by high-yielding metalation and hydrolysis steps, Hung and Diau synthesized a series of A2B2 porphyrin, in which 2 acceptors were used to enhance the dye-loading amount and control the adsorption geometry as well (Ambre et al., 2012, 2013; Luo et al., 2015). They placed the donors and acceptors at macrcycle's cis-position. ATR-FTIR spectroscopy confirmed that those A2B2 porphyrins chelated to the TiO₂ surface with a double-arm-binding mode. The highest efficiency was below 5% but still provided a solution to enhance dye attachment and photo-stability.