Synthesis and characterization of cyclopentadithiophene-based low bandgap copolymers for all-polymer solar cells

Three conjugated copolymers based on cyclopentadithiophene (CPDT) units, namely, poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzothiadiazole-4,7-diyl} (P1), Poly{4,4-bis(2-ethylhexyl)cyclopeanta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,1,3-benzoselenadiazole-4,7-diyl} (P2) and Poly{4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-N,N′-bis(2-ethylhexyl)-3,4,9,10-perylene diimide-1,7-diyl} (P3) have been synthesized via Stille coupling reaction. The polymers were characterized by ¹H NMR spectroscopy, gel permeation chromatography (GPC), UV–vis absorption spectroscopy, and cyclic voltammetry. These new polymers exhibit broad and strong absorption between 500 and 800 nm. The highest occupied molecular orbital energy levels of polymers vary between ?4.98 and ?5.27 eV and the lowest unoccupied molecular orbital energy levels range from ?3.43 to ?3.70 eV. By employing P1 and P2 as electron donors (D) and P3 as electron acceptor (A), all-polymer solar cells with bulk heterojunction structure have been fabricated. Preliminary results indicate that these devices show higher open circuit voltage (V _OC ) in comparison with the traditional polymer/fullerene systems of P1 and P2 blended with the acceptor (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM).     

Naphthalenediimide (NDI) polymers for all-polymer photovoltaics

Naphthalenediimide (NDI) polymers are an important class of electron-accepting (acceptor or n-type) semiconductors for organic photovoltaic (OPV) or organic solar cell (OSC) applications. Blending them with compatible electron-donating (donor or p-type) polymers yields an OPV device known as bulk-heterojunction (BHJ) all-polymer solar cells (all-PSCs). Compared to the more extensively studied OPVs using fullerene derivatives as the acceptor material, all-PSCs provide important benefits such as chemical tunability, mechanical flexibility and ambient/stress stability. Through an extensive research on materials design and device optimization in the last five years, all-PSCs employing NDI-based polymers have achieved remarkable improvement in device power conversion efficiency (PCE), now surpassing 10% – a number that approaches the state-of-the-art organic photovoltaic (OPV) cells using fullerenes. In this review, recent development of NDI-based conjugated polymers used in all-PSCs will be highlighted.     

Synthesis and characterization of new donor-acceptor type copolymers based on fluorene derivatives for photovoltaic solar cells

In this paper, we demonstrated the successful synthesis of newly designed copolymers, C1 and C2, with donor-acceptor type structure. Both C1 and C2 copolymers contained 9,9-dioctylfluorene-2,7-bis(trimethyleneboronate) as one constructional unit to improve the solubility in common organic solvents. The other constructional unit was 2,3-bis(5-bromothiophen-2-yl)acrylonitrile (DTDBAL) for C1, while 4,7-dibromobenzo[c][1,2,5]thiadiazole unit, 5,5'-dibromo-2,2'-bithiophene unit and N1, N1-bis(4-bromophenyl)-N4,N4-bis(4-(2-phenylpropan-2-yl)phenyl)benzene-1,4-diamine are for C2. We fabricated photovoltaic devices based on the C1 and the C2 copolymers with Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer, PC70BM layer, TiOx layer, and aluminum (Al) electrode. The bulk heterojuntion photovoltaic devices using these copolymers as electron donor and PC70BM as the acceptor exhibited good device performances when measured at 100 mW cm-2. The power conversion efficiency (PCE) of the C1 device reached 0.45% with Voc, Jsc and FF of 0.51, 2.50 and 35%, respectively. The PCE of the C2 device reached 0.34% with Voc, Jsc, and FF of 0.56, 2.01 and 30%, respectively.     

The use of nanoimprint lithography to improve efficiencies of bilayer organic solar cells based on P3HT and a small molecule acceptor

Nanoimprint lithography (NIL) was carried out on bilayer organic photovoltaics (OPVs) based on regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) as electron donor and 4,7-bis(2-(1-ethylhexyl-4,5-dicyano-imidazol-2-yl)vinyl)benzo[c]1,2,5-thiadiazole (EV-BT) as electron acceptor. The power conversion efficiency of the devices after nanoimprinting increased from 0.20% to 0.30%, closer to that of the bulk heterojunction device at 0.37%. As a result, NIL proved to be a feasible method for improving the bilayer OPV performance by increasing the donor/acceptor interfacial area.     

Molecular packing and solar cell performance in blends of polymers with a bisadduct fullerene

We compare the solar cell performance of several polymers with the conventional electron acceptor phenyl-C61-butyric acid methyl ester (PCBM) to fullerenes with one to three indene adducts. We find that the multiadduct fullerenes with lower electron affinity improve the efficiency of the solar cells only when they do not intercalate between the polymer side chains. When they intercalate between the side chains, the multiadduct fullerenes substantially reduce solar cell photocurrent. We use X-ray diffraction to determine how the fullerenes are arranged within crystals of poly-(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and suggest that poor electron transport in the molecularly mixed domains may account for the reduced solar cell performance of blends with fullerene intercalation.     

A Chemically Doped Naphthalenediimide‐Bithiazole Polymer for n‐Type Organic Thermoelectrics

The synthesis of a novel naphthalenediimide (NDI)‐bithiazole (Tz2)‐based polymer [P(NDI2OD‐Tz2)] is reported, and structural, thin‐film morphological, as well as charge transport and thermoelectric properties are compared to the parent and widely investigated NDI‐bithiophene (T2) polymer [P(NDI2OD‐T2)]. Since the steric repulsions in Tz2 are far lower than in T2, P(NDI2OD‐Tz2) exhibits a more planar and rigid backbone, enhancing π–π chain stacking and intermolecular interactions. In addition, the electron‐deficient nature of Tz2 enhances the polymer electron affinity, thus reducing the polymer donor–acceptor character. When n‐doped with amines, P(NDI2OD‐Tz2) achieves electrical conductivity (≈0.1 S cm^?1) and a power factor (1.5 µW m^?1 K^?2) far greater than those of P(NDI2OD‐T2) (0.003 S cm^?1 and 0.012 µW m^?1 K^?2, respectively). These results demonstrate that planarized NDI‐based polymers with reduced donor–acceptor character can achieve substantial electrical conductivity and thermoelectric response.     

Diethynylbenzene-based liquid crystalline semiconductor for solution-processable organic thin-film transistors

We report here the synthesis and characterization of novel diethynylbenzene-based liquid crystalline semiconductor (P1) for organic thin-film transistors (OTFTs). Compound P1 was synthesized by the Sonogashira coupling reaction between 2-bromo-5-(4-hexylthiophen-2-yl)thieno[3,2-b]thiophene and 1,4-bis(dodecyloxy)-2,5-diethynylbenzene. Top contact OTFTs were fabricated by spin casting with 2 wt% solution of P1 in chloroform and their best performance, which exhibited a hole mobility of 4.5 x 10(-5) cm2/Vs, was showed after annealing of the films at liquid crystalline temperature. Time-of-flight (TOF) mobility measured at liquid crystalline phase was observed to be 1.5 x 10(-6) cm2/Vs for both positive and negative carriers. These results indicate that the liquid crystallinity helps to improve the molecular packing and enhance charge mobility for P1. These advantages can be applicable to design and construct solution-processable OTFT materials for electronic applications.     

Synthesis of fluorene-based semiconducting copolymers for organic solar cells

Semiconducting polymers composed of 2,2'-(9,9-dioctyl-9H-fluorene-2,7-diyl)dithiophenes (F8T2s) and (2E,2'E)-3,3'-(2,5-bis(octyloxy)-1,4-phenylene) bis(2-(5-bromothiophene-2-yl)acrylonitrile)s (OPTANs) have been synthesized through Pd(O)-catalyzed Suzuki coupling polymerization by controlling the monomer ratio. The synthesized polymers were confirmed to exhibit good solubility in common solvents, simple processability, and thermal stability up to 350 degrees C. The highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and optical band-gap energies were determined using cyclic voltammetry (CV) and UV-visible spectrometry. The synthesized polymers showed their maximum absorption and edge at around 520 and 650 nm, respectively. The optical band-gap energies of the polymers were determined to be 1.89 eV. Bulk heterojunction organic solar cells were fabricated using the conjugated polymer as the electron donor, and 6,6-phenyl C61-butyric acid methylester (PC61BM) or 6,6-phenyl C71-butyric acid methylester (PC71BM) as the electron acceptor. The power conversion efficiencies (PCEs) of the solar cells based on polymer:PC71BM (1:1) and polymer:PC71BM (1:2) were 0.68% and 1.22%, respectively, under air mass 1.5 global (AM 1.5 G) illumination at 100 mW/cm2.     

A Narrow‐Bandgap n‐Type Polymer Semiconductor Enabling Efficient All‐Polymer Solar Cells

Currently, n‐type acceptors in high‐performance all‐polymer solar cells (all‐PSCs) are dominated by imide‐functionalized polymers, which typically show medium bandgap. Herein, a novel narrow‐bandgap polymer, poly(5,6‐dicyano‐2,1,3‐benzothiadiazole‐alt‐indacenodithiophene) (DCNBT‐IDT), based on dicyanobenzothiadiazole without an imide group is reported. The strong electron‐withdrawing cyano functionality enables DCNBT‐IDT with n‐type character and, more importantly, alleviates the steric hindrance associated with typical imide groups. Compared to the benchmark poly(naphthalene diimide‐alt‐bithiophene) (N2200), DCNBT‐IDT shows a narrower bandgap (1.43 eV) with a much higher absorption coefficient (6.15 × 10⁴ cm^?1). Such properties are elusive for polymer acceptors to date, eradicating the drawbacks inherited in N2200 and other high‐performance polymer acceptors. When blended with a wide‐bandgap polymer donor, the DCNBT‐IDT‐based all‐PSCs achieve a remarkable power conversion efficiency of 8.32% with a small energy loss of 0.53 eV and a photoresponse of up to 870 nm. Such efficiency greatly outperforms those of N2200 (6.13%) and the naphthalene diimide (NDI)‐based analog NDI‐IDT (2.19%). This work breaks the long‐standing bottlenecks limiting materials innovation of n‐type polymers, which paves a new avenue for developing polymer acceptors with improved optoelectronic properties and heralds a brighter future of all‐PSCs.     

Symmetric long alkyl chain end-capped anthracene derivatives for solution-processed organic thin-film transistors

Three new anthracene derivatives, 2,6-bis(4-decylphenyl)anthracene (DDPAnt), 2-decyl-5-(2-(5-decylthiophen-2-yl)anthracen-6-yl)thiophene (DDTAnt), and 2,6-bis(4-decyloxy phenyl) anthracene (DDPXAnt) were synthesized by Suzuki cross-coupling reaction. The obtained oligomers were characterized by 1H NMR, FT-IR, Mass, UV-visible spectroscopy, cyclovotammetry, differencial scanning calorimetry, and thermogravimetric analysis. The thermal studies show that these oligomers are stable up to 400 degrees C. The solution processed OTFTs were fabricated using synthesized oligomers by spin-coating and drop casting processes on Si/SiO2. OTFTs based on DDPAnt showed the mobility of 7.6 x 10(-3) cm2/Vs and on/off ratio of 10(5).     

Fill factor enhancement of organic solar cells based on a wide bandgap phosphorescent material and C60

Planar heterojunction organic solar cells using wide bandgap phosphorescent material bis[2-(4-tertbutylphenyl)benzothiazolato-N,C^2'] iridium (acetylacetonate) [(t-bt)₂Ir(acac)] as electron donor and fullerene (C₆₀) as electron acceptor were fabricated. A large open circuit voltage of 0.94 V was achieved due to low highest occupied molecular orbital level of (t-bt)₂Ir(acac). The effect of different hole transport layers and substrate heating were investigated to improve fill factor. It is shown that the open circuit voltage is strongly influenced by the interface energy barrier, whereas the fill factor is mainly limited by the charge carrier transport properties in active materials. The fill factor was significantly improved by either using hole transport layer with high carrier mobility or increasing the hole mobility of (t-bt)₂Ir(acac). A power conversion efficiency of 2.10% under AM 1.5 solar illumination at an intensity of 100 mW/cm² was achieved by heating the substrate during the deposition of active materials.     

Tailoring the optoelectronic properties of donor-acceptor-donor type π-conjugated polymers via incorporating different electron-acceptor moieties

Syntheses of donor-acceptor-donor type of π-conjugated monomers were performed to examine the effect of the acceptor units' strength on the electrochemical and optoelectrochemical properties of the resulting monomer and polymer. Palladium catalyzed Stille cross-coupling reaction of an organotin reagent with an organic electrophile was used for the synthesis of target monomers, 5,8-bis(4-hexylthiophen-2-yl)-2-(2,3-dihydrobenzo[b][1,4]dioxin-6-yl)-3-(2,3-dihydrobenzo[b][1,4]dioxin-7-yl)quinoxaline (DBQHT) and 10,13-bis(4-hexylthiophen-2-yl)dibenzo[a,c]phenazine (PHEHT).The presence of the strong electron-donating ethylenedioxy groups on pendant phenyl rings increased electron density on DBQHT, thus the oxidation potential of DBQHT shifts to a lower value than that of PHEHT. The π-π* absorption maximum of PPHEHT was about 40 nm red-shifted compare to that of PDBQHT, which can be attributed to the increase of the effective conjugation and coplanarity of PPHEHT relative to PDBQHT via using phenanthrene fused quinoxaline unit as the acceptor. The electronic band gap of polymer, defined as the onset of the π-π* transition, is found to be 1.65 eV for PPHEHT and 1.82 eV for PDBQHT. Both polymer films showed multi-color electrochromism. PDBQHT can be switched between a red neutral state and a green oxidized state with two intermediate states; purple and brown. PPHEHT also shows multicolored electrochromic behavior with three distinct states: a blue neutral state, a gray intermediate state, and a green oxidized state.     

Improvement of photovoltaic properties by addition of a perylene compound in P3HT:PCBM BHJ system

The synthesized n-type perylene derivative, N,N'-bis-(4-bromophenyl)-1,6,7,12-tetrakis(4-n-butoxy-phenoxy)-3,4,9,10-perylene tetracarboxdiimide (PIBr), was applied as an additive to polymer solar cells (PSCs) with P3HT [poly(3-hexylthiophene)]:PCBM [[6,6]-phenyl C61-butyric acid methyl ester] blend films. Without post thermal annealing, a considerable improvement of about 98% in power conversion efficiency was achieved by the addition of 1 wt% PIBr into a P3HT:PCBM layer, when compared with that of reference cell without the additive. The results, in combination with relevant data from UV-Vis. absorption, photoluminescence, X-ray measurements and carrier mobility studies, revealed that the addition of the perylene compound within active layer contributed to more effective charge transfer and enhanced electron mobility.     

A Narrow-Bandgap n-Type Polymer with an Acceptor–Acceptor Backbone Enabling Efficient All-Polymer Solar Cells

Narrow-bandgap polymer semiconductors are essential for advancing the development of organic solar cells. Here, a new narrow-bandgap polymer acceptor L14, featuring an acceptor–acceptor (A–A) type backbone, is synthesized by copolymerizing a dibrominated fused-ring electron acceptor (FREA) with distannylated bithiophene imide. Combining the advantages of both the FREA and the A–A polymer, L14 not only shows a narrow bandgap and high absorption coefficient, but also low-lying frontier molecular orbital (FMO) levels. Such FMO levels yield improved electron transfer character, but unexpectedly, without sacrificing open-circuit voltage (V_oc), which is attributed to a small nonradiative recombination loss (E_loss,nr) of 0.22 eV. Benefiting from the improved photocurrent along with the high fill factor and V_oc, an excellent efficiency of 14.3% is achieved, which is among the highest values for all-polymer solar cells (all-PSCs). The results demonstrate the superiority of narrow-bandgap A–A type polymers for improving all-PSC performance and pave a way toward developing high-performance polymer acceptors for all-PSCs.     

A highly efficient polymer non-fullerene organic solar cell enhanced by introducing a small molecule as a crystallizing-agent

Non-fullerene organic solar cells (OSCs) have attracted tremendous interest because of their potential to replace traditional expensive fullerene-based OSCs. To further increase the power conversion efficiency (PCE), it is necessary to offset the narrow absorption of the non-fullerene materials, which is often achieved by adding an additive (>10?wt%) to form a ternary blend. However, a high ratio of the third component can often be detrimental to the active layer morphology and can increase the complexity in understanding the device physics toward rationally designed improvements. In this work, we introduce 2,4-bis-[(N,N-diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyliminio) squaraine (ASSQ) in the poly [(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl) benzo [1,2-b:4,5-b′] dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl) benzo [1,2-c:4,5-c′] dithiophene-4,8-dione)] (PBDB-T): 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno [2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6-b′] dithiophene (ITIC) as an active layer “crystallizing-agent”. Through detailed morphology characterization, we find that the addition of 4?wt% ASSQ assists ITIC organization order and promotes PDBD-T:ITIC aggregation in the preferential face-on orientation. In addition, we demonstrate that the ASSQ and PBDB-T show efficient exciton dissociation in the ternary blend over Förster resonance energy transfer (FRET). We reveal using surface potential and solubility measurements that a ASSQ-ITIC co-crystalline structure forms which facilitates a significant improvement in the device PCE, from 8.98% to 10.86%.     

Cross‐Linkable and Dual Functional Hybrid Polymeric Electron Transporting Layer for High‐Performance Inverted Polymer Solar Cells

A cross‐linkable dual functional polymer hybrid electron transport layer (ETL) is developed by simply adding an amino‐functionalized polymer dopant (PN4N) and a light crosslinker into a commercialized n‐type semiconductor (N2200) matrix. It is found that the resulting hybrid ETL not only has a good solvent resistance, facilitating multilayers device fabrication but also exhibits much improved electron transporting/extraction properties due to the doping between PN4N and N2200. As a result, by using PTB7‐Th:PC₇₁BM blend as an active layer, the inverted device based on the hybrid ETL can yield a prominent power conversion efficiency of around 10.07%. More interestingly, photovoltaic property studies of bilayer devices suggest that the absorption of the hybrid ETL contributes to photocurrent and hence the hybrid ETL simultaneously acts as both cathode interlayer material and an electron acceptor. The resulting inverted polymer solar cells function like a novel device architectures with a combination of a bulk heterojunction device and miniature bilayer devices. This work provides new insights on function of ETLs and may be open up a new direction for the design of new ETL materials and novel device architectures to further improve device performance.     

Efficient and 1,8-diiodooctane-free ternary organic solar cells fabricated via nanoscale morphology tuning using small-molecule dye additive

The ternary strategy for incorporating multiple photon-sensitive components into a single junction has emerged as an effective method for optimizing the nanoscale morphology and improving the device performance of organic solar cells (OSCs).In this study,efficient and stable ternary OSCs were achieved by introducing the small-molecule dye (5E,5'E)-5,5'-(4',4″-(1,2-diphenylethene-1,2-diyl)bis(biphenyl-4',4-diyl))bis(methan-1-yl-1-ylidene)bis(3-ethyl-2-thioxothia zolidin-4-one) (BTPERn) into poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiopheneco-3-fluorothieno[3,4-b]thiophene-2-carboxylate] (PTB7-Th):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) blend films processed using a 1,8-diiodooctane (DIO)-free solvent.The incorporation of BTPE-Rn enhanced the short-circuit current density and fill factor of the ternary OSCs compared with those of binary OSCs.An investigation of the optical,electronic,and morphological properties of the ternary blends indicated that the third component of BTPE-Rn not only promoted the photon utilization of blends through the energy-transfer process but also improved the electron mobility of the blends owing to the fullerene-rich nanophase optimization.More importantly,this ternary strategy of utilizing a small-molecule dye to replace the photounstable DIO additive enhanced the operational stability of the OSCs.     

Design of a New Small‐Molecule Electron Acceptor Enables Efficient Polymer Solar Cells with High Fill Factor

Improving the fill factor (FF) is known as a challenging issue in organic solar cells (OSCs). Herein, a strategy of extending the conjugated area of end‐group is proposed for the molecular design of acceptor–donor–acceptor (A–D–A)‐type small molecule acceptor (SMA), and an indaceno[1,2‐b:5,6‐b′]dithiophene‐based SMA, namely IDTN, by end‐capping with the naphthyl fused 2‐(3‐oxocyclopentylidene)malononitrile is synthesized. Benefiting from the π‐conjugation extension by fusing two phenyls, IDTN shows stronger molecular aggregation, more ordered packing structure, thus over one order of magnitude higher electron mobility relative to its counterpart. By utilizing the fluorinated polymer (PBDB‐TF) as the electron donor, the corresponding device exhibits a high efficiency of 12.2% with a record‐high FF of 0.78, which is approaching the theoretical limit of OSCs. Compared with the reference molecule, such a high FF in the IDTN system can be mainly attributed to the more ordered π–π packing of acceptor aggregates, higher domain purity and symmetric carrier transport in the blend. Hence, enlarging the conjugated area of the terminal‐group in these A–D–A‐type SMAs is a promising approach not only for enhancing the electron mobility, but also for improving the blend morphology, and both of them are conducive to the fill‐factor breakthrough.     

Nanostructured nanorod arrays presenting TiO2 nanorods/poly(3-hexylthiophene) for solar cells application

We have fabricated inverted heterojunction solar cell devices incorporating titanium dioxide nanorod/poly(3-hexylthiophene) (P3HT) rod arrays using melt-assisted anodic alumina oxide template. Using transmission electron microscopy and conductance atomic force microscopy, we revealed that phase-separated TiO2 rich (n-type) and P3HT rich (p-type) regions presents in these rod arrays. The optimized composite rod array structure had a higher hole mobility than that of the blend film consisting of TiO2 nanorod and P3HT as determined by fitting the dark J-V curves into the space charge-limited current model. The more efficient carrier transport of the device incorporating the nanorod arrays provided it with both a higher short-circuit current density and power conversion efficiency.     

Molecular design of copolymers based on polyfluorene derivatives for Bulk-heterojunction-type solar cells

Poly{[2,7-(9,9′-dihexylfluorene)]-alt-[4,7-di(thiophen-2-yl)benzo[c][1, 2, 5]thiadiazole]} (PFDTBT) with low band gap was reported as an intriguing and promising donor in Bulk-heterojunction-type solar cells. In this paper, based on the structure of PFDTBT, three new kinds of donor materials: poly{[2,7-(9,9′-dihexylfluorene)]-alt-[4,7-di(thiophen-2-yl)-[1, 2, 5]thiadiazolo[3,4-d]pyridazine]} (PFDTTDP), poly{[2,7-(9,9′-dihexyloxyfluorene)]-alt-[4,7-di(thiophen-2-yl)-[1, 2, 5]thiadiazolo[3,4-d]pyridazine]} (POFDTTDP), and poly{[2,6-(4,4-dihexyl)-4H-cyclopenta[2,1-b;3,4-b’]-dithiophene)-alt-[4-(1,3,4-thiadiazol-2-yl)-7-(thiophen-2-yl)-[1, 2, 5]thiadiazolo[3,4-d]pyridazine]} (PCPTTTDP), were designed and computed by density function theory (DFT). The electronic, optical and photovoltaic properties, and charge transport rates were investigated. The reorganization energies for holes and electrons are around 0.11 and 0.08 eV, respectively. It indicates that PFDTTDP, POFDTTDP, and PCPTTTDP are good candidates for donor material. Especially, when 6,6-phenyl-C61-butyric acid methyl ester (PC₆₁BM) functions as acceptor, PCPTTTDP has the most appropriate highest occupied molecular orbital and lowest unoccupied molecular orbital energy, and has the broadest absorption in the near-infrared region.