•The strong intramolecular O–H⋯O can affect the whole π-conjugated property, thus morphology and photovoltaic performance.•The squaraine bearing two OH has obtained the best PCE, which is 3 times higher than that of the one bearing none of OH.•TLC analysis might be a simple, cheap, and rapid way to forecast the compatibility between donor materials and PC71BM.Three unsymmetrical squaraines (USQs) with different number of hydroxyl groups (BIISQ bearing two hydroxyl substituents, BIISQ-MOH bearing mono-one, and BIISQ-NOH bearing none) have been synthesized to understand of the insightful molecular structure-property relationship of photovoltaic materials. Our research shows that the introduction of strong intramolecular hydrogen bonding (HB) interactions between donor (D) and acceptor (A) units of squaraines can affect the whole π-conjugated properties, such as deepening the highest occupied molecular orbital (HOMO) energy level and thus enhancing the open-circuit voltage (Voc), increasing the dipole moment and improving hole mobility, weakening the electron-withdrawing ability of centric four-membered rings (as A unit in squaraines) and then improving the compatibility between USQ and [6,6]-phenyl-C71butyric acid methyl ester (PC71BM). Eventually, the bulk-heterojunction organic photovoltaic device (BHJ-OPV) based on BIISQ has obtained the best performance, which is approximately three times higher than that of BIISQ-NOH-based one. Interestingly, our research also finds that thin layer chromatography (TLC) analysis might be a simple, cheap, and rapid way to preliminarily forecast the compatibility between donor materials and PC71BM when the molecular skeletons of donor materials are relatively similar.The strong intramolecular hydrogen-bonding can significantly affect the compatibility between squaraines donor and PC71BM, thus film morphology and photovoltaic performance.Download high-res image (367KB)Download full-size image

•The performance of SQ-BP:PC71BM solar cells improved by adding Bis-PC71BM.•The PCE of ternary system with 5%wt Bis-PC71BM increased from 4.46% to 5.31%.•The TPV data exhibits a beneficial effect on reducing the charge recombination loss.The development of small molecule organic solar cells (SMOSCs) has attracted considerable attention and achieved comparable power conversion efficiency (PCE) with polymer solar cells. Here, we demonstrate a bulk heterojunction (BHJ) small molecular solar cell with PCE of 5.31% by incorporating Bisadduct of phenyl-C71-butyric acid methyl ester (Bis-PC71BM) as an additional acceptor material into the host binary blend of 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM). The short circuit current (JSC) and the fill factor (FF) of ternary SMOSCs are improved by decreasing the carrier recombination loss, increasing exciton dissociation and enhancing the carrier transport. The transient photovoltage (TPV) measurement indicates that the gradient HOMO energy alignment suppresses the charge recombination and leads to the increased charge carrier lifetime in ternary SMOSCs. As a result, the PCE of ternary devices with 5 wt% Bis-PC71BM is about 20% greater than that of SQ-BP: PC71BM based binary SMOSCs.Download high-res image (248KB)Download full-size image

•We fabricated various ternary devices by adding squaraine derivatives with different energy levels.•The large potential barrier between additional material and host material disrupts the charge carrier transport and leads to significant recombination.•The PCE of 10% SQ-3 ternary device is enhanced by 21% resulted from improved photon harvesting and changeless charge carrier characteristics.The ternary solar cell has been proved as an elegant strategy to realize efficient bulk heterojunction (BHJ) organic solar cells (OSCs) by harvesting a larger range of photons than binary devices. In order to design such an efficient system, the charge carrier transport and recombination in ternary OPVs must be considered. We fabricated various ternary devices by adding squaraine derivatives (SQ-1, SQ-2, SQ-3, SQ-4 and SQ-5) with complementary absorption spectrum in near infrared region as the additional donor material into Poly [[9-(1-octylnonyl)-9H-carbazole-2, 7-diyl]-2, 5-thiophenediyl-2, 1, 3-benzothiadiazole-4, 7-diyl-2, 5-thiophenediyl] (PCDTBT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host binary blend film in this paper. Through the charge carrier mobility and transient photovoltage (TPV) measurements, it is found that the incorporation of 10% SQ-5 with shallower Highest Occupied Molecular Orbital (HOMO) level hinders holes in transferring from SQ to PCDTBT, which leads to unbalanced charge carrier transport, significant charge recombination, and decrease of performance. On the contrary, the performance of ternary devices incorporated with 10% SQ-3 with deeper HOMO energy level is improved by 21% compared with binary devices, which can be attributed to increased photon harvesting and changeless charge carrier characteristics.Download high-res image (224KB)Download full-size imageTernary solar cells incorporated with various SQ dyes with different energy levels achieve different performance.

•Two different donor subunits substituted unsymmetrical squaraines exhibited a totally different aggregation.•The USQ-11/PC71BM blend film displayed quite smaller domains size than that of the USQ-12/PC71BM.•The PCE of USQ-11-based solution-processed BHJ-SMOSCs was much higher than that of USQ-12 (4.27% vs. 2.78%).•2,3,3-Trimethylindolenine unit should be an excellent D subunit for construction of squaraines photovoltaic materials.Two unsymmetrical squaraines (USQs) with different donor (D) subunits as photovoltaic materials, namely USQ-11 and USQ-12, were designed and synthesized to investigate the effect of different D subunits on the optoelectronic properties of USQs for the first time. The two USQs compounds were characterized for optical, electrochemical, quantum chemical and optoelectronic properties. By changing the two different D subunits attached to the squaric acid core from 2,3,3-trimethylindolenine to 2-methylbenzothiazole, the HOMO energy levels could be tuned with a stepping of 0.07 eV, and quite different solid state aggregations (H- or J-aggregation) were observed in the thin film by UV-Vis absorption spectra, which were attributed to their distinct steric effects and dipole moments. Solution-processed bulk-heterojunction small molecule organic solar cells fabricated with the USQ-11/PC71BM (1:5, wt%) exhibited extremely higher PCE (4.27%) than that of the USQ-12/PC71BM (2.78%). The much enhanced PCE should be attributed to the simultaneously improved Voc, Jsc and FF.

A novel asymmetrical squaraine derivative bearing a cyano-substituted indoline end-capping group, namely ASQ-5-CN, was designed and synthesized. In comparison with the noncyano-substituted ASQ-5, ASQ-5-CN showed an analogous absorption band-gap in the thin solid film state, but a 0.11 eV lowered HOMO energy level, which led to a higher Voc. Density functional theory calculation results revealed that the dipole moment of ASQ-5-CN was over double that of ASQ-5. Hence the stronger dipole–dipole interactions of ASQ-5-CN might trigger more intense intermolecular packing in ASQ-5-CN, which should account for the higher hole mobility of ASQ-5-CN than that of ASQ-5 (4.00 × 10−5vs. 1.67 × 10−5 cm2 V−1 s−1). Accordingly, solution-processed bulk-heterojunction small molecular organic solar cells using ASQ-5-CN as the electron donor exhibited a much higher PCE (5.24%) than that of the reference compound ASQ-5-based device (4.22%) due to its simultaneously enhanced Voc (0.92 vs. 0.82 V), Jsc (11.38 vs. 10.94 mA cm−2) and FF (0.50 vs. 0.47). Additionally, the PCE of the ASQ-5-CN-based device could be improved to be as high as 6.11% when measured at 80 °C, which is the record PCE among the hitherto reported squaraine-based solution-processed bulk-heterojunction organic solar cells.

Two novel asymmetrical squaraines based on the indoline unit, ASQ-5-F and ASQ-5-DF, with one and two fluorine substituents, have been developed to investigate the effect of fluorine substituted on small-molecule bulk-heterojunction (BHJ) organic solar cells (OSCs). In comparison with non-fluorine-substituted ASQ-5, both fluorine-substituted ASQ-5-F and ASQ-5-DF possess analogous absorption band gaps but 0.05 and 0.10 eV lowered highest occupied molecular orbital (HOMO) energy levels, respectively. Single-crystal analysis exhibits that ASQ-5-DF shows more desirable intermolecular packing patterns for the hole-carrier collection than ASQ-5 does; hence, higher hole mobility could be acquired. Therefore, solution-processed small-molecule BHJ OSCs fabricated with ASQ-5-F/PC71BM and ASQ-5-DF/PC71BM blends exhibit extremely higher power conversion efficiency (PCE; 5.0% and 6.0%, respectively) than that of ASQ-5/PC71BM (4.5%). The much improved PCE could be attributed to the simultaneously enhanced Voc, Jsc, and FF relative to those of the ASQ-5-based device. To our knowledge, this is the highest PCE (6.0%) among squaraine-based solution-processed BHJ OSCs and the highest PCE in OSCs based on the fluorinated donor segment of small molecules.Keywords: asymmetrical squaraines; fluorine-substituted; packing pattern; small-molecule organic solar cells; solution-processed;

An asymmetrical squaraine dye (Py-3) with its two electron-donating aryl groups directly linked to the electron-withdrawing squaric acid core possesses an ideal bandgap of 1.33 eV, together with an intense and broad absorption band in the range 550–950 nm. Hence, the resulting solution-processed solar cells display an impressive Jsc of 12.03 mA cm−2 and a PCE of 4.35%.

Small molecular neutral Ir(III) complexes have been demonstrated to be promising self-inclusive microcrystalline thin-film oxygen sensors with relatively high sensitivity (Ksv = 6.41), good stability, and linear Stern–Volmer behavior (R2 = 0.9979).

We demonstrate bulk heterojunction (BHJ) organic photovoltaics (OPVs) with a power conversion efficiency (PCE) of 6.39% by incorporating a small molecular compound 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP) as the additional donor material into a poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host binary blend. Incorporating SQ-BP into the PCDTBT:PC71BM host blend film increases the photon harvesting of a ternary photovoltaic device with a broad absorption spectrum from 300 nm to 750 nm, which results in an increased short-circuit current density (Jsc). In addition to efficient photon harvesting, Förster resonance energy transfer (FRET) between PCDTBT and SQ-BP is also the reason for the increase in Jsc. As a result, the PCE of ternary devices with 10 wt% SQ-BP is about 30% greater than that of PCDTBT:PC71BM-based binary OPVs.

A series of new asymmetrical squaraine derivatives bearing N,N-diarylamino substituents as end-capping groups, namely ASQAr-1–6, were designed and synthesized. In comparison with the reference compound ASQB bearing a N,N-diisobutylamino end-capper, all the six target compounds exhibit improved thermal stability, red-shifted and broadened absorption bands as well as lower HOMO and LUMO energy levels. Despite the hole mobility of most of the compounds still being lower than that of ASQB, solution-processed bulk-heterojunction small molecule organic solar cells (BHJ-SMOSCs) using ASQAr-1–6 as electron donor materials all show drastically higher power conversion efficiency (PCE, 3.08–3.69%) than that of the ASQB-based reference device (PCE = 1.54%). The much enhanced photovoltaic performance of BHJ-SMOSCs based-on ASQAr-1–6 could be attributed to the simultaneously enhanced open-circuit voltage (Voc, 0.81–0.87 V vs. 0.75 V), short-circuit current density (Jsc, 8.07–9.06 mA cm−2 vs. 5.40 mA cm−2), and fill factor (FF, 0.45–0.47 vs. 0.38) relative to those of the reference ASQB-based device.

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

A dual-channel naphthalimide-based chemosensor for rapid and sensitive detection of fluoride ion has been developed. Upon addition of F–, it undergoes deprotonation reaction through H-bonding interactions, and its maximum absorption wavelength is red-shifted for 214 nm to the far-red region, together with drastically quenched fluorescence. In addition, it shows high selectivity toward F– anion, thus could be used for practical applications to detecting F– in both solution and solid state. Furthermore, the fluorescence of NIM could be enhanced in protein-containing acidic environments, hence NIM could act as lysosome marker to differentiate cancer cells from normal ones in cell imaging.Keywords: cell imaging; chemosensor; dual-channel; fluoride ions; selectivity;

Three conjugated D–A copolymers were found to form well-defined nanopillar arrays through facile spin-casting process when blended with fullerene derivatives. Research results indicate that the presence of large coplanar segments and intramolecular S⋯O attractive interactions in the polymers are both crucial factors for achieving self-assembled nanopatterned pillar arrays.

By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothiazole ligand, two novel orange iridium(III) complexes, namely, (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac), have been synthesized. Compared with their parent compound (bt)222Ir(acac), both of them possess much enhanced thermostability and film amorphism, making them suitable candidates as guests for high performance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However, (4Phbt)222Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)222Ir(acac) due to spontaneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-processed PhOLEDs with (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac) as guests show peak current efficiency of 17.2 cd A−1 and 15.2 cd A−1, and maximum brightness of 28270 cd m−2 and 27900 cd m−2, respectively. Both are greatly improved compared to the devices employing (bt)222Ir(acac) (10.2 cd A−1 and 14350 cd m−2) and (4Phbt)222Ir(acac) (5.0 cd A−1 and 13790 cd m−2) as phosphors. Moreover, quite low efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.

•Two novel polymers bearing acenaphtho[1,2-b]quinoxaline as electron-deficient unit.•Strong S⋯O interaction(sum of van der Waals radii for S and O is 3.32 Å).•PB and PT possess deep HOMO energy level of −5.4 to −5.5 eV.•PB-based device showed maximum PCE value of 1.20% (Voc = 0.78 V).Dialkoxy substituted acenaphtho [1,2-b]quinoxaline (AQx) is demonstrated to be a promising electronic acceptor subunit for constructing high performance donor-acceptor (D–A) photovoltaic copolymers. Using it as acceptor and benzo[1,2-b:4,5-b′]-dithiophene (BDT) or thiophene (T) as donor units, two D–A conjugated polymers (abbr. as PB and PT respectively) are synthesized via Stille coupling reaction. The incorporation of alkoxy groups with quinoxaline moiety is found to be beneficial to both the enhanced solubility and the maintained coplanarity of the backbone due to strong S⋯O attractive interaction. UV–vis absorption and XRD characterization results indicate that the presence of planar AQx moiety could lead to well-ordered packing in solid state, and PB bearing planar BDT electron-donating subunit exhibits more intensified molecular self-organization. Both PB and PT possess deep HOMO energy level of −5.4∼−5.5 eV, and PB-based photovoltaic devices show maximum PCE value of 1.20% (Voc = 0.78 V, Jsc = 3.58 mA cm−2, FF = 0.43) under air mass 1.5 global (AM 1.5 G) irradiation of 100 mW cm−2.

A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)2(acac) (Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first example of a sulfur-free iridium complex for the detection of Hg2+ cations with high selectivity and sensitivity. Ir(pbi)2(acac) shows a multisignaling response towards mercury(II) ions through UV-vis absorption, phosphorescence and electrochemistry measurements. Upon addition of Hg2+ ions, solutions of this complex change from yellow to colorless, which could be observed easily by the naked eye, while its phosphorescence turns from bright green (λPLmax = 520 nm) into faint skyblue (λPLmax = 476 nm), and the detection limit is calculated to be 2.4 × 10−7 mol L−1. 1H NMR spectroscopic titration as well as ESI-MS results indicate that the decomposition of Ir(pbi)2(acac) in the presence of Hg2+ through rupture of Ir–O bonds is responsible for the significant variations in both optical and electrochemical signals.

Two novel blue light-emitting sextuple hydrogen-bonding self-assembly molecular duplexes bearing 4-phenoxy-1,8-naphthalimide fluorophores, namely PhNIHB and 2TPhNIHB, have been synthesized and characterized. Compared with their small molecular counterparts PhNI and 2TPhNI, the objective compounds exhibit 13–22 nm blue-shifted fluorescent emission, and much higher photoluminescence quantum yields (0.34 vs 0.18 for PhNIHB; 0.42 vs 0.27 for 2TPhNIHB) in solid state; and their thermal and morphological stability have been improved as well. Employing 2TPhNIHB or 2TPhNI as emitter, non-doped solution-processed light-emitting diodes with structure of ITO/PEDOT: PSS (40 nm)/PVK (40 nm)/blue emitter (70–80 nm)/CsF (1.5 nm)/Al (120 nm) have been fabricated. The 2TPhNI-based device gives yellow emission [CIE (0.38, 0.49)] with poor maximum luminous efficiency (LEmax) of 0.13 cd/A and external quantum efficiency (EQEmax) of 0.06%. The 2TPhNIHB-based device, however, gives blue-green emission [CIE (0.25, 0.34)], with much higher efficiency relative to 2TPhNI-based one (LEmax of 0.37 cd/A and EQEmax of 0.35%). The effective isolation of the naphthalimide fluorescent cores as well as the suppressed formation of exciplex at the PVK/emitter interface by these oligoamide motifs are suggested to be responsible for the improved EL performance.Graphical abstractHighlights► Blue multiple H-bonds self-assembly duplexes have been synthesized. ► They exhibit much enhanced PL and EL efficiency. ► They exhibit much improved thermal stability and film morphology.

Three novel conjugated polymers bearing 3,4-bis(4-hexylthiophen-2-yl)-3-cyclobutene-1,2-dione unit in their main chain have been synthesized successfully in good yields through Suzuki or Stille coupling reaction. Their molecular structures have been confirmed by FT-IR, 1H NMR and 13C NMR. All these copolymers exhibit broad and strong absorption bands in UV–vis region, and their optical band gaps are calculated to be 1.6–2.0 eV, suggesting that they have good coverage with the solar spectrum. These polymers have good thermostability and solubility in common organic solvents. Moreover, all these objective macromolecules possess high electron affinity of ∼3.8 eV determined from cyclic voltammetry measurement, implying that they are potential n-type polymeric photovoltaic materials.

Three electron-deficient conjugated polymers based on perylene diimide (PDI) units, namely, poly[(N,N′-didodecyl-3,4,9,10-perylene diimide-1,7-diyl)-alt-(9,9-dihexylfluorene-2,7-diyl)] (PPDIF), poly{(N,N′-didodecyl-3,4,9,10-perylene diimide-1,7-diyl)-alt-[N-(2-ethylhexyl) carbazole-3, 6-diyl]} (PPDIC) and poly{(N,N′-didodecyl-3,4,9,10-perylene diimide-1,7-diyl)-co-[N-(2-ethylhexyl) carbazole-3,6-diyl]-co-(9,9-dihexylfluorene-2,7-diyl)} (PPDICF) have been synthesized via Suzuki coupling reaction, and their chemical structures are confirmed by 1H NMR, 13C NMR and FT-IR. All these polymers show broad absorption bands in 250–700 nm, and their optical band gaps are calculated to be ~1.7 eV. Cyclic voltammetry results confirm that the objective macromolecules possess high electron affinity of ~3.9 eV. By employing poly-3-hexylthiophene (P3HT) as electron donor and PPDIC as electron acceptor, all polymer solar cells (aPSCs) with bulky heterojunction structure have been fabricated, preliminary results indicate they have one of the most highest open-circuit voltage (Voc) (0.86 V) reported so far in aPSCs with PDI-based polymers as electron acceptor.

Substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy or tert-butyl modified phenoxy groups, novel naphthalimide derivatives were obtained which emitted blue fluorescence with emission peaks of 425–444 nm in chloroform solution under UV irradiation, with highest relative photoluminescence quantum efficiency of 0.82. When in solid film, only compounds that contained ortho-tert-butylphenoxy substituents displayed blue photoluminescence of 438–451 nm, with highest absolute fluorescence quantum yield of 0.29; whereas other compounds showed greenish blue fluorescence at 471–478 nm, with highest absolute fluorescence quantum yield of 0.42. Cyclic voltammetry studies revealed that the molecules have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) ranging from −3.29 eV to −3.24 eV, and energy levels of the highest occupied molecular orbital (HOMO) ranging from −6.26 eV to −6.16 eV, suggesting they may possess good electron-transporting or hole-blocking properties. The findings indicate that the molecules offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue or greenish blue organic light-emitting diodes.

Two solution-processed asymmetrical squaraines (ASQs) with cyclopenta[b]indolinyl (1a) and cyclopenta[b]indolyl (1b) as end cappers have been designed and synthesized. Although the internal molecular structure variations are minimal, the presence of the cyclopenta[b]indolinyl group endows 1a more planar molecular structure, which results in a much more compact solid-state structure (density is 1.317 g cm−3 for 1a but is 1.187 g cm−3 for 1b), dramatically affecting charge transport in the thin films. The hole mobility of 1a:PC71BM blended film is about 7 times higher than that of 1b:PC71BM. Consequently, the maximum power conversion efficiency (PCE) value of the organic photovoltaic cells (OPVs) based on 1a of up to 4.1%, approximately 80% higher than that of 1b, is one of the highest PCEs achieved for ASQ-based bulk-heterojunction (BHJ) OPVs.

By introducing a phenyl substituent into the meta-site of the phenyl segment of the 2-phenylbenzothiazole ligand, two novel orange iridium(III) complexes, namely, (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac), have been synthesized. Compared with their parent compound (bt)222Ir(acac), both of them possess much enhanced thermostability and film amorphism, making them suitable candidates as guests for high performance solution-processed phosphorescent organic light-emitting diodes (PhOLEDs). However, (4Phbt)222Ir(acac) bearing para-phenyl possesses worse processability relative to (bt)222Ir(acac) due to spontaneous crystallization stemming from the intense intermolecular interactions. Single-layer solution-processed PhOLEDs with (3Phbt)222Ir(acac) and (3OMePhbt)222Ir(acac) as guests show peak current efficiency of 17.2 cd A−1 and 15.2 cd A−1, and maximum brightness of 28270 cd m−2 and 27900 cd m−2, respectively. Both are greatly improved compared to the devices employing (bt)222Ir(acac) (10.2 cd A−1 and 14350 cd m−2) and (4Phbt)222Ir(acac) (5.0 cd A−1 and 13790 cd m−2) as phosphors. Moreover, quite low efficiency roll-off is acquired in these devices at high luminance. The much improved electroluminescence performances of these objective complexes could be mainly attributed to the presence of a rigid phenyl on the appropriate substitution site of the cyclometallate ligand, which leads to improved thermostability with compatible alleviated intermolecular interactions, and consequently enhanced film amorphism.

Small molecular neutral Ir(III) complexes have been demonstrated to be promising self-inclusive microcrystalline thin-film oxygen sensors with relatively high sensitivity (Ksv = 6.41), good stability, and linear Stern–Volmer behavior (R2 = 0.9979).

An asymmetrical squaraine dye (Py-3) with its two electron-donating aryl groups directly linked to the electron-withdrawing squaric acid core possesses an ideal bandgap of 1.33 eV, together with an intense and broad absorption band in the range 550–950 nm. Hence, the resulting solution-processed solar cells display an impressive Jsc of 12.03 mA cm−2 and a PCE of 4.35%.

A neutral phosphorescent coordination compound bearing a benzimidazole ligand, Ir(pbi)2(acac) (Hpbi = 1,2-diphenyl-1H-benzo[d]imidazole; Hacac = acetylacetone), is demonstrated to be the first example of a sulfur-free iridium complex for the detection of Hg2+ cations with high selectivity and sensitivity. Ir(pbi)2(acac) shows a multisignaling response towards mercury(II) ions through UV-vis absorption, phosphorescence and electrochemistry measurements. Upon addition of Hg2+ ions, solutions of this complex change from yellow to colorless, which could be observed easily by the naked eye, while its phosphorescence turns from bright green (λPLmax = 520 nm) into faint skyblue (λPLmax = 476 nm), and the detection limit is calculated to be 2.4 × 10−7 mol L−1. 1H NMR spectroscopic titration as well as ESI-MS results indicate that the decomposition of Ir(pbi)2(acac) in the presence of Hg2+ through rupture of Ir–O bonds is responsible for the significant variations in both optical and electrochemical signals.

We demonstrate bulk heterojunction (BHJ) organic photovoltaics (OPVs) with a power conversion efficiency (PCE) of 6.39% by incorporating a small molecular compound 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP) as the additional donor material into a poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) host binary blend. Incorporating SQ-BP into the PCDTBT:PC71BM host blend film increases the photon harvesting of a ternary photovoltaic device with a broad absorption spectrum from 300 nm to 750 nm, which results in an increased short-circuit current density (Jsc). In addition to efficient photon harvesting, Förster resonance energy transfer (FRET) between PCDTBT and SQ-BP is also the reason for the increase in Jsc. As a result, the PCE of ternary devices with 10 wt% SQ-BP is about 30% greater than that of PCDTBT:PC71BM-based binary OPVs.

Among the multifarious kinds of small molecular photovoltaic materials, squaraines are quite attractive due to their facile and low-cost synthesis, intense and broad absorption in Vis-NIR spectral regions with very high molar extinction coefficients. Nevertheless, one key factor limiting the performance of squaraine-based organic solar cells (OSCs) is their low fill factor (FF), which is ascribed to the low hole mobility of the active layer. Here, we successfully reported a large π-extended bis(squaraine) electron donor, namely D-BDT-SQ, that showed one order of magnitude enhanced hole mobility in the donor–acceptor composite film, up to 10−4 cm2 V−1 s−1. As a result, an impressive power conversion efficiency (PCE) of 7.41% with a high FF of 0.60 was achieved for solution-processed bulk heterojunction OSCs when the working temperature of devices was 80 °C, which is, to the best of our knowledge, the record PCE among those reported for squaraine-based single-junction OSCs.

A novel asymmetrical squaraine derivative bearing a cyano-substituted indoline end-capping group, namely ASQ-5-CN, was designed and synthesized. In comparison with the noncyano-substituted ASQ-5, ASQ-5-CN showed an analogous absorption band-gap in the thin solid film state, but a 0.11 eV lowered HOMO energy level, which led to a higher Voc. Density functional theory calculation results revealed that the dipole moment of ASQ-5-CN was over double that of ASQ-5. Hence the stronger dipole–dipole interactions of ASQ-5-CN might trigger more intense intermolecular packing in ASQ-5-CN, which should account for the higher hole mobility of ASQ-5-CN than that of ASQ-5 (4.00 × 10−5vs. 1.67 × 10−5 cm2 V−1 s−1). Accordingly, solution-processed bulk-heterojunction small molecular organic solar cells using ASQ-5-CN as the electron donor exhibited a much higher PCE (5.24%) than that of the reference compound ASQ-5-based device (4.22%) due to its simultaneously enhanced Voc (0.92 vs. 0.82 V), Jsc (11.38 vs. 10.94 mA cm−2) and FF (0.50 vs. 0.47). Additionally, the PCE of the ASQ-5-CN-based device could be improved to be as high as 6.11% when measured at 80 °C, which is the record PCE among the hitherto reported squaraine-based solution-processed bulk-heterojunction organic solar cells.