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Herein, we have designed and synthesized two novel BODIPY dimer-based small molecules, denoted as ZMH-1 and ZMH-2, covalently linked and functionalized with triphenylamine (TPA) (ZMH-1) and carbazole (C Z ) (ZMH-2) units as the electron donor at the 3- and 5-positions of the BODIPY core, respectively. Their optical and electrochemical properties were investigated. We have fabricated all small molecule bulk heterojunction organic solar cells using these BODIPY-based small molecules as electron donors along with fullerene derivative (PC 71 BM) and medium bandgap non-fullerene acceptor IDT-TC as electron acceptors. The optimized OSCs based on ZMH-1:PC 71 BM, ZMH-2:PC 71 BM, ZMH-1:IDT-IC, and ZMH-2:IDT-IC attain overall PCEs of 8.91%, 6.61%, 11.28%, and 5.48%, respectively. Moreover, when a small amount of PC 71 BM as guest acceptor is added to the binary host ZMH-1:IDT-TC and ZMH-2:IDT-TC, the ternary OSCs based on ZMH-1 and ZMH-2 reach PCEs of 13.70% and 12.71%, respectively.

Two BODIPY derivatives with one (B2) and two (B3) carbazole moieties were synthesized and applied as electron-donor materials in organic photovoltaic cells (OPV). Their optical and electrochemical properties were systematically investigated. These BODIPY dyes exhibit excellent solubility in organic solvents and present high molar extinction coefficients (1.37-1.48 × 10 5 M -1 cm -1 ) in solutions with absorption maxima at 586 nm for mono-styryl groups and at 672 nm for di-styryl groups. The introduction of the styryl moieties results in a large bathochromic shift and a significant decrease in the HOMO-LUMO energy-gaps. The BODIPY dyes show relatively low HOMO energies ranging from -4.99 to -5.16 eV as determined from cyclic voltammetry measurements. Cyclic voltammetry measurements and theoretical calculations demonstrate that the frontier molecular orbital levels of these compounds match with those of PC 71 BM as the acceptor, supporting their application as donor materials in solution-processed small molecule bulk heterojunction (BHJ) organic solar cells. After the optimization of the active layer, B2:PC 71 BM and B3:PC 71 BM based organic solar cells showed an overall power conversion efficiency of 6.41% and 7.47%, respectively. The higher PCE of the B3-based OSC is ascribed to the more balanced charge transport and exciton dissociation, better crystallinity and molecular packing.

This is the first report on the highest efficiency NIR absorbing Aza-Bodipy small molecules. The molecular engineering of newly synthesized NIR absorbing Aza-Bodipy dyes consists of covalently linked phenothiazine (AZA-PTZ-BOD) and phenoxazine (AZA-POZ-BOD) moieties as terminal groups and Aza-Bodipy as a central core moiety. The highest efficiency for OPV devices of 8.23% is achieved for AZA-PTZ-BOD.

A new A-π-D-π-A small molecule with a Ni-porphyrin core (MV143) has been synthesized and employed as a donor, along with PC71BM, for the fabrication of solution-processed bulk heterojunction OSCs organic solar cells. The device exhibited an overall PCE of 9.14% (JSC = 13.87 mA cm-2, VOC = 1.08 V and FF = 0.61) and a low photon energy loss of 0.52 eV.

We have used two unsymmetrical small molecules, named phenothiazine 1 and 2 with a D-A-D-π-D configuration, where phenothiazine is used as a central unit, triphenylamine is used as a terminal unit and TCBD and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD are used as an acceptor between the phenothiazine and triphenylamine units, as a small molecule donor along with PC 71 BM as an acceptor for solution processed bulk heterojunction solar cells. The variation of acceptors in the phenothiazine derivatives makes an exciting change in the photophysical and electrochemical properties, hole mobility and therefore photovoltaic performance. The optimized device based on phenothiazine 2 exhibited a high power conversion efficiency of 7.35% (J sc = 11.98 mA cm -2 , V oc = 0.99 V and FF = 0.62), while the device based on phenothiazine 1 showed a low PCE of 4.81% (J sc = 8.73 mA cm -2 , V oc = 0.95 V and FF = 0.58) after solvent vapour annealing (SVA) treatment. The higher value of power conversion efficiency of the 2 based devices irrespective of the processing conditions may be related to the broader absorption and lower band gap of 2 as compared to 1. The improvement in the SVA treated active layer may be related to the enhanced crystallinity, molecular ordering and aggregation and shorter π-π stacking distance of the small molecule donors.

Three compounds consisting of the electron-donating triphenylamine-phenothiazine conjugate backbone and each of the electron-withdrawing groups 3-ethylrhodanine, malononitrile and 1,3-indandione have been synthesized and used as donors in blends with [6,6]-phenyl-C70-butyric acid methyl ester (PC 71 BM) for organic solar cell devices. After improvements of the active layer structure by a selected donor-to-acceptor weight ratio and a two-step solvent and thermal annealing, the organic solar cells showed power conversion efficiency (PCE) values in the range of 4.79-7.25%. The highest PCE was obtained for the bulk heterojunction device with the indandione compound, which can be attributed to its better absorption profile, higher crystallinity, more balanced electron and hole transport, higher charge collection efficiency and reduced recombination, in comparison with the photovoltaic cells from the other two compounds. DFT-calculated characteristics, absorption spectra and cyclic voltammetry of the compounds, along with X-ray diffraction patterns of the blend films, are used to validate the photovoltaic results.

Two new small molecule donors, namely ICT4 and ICT6 with D 1 -A-D 2 -A-D 1 architecture having 2,4-bis(2-ethylhexyl)-4H-dithieno[3,2-b:2',3'-d]pyrrole (EHDTP, D 1 ) and 4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b']dithiophene (OBDT, D 2 ) as the terminal and central donor, and benzo[c][1,2,5]thiadiazole (BT for ICT4) and 5,6-difluorobenzo[c][1,2,5]thiadiazole (F2BT for ICT6) as the acceptor (A) moieties, are synthesized and their optical, electronic and photovoltaic properties are investigated. Both ICT4 and ICT6 have considerable solubility in various solvents and possess efficient light absorption ability [ε (×10 5 mol -1 cm -1 ) is 0.99 and 1.06, respectively for ICT4 and ICT6] and appropriate frontier molecular orbital energy offsets with [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM). Bulk heterojunction solar cells (BHJSCs) are fabricated using ICT4/ICT6 and PC 71 BM as donors and acceptors, respectively and BHJSCs with two-step annealed (thermal followed by solvent vapor annealing) active layers of ICT4 and ICT6 show overall power conversion efficiencies (PCEs) of 5.46% and 7.91%, respectively. The superior photovoltaic performance of the ICT6 based BHJSCs is due to the favourable morphology with a nanoscale interpenetrating network in the ICT6:PC 71 BM active layer induced by the fluorine atoms on the BT acceptor, which significantly enhances the dissociation of excitons, charge transport and the charge collection efficiency, and suppresses bimolecular recombination in the BHJ. The observed higher PCE of 7.91% indicates that ICT6 is one of the best BT based donor material for small molecular BHJSCs.

We have designed and synthesized ferrocenyl (donor) bisthiazole linked triphenylamine (donor) based donor-π-acceptor-π-donor-acceptor (D-π-A) 2 -π-D-A type dyes D1 and D2 by using Pd-catalyzed Sonogashira cross-coupling and Knoevenagel condensation reactions. Their photophysical, electrochemical and computational studies reveal strong donor-acceptor interaction. Dye sensitized solar cells (DSSCs) based on D1 and D2 exhibit power conversion efficiencies (PCE) of 6.33% and 5.03%, respectively. The higher PCE value of the D1 based DSSC is attributed to its enhanced short-circuit current (J sc ) and open-circuit current (V oc ) and fill factor (FF) values because of the strong binding of the anchoring cyanoacrylic acid with the TiO 2 surface as compared to the dicyanovinyl unit in D2. Time dependent density functional theory (TD-DFT) calculations at B3LYP level on dyes D1 and D2 were performed, which reveal that both dyes show HOMO-1 → LUMO as a major transition. Computational photovoltaic calculations also reveal that dye D1 has better electron injection (ΔG inject ) from E LUMO to the conduction band (CB) of TiO 2 as compared to dye D2, which is in good agreement with experimental results.

A symmetrical D-π-A-π-D type small molecule consisting of thiophene flanked diketopyrrolopyrrole (DPP) as a core and an end capping ferrocene donor linked by an ethynyl bridge, denoted as Fc-DPP-Fc, was synthesized and its optical, thermal and electrochemical properties were investigated in order to explore its potential applicability as a donor for solution processed bulk heterojunction solar cells. The photophysical and electrochemical properties of this small molecule showed strong charge transfer interaction between the ferrocene donor and the DPP acceptor, and it is found to be suitable as a small molecule donor along with PC 71 BM as an electron acceptor for solution processed bulk heterojunction organic solar cells (OSCs). Although the open circuit voltage (V oc ) of the OSC based on as cast Fc-DPP-Fc : PC 71 BM (1 : 2 weight ratio) from a THF solvent is quite high (0.98 V), it showed an overall power conversion efficiency (PCE) of 2.55% with low values of short circuit current (J sc ) and fill factor (FF) of 6.35 mA cm -2 and 0.41, respectively. After using a solvent additive (SA), i.e. 3 v% DIO/THF solution for film deposition, the resultant OSC showed an improved overall PCE of 4.83% and it further improved up to 6.44%. The improvement in the PCE value is mainly attributed to the enhancement in J sc and FF, resulting from the increased light harvesting efficiency, balanced charge transport and favorable nanoscale morphology of the active layer, induced by SA and TSA.

Donor-acceptor-acceptor (D-A-A) type 1,8-naphthalimide based small molecules SM1 and SM2 functionalized with tetracyanobutadiene (TCBD) and dicyanoquino-dimethane (DCNQ) modules, showing strong absorption in the visible and near-infrared (NIR) region are reported. TCBD and DCNQ linked SM1 and SM2 exhibit multi-redox waves. The electrochemical and optical HOMO-LUMO gaps show similar trends. These SMs exhibit a broad absorption profile which is complementary to the D-A copolymer P donor and also possess an appropriate lowest unoccupied molecular orbital (LUMO) to serve as an acceptor with P with a LUMO level of -3.33 eV. The organic solar cells based on P : SM1 and P : SM2 exhibit a PCE of 4.94% and 6.11%, respectively. The higher value of the PCE for the SM2 based organic solar cells has been attributed to the broader absorption profile, more balanced charge transport and lower photon energy loss. The values of V oc of the organic solar cells for the SM1 acceptor (1.06 V and 1.02 V without and with solvent additive) are the highest values reported for devices based on non-fullerene acceptors to the best of our knowledge. The energy loss ( E loss ) of 0.56 eV and 0.48 eV for SM1 and SM2 based devices, respectively is one of the smallest reported for BHJ organic solar cells.

A novel, NIR absorbing organic small molecular donor material denoted as ICT3 with an A-D-D-D-A architecture having dithieno[3,2-b:2',3'-d]pyrrole (DTP) and butylrhodanine as donor and acceptor moieties, respectively, is synthesized and its thermal, photophysical, electrochemical and photovoltaic properties are explored. ICT3 has excellent stability over a broad range of temperatures with a decomposition temperature (T d corresponds to 5% weight loss) of 372 °C, soluble in most common organic solvents (solubility up to 30 mg mL -1 ) and suitable for solution processing during device fabrication. ICT3 has broad (520-820 nm) and intense visible region absorption (molar excitation coefficient is 1.69 × 10 5 mol -1 cm -1 ) and has suitable HOMO and LUMO energy levels with the [6,6]-phenyl-C 71 -butyric acid methyl ester (PC 71 BM) acceptor for efficient exciton dissociation and charge transfer. Bulk heterojunction solar cells (BHJSCs) with an indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/ICT3:PC 71 BM/poly(9,9-bis(3'-(N,N-dimethylamino)propyl)fluorene-2,7-diyl)-alt-(9,9-dioctylfluorene-2,7-diyl) (PFN)/aluminium (Al) structure are fabricated and the BHJSCs with the active layer as cast from chloroform solution displayed a power conversion efficiency (PCE) of 3.04% (J SC = 8.22 mA cm -2 , V OC = 0.86 V and FF = 0.43). Annealing the active layer significantly improved the PCE of these BHJSCs. While thermal annealing of the active layer improved the PCE of the BHJSCs to 4.94%, thermal followed by solvent vapour annealing enhanced the PCE to 6.53%. X-ray diffraction and atomic force microscopy analyses are carried out on the active layer and these results revealed that annealing treatment improves the crystallinity and nanoscale morphology of the active layer, enriches the device exciton generation and dissociation efficiency, charge transport and collection efficiency and reduces carrier recombination. The observed higher PCE (6.53%) of the BHJSCs having ICT3 with a DTP donor moiety broadens the scope to develop new, efficient DTP based small molecular donor materials for BHJSCs.

We report two acetylene-bridged small molecules DPP5 and DPP6 with low HOMO-LUMO gaps as donors along with PC71BM as an acceptor for the fabrication of solution-processed bulk heterojunction solar cells. After the optimization, i.e. weight ratio of donor to acceptor and surface treatment of the active layer, we achieved overall power conversion efficiencies up to 4.65% (Jsc = 8.19 mA cm(-2), Voc = 0.98 V and FF = 0.58) and 5.73% (Jsc = 9.58 mA cm(-2), Voc = 0.98 V and FF = 0.61), for DPP5:PC71BM and DPP6:PC71BM respectively, which are superior to those for the devices based on as-cast active layers. The significant change in the power conversion efficiency is attributed to the improvement in nanoscale morphology, balanced charge transport and charge collection efficiency, induced through the surface treatment.

Two organic small molecules based on diketopyrrolopyrrole (DPP) units having a D-A-D-π-D-A-D structure denoted as and were synthesized. Their optical and electrochemical properties relevant to organic solar cells were investigated. The wider optical absorption coverage from 450-800 nm, the highest occupied molecular orbital (HOMO) (-5.23 eV and -5.34 eV for and , respectively) and the lowest unoccupied molecular orbital (LUMO) (-3.47 and -3.45 eV for and , respectively) make these small molecules suitable as donors for bulk heterojunction organic solar cells. The bulk heterojunction (BHJ) organic solar cells based on an active layer consists of a blend of these small molecules as donors and PC71BM as an acceptor with an optimized weight ratio of 1 : 2 cast from chloroform (CF) showed overall power conversion efficiencies (PCEs) of 1.98% (with Jsc = 5.38 mA cm(-2), Voc = 0.84 V and FF = 0.42) and 1.85% (with Jsc = 4.56 mA cm(-2), Voc = 0.96 V and FF = 0.42) for and respectively. The relatively high Voc value based on the based device has been attributed to the deeper HOMO of compared to . The optimized  : PC71BM (1 : 2) and  : PC71BM (1 : 2) active layers were subjected to two step annealing (TSA), i.e. thermal annealing and subsequent solvent vapor annealing and the corresponding BHJ organic solar cells showed a PCE of 5.28% (Jsc = 11.53 mA cm(-2), Voc = 0.79 V and FF = 0.58) and 5.52% (Jsc = 10.84 mA cm(-2), Voc = 0.91 V and FF = 0.56), respectively. The enhancement in PCE is mainly due to the improvement in Jsc and FF, related to light absorption in an active layer, a better nanoscale morphology, and an increase in the crystalline nature of the active layer and balanced charge transport, induced by the TSA treatment.

Herein, benzothiadiazole (BTD), as an acceptor A1, has been used as a backbone to link triphenylamine (TPA) as donor and naphthalimide (NPI) as acceptor (A2) moieties through ethylene linkers to design a small molecule. The donor-π-acceptor-π-acceptor (D-π-A1-π-A2) type small molecule denoted as was synthesized. In order to use it as an electron donor for solution processed bulk heterojunction small molecule solar cells its photonic and electronic properties were explored. The small molecule organic solar cells based on the optimized blend of with PC71BM processed in chloroform showed a power conversion efficiency (PCE) of 2.21%, which was significantly improved up to 6.67%, when a two-step annealing (TSA) treated blend was used as an active layer. The increase in the PCE was due to the enhancement in both Jsc and FF. The improvement in Jsc was related to the enhancement in the light harvesting efficiency of a TSA treated active layer relative to the as-cast layer, which is reflected in a better IPCE and better charge collection. The TSA treatment also leads to better nanoscale morphology for exciton dissociation into free charge carriers and improved crystallinity for balanced charge transport.

We synthesized two novel ultra low bandgap donor-acceptor (D-A) copolymers (E(g) ≤ 1.2 eV), containing the thiadiazoloquinoxaline unit as the main electron accepting unit (A) and benzodithiophene (BDT) and dithienosilole (DTS) as different donor units (D), denoted as P1 and P2, respectively, using the cross-coupling Stille reaction. The copolymers possess light absorption ranging from UV (350 nm) to near-IR (1300 nm) with optical bandgaps of 1.16 eV and 1.08 eV, respectively. Quantum-chemical calculations and experimental data were compared for proposing a more detailed concept for the optical and electronic properties of these copolymers which can be used as donors for polymer solar cells (PSCs). The PSCs based on optimized P1:PC71BM and P2:PC71BM showed overall power conversion efficiencies (PCEs) of 4.32% and 3.48%, respectively. Although P2 possesses a broad absorption coverage of up to 1300 nm, the lower PCE may be attributed to the low J(sc), due to the poor driving force for exciton dissociation, since the LUMO offset with PC71BM is less than 0.3 eV. The PCE has been significantly increased to 7.27% and 6.68% for solvent vapor annealing (SVA) treated P1:PC71BM and P2:PC71BM active layers, respectively. This improvement arises from the appropriate nanoscale morphology and an increase in hole mobility, induced by the SVA treatment of the active layers.

Two unsymmetrical donor-acceptor-acceptor-π-acceptor type benzothiadiazoles (BTD3 and BTD4) functionalized with tetracyanobutadiene (TCBD) and dicyanoquinodimethane (DCNQ) modules, showing strong absorption in the visible region are reported. The bulk heterojunction solar cells based on BTD4:PC71BM and BTD3:PC71BM based active layers processed with chloroform (CF), thermal annealing and subsequent solvent vapor annealing, i.e. two step annealing (TSA), exhibited PCEs of up to 6.02% and 5.36%, respectively, which is significantly higher than those of the corresponding devices based on the as-cast blend active layer. This enhancement is related to the improvement in exciton dissociation efficiency and more balanced charge transport in the devices based on the active layer processed with TSA treatment.

In this study, we have used three D-A type carbazole substituted BODIPY (carbazole connected to the meso position of BODIPY) small molecules as donors along with PC71BM as an electron acceptor for the fabrication of solution processed bulk heterojunction organic solar cells. The devices based on the as cast active layer showed power conversion efficiency in the range of 2.20-2.70%, with high open circuit voltage (Voc) in the range of 0.94-1.08 V. The high Voc is related to the deeper highest occupied molecular orbital energy level of these small molecules. The power conversion efficiency (PCE) of devices based on thermally annealed and solvent vapor annealed (TSVA) :PC71BM and :PC71BM processed active layers improved up to 5.05% and 4.80%, respectively, attributed to the improved light harvesting ability of active layers, better phase separation for exciton dissociation and balanced charge transport, induced by the TA and TSVA treatment.

Two porphyrin-chromophores, i.e. triad PorZn-(PorCOOH)(2)-(piper)2 (GZ-T1) and dyad (PorZn)(2)-NMe2 (GZ-T1), have been synthesized and their photophysical and electrochemical properties have been investigated. The optical properties together with the appropriate electronic energy levels, i.e. the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels, revealed that both porphyrin assemblies can function as sensitizers for dye sensitized solar cells (DSSCs). The and -based DSSCs have been prepared and studied using 20 mM CDCA as coadsorbent and were found to exhibit an overall power conversion efficiency (PCE) of 5.88% and 4.56%, respectively (under an illumination intensity of 100 mW cm(-2) with TiO(2) films of 12 μm). The higher PCE of the -sensitized DSSC, as revealed from the current-voltage characteristic under illumination and the incident photon to current conversion efficiency (IPCE) spectra of the two DSSCs, is mainly attributed to its enhanced short circuit current (J(sc)), although both the open circuit voltage (V(oc)) and the fill factor are improved too. The electrochemical impedance spectra (EIS) demonstrated a shorter electron transport time, longer electron lifetime and higher charge recombination resistance for the DSSC sensitized with the dye as well as a larger dye loading onto the TiO(2) surface.

In this paper the three new narrow bandgap D–A conjugated copolymers P1, P2 and P3 based on different weak donor fused thiophene-imidazole containing derivatives and the same benzothiadiazole acceptor unit were synthesized by Stille cross-coupling polymerization and characterized by 1H NMR, elemental analysis, GPC, TGA, DSC. These copolymers exhibit intensive absorbance in the range 350–900 nm and the optical bandgap lies in the range of 1.50–1.61 eV, which corresponds to the maximum photon flux of the solar spectrum. The electrochemical bandgap derived from cyclic voltammetry varies within the limits 1.47–1.65 eV and is approximately very close to the optical bandgap. The highest occupied molecular orbital (HOMO) energy level of all copolymers is deep lying (−5.24 eV and −5.37 eV and −5.25 eV for P1, P2 and P2, respectively) which shows that copolymers have good stability in the air and assured a higher open circuit voltage (Voc) for polymer BHJ solar cells. These copolymers were used as donors along with PC71BM and the BHJ polymer solar cells based on P1:PC71BM, P2:PC71BM and P3:PC71BM processed from chloroform (CF) solvent with 3 v% DIO as an additive showed an overall PCE of 4.55%, 6.76% and 5.16%, respectively.

Two novel porphyrin dyads (9 and 11) consisting of two zinc-metallated porphyrin units, covalently linked at their peripheries through 1,2,3-triazole containing bridges and functionalized by a terminal carboxylic acid group, have been synthesized via "click" reactions, which are Cu-catalyzed Huisgen 1,3-dipolar cycloadditions between azide- and acetylene-containing porphyrins. Photophysical and electrochemical measurements, together with DFT calculations, showed that the two dyads possess suitable frontier orbital energy levels for use as sensitizers in DSSCs. The 9 and 11 based solar cells were fabricated resulting in power conversion efficiencies (PCEs) of 3.82 and 5.16%, respectively. As shown by photovoltaic measurements (J-V curves) and incident photon to current conversion efficiency (IPCE) spectra of the two solar cells, the higher PCE value of the latter is attributed to its enhanced photovoltaic parameters, and particularly its enhanced short circuit current (Jsc). This is related to the stronger absorption profile of the sensitizing dyad 11 (the dyad with the shorter triazole containing bridge) and the higher dye loading of the corresponding solar cell. Furthermore, electrochemical impedance spectra (EIS) demonstrated that the 11 based solar cell exhibits longer electron lifetime (τe) and more effective suppression of the recombination between the injected electrons and the electrolyte.

We have designed and synthesized a new thiocyanate-free sensitizer coded as SPS-01 and used it as the sensitizer in a TiO(2) based nanocrystalline dye-sensitized solar cell (DSSC). SPS-01 exhibits strong visible absorption properties with maximum peak around at 532 nm. The overall power conversion efficiency (PCE) of a DSSC sensitized with SPS-01 (7.96%) is higher than that of N719 (7.30%) under identical experimental conditions. This high PCE is attributed mainly due to the improvement in the short circuit current.