Subhayan Biswas

CuSCN as selective contact in solution-processed small-molecule organic solar cells leads to over 7% efficient porphyrin-based device

Two conjugated acceptor–π–donor–π–acceptor (A–π–D–π–A) small molecules with a zinc porphyrin donor core and 3-ethylrhodanine terminal acceptors connected at the meso positions by ethynylene groups and linked by one or two thienylenevinylene units denoted by 1a and 1b were synthesized and their optical and electrochemical properties were investigated. Bulk heterojunction organic solar cells fabricated with 1a : PC71BM and 1b : PC71BM processed with THF exhibited power conversion efficiencies of 2.75% (Jsc = 7.96 mA cm−2, Voc = 0.96 V and FF = 0.36) and 3.18% (Jsc = 8.84 mA cm−2, Voc = 0.90 V and FF = 0.40), respectively. Moreover, organic solar cells based on 1a : PC71BM and 1b : PC71BM processed with pyridine/THF solution displayed PCEs of 5.27% (Jsc = 10.61 mA cm−2, Voc = 0.92 V and FF = 0.54) and 5.78% (Jsc = 11.58 mA cm−2, Voc = 0.86 V and FF = 0.58), respectively. However, most important is the observation that the PCE was further increased to 7.24% for devices based on 1a : PC71BM and 1b : PC71BM active layers processed with pyridine/THF solution by employing CuSCN as a selective contact electrode instead of the most common hole transport material, namely, PEDOT:PSS.

Solution processed bulk heterojunction solar cells based on A–D–A small molecules with a dihydroindoloindole (DINI) central donor and different acceptor end groups

Four acceptor–donor–acceptor (A–D–A) small molecules with dihydroindoloindole (DINI) as the central donor unit and different acceptor end groups such as dicyanovinylene (DCV), indenedione (IND), cyanoacrylate (CA) and rhodanine (Rho) linked through bithiophene as the π-linker, DINI-DCV, DINI-IND, DINI-CA and DINI-Rho, were designed and synthesized for the application as donor materials along with PC71BM as an acceptor for solution processed organic bulk heterojunction solar cells. The effect of acceptor end groups on the photovoltaic performance was investigated. The organic solar cells (OSCs) based on as cast DINI-IND showed the highest power conversion efficiency of 3.04%, as cast DINI-CA showed the lowest PCE of 1.63% and the other two exhibit a PCE in between them. These results showed that acceptor end groups affect the overall performance of the cells. The PCE of OSCs has been further improved up to 7.04% and 6.16% employing two-step annealing (TSA) treated DIN-IND:PC71BM (1:2) and DIN-CN:PC71BM (1:2), respectively. The enhancement in the PCE of OSCs with TSA treated active layers is attributed to the better nanophase morphology, the increase in the crystalline nature and light harvesting efficiency and more balanced charge transport and charge collection probability.

S,N-Heteropentacene based small molecules with A–D–A structure for solution processed organic bulk heterojunction solar cells

Two small molecules with acceptor–donor–acceptor (A–D–A) structures, i.e. SN(BTTh2)2 and SN(BTAOTh2)2, denoted as J and K, respectively, containing an electron rich planar S,N-heteropentacene as the central core flanked with alkoxy substituted and un-substituted benzothiadiazole (BT) and end capped with hexyl-substituted bi-thiophene units, were designed and synthesized. Optical and electrochemical investigation of these two small molecules reveals that these compounds possess low optical bandgaps and broader absorption profiles to harvest solar photons, as well as suitable electrochemical molecular energy levels for application as donors in bulk heterojunction organic solar cells. The photovoltaic properties of these two small molecules as donors and PC71BM as the acceptor in solution processed bulk heterojunction cells were investigated. Devices based on cast J : PC71BM and K : PC71BM active layers showed power conversion efficiencies (PCE) of 1.96% and 1.57%, respectively; these efficiencies improved to 6.02% and 5.06% when the active layers were subjected to two steps annealing (TSA), i.e. combined thermal annealing followed by solvent vapor annealing. The increase in PCE was due to the enhancement of both the short circuit current (Jsc) and fill factor (FF) due to the enhancement of the light harvesting ability of the active layer, better phase separation between the small molecules and PC71BM, and more balanced charge transport induced by the TSA treatment.

BODIPY–diketopyrrolopyrrole–porphyrin conjugate small molecules for use in bulk heterojunction solar cells

Two small molecules denoted as BD-pPor and BD-tPor composed of a central BODIPY core surrounded with two DPP and two porphyrin units have been designed and synthesized. In BD-pPor and BD-tPor, porphyrins are linked to the central BODIPY by phenyl and thiophene bridges, respectively. The optical and electrochemical properties were systematically investigated in order to employ them as donors along with PC71BM as an acceptor for solution processed bulk heterojunction organic solar cells. After the optimization of the active layer, the organic solar cells based on BD-pPor and BD-tPor exhibit overall power conversion efficiencies of 6.67% and 8.98% with an energy loss of 0.63 eV and 0.50 eV. The low value of energy loss for BD-tPor may be related to the low LUMO offset between the BD-tPor and PC71BM (0.31 eV) as compared to that between BD-pPor and PC71BM (0.36 eV). The low energy loss also leads to a higher value of open-circuit voltage for the BD-tPor based OSC than its BD-pPor counterpart, despite the slightly deeper HOMO energy level of BD-pPor. The enhanced values of Jsc and FF of the BD-tPor based OSCs may be related to the better exciton dissociation and charge transport, as confirmed from the PL spectra and charge carrier mobility. These results indicate that the combination of BODIPY, DPP and porphyrin in the same conjugate is very promising for small molecule organic solar cells.

Study of Titanium Dioxide Nanotube Array for the Application in Dye-Sensitized Solar Cells

Abstract — Highly ordered, self-organized TiO 2 nanotube arrays (TNA) have been successfully prepared by anodization of titanium foil in ethylene glycol electrolyte containing 0.01% ammonium fluoride (NH4F). The effect of variation of applied anodization voltage ranging from 50V to 57 V on the morphology of the TNA has been studied using field emission scanning electron microscope. The increase in applied voltage enhances average pore size from 34nm to 58nm and reduces wall thickness. Diffuse reflectance spectroscopy has been used to evaluate the amount of dye absorption on the surface of various TNA which reveals direct correlation between the dye absorption and the morphology of the sample. Index Terms — Titanium dioxide, Dye-sensitized solar cell, anodization I. I NTRODUCTION In the recent years, vertically oriented, highly ordered titanium dioxide (TiO 2 ) nanotube array, prepared by anodization of titanium, and has attracted huge attention [1]-[5]. Highly ordered vertically oriented nanotube architecture of high surface to volume ratio exhibit unique properties. Nanotube offers a larger interfacial area due to their external and internal surfaces, which make them suitable for applications in dye-sensitized solar cells [6]. It also provides excellent electron pathways for charge transfer between interfaces. TiO

Synthesis and characterization of zinc carboxy–porphyrin complexes for dye sensitized solar cells

Two zinc porphyrins, 2 and 8, have been synthesized. Porphyrin 8 displays better electronic communication between the dye and the TiO2 electrode. Photophysical measurements and electrochemistry experiments suggest that both porphyrins are very promising sensitizers for dye-sensitized solar cells (DSSCs). It was found that their molecular orbital energy levels favor electron injection and dye regeneration in DSSCs. Solar cells sensitized by 2 and 8 were fabricated, and it was found that they show power conversion efficiencies (PCEs) of 5.27% and 7.13%, respectively. Photovoltaic measurements (J–V curves) together with the incident photon-to-electron conversion efficiency spectra of the two cells reveal that the higher PCE value of the DSSC based on 8 is ascribed to the higher short-circuit current (Jsc), open-circuit voltage (Voc), and dye loading values. Moreover, the larger charge recombination resistance, longer electron lifetime and shorter electron transport time for 8 also confirm the higher value of the Voc and the Jsc and the FF for the DSSC based on 8.

A non-fullerene all small molecule solar cell constructed with a diketopyrrolopyrrole-based acceptor having a power conversion efficiency higher than 9% and an energy loss of 0.54 eV

The synthesis of a new symmetrical A–D–π–A–D–π–A non-fullerene small molecule acceptor MPU3 is reported. MPU3 consists of a diketopyrrolopyrrole (DPP) central acceptor core coupled to terminal dicyanorhodanine acceptors via a thiophene donor and an ethynyl linker. The optical and electrochemical properties of this compound were investigated. MPU3 showed strong absorption in the 600–850 nm range with an optical bandgap of 1.52 eV and the material was used as an acceptor for the fabrication of solution-processed bulk heterojunction organic solar cells using an A–D–π–A–D–π–A small molecule donor (SMD) consisting of a 5,10-dihydroindolo[3,2-b]indole (DINI) central donor core and benzothiadiazole (BT) acceptor units. This donor has a complementary absorption profile to MPU3. The organic solar cell based on an optimized an SMD:MPU3 active layer afforded overall power conversion efficiencies as high as 9.05%, which is higher than that obtained with an organic solar cell using PC71BM as the acceptor (5.80%) fabricated under identical conditions. Interestingly, the energy loss in the organic solar cell based on SMD:MPU3 is very low, 0.54 eV, when compared to its SMD:PC71BM counterpart (1.02 eV). Low energy loss is an essential feature to achieve high PCE values in organic solar cells.

Micromagnetism of MnBi:FeCo thin films

MnBi:FeCo hard-soft bilayers are investigated using micromagnetic simulations with open boundary conditions and two-dimensional (2D) periodic boundary conditions (PBC). Open and PBC yield similar coercivities of about 1.01 T, in agreement with experiment, but the hysteresis-loop shape is very different in the two theoretical approaches. The difference is ascribed to edge effects, which occur in open boundary conditions but not in PBC and experiment. Near the nucleation field, a curling or vortex mode develops in dots with circular cross sections. The curling mode, which is caused by magnetostatic self-interaction, does not negatively affect the high coercivity of 1.01 T. The magnetostatic self-interaction contributes to the favorable second-quadrant behavior of the MnBi:FeCo thin films.

Polymer solar cells based on D–A low bandgap copolymers containing fluorinated side chains of thiadiazoloquinoxaline acceptor and benzodithiophene donor units

Three ultralow-bandgap D–A copolymers (P0F, P2F and P4F) based on fluorene-substituted thiadiazoloquinoxaline acceptor and BDT donor units were synthesized and characterized. The effect of the number of fluorine atoms substituted in the fluorene side chain on the optical, electrochemical, crystalline and photovoltaic properties of copolymers was compared. The results showed that the optical properties and lowest unoccupied molecular orbital energy levels of the copolymers are almost the same with an increase in fluorine atoms in the side chain, while the highest occupied molecular orbital (HOMO) energy level is shifted downward. P4F and P2F show better light harvesting abilities, higher crystallinities and low-lying HOMO energy levels than those of P0F. These copolymers were used as donors along with PC71BM as an acceptor for the bulk heterojunction polymer solar cells and optimized by adjusting the donor-to-acceptor weight ratios and solvent vapor treatment. Polymer solar cells based on optimized active layers based on P4F and P2F deliver a power conversion efficiency of 8.15% and 6.38%, respectively, which are higher than that of P0F counterpart (5.04%) due to an increase in all photovoltaic parameters, i.e., short circuit current, open circuit voltage and fill factor, and a very low voltage loss of 0.44 eV.