A grating-structured interface of a poly(3-hexylthiophene) (P3HT) and n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM)-based bulk-heterojunction (BHJ) photovoltaic (PV) cell was designed and fabricated to obtain a desirable thickness distribution of the deposited bathocuproine (BCP) buffer layer to efficiently utilize its potentials. As a master mold of the grating-structure, a commercially available recordable digital versatile disc (DVD-R) substrate was employed. The grating-structured surface of the P3HT:PCBM layer was successfully produced by duplication from a poly(dimethylsiloxane) secondary mold using the spin cast molding technique. From morphological observations of the grating-structured surface covered with vapor-deposited BCP, we roughly estimated the ratio of the BCP thickness at “walls” to that at “top” and “bottom” regions to be ∼0.5. The grating-type BHJ PV cell with a 5-nm-thick BCP layer exhibited the maximum power-conversion efficiency (ηp) of 3.51%. Compared with the conventional flat-type BHJ PV cell with a 20-nm-thick BCP layer, the performance of the grating-type BHJ PV cell with a 20-nm-thick BCP layer was remarkably improved, owing to the contribution of the wall side contact, which provides a lower-barrier path of the electrons toward the cathode through the thinner BCP layer.

The use of Langmuir-Blodgett (LB) monolayers to modify the indium tin oxide (ITO) work function and thus improve the performance of zinc phthalocyanine (ZnPc)/fullerene (C-60)-based and boron subphthalocyanine chloride (SubPc)/C-60-based small molecule organic photovoltaic devices (OPVs) was examined. In general, LB precursor compounds contain one or more long alkyl chain substituents that can act as spacers to prevent electrical contact with adjoining electrode surfaces. As one example of such a compound, arachidic acid (CH3(CH2)(18)COOH) was inserted in the forms of one-layer, three-layer or five-layer LB films between the anode ITO layer and the p-type layer in ZnPc-C-60-based OPVs to investigate the effects of the long alkyl chain group when it acts as an electrically insulating spacer. The short-circuit current density (Jsc) values of the OPVs with the three-and five-layer inserts (1.78 mA.cm(-2) and 0.61 mA.cm(-2), respectively) were reduced dramatically, whereas the Jsc value for the OPV with the single-layer insertion (2.88 mA.cm(-2)) was comparable to that of the OPV without any insert (3.14 mA.cm(-2)).The ITO work function was shifted positively by LB deposition of a surfactant compound, C9F19C2H4-O-C2H4-COOH (PFECA), which contained a fluorinated head group. This positive effect was maintained even after formation of an upper p-type organic layer. The Jsc and open-circuit voltage (V-oc) of the SubPc-C-60-based OPV with the LB-modified ITO layers were effectively enhanced. As a result, a 42% increase in device efficiency was achieved. (C) 2017 Author(s).

In this study, we have examined the correlation between work function (WF) of indium-tin-oxide (ITO) and open-circuit voltage (V-oc) of heterojunction photovoltaic (PV) cells based on different donor materials. Device configurations of ITO(modified)/SubPc/C60/BCP/Al and ITO(modified)/ZnPc/C60/BCP/Al were used in this study. Surface modification of ITO electrode through p-substituted benzenesulfonyl chlorides with different electron withdrawing terminal groups (C6H5-, Cl- and NO2-) has exploited to change the WF of ITO up to 5.28eV with NO2-. Chemically modified ITOs and SubPc, ZnPc were used as anode electrode and donor materials, respectively. In order to find the correlation of V-oc, two different donor materials with different HOMO levels were used in PV cells. When the SubPc was used as a donor in PV cells, the V-oc was strongly dependent on WF of ITO. However, for ZnPc, the V-oc was almost independent on WF of ITO due to its low HOMO level and formed ohmic contact at the interface between chemically modified ITO electrodes and ZnPc.

The photovoltaic (PV) characteristics of bulk-heterojunction (BHJ) solar cells based on poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PC61BM) were improved using indium-tin-oxide (ITO) anode electrodes modified chemically with CH3O-, H-, Cl-, CF3-, and NO2-terminated benzenesulfonyl chlorides as a self-assembled monolayer (SAM). The ITO electrode surfaces were easily treated through the chemical modification of the reactive –SO2Cl binding group, and the work function (WF) of the modified ITO was effectively changed depending on the permanent dipole moments introduced in the para-position of benzenesulfonyl chloride. We examined the correlation between the ITO WFs corrected by the change in the contact potential difference and the calculated dipole moments of the SAM models. Moreover, we examined the PV characteristics of the P3HT:PC61BM based BHJ organic PV cells using the SAMs or poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)-treated ITOs with different WFs lying within ±0.2 eV from the highest occupied molecular orbital (HOMO) level of P3HT. We found that the enhancement effect of the SAMs on the power-conversion efficiency (ηP) reached a maximum with Cl (ηP = 3.72%), and became larger than that of PEDOT:PSS (ηP = 3.62%). Two distinct Jsc dependencies, increasing and decreasing with the increasing WF of the anode ITO, were observed at higher and lower WFs than the HOMO level of the donor, respectively. Almost constant Voc values (around 0.6 V) were observed with different SAM-modified ITOs, which suggested that Fermi level pinning was achieved by aligning the anode Fermi level and positive polaronic level of the donor polymer.

In this work, we examine the effects of chemical modification of indium-tin-oxide (ITO) on the performance of organic solar cells (OSC)s. The OSCs are fabricated with the cell configuration of ITO/zinc phthalocyanine (ZnPc) (40 nm)/fullerene (C60) (40 nm)/ with and without bathocuproine (BCP) (10 nm) between C60 and Al. The device performances that change in work function of ITO and introduction of BCP thin layer between C60 and Al interface are dominant factors in improving photovoltaic (PV) characteristics in OSCs. The PV characteristics of OSCs were improved significantly by using ITO chemically modified with H and Clterminted benzenesulfonyl chlorides. This is attributed to the fact that the chemical modification of ITO by molecular self-assembly with a dipole moment of appropriate direction and magnitude was an effective way to change the work function of ITO and to decrease the injection barrier between ITO and the highest occupied molecular orbital (HOMO) level of ZnPc as electron-donor in OSCs. The results suggested that the work function of ITO chemically modified with H and Clterminated benzenesulfonyl chlorides were shifted down to the HOMO level of ZnPc resulting in formation of Ohmic contact at the ITO/ZnPc interface.

Small-molecule solar cells based on zinc phthalocyanine (ZnPc) fullerene (C-60) heterojunction structure were fabricated with a laminated top electrode. The photovoltaic (PV) performances of the laminated devices with and without a bathocuproine (BCP) buffer layer were investigated by comparing with those of the conventional devices with the top electrode deposited by successive vacuum evaporations. Improved PV performances by insertion of BCP buffer layer were observed with both the evaporation- and the lamination-type devices. These results suggest that the formation of the cathode-induced defect states below the lowest unoccupied molecular orbital (LUMO) of BCP would not be necessarily requisite for efficient charge transport across the BCP layer.

The possibility of the increase in open-circuit voltage of organic photovoltaic cells based primarily indium-tin oxide (ITO)/rubrene/fullerene/Al structure by changing the work function of ITO anodes and Al cathodes was described in this work. To change built-in potential preferably in order to increase the open-circuit voltage, the work function of ITO should be increased and work function of Al should be decreased. The correlation between the change in work functions of electrodes and performance of the organic photovoltaic cells before and after surface modifications was examined in detail. The enhancement of open-circuit voltage depends on a function of work function change of both ITO and Al electrode. We could show that the built-in potential in the cells played an important role in open-circuit voltage.

The properties of the modified surface of SnO2(110) with benzoic acid (Y-C6H4-COOH: Y is para position relative to -COOH group) derivatives were investigated using density functional theory. Zehner et al. mentioned that the modification of surface dipole moment made it possible to tune the work function of the system. The experiment of Ganzorig et al. showed that there was a linear relationship between the dipole moment of the binding molecule and the work function change of the system using the modified surface of indium-tin oxide (ITO) with some benzoic acid derivatives. To elucidate the relation between the dipole moment of the molecule and the work function change, we investigated the modified surface of SnO2(110) using Sn7O14 cluster model which was embedded in the fixed point charges. On the modification of the surface, benzoic acid derivatives were bound to SnO2 surface. By changing the terminal group of benzoic acid with H, Cl, F, CF3 and CCl3, the work function changed and the dipole moment of the binding molecules of the modified SnO2(110) were evaluated. The results showed that there was a linear relationship between the dipole moment of the binding molecules and the work function changed. From this relation, the average value of the dipole moments of Sn-OOC linkage at the surface was also evaluated.

To understand the origin of the open-circuit voltage of heterojunction photovoltaic (HJ-PV) cells with small-molecular-weight organic thin films, HJ-PV cells with ITO/donor (20 nm)/fullerene (40nm)/bathocuproine (10nm)/Al were studied using three kinds of donors and four types ITOs with different work functions.

The authors report the use of chemically modified indium tin oxide (ITO) with different binding groups (–COCl and –PO2Cl2) of p-chlorobenzene derivatives forming effective monolayers to control the work function of ITO and hence to enhance the hole injection. The enhanced hole injection is studied by measuring current density–voltage (J-V) characteristics. The behavior of J-V characteristics caused by varying the ITO work function in hole-only single-carrier devices with a hole transport layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine is examined. Upon grafting with p-chlorophenylphosphoryl dichloride, the J-V characteristics show a space-charge-limited conduction behavior. Such modified ITO anodes lead to improvements in the device properties.

The efficiency of the lateral photo-induced electron transfer between cyanine (electron donor) and viologen (electron acceptor) amphiphiles (Cy and V, respectively) was examined by observing the absorption and emission spectra of V doped Cy LB films. The sharp J-band observed in the absorption and fluorescence spectra indicated the efficient formation of J-aggregate of the cyanine chromophore even in the mixed LB monolayers with V. The fluorescence intensities of the J-band decreased monotonously with the increase of V concentration but the dependence of the electron transfer efficiency on the film composition was rather complicated. The efficiency of the lateral electron transfer estimated at the composition region of the lower V ratios, where V molecules are expected to disperse homogeneously into the Cy monolayer, was such that ca. 90 molecules of Cy would be quenched by one center molecule of V.

Transient absorption measurements of an acceptor–sensitizer–donor (A–S–D) triad molecule (ASD) that contained viologen, pyrene, and ferrocene as the A, S, and D moieties, respectively, were made in benzonitrile and aqueous micellar solutions, and in a Langmuir–Blodgett (LB) monolayer assembly. Intramolecular electron transfer reactions were initiated by photo-excitation of the S moiety, which gave rise to the long-lived final charge-separated A−–S–D+ state. The rate of the backward electron transfer from A− to D+ was determined from the absorption decay of the viologen cation radical monitored at 600–700 nm. The estimated lifetime of the final charge-separated state was ca. 60 ns, based on measurements of the solutions. Extremely slow decay (much longer than 600 μs) of the transient absorption of the viologen cation radical was observed for a mixed ASD LB film with 22-tricosenoic acid (TA). This slow decay observed on a LB film at higher ASD concentrations may be explained by lateral charge migration between adjacent molecules in the monolayer plane.

We first observed a unique pyrene (Py) emission from 10-(1-(6-hexyl)pyrenyl)decanoic acid (1,6-HPDA) Langmuir-Blodgett (LB) films. In the fluorescence spectra of the 1,6-HPDA LB films, the normal bands of monomer and excimer emission of Py were absent but the new structured emission band with peaks at 422, 448, and 473 nm was observed. Besides the intensity, the shape of the characteristic structured emission band at shorter wavelengths than normal excimers did not changed essentially on lowering the Py concentration in the LB monolayer from 100 to 1 mol%. The spectroscopic studies on HPDA LB films of a variety of Py concentrations suggested that a novel excimer responsible for the structured new emission would be efficiently formed by photo-excitation of a Py-Py pre-associated dimer in different manners than those of normal excimer formations. (c) 2005 Elsevier B.V. All fights reserved.

An indolinospiropyran possessing one long alkyl chain on the nitrogen atom of the indoline moiety and two nitro groups on the benzopyran ring, exhibiting negative photochromism in various organic solvents, showed negative photochromism in a Langnmir-Blodgett film. The change in the surface appearance was observed by atomic force microscopy after the visible light irradiation.

A coarse grain molecular dynamics simulation method was applied to polystyrenesulfonate in aqueous solution to investigate the conformational and orientational behavior of flexible polyion under an external electric field. When the electric field was applied, the deviation of the counter-ion distribution and the rotational orientation of the polyion toward the direction of the electric field were observed. The orientation behavior of flexible polyelectrolytes depended on the field strength, and two typical orientational motions were detected. That is, at low electric field strength, elongation of the whole polyion chain conformation occurs at first stage and subsequently rotationally orients toward the field direction. On the other hand, segment orientation occurs at the high field because of the fast displacement of the counter ions on each polymer segment. The ionic polarization of the flexible polyion was evaluated by the component analysis of the counter-ion deviation. Results showed that the loosely-bound counter ions contribute largely to the ionic polarization.

Photo-induced charge separation in mixed Langmuir–Blodgett (LB) films containing oriented A–S–D triads and inert matrix molecules was studied by using scanning Maxwell stress microcopy. A, S, and D stand for electron acceptor, sensitizer, and electron donor moieties of the triad, respectively. The change in surface potentials upon photo-excitation corresponded well with creation and disappearance of photo-induced dipole moments in the LB films due to the photo-induced charge separation in the oriented A–S–D triads followed by charge recombination. The observed lifetime of the separated charges in the LB films, however, was longer by several orders of magnitude than that of isolated A–S–D triads in solution observed by transient absorption spectroscopy. The magnitude of the photo-induced surface potentials, i.e. the content of the separated charges with the longer lifetime decreased drastically with the decrease in the concentration of A–S–D triads in the mixed LB films. Therefore, we concluded that the separated charges with the longer lifetime were created by lateral diffusion of photo-produced anion (electrons) and cation radicals (holes) among neighboring A and D moieties, respectively, in the LB films. We also found that addition of a second donor (D′) monolayer next to a layer of D end moieties of the mixed A–S–D monolayer in multilayered LB films enhanced the photo-induced charge separation with the long lifetime. When light harvesting (H) molecules acting as antenna was mixed with the A–S–D triads in place of the inert matrix molecules, the photo-induced charge separation was observed under irradiation of photons, which were absorbed by the H moieties.

An advanced Langmuir-Blodgett (LB) assembly designed to replicate the three key aspects (antenna, reaction center, and quinone pool) of a primary process in natural photosynthesis was successfully fabricated by alternate deposition of a mixed monolayer of an artificial reaction center and an antenna pigment and a pure bilayer of a second donor.