Certified efficiency of Pb perovskite solar cells with mixed cations is reported to be 19.6% (1 cm²). One of the recent research interests is on enhancing the solar cell efficiency further. Band gap of the present Pb perovskite is about 1.55 eV. Supposing that Voc loss is 0.4 eV, the best efficiency is obtained by using perovskite with about 1.4 eV band gap. One of the candidates for the narrow band gap perovskite material is SnPb mixed metal perovskite. In this paper, the relationship between Voc losses and hetero-interface structures is discussed in detail, leading to the conclusion that hetero-interface structures give serious effects on the solar cell efficiency. It is reported that the efficiency was enhanced from about 5% to 16% after optimization of the hetero-interface structure. In addition, research trend on Pb free perovskite solar cells consisting of Sn or Bi, which is another recent research interest, is also reviewed.

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We report herein the synthesis of new cobalt complexes tris[2-(1H-pyrazol-1-yl)-4-trifluoromethylpyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co1), tris[4-(1,1-difluoroethyl)-2-(1H-pyrazol-1-yl)pyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co2), bis[6-(1H-pyrazol-1-yl)-2,2′-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co3) and bis[4,4′-dimethyl-6-(1H-pyrazol-1-yl)-2,2′-bipyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (Co4), which we use as a dopant to create a hole-transporting material in perovskite solar cells. The addition of Co1–Co4 to 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD) in chlorobenzene changes the color of the material, which indicates that charge transfer occurs between spiro-OMeTAD and Co1–Co4. Devices made from spiro-OMeTAD doped by Co1, Co2, and Co4 perform better than those made from Co3 because of the presence of hydrophobic alkyl groups on the ligands. An overall power conversion efficiency of 14.8% is obtained by using Co2 as a dopant, which exceeds that of tris[2-(1H-pyrazol-1-yl)-4-tert-butylpyridine]cobalt(III) tris[bis(trifluoromethylsulfonyl)imide] (FK209) under the same conditions with spiro-OMeTAD as a hole-transporting material.

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The effect of the NiO_x thickness on the photovoltaic properties in planar type lead iodide perovskite solar cells consisted of Tin-doped indium oxide (ITO)/NiO_x/Perovskite/Phenyl-C61-Butyric-Acid-Methyl Ester (PCBM)/Ag was investigated. For films ranging from 35 nm to 120 nm in the thickness, the NiO_x thickness was found to be not crucial in fill factor and open circuit voltage. Calculations of the carrier flux in NiO_x, estimated from Einstein-Smoluchowski relation and Fick’s first law, found the carrier transport ability of the NiO_x to fully compensate the photo-generated current density in perovskite layers and the theoretical current density ca. 26 mA cm⁻². The favorable physical properties such as mobility and thickness were also estimated for effective carrier transport in perovskite solar cells assuming that the carrier density is 10¹⁵ cm⁻³.

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As next-generation printable solar cells, perovskite solar cell utilizing organometal halide perovskite crystalline semiconductor as the active layer has been considered as one of the most promising systems for its high efficiency (> ∼22%) and construction with low-cost materials. Annealing process of organometal halide perovskite crystalline films are, however, relatively time-consuming process for the perovskite solar cells. We propose the microwave heating applied to the annealing process of the CH₃NH₃PbI₃ perovskite crystalline films for converting the intermediate state of as-coated precursor films into perovskite poly-crystalline films. Observed rapid conversion process into perovskite crystals with higher crystallinity and even the degradation of perovskite to be PbI₂ by evaporation of methylammonium iodide at relatively lower temperature to the conventional hotplate heating imply a promising potential to achieve novel high throughput annealing process with high-quality crystallinity of perovskite films at low-temperature due to direct and effective heating of CH₃NH₃PbI₃ films by the microwave irradiation.

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Poly(3,4-phenylenedioxy-2,5-thienylenevinylene)s with a fluorine substituent at the phenylene moiety (PPhDOTFV) and without the substituent (PPhDOTV) were synthesized by Stille coupling polymerization. They exhibited good solubility in common organic solvents owing to a branched alkyl chain at the phenylene moiety and a narrow optical bandgap of 1.66 eV due to an increased conjugated length caused by the vinylene bridge. Both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PPhDOTFV were 0.2 eV deeper than those of PPhDOTV. Organic photovoltaic cells (OPVs) using PPhDOTV:PC₇₀BM exhibited a short-circuit current density (J_sc) of 3.30 mA cm⁻², an open-circuit voltage (V_oc) of 0.52 V, and a power conversion efficiency of 0.91%, as compared to 1.62 mA cm⁻², 0.68 V, and 0.45%, respectively, for OPVs using PPhDOTFV:PC₇₀BM. The J_sc value of PPhDOTV-based OPVs was higher than that of PPhDOTFV-based OPVs because of a large LUMO energy offset between PPhDOTV and PC₇₀BM. On the other hand, the V_oc value of PPhDOTV-based OPVs was lower than that of PPhDOTFV-based OPVs because of a small energy difference between the HOMO level of PPhDOTV and the LUMO level of PC₇₀BM.

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Solution-processable AgNW electrodes are prepared by our mechanical press process and applied for organic photovoltaic (OPV) cells. OPV cells using the AgNW electrodes exhibit good power conversion efficiencies of 3.05% even with that of ITO electrode (3.06%). Our mechanical press process enable to make OPV cell with standard device structure of Glass/AgNWs/PEDOT:PSS/Organic layer/Al. Conventional AgNW electrode fabrication methods were not able to apply the standard device structure because of their high roughness. Our methods open a new application route of AgNWs as the transparent electrodes for OPV cells. The OPV cell performances indicate their high potential the AgNWs for a photovoltaic electrode as an alternative material for the conventional ITO electrode. Furthermore, our AgNW fabrication process is suitable to make an electrode on organic-based substrate materials, especially for paper fabricated from natural cellulose. The results showed in this report will open the door of paper electronics.

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The electrical and physical properties at the interface between the ZnO electron transport/collection layer and an organic photoactive layer play important roles in determining photovoltaic performance. Therefore, we have fabricated ZnO nanorods and optimized the coating conditions used for deposition of the ZnO seed layer, which serves as a nucleation site for crystal growth. First, the relationship between the concentration of the ZnO precursor solution (zinc acetate dihydrate and hexamethylenetetramine in methanol) and diameter of the ZnO nanorods was examined, and we showed that the diameter continuously decreased with increasing concentration, reaching a minimum of 23 nm. Further, we investigated density of ZnO nanorods and photovoltaic performance of the inverted cell as a function of the rotation speed used during the spin coating process to deposit the ZnO seed layer. A low density of ZnO nanorods was obtained when the rotation speed was low, and this caused the short circuit current density, open circuit voltage, and fill factor of the organic photovoltaics to decrease. As a result, a maximum photoconversion efficiency of 3.0% was achieved, and this value was 1.6-fold greater than that measured using a reference device containing no ZnO nanorods (photoconversion efficiency: 1.9%).

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Microwave-assisted hydrothermal synthesis of ZnO nanocrystals has resulted in two distinctive structures: hexagonal bi-pyramidal (HBP) and nano-sheet (NS) shapes from Zn(CH₃COO)₂ solutions containing triethanolamine or benzene tetracarboxylic acid as structure directing agents, respectively. Observation by field-emission scanning electron microscope, high resolution transmission electron microscope as well as analysis of selected area electron beam diffraction patterns revealed that the predominantly exposed facets of HBP and NS ZnO are (102) and (100) faces, occupying 84 and 76% of the total surface area, respectively. Adsorption of D149 and eosin Y photosensitizer dyes onto HBP and NS ZnO was studied in comparison with a randomly structured commercial ZnO nanocrystal (MZ). Density of dye packing was found to be in the order of HBP > MZ > NS, for which HBP was 2 to 5 times higher than NS. Consequently, dye-sensitized solar cells employing HBP ZnO exhibited the highest short circuit photocurrent, but also the lowest open circuit voltage. By contrast, the DSSCs employing NS ZnO exhibited the highest voltage among the three.

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We present a novel insight on the light-soaking effect, in which the open-circuit voltage (V_oc) of inverted polymer solar cells improves with increase in light irradiation time. We previously have found that there are two kinds of the light soaking effects such as a short-circuit current density dependent type and a V_oc dependent type. The expression of these effects needed to irradiate the light containing UV component. However, for 4-(triphenylphosphonio)butane-1-sulfonate (TPPBS)-modified indium tin oxide (ITO)/poly(3-hexylthiophene-2,5-diyl) (P3HT): [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrene sulfonic acid) (PEDOT:PSS)/Au inverted cell, we first observed an increase in the V_oc by irradiating white light not containing UV component. After pre-treatment by applying forward bias under UV-cut light, the V_oc was improved even when the performance was evaluated by irradiating UV-cut light, that is, the V_oc values of 0.36, 0.42, 0.46 and 0.54 V were obtained by holding at forward voltages of 0, 0.3, 0.6, and 1.0 V, respectively. We discussed about this pre-treatment effect from the viewpoint of desorption of oxygen molecules adsorbing on ITO.

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We report the synthesis and characterization of a new thiazolothiazole (TzTz)-based semiconducting polymer incorporating the alkoxythiazole unit in the polymer backbone (PTzTTz-BOoHD). The introduction of the alkoxythiazole unit in a thiophene–TzTz polymer (PTzBT-BOHD) lowered the LUMO energy level while keeping the HOMO energy level, resulting in a narrower bandgap. The solar cell with PTzTTz-BOoHD showed photoresponse in a wider spectral range (∼740 nm) compared to that with PTzBT-BOHD and with a thiophene–TzTz polymer having the alkoxy thiophene unit (PTzBT-BOoHD). Although the fill factor (FF) of the cell based on PTzTTz-BOoHD was lower than the cell based on PTzBT-BOHD and -BOoHD, the maximum PCE of the cell based on PTzTTz-BOoHD resulted in 6.2%, which was comparable to and higher than the cell based on PTzBT-BOHD and -BOoHD, respectively. While PTzTTz-BOoHD showed good crystallinity in the polymer film, it was reduced in the film blended with PC₆₁BM, which is likely an origin of the drop of FF. As PTzTTz-BOoHD has more suitable electronic structures for solar cells such as a deep HOMO level and a narrow bandgap, compared to the other two polymers, we expect that PTzTTz-BOoHD would demonstrate higher PCE by further device optimization.

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We report a study of the dependence of the device performance of polymer solar cells with the blend films of regioregular poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) on the insertion of a layer of metal (Pd, Ag, Cu or Au) nanoparticles (NPs) at the electron-collecting electrodes. The short-circuit current density, the incident photo-to-current conversion efficiency, and the power conversion efficiency of the cells increased by the insertion of metal-NPs; the insertion of the Pd-NP layer has a large enhancement effect for the device performance. The light absorption was found to increase by forming the Pd-NP layer on the blend film without changing the light reflection. The formation of Au NPs on the blend films was directly observed by scanning tunneling microscopy (STM). The possible mechanism for such improvement is discussed using the results of the solar-cell parameters and the STM.

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The transient current analysis was conducted in order to reveal the mechanism of hysteresis phenomena in organolead halide perovskite solar cells. The change of the hysteresis in OPV-type device of CH₃NH₃PbI₃ was analyzed depending on scan condition. The change of time constant depending on the scan condition was observed. Our results suggest that the elimination of the hysteresis for slow scan speed cannot be explained by simple first-order relaxation model such as charging into capacitance. The model should include the parameters which depend on the scan speed and history of the bias scan.

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Organic multi-function diode with a function of light-emitting, photovoltaic, and color-sensing was demonstrated using a stack of 5,6,11,12-tetraphenylnaphthacene (rubrene) as an emitter/donor and N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) as an electron transporter/accepter. The device shows electroluminescent properties with a maximum luminance 840 cd/m² with yellow emission based on rubrene, photovoltaic properties with power conversion efficiency 0.15% under AM1.5, and green color sensing with high on/off ratio over 10⁴ under reverse bias of −1 V.

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