Identifying Molecular Orientation in a Bulk Heterojunction Film by Infrared Reflection Absorption Spectroscopy

The molecular orientation of organic molecules of zinc phthalocyanine (ZnPc) in single-component films on copper iodide (CuI) substrates can be controlled to achieve a molecular orientation lying flat on the substrate (flat-on) owing to π–d orbital interactions between the ZnPc molecules and the CuI. A 3-fold enhancement in the performance of organic photovoltaic cells has been reported by introducing a CuI interlayer between a ZnPc:fullerene (C60) bulk heterojunction (BHJ) film and the substrate. However, the mechanism underpinning the resultant solar cell performance enhancement was unclear. Herein, we report on the results of using in situ reflection absorption spectroscopy measurements during the vacuum deposition of coevaporated ZnPc:C60 BHJ films on various substrates to investigate the ZnPc molecular orientation. Our results revealed that the flat-on molecular orientation of ZnPc molecules in ZnPc:C60 BHJ films on CuI interlayers and flat-on ZnPc substrates can be successfully identified via the strong π–π interactions between the BHJ film and the substrate. The π–π interactions between individual ZnPc molecules are stronger than the π–d interactions between ZnPc molecules and CuI in coevaporated ZnPc:C60 films, as is evident from the molecular orientation of ZnPc, as determined by in situ reflection absorption spectroscopy. Our findings demonstrate that precisely controlling the molecular orientations of the films could enhance organic photovoltaic (OPV) performance. The present work provides important insights that will enable the design of higher performance OPV cells

Sexithiophene-Based Photovoltaic Cells with High Light Absorption Coefficient via Crystalline Polymorph Control

Herein, we report an efficient approach to control the crystalline polymorph of evaporated α-sexithiophene (6T) thin films by keeping them overnight (12 h) under vacuum. Further, we investigated the effects on the performance of organic photovoltaic devices of controlling the 6T polymorph via this vacuum technique so that the films take on the low-temperature (LT) polymorph (in which the backbones of the 6T molecules lie flatter on the substratethe so-called “lying-down” orientation). Our results revealed that when the organic layer was deposited directly onto a copper­(I) iodide interlayer, the angle between the organic backbone and the substrate was reduced in the LT polymorph compared with the high-temperature (HT) polymorph. The power conversion efficiency of solar cells could thus be enhanced from 0.58 to 1.77% via a change in the crystal polymorph of the 6T layer from HT to LT by simply keeping the films in vacuum for 12 h

Epistola medicinalis historica, exhibens argumenta quaedam refellentia res effusas in epistola insigniores, a doctore medico ad alium medicum scripta, kal. jan. an. 1697, et addita an. 1703 observationibus prelo subjectis de lue venerea

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Factors Affecting the Performance of Bifacial Inverted Polymer Solar Cells with a Thick Photoactive Layer

Photocurrent voltage curves and photocurrent action spectra of bifacial inverted polymer solar cells with a structure of ITO/ZnO/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM):regioregular poly­(3-hexylthiophene) (P3HT)/poly­(3,4-ethylenedioxylenethiophene):poly­(4-styrene sulfonic acid) (PEDOT:PSS)/Au were measured. High performance was obtained when light was irradiated from each side of the devices, even for those with a 500 nm thick PCBM:P3HT layer, but an optical filter effect of the photocurrent was somewhat larger for irradiation from the Au side than that for the ITO side. These results suggested that the efficiency of photocharge separation near the ZnO/PCBM:P3HT interface was higher than that near the PCBM:P3HT/PEDOT:PSS interface, although the photocharge separation and the charge transport were smooth in the whole PCBM:P3HT layer. Further, we found that the fill factor of the photocurrent voltage curves of these devices depended on the migration distance of holes with lower mobility in the PCBM:P3HT layer

Improved Reproducibility and Intercalation Control of Efficient Planar Inorganic Perovskite Solar Cells by Simple Alternate Vacuum Deposition of PbI₂ and CsI

Vacuum deposition is a simple and controllable approach that aims to form higher-quality perovskite films compared with those formed using solution-based deposition processes. Herein, we demonstrate a novel method to promote the intercalation control of inorganic cesium lead iodide (CsPbI₃) perovskite thin films via alternate vacuum deposition. We also investigated the effect of layer-by-layer deposition of PbI₂/CsI to fabricate efficient planar heterojunction CsPbI₃ thin films and solar cells. This procedure is comparatively simple when compared with commonly used coevaporation techniques; further, precise intercalation control of the CsPbI₃ thin films can be achieved by increasing the number of layers in the layer-by-layer deposition of PbI₂/CsI. The best control and the highest reproducibility were achieved for the deposition of four double layers owing to the precise intercalation control during the deposition of the CsPbI₃ thin film. A power conversion efficiency of 6.79% was obtained via alternating vacuum deposition of two double layers with a short-circuit current density (<i>J</i>_sc) of 12.06 mA/cm², an open-circuit voltage (<i>V</i>_oc) of 0.79 V, and a fill factor (FF) of 0.72. Our results suggest a route for inorganic precursors to be used for efficient perovskite solar cells via alternating vacuum deposition

Mechanism of Light-Soaking Effect in Inverted Polymer Solar Cells with Open-Circuit Voltage Increase

In this study, we present novel insights into the light-soaking effect of inverted polymer solar cells (PSCs), where the open-circuit voltage (<i>V</i>_oc) of the cells improves over time under light irradiation. The effect was investigated by electron spin resonance (ESR) studies of bare indium tin oxide (ITO) and piperazine derivative-modified ITO/regioregular poly­(3-hexylthiophene) (P3HT):[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM) substrates. These results were combined with alternating current impedance spectroscopy (IS) measurements of inverted PSCs based on the above substrates. In ESR experiments with the substrates under white light irradiation, with a UV light component, many P3HT^•+ radical cations were observed in the bare-ITO/P3HT:PCBM substrate. The number of radical cations was considerably suppressed in the ITO/P3HT:PCBM substrates with ITO modified by piperazine derivatives. This is because adsorbed oxygen molecules on the ITO acted as acceptor dopants for photoexcited P3HT, and the amount of adsorbed oxygen was decreased by modifying the ITO with piperazine derivatives. In IS measurements of the inverted PSCs under white light irradiation, a decrease in the electric capacitance (CPE2) of an electric double layer formed at the ITO/P3HT:PCBM interface was observed. A strong correlation was observed between the decrease of CPE2 and the increase of <i>V</i>_oc. From these results, the light-soaking behavior was attributed to the removal of an electron injection barrier formed between ITO and PCBM, under white light irradiation