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Abstract: Precise control of the molecular arrangements at the interface between the electron donor and acceptor in mixed bulk heterojunctions (BHJs) remains challenging, despite the correlation between structural characteristics and efficiency in organic photovoltaics (OPVs). This study reveals that the substitution patterns of linear and branched alkyl side chains on electron‐donating/‐accepting alternating copolymers can control the positions of an acceptor molecule (C60) around the π‐conjugated main chains in mixed BHJs. Two‐dimensional solid‐state NMR demonstrates a marked difference in the location of C60 in the blend films. A copolymer with an electron‐accepting unit positioned in close proximity to C60 demonstrated higher OPV performance in combination with various fullerene derivatives. This molecular design offers precise control over the interfacial molecular structure, thereby paving the way for overcoming the current limitations of OPVs comprising mixed BHJs. [ABSTRACT FROM AUTHOR]

In an ideal model, a p-n junction is formed by two stacked slabs of semiconductors. Although the construction of actual devices is generally more complex, we show that such a simple method can in fact be applied to the formation of organic heterojunctions. Two films of the organic semiconductors poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) can be connected by a simple film-transfer method without disturbing their flat surfaces. Each film can further be modified with a surface-segregated monolayer to tune the strength and direction of the surface dipole moment. Using this method, we fabricated bilayer organic photovoltaic devices with interfacial dipole moments that were selected to align the energy levels at the heterojunction. The open-circuit voltages of the P3HT/PCBM devices could be tuned over a wide range between 0.3 and 0.95 V, indicating that, even if the same combination of bulk materials is used, the interfacial properties drastically alter the performance of organic photovoltaic devices. [ABSTRACT FROM AUTHOR]

The introduction of an electron-blocking layer of vanadium oxide between the organic layer and the Ag electrode was studied as a means of improving the performance of bulk heterojunction photovoltaic devices based on poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester hybridized with vertically aligned ZnO nanorod arrays. The power conversion efficiency was further improved to 3.9% by applying a short circuit current density of 10.4 mA cm-2, an open circuit voltage of 0.58 V, and a fill factor of 65% under AM1.5 100 mW cm-2 irradiation, owing to the electron collection “antenna” effect of the ZnO nanorod arrays. [ABSTRACT FROM AUTHOR]

Abstract: Four 9-cyanomethylenefluorene derivatives were synthesized as a new class of electron-deficient building blocks for donor–acceptor (D–A) type alternating copolymers. UV–vis absorption spectroscopy, cyclic voltammetry and density functional theory calculations show that the substituent X at the cyanomethylene unit, depending on its electron-withdrawing ability (X=–H, –CONMe2, –COOiBu and –CN), can continuously control the LUMO energy level of the monomer. D–A type copolymers were synthesized using the derivatives and an oligothiophene as the electron-donating block. The photovoltaic performance of D–A copolymer:PCBM bulk heterojunction devices shows the importance of LUMO level alignment to achieve efficient charge separation at the donor/acceptor interface. After optimization, the power conversion efficiency of the device with X=–H reached 1.14% with V OC of 0.61V, J SC of 3.24mAcm−2 and FF of 0.58 under the irradiation of AM1.5 simulated solar light. The electronic properties of the copolymers correlated well with the electron-withdrawing ability of the monomers while the structure of the π-conjugated backbone was preserved, demonstrating the advantage of the chemically tunable monomer design. [Copyright &y& Elsevier]

Abstract: Use of a lamination process for the introduction of Au electrode, instead of conventional metal evaporation, improved the power conversion efficiency (PCE) of inverted-structure photovoltaic devices from 1.6% to 2.6% based on a bulk heterojunction of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). X-ray photoelectron spectroscopy indicated that a thin layer of P3HT is spontaneously formed at the air/polymer blend layer interface during the spin-coating process. It is suggested that the vacuum-evaporated gold could destroy the surface-segregated thin layer of P3HT, while the lamination process preserves the surface structure working as an electron-blocking layer. The insertion of a PEDOT:PSS layer, between the metal electrode and polymer layer, in the lamination process further improved the PCE to 3.3% with a short-circuit current density of 9.94mAcm−2, an open-circuit voltage of 0.60V, and a fill factor of 55% under AM1.5 100mWcm−2 irradiation. [Copyright &y& Elsevier]

Bulk heterojunction (BHJ) structures prepared by coating mixed solutions of a donor and an acceptor have been used for efficient organic photovoltaics (OPVs), but this approach allows little control of the molecular orientations in the films. Herein we show that a sequential deposition strategy, in which the molecular acceptor is deposited sequentially on a pristine polymer donor film, reliably controls the molecular orientations in the resulting BHJ films. Spectroscopic ellipsometry indicates that both the donor and the acceptor adopt higher degrees of orientation with the π-planes parallel to the film plane compared with mixed BHJ films. The sequentially deposited films enhance the power conversion efficiency of OPVs by 30% compared with mixed BHJ films, indicating the usefulness of this strategy for improving OPV performance. • Molecular orientations were controlled in bulk heterojunction films prepared by a sequential deposition process. • The sequential desposition showed 30% higher photovoltaic performance than the conventional mixed bulk heterojunction. • The film morphology, the molecular orientation, and the composition at the film surface were characterized. [ABSTRACT FROM AUTHOR]