In Situ Probing of the Charge Transport Process at the Polymer/Fullerene Heterojunction Interface

The polymer/fullerene interface (PFI) in polymer solar cells (PSCs) provides an energetic offset for exciton dissociation while at the same time influencing local transport of photocarriers adjacent to the interface. In this paper, we introduce a heterojunction field-effect transistor (FET) structure in charge modulation spectroscopy (CMS) to enable in situ probing of the charge transport process at PFIs. The PFIs formed by fullerene/crystalline polymer and fullerene/amorphous polymer systems are studied and compared, respectively. By correlating the steady-state and frequency-dependent CMS responses of pure polymer, polymer/fullerene bilayer, and polymer/fullerene blend FETs, we demonstrate that through different charge localization effects the interface fullerene molecules can influence the hole transport in both crystalline and amorphous polymer phases. We propose a trade-off between charge transfer and charge transport at PFIs with an aim to enhance the engineering of molecular orientation and packing at the donor–acceptor interface for high-performance PSCs

Enhancement of Photovoltaic Performance by Utilizing Readily Accessible Hole Transporting Layer of Vanadium(V) Oxide Hydrate in a Polymer–Fullerene Blend Solar Cell

Herein, a successful application of V₂O₅·<i>n</i>H₂O film as hole transporting layer (HTL) instead of PEDOT:PSS in polymer solar cells is demonstrated. The V₂O₅·<i>n</i>H₂O layer was spin-coated from V₂O₅·<i>n</i>H₂O sol made from melting-quenching sol–gel method by directly using vanadium oxide powder, which is readily accessible and cost-effective. V₂O₅·<i>n</i>H₂O (<i>n</i> ≈ 1) HTL is found to have comparable work function and smooth surface to that of PEDOT:PSS. For the solar cell containing V₂O₅·<i>n</i>H₂O HTL and the active layer of the blend of a novel polymer donor (PBDSe-DT2PyT) and the acceptor of PC₇₁BM, the PCE was significantly improved to 5.87% with a 30% increase over 4.55% attained with PEDOT:PSS HTL. Incorporation of V₂O₅·<i>n</i>H₂O as HTL in the polymer solar cell was found to enhance the crystallinity of the active layer, electron-blocking at the anode and the light-harvest in the wavelength range of 400–550 nm in the cell. V₂O₅·<i>n</i>H₂O HTL improves the charge generation and collection and suppress the charge recombination within the PBDSe-DT2PyT:PC₇₁BM solar cell, leading to a simultaneous enhancement in <i>V</i>_oc, <i>J</i>_sc, and FF. The V₂O₅·<i>n</i>H₂O HTL proposed in this work is envisioned to be of great potential to fabricate highly efficient PSCs with low-cost and massive production

Molecular Orientation and Performance of Nanoimprinted Polymer-Based Blend Thin Film Solar Cells

In this work, we have used synchrotron-based grazing incidence X-ray scattering to measure the molecular orientation and morphology of nanostructured thin films of blended poly­(3-hexylthiophene)/[6,6]-phenyl C61-butyric acid methyl ester blends patterned with nanoimprint lithography. Imprinting the blend films at 150 °C results in significant polymer chain orientational anisotropy, in contrast to patterning the film at only 100 °C. The temperature-dependent evolution of the X-ray scattering data reveals that the imprint-induced polymer reorientation remains at high temperatures even after the patterned topographic features vanish upon melting. Photovoltaic devices fabricated from the blend films imprinted at 150 °C exhibit a ∼21% improvement in power conversion efficiency compared to those imprinted at 100 °C, consistent with a polymer chain configuration better suited to charge carrier collection

Spectroscopic Study of Charge Transport at Organic Solid–Water Interface

Charge transport in an organic solid and its coupling with the neighboring aqueous biological environment dictates the performance of many organic bioelectronic devices. Understanding how the transport property at the solid–water interface is influenced by the surface structure characteristics of the organic solid is essential for rational design of organic bioelectronics and chemical sensors. However, <i>in situ</i> probing such structure–property relationships has been difficult due to lack of experimental techniques with sufficient sensitivity to the water-buried interface. Here, we demonstrate a charge accumulation spectroscopy (CAS)-based protocol, exploiting water-gated organic field-effect transistor as the testing platform, to investigate the structure-dependent localization of polaronic charge carriers at the organic semiconductor–liquid interface. Our results reveal that the degree of charge delocalization is reduced drastically when the charge carriers are moved from the bulk semiconductor to the semiconductor–water interface, suggesting the existence of a highly disordered surface layer in contact with water. It is also found that the charge delocalization could be further reduced by intercalation of chloride ions (from salt water) in the semiconductor surface layer. This study suggests that the spectroscopic signatures of polaronic charge carriers could be a sensitive probe to detect the structure-dependent charge localization at organic solid–liquid interfaces

Alkoxy-Induced Near-Infrared Sensitive Electron Acceptor for High-Performance Organic Solar Cells

We develop a fused-ring electron acceptor (IOIC3) based on naphtho­[1,2-<i>b</i>:5,6-<i>b</i>′]­dithiophene core with alkoxy side-chains and compare it with its counterpart (IOIC2) with alkyl side-chains. Change in the side-chains affects electronic, optical, charge transport, and morphological properties of the analogues. Because of π-conjugative effect and σ-inductive effect of the oxygen atoms, IOIC3 exhibits a slightly upshifted HOMO level (−5.38 eV) and a downshifted LUMO level (−3.84 eV) relative to IOIC2 (HOMO = −5.41 eV, LUMO = −3.78 eV), leading to red-shifted absorption and smaller optical bandgap of 1.45 eV than that of IOIC2 (1.54 eV). IOIC3 exhibits a higher electron mobility of 1.5 × 10^–3 cm² V^–1 s^–1 than IOIC2 (1.0 × 10^–3 cm² V^–1 s^–1). Organic solar cells (OSCs) based on PTB7-Th:IOIC3 exhibit power conversion efficiency (PCE) as high as 13.1%, significantly higher than that of PTB7-Th:IOIC2 (9.33%). The semitransparent OSCs based on PTB7-Th:IOIC3 afford PCEs of up to 10.8% with an average visible transmittance (AVT) of 16.4%, higher than those of PTB7-Th:IOIC2 (PCE = 7.32%, AVT = 13.1%)

Nanostructured Surfaces Frustrate Polymer Semiconductor Molecular Orientation

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C₆₁-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction

Effect of Core Size on Performance of Fused-Ring Electron Acceptors

We report 4 fused-ring electron acceptors (FREAs) with the same end-groups and side-chains but different cores, whose sizes range from 5 to 11 fused rings. The core size has considerable effects on the electronic, optical, charge transport, morphological, and photovoltaic properties of the FREAs. Extending the core size leads to red-shift of absorption spectra, upshift of the energy levels, and enhancement of molecular packing and electron mobility. From 5 to 9 fused rings, the core size extension can simultaneously enhance open-circuit voltage (<i>V</i>_OC), short-circuit current density (<i>J</i>_SC), and fill factor (FF) of organic solar cells (OSCs). The best efficiency of the binary-blend devices increases from 5.6 to 11.7%, while the best efficiency of the ternary-blend devices increases from 6.3 to 12.6% as the acceptor core size extends

Panchromatic Ternary Photovoltaic Cells Using a Nonfullerene Acceptor Synthesized Using C–H Functionalization

Panchromatic Ternary Photovoltaic Cells Using a Nonfullerene Acceptor Synthesized Using C–H Functionalizatio

Photo-Cross-Linkable Azide-Functionalized Polythiophene for Thermally Stable Bulk Heterojunction Solar Cells

We have synthesized photo-cross-linkable azide-functionalized poly­(3-hexylthiophene) to explore improvements in the thermal stability of bulk heterojunction solar cells. Exposing blends of photo-cross-linkable polythiophene and [6,6]-phenyl-C₆₁-butyric acid methyl ester to ultraviolet light preferentially cross-linked the polythiophene without degrading its optical or electrical properties. X-ray scattering measurements showed that cross-linking slightly compacted the polythiophene chain lamellar stacking while increasing the polymer crystal coherence length by 20%. Optimized solar cells having cross-linked active blend layers retained 65% of their initial photovoltaic power conversion efficiency after 40 h of thermal annealing at 110 °C, while devices using un-cross-linked commercial polythiophene underwent significant phase separation and retained less than 30% of their initial efficiency after annealing

A Medium Bandgap D–A Copolymer Based on 4‑Alkyl-3,5-difluorophenyl Substituted Quinoxaline Unit for High Performance Solar Cells

Development of high-performance donor–acceptor (D–A) copolymers has been indicated as a promising strategy to improve the power conversion efficiencies (PCEs) of organic solar cells (OSCs). In this work, a new medium bandgap conjugated D–A copolymer, HFAQx-T, based on 4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene (BDT-T) as donor unit, 4-alkyl-3,5-difluorophenyl substituted quinoxaline (HFAQx) as the acceptor unit, and thiophene as the spacer, was designed and synthesized. HFAQx-T is a well-compatible donor polymer; OSCs based on HFAQx-T exhibit excellent performance in both fullerene and fullerene-free based devices. The optimized conventional single junction bulk heterojunction (BHJ) OSCs of HFAQx-T:PC₇₁BM showed a PCE of 9.2%, with an open circut voltage (<i>V</i>_oc) of 0.9 V, a short circuit current (<i>J</i>_sc) of 14.0 mA cm^–2, and a fill factor (FF) of 0.74. Also, when blended with 3,9-bis­(2-methylene-(3-(1,1-dicyano­methylene)­indanone)-5,5,11,11-tetrakis­(4-hexylphenyl)­dithieno­[2,3-<i>d</i>:2′,3′-<i>d</i>′]-<i>s</i>-indaceno­[1,2-<i>b</i>:5,6-<i>b</i>′]-dithiophene (ITIC), the HFAQx-T-based device exhibited a PCE of 9.6%. HFAQx-T is among a few D–A copolymers that can deliver >9% efficiency in both fullerene and fullerene-free solar cells. This work demonstrates that the 4-alkyl-3,5-difluorophenyl substituted quinoxaline (Qx) is a promising electron-accepting building block in constructing ideal D–A copolymers for OSCs