Long-Term Stabilization of Organic Solar Cells Using Hindered Phenols as Additives

We report on the improvement of long-term stability of organic solar cells (OPV) using hindered phenol based antioxidants as stabilizing additives. A set of seven commercially available hindered phenols are investigated for use in bulk-heterojunction OPV. Polymer:fullerene films based on poly­(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are characterized with respect to the initial power conversion efficiency and the long-term stability improvement under illumination in ambient conditions. FTIR spectroscopy is used to trace chemical degradation over time. OPV performance is recorded under ISOS-3 conditions, and an improved long-term performance of OPV devices, manifested in increased accumulated power generation (APG), is found for octadecyl 3-(3,5-di-<i>tert</i>-butyl-4-hydroxyphenyl)­propionate. Using this additive, APG is increased by a factor of 3 compared to the reference. Observed differences in the stabilization of tested additives are discussed in terms of energetic trap states formation within the HOMO/LUMO gap of the photoactive material, morphological changes, and chemical structure

In Situ X‑ray Scattering Studies of the Influence of an Additive on the Formation of a Low-Bandgap Bulk Heterojunction

The evolution of the morphology of a high-efficiency, blade-coated, organic-photovoltaic (OPV) active layer containing the low band gap polymer poly­[(4,8-bis­[5-(2-ethylhexyl)­thiophene-2-yl]­benzo­[1,2-b:4,5-b′]­dithiophene)-2,6-diyl-<i>alt</i>-(4-(2-ethylhexanoyl)-thieno­[3,4-<i>b</i>]­thiophene))-2,6-diyl] (PBDTTT-C-T) is examined by in situ X-ray scattering. In situ studies enable real-time characterization of the effect of the processing additive 1,8-diiodoocatane (DIO) on the active layer morphology. In the presence of DIO, X-ray scattering indicates that domain structure radically changes and increases in purity, while X-ray diffraction reveals little change in crystallinity/local order. The solidification behavior of this active layer differs dramatically from those that strongly crystallize such as poly­(3-hexylthiophene) and small molecule containing systems, exposing significant diversity in the solidification routes relevant to high-efficiency polymer–fullerene OPV processing. In the presence of DIO, we find quantitative agreement between the evolution of the phase structure revealed by small-angle X-ray scattering and the binodal phase structure of a simple Flory–Huggins model

In Situ Characterization of Polymer–Fullerene Bilayer Stability

A consensus is emerging that mixed phases are present in bulk heterojunction organic photovoltaic (OPV) devices. Significant insights into the mixed phases have come from bilayer stability measurements, in which an initial sample consisting of material pure layers of donor and acceptor is thermally treated, resulting in swelling of one layer by the other. We present a comparative study of the stability of polymer/fullerene bilayers using two common OPV polymer donors poly­(3-hexylthiophene), P3HT, and poly­[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)], PCDTBT, and four fullerene acceptors phenyl-C61-butyric acid methyl ester, phenyl-C71-butyric acid methyl ester, [60]­PCBM bis-adduct, and indene C60 bis-adduct. Using in situ spectroscopic ellipsometry to characterize the quasi-steady state behavior of the films, we find that the polymer glass transition temperature (<i>T</i>_g) is critical to the bilayer stability, with no significant changes occurring below <i>T</i>_g of the high <i>T</i>_g PCDTBT. Above the polymer <i>T</i>_g, we find the behavior is irreversible and most consistent with swelling of the polymer by the fullerene, constrained by tie chains in the polymer network and influenced by the rubbery dynamics of the mixed region. The swelling varies significantly with the nature of the fullerene and the polymer. Across the eight systems studied, there is no clear relationship between swelling and OPV device performance. The relationship between the observed swelling and the underlying fullerene–polymer miscibility is explored via Flory–Rehner theory

Blade Coating Aligned, High-Performance, Semiconducting-Polymer Transistors

Recent demonstration of mobilities in excess of 10 cm² V^–1 s^–1 have energized research in solution deposition of polymers for thin film transistor applications. Due to the lamella motif of most soluble, semiconducting polymers, the local mobility is intrinsically anisotropic. Therefore, fabrication of aligned films is of interest for optimization of device performance. Many techniques have been developed to control film alignment, including solution deposition via directed flows and deposition on topologically structured substrates. We report device and detailed structural analysis (ultraviolet–visible absorption, IR absorption, near-edge X-ray absorption (NEXAFS), grazing incidence X-ray diffraction, and atomic force microscopy) results from blade coating two high performing semiconducting polymers on unpatterned and nanostructured substrates. Blade coating exhibits two distinct operational regimes: the Landau–Levich or horizontal dip coating regime and the evaporative regime. We find that in the evaporative deposition regime, aligned films are produced on unpatterned substrates with the polymer chain director perpendicular to the coating direction. Both NEXAFS and device measurements indicate the coating induced orientation is nucleated at the air interface. Nanostructured substrates produce anisotropic bottom contact devices with the polymer chain at the buried interface oriented along the direction of the substrate grooves, independent of coating regime and coating direction. Real time studies of film drying establish that alignment occurs at extremely high polymer volume-fraction conditions, suggesting mediation via a lyotropic phase. In all cases the final films appear to exhibit high degrees of crystalline order. The independent control of alignment at the air and substrate interfaces via coating conditions and substrate treatment, respectively, enable detailed assessment of structure–function relationships that suggest the improved performance of the nanostructure aligned films arise from alignment of the less ordered material in the crystallite interphase regions