Photocrosslinkable Polythiophenes for Efficient, Thermally Stable, Organic Photovoltaics

Abstract

Photocrosslinkable bromine‐functionalized poly(3‐hexylthiophene) (P3HT‐Br) copolymers designed for application in solution‐processed organic photovoltaics are prepared by copolymerization of 2‐bromo‐3‐(6‐bromohexyl) thiophene and 2‐bromo‐3‐hexylthiophene. The monomer ratio is carefully controlled to achieve a UV photocrosslinkable layer while retaining the π–π stacking feature of the conjugated polymers. The new materials are used as electron donors in both bulk heterojunction (BHJ) and bilayer type photovoltaic devices. Unlike devices prepared from either P3HT:PCBM blend or P3HT‐Br:PCBM blend without UV treatment, photocrosslinked P3HT‐Br:PCBM devices are stable even when annealed for two days at the elevated temperature of 150 °C as the nanophase separated morphology of the bulk heterojunction is stabilized as confirmed by optical microscopy and grazing incidence wide angle X‐ray scattering (GIWAXS). When applied to solution‐processed bilayer devices, the photocrosslinkable materials show high power conversion efficiencies (∼2%) and excellent thermal stability (3 days at 150 °C). Such performance, one of the highest obtained for a bilayer device fabricated by solution processing, is achieved as crosslinking does not disturb the π–π stacking of the polymer as confirmed by GIWAXS measurements. These novel photocrosslinkable materials provide ready access to efficient bilayer devices thus enabling the fundamental study of photophysical characteristics, charge generation, and transport across a well‐defined interface.

Grazing incidence wide angle x-ray scattering at the wiggler beamline BW4 of HASYLAB

“…37,38 The Kapton foil is inevitably present as a robust window of the ionization chamber right in front of the sample, and the molecular aggregation in the aromatic polyimide film ͑Kapton͒ contributes with scattering in the small and wide angle regimes. 38 The typical GIWAXS pattern for the measurement of a bare glass substrate in the absence of the knife edge is shown in Fig. 3͑a͒.…”

Reappraising the Need for Bulk Heterojunctions in Polymer−Fullerene Photovoltaics: The Role of Carrier Transport in All-Solution-Processed P3HT/PCBM Bilayer Solar Cells

“…30 Another recent alternate approach involved photocross-linking a derivative of P3HT to render it insoluble, so that a PCBM overlayer could be spun on top, producing devices with power conversion efficiencies of ∼2%. 31 In contrast, fabrication of fully solution-processed bilayer cells via spincoating of both components requires finding a set of so-called orthogonal solvents 32 such that the solvent used to spin-coat the fullerene overlayers does not affect the morphology of the polymer underlayers. For P3HT/PCBM devices, we find that the common organic solvent dichloromethane (DCM) meets this requirement: PCBM is sufficiently soluble in DCM that it is possible to spin-coat PCBM layers on top of P3HT, and as we show below, P3HT is so sparingly soluble in DCM that there is negligible redissolution of the P3HT underlayer during spincoating of the PCBM overlayer.…”

Combination of Indene-C60 Bis-Adduct and Cross-Linked Fullerene Interlayer Leading to Highly Efficient Inverted Polymer Solar Cells

“…Moreover, this crosslinking approach also allows for fabricating a bilayer solar cell device by solution processing. 9 Later on, we reported a cross-linkable fullerene material, [6,6]-phenyl-C 61 -butyric styryl dendron ester (PCBSD). 10 The formation of a cross-linked PCBSD (C-PCBSD, Figure 1) interlayer allows deposition of a sequential active layer on top of this interlayer without causing interfacial erosion and realizes a multilayer inverted device by all-solution processing.…”