Spin-coating is extensively used in the lab-based manufacture of organic solar cells, including most of the record-setting solution-processed cells. We report the first direct observation of photoactive layer formation as it occurs during spin-coating. The study provides new insight into mechanisms and kinetics of bulk heterojunction formation, which may be crucial for its successful transfer to scalable printing processes.
The absorbing layer in state-of-the-art colloidal quantum-dot solar cells is fabricated using a tedious layer-by-layer process repeated ten times. It is now shown that methanol, a common exchange solvent, is the main culprit, as extended exposure leaches off the surface halide passivant, creating carrier trap states. Use of a high-dipole-moment aprotic solvent eliminates this problem and is shown to produce state-of-the-art devices in far fewer steps.
Abstract:Blending of small-molecule organic semiconductors (OSCs) with amorphous polymers is known to yield high performance organic thin film transistors (OTFTs). Vertical stratification of the OSC and polymer binder into well-defined layers is crucial in such systems and their vertical order determines whether the coating is compatible with a top and/or a bottom gate OTFT configuration. Here, we investigate the formation of such blends prepared via spincoating in conditions which yield bilayer and trilayer stratifications, and use a combination of experimental and computational tools to study the competing effects of formulation thermodynamics and process kinetics in mediating the final vertical stratification. We show that trilayer stratification (OSC/polymer/OSC) is the thermodynamically favored configuration and that formation of the buried OSC layer can be kinetically inhibited in certain conditions of spincoating, resulting in a bilayer stack instead. Our analysis reveals that preferential loss of the Zhao et al., Adv. Func. Mater. 2016 2 OSC, combined with early aggregation of the polymer phase due to rapid drying, inhibit the formation of the buried OSC layer. We then moderate the fluid dynamics and drying kinetics during spin-coating to promote trilayer stratification with a high quality buried OSC layer which yields unusually high mobility >2 cm 2 V -1 s -1 in the bottom-gate top-contact configuration.
CuSCN is a highly transparent, highly stable, low cost and easy to solution process HTL that is proposed as a low cost replacement to existing organic and inorganic metal oxide hole transporting materials. Here, we demonstrate hybrid organic-inorganic perovskite-based p-i-n planar heterojunction solar cells using a solution-processed copper(I) thiocyanate (CuSCN) bottom hole transporting layer (HTL). CuSCN, with its high workfunction, increases the open circuit voltage (Voc) by 0.23 V to 1.06 V as compared with devices based on the well-known poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (0.83 V), resulting in a superior power conversion efficiency (PCE) of 10.8% without any notable hysteresis. Photoluminescence measurements suggest a similar efficiency of charge transfer at HTL/perovskite interface as PEDOT:PSS. However, we observe more efficient light harvesting in the presence of CuSCN at shorter wavelengths despite PEDOT:PSS being more transparent. Further investigation of the microstructure and morphology reveals differences in the crystallographic texture of the polycrystalline perovskite film, suggesting templated perovskite growth on the surface of CuSCN. The successful demonstration of the solution-processed
Despite the remarkable performance
progress being made, environmental
concerns remain for lead halide perovskite solar cells (PSCs) because
of the possible water dissolution of lead ions (Pb2+) into
the environment. Herein, we succeed in mitigating Pb leakage of PSCs,
for the first time, via implanting in situ polymerized networks into
perovskites. We strategically transform the dormant monomer additives
into chelating polymer networks within perovskite layers, which not
only passivate the defects of perovskite but also protect Pb2+ from water dissolution. The resultant perovskite–polymer
hybrids have successfully enabled state-of-art power conversion efficiencies
(PCEs) for inverted PSCs (PCE of 22.1%) and large-area modules (PCE
of 15.7%). More importantly, up to 94% rejection rate of Pb2+ dissolution is achieved upon directly immersing the unencapsulated
devices into water, which reasonably simulates the exposure of the
broken and unprotected panels to torrential rain for 24 h.
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