Abstract
The fabrication of organic solar cells with advanced multi-layer architectures from solution is often limited by the choice of solvents since most organic semiconductors dissolve in the same aromatic agents. In this work, we investigate multi-pass deposition of organic semiconductors from eco-friendly ethanol dispersion. Once applied, the nanoparticles are insoluble in the deposition agent, allowing for the application of further nanoparticulate layers and hence for building poly(3-hexylthiophene-2,5-diyl):indene-C60 bisadduct absorber layers with vertically graded polymer and conversely graded fullerene concentration. Upon thermal annealing, we observe some degrees of polymer/fullerene interdiffusion by means of X-ray photoelectron spectroscopy and Kelvin probe force microscopy. Replacing the common bulk-heterojunction by such a graded photo-active layer yields an enhanced fill factor of the solar cell due to an improved charge carrier extraction, and consequently an overall power conversion efficiency beyond 4%. Wet processing of such advanced device architectures paves the way for a versatile, eco-friendly and industrially feasible future fabrication of organic solar cells with advanced multi-layer architectures.
Highlights
As the field progresses and organic photovoltaics advance towards market-readiness, the deposition of functional layers by industrial printing or coating techniques comes into play.[1,2] Pivotal to the transfer strategy from lab to fab is the replacement of the commonly used toxic or chlorinated lab-solvents with eco-compatible agents.[3,4,5,6,7] The quest for eco-compatible solvents turned out to be very challenging since many organic materials do not dissolve in non-chlorinated aromatic hydrocarbons or exhibit very different processing properties
We demonstrate that, besides eco-friendly processing, future device fabrication may benefit from another unique feature of polymer:fullerene nanoparticles: layers that have been applied from nanoparticle dispersions are insoluble in the deposition agent, enabling the sequential deposition of the same material from the same agent and the fabrication of advanced multi-layer device architectures
Configuration A is a reference solar cell with an inverted device architecture that comprises an indium tin oxide/zinc oxide (ITO/ZnO) bottom cathode, a nanoparticulate P3HT: indene-C60 bisadduct (ICBA) BHJ photo-active layer that was deposited from ethanol (EtOH) dispersion, and a poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)/silver (PEDOT:PSS/Ag) top anode
Summary
As the field progresses and organic photovoltaics advance towards market-readiness, the deposition of functional layers by industrial printing or coating techniques comes into play.[1,2] Pivotal to the transfer strategy from lab to fab is the replacement of the commonly used toxic or chlorinated lab-solvents with eco-compatible agents.[3,4,5,6,7] The quest for eco-compatible solvents turned out to be very challenging since many organic materials do not dissolve in non-chlorinated aromatic hydrocarbons or exhibit very different processing properties. A promising approach to circumvent all issues arising from the deposition of organic bulk-heterojunctions (BHJs) from toxic solvents is the use of organic nanoparticle dispersions in water or alcohol.[8,9,10] Two synthetic routes to organic nanoparticles have been discussed in the literature: utilizing surfactants, organic nanoparticles can be synthesized in miniemulsions.[11] organic solar cells deposited from these aqueous dispersions do not yield high power conversion efficiencies (PCEs), which may be attributed to the surfactants remaining within the active layer where they hamper the photovoltaic performance. Nanoparticles from poly(3-hexylthiophene-2,5-diyl) (P3HT) and indene-C60 bisadduct (ICBA) can be precipitated in alcohol omitting any surfactants, thereby enabling organic solar cells with PCEs of 4%, almost matching the performance of organic solar cells deposited from chlorinated solvents.[12,13]
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