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We present a novel top-electrode spray-coating process for the solution-based deposition of silver nanowires (AgNWs) onto vacuum-processed small molecule organic electronic solar cells. The process is compatible with organic light emitting diodes (OLEDs) and organic light emitting thin film transistors (OLETs) as well. By modifying commonly synthesized AgNWs with a perfluorinated methacrylate, we are able to disperse these wires in a highly fluorinated solvent. This solvent does not dissolve most organic materials, enabling a top spray-coating process for sensitive small molecule and polymer-based devices. The optimized preparation of the novel AgNW dispersion and spray-coating at only 30 °C leads to high performance electrodes directly after the deposition, exhibiting a sheet resistance of 10.0 Ω □(-1) at 87.4% transparency (80.0% with substrate). By spraying our novel AgNW dispersion in air onto the vacuum-processed organic p-i-n type solar cells, we obtain working solar cells with a power conversion efficiency (PCE) of 1.23%, compared to the air exposed reference devices employing thermally evaporated thin metal layers as the top-electrode.

We measure basic network parameters of silver nanowire (AgNW) networks commonly used as transparent conducting electrodes in organic optoelectronic devices. By means of four point probing with nanoprobes, the wire-to-wire junction resistance and the resistance of single nanowires are measured. The resistance RNW of a single nanowire shows a value of RNW=(4.96±0.18) Ω/μm. The junction resistance RJ differs for annealed and non-annealed NW networks, exhibiting values of RJ=(25.2±1.9) Ω (annealed) and RJ=(529±239) Ω (non-annealed), respectively. Our simulation achieves a good agreement between the measured network parameters and the sheet resistance RS of the entire network. Extrapolating RJ to zero, our study show that we are close to the electrical limit of the conductivity of our AgNW system: We obtain a possible RS reduction by only ≈20% (common RS≈10 Ω/sq). Therefore, we expect further performance improvements in AgNW systems mainly by increasing NW length or by utilizing novel network geometries.

The surfactant polyvinylpyrrolidone (PVP) commonly used to synthesize silver nanowires (AgNW) in solution is known to negatively affect the performance of nanowire-based thin film electrodes. An insulating shell of the polymer hinders tight contact between the nanowires themselves and between the nanowires and substrate, resulting in high sheet resistance of the freshly prepared nanowire films. Here, we develop a simple low-temperature method allowing us to reduce the sheet resistance of AgNW networks and simultaneously improving the optical transmittance. The method is based on the capacity of PVP to absorb moisture which results in a strong decrease in the glass transition temperature of the polymer.The latter leads to softening effects, causing a reduced wire contact resistance already at 60 C for 90 nm thick AgNWs and even at 45 C for 35 nm thick AgNWs. As a result, the sheet resistances of the thin film electrodes treated by our method are near to the values conventionally obtained after thermal annealing at temperatures between 140-250 C. Our humidity assisted low temperature approach is especially advantageous for organic electronics and fabricating devices on thermally sensitive transparent flexible foils. † Electronic supplementary information (ESI) available: Changes in total transmittance DT vs. initial total transmittance; transmittance spectra of NW-35 electrode vs. annealing time; SEM images of a NW-35 electrode on glass; sheet resistance vs. annealing time at a constant temperature of 60 C and different RH values; SEM image, transmittance and reectance spectra of NW-90 electrode on PEN foil. See

a b s t r a c tPlanarization and filling voids between wires are key issues when using nanowire electrodes in flexible solar cells such as organic photovoltaics (OPV). For this purpose, we use poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) which leads to an electrically well connected silver nanowire (AgNW) network. Furthermore, the use of water based PEDOT: PSS leads to humidity assisted AgNW fusing, resulting in a maximum processing temperature of only 120 C. OPV cells using this AgNW/PEDOT: PSS transparent electrodes exhibit power conversion efficiencies up to 7.15%. Moreover, OPV devices on PET substrates with an alumina encapsulation and barrier adhesive show excellent mechanical flexibility.

fl exibility or light weight required for fl exible OPV. Atomic layer deposited (ALD) oxide thin-fi lms (e.g. SiO x , AlO x ) provide fl exibility and low WVTR rates at the same time. [22][23][24] This work is motivated by two factors: First, a lifetime study for cascade organic solar cells (CSCs) under ambient climate conditions has not been performed up to now, making the evaluation of their applicability diffi cult. Second, only a few publications show highly effi cient organic solar cells on silver nanowire electrodes. Beyond that, all devices exhibiting a PCE >5% on AgNW electrodes used polymer donors. [25][26][27] In this study, we show aging experiments on organic solar cells with the small molecules α-6T/SubNc/SubPc as cascade. [ 10 ] We compare rigid glass-glass encapsulated devices with ITO bottom electrodes to fully fl exible devices. AgNWs are utilized as transparent electrode, ALD thin-fi lms of alumina serve as fl exible ultra-barrier. The planarization of AgNWs is challenging, as they exhibit high surface roughness which usually causes the organic thin-fi lm device to shunt. Many different planarization techniques have been successfully employed using poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), metal oxides, or small molecules. [ 17,[28][29][30][31][32] Another method uses a peel-off process whereby the silver nanowires are buried in a polymer substrate. [ 25,[33][34][35] We employ the AgNW planarization concept recently demonstrated by Cui et al., [ 36 ] where the AgNWs are buried in the UV-curable optical adhesive "NOA63". This polymer serves as ultrathin and ultrafl exible substrate for the highly conductive AgNW network. Figure 1 shows the electrode fabrication and characterization by means of topographical and optical analysis. Silver nanowires with 35 nm diameter (NW35) are used to transfer the NOA63 into a conductive substrate (details can be found in the Experimental Section).Atomic force microscopy (AFM) measurements reveal that the root-mean-square roughness of the electrode is in the range of 1 to 2 nm (Figure 1 B). Furthermore, the strong phase contrast in Figure 1 C shows that AgNWs protrude from the NOA63 surface. This guarantees an electrical contact to adjacent conductive layers. A sheet resistance ( R S ) of (18.5 ± 0.2) Ω/sq is measured for this electrode even though the maximum processing temperature is only 80 °C. The electrode exhibits a total transmission of 84.5% at a wavelength of 550 nm which is comparable to NW35 on glass. However, plasmonic nanowire absorption, which is already present in the neat glass/NW35 electrode, broadens upon embedding in the polymer. This stronger plasmonic absorption in the spectral region between 350 and 500 nm reduces the average total transmission in the visible light spectrum T av to 82.8% as compared to 83.9% of NW35 on glass. Although ITO on glass exhibits a T av of 84.3%, its R S of 26 Ω/sq is higher than for the NW35 electrode.This ultrasmooth, highly conductive, and transparent AgNW electrode is well suitable for ...

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Improving the performance of Ag nanowire electrodes by adjusting the reaction conditions and the molar mass of PVP.