reference single-crystal silicon solar cell. All measurements were performed using a mask to confine the active area size to 1 cm2. During the exposure the current was measured in the voltage range
from —0.5e1.5 V using steps of 20 mV. External quantum effi- ciencies were recorded with an Enli Technology QE-R, which was also calibrated with a standard single-crystal silicon cell, between 300 nm and 900 nm using a step size of 10 nm. The transmittance data were acquired with a Lambda 950 from PerkinElmer. Trans- mission spectra were recorded between 300 and 800 nm with a step size of 1 nm using an integrating sphere. Sheet resistance measurements were performed with a four point probe R-Check from EDTM. A Confocal microscope msurf custom from NanoFocus as well as a scanning electron microscope Carl Zeiss Ultra Plus were used to image the AgNW layers.
3. Results and discussion
3.1. Semitransparent silver nanowire electrodes
An alcohol based ink formulation was developed for inkjet printing of AgNW meshes. The layer processing was done with a research printer (PiXDRO LP 50) using Spectra SE 128 industrial print heads with 35 mm nozzle diameter. The ink is based on iso- propanol dispersed nanowires from Cambrios (developmental ClearOhm ink-Y) diluted with 1-pentanol in a volume ratio of 1:2. Using this high boiling point alcohol, print head clogging due to evaporation of ink at the printing nozzles is inhibited and a stable printing process is achieved. Simulations of the drying behavior of modified and unmodified ink (Supporting Information Fig. S1)
suggest a sevenfold increase in drying time resulting from 1- pentanol addition. The length of the AgNWs is in the range of 10's of mm, which demonstrates that nanowires with similar di- mensions as the jetting nozzles can be printed.
With the alcohol based AgNW inkjet formulation, layers were printed on glass and optimized to reach sheet resistances below 20 U/,. These values were achieved by repeating the printing process four times. Furthermore, it was found that the sheet resistance parallel to the scanning direction of the printhead is only
¾ of that perpendicular to it (500 DPI printing resolution in both directions), independently of the layer thickness (Table 1). The difference can be attributed to the promotion of AgNW distribution along the printing direction. Fig. S2 demonstrates such distribution under optical microscope. Confocal images of the AgNW electrodes on glass show very homogeneous films with no visible anisotropy at the given scale (Fig. 1a). Length and diameter of the AgNWs as determined from scanning electron microscopy (SEM, Fig. 1b) are in the range specified by the manufacturer Cambrios. Obviously, the jetting process does not induce fracture of the nanowires, probably due to their flexibility or flow induced alignment [20].
The optical properties of the AgNW electrodes were determined by UV/Vis transmission measurements (Fig. 1). According to ISO 9005:2003, which takes the spectral sensitivity of the human eye into account, the transmittance of the AgNW electrode (trans- mittance of glass deducted) after four printing passes is about 94%. The haze of the AgNW electrode was calculated by dividing the diffuse transmission of the sample by the total transmission. Considering the sensitivity of the human eye, a mean value of 2.3% is obtained. The correlation between sheet resistance and optical