In situ electrical and thermal monitoring of printed electronics by two-photon mapping | Scientific Reports

Printed electronics is emerging as a new, large scale and cost effective technology that will be disruptive in fields such as energy harvesting, consumer electronics and medical sensors. The

performance of printed electronic devices relies principally on the carrier mobility and molecular packing of the polymer semiconductor material. Unfortunately, the analysis of such

materials is generally performed with destructive techniques, which are hard to make compatible with in situ measurements, and pose a great obstacle for the mass production of printed

electronics devices. A rapid, in situ, non-destructive and low-cost testing method is needed. In this study, we demonstrate that nonlinear optical microscopy is a promising technique to

achieve this goal. Using ultrashort laser pulses we stimulate two-photon absorption in a roll coated polymer semiconductor and map the resulting two-photon induced photoluminescence and

second harmonic response. We show that, in our experimental conditions, it is possible to relate the total amount of photoluminescence detected to important material properties such as the

charge carrier density and the molecular packing of the printed polymer material, all with a spatial resolution of 400 nm. Importantly, this technique can be extended to the real time

mapping of the polymer semiconductor film, even during the printing process, in which the high printing speed poses the need for equally high acquisition rates.

Non-linear optical characterization of a polymer semiconductor material roll coated on a flexible substrate. (a) Image of the flexible PET substrate with R2R printed source and drain silver

electrodes covered by P3HT polymer semiconductor stripes. (b) Schematic of the non-linear optical microscope. (c) Absorption spectrum of P3HT (blue curve), emission spectrum of P3HT (green

curve) and excitation laser spectrum (red coloured region). The green coloured region represents the amount of photoluminescence from P3HT detected in the experiment, considering that a 670 

nm short pass filter is used. The blue arrow represents the effective wavelength at which the two-photon absorption occurs.

Source and drain electrodes of the open transistor were roll-to-roll (R2R) flexo printed on PET foil at a web speed of 20 m min−1 using an anilox volume of 1.5 mL m2. The water-based silver

(Ag) nanoparticle ink (PChem PFI-722) was dried and sintered using hot-air (140 °C) and infrared lamps during printing. Further surface treatment, without major effect on the conductivity,

was carried out using photonic flashlight sintering (Xenon Sinteron 2000). P3HT was annealed at 60 °C for 10 min and a dielectric capping layer was annealed at 120 °C for 30 min. Both layers

were slot-die coated on a mini roll coater to simulate R2R environment. All processes were carried out at ambient conditions.

The two different curves in the graph of Fig. 2c correspond to the TPPL emission from P3HT acquired with and without the application of a current between source and drain. The transistor

supported current is applied by connecting a power supply to the electrodes with a fixed voltage of 1 V. In a semiconductor, this has the effect of rapidly separating the photo-created

charges (electron and holes), which no longer can recombine to produce photoluminescence. In the experiment indeed, upon application of a current the TPPL is highly quenched, as shown in the

graph Fig. 2c. The integrated TPPL signal, in the case of an applied current, is ~4 times lower than the one measured for zero current and is obtained for an incident power 10 times higher.

Considering that the TPPL scales as the square of the excitation power, we can estimate a decrease in the TPPL intensity of a factor of ~400 upon application of a current. The current

ON/OFF experiment demonstrates that carrier density and TPPL efficiently can be related in an in situ measurement. This confirms that the device works electrically and that our nonlinear

optical characterization is well suited to test the electrical properties of printed electronics devices.

We show here that, by introducing an experimental way to in situ vary and measure the temperature of the semiconductor, we can deterministically change its morphology in terms of molecular

packing and hence its charge carrier density. This in turn strongly influences both the supported current and the TPPL signal. The temperature of the open transistor is varied in the range

20–120 °C with a Peltier cell controlled in current and tuned by a Fluke thermal imager Ti 125 with an accuracy of ±2 °C +2% of the temperature in °C. For every applied temperature, we

acquire two-dimensional TPPL images of the sample. Examples of images, corresponding to temperatures of 30, 70 and 110 °C, are shown in Fig. 3a–c. The data for the three images is normalized

to the maximum of the 110 °C image. It is apparent that the TPPL signal increases for higher temperatures, which, for the colour scale used in Fig. 3a–c, results in brighter colours in the

optical images. We note here that, when changing the temperature of the sample, as a consequence of thermal dilatation, the focus of the microscope objective has to be slightly adjusted to

maximize the collected optical signal. In the graph of Fig. 3d we report the spatially integrated intensity from the two-dimensional optical images for every temperature and compare it to

the measured supported current characteristic. The behaviour of the two quantities is very similar, which strongly suggests that the TPPL intensity, for this material and in the current

experimental conditions, can be used as an indicator of the charge carrier density and molecular packing of the printed material. Moreover, the behaviour of the TPPL as a function of

temperature is reversible, as for the supported current: when decreasing the temperature form 110 °C to room temperature, the TPPL decreases to its minimum value, as demonstrated in Fig. 3d.

Temperature evolution of the two photon photoluminescence. (a–c) TPPL spatial map of P3HT at different temperatures, with a diffraction limited resolution (~400 nm). (d) Temperature

characteristic of the two-photon photoluminescence for increasing temperature (blue circles) and decreasing temperature (blue stars) and of the supported current (green curve) in the polymer

semiconductor transistor. The error bar in the measurement of the TPPL, obtained as the sum in quadrature of the error of each pixel in the spatial images (calculated as √N, where N is the

TPPL signal at each pixel), is always smaller than the size of the symbol used.

In conclusion, we have demonstrated that nonlinear optical microscopy is a powerful, versatile and non-destructive method to easily characterize printed electronics devices, with a spatial

resolution down to 400 nm. In our case, the level of TPPL can be used as an indicator of molecular packing. This might be extended to other materials whose optical properties depend on the

molecular packing, even if new experiments adjusted for each case will be necessary. We have shown that the different nonlinear signals generated by different components in the device can be

used to discriminate between semiconductor and NPs. Subsequently, we have demonstrated that the TPPL emitted by the active material can be quenched by the presence of an applied current

between source and drain. This is fundamental for in situ measurements where real devices can be electro-optically characterized with high spatial resolution. Most importantly, we have

provided evidence for a relation between the molecular packing of the active material, carrier density and the TPPL intensity. By studying the temperature dependence of the TPPL and

comparing it with the supported current in the device, we have shown that at 90 °C the semiconductor is characterized by a better interchain overlap and therefore higher mobility. This means

that nonlinear optical microscopy is able to extract fundamental information about printed electronics devices, such as free charges and NPs contents and, when combined with electrical

measurements, mobility and molecular interchain overlap in situ and in a completely non-destructive way. These information will allow printed electronics to emerge as a new, large scale and

cost effective technology that will be disruptive in fields such as energy harvesting, consumer electronics and medical sensors.

This work is part of F.P.’s project who has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no.

659747. N.F.v.H. acknowledges the financial support by the European Commission (ERC Adv. Grant 670949-LightNet); MINECO Severo Ochoa Programme SEV-2015-0522; CERCA Programme of

Generalitat de Catalunya; and Fundació CELLEX (Barcelona).

Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000, Roskilde, Denmark

ICFO - The Institute of Photonic Sciences, The Barcelona Institute of Science and Technology, 08860, Castelldefels (Barcelona), Spain

ICREA - Institució Catalana de Recerca i Estudis Avançats, 08010, Barcelona, Spain

F.P. conceived the experiments and fabricated the samples. M.J. and F.C.K. printed the electrode and contribute to the materials design. N.F.v.H. contributed to the analysis of the optical

characterization. N.A. and F.P. carried out the experiments, performed the data analysis and made all the figures. They wrote the manuscript with the participation of all authors.

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