All-printed diode operating at 1.6 GHzNegar Sania, Mats Robertssona, Philip Cooperb, Xin Wangc, Magnus Svenssonc, Peter Andersson Ersmanc,Petronella Norbergc, Marie Nilssonc, David Nilssonc, Xianjie Liud, Hjalmar Hesselbome, Laurent Akessob, Mats Fahlmand,Xavier Crispina, Isak Engquista, Magnus Berggrena,c,1, and Göran Gustafssonc,1

aDepartment of Science and Technology, Linköping University, SE-601 74 Norrköping, Sweden; bResearch and Development, De La Rue plc, Overton,Hampshire RG25 3SE, United Kingdom; cPrinted Electronics, Acreo AB, SE-602 22 Norrköping, Sweden; dDepartment of Physics, Chemistry and Biology,Linköping University, SE-581 83 Linköping, Sweden; and eHesselbom Innovation and Development HB, SE-14138 Huddinge, Sweden

Edited by Tobin J. Marks, Northwestern University, Evanston, IL, and approved June 13, 2014 (received for review January 28, 2014)

Printed electronics are considered for wireless electronic tags andsensors within the future Internet-of-things (IoT) concept. Asa consequence of the low charge carrier mobility of presentprintable organic and inorganic semiconductors, the operationalfrequency of printed rectifiers is not high enough to enable directcommunication and powering between mobile phones andprinted e-tags. Here, we report an all-printed diode operating upto 1.6 GHz. The device, based on two stacked layers of Si and NbSi2particles, is manufactured on a flexible substrate at low tempera-ture and in ambient atmosphere. The high charge carrier mobilityof the Si microparticles allows device operation to occur in thecharge injection-limited regime. The asymmetry of the oxidelayers in the resulting device stack leads to rectification of tunnel-ing current. Printed diodes were combined with antennas andelectrochromic displays to form an all-printed e-tag. The harvestedsignal from a Global System for Mobile Communications mobilephone was used to update the display. Our findings demonstratea new communication pathway for printed electronics within IoTapplications.

UHF | silicon particle

Printed electronics is the use of conventional printing techni-ques for patterning materials (semiconductors, conductors,

insulators) and manufacturing electronic components onto awide range of flexible and organic substrates, such as paper,plastic foils, and labels (1). A vision for printed electronics is toprovide an electronic label (e-label) for the Internet of things(IoT). For many targeted IoT applications, it would be of greatimportance if a smartphone operating in the ultra–high-frequency(UHF) band (300 MHz to 3 GHz) could communicate directlywith, or even power up, all-printed e-labels, and thereby serve asa communication base with e-labels as the outposts of the In-ternet. In pursuit of this goal, it is desirable to find low-cost andlow-temperature manufacturing processes for UHF electronicsto be compatible with organic carriers that are sensitive to hightemperature.One of the key electronic devices is the diode, which is widely

used in a variety of circuits such as rectifiers, voltage multipliers,and charge pumps, for applications such as AM radio and othertypes of receivers (2), and for energy harvesting circuits to extractenergy from radiofrequency (RF) electromagnetic waves (3, 4).Printed circuits and devices such as diodes, capable of operationin the UHF band, will be a cornerstone in interfacing mobiletelephony and printed functional objects, i.e., a “thing” inthe IoT.Until now, various diodes and transistors have been printed

using either organic (5–10) or inorganic (11–24) semiconductors.Organic semiconductors are promising candidates for printedelectronics due to their solution processability, which enablesprinting manufacturing of devices and circuits in an all-in-lineproduction scheme. The charge carrier mobility of organicsemiconductor thin films is directly related to the resulting op-erating frequency of the diode (25). High charge carrier mobilityup to (10) 9.0 cm2·V−1·s−1 has been obtained for highly crystallineorganic semiconductor films created via a vacuum deposition

technique. Following the demonstration of a pentacene-basedrectifier operating in the high-frequency range (∼50 MHz) (25),only a few studies have reported on the potential of organicdiodes for UHF applications. However, all of these requirevacuum deposition techniques: organic PIN diode with a cutofffrequency of 100 MHz (6), C60-metal oxide diode with a cutofffrequency of 800 MHz (5), and pentacene diode operating up to870 MHz (9). Charge carrier mobility values above 1 cm2·V−1·s−1 arechallenging to achieve via solution processing techniques. Utili-zation of printing techniques for the fabrication of organicdiodes leads to many technical challenges including the loss ofmolecular order that is a prerequisite for high mobility. State-of-the-art printed organic diodes operate below 10 MHz (7).In case of inorganic materials, the cutoff frequency depends

on the mobility and the concentration (11) of the charge carriers.Thus, another strategy for printed UHF electronics would be toconsider conventional inorganic semiconductors comprising highmobility and carrier concentration and modify the processingmethod to make it compatible with low-temperature processingand traditional printing techniques (13). One way to achieve thisis to use nanoparticle semiconductor inks, a route that wassuccessfully used to demonstrate fully printed rectennas, how-ever presently limited to an operational frequency of 13.56 MHz(16). With advances in chemistry and nanotechnology, solution-processed inorganic semiconductors are now available (23, 24).Using such materials, solution-processed films can be preparedthrough coating or printing processes. However, to improve theconductivity they usually need drastic or even extreme thermal

Significance

Printed electronic labels and stickers are expected to definefuture outposts of the communication web, as remote sensors,detectors, and as surveillance technology, within the Internet-of-things concept. It is crucial to couple such technology withstandard communication systems that commonly operate atgigahertz frequencies. To accomplish this, ultra–high-frequencyrectification components manufactured in a low-temperatureprinting process are necessary. Here, we report an all-printeddiode operating above 1 GHz, achieved using a combination ofSi and NbSi2 microparticles. The diode was integrated witha flexible antenna and a printed electrochromic display in-dicator to successfully demonstrate remote transfer of signaland power from a standard Global System for Mobile Com-munications phone to the resulting e-label.

Author contributions: N.S., M.R., P.C., X.W., M.S., P.A.E., P.N., D.N., H.H., L.A., X.C., I.E., M.B.,and G.G. designed research; N.S., M.R., X.W., M.S., P.A.E., M.N., D.N., X.L., and M.F. per-formed research; X.W. prepared the Si and NbSi2 powders; M.S., P.N., X.C., and I.E. super-vised part of the project; M.B. and G.G. supervised the project; N.S., X.L., M.F., X.C., and I.E.analyzed data; and N.S., X.C., and I.E. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

See Commentary on page 11917.1To whom correspondence may be addressed. Email: magnus.berggren@liu.se or Goran.Gustafsson@acreo.se.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1401676111/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1401676111 PNAS | August 19, 2014 | vol. 111 | no. 33 | 11943–11948

APP

LIED

PHYS

ICAL

SCIENCE

SSE

ECO

MMEN

TARY

Dow

nloa

ded

by g

uest

on

May

30,

202

0

treatments (high-temperature sintering or laser annealing),which are not compatible with flexible substrates such as poly(ethylene terephthalate) (PET) and paper (12, 14, 15, 17–19, 26).One solution suggested for dealing with this problem is to fab-ricate the device on a high-temperature tolerant substrate firstand then transfer it onto a flexible substrate. PIN diodes witha cutoff frequency of 1.9 GHz (22) and transistors with a cutofffrequency of 2.04 GHz (21) on flexible substrates have beendeveloped using this method. However, this technique involvesseveral steps, some that are certainly too complicated and timeconsuming for being feasible and cost-compatible with industrialmass production processes. However, another recent strategyinvolving inorganic semiconductors is a low-temperature aque-ous route for fabrication of thin-film transistors on plastic,yielding an average mobility of 2.6 cm2·V−1·s−1 (20). One of themajor challenges remaining here is to control the doping level vialow-temperature chemical growth.To the best of our knowledge, all-printed UHF devices, such

as transistors and diodes, achieved on flexible substrates, have sofar not been demonstrated. With such a technology at hand,direct communication and powering between a cellular phoneand printed e-labels would become possible.Here, we use silicon (Si) microparticles (μPs) in an organic

binder as a screen-printable semiconductor composite, which issandwiched between electrodes to form a diode. This allows us todirectly profit from the combination of the high charge carriermobility of the Si-crystal bulk (up to 1,400 cm2·V−1·s−1 forundoped Si), and the ability to use conventional printing pro-cesses. As a consequence, the printed Si-diode on a flexiblesubstrate operates up to 1.6 GHz, i.e., within the UHF range.In fact, the energy harvested from the antenna of a mobile phoneworking in the Global System for Mobile Communications (GSM)band, while making a call, can be rectified using our printedSi-diode to power a printed electrochromic polymer display. Thisdemonstrates the first (to our knowledge) direct communicationbetween an all-printed e-label and a cellular phone.

Results and DiscussionTo successfully use the printed diode, based on Si μPs, in futureIoT applications, a simple and high-throughput manufacturingprocess is needed. This forces us to consider well-establishedproduction techniques that makes use of manufacturing stepsperformed at low temperatures, in the ambience of a traditionalprinting house and that are compatible with common flexibleplastic and paper substrates. Moreover, for communication atvery high frequencies for power and signal transfer, the Si-diodemust possess a very fast response to a voltage pulse while a largecurrent rectification ratio is not as crucial as it would be inapplications such as active matrixes. Hence, tunnel diodes andSchottky diodes are more appropriate than p-n diodes, becauseof their advantages in high-speed operation (27, 28).We used ball milling to crush a Si wafer into Si particles in air.

Due to air exposure, the particles form a natural oxide shelllayer. As long as the thickness of this insulating shell layer in ametal–insulator–semiconductor (MIS) diode is less than 50 Å,the device should behave as a Schottky diode (28). Films fabri-cated from the Si μPs in an organic binder were studied using X-ray photoelectron spectroscopy. The Si 2p core level was probedto obtain an estimate of the thickness of the SiO2 outer shellcovering the Si μPs, using the relative intensities of the Si 2pfeatures (pure silicon, low binding energy peak) and silicon oxide(high binding energy peak), (Fig. 1A). The part of the Si 2pspectrum related to pure silicon showed an asymmetry (smallshoulder) on the low binding energy side and lacked the expec-ted distinct spin-orbit splitting, which we attribute to chargingeffects because not all of the Si particles in the film belong toa percolated network and thus will not maintain charge neu-tralization during the photoemission experiment. The Si 2pfeature belonging to pure silicon was hence fitted (using a Shirleybackground) with two peaks and the silicon oxide feature withonly one. The latter is the standard approach even for crystalline

silicon films (29). Assuming random order of the Si μPs in thefilm, we can exclude photoelectron diffraction effects and theoxide thickness doxy is then equal to 1.27 nm, following theprocedure in ref. 29 and using a photoelectron effective atten-uation length λoxy = 2.96 nm for the native silicon oxide.When considering the size of the Si particles, it is important to

remember that the electron mobility decreases considerably withthe size of the Si crystalline domains (1) because the wavefunction of the electron scatters at interfaces. Hence, the Si μPsare integrated in a polymer binder (SU8) to achieve a layer ofless than 5 μm in thickness and ensure that the electronic chargecarrier path between the two electrodes, sandwiching the Silayer, will pass through only one or just a few individual Si μPs.The measured size distribution of the Si particles is given inFig. 1B. To localize the current limitation of the diode to the Siparticle/electrode interfaces, we choose n-doped crystalline Si,with a doping level of ∼1018 cm−3, due to its high mobility andconductivity characteristics. The Si μPs are introduced along thesurface of the SU8 precursor layer.In the most simplified version, a diode device would be com-

posed of n-Si μPs sandwiched between two electrodes (Fig. 2 A,I). Compared with conventional MIS tunnel or Schottky diode,the geometry proposed is equivalent to two front-to-front con-nected MIS diodes. If the two MIS diodes are identical, very lowcurrent should pass through the structure in either forward orreverse bias. This is the case for a device composed of Si par-ticles, with their natural oxide (thickness, ∼12.7 Å), in SU8sandwiched between two aluminum electrodes (Fig. 2 A, I). Themeasured I–V characteristics (Fig. 2B) for this device show a lowcurrent level of around 10−7 A and practically no current recti-fication. To introduce current rectification (asymmetric devicestructure) in the I–V curve and to increase the current density,we replace the top Al/Al2O3 electrode with an oxide-free con-ductor. As a first candidate, we include conducting carbon that iseasily printable in the form of a screen-printing paste (Fig. 2 A,II). The top MIS contact diode includes only the thin siliconoxide layer (thickness, ∼12.7 Å), whereas the bottom contactdiode combines both the silicon and the aluminum oxide layers[thickness, 10–30 Å (30, 31)], coupled in series. However, be-cause the carbon paste can penetrate through cracks in theSi-in-SU8 layer, creating short circuits, the yield of thismanufacturing process is rather low (31%). Devices with highleakage currents, with charges running along the penetratingcarbon contact directly to the aluminum bottom contact, all havea high current density (>10−4 A) and an ohmic current vs.voltage behavior without any rectification (dashed curve, Fig.2B). To suppress the possibility for the carbon top contact topenetrate the Si μP-in-SU8 layer, we added a second similar

0

0.25

0.5

0.751

0 2 4 6Diameter (um)

Num

ber o

f par

ticle

s (a

rb. u

nits

)B

Inte

nsity

(arb

. uni

ts)

108 106 104 102 100 98 96Binding Energy (eV)

A

Fig. 1. Characterization of Si μPs. (A) XPS measurements from the topsurface of the Si/SU8 layer. The low binding energy peak represents pure Si,whereas the high-energy feature represents the SiO2. By comparing therelative intensities of these two peaks and taking the photoelectron atten-uation length into the account, we estimate the oxide thickness doxy to 1.27nm. (B) The size distribution of the Si μPs, measured using image analysisapproach using SEM images from particles.

11944 | www.pnas.org/cgi/doi/10.1073/pnas.1401676111 Sani et al.

Dow

nloa

ded

by g

uest

on

May

30,

202

0

layer on top of the first layer after cross-linking. With this thickerSi μP-in-SU8 layer, short circuit effects were significantly re-duced as indicated by the lower current level (III in Fig. 2B), butunfortunately the current density and rectification ratio are bothtoo low for practical applications. We conclude from theseexperiments that the required additional layer should be anoxide-free conductor that is printable on top of the first Si-in-SU8layer. Hence, we turn our attention to metal silicide, a family ofhighly conducting materials that are known to be resistant tooxidation (32, 33). We ground NbSi2 powder into microparticles,targeting a similar average μP size as for the Si μPs (Fig. S1), andthen printed a NbSi2-in-SU8 layer (less than 5 μm) on top of theSi-in-SU8 layer. The device was then finalized by printing a car-bon paste top electrode (Fig. 2 A, IV). Only 6% of these devicesare short circuited, 50% of them have a rectification ratio lowerthan 10. Hence, 44% of the carbon:NbSi2-in-SU8: Si-in-SU8:Aldiodes have a rectification >10, and of those, all devices showa cutoff frequency above 0.4 GHz. For a statistical comparisonbetween the different structures shown in Fig. 2 A, II–IV, batchesof 16 devices were manufactured for each of them. It is clearfrom these statistics that structure IV has the best yield andperformance (for a summary, see Table S1). The rectification ismeasured at 1 V; the amplitude required for demonstrating theconcept of an e-label in this paper.The cross-section of the final structure of the device is sche-

matically illustrated in Fig. 3A. The fabrication process startswith screen printing of dry Si particles on top of an inkjet-printedSU8 layer deposited on an Al substrate. The particles are thenpressed down with a laminating machine to contact the Al sub-strate through the binder. The dry NbSi2 particles are screen-printed via a similar process. The SU8 binder is cross-linked atthe end of each step. To finalize the diode, the carbon and Aginks are subsequently screen-printed and cured. The reason foradding the carbon contact is to form a contact for the NbSi2particles, whereas the extra Ag layer is printed to further in-crease the conductivity of the top contact. An SEM cross-sectionof the resulting device together with a top view is provided inFig. 3 B–D.The average I–V curve based upon 16 representative samples

is shown in Fig. 4A. A threshold current of 3 μA is defined as thelimit between on and off states of the diode because this is theminimum current needed for switching an electrochromic displayin a reasonably short time (tens of seconds). According to thisdefinition, the average turn-on voltage is about 1 V. The randomdistribution of particle sizes and distances and consequently theinsulator thickness in the path of charges will result in somevariation of the device characteristics, yielding a range of turn onvoltages for different devices and a very low current amplitudefor a percentage of the fabricated devices (that are discarded).The surface area of the samples ranges from 2 × 104 to 8 × 104

μm2. We report the resulting current amplitude rather than thecurrent density, because the current in the device is not homo-genously distributed over the whole surface area of the contactsand the μP-in-SU8 layers but rather follows a current filamentdynamics. Therefore, the current level does not scale linearlywith the surface area, at least not for the small variation of theactive device areas we use in our experiments (see further Fig.S2). By increasing the area of the diodes, the risk for short cir-cuits due to the cracks or pinholes of the binder increases andalso the operational bandwidth of the device decreases becausethe value of the parasitic capacitance is directly related to thesurface area.The average frequency response of a typical sample measured

by applying a 3-W input signal is shown in Fig. 4B. The estimatedcutoff frequency (half power point) of the device is about1.6 GHz.After discarding devices that exhibit low current character-

istics or a poor current rectification ratio, 77% of the remainingsamples show a cutoff frequency higher than 1 GHz, and 33%exhibit a cutoff frequency of around 1.6 GHz. DC performancemeasurements have been repeated after 2 y on a batch of 30devices. At 2 V, where all of the devices are certainly in “on”stage, 60% of the samples retain more than half of their initialcurrent and 64% of them retain more than half of their initialrectification ratio. To investigate the reliability of the devicesfurther, one of the devices was left running with 1-GHz input for1 h, which is 3.6 × 1012 cycles. No decrease of the output signalwas detected. Considering that these devices were not covered orencapsulated, the above data indicate a high reliability.As shown above, the operational frequency of our printed

diode reaches 1.6 GHz. The output signal of a mobile phoneworking in the GSM band, captured by a receiver antenna, couldthus in principle be rectified by our diode to produce a DCvoltage, which can be used to power printed electronics, suchas an electrochromic display indicator. To evaluate our printeddiode in such an application, a simple integrated circuit wasmanufactured. The fully printed circuit was manufactured on aPET label substrate and includes an Al-foil antenna, our diode,and an electrochromic polymer (EC) display (34), based on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (Fig. 5). Theantenna is designed to capture the electromagnetic signal ofa mobile phone working within the GSM band. The signaltransmitted from the mobile phone, while making a call, istransferred to the antenna of the label and then rectified by thediode and used to power the switching of the EC display. The

A B

Fig. 2. Examining different device architectures. (A) Different device struc-tures tested before designing the final structure of the device. (B) The I–Vcurves of the different device structures.

Al

Si

NbSi2

Carbon

Ag

50nm

4 um

4 um

6 um

3 um

PET

Carbon

Al

Al

A B

C D

2 μm Mag = 8.7 Kx

Si and NbSi 2

Carbon

Ag

Al Al

1 μm

Ag

Fig. 3. Device structure. (A) Schematic cross-section of the device. (B) TheSEM image of the cross-section of the device where all of the layers areclearly visible but the Al substrate. The Inset shows a cross-section of anotherdevice where the Al layer is visible. (C and D) Top view of the device.

Sani et al. PNAS | August 19, 2014 | vol. 111 | no. 33 | 11945

APP

LIED

PHYS

ICAL

SCIENCE

SSE

ECO

MMEN

TARY

Dow

nloa

ded

by g

uest

on

May

30,

202

0

maximum power produced by the mobile phone reaches 2 W, butonly a fraction of the emitted energy can be forwarded to thediode due to the path loss in wireless signal transfer. Despite this,the diode rectifies the signal and produces a DC voltage enoughto switch the EC display. DC measurements show an averagecurrent of 19 μA at 2-V forward bias, and a rectification factor ofabout 100 at 1 V. The cutoff frequency of our diodes is around1.6 GHz, and the transmission frequency of the GSM phone islocated at 1.8 GHz. Despite this, there is still enough energytransmitted to the antenna and rectified by the printed diode toenable a swift switch of the printed EC display element. At op-timum conditions, it takes less than 10 s to switch the color of theEC display indicator from transparent (Fig. 5A) to dark blue(Fig. 5C). The full switching of the printed electrochromic dis-play can be viewed in Movie S1.The semilogarithmic I–V characteristics of the device, recor-

ded at temperatures ranging from 150 to 325 K, are displayed inFig. 6A. The UHF operation indicates that the rectificationmechanism, more specifically the enhancement in current above0.5 V in forward bias (Fig. 6A) is due to a very fast charge in-jection or displacement process, possibly tunneling (35). Notethat the current below 0.7 V in forward bias and at all voltages inreverse bias is strongly dependent on the temperature. Con-versely, above 0.7 V in forward bias, the current level is higherand is almost entirely temperature independent (Fig. 6A). This isa feature that is characteristic for Fowler–Nordheim (FN) tun-neling current (28).Because the oxide thickness between the Si particles and

NbSi2 particles is thinner than the one between the Si particlesand the Al electrode, we believe that current is primarily limitedby the energetics of the Al–Si contact. This kind of MIS stack hasbeen studied theoretically and experimentally with a precisecontrol over the growth process and layer thicknesses in the past(36–43). However, the numerical values calculated for differentparameters in previous studies cannot be directly used formodeling and comparison here, because our device has a muchdifferent structure that includes particles with random shapes,oxide layers with inhomogeneous properties and the organicbinder. Although all of the theory regarding modeling of semi-conductor devices is based on planar contacts, we believe withsome simplifications the behavior of the device can be approxi-mated using classic semiconductor models. Due to the presenceof surface states at the Si–SiO2 interface, the Fermi level of Siis pinned and the barrier height cannot be calculated from thedifference between the work function of the metal and theelectron affinity of Si [as it would be in case of an intimatecontact between metal and semiconductor (28)]. A schematic

illustration of the band diagram of the Al–Si contact is given inthe Inset of Fig. 6A incorporating values for the Si/SiO2 andAl/Al2O3 barrier heights (3.1 and 2.8 eV, respectively) (44,45). Applying a positive bias to Al lowers EF of the metalcompared with Si and thus bends the semiconductor bandsdownward, causing the barrier height for the charges to de-crease. If the thickness of the insulator layer is below 50 Å,direct tunneling may occur. The current in this case wouldfollow the thermionic emission model multiplied by the tun-neling probability factor. We assume that direct tunnelingoccurs at low voltages at the contact area between the Siparticles and the soft Al film primarily across thin oxide pathsbecause none of the other tunneling mechanisms can producethe current levels as high as we obtain in the device with thisrange of voltage. At higher voltages, the current can also passthrough insulating regions with slightly larger Si–Al distance,as illustrated in the Inset of Fig. 4A, via other tunnelingmechanisms, mainly FN and Frenkel–Poole (FP) (46, 47).Moreover, in addition to the surface states at the interfacebetween the semiconductor and the insulator, there are othertrap states in the insulator layers and at the interface betweenthe insulators that can affect the current. However, based onthe fact that the current is temperature independent at highvoltages, we believe that FN tunneling is the dominatingmechanism. Therefore, a simplistic model based on directtunneling at low voltages plus an FN tunneling that is addedup at high voltages is presented here. Although other mech-anisms such as FP tunneling can certainly be responsible forpart of the current, especially in the intermediate voltagerange, we refrain from attempting a model including all pos-sible current mechanisms because the number of variables wouldbe too large.The current vs. voltage evolution for FN tunneling can be

expressed as follows (28, 48):

I =C1V 2exp�−C2

V

�; [1]

where C1 and C2 are functions of the electron effective mass andthe tunneling barrier. The Schottky model, based on the therm-ionic emission theory (28, 49), reproduces very well the experi-mental data for the low-voltage region where FN current isnegligible (the dashed line in Fig. 6 B and C), but it deviatesfrom the data as the voltage increases. A combination of therm-ionic emission and FN current (the dotted line in Fig. 6 B and C)reproduces our measured I–V evolution at high voltages.In the two-tunneling transport mechanism model, the total

current–voltage relation of the device can be expressed as thelinear combination of the tunnel assisted thermionic current (49,50) (first term) and FN tunneling current (second term):

-2 -1 0 1 2-2

0

2

4

6

8

10

12

14

16

18

20

Cur

rent

(μA

)

Voltage (V)

A

0,0 0,5 1,0 1,5 2,0 2,50,0

0,5

1,0

1,5

2,0

Out

put D

C v

olta

ge (V

)Frequency (GHz)

fc

B

Fig. 4. DC characteristics and frequency response. (A) The average I–V curvebased upon 16 representative devices. The error bars show the SD andthus represent the voltage range needed to obtain a certain current level.Using a threshold of 3 μA, which we have defined as the on/off limit forthe device, an average input voltage of 1 V is needed to switch the diodeon. The Inset shows two different current mechanisms for different in-sulator thicknesses. (B) Frequency response of the device with an inputpower of 3 W.

Fig. 5. Demonstration of e-label application (Movie S1). (A) The antenna–diode–display circuit. (B) The mobile phone is held close to the antennawhile making a call. The display starts to switch on. (C) The display isswitched on after 10 s.

11946 | www.pnas.org/cgi/doi/10.1073/pnas.1401676111 Sani et al.

Dow

nloa

ded

by g

uest

on

May

30,

202

0

I = α1TtI0exp�qðV − IRsÞ

nkT

��1− exp

�−qðV − IRsÞ

kT

��

+ α2C1V 2exp�−C2

V

�;

[2]

where α1 and α2 are constants representing the relative contri-bution of each tunneling mechanism to the total current and I0 isthe reverse saturation current, which is given by the following:

I0 = AApT2exp�−qΦb

kT

�; [3]

where Φb is the barrier height, A is the surface area, and A* is theeffective Richardson’s constant. This model fully reproduces thetemperature evolution of the I–V characteristics in forward bias(for plots, see Fig. S3).The tunnel-assisted thermionic forward bias current of a non-

ideal diode is characterized by the series resistance Rs and theideality factor n (k is the Boltzmann’s constant, q is the electroniccharge). In our case, this current should be multiplied by a tun-neling probability factor Tt. To extract the diode parameters,we started with determining the thermionic emission currentparameters at the low-voltage range, where the FN tunnelingcurrent is negligible. The procedure to extract those parametersis explained and outlined in detail in SI Text. The values of n, Rs,and Φb at room temperature are found to be 2.9, 0.3 MΩ, and0.21 eV, respectively. The coefficient of the thermionic emissionpart of the current, TtAA* is calculated to be 3.88 × 10−12 AK−2.Both barrier height and ideality factor are temperature de-pendent. As the temperature increases from 150 to 325 K, theideality factor decreases from 4.4 to 2.8 and the barrier heightincreases from 0.15 to 0.22 eV. Typically, a low barrier height is

expected in an Al/n-Si contact and the high values of the idealityfactor are expected due to the existence of the oxide layer (51).In summary, we demonstrated a printed diode with a vertical

structure operating in the UHF band. DC measurements showan average current of 19 μA at 2-V forward bias, and a rectificationfactor of about 100 at 1 V. The lifetime of the device is more than2 y. According to the measured frequency response of the diodes,their cutoff frequency is around 1.6 GHz, but they still have anoutput high enough to switch a printed organic display at the GSMfrequency 1.8 GHz. These diodes can be used in a simple circuit torectify an AC signal and convert it to DC, and because they canoperate at GSM frequencies, it is possible to simply use the signalof a mobile phone as the AC input signal source. The diodetogether with an antenna can be used to switch a printed organicdisplay using the signal of an ordinary mobile phone.

Materials and MethodsThe silicon powder is prepared using a Sb-doped silicon wafer with a resistivityof 0.01–0.02 Ω·cm. First, a 4-inch wafer is broken into small pieces usinga hammer. Then the parts are ball milled at 300 rpm for 16 h in RETSCH PM100 Planetary Ball Mill. The largest particles are separated away from thepowder dispersed in acetone by gravity sedimentation in a cylinder tube. Afterthis step, more acetone is added, and the dispersion is centrifuged with MSEMistral at 130 × g for 10 min. The suspension that contains the smallest par-ticles is discarded, and the bottom sedimentation is collected. The average sizeof the particles in the resulting powder is about 0.7 μm, but also includesparticles with a size of up to 5 μm and lots of small particles below 0.5 μm; thesmaller particles probably do not contribute to the current in the device butneither will cause any major performance degradation.

For fractioning of the niobium silicide powder, first 20 g of niobium silicide(99.85% metals basis) powder is mixed with 8 g of cyclohexanone and ballmilled for 16 h in RETSCH PM 100 Planetary Ball Mill. Afterward, gravitationalsedimentation and centrifugation are used to separate the smallest particlesaway from the rest of the powder having a targeted average size similar tothe silicon μPs.

The device is printed on top of an Al electrode on a PET substrate andconnected to another Al electrode next to the first one via conductive carbonpaste. The aluminum substrate pattern is prepared by photolithography froma 50-nm-thick aluminum evaporated on PET foil. The photoresist SU8 2010is diluted with isopropanol to 15% (wt/wt) and screen printed on the alu-minum electrode as a binder holding the silicon particles. The silicon par-ticles are then screen printed on top of the SU8 layer. The particles arepressed using a lamination machine with a pressure of 3 bar to make anintimate contact with the aluminum layer. The sample is then exposed to UVlight for 1 min and heated at 95 °C for 2 min to solidify the SU8. A layer ofNbSi2 particles with an average diameter size of 0.7 μm is screen printedonto a second layer of SU8 on top of the first Si layer in the same manner. Tofinalize the device, carbon paste is screen printed on top of the NbSi2-in-SU8layer to provide a bridge to an aluminum contact pad used for probing.After curing the samples at 105 °C for 5 min, a layer of Ag paste is screenprinted on top of the carbon to further decrease the impedance of thebridge. The structure of the final device is illustrated in Fig. 3 C and D. Thesurface area of the printed Si layer is 2 × 104 to 8 × 104 μm2.

DC measurements of diode performance are made using an Agilent 4155BSemiconductor Parameter Analyzer. For high-frequency measurements, asingle harmonic signal is generated using an Agilent 8665B signal generatorand applied to the device using a Cascade Microtech Air Coplanar Probe(ACP). The output signal from the device is then transferred to a low-passfilter using another ACP. The DC level of the output signal is measured usinga Tektronix TDS 3034 oscilloscope with 1-MΩ load while the frequency of theinput signal is swept between 10 MHz and 6 GHz. When the operationalfrequency reaches the HF and UHF range, the system should be analyzedaccording to transmission line theory. One of the most important problemswith this kind of measurement is the signal reflection at several points in thecircuit where the impedances of the two sides of a junction are not matched.In our case, the main cause of the signal reflection is the device itself, be-cause it is a nonlinear device with an unknown and obviously frequency-dependent impedance, whereas the rest of the circuit (cables and probes)has an impedance of 50 Ω. Hence, the output signal fluctuates as the fre-quency increases. Because it is impossible to completely eliminate the signalreflections in the wide range of frequency, we repeated the measurementwith several combinations of 1-dB attenuators and RC loads (R = 50 Ω andC = 100 pF) in the junctions with highest reflections, trying to diminish thereflected signals. One of the measurement setups used for high-frequency

−3 −2 −1 0 1 2 3

10−12

10−10

10−8

10−6

10−4

Voltage(V)

Cur

rent

(A)

325K300K275K250K225K200K175K150K

325 K

150 K

A

0 1 2 310−12

10−10

10−8

10−6

Voltage(V)

Cur

rent

(A)

ExperimentalThermionic emission (TE)FN TunnelingTE + FN

B C

EFM EFSi

EC

EV

2.8 eV3.1 eV

0 1 2 30

0.5

1

1.5

2

2.5

3x 10−5

Voltage(V)

Cur

rent

(A)

ExperimentalThermionic emission (TE)FN TunnelingTE + FN

Fig. 6. Evaluation of the model. (A) Temperature dependence of the current–voltage characteristics in a semilogarithmic graph. The Inset shows a sim-plistic band diagram of the Al–Si contact. As mentioned in the text, therewill also most likely be trap states at the interface between the insulators,but these are not indicated in the figure due to lack of quantitative in-formation. (B) The two current contributions (thermionic and tunnel cur-rents) as well as the sum of those current contributions modeling theexperimental data at T = 300 K. (C) The two current contributions in asemilogarithmic scale and the sum of those current contributions modelingthe experimental data at T = 300 K.

Sani et al. PNAS | August 19, 2014 | vol. 111 | no. 33 | 11947

APP

LIED

PHYS

ICAL

SCIENCE

SSE

ECO

MMEN

TARY

Dow

nloa

ded

by g

uest

on

May

30,

202

0

measurement is shown in Fig. S4. Despite the fact that the fluctuations in theVout vs. frequency vary depending on the measurement setup, all of themshow a similar trend, which is interpreted as the actual frequency responseof the diode using an averaging approach.

For characterization of the temperature dependence of the current, thesamples were probed in a cryogenic probe station and the I–V characteristicwas measured using a Keithley 4200 semiconductor characterization system.

X-ray photoelectron spectroscopy (XPS) was carried out on ex situ-prepared samples using a monochromatized Al Kα (hν = 1,486.6 eV) X-raysource and a Scienta-200 hemispherical analyzer. An electron flood gunwas used to neutralize the sample during the XPS measurements due tothe low conductivity of the sample. The reference energy position in theXPS spectra has been the C 1s peak, where the lowest binding energy ofC 1s was set to 284.8 eV (52).

For characterizing the size distribution of the Si μPs, the particleswere dispersedin hexamethyldisiloxane to form a hydrophobic shell, preventing them from

sticking to each other. The dispersion was then cast on an Al substrate and dried.A number of SEM images were taken, and particle size analysis was performedfor each image using the online tool SIMAGIS live for image analysis. The sizedistribution data of all of the images was gathered to generate the final sizedistribution statistics.

ACKNOWLEDGMENTS. We thank Lars Petterson and Duncan Platt (Acreo)and Prof. Robert Forchheimer, [Department of Electrical Engineering(Institutionen för Systemteknik), Linköping University] for assistance inhigh-frequency measurements and Anna Malmström (Acreo) for assis-tance in the laboratory and preparing the circuit layout. We acknowledgeVINNOVA (2012-01607), The European Regional Development Fund throughTillväxtverket (Project PEA-PPP), and The Knut and Alice Wallenberg Founda-tion (Power Paper Project, KAW 2011.0050). M.B. acknowledges The ÖnnesjöFoundation, X.C. acknowledges the Advanced Functional Material Program atLinköping University, and X.L. acknowledges funding from The Swedish Re-search Council Linnaeus Grant LiLi-NFM.

1. Sun J, Zhang B, Katz HE (2010) Materials for printable, transparent, and low-voltagetransistors. Adv Funct Mater 21:29–45.

2. Carr J (2002) RF Components and Circuits (Elsevier Science, Oxford).3. Nintanavongsa P, Muncuk U, Lewis DR, Chowdhury KR (2012) Design optimization

and implementation for RF energy harvesting circuits. IEEE J Emerging Sel Top CircuitsSyst 2:24–33.

4. Jabbar H, Song YS, Jeong TT (2010) RF energy harvesting system and circuits forcharging of mobile devices. IEEE Trans Consum Electron 56:247–253.

5. Im D, Moon H, Shin M, Kim J, Yoo S (2011) Towards gigahertz operation: Ultrafast lowturn-on organic diodes and rectifiers based on C60 and tungsten oxide. Adv Mater23(5):644–648.

6. Kleemann H, Schumann S, Jörges U, Ellinger F, Leo K, Lüssem B (2012) Organic pin-diodes approaching ultra-high-frequencies. Org Electron 13:1114–1120.

7. Lilja KE, Bäcklund TG, Lupo D, Hassinen T, Joutsenoja T (2009) Gravure printed organicrectifying diodes operating at high frequencies. Org Electron 10:1011–1014.

8. Park SK, Mourey DA, Subramanian S, Anthony JE, Jackson TN (2008) High-mobilityspin-cast organic thin film transistors. Appl Phys Lett 93(4):043301.

9. Steudel S, et al. (2008) Ultra-high frequency rectification using organic diodes. Elec-tron Devices Meeting, 2008. IEDM 2008. IEEE International (IEEE, New York), pp 1–4.

10. Wang M, et al. (2013) High-performance organic field-effect transistors based onsingle and large-area aligned crystalline microribbons of 6,13-dichloropentacene.Adv Mater 25(15):2229–2233.

11. Champlin KS, Eisenstein, Gadi (1978) Cutoff frequency of submillimeter Schottky-Barrier diodes. IEEE Trans Microw Theory Tech MTT-26:31–34.

12. Han S, et al. (2008) Printed silicon as diode and FET materials—preliminary results.J Non-Cryst Solids 354:2623–2626.

13. Johansson C, Wang X, Robertsson M (2008) Printable rectifying device using Si-composite. Electron Lett 44:53–55.

14. Kohno A, Sameshima T, Sano N, Sekiya M, Hara M (1995) High performance poly-SiTFTs fabricated using pulsed laser annealing and remote plasma CVD with lowtemperature processing. IEEE Trans Electron Dev 42:251–257.

15. Mitzi DB, Kosbar LL, Murray CE, Copel M, Afzali A (2004) High-mobility ultrathinsemiconducting films prepared by spin coating. Nature 428(6980):299–303.

16. Park H, et al. (2012) Fully roll-to-roll gravure printed rectenna on plastic foils forwireless power transmission at 13.56 MHz. Nanotechnology 23(34):344006.

17. Ridley BA, Nivi B, Jacobson JM (1999) All-inorganic field effect transistors fabricatedby printing. Science 286(5440):746–749.

18. Sameshima T, Usui S, Sekiya M (1986) XeCl Excimer laser annealing used in the fab-rication of poly-Si TFT’S. Electron Dev Lett 7(5):276–278.

19. Shimoda T, et al. (2006) Solution-processed silicon films and transistors. Nature440(7085):783–786.

20. Young Hwan H, et al. (2013) An “aqueous route” for the fabrication of low-temperature-processable oxide flexible transparent thin-film transistors on plasticsubstrates. NPG Asia Materials 5:e45.

21. Yuan HC, Celler GK, Ma Z (2007) 7.8-GHz flexible thin-film transistors on a low-temperature plastic substrate. J Appl Phys 102(3):034501.

22. Yuan HC, Ma Z, Celler GK (2007) Flexible RF/microwave switch-PIN diodes using single-crystal Si-nanomembranes. IEEE MTT-S International Microwave Symposium Digest2007, IEEE MTT-S International Microwave Symposium, June 3–8, 2007, Honolulu(Institute of Electrical and Electronics Engineers, Piscataway, NJ) pp 1027–1030.

23. Zhang X, Neiner D, Wang S, Louie AY, Kauzlarich SM (2007) A new solution route tohydrogen-terminated silicon nanoparticles: Synthesis, functionalization and waterstability. Nanotechnology 18(9):095601.

24. Härting M, Zhang J, Gamota DR, Britton DT (2009) Fully printed silicon field effecttransistors. Appl Phys Lett 94(19):193509.

25. Steudel S, et al. (2005) 50 MHz rectifier based on an organic diode. Nat Mater 4(8):597–600.

26. Zhou H, et al. (2013) Fast flexible electronics with strained silicon nanomembranes. SciRep 3:1291.

27. Singh J (2001) Semiconductor Devices: Basic Principles (Wiley, New York).

28. Sze SM, Ng KK (2006) Physics of Semiconductor Devices (Wiley, New York).29. Lu ZH, et al. (1997) SiO2 film thickness metrology by x-ray photoelectron spectroscopy.

Appl Phys Lett 71:2764–2766.30. Hunter MS, Fowle P (1956) Natural and thermally formed oxide films on aluminum.

J Electrochem Soc 103:482–485.31. Zähr J, Oswald S, Türpe M, Ullrich HJ, Füssel U (2012) Characterisation of oxide and

hydroxide layers on technical aluminum materials using XPS. Vacuum 86:1216–1219.32. Horache E, Fischer JE, Van Der Spiegel J (1990) Niobium disilicide formation by rapid

thermal processing: Resistivity-grain growth correlation and the role of native oxide.J Appl Phys 68:4652–4655.

33. Kurokawa K, Yamauchi A, Matsushita S (2005) Improvement of oxidation resistance ofNbSi2 by addition of boron. Materials Science Forum, International Conference on NewFrontiers of Process Science and Engineering in Advanced Materials, November 24–26,2004, Kyoto (Trans Tech Publications, Durnten, Switzerland), Vol 502, pp 243–248.

34. Andersson P, et al. (2002) Active matrix displays based on all-organic electrochemicalsmart pixels printed on paper. Adv Mater 14:1460–1464.

35. Chahal P, Morris F, Frazier G (2005) Zero bias resonant tunnel Schottky contact diodefor wide-band direct detection. IEEE Electron Device Lett 26:894–896.

36. Chattopadhyay P, Daw AN (1986) On the current transport mechanism in a metal-insulator-semiconductor (MIS) diode. Solid-State Electron 29:555–560.

37. Iwata S, Ishizaka A (1996) Electron spectroscopic analysis of the SiO2/Si system andcorrelation with metal-oxide-semiconductor device characteristics. J Appl Phys 79:6653–6713.

38. Card HC, Rhoderick EH (1971) Studies of tunnel MOS diodes I. Interface effects insilicon Schottky diodes. J Phys D Appl Phys 4:1589–1601.

39. Pananakakis G, Kamarinos G, El-Sayed M, Le Goascoz V (1983) Electrical character-ization of Al-SiO2-Si (N-type) tunnel structures. Influence of LPCVD and LPO2 oxidegrowth technologies on the properties of the Si-SiO2 interface. Solid-State Electron26:415–426.

40. Ng KK, Card HC (1980) Asymmetry in the Sio//2 tunneling barriers to electrons andholes. J Appl Phys 51:2153–2157.

41. Depas M, Van Meirhaeghe RL, Laflère WH, Cardon F (1994) Electrical characteristics ofAl/SiO2/n-Si tunnel diodes with an oxide layer grown by rapid thermal oxidation.Solid-State Electron 37:433–441.

42. Altindal S, Karadeniz S, Tuǧluoǧlu N, Tataroglu A (2003) The role of interface statesand series resistance on the I-V and C-V characteristics in Al/SnO2/p-Si Schottky diodes.Solid-State Electron 47:1847–1854.

43. Cova P, Singh A, Masut RA (1997) A self-consistent technique for the analysis of thetemperature dependence of current-voltage and capacitance-voltage characteristicsof a tunnel metal-insulator-semiconductor structure. J Appl Phys 82:5217–5226.

44. Aluguri R, Das S, Singha RK, Ray SK (2013) Size dependent charge storage charac-teristics of MBE grown Ge nanocrystals on surface oxidized Si. Curr Appl Phys 13:12–17.

45. Derrie J, Commandré M (1982) SiO2 ultra thin film growth kinetics as investigated bysurface techniques. Surface Science 118:32–46.

46. Frenkel J (1938) On pre-breakdown phenomena in insulators and electronic semi-conductors. Phys Rev 54:647–648.

47. Jensen KL (2003) Electron emission theory and its application: Fowler-Nordheimequation and beyond. J Vac Sci Technol B 21:1528–1544.

48. Chiou YL, Gambino JP, Mohammad M (2001) Determination of the Fowler-Nordheimtunneling parameters from the Fowler-Nordheim plot. Solid-State Electron 45:1787–1791.

49. Sze SM (2002) Semiconductor Devices: Physics and Technology (Wiley, New York).50. Saǧlam M, et al. (1996) Series resistance calculation for the metal-insulator-semi-

conductor Schottky barrier diodes. Appl Phys A Mater Sci Process 62:269–273.51. Kanbur H, Altındal S, Tataro�glu A (2005) The effect of interface states, excess ca-

pacitance and series resistance in the Al/SiO2/p-Si Schottky diodes. Appl Surf Sci 252:1732–1738.

52. Walther F, et al. (2007) Stability of the hydrophilic behavior of oxygen plasma acti-vated SU-8. J Micromech Microeng 17:524–531.

11948 | www.pnas.org/cgi/doi/10.1073/pnas.1401676111 Sani et al.

Dow

nloa

ded

by g

uest

on

May

30,

202

0