Natural and nature-inspired semiconductors for organic electronics

Eric Daniel Głowackia, Lucia Leonatb, Gundula Vossc, Marius Bodead, Zeynep Bozkurte, Mihai Irimia-Vladu*a,f, Siegfried

Bauerf, Niyazi Serdar Sariciftcia,

a Linz Institute for Organic Solar Cells (LIOS), Johannes Kepler University, 4040 Linz, Austria ; b National Institute for Electrical Engineering, ICPE-CA, Politehnica University of

Bucharest, Faculty of Applied Chemistry and Materials Science, Bucharest, Romania; c Department of Bioorganic Chemistry, University of Bayreuth, D-95440, Bayreuth, Germany;

d Experimental Physics, Johannes Kepler University Linz, Austria, 4040; e Dept. of Material Science and Engineering, Sabanci University, Istanbul, Turkey;

f Soft Matter Physics, Johannes Kepler University Linz, Austria 4040

ABSTRACT

Herein we report our recent efforts in employing natural materials and synthetic derivatives of natural molecules for organic field effect transistors (OFETs) and organic photovoltaics (OPVs). We evaluated dyes from the following chemical families: acridones, anthraquinones, carotenoids, and indigoids. These materials have proven effective in organic field effect transistors, with mobilities in the 4 × 10-4 – 0.2 cm2/V·s range, with indigoids showing promising ambipolar behavior. We fabricated complementary-like voltage inverters with high gains using indigoids. The photovoltaic properties of these materials were characterized in metal-insulator-metal (MIM) diodes, as well as in donor-acceptor bilayer devices. Additionally, we have found that indigoids and Quinacridone, show long-range crystalline order due to hydrogen binding, and have considerably higher relative permittivities (ε) compared to typical organic semiconductors. Higher permittivities result in lower exciton binding energies and thus may lead to high photocurrents in photovoltaic devices.

Keywords: organic field-effect transistors, organic solar cells, natural materials, high dielectric constant organic materials, biodegradable electronics, hydrogen-bonding semiconductors *To whom the correspondence should be addressed: E-mail: Mihai.Irimia-Vladu@jku.at

1. INTRODUCTION We have recently reported on the use of natural and biodegradable materials in the fabrication of organic electronic devices, showing suitable materials for substrates, dielectric layers, and semiconductors and demonstrating fully-biodegradable OFET devices.[1,2] A recent focus of our research is the development of fully biodegradable OPVs. Photovoltaics based on organic semiconductors have been a dynamic area of research in the past twenty years, and have recently evolved to the stage of commercialization.[3] Both OFET-based and OPVs offer the promise of low cost mass-produced devices. From the viewpoint of concern for the ultimate environmental sustainability of such products, as well as from the interest in evaluating unexplored but abundant materials, we chose to investigate natural and nature-based dyes. Many naturally occurring materials are large scale synthetically produced by the dye industry for textile coloring, paints, cosmetics, or food additives. Most of these materials are consequently environmentally safe and compatible, and are available at low cost. In this paper, we explored indigoids, anthraquinones, acridones, and carotenoids. The molecular structures of the investigated molecules are shown in figure 1. All materials were deposited in thin-films on glass/ITO substrates and first investigated via cyclic voltammetry; next optical and morphological data were obtained; subsequently organic field effect transistors (OFETs) and MIM diodes were fabricated, allowing both the estimation of field-effect mobility and the determination of the possible polarity of transport. [4] Basic energy level estimations, band gaps,

Invited Paper

Organic Semiconductors in Sensors and Bioelectronics IV, edited by Ruth Shinar, Ioannis Kymissis,Proc. of SPIE Vol. 8118, 81180M · © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.892467

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carrier mobilities, and relative permittivities for these materials are shown in table 1. Form our studies three concepts emerged: 1) intermolecular hydrogen bonding for supporting long-range ordering and thus effective transport in indigoids and Quinacridone, and 2) a high relative permittivity of indigoids and Quinacridone, resulting in high photocurrent yield in single-layer devices. 3) indigoids as high-mobility ambipolar organic semiconductors for complementary-type organic circuits.

Figure 1. Molecular structures of the materials evaluated in this work. In the field of organic electronics, it has been recognized that developing materials with high order and high-ε would allow many of the design principles of inorganic semiconductors to be applied to organic devices as well.

Table 1. Energy level estimates, mobilities, and permittivities of the materials evaluated in this work

Material HOMO (eV) LUMO (eV) Eg CV (eV) Eg optical (eV)

Carrier Mobility, (cm2/ V·s)

Relative permittivity (εr)

Indigo -5.5 -3.8 1.7 1.7 μe= 1 × 10-2, μh= 1 × 10-2

4.3

Tyrian Purple -5.8 -4.0 1.8 1.9 μe= 3 × 10-2, μh= 0.2

6.2

Cibalackrot -5.6 -3.5 2.1 2.0 μe = 9.3 × 10-3, μh= 5.3 × 10-3

4.8

Quinacridone -5.4 -2.9 2.5 2.0 μh= 0.1 5.2 Vat Yellow 1 -6.3 -3.6 2.7 2.3 μe = 4.2 × 10-2 3.8 Vat Orange 3 -6.2 -3.8 2.4 2.1 μe = 8.8 × 10-3 3.8 β-carotene 5.84* 3.54* NA 2.3 μh = 4 × 10-4 2.5

* Values from photoemission studies reported in [5]

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2. RESULTS AND DISCUSSION 2.1 Indigo and Indigoids

2.1.1 Morphology and transport –Indigo and Cibalackrot OFETs and diodes

Many derivatives in the indigoid family have reached a position of steady economic importance, with Indigo itself being the most mass-produced industrially dye (20 million tons per year, 3-4 USD/kg). Most of the produced Indigo is consumed for the coloring of blue jeans. Indigo can be extracted from the plants indigofera tinctoria and isatis tinctoria, which were cultivated for at least 4000 years in China, India and Egypt for coloring textiles. Indigo has an extremely low solubility and a high melting point (390-392oC), explained by stabilization from inter- and intramolecular hydrogen bonding. Intermolecular interactions of π domains also strongly influence the electronic and vibrational spectra of Indigo in solutions and dispersions. [6,7,8] Thin films of Indigo show a broad optical absorption with onset at ~730 nm. The ambipolarity of Indigo is explained by its reversible two-electron oxidation and reduction, shown in the cyclic voltammetry diagram (figure 2a). All indigoids have common this reversible electrochemistry due to reduction of ketone groups and oxidation of secondary amines. Transistors with Indigo on AlOx/TTC gate dielectrics afforded balanced electron and hole mobilities (figure 2b and table 1) with low threshold voltages. [9] Additionally we measured complementary-like voltage inverters with gains reaching 50-70 in the first and third quadrants for VDD voltages of +/-12V. [9] Cibalackrot is an Indigo derivative, first produced at Ciba in Switzerland by Von G. Engi in 1914 through a simple condensation reaction of Indigo with phenylacethylchlorine. [10] We found that the electrochemistry of Cibalackrot (figure 2a, red curve) is identical to that of Indigo, albeit with a wider band gap. OFETs with Cibalackrot channel showed ambipolarity with performance similar to the one of Indigo and balanced hole and electron mobilities: 5 × 10-3

cm2/ V·s for holes and 9 × 10-3 cm2/ V·s for electrons (figure 2c). In planar diodes however, with the structure featuring ITO or ITO|PEDOT:PSS as a bottom contact with various metal top contacts, we found that the transport in Indigo and indigoids was several orders of magnitude lower than the one recorded in transistors. Suspecting trap-limited conduction, we fabricated diodes with different thicknesses of Indigo, finding however that the current is independent on thickness, suggesting that traps do not represent a significant problem. In addition, frequency-dependent capacitance measurement of Indigo in MIM diode configuration indicated also that trap density is negligible. However, fabricating devices with Indigo on top of p-doped silicon substrates previously passivated by flash annealing in high vacuum, we found enhanced transport with mobility value on par with the one extracted from saturation regime in OFETs (as shown in a comparison of the two diode devices in figure 2d). In our recent work we found that Indigo formed thin films with a highly ordered crystalline texture and a single preferential orientation, displaying only one X-ray diffraction peak corresponding to a spot on the Debye diagram (instead of a ring characteristic to polycrystalline materials). [9] Interestingly, the excellent ordering of Indigo did not occur when the film was evaporated on every substrate. Specifically, Indigo thin films displayed a single peak only when deposited on highly aliphatic materials like polyethylene and tetratetracontane (TTC, a natural oligoethylene found in plants), while on other substrates (i.e. glass, poly(vinyl alcohol), poly(vinyl triazole), cross-linked benzocyclobutene, adenine, melamine) presented nearly or completely amorphous texture. These results show that interaction with the substrate material is critical for Indigo to adopt a morphology and an orientation requisite to good charge transport.

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Figure 2. (a) Cyclic voltammogram of Indigo (blue), Cibalackrot (red), and Tyrian Purple (purple) on ITO as the working electrode, Ag/AgCl as reference, and Pt as counter electrode, 20 mV/s scan rate, 0.1M tetrabutylammonium phosphorus hexafluoride in CH3CN as supporting electrolyte. (b) Transfer characteristics of an Indigo transistor with an AlOx (45nm) and tetratetracontane (40 nm) composite gate dielectric (C0d=43.2 nF cm-2) and gold source and drain electrodes, fabricated on a natural shellac resin substrate. (c) Transfer characteristics of a Cibalackrot channel transistor with an AlOx(45nm)|tetratetracontane(30 nm) composite gate dielectric (C0d=43.2 nF cm-2) and gold source and drain electrodes. (d) J-V characteristics in the dark and under simulated 1.5AM illumination for Indigo and Cibalackrot diodes. 2.1.2 Tyrian Purple OFETs and inverters Tyrian Purple (6,6’-dibromoindigo) is also a material known since ancient times, and is tipped to have been exploited previous to Indigo [11]. It was produced from sea snails and was considered throughout antiquity to be the most valuable pigment; Tyrian Purple-colored tunics were the sign of distinction of Roman senators and nobles. We prepared the dye via known synthetic procedures. [12] In the same architecture devices as Indigo, Tyrian Purple afforded mobilities an order of magnitude higher, µe = 0.03 cm2/Vs, µh = 0.2 cm2/Vs. Transfer characteristics are shown in figure 3a. We found that Tyrian Purple displays crystallinity, having a single-preferential orientation, showing also one spot in XRD Debye diagram, as in the case of Indigo. In fact, Indigo and Tyrian Purple are two of the most planar molecules ever known, with the deviation from planarity less than 4 picometers. [11, 13] From single crystalline diffraction, the interplanar spacing between adjacent parallel molecules in Indigo is 5.77 Å, while in Tyrian Purple is 4.82 Å. [14,15] We speculate that the tighter intermolecular packing is integral to the superior charge transport properties of Tyrian Purple. From

-9 -6 -3 0 3 6 9 12

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e C

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Vgs/ V

I ds

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Igs Ids

Vds = + 14.5 Vc

Vth-e = 10 VVth-h = -1.5 V

a b

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cyclic voltammetry measurements, we estimate for Tyrian Purple a LUMO level of -4.0 eV, which allowed Tyrian Purple transistors to operate in ambient air without degradation (figure 3b). [16] We measured complementary-like voltage inverters and recorded very high quasi-static gains of 255 in the first quadrant and 285 in the third quadrant, at VDD of +/- 15V. To our knowledge these are the highest gain values reported for single ambipolar material layers with a single type of contact electrodes. [17] The good transport properties of Indigoids are initially surprising, as the molecules alone have a low level of conjugation, two phenyl rings separated by an unconjugated bridge. We surmise that the inherent planarity of the molecules and the highly-ordered growth resulting in intermolecular π-stacking is integral for charge transport.

Figure 3. (a) Transfer characteristics of a Tyrian Purple transistor with an AlOx(45nm)|tetratetracontane(40 nm) composite gate dielectric (C0d = 43.2 nF cm-2) and gold source and drain electrodes, fabricated on a glass substrate. (b) comparison of transfer characteristics of these devices measured in N2 atmosphere and during repeated cycling in air. (c) Quasi steady-state transfer characteristics of complementary-like voltage inverters in the first quadrant, showing the corresponding voltage gain. (d) Quasi steady-state transfer characteristics of complementary-like voltage inverters in the third quadrant, shown with voltage gain. Each measurement is performed through 1000 points with hold time, step delay and delay time of 1 second each. Every point is measured using a high resolution A-D converter value (HR-ADC) of 127 and a high speed A-D converter value (HS-ADC) of 1023.

2.1.3 Photovoltaic properties of indigoids In MIM structures with Indigo, the photocurrent yield was low (a few µA·cm-2 at short circuit), due to the very rapid internal conversion of the Indigo molecule and thus inefficient dissociation of excitons. [18] The nonradiative rate constant (kNR) of Indigo is at least two to three orders of magnitude higher than its fluorescence rate constant (kF), and

0 2 4 6 8 10 120

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Gai

n

VDD = 15 V VDD = 12.5 V VDD = 10 V VDD = 7.5 V

V out (V

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I ds (A

) Ids-measured in Nitrogen Ids-measured in air

Vds = + 10 V

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10-11

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10-7

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Vth-e= 4.95 V

Sqrt

(Ids)

(A0.

5 )

Vgs (V)

I ds

(A)

Igs Ids

Vds = + 12 Va

Vth-h= 1.75 V

V in V out 2

1

V DD

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excited state lifetime is consequently very short. This property precludes the use of Indigo as an efficient donor material. The rapid deactivation of the S1 state in Indigo is a result of ultrafast intramolecular proton transfer. From the electrochemical data, we estimate a LUMO level of -3.8 eV, suggesting that Indigo could be a suitable acceptor material in donor-acceptor solar cells. Indeed, we found that structures featuring Indigo evaporated onto either P3HT, Quinacridone or β-carotene films resulted in photoinduced charge transfer (verified by means of photoluminescence quenching) of electrons to Indigo, thus generating donor-acceptor solar cells. However, absorption by Indigo produced a negligible contribution to photocurrent due to the rapid deactivation mechanism of the excited state. However, synthetic modification of Indigo molecule to block this deactivation pathway would markedly change the photophysics of Indigo. The molecule Cibalackrot was synthesized via a simple condensation of phenylacetylchoride with Indigo, resulting in a rigid C=C(Ph)-C=O bridge that replaces the carbonyl group of Indigo. [10, 19, 20] Cibalackrot possesses photophysical properties exactly opposite to those of Indigo: high fluorescence yield, nanoseconds S1 lifetime, and negligible internal conversion. [21] Cibalackrot proved to be a suitable acceptor material in solar cells with P3HT, ZnPc, and Quinacridone acting as donors. No attempts have been made to optimize these cells and performances were around 0.5% for all three devices. Cibalackrot also operated as a donor material in solar cells with the anthroquinones Yellow 1 and Orange 3 with similar performance. Aside from typical donor-acceptor bilayer structures, we explored single-layer MIM structures of Indigoids. The low dielectric constant of organic semiconductors in general, results in strongly-bound molecularly-localized excitons with a binding energy proportional to the square of the relative permittivity, ε, according to equation 1. [22]

22

4

2 εμh

eEB =

Different than typical highly-disordered organic materials that have low dielectric constant, the long-range order in Indigoids results in materials having high dielectric constant. Though Indigo or Tyrian Purple have an efficient excited-state deactivation pathway due to the proton-exchange mechanism, Cibalackrot has this pathway blocked. Nevertheless, in spite of the lack of hydrogen bonding, Cibalackrot retains a high dielectric constant (ε = 4.8) and in MIM diodes gives short-circuit photocurrents of around 300-400 μA cm-2, two orders of magnitude higher than typical polymer MIM devices [23,24] (figure 2d, red trace). 2.2 Quinacridone

Acridones are present in nature, and their synthetic derivatives have found pharmaceutical applications, such as antimalarial drugs. [25] Quinacridone (5,12-dihydro-quino[2,3-b]acridine-7,14-dione, vat violet 19) is a common pigment used in the dye industry. Many of its derivatives are also commercially available. Quinacridone is analogous to Indigo in its containing of amine and keto groups and therefore hydrogen bonding; not surprisingly it exhibits a very similar electrochemistry to Indigo. Quinacridone has however much lower electron affinity than Indigo or Cibalackrot, and therefore behaves as only a p-type (donor) material in transistors. We found a high hole mobility of 0.1 cm2/ V·s in transistors with Quinacridone channel on AlOx/TTC dielectric and gold as source and drain contact material. As with Indigo, Cibalackrot or Tyrian Purple, XRD studies of Quinacridone revealed a single diffraction peak, demonstrating a crystalline texture with a single preferential orientation. Structures with Quinacridone/Cibalackrot and Quinacridone/ Yellow 1 showed power conversion efficiencies of 0.22% and 0.25%, respectively. Similarly to the two aforementioned Indigoids, Quinacridone was found to have an unusually high dielectric constant of 5.2 and likewise MIM structures with Quinacridone (figure 4) were found to produce photocurrents as high as 1 mA·cm-2 which are considerably higher than the one obtained from typical organic materials. This is due to lower exciton binding-energy (equation 1). The open circuit voltages under illumination of Quinacridone diodes utilizing different metal top electrodes (Al, Ag, Au) were found to follow the MIM model closely. [26] Additionally, Quinacridone showed a substantial photoconductivity response as well. The total photocurrent was limited primarily by the low absorbance coefficient of Quinacridone, combined with its narrow absorbance band. At the peak of Quinacridone absorbance, ~580 nm, we measured an external quantum efficiency of 8%, a value much higher than those reported previously for MIM structures with conjugated polymers, where values were in the 0.1-1% range. [23,24]

(1)

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Figure 3. Transfer (left) and output (right) characteristics of Quinacridone channel transistors with an AlOx(45nm)|TTC(40 nm) composite gate dielectric (C0d=43.2 nF cm-2) and gold source and drain electrodes.

Figure 4. J-V characteristics of Quinacridone diodes (75 nm) in the dark and under simulated 1.5 AM illumination. The structures used were ITO|PEDOT:PSS(40nm)|Quinacridone(75 nm)|Top contact.

2.3 Anthraquinones

Numerous derivatives of anthraquinones are produced by plants, fungi, and animals. The derivative Carminic acid, for example, was extracted from cochineal insects and treated as a luxury pigment for centuries. Commercial textile dyes based on anthraquinone represent the largest chemical class of dyes used for this purpose. [27] We evaluated two commercially-available vat dyes, Yellow 1 and Orange 3, both of which are considered safe and nontoxic for use in cosmetics and textiles. Based on optical absorption and cyclic voltammetry we found that both materials are wide-band gap semiconductors with high electron affinity (LUMO level ~ -3.6 to -3.8 eV) and high oxidation potential. Consistent with this, in OFETs only electron transport was demonstrated, with electron mobilities of 4.2×10-2 cm2/V·s and 8.8×10-3

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cm2/V·s for Yellow 1 and Orange 3, respectively. Both dyes show a photovoltaic effect in MIM structures, as well as a pronounced photoconductivity (figure 5). Both materials likewise perform as electron acceptors in bilayer cells with ZnPc, with ITO|PEDOT:PSS|ZnPc(20nm)|Yellow 1 (30 nm)|Al affording power conversion efficiency of 0.5%. Vat yellow 1proved to be superior in terms of mobility and photovoltaic performance.

Figure 5. (a) Transfer characteristics of a Vat orange 3based transistor channel, on (45nm)|TTC(40 nm) composite gate dielectric (C0d=43.2 nF cm-2) and with gold source and drain electrodes. (b) Transfer characteristics with the same transistor architecture with Vat yellow 1channel. (c) Yellow 1 and Orange 3 in diodes of the structure ITO|PEDOT:PSS|Dye(75nm)|Al in the dark and under simulated 1.5 AM illumination.

2.4 β-Carotene

Over 600 carotenoids are known, most of which are naturally-occurring pigments present in chloroplasts of photosynthetic organisms. Commercially, many are used in food additives, coloration agents, and pharmaceutical applications. The material is highly soluble in organic solvents, and thus can be readily spin cast. Due to high solubility, thin films on ITO dissolved in solvent immediately, making cyclic voltammetry of thin films impossible. For HOMO and LUMO levels we refer to literature photoemission studies. [5] β−carotene has a narrow absorption between 400-500 nm with a high extinction coefficient (~1×105). In OFETs, we found that β−carotene behaves as a p-type semiconductor with a hole mobility of 4×10-4 cm2/ V·s. [2] In MIM structures, β−carotene performed similarly to conjugated polymers, generating a low short-circuit photocurrent of around 1µA cm-2. Mixing it with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) results in a two order of magnitude increase in photocurrent, and corresponding decrease in photovoltage, demonstrating that carotene can act as an electron donor. Rough film morphology due to macro-scale phase separation between PCBM and the beta-carotene resulted in poor reproducibility. Evaporation of C60 onto β−carotene proved considerably more reliable, producing the same photocurrent as in the case of PCBM, but with a higher open circuit

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voltage. Bilayer devices with Indigo evaporated onto β−carotene showed the same efficiency as cells with PCBM, suggesting that charge transfer also occurs between these two molecules. Though the photovoltage was high, around 1V, the short circuit photocurrent, however, was nearly 10 times lower than with the C60 or PCBM. Since the contribution to photocurrent from light absorption by the Indigo is negligible we attribute the gain in photocurrent to absorption by C60. The J-V and photovoltaic parameters are shown in figure 6.

Figure 6. J-V characteristics of β-carotene devices in the dark and under simulated 1.5 AM illumination.

3. CONCLUSIONS In this work, we have summarized recent progress in our work with natural and nature-inspired semiconducting materials. Indigoids and Quinacridone have proven especially promising, showing long-range order and crystallinity and consequently high dielectric constant. These materials offer good mobilities, and suggest their use in novel solar cell structures not reliant on the typical donor-acceptor architecture. Together with OFETs and high-performance inverter circuits, these devices exemplify the possibility of fabrication of high-performance biocompatible and biodegradable organic electronics.

REFERENCES

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