Introduction

Photovoltaic (PV) effects have long been studied for their main applications in solar energy and PV cells, which are energy-harvesting devices.1, 2, 3, 4, 5, 6 Organic and inorganic semiconductors have been used as active components of those energy-harvesting devices in a variety of diode forms, including Schottky diodes,7, 8, 9, 10 p-n diodes11, 12, 13, 14 and p-i-n diodes,15, 16, 17, 18 showing outstanding progress in enhancing their power efficiencies.19, 20 This study reports a rather different new application, dynamic PV switching and voltage generation by deep blue photons. Dynamic PV switching has so far been ignored in organic electronics and rarely attempted in inorganics despite its potential great usefulness as a dynamic energy-harvesting method.13, 21, 22, 23, 24, 25 In fact, dynamic PV switching has only been reported for its limited practical applications with inorganic nanostructure junctions, as seen in Supplementary Table S1.13, 21, 22, 23, 24, 25 Simply speaking, instantaneously built-in voltage can be used as a switching signal induced by energetic photons (ON/OFF: photo-induced charge collection/recombination), as any semiconductor thin film possesses electrodes for charge collection and recombination within a diode array. This type of self-powered voltage generation can have various applications, including ultraviolet (UV) and visible light detection, biological and chemical study in a wireless health-care platform, powerless communications and so on.26 Particularly, selective detection and absorption of deep blue and UV photons without any aid of electric power would be very important for eye protection applications in these days.27

The present study on PV switching and photo-induced dynamic energy harvesting used an organic semiconductor-based Schottky diode, as it is low cost and easy to fabricate on a glass or plastic substrate. Our organic PV switching diodes could be integrated into a tandem device array on a large area substrate to produce higher photo-voltage, which cannot yet be performed with inorganic nano-dimension diodes. The PV switching and voltage generation mechanisms are explained in detail. The most important property of the organic semiconductor thin film is its exciton-binding energy, which needs to be as small as possible for the easy/fast charge dissociation of photo-excited excitons and superior energy harvesting. In this context, we chose an heptazole (C26H16N2) thin film, as the latter showed a rather small exciton-binding energy of ~40 meV, while pentacene (C22H14) and dinaphtho[2,3-b:2′,3′-f]thieno[3,2-B]thiophene (DNTT; C22H12S2) revealed somewhat larger binding energies of 100 and 120 meV, respectively. (See Supplementary Figure S1 and Supplementary Table S2 for a detailed comparison of the respective exciton-binding energy properties of heptazole, DNTT28 and pentacene thin films based on photo-excited charge collection spectroscopic results.29) Besides its small exciton-binding energy, our organic heptazole with 2.95 eV energy band gap sensitively and selectively detects deep blue/UV photons in PV switching operations, which would be promising toward smart eye protection glass applications.27 Because of the intrinsic potential energy (or work function) difference between Al and indium-tin-oxide (ITO), our Schottky diode displayed an instantaneous voltage of 0.3 V by deep blue illumination from a laser diode, and four tandem diode cells could therefore produce ~1.2 V when integrated onto the same glass substrate. Using several diode circuitries, we demonstrate 50-ms short PV optical switching, logic gating and, ultimately, the gate switching of an organic field-effect transistor (FET).

Experimental Procedures

Fabrication of organic Schottky diode

A glass substrate was first cleaned with acetone, methanol and deionized water for 5 min each. A transparent ITO electrode was deposited onto the cleaned substrate using a DC sputtering system. Organic heptazole (Luminano Co., Seoul, Korea) and pentacene (Sigma Aldrich Co., St Louis, MO, USA) semiconductors were carefully grown onto the ITO electrode with an organic molecule beam deposition system, followed by Al Schottky electrode deposition using a thermal evaporation system. Every electrode and semiconductor layer was 50-nm thick and patterned with a conventional shadow mask. Finally, the device-to-device interconnection of Ti/Au was patterned with a DC sputtering system.

Fabrication of organic FET

An Al gate electrode was deposited on a precleaned glass substrate with a thermal evaporator. A 50-nm-thick Al2O3 gate insulator layer was then grown with an atomic layer deposition system using a trimethylaluminum precursor and water vapor reactant as usual. As the surface of the Al2O3 insulator was hydrophilic, the hydrophobic polymer of CYTOP (Asahi Glass Co., Tokyo, Japan) was coated and annealed in an oven at 180 °C for 2 h. Afterward, the heptazole small molecule was carefully grown again by organic molecule beam deposition with a deposition rate of 0.1 nm s−1, followed by Au source/drain electrodes with a thermal evaporation system. Every electrode and semiconductor layer was 50-nm thick and patterned with a conventional shadow mask.

Electrical and photoelectrical measurement

The static electrical behaviors of the Schottky diode and the FET were characterized using a semiconductor parameter analyzer (HP 4155C, Agilent Technologies, Santa Clara, CA, USA). The electrical dynamics measurements were investigated with a function generator (AFG 310, Sony, Tokyo, Japan/Tektronix Inc., Beaverton, OR, USA) and an oscilloscope (TDS210, Tektronix). The photoelectric measurements were taken by laser diodes with different wavelengths: red (656 nm), green (531 nm), and blue (406 nm). Their optical power density was 4, 2 and 1.5 mW cm−2 for the red, green and blue photons, respectively.

Results and Discussion

We prepared all devices on a glass substrate. Figures 1a and b, respectively, display the cross-section schematics and plan-view optical image of our heptazole film-based Schottky diode, while Figure 1c shows the diode circuit made up of two diodes along with a real circuit image. In the Figure 1c circuits, L1 and L2 represent same-intensity blue lights (406 nm laser diode, ~1.4 mW cm−2) as separately illuminated onto each Schottky diode. Figure 1d shows dark and photo-induced current plots as a function of the applied voltage (Va) obtained from the single diode of Figure 1b on both linear and log scales. The energy of the blue illumination was slightly higher than the band gap (~2.95 eV) of the crystalline heptazole film (a single heptazole molecule is shown in the inset).30 (We also conducted spectral responsivity measurements with the same diode in a UV/visible range as illustrated in Supplementary Figures S2a–d, where the unique light selectivity for photo detection is confirmed at 2.95 eV or 410 nm with a responsivity of 15–35 mA W−1). According to Figure 1d, an open circuit voltage (VOC) of ~0.3 V was achieved along with a good photo-induced current of 100 nA (at a reverse bias voltage of −2 V) under the blue light, while after illumination (turn off), the current–voltage (I–V) curve went back to its dark state behavior. The ideality factor was ~2.3 and the ON/OFF current ratio was ~105. A more interesting I–V curve behavior achieved from the circuit configuration of Figure 1c under the blue L1 and L2 combination can be observed in Figure 1e (for more precise VOC information, the corresponding I–V curves are provided on a linear scale in Supplementary Figures S3a and b). In the dark, the I–V curve appeared quite symmetric as the diode circuit behavior should always be forward biased: the current passes through the upper diode under (+) Va but chooses the down-side one under (−) Va. When L1 was off and L2 was on, a VOC of −0.3 V was obtained, while a VOC of 0.3 V was obtained in L1 on/L2 off conditions (this case was almost the same as in Figure 1d). However, when both L1 and L2 were on, the VOC reverted to 0 V and the I–V curve appeared symmetric with a large photocurrent. In this Schottky diode circuit, the behavior and size of the VOC were important as the VOC eventually turns into an instantaneous output voltage (VOUT) signal under dynamic illuminations, which means that the VOUT in the PV circuits can be extracted by optical switching (or illuminating photons onto spatially separated diodes).

Figure 1
figure 1

(a) Cross-section schematic and (b) optical image of our heptazole-based Schottky diode with circuit configuration. (c) Schematic circuit diagram and optical image for the circuit composed of two heptazole Schottky diodes (Blue and gray dashed lines in optical images indicate patterned heptazole semiconductor and ITO electrode, respectively.). I–V curves under dark and blue illumination for (d) single diode (inset: heptazole molecule structure) and (e) double diode circuit. L1 and L2 are representing the Light 1 on upper diode and Light 2 on downside diode, respectively, as shown in (c).

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For more detailed explanations, the energy band models of the two-diode circuit are presented in Figures 2a–g. Figure 2a shows the detailed energy-level information of the organic heptazole film, Al and ITO, allowing us to calculate a Schottky barrier height of 0.92 eV from the band diagram (this number was confirmed by the temperature-dependent I–V measurements presented in Supplementary Figures S4a and b). According to the band diagrams for a single diode in Figures 2a and b, the 0.3 V VOC originated from the work function difference between the ITO and Al (Schottky) electrodes:31, 32 when the blue light illuminated the heptazole thin film through the glass substrate, the electron charges moved to Al and the hole charges migrated to the ITO side until the heptazole band became flat. We then extended the single-diode band diagram to a double-diode band model, as shown in Figures 2c–f, which, respectively, picture dark, left diode-illuminating (L2), right diode-illuminating (L1) and simultaneous illuminating (L1, L2) situations. Figures 2d and e explain the way the two-diode PV circuit could gain a VOUT of 0.3 V and −0.3 V, respectively, while Figure 2f illustrates why our diode circuit reverted to a VOUT of 0 V. Figure 2f simply describes the recombination of the photo-induced electrons and holes in the midpoint (the connection electrode) between the two diodes so that the point had zero potential. This explained the photo-induced I–V curve behavior shown in Figures 1d and e comprehensively. Moreover, the phenomena could be applied to increase the VOUT or VOC by using multiple (tandem) diode PV circuits as shown schematically in Figure 2g, where four diodes connected in series produced a twice-larger VOUT of 0.6 V under light conditions.

Figure 2
figure 2

The band diagrams of a single diode (a) after and (b) before illumination (L). The band diagrams of two diode PV circuit (c) in the dark, under (d) left diode-illuminating (L1-off/L2-on), (e) right diode-illuminating (L1-on/L2-off) and (f) simultaneous illuminating (L1-on/L2-on). Each band diagram explains how VOUT signal is generated. (g) The band diagram of multiple (tandem) diode PV circuit produces a large VOUT of 0.6 V.

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Based on the above understandings, we measured the dynamic PV switching in a single-diode and a two-diode circuit. According to the time-domain VOUT plot of the single-diode circuit in Figure 3a, >0.3 V were repeatedly obtained under periodic blue illumination conditions, although the VOUT did not decrease to 0 V but almost stopped near 0.2 V. A similar phenomenon was observed in the two-diode circuit of Figure 3b, where L1 was periodically on while L2 stayed off all along. This slow switching-off behavior in the single diode was attributed to the long stay of floated hole charges (or slow discharge) in the ITO. This long stay was presumed to be caused by the hole barrier at the interface between the heptazole film and the ITO (Figure 2a). In the two-diode circuit, a long hole stay again occurred at the point between the two diodes, as evidenced by Figure 2e. Figure 3c displays the way to shorten the charge stay: this can be carried out by applying a short L2 pulse immediately after turning off L1. According to Figure 3c, the VOUT dropped closer to 0 V as the L2 pulse time grew longer. A 10-ms short L2 pulse seemed not to make a visible difference from the previous case (Figure 3b), while a 50-ms long L2 pulse achieved a very good VOUT drop without delay. In view of the band diagram of Figure 2f, the fast switch-off was easy to understand: the floated holes in the middle of the circuit rapidly recombined with the photo-electrons injected by the L2 pulse. When the L2 illumination time was long enough with L1 off, the photo-electrons were accumulated in the location between the two diodes even after recombination, eventually producing a (−) VOUT as observed in Figure 3d (which corresponds to the band model of Figure 2d). In addition, according to Figure 3e, if the L1 was always on, the periodic illumination of L2 could enable the circuit to repeat a (+) and 0 V VOUT through repeated turn-off (Figure 2e) and turn-on (Figure 2f), respectively. We therefore defined the phenomenon presented in Figure 3d as optical switching and that shown in Figure 3e as optical inversion. Figure 3f contains the example photos obtained during the PV circuit operations illustrated in Figure 3d (optical switching) and Figure 3e (optical inversion: NOT gating); the real-time records for those operations are provided in Supplementary Video File.

Figure 3
figure 3

Time domain VOUT plot obtained under periodic blue illuminations for (a) single diode and (b) two diode PV circuits. (Each inset shows schematic diode circuits.) The time domain VOUT plots obtained from two diode circuit with (c) short L2 pulse (ranging from 10 to 50 ms) after turning off L1, (d) alternate L1/L2 illuminations for optical switching phenomenon and (e) periodic L2 illumination for optical inversion (L1 always turns on during this operation). (f) Optical images of PV circuit operations for switching and inversion. (Watch Supplementary Video for more details.)

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In fact, PV optical switching could be directly applied to gate switching in a real organic FET, which also uses a heptazole thin film on glass as its active channel. Figure 4a shows a schematic device cross-section of our organic FET with a 30-nm CYTOP/50-nm atomic layer-deposited Al2O3 bilayer gate dielectric, while Figure 4c displays the drain current–gate voltage (ID–VGS) transfer curves of the FET.33 Figure 4b shows optical plan-view images of the two-diode FET circuit connected by an Al wire. Here a constant drain voltage (VDS=−1 V) was supplied to the FET, although its gating was operated by the optically switched VOUT signal of the PV diode circuit. Those PV-induced gatings are pictured in Figures 5a–i, where the PV-induced VOUT now works as a VGS. For instance, the first two-diode circuit of Figure 5a produced −0.3 and 0.3 V by periodic PV switching, as shown in the time-domain VOUT plot of Figure 5b. However, this input or gate voltage then modulated the ID from 0 to −0.7 nA on a linear scale (Figure 5c). The ID modulation became larger when multiplying the diode number in tandem mode: the second four-diode circuit of Figure 5d produced a VOUT and ID of −0.6 V and −1.4 nA, respectively (as seen in Figures 5e and f), while another circuit with eight diodes produced −1.2 V and ~−3.0 nA (as observed in Figures 5g–i). PV optical switching was again noted in a pentacene(C22H14)-based Schottky diode circuit with the same electrodes (Al and ITO), as shown in Supplementary Figures S5 and S6. According to Supplementary Figure S6, PV-induced optical inverting and switching visibly occurred even under red laser light (1.5 mW cm−2), as the energy gap of pentacene film is known to be 1.97 eV (photoelectric gap) or 1.85 eV (optical gap).34 However, the switching speed appeared to be one order of magnitude lower than that of the heptazole-based Schottky diode (fall/rise time ~0.5 s), although the switching speed improved under blue light (but remained slower than that of the heptazole device). The speed difference was believed to result from the different exciton-binding energies of the crystalline heptazole and pentacene films (40 vs 120 meV, respectively).29, 30, 34 As the recombination of the holes and electrons takes place within the electrode between the two organic diodes, efficient separation of the photo-excited charges or dissociation of the excitons in the semiconductor layer is required. Although the heptazole device must present advantages over other devices with a larger exciton-binding energy in terms of switching speed, PV-induced switching was in any case proven to be a working process. On the one hand, our organic semiconductor still had larger exciton-binding energy than its inorganic counterparts. Moreover, our organics had many traps/defects and grain boundaries causing low mobility, while most inorganic semiconductors are single crystalline. We believe that those aspects of exciton binding and defects/traps in organic semiconductors are key controlling elements of the response time delay, making it longer than those of inorganic ones (see Supplementary Table S1).21, 22, 23, 24, 25, 26

Figure 4
figure 4

(a) Schematic device cross-section of heptazole-based field effect transistor. (b) Optical images of our heptazole-based Schottky diode–transistor circuit. Blue and gray dashed lines in optical images indicate heptazole semiconductor and ITO electrode, respectively, while black curved line represents Al wire connection for transistor gating. (c) Transfer characteristics of heptazole-based field effect transistor.

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Figure 5
figure 5

(a) Schematic circuit diagram of the PV-induced transistor gating. Time domain (b) PV-induced VOUT plot from Schottky diode circuit and (c) ID plot obtained from the PV-induced gate voltage modulation. Tandem mode circuit diagrams composed of (d) four diodes and (g) eight diodes are also shown, and as a result, respective VOUT dynamics are demonstrated in panels (e) and (h) for transistor gating, which leads to ID dynamics in panels (f) and (i).

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As an additional experiment, we attempted to extend our diode circuit applications to OR and AND gate logic switching, as the optical inverting (NOT gating) in Figure 3e appeared successful. To study the OR and AND functions, we modified the circuits as shown in Figures 6a and b, using a 56-MΩ external resistor to rapidly discharge any accumulated carriers. In the case of the OR gate using a parallel diode circuit, the hole charges easily accumulated into the VOUT terminal under the blue light on both diodes for a (1,1) signal. Even when the light illuminated only one of the two diodes, the hole accumulation was still effective for a (0,1) or (1,0) signal. An issue was to discharge those hole charges for a (0,0) signal. However, this problem could be effectively resolved by using an external resistor. Figure 6c presents the time-domain plot of the OR gate operation. Using a similar principle in a series circuit, we obtained the AND gate operation shown in Figure 6d. Likewise, our PV-induced diode circuits with the heptazole film could achieve all the basic logic functions of NOT, OR and AND under blue light without using any electric power.

Figure 6
figure 6

Schematic circuit diagrams and optical images of PV logic gates for (a) OR and (b) AND functions. A 56-MΩ external resistor is connected to rapidly discharge any accumulated carriers. Time domain VOUT plots for respective logic gates are also shown in panels (c) and (d) for OR and AND.

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Conclusion

In conclusion, we fabricated an organic heptazole-based Schottky diode and related diode circuits for PV-induced switching applications, which are a kind of energy-harvesting device from blue illumination. The Schottky diode cell displayed an instantaneous voltage of 0.3 V as its VOC owing to the work function difference between two electrodes under blue illumination, and four tandem diode cells therefore produced ~1.2 V. As the photo-excited charges were accumulated into the central electrode of the diode array by illuminating the diodes in one half of the circuit, the VOC turned into photo-induced VOUT. Electron–hole recombination also took place within that electrode by illuminating the other half, thereby rapidly changing the VOUT to 0 V. Further illumination even changed the polarity of the VOUT. Based on those PV effects using an heptazole thin film with small exciton-binding energy, this study successfully demonstrated quite fast PV optical switching, logic gating and, ultimately, the gate switching of an organic FET. We therefore concluded that our self-powered PV-induced switching was novel and promising enough to open a new door for energy harvest-related device applications.