Use of an organic transistor with a permeable base as a memory or long-term photodetector and method of operating such a memory or long-term photodetector

Abstract

The invention relates to the use of an organic transistor with a permeable base (14) as a memory or as a long-term photodetector. The transistor is designed as a vertical transistor and has an emitter (11), a collector (12), an organic semiconductor (13) and a base (14). In a switched-off state, illumination with light to which the organic semiconductor is sensitive can be used to write information into the transistor, which information can, in a second operating state, then be read out on the basis of the shift in the threshold voltage of the transistor with respect to the nominal threshold voltage of the transistor. When further features of the transfer characteristic curve are evaluated, information about the illumination intensity and / or illumination duration can also be determined. In a third operating state, the transistor can be reset to an unilluminated state and the information stored by the illumination can be erased again.

Description

[0001] Use of an organic transistor with a permeable base as a memory or long-term photodetector and method for operating such a memory or long-term photodetector

[0002] The invention relates to the use of an organic transistor with a permeable base as a memory or long-term photodetector and to a method for operating such a memory or long-term photodetector.

[0003] Organic semiconductors offer several advantages over inorganic semiconductors. They are generally inexpensive, flexible, and their properties can be tailored. Furthermore, most exhibit high absorption of light in the visible range. Therefore, organic semiconductors are used both in electronics, e.g., in the form of organic transistors, and as photodetectors. On the other hand, organic semiconductors also have disadvantages compared to inorganic semiconductors, including low charge carrier mobility, undesirable background doping, and limited patterning options. Furthermore, it is difficult to achieve a large output signal for organic photodetectors.

[0004] To solve this problem, Guo et al. in [1] proposed operating a corresponding organic photodiode (OPD) under reverse bias, whereby holes at one of the electrodes or contacts of the photodiode are trapped in a hole blocking layer (HBL), so that an amplification effect for electrons can be achieved.

[0005] Another approach is to utilize the amplification in a field-effect transistor (FET), where charge storage in the gate dielectric is responsible for a significant increase in the output signal. However, such organic components usually have several disadvantages. For example, Ren & Chan in [2] describe a field-effect transistor and a photodetector array made of such field-effect transistors, which require a high operating voltage, while Kim et al. in [3] mention the low light sensitivity of the field-effect transistor, at most 1.6 A / W. Furthermore, such field-effect transistors often have only a short storage time and a slow response. This limits the efficient use of such components in conventional integrated circuits.In addition, most organic field-effect transistors exhibit a large lateral extension in the pm range, since lithography-based structuring options, as used for inorganic transistors, are difficult to implement.

[0006] Furthermore, organic transistors with a permeable base (organic permeable base transistor, OPBT) are known, which can be operated at low operating voltages down to a maximum of 2 V and with switching speeds of up to 100 MHz. Such a transistor is described, for example, by Dollinger et al. in [4]. The vertical structure of this transistor comprises a porous aluminum electrode within the channel, which acts as the base of the transistor. Guo et al. describe such a transistor in [5] with two superimposed porous aluminum electrodes separated by the organic semiconductor, which can be connected as two separate base electrodes. This allows logic elements, such as a NAND gate or an AND gate, to be realized using a single transistor. No other applications are currently known.

[0007] The object is to provide further applications of an organic transistor with a permeable base and a method for operating such an organic transistor, while overcoming some disadvantages of the prior art.

[0008] According to the invention, this object is achieved by using an organic transistor with a permeable base as a memory or long-term photodetector and by a method for operating such a transistor according to the independent claims. Advantageous embodiments of the invention are specified in the dependent claims.

[0009] A first aspect of the invention relates to the use of a known organic transistor with a permeable base as a memory or as a long-term photodetector. The organic transistor is designed as a vertical transistor and has an emitter, a collector, an organic semiconductor and a base. The emitter and the collector each comprise at least one electrically conductive material, e.g. aluminum or gold. The organic semiconductor is arranged between the emitter and the collector and comprises a first and a second sub-layer. The base consists of a permeable, porous aluminum layer which is surrounded by aluminum oxide and is arranged between the first and the second sub-layer of the organic semiconductor. The thin aluminum oxide layer has a thickness of less than 10 nm and acts as a dielectric, also referred to as a gate dielectric, wherein the aluminum oxide layer has a gate capacitance of, for example, approx.1 pF / cm. 2 generated.

[0010] In both applications, the ability of the organic semiconductor to absorb light and generate charge carrier pairs, i.e., electrons and holes, is utilized. This effect essentially only occurs in the second sublayer of the organic semiconductor, where a high electric field strength exists between the base and emitter when the transistor is off. Thus, in the transistor's off state, the majority charge carriers are discharged via the collector, while the minority charge carriers are transported to the base and trapped there in the aluminum oxide and / or at the interface between the organic semiconductor and the aluminum oxide. This leads to a change in the base potential and a shift in the transistor's threshold voltage compared to the actual nominal threshold voltage Uth (or V th) of the transistor, ie when no charges are trapped at the base. Thus, when holes are stored as minority charge carriers, the base potential is lowered and the threshold voltage is shifted to smaller values, while when electrons are stored as minority charge carriers, the base potential is increased and the threshold voltage is shifted to larger values. The amount of the threshold voltage shift results from the capacitance of the base or the aluminum oxide layer of the base and the number of trapped minority charge carriers according to the formula A th = e ■ Ntrapped / Cß, where Nt rapPed is the number of trapped charge carriers and CB is the capacitance of the base. Furthermore, the collector current increases by a factor of approximately 10 in saturation mode. Both effects naturally only occur if the organic semiconductor is sensitive to the incident light, with the strength of the effects increasing with increasing intensity and / or duration of the incident light until saturation of the aluminum oxide and / or the interface between the organic semiconductor and the aluminum oxide with minority charge carriers is reached. Furthermore, the thickness of the second sublayer, which is arranged between the base and the collector, can be used to adjust the sensitivity of the transistor to specific wavelengths of the incident light, since the base and the collector act like an optical cavity.Thus, the incidence of light of a specific wavelength and the intensity or duration of such light incidence can be determined from a shift in the threshold voltage and the magnitude of the collector current. In addition to high light sensitivity due to good photocurrent amplification and long storage times at low operating voltages, the organic transistor with a transparent base also offers the advantage of a structure that can be fabricated entirely by vapor deposition, precise adjustment of the wavelength to which the transistor is sensitive, and a small space requirement on a substrate, thus achieving a high degree of integration. This allows the production of optically writable memories and long-term photodetectors, as well as arrays of such memories or long-term photodetectors.For the purposes of this application, a memory is understood to be a component into which information can be written and read out multiple times without the written information being significantly changed. Deleting the information requires a process different from reading it out, i.e., an erase process. The ratio of the time required to write the information (also called the write time) to the time for which this information is retained in the memory without significant loss of information (also called the hold or storage time) is substantially less than 1. The ratio between write time and hold time can, for example, be less than 1:10, less than 1:100, less than 1:1000, or less than 1:10. 4 or less than 1 : 10 5 , where in any case the ratio is greater than 0. With such a memory, storage times in the range of greater than 1000 s or even in the range of greater than 10 5s feasible.

[0011] A long-term photodetector in the sense of this application is a photodetector that can detect light over a very long time and directly integrate the amount of light received over the entire time, whereby the realizable time duration of the detection is greater than 100 s, greater than 1000 s or even greater than 10 5 s. It is important to note that not all charge carriers generated by the incident light flow away or recombine in the photodetector, but that one type of the generated charge carriers remains in the photodetector throughout the entire detection period.

[0012] Known organic semiconductor materials, such as fullerene derivatives (e.g., Ceo), acene derivatives (e.g., pentacene), and thiophene- and benzothiophene-based semiconductors, can be used as the organic semiconductor. Transistors with fullerene derivatives as the organic semiconductor are referred to as n-type transistors, while transistors with acene derivatives as the organic semiconductor are referred to as p-type transistors. These semiconductor materials can also be doped, for example, with dopants for vacuum sublimation, such as W2(hpp)4 (n-type) or Fe-TCNNQ (p-type). Such doping is suitable both for improved charge carrier injection at the contacts and for presetting the transistor's threshold voltage.In some embodiments, the first and second sublayers of the organic semiconductor consist of the same semiconductor material, but can also consist of different organic semiconductor materials. They can also differ from one another in their doping with regard to the doping materials and / or their concentration, or they can be identical. The thickness of the second sublayer of the organic semiconductor is in the range between 50 nm and 300 nm in some embodiments. The thickness of the first sublayer is also in the range between 50 nm and 300 nm in some embodiments, with the thickness of the first sublayer being significantly greater, for example by a factor of 2 or 4, than the thickness of the second sublayer in specific embodiments.

[0013] In some embodiments, the base has a thickness in the range of 10 nm to 25 nm. The aluminum layer is not formed as a continuous, impermeable layer, but rather has pores (pinholes) through which electrons can pass when the transistor is switched on.

[0014] In addition to the layers or elements already mentioned, the transistor can also contain further layers or elements. For example, the emitter and / or the collector can be formed as a layer stack made of several layers, which, for example, in addition to aluminum or gold as the actual electrode material, also comprise an adhesion-promoting layer made of another material, e.g., chromium or MoOa. The adhesion-promoting layer is arranged between the actual electrode material and the organic semiconductor and also reduces the contact resistance to the organic semiconductor. In order to increase the efficiency of charge carrier generation in the semiconductor, transparent contact materials, such as indium tin oxide, can also be used. In addition, further layers of an organic semiconductor material can also be present, which, for example,serve to further reduce the contact resistance between the organic semiconductor and the emitter, or as a dedicated absorber to enhance the generation of charge carriers under irradiation of specific wavelengths.

[0015] In addition to the layers already mentioned, the organic transistor according to the present invention can also contain a further base, which is spaced from the previously described base by a further partial layer of the organic semiconductor. This further base is designed similarly to the base already described, i.e. it consists of a permeable, porous aluminum layer surrounded by aluminum oxide. The thickness of the further base also essentially corresponds to the thickness of the base already described. The further base is referred to below as the second base to distinguish it from the base described above and referred to below as the first base.The second base can either be provided with an external electrical connection so that it can be subjected to a defined potential, or it can have no such external electrical connection and is thus an insulated base with an undetermined potential, a so-called floating base. The second base can be arranged either between the first base and the emitter or between the first base and the collector. The thickness of the further sublayer of the organic semiconductor arranged between the first base and the second base is in the range of the thickness of the second sublayer of the organic semiconductor.

[0016] The second base also enables the capture and "storage" of minority charge carriers generated by incident light, as already described for the first base. Thus, the second base, in its floating base design, leads to a significant increase in the storage time of the information generated by the incident light, while in its design as a connected control electrode, it enables the light to influence the logical operations, as described, for example, by Guo et al. in [5].

[0017] Since, as described, several bases, i.e. two or more, are also possible in embodiments, “a base” in the sense of this application should be understood as “at least one base”.

[0018] The fabrication of an organic transistor with a permeable base is described in detail in the prior art, for example, in [4] and [5], and will therefore not be further described here. An organic transistor with multiple permeable bases is fabricated in a similar manner to a transistor with only one permeable base, with steps for depositing another organic semiconductor layer and for creating another aluminum-based permeable base surrounded by aluminum oxide being added. Steps for creating the first permeable base, e.g., anodizing or oxidation in air, can also be performed together with steps for creating the first permeable base.

[0019] A further aspect of the invention relates to a method for operating an above-described organic transistor with a transmissive base as a memory or as a long-term photodetector. All statements regarding the individual elements of the transistor apply as explained above. The method comprises operating the transistor in several states: a first, off state; a second state in which information written into the transistor by light incidence or no light incidence during the first state is retained or read; and a third state in which the transistor is returned to an unilluminated state, thus erasing information about light incidence.

[0020] In the first state, the transistor is in an off state. For this purpose, a first voltage between base and emitter (UBEI) and a first voltage between collector and emitter (UCEI) are applied, one of the two voltages being positive and the other negative. For an n-type transistor, the first voltage between base and emitter (UBEI) is negative and less than the threshold voltage (Uth), and the first voltage between collector and emitter (UCEI) is positive, while for a p-type transistor this is exactly the opposite. In embodiments, the first voltage between base and emitter (UBEI) is equal in magnitude to the first voltage between collector and emitter (UCEI). Preferably, both first voltages UBEI and UCEI have a magnitude in the range of 1 V to 2 V, e.g. UBEI = -2 V and UCEI = 2 V for an n-type transistor and UBEI = 2 V and UCEI = -2 V for a p-type transistor.If, in this state, light with a wavelength to which the organic semiconductor, in particular its second sub-layer, is sensitive strikes the organic semiconductor, electron-hole pairs are generated by the photoelectric effect in the organic semiconductor, in particular in the second sub-layer of the organic semiconductor, with the minority charge carriers, i.e. the holes in the case of an n-type transistor and the electrons in the case of a p-type transistor, being trapped in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer. If no corresponding light strikes the organic semiconductor, then correspondingly no minority charge carriers are trapped in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer.This means that during the first state of the transistor, information is written into the transistor or already written information is held.

[0021] In the transistor's second state, this information is read out and thus preserved via the light incident in the first state. This information can be purely qualitative, i.e., it can state whether or not light with a wavelength to which the organic semiconductor, in particular its second sublayer, is sensitive, has struck the organic semiconductor with a defined intensity or duration of light incidence. However, the information can also be quantitative, i.e., it can provide information about the intensity of the incident light or the duration of the light incidence, whereby the duration of the light incidence is of course limited by the duration of the transistor's first state. To read out the information, a second voltage is applied between base and emitter (UBE2) and the collector current is determined, while the first voltage between collector and emitter (UCEI) remains applied.The second voltage between base and emitter (UBE2) is smaller in magnitude than the first voltage between base and emitter (UBEI) and smaller in magnitude than or equal to the first voltage between collector and emitter (UCEI). For an n-type transistor, the second voltage between base and emitter (UBE2) is greater than the first voltage between base and emitter (UBEI), while for a p-type transistor, the second voltage between base and emitter (UBE2) is smaller than the first voltage between base and emitter (UBEI). For example, the second voltage between base and emitter (UBE2) can be 0 V (zero), while the voltage between collector and emitter (UCEI) remains 2 V (n-type transistor) or -2 V (p-type transistor). In the third state of the transistor, the information contained in it is reset to an unilluminated state, and the information written into it by light is thus erased.In the third state, a third voltage between base and emitter (UBES), which is greater in magnitude than the nominal threshold voltage of the transistor (Uth), and a second voltage between collector and emitter (UCE2), which is smaller in magnitude than the first voltage between collector and emitter (UCE1) and smaller in magnitude than the third voltage between base and emitter (UBES), are applied. For example, for an n-type transistor, the third voltage between base and emitter (UBES) can be 2 V and the second voltage between collector and emitter (UCE2) can be 0 V (zero), while for a p-type transistor, the third voltage between base and emitter (UBES) can be -2 V and the second voltage between collector and emitter (UCE2) can be 0 V (zero).

[0022] For the use of the transistor as a memory, the first state in embodiments exists not only during the writing of the information, but also during the holding of the information, i.e. between the writing and the first readout of the information, between two readout processes, and between a final readout process and an erase process. The second state always exists during the readout of the information, and the third state always exists during the erasure of the information. Since the transistor in embodiments remains in the first state, the off state, while the information is held, the power loss is low during this time. In addition, recombination of the minority charge carriers with majority charge carriers supplied from the outside and transported through the channel is prevented, so that the entire holding time, i.e. the time between the writing of the information and the complete removal of the information, e.g.by deleting, can become very long.

[0023] For the use of the transistor as a long-term photodetector, the first state in embodiments exists not only during the detection time of light, but also while maintaining the detected light quantity or light intensity after a readout process until the detected light quantity or light intensity is erased. The second state always exists during the readout of the detected light quantity or light intensity, and the third state always exists during the erasing of the detected light quantity or light intensity. Since the charge carriers corresponding to the detected light quantity or light intensity are not removed from the transistor by the readout, it is also possible to read out intermediate results over a long detection period, so that a temporal progression of the incident light quantity or light intensity can also be obtained.Furthermore, during the detection process, when the transistor is in the first state, the power loss remains low and recombination of the minority charge carriers with externally supplied majority charge carriers is prevented.

[0024] In embodiments, the incident light has one or more wavelengths in the range of 370 nm to 1000 nm, preferably in the range of 370 nm to 700 nm, and thus generates electron-hole pairs in the organic semiconductor.

[0025] In specific embodiments where qualitative information is to be obtained, the second base-to-emitter voltage (UBE2) is selected to be lower than the nominal threshold voltage of the transistor (Uth) for an n-type transistor and higher than the nominal threshold voltage of the transistor (Uth) for a p-type transistor, and the determined collector current is compared with a reference value corresponding to the nominal collector current of the transistor at the second base-to-emitter voltage (UBE2). In other words, it is determined whether there is a shift in the threshold voltage to smaller values ​​for an n-type transistor or to larger values ​​for a p-type transistor, which leads to an increase in the collector current at the second base-to-emitter voltage (UBE2) compared to the nominal collector current and allows the conclusion that there is a corresponding incidence of light on the transistor, in particular the organic semiconductor, in the first state of the transistor.The nominal collector current of the transistor is the collector current measured at a corresponding voltage between base and emitter when no light has hit the transistor.

[0026] "Determining a collector current" does not necessarily mean measuring the collector current precisely. As is common in logic circuits, determining whether a collector current is above or below a certain level is sufficient, which can also be achieved by triggering a subsequent component to respond accordingly. Especially for reading purely qualitative information (light incidence yes / no), a comparison with the nominal collector current is also possible using such logical evaluation by a subsequent component.

[0027] These embodiments are particularly useful when using the organic transistor as a memory, but also when used as a long-term photodetector when no quantitative information about the incident light is to be determined. For applications of the transistor as a long-term photodetector, it is useful in other embodiments to select the second voltage between base and emitter (UBE2) to be greater than the nominal threshold voltage of the transistor (Uth) for an n-type transistor and less than the nominal threshold voltage of the transistor (Uth) for a p-type transistor in order to achieve high photo amplification and thus a large output signal from the transistor, i.e. a high collector current. However, a specific measured value for the collector current can also be determined and compared with corresponding reference values ​​recorded in a storage medium.

[0028] In other embodiments where quantitative information is to be obtained, the second base-to-emitter voltage (UBE2) assumes at least one value in the range between the first base-to-emitter voltage (UBEI) and the third base-to-emitter voltage (UBES). If this value of the second base-to-emitter voltage (UBE2) is above the (known or expected) threshold voltage of the illuminated transistor for an n-type transistor and below the (known or expected) threshold voltage of the illuminated transistor for a p-type transistor, determining a specific measured value of the collector current makes it possible to obtain quantitative information if the illumination time, i.e., the time during which the information was written into the transistor, is known and equal to the time used to obtain the reference value.In specific embodiments, the second voltage between base and emitter (UBE2) assumes several values ​​in the range between the first voltage between base and emitter (UBEI) and the third voltage between base and emitter (UBES). In other words, values ​​of a current-voltage characteristic curve of the transistor in its current information state are determined. The respective collector currents determined are compared with reference values ​​known for known light intensities and / or known wavelengths of an incident light. Further details will be explained later with reference to the figures. The determined threshold voltage of the transistor and, if applicable, a hysteresis of the collector current when the voltage between base and emitter is increased and decreased can also be used to quantitatively determine the light intensity and / or the wavelength of the incident light.

[0029] The invention is not limited to the illustrated and described embodiments, but also encompasses all embodiments having the same effect within the meaning of the invention. Furthermore, the invention is not limited to the specifically described combinations of features, but can also be defined by any other combination of specific features of all the individual features disclosed as a whole, provided that the individual features are not mutually exclusive or a specific combination of individual features is not explicitly excluded.

[0030] The invention is explained in more detail with reference to the drawings, in which like reference numerals denote similar components or elements, and the dimensions of the individual elements and their relationship to one another are not shown to scale, but only schematically. The illustrated embodiments relate to an organic n-type transistor with a permeable base.

[0031] Fig. 1A shows a schematic structure of an organic transistor with exactly one permeable base, as used in embodiments of the invention, Fig. 1B shows a schematic structure of an organic transistor with two permeable bases, one of which is an isolated base,

[0032] Fig. 2A to 2C show different characteristics of such a transistor for different illumination intensities,

[0033] Fig. 3A to 3D show schematic band models to explain the processes in such a transistor during operation as well as the associated operating voltages, Fig. 4 shows a representation of the transfer characteristic of such a transistor after different operating states,

[0034] Fig 5 a photodetector array of several such used as photodetectors

[0035] transistors,

[0036] Fig. 6A an overview of properties of various memory transistors,

[0037] Fig. 6B an overview of properties of various phototransistors,

[0038] Fig. 7 shows the dependence of the threshold voltage shift on the intensity and

[0039] Duration of light incidence and

[0040] Fig. 8 shows the dependence of the current measured during the readout on the

[0041] Hold time and the voltage between base and emitter during the hold time of the transistor.

[0042] Figures 1A and 1B schematically show the structure of an organic transistor 10 or 100 with a transmissive base, which is used according to the invention as a memory or as a photodetector. The embodiments of Figures 1A and 1B differ in the number of bases present. In both cases, the organic transistor 10 or 100 is arranged and constructed on a substrate 1, which consists of any material suitable for the application of the transistor, such as a glass substrate, a plastic film, an insulator, or a metal foil, or which comprises or contains such materials. The transistor 10 of Figure 1A is designed as a vertical transistor in which the individual components of the transistor are arranged one above the other. It has an emitter 11, a collector 12, an organic semiconductor 13, and a base 14.The emitter 11 is formed, for example, from a 100 nm thick aluminum layer, but can also comprise other electrically conductive materials. The collector 12 is formed from a 50 nm thick gold layer in the illustrated example, but can also comprise other electrically conductive materials. The organic semiconductor 13 is Ceo in the illustrated example and has two sublayers: a first sublayer 131, which is arranged between the emitter 11 and the base 14, and a second sublayer 132, which is arranged between the collector 12 and the base 14. Both sublayers 131 and 132 consist of the same material in the illustrated example, while in other embodiments, different organic materials or identical organic materials with different dopants or doping concentrations can be used for the two sublayers.In the example shown, the first sublayer 131 has a thickness of 100 nm, while the second sublayer 132 has a thickness of 50 nm. The thickness of the second sublayer 132 determines the wavelength of the incident light to which the transistor 10 is sensitive and can be adjusted with a precision of a few nm during transistor manufacture. The base 14 comprises a porous aluminum layer surrounded by aluminum oxide (Al2O3) and has a thickness of 15 nm. The aluminum oxide layer has a thickness of less than 5 nm.

[0043] In addition to these essential elements of the transistor, the transistor 10 also has two adhesion-promoting layers 15 made of chromium, each 10 nm thick. Each adhesion-promoting layer 15 is arranged between the emitter 11 and the organic semiconductor 13, or between the collector 12 and the organic semiconductor 13, and improves the electrical transition between the respective electrode layer and the organic semiconductor. Furthermore, a doped semiconductor layer 16 consisting of CeO doped with 1 wt. % W2(hpp)4, with a thickness of 20 nm, is arranged between the adhesion-promoting layer 15, which adjoins the emitter 11, and the first sublayer 131 of the organic semiconductor 13. The adhesion-promoting layers 15 and, if appropriate, also the doped semiconductor layer 16 can each also be considered part of a layer stack that forms the emitter and the collector, respectively.

[0044] In Fig. 1, the electrical connections to the emitter 11, the collector 12, and the base 14 are also shown schematically and labeled E, C, and B. Furthermore, the voltage between the base and emitter and the voltage between the collector and emitter are also shown and labeled UBE and UCE, respectively. The base current l B and the collector current l c are shown.

[0045] The transistor 100 shown in Fig. 1B differs from the transistor 10 shown in Fig. 1A in that it has a first transmissive base 141 and a second transmissive base 142 as well as a further sub-layer 133 of the organic semiconductor 13. The first base 141 corresponds to the base 14 of the transistor 10 from Fig. 1A and is electrically connected to the outside. The second base 142 is basically designed similarly to the first base 141, i.e. in terms of material and thickness, but is not electrically connected to the outside and therefore represents an insulated or floating base. The further sub-layer 133 is arranged between the first and second bases 141, 142, is also made of Ceo and has the same thickness as the second sub-layer 132, as described with reference to Fig. 1A.

[0046] Using Figures 2A to 2C, various characteristics of transistor 10 for different light intensities are explained below. Fig. 2A shows the transfer characteristic of transistor 10, i.e., the dependence of the collector current lc (more precisely, the collector current density) on the voltage between base and emitter UBE, for illumination with an LED with intensities between 0 mW / cm 2 (zero, "dark") and 50 mW / cm 2for a duration of greater than 1 s, whereby the transistor 10 was first returned to the unlit state for the determination of the respective transfer characteristic. How this happens will be explained later with reference to Figures 3C and 3D. As can be seen from Fig. 2A, the threshold voltage of the transistor 10, which in the unlit state is approximately 1.1 V, which is the nominal threshold voltage Uth of the transistor 10, shifts to lower voltages, whereby the effect is already very clear for an illumination intensity of 0.5 mW / cm 2 , is. In addition to the shift in the threshold voltage, an increase in the collector current lc at a voltage between base and emitter UBE of 2 V by a factor of 10 for illumination intensities greater than or equal to 0.5 mW / cm 2to be recognized. The greater the illumination intensity, the greater the hysteresis in the transfer characteristic, whereby the ascending branch, ie for voltages between base and emitter that run from lower values ​​to higher values, has a lower threshold voltage than the descending branch, ie for voltages between base and emitter that run from higher values ​​to lower values.

[0047] Fig. 2B shows the dependence of the threshold voltage of transistor 10 on the illumination intensity. Two threshold voltage values ​​were determined for each of the tested illumination intensities, with the lower voltage value being determined in the ascending branch of the hysteresis and the higher voltage value in the descending branch of the hysteresis. Once again, the shift in the threshold voltage to lower values ​​for transistor 10 illuminated with a significant illumination intensity is clearly visible.

[0048] Figure 2C shows the dependence of the sensitivity of the transistor 10 on the voltage between base and emitter UBE for the different illumination intensities indicated in Fig. 2A.

[0049] The following explains the processes in transistor 10 during its operation as a memory device and the associated operating voltages using Figures 3A to 3D. Figures 3A to 3C schematically show a band model of the transistor with the respective energy levels (relative to each other, not absolute), while Figure 3D shows the operating voltages in the respective operating states Z1 to Z3.

[0050] Fig. 3A shows the process of storing information in transistor 10 by illumination with light of wavelength X, to which the organic semiconductor 13 of transistor 10 is sensitive. The light leads to the generation of electron-hole pairs, particularly in the second sublayer 132 of the organic semiconductor 13 of transistor 10, while the transistor is in an off operating state Z1. In this state Z1, the voltage between base and emitter is equal to a first, negative voltage UBEI , which is, for example, -2 V, while the voltage between collector and emitter is equal to a first, positive voltage UCEI , which is, for example, 2 V.The electrons generated in the second sublayer 132 flow to the collector, while the generated holes are trapped in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer, lowering the energy level of the base (indicated by the arrow). The resulting shift in the threshold voltage of transistor 10, or a lack of shift in the threshold voltage of transistor 10 when no light impinges on the organic semiconductor of transistor 10, represents information that can be read out in a second operating state Z2.

[0051] The band model of a previously illuminated transistor 10 is shown in Fig. 3B in the second operating state Z2. In this state, a second voltage UBE2 is applied between base and emitter, which is greater than the first voltage UBEI and less than or equal to the first voltage between collector and emitter UCEI, while the voltage between collector and emitter continues to correspond to the first voltage UCEI. To obtain purely qualitative information about the state of transistor 10, namely whether it was previously illuminated with a specified illumination intensity or not, it is sufficient that the second voltage UBE2 is less than the nominal threshold voltage U t h of the transistor 10, for example UBE2 = 0 V. A fixed illumination intensity according to Fig. 2C would be, for example, an intensity of greater than or equal to 0.5 mW / cm 2In state Z2, electrons can pass through the base, which is lowered due to the trapped holes, to the collector and generate a significant collector current. This process is referred to as reading the transistor 10. The holes trapped in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer are essentially retained. If the transistor is returned to the first operating state Z1 after the readout process, the written information remains 10 even for a longer period of time, e.g., over 1000 s. 5 s or more, and is thus held or stored.

[0052] Fig. 3C shows the band model of a previously illuminated transistor 10 in the third operating state Z3. In this state, a third voltage is applied between base and emitter UBES, which is greater than the nominal threshold voltage Uth of transistor 10, while a second voltage is applied between collector and emitter UCE2, which is smaller than the first voltage between collector and emitter UCEI and smaller than the third voltage between base and emitter UBES. For example, UBES is 2 V and UCE2 is 0 V. In this state, the holes generated during illumination and trapped at the base until then flow from the base to the collector. This erases the information stored in transistor 10 by the illumination and resets it to information corresponding to the unilluminated state.

[0053] Fig. 3D shows, by way of example, that states Z1 to Z3 follow one another directly, whereby after the information in Z3 has been erased, the transistor is returned to the first operating state Z1 in order to be able to write new information. However, the information can also be read out of transistor 10 several times before the information in state Z3 is erased again. In other words: state Z2 can last longer than the other two states, in particular than state Z3, or transistor 10 can first be returned to an off state (Z1) after reading (Z2) before being read again (Z2) or erased (Z3). The second variant mentioned has the advantage that the power loss remains low if the hold time is implemented in the off state of the transistor (Z1), and that the information can be retained for longer.The information remains until a deletion process (Z3) or at least for a period of 10. 3 s obtained. Figure 4 shows the transfer characteristic of the transistor 10 for different states: for a never-illuminated state, a first switching after illumination with an intensity of 50 mW / cm 2, a multiple switching of the transistor after such illumination and for an erased state. As can be seen, although the transfer characteristic curve shifts for multiple switching after illumination compared to the first switching after illumination, the characteristic curve remains stable even for multiple switching, so that the information in the transistor is retained even during repeated read operations. Furthermore, it can be seen that an erasing operation as described above actually leads to a resetting of the transfer characteristic curve to a state that has never been illuminated before, whereby the slight shift of the two curves is not significant for logical evaluation of the information.

[0054] With reference to Figures 3A to 4, the use of transistor 10 as a memory was demonstrated. However, transistor 10 can also be used as a photodetector, for example, for a specific wavelength of incident light. By specifically evaluating the threshold voltage and collector current lc of transistor 10, quantitative information about the illumination intensity or illumination duration of the incident light can also be obtained.

[0055] Figure 5 shows an array of several of the transistors 10 described above, which can be used either as long-term photodetectors or as memories. The transistors are arranged in three rows and four columns, with the emitters and collectors of all transistors in a column being electrically connected to one another, forming a common emitter E1 to E4 and a common collector C1 to C4, while the bases of all transistors in a row are connected to one another, forming a common base B1 to B3. Controlling and reading / erasing a single transistor is possible, as with other known arrays of memories or photodetectors, and is known from the prior art.

[0056] Figures 6A and 6B each show an overview of the properties of various organic transistors made of different materials and constructed as listed, i.e., as vertical or lateral memory or phototransistors. The properties listed include write time, hold time, response (also known as responsiveness or sensitivity), and specific detectivity. For ease of comparison, the memory transistors listed are optically writable components. The integration or write time is the maximum time in which a photodetector can integrate received light as information; after this time, the photodetector is saturated, as explained later with reference to Fig. 7. For memory components, this is the time required to write information into the component.The hold time is the maximum time that information can be held in the transistor before it can no longer be recognized using a normal readout method. Sensitivity is a measure of the input-output gain of a detector system, i.e. the ratio of electrical output to optical input for a photoelectric component such as a photodetector. The specific defectivity of a photodetector is a performance indicator that corresponds to the inverse of the noise equivalent power (NEP), normalized by the square root of the sensor area and the frequency bandwidth. Defectivity therefore provides information about how strong an optical signal must be for it to still be reliably received by the detector and interpreted as a measured value. The higher this indicator is, the weaker an optical signal can be.

[0057] All values ​​except those for the inventive organic transistor with a transmissive base are taken from various publications. Detective values ​​calculated based on dark currents are marked with "c," while defectivities based on noise measurements are marked with "m." Where necessary, write and hold times were estimated using the -3 dB cutoff frequency:

[0058] 3 5 f-3 dB ~ : ■ Non-determinable parameters are marked with

[0059] As can be seen from the overviews, the sensitivity and specific defectivity values ​​achieved with the inventive organic transistor with a transmissive base exceed many previously reported organic phototransistors and are comparable to the most powerful devices. When considering a combination of both values, the inventive transistors also perform exceptionally well, which is advantageous for the application of these transistors as long-term photodetectors. All organic phototransistors included in the overview exhibit higher sensitivity than conventional organic photodiodes. However, this usually comes at the expense of integration time. Nevertheless, most phototransistors cannot be operated as memory devices because their hold time is too short. The memory transistors shown in the overview, in contrast, exhibit hold times of > 10 3 s and often even > 10 5s, with write times of about 1 s. However, these components lack a large photo gain, ie the sensitivity is low. In contrast, the transistor according to the invention already shows excellent sensitivity at write times of 1 s, with a hold time of > 10 5 see

[0060] Components containing C60 as an organic semiconductor, possibly combined with an absorber material, offer particularly good performance both as long-term photodetectors and as memory. Pentacene, on the other hand, is particularly suitable as an organic semiconductor for use in the green and red wavelength range (550-700 nm) due to its stronger absorption there.

[0061] Figure 7 shows the dependence of the threshold voltage shift A th on the intensity and duration of the light incidence, where the duration of the light incidence corresponds to the write time. On the one hand, measured data from experiments with an exemplary transistor according to the invention are shown, and on the other hand, data calculated using a model, with the modeled data in good agreement with the experiments. For all measured write times, a linear increase in the threshold shift is observed for lower light intensities, which transitions to a constant saturation value of AVth, which is identical under all write conditions. This saturation is due to the fact that the number of captureable, i.e. storable, photogenerated minority charge carriers is limited by the number of trap sites in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer.In the low-intensity range, however, AVth is proportional to the light intensity. Thus, depending on the intensity of the incident light, a shorter or longer write time or detection time is possible or necessary before a further increase in the threshold voltage shift is achieved. For a memory application, high light intensities and short write times are therefore more suitable, whereas in a long-term photodetector application, even low light intensities can be detected and the detection time can be maintained during the relatively long detection time of, for example, 10 3 s can be integrated.

[0062] Figure 8 shows the dependence of the current measured during readout on the hold time and on the voltage between base and emitter during the transistor's hold time. For the measurements marked by symbols, the transistor was written with UCE = 2 V and UBE = -2 V for 1 s, held for various times and at various UBE voltages, and then read out at UCE = 0 V and UBE = 2 V, as described with reference to Fig. 3D. The dashed lines show continuous measurements of the collector current. All values ​​shown are normalized to the readout current. No illumination was applied during the hold time. For holding the component at the voltage values ​​of the write step (VBE = -VCE = -2 V), the information is available for > 10 5s stable. However, if the base voltage UBE is increased after writing, the state, i.e., the written or stored information, decays. The decay time, or rather, the rate of decay of the information, depends on the base voltage UBE. The main contribution to this information decay can be attributed to electron-hole recombination, with higher base voltages UBE facilitating electron injection into the channel and thus recombination with trapped holes. As a result, the transistor's hold time, i.e., the time until written information can still be reliably read, decreases with increasing base voltage UBE.

[0063] Cited non-patent literature:

[0064] [1] Guo, D., Yang, D., Zhao, J., Vadim, A. & Ma, D.: “Role of interfaces in controlling charge accumulation and injection in the photodetection performance of photomultiplication-type organic photodetectors”, J. Mater. Chem. 0 8, 9024-9031 (2020)

[0065] [2] Ren, X. & Chan, P. K. L: “23 bits optical sensor based on nonvolatile organic memory transistor”, Appl. Phys. Lett. 104, 113302 (2014)

[0066] [3] Kim, Y. et al.: “A Hemispherical Image Sensor Array Fabricated with Organic Photomemory Transistors”, Adv. Mater. 35, 2203541 (2023)

[0067] [4] Dollinger, F. et al.: “Vertical Organic Thin-Film Transistors with an Anodized Permeable Base for Very Low Leakage Current”, Adv. Mater. 2019, 31 , 1900917 (2019)

[0068] [5] Guo, E. et al.: „Organic Permeable Base Transistors - Insights and Perspectives“, Adv. Optical Mater. 2021 , 9, 2002058 (2021)

[0069] Bezugszeichen

[0070] I Substrat

[0071] 10, 100 transistors

[0072] II Emitter

[0073] 12 collector

[0074] 13 Organic semiconductor

[0075] 131 First sublayer of the organic semiconductor

[0076] 132 Second sublayer of the organic semiconductor

[0077] 133 Further sublayer of the organic semiconductor

[0078] 14 Base

[0079] 141 First Base

[0080] 142 Second Base

[0081] 15 Adhesion layer

[0082] 16 Doped semiconductor layer

[0083] B Basic connection

[0084] C Collector connection

[0085] E Emitter terminal

[0086] IB base current lc collector current

[0087] UBE voltage between base and emitter

[0088] UCE voltage between collector and emitter

[0089] Uth Nominal threshold voltage of the transistor

Claims

Patent claims 1. Use of an organic transistor which is designed as a vertical transistor, comprising an emitter, a collector, an organic semiconductor and a base, wherein the emitter and the collector each comprise at least one electrically conductive material, the organic semiconductor is arranged between the emitter and the collector and comprises a first and a second partial layer and the base consists of a permeable, porous aluminum layer which is surrounded by aluminum oxide and is arranged between the first and the second partial layer of the organic semiconductor, as a memory or as a long-term photodetector.

2. A method for operating an organic transistor, which is designed as a vertical transistor, comprising an emitter, a collector, an organic semiconductor, and a base, wherein the emitter and the collector each comprise at least one electrically conductive material, the organic semiconductor is arranged between the emitter and the collector and comprises a first and a second sub-layer, and the base consists of a permeable, porous aluminum layer surrounded by aluminum oxide and arranged between the first and the second sub-layer of the organic semiconductor, as a memory or as a long-term photodetector, wherein light incident on the organic semiconductor generates electron-hole pairs by the photoelectric effect in the second sub-layer of the organic semiconductor, which is arranged between the base and the collector,wherein the minority charge carriers are trapped in the aluminum oxide layer of the base and / or at the interface between the organic semiconductor and the aluminum oxide layer, while the organic transistor is in a first, off-state in which a first voltage between base and emitter (UBEI) and a first voltage between collector and emitter (UCEI) are applied, wherein the first voltage between base and emitter (UBEI) and the first voltage between collector and emitter (UCEI) are provided with different signs, qualitative or quantitative information about the incident light from the transistor is obtained by applying a second voltage between base and emitter (UBE2) and determining the collector current in a second state in which the first voltage between collector and emitter (UCEI) is applied, wherein the second voltage between base and emitter (UBE2) is smaller in magnitude than the first, Voltage between base and emitter (UBEI) and is less than or equal to the first voltage between collector and emitter (UCEI), and the information contained in the transistor about the incident light is reset to information corresponding to an unilluminated state in a third state in which a third voltage between base and emitter (UBES), which is greater than the nominal threshold voltage of the transistor (Uth), and a second voltage between collector and emitter (UCE2), which is less than the first voltage between collector and emitter (UCEI) and less than the third voltage between base and emitter (UBES), are present.

3. Method according to claim 2, wherein the information written by the light incident on the organic semiconductor and the minority charge carriers captured thereby is stored for a period of time greater than 10 3s or until the third state of the transistor is set.

4. The method according to claim 2 or 3, wherein the organic transistor is in the first, off state when no information is read out or the information is not reset.

5. The method according to any one of claims 2 to 4, wherein incident light having one or more wavelengths in the range of 370 nm to 1000 nm generates electron-hole pairs in the organic semiconductor.

6. The method according to any one of claims 2 to 5, wherein in the second state, qualitative information is obtained by comparing the second voltage between base and emitter (UBE2) for an n-type transistor less than the nominal threshold voltage of the transistor (Uth) or for a p-type transistor greater than the nominal threshold voltage of the transistor (Uth) and the determined collector current with a reference value corresponding to the nominal collector current of the transistor at the second voltage between base and emitter (UBE2).

7. Method according to one of claims 2 to 5, wherein in the second state a quantitative information is obtained by the second voltage between base and emitter (UBE2) assuming at least one value in the range between the first voltage between base and emitter (UBEI) and the third voltage between base and emitter (UBES) and comparing the respectively determined collector currents with reference values ​​which are suitable for known light intensities and / or known wavelengths of an incident light are known.

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