Organic N-type Electrets for Field-effect Transistors, Photo Memories and Artificial Synapses

: Electrets, typically with permanently-trapped charges, are widely used for electronic devices, actuators and medical filtering. Here, organic n-type electrets, generated from oxygen-degraded n-type semiconductors, are proposed as photo-induced electrets. The sheet charge densities of such n-type electret films, as high as 7.47 × 10 12 cm -2 , are used to provide steady built-in electric field to significantly improve the performance of organic field-effect transistors (OFETs). For example, OFETs made of p-type semiconductor 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT), in combination with n-type electret N,N ′ -Dioctyl-3,4,9,10-perylenedicarboximide (C 8 -PTCDI), show a hole field-effect mobility 13.3 cm 2 · V −1 · s −1 , along with a memory window over 100 V, a memory on/off ratio 10 6 and a stable retention life time over one month in ambient air. Subsequently, the generality of n-type electrets for transistors and photo memories applications are demonstrated. Furthermore, OFETs with organic n-type electrets are employed in artificial synapse, and the recognition rate of brain-inspired neuromorphic computing is 65%.


INTRODUCTION
Insulator materials with quasi-permanently trapped charges are generally named as insulator electrets 1 , which are widely used in transistors 2 , nanogenerators [3][4] , energy harvesters 5 , microphones 6 and filters 7 . For example, organic field-effect transistors (OFETs) are attracting sustainable interests for low-cost flexible electronics 8 . Typically, OFETs require high field-effect mobility, large on/off ratio, tunable threshold voltage, steep subthreshold swing, and good stability. To improve OFETs performance, insulator electrets, as an "container" for trapping neat charges, provide built-in electric field to manipulate the accumulation and depletion of mobilized charges within semiconductor layer, and thus to tune the charge transport in semiconductor layer. In fact, insulator electrets are not only used to manipulate threshold voltage 9 , but also contribute to improving on/off ratio of OFETs 10 . Moreover, with the built-in inhomogeneous electric field, provided by inhomogeneous insulator electrets, the measured field-effect mobility is improved, with optimized subthreshold swing 11 .
However, as compared with metal or semiconductor, insulator materials are simultaneously featured with low electron affinity potential (for electron storing) and high ionization potential (for hole storing), which means that the trapped charges are energetically unfavorable, tending to leak away. Moreover, the intrinsic low mobility of carriers in insulators implies that charge injection into the insulator requires substantial time-and/or energy-consuming, for example, by high voltage or at high temperature [12][13] . On the other hand, chemical dopants with suitable electron affinity or ionization potential provide alternative hosts for stable charge storage 14 . However, the chemical doping induced charge transfer from semiconductors to dopants is typically irreversible and usually independent on external stimulations (for example, light illumination), which thus is not easily applicable to electronic switches or memory devices.
Taking above considerations into account, in this work we propose a class of new electrets, namely oxygen-degraded n-type semiconductors. That is, after degradation by oxygen species, the n-type semiconductors become electrically insulating and charging of this "insulating" electret can be realized by light illumination, which endows the n-type electret with a unique characteristic as a photo-induced electret.
Actually, the oxidation of p-type semiconductors with high ionization potential usually induces extra holes to fill the traps, contributing to a higher electrical conductivity 9 . On the contrary, the oxidation induced degradation of n-type semiconductors typically induces new traps to immobilize electron carriers, significantly decreasing the electron mobility. Although these oxygen-degraded n-type semiconductors typically show several orders of magnitude lower electron mobility than the undegraded materials, their electrons affinities are almost kept and thus can stably accommodate sufficient trapped electrons. Consequently, this work provides an easily-accessible method to generate photo-induced organic n-type electrets, showing wide applications in OFETs, photo memories and artificial synapses.

RESULTS AND DISCUSSION
In this work, oxygen-degraded N,N'-dioctyl-3,4,9,10-perylene diimide (C8-PTCDI) is taken as an example for n-type electret (Fig. 1). For clarity, we use a n-type semiconductor/p-type semiconductor (n/p) heterojunction architecture for OFETs application, while p-type semiconductor is 2,7-diocty [1]benzothieno [3,2-b] [1]benzothiophene (C8-BTBT). The photoresponsive behavior of OFETs is commonly realized by photo-induced charge transfer from donor into the acceptor 15-16 . This behavior actually involves two processes including the electron excitation and charge transfer. Take n/p heterojunction of C8-PTCDI/C8-BTBT as an example. When the photons are absorbed by C8-BTBT molecules, the electrons are photo-excited from the highest occupied molecular orbit (HOMO) into the lowest unoccupied molecular orbit (LUMO), leaving holes in the HOMO and excitons are thus formed 17 . Then, the electric field, provided by the energy offset between the C8-BTBT donor and C8-PTCDI acceptor, affords a driving force for exciton separation. That is, the electrons excited into LUMO level of C8-BTBT are transferred to the LUMO level of C8-PTCDI (Fig.   1a). After light illumination, C8-PTCDI and C8-BTBT obtains equal number of electrons and holes, respectively. The increase of hole concentrations in C8-BTBT after light illumination improves drain current (Ids) of the OFETs, which refers to the wellknown photoconductivity. However, due to the intrinsic electron mobility of C8-PTCDI, the electrons tend to be instable under external stimulus (e.g. the sweep of gate voltage, Vgs), and finally are extracted from C8-PTCDI. The abovementioned instability makes C8-PTCDI a poor container for storing electrons, failing to act as the conventional electrets to stably supply built-in electric field to well regulate the accumulation or depletion of holes in the semiconducting layer. Consequently, the photoconductivity, which is a characteristic of such a n/p heterojunction, is actually not maximally achieved 18 .
Here, an easily accessible and repeatable method is proposed to suppress the unintended electron extraction from the C8-PTCDI molecules by utilizing the oxygeninduced degradation. In fact, most organic semiconductors can be gradually degraded by the ambient oxygen, especially for n-type semiconductors, leading to a significant decrease of electrical conductivity due to the formation of numerous traps [19][20] . This is the well-known ambient degradation of organic electronics. However, the much lower electron mobility of degraded n-type semiconductors enables themselves as good organic n-type electret materials for stably storing electrons, and the n-type electrets provide sustainable built-in electric field to effectively tune the accumulation of holes in the conductive channel. In a word, the electrons, photo-excited from C8-BTBT, are transferred and stably trapped (immobilized) inside the degraded C8-PTCDI. These trapped electrons inside C8-PTCDI, namely organic n-type electrets, provide additional electric field to to the gate-voltage-induced field and regulate the accumulation or depletion of holes in C8-BTBT (Fig. 1b), and thus the photoconductivity of OFETs are boosted and the conductivity can be kept for a longer time. Coincidentally, the oxidation-boosted charge trapping is recently reported in InSe field-effect transistors 21 , demonstrating that the degradation of semiconductors shows practical applications in electronic devices.
The bottom-gate top-contact OFETs with n/p heterojunction are demonstrated in Fig. 1a. The nominal layered interface of n/p heterojunction is in fact roughly presented, due to the polycrystalline feature of organic semiconductors 18 . Therefore, although C8-PTCDI was deposited on C8-BTBT layer via thermal evaporation, at the interface, the n-type semiconductors are actually partly mixed with the p-type semiconductors 22 .
Moreover, the nominal thickness of n-type semiconductors is crucial for the device performance. That is, the OFETs show substantial off-state current when the thickness of C8-PTCDI is over 2 nm (Supplementary Fig. 1). This can be explained by the effective electron transport through the interconnected network of n-type semiconductors. Therefore, the following 40 nm C8-BTBT-based OFETs, in combination with 2 nm n-type semiconductor, were fabricated unless specified. In a word, the top surface of the mixed semiconductors film is not always n-type semiconductors, and this key point will be referred later.
It is noted that the OFETs performance is poor when naked SiO2 without any modification is used as the dielectric layer 9 . Therefore, atactic poly(4-fluorostyrene) (FPS) and polystyrene (PS), acting as a passivation layer, are both used to modify the surface of SiO2, respectively. And their molecular structures are shown in Supplementary Fig. 2.
Subsequently, in-situ degradation of n-type semiconductors and quantitative analyses of degradation-dependent photoconductivity of OFETs with n/p configuration are both investigated. To in-situ degrade the OFETs fabricated with pure C8-PTCDI, five strategies including air exposure (with and without heating), oxygen plasma (low and high pressure) and ozone are adopted. The maximum Ids and on/off ratio are decreased by over an order of magnitude and over two orders of magnitude, respectively, after exposure to open air for over 100 days ( Supplementary Fig. 3a). When OFETs were annealed at 120 °C in ambient conditions, the degradation of C8-PTCDI is accelerated ( Supplementary Fig. 3b). Compared with oxygen doping in the ambient, high-pressure (150 Pa) oxygen plasma could easily deteriorate C8-PTCDI and the maximum Ids is significantly decreased in 30 s ( Supplementary Fig. 3c). Basically, longer durations of plasma treatment should further increase the on-state current.
However, plasma could also etch the organic films, 9 which is thus uncontrollable to reserve enough C8-PTCDI molecules in C8-BTBT. For low-pressure (30 Pa) oxygen plasma treatment, doping efficiency is limited ( Supplementary Fig. 3d).
Ozone, as a mild way to react with organic films, could effectively deteriorate the electron charge transport in n-type semiconductor molecules, which refers to degradation of n-type semiconductors. Therefore, Ids is reduced by two orders of magnitude in a few minutes (Fig. 2a). Therefore, in the following investigation, ozone is used as a model oxygen species in this work unless specified. Atomic Force Microscopy topographies (Fig. 2b-c) show minor changes in roughness as well as the morphology of semiconductor films after ozone treatment. Infrared (Fig. 2d) and ultra violet-visible (UV-vis) absorption spectra (Fig. 2e) also show little difference after ozone treatment, indicating major molecules are not degraded. Meanwhile, the glass coated by mixed semiconductors films with n/p heterojunction show a good optically transparency over 95% in visible region (Fig. 2f), demonstrating a potential application of n-type electret for transparent electronics. The height profile and transparency of the films are both shown as the inset of Fig. 2f.
To study the HOMO energy level of degraded C8-PTCDI, ultraviolet photoelectron spectroscopy spectra are investigated, which actually involves three parameters. That is, incident photon energy (hν = 21.2 eV), high binding energy cutoff (Ecutoff) and the onset of degraded C8-PTCDI thin film relative to the EF of Au (Eonset) 23 . From Fig. 2gh, Ecutoff = 17.68 ± 0.02 eV and Eonset = 2.46 ± 0.02 eV are obtained, respectively. The HOMO energy is thus calculated according to the following equation: Therefore, for degraded C8-PTCDI thin film, EHOMO = 5.98 ± 0.04 eV, while the LUMO energy of degraded C8-PTCDI thin film is: where Eg is the optical gaps. Eg = 1.98 eV (Fig. 2i) can be obtained from the UV-vis absorption spectra by using Tauc plot 24 . Therefore, ELUMO of 4 ± 0.04 eV and EHOMO of 5.98 ± 0.04 eV are calculated in this work, which is consistent with previous reports 22, [25][26] . Meanwhile, for as-prepared C8-PTCDI, ELUMO of 4.07 ± 0.06 eV and EHOMO of 6.02 ± 0.06 eV are obtained from Supplementary Fig. 4, demonstrating that the energy levels of the as-prepared C8-PTCDI and degraded C8-PTCDI are almost identical.
Actually, the degraded molecules of C8-PTCDI are solely a minority in terms of whole C8-PTCDI, which can be used to explain both the lowered electron mobility and unchanged energy levels of degraded C8-PTCDI. This part will be discussed later. Even though the ratio of degraded molecules is low compared with whole C8-PTCDI molecules, the electrical conductivity of C8-PTCDI-based OFETs is significantly decreased after ozone treatment, which is necessary for electret materials. Due to the fact that the energy levels of degraded C8-PTCDI are almost unchanged, electrons could be spontaneously transferred from C8-BTBT into degraded C8-PTCDI by energy offset.
On the other hand, poor conductivity makes the degraded C8-PTCDI could easily carry and store electrons even under cyclic external stimuli (e.g. the sweep of Vgs lower than 100 V). These trapped electrons appeal equal number of holes in C8-BTBT due to the Coulombic interaction. Finally, the electrical conductivity of C8-BTBT-based OFETs, affected by organic n-type electrets, is significantly enhanced under light illumination, indicating that organic n-type electrets contribute to improving the photoconductivity of OFETs.
In order to verify the role of organic n-type electrets for device applications, OFETs with configuration of C8-PTCDI/C8-BTBT is used as an example, and the relationship between OFETs photoconductivity and degrees of C8-PTCDI degradation is quantitatively investigated. For as-prepared OFETs, onset voltage (Von) is positively shifted from -4 V to 10 V, and the maximum Ids, referring the Ids at Vgs = -60 V, is increased twice in magnitude under light illumination (Fig. 3a), which is a sign of photoconductivity 27 . To quantitatively evaluate photoconductivity, the shift of onset voltage (∆Von) and Iphoto/Idark ratio will be discussed. That is, ∆Von, the Von discrepancy of transfer curves measured in the dark and light illumination, is 14 V. Iphoto/Idark ratio, the ratio of photocurrent and dark current at a specified Vgs, is 1.3×10 5 (Fig. 3a). Here, Iphoto and Idark is measured while keeping light on and in the dark, respectively. The specified Vgs is chosen as Von. After C8-PTCDI molecules are partially degraded, ∆Von and Iphoto/Idark ratio is increased to 120 V and 1. Meanwhile, Iphoto in different light intensities (varying from 10 to 850 μW·cm -2 ) are investigated, from which Iphoto is increased with the increase of light intensities (Fig.   3d). This is because the higher light intensity, the more photo-induced excitons are generated per unit time, and thus higher charge densities of organic n-type electrets are formed, leading to higher hole concentrations in C8-BTBT.
In the following, the relationship between ozone treatment time and ∆Von as well as Iphoto/Idark ratio are investigated from over 20 devices (Fig. 3e). Initially, ∆Von, as well as Iphoto/Idark ratio, increases with the increase of ozone treatment time, as longer durations of ozone treatment make more electron trap sites in C8-PTCDI, and thus more electrons are trapped in organic n-type electrets once light illumination is applied. As a result, the stronger electric field, induced by organic n-type electrets, is bound to accumulate more holes in C8-BTBT, and thus Iphoto is increased and meanwhile the transfer curves are shifted towards positive direction. However, ∆Von, as well as Iphoto/Idark ratio, no longer increase (even decrease) with the further increase of ozone treatment time. This is because longer durations of ozone treatment can damage OFETs performance by over doping 29 , which weakened the role of organic n-type electrets.
The fluorescence spectra of 40 nm C8-BTBT film upon excitation wavelength at 365 nm were investigated, as the absorption peak of C8-BTBT is ~ 365 nm (Fig. 2e).
Compared with absorption peak, the fluorescence peak shows red shift (~ 372 nm).
Compared with pure C8-BTBT film, the fluorescence intensity of 2 nm degraded C8-PTCDI/40 nm C8-BTBT film was quenched (Fig. 3f), indicating these electrons are efficiently transferred from C8-BTBT to degraded C8-PTCDI. This point is reasonable, and can be proved by referring to other systems 16 . Furthermore, Kelvin probe force microscopy (KPFM) was also used to investigate the process of charge transfer. In general, the increase of surface potential is induced by the aggregation of holes or the loss of electrons 30 . The surface potential is increased by light illumination, compared with the initial state ( Supplementary Fig. 5), indicating that electrons are efficiently transferred from C8-BTBT to degraded C8-PTCDI. From this result, we can conclude that in C8-BTBT/C8-PTCDI blend film, C8-PTCDI is not necessarily located on the top surface, and reasons have been discussed before. In this work, the decay of OFETs photoconductivity is also investigated by measuring the current after exposure OFETs to ambient conditions for several days (Fig. 3b).
In order to make photoconductivity more intuitive, a contour ( Fig. 3g) are used, of which the brightness of color is proportional to Iphoto/Idark. Here, a hollowed flake mask with "X" shape was used to partially filter incident light upon the device with 9×9 array.
The mask is placed between light source and device, and mask and device are parallel.
First, device with 9×9 OFETs were measured in the dark to obtain Idark. Second, by using the set-up shown in Fig. 3g, Iphoto was measured one by one under light illumination. By this way, the OFETs located in "X" shape show much larger Iphoto than those of the others. Iphoto/Idark, representing brightness level, is then calculated.
Afterwards, the above steps are repeated by using hollowed flake masks with "J", "T", "U" shape, respectively. Finally, photoconductivity is represented as the pattern of "XJTU" (Fig. 3h), and the decay of photoconductivity can be easily read out by the change of "XJTU" pattern after exposure devices to the ambient conditions (Fig. 3i).
By adding n-type electrets, OFETs photoconductivity is markedly improved from  Fig. 5). Therefore, higher operational voltage is required to drive the OFETs, and Von is shifted from 0 V to ~-55 V (Fig. 4a). In general, holes are injected into insulators under gate electric field 31 . On special occasions such as in the case of quantum well-like heterojunction, holes are transferred from p-type semiconductor to n-type semiconductor under gate electric field. 16 In this work, both the two routes are In a word, the increase of VMW is correlated with the increase of trapped electrons in electrets. The third strategy, synergistically using positive Vgs & light illumination for erasing operation, shows advantages, which will be discussed later.
After the in-situ degradation of C8-PTCDI molecules, VMW becomes larger without degrading memory on/off ratio by positive Vgs (Supplementary Fig. 6d) and light illumination ( Supplementary Fig. 6e-f) because more electrons are relatively stably trapped in degraded C8-PTCDI, resulting in the further increase of hole concentrations.
As has been mentioned above, both the two routes are occurred during programming operation, in terms of the OFETs configuration with n/p heterojunction. Similarly, in the case of erasing operation by positive Vgs, electrons, from source/drain electrodes, were partially injected into n-type semiconductors 16 , while the others were transferred into insulator materials 31 , which suffers from long durations 9,31 . In other words, the number of electrons, transferred in insulator materials, is limited in short time.
The relationship between memory performance and ozone treatment time is investigated ( Supplementary Fig. 8). Memory on/off ratio and VMW are both increased with the increase of ozone treatment time due to the fact that the number of organic ntype electrets is increased. That is, longer ozone treatment time is bound to generate more molecules of degraded organic n-type semiconductor, of which these degraded molecules are correlated with the organic n-type electrets. Once the number of organic n-type electrets is increased, Iphoto is thus increased, and ΔVon is well regulated in a wider range. Therefore, memory on/off ratio and VMW are thus increased with ozone treatment. However, the OFETs performance is inevitably damaged for longer durations of ozone treatment 29 , resulting in degradation of OFETs memory performance. Note that in erasing operation, the transfer curves are measured once light illumination is turned off, which is different from previous photoconductivity test.
To validate the role of organic n-type electrets in improving VMW, C8-BTBT-based OFETs, without C8-PTCDI, are fabricated. From programming state to erasing state, the as-prepared pure C8-BTBT based OFETs show photoresponsive features, namely Ids is increased under light illumination, which can be referred to the increase of hole concentrations. That is, once the OFETs are exposed to light illumination, electrons can be extracted out from semiconductor film under electric field, due to the weak binding between electron-hole pairs 33 . However, the transfer curves of erasing state are failed to be turned back to that of initial state, even the OFETs were pre-treated by ozone treatment (Supplementary Fig. 9), indicating organic n-type electrets are indeed critical.
Interestingly, VMW is increased after OFETs were pre-treated by ozone treatment, which can be attributed to the increase of film conductance, and exciton concentration is thus reduced.
The field-effect mobility and charge densities are one of the key factors of OFETs and OFETs memories, respectively, which is crucial in practical device applications.
Here, the statistics of field-effect mobility and charge densities are investigated from over 30 papers (Fig. 4c), showing that most OFETs memories suffer from the relative lower field-effect mobility. For example, OFETs with configuration of MH(APy)-b-PS /pentacene are featured with high charge densities of 7.05×10 12 cm -2 , of which the fieldeffect mobility is only 0.66 cm 2 · V -1 · s -1 34 . To improve field-effect mobility, without reduction in charge densities, C12-BTBT/FPS based OFETs were both achieved with high field-effect mobility of 11.2 cm 2 · V -1 · s -1 and high charge densities of 6.8×10 12 cm -gate stress 31 . To address these issues, organic n-type electrets are added in C8-BTBT/FPS based OFETs, and high field-effect mobility of 13.3 cm 2 · V -1 · s -1 and high charge densities with 7.47×10 12 cm -2 were both achieved within short time in this work.
The field-effect mobility in saturation regime can be calculated using the following equation: where W and L is channel width and channel length, respectively. In addition to fieldeffect mobility and charge densities, other crucial parameters are summarized in Supplementary Table 1. On the other hand, long-term memory stability is another key factor that determines whether the devices can be practically used. The OFETs memories, with configuration of degraded C8-PTCDI/C8-BTBT/FPS, show a good cycle repeatability and environmental stability in erasing state. That is, the maximum absolute value of Ids is not decreased even after Vgs sweep for 100 times (Fig. 4d). When OFETs are exposed in ambient for 4 weeks, the maximum absolute value of Ids is reduced by only half (Fig.   4e). Meanwhile, memory on/off ratio is well kept over 10 6 for 10 4 s. By extending fitting lines, memory on/off ratio is ~10 for over 10 years (Fig. 4f). All these measurements were taken in ambient conditions with humidity of ~45 %.
To further validate the universality of organic n-type electrets in tuning charge transport of p-type semiconductors, three strategies including insulator polymers that can replace FPS, a different p-type conjugated polymer (PCDTPT) with C8-PTCDI, and C8-BTBT with different degraded n-type semiconductors (PC71BM, Br-NDI) are investigated as follows.
PS, as the alternative to FPS, is used to fabricate OFETs with configuration of C8-PTCDI/C8-BTBT/PS. After ozone treatment, VMW is boosted from 8 V to 65 V ( Supplementary Fig. 10), indicating that PS doesn't affect the role of organic n-type electrets in tuning charge transport of OFETs. As has been reported before, FPS is featured with more hole traps than that of PS 31 Fig. 11). Finally, the conjugated polymer of PCDTPT is used as a p-type semiconductor. The OFETs, with configuration of PCDTPT/C8-PTCDI, demonstrate an increase of VMW (up to 40 V) after heating at 120 o C in ambient condition for one hour (Supplementary Fig. 12). In a word, organic n-type electrets are somehow universal for improving OFETs memories.
In addition to OFETs memories, organic n-type electrets also show their potential applications in light stimulated synaptic transistors, and several important neuronal/synaptic functions, such as excitatory postsynaptic current (EPSC), paired pulse facilitation (PPF), short term memory (STM) and long term memory (LTM) are studied.
The recognition of pictures through human eyes is actually a process of information transfer, learning and memory, while the carriers of information are neurotransmitters (Fig. 5a). The neurotransmitters, via neurons, are eventually transmitted to visual cortex and STM or LTM is built in brains. Due to the neurons are connected by synapses, the transmission of information necessarily passes through synapse, resulting in the release of excitatory neurotransmitters in synaptic gap and EPSC is thus formed 28 . Although the principles of information processing in human brains are relatively poorly understood, the human brains, can process large amounts of information in parallel with high efficiency and low power consumption, show huge superiority compared to conventional computers based on von Neumann architecture 35 .
Due to the challenge of "von Neumann bottleneck", bionic study of human brain using transistors attracts a wide range of research in academic community 36 . However, light stimulated synaptic transistors using n-type electrets is rarely reported.
As is shown in Fig. 5b, EPSC is low, while it is rapidly increased by light spike with pulse width (ΔT) of 1 s. When the light illumination is removed, EPSC is decreased and eventually stabilized, of which the stabilized EPSC is still larger than that of the initial state. This phenomenon becomes more pronounced when two consecutive light pulses with pulse width (ΔT1) of 1 s and spike time interval (ΔT2) of 1 s are applied (Fig. 5c). That is, once two consecutive light pulses are applied and then are removed, the stabilized EPSC is increased from 11 nA to 28 nA, which is STM. STM is bound to be appeared more frequently by more successive light pulses, and eventually STM leads to LTM. 37 Therefore, after light pulses is applied for hundreds of times, the steady-state EPSC becomes larger and larger (Fig. 5d). This is analogous to human learning and memory. For example, when an ordinary child tries to recite a new verse, short term synaptic plasticity is built in his mind, however, this memory is not solid and it is hard to recite fluently. After learning to recite for many times, synaptic weights about the verse are increased and long term synaptic plasticity is built. Once LTM is formed, the verse is engraved in a specific area of nervous system.
In terms of memory effectiveness, the number of times to study is important, but the frequency of study is equally crucial, because longer time intervals can lead to STM or even forgetting 38 . PPF, as a typical form of short term synaptic plasticity, is the ratio of the maximum EPSC value from the second spike to that of the first spike 28 , which is related to ΔT2 and can be fitted by a double exponential function: where A1 and A2 represents the value of initial fast and slow phase facilitation, respectively. The fitting result of τ1 = 66 ms and τ2 = 320 ms is obtained, which is equivalent to those of biological synapses (Fig. 5e). Long term synaptic plasticity is fundamental for LTM, while short term synaptic plasticity can lead to long term synaptic plasticity through repeated rehearsals [39][40] . In summary, the frequency of study plays a key role in LTM.
To further investigate learning ability based on synaptic behavior of light stimulated synaptic transistors, the Modified National Institute of Standards and Technology (MNIST) database is thus used 41 . As is known, learning ability is actually the iterative repetition by assigning weights and retraining 28 . In this work, Fig. 5f demonstrates total 100 channel conductance values, as the basis for increasing or decreasing the weights in artificial neural networks (Fig. 5g). The equations for conductance (G) and number of pulses (P) are 35 : where Gmax, Gmin and Pmax are taken directly from experimental data, which represent the maximum conductance value, the minimum conductance value and the maximum number of pulses, respectively. GP and GD denote conductivity enhancement and conductivity decrease, respectively. A is related to nonlinearity (NL) that controls weights update, and their relationship can be referred in previous report 37 . The weight updating method is available in previous reports [42][43] .
In this work, NL is low under illumination conditions (NLP = 0.05) because one photon induces an electron into n-type electrets and simultaneously induces a hole in p-type semiconductors, resulting in an approximately linear enhancement of conductance under light illumination. When positive Vgs is applied, the gate electric field causes holes in p-type semiconductors to inject into the n-type electrets, and these holes are recombined with the electrons previously stored in n-type electrets, resulting in those electrons in n-type electrets are decreased exponentially. Therefore, the conductance is rapidly decreased in the first few pulses. As the recombination of electrons-holes is on-going, the electron concentrations in n-type electrets are decreased, making the recombination relatively slower, and the decrease of conductance is relatively slower in the last few pulses. Therefore, the nonlinearity under positive Vgs is larger, and NLD = 2.44.
As shown in Fig. 5h, the overall recognition accuracy of artificial neural network grows fast at first and gradually saturates with the increase of training epochs. When the number of training epochs reaches 200, the overall recognition rate is 65%, which is comparable to previously reports of other systems 42,44 . To quantitatively illustrate the performance of artificial neural network, we calculated recognition rates for 0 to 9 digits, as shown in Fig. 5i. The recognition rates of "1" (the character with the highest recognition rate) and "8" (the character with the lowest recognition rate) are 0.93 and 0.08, respectively.

CONCLUSION
In conclusion, we propose a class of electrets, namely organic n-type electrets, made by oxygen-degraded n-type organic semiconductors. In this work, n-type semiconductors interact with oxygen species including air, ozone or oxygen plasma for in-situ degradation. After degradation, the electron mobility of n-type semiconductors is rapidly decreased, warranting an insulating characteristic of the oxygen-degraded semiconductor for organic n-type electrets. Afterwards, photo-induced electrons are transferred from p-type semiconductor to the degraded n-type semiconductor, and the charged degraded n-type semiconductors are n-type electrets. As compared with conventional insulator electrets, organic n-type electrets are featured with appropriate

ASSOCIATED CONTENT Supporting Information
The Supporting Information is available free of charge. The mechanism of n-type electrets for tuning the charge concentrations in p-type semiconductors. First, the n-type electret is generated upon exposing n-type semiconductors to oxygen species, which induces the material degradation. Then, the n-type electrets, carrying quasi-permanent neat charges, is obtained under light illumination in the n/p configuration: the photo-induced electron-hole pairs are split at the interface of n/p heterojunction, subsequently the electrons are transferred into and stably trapped in the n-type electrets (degraded n-type semiconductors).
Finally, the built-in electric field, supplied by the n-type electrets, is used to manipulate the charge transport inside p-type semiconductors for OFET applications.