Unveiling the Role of Side Chain for Improving Nonvolatile Characteristics of Conjugated Polymers‐Based Artificial Synapse

Abstract Interest has grown in services that consume a significant amount of energy, such as large language models (LLMs), and research is being conducted worldwide on synaptic devices for neuromorphic hardware. However, various complex processes are problematic for the implementation of synaptic properties. Here, synaptic characteristics are implemented through a novel method, namely side chain control of conjugated polymers. The developed devices exhibit the characteristics of the biological brain, especially spike‐timing‐dependent plasticity (STDP), high‐pass filtering, and long‐term potentiation/depression (LTP/D). Moreover, the fabricated synaptic devices show enhanced nonvolatile characteristics, such as long retention time (≈102 s), high ratio of G max/G min, high linearity, and reliable cyclic endurance (≈103 pulses). This study presents a new pathway for next‐generation neuromorphic computing by modulating conjugated polymers with side chain control, thereby achieving high‐performance synaptic properties.

The polymer film was spin-coated using PDPP3T solution on SiO2/Si substrate as a wafer scale.The film was patterned by RIE and the gate, source, and drain electrodes were deposited by thermal evaporation.The above image shows the device fabricated on a SiO2/Si substrate measuring 1.5 cm × 1.5 cm.Table S1.Melting point and melting enthalpy for cycle according to polymer.Differential scanning calorimetry (DSC) of PDPP3T series shows an endotherm with peak at the melting temperature (230-290 ℃) with enthalpy of fusion. [1]From PDPP3T-HD to OD and DT, the melting point tends to decrease in both 1-and 2-cycles due to the effect of the longer alkyl side chain. [2]The decrease in melting point and enthalpy of PDPP3T-DT (with a long chain) is more pronounced because of the large effect of structural relaxation. [3]As a result, the melting enthalpy of PDPP3T-DT is the lowest at 3.82 J g -1 , which showing that the degree of crystallization is low due to the long side chain.Table S2.The values used to calculate mobility and subthreshold swing (SS), as well as the calculated values.
We fabricated a synaptic device depending on PDPP3T-HD, PDPP3T-OD, and PDPP3T-DT and obtained the transfer curve.From this we could see that the hysteresis becomes wider depending on the branched alkyl chain length.With the forward sweep we could get threshold voltage and factors for calculating mobility, and subthreshold swing.Using these values, we calculated that HD has the largest mobility and OD has the smallest SS.As the drain voltage is -1 V and a 20 ms pulse is applied according to the gate voltage, the STP characteristics can be adjusted according to the PDPP3T material.Depending on the side chain length, the current reduction speed of OD is the slowest, followed by DT and HD.
It can be seen that the EPSC with increasing as the gate voltage increases.It can be seen that there can be adsorption on both sites 1-6.It can be seen that it is able to adsorb on both of the sites 1-6.The image above illustrates which part of the TFSI anion is involved in strong interactions, exchanging electrons with PDPP3T.We can identify that the oxygen within TFSI primarily engages in exchanging electrons with the PDPP3T chain.Table S3.d-spacing and CCL of PDPP3T based polymers.
The π-π stacking (010) and lamella (h00) reflection (d-spacing) obtained by the equation of d = 2π/q, where q is the corresponding x-coordinate of maximum diffraction peak intensity.The CCL was calculated quantitatively by using the Scherrer equation: CCL = 2πK/Δq, where K is the form factor (K = 1.0), and Δq is the FWHM of each diffraction peak.
The crystallites orientation was calculated through integration of (100) and (010) diffraction peak with calibration by multiplying intensity with sin(θ) according to azimuthal angle.(outof-plane (OoP, θ = 0 °) and in-plane (IP, θ = 90 °))  Figure S10a shows that increasing the interval between two consecutive pulses decreases the EPSC to a second changing pulse.Figure S10b-d     To determine the robustness of the PDPP3T-HD-based synaptic device, we conducted several electrical measurements.Figure S9a shows that the PPF index decreases as the pulse interval time (∆t) increases from 20ms to 3000ms.Figure S9b shows that the A10/A1 percent increases with increasing frequency, indicating the presence of high-pass filtering.Figure S11c and S11d show that the EPSC varies with changes in pulse width and pulse number, respectively.Figure S12 shows the additional synaptic performance of the PDPP3T-DT device.
Figure S12a shows that the PPF index decreases as the time interval increases.Figure S12b shows the high-pass filtering properties with increasing gain with increasing frequency.Figure S12c and S12d show that the EPSC increases with increasing width and number of pulses.To investigate the stability of our fabricated PDPP3T-OD devices, we observed the changing PSC values as a function of the application of 40 consecutive 100 pulses.This confirmed that the device was stable even after 4000 pulses had been applied.

Figure S1 .
Figure S1.An optical image of the fabricated PDPP3T-based synaptic transistors on the SiO 2 /Si substrate.The blue circle indicates the droplet region of the ionic liquid.

Figure S4 .
Figure S4.STP characteristics of PDPP3T-based synaptic transistor.EPSC responses to single pulse (20 ms width) with different amplitudes in a) PDPP3T-HD, b) PDPP3T-OD, and c) PDPP3T-DT.Input: Magnified view of the base current region.

Figure S5 .
Figure S5.Schematic illustrations for the adsorption energy calculation of TFSI anion at site 1 to site 6 (a-f) in PDPP3T-HD.

Figure S7 .
Figure S7.Schematic of adsorption energy calculation of the TFSI anion at site 1 to site 6 (af) in PDPP3T-DT.

Figure S8 .
Figure S8.The charge density difference between PDPP3T and TFSI anion for site 1 and 2. The unit of charge density is 0.002 e/Å 3 .

Figure S10 .
Figure S10.Synaptic characteristics of PDPP3T-OD synaptic devices.a) Change in EPSC over actual time as pulse interval increases.The change in EPSC as a function of time by b) pulse input frequency, c) pulse number, and d) pulse width when applying a series of pulses.

Figure
Figure S10 exhibits additional synaptic properties of PDPP3T-OD synaptic devices.
indicate that the EPSC response varies with changes in frequency, number of pulses, and pulse width, respectively.

Figure
FigureS11shows PPF behavior under low drain voltage to achieve low energyconsumption with PDPP3T-OD devices.As the results we observed that E was ~1.18 pJ per spike.

Figure S12 .
Figure S12.Robustness of PDPP3T-HD synaptic devices with the function of a) The PPF index is plotted as a function of time for an interval of two pulses.Inset: EPSC change with an interval of 20 ms.b) The current gain defined by A10/A1 is plotted as a function of frequency, indicating high-pass filtering.Inset: EPSC response to 10 pulses of 25 Hz (-4 V, 20 ms).EPSC value at 10th pulse by 10 consecutive pulses with different c) pulse width and d) pulse number.

Figure S13 .
Figure S13.Robustness of PDPP3T-DT synaptic devices.a) The PPF index, defined as the ratio of A2/A1, is plotted as a function of the gap difference (∆t) between a series of two pulses (-4 V, 20 ms).Inset: Conductance change with an interval of 20 ms.b) The gain, defined as A10/A1, is plotted as a function of frequency from 5 to 25 Hz to characterize high-pass filtering.Inset: EPSC change to 10 pulses of -4 V, 20 ms, with a frequency of 25 Hz.EPSC responses when applying a series of 10 pulses, c) with increasing pulse width from 20 to 150 ms, and d) with increasing number of pulses from 4 to 20.

Figure S14 .
Figure S14.LTP and LTD characteristics of a) PDPP3T-HD, b) PDPP3T-OD, c) PDPP3T-DT under 100 consecutive potentiation pulses and depression pulses for 2.5 Hz.The width is fixed to 20 ms.Normalized synaptic weight change characteristics as function of electrical pulses number of d) PDPP3T-HD, e) PDPP3T-OD, f) PDPP3T-DT.Blue(red) dots and lines indicated the updated conductance for each potentiated(depressed) pulse and fitted lines, respectively.

Figure
Figure S13a-c shows the LTP/LTD characteristics of the synaptic device fabricated

Figure S15 .
Figure S15.PSC response of a PDPP3T-OD synaptic device during 40 cycles of LTP/D with 100 consecutive pulses.