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Nobutaka Osakabe, Jeongeun Her, Takahiro Kaneta, Akiko Tajima, Elena Longhi, Kan Tang, Kazuhiro Fujimori, Stephen Barlow, Seth R. Marder, Shun Watanabe, [Jun Takeya](https://orcid.org/0000-0002-7003-1350), [Yu Yamashita](https://orcid.org/0000-0001-7966-3197)

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[Polymeric microwave rectifiers enabled by monolayer-thick ionized donors](https://mdr.nims.go.jp/datasets/189b3e49-13cb-4e50-b779-a9420e192df9)

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Polymeric microwave rectifiers enabled by monolayer-thick ionized donorsOsakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e1 of 8M AT E R I A L S  S C I E N C EPolymeric microwave rectifiers enabled by monolayer-thick ionized donorsNobutaka Osakabe1,2, Jeongeun Her1, Takahiro Kaneta1, Akiko Tajima1, Elena Longhi3, Kan Tang4, Kazuhiro Fujimori5, Stephen Barlow3,4, Seth R. Marder3,4,6, Shun Watanabe1,  Jun Takeya1,2*, Yu Yamashita1,2*Solution processing of polymeric semiconductors provides a facile way to fabricate functional diodes. However, energy barriers at metal-semiconductor interfaces often limit their performance. Here, we report rectifying poly-mer diodes with markedly modified energy-level alignments. The gold electrode surface was treated with a di-meric metal complex, which resulted in a shallow work function of 3.7 eV by forming a monolayer-thick ionized donor layer. When a polymeric semiconductor was coated on the treated electrode, most of the ionized donors remained at the metal-semiconductor interface. The confined ionized donors with the ideal thickness enabled fabrication of a polymer diode with a forward current density of over 100 A cm−2. Furthermore, a power conver-sion efficiency of 7.9% was observed for rectification at a microwave frequency of 920 MHz, which is orders ofmagnitude higher than that reported for organic diodes. Our findings will pave a way to solution-processed high-frequency and high-power devices.INTRODUCTIONSolution processing of polymeric semiconductors provides a facile way to fabricate vertical structures for high-power-density devices, including photovoltaic cells (1), light-emitting diodes (2,  3), and rectifiers (4, 5). The advantages of polymers for this purpose include tunable electronic properties by molecular design (6, 7) and high uniformity of thin films that suppress leakage problems. One of the target vertical devices based on polymeric semiconductors is a rec-tifying diode (5) that operates AC-to-DC power conversion. Micro-wave rectifiers serve as power supplies to wireless devices, which is essential for the development of device networks by leveraging the advantages of flexible printed electronics (8). Although pioneering works have demonstrated the rectification of voltages above 1 GHz (9, 10), the AC-to-DC power conversion efficiency needs to be con-sidered for wireless powering and communications. This value has not been reported for polymer diodes, and estimated to be low (<0.1%) based on available data on the literature as shown later. This raises questions regarding their practical applications considering the ideal half-wave rectification efficiency (40.5%).Advanced interface engineering is of key importance to develop efficient polymer diodes for energy harvesting and power manage-ment. Depending on the application, either hole or electron trans-port, or both, need to be considered. For rectifying diodes, transport of only one type of carrier is sufficient for device operation. The ac-tive material can be selected to facilitate vertical carrier transport, for which the semicrystalline electron-transporting polymer P(NDIOD-T2) (11) is a promising candidate due to its favorable face-on orienta-tion (12). On the basis of the typical electron-transporting energy levels of polymeric semiconductors including P(NDIOD-T2), a shal-low work function of electrodes (<4 eV) is necessary to decrease the energy barriers at the electrode/polymer interfaces. Because of the instability of shallow-work-function metals, such as Ca and Mg, mo-lecular modifications using self-assembled monolayers (SAMs) and interlayers have been used to tune the work function of electrodes (13–15). Although polyethyleneimine (PEI) has been reported to achieve a remarkably shallow work function (16) among interlayers, there seem to be considerable energy barriers between polymers and electrodes modified with PEI (4). This may be partly due to the low carrier concentrations and high trap density of states in polymers (1). In addition, the additional resistance of the interlayers can limit the attainable current density considering the exponentially increasing resistance with the thickness of insulating layers on electrodes (17).The use of redox agents and molecular electrical doping has emerged as a powerful tool for modifying energy-level alignments (3, 9, 18–24), and may offer an ideal solution to the above problems. Redox reactions between molecules and metal electrodes result in the formation of charged layers and shifts in work function (25, 26). While most redox agents do not have sufficient reducing strength and stability to achieve a low work function below 4 eV, dimeric do-nor molecules have been demonstrated to realize such low work functions (27, 28). This work function shift is predicted to occur with a monolayer-thick ionized donor layer on electrodes (29), suggesting the possibility of forming ideally thin and effective interlayers. How-ever, it has been unclear whether such a method is applicable to fab-ricate polymer diodes, where the vertical stacking structures need to be controlled during the solution process. The diffusion of donor and acceptor molecules has been a critical problem in the fabrication of advanced polymer devices (18, 30).In this study, we demonstrate polymeric microwave rectifiers us-ing the treatment of metal surfaces with dimeric donor molecules (Fig. 1, A and B). In our method, gold electrodes were exposed to a solution of the dimeric organometallic complex donor molecule 1Material Innovation Research Center (MIRC) and Department of Advanced Materi-als Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan. 2Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 205-0044, Japan. 3School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA. 4Renewable and Sustainable Energy Institute, Uni-versity of Colorado Boulder, Boulder, CO 80309, USA. 5Faculty of Environmental, Life, Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Kita-ku, Okayama 700-8530, Japan. 6Departments of Chemistry and of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA.*Corresponding author. Email: takeya@​k.​u-tokyo.​ac.​jp (J.T.); yamashita.​yu@​nims.​go.​jp (Y.Y.)Copyright © 2025 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. no claim to original U.S. Government Works. distributed under a creative commons Attribution license 4.0 (cc BY). Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025mailto:takeya@​k.​u-tokyo.​ac.​jpmailto:yamashita.​yu@​nims.​go.​jpmailto:yamashita.​yu@​nims.​go.​jphttp://crossmark.crossref.org/dialog/?doi=10.1126%2Fsciadv.adv9952&domain=pdf&date_stamp=2025-09-19Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e2 of 8(31), which decreased the work function of the gold electrode to 3.7 eV even after air exposure. X-ray photoelectron measurements suggest that the cationic donor molecules were present on the treated gold surface with a thickness close to one monolayer. To fabricate verti-cal diodes, solution-processed n-type polymeric semiconductor thin films were fabricated on the treated gold electrodes. Owing to the cationic donor layer present only around the bottom electrode, high forward current density and rectifying ratios were achieved, lead-ing to power conversion efficiency of 7.9% at 920 MHz. Notably, DC output power of 5 mW was obtained, which is orders of magnitude higher than previous reports. Our findings will pave a way to high-speed and high-power vertical polymer devices.RESULTSDiodes using surface-treated electrodesTo achieve the surface treatment of gold electrodes, the solution of do-nor molecule was spin-coated onto the electrodes followed by washing with pure solvents. The donor molecule (RuCp*Mes)2 (Cp* is penta-methylcyclopentadienyl and Mes is 1,3,5-trimethylbenzene) was cho-sen owing to its high reducing capability through cleavage and electron transfer reactions to form RuCp*Mes+ (31). Figure 1 (B and C) shows a possible model of the (RuCp*Mes)2 treatment, where (RuCp*Mes)2 and gold surface undergo redox reactions. This leaves cationic donor layers on the negatively charged gold surface, forming a dipole that would decrease the work function similar to what is reported in previ-ous literature (27–29).The work functions of the electrodes were evaluated by photo-electron yield spectroscopy (PYS). Compared to the untreated gold electrode, the electrode treated with (RuCp*Mes)2 showed smaller threshold photon energies to give rise to photoelectron yields, which confirms a decrease in the work function (Fig. 1D). We also com-pared the treatments with commonly used interlayers of PEI and poly[9,9-bis(3′-(N,N-dimethyl)-N-ethylammoinium-propyl-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br). In our measurements, the (RuCp*Mes)2 treatment resulted in a work func-tion of 3.7 eV, which is lower than the values obtained with either PEI or PFN-Br. This value is sufficiently small compared with the energy level of the lowest unoccupied molecular orbital (LUMO) of the target n-type polymer P(NDIOD-T2) (3.84 eV, Fig. 1E) (11, 32). Please see figs.  S1 to S3 for fittings of PYS measurements. Treat-ments with other donor molecules, 4-[1,3-dimethyl-2,3-dihydro-1H-benzoimidazol-2-yl)phenyl]dimethylamine (N-DMBI-H) and cobaltocene (CoCp2), resulted in work function higher than the case with (RuCp*Mes)2, which suggests the importance of the high re-ducing capability of (RuCp*Mes)2 (figs. S1 and S2). Note that our measurements were conducted after a short exposure of the samples to air. In addition, a work function of 3.7 eV after the (RuCp*Mes)2 treatment was observed even in PYS measurements in air (figs. S1 and S3), which shows moderate ambient stability of the treatments. Considering the low work function of this sample, the observed am-bient stability is remarkable (3) and may be attributed to the stable (33), hydrophobic, and bulky character of the RuCp*Mes+.The injection properties of the surface-treated electrodes were evaluated using polymer diodes. After the treatment of bottom elec-trodes using the interlayers or (RuCp*Mes)2, P(NDIOD-T2) was spin-coated with a thickness of ca. 80 nm (fig. S4), followed by evaporation of the top gold electrodes. Devices with the treated bottom electrodes showed higher current densities under a positive bias, where electrons were injected from the bottom electrodes (Fig. 1F). Furthermore, the D E FP(NDI2OD-T2)(RuCp*Mes) 2RuCp*Mes+A Bglassn-type polymerAuVacuumlevelpotentialAue−e−e−e−EFRuCp*Mes+AuRuCp*Mes+10−810−610−410−2100102Currentdensity(Acm2 )−4 −2 0 2 4Voltage (V)(RuCp*Mes)2PEIPFN-Bruntreated(Photoelectronyield)1/2(a.u.)4.84.44.03.6Photon energy (eV)(RuCp*Mes)2PEIPFN-Bruntreated2e–CFig. 1. Polymer diodes using surface treatments of bottom electrodes. (A) A schematic illustration of a polymer diode using (RuCp*Mes)2 treatment on the bottom electrode. (B) Illustrations of redox reactions of (RuCp*Mes)2 and (C) a model describing the shift in work function through (RuCp*Mes)2 surface treatment. (D) PYS mea-surements of Au electrodes treated with (RuCp*Mes)2 and conventional interlayers. The names of materials used are denoted as the legends. (E) The molecular structure of P(NDIOD-T2). (F) The current density-voltage characteristics of the fabricated polymer diodes with various treatments on the bottom electrodes. The diodes had an area of 2500 μm2 and a typical capacitance of 0.9 pF.Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e3 of 8maximum current density was higher for (RuCp*Mes)2-treated de-vice than for devices using PEI or PFN-Br. Here, a smaller work func-tion should be more advantageous for approaching ohmic contact, considering the LUMO energy level of P(NDIOD-T2) and the trap density of states in polymer thin films (1, 34). This highlights the im-portance of the robust control of work function by (RuCp*Mes)2, which realized larger shifts in work function compared to convention-al interlayers or SAM treatments (13, 35). We observed the highest current density of over 10 A cm−2 and a rectifying ratio of 106 when the (RuCp*Mes)2 treatment was used. These results support the suc-cessful confinement of the donor molecule in the vertical direction of solution-processed polymer diodes.Surface analysis of the treated gold electrodesThe surface elemental composition of the (RuCp*Mes)2-treated gold electrode was analyzed using x-ray photoelectron spectrosco-py (XPS). The treatment was conducted in the same manner as in the above experiments, where an n-butyl acetate (nBA) solution of (RuCp*Mes)2 was spin-coated on the electrode, followed by wash-ing with pure nBA. To understand the effect of the processing sol-vents, another sample was prepared, which was further washed with o-dichlorobenzene (oDCB). For the sample after nBA washing, Ru 3d5/2–derived peaks were observed at 281.1 and 282.0 eV in addi-tion to Au 4f and C 1s peaks (Fig. 2, A and B). The two different Ru 3d5/2 peaks originate from complexes with different charges, where the peak at the higher binding energy is ascribed to RuCp*Mes+ and the peak at the lower binding energy to (RuCp*Mes)2 (36). When the electrode surface was further washed with oDCB, only the peak originating from RuCp*Mes+ was observed. The above results sug-gest that while neutral (RuCp*Mes)2 was washed away by oDCB, RuCp*Mes+ was bound to the gold surface sufficiently tightly to en-dure such a solution process.Fitting of XPS spectra was conducted (see text S1), and the ratios of Ru atomic compositions to the total ones were evaluated to be 2.1% after washing with nBA and 0.9% after washing with oDCB. The value after washing with oDCB is consistent with one assuming a model with a monolayer of RuCp*Mes+ covering the gold surface (table. S10). For comparison, the XPS spectra of the gold electrodes with other interlayers and donor molecules were also measured (text S1). On the basis of the atomic composition and an overlayer model (37) (text S2), the thickness of the layers on the gold electrode was remarkably smaller for (RuCp*Mes)2 treatments (1.6 nm) than for conventional interlayers (over 3 nm, Fig. 2C). This is important considering the exponentially increasing resistance with the thick-ness of insulating layers on electrodes (17). Thus, the (RuCp*Mes)2 treatment realizes a marked decrease in the work function while minimizing the thickness and additional resistance of the interlayer.Distribution of ionized donorsThe distribution of RuCp*Mes+ in the vertical direction was inves-tigated using XPS depth profiles. The oDCB solution of P(NDIOD-T2) was spin-coated onto gold electrodes treated with (RuCp*Mes)2. As shown in Fig. 3A, an Ar ion gun was used to etch P(NDIOD-T2) and evaluate the atomic composition at varying depths. Figure 3 (B to D) shows representative XPS spectra at etching times of 0, 1080, and 2340 s. The Au 4f peaks in Fig. 3B were observed for an etching time of 2340 s, indicating that most of the P(NDIOD-T2) thin film was etched under this condition, where the gold electrode was close to the top surface. The Ru 3d peak at approximately 280 eV also ap-peared under this etching condition (Fig.  3D). The above results demonstrate that RuCp*Mes+ is present mostly at the interface be-tween P(NDIOD-T2) and the bottom gold electrode in our diode without diffusing into the entire P(NDIOD-T2) thin film.In diodes with low injection barriers at the bottom electrode and high injection barriers at the top electrode interfaces, band bending in opposite directions are predicted around the interfaces. This was verified in our diodes by ultraviolet photoelectron spectroscopy (UPS) measurements. Here, we used the (RuCp*Mes)2-treated and untreated gold electrodes covered with P(NDIOD-T2) thin films with varying thicknesses. Note that the donor molecules were con-fined around the electrode interface in the treated samples, which is verified by our XPS measurements of these samples (see text S3). Considering the high surface sensitivity of UPS measurements, UPS measurements of these samples indicate the work function and band bending at different vertical positions (38). Figure 4 (A and B) shows the cutoff regions of the UPS spectra.  Figure  4 (C and D) Overlayer thickness (Å)(RuCp*Mes)2PFN-BrPEI(RuCp*Mes)2o-DCB washedUntreated50403020100B CABinding energy (eV)Normalized intensity292 288 284 280C 1sRu 3d5/2Binding energy (eV)Normalized intensity92 90 88 86 84 82 80Au 4fnBAwashedoDCBwashedFig. 2. Evaluation of dopant distribution based on XPS measurements. XPS spectra of Au electrodes treated with (RuCp*Mes)2 after washing the surface with nBA (orange) and further with oDCB (red) in the (A) Au 4f, (B) C 1s, and Ru 3d regions. The black dashed lines show Gaussian peak shapes based on fittings. The gray dashed line shows the position of the peaks for the sample washed with nBA. The intensity of spectra was normalized on the basis of the total intensity of Au 4f peaks. The binding energy was shifted so that both spectra have the same binding energy for the C 1s main peaks to clarify the chemical shifts without effects of difference in work functions. (C) The overlayer thicknesses on the Au electrodes, which are evaluated on the basis of the Au contributions to XPS spectra. The error bar shows one SD. See text S2 for details of calculation.Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e4 of 8shows the work functions evaluated on the basis of the spectra (see text S4 for the details of the analysis). The whole UPS spectra are shown in fig. S20. For the samples with (RuCp*Mes)2 treatment, the work function increased with the increasing thickness of P(NDIOD-T2), while the opposite trend was observed for the samples with-out the treatment. These observations are consistent with the band bending model predicted for a semiconductor layer in contact with low- and high-work-function electrodes (Fig. 4E). The observed band bending extends over tens of nanometers, which is consistent with the previous studies for polymer thin films with low carrier concen-trations and trap density of states (39, 40).Rectification performance of diodesThe rectification performance of our diode using (RuCp*Mes)2 treat-ment was evaluated at the ultrahigh frequency (UHF) band. In our setup (Fig. 5A), the input power from the signal generator go through a power splitter that distributes ca. 40% of the input power to an oscil-loscope and the diode. The output voltage from the diode was mon-itored by the oscilloscope with 1-megohm termination. Figure 5B shows a photo of the fabricated diode, which is designed to be mea-sured with a high-frequency ground-signal-ground (GSG) probe. Fig-ure 5C shows the results obtained when the input signal was 920 MHz and 16 dBm. Under this condition, the input AC voltage monitored by the oscilloscope was ca. 6 V peak-to-peak ( Vpp ) and the output DC voltage from the diode was ca. 1.6 V. This is in contrast to previ-ous reports on polymer diodes, whose DC output voltages were less than 0.2 V at 1 GHz (9), and demonstrates that our diode converts UHF AC voltages to DC voltages with a small voltage loss under the above condition.AC DGlassAu 30 nmP(NDI2OD-T2)80 nmEtchingtime1000 s0 s2000 sBAu 4fIntensity92 90 88 86 84 82 80Binding energy (eV)0 s1080 s2340 sRu 3d5/2Intensity282 281 280 279 278 277Binding energy (eV)0 s1080 s2340 sC 1sIntensity292 288 284 280Binding energy (eV)0 s1080 s2340 sFig. 3. Evaluation of dopant diffusion based on depth profile. (A) Illustration of the depth profile measurements using Ar ion gun etching. (B to D) XPS spectra in Au 4f, C 1s, and Ru 3d5/2 regions with varying etching times. The etching times used are denoted as the legends.BEAC DIntensity18.017.016.0Binding energy (eV)Intensity18.017.016.0Binding energy (eV)0.2 wt %0.4 wt %0.6 wt %1 wt %4.64.44.24.03.8Work function (eV)100806040200Nominal thickness (nm)4.64.44.24.03.8Work function (eV)100806040200Nominal thickness (nm)VacuumLUMOHOMOTreatedAu Polymer4.8 eV3.7 eV 3.8 eVBefore contactUntreatedAuVacuumHOMOEFPolymerAfter contactTreatedAuUntreatedAuLUMOFig. 4. Energy level alignments at metal-semiconductor interfaces. UPS spectra of samples with varying thickness of P(NDIOD-T2) on (A) the (RuCp*Mes)2-treated and (B) untreated gold electrodes around the cut off regions. The concentrations of P(NDIOD-T2) solutions used were denoted as legends. The origins of x axis are the Fermi energy. Work functions evaluated for the samples with (C) the (RuCp*Mes)2-treated and (D) untreated gold electrodes. (E) Illustrations of the models of energy diagram and band bending before and after the contact. LUMO level of P(NDIOD-T2) polymer is according to the literature (32).A BC DSignalGeneratorPowersplitter Bias teeOscilloscopeInput50 ohmsOutput1 megohm−4−2024Voltage(V)−1.0 0.0 1.0Time (ns)InputOutput100101102103104105Impedance(ohm)107 108 109 1010Frequency (Hz)ZReZImrsCbRb200 mTop electrodeBottom electrodejunctionFig. 5. Rectification at the UHF band. (A) An illustration of the measurement set-up. (B) A photo of the fabricated diode using a pattern suitable for measurements at the UHF band. (C) The input and output voltages monitored by the oscilloscope using our polymer diode with the (RuCp*Mes)2-treated bottom electrode. The in-put voltage was an AC 920 MHz sine wave. The area of diode was 2500 μm2. (D) Real and imaginary parts of impedance of our polymer diode with the (RuCp*Mes)2-treated bottom electrode. The circuit model of a Schottky diode is shown as the inset. The capacitance of diode was estimated to be 0.85 pF.Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e5 of 8To operate diodes at frequencies above the HF band, we consid-ered requirements on device parameters. The first requirement is a sufficiently short RC delay, where the diode needs to possess low capacitance and resistance. This will be examined in the following section. The second requirement is that the semiconductor has suf-ficiently high mobility and low thickness that charge carriers can travel through the thin film (41). Vertical structures are advanta-geous for fulfilling this requirement, where the required mobility is the order of 0.1 cm2 V−1 s−1 when the thickness is 100 nm scale (See text S5 for details). This value may be attainable with semicrystalline P(NDIOD-T2) with preferential face-on-oriented packing (12).To understand the RC delay and circuit model of our diode at high frequencies, the one-port scattering parameters were mea-sured to evaluate real ( ZRe ) and imaginary ( ZIm ) parts of impedance (Fig. 5D). Considering the RC delay, the power efficiency of the cir-cuit will start to markedly decrease when the frequency exceeds a cutoff frequency ( fc ). fc is the frequency of the intersection of ZRe and ZIm , which was 2 GHz in our case. The equivalent circuit model of a Schottky diode can be represented by the circuit shown in the inset of Fig. 5D. rs is the sum of the series resistances including the resistances of electrodes. Rb is the parallel resistance and Cb is the parallel capacitance of the depletion layer. In this case, Cb contrib-utes to RC delay (42) and was estimated to be rather small value of 0.85 pF from the observed ZIm . On the basis of the geometry of our device and expected permittivity of the polymer, the depletion layer is 80 nm thick, which is almost identical to the total thickness of the polymer. This is consistent with the XPS measurements showing that dopants are present only around the bottom electrode interface. rs contributes to the RC delay, whose value can be estimated to be equal to ZRe at high frequencies (42). rs was ca. 45 ohms in this device, where the evaluated small capacitance and resistance ensure the short RC delay of our device. Note that, while decrease in rs in-creases fc , the use of rs close to 50  ohms is desirable considering impedance matching.We examined whether high output DC power and high AC-to-DC power conversion efficiency ( η ) are possible with our diodes. In this case, a reasonably large DC output voltage ( Vout ) needs to be developed even when the load resistance ( RL ) becomes rather small. η is defined as Pout ∕Pin , where Pin is the AC input power and Pout is the DC output power. Pout was calculated using RL and Vout . To elim-inate the effects of the power reflection that occurs mainly due to impedance mismatch, we also evaluated ηint , which is defined as Pout ∕Pin,eff . The effective input power ( Pin,eff ) and the reflected pow-er ( Pre ) were measured using the setup shown in  Fig.  6A, where Pin,eff = Pin − Pre . In these measurements, we used a 0.5 weight % (wt %) P(NDIOD-T2) solution to fabricate a diode, which resulted in a depletion layer thickness of ca. 35 nm (text S6). The capacitance of the diode was 3.3 pF based on the impedance measurement shown in fig. S22. When a 50-ohm system is used, fc of the system becomes 970 MHz, still higher than the used UHF frequency. Fig-ure 6B shows the current-voltage characteristics obtained with this diode, where a current level of over 1 mA and current density of over 100 A cm−2 at 2 V forward bias were observed. Even for the thin thickness, our diodes show moderate reproducibility and bias stability as discussed in texts S7 and S8. Figure 6 (C and D) shows Vout and ηint at different load resistances. While conventional organ-ic small molecular and polymer diodes showed a DC output of less than 20 μW (9, 10), our diode showed a DC output power reaching 5 mW. In contrast to previous reports, η of our diode reached 5.2%, approaching the ideal efficiency of 40.5% for a half-wave rectifier (Fig. 6E). For details on the efficiency calculation, see text S9. The high rectification performance of our device is ascribed to the high 10−11 10−9 10−7 10−5 10−3 Current (A)−2 −1 0 1 2Voltage (V)10−6 10−4 10−2 100 102mcA( ytisned tnerruC2−)A BD EC10−7 10−5 10−3 10−1 101 106 107 108 109 1010Frequency (Hz)This workPolymerSmall molecularUpper limitS5 S4S4S7S6S6S3S1S2P1P4P3P2P3 (%)PowermeterCoupler 2−10 dBVoutV RLBias teeCoupler 1Circulator AmplifierSignalgenerator−30 dB24 V DC+20 dB50 ohms +−Pin Pre3.02.52.01.51.00.50.0Output voltage (V)101103 105 20 dBm22 dBmPin1086420101 103 105 20 dBm22 dBmtni (%)Pin    Load resistance (ohm) Load resistance (ohm)Fig. 6. Measurements of AC-to-DC power conversion efficiency. (A) An illustration of the measurement setup. (B) The DC performance of the used polymer diode with the (RuCp*Mes)2-treated bottom electrode and P(NDIOD-T2) layer fabricated using a 0.5 wt % solution. The area of diode was 3750 μm2. The capacitance of the diode was 3.3 pF based on the measurement shown in fig. S22. (C) The output voltage and (D) AC-to-DC power conversion efficiency measured with our diode fabricated using 0.5 wt % P(NDIOD-T2) solution. The AC power from signal generator ( Pin ) were 20 and 22 dBm. (E) Comparison of the efficiency to those in literature studies of diodes based on small molecules [S1 (41), S2 (46), S3 (47), S4 (48), S5 (49), S6 (10), and S7 (50)] and polymers [P1 (4), P2 (5), and P3 (9)].Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e6 of 8current level at a small bias in the diode. When the DC power is consumed at the load resistor, the DC current needs to pass through the diode and load resistor, where the diode needs to flow a high current with a small voltage loss. From this point of view, the use of diodes with a thin depletion layer thickness is advantageous and our diode outperforms previously reported organic semiconductor di-odes based on small molecules and polymers (texts S10 and S11).DISCUSSIONConsidering all requirements, a microwave rectifying diode needs to be designed so that the junction shows a reasonable rectification ra-tio, small capacitance, and a high current at a small bias. Impedance matching may be achieved by designing the electrodes and circuits rather than junction of the diode (43). To design a diode with a spe-cific combination of materials, the depletion layer thickness and the junction area play key roles. In most organic diodes, while the for-ward current level is only proportional to the junction area, it shows larger dependence on the depletion layer thickness. For instance, in the space-charge limited current model following the Mott-Gurney’s law, the current is inversely proportional to the cube of the thickness (44). Considering this, it is reasonable to decrease the depletion layer thickness to increase the current density and then tune the junction area to fulfill the requirement on the capacitance. Our diode showed the reasonable rectification ratio with the thin depletion layer thick-ness of ca. 35 nm. Use of efficient donor molecules in such a thin vertical device was the key to achieving the observed high device performance, which highlights advantages of the used processes and materials. Here, the stable and highly reducing donor enabled facile fabrication of the monolayer-thick ionized donor layer, which fea-tures marked energy-level alignments, suppressed diffusion of do-nors, and minimized thickness and additional resistance of the interlayer. The attainable performance may be further enhanced by exploring molecular ions through the ion exchange doping and re-lated technologies to control dopant ions (20–24).In this study, the use of dimeric donor molecules in vertical poly-mer diodes was demonstrated, where a monolayer-thick RuCp*Mes+ layer was deposited to the surface of gold electrode. This method decreased the work function of gold electrode to 3.7 eV even after air exposure and effectively modified the energy-level alignment and the band bending when the electrode was in contact with P(NDIOD-T2). The fabricated diode exhibited a high current den-sity of over 100 A cm−2 and rectification at 920 MHz. The output DC power reached 5 mW owing to the high current levels at small for-ward voltages in our device, demonstrating a benchmark efficiency among diodes composed of printable semiconductors. These re-markable device performances owe to the monolayer-thick ionized donors, which feature marked energy-level alignments, suppressed diffusion of donors, and minimized thickness and additional re-sistance of the interlayer. Our findings will contribute not only to printed microwave rectifiers but also to polymer vertical optoelec-tronic devices in general, where the control of semiconductor-metal interfaces plays a key role.MATERIALS AND METHODSMaterialsThe polymeric semiconductor P(NDIOD-T2) was purchased from Ossila. Batch M1201A3 was used with the following molecular weights: weight-average molecular weight (Mw) = 202,261, number-average molecular weight (Mn) = 90,982, and polydispersity index = 2.22. (RuCp*Mes)2 was synthesized as described in the literature (45). CoCp2 was purchased from Sigma-Aldrich. N-DMBI-H was pur-chased from Tokyo Chemical Industry Co. Ltd. Anhydrous oDCB and nBA were purchased from FUJIFILM Wako Pure Chemical Corporation (FUJIFILM Wako). Methanol in the super dehydrat-ed grade was purchased from FUJIFILM Wako. Polyethylenimine was purchased from Sigma-Aldrich in the branched form whose Mw is claimed to be 25,000 by light scattering and Mn to be 10,000 by gel permeation chromatography (GPC). PFN-Br was pur-chased Sigma-Aldrich whose Mw is claimed to be 30,000 to 50,000 by GPC. EAGLE XG (Corning) glass substrates were used in this study.Fabrication of bottom electrodesBottom electrodes were fabricated on glass substrates. Three-nanometer Cr and 30-nm Au were thermally deposited. The samples used for the photoelectron measurements used electrodes deposited over the whole surface. The electrodes of the diodes used in Fig. 1 were pat-terned through shadow masks, and the electrodes of the diodes used in Figs. 5 and 6 were patterned through a lift-off process using a positive photoresist TLOR-P003 (Tokyo Ohka Kogyo Co. Ltd.). Af-ter the deposition of bottom electrodes, the samples were cleaned by ultraviolet/O3 for 15 min. The samples for diodes were further cleaned by sonication in acetone and isopropanol for 10 min each.(RuCp*Mes)2, CoCp2, and N-DMBI-H treatments were con-ducted in a N2 purged glove box. (RuCp*Mes)2 and N-DMBI-H were dissolved in nBA at a concentration of 2 mM. CoCp2 was dis-solved in acetonitrile at a concentration of 1 mM. These solutions were dropped on samples. After waiting for 30 s, spin coating at 2000 rpm was conducted. The samples were heated at 80°C for 5 min. Then, the samples were washed by spinning off nBA at 2000 rpm, followed by heating at 80°C for 5 min. PEI treatment was conducted in air. Aqueous solution with 0.2 wt % PEI was prepared. This solu-tion was dropped on samples. After waiting for 30 s, spin coating at 2000 rpm was conducted. The samples were heated at 80°C for 5 min. Then, the samples were washed by spinning off pure water at 2000 rpm, followed by heating at 80°C for 5 min. PFN-Br treatment was conducted in a N2 purged glove box. Methanol in the super de-hydrated grade was used to dissolve PFN-Br (0.5 mg/ml). This solu-tion was dropped on samples. After waiting for 30 s, spin coating at 2000 rpm was conducted. The samples were heated at 80°C for 5 min.Fabrication of diodesFabrications of P(NDIOD-T2) thin films were conducted in a N2 purged glove box. P(NDIOD-T2) was dissolved in oDCB. A concen-tration of 1 wt % was used unless indicated. Experiments shown in Fig. 4 used varying concentrations and ones shown in Fig. 6 used 0.5 wt %. Spin coating was conducted at 2000 rpm for 1 min using the P(NDIOD-T2) solutions preheated at 120°C. Then, samples were dried at 80°C for 20 min. The film thickness of P(NDIOD-T2) coated at a concentration of 1 wt % was determined to be 80 nm us-ing a Dektak Stylus Profiler (Bruker). The surface profile is shown in fig. S4.Top electrodes for diodes were fabricated by thermal deposition through shadow masks. Thirty-nanometer Au was deposited on the diodes used for Figs. 1 and 5. Thirty-nanometer Au and 300-nm Cu were deposited on ones used for Fig. 6. CYTOP CTL-809M diluted Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025Osakabe et al., Sci. Adv. 11, eadv9952 (2025)     19 September 2025S c i e n c e  A d v a n c e s  |  R e s e ar  c h  A r t i c l e7 of 8to one-fifth of the original concentration was spin coated on diode devices as an encapsulation layer. The spin coating condition was 2000 rpm for 1 min. Samples were then vacuum dried at room tem-perature for 5 min.XPS and UPS analysisXPS and UPS measurements were conducted using a KRATOS ULTRA 2 instrument with monochromatic Al Kα x-rays and He I α (21.2 eV). PYS measurements were conducted using SUMITOMO PYS-202 instrument with a xenon lamp. DC electrical measurements were conducted using Keithley 2450.Measurement setups for UHF rectificationFollowings are the list of components used in the experiments shown in Fig. 5: Agilent E4428C signal generator, Mini-circuit ZAPD-30-S+ power splitter, Mini-circuit ZFBT-4R2GW bias tee, and Tektronix MDO3014 oscilloscope. The probing to the samples was conducted using a Technoprobe TP40-GSG-200-A GSG probe.The following are the list of components used in the experiments shown in Fig. 6: Agilent E4428C signal generator, Digi-key D3C0802S circulator, mini-circuit ZHL-2-S+ amplifier, Pasternack PE2242-30 coupler as the coupler1, Pasternack PE2242-10 as the coupler2, Pasternack PE8013 power meter, Mini-circuit ZFBT-4R2GW bias tee, Agilent E5061B vector network analyzer, and Keithley 2000 and 2700 multimeters. 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Contributions to discussion of results and editing of the manuscript by S.B. and S.R.M. were supported by Office of Naval Research Grant N00014-24-1-2115. Author contributions:  N.O. performed PYS measurements, fabricated devices, and evaluated their AC properties.  J.H. performed UPS measurements. J.H., N.O., and Y.Y. performed XPS measurements. A.T. and T.K. fabricated devices and evaluated their DC properties. E.L. and K.T. synthesized (RuCp*Mes)2. K.F. contributed in high-frequency measurements. S.B. and S.R.M. contributed in synthesis of materials and discussion of the treatment process. S.W. and J.T. supervised electrical measurements. Y.Y. conceived of and designed the research. N.O. and Y.Y. wrote the manuscript. N.O., J.H., T.K., A.T., E.L., K.T., K.F., S.B., S.R.M., S.W., J.T., and Y.Y. discussed the results and reviewed the manuscript. Competing interests: N.O., S.W., J.T., and Y.Y. are inventors on a patent related to this work filed (no. WO2025053187A1, published on 13 March 2025). The other authors declare that they have no competing interests. Data and materials availability: Data supporting the plots within this study are available within the article and the Supplementary Materials. Requests for the synthesized materials should be addressed to  S.B. (stephen.​barlow@​colorado.​edu) and S.R.M. (seth.​marder@​colorado.​edu). Submitted 28 January 2025 Accepted 19 August 2025 Published 19 September 2025 10.1126/sciadv.adv9952Downloaded from https://www.science.org at National Institute for Materials Science on November 16, 2025mailto:stephen.​barlow@​colorado.​edumailto:seth.​marder@​colorado.​edu Polymeric microwave rectifiers enabled by monolayer-thick ionized donors INTRODUCTION RESULTS Diodes using surface-treated electrodes Surface analysis of the treated gold electrodes Distribution of ionized donors Rectification performance of diodes DISCUSSION MATERIALS AND METHODS Materials Fabrication of bottom electrodes Fabrication of diodes XPS and UPS analysis Measurement setups for UHF rectification Supplementary Materials This PDF file includes: REFERENCES AND NOTES Acknowledgments