High Rectiﬁcation Ratio in Polymer Diode Rectiﬁer through Interface Engineering with Self-Assembled Monolayer

: In this work, we demonstrate P3HT (poly 3-hexylthiophene) organic rectiﬁer diode both in rigid and ﬂexible substrate with a rectiﬁcation ratio up to 10 6 . This performance has been achieved through tuning the work function of gold with a self-assembled monolayer of 2,3,4,5,6-pentaﬂuorobenzenethiol (PFBT). The diode fabricated on ﬂexible paper substrate shows a very good electrical stability under bending tests and the frequency response is estimated at more than 20 MHz which is sufﬁcient for radio frequency identiﬁcation (RFID) applications. It is also shown that the low operating voltage of this diode can be a real advantage for use in a rectenna for energy harvesting systems. Simulations of the diode structure show that it can be used at GSM and Wi-Fi frequencies if the diode capacitance is reduced to a few pF and its series resistance to a few hundred ohms. Under these conditions, the DC voltages generated by the rectenna can reach a value up to 1 V. K.L., K.L., R.B.; funding


Introduction
Nowadays, diode rectifiers are a rapidly growing technology since it can be used in a range of applications such as radio frequency identification (RFID) tags [1], energy harvesting devices [2], and wireless communications [3]. In particular, rectifiers from organic materials have attracted considerable attentions since they offer many advantages of low cost [4], flexibility [1], and soft processing [5].
Interface properties between electrodes and semiconducting materials play an important role to improve the performance of electronic devices. To achieve good hole injection within the organic semi-conductor layer, it is necessary to diminish the energetic difference between the electrode work-function and the organic semiconductor's highest-occupied molecular orbital (HOMO). Even with the right choice of electrode material matching perfectly the semiconductor's HOMO level, molecular dipole forming at the interface can yield non-negligible energetic barriers for hole injection [22,23]. Many strategies have been investigated in the literature to decrease such injection barriers [18,24,25].
One major approach that has been employed to tune the metal work function is via the functionalization of a self-assembled monolayer (SAM) of molecules that bear large dipole moment when aligned and functionalized on a surface [26][27][28][29][30]. In particular, SAM of fluorinated thiols has attracted a lot of attention [26,28,31,32]. In addition to their ability to tune metals' work function and improve the charge injection, they can also improve conduction by affecting the molecular packing and crystallinity of organic semiconductors that are deposited either by vacuum evaporation [33], or from solution [34,35]. From molecules that has been widely used, 2,3,4,5,6-pentafluorobenzenethiol (PFBT) shows improved hole injection performances for a large range of applications such as organic field effect transistors (OFET) [26,27,[36][37][38][39], solar cells [31], memory devices [32,40], and diode rectifier [24,28]. Recently, Chang-mo Kang and coworkers have investigated the effect of these molecules on pentacene-based diodes and high rectification ratio of up to r = 10 7 has been obtained, with the ability to work in gigahertz frequency range [28].
In this work, we investigate the effect of interface engineering with PFBT SAM in P3HT polymer diode rectifier on rigid and paper substrate. We use this strategy to increase the imbalance between the forward and reverse current of the diode and thus to improve the rectification ratio. The diode is fabricated on a release paper and can be transferred on any other flexible substrate.
By reducing the thickness of the active layer of the diodes, we were also able to reduce the threshold voltages, which allows the diode to be sensitive to very low voltages of the ambient electromagnetic waves in energy harvesting applications.

Experimental
N type Si wafer (doped with phosphor, 380-400 µm thick, resistivity of 0.0001-0.0003 Ω·cm) with 200 nm thermal oxide layer was used as substrate. The substrate was ultrasonically cleaned with acetone and isopropanol for 10 min each, and then treated by ultra-violet ozone for 25 min to remove any residual contaminants on the surface. With a 5 nm titanium adhesion layer, 120 nm Au were deposited by thermal evaporation through shadow mask to serve as anode contact. For SAM functionalization on the anode, samples were immersed in the solution of 10 µL of PFBT (Sigma-Aldrich, Saint Louis, MO, USA, purity 97%, molecular weight M n = 15-45 kDA, polydispersity PDI ≤ 2) diluted in 10 mL of ethanol for 18 h, and followed by rinsing with pure ethanol and drying under nitrogen flow. After functionalization, the semiconductor was wet-deposited. The solution was prepared by dissolving 30 mg/mL of regioregular P3HT (Sigma-Aldrich 99.995%) in chlorobenzene, and coated in a glove box under a N 2 atmosphere at 1000 rpm for 30 s. The thickness was measured by profilometry to be 110 nm. The film was then backed for 1 h at 100 • C in vacuum to remove the remaining solvent. After the deposition of the semiconductor layer, 120 nm of Al was thermally evaporated through shadow mask to form top contacts. The overlap between Au and Al contacts defines the diodes' active area, ranging from 10 −4 to 10 −2 cm 2 . One sample of P3HT diode without PFBT treatment was fabricated as a reference.
Current-voltage characteristics were measured in a nitrogen-filled glove box (O 2 and H 2 O < 0.1 ppm) by using a microprobe station connected to an Agilent 4156C parameter analyzer (Keysight Technology, Ulis, France). The forward biasing is corresponding to a positive voltage at the Au electrode with respect to the grounded Al one. AFM was used to analyze the thin-film surface morphology. The modification of Au induced by PFBT was analyzed by contact angle measurement and X-ray photoelectron spectroscopy (XPS) (5600, Physical electronics GmBH, Feldkirchen, Germany). XPS analysis was performed by Physical Electronics system type 5600, under ultra-high vacuum, and by irradiating the surface by a beam of X-rays with a helium discharge lamp with an energy of 21.2 eV. Data were treated using the Multipak software (9.3, UlVAC-PHI, Chigasaki, Kanagawa, Japan).
AFM was performed on Bruker's (Dimension 3100, Santa Barbara, CA, USA), with employing silicon cantilevers (Bruker NCHV-A) in tapping mode with a spring constant of 40 N/m and a resonance frequency of 316 kHz: data were treated using the software Gwyddion.

Results and Discussion
The structure of the diode is shown in Figure 1a. The effect of PFBT was evaluated by mean of contact angle as shown in Figure 1b,c, and shows a decreased hydrophilicity with PFBT functionalization (θ = 87 • compared to 50 • for bare gold). Contact angle has been also performed on PFBT-treated Au after a backing at 100 • C for one hour where the measure do not show a significant change (θ = 89 • ). The later suggests that the temperature does not alter the formation of the monolayer. As the SAM treatment could affect the morphology of the semiconducting layer, AFM microscopy on both surfaces was measured as shown in Figure 1f,g. The PFBT treatment reduced also the roughness of the surface by a factor of 3, it has a value of 7.43 nm on Au surface and was lowered to a value of 2.27 nm with PFBT functionalization. Excess thiol potentially fills the voids on the evaporated Au nanostructured surface, it is worth highlighting that residual SAM molecules remaining at the surface as nanopuddles would have a rather detrimental effect on the current injection [41,42]. XPS spectroscopy was used to analyze the surface composition and further investigate the formation of PFBT on the Au surface as shown in Figure 1c. This spectrum shows a clear peak at 686.4 eV and a peak at 162.4 eV associated respectively to F1s and S2p, a clear signature of PFBT molecules, indicating that the PFBT monolayer is well deposited on gold.
AFM was performed on Bruker's (Dimension 3100, Santa Barbara, CA, USA), with employing silicon cantilevers (Bruker NCHV-A) in tapping mode with a spring constant of 40 N/m and a resonance frequency of 316 kHz: data were treated using the software Gwyddion.

Results and Discussion
The structure of the diode is shown in Figure 1a. The effect of PFBT was evaluated by mean of contact angle as shown in Figure 1b,c, and shows a decreased hydrophilicity with PFBT functionalization (θ = 87° compared to 50° for bare gold). Contact angle has been also performed on PFBT-treated Au after a backing at 100 °C for one hour where the measure do not show a significant change (θ = 89°). The later suggests that the temperature does not alter the formation of the monolayer. As the SAM treatment could affect the morphology of the semiconducting layer, AFM microscopy on both surfaces was measured as shown in Figure 1f,g. The PFBT treatment reduced also the roughness of the surface by a factor of 3, it has a value of 7.43 nm on Au surface and was lowered to a value of 2.27 nm with PFBT functionalization. Excess thiol potentially fills the voids on the evaporated Au nanostructured surface, it is worth highlighting that residual SAM molecules remaining at the surface as nanopuddles would have a rather detrimental effect on the current injection [41,42]. XPS spectroscopy was used to analyze the surface composition and further investigate the formation of PFBT on the Au surface as shown in Figure 1c. This spectrum shows a clear peak at 686.4 eV and a peak at 162.4 eV associated respectively to F1s and S2p, a clear signature of PFBT molecules, indicating that the PFBT monolayer is well deposited on gold.

Diode Rectifier on Silicon Substrate
Current-density-voltage (J-V) characteristics of P3HT diode rectifiers with and without PFBT are shown in Figure 2a. The current density is about 0.1 A/cm 2 at 4 V for the diode with PFBT one order of magnitude higher than the diode without PFBT. We can also evaluate from this figure the rectification ratio r defined as the ratio between ON

Diode Rectifier on Silicon Substrate
Current-density-voltage (J-V) characteristics of P3HT diode rectifiers with and without PFBT are shown in Figure 2a. The current density is about 0.1 A/cm 2 at 4 V for the diode with PFBT one order of magnitude higher than the diode without PFBT. We can also evaluate from this figure the rectification ratio r defined as the ratio between ON and OFF currents at a fixed voltage. r was measured for 13 devices as presented in the histogram of Figure 2b. The rectification ratio r reaches 10 6 at ±4 V for P3HT diodes with PFBT and spreads from 10 2 to 10 6 without SAM functionalization. Few diodes without SAM shows slightly higher rectification, but only due to the particularity low noise current under reversed polarization, but it is clear from the figure that the SAM treatment leads to increased ON/OFF ratio averagely for all the population of diodes. These values are two orders of magnitude higher than previously obtained for the same rectifying device stack [17][18][19]. In addition, we can see from these figures, that the diodes with PFBT monolayer are more reproducible. The turn on voltage is defined as the voltage when charge injection is taking place [28] and the current increases gradually, which is about 0.1 V.
Electron. Mater. 2021, 2, FOR PEER REVIEW 4 and OFF currents at a fixed voltage. r was measured for 13 devices as presented in the histogram of Figure 2b. The rectification ratio r reaches 10 6 at ±4 V for P3HT diodes with PFBT and spreads from 10 2 to 10 6 without SAM functionalization. Few diodes without SAM shows slightly higher rectification, but only due to the particularity low noise current under reversed polarization, but it is clear from the figure that the SAM treatment leads to increased ON/OFF ratio averagely for all the population of diodes. These values are two orders of magnitude higher than previously obtained for the same rectifying device stack [17][18][19]. In addition, we can see from these figures, that the diodes with PFBT monolayer are more reproducible. The turn on voltage is defined as the voltage when charge injection is taking place [28] and the current increases gradually, which is about 0.1 V. This performance improvement can be attributed to two different properties for the PFBT treatment: a first one is the energy barrier reduction between gold and HOMO level of organic semiconductor and the second one is a change in the semiconductor film morphology and molecular orientation. As it has been reported in the literature [32,44,45], the adsorption of PFBT monolayer on gold surface leads to interface dipole layers oriented downward to the metal substrate. Consequently, the work-function of gold increases that value to about 5.1 eV compared to 4.56 eV for untreated gold [43]. From the energy band diagram in Figure 2c, we can see that without PFBT modification, the barrier hole injection has a value of 0.34 eV, and decreases to 0.2 eV with the PFBT, (the value of HOMO and lowest unoccupied molecular orbital (LUMO) are taken from ref [18]). These confirm the reduction of the hole injection barrier and explains the increase of the ON current with PFBT modification.
In addition to tuning the work-function of gold, PFBT also affects the morphological properties of the film and the interface properties as shown in Figure 1. It has been reported in the literature that higher contact angle leads to enhanced crystallinity in the semiconductor and improved mobility near the electrodes [27]. This mechanism can also This performance improvement can be attributed to two different properties for the PFBT treatment: a first one is the energy barrier reduction between gold and HOMO level of organic semiconductor and the second one is a change in the semiconductor film morphology and molecular orientation. As it has been reported in the literature [32,44,45], the adsorption of PFBT monolayer on gold surface leads to interface dipole layers oriented downward to the metal substrate. Consequently, the work-function of gold increases that value to about 5.1 eV compared to 4.56 eV for untreated gold [43]. From the energy band diagram in Figure 2c, we can see that without PFBT modification, the barrier hole injection has a value of 0.34 eV, and decreases to 0.2 eV with the PFBT, (the value of HOMO and lowest unoccupied molecular orbital (LUMO) are taken from ref [18]). These confirm the reduction of the hole injection barrier and explains the increase of the ON current with PFBT modification.
In addition to tuning the work-function of gold, PFBT also affects the morphological properties of the film and the interface properties as shown in Figure 1. It has been reported in the literature that higher contact angle leads to enhanced crystallinity in the semiconductor and improved mobility near the electrodes [27]. This mechanism can also occur in our case with the PFBT-P3HT diode. In fact, poor wetting of P3HT solutions on low-hydrophilicity surface results in the formation of edge-on oriented film [46]. This molecular organization is more suitable for the charge transport and the Π-Π orbital overlapping. The performance improvement of P3HT film crystallinity can also be in- Electron. Mater. 2021, 2 449 duced by sulfur-fluorine interactions between PFBT and P3HT, which leads to a denser molecular packing of P3HT, and a higher ordering of the organic semiconductor layer. These results have been investigated by Jurchescu and co-workers on fluorinated 5,11bis(triethylsilylethynyl)anthradithiophene (diF-TESADT) films deposited on PFBT-Au electrodes [32,47]. Morphological effects such as roughness are also reported to affect the organic film growth and therefore electrical performances of the devices. In our configuration, the PFBT also leads to a decrease in the roughness of the surface. These multiple effects are proposed to be the origin of the performance improvement observed in the case of our PFBT-treated diodes.

Diode Rectifier on Flexible Substrate
On another level and to demonstrate the possible flexibility of our devices, the Au-PFBT/P3HT/Al diode has been fabricated on flexible substrate, which is a cardboard paper provided by Centexbel (ref WO84). This paper has the advantage to possess a small roughness of about 1.4 nm (Figure 3a) due to a protective coating of silicone, and showed high stability to the solvent that are employed in the diode fabrication process. The paper was first placed in a primary vacuum oven at 100 • C for 1 h to degas the moisture present in the fibers. The diode was then fabricated on this substrate with the same process was detailed in the experimental part for silicon oxide substrate. In addition, this paper substrate is a release paper that can allow an easy transfer to all other flexible substrates such as textiles. The electrical characteristic of the diode fabricated on the release paper is shown by Figure 3b. A density current of 0.026 A/cm 2 was obtained at 4 V and high rectification ratio r of 4 × 10 5 at ±4 V. The flexible diode rectifier is measured under bending tests (Figure 3c) with radius of 1.4 cm. The results show very good stability of the electrical characteristic except a little increase of the OFF current. This can be related to the direction of the bending that was made parallel to the aluminum electrode.

Frequency Response of the Rectifier Diode
The frequency response of the diode rectifier was evaluated by LT-Spice simulation tools using the basic circuit rectifier shown in the inset of Figure 4. This circuit is composed of a function generator which provides the AC input signal for various frequencies, the diode rectifier, and the load capacitance. The load capacitance value needs to be higher than the capacitance of the diode to reduce the output voltage ripples [48], a value of 10 nF was chosen in our case. The parameters of the diode used in simulation are extracted experimentally by impedance spectroscopy measurements [43]. The obtained values for the electrical equivalent circuit of the diode are C D = 174 pF (diode capacitance) and R D = 120 Ω (for the series resistance). cies, the diode rectifier, and the load capacitance. The load capacitance value needs to be higher than the capacitance of the diode to reduce the output voltage ripples [48], a value of 10 nF was chosen in our case. The parameters of the diode used in simulation are extracted experimentally by impedance spectroscopy measurements [43]. The obtained values for the electrical equivalent circuit of the diode are CD = 174 pF (diode capacitance) and RD = 120 Ω (for the series resistance). The results of the simulation are shown in Figure 4. Under a 500 kHz, 2 V peak-to-peak oscillating input signal, an output DC voltage of 1.51 V was obtained, value that decreases to reach a DC output voltage of 0.15 when the input frequency increases to 20 MHz which is compatible with RFID applications (13.56 MHz). For energy harvesting, the frequency response of the diode can be improved by optimizing the device structure with reducing the capacitance of the diode and its resistance [43].
The Figure 5 represents the effects of the capacitance and the resistance of the diode on the frequency response. As it can be seen, minimizing the capacitance and series resistance is critical to achieving high frequency response up to Gigahertz, which corresponds to the theoretical equations [49] evaluating the frequency of the diode as proportional to these two parameters. The resistance of our P3HT diode rectifiers is in the order of 100 Ω and as can be seen by Figure 5b may lead to achieve FM, Wi-Fi band frequency. However, the capacitance of the diode needs to be further decreased to the range of pF value. Value that can be obtained with the design of diode rectifier with low area of active layer at the range of 5 × 10 −5 cm 2 . In this case, a DC voltage of about 1 V is obtained at a frequency of 900 MHz. Even those values are small for energy harvesting systems, they can be increased by using other diode configurations, such as a voltage doubler or a Cockcroft multiplier [50,51]. The results of the simulation are shown in Figure 4. Under a 500 kHz, 2 V peak-to-peak oscillating input signal, an output DC voltage of 1.51 V was obtained, value that decreases to reach a DC output voltage of 0.15 when the input frequency increases to 20 MHz which is compatible with RFID applications (13.56 MHz). For energy harvesting, the frequency response of the diode can be improved by optimizing the device structure with reducing the capacitance of the diode and its resistance [43].
The Figure 5 represents the effects of the capacitance and the resistance of the diode on the frequency response. As it can be seen, minimizing the capacitance and series resistance is critical to achieving high frequency response up to Gigahertz, which corresponds to the theoretical equations [49] evaluating the frequency of the diode as proportional to these two parameters. The resistance of our P3HT diode rectifiers is in the order of 100 Ω and as can be seen by Figure 5b may lead to achieve FM, Wi-Fi band frequency. However, the capacitance of the diode needs to be further decreased to the range of pF value. Value that can be obtained with the design of diode rectifier with low area of active layer at the range of 5 × 10 −5 cm 2 . In this case, a DC voltage of about 1 V is obtained at a frequency of 900 MHz. Even those values are small for energy harvesting systems, they can be increased by using other diode configurations, such as a voltage doubler or a Cockcroft multiplier [50,51].
As the rectenna is compatible with large surface configuration, one can imagine a system with an overlay of many rectenna devices to increase the power conversion efficiency. Electron As the rectenna is compatible with large surface configuration, one can imagine a system with an overlay of many rectenna devices to increase the power conversion efficiency.

Conclusions
In this study, high rectification ratio up to 10 6 has been achieved for P3HT diode rectifier on silicon substrates and through the tuning of the work function of gold electrode anode by a self-assembled monolayer of PFBT. The P3HT rectifier diodes were also fabricated on flexible substrate (Centexbel release paper). They on paper demonstrated also a good rectification ratio and electrical stability under bending tests. Finally, to demonstrate the possibility of using our diodes in a rectenna circuits for the energy harvesting we study its frequency response. Operation frequency of 20 MHz can be achieved with the P3HT diode. This value can reach higher frequencies, in the GSM and Wi-Fi band by reducing the capacitance and series resistance of the diodes where continuous voltage of about 1 V can be recovered.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.
Acknowledgments: This research work has been partially undertaken with the support of IEMN fabrication (CMNF) and characterization (PCMP) facilities. We thank the French National Nanofabrication Network RENATECH, and the IEMN cleanroom staff for their support. We also thank CENTEXBEL for the release paper supply.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Conclusions
In this study, high rectification ratio up to 10 6 has been achieved for P3HT diode rectifier on silicon substrates and through the tuning of the work function of gold electrode anode by a self-assembled monolayer of PFBT. The P3HT rectifier diodes were also fabricated on flexible substrate (Centexbel release paper). They on paper demonstrated also a good rectification ratio and electrical stability under bending tests. Finally, to demonstrate the possibility of using our diodes in a rectenna circuits for the energy harvesting we study its frequency response. Operation frequency of 20 MHz can be achieved with the P3HT diode. This value can reach higher frequencies, in the GSM and Wi-Fi band by reducing the capacitance and series resistance of the diodes where continuous voltage of about 1 V can be recovered.