The photodiode performances of NDI-appended ruthenium complexes

The synthesis and investigation of photoelectric studies of simple organic compounds as organic interlayers are of significant importance and widely studied. As such, we synthesized naphthalene diimide (NDI)-appended ruthenium complexes (Ru-NDIs) to function as the interface layer, and have fabricated novel Al/NDIs or Ru-NDIs/p-Si devices (D1-D4) to investigate their photoelectric properties. Subsequently, we compared and discussed the photoelectric properties of these devices after synthesis and fabrication. According to this, the band-gap energy (E g ) values of organic materials were found to range from 2.95 eV to 3.14 eV, making them ideal for solar cells applications. Additionally, the photoresponse (Pr) values of Al/NDIs or Ru-NDIs/p-Si devices (D1-D4) were found to be 59.25, 1593.08, 198.77, and 134.47, respectively. Moreover, the Al/Ru-NDIs/p-Si D2 structure exhibited the highest Pr values. Experimental results indicate that since the four optoelectronic devices arranged with the derivation of synthesized compounds have good photoresponse characteristics, they can be utilized as a photosensor or photodiodes in different electronic and optoelectronic technologies.


Introduction
Scientists focus on optoelectronic devices (OEDs), such as photodiodes and solar cells, for other sustainable and renewable energies within the scope of environmental energy research.These optoelectronic devices have an important role in energy applications.To increase their performance, organic materials are preferred because they have interesting features such as relatively high thermal stability, transparency, lightweight, electrical conductivity, flexibility, low price, and molecular weights [1][2][3][4][5].The organic materials are generally used in the device as an interface layer, hole transport layer (HTL), or electron transport layer (ETL), to increase the number and efficiency of carriers that contribute to the photocurrent [6][7][8][9][10][11]. Recently, many researchers have been researching the development of organic-based OED as an alternative to silicon based optoelectronic devices [4,5,[12][13][14][15].Because of all these impressive properties, the production of new optoelectronic materials attracts a lot of attention.Among them, the most commonly used organic structure is organic compounds containing a naphthalene diimide (NDI) skeleton, which is used as a good electron donor or acceptor.Recently, NDIs have been used to create various types of molecular linkages that exhibit moderate rectification properties.
On the other hand, ruthenium complexes have been extensively studied due to their optoelectronic properties [16][17][18][19][20][21].In particular, ruthenium complexes have been identified as significant components in developing advanced photodiode systems [17,[22][23][24][25].The sensitivity and stability of ruthenium-based materials have improved the efficiency of photodiodes.Their photoconductive properties also make them popular for optoelectronic devices.As a result, due to their unique optoelectronic properties, ruthenium complexes play a significant role in the development of advanced photodiode systems that have the potential to contribute to a wide range of scientific and technological fields, such as medical imaging and remote sensing.From this perspective, the combination of ruthenium complexes and NDI could lead to the formation of donor-acceptor systems that exhibit multiple redox states.To explore these aspects, the synthesis of novel pairs for the creation of large-area molecular linkages is of great interest.
The synthesis and photoelectrical investigation of new simplistic organics as organic interlayers are very important and popular [26][27][28][29][30]. Therefore, a series of studies have been carried out by our group on the efficient synthesis and photoelectrical applications of simple structured perylenediimide (PDI) and NDI derivatives (scheme 1) [31,32].In these studies, the synthesis of novel symmetrical and asymmetrical PDIs and NDIs as simplistic interlayers for organic-based OED was carried out, and their photophysical properties were comparatively investigated with an analytical overview.Herein, NDI-appended ruthenium complexes (Ru-NDIs) were synthesized as the interface layer, and novel Al/NDIs or Ru-NDIs/p-Si devices (D1-D4) were fabricated (scheme 1).The photoelectric properties and performance of these structures were analyzed at various light intensities as well as optical and surface properties.Additionally, the frequency-dependent electrical properties of these structures were examined at various frequencies.Experimental results indicate that the fabricated four structures can be used as a photosensor or photodiode in different electronic and optoelectronic technologies.

Experimental details
2.1.The synthesis of naphthalene diimide-appended ruthenium complexes (Ru-NDIs) The synthesis of NDIs Pry-NDI-Ph (NDI-1) and Pry-NDI-Ph-PO 3 H 2 (NDI-2) to be used as starting molecules for the synthesis of target ruthenium complexes was successfully carried out in accordance with the literature [32].Following to synthesis of NDIs, we focused on the synthesis of target Ru-NDIs (scheme 2).For this purpose, the [RuCl 2 (p-cymene)] 2 (1) complex was added to the solution of Pry-NDI-Ph (NDI-1) in ethanol, and the reaction mixture was boiled at the boiling temperature of ethanol overnight.Then, the reaction mixture brought to room temperature was precipitated with diethyl ether, and as a result of the purification process, the Scheme 1.The strategies for synthetic NDIs and Ru-NDIs.synthesis of the targeted [Ru(p-cymene)(Pry-NDI-Ph)(Cl) 2 ] (Ru-NDI-1) was obtained with good yield.Thereupon, the synthesis of the other target ruthenium-complex [Ru(p-cymene)(Pry-NDI-Ph-PO 3 H 2 )(Cl) 2 ] (Ru-NDI-2) was also carried out under the same reaction conditions.At this stage, the 1 H-NMR spectrum of the starting molecule NDI-1 and the target Ru-NDI-1 was discussed comparatively (figure S1).Accordingly, looking at the 1 H-NMR spectrum of NDI-1, the first thing that stands out is the resonance at 8.81 (m, =CH, 2H, part A of AB system) ppm of the aromatic =CH (number 1) proton peak in the pyridine (Pry) ring in the structure of the molecule.In addition, while the four aromatic =CH (numbered 3 and 4) protons in the NDI skeleton were seen at 8.74-8.71(m, =CH, 4H) ppm, the five phenyls (Ph, =CH, numbered 5, 6, and 7) protons of NDI-1 were seen at 7.50-7.45(m, =CH, 5H) ppm.On the other hand, when the 1 H-NMR spectrum of the synthesized target Ru-NDIs [Ru(p-cymene)(Pry-NDI-Ph)(Cl) 2 ] (Ru-NDI-1) is examined, it is seen that the characteristic peaks of NDI-1 are present with small shifts.In addition, the four aromatic =CH (numbered 10 and 11) protons in the p-cymene group in the structure of Ru-NDI-1 resonate at 5.82-5.72 (m, =CH, 4H, AB system) ppm.In addition, the presence of resonance signals belonging to the aliphatic CH 3 (numbered 8), CH (numbered 9) and Ph-CH 3 (numbered 12) protons in the p-cymene group is also observed.On the other hand, the molecular mass of Ru-NDI-1 determined by LC-MS (C 36 H 30 C l2 N 3 O 4 Ru: [M+4H] + calculated for m/z: 744.10 and found: 744.15) supports the proposed molecular structure (figure S6 and S7).All data are concrete indications that the targeted products are achieved.In addition to the NMR and mass spectra, the FTIR spectra of the Ru-NDIs also support the structures.Accordingly, the presence of characteristic imide (C=O and N=C=O) peaks in the structure of the output molecules NDIs is also noticeable in the FT-IR spectrum of the synthesized Ru-NDIs with small shifts (figure S8).

Electronic background
The three-dimensional (3D) geometry optimizations and all other theoretical calculations of the complexes were carried out using the Gaussian software (09W version package) [33,34] by the effective core potential (ECP) in combination with the B3LYP hybrid functional [35,36] with 6-31G (d,p) basis set for H,O,C,N atoms and LANL2DZ [37] basis set for heavy atom (Ru).The geometry of Ru-NDIs has been optimized at the B3LYP and LANL2DZ level of theory.Besides, frontier molecular orbitals (HOMO and LUMO) surfaces and electronic properties were determined by the TD-DFT/CPCM method [38,39].Also, the absorption spectra and MEP scan were calculated.These parameters of Ru-NDIs were computed using the formulas shown below (1-10) [40][41][42].

Device fabrication and electrical measurements
Synthesized NDIs and Ru-NDIs were used as an interface layer in metal-semiconductor-based devices.A thermal evaporation system was used in the entire fabrication phase.First of all, the four p-Si (thickness: 525 mm, resistivity: 1-10 Ω.cm) semiconductor wafers of 1 × 1 cm size were chemically cleaned.In this step, the wafers were first cleaned with acetone (C 3 H 6 O) for 10 min and then rinsed in DI water for 10 min.After that, the wafers were cleaned with isopropyl alcohol (C 3 H 8 O) for 10 min and rinsed in DI water for 10 min.In order to prevent the formation of an oxide layer, after the wafer was kept in HF solution for 10 min, it was rinsed in DI water for 10 min again.Wafers were dried with N 2 to get rid of organic contamination.The cleaned wafers were taken to the thermal evaporation system.In here, Al was evaporated on the o back surface of the wafers and, ohmic contact with about 150 nm thickness was obtained.Then, 1 mg of each of NDIs and Ru-NDIs in powder form was formed on the polished surface of four different wafers via the thermal evaporation system.Finally, Al was formed by using a shadow mask to the interfacial coated surface to form rectifier contacts in the thermal system.Thus, four organic based photodiodes (OBPs) were fabricated.They were designated as D1 (with NDI-1), D2 (with NDI-2), D3 (with Ru-NDI-1), and D4 (with Ru-NDI-2), respectively.The current-voltage (I-V) measurements of all fabricated OBPs were carried out using Keithley 4200 current/voltage source meter at various light intensities.The XPS-150 solar simulator, which can produce solar radiation in the 290-400 nm range was used as the light source.The illumination-dependent electrical measurements were carried out using a 0-100 mW.cm −2 UV 16S-Series Solar Simulator, which can produce solar radiation in the 290-400 nm range.In here, A/W proportion is that how efficiently a photodetector responds to an optical signal.Figure 1 shows a schematic diagram of electrical measurement systems.The atomic force microscope (AFM) measurements were obtained by PSIA XE-100E model.

Electronic properties
Energies of the HOMO and LUMO orbitals play a crucial role in both photonic and electrical properties, and quantum chemistry [43].For this purpose, following the synthesis, to better understand how Ru-NDIs structural properties affect its reactivity, the energies of molecular orbitals were calculated using the DFT and TD-DFT methods (figure 2(A)).Additonaly, by using HOMO and LUMO energies, energy gap (ΔE), electron affinity (EA), ionization potential (IP), chemical potential (m), chemical softness (z ), electronegativity (χ), chemical hardness (h), optical softness (s o ), global electrophilicity index (ω), maximum charge transfer index (ΔNmax), nucleophilicity index (N) have been calculated, and listed in table 1.The energy gap (ΔE) is the difference between LUMO and HOMO, which also gives the excitation energy of a neutral system, and a small ΔE value indicates that the molecule is more reactive.The energy of HOMO and LUMO represents an electron donating ability and an electron acceptor ability, respectively [44].According to table 1, the high value of electronegativity and electrophilic index supports that Ru-NDI-2 exhibits more electrophilic behavior than Ru-NDI-1 [45].At the same time, the negative and large values of the chemical potential of Ru-NDIs confirmed the good stability of the molecules and showed their suitability for optoelectronic applications.Additionally, the energy gaps and some electronic properties of molecules designated as 1A and 2A were calculated by the DFT method in the gaseous state, while molecules with superscripts (1A * and 2A * ) were calculated in TD-DFT in excited states (in solvent environment).Looking at figure 2(A), in both Ru-NDIs, HOMO electrons are mostly delocalised on Ru(p-cymene)Cl 2 groups, while LUMO electrons are mostly delocalised on NDI groups.In the Ru-NDIs, the energy gaps calculated by the DFT and TD-DFT methods are 1.77 and 1.71 eV, 2.54 and 2.51 eV, respectively.The higher the energy gap, the higher the stability, and according to these results, we can say that Ru-NDI-1 is more stable than Ru-NDI-2.In addition to these, chemical reactivity increases with the increase of the softness value, while the increase in the hardness value decreases the chemical reactivity.The results support each other and show that Ru-NDI-2 is more chemically reactive than Ru-NDI-1.One of the useful ways of quantum chemical computation to determine the active sites of molecules is the measurement of MEP surface maps [46].The red colored region on this map shows the electrophilic attack region (negative potential), while the blue colored region shows the nucleophilic attack region (positive potential).Moreover, the green regions show the zero potential region.The computed 3D surface maps for Ru-NDIs are shown in figure 2  hydrogen and nitrogen atoms in the naphthalene diimide group (NDI) and the hydrogen atoms in the p-cymene group, in addition, for Ru-NDI-2, the most positive region are around the hydrogen atoms in the phosphoric acid group.As seen in figure 2(B) (right side), i.e. from the ESP plots, the red and yellow spots on the carbonyl groups and chlorine atoms indicate that the electron density is more localized to the oxygen and chlorine atoms.ESP maps confirm negative potential regions of Ru-NDIs on MEP surfaces.

Morphological and optical properties
Following, the NDIs or Ru-NDIs were coated on p-Si, the surface morphologies were scanned 40 × 40 m via atomic force microscopy (AFM), and were given both 2D and 3D images (figures 3 and S9).When the AFM results were analyzed, the root means square (RMS) roughness values of NDIs (D1 and D2) or Ru-NDIs (D3 or D4) appended devices were found to be 0.54 ± 0.08 nm (for D1), 1.87 ± 0.31 nm (for D2), 16.6 ± 3.4 nm (for D3) and 4.8 ± 0.07 nm (for D4).As known, if the interfacial layer is thin, it leads to low photocurrent generated by the absorber layer.Conversely, if the interfacial layer becomes too thick, a large series resistance arises, causing a decrease in photocurrent and storage of charge [47].According to the RMS values, D1 (with NDI-1), D2 (with NDI-2), and D4 (with Ru-NDI-2), except for D3 (with Ru-NDI-1), have acceptable effective RMS values.Furthermore, the D2 device (with NDI-2) has an RMS value that is neither thin enough to reduce photocurrent nor thick enough to store charge for photoelectrical behavior.Therefore, AFM images indicate that the material surfaces are adequately coated with Ru-NDIs having acceptable RMS values.
As shown in figure 4(A), the absorbance peak values of NDIs and Ru-NDIs were found to be 365, 360, 360, and 365 nm, respectively.The band-gap energy (E g ) values of the material were determined by using the equations given in the following equations (equations ( 11) and (12); where α and A represent absorbance coefficient, and the material constant.As shown in figure 4(B), the E g values of D1, D2, D3, and D4 organic materials were found to be 3.14 eV, 3.12 eV, 3.13 eV, and 2.95 eV, respectively.Thus, these materials can be used in different optoelectronic devices since they have these values.When the absorbance values of the NDIs and Ru-NDIs obtained are examined, it has been revealed that these materials, which peak in the visible region, can be used in optoelectronic application [48].It also means that the forbidden energy gap will restrict photovoltaic efficiencies between 2.7 eV and 3.2 eV [49].5(a), I-V plots of each structure deviate from the ideal behavior and exhibit non-ideal behavior due to factors such as the presence of an interfacial layer between metal and semiconductor and image force lowering effect [50][51][52].These structures have different slopes in forward bias region (0<V<2).The typical electrical properties of a diode can be frequently evaluated using Thermionic Emission (TE) theory [53].
where A is the diode area and A * is Richardson's constant.The n and Φ B0 values of D1, D2, D3, and D4 were obtained by using equations ( 14) and (15) with the linear and intercept parts of the ln(I)-V plot in dark and light (100 mW.cm −2 ) and were given in table 2. In this study, we carried out I-V measurements for all the structures at various light intensities, as shown in figures S10(a)-(d).As shown in these figures, especially in reverse bias, more electrons pass to the conduction band with increasing light intensities and this causes an increase in the current of the diode [54].The basic electrical parameters depending on the light intensities of these devices are listed in table S1.As shown in the table, all the electrical parameters have a strong light intensity dependence.While n and I 0 increase with increasing light intensities, Φ B0 , R s , and Rectifier Rate (RR) decrease.The elevated electric field during reverse bias and the existence of charge traps within the depletion region of photodiodes do not contribute to carrier multiplication.Consequently, this further diminishes the sensitivity of the photodiodes.In addition, it may be because the generation and separation of electron-hole pairs, and the interfacial states activate increases with light intensities [55][56][57].Such behaviors of these electrical parameters have been reported by different authors [54][55][56][57][58][59].The n value should be 1 for an ideal diode.But, n values obtained experimentally are greater than 2 for all diodes due to barrier inhomogeneity, the existence of an interfacial layer between metal and semiconductor, generation-recombination effect, interface states, etc [51,53,[60][61][62].
The series resistance (R s ) values that influence the performance of a diode can be obtained using Ohm's Law (R=V/I) [53].The R s values of all the diodes were obtained in dark and light (100 mW.cm −2 ) at −2 V and were given in table 2. R s values obtained for various light intensities are given in table S1.The R s dominate at a higher forward bias, causing the I-V plot to deviate from linearity.
In determining the diode characteristic, the Rectifier Rate (RR) and the photoresponse (P r ) value of the diode are used.The former is the ratio of the current in forward bias (I forward ) to the current in reverse bias (I reverse ) of the diode, while the latter is the ratio of the current under the light (I on ) to the current under the dark (I off ) of the diode.Both values should be maximum.The photocurrent-time in dark and under light (100 mW.cm −2 ) intensity of all the structures is given in figure 5(c).As can be seen in figure 5(c), all the structures have photoconductive behavior [54].This behavior is proof that all diodes have photodiode properties.The obtained various electrical values (n, I 0 , Φ B0 , R s , RR, and P r ) for D1, D2, D3, and D4 in dark and light (100 mW.cm −2 ) are given in table 2. It can be seen from these values that four different diodes have RR and P r values with different rates.These results show that all the obtained devices can be used in different electronic and optoelectronic applications.Additionally, the highest RR and P r values belong to the D2 structure when compared with other structures.In other words, D2 responds to light more than others besides exhibiting better diode characteristics than other structures.
The table 3 shows the various electrical parameters of the devices with various NDI derivatives as the interface layer in the literature [63,64].When the electrical parameters obtained for D2 (with Ru-NDI) in this study were compared with the literature, it was seen that the RR value was approximately 1750 times better, the n value was relatively close to the ideal compared to the studies, and the Φ B0 value was higher than the others.It also seems that the RMS value is at the appropriate value, as mentioned in section 3.1.
The changes in electrical properties under various light intensities are examined using a double logarithmic photocurrent (I ph ) versus light intensities (P) plot [65,66].where P and α represent light intensities and a constant value, respectively.The m is an exponent value that is calculated from the slope of log (I ph ) versus log (P). Figure 6 shows the log(I)-log(P) plot of D1, D2, D3, and D4 in various light intensities at −2 V.The m value determines whether the photoconduction behavior of a structure is linear or non-linear.If the m is between 0.5 and 1, the photoconduction behavior is linear.But, the photoconduction behavior is non-linear when m is greater than 1.The m values for D1, D2, D3, and D4 at −2 V were found to be 0.625, 1.262, 1.051, and 0.804, respectively.Thus, D2 and D3 have a non-linear behavior while D1 and D4 have a linear behavior.In other words, the trap levels localized in the band gap are irregularly distributed for D2 and D3, while they are regularly distributed for other structures [66][67][68].
The spectral responsivity (R), Detectivity (D * ), noise equivalent power (NEP), and the spectral external quantum efficiency (EQE) properties can use to evidence photoresponse properties.R is about the photocurrent produced per optical power of the incident light.D * gives a measure of the produced photodetector ability to detect weak signals.NEP express the power generated by a source of noise on a detector.The spectral EQE evaluates the conversion rate from photons to electrons/holes.

=
where I light , I dark , P in , and A denote the photo-current, dark current, the incident light power, and area of the structure, respectively.Table 4 shows some photodetector parameters for 350 nm within −2 V for D1, D2, D3 and D4 structures.These results show that D2 has R, D * , and EQE values higher than the other structures.

The frequency-dependent electrical properties
The C-V and G/ω-V give information about the charge storage of device.Thus, the C-V and G/ω-V measurements of D1, D2, D3, and D4 were carried out.The frequency-dependent various electrical parameters of D1, D2, D3, and D4 at 1 MHz in dark were calculated using the intercept, and slope of the C -2 -V plot and the following equations; In equation (3d), V D is the diffusion potential and equals V 0 +(kT/q), where V 0 is the x-axis point of the C -2 -V plot.Other parameters in the equations are defined in the literature [69][70][71].Thus, the C -2 -V plots at 1 MHz and at various frequencies for D1, D2, D3, and D4 were plotted and were given in

Conclusion
We synthesized NDIs and Ru-NDIs organic materials and fabricated the Al/NDIs or Ru-NDIs/p-Si (D1, D2, D3, and D4) structures using them as the interface layer.The various light intensities dependence of parameters of D1, D2, D3, and D4 were analyzed.Experimental results indicate that (i) all the structures have a good diode behavior, and (ii) since fabricated four structures have a good RR and P r values, they can be used as a photosensor, or photodiode in different electronic and optoelectronic technologies, (iii) the D2 structure exhibited the highest R, D * , EQE, RR and P r values.Additionally, the photoresponse of materials can depend on various factors, including the material's electronic structure, crystal structure, surface properties, and doping concentration.The tuning of the photoresponse properties of the materials is very important because this tuning can be used to optimize the optical and electronic properties of a material for specific applications.Therefore, in this work, we demonstrated that photoresponse characteristics of prepared optoelectronic devices are easily arranged with the derivation of synthesized compounds.

Figure 1 .
Figure 1.Schematic diagram of electrical measurement systems.
(B).Looking at figure 2(B) (left side), in both Ru-NDIs, the negative regions (red-yellow) are around oxygen and chlorine atoms due to the abundance of electrons around them, while the low positive regions (blue-light blue) are usually around the

Figure 2 .
Figure 2. Molecular orbital (A) and molecular electrostatic (B) surfaces of Ru-NDIs computed by TD-DFT method.

3. 3 .
The photoelectrical studies I-V plots in dark and light (100 mW.cm −2 ) at −2 V of the fabricated four OBPs are given in figures 5(a)-(b).As shown in figure

Figure 7
shows the C-V and G/ω-V plots of D1, D2, D3, and D4 at 1 MHz in dark.Additionally, the C-V and G/ω-V plots depending on various frequencies of D1, D2, D3, and D4 are shown in figures S11(a)-(d) and S12(a)-(d), respectively.As shown in these figures, the C-V and G/ω-V plots of D1, D2, D3, and D4 have the three regions, which are named inversion, depletion, and
figures 8 and S13(a)-(d), respectively.A comparison of various electrical parameters of D1, D2, D3, and D4 at 1 MHz is given in table 5.The obtained electrical value for D1, D2, D3, and D4 at various frequencies are also shown in tables S2(a)-(d).

Table 1 .
Electronic parameters calculated by DFT and TD-DFT a methods for Ru-NDI-1 and Ru-NDI-2.

Table 3 .
The various electrical parameters of the devices with various NDI derivatives as the interface layer.