Materials Science in Semiconductor Processing 130 (2021) 105805

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Materials Science in Semiconductor Processing

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The current transformer mechanism and structural properties of novel Al/ BODIPY/pSi and Au/BODIPY/pSi heterojunctions

¨ a b,* c,** a c Omer Sevgili , Lütfi Bilal Tasyürek , Sinan Bayindir , Ikram Orak , Eray Caliskan a Vocational School of Health Services, Bingol University, 12000, Bingol, Turkey b Department of Opticians, Darende Bekir Ilicak V.H.S, Malatya Turgut Ozal University, 44700, Malatya, Turkey c Department of Chemistry, Faculty of Science and Art, Bingol University, 12000, Bingol, Turkey

ARTICLE INFO ABSTRACT

Keywords: In this study, the performance of BODIPY dye-based organic layers was investigated with diode applications. Biphenyl Gold (Au) and aluminum (Al) rectifier contacts were evaporated on the BODIPY dye-based organic layers. The BODIPY dye effect of the Au and Al metal contacts were examined under different illumination and frequency conditions. The Electrical characteristics surface morphology of the organic layers was characterized with an atomic force microscopy. The electrical Organic interlayer measurements, such as current-voltage, capacitance-voltage and conductance-voltage, were measured in the Photodiode dark and at room temperature. The photovoltaic performance of the organic layers was investigated under various illumination conditions. Eight devices were made with four different organic materials using the Au and Al metal contacts. The effects of Al and Au on the devices were compared with the electrical measurements. It was concluded that the performance of the devices made with Au was better, as there was oxidation in the devices made with Al.

1. Introduction photodiodes is greater than the band gap energy of the semiconductor, electron-hole pairs are formed. The pairs of charges occurring in the In recent years, contact structures formed by the combination of space charge region separate from each other when exposed to the metals and semiconductors have been used in many technological ap electric fieldand move in opposite directions by being dragged into the plications due to their rectifierproperties [1,2]. Metal oxides, insulators neutral region. Drifting pairs of charges enables a photocurrent to occur and organic materials are used as interfaces to produce more ideal de [8,9]. vices by increasing the barrier height of metal-semiconductor (MS) The modern approach in organic chemistry allows the development rectifier structures [3]. In general, it is possible to improve the perfor of organic ligands fulfilling the above requirements as well as with the mance of Schottky barrier diodes (SBD) based on the materials used in necessary functionality to come off the targeted photovoltaic efficiency the interface layer [4]. Some polymers and organic molecules have [10]. In fact, there is a wide range of commercially available organic structural advantages such as low costs, renewability and easy produc dyes with emission/absorption bands plating almost the whole visible or tion. In recent years, these advantages have attracted attention, partic ultraviolet spectral area. Among these, borondipyrromethene (BODIPY) ularly in technological and optoelectronic device research [5]. Electrons dyes are the most important, due to their excellent photophysical and holes are the charge carriers in MS structures, which have an properties readily improvable to a plurality of synthetic ways [11,12]. interfacial layer of organic materials [6]. These structures are also used That is, BODIPY dyes can be specificallyand exhaustively functionalized in photodiode applications as they demonstrate photovoltaic properties with a great number of organic cores, and such a substitution model in addition to rectifier properties. The interface layer enables the for rules the photophysical properties such as energy transfer, electron mation of electrons and holes in optoelectronic devices and provides transfer, charge, etc. can induce [13,14]. In other words, after an effi advantages regarding device performance [7]. Photodiodes are popular cient molecular touch, custom made BODIPY dyes can be conceived with optoelectronic applications that convert illumination light into electric the specificrequirements of any application an organic dye (Fig. 1) [15]. current. If the energy of the illumination light coming into the The synthesis of organics containing the BODIPY core and

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L.B. Tasyürek), [email protected] (S. Bayindir). https://doi.org/10.1016/j.mssp.2021.105805 Received 26 November 2020; Received in revised form 26 February 2021; Accepted 10 March 2021 1369-8001/© 2021 Elsevier Ltd. All rights reserved. ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805

adsorption for 30 min under moist conditions, excess solution was removed by absorption onto filter paper. The resultant substrates were rinsed with solvent (2 × 50 μL) to remove the loosely bound monomers and the samples were stored in a desiccator in vacuum for 1 h before imaging. The scanning speed was at a line frequency of 1.0 Hz, and the original images were sampled at a resolution of 512 × 512 pixels. A Keithley 2400 Picoammeter/Voltage Source was used for the I–V mea surements. The photodiode performance was analyzed in a C01NC-16S- 150-002 solar simulator and an HP 4192 A LF impedance analyzer was used to measure the C–V and G-V characteristics of the device.

2.2. General procedure for the synthesis of BODIPY dyes

The target BODIPY dyes, namely B1, B2, B3 and B4, were synthesized by Fig. 1. The molecular structure of the BODIPY dye. the methods described recently [18]. Four drops of trifluoroaceticacid (TFA) were added to a stirring solution of pyrrole (2,4-dimethyl-1H-pyrrole or ′ investigating their potential applications are very popular [16,17]. 3-ethyl-2,4-dimethyl-1H-pyrrole) (2 equiv.) and aldehyde ([1,1 -biphe Thus, in this analytical study, the physical properties of BODIPY dyes nyl]-4-carbaldehyde or thiophene-2-carbaldehyde) (1 equiv.) in dichloro obtained by changing meso- or 2-positions of the BODIPY core were methane (DCM) at room temperature (rt) under an inert atmosphere. After compared. The primary goal of this study, in which the synthesis of 2- the completion of the reaction was checked using thin-layer chromatography and 8-substituted BODIPY dyes were carried out, was to examine the (TLC, 0.25-mm-thick precoated silica plates), 2,3-dichloro-5,6-dicyano-1, effect of the change of side groups on the photophysical properties of 4-benzoquinone (DDQ) (1.2 equiv.) was added to the reaction mixture, BODIPY dyes. For this purpose, the BODIPY dyes were synthesized and which was then stirred for 4–5 h. The mixture was washed (3 × 30 mL) with used as an interfacial layer in the photodiode and diode applications 0.1 M NaOH (aq), dried over Na2SO4, filtered, and combined with trime (Fig. 2). The photovoltaic performances of the Al/B1/pSi, Au/B1/pSi, . . thylamine (TEA) (1.6 mL, 10 equiv.) and boron trifluoride etherate (BF3 Al/B2/pSi, Au/B2/pSi, Al/B3/pSi, Au/B3/pSi, Al/B4/pSi and Au/B4/pSi OEt2) (1.4 mL, 10 equiv.) at rt. The obtained solution was once again washed structures were compared. Then, various electrical parameters were with water (3 × 30 mL) and dried over Na SO . The solvent was removed by – – 2 4 investigated using the I V measurements. In addition, the C V and G-V distillation under reduced pressure and the crude product was purifiedusing measurements of the devices were examined depending on the fre column chromatography (silica gel 60, 230–400 mesh ASTM). quency. In this way, it is aimed that producted devices are used in op The BODIPY dye B1. The compound B1 was purified by column toelectronic device applications. chromatography using hexane/chloroform (7:3, v/v) as an eluent to give 1 a 24% yield. H NMR (CDCl3, 400 MHz, ppm) 7.76–7.74 (m, =CH, 2H), 2. Experimental details 7.70–7.67 (m, =CH, 2H), 7.49 (t, J = 7.5 Hz, =CH, 2H), 7.41–7.35 (m, 13 =CH, 3H), 6.00 (s, =CH, 2H), 2.57 (s, CH3, 6H), 1.45 (s, CH3, 6H); C 2.1. General details NMR (CDCl3, 100 MHz, ppm) = 155.5, 143.1, 141.7, 141.5, 139.9, 133.9, 131.4, 128.9, 128.4, 127.9, 127.6, 127.0, 121.2, 29.7, 14.60, All of the chemicals were commercially purchased from Merck or 14.55; IR (cm 1): 2919, 2851, 1542, 1511, 1304, 1187, 1156, 1048, 1 13 + Sigma-Aldrich. The H NMR (400 MHz) and C NMR (100 MHz) spectra 973; ESI-MS m/z 400.4390 (M) (Figure S5). The spectroscopic results were recorded on a Bruker spectrometer. Infrared spectras were recor were compatible with those reported in the literature [19]. ded on a Mattson 1000 FT-IR spectrophotometer. The high-resolution The BODIPY dye B2. The compound B2 was purified by column mass spectrometry measurements were recorded on a Thermo Scienti chromatography using hexane/chloroform (6:4, v/v) as an eluent to give fic Exactive Plus Orbitrap LC-MS. The AFM images were collected on a 1 an 18% yield. H NMR (CDCl3, 400 MHz, ppm) 7.78–7.76 (m, =CH, 2H), NanoScope IIIa device at ambient temperature in tapping mode using 7.74–7.72 (m, =CH, 2H), 7.52–7.50 (m, =CH, 1H), 7.43–7.38 (m, =CH, silicon tips (NSC14/AIBS, MikroMasch). 10 μL of the sample solution 2H), 7.43–7.38 (m, =CH, 2H), 2.58 (s, CH3, 6H), 2.42–2.32 (m, CH2, (250 μM) was diluted 10-fold and placed on freshly cleaved mica. After 13 4H), 1.40 (s, CH3, 6H), 1.11–1.04 (m, CH3, 6H); C NMR (CDCl3, 100

Fig. 2. The examination of optoelectronic properties of the bodipy dyes containing modified groups to compare the photophysical properties.

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+ MHz, ppm) 153.7, 141.4, 140.1, 139.9, 138.4, 134.7, 132.8, 130.8, 1070, 968; ESI-MS m/z 331.1252 (M + H) (Figure S7). The spectro 128.9, 128.7, 127.8, 127.5, 127.0, 17.26, 17.06, 14.60, 14.56, 12.50, scopic results were compatible with those reported in the literature [18]. 11.82; IR (cm 1): 2961, 2926, 1535, 1473, 1312, 1179, 1056, 967; ESI- The BODIPY dye B4. The compound B4 was purified by column + MS m/z 457.4091 (M + H) (Figure S6). chromatography using hexane/ethyl acetate (7:3, v/v) as an eluent to 1 The BODIPY dye B3. The compound B3 was purified by column give a 17% yield. H NMR (CDCl3, 400 MHz, ppm) 7.49 (d, J = 3.9 Hz, chromatography using hexane/ethyl acetate (7:3, v/v) as an eluent to =CH, 1H), 7.13–7.11 (m, =CH, 1H), 6.97 (d, J = 3.9 Hz, =CH, 1H), 2.53 1 give a 15% yield. H NMR (CDCl3, 400 MHz, ppm) 7.50 (d, J = 3.9 Hz, (s, CH3, 6H), 2.41–2.28 (m, CH2, 4H), 1.48 (s, CH3, 6H), 1.08–1.00 (m, 13 =CH, 1H), 7.14–7.11 (m, =CH, 1H), 6.99 (d, J = 3.9 Hz, =CH, 1H), 6.00 CH3, 6H); C NMR (CDCl3, 100 MHz, ppm) 154.3, 138.7, 136.1, 135.58, 13 (m, =CH, 2H), 2.55 (s, CH3, 6H), 1.58 (s, CH3, 6H); C NMR (CDCl3, 133.1, 131.7, 127.8, 127.5, 127.2, 17.25, 17.06, 14.58, 14.56, 12.53, 100 MHz, ppm) 156.0, 143.5, 134.6, 134.0, 132.4, 127.8, 127.6, 127.4, 10.83; IR (cm 1): 2964, 2870, 1533, 1472, 1313, 1264, 1180, 1048, + 121.5, 14.63, 13.54: IR (cm 1): 2950, 1541, 1503, 1304, 1243, 1189, 972; ESI-MS m/z 387.1701 (M + H) (Figure S8).

Scheme 1. Synthesis and physical properties of the BODIPY dyes B1, B2, B3 and B4.

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2.3. The procedure of the photodiodes ratio of pure silicon atom. A ready-made pSi crystal with [100] a thickness of 400 μm and resistance of 1–10 Ω-cm was used to fabricate The crystal structure of pSi is obtained by adding atoms with three the devices. Firstly, in order to remove the impurities on the semi valence electrons such as aluminum, boron and gallium, to a certain conductor surfaces, the wafer was cleaned according to the RCA1

Fig. 3. The 2D and 3D AFM image of bodipy dyes B1 (a), B2 (b), B3 (c) and B4 (d).

4 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805 cleaning process. Then, the wafer was dried with nitrogen gas (N2). Al (99.999%) metal was evaporated using a thermal evaporation system at a pressure of about 10 6 Torr for ohmic contact. The wafer was annealed ◦ in 500 C under dry nitrogen gas flow for 3 min. Next, the eight sub strates were cut into small pieces, with two substrates each prepared for B1, B2, B3 and B4 under the same conditions. The organic layers, namely B1, B2, B3 and B4, were coated on the pSi with a spin speed of 1000 rpm for 30 s according to the spin coating method. Finally, Al and Au (99.999%) were thermally evaporated onto the surface of the organic layer-semiconductor pieces with approximately 10 6 Torr pressure in order to make the rectifier metal contacts.

3. Result and discussion

The purpose of this study was to endow BODIPY dyes with new functionalities and broaden their application areas. For this purpose, firstly,target BODIPY dyes, namely B1, B2, B3 and B4, were synthesized. The first step involved the preparation of BODIPY dyes according to a reported procedure [18]. The pyrrole (2,4-dimethyl-1H-pyrrole or 3-ethyl-2,4-dimethyl-1H-pyrrole) was initially reacted with an aldehyde ′ ([1,1 -biphenyl]-4-carbaldehyde or thiophene-2-carbaldehyde) in the existence of TFA in dry dichloromethane, then by oxidation with DDQ. Next, the resulting 8-aryl substituted dipyrrin intermediates formed . without purification steps were treated with trimethylamine and BF3 TFA, respectively and, as a result, the BODIPY dyes (B1, B2, B3 and B4) were obtained. It was observed that by connecting additional groups from the 8- and 2-positions to the BODIPY dye skeleton, the quantum Fig. 4. Reverse and forward bias current-voltage (I–V) characteristics of B1Al efficiency increased and, in this case, the energy efficiency increased and B1Au photodiodes dark and under 100 mW/cm2 light intensity. especially in the studies conducted on gold. On the other hand, it was observed that the energy efficiency of the BODIPY dyes obtained by derivatizing the meso position with arene or heteroarene derivatives was different. This was evident from both the naked eye color change and the spectroscopic studies (Scheme 1). The surface morphology of the organic layers, namely B1, B2, B3 and B4, were investigated with AFM. The two dimensional and three- dimensional AFM images are presented vertically and horizontally in Fig. 3. Moreover, the AFM surface images of the organic layer covering the p-type Si that were scanned at 20 × 20 μm2 are shown in this figure. It was observed that the surface homogeneities of BODIPY dyes B1, B2 and B4 were more suitable than BODIPY dye B3 for device fabrication. Due its organic material structure the surface morphology of BODIPY dye B3 was not homogeneous (Fig. 3(c)). The organic layer surfaces of BODIPY dyes B1, B2 and B4 had good homogeneity, were of uniform distribution and consisted of spherical micro crystals on the surface. However, the organic layer surface of BODIPY dye B3 was found to have irregular microstructures. It was clear that the best surface coating belonged to BODIPY dye B2. The surface roughness of BODIPY dyes B1, B2 and B4 were found to be 25 nm, 6.6 nm, 155 nm, 11 nm, respectively. Additionally, the thickness of the layers was measured with an ellips ometer and determined as 123 nm for B1, 96 nm for B2, 158 nm for B3 and 109 nm for B4. The I–V characteristics of the Au and Al/B1/pSi, Au and Al/B2/pSi, Au and Al/B3/pSi, Au and Al/B4/pSi devices in the dark and in 100 mW/cm2 light are shown in the graphs presented in Fig. 4, Fig. 5, Fig. 6 and Fig. 7, respectively. As can be seen from the figures, the rectifier ratio of the BODIPY dyes B1 and B2 devices were better than those of the Fig. 5. Reverse and forward bias current-voltage (I–V) characteristics of Al/B2/ BODIPY dyes B3 and B4 devices for the Au and Al metal rectifier pSi and Au/B2/pSi photodiodes dark and under 100 mW/cm2 light intensity. contacts. The TE model is used in the linear region of I–V curves to examine charge, V is the voltage, n is the ideality factor and I0 is the saturation electrical parameters at low voltages [20]. According to this model, the current and is expressed as: current is as follows; ( ) [ ( ) ] * 2 qΦb (qV) I0 = AA T exp 1 (2) I = I exp 1 kT 0 nkT (1) where A is the active diode field,A * is the effective Richardson constant 2 2 where k is the Boltzmann constant, T is the temperature, q is the electric (32 A/cm K for pSi) and Φb is the barrier height (BH) [21]. The n value

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( ) AA*T2 qΦb = kT ln (4) I0

Fig. 4 shows the I–V graphs of the Al/B1/pSi and Au/B1/pSi devices in the dark and under 100 mW/cm2 light at 300 K. Table 1 shows the values of n and Φb. According to Fig. 4 and Table 1, the Al/B1/pSi device showed better diode characteristics than the Au/B1/pSi device. Al metal contacts are more easily oxidized compared to gold ones, thus, their MS structure provides more ideal behavior. In both devices the value of n increased under light and the devices behaved like photodiodes. Electron-hole pairs are formed when illuminated with 100 mW/cm2 light and increase the current in the negative voltage region [22]. In addition, n values greater than 1 can be attributed to reasons such as the series resistance (Rs) effect, non-homogeneous BH, non-homogeneous distribution of the charge carriers in the interface and interface situa tions [23,24]. The reverse and forward bias of the I–V plots of the Al/B2/pSi and Au/B2/pSi devices in darkness and under 100 mW/cm2 light at 300 K are shown in Fig. 5. Table 1 shows the results obtained from the I–V characteristics. The results were consistent with the TE and the devices demonstrated rectifierproperties. The I–V characteristics of the devices were different in darkness and under 100 mW/cm2 light. This can be explained by the photoelectric effect in the depletion region [25]. The n and Φb values of the Al/B2/pSi and Au/B2/pSi devices are given in Table 1 and are showed photodiode behavior. The increase of the n value Fig. 6. Reverse and forward bias current-voltage (I–V) characteristics of Al/B3/ under light can be explained by the transition of the electrons in the pSi and Au/B3/pSi photodiodes dark and under 100 mW/cm2 light intensity. valence band excited by light to the conductivity band. It can also be attributed to the current increase in reverse bias, interface states and the interface layer between MS [26]. Fig. 6 shows the measured I–V properties of the Al/B3/pSi and Au/ B3/pSi devices in the dark and under light. It was observed that both devices reacted to light when they are illuminated. They demonstrated photodiode behavior, which can be explained by the formation of electron hole pairs at the interface with the effect of light, and the in crease in carrier concentration [27]. The use of organic interface ma terials between metal and semiconductors in photovoltaic applications is effective in the interface state and allows the absorption of photons [28]. Fig. 7 shows the I–V characteristics of the Al/B4/pSi and Au/B4/pSi devices in the dark and under 100 mW/cm2 light. As can be seen from the figure, both devices showed rectifying behavior in the dark. In addition, the Al/B4/pSi device demonstrated photodiode behavior under lighting, whereas the Au/B4/pSi device did not demonstrate

Table 1 Ideality factor and barrier height values of Al/B1/pSi, Au/B1/pSi, Al/B2/pSi, Au/B2/pSi, Al/B3/pSi, Au/B3/pSi, Al/B4/pSi and Au/B4/pSi photodiodes calculated using TE method from I–V characteristics in the dark and under 100 mW/cm2 illumination.

I–V

n Φb (eV)

Al/B1/pSi Dark 1.83 0.70 Al/B1/pSi 100 mW/cm2 2.44 0.68 Au/B1/pSi Dark 1.39 0.60 2 Fig. 7. Reverse and forward bias current-voltage (I–V) characteristics of Al/B4/ Au/B1/pSi 100 mW/cm 1.68 0.65 Al/B2/pSi Dark 1.33 0.70 pSi and Au/B4/pSi photodiodes dark and under 100 mW/cm2 light intensity. Al/B2/pSi 100 mW/cm2 2.43 0.79 Au/B2/pSi Dark 1.25 0.64 determined from the linear region in the low voltage region of the I–V Au/B2/pSi 100 mW/cm2 1.71 0.74 graph was calculated using Equation (1) given below: Al/B3/pSi Dark 2.31 0.91 Al/B3/pSi 100 mW/cm2 2.85 0.83 q dV Au/B3/pSi Dark 1.36 0.68 n = (3) 2 kT d(ln I) Au/B3/pSi 100 mW/cm 2.66 0.65 Al/B4/pSi Dark 2.46 0.88 2 The BH was calculated using Equation (2) given below: Al/B4/pSi 100 mW/cm 2.92 0.77 Au/B4/pSi Dark 1.28 0.71 Au/B4/pSi 100 mW/cm2 1.68 0.69

6 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805 photovoltaic behavior. The photo-current in the reverse bias region of frequencies ranging from 100 to 4000 kHz affected the C–V character the Al and Au contacts was increased by lighting. In the forward bias istics. In the measurements made between -2 V and +2 V, capacitance region of the I–V characteristics, photocurrent with photo-illumination decreased as frequency increased. The capacitance calculations are occurred only in the Al/B4/pSi device. Electron-hole pairs formed in carried out according to the equation given below: the Al contact structure. Under the influence of the electric field, elec 2(V + V) 2 = 0 trons move towards the organic interface, thus, the holes produced drift C 2 (5) εsε0eA Na towards the semiconductor along the barrier at the interface [29]. Therefore, it was determined that the Au/B4/pSi device was not suitable where V is the reverse bias voltage, V0 is the zero bias intersection, A is for photovoltaic applications. the effective diode field, Na is the acceptor concentration and q is the In this study, current density-voltage (J-V) curves were drawn to electrical charge. The diffusion potential (Vd) is calculated as follows determine photovoltaic properties. As can be seen in Fig. 8, the J-V 2 using the gradients of the V0 and C -V plots; characteristics of the Al/B1/pSi, Al/B2/pSi, Al/B3/pSi and Al/B4/pSi devices demonstrated photodiode properties under 100 mW/cm2 light. kT Vd = V0 + (6) Additionally, as can be observed in Fig. 9 the J-V characteristics of the q Au/B1/pSi, Au/B2/pSi and Au/B3/pSi devices demonstrated photo diode properties under 100 mW/cm2 light. The Au/B4/pSi device was additionally, the acceptor concentration (Na) and Fermi energy level (Ef) not included in the figureas it did not demonstrate photodiode behavior values were calculated from the equations given below: ( ) [5]. BODIPY dyes are paints that absorb rays in the visible area and are E E N = N exp f v resistant to light and chemicals [30]. In addition, BODIPY compounds a v kT (7) have nonlinear optical absorption measurements and can demonstrate ( ) different photon absorption properties [31]. The photovoltaic behavior Na Ef = kT ln (8) here can be explained by the effect of the organic layer between the MS Nv structure on the electrical and optical properties [32,33]. BODIPY dyes Φ have a wide absorption coefficient, good solubility and high quantum Barrier height ( b) was calculated as follows: efficiency. Furthermore, as they are fluorescent molecules with high Vd Φb = + Ef (9) photo-stability, they can be used efficiently in photodiode applications n [34]. Fig. 10(a) and (b) show the C–V plots of the Al/B1/pSi and Al/B2/pSi The capacitance-voltage (C–V), conductance-voltage (G-V) and C 2- structures. The capacitance values displaying different curves and peaks V graphs of the Al/B1/pSi, Au/B1/pSi, Al/B2/pSi, Au/B2/pSi, Al/B3/ may be the result of R and interface states [27,35]. Interface states can pSi, Au/B3/pSi, Al/B4/pSi and Au/B4/pSi structures are shown in s follow AC signals at low frequencies. As the frequency increases an Figs. 10–13. It was observed that the measurements carried out in the

Fig. 8. The current density-voltage (J–V) characteristic under 100 mW/cm2 light intensity for (a) Al/B1/pSi, (b) Al/B2/pSi, (c) Al/B3/pSi and (d) Al/B4/pSi photodiodes.

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Fig. 9. The current density-voltage (J–V) characteristic under 100 mW/cm2light intensity for (a) Au/B1/pSi, (b) Au/B2/pSi and (c) Au/B3/pSi photodiodes. excessive capacitance occurs, which is expressed only as a layer of space interface layer [41]. charge. Fig. 10(c) and (d) show the G-V graphs of the Al/B1/pSi and The C–V, G-V, C 2-V graphs used for the electrical characterization of Al/B2/pSi structures at different frequencies (between 100 and 4000 the Au/B1/pSi and Au/B2/pSi photodiodes are shown in Fig. 12. The kHz) in the range of -2 V to +2 V. When the graphs are examined, it can peaks observed in the forward bias region of the C–V graphs in both be seen that the conductivity values increased as the frequency devices became more pronounced as the frequency increased. This was increased. Medium and low frequencies are used to determine interfacial attributed to the localized interface state density and Rs between the state densities using conductance curves, while high frequencies are interface layer and pSi. The electrical parameters obtained from the C 2- 2 used to detect Rs [26]. Fig. 10(e) and (f) show the C -V graphs from the V graphs of the Au/B1/pSi and Au/B2/pSi photodiodes are shown in frequency-dependent C–V graphs of the Al/B1/pSi and Al/B2/pSi Table 3. The decrease in the Na values and the increase in BH values that structures. The electrical parameters such as diffusion potential (Vd), occcurred as the frequency increased, were attributed to the density acceptor concentration (Na), Fermi energy level (Ef) and barrier height distribution of the interface states and the interface states [42]. (Φb) calculated using these graphs are given in Table 2. As can be seen in Various electrical parameters of the Au/B3/pSi and Au/B4/pSi 2 the tables, the Vd and Φb values increased with the increase in frequency. photodiodes were obtained with the C–V, G-V and C -V graphs given in The Ef values differed slightly as the frequency changed, and were Fig. 13. The electrical parameters including Vd, Na, Ef, Φb, which were calculated to be close to each other. The differences in the calculations calculated as a function of the C 2-V graph, are given in Table 3. The can be attributed to reasons such as different interface thickness, load changes in the calculated values depending on the frequency may be due distribution in the interface and interface layer [36]. The experimental to trap centers. Deep traps and contact capacitance in the depletion zone values obtained showed that frequency and applied bias voltage were of photodiodes create a complex function for frequency [43]. The low effective, particularly in the reversal and accumulation zones [37]. BH values of the Au/B3/pSi device in Table 3 can be explained by the Fig. 11 shows the C–V, G-V and C 2-V graphics of the Al/B3/pSi and presence of the acceptor level and the excess capacitance between MS Al/B4/pSi devices between 100 and 4000 kHz. As the interface states at [39]. low frequencies can follow AC signals, an excessive capacitance occurs. When the frequency increases, the capacitance is expressed only as a 4. Conclusion space charge layer [38,39]. Fluctuations in the G-V graph can be attributed to the intensity of the interfacial state, Rs, thickness of the In this study, the performance of BODIPY dye-based organic layers interlayer as well as the inhomogeneity of the MS interface, BH and were investigated with diode applications. In addition to the differences carrier concentration [40]. The Vd, Na, Ef and Φb values obtained from in the BODIPY-dyes structures, the effects of Au and Al metal contacts the C 2-V graphs of the Al/B3/pSi and Al/B4/pSi photodiodes are given were examined under different lighting and frequency conditions. For in Table 2. As can be seen from the results, these parameters were a this purpose, initially, the surface morphologies of the organic layers, function of frequency. The electrical parameters of the Al/B4/pSi namely B1, B2, B3, and B4, were investigated with AFM, and the surface photodiode were higher than those of Al/B3/pSi photodiode. This can be roughness of the layers was found to be 29 nm, 6.6 nm, 155 nm, 11 nm, explained to the presence of surface charges in the traps and the respectively. In light of these values, it can be said that the best surface

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Fig. 10. (a–b) Capacitance-voltage (C–V), (c–d) conductance-voltage (G-V)and (e–f) 1/C2 –V graphics of Al/B1/pSi and Al/B2/pSi photodiodes at different frequencies.

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Fig. 11. (a–b) Capacitance-voltage (C–V), (c–d) conductance-voltage (G-V)and (e–f) 1/C2 –V graphics of Al/B3/pSi and Al/B4/pSi photodiodes at different frequencies.

10 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805

Fig. 12. (a–b) Capacitance-voltage (C–V), (c–d) conductance-voltage (G-V)and (e–f) 1/C2 –V graphics of Au/B1/pSi and Au/B2/pSi photodiodes at different frequencies.

11 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805

Fig. 13. (a–b) Capacitance-voltage (C–V), (c–d) conductance-voltage (G–V) and (e–f) 1/C2 –V graphics of Au/B3/pSi and Au/B4/pSi photodiodes at different frequencies.

12 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805 2.10 2.02 2.00 2.00 1.98 1.88 B4 N.D. 0.43 0.42 0.42 0.39 0.38 B4 N.D. 0.27 0.25 0.25 0.23 0.22 B3 0.81 0.67 0.64 0.63 0.60 0.59 B3 1.31 1.37 1.07 0.69 0.55 0.27 B2 1.85 1.49 1.38 1.31 1.22 1.07 B2 Ef Ef + + (eV) (eV) b b 2.36 1.22 0.79 0.74 0.65 0.56 B1 Φ Vd/n 5.15 1.93 1.26 1.19 0.93 0.79 B1 Φ Vd/n N.D. 0.50 0.48 0.48 0.45 0.44 B4 4.58 4.39 4.34 4.33 4.30 4.05 B4 N.D. 0.28 0.27 0.26 0.24 0.22 B3 1.15 0.92 0.87 0.85 0.80 0.79 B3 1.58 1.66 1.28 0.80 0.63 0.29 B2 2.37 1.90 1.74 1.66 1.54 1.34 B2 Ef Ef (eV) (eV) + b + b 3.18 1.62 1.01 0.95 0.81 0.69 B1 Vd ΔΦ 9.22 3.33 2.11 1.99 1.49 1.24 B1 Vd ΔΦ N.D. 0.19 0.19 0.18 0.18 0.17 B4 0.40 0.40 0.40 0.39 0.39 0.39 B4 N.D. 0.22 0.22 0.21 0.20 0.20 B3 0.29 0.30 0.30 0.30 0.30 0.30 B3 0.28 0.26 0.25 0.25 0.25 0.22 B2 0.25 0.25 0.25 0.25 0.25 0.25 B2 frequencies. frequencies. (eV) (eV) f f 0.26 0.22 0.23 0.23 0.23 0.22 B1 E 0.23 0.23 0.24 0.24 0.24 0.24 B1 E different different at at N.D. 1.52 1.97 2.40 2.76 3.51 B4 6.45 6.57 6.45 6.58 6.70 6.70 B4 photodiodes photodiodes Si 3.34 3.04 2.91 2.81 2.82 3.12 B3 Si N.D. 4.98 6.61 8.75 10.20 13.40 B3 p p ) ) 15 15 1.89 1.89 1.89 1.90 1.94 1.94 B2 0.66 1.37 1.53 1.47 1.82 6.19 B2 10 10 Al/Bodipy/ Au/Bodipy/ 3 3 of of (cm (cm a a 4.33 3.38 2.73 2.88 2.35 2.38 B1 N 1.14 5.15 4.55 4.65 4.68 5.11 B1 N measurements measurements 4.19 4.00 3.95 3.94 3.90 3.66 B4 N.D. 0.31 0.29 0.30 0.28 0.27 B4 V) V) – – (C (C 0.85 0.62 0.57 0.55 0.50 0.49 B3 N.D. 0.06 0.05 0.05 0.04 0.03 B3 2.12 1.65 1.50 1.41 1.29 1.09 B2 1.30 1.40 1.02 0.54 0.38 0.07 B2 capacitance-voltage capacitance-voltage (V) (V) d d 9.00 3.10 1.87 1.75 1.25 1.00 B1 V 2.92 1.39 0.78 0.72 0.59 0.47 B1 V using using Determined. 2 3 Not (kHz) (kHz) 4000 1000 4000 700 1000 500 700 500 300 300 100 100 f f Table Calculations Table Calculations N.D.:

13 ¨ O. Sevgili et al. Materials Science in Semiconductor Processing 130 (2021) 105805 coating belonged to B2. Following the surface morphology studies, the [2] K. Ejderha, A. Zengin, I. Orak, B. Tasyurek, T. Kilinc, A. Turut, Dependence of main electronic and photovoltaic properties of the new Al/B1/pSi, Au/ characteristic diode parameters on sample temperature in Ni/epitaxy n-Si contacts, Mater. Sci. Semicond. Process. 14 (1) (2011) 5–12. B1/pSi, Al/B2/pSi, Au/B2/pSi, Al/B3/pSi, Au/B3/pSi, Al/B4/pSi and [3] M.S.P. Reddy, J.-H. Lee, J.-S. Jang, Frequency dependent series resistance and Au/B4/pSi heterostructures were compared with the I–V and C–V interface states in Au/bio-organic/n-GaN Schottky structures based on DNA biopolymer, Synth. Met. 185 (2013) 167–171. characteristics. Basic electronic parameters, such as n and Φb, were ¨ [4] H.E. Lapa, A. Kokce,¨ M. Al-Dharob, I.˙ Orak, A.F. Ozdemir, S. Altındal, Interfacial calculated and compared with each other. The n values of the devices layer thickness dependent electrical characteristics of Au/(Zn-doped PVA)/n-4H- coated with Al varied between 1.33 and 2.46 in the dark and between SiC (MPS) structures at room temperature, Eur. Phys. J. Appl. Phys. 80 (1) (2017) 2.31 and 2.92 under light. On the other hand, the n values of the Au 10101. [5] O. Sevgili, S. Canlı, F. Akman, I. Orak, A. Karabulut, N. Yıldırım, Characterization coated ones varied between 1.28 and 1.39 in the dark and between 1.68 of aluminum 8-hydroxyquinoline microbelts and microdots, and photodiode and 2.66 under light. According to these results, while the Al/B1/pSi applications, J. Phys. Chem. Solid. 136 (2020) 109128. device showed better diode characteristics than the Au/B1/pSi device [6] R.T. Tung, Electron transport at metal-semiconductor interfaces: general theory, Phys. Rev. B 45 (23) (1992) 13509. under light, the Al/B2/pSi and Au/B2/pSi devices showed photodiode [7] K.L. Narayanan, M. Yamaguchi, “Photovoltaic effects of a: C/C60/Si (p–i–n) solar behavior in dark conditions. Finally, the effects of the metals used on cell structures, Sol. Energy Mater. Sol. Cells 75 (3–4) (2003) 345–350. device performance were compared using electrical measurements. The [8] H. Kacus, M. Yilmaz, A. Kocyigit, U. Incekara, S. Aydogan, Optoelectronic n values of Al/B2/pSi were calculated as 1.33 and 2.43 in lighting and properties of Co/pentacene/Si MIS heterojunction photodiode, Phys. B Condens. Matter (2020) 412408. dark, respectively, and Φb values were calculated as 0.70 eV and 0.79 eV [9] A. Tataroglu,˘ et al., Single crystal ruthenium (II) complex dye based photodiode, for lighting and darkness, respectively. The n values of Au/B2/pSi were Dyes Pigments 132 (2016) 64–71. calculated as 1.25 and 1.73 in lighting and dark, respectively, and Φ [10] F. de Moliner, N. Kielland, R. Lavilla, M. Vendrell, Modern synthetic avenues for b the preparation of functional fluorophores, Angew. Chem. Int. Ed. 56 (14) (2017) values were calculated as 0.64 eV and 0.74 eV for illumination and 3758–3769. darkness, respectively. As a result, it was determined that Al/B2/pSi and [11] A.C. Benniston, G. Copley, Lighting the way ahead with boron dipyrromethene – Au/B2/pSi had the best I–V characteristics among all eight devices. To (Bodipy) dyes, Phys. Chem. Chem. Phys. 11 (21) (2009) 4124 4131. [12] J. Banuelos, BODIPY dye, the most versatile fluorophore ever? Chem. Rec. 16 (1) summarize, the eight devices obtained were made with four different (2016) 335–348. organic materials using Au and Al metal contacts. It was concluded that [13] G. Ulrich, R. Ziessel, A. Harriman, The chemistry of fluorescent bodipy dyes: the performance of the devices made with gold was better, as there was versatility unsurpassed, Angew. Chem. Int. Ed. 47 (7) (2008) 1184–1201. [14] N. Boens, B. Verbelen, W. Dehaen, Postfunctionalization of the BODIPY core: oxidation in the devices made with aluminum. In addition to obtaining synthesis and spectroscopy, Eur. J. Org Chem. 30 (2015) 6577–6595, 2015. better results from the Au contact devices (Au/BODIPY/pSi), it was [15] V. Lakshmi, R. Sharma, M. Ravikanth, Functionalized boron-dipyrromethenes and ′ determined that the 8–1,1 -biphenyl-substituted BODIPY devices (Au/ their applications, Rep. Org. Chem. 6 (2016) 1–24. [16] N. Shivran, et al., Tuning of electron tunneling: a case study using BODIPY B1/pSi and Au/B2/pSi) gave better results compared to the 8-thiophene- molecular layers, Phys. Chem. Chem. Phys. 22 (4) (2020) 2098–2104. substituted BODIPY devices (Au/B3/pSi and Au/B4/pSi). Moreover, [17] B.T. Aksoy, et al., Solution-processable BODIPY decorated triazine photodiodes while the 2-ethyl-substituted BODIPY device Au/B2/pSi was determined and their comprehensive photophysical evaluation, New J. Chem. 44 (5) (2020) 2155–2165. to have a better effect than the 2-H-substituted BODIPY device Au/B1/ [18] L.C.D. de Rezende, M.M. Vaidergorn, J.C.B. Moraes, F. da Silva Emery, Synthesis, pSi, similarly the 2-ethyl-substituted BODIPY device Au/B4/pSi was photophysical properties and solvatochromism of meso-substituted tetramethyl determined to have a better effect than the 2-H-substituted BODIPY BODIPY dyes, J. Fluoresc. 24 (1) (2014) 257–266. device Au/B3/pSi. [19] N. Gupta, S.I. Reja, V. Bhalla, M. Gupta, G. Kaur, M. Kumar, A bodipy based fluorescent probe for evaluating and identifying cancer, normal and apoptotic C6 cells on the basis of changes in intracellular viscosity, J. Mater. Chem. B 4 (11) Author statement (2016) 1968–1977. [20] L.B. Tas¸yürek, S¸ . Aydogan,˘ M. Sevim, Z. Çaldıran, Temperature dependent ¨ electronic transport properties of heterojunctions formed between perovskite Omer Sevgili: Investigation, Lütfi Bilal Tasyürek: Investigation, SrTiO 3 nanocubes and silicon, J. Mater. Sci. Mater. Electron. (2020) 1–14. Writing – original draft, Writing – review & editing, Supervision, Sinan [21] R.K. Gupta, F. Yakuphanoglu, Analysis of device parameters of Al/In2O3/p-Si – – & Schottky diode, Microelectron. Eng. 105 (2013) 13–17. Bayindir: Investigation, Writing original draft, Writing review [22] A.S. Dahlan, et al., Photodiode and photocapacitor properties of Au/CdTe/p-Si/Al editing, Supervision, Ikram Orak: Investigation, Eray Caliskan: device, J. Alloys Compd. 646 (2015) 1151–1156. Investigation. [23] R.K. Gupta, R.A. Singh, Schottky diode based on composite organic semiconductors, Mater. Sci. Semicond. Process. 7 (1–2) (2004) 83–87. [24] A. Kocyigit, I. Orak, Z. Çaldıran, A. Turut, “Current–voltage characteristics of Au/ ZnO/n-Si device in a wide range temperature, J. Mater. Sci. Mater. Electron. 28 Declaration of competing interest (22) (2017) 17177–17184. [25] A. Karabulut, I.˙ Orak, A. 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