Comparative Electrical and Photoresponse Investigation of the Au/PVA/n-Si (MPS1) and Au/(CdTe:PVA)/n-Si (MPS2) Photodiodes, Both in the Dark and under Illumination

Abstract

In this current article, Au/n-Si (metal/semiconductor) MS devices with high-purity poly­(vinyl alcohol) (PVA) (99+% hydrolyzed) (MPS1) and Au/(CdTe:PVA)/n-Si (metal/polymer/semiconductor) (MPS2) are grown on the same n-type Si wafer by using the spin-coating technique and determine their effects on light sensitivity and basic electronic parameters like saturation current (I ₀), which is derived from the straight-line intercept of ln­(I) at V = 0, zero-bias barrier height (ϕ _{b 0}) at V = 0, ideality/quality factor (n), and both the shunt/series resistances (R _s , R _sh). For this purpose, the current–voltage (I–V) measurement is carried out over a wide voltage range (±4.5 V), both in the dark and under 100 mW/cm² conditions. Experimental results showed that the calculated electrical parameters were highly dependent on light, the organic interlayere, and voltage. The profile of interface states (N _ss ) dependent on energy and voltage-dependent resistance (R _i ), which significantly limit the performance of the photodiode, wwas obtained from the Card-Rhoderick model and Ohm’s law for both before and after illumination, respectively. In addition, voltage-dependent curves of the photosensitivity, photoresponsivity (R), and specific detectivity (D*) were obtained for the 100 mW·cm^–2 intensity. When the results obtained for MPS1 and MPS2 photodiodes were compared with each other and with the existing literature, it was observed that the MPS2 exhibited significantly better performance. Therefore, it was shown that MPS2 with the (CdTe:PVA) interlayer could be a good candidate for electrical, optical, and energy conversion applications.

1. Introduction

The metal and metal/oxide-doped polymer interlayered metal/interlayer/semiconductor (MIS)- type structures, like Schottky/photodiodes (SDs/PDs), solar cells (SCs), and capacitors, have usually formed the basis of electronic and optoelectronic technologies over the last two decades. − However, since there are many factors that determine the performance of these structures, either positively or negatively, there is no consensus on the current conduction/transport mechanisms (CCMs/CTMs), the nature of the barrier height (BH) formed at the metal–semiconductor interface, and the changes in their electrical and optical properties depending on light, temperature, and voltage. Since metal and semiconductors are contacted, the BH that forms at the M/S interface generally arises from the separation of charges at the junction. While electrons that are predominantly found on the front surface of an n-type semiconductor can diffuse to the front surface of the metal, leaving behind positively charged holes. When the system reaches thermal equilibrium, an internal electric field is generated from the semiconductor to the metal. The formation and nature of the BH at M/S interface are determined by the work functions of the chosen metal (ϕ _m ) and semiconductor (ϕ _s ), as well as the surface homogeneity of the semiconductor. − To obtain rectifier behavior in an n-Si-based SD, the value of ϕ _m must be higher than ϕ _s and the transport of electrons from the n-Si to the metal leads to band bending in the conduction (E _c ) and valence band (E _v ) until their Fermi energy (E _F ) are aligned. Then, in thermal equilibrium, a BH forms at the M/S interface, which limits the movement of electrons/holes from the metal to the semiconductor or semiconductor to the metal.

In general, the use of a high-permittivity (high-ε′) interlayer between M/S interfaces can increase the quality of these devices with respect to a decrease of leakage/reverse current, N _ss , R _s , an increase in BH, R _sh, and the rectification ratio (RR = I _f /I _r ). Although traditional insulating interlayers, such as SiO₂ and SnO₂, are both stable and long lasting, they cannot reduce the performance-limiting factors, such as high leakage currents, R _s , and N _ss in these devices to the desired level. − Additionally, some organic interlayers usually have lower conductivity, lower dielectric properties, and reduced stability, but this problem can be overcome by using some doping metals (Zn, Ni, Mn, Co, Cu), graphene, and metal oxides (CuO, MnO, ZnO). − In applications, unless specially fabricated, MS-type SDs, PDs, and SCs with and without an organic or oxide interlayer, both their n and R_s values can deviate significantly from ideal conditions even at room temperature (n > 1, R _s > 0). This indicates that these devices show significant deviations from the standard or pure thermionic emission (TE) theory. Furthermore, very high-order energy levels (N _ss ) (>10¹¹–10¹⁴ eV^–1/cm²) can occur at the interlayer/semiconductor interface, with energies corresponding to the bandgap of the semiconductor. − Moreover, these traps significantly affect the conduction mechanisms of these devices by capturing or releasing very high amounts of electrons under some external factors such as light and temperature.

There are similar studies in the literature on the electrical and optoelectrical characteristics of MS-type SDs with/without an interlayer. − For instance, Ata et al. showed that introducing an MWCNT-doped PVA-B­(OH)₃ layer in Au/n-Si SDs results in illumination-dependent variations in BH, n, and photocurrent, confirming their effective photosensing capability. Likewise, Surya Reddy et al. found that Au/V₂O₅/InP MIS diodes display systematic changes in key parameters with light intensity, demonstrating the photoactive contribution of the V₂O₅ interlayer. On the theoretical side, Di Bartolomeo et al. proposed a Landauer-based description of Schottky transport that captures tunneling effects and aligns well with experimental observations. Moreover, Beneldjemoui et al. reported that adding an ultrathin δ-GaN layer improves interface passivation in Au/δ-GaN/n-GaAs diodes, enhancing BH and producing a pronounced UV-visible photoresponse. Together, these findings underscore the importance of interface modification and advanced transport models in optimizing Schottky photodetectors. Karataş et al. have also studied the electrical and optical properties of MS- and MIS-type SDs. − They found that these parameters and charge transport mechanisms are a strong function of the applied bias voltage, illumination, interfacial layer, and surface states or traps. To summarize, today, scientists, such as physicists, chemists, electronics engineers, and materials scientists have two main goals: one is to improve performance, and the other is to reduce production costs. This problem can be an obstacle to developing a new generation of interfacial layered materials that are both simple and fast to create and inexpensive. In other words, instead of traditional oxide layers, inexpensive, flexible, high-strength, and high-dielectric pure organic materialseither metal or metal oxide-dopedcan be used. −

Therefore, in this manuscript, polyvinyl alcohol was chosen and doped with CdTe due to its some important features like direct bandgap, high absorption coefficient, and the creation of a high-mobility lifetime for electronic charges (electrons and holes), such as CdTe additive to the polymer, placed at M–S interface, and also leads to an increase in the performance of the device by passivating many unwanted interface states on the semiconductor surface and reducing both series resistance and leakage current. − All findings indicate that the (CdTe:PVA) at the junction interface leads to a significant increase in both the illumination sensitivity and the performance of the MS-type SDs with respect to low R _s , N _ss , leakage current, and higher RR, R _sh, and BH values, respectively.

2. Experimental Details

In this work, the (Cd­(CH₃COO)₂) and (Na₂TeO₃) precursors with 99% purity were provided by Merck Co. to grow CdTe nanostructures. After that, they were cleaned/filtered and then heated at 40 °C for 45 h. Au/(n-Si) (MS) devices with (pure-PVA) and (CdTe:PVA) thin interlayers (MPS1, MPS2) were fabricated on the n-type Si with P-doped single crystal in the same conditions, which has a <100> float zone, at about 300 μm thickness and 5 Ωcm resistivity. First, the wafer was cleaned by using acetone, methanol, H₂O₂, NH₄OH, and HF solutions in an ultrasonic cleaner/bath to pick off the natural SiO₂ and dried with nitrogen gas. Second, high-purity (99.999%) Al was evaporated onto the rear side of the wafer at 10^–6 Torr and heated at 500 °C for 5 min to perform a low-resistivity contact. In this study, the PVA used, with M_w 146,000–186,000 and 99+% hydrolyzed, was purchased from Aldrich-Chemistry company. Third, the (CdTe-PVA) composite was formed on the front of the n-Si substrate by the spin-coated technique. This technique is widely used to grow a thin interfacial film at the M/S interface as an interfacial layer, and its ability to produce highly uniform, smooth, and thickness-controlled films. Moreover, it offers some advantages, including low processing cost, operational simplicity, and suitability for a broad range of material systems, without the need for vacuum-based environments. Finally, high-purity Au (99.999%) rectifier contacts were also evaporated onto the polymer interlayer. The I–V–P measurements were done by Fytronix Co at RT. The schematic diagram of the MPS PDs and the measurement I–V system are represented in Figure a, and their energy diagram is given in Figure b, respectively.

1.

(a) Schematic representation of the PDs and (b) their energy diagram.

3. Results and Discussion

The ln­(I) vs V curves of the MPS1/MPS2 Schottky-type PDs in the dark and under 100 mW·cm^–2 between ±4.5 V are represented in Figure a and b, respectively. The ln­(I) vs V plots have a rectified behavior. While the value of the current shows a good rectifier feature at intermediate voltages, it deviates from linearity at high bias voltages because of the R _s and interlayer effects. Because the applied voltage on the diode is shared between the depletion regime, R _s , and the interfacial layer. Contrary, the non- or unsaturated current in the reverse bias voltages can be attributed to R _sh, generation and recombination, and the image lowering of the BH at junction. −

2.

Semilogarithmic I–V curves: (a) MPS1 and (b) MPS2 PDs in the dark/under illumination.

The I and V relation at V > (3kT/q), and the basic electronic features of these devices are calculated from the linear parts of the ln(I) vs V plot in the positive regime by using the TE model. According to TE, the current value for MS- and MIS-type SDs or PDs is given as follows:

$$\mathrm{I}=(A{A}^{*}{T}^2)\exp \left(\frac{-q{\mathrm{\Phi }}_{b0}}{kT}\right)\exp \left(\frac{q(V-I{R}_s)}{nkT}\right)$$ 1

where k is the Boltzmann constant, T is the temperature in K, and the prefactor expression in front of the brackets is the saturation current (I ₀ or I _s) and can be obtained from the interception point of the linear part of the ln(I)–V plot at V = 0. − In addition, the quantities A and A* are the area of the Schottky contact (7.85 × 10^–3 cm²) and the effective Richardson’s constant (=112 A/(cm·K)² for n-Si), respectively. After obtaining the I ₀ value and by using A, the value of Φ _b0 can be extracted as given in the following relation. ,

$${\mathrm{\Phi }}_{b0}=\left(\frac{kT}{q}\right)·\ln \left(\frac{A{A}^{*}{T}^2}{I_0}\right)$$ 2

Schottky barrier diodes exhibit a parameter called the barrier height, which represents the energy difference between the metal’s Fermi level and the semiconductor’s conduction band edge at the interface. In essence, the barrier height determines the ease with which charge carriers can move across the metal–semiconductor junction, thereby governing the diode’s I–V characteristics, rectification behavior, and overall electronic performance. The ideality factor was also obtained from the linear regime of ln(I)–V curve as given by the following relations. −

$$n(V)=\frac{q{V}_i}{kT(\tan \theta )}=\frac{q{V}_i}{kT(\mathrm{d}\ln I_i/\mathrm{d}{V}_i)}=1+\frac{d_i}{{\epsilon }_i}[\frac{{\epsilon }_s}{W_D}+q{N}_{ss}(V_i)]$$ 3

In eq , the quantities d _i , ε _i , ε _s , ε₀, and W _D are the interfacial layer thickness, permittivity of the semiconductor, permittivity of vacuum, and depletion layer width, respectively. The voltage dependence of resistance (ln­(R _i ) vs V) curve of this photodiode was also obtained from Ohm’s Law and is represented in Figure a,b. As seen in Figure , the R _i value becomes nearly constant at higher positive voltages and sufficiently lower negative voltages which correspond to the real R _s and R _sh values, respectively. The I ₀, Φ _b0, n, RR, R _s , and R _sh are tabulated in Table for MPS1- and MPS2-type photodiodes.

3.

R_s–V plot: (a) MPS1- and (b) MPS2-type PDs in the dark/illuminations.

1. Fundamental Electronic Parameters of Fabricated MPS1, MPS2 SDs and Literature-Obtained Values from Various Calculation Methods: A) in the Dark and B) under 100 mW/cm² .

	(A) In Dark
	Samples	TE			Ohm’s Law			Cheung’s				Norde
This work		I 0 (A)	n	Φ b0 (eV)	R s (Ω) (4.5 V)	R sh (Ω) (−4.5 V)	RR (±4.5 V)	n (d V /dln I )	R s (Ω) (d V /d ln I )	Φ b0  (eV)  H (I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/PVA/n-Si	5.33 × 10–7	5.27	0.52	4.11 × 103	2.57 × 105	61.88	8.53	7.13 × 103	0.72	5.26 × 103	0.65	2.47 × 104
	Au/CdTe:PVA/n-Si	2.00 × 10–9	4.59	0.66	1.89 × 104	6.79 × 105	91.77	7.36	9.05 × 103	0.70	1.05 × 104	0.78	8.39 × 103
Ref		I 0 (A)	n	Φ b0 (eV)	R s (kΩ) (3 V)	R sh (MΩ) (−3 V)	RR (±4.5 V)	n (d V /d ln I )	R s (kΩ) (d V /d ln I )	Φ b0  (eV)  H (I)	R s  (kΩ)  H (I)	Φ b0 (eV)	R s (kΩ)
	Au/n-Si	7.89 × 10–6	7.65	0.596	1.60	0.209	13.1	8.61	1.35	0.46	1.31	0.60	1.07
	Au/(Brushite + Monetite: PVC)/n-Si	1.28 × 10–9	2.46	0.822	2.05	308.00	1.5 × 105	6.55	1.58	0.70	1.17	0.862	46.62
Ref		I 0 (A)	n	Φ b0 (eV)	R s (Ω) (5 V)	R sh (MΩ) (−5 V)	RR (±3 V)	n (d V /d ln I )	R s (kΩ) (d V /d ln I )	Φ b0  (eV)  H(I)	R s  (kΩ)  H (I)	Φ b0 (eV)	R s (kΩ)
	Au/(Er 2 O 3 :PVC)/n-Si	6.93 × 10–10	1.93	0.838	858.39	78.183		7.25	0.90	0.664	0.72	0.959	17.73
Ref		I 0 (pA)	n	Φ b0 (eV)	R s (Ω) (5 V)	R sh (Ω) (−5 V)	RR (±4 V)	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0  (eV)  H (I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/P(EHA)/n-Si	8.3	4.15	0.92	1.84 × 105	4.89 × 107	2.70 × 102	4.41	2.08 × 105	0.97	1.63 × 105	-	-
	Au/P(EHA- co -AA)/n-Si	97.10	7.09	0.88	1.37 × 104	3.66 × 107	1.83 × 103	4.59	2.44 × 105	0.92	1.37 × 105	-	-
Ref		I 0 (A)	n		R s (Ω) (2 V)	R sh (MΩ) (−2 V)	RR (±2 V)	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0  (eV)  H (I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/(MWCNT:PVA-B(OH) 3 )/n-Si	1.37 × 10–9	2.36	0.819	429.89	36	15.5	3.67	337.82	0.62	266.56	-	-
Ref		I 0 (μA)	n		R s (Ω) (3 V)	R sh (Ω) (−3 V)	RR	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0 (eV) H(I)	R s (Ω) H(I)	Φ b0 (eV)	R s (kΩ)
	Al/p-Si	74.26	2.55	0.591	76.05	75.87	-	-	-	-	-	0.581	0.193
	Al/(PVP:ZnTiO 3 )/p-Si	0.08	2.75	0.768	1040	9921	-	-	-	-	-	0.725	411.70
Ref		I 0 (A)	n	Φ b0 (eV)	R s (kΩ) (3.5 V)	R sh (MΩ) (−3.5 V)	RR (±3.5 V)	n (d V /d ln I )	R s (kΩ) (d V /d ln I )	Φ b0  (eV)  H (I)	R s  (kΩ)  H (I)	Φ b0 (eV)	R s (kΩ)
	Au/n-Si	8.29 × 10–6	5.85	0.59	0.47	0.16	34	4.4	0.325	0.64	0.332	0.57	0.40
	Au/PVC/n-Si	1.12 × 10–7	3.98	0.7	1.02	0.46	455	5.77	0.41	0.75	0.475	0.72	1.68
	Au/(PVC:Sm 2 O 3 )/n-Si	7.58 × 10–10	2.27	0.84	0.35	1.39	3994	3.46	1.11	0.84	1.10	0.84	0.945
Ref		I 0 (A)	n	Φ b0 (eV)	R s (Ω)	R sh (Ω)	RR	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0  (eV)  H (I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/n-Ge	-	1.25	0.62	-	-	-	1.97	3027	0.61	2974	0.63	2458
	Au/MB/n-Ge	-	1.30	0.63	-	-	-	2.19	38	0.62	37	0.63	2666
Ref		I 0 (A)	n		R s (Ω)	R sh (Ω)	RR	n (d V /d ln I )	R s (MΩ) (d V /d ln I )	Φ b0  (eV)  H (I)	R s (MΩ) H(I)	Φ b0 (eV)	R s (MΩ)
	Au/n-InP	2.76 × 10–9	1.94	0.74	-	-	-	2.01	30.56	0.75	32.52	0.76	3.50
	Au/BST/n-InP	5.01 × 10–10	2.05	0.83	-	-	-	2.11	100.35	0.84	110.17	0.83	593.42
Ref		I 0 (A)	n		R s (Ω)	R sh (Ω)	RR	n (d V /d ln I )	R s (MΩ) (d V /d ln I )	Φ b0  (eV)  H (I)	Rs (MΩ) H(I)	Φ b0 (eV)	R s (Ω)
	Ni/PSR/n-Si	1.95 × 10–10	1.68	0.87	-	-	-	-	-	-	-	0.87	-
	Ni/PSR/p-Si	1.48 × 10–8	1.93	0.73	-	-	-	-	-	-	-	0.76	-

	(B) Under 100 mW/cm2
	Interlayer	TE			Ohm’s law			Cheung’s				Norde
This work		I 0 (A)	n	Φ b0 (eV)	R s (Ω) (4.5 V)	R sh (Ω) (−4.5 V)	RR (±4.5 V)	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0 (eV) H(I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/PVA/n-Si	6.87 × 10–7	8.3	0.51	1.11 × 103	6.82 × 103	5.96	9.33	3.61 × 102	0.60	2.84 × 102	0.63	4.85 × 102
	Au/CdTe:PVA/n-Si	4.59 × 10–6	9.14	0.46	1.02 × 103	1.37 × 104	13.67	9.15	2.48 × 102	0.42	3.69 × 102	0.59	2.13 × 102
Ref		I 0 (A)	n	Φ b0 (eV)	R s (kΩ) (3 V)	R sh  (MΩ) (−3 V)	RR (±3 V)	n (d V /d ln I )	R s (kΩ) (d V /dln I )	Φ b0 (eV) H(I)	R s  (kΩ)  H (I)	Φ b0 (eV)	R s (kΩ)
	Au/n-Si	1.83 × 10–5	8.77	0.57	0.63	0.122	19.5	9.74	0.34	0.42	0.37	0.59	0.30
	Au/(Brushite + Monetite:PVC)/n-Si	3.14 × 10–8	2.28	0.74	0.60	2.40	4 × 10–3	6.93	0.29	0.61	0.23	0.78	2.36
Ref		I 0 (A)	n	Φ b0 (eV)	R s (Ω) (5 V)	R sh  (MΩ) (−5 V)	RR	n (d V /d ln I )	R s (kΩ) (d V /d ln I )	Φ b0 (eV) H(I)	R s (kΩ) H(I)	Φ b0 (eV)	R s (kΩ)
	Au/(Er 2 O 3 :PVC)/n-Si	2.24 × 10–9	2.21	0.81	553.32	0.86	-	8.76	0.23	0.628	0.172	0.856	10.35
Ref		I 0 (nA)	nn	Φ b0 (eV)	R s (Ω) (5 V)	R sh (Ω) (−5 V)	RR	n (d V /d ln I )	R s (kΩ) (d V /d ln I )	Φ b0 (eV) H(I)	R s  (kΩ)  H (I)	Φ b0 (eV)	R s (Ω)
	Au/P(EHA)/n-Si	0.42	4.6	0.81	8.22 × 103	9.64 × 104	-	5.58	7.21	0.77	6.16	-	-
	Au/P(EHA- co -AA)/n-Si	268	6.99	0.68	4.47 × 103	5.34 × 104	-	5.08	5.41	0.68	4.61	-	-
Ref		I 0 (A)	n	Φ b0 (eV)	R s (Ω) (2 V)	R sh  (Ω) (−2 V)	RR (±2 V)	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0 (eV) H(I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Au/(MWCNT:PVA-B(OH) 3 )/n-Si	1.64 × 10–9	2.34	0.82	409.45	21	17.54	2.99	332.02	0.76	234.96	-	-
Ref		I 0 (A)	n	Φ b0 (eV)	R s (kΩ) (3 V)	R sh  (kΩ) (−3 V)	RR	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0 (eV) H(I)	R s (Ω) H(I)	Φ b0 (eV)	R s (kΩ)
	Al/p-Si	120.38	2.59	0.58	0.048	0.054	-	-	-	-	-	0.571	0.048
	Al/PVP:ZnTiO 3 /p-Si	8.23	5.69	0.65	0.40	14.11	-	-	-	-	-	0.671	46.17
Ref		I 0 (A)	n	Φ b0 (eV)	R s (Ω)	R sh (Ω)	RR	n (d V /d ln I )	R s (Ω) (d V /d ln I )	Φ b0 (eV) H(I)	R s (Ω) H(I)	Φ b0 (eV)	R s (Ω)
	Ni/PSR/n-Si	-	1.98	0.84	-	-	-	-	-	-	-	0.84	-
	Ni/PSR/p-Si	-	-	-	-	-	-	-	-	-	-	-	-

The fundamental electronic parameters (I ₀, n, ϕ _b0, R _s , R _sh) of the performed MPS1- and MPS2-type SBDs in the dark/under 100 mW/cm² are given in Table A and B, respectively, and compared with similar structures which have been carried out by different researchers in recent years. − When Table is examined, the performance of the samples we prepared appears generally better than that of the others. This shows that the organic interfacial layer used (CdTe:PVA) considerably increases the quality of the Au/n-Si (MS) structure. To determine how these parameters change with the applied voltage as well as the illumination intensity, and how the calculation system used affects the results, these parameters were also calculated from the Cheung and Norde functions and are given in Table . As can be clearly seen in Table , there are some discrepancies between basic electrical parameters due to their V dependence and the nature of the calculation models, which correspond to different voltages. For instance, TE theory corresponds to the intermediate bias voltages, Cheung’s functions correspond to higher bias voltages, and the Norde function corresponds to lower bias voltages.

According to Cheung & Cheung, they observed a deviation from linear behavior at higher positive voltages in the ln(I)–V features, which can be explained by the presence of R _s and the interlayer. Because of this, the applied voltage onto SDs will be shared between them. For this region, the basic electrical parameters were also calculated by using the following relations, as given in Figures and .

$$\frac{\mathrm{d}V}{\mathrm{d}\ln I}=\left(\frac{nkT}{q}\right)+I{R}_S$$ 4

$$H=V-n·\left(\frac{kT}{q}\right)\ln \left(\frac{I}{A{A}^{*}{T}^2}\right)=n{\mathrm{\Phi }}_b+I{R}_s$$ 5

4.

dV/d ln­(I) vs I and H­(I) vs I: (a) in the dark and (b) under illumination for MPS1 SBD.

5.

dV/d ln­(I) vs I and H­(I) vs I: (a) in the dark and (b) under illumination for MPS2 SBD.

As shown in these figures, the n and R _s value can be extracted from the intercept and slope of the dV/d ln­(I) vs I curve by using eq , respectively. The BH and R _s values (as a second way) can also be calculated from the intercept and slope of the H–I curve using eq , respectively. As can be clearly seen in Figures and , both the dV/d ln­(I)–I and H–I plots show good linearity over a wide range of voltages and currents. If the semilogarithmic I–V curve does not have a good linear region, then both the accuracy/reliability of the calculated basic electrical parameters (n, ϕ _b , R _s ) from its slope and intercept voltage at zero bias are questionable. In this case, Norde developed an eq given below, based on TE theory when the value of n is considerably higher than unity.

$$F(V)=\left(\frac{V_i}{\gamma }\right)-\left(\frac{kT}{q}\right)\left[\ln \left(\frac{I(V_i)}{A{A}^{*}{T}^2}\right)\right]$$ 6

Here, γ is a constant and must be bigger than the n value to get a minimum point in the F(V)–V plot (Figure ). As seen in Figure , the F–V plot has a distinctive minimum point for two types of PDs, and the corresponding voltage and current are called V _min and I _min. After that, the values of Φ _b and R _s were obtained using the following relation and are tabulated in Table .

6.

F(V) vs V: (a) for the MPS1- and (b) for the MPS2-type PDs.

$${\mathrm{\Phi }}_b=F(V_{\min })+\frac{V_{\min }}{\gamma }-\left(\frac{kT}{q}\right)$$ 7

$$R_s=\frac{kT(\gamma -n)}{q{I}_{\min }}$$ 8

As can be seen from Figure , the F­(V)–V plot for these PDs has a clear minimum point at the moderate bias voltage in dark/illumination conditions. The magnitude and position of the minimum point of the F–V plot decrease under the illumination effec and shift toward higher positive voltages, respectively. Such features can be explained by the creation of electron and hole pairs, which produce an additional bias voltage. Electrons that gain enough energy under the illumination effect can excite from states to states or conduction bands, causing an increase in conductivity or a decrease in R _s .

Thus, the Φ _b and R _s values of these SBDs are calculated from eqs and and Figure a,b by using the minimum point (F(V _min)) and corresponding to the V _min and I _min points. The obtained values of Φ _b and R _s of these two diodes, as seen in Table , are compared with the TE model and Cheungs’ functions. As shown in Table , there are some differences between them due to the calculation models and voltage dependence. Because these calculation methods are valid in different applied voltage ranges. That is, the Norde function is usually valid at lower forward bias voltages, the TE theory is valid at moderate bias voltages or the linear part or lnI–V plot, and the Cheung functions are valid at enough higher voltages or when lnI–V plot deviates from the linear part.

To determine the light sensitivity of the prepared MPS1 and MPS2 Schottky-type photodiodes, the V-dependent profile of fundamental PD parameters, like photosensitivity (S), photoresponsivity (R), and photoelectricity (R*), are calculated from eq a–c for 100 mW/cm² and are given in Figure a–c and Figure a–c, respectively. The photosensitivity of a photodiode is known as the ratio of its measurement of current in the dark to the photocurrent in the negative bias region rather than the forward bias region. Because in the reverse-bias condition, both the interior and applied electric fields at the junction have the same direction, the electric field is very strong, but under forward bias conditions, they have different directions, leading to a low electric field. Therefore, the created electron–hole pairs under illumination effect will be forced to move in different directions, giving a clear photocurrent in circuit. −

7.

(a) S vs V, (b) R vs V, and (c) D* vs V plot of the MPS1-type SBD for various voltages.

8.

(a) S vs V, (b) R vs V, and (c) D* vs V plot of the MPS2-type SBD for various voltages.

$$S(V)=\frac{I_{\mathrm{p}\mathrm{h}}-I_d}{I_d}$$ 9

$$R(V)=\frac{I_{\mathrm{p}\mathrm{h}}-I_d}{P^{*}A}$$ 10

$$(D^{*})=\frac{R·\sqrt{A}}{{(2q{I}_d)}^{0.5}}$$ 11

As can be seen in both Figure a–c and Figure a–c, the S, R, and D* values are sensitive to light for the two types of PDs, and they increase with an increase in voltage in the negative direction for MPS1 due to the increasing electric field. On the other hand, for MPS2, the S–V plot has a distinctive peak at around −1.5 V. In addition, as can be seen in Figure a–c, the use of 0.05 CdTe-doped PVA leads to a considerable increase in the S, R, and D* values. For example, the S value of CdTe-doped PVA increased by approximately 2 times when compared with pure PVA.

To determine the current transport mechanisms, the ln­(I) vs ln­(V) plot for the MPS1- and MPS2-type PDs has been drawn and represented in Figure a,b, respectively. As shown in these figures, the ln­(I _F ) vs ln (V _F ) plot both in the dark and under illumination, has two linear parts with different slopes, which correspond to moderate and high voltage ranges. The slope of these plots for the first region was found to be 1.638 (in the dark) and 1.244 (under 100 mW/cm²) which correspond to space-charge-limited current (SCLC) and ohmic behavior, respectively. For the second region, the value of the slope was found to be 2.661 (in the dark) and 3.395 (under 100 mW/cm²) which correspond to trap-charge-limited current (TCLC). As shown in Figure b, the ln­(I _F) vs ln­(V _F ) plot for Au/CdTe:PVA/n-Si SBD also has two different linear parts, like Au/PVA/n-Si SBD, with slopes of 5.300 and 3.174, respectively. The CTM was governed by TCLC for the two linear parts. Similarly, these slopes under illumination conditions were found to be 2.263 and 3.255, respectively. Thus, CTM was also governed by TCLC under illumination conditions.

9.

ln­(I_F )–ln­(V_F ) curves: (a) for MPS1 and (b) for MPS2 PDs in the dark/illumination.

According to Card and Rhoderick, when the interlayer width is higher than ∼3 nm, N _ss reaches equilibrium with the semiconductor rather than forming a Schottky contact, and both the n and BH values become strong functions at positive bias voltages. Thus, the energy density distribution profile of (N _ss vs (E _c –E _ss )) in both dark and illumination conditions for these two SBDs can be extracted from the I _F and V _F data by considering V-dependent ideality factor and BH, as demonstrated following eqs ,, and . −

$$N_{ss}(V)=\frac{{\epsilon }_0}{q}[\frac{{\epsilon }_i}{d_i}(n(V_i)-1)-\frac{{\epsilon }_s}{W_D}]$$ 12

$${\mathrm{\Phi }}_e(V)={\mathrm{\Phi }}_{b0}+(1-\frac{1}{n(V)})V$$ 13

$$q(E_c-E_{ss})=({\mathrm{\Phi }}_e-V)$$ 14

Thus, the N _SS –(E _c –E _ss ) curves of the MPS1 and MPS2 PDs were obtained from the I _F –V _F both in the dark/under 100 mW/cm² and are given in Figure a,b, respectively. As shown in Figure a,b, the N _ss vs (E _c –E _ss ) curves for the two PDs have an almost similar U-shaped behavior due to the specific distribution of interface traps with energies in the bandgap of Si, which are reordered and restructured under the effects of electric-field illumination. −

10.

N_ss vs (E_c−E_ss ) curves obtained from the I_F–V_F data: (a) for MPS1 and (b) for MPS2 PDs, respectively.

4. Conclusion

In the present study, both the MPS1 and MPS2 PDs were performed on the n-Si wafer by utilizing the spin-coating method to determine the effects of pure/(CdTe:PVA) interfacial layers on the fundamental electrical parameters and conduction mechanisms both in dark and 100 mW/cm² conditions. For this aim, some electric parameters were calculated from the I–V measurements and compared with each other in dark and under 100 mW/cm² illumination at RT in the voltage range of ±4.5 V. To determine the light sensitivity of the prepared two different SBDs, the voltage-dependent profiles S, R, and R* were also found under 100 mW/cm² condition and the S value of Au/CdTe:PVA/n-Si SBD increased by approximately 2 times when compared to pure PVA. Both the energy-dependent profiles of N _ss and voltage-dependent R _i were obtained by using I–V characteristics data by using Card-Rhoderick and Ohm’s law methods. The energy-dependent profile of N _ss shows an almost U-shaped behavior due to a special distribution of N _ss at the interlayer/Si interface with energies in the bandgap of Si, and the reordering and restructuring of them under electric field and illumination effects. All experimental findings were found to be strong functions of illumination as well as voltage. However, the MPS2 photodiode shows good performance and photosensitivity compared to the MPS1 photodiode.

All data supporting the conclusions of this study are presented within the article. Supplementary data sets produced or analyzed during the research are available from the corresponding author upon reasonable request.

It has been confirmed that all researchers made the same contribution.

ÇiĞdem Şükriye Güçlü, Latif Barış Akman, ÇiĞdem Bilkan, and Şemsettin Altındal declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.

The authors declare no competing financial interest.

Published as part of ACS Omega special issue “Energy Storage across Scales”.