Effect of various PCBM doping on the interfacial layer of Al/PCBM:ZnO/p-Si photodiodes

The light can be detected by various devices such as photoconductors, photodetectors, phototransistors, solar cells, and photodiodes, and the light can be employed energy harvesting as well as sensing applications. Various materials can be utilized to raising of the efficiency of light-based devices. In this present study, metal–semiconductor (MS) devices with various [6,6]-Phenyl C61-butyric acid methyl ester (PCBM)-doped zinc oxide (ZnO) interfacial layer were fabricated for photodiode applications. Commercially purchased PCBM and ZnO were mixed in order to obtain undoped, 3%, 5%, and 10% PCBM-doped ZnO interfacial layer for Al/PCBM:ZnO/p-Si devices. The obtained solutions were layered on the p-type silicon (p-Si) substrates by spin-coating method. The thermal evaporation technique was utilized to deposit Al metal electrodes both back and front sides. The morphological properties of the fabricated devices were characterized by AFM. The morphological images of the AFM revealed that PCBM doping affected the surface morphology as well as surface roughness of the ZnO layers. I–V measurements were carried out in the dark and under varying illumination conditions for electrical properties. The devices exhibited good rectifying properties at around 103 rates, both dark and various illumination conditions, according to I–V graphs. The junction parameters of the produced devices were determined by using thermionic emission, Norde, and Cheung models from the I–V characteristics of the devices. The devices have high ideality factors, and these values generally fluctuate with varying PCBM-doping amounts. The current transient measurements show that PCBM doping provide increasing in the light response capacity. According to the results, the devices can be employed as photodiode and photodetector applications for the further works in the industry.


Introduction
Metal and semiconductor (MS) junctions are employed in the diode, transistor, and capacitors for many years because of their essential junction properties [1][2][3][4]. The MS junctions are crucial in optoelectronic devices particularly in photovoltaic cells, photodetectors, and photodiodes [5][6][7]. The photodiodes work at reverse biases and provide detection of the light in a good sensitivity and use in a smoke detector, compact disk players, and switching-like applications [8][9][10][11][12]. Many materials for photodiode applications of the MS devices were studied by the researchers [13][14][15]. In general, a metal oxide or polymer layers are inserted in the interface of the MS junction to control electrical properties and to improve the detection amount of the light because of fabricating more powerful photodiode devices [16][17][18][19][20][21][22][23]. Metal-oxide-semiconductors (MOSs) have been tremendously studied due to their good optical, chemical, and electrical properties. Zinc oxide (ZnO) is a kind of popular MOS and has found wide employments in optoelectronic devices because of its distinctive optical, gas sensing, structural, and electrical properties. ZnO has some main advantages such as wide and direct band gap (3.37 eV), high exciton binding energy (60 meV), low cost, transparent conductivity, and non-toxicity, and also exhibits dual semiconducting and piezoelectric properties [24][25][26][27]. With these excellent properties, ZnO has found wide application in various devices such as phototransistors [28], nanometer gas sensors [29], dye-sensitized solar cells [30], photodetectors [31], and photodiodes [32]. Many methods such as chemical spray pyrolysis [33], SILAR [34], spin coating [35], hydrothermal [36], atomic layer deposition [37], thermal evaporation [38], and RF magnetron sputtering [39] are utilized to fabricate the ZnO thin-film layer. Among these methods, spin coating is an inexpensive and easy method to acquire good uniform film behaviors [40,41].
In order to improve electro-optic properties of ZnO-containing devices, it can be doped or composed easily with other transition metals or organic materials [42][43][44][45][46]. Organic materials have attracted much interest for optoelectronic applications due to low cost, flexibility, and wearability of organic materials [47][48][49][50]. Among the organics, carbonaceous materials (amorphous carbon, carbon nanotube, diamond, graphene, fullerene, etc.) draw great attention and are extensively used in optoelectronic device applications because of their unique structural and electronic properties. Nie et al. [51] fabricated a graphene-ZnO photodetector by coating ZnO nanorod arrays with a monolayer graphene layer. Cheng et al. [52] investigated a photodetector based on graphene/ ZnO/Si triple junctions. Shao et al. [53] produced a ZnO nanowires/p-Si heterojunction photodiode using a multiwalled carbon nanotubes. Bazargan et al. [54] employed a photodetector based on reduced graphene oxide (rGO) and ZnO thin film. Koç et al. [45] fabricated and investigated currentvoltage characteristics of ZnO-amorphous carbon photodiodes.
Among the carbonaceous materials, PCBM, which is a fullerene derivative, is one of the most studied materials in organic photovoltaic cells for the bulkheterojunction structure [55]. PCBM has n-type semiconducting property and is mainly used as electron acceptor for solar cells due to its high electron mobility [56][57][58][59]. Furthermore, PCBM can be used for dispersing of other material efficiently via blending and remain stable [60]. This behavior of PCBM provides the opportunity to use it with other organic and inorganic materials [61,62]. In the literature, PCBM was studied in p-Si/PCBM hybrid organic/inorganic semiconductor photodiodes by Yakuphanoglu [63]. PCBM:ZnO interlayer was used in a Al/PCBM:ZnO/p-Si heterojunction for photodiode applications by Gullu et al. [64].
This study focuses on the effect of various amounts of PCBM on Al/PCBM:ZnO/p-Si photodiodes. Various PCBM ratios can be doped to Al/PCBM:ZnO/p-Si devices to boost the performance of the devices and enhance their potential applications in optoelectronics, especially photodiode devices. With the intention to realize the effect of the various amounts of PCBM on the electrical properties of the Al/p-Si devices, undoped and various PCBM amounts-doped ZnO thin-film layers for interfacial materials in the Al/p-Si were manufactured by spin-coating method. The surface morphology of the PCBM-doped ZnO interfacial layers was investigated by AFM. Moreover, photodetection behaviors of the obtained various PCBM-doped ZnO Al/PCBM:ZnO/p-Si devices were investigated by employing I-V measurements.

Experimental details
PCBM was purchased from Sigma-Aldrich and used as received without further purification. First, PCBM was dissolved in 1-2 dichlorobenzene (25 g/L concentration) and stirred for 3 h at 60°C in a dried nitrogen atmosphere to obtain the PCBM solution. Second, the PCBM:ZnO organic blend was prepared using zinc acetate dehydrate (Zn(C 2 H 3 O 2 )2Á2H 2 O) as a precursor material. For this blend, the Zn(C 2 H 3-O 2 )2Á2H 2 O was dissolved in methanol as 0.2 M and mixed with PCBM to achieve undoped, 3%, 5%, and 10% molar ratio in different vessels, and then the mixtures were stirred for one night. Third, a p-type silicon substrate, which has 600 lm thickness, 5-10 X cm resistivity, and (111) orientation, was sliced into pieces with an area of 2 cm 2 and, then the parts were cleaned in acetone and propanol by an ultrasonic cleaner. Fourth, impurities and native oxide layer from the surface of the wafer pieces were removed with dumping into HF:H 2 O (1:1) solution for 30 s. Fifth, in order to place ohmic contact on the back surface of the wafer pieces, a 100 nm aluminum layer was deposited by thermal evaporation. Then the pieces were annealed in N 2 medium for 5 min at 570°C. Sixth, prepared PCBM:ZnO blend solutions were coated on p-Si substrates by Fytronix SC500 spin coater at 1500 rpm spinning rate for 30 s. Subsequently, to evaporate the solvents of the samples, they were thermally heated on a hot plate at 150°C for 15 min. At last, for achieving rectifying contact on the various PCBM-doped ZnO layers, 150 nm thicknesses Al layer was vaporized by thermal evaporator via a hole array mask for 3.14 9 10 -2 cm 2 contact area. The schematic drawing of the fabricated device as well as band diagram has been shown in Fig. 1. Here, the p-Si behaves substrate as well as semiconductor layer to collect carriers from metal to semiconductor. Thus, the rectifying behavior occurs due to their work function differences.
The surface morphologies of the obtained devices were taken by the PARK system XE 100E model AFM. I-V characteristics were measured by Fytronix FY-7000 photovoltaic characterization system in the dark and under varying illumination conditions in the range of -5 V and ? 5 V voltage range. The wavelength of the used light source was in between 400 and 1100 nm.
The I-V characteristics of the undoped and 3%, 5%, and 10% PCBM-doped Al/PCBM:ZnO/p-Si devices are exhibited in Fig. 3. The performed devices showed good rectifying property and their rectifying ratio (RR) values are given in Table 1. The 3% PCBMdoped Al/PCBM:ZnO/p-Si device has the highest RR value of 2.54 9 10 4 among the devices due to suitable amount of the PCBM in PCBM-ZnO layer. Thus, the PCBM-doped ZnO layer has more carriers and causes to higher current in forward biases. Furthermore, PCBM doping caused to decrease the leakage current at high voltages, especially 5% and 10% PCBM doping after 3% PCBM-doping level. The doping of the PCBM to the ZnO did not change the leakage current so much because the PCBM may serve only to increase the number of carriers in the interface of the device. Usually, low leakage current requires high barrier height, but there is deviation here due to having PCBM in the interface. The PCBM may cause to ease carrier transition in the interface and give low barrier height. The devices have displayed rectifying property even if they are under illumination conditions. This result can be assigned to the dielectric properties of the ZnO interfacial layer [65]. The reverse current of the Al/PCBM:ZnO/p-Si devices did not increase with increasing light illumination like Al/ZnO/p-Si device (displayed in Fig. 3a) because PCBM layers may behave electron acceptor layer in the interface of the devices. Thus, photocurrent of the devices did not increase with increasing light illumination at reverse biases. However, obtained all devices have a various responses to the light illumination at reverse biases. This behavior highlighted that Al/PCBM:ZnO/p-Si devices could be employed as photodiodes and photodetector applications [64]. Moreover, the increasing current with increasing light intensity as well as shifting of minimum current to positive side highlights that the fabricated devices can be viewed as photovoltaic devices, but their efficiencies are usually low [1,66,67] The I-V characteristics of the undoped and various PCBM-doped Al/PCBM:ZnO/p-Si devices were compared for dark and 100 mW/cm 2 light intensity in Fig. 4. The leakage currents increased with 3% PCBM doping and then slightly decreased with increasing PCBM doping in the dark. However, the leakage currents were almost stayed constant under 100 mW/cm 2 light intensity condition for various PCBM-doping levels. The forward bias currents increased with increasing PCBM doping, but maximum values were obtained for 3% PCBM-doped Al/ PCBM:ZnO/p-Si device both in the dark and under 100 mW/cm 2 light illumination. The slope of the second region in forward bias is highest for 5% PCBM doping, and the smallest ideality factor was obtained for 5% PCBM doping. However, the I-V characteristics of the 5% PCBM-doped Al/ PCBM:ZnO/p-Si device immediately bent at around 1.5 V forward bias, and it has highest series resistance. Both 3% and %10 PCBM-doped Al/  The obtained devices were characterized by I-V measurements for determining of the diode parameters such as ideality factor (n), barrier height ðU b Þ, and series resistance ðR s Þ by various methods. According to the results, the I-V measurements show   a non-linear attitude, and this kind of characteristics is interpreted via thermionic emission (TE) model [69]. n and U b can be acquired from I-V characteristics using TE model where the current (I) is given as the following equation [70]: where I 0 is the saturation current and expressed as follows [71]: where A is the contact area (A = 3.14 9 10 -2 cm 2 ), A * is the Richardson constant, T is temperature, k is the Boltzmann constant, q is the electronic charge, and V is the applied bias voltage. The n and U b are obtained by following relations when the V C 3kT/q [72]: and The calculated n and U b values as well as saturation currents are indicated in Table 1 for undoped and various PCBM-doped Al/PCBM:ZnO/p-Si devices. While the value of n increases from 4.47 to 5.47 with increasing PCBM-doping level from 0 to 3%, U b decreases from 0.74 to 0.69 eV. The value of n decreases to 3.89, but U b increases to 0.79 eV for a 5% PCBM-doping level. In the case of 10% PCBM doping, n increases up to 7.45, and U b decreases to 0.68 eV. In general, n values are expected close to unity; however, the obtained ideality factor values here are higher than unity due to barrier inhomogeneity and interface states as well as interfacial PCBM-doped ZnO layer [73,74]. The higher values of the n can also be attributed to non-uniform interfacial film layers of ZnO and PCBM. When the saturation current value of the PCBM-undoped Al/ PCBM:ZnO/p-Si device is 9.40 9 10 -9 A, it increased up to 5.47 9 10 -7 A by 10% PCBM doping. However, the minimum value of the saturation current was obtained for a 5% PCBM-doping level.
Light generally causes increasing of carriers in the interface of the MS devices, and this induces an increase in the current at reverse biases [75,76]. Here, the increasing light illumination generated charge carriers in the interface of the various PCBM-doped Al/PCBM:ZnO/p-Si devices, and current at reverse biases increased for all devices with increasing light illumination intensity. However, the devices still survived their rectifying behaviors, according to Fig. 4. These results revealed that the fabricated various PCBM-doped Al/PCBM:ZnO/p-Si devices can be used in optoelectronic devices such as photodiodes or photodetectors [77]. Furthermore, the increasing speed of the reverse current decreased with PCBM doping due to the electron acceptor behavior of the PCBM. Here, PCBM may have captured the electrons of the ZnO, which is induced by illumination, and transferred them to the Al electrode. However, the defect levels or interface states prevented this transferring process. Moreover, the current value at 5 V biases increased with a 3% PCBM-doping level and then decreased the lowest value for 5% PCBM doping. This kind of fluctuation can be found in the literature depending on the PCBM doping or mixing level amount due to the agglomeration effect of ZnO [78]. Furthermore, the responsivity values of the various PCBM-doped Al/ PCBM:ZnO/p-Si devices were calculated and are listed in Table 1. While the undoped Al/PCBM:ZnO/ p-Si device has minimum responsivity, 3% PCBMdoped Al/PCBM:ZnO/p-Si device has highest responsivity value. The obtained result verifies that the PCBM doping increases the responsivity value. Cheung or Norde models can be used to obtain once more junction parameters such as ideality factor and barrier height as well as series resistance [79,80]. The current, according to Cheung model, is given as follows: where IR s represents a voltage drop because of the series resistance in the junction. Eq. (5) can be rearranged, and below equations are obtained: where H(I) can be reorganized as follows: From these equations, it is obvious that dV/d(lnI) vs I and H(I) vs I graphs display straight-line profile. While dV/d(lnI) versus I graph gives n and R s values from the y-intercept and slope of the plot, the H(I) versus I graph provides to determine the U b and one of another R s values [81,82]. Figure 5a-d shows both the dV/d(lnI) and H(I) versus I graphs of the undoped and various PCBM-doped Al/PCBM:ZnO/ p-Si devices. The obtained graphs exhibited good linearity properly to Cheung model. The determined diode parameters have been arranged in Table 1. The obtained n and U b values are compatible with the results of the TE. The n values obtained Cheung method is slightly higher than the TE due to determination from different regions of I-V characteristic. The obtained R s values both the dV/d(lnI) and H(I) versus I plots are in agreement and verify the consistency of the Cheung model [83]. Furthermore, the R s values are high for undoped ZnO interfacial layer as 4.63 kX but decreased to 0.16 kX for 3% PCBM doping and increased up to 1.94 kX for 5% and decreased 0.79 kX for 10% PCBM doping again. The PCBM doping can alleviate the R s effect of the Al/PCBM:ZnO/p-Si devices [84]. The high values of the R s and the changes at the R s can be attributed to non-uniform interfacial film layers between the Al and p-Si [85].
Another way to determine the magnitudes of U b and R s is the implementation of the Norde model. Using this model, the Norde function is expressed as follows [86]: In this equation, c is a dimensionless integer value greater than TE ideality factor and I(V) is the current due to applied voltage experimentally. After the Norde function has been reorganized, the U b and R s formulas are obtained as the following equations: where V 0 represents the minimum value of voltage related to the minimum value of the F(V). F(V)-V plots of the undoped and different PCBM amount-doped Al/PCBM:ZnO/p-Si devices are depicted in Fig. 6a-d. The obtained plots are good agreement with the expected Norde plots. The values of U b and R s for each device were determined from the Norde model and are displayed in Table 1. The obtained U b and R s values are higher than other values that obtained from the TE and Chung models due to approximation differences of the methods or non-ideal junction structure [87]. However, their variation with various PCBM-doping amounts is the same as other models. The barrier height values were obtained as 0.82 eV, 0.72 eV, 0.93 eV, and 0.67 eV for the undoped and 3%, 5%, and 10% PCBM-doped Al/ PCBM:ZnO/p-Si devices.
The photoresponse properties of the devices can be studied by current transient measurements when the device is both at on and off positions [6]. Figure 7a-d indicates the current transient plots of the undoped and various PCBM-doped Al/PCBM:ZnO/p-Si devices for 20, 40, 60, 80, and 100 mW/cm 2 light intensities. All the devices have almost linearly increasing profile with growing light illumination intensity. The undoped, 3%, and 5% PCBM-doped Al/PCBM:ZnO/p-Si devices have small increase with increasing light intensity at low illumination power and a significant increase at 100 mW/cm 2 illumination, but 10% PCBM-doped Al/PCBM:ZnO/p-Si device usually has linear increase all illumination power range. When the devices are compared according to light response performances, the 10% PCBM-doped Al/PCBM:ZnO/p-Si device has the highest performance due to having high photocurrent in a transient period. This result highlights that the PCBM can be used in the photodiode with correct doping amount to increase the detection control [88].

Conclusion
The undoped and various PCBM-doped Al/ PCBM:ZnO/p-Si devices were produced by spincoating method as well as thermal evaporation method. The electrical characteristics of the obtained devices were performed by I-V measurements in the dark and under varying illumination conditions according to having various PCBM-doped ZnO interfacial layers. The PCBM doping changed the morphology of the ZnO interfacial layer according to AFM images. The obtained devices showed good photodiode properties and rectifying ratios in the vicinity of 10 3 rates in the dark and under varying illumination conditions. The junction parameters of the produced devices were calculated by using TE, Norde, and Cheung models from the I-V measurements. The ideality factor values determined from Cheung model are 4.74, 5.47, 3.60, and 7.72 for undoped, 3%, 5%, and 10% PCBM-doped Al/ PCBM:ZnO/p-Si devices, respectively. These results are compatible with the values found by TE method. The barrier height values calculated from the Norde model are 0.82, 0.72, 0.93, and 0.67 for undoped, 3%, 5%, and 10% PCBM-doped Al/PCBM:ZnO/p-Si devices, respectively, and these results are slightly bigger than the values found other methods. The series resistance values calculated from the Norde model are higher than other values that obtained from the TE and Chung method. The current transient results reveal that the PCBM doping can be used to increase the photoresponse capacity of the MS devices. According to the results, various PCBM doping helps us to understand the improvements in device performance, and such kind of photodiodes can find a place in future applications in the optoelectronics industry.