Nickel Oxide As Transperent Electrode Engineering Essay

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Researches on solar cells are consistently going on since its discovery in year 1839. Presently research more focused on increasing efficiency and reducing cost by changing device structure; using new and novel materials, surface texturing, light trapping layers, different substrates and technologies. Building integrated photovoltaics (BIPV) is one of the novel and demanding technology which focuses on integrating photovoltaic energy on to the structure of building (construction materials). BIPV has given room for development of Smart devices for several applications such as smart windows which allow only ambient light inside house and control of amount of light entering inside house.

K Nama Manjunatha, Emerging Technologies research Centre, Hawthron Building, De Montfort University, Leicester, UK, Tel: +447402422822,

Email: krishna_nm@live.com

Though there is huge growth in PV sector, still its utilization is only 0.1% in the world. This is mainly because of manufacturing, material and installation costs of crystalline PV devices. Even in BIPV there are challenges about how to integrate PV devices directly on materials used for building constructions.

A brick, tile, slate or window glass with added functionality of producing energy from sunlight sounds like good concept and contribute for attractive architecture with eco-friendly concepts of generating energy. Tile integrated with PV cells will serve its need by two ways; conventional function as barrier against elements and source of energy. More additional functions are; Photo-catalytic activity of TiO2 nanoparticles for decompositions of air pollutants [1]. Better aesthetics, integration, low cost and no installation cost make it possible for attractive concept of solar tile. In a way to realize PV tiles/brick (here termed as Solar tiles/brick), An extensive literature review gave few reports on thin film solar cells which can be integrated to different substrates [2] [3] [4] and previous work on thin film solar cells In EMTERC (Emerging Technologies Research Center) [5] gave an idea to test and realize BIPV models.

Goal of this work is to develop solar cells on Building construction materials. To achieve this goal, work is mainly divided into three tasks; 1) selection and optimization of transparent conductive oxide, 2) fabrication of thin film solar cell on test substrate (glass) 3) testing and Integration of thin film solar cells on thin film solar cells on building construction materials (brick, tile, slate, glass, wood and steel).

Firstly, transparent conductive material was selected by through research over literature and Nickel Oxide (NiO) is also alternative for transparent conductive material which shows attractive electrochemical, thermoelectric, electric and high chemical resistance properties [6] [7] [8]. Recently NiO films have shown 80% transmittance and above 1 S cm-1 conductivity [9]. This is satisfactory accepted values for TCO materials. Now a day's research on NiO has shown that it can be coated on windows to act as smart windows which will control intensity of light entering inside house by its electro-chromatic properties [9]. NiO is also used in decomposition of atmospheric pollutants [10]. Cheaply available resource and allows easy deposition, e.g. it can be deposited by thermal evaporation [11]. Hence when NiO is used as transparent electrode in solar cell; it acts as transparent electrode, allows light, decompose pollutants, can be used to control light intensity when solar cells are integrated on windows.

Secondly, Silicon based thin film solar cells is opted because of highest efficiency reported till today [12]. Silicon thin film technology is matured and can be easily adapted in industries. Conformability of silicon thin films are very high which allows for easy integration on tiles, slate and bricks with less impact on their texture and design [13]. Fabrication of thin film solar cells was employed with Plasma Enhanced Chemical Vapor deposition (PECVD) technique because it allows fabrication at low temperatures, utilizes less power and crystalline silicon can be deposited on substrates [14]. PECVD allows uniform deposition of film on curved substrates (traditional roof tiles). Importantly, considering environmental hazards, Silicon is found to have minimal effects when compared to CdTe and CIGS technologies [13].

This work provides broad understanding and framework for inorganic PV devices. Theory, principle, experimental methodology and results are described to understand properties of materials and their applications in use with transparent metal electrode and solar cells fabricated. Predicted structure of solar cell on building construction materials is shown in figure 1.

Top Contact (Transparent)

Thin Film (Silicon)

Bottom Contact

Tile

Glaze/ coating

Figure 1 Structure of thin film solar cell structure on building construction materials.

Solar cell structure

Nickel OxideSolar cell in this work mainly has thee functional layers, namely Top contact, Bottom contact and active layer (Silicon thin film) and one passive layer to hold these materials functioning as a substrate. Top contact is a transparent conductive layer which allows sunlight though it to reach active layer. Bottom contact is reflective which helps in light trapping by multiple reflections and also act as electrode. Active layer is sand witched between top and bottom contacts; active layer absorbs photons from sunlight and generates equal amount of charge carriers. Structure of solar cell is shown in figure 23.

Silicon Thin Film

Aluminium

Glass

Figure 2 Structure of solar cell showing three active layers and one passive layer.

Negative Electrode layer

Aluminum is used as schottky contact which also acts as back contact, anti reflective and cathode layer for solar cell. Barrier height of Silicon and Aluminum is 0.81ev which is high when compared to most of the metals except platinum and gold [22]. But gold is not used in fabrication labs as it alters density of states in silicon and platinum is a rare metal and expensive. Hence aluminum was chosen to form schottky contact which is available abundantly and cheaply. Barrier height is altered by many factors such as impurity, pre-deposition surface preparation, temperature, eutectic temperature, grain size in crystal and doping concentration [22]. Aluminum on poly-crystalline silicon also forms schottky contact [32] [33].

Positive electrode layer

In this work several materials have been used for investigation of anode material. Anode layer has to be transparent so as to allow light to pass through and should also be conductive to carry photo-generated carriers. Hence Transparent conductive oxides such as Indium Tin Oxide (ITO), Fluorine doped Tin Oxide (FTO) and Nickel Oxide (NiO) have been opted to investigate in this work. Work functions of all these materials are not fixed, since they are changed with amount of oxygen atoms present in the compound [8] [34] [35] [36], but work functions are higher than aluminum hence photo-generated carriers are easily collected at respective electrodes. Work functions of these materials are very close to work function of silicon there by establishing ohmic contact. Average work functions of ITO are 4.7eV, FTO is 5ev and NiO is 5.2eV which are high when compared with work function of Aluminum which is 4.08eV [37] [38] [39]. These work functions are almost near to work function of silicon with value 4.95eV [37].

Active layer - Silicon thin film

Silicon can be used undoped or doped with (p-type or n-type) material to act as active layer or absorber layer. Silicon can be used in different types of solar cells irrespective of schottky solar cell. Silicon is used in form of thin films as deposited, thereby limiting use of silicon wafers which are very expensive. Silicon deposited in form of film can be controlled by the by different parameters or depositions techniques so as to obtain crystalline, poly-crystalline or amorphous in nature, but their efficiency decreases from single crystal silicon to amorphous silicon [30]. In this work investigation is focused on un-doped and doped (phosphine n-type dopant) silicon thin films. Also it is extended to polycrystalline and amorphous silicon films. All types of silicon films will be importantly investigated on NiO positive electrode with aluminum negative electrode. Testing and comparison will be performed with ITO and FTO electrodes with aluminum used as cathode.

Experimental Details

This section will describe the sequential procedures used for fabrication of thin film solar cells which include all the process from cleaning to deposition of each layer which is associated in device structure.

Preparation of building construction materials

Several materials have been used in construction of buildings, but only those materials used for construction in which they are exposed to sunlight can be used for integration of PV devices on them. Materials like Steel, Brick, Roof tile, Slate, Vitreous tile, Plastic and wall tiles are used in this work for investigation (See figure 25). Bricks, granite and tiles were purchased from Marley Eternit Ltd. Slate, roof tile were purchase from Redland bricks. Steel and plastic is purchased from Parker steel company. To test integration of solar cells to these materials which act as substrate, they were cut into 3 X 3 cm2 and cleaned sequentially with water, soap water, de-ionised water and dried in air.

Figure 3 Several materials collected for investigation to be used as substrates for solar cells.

Cleaning of other substrates

There were four main test substrates used in this work, namely, corning glass slides, ITO coated glass slides and FTO coated Glass slides were purchased from Sigma Aldrich, P-type Silicon wafer. Substrate cleaning involved in this work is mainly chemical cleaning which was performed in Class 100 clean room in which drying was done in Laminar flow workbench to ensure dust free environment. Glass slides, FTO and ITO coated slides were cleaned separately by following similar sequential steps as explained. Substrates were placed in Beaker containing 5% Decon-90 soap detergent and de-ionised water which was placed in ultra sonic bath and sonicated for 30 minutes and 2minutes for five times respectively. This is followed by placing the samples in Acetone and Isopropyl alcohol separately in two different beakers and sonicated for 15 minutes. Then samples were taken out and rinsed in de-ionised water for 2 minutes and repeated for five times. Slides were then individually cleaned to dry by blowing nitrogen from gun. Once dried, samples were stored in slide box until needed.

Thermal Evaporation

Aluminum bottom contact and Nickel oxide top contact are deposited by using thermal evaporator. Nickel oxide (NIO) powder (99.99%) purchased from Sigma-Aldrich is deposited using tungsten boat under vacuum pressure of approximately 2 x 10-6 mbar by supplying 30 to 40 amperes of current for melting NIO initially and gradually increasing till 60 amperes for evaporation. Aluminum (99.999%) purchased from Sigma-Aldrich is deposited using tungsten filament under vacuum pressure of approximately 2 x 10­­-6 mbar by supplying 20 Amperes of current for evaporation. Evaporator was left unattended after evaporation of material for one hour so as to bring down the temperature within the chamber and then samples were taken out.

Radio Frequency - Plasma Enhanced Chemical Vapor Deposition (RF-PECVD)

This process is selected in this work mainly for low temperature process and cost effectiveness when compared to other techniques. This process started by placing substrates in PECVD chamber already deposited by catalysts on one side. In this work different substrates were used for investigation, testing and optimization, namely ITO coated glass, FTO coated glass, NIO coated glass, Corning glass, P-Type silicon wafer. Gallium is used as catalyst for ITO and FTO coated glass and P-type silicon wafer. Nitrogen is supplied and evacuated three times to clean the supply tubes and chamber. Chamber temperature is maintained at 4000 c, pressure is at 200mTorr, RF power is set to 25watt supplied at 13.6 MHz frequency. Plasma is initially created by passing Hydrogen gas into the chamber at the rate of 100 Standard Cubic Centimeter per minute (SCCM) for 5 minutes. Following this Silane (SiH4) gas is introduced into the chamber at 20 SCCM for growth of silicon nanostructures. Some silicon nanostructures were also grown by doping with phosphine so as to obtain n-type silicon nanostructures. This doping was done by simultaneous supply of saline gas with phosphine gas into the chamber. Later pressure within the chamber was reverted back and temperature was brought down to room temperature. Then samples were removed after sufficient cooling of chamber for further investigations and experiments.

Decomposition process in this experiment can be described by following chemical reaction:

SiH4(gas) Decomposition [SiHm] Si + m[H]

Above process is repeated for several times by altering parameters like doping, amount of gas flow, deposition time, doping concentration and pressure. These parameters are described individually

Results and Discussion

Optimization and characterization of transparent conductive nickel oxide

Optimization of Nickel oxide (NiO) is one of the important tasks in this work. As explained earlier, due to interesting properties of NiO this material was chosen to investigate and exploit more properties so as to achieve goal. Green granular powder of NiO (molecular weight - 74.69 g/mol, Density - 6.67g/cm3) was used in thermal evaporation process. It was quite difficult for evaporation since the need of high current to sublimate the material. Deposited films were then investigated using several techniques as discussed.

Transparency of NiO films were investigated using UV-Vis measurements from wavelengths ranging between 400nm to 700nm which is of only interest for solar cell.. Raw data obtained is plotted for transmittance (in percentage from 0 to 100) along Y-axis and wavelength in nanometers along X-axis (see figure 30 and 31).

Figure 4 transmittance of NiO film deposited at different thickness in visible region.

From the plot seen above it is observed that thickness of film is inversely proportional to transparency. Which means as thickness of film is increased, transmittance is decreasing. . It can be observed that transmittance is almost uniform between these wavelengths, which is good for absorption of all the light in these wavelengths from sunlight. Table 1 shows average transparency at visible wavelength. Transmittance values can be increased by annealing in presence of oxygen [9].

Table 1 Average transmittance at visible wavelength for NiO films at different thickness.

Thickness (nm)

10

30

42

55

62

143

162

Average Transmittance (%)

97.0

50.5

44.4

41.0

39.0

12.5

5.7

Bandgap calculations are performed by Tauc plot, which is plotted by photon energy (e) in electron volt on X-axis and product of absorption coefficient with photon energy (E) to the power of density of states (n), i.e. ((αE)n) on Y axis. Calculations for Tauc-gap also called as Bandgap is determined by using formula [42]:

(αE)n = A (E-Eg)

Where, α is absorption coefficient, Eg is band gap, h is Plank's constant, n = density of states (n = 2 for direct bandgap materials and n = ½ for indirect bandgap materials.

Considering value n = 2, average band gap of NiO is found to be 3.60 eV (see figure 32). Band gap value obtained is approximately the same as reported by [8] [9] [39]. Value of n= ½ did not give meaningful band gap value which is reported for NIO as a direct band gap semiconductor from literature.

FTIR spectroscopy is used to determine quality, chemical composition and structure of NiO layer. Figure 33 shows significant peaks of NiO film deposited on silicon wafer using thermal evaporation in the range of 400-4000cm-1. Peak arising at 1108 cm­-1 corresponds to SiO2 layer between Si wafer and NiO film and peak at 608 cm-1 is attributed for Si respectively [49]. Weak absorption at 662 cm-1 corresponds to Ni-OH vibration [50]. Some bands below 500 cm-1 (inset figure 7-4), namely 415 cm-1,458 cm-1 and 473cm-1 correspond to vibration of Ni-O [50] [51] [52]. Broad absorption band centered at 3700 cm-1 is attributed to O-H stretching vibrations and H-O-H bending vibrations is assigned to weak band across 1630 cm-1 [34]. Peaks of carboxyl and hydroxyl groups can arise from air which is present in the equipment since measurements are not performed in vacuum.

EDX investigation is performed on NiO films alone so as to check impurities or contaminations in the film. Since FTIR analysis showed CO2 and hydroxyl groups in the film; it is known from prior knowledge that FTIR spectrometer do not measure under vacuum conditions and these impurities which are present in air is being measured during instrumentation. Figure 40 shows EDX analysis of 55nm thickness NiO film which depicts different materials present in the film and their concentrations. Peaks of Oxygen and Nickel correspond to NiO conductive electrode, Peak of silicon and potassium corresponds to material present in glass. Only impurity present in this film was sodium otherwise quality of film is good and no presence of carboxyl and hydroxyl compounds.

Refractive indices (n) is useful for characterization of materials and to know composition in deposited film ,in this work NIO film refractive index was measured using Ellipsometery for all the above mentioned thickness in earlier sections. Average refractive index of NiO films in this work was measured to be 2.19 (± 0.08 measured at different spots across the film) which are almost same as those reported by [53] and [54] who had measured it to be 2.37. Difference observed is because of impurities present in film as shown in FTIR spectra and EDX analysis.

Electrical measurements for NiO films were performed to determine its resistance, sheet resistance, conductivity and contact resistance. Measuring these parameters will help in half a way to optimize NiO films. Firstly, I-V measurements are performed for deposited gap cells and slope of this plot will determine 1/R from which resistance is calculated (see figure 34). Looking at IV characterizes which is linear confirms that Al acts as ohmic contact with NiO (see figure 34). Graph for one sample in which one gap cell has been selected to show as an example and all other values for different samples are shown in table 3.

Measure of resistance in thin films is termed as Sheet resistance. This is accepted only for two-dimensional systems in which thin films are considered as two-dimensional entities. Hence flow of current is along the plane of film. Similarly sheet resistance is calculated by Transmission line method (TLM) for transparent conductive NiO film in this work. It is also important to characterize contact resistance (Rc) for devices having contacts or used as contacts. In this work NiO is used as top contact hence it is importance to understand contact resistance as how Rc would affect device performance and contacts might degrade device performance by injecting minority carriers.

Transmission line method is used to calculate different parameters in this work; because value of thickness is not required for calculations. Quartz microbalance reader and ellipsometer showed different thickness for the deposited film and this value when used in calculations would give wrong results. NiO film of 42nm thickness is taken as example and plot of resistance (R) verses contact spacing (d) from which sheet resistance and contact resistance can be extracted is shown in figure 35. Values for other samples are shown in table 3. In this work Sheet resistance of NiO film is measured to be 4.5KΩ/â-¡ for 42nm film thickness. From table 3 it is observed that as thickness increases sheet resistance and contact resistance decreases. Which means thickness of film is inversely proportional to sheet resistance.

Finally, after obtaining values of sheet resistance and transparency of NiO film at different thickness; next is to optimize best thickness at which sheet resistance and transparency is accepted. Figure 36 shows dependence of transparency and sheet resistance.

Figure 7-7 shows that as thickness increases transmittance and sheet resistance decreases. For a good conductor sheet resistance should be low, hence thicker film is preferred. Solar cell needs transparent conductive electrode with maximum transparency so as to absorb more light otherwise intensity of light reaching active layer would be less, as a consequence less photo-generated carriers. Hence thinner film is preferred having highest transparency. But thinner film has good transmittance but sheet resistance is more and vice versa. Hence in this work it is better to use film with 55nm thickness which has 41% of transparency in visible region which is almost same when compared to 42nm film with 44% transparency. But resistivity and sheet resistance is less in 55nm film when compared to 42nm film. Since there is no much difference in 55nm film when compared with 42nm; 55nm film is considered to be good and appreciable. Films with thickness 143nm and 162 nm has very less transparency; though sheet resistance is less, it is not accepted because amount of photo generated carriers are very less and low resistance does not have any effect on this. Because films with little increased sheet resistance can also carry these photo-generated carriers. When intensity of photon absorbed is very less, there is no necessity of having low sheet resistance.

Resistivity/Resistance of conductive metal oxides usually varies with temperature [34]. In this work Nickel oxide thin films are placed in PECVD chamber for growth of silicon nanostructures where temperature is 400oc. Hence change in resistivity with respect to increase in temperature is investigated and plot of the same is shown in figure 37.

In this work it is found that electrical properties of NiO films are governed by vacancies of oxygen and also from literature it has been seen that resistance varies in regard to change in temperature, amount of oxygen, heating and cooling. In slow temperature changes (Fig. 37) film resistance depends on temperature dependent oxygen content and not on cooling and heating conditions. Hence this measurement tells about oxygen in and out diffusion process. Three regions can be seen in the graph, in first region (temp < 425K) shown in black color corresponds to increase in temperature during which oxygen diffusion might be slow and vacancies concentration is unchanged. This can also be thought as characteristic of metallic conductivity. This metallic behavior at measured temperatures could be because of heavily doped semiconductors at which high self-doping of film might occur and oxygen vacancies which create impurity band will be overlapping with conduction band. Hence resistance is controlled by variations in carrier mobility which is consequence of scattering of electrons by vacancies of oxygen [40] [45] [6] [52] [57] [58].

Temperatures from 425K - 675K there is sharp increase in resistance with respect to temperature can be seen and it is explained by assumption that due to change in oxygen vacancies concentration, self-doping of material is governed by oxygen in-diffusion process. Hence it can be concluded that resistance at this temperature depends on exchange of oxygen between air and film which results in electron-hole carries but substituted by oxygen vacancies. In region three (red colour), resistance increases exponentially with reducing temperature. Then one should think of emerging temperature dependent energy gap between impurity band of oxygen vacancies and conduction band of semiconductor Nickel oxide. There may be chances of decrease in vacancy mobility, decomposition might have occurred at high temperature and deformation in contacts leading to change in contact resistance[40] [45] [6] [52] [57] [58]. Since this experiment was performed in open air; oxygen present in atmosphere was made easy to react with NiO films as a consequence change in resistance is observed. A NiO film when heated under vacuum in PECVD chamber, change in resistance is not absorbed in this work.

In material science, many phenomena are governed by thermally activated process, which means some thermal energy is required for some process to happen. This can occur by overcoming energy barrier referred as activation energy. By increasing temperature some amount of thermal energy is given to material in which atoms/molecules are made to surpass activation energy. Thermally activated process is determined by,

.

Where z = thermally activated phenomenon, EA= activation energy for the process, Ao= constant, R=universal gas constant (8.314 J·mol-1.K-1) and T= temperature (Kelvin). Activation energy divided by gas constant "R" is determined by slope of curve plotted with ln(z) on y-axis and (1/T) in X-axis. Hence Activation energy for NiO film is determined to be 0.090eV which is almost same when compared with reported value of 0.10eV and 0.085eV from literature [59].

Characterization and analysis of active layer

This section discusses second important task of depositing thin film silicon solar cells after successfulness of obtaining semi-transparent conductive NiO film. Results presented in this section are according to order of work that took place sequentially. Aim of this project was to deposit thin film solar cells on foreign substrates which can be done only if grown silicon structures show photoconductivity and obtaining working solar cell on test glass substrate.

Initially, silicon thin films were deposited separately on glass slide coated with ITO as transparent conductive electrode by RF-PECVD process. Silicon nanowires were grown using catalyst material (permission not granted to mention name) which is coated by simple paint brush on top of ITO and then placed in PECVD chamber. SiNW were grown in RF-PECVD chamber at 4000c temperature, 25watts RF power and 200mtorr pressure throughout the process. Hydrogen plasma was created by passing H2 at 100sccm for 5 minutes, along with hydrogen gas flow SiH4 is passed at 20sccm for 5 minutes and continuing with the flow of above mentioned gases; phosphine is then passed at 10sccm for one hour. Phosphine was passed so as to dope silicon nanowires thereby obtaining N-type SiNW. Totally deposition took place for one hour ten minutes. Later aluminum top contact of 150nm thickness was deposited using mask. Aluminum acts as schottky contact to n-type silicon. Final structure is ITO/i-Si/n-Si/Al. This sample was then investigated by IV characteristics and this indicated failure results with no current being measured for applied voltage. Reason for failure might be possibility of aluminum which has leaked in gap between nanowires and not much aluminum on top to act as electrode. Some possibility could be that catalyst/Si-SiNW might not be making good contact with ITO. It has also been reported that ITO films when exposed to 3000c, its resistance increase by three times. This is because of reduced oxygen vacancies at higher temperatures which actually functions as electron supplier [60].

Then an attempt was made to change Conductive oxide electrode from ITO to FTO, along with this catalyst was changed to gallium since it is good catalyst for Silicon nanowires growth [30]. ITO and FTO coated glass slides were deposited by 55nm thin film of gallium by thermal evaporator. These samples were then subjected to SiNW growth with same conditions mentioned earlier but process is carried out only for 20 minutes without doping phosphine. Again silicon structures deposited on ITO did not show diode characteristics and conductivity. Interestingly Si nanostructures deposited on FTO samples showed diode characteristics but again there was no photoconductivity. This characteristics shows that there is some schottky barrier which is formed by aluminum with SiNW because difference in work function of aluminum and silicon is large which creates schottky barrier [37]. It is also absorbed that threshold voltage is very high having the value of approximately 6V (± 0.4) measured for eight devices on same sample. Hence this is assumed to be photodiode showing similar characteristics. Since schottky diode was successful in fabrication, further investigation was carried to fabricate similar thin film schottky diode for NiO electrode instead of FTO under same conditions. This device showed non ideal schottky diode characteristics (double diode characteristics) which is not same when compared to earlier device fabricated as FTO/SiNW/Al. Increase in current can be observed in illuminant conditions but Voc and Isc characteristics of solar cell is not observed.

Finally, fabrication followed in following sequence with some changed parameters. Glass coated with 41nm NiO is deposited by 50nm gallium on top by thermal evaporation then Silicon nanostructures are grown for 5 min with saline at gas flow rate of 20sccm so as form schottky contact with NiO. Following Saline flow, phosphine (PH3) is introduced at 10sccm. Whole process was carried out 400deg temperature, 200mtorr pressure with 25watts RF power for 30 minutes. Altogether process was performed for 40 minutes. Later Aluminium was deposited on top at 200nm thickness. Finally structure is looks like NiO/i-Si/n-Si/Al. Fabricated structure was then investigated to observe the growth of silicon nanowires. Figure 43 shows SEM images of silicon nanowires grown on gallium catalyst.

Figure 43 SEM images of SiNWs morphology; nanowires standing vertical with gallium tips.

Nanowires had densely grown and have grown quite straight. It is also seen that several nanowires are coming out from single nanowire which looks like tree with branches. Reason for this could be that if catalyst is thicker it could benefit for additional growth or there might be possibility of catalyst flowing down the nanowire and could allow silicon to grow in sideways. Sometimes there might be possibility of gallium contamination in nanowires which act as catalyst during the growth process and allows silicon to grow at those particular sites. It was possible to zoom for one single nanowire and its thickness measured was 165.9nm.

Thin film of silicon is then analysed in Uv-Vis spectrometer for transmittance. Approximately 75% light is being absorbed by the active layer. Further investigations were carried on to investigate quality of film in regard to impurities by EDX analysis. Figure 44 depicts different materials present in the film. Peaks of Oxygen and Nickel correspond to NiO conductive electrode, Aluminum film corresponds to metallic electrode, Peak of silicon corresponds to silicon nanostructures and material present in glass. Peak of potassium corresponds to material of glass. Only impurity present in this film was sodium otherwise quality of film is good.

Figure 5 EDX analysis of fabricated solar cell on glass.

This structure is then investigated for electrical properties. Good characteristics of solar cell are observed from I-V characteristics but graph looks interesting with not having ideal diode characteristics during reverse bias (See figure 45) which had to be investigated further. Open circuit voltage and short circuit current are measured directly from graph (inset - Figure 45). Other characteristics of solar cell; Fill factor and Efficiency were calculated from the extracted values of I-V curve. All the plots shown here are explained in detail by taking example of one solar cell device whose parameters were explained above.

Curve which appears to be flat is because of effect caused from parasitic resistances which actually reduce the squareness of ideal diode. Above results are shown for device with area 0.0176cm2 and power output of light source is very less about 8.58mW/m2. This yields less efficiency which is shown in above table and transparency of NiO is also less and added some parasitic resistances which actually reduced the squareness of curve in fourth quadrant. Increasing thickness on NiO yields less efficiency, since transparency decreases as thickness increases which reduces absorption of light at active layer. Hence more investigations have to be carried on to optimize NiO thin films so as to enhance efficiency of solar cells. All measured parameters, namely Voc, Isc, FF, PCE, Vm, Im, and maximum power (Pm) are shown in table 6.

Open circuit voltage majorly depends on work function of schottky barrier metal. Low Voc value could also be because of barrier degradation by inter-diffusion [23]. Some complexities of materials, temperature, deposition rate, doping, etc. mainly determine the activity of active layer. It has been reported that silicon structures grown by PECVD process usually contain bonding defects, interstitial atomic and molecular hydrogen, some voids which actually affect the activity of photo-generation of carriers.

Curve in bottom right quadrant is flat, which indicates increased sheet resistance and decreased shunt resistance. An ideal solar cell should have infinite shunt resistance and series resistance should be as low as possible (close to zero). Shunt resistance is generally caused by leakage current which also affects by increasing slope of current component. Flattened region in fourth quadrant could be reason of reduced shunt resistance which is caused by more leakage current. This could be one reason for which current in third quadrant can be considered as leakage current. Usually leakage current arises from pinholes and recombination traps in active layer [4]. It is reported that leakage can also occur due to shunting of surface leakage along with junction leakage [62].

Until investigation on NiO film which is p-type semiconductor and its influence on silicon are understood reverse current can't be concluded as leakage current. NiO is a P-type semiconducting oxide and it could have formed junction with n-type silicon to form P-N junction or there might be possibility of Double schottky junction which might show I-V characteristics as seen in figure 46.

For a schottky contact, work function of metal should be greater than n-type semiconductor, similarly work function of silicon is 4.6 - 4.8eV and work function of nickel is 5.1 - 5.35 eV and addition of oxygen to nickel can change work function by ± 0.4 eV to that of nickel work function which in this work work-function might have increased and forming a schottky junction at NiO and n-type silicon interface [37] [22] [63] [64]. It is shown that work function of Nickel oxide can have value of 5.67eV [65] hence this clearly indicates that reverse bias current is not leakage current and it is forward bias of another schottky junction.

Hence this type of structure is usually referred as Double diode and their characteristics looks similar to the I-V characteristics obtained in this work. This double schottky diode will have no reverse bias since either of diode will work in forward bias at different conditions. For curiosity, aluminum dots with 0.176cm2 is used as front and back contacts for the same active layer and still it showed double diode characteristics. This is because n-type silicon with aluminum will form schottky junction and both the contacts acted as schottky junction.

Solar cells are exposed to sunlight for years; hence stability of solar cell is also important to measure if its efficiency and performance is consistent. In a way to understand stability, Voc is measured with respect to time for 1000 pulses under light conditions (Solar simulator with AM 1.5 light source). Figure 46 shows semi-logarithmic graph where voltage decay is observed with change in time. Little change is observed about 0.03V for 1000 pulses. Similarly change in Isc is measured with respect to time and it is observed to increase with time. Measured increase current for one hour is 6.5 nano-amperes. This shows that device is quite stable and further optimization could minimize the changes. When both are observed together with change in time; Voc decreases and Isc increases thereby fill factor will not change much since if one parameter decreases other parameter increases (change is not equally proportional and assumption is only for approximate values).

Conclusion

NiO transparent conductive oxide was optimized to integrate with solar cell, however efficiency in terms of transparency is not excellent when compared with ITO and FTO which are commercially used. Interestingly it showed low value of resistivity and from this work it is shown that deposition of NiO can be performed by thermal evaporation which is easiest and cheapest technique. Fabricated NiO films showed little increase in resistance with increase in temperature and EDX analysis has proved good quality of film with only contamination of sodium. Several spectroscopy and electrical techniques were performed and measured values are close to the values reported in literature. Nickel oxide has been investigated for the first time at Emerging technologies research Center in De Montfort university as part of this work.

Although, solar cell fabrication had several failures, it is finally integrated with NiO thin films and solar cell was fully fabricated. Several improvements are done in obtaining silicon nanostructures, finally doping with n-type impurity (phosphine) showed good schottky contact with aluminum. Grown silicon nanostructures were analyzed in SEM and it was observed that nanostructures were thick and dense. EDX analysis depicts only sodium impurity in the fabricated solar cell. Though it was not ideal schottky solar cell, a solar cell characteristic is obtained and was assumed to be double schottky solar cell or there could be possibility of more leakage current. Efficiency of solar cell was very less which has to be optimized in regard to NiO films to improve efficiency.

Future Scope

After the success of integration of NiO film for solar cell and fabrication of thin film solar cell next step was to integrate thin film solar cell on building construction materials. If provided more time; investigation of the same could have been performed. In later stage integration on to building construction materials has to be investigated and this is an interesting and emerging area of research.

Better optimization of NiO thin films would enhance transparency and thereby improving efficiency of solar cell. Research on NiO used as TCO for silicon thin film solar cells have to be more investigated to understand its influence in deviating from ideal diode characteristic. Investigation of thin film solar cells by using materials other than silicon and understanding its compatibility on NiO film can yield better efficiency or might give rise to cheaper solar cells.

Since Nickel acts as catalyst for growth of silicon nanowires, some research on NiO to be used as catalyst for the growth of nanowires would fetch cheaper solar cells, since another catalyst material is not required for growth of SiNWs and efforts to deposit catalyst separately are not necessary. Thereby NiO will provide double functionality as TCO and catalyst layer.

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