Preparation and Characterisation of Certain Ii Vi I Iii Vi2 Semiconductor Thin ﬁlms and Transparent Conducting Oxides

Malaya Journal of Matematik, Vol. S, No. 2, 3911-3915, 2020

https://doi.org/10.26637/MJM0S20/1008

Preparation and characterisation of certain ii vi i iii vi2 semiconductor thin ﬁlms and transparent conducting oxides

R. Rani1 and R. Velavan 2

Abstract Today certainly one of our main demanding situations to the arena scientific community is to find a maintainable deliver of electrical power. At gift, most of our energy come from fossil (i.E. Coal, liquefied petroleum, oil, herbal fuel) and nuclear sources. Not simplest are these assets of power non-renewable and in dwindling amounts, they also can be polluting to the surroundings. Burning of fossil fuels releases almost 7 billion lots of CO2 in line with yr, ensuing in environmental troubles inclusive of the greenhouse impact and worldwide warming. Burning of unrefined coal also consequences in acid rain, which is at once accountable for massive location wooded area and natural world destruction as well as soil pollutants. A collection of incidents at several nuclear electricity vegetation, combined with the shortage of an extended-time period waste disposal strategy, has resulted within the termination of nuclear power programmes inside the USA and most European nations. Keywords CO2m Electricity Vegetation, Acid Rain.

1,2Department of Physics, Bharath Institute of Higher Education and Research, Selaiyur, Chennai-600073, Tamil Nadu, India. Article History: Received 01 October 2020; Accepted 10 December 2020 c 2020 MJM.

Contents 2. The need to develop clean renewable energy resources to decrease the generation of greenhouse gases (CO2 1 Introduction...... 3911 and CH4) 2 Principle of solar cells...... 3912 3. Growing international demand for electric power, mainly 3 Thin movie solar cells...... 3913 in rural areas 4 Advantages of chalcopyrite thin films ...... 3913 5 Preparation and characterisation of cds thin films by Potential new strength assets include biomass, geothermal chemical bath deposition ...... 3914 electricity, hydroelectricity, ocean, thermal strength, wind en- 5.1 Cadmium sulphide buffer laver...... 3914 ergy and the direct conversion of sunlight into strength by way of the photovoltaic (PV) impact. Among these renew- 6 Experimental details...... 3914 able energies, the direct conversion of daylight is the most 7 Conclusion...... 3915 promising. The photovoltaic source of electricity, i.E. Sun References...... 3915 irradiation, has the advantage of being broadly dispensed over the arena, despite the fact that the most important call for does 1. Introduction not continually correlate with the deliver. The sun irradiation impinging on the planet’s surface isn’t always a proscribing These occasions have inspired hobby in clean renewable elec- issue and supersedes our desires. Future design of our strength tricity options. In standard these electricity systems do not device may be a mixture of various options. Solar cells by depend on assets, which can be restricted to our earth, but no means will or can represent the handiest solution. The at the constant radiation of the solar. There are three fun- useful resource have to be sustainable and the price have to be damental reasons for the development of alternative power in stage with today’s cost of energy. Furthermore we should resources: have a era to scale up and produce this gadget. Solar cell tech- 1. The speedy depletion of oil and gas resources nology is about to fulfill all of these requirements. Equally Preparation and characterisation of certain ii vi i iii vi2 semiconductor thin films and transparent conducting oxides — 3912/3915 crucial is the position of PV structures in assembly a number contemporary go with the flow. Illumination of the junction of the maximum vital desires of humanity. In India, by using (Fig.1.1d) creates electron-hole pairs, inflicting an increase in the end of 2002, 5084 sun PV water pumps were hooked up the minority carrier attention. The capacity electricity barrier in rural areas, with a total capacity of approximately five.Fifty decreases, allowing the current to waft, and a photovoltage five MW energy. And 2,four hundred villages and hamlets VOC (photovoltage underneath open circuit situations, or have been electrified in India with PV. This barely taps into the open circuit voltage) is generated across the junction [2, 4]. capacity for bringing sparkling water and light to the poor and Solar cells are characterised with the aid of current-voltage far flung populations in India, however it absolutely confirms (I-V) measurements inside the dark and beneath standardized the feasibility and advantages [1]. illumination that simulates the sunlight. Figure1.2 shows an example of diode characteristics of a sun cell inside the dark and underneath illumination. The maximum critical param- 2. Principle of solar cells eters that describe the overall performance of a solar mobile Solar cells, or photovoltaic gadgets, are gadgets that convert (open circuit voltage VOC, brief circuit present day density sunlight immediately into strength. The power generating part JSC and fill component FF) can be derived from the J-V curve of a strong-nation sun mobile consists of a semiconductor measured underneath illumination. that forms a rectifying junction both with every other semiconductor or with a metallic. Thus, the structure is largely a pn-diode or a Schottky diode. In a few junctions, a skinny insulator movie is positioned between the 2 semiconductors or between semiconductor and the metal, thereby forming a semiconductor – insulator– semiconductor or a metal – insulator – semiconductor junction. Moreover, pn-junctions can be categorized into homojunctions and heterojunctions in keeping with whether the semiconductor material on one side of the junction is similar to or unique from that on the alternative aspect. The open circuit voltage is restricted by way of the band hole strength Eg of the absorber fabric, and its most cost is calculated by dividing the band hole strength with the aid of the price of an electron (Eg /e). Because of electron-hole pair recombination, the open circuit voltages of actual solar cells are considerably under their maximum limits. The most cost of short circuit modern-day density, in flip, is the photogener- ated contemporary density Jph [3] that depends on the amount of absorbed mild. Fill factor, which describes the form of the illuminated I-V curve, is expressed in keeping with the following equation: VmpJmp FF = where Vmp represents the photo voltage and VOCJSC Jmp the photocurrent density at the maximum power point Pmax. The conversion efficiency η of a solar cell is simply the Figure 1 presents a schematic strength band diagram of a pn- ratio of the incoming power to the maximum power output heterojunction solar cell (a) at thermal equilibrium in dark, Pmax = Vmp Jmp that can be extracted from the device. V J (b) under a forward bias, (c) under a opposite bias, and (d) η = mp mp Based at the above concerns, the band gap Pin beneath illumination, open circuit conditions. Eci and Evi in price is one of the most vital houses of the absorber fabric of Fig.1.1 talk to the conduction and valence band energies of n a solar cellular. The most useful band gap fee for the absorber and p type semiconductor respectively. Egi and EFi are the fabric of a unmarried-junction sun cell is ready 1.Five eV, band gaps and Fermi degrees, respectively. In the absence of which ends up in a theoretical maximum performance of 30 an implemented potential (Fig.1.1a), the Fermi stages of the % [3]. This is because VOC and FF growth, and jsc decreases semiconductors coincide, and there’s no modern float. A for- with increasing band hole [2]. Even higher efficiencies can ward bias Vf (Fig.1.1b) shifts the Fermi degree of the n-type be done with tandem sun cellular systems or by way of us- semiconductor upwards and that of the p-kind semiconduc- ing sun radiation concentrators. Most commercial sun cells tor downwards, consequently lowering the ability electricity of these days are fabricated from mono- or polycrystalline barrier of the junction, and facilitating the current drift across silicon. Silicon is a totally plentiful and famous material of it. The impact of a reverse bias Vr (Fig.1.1c) is contrary: it which lots of revel in has been received over the decades - will increase the potential barrier and accordingly impedes the the primary pn-junction solar cell based on crystalline silicon

3912 Preparation and characterisation of certain ii vi i iii vi2 semiconductor thin ﬁlms and transparent conducting oxides — 3913/3915 turned into made already inside the 1950’s [5]. Silicon pho- tovoltaics owes loads to the microelectronics enterprise that has received the expertise of the cloth properties as well as advanced the manufacturing techniques. Additionally, rejects from microelectronics enterprise have served as a deliver for high best supply cloth that has accordingly been to be had at a particularly low rate [3, 6].

3. Thin movie solar cells Due to the constraints of crystalline silicon, other absorber materials have been studied considerably. These are semiconductors with direct band gaps and excessive absorption coefficients, and consequently they can be used in skinny movie shape. Thin movie sun cells have numerous blessings over crystalline silicon cells [6]. The intake of substances is less due to the fact the thicknesses of the energetic layers are just a few micrometers. Therefore, impurities and crystalline imperfections may be tolerated to a far better quantity as com- pared to crystalline silicon. Thin films may be deposited by a diffusion of vacuum and non-vacuum strategies on cheaper substrates including glass. Also curved and/or bendy substrates which includes polymeric sheets can be used, main to lighter modules. Furthermore, composition gradients can be received in a greater without problems controllable manner. The primary candidates for low-fee skinny film sun cell substances are amorphous hydrogenated silicon (a-Si:H), CdTe (cadmium telluride), CuInSe2 and its alloys with Ga and/or S [8,9] and CuInS2. Of those, amorphous silicon sun cells have currently the biggest marketplace percentage [10].

4. Advantages of chalcopyrite thin films Photovoltaic studies has moved beyond using unmarried crystalline substances such as Group IV elemental Si and Group III-V compounds like GaAs to an awful lot more complicated compounds of the Group I-III-VI2 with chalcopyrite structure. The ternary ABC2 chalcopyrites (A = Cu; B = In, Ga or Al; C= S, Se or Te) shape a large organization of semiconduct- ing materials with diverse structural and electrical properties. These substances are attractive for skinny movie photovoltaic application for some of motives. CuInSe2 has the very best optical absorption coefficient (α > 105 cm-1) of all acknowledged skinny movie materials (Fig. 1.4). This high price implies that ninety nine% of the incoming photons are absorbed within the first micrometer of the fabric. As a end result, best 1–2µm of this material is enough to efficiently take in the incoming photons in comparison to bulk Si in which at least 300µm of cloth is required. The parameter that depends most strongly on the choice of semiconductor fabric is the band hole strength. Table 1.2 indicates the very best efficiencies (η) produced with the aid of the skinny film laboratory-scale sun cells of CuInSe2 and its alloys. CuInSe2 based totally sun mobile tolerance of CuInSe2 thin movie is advanced to that of sin- devices have tested correct thermal, surroundings and electric gle crystalline Si or GaAs devices when tested underneath stability. Preliminary exams have indicated that the radiation excessive-energy electron and proton radiation [19]. Table 1.2

3913 Preparation and characterisation of certain ii vi i iii vi2 semiconductor thin ﬁlms and transparent conducting oxides — 3914/3915

Reported performances of laboratory-scale solar cells based hetero junction [1]. The role of the buffer laver is two fold: it on CuInSe2 and its alloys. affects the electrical properties of the junction and protects it from chemical reactions and mechanical damage. From the electronic point of view, the CdS layer optimizes the band alignment of the device [4,5] and builds a sufficiently wide depletion width that minimizes tunneling and establishes a higher contact potential that allows higher open circuit voltage [5] The buffer layer plays also a very important role as a ”mechanical buffer” since it protects the junction electroni- cally and mechanically against the damage that may otherwise be induced by the oxide deposition. Moreover, in large-area Figure 1. Solar cells based on CuInSe2 devices the electronic quality of the CIS film is not necessar- ily the same over the entire area, and recombination may be enhanced at grain boundaries or by local shunts. Together with the un doped ZnO. layer, CdS enables self limitation Device Structure The easy tool shape of any hetero- of electrical losses by preventing electrical inhomogeneities junction sun cell consists of the following layers; substrate, from dominating the open circuit voltage of the entire device window layer, active layer and the contacts to the external [1]. circuits. Substrate or superstrate this serves as the protecting layer for 5.1 Cadmium sulphide buffer laver the active substances of the solar cellular. The maximum not A variety of techniques are being used to deposit CdS thin unusual substrate is glass, but steel foils and a few bendy films, such as; molecular beam epitaxy (MBE) [6], metal plastic substrates also can serve the identical motive. organic chemical vapour deposition (MOCVD) [7], close spaced sublimation (CSS) [8,9], screen printing [10,11] phys- Window layer or buffer layer It’s miles a skinny layer ical wapour deposition [12,13], rf sputtering [14] , pulsed of a compound semiconductor, whose primary position is to laser ablation [15], spray pyrolysis [16,17] and chemical bath couple the light optically into the following layer, the absorber, deposition [8,9,18 − 25]. The characteristics of the CdS thin with minimal mirrored image losses. This residue additionally films required for the application as solar cell buffer layer constitutes the primary half of of the p-n junction. On the are, it should be conductive (∼ 1016 carriers/cm 3 ), thin to grounds that the position of this window layer is not to absorb allow high transmission (50 − 100nm) and uniform to avoid photons, it may be closely doped (usually to n-kind), which short circuit effects [26]. Thickness as well as the depo- reduces the general series resistance of the cellular. sition method of the CdS laver has a large impact on device properties. During the early days of the development of Absorber layer That is the place in which light is ab- CuInSe2/CdS junction, a thick (about 1 − 3Ωm ) CdS laver sorbed and the photocurrent is initiated. The band gap of the [27 − 29] was used as the buffer layer. The CdS layers of absorber must for this reason be appropriate for the absorption these devices were most often prepared by evaporation at sub- of photons. The absorber is commonly 100 times thicker than strate temperatures between RT and about 2000C, or in some the window layer, and of p-kind conductivity. cases by sputtering [28] The CdS film was often doped either with In[28] or Ga [30]. In some cases, a CdS bilaver was used 5. Preparation and characterisation of [31,32] , consisting of a thinner high-resistivity laver, prepared either by evaporation [31] or chemical bath deposition [31-33] cds thin films by chemical bath and a thicker lowresistivity layer, doped with 2% In [32] or Ga deposition [33]. Evaporated CdS has been used also in combination with Over the years cadmium sulphide thin films have been exten- the transparent conducting oxide laver [34-36]. Nowadays, [ , ], sively investigated as an n-type buffer layer to form thin film chemical bath deposition is used almost exclusively 37 38 heteroiunction solar cells with p-CdTe and p- CuInSe2 ab- and therefore this section of the thesis focuses mainly on the sorber lavers. Most of the highefficiency CIS based solar cells chemical bath deposited CdS buffer laver. Solar cell with of today have a thin (50nm or less) CdS buffer layer and an Mo/CIGS/CdS/ZnO device structure with the chemical bath . [ ] undoped ZnO. layer between the absorber and the transparent deposited CdS has shown a record efficiency of 19 2% 39 . conducting oxide (see Chapter 1 , Fig 1.6). The role of CdS and undoped ZnO, are related to some extent [1]. Although 6. Experimental details the open circuit voltages of high-efficiency CIS devices are mostly determined by the electronic quality of the bulk ab- Cadmium sulphide thin films were prepared by the chemical sorber material [2,3] , the cell performances are nevertheless bath deposition technique The films were smooth, reflecting, heavily influenced by the formation of the ZnO/CdS/CIS, and was bright yellow in appearance. The thickness of the film

3914 Preparation and characterisation of certain ii vi i iii vi2 semiconductor thin films and transparent conducting oxides — 3915/3915 was calculated using Tolansky’s multiple beam interferome- of the 17 th Eur. Photovolt. Sol. Energy Conf., ( 2001) try technique (see section 2.3.1). The thickness of the CdS 995 film on ITO substrate for a deposition time of 45 , minutes [14] K. Ramanathan K M.A. Contreras, C.L. Perkins, S. Asher, (single dip) was 52nm. In order to get thicker films, multiple F.S. Hasoon, J. Keane, D. Young, M. Roberto, W. Met- depositions were carried out with fresh reaction bath for each zger, R. Nouffi, J. Ward, A. Duda, Prog. Photovolt: Res. deposition. The measured thicknesses of the films are given Appl., 11 (2003) 225 in table 3.1. The structural studies of the films were perfomed [15] V.M. Fthenakis, S.C. Morris, P.D. Moskowitz, and D.L. using the x -ray diffractometer. The optical absorption and Morgan, Prog. Photovolt: Res. Appl., 7 (1999) 489 transmission spectra of the asdeposited and annealed samples [16] S. Siebentritt, Thin Solid Films, 403-404 (2002) 1 were recorded using UV-Vis-NIR spectrophotometer (Hitachi- [17] J. E. Jaffe, A. Zunger, Phys. Rev B, 29 (1984) 1882 . Model U3410). Four-probe method was used to study the [18] S.M. Sze, Wiley & Sons, Physics of Semiconductor De- electrical properties of the film (see section 2.3 .6 ). vices, (1981) 790 [19] R. A. Mickelsen et al., in: Proc. 18th IEEE Photovolt. Spec. Conf.,(1985) 1069. 7. Conclusion [20] W. Donald Aitken, Transitioning to a renewable energy Chalcopyrite polycrystalline CuInSe thin films were success- future, White paper by Intemational Solar Energy Society, fully prepared by the flash evaporation of the bulk material (2003) [21] with different Cu/In ratios. The as-deposited films showed T. Wada, N. Kohara, S. Nishiwaki and T. Negami, Thin higher band gap energies due to the presence of the binary Solid Films, 387 (2001) 118 phases, which might have formed due to the selenium defi- ciency in the films during flash evaporation. The as deposited ????????? films were found to be p-type. The selenisation of the as ISSN(P):2319 − 3786 deposited films resulted in stoichiometric films with a chal- Malaya Journal of Matematik copyrite structure showing a strong preferred orientation along ISSN(O):2321 − 5666 the (112) plane. The films showed electrical resistivity in the ????????? range 7 to 0.2Ωcm and an optical band gap energy of 1.18eV.

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