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Review of Photovoltaic Technologies, a Renewable Energy Resourse

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The confined resources of non-renewable energy like gas, petroleum, along with the increasing resistance to nuclear power, induced by worries regarding to protection disputes and radioactive trash neutralization, have directed to an exceeding requirement of safe, clean and above all considerations, renewable energy sources. Photovoltaic (PV) technologies propose similar solution and have been previously used for years. At the beginning, the use of PVs was for generation of power on satellites and air crafts and later for domestic or industrial utilities.

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Population explosion, which is especially large in off-grid village areas not concerned to the state electricity networks is one more factor supporting the growth of PV technology. The usefulness of PV cells as a substitutive power source for industrial/domestic applications is forwardly moved by social concerns regarding to present-day developed living standards along with the human inclination to save Rupees. Along with this, environment and human heath also require to be protected. It can be obtained by the use of power that is generated from environmentally much kind PV technology, rather than the non-renewable fossil fuels, which make environmentally-injurious gases.

The resounding PV potentials have increased research interests on continuity and exercise of PV systems to settle down power deficiencies in the world. These have influenced not only in increment of PV cell efficiencies with sufficient reduction in energy cost, also in the growth of novel PV materials and new PV cell structures. The improvement in production capacity and new inexpensive PV technologies, made this development possible in current energy generation scenario. Presently, many promising alternatives for future growth of solar cell, are available. The commercialization of solar cells have been obstructed by high cost/peak watt of solar cell system. Furthermore, scientists have done efforts over years, enhanced performance, cost effectiveness and reliability of solar cells, still a big concern.

Therefore, policy goals by state agents pertaining improved energy security and diversity, reduced emissions of greenhouse gases and increased levels of technology enhancement have spawned PV technologies in the past years.

Solar illumination

The sun is the core of majority of the energy, preserving life on this planet and generates the essential gravitational force to maintain earth in an almost circular orbit. It consists a mass of 1.99 x 1030 kg with having radius of 6.96 x 108 m [9]. The distance between earth and sun i.e. R is nearly 1.5 x 1011 m. A convenient representation admits that shape of sun is spherical and solar radiation spectrum is nearby to a blackbody with having temperature (surface) almost 6000 K. Energy produced by nuclear fusion of H (hydrogen) into He (helium) is responsible to maintain the surface temperature. The interior temperature of the sun is about 107K. Due to very high interior temperature, the surface illuminates EMW (electromagnetic waves) in each and every direction. Distribution of spectrum is changed extensively when the solar radiation enters through atmosphere of planet earth. Intensity of sunlight is weakened by 30% because of scattering by dust particles and other molecules and adsorption by various gases like H2O (vapour), O3 or CO2.

The Air Mass (AM)

The degree of reduction is vastly variable due to continuous realignment of the sun and the related shifting of the path of radiation from atmosphere. These type of effects are simply described by representing an Air Mass number (AMm). The mass of air between the sun and surface that influences sun illumination intensity and distribution of solar radiation is known as Air Mass. It is a comparative path length of radiation through atmosphere in relation with zenith point (Fig.1.2); zenith point is defined as upward path length (vertically) at 900 and denoted as AM1. Therefore, AM1 is the distribution of sunlight and illumination intensity on earth with the sun directly overhead and going through atmosphere. AM0 is the distribution of radiation and illumination intensity outside the atmosphere. Air mass 0 means above the atmosphere of earth along with the zenith point. PV cells are treated at AM1.5, which is related to the sun at an angle of 48.2° from zenith point, with temperature 25°C. If an angle θ is given, with regarding to overhead position, the AM takes the value AMm and in this value the air mass number, m, is denoted as m = 1/cosθ, in this way measures the atmospheric path length corresponds to path length when the sun is absolutely overhead. AM2 is the sunlight when the sun is at angle 60.1° directly above horizon. The most extensively used standard is distribution of solar spectrum at 1.5AM (angle θ = 48.2°). This standard permits a significant comparison of various PV cells treated at various places.

The Photovoltaic cell (solar cell)

An illustration of a PV cell is a p-n junction semiconductor that converts sunlight straight into electrical energy by the PV effect (photovoltaic effect). Solar cells are considered as being photo sensitive irrespective to if the light source is solar illumination or an artificial source.

Working of a solar cell depends mainly on the following steps:

  1. Absorption of light photons and generation of charge carriers;
  2. Separation of electron-hole pairs (excitons);
  3. Transportation of carriers to respective electrodes of the PV cell and then extraction of charge carriers to external circuitry.

Origination of PV cells:

The growth in PV cell technology is in order to make inexpensive, high power conversion efficiency and environmentally stable PV cell which is excellent alternative for energy generation from conventional energy sources. Research is going on, in order to achieve these supreme goals which has guided to the inventions of novel materials and novel techniques in PV cells manufacturing. PV cells are classified into 3 main categories called as generations on the basis of their order of attendance in commerce.

First generation

The PV cells in this group are generally made up of Si (silicon) substrate. Bell Laboratories produced the very first Si PV cell in 1954 with power conversion efficiency (PCE) of 6%. Eventual after, research on enhancing the PCE and reducing the cost of PV cells has been immense. Si PV cells are the vastly utilized of all PV cells, these are the most efficient considering single crystalline solar cells in PV devices. Along with this, silicon is one of the most abundant substance on planet. This type of PV cell is the most extensively used solar cell with the highest PCE of 28%. There are 3 types of silicon, used in 1st generation of PV cells: Amorphous type silicon (a-Si), Single crystalline type silicon (c-Si) and multicrystalline type silicon (m-Si). Nevertheless, c-Si is not cost effective and also related with complex fabrication process. This has amplified research devotion into the 2nd generation PV cells.

Second generation

It’s also called as thin film PV cells. In an endeavour to lower the fabrication cost of the existing engineering based on Si, to enhance material usefulness, thin films have been the centre of immense research. Materials for 2nd generation had been evolved to decrease cost of production of PV cells without risking their output power. There are three types of materials which are emerged as excellent promising nominees for 2nd generation of PV cells namely amorphous silicon (hydrogenated) (a-Si:H), cadmium telluride (CdTe) and Copper Indium Gallium diselenide (CIGS). The highest registered PCEs of CdTe and CIGS single cells are 17% and 20% respectively. Even though, thin film PV cells have an emulative strand on 1st generation PV cells attributed to reduced costs and excellent efficiencies, they have few loopholes. Many material that 2nd generation cells are made up of are both becoming extremely rare and not so cost effective (In) or extremely toxic (Cd). For industrial production, these PV cells would also need new techniques, which would effectively increase the cost of production. Due to these inconveniencies, a novel generation of PV cells has been prompted.

Third generation

Third generation of solar cells combines new theories in their advancement. These theories are developed to resolve the challenges being faced by above generations of PV cells which are mainly high production cost of 1st generation & toxicity and limited material availability of 2nd generation. This novel generation combines quantum dot/organic/perovskite/polymer/hybrid Nano polymer/dye-sensitized (DSSC) PV cells. Organic PV cell (OPV), uses polymers as an inexpensive material alternative to inorganic semiconductors i.e. thin films. The 3rd generation is the most cost effective of all the other PV cell offspring. The PCEs obtained yet are 11% for DSSCs and 8% for polymer solar cells. This denotes that PCE of OPV is normally very inferior. Further, OPV is engineeringly underdeveloped technology and its extended applications are limited by various stability concerns that are related to its degradation in different ambient conditions. Therefore, OPV and DSSCs are relatively low to make these cells emulative in a mercantile business. With the continuous effort of engineers in PVs, a novel PV cell which is basically based on organic-inorganic hybrid photovoltaic technology called as perovskite solar cell (PSC) was discovered by Miyasaka at el. in 2006. This novel material consists the highest registered PCE of 19.3%. These hybrid photovoltaic cells like PSCs are categorized as such due to their absorber layer is basically made up of organometallic substance.

This work is centred exclusively on organic inorganic perovskites which also can be limited to two particular types known as FASnI3 (Formamidinium tin iodide) and LaVO3 (Lithium vanadium oxide).

Motivation

In today’s scenario, PSCs are considered as excellent candidates of hybrid photovoltaic cells attributed to their low temperature processing, strong light absorption, low non-radiative rate of recombination and low production cost, along with this their calibre to capitalize on over many years of increment of associated DSSCs and OPVs. Recent outcomes in literature indicate that vapour and solution deposition techniques are desired to produce better quality thin films and high PCE (η ≥ 15%) photovoltaic devices for Pb (Lead) based perovskites. Yet, the existence of Pb in the most efficient solar cells which employ CH3NH3PbI3 (methyl ammonium lead iodide) contains a serious issue to the continued acceptance of Pb based PSCs for commercial applications because of toxicity of Pb which is a vital component in perovskite. This has directed to an investigation for a non- toxic alternate for Pb. Tin (Sn) can be a feasible replacement for Pb so far as it is non-toxic and both elements are in same group in periodic table. Comparatively high absorbance of Tin based PSCs have showed that performance isn’t related to the existence of Pb and also point out higher PCEs for non-toxic and inexpensive photovoltaic devices. The maximum PCE of Tin based perovskites as given by literature is 6.4%.

Objectives of the research

Even lead free PSCs are non-toxic as compared to Pb based still they cannot be straightly used to subrogate Pb PSCs. This is because of their low PCE which makes tin based PSCs not as emulative as Pb based PSC devices. Hence, tin PSCs demand efficiency enhancement before their acceptability for utilization in industrial/domestic power generation. Therefore, the role of this work is to unearth ways in which the PCE of tin perovskite can be increased. A numerical simulation technique will be utilized to study the properties of tin PSCs that are contingent for their performance increment. Our study is directed by the following main objectives:

  • Produce a planar structure of a tin halide perovskite cell
  • Regenerate the experimental outcome based our developed structure
  • To calibrate the effect of interlayer characteristics on photovoltaic device
  • Enhance the PCE of the achieved result (Device optimization)

Research Methodology

The method used in this study is purely computational modelling. Obtained results will be analysed with the experimental ones that were published in previous literatures [25]. Efforts will be made-up to optimize thickness of photon absorber layer to enhance performance of device. The modelling is performed using simulation software called as solar cell Capacitance Simulator (SCAPS). It is a 1D simulation software produced at University of Gent, Belgium. It is used to foreshow the variations in PSC performances related to the inclusiveness of the absorber and the electron transporting material. The optimized value of absorber layer will be established. Because of very short diffusion length (30nm) of carriers (electrons and holes) of the perovskite layer as suggested by there is limit of recombination of carriers produced in active layer, if the perovskite thickness is greater than diffusion length. In that scenario, the carriers recombine prior to get extracted at respective electrode. This accounts loss of carries, therefore low power conversion efficiency. By lowering the thickness of perovskite layer one can reduce the undesirable recombination impact. Further, various cell parameters will be calculated. These combine the PCE (η), fill factor (FF), short circuit current density (Jsc) and the open circuit output voltage (Voc).

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