Physics of Renewable Energy Systems - Lecture 5 - Origin of Band Structure & Energy Band Gap (PDF)

EL PT PHYSICS OF RENEWABLE ENERGY SYSTEMS Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 Solar power N Lecture 5 : Origin of Band Structure and Energy Band Gap CONCEPTS COVERED EL Origin of band gap...

EL PT PHYSICS OF RENEWABLE ENERGY SYSTEMS Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 Solar power N Lecture 5 : Origin of Band Structure and Energy Band Gap CONCEPTS COVERED EL Origin of band gap PT Classification of solid materials Metal, Semiconductors and Insulators N KEY POINTS EL ØBasics of free and extended free electron model ØBrillouin zone ØEnergy band gap PT N  EL In the previous lectures, we saw… PT N  Edmond Becquerel discovered the photovoltaic effect in 1839. A major increment in the performance came in 1950s, by the studies in Bell laboratories, EL where G. Pearson, D. Chapin and C. Fuller, Photo - Ref.: https://en.wikipedia.org › wiki › Edmond_Becquerel using doped silicon reported a solar cell with In 1894 , probably, the 5.7% efficiency first true solar cell was Efficiency (~ 1%) reported by Charles Fritts. PT Efficiency (~ 1%) In 1875 , William Grylls Adam and Richard Evans Day, using selenide as a solid material, showed that light can be used to generate electricity. We are now in 2021! N Lot of work still needs to be done and are being done Photovoltaic effect !" Grid electrode n-type region The process occurs EL in or close to the depletion layer Depletion layer Load v The resulting electron is not ejected from the surface, but is promoted to a higher-lying energy electron-hole pair PT level in the semiconductor, which enables it to move freely through the semiconductor material. v A metallic grid forms one of the electrodes of the cell, and a metallic layer on the back of the solar cell forms the other electrode. Back electrode p-type region v An incident photon must have sufficient energy to generate an electron-hole pair, i.e. !" > $%. N v The band gap of a semiconductor is ~1 eV, e.g. $%=1.1 eV for silicon. So, it is clear that semiconductors are playing the critical role! EL Hence: Quick revision on the basics of PT Semiconductors is essential. N  Conductor, semiconductor and insulator If we analyze the materials, we see every solid contains electrons. According to the nature of band occupation by electrons, all solids can be broadly classified into two groups viz.; 1) The ones which have completely filled valance band overlapping with partially filled EL conduction band (Metals) and 2) The ones which have empty conduction band lying over completely filled valance band Depending on the width of forbidden band, the second group can be divided into: Insulator (band gap E" > 3 eV) Semiconductor (band gap 0.1 < E" ≤ 3 eV) PT N From Metal à Semiconductor to Insulators [Mostly explained in terms of electrical conductivities and energy band] EL We will have to understand the concepts of energy bands, band gaps and interaction of the conduction PT electron waves with the ion cores of the crystal. N Brief revision and understanding of the origin of energy bands in solids W e w ill u se th e u n d e rsta n d in g to : Ø D istin gu ish b e tw e e n m e ta ls, se m im e ta ls, se m ico n d u cto rs a n d EL in su la to rs ØTo e xp la in re la tio n b e tw e e n co n d u ctio n a n d va la n ce e le ctro n s This can be developed using Extended free electron model or nearly free electron model PT Assumptions: ØSubjected to periodic lattice ØPotential energy of electron is not constant N ØVarying potential has periodicity similar to crystal lattice. Atom a 1 D Lattice EL PT N 2 D Lattice Small crystal EL Bigger crystal No of PT atoms are N more Simple cubic Body Centered Cubic (BCC) EL PT Cross- BCC crystal section of BCC N Assumptions of free electron model Ø A metal crystal consists of positive metal ions whose valance electrons are free to move between the ions. The crystal is held together by electrostatic forces of attraction between the EL Ø positively charged ions and negatively charged electron gas. Ø The mutual repulsion between the electrons is ignored. Ø The potential field due to positive ions is completely uniform Ø At any given temperature, the velocity of electrons could be determined PT according to Maxwell-Boltzmann distribution law. N  1.0 a EL Probability 0 Distance from nucleus Orbital electron Nucleus PT N  + a Attractive force FA Attraction Force F O EL Repulsive force FA Interatomic separation r The force exerted by the atoms (or molecules) Repulsion r0 on each other is if the derivative of the NET energy EN Lennard Jones (6-12) potential, w.r.t. the distance between them. - ' () '+ " # = 4& − + # # PT Repulsion Repulsive energy ER Potential energy E Weakly Interatomic separation r attractive O Repulsive potential NET energy EN potential Attraction ! σ, ε are parameters, which correspond to bond energy and length, N - respectively. They are obtained by Attractive energy EA fitting to the known properties of the system. Energy bands in solids One-dimensional case: Ø In the one-dimensional case, the Schrödinger equation EL can be written as: !" # "&∗ 0 a 2a 3a + " [* − *,-. / ]# = 2 !$ " ћ Ø Kronig and Penney suggested a simple model PT of periodically varying potential energy. Ø They assumed that the potential energy of an electron had the shape of a periodic array of square wells. Ø The distance between successive wells was assumed to be a+b, where b is the width of the N well.  Energy bands in solids One-dimensional case: Ø The Schrödinger equation for the two regions can be written as: EL !" # "&∗ + " [* − ,]# = , For, , < $ < 0 Region-I 0 a 2a 3a !$ " ћ !" # "&∗ " + " [* − *, ]# = , For, 0 < $ < 0 + 1 Region-II !$ ћ Ø In the case of a linear lattice with the spacing a, b is small. The solution of the Schrödinger equation, PT i.e. the eigenfunction for a given k value, can be written as: # $ = 234564(5) where 64 is the periodic Bloch function 64 5 = 64(5 + 9) N Ø The corresponding eigenvalue in terms of k depends on the lattice constant. Ø Both the eigenvalues and the wavevectors are quantized for a free electron. Energy bands in solids (One-dimensional case) ØFor the values of k, which are given by the conditions: # k=" %&': " = ±*, ±,, … $ the Schrödinger equation has no unique solutions. For each of these k values EL there are two eigenvalues, separated by an energy gap. Ø The above condition is analogous to Bragg’s law, applied to X-ray diffraction in crystals. PT Ø The Kronig–Penney energy curve shows bands of allowed energy values separated by energy gaps i.e. regions of forbidden energy values. Ø The eigenvalues and the k numbers are quantized. Ø The broken curve is not continuous but consists of closely spaced points. N Energy bands in solids E vs k relationship: Ø The k zones of allowed energies are called Brillouin zones. EL Ø The widths of the energy bands increase with increasing energy. Ø The stronger the electron is bound to the lattice ions, the narrower will be the widths of the energy bands. PT Ø The band theory of solids is able to explain properties of solids where the Sommerfeld model fails. N E vs k relationship For finite crystal of length !, " #+! =" # &'(# + !)*+'(#,!) = &'(#)*+'# EL *+'! = - = *+./0./0 '= !.3 63 83 1=± ,± ,… ± 4 4 4 PT Reduced zone scheme Energy vs wave vector for one dimensional lattice Major inferences: Ø The motion of electron in a periodic lattice is characterize by the bands of allowed energy separated by forbidden regions. N Ø The width of allowed energy band increases with the increasing energy. The band structure of crystalline solids EL Energy Energy level PT Energy level of Energy level for very N of an isolated five closely large number of atom spaced atom closely spaced atom  Conductor, semiconductor and insulator If we analyze the materials, we see every solid contains electrons. According to the nature of band occupation by electrons, all solids can be broadly classified into two groups viz.; 1) The ones which have completely filled valance band overlapping with partially filled EL conduction band (Metals) and 2) The ones which have empty conduction band lying over completely filled valance band Depending on the width of forbitten band, the second group can be divided into: Insulator (band gap E" > 3 eV) Semiconductor (band gap 0.1 < E" ≤ 3 eV) PT N Fermi level in Metal, Semiconductor and Insulators EL Electron energy Band gap Overlap Fermi level Metal PT Semiconductor Insulator N  CONCLUSION EL Ø The origin of energy band gap in solids was explained. Ø Based on the value of energy band gap, the materials can PT be classified as metals, semiconductors and insulators. N  REFERENCES EL “Photoelectrochemical Solar Cells” by Suresh Chandra (Gordon and Breach Publishers, 1985). Ø Physics of semiconductors devices (2nd Edition) by S,M. Sze (Wiley) PT “Physics of Energy Sources” by George C. King “Advance Renewable Energy Systems” by S. C. Bhatia N N PT EL  EL PT PHYSICS OF RENEWABLE ENERGY SYSTEMS Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 Solar energy Lecture 6 : Basics of Semiconductors N CONCEPTS COVERED EL ØSemiconductors ØTypes of semiconductors PT Øp-n junction ØUse of p-n junction in solar cells N KEY POINTS EL ØFabrication of p-n junction ØCharacteristics of p-n junction PT ØOperation of solar cell (p-n junction type) N Semiconductor v Conductors and insulators are distinguished by the enormous differences in their electrical resistivities. EL v For example, the resistivity of quartz is ∼1025 times larger than the resistivity of copper. v With respect to resistivity, semiconductors are lying between insulators and conductors. PT v For example, the resistivity of the semiconductor silicon is about 1010 times larger than that for copper. v The reason for such enormous differences is the variation in the number of free electrons that can carry electric N current in these kinds of material Types of semiconductor Semiconductor EL Intrinsic Semiconductor Extrinsic Semiconductor Silicon PT Germanium n-Type p-Type (ne=nh=ni) Pentavalent impurity Trivalent impurity (P, As, Sb etc.) (Ga, B, In, Al etc.) N Intrinsic semiconductor v Intrinsic semiconductor: Pure elements, pure Si or Ge v At temperatures above absolute zero, such intrinsic semiconductors have a EL Si Si Si finite number of thermally generated electrons in the conduction band and holes in valence band. Si Si Si v The densities of these charge carriers are PT small because the thermal energy ∼kT of the electrons is small compared with the band gap energy Eg. Si Si Si N  n-type semiconductor Conduction band Conduction band Si Si Si EL Ed Spare Donor level Si Si P Si Electron Donor atom PT Valance band Valance band Si Si Si T= 0 K T= 300 K v Impurity atoms that donate spare electrons called donor atom. N v They convert a pure (intrinsic) semiconductor into an n-type (extrinsic) semiconductor.  p-type semiconductor Created hole Conduction band Conduction band EL Si Si Si Acceptor level Ea Si Al Si Acceptor PT atom Valance band Valance band Si Si Si T= 0 K T= 300 K v In order to complete this bond, the Al atom accepts an electron from one of its Si N neighbours, creating a hole. v Acceptor atom Al converts the intrinsic semiconductor into p-type extrinsic one. Extrinsic semiconductor p-type n-type EL PT v The figure shows isolated pieces of p-type and n-type semiconductor. v For the sake of clarity, only the acceptor and donor ions and the respective majority carriers are shown. v In the p-type, there is an abundance of holes, which form the majority carriers. v In the n-type, there is an abundance of electrons. v In general, the concentration of holes in the p-type will be different from N the concentration of electrons in the n-type. The Fermi energy in a p-n junction p-type n-type v The Fermi energy in a p-type EL semiconductor lies just above the top of the valence band. Conduction band Conduction band v In an n-type semiconductor, Ecp Ecn the Fermi energy lies just PT EFn below the bottom of the conduction band. EFp Evp Evn Valance band Valance band N Device fabrication technology for p-type and n-type technology Type 1 Alloyed junction EL Type 2 Diffused mesa junction Type 3 Diffused planar junction on epitaxial substrate PT Type 4 Ion Implantation N Alloyed junction Al Molten metal Can act as ohmic contact EL Al p Cooling PT N-type Si n n Heat ~580 °C N Ohmic contact Diffused mesa junction (developed in 1956)Gives more control of the impurity profile; uses p-type impurities such as boron Diffusion P-n junction Contact EL formation making and cutting p-type p-type PT N-type Si N-type Si N-type Si Contact N Diffused planar junction on epitaxial substrate Gives more control over the lateral geometry of the diffused junction; uses insulating layer which prevents donor and acceptor impurities to diffuse through; utilized the technique of lithography. Heating at ~900- EL Diffusion Contact formation 1300 °C in presence of O2 SiO2 insulating p layer PT n n n N+ N+ N+ Doped semiconductor N  (gives most precise control of an impurity profile; can be done at room temperature; implantation induced Ion Implantation damages can be removed by annealing at higher temperatures; can work at lower temperatures than the diffusion process) EL Ion implantation Annealing and metallization p PT n n p N+ N+ N The p-n junction in equilibrium v Diffusion of electrons and holes across the Diffusion current, idiff junction and subsequent recombination Drift current, idrift produce a depletion layer that is devoid of mobile charge carriers. EL v The double layer of charge causes an electric field ε to be set up across the junction. p-type Depletion region n-type v At equilibrium, there must be no net current flow across a p-n junction. PT v The drift current is counter-balanced by diffusion current. Potential Vo, Contact Electric field potential ε N Distance The p-n junction with reverse bias V v The effect of the bias voltage V is to push the majority charge carriers Diffusion current, idiff away from the junction, increasing Drift current, idrift EL the width of the depletion region. v The bias voltage V drops across the high-resistance depletion layer, increasing the height of the potential p-type Depletion region n-type PT barrier from V0 to (V0 + V). V0 Potential v The diffusion current (V0+V) essentially reduces to zero. N Distance The p-n junction with forward bias V v The effect of the bias voltage V is to Diffusion current, idiff push the majority carriers toward the EL Drift current, idrift junction, reducing the width of the depletion region. v The height of the potential barrier is reduced to (V0-V). p-type Depletion n-type PT region v The diffusion current V0 increases, and becomes Potential larger than the drift current. (V0 -V) N Distance The current voltage characteristics of a p-n junction v As the voltage V is increased in the forward direction, corresponding to forward bias, the Current, i current i increases rapidly. EL v However, when the voltage V is increased in the reverse direction, corresponding to reverse bias, the reverse saturation current i0 remains essentially constant. PT i0 v The dependence of current i on bias voltage V is: Bias voltage, V %'& != !# (% () − +) N T is the absolute temperature and Reverse bias Forward bias k is the Boltzmann constant. The Fermi energy in a p-n junction v Fermi energy is constant throughout the semiconductor p-type n-type in an unbiased p-n junction. EL v There is a step change, eV0 , in energy between the n and p regions. Depletion region Ecp v This inhibits electrons eV0 PT Ecn from the n region EF EF diffusing into the p region. Evp Evn N The Fermi energy in a forward biased p-n junction v Energy difference is decreased by eV. V v It becomes easier for the electrons in the n region to EL p-type n-type diffuse into the p region and vice versa for the holes. v The Fermi level is not constant and PT Depletion region displaced by the Ecp magnitude of bias e(V0 - V) Ecn voltage V. EFp EFn Evp N Evn The Fermi energy in a reversed biased p-n junction V v The energy bands in the n-type are lowered relative to the p- type. EL p-type n-type v Electrons from the n region and holes from the p region have an even higher barrier to climb. Depletion region v Relatively few of these PT Ecp e(V0 + V) majority carriers diffuse across the junction. Ecn EFp Evp EFn N Evn Electron and hole concentration in a semiconductor v In order to describe the resulting electron (and hole) densities or concentrations in the energy bands, we need to know the following: i. the density of states in the bands EL ii. the probability of each of these states being occupied v The first factor is given by the density of states function, Z(E). - Defined as the number of energy states per unit energy per unit volume. " ! PT # = %& 1 #) #' #' < #( < #) #( N 0 !" ! Electron and hole concentration in a semiconductor v For the conduction band, the function Z(E) is given by: $% ' / ! " = ' ()*∗+).) ".) & EL where h is Planck’s constant, the energy E is measured from the bottom of the conduction band, and me is the effective mass of the electrons. v For fermions, the probability of a particular level at energy E being occupied is given by the Fermi- Dirac distribution: PT / 0 " = ("2"0 ). /++ 34 v If, (" − "0 ) ≫ 74, ("2"0 ). 0 " ≈ + 74 N  Electron and hole concentration in a semiconductor v For the concentration n of electrons in the conduction band, we can write ! = # $ % & % '% EL conduction band v Evaluation of the integral gives ! = () *,(%),%&)⁄/0 ∗ 76 2*/0 1 where, () = 1 14ℏ1 PT v Similarly, to determine the concentration of holes in valance band, we evaluate the integral: ! = # $ % [9 − & % ]'% valance band which gives < = (=*,(%&,%>)⁄/0 76 N 2∗? /0 1 where, (= = 1 14ℏ1 Electron and hole concentration in a semiconductor v The product of the electron and hole concentrations is given by !" = $% &((*%(*+)⁄-.×$0&((*+(*1)⁄-. EL = $% $0&(*2 ⁄-. [since, *2= *% - *0] v The product of n and p is fixed at constant temperature. v Also applicable for extrinsic semiconductor. v Thus, if we dope an intrinsic semiconductor with acceptor atoms to increase the concentration of holes, the concentration of electrons must decrease accordingly. PT v For the particular case of an intrinsic semiconductor: 54 (*24 !3 = ! = " = !" = ($% $0) 6 & 6-. (*2 4 !3 ∝ & 6-. N !" = !63 Electron and hole concentration in a semiconductor v The two shaded areas are the same, reflecting the fact Conduction E Z(E)F(E) that number of electrons in band the conduction band is EL Area = Electron equal to the number of concentration holes in the valence band in Ec intrinsic semiconductor. EF PT Ev Valance band Z(E)[1-F(E)] 1-F(E) Area = Hole concentration N Z(E) F(E) Carrier concentration Photon absorption at a p–n junction Conduction band v When a photon is incident upon a semiconductor material, an electron may be promoted from the EL valence band to the conduction band if the photon has an energy !" that is greater than the band gap Energy band Eg. !" gap, Eg v !" > $% PT v In terms of wavelength & !( Valance band &< = &( $% N Photon absorption at a p–n junction v The movement of electrons and holes across the junction results in a flow of electron Direction of electron flow current in an external circuit. !" v Electron-hole pairs that are EL p-type n-type generated within a diffusion length or so of the depletion layer may also contribute to the flow of charge carriers Depletion region across the junction. PT Ecp Ecn EF EF Evp N Evn  Si-solar cell operation and real cells el n Solar energy pa eal EL R Anti reflecting coating Thin film solar cell n-layer PT pn-junction P-layer Polycrystalline Back electrode as current collector Hole Mono N crystalline Electron CONCLUSION EL Ø Basics of semiconductors was discussed Ø How a p-n junction is formed must be clear now Ø The operation of p-n junction diode and its use in PT solar cells was also presented N  REFERENCES EL “Photoelectrochemical Solar Cells” by Suresh Chandra (Gordon and Breach Publishers, 1985). Ø Physics of semiconductors devices (2nd Edition) by S,M. Sze (Wiley) PT “Physics of Energy Sources” by George C. King “Advance Renewable Energy Systems” by S. C. Bhatia N N PT EL  EL PT PHYSICS OF RENEWABLE ENERGY SYSTEM Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 : Solar energy N Lecture 7 : Basics of Semiconductors CONCEPTS COVERED EL Ø Fabrication of Si Solar Cells Ø Mathematical formulation to characterize solar cells PT Ø Types of solar cells (based on materials) Ø Future direction N KEY POINTS EL Ø What is screen printing? Ø Various steps involved in fabrication of conventional solar cells PT Ø Calculation of solar cell parameters Ø Uses Ø Limitations N In the previous lectures, we have seen… p- and n-type semiconductors EL PT N © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 2019-20  p-n Semiconductor Energy Band Diagram Forward bias p-n junction energy band diagram EL p n Built in potential (eVbi) (a) PT (b) (a) Energy level of p and n-type semiconductors and N (b) band bending due to formation of p-n junction at equilibrium © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 2019-20 p-n diode under illumination EL PT N © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 2019-20  Complete Silicon Solar Cells EL Generation (m-3) PT p Thickness of the layer (µm) N-type layer thickness ~ < 100 micron N P-type ~300-500 micron Antireflection coating Fingers and bus bar to collect the charge carriers © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 2019-20 Steps involved during fabrications Grid electrode v Constructed on a thin single crystal of silicon. Anti-reflection film v Form a p-n junction. n-type Load v The thickness (~1 μm) of the n-type region is much EL less than the thickness (~300 μm) of the p-type region. v The n-type is more heavily doped. p-type v Typically a cell may be ~10 cm × 10 cm. Back electrode v A grid of wires and the semiconductor surface to PT provide electrical contact to the cell. v Light collection can be enhanced by texturing the front surface of the solar cell by a chemical etching N process. Before using the substrate, it is refined using the following steps” v Silica (SiO2) is reduced (oxygen removed) through a reaction with carbon in the form of coal, charcoal, and heating to 1500-2000 oC in an electrode arc furnace. !"#$ + & → !" + &#$ EL Result: 98% pure Silicon with slight amount of Fe, Al, and B. v What next? Further purification is required: !" + 3)&* → !")&*+ + )$ Result: Formation of halides viz., ,-&*+,.*&*+, /&*+. PT v !")&*+ has a low boiling point of 31.8 oC v Distillation process can be used to separate it from impurities. v Finally, the pure !")&*+ is reacted with hydrogen at 1100 oC for ~200-300 h, to produce a very pure form of silicon. !")&*+ + )$ → !" + 3)&* N Screen printing of Si solar cell v It’s a relatively simple process where a mesh is used to transfer ink onto a substrate. v The mesh structures prevents transfer of ink into the areas made impermeable by the EL blocking stencil. v A blade is moved across the screen to fill the open mesh apertures with ink. v A reverse stroke then causes the screen to PT - touch the substrate momentarily along a line of contact. - Ink once again wets the substrate. - Extra ink is pulled out of the mesh apertures N  The reactions take place inside large vacuum chambers. EL v v Silicon used for solar cell can be single crystalline, multi- crystalline, polycrystalline, or amorphous. PT N Steps for screen printing of Si solar cell Junction formation by doping: v Heating the wafer in a furnace (800-1000 oC). Phosphorous EL atoms producing v The phosphorous atmosphere causes a small p-type Bulk n-type layer amount of phosphorous to be incorporated in the outer layers of silicon. PT Screen printing the rear contact: v Rear of cell means that the wafer is upside down. v The screen printed rear contact is dried in an oven to drive off the organic solvents and binders. N Steps for screen printing of Si solar cell Firing the rear contact: v The cell is placed in a second furnace at a much higher temperature to fire the metal contact onto EL contact with the silicon. v The firing process destroys the rear n-layer so that the metal makes contact with the p-type bulk. v The cell is flipped over for printing on the front. Printing the front contact: PT v The front contact is printed in a similar manner to the rear contact. v A pattern of the lined is used to prevent shading of the cell. v A furnace heats the cell to a high temperature to fire the metal paste into the silicon. N v The finished cell is ready for encapsulation onto a module. Connecting the solar cells EL PT p N Drawbacks in conventional solar technology Low efficiency EL v Conventional solar cells can only achieve limited efficiencies. Introduction of nanotechnology might be able to PT increase the efficiency of solar cells. BUT Expensive to manufacturing N Advantages of nanotechnology in solar cell v Nanomaterials can be tuned for absorbing the photons. EL v Improved efficiency. v More flexibility in manufacturing. PT N Different from of nanomaterials used for solar cell EL PT Nanowires1 Nanotubes2 Nanocones3 Nanopillar array4 References: doi: 1. 10.1063/1.4916535 N 2. 10.1016/j.tsf.2006.10.056 3. 10.1007/s11671-010-9701-3 4. 10.1186/s11671-015-0891-6 Different from of nanomaterials used for solar cell EL PT Nanopagoda1 Nanocomb2 Nanorods3 Nanobelt4 References: doi: 1. 10.1021/cg801172h N 2. 10.1007/BF03010446 3. 10.1016/j.solener.2009.12.013 4. 10.1016/j.electacta.2009.07.065 Perovskite solar cells EL PT -2.23 e- -3.93 -4.0 CB hν HOMO VB -5.22 -7.3 -5.43 N h+ TiO2 CH3NH3PbI3 Spiro-OMeTAD  Perovskite Device The most studied method is Various fabrication Techniques spin coating as a one step 1. Spin coating process. The other method EL is sequential deposition 2. Spray coating 3. Screen printing 4. Thermal evaporation PT 5. Sputtering 6. Gas phase deposition N  One step process (a) MAI + PbI2 in (b) DMF EL Solvent used in these process: DMF (Dimethylformamide, (CH₃)₂NCH) PT Stoichiometric ratio of lead iodide and methylammonium iodide N DMSO (imethylsulfoxide, (CH₃)₂SO) Solution were vigorously stirred at 500-800 rpm. GBL (γ-Butyrolactone, C4H6O2) ACN (Acetonitrile, CH ₃CN) © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 20-2021  Two step process ITO/TiO2 substrates EL PT HTL N Sequential deposition method © IIT Kharagpur, Energy Materials ES60004: Trilok Singh Autumn Semester 2019-20  The other cations are Formamidinium (FA) Caesium (Cs) EL Rubidium (Rb) The other Anions are Chlorine PT Bromine Iodine And their combination in the precursor N N PT EL How to characterize a solar cell? Types of solar simulator Solar simulator: Newport EL Continuous or steady-state PT Pulsed simulator N  Photovoltaic Classifications 1st Generation d > 100 µm 4th Generation?? (Efficiency ~ 25% (±)) (Efficiency ~ 28% (±)) EL Crystalline and d < 1 µm Polycrystalline silicon Perovskite Solar cells 2nd Generation d> 1 µm PT (Efficiency ~ 22% (±)) 3rd Generation Amorphous silicon d < 1 µm for organic solar CdTe, CdS, GaAs cells CIGS d > 1 µm for DSSC (Efficiency ~ 12 % (±)) N * CIGS (Cu-In-Ga-Diselenide) * DSSC (Dye sensitized solar cells) CONCLUSION EL ØFabrication processes to obtain different types of solar cells was discussed. PT ØThe important of materials development was also introduced. N REFERENCES EL Lecture Notes, Prof. Trilok Singh, IIT Kharagpur (with due permission and acknowledgements) Papers mentioned in the slides “Photoelectrochemical Solar Cells” by Suresh Chandra (Gordon and Breach Publishers, 1985). PT Ø Physics of semiconductors devices (2nd Edition) by S,M. Sze (Wiley) “Physics of Energy Sources” by George C. King “Advance Renewable Energy Systems” by S. C. Bhatia N N PT EL  EL PT PHYSICS OF RENEWABLE ENERGY SYSTEM Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 : Solar energy N Lecture 8 : Characterization of Solar Cells and Future Direction CONCEPTS COVERED EL ØMathematical formulation to characterize solar cells ØTypes of solar cells based on materials PT ØFuture direction N KEY POINTS EL ØCalculation of solar cell parameters ØVarious types of solar cells PT ØFuture of solar cell technology N  In the previous class, we saw… EL PT p Generation (m-3) N-type layer thickness ~ < 100 micron N P-type ~300-500 micron Antireflection coating Fingers and bus bar to collect the charge carriers Thickness of the layer (µm) © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 2019-20 Solar cell equation V v The drift current idrift due to minority charge carriers that are Load thermally generated. v The diffusion current idiff due to the diffusion of majority icell EL charge carriers across the junction, opposite to drift current. v For an illuminated p–n junction, photocurrent iphoto due to the p n electron–hole pairs generated by the incident photons. !"#$ = !&!'' − !&)!'$ − !*+,$, A solar cell is conventionally considered to be PT #0/ a battery that delivers a current icell to an !"#$ = !- (# 12 − 3) − !*+,$, external load that becomes more positive as #0/ iphoto increases, defining 56788 = −597: !;#> 1 (below 100 mV) (mA) PT (Voltage) I-V curve in the first quadrant N Solar cell equation v A plot of solar cell current icell Vs. cell voltage V is shown as icell the solid curve in the Figure. v The shaded area is the EL quadrant in which the cell generates electrical power. v As the illumination of the cell increases; the icell–V curve moves upward as indicated by PT Increasing illumination the dashed curves. V N  !&'()( !"#$$ #.- v Considering, ! = !+ (# /0 − 1) ! We can write, !"#$$ = !&'()( − ! EL. LOAD If,. = + , the cell current flowing in the external circuit is !&'()(. 5677#8) 3!(4# Substituting for !&'()( = !9" 9(67"# PT #.- !"#$$ = !9" − !+ (# /0 − 1) The equivalent circuit Solar cell equation N  icell isc Maximum power delivery from a solar cell imp For open circuit solar cell, !"#$$ = &, we obtain, !'" + ) = ##+,"⁄-. !& EL V Usually, !'" ≫ !& , therefore P -. !'" Pmp +," ≈ $1 # !& The power P delivered by the solar cell is equal to the PT product of the cell current !"#$$ and the cell voltage +. V Substituting the value of !"#$$ Vmp Voc 2 = +!'" − +!& ##+⁄-. − ) The values of current and voltage that together provide the maximum power are called N 456 and 756 , respectively. Maximum power delivery from a solar cell For maximum power, "#⁄"$ = & '& +$ +$⁄,- '() − '& ++$⁄,- −. − + =& ,- +$ EL [As, +$⁄,- ≫.] '() − '& ++$⁄,-. + =& ,- +$ ⁄,- +$⁄,- +$ [substituting,'() = '& ++$1)⁄,-] '& + 1) = '& +.+ ,- ,- +$ PT [rearranging] $ = $1) − 23. + = $45 + ,- ,- +$1) $45 ≈ $1) − 23. + Final result: + ,- ,- '45 ≈ '().− N Similarly, +$45 The ratio of the maximum power !"# and the product $%& '(& is called the fill factor, FF EL !"# $"#'"# )) = = $%& '(& $%& '(& In practice, fill factors are ~80%. The ratio of the maximum electrical power !"# to the incident solar power !% is defined as the PT power conversion efficiency η The efficiency is generally measured under AM1.5 conditions !"# ))×$%& '(& at standard conditions (temperature += = 25oC, 100 mW/cm2) !% !% N For commercial solar cells, η ~ 20%. How to characterize a solar cell? Types of solar simulator Solar simulator: Newport EL Continuous or steady-state PT Pulsed simulator N  EL Few, more recent, types of Solar cells PT N  Mimicking the Nature EL PT N © IIT Kharagpur, Solar Photovoltaics ES61002: Trilok Singh Spring Semester 20-2021  DSSC: Mimicking the Nature e- DSSC: Dye-sensitized Solar Cells e- e- LUMO e- e- e- Light Dye Electrolyte Dye-sensitized EL e- TiO2 Solar Cells HOMO E PT e- Energy transfer in DSSC e Iodide ion gets oxidized - e- I- / I3- 3I- I3- 2e- + I3- + 2e- 3I- hv N FTO TiO2 Dye Electrolyte Back electrode  Record Efficiency EL Co-sensitization of two organic PT dyes; ADEKA-1 and LEG4 Cobalt-phenantroline as redox couple Top efficiency: 14.3% N  Disadvantages The most important issue of the dye-sensitized cells is the stability over the time and the temperature range, which occurs under outdoor conditions. Although it has been shown, that intrinsic degradation can considerably be reduced, but the EL behavior of the liquid electrolyte under extreme conditions is still unknown. For a successful commercialization of these cells, the encapsulation/ sealing, the coloration and the electrolyte filling has to be transferred into fully automated lines including the final closure of the PT filling openings. Therefore, a significant effort is taken in order to replace the liquid electrolyte by a gel electrolyte, a solid-state electrolyte or a p-type conducting polymer material. The suitable redox couple is still not optimally used/discovered N Perovskite solar cells EL PT -2.23 e- -3.93 -4.0 CB hν HOMO VB -5.22 -7.3 -5.43 N h+ TiO2 CH3NH3PbI3 Spiro-OMeTAD  Perovskite Perovskite is a calcium titanium oxide, with the chemical formula CaTiO3 (ABO3) The mineral was discovered in the Ural Mountains of Russia by Gustav Rose in 1839 and is named after Russian mineralogist: Count Lev Alekseevich Perovski (1992-1856) EL All minerals with the same crystal structure as CaTiO3 namely ABX3, are termed perovskites A= organic/Inorganic cations B= Inorganic cation X= halide (Cl, Br or I anions) PT A B X N O2- Ca2+ Ti4+ Perovskite: MPbX3 M Methylammonium (CH3NH3)+ EL M+ Formamidinium (CH3NH3)+ Pb2+ X X- PT Br, Cl, I N  First Article on Lead Halide Perovskite: 1893 A site ; CH3NH3+, Cs+ CH3NH3PbBr3 CH3NH3PbI3 Über die Cäsium- und Kalium-Bleihalogenide HC(NH2)2+ About the; Caesium B site Pb2+ or Sn2+and Potassium Lead halides EL X site ; I－, Br －, Cl － CH3NH3SnBr3 CsSnI3 H.L. Wells, Zeitschrift für anorganische und allgemeine Chemie, 3, 1893, 195 PT CH3NH3PbX3 (X=Cl, Br, I) CH3NH3SnBr3-xClx N K. Tanaka et. al Solid State Commun. 127, 2003, 619, Yamada et al. Bull. Chem. Soc. Jpn. 84, 2011, 926  Spectral Response Optical response of various solar cell materials Perovskite 22-25% 22-25% EL Perovskite GaAs GaAs 28-30% > 28% Solar spectral irradiance(W/m2nm) Relative spectral response CIGS PT N Wavelength (nm)  Application of Perovskite Lead and Lead free perovskite Water Splitting as a voltage Solar Cell Resistive luminescence supplier EL switching ReRAM Solar Cell application X-ray Perovskite detectors Tandem Solar radiation detector space Tandem solar cells Cell with Si, CIGS Good As a voltage Luminescence PT absorber for supplier for OPV Water Perovskite splitting multi- Resistive random access functionality memory (ReRAM) Many more N possibilities ?????? New Discoveries???  Device Configurations (n-i-p) ff eo ad Planar-Configuration Meso-Configuration Tr Absorption of light by perovskite EL Creation of e-h pairs Efficient separation of e-h Collection of e and h at respective electrode PT N  Device Configuration 25.2% KRICR/MIT low-temperature processes < 150oC (0.09 cm2) Power conversion efficiency / % EL 21.2% (0.25 cm2) 18.0% (1 cm2) 18.5% Planar18.3 %, TiOx (0.25 mm2) Mesoporous Planar configuration configuration PT 10.9% Toin & Oxford 18% 9% SKKU 6.5% (liquid junction) SKKU 2.2% (liquid junction) 3.8% (liquid junction) Toin Toin 0.4% (solid-state) Toin N 2019 Date of report (year) REFERENCES EL Lecture Notes, Prof. Trilok Singh, IIT Kharagpur (with due permission and acknowledgements) Papers mentioned in the slides PT “Photoelectrochemical Solar Cells” by Suresh Chandra (Gordon and Breach Publishers, 1985). Ø Physics of semiconductors devices (2nd Edition) by S,M. Sze (Wiley) “Physics of Energy Sources” by George C. King “Advance Renewable Energy Systems” by S. C. Bhatia N CONCLUSION EL Ø Evaluation of solar cell performance PT Ø Various parameters used to compare solar cells Ø Next generation solar cells Ø Importance of materials development N N PT EL  EL PT PHYSICS OF RENEWABLE ENERGY SYSTEM Prof. AMREESH CHANDRA DEPARTMENT OF PHYSICS, IIT KHARAGPUR Module 2 Solar energy N Lecture 9 : Solar Heaters  During the last few lectures, we have focussed our discussions on ‘Solar Cells’ 1st Generation d > 100 µm 4th Generation?? (Efficiency ~ 25% (±)) (Efficiency ~ 28% (±)) EL Crystalline and d < 1 µm Polycrystalline silicon Perovskite Solar cells 2nd Generation d> 1 µm PT (Efficiency ~ 22% (±)) 3rd Generation Amorphous silicon d < 1 µm for organic solar CdTe, CdS, GaAs cells CIGS d > 1 µm for DSSC (Efficiency ~ 12 % (±)) N * CIGS (Cu-In-Ga-Diselenide) * DSSC (Dye sensitized solar cells)  EL Let us now look at few other solar based devices… PT 1.0. Solar heaters N CONCEPTS COVERED EL Solar water heater PT Heat transfer processes Heat losses due to thermal conduction and convection N KEY POINTS EL The Selective Absorption of Radiation Process The Flat Plate Water heater PT Vacuum tube collectors Evacuated Tube collector Thermal Tube Collector N Maybe, you will like to design a solar water heaters for your house!  Solar heaters (SHs) v They collects the Sun’s radiation and transfers the absorbed energy, usually to a fluid. EL v Based on the temperature upto which they can heat a fluid, SHs are classified as can be classified: a) the solar water heaters that are seen on rooftops. (Range upto which they can heat the water: several tens of PT degrees above ambient temperature). b) solar thermal power systems. (These are able to attain much higher temperature). N  Basic idea used: Collect solar radiation over a large area using EL mirrors and, subsequently, concentrate the radiation into a much smaller area. PT N  The Flat Plate Water Heater v Area ~ few square meters. EL v Solar radiation captured by - flat plate collector. v Both direct and diffused solar radiation can be used. v What happens to the captured solar radiation? - It is transferred to water flowing through the copper pipes. v An electric pump is used to circulate the water between the flat plate heater and PT the insulated water tank. q What does the water tank do? - Stores heated water. - Its insulation prevents interaction with cool N air that can lead to loss of energy.  v The plate is mounted on a slab, which Schematic of solar water heater So is fabricated using a thermally lar insulating material. This helps to reduce loss due to conduction. Ra v The double glazed cover shelters the dia EL flat plate from losses due to wind tio convection. n le t out r in te r a te a w PT dw ld e a te Co H ppe r pipe work C o N Flat plate Side view of a flat plate heater t ion Double- dia a glazed cover la rR So EL Copper pipework Hot water storage Flat plate PT Slab with thermal N insulation The Flat Plate Water heater EL v Overall efficiency depends on several factors such as: incident solar power, heat loss processes, materials used, size, type of water used, etc. PT v However, under typical operating conditions, the overall efficiency is ~ 40-60%. N Heat transfer processes The plate can loose thermal energy to its surroundings by the following process : EL Radiation PT Conduction Convection N  Quick Revision… The Selective Absorption of Radiation Process v Radiation losses can be minimized by having a heater plate with a selective absorption EL surface. v What is known from earlier lectures? Solar spectrum peaks at a wavelength ~ 0.5μm. n ot e is se PT ur c au - Generally, the heater plate emits radiation in g v at fi le be much the far infrared, peaking at ~ 10 μm (see e th sca is not act rum adjacent blackbody spectra for 5800 and 350 se e x e ct e r a o p g Ple wn t n’s s h lar K, corresponding to temp. of Sun and a heater dra x. Su mu c plate). ma N vSo there is no overlap between the two spectra”. The Selective Absorption of Radiation Process Possible wish list: v The surface of the heater plate has absorption factor, which is nearly unity, in the region of solar spectrum. EL v The material has a spectral emissivity value close to zero, above 2.5 μm. Good news: v Some semiconductors have absorption/ emission characteristics that resembles this desired behaviour. PT v Some metal also exhibit an increase in absorption at shorter wavelengths and have low emission at long wavelengths. Therefore, a metal surface, coated with a semiconductor provides an ideal combination for a selective-absorption material. N Heat losses by thermal conduction and convection v Losses from thermal conduction is minimized by mounting the plate on a layer of thermally insulating material, such as polystyrene. EL v During the designing of thermal insulating systems, the concept of ‘thermal resistance (R)’ is used. (%& − %( ) + !=# where *= * , PT v Thermal conductivity, , depends only of the type of material. v R depends on the thickness of the material. R has the units of m2K W-1 v If two slabs of insulating material of area A and width L, with their own thermal conductance, are connected in parallel. Then the total heat flow: N Htot = H1 + H2 Heat losses by thermal conduction and convection v Convection is a complex process and there is no simple equation to describe it. EL v In the case, which we are discussing, it has been found that the convection is proportional to the surface area. v Further, it is also approx. proportional to the temperature difference PT between the surface and the main body of the fluid. Heat losses due to convection and conduction are eliminated in a vacuum tube collector. N Vacuum tube collectors v This has two glass tubes, which are held apart, with the space between them is evacuated. EL v Therefore, there is negligible thermal convection or conduction between the two glass tubes. v The outer surface of the inner glass tube is coated with a material with selective absorption. v The inner glass tube contains water and is connected to a water storage tank. PT v Typically, the water heater would have about 20 tubes, each 1-2 m long. v The efficiency is usually higher than that of a flat plate water heater of the same collecting area, and may be ~ 80%. N  Glass tube Storage tank Vacuum Heated water EL ~60 mm PT Selective absorption coating N ~1-2 m Vacuum tube collectors If the performance of the previous design has to be improved then the “efficiency” of heat-energy transfer to the fluid will have to be improved. EL What can be done? v Increase the contact area. PT v Use solar trackers so that radiation falls maximum duration. v Use thin transparent exterior glass/ cladding layer. v Increase the time available for the fluid to get heated. v Improve the storage tank efficiency, both in terms of storage and transmission. N v The distance between storage tank and heater collector system is minimal. Evacuated Tube collector Aim: To improve the interaction area of water/ fluid with the heating element EL v Inside the each glass tube, a flat or curved aluminum or copper fin is attached to a metal heat pipe running through the inner tube. v The fin is covered with a selective coating. PT Ref: http://www.alternative-energy-tutorials.com/solar- hot-water/evacuated-tube-collector.html v This sealed copper heat pipe transfers the solar heat via convection of its internal heat transfer fluid to a “hot bulb” that indirectly heats a copper within the header tank. v These copper pipes are all connected to a common manifold, which is then connected N to a storage tank.  Outer Selective Evacuated Tube collector Bracket with non condensable absorbent glass coating Absorber v It consists of a number of rows of parallel transparent cylindrical glass U tube EL tubes connected to a header pipe. These borosilicate glasses are strong, resistant to high temperatures and have a high Vacuum jet Copper fin transmittance for solar irradiation. PT Outer glass tube v Therefore, the angle of the sunlight Selective absorbing coating is always perpendicular to the heat Absorber tube absorbing tubes, which enables Copper fin these collectors to perform well Vacuum jacket even when sunlight is low. U tube N Glass ETC with U-tube (A) illustration of Ref.:https://www.sciencedirect.com/topics/engineering/evacuated- tube-collector the glass evacuated tube; (B) cross-section Evacuated Tube collector v Each individual tube varies in diameter from between 1" (25mm) to 3" (75mm) and between 5′ (1500mm) to 8′ (2400mm) in length. EL v Each tube consists of a thick glass outer tube and a thinner glass inner tube (called a “twin-glass tube”) or a “thermos-flask tube”, which is covered with a special coating that absorbs solar energy but inhibits heat loss. v There is vacuum between the two glasses. This vacuum acts as an insulator reducing any heat PT loss significantly to the surrounding atmosphere either through convection or radiation. v The water within the tubes is NOT directly heated. v There are a few different vacuum tube configurations: single wall tube, double wall tube, direct flow or heat pipe, and these differences can determine how the N fluid is circulated around the solar hot water panel. Evacuated Tube collector Advantages v More effective for domestic space heating, especially in areas where it is often cloudy. EL v Are overall more modern and efficient, in comparison to the standard flat plate collectors. v Due to the vacuum inside the glass tube, the total efficiency in all areas is higher and there is a better performance even when the sun is not at an optimum angle. PT Limitations v The panel can be a lot more expensive compared to standard flat plate collectors or solar batch collectors. N v Can be bulky and may require lot more space. v Corrosion affects remains an issue. Thermal Tube Collectors v The functioning is very similar to that discussed in the previous case. EL v The major difference comes from the design, which is slightly modified. v Instead of a U-tube, a thermal tube, is used. PT v The tube is filled with a fluidic substance, which vaporises under the influence of solar radiation. Therefore the choice of HTF (heat transfer fluid) becomes important. v The produced vapors rise to the top of the tube called, Ref.:https://www.sciencedirect.com/topi N which is called as condenser. This yields the latent heat cs/engineering/evacuated-tube-collector of condensation to HTF. Thermal Tube Collectors v The cooler liquid falls back into the thermal tube and the process restarts. v During the repeated process, the heat can be extracted and transferred to another fluid, which is EL to be stored in the tank. v The heat transfer fluids (HTF) should be able to operate upto high temperatures and have low freezing point. PT v Further, it will depend on the temperature range, which we are envisaging for the operation of TTC. Another wish list: low cost, cycling stability, long term shelf life, environmental friendliness, low corrosion effects and optimum viscosity. N Commonly used HTFs : their own advantages and disadvantages. Compressed Hydrocarbon Water Molten salts Silicones air oils EL (Synthetic, paraffin, ü Its availability ü Its availability aromatic oils, etc.) ü High boiling ü Low freezing ü Low cost ü Free of cost point point ü Leakage does ü Low viscosity ü Some are ü Nonflammable ü A very high not lead to inexpensive boiling point ü Non toxicity ü Abundantly pollution others are not! ü Noncorrosive ü High specific available ü Low freezing ü Long lasting PT ü It will freeze or heat ü Low vapor point X High viscosity boil X Low boiling pressure X High viscosity X Low heat ü Non corrosive point and high ü Pollution free X More pump capacities X Very low heat freezing point X High freezing energy needed, X Energy cost of capacity X PH need to be point pump X Low density hence increase controlled X Lower specific X Leak easily X High pressure in operational X Minor heat than through make the cost N deposition on water microscopic design more p