Introduction. Katarzyna Skorupska. Silicon will be used as the model material however presented knowledge applies to other semiconducting materials

Transcription

1 Introduction Katarzyna Skorupska Silicon will be used as the model material however presented knowledge applies to other semiconducting materials

2 2 June 26 Intrinsic and Doped Semiconductors 3 July 3 Optical Properties of Semiconductors 4 July 10 Semiconductor/Metal Schottky type junctions and Semiconductor/ Semiconductor Homojunctions 5 July 17 Semiconductor/Electrolyte Interface: Mechanisms of Chemical and Electrochemical Reactions at the Si Surface 6 July 24 Preparation of Si Surfaces: (Photo)(electro)chemical Passivation and Nanostructuring 7 July 31 Site-specific Adsorption of Metal Nanoparticles and Enzyme Molecules on Semiconductor Surfaces 8 August 7 Photovoltaic and Photoelectrochemical Solar Cells 9 August 14 A Photobiocathode 10 August 21 Bioinspired Systems From theoretical basics to applications

3 Technology in the good old days wood, glass/ceramics, cotton, leather, iron / metals, etc. and combination of these materials -compass and sextant (astronavigation) in use until late 1960s before GPS

4 Technology, lifestyle and materials science Full lines: demands of society on scientists Dashed lines: possibilities provided by basic research, materials science and engineering 4

5 Evolution of the energy sources Renewable energy sources Wood Turf / peat Nonrenewable energy sources Hard coal Petroleum / rock oil Earth gas / rock gas Depleted uranium Rapid increase of energy request Industrial-scientifical revolution: - steam engine - electricity - motorization - industry Increase of human population Increased production of pollutions -carbon dioxide -atmospheric pollutions... 5

6 Energy scenarios and exhaustion of fossil resources consideration of world energy consumption and energy consumption per capita and year Sources: Internat. Energy Annual; DOE; exploration of new resources only roughly included. Scenario A B C D E Population in billions 5 (1850) middle Europe 5 (1988) 6.5 presently presently USA Energy consumption/capita/ye ar Exhaustion of resources/ years A:nuclear fission B:coal / oil / gas world energy consumption 3000 kwh kwh presently kwh presently kwh presently kwh presently USA ~ x10 15 BTU 1x10 14 kwh 430x10 15 BTU 1.3x kwh british thermal units vs. kwh: 1 BTU 0.29x10-3 kwh 6

7 ????? Renewable energy sources

8 Renewable energy sources

9 Solar energy

10 SOLAR SPECTRUM The intensity of solar radiation in free space at the average distance of earth from the sun. solar constant 1353 Wm -2 surface of earth reaches about 50% 10

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12 Air Mass the degree to which the atmosphere affects the sunlight received at the earth s surface 12

13 Solar Irradiation - Definitions >Air mass definition: the degree to which the atmosphereaffects the sunlight received at the earth s surface. Air mass 0 (AM 0): Solar irradiation outside the earth s atmosphere, incident power I1353 Wm -2 Air mass 1(AM 1): Solar irradiation with sun in zenith at sea level at the equator, solar light travels through one atmosphere thickness, α90 o I925 Wm -2 Air mass 1.5 (AM 1.5): Customary most used. Solar irradiation at α 48.2, I844 Wm -2 Air mass 2 (AM 2): The sunlight travels through 2 atmosphere thicknesses, α 60 I 691 Wm -2 (spectrum is different from AM 1 due to atmospheric losses) The secant of the angel between the sun and the zenith is called the air mass and measures the atmospheric path lenght relative to the minimum path length when the sun is directly overhead

14 d l α γ cos 2 1 cos cos cos cos 1 1 cos cos 1 α α α α α α α α α α l for l for l for l l d d sin 2 1 sin sin sin sin 1 1 sin sin γ γ γ γ γ γ γ γ γ γ l for l for l for l l d

15 Atmospheric losses Solar spectra and theoretical efficiencies Rayleigh scattering (the intensity of the scattered radiation varies with I λ -4 for particles with diameter d particle << λ, i.e. < 50nm. electronic absorption of O 2, N 2 and O 3 (absorbs almost completely for λ < 290nm) infrared absorption by molecular rotational and vibrational modes of CO 2 and H 2 O scattering at aerosols and dust particles spectral changes due to changes in atmosphere refractory index (temperature, pressure, turbulences, water vapor content) changes by clouds, rain, turbulences Solar spectra and atmospheric losses wavelength /µm hν / ev

16 Rayleigh scattering (named after Lord Rayleigh) is the scattering of light, or other electromagnetic radiation, by particles much smaller than the wavelength of the light. It can occur when light travels in transparent solids and liquids, but is most prominently seen in gases. Rayleigh scattering of sunlight in clear atmosphere is the main reason why the sky is blue. Rayleigh scattering, as well as scattering by clouds both contribute to diffuse light (direct light being sunrays).

17 Standard industry metric for solar cell performance Power conversion efficiency (PCE) The ratio of electrical power produced by a solar cell (in watts) per unit area divided by the watts of incident light under certain specified conditions called Standard Test Conditions (STC). STC - Irradiance intensity of 1000 Watts per sq. meter - AM 1.5 solar reference spectrum - Cell/ module temperature during measurement of 25 o C PCE 2 Solar power output / m ( watts) AM1.5 Solar Spectrum light

18 Output power characteristics and terminology Definition of solar cell parameters based on a phenomenological photocurrent - voltage curve: short circuit current (i sc ) R0 in external circuit open circuit voltage (V oc ) R ~ maximum power point MPP: (i mp, V mp ) largest rectangle under i-v characteristic fill factor ff: rectangularity of photocurrent voltage curve: yellow area (i mp V mp ) divided by (i sc V oc ) rectangle efficiency η: power output divided by power input yellow area divided by the i sc x V oc rectangle Optimization: concerns three parameters - Short circuit current (quantum efficiency of system) - Open circuit voltage (contact potential difference and interface properties) - Fill factor (series of shunt resistance)

19 EFFICIENCY η P P output input power output i mp x V mp power input light intensity Fill factor- rectangularity of the i ph V curve ff i i mp sc V V pm OC η η η i i i sc sc mp I V I OC light V V light OC mp I ff i i light mp sc V V mp OC

20 1839 First photoeffect Becquerel A votage and a current are produced when a silverchloride electrode, immersed in an electrolyte and connected to a counter electrode is illuminated Photoeffect Se-Pt W. Adams, R. Day 0.5% efficiency 1930 Cu 2 O-Cu Cells (Schottky) High quality silicon J. Czochralski 1954 The first real solar cell Si pn (Bell Laboratory, Chapin, Fuller), 5% 1954 CdS-Cu 2 S cell (Reynolds) 1954 CdS thin film solar cell (Clevite Research Center) 1958 First silicon solar cell - satellite Vanguard 1963 CdTe 1975 CuInSe a-si (Carlson, Wronski) RWE/Schott Solar 1984 Organic solar cell 1991 Dye cell - Grätzel Little bit of history 20

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22 Photoelectrochemical cells Electrochemical Photovoltaic Cells Optical Energy converted into Electrical Energy Photoelectrosynthetic Cells Optical energy converted into Chemical Energy Schottky type cells: semiconductor and metal electrodes semiconductor : electrolyte p/n photo electrolysis cells: p-type semiconductor and n-type semiconductor

23 Principle of light-induced energy converting processes with semiconductors Semiconductor junctions n-semiconductor p-semiconductor differ. doping: semiconductor 1 semiconductor 2: semiconductor metal semiconductor redox electrolyte homojunction heterojunction Schottky junctions analogous to Schottky illumination with photons (energy?): absorption: electron-hole in semiconductor in semiconductor: space charge layer non-zero electrical field transport by drift neutral region charge transport by diffusion

24 Absorption of photon (hν>e g ): hole (h + ) : electron (e - ) pair is created in semiconductor material (SC) e - and h + residing in the E CB and E VB can recombine charge carriers (e - and h + ) diffuse away from each other: charge separation e - relax to edge of the bulk E CB and h + relax to edge of the bulk E VB During diffusion and charge separation, e - and h + can be captured by trap states; - surface defects - vacancies - it can occur in bulk as well. trapped e - can recombine with the E VB h + and trapped h + with E CB e - trapped electrons can also recombine with the trapped holes (this process is slow, unless the electron and hole traps are located close to each other) when the electrons (e - ) and holes (h + ) arrive at the surface reaction sites, they promote reduction and oxidation, respectively

25 VERY IMPORTANT PARAMETERS adsorption coefficient If light penetrates deep into material most light generated carriers have to reach the space charge region by diffusion. diffusion length For small diffusion length many carriers will be lost by recombination. low solar to electrical convention efficiency