Solar Cells Handout (PDF)

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices Lecture 4 Solar Cells Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 4 Learning Outcomes 1...

Frontiers in Chemistry CHEM1008 Electrochemistry and Devices Lecture 4 Solar Cells Darren Walsh Room A09, GSK Carbon Neutral Laboratory for Sustainable Chemistry Email: [email protected]  Lecture 4 Learning Outcomes 1. Understand why generating enough for future generations is a massive challenge 2. Appreciate that conventional methods cannot meet future demand and that solar power could 3. Understand (using band theory) why semi-conductors can conduct electricity 4. Understand how P-N junction photovoltaics work and what are their limitations 5. Be aware that other technologies are coming along that could improve efficiencies  The Energy Demand o Energy Consumption by the world is about 500×1018 joules, which is an average consumption of 16 terawatts (TW) o 1 TW = 1012 watts or 1012 joules per second o Expected to double by 2050 and triple by 2100 Where will the energy for 2050 come from? o Biomass? Imagine we consume all biomass from all agricultural land exclusively for energy = 7-10 TW o Nuclear? Imagine we build 8,000 new nuclear power plant (about 1 per day for 30 years) = 8 TW o Wind? Imagine we increase global land mass for wind turbines and saturate wind of appropriate speed = 2 TW o Hydroelectric? Imagine we place dams in all remaining rivers on the planet = 1.5 TW Our ambitious targets yield about 21 TW – not enough! So where will the energy come from? o Solar power provides ~1360 W m–2 outside our atmosphere o Less reaches us after passing through the atmosphere, and reflecting back to space, and amount depends on the Sun’s angle o The average flux hitting the earth’s surface is 175 W m–2 = 89,000 TW of potentially usable power o More in 1.5 hours than we need for a year  How much land would we need o If we covered 860,000 km 2 with 10% efficient solar-energy conversion systems, we would generate 15 TW power – enough for today’s needs o Approximately same as land area of Venezuela Harvesting solar energy – an old and legendary example o Syracuse, Sicily was a Hellenistic city attacked by Romans in 214-212 BC o According to a legend, Archimedes designed mirrors to focus solar power onto Roman ships o Rome ultimately conquered the city and 75-year old Archimedes was killed by Roman soldiers  Harvesting solar energy today Direct Methods Indirect Methods Photovoltaics Solar Thermal Ocean thermal Wave Wind Biomass Hydroelectric The solar spectrum “Air-mass” 1.5 spectrum is often used as a standard spectrum by solar industry Photovoltaics – start with band theory o MO theory describes the overlap of atomic orbitals to give bonding and anti- bonding orbitals o Think of the overlap of p-orbitals to form π-orbitals in ethene Imagine then adding many more atoms… lots  Band theory E o Each carbon atom contributes 1 electron to band so lowest energy N/2 Empty MOs above orbitals are occupied (according to the aufbau principle) Fermi level o The highest energy occupied molecular orbital (at 0 K) is the Fermi level Filled MOs below  Band gap width and effect on conductivity Insulator Semi- E Metal conductor Conduction band Semi-metal Conduction band Band overlap Narrow Wide band band gap gap Valence Fermi level band Valence band  Conductivity in semi-conductors Eg is small enough heat that some electrons can be promoted into the conduction CB electrons Narrow Eg band band gap VB “holes” h𝛖 o Conduction band electrons and valence band holes can carry charge, allowing a current to flow o Conductivity increases with increasing temperature, since it depends on the number of charge carriers  Intrinsic and extrinsic semiconductors o Intrinsic semi-conductors (materials such as Si and Ge) naturally have small Eg o Extrinsic semi-conductors are formed by doping materials o Guest atoms can donate electrons to the host material, resulting in N-type semi- conductors o Substituting a few Ge atoms (valence 4) with P atoms (valence 5) results in occupation of the conduction band by mobile electrons o Guest atoms can take electrons from the host material giving P-type semi- conductors o Substituting a few Ge atoms (valence 4) with Al atoms (valence 3) results in holes in the valence band that can conduct  P-N junction photovoltaics o If we put N- and P-type semi-conductors in contact, electrons and holes diffuse across the interface o A potential difference builds up that opposes any further diffusion (like in membrane potentials) o An electric field is spontaneously formed at the interface P-N junction photovoltaics o If a photon liberates an electron from its host atoms, an “electron-hole pair” is formed o The electrons go one way and the holes the other o If we provide a circuit for the holes and electrons to go through, sustained A current will flow as long as sunlight is shining  What should we make our cells from? o The maximum solar intensity occurs at an energy of between 1 and 1.4 eV, so semiconductors with similar band gaps will make the most efficient solar cells o Si (band gap = 1.1 eV) and GaAs (band gap = 1.4 eV) are good options o Single-crystal Si o 15-18% efficient but expensive to make a big Si single crystal o Polycrystalline Si o 12-16% efficient and slowly improving and cheaper to make than single- crystal Si o Amorphous Si o 4-8% efficient, cheapest per watt, called “thin film” easily deposited on range of surfaces Newer technologies can help push up efficiencies  What’s limiting our maximum efficiency? o Si transparent at 𝛌 > 1.1μm (1100 nm) o 23% of sunlight passes straight through o Only 51% of the photon energy of near IR light converted to electricity o About 33% of red light (700 nm) is converted o About 19% of blue light (400 nm) is converted o e−s go off in the wrong direction o Maximum efficiency of a Si solar cell is about 23% o Defeating “recombination” losses puts the limit in the low 30s  Using PVs to store energy o Single PV cell acts like a battery with a voltage of 0.58 V o Stacking cells in series gives us useful voltages o At higher voltage, we can deliver power at lower currents o smaller wiring, greater transmission efficiency o Typical panel has 36 cells for an open circuit voltage of about 21 V o Drops to about 16 V at peak power o Suited to charging a 12 V battery