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Solar Energy Conversion: Making a Dye-Sensitized TiO2 Solar Cell Background • A solar cell is a light-sensitive material that collects solar energy and converts it into fuel: electrical or chemical. Nature’s solar cell is a leaf on a plant as it undergoes photosynthesis. In photosynthesis, the chlorophyll dye in a leaf absorbs light from the sun, solar energy, and converts it into sugar, source of chemical energy. Making a solar cell • We want our solar cell to mimic photosynthesis, where solar energy does all the work, but our cell will produce electrical energy. Just like the leaf, we need to ensure that our cell can complete both (1) absorption and (2) conversion. Absorption • In this lab, we will used the dye found in blackberry juice and dried hibiscus leaves as our light absorber. • anthocyanins, occurs in many types of fruits and berries Conversion • We need a material that can take the light absorbed by the anthocyanin dye and convert it into a current, or moving electrons • Semiconductors • TiO2 Assembling the Electrodes TiO2 layer TiO2 layer dyed with blackberry juice Assembled sandwich Completed cell with electrolyte in between the layers Graphite counter electrode Testing the conductivity • Completed DSSC can be tested using a multimeter. Energy Atomic Energy Levels 2p 2s 1s atomic-orbital energy levels. Electrons populate these energy levels, and can be excited to higher energy levels. Extension of Energy Levels to DSSCs Energy 2p 2s 1s Extension of Energy Levels to DSSCs Energy 2p 2s 1s Energy I-/I3- Dye TiO2 Electron Transfer Energy In this scheme, we positioned the energy levels to spatially correspond to our materials’ locations. I-/I3Dye TiO2 But for our new energy diagram, there is no spatial xaxis dependence, so let’s rearrange the locations to see our analogy better. Electron Transfer Energy Although we’ve spatially rearranged the energy levels , they still sit at the same energies! Load Dye TiO2 I-/I3- We also added a load that the electrons pass through, as in the picture. Energy Electron Transfer Load Dye TiO2 I-/I3- Light excites the electron in the dye from the dye’s valence band to its conduction band Energy Electron Transfer Load Dye TiO2 I-/I3- The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Energy Electron Transfer Load Dye TiO2 I-/I3- The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Energy Electron Transfer Load Dye TiO2 I-/I3- The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Electron Transfer Energy Load Dye TiO2 I-/I3- The sun does all the work for us! It throws the electrons to the ‘top of the hill,’ while we simply make use of the electrons’ energy as it rolls down! This is our SOLAR ENERGY. Electron Transfer Energy Load Dye TiO2 I-/I3- Our load can be a light bulb or other electronic device. Today it is a multimeter. Using Multimeters DC = Direct Current Variable Units of Measurement Context Current ‘I’ Amps (A) = Coulomb/sec Electron travel rate Voltage ‘V’ Volts (V) = Joules/Coulomb ‘Push’ [or energy] per electron packet Resistance Ohms (Ω)= Volts/Amps Opposing force [like friction in mechanics] Watts (W) = Joules/ sec = Volts*Amps Energy transfer rate ‘R’ Power ‘P’ P = I*V V = IR Joule’s Law Ohm’s Law
