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 Graphite counter electrode TiO2 layer dyed with blackberry juice Assembled sandwich Completed cell with electrolyte in between the layers

Testing the conductivity Completed DSSC can be tested using a multimeter.

Atomic Energy Levels Energy atomic-orbital energy levels. 2p atomic-orbital energy levels. Electrons populate these energy levels, and can be excited to higher energy levels.

Extension of Energy Levels to DSSCs 2p Energy

Extension of Energy Levels to DSSCs 2p Energy Energy TiO2 Dye I-/I3-

Electron Transfer Energy In this scheme, we positioned the energy levels to spatially correspond to our materials’ locations. But for our new energy diagram, there is no spatial x-axis dependence, so let’s rearrange the locations to see our analogy better. Dye TiO2 Energy I-/I3-

Electron Transfer Energy Although we’ve spatially rearranged the energy levels , they still sit at the same energies! We also added a load that the electrons pass through, as in the picture. Energy I-/I3- Load Dye TiO2

Electron Transfer Energy Light excites the electron in the dye from the dye’s valence band to its conduction band Energy I-/I3- Load Dye TiO2

Electron Transfer Energy The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Energy I-/I3- Load Dye TiO2

Electron Transfer Energy The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Energy I-/I3- Load Dye TiO2

Electron Transfer Energy The electron then ‘rolls down the hill,’ passing through the load ‘knocking over dominos,’ then returns to the ground state in the dye Energy I-/I3- Load Dye TiO2

Electron Transfer Energy 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. Energy I-/I3- Load Dye TiO2

Electron Transfer Energy Our load can be a light bulb or other electronic device. Today it is a multimeter. I-/I3- Load Dye TiO2

Using Multimeters P = I*V V = IR 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 ‘R’ Ohms (Ω)= Volts/Amps Opposing force [like friction in mechanics] Power ‘P’ Watts (W) = Joules/ sec = Volts*Amps Energy transfer rate P = I*V Joule’s Law V = IR Ohm’s Law