The Physics of Organic Photovoltaics (OPV)
Study Questions
1. Waves and particles of light
a. What are some forms of electromagnetic radiation? What are the wavelengths of visible
light? What wavelengths of electromagnetic radiation are most prevalent on the
surface of Earth?
b. What is the relationship between the wavelength, frequency, and speed of light? What
is the speed of light in a vacuum? In air? In glass?
c. What is a photon?
i. What is the relationship between the energy and wavelength of a photon?
ii. What is the minimum amount of energy that a photoelectric cell can pick up
from a beam of blue light? Red light?
2. Light, lux, lumens, watts…
a. What is a watt?
b. What is intensity?
c. A green laser has an intensity 5 mW/cm2. It produces light of wavelength 532 nm. How
many photons does it produce per second? (Note: and “einstein” is a mole of photons.)
d. What is the definition of a lux?
e. What is the definition of a lumen?
f. What is the definition of efficacy of a light source?
g. How does one convert between measurements of luminous flux to power?
3. Energy transitions in atoms
a. What is meant by the “energy levels” of an atom?
b. What is the energy involved in the transition between the ground state (n = 1) and the
first excited state (n = 2) of a hydrogen atom? Is this energy in the range of visible light?
Is this energy in the range of what we expect for a photovoltaic cell? Why or why not?
c. What is the energy involved in the transition between the ground state (n = 1) and the
ionized state (n = ) of a hydrogen atom? Is this energy in the range of visible light? Is
this energy in the range of what we expect for a photovoltaic cell? Why or why not?
4. Interactions of light and matter
a. What are the atomic processes involved in fluorescence? As an example, a specific rock
glows orange (wavelength 590 nm) when it is irradiated with UV radiation of wavelength
365 nm. Explain what happens in this process, and what other wavelength of radiation
might be involved.
b. What is the difference between fluorescence and phosphorescence? Some phosphors
glow green when illuminated with a blue light but do not glow at all when illuminated
with a red light. Explain why this would be.
c. The greenhouse effect (in greenhouses, parked cars, and planets) occurs when visible
light enters a system, gets absorbed and re-emitted as IR radiation, and then gets
trapped (in the greenhouse, car, or atmosphere). The reverse process is when IR
radiation to be re-emitted as visible light and thus escape, which would cancel out the
greenhouse effect if it was as common as the greenhouse process. Why isn’t the
reverse process as common as the greenhouse process?
5. Lotsa atoms part I: Bands.
a. What happens to the energy levels of two identical atoms that are brought close
together? Do two energy levels that were equivalent when the atoms were separated
remain identical when the atoms are brought close together?
b. Explain the relationship to band formation in solids as this process is continued as a
million or a mole of identical atoms are brought closer together.
c. What is meant by the “Fermi level” of electrons in a solid?
d. What do we call a material which has a Fermi level in the middle of a band? Why?
e. What do we call a material which has a Fermi level at the top of one band and a large
energy gap between this energy level and the lowest energy level in the next band?
f. What do we call a material which has a Fermi level at the top of one band and a small
energy gap between this energy level and the lowest energy level in the next band?
6. Lotsa atoms part II: Organic molecules.
a. What is a pi bond? (Explain in terms of atomic orbitals.)
b. What is a pi-start bond? (Explain in terms of atomic orbitals for atoms that are brought
close together, as in part 5a, above.)
c. What would happen if all of the electrons in an atom went into pi-star bonds?
d. What is a conjugated double bond? What is a delocalized electron?
e. What is the structure of beta-carotene? (You don’t need to memorize it.) What is the
(approximate) length of the conjugated carbon chain? Over how much distance is a
delocalized electron free to move?
f. What is the formula for the energy levels of a (one dimensional) particle in a box? What
is the formula for the differences in energy levels for a particle in a box?
g. If the beta-carotene atom is acting as a box, what is the wavelength of the energy of a
photon absorbed between low-level energy states of an electron? What color of light is
associated with this transition?
h. Where are the conjugated carbon bonds in organic molecules associated with organic
photovoltaics?
7. Intro to photovoltaics:
a. If an electron is promoted in an organic molecule in a photovoltaic cell, what does it
leave behind?
b. When an electron is promoted in an organic molecule in a photovoltaic cell, what is the
most likely thing for that electron to do?
c. Why does this make it advantageous to make photovoltaic cells out of two kinds of
organic molecules?
d. What would be desirable characteristics of the energy levels of the Highest Occupied
Molecular Orbital in the molecule that gives up an excited electron and the Lowest
Unoccupied Molecular Orbital in a molecule that accepts an excited electron?