ELEG 867 - Devices for Light Detection from the Submillimeter to the Ultraviolet – Fall 2009
Homework #8 - due Friday, 30 October 2009, in class
1. Which is better – a p-on-n solar cell, or an n-on-p? Assume that the solar cell has the “classic” structure of a
thin emitter grown onto a thicker absorbing base. Answer this question in two parts by discussing two
collection processes. (a) First, address the differences in minority carrier diffusion length that would favor the
transport of one of the carriers in a thicker layer. (b) Second, discuss the effect of different carrier diffusivity in
a region where both electrons and holes are generated. The higher diffusivity carrier moves away more quickly,
producing a charge displacement and potential difference. This is called the Dember Effect, and the Dember
potential can affect Voc. 2. For the Ge cell in problem 1, estimate the optimum load resistance so that the cell
will operate near the maximum power point. Make reasonable estimates using the information provided and
justify your answer.
3. Design your favorite single junction solar cell from a real material (Si, GaAs, CdTe; your choice). Assume
that you have a perfect AR coating so that R = 0. Use the rule of thumb for maximum open circuit voltage (Eg/e
– 0.4 volts) and calculate the effective Io based on this Voc. (solar cell ideality factor is usually = 1) Make the
emitter thin (about 1 μm – your choice) and the base region one diffusion length. Using the real absorption
coefficient α at an effective photon energy of hνeff = 1.5xEg, calculate the amount of absorbed light and the
photocurrent. For Pinc, use AM1.5 conditions but only the energies available above your Eg (see the cumulative
plot below). Ignore the fact that photons with hν >> Eg, are not effective – assume that this is accommodated by
the Voc rule of thumb. Calculate the Vm graphically or iteratively; and the Im from the I-V equation, and Pm.
Conclude with your net conversion efficiency. Hint: use no back reflectors or texturing – just consider the
basic structure under these guidelines. An extra point goes to the highest ηconv in class!
70
Current (mA/cm2)
140
60
Power (mW/cm2)
120
50
100
40
80
30
60
20
40
10
20
0
Power (mW/cm2)
Current(mA/cm2)
Cumulative Power and Current in Solar
Spectrum
0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Photon Energy (eV)
Homework assignments will appear on the web at:
http://www.ece.udel.edu/~kolodzey/courses/eleg867f09.html.
Note: On each homework and report submission, please give your name, the due date, assignment number and
the course number.