Question

A solar cell is built from a semiconductor material, such as Si, in contact with a metal. As we will see later, when the semiconductor is doped, an internal voltage is created within the semiconductor. When photoelectrons are produced via the photoelectric effect, these freed electrons move due to this internal voltage, creating electric current.

(a) If silicon’s work function (i.e. the energy to free an electron from an atom) is 1.2 eV, what is the maximum photon wavelength that will produce current in a solar cell? Where is this wavelength in the EM spectrum? Will visible light produce current in a Si solar cell?

(b) The semiconductor cell acts as a current source, as each incoming photon produces a single photoelectron. If the current produced by a single cell is 1 A, how many photons per second are incident upon the solar cell? (Hint: this question is a simple review of the definition of electric current)

c) If the wavelength of these photons are the value you found in part (a), and the number of photons per second are the value you found in part (b), what is the power, in Watts, of the light incident on the solar cell?

Answer 1

Solution:

Work function = 1.2 eV = E

Maximum wavelength can be found using E = h c /

=> = h c / E = 1.04 × 10^-6 m

This wavelength belongs to the near infra red region of the electromagnetic spectrum.

b) Current = I = q/t = n e / t

=>  no of photons = 1 / 1.6 x 10^-19 = 6.25 x 10^18

c) power = no. of photons x energy / time =( 6.25 x 10^18) × (1.2x1.6x10^-19) ÷ 1

= 1.2 watts.