Photovoltaic (PV) cells convert sunlight energy directly into electricity, with no moving parts (recall Fig. 37.20 ). In a PV cell, photons incident on a semiconductor \(P N\) junction promote electrons to the conduction band, producing electron-hole pairs and driving current through an external circuit (Fig. 37.25 ). Commercially available PV cells are \(15-20 \%\) efficient, meaning they convert this fraction of incident sunlight into electrical energy; the theoretical maximum efficiency is around \(33 \%\) for silicon-based PV cells. An important limitation on PV efficiency is the relation between the solar spectrum (IMAGE CANNOT COPY) and PV cells' semiconductor band-gap energy. For silicon, the band gap is \(1.14 \mathrm{eV}\); photons with less energy can't promote electrons to the conduction zone and are thus unavailable for the PV energy conversion. Conversely, photons with more than the band-gap energy give up their excess energy as heat, also reducing PV efficiency. One way to improve PV efficiency is to make multi-layer cells with several \(P N\) junctions using semiconductors with different band gaps. For a multi-layer PV cell to be effective, a. the junction with the largest band gap should be closest to the top of the PV cell. b. the junction with the largest band gap should be closest to the bottom of the PV cell. c. the largest band gap should correspond to infrared wavelengths. d. the smallest band gap should correspond to ultraviolet wavelengths.

Step by step solution

01

Understand how PV cells work

PV cells convert sunlight into electricity by promoting electrons to the conduction band via photon incidence. This process of electron-hole pair creation drives current through an external circuit. The band-gap energy of the semiconductors used in these cells is crucial as it governs what quantity of sunlight can be converted into electricity. Photons with less energy than the band gap can't be used for this conversion while photons with more energy end up wasting their excess energy in the form of heat.

02

Consider multi-layer cells

Using multi-layer cells with semiconductors of different band gaps is proposed as a method to increase efficiency. Effectiveness of such a cell depends on the alignment of the \(P N\) junctions within the cell.

03

Evaluate the given options

The placement of a junction within the cell (options a and b) affects the intensity of light it will be exposed to. The junction with the highest band gap should be placed closest to the top of the cell (option a) as it can utilize high-energy, short-wavelength light, while the remaining light can then be used by the junctions beneath it. The relation of band gap to wavelength (options c and d) should be such that the largest band gap corresponds to the shortest wavelengths (UV light) and the smallest band gap to longer wavelengths (infrared light). Thus, the correct answer is option a.

Unlock Step-by-Step Solutions & Ace Your Exams!

• Full Textbook Solutions

  Get detailed explanations and key concepts

• Unlimited Al creation

  Al flashcards, explanations, exams and more...

• Ads-free access

  To over 500 millions flashcards

• Money-back guarantee

  We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!