Part I – Photovoltaic Cell Overview

Publish Date: Dec 04, 2009 | 56 Ratings | 3.55 out of 5 | Print

Overview

Solar energy technologies, which harness the sun’s energy to generate electrical power, are one of the fastest growing sources of renewable energy on the market today [1]. Around the world, engineers and scientists are collaborating to lower the material costs of solar cells, increase their energy conversion efficiency, and create innovative and efficient new products and applications based on photovoltaic (PV) technology.

This article is the first in a series of 3 tutorials on assessing the performance of photovoltaic cells through I-V characterization. This series includes an overview of PV cells, and describes the theory behind I-V characterization. The tutorials also include an example setup using National Instruments hardware and a free downloadable library of LabVIEW code for performing the I-V analysis.

The other two articles in this series are:

1. Introduction to Photovoltaic (Solar) Cells

Photovoltaic (PV) cells are made of semiconducting materials that can convert incident radiation in the solar spectrum to electric currents.  PV cells are most commonly made of silicon, and come in two varieties, crystalline and thin-film type, as detailed in Table 1.

Table 1 - Crystalline (Wafer-Based) and Thin-Film Photovoltaic Cells

When a photon is absorbed by a semiconducting material, it increases the energy of a valence band electron, thrusting it into the conduction band.  This occurs when the energy of incident photons is higher than the bandgap energy.  The conducting band electron then produces a current that moves through the semiconducting material. 


 Figure 1 – Cross-Section of a PV Cell

The amount of current generated by photon excitation in a PV cell at a given temperature is affected by incident light in two ways:

  • By the intensity of the incident light.
  • By the wavelength of the incident rays. 

The materials used in PV cells have different spectral responses to incident light, and exhibit a varying sensitivity with respect to the absorption of photons at given wavelengths.  Each semiconductor material will have an incident radiation threshold frequency, below which no electrons will be subjected to the photovoltaic effect.  Above the threshold frequency, the kinetic energy of the emitted photoelectron varies according to the wavelength of the incident radiation, but has no relation to the light intensity.  Increasing light intensity will proportionally increase the rate of photoelectron emission in the photovoltaic material.  In actual applications, the light absorbed by a solar cell will be a combination of direct solar radiation, as well as diffuse light bounced off of surrounding surfaces.  Solar cells are usually coated with anti-reflective material so that they absorb the maximum amount of radiation possible. 

PV cells can be arranged in a series configuration to form a module, and modules can then be connected in parallel-series configurations to form arrays.  When connecting cells or modules in series, they must have the same current rating to produce an additive voltage output, and similarly, modules must have the same voltage rating when connected in parallel to produce larger currents.


Figure 2 - Solar Panel Configurations

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2. Summary

This section contained a summary of photovoltaic (solar) cells.  More information about I-V characterization theory and test systems can be found in the subsequent parts:

The other two articles in this series are:

 

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