Measuring the Optical Power of your LED

Purpose of This Note

When performing experiments with dye solar cells, light is focused on a cell and its current or voltage response is measured. The generated power depends strongly on the intensity of the light.

In order to calculate the efficiency of a solar cell, the optical power of the incident light has to be known. This technical note explains how to measure and calculate the optical power of your light source.

Introduction

Optical Bench

Gamry’s Optical Bench allows performing various experiments with dye solar cells (DSCs) including cyclic voltammetry, potentiostatic EIS, and light pulse experiments as well as IMPS (intensity modulated photocurrent spectroscopy) and IMVS (intensity modulated photovoltage spectroscopy).

Note: For more information on experiments with DSCs, see Gamry’s application notes at www.gamry.com: Dye Solar Cells: Part 1 – Basic principles and measurements Dye Solar Cells – Part 2: Impedance measurements Dye Solar Cells – Part 3: IMPS and IMVS measurements

Figure 1 shows the setup of Gamry’s Optical Bench. It consists of a base which supports a rail system. Photodiode and light source can be mounted on optical posts. Dye solar cells can be mounted on the cage system using a special holder. The assembly allows simple and reproducible adjustment of distance and height between solar cell/photodiode and light source.

In order to perform experiments with dye solar cells, two synchronized potentiostats of the same family are needed. The “master” potentiostat controls the LED. The “serf” potentiostat is used for measuring.

The photodiode is used as dummy cell. It allows measuring and calculating the optical power of the light source. This parameter is needed to calculate the efficiency of your cell.

The active area of the photodiode has a diameter of 0.9 cm, which gives a sample area of 0.636 cm². It also includes an adapter cable for connecting a potentiostat.

 

Figure 1 – Gamry’s Optical Bench with LED and photodiode.

The light source in the setup is optional. You may either provide your own light source or use one of Gamry’s LEDs, see Table 1.

Table 1 – Gamry’s LEDs for the Optical Bench with corresponding Responsivity factors.

All LEDs are galvanostatically controlled with a maximum rated current of 1 A. Their power output is typically between 170 mW and 770 mW. Each LED provides four banana jacks for connecting a potentiostat.

Theory

Optical Power

The intensity I of a light source depends strongly on the distance. Imagine a light bulb that emits light equally in all directions (see Figure 2). The intensity is greatest at the center and decreases with increasing distance r.

Figure 2 –Schematic drawing illustrating the inverse‑square law of light. For details, see text.

The decrease of intensity follows the inverse‑square law of light (see also equation 1). This means that at a distance twice as far from the light source, light is spread over an area which is four times bigger. Hence intensity is only one‑fourth of the initial intensity I₀.

The same is true when measuring dye solar cells. Only a small portion of light that is emitted from the LED reaches the active area of the DSC.

However, the intensity of the incident light – from now on referred to as optical power P_in – is needed in order to calculate the efficiency h of the DSC (see also equation 2). 

P_max is the power maximum of a DSC at constant light intensity. This parameter can be obtained from I‑V curves.

Responsivity of a photodiode

A photodiode can be used to calculate the optical power P_in of light.

Similar to DSCs, photodiodes generate current when light shines on them. The amount of current depends on the light power as well as wavelength of the incident light. This relationship is called responsivity (R_PD). It is generally measured under short‑circuit conditions (0 V) and is typically indicated in the data sheet of a photodiode.

Figure 3 shows the responsivity curve of the photodiode used in Gamry’s Optical Bench. The graph is provided by Thorlabs Inc. Note that R_PD strongly depends on the wavelength of the light source.

Figure 3 – Spectral response curve of the photodiode used in Gamry’s Optical Bench  
(source: Thorlabs Inc).

Table 1 lists single responsivity values for all colored LEDs which Gamry currently offers. The responsivity R_PD can be used to calculate the optical power density p_PD of the light that shines on the active surface area of the photodiode.

Here i_PD is the generated current density from the photodiode under constant illumination.

The optical power P_in of the light that shines on a DSC can then be calculated (see equation 4). A_DSC is the active area of the dye solar cell.

Generally, photodiodes are only used for narrow‑band light sources (see Table 1) in order to measure the optical power.

Broadband light sources (e.g., warm-white LEDs) perform poorly with photodiodes because the responsivity depends on the wavelength. Only relative power changes can be measured. Typically, thermal power sensors are used for broadband LEDs to measure the optical power output.

Experiment

The photodiode replaces the dye solar cell when measuring p_PD. The distance between photodiode and light source should be similar to experiments with real DSCs. We recommend that you darken the environment around the setup in order to block ambient light which can falsify the results.

Figure 4 shows a series of potentiostatic experiments under constant illumination. The potential of the photodiode was set to 0 V (short‑circuit conditions) and its current response was measured. The distance between photodiode and light source was adjusted to 3 cm.

A red LED (625 nm) was used as light source. The LED was powered with a current of 100 mA, 300 mA, 500 mA, 700 mA, and 900 mA respectively.

Figure 4 – Current curves of the photodiode with increasing light intensities (from bright to dark). For details, see text.

The current density is automatically calculated by the Echem Analyst. As expected, the current increases with increasing light intensities.

Keep in mind that the photodiode is warmed up by the LED. This can lead to small changes in the measured current over time. We recommend running an experiment either until the measured current is constant at a given intensity or using only very small scan rates when linearly sweeping the applied current signal.

Table 2 lists the results from the previous measurements. The responsivity factor R_PD for this LED is 0.359 (see also Table 1).

Table 2 – Measured current densities i_PD and calculated optical power densities p_PD of the photodiode at different light intensities.

With these results, the optical power P_in (Eq. 4) of the incident light and the efficiency (Eq. 2) of your DSC can be calculated.

Summary

This technical note gives a short overview on Gamry’s Optical Bench.

It is described how to use the photodiode as dummy cell.  The photodiode allows measuring the optical power output of your light source. This parameter is needed in order to calculate the efficiency of your dye solar cell.

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