# I intend to investigate whether any correlation exists between the wavelength of light exerted upon a small solar cell impacts its rate of increase to response time

Solar Cell Response Times Quality Of Measurement

Introduction

For my quality of measurement coursework, I intend to investigate whether any correlation exists between the wavelength of light exerted upon a small solar cell impacts its rate of increase to ‘response time’ in any way. Response time in this scenario will be defined as the time required for the cell to reach its nominal voltage from 0v. The rate will be a measure of the response time divided by the voltage increase (measure of gradient). This will require the precise control of a number of variables; the largest being the co-ordination of light source start up and the voltage logging from the solar cell.

The practical applications of a wavelengths affect on solar cells are limited. In the case of data transfer, much more tailored methods exist. However, a hobbyist could use the information obtained here to build a very cheap receiver and transceiver for the transmission of data, given the low costs of solar cells and their abundance in consumable electronics (calculators, garden lights etc.). The cells response time will dictate the rate of data transfer, as bits can be signified by the rise and fall of the cells voltage. The wavelength of light pulsed to generate these bits will need to extract the highest transfer speeds possible by coaxing a smaller response time from the cell.

The relationship that I am investigating is one that is not commonly pursued. Different cells have different spectral responses; that is known. Red wavelengths tend to produce higher voltages from the cell; meaning more electrons are freed from their bonds. What is not researched as much (or is at least very hard to find) is how a photon’s energy alters the time taken to free electrons from these bonds.

Safety

I am working with relatively low voltages and currents (0-12v) meaning that the risk of shock is extremely low. Care should still be taken as the variable power supply I am using can ramp up to 40v and 5 amps. As a result, I will limit the output of the power supply by leaving the ‘High-Low’ switch on the latter setting; effectively limiting the power supply to 15 volts and low amperage. The switch will be taped over to avoid accidental changes.

The wavelength of the LED’s I am using poses no risk at all. They all form part of the visible spectrum so do not carry enough energy to ionise any bodily cells. This investigation therefore requires nothing in the way of safety equipment.

Physics

My investigation makes use of a photovoltaic solar cell, which works by having its electrons ‘knocked loose by photons’. The photons break apart electron-hole pairs, with each photon freeing exactly one electron. This provides current, and the cells electric field provides the voltage.

The more photons aimed at the cell the more electrons will be released; hence we have the commonly stated metric that the brighter the day, the more volts are produced by the solar panel. However, what I am investigating is how fast the electrons are freed from their bonds when given photons of higher energy (smaller wavelength).

I am unsure what to expect from the results of this experiment. Common sense dictates that the higher the photon energy, the faster the electrons will be released from their bonds. Ultimately, my level of physics knowledge is not advanced enough to be able to propose any hypothesis.  What I am expecting, are lower response times from the cell as the wavelength increases. Looking at spectral response graphs of various solar panels, it is apparent that lower voltages arise from lower wavelengths. What I do not know is how fast the cells will reach these nominal output voltages.

Equipment

The quantities we are working with are, by their very nature, extremely small; response times from solar cells can range between 0.5 and 4 milliseconds. Instantly this dictates that the investigation can’t have any human input, instead requiring specially configured logging equipment. When evaluating what would be required, there were two routes I could have chosen:

The first involved triggering a light source and starting a voltage logger simultaneously. My experiences with the schools provided GLX data logger indicated that a manual trigger was impossible without soldering a momentary push switch to the devices ‘Play’ button circuit board contacts. The GLX also seemed to have some degree of delay associated with pushing the ‘Play’ button, and data logging actually beginning.

I instead opted to use a custom programmed microcontroller. This had many added advantages, the main being that there was no extra overhead on the device, only my data logging code. This meant that when I requested data logging to begin, it actually did (with no delay). It also enabled me to trigger the light source at the exact same time that data logging began, allowing me to accurately identify the solar cells response times. Finally, the data outputted via the microcontroller would arrive back at the computer via a USB serial connection, meaning data manipulation was easier and faster to facilitate.

The GLX will still have its uses though. It will be required to measure the brightness’s of all available LED’s, to ensure they remain constant. This will be further covered in the ‘Calibration’ section of this document.

With all that said and done, here is my final equipment list

• Arduino 16Mhz Microcontroller
• 5v Relay
• Small Solar cell
• 9v Red LED
• 9v Green LED
• 9v Purple LED
• 12v Variable DC power supply
• Laptop
• Pasco GLX Data Logger
• Pasco GLX Light Sensor
• Digital Multimeter

Calibration:

Relay

I was limited in terms of LED driving power by the microcontroller. It can only supply the voltage it is given. Since it is powered over USB, this is 5v. The LED I am has a forward voltage of 2.5v, hence it is barely illuminated at all when driven directly from the Arduino. To work around this I purchased a 5v relay, which enabled me to switch a separate, higher voltage LED-driving circuit on.

By introducing the relay switching circuit, I introduced a delay in the start-up of the light source. By checking the relays data sheet, I discovered that the operation time (switching time) of the relay was about 10 milliseconds. ()

This has no tangible effect on the investigation. This is because only light sources available before data logging begins will effect measured response times of the cell; If the light starts before voltage logging begins, the nominal voltage from the cell will be reached faster. However, this delay would only serve to introduce the light after the data logging ...