Polaritat en els nivells de limitació funcional al llarg de l’e

Solar cell assessment requires the measurement of a range of values under specific conditions to enable comparisons between products and standardization of interpretation (210). To begin with the illumination for testing must be considered. AM is the air mass index defined as the path length which light takes through the atmosphere normalised to the shortest possible path length (211). This value quantifies the reduction in power of the incident light through absorption effects (212). Theta ( ) is the zenith angle of the sun to the earth. AM is calculated from equation (2. 9).

(2. 9)

– Air mass index – Zenith angle

The efficiency of a solar cell is sensitive to the power and incident spectrum of light, which varies throughout the position of the sun. Standardization of the spectrum was agreed upon so that comparative analysis between cell efficiencies can be conducted. AM1.5 was agreed upon for the standard spectrum of light, which gives approximately ⁄ (212).

Several important parameters are used to characterise the solar cell properties. The short- circuit current ( ), open-voltage ( ), fill factor ( ) and efficiency are all parameters determined using the IV curve (current-voltage curve) (213). The IV curve of a solar cell (Figure 9) is the superposition of the IV curve of the solar cell diode in the dark with the light generated current.

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Figure 9 - IV Curve for Ideal Solar Cell (214)

Short-circuit current is the current through the solar cell when the voltage across the solar cell is zero (essential the cell is short-circuited) (212). This current is due to the generation and collection of light-induced carriers. In ideal solar cells we would hope to generate a short circuit current equal to the light-induced current. Short circuit current is therefore the maximum current that may be drawn from a cell (212).

The short-circuit current is dependent on the solar active area, and it is more common to describe the current density ( ) of the cell. The short-circuit current depends directly on the illumination intensity, and the spectrum of light (this is standardised to AM1.5) (214).

Open-circuit voltage is the maximum voltage available from a solar cell which occurs when the current is zero (214). This voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-induced current (213) (214). is calculated from equation (2. 10)(212).

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( ) _{(2. 10)}

- Light Generated Current - Dark Saturation Current

depends on the saturation current and light-generated current of the cell (212) (214). Typically the short circuit current has small variations, the saturation current is critical as it can vary in orders of magnitude. Saturation current is dependent on the recombination in the device; therefore is a measure of the recombination in the device (212). may also be determined from the carrier concentration given in equation (2. 11)(212) (213).

[( ) ] _{(2. 11)} – Thermal Voltage – Doping Concentration – Excess Carrier Concentration

– Intrinsic Carrier Concentration

& describe the maximum current and voltage produced by a solar cell, however both these parameters are determined with no power generation from the cell. Fill factor in conjunction with & enables determination of the cells maximum power (214). Therefore the fill factor is defined as the ratio of the maximum power from the solar cell and the product of & .

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Figure 10 – Calculated Fill Factor (214)

Fill factor is determined by differentiating the power from the solar cell with respects to the voltage and determining where these equals to zero;

( )

(2. 12)

This enables the determination of the maximum voltage given by equation (2. 13) (212) (214).

(

⁄ ) (2. 13)

This equation only relates to the open voltage to the maximum voltage, implementation of this is required to find the maximum current using the fill factor. Therefore commonly the fill factor is expressed directly as given in equation (2. 14)(214).

( )

Page | 51 Where is defined as a normalized (212)(214);

(2. 15)

The efficiency is typically used to compare solar cell performance. It is defined as the ratio of energy output of the cell to the energy input from the sun. Efficiency is spectrum and intensity dependent and also dependent on the temperature of the cell. Terrestrial solar cells are measured using AM1.5 at 25°C, and amorphous cells go through a stabilization process to determine their stabilized efficiency. Maximum power of the cell is calculated from (2. 16) and

the resultant efficiency from equation (2. 17)(212).

(2. 16) (2. 17) – Maximum Power – Input Power – Efficiency

Quantum efficiency ( ) is the ratio of number of carriers collected by the cell to the number of photons of a given energy incident on the solar cell (214). The quantum efficiency is either represented as a function of wavelength or energy. If all photons of a certain wavelength are absorbed and the resulting minority carriers are collected, then the quantum efficiency for the given wavelength is unity (214).

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Figure 11 - External Quantum Efficiency, Ideal vs. Calculated

is typically reduced in most solar cells due to recombination effects, with the same mechanism that affect collection probability also affecting quantum efficiency (212). Front surface passivation affects carriers generated near the surface of the cell. Blue light is absorbed very close to the surface; high front surface recombination will define the lower end of the quantum efficiency (214). Green light is absorbed in the bulk of the solar cell and a low diffusion length will affect the collection probability, therefore the quantum efficiency for green (214). The quantum efficiency can be described as the collection probability due to the generation profile of a single wavelength, integrated over the device thickness and normalized to the incident number of photons.

External Quantum Efficiency ( ), for a solar cell is the ratio of extracted free charge carriers to incident photons (212). ( ) includes the optical losses of the cell. Internal quantum efficiency assesses the quantum efficiency post optical losses. Internal efficiency refers to the efficiency which photons that are not reflected or transmitted out of the cell can generate collectable carriers (212). By measuring the reflection and transmission of a device, curve can be corrected to acquire the internal quantum efficiency curve (212) (214).

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3 - Thin Film Deposition Techniques & Processes

Thin films are a continually growing area of research and commercial application. Developments of integrated circuits for micro-electronics has given birth to a whole wealth of thin film technology. Ranging from semiconductors for computing, displays devices (Fluorescent, LCD, LEDs, Plasma, OLED, Electrochromic) (20) (180) (215), optical coatings (anti- reflective, TCOs) (216) (88) and data storage (217) (magnetic materials, solid state devices, compact disc, digital versatile video, Blu-ray) (218)(216).

Thin film technology is tailored to the requirements of the application with consideration of industrial limitations. Suitability of a deposition technique is assessed during research and development processes to not only produce the required materials possessing the desired film characteristics, but explore the possibility of commercial exploitation. Described by Seshan (218)

is that deposition technologies may be classified into four major categories as shown in Table 5. The main process we are concerned with is chemical vapour deposition, an overview of this technique discussed through this section involving the nature of film deposition from nucleation to film development.

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Table 5 – Deposition technologies categories (218)

Vacuum Evaporation

Convential vacuum evaporation Molecular-beam epitaxy (MBE) Electron-beam evaporation Reactive evaporation

Sputtering Plasma Processes

Diode sputtering Plasma-enhacned CVD Reactive sputtering Plasma oxidation Bias sputtering (ion plating) Plasma anodization Magnetron Sputtering Plasma polymerization Ion beam deposition Plasma nitrdation Ion beam sputter deposition Plasma reduction

Reactive ion plating Microwave ECR plasma CVD Cluster beam deposition Cathodic arc deposition

Chemical Vapor Deposition Thermal Forming Processes

CVD epitaxy Thermal oxidation Atmospheric pressure CVD (APCVD) Thermal nitridation Low pressure CVD (LPCVD) Thermal polymerization Metalorganic CVD (MOCVD)

Photo enhanced CVD (PHCVD) Laser induced CVD (PCVD)

Electron enhacned CVD Ion implantation

Electro Processes Mechanical Techniques

Electroplating Spray pyrolysis Electroless plating Spray on techqiues Electrolytic anodization Spin on yechniques Chemical reduction plating

Chemical displacement plating

Electrophoretic deposition Liquid phase epitaxy

Evaporative Methods

Glow Discharge Processes

Gas Phase Chemical Processes

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