Chapter 1 Introduction and motivation
1.2 The Market of Solar Cell
In last section, it has introduced most kind of solar cells, including bulk silicon, thin film silicon, gallium arsenide multi-junction solar cell, CdTe, CIGS, and organic solar cells. But the mainstream of the market are still silicon base solar cells, which has already commercialized. The others are still under developed or partially commercialized. In recent years, the renewable energy has been taken more and more seriously and the demand of solar cells also increased significantly. The figure 2 shows the production of solar cells in recent years. It indicates that the production significantly increases year by year and silicon based solar cells dominates the majority of market. In figure 3, it shows
Figure 2 Comparison of Different Type of Solar Production
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Figure 3 The Growth Trend of Thin Films and Crystalline Silicon Solar Cell over the Years
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Chapter 2
Priciple of Semiconductor Solar Cells
Solar photovoltaic energy conversion is a one-step conversion process which generates electrical energy from light energy. The explanation relies on ideas from quantum theory.
Light is made up of packets of energy, called photons, whose energy depends only upon the frequency, or color of the light The energy of visible photons is sufficient to exite electrons, bound into solids , up to higher energy levels where they are more free to move. An extreme example of this is photoelectric effect, the celebrated experiment which was explained by Einstein in 1905, where blue or ultraviolet light provides enough energy for electrons to escape completely from the surface of a metal. Normally, when light is absorbed by matter, photons are given up to excite electrons to higher energy states within the material, but the exited electrons quickly relax back to their ground state. In a photovoltaic device, howere, there is some built-in asymmetry which pulls the excited electrons away before they can relax, and feeds them to an external circuit. The extra motive force. This force drives the electrons through a load in the external circuit to do electrical work. The effectiveness of a photovoltaic device depends on the choice of light absorbing materials and the way in which they are connected to the external circuit. In this chapter I will summarize the characteristics and discuss its physical function in detail.
2.1 Solar Radiation
The radiative energy output from the sun derives from a nuclear fusion reaction. In every second about 6x 1011 kg of H, is converted to He, with a net mass loss of about 4x103 kg, which is converted through the Einstein relation (E = mc2) to 4x 1020 J. This energy is emitted primarily as electromagnetic radiation in the ultraviolet to infrared and radio spectral ranges (0.2 to 3 um). The total mass of the sun is now about 2x1030 kg, and a reasonably stable life
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with a nearly constant radiative energy output over 10 billion (1010) years is projected. The intensity of solar radiation in free space at the average distance of the earth from the sun has a value of 1,353 W/m2. The atmosphere attenuates the sunlight when it reaches the earth's surface, mainly due to water-vapor absorption in the infrared, ozone absorption in the ultraviolet, and scattering by airborne dust and aerosols. The degree to which the atmosphere affects the sunlight received at the earth's surface is quantified by the air mass. The secant of the angle between the sun and the zenith is defined as the air mass (AM) number and it measures the atmospheric path length relative to the minimum path length when the sun is directly overhead. The AM0 thus represents the solar spectrum outside the earth's atmosphere.
The AM1 spectrum represents the sunlight at the earth's surface when the sun is at zenith ( ), see Figure 4, and the incident power is about 925 W/m2. The AM2 spectrum is for x=60o and has an incident power of about 691 W/m2, and so on. Figure 5 shows the solar spectrums at various AM conditions. The upper curve is the AM0 condition which can be approximated by a 5,800 K black-body radiation, as shown by the dashed curve. The AM0 spectrum is the relevant one for satellite and space-vehicle applications. The AM1.5 conditions (with sun at x=45o above the horizon) represent a satisfactory energy-weighted average for terrestrial applications. For solar-cell energy conversion, each photon produces an electron-hole pair, so the solar power has to be converted to photon flux. The photon flux density per unit energy for AM1.5 is shown in Fig. 6 together with wavelength to photon energy. The total incident power for AM 1.5 is 844 W/m2[1].
13 Figure 4 The Zenith Angle of the Sun
Figure 5 Solar Spectrum at Different Air-Mass Conditions
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Figure 6 Solar Spectrum in Photon Flux Density per Photon Energy for AM0 and AM1.5 Conditions[2]
2.2 PN Junction in Equilibrium
A solar cell is a pn junction device with no voltage directly applied across the junction. The solar cell converts photon power into electrical power and delivers this power to a load. In this section, I will introduce the primary characteristics of pn junction in equilibrium. Figure 7 schematically shows the pn junction. One region is doped with acceptor impurity atoms to form the p region and the adjacent region is doped with donor atoms to form the n region.
Majority carrier electrons in the n region will begin diffusing into the p region and majority carrier holes in the p region will begin diffusing into the n region. If we assume there are no external connections to the semiconductor, then this diffusion process cannot continue indefinitely. As electrons diffuse from the n region, positively charged donor atoms are left behind. Similarly, as holes diffuse from the p region, they uncover negatively charged
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acceptor atoms. The net positive and negative charges in the n and p regions induce an electric field in the region near the metallurgical junction, in the direction from the positive to the negative charge, or from the n to the p region[3].
Figure 7 The electric field and forces acting on the charged carrier in the space charge region
If we assume that no voltage is applied across the pn junction, then the junction is in thermal equilibrium which means the Fermi energy level is constant throughout the entire system.
Figure 8 shows the energy-band diagram for the pn junction in thermal equilibrium. The conduction and valance band energies must bend as we go through the space charge region, since the relative position of the conduction and valence bands with respect to the Fermi energy changes between p and n regions. This potential barrier is referred to as the built-in potential barrier and is denoted by
(1)
The built-in potential barrier maintains equilibrium between majority carrier electrons in the n region and minority carrier electrons in the p region, and also between majority carrier holes
n - type
Depletion Regionp - type
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in the p region and minority carrier holes in the n region. The potential barrier maintains equilibrium, so no current is produced by this voltage.
Figure 8 Energy band diagram of a pn junction in thermal equilibrium
In t he depletion region consists of a region of fixed charge corresponding to the ionized dopant atoms cores that lost their carriers due to the diffusion current. The depletion region tails off exponentially away from the junction edge. Assuming that the depletion region is zero a certain distance away from the junction edge (called the depletion region width ) greatly simplified analysis. Above assumption is called depletion region approximation: the depletion approximation assumes that the electric field is confined to a finite region. For constant doping it approximates the charge density as constant in the transition region, and zero everywhere else. The amount of charge on the two sides of the depletion region must be equal. The width of the depletion region can be calculated by integrating the charge density in the depletion region to get the electric field, and then integrating again to get an expression for the built-on voltage, whose value we already know from the difference in the Fermi levels.
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Figure 9 The space charge density, electric field, and electric field potential through the space charge region of a uniformly doped pn junction
(2)
Integrating twice and stting this equal to the built in voltage, allows us to find the maximum value of the electric field.
(3)
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The depletion region width is given by:
(4)
The maximum electric field increases as the doping increases and is controlled by the doping of the more lightly doped side. The depletion region width is also controlled by the more lightly doped side.
Ideal pn Junction Current
The total current in the junction is the sum of the individual electron and hole currents which are constant through the depletion region. Since the electron and hole currents are continuous functions through the pn junction, the total pn junction current will be the minority carrier hole diffusion current at x=xn plus the minority carrier electron diffusion current at x=xp The gradients in the minority carrier concentrations, as shown in Figure 10, produce diffusion currents, and since we are assuming the electric field to be zero at the space charge edges, we can neglect any minority carrier drift current component.
Figure 10 Minority carrier concentrations in a pn junction under forward bias.
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We calculated the minority carrier diffusion current densities at the edge of the space charge region. We can determine the minority carrier diffusion current densities as a function of distance through p and n regions. These results are
(5) The total current density in the pn junction is then
(6) We define a parameter Js as
(7)
So that, the ideal current-voltage equation of pn junction, see Figure 11, can be written as
(8)
20 Figure 11 Ideal I-V characteristic of a pn junction diode.
2.3 Under Illumination
Solar cell is based on the photovoltaic effect. As a cell is under illumination, absorption of light leads to generate electron-hole pairs. Figure 12 shows that light with different wavelength would be absorbed at the different depth. Then, the charged carriers would be separated by the electric field in depletion region. At last, the electrode collects the carriers.
Figure 12 Operation of Solar Cell
Con sider the pn junction with a resitive load. Even with zero bias applied to the junction, an electric field exists in the space charge region. Incident photon illumination can create electron-hole pairs in the space charge region that will be swept out p producing the photocurrent IL in the reverse-bias direction. The photo current IL produces a boltage drop across the resitive load which forward biases the pn junction. The forward-bias voltage produces a forward-bias current. The net pn junction current, in the reverse-bias direction, is
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Where the ideal diode equation has been used. Figure 13 shows the equivalent circuit of pn solar cell, where I is output current density, and V is output voltage on the load. As the diode becomes forward biased the magnitude of the electric field in the space charge region decreases, but does not go to zero or change direction. The photocurrent is always in the reverse-bias direction and the net solar cell current is also always in the reverse-bias direction.
There are two limiting cases of interest. The short-circuit condition occurs when Rload=0 so that V=0. The current in this case is referred to as the short-circuit current, or
L SC I I
I (10)
The second limiting case is the open-circuit condition and occurs whenRload∞. The net current is zero and the voltage produced is the open-circuit voltage. The photocurrent is just balanced by the forward –biased junction current so we have
)
A plot of the diode current I as a function of the diode voltage V from the above equation is shown in Figure 14. We may note the short-circuit current and open –circuit voltage points on the figure.
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Figure 13 Equivalent Circuit of pn Solar Cell (a) without (b) with parallel and series resistance
Figure 14 Illuminated I-V Characteristic
(a) (b)
23 setting the derivative equal to zero. We find
The conversion efficiency of a solar cell is defined as the ration of output electrical power to incident optical power. For the maximum power output, we can write
%
max 100
Pin
P (17)
The maximum possible current and the maximum possible voltage in the solar cell are Isc and Voc, respectively. The ration ImVm/IscVoc is called the fill factor and is a measure of the realizable power from a solar cell. Typically, the fill factor is between 0.6 to 0.8.
OC SCV I
FF Pmax (18)
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The conversion efficiency can be also written as
in
In real cells power is dissipated through the resistance of the contacts and through leakage currents around the sides of the device. These effects are equivalent electrically to two
parasitic resistances in series Rs and in parallel Rsh with the cell, see Figure 13(b). The series resistance arises from the resistance of the cell material to current flow, particularly through the front surface to the contacts, and from resistive contacts. Series resistance is a particular problem at high current densities, for instance under concentrated light. The parallel or shunt resistance arises from leakage of current through the cell, around the edges of the device and between contacts of different polarity. It is a problem in poorly rectifying devices. Series and parallel resistances reduce the fill factor as shown in Figure 15 For an efficient cell we want Rs to be as small and Rsh to be as large as possible. When parasitic resistances are included, the diode equation becomes
Figure 15 Effect of (a) incrising series and (b) reducing parallel resistances. In each case the outer curve has
25 Rs=0 .and Rsh=∞
Chapter 3
Experiment Instrument & Measurement System
In this chapter, I would introduce you our experiment instrument and measurement system we use in NDL. We use high density plasma chemical vapor deposition system(HDPCVD), E-gun evaporator deposition system and DC magnetrons sputter deposition system to fabricate thin film solar cell. Then, we use N&K measurement system and UV-Visible transmission spectrum system to measure the reflection rate, transmission rate, refractive index and absorption coefficient. At last, I will introduce how we measure the characteristic, including conversion efficiency and quantum efficiency of solar cell.
3.1 High Density Plasma Chemical Vapor Deposition System
We use high density plasma chemical vapor deposition system to fabricate amorphous or microcrystalline silicon thin film. High density plasma system has high fractional ionization capacity. The high dissociation capacity of HDPCVD can be used to yield high-density plasma and markedly increased electron temperature, promoting the diffusion capability of the reactive radicals and eventually yielding low-defect a-Si and microcrystalline Si (μc-Si) films at low temperatures. Unlike PECVD, the electrode of high density plasma chemical vapor deposition system is a coil of flat metal like a spiral, see Figure 16. The energy is supplied by electrical currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields. It ionized the reactive gases and produce glow discharge which lead to faster deposition rate. In the high density plasma chemical vapor deposition system, the reactive energy is from not thermal energy but inductively coupled plasma, so the temperature of the substrates is just about 100~300oC, which is one of its advantage.
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Figure 16 The High Density Plasma Chemical Vapor Deposition System
3.2 E-Gun Evaporator Deposition System
An E-gun evaporator fires a high-energy beam from an electron gun to boil a small spot of material; since the heating is not uniform, lower vapor pressure materials can be deposited.
The beam is usually bent through an angle of 270° in order to ensure that the gun filament is not directly exposed to the evaporant flux, see Figure 17. We can use the system to fabricate metal gate on our devices. The metal material includes Al, Ni and Ag. The base pressure and process pressure is 5E-7 torr and 8E-6 respectively.Typical deposition rates for E-gun evaporation range from 5 to 100 angstrom per second.
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3.3 DC Magnetrons Sputter Deposition
Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering, that is ejecting, material from a "target," that is source, which then deposits onto a substrate, such as a silicon wafer. Sputtered atoms ejected from the target have a wide energy distribution, typically up to tens of eV (100000 K). The sputtered ions (typically only a small fraction — order 1% — of the ejected particles are ionized) can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber (causing resputtering). Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk. The entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is often an inert gas such as argon. For efficient momentum
Figure 3. 1 E-gun Evaporator Deposition System Figure 17 E-gun Evaporator Deposition System
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transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used. Reactive gases can also be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters.
The availability of many parameters that control sputter deposition make it a complex process, but also allow experts a large degree of control over the growth and microstructure of the film.
3.4 N&K Analyzer
The system simultaneously determines film thickness, n and k in the spectral range of 190 to 1000 nm, and provides non-destructive, real time, high throughput measurements directly on the device. The n&k system is also equipped with an automated X-Y stage for full sample mapping.
3.5 UV-Visible Spectroscopy
The UV-Visible spectroscopy system could measure the absorption and transmission of the material in the range of 200~1000 nm. The ultraviolet and visible spectroscopy uses light in the visible and adjacent (near-UV and near-infrared (NIR)) ranges. The absorption in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state.
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3.6 Solar Simulator
The solar simulator we use is Sol3A Class AAA Solar Simulator, type No. 94063A made by Newport. Sol3A Class AAA Solar Simulator with a 1000 Watt Xenon source and 6 x 6 inch illuminated area. All Oriel Sol3A simulators are certified to IEC 60904-9 Edition 2 (2007), JIS C 8912, and ASTM E 927-05 standards for Spectral Match, Non-Uniformity of Irradiance, and Temporal Instability of Irradiance. The Oriel Sol3A simulators all use a single lamp design to meet not one or two, but all three performance criteria without compromising the 1 Sun output power, providing true Class AAA performance. The system uses a black non-reflective finish to minimize stray light and incorporates captive screws for all panels requiring user access to facilitate lamp replacement, alignment, and filter changes. Safety interlocks prevent inadvertent exposure to UV light.
After we fabricate the Si thin film solar cell, we use the light from solar simulator illuminate on the device which connects to Keithley 2440 to make a I-V measurement, see Figure 18.
Because of the incident light illuminates on the side of glass, we have to overturn the device
Because of the incident light illuminates on the side of glass, we have to overturn the device