EE 332 Photovoltaic Cell Design Iowa State University Electrical and Computer Engineering Dept

EE 332 Photovoltaic Cell Design Iowa State University Electrical and Computer Engineering Dept Authors: Bai Rui, Senior Electrical Engineering Cui Qiaoya, Senior Electrical Engineering Chris Krantz, Senior Electrical Engineering Professor: Dr. Vikram Dalal Page 1 of 20

Abstract: The following paper discusses the basic physics, device structures and designs of photovoltaic cells. The main design that will be discussed in depth is the Silicon Back Contact Solar Cell which is one of the most common solar cells manufactured today. The device physics, structures, and characteristics will be discussed thoroughly throughout this paper. The Silicon Solar Cell is usually operated between 400nm and 1100nm. Typical values used in the solar cell design are as follows: Cell thickness 100-500 µm, doping base 1 Ω.cm, Reflection control front surface textured, emitter dopant n-type, emitter thickness < 1 µm, doping level of emitter 100 Ω/sqr, and grid pattern fingers 20 to 200µm width, placed 1-5 mm apart. Effective absorption thickness of the cell has been measured to be about 30 times the physical thickness. A reflective coating is used to reduce the amount of absorption as well as a pyramid design. Page 2 of 20

Contents Abstract:... 2 Introduction:... 4 PV Device Physics... 4 Device Structures... 7 Characteristic of material or device:... 10 Figure 15 Diagram of Solar Cell [4]... 14 Table 2 Typical Values Used [4]... 14 Specific device design or structure... 14 Figure 16 Cross Section Silicon Solar Cell [7]... 15 Figure 17 Rear Two Dimensional Simulation Left and Rear Cell Right [3]... 15 Discussion about the future improvement:... 16 Figure 18 Photovoltaic Cell Production as report by PV industry [8]... 16 Equation 4 Open Ciruit Voltage Equation [5]... 18 Conclusion:... 18 Works Cited... 19 Page 3 of 20

Silicon Back contact Solar Cell Introduction: The science of photovoltaic energy is the study of the changing sunlight into electricity by the used of photovoltaic cells. Photovoltaic itself means first photo which is light and voltaic meaning electricity. PV cells are most commonly made of semiconductors such as silicon (Si). The idea of a PV cell is to absorb as much light as possible when it strikes the cell. The energy of the light is absorbed into the semiconductor device colliding with electrons which allows them to flow freely. The movement of electrons gives rise to a current and electric field. The electric field forces the electrons to flow in one direction. When placing metal contacts at the top and bottom of the cell the current can be extracted for external uses. The current and cell s voltage together define the power that the solar cell can produce [1].The following paper will be discussing photovoltaic energy and photovoltaic cell design. The discussions will include a brief discussion of physic of PV devices and device structures, specific device design of the silicon back contact solar cell, characteristics of the material, and future status of the solar cell and its industry. PV Device Physics When analyzing the physics of an organic photovoltaic cell four things must be accomplished: Absorption, diffusion, charge separation, and charge transport. In order to determine the current developed in a solar cell one must look at the number of charges created and collected at the electrodes. The way to calculate this is the fraction of photons absorbed, electron hole pairs dissociated, fraction of charges reaching the electrodes, which determines the photo current efficiency [2]. Equation 1 Photo Current Efficiency [2] There are two forces which allow the holes and electrons to migrate to the electrodes. The two forces are the internal electric fields and concentration gradients. These two forces lead to the drift current and diffusion current. Other important device physics are necessary to Page 4 of 20

consider in order understanding the operation of the devices. The model that is useful to understanding these conditions is the metal insulator metal model or MIM. The operation of the device under various conditions short circuit as seen in figure A, open circuit condition as seen in figure B, reverse bias as seen in figure C, and forward bias as seen in figure D [2]. Looking at figure 1 there is no applied voltage and no net current develops. The electric field which is a result from the built in work functions from the two different metals which is evenly distributed throughout the device. When the material has light shined on it the carriers will drift in the electric field to the contacts. Electrons will migrate towards the metal with the lower work function while holds do the opposite. In the short circuit condition seen in figure 2 there is an open circuit voltage. Since there is no net driving force the current is zero. Figure 4 shows a reverse bias condition and only a small amount of current can flow. Under forward bias the charges are efficiently injected into the semiconductor [2]. Figure 1 Open Circuit Conditon [2] Figure 2 Short Circuit Condtion [2] Page 5 of 20

Figure 3 Reverse Bias [2] Figure 4 Forward Bias [2] The equivalent circuit of a solar sell is show in figure 5 along with a descriptive equation of the solar cell. Io is the dark current, e is the elementary charge, U is the voltage applied, Rsh is the shunt resistance, Iph is photo current, and Rs is series resistance. The two requirements for this circuit are that the shunt resistance is very large to prevent leakage current, and series resistance is very low to get a sharp rise in forward current [2]. The circuit s efficiency is the power out over power in as seen in figure 6. Figure 5Equivalent Circuit of Solar Cell [2] Equation 2 Solar Cell Equation [2] Page 6 of 20

Equation 3 Efficiency of Solar Cell [2] Device Structures There are four device structures for PV cells which have different advantages and disadvantages when implemented. The four devices structures are as follows: Single layer, bilayer heterojunction, bulk heterojunction, and diffuse bilayer-heterojunction [2]. When considering the four structures there are several considerations that need to be addressed. One important issue is the separation of the electron hole pairs. The energy which holds the excitons or photo-excitations, is (0.1~1eV) in comparison to Si [2]. The built in electric fields are on the order of (10^6 ~10^7 V/m) which is unable to separate the excitons effectively [2]. A process to separate the electron hole pairs is then developed which can do this effectively. The two main issues that surround the different architectures are charge separation and transport [2]. The first structure to be discussed is the single layer solar cell. This device is placed between two metal contacts with different work functions. This is accurately described by looking at a p-typ Schottky device [2]. The region which is bending and labeled as W the depletion region is where the excitons can be dissociated. The use for this device is a photodetector. Page 7 of 20

Figure 6 Single Layer Device [2] The second device discussed is the bilayer hererojuntion. The device has donor and acceptor material, which will be stacked together between two electrodes. This is advantageous because the holes and electrons are effectively separated from each other. They travel independently in their own material i.e. holes travel in the p-type donor material and electrons will travel in the n-type acceptor material [2]. Advantages of this are larger fill factor is realized, the photo current dependency on illumination dependency is linear, and charge recombination is reduced [2]. Figure 7 Bilayer Heterojunction [2] The third device is the bulk heterojunction which mixes the donor and acceptor components so that there can be a disassociation of charges anywhere along the junction. Similarly to the heterojunction the charge recombination is reduced and the photo current follows light intensity linearly [2]. The most efficient devices today are based upon the Page 8 of 20

P3HT:PCBM blends with a power conversion efficiency of 3.5% with AM 1.5 [2]. Figure 8 Bulk Heterojunction [2] The final device structure is the diffuse bilayer heterojunction. This device tries to capture the advantages of an enlarged donor to acceptor interface and an uninterrupted pathway so that charge carriers reach the electrodes [2]. Power conversion efficiency is 2% with laminated polymer:polymer device in AM 1.5 conditions [2]. Figure 9 Bilayer Heterojunction [2] Page 9 of 20

Characteristic of material or device: 1) Optical absorption characteristics The best beginning to research the solar cell is to make more information about sunlight. The solar cell is used to convert the electromagnetic energy into electrical energy. The sunlight is magic if carefully observed the solar absorption spectra. There are some dark lines which are caused by atoms in the sun s and the earth s atmosphere absorbing precise energy of light, described by the Science of the Silicon Solar Cell website. There is a common method to characterize a light source, the function of photon wavelength. In the spectral irradiance, it is available to figure out the power density at a particular wavelength. The photon flux is really important in solar cell research; it can be inverted into the spectral irradiance, because there is an equation could reflect their relationship, it is F (λ) =Φ*E* Δλ. In this equation, F (λ) is the spectral irradiance; Φ is the photon flux in a certain number of photons and E and λ are the energy and wavelength of the photon. There is the spectral irradiance of artificial light sources in the left axis and the spectral irradiance from the sun in right axis [3]. If we have a good knowledge of a light source, it is will be really helpful for design the solar cell. Figure 10 Spectral Irradiance [3] The back contact solar cell is a well-known silicon solar cell. Therefore, it is necessary to learn about the optical features. To realized maximize the cell performance, it has to achieve that as much light as possible of useful wavelengths couple into cell and are absorbed by the cell [3]. Silicon solar cells could be usually operated between 400nm and 1100nm. Effective absorption thickness of the cell has been measured to be about 30 times the physical thickness. Page 10 of 20

(Encapsulation) Antireflection coating Front contact n-window layer p-absorber layer back contact Figure 11 Active Layers Solar Cells [4] In a silicon solar cell, silicon is weakly absorbing the light and the optical characteristics depend on many different materials [3]. To make the cell working as expected, it is essential to reach the each part optical requirement. From the observing the above figure, there are some basic stacks in a solar cell [4]. To analysis the process of the light absorption, it has to understand the each stack characteristics. The top part is termed Encapsulation which normally composed of glass plate and some organic glue [5]. When the light is coming into the solar cell, it will firstly strikes on this stack and then goes through the antireflection coating layer which function is to reduce the reflection loss. The optical losses are due to the reflection loss, the method to minimize this Figure 12 Stack Characteristics [5] loss is to established this stack and make it as large aspect ratio as possible. There are many design is to reduce the absorbing rate of silicon cell, which is called the staggered inverted pyramid and the layout is shown on the right hand. This approach could help the light reflected into a new direction which is leading it into the cell surface [4]. For the front contact layer, it should have the both transparency property for light and high electric conductivity property. The metal could not have these two properties, but the highly doped semiconductors like silicon has a good transparent for light and has certain conductivity. Page 11 of 20

Then the next layer is most important layer which is key part of pn-junction in silicon solar cell. In this layer, to obtain the relative lower absorption rate, it is better to use the wide band gap semiconductor. As for the p layer where is a place starting the absorption, because of its higher electrons mobility. For silicon, a kind of indirect band gap absorber, when the light is coming in, it could not be absorbed directly indirect band gap absorber, because there are no high absorption coefficients allowed the absorbed the photons. It is reason why in silicon solar cell, the thick absorbers and light trapping techniques are required. This property should be shown in the right hand chart. In this chart, the relationship between the absorption coefficient of silicon and the wavelength is clear. The drop in absorption at the band gap around 1100nm is sharper. The new photons coming inside and then created the holes pair, these holes pair are separated by the electric field to accumulate in electric contacts [4]. Figure 13 Wavelength vs. Absorption [4] The last layer is called the back contact. If the light reaches this layer, it will be reflected back into the solar cell. Therefore, making the back surface highly reflective will reflect many of the photons who is reaching this surface into the front surface and finally more and more photons has opportunity to generate the holes in order to produce electron hole pairs in pn junction [4]. 2) Physics characteristics of silicon and silicon solar cell parameters Silicon is the most popular material used for solar cell and in the solar cell discussed in this research paper is same as well. Silicon is the major material element in designing. For the properties of silicon, there are some basic silicon physics characteristics in the below table [6]: Band gap 1.1ev Valance electrons 4 Production cell efficiencies 12% Maximum efficiency 15% Page 12 of 20

Table 1 Basic Silicon Characteristics [6] Silicon atoms in the crystal are electrically neutral, when the light is pointing to it, if a photon of light Figure 14 Band Diagram [3] has enough energy, a valence electron can absorb this photon and then it will jump into the conduction band. If there are free electrons moving inside the conduction band, the current is being generated. Doping process is the process makes some impurities atoms replace the original silicon s place in crystal. n-type silicon is made by adding impurities, such as phosphorus which is called electron donor. As for the p-type silicon is making by adding impurities, such as boron which is called electron acceptor. When n-type and p-type silicon contact each other, they will make a pn junction. The electrons and holes can diffuse arcos this junction. Under the pn junction condition, the light is pointing into it; the electron will be excited from valence band into conduction band so that the electric field has been established. There will be positive and negative region formed inside the pn junction. If this physics characteristic is used into the solar cell, when the light is shining on the solar cell, there will be electric field generated and current will going through it. The solar cell is like a battery start to generate the power and make the device moving. For silicon solar cell parameters, because in the whole production processing, there are many constricts from silicon properties. The design should follow these principles depending on each important characteristic [3]. According to many people s measurement and calculation, some parameters are in summarized in the below table [4]: Page 13 of 20

Substrate Material Cell thickness Doping of Base Reflection control Emitter Dopant Figure 15 Diagram of Solar Cell [4] Silicon 100-500µm 1 Ω.cm Front surface typically textured n-type Emitter Thickness Less than 1 µm Doping level of Emitter Grid Pattern 100 Ω/sqr Fingers 20 to 200µm width, placed 1-5 mm apart Table 2 Typical Values Used [4] Specific device design or structure Back contacted Solar Cells has been talking and exploring for many years. The backcontacted solar cell is divided into three different classed. They are back-junction (BJ), emitter wrap through (EWT) and metallization wrap through (MWT). In this part, there are more detailed description about BJ device design and structure. The back-contact back junction solar cell is a two-dimensional modeling [3]. 1). Cross-section of the back-contact back-junction silicon solar cell [3] Page 14 of 20

Figure 16 Cross Section Silicon Solar Cell [7] 2). the symmetry element used in two-dimensional simulations on the left while a photograph of the rear cell side on the right are shown [3]. Figure 17 Rear Two Dimensional Simulation Left and Rear Cell Right [3] In the two dimensional simulations, there are some symmetry elements of the solar cell should be thought about. In the simulations, it is not true to simulate every corner of solar cell. The simulated part is just the active solar cell areas. In the simulation process, other related properties should be considered, such as the optical properties [3]. Page 15 of 20

Discussion about the future improvement: The world energy sector is facing two issues the decline of fossil fuels and the continuing production of greenhouse gasses. The types of energy available such as nuclear energy have reduced carbon emissions for electricity for production, but it is uncertain how long that will last and disposal of nuclear waste is becoming difficult. There are many types of renewable energy being considered today. Hydroelectric power is one of these renewable sources, but there are limitations on where the technology can be used. The other renewable source is wind energy however the total usable wind energy is in the 2-4 TeraWatts/Year (TWyr) [8]. Finally there is solar energy which strikes the earth with approximately 125,000 TW [8]. This could meet the worlds energy needs to end reliability on fossil fuels. In figure 19 there is a chart on the production of PV cells in major countries. It appears to be growing exponentially year to year. One issue that the PV market is having is the cost for electricity generation. The current cost for generation is about $0.25-$0.65/KWh while coal based fuel sources are about $0.04 /kwh [8]. There is research being done in order to lower the cost of production for solar cells as well as government subsidies for people who are utilizing solar cells. Another issue is a shortage in silicon. There are many price reductions which have occurred because of the achievements made with silicon. Higher efficiency cells are being created along with the development of thinner silicon wafers. Eventually though there is a limitation on how thin the wafers can become. Figure 18 Photovoltaic Cell Production as report by PV industry [8] There are many plans for future improvements of the current technology of the solar cells which will raise the efficiency above 25%. Some of those improvements to above technology include improved light trapping schemes, elimination of losses due to reflection from metallized and non-metallized regions, the reduction of top diffused region and contact recombination [5]. Page 16 of 20

The PERL cell configuration plays an important role in the improved light trapping scheme with the combination of the rear reflector. Using this kind of configuration will increase the path length of weakly absorbed light into the cell [5]. This configuration will improve the solar cells ability to absorb different wavelengths of light. The reflectance from this combination is dependent on the thickness of the oxide material and the angle of incidence of the light however it is typically about 95% as low as 90% [5]. In order to reduce the contact recombination the manufacturers use high quality material to reduce the recombination. The thinner the cell design the lower the bulk recombination will be. However there are tradeoffs when making the cell thinner in order to achieve the lower recombination. A couple tradeoffs include reduced mechanical strength of the material and light absorbing capability [5]. The improvement in material to reduce recombination, light trapping schemes, elimination of losses due to reflection from metallized and non-metallized regions, and the reduction of top diffused region are not the only improvements being done to the semi-conductor devices. New methods include improvement in current output, voltage output, and new design structures. Improvement in current output is realized by design regions of the solar cell to have a band gap which is lower than that of pure Si. The ways that the band gap can be reduced are methods such as doping, alloying and electric fields [5]. Of the three approaches to reduce the band gap alloying is the most promising. Reasoning is introducing large electric fields in homojunction cells will not allow for significant improvement in the band gap. If doping is used to change the band gap large quantities are require in order to achieve this change which also increases the likelihood of absorption [5]. Alloying germanium is one material where the band gap reduction is possible. This approach becomes the most effective when the alloyed region is kept from the cell surface and when it extends across the depletion region that is associated with the cell junction [5]. This means that the only way that efficiency can be improved is if solar cell performance is limited to surface effects. Another method is to improve voltage in the PERL cells. The way that this is done is by Auger recombination process within the bulk of the cell. The highest open circuit voltage for lightly doped mater is given in the following equation [5]: Page 17 of 20

( ) ( ) ( ( ) ) Equation 4 Open Ciruit Voltage Equation [5] Cn and Cp are the electron hole Auger coefficients in silicon, ni is intrinsic carrier concentration, W cell thickness, JL is the light generated current, k Boltzmann s constant, q is charge. The final method which will be discussed is new innovative structures which are being developed. One structure which will improve the cell efficiency is tandem cell. However there are difficulties in using Si for this method. There are very few materials in which are good matches of the lattice constant for the band gap of Si. In order to grow anything on top of Si another buffer is required to help with the lattice mismatch with Si. Currently the efficiency recorded by using this method is 20% [5]. One other method being researched is hydrogenated amorphous silicon cells on top of the silicon which has the appropriate band gap, but yields low open circuit voltage [5]. Other methods come from the use of crystalline silicon on crystalline silicon cell tandem. Blue light is readily absorbed and only a thin cell is required for this method on the top most cells in tandem. Finally, there is the idea of superlatttices though this is a largely new and unexplored field. The methods of improving current, voltage, and design are not the only things being explored. This research for improving PV devices is not cheap. The processes which these devices are made are also being researched in order to improve the fabrication processes. These processes include using thin film techniques which lower the cost of the substrates, material usage, and processing expenses [8]. Conclusion: The development of new energies over the next coming years will be increasing due to the decreasing of fossil fuels. Research into renewable energies is necessary in order to keep the needs of the world met. This research will point us to the improvement of solar cell efficiency and a growing market for engineers with knowledge of the function of solar cells. The methods, processes, designs, and techniques will be continually improved until the goals of higher efficiency and lower cost solar cells can be met. Page 18 of 20

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