Solar Cells Basic Concepts Materials and Device Structures Technical Issues System Design Modified Lecture based on the Original Presentation by J. M. Pearce,

Photovoltaic (PV) systems convert light energy directly into electricity. Commonly known as “solar cells.” The simplest systems power the small calculators we use every day. More complicated systems will provide a large portion of the electricity in the near future. PV represents one of the most promising means of maintaining our energy intensive standard of living while not contributing to global warming and pollution. What are Photovoltaics?

A Brief History Photovoltaic Technology 1839 – Photovoltaic effect discovered by Becquerel. 1870s – Hertz developed solid selenium PV (2%) – Photoelectric effect explained by A. Einstein. 1930s – Light meters for photography commonly employed cells of copper oxide or selenium – Bell Laboratories developed the first crystalline silicon cell (4%) – PV cells on the space satellite U.S. Vanguard (better than expected).

Things Start To Get Interesting... mid 1970s – World energy crisis = millions spent in research and development of cheaper more efficient solar cells – First amorphous silicon cell developed by Wronski and Carlson. 1980’s - Steady progress towards higher efficiency and many new types introduced 1990’s - Large scale production of solar cells more than 10% efficient with the following materials: –Ga-As and other III-V’s –CuInSe 2 and CdTe –TiO 2 Dye-sensitized –Crystalline, Polycrystalline, and Amorphous Silicon Today prices continue to drop and new “3 rd generation” solar cells are researched.

Types of Solar Photovoltaic Materials

Photovoltaic Materials

Solar PV Materials: Crystalline & Polycrystalline Silicon Advantages: –High Efficiency (14-22%) –Established technology (The leader) –Stable Disadvantages: –Expensive production –Low absorption coefficient –Large amount of highly purified feedstock

Amorphous Silicon Advantages: High absorption (don’t need a lot of material) Established technology Ease of integration into buildings Excellent ecological balance sheet Cheaper than the glass, metal, or plastic you deposit it on Disadvantages: Only moderate stabilized efficiency 7- 10% Instability- It degrades when light hits it –Now degraded steady state

How do they work? The physics view

p-n and p-i-n Junctions EfEf EfEf V bi

Barrier Changes Equilibrium means there is no net current Reduced barrier height is called forward bias (positive voltage applied to p-side) –Result- increases current through diode Increased barrier height is called reverse bias. –Result- decreases current to a very small amount..

Electric Currents in p-n Junction Under External Bias Diode I-V Characteristics

Current in a Solar Cell Output current = I = I l -I o [ exp(qV/kT)-1] –I l =light generated current –q = electric charge –V = voltage –k = Boltzman’s constant = × J/K When in open circuit (I=0) all light generated current passes through diode When in short circuit (V=0) all current passes through external load 2 Important points: 1) During open circuit the voltage of open circuit, V oc = (kT/q) ln( I l /I o +1) 2) No power is generated under short and open circuit - but P max = V m I m =FFV oc I sc

I-V Curve for Solar Cells Fourth quadrant (i.e., power quadrant) of the illuminated I-V characteristic defining fill factor (FF) and identifying Jsc and Voc

Light Absorption by a Semiconductor Photovoltaic energy relies on light. Light → stream of photons → carries energy Example: On a clear day 4.4x10 17 photons hit 1 m 2 of Earth’s surface every second. Eph( )=hc/ =hf –h = plank’s constant = x J-s – = wavelength –c = speed of light =3 x 10 8 m/s –f = frequency However, only photons with energy in excess of bandgap can be converted into electricity by solar cells.

The Solar Spectrum The entire spectrum is not available to single junction solar cell

Generation of Electron Hole Pairs with Light Photon enters, is absorbed, and lets electron from VB get sent up to CB Therefore a hole is left behind in VB, creating absorption process: electron-hole pairs. Because of this, only part of solar spectrum can be converted. The photon flux converted by a solar cell is about 2/3 of total flux.

Generation Current Generation Current = light induced electrons across bandgap as electron current Electron current:= Ip=qNA –N = # of photons in highlighted area of spectrum –A = surface area of semiconductor that’s exposed to light Because there is current from light, voltage can also occur. Electric power can occur by separating the electrons and holes to the terminals of device. Electrostatic energy of charges occurs after separation only if its energy is less than the energy of the electron-hole pair in semiconductor Therefore Vmax=Eg/q Vmax= bandgap of semiconductor is in EV’s, therefore this equation shows that wide bandgap semiconductors produce higher voltage.

Direct vs Indirect Bandgap Everything just talked about, where all energy in excess of bandgap of photons are absorbed, are called direct-bandgap semiconductors. More complicated absorption process is the indirect-gap series –quantum of lattice vibrations, of crystalline silicon, are used in the conversion of a photon into electron-hole pair to conserve momentum there hindering the process and decreasing the absorption of light by semiconductor.

The Solar Cell Electric current generated in semiconductor is extracted by contacts to the front and rear of cell. Widely spaced thin strips (fingers) are created so that light is allowed through. –these fingers supply current to the larger bus bar. Antireflection coating (ARC) is used to cover the cell to minimize light reflection from top surface. ARC is made with thin layer of dielectric material.

Different Types of Photovoltaic Solar Cells Diffusion Drift Excitonic

Diffusion n-type and p-type are aligned by the Fermi- level When a photon comes in n-type, it takes the place of a hole, the hole acts like an air bubble and “floats” up to the p-type When the photon comes to the p-type, it takes place of an electron, the electron acts like a steel ball and “rolls” down to the n-type

Drift There is an intrinsic gap where the photon is absorbed in and causes the electron hole pair to form. The electron rises up to the top and drifts downwards (to n-type) The hole drifts upwards (to p-type)

Excitonic Solar Cell Dye molecule –electron hole pair splits because it hits the dye –the electron shifts over to the electric conductor and the hole shifts to the hole conductor

Power Losses in Solar Cells

Recombination Opposite of carrier generation, where electron-hole pair is annihilated Most common at: –impurities –defects of crystal structure –surface of semiconductor Reducing both voltage and current

Series Resistance Losses of resistance caused by transmission of electric current produced by the solar cell. I-V characteristic of device: I = I l -I 0 [exp(qV+IRs / mkT) – 1] m= nonideality factor

Other Losses Current losses- called collection efficiency, ratio b/w number of carriers generated by light by number that reaches the junction. Temperature dependence of voltage –V decreases as T increases Other losses – light reflection from top surface –shading of cell by top contacts –incomplete absorption of light

Minimize Recombination Losses by Adapting the Device

Tandem Cells Tandem cell- several cells, – Top cell has large bandgap –Middle cell mid eV bandgap –Bottom cell small bandgap. Indium Tin Oxide p-a-Si:H Blue Cell i-a-Si:H n-a-Si:H p Green Cell i-a-SiGe:H (~15%) n p Red Cell i-a-SiGe:H (~50%) n Textured Zinc Oxide Silver Stainless Steel Substrate Silver Grid Schematic diagram of state-of-the- art a-Si:H based substrate n-i-p triple junction cell structure.

Examples of Photovoltaic Systems

Three Types of Systems Stand-alone systems - those systems which use photovoltaics technology only, and are not connected to a utility grid. Hybrid systems - those systems which use photovoltaics and some other form of energy, such as diesel generation or wind. Grid-tied systems - those systems which are connected to a utility grid.

Stand Alone PV System Water pumping

Examples of Stand Alone PV Systems PV powers stock water pumps in remote locations in Wyoming PV panel on a water pump in Thailand

Examples of Stand Alone PV Systems Communications facilities can be powered by solar technologies, even in remote, rugged terrain. Also, if a natural or human-caused disaster disables the utility grid, solar technologies can maintain power to critical operations

Examples of Stand Alone PV Systems This exhibit, dubbed "Solar Independence", is a 4-kW system used for mobile emergency power. while the workhorse batteries that can store up to 51 kW-hrs of electricity are housed in a portable trailer behind the flag. The system is the largest mobile power unit ever built

Examples of Stand Alone PV Systems Smiling child stands in front of Tibetan home that uses 20 W PV panel for electricity PV panel on rooftop of rural residence

Hybrid PV System

Examples of Hybrid PV Systems Ranching the Sun project in Hawaii generates 175 kW of PVpower and 50 kW of wind power from the five Bergey 10 kW wind turbines

Examples of Hybrid PV Systems A fleet of small turbines; PV panels in the foreground

Examples of Hybrid PV Systems PV / diesel hybrid power system - 12 kW PV array, 20 kW diesel genset This system serves as the master site for the "top gun" Tactical Air Combat Training System (TACTS) on the U.S. Navy's Fallon Range.

Grid-Tied PV System

Examples of Grid Tied Systems National Center for Appropriate Technology Headquarters

Examples of Grid Tied Systems The world's largest residential PV project

Designing a PV System 1.Determine the load (energy, not power) You should think of the load as being supplied by the stored energy device, usually the battery, and of the photovoltaic system as a battery charger. Initial steps in the process include: 2.Calculating the battery size, if one is needed 3.Calculate the number of photovoltaic modules required 4.Assessing the need for any back-up energy of flexibility for load growth Stand-Alone Photovoltaic Systems: A Handbook of Recommended Design Practices details the design of complete photovoltaic systems.

Determining Your Load The appliances and devices (TV's, computers, lights, water pumps etc.) that consume electrical power are called loads. Important : examine your power consumption and reduce your power needs as much as possible. Make a list of the appliances and/or loads you are going to run from your solar electric system. Find out how much power each item consumes while operating. –Most appliances have a label on the back which lists the Wattage. –Specification sheets, local appliance dealers, and the product manufacturers are other sources of information.

Determining your Loads II Calculate your AC loads (and DC if necessary) List all AC loads, wattage and hours of use per week (Hrs/Wk). Multiply Watts by Hrs/Wk to get Watt-hours per week (WH/Wk). Add all the watt hours per week to determine AC Watt Hours Per Week. Divide by 1000 to get kW-hrs/week

Determining the Batteries Decide how much storage you would like your battery bank to provide (you may need 0 if grid tied) – expressed as "days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar panels or the grid. Also consider usage pattern and critical nature of your application. If you are installing a system for a weekend home, you might want to consider a larger battery bank because your system will have all week to charge and store energy. Alternatively, if you are adding a solar panel array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated in needed for recharging.

Batteries II Once you have determined your storage capacity, you are ready to consider the following key parameters: –Amp hours, temperature multiplier, battery size and number To get Amp hours you need: 1.daily Amp hours 2.number of days of storage capacity ( typically 5 days no input ) –1 x 2 = A-hrs needed –Note: For grid tied – inverter losses

Temperature Multiplier Temp o F 80 F 70 F 60 F 50 F 40 F 30 F 20 F Temp o C 26.7 C 21.2 C 15.6 C 10.0 C 4.4 C -1.1 C -6.7 C Multiplier Select the closest multiplier for the average ambient winter temperature your batteries will experience.

Determining Battery Size Determine the discharge limit for the batteries ( between ) –Deep-cycle lead acid batteries should never be completely discharged, an acceptable discharge average is 50% or a discharge limit of 0.5 Divide A-hrs/week by discharge limit and multiply by “temperature multiplier” Then determine A-hrs of battery and # of batteries needed - Round off to the next highest number. –This is the number of batteries wired in parallel needed.

Total Number of Batteries Wired in Series Divide system voltage ( typically 12, 24 or 48 ) by battery voltage. –This is the number of batteries wired in series needed. Multiply the number of batteries in parallel by the number in series – This is the total number of batteries needed.

Determining the Number of PV Modules First find the Solar Irradiance in your area Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of Watts (or kiloWatts) per square meter (W/m 2 ) for irradiance. For detailed Solar Radiation data available for your area in the US:

How Much Solar Irradiance Do You Get?

Solar Radiation On any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Insolation (now commonly referred as irradiation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter squared (kW-hrs/m 2 ) - this value is equivalent to "peak sun hours".

Peak Sun Hours Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 W/m 2, that gives the same energy received from sunrise to sundown. Peak sun hours only make sense because PV panel power output is rated with a radiation level of 1,000W/m 2. Many tables of solar data are often presented as an average daily value of peak sun hours (kW-hrs/m 2 ) for each month.

Calculating Energy Output of a PV Array Determine total A-hrs/day and increase by 20% for battery losses then divide by “1 sun hours” to get total Amps needed for array Then divide your Amps by the Peak Amps produced by your solar module –You can determine peak amperage if you divide the module's wattage by the peak power point voltage Determine the number of modules in each series string needed to supply necessary DC battery Voltage Then multiply the number (for A and for V) together to get the amount of power you need –P=IV [W]=[A]x[V]

Charge Controller Charge controllers are included in most PV systems to protect the batteries from overcharge and/or excessive discharge. The minimum function of the controller is to disconnect the array when the battery is fully charged and keep the battery fully charged without damage. The charging routine is not the same for all batteries: a charge controller designed for lead-acid batteries should not be used to control NiCd batteries. Size by determining total Amp max for your array

Wiring Selecting the correct size and type of wire will enhance the performance and reliability of your PV system. The size of the wire must be large enough to carry the maximum current expected without undue voltage losses. All wire has a certain amount of resistance to the flow of current. This resistance causes a drop in the voltage from the source to the load. Voltage drops cause inefficiencies, especially in low voltage systems ( 12V or less ). See wire size charts here:

Inverters For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power –just the inverter. The Inverter changes the DC current stored in the batteries or directly from your PV into usable AC current. –To size increase the Watts expected to be used by your AC loads running simultaneously by 20%

Books for Designing a PV System Steven J. Strong and William G. Scheller, The Solar Electric House: Energy for the Environmentally- Responsive, Energy-Independent Home, by Chelsea Green Pub Co; 2nd edition, This book will help with the initial design and contacting a certified installer.

Books for the DIYer If you want to do everything yourself also consider these resources: –Richard J. Komp, and John Perlin, Practical Photovoltaics: Electricity from Solar Cells, Aatec Pub., 3.1 edition, (A layman’s treatment). –Roger Messenger and Jerry Ventre, Photovoltaic Systems Engineering, CRC Press, (Comprehensive specialized engineering of PV systems).

Photovoltaics Design and Installation Manual Photovoltaics: Design & Installation Manual by SEI Solar Energy International, 2004 A manual on how to design, install and maintain a photovoltaic (PV) system. This manual offers an overview of photovoltaic electricity, and a detailed description of PV system components, including PV modules, batteries, controllers and inverters. Electrical loads are also addressed, including lighting systems, refrigeration, water pumping, tools and appliances.

Solar Photovoltaics is the Future