EE580 – Solar Cells Todd J. Kaiser Lecture 08 Solar Cell Characterization Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Solar Cell Operation n Emitter p Base Rear Contact Absorption of photon creates an electron hole pair. If they are within a diffusion length of the depletion region the electric field separates them. Antireflection coating The electron after passing through the load recombines with the hole completing the circuit Front Contact - + External Load Montana State University: Solar Cells Lecture 8: Characterization 2

Solar Cell Electrical Model PV is modeled as a current source because it supplies a constant current over a wide range of voltages It has p-n junction diode that supplies a potential It has internal resistors that impede the flow of the electrons Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Input Circuit Diagram Front Contact I Rs Recombination Ohmic Flow RLoad Rsh I Current Source V External Load Rear Contact Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Electrical Losses Series Resistance (Resistance of Hole & Electron Motion) Bulk Resistance of Semiconductor Materials Bulk Resistance of Metallic Contacts and Interconnects Contact Resistance Parallel Resistance or Shunt Resistance (Recombination of Hole and Electron) PN junction Leakage Leakage around edge of Junction Foreign Impurities & Crystal Defects Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Power & IV Curve Power (Watts) is the rate at which energy (Joules) is supplied by a source or consumed by a load… It is a rate not a quantity The power output by a source is the product of the current supplied and the voltage at which the current was supplied Power output = Source voltage x Source current P=V x I (Watts = Joules/second) = (Volts)x(Amperes) By changing the resistance of the load different currents and corresponding voltages can be measured and plotted Montana State University: Solar Cells Lecture 8: Characterization

Solar Cell I-V Characteristics Current Dark Current from Absorption of Photons Voltage Light Twice the Light = Twice the Current Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Operating Point Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization IV Plot Short Circuit Current Maximum Power Imp Operating Point Current Power Vmp Voltage (V) Open Circuit Voltage Volt (V) 0.1 0.3 0.5 0.54 0.57 Current (A) 1.3 1.2 0.75 Power (W) 0.13 0.39 0.6 0.4 Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Resistance Effects ISC ISC Current Current Increasing RS decreasing RP VOC VOC Voltage Voltage Ideal case RS = 0 and RP = ∞ Both reduce the area of the maximum power rectangle therefore reducing the efficiency and fill factor Montana State University: Solar Cells Lecture 8: Characterization 10

Loss Mechanisms in Solar Cells Optical Electrical Ohmic Recombination Reflection Shadowing Unabsorbed Radiation Solar Cell Material Base Emitter Contact Material Finger Collection Bus Junction Metal – Solar Cell Emitter Region Solar Cell Material Surface Base Region Space Charge Region Montana State University: Solar Cells Lecture 8: Characterization

Photovoltaic Effect Separation of holes and electrons by Electric Field Voltage Creation of extra electron hole pairs (EHP) Absorption of Light Power = V x I Excitation of electrons Current Movement of charge by Electric Field Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Linking Cells Solar cells are not usually used individually because they do not output sufficient voltage and power to meet typical electrical demands The amount of voltage and current they output can be increased by combining cells together with wires to produce larger area solar modules Cells can be connected in a number of ways Strings – where cells are connected in series Blocks 2 or more strings connected together in parallel Joining 2 or more blocks together Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Solar Cell Panels Current Current Parallel connections increase the current output Voltage Voltage Blocks increase both current and voltage output Series connections increase the voltage output Current Voltage Montana State University: Solar Cells Lecture 8: Characterization 14

Calculating Voltage and Current Series connections are made by connecting one cell’s n-type contact to the p-type of the next cell Parallel connections are made by joining each cells n-type contacts together and p-type contacts together Series connections the voltages add Parallel connections the current add Series connections the current flow is equal to the current from the cell generating the smallest current (limited by poorest cell) Parallel connections the voltage is the average of the cells or string in parallel Montana State University: Solar Cells Lecture 8: Characterization

Example: Cells Series Connected Cell A V = 0.58 V I = 0.28 A Cell B V = 0.54 V I = 0.31 A Cell C V = 0.61 V I = 0.25 A 1 2 The voltage across terminals 12 is the sum of the voltages V12 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V) The current through the cells is restricted by the smallest current produce by any of the cells I12 = 0.25 (A) Montana State University: Solar Cells Lecture 8: Characterization

Example: Cells Parallel Connected 3 Cell A V = 0.58 V I = 0.28 A Cell B V = 0.54 V I = 0.31 A Cell C V = 0.61 V I = 0.25 A 4 The voltage across terminals 34 is the average of the voltages V34 = (VA + VB + VC )/3 = (0.58 + 0.54 + 0.61)/3 = 0.58(V) The current at the terminals 34 is the sum of the currents in each cell I34 = (IA + IB +IC) = (0.28 + 0.31 + 0.25) = 0.84(A) Montana State University: Solar Cells Lecture 8: Characterization

Example: Block Connected 5 A A A B B B C C C 6 The voltage across terminals 56 given by the series voltage already calculated: V56 = VA + VB + VC = 0.58 + 0.54 + 0.61 =1.73(V) The current at the terminals 56 is the sum of the currents in each string already calculated I56 = 3(Istring) = 3(0.25) = 0.75(A) Montana State University: Solar Cells Lecture 8: Characterization

Montana State University: Solar Cells Lecture 8: Characterization Summary Linking Cells Linking modules or batteries is similar to connecting PV cells Series Connections Voltages are added in series connections The current is restricted to the smallest current Parallel connections The currents are added in parallel connections The voltages are averaged from each string Solar Cells and Modules are Matched to improve the power generated Montana State University: Solar Cells Lecture 8: Characterization