Richard R. King, Chris M. Fetzer, Peter C. Colter, Ken M. Edmondson, James H. Ermer, Hector L. Cotal, Hojun Yoon, Alex P. Stavrides, Geoff Kinsey, Dimitri D. Krut, N. H. Karam Presented at the 29 th IEEE Photovoltaic Specialists Conference – New Orleans, Louisiana, May 20-24, 2002 Presented by Parth Bhide

 Multijunction cells employ a tandem of individual cells wired together each with its own bandgap.  The varying bandgaps allow for a wider range of absorbed photon wavelengths.  The bandgap is the amount of energy required to liberate an electron into the conduction band.  Measured in electron volts (eV). nt/postgraduate/regs/mpags/ex5/bandstructure/

 Photovoltaic (PV) Cells- The scientific name for solar panels.  Current (I)- The amount of particles moving in an electric circuit.  Voltage(V)- The strength of the electric circuit. King, R. R., A. Boca, X. Q. Liu, D. Bhusari, D. Larrabee, K. M. Edmondson, D. C. Law, C. M. Fetzer, S. Mesropian, W. Hong, and N. H. Karam. "Band-gap- engineered Architectures for High-efficiency Multijunction Concentrator Solar Cells." 24th European Photovoltaic Solar Energy Conference and Exhibition, Hamburg, Germany, Sep (2009): n. pag. Web.

 Doping- An impurity is intentionally added to a material.  Minority Carrier- The carrier of the electrical charge which is opposite to the doping of the layer in which the carrier is created.  Carrier Lifetime- The average amount of time before two carriers rejoin after separated.  Irradiance- The amount of incoming energy in an area. 1 sun is W/cm 3. Irradiance is adjusted by concentrator PV systems.

 Base  The bases are responsible for current generation.  The bases are GaInP/GaInAs/Ge.  Tunnel Junction  Areas with high doping and high bandgaps which limit photogeneration. These are situated between bases to act as a separator.  Other layers, such as the back-surface field and the window layers are similarly done as tunnel junctions.

 R. R. King, N. H. Karam, J. H. Ermer, M. Haddad, P. C. Colter, T. Isshiki, H. Yoon, H. L. Cotal, D. E. Joslin, D. D. Krut, R. Sudharsanan, K. M. Edmondson, B. T. Cavicchi, and D. R. Lillington, "Next-Generation, High-Efficiency III-V Multijunction Solar Cells," Proc. 28th IEEE Photovoltaic Specialists Conf. (IEEE, New York, 2000), p  The efficiency of multijunction solar cells benefits from a combination of varying bandgaps.  R. R. King, M. Haddad, T. Isshiki, P. C. Colter, J. H. Ermer, H. Yoon, D. E. Joslin, and N. H. Karam, "Metamorphic GaInP/GaInAs/Ge Solar Cells," Proc. 28th IEEE Photovoltaic Specialists Conf. (IEEE, New York, 2000), p  A greater current production can occur in the middle cell if the bandgap is lower due to the excess current in the bottom cell.

 M. W. Wanlass, T. J. Coutts, J. S. Ward, K. A. Emery, T. A. Gessert, and C. A. Osterwald, "Advanced High- Efficiency Concentrator Tandem Solar Cells," Proc. 22nd IEEE Photovoltaic Specialists Conf. (IEEE, New York, 1991), p. 38.  An optimal bandgap for the top layer is 1.9eV for the terrestrial spectrum.

 To improve the efficiency of multijunction PV cells by altering the material composition and the bandgap of the bases and of the auxiliary layers.

 Multijunction photovoltaic cells with a GaInP/GaInAs/Ge composition.  Different ratios were tested to determine which had the greatest efficiency.

 A sensor measured the outputs for both current and voltage.  Another sensor measured minority carrier lifetimes.  When photons hit the PV cell, the values were recorded.  These values determined efficiency. electric-appliances.html

 Results were measured by an I-V curve which uses both current and voltage to determine overall power and efficiency.  The standard AM1.5G atmosphere was used to simulate earth-like conditions.  A temperature of 25.0°C was maintained.  suns were used as irradiance levels.

 The highest recorded efficiency was 34%.  Relationships measured between  Bandgap and Efficiency  Minority Carrier Lifetime and Efficiency  Irradiance and Efficiency

 A mole fraction of 56%-In GaInP achieved the desired 1.9 eV bandgap for the upper cell.  A 1%-In GaInAs layer achieved the desired eV bandgap for the middle cell.  This bandgap functioned better than a 0%-In composition due to its use of current taken from the bottom Ge substrate.

 These bandgap levels allow for a maximum of open-circuit voltage (V OC ) and therefore efficiency.  Bandgap ratio is significant for the middle cell.  Total current and voltage levels must be balanced.

 The minority carrier lifetime is directly related to the efficiency.  If the lifetime is longer (a slower rate of recombination), then the efficiency will be greater.  A lifetime of 47ns was achieved for the GaInP layer.  A lifetime of 2450ns was achieved for the 1%- In GaInAs layer.

 Two main types of recombination are present.  Radiative recombination is natural. It depends on the material.  Shockley-Read-Hall (SRH) recombination is caused by material imperfections.  A longer minority carrier lifetime allows for more liberated electrons to complete the circuit, leading to higher efficiencies.

 Concentrator systems increase irradiance for the PV cell.  An increase in irradiance leads to an increase in efficiency, with a maximum at approximately 400 suns. quick-news-july-13-what-is-in-wind.html

 Properties of 4- and 5- junction cells.  Fine tuning bandgaps for three and more layers.  Increasing irradiance levels.  Controlling SRH recombination.  Maintaining voltage, current, and resistance values.

 King, Richard R., Chris M. Fetzer, Peter C. Colter, Ken M. Edmonson, James H. Ermer, Hector L. Cotal, Hojun Yoon, Alex P. Starvides, Geoff Kinsey, Dimitri D. Krut, and N. H. Karam. "High-Efficiency Space and Terrestrial Multijunction Solar Cells Through Bandgap Control in Cell Structures." 29th IEEE Photovoltaic Specialists Conference – New Orleans, Louisiana, May 20-24, 2002 (2002): n. pag. Print.  King, R. R., A. Boca, X. Q. Liu, D. Bhusari, D. Larrabee, K. M. Edmondson, D. C. Law, C. M. Fetzer, S. Mesropian, W. Hong, and N. H. Karam. "Band-gap-engineered Architectures for High-efficiency Multijunction Concentrator Solar Cells." 24th European Photovoltaic Solar Energy Conference and Exhibition, Hamburg, Germany, Sep (2009): n. pag. Web.  M. W. Wanlass, T. J. Coutts, J. S. Ward, K. A. Emery, T. A. Gessert, and C. A. Osterwald, "Advanced High-Efficiency Concentrator Tandem Solar Cells," Proc. 22nd IEEE Photovoltaic Specialists Conf. (IEEE, New York, 1991), p. 38.  R. R. King, M. Haddad, T. Isshiki, P. C. Colter, J. H. Ermer, H. Yoon, D. E. Joslin, and N. H. Karam, "Metamorphic GaInP/GaInAs/Ge Solar Cells," Proc. 28th IEEE Photovoltaic Specialists Conf. (IEEE, New York, 2000), p  R. R. King, N. H. Karam, J. H. Ermer, M. Haddad, P. C. Colter, T. Isshiki, H. Yoon, H. L. Cotal, D. E. Joslin, D. D. Krut, R. Sudharsanan, K. M. Edmondson, B. T. Cavicchi, and D. R. Lillington, "Next-Generation, High-Efficiency III-V Multijunction Solar Cells," Proc. 28th IEEE Photovoltaic Specialists Conf. (IEEE, New York, 2000), p. 998.

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