Part V. Solar Cells Introduction Basic Operation Mechanism Materials for Solar Cells Design Considerations of Solar Cell Various of Device Configurations Optical Concentration

Introduction to Solar Cells The solar cell is a semiconductor device that convert directly the solar energy into the electric energy by a photovoltaic (PV) effect. Solar cells are useful for both space and terrestrial applications. Long-duration power supply for satellites. An alternative terrestrial energy source (safe, convenient and clear) Advantages of solar power generation: Nearly permanent natural power source (~ 1010 years) Low operating cost (fuel and transportation cost are not needed) Virtually non-polluting Flexible module size Highly distributive

Solar Radiation Solar radiation is primarily as electro-magnetic radiation in the ultraviolet to infrared region (0.2 ~ 3 m), from a nuclear fusion reaction in the sun. Solar constant The intensity of solar radiation in free space at the average distance of the Earth from the sun. The value of the solar constant is 1353 W/m2 Air mass (AM) The degree to which the atmosphere affects the sunlight received at the Earth’s surface. AM0 : the solar spectrum outside the Earth’s atmosphere (1353 W/m2). AM1: the sunlight at the Earth’ s surface when the sun is overhead (at which point the incident is about 925 W/m2. Atmospheric attenuation of sunlight Ultraviolet absorption in the ozone Infrared absorption in the water vapor Scattering by airborne dust and aerosols. GaAs solar cells are better matched to the solar spectral and provide greater efficiencies than the Si solar cells.

Comparison between the Solar Cell and the Photodiode For a photodiode only a narrow wavelength range centered at the optical signal wavelength is important, whereas for a solar cell, high spectral responses over a broad solar wavelength range are required. Photodiodes are small to minimized junction capacitance, while solar cells are large-area devices One of the most important figures of merit for photodiodes is the quantum efficiency, whereas the main concern for solar cells is the power conversion efficiency.

Basic Operation Principles A p-n junction device operating at the 4th quadrant of I-V curve under illumination . In the 4th quadrant, the junction voltage is positive and the current is negative. Hence power is delivered to the external circuit. Photovoltaic effect : The appearance of a forward voltage across an illuminated junction. Ideal I-V characteristics: I = Is ( eqV/kT – 1) – IL Open-circuit voltage (Voc) Short-circuit current (Isc) Isc = IL

Maximum Output Power and The Fill Factor The output power P P = Is V( eqV/kT – 1) – ILV Pm is obtained when dP/dV = 0 The Fill Factor (FF) The fill factor is an important figure of merit for the solar cell design. The fill factor is about 0.7 ~ 0.83 for a Si cell and 0.8 ~ 0.9 for a GaAs cell.

Conversion Efficiency The power conversion efficiency of a solar cell is To maximize the efficiency, we should maximize all three items of FF, IL and Voc. The efficiency has a broad maximum and does not depend critically on Eg. Therefore, semiconductors with bandgaps between 1 ~ 2 eV can all be considered solar cell materials. The efficiency can be largely enhanced at an optical concentration of 1000 suns (C = 1000) A well-made Si cell can have about 10% efficiency (~ 100 W/m2 of electrical power under full illumination).

Degradation Effects of the Conversion Efficiency The series resistance Rs from the ohmic loss in the front surface and the recombination current in the depletion region are two of the major factors that degrade the ideal efficiency. The series resistance depends on the junction depth the impurity concentration of p-type and n-type regions the arrangement of the front surface. The efficiency for the recombination current case is found to be much less less than the ideal current case due to the degradation of both Voc and the fill factor. For Si solar cells at 300 K, the recombination current can cause 25% reduction in efficiency.

Materials of Solar Cells Material requirement: A bandgap matching the solar spectrum Having high carrier mobility Having long carrier lifetime Silicon Single-crystalline, poly-crystalline and amorphous Si. Although the a-Si solar cells (with an effective bandgap of 1.5 eV) has lower efficiency than the single-crystal Si cells, their production costs are considerably lower. Therefore, a-Si solar cell is one of the major candidates for large-scale use of solar energy. III-V compound semiconductors GaAs, GaP, InP, etc. heterojunction structures are used to enhance the conversion efficiency. II-VI compound semiconductors CdS, CdSe, CdTe, etc. Organic materials and others

Design Considerations of the Device Structures An ultra-thin (500-1000Å) window layer to minimize surface recombination and optical absorption in this layer Broadband antireflection coating on top to minimize reflection losses. – The refraction index of the AR coating must be near or higher than 1.87 – SiO2 (n = 1.5), Si3N4 (n = 2.0), Al2O3 (n = 1.86), Ta2O5 (n = 2.25), TiO2 (n = 2.2) The top finger stripes of contacts have to be properly designed to keep the cell series resistance to a low value. Use of solar concentrator systems for obtaining more power per solar cell.

Various Device Configurations of Solar Cells The “back surface field” (BSF) solar cell The “textured” solar cell The V-groove multi-junction solar cell The tandem-junction solar cell The vertical-junction solar cell Heterojunction solar cell Schottky-Barrier solar cell MIS solar cell Thin-Film solar cell Amorphous solar cell