Solar Cells Typically 2 inches in diameter and 1/16 of an inch thick Produces 0.5 volts, so they are grouped together to produce higher voltages. These groups can then be connected to produce even more output. In 1883 the first solar cell was built by Charles Fritts. He coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient.

Generations of Solar cells First generation – large-area, high quality and single junction devices. – involve high energy and labor inputs which prevent any significant progress in reducing production costs. – They are approaching the theoretical limiting efficiency of 33% – achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years. – Cost is not likely to get lower than $1/W.

Generations of Solar cells Second generation-Thin Film Cells – made by depositing one or more thin layers (thin film) of photovoltaic material on a substrate. – thickness range of such a layer varies from a few nanometers to tens of micrometers. – Involve different methods of deposition: Chemical Vapor deposition the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are removed by gas flow through the reaction chamber.

Thin Film deposition techniques Electroplating – electrical current is used to reduce cations (positively charged ions) of a desired material from a solution and coat a conductive object with a thin layer of the material. Ultrasonic nozzle – spray nozzle that utilizes a high (20 kHz to 50 kHz) frequency vibration to produce a narrow drop size distribution and low velocity spray over the wafer These cells are low cost, but also low efficiency

The Third Generation Also called advanced thin-film photovoltaic cell range of novel alternatives to "first generation” and "second generation” cells. more advanced version of the thin-film cell.

Third generation alternatives non-semiconductor technologies (including polymer cells and biomimetics) quantum dot technologies – also known as nanocrystals, are a special class semiconductors. which are crystals composed of specific periodic table groups. Size is small, ranging from 2-10 nanometers (10-50 atoms) in diameter. tandem/multi-junction cells – multijunction device is a stack of individual single-junction cells hot-carrier cells – Reduce energy losses from the absorption of photons in the lattice upconversion and downconversion technologies – Put a substance in front of the cell that converts low energy photons to higher energy ones or higher energy photons to lower energy ones that the solar cells can convert to electricity. solar thermal technologies, such as thermophotonics(TPX) – A TPX system consists of a light-emitting diode (LED) (though other types of emitters are conceivable), a photovoltaic (PV) cell, an optical coupling between the two, and an electronic control circuit. The LED is heated to a temperature higher than the PV temperature by an external heat source. If power is applied to the LED,, an increased number of electron-hole pairs (EHPs) are created.These EHPs can then recombine radiatively so that the LED emits light at a rate higher than the thermal radiation rate ("superthermal" emission). This light is then delivered to the cooler PV cell over the optical coupling and converted to electricity.

Efficiency and cost factors In 2002 average cost per peak watt was $2.90-$4.00. Coal fired plant is $1.00/watt. Efficiency is not great. – Recall, 77% of the incident sunlight can be used by the cell. – 43% goes into heating the crystal. – Remaining efficiency is temperature dependent – Average efficiency of a silicon solar cell is 14-17% The second and third generation technologies discussed are designed to increase these efficiency numbers and reduce manufacturing costs

Novel approaches UA astronomer Roger Angel Uses cheap mirrors to focus sunlight on 3 rd generation solar cells (triple junction cells) which handle concentrated light $1.00 per watt achievable- competitive with coal plants Potential: 1 solar farm 100 miles on a side could provide electricity to the whole nation Does not have to be all in one place

Solar Cooling Consider a refrigeration system with no moving parts. – Heat the coolant (say ammonia gas dissolved in water) and force it via a generator into an evaporator chamber where it expands into a gas and cools. Move it to a condenser and cool it back to a liquid and repeat the process. These systems actually have existed for a number of years, refrigerators in the 1950s were sold with this technology (gas powered and there was/is a danger of CO emissions). Energy to heat the coolant and drive it through the system comes from burning fuel or a solar cell to provide electricity to do the heating. – Need what is called a concentrating collector (lens or other system to concentrate more light on the solar cell). Ideally, you could do this with a flat plate collector system, though you do not obtain as much cooling. Devices are not widely used, due to the intermittency of sunlight