The Quest for Highly Efficient Solar Photovoltaics Cells


(T) Countries and regions with very high electricity costs and a lot of sunshine are the first adopters of solar energy. In California, the average cost over 30 years of electricity provided by a PV cell is $0.28/kW-hr while the average cost of electricity is $0.13/kW-hr and peak rates are $0.29/kW-hr (as always those do not include externalities dues to the impact on the environment of non-renewable energies).

So, we definitely need to lower the cost of solar energy and to that end, we need to improve existing technologies and looking forward to new disruptive technologies.

Conventional solar cells technology are based on a p-n junction that generates an electric field that will provide electricity when the light is absorbed. P-N junctions have a maximum solar light conversion efficiency of 31% called the Shockley-Queisser limit assuming a junction band gap (or energy gap) of 1.5 eV for the silicon.


Today’s multicrystalline (c-Si) silicon solar cells is the most popular technology achieving efficiencies from 15 to 18% and a module cost of $3 per watt (in addition to the cost of the DC to AC inverter and the installation on an existing home, the total cost reaches $7.5 per watt in the US). Chinese solar cell manufacturers are selling crystalline silicon PV modules for $2 per watt!

SunPower designs and markets the most efficient crystalline modules reaching 21.5% efficiency. SunPower’s technology, invented by Dick Swanson and called backside contact cell, is based on p-n junctions made in high-quality silicon that are located at the back of the solar module.

Competing with crystalline technologies are thin film solar cell technologies where only a thin film of semiconductor (silicon) is deposited by low-cost methods on a glass substrate.

The cells can be directly integrated into the roofing material. The p-n junction is based on Cadmium Telluride (CdTe) and thin film solar cells can achieve efficiency of 11% at a cost of $1 per watt. First Solar is the market leader for thin film solar cells. And, thin film solar cells have reached a market share of 20% while crystalline silicon solar cells have now a market share of 80%.

There are many start-ups which are working on improving existing technologies such as NanosolarSolyndraMiaSole, and many others. But while those start-ups are working on the next commercial products, there is a considerable amount of academic research on significant black swan technologies.

One of the first disruptive technologies is to increase the efficiency of a solar cell by stacking multi-junctions (at least three instead of two). High efficiency III–V compound semiconductors are good candidates for fabricating such multi-junction solar cells but the costs of manufacturing them are much more expensive obviously than two junction cells.

Another black swan is to design solar cells with very large numbers of single crystalline nano-wires, which are about 60 nanometers diameter and 20 micrometers in length, with unique electrical and optical properties. Nanowires have multiple ways to boost efficiency but the most critical one is that the path of the electrons through the wire is unaffected and thus suffers less energy loss which leads to higher efficiency.

Another nano-technology, like nano-wires, is photonic crystals for which certain wavelengths can pass through the photonic bandgap while light in other ranges is reflected. As such by “redirecting” unabsorbed photons back into the silicon, photonic crystals can improve efficiency.

Another disruptive technology is to generate electricity not only by light absorption but as well from the heat accumulated by rooftop solar panels. Stanford University pioneered that process called PETE and demonstrated that PETE could double the efficiency of existing solar cells.

The last black swan that could lead to even more efficiency is intermediate band solar cells that are single junction cells introducing multiple band gaps (see slide below) . Different options have been proposed to produce intermediate band solar cells in particular quantum dots, that are, simply said, extremely small semi-conductors more closely related to atoms than bulk material because of their discrete quantized energy levels. Unfortunately, no intermediate band materials have been realized as of today.

GTM Solar News
Stanford Center for Advanced Molecular Photovoltaics
Silicon Valley Photovoltaics Society (SVPVS)

Note 1: The first three slides are from Professor Mike McGehee from the Materials Science and Engineering department at Stanford University.

Note 2: The last slide is from Doctor Wladek Walukiewicz from the Solar Energy Materials Research Group at the Lawrence Berkeley National Laboratory.

Note 3: The picture above is a SunPower installation of a large-scale solar PV power plant for the US Air Force.

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Categories: Sustainability