|Fig. 1: Maximum Efficiencies of Photovoltaic Cells over the Past Six Decades. [2,4]|
Solar energy conversion technology if viable could completely revolutionize the power sector throughout the world. The amount of solar energy that strikes the Earth in an hour is more than enough energy to meet the solar demands of the global population for an entire year.  It is facts like this that continue to make improving this technology so alluring. The potential is incredible, but there are many barriers that prevent solar photovoltaic cells from contributing as a major source of global electricity generation. Solar photovoltaic cells have existed since 1954 and this technology has dramatically changed over time.  As energy collection efficiency has increased and the costs of manufacturing have decreased this technology has found different niches over the past several decades.  As the world searches for sources of carbon-neutral electricity generation it would be nice to have solar energy make-up part of the solution because of the large, widely accessible source of solar energy. Yet, in order for solar power to become a major source of world electrical energy it must become more cost-effectively captured, converted and stored. 
In 1954 photovoltaic technology was created by Daryl Chapin, Calvin Fuller, and Gerald Pearson at Bell Labs in the US.  This original cell had an efficiency of 4%, but this same style of silicon solar cell later achieved and efficiency of 11%.  The solar technology was utilized in its early years as a source of electricity for small office supplies that only required small amounts of electricity.  Yet, for the most part the attempt to commercialize solar cells in the 1950s and 60s was a failure.  They were too expensive and inefficient to provide much practicality for most domestic applications. However, in 1958 the Vanguard I, Explorer III, Vanguard II, and Sputnik-3 satellites were all launched with photovoltaic cells onboard as the powering energy source.  Solar photovoltaic cells are still the accepted energy source for satellites today. 
Then, in the 1970s Dr. Elliot Berman of the Exxon Corporation designed a solar cell that could produce electricity at $20 per watt versus the previous rate of $100 per watt.  Suddenly, solar technology became practical for technologies other than satellites. Photovoltaic cells began powering navigation warning lights, horns on offshore oil rigs, lighthouses, railroad crossings, and many small domestic applications.  Photovoltaics became present in households where connection to the traditional electricity grid was not affordable.
In 1982 the first photovoltaic megawatt power station was opened in Hisperia, California.  The push to create photovoltaic power across the world in the past thirty years has increasingly focused of domestic integration. In 1993 Pacific Gas and Electric started the first grid-supported photovoltaic system.  This was just one of many attempts at experimenting with solar electricity generation as a mainstream concept. In the 2000s many companies had begun focusing on large-scale manufacturing of solar panels in order to reduce cost.  Yet, the despite the industries best efforts this technology is still not a mainstream success. Throughout the past several decades the efficiency of photovoltaic cells has greatly increased, but domestic integration of this technology has not become feasibly on a large scale.
Why is this technology currently not feasible? The technology existing for efficiency of solar photovoltaic systems has surpassed the expected limit of efficiency predicted with the Shockley-Queisser efficiency limit.  The science behind creating efficient solar panels is excellent at this point in time, so the main issue with feasibility of this technology is cost. The future of this technology is dependent upon low-cost developments in the manufacturing of capture, conversion, and storage of sunlight.  The PV modules that are currently shipped and used domestically have an efficiency of approximately 20% and cost about $300 per square meter.  Notice how the efficiency of this system is significantly less than the most efficient technology that currently exists. This is because it is significantly less expensive to produce solar panels with this lower efficiency, which reiterates that manufacturing price rather than efficiency of solar panels is the limiting factor to the growth of this technology.  In order to recover the initial capital investment for a solar panel with this efficiency over the lifetime power generation costs must be $0.25 to $0.30 per kilowatt-hour.  Unfortunately, the current cost of electricity from utilities is around $0.03 to $0.05. 
The reason the purity is the main constraint for manufacturing technologies lies in the cost-thickness-purity constrain of a solar cell.  As a solar cell becomes thicker we are capable of collecting more incident sunlight.  However, with a thicker layer a higher purity of material is required. This is because impurities cause shortened lifetimes of photo-excited charge carriers.  Thus, with an impure material the layer needs to be thinner in order for a charge to be transmitted to the electrical junction, but the thinner layer will allow for less collection of incident sunlight.  In order to reduce the cost of the production of PV modules the cost-thickness-purity relationship needs to be improved and maximized.  As photovoltaic technology moves forward much attention will be focused on the low-cost production of high purity materials.
© Katy Ashe. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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