Efficiency in Solar Cells

The theoretical efficiency limit is calculated with some attention paid to the laws of physics ruling the limiting behavior of the system. Beginning with single gap solar cell systems, moving onward to tandem cells (solar cells containing several p-n junctions), and ending with solar cells with an intermediate energy band gap (created through an impurity in the system) progress towards maximum efficiency is revealed and the implications this may have on future energy consumption is briefly remarked upon. Alternate sources of energy are vital to the continuation of the human race. Conventional energy production is derived mainly through non-renewable sources. Of the total energy produced in the United States of America in 2011 82% was produced from oil, natural gas, and coal. 8% came from nuclear power. Only 9% of the total energy produced was from renewable sources such as geothermal, wind, or solar.[1] Every year as humanity expands 1 in population, in life expectancy, so too does the amount of energy we require continue to expand with it. Simply put, as the nonrenewable sources dwindle it is imperative to seriously look further into alternate, sustainable and renewable power sources. Research into maximizing the efficiency of solar cells is not a knew phenomenon, by as early as 1954 the potential of the silicon solar cell had begun to be explored in the scientific literature. [2] And it was in 1960 that the classical analysis of single-gap systems by Shockley and Quisser was published [3], an efficiency analysis that to this day remains the standard for measuring against when exploring new systems in the hope to maximize solar cell efficiency. Starting with Shockley and Quisser’s classical analysis this paper will attempt to step through some of the significant leaps in progress we have made in increasing the efficiency in solar cells. 1 Single Gap Solar Cells When studying the efficiency of solar cells it is important to understand exactly what it is happening. With that in mind, there are two terms that are important to distinguish between . The semiempirical limit of efficiency (calculated as a function of the energy gap) is based up empirical values for constants of the system while the detailed balanced limit is a theoretically justified limit.[3] At the time of Shockley and Queisser, efficiency of solar cells were based solely on the semiemperical limit. The walk through, following Shockley and Queisser’s steps, finds the maximum possible efficiency by deriving the theoretically justified limit, the detailed balanced limit. Since the detailed balanced limit is significantly higher than the semiemperical limit, at times up to 50% higher, the difference between the two gave scientists hope that real progress could be made in increasing the efficiency of pre-existing solar cells and bringing the experimentally found semiemperical limit up to to match the newly found theoretical limits. [3]