太陽能電池效率下降的原因

經過數十年的努力，科學家終於發現太陽能電池效率下降的原因

曼徹斯特大學（University of Manchester）的團隊解開了一個困擾 40 年的謎團，為太陽能電池提高效率邁出第一步。這項新研究發現作為太陽能電池原料的矽基半導體存在的缺陷，會導致光衰減（Light Induced Degradation , LID），讓太陽能電池在使用的前幾個小時內效率下降 2％。

謎團散去

研究人員東尼·匹克教授（Tony Peaker）表示：「因為環境和財務的影響，太陽能板『效率降低』一直是過去四十年來科學和工程界關注的主題，儘管有不少優秀的人才在這塊領域努力，但問題到現在仍未獲得解決。」

40 年間共有 270 篇研究論文專注在此領域，而這項新研究使用了一種稱為深階暫態能譜（deep-level transient spectroscopy, DLTS）的電學和光學技術來了解摻雜硼的矽基半導體缺陷。這項缺陷會處於休眠狀態，直到太陽能電池發熱，因溫度升高而解除休眠。

當太陽能電池受光照產生電子的同時，這項涉及分子結構轉變的缺陷也產生「陷阱」限制著電子流動，從而降低了電流。光電業界判斷材料品質標準的方式是量測產生電子的「壽命」，更高品質的材料產生的電子壽命更長，這支持了這個陷阱與效率降低有關的觀點。

另外研究人員也發現，在黑暗中加熱材料，可以逆轉衰減，但並非一勞永逸，因為該狀態是一個介穩狀態（metastable state）。

影響甚巨

匹克教授表示：「效率降低 2％ 可能看起來不是什麼大問題，但是當想到這些太陽能板負責提供世界大部分能量需求並以指數增長時，它就是一個重大的發電損失。」

另一位研究人員伊恩·克羅博士（Iain Crowe）表示：「太陽能電池可產生的電流取決於電子流動，阻礙它便是限制了太陽能電池的效率與發電量。我們已經證明缺陷存在，現在需要以工程方法修復。」

估計全球安裝的太陽能板因此短少的發電能力達數十吉瓦（GW），相當於全英國 15 座核電廠的發電量，而這部分能源缺口正倚靠較不環保的發電方式補足，例如火力發電。

參考資料：

• Vaqueiro-Contreras, M., Markevich, V. P., Coutinho, J., Santos, P., Crowe, I. F., Halsall, M. P., . . . Peaker, A. R. (2019). Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells. Journal of Applied Physics,125(18), 185704. doi:10.1063/1.5091759
• Robinson, B. (2019, June 03). Solar cell defect mystery solved after decades of global effort.Retrieved June 18, 2019.
• Nield, D. (2019, June 8). After 40 Years of Searching, Scientists Identify The Key Flaw in Solar Panel Efficiency. Retrieved June 18, 2019.
  

援引资料原文：

Solar cell defect mystery solved after decades of global effort

A team of scientists at the University of Manchester has solved a key flaw in solar panels after 40 years of research around the world.

Solar panels are among the most available system of generating energy through renewable sources due to their relative cost and consumer availability. However, the majority of solar cells only achieve 20 percent efficiency—for every kW of equivalent sunlight, about 200W of electrical power can be generated.

Now an international team of researchers have resolved a key fundamental issue of material defect which limits and degrades solar cell efficiency. The problem has been known about and studied for over 40 years, with over 270 research papers attributed to the issue with no solution.

The new research shows the first observation of a previously unknown material defect which limits silicon solar cell efficiency.

Prof Tony Peaker, who co-ordinated the research now published in the Journal of Applied Physics said: "Because of the environmental and financial impact, solar panel 'efficiency degradation' has been the topic of much scientific and engineering interest in the last four decades. However, despite some of the best minds in the business working on it, the problem has steadfastly resisted resolution until now."

"During the first hours of operation, after installation, a solar panel's efficiency drops from 20 percent to about 18 percent. An absolute drop of 2 percent in efficiency may not seem like a big deal, but when you consider that these solar panels are now responsible for delivering a large and exponentially growing fraction of the world's total energy needs, it's a significant loss of electricity generating capacity."

The energy cost of this shortfall across the world's installed solar capacity measures in the 10's of gigawatts, this is equivalent to more energy than is produced by the UK's combined total of 15 nuclear power plants. The solar shortfall has to be therefore met by other less sustainable energy sources such as burning fossil fuels.

The multi-disciplinary experimental and theoretical approach employed by the researchers identified the mechanism responsible for Light Induced Degradation (LID). Combining a specialized electrical and optical technique, known as "deep-level transient spectroscopy" (DLTS), the team have uncovered the existence of a material defect which initially lies dormant within the silicon use to manufacture the cells.

The electronic charge within the bulk of the silicon solar cell is transformed under sunlight, part of its energy generating process. The team found that this transformation involves a highly effective "trap" that prevents the flow of photo-generated charge carriers (electrons).

Dr. Iain Crowe said: "This flow of electrons is what determines the size of the electrical current that a solar cell can deliver to a circuit, anything that impedes it effectively reduces the solar cell efficiency and amount of electrical power that can be generated for a given level of sunlight. We've proved the defect exists, it's now an engineering fix that is needed."

The industry standard technique used to determine the quality of the silicon material measures the lifetime of charge carriers, which is longer in high quality material with fewer "traps." The researchers in Manchester lead by Prof Matthew Halsall found that their observations were strongly correlated with this charge carrier lifetime, which was reduced significantly after transformation of the defect under illumination. They also noted that the effect was reversible, the lifetime increased again when the material was heated in the dark, a process commonly used to remove the "traps."

The paper, "Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells," is published in the Journal of Applied Physics.