Poking a semiconductor Noncentrosymmetric crystal structure can lead to a peculiar kind of charge separation under illumination called the bulk photovoltaic (BPV) effect. Solar cells made of such materials, however, typically have low efficiency. Yang et al. expanded the class of materials capable of exhibiting the BPV effect by making ordinarily centrosymmetric materials, such as SrTiO3 and TiO2, lose their inversion symmetry. The authors accomplished this by applying a point force on the surface of the material. This induced a strain gradient and the loss of inversion symmetry, resulting in large photovoltaic currents under illumination. The mechanism, dubbed the flexo-photovoltaic effect, is expected to apply to most semiconductors. Science, this issue p. 904 A bulk photoelectric effect is induced in centrosymmetric semiconductors by applying a point force on the materials’ surfaces. It is highly desirable to discover photovoltaic mechanisms that enable enhanced efficiency of solar cells. Here we report that the bulk photovoltaic effect, which is free from the thermodynamic Shockley-Queisser limit but usually manifested only in noncentrosymmetric (piezoelectric or ferroelectric) materials, can be realized in any semiconductor, including silicon, by mediation of flexoelectric effect. We used either an atomic force microscope or a micrometer-scale indentation system to introduce strain gradients, thus creating very large photovoltaic currents from centrosymmetric single crystals of strontium titanate, titanium dioxide, and silicon. This strain gradient–induced bulk photovoltaic effect, which we call the flexo-photovoltaic effect, functions in the absence of a p-n junction. This finding may extend present solar cell technologies by boosting the solar energy conversion efficiency from a wide pool of established semiconductors.

中文翻译：

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柔印光伏效应

戳穿半导体非中心对称晶体结构会导致在光照下发生一种特殊的电荷分离，称为体光伏 (BPV) 效应。然而，由此类材料制成的太阳能电池通常效率低。杨等人。通过使普通的中心对称材料（如 SrTiO3 和 TiO2）失去其反转对称性，扩展了能够表现出 BPV 效应的材料类别。作者通过在材料表面施加点力来实现这一点。这引起了应变梯度和反转对称性的丧失，导致光照下产生大的光伏电流。这种被称为柔印光伏效应的机制有望应用于大多数半导体。科学，这个问题 p。904 通过在材料表面施加点力，在中心对称半导体中产生体光电效应。非常需要发现能够提高太阳能电池效率的光伏机制。在这里，我们报告了体光伏效应，它不受热力学肖克利-奎塞尔极限的限制，但通常仅在非中心对称（压电或铁电）材料中表现出来，可以通过弯曲电效应的中介在任何半导体（包括硅）中实现。我们使用原子力显微镜或微米级压痕系统来引入应变梯度，从而从钛酸锶、二氧化钛和硅的中心对称单晶产生非常大的光伏电流。这种应变梯度诱导的体光伏效应，我们称之为柔印光伏效应，在没有 pn 结的情况下起作用。这一发现可能会通过提高大量已建立半导体的太阳能转换效率来扩展现有的太阳能电池技术。