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Analysis of the intermediate-band absorption properties of type-II GaSb/GaAs quantum-dot photovoltaics

机译：II型GaSb / GaAs量子点光伏电池的中带吸收特性分析

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Quantum-dot (QD) intermediate-band (IB) materials are regarded as promising candidates for high-efficiency\udphotovoltaics. The sequential two-step two-photon absorption processes that take place in these materials\udhave been proposed to develop high-efficiency solar cells and infrared (IR) photodetectors. In this work, we\udexperimentally and theoretically study the interrelation of the absorptivity with transitions of carriers to and\udfrom the IB in type-II GaSb/GaAs QD devices. Our devices exhibit three optical band gaps with: EL = 0.49 eV,\udEH = 1.02 eV, and EG = 1.52 eV, with the IB located 0.49 eV above the valence band. These values are well\udsupported by semiempirical calculations of the QDs electronic structure. Through intensity-dependent two-photon\udphotocurrent experiments, we are able to vary the filling state of the IB, thus modifying the absorptivity of the\udtransitions to and from this band. By filling the IB with holes via E = 1.32 eV or E = 1.93 eV monochromatic\udillumination, we demonstrate an increase in the EL-related absorptivity of more than two orders of magnitude\udand a decrease in the EH-related absorptivity of one order of magnitude. The antisymmetrical evolution of those\udabsorptivities is quantitatively explained by a photoinduced shift of the quasi-Fermi level of the IB. Furthermore,\udwe report the observation of a two-photon photovoltage, i.e., the contribution of subband gap two-photon\udabsorption to increase the open-circuit voltage of solar cells. We find that the generation of the two-photon\udphotovoltage is related, in general, to the production of a two-photon photocurrent. However, while photons with\udenergy close to EL participate in the production of the two-photon photocurrent, they are not effective in the\udproduction of a two-photon photovoltage.We also report the responsivity of GaSb/GaAs QD devices performing\udas optically triggered photodetectors. These devices exhibit an amplification factor of almost 400 in the IR\udspectral region. This high value is achieved by minimizing—via doping—the absorptivity in the IR range of the QDs under equilibrium conditions.\ud©2017 American Physical Society
机译：量子点（QD）中带（IB）材料被认为是高效\光伏的有前途的候选材料。已经提出了在这些材料中进行的连续两步两光子吸收过程，以开发高效的太阳能电池和红外（IR）光电探测器。在这项工作中，我们在理论上研究II型GaSb / GaAs QD器件中吸收率与载流子向IB和从IB跃迁的相互关系。我们的器件表现出三个光学带隙：EL = 0.49 eV，\ udEH = 1.02 eV和EG = 1.52 eV，IB位于价带上方0.49 eV。这些值通过QDs电子结构的半经验计算得到了很好的支持。通过强度相关的双光子\ udphotocurrent实验，我们能够改变IB的填充状态，从而修改了\ udtransition到该频段和从该频段的吸收率。通过用E = 1.32 eV或E = 1.93 eV单色\反光填充IB的空穴，我们证明了EL相关的吸收率增加了两个数量级以上\ ud，而EH相关的吸收率降低了一个数量级数量级。 IB的准费米能级的光诱导位移可以定量地解释这些吸收系数的反对称演化。此外，\ udwe报告了对双光子光电压的观察，即子带隙双光子\吸收吸收对增加太阳能电池开路电压的贡献。我们发现，通常，双光子/双光子电压的产生与双光子光电流的产生有关。但是，虽然具有接近EL的能级的光子参与了双光子光电流的产生，但它们对产生双光子光电压却没有有效。我们还报告了执行udas的GaSb / GaAs QD器件的响应度光触发光电探测器。这些设备在IR \超光谱区域中表现出近400的放大系数。在平衡条件下，通过使QD的IR范围内的吸收率最小化（通过掺杂）来实现这一高价值。\ ud©2017 American Physical Society

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