†TU Delft, Chemical Engineering, Optoelectronic Materials, Julianalaan 136, 2628 BL Delft, The Netherlands.
§Kimika Fakultatea, Euskal Herriko Unibertsitatea (UPV/EHU) and Donostia International Physics Center (DIPC), P.K. 1072, 20018 Donostia, Euskadi, Spain.
Nano Lett. 2015 May 13;15(5):3056-66. doi: 10.1021/acs.nanolett.5b00050. Epub 2015 Apr 14.
Charge trapping is an ubiquitous process in colloidal quantum-dot solids and a major limitation to the efficiency of quantum dot based devices such as solar cells, LEDs, and thermoelectrics. Although empirical approaches led to a reduction of trapping and thereby efficiency enhancements, the exact chemical nature of the trapping mechanism remains largely unidentified. In this study, we determine the density of trap states in CdTe quantum-dot solids both experimentally, using a combination of electrochemical control of the Fermi level with ultrafast transient absorption and time-resolved photoluminescence spectroscopy, and theoretically, via density functional theory calculations. We find a high density of very efficient electron traps centered ∼0.42 eV above the valence band. Electrochemical filling of these traps increases the electron lifetime and the photoluminescence quantum yield by more than an order of magnitude. The trapping rate constant for holes is an order of magnitude lower that for electrons. These observations can be explained by Auger-mediated electron trapping. From density functional theory calculations we infer that the traps are formed by dicoordinated Te atoms at the quantum dot surface. The combination of our unique experimental determination of the density of trap states with the theoretical modeling of the quantum dot surface allows us to identify the trapping mechanism and chemical reaction at play during charge trapping in these quantum dots.
电荷俘获是胶体量子点固体中普遍存在的过程，也是限制基于量子点的器件（如太阳能电池、LED 和热电设备）效率的主要因素。尽管经验方法导致了俘获的减少，从而提高了效率，但俘获机制的确切化学性质在很大程度上仍未确定。在这项研究中，我们通过电化学控制费米能级与超快瞬态吸收和时间分辨光致发光光谱相结合的实验方法，以及通过密度泛函理论计算的理论方法，确定了 CdTe 量子点固体中的陷阱态密度。我们发现，在价带上方约 0.42 eV 的位置存在高密度的非常有效的电子陷阱。这些陷阱的电化学填充使电子寿命和光致发光量子产率增加了一个数量级以上。空穴的俘获速率常数比电子低一个数量级。这些观察结果可以用俄歇介导的电子俘获来解释。从密度泛函理论计算中我们推断，这些陷阱是由量子点表面的配位不足的 Te 原子形成的。我们独特的实验方法确定了陷阱态密度，并与量子点表面的理论建模相结合，使我们能够识别在这些量子点中发生电荷俘获时的俘获机制和化学反应。
