Origins of Stokes Shift in PbS Nanocrystals,Nano Letters

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Origins of Stokes Shift in PbS Nanocrystals
Nano Letters ( IF 9.6 ) Pub Date : 2017-11-02 00:00:00 , DOI: 10.1021/acs.nanolett.7b01843
Oleksandr Voznyy ₁ , Larissa Levina ₁ , Fengjia Fan ₁ , Grant Walters ₁ , James Z. Fan ₁ , Amirreza Kiani ₁ , Alexander H. Ip ₁ , Susanna M. Thon ₂ , Andrew H. Proppe ₁ , Mengxia Liu ₁ , Edward H. Sargent ₁

Affiliation

Stokes shift, an energy difference between the excitonic absorption and emission, is a property of colloidal quantum dots (CQDs) typically ascribed to splitting between dark and bright excitons. In some materials, e.g., PbS, CuInS₂, and CdHgTe, a Stokes shift of up to 200 meV is observed, substantially larger than the estimates of dark–bright state splitting or vibronic relaxations. The shift origin remains highly debated because contradictory signatures of both surface and bulk character were reported for the Stokes-shifted electronic state. Here, we show that the energy transfer among CQDs in a polydispersed ensemble in solution suffices to explain the excess Stokes shift. This energy transfer is primarily due to CQD aggregation and can be substantially eliminated by extreme dilution, higher-viscosity solvent, or better-dispersed colloids. Our findings highlight that ensemble polydispersity remains the primary source of the Stokes shift in CQDs in solution, propagating into the Stokes shift in films and the open-circuit voltage deficit in CQD solar cells. Improved synthetic control can bring notable advancements in CQD photovoltaics, and the Stokes shift continues to provide a sensitive and significant metric to monitor ensemble size distribution.

中文翻译：

PbS纳米晶体中斯托克斯位移的起源

斯托克斯位移是激子吸收和发射之间的能量差，是胶体量子点（CQD）的一种特性，通常归因于暗和亮激子之间的分裂。在某些材料中，例如PbS，CuInS ₂，和CdHgTe，观察到高达200 meV的斯托克斯位移，远大于暗-亮状态分裂或电子振动弛豫的估计值。移位起源仍然是一个备受争议的问题，因为据报道，斯托克斯移位电子状态的表面和体相特征都相互矛盾。在这里，我们显示了溶液中多分散集成体中CQD之间的能量转移足以解释多余的斯托克斯位移。这种能量转移主要归因于CQD聚集，并且可以通过极端稀释，更高粘度的溶剂或更好分散的胶体来基本消除。我们的发现突出表明，整体多分散性仍然是溶液中CQD斯托克斯位移的主要来源，传播到薄膜的斯托克斯位移和CQD太阳能电池的开路电压不足。

更新日期：2017-11-02

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