Strain Induced Type-II Band Alignment Control in CdSe Nanoplatelets / ZnS Sensitized Solar Cells
Miri Kazes a, Songping Luo b, Dan Oron a, Hong Lin b
a School of materials science and engineering, Tsinghua University, Room 2421, Yifu Science and Technology Building, Tsinghua University, Beijing, 100084, China
Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe September Meeting 2017 (NFM17)
SE2: Opto-electronics of 2-D Nanostructured Semiconductors: Parabolic vs. Linear Dirac Bands
Barcelona, Spain, 2017 September 4th - 9th
Organizers: Daniel Vanmaekelbergh and Cherie Kagan
Oral, Miri Kazes, presentation 072
Publication date: 20th June 2016

The use of colloidal semiconductor nanoparticles (NPs) as sensitizers in solar cells have long been desired owing to their low cost and ease of processing. Colloidal CdSe nanoplatelets (NPLs) are a new class of NPs that have a well-defined thickness of only a few atomic monolayers and lateral dimensions of tens of nanometers. This geometry gives rise to a 2D electronic confinement with a continuous density of states allowing for an inherently high carrier density upon optical excitation and a reduced non-radiative Auger recombination rates. In addition, the high aspect ratio of NPLs leads to a significantly increased intrinsic linear absorption compared to quantum dots or rods making NPLs more efficient light absorbers as compared to their counterparts.

Here, colloidal CdSe nanoplatelets (NPLs) deposited on TiO2 and overcoated by ZnS were explored as light absorbers in semiconductor sensitized solar cells (SSSCs). Significant red-shifts of both absorption and steady-state photoluminescence (PL) along with rapid PL quenching suggest a type-II band alignment at the interface of the CdSe NPL and the ZnS barrier layer grown on the NPLs layer, as confirmed by energy band measurements. The considerable red shift leads to enhanced spectral absorption coverage and the sharp band edge absorption suggests a defect free interface. Cell characterization show an increased open-circuit voltage of 664 mV using a polysulfide electrolyte, which can be attributed to a photo-induced dipole (PID) effect created by the spatial charge separation across the nanoplatelet sensitizers. The observed short-circuit current density of 11.14 mA cm-2 approaches the maximal theoretical current density for this choice of absorber, yielding an internal quantum efficiency (IQE) of close to 100%, a clear signature of excellent charge transport and collection yields. An improved cell design that will offer a controlled orientation of NPLs arrays is expected to realize the full advantage of this material.

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