Semiconductor Nanocrystals as Luminescent Down-Shifting Layers To Enhance the Efficiency of Thin-Film CdTe/CdS and Crystalline Si Solar Cells Sergii Kalytchuk, †,‡ Shuchi Gupta, † Olga Zhovtiuk, † Aleksandar Vaneski, † Stephen V. Kershaw, † Huiying Fu, § Zhiyong Fan, § Eric C. H. Kwok, ∥ Chiou-Fu Wang, ∥ Wey Yang Teoh, ‡ and Andrey L. Rogach* ,† † Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City University of Hong Kong, 88 Tat Chee Avenue, Kowloon, Hong Kong SAR ‡ Clean Energy and Nanotechnology (CLEAN) Laboratory, School of Energy and Environment, City University of Hong Kong, Shatin, N.T., Hong Kong SAR § Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR ∥ Du Pont Apollo Limited, 8 Science Park West Avenue, Hong Kong Science Park, Shatin, N.T., Hong Kong SAR * S Supporting Information ABSTRACT: A simple optical model is presented to describe the influence of a planar luminescent down-shifting layer (LDSL) on the external quantum efficiencies of photovoltaic solar cells. By employing various visible light-emitting LDSLs based on CdTe quantum dots or CdSe/CdS core−shell quantum dots and tetrapods, we show enhancement in the quantum efficiencies of thin-film CdTe/CdS solar cells predominantly in the ultraviolet regime, the extent of which depends on the photoluminescence quantum yield (PLQY) of the quantum dots. Similarly, a broad enhancement in the quantum efficiencies of crystalline Si solar cells, from ultraviolet to visible regime, can be expected for an infrared emitting LDSL based on PbS quantum dots. A PLQY of 80% or higher is generally required to achieve a maximum possible short-circuit current increase of 16 and 50% for the CdTe/CdS and crystalline Si solar cells, respectively. As also demonstrated in this work, the model can be conveniently extended to incorporate LDSLs based on organic dyes or upconverting materials. 1. INTRODUCTION Crystalline silicon and CdTe-based photovoltaics are among the leading technologies for solar power conversion, with respective laboratory-based solar cells demonstrating 25 and 18.3% efficiency and large-scale production modules reaching efficiencies of 22 and 15%. 1 One possible strategy for further efficiency improvements is to optimize the response of the photoactive materials at shorter wavelengths (blue and UV), as illustrated in Figure 1 for the CdTe-based solar cells. A commonly used thin-film CdTe/CdS solar cell is transparent to photons with energies below the bandgap energy, and an additional portion of sunlight is lost because of absorption of high-energy UV photons in the CdS buffer layer. One way to increase solar cell efficiency is to employ a luminescent down- shifting layer (LDSL), which allows for conversion of high- energy photons, which are inefficiently utilized by the photovoltaic material, to photons with energy that can be efficiently converted to electricity. In the most straightforward practice, an LDSL can be conveniently deposited on top of the existing photovoltaic material layer so that no adjustment in the overall architecture and the already optimized electrical properties is required. In recent years, several classes of Special Issue: Michael Grä tzel Festschrift Received: October 17, 2013 Revised: January 6, 2014 Published: January 17, 2014 Figure 1. Global solar spectrum at air mass 1.5 showing the fraction absorbed by a typical CdTe based photovoltaic device (green) and the spectral region that can be utilized through down-shifting process (highlighted in blue). Article pubs.acs.org/JPCC © 2014 American Chemical Society 16393 dx.doi.org/10.1021/jp410279z | J. Phys. Chem. C 2014, 118, 16393−16400