Enhanced Power-conversion-efficiency of Silicon Solar Cells Using Functional Quantum Dots as Energy-down-shift Layer Having Energy-tuning-effect

Abstract

Enhancing the power conversion efficiency (PCE) of solar cells via quantum dots (QDs) as an energy-down-shift (EDS) layer is a new approach to utilize the wasted energy in the ultra-violet (UV)-light, but the smaller Stokes shift in QDs leads towards partial self-re-absorption losses among QDs due to which the overall enhancement is limited. Therefore, we investigated the PCE enhancement for silicon solar cell implemented with EDS via Mn2+-doped Cd0.5Zn0.5S/ZnS core/shell QDs. The Mn2+-doped QDs were deposited as an EDS with a higher Stokes shift on the front surface of monocrystalline p-type silicon (mc-p-Si) solar cells through straightforward and cost-effective spin-coating technique. The Mn2+-doped QDs confirmed a typical EDS which absorb UV light and re-emit visible yellow-orange light at 583 nm. Application of these QDs into the mc-p-Si solar cells clearly exhibited a larger Stokes shift (> 200 nm), generated through a functional energy tuning effect (ETE) of Mn2+ dopant. Through this EDS having an ETE (EDS/ETE) QDs, a remarkable enhancement of the external-quantum-efficiency (EQE) in the UV region was observed. The EQE was improved by ~22.5% in the wavelength range of 300-450 nm and the short-circuit-current-density (JSC) was enhanced about 1.42 mA/cm2 (+4.02%). In particular, the PCE was improved by ~3.22%, compared to its reference value recorded in the absence of an EDS/ETE QD layer. However, although Mn2+-doped Cd0.5Zn0.5S/ZnS QDs have been proved as an excellent EDS-QD layer having free-self-reabsorption, but still with environmentally hazardous Cd material. The European Union's Restriction of Hazardous Substances Directive (RoHS) restricts the use of certain hazardous substances, including Cd, in electrical and electronic equipment. As a result, the quest for nontoxic high photoluminescence-quantum yield (PLQY) and the zero-self-reabsorption EDS/ETE QD layer is a priority for their commercial applications. Therefore, We present for the first time nontoxic CuGaS2/ZnS QDs with free-self-reabsorption losses and large Stokes shift (>190 nm) synthesized on an industrially gram-scale as an alternative for Cd-based EDS-QD layers. The QDs exhibited also a typical EDS that absorbs only UV light (< 407 nm) and emits the whole range of visible light (400–800 nm) with a high PLQY of ~76%. The straightforward application of these EDS/ETE-QDs on the front surface of a mc-p-Si solar cell significantly enhanced the JSC by ~1.64 mA/cm2 (+4.20%); thereby, improving the PCE by ~4.11%. The significant improvement in the EQE increased by ~35.7% and that in the surface reflectance decreased by ~14.1% in the UV region (300–450 nm) clearly manifest the photovoltaic enhancement. Such promising results together with the simple (one-pot core/shell synthesis), cost-effective (reduction in a bill of material–system, BoM–S, by ~2.62%), and scalable (2000-mL three-neck flask, 11 g of QDs) preparation process might encourage the manufacturers of solar cells and other optoelectronic applications to apply these nontoxic EDS/ETE-QDs to different broader eco-friendly applications.