Industrially scalable and cost-effective Mn²⁺ doped Zn_xCd_1−xS/ZnS nanocrystals with 70% photoluminescence quantum yield, as efficient down-shifting materials in photovoltaics (original) (raw)

2016, Energy and Environmental Science

We present colloidally stable and highly luminescent Zn x Cd 1Àx S:Mn/ZnS core-shell nanocrystals (NCs) synthesized via a simple non-injection one-pot, two-step synthetic route, which can be easily upscaled. A systematic variation of the reaction component, parameters and thickness of the ZnS shell yielded doped nanocrystals with a very high photoluminescence quantum yield (F pl) of 70%, which is the highest value yet reported for these Mn-doped sulfide-semiconductor NCs. These materials can be synthesized with high reproducibility in large quantities of the same high quality, i.e., the same F pl using accordingly optimized reaction conditions. The application of these zero-reabsorption high quality NCs in the light conversion layers, deposited on top of a commercial monocrystalline silicon (mono-Si) solar cell, led to a significant enhancement of the external quantum efficiency (EQE) of this device in the ultraviolet spectral region between 300 and 400 nm up to ca. 12%. EQE enhancement is reflected by an increase in the power conversion efficiency (PCE) by nearly 0.5 percentage points and approached the theoretical limit (0.6%) expected from down-shifting for this Si solar cell. The resulting PCE may result in a BoM (bill of materials) cost reduction of app. 3% for mono-Si photovoltaic modules. Such small but distinct improvements are expected to pave the road for an industrial application of doped semiconductor NCs as cost-effective light converters for silicon photovoltaic (PV) and other optoelectronic applications. Broader context The low efficiencies of inorganic solar cells in the ultraviolet (UV) and blue spectral region are key factors which restrict their power conversion efficiency (PCE). A simple and reliable technological step to overcome these limitations, which can be easily introduced into the manufacturing process, is the coating of the cover glass or the solar cells/modules by a light-converting material with high absorption in the UV-blue, luminescence in the active solar cell region, and a high photoluminescence quantum yield/efficiency. The resulting conversion of UV/blue light to the active spectral region of the solar cell can improve PCE by several percentage points, especially in the case of thin-film solar cells based on CuInGaSe 2 or CdTe. Due to the already relatively high efficiency of the monocrystalline silicon (mono-Si) solar cells in the UV (about 45% at 300 nm) the expected improvement of the PCE of these cells is about 0.6 percentage points. Due to the high market share of the mono-Si modules, this improvement is expected to be nevertheless very profitable. To demonstrate the potential of this concept using the most promising type of light converters, that is, zero-reabsorption doped semiconductor quantum dots (QDs), we developed a simple non-injection one-pot, two-step synthetic route towards Zn x Cd 1Àx S:Mn/ZnS core-shell nanocrystals showing a broad and efficient emission centered at 598 nm, which can be easily upscaled, and a simple coating technique to generate down shifting layers applicable to conventional mono-Si solar cells. With this approach, we could improve the efficiency of a commercial Si solar cell by up to ca. 12% in the UV spectral region which led to an enhancement of PCE by nearly 0.5 percentage points and will enable a cost reduction of ca. 3.3%. Our simple strategy is expected to encourage solar module manufacturers to also equip their photovoltaic modules with light converting layers and pave the road to a broader application of solar cells.