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Key Results

In the Nanospec project seven European partners joint their forces to develop an advanced upconverting system that significantly enhances solar cell efficiencies. Key developments were the upconverter material, the combination with a second luminescent material to enlarge the used spectral region, photonic structures for photon management and efficient solar cells.

Embedded upconverter materials

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Left: Strong blue upconversion light is emitted from b-NaYF4:Yb,Tm after Yb excitation at 980 nm.
Right: Diffuse scattering originates from the cation disorder in b-NaYF4.

Different lanthanide ions based upconverter materials were investigated to find the host lattice, the dopant and the dopant concentration that yielded the highest external UC quantum yield (eUCQY). Efficient IR to NIR UC was obtained for Er3+ in fluoride and oxysulfide hosts, while less efficient UC was obtained for Nd3+, Sm3+, Dy3+, Ho3+ and Tm3+. The material β-NaYF4 : 25% Er3+ with a particle size of 63-125 μm was identified as the best upconverter for broadband IR excitation. Improvements of the synthesis increased the eUCQY significantly by about 50%. As β-NaYF4 : 25% Er3+ is a microcrystalline powder, it needs to be incorporated into a transparent matrix for integration into an upconverter solar cell device (UCPVD) (see detailed results on upconversion). Perfluorocyclobutyl (PFCB) was found to be a good matrix material due its low optical losses, low polymerization temperature, suitable refractive index and good thermal stability (see detailed results on matrix materials).

 

Luminescent material to enlarge the used spectral region

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TEM images of PbSe-QDs with diameters of ~ 2.5 nm, 5 nm and 6.5 nm (from left to right)

As second luminescent materials, nano-crystalline semiconductor quantum dots (NQD) were investigated. The NQD should absorb the light, which is neither absorbed by the solar cell nor by the upconverter, and should emit in the absorption range of the upconverter. NQD with emission around 1520 nm und suitable absorption were realized as PbSe quantum dots (QD), PbS shell on PbSe core QDs, PbSeXS1-X alloy shells on PbSe QDs and PbSe-CdSe core-shell QDs. Synthesis with high chemical yield and amounts produced from one reaction in the range of 1-3 g could be realized. A very high, stable photoluminescence quantum yield (PLQY) of up to 25% was determined for PbSe-CdSe core-shell QDs in solution. Embedded in PFCB, the highest PLQY value found was around 20%. Furthermore, the absorption and PL spectra of ensembles of PbSe QDs and of PbSe-PbS core-shell QDs were simulated using a quantum-kinetic model (see detailed results on NQDs).

 

Photonic structures for photon management

Spectrally selective photonic structures were realized by plasma enhanced chemical vapor deposition of multi-layer structures from two different amorphous silicon carbides. A reflectance exceeding 95% and transmittance of up to 90% in the desired spectral regions was achieved. As a potential cheaper alternative, broadband reflectors based on cholesteric liquid crystals were investigated (see detailed results on photonic structures).

 

Upconverter solar cell devices

Adapted bifacial microcrystalline Si thin-film solar cells, with optimized contact grid configuration, and adapted bifacial monocrystalline wafer-based Si solar cells, with optimized anti-reflection coatings on front and rear, were produced. In such a configuration, the predicted short-circuit current density of the solar cell of 40.6 mA/cm2 was modelled to increase by 1.5 mA/cm2 by adding a realistic Er-based upconverter in conjunction with idealized NQD under one sun-illumination.

We realized experimentally planar, bifacial monocrystalline n-type Si solar cells with optimized ARCs. UCPVDs were formed by coupling embedded upconverter materials to the solar cells. For an UCPVD, in which β-NaYF4 : 25% Er3+ embedded in PFCB with a powder to polymer concentration of 75.7 w/w% was coupled to the solar cell, an additional short-circuit current density due to UC of sub-band-gap photons ΔJSC,UC of 2.2 ±0.3 mA/cm2 was determined under broadband excitation ranging from 1450 nm to 1600 nm and an equivalent solar concentration of 78±6 suns. In solar simulator measurements a ΔJSC,UC of 13.3 ±3.0 mA/cm2 was achieved for a concentration of 207±86 suns. This is the highest ΔJSC,UC value determined so far. Taking a broader picture into account, we could show that the materials used in the Nanospec system are relatively abundant, and the concept is not limit by resource availability. Furthermore, procedures were developed to safely handle all involved materials. A cost analysis showed that Erbased upconverters have the potential to increase the cost efficiency of a concentrator module.

As a conclusion, the realized upconverter devices and also its components achieved world record performance levels. However, the impact of upconversion on the device performance is still relatively low. As a consequence, the concept is still not commercially viable. Based on our results, however, we estimate that with additional research it should be possible to realize devices, in which upconversion increases the efficiency of a 23% efficient silicon solar cell to up to 25% thereby increasing the cost-efficiency of such systems.

Funding

The Nanospec Project is funded by the European Community's Seventh Framework Program (FP7/2007-2013) under grant agreement no. [246200].

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