We report on the efficiency improvement of Cu(InGa)Se-2 (CIGS) based solar cells obtained upon coating the cell with a Yb-doped SnOx layer. This layer is deposited by reactive sputtering and serves as a photon down-shifting converter. The direct excitation of the SnOx host matrix with UV photons leads to a strong emission of near infrared photons from the Yb3+ ions suggesting an efficient energy transfer from SnOx to the Yb3+ ions. The deposition of the Yb:SnOx films at higher temperatures results in an enhancement of the photoluminescence (PL) emission as well as in an improvement of the transport properties. The optimized films exhibit a transmittance around 80% in the visible region, a resistivity of 6 x 10(-3) Omega cm, and a mobility as high as 50.1 cm(2)/(V s). Such SnOx layers doped with 1.3 at % of Yb were deposited at 100 degrees C on conventional CIGS based solar cells to replace the standard ZnO n-type conductive layer. The performances of the solar cells are noticeably improved. This is witnessed by a net gain of 10% of the external quantum efficiency (EQE) at 360 nm. The short-circuit current (J(SC)) increased by about 0.56 mA/cm(2) while the fill factor reaches 64.4%. As an overall result, the best solar cell exhibited a remarkable enhancement in efficiency of about 0.6%. This improvement of the photovoltaic efficiency by a simple substitution of i-ZnO for Yb:SnOx for CIGS cells offers possible applications to other solar cells. These results are encouraging toward the enhancement of the efficiency of solar cells at low cost which will contribute to the larger deployment of clean energy.

SnO2 is a transparent large band gap semiconductor, particularly interesting for optoelectronic and photovoltaic devices, mainly because its conduction can be easily tuned by doping or by modulating the amount of oxygen vacancies. Besides, rare earth doping was successfully exploited for up conversion properties. Here we report on the functionalization of SnO2 nanoparticles with optically active Yb3+ ions using the sol-gel method, which allows UV to NIR spectral (down) conversion. As starting solutions we used stable non-alkoxide metal-organic compounds, which is rather uncommon. Transmission electron microscopy analysis demonstrated the formation of small well-crystallized nanoparticles while X-ray photoelectron spectroscopy measurements have revealed that the Yb is well inserted in the host matrix and has a 3+ valence state. All nanoparticles present large absorption in the UV-visible range (250 to 550 nm) and a band gap that decreases down to 2.72 eV upon doping. The UV energy converted into NIR on the basis of efficient energy transfer from SnO2 to the Yb3+ ions ranges between 250 and 400 nm. Reference undoped SnO2 nanoparticles with a mean size of 20 nm allow converting UV light into broad visible emission centered at 650 nm. The incorporation of up to 3.5 at% of Yb3+ ions into the SnO2 host matrix results in a spectacular decrease of the nanoparticle size down to 6.6 nm. This allowed also the shift of the photoluminescence to NIR in the 970-1050 nm range. The energy level structure of Yb3+ in SnO2 was successfully determined from the deconvolution of the Yb emission. This emission is significantly enhanced by increasing the doping level. All optical measurements suggest that these nanoparticles can be efficiently used as down-shifting converters.

Nd-doped SnO2 (SnO2:Nd) and ZnO (ZnO:Nd) thin films were deposited by radio-frequency magnetron sputtering of pure Sn and Zn targets embedded with small-sized Nd discs, in an Ar/O-2 ambient. The structural and optoelectronic properties of the SnO2:Nd and ZnO:Nd thin films deposited at different temperatures were investigated and compared with those of the undoped films. X-ray diffraction analysis suggested that both tetragonal SnO2 and hexagonal ZnO crystal lattices were expanded after Nd doping. Radiative transition between the energy states of Nd3+ (F-4(3/2) -> I-4(9/2)) in the SnO2:Nd thin films was identified by the photoluminescence at similar to 885 nm, which was enhanced with increased deposition temperatures (150 to 300 degrees C). The Cu(InGa)Se-2-based solar cell with SnO2:Nd deposited at 300 degrees C demonstrated enhanced cell efficiency (12.49%), mainly with increased short circuit current (J(SC) = 36.48 mA/cm(2)), compared to those of the cell with SnO2 (J(SC) = 31.98 mA/cm(2) and efficiency = 11.24%), attributed to the down-conversion effect of the doped Nd, which is also supported by the quantum efficiency values. However, ZnO:Nd deposited at room temperature did not improve the Cu(In,Ga)Se-2 cell performance, presumably owing to the inactivation of Nd by the low deposition temperature. However, the key device characteristics of the Cu(In,Ga)Se-2 cell with SnO2:Nd were superior to those of the reference Cu(In,Ga)Se-2 cell with an intrinsic ZnO.

p and n type SnOx thin films are successfully functionalized with optically active Nd3+ ions for efficient UV photon conversion.

The use of photon conversion layers is an interesting way to improve the overall efficiency of solar cells. Herein we report on Nd-doped SnO2 thin films with photon management property inserted further into CIGS based solar cells. The functionalized layers were deposited by reactive magnetron sputtering whose structural, optical and electrical properties were tuned by varying the deposition temperature. Careful analysis of the structure using XRD and XPS showed that the tetragonal rutile SnO2 phase can be obtained at a deposition temperature as low as 100°C. Transparency was found to be as high as 90% for all layers while the absorption edge is found to increase when increasing the deposition temperature up to 300°C. The photoluminescence measurements under 325nm UV laser excitation showed that 100°C is needed for the optical activation of the rare earth. Despite the small amounts of Nd (around 0.62at%), intense and narrow emission bands have been collected in the Near Infrared Region (NIR) which are characteristics of Nd3+ ions whose the ionic state was confirmed by the 3d XPS core levels. Thus, the emission spectra cover a good part of the spectrum useful to the solar cell. Photoluminescence excitation spectroscopy experiments were also carried out on Nd:SnO2 samples to get insights on the energy transfer. By exciting in the deep UV from 250 to 400nm intense Nd emission was collected giving an experimental evidence of the down-shifting process through a resonant energy transfer from the SnO2 host matrix to the Nd3+ ions. Hall Effect measurements showed that the n-type character and good conductivity of the Nd doped SnO2 films can be correlated to the highest optical activity of Nd in the matrix. An optimal condition is found for the Nd-doped SnO2 film grown at 300°C for which the highest PL and the best electrical data were measured. Finally, we show that the implementation of such optimized Nd–SnO2 films on CIGS based solar cells serving as a transparent conducting oxide and a down shifting converter results in the best power conversion efficiency.

UV excitation of Nd doped SnO2 leads to visible & NIR emission: Nd3+ ions are optically active in SnO2 nanoparticles.