1. Solar cells conversion
Multifunctional metal oxide semiconductors presenting simultaneously transparency, conductivity and luminescent properties – Luminescent TCOs
S. H. Ferreira*,1, L. Rebouta 2, R. Martins 1, E. Fortunato* ,1
1i3N/CENIMAT, Dep. de Ciência dos Materiais, Faculdade de Ciências e Tecnologia – Universidade NOVA de Lisboa and CEMOP-UNINOVA, Caparica, Portugal
2CFUM, Dep. De Física, Universidade do Minho, Braga, Portugal *corresponding author:
Objectives
To use for the first time transparent and conducting oxides (TCOs), such as zinc oxide and tin oxide, as hosts for rare earth elements (REEs) to be used as photon converters via luminescence, thus creating luminescent TCOs (L-TCOs).
The optimized L-TCOs will work as light emitters, without loosing the conductivity and transparency, in conventional amorphous silicon thin film solar cells in order to increase their energy conversion efficiency.
Introduction
The ever growing demand for green and sustainable energy sources and technologies promotes a continuing development of cost-effective devices capable of converting renewable energies into electricity. One hour of sunlight provides Earth with as much energy as human civilization uses in one year [1]. For this reason, photovoltaic (PV) solar cells are the prime candidates for large-scale capture and conversion of sunlight, as they have the ability to directly convert sunlight into electricity in a sustainable way. However, the efficiency and cost- effective conversion of these solar cells are still the two major challenges in photovoltaics.
The conversion efficiency in single-junction solar cells is fundamentally limited by the bandgap energy (Eg) of the semiconductor from which it is fabricated, as they only convert efficiently the incident photons that have energies close to the bandgap of the semiconductor material. The theoretical efficiency limit for a single junction crystalline silicon solar cell with Eg equal to 1.1 eV is about 30% [2].
The efficiency loss processes in a single-junction solar cell are mainly due to the thermalization of the excited carriers and to the transmission of photons with energies below the Eg. There are two ways that can be approached to reduce these losses: designing a novel PV device that can make better use of the incident solar spectrum, or modifying the solar spectrum to better match the bandgap of the solar cells. The former consists in constructing the so-called multijunction tandem solar cells, but it is a complex and expensive technology. The latter consists in converting or shifting the incident solar radiation to better match the absorbed spectrum of the semiconductor material, through downshifting (DS), downconversion (DC) or upconversion (UC) processes. Photon conversion has a major advantage over the tandem solar cells technology as it can be employed to existing solar cells by incorporating layers on the front or on the rear side of the PV device, allowing the conversion layers and solar cells to be optimized separately [1].
REEs are the preferential candidates for photon conversion because of their rich energy level structures that allow them to absorb and emit photons in the NIR, VIS and UV spectral range when doped in solid hosts, such as TCOs.
Motivation
Solar cells conversion
efficiency
losses
Ephoton > Ebandgap
Ephoton < Ebandgap
Thermalization of the excited carriers
Transmission of the sub-bandgap photons
Solutions
Photon Conversion
Downshifting (λexc<λem)
Downconversion (λexc<λem)
Upconversion (λexc>λem)
Multijunction Solar Cells
λexc
λem
Methodology
Deposition of L-TCOs
Characterization of L-TCOs
Structural and Morphological
RF Sputtering
Spin-coating
Target
Substrate
Plasma
Depositing
Thin Film
Ar+
Sputtered material
XRD Panalytical
SEM-FIB-EDS-EBSD Zeiss Auriga
AFM Asylum MFP3D
Electrical
Optical
Doping with REEs
Co-sputtering using a TCO target and a REE target
REEs pellets on the TCO target
Ion implantation of the as-deposited TCO
TCO precursor solution
with REEs precursors
Hall-effect Biorad HL500PC
Spectrophotometer Perkin-Elmer Lambda 950 and Spectrofluorometer Perkin-Elmer LS55
Application of L-TCOs
The optimized L-TCOs will be applied to amorphous silicon solar cells, under development at CEMOP/CENIMAT, using either glass substrates or paper substrates.
Examples of a-Si solar cells developed at CEMOP/CENIMAT
H. Águas et al [3]
A. Vicente et al [4]
H. Águas et al [5]
References
Acknowledgments
[1] D. Chen, Y. Wang, and M. Hong, “Lanthanide nanomaterials with photon management characteristics for photovoltaic application,” Nano Energy, vol. 1, no. 1, pp. 73–90, 2012.
[2] W. Shockley and H. J. Queisser, “Detailed balance limit of efficiency of p-n junction solar cells,” J. Appl. Phys., vol. 32, no. 3, pp. 510–519, 1961
[3] H. Águas, T. Mateus, A. Vicente, D. Gaspar, M. J. Mendes, W. a. Schmidt, L. Pereira, E. Fortunato, and R. Martins, “Thin Film Silicon Photovoltaic Cells on Paper for Flexible Indoor Applications,” Adv. Funct. Mater., p. n/a-n/a, 2015.
[4] A. Vicente, H. Aguas, T. Mateus, A. Araujo, A. Lyubchyk, S. Siitonen, E. Fortunato, and R. Martins, “Solar cells for self-sustainable intelligent packaging,” J. Mater. Chem. A, vol. 3, no. 25, pp –13236, 2015.
[5] H. Águas, S. K. Ram, A. Araujo, D. Gaspar, A. Vicente, S. A. Filonovich, E. Fortunato, R. Martins, and I. Ferreira, “Silicon thin film solar cells on commercial tiles,” Energy Environ. Sci., vol. 4, no. 11, pp , 2011.
Author S. H. Ferreira acknowledges the support from the Portuguese Foundation for Science and Technology through the AdvaMTech PhD program scholarship PD/BD/114086/2015.