Cu(InGa)Se 2 Solar Cell Efficiency Enhancement Using a Yb-Doped SnO x Photon Converting Layer
Abstract
We report on the efficiency improvement of Cu(InGa)Se 2 (CIGS) based solar cells obtained upon coating the cell with a Yb-doped SnO x layer. This layer is deposited by reactive sputtering and serves as a photon down-shifting converter. The direct excitation of the SnO x host matrix with UV photons leads to a strong emission of near infrared photons from the Yb 3+ ions suggesting an efficient energy transfer from SnO x to the Yb 3+ ions. The deposition the Yb:SnO x films at higher temperatures results in an enhancement of the 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 6×10 −3 Ωcm and a mobility as high as 50.1 cm 2 /Vs. Such SnO x layers doped with 1.3 at. % of Yb were deposited at 100 • C on conventional CIGS based solar cells to replace the standard ZnO n-type conductive layer. The solar cells performances 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 by Yb:SnO x for CIGS cells offers possible application to other solar cells. These results are encouraging towards enhancing the efficiency of solar cells at low cost which will contribute to the large deployment of clean energy.
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