Cu(InGa)Se2 Solar Cell Efficiency Enhancement Using a Yb-Doped SnOx Photon Converting Layer

We report on the efficiency improvement of Cu(InGa)Se2 (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 × 10–3 Ω cm, and a mobility as high as 50.1 cm2/(V s). Such SnOx 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 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 (JSC) increased by about 0.56 mA/cm2 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.