Overcoming Defects in Undoped Sub-Micron CdSeTe-Only Devices to Achieve 13% Efficiency

One of the major advantages of CdTe based solar cells has been the reduction of absorber material compared to other technologies (i.e. Si). With the introduction of Se alloying to create CdSe<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf>Te<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> layers, graded devices with thick p-doped CdTe layers (~2-3 microns) achieved higher efficiencies. CdSeTe-only devices have suffered from low carrier extraction, resulting in low current densities and limited performance. It can be challenging to fabricate ternary polycrystalline alloys, without incorporating high defect densities. One hypothesis is these densities come from vacancy Se (V<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Se</inf>) sites within the absorber. The presence of a defect state is in agreement with photoluminescence measurements, which exhibit a sub-band gap emission around 1.1 eV. A discovered method of limiting the impact of the defects dovetails with another goal of the community, reduction of material. The thinner nature also allows analysis of the absorber and interfaces as opposed to a thicker device, which can hide effects such as back-interface issues. Through this, we have achieved over 13% efficient cells with a sub-micron absorber of unintentionally doped CdSeTe-only.