Fabrication of Characterization of PERC-Like CdTe Solar Cells

Remarkable progress has been achieved in CdTe photovoltaics (PVs) to improve cell performance while reducing manufacturing costs. At the best efficiency of 22 %, researchers have demonstrated the short-circuit current (J_sc) and fill factor (FF) close to the limit of their maximum values via band-gap engineering and front-contact optimization. Considerable efforts have been devoted to increasing the carrier lifetime and the doping density (N_A &gt; 10¹⁶ cm^-3) to improve the open-circuit voltage (V_oc) of CdTe-based solar cells. Recent studies suggested that the Voc improvement in polycrystalline CdTe PVs requires well-passivated back contact. One possible strategy is to engineer the band-gap offset (e.g., CdMgTe; △E_CB ≈ 0.2 eV) to reflect minority carrier electrons, thereby decreasing surface recombination. Another approach is to utilize a stable Al₂O₃ layer as an electron reflector; experimental works confirmed the improved performance with a conformal Al₂O₃ layer. In this configuration, the precise control of Al₂O₃ (≈ 1 nm) over the entire CdTe layer is essential because the J_sc greatly depends on the tunneling current. While promising, both CdMgTe and fully covered Al₂O₃ passivation on CdTe introduce unfavorable valence band offset that blocks the hole transport.<br/><br/>Here, we describe the design and fabrication of PERC-like (passivated emitter and rear contact) CdTe solar cells. Conventional lithography and wet-etching processes were used to pattern variable hole arrays on 20 nm-thick Al₂O₃ coated CdSe_(1-x)Te_x (CST) solar cells. The CST absorber materials were doped with traditional copper elements or group-V dopants (e.g., P, As, Sb; GrV). We performed quantitative and qualitative PV analysis for a series of Cu-doped and GrV doped PERC devices. In contrast to the similar behaviors of J_sc and FF, we show a significant difference in the V_oc behaviors for Cu-doped and GrV-doped CdTe PERC solar cells. The V_oc of GrV-doped CdTe PERC remains constant, whereas the V_oc of the Cu-doped PERC shows a radical decrease with an increase in the exposed CdTe area. The defect chemistry of GrV-doped CdTe can be significantly different from that of traditional Cu-doped CdTe absorber materials. We will discuss the possible mechanisms responsible for these Voc changes of the CdTe PERC devices.