Back-Surface Passivation of CdTe Solar Cells Using Solution-Processed Oxidized Aluminum

Hierarchical Transparent Back Contacts for Bifacial CdTe PV

A hierarchical transparent back contact leveraging an AlGaOx passivating layer, Ti3C2Tx MXene with a high work function, and a transparent cracked film lithography (CFL) templated nanogrid is demonstrated on copper-free cadmium telluride (CdTe) devices. AlGaOx improves device open-circuit voltage but reduces the fill factor when using a CFL-templated metal contact. Including a Ti3C2Tx interlayer improves the fill factor, lowers detrimental Schottky barriers, and enables metallization with CFL by providing transverse conduction into the nanogrid. The bifacial performance of an AlGaOx/Ti3C2Tx/CFL gold contact is evaluated, reaching 19.5% frontside efficiency and 2.8% backside efficiency under 1-sun illumination for a copper-free, group-V doped CdTe device. Under dual illumination, device power generation reached 200 W/m2 with 0.1 sun backside illumination.

Cracked Film Lithography with CuGaOx Buffers for Bifacial CdTe Photovoltaics

Bifacial CdTe solar cells with greater power density than the monofacial baselines are demonstrated by using a CuGaO_x rear interface buffer that passivates while reducing sheet resistance and contact resistance. Inserting CuGaO_x between the CdTe and Au increases mean power density from 18.0 ± 0.5 to 19.8 ± 0.4 mW cm^-2 for one sun front illumination. However, coupling CuGaO_x with a transparent conductive oxide leads to an electrical barrier. Instead, CuGaO_x is integrated with cracked film lithography (CFL)-patterned metal grids. CFL grid wires are spaced narrowly enough (≈10 µm) to alleviate semiconductor resistance while retaining enough passivation and transmittance for a bifacial power gain: bifacial CuGaO_x /CFL grids generate 19.1 ± 0.6 mW cm^-2 for 1 sun front + 0.08 sun rear illumination and 20.0 ± 0.6 mW cm^-2 at 1 sun front + 0.52 sun rear-the highest reported power density at field albedo conditions for a scaled polycrystalline absorber.

Design optimization and efficiency enhancement of axial junction nanowire solar cells utilizing a forward scattering mechanism

We report the design, optimization, and performance analysis of three axial junction nanowire solar cells (NW SCs) based on cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and copper zinc tin sulfide (CZTS) with significant improvement in their optical and electrical characteristics compared to their planar counterparts. It is shown that the performance of these NW SCs can be further improved by incorporating a hemispherical indium doped tin oxide (ITO) forward scatterer on top of the ITO front contact of the solar cells. We also compare forward scatterer incorporated NW SCs with forward scatterer incorporated planar solar cells (PSCs) and observe that forward scatterers significantly enhance the absorption in both cases. We further study the optimum size and arrangement of ITO hemispheres that result in improved photocurrent. In optimum cases, the incorporation of forward scatterers leads to absorption enhancement of 7.8%, 5.36%, and 8.8% in PSCs, and 21.4%, 7.36%, and 6.02% in NW SCs, respectively, for CdTe, CIGS, and CZTS absorbers in the same order. From the absorption profile at various wavelengths, it is found that forward scatterers enhance absorption in the 450-600 nm wavelength range, while nanowires improve absorption in the 600-800 nm range, and their combination results in an improved absorption profile for the entire visible wavelength range. We also observe increased electron-hole-pair (EHP) generation rate due to increased field-scattering and light concentration at the center of the nanowire below forward scattering hemispheres, leading to 46%, 32%, and 82.5% improvement in power conversion efficiency (PCE) for the three absorber layers, respectively. The effects of Al₂O₃ and SiO₂ passivation layers surrounding the nanowires of the optimized cells are observed, and we conclude that the CIGS absorber benefits the most when the SiO₂ passivation layer is used, increasing its PCE from 29.72% to 32.43%, while the PCEs of CdTe and CZTS are unaffected by the passivation layer due to competing effects of reduced absorption and reduced surface recombination.