Determination of Defect Densities in Thin (i) a‐Si:H Used as the Passivation Layer in a‐Si:H/c‐Si Heterojunction Solar Cells from Static Planar Conductance Measurements

Abstract

A set of (p) a‐Si:H/(i) a‐Si:H/(n) c‐Si heterostructures is investigated by coplanar conductance measurements. The thickness of the (i) a‐Si:H buffer layer is varied between 2 and 50 nm, well beyond the values used in heterojunction solar cells. The change in this thickness plays a role on band bending at the heterointerface and therefore impacts the level of inversion of carrier population at the c‐Si surface. Measurements are compared with 1D analytical calculations and 2D electrical modeling. It is demonstrated that the deep defect density, related to silicon dangling bonds, in the (i) a‐Si:H layer strongly increases from 1 × 1017 to 4 × 1018 cm−3 when the (i) a‐Si:H layer thickness is decreased from 50 to 2 nm. This result is interpreted in terms of defect formation and dependence of the defect density upon the position of the Fermi level with respect to the valence band edge. Quantitative analysis in the framework of the defect‐pool model demonstrates that the strong increase of defect density is also promoted by an increase in the width of the valence band tail in the thin (i) a‐Si:H layer, suggesting that a very thin layer also suffers from increased disorder.

Optoelectrical modeling of solar cells based on c-Si/a-Si:H nanowire array: focus on the electrical transport in between the nanowires

“…The structure of interest is depicted in figures 1(a) and (b), and is similar to the one studied in our previous work [20]. The array of nanowires has been designed from experimental achievements [11][12][13].…”

“…The array of nanowires has been designed from experimental achievements [11][12][13]. For the modeling, a periodic square mesh arrangement was considered with a nanowire density of 2.6×10 8 cm −2 roughly equivalent to a period of 620 nm [12,20]. Each nanowire, described in figure 1(b), consists of a radial p-i-n junction with 20 nm p-type crystalline silicon core, (p) c-Si, covered by 100 nm of intrinsic hydrogenated amorphous silicon absorber, (i) a-Si:H, surrounded by 10 nm of n-type hydrogenated amorphous silicon shell, (n) a-Si:H. The 1 μm long c-Si core allows the radial collection of holes which then flow to the back contact made of aluminum-doped zinc oxide (AZO).…”

“…The input parameters for the DOS of the a-Si:H layers are reported in table 2. More details about the electrical parameters of a-Si:H layers can be found elsewhere [20]. In the (p) c-Si core, recombination using both standard Shockley-Read-Hall and Auger models were taken into account and the electron and hole mobilities were also considered as doping dependent using a standard model [23,24].…”

“…In contrast to the electrical simulation where ITO and AZO are replaced by electric contacts modeled by specific boundary conditions (work function and surface recombination velocities values), the optical simulation considers the bulk of both ITO and AZO materials (with optical indices). Photogeneration profile of a single nanowire extracted from optical simulation is then flattened to a 2D plane to be input in the electrical simulations [19,20,28,29]. The photogeneration profile is shown in figure 1(c).…”

“…Very few articles rigorously take into account both the optical and electrical aspects [19]. Recently, we have developed a tool to couple electrical and optical simulations and we applied it on p-i-n radial nanowires based on c-Si core/a-Si:H shell heterojunction in order to simulate the electrical performance of solar cells when the light harvesting effect is taken into account in the photogeneration rate distribution of vertical nanowire arrays [20]. It was emphasized that the planar region between the nanowires has to be taken into account because it strongly influences the performance of the entire solar cell.…”