Measuring carrier injection from amorphous silicon into crystalline silicon using photoluminescence

Abstract

In devices with intrinsic amorphous silicon layer on a crystalline silicon substrate, the light absorbed in the amorphous layer can be weakly electronically coupled into the silicon base. Such carrier injection has previously been reported from measurements on finished devices containing stacks of intrinsic and doped amorphous silicon layers. Here, we use spectral response of photoluminescence, a contactless approach, to investigate this carrier injection on significantly simpler structures. In such devices, the effect of absorption in the front layer can be measured by the internal quantum efficiency. A highly absorbing front layer is expected to cause a drop in the quantum efficiency at short wavelengths. However, if electron‐hole pairs that are generated in the front layer are subsequently injected into the base, the optical losses will be reduced, resulting in a partial recovery of the quantum efficiency at short wavelengths. Here, we quantify the efficiency of carrier injection from the intrinsic amorphous silicon front layer to the crystalline silicon base, by measuring the spectral response of photoluminescence heterojunction test structures. For devices with just an intrinsic amorphous silicon layer, the carrier injection from the layer was found to be close to unity.

Potential of very thin and high‐efficiency silicon heterojunction solar cells

“…The thickness of the ITO layers and the front a‐Si:H layer were set to 80 and 6 nm, respectively. The latter was chosen to be smaller than the actual thickness in the fabricated cells (~10 nm as a sum of p ‐layers and i ‐layers), by taking the partial charge‐carrier collection within the a‐Si:H layer into consideration . For simplicity, the AR film used in Figure C was not applied here.…”

“…23 In the simulation, we ation. 36,37 For simplicity, the AR film used in Figure 4C was not applied here. Under these assumptions, we simulated the spectral absorptance within the Si wafer, which is equivalent to EQE when assuming perfect carrier collection.…”

Silicon heterojunction solar cells: Recent technological development and practical aspects - from lab to industry

“…Holman et al state a J SC loss of 2 mA/cm 2 in the wavelength range between 300 nm and 800 nm due to parasitic absorption at the front-side for a typical SHJ solar cell [53] with ITO as TCO. While charge carriers generated in the doped a-Si:H can be considered as fully lost due to the short diffusion length in the material, it is reported that at least parts of the carriers generated in the a-Si:H(i) layer are injected in the c-Si absorber and thus contribute to J SC [53,54] Holman et al estimate that 30% of the carriers contribute to J SC [53], while Paduthol et al report even 40% [54].…”

Ultra‐Thin Poly‐Si Layers: Passivation Quality, Utilization of Charge Carriers Generated in the Poly‐Si and Application on Screen‐Printed Double‐Side Contacted Polycrystalline Si on Oxide Cells

“…Thus, the IQE with carrier injection efficiency β (IQE β ) can be defined as , where is the absorption in the Si absorber layer, is the absorption in poly‐Si layer, R is the external front reflectance of the finished cell, and λ is the wavelength of incident light. [ 23 ] Note that IQE β = 0 refers to a collection efficiency of 1 for all absorbed photons in the c‐Si base without recombination losses. It is thus to be taken as the maximum possible IQE under the assumption that only photons absorbed in the c‐Si base contribute to the photocurrent of the solar cell.…”

“…It has previously been shown in literature for silicon heterojunction solar cells that carriers that are photo‐generated in the amorphous silicon can be efficiently electronically injected into the crystalline silicon, adding to the cell's photocurrent. [ 23–25 ] Here we analyze this effect and vary the poly‐Si thickness on the front, including ultra‐thin (down to 10 nm) values for the cells with POLO junctions. This effect would reduce the parasitic absorption in the poly‐Si as compared with the expectations based on ray‐tracing simulations.…”