Imaging of Bandtail States in Silicon Heterojunction Solar Cells: Nanoscopic Current Effects on Photovoltaics

Abstract

Silicon heterojunction (SHJ) solar cells represent a promising technological approach towards higher photovoltaics efficiencies and lower fabrication cost. While the device physics of SHJ solar cells have been studied extensively in the past, the ways in which nanoscopic electronic processes such as charge-carrier generation, recombination, trapping, and percolation affect SHJ device properties macroscopically have yet to be fully understood. We report the study of atomic scale current percolation at state-of-the-art a-Si:H/c-Si heterojunction solar cells under ambient operating conditions, revealing the profound complexity of electronic SHJ interface processes.Using conduction atomic force microscopy (cAFM), it is shown that the macroscopic currentvoltage characteristics of SHJ solar cells is governed by the average of local nanometer-sized percolation pathways associated with bandtail states of the doped a-Si:H selective contact leading to above bandgap open circuit voltages (V OC ) as high as 1.2 V(V OC > 𝑒 • 𝐸 gap Si ). This is not in violation of photovoltaic device physics but a consequence of the nature of nanometer-scale charge 2 percolation pathways which originate from trap-assisted tunneling causing dark leakage current.We show that the broad distribution of local photovoltage is a direct consequence of randomly trapped charges at a-Si:H dangling bond defects which lead to strong local potential fluctuations and induce random telegraph noise of the dark current.

Relaxation processes in silicon heterojunction solar cells probed via noise spectroscopy

“…To avoid confusion, we note that in our model, process 2 contributes to both shot noise and 1/f noise: each conduction channel contributes to shot noise due to the discreteness of the charge, but it is the fluctuation in the conduction of these channels over time that is responsible for 1/f noise. This assertation is consistent with the recent observation of trap-assisted tunneling in conjunction with random telegraph noise by conductive atomic force microscopy in the a-Si:H 40 . However, we cannot completely reject a possibility that at least part of the 1/f noise comes from current-induced migration of hydrogen atoms as such a process could cause local fluctuations in device conductance as the percolation path of carriers is modulated by slight shifts in crystal structure 41 .…”

Silicon Heterojunction Solar Cells Utilizing Dual-Layer Intrinsic Amorphous Silicon with Intermediate Hydrogen Plasma Treatment as the Passivation Layer

“…First, with the extension of HPT time, due to the relatively thin passivation layer in the solar cell, the hydrogen etching effect becomes more pronounced compared to the bulk material, resulting in the passivation layer becoming thin and thereby facilitating tunneling. , Second, the introduction of HPT allows hydrogen to diffuse to the interface between c-Si and a-Si:H, further passivating dangling bonds, leading to a decrease in the interface defect state density and recombination, thereby promoting charge carrier transport. Lastly, with prolonged HPT, the hydrogen content within the film increases, along with an increase in Si–H bond density, introducing additional strain into the amorphous silicon network, ultimately broadening the tail states in the valence band, which enhance the nanoscale charge percolation and promote charge carrier transport. , Longer HPT durations up to 40 s did not further increase PCE (not shown); this may be attributed to the deterioration in film quality, leading to a diminished passivation effect.…”