Origins of Structural Hole Traps in Hydrogenated Amorphous Silicon

Abstract

The inherently disordered nature of hydrogenated amorphous silicon (a-Si:H) obscures the influence of atomic features on the trapping of holes. To address this, we have created a set of over two thousand ab initio structures of a-Si:H and explored the influence of geometric factors on the occurrence of deep hole traps using density-functional theory. Statistical analysis of the relative contribution of various structures to the trap distribution shows that floating bonds and ionization-induced displacements correlate most strongly with hole traps in our ensemble.

Hole-mobility-limiting atomic structures in hydrogenated amorphous silicon

“…[13] show that ionization displacement (ID) defects are predominately responsible for the deep band-tail traps in an a-Si:H ensemble with ∼10% H and neutral stress. It is known that both the stress and hydrogen content vary depending upon the deposition condition [31].…”

“…The procedures are identical to those discussed and tested in our previous work [13], with the only modifications being the targeting of specific stress states, and an additional ensemble created with a lower, ∼5% hydrogen concentration. Hole trap depth (HTD) analysis also proceeded identically to the above-mentioned work, investigating ensembles of 1200 a-Si:H structures, allowing full relaxations both before and after introduction of the hole into the system, converging these structures to ensure the hole-induced relaxations are fully reversible, and taking the energy difference between these structures as the ionization potential of the structure.…”

“…3(b), we investigate the influence of structures containing one or more floating or dangling bonds in our ensembles, with bonded silicon defined as atoms with a separation of less than 2.75Å [13]. We plot density functions of the probability of finding hole trap depths in the ensemble when filtered by said conditions (e.g., contains FBs).…”

“…Our own previous computational investigation into structures influencing hole trapping in a-Si:H found that a particular self-trapping defect showed the strongest correlation with deep hole traps: it was shown that local rearrangement of the a-Si:H atomic structure under the addition of a hole to the system allowed for more energetically favorable localization of a hole in the proximate region (and was thus referred to as an ionization displacement defect) [13]. Floating bonds, or overcoordinated Si atoms, showed the second largest correlation in our ensembles, while DBs contributed little to the band-tail trap states.…”

Towards a Multi-scale Approach to the Simulation of Silicon Hetero-junction Solar Cells

“…The a-Si:H side of the system is generated using a quench from the melt simulation protocol, currently used to generate high-quality amorphous samples [7]. It is composed of 128 Si atoms and 16 H atoms to have a concentration percentage of about 11% that is the nominal concentration targeted in experimental materials optimized for PV performance [13]. The total length of the system is L z = 38.70 Å, while in the x and y direction the system has L x = L y = 15.48 Å.…”