• Schulze, Tim Ferdinand: Structural, electronic and transport properties of amorphous/crystalline silicon heterojunctions. , Diss., Techn. Univ. Berlin, 2011

10.5442/d0009

Abstract:
Solar cells based on heterojunctions between hydrogenated amorphous (a-Si:H) and crystalline silicon (c-Si) are an active field of research. a-Si:H/c-Si heterojunction solar cells combine the high efficiency potential of wafer-based photovoltaics (PV) with simple and low-temperature processing using thin-film deposition with PECVD. The company Sanyo has reached a conversion efficiency of 23 % with this concept, which is considerably more than delivered by typical 'classical' c-Si based homo-junction solar cells (< 19 %), with a potentially simplified processing. Also from a scientific point of view, a-Si:H/c-Si heterojunctions are of great interest, as fundamental properties of the heterojunction remain under dispute. This e.g. concerns the lineup of the electronic bands, the charge carrier transport across the heterojunction, or the outstandingly effective passivation of c-Si surface defects by a-Si:H. The present dissertation is concerned with the physical aspects of the a-Si:H/c-Si heterojunction in the context of PV research. In a first step, the technological development which took place in the framework of the thesis is summarized. Its main constituent was the development and implementation of ultrathin (10 nm) undoped a-Si:H [(i)a-Si:H] layers to improve the passivation of the c-Si surface with the goal of increasing the open-circuit voltage of the solar cell. It is shown that the effect of (i)a-Si:H interlayers depends on the c-Si substrate doping type, and that challenges exist particularly on the technologically more relevant (n)c-Si substrate. A precise optimization of (i)a-Si:H thickness and the doping level of the following a-Si:H top layers is required to realize an efficiency gain in the solar cell. In this chapter, the key scientific questions to be tackled in the main part of the thesis are brought up by the technological development. In the next chapter, the charge carrier transport through a-Si:H/c-Si heterojunctions is investigated making use of current-voltage (I/V) characteristics taken at different temperatures. The dominant transport mechanisms in a-Si:H/c-Si heterojunctions are identified, and the relevance for solar cell operation is discussed. It is found that in the bias regime relevant for solar cell operation, the theoretical framework for the description of carrier transport in classical c-Si solar cells applies as well, which enables to use I/V curves for a simple characterization of a-Si:H/c-Si structures. The next chapter deals with the microscopic characterization of ultrathin a-Si:H layers. Employing infrared spectroscopy, spectroscopic ellipsometry, photoelectron spectroscopy and secondary ion mass spectroscopy, the structural, electronic and optical properties of (i)a-Si:H are analyzed. It is found that ultrathin a-Si:H essentially behaves like layers of 10...100 times the thickness. This represents the basis for the application of established concepts for the physical description of a-Si:H in the following chapters. Further, the impact of the PECVD deposition parameters on the properties of the resulting layers is explored and discussed. Next, the lineup of the electronic bands at the heterojunction is elucidated in device-relevant a-Si:H/c-Si heterostructures. To this end, a novel method combining photoelectron spectroscopy and surface photovoltage measurements is developed and employed. It is found that upon widening the a-Si:H optical band gap by controlling its hydrogen content, predominantly the valence band offset is increasing while the conduction band offset stays constant. This result is consistent with established theories on the a-Si:H electronic structure, but was not experimentally observed to date to the author's knowledge. The significance of the valence band offset for solar cell operation and possible pathways for tailoring the electronic properties of the heterojunction are discussed.