Hole-mobility-limiting atomic structures in hydrogenated amorphous silicon

Low hole mobility currently limits the efficiency of amorphous silicon photovoltaic devices. We explore three possible phenomena contributing to this low mobility: coordination defects, self-trapping ionization displacement defects, and lattice expansion allowing for hole wave-function delocalization. Through a confluence of experimental and first-principles investigations, we demonstrate the fluidity of the relative prevalence of these defects as film stress and hydrogen content are modified, and that the mobility of a film is governed by an interplay between various defect types

A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction

With the advent of efficient high-bandgap metal-halide perovskite photovoltaics, an opportunity exists to make perovskite/silicon tandem solar cells. We fabricate a monolithic tandem by developing a silicon-based interband tunnel junction that facilitates majority-carrier charge recombination between the perovskite and silicon sub-cells. We demonstrate a 1 cm[superscript 2] 2-terminal monolithic perovskite/silicon multijunction solar cell with a V [subscript OC] as high as 1.65 V. We achieve a stable 13.7% power conversion efficiency with the perovskite as the current-limiting sub-cell, and identify key challenges for this device architecture to reach efficiencies over 25%.Bay Area Photovoltaic Consortium (Contract DE-EE0004946)United States. Dept. of Energy (Contract DE-EE0006707

Origins and implications of intrinsic stress in hydrogenated amorphous silicon thin films

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis. Page 62 blank.Includes bibliographical references (p. 57-61).Despite decades of research on hydrogenated amorphous silicon (a-Si:H), there remains much to be understood about the relationship between deposition conditions and the resulting structural, optical, and bulk properties of the material. In this work we investigate these correlations for a-Si:H films created using plasma enhanced chemical vapor deposition (PECVD), focusing on the creation of intrinsic stresses within the films. Through experimental examination of the deposition process pressure, we model the plasma ion momentum using a combination of theoretical models and empirical trends. We find that compressive stress is controlled by ion bombardment causing of peening the film, and leading to lattice distortion in the material. Conversely, tensile stress is created through bombarding ions collapsing nano-sized voids within the material, which are formed during the vapor-phase deposition. Combining our model of ion momentum with the theory of ion peening creating compressive stress, we are able to fit the process conditions to the observed the compressive regime of our films. Furthermore, by analyzing the hydrogen content in voids within our films, we are able to predict the film porosity, and thereby model the void collapse, yielding the tensile stresses. The balance between these compressive and tensile stress forces determines the final intrinsic stress state, and allows our refined model to fit the entire range of highly compressive to highly tensile film stresses. Finally, we present correlations between film structural properties and observed optical properties, real and imaginary refractive indices and optical band gap, factors important for the creation of a-Si:H based devices.by Eric Johlin.S.M

Understanding and improving hole transport in hydrogenated amorphous silicon photovoltaics

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 133-141).While hydrogenated amorphous silicon (a-Si:H) solar cells have been studied extensively for the previous four decades, the low performance of the devices is still not well understood. The poor efficiency (below 10%, even in research-scale devices) is believed to be mainly due to the low hole mobility of the bulk material, but there is little is known about the physical phenomena responsible for this deficient mobility. This work explores the atomic structures causing the inefficient hole transport in a-Si:H, as well as a novel rout toward improvement. First, a large ensemble of computational a-Si:H structures (216 Si atoms, with ~10% H) is created, representing over 600,000 atoms. This ensemble is analyzed using density-functional theory (DFT) calculations, and statistical correlations are made between discovered defective atomic structures and strong hole trapping. It is observed that a self-trapping defect arising from a reversible atomic rearrangement in the presence of a hole is most strongly correlated with deep trapping, followed by floating bonds, or over-coordinated silicon defects. Dangling bonds, or under-coordinated silicon defects, despite their traditional indictment for the responsibility of trapping in the literature, are found not to correlate with strong hole traps. Experimental films are produced using plasma enhanced chemical vapor deposition (PECVD), in p-i-n solar cell device configurations. By varying the chamber pressure, a set of devices with widely ranging properties are produced, varying hydrogen content and bonding configuration, stress, and density. These devices are then characterized using time-of-flight (ToF) photocurrent-transient measurements, allowing the direct measurement of hole transport though the intrinsic layer, and thereby the calculation of the material hole mobility. It is found that the peak mobility occurs at both intermediate (compressive) stress and hydrogen contents, with rapid linear declines in mobility as this maximum is deviated from (with respect to film stress). The computational ensemble of a-Si:H is then extended to include the experimentally-observed variables of stress and hydrogen concentration. A second ensemble of lower hydrogen content (~5%) is created, and both hydrogen contents are relaxed at three differing stress states (-1, 0 and +1 GPa), extending the full simulation to approximately 2 million atoms across over 8 thousand structures. It is found that the modification of the stress and hydrogen content of computational samples correlates to shifting regimes of defect prevalence - increased hydrogen content and increasing compressive stress are both correlated with increased floating bond concentration. Low absolute values of stress correspond to increased ionization displacement defects. High tensile stress is observed to increase strong hole traps, without substantial increases in any of the previously explored defects, which is attributed to lattice expansion allowing further hole delocalization around trapping structures which would be otherwise less favorable due to the high kinetic penalty of strong wavefunction confinement. These relationships are then correlated to the aforementioned experimental results, and further experimentally vetted, where possible. Finally, as the observed shifting nature of defects in a-Si:H makes the further improvement of the bulk material untenable, methods are explored for utilizing the beneficial properties of the material (namely the strong bulk absorption and robust surface states) to achieve improved hole extraction (or effective mobility) from devices. Specifically, nanohole structured hydrogenated amorphous silicon (nha-Si:H) devices are created as a proof-of-concept, showing up to 50% increases in efficiency over equivalent planar devices, for low-performing materials.by Eric C. Johlin.Ph. D

Grain Boundary Engineering for Improved Thin Silicon Photovoltaics

In photovoltaic devices, the bulk disorder introduced by grain boundaries (GBs) in polycrystalline silicon is generally considered to be detrimental to the physical stability and electronic transport of the bulk material. However, at the extremum of disorder, amorphous silicon is known to have a beneficially increased band gap and enhanced optical absorption. This study is focused on understanding and utilizing the nature of the most commonly encountered Σ[subscript 3] GBs, in an attempt to balance incorporation of the advantageous properties of amorphous silicon while avoiding the degraded electronic transport of a fully amorphous system. A combination of theoretical methods is employed to understand the impact of ordered Σ[subscript 3] GBs on the material properties and full-device photovoltaic performance.King Fahd University of Petroleum and Minerals (Project R1-CE-08

Origins of hole traps in hydrogenated nanocrystalline and amorphous silicon revealed through machine learning

Genetic programming is used to identify the structural features most strongly associated with hole traps in hydrogenated nanocrystalline silicon with very low crystalline volume fraction. The genetic programming algorithm reveals that hole traps are most strongly associated with local structures within the amorphous region in which a single hydrogen atom is bound to two silicon atoms (bridge bonds), near fivefold coordinated silicon (floating bonds), or where there is a particularly dense cluster of many silicon atoms. Based on these results, we propose a mechanism by which deep hole traps associated with bridge bonds may contribute to the Staebler-Wronski effect.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R1-CE-08)National Science Foundation (U.S.) (Grant 1035400

Process-to-panel modeling of a-Si/c-Si heterojunction solar cells

The cell-to-panel efficiency gap observed in a-Si/c-Si heterojunction solar cells is one of the key challenges of this technology. To systematically address this issue, we describe an end-to-end modeling framework to explore the implications of process and device variation at the module level. First, a process model is developed to connect the a-Si deposition parameters to the device parameters. Next, a physics based device model is presented which captures the essential features of photo-current and diode injection current using the thermionic-diffusion theory. Using the process and device models, the effects of process conditions on cell performance are explored. Finally, the performance of the panel as a function of device and process parameters is explored to establish panel limits. The insights developed through this process-to-panel modeling framework will improve the understanding of the cell-to-panel efficiency gap of this commercially promising cell technology.United States. Department of Energy. Solar Energy Research InstituteNational Science Foundation (U.S.). Nano-Engineered Electronic Device Simulatio

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices, attempts to improve mobility of the material directly have stagnated. Herein, we explore a method of utilizing nanostructuring of a-Si:H devices to allow for improved hole collection in thick absorber layers. This is achieved by etching an array of 150 nm diameter holes into intrinsic a-Si:H and then coating the structured material with p-type a-Si:H and a conformal zinc oxide transparent conducting layer. The inclusion of these nanoholes yields relative power conversion efficiency (PCE) increases of ∼45%, from 7.2 to 10.4% PCE for small area devices. Comparisons of optical properties, time-of-flight mobility measurements, and internal quantum efficiency spectra indicate this efficiency is indeed likely occurring from an improved collection pathway provided by the nanostructuring of the devices. Finally, we estimate that through modest optimizations of the design and fabrication, PCEs of beyond 13% should be obtainable for similar devices

Optical loss analysis of monolithic perovskite/Si tandem solar cell

Coupling perovskite and silicon solar cells in a tandem configuration is considered an attractive method to increase conversion efficiency beyond the single-junction Shockley-Queisser limit. While a mechanically-stacked perovskite/silicon tandem solar cell has been demonstrated, a method to electrically couple perovskite and silicon solar cell in a monolithic configuration has not been demonstrated. In this contribution, we design and demonstrate a working monolithic perovskite/silicon tandem solar cell, enabled by a silicon tunnel junction, with a VOC of 1.58 V. We further discuss possible efficiency loss mechanisms and mitigation strategies