Hole-mobility-limiting atomic structures in hydrogenated amorphous silicon

Johlin, Eric; Simmons, C. B.; Buonassisi, Tonio; Grossman, Jeffrey C.

doi:10.1103/physrevb.90.104103

Cited by 5 publications

(6 citation statements)

References 38 publications

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“…In order to verify the presence of the intended improvement in collection pathways, we perform time-of-flight hole drift mobility measurements (procedure in accord with those performed in ref 7). The nanohole samples were fabricated similar to those used for the absorption measurements, but with an additional deposition of indium tin oxide sputtered onto the quartz slides before the a-Si:H deposition.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…It is known that the low hole mobility (due mainly to the extended band-tail states) in hydrogenated amorphous silicon (a-Si:H) prevents effective hole transport and thus limits current collection in these photovoltaic devices. , The practical implications of this, other than limiting the efficiency of a-Si:H photovoltaics, is that the devices are made much thinner than would absorb the majority of the available (above-band gap) solar illumination . The high number of previous studies into the optimization of a-Si:H devices, − coupled with recent improvements in understanding of the innate limitations to transport in the currently possible materials, makes the further improvement of the bulk material especially challenging. However, we herein demonstrate the ability of nanostructuring to utilize the advantageous properties of the material, namely, the strong optical absorption as well as the low surface recombination, critical for maintaining performance of nanostructured devices, to counteract this inherently low bulk mobility and achieve a significant improvement in hole extraction (effective mobility) in photovoltaic devices.…”

Section: Introductionmentioning

confidence: 99%

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Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

Johlin

Al-Obeidi

Nogay

et al. 2016

ACS Appl. Mater. Interfaces

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While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (aSi: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 ptype 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.

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Section: ■ Results and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

Johlin

Al-Obeidi

Nogay

et al. 2016

ACS Appl. Mater. Interfaces

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“…In a-Si:H thin film, the diffusion length of the carriers is very short, nearly <1 μm, so the transport is considered to be drift assisted where the collection length of the carriers depends on the strength of the built-in-electric field (E b ) and mobility of the minority carrier (i.e. holes, have a key role in the uniformity of the electric field within the i layer of a p-i-n a-Si:H solar cell) [6,7]. If the E b is strong, the carrier transport depends on the drift length, but if the E b is weak, carrier transport depends on diffusion length.…”

Section: Introductionmentioning

confidence: 99%

Effect of Pressure on Bonding Environment and Carrier Transport of a-Si:H Thin Films Deposited Using 27.12 MHz Assisted PECVD Process

Chaudhary

Sharma

Sudhakar

et al. 2016

Silicon

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Investigation of carrier transport in hydrogenated amorphous silicon (a-Si:H) thin films deposited at various pressures (0.03 -0.53 Torr) using 27.12 MHz assisted high frequency Plasma Enhanced Chemical vapor Deposition (PECVD) process is presented. From results of Steady State Photocarrier Grating (SSPG) the carrier diffusion length was found to vary from 0.098 -0.189 μm. Moreover a direct influence of ambipolar diffusion length was observed with the transport mechanism for deposition pressure in the range (0.13 -0.53 Torr). There was a correlation observed for photosensitivity and microstructure parameter with mobility lifetime (μτ ) product and diffusion length of carriers. Diffusion length and μτ product were observed to be maximum (0.189 μm and 0.471 x 10 −8 cm 2 V −1 ) for the film having high photosensitivity (7.2x10 3 ) deposited at a rate ∼1.39Å/s at 0.53 Torr deposition pressure. In addition to electrical transport properties, the effect of deposition pressure on structural and optical properties was also studied using various characterization tools such as Raman, UV-Vis and infrared spectroscopy.

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“…All these experimental procedures result in a-Si samples with different properties and defect concentrations [5,18]. Defects present in a-Si interact with charge carriers [19,20], affect dopant diffusion [21] and limit the efficiency of fabricated devices [22]. Experimentally observed defects in a-Si are associated to non four-fold coordinated Si atoms (like dangling or floating bonds, which are three-and five-folded Si atoms, respectively) or highly stretched Si-Si bonds in the material [23].…”

Section: Introductionmentioning

confidence: 99%

Generation of amorphous Si structurally compatible with experimental samples through the quenching process: A systematic molecular dynamics simulation study

Santos

Aboy

Marqués

et al. 2019

Journal of Non-Crystalline Solids

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The construction of realistic atomistic models for amorphous solids is complicated by the fact that they do not have a unique structure. Among the different computational procedures available for this purpose, the melting and rapid quenching process using molecular dynamics simulations is commonly employed as it is simple and physically based. Nevertheless, the cooling rate used during quenching strongly affects the reliability of generated samples, as fast cooling rates result in unrealistic atomistic models. In this study, we have determined the conditions to be fulfilled when simulating the quenching process with molecular dynamics for obtaining amorphous Si (a-Si) atomistic models structurally compatible with experimental samples. We have analyzed the structure of samples generated with cooling rates ranging from 3.3 10 10 to 8.5 10 14 K/s. The obtained results were compared with experimental data available in the literature, and with samples generated by other state-of-the-art and more sophisticated computational procedures. For cooling rates below 10 11 K/s, a-Si samples generated had structural parameters within the range of experimental samples, and comparable to those obtained from other refined modeling procedures. These computationally slow cooling rates are of the same order of magnitude than those experimentally achieved with pulsed energy melting techniques. Samples obtained with faster cooling rates can be further relaxed with annealing simulations, resulting in structural parameters within the range of experimental samples. Nevertheless, the required annealing times are on the order of microseconds, which makes this annealing step non practical from a computational point of view.

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Hole-mobility-limiting atomic structures in hydrogenated amorphous silicon

Cited by 5 publications

References 38 publications

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

Effect of Pressure on Bonding Environment and Carrier Transport of a-Si:H Thin Films Deposited Using 27.12 MHz Assisted PECVD Process

Generation of amorphous Si structurally compatible with experimental samples through the quenching process: A systematic molecular dynamics simulation study

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