Dissertation/Thesis Abstract

Theoretical Modeling of Polycrystalline Thin-Film Photovoltaics
by Attygalle, Muthuthanthrige Lilani C., Ph.D., The University of Toledo, 2008, 151; 10630896
Abstract (Summary)

This work is aimed at developing realistic theoretical models of major thin-film photovoltaics (PV) which currently are CdTe based and CuIn(Ga)Se 2 based materials. We emphasize the physical and technological aspects that are unique to these types of PV and are not adequately reflected in the existing modeling software. They are: (1) the presence of CdS layer whose role remained poorly understood; (2) the electrostatic screening length larger than or comparable to the device thickness; (3) high density of states in the forbidden gap conducive of efficient hopping transport. We note however that the problem of lateral nonuniformities typical of thin-film PV is left beyond the present scope to avoid too much interference with already published results on that topic.

We collected and critically analyzed many facts indicative of the physics of operations of CdS-based thin-film photovoltaic junctions, including their major types with CdTe and CIGS absorber layers. Based on these observations we proposed a realistic physical model of CdS based junctions. Our model allows for field reversal in the depleted CdS layer. It is solved analytically and numerically, and predicts a variety of phenomena, such as the lack of carrier collection from CdS, buffer layer effects, light to dark current-voltage curve crossing, and rollover.

Furthermore, we have numerically modeled the current-voltage characteristics and quantum efficiencies of devices with different electric field profiles including the standard p-n junction case and field reversal. Our quantitative modeling shows that the CdS polarization and its related field reversal turn out to be beneficial for photovoltaic technology making it more forgiving and reliable. Our understanding points at new venues in thin-film photovoltaic technology.

As another step in exploring more realistic models of thin-film photovoltaics we have developed a theory of long-range random potential caused by charge density fluctuations in thin nonmetal structures sandwiched between two conducting electrodes. This model applies to many practical systems including not only thin film photovoltaics but also classical p-n, and Schottky junctions. The lateral screening due to conducting electrodes leads to the screening length close to the structure thickness. We have analytically calculated the random potential amplitude for practically important cases of point defects, spherical grains, and columnar grains. Implications of our findings for polycrystalline devices are discussed.

As one other realistic feature we have investigated the defect-assisted tunneling (hopping) electron transport through non-crystalline Schottky barriers and junctions. We have shown that it can be a dominant mechanism winning over the barrier activated transport. A non-trivial interplay between the transverse tunneling and lateral device resistance is found explaining some phenomena that remained poorly understood, such as lay-down current-voltage characteristics and their light- to dark crossovers.

The above described findings are combined to elucidate the physical properties of ultrathin photovoltaics with thickness (<∼ 1 μm) smaller than both the depletion width and diffusion length, applicable to the cases of amorphous, polycrystalline, and nano-structured devices. We show that three phenomena underlie the unique physics of such systems: (1) lateral screening by conducting electrodes, (2) leakiness due to defect assisted tunneling, and (3) gigantic capacitive energy conducive to shunting breakdown. These phenomena overlooked in the classical theory of photovoltaic operations need to be taken into account in the emerging technology of ultra-thin photovoltaics.

Indexing (document details)
Advisor: Karpov, V. G.
Commitee: Compaan, Alvin D., Gao, Bo, Jayamaha, Uplai, Khare, Sanjay
School: The University of Toledo
Department: Physics
School Location: United States -- Ohio
Source: DAI-B 78/11(E), Dissertation Abstracts International
Subjects: Physics
Keywords: CdTe, Device physics, Photovoltaics, Polycrystalline, Theoretical modeling, Thin-film
Publication Number: 10630896
ISBN: 9780355012224
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