Photo and dark current mechanisms in organic heterojunction solar cells
Abstract/Contents
- Abstract
- Organic photovoltaic (OPV) cells can potentially become the renewable energy source of choice because of their advantages such as flexibility and low-cost. Over the past decade, extensive research attention has focused on increasing the power conversion efficiency of OPV cells with record efficiencies near 6%, still falling far short of the efficiency achieved in traditional inorganic solar cells. Further improvements in OPV cell performance will require a thorough understanding of the physical processes that govern photocurrent generation. It is usually assumed that non-geminate recombination is the most important loss mechanism that can be minimized by increasing the carrier mobilities. We have modeled the separation of the geminate charge-pair at a donor-acceptor interface of arbitrary geometry using kinetic Monte Carlo simulations. We find that the geminate carrier recombination process that takes place at the donor-acceptor immediately following exciton dissociation determines the shape of the photocurrent-voltage characteristics and contributes significantly to losses in organic donor-acceptor solar cells. The ratio of the electron mobility in the acceptor material over the hole mobility of the donor material (or vice versa), and not the absolute carrier mobility, determines the geminate separation probability and fill factor. These results are confirmed by intensity, voltage, intentional doping, and temperature dependent photocurrent measurements on planar and bulk heterojunctions. We performed capacitance-voltage measurements on simple bilayer organic solar cells as a function of temperature. These measurements provide information about the electrically active doping concentration and the ionization energy of these dopants. Band structures were calculated for the doping density and ionization energy typically found for various temperatures, and Monte Carlo simulations of the geminate pair separation process were performed, providing a complete model for photocurrent generation that matches experimental observations closely. The Shockley diode equation has been used extensively to explain the dark current in donor-acceptor organic heterojunctions, but without thorough justification. We show experimentally that these devices cannot be modeled accurately using Shockley's model. The fit to Shockley's diode equation is coincidental and holds for a single temperature only. A new model was proposed and shown to fit experimental data. The work provides precise guidelines for increasing the efficiency of organic heterojunction photovoltaic cells.
Description
| Type of resource | text |
|---|---|
| Form | electronic; electronic resource; remote |
| Extent | 1 online resource. |
| Publication date | 2010 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Associated with | Zhao, Shanbin |
|---|---|
| Associated with | Stanford University, Department of Materials Science and Engineering |
| Primary advisor | Peumans, Peter, 1975- |
| Thesis advisor | Peumans, Peter, 1975- |
| Thesis advisor | Brongersma, Mark L |
| Thesis advisor | McGehee, Michael |
| Advisor | Brongersma, Mark L |
| Advisor | McGehee, Michael |
Subjects
| Genre | Theses |
|---|
Bibliographic information
| Statement of responsibility | Zhao Shanbin. |
|---|---|
| Note | Submitted to the Department of Materials Science and Engineering. |
| Thesis | Ph.D. Stanford University 2010 |
| Location | electronic resource |
Access conditions
- Copyright
- © 2010 by Zhao Shanbin
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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