Quantum efficiency of organic solar cells: electro-optical cavity considerations

Armin, Ardalan, Velusamy, Marappan, Wolfer, Pascal, Zhang, Yuliang, Burn, Paul L., Meredith, Paul and Pivrikas, Almantas (2014) Quantum efficiency of organic solar cells: electro-optical cavity considerations. ACS Photonics, 1 3: 173-181. doi:10.1021/ph400044k

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Author Armin, Ardalan
Velusamy, Marappan
Wolfer, Pascal
Zhang, Yuliang
Burn, Paul L.
Meredith, Paul
Pivrikas, Almantas
Title Quantum efficiency of organic solar cells: electro-optical cavity considerations
Journal name ACS Photonics   Check publisher's open access policy
ISSN 2330-4022
Publication date 2014-03
Year available 2014
Sub-type Article (original research)
DOI 10.1021/ph400044k
Open Access Status
Volume 1
Issue 3
Start page 173
End page 181
Total pages 9
Place of publication Washington, DC United States
Publisher American Chemical Society
Collection year 2015
Language eng
Formatted abstract
Organic solar cells (OSCs) are composed of one or more layers of order 100 nm thickness sandwiched between metallic and transparent electrodes. As such, they are low finesse, multilayer optical cavities where the optical field distribution is governed by the complex refractive indices and thicknesses of all layers in the “solar cell stack”. Optical interference and parasitic absorbance in nonactive layers can have a dramatic effect on the shape of the measured external quantum efficiency (EQE), the parameter often used to optimize device structure and derive critical insight regarding charge generation and extraction. In this communication, we study a model high efficiency OSC system (PCDTBT/PC70BM) as a function of active layer thickness, blend composition and processing. The spectral shapes of the measured EQEs show strong thickness and blend ratio dependence. However, when correctly determined, the internal quantum efficiencies (IQEs) are spectrally flat. The differences in EQE spectral shape predominantly originate from optical interference and parasitic absorptions rather than charge generation or transport phenomena. We also demonstrate similar results for a second model system (PCPDTBT/PC60BM) in which an energy-dependent “IQE-like” response has recently been used to justify the existence of hot excitons. Once again, we show the origin of these spectral phenomena to be optical, not electronic. These cases highlight the fact that thin film organic solar cells (even single junction) must be properly considered as low finesse electro-optical cavities, a point that is not universally appreciated.
Keyword Organic solar cells
Low finesse electro-optical cavities
Internal quantum efficiency
Hot exciton
Parasitic absorption
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

 
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