Molecular Multilayer Organic Solar Cells with Large Excitonic Diffusion Length
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0965-S11-01
Molecular Multilayer Organic Solar Cells with Large Excitonic Diffusion Length Seunghyup Yoo1, William J Potscavage1, Benoit Domercq1, Sung-Ho Han2, Dean Levi2, and Bernard Kippelen1 1 School of Electrical and Computer Engineering, Georgia Institute of Technology, 777 Atlantic Dr., Atlanta, GA, 30332-0250 2 NREL, Golden, CO, 80401 ABSTRACT We report on the photovoltaic properties of organic solar cells based on pentacene and C60 thin films. A peak external quantum efficiency (EQE) of 69 % at a wavelength of λ = 668 nm is achieved upon optimization of the exciton blocking layer (EBL) thickness. Complex optical functions of pentacene films are measured as a function of wavelength by spectroscopic ellipsometry and used to analyze the EQE spectra. Detailed analysis of the EQE spectra indicate that the pentacene layers exhibit large excitonic diffusion lengths of ~70 nm and that the performance improvement in EQE can be attributed to the influence of the thickness of the EBL layer on the carrier collection efficiency.
INTRODUCTION Organic photovoltaic (OPV) cells are rapidly gaining attention due to their potential to be fabricated at low cost in thin, flexible, and light-weight form factors. Recent developments of OPV cells with ~ 5 % power conversion efficiencies are regarded as an important step towards the demonstration of their practical viability [1,2]. In general, power conversion efficiencies of PV cells are proportional to the product of a short-circuit current density JSC, an open-circuit voltage VOC, and a fill factor FF. Among them, JSC represents the photocurrent density generated by a given PV cell and is directly related to a cell’s light absorbing properties and quantum efficiencies associated with carrier generation and collection. In most OPV cells, a relatively large exciton binding energy makes photo-generated excitons need to migrate to a donor/acceptor junction to dissociate into free carriers [3]. However, because of the limited excitonic diffusion length L, or the characteristic distance over which excitons migrate before recombining, the thickness of a photoactive layer in multilayer-type organic solar cells is often limited to values typically of the order of L. Excitonic diffusion lengths in common organic materials used in OPV, typically less than a few tens of nanometers, are often smaller than their optical penetration depths 1/α in which α is the absorption coefficient. Such thin photoactive materials cannot absorb a large portion of the incoming photons and thus are subject to a small JSC, limiting power conversion efficiencies. In this respect, the product of L and absorption coefficient α is regarded as a figure of merit for JSC [4]. While the excitonic diffusion length can depend on many different factors, one way to improve it may be to increase the molecular ordering in the organic films. In this case, however, the challenge is to retain ease of processing and scalability. Solution-processible
columnar discotic liquid crystals were proposed in that respect [5], but it
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