Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-08-19
2001-10-02
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C136S263000, C430S059400
Reexamination Certificate
active
06297495
ABSTRACT:
FIELD OF INVENTION
The present invention generally relates to organic thin-film photosensitive optoelectronic devices. More specifically, it is directed to organic photosensitive optoelectronic devices, e.g., solar cells and photodetectors, having transparent electrodes. Further, it is directed to organic photosensitive optoelectronic devices having a transparent cathode which forms a low resistance interface with the adjacent organic semiconductor layer. Further, it is directed to an organic photosensitive optoelectronic device consisting of a plurality of stacked photosensitive optoelectronic subcells, each subcell having one or more photosensitive optoelectronically active layers and transparent charge transfer layers.
BACKGROUND OF THE INVENTION
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photosensitive optoelectronic devices convert electromagnetic radiation into electricity. Solar cells, also known as photovoltaic (PV) devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating, or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available. As used herein the term “resistive load” refers to any power consuming or storing device, equipment or system. Another type of photosensitive optoelectronic device is a photoconductor cell. In this function, signal detection circuitry monitors the resistance of the device to detect changes due to the absorption of light. Another type of photosensitive optoelectronic device is a photodetector. In operation a photodetector has a voltage applied and a current detecting circuit measures the current generated when the photodetector is exposed to electromagnetic radiation. A detecting circuit as described herein is capable of providing a bias voltage to a photodetector and measuring the electronic response of the photodetector to ambient electromagnetic radiation. These three classes of photosensitive optoelectronic devices may be characterized according to whether a rectifying junction as defined below is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
Traditionally, photosensitive optoelectronic devices have been constructed of a number of inorganic semiconductors, e.g. crystalline, polycrystalline and amorphous silicon, gallium arsenide, cadmium telluride and others. Herein the term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct, i.e., transport, electric charge in a material. The terms “photoconductor” and “photoconductive material” are used herein to refer to semiconductor materials which are chosen for their property of absorbing electromagnetic radiation of selected spectral energies to generate electric charge carriers. Solar cells are characterized by the efficiency with which they can convert incident solar power to useful electric power. Devices utilizing crystalline or amorphous silicon dominate commercial applications and some have achieved efficiencies of 23% or greater. However, efficient crystalline-based devices, especially of large surface area, are difficult and expensive to produce due to the problems inherent in producing large crystals without significant efficiency-degrading defects. On the other hand, high efficiency amorphous silicon devices still suffer from problems with stability. More recent efforts have focused on the use of organic photovoltaic cells to achieve acceptable photovoltaic conversion efficiencies with economical production costs.
PV devices typically have the property that when they are connected across a load and are irradiated by light they produce a photogenerated voltage. When irradiated without any external electronic load, a PV device generates its maximum possible voltage, V open-circuit, or V
OC
. If a PV device is irradiated with its electrical contacts shorted, a maximum short-circuit current, or I
SC
, is produced. When actually used to generate power, a PV device is connected to a finite resistive load and the power output is given by the current voltage product, I×V. The maximum total power generated by a PV device is inherently incapable of exceeding the product, I
SC
×V
OC
. When the load value is optimized for maximum power extraction, the current and voltage have values, I
max
and V
max
respectively. A figure of merit for solar cells is the fill factor ff defined as:
ff
=
I
max
V
max
I
SC
V
OC
(
1
)
where ff is always less than 1 since in actual use I
SC
and V
OC
are never obtained simultaneously. Nonetheless, as ff approaches 1, the device is more efficient.
When electromagnetic radiation of an appropriate energy is incident upon a semiconductive organic material, for example, an organic molecular crystal (OMC) material, or a polymer, a photon can be absorbed to produce an excited molecular state. This is represented symbolically as S
0
+hv→S
0
*. Here S
0
and S
0
* denote ground and excited molecular states, respectively. This energy absorption is associated with the promotion of an electron from a bound state in the valence band, which may be a &pgr;-bond, to the conduction band, which may be a &pgr;*-bond, or equivalently, the promotion of a hole from the conduction band to the valence band. In organic thin-film photoconductors, the generated molecular state is generally believed to be an exciton, i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. The excitons can have an appreciable life-time before geminate recombination, which refers to the process of the original electron and hole recombining with each other as opposed to recombination with holes or electrons from other pairs. To produce a photocurrent the electron-hole pair must become separated. If the charges do not separate, they can recombine in a geminate recombination process, either radiatively—re-emitting light of a lower than incident light energy—, or non-radiatively—with the production of heat.
Either of these outcomes is undesirable in a photosensitive optoelectronic device. While exciton ionization, or dissociation, is not completely understood, it is generally believed to occur in regions of electric field occurring at defects, impurities, contacts, interfaces or other inhomogeneities. Frequently, the ionization occurs in the electric field induced around a crystal defect, denoted, M. This reaction is denoted S
0
*+M→e
−
+h
+
. If the ionization occurs at a random defect in a region of material without an overall electric field, the generated electron-hole pair will likely recombine. To achieve a useful photocurrent, the electron and hole must be collected separately at respective opposing electrodes, which are frequently referred to as contacts. This is achieved with the presence of an electric field in the region occupied by the carriers. In power generation devices, i.e., PV devices, this is preferably achieved with the use of internally produced electric fields that separate the generated photocarriers. In
Bulovic Vladimir
Forrest Stephen R.
Lee John R.
The Trustees of Princeton University
LandOfFree
Organic photosensitive optoelectronic devices with a top... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Organic photosensitive optoelectronic devices with a top..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Organic photosensitive optoelectronic devices with a top... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2559795