Organic photovoltaics, the technology to convert sunlight into electricity by employing thin films of organic semiconductors, has been the subject of active research for more than 20 years and has recently seen increased interest due to global energy challenges. Solar technologies are currently dominated by wafer-size, single-junction solar cells based on crystalline silicon, which are assembled into large-area modules. However, other technologies based on thin films are under active investigation because thin-film cells absorb light more efficiently, reducing the amount of absorbing material needed, and they can also be processed directly onto large-area substrates, significantly lowering the assembly costs of modules.
The low-temperature processing of organic small molecules from the vapor phase (or of polymers from solution) can confer organic semiconductors with a critical advantage over their inorganic counterparts. This is because the high-temperature processing requirements of the latter limit the range of substrates on which they can be deposited. A particularly attractive application for organic semiconductors is flexible plastic substrates that can be used in lightweight and low-cost consumer products with highly flexible form factors. However, despite recent advances, the power conversion efficiency of organic solar cells remains rather small, with maximum values in the range of 5–6%.1 In order to compete with other thin-film technologies, it is critical to increase the efficiency of organic photovoltaic cells.
The power conversion efficiency η, which represents the most important metric for a photovoltaic cell, is defined as:
where Pinc is the incident power density, Jsc is the short-circuit current, Voc is the open-circuit voltage, and FF denotes the fill factor. These parameters are illustrated in Figure 1. While maximum values for Jsc can be estimated from the light-harvesting capabilities of the molecules or polymers employed in solar cells, the physical processes that determine Voc in organic solar cells remains the subject of active research. The conventional wisdom of using organic semiconductors with smaller band gaps to harvest a larger portion of the solar spectrum does not always increase efficiency because Voc generally decreases in devices employing materials with smaller band gaps, as is the case with inorganic semiconductors.
Figure 1. Illustration of the current-voltage characteristic of a solar cell and corresponding electrical power density output. Pmax: Maximum power. Jmax: Maximum current. Jsc: Short-circuit current. Vmax: Maximum voltage. Voc: Open-circuit voltage.
The operation of organic solar cells relies largely on the heterojunction between the donor and acceptor (like molecules/polymers) as well as the selective electrodes that extract either holes or electrons from the device. These heterojunctions are responsible for the efficient dissociation of the excitons formed after light absorption into free carriers that lead to electrical current. Studies have shown that the value of Voc depends largely on the relative energy levels of the donor and acceptor species that form the essential heterojunction.
An analysis of solar cell properties using equivalent-circuit methods reveals that a first approximation of their electrical characteristics can be described by a single diode in parallel with a current source. Within this model, Jsc and Voc can be parameterized by a limited set of conventional diode parameters (an ideality factor, n, and a reverse saturation current, J0), and Voc is proportional to the logarithm of the ratio of the photocurrent density, Jph, divided by the J0 density. Hence, an understanding of the physical origin of J0 directly yields information as to what limits Voc.
Our work has investigated the electrical characteristics of model solar cells that incorporate materials with different relative energy levels and how these properties relate to temperature. We have shown that J0 is thermally activated with an energy barrier height that corresponds to the difference in energy between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor (corrected for vacuum level misalignments and the presence of charge-transfer states).2 Thermal excitation of charge carriers from the charge-transfer states formed between the donor and the acceptor molecules at the heterojunction is thought to determine values for J0. Consequently, values for Voc can be derived.
To evaluate the potential of organic solar cells in terms of maximum achievable power conversion efficiency, physical models are needed to define design guidelines for the synthesis of organic semiconductors with optimized optical and electrical properties. So far, our results have shown that the maximum Voc is limited by the value of J0, which in turn is thought to originate from the thermal excitation of charge transfer states formed between the donor and acceptor species that comprise the essential heterojunction in organic solar cells. The challenge of achieving higher power conversion efficiency from organic solar cells will rely on controlling the energetics of these states through molecular engineering.
This work was funded in part by the STC program of the National Science Foundation (agreement number DMR-0120967), the Office of Naval Research, the Georgia Research Alliance, and the AtlanTICC Alliance.
School of Electrical and Computer Engineering
Bernard Kippelen received his PhD in solid-state physics from the University Louis Pasteur, Strasbourg, France. He is currently a professor of electrical and computer engineering at the Georgia Institute of Technology, Atlanta. He is a fellow of the Optical Society of America and SPIE.