Photosynthesis Research (2006) 87: 73–81 Ó Springer 2006 DOI: 10.1007/s11120-005-6397-9 Review Organic solar cells: An overview focusing on active layer morphology Travis L. Benanti & D. Venkataraman* Department of Chemistry, University of Massachusetts – Amherst, 710 North Pleasant Street, Amherst, MA 01003, USA; *Author for correspondence (e-mail: [email protected]) Received 10 March 2005; accepted in revised form 25 April 2005 Key words: bulk heterojunction, conducting polymers, donor/acceptor blend, morphology, photovoltaic devices, plastic solar cells, thin films Abstract Solar cells constructed of organic materials are becoming increasingly efficient due to the discovery of the bulk heterojunction concept. This review provides an overview of organic solar cells. Topics covered include: a brief history of organic solar cell development; device construction, definitions, and character- istics; and heterojunction morphology and its relation to device efficiency in conjugated polymer/fullerene systems. The aim of this article is to show that researchers are developing a better understanding of how material structure relates to function and that they are applying this knowledge to build more efficient light- harvesting devices. Abbreviations: AFM – atomic force microscopy; C60 – fullerene; FF – fill factor; HOMO – highest occupied molecular orbital; Isc – short circuit voltage; IPCE – incident photon to current efficiency; ITO – indium tin oxide; KFM – Kelvin force microscopy; LUMO – lowest unoccupied molecular orbital; MDMO-PPV – poly[2-methoxy-5-(3070-dimethyloctyloxy)-1,4-phenylene vinylene]; MEH-PPV – poly[(2-methoxy- 5-((2-ethylhexyl)oxy)-1,4-phenylene)vinylene]; MPP – maximum power point; OSC – organic solar cell; P3HT – poly(3-hexylthiophene); P3OT – poly(3-octylthiophene); Pc – phthalocyanine; PCBM – (6,6)-phenyl-C61-butyric-acid methyl ester; PCE – power conversion efficiency; PEDOT–PSS – poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate); QE – quantum efficiency; SEM – scanning electron microscopy; TEM – transmission electron microscopy; Voc – open circuit voltage Introduction organic solar cells on flexible substrates. Such flexible cells, it is proposed, could be used in Interest in organic solar cells stems primarily from countless ways, from handheld electronics to the promise of ease of processing. This is because, commercial power production. to date, many organic solar cell devices have used However, basic organic solar cell research and polymers as integral parts of their construction. device development still have a long way to go to For example, conjugated polymers often partici- compete with inorganic solar cells. The efficiency pate as electron donors and hole conductors in the of inorganic solar cells can top 20% and the active layer of organic solar cells. Since the science development of inorganic thin-layer and multi- of polymer processing is well-developed, it is junction devices will likely lead to even better hoped that one day conventional processing steps, performance. In contrast, the best organic solar such as roll-to-roll processing and doctor-blading cells, based on the bulk heterojunction concept, can be employed to make large-area, inexpensive operate at 3.0–3.5% efficiency. Nevertheless, 74 progress is being made and much research effort is each other in the form of excitons. The excitons being spent to better understand the operation of only dissociate at interfaces, such as at electrodes organic solar cells and their structure/property or, as in the case of heterojunction devices, at the relationships. interface between donor and acceptor organic This brief review aims to summarize the materials. development and characteristics of organic solar In addition to, or as a consequence of, the cells and highlight recent research in the area. properties outlined above, there are other differ- First, a short history of organic solar cells is pre- ences between conventional and organic solar sented. Pertinent definitions and device construc- cells. Conventional devices are so-called minority tion follow, along with materials that are carrier materials. The diffusion of the minority commonly used. Next, recent efforts to understand carriers in the built-in electric potential (electric the relationship between active layer morphology field) creates the photovoltaic current. On the and device performance are highlighted. other hand, organic cells are majority carriers because holes exist primarily in one phase, elec- trons exist primarily in the other phase, and their Background movements result directly in current flow. This is illustrated in Figure 1 below. In 1959, Kallmann and Pope observed a photo- Also, the distribution and diffusion of carriers voltaic effect in a single crystal of anthracene when within the materials operate under different sandwiched between two identical electrodes and mechanisms (Gregg 2003; Gregg et al. 2003). In illuminated from one side (Kallmann et al. 1959). conventional cells, holes and electrons are gener- While they could not completely explain the phe- ated together, in the same phase of the material, nomenon, they postulated that different exciton and the photoinduced chemical potential gradient dissociation mechanisms must occur at the light tends to drive them in the same direction. This and dark electrodes. Later, they also observed a effect is greater on the minority carriers than on photovoltaic effect in a tetracene–water system the majority carriers. In addition, the built-in (Geacintov et al. 1966). Since this device was also electric potential of inorganic devices drives the completely symmetrical, except for illumination, separation and flow of holes and electrons. In they thought that exciton dissociation via electron contrast, in organic heterojunction devices, exci- injection into the water, and hole transport by the tons dissociate at interfaces. So, the hole is gen- organic material away from the interface, could erated in one phase (the donor phase) and the explain the observed behavior. These studies, electron is generated in the other phase (the along with later studies on liquid crystalline por- acceptor phase). As a consequence of the free phyrins by Gregg (1989), show how a photovoltaic carriers being spatially separated and existing in effect can arise in a symmetrical organic device. different phases, the photoinduced chemical Furthermore, they highlight the differences between conventional (inorganic) solar cells, which are usually based on silicon or other inorganic semiconductors, and organic solar cells. According to Gregg, in a conventional device, charge carriers (electrons and holes) are generated in the bulk of the material and the electrons and holes are not tightly bound to each other (Gregg 2003; Gregg et al. 2003). The charge carriers are separated from each other by the built-in electric field of the device and travel to their respective electrode where they are transported out of the semiconducting mate- rial. Devices made from organic semiconductors do not operate this way. In contrast to the free Figure 1. Schematic diagrams of a conventional p–n junction solar cell (left) and an organic heterojunction solar cell (right). carriers in inorganic materials, the charge carriers The diagram highlights differences in carrier generation in organic semiconductors are tightly bound to between the two types of devices. 75 potential drives them in opposite directions. In heterojunction organic devices, built-in electrical fields may play a smaller role in carrier movement, depending on device construction (solid state or dye-sensitized). Organic solar cell basics In this section, some basics of organic solar cells are outlined. First, device construction is outlined and the difference between a bilayer heterojunction and a bulk heterojunction is emphasized. Then, some characteristics, or properties of organic solar cells Figure 2. Schematic diagram of the band structure of an are reviewed. These properties, such as fill factor organic solar cell having only one material in the active layer and various types of efficiencies, are often quoted in and different types of metal electrodes. the literature. However, it is important to under- stand the meaning of these terms, how they arise, planar. In a bulk heterojunction device, attempts and what material parameters affect them. For have been made to maximize/optimize the inter- example, the term ÔefficiencyÕ is regularly used to face between phases. In organic solar cells made describe devices, but there are many types of effi- from blends of conjugated polymers (donor) and ciencies used in the literature. An attempt will be fullerenes (acceptor), it is the conjugated polymer made to clear up the many definitions. that absorbs the incident light. The absorption It is interesting to note that, in organic devices, process generates an exciton that can either relax a photovoltaic current can be observed even in a back to the ground state or dissociate into an symmetrical device – a setup with only one pho- electron and a hole. Since, in organic cells, exciton toactive material and electrodes constructed of the diffusion lengths are small and the dissociation same material top and bottom. In this type of process only occurs at the donor/acceptor inter- device, the excitons must remain intact (not relax) face, controlling the structure of the active layer is long enough to reach an electrode and dissociate. very important to constructing efficient devices. Since the electrons and holes are so tightly bound, A complete bulk heterojunction