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| Fig. 1: Basic device architecture for a conventional OPV. (Source: Wikimedia Commons) |
The quest for energy independence and the evolution beyond fossil fuels has made substantial progress since the turn of the century. Solar energy is one promising candidate to replace non- renewable energy sources. The sun alone provides 120,000 TW of power, far exceeding the world's current needs of ~30 TW. Amazingly, Earth receives enough solar energy to fulfill the planet's annual energy needs in just about two hours. Using current solar cell technology with 15% power- conversion efficiency (PCE), covering about 1% of the United States would generate 30 TW. Incidentally, this is about the same amount of space already covered by roads. The execution of such a project is limited by economics. The relatively high costs of solar cells traditionally are the result of an expensive manufacturing process involving semiconductor deposition and subsequent panel installation. The first solar cells were developed using monocrystalline Si at Bell Labs in the 1950s, and these devices were very stable and resistant to environmental failure modes. [1] However, the cost was extensive, and the thin-film revolution that followed led to more cost-effective solar cells using less material. Along with this discovery came the Staebler-Wronski effect, where a significant decrease in solar cell performance was observed upon device exposure to sunlight, and attention was drawn to device reliability. [2]
Over the past decade, advances in polymer science have led to the development of organic photovoltaics (OPVs), which use carbon-based conjugated polymers as a substitute for inorganic materials. Fig. 1 shows a standard geometry for a conventional OPV device on a polyethylene substrate. Illumination occurs from the bottom of the sample, and the operation is based on the photoactive polymer blend positioned between two electrodes with different work functions. The front electrode (ITO) has a higher work function compared to the back electrode (Al), and a hole collection layer (PEDOT:PSS). The operational principle of OPVs is identical to that of traditional Si-based solar cells, namely through the creation of electron-hole pairs that are extracted to generate electricity. OPVs offer a cost advantage based on the lightweight and flexible nature of the polymer materials, ease of manufacturing using roll-to-roll printing methods, and the ability to produce devices on low-cost plastic substrates. [3] Current research focuses on improving the PCE, as current state-of-the-art OPVs are just reaching about 10% PCE. [4] This estimate must be taken with a grain of salt, however, as recent evidence exposed the widespread mischaracterization of solar cell power conversion efficiencies, where nearly half of all publications misreport or overestimate solar cell efficiency. [5] Even putting these potential exaggerations aside, Fig. 2 shows the best research-cell efficiencies for various device architectures, and the highest efficiency OPVs have PCE values that are less than half of the best multijunction inorganic cells.
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| Fig. 2: Compiled values of highest confirmed PCEs for research solar cells, from 1976 to 2014, for a wide variety of photovoltaic technologies. [4] (Courtesy of the U.S. Department of Energy) |
A further concern for PV technologies is the lifetime of such devices, and the task of increasing lifetime carries just as much importance as creating more efficient cells. The lifetime is determined by the rate of thermomechanical degradation of solar cell and module materials, along with all of the internal interfaces. It is controlled by synergistic contributions of mechanical stress, temperature, moisture, and UV radiation. These accelerate the evolution of internal defects causing delamination of layered structures, which leads to degradation of device performance and ultimately device failure. [6] OPVs are particularly sensitive to environmental stresses, where there are significant concerns about the reliability of polymer cells. [6] The fact that most OPVs contain toxic solvents is particularly unsettling based on their potential to degrade. Perovskite cells, for instance, are an emergent class of OPVs with promising efficiency ranges, but their rapid dissociation in the presence of moisture and UV radiation along with containing toxic Pb-based compounds means that they will not be on residential rooftops in the near future. [7] In essence, the incredible complexity of OPVs has opened the door for unforeseen failure modes and presents enormous variability in all aspects. [8]
The most common source of efficiency loss for OPVs is attack by oxygen-containing compounds. The diffusion of molecular oxygen and water into the device cause photo-oxidation of the polymer layers and boundaries. [9] In order to counteract this chemical reaction, various factors are involved such as the type/quality of encapsulant sealing the cell, the barrier effects from the resiliency of layers to oxygen incorporation and the layer stack configuration which control the interfaces of the device. [8] The primary degradation mechanism as a result of oxidation is the corrosion of the metal-electrode surface. [10] Conventional OPV architectures as shown in Fig. 1 use a low work function metal electrode that is vulnerable to oxidation. [11] More recent OPVs have successfully used inverted geometries to increase stability by using a higher work function electrode, which is feasible since the order of the layers is reversed. [8]
The fundamental connection between material degradation and stressing parameters, together with their mechanistic origins, remains largely uncharacterized. For example, the synergistic effect of UV photon energy and flux has never been directly characterized on the kinetics of interface debonding or defect growth in any material. Yet these photo-chemo-mechanical processes are central in the degradation of a wide range of materials exposed to solar environments from external protective coatings, through collection optics and encapsulation, and extending into the active device layers themselves. There may well be new physics to be discovered that are associated with the interaction of photons with such highly strained atomic bonds. The bottom line is that OPV lifetimes and efficiencies cannot currently compete with the conventional Si-based cells. Thus, the potential of cheaper fabrication methods for OPVs is a moot point until reliability is better characterized and new generations of devices is engineered that can function without such susceptibility to environmental degradation.
© Nick Rolston. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[9] N. Sai et al., "First Principles Study of Photo-Oxidation Degradation Mechanisms in P3HT for Organic Solar Cells," Phys. Chem. Chem. Phys. 16, 8092 (2014).
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