Stability and Degradation of Organic and Polymer Solar Cells

Frederik C. Krebs

• Publisher: Wiley
• Year: 2012
• Print ISBN: 9781119952510
• eBook ISBN: 9781118312230
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Organic photovoltaics (OPV) are a new generation of solar cells with the potential to offer very short energy pay back times, mechanical flexibility and significantly lower production costs compared to traditional crystalline photovoltaic systems. A weakness of OPV is their comparative instability during operation and this is a critical area of research towards the successful development and commercialization of these 3^rd generation solar cells.

Covering both small molecule and polymer solar cells, Stability and Degradation of Organic and Polymer Solar Cells summarizes the state of the art understanding of stability and provides a detailed analysis of the mechanisms by which degradation occurs. Following an introductory chapter which compares different photovoltaic technologies, the book focuses on OPV degradation, discussing the origin and characterization of the instability and describing measures for extending the duration of operation.

Topics covered include:

• Chemical and physical probes for studying degradation
• Imaging techniques
• Photochemical stability of OPV materials
• Degradation mechanisms
• Testing methods
• Barrier technology and applications

Stability and Degradation of Organic and Polymer Solar Cells is an essential reference source for researchers in academia and industry, engineers and manufacturers working on OPV design, development and implementation.

1

The Different PV Technologies and How They Degrade

Frederik C. Krebs

Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde, Denmark

1.1 The Photovoltaic Effect and the Overview

Shining electromagnetic radiation on matter has been employed by scientists to make observations and study the fundament of nature for centuries and the list of experiments that has been carried out is almost endless. Some of the experiments have led to deep understanding of our world and others have led to discoveries that have been reduced to practical applications that serve our society today. One particular effect is where light with wavelengths from the ultraviolet (UV) to the infrared (IR) interact with matter to create an electrical current in an external circuit. This effect is called the photovoltaic effect and there have been many experiments that documented the phenomenon very early on. One of the best known examples is that of Becquerel [1] and even if this is considered by many the first proof of principle it is difficult to extrapolate this early description to any useful application.

It was not until the bulk semiconductors arrived in the early 1950s that the photovoltaics developed into the more useful form of a solar cell as we know them today. Chapin, Fuller and Pearson [2] made the first solar cell and many applications were envisaged very shortly thereafter. Progress has been massive and today solar cells represent a large multifaceted industry, even if solar cells still contribute little to the overall production of electrical energy when viewed globally. This is likely to change radically in years to come as the annual production capacity in terms of Wpeak (Wp) increases. In the year 2010 the annual production grew by more than 15 GWp alone [3]. The early solar cell has over the past 60 years developed into several different technologies that are fundamentally different in their manufacture, use, operation, mechanism and stability, to a degree that the only common point is that they convert light (sunlight) into electricity.

In this introductory chapter the evolution of the photovoltaic technologies is briefly outlined and exemplified with some of the most important examples. The overview is meant to provide you with enough knowledge on the different technologies and to understand how they differ in the context of degradation and stability. This implies that not all solar cell technologies and disciplines will be mentioned and the literature covered is exemplary rather than exhaustive. Several books have been dedicated to general aspects of solar cells and the reader is referred to those [4–6].

1.2 The Photovoltaic Technologies

Broadly speaking the development of photovoltaic technologies has been driven with the aim of providing a stable and low-cost source of electrical energy from light (sunlight). The first solar cells that fulfilled this are often called the 1st-generation solar cells and the monocrystalline silicon solar cells should be considered prototypical for this type comprising a semiconductor p-n junction. In terms of stability the 1st generation had few problems and emerged as an intrinsically very stable technology. The energy requirements in its making and the relatively large amounts of bulk material needed resulted in the desire to develop new technologies with lower energy and material requirements. This next generation (naturally named the 2nd generation) solar cells generally encompass all the thin-film solar cells. Generally speaking the 2nd generation of solar cells solved the problems but very early on new problems of stability emerged, at least when compared to the 1st generation. The 2nd generation became much more diverse, while still being exclusively based on inorganic materials and in terms of speed of development and performance they quickly rivaled the 1st generation. The 2nd generation, however, proved remarkably slow in being upscaled reliably and this made room for the 3rd generation of solar cells that is broadly different in the sense that they encompass multijunction tandem cells and a diverse set of materials such as for instance organic polymers. The polymer or small-molecule organic solar cell is thus similar to the 2nd generation of solar cells and in essence qualify as a thin-film solar cell except that its constitution comprise organic materials. The second most preponderant organic solar cell is the dye-sensitized solar cell that is a thicker solar cell relying on the interplay between an organic and an inorganic material. Hybrid cells that are a mix of organic and inorganic material are also a 3rd-generation type of solar cell. The 3rd generation of solar cells elegantly addressed the problem of manufacturing complexity and can in essence be prepared with reasonable efficiency and very modest equipment. They also possess the potential for inherently low-cost and fast manufacture using only abundant elements. Few of the 1st- and 2nd-generation solar cells share this latter point (essentially only silicon). The 3rd-generation solar cells, however, had several weaknesses in their generally low performance and also a significantly more pronounced tendency for degradation. The diversity of the 3rd generation of solar cells is even larger than the preceding generations, but this diversity is not only linked to the constitution but also to the manners in which they degrade and this is what serves as the basis of this book. There has been discussion of whether a 4th generation of solar cells can be identified but in essence these recent types of solar cells (quantum dots, plasmonics, etc.) either fit under the hat of the 3rd generation or have a degree of esotericism that makes it difficult to pull a classification together. Another rough distinction between the generations is that the 1st generation is processed from a solid block of semiconductor by sawing it into thin slices, whereas the 2nd generation is prepared by depositions of the materials from the gas phase, and the 3rd generation is processed from solution by coating and printing. This processing evolution has transcended back and forth and today there are examples of 2nd-generation solar cells (i.e. CIGS) that can be processed from solution and 3rd generation (i.e. small molecule) prepared by evaporation.

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1.3 Intrinsic Versus Extrinsic Stability

When considering the stability of any photovoltaic the question of where the stability (or instability) comes from arises. There are several examples of solar cells that prove unstable in operation while their constituents are stable. On the other hand, there are no examples of solar cells where stable operation is achieved while their constituents are unstable. One may then ask why raise the question at all and the answer is that during development of a solar cell technology one of course strives to achieve stable operation, but when, for one reason or another, this is not reached and the cause to degradation is established it is useful to know whether the source of degradation is something you can solve or whether the degradation is fundamentally linked to the materials and the approach. A poor intrinsic stability can for instance be linked to an interface inside the working device, whereas a poor extrinsic stability can be caused by corrosion or crack formation causing failure of an otherwise well-operating solar cell.

1.3.1 Intrinsic Stability

A good example of intrinsic stability for a solar cell is the pn-heterojunction in monocrystalline silicon solar cells. Being a single-crystalline material that is passivated at the surfaces with stable materials and interfaces yields a solar cell where the part that converts sunlight into an electrical current is intrinsically stable during operation.

1.3.2 Extrinsic Stability

Taking a monocrystalline silicon solar cell module as an example it was sometimes observed for the early versions of the technology that the module performance failed quickly due to corrosion of the interconnections or dropped significantly due to yellowing of the encapsulation material upon exposure to sunlight without proper UV-blocking using, e.g., cerium ions in the front window.

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1.4 Degradation – The Culprits, the What, the Why and the How

When approaching the stability of solar cells, it is most useful to examine degradation as this, from a scientific point of view, is more easily studied and characterized. A very stable solar cell is of course of great technological relevance but does not leave a lot to be studied as there ideally is no change in performance or appearance over time, regardless of the conditions the solar cell or module is subjected to. For this reason failure modes or sources of degradation are often deliberately sought to enable observations to be made. For the more novel technologies that do present significant instabilities this is straightforward. For the more stable solar cells special conditions are employed to accelerate the occurrence of failure modes or degradation. Typical stress conditions are high temperatures, high/low humidity, salt-spray, electrical stress, mechanical stress, intense light, ionizing radiation or strong UV-light. Very often combinations of those stress conditions are employed to provoke the preponderance of a particular failure type or a cycling of parameters between for instance light/dark, dry/wet, hot/cold, etc. When employing these conditions (that can be viewed as environmental or surrounding conditions) to deliberately observe changes in the performance (or even catastrophic failure) it often becomes possible to identify “what causes degradation”. This is the first important step but to find a remedy for the problem it is necessary to establish answers to the two more elaborate questions; “why it degrades” and “how it degrades”. With those three answers at hand one is left in a powerful situation where decisions on a technology can be made, further research can be planned or the technology abandoned. The stress factors described above combined with careful analysis of the results obtained when using them can be used to gain insight into intrinsic instabilities even though the conditions are external to the device. It is thus poss...

Table of contents

• Cover
• Title Page
• Copyright
• Preface
• Acknowledgements
• List of Contributors
• 1: The Different PV Technologies and How They Degrade
• 2: Chemical and Physical Probes for Studying Degradation
• 3: Imaging Techniques for Studying OPV Stability and Degradation
• 4: Photochemical Stability of Materials for OPV
• 5: Degradation of Small-Molecule-Based OPV
• 6: Degradation of Polymer-Based OPV
• 7: Test Equipment for OPV Stability
• 8: Characterization and Reporting of OPV Device Lifetime
• 9: Concentrated Light for Organic Photovoltaics
• 10: Barrier Technology and Applications
• 11: Summary and Outlook
• Color Plate
• Index