Stability and Degradation of Organic and Polymer Solar Cells

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 3rd 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.

• Language: English
• Publisher: Wiley
• Release date: Apr 2, 2012
• ISBN: 9781118312230

Book preview

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.

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.

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 possible to gain knowledge on instabilities rooted in the materials, interfaces and instabilities linked to processing and preparation of the solar cell or module. In this book you should find the methodologies that cover most of it (if not all) in the context of polymer and organic solar cells. To round off this chapter some examples of some of the most distinct solar cell technologies are given with special focus on some of their most well-known degradation paths and failure mechanisms.

1.5 Some Representative Technologies and How They Degrade

Even though this book deals only with the degradative behavior of a branch of the 3rd-generation solar cells (polymer and organic solar cells excluding dye-sensitized solar cells) it is considered instructive to present well-known technologies and their most preponderant failure modes and degradation paths. Also, reflection is given on why a given solar cell type may be particularly stable under a given set of conditions, whereas another technology might be very sensitive under that set of conditions. One very obvious overall observation that is also summarized in Figure 1.1 is that when the operating temperature for a solar cell is much lower than the processing temperature the stability generally seems to be larger. Also, the more degrees of freedom that there are in the constitution the more sources of degradation are found. While this corroborates well with the observation that the newer the generation the more challenges there are with ensuring operative stability, it does not mean that it is fundamentally impossible to make a polymer solar cell that will work well for 25 years. We just do not know how to get there yet. Since stability studies on polymer solar cells started 10 years ago the stability under ambient conditions was measured in minutes or even within the time span of recording one or a few I–V curves and until today where we have continuous operation outdoor on the order of years, it is not unreasonable to expect that we can improve it by the remaining factor of 10–20 when considering that we have already improved it by a factor of 10 000–1 000 000 under ambient conditions.

Figure 1.1 An arbitrary scale illustration of how the different solar cell generations have evolved in time with respect to stability, processing temperature, cost, simplicity and materials use.

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1.5.1 Mono- and Polycrystalline Silicon Solar Cells

Monocrystalline silicon solar cells can be viewed as the first ‘real’ solar cell. It was developed in 1954 by Chapin et al. [2] and came out with a power-conversion efficiency of 6% with fast development to 10% and in excess of 20% today even for multicrystalline silicon [7]. Already at birth operation was exceptionally stable with little concern being raised over operation for extended periods of time. A schematic of a typical crystalline silicon solar cell is shown in Figure 1.2 where a few of the degradation paths are shown. Possibly the first real surprise regarding the stability of crystalline silicon came with space exploration where solar panels were employed to power electronics in satellites. Here, radiation damage (from ionizing radiation) was found to be a significant source of gradual degradation in performance with increasing dose [8,9]. On earth operation proved to be very stable and radiation damage cannot be viewed as a significant cause of performance degradation for crystalline silicon. The largest problem with outdoor deployment and operation of silicon solar cells were external to the device and linked to mechanical cracking [10] of the silicon wafer or corrosion of electrodes and interconnections [11]. The encapsulation techniques quickly developed into a form where a thermoplastic polymer such as ethylvinylacetate (EVA) was employed to embed the wafer and wiring. The use of EVA has been extensively studied with respect to use stability, yellowing, water ingress, etc. [12–16].

Figure 1.2 A schematic view of a typical crystalline silicon solar cell with an indication of where the most preponderant failure modes are observed. The processing starts with a silicon wafer that is processed on both sides. The completed wafers with front electrodes are stringed together and hot-melt laminated between two sheets of EVA (0.5 mm thick) onto an antireflective glass plate and a Tedlar™ or glass back plate.

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Typically, a glass plate has been employed and the mechanical problems were mostly solved and are a guarantee of being durable to weathering outside for many years. The use of a printed silver grid on the front of the wafer along with stringing a tape wire across the front even solved problems related to complete failure when the devices crack. Early on some yellowing of EVA was observed outside but once cerium-doped glass was employed, thus eliminating UV-B admission to the EVA, operation for 25 years or more would seem to be the general expectancy for silicon solar cells. In terms of reports dedicated to degradation of crystalline silicon solar cell modules studies on EVA degradation have been accounting for most of the recent literature [13–16] simply because the crystalline silicon solar cells are so stable that anything you develop has to comply with that stability.

1.5.2 Amorphous, Micro- and Nanocrystalline Silicon Solar Cells

Amorphous silicon was developed with the aim of reducing the large materials usage and thermal budget of crystalline silicon solar cells and this was achieved by Carlson and Wronski in 1976 [17].

Even though the performance was lower the excitement was enormous and expectations were high. It did not, however, take long before the first surprise of degradation came and this was reported by Staebler and Wronski in the following year [18] and became known as the Staebler–Wronski effect. The Staebler–Wronski effect is what happens when an amorphous silicon solar cell degrades in performance when subjected to illumination. The typical amorphous silicon solar cell is shown in Figure 1.3. The studies of the Staebler–Wronski effect led to the relatively quick development of doped versions of a-Si:H and more importantly by controlling the ratio between SiH4 and H2 in the feed gas during PECVD deposition of the a-Si:H layer it was found that nano- and microcrystallinity could be induced and that this was an efficient remedy for the Staebler–Wronski effect [19–21] whereas chemical means have also been attempted [22]. This has also led to the further development of stacked junctions giving higher efficiencies that today rival that of polycrystalline silicon wafers. Similarly to crystalline silicon a:Si:H is also sensitive to ionizing radiation to a certain degree. It must have been a fact that was emotionally difficult to handle as everyone was accustomed to the stability of the silicon solar cells and the means available to stabilize crystalline silicon (extrinsically), while handy, would not solve this intrinsic stability problem that amorphous silicon presented. This naturally opened a whole new research discipline on what the underlying physical reasons were [23–29] and also a large technological development on how to eliminate it [30–32]. Thus, amorphous silicon became the first example of a solar cell technology that was not simply dismissed because it presented degradation to certain conditions but where scientists invested a lot of time in fixing the problem. The reason for this was of course (as always) the potential financial gain but also because amorphous silicon (potentially at least) efficiently addressed so many challenges that one would be able to sacrifice some performance to get something else.

Figure 1.3 A schematic view of the amorphous silicon solar cell with an indication of where the iconic Staebler–Wronski failure mode is observed.

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1.5.3 CIS/CIGS Solar Cells

Just like amorphous silicon solar cells are prepared by a low-temperature (relative to crystalline Si) gas phase deposition technique so is the copper indium diselenide (CIS) and copper indium gallium diselenide (CIGS) solar cells, as illustrated for the CIGS solar cell in Figure 1.4. The CIGS solar cell relies on the codeposition of all the components onto a molybdenum electrode and thus provides a solution similar to the a-Si:H solar cells and even presents a very significant power-conversion efficiency of around 20%. It has some drawbacks in that very thick active layers are required and also that some of the elements are not the most abundant. The largest degradation path for CIGS is their sensitivity to humidity where especially the front electrode suffers. This is viewed as an extrinsic stability problem and has been addressed by encapsulation techniques well known from the crystalline silicon solar cells such as EVA and a glass front with special edge sealing [33]. The CIGS solar cells presented great difficulty in passing a damp heat test for 1000 h and it is likely that further development of the front electrode and encapsulation will enable an improvement in the technology [34–42]. One particular advantage of CIGS solar cells are their resistance to radiation [43] which makes them particularly suited for space application, granted their high efficiency and the possibility of a thin lightweight outline. In space, the humidity is not a problem and the CIGS cell is well suited for space application from that point of view. Several reports have suggested that CIGS solar cells present self-healing properties towards defects in the bulk [44,45].

Figure 1.4 A schematic view of the typical CIGS solar cell illustrating the most dominant failure mode associated with humidity is highlighted.

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1.5.4 CdS/CdTe Solar Cells

The cadmium sulphide–cadmium telluride solar cell is also prepared by deposition from the gas phase and is typically constituted as shown in Figure 1.5. It employs thin films and was possibly the first of the thin-film solar cell technologies that very quickly reached a very large production capacity (First Solar claims in excess of 1 GWp/year). Most importantly, a cost of < 0.7 USD/Wp is what has warranted such intense technological development and investment in increased production capacity [46]. There have been claims of uncharted long-term toxicity issues with cadmium emission from decommissioned solar panels and also tellurium is one of the rarer elements. The debate is ongoing and has so far been warded off and has at least satisfied investors in the technology well enough to consider large-scale production (First Solar). In terms of stability CdTe solar cells present a significant stability towards ionizing radiation but has been deemed unsuited for space application [47]. The most pertinent degradation path for CdTe solar cells is linked to the back electrode that traditionally comprised a copper–carbon electrode that had a tendency to induce diffusion of copper into the bulk and creation of a transport barrier and degradation in performance, seen as the appearance of an inflection point in the I–V curve [48]. The CdTe solar cells also present a significant sensitivity towards humidity at the back electrode and this has resulted in a concentrated effort towards development of alternative back electrodes that are more stable towards humidity and prevents diffusion into the CdTe bulk [49–53], In particular, Sb/Mo back electrodes have shown that this intrinsic problem is one where a solution exists or at least significant reduction of the degradation mode can be produced [54].

Figure 1.5 A schematic view of the typical CdTe solar cell with an indication of the two commonly observed failure modes are indicated.

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1.5.5 Dye-Sensitized Solar Cells (DSSC)

The dye-sensitized solar cell appeared in 1991 as reported by O’Regan and Grätzel and came with a reported efficiency that competed with polycrystalline silicon and for sure surpassed amorphous silicon (∼11%) [55]. It presents an elegant solution to principally all of the disadvantages that one can think of when it comes to preparing solar cells. It is simple to make and has proven to be a fantastic education tool and has been employed by many teachers of school children. One would have thought that the DSSC would have industrialized more quickly. The intrinsic stability of electrolytes dye and the construct have been convincingly reported [56–72].

The Achilles heel of the DSSC, as shown if Figure 1.6, is the fact that a liquid electrolyte is required for most efficient operation and while solid-state electrolytes have been reported as an efficient solution this has not made it beyond a scientific curiosity and being a great teaching tool. The absence of successful commercialization of the DSSC must at this point be considered as an abysmal failure.

Figure 1.6 A schematic view of a DSSC that is the only solar cell technology to comprise a liquid layer (an electrolyte). Some of the identified sources of degradation have been labeled.

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1.5.6 Organic and Polymer Solar Cells (OPV)

Finally, we turn towards the solar cell type that is the topic of this book, namely the organic and polymer solar cells that do present significant degradation when operated. The degradative behavior is in fact so extensive that an entire book (this one) can be dedicated towards it. The OPV has had a slow start and the evolution this far could justify the view that OPV is the ugly duckling of solar cells and perhaps also of organic electronics taken as a whole. No matter how OPV has been examined it always came out inferior, mediocre or downright poor in performance. The only reason it has not been dismissed as a possibility is that it so elegantly promises low cost, use of only abundant materials and facile and fast manufacture to an extent that would make all other PV technologies seem like a necessary step on the path towards development of the ultimate energy technology. Of course, provided that a few challenges could be solved first, i.e. stability and power-conversion efficiency. OPV has consistently improved and now approaches performance at all levels that justifies its consideration as an industrially relevant technology, even if it is still in the low end in terms of performance (at all levels). Considering the consistent improvement that OPV has undergone there is no reason to believe that the development will stop.

OPV is undoubtedly the most diverse solar cell type with a myriad of different materials and reported device geometries Figure 1.7. It is also characterized by being the solar cell where almost all materials and every interface present intrinsic stability. Most often this is due to a poor choice of combining device structure, processing and materials. Fortunately, the diversity is very large and most often a particular choice made in the aim of solving one problem, causes another one [73]. This, however, can gradually be rectified in an iterative process where modification followed by analysis is applied until all failure modes have been weeded out.

Figure 1.7 A schematic view of one type of organic or polymer solar cell that comprises a multilayer structure with identified degradation problems at all interfaces and in the bulk of all layers. Some failure modes have been highlighted.

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2

Chemical and Physical Probes for Studying Degradation

Birgitta Andreasen and Kion Norrman

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

2.1 Introduction

Degradation has a detrimental effect on efficiency and lifetime for organic solar cells [1]. It is therefore highly desirable to prevent (or delay) degradation processes to an extent that will make the technology attractive from a commercialization point of view so that the large-scale vision can be carried out. In order to prevent degradation one needs to understand the degradation mechanisms that are in play. The most common approach to study OPV degradation is based on trial and error. Photovoltaic properties (JSC, VOC, FF, and PCE) are monitored as a function of lifetime (described in Chapter 8) and modifications to prevent degradation is based on guessing what the problem is, typically by exposing the OPV devices to various experimental conditions. This approach is empirical and indirect. However, it is nevertheless the approach that is responsible for the majority of progress within OPV degradation research.

Physical and chemical analytical techniques are more direct methods used to obtain a greater insight into degradation processes in OPV devices and materials from a physical and chemical point of view. There are two overall