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the blend morphology can be analyzed at dierent stages of this optimization proce- dure [102, 126, 127].

However, for a given donor-acceptor couple, the inuence of such post production treat- ments on morphology is limited: One lacks a method to directly and precisely tune the size of the individual domains in the blended structure and also the intermixing at a molecular level can commonly not be altered [19]. Furthermore, the formation of iso- lated islands of donor or acceptor material can sometimes not be avoided [128]. As one lacks precise and direct inuence, it also remains very dicult to study the impact of nanoscale phase separation in the active layer of blend devices on resulting device characteristics and performance in more detail [114, 128].

It is worth mentioning here, that the photophysical properties of liquid crystalline mate- rials (as discussed in Section 2.3.3) can be aected very signicantly by post-production annealing treatments. In fact, the self-assembly of LC materials can be used to drive an alignment into macroscopically sized domains with high supramolecular order and outstanding bulk material properties [85].

2.7 Loss mechanisms and degradation of devices

Figure 2.17: Recombination mechanisms for free charges in OPV devices. Mono-molecular and bi-molecular recombination of free charges are exemplarily shown for bi-layered and blend de- vices, respectively.

Analyzing and reducing recombination pathways present in organic photovoltaic devices can be very instructive to improving device eciency: Not only the current generation but also ll factor and open circuit voltage may be strongly aected in devices where re-

combination loss plays a major role [58, 129]. As has been discussed previously (Section 2.2.2) the exciton binding energy can be overcome at the interface of organic donor and acceptor materials provided an appropriate oset in the energy levels. As a consequence exciton recombination can be minimized at the cost of a certain loss in maximum at- tainable open circuit potential.

After the formation of charged species at the interface of the donor and acceptor mate- rials we can distinguish two distinct types of loss mechanisms: mono-molecular (gem- inate) and bi-molecular (bulk) recombination mechanisms may lead to charge carrier annihilation in the active layer of the photovoltaic device [58].

If mono-molecular recombination of electron-hole pairs is the only loss mechanism a strictly linear increase of current generation with increased incident light intensity is expected. This is commonly found to be the major loss mechanism in bi-layered and morphologically optimized blend devices at low to moderate light intensities [58]. On the contrary, free charges originating from dierent exciton separation processes may accumulate in the active layer of devices and subsequently recombine with one another. This non-geminate recombination occurs if charges are not eciently extracted - a fact that can sometimes not be avoided in highly intermixed active layers, especially at high illumination intensities. A sub-linear increase in the current generation will be observed as a consequence. The common recombination routes are schematically shown in Fig- ure 2.17.

When analyzing the continuous competition of charge separation and recombination in an OPV device under illumination it may be instructive to sketch a schematic energy level alignment as exemplarily shown in Figure 2.18: Possible charge carrier migration pathways after photo-excitation are indicated with arrows and termed in the gure cap- tion. It is important to note, that the exact alignment of the energy levels which are shown only exemplarily in Figure 2.18 has a strong impact on the population of the indi- vidual states and thus the probability of charge separation at the interface after photo- excitation. A detailed analysis of this energy landscape has been conducted for several material combinations only recently and remains a hotly debated topic [51, 52, 130]. It is also important to mention here, that - besides the energetic landscape discussed - entropy plays an important role and helps to stabilize the charge separated state with two separated species (electron and hole) with respect to the single species of a charge transfer state (bound electron-hole pair).

2.7 Loss mechanisms and degradation of devices

Figure 2.18: Schematic energy diagram of charge formation and recombination in organic donor-acceptor systems. After Photo-excitation (1) a singlet exciton (S1) is formed. The singlet

exciton can be transferred to a charge transfer (CT) state (2). Depending on the kinetics and

energy level alignment the CT state may undergo geminate recombination to a triplet exciton (T1)

(3), relax to the singlet ground state (S0) (4) or dissociate into separated charges (CS) (5). If

charges are not extracted bimolecular recombination of separated charges may occur (6).

2.7.2 Up-scaling and stability of OPV devices

To allow commercial success of OPV devices not only the device eciency but also applicability of fabrication processes for large scale production and lifetime of devices will have to be considered in greater detail in the future.

For solution processed devices deposition techniques such as spray coating and doctor- blading should therefore be chosen for the application of the organic materials rather than rotatory spin-coating. Time and cost intensive production steps involving high vacuum or even localized electron-beam lithography should be avoided, instead. The device degradation is mainly caused by the exposure of organic compounds and contact materials to oxygen, water and strong illumination [131, 132]. However, tem- perature and mechanical stress can also signicantly aect the device properties. Com- monly devices are either fabricated and tested in an inert atmosphere or encapsulation

methods are used in order to avoid a direct exposure of the sensitive compounds to environmental conditions. On the other hand, resistivity of the organic compounds and contact materials to environmental conditions is of great importance to facilitate exible and cost eective encapsulation methods and to improve the lifetime of devices. It is worth mentioning, that interfacial layers may also have a signicant impact on the device lifetime [133]. For example, in inverted devices TiO2 is used as transparent

electrode with a low work function in combination with more noble metal top contacts. The reduced risk of contact oxidation allows the fabrication and storage of devices in ambient conditions even without device encapsulation [112, 134].