Formación en competencias

Solution processable blend mixtures of fullerenes with strongly absorbing polymers have repeatedly shown record eciencies as active layer materials for OPV devices. For example, in the year 2001 a 2.5% power conversion eciency was reported for a poly(para-phenylenevinylene) polymer (MDMO-PPV) in combination with the solubi- lized fullerene derivative [6,6]-penyl-C61 butyric acid methyl ester (PC61BM) [135].

The blend mixtures of MDMO-PPV:PC61BM showed this high power conversion ef- ciency only when appropriate solvents were used for processing. A domain size in the order of several nanometers with bi-continuous pathways could be accomplished in these blend mixtures.

Similarly for blends of the polythiophene P3HT with PC61BM strongly morphology de-

pendent eciency values have been reported. In fact, highly optimized P3HT:PC61BM mixtures have shown certied eciencies as high as 5% repeatedly [102, 136]. This material combination has dominated the sector of organic solar cell research during the last years [16]. Numerous studies have analyzed morphology, loss mechanisms, theoret- ical limitations and photophysical processes of these blend mixtures in great detail [16, 102].The theoretical maximum attainable current generation yield of a P3HT:PC61BM

blend device has been calculated recently (ISCmax=18.7 mA cm−2); Instead, for a realis-

2.8 State of the art organic photovoltaic systems

yield (IQE=80 %), the maximum attainable I_SC for these blend devices is signicantly

lower11.5 mA cm−2 [16]. Experimental results have come very close to this theoretical

value.

Several other types of polymers have been successfully used for the application in OPV devices in conjunction with PC61BM. For example, polymers having donor-acceptor

push-pull units like PCPDTBT show a lower HOMO-LUMO bandgap compared to P3HT resulting in an absorption onset shifted towards the near-IR region. As such, light absorption and photon harvesting can be accomplished over a wide spectral range and high short circuit currents of up to15 mA/cm2 have been shown in experimental

studies [103].

Lately also other fullerene derivatives like PC71BM and Indene-PC61BM bisadducts

have been studied in conjunction with several donor polymers [137, 138]. Better photon absorption and more suitable energy levels in conjunction with common polymers are believed to be the major reasons for improved device eciencies using these molecules.

Figure 2.19: Schematic representation of a polymer-fullerene blend before and after anneal- ing. The dimensions of fullerene molecules and the well studied donor-polymer P3HT are shown. Incompatibility in size allows the formation of bi-continuous networks after an annealing treatment. Reprinted with permission from [19] Copyright 2010 American Chemical Society.

Favorable properties of polymer-fullerene blends The common electron acceptor PC61BM shows outstanding electron conduction properties [110], suitable energy lev-

els [11] and a long exciton diusion length of about 40 nm [111]. Pristine and highly symmetric donor polymers like regio-regular P3HT (see Section 2.5.1) spun from so-

lution form crystalline phases and a layered stacking which allows high charge carrier mobility.

In fact, most important for the performance of the heterojunction device will be the interplay of both the polymer and the fullerene molecules in the active layer of the de- vice. A high degree of intermixing of both donor and acceptor compounds is necessary for ecient exciton harvesting but has to be balanced by a certain degree of phase seg- regation between the materials to allow the extraction of the charges generated [128]. As cast lms, obtained directly after solution deposition of the blended materials, are commonly not optimized for charge extraction. Charges may become trapped in the highly intermixed phases with no interconnection pathways.

However, post production treatments (e.g. thermal annealing, see Section 2.6.3) allow the formation of interconnected polymer networks within the blended material which may become lled with aggregates of the fullerene derivative. Figure 2.19 shows a schematic representation of the circular shaped fullerene molecules and the well studied donor polymer P3HT. A incompatibility in size and shape drives the formation of in- terconnected but well segregated domains throughout the annealing treatment. P3HT tends to assemble into highly ordered lamella sheets with a typical and characteristic layer spacing of d = 3.5Å. Instead, the fullerene derivative tends to form clusters of

nano-crystals with a molecular spacing of slightly more than 10Å, which themselves

are of appropriate size for exciton separation and exhibit high charge carrier mobility and good transport pathways [139].In summary, after annealing a high degree of donor- acceptor intermixing is maintained and percolation pathways are established allowing ecient extraction of charges [140].

Drawbacks and limitations of polymer-fullerene blends Record eciency has been reported repeatedly for novel donor polymers blended with electron accepting fullerene derivatives. However, the material combination shows some intrinsic problems and some limitations apply for these photovoltaic devices.

Fullerenes, especially C60-derivatives show only little absorbance in the visible wave-

length range [141]. As such, best power conversion eciency can commonly only be achieved when blending the fullerene with polymers showing a pronounced and wide- band photon absorption over the entire visible spectral range.

Furthermore, various polymers with appealing optical end electronic properties exhibit energy levels that are not ideal for a heterojunction with PC61BM: in many cases a

high oset of HOMO levels leads to a signicant loss in the VOC [11].

2.8 State of the art organic photovoltaic systems

cerns the thickness of the devices. Peak eciencies are reported frequently for an active layer thickness around or even below100 nm[103, 142].Instead, an increased active layer

thickness (d≥200 nm) is conductive for upscaling the device fabrication process2.7.2

and to reduce the risk of device shortening [143].

Furthermore, most known polymer-fullerene blends need post-processing treatments like thermal or solvent annealing in order to optimize the donor-acceptor morphology [103]. Long term stability and the operation under stress of diering environmental conditions might be critical for these devices. The morphology obtained after the specialized lab- oratory treatments used when high eciency values are reported often diers from the entropically favored morphology. As such, decrease of device performance can often not be avoided when devices are used in ambient conditions. Furthermore, The inuence on the exact blend morphology is only very limited - ideal inter-penetrating networks can often not be achieved and a certain degree of bi-molecular recombination can not be avoided [128].

After having seen tremendous improvements in the early years, the reports on new record eciency values achieved for polymer-fullerene blends have become less frequent. Distinct concepts, e.g. using small molecular weight or oligomeric materials, vacuum processing techniques or more advanced processing techniques are drawing an increasing attention at present.