ABSTRACT

During the last few years solution-processed organic photovoltaics (OPVs) have gained huge attention and are one of the future photovoltaic technologies for low-cost production. The first OPV products entered the market after the power conversion efficiency increased from 1% in 1999 to almost 7% in 2009 and beyond 12% nowadays. This significant progress shows the potential of this technology to be one of the key technologies for future power generation. Most of the scientific literature in the field of OPV is very focused on the fundamental science, the materials, and their optical and electrical properties, as well as device physics. These disciplines are essential to continue driving the OPV technology toward high efficiencies and high device stability. This chapter addresses the basic state-of-the-art technologies around the OPV science. Section

7.1 will explain the basic principles of OPV. Manufacturing and production technologies will be described in Section 7.2, including solution-processed technologies such as printing and coating and finally vacuum deposition. The various aspects of material choices will be discussed in Section 7.3 by addressing materials for interfacial layers; photoactive materials, including small molecules and polymers; and design and criteria for electrode materials and encapsulation materials. Device architectures and concepts of multilayer solar cells (tandem solar cells) are demonstrated in Section 7.4. The metrology and electrical characterization methods of organic solar cells are demonstrated in Section 7.5. Solar cell device stability, degradation, and device lifetime will be addressed in Section 7.6. Moreover, examples of applications of OPV are shown in Section 7.7. A summary and outlook are given in Section 7.8. 7.1 Basic Principles of Organic Solar Cells

The first research activities on organic solar cells concentrated on pure layers of conjugated polymers. The main chain of these polymers can be represented by an alternating arrangement of single and double bonds along the polymer backbone. The double bonds are in fact only a simplified representation of the state of the pz electrons in the polymer. They are highly delocalized along the polymer backbone, forming a molecular orbital. In monomers, the energy levels of all molecular orbitals are clearly defined. For longer polymer chains, the orbitals of all included monomers combine to yield a band structure. The upper energy limit of the bonding molecular orbital, the π-band, is known as the level of the highest occupied molecular orbital (HOMO). In the classical theory of semiconductors, this corresponds to the valence band. At the same time, the antibonding π* orbitals form a π* band, which is known as the lowest unoccupied molecular orbital (LUMO). It is comparable to the classical conduction band. The band gap between the energy levels of the HOMO and the LUMO is small enough to allow semiconducting behavior. As a result, conjugated polymers are sensitive toward excitation, which can be caused by the incident light. A breakthrough for polymer solar cells was the introduction of the bulk heterojunction as the photoactive layer. The bulk hetero-

junction is based on the photo-induced charge transfer between an electron donor (conjugated polymer, for example, poly(3-hexylth-iophene) [P3HT]) and an electron acceptor (fullerene or fullerene derivative, for example, PC61BM). The photo-induced charge trans-fer proceeds at the interface of the conjugated polymer donor and the fullerene acceptor on a femtosecond timescale. The bulk het-erojunction is a mixture of the donor and acceptor materials on the nanometer scale. Mixing of conjugated polymers and fullerenes leads to a three-dimensional heterojunction and therefore, efficient charge generation within the entire bulk occurs. The morphology of the bulk, consisting of the donor and acceptor domains, affects the charge separation, the photogenerated current, and the total power conversion efficiency (PCE) significantly. Too large domains result in insufficient charge separation, whereas too small domains favor re-combination of free charges and charge transportation is hindered. The conversion of light into electricity occurs in several successive steps, which is demonstrated in Fig. 7.1: ∑ Absorption of photons ∑ Exciton generation ∑ Charge separation ∑ Selective transportation and extraction of the charges to the opposite contacts

Photons with energy smaller than the band gap, Eg, will not be absorbed by the polymer, leading to reduced charge generation. With the absorption of photons by the polymer, having an energy larger than the band gap, Eg, electrons can be excited over the band gap from the valence band to the conduction band. In a conjugated polymer, this excitations are the π-π* or HOMO-LUMO excitations. The so-called excitons are generated. An exciton is a bounded electron-hole pair. For efficient charge generation, an exciton generated anywhere in the blend has to reach, by drift and diffusion, a donor-acceptor interface within its lifetime. The separation is determined by the diffusion length of the excited electron, which is in the range of 10 nm. The diffusion length is the distance that the electron-hole pair travels prior to recombination occurs. For organic materials the binding energy is in the excess of 0.5 eV, resulting in metastable excitons with lifetimes up to several microseconds at room temperature. The excitons have to be separated at the interface between the donor and the acceptor prior to recombination within the active layer. The electric field across the polymer layer is the driving force for the separation. In the bulk heterojunction the interface between the donor and the acceptor is distributed throughout the photoactive layer. With incident light, the charge transfer on the interface takes place and the electron is transferred from the LUMO of the conjugated polymer to the LUMO of the fullerene. This charge separation occurs due to an energetic difference of the LUMO levels of the polymer and the fullerene. Through this step, the exciton is separated into a free hole on the polymer and a free electron on the fullerene. The charge transport competes with recombination of charge carriers. The charges may encounter an opposite charged carrier and recombine with this carrier. Conjugated polymers show high mobilities along the polymer chain. However, the mobility is limited by the hopping process between the chains. The mobility is a dimension for the velocity of the charge carriers in the electric field. For a conversion to current the generated charges have to be transported selectively to the metal contacts. The holes are transported through a hopping process by the polymer to the cathode and, respectively, the electrons to the anode by the fullerene. The charge carriers are collected at the metal contacts. The amount of absorbed light is directly related to the short-circuit current.