Motivation: Organic Photovoltaics

The future supply of energy for humankind is currently a heavily discussed topic in politics and society. While the worldwide energy demand increases and global warming is continuing, the acceptance of traditional energy sources like fossil fuels and nuclear power is shrinking. Hence, providing alternative, environmental friendly and renewable energy in sufficient quantity and at low costs is one of the biggest challenges that our society now faces. Among all “green” energy sources, the solar energy is of special importance due to the huge amount of energy that is provided in the form of irradiation every day. It is worth mentioning that the amount of solar energy reaching the earth surface during 1 h (1.4·10³⁰ J) is equivalent to the annual worldwide energy consumption.^{1, 2} In a solar cell, the energy of light absorbed by a semiconducting material, can directly be converted into electricity using the photovoltaic effect, which was discovered by Alexandre-Edmond Becquerel in 1839.³ Today, the most efficient photovoltaic (PV) cells are based on monocrystalline silicon as semiconductor, where power conversion efficiencies (PCE) of 25 % can be reached in single junction devices.⁴ The high material and production costs of crystalline silicon cells however limit their wide-scale use at present which is why other PV technologies are emerging.

Among the most promising types in this context are organic photovoltaic (OPV) cells. Since the discovery of electrical conductivity in conjugated polymers by Heeger, MacDiarmid and Shirakawa in the 1970s (which was awarded with the Nobel Prize in chemistry in 2000), semiconductors based on organic molecules and polymers have attracted great attention.

When compared to inorganic silicon, the key benefit in using organic or polymeric (plastic) semiconductors in PV is its solution processability, which allows for cost efficient, high-throughput, roll-to-roll printing of multilayered structures on a large area. As a consequence, production cost can be minimized significantly. Crucial for the success of a ”green” energy technology is the time it takes to get back the energy that was invested during production,

which is known as the energy payback time. For crystalline silicon solar cells, the energy payback time is approximately 1-2 years, whereas for OPV energy payback times of only 1 day can be achieved.⁵ In addition, the feasibility of large area printing on flexible substrates opens up new possible applications of portable and light-weight solar power for integration into clothes, bags, etc.. Also, since only very thin films of a few hundreds of nanometers are required as the active layer, semitransparent cells can realize building integrated photovoltaics. Another advantage of OPV is their higher sensitivity at low light intensities, which enables better efficiencies for indoor applications or at diffuse light conditions.¹ Furthermore, the color of the devices, which is also an important factor from a marketing point of view, can be tuned by changing the chemical structure of the materials used. The huge interest in OPV technology is also attested by the growing number of scientific papers being published each year, as illustrated in Figure 1a. An image of a flexible and light weight organic solar panel is shown in Figure 1b.

Figure 1: a) Number of publications on organic solar cells since 1992;⁶ b) flexible organic solar panel.⁷

Organic Photovoltaik Concepts

An electrical isolator is roughly defined as a material with a wide band gap (Eg) larger than 3 eV. For organic materials, the band gap is given by the difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

The majority of organic molecules and polymers are considered as isolators. To become

a) b)

semiconducting, a material needs to exhibit the following important features. Organic semiconductors mainly consist of alternating C-C single and C=C double bonds, resulting in sp²-hybridization of their carbon atoms. Electrons in the pZ-orbital of each sp²-hybridized carbon atom form π-bonds with neighboring pZ-electrons, which then results in delocalization of these π-electrons. As a result of π-electron delocalization, the energy levels of the orbitals merge and the band gap energy is reduced. Thus the band gap in organic semiconductors reaches values of approximately only 1-3 eV. Electrons can be excited from the HOMO into the LUMO by thermal excitation or by absorption of light with suitable energy. Therefore, organic semiconducting materials are able to absorb light in the UV/visible region of the solar spectrum and transport electric current.⁸ From the onset of the absorption spectrum, one can calculate the optical band gap. An onset at longer wavelengths is equivalent to a band gap decrease. The often used term ionization potential (IP) can be related to the HOMO, electron affinity (EA) refers to the LUMO of a material. Concerning the mechanism of charge transport in organic materials, a direct comparison to the band-like transport in inorganic semiconductors is inaccurate. Since there is no long range order in organic materials, electronic states are localized on individual molecules or segments of molecules and charge carrier mobility is commonly orders of magnitude lower. The implications on charge transport are discussed in more details in Section 1.5. Another important property of organic semiconductors is their high absorption coefficients, which enables efficient light absorption in films with a thickness of only hundreds of nanometers. These intrinsic properties of organic semiconductors dictate the design of OPV cells. In general, organic semiconductors can be classified as donors (p-type) and acceptors (n-type). Further they are divided into polymer and small molecule based materials. Examples for both can be found in Section 1.4.

The fundamental processes of photoinduced charge generation in an organic solar cell, comprise of absorption of a photon, formation of an exciton, exciton diffusion, charge transfer, charge separation, charge transport and charge collection, which are outlined in the following. The efficiency of light absorption in the active layer depends on its absorption spectrum, the extinction coefficient and the active layer thickness. Absorption of light within the semiconductor leads to the excitation of an electron from the HOMO into the LUMO of

the material, resulting in an electron-hole pair or exciton. The electron-hole pair is coulombically bound with the exciton binding energy typically being in the range of 0.1-1 eV.^{9, 10} At room temperature the binding energy is larger than kBT and hence electron and hole cannot dissociate directly.¹¹ To overcome the exciton binding energy, an electrical field is required, so that charges can separate. Therefore, the exciton needs to diffuse to a region, where such a driving force is provided. Exciton diffusion lengths however, are usually limited to approximately 10 nm due to the short lifetime and slow diffusion coefficient of this state.^12-14 Several ways of providing the driving force for charge separation and avoiding recombination of the exciton will be described for each type of OPV cell below. Once electron and hole are separated, the free charges need to be transported to the respective electrodes, holes to the anode, electrons to the cathode. Finally, charges are collected at the electrodes generating a photocurrent. In an organic solar cell, most of the steps described here compete with various recombination processes, resulting in energy losses. Depending on device architecture and materials used, the reason for recombination can be different. Details will be discussed in the following sections for each kind of OPV device. Understanding the cause and minimizing recombination losses is the focus of extensive studies worldwide in order to improve the efficiency of OPV.¹⁵ Researchers commonly use the following methods to characterize organic solar cells.