Organic solar cells

Riccardo Volpi and Mathieu Linares

DOI: 10.1039/9781782626862-00001

1 Introduction

As climate change and energy sufficiency are progressively becoming more pressing issues, environmentally friendly and cheap energy sources need to be developed. Solar energy production, with inorganic solar cells, has progressed greatly in recent years and is already able to compete with traditional energy sources. Photovoltaic solar cells convert photons into electric current. Although traditional photovoltaic technology based on inorganic materials has become commercially successful, it faces some limitations that can be overcome by organic solar cells. Organic solar cells are low-cost and easy to process, furthermore they possess innovative properties being potentially lightweight, flexible, and transparent. However, organic solar cells still have lower efficiencies and shorter lifetimes than traditional inorganic solar cells. While the best organic solar cells have reached around 11% efficiency, the best single junction crystalline silicon solar cells and thin film CdTe cells have efficiencies of around 25% and 22%, respectively. Furthermore, the lifetime of organic solar cells is still short in comparison to the lifetimes of inorganic solar cells, so stability challenges must be addressed for organic solar cells to compete with conventional photovoltaics on the market. The efficiency of an organic solar cell is determined by the efficiency of the different steps from photon absorption to charge collection, as shown in Fig. 1. Photons are absorbed in either the acceptor or the donor phase with efficiency ?abs, generating excitons (1). These excitons then either diffuse toward the interface and form a Charge Transfer (CT) state (2) or decay (6). The CT state is defined as an electron-hole pair Coulombically bound, composed usually by an electron on an acceptor molecule and a hole on a donor molecule. In an organic solar cell CT states (3) are formed at the interface from exciton dissociation. The efficiency of the excitons forming CT states is ?ex, and it is high when the distance to the interface is small. The CT states then split (4) into free electrons and holes with efficiency ?diss. After the charge carriers are freed, they may still move back to the interface and recombine in a CT state (8), or even to the ground state (7) and (9). ?trans is the efficiency of the free charge carriers being collected at the electrodes (5). The product of these different efficiencies gives the external quantum efficiency (EQE), the ratio of charge carriers collected to the number of incoming photons.

EQE(E) = ?abs?ex?diss?trans (1)

The efficiency of organic solar cells is greatly impacted by the processes that occur at the interface between the donor and acceptor. The electron and hole have a strong Coulombic attraction, which must be overcome for the charges to separate. Organic solar cells have much lower dielectric constants and more localized electronic states than inorganic solar cells, so excited states are localized and the Coulombic barrier to overcome for the charges to dissociate is large.

The amorphous nature of these materials make analytical studies difficult and theoretical investigations of mobility and charge transport are to a large extent based on simulations: drift diffusion and Kinetic Monte-Carlo simulations (KMC). Drift-diffusion models employ a classical picture, they are very suitable to simulate the whole solar cell and give some macroscopic information. Kinetic Monte-Carlo simulations have potentially the capability to model the quantum phenomena happening at the nanoscale, but the more details are included in the KMC scheme the more computational effort is required. Detailed KMC schemes are thus mainly limited in studying some interesting portions of a solar cell. Several studies employing both approaches showed the significant contribution of morphology to the efficiency of solar cells. Different structures for organic solar cells have been researched, the simplest consisting of a single layer of an organic semiconductor between two electrodes. However, solar cells with this structure have low efficiency, and the performance of the device is improved by a bilayer structure of two organic materials: a donor and an acceptor materials. With this design, a trade-off must be made between light absorption and exciton dissociation. The thicker the layers are, the more light will be absorbed, increasing the efficiency of the solar cell. However, the excitons formed in the single material, will have in average a greater distance to the donor-acceptor interface, where they can dissociate in a CT state. If the domain size is too large, the exciton will decay before reaching the interface, thereby lowering the efficiency of the solar cell. A solution to this problem is to combine the donor and acceptor phases so that the distance to the interface is short even with thick films. This can be best achieved with interdigitated structures, providing good exciton dissociation and at the same time a clear path to the electrodes for the free charge carriers. The interdigitated interface is very promising but also very difficult to obtain in practice. Another type of interface commonly used in the lab employs a blend of donor and acceptor materials forming a three-dimensional interpenetrated structure called bulk heterojunction (BHJ) solar cell.

In the present chapter we will focus on the KMC approach used to model charge transport in organic materials. The properties of organic materials are different from those of crystals; their intrinsic disorder tends to localize the charge carriers on one or few molecules. The conduction in these materials is therefore temperature activated, in contrast to the band conduction of crystalline materials. Two methods are predominantly used in literature to study charge transport using KMC simulations: the Miller-Abrahams and the Marcus hopping rates. The Miller-Abrahams formula is one of the simplest ways to couple temperature to the hopping rate. This is achieved by means of a Boltzmann factor helping to overcome the energy barrier for the transport. The Marcus hopping rate can be derived from the Fermi golden rule and takes into account also the reorganization energy after each hop.

In this book chapter, we will show how to model the steps (3), (4) and (5) of Fig. 1, namely, how the CT state split and the charges are subsequently transported to the electrodes. We will thus not consider the transport of excitons or the coupling between the excited...