Experimental Physics VI

Overview

The promising photophysical and electronic properties of organic semiconductors such as polymers call for new concepts in their application in photovoltaics. We focus our research on concepts based on the so-called bulk heterojunction approach, where one electron and one hole conducting material are closely intermixed, leading to a distributed interface between these two phases. This concept accounts for the absorption of light leading to strongly bound electron-hole pairs, excitons, in the donor material (e.g., a conjugated polymer). They can travel only short distances (about 10-20nm) by diffusion before they recombine. Alternatively, if an acceptor material (e.g., a fullerene derivative) is within this range, the exciton can dissociate into a polaron pair, being a precursor of free charges. These free charges ultimately lead to the photocurrent.

Here, we a brief overview on three research topics related to optoelectronics devices, bulk heterojunction solar cells, hybrid solar cells, and investigations concerning the dominant recombination mechanism in organic solar cells.

Organic Bulk Heterojunction Solar Cells

An alternative approach in order to reach an efficient charge carrier generation utilises a mixture of electron donating and accepting materials - for instance, a polymer-fullerene blend. This active layer consists of two components forming disordered, mutually penetrating networks of the donor and acceptor phase. In such a bulk heterojunction, the interface is not planar but spatially distributed over the entire volume of the active layer. After the light induced exciton generation, the electron is transferred from the donor (for instance, a conjugated polymer) to the highly electron accepting fullerene. This ultrafast photoinduced charge transfer takes place on a time scale of <100fs and has an efficiency close to unity. Now, the separated charges are not strongly bound any more, and are transported selectively in the respective semiconductor phases: the electrons travel along fullerene pathways, whereas the positive charges move within the polymer network. The efficiency of such a bulk heterojunction solar cell depends substantially on the internal structure (morphology) of the absorber layer. Upon a spin-coating deposition of the blended organic semiconductors from solution, a (partial) phase separation takes place, depending on the choice of materials, solvents, and process parameters.
In order to extract the photogenerated charge carriers via drift selectively to the respective electrodes, an energetically asymmetric structure is essential for an efficient solar cell. The difference of the electron affinities of the two electrodes induces a built-in electrical field, which causes a preferred direction for the charge transfer. As window electrode, a layer of indium tin oxide (ITO) is used, which is a strongly doped transparent conductive oxide (TCO). The selectivity of this electrode is enhanced by depositing a p-conducting transport layer (here the conjugated polymer PEDOT:PSS) on top of the ITO layer (not shown). The backcontact consists of a low workfunction metal (e.g. aluminum), forming an ohmic contact to the acceptor phase.

Hybrid Solar Cells: Organics and Inorganics

In order to combine the advantages of inorganic materials (high carrier mobility, high dielectric constant) with the advantages of organics (high absorption coefficient, solution-based processing), the novel concept of so-called hybrid photovoltaics is investigated in our group. The operating principle is the same as in organic solar cells, with the only difference that instead of organic acceptors, TiO₂ or ZnO are used. The advantage of the metal oxides are the possibility to process them in a sol-gel route which leads to defined porousity or size distributions - a first step to control the nanomorphology properly. For solar cells made of a porous acceptor layers or a blend solution, we use Al-doped ZnO nanocrystals (nc-ZnO:Al). For ordered acceptor matrices, we use TiO₂, which is derived using structure-directing block-copolymers and evaporation induced self assembly techniques. In order to determine the exciton quenching and charge carrier generation, we apply the techniques of photoluminescence and photoinduced absorption (see Optics), as well as the spin-sensitive magnetic resonance spectroscopy. The mobility of our metal oxides is investigated by field effect transistor measurements. The photovoltaic cells are characterised by current voltage measurements in the dark and under AM1.5g irradiation. Additionally, we are able to record the external quantum efficiency, which indicates the number of electrons extracted per incident photon at a given wavelength. Besides the application as acceptor in solar cells, sol-gel derived metal oxides can also act as recombination layer in polymer-based tandem solar cells, presenting another route to high power conversion efficiencies.

Bimolecular Recombination in Polymer:Fullerene Solar Cells

In contrast to inorganic photovoltaics like silicon, no free carriers but bound excitons are generated in organic semiconductors like P3HT. Thus they are often called excitonic semiconductors. In organic solar cells the photo-generated excitons get split at an interface of electron donor (P3HT) and electron acceptor (PCBM). However, the two separated carriers, electron and hole (also called negative and positive polaron), are still bound to each other due to the large Coulomb interaction (low dielectric constant). The bound polaron pair now needs to be separated in a competition of hopping processes away from the interface. If the carriers cannot overcome the Coulomb barrier, they recombine and can no longer participate at an efficient current flow. Usually we distinguish between mono- and bimolecular recombination. The former process takes place in the first step of photogeneration whereas the second is determined by the lifetime of the polarons. It has to be mentioned that of course not all separated polarons reaches their electrode. They can also meet their counter part at another interface and recombine. It is important to understand the loss mechanism of carriers in an organic solar cells due to recombination process. Its understanding will help improving the overall efficiency of the organic photovoltaics.

 

Experiments

We apply different methods to examine efficiency and fundamental properties of our photovoltaic devices

• Solar Cell Characteristics
  • Current-Voltage Characteristics
  • External Quantum Efficiency
• Recombination
  • Photoinduced Charge Extraction by Linearly Increasing Voltage (photo-CELIV)

Simulations

We implemented two different kinds of simulations, complementary in view of the applicability to physical problems:

• Macroscopic Simulations
  • current–voltage characteristics of organic bulk heterojunction solar cells in dependence of illumination intensity and temperature
  • optical simulation of the interference effects in organic thin films
  • open-circuit voltage of organic bilayer solar cells

Contact

Dr. Carsten Deibel, phone +49 931 888 3119

Andreas Baumann, phone +49 931 888 3115