Device physics of donor/acceptor-blend solar cells

Harvesting energy directly from the Sun is a very attractive, but not an easy way of providing mankind with energy. Efficient, cheap, lightweight, flexible, and environmentally friendly solar panels are very desirable. Conjugated polymers bear the potential of fulfilling these requisites. Due to their unique chemical makeup, these polymers can be used as optoelectronically active materials, e.g., they can be optically excited and can transport charge carriers. As compared to inorganic materials, polymers have (at least) one serious drawback: upon light absorption excitons are formed, rather than free charge carriers. A second material is needed to break up these excitons. A much used way of achieving this is to mix the polymer with a material that readily accepts the electrons, leaving the holes in the polymer phase. As excitons in the polymer phase only move around for a couple of nanometers before they decay to the ground state, it is vital to induce a morphology that is characterized by intimate mixing of both materials (a so-called bulk heterojunction or BHJ). A typical BHJ solar cell consists of a glass substrate coated with a transparent electrode, the active layer, and a metallic top electrode. The active layer is formed by spin casting a co-solution of the polymer and the electron accepting material. The voltage for which the current in the external circuit is zero is called the open-circuit voltage Voc. The current density that flows out of the solar cell at zero bias is named the short-circuit current density Jsc. These two important quantities are described in the following. Although significant progress has been made, the efficiency of current BHJ solar cells still does not warrant commercialization. Targeted improvement is hindered by limited understanding of the factors that determine the performance. The main theme of this thesis is to introduce a simple model for the electrical characteristics of BHJ solar cells relating their performance to basic physics and material properties such as charge carrier mobilities. The metal-insulator-metal (MIM) model, as introduced in this work, describes the generation and transport processes in the BHJ as if occurring in one virtual semiconductor. Drift and diffusion of charge carriers, the effect of charge density on the electric field, bimolecular recombination, and a temperature- and field-dependent generation mechanism of free charges are incorporated. By using (values close to) measured charge carrier mobilities, the experimental current-voltage characteristics are regained by the MIM model, showing the soundness of this approach. Although bimolecular recombination in organic semiconductors can be adequately described by Langevin’s equation, meaning that the recombination strength depends on the sum of the charge carrier mobilities, BHJs behave differently. As is known from direct measurements, the bimolecular recombination strength in BHJs is significantly smaller than predicted by the Langevin equation. From the modeling of current-voltage characteristics, it is found that the bimolecular recombination strength is indeed significantly reduced, and is governed by the mobility of the slowest charge carrier and not by the sum of the mobilities. The MIM model sheds new light on two key parameters of BHJ solar cells: the open-circuit voltage and the short-circuit current. By studying the dependence of Voc on incident light intensity, it is established that BHJs behave differently than inorganic p-n junctions. Within the framework of the MIM model, an alternative explanation for the open-circuit voltage is presented. Based on the notion that the quasi-Fermi potentials are constant throughout the device, a formula for Voc is derived that consistently describes the open-circuit voltage. In short, if suitable electrodes are applied to the active layer, Voc is determined by the energy levels of both materials. The energy needed to dissociate excitons represents an important loss in Voc. Simple analytical expressions for the current that can be drawn from a photoconductor indicate that the short-circuit current density should be equal to qGL, where q is the elementary charge, G the generation rate of free electrons and holes, and L is the thickness of the active layer. In this case, Jsc is proportional (through G) to the intensity I of light incident on the device. This linear dependence has been observed in many systems. A small deviation from linearity, in which case Jsc µ Iα with 0.85 ≤ α ≤ 1, was also reported for various systems. This sublinear behavior was ascribed to bimolecular recombination. In the 1970’s Goodman and Rose pointed out that the photocurrent can become limited by space charge, provided that the active layer be thick enough and there exists a difference between electron and hole mobilities. Under these premises, the photocurrent is expected to be proportional to I0.75. This suggests that the exponent α is a function of the charge carrier mobilities and that the sublinear behavior is caused by space-chargebuildup. Numerical modeling and measurements on a suitable BHJ system confirm that the intensity dependence of Jsc is indeed governed by space charge rather than by bimolecular recombination per se. Hybrid organic/inorganic solar cells, are an auspicious alternative to polymer/ fullerene devices. In this case, an inorganic semiconductor, either titanium dioxide or zinc oxide, is used as the electron acceptor. One way of making these cells is the precursor route: A precursor for the inorganic semiconductor is mixed with the solution of the polymer. Upon spin casting of the active layer in ambient conditions, the precursor reacts with moisture from the air and the inorganic semiconductor is formed. Although promising, this method seems to harm the transport of holes through the polymer phase in the active layer. Alternatively, the inorganic semiconductor, in this case zinc oxide, can be formed ex situ in the form of nanoparticles. This enables one to control the reaction conditions and purity of the material better. It is demonstrated that the hole transport through the thus-formed blends is not affected by the presence of the zinc oxide nanoparticles. The electron mobility in blends with the often used conjugated polymer MDMO-PPV is quite decent and, consequently, the hole transport through the polymer phase is identified as the limiting factor in these devices. A much pursued way to increase the performance is to increase the amount of photons absorbed by the film by decreasing the band gap of the polymer. Calculations based on the model presented in this work confirm that this would indeed enhance the performance. However, it is demonstrated that the effect of minimizing the energy loss in the electron transfer from the polymer to the acceptor phase is even more beneficial. By combining these two effects—under the premise of just sufficient driving force for exciton dissociation—it turns out, that the optimal band gap of the polymer is 1.9 eV. This is significantly higher than what is predicted for p-n junction solar cells (1.4 eV). With balanced charge transport, polymer/fullerene solar cells can reach power conversion efficiencies of at least 10.8%.