Organic Solar Cells

This chapter provides an introduction to the basic principles of solar energy conversion including its thermodynamic limits. We discuss the optical and electrical requirements for an ideal photovoltaic device and show examples of possible realizations based on semiconductors. To recall the basics, a brief review on semiconductor physics with emphasis on the p-n junction is given. We discuss the role of the electrochemical potential as driving force for the conversion of sunlight into electricity. We conclude with estimations on the maximum power-conversion efficiency for a single band-edge absorber and introduce approaches for achieving it or even going beyond it. Readers without any background in solid state physics might consider consulting an introductory textbook while reading this chapter. If the reader comes to the conclusion that his/her background in physics is not sufficient, he might consider to skip this chapter and directly start with Chap. 3, as a detailed understanding of thermodynamics is not required to follow most of the elaborations on the working principle of organic solar cells in subsequent chapters. The reader interested in the fundamental laws of solar energy conversion should follow this chapter and be able to answer the following questions afterwards: (a) What is the thermodynamic limit of solar-thermal energy conversion with a device located on the earth surface? What is the role of entropy? (b) Why is the power-conversion efficiency of a solar cell based on a single semiconductor limited to 33 %? What tradeoffs have to be made? (c) Where is the “maximum” of the solar spectrum located? What are possibilities of expressing spectra (e.g. from the sun) considering energy versus wavelength or photon fluxes versus intensity fluxes (irradiance)? (d) What are the main optical and electrical properties of semiconductors and how can they be derived? (e) What are the relations between Fermi levels and charge carrier densities? (f) What are the driving forces for the movement of charge carriers? What is the concept of quasi-Fermi levels? (g) What is the effect of recombination on the photovoltage of a solar cell? Which loss processes are unavoidable? (h) How does a p-n junction solar cell work? Are there alternative architectures? (i) What are the basic requirements for a solar cell? Consider the role of selective contacts and the built-in electric field. (j) Why should a good solar cell show a high electroluminescence quantum yield, i.e. large emission? (k) What are the main concepts for overcoming the so-called Shockley-Queisser limit?

This chapter starts with a description of the characteristic electronic structure and charge transport properties of organic semiconductors. These introductory parts are followed by detailed elaborations on the working principles of organic solar cells based on the considerations of Chap. 2. The discussions focus on processes which limit the quantum efficiency and the maximum extractable energy. Beginning with a description of the general steps involved in energy conversion, the chapter shifts focus towards real structures and materials. It concludes with more experimental aspects concerning fabrication and characterization. Whereas the first sections review very basic chemistry knowledge, the solar-cell section gives a detailed evaluation of currently used models and ideas to explain the current-voltage characteristics of organic solar cells. Some basic questions addressed are: (a) What are the major prerequisites for organic materials to show semiconducting properties? (b) What happens when molecules form a solid? What are the main differences between organic and crystalline inorganic semiconductors? (c) What is the role of the reorganization energy considering optical and electrical properties of organic semiconductors? (d) What approaches exist to describe the charge-carrier mobility in organic solids? (e) What are the steps of energy conversion in an excitonic solar cell? What is the role of the bulk heterojunction? (f) How can different recombination mechanisms be distinguished experimentally? (g) What is (the role of) the charge-transfer state? (h) Can the open-circuit voltage be related to other optical and electrical quantities accessible by experiment? (i) Does a stricter limit exist for the maximum power-conversion efficiency of organic solar cells compared to their inorganic counterparts? (j) Why does a single-layer organic solar cell show diode behavior? (k) What determines the fill factor? (l) What is the role of metal electrodes? What kind of energetic situations are possible when a metal-organic contact is formed? (m) What are common solar-cell stacks? How can they be fabricated? (n) What are the remaining challenges for making organic solar cells competitive?

After having discussed general principles of solar energy conversion and the elementary processes in organic solar cells, we focus on modeling and simulation in this chapter. The first part deals with drift-diffusion simulation in general including the Einstein relation. In the second part, specific models for physical processes are discussed, which range from mobility and recombination models to the description of CT states, traps, interface barriers, and a Gaussian-shaped density of states. In a third part, an optical thin-film model based on the transfer-matrix approach is described. The final part contains discussion on exemplary devices visualized by simulation results. Readers, who are not interested in the details of drift-diffusion simulation, can simply skip the technical parts. However, they are encouraged to follow the descriptions of the ideas and models, as almost all analytical approaches to understand the current-voltage relation of organic solar cells are based on special cases of these equations. In particular, this chapter should help the reader to answer the following questions: (a) Which approaches exist to model organic solar cells? (b) What are the main assumptions for a drift-diffusion model? Where are difficulties in applying it to organic solar cells? (c) What are the basic equations and input parameters? (d) Charge carrier mobility and recombination in organic semiconductors: How can they be described? Are they interrelated? (e) What is the role of the contacts—mathematically and physically? (f) Why is the Lambert-Beer law incapable of describing absorption in organic solar cells? What is coherence? (g) What is the basic idea of the transfer-matrix model? (h) What are the different working regimes of a single-carrier device?

This chapter contains a detailed simulation study on a single-layer bulk heterojunction sandwiched between two metal contacts. We apply the simulation approach discussed in the previous chapter and compare the results to the predictions of approximated analytical solutions introduced in Chap. 3. In particular we investigate the interplay between mobilities and different recombination mechanisms and their effect on the fill factor and the open-circuit voltage. In the calculations we combine various properties of the bulk heterojunction with characteristics of the contacts. We point out the role of injection barriers, built-in potential, bending of energy levels, and contact selectivity. The quest for an optimum mobility will be discussed as well. In the final part of the chapter we analyze the photocurrent (dark current subtracted from current under illumination) as a function of voltage to identify processes that limit the photocurrent. In particular, we want to clarify the following points: (a) Is there an optimum mobility for charge carriers in the active layers of an organic solar cell? (b) What order of magnitude for the mobility is required for a high-performance solar cell? (c) Does the mobility influence the open-circuit voltage? (d) What does selectivity of the contacts mean and why is it important? (e) What is the role of injection barriers at the electrodes? (f) Which processes reduce the open-circuit voltage and which do not? (g) What governs the temperature dependence of the open-circuit voltage? (h) What can be learned from the intensity dependence of the photocurrent-voltage relation? (i) What is the meaning of a point of intersection between the J-V curves in dark and under illumination? (j) What does the fill factor as a function of device thickness tell?

This chapter contains a detailed study on the effects of energy barriers at the electrode. To vary these barriers in a controlled and systematic way, the interface between a hole-transport layer and the donor is selected. This interface serves as a model system for a contact between active layer and electrode in general. Different values for the HOMO levels of both organic materials give rise to barriers for injection or extraction of charge carriers. We discuss through experiment and simulation how these barriers influence the current-voltage characteristics, in particular the open-circuit voltage. One main observation are S-shaped current-voltage curves, which severely limit the fill factor. Such “S-kinks” are frequently observed when characterizing organic solar cells. That is why we develop methods to identify the reasons for S-kinks. Amongst those methods are measurements as a function of temperature or light intensity. Another method is based on transient photocurrent measurements, which reveal redistributions of the electric field or a pile-up of charge carriers. The following questions are of particular relevance: (a) What are the definitions of injection and extraction barriers for charges? What causes such barriers? (b) Why, and under what conditions does an injection barrier change the open-circuit voltage? (c) Why are injection barriers detrimental for efficient extraction of charge carriers? (d) What experimental approaches exist to distinguish S-kinks introduced by extraction barriers from those caused by injection barriers? (e) In which case does the illumination intensity influence the S-kink and why? (f) What can transient photocurrent measurements close to open-circuit voltage tell about the driving forces for charge carrier extraction?

This chapter is dedicated to S-kinks in the J-V curve of organic solar cells. They are very interesting because they represent a pronounced feature in the J-V curve, which is related to a change in the working regime of the device. Often, they are manifest with decreased solar-cell performance. Consequently, it is desired to remove them based on the knowledge of their origin. In the previous chapter, barriers at electrodes have been discussed as possible reasons for S-kinks. In this chapter further sources of S-kinks are presented. We describe how a strong imbalance in mobilities (hole mobility in donor, electron mobility in acceptor) in flat heterojunction devices or a field-dependent dissociation of geminate pairs at the donor/acceptor interface causes S-kinks. In the final parts of this chapter “unconventional” kinks with turning points at voltages higher or lower than the open-circuit voltage are discussed. The following questions cover the content of this chapter: (a) Under what conditions can imbalanced mobilities cause S-shaped J-V curves? (b) What is the mechanism in which imbalanced mobilities cause S-shaped J-V curves? (c) How can this effect be identified and distinguished from others? (d) How can kinks in the forward current be explained? (e) What might be the reason for “unconventional” distortions of the J-V curve?

The topic of this chapter will be a detailed investigation of current-voltage data of the model system ZnPc:C\(_{60}\). We start by explaining the influence of the donor:acceptor mixing ratio on the open-circuit voltage. Then, we investigate a possible optimization of the morphology in the bulk heterojunction by using a vertical concentration gradient of donor and acceptor. The main focus of the further parts of this chapter is on the role of recombination and charge transport governing the power-conversion efficiency. By manipulating spatial absorption profiles in the bulk heterojunction we conclude that charge extraction and recombination between charges are the competing processes limiting the fill factor. An investigation of the open-circuit voltage as a function of illumination intensity allows us to discriminate between direct, indirect, and surface recombination. In particular, this chapter addresses the following questions: (a) What are possible explanations for different open-circuit voltages of devices consisting of the same active material system (e.g. when changing the mixing ratio)? (b) What are simple experiments to identify the correct reason? (c) Which explanations exist for a changed donor-acceptor gap in a bulk heterojunction based on the same materials? (d) Why is it interesting to investigate graded junctions? (e) What are possible explanations for the fill factor depending on the color of the illumination? What is the role of charge-carrier mobility? (f) Why does the internal quantum efficiency depend on the wavelength of the incident light and on the thickness of the optical spacer? What is the consequence for the spectral mismatch factor? (g) What point of the J-V curve is a good choice to investigate the dominating recombination mechanisms in a solar cell? How can this be done? (h) What does the diode ideality factor tell?

This chapter summarizes the main effects and parameters influencing the performance of organic solar cells in very brief statements. On the one hand, this is done by comparing J-V data of a fictitious device X with its reference device. On the other hand, the influence of changes in specific properties of the device is visualized by a series of calculated J-V curves. These two approaches contain a high risk for an oversimplification. For this reason, the given references pointing back to the main chapters of this book should be consulted for a detailed understanding. However, used in the right way, this chapter offers a very compressed set of guidelines on how to interpret J-V curves of organic solar cells from a drift-diffusion point of view. Simple experimental methods are suggested to distinguish several reasons for detrimental device properties such as a low \(V_{\mathrm{{oc}}}\), low FFs from S-shapes, and low photocurrents. The chapter closes with concluding thoughts about the applicability of drift-diffusion simulations to solar cells based on disordered organic thin films.

In the preface of this book I brought up the issue of sustainability. In the long run this will determine the fate of organic photovoltaics. Sustainability is a complicated function of physical and technological factors like device efficiency and lifetime. For photovoltaics as a technology, which is supposed to be scaled up to the terawatt, i.e. square kilometer range, sustainability contains questions on the abundance of the required materials, energy input, and the feasibility of closed life cycles. Those define a technological potential of a given system. Finally, social and economic constraints are of significance as well. This chapter tries to touch upon those questions. Some of them might be formulated in the following way: (a) Why will organic solar cells with a low power-conversion efficiency not be successful, even if they are very cheap? (b) What are conventional approaches to increase the power-conversion efficiency? (c) What are the main challenges in material design to reach a new working and efficiency regime of organic solar cells? (d) Nominal power-conversion efficiency does not tell the complete story. What is the main figure of merit? Why and under which conditions can organic solar cells benefit from it? (e) What are major technological challenges? (f) What are estimated values for energy payback times, material and manufacturing costs, and lifetimes?

The discussed simulation results were obtained using a self-made drift-diffusion solver.