Analysis of the different kinetic processes in perovskite solar cells

Resumen

In order to satisfy the increasing energy demands around the world, it is necessary to look into the different energy resources that are available. Photovoltaic technologies are one of the most promising renewable energy sources to produce electricity. They can directly transform the sunlight energy into electrical energy without the use of any intermediates. Silicon is still the most used material to fabricate solar panels to produce electricity. Although the production costs are getting lower and lower, silicon still presents some disadvantages, for example, they require lots of energy in their production or they cannot be implemented into flexible substrates, which made researchers look for alternatives to silicon. From the emerging photovoltaic technologies, lead halide perovskites have received the focus of most research groups since their implementation as light harvesters in 2009. Huge efforts were directed into developing fabrication methods and techniques to get higher power conversion efficiencies. These efforts have been very fruitful, as efficiencies have increased from 3.8% to a very recent 25.2% in just ten years. However, there is still room for debate about the working mechanisms and the physics that govern perovskite solar cells. The principal aim of this thesis is to shed light on the charge carrier processes that take place at different timescales, from the charge carrier generation to the charge carrier recombination. For this purpose, it is necessary first to develop efficient solar cells based on lead halide perovskites. Once this is accomplished, a complete physical-chemical characterization of the material should be carried out. After having characterized both films and devices, we move on the study using advanced characterization techniques, such as transient photovoltage (TPV), transient photocurrent (TPC), and charge extraction (CE) for complete devices; and femtosecond transient absorption spectroscopy (fsTA). We use these techniques to study charge carrier processes in the bulk of the perovskite material and the different perovskite/selective contacts interfaces. In this chapter, we will briefly describe the objectives and contents of every chapter. Chapter 1 includes an introduction to perovskite solar cells, and a description of the state-of-the-art of the fabrication of perovskite solar cells. A timeline describing the different charge carrier processes, starting from the femtosecond timescale with the description of charge carrier generation to the second timescale, with the description of the influence of ionic migration on perovskites is given. In Chapter 3, we give a detailed description of the experimental procedures for the fabrication of perovskite. All the characterization techniques used in this thesis are described focusing on the advanced characterization techniques (TPV, TPC, CE, and fsTA). In Chapter 4, we focus on the charge carrier processes happening at the interface perovskite/HTM. With this purpose, we used a series of semiconductor polymers, namely PTB7, P3HT, and PCPDTBT, widely used in organic solar cells, and compared them as the standard HTM, spiro-OMeTAD. In the first part, we fabricate and characterize perovskite solar cells with different HTM. Using transient optoelectronic techniques, we study which processes govern the final open-circuit voltage. In the second part, in collaboration with Prof. Dirk M. Guldi and Dr. Bianka M.D. Puscher from the Friedrich-Alexander Universität at Erlangen (Germany), as part of my research stay, we study the charge transfer processes at the interface perovskite/HTM using fsTA spectroscopy. Chapter 5 reflects the results obtained from the study of the other interface, perovskite/ETM. Here, we modified the interface TiO2/perovskite with the deposition of a thin layer of C60 fullerene. Again, first, complete devices were prepared and characterized. Using fullerenes was early showed as a strategy to reduce the current-voltage hysteresis observed in perovskite solar cells. In this chapter, again in collaboration with Prof. Guldi and Dr. Puscher, we studied how fullerene derivatives have an influence over charge carrier processes at different timescales. In the next chapter, Chapter 6, we focus on charge carrier processes in the bulk of the perovskite. We monitor the influence of the aging of perovskite solar cells in dry air conditions (< 10% H2O) in the photovoltaic parameters and in the charge carrier recombination kinetics. With this purpose, we used transient optoelectronic techniques (TPV, TPC, and CE) and a modification of TPV, which is called transient of the transient photovoltage (TRoTR). Finally, Chapter 7 summarizes the main conclusions obtained in the previous chapters, which are: In Chapter 4, we fabricated efficient solar cells with different low band gap semiconductor polymers that act as HTM and compare them with spiro-OMeTAD. The VOC of such devices is not correlated with the HOMO levels of the HTM and that was corroborated with DiffCap measurements. With fsTA measurements, we found that the injection of hot carriers into semiconductor polymers with a LUMO level close to the conduction band minimum of the perovskite suppose a carrier loss pathway that may be detrimental to the final device performance. This conclusion makes us suggest that a high LUMO level, certifying the electron blocking layer properties of the HTM, is desirable for the future design and synthesis of new materials for HTM. From Chapter 5, we determined that the modification of the interface MAPbI3/TiO2 with a thin layer of C60 effectively reduces the current-voltage hysteresis in these devices. This is caused by defect passivation at the grain boundaries that reduces the ionic motion in the device. Additionally, using fsTA we have confirmed that the use of C60 avoids charge carrier accumulation at the interface, and that also is capable of hot carrier extraction. Finally, in Chapter 6, we monitored the reduction of current-voltage hysteresis of perovskite solar cells with the aging of the devices under dry air conditions (< 10% H2O). Using transient optoelectronic techniques, we have assigned this behavior to a reduction of the ionic influences over carrier recombination over time, which supposed an increased in both VOC and fill factor.