Abstract
The extraction of photogenerated holes from CH3NH3PbI3 is crucial in perovskite solar cells. Understanding the main parameters that influence this process is essential to design materials and devices with improved efficiency. A series of vacuum deposited hole transporting materials (HTMs) of different ionization energies, used in efficient photovoltaic devices, are studied here by means of femtosecond transient absorption spectroscopy. We find that ultrafast charge injection from the perovskite into the different HTMs (<100 fs) competes with carrier thermalization and occurs independently of their ionization energy. Our results prove that injection takes place from hot states in the valence band making this efficient even for HTMs with higher ionization energy than that of the perovskite. Moreover, a new trapping mechanism is observed after the addition of HTMs, which is attributed to interfacial electron traps formed between the CH3NH3PbI3 and the HTMs, in addition to traps in the neat perovskite. Interfacial electron trapping is slower compared to the ultrafast hole injection, which contributes to the high efficiency obtained when these HTMs are employed in solar cells.
Highlights
Since up to now most of the research has focused on SpiroOMeTAD, our study provides useful insights into the aforementioned processes for novel hole transporting materials (HTMs) with different ionization energy (IE) and the additional benefit of being compatible with vacuum-deposition
The slower recombination is observed only for TaTm, while the rest of the bilayers show the opposite trend. This behavior can be explained by increased additional trapping in deeper interfacial states—not directly probed here—in TCTA/MAPI and m-MTDATA/MAPI that causes a reduction in the population of electrons in the conduction band (CB), which results in the faster decay of the vis GSB compared to the neat perovskite
Our results indicate that hole injection to the different HTMs takes place within 100 fs following photo-excitation
Summary
Metal halide perovskites that have the general chemical formula ABX3, where A is an inorganic or organic monovalent cation, B is a divalent metal cation, and X is a halogen atom, are proving themselves as efficient light-harvesters for solar cell applications. Key benefits over other thin-film alternatives are their high absorption coefficient, low exciton binding energy, and long carrier diffusion lengths. In addition, the versatility of the material allows modifications to form perovskites with higher exciton binding energies, tuneable bandgap, and narrow emission bandwidth for light-emitting diodes (LEDs). Concerning perovskite solar cells (PSCs), the exceptionally steep increase in their performance within the last few years situates them amongst the most promising candidates to provide a solution for modern energetic demands, either in the single junction or, as recently shown, in multijunction tandem devices. Standard single junction devices consist of an absorber perovskite, such as CH3NH3PbI3 (MAPI), interposed between an electron-transporting material (ETM) and a hole-transporting material (HTM). In these devices, the favorable processes of charge formation and subsequent electron and hole extraction are in kinetic competition with undesirable processes, such as trapping, that can lead to recombination losses. the main role of the ETM and HTM layers is to facilitate the charge transport to the electrodes, block undesired back injection, and help the long-term stability of the PSCs.. Additional surface states can be formed at the interface with an ETM or HTM, which can act as active interfacial traps if their energy lies in the bandgap. Along this line, an increased crystallinity of the MAPI perovskite can result in a Voc enhancement, highlighting the effect of the trap states.. We use femtosecond transient absorption (TA) spectroscopy to determine the injection dynamics between MAPI and organic HTMs with different IEs, as well as the interfacial trapping processes This analysis helps to elucidate the main mechanism of hole injection, while highlighting the importance of the interfacial traps. An additional mechanism due to interfacial trapping is revealed upon the addition of the HTMs in contact with the perovskite
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