Unravelling the mechanism of photoinduced charge transfer processes in lead iodide perovskite solar cells

Lead halide perovskites have recently been used as light absorbers in hybrid organic–inorganic solid-state solar cells, with efficiencies as high as 15% and open-circuit voltages of 1 V. However, a detailed explanation of the mechanisms of operation within this photovoltaic system is still lacking. Here, we investigate the photoinduced charge transfer processes at the surface of the perovskite using time-resolved techniques. Transient laser spectroscopy and microwave photoconductivity measurements were applied to TiO2 and Al2O3 mesoporous films impregnated with CH3NH3PbI3 perovskite and the organic hole-transporting material spiro-OMeTAD. We show that primary charge separation occurs at both junctions, with TiO2 and the hole-transporting material, simultaneously, with ultrafast electron and hole injection taking place from the photoexcited perovskite over similar timescales. Charge recombination is shown to be significantly slower on TiO2 than on Al2O3 films.

A Time-Resolved Photophysical Study of Hybrid Organic-Inorganic Perovskite Photovoltaic Materials

Arun Aby Paraecattil

The search for new photovoltaic materials has been driven by the combined need to exploit sources of energy that are clean and sustainable, while simultaneously doing it in a cost effective manner. In this light, hybrid organic-inorganic perovskites have recently emerged as an extremely promising photonic material, and their application as a functional photovoltaic layer has resulted in device efficiencies that rival long established silicon based technologies. Rapid progress in device efficiencies have occurred over the last years (22.1% being the current record). However, the simultaneous growth in basic studies of the material have not resulted in a conclusive understanding of the fundamental process that occur subsequent to photoexcitation. Hence, there is a pressing need to identify the pathways that currently limit device performance and provide direction for future work in materials and device engineering. Towards this goal, we investigate two perovskite compositions (CH3NH3PbI3 and (FAPbI3)0.85(MAPbBr3)0.15) using time-resolved (THz and electroabsorption) spectroscopic techniques. Chapter 1 and 2 provide a general introduction into the investigated system and the experimental techniques that have been used. In chapter 3, we detail time-resolved THz measurements and report on experimental evidence for carrier recombination through an indirect transition, as well as a direct recombination pathway that is present at higher carrier densities. We calculate temperature dependent carrier mobilities (at THz frequencies) and bimolecular recombination constants. Through which we identify phonon scattering as the primary limiting mechanism for carrier transport, and temperature dependent bimolecular recombination that is mediated by the relative mobility of the charge carriers. Analysis of the complex photoconductivity spectra using the Drude-Smith model revealed a large difference in carrier scattering between the two perovskite films that could be attributed to the significantly different morphologies. In chapter 4 we apply time-resolved electroabsorption spectroscopy (TREAS) to insulated CH3NH3PbI3 layers and investigate the macroscopic carrier transport dynamics under applied electric fields. Transport within the 40nm perovskite grain was discovered to be diminished by a factor ¿ 2 relative to the high frequency mobility obtained through THz spectroscopy. The averaged carrier mobility across the 280nm film length was reduced by a factor ¿ 4, due to the presence of grain boundaries and defects. Preliminary investigations also identified spectral signatures associated with carrier accumulation at the perovskite interface and delayed extraction at the contacts. Chapter 5 deals with complete solar cells formed using (FAPbI3)0.85(MAPbBr3)0.15 as the active layer and we report the first application of the TREAS technique to complete perovskite devices. Our results reveal that the improved morphology of the film results in film averaged mobilities that are near the intrinsic values obtained using THz spectroscopy. Analysis of transient absorption spectra revealed an electroabsorption signature, its dynamics can be correlated with the disassociation of a transient excitonic species to form free charge carriers.

EPFL2017

Composition and Interface Engineering of Organic-Inorganic Hybrid Perovskites to Improve Photovoltaic Performance and Stability

Kyung Taek Cho

Faced with the growing demands for energy in modern society, organic-inorganic metal halide perovskite materials have recently fascinated the photovoltaics (PV) research community due to a combination of their high quality optoelectronic properties and their ease in the fabrication proceed. As a result, solar cells employing the perovskite materials have been researched exponentially on their way and now the power conversion efficiency (PCE) of perovskite solar cells have been improved over 23% since the first report showing the PCE of 3% in 2009. In this thesis, I investigate compositional modification of perovskite materials and optimization of the charge transporting materials to produce high efficiency, stable and reproducible perovskite solar cells. First of all, after the reasonable performance was achieved, we have innovated a new approach of interface engineering in a perovskite layer to boost efficiency of devices. Engineering a compositional gradient with formamidinium bromide (FABr) at the rear interface between a pristine mixed perovskite ((FAPbI3)0.85(MAPbBr3)0.15) film and a hole transporting material (spiro-OMeTAD) demonstrated that charge collection is improved and charge recombination is reduced at the interface, which leads to a striking enhancement in open-circuit voltage (VOC). This result shed light on the importance of the passivation engineering on the rear surface of perovskite layers. However, beyond the improvement of efficiency, a long-term stability under moisture and continuous illumination is still remained as another challenge for market deployment of the perovskite solar cells. To develop the stability, we have developed the engineering by the surface growth of a two-dimensional (2D) perovskite, the crystal structure of A2BX4, on top of a bulk three-dimensional (3D) perovskite ABX3 film. It is well-known that the 2D perovskite has the superior stability, but suffered because of their low efficiency in the application of photovoltaics. The formation of a distinct 2D perovskite on top of the 3D perovskite (Cs0.1FA0.74MA0.13PbI2.48Br0.39) layer was proved by investigation of structural and optical properties of the stack. This embodying two different type of perovskite layer in one film had never been shown. Finally, this innovative approach led to the PCE of 21% and enhanced stability sustaining 85% of the initial value after 800 hours under full illumination. Thus, my approach of coating 2D perovskite layer make the perovskite solar cells more effective and stable for commercialization. Besides, to avoid the toxic chemical the lead free perovskite (Cs2AgBiBr6) was explored as photovoltaics and the further improvement can be expected. The last results presented in this thesis are related to optimization of the electron transporting layer (ETL) and hole-transporting materials (HTM) for efficient perovskite solar cells. It is shown that the ETL and the HTMs are playing a significant role in realizing efficient and stable perovskite solar cells.

EPFL2018

Power output stabilizing feature in perovskite solar cells at operating condition: Selective contact-dependent charge recombination dynamics

Seckin Akin, Ulf Anders Hagfeldt, Hui-Seon Kim, Jiyoun Seo, Shaik Mohammed Zakeeruddin

Stabilized power output at maximum power point (mpp) has been considered as one of the most reliable parameters as it provides a key performance indicator for perovskite solar cells (PSCs) revealing the operational stability of the photovoltaic device. Here, we show the effect of selective contact on the power output change under mpp tracking, which closely correlates with the charge recombination dynamics with a time scale of minutes. The normal n-i-p cell architecture comprising cp-TiO2/mp-TiO2/perovskite/spiro-MeOTAD (doped by either Li-TFSI or Zn-TFSI2) and the inverted p-i-n structure, NiOx/perovskite/PCBM, are examined to investigate the specific effect of the nature of the interface on operational stability. The normal structure with Li-TFSI shows a gradual performance decrease at mpp owing to the enhanced recombination at the interface between the perovskite and the spiro-MeOTAD, becoming the dominant recombination process, although the bulk-related recombination is suppressed. On the other hand, the inverted structure demonstrates an improved photocurrent at mpp due to the effectively suppressed recombination both in bulk and at the interface. Remarkably, the deteriorating performance of the normal structure with Li-TFSI at mpp is successfully avoided by replacing Li-TFSI with Zn-TFSI2, leading even to an increased power output with stable performance at mpp.