Single-source pulsed laser deposition of hybrid halide perovskite for solar cells
Tatiana Soto Montero is a PhD student in the department Inorganic Materials Science. Promotors are prof.dr. M. Morales Masis and prof.dr.ing. A.J.H.M. Rijnders from the faculty of Science & Technology.
The world is rapidly shifting towards renewable and sustainable energy as we face concerns about climate change. In such times, the abundant energy from the sun is crucial in aiding this transition. The devices responsible for the conversion of solar energy into electricity are termed solar cells. Nowadays, the well-established photovoltaic (PV) industry belongs to silicon PV. Nevertheless, new materials are being researched to complement silicon PV technologies. Metal halide perovskites (MHPs) are one of the emerging solar cell technologies that have fascinated researchers due to their versatility in terms of both composition and fabrication methods, delivering power conversation efficiencies in pair-to-crystalline silicon cells, making them one of the best candidates for the next generation of photovoltaics. The construction of these emerging solar cell devices involves heterostructures containing an absorber material sandwiched between carrier-selective layers and electrodes.
Despite these advantages, challenges remain regarding upscalable fabrication methods compatible with integrating complex perovskite materials within heterostructures. Therefore, one of the main challenges addressed by the research within this PhD is the demonstration of an alternative physical vapor deposition (PVD) method known as pulsed laser deposition (PLD) for the growth of MHPs. The main motivation to employ PLD for growing MHPs is its unique capability to transfer highly complex chemical compositions from a single-source target to the substrate or a partial solar cell stack.
The first approach consisted of exploring the growth of the archetypical methylammonium lead iodide (CH3NH3PbI3 or MAPbI3), utilizing a PLD coupled with a UV-excimer laser (248 nm). In Chapter 4, we demonstrated the control achieved in the growth of MAPbI3, a material comprising a diverse mix of light and heavy elements with dissimilar volatilities (H, C, N, I, Pb). To address this complexity, we successfully tested different off-stoichiometric PLD targets, incorporating a substantial excess of the organic component to compensate for the dissimilar atomic masses. Moreover, we not only emphasized the importance of a customized target but also investigated the impact of other key deposition parameters, such as deposition pressure, fluence, spot size, and substrate type. This comprehensive exploration aimed to understand their influence on the ultimate thin film microstructure, morphology, and optical properties. For this, we employed amorphous substrates such as glass, quartz, and silicon, along with device-relevant substrates featuring different surface polarities. Finally, the demonstration of thin film conformal growth on micrometer-thick textured silicon substrates highlights the potential of PLD for future thin film integration into silicon/perovskite tandem solar cells.
The second approach involved achieving precise control over the growth of the double organic cation MA1-xFAxPbI3 (x = 0-1) from a single-source PLD target while preserving the integrity of the organic molecules. In Chapter 5, this was successfully achieved by fine-tuning the cation ratio of the PLD target while maintaining optimized values for fluence, deposition pressure, and spot size. Our findings illustrate that near-stoichiometric transfer of the cation ratio can be attained when utilizing MHP targets having an 8-fold excess of the organic components compared to the inorganic constituent. Furthermore, we extended the capabilities of PLD by taking the initial steps toward large-area deposition of MHP. In pursuit of this, we systematically scanned over a larger substrate surface to ensure homogeneous thickness and composition distribution on a 30 x 30 mm2 substrate. Further, we integrated the most promising PLD-deposited MA1-xFAxPbI3 (x = ~ 0.5) thin films in an n-i-p architecture, demonstrating proof-of-concept devices with 14% PCE.
In Chapter 6, we introduce the use of stoichiometric pellets of MA1-xFAxPbI3 (x = 0-1) to investigate the best mechanochemical synthetic conditions later to fabricate PLD targets with high-quality and controlled stoichiometry. Moreover, we subjected these materials in powder or pellet form to examine the actual organic cation ratio using ssNMR and XPS techniques. This not only allowed us to verify the actual cation ratio of the mechanochemically synthesized materials but also enabled the development of a calibration procedure for ex-situ estimating the organic cation ratio of PLD-grown thin films. This analysis lays the groundwork for the optimization and correlation between the cation ratio in the PLD target and the final thin film, depending on the deposition parameters.
The third approach focused on enhancing both the overall quality of the bulk thin film and refining crucial device interfaces to yield high-performance solar cells. In Chapter 7, we delved into strategies for both bulk and interface passivation applied to PLD-grown MA1-xFAxPbI3 (x = ~ 0.5) thin films. To begin with, we transitioned to a p-i-n device architecture, where the p-type contact involved a self-assembly monolayer (2PACz) to minimize parasitic absorption losses. In addition, we make use of the unique PLD property of transferring complex compositions from a single source, incorporating PbCl2 into a target comprising PbI2, MAI, and FAI.
Consistent with prior reports, the incorporation of PbCl2 as a bulk passivator brought forth multiple benefits. Subsequently, we focus on controlling interfaces as a key to improving charge transport, device stability, and overall solar cell performance. To address this, we examined the passivation effects of OAIm on the PLD-grown Cl-passivated MA1-xFAxPbI3 thin film. The resulting devices yield up to 19.7% PCE, attributed to a reduction of non-radiative recombination losses. Further, the increased stability is possibly facilitated by the bulky characteristics of the 2D-layered perovskite. These outcomes underscore the ability of PLD to facilitate the formation of complex MHP compositions while delivering high-performance solar cells.
In this thesis, we have demonstrated the utility of PLD as an alternative PVD method for depositing complex MHP thin films with precise stoichiometry. Notably, this method exhibits compatibility with heterostructures and potential for scalability. This compatibility can be further enhanced by improving the hardware configuration of the PLD for wafer-scale area coatings and superior deposition rates. Additionally, we demonstrate PLD as an appealing deposition method for studying the growth of low- and wide-bandgap MHP. These materials pose challenges with alternative deposition methods due to constraints regarding the solubility of different precursors, varying solvent evaporation rates, or difficulties in reproducibility arising from the need to control four or more sublimation sources with significant differences in volatilities.
