In a partially shaded photovoltaic module, the shaded cells experience reverse bias conditions that may lead to significant degradation, especially in perovskite solar cells. A thorough understanding of the behavior of perovskite solar cells under reverse bias is therefore required for designing modules that are suitable for outdoor operation.
This thesis combines literature data with experimental results obtained from spin-coated, planar inverted perovskite solar cells.
In the first part of the thesis, measurement parameters and a definition of the breakdown voltage are proposed. The resulting breakdown voltages can be used as scaling factor to correct cell-to-cell variation in the reverse bias behavior. In the second part of this thesis, the effects of illumination on the reverse bias behavior are explained through a mechanism based on ion migration and an electrochemical reaction. Additionally, an imaging technique (ReBEL) is introduced and employed to reveal that two distinct breakdown mechanisms occur, depending on the current injection level.
These findings indicate that further research is required to gain a complete understanding of the reverse bias behavior of perovskite solar cells. However, by standardizing measurement procedures, investigating the breakdown mechanisms, and developing novel characterization techniques, meaningful progress towards stable perovskite modules can be achieved. ... Read more

Perovskite Light-Emitting Diodes (PeLEDs) represent a highly promising LED technology due to their exceptional color purity, tunable emission color, and low manufacturing cost. However, the current record external quantum efficiency (EQE) for PeLEDs is limited to 32\%, still below the 40\% achieved by organic LEDs (OLEDs). This limitation primarily stems from defects within the perovskite layer and at interfaces between different layers in the device. Effective passivation of these defects is essential for further advancing PeLED efficiency.

In this thesis, we established the first operational PeLEDs within the PVMD group, leveraging its perovskite solar cell production line. We developed the fabrication process from the ground up and characterized the resulting device performance.

The first part of this thesis addresses the stability of perovskite layers during optical testing in ambient air. We observed that carrier lifetime measurements depend heavily on the perovskite's exposure time to air and the presence of protective overlayers, rather than solely on its inherent optoelectronic properties. This instability arises from rapid perovskite degradation in air. To mitigate this, we employed spin coating to deposit a protective PMMA layer over the perovskite and optimized this coating process. Characterization confirmed that PMMA significantly slows degradation. Additionally, we identified and corrected alignment issues in the photoluminescence quantum yield (PLQY) measurement setup, which had introduced substantial uncertainty. We refined the PLQY testing procedure to enhance result reliability.

The second part focuses on developing a quasi-2D perovskite emissive layer (EML) with low bulk defect and superior optical properties. Based on quasi-2D perovskites (PEA2(FAPbBr3)n−1PbBr4) typically exhibit better emission characteristics than bulk counterpart (FAPbBr3) We first confirmed the phase composition of our synthesized perovskite, verifying the formation of quasi-2D perovskite with the targeted phase distribution (n = 3). We then introduced two additives: [2-(9H-Carbazol-9-yl)ethyl]phosphonic Acid (2PACz) and KBr to enhance the optoelectronic property of EML. Photoluminescence (PL) testing revealed that 2PACz significantly enhances PL intensity, while KBr showed no such effect. Subsequently, we applied different antisolvents to improve perovskite film morphology. Results demonstrated that ethyl acetate yielded the highest PLQY. Our champion quasi-2D perovskite sample achieved a PLQY of up to 49\%.

The third part details the development and characterization of carrier transport layers for PeLEDs. For the electron transport layer (ETL), we employed 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) and used simulations to study how its thickness affects optical performance. Simulations indicated negligible performance variation for TPBi thicknesses between 40 nm and 80 nm. For the hole transport layer (HTL), we fabricated a self-assembled monolayer (SAM) HTL using [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic Acid (MeO-2PACz) and [4-(3,6-Dimethoxy-9H-carbazol-9-yl)butyl]phosphonic Acid (MeO-4PACz), which could reduce interfacial defect at HTL/ETL interface. However, devices with this SAM-HTL structure didn't emit light during voltage sweeps and exhibiting low turn-on voltages in J-V characteristic. We attribute this to electrical breakdown caused by electron tunneling through the extremely thin (1-2 nm) SAM layer. To suppress tunneling, we introduced an underlying NiO$_{x}$ layer. This modified structure demonstrated to be successfully prevented tunneling and enabled functional PeLEDs. To further optimize hole transport while minimizing thickness, we modeled the relationship between HTL thickness and electron tunneling probability. Calculations revealed that increasing the HTL thickness by approximately 3 nm effectively shields against tunneling. This optimized, ultra-thin HTL design paves the way for PeLEDs operating at low voltages with state-of-the-art optical performance. ... Read more

Silicon heterojunction (SHJ) technology is gaining increasing attention due to its lowtemperature and simple fabrication process. Intrinsic hydrogenated amorphous silicon
((i)a-Si:H) serves as a passivation layer, providing excellent chemical passivation, while
doped a-Si:H offers good field passivation and selective contact. Thanks to these features,
SHJ has achieved a high conversion efficiency of 27.09%, approaching the theoretical
limit for silicon-based solar cells. However, achieving excellent passivation performance
results in optical and electrical losses, as the band gap of doped a-Si:H causes parasitic
absorption. One strategy to address this issue is to replace the doped layer with transition
metal oxides (TMOs), which can enable selective contact due to their high or low work
functions. Few researchers have integrated TMOs in silicon-based solar cells to replace
silicon-based doped thin films as selective contact layers. Wide-bandgap TMOs improve
the optical performance of the cells. However, interface reaction between TMOs and the
silicon substrate becomes an issue limiting the electronic properties of the device.

In this research, we first present three different interface engineering methods: no
plasma treatment (noPT), plasma treatment (PT), and plasma treatment with boron
(PTB). We applied these methods to SHJ solar cells with MoOx (2.9 < x < 3) as the hole
transport layer (HTL).MoOx thin-film is deposited by thermal evaporation. The methods
were implemented at theMoOx/(i)a-Si:H interface. Additionally, to address sustainability
concerns related to indium consumption and fully exploit the optical advantages of
MoOx, we propose bifacial SHJ solar cells withMoOx as the HTL to reduce the thickness
indium doped tin oxide (ITO) films. Furthermore, to test the capability of these interface
engineering methods with other TMO materials, we usedWOx (2.9 < x < 3) and V2Ox (2.9
< x < 3) as the hole transport films in SHJ solar cells. The TMO thin-films are deposited by
thermal evaporation. The specific results are summarized as follows.

Chapter 3 explores using MoOx as a HTL to address these issues. By tailoring the
interface betweenMoOx and (i)a-Si:H using interface engineeringmethods, the oxygen
content in MoOx layers has been successfully optimized. The PTB method reduces
the formation of SiOx layer resulting in improved electronic properties and low contact
resistivity. PTB treated samples showed the best performance, with high open-circuit
voltage (VOC) and fill factor (FF). This approach achieved a certified conversion efficiency
of 23.83% with an ultra-thinMoOx layer of 1.7 nm. Notably, a short-circuit current density
(JSC) above 40 mA/cm² has been achieved.

Chapter 4 explores reducing indium consumption by optimizing n-contacts as electron
collector layer andMoOx as a hole collector layer. Bifacial SHJ solar cells withMoOx
as the HTL and various electron transport layers (ETL) were fabricated and optimized
using optical simulations. The results showed that bilayer ((n)nc-Si:H/a-Si:H) and trilayer
((n)nc-SiOx:H/nc-Si:H/a-Si:H) better than monolayer (a-Si:H) in electronic and optical
performance. The use of ultra-thin transparent conductive oxide (TCO) layers combined
withMgF2 as double layer antireflection coatings (DLARC) significantly reduced TCO consumption whilemaintaining high performance. Devices exhibiting 10-nm thick indium
tungsten oxide (IWO) on both side and bilayer n-contact achieved certified efficiencies
of 21.66% and 20.66% when measured from theMoOx and n-contact side, respectively.
This device realizes 90% TCO-reduction. The best-performing bifacial cells exhibited
conversion efficiencies of 23.25% and 22.75% measuring from front and back side, respectively.
The bifaciality factor of the champion device is 0.96. It demonstrates over 67%
reduction in TCO usage compared to traditional SHJ solar cells. This study successfully
shows that reducing TCO thickness and optimizing the interface withMoOx can lead to
high-efficiency bifacial SHJ solar cells, contributing to sustainability challenges related to
indium consumption.

Chapter 5 investigates the application of TMOs likeWOx, and V2Ox as HTLs combined
with interface engineering methods in SHJ solar cells. The study aims to investigate the
applicability of interface engineering methods to other TMOs. X-ray photoelectron
spectroscopy (XPS) is employed to measure oxygen content and defects within TMO films.
The XPS results demonstrate that PTB resulted in higher oxygen content and fewer oxygen
vacancies. The optimal WOx thickness was found to be 2 nm, achieving a champion
cell with 23.30% efficiency with PTB and improved FF of over 80%. Similarly, with PTB
method, the highest efficiency of 22.04% has been realized with 3-nmthick V2Ox layer.
The PTB method effectively controlled interface reactions, leading to better electronic
properties of TMOs. The study concludes that the PTB method offers suitable interface
conditions for depositing TMOs, enhancing SHJ solar cell performance.

Our findings in this study may provide valuable insights into applying TMOs in SHJ
solar cells to achieve high performance. With optimized interface engineeringmethods,
we can significantly reduce the optimal thickness of TMOs and achieve world-record
efficiency. Furthermore, by applying TMOs in bifacial SHJ solar cells, we can decrease the
thickness of indium-based TCOs, realizing both high efficiency and high bifaciality factor.
This approach makes it possible to develop sustainable and efficient solar cells. ... Read more

For centuries, society has relied on fossil fuels for development, leading to the problem of global warming and significant environmental changes. To address these environmental issues, cleaner and more cost-effective energy productions are required. Solar energy, harnessed through well-developed photovoltaic (PV) technology, offers a promising solution. In the PV research field, perovskite (PVK)-based devices offer a feasible processing and have exhibited a fast increase in efficiency. Despite advancements in both the efficiency and stability of perovskite solar cells, there still is a long way to go towards industrialization due to the formation of pinholes during large area film deposition, nonuniformity, and poor reproducibility. Thermal evaporation technology has shown potential for the commercialization of perovskite solar cells, owing to its compatibility with large areas and textured substrates. In this thesis, we focused on the sequential thermal evaporation of perovskite. Through this approach, post-annealing and precursor mixing processes were investigated. Additionally, crystal orientation was tuned by applying different intermediate annealing temperatures. The optimized process was then applied to upscale both absorber films and cells from 0.09 cm2 to 1 cm2... ... Read more

As global temperatures rise and energy demands increase, the need for clean, renewable
energy sources is more critical than ever. Solar energy is one of the key solutions, with the
majority of solar panels currently on the market being made from crystalline silicon. However, emerging photovoltaic (PV) technologies such as perovskite solar cells have already demonstrated efficiencies comparable to those of silicon solar cells, making them a promising contender to achieve even higher efficiencies.

Most of the layers in perovskite solar cells are deposited via spincoating, which is a fast and easy process but can only be done on laboratory-scale. However, deposition through thermal evaporation offers significant advantages, enabling fabrication of nanometer-thin films and facilitating large-scale fabrication needed for future industrialization of perovskite solar cell. Therefore, this research aims to develop perovskite solar cells entirely through thermal evaporation.

The reported number of hole transport materials deposited through thermal evaporation is limited. Recently, fully thermally evaporated perovskite solar cells have been created using the hole transport materials MoOx and TaTm, and these hole transport materials will be studies in this thesis.

The MoOx and TaTm were used as single and double hole transport layer to replace the
reference layer of spincoated PTAA. It was found that the MoOx in direct contact with the pervovskite resulted in a chemical reaction, which negatively affected the energy alignment. The MoOx also showed poor charge carrier selectivity, resulting in high interfacial recombination. Great hole extraction from the perovskite was observed for TaTm, however, a misalignment of the band energy with the electrode hindered the hole collection. Improved hole transfer was found with MoOx and TaTm being used a double hole transport layer. Here, the TaTm functions as a passivation layer between the MoOx and perovskite, while effectively blocking the electrons. In turn, the MoOx improved the energy alignment from the TaTm to the electrode to improve the hole collection.

A thickness optimization of the hole transport layers was also performed. For MoOx as
single hole transport layer, it was found that number of oxygen vacancies decreased with
thickness, leading to less recombination. No change was observed for TaTm as single hole
transport layer when varying the thickness. However, as a double hole transport layer with MoOx, increasing the thickness of TaTm led to an increase in Voc . Ultimately, a thin layer of 2 nm MoOx with a 5-nm thick TaTm showed the most promising results, demonstrating a final efficiency of 4.73%.
... Read more

Silicon heterojunction solar cells (SHJ) showed a record efficiency of 26.81%, approaching the theoretical limit of single-junction crystalline silicon (c-Si) solar cells. To further improve the efficiency, a wide bandgap perovskite top cell can be stacked on top of the SHJ bottom cell forming tandem solar cells, which utilize better the solar spectrum. Recently, a record efficiency of 33.70% was achieved for a monolithic two-terminal perovskite/SHJ tandem solar cell. Typically, a transparent conductive oxide (TCO) layer, functioning as the recombination junction, is used to connect the two sub-cells. However, tandem solar cells with this conventional TCO recombination junction often feature high reflection losses originating from the intermediate interfaces between the two sub-cells. Therefore, this master thesis focused on minimizing these intermediate reflection losses by substituting the TCO-based recombination junction with proposed TCO-free recombination junctions.

Firstly, comprehensive optical simulation studies that compared 2T tandem solar cells with various recombination junctions were performed. In the case of single-side-textured (front-side-flat) tandem configuration, as compared to the reference cell with tin-doped indium oxide (ITO) recombination junctions, the use of the more transparent tungsten-doped indium oxide (IWO) allowed an improved implied photocurrent density in the bottom cell (Jimp,bottom) from 18.30 mA/cm2 to 18.70 mA/cm2. Further, by using the TCO-free recombination junction composed of (p)nc-SiOx:H/(n)nc-SiOx:H or (p)nc-Si:H/(n)nc- Si:H, ranges of optimum thickness combinations were discovered, which allowed high Jimp,bottom values of 20.30 mA/cm2 or 19.80 mA/cm2, respectively. Both TCO-free recombination junctions demonstrated enhanced light coupling to the bottom cell thanks to the optimized interference effect at the intermediate interfaces between two sub-cells, minimizing the associated reflection losses. Furthermore, the designs of tandem solar cells featuring various recombination junctions were optimized to reach maximum matched tandem current density. For the reference cell with ITO recombination junction, a matched tandem current density of 19.40 mA/cm2 was obtained, while the use of TCO-free recombination junctions, for instance, 60 nm (p)nc-SiOx:H/ 70 nm (n)nc-SiOx:H or 30 nm (p)nc-Si:H/75 nm (n)nc-Si:H, demonstrated high Jimp,bottom values of 19.80 mA/cm2 and 19.80 mA/cm2, respectively. These results highlight the optical advantageous implementations of proposed TCO-free recombination junctions for monolith tandem solar cells. Similar observations but less significant improvement by using the proposed TCO-free recombination junctions were found in double-side-textured tandem solar cells. This is due to the already minimized reflection losses of the (p)nc-SiOx:H/(n)nc-SiOx:H or (p)nc-Si:H/(n)nc- Si:H configurations (1.3 mA/cm2 and 1.4 mA/cm2 respectively) as a result of the textured front surface.

Based on optical simulation studies conducted on 2T tandem solar cells, the electrical effectiveness of proposed TCO-free recombination junctions was examined by fabricating proof-of-concept single junction single-side-textured SHJ solar cells. First, we focused on the passivation optimization of the flat (100) c-Si surface as it is prone to detrimental epitaxial growth. An impressive minority carrier lifetime of 16.87 ms was achieved by combing (n)nc-Si: H and (i)a-Si: H bi-layer in a symmetrical configuration. Moreover, we also observed, in general, better conductivity when increasing thicknesses of doped nc- SiOx: H layers when they were deposited on glass or (i)a-Si: H coated glass substrates. Eventually, proof-of-concept single junction single-side-textured SHJ solar cells featuring the proposed TCO-free recombination junction were fabricated. According to the optical simulations, various optimum thickness combinations of (p)nc-SiOx: H and (n)nc-SiOx: H or (p)nc-Si: H and (n)nc-Si: H that composes the recombination junction were tested. Overall, the optically promising TCO-free recombination junctions in 2T tandem solar cells also delivered high FF values in proof-of-concept single-junction SHJ solar cells, demonstrating their potential to be implemented to fabricate high-efficiency monolithic 2T tandem solar cells.
... Read more

Perovskite solar cells (PSCs) are an emerging and promising photovoltaic technology that have demonstrated impressive power conversion efficiencies exceeding 25% after only fourteen years of development. This rapid progress can mostly be attributed to the development and optimisation of PSCs fabricated by solution-based methods, because of their easy processing and inexpensive equipment. However, to commercialise PSCs, alternative methods are required. Among these methods, the vacuum-based technique thermal evaporation is a suitable candidate. In thermal evaporation, solid precursors are evaporated and deposited onto a substrate in a high vacuum chamber. Thermal evaporation is a mature technique in the semiconductor industry to fabricate pinhole-free and highly uniform thin films at a large scale on different types of substrates. To date, the efficiency of PSCs by thermal evaporation is lagging behind their solution-based counterpart, partly due to limited research on vacuum deposited PVK films and several challenges unique to thermal evaporation.
In this MSc thesis project, organic-inorganic metal halide perovskite absorbers based on thermally evaporated FAxCs1-xPb(IyBr1-y)3 and spin-coated MAPbI3 have been developed together with the additional supporting layers for device fabrication. Several PSCs with different p-i-n architectures have been fabricated and characterised. The device comprises of: ITO as front electrode, spiro-TTB, spiro-OMeTAD and/or MoOx as hole transport layer, PVK absorber layer, C60 as electron transport layer, BCP as buffer layer, and silver and aluminium as metallic back electrode. The goal of this work is to develop and optimise the structural and opto-electronic properties of different device layers and demonstrate a working perovskite solar cell.
PVK engineering is carried out on both thermally evaporated and spin-coated PVK thin films with the aim at obtaining perovskite that possesses desirable opto-electronic properties. For FAxCs1-xPb(IyBr1-y )3 fabricated by sequential layer thermal evaporation, the uniformity and tooling factors were optimised to grow layers with a correct precursor ratio showing a high crystallinity and absorptance. No clear effect of post annealing could be detected for the temperatures and times tested in this work. Achieving the target thickness and obtaining a satisfactory reproducibility were not fully reached. For spin-coated MAPbI3, it was found that increasing the precursor solution concentration helped to achieve a desirable thickness for PSCs. Moreover, an adequate bandgap, diffractogram and absorptance were measured. However, based on the small crystal size determined with SEM, and poor device performance, further improvement is needed.
Transport, contact, and buffer layers were successfully fabricated by developing new deposition recipes for silver, BCP, and spiro-TTB. Furthermore, spectrophotometry measurements show that thermally evaporated HTLs (MoOx and spiro-TTB) exhibit much lower parasitical absorption losses compared to spin-coated spiro-OMeTAD. An optical model to fit spectroscopic ellipsometry data measured on spiro-TTB thin films is created to determine its thickness and optical properties. Moreover, time resolved microwave conductivity results indicate that C60 effectively extracts electrons from MAPbI3. In contrast, spiro-TTB films did not appear to possess any hole extracting abilities. Strong quenching of the steady-state PL signal is observed for bi-layers of MAPbI3 with either C60, spiro-OMeTAD and MoOx . This quenching is possibly caused by extraction of either electrons or holes and/or enhanced non-radiative recombination at the interface.
The optimised films were combined in complete PSCs with most layers fabricated by thermal evaporation through metal masks structuring cells with active areas of 0.16 and 0.36 cm2. Solar cells were characterised in the dark and under illumination using a solar simulator integrated with a glove box to prevent degradation. A wide range of J-V characteristics are identified and classified: (1) ohmic responses, possibly caused by poor quality PVK absorber layers with pinholes filled with thermally evaporated metal, (2) S-shaped curves under illumination, possibly caused by a mismatch in energy band alignment or increased non-radiative recombination at the MoOx/MAPbI3 interface, (3) diode behaviour in the dark, and (4) a high series resistance and low shunt resistance, possibly caused by low mobility, non-optimal thicknesses, internal currents, and a mismatches in band alignment. The most promising PSC featuring the ITO (200 nm)/Spiro-OMeTAD/MoOx (5 nm)/MAPbI3/C60 (20 nm)/BCP (4 nm)/Ag (150 nm) architecture, had a short circuit current of 10.0 mA/cm2 and an open circuit voltage of 0.67 V.
The experiments carried out in this MSc thesis project supported the development of several steps of PSC fabrication process. This work contributes towards understanding thermal evaporation processes of perovskite films and fabricating fully evaporated devices with high efficiency and a large area.
... Read more

Silicon heterojunction (SHJ) solar cells have exhibited efficiencies well above 25%. To further boost the efficiencies of c-Si-based solar cells, high-bandgap perovskite cells are stacked on top achieving a record efficiency of 29.52%. However, as most of the high-quality perovskite films are solution-processed, the front surface of the bottom device should be flat. Therefore, in this work SHJ bottom c-Si cells featuring front-side-flat and rear-side-textured morphology, which delivers high V_OC together with excellent near-infrared response, have been optimized as bottom cells for tandem configurations.

Firstly, RF-PECVD deposition conditions of a (i)a-Si: H monolayer for symmetric <100> flat c-Si surfaces were optimized. The optimized (i)a-Si:H monolayer ( 10-nm-thick) was obtained using pure SiH₄, which results in rather moderate passivation performances (teff = 1.2ms, i-V_OC = 701 mV).
To improve further the passivation quality of monolayer (i)a-Si:H on flat <100> surface, other passivation approaches aiming at incorporating more H without promoting detrimental epitaxial growth have been investigated.
With a bilayer deposition approach, which features firstly a less H-containing (i)a-Si:H to prevent epitaxial growth and then a second H-rich (i)a-Si:H layer, the passivation properties were slightly enhanced to τeff=1.4 ms and i-V_OC=704 mV. Subsequently, by combining the bilayer approach with a post HPT, τeff of 2.0
ms and an i-V_OC of 714 mV were achieved. Finally, by combining the bilayer approach with an intermediate HPT, the optimal passivation sample was deposited, with τeff of 2.4 ms and an i-V_OC of 720 mV on the flat <100> surface.
To gain a better understanding of the correlation between passivation qualities and the microstructure properties of (i)a-Si:H on flat <100> surface, the layers have been characterized mainly via Fourier-transform infrared spectroscopy (FTIR). From the analysis, it can be concluded that the passivation layer that contains
sufficient H and a higher fraction of monohydrides is beneficial for achieving a better passivation quality.

For the two-terminal tandem solar cells, bottom cells with (n)-contact on top are preferred due to the optical advantage of the perovskite top cells with the p-i-n configuration. Therefore, a first tandem cell with (n)a-Si:H has been fabricated in collaboration with TU Eindhoven resulting in 22.2% efficiency. Starting from
this first fabricated tandem cell, its main optical limitations have been identified by performing advanced optical simulations using GenPro4, and the main strategies to overcome these optical drawbacks have been defined. By optimizing the front anti-reflection layers (MgF₂ and ITO) thicknesses (at 100 nm and 20 nm, respectively), and reducing C₆₀ thickness from 20 to 10 nm, front reflections, and parasitic absorption can be minimized. Thus a gain of implied photocurrent density of 1.8 mA/cm² for the tandem cell was obtained.
Further, by implementing (n)nc-SiOx:H doped layer in the SHJ bottom cell, instead of standard (n)a-Si:H layer the reflection between the top and bottom cell is also reduced, and enhanced light incorporation into the bottom cell is obtained. By adopting all the above optimizations and also adjusting the perovskite
layer from 473 nm to 530 nm, a total improvement of 2.7 mA/cm² in implied photocurrent density with respect to the initial 22.2% tandem cell can be achieved.
After having identified different optically optimized SHJ bottom cells for tandem applications, both rear junction and front junction single-side-textured SHJ solar cells were fabricated. Firstly, the passivation quality of (i)a-Si:H/(n)-layer and (i)a-Si:H/(p)-layer on different (i)a-Si:H were investigated. Then RJ solar cells
with three different (n)-type layers [(n)nc-SiOx:H;(n)nc-Si:H;(n)a-Si:H)] have been fabricated with optimal thicknesses individuated from the tandem optical simulations. Furthermore, a tunnel recombination junction SHJ solar cell with a layer stack of (n)nc-Si:H/(p)nc-SiOx:H/(p)nc-Si:H has been fabricated and measured as well.

In conclusion, various doped contacts (both n- and p-type) were successfully implemented into SHJ solar cells, which delivered V_OCs range from 700 to 714 mV and FFs range from 77.8% to 80.9%. Therefore, different well-functioning SHJ solar cells have been developed and are ready to be implemented as bottom cells for high-efficiency tandem devices. ... Read more

Perovskite Solar Cells are an increasing attraction in research with great strides in performance efficiencies. But their commercialization is far from fruition, due to persistent issues. Stability under prolonged exposure to external stresses is a major concern. Furthermore, the adaption of upscaling technologies in research is presently slow. This research is done on a fully scalable architecture, fabricated with technologies adaptable to mass-production. Variations in active layer (dual cation and triple cation) and Electron Transport Layer are designed into different stack combinations to discriminate the more resilient stack. Long-term stability of these stacks is tested by accelerated thermal test and light-soaking test. Light-soaking test is done from different sides and with a filter to determine the difference in behaviour of samples. From thermal testing, the dual cation perovskite was found to be more resilient. In light-soaking tests done from side of glass, all stacks failed, while the stacks illuminated from side of ETL showed a far better performance. The dual cation perovskite was again found to be more resilient. Despite the performance loss, none of the stacks demonstrated significant degradation in perovskite layer under XRD or photoluminescence. Therefore, recombination mechanisms were studied under light-intensity measurements. The presence of carrier collection problems across stacks and degradation of HTL particularly in samples of glass-side exposure are determined to be the probable cause. ... Read more

Among the emerging photovoltaic (PV) technologies under development, perovskite solar cells (PSCs) stood out in the last decade as one the fastest advancing solar technology to date, reaching a power conversion efficiency of 25.5% in 2020. Perovskite (PVK) absorbers show great versatility thanks to compositional and bandgap engineering. In this MSc Thesis Project, multications mixed-halides PVK thin-films of composition CsxFA1-xPbI3-xBrx are developed. Multiple-source layer-by-layer thermal evaporation and sequential thermal evaporation/spin coating hybrid synthesis techniques are investigated because of their compatibility with non-flat substrates. As they would enable the conformal growth of PVK on the microsized pyramidal textured surface of a c-Si- based solar cell, the long-term goal is the application in a monolithic (2-T) PVK/Si tandem. PVK engineering is carried out through tuning the main processing parameters with the aim of obtaining high-quality absorber layers. The optimisation of the PVK thin-films is based on criteria such as composition, phase, bandgap, crystallinity, homogeneity, uniformity and charge carrier transport properties. After optimising the annealing thermal treatment, the homogeneous and uniform thin-films resulting from the thermal evaporation of CsBr, PbI2 and FAI show a highly crystalline photoactive α phase of PVK, as well as a sharp-edge absorption onset corresponding to a bandgap (Eg) of Eg=1.60 eV and high absorption coefficient (α) in the order of α=10^4-10^5 cm^-1. Then, the two-step spin coating technique is explored as a preliminary study to develop the hybrid method, with the aim of analysing the surface wettability and reactivity of an inorganic layer of PVK inorganic precursors (CsBr and PbI2) when an organic solution (FAI in IPA) is spun on it. In spite of the optimisation of the FAI solution spun volume and concentration and the annealing parameters, the spin-coated thin-films present a mixture of photoactive and non-photoactive phases, i.e. PbI2-xBrx and α-, γ- and δ-PVK phases. The ineffective interdiffusion reaction leading to inhomogeneous and non-uniform PVK thin-films shows that the processing parameters need further fine-tuning. However, as spin coating does not allow to deposit PVK on top of non-flat substrates, the thermal evaporation/spin coating hybrid method is developed. The solution of FAI in IPA has been spun on a thermally evaporated inorganic bilayer of CsBr/PbI2. Albeit incomplete conversion to PVK is under suspicion, the resulting α-PVK thin-film shows good crystallinity and homogeneity and acceptable uniformity. It also presents high absorption (A=70%) for λ<500 nm (blue light) and low absorption for λ>500 nm (from green to red light), although the bandgap of Eg=1.53 eV needs to be further optimised by adjusting the content of Cs+ and most importantly Br–. On top of that, for all the synthesis techniques under research some unsuccessfully deposited PVK thin-films show signs of thermal and atmospheric degradation, leading to the decomposition of the α phase of PVK and formation of PbI2 and other non-photoactive phases, along with poor optical properties. Such degradation phenomena, which might be ascribed to the ineffective incorporation of Cs+ leading to structural instabilities, highlights the difficulties and limitations of PVK engineering. ... Read more

Perovskite materials gain a huge interest in the photovoltaic (PV) community due to its unique characteristics, including long carrier diffusion length, widely tunable bandgap, light absorption potential, and low processing cost. Nowadays, most perovskite fabrication methods employ a solution-based process due to its simplicity and production speed. However, this deposition method provides a non-uniform structure and uses highly toxic solvents, posing the risk of contamination and adverse effects on the environment. On the other hand, a solvent-free method like thermal evaporation can produce a uniform and conformal layer. This method can be used to produce not only the perovskite absorber layer but also the contact layers and transport layers. Depending on the deposition parameters, the resulting morphological properties also change. Therefore, it becomes interesting to understand the detailed knowledge of the film growth and the effects of the deposition parameters on the exact kinetics and the optical properties. Hence, the first objective of this study focuses on developing C60 electron transport layers (ETL) for application in all-evaporated perovskite solar cells (PSCs). The C60 thin film was deposited with different thicknesses of 20, 30, 40 nm and deposition rates 0.3, 0.5, and 1 Å/s on top of the silicon wafer substrate. The resulting surface morphology is obtained from scanning electron microscopy (SEM) and atomic force microscopy (AFM). It indicates that C60 with 40 nm thickness and 0.3 Å/s deposition rate shows a pinhole-free layer with an average surface roughness of 1.05 nm and thickness uniformity of more than 94%. The X-ray diffraction (XRD) measurement shows that decrease of peak intensity as the thickness is reduced from 40 to 20 nm. Moreover, with different deposition rates, 1 Å/s of deposition rate exhibits an asymmetric broadening peak which attributes to the small grain size and the presence of a planar defect in the structure of C60.
The optical analysis has also been performed to get the complex refractive index C60 and identify the effect of deposition rates and layer thicknesses on optical constants. A procedure to extract optical constant for the perovskite absorber layer has been developed during this thesis project using a combination approach of b-spline and Tauc-Lorentz dispersion model. The obtained results were found to be in excellent agreement with experimental work and literature data.
Furthermore, the complete solar cells with p-i-n configuration and semi-transparent perovskite solar cells (ST-PSCs) were optically simulated using GenPro4 software. This simulation aims to identify both the photocurrent density of the perovskite absorber layer and the optical losses caused by parasitic absorption in the supporting layers. In the p-i-n structure, ITO and MoOx layer located on the illuminated side contribute to the main portion of optical loss. Simulations suggest that 40-nm-thick ITO and 10-nm-thick MoOx is an ideal layer stack to deliver high implied photocurrent (22.14 mA/cm2). On the other hand, the optical loss in semi-transparent perovskite solar cells is investigated in two different wavelength regions (i) 300 – 800 nm and (ii) 800 – 1200 nm. In this investigation, the metal back contact is replaced with ITO and cells illuminated from the ETL side. The results show that, in the first wavelength range, the main optical losses are due to reflection, parasitic absorption in the C60 and top ITO layer. These losses are reduced by applying 120-nm-thick anti-reflective coating MgF2 and decreasing the thickness of C60 to 10 nm. Moreover, in the wavelength region of 800 – 1200 nm, the optical losses are mainly affected by the top and bottom ITO, MoOx layer, and reflected light. After optimizing top ITO and MgF2 thickness to 50 and 120 nm, respectively, a 17.07 mA/cm2 of photocurrent transmitted through the cells can be achieved. The light transmittance is ~88%, indicating the potential of semi-transparent perovskite solar cells to be applied in perovskite/silicon tandem devices.

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Carrier-selective passivating contacts (CSPC) are very promising contact structures for highly efficient silicon solar cell. They provide passivation of silicon surface and high carrier selectivity. So far, superior results have been achieved through the use of a stack of poly-Si with SiO2. The objective of this project is the optimization of an alternative passivation stack based on carbon alloyed poly-Si (poly-SiCx). The alloy is selected since carbon provides improved material resilience against blistering and wet-chemical stability in commonly used chemicals in the silicon industry. The optimized poly-SiCx contacts are implemented in fornt back contacted (FBC) solar cell. We improve the passivation quality of SiOx/poly-SiCx contacts optimizing several parameters. Our main focus is on the buffer layer optimization and its deposition methods comparing LPCVD and PECVD technique. We also turn our attention to identify the optimum annealing temperature and thickness of doped layers. Finally, we focus on the influence of the different capping layer composition on hydrogeneation process. With the optimization of the annealing temperature, and thickness of (i)a-Si layer deposited by LPCVD, we obtain high passivation quality on cell precursor structure: i-Voc of 713 mV, τeff of 2.14 ms and Jo of 9.5 fA/cm2. On the contrary, during the optimization of buffer layer deposited by PECVD, we firstly focus on the material properties and the bonds present in the deposited layers to minimize the hydrogen contact. After the optimization of the (i)a-Si:H layer thickness, deposition parameters and annealing temperature on (p)poly-SiCx symmetrical sample, we obtain the remarking results: i-Voc of 681 mV, τeff of 1.15 ms and Jo of 31.3 fA/cm2.
The potential of SiOx/poly-SiCx passivation contacts is checked on the device level of FBC solar cells. On FBC solar cells with buffer layer deposited by the PECVD, after an effective post annealing of the cell performed at a temperature of 350°C, we achieve the remarking parameters; Voc of 659 mV, Jsc of 34.36 mA/cm2, FF of 77.58 % and ηact of 17.56 %. We check also the the influence of different SiNx capping layers on the hydrogenation process. We obtain the best results on the FBC solar cell on which the capping layer is stoichiometric. These results, after the high-temperature port annealing, are Voc of 690 mV, Jsc of 36.18 mA/cm2, FF of 80.38 %, and ηact of 20.06 %. These are also the best FBC poly-SiCx results which we have obtained in this project.
The application of SiOx/poly-SiCx passivation contacts is investigated also in terms of the IBC solar cell concept. We proposed two alternative fabrication methods based on photolithography, which is crucial for contact formation of IBC solar cells. One of the fabrication methods focuses on a buffer layer deposited by LPCVD as this layer is of higher quality and homogeneity than the interlayer deposited by PECVD. The second developed method focuses on the buffer layer deposited by LPCVD and PECVD. Despite the lower passivation quality achieved on contact with the buffer layer deposited by PECVD, this method has a significant advantage because it requires fewer photolithography steps when compared to the fabrication method, which makes use only of buffer layers deposited by LPCVD. In the view of the photolithography processes, we have performed etching tests on (i)a-Si, (i)a-Si: H, and amorphous doped layers. Thanks to these tests, we have identified etching rates of these layers in various prepared poly-Si etching solutions.
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