Rubidium Multication Perovskite with Optimized Bandgap for Perovskite‐Silicon Tandem with over 26% Efficiency

Effects of Atomic Oxygen Irradiation on Transparent Conductive Oxide Thin Films

This project aims at developing highly efficient perovskite/silicon tandem solar cells by developing efficient perovskite top cells, silicon bottom cells, and monolithically integrating the subcells for highly efficient tandem solar cells. The project focused on increasing the efficiency of perovskite solar cells to 20% and understanding related materials science and device physics, as well as developing a compatible bottom high-efficiency c-Si solar cell. The perovskite cells with a wide bandgap of 1.61 eV and 1.84 eV are fabricated by a low-cost, low-temperature, scalable solution process on lower bandgap silicon bottom cells. The bandgaps of perovskite top cells have been tuned between 1.61 eV and 1.84 eV by composition engineering mainly through controlling the halide mixing ratio in perovskite materials. The efficiencies of the wide bandgap perovskite solar cells have been improved to an excellent efficiency of 21.0% for perovskites with a bandgap of 1.63 eV with increased grain size, reduced energy disorder at cathode contact, and defect passivation. We invented a solvent annealing approach with solvent vapor during thermal annealing to enlarge grain size of perovskite film which reduces the charge trap density and improves the device efficiency. Moreover, we discovered that the non-wetting surface could further increase the grain size for perovskite films, because non-wetting surface could prevent the formation of too dense nuclei from heterogeneous nucleation process. We proposed to reduce the energy disorder of organic charge extraction layer to reduce the power loss in perovskite cells. We pioneered defect passivation concept and techniques to suppress the charge non-radiative recombination in perovskite solar cells to boost the efficiency. For the silicon bottom cell development, different from the conventional silicon cell work under full illumination, the light available for the silicon bottom cell in tandem structure is mainly NIR region. We developed a NIR enhanced silicon cell with efficiency of 21.2% by applying a double-layer antireflection coating at the front side and a MgF2 back reflector layer at the rear side. For the perovskite/silicon tandem device, we achieved a highest reported power conversion efficiency of 25.4% in 2018, and further increased the efficiency up to 26.0% in 2019. The research on tandem solar cells focused on reducing reflection loss to <5% at spectrum of 300-1100 nm by antireflection layer, tuning the bandgap of perovskite film for matched current density between sub-cells, and utilizing a ITO layer between perovskite top cell and silicon bottom cell as interconnect layer without Voc loss. Finally, we developed a blading technique to fabricate perovskite films which produces perovskite solar cells with comparable or even superior efficiency to those made by non-scalable spin coating method. This paves the way for scaling up perovskite/silicon tandem solar cells using solution approach.

ABX3 Perovskites for Tandem Solar Cells

At an infra-red (IR) wavelength of 1,064 nm, a diode-pumped Q-switched laser was used for the direct patterning of various transparent conductive oxide (TCO) thin films on glass substrate. With various laser beam conditions, the laser ablation results showed that the indium tin oxide (ITO) film was removed completely. In contrast, zinc oxide (ZnO) film was not etched for any laser beam conditions and indium gallium zinc oxide (IGZO) was only ablated with a low scanning speed. The difference in laser ablation is thought to be due to the crystal structures and the coefficient of thermal expansion (CTE) of ITO, IGZO, and ZnO. The width of the laser-patterned grooves was dependent on the film materials, the repetition rate, and the scanning speed of the laser beam.

Perovskite material with a bandgap of 1.7-1.8 eV is highly desirable for the top cell in a tandem configuration with a lower bandgap bottom cell, such as a silicon cell. This can be achieved by alloying iodide and bromide anions, but light-induced phase-segregation phenomena are often observed in perovskite films of this kind, with implications for solar cell efficiency. Here, we investigate light-induced phase segregation inside quadruple-cation perovskite material in a complete cell structure and find that the magnitude of this phenomenon is dependent on the operating condition of the solar cell. Under short-circuit and even maximum power point conditions, phase segregation is found to be negligible compared to the magnitude of segregation under open-circuit conditions. In accordance with the finding, perovskite cells based on quadruple-cation perovskite with 1.73 eV bandgap retain 94% of the original efficiency after 12 h operation at the maximum power point, while the cell only retains 82% of the original efficiency after 12 h operation at the open-circuit condition. This result highlights the need to have standard methods including light/dark and bias condition for testing the stability of perovskite solar cells. Additionally, phase segregation is observed when the cell was forward biased at 1.2 V in the dark, which indicates that photoexcitation is not required to induce phase segregation.

Sputtered transparent electrodes for optoelectronic devices: Induced damage and mitigation strategies

Decoupling the effects of defects on efficiency and stability through phosphonates in stable halide perovskite solar cells

Revealing fundamentals of charge extraction in photovoltaic devices through potentiostatic photoluminescence imaging

Diversifying Progress in Solar

Silicon (Si) solar cells are the dominant and well-developed solar technology holding more than 95% share of the photovoltaic market with efficiencies over 26%. Still, this value is far away from the Shockley–Queisser limit of 33.15% for single-junction devices. A prominent way to surpass this limit and boost the device performance is to combine different bandgap materials in a tandem configuration. The rapid emergence of perovskite solar cells (PSCs) as one of the most exciting research fields with over 25% efficiency has attracted the focus of the scientific community. The solution-processability, bandgap tunability, and outstanding optoelectronic properties of perovskites mark them a potential pair with silicon to develop tandem solar cells (TSCs). In nearly seven to eight years of development, Si/perovskite TSCs have achieved record certified efficiencies of over 29%. This review emphasizes on two and four-terminal Si/perovskite TSCs. Initially, the advancement of efficiencies to date is discussed, including a comparison of numerous perovskite and silicon material choices. Then, the evolution of PSCs with Si (homojunction and heterojunction) bottom devices and their impact on the performance of TSCs is summarized. The suitable candidates for the perovskite and Si cells are proposed for Si/perovskite TSCs. Next, factors influencing the performance of tandems, such as fabrication issues on textured surfaces, parasitic absorption, reflection losses, nonideal perovskite absorber layer bandgap, device instability, and large-area fabrication, are highlighted. To reduce the electrical and optical losses for highly efficient tandems, an investigation of anti-reflecting coatings, current matching mechanisms, transparent electrodes, and recombination layers is discussed. Finally, based on these findings, future guidelines are proposed to boost the efficiencies beyond 30%. To the best of our knowledge, this is the first detailed study on two and four-terminal Si/perovskite TSCs. These findings would open new avenues for the research community with detailed information on Si/perovskite tandem cells.

In solar cells, efficiency is the most critical parameter. In single‐junction solar cells, efficiency is always limited, so to increase the efficiency beyond a specific range, we need to switch to a tandem configuration. Tandem configuration is the stacking of two different cells together, termed as top and bottom cells, where the electrical properties of these two cells are different. ln this paper, free perovskite material with a wide band gap of 1.5 eV (CsSnGeI 3 ) is used to make the top cell, and a narrow band gap of 1.12 eV crystalline silicon is used to create a bottom cell. The advantage of using perovskite material is its high light conversion rate, which leads to better efficiency. Power conversion efficiency (PEC) for perovskite cells is up to 30%. The proposed cell is simulated by SCAPS‐1D. In this process, the top and bottom cells are calibrated in the standalone conditions before analyzing the tandem configuration, where the optimized thickness for the top cell is 100 nm and that of the bottom cell is 200 nm. To combine both cells, the current of both cells should be matched, where the top cell current acts as a limiting factor. By changing the width of the top cell for a filtered spectrum and integrated filtered power, a matched current of 18.34 mA cm −2 is obtained for the top/bottom cell. The resultant parameters of the proposed tandem solar cell are PCE of approximately 26%, open circuit voltage ( V oc ) of 1.576 V, fill factor (FF) of 90%, and short circuit current ( J sc ) of 18.34 mA cm −2 . The proposed device can help develop low‐cost applications and high‐efficiency tandem solar cells.

Analysis of thermal effect on transparent conductive oxide thin films ablated by UV laser

Hybrid inorganic-organic thin-film solar cells, such as dye-sensitized solar cells (DSCs), can be assembled with low-cost materials and manufactured with cost-effective methods, and are considered promising photovoltaic technologies. A typical DSC is composed of a layer of mesostructured wide-band gap metal oxide semiconductor on which is adsorbed a light-absorbing sensitizer. Upon illumination, the adsorbed sensitizer is photoexcited and injects an excited electron into the conduction band of the semiconducting oxide. The oxidized dye is then regenerated by electron donation from a redox couple present in the electrolyte. The redox couple is regenerated at the counter electrode with electrons that have migrated through an external circuit. Despite over 20 years of development, the performance and long-term stability of these solar cells are still lagging behind major photovoltaic technologies, such as silicon-based solar cells. This is partly because some of the factors limiting device performance are not fully understood. Moreover, novel photoactive materials that have both high light absorption capability and fast carrier mobility are highly desired. The aim of the research presented in this thesis was to contribute to the development of high-performance hybrid thin-film solar cells by designing novel device architectures and exploiting hybrid perovskite materials as novel light absorbers along with providing a deeper understanding of device operation mechanisms. In order to enhance the charge transport in DSCs, a nanostructured collector–shell electrode was developed. A collector–shell electrode consists of a porous scaffold material having high electronic conductivity and a thin metal oxide shell. The shell provides sites for dye adsorption and photoelectron injection while the conductive backbone allows fast electron transport. DSCs fabricated using the collector–shell electrodes along with a cobalt redox couple and an organic sensitizer, MK-2, exhibited a promising power conversion efficiency (PCE) of 3.3% and a charge transport rate that was 2.6 times faster than observed for devices utilizing P25-based TiO2 electrodes. In the quest to explore novel efficient light absorber materials, an organic-inorganic hybrid perovskite material, viz., CH₃NH₃PbI₃, was utilized in planar structured thin film solar cells. The CH₃NH₃PbI₃ perovskites have been shown to exhibit excellent light harvesting, high carrier mobility and facile solution processability. A one-step, solvent-induced fast crystallization method was developed which produced high quality CH₃NH₃bI₃ perovskite thin films. These thin films exhibit full surface coverage and are composed of micron-sized grains. The application of these films in solar cell construction led to highly efficient devices with an average PCE of 13.9±0.7% and a steady state efficiency of 13%. The champion device fabricated using this deposition method achieved a PCE of 16.2%. In an attempt to further understand the perovskite-based device working mechanism, inverted structure perovskite-based solar cells with single or double selective contacts were fabricated which employed CH₃NH₃PbI₃ as light absorber and different inorganic metal oxides as interlayers. Solar cells fabricated on metal oxide layer coated substrates exhibit promising PCEs of over 10%. In addition, solar cells fabricated on ITO substrate without any metal oxide layer also exhibit a high PCE of 11.5%. This result indicates that perovskite solar cells can also perform well with single selective contact. The inverted structure perovskite solar cells, with or without the metal oxide interlayers, all exhibit very weak hysteresis in J–V measurements. This finding suggests that the hysteresis effect is not an intrinsic characteristic of the CH₃NH₃PbI₃ perovskite material and can be alleviated by optimization of the cell structure and judicious selection of the contact material. To understand the perovskite crystallization process and hence obtain a better control of film morphology for device fabrication, a series of perovskite materials including FAPbI₃, MAPbBr₃, MASnI₃ and mixed perovskites were investigated. Tuning of the relative rates of nucleation and crystal growth is crucial to achieve control over the final film morphology. For the FAPbI₃ system, smooth and uniform perovskite films were obtained over a large area by simultaneously applying a gas-assisted deposition method and adding HI solution in the perovskite precursor solution. Optimization of fabrication process resulted in a solar cell with a best performance of 12.0%. For MAPbBr₃ system, planar structure photovoltaic devices using these MAPbBr3 films achieved a PCE of 0.5%. The device performance is further increased to 2.2% by deposition of a mesoporous TiO₂ layer on top of the dense TiO2 blocking layer. For MASnI₃ system, application of a precursor solution combining MASnI₃ with MAPbI3 in a molar ratio of 1:1, and using the gas-assisted deposition method, smooth MASn₀.₅Pb₀.₅I₃ films over large area were obtained. In summary, high performance hybrid thin film solar cells have been developed by engineering the device architecture and employing an alkylammonium metal halide perovskite material as light absorber. The studies presented herein highlight the potential of hybrid mesoscopic thin film solar cells to become a promising photovoltaic technology.

In this study, a method combining room temperature pulsed laser deposition (PLD) and direct pulsed laser recrystallization (DPLR) are introduced to deposit superior transparent conductive oxide (TCO) layer on low melting point flexible substrates. As an indispensable component of thin film solar cell, TCO layer with a higher quality will improve the overall performance of solar cells. Alumina-doped zinc oxide (AZO), as one of the most promising TCO candidates, has now been widely used in solar cells. However, to achieve optimal electrical and optical properties of AZO on low melting point flexible substrate is challenging. Recently developed direct pulsed laser recrystallization (DPLR) technique is a scalable, economic and fast process for point defects elimination and recrystallization at room temperature. It features selective processing by only heating up the TCO thin film and preserve the underlying substrate at low temperature. In this study, 250 nm AZO thin film is pre-deposited by pulsed laser deposition (PLD) on flexible and rigid substrates. Then DPLR is introduced to achieve a uniform TCO layer on low melting point flexible substrates, i.e. commercialized Kapton polyimide film and micron-thick Al-foil. Both finite element analysis (FEA) simulation and designed experiments are carried out to demonstrate that DPLR is promising in manufacturing high quality AZO layers without any damage to the underlying flexible substrates. Under appropriate experiment conditions, such as 248 nm in laser wavelength, 25 ns in laser pulse duration, 15 laser pulses at laser fluence of 25 mJ/cm2, desired temperature would result in the AZO thin film and activate the grain growth and recrystallization. Besides laser conditions, the thermal conductivity and crystallinity of the substrate serve as additional factors in the DPLR process. It is found that the substrate’s thermal conductivity correlates positively with the AZO crystal size; the substrate’s crystallinity correlates positively with the AZO film’s crystallinity. The thermal expansion of substrate would also contribute to the film tensile stress after processed by DPLR technique. The overall results indicate that DPLR technique is useful and scalable for flexible solar cell manufacturing.

Halide Perovskites – Optoelectronic and Structural Characterization Methods