Suppressed silver iodide formation through buffer layer and polymer encapsulant to increase stability in the monolithic two-terminal perovskite/c-Si cell with a fully textured heterojunction Si cell
The sun has provided energy and supported the growth of all forms of life on earth since the dawn of time, either through direct or indirect forms. However, the dependence on merely fossil fuels cannot keep up the pace with the ever-growing population, the fast development of society as well as irreversible environmental pollution. To continue utilizing and benefiting from the sun, solar photovoltaic (PV) technology has inevitably become the solution to solve the world’s energy crisis and sustain life on earth for generations to come.
Among various forms of solar PV, crystalline silicon (c-Si) solar cells have dominated the market since its first invention in Bell Labs in 1954, due to its abundance of raw materials, favored bandgap of 1.12 eV, and matured processing. Through many technology advancements, such as surface texturing, antireflection coating, doping, surface passivation and the use of carrier selective contacts, multiple high-efficiency silicon solar cells, such as back surface field (BSF), interdigitated-back contact solar cells (IBC), silicon heterojunction cell (SHJ) have reached power conversion efficiency above 25%, quickly approaching the theoretical Auger efficiency limit for the single-junction c-Si cell of 29.4%.
To further reduce the Levelized cost of electricity (LCOE) and increase efficiency above the single-junction limit, the most promising approach is to stack absorbers with different bandgaps on top of each other creating a tandem solar cell. While conventional multijunction cells using costly III-IV materials and c-Si reached an efficiency of 32.8%, the rapidly emerging perovskite has become a capable alternative as the top cell in a tandem device with c-Si, thanks to its high efficiency, low processing cost, and most importantly, wide tunable bandgap within 1.2 – 2.3 eV.
From a device standpoint, the conventional tandem architectures include mechanically stacked four-terminal tandem, monolithic two-terminal tandem, and four-terminal spectrum splitting tandem. Monolithic two-terminal tandem stands out due to its lower manufacturing cost and reduced absorption. In 2016, a monolithic device with Cs0.17FA0.83Pb(Br0.17I0.83)3 (CFA) as top cell and an SHJ bottom cell reached an efficiency of 23.6%. Later, improvements were made to change the front-side of the SHJ cell from a polished surface to a textured one, which increased light trapping and decreased production cost. This device, however, suffered from low stability (efficiency decreased 10% after 270 hours), which is crucial for any commercial considerations. PTFE encapsulation
Here I propose to suppress silver iodide formation through the incorporation of the buffer layer and polymer encapsulant to increase the stability in a monolithic two-terminal perovskite/c-Si cell with a fully textured heterojunction Si cell
The bottom c-Si cell is chosen as an a-Si/c-Si heterojunction solar cell due to its high open-circuit voltage (Voc) and excellent surface passivation. The fabrication process includes surface texturing of monocrystalline Si using potassium hydroxide (KOH) to produce randomly sized square pyramids, plasma-enhanced chemical vapor deposition (PECVD) of the thin layer of intrinsic hydrogenated a-Si with 5 mm in thickness for passivation. Doped a-Si with boron and phosphorus will then be deposited onto the intrinsic layer as corresponding hole and electron transport layer with a thickness of 10 nm. Transparent conductive oxides (TCOs) is introduced between the top and bottom cell to reduce optical loss using doped hydrogenated nanocrystalline silicon (NC-Si: H), based on a previous study focusing on the tandem device with front side polished bottom cell. A 300-nm-thick silicon nanocrystal layer will then be spray coated through a stainless-steel mesh to increase internal rear reflectance. For the TCOs at the back, indium tin oxide (ITO) will be sputtered to simultaneously act as a good charge collection and an anti-reflection coating.Fig.1 cross-sectional area of proposed two-terminal perovskite/Si tandem cell
To form a two-terminal tandem device, the perovskite layer will be deposited directly on top of the complete bottom cell in an inverted p-i-n structure with an electron transport layer (ETL) of tin oxide (SnO2) and zinc tin oxide (ZTO) as the window layer. This bilayer selection is chosen due to its ability to be deposited under 150 °C through low-temperature atomic layer deposition (ALD) to prevent damage to the underlying layers. For hole transport layer (HTL), nc-Si:H is first deposited on the textured surface, then a 2,2?,7,7?-tetra(N, N-di-tolyl)amino-9,9-spiro-fluorene (spiro-TTB) will be deposited by thermal evaporation, followed by a two-step process consisting of co-evaporating cesium iodide (CsBr) and lead bromide (PbI2) with the proper molar ratio. The spin-coating of formamidinium iodide (FAI) and format aclidinium bromide (FABr) will then be performed.
This improvement of vapor deposition could fabricate thin film on various different textured surfaces, compared to the conventional spin coating. Additionally, 150 nm buffer layers of lithium fluoride are thermally evaporated to reduce shunts and prevent iodine migration. Finally, PTFE (Teflon) is chosen as good encapsulation material due to its excellent hydrophobic and good light transmittance abilities. It will be deposited via initiated chemical vapor deposition (iCVD) according to a previous study. To determine the perovskite film structure and ensure the mass ratio of each species, X-ray diffraction spectra will be measured on a diffractometer. Absorption spectra will be determined by ultraviolet-visible spectroscopy (UV-Vis) to get the bandgap of the film.
In addition, ambient air stability testing under continuous AM 1.5 G illumination will be performed. Furthermore, a damped heat test which is standard for a single-junction perovskite cell will also be tested at a temperature of 85°C and relative humidity of 85%. Absorption data will be collected as a function of time to monitor its degradation. The data will then be compared to the published stability data for CSFA.
To visualize the film deposition of areas of interest, scanning electron microscopy (SEM) and atomic force microscopy (AFM) will be used to determine the surface morphology and confirm the uniform and conform growth of perovskite on the textured pyramids. The optical quality of the device will be determined via the standard J-V curve and compared to the NREL certified champion tandem cell with an efficiency of 23.6%, Voc of 1.65V, Jsc of 18.1mA/cm2, and fill factor of 79%. In addition, the absorbance and external quantum efficiency (EQE) of each sub-cell will be obtained through a lamp excitation source and a monochromator at a specific frequency. The stability of this device will be tested via ambient air test and damp heat degradation test of 1000 hours at 85°C and 85% relative humidity, according to the standard testing in ref. 7. Moreover, the thickness of each layer will be measured by a stylus profilomemter.
The deposition of the perovskite layer should yield a uniform and pinhole-free film, and by allowing the front surface of the bottom cell to be fully textured, we should see a direct optical gain from the light trapping of the pyramids, which could be observed by a higher Voc and Jsc compared to a polished front side. The results will be compared with the published data and the NREL-certified cell. After the incorporation of the buffer layer and PTFE encapsulant, the device should deliver stable performance under the damped heat degradation test with less than 10% efficiency loss after 1000 hours.Fig.2 Total absorbance (1 ? R, where R is the reflectance; dashed grey line), EQE of the perovskite top cell (solid blue line), and EQE of the silicon bottom cell (solid red line)
The main challenge of this proposal is the extra processing step involved will add more cost on top of the tandem structure. In addition, the efficiency is still not very high compared to the single junction SHJ cell. However, we are still currently at the phase of understanding the necessary components to achieve a highly efficient and stable perovskite/Si tandem cell. With that being said, when the technology has matured enough, tandem cells have a potential of achieving efficiency above 30%.
Additional optimization such as replacing 200 ?m evaporated Ag contact with 50 ?m wide screen-printed ones will help increasing short circuit current to further increase efficiency. Some remaining issues with the commercializing perovskite/Si tandem cell include the upscaling of perovskite top cell and reducing processing cost.
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