الفهرس 
Toward Thin-Film SingleJunction and Tandem Solar CellsOnly 14 pages are availabe for public view
 |  | from | 222 |  |  |
| | | from | 222 | | |
Abstract
Photovoltaic (PV)-based renewable energy technologies play an important role in meeting global electricity demand in the future. To fulfil this, the power conversion efficiency (PCE) of crystalline silicon (c-Si), the market-leading solar technology due to its abundance, non-toxicity, and suitable energy bandgap, must be enhanced. However, the PCE of c-Si solar cells is now close to its limit of 29%, making further advances scientifically problematic. Recently, manufacturing thin-film c-Si cells with wafer’s thickness less than 50 μm has been attempted. These cells offer substantial advantages concerning flexibility, ease of fabrication, and cost-competitiveness, using traditional methods similar to thick c-Si cells. On the other hand, thin-film c-Si devices draw a poorer PCE than the traditional thick c-Si cells. A potential approach to enhance the PCE of a PV cell involves creating a tandem cell configuration, using a wide-bandgap top cell positioned above the thin-film c-Si bottom cell. Given the silicon bandgap of 1.12 eV, the top cell materials require bandgap of (1.7-1.8) eV to current match both top and bottom cells. Suitable wide-bandgap top cells for thin-film c-Si bottom cell that offer low-cost, flexibility, high performance and stability have been a challenge. One of the preferable materials applied as an absorber of the top cell is the III-V semiconductor materials. The III-V solar cells have been successfully implemented via the approach of tandem cells since III-V semiconductors have a proper range of bandgaps and lattice constants to choose from. Over the past years, organic-inorganic metal halide perovskite (PVK) solar cells occupied a remarkable consideration owing to their high PCEs and low-cost processes. Additionally, one attractive property for the perovskite material that makes it a promising material as a top sub-cell is its tunable bandgap (1.48 to 2.3 eV). These properties make perovskites ideal candidates as wide-bandgap material for a tandem cell. Usually, a perovskite thin-film solar cells (TFSC) adopt p-i-n heterojunction configuration by employing carrier transport materials along with the absorber material in order to extract the photogenerated electrons and holes by realizing a built-in electric field. Eventually, this dependency of conventional heterojunction perovskite cells on carrier transport layers (CTLs) results in cost-ineffective cells and increases the possibility of device instability and interface problems. Thus, the design of homojunction perovskite cells is highly desirable as an essential direction of structural innovation to realize efficient solar cell operation.
To provide a literature review, different technologies to construct Si-based tandem solar cells, including the III-V/Si and PVK/Si tandem solar cells, are firstly reported along with their advantages and challenges, illustrating that the PVK/Si is a promising candidate to overcome the limitations of Si single-junction solar cells. Then, we discuss, in detail, some challenges and concerns of homojunction perovskite TFSCs that should be taken into consideration to enhance their PCE.
This thesis focuses on design and simulation of flexible monolithic perovskites/c-Si tandem cells and 4-T III-V/Si tandem cells. As a starting point, a state-of-art self-protected thin-film c-Si solar cell against reverse bias is designed and simulated by introducing a P+ reverse-conducting layer (RCL) as an extra layer just below the emitter, that can be introduced before the formation of the emitter. The study investigates the electrical and optical characteristics of both forward and reverse bias conditions. At forward biasing, the device operates as an efficient solar cell while it operates as a backward diode when applying reverse biasing. Thus, there is no need for bypass diodes or modified protection circuits. The performance of such a novel device is compared with the performance of a normal design without RCL. In addition, improvement techniques are applied to optimize the performance of the proposed device. Further, a thorough investigation of four distinct structures for the n-p homojunction PVK cell is conducted. The four structures include a complete cell, electron transport layer (ETL)-free, hole transport layer (HTL)-free, CTL-free structures. The results show that the CTL-free structure has significant potential after applying certain optimization techniques that result in reducing surface recombination, enhancing the built-in electric field, and improving light absorption. The top homojunction PVK cell that has a bandgap of 1.72 eV, and the bottom sub-cell uses thin c-Si with a bandgap of 1.12 eV are connected via a p++/n++ silicon tunnel diode. With the current-matching condition achieved, the tandem efficiency reaches 36.37%. Finally, the design of III-V/Si tandem solar cells is discussed to obtain the highest possible conversion efficiency using different top absorber materials. The study highlights the effect of top absorber thickness in gathering the light photons. In addition, the impact of the energy bandgap of the top absorber on the performance of the 4-T III-V/Si is investigated. A PCE of 35.1%, 35.6%, and 36.71% are achieved for GaAs/Si, Al(x)Ga(1-x)As/Si, and InGaP/Si 4-T all-thin-film tandem cells, respectively.
All-Thin-film solar cell