Nano-structures and materials for wafer- scale solar cells

This thesis addresses two of the main materials for solar cells, namely silicon and the family of halide perovskites. For silicon, light trapping structures are investigated for solar cell applications while perovskite materials are investigated as a gain material for optoelectronic applications. Light trapping allows the capture of photons that might otherwise be lost, especially at the red end of the spectrum where silicon is less absorptive. The key is to enhance the efficiency of silicon cells by thinning down the wafer and reducing the bulk recombination losses and to achieve a higher Voc while maintaining strong light absorption (represented by a high short circuit current, Jsc) by applying efficient light trapping schemes. It is still an open question whether nanostructures are beneficial for real devices, especially since highly efficient solar cells employ >100 μm thick absorber materials and use wet etched micron-sized pyramids for light trapping. In this thesis, I conduct a study which compares nanostructures and pyramid microstructures on wafer-based silicon solar cells. This study is important because (1) most light trapping nanostructures are investigated only in the optical regime, while I realize them on silicon devices to analyze both their optical and their electrical character; (2) nanostructures perform better than microstructures in wafer based silicon solar cells, highlighting the effectivity of nanostructures even in wafer-based silicon. Here, the nanostructures comprise wet and dry-etched quasi-random structres and they are compared with pyramidal microstructures. A photocurrent as high as 38 mA/cm2 for a dry etched quasirandom nanostructure is attained experimentally, which is 3.2 mA/cm2 higher than wet etched pyramids fabricated in the same batch. The other material which is now becoming very popular in the solar cell community is the family of metal halide perovskite materials that are increasingly attracting the attention of optoelectronics researchers, both for solar cell and for light emission applications. The ultimate is in simplicity, however, is to observe lasing from a continuous thin film, which has not been aimed before. Here, I show perovskite random lasers; they are deposited at room temperature on unpatterned glass substrates and they exhibit a minimum threshold value of 10 μJ/cm2. A rather special feature is that some of the films exhibit single and dual mode lasing action, which is rarely observed in random lasers. This work fully exploits the simplicity of the solution-based process and thereby adds an important capability to the emerging field of perovskite-based light emitters

Random lasing in uniform perovskite thin films

Following the very promising results obtained by the solar cell community, metal halide perovskite materials are increasingly attracting the attention of other optoelectronics researchers, especially for light emission applications. Lasing with both engineered and self-assembled resonator structures, such as microcrystal networks, has now been successfully observed, with the low cost and the simple solution-based process being a particular attraction. The ultimate in simplicity, however, would be to observe lasing from a continuous thin film, which has not been reported yet. Here, we show random lasing action from such a simple perovskite layer. Our lasers work at room temperature; they are deposited on unpatterned glass substrates and they exhibit a minimum threshold value of 10 µJ/cm2. By carefully controlling the solution processing conditions, we can determine whether random lasing occurs or not, using identical precursors. A rather special feature is that some of the films exhibit single and dual mode lasing action, which is rarely observed in random lasers. Our work fully exploits the simplicity of the solution-based process and thereby adds an important capability into the emerging field of perovskite-based light emitters

Controlled Morphology and Its Effects on the Thermoelectric Properties of SnSe2 Thin Films

In the last few years, the thermoelectric properties of tin selenide (SnSe) have been explored in much detail due to its high efficiency and green nature, being free of Te and Pb. In the same chalcogenide family, SnSe2 is also a layered structured material, but its thermoelectric potential has not been widely explored experimentally. Since SnSe2 has the layered structure, its electrical transport properties may strongly be affected by its microstructure and morphology. Here, we report the effect of reaction time on the structure, phase, and morphology of the SnSe2 during solvothermal synthesis process. We have studied four SnSe2 samples with different reaction times. The sample obtained after 16 h of reaction time was named as M1, for 20 h M2, similarly for 24 h was M3 and for 48 hours’ time, the sample was named as M4. We investigated its thermoelectric properties and found that phase purity and morphology can affect the thermoelectric performance of the synthesized samples. The peak power factor (PF) value along the ab plane was (0.69 μWcm−1K−2) for the M4 sample at 575 K, which was the highest among all the measured samples. The comparatively larger PF value of sample M4 can be related to the increase in its electrical conductivity due to increase in phase purity and band gap reduction

Interplay between Optical and Electrical Properties of Nanostructured Surfaces in Crystalline Silicon Solar Cells

Light trapping has now been recognized as an essential element of highly efficient solar cells. A large number of sophisticated nanostructures have been developed and optically characterized, many of which have been aimed at thin-film silicon technology. It is still an open question whether such nanostructures are beneficial for thick devices, however, especially, since highly efficient solar cells employ >100 μm thick absorber materials and wet etched micron-sized pyramids for light trapping. In this paper, we study and compare the optical and electrical performances of binary quasirandom nanostructures with pyramidal structures to address this question. We show that, while simulations indicate that pyramids have better optical performance, the best overall performance observed experimentally was achieved with binary nanostructures. We found that the experimental short-circuit current for a solar cell patterned with a quasirandom nanostructure is 3.2 mA/cm2 higher than the current observed with pyramids. We attribute this higher current to a better balance between optical performance and surface recombination achieved by the binary nanostructures. This result indicates that binary nanostructures may be beneficial even for thick solar cells

A Clustered PD-NOMA in an Ultra-Dense Heterogeneous Network with Improved System Capacity and Throughput

In the current era of exponentially growing demand for user connectivity, spectral efficiency (SE), and high throughput, the performance goals have become even more challenging in ultra-dense 5G networks. The conventional orthogonal frequency division multiple access (OFDMA) tech-niques are mature but have not proven sufficient to address the growing user demand for high data rates and increased capacity. Therefore, to achieve an improved throughput in an ultra-dense 5G network with an expanded network capacity, the unified non-orthogonal multiple access (NOMA) technique is considered to be a more promising and effective solution. Throughput can be im-proved by implementing PD-NOMA, as the interference is managed with the successive inter-ference cancellation (SIC) technique, but the issue of increased complexity and capacity with compromised data rate persists. This study implements the clustered PD-NOMA algorithm to enhance user association and network performance by managing the users in clusters with fewer users per cluster with the implementation of the cooperative PD-NOMA within the clusters. In this study, we enhanced the user association in a network and ultimately improved the throughput, sum rate, and system capacity in an ultra-dense heterogeneous network (HetNet). By imple-menting the proposed clustered PD-NOMA scheme, the system throughput has improved by 23% when compared to the unified PD-NOMA scheme and 65% when compared to the OFDMA scheme with a varied number of randomly deployed users, along with an improvement in system capacity of 8% as compared to the unified PD-NOMA and almost 80% as compared to the conventional OFDMA scheme in a randomly deployed ultra-dense multi-tier heterogeneous network. Thus, we improved the network performance with the proposed algorithm and achieved increased capacity, throughput, and sum rate by outperforming the unified PD-NOMA scheme in an ultra-dense heterogeneous network

Optical Optimization of Tandem Solar Cells: A Systematic Review for Enhanced Power Conversion

Tandem solar cells (TSCs) perform a better adaptation of the incident photons in different-energy-level bandgap materials, and overcome the Shockley–Queisser limit, but they require advanced control over the management of light for optimum performance. Nanomaterials and nanostructures offer a vastly improved control over the management of light. Through different optimization techniques, researchers can gain valuable insights regarding the optimization of various parameters of nano-optical designs. Over the past years, the number of studies on this topic has been continuously increasing. The present study reviews various current state-of-the-art optical designs, and provides an overview of the optimization techniques and numerical modeling of TSCs. This paper collected and analyzed different studies published within the years 2015–2022, using systematic literature review techniques, such as specific protocol screening and a search strategy. Seven different optical designs were extracted, along with their advanced local and global optimization methods, which offer a solution to the optical limitations of TSCs

Clustering Approaches for Efficient Radio Resource Management in Heterogeneous Networks

5G telecommunication industry promises to manage and accomplish the massive data traffic and growing network requirement complexities in heterogeneous networks (HetNets). HetNets are K-tier networks and are expected to be seamlessly connected networks with robust services for users anywhere at any time. In near future, the significance of 5G/B5G cellular networks; in both indoor and outdoor environments will be greater than before and it would add up to an exhaustive level. However, as a result of the increased density of networks, a rise in interference within these ultra-dense networks (UDN) will have an alarming impact on throughput, interference and latency.  To ensure high throughput with reduced interference in UDNs a clustered architecture is required. A HetNet with clustered approach enables the network to mitigate interference effectively and achieve efficient radio resource management (RRM). In this paper, we analyzed different clustering classifications and existing clustering techniques that are used for proficient radio resource management. The centralized clustering techniques and decentralized clustering techniques are analyzed and as a result, it is assumed that improved performance can be achieved by emphasizing on hybrid clustering approaches. In addition to this, performed a thoughtful review of existing hybrid clustering techniques to achieve improved throughput and mitigate interference in dense heterogeneous networks.  Our analysis shows that improved radio resource management and increased throughput in HetNets is achieved by applying hybrid clustering techniques with reduced inter and intra tier interference.

The Potential Effect of Annealing Mesostructured Titanium Dioxide Electrode in a Closed Box Furnace on the Concentration of Lead (II) Iodide Solution Required for Optimal Performance of Mesoscopic Perovskite Solar Cells

Highly reproducible mesoscopic perovskite solar cells (PSCs) can be fabricated using two-step sequential deposition of organo-lead halide (perovskite) active layer. However, differences in the processing conditions of individual layers which are subsequently assembled to construct the ultimate device can result in variations in the solar cell performance. For instance, here we report trends in the device performance as a function of PbI2 solution concentration, where the compact and mesoporous TiO2 layers were annealed in a closed box furnace (instead of doing it in open air). We observed that the devices prepared using 1.2 M PbI2 solution concentration performed better than those prepared from 0.8 M and 1 M PbI2 solutions. Generally, the researchers use the hot plate in an open-air environment or use a special hot plate where a continuous flow of air is ensured while annealing TiO2 electron selective layers (ESL) for perovskite solar cells. In this case, the highest possible device efficiencies are achieved using 1 M concentration of PbI2 solution. Although the influence of PbI2 solution concentration has been previously studied in detail, here our prime focus is to briefly comment on slight differences in the device performance trends which we observed in comparison to the previously reported results, where TiO2 layers were calcined in open air