Remote plasma chemical vapor deposition for high efficiency heterojunction solar cells on low cost, ultra-thin, semiconductor-on-metal substrates

textIn the crystalline Si solar cell industry, there is a push to reduce module cost through a combination of thinner substrates and increased cell efficiency. Achieving solar cells with sub-100 µm substrates cost-effectively is a formidable task because such thin substrates impose stringent handling requirements and thermal budget due to their flexibility, ease of breakage, and low yield. Moreover, as the substrate thickness decreases the surface passivation quality dictates the performance of the cells. Crystalline Si heterojunction (HJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) have attracted significant interest in recent years due to their excellent surface passivation properties, potential for high efficiency, low thermal budget and low cost. HJ cells with ultra-passivated surfaces showing > 700 mV open-circuit voltages (Voc) and > 20% conversion efficiency have been demonstrated. In these cells, it has been identified that high-quality a-Si:H films deposited by a low-damage plasma process is key to achieving such high cell performance. However, the options for low-damage plasma deposition process are limited. The main objectives of this work are to develop a low-plasma damage a-Si:H thin film deposition process based on remote plasma chemical vapor deposition (RPCVD) and to demonstrate high efficiency HJ solar cells on bulk substrates as well as on ultra-thin silicon and germanium substrates obtained by a novel, low-cost semiconductor-on-metal (SOM) technology. This manuscript presents a detailed description of the RPCVD system and the process leading to the realization of high quality a-Si:H thin films and high efficiency HJ solar cells. First, p-type a-Si:H thin films are developed and optimized, then HJ solar cells are subsequently fabricated on bulk and ultra-thin Si and Ge SOM substrates without intrinsic a-Si:H passivation. Single HJ cells on ~ 500 µm bulk Si and ~25 µm ultra-thin substrates exhibited conversion efficiencies of η = 16% (Voc = 615 mV, Jsc = 34 mA/cm2, and FF = 77%) and η = 11.2% (Voc = 605 mV, Jsc = 29.6 mA/cm2, and FF = 62.8%), respectively. The performance of the ~25 µm cell was further improved to η = 13.4% (Voc = 645 mV, Jsc = 31.4 mA/cm2, and FF = 66.2%) by implementing the dual HJ architecture without front side i-layer passivation. For single HJ cells based on Ge substrates, the results were η = 1.78 % (Voc = 148 mV, Jsc = 35.1 mA/cm2, and FF = 1.78%) on ~500 µm bulk Ge, compared to η =5.3% (Voc = 203 mV, Jsc = 44.7 mA/cm2, and FF = 5.28%) on ~ 50 µm Ge SOM substrates. Respectively, the results obtained on ultra-thin SOM substrates are among the highest reported in literature for based on comparable architecture and substrate thickness. In order to achieve improved cell performance, dual HJ cells with i-layer passivation of both surfaces were fabricated. First, optimized RPCVD-based i-layer films were developed by varying the deposition temperature and H2 dilution ratio (R). It was found that excellent surface passivation on planar substrates with as-deposited minority carrier lifetimes > 1 ms is achievable by using deposition temperature of 200 ºC and moderate dilution ratio 0.5 ≤ R ≤ 1, even without the more rigorous RCA pre-cleaning process typically used in literature for achieving comparable results. Subsequently, dual HJ solar cells with i-layer films were demonstrated on planar and textured bulk Si substrates showing improved conversion efficiencies of η = 17.3% (Voc = 664 mV, Jsc = 34.34 mA/cm2 and FF = 76%) and η = 19.4% (Voc = 643 mV, Jsc = 38.99 mA/cm2, and FF = 77.5%), respectively.Electrical and Computer Engineerin

The association of long working hours and the use of prescription sedatives among U.S. workers.

BACKGROUND: Meeting the needs of a round-the-clock and globalized society has led to an increase in long working hours. This trend has been accompanied by a corresponding rise in sleep disorders and subsequent use of sedating medications. Overtime hours have been associated with adverse health outcomes such as cardiovascular diseases, symptoms of psychological distress, and health behaviors, including risky intake of alcohol and smoking. Hence, the main objectives of this three-paper dissertation were to examine the multi-faceted relationship between working hours, the use of prescription sleep aids, the onset of psychological distress, and the use of health care services. METHODS: The 2010-2019 Medical Expenditure Panel Survey (MEPS) data was utilized. The first paper investigated the relationship between working hours and the use of prescribed sedating medications. Different regression models were employed, ranging from multivariable linear regression, Tobit regression, Heckman regression, and multivariable logistic regression. The second paper utilized a fixed-effect linear model in evaluating the relationship between working hours and the onset of psychological distress. The third paper also employed fixed-effect estimators in modeling the association between working hours, health care utilization, and the use of sedating medications. FINDINGS: Long working hours were associated with increased odds of using sleep aids and medications with sedative properties. Females had a higher likelihood of using sleep aids when compared to males. Also, professional services had the highest likelihood of using sleep medications. Over time hours was associated with the onset of psychological distress, with differences in the risk of onset across gender. Respondents working very long hours had the highest odds of using outpatient medical services. This association between very long hours and the use of outpatient services was significantly more pronounced in respondents using medications with sedating side effects. CONCLUSIONS: Long working hours were associated with an elevated risk of using sedating medications, developing psychological distress, and healthcare utilization. This highlights the probable negative impact of overtime hours on the health status of individuals. Implementing policies that encourage work-life balance and aid interventions that decrease work-related stress may help in mitigating risks associated with long work hours

Magic composite pulses

I describe composite pulses during which the average dipolar interactions within a spin ensemble are controlled while realizing a global rotation. The construction method used is based on the average Hamiltonian theory and rely on the geometrical properties of the spin-spin dipolar interaction only. I present several such composite pulses robust against standard experimental defects in NRM: static or radio-frequency field miscalibration, fields inhomogeneities. Numerical simulations show that the magic sandwich pulse sequence, a pulse sequence that reverse the average dipolar field while applied, is plagued by defects originating from its short initial and final \pi/2 radio-frequency pulses. Using the magic composite pulses instead of \pi/2 pulses improves the magic sandwich effect. A numerical test using a classical description of NMR allows to check the validity of the magic composite pulses and estimate their efficiency.Comment: 22 pages, 6 figure

The Use and Abuse of Police Powers and Extrajudicial Killings in Nigeria

The paper examines the use and abuse of police powers in Nigeria, with emphasis on extrajudicial killing. It argues that extrajudicial killing is the most serious abuse of power and violation of victims’ rights to life and fair hearing. Using four typical case studies of live events from Nigerian newspapers, the paper shows that the police in Nigeria not only adopt the doctrine of maximum force in crime control, they often kill suspects and non-suspects at the flimsiest excuse because of an institutionalized culture of impunity. This culture of impunity is derived largely from Government’s authorization of a policy of shoot-to-kill, when in confrontation with armed robbers and other dangerous criminal

ISM studies of GRB 030329 with high resolution spectroscopy

We present a series of early UVES/VLT high resolution spectra of the afterglow of GRB 030329 at redshift z=0.16867+-0.00001. In contrast to other spectra from this burst, both emission and absorption lines were detected. None of them showed any temporal evolution. From the emission lines, we determine the properties of the host galaxy which has a star formation rate (SFR) of 0.198 M_solar yr^-1 and a low metallicity of 1/7 Z_solar. Given the low total stellar host mass M_star=10^7.75+-0.15 M_solar and an absolute luminosity m_V=-16.37, we derive specific SFRs (SSFR) of log SFR/M = -8.5 yr^-1 and SFR/L = 14.1 M_solar yr^-1 L_*^-1. This fits well into the picture of GRB hosts as being low mass, low metallicity, actively star forming galaxies. The MgII and MgI absorption lines from the host show multiple narrow (Doppler width b=5-10 km/s) components spanning a range of v about 260 km/s, mainly blueshifted compared to the redshift from the emission lines. These components are likely probing outflowing material of the host galaxy, which could arise from former galactic superwinds, driven by supernovae from star forming regions. Similar features have been observed in QSO spectra. The outflowing material is mainly neutral with high column densities of log N(MgII)=14.0+-0.1 cm^-2 and log N(MgI)=12.3+-0.1 cm^-2.Comment: 11 pages, 4 figures, submitted to Ap

Enterprise Systems Network: SecurID Solutions, the Authentication to Global Security Systems

Enterprise systems need reliable, flexible and secure means for making public and confidential information available to users in a secured and trusted manner. Although enterprise systems have variety of choice to authenticate these users, organizations face significant issues when granting access and providing a manageable structure for valuable access control. Logon functionalities such as user name and password algorithm have been used to grant authentication and authorization into enterprise systems network resources. Since most systems clients prefer the ease of using passwords, and since passwords are easily compromised, the urgency for a stronger authentication process becomes paramount. This study performed an internal evaluation of enterprise systems such as rating the effectiveness of a security platform as well as an external evaluation; i.e., analyzing how a security system is been rated by external entities. The study will examine correlations between system security best practices and reported or observed outcomes. The study concluded by evaluating the use of added protective layers to the two or multi-factor authentication security system

Effect of Virtualization on Enterprise Network, Server/Desktop Systems on Small and Mid-Size Businesses (SMB)

Enterprise small and mid-size businesses (SMB) are embracing virtualization because of the need to reduce risks associated to IT outages and data loss. Most of these establishments have loss critical enterprise data due to systems failures, accidents or natural causes. Virtualization platforms increase application availability which can shorten disaster recovery time and improve SMBs business continuity preparedness. This study will explore these benefits to find critical issues that can enable SMBs to maintain competiveness by utilizing less to do more

Enterprise Integrated Security Platform: A Comparison of Remote Access And Extranet Virtual Private Networks

The Internet has created unprecedented opportunities for both organizations and individuals. However, these opportunities also have created a double-edge sword as organizations attempt to connect trading partners, customers, and remote users while providing adequate security measures that are flexible and cost-effective. This paper explores why secured socket layer (SSL) may be better tool for secured remote access and extranets by comparing it to internet protocol security virtual private networks (IPSec-based VPNs)

Social Networking Sites As Tools For Sexual Perversion Among Students Of University Of Nigeria, Nsukka

The use of online social networking tools like blackberry messenger, face book, 2go, Nimbuzz, yahoo messenger, etc, continues to increase rapidly among all age groups and segments of the society, presenting new opportunities for the exchange of sexual information  as well as for potentially unsafe encounters. This study using the survey research design and hinged on questionnaires and personal interview as instruments, explored the influence of social networking sites as tools for sexual perversion among students of University of Nigeria, Nsukka (UNN). Anchored on the technological  determinism and uses and gratifications theories, the study found out that social networking tools  actually contribute to UNN students sexual perversion and that there is an increasing adoption of the use of these tools, especially black berry messenger, 2go and facebook in that order. The study also discovered that majority of the students do not consider contents on these sites as being offensive. The study recommended that the use of SNSs should be regulated and regulators should device ways of detecting, exposing and punishing anybody found exchanging sexually explicit or implicit content or engaging in online sex. Keywords: Sexting, sexual perversion, social networking sites, Online- Offline contact, Nigeria