Advantages of N-Type Hydrogenated Microcrystalline Silicon Oxide Films for Micromorph Silicon Solar Cells

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) in hydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/μc-Si:H) micromorph solar cells. The n μc-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO2) to silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n μc-SiO:H films in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of the films. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top μc-SiO:H layer enhanced with increasing the CO2/SiH4 ratio up to 0.23, where the highest initial cell efficiency of 10.7% was achieved. The enhancement of the open circuit voltage (Voc) was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and a better tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency (QE) results also have demonstrated intermediate-reflector function of the n μc-SiO:H films

Study of an Amorphous Silicon Oxide Buffer Layer for p-Type Microcrystalline Silicon Oxide/n-Type Crystalline Silicon Heterojunction Solar Cells and Their Temperature Dependence

Intrinsic hydrogenated amorphous silicon oxide (i-a-SiO:H) films were used as front and rear buffer layers in crystalline silicon heterojunction (c-Si-HJ) solar cells. The surface passivity and effective lifetime of these i-a-SiO:H films on an n-type silicon wafer were improved by increasing the CO2/SiH4 ratios in the films. Using i-a-SiO:H as the front and rear buffer layers in c-Si-HJ solar cells was investigated. The front i-a-SiO:H buffer layer thickness and the CO2/SiH4 ratio influenced the open-circuit voltage (Voc), fill factor (FF), and temperature coefficient (TC) of the c-Si-HJ solar cells. The highest total area efficiency obtained was 18.5% (Voc=700 mV, Jsc=33.5 mA/cm2, and FF=0.79). The TC normalized for this c-Si-HJ solar cell efficiency was −0.301%/°C

Effect of the CO 2

This paper reports the preparation of wide gap p-type hydrogenated microcrystalline silicon oxide (p-μc-SiO:H) films using a 40 MHz very high frequency plasma enhanced chemical vapor deposition technique. The reported work focused on the effects of the CO2/SiH4 ratio on the properties of p-μc-SiO:H films and the effectiveness of the films as an emitter layer of crystalline silicon heterojunction (c-Si-HJ) solar cells. A p-μc-SiO:H film with a wide optical band gap (E04), 2.1 eV, can be obtained by increasing the CO2/SiH4 ratio; however, the tradeoff between E04 and dark conductivity must be considered. The CO2/SiH4 ratio of the p-μc-SiO:H emitter layer also significantly affects the performance of the solar cells. Compared to the cell using p-μc-Si:H (CO2/SiH4 = 0), the cell with the p-μc-SiO:H emitter layer performs more efficiently. We have achieved the highest efficiency of 18.3% with an open-circuit voltage (Voc) of 692 mV from the cell using the p-μc-SiO:H layer. The enhancement in the Voc and the efficiency of the solar cells verified the potential of the p-μc-SiO:H films for use as the emitter layer in c-Si-HJ solar cells

Analysis of initial stabilization of cell efficiency in amorphous silicon photovoltaic modules under real outdoor conditions

[EN] This contribution presents a field study in which the initial stabilization of thin-film amorphous silicon (a-Si:H) is investigated. Two grid-connected a-Si:H photovoltaic plants have been monitored and analyzed under real outdoor conditions. A per-unit approach is proposed to compare PV plants with differences in their electrical characteristic and the start-up date. The representation of a normalized per unit PV power versus the accumulated incoming irradiation reveals an evolution that can be characterized through an exposure-response function. By this function, two populations of defects in the cells are detected. It is found that the stabilization process in the first year of operation produces a decrease of 10% in the peak power, equivalent to a decrease of 0.5% in cell efficiency. The use of the accumulated PSH for conducting the analysis of the initial stabilization produces similarities that cannot be obtained if a time scale is used. These results provide a powerful tool for PV plant designers because they enable a prediction to be made of the time-scale stabilization response in terms of unitary power, correlated with the peak sun hours received. (C) 2017 Elsevier Ltd. All rights reserved.This work was supported by Generalitat Valenciana (PROM-ETEOII/2014/059) and Spanish MINECO (Ministry of Economy and Competitiveness TEC2014-53727-C2-1-R).Mateo-Guerrero, C.; Hernández Fenollosa, MDLÁ.; Montero Reguera, ÁE.; Segui-Chilet, S. (2018). Analysis of initial stabilization of cell efficiency in amorphous silicon photovoltaic modules under real outdoor conditions. Renewable Energy. 120:114-125. https://doi.org/10.1016/j.renene.2017.12.054S11412512

Resource allocation in cognitive radio networks

© 2012 Dr. Athipat LimmaneeThis thesis focuses on optimal power allocation problems for various types of spectrum-sharing based cognitive radio networks in the presence of delay-sensitive primary links. To guarantee the quality of service in the delay-sensitive primary network, primary user’s outage probability constraint (POC) is imposed such that the transmission outage probability of each primary user is confined under the predefined threshold. We first consider a cognitive radio network consisting of a secondary user (SU) equipped with orthogonal frequency-division multiplexing (OFDM) technology able to access N randomly fading frequency bands for transmitting delay-insensitive as well as delay-sensitive traffic. Each band is licensed to an individual single-antenna and delay-sensitive primary user (PU) whose quality of service is assured by a POC. Assuming full channel state information (CSI) is available at the secondary network, we solve the SU’s ergodic capacity maximization problem subject to SU’s average transmit power, SU’s outage probability constraints (SOC) and all POCs by using a rigorous probabilistic power allocation technique. A suboptimal power control policy is also proposed to reduce the high computational complexity when N is large. Next, we study cognitive broadcast channels with a single-antenna secondary base station (SBS) and M single-antenna secondary receivers (SRs) sharing the same spectrum band with one single-antenna and delay-sensitive PU. The SBS aims to maximize the ergodic sum downlink throughput to all M SRs subject to a POC and a transmit power constraint at the SBS. With full CSI available at the secondary network, the optimal solution reveals that at each timeslot SBS will choose the SR with the highest direct channel power gain and allocate the timeslot to that user. The opportunistic scheduling aspect from the optimality condition allows us to further analyze the downlink throughput scaling behavior in Rayleigh fading channel as M grows large. We then examine a cognitive multiple-access channels with a single-antenna SBS and M single-antenna secondary transmitters sharing the same spectrum band with a single-antenna and delay-sensitive PU. Under an average transmit power constraint in each secondary transmitters and a POC at the primary link, we characterize the ergodic capacity region and two outage capacity regions, i.e. common outage capacity region and individual outage capacity region, in the secondary uplink network by exploiting the polymatroid structure of the problems. Also, the derivation of the associated optimal power allocation schemes are provided. The optimal solutions for the problems demonstrate that successive decoding is optimal and the decoding order can be solved explicitly as a function of joint channel state. Finally, we investigate a transmit power allocation problem for minimizing outage probability of a single-antenna SU subject to a POC at a delay-sensitive and single-antenna PU and an average transmit power constraint at the SU, providing that the SU has quantized channel side information via B-bit feedback from the band manager. By using nearest neighbourhood condition, we can derive the optimal channel partition structure for the vector channel space, making Karush-Kuhn-Tucker condition applicable as a necessary condition for finding a locally optimal solution. We also propose another low-complexity suboptimal algorithm. Numerical results show that the SU’s outage probability performance from the suboptimal algorithm approaches the SU’s outage probability performance in the locally-optimal algorithm as the number of feedback bits, B, increases. Besides, we include the asymptotic analysis on the SU’s outage probability when B is large

Optimal Power Policy and Throughput Analysis in Cognitive Broadcast Networks under Primary’s Outage Constraint

Abstract—This paper focuses on a spectrum-sharing based cognitive radio fading broadcast channel (BC) with a singleantenna secondary base station (SBS) and M secondary receivers (SRs) concurrently utilizing the same spectrum band with one delay-sensitive primary user (PU). The quality-of-service requirement in the primary network is given by the primary user’s outage probability constraint (POC). We address the optimal power allocation problem for the ergodic sum capacity (ESC) maximization in the secondary BC network subject to a POC and an average transmit power constraint at SBS. Optimality conditions reveal that in each fading block SBS will choose only one SR with the highest channel power gain and allocate the block to that user. Furthermore, if PU’s power strategy is assumed to be ON-OFF with constant power when ON, the secondary network throughput scaling for large M in Rayleigh fading is also investigated. It is shown that the secondary sum throughput in Rayleigh fading BC scales like O(log(log M)). Numerical results support the theoretical results derived in the paper. I

Distortion minimization via multiple sensors under energy harvesting constraints

We consider a wireless sensor network equipped with energy harvesting technology. It contains M sensors that observe a random process and transmit an amplified uncoded analog version of the observed signal through fading wireless channels to a remote station. The remote station, often called the fusion center, estimates the realization of the random process by using a best linear unbiased estimator. In this paper, we consider the optimal energy allocation policy that minimizes total distortion over a finite time horizon subject to energy harvesting constraints at the sensors. We focus on two types of available side information at the sensor, i.e. (1) causal side information involving the present and previous channel states and the previous values of the harvested energy and (2) full (non-causal) side information, under both finite and infinite energy storage capacity at each sensor's battery. The derivations and some structural properties of the optimal energy allocation schemes are discussed, and numerical results presented

Optimal power policies and throughput scaling analyses in fading cognitive broadcast channels with primary outage probability constraint

This paper focuses on a spectrum-sharing-based fading cognitive radio broadcast channel (BC) with a single-antenna secondary base station (SBS) and M single-antenna secondary receivers (SRs) utilizing the same spectrum band with a delay-sensitive primary user (PU). The service-quality requirement for the primary user is set by an outage probability constraint (POC). We address the optimal power allocation problem for the SBS ergodic sum capacity (ESC) maximization in the secondary BC network subject to POC and a transmit power constraint at SBS specified by either a long-term or a short-term power constraint. The optimality conditions reveal that in each joint channel state, the SBS allocates transmission power to the only one selected SR with the highest value of a certain metric consisting of the ratio of the SR's direct channel power gain and the sum of interference power and noise power at the SR. Then, the secondary network throughput scaling analysis as the number of SRs becomes large, is also investigated, showing that if PU applies a truncated channel inversion (TCI) power policy, the SBS ESC scales like epsilon(p) log(log M) where epsilon(p) is the PU outage probability threshold. To reduce the amount of channel side information (CSI) transferred between the two networks, we propose a suboptimal transmission scheme which requires only 1-bit feedback from the delay-sensitive PR (partial CSI). We show that the new power control policy is asymptotically optimal, i.e. the SBS ESC under this reduced feedback scheme still scales like epsilon(p) log(log M)

Service-Outage Capacity Maximization in Cognitive Radio

In spectrum sharing based cognitive radio networks, unlicensed users (secondary users) are allowed to communicate over the same frequency band as the licensed users (primary users) as long as the required quality-of-service (QoS) of the licensed users is guaranteed. This paper focuses on a cognitive radio network, where a secondary user (SU) sharing the same frequency band with a primary user (PU) wishes to transmit delay-insensitive as well as delay-sensitive data while the PU is interested in meeting a minimum rate guarantee for delay-sensitive services. Typically, PU's are oblivious to the SU's existence and has its own power policy based on channel side information (CSI) of its direct gain between PU transmitter and PU receiver. Under the assumption that SU knows PU's power policy and CSI of the entire network, we solve the optimal power allocation problem of maximizing SU's ergodic capacity subject to PU's outage probability constraint (POC), SU's outage probability constraint (SOC), and SU's average power constraint. We generalize earlier results which considered either ergodic capacity maximization or outage probability minimization for SU with POC, to the so-called service-outage based capacity optimization for SU with POC. We use a rigorous probabilistic power allocation technique that allows us to derive optimal power policies that are applicable to both continuous and discrete fading channels