Effects of Zn Diffusion from (Zn,Mg)O Buffer to CIGS Film on the Performance of Cd-Free Cu(In,Ga)Se₂ Solar Cells

Lee, Chang-Soo,Kim, Suncheul,Al-Ammar, Essam A.,Kwon, HyuckSang,Ahn, Byung Tae The Electrochemical Society 2014 ECS journal of solid state science and technology Vol.3 No.6

<P>A diffusion of Zn into the CIGS film and its effects on the device performance were investigated when the atomic layer deposited (Zn,Mg)O film was used as a buffer layer of CIGS solar cells. SIMS and ICP mass analysis showed that the Zn concentration in the CIGS film was increased with increasing ALD process time. The hole concentration in the CIGS film was decreased by Zn incorporation. It was suggested that open circuit voltage degradation in CIGS cell with Zn incorporation was mainly due to enhanced recombination by Zn defects instead of Fermi-level change. The results suggest that the ALD process should be as short as possible to avoid Zn diffusion. On the other hand, a small amount of Zn incorporation on the surface of the CIGS film by post rapid thermal annealing of a (Zn,Mg)O/CIGS cell at 200°C showed an increased cell performance. However, excess Zn incorporation on the surface of the CIGS film caused degradation of cell performance due to the generation of a new defect. The generation of a deep-level defect and the change of the minority carrier lifetime were characterized by low-temperature PL and time-resolved PL, respectively. The cell performance was correlated with these results.</P>

Synchronous Reference Frame Theory Based Intelligent Controller for Current THD Reduction

Al-Ammar Essam A.,Ul Haq Azhar,Iqbal Ahsan,Ko Wonsuk,Jalal Marium,Anjum Muhammad Almas,Choi Hyeong-Jin,Kang Hyun-Koo 대한전기학회 2021 Journal of Electrical Engineering & Technology Vol.16 No.6

Concerns related to current harmonics have gained increased attention in recent years due to technological advancement in power electronics and the proliferation of non-linear loads in power distribution system. It has become challenging to limit current total harmonic distortion (THD) at consumer end mainly due to increased penetration of intermittent switching based non-linear loads. Shunt active power fi lter (SAPF) is considered a marvelous power electronics device to tackle consumer side current harmonics. This paper presents a synchronous reference frame (SRF) based improved control method for compliant working of SAPF to reduce current harmonics in the distribution network with same and diff erent THD levels in three phases. This research work investigates an eff ectual DC link voltage control technique linked with SRF based control of SAPF to eff ectively mitigate current harmonics. A fuzzy logic-based automatic switch (FLBAS) is designed according to the THD standards of IET and IEEE for real-time controlling of SRF based control of SAPF. In addition, S-plane stability analysis of the proposed control scheme is performed using the model of SAPF’s inverter. It is established that the closed-loop system is asymptotically stable with the proposed control scheme. The presented simulation results validate the eff ectiveness of the proposed control technique for SAPF robustly.

Performance improvement in CdTe solar cells by modifying the CdS/CdTe interface with a Cd treatment

윤재호,차은석,안병태,권혁상,Essam A. Al-Ammar 한국물리학회 2014 Current Applied Physics Vol.14 No.4

The performance of n-CdS/p-CdTe solar cells is often degraded under light soaking or thermal stress, even though the technology of CdTe solar cells is close to a commercial level. The Cu diffusion from a Cu back contact to a CdS window layer might degrade the cell’s performance. To prevent the Cu diffusion, a very-thin intrinsic CdTe layer was introduced at the n-CdS/p-CdTe interface by depositing a very-thin Cd metal layer on the CdS film and converting the Cd metal into intrinsic CdTe during p-CdTe deposition at high temperature. By the Cd treatment on CdS surface, pinholes or voids were eliminated at the CdS/CdTe and the intermixing of Te and S at the interface was much suppressed. The depletion width was much increased and the intensity of LTPL peak was increased. The analysis suggested that an intrinsic CdTe interlayer was formed and the surface recombination rate was suppressed by the intrinsic interlayer. As a result, the short circuit current of the CdTe solar cell was significantly increased due the increased current gain in the blue wavelength region. The thermal stability of the CdTe solar cell was also greatly improved and the Cu diffusion was retarded by the intrinsic CdTe interlayer at the n-CdS/p-CdTe.

Characterization of Cu(In,Ga)Se₂ Solar Cells Grown on Na-Free Glass with an NaF Layer on a Mo Film

Shin, Young Min,Lee, Chang Soo,Shin, Dong Hyeop,Ko, Young Min,Al-Ammar, Essam A.,Kwon, Hyuck Sang,Ahn, Byung Tae The Electrochemical Society 2013 ECS journal of solid state science and technology Vol.2 No.6

<P>Cu(In,Ga)Se<SUB>2</SUB> (CIGS) solar cells fabricated on soda-lime glass (SLG) exhibited high efficiency due to the supply of Na from the SLG substrate. As a simple doping method, Na can be supplied into CIGS films using a Na compound deposited on Mo electrodes. In this study, the authors compared the properties of CIGS thin films grown on a NaF layer deposited on Mo-coated Na-free glass with those of CIGS films on Mo-coated SLG by a standard through high-resolution transmission electron microscopy and low-temperature photoluminescence analyzes. After NaF deposition on the Mo film, an amorphous interlayer was detected at the CIGS/MoSe<SUB>2</SUB> interface, and the MoSe<SUB>2</SUB> layer that formed on the Mo surface was thin. The photoluminescence study showed that NaF doping did not effectively prevent the formation of deep donors in the CIGS film, whereas Na supplied by SLG effectively prevents their formation. We concluded that the poor performance of CIGS solar cells incorporating a NaF precursor is due to the amorphous layer at the CIGS/Mo interface and different luminescence characteristic with standard CIGS films, resulting in a low minority carrier collection.</P>

Design of energy band alignment at the Zn_1−<i>x</i>Mg_<i>x</i>O/Cu(In,Ga)Se₂ interface for Cd-free Cu(In,Ga)Se₂ solar cells

Lee, Chang-Soo,Larina, Liudmila,Shin, Young-Min,Al-Ammar, Essam A.,Ahn, Byung Tae The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.14

<P>The electronic band structure at the Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O/Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> interface was investigated for its potential application in Cd-free Cu(In,Ga)Se<SUB>2</SUB> thin film solar cells. Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O thin films with various Mg contents were grown by atomic layer deposition on Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn<SUB>0.9</SUB>Mg<SUB>0.1</SUB>O and Zn<SUB>0.8</SUB>Mg<SUB>0.2</SUB>O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O (<I>x</I> = 0.1 and <I>x</I> = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O films results in the flat (0 eV) and cliff (−0.23 eV) conduction band alignments at the Zn<SUB>0.8</SUB>Mg<SUB>0.2</SUB>O/Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> and Zn<SUB>0.9</SUB>Mg<SUB>0.1</SUB>O/Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> interfaces, respectively. The experimental results suggest that the bandgap energy of Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O films is the main factor that determines the conduction band offset at the Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O/Cu(In<SUB>0.7</SUB>Ga<SUB>0.3</SUB>)Se<SUB>2</SUB> interface. Based on these results, we conclude that a Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn<SUB>1−<I>x</I></SUB>Mg<SUB><I>x</I></SUB>O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.</P> <P>Graphic Abstract</P><P>The electronic structure at the ALD Zn1-xMgxO/Cu(In0.7Ga0.3)Se2 interfaces was investigated by XPS and UPS depth profiles for its potential application in Cu(In,Ga)Se2 thin film solar cells.” <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2cp40355b'> </P>

Jointless Pancake Coil Winding for Minimizing Electrical Loss in HTS SMES for Wind Power

Jaesik Kang,Tae Kuk Ko,Al-Ammar, Essam A.,Kyeon Hur Institute of Electrical and Electronics Engineers 2015 IEEE transactions on applied superconductivity Vol.25 No.3

<P>Many different applications of superconducting devices have been designed using a modular pancake arrangement. It has been reported that this modular pancake structure provides excellent mechanical holding, effective cooling, and high magnetic flux density for stability of the superconducting coil. However, soldering for interconnecting each double pancake causes current decay, cryogenic coolant loss, and thermal stress. Thus, this paper investigates an improved double pancake winding, named the “jointless double pancake coil winding” and demonstrates the efficacy for minimizing the electrical loss of the high-temperature superconducting (HTS) coil, assembled in the modular double pancake types. It presents a winding method that avoids soldering between the pancake connections and thus removes the primary cause of the electrical loss generated in the existing design. This proposed winding method can effectively achieve zero electrical loss. This paper particularly presents the application of the proposed winding for the HTS-SMES and demonstrates the feasibility and improved functional benefits with reference to the conventional coil. This paper should maximize the operational benefits of using the SMES coil employing the proposed winding method in charge and discharge operations and should increase the effectiveness and efficiency in cryogenic systems.</P>

Understanding the light soaking effect of ZnMgO buffer in CIGS solar cells

Kim, Suncheul,Lee, Chang-Soo,Kim, Seungtae,Chalapathy, R. B. V.,Al-Ammar, Essam A.,Ahn, Byung Tae The Royal Society of Chemistry 2015 Physical chemistry chemical physics Vol.17 No.29

<P>This study investigated the mechanism underlying the light soaking effect of a ZnMgO buffer in Cu(In,Ga)Se<SUB>2</SUB> (CIGS) solar cells, where the cell efficiency increased with an increase of light soaking time. The ZnMgO buffer layer was deposited by an atomic layer deposition method. With light soaking, the cell efficiency of ZnMgO/CIGS cells increased mainly by the increase of the fill factor and partly by the increase of the open-circuit voltage. With light soaking, the electron carrier concentration of the ZnMgO layer increased and the XPS intensity of the hydroxyl bond in the ZnMgO layer decreased. Based on the above results and the comparison of other buffers in literature, we assumed that the hydrogen atoms broken away from the hydroxyl bond by photon irradiation occupied the interstitial sites of the ZnMgO layer as a donor atom and also passivated the defects at the ZnMgO/CIGS interface. The increase of the fill factor and open circuit voltage was explained based on H doping in the ZnMgO layer and H passivation at the ZnO/CIGS interface.</P> <P>Graphic Abstract</P><P>This study investigated the mechanism underlying the light soaking effect of a ZnMgO buffer in Cu(In,Ga)Se<SUB>2</SUB> (CIGS) solar cells, where the cell efficiency increased with an increase of light soaking time. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5cp01758k'> </P>