Point Contacts in Chalcopyrite Solar Cells

Recent advancements in Cu(In,Ga)Se2 based solar cells with alkaline fluoride treatment have suggested the role of the recombination at the absorber/buffer interface as a limiting factor of open-circuit voltage (Voc ) in high-efficiency CIGSe solar cells. Therefore, this work focuses on the interface engineering at the CIGSe/buffer of 2 μm standard absorbers. Furthermore, as the influence of recombination at the back interface becomes noticeable with thin absorbers, the back interface Mo/CIGSe of thinner absorbers is also optimised. Both use the concept of point junctions through a passivation layer (PaL). A low-cost versatile technique for point contact fabrication is developed by using a nanolithographic technique employing a sacrificial template of low temperature synthesised, mono-dispersed, self-assembled and size-tunable CdS nanoparticels (NP’s), and a PaL of aluminium oxide (Al2O3), thereby achieving 60 nm point contact radius and a maximum of 89% PaL coverage on CIGSe. The impact of point contacts at the CIGSe/buffer interface on solar cell performance is theoretically analysed using three-dimensional simulations on the point contact radius, coverage area, defect density and interface quality. An efficient PaL should create positive surface charge, which induces band bending at the CIGSe/PaL and influences the contact junction properties; its beneficial effect on Voc and efficiency reaches a maximum when the coverage area of PaL is more than 95% and the interface charge density is greater than 10^12/cm2 . The point contact technology is experimentally validated by incorporating it into tangible CIGSe devices, featuring CdS and Zn(O,S) buffer layers. A positive impact of +10.4% is seen on the Voc of the point contact devices with sputtered Zn(O,S) compared to the unpassivated reference cells. However the power conversion efficiency (PCE) didn’t follow the same trend, which might be due to an upward bandbending created by Al2O3 at the interface, impeding the current flow. Nonetheless, at the Mo/CIGSe interface of thinner CIGSe absorbers, the technology led to a significant reduction in the surface recombination velocity, due to the back surface field from the Al2O3 layer. Consequently, all cell parameters of point contact devices showed a relative improvement to the unpassivated reference devices: open-circuit voltage (Voc : +21%), short-circuit current (Jsc : +2.6%), fill-factor (FF: +4.9%), and efficiency (η: +31%).Jüngste Fortschritte bei Cu(In,Ga)Se2 -basierten Solarzellen durch eine Alkalimetallfluorid Behandlung haben dazu geführt, dass die adungsträgerrekombination an der Absorber/Puffer-Grenzfläche als limitierender Faktor für die Leerlaufspannung (Voc) in hocheffizienten CIGSe-Solarzellen diskutiert wird. Daher konzentriert sich diese Arbeit auf die Optimierung der Grenzfläche CIGSe/Puffer von Solarzellen mit 2 μm Standard absorbern. Außerdem wird auch der Rückkontakt Mo/CIGSe von Solarzellen mit dünneren Absorbern optimiert. In beiden Fällen wird das Konzept der Punktkontakte durch eine Passivierungsschicht (PaL) verwendet. Eine vielseitige, kostengünstige nanolithografische Technik zur Herstellung von Punktkontakten wird unter Verwendung einer Opferschicht realisiert, die als Schablone eingesetzt wird. Diese besteht aus bei niedrigen Temperaturen synthetisierten, monodispersen und größenabstimmbaren CdS-Nanopartikeln (NP’s) und einem PaL aus Aluminiumoxid (Al2O3). Damit lässt sich ein Punktkontaktradius von 60 nm und eine Oberflächenbedeckung von maximal 89% PaL auf CIGSe erreichen. Der Einfluss von Punktkontakten an der CIGSe/Puffer-Grenzfläche auf die Leistung der Solarzelle wird mittels dreidimensionaler Simulationen unter Berücksichtigung des Punktkontaktradius, der Oberflächenbedeckung, der Defektdichte und der Qualität der Grenzfläche analysiert. Eine effiziente PaL sollte eine positive Oberflächenladung erzeugen, die am CIGSe/PaL eine Bandverbiegung induziert und damit die Eigenschaften der Grenzfläche beeinflusst; ihre positive Wirkung auf Voc und den Wirkungsgrad erreicht ein Maximum, wenn die Oberflächenbedeckung der PaL mehr als 95% beträgt und die Grenzflächenladungsdichte größer als 10^12/cm2 ist. Die Punktkontakttechnologie wird mithilfe von CIGSe-Solarzellen mit CdS- und Zn(O,S)- Pufferschichten experimentell validiert. Eine Steigerung der Voc um 10,4% gegenüber den unpassivierten Referenzzellen wird mit der Punktkontaktsolarzelle erzielt. Allerdings wird keine Erhöhung des gesamten Wirkungsgrads erreicht, was auf eine von Al2O3 erzeugte, nach oben gerichtete Bandverbiegung an der Grenzfläche hindeuten könnte, die den Stromfluss behindert. Am Mo/CIGSe-Rückkontakt von dünneren CIGSe absorbern führt die Einführung der Punktkontakte jedoch zu einer signifika nten Senkung der Oberflächen rekombinationsgeschwindigkeit, bedingt durch das durch Ladungen in der Al2O3 -Schicht induzierte elektrische Feld. Folglich zeigten alle Zellparameter von unktkontaktsolarzellen eine relative Verbesserung gegenüber den unpassivierten Referenzzellen: Leerlaufspannung (Voc : + 21%), Kurzschlussstrom (Jsc: + 2,6%), Füllfaktor (FF: + 4,9%) und Wirkungsgrad (FF+ 31%)

INVESTIGATION OF THE OPTICAL PROPERTIES OF THE ZN DOPPED CDS THIN FILM DEPOSITION AND CHARACTERIZATION

For the present work we employ Chemical Bath deposition Technique. Initially the bathtubparameters like temperature, pH of the precursors, polarity of the solutions, deposition timewas optimized for synthesis of CDS thin films. Using above optimized parameters, weprepare CDS thin films for the study of effect of Zn concentration on various properties of thefilms. The structural, Morphological, and optical characterization were investigated. TheXRD spectra of the CDS films exhibit the hexagonal crystal structur

Three-dimensional nanotube arrays for solar energy harvesting and production of solar fuels

Over the past decade extensive research has been carried out on photovoltaic semiconductors to provide a solution towards a renewable energy future. Fabricating high-efficiency photovoltaic devices largely rely on nanostructuring the photoabsorber layers due to the ability of improving photoabsorption, photocurrent generation and transport in nanometer scale. Vertically aligned, highly uniform nanorods and nanowire arrays for solar energy conversion have been explored as potential candidates for solar energy conversion and solar-fuel generation owing to their enhanced photoconversion efficiencies. However, controlled fabrication of nanorod and especially nanotube arrays with uniform size and shape and a pre-determined distribution density is still a significant challenge. In this research work, we demonstrate how to address this issue by fabricating nanotube arrays by confined electrodeposition on lithographically patterned nanoelectrodes defined through electron beam as well as nanosphere photolithography. This simple technique can lay a strong foundation for the study of novel photovoltaic devices because successful fabrication of these devices will enhance the ability to control structure-property relationships. The nanotube patterns fabricated by this method could produce an equivalent amount of photocurrent density produced by a thin film like device while having less than 10% of semiconducting material coverage. This project also focused on solar fuel generation through photoelectrocatalytic water splitting for which efficient electrocatalysts were developed from non-precious elements. Lastly, a protocol was developed to disperse these electrocatalysts into a butadiene based polymeric catalytic ink and further processing to yield free-standing catalytic film applicable for water electrolysis”--Abstract, page iv

Activating ZnO nanorods photoanodes in visible light by CdS surface sensitiser

Thin films of c-axis aligned uniform ZnO nanorods (NRs) were fabricated on to fluorine-doped tin oxide-coated soda lime glass substrate by a two-step chemical route. Thereafter ZnO NRs/CdS core shell structures were successfully synthesised by depositing CdS layer on top of vertically aligned ZnO NRs using less hazardous nanocrystal layer deposition technique. The presence of CdS in ZnO NRs/CdS core shell structures was confirmed by energy dispersive X-ray analysis. Examination of structure and morphology of the fabricated films by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) revealed that both films have one-dimensional hexagonal wurtzite structure. Optical properties evaluated from ultraviolet-visible and photoluminescence spectra demonstrated better photo response of ZnO NRs/CdS core shell structure with respect to bare ZnO NR structure. Optical to chemical conversion efficiency of ZnO NRs/CdS photoanode was found to be similar to 1.75 times higher than bare ZnO NRs photoanode in photo electrochemical water splitting under visible light

Integrated p-n Junctions for Efficient Solar Water Splitting upon TiO2/CdS/BiSbS3 Ternary Hybrids for Improved Hydrogen Evolution and Mechanistic Insights

The development of efficient and novel p-n heterojunctions for photoelectrochemical (PEC) water splitting is still a challenging problem. We have demonstrated the complementary nature of (p-type) BiSbS3 as a sensitizer when coupled with (n-type) TiO2/CdS to improve the photocatalytic activity and solar to hydrogen conversion efficiency. The as-prepared p-n heterojunction TiO2/CdS/BiSbS3 exhibits good visible light harvesting capacity and high charge separation over the binary heterojunction, which are confirmed by photoluminescence (PL) and electrical impedance spectroscopy (EIS). The ternary heterojunction produces higher H-2 than the binary systems TiO2/CdS and TiO2/BiSbS3. This ternary heterojunction system displayed the highest photocurrent density of 5 mA center dot cm(-2) at 1.23 V vs. reversible hydrogen electrode (RHE) in neutral conditions, and STH of 3.8% at 0.52 V vs. RHE is observed. The improved photocatalytic response was due to the favorable energy band positions of CdS and BiSbS3. This study highlights the p-n junction made up of TiO2/CdS/BiSbS3, which promises efficient charge formation, separation, and suppression of charge recombination for improved PEC water splitting efficiency. Further, no appreciable loss of activity was observed for the photoanode over 2500 s. Band alignment and interfaces mechanisms have been studied as well

Electrochemical synthesis of core-shell ZnO/CdS nanostructure for photocatalytic water splitting application

We have successfully synthesized ZnO NRs and ZnO/CdS core-shell structures by a facile two step chemical routes viz. electrodeposition and chemical bath deposition. Plane ZnO nanorods films were deposited by using three electrode electrodeposition on FTO glass substrates. The ZnO/CdS core-shell structures were deposited by immersing plane ZnO nanorod films into a bath containing precursor solution of CdS in chemical bath deposition. Formation of ZnO NRs and ZnO/CdS core-shell structures has been confirmed by UV-Visible absorption, Raman spectroscopy and scanning electron microscopy. The synthesized ZnO NRs and ZnO/CdS core-shell structures has been also characterized for photoelectrochemical (PEC) properties, Mott-Schottky analysis, electrochemical impedance spectroscopy (EIS) and efficiency measurements of PEC system. It has been found that the photocurrent conversion efficiency in water splitting is higher for ZnO/CdS core-shell photoanode than ZnO NRs photoanode. These results suggest that addition of CdS with ZnO NRs is beneficial in increasing the visible light absorption and to enhance the photocurrent conversion efficiency in water splitting. Thus, ZnO/CdS core-shell configuration can be a prospective candidate for efficient PEC splitting of water

A CdS/CdSeTe with Carbon Nanospheres Sensitized Solar Cell with a Reduced Graphene Oxide Counter Electrode: Fabrication and Characterization

To make Quantum Dot Sensitized Solar Cells (QDSSCs) competitive, it is necessary to achieve power conversion efficiencies comparable to other solar cell technologies. In this work, a photoactive electrode consisting of CdS (visible light absorbing) and CdSeTe (alloyed quantum dots with an excitonic absorption onset at 859 nm) and Carbon Nanospheres (CNS) were prepared by assembling them onto a mesoporous TiO2 electrode for the first time. Carbon Nanospheres were prepared from starch by an acid treatment and they were dissolved in ethanol and deposited onto TiO2 electrophoretically. CdS QDs were formed over TiO2 and the TiO2/CNS electrodes by SILAR (Successive Ionic Layer Absorption and Reaction) method and CdSeTe QDs were deposited over them by an electrophoretic deposition method. Both the electrodes (photoanode and cathode) were characterized by electron microscopy, cyclic voltammetry, Raman and X-ray diffraction analyses. QDSSCs were constructed with photoanodes of different compositions, and with a 0.1 M Na2S in water-methanol (3:7) solution as electrolyte and a Reduced Graphene oxide based counter electrode. The best overall power conversion efficiency was achieved for the TiO2/CNS/CdS/CdSeTe photoanode-based cell and it is 2.82%. The use of a near-infrared absorber like CdSeTe and electron acceptors like Carbon nanospheres in realizing remarkable improvements in solar-cell performance metrics is demonstrated

Copper–antimony and copper–bismuth chalcogenides—Research opportunities and review for solar photovoltaics

The ternary Cu-Sb- and Cu-Bi-chalcogenides present a rich range of compounds of potential use for large-scale photovoltaics from Earth abundant elements. This paper reviews the state of fundamental knowledge about them, and their technological status with regard to solar cells. Research targets and missing data are highlighted, which may provide opportunities to help realize the goal of sustainable photovoltaics. The family of ternary Cu-Sb- and Cu-Bi-chalcogenides and their solid solutions present a rich selection of potential candidates for Earth-abundant low toxicity photovoltaic (PV) absorber materials. Moreover, they have some novel features imparted by the ns2 lone pair of electrons on the Sb and Bi ions. This review evaluates them as electronic materials, including experimental and theoretical evaluations of their phases, thermodynamic stability, point defects, conductivity, optical data, and PV performances. Formation of the materials in bulk, thin film, and nanoforms and the properties of the materials are critically assessed with relevance to their suitability for PV devices. There is special emphasis on CuSbS2 and CuSbSe2 which form the mainstay of the device literature and provide the most insights into the present-day limitation of the device efficiencies to 3 or 4%. Missing features of the literature are highlighted and clear statements recommending potential research pathways are made, which may help advance the technological performance from its present stuck position