Ricardo Poeira

---

Main goal
The general topic of the PhD project involves the development of Cu(In,Ga)Se₂ micro-solar cells for high efficiency applications.

Abstract
The scope of the project extends from the synthesis and monitoring of the semiconductor absorber by a two-step process, namely, the electrodeposition of the metal precursors followed by selenization, up to the characterization of the complete device. The focus is to investigate the selenization process in order to control the quality of the absorber layer. Key to this will be the controlled distribution of gallium and the presence of dopants such as sodium and potassium.

Techniques used
– Some key characterization techniques used are Raman, XRD and Photoluminescence.

Longfei Song

---

Main goal
To develop a low-cost process for fabrication of high-quality transparent oxides conductors, which is compatible with micro-solar cells.

Abstract
To develop a high-efficiency micro-concentrator solar cell, a transparent oxide conductor is needed as top electrode. Among oxide film fabrication technologies, chemical solution deposition (CSD) exhibits many advantages in terms of low cost, high efficiency, and flexibility of composition adjustment. However, the obstacle of CSD based oxides conductors have high thermal budget (>400 ^oC), which can damage the absorb layer (GIGS in our case). To address this issue, a low temperature processing is necessary. Also, among the solution-based technologies, inkjet printing offers an opportunity to directly pattern electrode instead of lithography.

Techniques used
– Inkjet printing is employed as our main technology to fabricate transparent conducting oxide films.

Taowen Wang

---

Main goal
Improving efficiency of CuInSe₂ by introducing a hole transport layer to reduce back surface recombination and developing a tandem solar cell.

Abstract
Copper indium diselenide (CuInSe₂) with a bandgap of around 1.0 eV is ideal for bottom cells in tandem applications. However, CuInSe₂ solar cell suffers from a very high back surface recombination velocity of more than 10⁵ cm/s, which can be mitigated by a Gallium (Ga) back grading. Ga back grading is very efficient to reduce the back-surface recombination and has already shown some impressive results. However, the complex manufacturing process of controlling Ga grading is rather unbeneficial for industrialization. To solve this problem, we propose a selective transport structure for CuInSe₂ solar cell where we introduce a hole selective transport layer (HTL) at the backside to promote the collection of holes and drive the minority carries (electrons) away, resulting in reduced back surface recombination.

Techniques used
– Physical Vapor Deposition (PVD) will be used to prepare 1-stage or 3-stage CuInSe₂ as an absorber of the solar cells.
– Photoluminescence (PL) will be performed to characterize passivation effects of HTL layer by measuring quasi-fermi level splitting (QFLS) of the CuInSe₂.

Alfredo Blázquez Martínez

---

Main goal
The main aim of the project is to answer the question of the physical mechanism driving light-induced charge transport in BFO films. If possible, a way to increase the conversion efficiency to a degree that allows the implementation of BFO as a wide-bandgap material for the top cell in a photovoltaic tandem cell arrangement should be devised.

Abstract
The project is concerned with the investigation of the photovoltaic and photo-ferroelectric properties of solution-deposited films of BiFeO3 (BFO). Particular attention is paid to the role of ferroelectric domain structures in the photovoltaic response of BFO and the influence of stress, either directly through an intrinsic piezo-photovoltaic contribution, or indirectly by a modification of the domain structure. Besides, other properties that depend critically on the domain structure and stress state will be investigated. This also includes electrooptical and nonlinear optical properties such as photorefractive or photostrictive behaviour.

Techniques used
– Processing techniques: Chemical solution deposition, UV photolithography, sputtering of metal and oxide electrodes.
– Characterization techniques: X-ray diffraction, X-ray reflectivity, electrical and opto-electronic characterization, dielectric and impedance spectroscopy, SEM, UV-Vis spectroscopy

Poorani Gnanasambandan

---

Main goal
The dissertation topic concerns the design, fabrication and characterisation of high performance transparent conducting materials as electrodes for the low band gap solar cell.

Abstract
The challenges of the PhD thesis will be to design materials combining appropriate band alignment, conductivity and transparency, to be adapted to low band-gap CIGS solar cells. The new concept of this structure will require to develop new oxide or oxychalcogenide materials to serve as hole and electron extraction layers, with reduced interface defect to avoid recombination, and well controlled work function and doping. Moreover, the developed materials will require low temperature vapor phase deposition compatible with the cell processing. At last, in order to ensure high in-plane conductivity for the top transparent electrode, the concept of hybrid electrode combining transparent conducting materials and silver nanowires will be investigated.

Techniques used
– Atomic Layer Deposition (ALD) will be used to perform low temperature deposition of transparent conducting materials on the solar cell absorbers and achieve high degree of composition control.
– Analytical techniques will be performed to characterize material properties of the films with focus on optical and electrical properties.

Himanshu Phirke

---

Main goal
To correlate the optical, electrical and compositional properties of hybrid organic/inorganic solar cells using different characterization techniques (AFM, KPFM, PL, HIM-SIMS).

Abstract
To boost the efficiency of hybrid organic/inorganic solar cells, it is very important to know about their properties in different regimes. Because some techniques provide information about the only surface and not the bulk while others provide information about the bulk and not the surface, it is difficult to correlate the properties directly and reach a robust conclusion. In this project, the goal will be to do correlative studies using microscopy techniques combining structural, chemical, optical and electrical information at identical location.

Techniques used
– Kelvin Probe Force Microscopy (KPFM) will be used in combination with Helium Ion Microscope (HIM) coupled with Secondary Ion Mass Spectrometry (SIMS) analyzer to gain access to surface compositions, work functions and surface photovoltage at nanometer resolution.
– Time-resolved hyperspectral imaging will be used on various length scales to correlate the electrical and chemical properties to the optical properties such as minority carrier lifetimes and Quasi-fermi level splitting. Hyperspectral lifetime mapping produces a homogeneous illumination of the sample (in contrast to confocal setups). Inhomogeneities are likely to influence the transients (on the local scale via changes in defect densities and doping). The inhomogeneities will be transferred into the average signal acquired on different length scales.

Saba Tabean

---

Main goal
The PhD thesis work involves investigation of fundamental physics of ion interaction with solid material in transmission geometry. In parallel, structural and chemical analyses of materials will be performed usingMAIN_ELEMENT_TD SPAN:3 WIDTH:ELEMENT BEGIN Section=1 Question ID=44644 presentation_type=textarea abstract_data_type=texttextarea besideBEGIN secondary electron imaging and Secondary Ion Mass Spectrometry (SIMS) using ion and electron beam microscopy methods.

Abstract
Knowledge about the nanoscale structure and chemical composition is important to understand the macroscale properties and improve the design and fabrication methods of energy materials including photovoltaics. This thesis develops new analytical and data treatment methods for high-resolution Secondary Ion Mass Spectrometry (SIMS) imaging to obtain nanoscale chemical maps of the samples developed by the project partners. Furthermore, the project explores new contrast mechanisms that are possible using transmitted ions for materials characterization.

Techniques used
– Secondary Ion Mass Spectrometry (SIMS)

Henry Fried

---

Main goal
Development of a “second principles” tight-binding approach for isolated defect levels.

Abstract

The calculation of isolated shallow point defect levels requires large supercells containing more that 1000 atoms. Such a high number of atoms is needed to reduce the interaction between the defects and therefore the resulting dispersion. This is prohibitively large for first-principles (ab-initio) calculations. Therefore, we are developing a tight-binding approach for defect states with n-th nearest neighbor interaction. The generation of tight-binding parameters can be achieved by different approaches based on first-principles calculations: fitting of band structures, Wannierisation or machine learning. The final goal is to construct a tight-binding model to simulate bigger and more complicated structures..

Techniques used

• DFT
• Tight-binding.