II. Design and demonstration of interfaces and device structures

II.1 Bulk and interface characteristics of alternative inorganic HaP material systems with widely tunable optoelectronic properties

  • Supervising PIs: M. Bär (HZB), D. Cahen, G. Hodes (WIS)
  • Collaboration with projects: I.1, I.2, II.2., III.1

By varying the Pb/Sn and/or the halide composition, the optical band gap energy of inorganic ABX3 HaP (HaP) materials with A = Cs+ (Rb+), B = Pb2+ and/or Sn2+, and X = I, Br-, Cl- can be tuned between 1.3-1.4 and 3.0 eV. HaP variants thus also fulfil the wide-bandgap (1.5 – 2.0 eV) criterion that fits to, and is necessary for this graduate school initiative, aiming at efficient tandem devices for solar energy conversion in photovoltaics and for solar fuels. Besides synthesis the study of the HaP interface structure to the adjacent charge selective contacts is the main focus of the proposed research. Currently, it is far from clear whether the currently used standard materials do also form good contacts to these HaP materials.

The HZB PhD student will deposit Cs-based HaP thin films by evaporation using HZB’s Energy Materials In-Situ Laboratory Berlin (EMIL) facility, a combined deposition and analysis UHV system. Furthermore, he/she will make use of the full analytical suite at EMIL to study HaP’s surface and interface properties. During his/her visit of WIS, the student will get acquainted with the wet-chemical deposition route established at WIS and will be introduced into complementary (bulk-sensitive) characterization techniques.

The WIS PhD student will prepare HaP films and characterize them per se and with contacting layers. The latter will be screened and chosen in close consultation with HZB. Particular emphasis will be on finding ways to prepare structures that will allow (@HZB, after preliminary screening experiments at WIS) to follow the energy level alignment evolution as the interface is formed. 

Through this activity – which addresses the graduate school’s main research themes I and II – the WIS will have direct access to the EMIL infrastructure and HZB will profit from the WIS group’s expertise in preparation, characterization, and optimization of HaP materials and device structures.

II.2 Comprehensive understanding of the interface electronic structure in perovskite-based solar cells

  • Supervising PIs: Norbert Koch (HUB, HZB), David Cahen (WIS), Leeor Kronik (WIS)
  • Collaboration with projects: I.1, I.4, II.1

Many perovskites exhibit structural and compositional changes during operation in devices, and particularly interfaces, which are crucial for device function and efficiency are affected. Therefore, we will aim at (i) providing a fundamental understanding of the electronic structure of perovskites, (ii) unraveling the nature and formation pathways of defects, and (iii) explore methods to avoid defect formation or at least to passivating them. For that purpose, the Ph.D. students (one at HUB/HZB and one at WIS) will perform coordinated and complementary experimental and theoretical work encompassing electronic structure determination and photovoltaic cell performance.

  1. We will study the electronic structure of perovskite single crystal surfaces of different crystallographic orientation. We will employ angle-resolved ultraviolet photoelectron spectroscopy (AR-UPS), the benchmarked against state-of-the-art density functional theory (DFT) modeling.
  2. To unravel the nature of defects, we will employ scanning tunneling microscopy to determine the structural features of defects, and X-ray photoelectron spectroscopy (XPS) to identify their chemical nature. The impact of defects on the valence electronic structure will be assessed by combined (AR-)UPS experimental and theoretical modeling. The effect of external stimuli (light, temperature, cur-rent) on defect formation will be investigated primarily.
  3. Preliminary data indicate that some of the defects at perovskite surfaces are charged. We will explore to what extent these can be passivated by the use of molecular electron acceptor and donors, and how these extend the lifetime of perovskite-based photovoltaic devices.

The reach the project goals, the students will gain and use the complementary expertise in preparation and characterization of photovoltaic materials and devices (Cahen), photoelectron spectroscopy and interfaces of electronic materials (Koch), and modern DFT modeling of materials & interfaces (Kronik).

II.3 Growth and in situ/high throughput characterization of thin-film oxides for silicon/perovskite tandem solar cells with novel electron-spin resonance on-a-chip

  • Supervising PIs: Klaus Lips (HZB, FU Berlin), Steve Albrecht (HZB), Aharon Blank (Technion), Arie Zaban (BIU)
  • Collaboration with projects: I.I, I.3, II.1, III.3

The scope of this proposal is to develop thin oxide films such as NiOX, WO3 or MoO3 using PVD, PLD and ALD in conjunction with printing technologies that serve as selective contacts in a silicon/perovskite tandem solar cell developed in project I.I., I.3 and III.1. We will focus on covering a wide range of possible material’s compositions in a single growth process. The characterization of the oxide and the interface will be performed in two different approaches.

  • We will investigate the structure-property relationships in collaboration with project and utilize high-throughput deposition to accelerate feedback times.
  • A novel electron-spin resonance on-a-chip (ESRoC) approach will be developed and applied that will allow in situ characterization of film and interface formation by monitoring the evolution of paramagnetic probes during growth.

The compact ESRoC system will be battery driven and operated with a permanent magnet. The ESRoC will be implemented in the deposition units for in-operando detection of film growth. It will deliver information of the film growth and post film processing such as interface quality or doping efficiency by monitoring observer spin states in the film.

The PhD student at HZB will deposit and characterize the oxides and develop the EPRoC technology. The student will visits Technion to implement the magnet technology in the ESRoC setup. At BIU he will grow and characterized further specific oxides by ALD or PLD.

The PhD student at Technion will contribute to the ESRoC project by designing, constructing and testing a compact permanent magnet that will be used inside the processing environment in conjunction with the ESRoC system.

The PhD student at BIU will establish the growth protocols for the oxide libraries.

The PhD candidate at Univ. Ulm will design and fabricate the specific silicon chips for the in-situ ESRoC technology.

We note that the partners in the present project have already successfully collaborated in recent joint research work.[1]

[1] Katz, I., Fehr, M., Schnegg, A., Lips, K., and Blank, A., J. Magn. Reson. 251, (2015) 26–35; Dikarov, E., Fehr, M., Schnegg, A., Lips, K., and Blank, A., Meas. Sci. Techn. 24 (2013) 115009.

II.4 Advanced halide-perovskite materials and device characterization

  • Supervising PIs: Daniel Abou-Ras (HZB, TU Berlin), Eva Unger (HZB, HU Berlin), David Cahen, Dan Oron, Omer Yaffe,  Leeor Kronik (WIS)
  • Collaboration with projects: I.1, II.5

This project is in close relationship to Project I.1, “Materials synthesis and device optimization for wide-gap HaPs” and will also provide answers to the question “What limits the photovoltage in high-bandgap HaP-based materials?”. The main approach will be correlative scanning probe and electron microscopy in order to elucidate the limiting issues with respect to the performance of the corresponding solar cells. Microstructure and composition will be correlated with electrical and optoelectronic properties by applying corresponding microscopy techniques on identical specimen areas. Scanning electron microscopy studies employing imaging, energy-dispersive X-ray spectrometry (EDX), electron-beam-induced current (EBIC), and cathodoluminescence (CL) measurements at various temperatures (100-300 K) will be conducted. The work will be carried out by two PhD students, one at HZB and one at WIS.

The HZB student will focus on the microscopy studies and collaborate closely with WIS on these measurements. The WIS PhD student will work on EBIC under bias and illumination as function of temperature to search for features and characteristics that can shed light on the photovoltage issue. In addition scanning probe microscopy-based experiments, also at applied voltages and under illumination will be employed to that end. In their research stays at HZB and WIS, the WIS and HZB students will work on EDX/EBIC/CL studies and EBIC/SPM, respectively. The microscopic electrical / optoelectronic properties will be correlated with macroscopic materials & device characteristics. A critical role is foreseen for electronic structure computations to help understand and predict consistent structure-property relationships. Additional transmission electron microscopy investigations will give insight to HaP-containing interfaces and provide input parameters for electronic structure computations. The results of this work will be directly fed back into project I.1.

The HZB and WIS PIs have already collaborated successfully in a joint research work.[1]

[1] Katz, I., Fehr, M., Schnegg, A., Lips, K., and Blank, A., J. Magn. Reson. 251, (2015) 26–35; Dikarov, E., Fehr, M., Schnegg, A., Lips, K., and Blank, A., Meas. Sci. Techn. 24 (2013) 115009.
[2] N. Kedem, D. Abou-Ras, D. Cahen, et al., J. Phys. Chem. Lett. 6 (2015) 2469-2476

II.5 Effect of secondary phases on the electrical properties of HaP thin films and solar cells

  • Supervising PIs: Daniel Abou-Ras (HZB, TU Berlin), Christoph Koch (HU Berlin), Omer Yaffe, Dan Oron, David Cahen (WIS)
  • Collaboration with projects: I.1, II.4

The presence of secondary phases in HaP thin films, influenced by adjusting the stoichiometry of the compounds during the synthesis, has a substantial impact on the device performance. For the development of high-efficient HaP devices, it is essential to obtain enhanced knowledge about which secondary phases are present on what scales and how they affect the electrical properties of the thin films and the corresponding solar-cell devices. The present project aims at correlating the results from macroscopic vibrational spectroscopy (WIS) and microscopic electron energy-loss spectroscopy in scanning transmission electron microscopy (HZB) acquired on HaP thin films to obtain enhanced insight into spatial phase distributions. Complementary cathodoluminescence (CL), electron-beam-induced current (EBIC), and atomic-force microscopy (AFM) measurements as well as optical modulation spectroscopy (WIS), low-frequency THz and 2-photon spectroscopy (WIS), and macroscopic transport measurements at variable temperature will provide the means to correlate the structure and composition with microscopic electrical and optoelectronic properties. A MatSEC-funded PhD student has recently started on this project. The WIS PhD student will focus on low-frequency THz and 2-photon, and on optical modulation spectroscopy.

During the research stays of these two students at WIS and HZB, the HZB student will perform ThZ and 2-photon spectroscopy, while the WIS student will conduct analyses in SEM by EDX, EBIC, and CL.