I. Development of novel wide-bandgap absorbers
I.1 Materials synthesis and device optimization for wide-gap HaPs
- Supervising PIs: Eva Unger (HZB, HU Berlin), Marcus Bär (HZB, BTU Cottbus), Leeor Kronik, (WIS), David Cahen, (WIS)
- Collaboration with projects: I.2, I.3, II.1, II.2, II.4
The aim of this project is to elucidate the fundamental limitations of the photovoltage in high-bandgap HaP-based materials by combining computational, synthetic and characterization efforts and contrast this to the limitations induced by the chosen device design. The task of this project will be the synthesis of high-bandgap HaP materials such as CsPbBr3, MAPbBr3, FAPbBr3, CsSnBr3, MASnBr3 and FASnBr3. Collaborators at WIS have advanced expertise in the fabrication routines of high quality high band gap lead halide materials. Consecutively, we will vary the band gap by compositional tuning making mixed cation/anion alloys such as (CsyFA1-y)Pb(BrxI1-x)3 or (CsyFA1-y)(SnzPb1-z)(BrxI1-x)3.
The HZB PhD student will focus on the synthesis of higher band gap HaP materials correlating fundamental properties such as the band gap and lattice parameters established from XRD. More advanced characterization methods will be employed for the characterization of these materials such as SPV and KPFM with project partners at HZB, WIS and BGU (I.3), XPS (II.2) and electron microscopy (II.4). Trends will be rationalized using computational methods (Kronik).
The 2 WIS PhD students (each of which will be involved to ~50% in HI-SCORE) will use solution-based methods to prepare and characterize high band gap HaP materials to make high voltage and high voltage efficiency HaP-based devices. Characterizations of materials include optical and optoelectronic ones (SSPG, PR, PL, TR-PL). Partial devices will be studied by CPD, SPV and SPS. Computational research will focus on structural analysis, bandgaps, and band-structure. The results of these approached will be combined with those from HZB to rationalize potential losses in voltage due to intrinsic limitations in the absorber materials and losses induced by interfacial offsets in cooperation with project II.1 and II.2.
I.2 Combinatorial synthesis and material property mapping of HaPs
- Supervising PIs: Thomas Unold (HZB), Susan Schorr (HZB, FU Berlin), Arie Zaban (BIU)
- Collaboration with projects: I.1, II.3, II.4
In this project we will explore new perovskite-based absorber materials by coevaporation methods. This method is advantageous for such studies as it can overcome solvent-limitations and also allows the generation of lateral compositional and doping gradients, thus enabling a combinatorial study of structure-property relationships and optoelectronic performance. Such a combinatorial approach will increase significantly the speed of materials characterization greatly facilitating the discovery and optimization of new promising absorber materials. Examples for a combinatorial material screening approach to be followed within this project are (1) compositional screening for band gap tuning of metal HaP semiconductors (2) fully inorganic perovskite materials (3) lead-free perovskite materials (4) Screening different contact layer materials in conjunction with project II.3.
For the combinatorial analysis a number of different recently developed characterization methods will be applied, which allow to map the most relevant material properties and to identify promising candidates for further device research, among these (1) XRD and XRF mapping for identification of material phases and their composition (2) quantitative luminescence imaging (3) time-resolved photoluminescence spectroscopy to identify minority carrier lifetimes and doping levels (4) optical pump-terahertz probe spectroscopy for the analysis of charge carrier mobilities.
An important part of the project will be to generate material libraries as well as develop and apply data mining methods in order to identify promising materials in such still rather novel combinatorial approaches. In this project the strong expertise of Arie Zaban’s group in combinatorial materials research and HZB’s group in coevaporation and structural and optoelectronic characterization is expected to generate the synergies needed to advance on these outlined ambitious tasks. The project will also naturally integrate with effort to synthesize and analyze wide-gap perovskites by diverse non-combinatorial methods (I.1), advanced microstructural and spatially resolved characterization (II.4), and combinatorial approaches to contact layers pursued in subproject II.3.
I.3 Scaling the deposition of high band gap perovskites to larger areas with solution-processable methods
- Supervising PIs: Eva Unger (HZB, HU Berlin), Iris Visoly-Fisher (BGU), Lioz Etgar (HUJ)
- Collaboration with projects: I.1, I.2, III.1
The Young Investigator Group of E. Unger develops scalable solution-based deposition methods such as inkjet printing and slot-die coating for the deposition of metal HaP (HaP) semiconductor layers on larger areas. In close collaboration with project I.1 and I.2, we aim to scale the deposition of stable high band gap HaPs to large areas for the development of HaP/Si tandem devices (project III.1) on the modul level. Project partners at HUJ (Etgar) are developing spray-coating as alternative scalable deposition methods and project partners at BGU (Visoly-Fisher) work on polymer/HaP blends as a potential approach to make high quality and coverage HaP layers.
The HZB PhD student will focus on scalable solution-based deposition methods such as slot-die coating and ink-jet printing at HZB. The morphology of larger area samples will be compared with layers deposited by spray-coating at HUJ and PVD as described in project I.4 through exchange of samples and project placements to HUJ. The student will go to BGU (Visoly-Fisher) to perform complementary high-resolution KPFM measurements to characterize the homogeneity and photoelectric properties of thin films.
The HUJ PhD student will prepare HaP films using spray-coating as a generic scalable deposition technique and explore material blends containing larger organic molecules. The PhD student will go to HZB for comparing coating results using different scalable deposition methods and characterization methods with spatial resolution such as PL mapping.
The BGU PhD student will focus on brominated polyaniline as hole conductor for HaP-based solar cells as well as hybrid layers of HaP/Br-polyaniline. The israeli PhD student will visit HZB for complementary charge transfer characterization using optical spectroscopic methods.
I.4 Light induced degradation – material stability
- Supervising PIs: Norbert Nickel (HZB, TU Berlin), Bernd Rech (HZB, TU Berlin), Iris Visoly-Fisher, Eugene Katz (BGU)
- Collaboration with projects: I.1, I.2, III.2
In this work we plan to investigate the stability of methylammonium (CH3NH3+ - MA) and formamidinium (HC(NH2)2+ - FM) lead iodide perovskite films using visible and ultra violet light in oxygen atmosphere and in vacuum. Insight into the degradation mechanisms will be obtained from in-situ Fourier-transform infrared absorption (FT-IR), photoluminescence, and gas effusion measurements.
Recently, we have revisited the light-induced degradation of MAPbI3 in the presence of oxygen. Illumination in O2 atmosphere results in a swift degradation. Isotope experiments clearly show that O2 acts as a catalyst decomposing MA ions into CH3NH2 and hydrogen. In case of FMPbI3 perovskites illumination in the presence of O2 results in a more complex reaction; decomposition of the FM ions occurs at the N–C–N bonds and as a result CO2 and C = O molecules are formed that rapidly diffuse out of the crystalline lattice. In addition, we found experimental evidence of a hitherto unknown but fundamental degradation mechanism of MAPbI3 and FMPbI3 perovskite layers due to exposure to visible and ultra violet light. This degradation mechanism does not require the presence of oxygen or other constituents. Prolonged illumination causes the dissociation of MA ions into molecular hydrogen and CH3NH2. Interestingly, FM ions also decompose into CH3NH2. The resulting molecules are highly mobile at room temperature and diffuse out of the perovskite layer. As a result, the concentration of localized defects increases and quenches the photoluminescence.
The planned PhD thesis will study the stability of different perovskite based materials with different bandgaps grown at HZB, Weizmann and Hebrew University. Aim is to reveal, understand and eliminate degradation mechanisms in the material. There is a strong link to projects I.1 and I.2 to study effects on different materials and to project III.2 addressing stability on the device level. 1-2 PhD students will address the topic from the Israelian side (WIS and BGU).
I.5 Understanding carrier dynamics in wide-bandgap absorbers
- Supervising PIs: Roel van de Krol (HZB, TU Berlin), Avner Rothschild (IIT)
- Collaboration with projects: I.5, II.3, II.5
Metal oxide semiconductors are attractive wide-bandgap candidates for water splitting applications because of their low cost, easy processing, and good chemical stability. Despite impressive progress in the past years, their performance is still 2-3 times below the theoretical limit. Preliminary evidence suggests that this is due to poor carrier mobilities. The aim of this proposal is therefore to develop a robust understanding of the influence of carrier dynamics on the overall performance characteristics. To achieve this, we will:
- Explore the influence of illumination conditions on the carrier dynamics and performance limitations of representative metal oxides. Such studies are missing in the literature, and will provide key insights.
- Develop a combined opto-electrical model to describe the data and get a more detailed insight in the mechanisms involved. The model will allow us to predict the behavior of wide-bandgap, low-mobility absorbers under practically-relevant conditions.
- Construct and characterize a tandem device based on a metal oxide top absorber and a small-bandgap bottom absorber in order to validate the results.
- Use the insights gained to specify critical requirements for wide-bandgap absorbers in efficient tandem devices.
The groups of Van de Krol and Rothschild have been collaborating in EU projects since 2009 and complement each other very well. The PhD student at HZB will focus his/her efforts on time resolved spectroscopy and development of the tandem device, and will visit IIT to develop expertise in modeling. The PhD student at IIT will focus on opto-electrical modeling, and will visit HZB to learn more about time-resolved spectroscopy. The project will strongly benefit from synergies with projects I.5 (oxide-based tandem device), II.3, and II.5 (electrical and defect characterization).
 Zachäus et al., Chem. Sci. (in press); Abdi et al., Nat. Commun. 4:2195 (2013)
 Dotan et al., Nat. Mater. 12, 158 (2013)