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III. Integration in photovoltaic devices and solar fuel generators

III.1 Stable semitransparent top cells and tandem cell integration

  • Supervising PIs: Steve Albrecht (HZB, TU Berlin), Dieter Neher (U Potsdam), Bernd Rech (HZB, TU Berlin), David Cahen (WIS), Gary Hodes (WIS), Lioz Etgar (HUJ)
  • Collaboration with projects: I.1, II.1, II.4, III.2

The perovskite-based tandem cell design imposes special requirements on the transparent electrodes used for lateral current collection and the charge selective layers. The transparent front contact is deposited on the perovskite top cell, therefore, damage to the underlying selective (mostly organic) contact layer or the perovskite absorber has to be avoided. Additionally, charge selective layer schemes need to be developed and investigated in detail to enable high transmission and good interface properties reaching high open circuit voltages and fill factors in parallel.

For silicon solar cells reflection losses can be significantly decreased when applying light trapping schemes on the front of the wafer. Such schemes involve textured surfaces with randomly distributed pyramids with several microns height. This surface texture is not compatible with recent solution processing utilized for perovskite solar cells. Therefore, strategies need to be developed that allow for conformal growth of the perovskite on textured silicon wafers. Then standard silicon texture could be utilized that enable highly efficient light management and with that high efficiency in the tandem device.

Based on the proposal submitted recently by several of the Israeli partners (WIS, BGU, HUJ, BIU), including a letter of interest from HZB by Albrecht/Rech. 1 PhD student from the Israel side (an at least one more if the proposal receives funding) will complement the two HZB based PhD student. The existing PhD student works on the semitransparent top cell design. The HyperCell funded student will target the role of the nature of the interface, namely the topology and energetics, in determining the mechanism and rate of interfacial recombination losses.

With the complementary experience of the participating groups, the investigation of different organic and inorganic CTLs and its interface dynamics with the perovskite will potentially lead to highly efficient contact designs that can be implemented in semitransparent top- and tandem-cells.The teams in Israel and Germany will jointly develop such tandem cells using silicon bottom cells designed at HZB

III.2 Long-term device reliability of halide-perovskite-based solar cell devices

  • Supervising PIs: Antonio Abate (HZB), Rutger Schlatmann (HZB, HTW),
    Iris Visoly-Fisher (BGU), Eugene Katz (BGU)
  • Collaboration with projects: I.4, II.5, II.4, III.1

An operational lifetime of 25 years is a prerequisite for HaP-based solar cells to become a viable technological option for solar energy conversion. In this project, we will be running long-term device reliability testing on HaP-based solar cell devices, tracking the performance of 100 devices in parallel over time under constant illumination in a controlled atmospheric conditions, compared to outdoor testing of encapsulated cells with alternating light/ dark periods and under accelerated stress conditions using concentrated sunlight. The long-term performance of hybrid tandem devices will be evaluated as a function of device architecture, selective contact layer materials and the composition of the HaP absorber layer. After long-term stability testing, devices will be analyzed using imaging and mapping methods available at HZB, such as photoluminescence, electroluminescence and imaging and thermography. These experiments will be carried out by a PhD student at HZB. Complementarily KPFM measurements will be carried out by the PhD student at BGU to investigate spatial effects in device degradation. We will directly compare and correlate the device degradation pathways and failure evolution of devices tested under simulated 1 sun conditions in a protected environment and devices tested outdoors at BGU (in the Negev desert, Israel) and under accelerated stress conditions using concentrated sunlight. As light and heat induced degradation proceeds differently compared to bias induced degradation, we are expecting significant differences between these data sets. Extended research stays of both students at the partner institute will be utilized for result comparison and measurement coordination, as well as for using characterization methods not available in the respective home institutions (KPFM and other AFM-based characterization will be used in BGU, transient absorption, PL and other light spectroscopies will be used in HZB).

III.3 Structure-property relationships in halide-perovskite/Cu(In,Ga)Se2 tandem devices

  • Supervising PIs: Daniel Abou-Ras (HZB, TU Berlin), Oded Millo (HUJ)
  • Collaboration with projects: II.5, II.6, III.1

Cu(In,Ga)Se2 solar cells are promising devices for the use as bottom cells for tandem devices in combination with halide-perovskite top cells.  The topic for the proposed PhD thesis will focus on the analysis of materials properties of Cu(In,Ga)Se2 and halide-perovskite in completed tandem structures on the performance of the corresponding devices. The aim is to employ experimental “nano-tools" in conjunction with the macroscopic transport and phototransport measurements. The nano-tools include scanning probe microscopy techniques such as conductive atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM) as well as scanning tunneling microscopy and spectroscopy (STM and STS). This is in addition to other nanoscale techniques in scanning electron microscopy such as electron-beam induced current (EBIC) in combination with cathodoluminescence, electron backscatter diffraction (EBSD), and energy-dispersive X-ray spectrometry (EDS) on identical positions, as well as nano-lithography for local (‘mesoscopic scale’) transport measurements. The proposed study will not only address local doping densities and charge-carrier lifetimes, but also investigate the role of the GBs and other types of extended defects by using a combination of all the aforementioned local techniques and correlate the results with macroscopic measurements on these films and on the corresponding solar-cell devices.

In the present project, one student (already employed) is co-supervised by HZB and HUJ partners. She will perform SPM studies at HUJ and SEM analyses at HZB. The partners at HZB and HUJ have already successfully collaborated in the field of the present project.1

1 D. Azulay, D. Abou-Ras, O. Millo, et al., phys. stat. sol. (RRL) 10 (2016) 448-452

III.4 Towards an Oxide-Based Buried Junction Tandem Device for Solar Water Splitting

  • Supervising PIs: Roel van de Krol (HZB, TU Berlin), Sonya Calnan (HZB), Rutger Schlatmann (HZB, HTW), Menny Shalom (BGU)
  • Collaboration with projects: I.6, III.1

To construct a complete water splitting device based on wide-bandgap metal oxide absorbers, three main challenges need to be addressed. First, one must create a junction within the oxide absorber that allows efficient charge separation and a sufficiently large photovoltage. Second, suitable catalysts for the hydrogen and oxygen evolution reactions need to be developed. Finally, these components need to be combined with a suitable bottom absorber and integrated into a clever device design. The aim of this proposal is to tackle all three challenges and demonstrate a complete oxide-based tandem device for solar water splitting. To achieve this, we will:

  1. Develop efficient solid state junctions for selected metal oxide absorbers. More than 99% of the efforts in the field have been on semiconductor / liquid junctions, but these almost invariably suffer from large voltage losses. We will leverage the latest developments in thin film growth methods to create high-quality solid state ‘buried’ junctions in which such losses can be avoided.
  2. Develop highly efficient earth-abundant co-catalysts based on novel ceramic nickel compounds to enhance the kinetics of the electrochemical reactions.
  3. Combine the components above with a small-bandgap bottom absorber, based on an optimized silicon HIT design, in an integrated tandem device. The device will be designed to minimize electrical and optical losses and to ensure manufacturability and scalability to larger areas.

The oxide absorbers and Ni-based catalysts will be developed by two PhD students, one in the group of van de Krol (HZB) and one in the group of Shalom (BGU). The integration of these components with a Si-based bottom absorber in a tandem device will be done by a third PhD student in the group of Schlatmann (HZB). Exchange visits are planned between the groups and are essential to develop the necessary expertise for component integration.