Metal halide perovskites attract considerable attention for application in photovoltaic cells. These materials have an ABX3 crystal structure where A is a monovalent cation, B is metal dication, and X a halide anion. The first perovskites used for solar cells consisted of methylammonium, lead, and iodide (CH3NH3PbI3) but in the meantime many more complex metal halide perovskites have been developed in which other organic or inorganic cations, different metals, and multiple halides are used. These perovskites can be processed from solution into thin films in one- or two-step procedures and afford very efficient solar cells. The highest reported efficiencies already exceed 23%. However, the materials pose several scientific and technological questions, regarding their operational mechanism, stability, and opportunities to further increase the efficiency.
With the M2N group we investigate perovskite solar cells from different perspectives:
Band gap tuning
The CH3NH3PbI3 perovskite has a band gap of about 1.55 eV. Tuning of the band gap can be accomplished in several ways and can be very beneficiary. For example for the development of tandem solar cells two different band gap materials are needed and this can be accomplished by tuning the perovskite structure. Also indoor photovoltaic applications will require tailored absorber layers. By partially replacing lead by tin one can achieve band gaps as low as 1.21 eV, while by replacing iodide by bromide, the band gap can be increased to 2.32 eV. Another way of tuning the perovskite is by changing the dimensionality of the crystal. These are known as 2D Ruddlesden-Popper perovskites in which one or more perovskites layers are isolated from each other by the large spacer cations. Each new perovskite poses new challenges with respect to processing into thin and smooth layers, stability, and charge transport layers that allow extraction of electrons and holes without energy loses.
We develop new perovskites semiconductors, investigate these with a range of techniques such as X-ray diffraction, scanning electron microscopy, photoelectron spectroscopy and fabricate and characterize solar cells based on these materials.
Cells on opaque substrates
Traditionally perovskite solar cells are made on a transparent substrate such as glass or polymer layer. We are also interested in making cells on opaque substrates such as metal foils that are inherently very stable and allow for new applications. The challenge to be solved is making a transparent electrically conductive top contact, by which light can enter the cell. We are developing dielectric-metal-dielectric layers to accomplish this.
Multi-junction solar cells
To eventually surpass the 33.7% Shockley-Queisser limit for single solar cells junctions a well-knows strategy is to stack multiple cells with different band gaps. Obviously this requires absorber layers with different band gaps. We investigate both perovskite-perovskite and perovskite-silicon tandem cells in two-terminal or four-terminal device configurations. To maximize light absorption and power conversion efficiency, we perform optical simulations on to design stack in order to reduce parasitic absorption and reflection losses. Especially for a two-terminal configuration a challenge is achieve current matching but also to fabricate the cells. Sometimes ten or even more layers must be stacked on top of each other. In case of solution processing this requires careful optimization and design of materials a processing steps.
Detection of defect states
Perovskites are known to have defects that may come as interstitials, vacancies, impurity atoms, lattice defects, phase separation, grain boundaries and others. On top of this, these defects are known to be dynamic: they can be mobile or change with time or operating conditions. Some of these defects are important to the operation of the solar cells because they cause non-radiative recombination of electrons and holes that were generated by light. This non-radiative recombination causes a loss in the open-circuit voltage and to mitigate these effects it is necessary to understand in detail their nature. For this we employ different techniques such as pulse electroluminescence, impedance spectroscopy, photothermal deflection spectroscopy, and sensitive measurements of sub band gap quantum efficiency.
Perovskite nanocrystal can also be used to make solar cells. We have just started working on this topic with the aim of generating new materials that do not exist in the bulk. A well-known example in this respect is cubic a-CsPbI3 which is thermally instable and rapidly coverts into a yellow orthorhombic phase. CsPbI3 nanocrystals, on the other hand, retain their cubic crustal structure.