Interface Energetics: The key to efficient organic and perovskite solar cells

Abstract

As demand for cheap, carbon-neutral energy continues to grow, so too does the search for novel photovoltaic (PV) technologies that can complement or outperform silicon. Both organics and metal-halide perovskites are materials that have strong, tunable absorption and can be processed at low temperature from solution. In tandem with silicon, they could lead to inexpensive solutions that boost the efficiency of traditional solar cells. On their own, they are ideal candidates for thin, flexible, or transparent PV applications. Organic and perovskite solar cell (OSC and PSC) efficiencies have grown rapidly, yet knowledge of the electronic states within these systems is necessary to continue this trend. This work establishes the fundamental energy levels for charge transport in materials central to two high-performance solar cells, an organic energy cascade cell and a perovskite-polymer hybrid cell, using the techniques of photoemission spectroscopy (PES).

The electron and hole transport levels are responsible for charge separation and flow within a device, but they are difficult to probe, especially at the interfaces critical to device performance. By combining layer-by-layer deposition with a full PES characterization, we were able to measure these levels directly in the organic energy cascade cell. For perovskites, we developed a combined theoretical/experimental approach to extract the transport levels from the low density of states at the band edges. These were then linked to X-ray photoemission spectroscopy (XPS) core level measurements so they could be tracked at the buried perovskite/polymer interface.

In the energy cascade cell, we discovered a dipole at the α-6T/SubNc (donor/acceptor) interface which enables high open circuit voltages in these devices. However, excess chlorine was detected in SubNc. This was likely behind a mismatch found in the electron transport levels at the SubNc/SubPc interface which presented a barrier to electron extraction. We propose that partially chlorinating SubPc may remove this barrier and improve efficiency. In our study of metal-halide perovskites, we discovered significantly lower ionization energies and electron affinities than previous experimental methods suggested. We also found poor alignment between the hole transport levels of the perovskite, CsPbBr3, and the polymer, PTAA. However, this was corrected by p-doping the polymer.