Enhanced charge carrier transport in perovskite solar cells
Methylammonium lead halide perovskites (CH3NH3PbX3) are considered promising photovoltaic materials because of their high absorption coefficient and charge carrier mobility. Furthermore, the alignment of the energy band edges in these materials enables charge carrier delivery across the band gap with minimal loss of particles or energy. The perovskite triiodide absorber has an electron-hole diffusion length of 100nm or longer, and a planar heterojunction perovskite solar cell of mixed halide perovskite has a 1μm diffusion length. Electron-hole pairs therefore separate easily and are likely to become free carriers at room temperature. In addition, perovskite solar cells have low production costs and are compatibile with flexible substrates. The power conversion efficiency of perovskite cells has increased from 3.8% in 2009 to 20.1% in 2015 as a result of optimized cell structure and chemical composition.
Recent studies by Yang and Green1,2 suggest that it is the polycrystalline grain boundaries (GBs) in perovskite solar cells that have a benign effect on carrier transport, as is seen in chalcopyrite and kesterite solar cells.3, 4 One theory is that bending of surface potentials near the GBs helps carrier separation because the photoexcited electrons tend to gather at the boundaries, and the holes are likely to be expelled from the facets (which are induced by downward band bending). Another suggestion is that chemicals surrounding the lead(II) iodide (PbI2) phase at the methylammonium lead iodide (MAPbI3) GBs enable improved carrier behavior due to reduced recombination in the GBs and the titanium dioxide (TiO2)/perovskite surface.
However, beyond these studies there is still insufficient understanding of the effects of GBs on the electrical properties of some perovskite semiconductor materials. One example is mesoporous CH3NH3Pb(I,Br)3 films (where I is iodide, and Br bromide), which potentially offer a cost-effective material for optoelectronic devices. Therefore, we have further investigated the role that the GBs play in mesoporous perovskite solar cells.
We considered the effects of passivation on the GBs, whereby treatments such as chemical coatings of buffer materials, inherent effects of fundamental charged particles, or artificial energetic treatment of surface ions or plasma could assist carrier separation. Specifically, we focused on inherent processes obtained by band potential bending, which we can achieve by introducing an inhomogeneous distribution of elements, such as sodium in copper indium gallium selenide cells, and PbI2 in perovskites.
We characterized the GBs of the perovskite film by Kelvin probe and conductive atomic force microscopy to investigate their roles in a perovskite heterojunction device with different bromide ratios. We fabricated a mesoporous perovskite solar cell with an ∼14% conversion efficiency for CH3NH3Pb(I,Br)3 solar cells. The charged GBs play a beneficial role in electron-hole de-pairing and in suppressing recombination, and enable high-efficiency perovskite solar cells.5 Potential bending of a few hundred millivolts was sufficient to separate the electrons and the holes, because the positive potential repelled the holes and attracted the electrons. This de-paring reduced the chances of recombination in the GBs, and therefore enhanced the collection of carriers and increased the photoconversion efficiency. We also investigated the polarization and hysteretic properties of perovskite thin films by measuring the local piezo response using piezo response force microscopy (PFM), which is commonly used to image ferroelectric materials. Methylammonium lead halide (MAPbX3) is not ferroelectric. However, we considered PFM to be a suitable tool to assist investigation of the hysteretic I–V (current-voltage) curves of the polycrystal MAPbX3-based solar cell. It is very likely that perovskite has at least some bound charges or dipoles, which may even form some domain structures on the surface, and we concluded that there is a need for further study of ferroelectricity or related properties in perovskites. Jiang and coworkers recently reported surface potential profiles of a variety of planar-type perovskite solar cells.6
To summarize, we achieved passivation of GBs in perovskites experimentally by inherent positive bending of the surface potentials near the GBs, and confirmed the results by Kelvin probe force microscopy. It is possible that the passivation process could enhance the perovskite material's photoconversion efficiency. Our future work will focus on elucidating the role of free carriers and bound charges (if any) in perovskite devices and materials.
William Jo is a professor in the Physics Department and director of the Renewable Research Center. He is currently studying thin-film and perovskite solar cells. His research group also works on oxide-based electronics.
Gee Yeong Kim is a PhD student whose research is in characterizing thin-film solar energy materials using Kelvin probe force microscopy, and in fabrication of the photovoltaic devices.
Bich Phuong Nguyen received her BS and MS in Vietnam, and recently joined as a PhD student. Her work is focused on stability issues in lead- and tin-based perovskite solar cells.