Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
A novel type of thin film photovoltaic device, known as perovskite solar cell, has entered the field during the last 2 years based on a solution-processable layered semiconductor as the absorber, which combines a range of unique properties such as large optical absorption cross section, low exciton binding energy and high crystallinity. Quite notably, this absorber was successfully incorporated in different solar cell configurations (sensitized solar cells, mesosuperstructured solar cells, planar heterojunction solar cells and hole conductor-free solar cells) providing maximum overall power conversion efficiencies ranging from around 11% up to over 16% under 1 sun AM1.5G solar illumination [1]. However, despite the high performance of this type of solar cell, the physics behind the operation of the solar cells are far from being well understood.
Electrochemical Impedance Spectroscopy (EIS) is a powerful technique due to its ability to distinguish the dielectric properties of individual contributions of the components of a solar cell. Additionally, in the case of dye solar cells (DSCs) where modelling of the operation with equivalent circuits is now well established, it provides the ability to investigate the parameters of the electronic processes defining the cells' operation, such as the chemical capacitance (Cμ), the recombination resistance (Rrec) and the diffusion length (Ln). However, in the case of perovskite solar cells where the absorber does not “charge” TiO2 with electrons, things seem more complicated. Even though values of diffusion lengths and conductivities have been already presented [2] in agreement with those found in literature estimated by other techniques [3], there is still a debate whether or not a transmission line, observed at frequencies lower than 100 Hz, can be correctly attributed to diffusion of carriers through the perovskite layer.
In this work, we examine two types of solar cells based on the CH3NH3I3-xClx perovskite, previously invented in our lab, i.e. mesosuperstructured solar cells and planar heterojunction solar cells which easily provide efficiencies over 10% [4]. We deconstructed the cells (by fabricating a variety of different cell architectures with or without perovskite, having much simpler structures than the whole cell) and tried to distinguish the contribution by each interface. The analysis of the results points to a significant difficulty to assess correctly parameters such as depletion capacitance, perovskite’s conductivity, effective diffusion length and minority carrier lifetime.
[1] Snaith H. J. Perovskites: The Emergence of a New Era for Low-Cost, High-Efficiency Solar Cells, J. Phys. Chem. Lett. 2013, 4, 3623–3630. [2] Gonzalez-Pedro V.; Juarez-Perez E. J.; Arsyad W. -S.; Barea E. M.; Fabregat-Santiago F.; Mora-Sero I.; Bisquert J. General Working Principles of CH3NH3PbX3 Perovskite Solar Cells, Nano Lett. 2014, 14, 888–893. [3] Stranks S. D.; Eperon G. E.; Grancini G.; Menelaou C.; Alcocer M. J. P.; Leijtens, T.; Herz L. M.; Petrozza A.; Snaith H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber, Science 2013, 342, 341-344. [4] Lee M. M.; Teuscher J.; Miyasaka T.; Murakami T. N.; Snaith H. J. Efficient Hybrid Solar Cells Based on Meso-superstructured Organometal Halide Perovskites, Science 2012, 338, 643-647. [5] Liu M.; Johnston M. B.; Snaith H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition, Nature 2013, 501, 395-398.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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