Research on Charge Accumulation and Transport in Hole-Conductor-Free Perovskite (CH3NH3PbI2Cl) -based Nanostructure Solar Cells

Organic-inorganic lead halide perovskite, initially applied as active layer in light-emitting diode (LED) and field-effect transistor ( FET) devices^[1], has been gradually introduced into the development of solar cells and plays such critical role as either light absorber or hole conducting material (HCM). This kind of  hybrid layer structure makes perovskite as a promising candidate in photovoltaic applications, mainly due to its attractive chemical versatility that can readily tune the electronic coupling between inorganic sheets, and eventually tailor its optical/electronic properties^[2]. In order to understand the performance of a photovoltaic device especially involving such novel material for its further optimization consideration, exploring the photogenerated charge accumulation and transport is imperative to be highlighted. Here we report the work on  perovskite (CH₃NH₃PbI₂Cl^[3])-based nanostructure solar cell, including the cell fabrication and characterization,  and particularly provide the analysis on charge carrier properties on the basis of  dynamic electrochemical measurements. In this research stage, we targets more than achieving relatively fair energy conversion efficiency, instead we hope to uncover internal mechanisms of charge carrier movement and subsequently the substantial function of perovskite in photovoltaic device.

The stack structure of the mesoscopic solar cell we developed for this investigation is shown in Fig. 1(a). The nanoporous semiconductor oxide membrane was fabricated by screen-printing technique on fluorine doped tin oxide (FTO) glass substrate with terpineol-based paste made by commercial-available TiO₂ powder (Aldrich, d=25nm). The work electrode coated with the TiO₂ paste was sintered under airflow at 550^oC for 3 h. Iodide-chloride mixed-halide perovskite was synthesized from a N,N-Dimethylformamide solution of CH₃NH₃I and PbCl₂  (3:1 molar ratio) ^[3]. The precursor solution was deposited on porous TiO₂ surface via spin-coating or dipping process, and then was dried at 100^oC. After drying, obvious color variation indicated the formation of perovskite layer. The function of the perovskite layer in nanostructure solar cells strongly depends on the deposition method. In particular, the shape of nanocrystalline CH₃NH₃PbI₂Cl infiltrating into TiO₂ porous membrane, probably with quantum-dot structure, is confirmed by electron microscopy micrographs. One thing should be noted is that, combining the band diagram shown in Fig.1(b), the extremely thin CsSnI_2.95F_0.05 layer (referring our previous work for its synthesis^[4]) deposited on counter electrode performs more like energy ‘staircase’ for hole transport rather than pure hole conducting material. Meanwhile, a heterojunction-like cell (called ‘blank’ cell) containing only CH₃NH₃PbI₂Cl layer on the TiO₂film is produced for comparison.

In order to thoroughly understand the decisive mechanisms of charge accumulation and transport behaviors on photovoltaic conversion, a systematic characterizations including photocurrent transient, open-circuit voltage decay (OCVD) and electrochemical impedance spectroscopy (EIS) were carried out under different operation conditions. Correspondingly the effective lifetime and diffusivity of charge carriers are extracted to investigate the transport properties of photon-generated electrons. Particularly, chemical capacitance (C_μ) is examined since it assists to monitor the Fermi level of electrons and density of states (DoS) in perovskite absorber, which significantly differentiates from conventional dye (e.g. N719) sensitized solar cells on the aspect of charge generation and accumulation in light absorber material.

Consequently, our research demonstrates the promising HCM-free nanostructure solar cells involving solution-processed perovskite material. Fair light absorption and charge accumulation capability make self-assembled CH₃NH₃PbI₂Cl behave as both sensitizer and charge transport material, which helps to constitute new kind of solid-state sensitized solar cells with a much simpler configuration.

Reference

1.         D. B. Mitzi, K. Chondroudis, and C.R. Kagan, Organic–inorganic electronics. IBM J. Res. & Dev. , 2001. 45(1): p. 29-45.

2.         Etgar, L., Semiconductor Nanocrystals as Light Harvesters in Solar Cells. Materials 2013. 6: p. 445--59.

3.         Lee, M.M., et al., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science, 2012. 338 ( 6107): p. 643-647.

4.         Sun, X., et al., Effects of calcination treatment on the morphology, crystallinity, and photoelectric properties of all-solid-state dye-sensitized solar cells assembled by TiO2 nanorod arrays. Phys. Chem. Chem. Phys., 2013. 15: p. 18716-18720.