Elsevier

Materials & Design

Volume 135, 5 December 2017, Pages 112-121
Materials & Design

Properties of perovskite ferroelectrics deposited on F doped SnO2 electrodes and the prospect of their integration into perovskite solar cells

https://doi.org/10.1016/j.matdes.2017.09.013Get rights and content

Highlights

  • Novel perovskite solar cells were fabricated by replacing the TiO2 layer with PbTiO3 and BaTiO3 perovskite ferroelectrics

  • Power conversion efficiency of nearly 1% was obtained by lowering the serial resistance of the ferroelectric layer

  • Short-circuit current increases with the magnitude of the ferroelectric polarization oriented towards the FTO electrode

Abstract

The integration of ferroelectrics in perovskite solar cells is proposed as possible way to enhance charge collection efficiency. First results on solar cell manufactured with PbTiO3 (PTO) instead of TiO2 have shown negligible values for the power conversion efficiency (PCE). This is explained by the high serial resistance of sol-gel deposited PTO on F:SnO2 electrodes (FTO). Although PTO layer has remnant polarization of 22 μC/cm2, the high potential barrier (0.25 ± 0.05 eV) at the FTO/PTO interface and low carrier mobility (10 8 cm2 V 1 s 1) compared to TiO2 leads to high serial resistance. Better results were obtained with thinner PTO layers grown by pulsed laser deposition, with PCE values up to 0.6%. Further enhancement was obtained by replacing PTO with BaTiO3 (BTO), with PCE value reaching about 0.8% after poling the cell with + 3 V. The most important finding was that the magnitude of the short circuit current increases with the amplitude of the poling voltage while the value of the open-circuit voltage remains about the same, around 0.9 V. This is explained through more efficient collection of the charges generated under illumination in the absorber layer due to the polarization that is present in the ferroelectric film.

Introduction

The perovskite solar cells based on hybrid organic-inorganic halides with perovskite structure, such as methylammonium lead halides (MALHs), have achieved impressive performances in term of power conversion efficiency (PCE) reaching values higher than 20% for standard illumination conditions (100 mW/cm2) [1], [2], [3]. The chemical formula is CH3NH3PbX3 (MAPbX3), where X = I, Br or Cl. The most studied is methylammonium lead iodide (MAPbI3 or shortly MAPI). The original structure of the solar cell comprised a transparent conductive oxide deposited on a glass substrate (usually fluorine doped tin oxide, shortly FTO), a compact TiO2 layer, a mesoporous TiO2 layer (eventually), the active (absorber) layer of halide perovskite, a hole transporter layer (e.g. spiro-OMETAD), and a counter-electrode [4], [5]. A significant research effort is allocated for understanding the origin of this high PCE value and to find ways to further enhance the performances for this type of solar cells by refining the technology or by changing the component materials [6], [7], [8], [9], [10]. It was speculated that, among the phenomena concurring to the high values of PCE in perovskite solar cells can be the possible presence of the polar order (ferroelectricity) in the halide perovskite [11], [12], [13]. There are theoretical studies showing that dipoles are present in MAPI, and that these dipoles may order on small nanoregions helping to the efficient separation of the photogenerated charge carriers due to the induced internal electric field [14]. Other theoretical and experimental studies have shown that there are halide perovskites that can be ferroelectric at room temperature [15], [16]. On the other hand, ferroelectrics as such are considered as potential candidates for future generation of solar cells [17], [18]. Recently it was shown that PCE up to about 8% can be achieved in simple ferroelectric capacitors by careful band-gap engineering [19]. Other studies have shown that deposition of a thin ferroelectric layer on Si surface or on Si p-n junctions may enhance their photovoltaic properties by a more efficient carrier collection at the electrodes [20], [21], [22], [23]. Some attempts were made to combine ferroelectrics with halide perovskites, one reporting enhancement of open circuit voltage in a BiFeO3-CH3NH3PbI3 structure and one reporting better crystallization of CH3NH3PbI3 if P(VDF-TrFE) is used as additive [24], [25]. However, to our best knowledge, no attempts were made to integrate ferroelectrics in the structure of the perovskite solar cells.

The present study is addressing the possible integration of perovskite ferroelectrics in perovskite solar cells. The idea that triggered the study was to replace the TiO2 layer with a ferroelectric one assuming that the internal electric field associated with the presence of the ferroelectric polarization will enhance the charge collection at the electrodes by attracting more electrons towards the FTO transparent contact. Possible enhancement of the short-circuit current and PCE values may be obtained using this approach. The study was conducted on standard structure of perovskite solar cells, involving FTO electrode, ferroelectric or TiO2 layer, MAPI, spiro-OMETAD and an Au back contact. The first deposition method was chosen to be sol-gel, since it is low-cost and has been already used to fabricate ferroelectric layers and solar cell structures [26], [27], [28], [29], [30], [31], [32]. In our initial attempt, PbTiO3 (PTO) was the preferred ferroelectric, due to its high polarization value in polycrystalline form (up to 30–40 μC/cm2 in PTO [32]) compared to other ferroelectrics (2–3 μC/cm2 in BaTiO3 (BTO) [33] and 10 μC/cm2 in doped BiFeO3 [34]). However, we found that replacement of TiO2 with sol-gel deposited PTO in perovskite solar cells leads to negligible PCE values. This is explained by the high thickness of the ferroelectric layer and by the differences in electrical properties compared to TiO2. In order to reduce the thickness, the deposition method was changed to pulsed laser deposition (PLD), which allows more accurate control of the layer thickness compared to sol-gel. Another advantage of using PLD over sol-gel is a lower serial resistance through the introduction of oxygen vacancies acting as donors. PTO and BTO layers of the same thickness were deposited by PLD and integrated into solar cell devices. It was found that BTO was the better performer, leading to about 0.8% value for PCE. However, the most important result of the present study is that the magnitude of the short-circuit current increases as the polarization value inside the ferroelectric layer is increased by poling the solar cell, while maintaining the open-circuit voltage value unchanged. This is a promising result, showing that ferroelectric polarization may actually lead to a more efficient collection of the charge carriers generated under illumination, leading to larger short-circuit currents, with the amendment that more work is required to increasing the PCE value.

Section snippets

Sol-gel PTO film deposition

The following starting reagents were acquired for the synthesis of the precursor solution: lead acetate Pb(CH3COO)2  3H2O (Sigma-Aldrich); titanium isopropoxide Ti[OCH(CH)2]4 (Sigma-Aldrich); 2-methoxyethanol CH3OCH2CH2OH (2ME) (Sigma-Aldrich) as solvent. An excess of 5 wt% lead acetate was added in order to compensate the loss of lead during heat treatments. Dimethyformamide was used as drying agent. The preparation of precursor solution was carried out in an inert atmosphere (N2). Florine doped

Results and discussions

As mentioned in the Introduction section, the driving idea of this study was to replace the TiO2 layer in perovskite solar cells with a ferroelectric material (e.g. PTO), in order to take advantage of the presence of the ferroelectric polarization to enhance charge collection at the electrodes and the PCE value. Because the PTO is replacing the TiO2, it should act as the electron transporting layer. Therefore, the PTO film is assumed to be of n-type due to the presence of some oxygen deficit

Conclusions

A comprehensive study regarding the possible integration of perovskite ferroelectrics into perovskite solar cells was performed. First attempts to fabricate solar cells using sol-gel deposited PTO no enhancement of PCE, instead they have shown almost total destruction of the signal most probably due to the very large serial resistance introduced by the PTO film. Therefore, electric properties of sol-gel deposited PTO were investigated and compared with those of TiO2 layers, both with bottom FTO

Acknowledgement

The authors wish to acknowledge the financial support through EEA Financial Mechanism 2009-2014 under the project contract no 8SEE/30.06.2014 (project with acronym PERPHECT). The help of Dr. Monica Enculescu with SEM photos is also acknowledged.

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