Low temperature reactively sputtered crystalline TiO2 thin film as effective blocking layer for perovskite solar cells
Introduction
Solar cells employing organometal perovskite (e.g. CH3NH3PbI3) light absorber instead of dye in dye sensitised solar cells (DSSCs) have been recently recognised as one of the promising next generation photovoltaics [1], [2]. The perovskite materials possess superior photovoltaic properties, namely, direct bandgaps, high absorption coefficient, long exciton diffusion lengths and excellent charge transport properties [3], [4]. The power conversion efficiency of lead halide perovskite solar cells (PSCs) have sky rocketed from 3.8% to about 22.1% within a short time span [5], [6], [7].
At first the perovskite materials were used in conjunction with mesoporous TiO2, similar to that of DSSC architecture, where, an absorber layer with mesoporous semiconductor oxide layer is sandwiched between the electron transport layer and hole transport medium (HTM) [8]. However, it has been demonstrated that perovskite absorber is capable of operating in a much simpler planar heterojunction architecture [9], [10], where a perovskite layer is sandwiched between a HTL and a compact TiO2 thin film which is coated over fluorine doped tin (FTO) glass substrate [11]. Upon absorption of sunlight the perovskite generates charge carriers, the electron is transported to TiO2 whereas the hole is conducted through the perovskite to the HTM. The TiO2 compact layer plays two major roles (i) it blocks the direct contact between the perovskite and FTO, which otherwise could lead to charge recombination (ii) it serves as an electron selective contact that collects the photo generated electrons from the perovskite layer [12], [13].Therefore, the blocking layer is requested to be uniform without cracks and pinhole, otherwise the blocking function will be strongly impaired [14]. So far, many methods have been used for the synthesis of TiO2 blocking layer like spray pyrolysis [14], electrochemical deposition [15], atomic layer deposition [16], [17], sol gel [18] and DC magnetron sputtering [19]. The last method is attractive not only for the ease of control over the process parameters but also it is an industrial scalable technique which allows reproducible results, over large surface area [20], [21]. Most of these methods require the use of high temperature (500 °C) sintering and thereby limiting the choice of substrates. High temperature sintering has several drawbacks like higher cost and slower production, temperature resilient substrates, preventing the use of plastic and pliable soft substrates. The use of other oxides like Al2O3 [22], Al-ZnO [22] and SnO2 [17], [23] as a block layer has been reported in different cell architectures and mixed perovskite cells respectively. However, TiO2 is the material of choice for this study because of its energy band alignment with that of the CH3NH3PbI3 perovskite layer [24].
Therefore, the main objective of this study was to reactively sputter crystalline TiO2 thin film at low temperature which can be potentially used as a blocking layer in PSC. Generally, TiO2 thin film prepared by sputtering is very compact which is required for a good blocking layer. This low temperature synthesis process will extend the choice of the substrates to cheaper and flexible polymer substrates. Polymer substrates give the devices light weight, flexibility which open new avenues like flexible electronics and power generating textiles [20]. Here, it is reported that planar heterojunction perovskite solar cells consisting of FTO coated substrate, thin TiO2 blocking layer reactively sputtered at 150 °C, CH3NH3PbI3 perovskite layer, 2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (Spiro-OMeTAD) HTL and silver contact with the power conversion efficiency of 8.7% which demonstrates the potential use of heat resilient substrates. Furthermore, it is also found that the thickness of the blocking layer plays a crucial role in determining the photovoltaic performance of the cell. Herein, the effect of plasma treating of the FTO substrate, prior to the blocking layer deposition, on the photovoltaic performance of the solar cell was also investigated.
Section snippets
Materials
Unless specified otherwise, all chemicals were purchased from either Alfa Aesar (USA) or Sigma–Aldrich (USA) and used as received. 2, 2′, 7, 7′-tetrakis (N, N-di-p-methoxyphenylamine) - 9, 9′-spirobifluorene (spiroOMeTAD) was purchased from Merck KGaA (Germany). CH3NH3I was synthesised by mixing 24 mL of CH3NH3 (33% in ethanol) and 10 mL of HI (57% in water) in 100 mL ethanol. After stirring for 2 h, the solvent was removed using a rotary-evaporator. The white crystals were washed with diethyl
Morphology of the reactively sputtered TiO2 thin film
The morphology of a bare FTO and FTO with TiO2 thin films that were reactively sputtered at various deposition time with a substrate temperature of 150 °C were analysed using SEM and the images are presented in Fig. 1. The bare FTO (Fig. 1A & a) shows characteristic morphology of tin oxide crystals. The thickness of the FTO layer was estimated to be 500 nm from the cross-sectional analysis. The FTO surface with 10 min (Fig. 1B & b) TiO2 deposition appeared almost similar to bare FTO, indicating
Conclusion
A crystalline TiO2 thin film was successfully grown at a low temperature (150 °C) by a pulsated DC magnetron reactive sputtering system. Plasma treatment time of the FTO substrates had an influence on the solar-to-electric energy conversion efficiency of PSC. 5 min plasma treatment of FTO was found to give better efficiency when compared to 15 min and 30 min treatment. The reactively sputtered TiO2 thin films were used as a blocking layer in perovskite solar cells. Based on our results, the optimum
Acknowledgements
The authors acknowledge the XPS facilities at RMIT and the solar cell testing facility at Monash University.
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