Enhancing efficiency and decreasing photocatalytic degradation of perovskite solar cells using a hydrophobic copper-modified titania electron transport layer

Highlights

• Copper cations successfully employed to modify the electron transport layer (ETL).

• Advanced engineering strategy focusing on the ETL/perovskite interface optimization. Hydrophobic Cu-TiO₂ compact layer with mitigated photocatalytic action.

• Perovskite solar cells with high power conversion efficiency.

• Devices with low hysteresis and enhanced stability under light stress.

Abstract

The electron transport layer (ETL) plays a pivotal role in obtaining perovskite solar cells (PSCs) with high power conversion efficiency (PCE). Titanium dioxide is a widely used ETL, however it suffers from low electron mobility, poor conductivity and may act as a photocatalyst of chemical reactions leading to degradation of the perovskite. Herein, copper cations employed to modify the titania ETL of PSCs, mitigate the photocatalytic action of the compact layer, increase its conductivity and electron mobility, adjust favorably the energy levels, improve the ETL/perovskite interface, thus enhance perovskite’s light absorption and provoke passivation of the perovskite surface trap states. The investigation of structural and surface chemistry properties revealed the uniform distribution of Cu¹⁺ in TiO₂ and confirmed the determining role of copper in the fast extraction of the photogenerated charge carriers to the adjacent electrode. As a result, the champion planar PSCs based on the hydrophobic Cu-TiO₂ ETL showed a 18.15 % PCE outperforming the reference devices (based on pristine TiO₂ ETL) which showed a PCE equal to 15.78 %. The results followed the same trend also in the case of mesoporous PSCs, proving the universality of our approach. Finally, aging tests confirmed that the copper-modified devices showed higher stability in comparison with the non-modified ones retaining the 53 % of the initial PCE value after 51 days of storage in relative humidity (RH = 25 %) and dark conditions. Photostability experiments proved that the Cu-TiO₂ based devices showed remarkable robustness and retained approximately 91 % of their initial PCE, even after 5 h under continuous UV stress. These results are associated with the suppression of the photocatalytic activity of the ETL and open new perspectives for improving the performance of TiO₂-based PSCs.

Introduction

The field of hybrid halide perovskites has been witnessing a surge of interest over the past few years across the breadth of nanoscience and nanotechnology. Of particular interest, controlling the nanomorphology of the hybrid perovskite absorber can modify intrinsically different properties (crystallinity, defects, grain boundaries) and optimize charge transport in the bulk structures and at the corresponding interfaces, thus leading to highly performing devices. Perovskite solar cells (PSCs), were recently developed and rapidly optimized due to the intriguing optoelectronic characteristics (long diffusion lengths, long charge carrier lifetime and high dielectric constant) of the perovskite polycrystals [[1], [2], [3], [4], [5], [6]]. The latter, employed in the solar cells primarily as sunlight absorbers, are described by the general formula ABX₃, where A is organic and/or inorganic cations, B is a metal cation, and X refer to halide anions. PSCs are characterized by low cost and facile fabrication techniques along with high power conversion efficiency (PCE), which after thorough research has raised within a few years to 25.2 % [[7], [8], [9]]. However, there are still unsettled issues restraining their further commercialization, such as further improvement of efficiency and long-term stability [[10], [11], [12], [13], [14], [15], [16], [17], [18]]. The structure of PSCs consists of a perovskite absorber sandwiched between two charge extraction/transport layers, which lead the photogenerated charge carriers (electrons and holes) to the corresponding electrodes. In this context, the electron transport layer (ETL) plays a vital role in obtaining efficient and stable PSCs [[19], [20], [21], [22]], thus various materials have been used as ETLs including ZnO [[23], [24], [25], [26], [27]], TiO₂ [28], SnO₂ [[29], [30], [31]], WO_x, [32] and PbTiO₃ [33]. Among transition metal oxides, titanium dioxide is the most popular one due to the large band gap, long electron lifetimes, low-cost and facile fabrication methods and its suitably located energy bands against the most common sunlight absorbers [[34], [35], [36], [37]]. However, the TiO₂ conductivity and electron mobility is much lower compared to the corresponding value of the hole transporter, provoking a significant imbalance in the electron flow towards the anode [38,39].

Titania is a well-known photocatalyst [[40], [41], [42], [43], [44], [45]]. Different strategies (including doping) were used to modify the optoelectronic properties of TiO₂ [46]. Doping is usually associated with defects which have the ability to alter the mechanisms of photocatalytic reactions, although the triggering of undesired thermodynamically favored back reactions by excess defects can lead to diminished photocatalytic activity [47,48]. Thus, the TiO₂ layer may act as a photocatalyst of chemical reactions leading to degradation of the perovskites and to a permanent polarization of the film. Ito et al. also found that CH₃NH₃PbI₃ degraded on TiO₂ rapidly under visible and UV–vis light for 24 h in air but on the contrary, Al₂O₃ successfully protected the perovskite, which was verified by UV–vis and XRD measurements [49]. Therefore, ETLs with improved conductivity and increased electron mobility are required [50] in order to reduce the charge carriers accumulation and favor the electron passage to the ETL/perovskite interface. The latter can be achieved, among other ways, through ETL’s dye sensitization approach [50,51] or by ETL doping/modification with metal ions such as Y³⁺, Zn²⁺, Fe³⁺, Mg²⁺, Nb⁵⁺, Ta⁵⁺ and Li⁺ [[52], [53], [54], [55], [56], [57]].

Recently, Soultati et al. confirmed that appropriate lithium doping of the ETL layer is an effective approach to suppress the photocatalytic activity of the titania underlayer, mitigate the interfacial degradation and increase the stability of fullerene and non-fullerene organic solar cells [58]. In the literature of perovskite solar cells, there are a few papers adopting copper materials as additives in charge transport or perovskite layers with positive effects in the performance of the corresponding PSC devices however, these works are focusing on their role as hole transporting materials [[59], [60], [61]]. On the other hand, copper has been extensively used to modify the photocatalytic properties of TiO₂ [[62], [63], [64], [65]] however, as far as we know, the corresponding effects as additive in the ETL of PSCs have not been studied. Herein, a novel interface engineering approach was adopted and Cu-modified TiO₂ was incorporated in planar perovskite solar cells in order to improve ETL/perovskite interface. The Cu-TiO₂ solution was obtained by blending Cu (NO₃)₂.3H₂O with the TiO₂ precursor solution. The presence of copper significantly advanced the quality of the TiO₂ compact ETL by mitigating its photocatalytic action, increasing its conductivity and improving the adjacent interface with the perovskite layer. XPS analysis revealed the presence of copper in the form of oxide Cu₂O, J–V measurements for modified electron-only devices indicated improved electron mobility and conductivity, PL spectroscopy evidenced the advanced electron transport at the Cu-ETL/perovskite interface, while trap density measurements implied the suppression of surface perovskite trap states. Therefore, PSCs with higher open circuit voltage (V_oc) of 1.12 V, higher short circuit current density (J_sc) of 23.15 mA cm⁻² and a higher fill factor (FF) of 73 % were obtained after copper addition, resulting in a champion PCE of 18.15 %. Improved results against reference devices, were also obtained for mesoporous PSCs when copper was added in the mesoporous titania scaffold, indicating the universality of our approach. The findings of this study showed that PSCs performance can be directly enhanced by modifying TiO₂ ETL with Cu cations.