Electrically Sorted Single-Walled Carbon Nanotubes-Based Electron Transporting Layers for Perovskite Solar Cells

摘要

class="head no_bottom_margin" id="sec1title">IntroductionOrganic−inorganic halide perovskite solar cells (PSCs) have received a great deal of attention from the photovoltaic (PV) community owing to the remarkable progress in the performance, which has recently exceeded 23% (). Hybrid perovskite-based light absorbers exhibit an outstanding combination of desirable properties, including high absorption coefficients, high ambipolar carrier transport, and long charge-diffusion lengths, in addition to their attractive features such as ease of fabrication, low cost, and rigid/flexible substrate compatibility (, , , , ). A typical state-of-the-art PSC that exhibits high device efficiency consists of a transparent conducting oxide substrate, compact layer, semiconducting metal oxide scaffold layer, perovskite light absorber, hole transporting layer (HTL), and metal electrode. In such a device architecture, the semiconducting metal oxide nanoparticles (NPs) play an important role in extracting and transporting electrons from the perovskite to the conductive electrode (, ). The most commonly used electron transporting material (ETM) is the semiconducting metal oxide TiO2. TiO2 nanostructure materials have been intensively used in various applications such as photovoltaics () and photocatalysis (, ) because of their good chemical stability, wide band gap, lack of toxicity, and high transparency (). However, the TiO2-based ETM suffers from intrinsic drawbacks including low electron mobility, numerous grain boundaries, and heterogeneous spreading of TiO2 nanocrystallite network, which introduces a random transit path for the electrons, leading to rapid charge recombination and thus a deterioration of device performance (). Therefore, there is a real necessity to discover and investigate alternative materials to replace or improve the properties of TiO2-based ETM for further enhancement in the PCE of the PSCs. In this context, various highly conducting carbon-based nanomaterials such as graphene and its derivatives (, , href="#bib56" rid="bib56" class=" bibr popnode">Wang et al., 2013), fullerenes (href="#bib60" rid="bib60" class=" bibr popnode">Wojciechowski et al., 2014, href="#bib22" rid="bib22" class=" bibr popnode">Hou et al., 2018, href="#bib61" rid="bib61" class=" bibr popnode">Yoon et al., 2016), and carbon nanotubes (href="#bib17" rid="bib17" class=" bibr popnode">Habisreutinger et al., 2014b, href="#bib18" rid="bib18" class=" bibr popnode">Habisreutinger et al., 2017, href="#bib36" rid="bib36" class=" bibr popnode">Li et al., 2014, href="#bib39" rid="bib39" class=" bibr popnode">Luo et al., 2017b, href="#bib40" rid="bib40" class=" bibr popnode">Luo et al., 2018) have been widely used in PSCs and are recognized as promising candidates to facilitate charge extraction/transportation and extend electron lifetime. Among these nanomaterials, SWCNTs have shown remarkable properties, such as high aspect ratio, excellent electron mobility, high chemical stability, outstanding mechanical and thermal properties, high transparency, and suitable electronic structure (href="#bib63" rid="bib63" class=" bibr popnode">Zhou et al., 2009), making them ideal for improving the performance of PSCs (href="#bib2" rid="bib2" class=" bibr popnode">Aitola et al., 2016, href="#bib25" rid="bib25" class=" bibr popnode">Jeon et al., 2015, href="#bib35" rid="bib35" class=" bibr popnode">Li et al., 2016). Recently, our group reported for the first time the successful fabrication of efficient PSCs by incorporating SWCNTs into the TiO2 photoelectrodes (href="#bib8" rid="bib8" class=" bibr popnode">Batmunkh et al., 2017b). This work highlights the potential of SWCNTs to enhance the properties of the electron transporting layer (ETL) in PSCs. It is worth noting that pristine SWCNTs (p-SWCNTs) (before separation) are commercially available as a mixture of two-thirds semiconducting (s-SWCNTs) and one-third metallic (m-SWCNTs) nanotubes. Such a variation in the nanotube properties is detrimental to device performance and reproducibility. Driven by the recent advances made in the separation and controlled growth methods of SWCNTs, it is now viable to tailor the optoelectronic properties of this fascinating material to maximize the charge extraction and transport in PSCs (href="#bib6" rid="bib6" class=" bibr popnode">Bati et al., 2018). For example, Snaith's group employed a double HTL composed of P3HT-SWCNTs network and poly(methyl methacrylate) (PMMA) matrix (href="#bib16" rid="bib16" class=" bibr popnode">Habisreutinger et al., 2014a). Comparing the performance of the devices when the as-produced SWCNTs (2:1 s-:m-SWCNTs) were replaced with highly enriched m-SWNTs, an improvement was evident boosting the PCE from a maximum of 14.2% to 15.3%. This efficiency improvement is attributed to the conductivity enhancement resulting from the incorporation of m-SWCNTs with their high conductivity as compared with their s-SWCNTs counterparts. Blackburn and co-workers reported the improved efficiency, stability, and reduced hysteresis when a pure s-SWCNTs layer highly enriched with (6,5) nanotubes was inserted as an interfacial layer between the perovskite layer and Spiro-OMeTAD (href="#bib24" rid="bib24" class=" bibr popnode">Ihly et al., 2016). The PCE of such a device configuration exhibited a dramatic improvement from 14.7% to 16.5%.Although PSCs based on TiO2 with pristine SWCNTs incorporated have shown improved performance and stability, a crucial fundamental question regarding whether integrating a pure single electronic-type SWCNT into the TiO2 photoelectrodes would provide better charge transfer still remains. Moreover, further insight into the effect of the distribution of carbon nanotube (CNT) electronic types would be very valuable.Herein, we report the implementation of SWCNTs separated by electrical type (i.e., semiconducting and metallic) into TiO2 at precisely tuned ratios as an efficient ETL for PSCs. At the optimum SWCNT loadings, notable enhancement in the PV efficiency was achieved regardless of the nanotube types incorporated. Interestingly, (2:1 w/w) s-:m-SWCNTs showed a remarkable PCE of 19.35%, whereas the best performing TiO2-only reference device exhibited an efficiency of 17.04%. This significant efficiency enhancement is attributed to the reduced charge transfer resistance (Rct) resulting from the favorable energy level alignment at the perovskite/ETL interface. In addition, our devices with nanotubes showed improved stabilities under continuous light illumination in ambient environment and exhibited less hysteresis behavior.