Low-temperature processed highly efficient hole transport layer free carbon-based planar perovskite solar cells with SnO2 quantum dot electron transport layer

https://doi.org/10.1016/j.mtphys.2020.100204Get rights and content

Highlights

  • Fully low-temperature processed planar n-i-p structured perovskite solar cells has a hole transport layer free carbon electrode.

  • SnO2 quantum dot electron transport layer was used and improved power conversion efficiency (PCE) was demonstrated.

  • Efficient solar cells with a PCE of 13.4% have been achieved with carbon as the back contact.

Abstract

The use of expensive hole transport layers (HTLs) and back contact along with the stability issue of perovskite solar cells (PSCs) has been a detrimental factor when it comes to commercialization of the technology. In addition, high temperature and long annealing time processed electron transport layers (ETLs, e.g. TiO2) prevent the flexible solar cell application in most polymer substrates. Herein, we opted for HTL-free carbon electrodes owing to their low-cost production and superior stability in air, compared with their noble metal counterparts. In this work, we fabricate planar PSCs using low-temperature solution processed SnO2 quantum dots (QDs) as ETLs, which offers significant advantages over high-temperature processed ETLs because of its excellent electron extraction and hole blocking ability. In addition, by integrating a low-cost and stable carbon electrode, an impressive energy conversion efficiency of 13.64% with a device architecture glass/In doped SnO2/QD-SnO2/perovskite/carbon under 1 sun illumination at ambient conditions have been achieved. This work paves the way to achieve fully low-temperature processed printable PSCs at an affordable cost by integrating the QD SnO2 ETL and carbon electrodes.

Introduction

Photovoltaic devices such as solar cells are used to efficiently convert solar energy into useful electrical energy. More than 90% of the installed solar modules use crystalline silicon panels [1]. In the last few decades, new technologies such as organic photovoltaics, excitonic solar cells, and thin-film modules have been developed to keep costs low and to boost the device performance beyond conventional efficiency limits [[2], [3], [4], [5], [6], [7], [8], [9]]. Perovskite solar cells (PSCs), with an inorganic-organic hybrid absorber (e.g. CH3NH3PbI3), have demonstrated a power conversion efficiency (PCE) from ~3.8% to an impressive 25.2% in less than one decade [[10], [11], [12], [13]]. The rapid development of PSCs has turned heads in the solar industry as it has the potential for replacing the conventional photovoltaic devices, such as Si and CdTe. Going by the theoretical prediction, a single-junction PSC can attain PCEs as high as 33.5%, surpassing the well-established copper indium gallium selenide and CdTe-based solar cells [14]. The hybrid PSCs have gathered attention not only because of their sky-rocketing PCEs but also because of the low-cost materials and synthesis techniques involved as it makes use of simple solution-processing techniques, such as roll-to-roll processing [15,16].

For the highly efficient planar solar cell architecture, either the regular or inverted structure, the PSC layer was switched between an ETL and a HTL. The ETL is an indispensable component to select electrons and block holes while the HTL is desired to collect the holes and form a barrier for the electron transport. Thus, improving the performance of PSCs starts with proper choice of ETLs and HTLs and optimizing their processing conditions. To date, most PSCs use compact layers and mesoporous scaffolds of TiO2 as an ETL, and it has resulted in an impressive PCE (up to 25.2%) through various material and structural optimizations [[15], [16], [17], [18], [19], [20]]. Despite numerous advantages that TiO2 possesses, one major disadvantage to scale up the TiO2-based PSC is its high processing temperature (>450 °C) and long durations for sintering to fully crystallize into the desired polymorph, which translates into high costs and high energy consumption while also hampering the possibility of developing flexible PSCs on most of the low-melting-point polymer substrates [21]. TiO2 also suffers from poor optoelectronic properties including poor conductivity and mobility [[22], [23], [24]]. Moreover, using TiO2 compromises long-term stability as it is prone to photocatalysis under UV illumination [22,25,26]. To address these concerns, replacing TiO2 with SnO2 in planar PSCs plays as an effective way to reduce the annealing time and temperature and to improve the stability and efficiency [[27], [28], [29]]. In particular, photophysical benefit from SnO2 ETL includes excellent charge mobility, wider bandgap, low-temperature synthesis, and a favorable band energy alignment [30,31]. The excellent electron mobility and wider bandgap contributes to more efficient charge transport from the absorber layer to SnO2, which contributes toward improved PCEs. However, SnO2 comes with its own set of issues. For instance, solution-processed SnO2 cannot be fully crystallized at low annealing temperatures (<200 °C) which in turn takes a toll on the electron mobility. If the annealing temperature is increased to fully crystallize the film, it may result in film breakdown [32]. The annealing temperature can be reduced by using the low-temperature processed colloidal nanoparticles, thereby overcoming this dilemma. While the colloidal SnO2 solution may suffer from long-term stability issues, SnO2 quantum dots (QDs) have been extensively explored in PSCs, due to their higher molar extinction coefficients, tunable photoresponse, strong light scattering ability, fast electron transport, and slow recombination, resulting in a dramatic increase in PCE [[33], [34], [35], [36], [37]].

Most commonly used HTLs are organics, such as spiro-OMeTAD and P3HT, and are usually unstable and much more expensive than perovskite materials. Hence, it is desired to eliminate HTLs from PSC architecture to reduce the complexity and the cost of the device fabrication process. By reducing one step of the standard coating process, at least one-third of the manufacturing cost and time can be saved, in addition to savings of material cost for HTLs [[38], [39], [40], [41], [42]]. The noble metal electrode, for example, Au and Ag, also adds to the cost of the device as the metal itself is expensive and it requires to be thermally evaporated in vacuum. These mounting costs will hinder the progress of PSCs toward affordable commercialization. Hence, it becomes imperative to address these challenges by combinations of HTLs with back contact that can also be efficiently used for large-scale deployment of PSCs [[43], [44], [45]]. Fortunately, perovskites are so unique that it can also serve as an HTL because of its ambipolar nature, thereby enabling HTL-free PSCs [[46], [47], [48], [49], [50]]. And these aforementioned issues prompted the researchers to explore the promise offered by carbon-based perovskites as it is earth abundant, low cost, environmentally stable, and conductive [20,[51], [52], [53], [54]]. In particular, the conventional precious HTL-free device is desired to reduce the cost of the PSCs.

Carbon electrode is considered to be the most promising electrode for the back contact as it is cheap, stable, inert to ion migration from perovskite, and inherently water resistant which acts as a protection for the perovskite layer from the moisture [55,56]. But carbon comes along with a set of disadvantages such as low conductivity and the need for HTLs for highly efficient carbon-based PSCs. Although the stability of the carbon-based PSCs is far superior than that of the traditional PSCs, the long-term stability still lags when compared with that of the well-established inorganic solar cells, such as Si, and CdTe solar cells. Ku et al. [52] were the first to use spheroidal graphite and carbon black as a substitute for HTLs and the metal cathode. The fabricated cell reported a PCE of 6.6%, and it was a breakthrough in PSCs that resulted in the evolution of HTL and metal electrode-free PSCs. Since then several forms of carbon such as single-walled and multi-walled carbon nanotubes, graphite powder, carbon cloth carbon black, and so on have been used in HTL and metal electrode-free PSCs [[57], [58], [59], [60], [61]]. Within a short span of time, the PCE of carbon-based PSCs have drastically increased from 6.6% to 17%. But most of the reported work requires the HTL along with carbon [62,63].

Based on the well-established fabrication techniques for high-efficiency PSCs, the fabrication process can be made scalable for flexible devices via roll-to-roll processing by opting for materials that require low processing temperatures (<200 °C) and printing or coating of the back electrode with good contact to the absorber layer without damaging it. Through our work, we address the usage of low-temperature processed materials by replacing the commonly used mesoporous TiO2 as the ETL with SnO2 QDs. And we substitute the commonly used vacuum-deposited back electrode with low-temperature curable carbon electrode. In this work, we develop a fully low-temperature processed carbon-based HTL-free planar PSC with a PCE of ~13.64% by integrating SnO2 QD ETLs. The processing temperature for the whole device, glass/ITO/SnO2/perovskite/carbon, never exceeded 180 °C. This study demonstrates printability for the flexible carbon-based low-cost PSCs on the polymer substrate.

Section snippets

Materials

PbI2 (Sigma-Aldrich, 99.999%), PbBr2 (Alfa Aesar, 99.98%), formamidinium iodide (FAI, GreatCellSolar), methylammonium bromide (MABr, GreatCellSolar), CsI (BeanTown Chemical, 99.9%), SnCl2.2H2O (Acros Organics, 97%), and thiourea (Alfa Aesar, 99%) were used as received. Dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) were purchased from Sigma-Aldrich and used without further purification.

Perovskite precursor preparation

The Cs0.05FA0.81MA0.14PbI2.55Br0.45 precursor solution was prepared with corresponding molar ratios of

Results and discussion

X-ray diffraction (XRD) pattern of the photoanode (ITO, SnO2, and perovskite layers) is shown in Fig. 1a. The (200) peak at 2θ ~ 37.4° indicates that the SnO2 QDs ETL is highly crystalline with a tetragonal rutile structure. The perovskite film presents cubic crystalline structure and is in agreement with the reported triple perovskite structure. The presence of the XRD peaks of photoinactive cubic PbI2 ~ 12.38° suggests that extra PbI2 exist in the perovskite films, which is due to the excess

Conclusions

In conclusion, we successfully fabricated highly efficient and reproducible PSCs by replacing TiO2 with low-temperature processed SnO2 QD ETLs and carbon electrodes. Using the optimized parameters for all the functional layers in the device, we fabricated a planar PSC with a champion PCE of 13.64%. The cells fabricated with SnO2 QD precursors of different concentrations showed similar impressive performance while SnO2 colloid could not match the same. The experimental results reveal that the

CRediT authorship contribution statement

S.N. Vijayaraghavan: Conceptualization, Methodology, Writing - original draft. J. Wall: Conceptualization, Methodology. L. Li: Writing - review & editing, Supervision. G. Xing: Writing - review & editing, Supervision. Q. Zhang: Writing - review & editing, Supervision. F. Yan: Conceptualization, Writing - review & editing, Project administration, Supervision, Funding acquisition.

Declaration of Competing Interest

The authors declare no conflict of interest.

Acknowledgements

S.N.V, J. W, L.P. G, and F. Y acknowledge the support from the Alabama Water Institute inThe University of Alabama and National Science Foundation under Grant No. 1844210.

References (89)

  • W.Q. Wu et al.

    Integrated planar and bulk dual heterojunctions capable of efficient electron and hole extraction for perovskite solar cells with > 17% efficiency

    Nanomater. Energy

    (2017)
  • H.Y. Wei et al.

    Free-standing flexible carbon electrode for highly efficient hole-conductor-free perovskite solar cells

    Carbon

    (2015)
  • M. Batzill et al.

    The surface and materials science of tin oxide

    Prog. Surf. Sci.

    (2005)
  • Y. Luan et al.

    High-performance planar perovskite solar cells with negligible hysteresis using 2,2,2-trifluoroethanol-incorporated SnO2

    Iscience

    (2019)
  • A. Mishra et al.

    Effect of annealing temperature on the performance of printable carbon electrodes for perovskite solar cells

    Org. Electron.

    (2019)
  • Y. Jiang et al.

    Reduced-dimensional α-CsPbX3 perovskites for efficient and stable photovoltaics

    Joule

    (2018)
  • Y.L. Lin

    Organic Photovoltaics Using Multiple Exciton Effects

    (2019)
  • T. Sogabe et al.

    Recent progress on quantum dot solar cells: a review

    J. Photon. Energy

    (2016)
  • S.N. Vijayaraghavan et al.

    All spray pyrolysis-coated CdTe–TiO2 heterogeneous films for photo-electrochemical solar cells

    Mater. Renew. Sustain Energy

    (2018)
  • A. Polman et al.

    Photovoltaic materials: present efficiencies and future challenges

    Science

    (2016)
  • A. Kojima et al.

    Organometal halide perovskites as visible-light sensitizers for photovoltaic cells

    J. Am. Chem. Soc.

    (2009)
  • L. Yang et al.

    Novel insight into the role of chlorobenzene antisolvent engineering for highly efficient perovskite solar cells: gradient diluted chlorine doping

    ACS Appl. Mater. Interfaces

    (2019)
  • H. Tsai et al.

    High-efficiency two-dimensional Ruddlesden-Popper perovskite solar cells

    Nature

    (2016)
  • D.Q. Bi et al.

    Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%

    Nat. Energy

    (2016)
  • W.S. Yang et al.

    Iodide management in formamidinium-lead-halide-based perovskite layers for efficient solar cells

    Science

    (2017)
  • F. Wang et al.

    Activating old materials with new architecture: boosting performance of perovskite solar cells with H2O-assisted hierarchical electron transporting layers

    Adv. Sci.

    (2019)
  • J.H. Heo et al.

    Efficient inorganic-organic hybrid heterojunction solar cells containing perovskite compound and polymeric hole conductors

    Nat. Photon.

    (2013)
  • H. Chen et al.

    Efficient panchromatic inorganic-organic heterojunction solar cells with consecutive charge transport tunnels in hole transport material

    Chem. Commun.

    (2013)
  • A.Y. Mei et al.

    A hole-conductor-free, fully printable mesoscopic perovskite solar cell with high stability

    Science

    (2014)
  • J.M. Ball et al.

    Low-temperature processed meso-superstructured to thin-film perovskite solar cells

    Energy Environ. Sci.

    (2013)
  • T. Leijtens et al.

    Overcoming ultraviolet light instability of sensitized TiO(2) with meso-superstructured organometal tri-halide perovskite solar cells

    Nat. Commun.

    (2013)
  • K. Wojciechowski et al.

    C60 as an efficient n-type compact layer in perovskite solar cells

    J. Phys. Chem. Lett.

    (2015)
  • P. Tiwana et al.

    Electron mobility and injection dynamics in mesoporous ZnO, SnO(2), and TiO(2) films used in dye-sensitized solar cells

    Acs Nano

    (2011)
  • E.H. Anaraki et al.

    Highly efficient and stable planar perovskite solar cells by solution-processed tin oxide

    Energy Environ. Sci.

    (2016)
  • G. Yang et al.

    Interface engineering in planar perovskite solar cells: energy level alignment, perovskite morphology control and high performance achievement

    J. Mater. Chem.

    (2017)
  • L. Zuo et al.

    Tailoring the interfacial chemical interaction for high-efficiency perovskite solar cells

    Nano Lett.

    (2017)
  • J.P.C. Baena et al.

    Highly efficient planar perovskite solar cells through band alignment engineering

    Energy Environ. Sci.

    (2015)
  • H.J. Snaith et al.

    SnO2-based dye-sensitized hybrid solar cells exhibiting near unity absorbed photon-to-electron conversion efficiency

    Nano Lett.

    (2010)
  • W.J. Ke et al.

    Effects of annealing temperature of tin oxide electron selective layers on the performance of perovskite solar cells

    J. Mater. Chem.

    (2015)
  • G.L. Shang et al.

    Facile synthesis of mesoporous tin oxide spheres and their applications in dye-sensitized solar cells

    J. Phys. Chem. C

    (2012)
  • G.L. Shang et al.

    Preparation of hierarchical tin oxide microspheres and their application in dye-sensitized solar cells

    J. Mater. Chem.

    (2012)
  • G. Yang et al.

    Effective carrier-concentration tuning of SnO2quantum dot electron-selective layers for high-performance planar perovskite solar cells

    Adv. Mater.

    (2018)
  • V.T. Chebrolu et al.

    Recent progress in quantum dot sensitized solar cells: an inclusive review of photoanode, sensitizer, electrolyte, and the counter electrode

    J. Mater. Chem. C

    (2019)
  • S. Ameen et al.

    Perovskite solar cells: influence of hole transporting materials on power conversion efficiency

    ChemSusChem

    (2016)
  • Cited by (39)

    • Context and prospects of carbon quantum dots applied to environmental solutions

      2023, Environmental Nanotechnology, Monitoring and Management
    • Interfacial engineering with NiO<inf>x</inf> nanofibers as hole transport layer for carbon-based perovskite solar cells

      2021, Solar Energy
      Citation Excerpt :

      After the spinning process, the fibers formed on the aluminum foil directly beneath the spinneret tip were collected and annealed at 400 °C for 30 min in a box oven with an adequate supply of oxygen flow. PSCs were fabricated as mentioned in our previous work (Vijayaraghavan et al., 2020). In brief, In doped SnO2 (ITO) substrates were successively sonicated in detergent solution, DIW, acetone, and IPA, followed by a 30 min UV-Ozone treatment.

    View all citing articles on Scopus
    1

    Contributions: S. N. Vijayaraghavan and J. Wall contributed equally to this work.

    View full text