Elsevier

Organic Electronics

Volume 12, Issue 12, December 2011, Pages 2131-2139
Organic Electronics

Structural templating of chloro-aluminum phthalocyanine layers for planar and bulk heterojunction organic solar cells

https://doi.org/10.1016/j.orgel.2011.08.031Get rights and content

Abstract

Chloro-aluminum phthalocyanine (ClAlPc) film growth on 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and MoO3 is studied and correlated to ClAlPc/C60 solar cell performance for both planar and bulk heterojunction (HJ) architectures. On top of unheated substrates, ClAlPc films grow amorphous independent of the substrate surface. When heated to 105 °C, ClAlPc grows with a face-on orientation on MoO3, with a crystalline phase I-like absorption profile. On FDTS, the film is optically characterized as phase II, and adopts an edge-on orientation. Implemented in planar HJ cells, the latter films show a substantially higher current compared to the other growth conditions, leading to 3% efficient cells. This current increase is investigated with spectral response and reflectivity measurements, and is found to be related to a more efficient exciton dissociation. Next, ClAlPc and C60 are co-evaporated on FDTS and MoO3 modified ITO substrates to fabricate bulk HJ devices. Notably, we find that when a thin pure “templating” layer of ClAlPc is grown first, the subsequently grown ClAlPc:C60 bulk HJ propagates the templating effect, and films show a higher crystallinity than without this templating layer, with higher fill factors as a result. On MoO3, this approach yields efficiencies above 4%.

Highlights

► ClAlPc/C60 planar and bulk heterojunction (HJ) solar cells grown on MoO3 and on FDTS. ► On FDTS, a phase II, edge-on orientation leads to higher currents for planar HJs. ► On MoO3, the phase I, face-on orientation results in a greater open-circuit voltage. ► A pure ClAlPc layer prior to ClAlPc:C60 co-evaporation improves crystallinity. ► Use of such a templating layer leads to +4% efficient bulk heterojunction solar cells.

Introduction

Progress [1] in the power conversion efficiency of organic photovoltaic cells (OPVCs) has generated significant interest for the generation of low-cost, renewable energy. The first critical improvement of OPVC device architecture was the introduction of the donor/acceptor (D/A) interface [2] that functions as a dissociation site for strongly bound excitons that are the product of light absorption in an organic semiconductor. The next major development was the bulk heterojunction (BHJ) architecture, whereby an interpenetrating D/A network ensures that a high percentage of excitons are able to diffuse towards a nearby D/A interface [3], [4]. In solution-processed solar cells, such a BHJ is usually made by mixing donor and acceptor in one solution prior to deposition. In small-molecule evaporated OPVCs, BHJs are typically formed via co-evaporation of the donor and acceptor materials.

The solar cells investigated here are based on chloro-aluminum phthalocyanine (ClAlPc)/fullerene C60 as a D/A system. This combination has shown promise as it produces a high open-circuit voltage (Voc) of up to 0.85 V with respect to its near infrared absorption edge of 1.5 eV [5], [6]. However, ClAlPc/C60 cells seem very sensitive to growth conditions, as varying the ClAlPc evaporation rate from 0.1–1.5 Å s−1 leads to a variation in Voc from 0.49–0.71 V. Later, it was found that the deposition rate affects the molecular orientation of the ClAlPc molecules relative to the indium tin oxide (ITO) substrate, and that this in turn modifies the energy level alignment at the organic heterojunction interface [7]. Another effective way to control the growth is by modifying the substrate surface properties. To template the growth of subsequent ClAlPc layers [8], [9], an interlayer composed of 3,4,9,10-perylene tetracarboxylic acid (PTCDA) has been used at the anode, whereby the improved stacking of the ClAlPc molecules led to an increase in short-circuit current density Jsc of 25% [10]. For organic transistors, the use of self-assembled monolayers (SAMs) is a well-established technique for the control of the growth [11]. Using this approach, the molecular orientation of ClAlPc molecules could be changed from face-on (parallel to the substrate) to edge-on (normal to the substrate) by treating a SiO2 substrate with octadecyltrichlorosilane [12]. For OPVCs, SAMs have been used to treat the anode, though here the aim has typically been to adjust the ITO work function, rather than adjusting the growth of subsequent layers [13], [14], [15], [16].

To obtain a BHJ that combines good transport properties with efficient exciton separation, the control of morphology and growth is even more critical. For example, the co-evaporation of a planar phthalocyanine (Pc) and C60 at room temperature will typically lead to an amorphous film [17], [18]. Compared to the pure CuPc layer these amorphous films have poor hole mobilities, limiting cell efficiency. By applying an elevated substrate temperature (Tsub) during Pc:C60 deposition, the efficiency can be improved compared to deposition at room temperature [19]. Also here, the growth surface can have a large impact on the solar cell performance, with films grown on 3,4-polyethylenedioxythiophene:polystyrenesulfonate (PEDOT:PSS) showing a different morphology and lower efficiency compared to devices with films grown directly on ITO [20].

For ClAlPc, Li et al. showed that the surface area of the films could be increased by using oblique angle vacuum deposition [21]. This resulted in an increase in Jsc in their ClAlPc/C60 cells, thereby showing that ClAlPc cells could be improved with a bulk heterojunction approach. First promising results for co-evaporated ClAlPc:C70 BHJ cells have already been reported [22], where Tsub was shown to have a strong impact on the growth of the blend, with nanocrystal formation at an elevated temperature from 370–390 K. Consequently, Jsc and FF were enhanced significantly, resulting in a power conversion efficiency of 4.1%.

In this article, we will further investigate the growth of ClAlPc for BHJ solar cells. We will focus on the use of structural templating layers, an approach which has already proven successful in planar heterojunction cells, but has not been thoroughly investigated for their role to improve BHJ-based cells. The two different surfaces we compare as templating layers are molybdenum oxide (MoO3) and MoO3 treated with a SAM of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS). First, pure ClAlPc films were grown on top of these surfaces, and the influence of Tsub on the film properties was investigated. Planar HJ solar cells were then produced from these pure films to probe how the different growth properties affect the ensuing solar cell performance.

Next, we proceed by growing co-evaporated ClAlPc:C60 films at elevated Tsub in order to grow crystalline BHJ layers. Here, we observe a significant device performance improvement depending on the presence of an initial pure ClAlPc templating layer underneath the BHJ layer. Bulk heterojunction solar cells produced with these blends all show increased current with respect to the planar HJ devices, with power conversion efficiencies (η) greater than 4%.

Section snippets

Experimental

Glass substrates coated with ITO (Kintec, 85 nm, sheet resistance <20 Ω □−1) are cleaned by subsequent ultrasonic treatment in detergent, de-ionized water, acetone, and iso-propanol, for 5 min each, followed by an ultraviolet-ozone treatment for 900 s. On the samples for film characterization, a 25 nm thick layer of PEDOT:PSS (HC Starck Clevios P) was deposited. Prior to FDTS surface modification, 10 nm of MoO3 is evaporated at a rate of ∼1 Å s−1 and pressure of ∼10−7 Torr in a vacuum chamber. Then FDTS

Characterization of pure ClAlPc films

Four samples are prepared to study the influence of temperature and substrate surface on the film growth of ClAlPc:

  • a)

    ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = room temperature (RT))

  • b)

    ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = RT)

  • c)

    ITO/25 nm PEDOT:PSS/5 nm MoO3/50 nm ClAlPc (Tsub = 105 °C)

  • d)

    ITO/25 nm PEDOT:PSS/5 nm MoO3/FDTS/50 nm ClAlPc (Tsub = 105 °C)

To study the orientation of the ClAlPc molecules with respect to the substrate, an X-ray reflectivity (XRR) measurement is performed. The PEDOT:PSS in

Conclusion

In conclusion, we have investigated ClAlPc films grown on either FDTS or MoO3. At RT, the ClAlPc films grow amorphous, independent of the surface. When heated to 105 °C, ClAlPc grows face-on on MoO3, with a phase I-like absorption profile. On FDTS, the film is characterized as edge-on, phase II. These films are then used to fabricate planar HJ cells. On FDTS, the induced phase is accompanied by a lower Voc, but also by a higher current, leading to 3% efficient planar HJ cells. The higher current

Acknowledgements

The research leading to these results has received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under grant agreement 212311 of the ONE-P project. K.V. acknowledges the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for financial support.

References (39)

  • D. Janssen et al.

    Thin Solid Films

    (2006)
  • T. Sakurai et al.

    Org. Electron.

    (2007)
  • M.A. Green et al.

    Prog. Photovolt: Res. Appl.

    (2011)
  • C.W. Tang

    Appl. Phys. Lett.

    (1986)
  • G. Yu et al.

    Science

    (1995)
  • P. Peumans et al.

    Nature

    (2003)
  • D. Cheyns et al.

    Appl. Phys. Lett.

    (2010)
  • R.F. Bailey-Salzman et al.

    Appl. Phys. Lett.

    (2007)
  • S.W. Cho et al.

    J. Phys. Chem. C

    (2010)
  • C.H. Cheng et al.

    Appl. Phys. Lett.

    (2010)
  • B. Yu et al.

    Adv. Mater.

    (2010)
  • K.V. Chauhan et al.

    J. Phys. Chem. C

    (2010)
  • S.A. DiBenedetto et al.

    Adv. Mater.

    (2009)
  • L. Li et al.

    Adv. Energy Mater.

    (2011)
  • A. Sharma et al.

    J. Appl. Phys.

    (2009)
  • N. Beaumont et al.

    Energy Environ. Sci.

    (2011)
  • S. Khodabakhsh et al.

    Adv. Funct. Mater.

    (2006)
  • R.M. Cook et al.

    Adv. Eng. Mater.

    (2011)
  • J. Xue et al.

    J. Appl. Phys.

    (2005)
  • Cited by (37)

    • Transparent photodetectors with ultra-low dark current and high photoresponse for near-infrared detection

      2021, Organic Electronics
      Citation Excerpt :

      To avoid absorption in the visible region, chloroaluminum phthalocyanine (ClAlPc) is a suitable candidate due to its absorption is in the near-infrared (NIR) region [10,11]. For instance, application of fullerene (C60) material as an acceptor and ClAlPc as a donor for the bulk-heterojunction of ClAlPc:C60 as fabricated by Li et al. successfully demonstrated a transparent organic photovoltaic (OPV) with AVT up to 77.45%, while Verreet et al. achieved 3% cell efficiency with ClAlPc:C60 based OPVs by the aid of structural templating method [12,13]. Although the bulk-heterojunction of ClAlPc:C60 was applied in the OPVs, this kind of active layer is promising to be used in the structure of transparent OPDs, since OPDs can be built based on the structure of the OPV as demonstrated recently by Lee et al. [10].

    • Analysis of ITO cleaning protocol on surface properties and polymer: Fullerene bulk heterojunction solar cell performance

      2013, Solar Energy Materials and Solar Cells
      Citation Excerpt :

      The IPA, acetone, and methanol series then allows structured transition from a hydrophobic solution to a highly polar, low boiling point media. For direct comparison with literature protocols [5,13,14,16–19], several substrates were sonicated in sequential acetone and IPA baths (S2) or detergent solution, water, acetone, and IPA (S3). Since it is typically included in substrate cleaning procedures, UVO exposure was also evaluated following the solvent-only protocols.

    View all citing articles on Scopus
    View full text