L3 group

Research Activities







Maurizio Martino

University of Salento,

Dipartimento di

Matematica e Fisica

"Ennio De Giorgi"

Via Arnesano, 73100 Lecce, Italy ph: (39) 0832 297495

fax: (39) 0832 297505


 P3HT and PCBM bilayer

 modified on 17/06/2012


  • The first single step MAPLE (ss-MAPLE) deposition of a bilayer structure consisting of the most-studied donor-acceptor combination for polymer solar cells, namely, the electron donating polymer [regioregular polymer poly-(3-hexylthiophene) (P3HT) and the electron accepting fullerene [6,6]-phenyl-C61-butyric-acid methylester (PCBM). Furthermore, the same solvent has been used for both polymers, thus overcoming the typical drawback of re-dissolution of the bottom layer occurring in the conventional solution-based deposition techniques.

    A polymeric solar cell based on the ss-MAPLE deposited P3HT/PCBM heterojunction has been fabricated and characterized. The two polymers, P3HT and PCBM (Sigma Aldrich), were both dissolved in toluene with the weight concentration of 0.3 wt.%.

     MAPLE depositions of each material were performed using a pulsed Lambda Physik (LPX-305i) KrF excimer laser (l=248 nm, t=20 ns, pulse rate=10 Hz). To deposit P3HT and PCBM layers, 6000 and 4000 laser pulses were applied obtaining a corresponding final thickness of 68 nm and 25 nm, respectively.  UV-Vis absorption spectra were acquired for the single polymer layers and for the bilayer structure, deposited on silica slabs, using a Perkin Elmer spectrophotometer in the wavelength range 350-750 nm with a resolution of 2 nm. Atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive x-ray spectrometry (EDS) were used to characterize the morphological and compositional properties of the deposited samples. In particular, SEM and EDS analyses were performed by using a NVISION 40 Focussed Ion Beam (FIB) system equipped with a high resolution SEM Field Emission Gun (FEG) Gemini column and an Inca Energy 350 X-ACT Oxford EDS spectrometer.

    Morphological and chemical investigations were carried out both in plan view and cross-sectional geometry. AFM measurements were performed in non contact mode (Park XE 70 instrument). Finally, a bilayer solar cell was fabricated.

     A poly(3.4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) layer was spin-coated onto an ITO/glass substrate. Then, the active layer was fabricated by depositing the bilayer structure onto the PEDOT:PSS layer with 8000 laser pulses for P3HT layer (90 nm) and 4000 for PCBM layer (25 nm). To complete the device structure, LiF (0.6 nm-thick) and Al (ca.100 nm-thick) were thermally evaporated at low pressure (<1x10-6 Torr). The active area of the devices was 0.03 cm2). Current density versus voltage (J-V) characteristics were measured using a Keithley 2400 sourcemeter both in dark and under AM 1.5-G illumination, filtered irradiation with an incident power of 100 mW/cm2). All of the measurements were carried out under ambient conditions


    Figure 1. Color online) Cross-section (a), top view (b), and EDS spectrum (c) of the P3HT/PCBM bi-layer with a 68 nm thick layer of P3HT and a 25 nm thick layer of PCBM

    UV absorption spectra of the single layers of P3HT and PCBM and of the bilayer  P3HT/PCBM, are reported in Fig. 2., thus excluding intermixing or damaging phenomena related to the bilayer deposition. As a preliminary application, we tested the ss-MAPLE bilayer production in fabricating a solar cell geometry based on the configuration ITO/PEDOT:PSS/P3HT(90 nm)/PCBM(25 nm)/LiF/Al. The PEDOT:PSS was deposited by spin-coating according to the standard procedure for the preparation of conventional solar cells.

    Figure 2. (Color online) UV-visible absorption spectra of the P3HT single layer (dotted line), PCBM single layer (dashed line), and P3HT/PCBM bilayer (full line). The arithmetical sum of the absorption spectra of the two single layer polymers is reportedtoo (dot-dashed line).

    The layered structure of the device is sketched in Fig. 3, where it is shown the current density-voltage curves (dark and illuminated) of the fabricated bilayer. The open circuit voltage is Voc=0.32, the short circuit current is Jsc=0.33 mA/cm2), the fill factor is FF=28%, and PCE=0.03%. Our ss-MAPLE deposition procedure also rules out the contamination consequences related to the MAPLE deposition experiments of organic bilayers, already reported in the literature.

    Figure 3. Current-voltage characteristics of the bi-layer photovoltaic device in dark (open circles) and under one-sun illumination (AM 1.5 G; filled circles). Inset image: Sketch of the bilayer device structure fabricated by ss- MAPLE technique.


Lecce Laser Laboratory

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