Small-bandgap polymer photovoltaic devices

Lu Shengli , Yang Mujie
(Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027)

1 INTRODUCTION
Solar light is the most important source of regenerative energy and represents mankind's only inexhaustible energy source. In the last couple of years, the development of solar cells based on organic molecules^[1] and conjugated polymers^[2-4] has progressed rapidly. Semiconducting polymers in combination with electron accepting fullerenes were found to show an ultrafast photoinduced charge transfer reaction^[5-6] at the donor-acceptor interface, which results in a metastable charge-separated state. Energy conversion efficiency exceeding 3.3% under AM1.5 illumination has recently been reported^[7] for bulk heterojunction polymer photovoltaic devices. One of the limiting parameters in these polymer photovoltaic devices is the mismatch of their absorption to the terrestrial solar spectrum. At present, substituted poly(p-phenylene vinylene)s and polythiophenes are typically used in the construction of polymer photovoltaic devices. The optical bandgap of these conjugated polymers (Eg=2.0-2.2eV) is not optimized with respect to the solar emission, which has the maximum photon flux around 1.8eV. For efficient harvesting the terrestrial solar spectrum in conjugated polymer based photovoltaic devices, small-bandgap polymers are needed. Poly(heteroarylene methines) are known as small-bandgap conjugated polymers^[8,9], because they contain alternating aromatic and quinoid segments in the main chain. The use of small-bandgap polymers expands the spectral region of bulk heterojunction solar cells and is a viable route to enhance the number of photons absorbed.

2 EXPERIMENTAL       
PTNBQ and PTBBQ were synthesized by acid-catalyzed polymerization of p-nitrobenzaldehyde and m-bromobenzaldehyde with thiophene, respectively^[8,9]. PCBM was synthesized according to Ref [10]. The chemical structure of them are shown in Scheme 1.
    Absorption spectra were obtained using HEWLETT Packard 8453 spectrophotometer.
    Photovoltaic devices were fabricated using PTNBQ+PCBM(1/4,w/w) and PTBBQ+PCBM(1/4,w/w) as the active layer, respectively, which was spin-coated from a toluene solution. The thickness of the active layer of PTNBQ+PCBM was 50nm and PTBBQ+PCBM was 85nm, respectively, as determined by the Surface profilometer(Tencor Alpha-500). Indium tin oxide (ITO) was used as the anode and 100nm thick poly(ethylene dioxythiophene)-poly(styrene sulfonic acid)(PEDOT-PSS) layer(Bayer Batron-P) was incorporated between ITO and the active layer to reduce device leakage. The PEDOT-PSS layer was dried in a vacuum oven at 80ºC for 1h. Ba(barium) metal was thermally evaporated onto the top of the active layer, finally, 170nm thick Al(aluminum) was thermally evaporated onto the top of Ba. The deposition rates and the thickness of the evaporation layers were monitored by a thick/rate meter(Sycon, STM100). The deposition rates were usually 0.5-1nm/s. The cathode area defined the size of the active area was 0.15cm². Except for spin-casting PEDOT-PSS layer, all the fabrication steps were carried out in a nitrogen glove box. Dark and photo I-V characteristics of the devices were measured with a Keithley 236 source-measure unit. Photo current was measured under illumination (78.2mW/cm²,AM1.5) with a tungsten lamp. The photosensitivity was measured with a commercial photomodulation spectroscope setup(Merlin digital lock-in, Oriel) including a xenon lamp, an optical chopper, a monochromator and lock-in amplifier operated by PC computer. Calibrated Si photodiode was used as the standard in the determination of photosensitivity.

3 RESULTS AND DISCUSSION         
Poly(heteroarylene methines) are known as small-bandgap conjugated polymers, because they contain alternating aromatic and quinoid segments in the main chain^[8,9,11]. From Figure 1 and Figure 2, it is seen that both PTNBQ and PTBBQ have the broad absorption in the visible region from 400 to 600nm. The corresponding absorption edges of the spectra are at 690 and 660nm, hence the optical bandgaps are 1.79 and 1.88eV, respectively.
    Figure 1 and Figure 2 also show the absorption of PCBM, the composite of PTNBQ with PCBM and the composite of PTBBQ with PCBM, respectively. The absorption spectrum of PCBM is similar to that of C₆₀, with the absorption peak at 343nm and smaller peaks located at 420-500nm and 550-720nm, respectively. The absorption peaks at 336nm of the composite of PTNBQ+PCBM and 337nm of the composite of PTBBQ+PCBM are assigned to PCBM absorption, the other absorption of PCBM in visible region is almost overlapped with those of the PTNBQ or PTBBQ. So we can deduce that the absorption of the composite is just superposition of the peaks of them and PCBM without any indications of extra spectral features. This result confirms that there is no significant interaction between the two materials in the ground state.

Figure 1 The UV-Vis spectra of PTNBQ (in THF), PCBM (in toluene) and PTNBQ+PCBM (1/4,w/w) composite (in toluene)

Figure 4 I-V characteristics of device ITO/PEDOT-PSS/PTBBQ+PCBM (1/4,w/w)/Ba/Al

Table 1 Photovoltaic performance parameter of the devices under AM1.5

    Compared Figure 5 with Figure 1 and Figure 2, the photosensitivity form of these two devices are almost identical and coincide quite well with their absorption spectra. The photosensitivity peak at about 330nm in Figure 5 comes from the absorption by PCBM, while the broadened peaks at 400-500nm originate from combination of PCBM and PTNBQ or PTBBQ. A small shoulder in the ranger 600-800nm comes from PCBM absorption. The photosensitivity and incident photo-current conversion efficiency of PTBBQ+PCBM device is larger than that of PTNBQ+PCBM in the whole spectral region probably due to the reason discussed above. The photosensitivity and IPCE data are also summarized in Table 1.

Figure 5 Photosensitivity of photovoltaic devices with PTNBQ+PCBM(1/4,w/w) and PTBBQ+PCBM(1/4,w/w) at zero bias

4 CONCLUSION
Polymer photovoltaic devices using the small-bandgap polymers combined with PCBM as the active layer were fabricated. They show photovoltaic effect. In the future, synthesizing new small-bandgap conjugated polymers with high molecular weight maybe the way to obtain high energy conversion efficiency of photovoltaic devices.

REFERENCES
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