Effect of sorbitol doping in PEDOT:PSS on the electrical performance of organic photovoltaic devices
Highlights
► We discuss the direct current (DC) conductivity of PEDOT:PSS films containing various weight ratios of sorbitol dopant. ► The work function is shown to steadily decrease with increasing dopant content. ► The sorbitol dopant is able to improve the efficiency of the device. The charge collection at the anode influenced the open circuit voltages.
Introduction
Solar cells based on organic materials have recently been intensively investigated in order to increase their efficiency. Inserting a poly(3,4-ethylene dioxythiophene):poly(styrene-sulfonate) PEDOT:PSS layer can lower or raise the work functions of the cathode and anode, or enhance the cohesion, and thus lower the interfacial series resistance. Moreover, PEDOT:PSS layers have been widely used to improve the efficiency of organic cells. The conductivity of the PEDOT:PSS layer is a particularly important factor for device performance. It is known that the conductivity of the PEDOT:PSS film can be increased by the addition of polyols, such as sorbitol, mannitol or glycerol, to the dispersion from which the film is produced [1], [2], [3], [4]. These additives have also been reported to improve the hole extraction from polymer solar cells [5]. The enhanced conductivity is believed to be related to the interaction between the sorbitol and the PEDOT:PSS. Scanning-tunneling microscopy studies by Timpanaro et al. have shown that the sorbitol influences the nanometer-scale morphology of the PEDOT:PSS [6]. According to their research, the sorbitol interacts with both the PEDOT and the PSS, causing their chains to separate. As the PEDOT:PSS film is heated up, the sorbitol evaporates but the already separated chains have more freedom in comparison to the chains in samples which had no sorbitol incorporated [2]. This favorable effect reduces the hopping distance between the PEDOT chains and enhances the conductivity of the PEDOT:PSS film. A similar effect has been observed by Ko et al., who found that doping PEDOT:PSS with mannitol, a structural isomer of sorbitol, improves the efficiencies of organic solar cells [7]. The temperature-dependent conductivity of doped non-crystalline materials like PEDOT:PSS can be described in the context of variable range hopping (VRH) as follows:
In this equation, is the conductivity at infinite temperature, T0 is the characteristic temperature, and α is the exponent that in standard VRH theory is equal to 1/(1 + D), where D is the dimensionality of the system. Nardes et al. have studied the conductivity of sorbitol-containing PEDOT:PSS thin films made by spin coating. They found that incorporating the sorbitol increased the conductivity by several orders of magnitude. This was accompanied by a transition from a 3D VRH to a quasi-1D VRH model. This effect may be due to the PEDOT-rich grains changing the conduction mechanism from 3D hopping to 1D hopping, which means that the sorbitol resulted in a conductivity increase by reducing the distances between the PEDOT chains [8].
In this paper, we report the effects of varying the sorbitol content in a PEDOT:PSS film on the performance (I–V curve) of an organic solar cell. Adding sorbitol to the PEDOT:PSS affects its work function, which in turn has an effect on the open circuit voltage of the solar cell.
Section snippets
Experiment
Cz 1–5 wt% of D-sorbitol 97% (Sigma–Aldrich) was added to an aqueous dispersion of PEDOT:PSS. This mixture was stirred for at least 10 h using an ultrasonic bath, after which it was filtered using a 0.5 μm filter. The resulting PEDOT:PSS was deposited on ITO by spin coating. The samples were then moved into a glove box and annealed on a hot plate at 150 °C for 1 h in order to remove the water and most of the sorbitol.
Bulk heterojunction solar cells were prepared using an active layer of
Results and discussion
The electrical properties of conducting polymers are strongly dependent on their chemical and physical structures. The conductivity of the pristine and sorbitol-doped PEDOT:PSS films were measured using a 4-point probe. The results are presented in Table 1. It is clear that the conductivity of the PEDOT:PSS film is affected by the sorbitol content. In the case of a sorbitol content of 8 wt%, the conductivity reached over 70 S/cm. However, for sorbitol contents of over 10 wt%, the conductivity
Conclusion
In conclusion, varying the sorbitol concentration in the PEDOT:PSS anode buffer layer influences the efficiency of the organic solar cells. The improvements in conductivity after doping are attributed to the size changes of the inter PEDOT grains that lead to lower intergrain hopping barriers. The work functions of the sorbitol-doped PEDOT:PSS films are lower than those of the pristine films and they decrease with increasing sorbitol concentration. The charge collection at the anode influenced
Acknowledgement
This work was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2008-N-PV12-P-09-0-000).
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