| In recent decades, the growing demand for energy is driving the development of solar cells and other renewable energy technologies. Due to its low cost, rich material resources, simple preparation process(spin-coating method), roll to roll mass production on flexible substrates and other advantages, polymer solar cells(PSCs) has become the focus of attention of many researchers. The selective light absorption characteristics of organic materials makes PSCs to be applied to transparent or semi-transparent solar cells. Transparent/semi-transparent PSCs have great potential in buildings, windows and tandem cells, which is the reason for choosing this type of PSCs as the object of this study. This paper mainly focused on the preparation and optimization of semi-transparent PSCs and their stability under light illumination. The work can be divided into two parts:(1) Polyethylene glycol(PEG) doped PEDOT:PSS as transparent electrode in semi-transparent PSCsConductive polymer material PEDOT:PSS has a high transmittance in the visible range, and its conductivity can be improved by doping with some polar solvents. In this study, we use PEG doped PEDOT:PSS solution to fabricate the transparent anode in inverted semi-transparent devices with the structure of ITO/PEIE/P3HT:PCBM/ PEDOT:PSS(PEG). In order to form a uniform transparent top electrode on polymer active layer by spin-coating aqueous PEDOT:PSS(PEG) solution, the surface hydrophilicity of the active layer was improved by plasma treatment first. Experimental optimization of PEG concentration was conducted by testing the I-V performance of the semi-transparent devices under the same conditions. The results showed that when doped with 2% PEG, PEDOT:PSS can be used as the best transparent electrode to get the highest conversion efficiency of 0.98%. Semi-transparent cell prepared by all spin-coating method has the average transmittance of 47.7% within the range of 400-760 nm in the visible light, avoiding the use of vacuum system at the same time.(2) The stability of the PEDOT:PSS(PEG) based semi-transparent PSCs under light illuminationStability is an important paprameter of the solar cells. We study the stability of PEDOT:PSS(PEG) based semi-transparent PSCs under the illumination of simulated sunlight of AM 1.5 G(100 mW/cm2). It turns out to be that PEDOT:PSS(PEG) based semi-transparent PSCs performance becomes even better together along with the first 15 minutes illumination and then reaches a saturation conversion efficiency of 1.58%. By using band filters to narrow the wavelength range of light and comparing those semi-transparent PSCs with control inverted PSC devices, we can draw a conclusion that ultraviolet light whose wavelength is less than 450 nm makes the major contribution to this performance enhancement. This performance improvement is attributed mainly to the conductivity enhancement of PEDOT:PSS(PEG) due to the conformation change under UV light irradiation and partially to the better interface between PEDOT:PSS(PEG) and the active layer. This is further verified by the electrochemical impedance spectroscopy(EIS) measurements before and after light illumination. AFM studies also show the chain conformation change of PEDOT when exposing under ultraviolet light. Thus the more effective link among PEDOT chain makes a better conductivity of PEDOT:PSS and then higher performance of our semi-transparent PSCs under light illumination. Another reason for the performance improvement is the better interface between the PEDOT:PSS(PEG) and active layer after light irradiation. This might not only reduce the interfacial contact resistance but also the leakage current in cells. The observed performance recovery for long stored device after light irradiation indicates the reversibility of the conformation change of PEDOT chains. |
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