A Study on the Development of Organic Thin Film Solar Cell Device With Optimized Hole Transfer Layer of PEDOT : PSS

Organic thin film solar cells show generally lower power conversion efficiencies than those of the solar cells based on inorganic active materials. To solve the problem diverse research works have been tried: development of new organic semiconductors; modification of internal structure of active layers by means of organic solvent; design and fabrication of vertical device structure; introduction of buffer layer on the side of cathode electrode. Nevertheless, the buffer layer on the side of anode has not been fully optimized. This study is focused on the representative anode-side-buffer layer of Poly(3,4-ethylenedioxythiophene)(PEDOT): poly(styrenesulfonate) (PSS). PEDOT:PSS is an excellent conducting polymer material with favorably high optical transparency for almost entire range of visible wavelength, which is widely used for organic thin film solar cells as well as for other organic material based electronic devices such as organic light emitting diodes. Aiming for further performance enhancement of organic thin film solar cell device, an optimized condition was studied on the PEDOT:PSS by means of the thickness control and modification of the internal structure via addition of different organic solvents in mixture solution for thin film process.


Introduction
Today the exhaustion of energy sources is the most critical issue for modern civilization, which is mainly due to drastic decrease of the remaining fossil fuel based energy resources by continuous and increasing utilization for the conventional electric power generation system.Moreover, environment pollution is another severe problem contributed by carbon dioxide emission from the thermoelectric power generation and radioactive wastes from the nuclear power generation.Focused on these problems of environment and being exhausted energy resources, an environment-friendly generation technology of electricity has been developed since decades, and solar-energy based solar cells have attracted much attention.Enormous amount of the solar light is emanated to the earth and it could be utilized to generate electricity without emission of harmful wastes, and therefore, the solar cell is regarded as an environment-friendly energy generation technology.Nevertheless, current supply price of the electricity via Si-based inorganic solar cell is still relatively expensive (e.g., 30 Yen/kWh in Japan) compared to that (e.g., 22 Yen/kWh in Japan) of the electricity supplied by conventional electric power company.Therefore, price cutting for consumer electricity by reduced production cost would be a critical issue for popularization of the environment-friendly electricity based on solar cell technology.Topics of almost all the solar cell researches are nowadays focused on the low-cost generation of electric power.
Recently, organic thin film solar cell (OTFSC) based on organic semiconductor materials have attracted much attention to solve the above-mentioned problems of environment-friendly and low-cost power generation.Easier and low-cost fabrication technologies distinguished from those of conventional inorganic counterparts would be a typical advantage for the OTFSCs (Pivrikas, Neugebauer, & Sariciftci, 2011).In addition, light-weight originated from the organic material itself, flexibility, and three-dimensional layout could be other merits for OTFCs.If the OTFSCs combined with such advantages could be realized, then wearable or portable power source units could be popularized, and the place for its utilization could be much extended: although the entire amount of the solar energy emitted to the earth is huge enough, but it is strictly limited to the size of being emanated area (e.g., 1.37 kW/m 2 ).An accelerated supply of the solar light power generation could only be

Prepar
Organic th transfer la device: (a) cleaned ul min, respe were then solutions.process fo were also additive of ml).The a PC 71 BM ( thermal ev layer of 2 employing filter.Ope efficiency of the org morpholog orientation     Deeper insight of the thickness-effect was studied by investigation of the light absorbance characteristic (UV-vis) and surface morphology (AFM) on the three varieties of PEDOT:PSS.Figure 7 shows the light absorbance characteristic of the PEDOT:PSS layers analyzed by UV-vis spectrophotometer.Figure 8 shows an image of surface morphology of the PEDOT:PSS prepared by a rotation speed of 5000 rpm compared with that of the ITO anode used for fabrication of organic thin film solar cell device.Difference in surface roughness was obtained from the AFM analysis for the ITO and PEDOT:PSS (5000 rpm; 58 nm), and root-mean-square (rms) and peak-to-valley (P-V) values are summarized in Table 3.The light absorbance characteristic curves (Figure 7) reveal that there is no distinctive variation of light-absorption-peak and light-absorption-wavelength for ITO and PEDOT:PSS layers.It can be thought that the PEDOT:PSS thin films have fairly high optical transmittance characteristic in visible wavelength range.The surface roughness data (Table 3) indicate that the PEDOT:PSS resulted in drastic decrease of surface roughness for both of rms and P-V values.The decreased surface roughness could be contributed to improvement of surface interface with active layers.The increased J sc of the organic thin film solar cell device (Table 1) might be originated by the enhanced charge carrier extraction due to improved surface interface (Gholamkhass & Servati, 2013).Considering the above mentioned factors, an optimized process condition was decided for the preparation of PEDOT:PSS by spin coating technique: rotation speed of 5000 rpm; rotation time of 30 s.

Modification of the Property of PEDOT:PSS by Two Different Additives
Further improvement of hole transport property of the PEDOT:PSS was tried by adding two different additives in the precursor mixture solution for preparation of the hole transfer layer of PEDOT:PSS by spin coating process (Xiao, Cui, Anderegg, Shinar, & Shinar, 2011;Ochiai, Kumar, Santhakumar, & Shin, 2013).4. To clarify the effect of the additive of EG or DMSO in PEDOT:PSS on the characteristic of the resulting OSC device, surface morphology (AFM), light absorption characteristic (UV-vis spectrophotometer), and molecular orientation (XRD) of the PEDOT:PSS layers were investigated.Figure 11 shows images of surface morphology analyzed by AFM.Surface roughness parameters obtained from the AFM analysis are summarized in Table 5: rms [nm]; P-V [nm].The pristine PEDOT:PSS reveals relatively superior surface smoothness than those of the PEDOT:PSS with additives, which is originated that PEDOT:PSS is a conducting polymer with a nature of colloid-dispersive-molecules.The PEDOT:PSS with additive of DMSO shows grains with elongated shape on the surface, which indicates that DMSO effected to promote a condensation of colloid particles.On the other hand, the PEDOT:PSS with additive of EG shows no particles on the surface, which implies that EG did not effect to hinder a condensation of particles of PEDOT:PSS but rather to be formed as needle-shapes (Nano-morphology).Although the PEDOT:PSS with additive of EG reveals clearly increased surface roughness (rms and P-V value) compared to that of the pristine PEDOT:PSS, such a change of the surface morphology might be contributed to modification of internal structure of active layers for improved power conversion efficiency: increase of FF and parallel resistance; decrease of serial resistance.Thickness of the PEDOT:PSS layers are given in Table 6.Thicknesses of the PEDOT:PSS with additives are thinner than that of the pristine PEDOT:PSS, which is possibly due to higher melting point of the organic solvent including the additives of EG (T m = 189 C) or DMSO (T m = 197 C): drying after spin casting might be done slowly.Although too thin layer of PEDOT:PSS could not be worked effectively as buffer layer, the PEDOT:PSS with additive of EG revealed suppressed decrease of FF (Table 4).That might be closely related to the surface morphology analyzed by AFM, so that increase of specific surface area due to additive effected to suppress the Fig

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Table 4 .
Performance parameters of the organic thin film solar cell devices with three different PEDOT:PSS as