The Study On PAA-PEG Thermosetting Gel Electrolyte For Quasi-Solid-State Dye-Sensitized Solar Cell

Since the prototype of a dye-sensitized solar cell (DSSC) was reported in 1991 by O'Regan and M Gratzel, it has aroused intensive interest over the past decade. The highest light-to-electricity conversion efficiency of 11 % has already been achieved. However, the long-term performance of DSSC is poor due to the leakage, volatilization and other potential problems in liquid electrolytes. In order to overcome these problems, we carried out several studies on the preparation of high long-term stability and high photovoltaic performance of quasi-solid-state dye-sensitized solar cell (QS-DSSC) based on PAA-PEG thermosetting gel electrolyte (TSGE). The study is including the following aspects:(1) The solvents in liquid electrolyte not only play as solution for dissolving I-/I3- redox couples, but their some physical characteristics, especially the donor number values, also have some important influences on the photovoltaic performance of DSSCs owing to the existed donor-acceptor interaction among the solvents, I-/I3- redox couples and the active nanocrystal TiO2 film. The study selected GBL, NMP and PC as the basic solvents for liquid electrolyte due to their suitable donor number for normal light-to-electricity conversion processes in DSSC. The influence of donor number on the Voc and Isc values of DSSC was carried out by mixing GBL with NMP or PC to form different donor number and measuring the photovoltaic performance of DSSCs with different solvent components in liquid electrolyte. It is found that solvents with larger donor number enhance the Voc but usually decrease the Isc. According to this mechanism, DSSC with conversion efficiency of 5.73 % containing the optimized components with 30 vol. % NMP and 70 vol. % GBL, 20.79 donor number was obtained. The value is 14 % higher than that of DSSC with pure GBL. For further addition of 0.4 M PY into the above optimized solvents, the light-to-electricity conversion efficiency of DSSC enchanced to 6.70 %.(2) There should be existed some interaction between the matrix of TSGE and liquid electrolyte for well swelling of the matrix to form thermosetting gel electrolyte.It is found that DSSCs with liquid electrolyte containing some higher donor number of solvents show better photovoltaic performance. According to Lewis acid-basic theory, the higher donor number of the solvents show higher Lewis basic values. Therefore, the study selected 3D network structure acid polymeric hybrid PAA-PEG as the matrix of TSGE, which is formed through the acid-basic interaction between PAA-PEG matrix and liquid electrolyte. The liquid electrolyte absorbency and ionic conductivity of TSGE can be well controlled by mixing different volume ratio of GBL with NMP or PC in mixed solvents. TSGE with the optimized liquid electrolyte composition (30 vol. % NMP, 70 vol. % GBL and 0.4 M PY) shows high liquid electrolyte absorbency and ionic conductivity. Finally, 4.74 % of light-to-electricity conversion efficiency of QS-DSSC with this kind of TSGE is obtained.(3) The ionic additives NaI/I2 do not simply play as charge carriers in TSGE, they also show determining influence on liquid electrolyte absorbency of TSGE, which further affects ionic conductivity of TSGE and photovoltaic performance of QS-DSSC. It is a dynamic factor for the swelling of PAA-PEG matrix in liquid electrolyte through the complexation interaction between Na+ and the functional groups existed in PAA-PEG matrix such as carboxyl and hydroxyl groups. Furthermore, the changed concentrtion of I2 can adjust the mole ratio of I-/I3- redox couples, ionic conductivity of TSGE and photovoltaic performance of QS-DSSC. Taking into account of all the above factors, the optimized concentration of NaI/I2 is 0.5 M and 0.05 M.(4) Although the PAA polymer is acid, due to the hydrophilic property, it can not swell in organic liquid electrolyte, even in the Lewis basic electrolyte. So it needs another polymeric component with amphiphilic or hydrophobic property, which will have some important influences on the function of TSGE and QS-DSSC. It is found that with the increase of molecular weight and added amount of PEG in PAA-PEG matrix, the 3D network structure of TSGE is formed gradually. The liquid electrolyte absorbency and ionic conductvity of TSGE are also increased at the same time. In the case of TSGE with low liquid electrolyte absorbency, the diffusion of ions is mainly dependent on the chain segment in PAA-PEG matrix. The speed and ionic conductivity are relatively low. While for TSGE with high liquid electrolyte absorbency, the transportation of ions is mainly through the solvent channel in gel electrolyte, which is efficient, so the ionic conductivity is relatively high. The state of TSGE is also dependent on the liquid electrolyte absorbency. With the increase of liquid electrolyte absorbency, the state of TSGE turns from hard to soft, which results in the improved interfacial contact property among nanocrystal TiO2 film, electrolyte layer and counter electrode in QS-DSSC. The photovoltaic performance of QS-DSSC is also increased. QS-DSSC with the optimized composition of TSGE shows 6.10 % light-to-electricity conversion efficiency. Additionally, QS-DSSC with the 3D network structure of TSGE shows higher long-term stabiltiy than that of DSSC.(5) The dissolution of whole or part of matrix in electrolyte hinders the diffusion of ions and causes low ionic conductivity. Meanwhile, high ionic concentration of electrolyte will cause serious shrinkage or phase-separation between matrix and electrolyte in traditional gel electrolyte. In order to solve these problems, a new kind of gel electrolyte with independent microporous structure is designed. It is named as thermosetting closed microporous polymer gel electrolyte (MPGE). The novel structure is formed through in situ polymerization and microphase separation of one component in matrix. The large amount of low viscosity liquid electrolyte can be hold in the closed microporous. The wall of microporous can sufficiently swell to form gel state, which is benefit for the ionic transportation among closed microporous. In this paper, PPy is in situ synthesized in PAA-PEG matrix to form ternary matrix. It is well known that PPy can not dissolve in most organic solvents, which will separate from ternary matrix. The interpenetrate structure and hydrogen interaction among PAA, PEG and PPy hinder the macrophase separation of PPy from ternary matrix, so phase-separation can only occur in microphase. MPGE with this novel structrue shows high ionic tolerance and ionic conductivity. Also the I2 optimal concentration for normal light-to-electricity conversion processes in QS-DSSC is increased 3 times than that of TSGE, which results in big increase of Isc to 15.28 mA·cm-2 , a little higher than that of DSSC 13.34 mA·cm-2. The I2 doped PPy has a negative influence on Voc of QS-DSSC, which can be suppressed through modification of active nanocrystal TiO2 film with CaCO3. QS-DSSC with MPGE shows 7.01 % light-to-electricity conversion efficiency and high long-term stability after the above modification.