Quantitative Morphology Analysis Study for Improving Polymer Solar Cell Efficiency
Date Issued
2011
Date
2011
Author(s)
Lin, Tsung-Han
Abstract
In this thesis, there are two different methods we used to optimize the performance of polymer solar cell, and we study the enhancement of efficiency by improved quantitative analysis. The organic photovoltaic devices we studied are based on the system of poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT: PC61BM).
In the first part, we utilized the method of two-stage annealing to enhance the power conversion efficiency (PCE) (31% compared to one-stage annealing; from 2.91% to 3.80%) of the organic photovoltaic devices and constructed a scheme of the evolution of morphology after two-stage annealing by the measurement of grazing-incidence wide-angle X-ray diffraction (GIWAXD) and grazing-incidence small-angle X-ray scattering (GISAXS). From GIWAXD, we can get the information of the crystallinity of P3HT (100) planes and we figured out that the intensity increased apparently once the devices were annealed. But there were almost no change between the different thermal conditions (90, 110, 150℃). As a result, we needed in-depth information of PCBM so that we did the GISAXS measurement to know the difference of aggregation of PCBM after two-stage annealing. In order to understand the change of PCBM in the procedure of annealing, we also did the in-situ GISAXS measurement. To explain these data from GISAXS measurement, we combined the Debye-Anderson-Brumberger (DAB) model and Polydispersed hard-sphere model to fit the GISAXS data of our devices under different thermal conditions, include one-step annealing and two-step annealing. Besides, we can get the specific surface area of the PCBM clusters in these devices by Porod law and integrate with the model we use. From the model we established, we can figure out the evolution of the structure of P3HT and PCBM two phase system and hence provide an efficient way to improve the PCE of organic photovoltaic devices by controlling the nanostructure inside.
In the second part, we incorporated copper sulfide (Cu2S) into the P3HT/PCBM two phase system in order to enhance the performance of devices. We found out that if we added adequate amount of Cu2S (0.28 wt%) to the system, the power conversion efficiency increases ~23% compared to the devices with the same thermal condition in the first part (from 3.5% to 4.3%). To precisely know the nanostructure of nanoparticles and PCBM, we use two different Polydispersed Hard-Sphere models to fit the GISAXS data of the active layer P3HT/PCBM/Cu2S. By this combined model, we can figure out the effect of adding nanoparticles in the organic solar cell, and provide a new way to make high efficiency organic solar cell.
In conclusion, we use two-stage annealing method and additive incorporation methods to enhance the performance of devices up to about 4%. The improvement mainly comes from the change of morphology by the measurement of GISAXS. By this analysis, the loose-packing PCBM clusters formed when the annealing temperature below the Tc of PCBM (~130℃), while the dense-packing PCBM clusters formed when the annealing temperature above the Tm of PCBM (~149℃). As a result, if we use the temperature of 150 ℃ to do the two-stage annealing process, the values of mean radius and volume fraction of PCBM clusters we attained from model fitting are the largest among samples we studied. Hence the sample PR_150+PO get the best power conversion efficiency (3.8%) in the first part. Besides, we also incorporate adequate amounts of nanoparticles in the system, and we find out that the effect of adding nanoparticles is like the annealing effect. But once we add too much nanoparticles in the system, the self-aggregation caused by nanoparticles may lead the efficiency to decrease. The best power conversion efficiency we get is 4.3% if we add 0.28 wt% nanoparticles in the system.
Subjects
Polymer solar cell
grazing-incidence X-ray scattering
annealing
inorganic nanoparticles
quantitative analysis
SDGs
Type
thesis
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