Investigation on The Thermal Stability of Polymer/Fullerene Derivatives Bulk Heterojunction Solar Cells
Date Issued
2014
Date
2014
Author(s)
Liu, Wei-Shin
Abstract
In recent years, there is a tremendous growth in the performance of polymer solar cells. The lifetime and durable under prolonged exposure to sunlight and weather of polymer solar cells are much more important for commercialization. In this research, it will be discussed with the factors and mechanisms of thermal stabilities in the morphology of bulk heterojunction polymer solar cells.
In the first aspect, we have designed and synthesized a series of fullerene derivatives with four visible-region absorption compounds which are having a similar structure with P3HT and conjugated groups with absorption in visible region, to improve the light-absorptivity of C60 and the thermal stability of the active layer. The best power conversion efficiency (PCE) is 3.14% in the polymer solar cells which based on 4,6-bis(4-(2-ethylhexyl)thiophen-2-yl)thieno[3,4-b]thiophene-fulleropyrrolidine (2TTT-EH-C60) as electron accepter blending with P3HT. Although the PCE is not better than the devices fabricated with P3HT:PCBM, we have successfully constructed a devices with high thermal stability. Both OM and TEM images of the thermally aged blend film show the amorphous nature of 3T-EH-C60 and 2TTT-EH-C60 which effectively suppresses the thermal-driven aggregation of C60 adducts during aging process, leading to a extremely stable morphology. Consequently, the PCE is stable on annealing at 130℃ for 480 minutes. Besides, the conjugated groups have similar structure with P3HT, it may act as the compatibilizer of P3HT and PCBM to avoid the phase separation during long time thermal annealing inorder to achieve the superior thermal stability of polymer solar cells.
Second aspect, is mainly focused on the mechanism and factors in phase separation with long time thermal annealing in the active layer. We have carried out, using P3HT with organic impurities, without organic impurities, with higher RR value or lower RR value, as electron donor to blend with PCBM. After long time thermal annealing, the active layers may form the large-scale PCBM aggregations, but the stabilities have the large difference. GISAXS study revealed that the large-scale PCBM aggregations is not the major reason for thermal instability. The critical factor on thermal stabilities is the nanoscale phase separation in the active layer. Materials with low molecular weight and low RR value may have the poor crystallinity, it may form the PCBM clusters with a small amounts of grains with large size. After annealing, the stability may decrease because of the huge distance between the clusters with increasing resistance and pathways. Otherwise, the P3HT without organic impurities or high RR value may shape a large amounts of grains with smaller size. It observed that the photovoltaic performance is stable because of the large amount of clusters forming the effective electron transporting pathways.
Third aspect, it is observed that the PTB7-Th:PCBM may shape the large-scale PCBM aggregations after annealing at 100℃ for 900 minutes. The aggregation may decrease the interface area with donor and accepter, hole mobility and photovoltaic performances from 6.41% to 2.85%. Furthermore, while adding bis-PCBM as phase separation inhibitors, it is observed that it may decrease the PCBM aggregation with long time annealing, in order to get better thermal stability. Besides, the device performance i.e., the PCE can still remain at 75% after annealing at 100℃ for 900 minutes.
Subjects
高分子太陽能電池
形態熱穩定性
聚(3-己基噻吩)
富勒烯衍生物
bis-PCBM
SDGs
Type
thesis
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