Abrupt Morphology Change upon Thermal Annealing in Poly(3‐Hexylthiophene)/Soluble Fullerene Blend Films for Polymer Solar Cells

Abstract

The in situ morphology change upon thermal annealing in bulk heterojunction blend films of regioregular poly(3‐hexylthiophene) (P3HT) and 1‐(3‐methoxycarbonyl)‐propyl‐1‐phenyl‐(6,6)C61 (PCBM) is measured by a grazing incidence X‐ray diffraction (GIXD) method using a synchrotron radiation source. The results show that the film morphology—including the size and population of P3HT crystallites—abruptly changes at 140 °C between 5 and 30 min and is then stable up to 120 min. This trend is almost in good agreement with the performance change of polymer solar cells fabricated under the same conditions. The certain morphology change after 5 min annealing at 140 °C is assigned to the on‐going thermal transition of P3HT molecules in the presence of PCBM transition. Field‐emission scanning electron microscopy measurements show that the crack‐like surface of blend films becomes smaller after a very short annealing time, but does not change further with increasing annealing time. These findings indicate that the stability of P3HT:PCBM solar cells cannot be secured by short‐time annealing owing to the unsettled morphology, even though the resulting efficiency is high.

Initial Performance Changes of Polymer/Fullerene Solar Cells by Short‐Time Exposure to Simulated Solar Light

“…This result supports the notion that continuous illumination with simulated solar light increases the temperature inside the sample holder (sealed tightly), leading to a minimal phase tran- www.chemsuschem.org sition at the surface of the blend film. In the case of metastable domains (phases) caused inevitably by thermal annealing, as often observed from the P3HT:PCBM blend films, [28,29,31] their phase transition could be easily accelerated by even small (thermal) impacts. Hence, we consider that the surface of the P3HT:PCBM blend film might be affected by heating effects during 11 h exposure.…”

“…In contrast, the J-V curves of C-2 devices showed a relatively larger change, that is, a lowered V oc , after 11 h exposure ( Figure 2 b and d), which may be attributed to the imperfect setting of the blend morphology in comparison with the C-1 devices. [28,29] In Figure 3, detailed trends of cell performances as a function of exposure time are given. For C-1 devices (Figure 3 a), the J sc value initially increased as the exposure time increased up to 2 h, but then showed a gradual decay with up-anddown fluctuations.…”

Real‐Time Investigation of Crystallization and Phase‐Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing

“…Hence we checked if the above dynamics hold in a devicelike structure by comparing with glass/ITO/PEDOT:PSS/P3HT:PCBM/Al (15-nm thick) structures. The Al layer was suffi ciently thin for the diffraction from the active layer not to be obscured, though the presence of glass and ITO leads to increased background scattering (diffuse scattering for glass around q = 1.5 Å − 1 , and sharp rings at higher q values for ITO) [ 13 ] that make detailed analysis more diffi cult ( Figure 5 ). The OOP-(100) P3HT peak is the only clearly visible peak before annealing, which indicates wileyonlinelibrary.comdensity in the fi rst minutes correlates well with the rapid initial growth in reciprocal FWHM observed using GIXRD (Figure 2 ).…”

“…For thermal annealing, device performance, morphology, and crystallization have been monitored as a function of the annealing time by quenching individual samples and comparing the results before and after annealing. [ 6 , 11 ] However, there have been few studies of changes in i) crystallization, [12][13][14] ii) density, and iii) carrier mobilities [ 15 ] in situ during annealing.…”

Influence of Physical Load on the Stability of Organic Solar Cells with Polymer : Fullerene Bulk Heterojunction Nanolayers

“…A thermal annealing method has been proposed for crystalline polymerbased BHJ systems [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] , while an additive method is currently used for less-crystalline (amorphous) polymer-based BHJ layers [37][38][39][40][41][42][43][44][45][46][47] . However, it is doubtable that such an optimized BHJ layer (system) can withstand external physical loads such as pressing, hitting, rubbing and the like when it comes to the fullerene nano-domains dispersed in the polymer:fullerene mixture structures.…”