Controlling Morphology in Polymer–Fullerene Mixtures

Abstract

In the past several years, polymer-fullerene mixtures have been intensely studied for use in organic solar cells because they can be deposited from solution, are compatible with lowcost roll-to-roll fabrication technology, and have shown high power conversion efficiency (g), up to 4-5%. [1][2][3] The best devices consist of a single bulk-heterojunction active layer, in which the polymer (donor) and fullerene (acceptor) are deposited from a common solvent. As the solvent dries the donor and acceptor components separate into domains. The final efficiency of the solar cell has been shown to be extremely sensitive to the size, composition, and crystallinity of the formed domains. [4,5] Improvement of the morphology in devices fabricated with a mixture of [6,6]-phenyl C 61 -butyric acid methyl ester (PCBM) and regioregular poly(3-hexylthiophene) (P3HT) has been achieved by using heat-treatment techniques [2,6] and long-time solvent curing, [1] with resulting record efficiencies. More recently, a method for increasing the crystallinity of the P3HT component has been introduced which involves filtering preformed nanofibers of P3HT out of solution and mixing the prepared nanofiber dispersion with PCBM to increase the efficiency of as-cast devices. [7] Interestingly, the best device performance was achieved by mixing lower-molecular-weight (M W ) amorphous P3HT back into the solution to reduce the crystalline content of the active layer and, thereby, to increase connection between crystalline domains. Studies of the M W impact on P3HT/PCBM solar cells have indicated that a large polydispersity and number-average molecular weight (M n ) over 19000 g mol -1 leads to improved efficiency. [8,9] Morphology studies of organic field-effect transistor (OFET) devices indicate that the increased M W leads to better network formation between crystalline domains. [10,11]

Long time thermal annealing effects on the film morphology and performance of polymer solar cells with calcium electrode

“…[5][6][7] However, a long time (> 2 h) annealing has not yet been studied for P3HT:PCBM solar cells with a thin calcium electrode of which thickness is typically in the range of 5-30 nm. 6,11 In this study we have attempted to anneal P3HT:PCBM blend films and corresponding devices for up to 5 h (the longest time ever reported). To understand the long time annealing effect on the device performance, the blend film morphology was measured with the annealing time.…”

Monitoring degradation mechanisms in PTB7:PC71BM photovoltaic cells by means of impedance spectroscopy

“…However, there are still some issues, 35 such as device stability and life time that must be solved before OPV turns into a real 36 alternative to inorganic devices [6]. 37 Regarding materials, poly (3-hexylthiophene) P3HT and fullerene [6,6]-phenyl-C60 38 butyric acid methyl ester PC 60 BM blend has been the most studied and understood 39 system, and it has achieved efficiencies up to 4 % [7]. This value has been far exceeded …”

P3HT-Based Solar Cells: Structural Properties and Photovoltaic Performance

“…The realization that morphology and device efficiency are 211 intimately related resulted in rapid advances in the use of new instrumentation to 212 study organic nanostructure [40,41]. At the same time, further OPV device 213 efficiency increases were realized by the use of optimized morphology-controlling 214 strategies including the use of thermal annealing [42], solvent annealing [43], and 215 the application of various co-solvent additives [44,45]. The most cited paper in all 216 of OPV research (>2,900 total) was published by Li et al [43].…”

“…In 579 subsequent research that compared several PCBM selective additives, it was deter-580 mined that 1,8-di-iodo-octane (DIO) produced BHJ layers with the highest PCE 581 [137]. Moulé et al published the use of nitrobenzene (NB) as a nonsolvent additive 582 for both P3HT and PCBM [45]. Both DIO and NB produce unannealed P3HT: 583 PCBM devices with PCE near 4% [45,138].…”

“…Moulé et al published the use of nitrobenzene (NB) as a nonsolvent additive 582 for both P3HT and PCBM [45]. Both DIO and NB produce unannealed P3HT: 583 PCBM devices with PCE near 4% [45,138]. DIO has been shown to be an essential 584 solvent additive for many copolymer donors in mixtures with PCBM and PC 71 BM.…”