Morphology of All‐Solution‐Processed “Bilayer” Organic Solar Cells

Abstract

Organic photovoltaic (OPV) cells have attracted substantial commercial and scientifi c interest as they provide a pathway for a renewable energy source that is portable and potentially inexpensive. [ 1,2 ] Current state-of-the-art solution-processed OPV cells are based on the so-called bulk heterojunction (BHJ) architecture, consisting of an active layer that is a "blend" between an electron donor and acceptor forming an interpenetrating network structure due to self-organized phase separation. [3][4][5] Compared to a simpler planar heterojunction (i.e., a bilayer architecture), the BHJ geometry provides a greater total surface area for charge separation and reduced recombination, and these advantages should theoretically translate to dramatically enhanced power conversion effi ciency. [ 6,7 ] BHJ cells based on a donoracceptor pair of poly(3-n -hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) have shown effi ciencies approaching 5%, [ 1 ] while effi ciencies of 6.77% have been reported with "low-bandgap" polymer donors. [ 8 ] Given that blending and morphology are critical for device performance, signifi cant research effort has focused on studying the phase separation mechanism and optimum conditions to permit effi cient splitting of excitons into charge carriers. [9][10][11][12] This is widely acknowledged as a delicate balance between forming the interpenetrating network and excessive phase segregation causing carrier traps. [ 13 ] Additionally, due to the short exciton diffusion length in organic semiconductors (approximately 10 nm), [ 14 ] the nanoscale morphology presents characterization and processing challenges. [15][16][17] In contrast, sequential processing to form the active bilayer is conceptually more straightforward. Additionally, as discussed in Ayzner et al. [ 18 ] sequential processing allows the use of organic electron acceptors that may not survive thermal evaporation and in principle allows each layer to be controlled and optimized independently. Such a strategy also lends itself more readily to multiple junction devices which are widely believed to be the pathway for OPV cells to be competitive with inorganic systems (conversion effi ciencies > 10%). However, until very recently, no solution processed bilayer device has performed anywhere near as well as its BHJ counterpart. [ 19 ] In this regard, Ayzner et al. [ 18 ] have reignited interest (and much debate) by reporting all-solution-processed P3HT/PCBM bilayer cells with performance approaching that of a BHJ. This performance was somewhat surprising given the individual active layer thicknesses were larger than the expected exciton diffusion lengths in either component and the planar heterojunction yields a reduced interfacial area for exciton separation. Motivated by these potentially very signifi cant, yet somewhat controversial observations, we have undertaken a detailed study of the morphology of solution processed P3HT/ PCBM devices formed by sequential layer deposition using a very similar approa...

Room-temperature organic ferromagnetism in the crystalline poly(3-hexylthiophene): Phenyl-C61-butyric acid methyl ester blend film

“…The PCBM material was dissolved in dichloromethane which is an orthogonal solvent for P3HT. The film was thermally annealed so that PCBM can diffuse into the P3HT film, which has been proved to form excellent interpenetrating bulk heterojunctions with a large contact area [18,19]. Under illumination of a 650 nm laser beam of 0.3 mW mm −2 , the magnetization nearly doubled after annealing the sample for 10 min, as shown in Figure 3a.…”

On the morphology of polymer‐based photovoltaics

“…56 In fact, the device performances achieved with this postdiffusion route were as good as when the P3HT and PCBM were initially uniformly mixed. [56][57][58][59] Solvent Annealing Approach Solvent annealing is another effective method that has been used to induce phase separation in P3HT:PCBM blends. When the solvent vapor enters the mixture, the mobility of both the P3HTand PCBM increases, enabling the P3HT to crystallize and drive the segregation of the PCBM.…”

Mixing in PCBM/P3HT bilayers, using in situ and ex situ neutron reflectivity

“…It is hoped that the development of further understanding of the morphology of OPV materials in thin-film architectures will underpin the successful commercial application of OPVs. 2,5,10,11 This paper describes a study that focusses on conjugated polymer/fullerene mixing, by looking at the evolution of morphology within a simplified (bilayer) geometry, as a function of the polymer film thickness and temperature.…”

“…[11][12][13][14][15][16][17][18][19][20] These bilayers initially consisted of pure P3HT and PCBM layers, with thermal annealing used as a means to study the intermixing of the PCBM and P3HT. A number of studies have shown that PCBM and P3HT can rapidly intermix, 11,14,18,19 and that this mixing involves diffusion of PCBM into amorphous regions of the P3HT. However, in PCBM/regioregular-P3HT bilayers, there are significant quantitative differences between the composition profiles in some of these studies.…”