In Situ Morphology Studies of the Mechanism for Solution Additive Effects on the Formation of Bulk Heterojunction Films

Abstract

and renewable or earth abundant materials. [ 1,2 ] For photovoltaic panels, the energy recovery time is the length of operation of a panel required to generate the energy used to manufacture the panel. In thin fi lm inorganic panels, this time has dropped to about one year. [ 3 ] One of the revolutionary appeals of roll-to-roll manufacturing of organic photovoltaics (OPV) is the potential to achieve energy recovery times as low as 10 d. [ 4 ] Tremendous advances have recently been made in OPV effi ciencies, [ 5 ] all based on optimization of the bulk-heterojunction (BHJ) motif. [ 6,7 ] The performance of a BHJ requires the optimization of both its molecular-scale order and its nanometer-scale domain structure. [ 8 ] In general, the optimized structure is not at equilibrium. Many techniques have been employed for BHJ fi lm optimization, [ 9 ] including solvent choice and post deposition thermal [ 10 ] or vapor annealing. [ 11 ] Recently, formulations using low volatility liquid additives to achieve higher effi ciencies without thermal treatment have become popular. [12][13][14] Although additives such as 1,8-octanedithiol (ODT) and 1-chloronaphthalene (CN) have almost ubiquitous benefi cial effects, there is no consensus on either the origin or mechanism of their effi cacy. In some systems, additives cause smaller domains, [ 15,16 ] while in others they cause larger domains. [ 12,17 ] The effi cacy of ODT and diiodooctane has been attributed to selective solvation of the fullerene, enabling improved polymer order. CN, however, is typically a good solvent for both polymer and fullerene and thus must act via a different mechanism.We recently employed real-time optical techniques to study the mechanism by which the additives ODT and CN infl uence the solidifi cation of the poly(3-hexylthiophene):phenyl-C61butyric-acid-methyl-ester (P3HT:PCBM) BHJ. [ 18 ] Both additives promoted a greater than 5× improvement in device effi ciency over the additive-free fi lm in the absence of thermal annealing. Post-deposition characterization with ultraviolet-visible absorption spectroscopy (UV-Vis) and grazing incidence X-ray diffraction (GIXD) established that the additives increased the polymer's local order and crystallinity. Additionally, energy-fi ltered transmission electron microscopy (EF-TEM) revealed that the The most successful active fi lm morphology in organic photovoltaics is the bulk heterojunction (BHJ). The performance of a BHJ arises from a complex interplay of the spatial organization of the segregated donor and acceptor phases and the local order/quality of the respective phases. These critical morphological features develop dynamically during fi lm formation, and it has become common practice to control them by the introduction of processing additives. Here, in situ grazing incidence X-ray diffraction (GIXD) and grazing incidence small angle X-ray scattering (GISAXS) studies of the development of order in BHJ fi lms formed from the donor polymer poly(3-hexylthiophene) and acceptor phenyl-C61-butyric acid methyl e...

Real-time evaluation of thin film drying kinetics using an advanced, multi-probe optical setup

“…during the drying process. 7,8 Therefore, gaining knowledge over the drying properties of polymer/fullerene thin films and correlating this information with the performance of lab-scale BHJs are essential in order to design better performing organic photovoltaic devices and transfer the manufacture protocols to large scale production. Over the past decade, a considerable amount of in situ characterization methods has been used to study drying dynamics and microstructure evolution of BHJs.…”

“…9 One of the most popular methods is white-light/laser reflectometry, using which the evolution of drying film thickness can be determined. 7,8,[10][11][12][13][14][15][16][17][18] Instead of reflectometry, some investigations have employed spectroscopic ellipsometry (SE), which is more sensitive to sub-nanometer regions, allowing the user to probe final stages of thin film drying. [19][20][21] Additionally, laser light and X-ray scattering techniques are used either as stand-alone methods or in combination with other optical methods.…”

Reducing the efficiency–stability–cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells

“…The crystalline correlation length (CCL) and the relative crystallinity of the P3HT phase for each of the ternary blends have been calculated in Table S2. 39 The as-cast P3HT:IDTBR:60PCBM blend exhibits the lowest P3HT crystallinity and CCL (corresponding to the smallest crystallite size). P3HT:IDTBR:IDFBR exhibits the highest relative crystallinity amongst ternary films, indicating more pronounced polymer:small molecule phase separation for both the as-cast and annealed films.…”

Combining theory and experiment for X-ray absorption spectroscopy and resonant X-ray scattering characterization of polymers

“…Although examining structure preand post-processing is critical, a full understanding of structure development and chemical changes requires advanced in situ or in operando characterization that can probe dynamics and chemical kinetics. In situ and time-resolved investigations are more frequently done with hard X-rays [66,[100][101][102][103] since experiments do not need to be done under vacuum at these energies. The high vacuum chambers used for soft X-ray experiments makes in situ studies much more challenging, and further complications arise for scattering due to limitations on sample-detector distance.…”