Peculiarity of Two Thermodynamically-Stable Morphologies and Their Impact on the Efficiency of Small Molecule Bulk Heterojunction Solar Cells (original) (raw)

Self-Assembly of Selective Interfaces in Organic Photovoltaics

Advanced Functional Materials, 2013

The composition of polymer-fullerene blends is a critical parameter for achieving high efficiencies in bulkheterojunction (BHJ) organic photovoltaics. Achieving the "right" materials distribution is crucial for device optimization as it greatly influences charge-carrier mobility. The effect of the vertical concentration profile of materials in spin-coated BHJs on device properties has stirred particularly vigorous debate. Despite available literature on this subject, the results are often contradictory and inconsistent, likely due to differences in sample preparation and experimental considerations. We attempt to reconcile published results by studying the influence of heating, surface energy, and solvent additives on vertical segregation and doping in polymer-fullerene BHJ organic photovoltaics using neutron reflectometry and near edge x-ray absorption fine structure spectroscopy. We show that surface energies and solvent additives greatly impact heat-induced vertical segregation. We also find that interface charging due to Fermi level mismatch increases PCBM-enrichment at the BHJ-cathode interface. Current-voltage measurements show that self-assembly of interfaces affects the open circuit voltage, resulting in clear changes to the power conversion efficiency.

Importance of Nucleation during Morphology Evolution of the Blade-Cast PffBT4T-2OD-Based Organic Solar Cells

Macromolecules, 2018

Different temperatures, solvents, and additives are used as influencing parameters to drive molecular packing and phase separation for the development of the controlled evolution of nanostructures for organic solar cells (OSCs). The temperature-dependent aggregation (TDA) features of polymers are explored by investigating aggregation in solution and solid thin-film states using solution small-angle neutron scattering (SANS), soft/hard X-ray scattering, and transmission electron microscopy (TEM) characterizations. In situ grazing incidence wide angle X-ray scattering (GIWAXS) reveals that the nucleation process is highly significant and responsible for the ultimate film morphology. Processing conditions such as temperatures, solvents, and additives were used to influence the nucleation and evolution of film morphology. The nucleation process may improve the polymer packing and phase separation. It may translate into optimized multilength scale domains and efficient charge percolation pathways, a strong implication to control the nucleation process for the efficient separation and transportation of charges in bulk heterojunction (BHJ) devices.

Nvestigation of Bulk Heterojunction Organic Photovoltaics †

2017

A key parameter to improve the performance of organic solar cells is the optimization of electronic phenomena at donor–acceptor interfaces through the optimization of the morphology of the bulk heterojunction. The correlative mapping of morphological, electrical and mechanical properties at the nanoscale by advanced scanning probe microscopy techniques allows for a detailed characterization of the local structure–property relationships in bulk heterojunctions solar cells. Unique opportunities for the investigation of these photoactive films are shown here, ultimately suggesting fundamental guidelines toward the accurate engineering of these materials at the nanoscale.

Symmetry and coplanarity of organic molecules affect their packing and photovoltaic properties in solution-processed solar cells

ACS applied materials & interfaces, 2014

In this study we synthesized three acceptor-donor-acceptor (A-D-A) organic molecules, TB3t-BT, TB3t-BTT, and TB3t-BDT, comprising 2,2'-bithiophene (BT), benzo[1,2-b:3,4-b':5,6-d″]trithiophene (BTT), and benzo[1,2-b;4,5-b']dithiophene (BDT) units, respectively, as central cores (donors), terthiophene (3t) as π-conjugated spacers, and thiobarbituric acid (TB) units as acceptors. These molecules display different degrees of coplanarity as evidenced by the differences in dihedral angles calculated from density functional theory. By using differential scanning calorimetry and X-ray diffractions for probing their crystallization characteristics and molecular packing in active layers, we found that the symmetry and coplanarity of molecules would significantly affect the melting/crystallization behavior and the formation of crystalline domains in the blend film with fullerene, PC61BM. TB3t-BT and TB3t-BDT, which each possess an inversion center and display high crystallinity in ...

Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells

Journal of Physical Chemistry C, 2010

Structure and morphology play a critical role in determining the performance of organic photovoltaic devices. In this paper, variation of the postannealing cooling rate is used to create a series of "snapshots" of the vertical and interfacial reorganization processes that occur upon annealing. The data show that slower cooling rates result in significantly enhanced device efficiencies primarily driven by increased short circuit current and fill factor. UV-vis spectroscopy, X-ray diffraction (XRD), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), atomic force microscopy (AFM), and contact angle measurements are used to probe the origin of these improvements. Our results show evidence for a distinct and changing vertical stratification and interfacial structure in the device throughout the annealing process, with both composition and crystallinity varying through the active layer. The implications of these changes are discussed in terms of device properties.

Elongated Nanodomains and Molecular Intermixing Induced Doping in Organic Photovoltaic Active Layers with Electric Field Treatment

ACS Applied Polymer Materials

The effects of the electric-field-assisted annealing on the bulk heterojunction nanomorphology in the P3HT/PCBM active layer of the organic photovoltaic cells (OPVCs) are presented here. It was widely accepted that the electric-field-assisted annealing will facilitate the P3HT, the polar polymer, to be better crystalline to enhance the charge mobility, hence the improvement of the OPVC performance. The influences on the nano-morphology of the electron donor and accepter domains are not well understood. Here, using the cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S), the electric-field-assisted annealing treatment is found to influence the molecular domains to be elongated with the orientation near the direction of the external electric field. The elongation of the molecular domains is believed to facilitate the domain percolation, which causes higher charge mobility, hence the higher short-circuit current density (Jsc). On the other hand, it was also observed that the electronic properties of the P3HTrich and PCBM-rich domains in the electric-field-assisted annealed samples showed smaller 2 energy band gaps and smaller molecular orbital offset between the two domains, which is argued to decrease the open circuit voltage (Voc) and negatively impact the OPVC performance. Based on the X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results, the altered electronic properties are argued to be due to the molecular intermixing induced doping effects. These results point out competing factors affecting the OPVC performance with the electric-field-assisted annealing treatment.

Morphology versus Vertical Phase Segregation in Solvent Annealed Small Molecule Bulk Heterojunction Organic Solar Cells

The deep study of solvent annealed small molecules bulk heterojunction organic solar cells based on DPP(TBFu) 2 : PC 60 BM blend is carried out. To reveal the reason of the solvent annealing advantage over the thermal one, capacitance-voltage measurements were applied. It was found that controlling the vertical phase segregation in the solar cells a high fullerene population in the vicinity of the cathode could be achieved. This results in increase of the shunt resistance of the cell, thus improving the light harvesting efficiency.

Reticulated Organic Photovoltaics

Advanced Functional Materials, 2012

properties at the molecular level through synthesis, have the potential to improve photovoltaic device efficiency beyond present levels and make organic photo voltaics (OPV) an important component of future solar energy conversion technology.