Morphology Evolution Mechanisms of Low Band Gap Polymer-Based Photovoltaics

An optimal nanoscale phase separation between the donor (generally, a conjugated polymer) and the acceptor (generally, a fullerene derivative) materials is one of the major requirements for obtaining high efficiency organic photovoltaic (OPV) device. Recent methods of controlling such nanostructure morphology in a bulkheterojunction (BHJ) OPV device involve addition of a small amount of solvent additive to the donor and acceptor solutions. The idea is to retain the acceptor materials into the solution for a longer period of time during the film solidification process, thus allowing the donor material to crystallize earlier. The ultimate morphology resulting from the solvent casting process of such multicomponent active layers involves a complex interplay of interactions between polymer/solvent, polymer/additive, fullerene/solvent, fullerene/additive, polymer/fullerene, and solvent/additive. In addition, multiple kinetic processes occur including solvent evaporation, phase separation, as well as polymer crystallization that lead to the final morphology of the active layer. Disentangling these different contributions is the key for optimization of the active layer morphology, and has been a primary emphasis of this dissertation. Accordingly, the major focus of this dissertation is twofold: to understand the parameters and interactions of solvent additives that govern the morphology evolution process of different low band-gap polymer/fullerene systems, as well as developing a laboratory-scale slot-die coating methodology, which not only mimics the large area roll-to-roll device fabrication process, but also plays an integral part on investigating the morphology evolution process of the polymer/fullerene blends. Two different low band-gap polymers (PDPPBT and PTB7) are investigated. Detail descriptions of the mechanisms leading to the final morphology are also provided

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater

Effect of Pendant Functionality in Thieno[3,4‑<i>b</i>]thiophene-<i>alt</i>-benzodithiophene Polymers for OPVs

The performance of organic photovoltaics (OPVs) is heavily dependent on the structure and functionalization of the conjugated polymer used in the active absorbing layer. Using a set of materials based on poly­(thieno­[3,4-<i>b</i>]­thiophene-<i>alt</i>-benzodithiophene) with different alkyl, aryl, perfluoroalkyl, and perfluoroaryl pendant functionalities, we have studied the correlation between absorbance, morphology, crystallinity, charge mobility, and the OPV performance in an effort to identify structure-performance relationships. The perfluorinated pendants on PTF8B and PTFPB were shown to significantly enhance the <i>V</i>_oc in the OPV devices (by ∼0.2 V), but also induced the formation of larger phase separated PCBM-rich domains. PT8B and PTFPB devices reached average efficiencies of ∼3.2%

Sequential Deposition: Optimization of Solvent Swelling for High-Performance Polymer Solar Cells

Organic solar cells based on a typical DPP polymer were systematically optimized by a solvent swelling assisted sequential deposition process. We investigated the influence of solvent swelling on the morphology and structure order of the swollen film and the resultant device performance. Morphological and structural characterization confirmed the realization of ideal bulk heterojunctions using a suitable swelling solvent. A trilayered morphology was also found with the conjugated polymer concentrated bottom layer, PC₇₁BM concentrated top layer, and interpenetrated networks of donor and acceptor in the middle by solvent swelling instead of thermal annealing in the sequential solution processing method. We proposed a simple strategy to optimize the sequential deposition fabricated devices by tuning the concentration of the PC₇₁BM solution instead of thermal annealing. The best device showed a PCE of 7.59% with a <i>V</i>_oc of 0.61 V, <i>J</i>_sc of 17.95 mA/cm², and FF of 69.6%, which is the highest reported efficiency for devices fabricated by a sequential processing method and among the best results for DPP polymers

New Insights into Morphology of High Performance BHJ Photovoltaics Revealed by High Resolution AFM

Direct imaging of the bulk heterojunction (BHJ) thin film morphology in polymer-based solar cells is essential to understand device function and optimize efficiency. The morphology of the BHJ active layer consists of bicontinuous domains of the donor and acceptor materials, having characteristic length scales of several tens of nanometers, that reduces charge recombination, enhances charge separation, and enables electron and hole transport to their respective electrodes. Direct imaging of the morphology from the molecular to macroscopic level, though, is lacking. Though transmission electron tomography provides a 3D, real-space image of the morphology, quantifying the structure is not possible. Here we used high-resolution atomic force microscopy (AFM) in the tapping and nanomechanical modes to investigate the BHJ active layer morphology that, when combined with Ar⁺ etching, provided unique insights with unparalleled spatial resolution. PCBM was seen to form a network that interpenetrated into the fibrillar network of the hole-conducting polymer, both being imbedded in a mixture of the two components. The free surface was found to be enriched with polymer crystals having a “face-on” orientation and the morphology at the anode interface was markedly different