The efficiency of organic bulk heterojunction solar cells strongly depends on the multiscale morphology of the interpenetrating polymer-fullerene network. Understanding the molecular assembly and the identification of influencing parameters is essential for a systematic optimization of such devices. Here, we investigate the molecular ordering during the drying of doctor-bladed polymer-fullerene blends on PEDOT:PSS-coated substrates simultaneously using in situ grazing incidence X-ray diffraction (GIXD) and laser reflectometry. In the process of blend crystallization, we observe the nucleation of well-aligned P3HT crystallites in edge-on orientation at the interface at the instant when P3HT solubility is crossed. A comparison of the real-time GIXD study at ternary blends with the binary phase diagrams of the drying blend film gives evidence of strong polymer-fullerene interactions that impede the crystal growth of PCBM, resulting in the aggregation of PCBM in the final drying stage. A systematic dependence of the film roughness on the drying time after crossing P3HT solubility has been shown. The highest efficiencies have been observed for slow drying at low temperatures which showed the strongest P3HT interchain π-π-ordering along the substrate surface. By adding the "unfriendly" solvent cyclohexanone to a chlorobenzene solution of P3HT:PCBM, the solubility can be crossed prior to the drying process. Such solutions exhibit randomly orientated crystalline structures in the freshly cast film which results in a large crystalline orientation distribution in the dry film that has been shown to be beneficial for solar cell performance.
Organic bulk heterojunction (BHJ) solar cells comprising conjugated polymers (as electron donor) and fullerene derivatives (as electron acceptor) deposited from solution make low cost photovoltaic-energy conversion feasible. [ 1 , 2 ] The active layer of BHJ solar cells comprises an interpenetrating network of polymer and fullerene domains that forms during deposition and drying. The energy-conversion process involves the creation of excitons upon light absorption which are dissociated into free charges at the donor/acceptor interface and are further transported towards the respective electrodes. Thus, for a given combination of donor and acceptor materials, the effi ciency of a BHJ critically depends on its nanoscale structural properties. Ideally, BHJs must have a large interface with domain sizes comparable to the exciton diffusion length (a few nano meters) and effective percolation paths to the electrodes, with balanced transport of holes and electrons. In reality, because of the variety and complex interplay of thermodynamic and kinetic factors that infl uence the resulting nanomorphology, optimization of the microstructure and the morphology is a laborious task that involves trialling many processing protocols. [3][4][5] Over recent years numerous studies have been performed on the blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C 61 butyric acid methyl ester (PCBM) with the aim of advancing the understanding of the structure-property-performance relationships. The development of P3HT crystalline order with a π -π stacking of the molecules has been found to be an important factor in ensuring suffi cient hole-carrier mobility for charge extraction. [6][7][8] Other structural issues of importance are the relative orientation of P3HT crystallites (due to their anisotropic electrical charge mobility) and the distribution of amorphous P3HT regions (which are expected to affect the electronic interconnection among P3HT domains). [ 6 , 9 ] However, the relative importance of the many implied microstructural features on solar-cell performance and the factors determining the development of specifi c structures during blend solidifi cation are still poorly understood. As many results of studies into the effect of processing conditions on nanomorphology cannot be transferred to an industrially relevant process (because the coating shear forces and solvent-evaporation conditions are different), we have chosen doctor-blading as a lab-scale fabrication method which is scalable to roll-to-roll (R2R) processing. [ 10 , 11 ] Here, the effect of substrate temperature on the structural evolution of P3HT:PCBM blends has been investigated using in situ grazing incidence X-ray scattering (GIXS). In situ X-ray measurements allow real-time observation of the emergence and evolution of the blend microstructure during solvent evaporation. The characterization of the dried fi lms is complemented with ex situ X-ray measurements, optical-absorption data, and the application of atomic force microscopy (AFM) to explore the de...
We demonstrate highly-efficient solution processed small molecule solar cells with the best power conversion efficiency (PCE) of more than 5%. The active layer consists of a diketopyrrolopyrrole based donor molecule (DPP(TBFu)2) and a fullerene derivative (PC71BM) that is spin cast and subsequently treated with solvent vapor annealing (SVA) in air. We find not all solvent vapors leads to the best PCE. Solvents of high vapor pressures and medium donor solubilties, such as tetrahydrofuran or carbon disulfide, are most suitable for SVA in the context of organic solar cell application. On the other hand, acceptor solubility plays an insignificant role in such a treatment. Active layer treated with ideal solvent vapors develops desirable phase separation in both lateral and vertical directions, as revealed by AFM, TEM and TEM tomography. The SVA also leads to enhanced hole mobility. We believe the fast SVA treatment performed in air is a viable way to tune the active layer morphology for printed solar cells.
The efficiency of polymer based bulk heterojunction (BHJ) solar cells mainly depends on the film morphology of the absorption layer and the interface properties between the stacked layers. A comparative study using atomic force microscopy (AFM) and optical in situ thin film drying measurements is performed. The strong impact of distinct drying scenarios on the polymer:fullerene BHJ layer morphology is investigated by AFM. The AFM images show a systematic dependency of structure sizes at the surface on drying kinetics. In addition thin film optical measurements for the determination of thin film drying kinetics and parameters are performed using a dedicated experimental setup. The data are used as the input for a quantitative simulation of the drying process. The film thickness decreases linearly during drying while the solvent mass fraction decreases moderately over a wide range until it drops rapidly. Subsequently the remaining solvent fraction evaporates considerably slower. Our work gives a fundamental understanding of the film formation kinetics and prerequisites for the systematic optimization of the film morphology in solution processed organic photovoltaic devices.
We report an in situ X-ray investigation of the composition dependence on the structural evolution during drying of doctor-bladed blends of poly-(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). This study enables an observation of the microstructure evolution in real time during blend crystallization. P3HT:PCBM blends with ratios of 1:0.5, 1:0.8, and 1:2 exhibit differing structural evolution during the course of solvent evaporation resulting in a different microstructure of the blends upon solidification. Large excess of PCBM over the eutectic composition impedes the π−π packing of P3HT chains and leads to a not yet observed diffraction feature with an associated spacing of 12.6 Å, which might originate from a disordered phase of intimate mixed P3HT and PCBM molecules. This work provides a microscopic understanding of the composition dependence of the film formation from solution. The structural results are discussed in relation to the composition dependence of photovoltaic performance previously reported.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.