In Situ X‐Ray Study of Drying‐Temperature Influence on the Structural Evolution of Bulk‐Heterojunction Polymer–Fullerene Solar Cells Processed by Doctor‐Blading

Abstract

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...

Carrier Transport Enhancement in Conjugated Polymers through Interfacial Self-Assembly of Solution-State Aggregates

“…11,13 This can be challenging to investigate, as optical and X-ray scattering techniques previously developed to investigate film formation probe mainly the bulk and surface of the drying film, rather than its buried interface. [48][49][50][51][52][53][54][55] We have recently shown that thin film deposition at the solid-liquid interface, solvent vapour annealing and ligand exchange in colloidal quantum dot solids can be monitored using a quartz crystal microbalance with dissipation (QCM-D) capability. [56][57][58] QCM-D measures the change of frequency (∆f) and dissipation (∆D) at the surface of the microbalance in relation to mass deposition or loss and changes in the viscoelastic properties of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 10 adsorbed mass, respectively.…”

One-dimensional self-confinement promotes polymorph selection in large-area organic semiconductor thin films

“…In situ X-ray diffraction has been previously used to provide insight to a variety of solution deposition processes, for example, the formation of OSCs films by drop casting, the formation of the photoactive layer in organic photovoltaics using spin coating and knife coating, the film formation of dip-coated nanoparticles and the formation of microstructures of collagen [31][32][33][34][35][36] . We have combined high-speed in situ microbeam grazing incidence wide-angle X-ray scattering (mGIWAXS) with high-speed in situ polarized optical microscopy (POM) to investigate the growth sequence of TIPS-pentacene polymorphs during solution shearing 28 .…”

High-throughput roll-to-roll X-ray characterization of polymer solar cell active layers

“…It was found for bulk heterojunctions of poly(3-hexyl)thiophene and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) that the nanomorphology, phase separation, and crystallinity could be controlled by drying temperature through reduced molecular kinetics. 7 The effect of the P3HT:PCBM blend ratio on structural evolution of a drying doctor-bladed film indicated a strong molecular interaction between the two compounds, inhibiting P3HT p-p packing at a large excess of PCBM. 8 At an excess of P3HT it was found that crystallization of PCBM is inhibited long past the solubility limit.…”

“…Of relevance, though, are the recent intensive studies by Schmidt-Hansberg et al of the drying process of polymer solar cell active layer solutions, in a knifecoating setup (''doctor-blading''). [7][8][9] In these studies, X-ray diffraction of the drying film is measured in grazing incidence, while simultaneously monitoring the film thickness with a visiblelight reflectometer. It was found for bulk heterojunctions of poly(3-hexyl)thiophene and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT/PCBM) that the nanomorphology, phase separation, and crystallinity could be controlled by drying temperature through reduced molecular kinetics.…”