Direct Correlation of Nanoscale Morphology and Device Performance to Study Photocurrent Generation in Donor-Enriched Phases of Polymer Solar Cells

Abstract

The nanoscale morphology of polymer blends is a key parameter to reach high efficiency in bulk heterojunction solar cells. Thereby, research typically focuses on optimal blend morphologies while studying non-optimized blends may give insight into blend design that can be more robust against morphology defects. Here we focus on the direct correlation of morphology and device performance of PTB7:PC71BM bulk heterojunction (BHJ) blends processed without additive in different donor:acceptor weight ratios. We show that while blends of a 1:1.5 ratio are composed of large donor enriched and fullerene domains beyond exciton diffusion length, reducing the ratio below 1:0.5 leads to blends composed purely of polymer enriched domains. Importantly photocurrent density in such blends can reach values between 45 to 60% of those reached of fully optimized blend using additives. We provide here a direct visual evidence that fullerenes in the donor enriched domains are not distributed homogeneously but fluctuate locally. To this end, we performed compositional nanoscale morphology analysis of the blend using spectroscopic imaging of low energy-loss electrons in the transmission electron microscope. Charge transport measurement in combination with molecular dynamics simulations show that the fullerene sub-structures inside the polymer phase generate efficient electron transport in the polymer enriched phase. Furthermore, we show that the formation of densely 3 packed regions of fullerene inside the polymer phase is driven by the PTB7:PC71BM enthalpy of mixing. The occurrence of such a nanoscale network of fullerene clusters leads to a reduction of electron trap states and thus efficient extraction of photocurrent inside the polymer domain. Suitable tuning of the polymer acceptor interaction can thus introduce acceptor sub-networks in polymer enriched phases improving the tolerance for high efficiency BHJ towards morphological defects such as donor enriched domains exceeding exciton diffusion length.

A Theoretical Perspective on the Thermodynamic Stability of Polymer Blends for Solar Cells: From Experiments to Predictive Modeling

“…This has been carried out by different groups in a wide range of applications, such as the prediction of the mixing of impurities with crude oil, [ 81,82 ] the study of drug–polymer formulations for pharmaceutical use, [ 80,122–125 ] and the study of the solubility of conjugated polymers for organic electronics [ 83,84 ] and the thermodynamics of mixing in organic blends for solar cells. [ 86–88 ]…”

“…This has been carried out by different groups in a wide range of applications, such as the prediction of the mixing of impurities with crude oil, [81,82] the study of drug-polymer formulations for pharmaceutical use, [80,[122][123][124][125] and the study of the solubility of conjugated polymers for organic electronics [83,84] and the thermodynamics of mixing in organic blends for solar cells. [86][87][88] The field of drug delivery is the one where the method has found most of its applications: as many newly developed drugs possess poor water solubility, delivery of these drugs within the human body is a major and challenging problem in pharmaceutical technology. Among the approaches to solve this problem there is the incorporation of drugs into nanoparticles, liposomes, emulsions, or polymers.…”

“…An ideal computational-theoretical method for material screening would use the chemical structure as the only input (no need for experimental information) and would be able not only to discriminate between miscible and immiscible materials, but also to predict their volume/weight ratios in the different phases. Computational methods to investigate the miscibility for different classes of materials have been widely used across the literature [79][80][81][82][83][84][85][86][87][88] but their potential in the study of blends specific for photovoltaics is still largely unexplored.…”

Theoretical insight on PTB7:PC71BM, PTB7-th:PC71BM and Si-PCPDTBT:PC71BM interactions governing blend nanoscale morphology for efficient solar cells

“…In the light of these findings, as a first-step towards the modeling of complex ternary blends, it is of primary importance to study comparatively the binary interactions and miscibility of different polymers with fullerenes and to clarify the physical quantities (such as the concentration and density) that control the miscibility and the interfaces. Atomistic simulations, thanks to the available computing power, are nowadays able to study polymer solutions [34,35,36,37,33,38,39]. For example, Wang et al [33] have investigated the polymer self-aggregation, polymer-fullerene packing and fullerene connecting networks in three different blends by means of model potential molecular dynamics (MPMD).…”

Tunable Porosity and Conjugation in Ferrocene Polymers of Intrinsic Microporosity

“…For example, intrinsic microporosity in the conjugated MPIM electron donor material could potentially enable the acceptor to be intimately interspersed within the layer, limiting the phase separation that remains an outstanding challenge with non-porous compounds. [23][24][25][26][27][28][29] In this work, we systematically characterize the structural and optoelectronic properties of a set of MPIM models, providing the necessary foundation for further exploration of these materials and their potential application in sustainable energy technologies. In particular, we examine how Fc can be used to support porosity in a 1,4-diethynylbenzene-Fc polymer (Poly-Fc1) while maintaining conjugation across the polymer backbone.…”