Dylan Kipp

Influence of Block Copolymer Compatibilizers on the Morphologies of Semiflexible Polymer/Solvent Blends

We study the influence of block copolymer (BCP) compatibilizers on the domain and interfacial characteristics of the equilibrium morphological structures of semiflexible polymer/solvent blends. Our study is motivated by the question of whether block copolymer compatibilizers can be used to influence the phase separation morphologies resulting in conjugated polymer/fullerene blends. Toward this objective, we use a single chain in mean field Monte Carlo simulations for the phase behavior of semiflexible polymer/solvent blends and study the influence of BCP compatibilizers on the morphologies. Our results reveal a range of blend compositions and molecular chemistries that result in equilibrium structures with domain sizes on the order of 5-20 nm. To elucidate the morphological characteristics of these structures, we first present a series of ternary phase diagrams and then present results demonstrating that the blend composition, semiflexible chain rigidity, BCP composition, and component miscibility each provide unique handles to control the phase separation morphologies and interfacial characteristics in such blends.

A kinetic Monte Carlo model with improved charge injection model for the photocurrent characteristics of organic solar cells

We develop a kinetic Monte Carlo model for photocurrent generation in organic solar cells that demonstrates improved agreement with experimental illuminated and dark current-voltage curves. In our model, we introduce a charge injection rate prefactor to correct for the electrode grid-size and electrode charge density biases apparent in the coarse-grained approximation of the electrode as a grid of single occupancy, charge-injecting reservoirs. We use the charge injection rate prefactor to control the portion of dark current attributed to each of four kinds of charge injection. By shifting the dark current between electrode-polymer pairs, we align the injection timescales and expand the applicability of the method to accommodate ohmic energy barriers. We consider the device characteristics of the ITO/PEDOT/PSS:PPDI:PBTT:Al system and demonstrate the manner in which our model captures the device charge densities unique to systems with small injection energy barriers. To elucidate the defining characteristic...

Parallel bulk heterojunction photovoltaics based on all-conjugated block copolymer additives

In recent studies, we demonstrated that the addition of block copolymers to binary donor–acceptor blends represents an effective approach to target equilibrium, co-continuous morphologies of interpenetrating donors and acceptors. Here, we report a study of the impact of all-conjugated poly(thieno[3,4-b]-thiophene-co-benzodithiophene)-b-polynaphthalene diimide (PTB7-b-PNDI) block copolymer additives on the electronic properties and photovoltaic performance of bulk heterojunction organic photovoltaic active layers comprised of a PTB7 donor and a phenyl-C61-butyric acid methyl ester (PCBM61) acceptor. We find that small amounts of BCP additives lead to improved performance due to a large increase in the device open-circuit voltage (VOC), and the VOC is pinned to this higher value for higher BCP additive loadings. Such results contrast prior studies of ternary blend OPVs where either a continuous change in VOC or a value of VOC pinned to the lowest value is observed. We hypothesize and provide evidence in the form of device and morphology analyses that the impact of VOC is likely due to the formation of a parallel bulk heterojunction made up of isolated PCBM and PNDI acceptor domains separated by intermediate PTB7 donor domains. Altogether, this work demonstrates that all-conjugated block copolymers can be utilized as additives to both dictate morphology and modulate the electronic properties of the active layer.

Block copolymer compatibilizers for ternary blend polymer bulk heterojunction solar cells – an opportunity for computation aided molecular design

In this review article, we discuss some recent developments in the context of the use of block copolymers (BCP) as compatibilizers in ternary blend organic photovoltaic devices. At the outset, we review some of the salient experimental studies and highlight the successes achieved in using BCPs to improve the long-term stability of the donor–acceptor based devices. We suggest that the physico-electro-chemical properties of BCP compatibilizers may be targeted as a hitherto less-explored handle to modulate the morphology and device characteristics of such systems. In this regard, we propose that coarse-grained computer simulation tools, despite being limited in their ability to capture complex interaction features and crystallization phenomena, can serve as a guide to narrow and/or identify parametric regions where morphologies desirable for better exciton and charge transport can be achieved. Furthermore, we suggest that the output of the morphological predictions can be combined with coarse-grained device simulation approaches to identify the overall photovoltaic properties of the ternary blends containing such BCP compatibilizers. Together, such computational studies can serve to connect the molecular structure and electronic properties of the BCP to the overall device properties and, thereby, to enable a rational molecular design of BCP compatibilizers for organic photovoltaic devices.