Maxime Babics

Mixed domains enhance charge generation and extraction in small-molecule bulk heterojunction solar cells (Conference Presentation)

It is established that the nanomorphology plays an important role in performance of bulk-heterojunction (BHJ) organic solar cells. From intense research in polymer-fullerene systems, some trends are becoming apparent. For example, small ~10 nm domains, high crystallinity, and low miscibility are typically measured in high-performance systems. However, the generality of these concepts for small-molecule (SM) BHJs is unclear. We present a comprehensive study of performance, charge generation and extraction dynamics, and nanomorphology in SM-fullerene BHJ devices to probe these critical structure-property relationships in this class of materials. In the systems investigated, small domains remain important for performance. However, devices composed of highly mixed domains with modest crystallinity outperform those consisting of pure/highly crystalline domains. Such a result points to an alternative ideal morphology for SM-based devices that involves a predominant mixed phase. This stems from SM aggregation in highly mixed domains that both maximize interface for charge generation and establish continuous pathways for efficient charge extraction. Such a morphological paradigm should be considered in future SM systems in pursuit of high-efficiency large-scale solar power production.

F-Substituted oligothiophenes serve as nonfullerene acceptors in polymer solar cells with open-circuit voltages >1 V

We report on a set of two analogous, F-substituted oligothiophene derivatives (D5T6F-M and D7T8F-M), in which systematic fluorination at the thienyl moieties is shown to induce a sufficient increase in electron affinity (EA; i.e. suppressed LUMO level) and in ionization potential (IP; i.e. deeper HOMO) to (i) impart the molecules with electron-accepting and -transporting characteristics, and (ii) yield high open-circuit voltages >1 V in BHJ solar cells when combined with a low-bandgap polymer donor (PCE10) commonly used with fullerenes. The nonfullerene BHJ devices with the SM acceptor D5T6F-M achieve power conversion efficiencies (PCEs) of up to ca. 4.5% (vs. ca. 2% for D7T8F-M-based devices). Our study shows that –F substitutions in SM systems – otherwise used as donors – is an effective approach in the design of nonfullerene acceptors for efficient BHJ solar cells with polymer donors.

Atomic-layer-deposited AZO outperforms ITO in high-efficiency polymer solar cells

Tin-doped indium oxide (ITO) transparent conducting electrodes are widely used across the display industry, and are currently the cornerstone of photovoltaic device developments, taking a substantial share in the manufacturing cost of large-area modules. However, cost and supply considerations are set to limit the extensive use of indium for optoelectronic device applications and, in turn, alternative transparent conducting oxide (TCO) materials are required. In this report, we show that aluminum-doped zinc oxide (AZO) thin films grown by atomic layer deposition (ALD) are sufficiently conductive and transparent to outperform ITO as the cathode in inverted polymer solar cells. Reference polymer solar cells made with atomic-layer-deposited AZO cathodes, PCE10 as the polymer donor and PC71BM as the fullerene acceptor (model systems), reach power conversion efficiencies of ca. 10% (compared to ca. 9% with ITO-coated glass), without compromising other figures of merit. These ALD-grown AZO electrodes are promising for a wide range of optoelectronic device applications relying on TCOs.