Charge Carrier Dynamics of Vapor-Deposited Small-Molecule/Fullerene Organic Solar Cells

Abstract

Although small-molecule organic solar cells (SMOSCs) have shown increasingly promising prospects as a source of solar power, there have been few studies concerning the photophysics of these systems. Here, we report the time scale and efficiency of charge separation and recombination in a vapor-deposited SMOSC material that produces 5.81% power conversion efficiency. Transient absorption and time-resolved photoluminescence (trPL) studies of thin film blends comprising DTDCTB, a narrow-band gap electron donor, and either C60 or C70 as an electron acceptor show that charge separation occurs in ~100 fs, while charge recombination takes place over sub-ns and ns time scales. trPL indicates a donor electron-hole pair lifetime of ~33 ps in the neat film and reveals that ~20% of donors fail to charge separate in donor-acceptor mixed films, likely owing to some spatially extended donor-rich regions that interact poorly with acceptors. Our results suggest that morphological manipulations of this material could further improve device efficiency.

Deposition Growth and Morphologies of C60 on DTDCTB Surfaces: An Atomistic Insight into the Integrated Impact of Surface Stability, Landscape, and Molecular Orientation

“…Over the past a few years, a series of small‐molecule donors with a simple D‐A‐A structure have been synthesized by Wong and co‐workers for high‐efficiency vacuum‐deposition OSCs; in conjunction with the C 60 or C 70 acceptor, the highest power conversion efficiency (PCE) has reached 6.8% for a single junction cell, 10.0% and 11.1% for a tandem and a triple junction cell, respectively. Among these donor materials, 2‐{[7‐(5‐ N , N ‐ditolylaminothiophen‐2‐yl)‐2,1,3‐benzothiadiazol‐4‐yl]methylene}malononitrile (DTDCTB, see Figure a) has gained intensive attention . Here, interesting to us is the antiparallel packing mode in the DTDCTB crystal (see Figure b), which can provide a variety of model surfaces with different features to systematically probe the influence of donor surfaces on the fullerene packing and interface morphologies.…”

Aggregate‐Induced Self‐Assembly and Ultrafast Dynamics of Light‐Harvesting D‐A‐A Polymorphs

“…Structurally defined organic molecules composed of short, π‐conjugated electron donating (D) and electron accepting (A) groups as covalent D–A, D–π–A, D–A–A, A–D–A represent a broadly studied class of dipolar (push–pull) chromophores used in sensing, light‐harvesting, and in photonics and optoelectronic applications, such as organic light‐emitting diodes (OLEDs), small‐molecule organic solar cells (SMOSCs), and second‐order nonlinear optics (NLO) . The realization of SMOSC technologies using dipolar molecules relies on the advancement of research beyond the cyclical thin film preparation—device testing—chemical structural modification in order to explain how to reach efficiencies beyond 10%.…”

“…This important scientific problem is highlighted profoundly by Zhan and coworkers for SMOSCs prepared solely from triphenylamine, (D), derivatives with power conversion efficiencies consistently below 10% for over 90+ chemical structures tested in blend films. Therefore, a combined morphological and photophysical description of the photoactive layer in solar cells that monitor the presence and amount of charges formed is critical for developing a fundamental understanding of energy loss in the new efficiency regime . Here, we report new chemical structures synthesized and studied spectroscopically of a class of dipolar light harvesting molecules that have been previously tested in vapor‐deposited films with solar cell efficiencies up to 9.6%, of the methyl‐triphenylamine functionalized D–A 1 –A 2 structure with C 70 prepared as blend films .…”

Theoretical investigations on enhancing the performance of terminally diketopyrrolopyrrole-based small-molecular donors in organic solar cell applications

“…Organic solar cells (OSC) have gained much attention over traditional inorganic solar cell devices due to their advantages of low cost of manufacture, light weight and scalability [1][2][3]. At present, OSCs based on bulk heterojunction (BHJ) solar cells (BHJSC), which consist of donor polymers and acceptor fullerene derivatives, have enjoyed enormous success, with high power conversion efficiencies (PCE) of >8 % [4,5].…”

“…In contrast, small-molecular OSCs (SMOSCs), based on small molecule materials as electron donors, have attracted less attention because their PCE lags behind that of polymer-based BHJSCs. Actually, in comparison to polymer donor materials, small molecular materials in OSC applications have many advantages, such as well-defined molecular structures, tunable electronic properties, and better batch-to-batch reproducibility [3,[6][7][8][9]. To date, a high PCE in excess of 7 % has been reported from single-junction SMOSCs [10][11][12][13].…”