• Energy Research

An efficient photogeneration of free charge carriers has long been recognized as the paramount challenge in organic photovoltaic (OPV) devices. The low dielectric constant organic semiconductors fall short to reduce strong Coulombic interaction of tightly bound exciton and hence lead to a loss mechanism in OPVs due to charge-carrier recombination. To circumvent this problem, we adopt a strategy to enhance the dielectric constant of organic semiconductors by incorporating tetraethyleneglycol (TEG) side-chains. We report synthesis of three new semiconducting copolymers by combining thiophene substituted diketopyrrolopyrrole (TDPP) monomer with three other monomeric units with varying electron donating strength: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP) and naphthalene diimide (PNDITEG-TDPP). BBT-3TEG-TDPP and PNDITEG-TDPP showed highest dielectric constants (~ 5) at 1MHz frequency suggesting efficient contribution of dipolar polarization from TEG side-chains. To understand the electronic contribution of the polymer backbone and the polarity of TEG side-chains, and the resulting enhancement of the dielectric constant, we further performed first-principles density functional theory calculations. Single-component organic solar cells (OSC) fabricated utilizing these polymers resulted in poor performance which is attributed to the absence of free charge generation. Furthermore, transient absorption spectroscopy studies show low exciton diffusion length as observed in donor-acceptor type conjugated polymers. Our results suggest that, the strategy of enhancing dielectric constant with polar side-chains is not sufficient to reduce Coulombic interaction between hole and electron in OSCs.

Phosphorene nanoribbons (PNRs) have been widely predicted to exhibit a range of superlative functional properties; however, because they have only recently been isolated, these properties are yet to be shown to translate to improved performance in any application. PNRs show particular promise for optoelectronics, given their predicted high exciton binding energies, tunable bandgaps, and ultrahigh hole mobilities. Here, we verify the theorized enhanced hole mobility in both solar cells and space-charge-limited-current devices, demonstrating the potential for PNRs improving hole extraction in universal optoelectronic applications. Specifically, PNRs are demonstrated to act as an effective charge-selective interlayer by enhancing hole extraction from polycrystalline methylammonium lead iodide (MAPbI3) perovskite to the poly(triarylamine) semiconductor. Introducing PNRs at the hole-transport/MAPbI3 interface achieves fill factors above 0.83 and efficiencies exceeding 21% for planar p-i-n (inverted) perovskite solar cells (PSCs). Such efficiencies are typically only reported for single-crystalline MAPbI3-based inverted PSCs. Methylammonium-free PSCs also benefit from a PNR interlayer, verifying applicability to architectures incorporating mixed perovskite absorber layers. Device photoluminescence and transient absorption spectroscopy are used to demonstrate that the presence of the PNRs drives more effective carrier extraction. Isolation of the PNRs in space-charge-limited-current hole-only devices improves both hole mobility and conductivity, demonstrating applicability beyond PSCs. This work provides primary experimental evidence that the predicted superlative functional properties of PNRs indeed translate to improved optoelectronic performance.

In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells.

Organic semiconductor photocatalysts for the production of solar fuels are attractive as they can be synthetically tuned to absorb visible light while simultaneously retaining suitable energy levels to drive a range of processes. However, a greater understanding of the photophysics that determines the function of organic semiconductor heterojunction nanoparticles is needed to optimize performance. Here, we show that such materials can intrinsically generate remarkably long-lived reactive charges, enabling them to efficiently drive sacrificial hydrogen evolution. Our optimized hetereojunction photocatalysts comprise the conjugated polymer PM6 matched with Y6 or PCBM electron acceptors, and achieve external quantum efficiencies of 1.0% to 5.0% at 400 to 900 nm and 8.7% to 2.6% at 400 to 700 nm, respectively. Employing transient and operando spectroscopies, we find that the heterojunction structure in these nanoparticles greatly enhances the generation of long-lived charges (millisecond to second timescale) even in the absence of electron/hole scavengers or Pt. Such long-lived reactive charges open potential applications in water-splitting Z-schemes and in driving kinetically slow and technologically desirable oxidations.

Increasing the interface area between organic semiconductor photocatalysts and electrolyte by fabricating nanoparticles has proven to be an effective strategy to increase photocatalytic hydrogen production activity. However, it remains unclear if increasing the internal interface by the introduction of porosity has as clear benefits for activity. To better inform future photocatalyst design, a series of polymers of intrinsic microporosity (PIMs) with the same conjugated backbone were synthesized as a platform to independently modulate the variables of porosity and relative hydrophilicity through the use of hydrophilic alcohol moieties protected by silyl ether protecting groups. When tested in the presence of ascorbic acid and photodeposited Pt, a strong correlation between the wettable porosity and photocatalytic activity was found, with the more wettable analogue of two polymers of almost the same surface area delivering 7.3 times greater activity, while controlling for other variables. Transient absorption spectroscopic (TAS) investigation showed efficient intrinsic charge generation within 10 ps in two of the porous polymers, even without the presence of ascorbic acid or Pt. Detectable hole polarons were found to be immediately extracted by added ascorbic acid, suggesting the generation of reactive charges at regions readily accessible to electrolyte in the porous structures. This study directs organic semiconductor photocatalysts design toward more hydrophilic functionality for addressing exciton and charge recombination bottlenecks and clearly demonstrates the advantages of wettable porosity as a design principle.

Understanding charge carrier extraction from the perovskite photoactive layer is critical to optimizing the design of perovskite solar cells. Herein, we demonstrate a simple time-resolved photoluminescence method to characterize the kinetics of charge transport across the bulk perovskite and charge transfer from the perovskite layer to the interlayers, elucidating the dependence of these dynamics on film thickness, grain boundaries (GBs), and interlayers. Using asymmetric laser excitation, we selectively probe charge transport by generating charges both close to and far from the heterojunction interface and correlate these results with device performance. We observe that both film thickness and GBs affect the asymmetry between electron and hole charge transport across the bulk perovskite and charge transfer from the bulk perovskite to the respective interlayers.