The combination of donor (D) and acceptor (A) materials in organic solar cells (OSC) determines the corresponding D:A morphology in solar cells and the so-called golden triangle of OSC, that is, cost, power conversion efficiency (PCE), and stability. However, despite the recent advancement in OSC, determining the optimal material combination for industrialization is still a challenge. Herein, we unveil the optimal material combination that exhibits maximum industrial viability. Specifically, the industrial figure of merit (i-FoM) of 7 OSC categories is calculated and further analyzed, including small molecule donor (SMD):fullerene acceptor, SMD:non-fullerene acceptor (NFA), oligomer donor:NFA, terpolymer:NFA, polymer donor:NFA, polymer donor:polymer acceptor, and single-component materials. Since OSC is approaching wide-scale industrialization, our insights into the successes and challenges of these material combinations, particularly their PCE, photostability, and synthetic complexity (SC) index, offer guidance toward accelerating the industrialization of OSC.
In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will primarily address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive layers using state-of-the-art Y-type acceptors.
Shahidul Alam, Aman Anand, Md Moidul Islam, Rico Meitzner, Aurelien Sokeng Djoumessi, Josef Slowik, Zekarias Teklu, Peter Fischer, Christian Kästner, Jafar Khan, Ulrich Schubert, Frédéric Laquai, Harald Hoppe
Here, we studied the influence of pre- and post-thermal annealing on the performance of polymer:fullerene bulk heterojunction solar cells using the conventional architecture, comprising a conjugated polymer, namely, poly(3-hexylthiophene-2,5-diyl) (P3HT) and a fullerene derivative [6,6]-phenyl-C60-butyric acid methyl ester (PC60BM) as a photoactive layer. The non-annealed active layer device exhibited a power conversion efficiency of <1 % , which was significantly lower than the pre- and post-annealed devices. To investigate the impact of pre- and post thermal annealing on the natural morphological state of the polymer, regiorandom (P3HT-I) and regioregular (P3HT-II) type P3HT were compared in photoactive layers. In general, P3HT-I is amorphous, whereas P3HT-II is semi-crystalline. Changes in solar cell performance were associated with changes in carrier extraction efficiencies influenced by the annealing conditions. The charge photogeneration processes were investigated using spectroscopic techniques, including electroluminescence, steady-state, and time-resolved photoluminescence spectroscopy. Finally, to explore the morphological changes upon annealing, atomic force microscopy and electroluminescence imaging measurements were performed on films and solar cells, respectively.
Solution-processed metal-halide perovskite solar cells (PSCs) have received immense attention in the field of photovoltaic research due to their outstanding power conversion efficiency (PCE), which has surpassed 24% in a relative short time. Understanding carrier losses at metal halide perovskite/charge transport layer interfaces is a pre-requisite to bring the efficiency closer to the Shockley-Queisser limit. Ultrafast transient absorption spectroscopy is a vital tool to study such a processes and specifically interfacial recombination can accessed through these measurements, and further in-sights into losses associated with the open circuit voltage Voc are gained. Transient spectroscopy techniques will be used to unravel the dynamics of processes limiting the photoluminescence quantum efficiency and thus the Voc. Employing both transient photoluminescence and transient absorption techniques, enables differentiation be-tween various recombination processes. Here we study the impact of the different hole transport layers, namely, PDPP-3T, NiO and PTAA hole transport layers and reveal the charge carrier recombination. We report the direct observation of hole extraction and carrier recombination dynamics of mixed-cation lead mixed-halide perovskite layers interfacing with a polymeric hole transport layer: PDPP-3T. The dynamics of the ground state bleach of the polymer, which directly reveals the hole extraction and re-combination at the perovskite/polymer interface. The perovskite hole mobility was found to be 3.08 cm2 V-1 s-1. To gain further insight into the hole extraction dynamics, we vary the thickness of the perovskite film. We observe that the hole extraction time is slower with increasing the perovskite thickness following optical excitation from the perovskite side. Mimicking the device architecture via introducing an electron transport layer to the perovskite/PDPP-3T stack resulted in slower carrier recombination dynamics due to decreased charge carrier recombination in the perovskite.
KEYWORDS: Solar cells, Internal quantum efficiency, Perovskite, Luminescence, Terahertz spectroscopy, Quantum efficiency, Time resolved spectroscopy, Spectroscopy, Diffusion, Probability theory
Internal quantum efficiency (IQE) is a key parameter determining solar cell power conversion efficiency. While reported IQEs of metal-halide perovskite solar cells are often close to one, the contributions of photoluminescence reabsorption (PLr) and surface recombination (SR) to IQE has not been elucidated. In this work, both effects are examined by photoluminescence spectroscopies and time-resolved terahertz spectroscopy (TRTS). Then PLr rate and SR velocity are extracted from TRTS kinetics by diffusion theory. At last a model is proposed to calculate the carrier-collection probability and discuss contributions of PLr and SR on the IQE.
Brian Collins, Obaid Alqahtani, Maxime Babics, Julien Gorenflot, Victoria Savikhin, Thomas Ferron, Ahmed Balawi, Andreas Paulke, Zhipeng Kan, Michael Pope, Andrew Clulow, Jannic Wolf, Paul Burn, Ian Gentle, Dieter Neher, Michael Toney, Frédéric Laquai, Pierre Beaujuge
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.
In organic photovoltaics (OPV), perylene diimide (PDI) acceptor materials are promising candidates to replace the commonly used, but more expensive fullerene derivatives. The use of alternative acceptor materials however implies new design guidelines for OPV devices. It is therefore important to understand the underlying photophysical processes, which either lead to charge generation or geminate recombination. In this contribution, we investigate radiative losses in a series of OPV materials based on two polymers, P3HT and PTB7, respectively, which were blended with different PDI derivatives. Our time-resolved photoluminescence measurements (TRPL) allow us to identify different loss mechanisms by the decay characteristics of several excitonic species. In particular, we find evidence for unfavorable morphologies in terms of large-scale pure domains, inhibited exciton transport and incomplete charge transfer. Furthermore, in one of the P3HT-blends, an interfacial emissive charge transfer (CT) state with strong trapping character is identified.
We report the photovoltaic performance of a low-bandgap polymer:perylene diimide (PDI) photovoltaic blend and study
the exciton to charge carrier conversion in the photoactive layer by Vis-NIR broadband transient absorption spectroscopy
over a dynamic range from pico- to microseconds. Power conversion efficiencies of 1.2 % are obtained from the
polymer:PDI blends with a maximum EQE of about 30 %, which is significantly below the performance of the same
polymer with fullerene as acceptor indicating that severe loss processes exist that limit the photocurrent. From the
evolution of the transient absorption spectra we conclude that the photovoltaic performance of the polymer:PDI blends is mainly limited by inefficient exciton harvesting and dissociation at the interface. However, once free charge carriers are generated in the blend they can be extracted as photocurrent as their recombination occurs on a timescale similar to the time typically needed for charge extraction from the photoactive layer. Hence, strategies to improve the efficiency of polymer:PDI blends should aim at increasing exciton harvesting at the heterojunction and the dissociation efficiency into free charges at the interface.
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