The role of spin in the kinetic control of recombination in organic photovoltaics.

In biological complexes, cascade structures promote the spatial separation of photogenerated electrons and holes, preventing their recombination. In contrast, the photogenerated excitons in organic photovoltaic cells are dissociated at a single donor-acceptor heterojunction formed within a de-mixed blend of the donor and acceptor semiconductors. The nanoscale morphology and high charge densities give a high rate of electron-hole encounters, which should in principle result in the formation of spin-triplet excitons, as in organic light-emitting diodes. Although organic photovoltaic cells would have poor quantum efficiencies if every encounter led to recombination, state-of-the-art examples nevertheless demonstrate near-unity quantum efficiency. Here we show that this suppression of recombination arises through the interplay between spin, energetics and delocalization of electronic excitations in organic semiconductors. We use time-resolved spectroscopy to study a series of model high-efficiency polymer-fullerene systems in which the lowest-energy molecular triplet exciton (T1) for the polymer is lower in energy than the intermolecular charge transfer state. We observe the formation of T1 states following bimolecular recombination, indicating that encounters of spin-uncorrelated electrons and holes generate charge transfer states with both spin-singlet ((1)CT) and spin-triplet ((3)CT) characters. We show that the formation of triplet excitons can be the main loss mechanism in organic photovoltaic cells. But we also find that, even when energetically favoured, the relaxation of (3)CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of (3)CT states back to free charges, thereby reducing recombination and enhancing device performance. Our results point towards new design rules both for photoconversion systems, enabling the suppression of electron-hole recombination, and for organic light-emitting diodes, avoiding the formation of triplet excitons and enhancing fluorescence efficiency.This work was supported by the Engineering and Physical Sciences Research Council (EPSRC)This is the accepted version of the original publication available at: http://www.nature.com/nature/journal/v500/n7463/full/nature12339.html

Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells

State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar cells (OSCs) to achieve highly efficient charge generation with extended near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise molecular-level mechanism that enables low-energy excited states in Y6-type acceptors to achieve nanosecond lifetimes has remained elusive. Here, we demonstrate that the distinct packing of Y6 molecules in film leads to a strong intermolecular charge-transfer (iCT) character of the lowest excited state in Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton energies are greatly reduced by up to ~0.25 eV with respect to excitons formed in separated molecules. Importantly, despite their low energies, the iCT excitons have reduced non-adiabatic electron-vibration couplings with the electronic ground state, thus suppressing non-radiative recombination and allowing Y6 to overcome the well-known energy gap law. Our results reveal the fundamental relationship between molecular packing and nanosecond excited-state lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding performance of Y6-based OSCs

Long-lived and disorder-free charge transfer states enable endothermic charge separation in efficient non-fullerene organic solar cells

Funder: HKU | University Research Committee, University of Hong Kong (HKU Research Committee); doi: https://doi.org/10.13039/501100003802Abstract: Organic solar cells based on non-fullerene acceptors can show high charge generation yields despite near-zero donor–acceptor energy offsets to drive charge separation and overcome the mutual Coulomb attraction between electron and hole. Here, we use time-resolved optical spectroscopy to show that free charges in these systems are generated by thermally activated dissociation of interfacial charge-transfer states that occurs over hundreds of picoseconds at room temperature, three orders of magnitude slower than comparable fullerene-based systems. Upon free electron–hole encounters at later times, both charge-transfer states and emissive excitons are regenerated, thus setting up an equilibrium between excitons, charge-transfer states and free charges. Our results suggest that the formation of long-lived and disorder-free charge-transfer states in these systems enables them to operate closely to quasi-thermodynamic conditions with no requirement for energy offsets to drive interfacial charge separation and achieve suppressed non-radiative recombination

Device Performance of Emerging Photovoltaic Materials (Version 3)

Following the 2nd release of the “Emerging PV reports,” the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2021. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application, and are put into perspective using, e.g., the detailed balance efficiency limit. The 3rd installment of the “Emerging PV reports” extends the scope toward triple junction solar cells

Device Performance of Emerging Photovoltaic Materials (Version 2)

Following the 1st release of the “Emerging photovoltaic (PV) reports”, the best achievements in the performance of emerging photovoltaic devices in diverse emerging photovoltaic research subjects are summarized, as reported in peer-reviewed articles in academic journals since August 2020. Updated graphs, tables, and analyses are provided with several performance parameters, e.g., power conversion efficiency, open-circuit voltage, short-circuit current density, fill factor, light utilization efficiency, and stability test energy yield. These parameters are presented as a function of the photovoltaic bandgap energy and the average visible transmittance for each technology and application and are put into perspective using, e.g., the detailed balance efficiency limit. The 2nd instalment of the “Emerging PV reports” extends the scope toward tandem solar cells and presents the current state-of-the-art in tandem solar cell performance for various material combinations.</p

Roadmap on commercialization of metal halide perovskite photovoltaics

Perovskite solar cells (PSCs) represent one of the most promising emerging photovoltaic technologies due to their high power conversion efficiency. However, despite the huge progress made not only in terms of the efficiency achieved, but also fundamental understanding of the relevant physics of the devices and issues which affect their efficiency and stability, there are still unresolved problems and obstacles on the path toward commercialization of this promising technology. In this roadmap, we aim to provide a concise and up to date summary of outstanding issues and challenges, and the progress made toward addressing these issues. While the format of this article is not meant to be a comprehensive review of the topic, it provides a collection of the viewpoints of the experts in the field, which covers a broad range of topics related to PSC commercialization, including those relevant for manufacturing (scaling up, different types of devices), operation and stability (various factors), and environmental issues (in particular the use of lead). We hope that the article will provide a useful resource for researchers in the field and that it will facilitate discussions and move forward toward addressing the outstanding challenges in this fast-developing field

Multi-junction polymer solar cells

We discuss the materials challenges for increasing cell efficiencies of polymer solar cells via the route of stacking several cells on top of each other in multi-junction solar cells. For such cells, the recombination layer between the two sub cells is crucial. Typically, the recombination layer consists of a semi-transparent n- and p-type semiconductor bilayer. Several options for the recombination layer will be discussed. Furthermore, we discuss how the workfunction and the conductivity of the semi-transparent semiconductors may influence the performance of the cells. We also discuss requirements for accurate measurements of spectral response and efficiency. Keywords: tandem, polymer film, multi-junction solar cel