Chang-Zhi Li

Recent progress and perspective in solution- processed Interfacial materials for efficient and stable polymer and organometal perovskite solar cells

In this review, we summarize the latest developments in solution-processed interfacial layers that have contributed to the significantly improved performance of polymer and perovskite solar cells (PSCs and PVSCs). The solution-processed interfacial materials, including organic electrolytes, organic–inorganic hybrids, graphene oxides (GOs), transition metal oxides (TMOs), and self-assembled functional materials, along with their integration into efficient PSCs, polymer tandem cells (PTCs), and the emerging perovskite solar cells (PVSCs) are discussed. Regarding the rapid progress of PSCs and PVSCs, strategies and perspectives of further improving solution-processed interfacial materials are also discussed to help readers understand the challenges and opportunities in transitioning from scientific curiosity into technology translation for realizing low-cost, printable, and high-efficiency flexible solar cells to address the scalability issues facing solar energy.

Integrated Molecular, Interfacial, and Device Engineering towards High‐Performance Non‐Fullerene Based Organic Solar Cells

High-performance non-fullerene OSCs with PCEs of up to ca. 6.0% are demonstrated based on PBDTT-F-TT polymer and a molecular di-PBI acceptor through comprehensive molecular, interfacial, and device engineering. Impressive PCEs can also be retained in devices with relatively thick BHJ layer and processed through non-halogenated solvents, indicating these high-performance non-fullerene OSCs are promising for large-area printing applications.

Functional fullerenes for organic photovoltaics

Tremendous progress has been made on the design and processing of new active and interfacial materials to enable organic photovoltaics to achieve high power conversion efficiencies of >10%. In this Feature Article the development of functional fullerenes as (1) acceptors, (2) electron selective layers, and (3) morphology stabilizers for bulk heterojunction polymer solar cells is reviewed. In addition to the standard PCBM based acceptors, a wide variety of newly developed fullerene-derived molecules have appeared in the past few years and started to show very encouraging photovoltaic performance when they were blended with low bandgap conjugated polymers. New fullerene derivatives with proper molecular design can also serve as electron selective interfacial materials and morphology stabilizers for the bulk heterojunction layer, which are essential to improve the interfacial property and long term stability of polymer solar cells. Although there still are many challenges ahead before practical polymer solar cells will arrive in the market place, the research in functional fullerenes deserves to have more attention in order to expedite this development process.

Improved Charge Transport and Absorption Coefficient in Indacenodithieno[3,2‐b]thiophene‐based Ladder‐Type Polymer Leading to Highly Efficient Polymer Solar Cells

A novel ladder-type donor (IDTT) is developed by substituting the two outward thiophenes of the IDT donor with two thieno[3,2-b]thiophenes. The polymer derived from this donor possesses longer effective conjugation and better planarity, which improves electron delocalization along the polymer backbone and charge mobility. The polymer solar cell device using PIDTT-DFBT shows a high power conversion efficiency of 7.03% with a large open-circuit voltage of 0.95 V without using any additives or post-solvent/thermal annealing processes.

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 (1CT) and spin-triplet (3CT) 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 3CT states to T1 states can be strongly suppressed by wavefunction delocalization, allowing for the dissociation of 3CT 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.

A spirobifluorene and diketopyrrolopyrrole moieties based non-fullerene acceptor for efficient and thermally stable polymer solar cells with high open-circuit voltage

In this study, we design and synthesize a new non-fullerene electron acceptor, SF(DPPB)4, in which a spirobifluorene (SF) core is installed with four benzene endcapped diketopyrrolopyrrole (DPP) arms. SF(DPPB)4 exhibits energy levels matching perfectly with those of the commonly used poly(3-hexyl thiophene) (P3HT) donor in polymer solar cells (PSCs). Furthermore, a designed cross-shaped molecular geometry helps in suppressing strong intermolecular aggregation in the P3HT : SF(DPPB)4 blend, leading to efficient non-fullerene PSCs. The resultant devices give a maximum power conversion efficiency (PCE) of 5.16% with an extremely high open-circuit voltage (Voc) of 1.14 V. In contrast, the devices based on P3HT : PC61BM blends provide a PCE of 3.18% with a Voc of 0.62 V. Finally, we observe that the P3HT : SF(DPPB)4 devices exhibit significantly improved thermal stability from that of the P3HT : PC61BM devices; upon thermal treatment at 150 °C for 3 h, the PCEs of P3HT : SF(DPPB)4 devices remain unchanged, whereas those of the P3HT : PC61BM devices drop drastically to below 1%. The abovementioned results demonstrate that the new design strategy of employing a high-performance non-fullerene acceptor, SF(DPPB)4, is promising for the future practical application of PSCs.

Dopant-Free Hole-Transporting Material with a C3h Symmetrical Truxene Core for Highly Efficient Perovskite Solar Cells.

Herein we present a new structural design of hole-transporting material, Trux-OMeTAD, which consists of a C3h Truxene-core with arylamine terminals and hexyl side-chains. This planar, rigid, and fully conjugated molecule exhibits excellent hole mobility and desired surface energy to the perovskite uplayer. Perovskite solar cells fabricated using the p-i-n architecture with Trux-OMeTAD as the p-layer, show a high PCE of 18.6% with minimal hysteresis.

Suppressed Charge Recombination in Inverted Organic Photovoltaics via Enhanced Charge Extraction by Using a Conductive Fullerene Electron Transport Layer

Conductive fullerene electron-transporting layers (ETLs) are developed to facilitate the solution processing of highly efficient inverted OSCs with power conversion efficiency (PCE) reaching 9.6%. Its high conductivity also allows devices to be fabricated independently of the ETL thickness (up to ca. 50 nm). Transient photovoltage (TPV) measurements are used to shed light on how these conductive ETLs help suppress charge recombination in solar cells.

Highly Efficient Organic Solar Cells with Improved Vertical Donor-Acceptor Compositional Gradient Via an Inverted Off-Center Spinning Method.

A novel, yet simple solution fabrication technique to address the trade-off between photocurrent and fill factor in thick bulk heterojunction organic solar cells is described. The inverted off-center spinning technique promotes a vertical gradient of the donor-acceptor phase-separated morphology, enabling devices with near 100% internal quantum efficiency and a high power conversion efficiency of 10.95%.

C60 as an Efficient n-Type Compact Layer in Perovskite Solar Cells

Organic-inorganic halide perovskite solar cells have rapidly evolved over the last 3 years. There are still a number of issues and open questions related to the perovskite material, such as the phenomenon of anomalous hysteresis in current-voltage characteristics and long-term stability of the devices. In this work, we focus on the electron selective contact in the perovskite solar cells and physical processes occurring at that heterojunction. We developed efficient devices by replacing the commonly employed TiO2 compact layer with fullerene C60 in a regular n-i-p architecture. Detailed spectroscopic characterization allows us to present further insight into the nature of photocurrent hysteresis and charge extraction limitations arising at the n-type contact in a standard device. Furthermore, we show preliminary stability data of perovskite solar cells under working conditions, suggesting that an n-type organic charge collection layer can increase the long-term performance.