ZiQi Sun

On page 1602969, S. Coloredo, J. Martorell, and co-workers describe a new methodology to attain high performance free-standing organic photovoltaic cells. As depicted in the cover image, a sacrificial nanoparticle layer incorporated in between the substrate and the rest of the cell architecture is used to grow high efficiency cells detachable from the rigid glass substrate.

Over the last 5 years, research on the synthesis, device engineering, and device physics of solution-processed small molecule solar cells (SMSCs) has rapidly expanded. Improvements in molecular design and emergent device processing techniques have helped solution-processed SMSCs overcome earlier difficulties in controlling active layer morphology, such that many systems are now at—or approaching—10% power conversion efficiency. In this review, details of the highest performing blend systems are presented in order to identify key trends and provide perspective on current progress in the field. Among the best systems, a planarized molecular structure is prevalent, which can be achieved using large fused-ring moieties, intermolecular non-bonding interactions, and side chain engineering. To obtain efficient devices, the highest performing systems have been optimized through the careful combination of thermal and solvent annealing procedures. Even without additional processing, some systems have been able to obtain interconnected morphologies and efficient charge generation and charge transport. Ultimately, the design of more efficient materials also requires additional understanding of the device physics and loss mechanisms. After highlighting what is known to date on processes limiting device efficiency, an outlook on the most important challenges remaining to the field is provided.

Bulk heterojunction photovoltaics based on small molecule donors have greatly improved their performance thanks to a growing multidisciplinary effort. This review presents recent advances in molecular design, control and characterization of bulk heterojunction film morphology, and understandings of charge generation and recombination in these devices. The review is concluded with a discussion of questions that need further research to push small molecule photovoltaics higher.

The solidification of hybrid perovskite (MAPbX₃, X = I, Br, Cl) inks is studied in situ by A. Amassian and co-workers in article 1604113, revealing a remarkably complex process mediated by strong solvent-solute interactions, the outcome of which is halide-dependent. The ink is shown to solidify within 15–20s of spinning, forming a highly solvated (60–70 vol%) precursor phase. Crystalline order of the precursor, or lack thereof, strongly impacts the morphological outcome of thermal conversion and the reproducibility of solar cells.

Charge transport in organic photovoltaic (OPV) devices is often characterized by space-charge limited currents (SCLC). However, this technique only probes the transport of charges residing at quasi-equilibrium energies in the disorder-broadened density of states (DOS). In contrast, in an operating OPV device the photogenerated carriers are typically created at higher energies in the DOS, followed by slow thermalization. Here, by ultrafast time-resolved experiments and simulations it is shown that in disordered polymer/fullerene and polymer/polymer OPVs, the mobility of photogenerated carriers significantly exceeds that of injected carriers probed by SCLC. Time-resolved charge transport in a polymer/polymer OPV device is measured with exceptionally high (picosecond) time resolution. The essential physics that SCLC fails to capture is that of photo­generated carrier thermalization, which boosts carrier mobility. It is predicted that only for materials with a sufficiently low energetic disorder, thermalization effects on carrier transport can be neglected. For a typical device thickness of 100 nm, the limiting energetic disorder is σ ≈71 (56) meV for maximum-power point (short-circuit) conditions, depending on the error one is willing to accept. As in typical OPV materials the disorder is usually larger, the results question the validity of the SCLC method to describe operating OPVs.

Detailed comparison of steady-state and ultrafast time-resolved experiments reveals that photogenerated carrier mobility in organic solar cells is significantly higher than that probed by space-charge limited currents (SCLCs). The SCLC method is only valid for materials with a sufficiently low energetic disorder, where photogenerated carrier thermalization can be neglected. The over-simplified use of quasi-equilibrium mobility data in literature requires re-evaluation.

Organometal halide perovskite materials have become a superstar in the photovoltaic (PV) field because of their advantageous properties, which boost the power conversion efficiency (PCE) of perovskite solar cells (PSCs) from about 3.8% to above 22% in just seven years. Most importantly, such promising achievement is mainly based on its low-cost and solution-processed fabrication technique. One of the most promising and famous approaches to fabricating perovskite is a two-step sequential deposition method because precursor (e.g., PbI₂) deposition is controllable, versatile, and flexible. Due to tremendous efforts, great progress has been achieved on the two-step sequential deposition method, which helps to promote the development of PSCs. Herein, the progresses on the two-step sequential deposition method of perovskite layers is reviewed thoroughly. At first, the reaction process and principle is introduced and discussed. Then, the research on the deposition techniques, structures, and compositions of precursors (the first step) is presented. Subsequently, the developments on the conversion techniques, conversion solutions, and growth of large crystals at the second step are introduced. Finally, four important issues on the two-step sequential deposition method will be stated, accompanied with proposed solutions.

The two-step sequential deposition method is one of the most important deposition methods for the organometal halide perovskite layer in photovoltaic application. So far, great progress has been made on this method, which helps to promote the development of PSCs. Herein, its recent achievements are reviewed and the important issues are highlighted while proposing the corresponding solutions.