Liuyanfeng

Recent years have seen a substantial efficiency improvement for a variety of solar cell technologies as well as the rise of a new class of photovoltaic absorber materials, the metal-halide perovskites. Conversion efficiencies that are coming closer and closer to the thermodynamic limits require a physical description of the corresponding solar cells that is compatible with those limits. This progress report summarizes recent work on the interdependence of basic material properties of semiconductor materials with their efficiency potential as photovoltaic absorbers. The connection of the classical Shockley–Queisser approach, with the band gap energy as the only parameter, to a more general radiative limit and to situations where nonradiative recombination dominates is discussed. The authors delineate a consistent loss analysis that enables a quantitative comparison between different solar cell technologies. In a next step, bulk material properties that influence the photovoltaic performance of a semiconductor like absorption coefficient, densities of states of the free carriers, or phonon energies are considered. It is shown that variations of these properties have a big influence on the optimized design of a solar cell but not necessarily on the achievable efficiency.

This progress report explains how microscopic properties of solar cell absorber materials affect properties such as absorption coefficient, mobility, and charge carrier lifetime and how these properties affect photovoltaic performance. The report provides the necessary theoretical background to describe solar cells on different levels of abstraction which helps our understanding of what makes some materials good solar cell materials.

The commercialization of nonfullerene organic solar cells (OSCs) critically relies on the response under typical operating conditions (for instance, temperature and humidity) and the ability of scale-up. Despite the rapid increase in power conversion efficiency (PCE) of spin-coated devices fabricated in a protective atmosphere, the efficiencies of printed nonfullerene OSC devices by blade coating are still lower than 6%. This slow progress significantly limits the practical printing of high-performance nonfullerene OSCs. Here, a new and relatively stable nonfullerene combination is introduced by pairing the nonfluorinated acceptor IT-M with the polymeric donor FTAZ. Over 12% efficiency can be achieved in spin-coated FTAZ:IT-M devices using a single halogen-free solvent. More importantly, chlorine-free, blade coating of FTAZ:IT-M in air is able to yield a PCE of nearly 11% despite a humidity of ≈50%. X-ray scattering results reveal that large π–π coherence length, high degree of face-on orientation with respect to the substrate, and small domain spacing of ≈20 nm are closely correlated with such high device performance. The material system and approach yield the highest reported performance for nonfullerene OSC devices by a coating technique approximating scalable fabrication methods and hold great promise for the development of low-cost, low-toxicity, and high-efficiency OSCs by high-throughput production.

A new nonfullerene combination composed of a high-performance polymer and a nonfluorinated small molecule is presented. It holds great potential for additive-free and halogen-free processing. Small and pure domains and face-on molecular packing collectively enable the first demonstration of ≈11% efficiency air-processed and stable nonfullerene solar cells by blade-coating techniques. Additionally, complete solvent–morphology–performance relations are established for further improvements.

A novel small molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits a higher lowest unoccupied molecular orbital (LUMO) level value, tighter molecular packing, better crystallites quality, and stronger absorption in the range of 520–740 nm. The MeIC-based polymer solar cells (PSCs) with J71 as donor, achieve high power conversion efficiency (PCE), up to 12.54% with a short-circuit current (J_SC) of 18.41 mA cm⁻², significantly higher than that of the device based on J71:ITCPTC (11.63% with a J_SC of 17.52 mA cm⁻²). The higher J_SC of the PSC based on J71:MeIC can be attributed to more balanced μ_h/μ_e, higher charge dissociation and charge collection efficiency, better molecular packing, and more proper phase separation features as indicated by grazing incident X-ray diffraction and resonant soft X-ray scattering results. It is worth mentioning that the as-cast PSCs based on MeIC also yield a high PCE of 11.26%, which is among the highest value for the as-cast nonfullerene PSCs so far. Such a small modification that leads to so significant an improvement of the photovoltaic performance is a quite exciting finding, shining a light on the molecular design of the nonfullerene acceptors.

A novel small-molecule acceptor MeIC with a methylated end-capping group is developed. Compared to unmethylated counterparts (ITCPTC), MeIC exhibits higher lowest unoccupied molecular orbital (LUMO) level, tighter molecular packing, and better crystallite quality. MeIC-based polymer solar cells with J71 as donor achieve high power conversion efficiency up to 12.54%, significantly higher than that of the device of ITCPTC.

A fused tris(thienothiophene) (3TT) building block is designed and synthesized with strong electron-donating and molecular packing properties, where three thienothiophene units are condensed with two cyclopentadienyl rings. Based on 3TT, a fused octacylic electron acceptor (FOIC) is designed and synthesized, using strong electron-withdrawing 2-(5/6-fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile as end groups. FOIC exhibits absorption in 600–950 nm region peaked at 836 nm with extinction coefficient of up to 2 × 10⁵m^–1 cm^–1, low bandgap of 1.32 eV, and high electron mobility of 1.2 × 10^–3 cm² V^–1 s^–1. Compared with its counterpart ITIC3 based on indacenothienothiophene core, FOIC exhibits significantly upshifted highest occupied molecular orbital level, slightly downshifted lowest unoccupied molecular orbital level, significantly redshifted absorption, and higher mobility. The as-cast organic solar cells (OSCs) based on blends of PTB7-Th donor and FOIC acceptor without additional treatments exhibit power conversion efficiencies (PCEs) as high as 12.0%, which is much higher than that of PTB7-Th: ITIC3 (8.09%). The as-cast semitransparent OSCs based on the same blends show PCEs of up to 10.3% with an average visible transmittance of 37.4%.

A fused tris(thienothiophene)-based electron acceptor with strong near-infrared absorption and high electron mobility is designed, synthesized, and applied in as-cast organic solar cells and as-cast semitransparent organic solar cells, which exhibit efficiencies of 12.0% and 10.3%, respectively.

The power conversion efficiencies (PCEs) of state-of-the-art organic solar cells (OSCs) have increased to over 13%. However, the most commonly used solvents for making the solutions of photoactive materials and the coating methods used in laboratories are not adaptable for future practical production. Therefore, taking a solution-coating method with environmentally friendly processing solvents into consideration is critical for the practical utilization of OSC technology. In this study, a highly efficient PBTA-TF:IT-M-based device processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, is fabricated; a high PCE of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. More importantly, a blade-coated non-fullerene OSC processed with THF/IPA is demonstrated for the first time to obtain a promising PCE of 11.7%; even for the THF/IPA-processed large-area device (1.0 cm²) made by blade-coating, a PCE of 10.6% can still be maintained. These results are critical for the large-scale production of highly efficient OSCs in future studies.

Highly efficient non-fullerene organic solar cells (OSCs) are fabricated, processed with environmentally friendly solvents, tetrahydrofuran/isopropyl alcohol (THF/IPA) and o-xylene/1-phenylnaphthalene, respectively. The highest power conversion efficiency (PCE) of 13.1% can be achieved by adopting the spin-coating method, which is the top result for OSCs. When the blade-coating method is used in an ambient atmosphere, the THF/IPA-processed device maintains a high PCE of 11.7%.

Nonfullerene acceptors (NFAs) in blends with highly crystalline donor polymers have been shown to yield particularly high device voltage outputs, but typically more modest quantum yields for photocurrent generation as well as often lower fill factors (FF). In this study, we employ transient optical and optoelectronic analysis to elucidate the factors determining device photocurrent and FF in blends of the highly crystalline donor polymer PffBT4T-2OD with the promising NFA FBR or the more widely studied fullerene acceptor PC₇₁BM. Geminate recombination losses, as measured by ultrafast transient absorption spectroscopy, are observed to be significantly higher for PffBT4T-2OD:FBR blends. This is assigned to the smaller LUMO-LUMO offset of the PffBT4T-2OD:FBR blends relative to PffBT4T-2OD:PC₇₁BM, resulting in the lower photocurrent generation efficiency obtained with FBR. Employing time delayed charge extraction measurements, these geminate recombination losses are observed to be field dependent, resulting in the lower FF observed with PffBT4T-2OD:FBR devices. These data therefore provide a detailed understanding of the impact of acceptor design, and particularly acceptor energetics, on organic solar cell performance. Our study concludes with a discussion of the implications of these results for the design of NFAs in organic solar cells.

Charge separation and recombination dynamics in relation to film morphology and energetics are reported in nonfullerene-based PffBT4T-2OD:FBR solar cell. PffBT4T-2OD shows efficient exciton diffusion to the interface between the electron donors and the acceptors. Its small energetic offset explains relatively lower current density and fill factor correlated with geminated recombination and field-dependent photogeneration.