The emergence of nonfullerene small-molecule acceptors (NFSMA) with the advantages of synthetic versatility, high absorption coefficient in wide wavelength range, and high thermal stability has attained the power conversion efficiency (PCE) exceeding 19% for resulted organic solar cells (OSCs) with the optimization of interface engineering and active layer morphology. Interfacial layers including both hole transporting layer (HTL) and electron transporting layer (ETL) are equally important in the OSCs for facilitating electron and hole extraction from the bulk heterojunction (BHJ) photoactive layer by the respective electrodes. In this Review, we summarize the recent progress in the materials used as HTL and ETL in conventional and inverted OSCs on the basis of their effect on the PCE. Finally, the prospects of HTL and ETL materials for NFSMA-OSCs will be provided.

This article presents recent advances in the ternary organic solar cell (TOSC), such as technological interventions from the material design to the device performance, which led to more than 19% power conversion efficiency (PCE). The research and development in TOSCs reported in the past decade have been inspiring and promising in terms of molecular processing to device properties for low-power devices and building integrated photovoltaics applications. Many of these are still in the early phase, enabling researchers to explore more and flourish. The research community has made numerous efforts to address the different aspects to enhance the efficiency and stability of the TOSCs. Therefore, the objective of this article is to review the recent advances in TOSCs and present a comprehensive discussion in this regard. This review also identifies and suggests the possible outlook for the critical issues associated with TOSCs, along with future perspectives.

Understanding the linear and nonlinear optical responses of two-dimensional nanomaterials is essential to effectively utilize them in various optoelectronic applications. Here, few-layer MoS2 and WS2 nanoflakes with lateral size less than 200 nm were prepared by liquid-phase exfoliation, and their linear and nonlinear optical responses were studied simultaneously using experimental measurements and theoretical simulations. Finite-difference time-domain (FDTD) simulations confirmed the redshift in the excitonic transitions when the thickness was increased above 10 nm indicating the layer-number dependent bandgap of nanoflakes. WS2 nanoflakes exhibited around 5 times higher absorption to scattering cross-section ratio than MoS2 nanoflakes at various wavelengths. Open aperture Z scan analysis of both the MoS2 and WS2 nanoflakes using 532 nm nanosecond laser pulses reveals strong nonlinear absorption activity with effective nonlinear absorption coefficient (βeff) of 120 cm/GW and 180 cm/GW, respectively, which was attributed to the combined contributions of ground, singlet excited and triplet excited state absorption. FDTD simulation results also showed the signature of strong absorption density of few layer nanoflakes which may be account for their excellent nonlinear optical characteristics. Optical limiting threshold values of MoS2 and WS2 nanoflakes were obtained as ~ 1.96 J/cm2 and 0.88 J/cm2, respectively, which is better than many of the reported values. Intensity dependent switching from saturable absorption to reverse saturable absorption was also observed for MoS2 nanoflakes when the laser intensity increased from 0.14 GW/cm2to 0.27 GW/cm2. The present study provides valuable information to improve the selection of two-dimensional nanomaterials for the design of highly efficient linear and nonlinear optoelectronic devices.

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Recently, the power conversion efficiency (PCE) of organic solar cells (OSCs) has significantly progressed with a rapid increase from 10 to 19% due to state-of-theart research on nonfullerene acceptor molecules and various device processing strategies. However, OSCs still exhibit significant open circuit voltage loss (ΔV OC ∼ 0.6 V) due to high energetic offsets and molecular disorder. In this work, we present a systematic investigation to determine the effects of energetic offset and disorder on different recombination losses in open circuit voltage (V OC ) using 13 different photoactive layers, wherein the PCE and ΔV OC vary in the ranges of 2.21−14.74% and 0.561−1.443 V, respectively. The detailed voltage loss analysis of all these devices was carried out, and voltage losses were correlated with energetic offset and disorder. This has enabled us to identify the key features for minimizing the voltage loss like: (1) a low energy offset between the donor and acceptor molecular states is essential to attain a nonradiative voltage loss (ΔV OC, nrad ) as low as ∼200 meV and (2) Urbach energy, which is a measure of the materials' disorder and packing, should be low for the minimization of the radiative voltage loss (ΔV OC, rad ). In addition, time-resolved photoluminescence spectroscopy was employed to further understand the exciton dynamics of pristine materials and donor−acceptor blends. It was observed that the absorbers with ultralong exciton lifetime (∼1000 ps) produce higher efficiencies. The current study emphasizes the importance of simultaneously testing photovoltaic performance and active layer exciton dynamics for rational device optimization and opens new prospects for designing novel molecules with fine-tuning of energetic offset and disorder with longer exciton lifetime which is the effective strategy to boost the efficiency of OSCs to their modified Shockley−Queisser (SQ) limit by minimizing radiative and nonradiative voltage losses.

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Two donor–acceptor (D–A) conjugated polymers designed on same 8,10-bis­(2-octyldodecyl)-8,10-dihydro-9H-bisthieno­[2′,3′:7,8;3″,2″:5,6]­naphtho­[2,3- d]­imidazole-9-one donor and dissimilar acceptor units, i.e., benzothiadiazole (P104) and fluorinated benzothiadiazole (P105), were synthesized, and their photophysical and electrochemical properties were investigated. The influence of the incorporation of fluorine atoms into the benzothiadiazole (BT) acceptor moiety in the polymer backbone on the photovoltaic performance when combined with the low bandgap nonfullerene acceptor ITIC-F was explored. The polymer solar cells based on P105:ITIC-F exhibited higher PCE (10.65%) as compared to P104:ITIC-F (8.32%), resulting from the improved values of all the photovoltaic parameters. A high value of V oc is linked with the deeper highest occupied molecular orbital energy level of P105, and the larger values of both short circuit current and fill factor are endorsed to the efficient exciton separation into charge carriers, their subsequent transfer owing to the increased value of dielectric constant and reduced value of exciton dissociation and energy loss, and promoted balanced charge transportation. The intra/interchain interaction can be modulated by F atom substitution in the BT unit, resulting reduction in π–π stacking distance, and increase in the crystal coherence length, benefiting the charge transportation in the active layer. These results offer a simple effective approach to regulate the optical and electrochemical properties and therefore increase the overall photovoltaic response.