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An overshoot of the open‐circuit voltage (VOC) after switching off the illumination is observed for perovskite solar cells, while the simultaneously measured photoluminescence (PL) intensity decreases continuously. Similarly, a dip in the photovoltage transient is detected at the beginning of a light pulse added to a continuous bias light, while the PL increases. This decoupling of external and implied VOC (as derived from the PL data) originates from a strong gradient of the majority charge carrier quasi‐Fermi level in the vicinity of a nonideal contact. This gradient reduces the external voltage much more than the implied voltage. The VOC overshoot is observed whenever the gradient decreases faster than when the separation of the quasi‐Fermi levels is reduced by charge carrier recombination. As shown in previous work, in perovskite solar cells, the magnitude of the gradient is strongly influenced by mobile ionic species and it decreases upon light soaking. This is why a fully light‐soaked device does not show this kind of VOC overshoot. [ABSTRACT FROM AUTHOR]

A bandgap gradient at the front surface of solar absorbers can effectively suppress surface recombination while not affecting photocurrent. Herein, it is demonstrated that a front‐surface gradient can be formed in an inverted perovskite cell by introducing perovskite quantum dots (QDs) between the hole‐transporting layer (HTL) and the remaining absorber. Ultraviolet photoelectron spectroscopy reveals that, with the addition of CsPbBr3 QDs onto the HTL substrate, the subsequently deposited MAPbI3 is converted from mild p‐type to n‐type, and the resultant band alignment can effectively reduce the electron concentration at the front surface without significantly affecting hole extraction. Multiple independent characterizations further confirm the reduction of surface recombination. As a result, the inverted MAPbI3 cells exhibit an open‐circuit voltage of 1.154 V, which translates to a nonradiative recombination loss of 0.15 V and a power conversion efficiency of 20.51%. [ABSTRACT FROM AUTHOR]

The performance of thermally deposited Sb2Se3 solar cells are severely limited by various bulk and interfacial recombination, instigating a large open‐circuit voltage (VOC) deficit. Ternary Sb2(S,Se)3 is considered as a remedy, however, it is also subjected to a dilemma that improvement in VOC will be escorted by JSC loss due to the shrinkage of light harvest. Thus, a gradient of S/Se across the film is a prerequisite to avoid this detrimental compromise. Herein, the incorporation of S in the Sb2Se3 absorber layer evaporated from a CdS buffer layer during vapor transport deposition (VTD) process, and its further self‐activated diffusion at the interface upon ambient storage is explored. For the gradient indium tin oxide (ITO)/CdS/Sb2(S,Se)3/Sb2Se3/Au solar cell, the large bandgap Sb2(S,Se)3 at the heterojunction side contributes to high VOC, while the narrow bandgap Sb2Se3 at the top side confirms high JSC. Sulfur diffusion at the CdS/Sb2Se3 interface also improves the junction quality with an enlarged Vbi, reduced interfacial defects and recombination loss, thus improving VOC from 393 to 430 mV. Such VOC represents the highest value for that of thermally deposited Sb2Se3 solar cells. The champion device also delivers an interesting efficiency of 7.49%. This research provides substantial guidance in exploring efficient approaches to improve the performance of Sb2Se3 solar cells. [ABSTRACT FROM AUTHOR]

The development of small molecule-based (SMs) organic photovoltaic (PV) materials is under intense attention by the researchers for organic solar cells (OSCs) to constantly revise their power conversion efficiencies (PCEs). Therefore, we have designed a series of Z-shaped non-fullerene acceptors (NFAs) (SNAA1-SNAA6), and characterized them theoretically for the possible use of in efficient OSCs devices. We have designed this Z-shaped series after employing several end-capped modifications over reference molecule (COi5DFIC (R)) to enhance their photovoltaic parameters further. We have extensively characterized these Z-shaped NFAs and explored their optoelectronic, photovoltaic and structure-property relationship with density functional theory (DFT) and time-dependent (TD-DFT). As a result, this Z-shaped NFA series (SNAA1-SNAA6) showed attractive narrower bandgaps (E g) compared to parent molecule R, and all are extremely red-shifted in the UV-Visible absorption spectrum. Moreover, we have also performed a complex study of donor:acceptor (D:A) blend (SNAA1:PTB7-Th) to show their compatibility in fabricating highly efficient OSCs devices. • Z-Shaped Non-fullerene acceptors (SNAA1-SNAA6) are designed for organic solar cells. • Highly red-shifted absorption, and an attractive narrow band-gaps are observed. • The non-fullerene acceptors showed an outstanding electron/hole reorganizational energy. • The designed materials (SNAA1-SNAA6) exhibited better optoelectronic properties as compared with reference R. [ABSTRACT FROM AUTHOR]

The key strategy to enhance the intra-molecular push-pull effects by broadening the optial absorption of small molecule based organic photovoltaic (SM-OPV) materials is considered an effective approach to enhance the power conversion efficiencies (PCEs) of SM-OPV devices. However, in case of acceptor materials, the highly desirable molecular modelling strategy of halogenation generally effects in downward-shifting of molecular energy levels, resulting decrease in open-circuit voltages (V oc) in the devices. Herein, we investigate a fluorinated, chlorinated and cyanide (CN) based end-capped acceptor materials, which shows a broader optical absorption phenomenon and exhibited a good voltages than it chlorinated counterparts. These new molecularly engineered SM-OPV were characterize theoretically by density functional theory (DFT) and time-dependent (TD-DFT) approaches. The estimation of electron/hole mobility, and V oc was done by calculating the geometric parameters, electronic structures, frontier molecular orbitals (FMOs), charge transfer rates, and exciton binding energies of the designed OPV materials. The outcomes of these investigations revealed that all newly engineered SM-OPV acceptor materials displays an enhanced exciton dissociation and absorption efficiency and underneath LUMO levels which might be responsible to improve the V oc , reorganization energies, and photo-current density parametrs, resulting enhancement in the PCEs of the organic solar cells (OSC) devices. [Display omitted] • Non-fullerene acceptor molecules (SiOA1-SiOA5) are studied for organic solar cells applications. • Significant lowering of energy gap with concomitant red shifting of the absorption spectra is achieved. • All acceptor molecules have remarkable optoelectronic properties compared to R. [ABSTRACT FROM AUTHOR]