In this study, we investigate the underlying origin of the high performance of PM6:Y6 organic solar cells. Employing transient optoelectronic and photoemission spectroscopies, we find that this blend exhibits greatly...

Organic solar cells usually utilise a heterojunction between electron-donating (D) and electron-accepting (A) materials to split excitons into charges. However, the use of D-A blends intrinsically limits the photovoltage and introduces morphological instability. Here, we demonstrate that polycrystalline films of chemically identical molecules offer a promising alternative and show that photoexcitation of α-sexithiophene (α-6T) films results in efficient charge generation. This leads to α-6T based homojunction organic solar cells with an external quantum efficiency reaching up to 44% and an open-circuit voltage of 1.61 V. Morphological, photoemission, and modelling studies show that boundaries between α-6T crystalline domains with different orientations generate an electrostatic landscape with an interfacial energy offset of 0.4 eV, which promotes the formation of hybridised exciton/charge-transfer states at the interface, dissociating efficiently into free charges. Our findings open new avenues for organic solar cell design where material energetics are tuned through molecular electrostatic engineering and mesoscale structural control.

have especially attracted great attention as the demand for low-power electronic devices is increasing rapidly with the advent of the Internet of Things, radiofrequency identification, Bluetooth low energy, etc. requiring ≈10 µW to ≈1 mW of electrical power to communicate between wireless electronic devices. [8] Indoor OPVs utilize organic semiconductors as the photoactive material in indoor energy harvesting devices. This allows for optical band gap control to ensure a good match with the visible emission spectra (300-800 nm) of indoor lighting, such as light emitting diodes (LEDs), fluorescent lamps, or halogen lamps. [7,9] Freunek et al. reported theoretical maximum PCE limits of photovoltaic devices under indoor lighting conditions as a function of optical band gap of photoactive materials. [10] In case of white RGB LEDs, for example, theoretical maximum PCE limits of over 50% can be achieved when a photoactive material with an optical band gap of 1.90 eV is used. This emphasizes the importance of the spectral overlap to achieve high PCEs in indoor photovoltaics, exemplifying the applicability of OPVs to indoor applications. Additional to the optical band gap, the frontier molecular orbital energy levels of organic semiconductors can be controlled by adjusting molecular structure. Therefore, unlike inorganic photovoltaic devices, both a high short-circuit current density (J SC ) and opencircuit voltage (V OC ) can be achieved. Although promising, there are some important considerations when using OPVs for indoor light applications. For example, unlike use under solar radiation (1 Sun), typical light intensities of indoor conditions (e.g., office, supermarket, etc.) are very low, ≈1000 lux. Due to the low light intensity, the photocurrent density of OPVs is also extremely low, typically around hundreds of µA cm −2 . Therefore, minimizing leakage currents and reducing recombination losses are essential strategies to achieve highly efficient indoor OPVs. [5,6,11,12] Most research related to indoor OPVs utilizes a bulk-heterojunction (BHJ) photoactive layer. BHJs have been widely used to overcome the limitations of organic semiconductors, namely a large exciton binding energy, and short exciton diffusion lengths (L D ). [13] Randomly intermixed donor and acceptor domains in BHJs facilitate exciton dissociation at the interface between donor and acceptor, leading to high photocurrent generation. However, this nano-structured morphology can induce unwanted energy losses by trapped charge carriers Indoor organic photovoltaics (OPVs) are a potential niche application for organic semiconductors due to their strong and well-matched absorption with the emission of indoor lighting. However, due to extremely low photocurrent generation, the device parameters critical for efficient indoor OPVs differ from those under 1 Sun conditions. Herein, these critical device parameters-recombination loss and shunt resistance (R sh )-are identified and it is demonstrated that bilayer OPVs are suitable for indoor PV applications. Co...

BiVO 4 has attracted wide attention for oxygen-evolution photoanodes in water-splitting photoelectrochemical devices. However, its performance is hampered by electron-hole recombination at surface states. Herein, partially oxidized two-dimensional (2D) bismuthene is developed as an effective, stable, functional interlayer between BiVO 4 and the archetypal NiFeOOH co-catalyst. Comprehensive (photo)electrochemical and surface photovoltage characterizations show that NiFeOOH can effectively increase the lifetime of photogenerated holes by passivating hole trap states of BiVO 4 ; however, it is limited in influencing electron trap states related to oxygen vacancies (V O ). Loading bismuthene on BiVO 4 photoanodes increases the density of V O that are beneficial for the oxygen evolution reaction via the formation of oxy/hydroxyl-based water oxidation intermediates at the surface. Moreover, bismuthene increases interfacial band bending and fills the V O -related electron traps, leading to more efficient charge extraction. With the synergistic interaction of bismuthene and NiFeOOH on BiVO 4 , this composite photoanode achieves a 5.8-fold increase in photocurrent compared to bare BiVO 4 reaching a stable 3.4 (±0.2) mA cm -2 at a low bias of +0.8 V RHE or 4.7(±0.2) mA cm -2 at +1.23 V RHE . The use of 2D bismuthene as functional interlayer provides a new strategy to enhance the performance of photoanodes.

PEDOT:PSS is widely used as a hole transport layer (HTL) in perovskite solar cells (PSCs) due to its facile processability, industrial scalability, and commercialization potential. However, PSCs utilizing PEDOT:PSS suffer from strong recombination losses compared to other organic HTLs. This results in lower open-circuit voltage ( V OC ) and power conversion efficiency (PCE). Most studies focus on doping PEDOT:PSS to improve charge extraction, but it has been suggested that a high doping level can cause strong recombination losses. Herein, we systematically dedope PEDOT:PSS with aqueous NaOH, raising its Fermi level by up to 500 meV, and optimize its layer thickness in p-i-n devices. A significant reduction of recombination losses at the dedoped PEDOT:PSS/perovskite interface is evidenced by a longer photoluminescence lifetime and higher magnitude of surface photovoltage, leading to an increased device V OC , fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses.

The non-fullerene acceptors (NFAs) employed in state-of-art organic photovoltaics (OPVs) often exhibit strong quadrupole moments which can strongly impact on material energetics. Herein, we show that changing the orientation of Y6, a prototypical NFA, from face-on to more edge-on by using different processing solvents causes a significant energetic shift of up to 210 meV. The impact of this energetic shift on OPV performance is investigated in both bilayer and bulk-heterojunction (BHJ) devices with PM6 polymer donor. The device electronic bandgap and the rate of non-geminate recombination are found to depend on the Y6 orientation in both bilayer and BHJ devices, attributed to the quadrupole moment-induced band bending. Analogous energetic shifts are also observed in other common polymer/NFA blends, which correlates well with NFA quadrupole moments. This work demonstrates the key impact of NFA quadruple moments and molecular orientation on material energetics and thereby on the efficiency of high-performance OPVs.

Enhanced Oxygen‐Evolution Photoanodes In article number 2207136, Salvador Eslava and co‐workers demonstrate a successful design of BiVO4/bismuthene/NiFeOOH composite photoanodes for enhanced solar water oxidation. They found that bismuthene increases the density of oxygen vacancies on BiVO4, increases interfacial band bending, and fills vacancy‐related electron traps with charges, whereas NiFeOOH significantly increases surface hole concentration by passivating recombination surface states.

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