Origin of the switchable photocurrent direction in BiFeO3 thin films

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO3) photoactive thin films. Depending on the applied bias our BiFeO3 films exhibit photocurrents more usually found in p- or n-type semiconductor photoelectrodes. In order to understand the anomalous behaviour ambient photoemission spectroscopy and Kelvin-probe techniques have been used to determine the band structure of the BiFeO3. We found that the Fermi level (Ef) is at −4.96 eV (vs. vacuum) with a mid-gap at −4.93 eV (vs. vacuum). Our photochemically determined flat band potential (Efb) was found to be 0.3 V vs. NHE (−4.8 V vs. vacuum). These band positions indicate that Ef is close to mid-gap, and Efb is close to the equilibrium with the electrolyte enabling either cathodic or anodic band bending. We show an ability to control switching from n- to p-type behaviour through the application of external bias to the BiFeO3 thin film. This ability to control majority carrier dynamics at low applied bias opens a number of applications in novel optoelectronic switches, logic and energy conversion devices

Bi2Fe4O9 thin films as novel visible-light-active photoanodes for solar water splitting

We report the chemical solution deposition (CSD) of a phase-pure Bi2Fe4O9 thin film for use as a photoanode in photoelectrochemical (PEC) water splitting. The energy levels of Bi2Fe4O9 films have been measured and n-type characteristics have been confirmed. With band gaps determined as 2.05 eV (indirect) and 2.80 eV (direct) and valence and conduction bands straddling the water oxidation and reduction potentials, this material is highly promising as a photocatalyst for solar water splitting. The photocurrent of a planar photoanode reached 0.1 mA cm−2 at 1.23 VNHE under AM1.5G illumination. The addition of H2O2 as a hole scavenger increased the photocurrent to 0.25 mA cm−2, indicating hole injection is one limiting factor to the performance. The performance was enhanced by nearly 5-fold when the Bi2Fe4O9 photoanode is coupled to a Co–Pi surface co-catalyst. The photoanode also shows excellent stability with no change in photocurrent over three hours of continuous illumination. These results indicate that this material represents a promising addition to the growing selection of low-cost, stable photocatalysts for use in solar water splitting

Towards Efficient Integrated Perovskite/Organic Bulk Heterojunction Solar Cells: Interfacial Energetic Requirement to Reduce Charge Carrier Recombination Losses

Integrated perovskite/organic bulk heterojunction (BHJ) solar cells have the potential to enhance the efficiency of perovskite solar cells by a simple one‐step deposition of an organic BHJ blend photoactive layer on top of the perovskite absorber. It is found that inverted structure integrated solar cells show significantly increased short‐circuit current (Jsc) gained from the complementary absorption of the organic BHJ layer compared to the reference perovskite‐only devices. However, this increase in Jsc is not directly reflected as an increase in power conversion efficiency of the devices due to a loss of fill factor. Herein, the origin of this efficiency loss is investigated. It is found that a significant energetic barrier (≈250 meV) exists at the perovskite/organic BHJ interface. This interfacial barrier prevents efficient transport of photogenerated charge carriers (holes) from the BHJ layer to the perovskite layer, leading to charge accumulation at the perovskite/BHJ interface. Such accumulation is found to cause undesirable recombination of charge carriers, lowering surface photovoltage of the photoactive layers and device efficiency via fill factor loss. The results highlight a critical role of the interfacial energetics in such integrated cells and provide useful guidelines for photoactive materials (both perovskite and organic semiconductors) required for high‐performance devices

High efficiency blue organic light-emitting diodes with below-bandgap electroluminescence

Blue organic light-emitting diodes require high triplet interlayer materials, which induce large energetic barriers at the interfaces resulting in high device voltages and reduced efficiencies. Here, we alleviate this issue by designing a low triplet energy hole transporting interlayer with high mobility, combined with an interface exciplex that confines excitons at the emissive layer/electron transporting material interface. As a result, blue thermally activated delay fluorescent organic light emitting diodes with a below-bandgap turn-on voltage of 2.5 V and an external quantum efficiency of 41.2% were successfully fabricated. These devices also showed suppressed efficiency roll-off maintaining an EQE of 34.8% at 1000 cd m-2. Our approach paves the way for further progress through exploring alternative device engineering approaches instead of only focusing on the demanding synthesis of organic compounds with complex structures

Suppressing PEDOT:PSS doping-induced interfacial recombination loss in perovskite solar cells

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 (VOC) 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 VOC, fill factor, and PCE. These results provide insights into the relationship between doping level of HTLs and interfacial charge carrier recombination losses

Scalable all-inorganic halide perovskite photoanodes with >100 h operational stability containing earth-abundant materials

Abstract The application of halide perovskites in the photoelectrochemical generation of solar fuels and feedstocks is hindered by the instability of perovskites in aqueous electrolytes and the use of expensive electrode and catalyst materials, particularly in photoanodes driving kinetically slow water oxidation. Here, solely earth-abundant materials are incorporated to fabricate a CsPbBr3-based photoanode that reaches a low onset potential of +0.4 VRHE and 8 mA cm−2 photocurrent density at +1.23 VRHE for water oxidation, close to the radiative efficiency limit of CsPbBr3. This photoanode retains 100% of its stabilized photocurrent density for more than 100 h of operation by replacing once the inexpensive graphite sheet upon signs of deterioration. The improved performance is due to an efficiently electrodeposited NiFeOOH catalyst on a protective self-adhesive graphite sheet, and enhanced charge transfer achieved by phase engineering of CsPbBr3. Devices with >1 cm2 area, and low-temperature processing demonstrate the potential for low capital cost, stable, and scalable perovskite photoanodes

Origin of the switchable photocurrent direction in BiFeO3 thin films.

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO3) photoactive thin films. Depending on the applied bias our BiFeO3 films exhibit photocurrents more usually found in p- or n-type semiconductor photoelectrodes. In order to understand the anomalous behaviour ambient photoemission spectroscopy and Kelvin-probe techniques have been used to determine the band structure of the BiFeO3. We found that the Fermi level (Ef) is at -4.96 eV (vs. vacuum) with a mid-gap at -4.93 eV (vs. vacuum). Our photochemically determined flat band potential (Efb) was found to be 0.3 V vs. NHE (-4.8 V vs. vacuum). These band positions indicate that Ef is close to mid-gap, and Efb is close to the equilibrium with the electrolyte enabling either cathodic or anodic band bending. We show an ability to control switching from n- to p-type behaviour through the application of external bias to the BiFeO3 thin film. This ability to control majority carrier dynamics at low applied bias opens a number of applications in novel optoelectronic switches, logic and energy conversion devices

A versatile star-shaped organic semiconductor based on benzodithiophene and diketopyrrolopyrrole

We report the synthesis of a new star-shaped π-conjugated oligomer, BDT(DPP)4, containing a benzodithiophene core and four diketopyrrolopyrrole arms. The thermal, electrochemical and optical properties are characterized and the results complemented by computational studies. The utility of the molecule is demonstrated in both solar cell and field-effect transistor devices. In the former, BDT(DPP)4 displays low efficiency when used as an acceptor in blends with poly(3-hexylthiophene) but exhibits promising performance as a donor, in blends with either a fullerene or a non-fullerene acceptor. In field-effect transistors BDT(DPP)4 exhibits typical p-type transistor behavior, which is in accordance with its better donor performance in solar cell devices

Determining Out-of-Plane Hole Mobility in CuSCN via the Time-of-Flight Technique To Elucidate Its Function in Perovskite Solar Cells.

Copper(I) thiocyanate (CuSCN) is a stable, low-cost, solution-processable p-type inorganic semiconductor used in numerous optoelectronic applications. Here, for the first time, we employ the time-of-flight (ToF) technique to measure the out-of-plane hole mobility of CuSCN films, enabled by the deposition of 4 μm-thick films using aerosol-assisted chemical vapor deposition (AACVD). A hole mobility of ∼10-3 cm2/V s was measured with a weak electric field dependence of 0.005 cm/V1/2. Additionally, by measuring several 1.5 μm CuSCN films, we show that the mobility is independent of thickness. To further validate the suitability of our AACVD-prepared 1.5 μm-thick CuSCN film in device applications, we demonstrate its incorporation as a hole transport layer (HTL) in methylammonium lead iodide (MAPbI3) perovskite solar cells (PSCs). Our AACVD films result in devices with measured power conversion efficiencies of 10.4%, which compares favorably with devices prepared using spin-coated CuSCN HTLs (12.6%), despite the AACVD HTLs being an order of magnitude thicker than their spin-coated analogues. Improved reproducibility and decreased hysteresis were observed, owing to a combination of excellent film quality, high charge-carrier mobility, and favorable interface energetics. In addition to providing a fundamental insight into charge-carrier mobility in CuSCN, our work highlights the AACVD methodology as a scalable, versatile tool suitable for film deposition for use in optoelectronic devices