CuSCN Nanowires as Electrodes for p-Type Quantum Dot Sensitized Solar Cells: Charge Transfer Dynamics and Alumina Passivation

Quantum dot sensitized solar cells (QDSSCs) are a promising photovoltaic technology due to their low cost and simplicity of fabrication. Most QDSSCs have an n-type configuration with electron injection from QDs into TiO2, which generally leads to unbalanced charge transport (slower hole transfer rate) limiting their efficiency and stability. We have previously demonstrated that p-type (inverted) QD sensitized cells have the potential to solve this problem. Here we show for the first time that electrodeposited CuSCN nanowires can be used as a p-type nanostructured electrode for p-QDSSCs. We demonstrate their efficient sensitization by heavy metal free CuInSxSe2-x quantum dots. Photophysical studies show efficient and fast hole injection from the excited QDs into the CuSCN nanowires. The transfer rate is strongly time dependent but the average rate of 2.5 × 109 s–1 is much faster than in previously studied sensitized systems based on NiO. Moreover, we have developed an original experiment allowing us to calculate independently the rates of charge injection and QD regeneration by the electrolyte and thus to determine which of these processes occurs first. The average QD regeneration rate (1.3 × 109 s–1) is in the same range as the hole injection rate, resulting in an overall balanced charge separation process. To reduce recombination in the sensitized systems and improve their stability, the CuSCN nanowires were coated with thin conformal layers of Al2O3 using atomic layer deposition (ALD) and fully characterized by XPS and EDX. We demonstrate that the alumina layer protects the surface of CuSCN nanowires, reduces charge recombination, and increases the overall charge transfer rate up to 1.5 times depending on the thickness of the deposited Al2O3 layer

Role of lattice distortion and A site cation in the phase transitions of methylammonium lead halide perovskites

The rapid increase in power conversion efficiencies of photovoltaic devices incorporating lead halide perovskites has resulted in intense interest in the cause of their excellent properties. In the present paper, resonant ultrasound spectroscopy has been used to determine the elastic and anelastic properties of CH3NH3PbX3(where X=Cl, Br, or I) and CD3ND3PbI3 perovskites in the 5–380 K temperature range. This is coupled with differential scanning calorimetry, variable temperature neutron powder diffraction, and variable temperature photoluminescence studies to provide insights into the underlying processes and structural instabilities in the crystal structure. By comparing measurements on CH3NH3PbI3 with the deuterated equivalent, it has been possible to distinguish processes which are related to the hydrogen bonding between the methylammonium cation and the perovskite framework. We observe that replacing hydrogen with deuterium has a significant impact on both the elastic and photophysical properties, which shows that hydrogen bonding plays a crucial role in the material performance. Temperature-dependent photoluminescence studies show that the light emission is unaffected by the tetragonal-orthorhombic phase transition, but a blueshift in the emission and a steep increase in photoluminescence quantum yield are seen at temperatures below 150 K. Finally, observations of peaks in acoustic loss occurring in CH3NH3PbCl3 have revealed freezing processes in the vicinity of ∼150−170K, with activation energies in the range of 300 to 650 meV. These processes are attributed to freezing of the motion of methylammonium cations, and could explain the changes in photoluminescence seen in CH3NH3PbI3 at the same temperature. © 2018 American Physical Society. This work has been made available online in accordance with the publisher’s policies. This is the author created, accepted version manuscript following peer review and may differ slightly from the final published version. The final published version of this work is available at: https://doi.org/10.1103/PhysRevMaterials.2.06540

CuSCN nanowires as electrodes for p-type quantum dot sensitized solar cells : charge transfer dynamics and alumina passivation

Funding: European Research Council (grant number 321305) and the EPSRC (grant numbers EP/L017008/1 and EP/M506631/1). IDWS is a Royal Society Wolfson Research Merit award holder.Quantum dot sensitized solar cells (QDSSCs) are a promising photovoltaic technology due to their low cost and simplicity of fabrication. Most QDSSCs have an n-type configuration with electron injection from QDs into TiO2, which generally leads to unbalanced charge transport (slower hole transfer rate) limiting their efficiency and stability. We have previously demonstrated that p-type (inverted) QD sensitized cells have the potential to solve this problem. Here we show for the first time that electrodeposited CuSCN nanowires can be used as a p-type nanostructured electrode for p-QDSSCs. We demonstrate their efficient sensitization by heavy metal free CuInSxSe2-x quantum dots. Photophysical studies show efficient and fast hole injection from the excited QDs into the CuSCN nanowires. The transfer rate is strongly time dependent but the average rate of 2.5 x 109 s-1 is much faster than in previously studied sensitized systems based on NiO. Moreover, we have developed an original experiment allowing us to calculate independently the rates of charge injection and QD regeneration by the electrolyte and thus to determine which of these processes occurs first. The average QD regeneration rate (1.33 x 109 s-1 ) is in the same range as the hole injection rate, resulting in an overall balanced charge separation process. To reduce recombination in the sensitized systems and improve their stability, the CuSCN nanowires were coated with thin conformal layers of Al2O3 using atomic layer deposition (ALD) and fully characterized by XPS and EDX. We demonstrate that the alumina layer protects the surface of CuSCN nanowires, reduces charge recombination and increases the overall charge transfer rate up to 1.5 times depending on the thickness of the deposited Al2O3 layer.PostprintPeer reviewe

Role of lattice distortion and A site cation in the phase transitions of methylammonium lead halide perovskites

The rapid increase in power conversion efficiencies of photovoltaic devices incorporating lead halide perovskites has resulted in intense interest in the cause of their excellent properties. In the present paper, resonant ultrasound spectroscopy has been used to determine the elastic and anelastic properties of CH3NH3PbX3 (where X=Cl, Br, or I) and CD3ND3PbI3 perovskites in the 5–380 K temperature range. This is coupled with differential scanning calorimetry, variable temperature neutron powder diffraction, and variable temperature photoluminescence studies to provide insights into the underlying processes and structural instabilities in the crystal structure. By comparing measurements on CH3NH3PbI3 with the deuterated equivalent, it has been possible to distinguish processes which are related to the hydrogen bonding between the methylammonium cation and the perovskite framework. We observe that replacing hydrogen with deuterium has a significant impact on both the elastic and photophysical properties, which shows that hydrogen bonding plays a crucial role in the material performance. Temperature-dependent photoluminescence studies show that the light emission is unaffected by the tetragonal-orthorhombic phase transition, but a blueshift in the emission and a steep increase in photoluminescence quantum yield are seen at temperatures below 150 K. Finally, observations of peaks in acoustic loss occurring in CH3NH3PbCl3 have revealed freezing processes in the vicinity of ∼150−170K, with activation energies in the range of 300 to 650 meV. These processes are attributed to freezing of the motion of methylammonium cations, and could explain the changes in photoluminescence seen in CH3NH3PbI3 at the same temperature

Role of lattice distortion and A site cation in the phase transitions of methylammonium lead halide perovskites

This paper was supported by Engineering and Physical Sciences Research Council (EPSRC) Grants No. EP/K503162/1, No. EP/M506631/1, No. EP/M025330/1, No. EP/L017008/1, No. EP/K022237/1, No. EP/K015540/1, and No. EP/I036079/1. RUS facilities in Cambridge were established through grants to M.A.C. from the EPSRC and the Natural Environment Research Council (Grants No. NE/B505738/1 and No. NE/F17081/1). A portion of this research used resources at the Spallation Neutron Source, a U.S. Department of Energy Office of Science User Facility operated by the Oak Ridge National Laboratory (ORNL). I.D.W.S. acknowledges funding from a Royal Society Wolfson research merit award.The rapid increase in power conversion efficiencies of photovoltaic devices incorporating lead halide perovskites has resulted in intense interest in the cause of their excellent properties. In the present paper, resonant ultrasound spectroscopy has been used to determine the elastic and anelastic properties of CH3NH3PbX3 (where X = Cl, Br, or I) and CD3ND3PbI3 perovskites in the 5-380 K temperature range. This is coupled with differential scanning calorimetry, variable temperature neutron powder diffraction, and variable temperature photoluminescence studies to provide insights into the underlying processes and structural instabilities in the crystal structure. By comparing measurements on CH3NH3PbI3 with the deuterated equivalent, it has been possible to distinguish processes which are related to the hydrogen bonding between the methylammonium cation and the perovskite framework. We observe that replacing hydrogen with deuterium has a significant impact on both the elastic and photophysical properties, which shows that hydrogen bonding plays a crucial role in the material performance. Temperature-dependent photoluminescence studies show that the light emission is unaffected by the tetragonal-orthorhombic phase transition, but a blueshift in the emission and a steep increase in photoluminescence quantum yield are seen at temperatures below 150 K. Finally, observations of peaks in acoustic loss occurring in CH3NH3PbCl3 have revealed freezing processes in the vicinity of ∼150-170 K, with activation energies in the range of 300 to 650 meV. These processes are attributed to freezing of the motion of methylammonium cations, and could explain the changes in photoluminescence seen in CH3NH3PbI3 at the same temperature.PostprintPeer reviewe

Optoelectronic applications of lead halide perovskites

Hybrid perovskites are a new class of semiconductor which have proven to be an ideal material for making thin film solar cells. They have the advantages of flexibility, low cost, and easy processing, whilst achieving efficiencies competitive with monocrystalline silicon. Many of the properties which make them ideal for solar cells are also applicable to light emitting devices, and there is now increasing interest in their application for light emitting diodes (LEDs) and lasers. This thesis aims to use a range of novel spectroscopy techniques to investigate the origin of these favourable properties, and to exploit these properties to produce high performance distributed feedback lasers. A detailed understanding of the origins of the excellent properties of hybrid perovskites is of crucial importance in the search for new variations with improved performance or lowered toxicity. This thesis uses Kelvin probe, air photoemission, and resonant ultrasound spectroscopy to probe deeply into the underlying physics of hybrid perovskite single crystals and devices. Using these techniques, we are able to produce detailed maps of the energy levels in a common perovskite solar cell, and we also gain strong insight into the underlying strains and instabilities in the perovskite structure that give rise to their elastic properties. The strong light emission of hybrid perovskites is then exploited to produce high quality distributed feedback lasers emitting in the green and infrared part of the spectrum. These lasers are observed to have superior stability, good thresholds, and many interesting beam parameters owing to their high refractive index. We explore a wide range of processing methods in order to achieve the lowest lasing threshold and the best stability. Finally, we investigate the properties of low dimensional perovskites and investigate their potential in optoelectronic applications."This work was supported by the Engineering and Physical Sciences Research Council (grant number EP/M506631/1)" -- Fundin

An investigation of the energy levels within a common perovskite solar cell device and a comparison of DC/AC surface photovoltage spectroscopy Kelvin Probe measurements of different MAPBI₃ perovskite solar cell device structures

We present a study of the energy levels in a FTO/TiO2/CH3NH3PbI3/Spiro solar cell device. The measurements are performed using a novel ambient pressure photoemission (APS) technique alongside Contact Potential Difference data from a Kelvin Probe. The Perovskite Solar Cell energy band diagram is demonstrated for the device in dark conditions and under illumination from a 150W Quartz Tungsten Halogen lamp. This approach provides useful information on the interaction between the different materials in this solar cell device. Additionally, non-destructive macroscopic DC and AC Surface Photovoltage Spectroscopy (SPS) studies are demonstrated of different MAPBI3 device structures to give an indication of overall device performance. AC-SPS measurements, previously used on traditional semiconductors to study the mobility, are used in this case to characterise the ability of a perovskite solar cell device to respond rapidly to chopped light. Two different device structures studied showed very different characteristics: Sample A (without TiO2): (ITO/PEDOT:PSS/polyTPD/CH3NH3PbI3/PCBM) had ∼4 times the magnitude of AC-SPS response compared to Sample B (including TiO2): (ITO/TiO2/ CH3NH3PbI3/Spiro). This demonstrates that the carrier speed characteristics of device architecture A is superior to device architecture B. The TiO2 layer has been associated with carrier trapping which is illustrated in this example. However, the DC-SPV performance of sample B is ∼5 times greater than that of sample A. The band gap of the MAPBI3 layer was determined through DC-SPS (1.57 ± 0.07 eV), Voc of the devices measured and qualitative observations made of interface trapping by DC light pulsing. The combination of these (APS, KP, AC/DC-SPV/SPS) techniques offers a more general method for measuring the energy level alignments and performance of Organic and Hybrid Solar Cell Devices.</p