Matthew J. Carnie

Highly efficient, flexible, indium-free perovskite solar cells employing metallic substrates

Flexible perovskite solar cells with power conversion efficiencies of up to 10.3% have been prepared using titanium foil as an electrode substrate. Our method uses an indium-free transparent counter electrode which allows device performance to remain high despite repeated bending, making it suitable for roll-to-roll processing.

Perovskite processing for photovoltaics: a spectro-thermal evaluation

Thermal analysis (TGA and DSC), coupled with evolved gas FTIR spectroscopy, has been used to study the changes occurring during, and differences between materials after, the annealing step of mixed-halide methylammonium lead halide perovskites. This is important because, to date, the material is the most efficient light harvester in highly efficient, 3rd generation perovskite photovoltaic devices, and processing plays a significant role in device performance. TGA-FTIR data show only solvent evolution during the annealing step, whilst post-annealing analysis shows that the resulting material still contains a significant amount of residual solvent; however, efficient DMF removal was possible using a silica gel desiccant for a period of 3 days. The data also show that methylammonium halide decomposition does not occur until temperatures well above those used for perovskite processing, suggesting that this is not a significant issue for device manufacture. The absence of a well-defined, reversible tetragonal – cubic phase change around 55 °C in the DSC data of the annealed material, and the presence of HCl in evolved gas analysed following thermal decomposition, demonstrates that CH3NH3I3−xClx does retain some Cl after annealing and does not simply form stoichiometric CH3NH3PbI3 as has been suggested by some workers.

Ultra-fast sintered TiO2 films in dye-sensitized solar cells: phase variation, electron transport and recombination

With the application of near-infrared radiation (NIR), TiO2 films for dye-sensitized solar cells (DSCs) on metallic substrates can be sintered in just 12.5 seconds. The photovoltaic performance of devices made with NIR sintered films match those devices made with conventionally sintered films prepared by heating for 1800 seconds. Here we characterise the electron transport, electron lifetime and phase-morphological properties of ultrafast NIR sintered films, using impedance spectroscopy, transient photovoltage decay and X-ray diffraction measurements. An important factor in NIR processing of TiO2 films is the peak metal temperature (PMT) and we show that during the 12.5 second heat treatment, a PMT of around 635 °C gives near identical electron transport, electron lifetime and morphological properties, as well comparable photovoltaic performance to a conventionally sintered (500 °C, 30 min) film. We demonstrate that the rapid heating of the TiO2 (to temperatures of up to 785 °C) does not lead to a large scale rutile phase transition. As such photovoltaic performance of resultant DSC devices is maintained since the heating period is insufficient to induce a significant transition from anatase to rutile or morphology changes which result in a loss of photocurrent.

Rapid processing of perovskite solar cells in under 2.5 seconds

A rapid annealing technique for CH3NH3PbI3 perovskite solar cells is presented. We report a co-deposited Al2O3–perovskite device annealed in under 2.5 seconds with a PCE of 10.0% compared to 10.9% for a 45 minute oven-annealed device.

Performance enhancement of solution processed perovskite solar cells incorporating functionalized silica nanoparticles

High efficiency, solution processed organic–inorganic trihalide perovskite solar cells are now a reality, meaning that perovskite photovoltaics have the potential to challenge more established photovoltaic technologies. To date, some of the most efficient solution processed perovskite solar cells feature a pre-deposited Al2O3 scaffold and we have shown in a previous communication, that it is possible to make efficient devices by co-depositing the Al2O3 nanoparticles with the perovskite precursor solution. In this work, we have substituted the alumina nanoparticles with 3-aminopropyl (3-oxobutanoic acid) functionalized silica nanoparticles (f-SiO2). We observe performance enhancements in planar heterojunction (PHJ) devices made with up to 0.75 wt% f-SiO2 nanoparticles present in the precursor solution, yielding power conversion efficiencies (PCE) of up to 12.4%, compared to the maximum PCE of 10.5% in the equivalent PHJ devices made without f-SiO2 nanoparticles. The performance enhancement arises in part from an average increase to VOC by up to 50 mV when the nanoparticles are present in the precursor solution and is attributed to substrate passivation within pinholes formed in the perovskite film during processing.

Photocatalytic Oxidation of Triiodide in UVA-Exposed Dye-Sensitized Solar Cells

UVA irradiation of glass mounted dye-sensitized solar cells without UV filtration causes failure within 400 hours of light exposure. The failure mode is shown to relate to consumption of I3−, which is directly related to TiO2 photo-catalysis. The onset of failure is easily determined from electrochemical impedance data where the recombination resistance of the TiO2/electrolyte back reaction drops markedly prior to the onset of degradation. At the point of complete cell failure this impedance value then dramatically increases as there is no longer an interfacial reaction possible between the TiO2 and the I3− depleted electrolyte. Device failure is most rapid for cells under electrical load indicating that the degradation of the electrolyte is related to photogenerated hole production by excitation of the TiO2. Once depleted by UV exposure, the I3− can be regenerated by simple application of a reverse bias which can restore severely UV degraded devices to near original working conditions.

Near Infrared Radiation as a Rapid Heating Technique for TiO2Films on Glass Mounted Dye-Sensitized Solar Cells

Near infrared radiation (NIR) has been used to enable the sintering of TiO2 films on fluorine-doped tin oxide (FTO) glass in 12.5 s. The 9 µm thick TiO2 films were constructed into working electrodes for dye-sensitized solar cells (DSCs) achieving similar photovoltaic performance to TiO2 films prepared by heating for 30 min in a convection oven. The ability of the FTO glass to heat upon 12.5 s exposure of NIR radiation was measured using an IR camera and demonstrated a peak temperature of 680°C; glass without the 600 nm FTO layer reached 350°C under identical conditions. In a typical DSC heating step, a TiO2 based paste is heated until the polymeric binder is removed leaving a mesoporous film. The weight loss associated with this step, as measured using thermogravimetric analysis, has been used to assess the efficacy of the FTO glass to heat sufficiently. Heat induced interparticle connectivity in the TiO2 film has also been assessed using optoelectronic transient measurements that can identify electron lifetime through the TiO2 film. An NIR treated device produced in 12.5 seconds shows comparable binder removal, electron lifetime, and efficiency to a device manufactured over 30 minutes in a conventional oven.

UV Filtering of Dye-Sensitized Solar Cells: The Effects of Varying the UV Cut-Off upon Cell Performance and Incident Photon-to-Electron Conversion Efficiency

With current technology, UV filters are essential to ensure long-term dye-sensitized solar cell (DSC) stability. Blocking photons, however, will have an obvious effect on device performance and upon its incident photon-to-current conversion efficiency (IPCE). Filters have been applied to DSC devices with a range of cut-off wavelengths in order to assess how different levels of filtering affect the performance and IPCE of devices made with three different dyes, namely N719, Z907, and N749. It is shown that dyes that extend their IPCE further into the NIR region suffer lesser relative efficiency losses due to UV filtering than dyes with narrower action spectra. Furthermore, the results are encouraging to those working towards the industrialisation of DSC technology. From the results presented it can be estimated that filtering at a level intended to prevent direct band gap excitation of the TiO2 semiconductor should cause a relative drop in cell efficiency of no more than 10% in forward illuminated devices and no more than 2% in reverse illuminated devices.

Low temperature sintering of binder-containing TiO2/metal peroxide pastes for dye-sensitized solar cells

Nano-structured metal oxide films are key components of dye-sensitized (DSC) solar cells. Scaling such devices requires lower temperature processing to enable cheaper substrates to be used. In this context, we report a new and scalable method to sinter binder-containing metal oxide pastes to make DSC photo-electrodes at lower temperatures. Metal peroxide powders (CaO2, MgO2, or ZnO2) were added to terpineol-based P25 pastes containing ethyl cellulose binder or to commercial TiO2 paste (DSL18NR-T). Thermal analysis shows that binder decomposition occurs at 300 °C instead of the standard 450 °C for a TiO2-only paste and suggests that the metal peroxides act as combustion promoters releasing heat and oxygen within the film while heating. The data show that this heat and oxygen release coincide best with binder combustion for ZnO2 and DSC device tests show that adding ZnO2 to TiO2 pastes produces the best performances affording η = 7.5% for small devices (0.26 cm2) and η = 5.7% at 300 °C or 450 °C for DSL18NR-T/ZnO2 for larger (1 cm2) devices. To the best of our knowledge, the performance of the (0.26 cm2) cells is comparable to the highest efficiency devices reported for DSCs fabricated using low temperature methods. The device efficiency is most strongly linked with Jsc; BET and dye sorption measurements suggest that Jsc is linked with the metal oxide surface area and dye loading. The latter is linked to the availability of surface sorption sites for dye molecules which is strongly negatively affected by any residual organic binder which resulted from incomplete combustion.

High throughput fabrication of mesoporous carbon perovskite solar cells

The screen printed mesoporous carbon perovskite solar cell has great potential for commercialisation due to its scalable deposition processes and use of inexpensive materials. However, each layer requires long high temperature heating steps to achieve the necessary sintering and porosity, which is very time and energy intensive for large scale production. Near infrared processing is demonstrated here to reduce the heating time of mesoporous layers within a fully printed lead halide perovskite solar cell from 2 hours to 30 seconds. A stabilised efficiency of 11% was achieved by processing in 30 seconds, identical to that of devices heated in 2 hours. For the first time the effect of residual binder in the carbon electrode on the electron lifetime and charge transfer within devices has been investigated. Furthermore cross section EDX mapping of perovskite infiltration provides a greater understanding into the processing requirements of these devices vital to enable commercialisation.