We present a density functional theory (DFT) study aimed at understanding the injection and recombination processes that occur at the interface between PbS QDs and TiO2 oxide nanoparticles with different morphologies. The calculated injection rates fall in the picosecond timescale in good agreement with the experiments. In addition, our simulations show that the (101) facet of TiO2 more favourably accommodates the QD, resulting in stronger electronic couplings and faster electron injections than the (001) surfaces. Despite this, the (101) slab is also more prone to faster electron recombination with the valence band of the QD, which can lead to overall lower injection efficiencies than the (001) surface.
This paper presents for the first time Sb2S3-based solar cells operating on scaffold film. The scaffolds studied are Al2O3 and ZrO2, for which no electron injection from the Sb2S3 to the Al2O3 or ZrO2 is possible. As a result, one of the highest open circuit voltages (Voc) of 0.712 V was observed for this solar cell configuration. Electron dispersive spectroscopy (EDS) was performed, revealing complete pore filling of the Sb2S3 into the metal oxide pores (e.g., Al2O3 or ZrO2); the complete pore filling of the Sb2S3 is responsible for the photovoltaic performance (PV) of this unique solar cell structure. In addition, intensity modulated photovoltage and photocurrent spectroscopy (IMVS and IMPS) were performed to extract the electron diffusion length. Electron diffusion length in the range of 900 nm to 290 nm (depending on the light intensity) was observed, which further supports the operation of metal oxide/Sb2S3 solar cell configuration. Moreover, the Al2O3-based cells have longer electron diffusion length than the TiO2-based cells, supporting the higher open circuit voltage of the noninjected metal oxide-based cells. This work demonstrates the potential of Sb2S3 to gain high voltage and to perform on a scaffold substrate without requiring electron injection.
Recent discoveries have revealed a breakthrough in the photovoltaics (PVs) ! eld using organometallic perovskites as light harvesters in the solar cell. The organometal perovskite arrangement is self-assembled as alternate layers via a simple low-cost procedure. These organometal perovskites promise several bene! ts not provided by the separate constituents. This overview concentrates on implementing perovskites in PV cells such that the perovskite layers are used as the light harvester as well as the hole-conducting component. Eliminating hole-transport material (HTM) in this solar-cell structure avoids oxidation, reduces costs, and provides better stability and consistent results. Aspects of HTM-free perovskite solar cells discussed in this article include (1) depletion regions, (2) high voltages, (3) panchromatic responses, (4) chemical modi! cations, and (5) contacts in HTM-free perovskite solar cells. Elimination of HTM could expand possibilities to explore new interfaces in these solar cells, while over the long term, these uniquely structured HTM-free solar cells could offer valuable bene! ts for future PV and optoelectronics applications.
In the recent years, the heterojunction solar cells based on quantum dots (QDs) have attracted attention due to strong light absorbing characteristics and the size effect on the bandgap tuning. This paper reports on the kinetics of interfacial charge separation of PbS QDs/(001) TiO 2 nanosheets heterojunction solar cells. PbS QDs are deposited using a bifunctional linker molecule on two different TiO 2 fi lms, i.e., TiO 2 nanosheets (with 001 dominant exposed facet) and TiO 2 nanoparticles (with 101 dominant exposed facet). Upon bandgap excitation, electrons are transferred from the PbS QDs conduction band to the lower lying conduction band of TiO 2 . Based on the ultrafast pump-probe laser spectroscopy technique, the kinetics of charge separation is scrutinized at the PbS/TiO 2 interface. The interfacial charge separation at PbS/TiO 2 nanosheets films made of (001) dominant exposed facets is fi ve times faster than that on (101) dominant exposed facets TiO 2 nanoparticles. The quantum yields for charge injection are higher for the (001) TiO 2 nanosheets than the (101) TiO 2 nanoparticles due to enhanced interfacial interaction with (001) surface compared to the (101) nanoparticles. The superior interfacial charge separation at PbS/(001) nanosheets respect to PbS/(101) nanoparticles is consistent with the higher photocurrent and enhanced power conversion effi ciency in the PbS QDs/(001) TiO 2 heterojunction solar cell. The use of (001) TiO 2 nanosheets can be a better alternative to conventional mesoporous TiO 2 fi lms in QD heterojunction solar cells and perovskites-based heterojunction solar cells.
In this work we demonstrate the planar configuration on hole conductor (HTM) free perovskite based solar cells. The CH3NH3PbI3 perovskite was deposited using the spray technique to achieve micrometer size perovskite crystals. The number of spray passes changes the CH3NH3PbI3 film thickness; for example, 10 spray passes achieved a film thickness of 3.4 μm of perovskite. Surprisingly, power conversion efficiency of 6.9% was demonstrated for this novel, simple solar cell structure with thick perovskite film that has no HTM. Capacitance−voltage measurements reveal charge accumulation at the CH3NH3PbI3/Au interface while the compact TiO2/CH3NH3PbI3 junction showed a space charge region, which inhibits the recombination. Studying these interfaces is key to understanding the operation mechanism of this unique solar cell structure. This simple planar HTM free perovskite solar cell demonstrates the potential to make large-scale solar cells while maintaining a simple, low-cost architecture.
This work reports on the preparation of semitransparent perovskite solar cells. The cells transparency is achieved through a unique wet deposition technique that creates perovskite grids with various dimensions. The perovskite grid is deposited on a mesoporous TiO 2 layer, followed by hole transport material deposition and evaporation of a semitransparent gold fi lm. Control of the transparency of the solar cells is achieved by changing the perovskite solution concentration and the mesh openings. The semitransparent cells demonstrate 20–70% transparency with a power conversion effi ciency of 5% at 20% transparency. This is the fi rst demonstration of the possibility to create a controlled perovskite pattern using a direct mesh-assisted assembly deposition method for fabrication of a semitransparent perovskite-based solar cell.
We report on accelerated degradation testing of MAPbX3 films (X = I or Br) by exposure to concentrated sunlight of 100 suns and show that the evolution of light absorption and the corresponding structural modifications are dependent on the type of halide ion and the exposure temperature. One hour of such exposure provides a photon dose equivalent to that of one sun exposure for 100 hours. The degradation in absorption of MAPbI3 films after exposure to 100 suns for 60 min at elevated sample temperature (∼45−55 °C), due to decomposition of the hybrid perovskite material, is documented. No degradation was observed after exposure to the same sunlight concentration but at a lower sample temperature (∼25 °C). No photobleaching or decomposition of MAPbBr3 films was observed after exposure to similar stress conditions (light intensity, dose, and temperatures). Our results indicate that the degradation is highly dependent on the hybrid perovskite composition and can be light- and thermally enhanced.
Cross-sections of a hole-conductor-free CH3NH3PbI3 perovskite solar cell were characterized with Kelvin probe force microscopy. A depletion region width of about 45 nm was determined from the measured potential profiles at the interface between CH3NH3PbI3 and nanocrystalline TiO2, whereas a negligible depletion was measured at the CH3NH3PbI3/Al2O3 interface. A complete solar cell can be realized with the CH3NH3PbI3 that functions both as light harvester and hole conductor in combination with a metal oxide. The band diagrams were estimated from the measured potential profile at the interfaces, and are critical findings for a better understanding and further improvement of perovskite based solar cells.
Organometal halide perovskite is a promising material in photovoltaic (PV) cells. Within a short time, its performance has increased dramatically to become a real competitor to silicon solar cells. Here we report on the temperature dependence (annealing temperature and the dependence of the photovoltaic parameters on temperature) of formamidinium (FA) lead iodide (FAPbI3), methylammonium (MA) lead iodide (MAPbI3) and their mixture (MAPbI3 : FAPbI3) in hole conductor free perovskite solar cells. These three types of perovskites function both as light harvesters and as hole conductors. Surface photovoltage and optical characterization reveal the p-type behavior and the band gap of the different perovskites. We observed that the ratio between the MA and FA cations might change during the annealing process, affecting the band gap and the stability of the layers. The PV parameters at different temperatures show better stability for the pure MAPbI3 and FAPbI3 solar cells compared to their mixture. Using intensity modulated photovoltage/photocurrent spectroscopy, we found that the diffusion length is weakly dependent on the light intensity, while the charge collection efficiency drops with light intensity for the FAPbI3-based cells. However, for MAPbI3 and the mixture, the charge collection efficiency remains constant for a wide range of light intensities.