High time resolution broadband pump-probe experiments on CH3NH3PbI3 and CH3NH3PbBr3 films are described. The improved time resolution delineates instantaneous and delayed relaxation related effects on sample absorption and assists in clarifying controversial assignment of the underlying mechanisms. Analysis of the data in terms of finite difference spectra and spectral band integrals reveals that photoexcitation is high in the inter-band continuum leading to partial bleaching and red-shifts of the exciton band just below the absorption-edge instantaneously. Increased pump intensity saturates the exciton bleach and progressively reduces inter-band absorption in a broad range extending from the band edge to higher photon energies. Both effects are attributed to reduced Coulomb enhancement due to hot carrier screening. The spectral extent of the inter-band absorption attenuation provides estimated binding energies in the range of 20–30 meV in both materials. Sub-picosecond carrier cooling reverses the initial exciton transition red-shift and induces transmission near the band edge due to state filling and stimulated emission. Finally, 1–100 ps signals are dominated by reverse state filling due to non-geminate recombination. These results demonstrate that both inter-band and exciton absorptions are essential for unraveling photo-induced dynamics in these materials, and that insights obtained from many-body theoretical analysis of dynamic screening are essential for correctly assigning the recorded spectral evolution.
In this work we study the kinetics of cesium lead halide perovskite nanoparticle (NP) growth; the focusing and de-focusing of the NP size distribution. Cesium lead halide perovskite NPs are considered to be attractive materials for optoelectronic applications. Understanding the kinetics of the formation of these all-inorganic perovskite NPs is critical for reproducibly and reliably generating large amounts of uniformly sized NPs. Here we investigate different growth durations for CsPbI3 and CsPbBr3 NPs, tracking their growth by high-resolution transmission electron microscopy and size distribution analysis. As a result, we are able to provide a detailed model for the kinetics of their growth. It was observed that the CsPbI3 NPs exhibit focusing of the size distribution in the first 20 seconds of growth, followed by de-focusing over longer growth durations, while the CsPbBr3 NPs show de-focusing of the size distribution starting from the beginning of the growth. The monomer concentration is depleted faster in the case of CsPbBr3 than in the case of CsPbI3, due to faster diffusion of the monomers, which increases the critical radius and results in de-focusing of the population. Accordingly, focusing is not observed within 40 seconds of growth in the case of CsPbBr3. This study provides important knowledge on how to achieve a narrow size distribution of cesium lead halide perovskite NPs when generating large amounts of these promising, highly luminescent NPs.
This work demonstrates antisolvent treatment of organo-metal halide perovskite film in hole conductor-free perovskite-based solar cell, achieving impressive power conversion efficiency of 11.2% for hole-conductor-free cells with gold contact. We found that antisolvent (toluene) surface treatment affects the morphology of the perovskite layer, and importantly, it also affects the electronic properties of the perovskite. Conductive atomic force microscopy (cAFM) and surface photovoltage show that the perovskite film becomes more conductive after antisolvent treatment. Moreover, the antisolvent treatment suppresses the hysteresis commonly obtained for perovskite-based solar cells. When the perovskite alone is characterized, a I−V plot of a single perovskite grain measured by cAFM shows that hysteresis vanishes after toluene treatment. During toluene treatment, excess halide and methylammonium ions are removed from the perovskite surface, leading to a net positive charge on the Pb atoms, resulting in a more conductive perovskite surface, which is beneficial for the hole-conductor-free solar cell structure. The reliability of the surface treatment was proved by calculating the statistical parameters Z score and p value, which were 2.5 and 0.012, respectively. According to these values, it can be concluded with 95% confidence that the average efficiency of cells fabricated via surface treatment is greater than the average efficiency of cells without surface treatment. The statistical data support the impact of surface treatment on the photovoltaic performance of perovskite 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.
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.
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.
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.
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.
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.
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.
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.
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.
Organo metal halide perovskite has attracted considerable attention recently due to its distinctive properties that make it especially useful in photovoltaic solar cells. In this work we demonstrate high open circuit voltage of 1.35 V using Al2O3/CH3NH3PbBr3 perovskite solar cells without a hole conductor. The contact potential difference under light measured by surface photovoltage spectroscopy of CH3NH3PbBr3 was more than twice that of CH3NH3PbI3, which results in smaller surface potential for the Al2O3/CH3NH3PbBr3 cells. Incident modulated photovoltage spectroscopy shows a longer recombination lifetime for the Al2O3/CH3NH3PbBr3 cells than for the TiO2/CH3NH3PbI3 cells or for the TiO2/CH3NH3PbBr3 cells, further supporting the high open circuit voltage. The possibility to gain high open circuit voltage even without a hole transport material in perovskite solar cells shows that the perovskite/metal oxide interface has a major effect on the open circuit voltage in perovskite based solar cells.
We report for the first time on co-sensitization between CH3NH3PbI3 perovskite and PbS quantum dots (QDs) in a heterojunction solar cell to obtain a panchromatic response from the visible to near IR regions. Following the deposition of the sensitizers on a TiO2 film, an Au thin layer is evaporated on top as a back contact. Importantly, the CH3NH3PbI3 nanoparticles and the PbS QDs used here simultaneously play both the role of a light harvester and a hole conductor, rendering superfluous the use of an additional hole transporting material. The mesoscopic CH3NH3PbI3 (perovskite)–PbS (QD)/TiO2 heterojunction solar cell shows an impressive short circuit photocurrent (Jsc) of 24.63mA cm2, much higher than those of the individual CH3NH3PbI3 perovskite and the PbS QD solar cells. The advent of such co-sensitization mesoscopic heterojunction solar cells paves the way to extend the absorbance region of the promising low cost, high-efficiency perovskite based solar cells.
Organo-metal halide perovskites are composed of an ABX3 structure in which A represents a cation, B a divalent metal cation and X a halide. The organo-metal perovskite shows very good potential to be used as a light harvester in solar cells due to its direct band gap, large absorption coefficient, high carrier mobility and good stability. However, there is an important question in the photovoltaic field regarding the most advantageous architecture for perovskite based solar cells. Several studies showed sensitized perovskite solar cells achieving high performance, while high efficiency was also observed with the planar architecture. Consequently, it is still an open question regarding which operation mechanism and which architecture offers better results. This review describes both architectures, based on studies in the field. In the case of the sensitized structure, there are more difficulties in pore filling, naturally more recombination, and the possibility to use thicker metal oxide films. In the planar structure, thin metal oxide films are used, less recombination was observed and there are no infiltration problems. Both architectures exhibit long-range diffusion length and meet the demand for excellent coverage of the perovskite film.
In this work we used CH3NH3PbInBr3−n (where 0 ≤ n ≤ 3) as hole conductor and light harvester in the solar cell. Various concentrations of methylammonium iodide and methylammonium bromide were studied which reveal that any composition of the hybrid CH3NH3PbInBr3−n can conduct holes. The hybrid perovskite was deposited in two steps, separating it to two precursors to allow better control of the perovskite composition and efficient tuning of its band gap. The X-ray diffraction reveals the change in the lattice parameter due to the introducing of the Br− ions. The hybrid iodide/bromide perovskite hole conductor free solar cells show very good stability, their power conversion efficiency achieved 8.54% under 1 sun illumination with current density of 16.2 mA/cm2. The results of this work open the possibility for graded structure of perovskite solar cells without the need for hole conductor.
The inorganic–organic perovskite is currently attracting a lot of attention due to its use as a light harvester in solar cells. The large absorption coefficients, high carrier mobility and good stability of organo-lead halide perovskites present good potential for their use as light harvesters in mesoscopic heterojunction solar cells. This work concentrated on a unique property of the lead halide perovskite, its function simultaneously as a light harvester and a hole conductor in the solar cell. A two-step deposition technique was used to optimize the perovskite deposition and to enhance the solar cell efficiency. It was revealed that the photovoltaic performance of the hole conductor free perovskite solar cell is strongly dependent on the depletion layer width which was created at the TiO2–CH3NH3PbI3 junction. X-ray diffraction measurements indicate that there were no changes in the crystallographic structure of the CH3NH3PbI3 perovskite over time, which supports the high stability of these hole conductor free perovskite solar cells. Furthermore, the power conversion efficiency of the best cells reached 10.85% with a fill factor of 68%, a Voc of 0.84 V, and a Jsc of 19 mA cm2, the highest efficiency to date of a hole conductor free perovskite solar cell.
Lead halide perovskite is an excellent candidate for use as a light harvester in solar cells. Our work presents a depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell using a thick CH3NH3PbI3 film. The CH3NH3PbI3 formed large crystals which function simultaneously as light harvesters and as hole transportmaterials. We performed capacitance voltage measurements, which show a depletion regionwhich extends to both n and p sides. The built-in field of the depletion region assists in the charge separation and suppresses the back reaction of electrons from the TiO2 film to the CH3NH3PbI3 film. This depleted hole conductor free CH3NH3PbI3/TiO2 heterojunction solar cell provides a power conversion efficiency of 8% with a current density of 18.8 mA cm2, the highest efficiency achieved to date for perovskite based solar cells without a hole conductor.
We prepared a quasi-solid electrolyte for dye-sensitized solar cells (DSSCs) that consist of ionic liquid and modified silica particles. Commercial bare silica F5 particles and modified silica F5 by NH2 and NH3+ groups were prepared, and fully characterized. The best photovoltaic performance was observed using the NH2 modified silica particles giving an open circuit voltage (Voc) of 815 mV, a short-circuit current (Jsc) of 11.23 mA cm-2, and a fill factor (FF) of 0.75 corresponding to an overall power conversion efficiency of 7.04% at 100 mW cm-2 AM 1.5. The modification of the silica particles by NH2 groups increases the Voc of DSSCs by around 60 mV compared to pure ionic liquid electrolyte based DSSCs.
Photovoltaic cells use semiconductors to convert sunlight into electrical current and are regarded as a key technology for a sustainable energy supply. Quantum dot-based solar cells have shown great potential as next generation, high performance, low-cost photovoltaics due to the outstanding optoelectronic properties of quantum dots and their multiple exciton generation (MEG) capability. This review focuses on QDs as light harvesters in solar cells, including different structures of QD-based solar cells, such as QD heterojunction solar cells, QD-Schottky solar cells, QD-sensitized solar cells and the recent development in organic-inorganic perovskite heterojunction solar cells. Mechanisms, procedures, advantages, disadvantages and the latest results obtained in the field are described. To summarize, a future perspective is offered.