FAPb0.5Sn0.5I3: A Narrow Bandgap Perovskite Synthesized through Evaporation Methods for Solar Cell Applications

The tunability of the optoelectrical properties upon compositional modification is a key characteristic of metal halide perovskites. In particular, bandgaps narrower than those in conventional lead‐based perovskites are essential to achieve the theoretical efficiency limit of single‐absorber solar cells, as well as develop multijunction tandem devices. Herein, the solvent‐free vacuum deposition of a narrow bandgap perovskite based on tin-lead metal and formamidinium cation is reported. Pinhole‐free films with 1.28 eV bandgap are obtained by thermal codeposition of precursors. The optoelectrical quality of these films is demonstrated by their use in solar cells with a power conversion efficiency of 13.98%

Charge injection and trapping at perovskite interfaces with organic hole transporting materials of different ionization energies

The extraction of photogenerated holes from CH3NH3PbI3 is crucial in perovskite solar cells. Understanding the main parameters that influence this process is essential to design materials and devices with improved efficiency. A series of vacuum deposited hole transporting materials (HTMs) of different ionization energies, used in efficient photovoltaic devices, are studied here by means of femtosecond transient absorption spectroscopy. We find that ultrafast charge injection from the perovskite into the different HTMs (<100 fs) competes with carrier thermalization and occurs independently of their ionization energy. Our results prove that injection takes place from hot states in the valence band making this efficient even for HTMs with higher ionization energy than that of the perovskite. Moreover, a new trapping mechanism is observed after the addition of HTMs, which is attributed to interfacial electron traps formed between the CH3NH3PbI3 and the HTMs, in addition to traps in the neat perovskite. Interfacial electron trapping is slower compared to the ultrafast hole injection, which contributes to the high efficiency obtained when these HTMs are employed in solar cells

Vacuum-Deposited Multication Tin-Lead Perovskite Solar Cells

The use of a combination of tin and lead is the most promising approach to fabricate narrow bandgap metal halide perovskites. This work presents the development of reproducible tin and lead perovskites by vacuum codeposition of the precursors, a solvent-free technique which can be easily implemented to form complex stacks. Crystallographic and optical characterization reveal the optimal film composition based on cesium and methylammonium monovalent cations. Device optimization makes use of the intrinsically additive nature of vacuum deposition, resulting in solar cells with 8.89% photovoltaic efficiency. The study of the devices by impedance spectroscopy identifies bulk recombination as one of the performance limiting factors

Hole Transport and Recombination in All-Solid Sb2S3-Sensitized TiO2 Solar Cells Using CuSCN As Hole Transporter

All-solid semiconductor-sensitized solar cells lack models allowing their characterization in terms of the fundamental processes of charge transport and recombination. Nanostructured TiO 2/Sb 2S 3/CuSCN solar cells were characterized by impedance spectroscopy, and a model was proposed for this type of cells. One important feature resulting from this analysis was the hole transport diffusion, which could be assimilated to a series resistance affecting the cell fill factor. The other important feature was the recombination rate, which could be described in a similar manner as other cells using nanostructured TiO 2 electrodes and which had an important impact on the open circuit. A simulation of the current-voltage curves using such model allowed us to get an approximate quantification of the losses caused by each process and to evaluate the possible improvements on the performance of this kind of cell

Effects of Frequency Dependence of the External Quantum Efficiency of Perovskite Solar Cells

Perovskite solar cells are known to show very long response time scales, on the order of milliseconds to seconds. This generates considerable doubt over the validity of the measured external quantum efficiency (EQE) and consequently the estimation of the short-circuit current density. We observe a variation as high as 10% in the values of the EQE of perovskite solar cells for different optical chopper frequencies between 10 and 500 Hz, indicating a need to establish well-defined protocols of EQE measurement. We also corroborate these values and obtain new insights regarding the working mechanisms of perovskite solar cells from intensity-modulated photocurrent spectroscopy measurements, identifying the evolution of the EQE over a range of frequencies, displaying a singular reduction at very low frequencies. This reduction in EQE is ascribed to additional resistive contributions hindering charge extraction in the perovskite solar cell at short-circuit conditions, which are delayed because of the concomitant large low-frequency capacitance

Enhanced operational stability through interfacial modification by active encapsulation of perovskite solar cells

Encapsulates are, in general, the passive components of any photovoltaic device that provides the required shielding from the externally stimulated degradation. Here we provide comprehensive physical insight depicting a rather non-trivial active nature, in contrast to the supposedly passive, atomic layer deposition (ALD) grown Al2O3 encapsulate layer on the hybrid perovskite [(FA0.83MA0.17)0.95Cs0.05PbI2.5Br0.5] photovoltaic device having the configuration: glass/FTO/SnO2/perovskite/spiro-OMeTAD/Au/(±) Al2O3. By combining various electrical characterization techniques, our experimental observations indicate that the ALD chemistry produces considerable enhancement of the electronic conductivity of the spiro-OMeTAD hole transport medium (HTM), resulting in electronic modification of the perovskite/HTM interface. Subsequently, the modified interface provides better hole extraction and lesser ionic accumulation at the interface, resulting in a significant lowering of the burn-in decay and nearly unchanged charge transport parameters explicitly under the course of continuous operation. Unlike the unencapsulated device, the modified electronic structure in the Al2O3 coated device is essentially the principal reason for better performance stability. Data presented in this communication suggest that the ionic accumulation at the spiro-OMeTAD/perovskite interface triggers the device degradation in the uncoated devices, which is eventually followed by material degradation, which can be avoided by active encapsulation

Oxygen doping-induced photogeneration loss in P3HT:PCBM solar cells

This work investigates the loss in performance induced by molecular oxygen in bulk heterojunction solar cells. We observe that upon exposure to molecular oxygen both formation of P3HT+:O2− complex and metal oxidation at the interface between the active layer and metallic contact occur. These two different effects were separately investigated using NOBF4 as an oxidant. Our procedure has allowed studying p-doping of the active layer independently from contact degradation. A loss in photocurrent is associated with formation of P3HT+:O2− complex, which reduces the concentration of neutral P3HT present in the film in accordance with absorption and external quantum efficiency spectra. This complex is regarded as a source of a pathway of reversible degradation. Capacitance–voltage measurements allow for an accurate extraction of p-doping levels of the active layer produced by the presence of charged O2− species. In addition, one of the irreversible degradation pathways is identified to be oxidation of the metallic contact to form CaO. This oxide forms a thin dipole layer producing a voltage drop across the active layer/Ca interface, which has a direct impact on the open circuit voltage and fill factor

Pressure-Induced Phase Transition Versus Amorphization in Hybrid Methylammonium Lead Bromide Perovskite

The crystal structure of CH3NH3PbBr3 perovskite has been investigated under high-pressure by synchrotron-based powder X-ray diffraction. We found that after the previously reported phase transitions in CH3NH3PbBr3 (Pm-3m->Im-3->Pmn21), which occur below 2 GPa, there is a third transition to a crystalline phase at 4.6 GPa. This transition is reported here for the first time contradicting previous studies which reported amorphization of CH3NH3PbBr3 between 2.3 and 4.6 GPa. Our X-ray diffraction measurements show that CH3NH3PbBr3 remains crystalline up to 7.6 GPa, the highest pressure covered by experiments. The new high-pressure phase is also described by the space group Pmn21, but the transition involves abrupt changes in the unit-cell parameters and a 3% decrease of the unit-cell volume. Our conclusions are confirmed by optical-absorption experiments and visual observations and by the fact that changes induced by pressure up to 10 GPa are reversible. The optical studies also allow for the determination of the pressure dependence of the band-gap energy which is discussed using the structural information obtained from X-ray diffraction.Comment: 15 pages, 4 figure

Colloidal PbS and PbSeS Quantum Dot Sensitized Solar Cells Prepared by Electrophoretic Deposition

Here we report the developement of quantum dot sensitized solar cells (QDSCs) using colloidal PbS and PbSeS QDs and polysulfide electrolyte for high photocurrents. QDSCs have been prepared in a novel sensitizing way employing electrophoretic deposition (EPD), and protecting the colloidal QDs from corrosive electrolyte with a CdS coating. EPD allows a rapid, uniform and effective sensitization with QDs, while the CdS coating stabilizes the electrode. The effect of electrophoretic deposition time and of colloidal QD size on cell efficiency is analyzed. Efficiencies as high as 2.1±0.2% are reported

Efficient Vacuum Deposited P-I-N Perovskite Solar Cells by Front Contact Optimization

Hole transport layers (HTLs) are of fundamental importance in perovskite solar cells (PSCs), as they must ensure an efficient and selective hole extraction, and ohmic charge transfer to the corresponding electrodes. In p-i-n solar cells, the ITO/HTL is usually not ohmic, and an additional interlayer such as MoO3 is usually placed in between the two materials by vacuum sublimation. In this work, we evaluated the properties of the MoO3/TaTm (TaTm is the HTL N4,N4,N4″,N4″-tetra([1,1′-biphenyl]-4-yl)-[1,1′:4′,1″-terphenyl]-4,4″-diamine) hole extraction interface by selectively annealing either MoO3 (prior to the deposition of TaTm) or the bilayer MoO3/TaTm (without pre-treatment on the MoO3), at temperature ranging from 60 to 200°C. We then used these p-contacts for the fabrication of a large batch of fully vacuum deposited PSCs, using methylammonium lead iodide as the active layer. We show that annealing the MoO3/TaTm bilayers at high temperature is crucial to obtain high rectification with low non-radiative recombination, due to an increase of the electrode work function and the formation of an ohmic interface with TaTm