Light-Induced Space-Charge Accumulation Zone as Photovoltaic Mechanism in Perovskite Solar Cells

We fabricated formamidinium lead iodide perovskite solar cell for analysis of the photovoltaic mechanism based on the interpretation of the capacitance variation under illumination. It was shown that the low-frequency capacitance increases proportional to incident light intensity, and in addition it increases proportional to absorber thickness. Furthermore, the voltage dependence of capacitance is exponential with slope 1/2 (thermal energy). We conclude that the large photovoltage and capacitance are associated with electronic accumulation zone at the interface with the metal oxide contact. While this type of accumulation capacitance is common in many devices as transistors, the perovskite solar cell shows a singular behavior in that under light the electronic carrier accumulation grows unlimited by another series capacitance, reaching values as large as 10 mF cm–2 at one sun illumination.The work was supported by Generalitat Valenciana project PROMETEO/2014/020 and MINECO of Spain under project MAT2013-47192-C3-1-R. I.Z. thanks CONACYT for the international fellowship

In situ ethanolamine ZnO nanoparticle passivation for perovskite interface stability and highly efficient solar cells

Zinc oxide (ZnO) has interesting optoelectronic properties, but suffers from chemical instability when in contact with perovskite interfaces; hence, the perovskite deposited on the top degrades promptly. Surface passivation strategies alleviate this instability issue; however, synthesis to passivate ZnO nanoparticles (NPs) in situ has received less attention. Here, a new synthesis at low temperatures with an ethanolamine post treatment has been developed. By using ZnO NPs prepared with ethanolamine and butanol (BuOH), (E-ZnO), the stability of the FA0.9Cs0.1PbI3 (FACsPI)–ZnO interface was achieved, with a photoconversion efficiency of >18%. Impedance spectroscopy demonstrates that the recombination at the interface was reduced in the system with E-ZnO/perovskite compared to common SnO2/perovskite and that the quality of the perovskite on the top is clearly due to the ZnO in situ passivation with ethanolamine. This work extends the use of E-ZnO as an n-type charge extraction layer and demonstrates its feasibility with methylammonium perovskite. Moreover, this study paves the way for other in situ passivation methods with different target molecules, along with new insights regarding the perovskite interface rearrangement when in contact with the modified electron transport layer (ETL)

Effect of the electrophoretic deposition of Au NPs in the performance CdS QDs sensitized solar cells

Solution-processed mesoscopic oxide semiconductor-based materials offer potentially low-cost and high stability alternative for next generation of solar cells, and metallic nanoparticles had shown to be a good alternative to improve specific parameters in such kind of devices. In the present work, it is showed the systematic study of the effect of electrophoretic gold nanospheres (Au NPs) with cadmium sulfide Quantum Dots (CdS QDs) sensitized TiO2 solar cells. Au NPs were added by electrophoretic deposition at several times (0.5, 2.5 and 7.5 minutes) and CdS QDs were deposited by a Successive Ionic Layer Absorption and Reaction (SILAR) method. Electrophoretic deposition allowed to significantly decrease the Au NPs deposition times respect previously reported methods. The results show that Au NPs reduce the photocurrent (from 9.85 to 9.44 mA/cm2) at the same time that increase the open circuit voltage (Voc) (from 575 to 618 mV) and the Fill Factor (FF) (from 46 to 51%) which result in a final increase of the photoconversion efficiency (η) (from 2.63 to 2.96% for 0.5 min of deposit). A systematic characterization permitted to identify the origin of the variations observed in the solar cell parameters with and without Au NPs. Incident Photon to Current conversion efficiency (IPCE) demonstrate that the Au NPs reduces the amount of light that reach the CdS QDs and Impedance Spectroscopy (IS) analysis, indicates a downshift in the TiO2 conduction band (CB) and decreases the recombination processes, resulting in the observed increase in the FF and Voc.We acknowledge financial support from CONACYT through grant 134111, the UC-MEXUS program grant 00007, the European Community Seven Framework Program (FP7- 428 NMP-2010-EU-MEXICO), CIO-UGTO 2013–2015 and the CEMIE-Solar (04002) consortium. D. Esparza, and A. Ceja acknowledge scholarship from CONACYT and thanks to Maria Christian Albor for SEM and EDS analysis. Isaac Zarazúa thanks to CONACYT for the postdoctoral fellow

Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis

The large diffusion lengths recurrently measured in perovskite single crystals and films signal small bulk nonradiative recombination flux and locate the dominant carrier recombination processes at the outer interfaces. Surface recombination largely determines the photovoltaic performance, governing reductions under short-circuit current and open-circuit voltage. Quantification of recombination losses is necessary to reach full understanding of the solar cell operating principles. Complete impedance model is given, which connects capacitive and resistive processes to the electronic kinetics at the interfaces. Carrier collection losses affecting the photocurrent have been determined to equal 1%. Photovoltage loss is linked to the decrease in surface hole density, producing 0.3 V reduction with respect to the ideal radiative limit. Our approach enables a comparison among different structures, morphologies, and processing strategies of passivation and buffer layers.We acknowledge financial support by Ministerio de Economía y Competitividad (MINECO) of Spain under project (MAT2016-76892-C3-1-R) and Generalitat Valenciana (Prometeo/2014/020). SCIC at UJI is also acknowledged

Operating Mechanisms of Mesoscopic Perovskite Solar Cells through Impedance Spectroscopy and J-V Modeling

The performance of perovskite solar cell (PSC) is highly sensitive to deposition conditions, the substrate, humidity, and the efficiency of solvent extraction. However, the physical mechanism involved in the observed changes of efficiency with different deposition conditions has not been elucidated yet. In this work, PSCs were fabricated by the antisolvent deposition (AD) and recently proposed air-extraction antisolvent (AAD) process. Impedance analysis and J–V curve fitting were used to analyze the photogeneration, charge transportation, recombination, and leakage properties of PSCs. It can be elucidated that the improvement in morphology of perovskite film promoted by AAD method leads to increase in light absorption, reduction in recombination sites, and interstitial defects, thus enhancing the short-circuit current density, open-circuit voltage, and fill factor. This study will open up doors for further improvement of device and help in understanding its physical mechanism and its relation to the deposition methods

Preferred Growth Direction by PbS Nanoplatelets Preserves Perovskite Infrared Light Harvesting for Stable, Reproducible, and Efficient Solar Cells

Formamidinium‐based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm−2, and a stable incident photon to current efficiency in the infrared region of the spectrum for 4 months is obtained, one of the best stability achievements for planar PSCs. Moreover, a better reproducibility than the control device, by optimizing the PbS concentration in the perovskite matrix, is achieved. These outcomes validate the synergistic use of PbS nanoplatelets to improve formamidinium long‐term stability and performance reproducibility, and pave the way for using metastable perovskite active phases preserving their light harvesting capability

Noncapacitive hysteresis in perovskite solar cells at room temperature

The current density−voltage (J−V) curves of perovskite solar cells have been found to present a hysteresis-like distortion when the measurement is done by sweeping the applied voltage at different scan conditions. Hysteresis has raised many concerns about the feasibility and long-term stability of this kind of photovoltaic technology. However, there is a lack of distinction among different hysteretic phenomena which is necessary to unravel its underlying physical and chemical mechanisms. Here we distinguish between capacitive and noncapacitive currents giving rise to specific hysteretic responses in the J−V curves of PSCs. It is reported that capacitive current causing hysteresis dominates in regular structures with TiO2 as bottom electron selective layer. This is mainly caused by the charge, both ionic and electronic, accumulation ability of the TiO2/perovskite interface but has no influence on the steady-state operation. Noncapacitive hysteresis is observable at slow enough scan rates in all kind of architectures. Inverted structures, including organic compounds as bottom hole selective layers and fullerene materials as top contact, exhibit larger noncapacitive distortions because of the inherent reactivity of contact materials and absorber perovskitesWe acknowledge financial support by Ministerio de Economia ý Competitividad (MINECO) of Spain under Project MAT2013- 47192-C3-1-R, and Generalitat Valenciana (PrometeoII/2014/ 020). O.A. acknowledges Generalitat Valenciana for a grant (GRISOLIAP2014/035). A.G. thanks MINECO for a Ramon y ́ Cajal Fellowship (RYC-2014-16809). SCIC at UJI are also acknowledged

Capacitive dark currents, hysteresis, and electrode polarization in lead halide perovskite solar cells

Despite spectacular advances in conversion efficiency of perovskite solar cell many aspects of its operating modes are still poorly understood. Capacitance constitutes a key parameter to explore which mechanisms control particular functioning and undesired effects as current hysteresis. Analyzing capacitive responses allows addressing not only the nature of charge distribution in the device but also the kinetics of the charging processes and how they alter the solar cell current. Two main polarization processes are identified. Dielectric properties of the microscopic dipolar units through the orthorhombic-to-tetragonal phase transition account for the measured intermediate frequency capacitance. Electrode polarization caused by interfacial effects, presumably related to kinetically slow ions piled up in the vicinity of the outer interfaces, consistently explain the reported excess capacitance values at low frequencies. In addition, current–voltage curves and capacitive responses of perovskite-based solar cells are connected. The observed hysteretic effect in the dark current originates from the slow capacitive mechanisms.Financial support by MINECO of Spain (project MAT2013-47192-C3-1-R), and Generalitat Valenciana (project ISIC/2012/008 Institute of Nan otechnologies for Clean Energies). E.M.-M. thanks the Ramo n y Cajal program, and I. Z. thanks CONACYT for a postdoctoral fellow. SCIC services at UJI are also acknowledged

Panchromatic Solar-to-H2 Conversion by a Hybrid Quantum Dots-Dye Dual Absorber Tandem Device

Solution-processed mesoscopic oxide semiconductor-based materials offer potentially low-cost and high stability alternative for next generation of water to hydrogen conversion photoelectrochemical cells (PEC). In the present study, we demonstrate the effective unassisted H2 generation by a tandem device based on a quantum dot (QD)-dye dual absorber system. These systems are constituted by a TiO2 mesoscopic photoanode sensitized with CdS QDs and a dye sensitized solar cell (DSSC), based on ruthenium dye, connected in series. This solar cell supplies the needed photovoltage to induce photodriven hydrogen production. Opto-electrochemical characterization of the single components allows the prediction of the operational photocurrents and a reliable estimation of the theoretical power conversion efficiencies of tandem systems. Evolved hydrogen under simulated solar illumination was collected, and solar to hydrogen conversion efficiencies (STH) were obtained. The tandem devices have demonstrated high stability in aqueous medium and solar-to-hydrogen conversion efficiency of (0.78 ± 0.04)%, near tripling the efficiency of single QD based photoanodes. These results highlight the importance of the design of hybrid photoanodes combining the effect of different light absorbers working in parallel tandem devices for the development of efficient H2 generation QD-based photoelectrochemical cells. © 2013 American Chemical Society

Degradation analysis of perovskite solar cells doped with MABr3 via electrochemical impedance

In this study, perovskite solar cells with a structure CsyFA1-xMAxCS0.05Pb(I1.05-xBrx)3, with X = 0 (no doping was added to the base structure CsFAPbI3) and X = 0.085 (CsFAPbI3 was doped with 8.5% MABr3) were prepared and were subjected to a 60 days ageing process (15 days under vacuum conditions followed by 45 days under room conditions) to study their stability. J-V curves showed that after the 60 days, the cells with no MABr3 only had 50.32% of the original efficiency remaining, while the samples with 8.5% MABr3 had 92.89% of the original efficiency, being the Jsc the main parameter that was affected during the ageing period. IPCE measurements suggest the formation of Cs segregates, that maintain or even increase the Jsc. According to electrochemical impedance studies, these segregates could be promoted by the lower ionic movement resistance in the 8.5% MABr3 samples, which may allow to re-arrange the atoms of the crystalline structure in a benefic way. This work is of great importance because it helps to understand how the doping affects the degradation processes, while showing how Impedance Spectroscopy can be used to characterize the changes in the electrical and ionic dynamics of the solar cells