November 30-December 5, 2014 | Boston
Meeting Chairs: Husam N. Alshareef, Amit Goyal, Gerardo Morell, José A. Varela, In Kyeong Yoo
Recently, organic-inorganic perovskites were identified as promising absorbers for solar cells.1 In the three years since, the performance of perovskite-based solar cells has improved rapidly to reach efficiencies as high as 15 %.2-4 We developed metal oxide free methylammonium lead iodide perovskite cells with high power-conversion efficiencies.5 The effect of the organic charge transporting layers on the performance of these solar cells will be presented as well as the effect of different layer thicknesses. The power conversion efficiency increases from 4.7 % for a device with only an organic hole transporting/electron blocking layer to 12 % when the perovskite layer is sandwiched in between suitable organic electron and hole blocking layers. We will present recent developments in these metal oxide free perovskite solar cells, such as semi-transparent, flexible and large area cells as well as insight in to their operational mechanism.1 Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. JACS131, 6050-6051 (2009).2 Liu, D. & Kelly, T. L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat Photon8, 133-138 (2014).3 Liu, M., Johnston, M. B. & Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature501, 395-398 (2013).4 Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature499, 316-319 (2013).5 Malinkiewicz, O. et al. Perovskite solar cells employing organic charge transport layers. Nature Photonics8, 128 (2014).
An intense effort is boosting the development of third generation photovoltaic (PV) cells, to obtain cheap, high efficiency and environmentally friendly devices. One of the most promising solar cell architectures is based on quantum dots (QDs). The photoconversion efficiency (PCE) have reached to above 7%, by using near infrared (NIR) PbS QDs. Electrophoretic deposition (EPD) has been demonstrated for preparation of high efficiency photo-anodes for QD solar cells, in which QDs are grafted to a mesoporous TiO2 NP thin film. As the performance of QD solar cell is highly dependent on not only the loading amounts, but also the QDs dispersion in TiO2 film, it is very important to control the QDs loading process.Here, for the first time, we report a systematic investigation and modeling of the dynamics of NIR QDs loaded into TiO2 mesoporous film via EPD. We used PbS@CdS core@shell QDs and investigated the influence of EPD time, QD&’s concentration and voltage on the QD uptake process via Rutherford backscattering for Pb depth profiling. The optical density of the obtained film is strongly dependent on the applied voltage, the deposition time and the concentration of solution containing the QDs. We modeled the deposition process using Fick&’s diffusion law and explained the observed trends as a fast (and depth-independent) QD uptake induced by the presence of the electric field, followed by a diffusion-induced QD migration from outside the film, due to the fast creation of a QD concentration gradient. In addition, we demonstrated the increased stability of the core@shell structure compared to PbS QDs in terms of structure and optical property, based on X-ray photoelectron spectrometry and photoluminescence measurements. Thanks to the much higher stability of the core@shell QDs as compared to pure PbS QDs, our findings suggest that the PbS@CdS QDs loaded with EPD can be profitably used for the development of highly efficient and stable light absorbers in PV devices.[] Salant, A.; Shalom, M.; Hod, I.; Faust, A.; Zaban, A.; Banin. “U. Quantum Dot Sensitized Solar Cells with Improved Efficiency Prepared Using Electrophoretic Deposition”, ACS Nano, 4, 2010, pp. 5962minus;5968
Recently, the low-cost organolead halide perovskites have emerged as the most promising absorber materials for the development of next generation high efficiency and cost-effective photovoltaic devices. Owing to its impressive properties such as high absorption coefficient over a broad region of visible light spectrum and extremely long carrier diffusion lengths, a device power conversion efficiency (PCE) as high as 25 % could well be within reach in the future. This is comparable to the best commercial single-crystalline silicon solar cells which are substantially more expensive than the perovskite materials. Although there have been a number of reports on the development of high efficiency perovskite-based solar cells demonstrate significant potential in achieving high device efficiency, the basic understanding of the materials, device properties, working mechanisms as well as the manufacturing processes are still at the early stages of development. In this work, we report on the fabrication and systematic investigations of high efficiency planar CH3NH3PbI3-based solar cells (FTO/TiO2 compact layer/CH3NH3PbI3/spiro-MeOTAD/metal electrode). A two-step spin coating technique was used to fabricate the devices. Through careful optimization of the fabrication and film formation processes we have achieved a high PCE of 15.4% measured under the calibrated ABET Technologies SUN 2000 solar simulator equipped with AM 1.5 filter at 100mW/cm2, which is a record efficiency, at the time of the composition of this abstract, for all-solution processed CH3NH3PbI3-based devices with a planar structure. Detailed investigations, including I-V characteristics, external quantum efficiencies, carrier lifetimes, impedance spectroscopy and low-frequency noise measurements, were performed on the devices to examine the underlying mechanisms responsible for the observed improvements in the PCEs of the devices. In particular, systematic studies on the impact of the optimized fabrication process on the density of the localized states and their effects on the performance of the devices were performed. From the experimental results, it is observed that performance of perovskite solar cells is strongly affected by concentration of the material defects which could be highly sensitive not only to the processing parameters but also the post-deposition treatments of the films. The results of our investigations point to a direction for future improvements of perovskite-based solar cells.
Mixed-halide hybrid perovskites such as CH3NH3Pb(BrxI1-x)3 are a promising family of photovoltaic absorber materials that have achieved power conversion efficiencies of over 17%. By varying the halide composition, the optical bandgap can be tuned over the range 1.6-2.3 eV, making this family of materials a suitable candidate for both single-junction solar cells as well as the large bandgap absorber of a tandem solar cell. However, reports of mixed CH3NH3Pb(BrxI1-x)3 devices with higher bromine content have so far not been able to achieve the increase in open circuit voltage that may be expected from the larger bandgap of these materials. We observe photo-induced halide segregation in bromine-rich CH3NH3Pb(BrxI1-x)3 and other mixed-halide perovskites as evidenced by the appearance of intense photoluminescence and absorption features from a new iodide-rich phase upon continuous illumination and the disappearance of these features with time in the dark. We suggest that photoexcitation may induce halide migration, resulting in iodide-rich domains that act as traps and pin the open circuit voltage at a lower energy. The kinetics of this process have a similar temperature dependence to the hysteretic behavior in planar CH3NH3PbI3-xClx solar cells which is suggestive of a prominent role of halide migration in perovskite photovoltaic hysteresis. These observations are reminiscent of photo-initiated halide migration in lead halides and other metal halides, which has been proposed to occur via a halide-vacancy diffusion mechanism from surface sites. This suggests that improved control of the perovskite stoichiometry, crystallinity and surface passivation are potential strategies towards reducing halide migratory effects and improving the stability of halide perovskite optoelectronic devices.
High voltage output is a central feature of the high efficiency of perovskite photovoltaic cells. Open-circuit voltage can reach 1.1V or more by constructing multilayer structures of uniform thickness and minimized interfacial resistance with use of suitable compact layers and hole transport materials. For the narrow band gap tri-iodide perovskite (Eg=1.55 eV ), relatively high voltage (Voc >1.05V) is obtained with use of Al2O3 as a mesoporous and insulating scaffold when compared to semiconductive TiO2. Planar structured perovskite cells without using mesoporous scaffold is also able to generate high voltage. Here a common point of their structures is that electron transfer interface is formed at the peroskite/compact layer junction rather than perovskite/mosoporous TiO2 interface. In the latter case, number of hetero interfaces for electron transfer becomes larger (perovskite/mesoTiO2 and meso-TiO2/compact TiO2); this increases internal resistance to reduce Voc.Apart from high voltage, I-V characteristics of perovskite cell are often accompanied by serious hysteresis, which impairs the reliability of cell perfo