1:30pm - 2:00pmInvited
Improving Perovskite Solar Cells: Insights from a Validated Device Model
Zernike Institute for Advanced Materials, The Netherlands
To improve the performance of existing perovskite solar cells (PSCs), a detailed understanding of the underlying device physics during their operation is essential.
As a first step, we have developed and validated a device model that describes the operation of PSCs and quantitatively explains the role of contacts and of (doped) transport layers, carrier generation, drift and diffusion of carriers and recombination. We fit the simulation to experimental data of vacuum deposited CH3NH3PbI3 solar cells over multiple thicknesses. By doing so, we identify a unique set of parameters and physical processes that describe these solar cells. Recombination at material interfaces (HTL/perovskite and perovskite/ETL) is the dominant loss channel limiting the device performance and passivation of these interfaces increases the power conversion efficiency (PCE) of these devices by 40%. Finally, we issue guidelines to increase performance and show that a PCE beyond 25% is within reach.
Grain boundaries are ubiquitous in polycrystalline films and are studied extensively in CIGS, poly-Si and CdTe solar cells. The Seto model is able to successfully describe the grain boundary physics in these doped solar cells. However, PSCs are different. Perovskites are lightly doped materials and due to the presence of ionic defects it is likely that the grain boundaries are charged when empty and neutral when filled, in contrast to the basis of the classic Seto model. Therefore, a different perspective on defect physics is essential for PSCs. We include grain boundaries in our model and fit the simulation to vacuum deposited CH3NH3PbI3 solar cells in p-i-n and n-i-p configuration. Our model quantitatively explains (for both p-i-n & n-i-p cells) the light intensity dependence of the open-circuit voltage and fill-factor, delineating the recombination dynamics at grain boundaries and interfaces under different operating conditions. We find that despite the presence of traps at grain boundaries, their neutral (when filled) disposition along with the long-lived nature of holes leads to the high-performance of PSCs. We also give an estimate of the defect ion density in these solar cells. Furthermore, we shed light on the role of charged grain boundaries which may exist under some conditions (under/over stoichiometric preparation).
 T.S. Sherkar, C. Momblona, L. Gil-Escrig, H.J. Bolink, and L.J.A. Koster, Adv. Energy Mater. 1602432 (2017).
 J. Y. Seto, J. Appl. Phys. 1975, 46, 5247-5254.
 T.S. Sherkar, C. Momblona, L. Gil-Escrig, J. Ávila, M. Sessolo, H. Bolink, and L.J.A. Koster(submitted)
2:00pm - 2:30pmInvited
Organic Cation Dynamics in Mixed Hybrid Perovskite Materials
Imperial College London, United Kingdom
Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed tri-halide perovskite materials. For pure MAPbX3 (X=I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilisation of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.
2:30pm - 2:45pmOral
Accelerated Stability Testing of Perovskite Photovoltaic Materials
1Ben-Gurion University of the Negev, Israel; 2Casali Center for Applied Chemistry, The Institute of Chemistry, The Hebrew University of Jerusalem, Israel; 3School of Electronic Engineering, Bangor University, United Kingdom
The greatest challenge facing the development of organic and perovskite photovoltaics is combining high efficiency and long-term stability. We recently demonstrated the first realization of stability testing methodology using concentrated sunlight that allows independent control of light intensity, the sample temperature and environment during the exposure. Accelerated photo-stability testing of hybrid perovskite MAPbX3 films (X = I or Br) by exposure to 100 suns showed that the evolution of light absorption and the corresponding structural modifications were dependent on the type of halide ion and the sample temperature. The degradation in absorption of MAPbI3 films after exposure at elevated sample temperature, due to decomposition of the hybrid perovskite material, was documented, while no photobleaching or decomposition of MAPbBr3 filmswere observed after exposure to similar stress conditions. This improved stability was related to differences in Br-related bond strengths and in the perovskites' crystalline forms. The stability of pure MAPbI3 and MAPbBr3 films was found to be superior to that of mixed halide compositions MAPb(I1-xBrx)3, possibly due to stressing its crystal structure, inducing more structural defects and/or grain boundaries compared to pure halide perovskites. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials. Furthermore, the synthesis sequence of the Perovskite deposition process was found to affect its stability, due to the effect of PbI2 residue in the film.
2:45pm - 3:00pmOral
Study for Realizing the Role of Cs2SnI6 in Perovskite based Solar Cells
1The University of Tokyo, Japan; 2Kyushu Institute of Technology, Japan; 3University of Miyazaki, Japan; 4University of Electrocommunication, Japan; 5Ritsumeikan University, Japan; 6National Institute of Advanced Industrial Science &Technology, Japan
Organic-Inorganic lead (Pb) based perovskite solar cells have successfully attained the photoconversion efficiency (PCE) of 22.1 % owing to its high absorption coefficient, ambipolar charge transport with large diffusion length and large carrier life time decay .However, implementation of the solar modules based on Pb concerns with the toxicity and longtime device stability. Therefore, interest for Pb free new perovskite and perovskite related materials such as Cs2SnI6 has become an area of deep study. Cs2SnI6 works as a good hole transport material (HTM) in dye sensitized solar cells (DSSCs) and recently there are some reports on Cs2SnI6 as an absorber also, with PCE less than 1% . In our work, we have focused on finding the correct role of Cs2SnI6 by using techniques such as femtosecond and nanosecond transient absorption spectra (fs-TAS & ns-TAS). These results support the fast hole mobility inside Cs2SnI6 compared to electron mobility, which supports for its role as HTM. Also, TAS result encourages for using Cs2SnI6 as an absorber due to presence of ambipolar charge transport as confirmed by it. However, both electron-transport layer (ETL) and hole-transport layer (HTL) are needed for exciton splitting. Here, we successfully fabricated solar cells in two configurations, (a) ETL/ light-absorber/ HTL(Cs2SnI6) and (b) ETL/ light-absorber(Cs2SnI6)/HTL. The photovoltaic performances with different role of Cs2SnI6 will be discussed in detail in the light of HTL and ETL property correlation.
 www.nrel.gov/ncpv/images/efficiency_chart. jpg
 X. Qiu, and M. G. Kanatzidis et al, Sol. Energy Mater. Sol. Cells, 2017, 159, 227–234.
3:00pm - 3:15pmOral
Novel Materials for Stable Perovskite Solar Cells
Helmholtz-Zentrum Berlin, Germany
Organic-inorganic perovskites are quickly overrunning research activities in new materials for cost-effective and high-efficiency photovoltaic technologies. Since the first demonstration from Kojima and co-workers in 2009, several perovskite-based solar cells have been reported and certified with rapidly improving power conversion efficiency. Recent reports demonstrate that perovskites can compete with the most efficient inorganic materials, while they still allow processing from solution as a potential advantage to deliver a cost-effective solar technology.
Compare to the impressive progress in power conversion efficiency, stability studies are rather poor and often controversial. An intrinsic complication comes from the fact that the stability of perovskite solar cells is strongly affected by any small difference in the device architecture, preparation procedure, materials composition and testing procedure.
In the present talk, we will focus on the stability of perovskite solar cells in working condition. We will discuss a measuring protocol to extract reliable and reproducible ageing data. We will present new materials and preparation procedures which improve the device lifetime without giving up on high power conversion efficiency.
3:15pm - 3:30pmOral
A Facile Method to Reduce Surface Defects and Trap Densities in Perovskite Photovoltaics
Nanyang Technological University, Singapore
Owing to the morphology, crystallization process optimization and compositional design, the power conversion efficiency of perovskite solar cell has increased from 3.8% to 22.1% in a period of five years. Nearly defect-free crystalline films and slow recombination rate enable polycrystalline perovskite have comparable efficiency with multi crystalline silicon solar cells. Volatile low melting point components and anti-solvent treatments essential for processing of dense and smooth films, often lead to surface defects. Therefore, to yield high photovoltaic performance, it is critical to minimize formation of defects. In this study, we investigate methylammonium bromide (MABr) surface treatments on perovskite films to compensate for the volatility of the cations during annealing process and also to passivate the surface defects. This facile method did not change the phase and bandgap of perovskite films, however resulted in significant increase in the open circuit voltages of the devices. The power conversion efficiency shows 2% improvement over the control sample with 10 mM MABr treatment; with the treatment attributable to surface passivation induced lower trap densities.