4:00pm - 4:30pmInvited
Highly Stable and Efficient Perovskite Solar Cells Via Multication Engineering
École Polytechnique Fédérale de Lausanne, Switzerland
The perovskites used for solar cells have an ABX3 structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); B is Pb or Sn; and X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from instabilities. This is particularly noteworthy for CsPbX3 and FAPbX3 which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures.
Recently, double-cation perovskites (using MA, FA or Cs, FA) exhibited a stable “black phase” at room temperature. These perovskites have unexpected; e.g. Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon tandems. In general, adding more components increases entropy that can stabilize otherwise unstable materials. Here, we investigate triple cation (Cs, MA, FA) perovskites resulting in significantly improved reproducibality and stability. We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (never shows a black phase as single-cation perovskite) to study novel multication perovskites.
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The Voc of 1.24V at a band gap of 1.63eV leads to a very small loss-in-potential of 0.39V. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500hours under full illumination and maximum power point tracking.
 Lee et al., Formamidinium and Cesium Hybridization for Photo- and Moisture-Stable Perovskite Solar Cell. Advanced Energy Materials (2015)
 McMeekin et al., A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells. Science (2016)
 Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
 Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016)
4:30pm - 5:00pmInvited
Recombination and Hysteresis in Perovskite Solar Cells
Laboratory of Photonics and Interfaces (LPI), École Polytechnique Fédérale de Lausanne, Switzerland
Solar cells based on lead halide perovskites have recently emerged showing a tremendous increase of power-conversion efficiency which exceeded 20 %. In this talk, the device physics of perovskite solar cells is addressed. The focus is on recombination of charge carriers because this process is ultimately limiting open-circuit voltage and fill factor in perovskite solar cells. Different architectures such as planar and mesoporous-TiO2 based devices are presented.
The origin of the open-circuit voltage is discussed based on the reciprocity relation between electroluminescence and photovoltaic quantum efficiency.1,2 Different recombination mechanisms (radiative, trap-mediated) are investigated and related to the device performance.
Hysteresis in the current-voltage curve is related to recombination as well. It is shown how different prebias voltages influence recombination rates.3 The results are explained by the mixed ionic and electronic conductivity of the material, where displaced ions change interface and defect recombination. A recently discovered inverted hysteresis and reversible photo-induced degradation mechanisms on the timescale of minutes to hours are put into the framework of ion migration as well.4
An outlook is given on strategies aiming for a further improvement of the open-circuit voltage.
 Tress, W. et al. Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination. Adv. Energy Mater. 5, 140812 (2015).
 Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Sci. Adv. 2, e1501170 (2016).
 Tress, W. et al. Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy Environ. Sci. 8, 995–1004 (2015).
 Tress, W. et al. Inverted Current–Voltage Hysteresis in Mixed Perovskite Solar Cells: Polarization, Energy Barriers, and Defect Recombination. Adv. Energy Mater. 6, 1600396 (2016).
5:00pm - 5:15pmOral
Manufacturability Challenges of Perovskite Solar Cells
1Advanced Technology Institute, University of Surrey, Guildford, United Kingdom; 2DZP Technologies Ltd., Cambridge, United Kingdom
Printable solar cells based on halide Perovskites is being proposed as a low cost and scalable thin film PV technology. However, there are materials challenges to be overcome in moving this technology towards commercial feasibility. In this paper we present an overview of collaborative research undertaken towards the up-scaling of printable Perovskite PV technology.
One of the challenges in moving Perovskites towards large scale manufacture is the sensitive nature of the Perovskite active layer films to ambient humidity during production. In our previous work  we conducted a systematic study of the combined effects of humidity and thermal annealing time, which is key for moving towards up-scaled production. We reveal the trade-off between these two variables, with the optimum being a moderate level of humidity and a moderate annealing time. Based on these results we propose a strategy to reduce annealing times by control of humidity, allowing for cost reductions.
A further challenge is the use of highly toxic solvents for solution processing of Perovskites. In this work we go beyond the standard one- or two-step solution processing techniques to study a novel powder-based method, whereby the Perovskite material is synthesised under controlled conditions before dispersion in an ink and subsequent deposition. Using this method we are able to synthesise highly pure CH3NH3PbI3 material, which is then dispersed in non-toxic solvents to manufacture colloidal Perovskite inks. This allows us to both avoid the sensitivity to ambient conditions and to select less toxic solvents to allow for industrial scale-up. These Perovskite inks are stable on the timescale of months, and Perovskite active layers formed by doctor blade coating show no degradation after two months’ storage in ambient conditions. Such colloidal inks represent a highly promising avenue for future Perovskite research.
 Cronin, H. M. et al., Nanotechnology, 2017, doi.org/10.1088/1361-6528/aa5bec
5:15pm - 5:30pmOral
Crystallinity Preservation and Ion Migration Suppression through Dual Ion Exchange Strategy for Stable Perovskite Solar Cells
The Chinese University of Hong Kong, Hong Kong S.A.R. (China)
The mixed perovskite (FAPbI3)1-x(MAPbBr3)x prepared from directly mixing (DM) of different perovskite components suffers from phase competition and low-crystallinity character resulting in instability despite the high efficiency. Here, we developed a dual ion exchange (DIE) method by treating the pre-deposited FAPbI3 with MABr/tert-butanol solution. The converted perovskite shows an optimized absorption edge at 800 nm after reaction time control. Besides, the high crystallinity can be maintained after MABr incorporation, which can be attributed to the firstly established and then well-preserved inorganic framework during the ion exchange process. More importantly, we further found that the threshold electrical field to initiate ion migration was also greatly increased in DIE perovskite because the excess MABr on the surface can effectively heal the structural defects located on grain boundary during the ion exchange process, contributing to the over one-month moisture stability under ~65% RH and greatly enhanced light stability. As the result of preserved high crystallinity and the simultaneous grain boundary passivation, the perovskite solar cell devices by DIE method demonstrates reliable reproducibility with an average power conversion efficiency (PCE) of 17% and the maximum PCE of 18.1%, with negligible hysteresis.
5:30pm - 5:45pmOral
Efficient Semi-Transparent Triple Cation Perovskite Solar Cells with Optimized Buffer Layer Yielded 20.7% Efficiency in 4-Terminal Tandem Configuration with CIGS
Nanyang Technological University, Singapore
The development of high efficiency semi-transparent perovskite solar cells will see a wide range of application in integrated photovoltaics, such as solar windows, buildings facades and green houses, and is also an important component in tandem solar cells. However, material sensitivity to temperature and solvents imposes a restriction on the deposition process for the transparent contacts that necessitates the use of sputtering or evaporation deposition process. Thus, a need arises for the development of a proper buffer layer to protect the absorber and charge transport layers from damage during contact deposition, while ensuring good adhesion and conductivity of the contact and high device transparency. In our approach, we have done a comparative study on Ag and MoOx buffer layers for the deposition of Indium Tin Oxide (ITO) transparent contacts. The usage of thin Ag as a buffer layer demonstrated ITO contacts that were resistant to delamination and yielded a semi-transparent perovskite solar cell with power conversion efficiency of 15.99%. Average transparency of the device was 12% in visible range and more than 50% in the near infra-red (800-1200nm). Further application in tandem with Cu(In,Ga)Se photovoltaic cells shows an overall tandem efficiency of 20.7% in a 4-terminal (4T) configuration.