Following lunch, attendees returned for a further two afternoon technical sessions. In the first set, from 1.30 to 3.00 pm in Areas 1, 4, 5, 6, 7, and 9, with the following highlights provided by the session chairs:
Area 6 - Examining Perovskite Stability
Rongrong Cheacharoen from Stanford presented encouraging results of perovskite solar cell stability, showing encapsulated perovskite solar cells passed the IEC standard tests (damp heat and thermal cycle test etc.). The mechanical strength of perovskite solar cell, which is another important aspect of stability, was well discussed by Nick Rolston from Stanford. Rongrong Cheacharoen presented promising results for perovskite stability, passing 1000 hrs of damp heat, 250 thermal cycles, and UV irradiation by using industry standard encapsulation. Professor Shengzhong Liu presented 21.5% efficient planar perovskite solar cells with organic molecule treatment of SnO2. The Duong showed how standard stability testing may be insufficient and specifically, metal induced degradation can be accelerated under heat and light. However, metal induced degradation can be mitigated by using transparent conducting oxides. Nicholas Rolston showed how mesoporous scaffolds can significantly improve the fracture energy of perovskite solar cells. Valerio Zardetto showed minimal loss in performance after >2500 hours at 85C with the introduction of a ZnO barrier layer deposited by spatial ALD.
Area 7 - Space PV Systems
Brian Spence of Deployable Space Systems gave a presentation on the recent successes with ROSA, or roll-out solar array, including the first ever flight experiment of ROSA on the International Space Station (ISS). ROSA is a boom/flexible blanket array geared towards improving W/kg metrics. Bao Hoang of SSL presented ROSA from a different perspective as SSL show the benefits of ROSA at the system level. ROSA can enable on orbit servicing, persistent platforms and high power interplanetary missions. SSL is presently qualifying a large scale ROSA product. John Gibb from Lockheed Martin gave an overview of "flex" solar arrays of the past, present, and the future. He announced the MMSA, multi- mission modular solar array that will be flown later this year, 2018. Christian Ruud of Penn State University makes the case for a novel space CPV design that leverages transfer printed microcells. These microcells are lightweight, very high efficiency, and when integrated into their molded reflective mirror array CPV design can lead to a module efficiency up to 32%, and 350 W/kg. Dave Scheimann of the Naval Research lab presented work on their Solar UAV program. The UAV needs about 70 watts to cruise and they have compared several wings populated with several different cell designs. Solar power has been shown that it can double flight duration of UAVs. Raymond Chan of Microlink Devices gave a talk detailing their work related to lightweight flexible sheets for solar flight. HALE, or High Altitude Long Endurance UAVs are a new class of aircraft that are able to stay aloft for weeks or potential months at a time. MLDs high specific power enable these aircraft to remain in flight. MLDs IMM solar sheets are currently operating at 1500 W/kg.
Area 1 - Quantum-well, Wire, and Dot-Architectured Devices / Advanced Light Management and Spectral Shaping
Stephen Bremner presented the effects of Sb in AlAs capped QDs that improves the transport for carriers photogenerated in the bulk. S.Asahi presented progress on improvements in the efficiency of 2-step photoabsorption in a heterojunction device. R.Vismara presented work on how decoupled pyramid textures deliver absorption enhancement beyond the Lambertian limit and simulation results presented on BaSi2 as well as nc-Si:H. S. Collin showed new calculations on multi-resonant absorption that can increase absorption by 4π n
higher than the traditional 4n
for more standard structures. Jose Ulloa working on type II strain-balance superlattices demonstrated that superlattice periods must be less than 6nm to obtain a 120% conversion efficiency enhancement over bulk material in GaAsN/GaAsSb devices. Haley Bauser presented the design of a high contrast metasurfaces (AlSb on glass array) to be included in tandem (colloidal quantum dots/InGaP and colloidal quantum dots/Si calculations give eff 24%) luminescent concentrator.
Area 5 - Module/Cell Characterization 2
Two excellent talks using statistical methods to extract influence of process variables in manufacturing for process optimization, by measuring large numbers of cells in process. Two excellent talks using imaging methods to extract physical quantities: IR imaging of water vapor transport; imaging of stress in Si wafers for determining effect of process variables on stress.
Area 4 - Passivated Contacts, Carrier Selective Contacts: Fundamentals
F. Feldmann presented convincing results showing that MIS theory can well explain some of the high performance passivated contacts, but also show that contacts including holes in the SiOx layer can perform well. Not all good contacts work the same! A. Morales demonstrated 2x2 cm
heterojunction cells with ZnO front and ZnO/Ag with 23.02% efficiency, an excellent result when using no Indium containing TCO. M.Koehler demonstrated a promising front heterojunction contact based on tunnel SiOx and CAT/CVD Sic. The QEE shows impressive current gain in the blue and implied Voc of 728 mV. ITO sputtering damages prevented good device fabrication. A. Kahle discussed in a detailed investigation of possible pinholes, in particular using EBIC measurements, he showed how thinned oxide location can lead to strong local current flow.1062. T. de Vrijer discussed the degradation in thin film silicon solar cells and pointed out that the degradation is caused by p-SiOx:H degradation. J. Bullock presented on passivated contacts based on TiOx/LiFx/Al applied locally on the rear-side with efficiencies exceeding 23%.
Area 9 - Adhesion and Corrosion
Nick Bosco, presenting on behalf of J. Tracy, reports on work to build a model for encapsulant to cell, and encapsulant to glass adhesion, using a combination of proposed chemical degradation mechanisms. In a second presentation, Bosco showed that temperature and RH during an extended adhesion test impacted the measured adhesion values of EVA/glass. Xin Hi described Finite Element Analysis using a cohesive zone model to simulate mixed mode delamination of EVA to glass, which showed the delamination mode shifts with temperature. Tadanori Tanahashi described AC impedance measurement technique used to localize source of degradation between the front contact and Si-emitter using acetic acid vapor. Yuji Ino presented an accelerated method using "pressure cooker" and extended damp heat tests to study the influence of Pb and non-PB solder and various solder fluxes on EL darkening that were responsible for approximately 20% series resistance increase. To close the session, Michael Kempe discussed using electric leakage currents to understand electrochemical and ionic species related corrosion.
After a quick re-charge the final oral technical sessions for the day covering work in Areas 2, 3, 4, 5, 8, 9, and 10 took place. Some of the highlights provided by the session chairs are given below:
Area 10 - Solar Variability and Hosting Capacity
Andrew Blakers presented on how off river pumped storage sites offer the option to move to 100% renewable power system over the next 15 years. An analysis of possible storage sites worldwide revealed a vast number of available sites which could be used. The associated costs of balancing PV and wind generation with such technology including HVDC transmission would add about 20 USD/MWh.
Area 3 - Hybrid Tandems: Battle Royale, Part 2
Tyler Grassman from Ohio State presented results on the development of a GaAsP/Si epitaxially grown tandem cell with a certified efficiency of 20%. Florent Sahli from EPFL presented work on monolithically integrated perovskite/Si tandem cells using fully textured Si cells, with a certified efficiency of over 25% and high quantum efficiency for the Si subcell. Markus Feifel from Fraunhofer ISE presented the former record (only broken 30 minutes earlier) for an epitaxially grown III-V on Si device with an efficiency of 19.7% from a GaInP/GaAs/Si multijunction cell. Subsequent growth improvements have enabled nucleation of GaP on Si with a threading dislocation density of 10^5 cm^-2, which has great potential to improve the efficiency of future devices. Valerio Zardetto showed the progress that Solliance has achieved in 4-terminal integration of high-transparency perovskite top cells with IBC and HIT Silicon base cells, reaching up to 26.3% efficiency. Adele Tamboli from NREL presented a transparent conductive bonding method to optically and electrically couple III-V top cells with IBC Si tandems. GaInP/Si cells reached 27.3% efficiency in 3T operation. Manoj Jaysankar from IMEC presented 26% and 23% efficient 4T tandems on Si and CIGS base cells respectively (0.13cm2 aperture area) and 20% from PVSK/Si at 16 cm
Area 2 - Industrial Aspects
Gang Xiong First Solar Invited presentation was a well presented, balanced presentation providing commercial status as well as technology and research direction for improvement of performance. Thrust is on Voc improvement with group V doping optimization. He underscored the move away from using Copper. Yoshiaki Hirai Solar Frontier: 22.9% efficiency CIGSSe Solar Cell. The speaker discussed how bandgap engineering and a treatment based on Cs lead to new record CIGSeS efficiency of 22.9%. Cs alkali addition, enhanced sulfurization and modified absorber contribute to the new record. Phillip Kratzert Solibro Hi-Tech: New world record for full size CIGS module18.7 % Aperture area efficiency full size module, new world record for thin film. Explained contributing factors to this record as optimized CIGS, Rb alkali addition and optimized metallization grid. Jinwoo Lee Global Solar Energy: Na interface accumulation in CIGS. The speaker discussed a loss mechanism (up to 2% in efficiency) in CIGS modules related to Na humps observed in CIGS films. The Na humps correlated with interfaces of the 3-stage process. Veronique Gevaerts Solliance (ECN and TNO) Backend Monolithic interconnect for electroplated CIGS. The speaker described a novel back end technique for laser scribing combined with inkjet printing for the monolithic interconnects.
Area 4 - Device Modeling and Power Loss Analysis
Andreas Fell gave an excellent comprehensive overview of the capabilities of an MIS (metal-insulator-semiconductor) model to describe passivated and even HJT contacts. This model is now included in Quokka3 for everyone to use. Olindo Isabella presented his implementation of passivated contacts in Synopsys Sentaurus and gave a detailed analysis of the behavior of different contact realizations in order to optimize devices and estimate their efficiency potential. In Sebastian Meier's comparison of the potential of bifacial p-PERT and PERC there was no clear winner. Only when turning to bifacial cells, p-PERT gained a significant advantage over PERC. Julian Weber used meta model device simulations to optimize the design of laser doped selective emitters.
Area 8 - PV Modules: Back Contact cells and Partial Shading
Two papers (by Ian Bennett from DSM and Bonna Newman from ECM) discussed the development of a new integrated conductive backsheet for back contact cells. Newman showed the application of this new capability in combination with integrated light management thinner cells to achieve a 15% module cost reduction.
That ended our technical program for Thursday. It was then time to get out your most tropical outfit and get along to the Luau for the Conference Banquet. After piling up their plates with delicious food attendees were treated to some traditional dancing from the South Pacific including Samoa and Hawai’i. Along with a thrilling fire dance there was also the challenge taken up by the conference Chair Alex Freundlich. We were even lucky to once again experience a fly-over of the International Space Station during our banquet. There was even the celebration of a sixtieth wedding anniversary, at the exact same spot! A great time was had by all as the conversations and fun flowed into the night.