4:00pm - 4:30pmInvited
Hybrid Perovskite Thin Film Photovoltaics: The Importance of Precursor Intermediates
King Abdullah University of Science and Technology, Saudi Arabia
Printed semiconductors have made remarkable inroads in recent years, raising the profile of solution-based manufacturing as an increasingly viable and potentially low-cost platform for making electronic, optoelectronic and photovoltaic (PV) devices more pervasive in our lives. For instance, solar cells using solution-processed lead-based organohalide perovskites have skyrocketed in efficiency in the past few years. A great deal of this progress has been achieved by brute force optimization, leaving the understanding and control of semiconductor thin film microstructure and morphology in its infancy as compared with conventional semiconductors used in everyday technologies. Little attention has been paid to the phase transformation mechanisms and pathway leading to these highly performing solid-state materials. This presentation will discuss our latest understanding of the ink-to-solid transformation underpinning perovskite thin film formation. The approach we use relies heavily on in situ investigations of solution processing and process diagnostics. We systematically map the spin-coating and blade-coating processes, starting with solution thinning to precursor phase formation and subsequent perovskite conversion. We find the phase transformation of perovskite inks to be complex and is largely mediated through formation of intermediate precursor phases and their structural properties. The state of the precursor strongly dictates the conversion, microstructure and morphology of the perovskite phase and we show that successful processing strategies work by managing the intermediate state of perovskite films first and foremost.
4:30pm - 5:00pmInvited
Solution Approach to pn-Junction and Perovskite Solar Cells
Indian Association for the Cultivation of Science, India
In the research arena of photovoltaics, achievement of high efficiency has not diluted the urge of searching newer materials, which should be composed of earth-abundant, atoxic, and inexpensive elements. Emergence of quaternary chalcogenides has been a noteworthy step in this trend. To fabricate devices, several vacuum and non-vacuum based methods are being used. Amongst them, solution-based deposition techniques are particularly attractive for their low-cost and easy-processing approach.
We will present formation of Cu2FeSnS4 (CFTS) thin-films through successive ionic layer adsorption and reaction (SILAR) method. With p-type CFTS, we have considered a range of chalcogenides as n-type compound semiconductors, also formed through SILAR method, to fabricate pn-junction solar cells. The deposition method, which is a low temperature and non-vacuum one and is also suitable to form large-area films, has maintained a balance between fabrication cost and phase purity. From scanning tunneling spectroscopy (STS) and correspondingly density of states (DOS) of the semiconductors, we have estimated their band-edges to draw energy-level diagram of the heterojunctions. This has led to establish a correlation between the band-alignment in pn-junctions and energy conversion efficiency (h) of solar cells based on the junctions. An h of 2.9 % with promising reproducibility could be achieved in CFTS/Bi2S3 junctions formed through this room-temperature film deposition route.
We also formed Cu2O thin-films through SILAR method to introduce it as a hole-transport layer in planar perovskite solar cells. With methylammonium lead triiodide (MAPbI3), we have formed a p−i−n structure with low work-function aluminum as an electron-collecting electrode. From the band-diagram drawn from STS, we have observed that energy levels of the materials formed type−II band-alignment at both p−i and i−n interfaces for charge separation and uninterrupted carrier transport upon illumination. The planar p−i−n structured perovskite solar cells (Cu2O/MAPbI3/PCBM) yielded an η of 8.23 % under 1 Sun illumination.
5:00pm - 5:15pmOral
Visible to Near Infrared Absorption of Thermally Evaporated Silver Sulfide (Ag2S) for Hybrid Solar cells
Research Center for Applied Sciences, Academia Sinica, Taiwan
The use of toxic compounds will be eventually terminated due to the increasingly stringent environmental requirement. Therefore, current challenge is to achieve low toxic materials with extended light absorption toward the near-infrared (NIR) region of the solar spectrum. Silver sulphide (Ag2S) can be a promising material for conversion of solar energy into electricity as its band gap (Eg) is ~1 eV. In this work, we successfully co-evaporated the Ag2S and sulphur (S) to form stoichiometric Ag2S thin film by thermal vapour deposition method. UV-Visible absorption spectra of evaporated Ag2S thin film showed wide absorption upto NIR (~1260 nm) region. The band gap of thermally evaporated Ag2S film was calculated as 0.984 eV from Tauc plot, which is similar with already reported one. X-ray diffraction pattern confirms the formation of crystalline Ag2S film with no impurities. The planar architecture (Glass/ITO/PEDOT:PSS/P3HT/Ag2S/Al) of hybrid photovoltaic device exhibits the power conversion efficiency of 0.79% with a superior short circuit current density of 13.73 mA cm-2 under AM 1.5 illumination at 100 mW cm-2.
5:15pm - 5:30pmOral
Efficient Silicon Heterojunction Solar Cells with Carrier-selective and Dopant-free Contacts
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, China
Enormous amount of effort and resources have been invested on seeking new generation photovoltaic technologies, which can reach both materials saving and procedures simplification. Compared with commercially available crystalline silicon (c-Si) solar cells achieved by doping technique in near-surface regions, heterojunction solar cells consisted with hole-transporting layers of MoOx (x<3), poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), etc. and/or electron-transporting layers such as titanium oxide (TiOx), lithium fluoride (LiF), etc. on n-type c-Si provide a promising new concept with dopant-free and carrier-selective heterocontacts. In combination with advanced light-trapping structures on c-Si surface, the simplified heterojunction of hybrid PEDOT:PSS/c-Si can achieve power conversion efficiencies (PCE) approaching 13%. For this heterojunction solar cell, interface engineering is crucial to promote the performance of photovoltaic devices due to the abilities to optimize the separation of carries and minimize the interfacial recombination. Here, efficient PEDOT:PSS/c-Si heterojunction solar cells with pyramidal configurations are present and physical method is firstly used to minimal the interfacial recombination at PEDOT:PSS and c-Si interface. Benefit from the acting force on PEDOT:PSS, perfect interfacial contact is achieved on nanostructured c-Si surface, resulting in an open-circuit voltage beyond 660 mV for this heterojunction solar cell and finally a record PCE over 16.2%. A superior PCE over 21% is further predicted by considering more optimal contact resistance as well as doping concentration of Si. In addition, our recent achievements on MoOx/c-Si and c-Si/TiOx, etc. will also be shared.
 Pingqi Gao, et al., ACS nano, 2015, 9(6), 6522-6531.
 Pingqi Gao, et al.,Nano letters, 2015, 15(7), 4591-4598.
 Pingqi Gao, et al., Advanced Energy Materials, 2016, 6, 1501793.
 Pingqi Gao, et al., Advanced Materials, 2017, DOI:10.1002/adma.201606321.
 Pingqi Gao, et al., ACS Nano, 2016, 10, 11525.
5:30pm - 6:00pmInvited
Nanostructured Metal Oxide Interlayer for Efficient Organic and Perovskite Solar Cells
The University of Melbourne, Australia
Organic photovoltaic (OPV) cells have emerged as a potential alternative to conventional silicon based photovoltaic cells for a viable green energy sources, owing to their distinctive advantages of solution processability, low-cost, flexibility and roll-to-toll production. Over the past few years, power conversion efficiencies (PCEs) of over 11% have been achieved for single-junction OPV devices with bulk heterojunction (BHJ) architectures. This progress can be attributed to improvements in device architecture, device processing, and the development of new electron-donor and electron-acceptor materials. However, the interface between electrode and active layer plays a critical role in determining the performance of OPV devices. Among various interface materials, metal oxide based interlayer provide efficient charge extraction and enhance the stability of the OPV devices.
Recently, we have reported polymer materials (PBDT-BT and PTB7-Th) with OPV device efficiency of close to 10%. Also, we have obtained the perovskite devices with PCE of over 15% using inverted device geometry. Here, we achieved the high performance OPV and Perovskite devices using interface engineering of electrode. In this talk, our recent work on the fabrication of efficient organic and perovskite solar cells using various nanostructured metal oxide interlayers and the effect of the interlayer materials on the device performance will be discussed.
6:00pm - 6:30pmInvited
Controlled Inorganic Surface by Annealing Process for Organic Photovoltaic Cells
Saitama University, Japan
An metal oxide, such as ZnO and MoOx thin film has been investigated for the electron and hole transport layers of organic photovoltaic cells, and the superior long term stability has been already demonstrated compared to the conventional device structure with only the organic material. In addition, the carrier transport properties in the photovoltaic device is sensitive to surface/interface defects of ZnO and MoOx. Because the photo-induced carriers are trapped between the inorganic/organic surface due to the trap site. In this study, we investigated the relationship between the annealing temperature for the inorganic layer and the photovoltaic performance.
In the case of MoOx, the surface morphology and the energy level were not changed by the annealing process at 160℃. However, the surface defect can be removed by the annealing process, and was evaluated by the X-ray photoelectron spectroscopy. As a result, the photovoltaic performance was drastically improved for organic photovoltaic cells with Poly(3-hexylthiophene-2,5-diyl)] (P3HT), poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), and poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th). Finally, the photoconversion efficiency of 10.1% was demonstrated by optimizing the active layer thickness.
The ZnO thin film has been utilized for the inverted device structure, and the smooth surface morphology can be realized at the low annealing temperature of 200℃ for the ZnO layer. By investigating the influence of annealing temperature on the photoconversion efficiency, the high photovoltaic performance was realized at annealing temperature ranging from 125 to 200℃. However, the crystalline phase was appeared at 250℃, resulting in the reduced photovoltaic performance. This indicates that the amorphous phase of ZnO is an important issue for realizing high device performances of inverted photovoltaice cells.
These researches demonstrated the surface condition of inorganic thin film shows important roles for normal and inverted organic photovoltaic cells.