Dechan Angmo

Roll-to-roll fabrication of polymer solar cells

As the performance in terms of power conversion efficiency and operational stability for polymer and organic solar cells is rapidly approaching the key 10-10 targets (10 % efficiency and 10 years of stability) the quest for efficient, scalable, and rational processing methods has begun. The 10-10 targets are being approached through consistent laboratory research efforts, which coupled with early commercial efforts have resulted in a fast moving research field and the dawning of a new industry. We review the roll-to-roll processing techniques required to bring the magnificent 10-10 targets into reality, using quick methods with low environmental impact and low cost. We also highlight some new targets related to processing speed, materials, and environmental impact.

Solar cells with one-day energy payback for the factories of the future

Scalability is a requirement before any new energy source can be expected to house a possible solution to the challenge that mankind’s increasing energy demand presents. No renewable energy source is as abundant as the Sun and yet efficient and low-cost conversion of solar energy still has not been developed. We approach the challenge by firstly taking a technology that efficiently addresses the need for daily production of 1 GWp on a global level, which does not employ elements with critically low abundance and has a low thermal budget. We then applied life cycle assessment methodologies to direct research and developed such technology in the form of a polymer solar cell that presents a significant improvement in energy payback time (EPBT) and found that very short energy payback times on the order of one day are possible, thus potentially presenting a solution to the current energy gap of >14 TW by year 2050.

High‐Volume Processed, ITO‐Free Superstrates and Substrates for Roll‐to‐Roll Development of Organic Electronics

The fabrication of substrates and superstrates prepared by scalable roll‐to‐roll methods is reviewed. The substrates and superstrates that act as the flexible carrier for the processing of functional organic electronic devices are an essential component, and proposals are made about how the general availability of various forms of these materials is needed to accelerate the development of the field of organic electronics. The initial development of the replacement of indium‐tin‐oxide (ITO) for the flexible carrier materials is described and a description of how roll‐to‐roll processing development led to simplification from an initially complex make‐up to higher performing materials through a more simple process is also presented. This process intensification through process simplification is viewed as a central strategy for upscaling, increasing throughput, performance, and cost reduction.

Solution processed large area fabrication of Ag patterns as electrodes for flexible heaters, electrochromics and organic solar cells

A simple method for producing patterned Ag electrodes on transparent and flexible substrates is reported. The process makes use of a laser printed toner as a sacrificial template for an organic precursor, which upon thermolysis and toner lift off produced highly conducting Ag electrodes. Thus, the process takes only a few minutes without any expensive instrumentation. The electrodes exhibited excellent adhesion and mechanical properties, important for flexible device applications. Using Ag patterned electrodes, heaters operating at low voltages, pixelated electrochromic displays as well as organic solar cells have been demonstrated. The method is extendable to produce defect-free patterns over large areas as demonstrated by roll coating.

Fullerene alloy formation and the benefits for efficient printing of ternary blend organic solar cells

Composition average dependent properties for blends of the conjugated polymer P3HT and the fullerenes [60]PCBM, [60]ICBA and their mixtures were studied using cross-polarization magic-angle-spinning solid-state NMR techniques. We found that the blended fullerenes form an alloy and that when mixed with a third polymer component, the system exhibits pseudo-binary phase behaviour instead of the expected ternary phase behaviour. Our results experimentally confirm the earlier hypothesis that the unexpected composition average dependent IV-behaviour for these supposed ternary mixtures are indeed due to them behaving as pseudo-binary mixtures due to alloying of the fullerene components. This finding has vast implications for the understanding of polymer–fullerene mixtures and quite certainly also their application in organic solar cells where performance hinges critically on the blend behaviour which is also investigated in this study.

Medium area, flexible single and tandem junction solar cells based on roll coated semi-random copolymers

We report on medium area (1 cm2) slot-die coated organic photovoltaic devices (OPVs) of a recently developed semi-random copolymer of poly-3-hexylthiophene and diketopyrrolopyrrole (P3HTT–DPP-10%) mixed with phenyl-C61-butyric acid methyl ester ([60]PCBM). The devices were prepared using a compact laboratory roll-coater using only slot-die coating and flexographic printing under ambient conditions on a flexible ITO-free substrate. In order to overcome a low JSC and FF obtained for single junction devices, devices were also prepared in a tandem geometry making it possible to employ thinner junction films. Power conversion efficiencies of up to 1.36% and 1.31% were achieved for the tandem and single junction geometries, respectively.

Roll coated large area ITO- and vacuum-free all organic solar cells from diketopyrrolopyrrole based non-fullerene acceptors with molecular geometry effects

In this paper, we investigate three diketopyrrolopyrrole (DPP) based small molecular non-fullerene acceptors, namely Ph(DPP)3, Ph(DPP)2, and PhDMe(DPP)2, focusing on molecular geometry effects on the frontier orbital level, light absorption, molecular configuration, electron mobility, thin film morphology, and photovoltaic performance of both spin-coated ITO based and roll coated large area, ITO- and vacuum-free organic solar cells (OSCs). For spin-coated devices based on P3HT as the donor polymer the solar cells gave power conversion efficiencies (PCEs) in the following order for (P3HT:PhDMe(DPP)2, 0.65%) > (P3HT:Ph(DPP)2, 0.48%) > (P3HT:Ph(DPP)3, 0.31%). All devices present an open circuit voltage (Voc) higher than 1.0 V. For the roll-coated devices, the PCEs were found to fall in another order and with lower values (P3HT:Ph(DPP)3, 0.54%) > (P3HT:Ph(DPP)2, 0.43%) > (P3HT:PhDMe(DPP)2, 0.04%) and the highest Voc was 0.82 V. Our preliminary results highlight the influence of geometry, structure and processing on the performance of non-fullerene acceptors.

Novel high band gap pendant-borylated carbazole polymers with deep HOMO levels through direct +N B− interaction for organic photovoltaics

In this communication, we investigate the direct and still conjugated intramolecular +NB− interactions in novel high band gap borylated carbazole containing polymers, namely, poly(3,6-(N-di(2,4,6-trimethyl)phenylboryl-carbazole)-alt-4,8-di(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene) (P(3,6-BCBDT)) and poly(3,6-(N-di(2,4,6-trimethyl)phenylboryl-carbazole)-alt-3,3′′′-didodecyl-2,2′:5′,2′′:5′′,2′′′-quaterthiophene) (P(3,6-BCQT)), which result in ambipolarity, high electron affinity, and deep HOMO levels. The quasi-donor–acceptor nature of the two polymers was confirmed by UV-Vis absorption, electro-chemical property studies, and computer modelling. Band gaps of 2.07 eV for P(3,6-BCBDT) and 2.23 eV for P(3,6-BCQT) were obtained. P(3,6-BCQT) afforded a power conversion efficiency of 1.44%, with a Jsc of 4.82 mA cm−2, a Voc of 0.79 V and a FF of 37%, and P(3,6-BCBDT) performed better with an efficiency of 3.82%, with a Jsc of 8.31 mA cm−2, a Voc of 1.0 V based on its low lying HOMO level, and a FF of 45%.

Indium Tin Oxide-Free Polymer Solar Cells: Toward Commercial Reality

Polymer solar cell (PSC) is the latest of all photovoltaic technologies which currently lies at the brink of commercialization. The impetus for its rapid progress in the last decade has come from low-cost high throughput production possibility which in turn relies on the use of low-cost materials and vacuum-free manufacture. Indium tin oxide (ITO), the commonly used transparent conductor, imposes the majority of the cost of production of PSCs, limits flexibility, and is feared to create bottleneck in the dawning industry due to indium scarcity and the resulting large price fluctuations. As such, finding a low-cost replacement of ITO is widely identified to be very crucial for the commercial feasibility of PSCs. In this regard, a variety of nanomaterials have shown remarkable potential matching up to and sometimes even surpassing the properties of ITO. This chapter elaborates the recent developments in ITO replacement which include, but are not limited to, the use of nanomaterials such as metal nanogrids, metal nanowires, carbon nanotubes, and graphene. The use of polymers and metals as replacement to ITO are described as well. Finally, recent progress in large-scale experiments on ITO-free PSC modules is also presented.

Organic solar cells (OSCs)

Organic-based photoconverters are subject to a considerable interest due to their promising functionalities and their potential use as alternatives to the more expensive inorganic analogues. We introduce the basic operational mechanisms, limitations and some ideas towards improving the efficiency of organic solar cells by focusing on probing the morphological/structural, dynamical, and electronic aspects of a model organic material consisting of charge-transfer discotic liquid-crystal system hexakis(n-hexyloxy)triphenylene/2,4,7 trinitro-9-fluorenone (HAT6/TNF). For the electronic ground-state investigations, neutron-scattering techniques play a key role in gaining deeper insight into structure and dynamics. These measurements are complemented by Raman and nuclear magnetic resonance probes, as well as resonant Raman and UV-vis spectroscopies that are used to explore the low-lying excited states, at the vibronic level. Synergistically, numerical simulations, either classical via empirical force fields, or first-principles via density functional theory, are used for the analysis, interpretation and predictions.