Skip to main content

New materials for highly efficient and reliable organic solar cells

Final Report Summary - MATHERO (New materials for highly efficient and reliable organic solar cells)

Executive Summary:
Over the last couple of years, the global research community has been experiencing an Organic Photovoltaics (OPV) boom, triggering a fast growing interest in industry in this young and disruptive technology, since OPV devices enable various new applications that cannot be served by classical silicon solar cells. In particular, OPV opens up new opportunities for design in architecture, e.g. the integration of solar cells into facades, overhead glazing or windows.
Major challenges associated with bringing organic photovoltaics to the market are: Increasing the power conversion efficiency, reducing the production costs and increasing the material and device long-time stability. A key to meeting all those objectives is the utilization of environmentally friendly (“green”) synthesis of materials and green deposition techniques, since only green processes allow for advancing to large scale material synthesis and device fabrication.
The MatHero project was designed to address those key parameters in order to enable devices with power conversion efficiencies exceeding 10%, a cost reduction below 0.5 €/Wp and a life-time of more than 10 years by developing disruptive green synthesis and fabrication techniques.

Main objectives from MatHero and the achievements within the project duration are:
1. Developing green materials with higher power conversion efficiency (PCE) by developing (i) high efficiency donor polymers, (ii) green inks utilizing green solvents and (iii) materials with green and up-scalable chemistry.
Benzodithiophene-quinoxaline copolymers were synthesized by the traditional STILLE cross coupling polymerization, performing with more than 8% PCE in laboratory scale organic solar cells. Moreover, by passing to direct arylation which is a green and up-scalable chemistry route, the resulting polymers reached up to 6% PCE. Furthermore, green inks were developed using non-halogenated non-toxic solvents, providing as efficient solar cells as chlorinated solvents commonly used in research labs, with PCEs exceeding 10%.

2.Enhancing cell lifetime (up to 10 years)
A disruptive monolithic encapsulation process fully from solution was achieved within MatHero. New materials and processes were developed, with a final monolithic encapsulation stack of a planarization layer and three dense layers and gas barrier properties in the range of OPV requirements, without the need for adhesives or energy-consuming vacuum techniques that are typically used for high quality gas barrier foils. The solution-processed monolithic encapsulation is compatible with the deposition on top of organic solar modules.

3. Demonstrating cost effectiveness of OPV modules with environmental & sustainable technologies.
Flexible substrates, photo-active polymers from an up-scalable green synthesis, sustainable production using green solvents, inexpensive interlayers and the disruptive monolithic encapsulation enable substantial cost reduction concerning bill of materials, the main cost factor for large scale OPV module fabrication.

The MatHero consortium comprises world leading research groups and companies that were capable of reaching the challenging goals of MatHero. The companies are directly exploiting the results and are bringing organic solar cells or related components to the market.
Project Context and Objectives:
Organic bulk-heterojunction solar cells are on the cusp of commercialization. Printing and coating techniques are widely considered enablers of low-cost solar module fabrication with excellent carbon footprints. For the transfer of lab-scale processes to an environmentally friendly and sustainable industrial large-scale fabrication of polymer solar cells by printing, non-halogenated solvents and processing additives are mandatory prerequisites. As the choice of solvents is pivotal to the complex formation of the bulk-heterojunction and hence to the device performance, both academia and solar industry have fostered strong research on “green” device processing in order to advance the market-readiness of organic solar modules. The eco-friendly fabrication objectives are furthermore extended to the materials synthesis. Large-scale printing of organic solar cells requires large amounts of materials that require advanced synthesis. Toxic reagents object the large-scale synthesis of organic semiconductors. Thus the investigation and subsequent use of non-toxic material synthesis routes is pivotal to the success of the technology, ruling out common synthetic lab processes such as Stille coupling. In addition to green synthesis of efficient materials and environmentally friendly fabrication of solar modules, the ability to convert the energy of light into electrical energy must be guaranteed for several years, for which the importance of protective barrier materials as encapsulation comes into play. The longer the lifetime of a solar module, the higher is its integrated energy output, reducing cost of energy and saving valuable resources.
In light of these important considerations to bring organic solar cells to the market in the future, the following objectives were pursued deliberately.

Objective 1: Development of materials with high power efficiency
In order to improve efficiency, the MatHero consortium investigates polymer solar cells having targeted 8% power conversion efficiency in the first period of the project, ultimately improving its efficiency beyond 10% in the second project period. Therefore, new materials are synthesized by the chemistry partners and evaluated for high power conversion efficiencies by the device engineering partners. Providing many handles to structural and hence optoelectronic modifications, the consortium deliberately has decided to investigate photo-active benzodithiophene-quinoxaline copolymers. After the decision on a hero polymer around half time of the project, up-scaling activities started to provide sufficient quantity of efficient materials for the demonstration of their feasibility in organic solar modules and final demonstrators. In order to obtain green materials that enable upscaling, MatHero follows two paths: (I) develop green inks based on a readily available polymer (shelf-polymer) and later using the hero-polymer; and (II) use green chemistry to synthesize the hero-polymer.
I. Green inks
In order to apply organic bulk-heterojunctions by printing or coating techniques and to ensure non-toxic processing and hence low-production cost and upscaling, the materials have to be dissolved in non-hazardous solvents. This formulation of suitable printing inks is pivotal to the formation of the two-phase bulk-heterojunctions during coating and drying. In the first funding period, the consortium worked on a protocol to analytically determine suitable non-toxic solvents or binary solvent systems that have similar properties as compared to the widely used, toxic chlorinated solvents, for which the shelf polymer/ fullerene blend system was studied in detail. In the second period, the approach is transferred to analyzing the hero polymer/ fullerene blend and providing non-toxic green solvents for the fabrication of efficient solar cells and modules.
II. Green synthesis
On the basis of the hero-polymer synthesized by eco-toxic STILLE coupling during the first period of the project, the chemistry partners advance their polymer synthesis towards green chemistry (i.e. direct arylation). This substantially reduces toxic by-products and is therefore a prerequisite for an industrialization and exploitation of the polymer synthesis on truly large scale.

Objective 2: Enhancement of cell lifetime by improved oxygen/moisture barriers
A widely disregarded objective in the field of organic photovoltaics is the long-time stability of devices. In the long run, the consortium aims at solar module life-times beyond 10 years. Therefore, a new and disruptive encapsulation technique was developed which allows for printing of the oxygen and moisture barriers directly atop the organic solar cell rather than laminating the device between barrier foils, which itself are typically fabricated by expensive vacuum techniques. This approach saves materials, costs and time during the future fabrication of organic solar cells. At first, materials for the encapsulation layers were developed and aging protocols were designed. In the second half of the project, the monolithic encapsulation stack was established, making use of direct coating of the solution processed barriers on top of the organic solar device, testing for its compatibility. Barrier properties better than 10^-4 g/(m^2.d) as water vapor transmission rate (WVTR) were targeted. This disruptive encapsulation process makes the use of adhesive materials obsolete and considerably reduces problems of lateral permeation and delamination which reduces long lifetime applications. In addition, it will allow decreasing the environmental impact of the encapsulation by avoiding the use of energy consuming processes.

Objective 3: Cost effectiveness demonstration of OPV environmental & sustainable technologies
I) Up-scaling of materials synthesis
MatHero develops polymers suited for a later up-scaling of the synthesis that allows further cost reduction. A MatHero synthetic route for the hero-polymer was developed to synthesize a pilot batch. This batch was targeted to be used to facilitate the implementation of industrial production after the project. The final goal of MatHero was to utilize eco-friendly chemical methodology for the polymer synthesis (direct arylation instead of the commonly used STILLE coupling) and green inks (instead of chlorinated solvents). STILLE coupling limits the scalability, hence, the MatHero green approach will not only reduce the environmental impact but also allows exploitation of the materials in an industrial environment. Besides the cost reduction potential for light absorbing polymers, the consortium aimed at reducing future device fabrication costs by utilizing inexpensive metal oxides for charge carrier extraction buffer layers. Scalability of materials was also considered in the case of encapsulation processes developed during the project.
II) Upscaling of the OPV manufacturing process
Flexible plastic substrates replace the energy-intensive glass substrates, at the same time reducing the environmental impact. At the end of the project, the consortium targeted and successfully presented a 100cm^2 flexible solar cell carrying an organic photovoltaic sub-module and a roll-to-roll coated demonstrator with an output power under standard conditions of more than 0.5W. It was fabricated for field trials in off-grid solar systems in operational conditions. The fabrication of solar modules was accompanied by the development of a new oxygen and moisture barrier encapsulation that can be applied in roll-to-roll coating processes directly atop the solar cell.
Project Results:
The main achievements within the MatHero project can be best summarized along the individual work packages for materials (WP1), devices (WP2), stability (WP3) and final demonstration (WP4) as follows:

Work Package 1 (Materials)
(Polymer synthesis at Fraunhofer Institute for Applied Polymer Research and ADVENT Technologies, ink formulation at LEITAT Technological Center)

Synthesis of highly efficient polymers
For the prospected benzodithiophene-quinoxaline copolymers a theoretical PCE of 10% was calculated. In solar cells, the maximum PCE obtained with the hero polymer (WF3) was 7.6%. Further synthetic advancements on the hero polymer structure enabled a maximum solar cell PCE of > 8%.

Green synthesis of highly efficient polymers by direct arylation
The eco-friendly methodology of direct arylation was established to avoid toxic STILLE polymerisation. Employing direct arylation, the hero polymer and an additional copolymer were synthesized, achieving 80% of the PCE of the corresponding STILLE copolymers. These copolymers are among the most efficient direct arylation polymers reported so far. The green polymerisation was upscaled to a 1 g-batch by ADVENT.

Synthesis upscaling to multi-gram scale
The hero copolymer was upscaled to multi-gram scale at ADVENT. It was synthesized in several batches of up to 12 grams per batch. Therefore the multi-step monomer synthesis of both monomers (benzodithophene and quinoxaline derivatives) was also carried out on 10 gram-scale.

Solubility of highly efficient polymers in green solvents
At LEITAT, Hansen solubility parameters (HSP) were successfully used to identify suitable green solvents for the deposition of blends of the hero polymer and fullerenes (PCBM). Starting from a list of over 50 solvent candidates, complementary criteria such as processability, toxicity and price were used for selecting a final list of 10 green solvents candidates for further experimental evaluation via thin-film properties and device performance.

Formulation of green inks
Four green solvent candidates were selected with the best potential to be used in the optimal green ink formulation. After evaluation via device performance the formulation based on polymer:PCBM active blend using o-xylene as solvent and p-anisaldehyde as additive, was chosen as the optimal green ink formulation with respect to highest power conversion efficiency around 8%.

Cross-linkable polymers for enhanced device stability
In order to further enhance the thermal stability of organic solar cells, crosslinking of the photo-active layer was targeted, were two different approaches were established. The first approach used a bisazide crosslinking agent which leads to a crosslinking of the fullerene derivative. With this, the blend morphology was stabilized by avoiding fullerene agglomeration during annealing of the solar cell. The second approach established a cross-linkable absorber copolymer. This cross-linkable absorber polymer carries octenyl side groups which undergo thermal cross-linking. By that the solvent stability of the active layer could be stabilized. Both approaches were additionally applied to direct arylation copolymers.

Work Package 2 (Devices)
(Lab-scale investigation of materials and up-scaling concepts at Karlsruhe Institute of Technology, large area and roll to roll fabrication of solar modules at Eight19)

Fabrication and investigation of highly efficient organic solar cells from green solutions on lab-scale
Multiplicity of donor polymers investigated for their performance in solar cells, with PCEs of >8% from polymers developed in WP1 and >10% from reference polymers. Using green inks developed in WP1 was found to result in as efficient solar cells as compared to using typical halogenated solvents.

Investigation of solution processable metal oxides as reliable low-cost buffer layers
Architectures were developed using a variety of solution processed metal oxides as hole transport material (MoO3, WO3, NiO) and ZnO as electron material, both in regular and inverted architecture. While for some of the precursors (WO3, MoO3, ZnO) the moderate conversion temperatures required are compatible with flexible PET substrates, high temperature annealing typically needed for NiO precursor conversion could be avoided using nanosecond laser irradiation. Solar cells employing metal oxide transport layers and project developed photo-active materials obtained similar efficiencies as compared to PEDOT:PSS reference cells.

Up-scaling concepts towards printable large-area photovoltaic devices
Solar sub-modules were fabricated employing the hero polymer and green ink formulations for the deposition of the photo-active layer, resulting in up to 5.8% PCE by doctor blading and 4.8% by slot-die coating at Eight19, paving the way for industrial large area fabrication.

Development of roll to roll processes
Fully roll to roll processing was employed to fabricate organic solar modules, using green inks and the project hero polymer, with a final demonstrator of sufficient power output >0.5W and good low light performance.

Work Package 3 (Stability)
(Monolithic encapsulation and barrier property determination by Arkema and CEA, aging studies complemented by Eight19, LEITAT and KIT)

A better understanding of OPV stack degradation
An extensive aging protocol was installed to determine the degradation mechanisms and discriminate between intrinsic and extrinsic impact. Solar cells employing the hero polymer or reference polymers were studied according to the aging protocol, and the main stress factors (light, H2O) were identified and quantified.

The development of a disruptive monolithic encapsulation procedure fully from solution
Materials for the solution processed monolithic encapsulation (planarization layer, silica dense layers) were developed. The encapsulation stack comprising the planarization layer and three dense layers results in gas barrier properties in the range of OPV requirements (<10^-3 g/(m^2.d)) and is compatible with the deposition on top of organic solar modules.

Life-time study of organic solar cells and modules in order to prove reliability during 10 years
Apart from an initial burn-in loss, it could be shown that solar cells comprising the hero polymer are stable at 5% PCE in >5000h outdoor conditions. Because of the stabilized performance in outdoor conditions, it was not possible to calculate accelerating factors based on the comparison of accelerated aging with outdoor aging, but it is reassuring that a lifetime of 10 years can in fact be achieved.

Work Package 4 (Demonstration)
(Eight19, CEA, Arkema and KIT)

Demonstration of a solar module with a power output >500mW for off-grid applications, fabricated on large-area deposition equipment
A fully functional solar module demonstrator comprising the hero polymer was fabricated using R2R deposition of all layers, and configured as an off-grid charging unit, complete with USB connector. The module delivered over 500 mW at 1 Sun and showed that charging was also possible at much lower light levels, down to 1000 Lux and below.

Demonstration of life-time >10 years in accelerated life-time measurements.
Accelerated aging data from round robin studies between four consortium partners and outdoor measurement data from two sites were evaluated with the target of determining a life-time estimate. The objective of demonstrating 10 years lifetimes based on monolithically encapsulated hero polymer solar modules remained too challenging for the time being even though the compatibility of the monolithic processing and the encapsulation stack itself were proven successful in WP3.
Potential Impact:
Dissemination and Exploitation was addressed in the Work Package 5.
Its main objectives and achievements can be summarized as follows:

Analyze market barriers and technological needs, the economic viability of the proposed methodology and develop a dissemination plan that ensures a rapid time to market:
Six key exploitable results were identified as exploitation strategy. A Plan for the Use and the Dissemination of Foreground (PUDF) document was created.

A standardization workshop was organized, raising the importance of standards within the OPV community and discussing a standard for low-light applications.

Presentation of project results on international conferences
As an important aspect of project dissemination, project results or the MatHero project itself were presented on a variety of renowned European scientific conferences, such as LOPE-C, ICOE, ISFOE, ICME, EU-PVSEC, E-MRS Fall, HOPV, ISFOE, ElecMol (presented by coordinator or project partners). The project was also presented on European workshops such as the EU PV Cluster meeting (2nd and 3rd workshop and general assembly), workshops from other European projects, and international summer schools.

2 peer-reviewed articles about MatHero research results were published in top-level journals, and more than 5 are currently in preparation. One patent was filed, 2 more are currently under consideration.

Summer school for PhD and master student education:
Education of students was considered as being one of the major targets for dissemination: apart from the training of students and supervision of student theses, a summer school was organized by KIT in 2015, with students learning about OPV from and discussing with and high-level internal speakers.

Public workshop with invited speakers for dissemination of MatHero results and OPV recognition in general:
A public industrialization workshop was hosted by LEITAT, key exploitable results of MatHero were presented and representatives from different industrial sectors covering the whole OPV value chain participated.

Potential Impact of the MatHero Project:
MatHero´s main impact is the development of long-term stable and cheap organic photovoltaic cells using green materials processed from green inks, and therefore avoiding any hazardous constituents in the manufacturing of these device architectures. In addition, new encapsulation concepts were investigated in order to improve the device stability and to achieve a high protection level.
The impacts of MatHero are consistent with the guidelines of the sustainable development paradigm, public health, worker safety, environmental protection and the societal dimensions. Besides, the MatHero project obtained research results aim to enlarge the optimal normalisation and standards needed in new materials for OPV technologies and barrier materials for printed electronics.
The objectives to consolidate the leading position of European research in organic, and more particularly OPV topics, and to develop industries in relation with these promising technologies are consistent with the Low Carbon Energy Technologies set plan. To fulfil these objectives, the present Work Programme includes the following expected impacts:
1. Efficiency of OPV modules of at least 15% in a relevant environment, with a considerable improvement in the service life-time and performance of the materials by 2030.
2. Enhancement of the efficiency of material use and/or in the OPV production processes.
3. More favourable cost/efficiency ratio compared to inorganic PV.
4. Contributions to the implementations of the SET plan, in particular to the Materials Roadmap enabling Low Carbon Energy Technologies.
5. Potential areas and markets of application: Besides the OPV market, the processes developed herein may also impact on the fabrication of printed organic electronics (e.g. transistors), organic photodetectors or organic light emitting diodes (OLEDs). Even some inorganic devices may benefit from the project results (e.g. printable metal oxides)
6. Environmental and societal impact: Mankind does have to overcome an energy challenge which will become even more pronounced in the upcoming decades. Organic photovoltaics aim at low-cost and eco-friendly energy harvesting, being one of the major challenges of the society.
List of Websites:

Coordinator contact details

Alexander Colsmann
Karlsruhe Institute of Technology (KIT)
Light Technology Institute (LTI)
Engesserstrasse 13
76131 Karlsruhe, Germany