G8-2012 Ink-jet printed single-crystal organic photovoltaics (IPSOP)

Lead Research Organisation: University of Cambridge
Department Name: Physics


87% of global energy production in 2010 was derived from unsustainable fossil fuels and as energy consumption grows, we urgently need to move towards renewable, clean energy sources to ensure national and international energy security. Only ~36 kWh/day/person could realistically be generated by non-solar renewables, falling short of the global target for energy requirements of 80 kWh/day/person. Therefore, without relying on nuclear energy, we must ensure that solar energy fills the gap. To meet demand, we require as many on- and off-grid photovoltaic (PV) technologies as possible and development of sustainable, low-energy and material-light technologies should be prioritized. In this context, ink-jet printing organic semiconductors is highly attractive.

Our project aims to use similar processing techniques to demonstrate a dramatic step change in PV efficiency to
match that of transistors. Such a step change requires a global consortium such as ours, including world-leading chemists (USA), physicists/engineers (UK) and material scientists (Japan) as well as knowledge of market requirements (Organic PV company). We aim to tackle the whole PV cycle, from materials to exploitation. In particular concentrating on
(i) sustainable approaches to organic semiconductor synthesis, potentially allowing organic PVs to be made out of bio waste such as corn stover,
(ii) creating highly ordered PVs using novel adapted ink-jet printing and vapor-phase deposition (and comparing the two techniques)
(iii) quantitatively characterising the single crystal structure, physics and morphology
(iv) characterizing the finished PVs, including stability and lifetime.

Constant feedback between the groups will be used to optimize the materials and processing techniques to develop revolutionary >10% efficient sustainable PVs.

Planned Impact

With our proposed approach we are aiming to lift the efficiency of printed organic solar cells to levels comparable to those of competing inorganic thin film PV technologies. Our target efficiency for the project is to realize an efficiency of 10% with our novel bilayer, molecular crystal architecture and to demonstrate the potential for achieving efficiencies of 15%. This would constitute the necessary scientific breakthrough that could pave the way for organic solar cell technology to achieve lower cost of electricity than inorganic PV technologies and ultimately to match the cost of electricity achievable with burning fossil fuels.


10 25 50
Description The IPSOP project aimed to develop high efficiency solar cells based on single crystal organic semiconductors deposited by printing techniques, such as inket printing. It involved a close and interdisciplinary collaboration between a synthetic chemistry group (Anthony, University of Kentucky, USA), a group specializing in the growth and characterization of molecular single crystals (Hotta, University of Kyoto, Japan), a group specializing in the device and photophysics of organic solar cells (Clarke/Sirringhaus, University of Cambridge, UK) and an industrial end-user partner (Eight 19, UK).
At an early stage we discovered that in the class of materials targeted by the project, the photophysics is dominated by the process of singlet fission. This process causes the photoexcited species generated by absorption of sunlight, which are singlet (spin-0) excitons, to undergo a spin-allowed transition into a pair of spin-1 triplet states. In some of the materials this process was found to occur on an ultrafast timescale and to be highly efficient.

The process of singlet fission provides a very powerful way to improve the performance of a solar cell: Whereas in a conventional solar cell each photon generates only one electron-hole pair and the excess of the photon energy above the band gap energy of the semiconductor is lost as heat, in a solar cell utilizing a singlet fission material each photon can create two electron-hole pairs. If these can be harvested efficiently, this provides a route to achieving up to 200% quantum efficiency and improving the power efficiency of the solar cell beyond the Schottky-Queisser limit.

We investigated systematically the singlet fission process in a wide range of soluble acenes, anthradithiophenes and other molecules with band gaps tuned to the solar spectrum that were synthesized by Prof. Anthony's group. We discovered that the timescale and efficiency with which singlet fission occurs can be tuned by molecular design and side chain substitution. Most importantly, we were able to understand in more detail the microscopic mechanism for singlet fission by being able to spectroscopically resolve a quantum mechanically correlated triplet pair state which acts as an intermediate state between the initial photoexcited singlet state and the two spatially separated triplet states that are ultimately harvested in the solar cell. Identifying the nature of this intermediate state is crucial for the design of molecules that are optimum for exploiting the singlet fission process. As a result we were able to design and fabricate a singlet fission solar cell based on a crystalline molecular semiconductor film combined with a fullerene acceptor that has a quantum efficiency in excess of 140%.
Exploitation Route Follow-on research to enhance further the efficiency of solar cells
Sectors Energy

Description The project has developed an improved scientific understanding of the process of singlet fission in molecular semiconductors. This process is currently being actively developed as a mechanism for enhancing the efficiency of solar cells.
First Year Of Impact 2015
Sector Energy
Impact Types Societal,Economic

Title Research data supporting: The Entangled Triplet Pair State in Acene and Heteroacene Materials 
Description The data comprise the underpinning data of the main text figures and supplementary figures of a paper accepted for publication in Nature Communications. The paper investigates the role of an entangled triplet pair state in the singlet fission process occurring in organic semiconductors. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes