Device Degradation and Lifetime of OPVs Utilising a Solution Processed MoOx Interface
Alastair Buckley a, Jon Griffin a, Edward Bovill a, David Lidzey a
a University of Sheffield, Hounsfield Road, United Kingdom
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Ecublens, Switzerland, 2014 May 11th - 14th
Organizers: Michael Graetzel and Mohammad Nazeeruddin
Oral, Edward Bovill, presentation 191
Publication date: 1st March 2014

Here we investigate the lifetime and degradation mechanisms of Organic Photovoltaic (OPV) devices, based on solution processed MoOx hole transport layers (HTLs) deposited in air, using a number of techniques. The effects on device lifetime of thermally annealing the solution processed MoOx are studied, and are compared to other devices that utilise thermally evaporated MoOx or PEDOT:PSS HTLs.

Introduction

MoOx has been studied extensively as a HTL in OPVs due to its lower lying valence band (in the region of -5.30 to -6.86 eV [1]), which makes it more compatible with low energy gap polymers. MoOx and other transition metal oxides used as hole, or electron, transport layers are often deposited via thermal evaporation. Unfortunately, for low cost and large scale production of OPVs, thermal evaporation steps need to be avoided and, therefore, alternative methods of deposition need to be found.

Solution processing of metal oxide films presents a promising alternative, as this process offers the potential for low temperature, low cost manufacturing. Several groups have reported some success with solution processed MoOx HTLs [2-4], though many require high temperature annealing to form the film.

The method used herein uses a facile synthetic process based on work by S. Murase [4]. A precursor of Ammonium Molybdate Tetrahydrate was dissolved in DI water and Acetonitrile - thermal decomposition of which forms water, gaseous ammonia and MoOx in solution. Thin layers (~5nm) of MoOx were produced by spin coating in air, and were left unannealed or were thermally annealed in air. OPV devices based on an PCDTBT/PC70BM active layer were fabricated using these MoOx HTLs, and their efficiency, lifetime and degradation mechanisms were measured by a combination of device characterisation and impedance spectroscopy.

 

Results

Initial results show that OPV devices made with a solution processed MoOx HTL and a PCDTBT/PC70BM active layer can achieve efficiencies of up to 5.5%. This is comparable to devices with a thermally evaporated MoOx or PEDOT:PSS HTL.

Preliminary lifetime data shows that devices incorporating solution processed MoOx have shorter lifetimes than devices with thermally evaporated MoOx, with devices with thermally annealed solution processed MoOx having improved lifetimes over those with unannealed solution processed MoOx.

Impedance spectroscopy has also been used to explore the mechanisms that result in device degradation and poor device lifetimes. 



[1] I. Irfan, A. James Turinske, Z. Bao, and Y. Gao, Appl. Phys. Lett. 101, 093305 (2012). [2] L. Chen, P. Wang, F. Li, S. Yu, and Y. Chen, Sol. Energy Mater. Sol. Cells 102, 66 (2012). [3] C. Girotto, E. Voroshazi, D. Cheyns, P. Heremans, and B.P. Rand, ACS Appl. Mater. Interfaces 3, 3244 (2011). [4] S. Murase and Y. Yang, Adv. Mater. 24, 2459 (2012).
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