Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer

Significance Statement

Organic photovoltaic diodes appear to be a promising technology because of their potential use in synthesizing low-cost, flexible, large area and lightweight electronics. Unfortunately, the limited lifetime of organic photovoltaic diodes hinders their commercialization. For example, a thousand hours lifetime reported for poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester solar cells is inferior to that of silicon photovoltaics whose lifetime extends up to 25 years.

The degradation of these solar cells remains a limitation and many researchers are still looking for ways to improve their performance. Organic photovoltaic diodes degrade owing to chemical and physical processes. Chemical degradation results from oxygen, light, water and temperature. However, water and oxygen are considered the principle factors; these oxidize the organic photovoltaic materials as well as electrodes leading to poor performance and electronic traps.  Degradation may also result from the bulk heterojunction morphology since the constituent materials may aggregate with time, leading to reduced exciton dissociation and poor performance.

Blending the active semiconductor with an inert polymer appears to improve the lifetime of the organic photovoltaic diodes. A team of researchers under the guidance of Professors Chris Groves and Michael Petty at Durham University in United Kingdom investigated the use of an insulating polymer, poly(methyl methacrylate), as a ternary component in poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester solar cells as a way of enhancing their lifetime and reducing degradation. Their research work is now published in Solar Energy Materials & Solar Cells.

The authors separately prepared poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester solutions before mixing in a 1:1 weight ratio. In a bid to make the ternary blend, they first prepared poly(methyl methacrylate) solution by dissolving in anhydrous 1,2-dichlorobenzene to obtain a clear solution.

The authors stirred both the ternary and binary organic photovoltaic blends before spin coating them onto the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate). They annealed all devices before lifetime tests.

The research team observed that the addition of poly(methyl methacrylate) improved both the initial performance as well as the lifetime of the solar cells. Measurement of the different relative humidity values suggested that poly(methyl methacrylate) absorbed water, thereby reducing the rate of chemical degradation in the solar cell. The lifetime improvement with poly(methyl methacrylate) reduced with decreasing humidity. This suggested that the poly(methyl methacrylate) becomes saturated. A number of studies revealed that the addition of poly(methyl methacrylate) led to a morphology containing poly(methyl methacrylate) pillars dissimilar to the morphology seen in the binary film.

Electrical conductivity did not degrade at different rates across the ternary film. This suggested that water diffused microns through the film before reacting with the active material. The rate of conductivity degradation was similar for binary and ternary devices. This indicated that various degradation mechanism were present, and that poly(methyl methacrylate) only assisted in extending the lifetime associated with a selected degradation pathways.

Power conversion efficiency of the poly(3-hexylthiophene): phenyl-C61-butyric acid methyl ester solar cells is severely limited by reaction with water. The incorporation poly(methyl methacrylate) slows down this degradation process through water absorption. The results of their study suggest that electrically inert and hydroscopic polymers can be blended with an organic photovoltaic active layer to extend the device lifespan.

Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer

The addition of PMMA to a P3HT:PCBM solar cell slows down the device degradation. Left: Power conversion efficiency (PCE) as a function of time for binary P3HT:PCBM (1:1) blend and ternary (1:1:0.3) P3HT:PCBM:PMMA blend solar cells, stored at a relative humidity (RH) level of 1%. Right: AFM topography image of as-deposited (1:1:0.3) ternary P3HT:PCBM:PMMA blend film. The PMMA is in the form of circular islands.

About The Author

Mrs Zakiya AL-Busaidi received her M.Sc. degree in physics at Sultan Qaboos University, Oman.  She is currently a PhD student in the School of Engineering and Computing Sciences, Durham University, UK. Her current research focuses on how to enhance the lifetime of organic photovoltaics by using insulating polymers.

About The Author

Dr Chris Groves is an Associate Professor at the School of Engineering and Computing Sciences at Durham University, UK, where he is also a Director of the Durham Centre for Molecular and Nanoscale Electronics. He completed his PhD on III-V photodetectors at Sheffield University in 2004, before undertaking postdoctoral positions in the field of organic electronics at the Cavendish Laboratory and the University of Washington.

His research interests focus on the use of experiment and simulation to examine the relationship between charge transport and the performance of electronic devices. Recently, this has involved the development of Monte Carlo models and experimental techniques to reveal how morphology influences charge transport, and ultimately performance, in organic photovoltaic diodes.

About The Author

Dr Christopher Pearson received the Degree in Physical Electronics from Newcastle Polytechnic, Newcastle, UK, and a PhD, focusing on thin films of organic charge-transfer materials, from Durham University, Durham, UK, in 1997. Since 1981, he has been working at the University of Durham. Currently, he is an Experimental Officer with the Durham Centre for Molecular and Nanoscale Electronics, providing support for the group and carrying out research on organic thin films prepared using a variety of techniques.

About The Author

Professor Michael Petty’s higher education was at Sussex University, UK (BSc and DSc) and Imperial College, London (PhD Electronic Materials.) He has been at Durham University, UK since 1976 progressing to Professor in 1994, then Chairman of the School of Engineering from 1997 to 2000.

His research activities focus on the properties of thin films of organic materials (Langmuir-Blodgett, self-assembled, electrostatically deposited, evaporated). He has a special interest in the application of these layers to electronic and opto-electronic devices. Professor Petty has lectured extensively worldwide and published over 400 papers/books/patents in these subjects.

He is a member of Durham University Centre for Molecular and Nanoscale Electronics

Reference

Zakiya AL-Busaidi, Christopher Pearson, Christopher Groves, Michael C. Petty. Enhanced lifetime of organic photovoltaic diodes utilizing a ternary blend including an insulating polymer. Solar Energy Materials & Solar Cells, volume 160 (2017), pages 101–106.

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