University of Minnesota
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Electrical and Computer Engineering

Electrode Surface Modifications for Efficient Polymer Bulk
Heterojunction Photovoltaic Devices

Paul R. Berger
Ohio State University

Key advantages of polymer bulk heterojunction photovoltaic (PV) technology is that polymer based PV versus traditional inorganic semiconductors are inherently inexpensive; typically have very high optical absorption coefficients (>105 cm-1); are compatible with plastic substrates; and can be fabricated using high-throughput low temperature processes for low-cost roll-to-roll manufacturing. Although polymer solar cells have improved rapidly from very low efficiencies to moderate efficiencies of ~5-6%, the overall performance of polymer PVs is not yet high enough for commercial opportunities.

Among recent advances towards improving the efficiency in polymer PV devices, various interface and surface engineering techniques to the anode have demonstrated improved efficiencies for organic PV devices by an optimization of the short-circuit current, the open-circuit voltage and the fill factor. Although direct surface modifications to the anode have been successfully implemented for organic light emitting diodes (OLED) to improve their device performance by creating an interface energy step between the anode and the hole-transporting layer, thereby enhancing hole injection by effectively lowering the hole injection barrier, this approach has not been fully explored for polymer PV yet. Recently it has been reported that the creation of an interfacial energy step could promote the exit on dissociation in polymer PV devices, leading to an increase of the overall efficiency by improving the short-circuit current without any significant change to open-circuit voltage and fill factor. In this talk, we will present efficient PV devices through tailored modifications to electrode surfaces. Then we will discuss the related overall physical mechanisms behind this improvement.

In order to improve the efficiency of polymer PV devices, one approach, addressed here, will be to yield increased optical absorption and photocurrent generation in the photoactive layer over a broad range of visible wavelengths by inducing surface plasmons through careful control of metallic nanoparticle’s properties. It is well known that the optical absorption spectra of metal nanoparticles are dominated by localized surface plasmons. The application of plasmonic materials to various PV devices has been widely utilized for improving the PV device performance. However, these metal nanoparticles were actually random shapes formed during the early stages of electron-beam evaporation, forming a discontinuous metal film of small islands with non-uniform diameters distributed on the ITO anode. As a result, their particle size and shape varied over a wide distribution which could distort the plasmonic effects by broadening their spectral enhancement. In this work, a unique colloidal silver nanoparticle solution with the presence of suitable organic capping groups that stabilize the nanoparticles and inhibit their propensity to agglomerate is applied to organic bulk heterojunction PV devices. An improved optical absorption and photocurrent for PV devices was demonstrated due to the increased electric field in the photoactive layer by excited localized surface plasmons of Ag nanospheres.

Biographical Sketch:

Paul R. Berger received the B.S.E. degree in engineering physics in 1985, and the M.S.E. and Ph.D. degrees in electrical engineering in 1987 and 1990, respectively, all from the University of Michigan, Ann Arbor. He is currently a Professor in both the Electrical and Computer Engineering Department and the Physics Department at the Ohio State University (2000-present). He is the Founder of Ohio’s Nanoscale Patterning Laboratory.

Formerly, he worked at Bell Laboratories, Murray Hill, NJ (1990-’92) and taught at the University of Delaware in Electrical and Computer Engineering (1992-2000). In 1999, Prof. Berger took a sabbatical leave while working first at the Max-Planck Institute for Polymer Research, Mainz, Germany while supported by Prof. Dr. Gerhard Wegner and then moved on to Cambridge Display Technology, Ltd., Cambridge, United Kingdom working under Dr. Jeremy Burroughes. In 2008, Prof. Berger spent an extended sabbatical leave at IMEC (Interuniversity Microelectronics Center) in Leuven, Belgium while appointed as a Visiting Professor in the Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, Belgium.

Currently, Dr. Berger is actively working on conjugated polymer-based optoelectronic and electronic devices; molecular electronics; Si/SiGe nanoelectronic devices and fabrication processes; Si-based resonant interband tunneling diodes and quantum functional circuitry; and semiconductor materials, fabrication and growth.

Dr. Berger has co-authored >90 archival refereed journal articles, >90 invited and regular conference presentations, 3 book sections and been issued 12 patents with 5 more pending. Some notable recognitions for Dr. Berger were an NSF CAREER Award (1996), a DARPA ULTRA Sustained Excellence Award (1998), a Lumley Research Award (2006), and a Faculty Diversity Excellence Award (2009). He has been on the Program and Advisory Committees of numerous conferences, including the IEDM, ISDRS meetings. He currently is the Chair of the Columbus IEEE EDS/LEOS Chapter and Faculty Advisor to Ohio State’s IEEE Student Chapter. He is a Senior Member of IEEE and OSA.