Acetylferrocene. Dimethylferrocene. Decamethylferrocene. Ferrocene. 1,1'-Diacetylferrocene. Dicarbomethoxy- cobaltacene. Octamethylnickelocene. qE(A/Aâ).
Testing the photovoltage-potential of PbS nanocrystal-solid photovoltaics Valentin Uzunov, Vitalii Dereviankin, Dr Eric Johansson, Chemistry Department, Portland State University, Oregon , USA Lead-chalcogenide quantum dots have been identified as a promising class of semiconductor materials for photovoltaic applications. The properties of PbS QD’s in particular, offer several advantages over alternative semiconductor materials. Band gap tuning and a large molar absorption coefficient offer the potential for optimal solar spectrum radiation absorption with high photo conversion efficiencies [1]. Of equal importance to their advantageous chemical properties, is that solution processing is possible, thereby allowing for cost-effective large scale application [1]. Despite their promising properties, devices consistently exhibit photovoltages significantly lower than expected based on thermodynamic considerations. It is believed that by being able to better understand the ligand chemistry of the QD films, the material promises can be unlocked. In order to study the photovoltage potential of the photo-active quantum dot films, we investigated the use of liquid contacts as a diagnostic tool. Liquid contacting is ‘soft’ in comparison to traditional methods such as metal evaporation. The energetics of liquid contacts can be smoothly tuned across a wide energetic window which allows us to decouple complications related to contacting via metal evaporation from the intrinsic properties of the quantum-dot solid. Using a sandwich architecture device, a selected range of potentially suitable solution redox couples were tested and their response was evaluated against expected theoretical expectations as a preliminary step.
MATERIALS and METHODS To test our hypothesis, colloidal PbS QD capped in Oleic acid were spun coated on a transparent conductive oxide (TCO) glass. Upon deposition, solid state ligand exchange with 3-mercaptopropanoic acid (MPA) is performed. MPA is a much shorter bifunctional ligand believed to promote close packed ordered assembly of film, which is desired. A surlyn spacer is used to form a cavity between photo-active film substrate and a gold coated TCO counter electrode. The cavity was filled and tested separately using 20 mM Ferrocene ( Fc+/0 ) and 5 mM Decamethyferrocene ( DmFe 0/+ ) redox couples (Figure 1). These redox couples were selected based on their redox potential in reference to the PbS QD film. Cells were illuminated using 100 mW cm −2 of Air Mass 1.5 Global sunlight lamp, and I-V recorded with a Gamry Reference 600 potentiostat/galvanostat/ZRA.
RESULTS 0
Evac
-3.5
Energy (eV)
INTRODUCTION
-4
Dimethylcobaltacene
Ecb
Octamethylnickelocene Methyl viologen Dicarbomethoxycobaltacene Decamethylferrocene
EF Evb
Dimethylferrocene Ferrocene Acetylferrocene
-4.5
-5 -5.5
qE(A/A–)
1,1’-Diacetylferrocene
quantumdot solid liquid
0/+
FIGURE 2 - Energy diagram shows relative redox potential energies of various electrolytes, including Fc+/0 and DmFe0/+, relative to band gap of bulk PbS quantum-dot solid, in reference to vacuum level.
FIGURE 5 - Chronoampometry of device filled with DmFe shows reduced current density over time under illumination
FIGURE 3 - I-V curves show expected decreasing Voc (when current dencsity is zero) and Isc’s (when potential is zero) with decreasing redox couple potential see in energy digram Figure 2.
CONCLUSION
Preliminary results using an unoptimized system, demonstrated reduced device performance compared to standards made using Au deposition; however, selected redox couples resulted in predictable photovoltage trends, supporting the use of the approach as a viable diagnostic tool to improve our understanding of quantum-dot solid-based photovoltaics.
The current preliminary investigation demonstrated reduced device performance, compared to standards made using direct Au deposition on QD films. It was noted that, the open-circuit voltages achieved were lower than expected, which is may be attributed various machenism beyond scope of current investigation. The overall device performance based on contact energetics was within expectation. Further investigation is now possible, to probe responsible machenism for reduced Voc measured. The results give support to the viability of the current methods as a diagnostic tool for probing the photovoltage potential of current quantum-dot solids, without the limitations imposed by common electrical contacting schemes. Future work will continue to examine the open circuit potentials as a function of contacting energetics, and system design optimization design.
From the energetic alignment of the redox potentials for Fc+/0 and DmFe0/+ relative to the quantumdot solid (QD solid), it was expected that the open circuit otentials (Voc) will decrease proportionally with the difference between the redox potential energy of the solutions and the QD solid Fermi energy (EF) (Figure 2). This trend was indeed observed in the the current-voltage (I-V) cell characteristics studies (Figure 3). Figure 4 and 5 showed that photovoltage response of the cell using a liquid contact were stable, but current-densities declined over time. 0/+,
FIGURE 4 - Chronovoltametry profile of functioning device filled with DmFe
Of note, the Voc values measured are lower than the maxium potential possible, based on energy differences alone (Figure 6). The benefit of using liquid contacts is that this kind of behavior can be directly studied using more advanced electrochemical methods and models, which will be explored in future work [3]. 0
0 PbS
-3.5
-3.5
PbS
-4 -4.5
-5 -5.5
FIGURE 1 - Schematic cross section illustration of sandwich cell architecture and design
-4 -4.5
TCO Ferrocene
Voc = 0.05 V Predicted
TCO
Decamethyferrocene
-5 -5.5
Voc = 0.40 V Predicted
FIGURE 6 - Band diagrams show the change in the potential gradient (bending) of the solid/liquid junction of a functioning device. This change in potential is proportional to predicted Voc
LITERATURE 1- Giansante, C, Carbone, L & Giannini, C. Colloidal arenethiolate-capped PbS quantum dots: optoelectronic properties, self-assembly, and application in solution-cast photovoltaics. (2013). doi:10.1021/jp403066q 2 - Tan, M. X., Laibinis, P. E., Nguyen, S. T., Kesselman, J. M., Stanton, C. E., & Lewis, N. S. (1994). Principles and applications of semiconductor photoelectrochemistry. Progress in Inorganic Chemistry, Volume 41, 21-144 3- Lewis, Nathan S. (1984). A Quantitative Investigation of the OpenCircuit Photovoltage at the Semiconductor/Liquid Interface." Journal of The Electrochemical Society 131.11 (1984): 2496-2503.
ACKNOWLEDGMENTS I would like to thank all the memebers of Johansson Research Group for this summer, Dr. Johansson, Chase Reinhart, Qi Tong, Eric Young, and Vitalii.
