58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Scheme 2. Preparation of PS-b-P4VP film and its reconstruction into a nanostructured template for electrochemical deposition.

Scheme 1. Chemical functionalization of self-assembled PS-b-P4VP films with habit modifiers [e.g. Br-(CH2)y-X] for the templated electrochemical growth of shape-tunable semiconductor nanocrystals.

The annual energy demand of the US could be supplied by covering 2% of its landmass with current photovoltaic technology.¹ The main obstacle in this approach however is the manufacturing cost of silicon solar cell modules. New photovoltaic technology based on organic and inorganic semiconductors is an attractive alternative due to significantly reduced fabrication costs.²  A recent evaluation of several inorganic semiconductors has identified cuprous oxide (Cu₂O) as a possible candidate for contributing to future solar energy infrastructure.³ Accordingly, we have investigated a new route to arrays of morphologically controlled Cu₂O nanoparticles for solar energy technology. The route employs self-assembling polyvinylpyridine (PVP) block copolymers as a template capable of (i) chemical functionalization and (ii) directing the location, size and structure of Cu₂O (Scheme 1).

The precursor template films are formed by spin coating micellar solutions of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) onto an electrode (Scheme 2). An array of nanoscaled domains lined with P4VP chains is formed when the template is exposed aqueous or methanolic solution (atop in Scheme 1). For chemical functionalization, we have confirmed a quaternization reaction of pyridine groups with methanolic alkylhalide. These self-assembled nanodomains represent a well-defined array of electrochemical nanoreactors suitable for controlled electrodeposition as the P4VP linings contain built-in chemical functionality that can be exploited to control the nanoparticle morphology during electrodeposition (middle and bottom in Scheme 1).

Figure 2.  J-V characteristic of Al/PCBM+P3HT/Cu2O/ITO solar cell.

Figure 1. XPS of Cu 2p signal for templated Cu2O. Inset: 0.5x0.5 µm2 AFM image of nanoring morphology of Cu2O (color scale = 25 nm).

Figure 3.  FTIR spectra for as-cast (top) and methyliodide functionalized PS-b-P4VP film (bottom). C---N ring stretch at 1600 cm-1 and C---N+-Me ring stretch at 1640 cm-1.

Our initial investigations have focused on understanding the templated electrodeposition of Cu₂O using the unfunctionalized parent PS-b-P4VP template. A thermostatically controlled galvanostatic electrodeposition procedure has been developed that produces convenient access to a novel nanoring-type Cu₂O. The structure and composition of arrays of Cu₂O nanorings have been confirmed by atomic force microscopy (AFM; Figure 1 inset) and high resolution X-ray photoelectron spectroscopy (XPS; Figure 1).  By deconvolution of the XPS signal, the nanorings were found to be 97% Cu(I)  oxide indicating excellent stoichiometric control of the oxidation state of the copper during the electrochemical deposition procedure.

Collaborative investigations (Prof. Janelle Leger, WWU Physics) are currently ongoing to better understand the photooptical properties of these Cu₂O. As a preliminary assessment of the photovoltaic potential of this material, we have introduced electrodeposited Cu₂O into a photovoltaic cells that employ a photoactive bulk heterojunction layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM). In this device architecture, electrodeposited Cu₂O acts a buffer layer for charge extraction and is anticipated to assist in the function of these devices (See Figure 2).⁴

We have recently shown that the chemical functionalization of the templates used to create the above Cu₂O can be elaborated by quaternization (Figure 3). We have also found that a tunable level of quaternization is possible and is set by an exposure time factor when template films are treated to vapor phase quaternization procedure.

This work has provided the basis for an ongoing research program aimed at establishing fabrication routes to inexpensive and environmentally low-impact materials for enabling wide-spread solar energy technology. Many of these initial results have presented by the PI and student participants at local and regional meetings. Additionally, two student researchers supported by ACS PRF 51559-UNI10 have recently gone on to pursue a graduate degrees in chemistry at Kent State University (Leah Bergquist) and University of Massachusetts – Amherst (Fumitoshi Kato).

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REFERENCES:

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3. Wadia, C.; Alivisatos, A. P.; Kammen, D. M., Materials Availability Expands the Opportunity for Large-Scale Photovoltaics Deployment. Environmental Science & Technology 2009, 43, 2072-2077.

4. Shao, S.; Liu, F.; Xie, Z.; Wand, L. High Efficiency Hybrid Polymer Solar Cells with Inorganic P- and N-type Semiconductor Nanocrystals to Collect Photogenerated Charges. J. Phys. Chem. C 2010, 114, 9161-9166.