SUMMARY

We report on a novel approach to synthesize hybrid nanostrucutres of meso porous silicon (PS) and conjugated organic polymers that are suitable for solar cell applications. Meso-PS substrates with relatively large pores have been exploited for electrochemical polymerization of organic monomers that were introduced into the PS matrices. We present electrical characteristics of a device having relatively thin films of the hybrid medium, which demonstrate a photovoltaic mode of operation of the device.

1. INTRODUCTION

Developing renewable energy sources that are secured for long term supply at affordable cost and do not depend on hydrocarbon fossil fuels, is one of the most challenging research topics of the 21st century[1-2]. Solar energy in general and particularly photovoltaic (PV) energy conversion is expected to play a major role in this area. Currently, the dominant PV technology is based on crystalline silicon with bulky silicon wafers used to create photovoltaic p-n diode-junctions [3]. However, silicon based solar cells suffer from a poor efficiency-to-cost ratio (usually defined in units of $ per kW/h) and there is an urgent need for alternative PV technologies with improved efficiency-to-cost ratio. One of the most promising approaches is using other photo-active compounds such as semiconductor quantum dots [4-6], metallic nanoparticles [7] and organic polymers or dye molecules for light harvesting [8-10]. In this case, due to the considerably larger light absorption coefficient of these compounds (relative to silicon), one needs a much thinner film for absorbing the solar energy. On the other hand, thin film compounds require interfacing with other semiconductors to create hybrid, thin film solar cells (sometimes called "third generation solar cells" [11]).A porous matrix, particularly a porous silicon matrix [12-15], seems to be an ideal matrix for this purpose. The ability to control the size of the pores, their morphology and the conductivity of the PS layers make PS a very promising candidate for PV applications.

Here, we report on the extension of our previous work on developing hybrid structures of meso-PS and conjugated polymers [16-17] for PV applications. We have chosen to use poly-vinyl-carbazole (PVK) due to its excellent photoconductivity [18-19] as a test medium that demonstrate the concepts of the device.

2. EXPERIMENTAL

To allow introducing long polymer chains into the porous matrix, we have prepared meso-PS matrices (average pore's diameter ~ 50 nm) using electrochemical solution containing a strong oxidizer [20]. The PS structure has typical columnar pores of about 3 m in depth. Next, electrochemical polymerization using the method of cyclic voltammetry (CV) has been applied to create hybrid matrices of PS-PVK. In addition, this method has been used to identify the reduction-oxidation states of the polymer. Similar to the results reported in [16-17] we were able to observe larger and larger PVK oxidation peaks with the increasing number of cycles until saturation has been achieved.

To demonstrate that PVK molecules infiltrated into the pores and uniformly fulfilled the pore walls with polymer, we have measured the EDX spectrum from the samples before and after polymerization. The spectra,  reveal considerable amounts of carbon and oxygen in the hybrid structure, which do not exist prior to polymerization. The carbon peak, which can be considered as a marker of the polymer, has been used for EDX mapping along the cross-section of the hybrid PS-polymer structure.

In order to create electrical junction between the polymer and the n-type meso-PS we have made the following. After completion of the CV polymerization stage we have performed an additional oxidation cycle, which has been terminated in the doped state of the polymer. This oxidized state of the polymer is known to be a good photoconductive state [21-22] with the conductivity being dominated by holes transport (in this respect, it is equivalent to a formation of a "p-type" semiconductor). Finally, top, 17 nm thin film gold electrodes, have been defined to the hybrid PS by evaporation while a standard Al backside electrode has been defined to the silicon substrate.

3. DISCUSSION

The current-voltage characteristics of the hybrid PS-PVK structure have been measured in the dark and under illumination. The dark current of the device is fairly low for the backwards bias polarity and rapidly increases in the forward bias polarity. Under illumination of an AM 1.5 solar simulator, a considerably large open circuit voltage appears, Voc  250 mV, demonstrating the photovoltaic effect in our device. The short circuit current density, defined as the current per unit area under illumination at a zero voltage, is about ~ 8.2 mA/cm^2. The total energy conversion efficiency of the device is about 0.75%.

4. CONCLUSIONS

In conclusion, we have demonstrated a novel approach to fabricate thin films of hybrid PS-PVK structures that can be exploited for PV applications. Preliminary results indicate the feasibility of our approach, yet the efficiency (~0.75%) of the device (having a fill factor of about 36%) is still too low for practical applications. However, future works where other types of polymers, PS media and nano-particles that can be incorporated into the matrix, are expected to considerably improve the performances of these devices.

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