The largest single loss of energy in a single-junction silicon solar cell is the relaxation of photoexcited “hot carriers” to the band edges. This accounts for a loss of over 30% of the incident photon energy. In principle, this energy may be captured using a single-junction device based on carrier multiplication. In such devices, a single photon generates more than one carrier resulting in a quantum yield greater than 100%. Theoretical calculations based on a detailed balance shows that the limiting efficiency of these devices is over 44% [1], exceeding the Shockley-Queisser limit. Carrier multiplication has been observed experimentally in PbSe quantum dots and other semiconductor nanocrystals by transient absorption spectroscopy [2]. However, no photovoltaic devices have yet been developed that utilize this new physical phenomenon since the quantum dots are inherently 0D – once generated, the carriers have no way out of the quantum dot. Here, we present the development of PbSe nanowire array films. These unique nanomaterials are fabricated using highly ordered nanoporous films as a nanowire template. A key element in this fabrication method is our recent development [3] of self-assembled nanoporous films with suitably small diameter pores that provide direct access an underlying electrode through the nanopore. The nanoporous films are highly oriented and ordered. Grazing incidence x-ray scattering (GISAXS) results in 96 diffraction peaks that can all be indexed with a 211 oriented film that exhibits systematic extinctions based on Ia-3d symmetry. Comparison on observed and simulated transmission electron microscopy (TEM) images show that the pore topology is based on the zero mean curvature G-surface (the tricontinuous “double-gyroid” topology) with a pore diameter of approximately 4 nm. These nanoporous film coated electrodes have been filled by electrochemical deposition of PbSe with high fidelity. The pore fill fractions are over 80%, and the 700 nm thick PbSe nanowire arrays are opaque, black and uniform in appearance. GISAXS and TEM images of the PbSe/SiO2 hybrids and of the free standing PbSe nanowire arrays show that that the PbSe has the topology of the inverse double-gyroid – 4 nm diameter PbSe nanowires interconnected with Ia-3d symmetry [4]. These diameters are small enough to exhibit quantum size effects and yet are interconnected to allow carrier extraction. The presentation will focus on the fabrication method, spectroscopy results investigating carrier multiplication, and perhaps preliminary solar cell measurements.

References:

[1] Hanna MC & Nozik AJ, “Solar conversion efficiency of photovoltaic and photocatalytic cells with carrier multiplication absorbers,” J. Appl. Phys. 100 (7), Art. No. 074510 (2006).

[2] Schaller RD & Klimov VI, “High efficiency carrier multiplication in PbSe nanocrystals: Implications for solar energy conversion,” Phys. Rev. Lett. 92 (18): Art. No. 186601 (2004).

[3] Urade VN, Wei TC, Tate MP, Kowalski JD, & Hillhouse HW, “Nanofabrication of double-gyroid thin films,” Chem. Mater. 19 (4), 768 (2007).

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