10.1117/2.1201107.003803 A novel nanowire photovoltaic architecture Cun-Zheng Ning and Derek Caselli Simulation and design of a new solar cell, which can potentially be fabricated at low cost, demonstrate its ability to achieve efficiencies competitive with the best cells on the market. In recent years, concern over global climate change, rising gaso- line prices, the British Petroleum oil spill in the Gulf of Mexico, and the disaster at the Fukushima nuclear reactor in Japan have intensified interest in renewable energy technologies. Solar cells are particularly attractive because they are clean, safe, and easily integrated into urban environments. However, the high cost of solar electricity continues to limit its widespread use. Increasing the efficiency of a solar cell decreases the cost of the electricity it generates if the price of the cell remains fixed. However, in practice, highly efficient solar cells—called tandems because they act like three solar cells in one, each absorbing different components of the solar spectrum—are prohibitively expensive. This is because stacking three different solar-cell materials on top of each other requires a complex fabrication process and materials that meet demanding specifications. Our aim is to create a new type of solar cell that can achieve the high efficiencies of tandems without the high cost. Our approach 1 is to put the different solar cell materials side- by-side, rather than in a vertical stack, and split the sun’s light so that each photon is absorbed by the most suitable cell. We call this type of device a laterally arranged multiple-bandgap (LAMB) solar cell. Even though this concept is nearly 30 years old, 2 and inexpensive dispersive optics have been available for some time, the key missing element has been the ability to produce absorption materials for such lateral cells on a single platform. Past attempts to implement this concept had to use discrete solar cells fabricated on separate substrates, but this ap- proach is prohibitively complex and expensive for most appli- cations. Recent progress in material technology in our group 3, 4 and elsewhere 5 has allowed inexpensive growth of such absorp- tion materials, which could enable relatively simple, low-cost fabrication of a LAMB cell on a single substrate. Figure 1. Schematic design of a laterally arranged multiple-bandgap (LAMB) solar cell with six subcells consisting of intrinsic CdPbS (cadmium lead sulfide) nanowires and doped n- and p-type contact layers: doped zinc sulfide, n-ZnS; doped zinc telluride, n-ZnTe; and doped germanium, p-Ge. Ag: Silver. Au: Gold. Cu: Copper. ITO: Indium tin oxide. By continuously varying the composition of semiconductor alloy nanowires over the surface of a substrate, we have demon- strated the ability to vary the band gap energy—the minimum energy photons must have to be usefully absorbed—over a wide range with only a single growth step. 3, 4 This is important because the band gap also determines the maximum amount of energy we can extract from a photon, so photons with energy near this value are absorbed the most efficiently. 6 Like tandem cells, ours use different band gaps to absorb different parts of the solar spectrum. But unlike tandems, which effectively split the solar spectrum by sequentially absorbing longer wavelengths with smaller band gap cells from top to bottom, in our devices this task is performed with an optical element such as a holo- graphic dispersive concentrator. 7, 8 We have designed 1 a LAMB solar cell (see Figure 1) for use with CdPbS (cadmium lead sulfide) nanowires, which serve as the absorbing materials for six separate subcells. Each subcell consists of intrinsic CdPbS nanowires with different Cd/Pb ra- tios to produce different band gaps, and heavily doped n- and Continued on next page