Full-Spectrum Solar Cells
One of our efforts is to grow semiconductor alloy nanowires with composition graded spatially such that the bandgaps of these alloys vary over a large range across a single substrate. [1,2]. This unique material capability allows many unique applications be designed. One of such applications is solar cells. In a typical single junction solar cell with a given bandgap, there are two significant loss mechanisms that reduce the conversion efficiency of solar cells. The photons with energy smaller than the bandgap will be transmitted without converting to electrons. The photons with energy larger than the bandgap will lose the excess energy over bandgap to heat generation. Thus an ideal material for solar cells without subjecting to these two loss mechanisms would be the one that absorbs photons of any frequency and to collect the generated charges at the same generation energy without thermal relaxation. Since thermalization occurs on an ultrafast time scale, faster than the typical collection process, such a material would ideally need to have bandgaps covering the entire solar spectrum. Solar cells based on such ideal materials would be able to reach the highest theoretical efficiency limit. This is why efforts are being made to make more and more junctions in the multi-junction approach. But due to the same lattice mismatch problem discussed earlier, such multijunction cells with more than 3 junctions turn to be very difficult in the vertically-staggered design. In contrast to such vertical tandem cells, there is also a multijunction approach with different junctions (or cells) arranged side by side. This approach is often associated with the dispersive concentration photovoltaics (DCPV) [3,4]. The key difference is that broadband solar energy is spectrally split first before being absorbed by solar cells. In this way, photons of a given wavelength band are absorbed by the “right” bandgap, so that the transmission and thermalization losses are both minimized.
Currently the prevailing approach in the DCPV is to use dichroic mirrors or beam splitters to spectral split the solar spectrum into two or multiple bands and then direct these bands onto different cells . While holographic thin film technology  can replace the bulky optical elements for spectral splitting and concentrating, the absorption cells remain discrete and dissimilar cells such as separate GaAs and Si cells are used . One possible application of our spatially-composition graded materials is to be used as a monolithic absorption cell-layer . As shown in Fig.1, the composition graded alloy nanowires are grown on pre-patterned substrate with sub-cells defined based on the dispersive optical layer. The number of sub-cells and the specific division into different alloy composition ranges (bandgaps) can all be optimized for the given optical layer to achieve maximum efficiency. Different lateral subcells can then be separately contacted with currents and voltages collected individually. There are several advantages of using this approach. First the number of subcells can be much higher than one can achieve using the vertical multijunction approach. This would help to achieve higher efficiency. Second, the issue of current matching as in the vertical approach is non-existent. Instead, different subcells can be reconnected based on the IV characteristics of them. Third, this approach potentially allows low cost manufacturing, MLJ_Design1_redsince the nanowire growth is CVD based and the cost should be similar to the thin-film solar cell technology. We have performed extensive simulation and design study using this approach and currently in the process of fabricating simple devices to demonstrate this approach.
The similar approach can be also applied for multispectral detection in the infrared wavelength range, if narrow gap semiconductors are used for the alloy composition graded growth. Potentially spectrometer on a chip can be also envisioned using similar approach as depicted in Fig.1.
References A. Pan, W. Zhou, E. Leong, A. Chin, R. Liu, B. Zou, C. Z. Ning, Continuous Alloy-Composition Spatial Grading and Superbroad Wavelength-Tunable Nanowire Lasers on a Single Chip, Nano Letters, 9, 784 (2009).
 Pan, R.B. Liu, M.H. Sun and C.Z. Ning, Spatial composition grading of quaternary alloy ZnCdSSe nanowires with tunable light emission between 350 nm and 710 nm on a single substrate, ACS Nano, 4, 671-680(2010).  W.H. Bloss, M. Griesinger, and E.R. Reinhardt, Appl. Opt., 21, 1982, 3739.  J.E. Ludman, “Holographic solar concentrator”, Appl. Opt., 21, 1982, 3057.  A. Barnett et al., “Milestones toward 50% efficient solar cell modules”, 22nd European Photovoltaic Solar Energy Conference, Milan, Italy, 3 September 2007.  C.Z. Ning, A.L. Pan, and R.B. Liu, “Spatially composition-graded alloy semiconductor nanowires and wavelength specific lateral-multijunction full-spectrum solar cells,” in Proc. of the 2009 34th IEEE Photovoltaic Specialists Conference (PVSC 2009), 2009, p 001492-5.