Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics

Abstract

Silicon nanowires (NWs) could enable low-cost and efficient photovoltaics, though their performance has been limited by nonideal electrical characteristics and an inability to tune absorption properties. We overcome these limitations through controlled synthesis of a series of polymorphic core/multishell NWs with highly crystalline, hexagonally-faceted shells, and well-defined coaxial p-type∕ n-type (p∕n) and p∕intrinsic∕n (p∕i∕n) diode junctions. Designed 200-300 nm diameter p∕i∕n NW diodes exhibit ultralow leakage currents of approximately 1 fA, and open-circuit voltages and fillfactors up to 0.5 V and 73%, respectively, under one-sun illumination. Single-NW wavelength-dependent photocurrent measurements reveal size-tunable optical resonances, external quantum efficiencies greater than unity, and current densities double those for silicon films of comparable thickness. In addition, finite-difference-time-domain simulations for the measured NW structures agree quantitatively with the photocurrent measurements, and demonstrate that the optical resonances are due to Fabry-Perot and whispering-gallery cavity modes supported in the high-quality faceted nanostructures. Synthetically optimized NW devices achieve current densities of 17 mA∕cm 2 and power-conversion efficiencies of 6%. Horizontal integration of multiple NWs demonstrates linear scaling of the absolute photocurrent with number of NWs, as well as retention of the high open-circuit voltages and short-circuit current densities measured for single NW devices. Notably, assembly of 2 NW elements into vertical stacks yields short-circuit current densities of 25 mA∕cm 2 with a backside reflector, and simulations further show that such stacking represents an attractive approach for further enhancing performance with projected efficiencies of >15% for 1.2 μm thick 5 NW stacks.nanodevices | nanomaterials | nanophotonics | optical nanocavities | solar cells N anostructures and nanostructured materials may enable next-generation solar cells by providing for efficient charge separation (1-16) and tunable optical absorption (11,(17)(18)(19). Semiconductor nanowires (NWs) have exhibited promising efficiencies as single NW photovoltaic elements (8)(9)(10)(11)(20)(21)(22) and as vertical arrays configured as photovoltaic (23-26) and photoelectrochemical (27, 28) solar cells, where the vertical array has been used to enhance light absorption (29). In the case of Si-based nanostructures, where Si photovoltaics represent benchmark systems with attractive material abundance and cost (30), the efficiency of NW devices has typically been limited by poor electrical performance and lack of tunable control of absorption properties at specific and broadband wavelengths. For example, previous reports of coaxial (8) and axially modulated (9) p-i-n single-NW photovoltaic devices yielded open-circuit voltages (V OC ) below 0.29 V, and, for coaxial devices, large leakage currents >1 pA. Furthermore, to accurately identify potentially unique absorption modes through photocurrent spectra ...

Grain size tuning of nanostructured Cu2O films through vapour phase supersaturation control and their characterization for practical applications

“…3 shows the fitting of eqn. (2) to the experimental data points which confirm that the grain size of nanostructured Cu 2 O films could be tuned through vapour phase supersaturation control and the quantity (p/m) can replace the supersaturation (C/C 0 ) in equation (1). The HRTEM image in Fig.…”

“…Nanostructured semiconductor films composed of thin layers of nanostructured objects such as nanoparticles, nanorods, nanowires or nanoporous networks with variable grain size and controlled porosity can exhibit properties superior to conventional thin films due to the deliberate engineering of nanoscale features into their structure and these films have practical applications in areas such as photovoltaics, electrochemical sensors, photoelectrochemical water splitting, biosensors and as antireflection layers. [1][2][3][4][5] Techniques such as thermal evaporation, 6 pulsed laser deposition, 7 DC and RF sputtering, 8,9 plasma evaporatihon, 10 sol-gel technique, 11 molecular beam epitaxy, 12 electrodeposition 13 and chemical vapor deposition 14 have been employed for the synthesis of nanostructured thin films of different semiconductors. Another technologically logical strategy for creating nanostructured films is the alignment of nanoparticle building blocks into ordered superstructures with non-resistive inter-particle contacts by bottom-up approaches and is one of the recent key topics in the area of nanomaterials.…”

Single-nanowire solar cells beyond the Shockley–Queisser limit

“…3a (see Supplementary Section S1 for more details). Lying nanowires exhibit EQE values up to 2 due to Mie resonances 37 , while for standing nanowires values of up to 14.5 are reached. This further confirms that the absorption cross-section is several times larger than the apparent crosssection of the wire, especially at wavelengths close to the bandgap.…”

“…For low-absorbing materials (for example, indirect bandgap materials such as silicon), waveguiding effects plays a key role 23,24 , whereas highly absorbing semiconductors (such as direct-bandgap GaAs) exhibit resonances that increase the total absorption several times. Nanowires lying on a substrate also exhibit such resonances, often described by Mie theory 25,26 , although the total absorption rate is significantly lower 27,28 . Even though the optical absorption of nanowires arranged in an array has been shown to be far more complex than in thin films, nanowire vertical arrays currently seem to be the most reasonable device proposal.…”

All-back-contact ultra-thin silicon nanocone solar cells with 13.7% power conversion efficiency

“…The conventional surface texturing with alkaline or acidic solution for sub-10-mmthick Si substrates requires additional masking steps including photolithography 15 , and it is hard to implement on thin substrates with high yield 16 . In the past several years, significant effort has been focused on enhancing the light absorption by nanoscale light trapping using nanowires 8,[17][18][19] , nanocones [20][21][22] , nanodomes 7 and nanoholes [23][24][25][26] . Despite the exciting success in light trapping, the power conversion efficiencies of nanostructured Si solar cells, however, remain below 19% for thick devices 26 and below 11% for thin devices 27 .…”