Jess Kaneshiro

Improved current collection in WO 3 :Mo/WO 3 bilayer photoelectrodes

We report on the incorporation of molybdenum into tungsten oxide by co-sputtering and its effect on solar-powered photoelectrochemical (PEC) water splitting. Our study shows that Mo incorporation in the bulk of the film (WO 3 :Mo) results in poor PEC performance when compared with pure WO 3 , most likely due to defects that trap photo-generated charge carriers. However, when a WO 3 :Mo/WO 3 bilayer electrode is used, a 20% increase of the photocurrent density at 1.6 V versus saturated calomel reference electrode is observed compared with pure WO 3 . Morphological and microstructural analysis of the WO 3 :Mo/WO 3 bilayer structure reveals that it is formed by coherent growth of the WO 3 :Mo top layer on the WO 3 bottom layer. This effect allows an optimization of the electronic surface structure of the electrode while maintaining good crystallographic properties in the bulk.

Measurement of the sodium concentration in CIGS solar cells via laser induced breakdown spectroscopy

The control of sodium in CIGS solar cells is critical to achieve high efficiency devices, but to date composition measurement techniques either cannot detect the sub-one atomic percent levels (for example, x-ray fluorescence (XRF)) or are expensive, time consuming, and destructive (examples include SIMS, XPS/ESCA). We employed an inexpensive, fast, and minimally destructive method to measure the concentration of sodium in CIGS solar cells fabricated at the Hawaii Natural Energy Institute (HNEI). Laser induced breakdown spectroscopy (LIBS) was used to determine the relative concentration of sodium. Two different analysis methods of the LIBS data were explored: the first assumes local thermal equilibrium (LTE) of the plasma and is calibration-free while the second employs comparison of relative peak heights after calibration to determine the concentration. Analysis is presented for solar cells produced on thin titanium foils where sodium fluoride is included in the deposition process to incorporate sodium into the CIGS layer.

Copper-silver chalcporyites as top cell absorbers in tandem photovoltaic and hybrid photovoltaic/photoelectrochemical devices

Bandgap tuneability with copper chalcopyrites offers convenient opportunities to use this material in tandem solar cells. While single-junction photovoltaic cells have performed remarkably well, the use of tandem cells can more efficiently utilize the solar spectrum by converting different portions of it with multiple cells, resulting in devices with higher voltages and possibly greater efficiencies. While it might be expected to be analogous to lower-bandgap materials, higher-bandgap copper chalcopyrites (like CuGaSe2 where Eg∼1.65eV) do not form efficient junctions with back contacts and buffer/window layers due to band edge misalignments and material differences (like lattice mismatching). The addition of silver to replace part or all of the copper in chalcopyrites may offer an avenue of improvement providing higher bandgaps, more favorable band edge alignments, and better junctions with back contacts and buffer/window layers in tandem PV devices. The different band edge positions resulting from the addition of silver also offers itself well to tandem hybrid photovoltaic/photoelectrochemical cells where the top photoelectrochemical cell drives a chemical reaction like splitting water into hydrogen and oxygen gases.

Low-temperature indium molybdenum oxide as a window layer in CIGS photovoltaic devices

High mobility transparent conductive oxides (HMTCO) have the distinguishing property of a higher near infrared (NIR) transmittance when compared with ordinary transparent conducting oxides (TCO). The use of HMTCO as a window layer in CIGS PV cells is expected to increase device efficiency by extending the spectral response and increasing photocurrent. In this work, low temperature sputter-deposited IMO (LTIMO) has been developed with resistivitiy of 10−3 Ω-cm, and an average optical transmittance of nearly 80% over the visible range. An investigation into the use of this LTIMO as a window layer in CIGS solar cell applications is presented. Results from a comparative study of CIGS PV devices shows that incorporating indium-tin-oxide (ITO) versus LTIMO yields modest improvements in photovoltaic conversion efficiency attributable to the benefits of LTIMO. Follow-on investigations are currently underway to further optimize the LTIMO sputtering process for additional PV device improvements.