Technical Insight
Abolishing unwanted fields in nitride solar cells
Growth on a-plane sapphire enables nitride cells to deliver record-breaking short-circuit current densities
THE performance of conventional nitride solar cells are held back by piezoelectric fields that oppose built-in electric fields and hamper carrier ejection out of the active region. But it is possible to negate these piezoelectric fields and increase efficiency by making nitride photovoltaic on the non-polar a-plane, according to a Japanese research team from Meijo University and Nagoya University.
Wide bandgap non-polar devices have the potential to span a vast spectral range are yet to deliver high efficiencies – the team’s latest a-plane nitride cells convert just 1.6 percent of the sun’s radiation into electricity, compared to 2.9 percent for variants grown on the c-plane. But far higher efficiencies on non-polar planes will follow through improvements in crystal quality, which should be possible by switching the substrate from r-plane sapphire to bulk GaN.
Non-polar nitride solar cells were fabricated by placing an r-plane sapphire substrate into an MOCVD reactor, thermally cleaning its surface in a hydrogen atmosphere at 1100 °C and depositing a stack of nitride films: First a 150 nm layer of AlN and then a 500 nm film of Al0.5Ga0.5N, a 1.2 ìm-thick layer of undoped GaN, an n-doped 2.5 ìm layer of GaN, a superlattice active region and a 100 nm-thick, p-type GaN cap. The active region comprised 30 periods of 3 nm-thick Ga0.85In0.15N and 1 nm-thick GaN.
Reactive ion etching defined mesa areas in the substrate, and electron beam evaporation added n-type and p-type contacts to the devices, which have dimensions of 350 ìm by 350 ìm.
External quantum efficiency peaked at 62 percent at 400 nm. This corresponds to an internal quantum efficiency of 94 percent, assuming transmissivity of 67 percent for the Ni/Au electrode. The open-circuit voltage, short-circuit current density and fill factor for the cell were 0.9 V, 4.8 mA/cm2and 57 percent.
The team claims that its device sets a new benchmark for the short-circuit current density for a nitride cell. This record breaking current may stem from the longer wavelength of the absorption edge or the reduction in internal electric field.
Open circuit voltage is lower than what would be expected from the bandgap of the InGaN active layer. Team member
Hiroshi Amano says that this could be due to a defect-related leakage pass or a low shunt resistance. The non-polar device has a very high level of defects: Densities of threading dislocations and stacking faults are 1 x 1010 cm-2 and 1 x 105 cm-1, respectively. The team plans to address this by producing devices on a high-quality aplane GaN substrate, which should improve material quality and boost cell performance.
T. Nakao et. al. Appl. Phys Express 4
101001 (2011)
