research review
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 Al0.5
Ga0.5N, a 1.2 µm-thick layer of
undoped GaN, an n-doped 2.5 µm 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 Ga0.85
In0.15 N 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 µm by 350 µm.
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/cm2 and 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
The team plans to address this by producing devices on a high-quality a- plane GaN substrate, which should improve material quality and boost cell performance.
T. Nakao et. al. Appl. Phys Express 4 101001 (2011)
, respectively.
52
www.compoundsemiconductor.net November/December 2011
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146 |
Page 147 |
Page 148 |
Page 149 |
Page 150 |
Page 151 |
Page 152 |
Page 153 |
Page 154 |
Page 155 |
Page 156 |
Page 157 |
Page 158 |
Page 159 |
Page 160 |
Page 161 |
Page 162 |
Page 163 |
Page 164 |
Page 165 |
Page 166 |
Page 167 |
Page 168 |
Page 169 |
Page 170 |
Page 171 |
Page 172 |
Page 173 |
Page 174 |
Page 175 |
Page 176 |
Page 177 |
Page 178 |
Page 179 |
Page 180 |
Page 181 |
Page 182 |
Page 183 |
Page 184 |
Page 185 |
Page 186 |
Page 187 |
Page 188 |
Page 189 |
Page 190 |
Page 191 |
Page 192 |
Page 193 |
Page 194 |
Page 195 |
Page 196 |
Page 197 |
Page 198 |
Page 199 |
Page 200 |
Page 201 |
Page 202 |
Page 203 |
Page 204 |
Page 205 |
Page 206 |
Page 207 |
Page 208 |
Page 209 |
Page 210 |
Page 211 |
Page 212 |
Page 213 |
Page 214 |
Page 215 |
Page 216 |
Page 217 |
Page 218 |
Page 219 |
Page 220 |
Page 221 |
Page 222 |
Page 223 |
Page 224 |
Page 225 |
Page 226 |
Page 227 |
Page 228 |
Page 229 |
Page 230 |
Page 231 |
Page 232 |
Page 233 |
Page 234 |
Page 235 |
Page 236 |
Page 237 |
Page 238 |
Page 239 |
Page 240 |
Page 241 |
Page 242 |
Page 243 |
Page 244 |
Page 245 |
Page 246 |
Page 247 |
Page 248 |
Page 249 |
Page 250 |
Page 251 |
Page 252 |
Page 253 |
Page 254 |
Page 255