Cu(In, Ga)Se₂ microcells: High efficiency and low material consumption

Using solar cells under concentrated illumination is known to improve the conversion efficiency while diminishing the active area and thus material consumption. Recent concentrator cell designs tend to go miniaturized devices, in the 0.5-1 mm range, enabling a better thermal evacuation due to higher surface to volume ratio. If the cell size is further reduced to the micrometric range, spreading resistance losses can be made vanishingly small. This is particularly interesting for the thin film technology which has been limited up to now to very low concentration systems, from ×1 to ×10, due to excessive resistive losses in the window layer and difficult thermal management of the cells, grown on glass substrates. A new solar cell architecture, based on polycrystalline Cu(In,Ga)Se₂ (CIGS) absorber, is studied: microscale thin film solar cells. Due to the reduced lateral dimension of the microcells (5 to 500 μm in diameter), the resistive and thermal losses are drastically decreased, enabling the use of high concentration (>×100). This results in a breakthrough for concentration on this type of devices, which were previously limited to the low concentration range (about ×10). Due to light concentration, the open circuit voltage increases up to several thousand suns equivalent, to reach over 900 mV. The temperature increase is limited to less than 20 °C over the ambient at concentration around ×1000. A 5% absolute efficiency increase on microcells at ×475 is observed and a 21.3% ± 0.2% equivalent efficient microcell of 50 μm of diameter is measured.