Multi-junction concentrator solar cells incorporate tunnel diodes that undergo a transition from high-conductance tunneling to low-conductance thermal diffusion behavior, typically at threshold current densities of the order of 102-103 mA/mm2. We present experimental evidence of a prominent heretofore unrecognized dependence of threshold current density on the degree of localized irradiation, in different solar cell architectures. We also show that solar cells with non-uniform metallization can exhibit an additional spatial dependence to the tunnel diode threshold current density. These previously undiscovered phenomena - which should be observable in all non-uniformly irradiated photovoltaic tunnel diodes - are interpreted as being derived from the lateral spreading of excess majority carriers (analogous to current spreading in light-emitting diodes (LEDs)). The consequences for concentrator photovoltaics are addressed.
Bibliographical noteFunding Information:
J.M.G. expresses appreciation to SolFocus Inc. of Mountain View, CA for partial funding of this work. B.H. acknowledges Ronen Vaknin of the Israel Electric Corp. for assistance in preparing the 0.2 mm optical fibers. W.G. and A.W.B. gratefully acknowledge partial funding by the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) under the ProKonPV project (Contract 0327567). W.G. is supported by a scholarship from the Deutsche Bundesstiftung Umwelt (DBU).
- Tunnel diode