Quantifying Large Lattice Relaxations in Photovoltaic Devices

Abstract

Temporal variations of Cu(In,Ga)Se2 photovoltaic device properties during light exposure at various temperatures and voltage biases for times up to 100 h were analyzed using the kinetic theory of large lattice relaxations. Open-circuit voltage and p-type doping increased with charge injection and decreased with temperature at low injection conditions. Lattice relaxation can account for both trends and activation energies extracted from the data were approximately 0.9 and 1.2 eV for devices with lower and higher sodium content, respectively. In these devices, increased sodium content resulted in higher initial p-type doping with greater stability. First principles calculations providing revised activation energies for the (VSe − VCu) complex suggest that this defect does not account for the metastability observed here.

Behavior of Na and RbF‐Treated CdS/Cu(In,Ga)Se2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

“…One exception is our recent work which indicated a metastable increase in doping and V OC with forward‐voltage bias heat–light stress and losses in doping and V OC with V = 0 stress. [ 47 ]…”

“…[ 13 ] Other mechanisms that have been used to explain metastability in CIGS solar cells include the following: a) lattice relaxation around the ( V Se – V Cu ) divacancy defect complex, which has also been associated with persistent photoconductivity (PPC) of bulk CIGS material; [ 36–40 ] b) fast diffusion of Cu as interstitial Cu i + ions leads to increase in V oc upon forward bias soaking of the CIGS devices; [ 41,42 ] c) formation of In Cu DX‐center defects [ 43,44 ] which has been reported to have metastable shallow and deep donor configurations; d) photodoping of CdS or ZnS buffers; and [ 45,46 ] e) large lattice relaxation in the bulk CIGS driven by charge injection, similar to the ( V Se – V Cu ) mechanism, but with much longer characteristic times. [ 47 ] Although these studies have explored the effect of light soaking on Na‐PDT, RbF‐PDT, and CsF‐PDT devices, the combined effect of junction bias and light soaking on the metastability in CIGS solar cells remains relatively unexplored. One exception is our recent work which indicated a metastable increase in doping and V OC with forward‐voltage bias heat–light stress and losses in doping and V OC with V = 0 stress.…”

“…Low‐Na devices showed the largest decrease in V OC and significant increase in depletion widths. The observed variations in acceptor density with OC and SC stress may be due to large lattice relaxation mechanisms [ 47 ] driven by charge injection and/or drift‐diffusion of ions under the different electric field conditions. Devices treated with RbF (Type 4) exhibited stable V OC under SC AST, and are robust against losses in acceptors due to an additional negative charge near CIGS/CdS interface.…”

Comparative Study of Al2O3 and HfO2 for Surface Passivation of Cu(In,Ga)Se2 Thin Films: An Innovative Al2O3/HfO2 Multistack Design

“…in which V m ¼ φ m /q, with φ m the work function of the silver contact considered equal to 4.5 eV in this work [21] ; χ CIGS and E g,CIGS are, respectively, the electronic affinity and bandgap of CIGS, equal to 4.36 and 1.18 eV [22] ; N A is the density of shallow acceptor dopants in the CIGS layer to be determined with CV measurements; n i is the intrinsic density of carriers in the CIGS layer and equal to 1.4 Â 10 10 cm À3 [22] ; kT/q is the thermal energy. In a real case, the charged defects induce a voltage shift of the CV curves.…”

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

“…Furthermore, Nordone et al. suggested that the activation energies for lattice relaxiation are sensitive to alkaline metal concentration and heating temperature …”