Microscopic origins of performance losses in highly efficient Cu(In,Ga)Se2 thin-film solar cells

Abstract

Thin-film solar cells based on polycrystalline absorbers have reached very high conversion efficiencies of up to 23-25%. In order to elucidate the limiting factors that need to be overcome for even higher efficiency levels, it is essential to investigate microscopic origins of loss mechanisms in these devices. In the present work, a high efficiency (21% without anti-reflection coating) copper indium gallium diselenide (CIGSe) solar cell is characterized by means of a correlative microscopy approach and corroborated by means of photoluminescence spectroscopy. The values obtained by the experimental characterization are used as input parameters for two-dimensional device simulations, for which a real microstructure was used. It can be shown that electrostatic potential and lifetime fluctuations exhibit no substantial impact on the device performance. In contrast, nonradiative recombination at random grain boundaries can be identified as a significant loss mechanism for CIGSe solar cells, even for devices at a very high performance level.

Understanding Performance Limitations of Cu(In,Ga)Se2 Solar Cells due to Metastable Defects—A Route toward Higher Efficiencies

“…[ 1 ] With an efficiency of 23.35%, [ 2 ] Cu(In,Ga)(S,Se) 2 (CIGS) solar cells reach the highest efficiencies among the thin‐film technologies. [ 3 ] The efficiency for these solar cells is limited by nonradiative recombination, [ 4 ] in particular originating from grain boundary recombination, [ 5 ] which limits the open circuit voltage.…”

“…[1] With an efficiency of 23.35%, [2] Cu(In,Ga)(S,Se) 2 (CIGS) solar cells reach the highest efficiencies among the thin-film technologies. [3] The efficiency for these solar cells is limited by nonradiative recombination, [4] in particular originating from grain boundary recombination, [5] which limits the open circuit voltage.Another important parameter is the diode factor, which describes the voltage dependence of the diode current. [6] A higher diode factor reduces the fill factor (FF) of the current-voltage characteristics and therefore the efficiency of the solar cell.…”

Optoelectronic Inactivity of Dislocations in Cu(In,Ga)Se2 Thin Films

“…For SEM imaging, which uses a channeling contrast, it is known that dislocations can be resolved clearly (see the study by Zaefferer and Elhami [10] ); thus, we can assume a spatial resolution on the order of few 10 nm. As for the corresponding value of the CL signals, they can be estimated to about 100 nm (order of magnitude), from various CL analyses on grain boundaries in polycrystalline CIGS layers, [3,12] in which the contrasts of grain boundaries were clearly detectable, in spite of at times small grain diameters of down to about 100 nm. Although the diffusion of excited charge carriers prior to the radiative recombination has to be considered, the recombination rates of excited electrons and holes are rather high due to probable high-injection conditions during the CL experiment at 10 keV and 1 nA.…”

“…We face a different scenario at (randomly oriented) grain boundaries, where strain accumulates [24] and where despite the atomic reconstruction of grain-boundary planes [25,26] considerable recombination velocities of several 100 cm s À1 even for high-efficiency devices are measured. [3] Nevertheless, CIGS solar cells with very high conversion efficiencies can be obtained by increasing the average grain sizes, because dislocations-that might have higher densities in larger grains-do not exhibit effective centers of enhanced recombination.…”

“…[2] It was shown that also in highly efficient devices, enhanced nonradiative recombination at grain boundaries leads to losses in open-circuit voltage of 20-30 mV. [3] In contrast, the optoelectronic properties of dislocations in CIGS thin films have not yet been assessed unambiguously.…”