Diode Factor in Solar Cells with Metastable Defects and Back Contact Recombination

Abstract

To achieve a high fill factor, a small diode factor close to 1 is essential. The optical diode factor determined by photoluminescence is the diode factor from the neutral zone of the solar cell and thus a lower bound for the diode factor. Due to metastable defects transitions, the optical diode factor is higher than 1 even at low excitation. Here, the influence of the backside recombination and the doping level on the optical diode factor are studied. First, photoluminescence and solar cell capacitance simulator (SCAPS) simulations are used to determine the back surface recombination velocity of Cu(In, Ga)Se2 with various back contacts and different doping levels. Then, experimental results and simulations show that both back surface recombination and high doping density reduce the optical diode factor. The back surface recombination reduces the optical diode factor with undesirable extra nonradiative recombination. The smaller value achieved by higher doping can increase quasi‐Fermi level splitting at the same time. The simulations show that the back surface recombination reduces the optical diode factor due to an illumination‐dependent recombination rate. In addition, a higher majority carrier doping reduces the influence of majority carrier gain from metastable defect transitions, thus reducing the optical diode factor.

Photoluminescence assessment of materials for solar cell absorbers

“…The band gap determination is further hampered by band gap gradients along the depth of the sample, which are often intentional to reduce interface recombination. 27–31 Similar to tail states, these gradients broaden the absorption onset. If the absorptance spectrum is known, the radiative qFls rad can be determined, using eqn (3).…”

Organic Passivation of Deep Defects in Cu(In,Ga)Se 2 Film for Geometry-Simplified Compound Solar Cells

“…Such improvements in carrier lifetime allow us to roughly extract the quasi Fermi level splitting ( qFLs ) of the passivated CIGS absorber by using the method ( Eq. 1 ) recently reported by Siebentritt et al [ 48 – 50 ], …”

Chalcopyrite solar cells —state-of-the-art and options for improvement

“…We have recently shown quantitatively that a Ga gradient reduces the backside recombination as effectively as a dielectric layer [150]: both increase the quasi-Fermi level splitting by almost 50 meV for a mediocre absorber with a bulk lifetime of 40 ns. In a good absorber with a lifetime of 200 ns [151] the improvement increases to 80 meV [150]. These experiments and simulations were performed with rather thick absorbers of 3 μm thickness.…”