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

Abstract

Thin‐film Cu(In,Ga)Se2 solar cells reach power conversion efficiencies exceeding 23% and nonradiative recombination in the bulk is reported to limit device performance. The diode factor has not received much attention, although it limits the fill factor, and therefore the efficiency, for state‐of‐the‐art solar cells. Herein, the diode factor of Cu(In,Ga)Se2 absorbers, measured by photoluminescence spectroscopy, and of solar cells, measured by current–voltage and capacitance–voltage characteristics, are compared, supported by simulations using rate equations of generation and recombination. It is found that the diode factor is already increased in the neutral zone of the absorber due to metastable defects, such as the VSe–VCu defect found in Cu(In,Ga)Se2, because of an increased net acceptor density upon minority‐carrier injection. The metastable and persistent increase of the net acceptor density has a detrimental effect on the device performance. Diode factors of 1 and efficiencies exceeding 24% are expected when, in current state‐of‐the‐art Cu(In,Ga)Se2 solar cells, the formation of metastable defects is suppressed.

Charge Carrier Lifetime Fluctuations and Performance Evaluation of Cu(In,Ga)Se2 Absorbers via Time‐Resolved‐Photoluminescence Microscopy

“…Optical ideality factors higher than one have been recently associated to metastable defects whose origin is still under investigation. [46] Therefore, despite that about 30% of the recombination is radiative, the voltage loss analysis and optical ideality factors suggest the material is still dominated by nonradiative recombination (in agreement with the results of Figure 4). Optimized back reflectors to maximize the reabsorption probability would provide negligible V OC benefits if applied to these structures.…”

Photoluminescence assessment of materials for solar cell absorbers

“…Even the maximum ll factor of which the absorber is capable can be determined by PL. 4,13,14 Another unavoidable loss is the fact that the absorption edge of a real material is not a step function, as assumed in the Shockley-Queisser model. As discussed in detail below, the gradual increase in absorptance in a real semiconductor causes difficulties in determining the band gap of the material.…”

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

“…The radiation flux R r is empirically found to follow a power law over many orders of generation flux (given by the illumination intensity) G, R r ∝G A . The exponent A is the ODF and is directly determined by the derivation of the logarithmic Radiation-Generation characteristic, which is: [8] d ln d ln…”

“…More details of the derivation are shown in Section S11 (Supporting Information). [8] For doped semiconductors, the ODF depends on changes in the doping density N A upon illumination, which can be explained by metastable defects. [1,8] If there were no metastable defect transitions, 0 A dlnN dlnG = , which gives A = 1 in low injection conditions, as expected.…”

“…Investigation of the absorber alone helps to disentangle the different effects.Recombination not only influences the V oc or ΔE F but also the fill factor of the solar cells via the diode factor, for which a small diode factor close to 1 is generally desirable. [8,9] For a sufficiently doped semiconductor in low injection conditions, 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.…”