Near surface defects: Cause of deficit between internal and external open‐circuit voltage in solar cells

Abstract

Interface recombination in a complex multilayered thin‐film solar structure causes a disparity between the internal open‐circuit voltage (VOC,in), measured by photoluminescence, and the external open‐circuit voltage (VOC,ex), that is, a VOC deficit. Aspirations to reach higher VOC,ex values require a comprehensive knowledge of the connection between VOC deficit and interface recombination. Here, a near‐surface defect model is developed for copper indium di‐selenide solar cells grown under Cu‐excess conditions. These cell show the typical signatures of interface recombination: a strong disparity between VOC,in and VOC,ex, and extrapolation of the temperature dependent q·VOC,ex to a value below the bandgap energy. Yet, these cells do not suffer from reduced interface bandgap or from Fermi‐level pinning. The model presented is based on experimental analysis of admittance and deep‐level transient spectroscopy, which show the signature of an acceptor defect. Numerical simulations using the near‐surface defects model show the signatures of interface recombination without the need for a reduced interface bandgap or Fermi‐level pinning. These findings demonstrate that the VOC,in measurements alone can be inconclusive and might conceal the information on interface recombination pathways, establishing the need for complementary techniques like temperature dependent current–voltage measurements to identify the cause of interface recombination in the devices.

Sulfide Chalcopyrite Solar Cells––Are They the Same as Selenides with a Wider Bandgap?

“…If the solar cell is dominated by bulk recombination, the opencircuit voltage will be similar to the quasi-Fermi-level splitting, measured by PL. [78,105] Thus, Figure 7 can give insights into Figure 10. Ratios of open-circuit voltage, short-circuit current, and fill factor of the best reported Cu(In,Ga)S 2 solar cells [17,[19][20][21]70] with respect to the corresponding Shockley-Queisser values.…”

“…[76,77] The defect was shown to reside in a thin layer (a few tens of nm thick) just below the absorber surface. [78] Modeling shows that such a layer results in the typical signatures of interface recombination: activation energy of the reverse saturation current lower than the bandgap and a V OC considerably smaller than the quasi-Fermi level splitting. [78] The question arises if a similar mechanism causes the interface recombination in Cu-rich sulfide chalcopyrites.…”

“…[78] Modeling shows that such a layer results in the typical signatures of interface recombination: activation energy of the reverse saturation current lower than the bandgap and a V OC considerably smaller than the quasi-Fermi level splitting. [78] The question arises if a similar mechanism causes the interface recombination in Cu-rich sulfide chalcopyrites. So far, it has not been possible to detect a corresponding defect signature in admittance spectroscopy on Cu-rich CuInS 2 or Cu(In,Ga)S 2 devices, because the spectra are dominated by the high doping level in Cu-rich absorbers.…”

“…[ 76,77 ] The defect was shown to reside in a thin layer (a few tens of nm thick) just below the absorber surface. [ 78 ] Modeling shows that such a layer results in the typical signatures of interface recombination: activation energy of the reverse saturation current lower than the bandgap and a V OC considerably smaller than the quasi‐Fermi level splitting. [ 78 ]…”

“…[ 95 ] 2) a significant difference between qFls and V OC also indicates severe interface recombination. [ 78 ] The difference is, therefore, labeled the interface V OC loss.…”