Combinatorial study of NaF addition in CIGSe films for high efficiency solar cells

Abstract

We report on a sodium fluoride (NaF) thickness variation study for the H2Se batch furnace selenization of sputtered Cu(In,Ga) films in a wide range of Cu(In,Ga) film compositions to form Cu(In,Ga)Se2 (CIGSe) films and solar cells. Literature review indicates lack of consensus on the mechanisms involved in Na altering CIGSe film properties. In this work, for sputtered and batch‐selenized CIGSe, NaF addition results in reduced gallium content and an increase in grain size for the top portion of the CIGSe film, as observed by scanning electron microscopy and secondary ion mass spectrometry. The addition of up to 20 nm of NaF resulted in an improvement in all relevant device parameters: open‐circuit voltage, short‐circuit current, and fill factor. The best results were found for 15 nm NaF addition, resulting in solar cells with 16.0% active‐area efficiency (without anti‐reflective coating) at open‐circuit voltage (VOC) of 674 mV. Copyright © 2013 John Wiley & Sons, Ltd.

Fulfilling the promise of the materials genome initiative with high-throughput experimental methodologies

“…Such HTE research has been performed for a range of solar cell absorber materials, from quite mature (e.g., Cu(In,Ga)Se 2 , 40 ). For some of these absorbers, the PV efficiency was correlated with quasi-Fermi level splitting determined from photoluminescence mapping.…”

Ultrafast pump-probe reflectance spectroscopy: Why sodium makes Cu(In,Ga)Se2 solar cells better

“…Selenisation was performed at 250 °C for 60 min followed by 1-5 min at 500 °C, giving the composition range of Cu/(In+Ga)=0.8-1 and Ga/(In+Ga)=0.30-0.45. All detailed preparations and characterizations of CIGS layers have been previously reported [11]. Eid et al [11] demonstrated that the thickness of added NaF to the CIGS layers changes the efficiency of solar cells: increasing NaF thickness from 0 to 15 nm increased solar cell efficiencies from <10% to 16.0% (without antireflective coatings), but increasing NaF thickness from 15 to 30 nm decreased solar cell efficiencies.…”

“…All detailed preparations and characterizations of CIGS layers have been previously reported [11]. Eid et al [11] demonstrated that the thickness of added NaF to the CIGS layers changes the efficiency of solar cells: increasing NaF thickness from 0 to 15 nm increased solar cell efficiencies from <10% to 16.0% (without antireflective coatings), but increasing NaF thickness from 15 to 30 nm decreased solar cell efficiencies. Here, CIGS layers grown with 0-, 10-, 15-, 20-, or 30-nm NaF (CIGS-0NaF, CIGS-10nmNaF, CIGS-15nmNaF, CIGS-20nmNaF, CIGS-30nmNaF, respectively) were chosen in our time-resolved spectroscopic study.…”

“…This is predicted to result from increased net carrier concentration, which is typically one order of magnitude due to either an increased p-type doping concentration (potentially by a reduction of compensating donors) and/or reduced recombination. Several mechanisms have been proposed on the electronic impact of Na via both grain interior and grain boundaries [10][11][12][13][14][15][16].…”

“…In light of the important role that Na plays in CIGS layers, several groups have examined the growth and performance of CIGS layers by systematically varying Na content [9][10][11][12]. Typically, exposure to Na during growth of CIGS layers has been observed to change interdiffusion of Cu, In, and Ga [9,12] to modify grain size and orientation [9][10][11], which may result in a redistribution of oxygen [9,12]. Solar cells grown in optimal Na concentrations are typically more efficient because they have a higher open-circuit voltage (V OC ) and a higher fill factor (FF) [12,13].…”