Lock-in Thermography - Special Investigation Techniques Otwin Breitenstein Max Planck Institute for Microstructure Physics, Halle, Germany
Outline 1. Introduction 2. The „Local I - V“ method 3. DLIT- versus ILIT-based local efficiency analysis 4. ILIT- and DLIT-based J sc Imaging 5. A new DLIT method for depth-dependent investigations 6. Conclusions efficiency(1 sun) 12 to 17 % DLIT-based 0 ° front-minus- back difference ILIT-based DLIT image 2 cm 10 mm 2
1. Introduction • Dark lock-in thermography (DLIT) is the technique of choice for shunt imaging, but LIT can do much more on solar cells. • DLIT results can easily be quantified in terms of local current densities, which is used in the „Local I - V“ method providing local efficiency analysis. • The locally contributing efficiency in a cell can alternatively be imaged directly under realistic conditions by ILIT. • The local short circuit current density J sc , which is important for local efficiency analysis, can be imaged both by DLIT and ILIT. • Recently a new method has been developed for distinguishing heat sources in different depths in a solar cell. • In this talk all these special methods will be introduced. 3
2. The „Local I - V“ method • Voltage-dependent DLIT signal evaluation (images for 3 forward biases + one reverse bias), each pixel is fitted to a local 2-diode model 1 . V R J ( V ) V R J ( V ) V R J ( V ) s s s ( ) exp 1 exp 1 J V J J J 01 02 sc n V n V R 1 T 2 T p R s V V d J sc • This method is based on the model of independent diodes. J 02 R p J 01 n 2 • Result of the ‘Local I -V ’ procedure: images of local diode parameters J 01 , J 02 , n 2 , and G p = 1/ R p . n 1 is assumed to be homogeneous, but can be > 1. • Local series resistance or local diode voltage must be known, e.g. from PL / EL analysis. • After the local diode parameters are known, the software calculates images of the local cell parameters ( V oc , J sc , FF, h , n eff , V oc;mpp , V d ( V oc;mpp ), J d ( V oc;mpp ) ...) 2 . potential • This software is commercially available 3 . in-circuit (expectation) values values 1 O. Breitenstein, Solar En. Mat. & Solar Cells 95 (2011) 2933 2 O. Breitenstein, Solar En. Mat. & Solar Cells 107 (2012) 381 3 www.maxplanckinnovation.com 4
2. The „Local I - V“ method Input images 0.6 V, 0 to 5 mK 0.5 V, 0 to 0.5 mK 0.55 V, 0 to 1 mK -1 V, 0 to 5 mK max B D D A C 2 cm min RESI- R s , 0 to 3 W cm 2 The dark spots in EL (0.6V), a.u. V d (0.6V), 0.575 to 0.6 V the RESI- R s image [1] in defect positions are a natural result of the assumed model of indepen- dent diodes [1] K. Ramspeck et al., APL 90 (2007) 153502 5
2. The „Local I - V“ method Dark current data, local characteristics J rec (0.6 V) log( J 02 ), -8 to -2 n 2 , 0 to 10 J 01 , 0 to 3*10 -12 A/cm 2 0 to 20 mA/cm 2 B B D D A A C C 2 cm max 35 35 EL, 600 mV region A 2 ] 2 ] 30 30 • A: J 02 -shunt current density [mA/cm current density [mA/cm region B 25 25 J diff J diff 20 J rec 20 J rec J sum 15 J sum 15 • B: J 01 -shunt J illum J illum 10 10 DLIT DLIT 5 5 0 0 • C: good 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 voltage [V] voltage [V] 35 35 region region D 2 ] 30 2 ] current density [mA/cm 30 current density [mA/cm region C J diff 25 25 J diff min J rec 20 • D: ohmic 20 J rec J shunt • Only J 01 correlates with 15 15 J sum J total shunt 10 J illum 10 J illum crystal defects (EL 5 DLIT J meas 5 image), not J rec 0 0 6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 voltage [V] voltage [V]
2. The „Local I - V“ method Local efficiency parameter potential data V oc (1 sun) V oc (0.2 sun) FF(1 sun), 65 to 85 % efficiency(1 sun) 0.55 to 0.65 V 0.51 to 0.61 V 12 to 17 % D A B C • Region A ( J 02 shunt): influences mostly FF und V oc (0.2 suns) max • Region B ( J 01 -Shunt): influences mostly V oc • Region C (good region): best efficiency parameters • Region D (ohmic shunt): influences mostly FF and V oc (0.2 suns) • R s inhomogeneities: influence only FF min 7 O. Breitenstein, Solar En. Mat. & Solar Cells 107 (2012) 381
2. The „Local I - V“ method Global characteristics dark characteristic illuminated characteristic 2 ] current density [mA/cm 10 simulated 2 ] current density [mA/cm 30 measured DLIT 1 mpp 28 simulated measured 0.1 26 0.01 24 0.1 0.2 0.3 0.4 0.5 0.6 0.46 0.48 0.50 0.52 0.54 0.56 0.58 voltage [V] voltage [V] intensity-dependent efficiency measured versus simulated global cell parameters 16 Producer 850 nm simulated simulated 14 data measured whole cell best region efficiency [%] 12 J sc [mA/cm 2 ] 31.8 31.8 31.8 31.8 10 V oc [mV ] 625 624 625 632 region A 8 region B FF [%] 76.5 77.6 78.0 81.6 6 region C h [%] region D 15.2 15.4 15.5 16.4 4 cell 2 0.2 0.4 0.6 0.8 1.0 intensity [suns] 8 O. Breitenstein, Solar En. Mat. & Solar Cells 107 (2012) 381
2. The „Local I - V“ method Demonstration of the „cut shunt“ option T rev (-5 ... 50 mK) efficiency pot. (0 ... 22 %) illuminated I-V (1 sun) • Shunted cell (SiC) • V oc : 607mV FF: 74.8% h : 15.6% • All strong ohmic shunts cut • V oc : 609mV FF: 78.1% h : 16.4% 9
2. The „Local I - V“ method • This available DLIT evaluation method allows to perform a quantitative local efficiency analysis of solar cells. • The obtained local efficiency parameter images allow to judge about the influence of different defect types on solar cell parameters. • The possibility to evaluate selected regions (e.g. cell without edge, best region of a cell) gives quantitative information on the influence of certain regions on the efficiency (see talk: „The role of inhomogeneities ...“). • The „cut shunt“ option allows to virtually cut out shunts and replace their properties by that of the surrounding. This allows to measure the influence of single shunts or other defect regions on the efficiency. • The „Local I - V 2“ software is used already in various PV labs (Fraunhofer ISE, Fraunhofer CSP, SolarWorld, Hanwha Q-Cells, NREL, RWTH Aachen), UNSW is still missing ;-). 10
3. DLIT- versus ILIT-based local efficiency analysis • In „Local I - V“, the operation of a solar cell is simulated, based on dark current measurements and the two-diode model (superposition principle). • Already in 2008 K. Ramspeck et al. [1] (ISFH) have proposed an illuminated lock-in thermography (ILIT-) based method for imaging the locally contributing (in-circuit) efficiency. • This measurement is performed under realistic illuminated mpp condition and does not assume any solar cell model. • This method was originally restricted to measuring the internal (reflection- corrected, irradiation intensity-independent) monochromatic efficiency. • We have extended this method to measuring also external and AM 1.5 efficiencies. [2] [1] K. Ramspeck et al., J. Mater. Sci: Mater. Electron. 19 (2008) S4-S8 [2] F. Frühauf and O. Breitenstein, SOLMAT 169 (2017) 195-202 11
3. DLIT- versus ILIT-based local efficiency analysis Efficiency potential versus in-circuit efficiency • It is assumed that each pixel is • It describes the locally contributing cell . electrically isolated from its efficiency, if the cell is at its V mpp cell surrounding and works at its 𝜃 ic (𝑦, 𝑧) = 𝐾 𝑦, 𝑧 𝑊 mpp cell ) 𝑊 oc,ic = 𝑊 d (𝑊 oc individual (local) mpp. 𝑞 ill • The in-circuit V oc,ic is the local diode • This definition needs a solar cell cell . voltage, if the cell is at its V oc model (e.g. one- or two-diode). • In inhomogeneous cells, local • The local efficiency potential is variations of the V oc -potential are always positive. always larger than that of V oc,ic • The efficiency potential parameters • These in-circuit definitions needs no ( V oc , FF, h ) in position ( x,y ) mean cell model. that an extended cell showing the parameters of position ( x,y ) would • The in-circuit efficiency may have these parameters. become negative in shunt positions 12
3. DLIT- versus ILIT-based local efficiency analysis • Basic ideas of the Ramspeck ILIT method: At J sc condition the complete locally irradiated power is internally converted into heat (DLIT( J sc ) ~ p ill ). • At mpp some fraction of the irradiated power is converted into electric energy, the local heating becomes correspondingly lower. 𝜃 ic,ext = p el 𝑞 el = 𝐷[𝐽𝑀𝐽𝑈 𝐾 sc − 𝐽𝑀𝐽𝑈(mpp) ] 𝑞 rad • One can get rid of the proportionality factor C by defining the internal (in- circuit) efficiency: [1] 𝜃 𝑗𝑑,int = 𝐷[𝐽𝑀𝐽𝑈 𝐾 sc − 𝐽𝑀𝐽𝑈(mpp) ] = 𝐽𝑀𝐽𝑈 𝐾 sc − 𝐽𝑀𝐽𝑈(mpp) 𝑞 abs = 𝐷 < 𝐽𝑀𝐽𝑈 sc > 𝑞 abs 𝐽𝑀𝐽𝑈 𝐾 sc • This magnitude refers to the irradiated wavelength (monochromatic efficiency) and is independent of the irradiation intensity. [1] K. Ramspeck et al., J. Mater. Sci: Mater. Electron. 19 (2008) S4-S8 13
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