1. Theory

Definition: The CIGS cell uses a polycrystalline p-type absorber of the chalcopyrite Cu(In,Ga)Se $_{2}$ , usually with a copper deficit (Cu/(In+Ga) of 0.80 to 0.95, Intal and Ebong). The record stack of Keller et al. is glass, molybdenum back contact, an NaF precursor layer, the CIGS absorber (2.0 to 2.4 µm in their earlier work), an RbF treatment, a CdS buffer, i-ZnO (60 nm), ZnO:Al (200 nm, about 50 $Ω$ /sq) and a MgF $_{2}$ anti-reflection layer of about 105 nm. The soda-lime glass supplies sodium.
Alloy: CuInSe $_{2}$ has a gap of about 1.04 eV; gallium raises it to 1.2 to 1.3 eV (Intal and Ebong) and above 1.4 eV for GGI above 0.6 (Keller et al., Solar RRL). The record cell of 2024 also contains silver ([Ag]/([Ag]+[Cu]) = 0.19) and has a gap close to the local optimum of 1.15 eV.
Deposition: Three-stage co-evaporation with grain growth (growth up to 550 °C in Keller et al.), low-temperature co-evaporation for flexible cells on polyimide (Empa), and roll-to-roll or ink routes for modules (Intal and Ebong).
Alkali treatment: Sodium is added from the glass or from an NaF layer of 10 to 15 nm and the heavy alkalis K, Rb and Cs by a post-deposition treatment at 350 to 400 °C in selenium (Keller et al.; Sun et al.). They form KInSe $_{2}$ or RbInSe $_{2}$ -type surface phases with a gap above 2 eV and passivate grain boundaries and the buffer interface; ZSW reported 22.6 % with Rb in 2016.
Records: Green et al. list 23.35 % for Solar Frontier (1.043 cm $^{2}$ , 734 mV, 39.58 mA/cm $^{2}$ , 80.4 %, Cd-free) and 23.6 % for Evolar/Uppsala (0.899 cm $^{2}$ , 767.1 mV, 38.30 mA/cm $^{2}$ , 80.5 %, Fraunhofer ISE); the Nature Energy paper of 2024 reports 23.64 % certified, 23.75 % without a mask on 1.03 cm $^{2}$ and an external radiative efficiency of 1.6 %. Modules: 19.2 % (Solar Frontier, 841 cm $^{2}$ ), 20.3 % for an Avancis CIGSSe submodule and 18.6 % for a large Miasole module. The best flexible cell is 22.2 % (Empa).
Tandems and thin absorbers: Perovskite/CIGS reaches 24.2 % (HZB) and GaInP/AlGaAs/CIGS 28.1 %; Rath et al. reach 11.72 % with an absorber of only 500 nm.
Market: Intal and Ebong give a 2024 revenue of 2.8 billion dollars and a share of about 1.1 %, with the supply of indium and gallium as a bottleneck.

2. Working Principle

Junction and collection: The p-type absorber and the n-type CdS/ZnO form a heterojunction; a thin CdS gives a stronger buried homojunction and more current without a voltage loss (Sun et al.). Near stoichiometry ([I]/[III] close to 0.97) the absorber is nearly undoped and fully depleted and collects well, while off-stoichiometric absorbers are doped and have a space charge region below 550 nm (Keller et al.), which needs an acceptor density above about 2.6 × 10 $^{15}$ cm $^{- 3}$ (computed, assumed built-in voltage).
Grading: A gallium gradient widens the gap towards the molybdenum, which acts as a back-surface field; for 0.3 eV over 0.5 µm the field is 6 kV/cm (assumed). The hockey-stick profile of the record cell keeps the gallium constant and low near the CdS to reduce lateral and in-depth gap fluctuations (Keller et al.).
Voltage: The record cell has 767 mV, 86 % of the radiative limit for 1.15 eV (computed), and a wide-gap silver-alloyed cell has 926 mV at 1.44 eV, 80 % of the limit (computed). An external radiative efficiency of 1.6 % costs 106 mV (computed). Keller et al. attribute the large deficit at wide gaps mainly to the bulk lifetime, not to the interfaces.
Heavy alkalis: RbF raised the voltage by about 25 mV on average and by up to 58 mV in a matched pair of wide-gap cells, and KF at ZSW cut J $_{0}$ by one order of magnitude with a fill factor above 79 % (Keller et al.; Sun et al.). The balance of sodium and rubidium is important (Uppsala).
Buffer and window: A buffer thinner by 13 nm gave 0.7 mA/cm $^{2}$ more; the record cell has a CdS of 25 nm (First Solar) against 50 to 60 nm in the standard stack (Intal and Ebong).

3. Applications

Rigid modules: Solar Frontier and Avancis modules of about 19 to 20 % on glass, with a capex below 0.25 dollars per W (Intal and Ebong).
Flexible and lightweight: Cells on polyimide (22.2 %) and stainless-steel foil, roll-to-roll modules of 350 W and facade cladding below 3 kg/m $^{2}$ (Intal and Ebong).
Tandem bottom cell: The narrow gap of 1.0 to 1.2 eV suits the bottom cell of a perovskite/CIGS tandem (24.2 %) and the flexible tandem of 23 % (MiaSole and Solliance, 2019).

Absorber, Gap and Detailed Balance

Name	Formula	Related Formulae	Variables/Constants
Composition ratios of the chalcopyrite absorber	$GG I = \frac{[ G a ]}{[ G a ] + [ I n ]}$	$C G I = \frac{[ C u ]}{[ G a ] + [ I n ]}, \frac{[ I ]}{[ I I I ]} = \frac{[ C u ] + [ A g ]}{[ G a ] + [ I n ]}$ $AA C = \frac{[ A g ]}{[ A g ] + [ C u ]}$	$GG I : g a l l i u m - t o - g r o u p - I I I r a t i o$ $C G I : co pp er - t o - g r o u p - I I I r a t i o, [I] / [I I I] : r a t i o o f t h e g r o u p I t o t h e g r o u p I I I m e t a l s$ $AA C : s i l v er f r a c t i o n o f t h e g r o u p I m e t a l s, t h e d e f ini t i o n s a r e t h e u s u a l o n es, K e l l er e t a l . (S o l a r R R L) u se GG I an d [I] / [I I I], t h e r ecor d p a p er g i v es [A g] / ([A g] + [C u]) = 0.19$ $(so u r ces : C u / (I n + G a) = 0.80 t o 0.95 f or C u - p oor ab sor b er s (I n t a l an d E b o n g); c l ose - s t o i c hi o m e t r i c [I] / [I I I] \approx 0.97 g i v es a w i d e s p a ce c ha r g e r e g i o n an d hi g h co l l ec t i o n, an d [I] / [I I I] < 0.90 g i v es a d o p e d ab sor b er w i t h a s p a ce c ha r g e r e g i o n b e l o w 550 nm (K e l l er e t a l .); GG I = 0.3 f or t h e 500 nm ce l l s o f R a t h e t a l . an d 0.69 t o 0.74 f or t h e w i d e - g a p ce l l s o f K e l l er e t a l .)$
Band gap of the alloy	$E_{g} (x) = (1 - x) E_{C I S} + x E_{C GS} - b x (1 - x)$	$\frac{d E _{g}}{d x} = E_{C GS} - E_{C I S} - b (1 - 2 x)$	$x : g a l l i u m f r a c t i o n (GG I)$ $E_{C I S}, E_{C GS} : g a p s o f C u I n S e_{2} an d C u G a S e_{2}$ $b : b o w in g p a r am e t er, I n t a l an d E b o n g g i v e 1.04 e V f or C u I n S e_{2} an d a r i se t o 1.2 t o 1.3 e V w i t h g a l l i u m, t h e o t h er so u r ces g i v e g a p s f or g i v e n GG I (R a t h e t a l ., K e l l er e t a l .), t h e q u a d r a t i c f or m, 1.68 e V an d b a r e m y a ss u m pt i o n s f i tt e d t o t h e m$ $(co m p u t e d w i t h E_{C I S} = 1.04 e V, E_{C GS} = 1.68 e V (ba c k g r o u n d) an d b = 0.21 (f i tt e d) : GG I = 0.3 g i v es 1.19 e V (R a t h e t a l . : 1.14 t o 1.25 e V), 0.5 g i v es 1.31 e V an d 0.7 g i v es 1.44 e V (K e l l er e t a l . : 1.43 t o 1.44 e V f or GG I = 0.69 t o 0.74 an d GG I > 0.6 f or g a p s ab o v e 1.4 e V); b = 0.15 an d 0.24 c han g e t h e v a l u e a t GG I = 0.7 t o 1.457 an d 1.438 e V)$
Detailed-balance limit at the gap	$V_{oc, S Q} = \frac{k _{B} T}{q} ln (\frac{J _{sc, ma x}}{J _{0, r a d}} + 1)$	$r_{V} = \frac{V _{oc}}{V _{oc, S Q}}, Δ V = \frac{E _{g}}{q} - V_{oc}$	$V_{oc, S Q} : v o l t a g e a t t h e r a d ia t i v e l imi t$ $J_{sc, ma x} : p h o t oc u r r e n t ab o v e t h e g a p u n d er A M 1.5 G$ $J_{0, r a d} : r a d ia t i v e s a t u r a t i o n c u r r e n t$ $r_{V} : f r a c t i o n o f t h e l imi t$ $Δ V : d e f i c i t f r o m t h e g a p, t h e m e t h o d i s t h e o n e o f t h e S o l a r C e l l F u n d am e n t a l s p a g e, K e l l er e t a l . (S o l a r R R L) q u o t e 80 % o f V_{oc, S Q} f or t h e i r r ecor d w i d e - g a p ce l l an d 0.90 f or t h e b es t l o w - g a p c ha l co p y r i t e$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d a t 298.15 K, e mi ss i o n f r o m b o t h f a ces : E_{g} = 1.15 e V g i v es V_{oc, S Q} = 0.888 V, J_{sc, ma x} = 42.43 m A / c m^{2} an d 32.8 %; 1.19 e V 0.925 V an d 40.43; 1.25 e V 0.981 V an d 37.52; 1.44 e V 1.159 V an d 31.24; t h e ce l l o f 767.1 mV ha s r_{V} = 0.864 (a t 1.15 e V), Δ V = 383 mV an d J_{sc} / J_{sc, ma x} = 0.903; 926 mV a t 1.44 e V g i v es r_{V} = 0.799)$
Quasi-electric field from the gallium grading	$F = \frac{Δ E _{c}}{q d}$	$Δ E_{c} \approx Δ E_{g} (assumed)$	$F : d r i f t f i e l d f or e l ec t r o n s$ $Δ E_{c} : c han g e o f t h e co n d u c t i o n - ban d e d g e a cr oss t h e g r a d e d r e g i o n$ $d : w i d t h o f t h e r e g i o n$ $Δ E_{g} : c han g e o f t h e g a p, K e l l er e t a l . d escr ib e t h e h oc k ey - s t i c k p r o f i l e (hi g h g a l l i u m a t t h e m o l y b d e n u m, l o w er an d co n s t an t g a l l i u m a t t h e C d S) an d a n o t c h i s d escr ib e d b y I n t a l an d E b o n g (0.9 t o 1.0 e V a t t h e C d S, 1.4 t o 1.5 e V a t t h e M o), n o w i d t h s or f i e l d s a r e p r in t e d$ $(a ss u m e d : Δ E_{g} = 0.3 e V o v er 0.5 μ m g i v es F = 6 k V / c m, o v er 1 μ m 3 k V / c m, an d 0.2 e V o v er 0.5 μ m 4 k V / c m, co m p u t e d)$

Junction, Voltage and Current

Name	Formula	Related Formulae	Variables/Constants
Space charge width and doping	$W = \frac{2 ε _{s} V _{bi}}{q N _{A}}$	$N_{A} = \frac{2 ε _{s} V _{bi}}{q W ^{2}}$	$W : w i d t h o f t h e s p a ce c ha r g e r e g i o n$ $ε_{s} : p er mi tt i v i t y$ $V_{bi} : b u i l t - in v o l t a g e$ $N_{A} : a cce pt or d e n s i t y, K e l l er e t a l . a ss u m e ε_{r} = 10 in t h e i r c a p a c i t an ce ana l y s i s an d f in d w i d t h s b e l o w 550 nm f or o f f - s t o i c hi o m e t r i c ab sor b er s an d f u l l d e pl e t i o n n e a r s t o i c hi o m e t r y, t h e o n e - s i d e d f or m i s ba c k g r o u n d k n o w l e d g e$ $(co m p u t e d w i t h ε_{r} = 10 : W = 550 nm n ee d s N_{A} = 2.2, 2.6 an d 2.9 \times 1 0^{15} c m^{- 3} f or V_{bi} = 0.6, 0.7 an d 0.8 V (a ss u m e d); w i t h 0.7 V N_{A} = 1 0^{15}, 5 \times 1 0^{15} an d 1 0^{16} c m^{- 3} g i v e 0.88, 0.39 an d 0.28 μ m)$
Voltage from the external radiative efficiency	$V_{oc} = V_{oc, r a d} + \frac{k _{B} T}{q} ln Q_{e}$	$Δ V_{n r} = - \frac{k _{B} T}{q} ln Q_{e}$	$V_{oc, r a d} : v o l t a g e a t t h e r a d ia t i v e l imi t$ $Q_{e} : e x t er na l r a d ia t i v e e f f i c i e n cy$ $Δ V_{n r} : n o n - r a d ia t i v e l oss, t h e r e l a t i o n i s s t an d a r d (ba c k g r o u n d k n o w l e d g e), t h e p v ma g a z in e a r t i c l e g i v es Q_{e} = 1.6 % f or t h e r ecor d ce l l$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : 1.6 % g i v es - 106 mV; w i t h V_{oc, r a d} = 0.888 V (1.15 e V) t hi s i s 0.782 V a g ain s t 0.767 V m e a s u r e d, i . e . t h e ce l l i s 15 mV b e l o w t h e p r e d i c t i o n, t h e g a p o f t h e ce l l i s n o t p r in t e d an d t h e l imi t i s f or a s t e p ab sor b er)$
Buffer thickness and photocurrent	$Δ J_{sc} = - \int q Φ (λ) (1 - e^{- α_{b} t_{b}}) d λ \approx - κ t_{b}$	$κ = \frac{J _{sc} ( t _{1} ) - J _{sc} ( t _{2} )}{t _{2} - t _{1}}$	$Δ J_{sc} : c u r r e n t l os t in t h e b u f f er$ $Φ : p h o t o n f l ux$ $α_{b} : ab sor pt i o n coe f f i c i e n t o f t h e C d S$ $t_{b} : b u f f er t hi c k n ess$ $κ : l oss p er nm f r o m tw o s am pl es, t h e l in e a r a pp r o x ima t i o n i s min e, K e l l er e t a l . (S o l a r R R L) g i v e 44 nm a g ain s t 57 nm w i t h 0.7 m A / c m^{2} m or e c u r r e n t, t h e p v ma g a z in e a r t i c l e g i v es 25 nm f or t h e r ecor d ce l l$ $(co m p u t e d : κ = 0.054 m A / c m^{2} p er nm; e x t r a p o l a t e d l in e a r l y 25 nm w o u l d g i v e 1.0 m A / c m^{2} m or e t han 44 nm an d 1.7 m A / c m^{2} m or e t han 57 nm, an es t ima t e b ec a u se t h e ab sor pt i o n i s n o t l in e a r an d t h e C d S d i f f er s)$
Current of an ultra-thin absorber	$J_{sc} \approx q \int Φ (λ) (1 - e^{- α t}) d λ$	$\frac{J _{sc}}{J _{sc, ma x}}$	$t : ab sor b er t hi c k n ess$ $α : ab sor pt i o n coe f f i c i e n t, t h e s in g l e - p a ss f or m i s t h e s im pl es t es t ima t e (ba c k g r o u n d k n o w l e d g e), R a t h e t a l . m e a s u r e 24.58 m A / c m^{2} f or ab o u t 500 nm o f C I GS e o n M o, t h e r ecor d ce l l s ha v e 2.0 t o 2.4 μ m (K e l l er e t a l .)$ $(co m p u t e d : 24.58 m A / c m^{2} i s 61 % o f t h e l imi t o f 40.43 m A / c m^{2} a t 1.19 e V; f or an a ss u m e d α = 1 0^{5} c m^{- 1} a s in g l e p a ss in 0.5 μ m ab sor b s 99.3 % an d f or 1 0^{4} c m^{- 1} 39 %, so t h e l oss co m es main l y f r o m t h e w e ak l y ab sor b e d l o n g w a v e l e n g t h s; R a t h e t a l . ha v e 658.3 mV an d a d e f i c i t o f 532 mV)$

Efficiency Checks

Name	Formula	Related Formulae	Variables/Constants
Efficiency from the reported parameters	$η = \frac{V _{oc} J _{sc} F F}{P _{in}}, P_{in} = 100 mW/c m^{2}$	$η_{c h ec k} = \frac{V _{oc} J _{sc} F F}{1000}$ $Δ η = η_{p r in t e d} - η_{c h ec k}$	$η : e f f i c i e n cy, V_{oc} in mV, J_{sc} in m A / c m^{2}, F F a s a f r a c t i o n (η_{c h ec k} i s in p er ce n t : t h e 1000 t u r n s mV in t o V, an d 1 mW / c m^{2} i s 1 p er ce n t o f P_{in}), t h e v a l u es a r e f r o m t h e e f f i c i e n cy t ab l es (v er s i o n 64) o f G r ee n e t a l . an d f r o m R a t h e t a l .$ $(co m p u t e d : S o l a r F r o n t i er 734, 39.58, 80.4 % g i v e 23.36 % (23.35 \pm 0.5, 1.043 c m^{2}); E v o l a r / U pp s a l a 767.1, 38.30, 80.5 % g i v e 23.65 % (23.6 \pm 0.4, 0.899 c m^{2}); R a t h e t a l . 658.3, 24.58, 72.46 % g i v e 11.72 % (11.72); t h e t an d e m p er o v s k i t e / C I GS 1768, 19.24, 72.9 % g i v es 24.80 % a g ain s t 24.2 \pm 0.7 p r in t e d; G a I n P / A l G a A s / C I GS 2952, 11.72, 81.1 % g i v es 28.06 % (28.1))$
Module efficiency from voltage, current and area	$η_{m o d} = \frac{V _{oc} I _{sc} F F}{G A}$	$P_{ma x} = V_{oc} I_{sc} F F$	$η_{m o d} : m o d u l e e f f i c i e n cy$ $V_{oc}, I_{sc} : m o d u l e v o l t a g e an d c u r r e n t$ $F F : f i l l f a c t or$ $G : i r r a d ian ce o f 1000 W / m^{2}$ $A : a r e a (a p er t u r e f or t h e e n t r i es b e l o w), t h e e n t r i es a r e f r o m G r ee n e t a l . (v er s i o n 64)$ $(co m p u t e d : S o l a r F r o n t i er 48.0 V, 0.456 A, 73.7 % o n 841 c m^{2} g i v e 16.13 W an d 19.18 % (19.2 \pm 0.5); M ia so l e 58.00 V, 4.545 A, 76.8 % o n 10858 c m^{2} g i v e 202.5 W an d 18.65 % (18.6 \pm 0.6); t h e A v an c i s s u bm o d u l e o f 20.3 % ha s 100 ce l l s o f 683.4 mV an d an a r e a o f 526.7 c m^{2})$

Important Questions

Ques: Which CIGS record is right: 23.35, 23.6, 23.64 or 23.75 %?

Green et al. list 23.35 ± 0.5 % for Solar Frontier on 1.043 $c m^{2}$ and 23.6 ± 0.4 % for Evolar/Uppsala on 0.899 $c m^{2}$ (767.1 mV, 38.30 $mA/c m^{2}$ , 80.5 %), whose product is 23.65 % (computed). The Nature Energy paper and press release give 23.64 % certified by Fraunhofer ISE with a shadow mask of 0.9 $c m^{2}$ and 23.75 % measured in-house without a mask on 1.03 $c m^{2}$ . Intal and Ebong credit the 23.64 % to Solar Frontier in one section, which I take as an error, since Solar Frontier’s record is 23.35 %. The numbers describe one cell measured in two ways: 23.6 is the rounded title, 23.64 the certified value on the masked area and 23.75 the unmasked in-house one. The certified 23.64 % is the figure to quote. Reference: Green, Dunlop, Yoshita, Kopidakis, Bothe, Siefer, Hinken, Rauer, Hohl-Ebinger and Hao, ‘Solar cell efficiency tables (Version 64)’, Prog. Photovolt. 32(7), 425-441 (2024); Keller, Kiselman, Donzel-Gargand, Martin, Babucci, Lundberg, Wallin, Stolt and Edoff, ‘High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency’, Nature Energy 9(4), 467-478 (2024) (abstract page); Uppsala University press release, ‘Uppsala University sets new world record for CIGS solar cells’ (26 February 2024); Phys.org feature on the strategy to boost the efficiency of copper indium gallium selenide solar cells (2 March 2024); Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025).

Ques: Which flexible CIGS record is right: 21.7, 22.2 or 23 %?

Empa presented 22.2 % on polymer film at WCPEC-8 on 26 September 2022, confirmed by Fraunhofer ISE, after 21.4 % in 2021 (record steps 12.8 % in 1999 up to 21.4 %). Intal and Ebong give 21.7 % for a flexible polyimide cell but say in the next sentence that the same value is a rigid-substrate record, so the label is unclear. MiaSole and Solliance announced 23 % in September 2019, but that is a flexible tandem of a CIGS cell and a perovskite top cell, and NREL confirmed 20.56 % for the MiaSole CIGS cell alone. The records therefore refer to different devices and the single-junction flexible value is 22.2 %. The 21.7 % is more likely an earlier value, and no certified area is printed for any of them in the extracts. Reference: Empa, ‘Gaining power’ (flexible CIGS cell of 22.2 %, presented 26 September 2022) and pv magazine, ‘Swiss scientists achieve 22.2% efficiency for flexible CIGS solar cell’ (11 October 2022); Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); eepower.com, ‘New world record 23% efficiency with flexible CIGS solar cell’ (20 September 2019, MiaSole and Solliance).

Ques: What is the best CIGS module efficiency: 19.2, 19.5, 19.64 or 20.3 %?

Green et al. give 19.2 ± 0.5 % for the Solar Frontier module of 841 $c m^{2}$ (48.0 V, 0.456 A, 73.7 %), which the product confirms as 19.18 % (computed), and 18.6 % for a large Miasole module; the Avancis CIGSSe submodule of 526.7 $c m^{2}$ has 20.3 %. Intal and Ebong print 19.2 % in a table, ‘above 19.5 %’ in the text and 19 to 20 % in the conclusions, and pv magazine quotes 19.64 % for Avancis. The sources compare different areas and dates: a submodule of about 500 $c m^{2}$ is not a full module, and aperture-area numbers are not comparable with total-area numbers. For a full-size module 19.2 % is the printed value and 19.5 to 19.64 % is more likely a later or aperture-based result; the 20.3 % is a submodule. Reference: Green, Dunlop, Yoshita, Kopidakis, Bothe, Siefer, Hinken, Rauer, Hohl-Ebinger and Hao, ‘Solar cell efficiency tables (Version 64)’, Prog. Photovolt. 32(7), 425-441 (2024); Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Empa, ‘Gaining power’ (flexible CIGS cell of 22.2 %, presented 26 September 2022) and pv magazine, ‘Swiss scientists achieve 22.2% efficiency for flexible CIGS solar cell’ (11 October 2022).

Ques: Is the CdS buffer 25, 44, 57 or 50 to 60 nm thick?

Intal and Ebong give 50 to 60 nm for the standard stack. Keller et al. (Solar RRL) use 44 nm with a rubidium treatment and 57 nm without, and the thinner layer gave about 0.7 $mA/c m^{2}$ more current. The pv magazine article states that the rubidium treatment allows a CdS of only 25 nm in the 23.64 % cell. These values describe different cells: a standard layer, a wide-gap silver-alloyed cell and the record. The two samples give 0.054 $mA/c m^{2}$ per nm (computed), so 25 nm would give about 1.0 $mA/c m^{2}$ more than 44 nm if the loss were linear. The estimate ignores that CdS also affects the junction and the voltage, so it is only an upper guide. For the record cell 25 nm is more likely, and for a standard device 50 to 60 nm. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Keller, Aboulfadl, Stolt, Donzel-Gargand and Edoff, ‘Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells’, Solar RRL 6(6), 2200044 (2022); pv magazine, ‘First Solar’s unit, Uppsala University present CIGS solar cell with world record efficiency of 23.64%’ (7 March 2024).

Ques: Is the gap of the record CIGS cell 1.04, 1.15 or 1.19 eV?

Intal and Ebong give 1.04 eV for CuInSe $_{2}$ and 1.2 to 1.3 eV with gallium. Keller et al. say that the record cell sits close to the local optimum of 1.15 eV, and Rath et al. measure 1.14 to 1.25 eV for GGI of 0.3 on thin absorbers. A bowing model with 1.04 and 1.68 eV and b = 0.21 gives 1.19 eV at GGI = 0.3 (computed). The values belong to different compositions, and the record cell has a graded gap, so 1.15 eV is a local value. The gap sets the voltage ratio: 767 mV is 86 % of the limit at 1.15 eV, whereas Keller et al. (Solar RRL) quote 0.90 for the best low-gap chalcopyrite, which would imply a gap near 1.10 eV (computed). The 1.15 eV is more likely for the record cell. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Keller, Kiselman, Donzel-Gargand, Martin, Babucci, Lundberg, Wallin, Stolt and Edoff, ‘High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency’, Nature Energy 9(4), 467-478 (2024) (abstract page); Rath, Koehler and Schmid, ‘Optimizing Na and Rb Incorporation Along With Bandgap Grading for High Efficiency Ultra-Thin CIGSe Solar Cells’, Prog. Photovolt. (2026), DOI 10.1002/pip.70126; Keller, Aboulfadl, Stolt, Donzel-Gargand and Edoff, ‘Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells’, Solar RRL 6(6), 2200044 (2022).

Ques: Is the Voc of the wide-gap cell 80 % of the limit?

Keller et al. state that their wide-gap silver-alloyed cell (926 mV at 1.44 eV) reaches about 80 % of the radiative limit. The detailed-balance calculation for the AM1.5G spectrum gives 1.159 V at 1.44 eV, so the ratio is 0.799 (computed), and for 1.43 eV 0.805, which agrees. For the record cell of 767.1 mV the ratio is 0.864 at 1.15 eV, and for Solar Frontier (734 mV) 0.748 at 1.25 eV, assumed gap. The ratio depends strongly on the gap used, and the gaps of the cells are not all printed. The paper also says that alternative buffers cannot reach more than a loss of 20 % in relative voltage above 1.3 eV, which is consistent with 0.80. Gap-dependent ratios should be compared at the same gap only; the 0.80 for 1.44 eV is the more reliable number. Reference: Keller, Aboulfadl, Stolt, Donzel-Gargand and Edoff, ‘Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells’, Solar RRL 6(6), 2200044 (2022); Green, Dunlop, Yoshita, Kopidakis, Bothe, Siefer, Hinken, Rauer, Hohl-Ebinger and Hao, ‘Solar cell efficiency tables (Version 64)’, Prog. Photovolt. 32(7), 425-441 (2024); Solar Cell Fundamentals page of this vault.

Ques: What is the surface gap after the KF treatment: 1.6, 2.5 or 2.7 eV?

Sun et al. quote Handick et al. for a near-surface gap of 1.61 eV after an NaF treatment and 2.52 eV after NaF plus KF, with an uncertainty of +0.14 and -0.51 eV, and 2.7 eV for KInSe $_{2}$ ; Intal and Ebong say that the KF treatment forms a KIn $_{1 - y}$ Ga $_{y}$ Se $_{2}$ surface layer with a gap above 2 eV, and Rb and Cs form RbInSe $_{2}$ and CsInSe $_{2}$ . The measured 2.52 eV is lower than the 2.7 eV of the pure compound, which is consistent with a mixed layer of a few nm. The 1.61 eV belongs to the sodium-only surface and the difference of 0.91 eV between the two is the effect of the potassium (computed). Keller et al. say that a thin RbInSe $_{2}$ -like layer (below 5 nm) is proposed but not proven. A gap of 2.5 eV is more likely for the potassium-treated surface. Reference: Sun, Lin, Li, Cheng, Zhang, Liu and Liu, ‘Review on Alkali Element Doping in Cu(In,Ga)Se2 Thin Films and Solar Cells’, Engineering 3(4), 452-459 (2017); Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Keller, Aboulfadl, Stolt, Donzel-Gargand and Edoff, ‘Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells’, Solar RRL 6(6), 2200044 (2022).

Ques: Does the perovskite/CIGS tandem entry add up?

Green et al. list a perovskite/CIGS two-terminal cell from HZB with 24.2 ± 0.7 % on 1.045 $c m^{2}$ , 1.768 V, 19.24 $mA/c m^{2}$ and a fill factor of 72.9 %. The product is 24.80 % (computed), 0.6 points more than printed, though inside the stated uncertainty. The GaInP/AlGaAs/CIGS entry (2.952 V, 11.72 $mA/c m^{2}$ , 81.1 %) gives 28.06 % against 28.1 %, which agrees. For tandems the printed current is usually that of the limiting subcell, and a difference can arise when the certified current differs from that of the J-V curve, but the extract gives no explanation. The efficiency of 24.2 % is the certified value and is more likely; the 24.8 % product should not be used as a record. The tandem is treated further on the perovskite-silicon and perovskite pages of the plan. Reference: Green, Dunlop, Yoshita, Kopidakis, Bothe, Siefer, Hinken, Rauer, Hohl-Ebinger and Hao, ‘Solar cell efficiency tables (Version 64)’, Prog. Photovolt. 32(7), 425-441 (2024).

Ques: Which absorber composition is right: Cu-poor 0.80 to 0.95 or close-stoichiometric 0.97?

Intal and Ebong state that CIGS absorbers are Cu-poor with Cu/(In+Ga) of 0.80 to 0.95. Keller et al. (Solar RRL) find that carrier collection is high only near stoichiometry, [I]/[III] close to 0.97, where the space charge region is wide and the doping very low, and that absorbers with [I]/[III] below 0.90 have a doping that saturates and a space charge region below 550 nm. The quantities differ slightly (Cu only against Cu plus Ag), and the two ranges overlap near 0.90 to 0.95. The silver in the record cell raises the group I content, with [Ag]/([Ag]+[Cu]) = 0.19. For the record cells the near-stoichiometric range is more likely; for general production material the Cu-poor range of the review is, but it is a general statement and the extract gives no process window. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Keller, Aboulfadl, Stolt, Donzel-Gargand and Edoff, ‘Rubidium Fluoride Absorber Treatment for Wide-Gap (Ag,Cu)(In,Ga)Se2 Solar Cells’, Solar RRL 6(6), 2200044 (2022); Keller, Kiselman, Donzel-Gargand, Martin, Babucci, Lundberg, Wallin, Stolt and Edoff, ‘High-concentration silver alloying and steep back-contact gallium grading enabling copper indium gallium selenide solar cell with 23.6% efficiency’, Nature Energy 9(4), 467-478 (2024) (abstract page).

Ques: Which was the CIGS record before 2019: 22.3, 22.6 or 22.9 %?

Intal and Ebong list 22.3 % for Solar Frontier in 2015, Sun et al. report 22.6 % from ZSW with a heavy-alkali treatment in June 2016, and the Uppsala press release says that the 23.35 % of Solar Frontier was preceded by 22.9 % from ZSW. The steps are consistent in order: 22.3 (2015), 22.6 (2016), 22.9 (ZSW), 23.35 (2019), 23.64 (2024), with increments of 0.3, 0.3, 0.45 and 0.29 points (computed). They are successive records of different institutes, not a conflict, and the dates of the 22.9 % are not given in the extracts. A source that quotes only one of these values is therefore dated rather than wrong, and the 23.64 % of 2024 is the certified figure of Keller et al. that replaced the 23.35 % of Solar Frontier. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Sun, Lin, Li, Cheng, Zhang, Liu and Liu, ‘Review on Alkali Element Doping in Cu(In,Ga)Se2 Thin Films and Solar Cells’, Engineering 3(4), 452-459 (2017); Uppsala University press release, ‘Uppsala University sets new world record for CIGS solar cells’ (26 February 2024).

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Copper Indium Gallium Selenide (CIGS) Thin-Film Solar Cell