1. Theory

Definition: Kesterite is the quaternary Cu $_{2}$ ZnSnS $_{4}$ (CZTS) and its selenium alloy Cu $_{2}$ ZnSn(S,Se) $_{4}$ (CZTSSe), made of the abundant elements copper, zinc, tin, sulfur and selenium. Intal and Ebong give a direct gap of 1.4 to 1.6 eV for CZTS and an absorption coefficient above 10 $^{4}$ cm $^{- 1}$ for photon energies above 1.6 eV in films of 1 to 2 µm; Chen et al. give 1.0 to 1.5 eV for the family. Selenium alloying lowered the gap to about 1.1 eV in 2010, and a graded absorber of 2014 ran from about 1.0 eV at the molybdenum to 1.15 eV near the CdS (Intal and Ebong).
Defects: Cu and Zn have ionic radii that differ by only 0.06 Å, so they exchange sites (Cu-Zn disorder) and form deep Sn $_{Z n}$ antisites, which cause a voltage deficit above 0.40 V (Intal and Ebong). Even after process optimisation at least 30 % disorder stays on the Cu-Zn plane and band tails are seen in all cases (IBM review). Chen et al. calculate an order-disorder temperature of 470 K, against 533 ± 10 K measured.
Processing: Routes include electron-beam evaporation of metals with sulfurization at 520 °C (1996), vacuum thermal evaporation, hydrazine and thiourea inks (the latter suited to roll-to-roll), a chloride ink with (Ag,Na) doping, and graded sulfur-selenium annealing (Intal and Ebong). Recent steps are a sodium-activated selenization (IoP-CAS), a Li $_{2}$ SnS $_{3}$ interphase (CAS), a Cu-then-Cu $_{2}$ S co-sputtered precursor and a hydrogen anneal (UNSW).
Records (efficiency tables, version 64): Green et al. list CZTSSe 13.45 ± 0.3 % (1.101 cm $^{2}$ , 510.9 mV, 37.90 mA/cm $^{2}$ , 69.5 %, IoP-CAS, NPVM April 2024), a notable exception of 15.1 ± 0.3 % (0.2697 cm $^{2}$ , 529.9 mV, 38.44 mA/cm $^{2}$ , 74.0 %, NPVM April 2024, previous value 14.9 %), a minimodule of 10.1 ± 0.3 % (10.48 cm $^{2}$ , six cells of 530.9 mV, 32.77 mA/cm $^{2}$ , 57.9 %, NJUPT, NREL January 2024), and for the pure sulfide CZTS 10.0 ± 0.2 % (1.113 cm $^{2}$ , 708.3 mV, 21.77 mA/cm $^{2}$ , 65.1 %, UNSW, NREL March 2017) and a notable exception of 12.1 ± 0.3 % (0.2021 cm $^{2}$ , 749.0 mV, 23.40 mA/cm $^{2}$ , 68.9 %, UNSW, NPVM May 2024, listed under a rule for gaps above 1.5 eV).
Later reports (press): CAS reports 15.45 % in-house and 15.04 % certified in February 2026 (voltage above 600 mV at 1.10 eV); UNSW announced 13.2 % in January 2025 and in August 2026 12.6 % (certified 12.36 %) with 852.8 mV.
Timeline of Intal and Ebong: 0.66 % (1996), 2.3 % (1997), 5.7 % (2005), 9.7 % (2009), above 10.1 % (2010), 11.1 % (2012), 12.6 % (2014), 13.0 % certified (2021) and 14.9 % (2023); several text years differ from the years of the references.
Market: Intal and Ebong give a 2024 market of 0.0784 billion dollars, rising to 0.264 billion dollars in 2032 to 2034 (15.7 % a year), no major module producer and pilot lines with mini-modules of 11 to 12 %.

2. Working Principle

Junction: A p-type kesterite absorber forms a heterojunction with an n-type buffer and window; the early devices used a CdS buffer (Intal and Ebong). Capacitance-voltage doping of the CZTSSe cells of Azzouzi et al. is of the order of 10 $^{16}$ cm $^{- 3}$ .
Voltage loss: The record CZTSSe cell has 529.9 mV at an assumed gap of 1.10 eV, 63 % of the radiative limit and a deficit of 0.570 V (computed), whereas Bourdais et al. found at most 60 % in 2016. Azzouzi et al. split the deficit of their cells into the absorption-edge loss and a non-radiative loss of 376 to 439 mV (computed), and their radiative efficiencies of 10 $^{- 5}$ % are far below those of CIGS (0.03 %) and silicon (0.13 %).
Disorder and tails: Disorder lowers the gap by about 130 meV (experiments 40 to 200 meV), and gap fluctuations of 60 meV cost only about 70 meV, 10 % of the deficit. The larger effect of disorder is on defects: formation energies fall by 0.5 to 1 eV, the lifetime of Sn $_{Z n}$ falls from 2.0 × 10 $^{4}$ to 3.2 ns, and the limit set by recombination via defects falls from about 25 % to about 14 % (Chen et al.).
Process effects: Selenization at a positive pressure delays the phase evolution above 400 °C and cut the bulk defect density by about one order of magnitude (IoP-CAS, 2023). The sodium-activated selenization forms Cu $_{2}$ SnSe $_{3}$ at a lower temperature together with ZnSe and reduces Zn segregation, the Li $_{2}$ SnS $_{3}$ interphase lowers the Zn-Sn migration barrier difference from 0.41 to 0.21 eV (CAS), and the hydrogen anneal passivates defects (UNSW).
Current: The 15.1 % cell collects 38.44 mA/cm $^{2}$ , 87 % of the limit at 1.10 eV (computed), so the voltage and fill factor limit the efficiency, and the 15.1 % is 47 % of the limit of 32.2 % (computed).

3. Applications

Tandem partner: The wide-gap sulfide (1.5 to 1.6 eV, with 852.8 mV) is proposed by UNSW as an add-on or replacement for a silicon cell in tandems, with 15 % expected within a year and commercialisation by 2030 as stated targets.
Thin-film modules: Mini-modules of 10.1 % (certified) and 11 to 12 % (pilot lines, unreferenced), and a 10.1 % module with a fill factor of 57.9 % against 74.0 % for the best cell (Green et al.; Intal and Ebong).
Earth-abundant alternative: The absorber avoids indium and, apart from the buffer, cadmium; a UNSW cell kept its performance after 231 days in a nitrogen desiccator (pv magazine, 2026).

Absorber, Gap and Detailed Balance

Name	Formula	Related Formulae	Variables/Constants
Composition ratios and anion fraction	$\frac{[ C u ]}{[ Z n ] + [ S n ]}$	$\frac{[ Z n ]}{[ S n ]}$ $y = \frac{[ S e ]}{[ S ] + [ S e ]}$	$[X] : a t o mi c f r a c t i o n o f e l e m e n t X$ $y : se l e ni u m f r a c t i o n o f t h e ani o n s u b l a tt i ce, t h e r ecor d ce l l o f 14.9 % w a s ma d e w i t h a C u - p oor an d Z n - r i c h s u r f a ce d u r in g se l e ni z a t i o n (I n t a l an d E b o n g), t h e u s u a l co m p os i t i o n w in d o w i s ba c k g r o u n d k n o w l e d g e, an d n o r a t i os o f t h e r ecor d ce l l s w er e f o u n d$ $(co m p u t e d : s t o i c hi o m e t r i c C u_{2} Z n S n S_{4} ha s [C u] / ([Z n] + [S n]) = 1 an d [Z n] / [S n] = 1; t h e h y d r a z in e ce l l o f 11.1 % i s C Z T S_{0.3} S e_{0.7}, y = 0.7)$
Gap of the sulfoselenide alloy	$E_{g} (y) = (1 - y) E_{S} + y E_{S e}$	$y = \frac{E _{S} - E _{g}}{E _{S} - E _{S e}}$	$E_{g} : g a p o f C u_{2} Z n S n (S_{1 - y} S e_{y})_{4}$ $E_{S}, E_{S e} : g a p s o f t h e s u l f i d e an d se l e ni d e, b o w in g i s n e g l ec t e d, I n t a l an d E b o n g g i v e 1.4 t o 1.6 e V f or C Z T S an d ab o u t 1.1 e V f or C Z T S S e, C h e n e t a l . g i v e 1.0 t o 1.5 e V f or t h e f ami l y, t h e e n d - m e mb er v a l u es o f 1.5 an d 1.0 e V an d t h e l in e a r f or m a r e m y a ss u m pt i o n s$ $(co m p u t e d w i t h E_{S} = 1.5 e V an d E_{S e} = 1.0 e V (a ss u m e d) : y = 0.7 g i v es 1.15 e V, y = 0.8 g i v es 1.10 e V an d y = 0.9 g i v es 1.05 e V; 1.10 e V cor r es p o n d s t o a w a v e l e n g t h o f 1127 nm an d 1.5 e V t o 827 nm)$
Detailed-balance limit and voltage ratio	$r_{V} = \frac{V _{oc}}{V _{oc, S Q}}$	$Δ V = \frac{E _{g}}{q} - V_{oc}, \frac{J _{sc}}{J _{sc, ma x}}, r_{η} = \frac{η}{η _{S Q}}$	$r_{V} : f r a c t i o n o f t h e r a d ia t i v e v o l t a g e$ $V_{oc, S Q} : v o l t a g e a t t h e d e t ai l e d - ba l an ce l imi t$ $Δ V : d e f i c i t$ $J_{sc, ma x} : p h o t oc u r r e n t ab o v e t h e g a p$ $r_{η} : f r a c t i o n o f t h e l imi t in g e f f i c i e n cy, t h e m e t h o d i s t h e o n e o f t h e S o l a r C e l l F u n d am e n t a l s p a g e, t h e g a p s o f t h e t ab l e ce l l s a r e a ss u m e d (1.10 e V f or C Z T S S e a s s t a t e d b y C A S, 1.5 an d 1.6 e V f or C Z T S), B o u r d ai s e t a l . f o u n d r_{V} \leq 0.60 f or ce l l s ab o v e 9 % in 2016$ $(co m p u t e d a t 298.15 K, b o t h f a ces : 1.10 e V g i v es V_{oc, S Q} = 0.842 V, J_{sc, ma x} = 44.23 m A / c m^{2} an d 32.2 %; 1.50 e V g i v es 1.215 V, 28.94 an d 31.6 %; 1.60 e V g i v es 1.308 V, 25.40 an d 30.1 %; t h e C Z T S S e ce l l o f 15.1 % (529.9 mV, 38.44) ha s r_{V} = 0.630, Δ V = 0.570 V, J_{sc} / J_{sc, ma x} = 0.869 an d r_{η} = 0.469; t h e ce l l o f 13.45 % ha s r_{V} = 0.607; t h e C Z T S ce l l o f 12.1 % (749 mV, 23.40) ha s r_{V} = 0.616 an d J_{sc} / J_{sc, ma x} = 0.809 a t 1.5 e V, an d 0.572 an d 0.921 a t 1.6 e V; t h e C Z T S ce l l o f 10.0 % (708.3 mV) ha s r_{V} = 0.583 a t 1.5 e V)$

Voltage Deficit, Disorder and Defects

Name	Formula	Related Formulae	Variables/Constants
Decomposition of the voltage deficit	$\frac{E _{g}}{q} - V_{oc} = (\frac{E _{g}}{q} - V_{oc, S Q}) + Δ V_{ab s} + Δ V_{n r}$	$Δ V_{ab s} = V_{oc, S Q} - V_{oc, r a d}$ $Δ V_{n r} = V_{oc, r a d} - V_{oc} = - \frac{k _{B} T}{q} ln Q_{e}$	$Δ V_{ab s} : l oss f r o m t h e n o n - i d e a l ab sor pt i o n e d g e (ban d t ai l s)$ $Δ V_{n r} : n o n - r a d ia t i v e l oss$ $V_{oc, r a d} : r a d ia t i v e v o l t a g e f r o m t h e m e a s u r e d e x t er na l q u an t u m e f f i c i e n cy$ $Q_{e} : e x t er na l e l ec t r o l u min esce n ce e f f i c i e n cy, A z z o u z i e t a l . g i v e f o u r C Z T S S e ce l l s o f 27.6 t o 32.9 a t . % S n w i t h E_{g} = 1.04 t o 1.12 e V (e l ec t r o l u min esce n ce) or 1.13 t o 1.15 e V (q u an t u m e f f i c i e n cy), V_{oc} = 423, 453, 485 an d 465 mV, V_{oc, r a d} = 799, 840, 888 an d 904 mV, an d Q_{e} = 5.2 \times 1 0^{- 5}, 3.3 \times 1 0^{- 5}, 1.8 \times 1 0^{- 5} an d 4.6 \times 1 0^{- 6} %$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d f r o m t h e t ab l e : V_{oc, r a d} - V_{oc} = 376, 387, 403 an d 439 mV, e q u a l t o t h e t ab u l a t e d Δ V_{n r}; E_{g}^{E L} - V_{oc} = 617, 617, 625 an d 655 mV an d E_{g}^{E QE} - V_{oc} = 707, 687, 655 an d 685 mV, e q u a l t o t h e t ab u l a t e d d e f i c i t s; - (k_{B} T / q) ln Q_{e} w i t h 25.693 mV g i v es 372, 383, 399 an d 434 mV, ab o u t 1 % b e l o w t h e t ab u l a t e d v a l u es; w i t h V_{oc} = 465 mV f or d e v i ce C t h e d i f f er e n ces w o u l d b e 423 an d 645 mV)$
Loss from gap fluctuations	$Δ V_{σ} = \frac{σ _{g}^{2}}{2 k _{B} T}$	$α (h ν) \propto exp (\frac{h ν}{E _{U}})$	$σ_{g} : s t an d a r d d e v ia t i o n o f t h e l oc a l g a p (in e V)$ $k_{B} T : t h er ma l e n er g y in e V, Δ V_{σ} : v o l t a g e l oss in V$ $E_{U} : U r ba c h e n er g y, t h e G a u ss ian - f l u c t u a t i o n f or m i s ba c k g r o u n d k n o w l e d g e, C h e n e t a l . g i v e σ_{g} \approx 60 m e V, a v o l t a g e l oss o f ab o u t 70 m e V (10 % o f t h e d e f i c i t), E_{U} \approx 30 m e V f or or d er e d C Z T S an d a t m os t 10 m e V m or e w i t h d i sor d er$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : 0.0 6^{2} / (2 \times 0.025852) = 69.6 mV a t 300 K, w hi c h a g r ees w i t h t h e 70 m e V o f C h e n e t a l .; σ_{g} = 40 m e V g i v es 31 mV an d 30 m e V g i v es 17 mV; 70 m e V b e in g 10 % m e an s a t o t a l d e f i c i t o f ab o u t 700 mV)$
Defect concentration and lifetime	$N_{d} = N_{s} exp (- \frac{E _{f}}{k _{B} T})$	$τ_{S R H} = \frac{1}{σ v _{t h} N _{t}}$	$N_{d} : d e n s i t y o f d e f ec t s$ $N_{s} : d e n s i t y o f s i t es$ $E_{f} : f or ma t i o n e n er g y$ $T : t e m p er a t u r e o f e q u i l ib r a t i o n$ $τ_{S R H} : l i f e t im e f r o m a t r a p$ $σ : c a pt u r e cr oss - sec t i o n$ $v_{t h} : t h er ma l v e l oc i t y$ $N_{t} : t r a p d e n s i t y, b o t h f or m s a r e ba c k g r o u n d k n o w l e d g e, C h e n e t a l . f in d t ha t C u - Z n d i sor d er l o w er s t h e f or ma t i o n e n er g i es b y 0.5 t o 1 e V, r ai ses t h e e q u i l ib r i u m d e f ec t co n ce n t r a t i o n s b y f a c t or s o f 10 t o 1 0^{4}, an d s h or t e n s t h e l i f e t im es o f S n_{Z n} f r o m 2.0 \times 1 0^{4} t o 3.2 n s an d o f [S n_{Z n} + C u_{Z n}] f r o m 3.2 \times 1 0^{3} t o 1.0 n s$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : e x p (0.5/ k_{B} T) i s 1.3 \times 1 0^{3} a t 813 K (540 ° C, a ss u m e d) an d 1.6 \times 1 0^{4} a t 600 K, an d e x p (1/ k_{B} T) i s 1.6 \times 1 0^{6} a t 813 K, so t h e f a c t or s o f C h e n e t a l . (10 t o 1 0^{4}) a r e s ma l l er t han a s im pl e B o l t z mann f a c t or f or 0.5 t o 1 e V, an d t h e e x t r a c t d oes n o t s a y w h y; t h e l i f e t im e r a t i os a r e 6.3 \times 1 0^{3} an d 3.2 \times 1 0^{3}; t h e ba r r i er d i f f er e n ce o f t h e Z n - S n mi g r a t i o n f a l l s f r o m 0.41 t o 0.21 e V w i t h t h e L i_{2} S n S_{3} in t er p ha se (C A S), a f a c t or e x p (0.2/ k_{B} T) = 17 a t 813 K)$

Efficiency Checks

Name	Formula	Related Formulae	Variables/Constants
Efficiency from the reported parameters	$η = \frac{V _{oc} J _{sc} F F}{P _{in}}, P_{in} = 100 mW/c m^{2}$	$η_{c h ec k} = \frac{V _{oc} J _{sc} F F}{1000}$ $Δ η = η_{p r in t e d} - η_{c h ec k}$	$η : e f f i c i e n cy, V_{oc} in mV, J_{sc} in m A / c m^{2}, F F a s a f r a c t i o n (η_{c h ec k} i s in p er ce n t : t h e 1000 t u r n s mV in t o V, an d 1 mW / c m^{2} i s 1 p er ce n t o f P_{in}), t h e e n t r i es a r e f r o m t h e e f f i c i e n cy t ab l es (v er s i o n 64) o f G r ee n e t a l . an d f r o m t h e p v ma g a z in e r e p or t o n U N S W$ $(co m p u t e d : C Z T S S e 510.9, 37.90, 69.5 % g i v e 13.46 % (13.45); 529.9, 38.44, 74.0 % g i v e 15.07 % (15.1); minim o d u l e 530.9, 32.77, 57.9 % g i v e 10.07 % (10.1); C Z T S 708.3, 21.77, 65.1 % g i v e 10.04 % (10.0); 749.0, 23.40, 68.9 % g i v e 12.08 % (12.1); U N S W 852.8, 21.0, 70.1 % g i v e 12.55 % (12.6); t h e cer t i f i e d 12.36 % a t 846.7 mV n ee d s J_{sc} F F = 14.6 m A / c m^{2}; 15.45 % a t 600 mV n ee d s J_{sc} F F = 25.75 a g ain s t 28.45 f or t h e 15.1 % ce l l)$

Important Questions

Ques: Which kesterite record is right: 15.1, 15.45 or 15.04 %?

Green et al. list 15.1 ± 0.3 % for a CZTSSe cell of IoP-CAS (0.2697 $c m^{2}$ , 529.9 mV, 38.44 $mA/c m^{2}$ , 74.0 %, NPVM April 2024), whose product is 15.07 % (computed). The IoP page of March 2026 gives 15.5 % for the champion device certified at 15.1 %. pv magazine (27 February 2026) headlines 15.45 % as a world record for a CAS cell with a Li $_{2}$ SnS $_{3}$ interphase and over 600 mV at 1.10 eV, but gives 15.04 % as the certified value. The ratios are alike, 15.5/15.1 = 1.026 and 15.45/15.04 = 1.027 (computed), yet the extracts do not say whether it is the same cell. The certified 15.1 % of 2024 is the highest certified figure, 15.04 % is 0.06 points lower, and 15.45 % is an in-house value that should not be quoted as a certified record. 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); Institute of Physics, Chinese Academy of Sciences, news item on the 15.1% certified kesterite record (20 March 2026; Joule paper of Meng’s group, DOI 10.1016/j.joule.2026.102345); pv magazine, ‘Chinese Academy of Sciences achieves world record efficiency of 15.45% for kesterite solar cell’ (27 February 2026); Interesting Engineering, report on the CAS kesterite record of 15.45 % (about 27 February 2026).

Ques: Was the previous kesterite record 14.9, 14.2, 14.1 or 13.8 %?

Green et al. give 14.9 % as the previous value before the 15.1 %, and Intal and Ebong list 14.9 % (text year 2023, reference 2024) after 13.0 % certified in 2021 (reference 2022). The IoP news of April 2023 reports 13.8 % certified and 14.1 % not certified. pv magazine of February 2026 calls 14.2 % the earlier record of the same team ‘in June 2024’, which lies below the 15.1 % certified in April 2024. The steps are therefore 13.0 % (2021), 13.8 % (certified 2023), 14.1 and 14.2 % (not certified or lab-scale), 14.9 % and 15.1 % (certified tables), with increments of 0.2 and 0.35 points to the later values (computed). The June 2024 date is more likely the date of a paper than of a record (my reading), and 14.9 % is the certified predecessor. 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); PV Buzz article on the 13.8% certified kesterite cell of IoP-CAS and NJUPT, Nature Energy (18 April 2023); pv magazine, ‘Chinese Academy of Sciences achieves world record efficiency of 15.45% for kesterite solar cell’ (27 February 2026).

Ques: Which pure-sulfide CZTS record is right: 10.0, 11.4, 12.1, 12.36, 12.6 or 13.2 %?

Green et al. list UNSW cells of 10.0 ± 0.2 % (1.113 $c m^{2}$ , 2017) and 12.1 ± 0.3 % (0.2021 $c m^{2}$ , 749.0 mV, May 2024). UNSW announced 13.2 % on 28 January 2025 for a hydrogen-annealed cell, called 11.4 % the first record after about six years near 11 %, and did not state area, voltage or certification. In August 2026 pv magazine reports 12.6 % (852.8 mV, 21.0 $mA/c m^{2}$ , 70.1 %) with a certified 12.36 % (846.7 mV) on 0.2021 $c m^{2}$ , the area of the 12.1 % cell. The 13.2 % is higher than the 12.6 % of 2026 and the extracts do not explain this. The certified 12.36 % is the figure I use for a sulfide cell, the 12.1 % is the earlier certified value, and the 13.2 % is an uncertified claim until a certificate is found. 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); UNSW newsroom, ‘World-record photovoltaic efficiency achieved for kesterite solar cell’ (28 January 2025) and PV Tech, ‘UNSW claims world-record efficiency for kesterite solar cell’ (28 January 2025); pv magazine, ‘UNSW researchers build 12.6%-efficient kesterite solar cell with lower open-circuit voltage losses’ (18 August 2026).

Ques: What is the gap of the sulfide CZTS cell: 1.4, 1.5 or 1.6 eV?

Intal and Ebong give 1.4 to 1.6 eV for CZTS, Chen et al. an ordered gap of 1.4 eV, and Green et al. place the 12.1 % cell under a rule for gaps above 1.5 eV. pv magazine says that the 2026 UNSW cell reaches 65.3 % of the Shockley-Queisser voltage at 852.8 mV, which needs a limit of 1.306 V and so a gap of 1.60 eV (computed); at 1.5 eV the ratio would be 0.702. The 12.1 % cell has r $_{V}$ = 0.572 and a current of 92 % of the limit at 1.6 eV, but 0.616 and 81 % at 1.5 eV (computed). A current ratio of 92 % seems too high for a thin-film cell (my reading), so 1.5 eV is more likely for that cell and 1.6 eV for the 2026 cell; no gap is printed for either. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Chen, Dahliah, Rignanese and Hautier, ‘Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder’, Energy Environ. Sci. 14, 3567-3578 (2021); 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); pv magazine, ‘UNSW researchers build 12.6%-efficient kesterite solar cell with lower open-circuit voltage losses’ (18 August 2026).

Ques: What is the gap of CZTSSe: 1.0, 1.04 to 1.12, 1.10 or 1.15 eV?

Intal and Ebong report that selenium alloying lowered the gap to about 1.1 eV and that a graded absorber of 2014 ran from 1.0 eV at the molybdenum to 1.15 eV near the CdS. CAS gives 1.10 eV for the cell of 15.45 %, and Chen et al. give 1.0 to 1.5 eV for the family. Azzouzi et al. find 1.13 to 1.15 eV from the quantum-efficiency edge and 1.04 to 1.12 eV from electroluminescence, so the deficit of their device A is 707 or 617 mV (computed from the table), and they argue that gap-based deficits are unreliable. Linear interpolation with assumed end members of 1.5 and 1.0 eV puts 1.10 eV at 80 % selenium (computed). The values describe graded layers and different definitions, and 1.10 eV is the more likely effective gap of the record cell. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Institute of Physics, Chinese Academy of Sciences, news item on the 15.1% certified kesterite record (20 March 2026; Joule paper of Meng’s group, DOI 10.1016/j.joule.2026.102345); pv magazine, ‘Chinese Academy of Sciences achieves world record efficiency of 15.45% for kesterite solar cell’ (27 February 2026); Azzouzi, Cabas-Vidani, Haass, Roehr, Romanyuk, Tiwari and Nelson, ‘Analysis of the Voltage Losses in CZTSSe Solar Cells of Varying Sn Content’, J. Phys. Chem. Lett. 10, 2829-2835 (2019); Chen, Dahliah, Rignanese and Hautier, ‘Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder’, Energy Environ. Sci. 14, 3567-3578 (2021).

Ques: Does Cu-Zn disorder lower the gap by 130 meV or by 400 meV?

Chen et al. give the calculated gap of ordered CZTS as 1.4 eV and of a disordered 288-atom supercell as 1.0 eV, a difference of 0.4 eV (computed), but state elsewhere that disorder lowers the gap by about 130 meV, against 40 to 200 meV measured. Bourdais et al. say that Cu/Zn order can change the gap by more than 10 %, which is 0.14 eV for 1.4 eV (computed) and agrees with the 130 meV. The 1.0 eV probably belongs to a single fully disordered cell and the 130 meV to a thermal average (my reading), since the same paper has only about 20 % of the tetrahedra off-stoichiometry at an order parameter of 0.8. The 130 meV and the measured 40 to 200 meV are more likely for real films, and the extract does not explain the 0.4 eV. Reference: Chen, Dahliah, Rignanese and Hautier, ‘Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder’, Energy Environ. Sci. 14, 3567-3578 (2021); Bourdais et al., ‘Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?’, Adv. Energy Mater. 6(12), 1502276 (2016) (abstract page).

Ques: Is Cu-Zn disorder the main cause of the voltage deficit?

Bourdais et al. (2016) conclude that Cu/Zn disorder stays within the radiative limit and is not the main cause, because cells above 9 % never exceed 60 % of the radiative voltage. Chen et al. (2021) find that gap fluctuations of 60 meV cost about 70 meV, 10 % of the deficit (computed: 69.6 mV), and that the Urbach energy of 30 meV grows by at most 10 meV. Their title still speaks of the actual role of cation disorder: it lowers defect formation energies by 0.5 to 1 eV, cuts the lifetime of Sn $_{Z n}$ from 2.0 × 10 $^{4}$ to 3.2 ns and the recombination-limited efficiency from 25 to 14 %. The papers differ in the mechanism, not in the data: disorder acts through defects rather than through gap fluctuation. The defect route is more likely, and the IBM review notes at least 30 % disorder remaining. Reference: Bourdais et al., ‘Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?’, Adv. Energy Mater. 6(12), 1502276 (2016) (abstract page); Chen, Dahliah, Rignanese and Hautier, ‘Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder’, Energy Environ. Sci. 14, 3567-3578 (2021); IBM Research publication list entry ‘Unconventional kesterites: the quest to reduce band tailing in CZTSSe’ (abstract of the first listed entry, by Antunez, Bishop et al., Adv. Energy Mater.; year not stated).

Ques: Which voltage is the kesterite record: 530, above 600 or above 650 mV?

Green et al. give 529.9 mV for the 15.1 % CZTSSe cell and 510.9 mV for the 13.45 % cell; CAS reports above 600 mV at 1.10 eV for its 15.45 % cell; Intal and Ebong state a record voltage above 650 mV with a band-tail energy below 30 meV (2023). With 0.842 V as the limit at 1.10 eV these are 63 %, above 71 % and above 77 % (computed), against ‘never better than 60 %’ in 2016. The values need not conflict, because the cell of highest voltage is not the cell of highest efficiency: 600 mV and 15.45 % need a product J $_{sc}$ FF of 25.8 $mA/c m^{2}$ , against 28.4 for the 15.1 % cell (computed). No device is named for the 650 mV. For the certified cell of 2024 530 mV is the value, and the above 600 mV is the more recent one. 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); pv magazine, ‘Chinese Academy of Sciences achieves world record efficiency of 15.45% for kesterite solar cell’ (27 February 2026); Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Bourdais et al., ‘Is the Cu/Zn Disorder the Main Culprit for the Voltage Deficit in Kesterite Solar Cells?’, Adv. Energy Mater. 6(12), 1502276 (2016) (abstract page).

Ques: Is the best voltage of the Sn series 465 or 485 mV?

The abstract of Azzouzi et al. gives a voltage range of 423 to 465 mV for the four Sn contents, and the discussion repeats it, but Table 2 gives 485 mV for device C (31.2 at.% Sn) and the text says that the voltage rises from 423 to 485 mV. The table is consistent: V $_{oc, r a d}$ - V $_{oc}$ = 888 - 485 = 403 mV (computed), equal to the tabulated non-radiative loss, and 1110 - 485 = 625 mV equals the tabulated deficit. With 465 mV the differences would be 423 and 645 mV, which do not match. Device C also has the best efficiency, 8.3 %. The 485 mV is therefore more likely correct, and 465 mV is the voltage of device D, which falls because of a Sn(S,Se) $_{x}$ secondary phase. Reference: Azzouzi, Cabas-Vidani, Haass, Roehr, Romanyuk, Tiwari and Nelson, ‘Analysis of the Voltage Losses in CZTSSe Solar Cells of Varying Sn Content’, J. Phys. Chem. Lett. 10, 2829-2835 (2019).

Ques: Is a kesterite mini-module 10.1 or 11 to 12 % efficient?

Green et al. list a CZTSSe minimodule of NJUPT with 10.1 ± 0.3 % (10.48 $c m^{2}$ , six cells of 530.9 mV, 32.77 $mA/c m^{2}$ , 57.9 %, NREL January 2024), whose product is 10.07 % (computed); its fill factor is far below the 74.0 % of the best cell. Intal and Ebong say that pilot lines (they name SolAero and Hitachi Chemical) report 11 to 12 % mini-modules without a reference, and that no major manufacturer sells kesterite modules. The two figures differ in source type (a certified table and an unreferenced industry statement) and the preprint gives no module size. Its market of 0.0784 billion dollars in 2024 is also written as below 0.1 billion. The certified 10.1 % is the more reliable value; 11 to 12 % is possible but not verified. 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).

Ques: Is kesterite cadmium-free?

Intal and Ebong describe CZTS as free of cadmium and indium, but the same preprint notes that early devices used CdS buffers and that a Zn $_{1 - x}$ Cd $_{x}$ S buffer appears in 2016 work. Chen et al. report Cd-alloyed CZTS above 12 %, with cadmium in the absorber, and the IBM review concentrates on Cd and Ag substitution for reducing band tailing. The absorber elements Cu, Zn, Sn, S and Se contain neither Cd nor In, so the statement holds for the absorber. A device with a CdS buffer or a Cd-alloyed absorber is not Cd-free, whereas for CIGS a Cd-free record exists (Solar Frontier, CIGS page). The buffers of the best kesterite cells are not given in the extracts, so I treat the claim as true for the absorber only. Reference: Intal and Ebong, ‘Thin-Film Solar Photovoltaics: Trends and Future Directions’, arXiv:2508.05589 (2025); Chen, Dahliah, Rignanese and Hautier, ‘Origin of the low conversion efficiency in Cu2ZnSnS4 kesterite solar cells: the actual role of cation disorder’, Energy Environ. Sci. 14, 3567-3578 (2021); IBM Research publication list entry ‘Unconventional kesterites: the quest to reduce band tailing in CZTSSe’ (abstract of the first listed entry, by Antunez, Bishop et al., Adv. Energy Mater.; year not stated); Copper Indium Gallium Selenide (CIGS) Thin-Film Solar Cell page of this vault.

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Kesterite (CZTS and CZTSSe) Thin-Film Solar Cell