1. Theory

Definition: An organic solar cell uses a thin film of organic semiconductors between a transparent anode (indium tin oxide) and a metal cathode. In a bulk heterojunction the electron donor (a polymer such as P3HT, PTB7-Th, PM6 or D18) and the electron acceptor (the fullerenes PCBM and PC $_{71}$ BM, or non-fullerene molecules such as Y6, N3, BO-4Cl and IHIC) are mixed, so that every exciton is close to an interface (Wikipedia; Nature Materials).
Materials parameters: The gap of the molecules is 1 to 4 eV, the exciton binding energy 0.1 to 1.4 eV, the exciton diffusion length about 10 nm, the charge carrier diffusion length 3 to 10 nm in amorphous films and the domain size several nm; the hole mobility is at least a factor of ten below the electron mobility and at least 100 nm of film is needed for absorption (Wikipedia).
Absorption: Non-fullerene acceptors absorb up to $1.5 \times 1 0^{5}$ cm $^{- 1}$ against 1.0 to $5.0 \times 1 0^{4}$ cm $^{- 1}$ for fullerenes (Liu and Vandewal).
Voltage loss: The best non-fullerene cell of Liu and Vandewal has a gap of 1.45 eV (about 1.4 eV in the text), 0.89 V, 26.7 mA/cm $^{2}$ , 80.8 % and 19.2 %, so the deficit is 0.56 V against 0.31 V for GaAs and 0.36 V for silicon; the non-radiative part is 240 mV. Azzouzi et al. say that the deficit is seldom below 0.6 eV and that it is divided into a charge-separation loss (below 0.1 eV possible), an absorption-onset loss (down to about 0.01 eV in the best cells) and a non-radiative loss (typically 0.3 to 0.4 V).
Records (efficiency tables, version 64): Green et al. list 15.8 % for a cell of 1.064 cm $^{2}$ (0.8513 V, 25.11 mA/cm $^{2}$ , 73.9 %; test centre listed as FhG-ISE/FMF), 15.7 % for a minimodule of 19.31 cm $^{2}$ (Zhejiang University, 0.8771 V, 24.37 mA/cm $^{2}$ , 73.4 %), 14.5 % for a submodule of 204.11 cm $^{2}$ with 38 cells (FAU and Forschungszentrum Jülich), 13.1 % for a module of 1475 cm $^{2}$ (Waystech and Nanobit) and 19.2 ± 0.3 % for a small-area cell of 0.0326 cm $^{2}$ (Shanghai Jiao Tong University, NREL, March 2023, 0.9135 V, 26.61 mA/cm $^{2}$ , 79.0 %).
Later records: A Nature Materials work with D18-Cl as donor, N3 as acceptor and a regulator $AT - β 2O$ reports 20.82 % in its title and “over 20 %” certified, a thickness tolerance of 100 to 400 nm and a module of 18.04 % (certified); infinityPV gives 20.82 % in one passage and 20.43 % in another, a module of 15.03 cm $^{2}$ with 18.36 % (highlights) or 18.04 % (certified), more than 19 % for thicker layers and 83 % retention after 1200 h. Wikipedia gives 19.3 % for a 2023 cell of Hong Kong Polytechnic University.
Fullerene and tandem history: PTB7-Th:PC $_{71}$ BM reached 10.61 % and fullerene cells about 12 % in 2016; Azzouzi et al. quote 14 % for single junctions and 17 % for tandems by 2018 and Liu and Vandewal 20 % for two-terminal tandems.
Stability: Ossila gives a target of 10 % efficiency and 10 years, accelerated tests extrapolated to 15 years, a burn-in within the first 100 h and a T $_{80}$ counted after the burn-in; the Nature Materials cell keeps 83 % after 1200 h (infinityPV).

2. Working Principle

Steps: A photon creates an exciton in the donor or the acceptor, the exciton diffuses to the interface (about 10 nm), dissociates into a charge-transfer state and then into free charges, and the electrons and holes are carried through the acceptor and donor phases to the electrodes (Wikipedia; background knowledge for the sequence).
Absorption: A film of 100 nm absorbs 77.7 % of the photons at $1.5 \times 1 0^{5}$ cm $^{- 1}$ and 250 nm absorbs 97.6 % (computed, single pass without reflection).
Domain size: With a diffusion length of 10 nm, 20 nm domains collect 0.762 of the excitons and 100 nm domains 0.200 (computed, uniform generation and perfect sinks at the boundaries), which is why a blend with domains of several nm is used.
Dissociation: For PM6:Y6 the calculated dissociation rate of $5 \times 1 0^{12}$ s $^{- 1}$ is far above the decay rate of $7.26 \times 1 0^{8}$ s $^{- 1}$ , so the branching is 0.99985 (computed from the computational study).
Offset: The energy offset between the local exciton and the charge-transfer state is 170 meV (calculated) or 140 meV (measured) for PM6:Y6, and Liu and Vandewal report a negligible offset in the best non-fullerene blends.
Voltage: The voltage is set by the gap of the interface and the non-radiative recombination; 240 mV of loss means an electroluminescence efficiency of $8.8 \times 1 0^{- 5}$ (computed). The record cells have deficits of 0.49 to 0.60 V against 0.31 to 0.36 V for the perovskite cells of page 89 (computed, with assumed gaps of 1.40 to 1.45 eV).
Fill factor and current: The fill factors of 73.4 to 79.0 % are 0.84 to 0.90 of the ideal values and the current of 26.61 mA/cm $^{2}$ is 0.81 to 0.86 of the radiative limit (computed).

3. Applications

Printed and flexible modules: The large-area cells of the Nature Materials work are slot-die coated at 20 mm/s with a layer of about 250 nm (infinityPV), and Green et al. list modules up to 1475 cm $^{2}$ (13.1 %); the layers are thin, and Wikipedia names a fracture strain of 20 to 30 % as a mechanical target for flexible cells.
Semi-transparent and infrared-transparent cells: Wikipedia lists a semi-transparent PTB7-Th:IHIC cell of 9.77 % (2017), an infrared-transparent cell with 80 % transmittance from 670 to 1200 nm (2010) and a cell with 70 % visible transparency and 4 % efficiency (2012).
Tandem partners: Green et al. list a perovskite/organic tandem of 23.4 ± 0.8 % (2136 mV, 14.56 mA/cm $^{2}$ , 75.6 %), in which the organic cell is the low-gap cell (my reading, as the entry gives no gap).
Commercial position: Wikipedia puts the commercial threshold at 10 to 15 % and reports modules of 3 to 5 % from Konarka that lasted a few years; Ossila gives 10 years as the target.
Next steps: The thickness tolerance, the non-radiative loss near 0.15 V and the module records are the three items named in the sources (Nature Materials; Liu and Vandewal; Green et al.).

Exciton Generation, Diffusion and Dissociation

Name	Formula	Related Formulae	Variables/Constants
Absorbed fraction of the active layer	$A = 1 - e^{- α d}$	$d_{90} = \frac{l n 10}{α}$	$A : f r a c t i o n o f t h e p h o t o n s ab sor b e d in a s in g l e p a ss w i t h o u t r e f l ec t i o n (m y m o d e l)$ $α : ab sor pt i o n coe f f i c i e n t, u p t o 1.5 \times 1 0^{5} c m^{- 1} f or n o n - f u l l er e n e a cce pt or s an d 1.0 t o 5.0 \times 1 0^{4} c m^{- 1} f or f u l l er e n es (L i u an d V an d e w a l)$ $d : l a y er t hi c k n ess$ $d_{90} : t hi c k n ess f or 90 % ab sor pt i o n, W ik i p e d ia a s k s f or a t l e a s t 100 nm an d t h e N a t u r e M a t er ia l s w or k r e p or t s a t o l er an ce o f 100 t o 400 nm$ $(co m p u t e d : f or α = 1.5 \times 1 0^{5} c m^{- 1} d_{90} = 154 nm an d 100, 200, 250, 300 nm ab sor b 77.7, 95.0, 97.6, 98.9 %; f or 1 \times 1 0^{5} c m^{- 1} d_{90} = 230 nm an d 63.2, 86.5, 91.8, 95.0 %; f or 5 \times 1 0^{4} c m^{- 1} d_{90} = 461 nm an d 39.3, 63.2, 71.3, 77.7 %)$
Exciton collection efficiency of a domain	$η_{d i f f} = \frac{2 L}{w} tanh (\frac{w}{2 L})$	$L = D τ$	$L : e x c i t o n d i f f u s i o n l e n g t h, ab o u t 10 nm (W ik i p e d ia)$ $w : w i d t h o f a d o n or or a cce pt or d o main (W ik i p e d ia : se v er a l nm)$ $D : e x c i t o n d i f f u s i o n coe f f i c i e n t$ $τ : e x c i t o n l i f e t im e (Y 6 : u p t o 1 n s, L i u an d V an d e w a l), u ni f or m g e n er a t i o n an d p er f ec t s ink s a t b o t h d o main b o u n d a r i es a r e m y m o d e l a ss u m pt i o n s (ba c k g r o u n d k n o w l e d g e)$ $(co m p u t e d f or L = 10 nm : w = 10, 20, 40 an d 100 nm g i v e η_{d i f f} = 0.924, 0.762, 0.482 an d 0.200)$
Exciton dissociation branching	$η_{d i s} = \frac{k _{d}}{k _{d} + k _{r} + k _{n r}}$	$τ = \frac{1}{k _{r} + k _{n r}}$	$k_{d} : r a t e o f d i ssoc ia t i o n o f t h e l oc a l e x c i t o n in t o a c ha r g e - t r an s f er s t a t e, ab o u t 5 \times 1 0^{12} s^{- 1} f or P M 6 : Y 6 (c a l c u l a t e d)$ $k_{r}, k_{n r} : r a d ia t i v e an d n o n - r a d ia t i v e d ec a y r a t es o f t h e Y 6 s in g l e t, 3.6 \times 1 0^{7} an d 6.9 \times 1 0^{8} s^{- 1} (e x p er im e n t a l), a l l f r o m t h e co m p u t a t i o na l s t u d y in E n er g y an d E n v i r o nm e n t a l S c i e n ce (d 4 ee 03815 k)$ $(co m p u t e d : k_{r} + k_{n r} = 7.26 \times 1 0^{8} s^{- 1} g i v es τ = 1.38 n s, a g ain s t u p t o 1 n s o f L i u an d V an d e w a l, an d η_{d i s} = 0.99985)$

Voltage Loss, Fill Factor and Efficiency Checks

Name	Formula	Related Formulae	Variables/Constants
Radiative limit at the optical gap	$η_{r a d} = \frac{V _{oc, r a d} J _{sc, r a d} F F _{0}}{P _{in}}$	$J_{sc, r a d} = q \int_{E_{o pt}}^{\infty} Φ d E$	$E_{o pt} : o pt i c a l g a p, 1.45 e V in T ab l e 1 o f L i u an d V an d e w a l an d ab o u t 1.4 e V in t h e i r t e x t (t h e g a p s o f t h e r ecor d ce l l s a r e n o t g i v e n, so b o t h a r e a ss u m e d)$ $Φ : p h o t o n f l ux o f t h e A M 1.5 G s p ec t r u m, P_{in} = 1000.37 W / m^{2}, k_{B} T / q = 25.693 mV, e mi ss i o n in t o 2 π or, w i t h a mi r r or, π, s t e p ab sor pt an ce a s o n t h e e a r l i er p a g es$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d, π (2 π) : 1.40 e V 33.49 (32.92) %, V_{oc, r a d} 1.1397 (1.1219) V, J_{sc, r a d} 32.88 m A / c m^{2}; 1.45 e V 32.81 (32.27) %, 1.1863 (1.1685) V, 30.84 m A / c m^{2}; t h e 19.2 % ce l l s a r e 0.573 t o 0.585 o f t h e π l imi t; t h e m e a s u r e d 26.61 m A / c m^{2} i s 0.809 (1.40 e V) or 0.863 (1.45 e V) o f t h e l imi t c u r r e n t)$
Voltage deficit	$Δ V = \frac{E _{o pt}}{q} - V_{oc}$	$r_{V} = \frac{V _{oc}}{V _{oc, r a d}}$	$Δ V : d e f i c i t, L i u an d V an d e w a l g i v e 0.56 V (t e x t : ab o u t 0.55 V) f or t h e b es t n o n - f u l l er e n e ce l l a g ain s t 0.31 V f or G a A s an d 0.36 V f or cr y s t a l l in e s i l i co n, A z z o u z i e t a l . s a y t ha t i t i s se l d o m b e l o w 0.6 e V, t h e ce l l s a r e t h e G r ee n e t a l . e n t r i es (0.8513 V, 0.8771 V an d 0.9135 V) an d t h e 0.89 V ce l l o f L i u an d V an d e w a l, w i t h a ss u m e d g a p s$ $(co m p u t e d f or E_{o pt} = 1.45 (1.40) e V : 15.8 % ce l l 0.599 (0.549) V, 15.7 % minim o d u l e 0.573 (0.523) V, 19.2 % ce l l 0.537 (0.487) V, L i u an d V an d e w a l 0.560 (0.510) V; r_{V} (2 π) 0.729 (0.759), 0.751 (0.782), 0.782 (0.814) an d 0.762 (0.793); t h e p er o v s k i t e ce l l s o f p a g e 89 ha v e 0.307 t o 0.357 V)$
Non-radiative voltage loss	$Δ V_{n r} = \frac{k _{B} T}{q} ln \frac{1}{E Q E _{E L}}$	$V_{oc} = V_{oc, r a d} + \frac{k _{B} T}{q} ln Q_{l u m}$	$E Q E_{E L} : e x t er na l q u an t u m e f f i c i e n cy o f e l ec t r o l u min esce n ce$ $Q_{l u m} : l u min esce n ce q u an t u m e f f i c i e n cy (t h e tw o a r e t h e s am e q u an t i t y)$ $V_{oc, r a d} : r a d ia t i v e o p e n - c i r c u i t v o l t a g e, a c han g e o f 60 mV p er d ec a d e i s g i v e n b y L i u an d V an d e w a l, w h o r e p or t 240 mV (E Q E_{E L} ab o u t 0.01 %) f or t h e b es t ce l l, an d b y t h e co m p u t a t i o na l s t u d y 0.25 t o 0.27 V f or P M 6 : Y 6, 0.24 V f or P M 6 : L 8 - B O, 0.17 V f or P M 6 : Y 11 an d 0.15 V f or P B D B - T : S M 16$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : k_{B} T / q l n 10 = 59.16 mV p er d ec a d e; 240 mV g i v es E Q E_{E L} = 8.77 \times 1 0^{- 5}, 250 mV 5.95 \times 1 0^{- 5}, 270 mV 2.73 \times 1 0^{- 5}, 170 mV 1.34 \times 1 0^{- 3}, 150 mV 2.91 \times 1 0^{- 3}; Q_{l u m} = 1 0^{- 7} an d 1 0^{- 4} g i v e 0.414 an d 0.237 V (A z z o u z i e t a l . : 0.42 an d 0.24 V a t 300 K); w i t h V_{oc, r a d} = 1.1685 (1.1219) V t h e f o u r ce l l s ab o v e ha v e Δ V_{n r} = 0.317 (0.271), 0.291 (0.245), 0.255 (0.208) an d 0.279 (0.232) V)$
Ideal fill factor	$F F_{0} = \frac{v - l n ( v + 0.72 )}{v + 1}$	$v = \frac{V _{oc}}{k _{B} T / q}$	$F F_{0} : f i l l f a c t or w i t h o u t r es i s t i v e l osses (ba c k g r o u n d k n o w l e d g e, a s o n t h e e a r l i er p a g es), o n e j u n c t i o n (N = 1), t h e ce l l s a r e t h e e n t r i es o f G r ee n e t a l ., t h e 0.89 V ce l l o f L i u an d V an d e w a l (80.8 %) an d t h e P M 6 : Y 6 ce l l c i t e d in t h e co m p u t a t i o na l s t u d y (0.83 V, 25.3 m A / c m^{2}, 74.8 %)$ $k_{B} : B o l t z mann co n s t an t$ $(co m p u t e d : F F_{0} = 0.8675 f or t h e 15.8 % ce l l an d 73.9 % i s 0.852 o f i t; 0.8705 an d 0.843 f or t h e minim o d u l e; 0.8744 an d 0.903 f or t h e 19.2 % ce l l; 0.8719 an d 0.927 f or L i u an d V an d e w a l; 0.8650 an d 0.865 f or P M 6 : Y 6)$
Efficiency from the reported parameters	$η = \frac{V _{oc} J _{sc} F F}{P _{in}}, P_{in} = 100 mW/c m^{2}$	$η_{1} = \frac{V _{oc, 1} J _{sc, 1} F F _{1}}{V _{oc, 2} J _{sc, 2} F F _{2}} η_{2}$	$η : e f f i c i e n cy, V_{oc} in mV, J_{sc} in m A / c m^{2}, F F a s a f r a c t i o n (e f f i c i e n cy in p er ce n t = V_{oc} [mV] \cdot J_{sc} [m A / c m^{2}] \cdot F F /1000), t h e e n t r i es a r e f r o m G r ee n e t a l . (v er s i o n 64), L i u an d V an d e w a l (T ab l e 1) an d t h e co m p u t a t i o na l s t u d y (P M 6 : Y 6)$ $(co m p u t e d : 851.3 mV, 25.11, 73.9 % g i v e 15.80 %; 877.1 mV, 24.37, 73.4 % g i v e 15.69 %; 913.5 mV, 26.61, 79.0 % g i v e 19.20 %; 890 mV, 26.7, 80.8 % g i v e 19.20 %; 830 mV, 25.3, 74.8 % g i v e 15.71 %; t h e 15.8 % ce l l ha s 0.932 o f t h e v o l t a g e, 0.944 o f t h e c u r r e n t an d 0.935 o f t h e f i l l f a c t or o f t h e 19.2 % ce l l, p r o d u c t 0.823)$

Stability and Area Scaling

Name	Formula	Related Formulae	Variables/Constants
Burn-in, retention and lifetime	$R (t) = \frac{η ( t )}{η ( 0 )}$	$T_{80} = \frac{0.20 η ( 0 )}{∣ d η / d t ∣}$	$R : r e t ain e d f r a c t i o n$ $T_{80} : t im e t o 80 %, t h e seco n d e x p r ess i o n a ss u m es a l in e a r d ec a y f r o m t = 0 w i t h o u t b u r n - in (a ss u m e d), O ss i l a d escr ib es a b u r n - in in t h e f i r s t 100 h f o l l o w e d b y a l in e a r d ec a y an d a T_{80} co u n t e d a f t er t h e b u r n - in, t h e N a t u r e M a t er ia l s w or k r e p or t s 83 % a f t er 1200 h, O ss i l a g i v es a t a r g e t o f 10 y e a r s an d an e x t r a p o l a t e d l i f e t im e o f u p t o 15 y e a r s$ $(co m p u t e d : a l oss o f 17 % in 1200 h i s 0.0142 % p er h o u r an d a l in e a r T_{80} o f 1412 h; 10 y e a r s i s 87600 h an d 15 y e a r s 131 400 h, so 1200 h i s 0.0137 o f 10 y e a r s)$
Module power and area scaling	$P = η G A$	$\frac{η _{m o d}}{η _{ce l l}}$	$P : p o w er$ $G : 100 mW / c m^{2}$ $A : d es i g na t e d i l l u mina t i o n a r e a, t h e e n t r i es a r e t h e N a t u r e M a t er ia l s m o d u l e (15.03 c m^{2}, 18.04 % cer t i f i e d, 18.36 % in an o t h er p a ss a g e) an d t h e m o d u l es o f G r ee n e t a l . (19.31 c m^{2} 15.7 %, 204.11 c m^{2} 14.5 % w i t h 38 ce l l s, 1475 c m^{2} 13.1 %)$ $(co m p u t e d : 18.04 % o n 15.03 c m^{2} i s 271.1 mW an d 18.36 % i s 276.0 mW; t h e r a t i os t o t h e 19.2 % ce l l a r e 0.823 (15.8 %), 0.818 (15.7 %), 0.755 (14.5 %), 0.682 (13.1 %) an d 0.940 (18.04 %); 20.43/20.82 = 0.981 an d 18.04/20.43 = 0.883)$

Important Questions

Ques: What is the organic solar cell record: 15.8, 19.2, 19.3, 20.43 or 20.82 %?

Green et al. (version 64) list 15.8 % for a 1.064 $c m^{2}$ cell (0.8513 V, 25.11 $mA/c m^{2}$ , 73.9 %) and 19.2 ± 0.3 % for a small-area cell of 0.0326 $c m^{2}$ (Shanghai Jiao Tong, NREL, March 2023, 0.9135 V, 26.61 $mA/c m^{2}$ , 79.0 %). Wikipedia gives 19.3 % for a 2023 cell of Hong Kong Polytechnic, Liu and Vandewal give 19.2 % (0.89 V, 26.7 $mA/c m^{2}$ , 80.8 %), and the Nature Materials work has 20.82 % in its title with only ‘over 20 %’ certified. The 15.8 % entry has a larger area, so it does not compete with the small-area values; 19.2 to 19.3 % are one level and 20.4 to 20.8 % the later one. Wikipedia’s statement that these cells ‘lie near 10 %’ is out of date, and I take about 20.4 % as the certified level (my reading). 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); Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023); Nature Materials, ‘Organic solar cells with 20.82% efficiency and high tolerance of active layer thickness through crystallization sequence manipulation’ (abstract page, article s41563-024-02062-0); Wikipedia, ‘Organic solar cell’.

Ques: Is the certified Nature Materials cell 20.82 or 20.43 %, and the module 18.36 or 18.04 %?

The infinityPV page gives 20.82 % as certified in its highlights and 20.43 % as certified in its results, and 18.36 % for a 15.03 $c m^{2}$ module in the highlights against 18.04 % certified in the large-area section. The Nature Materials abstract page gives 20.82 % only in the title and says that the certified efficiency is over 20 %, with 18.04 % certified for the module. The title often carries the best in-house value and the certified value is lower (background knowledge), so 20.43 % is the more likely certified cell and 20.82 % the champion value (my reading). The module is more likely 18.04 %, which is the one called certified. The two module values correspond to 271.1 and 276.0 mW, and the ratios are 0.981 for 20.43 to 20.82 % and 0.883 for 18.04 to 20.43 % (computed). Reference: Nature Materials, ‘Organic solar cells with 20.82% efficiency and high tolerance of active layer thickness through crystallization sequence manipulation’ (abstract page, article s41563-024-02062-0); infinityPV, ‘Organic Solar Cells Breakthrough: 20.82% Efficiency and High Thickness Tolerance’.

Ques: Is the non-radiative voltage loss 0.15, 0.24, 0.25 or 0.4 V?

Liu and Vandewal give 240 mV for the best non-fullerene cell (an electroluminescence efficiency of about 0.01 %), 0.15 to 0.35 V over more than 50 Y-series devices and about 0.15 V for a near-infrared acceptor. The computational study gives 0.25 to 0.27 eV for PM6:Y6, 0.24 V for PM6:L8-BO, 0.17 V for PM6:Y11 and 0.15 V for PBDB-T:SM16, and 0.4 to 0.6 eV for fullerene cells. Azzouzi et al. give 0.3 to 0.4 V and a historical floor near 250 mV. The sources mix eV and V and refer to different blends and years. At 298.15 K, 240 mV corresponds to an electroluminescence efficiency of $8.8 \times 1 0^{- 5}$ , and 60 mV per decade is 59.2 mV (computed). The best blends reach 0.15 to 0.17 V, and 0.24 V is the level of the leading cells (my reading). Reference: Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023); Energy and Environmental Science article d4ee03815k (2025), computational study of PM6:Y6 charge-transfer and local-excitation states (RSC open access); Azzouzi et al., review of open-circuit voltage losses in organic solar cells, Trends in Chemistry (2019), accepted manuscript (Imperial College repository).

Ques: Does an organic cell need an energy offset to split excitons: 0.17, 0.09 or about 0 eV?

The computational study gives 170 meV between the local exciton and the charge-transfer state (140 meV in an electro-absorption measurement) and 0.09 eV between the exciton and the charge-transfer state of PM6:Y6. Liu and Vandewal write that the offset is negligible in the best non-fullerene blends and that a HOMO offset of about 0.2 eV is an empirical threshold below which the quantum efficiency and the voltage loss trade off. The calculation uses a dielectric constant of 3, and the authors say that experiments suggest more than 5, which they name as a likely source of the calculated offset. The dissociation rate of $5 \times 1 0^{12}$ $s^{- 1}$ against a decay rate of $7.26 \times 1 0^{8}$ $s^{- 1}$ gives a branching of 0.99985 (computed), so a small offset still splits the excitons. The numbers are different offsets, and a small one is the likely best (my reading). Reference: Energy and Environmental Science article d4ee03815k (2025), computational study of PM6:Y6 charge-transfer and local-excitation states (RSC open access); Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023).

Ques: How thick should the active layer be: 100, 250 or 400 nm?

Wikipedia says that at least 100 nm is needed for absorption. The Nature Materials work reports a thickness tolerance of 100 to 400 nm and more than 19 % for thicker layers, and its slot-die coated modules have about 250 nm. Liu and Vandewal give an absorption coefficient of up to $1.5 \times 1 0^{5}$ $c m^{- 1}$ for non-fullerene acceptors and 1.0 to $5.0 \times 1 0^{4}$ $c m^{- 1}$ for fullerenes. In a single pass without reflection, 100 nm absorbs 77.7 % and 250 nm 97.6 % at $1.5 \times 1 0^{5}$ $c m^{- 1}$ , but only 39.3 % and 71.3 % at $5 \times 1 0^{4}$ $c m^{- 1}$ (computed). The thickness tolerance helps coating, and the charge extraction limits thicker films (background knowledge). I take 100 nm as the minimum and 250 nm as the practical value for coated modules (my reading). The sources give no efficiency for each thickness. Reference: Wikipedia, ‘Organic solar cell’; Nature Materials, ‘Organic solar cells with 20.82% efficiency and high tolerance of active layer thickness through crystallization sequence manipulation’ (abstract page, article s41563-024-02062-0); infinityPV, ‘Organic Solar Cells Breakthrough: 20.82% Efficiency and High Thickness Tolerance’; Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023).

Ques: What domain size is needed: 5, 10 or 20 nm?

Wikipedia gives an exciton diffusion length of about 10 nm, domains of several nm and a carrier diffusion length of 3 to 10 nm in amorphous films, and the films are at least 100 nm thick. Liu and Vandewal give an exciton lifetime of up to 1 ns for Y6, while the computational study gives decay rates of $3.6 \times 1 0^{7}$ and $6.9 \times 1 0^{8}$ $s^{- 1}$ , which means 1.38 ns (computed). With L = 10 nm and perfect sinks at both boundaries, the collection efficiency of a domain of width w is $(2 L / w) tanh (w /2 L)$ , which gives 0.924 for 10 nm, 0.762 for 20 nm, 0.482 for 40 nm and 0.200 for 100 nm (computed, background knowledge for the model). A domain width of 10 to 20 nm is therefore the likely target, and the diffusion length of Y6 itself is not given in the sources (my reading). Reference: Wikipedia, ‘Organic solar cell’; Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023); Energy and Environmental Science article d4ee03815k (2025), computational study of PM6:Y6 charge-transfer and local-excitation states (RSC open access).

Ques: What fill factor do organic cells reach: 65, 70, 75 or 81 %?

Wikipedia says that the typical fill factor is below 70 %, that more than 75 % was reported by 2013 and that a bulk heterojunction needs above 0.8 with an external quantum efficiency above 90 % (simulation). Azzouzi et al. say that it seems capped near 65 % for Voc above 1 V against about 80 % in leading devices. Green et al. list 73.9, 73.4 and 79.0 %, Liu and Vandewal 80.8 % and the cited PM6:Y6 cell 74.8 %. The ideal values at these voltages are 0.868, 0.871, 0.874, 0.872 and 0.865, so the ratios are 0.852, 0.843, 0.903, 0.927 and 0.865 (computed). ‘Typically below 70 %’ is out of date; 73 to 75 % is typical for the larger cells and 79 to 81 % for the best small ones (my reading). Voltages above 1 V were not found in the records read here. Reference: Wikipedia, ‘Organic solar cell’; Azzouzi et al., review of open-circuit voltage losses in organic solar cells, Trends in Chemistry (2019), accepted manuscript (Imperial College repository); 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); Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023); Energy and Environmental Science article d4ee03815k (2025), computational study of PM6:Y6 charge-transfer and local-excitation states (RSC open access).

Ques: How does the efficiency depend on the area: 0.03 cm2, 1 cm2, 20 cm2 or 1475 cm2?

Green et al. list 19.2 % at 0.0326 $c m^{2}$ , 15.8 % at 1.064 $c m^{2}$ , 15.7 % for a minimodule of 19.31 $c m^{2}$ , 14.5 % for a submodule of 204.11 $c m^{2}$ with 38 cells and 13.1 % for a module of 1475 $c m^{2}$ . The Nature Materials work gives 18.04 % certified at 15.03 $c m^{2}$ . The ratios to 19.2 % are 0.823, 0.818, 0.755 and 0.682, and 18.04 % is 0.940 (computed). The 15.8 % cell has 0.932 of the voltage, 0.944 of the current and 0.935 of the fill factor of the 19.2 % cell (computed), so the loss is shared. The materials of the larger entries are not given, so the fall cannot be assigned to the area alone. The 18.04 % module is the best large-area value read here, and the module records lag the cell records by 4 to 6 points (my reading). 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); Nature Materials, ‘Organic solar cells with 20.82% efficiency and high tolerance of active layer thickness through crystallization sequence manipulation’ (abstract page, article s41563-024-02062-0); infinityPV, ‘Organic Solar Cells Breakthrough: 20.82% Efficiency and High Thickness Tolerance’.

Ques: How long do organic cells last: 1200 hours, 10 years or 15 years?

Ossila states a commercial target of 10 % efficiency and 10 years, extrapolated lifetimes of up to 15 years from accelerated tests (conditions not given), outdoor studies of about one year and a burn-in in the first 100 h, after which 25 to 50 % of the efficiency is said to remain (the page’s own summary flags this as inconsistent). The Nature Materials work reports 83 % after 1200 h, and Wikipedia says that Konarka modules lasted a few years. A loss of 17 % in 1200 h is 0.0142 % per hour and a linear T $_{80}$ of 1412 h, and 1200 h is 0.0137 of 10 years (computed, no burn-in assumed). The 15 years is an extrapolation and the 83 % a measurement; no source gives an outdoor lifetime for a record cell, so 10 years is a target and not a result (my reading). Reference: Ossila, ‘Factors Influencing OPV Stability’ (web page, based on Rafique et al.); infinityPV, ‘Organic Solar Cells Breakthrough: 20.82% Efficiency and High Thickness Tolerance’; Wikipedia, ‘Organic solar cell’.

Ques: How good is an organic tandem: 10.6, 17, 20 or 23.4 %?

Wikipedia reports a polymer tandem of 10.6 % (the cited paper is dated 2013 while the text says 2015), Azzouzi et al. 17 % for tandems by 2018 and Liu and Vandewal 20 % for two-terminal tandems. Green et al. list 23.4 ± 0.8 % for a perovskite/organic tandem (2136 mV, 14.56 $mA/c m^{2}$ , 75.6 %), and the parameters give 23.51 % (computed). The first three are all-organic and the last is a hybrid with a wide-gap perovskite (page 90), so they do not compete; the 10.6 % is old. The all-organic value of 20 % is about the same as the single-junction values of 19.2 to 20.8 %, so the sources give no like-for-like comparison of a tandem against its best subcell. The perovskite/organic value is the highest tandem number read here, and the lower values belong to earlier years (my reading). Reference: Wikipedia, ‘Organic solar cell’; Azzouzi et al., review of open-circuit voltage losses in organic solar cells, Trends in Chemistry (2019), accepted manuscript (Imperial College repository); Q. Liu and K. Vandewal, ‘Understanding and Suppressing Non-Radiative Recombination Losses in Non-Fullerene Organic Solar Cells’, Adv. Mater. 35, 2302452 (2023); 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); Perovskite-Silicon Tandem Solar Cell page of this vault.

Quartz 5

Explorer

Organic Solar Cell (Bulk Heterojunction)