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Andrew M Telford Junior Research Fellow Imperial College London

1

Jenny Nelson’s group

• + -

+

New generation solar cells

greenoptimistic.com sigmaaldrich.com

Credit: University of California, Berkeley

• zbiosciences.com

The next step: biological electronics

Transients of the transient photovoltage spectroscopy (TrOTTr TPV)

Dan Bryant (ICL, Chemistry) Xiaoe Li (ICL, Chemistry) Jenny Nelson (ICL, Physics) James Durrant (ICL, Chemistry) Matt Carnie (Swansea, Specific) Joel Troughton (Swansea, Specific)

Phil Calado Brian O’Regan Piers Barnes

Hysteresis in perovskite solar cells

0.75 V

• Current-Voltage scans exhibit hysteresis

between forward and reverse scans

• Short circuit current and open circuit

voltage exhibits relaxation on the timescale of seconds

CH3NH3PbI3 (MAPI)

6

Ion migration

+ + +

• +

Eions

• +

C a t h

• d

e A n

• d

e

Ecell

Anode Cathode

Dark Light Voc Time Voltage

7

C a t h

• d

e A n

• d

e

Ion migration

+ + +

• +

Eions

• +

Ecell

Anode Cathode

Dark Light Vprecon < Voc Voc Time Voltage

8

Transients of the transient (TrOTTr) photovoltage measurements

9

The TrOTTr rig

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0V

TiO2 bottom cathode architecture

11

E E + +

• +

+

• +
• +

a) t = -60 s, Short Circuit b) t = -1 s, Short Circuit c) t = 0 s, Open Circuit d) t = 45 s, Open Circuit

+ +

• E

E field

Ionic charge accumulation

Tress et al. EES 2015; Zhang Mater Horiz RSC 2015; Xiao et al. Nat Mater. 2014; Eames et al. Nat. Comm. 2015 Anode Anode Anode Anode Cathode Cathode Cathode Cathode

12

Hysteresis-free devices!

• Changing contact materials

appears to alter J-V hysteresis

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Hysteresis depends on the contact materials

Top cathode - PCBM Bottom cathode – TiO2 Top cathode - ZnO

ZnO 14

0V

PCBM top cathode architecture

15

The role of interfacial recombination

• Without surface recombination, photogenerated charge carriers flood device and screen

ionic charge

Cathode Anode

ETL HTL

E

ETL HTL

E

Cathode Anode

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The role of interfacial recombination

• With surface recombination, photo-carrier concentrations are low- ionic charge dominates

E-field distribution

ETL HTL

E

ETL HTL

E

Cathode Anode Cathode Anode

Van Reenen et al. JPCL 2015, DOI: 10.1021/acs.jpclett.5b01645

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Jump-to-voltage photocurrent current transient

Positive transient photocurrent observed in both architectures indicating reverse field No bias light  few background carriers

E field

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Conclusions

• Hysteresis can be

reproduced in JV curves by switching ON or OFF the surface recombination, while allowing for ion migration.

Recombination centres

• Surface recombination

determines whether a reverse electric field at Voc is detectable or not. It also affects the extraction efficiency at 0<V<Voc.

• Mobile ions are present in

the device regardless of hysteresis.

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Ho How D w Does

• es the Ph

the Photo

• to-oxidation
• xidation of
• f Ful

Fuller lerenes enes Af Affe fect ct the the Beh Behav aviou iour r of

• f OP

OPV D V Dev evic ices? es?

James R. Durrant (Imperial, Chemistry) Jiaying Wu (Imperial, Chemistry Jason Röhr Beth Rice Alexandre De Castro Maciel Jenny Nelson Wing C. Tsoi (Swansea) Zhe Li (Swansea) Harrison K. H. Lee (Swansea) Emily Speller (Swansea)

Photo-oxidation of fullerene

0.0 0.2 0.4 0.6 0.8 1.0

• 8
• 6
• 4
• 2

3.6% 1.8% 0.9% 0.4% 0.2%

Current density (mA/cm

2)

Voltage (V)

PCDTBT:PCBM:O-PCBM 0%, Fresh

Ca/Al Glass ITO PEDOT:PSS PCDTBT:PCBM

0.0 0.2 0.4 0.6 0.8 1.0

• 8
• 6
• 4
• 2

0 min, fresh 10 mins 30 mins 60 mins

Current density (mA/cm

2)

Voltage (V)

Degraded PCDTBT:PCBM 600 mins

1 2 3 4 5 1 2 3 4 5 6 60 600 mins 30 10 Degraded PCDTBT:PCBM PCDTBT:PCBM:O-PCBM PCE (%) Relative O-PCBM (%) Fresh

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Harrison K. H. Lee

Photo-oxidation of PCBM

PCBM

O2

PCBM

N2

PCDTBT

PCDTBT:PCBM:O-PCBM

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LUMO HOMO

Photo-oxidation products

Xiao et al. JACS 2007, DOI: 10.1021/ja0763798 Matsuo et al. Chem Comm 2012, DOI: 10.1039/c2cc30262d

Epoxide Di-carbonyl

Beth Rice

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Mass spectrometry

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910.1 100 Relative Intensity (%) 850 900 950 1000 1050 1100 m/z

(a)

100 Relative Intensity (%) 850 900 950 1000 1050 1100 m/z

(b)

926.1 958.1 974.1 910.1 100 Relative Intensity (%) 850 900 950 1000 1050 1100 m/z

(c)

926.1 958.1 974.1 990.1 910.1 920 980 1040 942.1 958.1 974.1 926.1 990.1 1006.1 1022.1 920 980 1040 942.1 958.1 974.1 926.1

Fresh PCBM Degraded PCBM in blend with PCDTBT Degraded PCBM in solution

Mark F. Wyatt

IR spectroscopy

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1650 1700 1750 1800 1850 0.0 0.5 1.0

1782 cm

• 1

Normalised Absorbance Wavenumber (cm

• 1)

Photo-oxidation time 0min 10min 40min 100min 280min 610min 970min 1950min 1737 cm

• 1

Emily Speller

Sub-band gap states in PCBM

+ +

• 3.6 eV
• 5.5 eV
• 4.3 eV
• 6.0 eV

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Effect on RECOMBINATION #1: solar cell

+ +

• 3.6 eV
• 5.5 eV
• 4.3 eV
• 6.0 eV

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Time [μs] Light Intensity Cell Voltage Total Current Bias ON OC Bias OFF SC

ΔN

Bias ON OC Pulse Light Intensity Cell Voltage Δ Voltage Time [ns-μs]

τΔn

What happens in the device?

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Charge Extraction Transient Photovoltage

5.0x10

16

1.0x10

17

1.5x10

17

1E-6 1E-5 1E-4

0% O-PCBM 0.4% O-PCBM 1% O-PCBM

Charge carrier lifetime  [s] Charge carrier density n [cm

• 3]

Charge extraction and Transient Photocurrent/Photovoltage Spectroscopies

Energy Ln DoS Energy Ln DoS Energy Ln DoS Voc Voc Voc CB CB CB

0.70 0.75 0.80 0.85 0.90 2E16 4E16 6E16 8E16 1E17 1.2E17 1.4E17

0% O-PCBM 0.4% O-PCBM 1% O-PCBM

Charge Density n [cm-3] Voc [V]

Jiaying Wu

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Voc reconstruction

1 2 3 4 5 0.8 1.0

0% measured 0% reconstructed 0.4% measured 0.4% reconstructed 1% measured 1% reconstructed

Voc (V) light intensity (sun equivalents)

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Effect on RECOMBINATION #2: LED

+ +

• 3.6 eV
• 5.5 eV
• 4.3 eV
• 6.0 eV

31

Electroluminescence

1.0 1.2 1.4 1

O-PCBM 0% 0.2% 0.4% 0.9% 1.8% 3.6%

Normalised EL Energy [eV]

32

EL of pure blend components

Red-shift or change in oscillator strengths?

Ca/Al PCBM Glass ITO PEDOT:PSS Ca/Al PCDTBT Glass ITO PEDOT:PSS

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 0.0 0.2 0.4 0.6 0.8 1.0

Fresh PCDTBT Degraded PCDTBT Normalised EL Energy [eV] 1.0 1.2 1.4 1.6 1.8 0.0 0.5 1.0

PCBM fresh PCBM degraded

Normalised EL Energy [eV]

33

1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.01 0.1 1

O-PCBM 0% 0.2% 0.4% 0.9% 1.8% 3.6% Normalised EL Energy [eV]

1E-15 1E-13 1E-11 1E-9 1E-7 1E-5 1E-3 0.1

EQE

Electroluminescence and external quantum efficiency

Eg 𝐾0,𝑠𝑏𝑒 = 𝑟 න

∞

𝐹𝑅𝐹 ∙ 𝜚𝑐𝑐𝑒𝐹 𝑊

𝑃𝐷,𝑠𝑏𝑒 = 𝑙𝑈

𝑟 𝑚𝑜 𝐾𝑡𝑑,𝑠𝑏𝑒 𝐾0,𝑠𝑏𝑒 + 1

1.0 1.5 0.01 0.1 1

Normalised EL Energy [eV]

1E-13 1E-11 1E-9 1E-7 1E-5 1E-3

EQE 34

0% 0.2% 0.4% 0.9% 1.8% 3.6% 0.0 0.5 1.0 1.5 2.0

Voltage [V] O-PCBM content

Voc non-rad. loss

• abs. loss

SQ loss

Open-circuit voltage losses

EL-QE Solar Simulator SQ limit Eg/q

35

Conclusions

TRAPS!

HOMO-LUMO simulations Recombination at the D/A interface Voltage losses from non-radiative recombination Transport

36

Thanks

• Piers Barnes (Imperial)
• James Durrant (Imperial)
• Dan Bryant (Imperial)
• Xiaoe Li (Imperial)
• Xuhua Wang (Imperial)
• Saif Haque (Imperial)
• Irene Sanchez (Imperial)
• Emily Speller (Swansea)
• Harrison K. H. Lee (Swansea)
• Emily Speller (Swansea)
• Wing C. Tsoi (Swansea)
• Zhe Li (Swansea)
• Matt Carnie (Swansea)
• Joel Troughton (Swansea)
• Josep Sancho (Valencia)
• Brian Saunders (Manchester)
• Paul O’Brien (Manchester)
• Paul McNaughter (Manchester)
• Brian O’Regan (Sunlight

Scientific)