Fundamental Processes in Organic and Hybrid Solar Cells

www.tu-chemnitz.de/physik/OPKMLHP15, Kloster Banz · 12. März 2015 · [email protected]

Carsten Deibel

Optik und Photonik kondensierter Materie

Institut für Physik

Technische Universität Chemnitz

Fundamental Processes in Organic and Hybrid Solar Cells

www.tu-chemnitz.de/physik/[email protected]

How Do Organic Solar Cells Work?

Step 1: Light Absorption ➟ Exciton Generation in Polymer

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current



Step 2: Exciton Diffusion ➟ to Acceptor Interface

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet losses


Step 3: Exciton Dissociation ➟ Polaron Pair Generation

How Do Organic Solar Cells Work?Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

charge transfer: very fast and very efficient

Voltage

Current

singlet losses



Step 4: Polaron Pair Dissociation ➟ Free Electron–Hole Pairs!

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet lossesgeminate losses



Step 5: Charge Transport ➟ Photocurrent

Fulle

rene

Aluminium Cathode

Transparent Anode

Polymer

Voltage

Current

singlet lossesgeminate losses

nongeminate losses


Outline

photogeneration vs geminate recombination

experiment: effect of intercalation

nongeminate recombination

simulation: effect of phase separation

radiative efficiency

experiment: perovskite photovoltaics


Model System: Intercalation

OCH3O

SS

SS

R

Rn

pBTTT pBTTT:PCBM

1:4

1:0.7

1:1

OCH3O

H3COO

pBTTT:bisPCBM

1:1

PCBM

c.f. Cates et al, Nano Lett. 9, 4153 (2009) and Nano Lett.12, 1566 (2012)


Intercalation→ Microstructure

pBTTT:PCBM

1:4

1:1

100

80

60

40

20

Cou

nts

/ a.u

.

4.54.03.53.02.52.0

2θ / °

neat pBTTT pBTTT:PCBM, 1:1 pBTTT:PCBM, 1:4 pBTTT:bisPCBM, 1:1

3.1 nm

2.3 nm85 °C, 30 min

pBTTT:bisPCBM

1:1

OCH3O

OCH3O

H3COO

SS

SS

R

Rn


Formation of Charge Transfer Complex

pBTTT:PCBM

1:4

1:1

pBTTT:bisPCBM

1:1

101

102

103

104

105

106

107

Abs

orpt

ion

/ cm

-1

1000 800 700 600 500

Wavelength / nm

3.02.52.01.51.0

Energy / eV

neat pBTTT pBTTT:PCBM, 1:1 pBTTT:PCBM, 1:4 pBTTT:bisPCBM, 1:1

CT

groundstate

singletexcitons

CT

freecarriers

c

h


Time Delayed Collection Field (TDCF)

0

td tcoll

-V

Vpre

Vcoll

t

laserpulse

0

j

Qpre

Qcoll

t

Qtot = Qpre+Qcoll

Laser excitation: 532 nm, 80 ps

typical delay time td=20ns

c.f. Mort et al, PRL 45, 1348 (1980)


Field Dependent Photogeneration

In Dependence of Intercalation

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

cha

rge

q tot

-4 -3 -2 -1 0

Prebias / V

pBTTT:PCBM 1:1 1:4

532nm

pBTTT:PCBM

1:4

1:1


Hot or not? Scenarios:

ground state

singletexcitons

CTcomplexes

freecarriers

fastrelaxation

ground state

singletexcitons

CTcomplexes

freecarriers

hotdissociation

Photon energy dependence

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

cha

rge

q tot

-4 -3 -2 -1 0

Prebias / V

pBTTT:PCBM 1:1 1:4

532nm


Hot or not?

P=P(λ)

P≠P(λ)ground state

singletexcitons

CT

freecarriers

fastrelaxation

ground state

singletexcitons

CT

freecarriers

hotdissociation


532

nm53

2 nm

1064

nm

1064

nm

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

cha

rge

q tot

-4 -3 -2 -1 0

Prebias / V

pBTTT:PCBM 1:1 1:4

532nm


Hot or not?


Only weak photon energy dependence on photogeneration yield

P=P(λ)

P≠P(λ)ground state

singletexcitons

CT

freecarriers

fastrelaxation

ground state

singletexcitons

CT

freecarriers

hotdissociation

532

nm53

2 nm

1064

nm

1064

nm

1.0

0.8

0.6

0.4

0.2

0.0

Nor

mal

ized

cha

rge

q tot

-4 -3 -2 -1 0

Prebias / V

pBTTT:PCBM 1:1 1:4

532nm1064nm


Subsequent Charge Extraction

1.00

0.95

0.90

0.85

0.80

Extraction efficiency

-4 -3 -2 -1 0

Prebias / V

1.00

0.95

0.90

0.85

0.80

0.75

0.70

Nor

mal

ized

cha

rge

q tot

pBTTT:PCBM 1:4

pBTTT:bisPCBM 1:1

pBTTT:PCBM

1:4

pBTTT:bisPCBM

1:1

A. Zusan, K. Vandewal, …, C. Deibel. Adv. Ener. Mater. 4, 1400922 (2014)


Intermediate Conclusions on Photogeneration

Photogeneration depends on electric field • strong vs weak field dependence

(pBTTT 1:1→1:4): role of delocalisation!

Photogeneration is approx. photon energy independent • Photogeneration via

relaxed intermediate state (charge transfer complex)

ground state

singletexcitons

CT

freecarriers

fastrelaxation

A. Zusan, K. Vandewal, …, C. Deibel. Adv. Ener. Mater. 4, 1400922 (2014)


Outline


experiment:






LUMO

HOMO

(1)

(2)

(1)

Nongeminate Recombination by Langevin

1st guess for Recombination in low-mobility materials

}R(n) / (µe + µh)n2

(1) finding of charge carriers → mobility μ

(2) recombination event (faster than (1))


Deviations from Langevin Recombination

300

200

100

0

-100

curre

nt d

ensi

ty [

A/m

2 ]

0.80.60.40.20.0

voltage [V]

dark 1 sunw/o add with add

PCE [%] FF [%]w/o add 3.8 51with add 7.1 69

• „reduced Langevin“, c.f. Pivrikas et al,

PRL 94, 176806 (2005)

• R(n) due to μe(n)+ μh(p)?

PC70BM

F

S

S

O

S

S

O

O

n

O

PTB7


(a) (b) (c)

Impact of Morphology

Phase separation with bulk traps

Phase separation, bulk & interface traps

Classical Langevin case, R ∝ (μe+μh)⋅n2

R ∝ f(μe,μh)⋅n2 ? In addition, deep interface traps?

Homogeneous, „effective medium“


Kinetic Monte Carlo Simulation

• hopping transport

• energetic disorder, e.g. Gaussian

density of states distribution

• blend morphology

• determine μe, μh, and R=k(μe, μh)npAcceptor

Donor

+

E-Field

−

c.f. Bässler, PSSB 175, 15 (1993)


Simulated Nongeminate Recombination

small domains: almost as homogeneous system, Langevin

R = knp

k =e

✏(µe + µh) = kL



large domains: min mobility, Koster et al, APL 88, 052104 ︎2006︎

R = knp

k =e

✏min (µe, µh)



general case: „mix of both“!



Result:

power law mean

R = knp

k =e

✏f1(d)2Mg(d) (µe, µh)

where Mg =

✓µge + µg

h

2

◆1/g

d (nm) f1 g Case

small domains < 5nm ≈ 1 1 arithmetic mean

typical case 10–35 0,5–1 0 geometric mean

large domains > 50nm < 0,5 -1 harmonic mean k /✓

1

µe+

1

µh

◆�1

k /✓µe + µh

2

◆

k / (µeµh)1/2

M. Heiber, V. Dyakonov, C. Baumbach, C. Deibel. Accepted by PRL (2015). arXiv:1503.02848


Intermediate Conclusions on Nongeminate Recombination

Charge carrier recombination in dependence of phase separation • most organic bulk heterojunction

solar cells: geometric mean of mobilitiesbetter approximation of nongeminate losses than Langevin’s arithmetic mean

• reduction factor partly explained for asymmetric mobilities

M. Heiber, V. Dyakonov, C. Baumbach, C. Deibel. PRL, accepted (2015). arXiv:1503.02848


Outline


experiment:






Absorption tail of Inorganics

De#Wolf.#et.#al.#JPCL%5%1035%(2014)%


…of an OPV

De#Wolf.#et.#al.#JPCL%5%1035%(2014)%

PPV:PCBM((EQE%shape)


De#Wolf.#et.#al.#JPCL%5%1035%(2014)%EU=15#meV

…and a Perovskite


MAPI Perovskite cells

hence relates to the offset between band gap and open circuit voltagedefined by: Woc ; (Egap/q) – Voc.

A highly relevant figure of merit to compare different PV tech-nologies is therefore to determine how far they are from their respect-ive radiative upper limits8 and hence also what we focus on here. Thiscan be determined in two separate ways: A) One basically measuresthe EQEEL of the cell in absolute terms. This is performed by injectinga small forward current, as close as possible to the short circuitphotocurrent (determined under solar illumination) and then mea-sure how many photons are emitted from the device. The procedureis hence analog to measuring the efficiency of a LED close to the veryonset of emission. With the EQEEL determined and the device darksaturation current (J0), obtained from Eqn. 4, one can calculate thedark saturation current in the radiative limit (J0,Rad) via their simplerelation

J0,Rad~EQEEL:J0 ð6Þ

and then also the corresponding open circuit voltage in the radiativelimit (VOC,Rad). B) The second method to determine radiative effi-ciency relies on measuring the EQEPV(E) at the band edge and thencalculate J0,Rad via Eqn. 2. Since J0,Rad calculated this way is fullydetermined by the shape of the EQEPV(E) at very low energies andthe earth blackbody spectrum is such a strongly decreasing exponen-tial function, it is essential to measure the spectral shape of EQEPV(E)very accurately with high sensitivity to minimize measurement noisein the lowest energy regime. By rearranging Eqn. 5 one obtains:

EQEEL~eqVOC

kT : J0,Rad

JSC

! "ð7Þ

so that with a reliable value of J0,Rad available, one can instead cal-culate the radiative efficiency. Hence two methods are available andhere we have pursued both methods to verify their accuracy in deter-mining the radiative efficiency and as such we assess if the R.R.R. isvalid for the new perovskite based solar cells.

Devices. The device structures of the two types of solar cells used inthis study are shown schematically in Figure 2. We first emphasizethat the I–V characteristics of the fresh MAPI cells showed negligiblehysteresis, meaning that concerns related to voltage sweep directionand time dependence, could be safely disregarded22. Such depen-dencies would likely affect the illuminated and dark I–V sweeps

differently, and may therefore also invalidate the superpositionassumption in the R.R.R. A short circuit current of 189 A/m2, anopen circuit voltage of 1080 mV and a fill factor (FF) of 55% wereobtained for the MAPI cell. The OPV showed, as expected,substantially worse performance with a JSC limited to only 32 A/m2, a VOC of 830 mV and a FF of 51%.

Quantum efficiency of radiative recombination. The observedpeak position of electroluminescence from the MAPI cell is similarto that detected earlier by photoluminescence23 and is located at760 nm or 1.61 eV (Figure 3A) and the shape is independent ofthe injection current density. The very sharp peak with a full-width-at-half-maximum of only 81 meV indicates that the emis-sion comes from a narrow distribution of states likely located atthe band edge of this perovskite semiconductor24. The peak of theEL coincides with the location of the steep slope of the EQEPV(E)spectra, highlighting the complete absence of any Stokes shift in theMAPI material. Hence, the assumption that this MAPI structure is adirect band gap semiconductor with its band gap energy located atthe peak of EL emission is highly justified, but actually somewhat indisagreement with recent published values18,25,26 of the band gap ofMAPI cells. Those were obtained from diffuse reflectance spectro-scopy or UPS, and reported to be 1.50–1.57 eV. This implies thateither the earlier band gap determination is not fully justified, or thehere observed radiative recombination does not occur over the bandgap, but from higher lying electronic states, which we deem less likelyto occur. It must be pointed out that the absorptance andcorresponding EQEPV(E) spectra actually do extend to slightlylower energies than the peak position of the EL spectra, indicatingthat absorption does indeed occur slightly below the here determinedband gap from EL, also in our device.

A typical EL spectrum obtained by driving the perovskite cells at acurrent density similar to its JSC is depicted in Figure 3A, showing themeasured photon flux distribution (in eV) (black line). When thisflux distribution is integrated over all energies the total EL flux isobtained, which when divided with the injected current provides thevalue of the radiative efficiency EQEEL under solar flux charge carrierdensity.

Figure 3B shows the measured EQEPV(E) spectra, the spectraldistribution of the 300 K BB environment radiation, and thesought-after product of these two spectra. This spectrum, in accord-

Figure 2 | Device layout of the studied solar cells. (A) Perovskite (MAPI) and (B) Organic (MDMO-PPV:PCBM) cells.

www.nature.com/scientificreports

SCIENTIFIC REPORTS | 4 : 6071 | DOI: 10.1038/srep06071 3

Jsc 18,9 mA/cm2

Voc 1080 mVFF 55 %η 11,2 %

Solar cell processing including perovskite coevaporation of the two starting materials PbI2 and CH3NH3I by Bolink group in Valencia

c.f. Reviews by Snaith, Grätzel, etc


Photovoltaic Quantum Efficiency

1.5 2.0 2.5 3.0 3.5Energy (eV)

0.0

0.2

0.4

0.6

0.8

1.0 EQE PV

EQ

EP

V

normalised


and Emission (Electroluminescence)

1.5 2.0 2.5 3.0 3.50.0

5.0x1017

1.0x1018

1.5x1018

Energy (eV)

EQEEL=1.2E-4

Pho

ton

Flu

x nr

/(s*e

V*m

2 ) Emission Flux(Jinj=189 A/m2)

0.0

0.2

0.4

0.6

0.8

1.0 EQE PV

EQ

EP

V

Electroluminescence Yield = 0,012 % for Jinj = Jsc (1 sun)


EQEPV vs EL


Black Body Radiation: Sun and Sample

EQEPV (E) = EL(E)E�2exp

✓E

kT

◆Reciprocity: G. Kirchhoff (1860); P. Würfel (1982); U. Rau, PRB (2007)

SunEarth


EQEPV vs EL: Reciprocity Relation


A figure of merit to judge solar cell performance.

External Radiative Efficiencies (ERE)

M. A. Green, Prog. Photovolt: Res. Appl. 20:472 (2012)


A figure of merit to judge solar cell performance

External Radiative Efficiencies (ERE)

K. Tvingstedt, Malinkiewicz, Baumann, Deibel, Snaith, Dyakonov, Bolink. Sci Rep, 4:6071, 2014

M. A. Green, Prog. Photovolt: Res. Appl. 20:472 (2012)


Intermediate Conclusions on Radiative Efficiency in Perov’SC

Perovskite Hybrid Solar Cells • steep absorption onset:

high degree of order • reciprocity fulfilled • EL yield: promising • voltage induced PL quenching:

dominantly free carriers

K. Tvingstedt, Malinkiewicz, Baumann, Deibel, Snaith, Dyakonov, Bolink. Sci Rep, 4:6071, 2014

hence relates to the offset between band gap and open circuit voltagedefined by: Woc ; (Egap/q) – Voc.

A highly relevant figure of merit to compare different PV tech-nologies is therefore to determine how far they are from their respect-ive radiative upper limits8 and hence also what we focus on here. Thiscan be determined in two separate ways: A) One basically measuresthe EQEEL of the cell in absolute terms. This is performed by injectinga small forward current, as close as possible to the short circuitphotocurrent (determined under solar illumination) and then mea-sure how many photons are emitted from the device. The procedureis hence analog to measuring the efficiency of a LED close to the veryonset of emission. With the EQEEL determined and the device darksaturation current (J0), obtained from Eqn. 4, one can calculate thedark saturation current in the radiative limit (J0,Rad) via their simplerelation

J0,Rad~EQEEL:J0 ð6Þ

and then also the corresponding open circuit voltage in the radiativelimit (VOC,Rad). B) The second method to determine radiative effi-ciency relies on measuring the EQEPV(E) at the band edge and thencalculate J0,Rad via Eqn. 2. Since J0,Rad calculated this way is fullydetermined by the shape of the EQEPV(E) at very low energies andthe earth blackbody spectrum is such a strongly decreasing exponen-tial function, it is essential to measure the spectral shape of EQEPV(E)very accurately with high sensitivity to minimize measurement noisein the lowest energy regime. By rearranging Eqn. 5 one obtains:

EQEEL~eqVOC

kT : J0,Rad

JSC

! "ð7Þ

so that with a reliable value of J0,Rad available, one can instead cal-culate the radiative efficiency. Hence two methods are available andhere we have pursued both methods to verify their accuracy in deter-mining the radiative efficiency and as such we assess if the R.R.R. isvalid for the new perovskite based solar cells.

Devices. The device structures of the two types of solar cells used inthis study are shown schematically in Figure 2. We first emphasizethat the I–V characteristics of the fresh MAPI cells showed negligiblehysteresis, meaning that concerns related to voltage sweep directionand time dependence, could be safely disregarded22. Such depen-dencies would likely affect the illuminated and dark I–V sweeps

differently, and may therefore also invalidate the superpositionassumption in the R.R.R. A short circuit current of 189 A/m2, anopen circuit voltage of 1080 mV and a fill factor (FF) of 55% wereobtained for the MAPI cell. The OPV showed, as expected,substantially worse performance with a JSC limited to only 32 A/m2, a VOC of 830 mV and a FF of 51%.

Quantum efficiency of radiative recombination. The observedpeak position of electroluminescence from the MAPI cell is similarto that detected earlier by photoluminescence23 and is located at760 nm or 1.61 eV (Figure 3A) and the shape is independent ofthe injection current density. The very sharp peak with a full-width-at-half-maximum of only 81 meV indicates that the emis-sion comes from a narrow distribution of states likely located atthe band edge of this perovskite semiconductor24. The peak of theEL coincides with the location of the steep slope of the EQEPV(E)spectra, highlighting the complete absence of any Stokes shift in theMAPI material. Hence, the assumption that this MAPI structure is adirect band gap semiconductor with its band gap energy located atthe peak of EL emission is highly justified, but actually somewhat indisagreement with recent published values18,25,26 of the band gap ofMAPI cells. Those were obtained from diffuse reflectance spectro-scopy or UPS, and reported to be 1.50–1.57 eV. This implies thateither the earlier band gap determination is not fully justified, or thehere observed radiative recombination does not occur over the bandgap, but from higher lying electronic states, which we deem less likelyto occur. It must be pointed out that the absorptance andcorresponding EQEPV(E) spectra actually do extend to slightlylower energies than the peak position of the EL spectra, indicatingthat absorption does indeed occur slightly below the here determinedband gap from EL, also in our device.

A typical EL spectrum obtained by driving the perovskite cells at acurrent density similar to its JSC is depicted in Figure 3A, showing themeasured photon flux distribution (in eV) (black line). When thisflux distribution is integrated over all energies the total EL flux isobtained, which when divided with the injected current provides thevalue of the radiative efficiency EQEEL under solar flux charge carrierdensity.

Figure 3B shows the measured EQEPV(E) spectra, the spectraldistribution of the 300 K BB environment radiation, and thesought-after product of these two spectra. This spectrum, in accord-

Figure 2 | Device layout of the studied solar cells. (A) Perovskite (MAPI) and (B) Organic (MDMO-PPV:PCBM) cells.

www.nature.com/scientificreports

SCIENTIFIC REPORTS | 4 : 6071 | DOI: 10.1038/srep06071 3


Acknowledgments…

Andreas Zusan Michael Heiber → Kristofer Tvingstedt Alexander Wagenpfahl → Alexander Förtig Jens Pflaum Vladimir Dyakonov

Martin Heeney

Koen Vandewal (→TUD) Alberto Salleo

Juliane Kniepert Thomas Brenner Dieter Neher

Arved HüblerBystrik TrnovecTino Zillger et al.

Fundamental Processes in Organic and Hybrid Solar Cells

Science

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