Solar Photovoltaics & Energy Systems · Summary: Organic Semiconductors... have a huge potential...

Solar Photovoltaics & Energy SystemsLecture 6. Emerging Technologies

ChE-600

Wolfgang Tress, May 2016

1

Energy Payback Time (EPBT)

2

Record Efficiencies

3

NREL Chart

4

Organic Solar Cells

5https://3dprint.com

10..100 nm

Steps of Energy Conversion

[1] Tang, Appl. Phys. Lett. 48, 183 (1986)[2] Hiramoto et al. Appl. Phys. Lett. 58, 1062 (1991)[3] B. Maennig et al., Appl. Phys. A 79, 1 (2004)

Steps of energy conversion

Exciton generation via light absorption

Exciton migration (diffusion)

Exciton separation and dissociation

Charge carrier transport

Charge carrier collection/injection

[3]

Donor Acceptor

energy

-

+

-

-

+

electrode

electrode

Challenges

• Disorder, poor charge transport

• Excited states hard to split

• Low fill factor

• High recombination, low voltage

• Narrow absorption, but broad onset

• Low long-term stability

6

Bulk Heterojunction: Morphology

• Tune amount of mixing/demixing

• Energy vs. entropy

• Annealing

• Solvents

7

TEM tomography

Nano Lett., 2009, 9 (2), pp 853–855 DOI: 10.1021/nl803676e

Absorption and EQE

8

• Narrow absorption bands

EQE and JV-curve

9

Jph depends on voltage

Charge collection problem

Reduces fill factor

p-HTL

n-ETL

Light Management

10

Thin film optics: interference

Beer-Lambert law does not apply

Meta

l

HTL

D:A

ETL

ITO

Meta

l

HTL

D:A

ETL

50 nm

Microcavity and Plasmonics:Marc Schiffler

p-HTL

n-ETL

Tandem Solar Cells

• Current matching

11

How to Increase Current and Voltage

12

EFn

EFp

eVoc

+

-

“HOMO”

“LUMO”

E

+

-

EgDA

Donor Acceptor

Current: Low band gap materials Broad absorption spectra Complementary

absorption of donor and acceptor

push-pull molecules

Voltage: Reduce “offset” energies Combination of donor

and acceptor Decrease recombination

Tuning Absorption in Polymers

13

Working Principle and Role of Contacts

14

→ Provide built-in potential due to difference in work functions

E

+

-

HTL Donor Acceptor Al

HTL = hole

transport layer

Energy diagrams at short circuit:

ITO Donor Acceptor Al

+

-

EVac

EF


+

-

Donor Acceptor

EVac

WF

anode

EF

WF

cathode

EF

Model-system

Flat vs. Bulk Heterojunction

15

→ Build selective device!

E

+

-


+

-


EVac

HTL = hole

transport layer

Energy diagrams at short circuit:

+

-

Acceptor 40 nm

Donor 8 nm

D:A 30 nm

diffusion

recombination

blocked

HTL Donor:Acceptor Al

no changes of Voc

decrease of Voc

Missaligned contacts → S-Shape

Geminate Recombination

Donor Acceptor

-

+

-

-

+

electrode

electrode

Geminate recombination

+

-

Non-geminaterecombination

16

Charge Transfer States: hot, (de)localized?

Donor Acceptor

-

+

-

-

+

electrode

electrode

Coulombically bound?

hot

energy

Vandewal et al. Nature Materials 2014

17

Charge Transfer State: Delocalized?

18

Compare to TD DFT simulations

In collaboration with U. Roethlisberger, EPFL

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

10-4

10-3

10-2

10-1

100

energy / eV

EQ

E F

TP

S s

cale

d

ZnPc

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

10-4

10-3

10-2

10-1

100

energy / eV

EQ

E F

TP

S s

cale

d

0.0 : 2.0 : 3

ZnPc

ZnF4Pc:ZnPc:C

60

Charge Transfer State and Voc

19

Charge-transferabsorption

Donor absorption

𝑉oc,rad = 1.0 V

𝑉oc,rad = 1.2 V

𝐽ph = 𝑒 EQE 𝐸 𝛷AM1,5g 𝐸 𝑑𝐸

𝑉oc,rad =𝑘B𝑇

𝑒ln

𝐽ph

𝐽em,0+ 1

𝐽em,0 = 𝑒 EQE 𝐸 𝜙BB 𝑇0 𝑑𝐸

State-of-the-Art Efficiencies

• Single junction η > 11 %

• Triple η > 13 %20

Fullerene free

Jsc = 16 mA/cm2

Fabrication

• Low-Cost: Roll-to-roll

• Printing

• Slot die coating

• Evaporation

21

popupcity.net

penscetriaru.5hark.net

https://www.tu-ilmenau.de/

http://penscetriaru.5hark.net/roll-out-solar-panel-9.html

Organic Photovoltaics State of the Art

Building integrated, Flexible, Colorful, Semitransparent

Record 13.2 %

o Stability

- In module ? %

- Not really in market yet

- Efficiency

22

Future Applications

23

Pictures: heliatek and Konarka

Summary: Organic Semiconductors

... have a huge potential• low cost roll to roll and solution processability• large area and ultra-thin devices• variety of different hydrocarbons

… are well suited for solar cells• low energy input• different dyes• abundant

… are different• film consists of weakly bound molecules• mostly amorphous and even intermixed films• optical excitation is localized• charge transport occurs mainly via hopping• optical and electrical gap decoupled

24

Dye-Sensitized Solar Cells

25

www.asknature.org

http://www.asknature.org/strategy/ee4e268a5a0fe3861f6d1f5ae21ea608

Dye-Sensitized Solar Cell: Ingredients

26Hardin, Slideshare

Dye-Sensitized Solar Cell: Schematics

• Absorption on monolayer of dye, decoupled from transport

• Meso or nanostructure increased surface area27

Dye-Sensitized Solar Cell: Energetics

• Redox potential of electrolyte fixed28

Maçaira, J., Andrade, L. & Mendes, A. Modeling, simulation and design of dye sensitized solar cells. RSC Adv. 4, 2830–2844 (2013).

History and State of the Art

29

• 1980s sensitization of TiO2

• 1991 Nature paper: η = 7 %

• Solid state DSC

• Now: η = 13 %

• Dye with designated (broad or IR) spectrum

• Electrolyte for high voltage (I Co)

Applications

30

www.connect-green.com

• Consumer: indoor?

• Building integrated

• Challenges: Efficiency, Stability, Upscaling

http://www.connect-green.com/tag/alternative-energy/page/4/

Is there room for further technologies?

31

Perovskite Solar Cells

32http://cleantechnica.com

What is Perovskite?

• 1839: perovskite = CaTiO3 discovered

• 1958: CsPbX3 (X = Cl, Br, or I) perovskite structure determined

• 1978 Cs cation replaced by methylammoniumcations CH3NH3

+ organic–inorganic hybrid

perovskites

• Last two decades: perovskite researched in electronics

Møller, C. K. Nature 182, 1436 (1958).

Mitzi, D. B. Synthesis, Structure and Properties of Organic–Inorganic Perovskites and Related Materials: Progress in Inorganic Chemistry Vol. 48 (ed. Karlin, K. D.) 1–121 (J. Wiley & Sons, 1999).

Ishihara, T. Optical properties of PbI-based perovskite structures. Journal of Luminescence 60–61, 269–274 (1994).

33

Weber, D. Z. Naturforsch. 33b, 1443–1445 (1978). Weber, D. Z. Naturforsch. 33b, 862–865 (1978).

History

1. Perovskite replaces dye

Solution processing route on TiO2

2009 2012 2013 2014 2015

Miyasaka, Park, Graetzel, Snaith, Seok, Bolink …

Fabrication:PbI2 + CH3NH3I

3.5 %6.5 %

12 %15 %

20 %

16 %18 %

34

The First Solar Cells

• perovskite replaces the molecular sensitizer in dye sensitized solar cell (DSSC)

• unstable (10 minutes operation), dissolves in liquid electrolyte

35

Kojima, A., Teshima, K., Shirai, Y. & Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 131, 6050–6051 (2009).

CH3NH3PbI3

CH3NH3PbBr3

3.8 %2009

Structure of dye-sensitized solar cell

CH3NH3Pb-Halide as Pigment in DSSC

36

Im, J.-H., Lee, C.-R., Lee, J.-W., Park, S.-W. & Park, N.-G. 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 3, 4088–4093 (2011).

CH3NH3PbI36.5 %

2011

• perovskite replaces the molecular sensitizer in dye sensitized solar cell (DSSC)

• unstable (10 minutes operation), dissolves in liquid electrolyte

History

37


2. Solid state device


2009 2012 2013 2014 2015



3.5 %6.5 %

12 %15 %

20 %

16 %18 %

3. Sequential deposition

Sequential Deposition

• Better control of crystal morphology and conformal coating

• Tuning of dipping time, concentration, solvent, temperature etc.

• Correlation of perovskite loading, conversion and thickness

Use of a two-step technique to form the hybrid perovskite:

38

Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316–319 (2013)

PbI2 + CH3NH3I CH3NH3PbI3

Cross Sectional SEM

P

i

nanocomposite

n

39

Burschka, J. et al. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 499, 316–319 (2013).

sublimation in vacuum

History

40


2. Solid state device

4. Solvent engineering

5. Composition engineering


2009 2012 2013 2014 2015



3.5 %6.5 %

12 %15 %

20 %

16 %18 %

Co

mp

act

film

s w

ith

larg

e cr

ysta

ls

planar architectures

3. Sequential deposition

CH3NH3PbI3: A Good Solar Cell Material

*Stranks, S. D. et al. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide PerovskiteAbsorber. Science 342, 341–344 (2013); Xing, G. et al. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 342, 344–347 (2013).

• High crystallinity• Absorption coefficient of 104 … 105 cm-1

• Band gap of 1.6 eV, valence band edge at -5.4 eV • Ambipolar semiconductor with high charge carrier mobilities*• Wannier excitons with fast dissociation at room temperature, high dielectric

constant• Low defect density, even if solution processed• Characterized in stacks similar to solar cells many intrinsic parameters and

influence of morphology and grain boundaries not extensively quantified

41

Goldschmidt Tolerance Factor

t = 0.89–1.0 cubic structure

t = 0.8-1.0 3 D perovskite

42

ABX3

NATURE PHOTONICS DOI: 10.1038/NPHOTON.2014.134

MA: methylammonium (CH3NH3+)

EA: ethylammonium (CH3CH2NH3+)

FA: formamidinium (NH2CH=NH2+)

Band Gap Engineering

Valence band formed by orbitals from B and X43

http://onlinelibrary.wiley.com/doi/10.1002/smll.201402767/full#smll201402767-fig-0002

http://onlinelibrary.wiley.com/doi/10.1002/smll.201402767/full#smll201402767-fig-0002

Certification

44

PCE of 21 %> 1.1 V

19.5 % on 1 cm2

Luminescence and Open-Circuit Voltage

• If there is absorption, there will be emission.

• The solar cell in the dark is in thermal equilibrium with its surroundings,

meaning that it absorbs and thus emits:

𝑎 𝐸 𝛷BB 𝑇 = 300K, 𝐸

Emission spectrum can be predicted from absorption

45

absorber Black body emitter at 300 K

0 0.2 0.4 0.60

1

2

3

4

x 1023

energy /eV

photo

n flu

x / m

-2 s

-1

Black body radiation at T = 300 K

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d s

pe

ctr

al p

ho

ton

flu

x

1.5 1.6 1.7 1.80

0.5

1

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d E

QE

PV

experimental data

model

experimental data

model T = 320 K

model T = 300 K

model T = 350 K

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d s

pe

ctr

al p

ho

ton

flu

x

1.5 1.6 1.7 1.80

0.5

1

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d E

QE

PV

experimental data

model T = 320 K

model T = 300 K

model T = 350 K

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d s

pe

ctr

al p

ho

ton

flu

x

1.5 1.6 1.7 1.80

0.5

1

1.5 1.55 1.6 1.65 1.7 1.75 1.8

10-3

10-2

10-1

100

energy / eV

no

rma

lize

d E

QE

PV

experimental data

model T = 320 K

model T = 300 K

model T = 350 K

Spectra of Perovskite Device

46

Tress, W. et al. Predicting the Open-Circuit Voltage of CH3NH3PbI3 Perovskite Solar Cells Using Electroluminescence and Photovoltaic Quantum Efficiency Spectra: the Role of Radiative and Non-Radiative Recombination. Adv. Energy Mater. (2015). doi:10.1002/aenm.201400812

emission EQE ∝ absorption

Steep absorption onset, Urbach energy of approx. 15 meV



• The solar cell in the dark is in thermal equilibrium with its surroundings,

meaning that it absorbs and thus emits:



• Under illumination (thermodynamic limit):


𝑒ln

𝐽ph

𝐽em,0+ 1

47

𝑒 𝑎 𝐸 𝛷BB 𝑇 = 300K, 𝐸 𝑑𝐸

Rau, U. Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells. Phys. Rev. B 76, 085303 (2007).

1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

10-3

10-2

10-1

100

energy / eV

scale

d E

QE

PV

Compared to Organics

Steeper absorption onset higher Voc

Charge-transfer-state absorption

Donor (ZnPc)absorption

𝑉oc,rad = 1.0 V

𝑉oc,rad = 1.2 V

𝑉oc,rad = 1.33 V

48

Shockley Queisser Limit

In theory:

CH3NH3PbI3 can reach 30 %

49

0.5 1 1.5 2 2.5 3 3.5 4 4.50

5

10

15

20

25

30

35

bandgap Eg [eV]

eff

icie

ncy

[%

]

(c)

SiGaAs

CIGS perovskite

0 0.2 0.4 0.6 0.8 1 1.2

-25

-20

-15

-10

-5

0

5

10

voltage [V]

cu

rre

nt d

en

sity [m

A / c

m 2]

0 0.2 0.4 0.6 0.8 1 1.2

-60

-40

-20

0

20

extr

acta

ble

po

we

r [m

W/c

m2]

(a)

Eg = 1.6 eV

VOC

= 1.32 V

JSC

= 25 mA/cm2

FF = 90.5 % = 30 %

MPP

𝑉oc =𝐸g

𝑒−𝑘B𝑇

𝑒ln

𝑁𝐶𝑁𝑉

𝑛𝑝

𝜂 = 𝑉oc𝐽scFF

𝑉oc

𝐽sc



• The solar cell in the dark is in thermal equilibrium with its surroundings, meaning that it absorbs and thus emits:





𝑒ln

𝐽ph

𝐽em,0+ 1

50



Recombination

-

+

Φem

-

+

-

via defects (SRH type)

trap

CB

VB

Au

-

at surfaces (wrong electrode)

radiativeelectron-hole

-

+

CB

VB

Auger recombination

-

non-radiative recombination

enters thermodynamic

limit

pure, defect-free material

passivation, reduction of contact area

material property (indirect

semiconductor)

51

to be avoidedunavoidable in absorber



• The solar cell in the dark is in thermal equilibrium with its surroundings, meaning that it absorbs and thus emits:





𝑒ln

𝐽ph

𝐽em,0+ 1

𝑉oc,real=𝑘B𝑇

𝑒ln EQEEL

𝐽ph

𝐽0+ 1

52


solarcell

light

operate as LED

EQEEL =


Driving the solar cell as LED

EL reaches 0.5 %, non-rad. recombination dominant

53

JV curve in the dark, emitted photon flux and the resulting quantum efficiency of EL

𝐽inj = 𝐽0 exp𝑒𝑉

𝒏 𝑘B𝑇− 1

Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Science Advances 2, e1501170 (2016).

Arun Paraecattil

ideality factor

Recombination Mechanisms

54

𝑉oc =𝐸g

𝑒−𝑘B𝑇

𝑒ln

𝑁𝐶𝑁𝑉

𝑛𝑝

𝛽𝑛𝑝

𝐺0 = 𝛼 𝐸 𝛷BB 𝐸 𝑑𝐸 = 𝛽𝑛0𝑝0 = 𝛽𝑛i2

𝛽 ≈ 10−11cm3s−1

radiativeelectron-hole

-

+

Φem

-

+

-

via defects (SRH type)

trap

CB

VB

n = 1 n = 2

𝑘𝑛 =1

𝜏𝑛

𝑑𝑛

𝑑𝑡= −𝛽𝑛2(𝑡) −

1

𝜏𝑛(𝑡)

Ideally:

Measure Recombination Rates

Long PL lifetime reduced non-radiative recombination

55

𝑑𝑛

𝑑𝑡= −𝛽𝑛2(𝑡) −

1

𝜏𝑛(𝑡)

𝛽 ≈ 10−11cm3s−1

PL decay

Bi, D. et al. Efficient luminescent solar cells based on tailored mixed-cation perovskites. Science Advances 2, e1501170 (2016).

𝜏 = 350 ns

Hysteresis: Backward and Forward Scan

-1 -0.5 0 0.5 1

-20

-15

-10

-5

0

voltage / V

curr

ent density / m

A c

m-2

10 mV/s

100 mV/s

1,000 mV/s

10,000 mV/s

100,000 mV/s

(a)

1 sun

• Result depends on• preconditioning

• scan direction

-1 -0.5 0 0.5 1

-20

-15

-10

-5

0

voltage / V

cu

rre

nt d

en

sity / m

A c

m-2

JV curve not sufficient to accurately determine efficiencyHysteresis related to long-term instability and device degradation?

56

-1 -0.5 0 0.5 1

-20

-15

-10

-5

0

voltage / V

cu

rre

nt d

en

sity / m

A c

m-2

10 mV/s

100 mV/s

1,000 mV/s

10,000 mV/s

100,000 mV/s

1 sun

Hysteresis: Voltage Sweep Rates

• Result depends on• preconditioning• scan direction• scan rate

Device has “short-term memory”

• 2 possible reasons:• Displacement

current• Modified

photocurrent

57

-0.2 0 0.2 0.4 0.6 0.8

-20

-15

-10

-5

0

5

10

15

voltage / V

curr

ent density / m

A c

m-2

100 mV/s

1,000 mV/s

10,000 mV/s

30,000 mV/s

100,000 mV/s

1 sun

(a)

Charge carrier collection efficiency recombination

Tress, W. et al. Understanding the rate-dependent J–V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy Environ. Sci. 8, 995–1004 (2015).

without TiO2 blocking layerLight induced structural changes?

Deep (surface) traps?Ferroelectrics?Ion migration?

…?

Hysteresis: What Happens?

58

Solar cell works as a pin device with built-in potential Vbi dropping over the approx. intrinsic perovskite layer**Bergmann, V. W. et al. Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell. Nat Commun 5, (2014).


CH3NH3PbI3

perovskite

-

+

CB -3.8 eV

VB -5.4 eV

-5.2 eV

-4.0 eV

1.6 eV

TiO2

HTM


-1 -0.5 0 0.5 1

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

voltage / V

curr

ent density / m

A c

m-2

-0.5 0 0.5 1

-20

-15

-10

100,000 mV/s

100,000 mV/s

10,000 mV/s

200,000 mV/s

50,000 mV/s

10 mV/s

(a)

100,000 mV/s

59

Solar cell works as a pin device with built-in potential Vbi dropping over the approx. intrinsic perovskite layer**Bergmann, V. W. et al. Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell. Nat Commun 5, (2014).


Solar cell works as a pin device with built-in potential Vbi dropping over the approx. intrinsic perovskite layer*


-1 -0.5 0 0.5 1

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

voltage / V

curr

ent density / m

A c

m-2

-0.5 0 0.5 1

-20

-15

-10

100,000 mV/s

100,000 mV/s

10,000 mV/s

200,000 mV/s

50,000 mV/s

10 mV/s

(a)

100,000 mV/s

-0.2 0 0.2 0.4 0.6 0.8-20

-15

-10

-5

0

5

10

voltage / V

curr

ent density / m

A c

m-2

-0.3 V

0.25 V

0.5 V

0.75 V

(b)

100,000 mV/s

*Bergmann, V. W. et al. Real-space observation of unbalanced charge distribution inside a perovskite-sensitized solar cell. Nat Commun 5, (2014).

60

without TiO2 blocking layer

Perovskite Solar Cells: Outlook

61

Costs• Solution processing

• Towards low-T processes• Absorber material low-cost

• Encapsulation?

Lifetime / Stability

Efficiency

• Theoretical SQ limit with 1.57 eV gap: η = 31 %

1 2 3 40

5

10

15

20

25

30

35

bandgap Eg [eV]

eff

icie

ncy

[%

]

• Considering the predicted max. Voc (incl. tail) of 1.32 V, 24 mA/cm2

η = 29 %, (VMPP = 1.2 V, FF ≈ 90 %)

• Considering state-of-the-art recombination (SRH): Voc = 1.2 V, FF = 83 % η = 24 %

CH3NH3PbI3

• Toxicity?• Acceptance?

• Sensitive to humidity• Stable when encapsulated?

• Upscaling?

Third Generation Concepts

62

http://www.solarroadways.com/

Overcoming the SQ Limit

63

http://www.intechopen.com/books/solar-cells-research-and-application-perspectives/optimization-of-third-generation-nanostructured-silicon-based-solar-cells

Intermediate Band Solar Cells

• Experimental trials: InAs QDs in GaAs not yet very successful

64

cstec.engin.umich.edu

• Maintain high voltage

• Increase current

• 3 quasi Fermi levels

http://cstec.engin.umich.edu/highlights/highlights_archive2010/

Hot Carrier Solar Cells

• Avoiding thermalization by introducing “phonon bottleneck”

• Energy selective contacts

not yet successfully realized

65

web.stanford.edu

http://web.stanford.edu/group/gcep/cgi-bin/gcep-research/all/hot-carrier-solar-cell-implementation-of-the-ultimate-photovoltaic-converter/

Up- and Down Conversion

• Effects shown, however with low yield66

www.intechopen.com

Fraunhofer ISE

http://www.intechopen.com/

There is always room for new ideas…

67

http://www.solarroadways.com/

Wattways

World Solar Challenge Australia

http://www.colas.com/

Solar Photovoltaics & Energy Systems · Summary: Organic Semiconductors... have a huge potential...

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Transcript of Solar Photovoltaics & Energy Systems · Summary: Organic Semiconductors... have a huge potential...