The Current Status of Perovskite Solar Cell Research at UCLA

Department of Materials Science and Engineering University of California, Los Angeles, CA, USA

Lijian Zuo, Sanghoon Bae, Lei Meng, Yaowen Li, and Yang Yang*

1

What is a Solar Cell?

Diffusion potential to contacts is typically <1mVolt. k

A GOOD Solar Cell does not require a p-n junction!*

hνE

EFn

EFp

Voc

e- h

h+ e-e-

e - h+ e- h+

e-

h+ +

h + h +Wider Bandgap top,

bottom and sides

double hetero-structure Selective electron contact

Selective hole

contact

+ -

-

+

e- e- e-

h+h+ h+

Courtesy of Prof. Eli Yablonovitch

*http://energyseminar.stanford.edu/node/369

Lev Perovski (1792 – 1856) Structure

History of hybrid perovskite materials and solar cells

3Courtesy of Prof. Yanfa Yan

Perovskite Si CIGS GaAsBand gap (eV) 1.5 (tunable) 1.1 1.12 1.43

Absorption coefficient 104-5 103 104-5 104-5

Carrier mobility cm2/(V·s) 100 1500 < 10 8500

Carrier lifetime > 100 ns ms 50-200 ns <100 ns

Essential physical properties of major PV materials

Long Diffusion length

Low recombination rate

and high PL

Electron/hole transportation

Long carrier lifetime

4

Perovskite is a great PV material

Perovskite Si CIGS GaAsVOC 1.1 V 0.706 0.68V 1.12 V

VOC deficit 0.3 - 0.45 V 0.3-0.4V > 0.4 V ~0.3 V

JSC (mA/cm2) ~ 22 42.7 36 29.5FF ~80% ~ 80% ~80% >85%

Film thickness ~350 nm 100-200 um 1-2 um 4 um

Device parameters of different solar cells

5

Device parameter: A promising PV material

Yan et al, Adv. Mater. 2014, 26, 4653–4658

Planar Device structure: PiN & NiP

ITO

TiOx (N)

Perovskite (i)

Spiro (P)

Au

ITO

PEDOT or NiOx (P)

Perovskite (i)

PCBM or ZnO(N)

Au

N-i-P device structure (regular structure)

P-i-N device structure* (inverted structure)

At UCLA , we work on both PiN and NiP planar perovskite solar cells.

6

*Prof. T.F. Guo (Taiwan), (1) Adv. Mat. 25, 3727, 2013; (2) SPIE Solar and Alternative Energy, 2013

Park et al, APL Mater. 2, 081510 (2014)

• Solution-Based: one-step or two-Step • Annealing to evaporate solvents and to crystallize perovskites • Challenge: large-area, pinhole-free, with thickness/composition

control 10

Film formation via solution: one-step v.s. two-step

7Courtesy of Prof. Kai Zhu

8/16/17 8

Importance of crystal growth

Solution grown polycrystalline PVSK

[1] H. J. Snaith et al. Science 2015, 348, 683-686

500 nm

Fluorescence image of PVSK film

Boundary

Boundary

Local PL decay profiles

Recombination at GBs

Formation of trap statesStructural defects at GBs

non-radiative recombination loss at GBs

8/16/17 9

[1] Y. Yan et al. Adv. Electron. Mater. 2015, 1, 1500044 [2] D. K. et al. J. Phys. Chem. C 2017, 121, 3143−3148

→ structural defects at GB induce charge recombination loss

8/16/17 10

Ion migration through GBs Current-voltage hysteresis

Ion migration at GBs

ions

→ ion migration through GBs results in I-V hysteresis and poor stability

[4] J. Huang et al. Energy Environ. Sci., 2016, 9, 1752-1759

8/16/17 11

Ingression of moisture through GBs

Moisture ingression through GBs

→ We need to grow highly crystalline PVSK with less grain boundaries

[1]J. Huang et al. Energy Environ. Sci. 2017, 10, 516-522

8/16/17 12

1) Intermediate adduct method using a Lewis base additive

Manuscript under revision

Intermediate phase : adduct

8/16/17 13

MAPbI3

MAI•PbI2•DMSO MAI+PbI2+DMSO in DMF

After heatingIntermediate phase

X + :Y → X⋅Y acid base adduct

[1] N.-G. Park et al. J. Am. Chem. Soc. 2015, 137, 8696−8699 [2] N.-G. Park et al. Acc. Chem. Res. 2016, 49, 311−319

10 µm 10 µm

w/o intermediate phase w/ intermediate phase

DMSO (g)↑

EaIntermediate adduct

PVSK

 

[1] N.G. Park et al. Nat. Nanotech. 9, 2014, 927-932

fast growth → formation of small grains

slower growth→ formation of large grains

Crystallization kinetics

→To enhance the grain size, we need to slow down the reaction by Ea↑

8/16/17 14

MAI•PbI2•Lewis base

MAPbI3

Urea additive for higher Ea

DMSO µ=3.96D

Urea µ=4.56D

···· ····

0 s 10 s 30 s 60 s 70 s 90 s 120 s

65 oC 100 oC

ref

w/ 10 mol% urea

MAI•PbI2•DMSO

MAI•PbI2•DMSO•urea0.1

σ-

σ+

σ-

σ+

→ stronger interaction of urea with PVSK precursor, Ea↑, k↓

8/16/17 15[1] Manuscript accepted.

ref 1 mol% 2 mol% 4 mol% 6 mol%14161820

PCE

(%)

0.6

0.7

0.8

FF

20

22

J SC (m

A/cm

2 )

1.0

1.1

1.2

V OC (V

)

0.0 0.2 0.4 0.6 0.8 1.0 1.20

5

10

15

20

25

RefJ

SC: 21.47 mA/cm2

VOC

: 1.048 VFF: 0.77PCE: 17.34%

w/ urea 4 mol%J

SC: 21.68 mA/cm2

VOC

: 1.092 VFF: 0.78PCE: 18.55%

Curre

nt d

ensit

y (m

A/cm

2 )

Voltage (V)

300 400 500 600 700 8000

20

40

60

80

100

EQE

(%)

Wavelength (nm)

Ref (20.91 mA/cm2) w/ 4 mol% urea (21.26 mA/cm2)

Effect of urea on PV performance

8/16/17 16[1] Manuscript accepted.

ref

w/ 4mol% urea

500 nm

ref

500 nm

w/ 4mol% urea

ITO+SnO2

PVSK

spiro-MeOTAD

Ag

Effect of urea on morphology

8/16/17 17

800 nm

800 nm

[1] Manuscript accepted.

Presence of urea in the film

8/16/17 19

4000 3000 2000 1000

Tran

smitt

ance

Wavelength (nm)

MAPbI3+ureaMAPbI3urea

1800 1500 1200

C-NN-HC=O

10 20 30 40 50 Two theta (degree)

urea (powder)

# # (314)(224

)(222

)

(220

)

(202

)(2

11)

(112

)

4 mol% urea

Ref

(110

)

*

50 100 150 200 250 3000

2040

60

80

100urea

TG (%

)

Temperature (oC)

DMSO

→ urea exist in MAPbI3 film

→ no additional peaks and peak shift[1] Manuscript accepted.

8/16/17 20

Urea at GBPb I

ONC

A B C

D E F

4 mol% →with 50 mol% urea →crystallization of urea at GBs[1] Manuscript accepted.

8/16/17 21

Stability

0 5 10 15 20 25 300

5

10

15

20

PCE

(%)

Time (days)

ref 4 mol%

→ Both ex-situ and in-situ stability were improved after addition of urea

On shelf test Under 1 sun

[1] Manuscript accepted.

N-I-P Structure P-I-N Structure

1. Instability of Perovskite: CH3NH3PbI3 ! CH3NH3I+PbI2 2. Instability of Interface: Organic transport layers

Two major reasons of device instability：

Pursue of highly stable perovskite solar cells

ITO Glass

compact-TiO2

CH3NH3PbI3

Auspiro-MeOTAD

ITO GlassPEDOT:PSS

CH3NH3PbI3

Al

PCBM

22

ITO Glass

PEDOT:PSS

CH3NH3PbI3

Al

PCBM

NiO

ZnO

Organic: • Less stability; • Unable to block carriers;

“Replacement”

Metal oxide: • Ambient stability; • Prevent carrier leakage; • Optical transparency

Materials selection for charge transport layers

23

NiO

ITO Glass NiO

CH3NH3PbI3

Al

ZnO

ZnO

Transmission and AFM images of NiOx and ZnO layers

24J. You, et. al, Nature Nanotech. 11, 75 (2016).

NiOx ZnO

Device performance >16% PCE

Performance of Perovskite Solar Cells

25Y. Yang* et. al, Nature Nanotech. 11, 75 (2016).

Comparison of device stability using inorganic and organic charge transport layers

Air stability of devices with all metal oxide layers

26Y. Yang* et. al, Nature Nanotech. 11, 75 (2016).

Summary & outlooks

27

1. We report our studies on (1) UCLA PVSK progress, (2) the intermediated phase engineering, (3) carrier transport layers (All metal oxides).

2. Many issues: physical mechanism(s), hysteresis, Pb-containing, water soluble, stability still require much more understanding

3. We are still in the early stage of the perovskite PV research, and we like to establish collaborations with others who are interested in this topic.

4. How about OPV? We are continuing to work on it, and please be patience, more ( and important) results to come.

Yang’s group, UCLA, summer 2016

Acknowledgments

Thank you for your attention

UC-Solar Program