The Current Status of Perovskite Solar Cell Research at UCLA Lijian Zuo, Sanghoon Bae, Lei Meng, Yaowen Li, and Yang Yang* Department of Materials Science and Engineering University of California, Los Angeles, CA, USA 1
What is a Solar Cell? E hν - + e - e - e h + h + h + - Voc EFn EFp k Wider Bandgap top, bottom and sides Selective hole contact + + e - h h h + e - e - h + e - h + e - h + h + e - double hetero-structure + - Diffusion potential to contacts is typically <1mVolt. Selective electron contact A GOOD Solar Cell does not require a p-n junction!* *http://energyseminar.stanford.edu/node/369 Courtesy of Prof. Eli Yablonovitch
History of hybrid perovskite materials and solar cells Lev Perovski (1792 1856) Structure Courtesy of Prof. Yanfa Yan 3
Perovskite is a great PV material Perovskite Si CIGS GaAs Band gap (ev) 1.5 (tunable) 1.1 1.12 1.43 Absorption coefficient Carrier mobility cm 2 /(V s) Essential physical properties of major PV materials 10 4-5 10 3 10 4-5 10 4-5 100 1500 < 10 8500 Carrier lifetime > 100 ns ms 50-200 ns <100 ns Electron/hole transportation Low Long recombination rate Diffusion length and high PL Long carrier lifetime 4
Device parameter: A promising PV material Perovskite Si CIGS GaAs V OC 1.1 V 0.706 0.68V 1.12 V V OC deficit 0.3-0.45 V 0.3-0.4V > 0.4 V ~0.3 V J SC (ma/cm 2 ) ~ 22 42.7 36 29.5 FF ~80% ~ 80% ~80% >85% Film thickness Device parameters of different solar cells ~350 nm 100-200 um 1-2 um 4 um Yan et al, Adv. Mater. 2014, 26, 4653 4658 5
Planar Device structure: PiN & NiP Au Spiro (P) Perovskite (i) TiOx (N) ITO N-i-P device structure (regular structure) Au PCBM or ZnO(N) Perovskite (i) PEDOT or NiOx (P) ITO 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
Film formation via solution: one-step v.s. two-step 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 10 control Courtesy of Prof. Kai Zhu 7
Importance of crystal growth Solution grown polycrystalline PVSK 500 nm Boundary Boundary Fluorescence image of PVSK film [1] H. J. Snaith et al. Science 2015, 348, 683-686 Local PL decay profiles 8/16/17 8
Recombination at GBs Structural defects at GBs Formation of trap states non-radiative recombination loss at GBs structural defects at GB induce charge recombination loss [1] Y. Yan et al. Adv. Electron. Mater. 2015, 1, 1500044 [2] D. K. et al. J. Phys. Chem. C 2017, 121, 3143 3148 8/16/17 9
Ion migration at GBs ions Ion migration through GBs Current-voltage hysteresis 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 10
Moisture ingression through GBs Ingression of moisture 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 11
1) Intermediate adduct method using a Lewis base additive Manuscript under revision 8/16/17 12
Intermediate phase : adduct X + :Y X Y acid base adduct MAI+PbI 2 +DMSO in DMF MAI PbI 2 DMSO DMSO (g) MAPbI 3 Intermediate phase After heating 10 µm 10 µm w/o intermediate phase w/ intermediate phase [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 8/16/17 13
Crystallization kinetics MAI PbI 2 Lewis base Intermediate adduct E a MAPbI 3 PVSK fast growth formation of small grains slower growth formation of large grains To enhance the grain size, we need to slow down the reaction by E a [1] N.G. Park et al. Nat. Nanotech. 9, 2014, 927-932 8/16/17 14
Urea additive for higher E a σ - σ - DMSO µ=3.96d σ + Urea µ=4.56d σ + MAI PbI 2 DMSO ref MAI PbI 2 DMSO urea 0.1 w/ 10 mol% urea 0 s 10 s 30 s 60 s 70 s 90 s 65 o C 100 o C stronger interaction of urea with PVSK precursor, E a, k [1] Manuscript accepted. 8/16/17 15
Effect of urea on PV performance J SC (ma/cm 2 ) V OC (V) FF PCE (%) 22 20 1.2 1.1 1.0 0.8 0.7 0.6 20 18 16 14 ref 1 mol% 2 mol% 4 mol% 6 mol% Current density (ma/cm 2 ) EQE (%) 25 20 15 10 5 100 Ref J SC : 21.47 ma/cm 2 V OC : 1.048 V FF: 0.77 PCE: 17.34% w/ urea 4 mol% J SC : 21.68 ma/cm 2 V OC : 1.092 V FF: 0.78 PCE: 18.55% 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 80 60 40 20 Voltage (V) Ref (20.91 ma/cm 2 ) w/ 4 mol% urea (21.26 ma/cm 2 ) 0 300 400 500 600 700 800 Wavelength (nm) [1] Manuscript accepted. 8/16/17 16
Effect of urea on morphology ref ref 800 nm 500 nm w/ 4mol% urea w/ 4mol% urea Ag spiro-meotad PVSK ITO+SnO 2 500 nm 800 nm [1] Manuscript accepted. 8/16/17 17
TG (%) 100 80 60 40 20 0 Transmittance [1] Manuscript accepted. Presence of urea in the film MAPbI 3 +urea MAPbI 3 urea 4000 3000 2000 1000 DMSO urea 50 100 150 200 250 300 Temperature ( o C) Wavelength (nm) * (110) (112) (211) (202) C=ON-HC-N 1800 1500 1200 (220) 4 mol% urea # # (314) urea (powder) 10 20 30 40 50 Two theta (degree) (222) (224) urea exist in MAPbI 3 film Ref no additional peaks and peak shift 8/16/17 19
Urea at GB A B Pb C I D C E N F O 4 mol% with 50 mol% urea crystallization of urea at GBs [1] Manuscript accepted. 8/16/17 20
Stability 20 On shelf test Under 1 sun 15 PCE (%) 10 5 ref 4 mol% 0 0 5 10 15 20 25 30 Time (days) Both ex-situ and in-situ stability were improved after addition of urea [1] Manuscript accepted. 8/16/17 21
Pursue of highly stable perovskite solar cells Two major reasons of device instability: 1. Instability of Perovskite: CH 3 NH 3 PbI 3! CH 3 NH 3 I+PbI 2 2. Instability of Interface: Organic transport layers Au spiro-meotad CH3NH3PbI3 Al PCBM CH3NH3PbI3 compact-tio 2 ITO Glass PEDOT:PSS ITO Glass N-I-P Structure P-I-N Structure 22
Materials selection for charge transport layers Al PCBM CH3NH3PbI3 ZnO Replacement NiO PEDOT:PSS ITO Glass Organic: Less stability; Unable to block carriers; Metal oxide: Ambient stability; Prevent carrier leakage; Optical transparency 23
Transmission and AFM images of NiOx and ZnO layers NiOx ZnO ZnO CH3NH3PbI3 Al NiO ITO Glass ZnO NiO J. You, et. al, Nature Nanotech. 11, 75 (2016). 24
Performance of Perovskite Solar Cells Device performance >16% PCE Y. Yang* et. al, Nature Nanotech. 11, 75 (2016). 25
Air stability of devices with all metal oxide layers Comparison of device stability using inorganic and organic charge transport layers Y. Yang* et. al, Nature Nanotech. 11, 75 (2016). 26
Summary & outlooks 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. 27
Yang s group, UCLA, summer 2016
Acknowledgments UC-Solar Program Thank you for your attention