The Importance of Perovskite Pore Filling in Organometal Halide Sensitized TiO2 – based Solar Cells
Tomas Leijtens a b, Giles Eperon b, Henry Snaith b
a Condensed Matter Physics, University of Oxford, Parks Road, Oxford OX1 3PU
International Conference on Hybrid and Organic Photovoltaics
Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Ecublens, Switzerland, 2014 May 11th - 14th
Organizers: Michael Graetzel and Mohammad Nazeeruddin
Poster, Tomas Leijtens, 101
Publication date: 1st March 2014

 

 

 

Long considered a promising technology for sustainable energy production, solid state dye sensitized solar cells (ssDSSCs) have recently begun to fulfill their promise of high performances by substituting the traditional sensitized dyes for organometal halide perovskites.1,2Both the commonly used CH3NH3PbI3-xClx and CH3NH3PbI3 perovskite absorbers have been shown to be effective charge transporters, negating the requirement to infiltrate the organic hole transporter, spiro-MeOTAD, into the TiO2 mesopores, depdening on the degree to which the perovskite itself fills the pores.2–7

Here, we quantify the degree of pore filling of the CH3NH3PbI3-xClx perovskite in the TiO2 mesopores and explore its effect on device performance.  We gradually change the solar cell architecture from one where the perovskite truly acts as a sensitizer (< 50 % pore filling), to one where the perovskite completely fills the TiO2 pore volume uniformly and forms a capping layer of perovskite on top of the mesostructure.

The solar cell performance is strongly correlated with the degree of perovskite pore filling, and solar cells with perovskite capping layers exhibit by the highest performance.  By performing steady state photoinduced absorption measurements as well as small perturbation photocurrent and photovoltage measurements, we can identify the mechanism responsible for this increase in performance and also show that electron transport occurs through the TiO2 in all architectures studied here.  As the perovskite begins to completely fill the mesopores, the contact between the spiro-OmeTAD hole transporter and the TiO2 nanoparticles is decreased, drastically reducing the recombination rate and fundamentally changing the recombination pathway (Figure 1), allowing higher electron densities to be maintained in the mesoporous TiO2.  This raising of the electron quasi Fermi level increases the electron transport rate without a coupled increase in the recombination rate, unlike what is generally observed in traditional ssDSSCs.  These findings shed light on the operating principles of some of the highest reported performances for such solar cells.7,8

We will moreover present the results of stability studies on such solar cells under continuous illumination, identifying that TiO2-based solar cells suffer from a fundamental instability to ultraviolet light.9  We ascribe this instability to the de-passivation of surface traps in the mesoporous TiO2, and show that increasing the perovskite pore filling fraction slows the degradation, because recombination processes are slowed in solar cells with high degrees of perovskite pore filling.


Figure 1. Recombination pathways in solar cells with low (a) and high (b) perovskite filling fractions.
(1) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; Grätzel, M.; Park, N.-G. Sci. Rep. 2012, 2. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science (80-. ). 2012, 338, 643. (3) Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. J. Energy Environ. Sci. 2013. (4) Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M. J. Am. Chem. Soc. 2012, 134, 17396. (5) Laban, W. A.; Etgar, L. Energy Environ. Sci. 2013, 6, 3249. (6) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Science (80-. ). 2013, 342, 341. (7) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C.-S.; Chang, J. A.; Lee, Y. H.; Kim, H.; Sarkar, A.; Nazeeruddin, M. K.; Graetzel, M.; Seok, S. Il. Nat. Photonics 2013, 7, Ahead of Print. (8) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. Il. Nano Lett. 2013, 13, 1764. (9) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Nat. Commun. 2013, 4, 2885.
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