High-Performance Perovskite-Polymer Hybrid Solar Cells via

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High Performance Perovskite-Polymer Hybrid Solar Cells via Electronic Coupling with Fullerene Monolayers Agnese Abrusci, Samuel David Stranks, Pablo Docampo, Hin-Lap Yip, Alex K. -Y. Jen, and Henry J. Snaith Nano Lett., Just Accepted Manuscript • DOI: 10.1021/nl401044q • Publication Date (Web): 17 Jun 2013 Downloaded from http://pubs.acs.org on June 18, 2013

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High Performance Perovskite-Polymer Hybrid Solar Cells via Electronic Coupling with Fullerene Monolayers Agnese Abrusci1‡, Samuel D. Stranks1‡, Pablo Docampo1, Hin-Lap Yip2, Alex K.-Y. Jen2 and Henry J. Snaith1* 1

Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom, 2Department of Materials Science & Engineering, Seattle, WA 98195, United States

ABSTRACT A plethora of solution processed materials have been developed for solar cell applications. Hybrid solar cells based on light absorbing semiconducting polymers infiltrated into mesoporous TiO2 are an interesting concept, but generating charge at the polymer:metal oxide heterojunction is challenging. Metal-organic Perovskite absorbers have recently shown remarkable efficiencies but currently lack the range of color tunability of organics. Here, we have combined a fullerene self-assembled monolayer (C60SAM) functionalised mesoporous titania, a perovskite absorber (CH3NH3PbI3-xClx) and a light absorbing polymer hole-conductor, P3HT, to realise a 6.7% hybrid solar cell. We find that photoexcitations in both the perovskite and the polymer undergo very efficient electron transfer to the C60SAM. The C60SAM acts as an electron acceptor, but inhibits further electron transfer into the TiO2 mesostructure due to energy level misalignment

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and poor electronic coupling. Thermalized electrons from the C60SAM are then transported through the perovskite phase. This strategy allows a reduction of energy loss, whilst still employing a “mesoporous electron acceptor”, representing an exciting and versatile route forward for hybrid photovoltaics incorporating light absorbing polymers. Finally, we show that we can use the C60SAM functionalization of mesoporous TiO2 to achieve an 11.7% perovskitesensitized solar cell using Spiro-OMeTAD as a transparent hole transporter.

KEYWORDS: perovskite, fullerene, hybrid photovoltaics, self-assembled monolayer, photoinduced absorption, photoluminescence

MAIN TEXT Solution-processable organic and hybrid photovoltaic devices should eventually compete with conventional thin-film technologies on both cost and efficiency. However, their performance is usually limited by the energetic cost of separating or transferring the bound charges from the absorber following photoexcitation. A route to overcome this limitation can be to employ solution processable thin-film absorbers, within which the photoinduced electrons and holes can separate with minimal cost in energy. One interesting family of solution-processable materials suitable for optoelectronic applications are the inorganic-organic perovskites1-6. Until very recently, interest in the use of perovskites in photovoltaic applications has been relatively limited4. However, when processed in a mesoporous architecture in combination with the wide band gap p-type organic hole-conductor , 2’-7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’spirobifluorene (spiro-OMeTAD), they have recently been shown to compete with the very best state-of-the-art solution-processable solar cells1, 6. This perovskite can be processed from


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solution in air and absorb light very strongly from the visible out to the near-infrared in submicron thick films. Another approach to improve light absorption in thin hybrid devices has been to use light absorbing polymer hole transporters, which increases the optical density of the film as compared to dye-sensitized solar cells, due to the additional contribution to light harvesting by the hole-conductor within the pores7, 8. Although this offers great material versatility and spectral tunability, photoinduced electron transfer from the polymer to the metal oxide rarely occurs efficiently9, 10. It has recently been reported that, by functionalizing the meosporous TiO2 9 or ZnO11 with a fullerene based organic electron acceptor self-assembled monolayer (C60SAM), photo-induced electron transfer from a polymer hole-conductor to the oxide is significantly enhanced. However, in order to prevent electrons being “trapped” on the C60SAM that is functionalizing the TiO2, excessive concentrations of potential-determining ions (Li-TFSI) were required to shift the surface potential of the TiO2 to allow forward electron transfer to the TiO2 9. This resulted in very low open-circuit voltages and fundamentally inefficient devices. Here, we combine an inorganic-organic perovskite (CH3NH3PbI3-xClx) with C60SAM functionalization of TiO2 to produce a dual absorbing, perovskite:polymer hole-conductor hybrid solar cell. The presence of the C60SAM significantly enhances the contribution to photocurrent by light absorbed in the poly(3-hexylthiophene) (P3HT) polymer, but in this configuration the SAM also unexpectedly enhances the open-circuit voltage in comparison to the “bare” TiO2perovskite sensitized solar cell. This is in contrast to when previously employed in the dyesensitized polymer hybrid solar cells where a significant voltage drop was observed. Through optical spectroscopy and electronic characterization, we deduce that electrons become “trapped” on the fullerene SAM, which consequently blocks further forward electron transfer into the TiO2. Subsequent transport occurs via thermal population of the perovskite conduction band (not the


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TiO2), offering an efficient route for charge generation and a fast path for transport of charges originating from photoexcitations on both the perovskite and polymer. Notably, excitons on the P3HT are not quenched efficiently at the P3HT:perovskite heterojunction, but electron transfer proceeds via the C60SAM. This peculiar but effective mechanism, in what is essentially an organic-acceptor:perovskite-absorber:organic-donor distributed heterojunction, is an interesting architecture for efficient hybrid solar cells, and may prove useful in both fundamental studies and technology development. The combination of highly efficient charge generation and transport in the perovskite, with color tunability of the organic component, could lead to efficient and colorful solar cells for building-integrated photovoltaics. The samples used in this work were prepared as follows (see Supporting Information for full experimental details). TiO2 compact ‘blocking’ layers (100 nm) were spin-coated on glass or fluorine doped tin oxide (FTO) coated glass sheets. TiO2 paste (Dyesol, 18NR-T) was then screen-printed to get an average TiO2 thickness of 600 nm, and the substrates sintered at 500 ºC for 30 mins. The C60-substituted benzoic acid self-assembled monolayer (C60SAM) material was synthesized as described elsewhere12,

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, and functionalization of the internal surface of the

mesoporous TiO2 was achieved by immersing the substrates in the C60SAM solution for 24 hrs. CH3NH3PbI3-xClx was employed as the perovskite absorber and the precursor solution was spincoated, and the substrates subsequently heated at 100 ºC for 45 min to allow perovskite crystal formation. Finally, the hole transporting polymer P3HT was spin-coated on top and silver electrodes (150nm) thermally evaporated under high vacuum to complete the devices. Fig. 1(a) shows the absorption spectra of the various components processed on glass substrates. The relative energy levels are also shown (see Supporting Information for details), and the complete device structure is shown schematically in Fig. 1(b). The schematic and energy levels


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indicate that there are both C60SAM/perovskite and C60SAM/P3HT heterojunctions within the photoactive region which can dissociate charge. We observe that, with and without C60SAM, there is no significant change to the light absorption of the perovskite. The marginal reduction in absorption strength for the films with the C60SAM interlayer may be due to the C60SAM reducing the porosity of the TiO2 films, and hence overall coverage with the perovskite absorber after casting.

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Figure 1. (a) Absorption spectra of P3HT and perovskite films with and without C60SAM fullerene functionalization. The materials were cast on glass substrates which had been coated in a 600 nm thick mesoporous titania layer. Inset: Energy levels of the system components relative to vacuum. (b) Schematic of the device structure. In Figure 2, we show the J-V curves for a set of titania:perovskite:P3HT-based devices, prepared with and without C60SAM. The power conversion efficiency increases from 3.8% to 6.7% in the presence of the C60SAM functionalization, with an increase in both open-circuit voltage (0.68 V to 0.81 V) and short-circuit current (10.1 to 14.9 mA∙cm-2). We also observe a significant improvement in performance with the low bandgap polymer PCPDTBT (see Supporting


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Information). The external quantum efficiencies (EQEs) for the P3HT devices are shown in the inset of Fig. 2. We observe a decrease in the photocurrent at ~600nm, corresponding to parasitic light absorption in P3HT. However, when the titania is functionalized with the C60SAM, we observe a restored contribution to the photocurrent in this region, demonstrating ‘photoactivation’ of the polymer.

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Voltage (V) Figure 2. J-V characteristics taken under AM1.5 simulated sunlight (100 mW∙cm-2 irradiance) and in the dark for devices with and without C60SAM fullerene functionalization. Inset: External quantum efficiencies (EQE) for the same devices. To examine the generation of free charges in these devices, photoinduced absorption (PIA) spectra were taken from samples prepared on glass substrates without the P3HT hole transporter or electrodes (see Supporting Information for experimental details). The PIA results are presented in Fig. 3(a), where an excitation wavelength of 496.5 nm was used to predominantly excite the perovskite (Fig. 1(a)). The perovskite sample coated on the mesoporous titania shows a broad absorption feature in the region >1100nm attributed to free electrons in the titania1. The


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addition of the C60SAM inter-layer to the oxide/perovskite interface leads to the emergence of a distinct PIA peak at ~1030 nm concomitantly with a decrease in the peak corresponding to free electrons in the TiO2 (>1100 nm). The peak at 1030 nm has been previously attributed to longlived electrons on the C60 fullerene16. The steady-state and time-resolved photoluminescence (PL) measurements in Fig. 3(b) reveal that the addition of the C60SAM inter-layer also leads to strong quenching of the perovskite PL peak (675 - 825nm). The results taken together show that the photoinduced electrons in the perovskite are quenched by electron injection into the C 60SAM monolayer upon which the electrons are stabilized, thereby inhibiting or blocking injection into the titania. By contrast, a titania sample coated with a layer of benzoic acid mimicking the attachment group on the fullerene exhibits significant electron density on the titania17 and no peak at 1030 nm. These results indicate that the fullerene itself is acting as a strong electron acceptor from the perovskite and at least partially blocks electron transfer into titania. The PL results in Fig. 3(b) also show that the C60SAM functionalization leads to a quenching of the P3HT emission, which is seen in the region 600–750nm, even when the perovskite is in place. This is consistent with the earlier observation of the increased contribution that P3HT makes to device photocurrent with the insertion of a C60SAM monolayer (Fig. 2 inset). We note that this indicates that the perovskite is not entirely coating the C60SAM, but there are sufficiently regular patches of bare C60 to which the P3HT can make direct contact.


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TiO2/PEROVSKITE/P3HT TiO2/C60SAM/PEROVSKITE/P3HT

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Figure 3. (a) Photoinduced absorption (PIA) spectra of mesoporous samples sensitized with perovskite in the presence and absence of a C60SAM or benzoic acid inter-layer. Samples were excited using a 496.5nm laser line, chopped at 23 Hz. Data have been removed from the region 690-830nm, which corresponds to perovskite PL. (b) Steady-state photoluminescence


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measurements from samples photoexcited at 525nm. Inset: Time-resolved measurements of the perovskite PL (770nm) following pulsed excitation at 507nm. The instrument response function (IRF) is also shown. (c) Transient photocurrent decays trace for two sets of devices with and without C60SAM. The spectroscopy suggests that the C60SAM predominantly blocks forward electron transfer into the TiO2. However, in “dye-sensitized” analogues, the TiO2 is responsible for electron transport18. In order to study the effects of the addition of the C60SAM to the metal oxide structure on the charge transport characteristics, the devices were characterized via a photocurrent decay technique (see Supporting Information for experimental details)

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3(c) we show the decay traces for devices fabricated both with and without the addition of the C60SAM, following a short light pulse with a constant background light intensity. Upon the addition of the C60SAM on the TiO2 surface, transport is observed to be a factor of 4-5 times faster, and the transport lifetime approaches that in a perovskite meso-superstructured solar cell (MSSC), where the perovskite absorber is coated upon an insulating alumina scaffold1. From the PIA measurements shown in Fig. 3(a), we can infer that electrons are being transferred into the fullerene molecules rather than into the TiO2. Additionally, the measured density of states for TiO2 devices incorporating the C60SAM interlayer (Fig. 4(a)) show a marked reduction of subbandgap states (see Supporting Information for experimental details). This is again consistent with charge now being “stored” in these adsorbed fullerene molecules, which have a comparatively narrower density of states than the exponential tail to the density of sub-bandgap states in the TiO2. However, the faster charge transport is a surprise since we would not expect the C60SAM to be able to sustain rapid electron conduction. Even though crystalline C60 films can exhibit very high


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mobilities, in the range of 0.2 to 6 cm2/V∙s

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nanocrystalline TiO220-22, care must be taken to achieve anywhere close to these mobilities in self-assembled monolayers across extremely narrow channels23,

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C60SAM coated TiO2-P3HT solar cells which performed extremely poorly and exhibited appreciably slower transport (see Supporting Information), suggesting that the organic acceptor alone is not sufficiently conductive to operate as the electron transporter in a solar cell.

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Figure 4. (a) Differential capacitance as a function of open-circuit voltage for devices with and without C60SAM functionalization. Solid lines are exponential fits to the data. (b) Energy diagram of the proposed mechanisms for electron transfer from perovskite. The shaded regions in the Density of States (DOS) of the TiO2 and fullerene represent filled states.


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We propose a mechanism, summarized in Fig. 4(b), in which electrons transfer from both the perovskite and P3HT to the fullerene molecules, on which the electrons are then “trapped”. Given the suitable energy level alignment (Figure 1(a), inset), it is feasible that the thermal population of electrons in the fullerene LUMO levels at the energy of the conduction band of the perovskite are free to transfer back and forth into the perovskite conduction band, such that the C60SAM effectively acts as an “electron reservoir”. The thermal electrons at this level will then be free to be transported through the crystalline perovskite absorber until they trap again in the C60SAM. This is in essence the multi-trapping model usually used to describe charge transport in TiO2 25, but in this case the “traps” are the fullerene molecules and the conduction band is that of the perovskite. The electrons on the C60SAM are blocked from transferring onwards to the titania because of the poor electronic coupling across the non--conjugated benzoic acid group binding the two (Fig. 1b). By contrast, the perovskite and polymer electronically couple well to the C60SAM, as is inferred from the PL quenching measurements, likely to be due to the formation of a close interface between the materials. The proposed mechanism is consistent with the transport measurements presented in Fig. 3(c), since charge transport through the perovskite absorber is more than an order of magnitude faster than through the TiO2 1. Indeed it is also worth noting that perovskite-sensitized TiO2 solar cells exhibit significantly faster charge transport than dye-sensitized TiO2 solar cells, again consistent with the thermal population of electrons in the TiO2 preferentially transporting through the perovskite conduction band. The mechanism is also consistent with the increased voltages observed in devices functionalized with C60SAM. The inhibition of electron transfer into the TiO2, which contains sub-bandgap states, reduces the chemical capacitance in the device and


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moves the quasi-Fermi level for electrons (EF) closer to the perovskite conduction band for any given charge density1.

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Voltage (V) Figure 5. J-V characteristics taken under AM1.5 simulated sunlight (100 mW∙cm-2 irradiance) and in the dark for optimized Spiro-OMeTAD devices with and without C60SAM fullerene functionalization (see Supporting Information for complete device description). In order to assess if the C60SAM functionalization can also enhance the state-of-the-art perovskite-sensitized devices, we have fabricated devices which employ spiro-OMeTAD as the hole transporting material (Figure 5). There is a notable increase in the fill factor (0.65 to 0.72) with the addition of the C60SAM, a marginal increase in short-circuit current (19.4 to 19.6 mA∙cm-2), and a slight increase in open-circuit voltage (0.82 to 0.84V). The power conversion efficiency increases from 10.2% to 11.7% in the best devices, representing a substantial increase over our previously reported results with titania and this mixed halide perovskite.1 The relative increase in performance with the C60SAM is reduced compared to P3HT devices. The open-


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circuit voltage is only slightly increased, suggesting that the addition of lithium additives to the hole transporter, which is required to improve spiro-OMeTAD conductivity, enables a certain degree of forward electron transfer to the titania.9,22 The improved fill factor suggests that there is reduced recombination under working conditions. In summary, we have demonstrated that, by using perovskite as an inorganic absorber and C60SAM as an interlayer in a P3HT-based hybrid device, electron transfer from the perovskite to the titania can be blocked and the open circuit voltage loss reduced. The C60SAM acts as a very effective electron acceptor from the perovskite and the P3HT polymer, additionally providing polymer photoactivation. Notably, although long range electron transport and photocurrent collection occurs through the perovskite, the electrons are mostly mediated through the C60SAM, regardless of whether they originate from light absorbed in the perovskite or P3HT. We were able to fabricate devices with power conversion efficiencies exceeding 6.7% and open-circuit voltages over 0.8 V, compared to devices with ~4% efficiency and ~0.7 V without C60SAM functionalization. This strategy can allow enhanced electronic coupling between perovskites and polymer semiconductors and hence represents an exciting route forward for hybrid photovoltaics. The architecture was also used with Spiro-OMeTAD as the hole transporter to produce hybrid solar cells with power conversion efficiencies of 11.7%. Finally, the strategy offers a novel and versatile system to study charge transport within a multi-trapping framework, as well as other fundamental photovoltaic phenomena. ASSOCIATED CONTENT Supporting Information. Full experimental methods, additional absorption spectra, photoluminescence spectra, and device J-V curves. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions ‡These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This project was funded by the Engineering and Physical Sciences Research Council (EPSRC), the European Research Council (ERC-StG 2011 HYPER Project no. 279881), and the European Community’s Seventh Framework Programme (agreement number 246124 of the SANS project). ACKNOWLEDGMENT The authors thank Mr. A. S. Hey for device schematics, Dr A. Petrozza and Dr G. Grancini for useful discussion, and Dr E. Johansson for XPS measurements. Dr. S. Stranks thanks Worcester College, Oxford, for additional financial support. ABBREVIATIONS SAM, self-assembled monolayer; P3HT, poly(3-hexylthiophene); JSC, short-circuit current density; VOC, open-circuit voltage; DOS, density of states; PL, photoluminescence; PIA, photoinduced absorption; PCPDTBT, Poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4b′]dithiophene)-alt-4,7(2,1,3-benzothiadiazole)]; SEM, scanning electron microscope; MSSC, meso-superstructured solar cell; Spiro-OMeTAD, 2,2’-7,7’-tetrakis(N,N-di-pmethoxyphenylamine)-9,9’-spirobifluorene


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10. Abrusci, A.; Ding, I. K.; Al-Hashimi, M.; Segal-Peretz, T.; McGehee, M. D.; Heeney, M.; Frey, G. L.; Snaith, H. J. Energy & Environmental Science 2011, 4, (8), 3051-3058. 11. Vaynzof, Y.; Kabra, D.; Zhao, L.; Ho, P. K. H.; Wee, A. T. S.; Friend, R. H. Applied Physics Letters 2010, 97, (3), 033309-3. 12. Hau, S. K.; Cheng, Y.-J.; Yip, H.-L.; Zhang, Y.; Ma, H.; Jen, A. K. Y. ACS Applied Materials & Interfaces 2010, 2, (7), 1892-1902. 13. Hau, S. K.; Yip, H.-L.; Acton, O.; Baek, N. S.; Ma, H.; Jen, A. K. Y. Journal of Materials Chemistry 2008, 18, (42), 5113-5119. 14. O'Regan, B. C.; Lenzmann, F. The Journal of Physical Chemistry B 2004, 108, (14), 4342-4350. 15.

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Nano Letters

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Nano Letters

TABLE OF CONTENTS FIGURE Current (mA/cm2)

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20 0

+ C60SAM P3HT

Perovskite

-20 0.0

TiO2

0.5 Voltage (V)

1.0


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High-Performance Perovskite-Polymer Hybrid Solar Cells via

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