PCBM Interface Probed

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C: Energy Conversion and Storage; Energy and Charge Transport

Charge Transfer and Diffusion at Perovskite/PCBM Interface Probed by Transient Absorption and Reflection Meng Zhou, Julio S. Sarmiento, Chengbin Fei, and He Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07591 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry

Charge Transfer and Diffusion at Perovskite/PCBM Interface Probed by Transient Absorption and Reflection Meng Zhou,† Julio S. Sarmiento,† Chengbin Fei, He Wang* Department of Physics, University of Miami, Coral Gables, Florida 33146, USA Corresponding Author *To whom correspondence should be addressed. Email: [email protected] †These

authors contributed equally

ABSTRACT Interfacial charge transfer (CT) and diffusion are crucial processes that largely affect the device performance of perovskite solar cells. Here, we investigated the interfacial charge transfer and diffusion in the CH3NH3PbI3/PCBM film by transient absorption (TA) and transient reflection (TR) spectroscopies. A sub-picosecond CT coupled with carrier diffusion and hot carrier cooling is observed, which is almost independent on temperature. It is also found that the interfacial CT is slowed down to 11 picoseconds in a thinner perovskite film. A diffusionbased model is further built to fit the TR dynamics and we find that the more significant surface recombination in thinner films can compete with interfacial CT, which lowers CT efficiency.

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Introduction Hybrid perovskites have received considerable research interests for their applications in solar cells, with a new remarkable record efficiency over 25%.1 A typical perovskite solar cell is made of multilayer layers, with perovskite sandwiched by electron transporting layer (ETL) and hole transporting layer (HTL).2-4 Efficient charge transfer (CT) between perovskite and electron/hole extracting layer is a key step in a working device.5,6 Fullerenes have been used as a typical ETL in perovskite solar cells7-10 while understanding the CT between perovskite and fullerene acceptor is still far from complete. On the other hand, surface defects in the perovskite film lead to interfacial recombination and it has been found that surface passivation is an important strategy to reduce the surface recombination and improve device efficiency and stability.8,11-14 Therefore, direct probing surface dynamics is necessary to understand how film morphology affects the interfacial CT from perovskite to electron transport layer. Time resolved spectroscopy has been widely used to study the CT between perovskite and different electron or hole acceptors and the reported CT time in literatures varies from subpicoseconds to nanoseconds.5,6,15-22 For example, interfacial charge transfer in tens of picoseconds was observed from CsPbBr3 quantum dot to organic molecules or metal oxide.23,24 In polycrystalline thin film, on the other hand, electrons and holes on the far side from the acceptor will experience diffusion to the interface before CT occurs.25 It has been reported that the CT in CH3NH3PbI3/PCBM films involve both interfacial electron transfer and charge diffusion across the whole film and the interfacial CT was determined to be several picoseconds using a diffusion-coupled charge-transfer model.26 Moreover, hot carrier diffusion in subpicosecond to tens of picoseconds along the horizontal surface was also observed by transient


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absorption microscopy.27 Therefore, it is not straightforward to directly observe interfacial CT from perovskite film to ETL because it is typically coupled with carrier diffusion. Most of previous studies used transient absorption spectroscopy (TA) to study the CT dynamics between perovskite and acceptor.16,17,26,28,29 In TA measurements, the pump and probe pulses travel through the entire film and the obtained signal reflects the bulk properties of the sample.30,31 Transient reflection (TR) spectroscopy, on the other hand, probes the difference of reflected probe light (ΔR/R) with and without pump pulse.32-36 Unlike TA, TR detects the change in the photoinduced reflection of the sample surface so that it will reflect surface recombination dynamics.33,35 Therefore, TR spectroscopy is more suitable for detecting the interfacial charge transfer and energy transfer in thin films and there has been several successful examples.37-39 Moreover, establishing a suitable model to resolve the surface recombination rate and diffusion coefficient is of importance to understand how interfacial CT is determined by surface morphology. In this work, we combined TA and TR spectroscopies to study the interfacial CT and diffusion dynamics between CH3NH3PbI3 (MAPbI3) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Steady state and time resolved photoluminescence indicates that there should be efficient CT from MAPbI3 to PCBM. Although there is no additional ultrafast CT observed in TA, a sub-picosecond charge transfer coupled with carrier diffusion was observed in TR kinetics after coating a MAPbI3 film (1 M) with PCBM. The interfacial CT observed in TR was slightly dependent on temperature, which indicates that the CT has a very small activation energy. Interestingly, the interfacial CT process significantly slows down in a thinner film (0.25 M), which was explained by more significant surface recombination and carrier diffusion. A diffusion-based model was further established to fit the TR dynamics, and surface recombination


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rate and diffusion coefficient of different films was compared. The observation of diffusion coupled CT from MAPbI3 film to PCBM will stimulate future work on the CT study on perovskites. Results and Discussions MAPbI3 films were fabricated by spin coating the precursor solution (MAI and PbI2 dissolved in dimethylformamide, 1 M) onto the glass substrate in the nitrogen filled glovebox. Figure 1A shows the steady state absorption spectra of pristine film and film spin-coated with PCBM (dissolved in chlorobenzene). One can observe that the UV-vis absorption spectra show very similar profiles before and after adding PCBM, and only a slight decrease in the subbandgap region is observed, which is probably due to the surface passivation by PCBM. It is found that the photoluminescence intensity of MAPbI3 (Figure 1B, centered at 770 nm) decreased significantly (by over 10 times) after the perovskite film was coated with PCBM, which indicates that there exists efficient CT from MAPbI3 to PCBM. The total carrier lifetime of films with and without PCBM can be obtained by PL lifetime measurements. One can observe that the PL lifetime were shortened from 20 ns to 1.5 ns after coating the film with PCBM (Figure 1C), which is consistent with the trend in the steady state PL measurement (Figure 1B). From scanning electron microscope (SEM) image, the grain size of the film was determined to be 200-300 nm (Figure 1D).


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Figure 1. (A) UV-vis absorption spectra of MAPbI3 films with and without PCBM; (B) steady state photoluminescence spectra of two films with 405 nm excitation; (C) PL decay and corresponding fitting of pristine film and film coated with PCBM; (D) SEM image of 1 M film; (E) diagram of electron diffusion and charge transfer process in MAPbI3/PCBM films, TA and TR; (F) energy diagram of CT between MAPbI3 and PCBM. To further understand the CT dynamics, we performed TA and TR measurements on films with and without PCBM. As shown in Figure 1E, with front excitation (laser from right to left), CT from perovskite to PCBM and carrier diffusion from perovskite film surface to interior should occur at the same time. It would be important to see the time-scales of both CT and carrier diffusion. According to the Beer Lambert law, the absorption will decrease exponentially as the light travels inside the material. Pump pulse with different energies have different absorbance and thus different penetration depth. It is found that for MAPbI3, shorter wavelength has much shorter penetration depth than that of longer wavelength (see Figure S1). The thickness of the film (1M) we prepared was measured to be around 280 nm (Table S1). With 400 nm


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excitation, the penetration depth was calculated to be 32 nm (see Supporting Information Figure S1), which allows us to mainly probe the surface of the film.32 The energy level of perovskite and PCBM was shown in Figure 1F and there should be efficient CT from MAPbI3 to PCBM. During both TA and TR measurements, the pump fluence was kept as low as 3 μJ/cm2 and the initial carrier density (t=0) over the illuminated area was calculated to be 1.2×1017 cm-3. With such a low carrier density (10-17 cm-3), the thermal effect and Auger recombination should be eliminated.40 The TA results with 400 nm excitation were shown in Figure 2 A-D. In the TA spectra (ΔT/T), the positive signal represents ground state bleaching (GSB) and the negative signal stands for excited state absorption (ESA). Before ~1 ps, one can observe a broadened GSB tail to the shorter wavelength and a negative ESA to the longer wavelength of the maxima at 754 nm (Figure 2A). The ESA observed at 780 nm disappeared within 1 ps, which was attributed to a photoinduced absorption caused by the renormalization of bandgap.34 On the other hand, the narrowing process of the GSB tail indicates the cooling of the hot carriers.31,41 In the film coated with PCBM, similar spectra features can be observed in the initial 1 ps (Figure 2B). The total intensity of the GSB is weaker for the film coated with PCBM (Figure 2C), which can be explained by the absorption of PCBM at 400 nm, which reduced the initial carrier density. On the other hand, the TA decay after 100 ps accelerates significantly in film coated with PCBM (Figure 2D), which agrees with the PL lifetime measurements and indicates efficient charge transfer.


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Figure 2. (A-B) TA data map of 1 M films with 400 nm (3.1 eV) excitation; (C) TA spectra of two films probed at 10 ps; (D) Normalized TA decay traces of two films probed at GSB maxima; (E-F) TR data map of two films with 400 nm (3.1 eV) excitation; (G) TR spectra of two films probed at 10 ps; (H) Normalized TR decay traces of two films probed at GSB maxima. In contrast to TA, TR probes the change in the real part of the refractive index, which is relatively more sensitive to the film surface.33 The TR measurements were performed under the same carrier density, with incident angle of 45 degree. It is found that ΔR/R shows opposite spectral features to that of ΔT/T, with GSB showing negative signal (Figure 2 E, F). The positive peak at 775 nm corresponds to the ESA at similar position in TA (Figure 2 A, B), which disappeared within 1 ps due to bandgap renormalization.34 Interestingly, the positive signal at 775 nm slowly rises again between 10 ps and 100 ps. In general, the TR signal decays faster than TA, which could be explained by both carrier diffusion into the film and stronger surface recombination. As shown in Figure 2G, stronger TR signal in PCBM coated film could be explained by the change in the refractive index after coating PCBM. A closer look at the decay of ΔR/R at the GSB maxima indicates that in the film with PCBM, a stronger sub-picosecond


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decay can be clearly observed (Figure 2H). We have performed TR measurements on multiple samples and the stronger sub-picosecond relaxation in MAPbI3/PCBM films can be well reproduced (Figure S2). By performing multi-exponential decay convoluted by instrument response, the sub-picosecond decay in MAPbI3:PCBM film is determined to be 450 fs (Figure S3). We further prepared MAPbI3/ZnO and MAPbI3/spiro-OMeTAD and found that similar process can be observed regardless of the ETL or HTL used (Figure S4). Global fitting on TA and TR dynamics were performed to extract the time constants of multiple decay components (Figure 3). Figure 3A and B shows the decay associated spectra (DAS) of TA spectra with and without PCBM, respectively. In both films, two decay components are required to fit the dynamics and the profiles of DAS remain the same in two films. Based on previous study on the photophysics of MAPbI3, the sub-picosecond relaxation should arise from hot carrier relaxation. The hot carrier relaxation time (260 fs) remains the same in two films while the second decay is shortened from 17 ns to 11 ns after adding PCBM. In contrast to TA, three decay components are required to fit the TR spectra (Figure 3C and D). Unlike the case in TA, the first sub-picosecond DAS show different profile in two films. After coating PCBM, the first DAS shows strong amplitude around 750 nm, which should explain the more significant sub-picosecond decay observed in Figure 2H. Ultrafast diffusion, hot carrier relaxation and hot carrier charge transfer can all happen within 1 ps. Theoretically, when we add electron or hole transport layer, perovskite film does not change, so the ultrafast diffusion and hot carrier relaxation within the perovskite film should not be affected. With the existence of charge transfer from perovskite to PCBM, the diffusion into the deeper film should remain similar or be suppressed. The difference in the spectra (amplitude of ultrafast decay increases with PCBM) should originate from hot carrier charge transfer. It is interesting to see there is an


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additional 20 ps component observed in TR data of both two films (blue DAS in Figure 3C and D). Such a slow relaxation could arise from slow carrier diffusion from surface to interior of the film, which also explained the slow rise of 770 nm ESA in TR (Figure 2 E and F).

Figure 3. Decay associated spectra (DAS) obtained from global fitting on the TA (A and B) and TR (C and D) spectra of perovskite films. It is worth noting that, the ultrafast interfacial CT was not observed after we repeated the TA and TR experiment with 600 nm and 710 nm excitation (Figure S5 and S6). There are two possible explanations: First, the sub-picosecond CT we observed in TR could be hot electron transfer, which becomes weaker with 600 nm excitation and disappears with 710 nm (1.75 eV) excitation because the pump energy is close to the bandgap of MAPbI3 (1.55 eV). Second, it is found that 710 nm light has a penetration depth of 240 nm in the 1 M film (Figure S1), much longer than that of 400 nm excitation (32 nm). Longer pump penetration depth indicates that


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electrons far away from the surface could also contribute to the TR signal so that the interfacial CT could be hidden in the average TR signal. It is agreed that intramolecular charge transfer requires activation energy (Ea)42 while it remains unknown whether Ea is needed in the intermolecular charge transfer between significantly different solid thin films. To estimate the activation energy of the CT, we further probed the CT process at different temperatures. It is known that at low temperatures, MAPbI3 will experience phase transition from Tetragonal phase (T phase for short) to Orthorhombic phase (O phase for short).43,44 To estimate the phase transition temperature, TA measurements from 78 K to 295 K were first carried out (Figure S7, S8). It was found that O phase started to show up at 140 K for pristine film while after adding PCBM the phase transition started to occur at 120 K. We then performed TR measurements on these two films (Figure S9, S10) and obtained the CT kinetic traces at different temperatures (Figure 4A). We compared the kinetic decays between room temperature and 120 K in Figure 4A to see the CT dynamics before the phase transition occurs. It is found that between 120 and 295 K, there is always an ultrafast decay in the first 2 ps. After performing multiexponential fitting, the extracted time constant slightly increases as temperature decreasing and the activation energy is determined to be 0.36 kJ/mol (Figure 4B).


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Figure 4. (A) TR kinetics of MAPbI3/PCBM film at different temperatures. From bottom to top:120, 140, 200, 240, 295 K. Solid lines are fits to the kinetic traces. (B) Time constants obtained from fitting at different temperatures and a linear fit of ln k over 1/T. The efficiency of perovskite solar cells is reported to be largely dependent on the film morphology and surface passivation.8,13 To further understand the effect of film thickness on the CT, we repeated TA and TR experiments in 0.25 M films. The 0.25 M film is thinner (60 nm) than 1 M film (280 nm) and it showed weaker absorption (Figure 5A). Moreover, one can observe relatively large grain size as well as pinholes in the SEM image of the 0.25 M films (Figure 5B).

Figure 5. (A) UV-vis absorption spectra of 0.25 M MAPbI3 film with and without PCBM; (B) SEM image of the 0.25 M perovskite film. As shown in Figure 6A, the TA data of 0.25 M pristine film shows much shorter lifetime compared to that of 1 M (Figure 2A) and it decays to less than 10% of the initial intensity within 8 ns. Moreover, the TA spectra at 10 ps of 0.25 M show a shoulder around 720 nm (Figure 6A and B), which is not observed in 1 M film (Figure 2AB). Such a shoulder has been observed in previous experiments and was explained as the contribution from TR signal.45 It is found that adding PCBM further accelerated the decay while the spectral features remained the same


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(Figure 6B-D). Compared to the TA kinetics of pristine film, there is an additional fast decay between 1 ps and 100 ps in the MAPbI3/PCBM film and the decay time constant is around 11 ps (see Figure S11A), much slower than that in 1 M film. Such an additional decay should be ascribed to the CT from perovskite to PCBM.

Figure 6. (A-B) TA data map of 0.25 M films with 400 nm (3.1 eV) excitation; (C) TA spectra of two films probed at 10 ps; (D) Normalized TA decay traces of two films probed at 750 nm; (E-F) TR data map of 0.25 M films with 400 nm (3.1 eV) excitation; (G) TR spectra of two films probed at 10 ps; (H) Normalized TR decay traces of two films probed at 750 nm. As shown in Figure 6E-G, the TR spectra of 0.25 M film is drastically different from those of 1 M film. In the first 1 ps, a strong negative peak at 765 nm can be observed in both pristine and PCBM coated films (Figure 6E, F), which decays rapidly to give rise to a broad positive band around 740 nm and a negative band around 780 nm. The initial fast decay should be the hot carrier cooling and the slower decay should be the band edge recombination. In contrast to the TR in 1 M film, there is no ESA at 770 nm and the hot carrier lifetime is much shorter. Moreover, we didn’t observe any additional ultrafast process in PCBM coated film in 0.25 M


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film, which indicated no ultrafast interfacial CT was observed. Instead, there is an additional 11 ps decay in PCBM coated perovskite films (Figure S11B), which is similar to that observed in TA and should be assigned to interfacial CT. If one compares the kinetic decays of TA at 760 nm and TR at 740 nm, it is found that the normalized TA and TR kinetics are almost identical for both pristine film and PCBM coated film (Figure S12). These observations further indicate that TA signal has strong contribution from TR and thus surface recombination dominated the TA signal. Those pinholes observed in 0.25 M should explain the stronger surface recombination (Figure 5B). Global fitting was also performed on the TA and TR spectra 0.25M film to extract the decay time constants (Figure 7). In the TA data of two films, three decay components are required for the best fitting while an additional 11 ps decay component was observed in PCBM coated film (Figure 7 A and B), which agrees with the additional decay observed in Figure 6 D and H. Similarly, in the DAS of TR data (Figure 7 C and D), an additional 11 ps decay was observed in PCBM coated film. Besides the 11 ps component, other DAS profiles are similar in two films. These observations suggest that the 11 ps process should be charge transfer from perovskite to PCBM.


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Figure 7. Decay associated spectra (DAS) obtained from global fitting on the TA (A and B) and TR (C and D) spectra of 0.25 M perovskite films. Carrier dynamics and distribution of perovskite film is determined by carrier diffusion, surface and bulk recombination.33 To quantitatively extract the diffusion coefficient and surface recombination rate in 1 M and 0.25 M films, we further performed fitting based on the following equations:26

dN d 2 N N ( x, t ) D 2   dt dx

(1)

S dN ( x, t )  front N (0, t ) dx x 0 D

(2)

S dN ( x, t )   back N ( L, t ) dx x  L D

(3)


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N ( x,0)  N 0e x

(4)

where N(x,t) describes the carrier density as a function of the film depth ‘x’ and of the time ‘t’, D is the diffusion coefficient, τ is the bulk carrier lifetime, Sfront and Sback are front and back surface recombination rates, respectively. Equation 2) and Equation 3) are the Neumann boundary conditions for the carrier density with L being the thickness of the film (L = 280 nm for 1 M film and L = 60 nm for 0.25 M). As the equation and boundary conditions are written, there is no explicit solution to describe the carrier density. To fit our model to the data, we used the finite backward difference method on Equation 1), and the finite central difference method on Equation 2) and Equation 3). Equation 4) describes the instantaneous carrier density distribution where N0 is the initial carrier density, and α is the absorption coefficient of the film at the wavelength in question. The values for α is calculated by

  2.303

A l

(5)

where A is the absorbance at the excitation wavelength and l is the film thickness. The fitting of kinetic traces of two films based on the proposed model leads to two distinct sets of constants for the 1 M and 0.25 M films as shown in Table 1. Since the TR dynamics is strongly dependent on probe wavelength, we choose to fit the dynamics with most significant diffusion decay component based on the global fitting results (Figure 3 and 7). Table 1. Lists of best fitting parameters for 1 M and 0.25 M films. Concentration

D (cm2/s)

Sfront(cm/s)

Sback (cm/s)

τ (nanosecond)

1M

0.05

420

420

200

0.25 M

0.005

850

460

20


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In 1M film, the front and back excitation gives similar decay dynamics so that Sfront and Sback are the same (Figure S14A). In 0.25 M film, front excitation shows faster decay than that of back excitation so that Sfront is higher than Sback (Figure S14B). As shown in Figures 8A and B, the decay in the carrier density of the 0.25 M film is more rapid than in the 1 M film. That is attributed to the 0.25 M films having more surface defects than the 1 M films; this is also reflected in their values for the surface recombination rate where S for the 0.25 M films is almost two times larger than the S for the 1 M film. The bulk carrier lifetime of the film is also ten times longer in the 1 M film than in the 0.25 M film, allowing for a slower decay. Those pinholes observed in the 0.25 M film (Figure 5B) should account for more trap state and strong surface recombination, which would compete with interfacial CT and reduce the CT efficiency. Based on the diffusion coefficient D and surface recombination rate S in Table 1, we further plot normalized N(x,t), the carrier density as a function of time and position, in Figure 8 C and D. In the 1 M film, it is found that the relaxation dynamics is significantly different for front (x=0 nm) and back (x=280 nm) surface (Figure 8C). The front surface saw a rapid decay in the first 0.2 ns and a very slow decay in the following 6 ns, while the carrier density at x=100 nm saw a rise in the initial 0.2 ns followed by a slow decay. The initial rapid decay in the front surface and the initial rise in the dynamics at x=100 nm, respectively, should be explained by the diffusion of carriers from higher to lower carrier concentration region. In the 0.25 M film, on the other hand, the front surface saw a rapid decay in the first 1 ns followed by a relatively slower decay to almost zero between 1 and 6 ns while the back surface saw a rise in the first 1 ns followed by a slow decay.


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Figure 8. (A) TR decay trace and fit for 1 M film probed at 768 nm. (B) TR decay trace and fit for 0.25 M film probed at 769 nm. In both figures the blue circles represent the transient reflection data while the bold red line represents the fitted curve. To avoid the contribution of hot carriers, the fit was performed starting at 1 picosecond after initial excitation for both films. (C) Carrier density as a function of time and film depth in the (C) 1 M film and (D) 0.25 M film.

Conclusions. In summary, we have probed the interfacial hot CT and diffusion in MAPbI3/PCBM films by femtosecond transient absorption and reflection spectroscopies. The interfacial CT time constant is determined to be

PCBM Interface Probed

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