Role of Intrinsic Ion Accumulation in the Photocurrent and

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On the role of intrinsic ion accumulation in the photocurrent and photo-capacitive responses of MAPbBr photodetectors 3

Evan Lafalce, Chuang Zhang, Xiaojie Liu, and Zeev Valy Vardeny ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11925 • Publication Date (Web): 05 Dec 2016 Downloaded from http://pubs.acs.org on December 5, 2016

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ACS Applied Materials & Interfaces

On the role of intrinsic ion accumulation in the photocurrent and photo-capacitive responses of MAPbBr3 photodetectors Evan Lafalce, Chuang Zhang, Xiaojie Liu, Zeev Valy Vardeny* Department of Physics & Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, UT 84112

Keywords Hybrid Perovskites, Photodetectors, Ionic impurities, Charge-carrier transport, I-V Hysteresis.

Abstract We studied steady state and transient photocurrent in thin film and single crystal devices of MAPbBr3, a prototype organic-inorganic hybrid perovskite. We found that the devices’ capacitance is abnormally large, which originates from accumulation of large densities of Pb+2 and Br- in the active perovskite layer. Under applied bias, these ions are driven towards the opposite electrodes leading to space-charge fields close to the metal/perovskite interfaces. The ion accumulation, in turn causes photocurrent reversal polarity that depends on the history of the applied bias and excitation photon energy with respect to the optical gap. Furthermore, the large

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capacitive response dominates the transient photocurrent, and therefore obscures the weaker contribution from the photocarriers drift. We show that these properties depend on the ambient conditions in which the measurements are performed. Understanding these phenomena may lead to better control over the stability of perovskite photo-detectors for visible light.

Introduction The optical, electronic and transport properties of the organic-inorganic hybrid trihalide perovskites (OTP) are a matter of intense research, in parallel with promising developments in semiconductor device applications. The wide versatility of compositional, structural and morphological forms that these materials may take is a tantalizing frontier for the development of new and better optoelectronic devices. Optimizing the fabrication and device architecture has unlocked their potential as world-class active layers in photovoltaics1-3, yet many other applications are still in early development stages including light sources4 with tunable emission including LED’s5,6, lasers7, and optical amplifiers8, spintronic applications9,10, and photodetectors that span the spectral range from near-IR to UV, to soft X-rays11-14. Meanwhile, the fundamental properties of these compounds are far from being understood. While it seems clear that the band edge absorption coefficient of the OTPs is the result of strong excitonic enhancement effects15, the more subtle influence of electronic traps and disorder on the belowgap absorption spectrum and its consequent on carrier photogeneration and transport have not been well studied. The OTP have been shown to have exceptionally long photocarriers lifetime leading to long diffusion length16-20, but reports of carrier mobility are scarce and show a significant scatter in the obtained values using different techniques and preparation methods of the same compound18-28.


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One reason for the complexity of the photocarriers transport properties in the OTP is the influence of morphology, defects, grain boundaries, and environmental effects. Many reports indicate a higher density of traps at the grain boundaries compared to that in the bulk7,14, even that the overall trap density in the OTP is found to be relatively low. Interestingly, theoretical calculations indicate that such defects form mainly shallow traps29. In spite of this, the OTP optoelectronic properties have been shown to be sensitive to the size and distribution of grain boundaries, as well as the stoichiometry of chemical precursors and preparation conditions. Thus it is considered that ion vacancies play a significant role in determining sample properties and ultimately device performance. Accordingly, single crystals OTP18-22,30,31 have been studied as both a means of investigating the ‘intrinsic’ optoelectronic properties free from the effects of grain boundaries, and also for their unique potential as photo-detectors14,32-34. We have been interested in using the large transport lengths provided by such crystals in hope to clearly distinguish between surface effects and bulkdrift transport. Despite this interest a great deal of ambiguity remains in the fundamental mechanism of carriers photogeneration and transport, in view of variability in values of the optical band gap, Eopt19,35,36, carrier mobility18-21, as well as noticeable differences in photoluminescence and photocurrent (PC) excitation spectra reported in the literature14,18,19,21,30,33-37. In the present work we have investigated photocarriers generation and transport dynamics in methyl-ammonium (MA) MAPbBr3 photo-detectors made from both solution-grown single crystals and solution-processed thin films. We show that the primary photo-response in these devices arises from surface effects including the extraction of carriers in the presence of


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depletion regions of reversible polarity, and changes in the sample capacitance upon illumination into band-tail states. These effects strongly suggest the important role of intrinsic ionic impurities such as Pb2+ and Br- in controlling the PC response. We show that both the excitation photon energy with respect to the optical gap, Eopt and sample environment modulate the abnormally high sample capacitance under photo-excitation. These effects have a strong influence on the photo-response in the low-fluency regime, where high detectability is of utmost importance and benefit.

Experimental MAPbBr3 bulk crystal growth and device fabrication. The precursor solution for this growth was prepared from 0.37 g PbBr2 and 0.125 g MABr (Sigma Aldrich) in 2 mL N,Ndimethylformamide. The solution was placed on a hot plate at 50 ⁰C and stirred for 30 mins. before use. After cooled down to room temperature, the solution was filtered and transferred to an open-top glass vial. The bulk crystal was grown by a solvent-exchange method, in which the vial was placed inside a beaker with 10 mL 2-propanol as the anti-solvent. The beaker was then capped and kept undisturbed for 48 hours. The red cubic crystals appeared at the bottom after the anti-solvent diffused into the vial, and were collected for further device fabrication. Subsequently a thin layer of Au electrode (30 nm) was evaporated on top of the crystal, and a conductive silver epoxy was used to fasten the crystal to a glass substrate and serves as the bottom contact. MAPbBr3 thin film device The precursor solution was the same as for the bulk crystal growth. The thin film devices were fabricated on patterned indium tin oxide (ITO) coated glass (15-20 Ohm/Sq, Lumin. Tech.). The ITO glass was cleaned and treated with O2 plasma for 15 mins. Subsequently the PEDOT:PSS (Clevios P VP AI 4083) was spin coated on top of it at 5000 rpm and annealed for 20 minutes at 150°C. The MAPbBr3 layer was prepared by spin coating the


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precursor solution at 2000 rpm, and 0.2 mL chloroform was dripped onto the film during this process for nanocrystal pinning. The MAPbBr3 film was then annealed at 90°C for 20 mins, and once cooled a 20 mg/mL solution of PC71BM (American Dye Source) dissolved in chlorobenzene it was spin coated at 1500 rpm if needed. The devices were finally capped with an Al or Au electrode (50 nm) that was evaporated through a shadow mask. Cw-photocurrent excitation (PCE) and dark-current characterization: The devices were illuminated through the ITO or the semi-transparent metal contact using a Xenon arc lamp predispersed through a ¼ meter monochromator. The light beam output was modulated by a mechanical chopper and focused on the device using a 4x (NA = 0.10) objective lens. An iris was used to ensure that the beam spot was smaller than the 2mm2 area of the device, defined by the overlap of ITO and metal electrodes or area of the crystal surface (typically 4mm2). Bias voltage was applied and the steady-state dark current was measured by Keithely238 sourcemeasure unit while the photocurrent was measured as the voltage drop across an external resistance of a 50k resistance by a Sr-830 lock-in amplifier referenced to the frequency of the chopper (330Hz) (see Supplemental, Fig. S1).

The resulting photocurrent spectrum was

normalized by the transmitted photon intensity through the respective electrodes. The latter was determined by measuring the transmission of the Xenon lamp through reference electrodes deposited on glass using Si photodiode. Measurements were performed with the devices housed in a cryostat with optical windows and electrical feed-through, where the pressure could be reduced to 100 mTorr using a mechanical pump. Transient photocurrent (t-PC) measurement: Excitation of the single crystal devices was through the evaporated top Au contact using the tunable output of an OPO seeded by the third-harmonic of an Q-switched Nd:YAG laser. This system supplied 5 ns pulses at a repetition rate of 10 Hz


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in the spectral range of 0.6 eV to 3 eV. The pulse energy transmitted through the Au electrode was ~10 nJ. The transient photocurrent was measured as the voltage drop across an external resistor using an oscilloscope. The applied bias was from a Kiethley238 source/measure unit and measurements were performed in a cryostat as described for the cw-measurements (see Supplemental, Fig. S1).

Results & Discussion EFFECT OF τRC IN TIME-OF-FLIGHT PC MEASUREMENTS The transient photocurrent, t-PC of MAPbBr3 single crystal photodetectors is well described by a single exponential decay. Rather than reflecting a material property, however we show in Fig. 1a that the early time dynamics are instrument limited by the RC-time constant, τRC of the experimental circuit by varying the external resistance, Rext across which t-PC is measured (see Experimental and Supplemental, Fig. S1). The t-PC plotted on a semi-logarithmical scale displays exponential dynamics in the form of a line with slope of -(1/τRC) at early times; in the Supplemental (Fig. S2) the t-PC is plotted on a linear scale with RC time constants extracted by fitting the response with an exponential decay function. The extracted RC time constants are plotted vs. Rext in Fig. 1b, showing a linear dependence of τRC on Rext, that allows us to determine the capacitance of the electrical circuit, Cc immediately following photo-excitation. We found Cc~600 pF, after subtracting off the parallel capacitance of the external experimental circuit. It is worth noting that if this value is attributed to the device geometric capacitance, Cg, where Cg = ε0εrA/L, this would imply a value of εr ~ 105. This is a colossal value that is


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significantly higher than has been used in pervious transport studies18,19. While giant photoinduced dielectric responses have been reported in the OTP38, more detailed understanding of the charge trap concentrations, as well as the interface between the crystal and the various applied contacts is prerequisite for understanding the origin of this anomalously high capacitance value. More importantly, we observe that in fact τRC dominates the t-PC response down to time scales of ~1 µs revealing an apparent rapid carriers transport process that is difficult to attribute to carriers drift. This is especially true when considering the large transport length, L of the crystal sample and relatively low applied field. Additionally, we note that in thin film devices, where L is 104 times smaller than in crystal samples, similar capacitance values are extracted (see Fig. S3), which is, again, inconsistent with a simple geometric capacitance, Cg. On the contrary, this relatively high capacitance value obtained in the crystal samples suggests that charges stored at the crystal surfaces (rather than in bulk) play a major role, as will be further substantiated below. In previous works13,14,18,19, the carrier mobility was extracted from the long-tail of the t-PC response within the context of a dispersive transport model of carrier transport in disordered semiconductors39,40. This amounts to plotting the t-PC response on a log-log scale as displayed in Fig. 1c, for observing the carrier time of flight formed by their drift to the opposite electrode.


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µ e = 6.4 ± 0.4 cm2/Vs

Figure 1. (a) Transient photocurrent, t-PC in MAPbBr3 single crystal photodetector measured with various load resistance, Rext that is generated using 5ns pulse laser excitation at ħω = 2.85 eV, plotted on semi-natural logarithmical scale. (b) The obtained RC time constant, τRC for various Rext with a linear fit for extracting the sample capacitance. (c) t-PC response of electrons plotted on a log-log scale at different values of the applied bias, Vbias. The transit time, ttr is extracted from the intersection of the asymptotes of two different t-PC power laws, as indicated by the squares. (d) Fit to the obtained ttr vs. Vbias for extracting the electron mobility, µe using the relation ttr=L2/Vbiasµe; its value is indicated. The electric field polarity in our devices drives electrons from the illuminated Au contact through the bulk of the crystal to the Ag back electrode. In the absence of a clear t-PC plateau followed by a sharp drop-off in the PC, which would signal photocarriers drift at a constant velocity and extraction at the back electrode, respectively; typically in this case, the “kink” in the log-log plot that represents a change in t-PC power law is then used as a measure of the transit time, ttr. Consequently from the dependence of ttr on the applied bias voltage, Vbias we may extract an electron mobility, µe using the relation: ttr = L2/Vbiasµe. Following this procedure we


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2 get from the ‘TOF response’ µe = 6.4 cm /Vs (Fig.1d). However this procedure admits that the group velocities of carriers experience significant dispersion as they traverse the sample, and therefore µ is no longer a well-defined physical quantity since it depends on the applied voltage and sample length to some power. We emphasize the fact that this “dispersive drift” component in the observed t-PC response only accounts for 5% of the total PC. This may result from the more efficient extraction of holes at the illuminated electrode compared to the electrons that must traverse the bulk and are either lost to or significantly slowed through multiple trapping. Therefore a second contribution might be buried under τRC, such as the change in sample capacitance. Furthermore, the low effective field applied here (which is similar as in previous reports) suggests a non-negligible contribution from diffusion that has yet to be accounted for in TOF measurements of the hybrid perovskites. We therefore conclude that a more detailed analysis of the t-PC long-tail component is necessary in order to extract useful information about the carriers drift in the family of hybrid perovskites from TOF measurements. CW-PHOTOCURRENT EXCITATION SPECTRA OF CRYSTAL AND THIN FILM DEVICES In order to learn more about the photocurrent generation mechanism at the perovskite/metal interface we conducted PC excitation photon energy dependent studies in both the transient and steady-state regimes. Figure 2 shows the cw-photocurrent excitation (PCE) spectrum from MAPbBr3 crystal device at various values of Vbias. At negative bias, the device is polarized to extract photogenerated holes at the Au interface, whereas the photogenerated electrons must traverse the entire crystal and are then extracted at the opposite, Ag electrode. The obtained PCE spectrum at negative bias peaks sharply at the crystal absorption edge, namely at ħω = 2.19 eV,


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whereas PCE at higher energies, PCE(plateau) is relatively suppressed. Fang et al. attributed such spectral response to carrier trapping at the illuminated crystal/electrode interface, that results in the PC suppression at high ħω, whereas low-energy absorption with larger penetration depth was considered to be the origin of the low-energy PCE sharp band, PCE(peak) 13. This interpretation however needs to be modified in light of our results presented below.

Firstly we note that PCE(peak) at negative bias occurs at ħω below the nominal optical band gap, Eopt of MAPbBr3. In thin MAPbBr3 films, where Eopt may be reliable determined through transmission measurements, the onset of absorption occurs at ~2.30 eV, followed by an excitonic band peaking at 2.35eV (see Fig. S4). Whereas many reports have shown decreased Eopt values in single crystals13,19 the absorption spectrum determined by ellipsometry was nearly identical to the results from thin films35. Growing evidence suggests that PCE(plateau) is associated with the crystal surface, while the PCE(peak) at lower ħω may be ascribed to the bulk19,30,37 as originally suggested by Shi et al19. However PCE(peak) occurs at ħω significantly below the lowest Eopt values extracted from MAPbBr3 single crystal ellipsometry, and therefore cannot be ascribed to exciton dissociation, or to states in the gap where donor-acceptor emission was observed36. Secondly we show in Fig. 2 that the PCE spectrum at hω > Eopt measured at forward bias is much higher than that obtained at negative bias, while PCE(peak) in the spectrum is still well resolved (although it is clearly broadened on the high energy side at large forward bias). Overall the PCE value is significantly higher for forward bias compared to that obtained at negative bias. We note that at positive bias electrons are extracted at the Au-interface while holes must traverse the entire crystal to the opposite Ag electrodes. Even that Au is traditionally thought of as a holeselective contact, the ionization energy of MAPbBr3 films has been reported to be at ~6.5 eV41,


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placing the Au Fermi level near mid-gap, or possibly closer to the conduction band. Indeed, the I-V response is nearly symmetric at the values of the applied voltages in our studies (Fig. S5). We also found that the short-circuit PCE spectrum measured at Vbias=0 depends on the history of the applied bias. When measured after biasing the crystal at +5V, PCE at Vbias = 0 (0V(+), blue solid line) resembles those measured at negative bias (-1V and -5V, red and green solid lines, respectively) but PCE(peak) and PCE(plateau) components have opposite polarity. On the contrary, when measured after biasing at -5V, PCE at Vbias = 0 (0V(-), blue dotted line) resembles that obtained with forward bias (+1V and +5V, red and green dotted lines, respectively), except that PCE(peak) is significantly reduced.

Figure 2. Photocurrent excitation (PCE) spectrum obtained from a MAPbBr3 single crystal device measured at various applied biases, Vbias as labeled. Note that the PCE spectrum measured at Vbias=+5V is divided by six for easy comparison. 0V+(-) refers to measurements at Vbias=0 immediately following the measurements at Vbias=+(-)5V. Negative bias refers to the illuminated Au contact held at negative potential with respect to the grounded Ag contact.


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The above observations can be explained in terms of a built-in voltage, or space-charge field (depletion region) that arises at the perovskite/Au interface due to ions accumulation. For instance, under forward bias Pb2+ ions drift towards the Ag interface while Br- ions are pushed towards the Au electrode until a resulting space charge field balances their respective motion, while large accumulation of ion densities is formed. After the bias is turned off, the steady state space charge remains due to the impeded diffusion of heavy ions, and the device acts as if it is negatively biased, at least near the interfaces. This was experimentally verified recently by surface voltage measurements42. Excitation hω< Eopt into surface trap states may then result in photocurrent due to sweep out of the free carrier; in this case photogenerated holes, based on consideration of the applied bias polarity. For Vbias > 1V the device shows photocurrent of the expected polarity, apparently overcoming the built-in potential. That PCE(peak) is barely observable (and still slightly negative) upon switching from -5V to 0V indicates that the trap states are donor-like, excitation into which leaves a trapped electron and free hole in the valence band that may be extracted at the Au interface only when the interface is negatively charged, or requires a significant forward bias to enable hole-drift to the back contact. The enhanced PCE response at positive bias, particularly at hω > Eopt, strongly suggests predominance of positive ions, consistent with theoretical predictions29. However, the observed PCE(peak) broadening may also reveal hole trap states with a higher concentration near the valence band minimum. The proposed scenario of bias-driven accumulation of semi-mobile ions is supported by the largereversible I-V hysteresis known to exist in these devices (Fig. S5). We note that while it is also possible that free MA+ ions (as compared to Pb2+ ions discussed above) contribute to the externally switchable built-in field, we focus on Pb2+ ions for two reasons. First, according to the predictions by Yan et al., the Pb-vacancy defect has lower formation energy in the Br-rich


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conditions in which the samples were grown compared to the MA-vacancy29, and thus we expect a higher density of the former. Secondly, the larger ionic charge and smaller volume of the Pb2+ ion both make it more mobile under externally applied bias compared to MA+. We found a similar behavior in “inverted device” architectures of thin MAPbBr3 polycrystalline films at short-circuit conditions (see Fig. 3a). We measured PCE in devices having the architecture of Y/MAPbBr3 (120nm)/PEDOT:PSS/ITO, where Y stands for the following three top electrodes; Al/PCBM, Au, and Au/PCBM. The bias polarity convention is the same as in Fig. 2, where negative bias refers to the ITO contact held at negative potential with respect to the grounded Y contact. In this case the ‘poling’ effect was not required for observing the two distinct PCE components having opposite polarities; most likely as a result of the larger work function difference of the device electrodes. Here we plot the typical minority carrier current (electrons extracted at the Al/PCBM interface, holes extracted at the PEDOT/ITO interface) as negative, consistent with the typical sign chosen in photovoltaic I-V response. Now the sharp PCE band, PCE(peak) results from hole extraction at the Al/PCBM interface in a manner identical to reverse bias in the single crystal device. The increased Eopt of thin films is apparent here, as is the fact that PCE(peak) is significantly broader compared to that in the single crystal devices. This may be due to inhomogeneous broadening that originates from the small crystallite size in the film. Interestingly, we found that in devices with Au as evaporated top electrode no PCE at hω Eopt is significantly reduced due to recombination with injected majority carriers. However, the hole extraction through the trap-state excitation increases. This effect is even more pronounced when illuminating the device through the Al/PCBM interface, where even the PCE(plateau) at hω > Eopt reverses sign due to the increase of the depletion region through the applied bias, highlighting the localized interfacial nature of this effect.


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Figure 3. (a) PCE spectra measured on Y/MAPbBr3 (120nm)/PEDOT:PSS/ITO photodetector devices, where Y stands for the following three top electrodes; Al/PCBM, Au, and Au/PCBM. The bias polarity convention is the same as in Fig. 2, where negative bias refers to the ITO contact held at negative


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potential with respect to the grounded Y contact. (b) PCE spectra at various Vbias for Y = Al/PCBM device illuminated through the ITO contact, and at Vbias = +2V when illuminated through the Al/PCBM contact (dashed line). (c) PCE spectra at Vbias = 0 from Y= Al/PCBM device measured at ambient conditions (atm) and in vacuum (100 mTorr).

In Fig. 3c we show that PCE(plateau) at hω > Eopt is sensitive to ambient. When the device is characterized in vacuum conditions then PCE(peak) disappears and PCE(plateau) component increases; this is also observed in devices having Au top contact (see Fig. S6). We found that this effect is reversible, echoing recent observations of reversible modulation of photoluminescence and photocurrent in MAPbBr3 crystals due to sensitivity to oxygen and moisture30. In that work, the photocurrent was found to increase in a surface-contact and transport channel configuration, although it was not spectrally resolved30. We did not observe an experimentally significant difference in PCE(plateau) in our sandwich-structure MAPbBr3 crystal devices when measuring in vacuum or oxygen, as the interaction with the gases occurs primarily at the surface. For the thin film device, we expect that the interaction occurs via diffusion through grain boundaries.

EXCITATION AND ATMOSPHERIC EFFECTS ON THE TRANSIENT PHOTOCURRENT DYNAMICS Figure 4 displays the effects of the applied bias and ambient conditions on the t-PC response when the MAPbBr3 crystal device is illuminated with pulsed excitation above-gap (hω = 2.85 eV) and below-gap (hω = 2.16 eV). In almost every case, we found that the application of positive bias leads to enhanced long-lived t-PC at t > τRC. This is consistent with the cw PCE spectrum displayed in Figure 2. Additionally, it is seen that τRC is independent of bias for abovegap excitation (Fig. 4a), while for below-gap excitation τRC depends on both Vbias polarity and


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the ambient conditions of the measurements (Fig. 4b) providing more evidence that when exciting at this energy charged carrier trap states are directly populated. In air, τRC is smaller under the application of negative bias suggesting a decreased photo-capacitive response, due to compensation of positive charges at the interface. In vacuum, this situation is reversed and provides the only example at which the longer t-PC component is observed to be less at positive bias then at negative bias.

Figure 4. Effect of bias and environment on Iph(t) transient response in MAPbBr3 single crystal photodetector device, measured at various excitation photon energies. (a) 2.85 eV excitation at ± 100 V in air (solid lines) and vacuum (100 mTorr; dashed lines). (b) Same as in (a) but for excitation at 2.16 eV (c) Comparison of excitation at 2.85 eV (blue) and 2.16 eV (orange) in air (solid lines) and in vacuum (dashed lines) at +100V. (d) Log-log plot of the hole photocurrent transient for hω = 2.16 eV at +100V, showing the extracted hole mobility, µh and dispersion parameter, α.

In Fig. 4c we summarize the atmospheric effects at the different excitation energies. Clearly τRC is similar for both below-gap and above-gap excitation in air, showing that the capacitance is


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smaller at Vbias=100V for hω ttr with the obtained dispersion parameter α = 0.32. For the t-PC response in Fig. 1c we obtained p=1.32 for t > ttr, however, for t < ttr p=1.1 was measured, and thus the transients cannot be described by a single dispersive transport parameter α. This is a result of the capacitive response influencing the current decay at early times, requiring a deconvolution of the t-PC response with the RC response to obtain the true dispersion parameter. This last feature highlights the necessity of understanding the dynamics photo-capacitive response in order to obtain the true drift transport properties of OTP through the time-of-flight technique.


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Figure 5. Comparison of the normalized t-PC response response at Vbias= +100V for one-photon (hω = 2.85 eV) excitation (blue) and two-photon excitation (hω =1.50 eV) (green). In Fig. 5 we compare the t-PC dynamics under one and two-photon excitation that occurs due to two-photon absorption in the crystal. For the two-photon excitation process, the laser pulse penetrates the bulk of the crystal and thus the initial carrier density should be nearly uniform through the volume of the crystal; in contrast to the one-photon excitation process where carries are photogenerated close to the surface. The expected result in the case of drift-current would be an increased decay of photocurrent due to the reduced transport distance required for carriers to reach the back contact. For dispersive current this effect will be magnified by the dependence of the mobility on sample thickness45. In contrast, we see that the overall t-PC dynamics are very similar in case of one photon excitation when measured in air. This provides further evidence that the photocurrent in these devices is dominated by a surface contribution, with bulk transport contributing only a minor role.

Conclusion


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In conclusion, we have shown that the photocurrent response in MAPbBr3 single crystal and thin film photodetectors is dominated by surface charge effects generated by accumulation of intrinsic ionic impurities such as Pb2+ and Br- leading to space-charge fields at the interface and photo-active trap states. The trap states are observed via photocurrent response obtained when exciting well below the MAPbBr3 optical gap. From the transient photocurrent investigation we showed that the device capacitance could be modulated by photoexcitation into these trap states, in addition to the applied bias and atmospheric conditions. These observations may help to rationalize both the spectral and temporal response of OTP photo-detectors, and should be useful in improving their stability and consistency. Additionally, this work highlights certain aspects of the dispersive transport nature of the charge transport in OTP, a phenomenon that has been observed rather ubiquitously but is yet poorly understood.


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Supporting Information. Supporting Information includes a diagram of the measuring circuit, the results of Fig. 1 plotted on a linear scale with results from an exponential fit to the dynamics, transient photocurrent from a thin film device Al/PCBM thin film device, absorption spectrum of thin film, dark current vs. voltage measurements, and comparison of PCE obtained in ATM and vacuum from Au/PCBM thin film device. (Word Document) Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded by the DOE, Office of Science, grant DE-SC0014579 (film and crystal growth, device fabrication); ONR grant N00014-15-1-2524 (device characterization) and NSF ECCS 1607516 (transport measurements); also the NSF-MRSEC Grant No. DMR-1121252 (device fabrication and film deposition facilities).

ACKNOWLEDGMENT This work was funded by the DOE, Office of Science, grant DE-SC0014579 (film and crystal growth, device fabrication); NSF grant ECCS 1607516 ONR grant N00014-15-1-2524 (device


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characterization) and NSF grant ECCS 1607516 (transport measurements). We also acknowledge the NSF-MRSEC program at the University of Utah for supporting the device fabrication and film deposition facilities, under Grant No. DMR-1121252.

ABBREVIATIONS MAPbBr3, methyl-ammonium lead bromide; TOF, time-of-flight; PCE, photo-current photon energy excitation spectra, t-PC, transient photocurrent.

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Table of Contents Figure


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Role of Intrinsic Ion Accumulation in the Photocurrent and

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