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Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces Kateryna Kushnir, Kefan Chen, Lite Zhou, Binod Giri, Ronald L. Grimm, Pratap Mahesh Rao, and Lyubov V. Titova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02474 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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The Journal of Physical Chemistry

Dynamics of Photoexcited Carriers in Polycrystalline PbS and at PbS/ZnO Heterojunctions: Influence of Grain Boundaries and Interfaces Kateryna Kushnir1, Kefan Chen2, Lite Zhou2, Binod Giri2, Ronald L. Grimm3, Pratap M. Rao2, and Lyubov V. Titova1* 1 Department of Physics, Worcester Polytechnic Institute, Worcester MA, 01609 2 Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA 01609 3 Department of Chemistry and Biochemistry, Life Science and Bioengineering Center, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609 *E-mail: [email protected] Abstract We investigate the impact of grain boundaries and interfaces on dynamics of photoexcited charge carriers in polycrystalline lead sulfide (PbS) films and at interfaces between polycrystalline PbS and ZnO by studying transient photoconductivity over sub-picoseconds to microseconds time scales using time-resolved terahertz (THz) spectroscopy and time-resolved microwave conductivity measurements. Narrow band gap bulk-like polycrystalline PbS with high absorption in the infrared paired with wide band gap metal oxide current collectors holds promise for infrared photodetectors and photovoltaics for converting infrared radiation to electricity. We find that grain boundaries in polycrystalline PbS suppress long-range conductivity and confine photoexcited carriers within individual crystallites. The mobility of photoexcited holes inside the ~ 150 nm crystallites reaches 750 cm2/Vs, and their lifetime exceeds hundreds of microseconds, while electrons get rapidly trapped at grain boundary states. The presence of PbS/ZnO interfaces dramatically reduces lifetime of the photoexcited free holes in the PbS crystallites. However, trapping and recombination of carriers at PbS/ZnO interfaces suppresses injection of free electrons from PbS into ZnO. Optimal transfer of photoexcited electrons as is needed for optoelectronic devices with PbS/ZnO heterojunctions may require engineering PbS/ZnO heterojunctions with buffer layers or organic ligands to passivate deleterious interface states.

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Introduction Narrow band gap in the bulk makes lead sulfide (PbS) an attractive choice for infrared optoelectronic devices including photodetectors and solar cells that are efficient in utilizing of infrared range of solar spectrum.1-6 he possibility of generating multiple carriers for one absorbed photon has been demonstrated in both bulk PbS and PbS quantum dots, bringing forth the possibility of PbS photovoltaics with efficiencies that exceed the Shockley-Queisser limit.6 Efficient carrier extraction in PbS optoelectronic devices can be achieved by forming p-n heterojunctions between a p-type PbS absorber and n-type wide band gap transparent metal oxide semiconductors such as ZnO or TiO2, where the band alignment favors rapid injection of electrons from PbS into the oxide.7-14 Heterojunction interfaces play a crucial role in device performance as they often harbor a variety of trap states and recombination centers. Surface states at PbS grain boundaries serve as efficient electron traps, and trap-mediated carrier recombination at PbS/ZnO interfaces limits the performance of PbS/ZnO heterojunction photovoltaics and other optoelectronic devices.2, 8-9, 15-18 Recent studies using transient optical absorption and time-resolved photoluminescence have demonstrated that lifetimes of photoexcited carriers in PbS quantum dots are reduced in the vicinity of PbS/ZnO interfaces, suggesting carrier injection from PbS quantum dots into ZnO.7, 16 However, the ultrafast dynamics of photoexcited carriers in bulk-like, small band gap polycrystalline PbS and at the interfaces of polycrystalline PbS with metal oxide electron collectors have not yet been elucidated. Revealing such photoexcited carrier dynamics motivates the present investigation. Here, we employ time-resolved terahertz (THz) spectroscopy (TRTS) and time-resolved microwave photoconductivity (TRMPC), contact-free techniques that are sensitive to free (mobile) carriers, to probe the dynamics of free photoinjected carriers in polycrystalline PbS and at PbS/ZnO interface in the picosecondto-millisecond time domain. Crystallite sizes of ~ 150 nm that are significantly larger than 20 nm exciton Bohr radius,19 and weak exciton binding in PbS ensure that excitonic effects do not play a role in the studied system. Minimial excitonic effects allows us to focus on the properties of free carriers in bulk-like PbS, which is necessary for implementation for PbS photovoltaics with high efficiency in the infrared range. Furthermore, PbS crystallites were not capped by organic or inorganic ligands, as the goal of this study is to elucidate the role of PbS grain boundaries and PbS/ZnO interfaces on carrier dynamics. We compare transient photoconductivity in polycrystalline PbS films that are deposited either on quartz or on a ZnO film. We find that the lifetime of photoexcited carriers in the polycrystalline PbS film deposited on quartz is extremely long, reaching 1 ms, and the intrinsic carrier mobility within the crystallites is large, at ~ 750 cm2/Vs. At the same time, inter-crystallite transport in the film is limited to a short ~10 ps time window after excitation. At longer times, inter-grain interfaces impede long-range conductivity. On the other hand, the population of photoexcited free carriers in the polycrystalline PbS film deposited on ZnO decays on timescales shorter than our experimental time resolution of 100 µs, lifetime of photoconductivity as revealed in Fig. 2(b) is consistent with previous reports of electron traps exist in PbS grains, which rapidly trap photoexcited electrons and leave holes free to conduct.20 Insight into the mechanisms that govern photoexcited carrier dynamics can be gained by examining the timeevolution of frequency-resolved complex conductivity following photoexcitation with varying fluence. The complex conductivity can be extracted from photoexcitation-induced changes in amplitude and phase of the THz pulse waveform transmitted through the sample.27-30 In Fig. 3 (a), we compare the transient THz photoconductivity spectra of the polycrystalline PbS at different time delays after excitation with 25 µJ/cm2 and 255 µJ/cm2 fluence of the 800 nm pump. At early times (5 ps) after photoexcitation, there is a pronounced difference in the photoconductivity spectra for different fluence values. But by 100 ps the spectra are indistinguishable despite the order-of-magnitude difference in excitation fluence. Furthermore, 5 ACS Paragon Plus Environment


photoconductivity at all fluences and time delays exhibits strong suppression of real conductivity at low frequencies and negative imaginary conductivity in the experimental frequency range (< 1.55 THz). These features have been observed in a wide variety of nanocrystalline and granular materials where long-range carrier transport is inhibited by the grain boundaries. They can be described by the Drude-Smith model, a phenomenological modification of the Drude model for free carrier conductivity, that takes into account the effects of charge carrier confinement on the length scales commensurate with carrier mean free path, as in the case of PbS crystallites in our sample.25, 29, 31, 36, 39-44 In the Drude-Smith model, the complex conductivity is given to the first order by

/∗ 1

,

(1)

where N is the charge carrier density, m* is the carrier effective mass, and the c-parameter characterizes the degree of carrier localization. For c = 0, this model reduces to the Drude model for free carriers, while c = −1 describes situations in which carriers are completely localized within individual grains, and long-range, intergranular DC conductivity 0 is completely suppressed. The effective scattering time "#$ takes into account intrinsic electron-lattice scattering "%& , collisions with grain boundaries ("()*%+,-. ) and carrier-carrier scattering:26, 45 12 1 1 1 τ01 2τ345 2τ67849:;< 2",---,---.

(2)

Drude-Smith fitting parameters (N, c, and τ01 ) at different times after excitation as a function of excitation

Figure 3. a) Real (black and green) and imaginary (red and blue) photoconductivity for two different values of pump fluence in PbS on quartz; (b) Real (green) and imaginary (blue) photoconductivity for high pump fluence in PbS on ZnO. The lines in (a) and (b) represent a global fit of real and imaginary components of the conductivity for each fluence and time delay value to the Drude-Smith model (Eq. 1). (c) Drude-Smith fitting parameters as a function of the excitation fluence for different times6after excitation. Dotted blue lines in the graph of τDS as a function of fluence show extrapolation of the scattering time to zero fluence. (d) Schematic energy level diagram ACS Paragon Plus Environment at the interface between two PbS grains.

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fluence are plotted in Fig. 3(b). The density of photoexcited carriers at 5 ps after excitation ranges from ~2 x 1017 cm–3 to ~ 5 x 1017 cm–3 depending on excitation fluence, which is nearly two orders of magnitude lower than the photoinjected carrier density that would be expected if each absorbed photon resulted in an electron-hole pair. This suggests that a large portion of the carriers become trapped or recombine nonradiatively over very short time scales that are shorter than our experimental resolution of ~ 300 fs. After that, most photoexcited electrons are trapped, and the remaining holes exhibit long lifetimes, as discussed earlier. The c-parameter in all the measured spectra is close to -1, indicative of a high degree of photoexcited carrier localization within the grains. However, at early times after excitation (5 ps), the c-parameter is less negative, and the long-range conductivity #> is non-zero at higher excitation fluences, suggesting that some carriers can travel across inter-grain boundaries. This transient occurrence of long-range transport correlates with high (> 3 x 1017 cm-3) carrier density, and can be explained by a previously proposed model of PbS grains and grain boundaries20, 46 (Fig. 3 (d)). Positively-charged defects at the boundaries result in an ~ 80 meV energy barrier for holes, across which holes can travel by thermally-activated transport or tunneling.20 At high excitation fluence, large initial carrier density increases the probability of electrons becoming trapped at the grain boundary defects, temporarily reducing the energy barrier for hole transport and resulting in increased long-range conductivity. The barriers are restored by ~ 8-10 ps as the defects at the boundaries are again filled by holes. Slowing of the fast (8-10 ps) decay components at higher excitation fluence suggests transient saturation of the electron traps in the PbS grains, which leaves more electrons to be trapped at grain boundaries and lower inter-grain barriers for mobile holes. Transient conductivity at times >100 ps after excitation varies slowly and is independent of initial injected carrier density. When the density of free carriers falls to ~ 0.8 x 1017 cm–3, the system reaches quasi-equilibrium which persists for hundreds of microseconds. This carrier density corresponds to ~ 300 carriers (holes) per PbS crystallite of 150 nm size, which can be used as an estimate of the electron trap density in the PbS grains. Slow decay of photoconductivity over hundreds of picoseconds and slower time scales then proceeds by recombination of trapped electrons and mobile holes. High carrier density at early times after excitation, particularly with high pump fluence, also manifests in reduced "#$ as carrier-carrier scattering contributes to scattering time as described by Eq. 2.40, 47-48 Extrapolating "#$ to zero fluence (dashed lines in the bottom graph of Fig. 3(b)) yields the lower limit to "#$ = 97 ± 10 fs. Using the average grain size of 150 nm to estimate contribution of grain boundary scattering, we find that the intrinsic mobility of carriers is ?%& = ∗ "%& ~ 750 cm2/Vs, comparable to that in single

crystalline PbS.49 At the same time, long range mobility is negligibly small at ?@)%A-,%A ?%& 1 B due to the grain boundaries.

Concluding, we found that room temperature chemical bath deposition results in polycrystalline PbS films that consist of cubic crystallites with high optical absorption and large, ~ 750 cm2/Vs intrinsic carrier mobility but suppressed long-range, inter-grain transport. Inter-grain mobility can be transiently increased by photoexciting the film using high, > 100 µJ/cm2 fluence. However, the question remains: can photoexcited electrons be efficiently extracted from PbS crystallites into a wide-bandgap, conductive material like ZnO?

Part II. Polycrystalline PbS on ZnO: Role of the PbS/ZnO Interface ZnO has been suggested as an electron collector in ZnO/PbS quantum dot heterojunction solar cells, as quantum confinement increases the PbS band gap and results in band alignment favoring injection of electrons from PbS quantum dots into ZnO.7, 9, 11, 16, 50 As discussed earlier, studies using transient optical absorption, time-resolved photoluminescence and time-resolved surface photovoltage spectroscopy have demonstrated rapid decay of photoexctited carrier density over sub-picosecond time scales in PbS nanocrystals in the presence of ZnO.7-8, 16 In the case of bulk-like, small band gap PbS, Fermi level equilibration may either in injecting result in a non-injecting (1) or an injecting (2) junction, as illustrated 7 ACS Paragon Plus Environment


schematically in Fig. 4(a). In both cases, electrons in the conduction band of PbS can get trapped interface defect states. Even in the case of injecting junction (2), interface trapping may be sufficiently rapid to prevent appreciable injection of mobile electrons into ZnO. The electrons trapped at PbS/ZnO interfaces attract mobile holes and result in rapid carrier recombination. Given the unique sensitivity of THz spectroscopy to only free carriers, comparing transient THz photoconductivity of the polycrystalline PbS film on ZnO and on quartz allows us to assess whether the injection of free, conductive electrons into ZnO occurs following photoexcitation.

Figure 4 (a) The band diagram at PbS/ZnO interface (after Sun et al.9 and Ehrler et al.51). (b) shows the normalized change in microwave conductivity for PbS-on-quartz (red symbols) and PbS-on-ZnO (blue symbols) samples for 0.5 ms following a ~10 ns illumination pulse at 905 nm. (c) Left: timeresolved photoconductivity of PbS polycrystalline film on quartz (red) and PbS film on top of ZnO layer (blue) of 800 nm 100 fs pump pulse measured for different fluence. Normalized decays are shown on the right.

Both microsecond and ultrafast carrier dynamics reveal further insight into the recombination processes. The intensity-normalized microwave photoconductivity (TRMPS) results in Fig. 4(b) demonstrate significantly faster recombination for PbS on ZnO (blue symbols) as compared to recombination of PbS on quartz (red symbols). Fig. 4(c) shows the ultrafast photoconductivity decay dynamics in polycrystalline PbS film on quartz (red) and on ZnO (blue), with absolute values of –∆T/T on the left, and normalized –∆T on the right. The presence of ZnO decreases the peak photoconductivity by over an order of magnitude at all fluence values, and speeds up carrier trapping. The small residual photoconductivity in the PbS/ZnO system after the first few picoseconds is due to the photocarriers in a small fraction of the PbS grains that are not in contact with ZnO, as can be seen from transient frequency-resolved THz conductivity spectra (Fig. 3 (b)). The complex conductivity in PbS/ZnO system is qualitatively the same as in polycrystalline PbS on quartz (Fig. 3 (a)), with fully suppressed long range transport at times ≥ 100 ps and small non-zero σ (ω=0) at 5 ps, and 8 ACS Paragon Plus Environment

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differs only by the magnitude that is ~ 10 times smaller due to the lower carrier density. Specifically, we do not observe injection of mobile carriers into the ZnO layer, which would result in significant long-range conductivity at longer times, as shown in the supplementary information for ZnO photoexcited with a 400 nm pump (Supplementary Fig. S1). Instead, the only mobile carriers in both systems are found within PbS crystallites that are not in contact with ZnO, again characterized by strong localization by PbS grain boundaries and an intrinsic mobility of ~ 750 cm2/Vs. We therefore conclude that the photoexcited electrons are trapped at the interface between PbS and ZnO rather than injected into ZnO, and that this occurs over scales below our time resolution of ~ 300 fs. As a result, highly mobile photoexcited holes are attracted to the PbS/ZnO interface, where they recombine with trapped electrons.51

Conclusion In summary, we have studied transient THz photoconductivity in polycrystalline PbS films deposited on ZnO and on insulating quartz. Photoconductivity of PbS film on quartz lasts for milliseconds after excitation, with high, ~ 750 cm2/Vs intrinsic mobility of carriers within individual crystallites but suppressed long-range, inter-grain transport. We also find that PbS/ZnO interface states facilitate rapid trapping and recombination of carriers photoexcited in PbS, reducing the lifetime of mobile carriers by over an order of magnitude compared to the PbS film on quartz. We do not observe injection of mobile carriers into the ZnO layer over time scales of hundreds of picoseconds. Instead, the only mobile carriers in both systems are found within PbS crystallites, characterized by strong localization by PbS grain boundaries and intrinsic mobility of ~ 750 cm2/Vs. This observation underscores the need for chemical passivation of PbS/ZnO interfaces in order to achieve efficient injection of photoexcited carriers into ZnO for application of room temperature CBD-grown polycrystalline PbS on ZnO in photovoltaics and photodetectors. Increase of efficiency of electron injection from PbS quantum dots into ZnO has been demonstrated using inorganic buffer layers,11, 15 organic ligands,52 or self-assembled monolayers.53 Future work is needed to establish whether similar approaches of using buffer layers to minimize interfacial recombination and enable transfer of photoexcited electrons from PbS into ZnO electrodes are also applicable to small band gap, polycrystalline PbS.

Acknowledgements: This work was supported by the National Science Foundation under award DMR1609538 and by the Massachusetts Clean Energy Center (MassCEC).

Supporting Information for Publication. Supporting information includes transient photoconductivity of ZnO film on quartz without PbS.



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