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Towards practical carrier multiplication: Donor/ acceptor co-doped Si nanocrystals in SiO2 Nguyen Xuan Chung, Rens Limpens, Chris de Weerd, Arnon Lesage, Minoru Fujii, and Tom Gregorkiewicz ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00144 • Publication Date (Web): 17 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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Towards practical carrier multiplication: Donor/acceptor co-doped Si nanocrystals in SiO2 Nguyen Xuan Chung1*, Rens Limpens2*#, Chris de Weerd3, Arnon Lesage3, Minoru Fujii4, and Tom Gregorkiewicz3# 1

Royal Institute of Technology (KTH)

Kistagången 16, 164 40 Kista, Sweden. 2

National Renewable Energy Laboratory,

15013 Denver West Parkway, Golden, CO 80401, 303-275-3000, USA. 3

Van der Waals-Zeeman Institute, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, the Netherlands.

4

Department of Electrical and Electronic Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan.

* equal contribution. # corresponding authors: Rens Limpens: [email protected] & Tom Gregorkiewicz: [email protected].

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Abstract Carrier multiplication (CM) is an interesting fundamental phenomenon with application potential in optoelectronics and photovoltaics, and it has been shown to be promoted by quantum confinement effects in nanostructures. However, mostly due to the short lifetimes of additional electron-hole (e-h) pairs generated by CM, major improvements of quantum dot devices that exploit CM are limited. Here we investigate CM in SiO2 solid state dispersions of phosphorus and boron co-doped Si nanocrystals (NCs): an exotic variant of Si NCs whose photoluminescence (PL) emission energy - the optical bandgap - is significantly red-shifted in comparison to undoped Si NCs. By combining the results obtained by ultrafast induced absorption (IA) with PL quantum yield (PL QY) measurements, we demonstrate CM with a long (around 100 µs) lifetime of the additional e-h pairs created by the process, similar as previously reported for undoped Si NCs, but with a significantly lower CM threshold energy. This constitutes a significant step towards the practical implementation of Si based NCs in optoelectronic devices: we demonstrate efficient CM at the energy bandgap optimal for photovoltaic conversion. Keywords: Carrier multiplication, carrier generation rate, co-doped silicon nanocrystals, photoluminescence quantum yield. Introduction The quest for realization of efficient carrier multiplication (CM) has been the primary goal of many investigations in nanomaterials, and specifically in semiconductor nanocrystals (NCs) for more than a decade1-5. From the point of view of potential application in photovoltaics, the most important aspects are the (i) energy of the electron-hole (e-h) pairs and the (ii) threshold energy for CM. Indeed, for the solar spectrum AM 1.5, the optimal e-h pair energy for CM yielding the highest conversion efficiency of approx. 40 % is around 0.8 eV6. This low energy cannot be attained in Si NCs and that partly explains the wide investigation of CM in toxic Pb compounds, such as PbS and PbSe – we do think it is important to mention the low bandgap energy allotropes of Si

7, 8

. Experimental investigations of CM in these materials have

not been reported, to our knowledge, but would certainly be interesting. To this end we note that while some investigations focus on band structure engineering in order to promote the CM process itself by, e.g., making use of core-shell NCs9,

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, others try to optimize the

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harvesting of the additional e-h pairs generated by CM . The latter is essential since, whereas increased Coulomb interactions and decelerated hot-carrier cooling rates significantly enhance the CM potential, as has been shown for multiple NC systems1, 2, 4, 12, 13, the main bottleneck remains the utilization of the additional e-h pairs. Their extraction (as well as radiative recombination) is dramatically hindered by the efficient Auger recombination (AR, ps-range), which is also enhanced in nanostructures by quantum confinement. While it has been shown that the additional e-h pairs generated by CM can escape rapid AR by dedicated device optimization of PbS NCs14 and hydrazine surface treated PbSe NC films11, the problem is severe. In this study we address both challenges by optimizing the CM threshold and the extraction feasibility at the same time. We do this by making use of P and B co-doped Si NCs. Previously we have demonstrated that AR can be suppressed in dense solid state dispersions of Si NCs in SiO2 by spatially separating the e-h pairs into neighboring NCs15, in analogy to the singlet fission process in organic chromophores16. This is the essential reason why we picked Si NCs for this study. At the same time, we expect that by P and B co-doping, the threshold energy of CM might decrease, as a consequence of the optical bandgap reduction, while still maintaining the single-exciton recombination of the additional e-h pairs that are created by the process. Doping of NCs has been investigated as an interesting option to influence their optical and electronic properties17. While mostly explored for one type of dopants only18,

19

, the simultaneous doping with electron and hole donating species, hereby creating neutrally charged

nanoparticles, is particularly interesting. This has been explored for Si NCs and shown to create new possibilities in terms of the surface functionalization20, dispersibility21,

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and electronic capabilities23-25. One specific advantage was the lowering of the optical bandgap26,

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,

below that of bulk Si, facilitated by the radiative recombination between the donor and acceptor states that are introduced by the P/B doping, as recently confirmed by a scanning tunneling microscopy study28. As explained before, this makes co-doped Si NCs interesting for CM and in our preliminary report we have indeed presented a fingerprint of CM in these materials29. In light of this study it is important to realize that CM efficiencies are traditionally inferred from ultrafast transient absorption techniques, which monitor the fast nonradiative recombination of the multiple excitons by Auger recombination30. As shown previously31, this method fails in case of closely-packed SiO2 embedded Si NCs due to the delocalized nature of the CM process. Instead, here we derive the carrier generation efficiency by rigorously scaling the number of photons emitted in photoluminescence (PL)15 and the induced absorption (IA) amplitude31 to the number of absorbed photons, analogously to THz investigations of CM in bulk PbS and PbSe4. In this study we cross correlate both the IA and the PL approach to quantify the carrier generation rate and to investigate the carrier dynamics of the additional e-h


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pairs in the co-doped Si NCs. In that way, the CM efficiency at particular excitation energies is found and the nature of CM and its threshold energy can be determined. Materials and Methods Materials The thin films containing Si NC samples are deposited by a magnetron RF co-sputtering system, with a consequent annealing process as follows. Full details of the production procedure can be found in Ref.

32

. Two samples are chosen for this study. P and B co-doped NCs

prepared by sputtering a phosphosilicate glass target partially covered with pieces of silicon and B2O3 tablets. An undoped reference Si NC sample is produced by using a SiO2 target covered with pieces of silicon. For each sample, a layer of approximately 1.5 µm is deposited on top of a fused silica substrate. The samples are then annealed for 30 min at 1150 oC in nitrogen atmosphere. This high-temperature heat treatment induces phase separation between Si and SiO2, and Si NCs (either un- or co-doped) are formed. Based on this same previous work, also the concentration of dopants could be determined for the used preparation procedure, and is determined to contain 1.25 % atomic concentration of both P and B. The excess silicon concentration is set in both samples at 18 %.

Methods Standard PL spectra are investigated by making use of the 3.64 eV excitation of a xenon lamp coupled to a Solar LS MSA130 monochromator. For the pure Si NCs, the PL spectrum is detected with a Hamamatsu CCD (S7031-1008S) integrated in a Solar LS M266 monochromator, whereas the near infrared (NIR) PL spectrum of the co-doped NCs is investigated on a setup including a Spex 1000M monochromator with a high resolution grating for the NIR detection, an Edinburgh EI-A germanium detector and a DSP 7280 lock-in amplifier. Furthermore, linear absorption has been measured with a dual beam mode Perkin Elmer Lambda 950 spectrometer. IA measurements are performed at two different systems. To reach the nonlinear excitation regime and verify the single-exciton dynamics the pump-probe data is collected using a femtosecond transient absorption spectrometer (Helios, Ultrafast Systems). The laser source is a 4 W Ti-sapphire amplifier (Libra, Coherent), operating at 1 kHz and 100 fs pulse width (see Ref.

31

for further details). To investigate the carrier

generation rates, a 1028 nm fundamental laser is produced by Yb:KGW oscillator (Light Conversion, Pharos SP) with a repetition rate of 5 kHz and a duration of 180 fs and then is split into two beams. The first beam is non-linear mixed in an Optical Parametric Amplifier (OPA) and passes a second harmonic module (Light Conversion, Orpheus), pump beam with varied wavelengths between 310 and 1500 nm is produced. Being modulated by a chopper, the repetition frequency of the pump beam is 2.5 KHz. The second beam passes a sapphire crystal and produces a broadband probe (500-1600 nm). The probe spectra are recorded on a CCD (Ultrafast Systems, Helios). Relative PL quantum yield (QY) of the co-doped sample is measured by using two different setups in combination with two excitation sources. The sample is excited inside a Newport integrating sphere (with a diameter of 7.5 cm) in order to collect emitted photons in all directions and to avoid the scattering effects. For excitation energies above 2.1 eV, a highly stable L2272 Hamamatsu xenon lamp has been used as excitation source. For excitation energies below 2.3 eV, because of low absorption and low emission in this region, an optical parameter oscillator (OPO) producing a varied wavelength laser with high power is employed as an excitation source. In order to evaluate the number of emitted photons, the same combination of the Edinburgh germanium detector EI-A with the DSP 7280 lock-in amplifier is used.


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Results and Discussion Sample characterization

Figure 1. Normalized PL spectra of undoped (thick cyan) and P and B co-doped (thick red curve) Si NCs upon an excitation energy of 3.64 eV. Normalization is performed by scaling the PL maxima to one. The dark red curve represents a log-normal fit of the PL spectrum of the co-doped NCs. Absorption spectra of the undoped and the co-doped NCs (thin cyan and thin red curves, respectively) are investigated in the excitation energy region below 2 eV. Parasitic absorption (evaluated by the level of absorption in the NIR regime) of the pure (cyan dashed line) and co-doped (red dashed line) NCs is found to be around 0.3 and 2.5 %, respectively. The missing regions of absorption spectra indicate a detector switch. Before investigating CM, we start with the optical characterization of the samples. In figure 1 we present PL and absorption spectra of the P and B co-doped Si NCs and compare them to those of the undoped NCs as a reference. In agreement with previous reports33, 34, we confirm that the co-doping significantly red-shifts the optical bandgap to a value below the bandgap of bulk silicon (i.e., 1.12 eV). At the same time, the PL lifetime is of the order of 100 µs 20. The spectral downshift of 0.45 eV is ascribed to donor-to-acceptor transitions in fully compensated NCs33, and is perfectly in line with a recent scanning tunneling microscopy (STM) study of similar sized co-doped Si NCs Ref.28 Here it is shown that these co-doped Si NCs with donor-acceptor transitions of around 1eV have an intrinsic bandgap of around 1.5 eV (similar to the undoped NCs in this work). We further note considerable (≈ 2.5 %) absorption in the co-doped sample for below-band gap energies. This we ascribe to “parasitic” absorption (i.e., not contributing to PL), which most probably originates from the excitation of free carriers in not fully compensated NCs (as discussed further on), with a possible contribution of defect states unavoidably introduced by doping. Furthermore, the overall linear absorption in the VIS regime is significantly enhanced in the co-doped NCs, which is related to a higher Si excess and consequently, a somewhat higher concentration of Si NCs in the co-doped sample.


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Ultrafast carrier dynamics

Figure 2. Normalized IA traces of the co-doped Si NCs, for pump energies below and above 2 Eg, e.g., the lowest possible threshold energy for CM, at Nabs « 1. The traces are averaged over a range (1200-1350 nm) of probing wavelengths. b): IA traces for three different excitation power densities, ranging from 21 to 127 mW/cm2, for a pump energy of 2.27 eV. The inset shows the derived A-to-B ratios (representing the ratio of the trace amplitude at 1 ps and at 500 ps, respectively) as a function of the excitation power density, averaged over a probing range of 1100-1200 nm, verifying the linear excitation regime (Nabs « 1). Note that the non-unity baseline of the A/B ratio is indicative of ultrafast nonradiative recombination channels, in the absence of AR. In figure 2a we display IA decay transients for the co-doped NCs, for pump energies from below to above two times the optical bandgap (Eg). These have been obtained in the linear regime, so with significantly less than one photon absorbed per NC, Nabs « 1. The traces are averaged over a range of probing wavelengths (1200-1350 nm) to increase the signal-to-noise ratio - this is possible since no probing energy dependence is detected in this domain. The traces are normalized at 3 ps. As can be concluded, identical IA dynamics are obtained regardless of the excitation energy, below and well above 2 Eg, i.e., where eventual CM might be expected. As mentioned in the introduction, the existence of CM is traditionally inferred from a change in IA dynamics, induced by rapid AR of multiple excitons co-localized in the same NC. Consequently, the absence of an ultrafast decaying component for high energy pumping indicates an absence of AR, and therefore also the absence of the CM process in the traditional sense of the term. We therefore conclude that we do not observe carrier multiplication within the same NC (also referred to as multiple exciton generation). In the next part, by monitoring the carrier generation yield, we investigate the potential occurrence of the spatially separated form of CM. In order to analyze the IA dynamics further we recall that oxidized Si NCs typically suffer from efficient trapping of free carriers, with the effect being enhanced at higher energies, i.e., for “hot” carriers30,

35, 36

, thus quenching the IA amplitude on the time-scales of tens of

picoseconds. As a result, even at pump energies below 2 Eg (excluding possible CM) and in linear excitation regime with Nabs « 1 (so under conditions precluding a possibility of generation of multiple excitons in a single NC) the carriers are prone to nonradiative recombination. This manifests itself by the non-unity baseline of the A-to-B ratios for low pump power densities, depicted in the inset to figure 2b (for pump energy of 2.27 eV). In our analysis, we take the intensity of the IA traces at 1 ps (defined as A) and compare it to the intensity at 500 ps (B). Furthermore, as the number of absorbed photons is increased (by changing the pump power) we observe increasing A-to-B ratios - being the result of Auger-induced quenching of the multiple excitons that are created by multiple-photon-absorptions in the same NC. This behavior independently verifies that all traces in figure 2a are representing the single-exciton regime.


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Photoluminescence quantum yield

Figure 3. Absolute PL QY of the co-doped Si NCs, measured by using a xenon lamp (blue circles) and a laser (orange triangles) as excitation sources, as described in the Methods. The error bars for the data obtained under laser excitation are larger due to the significant power fluctuations and the low linear absorption in this energy range. Relative PL QY of the co-doped Si NCs has been investigated in two different setups whose details are given in the Supporting Information. For the measurements with excitation energies below 2.3 eV (depicted by the orange symbols), the experimental error is governed by the laser power fluctuations, of 5-10 %. Moreover, the PL QY values measured in that range have to be corrected for the strong parasitic absorption – see figure 1. The correction procedure is discussed in detail in the Supporting Information. As such, and in combination with the rather low absorption cross section at excitation energies below Eexc = 2 eV (as a result of the indirect bandgap of Si), it is extremely challenging to perform PL QY measurements at low excitation energies. We direct the interested reader to a recent paper of ours (Ref 37. ). In spite of these limitations and forthcoming larger scatter of carrier yield data we managed to go as low as Eexc = 1.5 eV (Figure 3). For the high excitation energies above 2.1 eV (blue symbols) the linear absorption of the NCs is fairly high, and a xenon lamp can be used; its power fluctuations are low (around 0.25 %) reducing the error bars to within the symbol size. In that range also the absolute value of the PL QY can be determined - see Ref. 29 for details. We end up having two sets of partially overlapping PL QY data, with the one at high excitation energy containing information on the absolute PL QY. Therefore, by re-scaling, the relative PL QY in the low excitation energy range can be converted to the absolute values. The results are shown in figure 3. Inspecting figure 3, we note that (i) the PL QY increases for higher excitation energies and (ii) the absolute PL QY is quite low, between 0.5 and 1.5 % indicating that the optical activity of the ensemble of co-doped Si NCs is considerably lower than 5-35 % typically observed for undoped NCs15, 37-39, with the reference sample featuring a value around 5 %, as shown in the Supporting Information. This low PL QY we attribute to the unavoidably large fraction of not perfectly compensated NCs where the nonradiative Auger recombination will prevail. While compensated doping - with an equal number of donors and acceptors per NC is energetically favorable40, this advantage of formation energy diminishes for a higher number of dopants per NC. Therefore, within our sample (where the average nominal number of P and B dopants per NC is relatively high41, >10) we do expect a significant amount of uncompensated NCs. Although it is at the moment unclear what the actual fraction of uncompensated NCs should be (especially so, since the number of electrically active dopants is lower than the nominal doping level), the large drop of the PL QY (~factor 5 with our reference sample) suggest a significant fraction. In result, while the radiative recombination will dominate fully for compensated NCs it will be quenched in those with unbalanced doping.


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Carrier generation rate In figure 4, we combine the carrier generation rates calculated from both the PL QY and the IA measurements. The rate is defined as the number of e-h pairs created upon the absorption of a single photon. The baseline values of the absolute PL QY for pump energies below 2 Eg (which are presented in figure 3) are normalized to 1. To derive the carrier generation rate from the IA dynamics we normalized all traces to the number of absorbed photons at that specific pump condition, in a similar way to our previous study for pure Si NCs31, and used the amplitude (at 3 ps) from the normalized traces as the relative carrier generation rate. To optimize the result, we used three different excitation powers (all in the linear regime), for each pump energy, following Ref.

42

– the details of this procedure can be found in the

Supporting Information. These relative IA carrier generation rates scaled to those derived from the PL QY are presented in figure 4.

Figure 4. Carrier generation yield in the co-doped Si NCs, obtained from PL QY (black markers) and IA (red markers) measurements. The effect of ideal CM (i.e. commencing at the energy conservation limit and with 100 % efficiency15, see text for further explanation) is presented by the blue curve. The error bars of the IA-based data are derived from the noise level of the IA amplitude at 3 ps and the uncertainty in the estimation of the number of absorbed photons per NC. b): Magnification of panel (a) for excitation energies below 2.5 eV. Detailed interpretation The first observation we make from figure 4 is that the carrier generation rates as determined from IA and PL QY data are identical. Since both techniques measure the free carrier population at very different time scales of ps and near ms, respectively, this implies that the free carrier population at long delay times is proportional to that shortly after the pump pulse, for all excitation energies. We can therefore exclude that the rise of the PL QY at high excitation energies is resulting from decreased nonradiative recombination channels; if that would be the case, the change in the nonradiative recombination rate would be visible in the short time-scales through the ultrafast IA spectroscopy measurements, lowering the IA-derived carrier generation rate, which is not observed. Hence, we conclude that the data depicted in figure 4 indeed correspond to the real “carrier generation rate”, which then rightfully labels the y-axis. Secondly, we note that the carrier generation rate increases for higher excitation energies, as already indicated when discussing the PLQY. Bearing in mind the afore mentioned temporal considerations, this is a direct sign of the delocalized CM process, as shown by us in the past15. The process, known as space-separated quantum cutting, suppresses AR and enables radiative recombination of the additional e-h pairs, i.e., yields single-exciton lifetimes of all the carriers photogenerated in the linear pump regime. These single-exciton dynamics are indeed confirmed by the identical traces of the ultrafast spectroscopy measurements depicted in figure 2a. We then estimate the maximum possible effect of CM on the excitation energy dependence of the carrier generation. This is obtained by assuming a CM process with 100 % efficiency and in the energy conservation limit, i.e., every absorbed photon with the energy of Eg < hν < 2 Eg, 2 Eg < hν < 3 Eg, 3 Eg < hν < 4 Eg, etc. generates 1, 2, 3 Z electrons, respectively. The NC size dispersion is included by using accumulation of multiples of the actual PL spectrum of the investigated sample – similar to our earlier work15 for undoped Si NCs. The result is shown by the blue curve in figure 4. As can be concluded, for excitation energies below 2.5 eV this crude simulation satisfactorily reproduced the experimentally derived carrier generation yield. In particular, already for Eexcൎ2.15 eV we observe a carrier generation yield


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of 190% in comparison to the baseline yield (Figure 4). Following the semi-empirical model developed by Hanna and Nozik43, this agreement implies a near 100 % efficiency η ≈ 100 % and near zero excess energy that is necessary to induce CM ∆ ≈ 043, 44. The latter represents a considerable improvement in comparison to the undoped Si NCs for which ∆ ≈ 0.3 eV has been determined15. In direct comparison with this previous work, here we show similar step-like CM features, resembling the high CM efficiencies found for the undoped Si NCs. With the differences being that these co-doped NCs report a ∆ ≈ 0 eV, a much lower absolute value of the CM energy onset (~2 eV, in comparison to ~3.1 eV for undoped NCs), lack of further enhancement of the carrier generation yield at excitation energies above approximately 3Eg, and the generally lower PL QY, indicating presence of prominent nonradiative recombination channels. The latter aspect is directly addressed in a recent publication of us 45, that investigates the effect of substitutional co-doping on the photo-excited electron-hole pair carrier dynamics. Combined with this recent work, and the tools it brings to optimize the optical quality of co-doped Si NCs, the present results could constitute an important step for future photovoltaic applications of CM. Finally, we make the following three notions. (i) For pump energies hν > 2.5 eV, the experimentally measured carrier generation is considerably lower than predicted by the simulation. Obviously in that range detrimental processes leading to carrier loss (such as, e.g., the increased role of high-energy traps) appear, effectively counterbalancing the positive effect of CM - as discussed in reference37. (ii) As mentioned, the investigated doped NC ensemble consists of a combination of compensated, uncompensated and doping-induced defected NCs. Although based on the results at hand we cannot separate the respective contributions of these specific fractions of the total NC ensemble, they obviously affect the optical activity, being responsible for the relatively low PL QY yield observed in our materials. However, it is important to realize that this does not influence the experimental method used to determine the delocalized form of carrier multiplication, since the effect of these different fractions on the ensemble optical properties is already accounted for in the “baseline” carrier generation efficiency, for excitation energies below 2 times the optical bandgap ( 100 µs). This is beneficial for optoelectronic applications and is in contrast to the ps extraction-window typical for CM-generated carriers in traditional CM. Moreover, the donor and acceptor levels in these co-doped nanoparticles significantly decrease the CM threshold to 2 Eg, to below hν ≈ 2 eV on the absolute energy scale. Therefore, the current results show that more than half of all photons of the solar spectrum are available for CM.

Acknowledgments The work was financially supported by the Technology Foundation STW, by NanoNextNL, by Stichting voor Fundamenteel Onderzoek der Materie (FOM), and RL acknowledges the National Renewable Energy Laboratory (NREL) LDRD program for the award of the Nozik postdoctoral Fellowship to perform this research. Authors’ contribution NXC, RL and TG conceived the project, provided experimental interpretation, and wrote the manuscript. NXC and RL conducted the induced absorption and photoluminescence measurements. NXC performed the linear absorption and quantum yield measurements. AL prepared the co-sputtered samples under supervision and facilitation of MF, and CdW guided the initial induced absorption measurements. Associated Content, Supporting information The Supporting information contains the setup to measure the PL QY, transmission, relative PL QY, absolute PL QY, and the parasitic absorption. In addition, we provide IA dynamics for six excitation energies.


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For table of contents use only. Title: Towards practical carrier multiplication: Donor/acceptor co-doped Si nanocrystals in SiO2 Authors: N.X. Chung1*, R. Limpens2*, C. de Weerd3, A. Lesage3, M. Fujii4 and T. Gregorkiewicz3# This TOC-graphic illustrates the delocalized form of CM, observed in P-B co-doped Si NCs. Electronic coupling of the Si NCs (illustrated by their overlapping halo) facilitates spatial separation of the multiple e-h pairs. This suppresses detrimental Auger recombination, and as a result, radiative recombination of both e-h pairs is possible (indicated by the red arrows). The donor-acceptor levels decrease the optical bandgap and the energy onset for CM to occur. In particular we achieve 190% efficient CM at an excitation energy of Eexc=2.15 eV.


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acceptor co-doped Si

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