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Charge Carrier Generation and Extraction in Hybrid Polymer/Quantum Dot Solar Cells Valentas Bertasius, Rosanna Mastria, Aurora Rizzo, Giuseppe Gigli, Carlo Giansante, and Vidmantas Gulbinas J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02965 • Publication Date (Web): 08 Jun 2016 Downloaded from http://pubs.acs.org on June 12, 2016

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The Journal of Physical Chemistry

Charge Carrier Generation and Extraction in Hybrid Polymer/Quantum Dot Solar Cells

Valentas Bertasius,1 Rosanna Mastria,2,3 Aurora Rizzo,2 Giuseppe Gigli,2,3 Carlo Giansante,2,3,4,* Vidmantas Gulbinas1,*

1

Center for Physical Sciences and Technology, Savanorių Ave. 231, Vilnius LT-02300, Lithuania

2

CNR NANOTEC–Istituto di Nanotecnologia, Polo di Nanotecnologia c/o campus Ecotekne, Via Monteroni, 73100

Lecce, Italy 3

Dipartimento di Matematica e Fisica ‘E. De Giorgi’, Università del Salento, via per Arnesano, 73100 Lecce,

Italy 4

Center for Biomolecular Nanotechnologies @UNILE, Istituto Italiano di Tecnologia, via Barsanti 1, 73010

Arnesano (LE), Italy

* Corresponding authors: E-mail: [email protected]; Tel.: 0039 0832 1816201 E-mail: [email protected] ; Tel.: 00370 5 2641236

1 ACS Paragon Plus Environment


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Abstract Here we investigate charge carrier generation and extraction processes in hybrid polymer/nanocrystal solar cells by means of time-resolved optical and photoelectrical techniques. We addressed the role of both poly(3-hexyltiophene) and colloidal arenethiolate-capped PbS quantum dots, which constitute the hybrid composite nanomaterial, in the photo-induced processes most relevant to device operation by changing the compositional ratio and applying chemical and thermal post-deposition treatments. The carrier generation processes were found to be wavelengthdependent: excitons generated in the polymer domains led to long-lived weakly bound charge pairs upon electron transfer to PbS nanocrystals; whereas charge carrier generation in the nanocrystal domains is highly efficient, although effective separation required the application of external electric field. Overall, charge carrier generation was found efficient and almost independent on the strength of applied electric field, therefore competition between separation of electron-hole pairs into free carriers and geminate recombination is the major factor limiting the fill factor of nanocomposite-based solar cells. Device efficiency improvements thus require faster interfacial charge transfer processes, which are deeply related to the refinement of nanocrystal surface chemistry.


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1. Introduction Organic and hybrid photo-active nanomaterials for solar cell applications attract attention of researchers as potential alternatives to the energivorous production of devices based on bulk inorganic semiconductors. So far, solar cells based on conjugated polymer as light-harvesters/electron donors and fullerene derivatives as electron acceptors were among the most intensively investigated systems, leading to efficiencies that exceed 10 %.1,2 Notwithstanding the great progresses, their further improvement and possible implementation is still limited by the use of optically transparent and rather expensive fullerene derivatives. An attractive alternative to fullerenes is represented by colloidal inorganic semiconductor nanocrystals (commonly referred to as quantum dots, QDs). The compatibility with solution-phase processing and the possibility of bandgap tuning by material choice and synthetic size control make QDs plausible electron-withdrawing materials in heterojunction solar cells;3-5 moreover, their inherently large surface-to-volume ratio permits to further improve light-harvesting properties and tuning of relevant band energies upon suitable surface chemistry post-synthesis modification.6,7 Among other nanomaterials, lead chalcogenide QDs represent benchmark systems for photovoltaic applications, due to narrow bulk bandgap, large optical absorption coefficients, and small effective masses for charge carriers.8-9 Albeit the aforementioned potential advantages, power conversion efficiencies (PCEs) for solar cells incorporating conjugated polymers and lead chalcogenide QDs reached 5.5% for alloyed PbSxSe1-x QDs,10 5.3% for PbSe QDs,11 and 4.8% for PbS QDs,12 among others,13-16 which are about twice lower than the best values obtained with fullerene derivatives.1,2 In order to improve the PCE of such polymer/QD devices, is deemed necessary to deepen the understanding of photo-induced charge carrier generation/extraction mechanisms that are still not exhaustively described and explained in these hybrid systems. Indeed, light absorption in polymer/QD solar cells initiates a number of electronic processes. Photon absorption in the polymer matrix may lead to electron transfer to QD domains creating pairs of charge carriers separated at the hybrid heterojunction,17-19 eventually in concomitance with resonant exciton transfer to QDs and other, often neglected, exciton relaxation pathways. On the other hand, excitation of QDs may lead to charge separation between adjacent QDs20 or to hole transfer from QD to polymer domains.21 Such disparate and eventually concurrent processes lead to different charge carrier separation efficiencies, with geminate and nongeminate recombination limiting effective carrier extraction.22



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In this work, we investigate charge carrier generation, separation, and extraction processes in hybrid solar cells comprising poly(3-hexylthiophene), commonly referred to as P3HT, and arenethiolate-capped PbS QDs with different compositional ratios and upon applying chemical and thermal post-deposition treatments (ligand exchange and annealing, respectively). To this aim, a combination of time-resolved photoluminescence, transient photocurrent and delayed collection field techniques were exploited to identify the major photo-induced processes involved in hybrid device operation. We found that photo-induced processes are substantially different if the incident light is prevalently absorbed by P3HT or QD components: i) excitons photo-generated in the polymer matrix dissociate into separated charge carriers upon interfacial electron transfer to QDs, although the efficiency of this process is limited by the barrier to electron transfer constituted by the QD ligand shell; ii) excitons photo-generated in the QD domains yield separated charge carriers, although the application of an external electric field is required to prevent geminate recombination. In addition, we observed improved electron mobility up to ~ 4x10-4 cm2/Vs for post-deposition treated samples, although high QD volume fractions are necessary to ensure efficient polymer exciton dissociation. Charge carrier photo-generation efficiency in all of the samples is almost independent on the applied electric field strength, therefore competition between charge carrier extraction and recombination appears as the most relevant limitation to the fill factor of such hybrid solar cells. As a result, we suggest QD-to-P3HT hole transfer and inter-QD charge transport as the major issues to be addressed, most likely via improved QD surface chemistry, for further development of efficient hybrid nanocomposite-based solar cells.

2. Experimental Section Synthesis of PbS QDs. The synthesis of colloidal (Pb-)oleate-capped PbS QDs was carried out following a modified reported procedure.23 In a typical synthesis, 2 mmol of PbO, 6 mmol of oleic acid, and 10 g of 1-octadecene were mixed and degassed via repeated vacuum-nitrogen cycles. Subsequently, the mixture was heated to allow dissolution of PbO (~ 100 °C) until the solution became optically transparent and colorless indicating the formation of lead(II)oleate complex(es). The solution was then cooled at 80 °C and switched again under vacuum in the attempt to remove water formed upon lead(II)-oleate complex formation. At this point, the solution was heated and kept under nitrogen flow at 110°C and then 1 mmol of bis(trimethylsylil)sulfide in 2 mL of 1-octadecene was swiftly injected. The heating mantle was immediately removed and the resulting colloidal dispersion was left to cool to room temperature. The obtained QDs were purified by using a non-solvent precipitation procedure, carried out by adding


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to the reaction product excess acetone, centrifuging at 4000 rpm and dissolving in toluene. The purification step was repeated two times using methanol as non-solvent and finally the solution was filtered through a 0.2 µm polytetrafluoroethylene (PTFE) filter. All manipulations were performed by using air-free techniques. Solution-based ligand exchange procedure. In order to produce colloidally stable arenethiolate-capped PbS QDs, a slight excess of equimolar p-methylbenzenethiol/triethylamine solution was added to (Pb-)oleate-capped QDs following an already published protocol.24 In a typical ligand exchange procedure, 300 equivalents of pmethylbenzenethiol/triethylamine were added slowly to a 1 mM solution of as-synthesized PbS QDs in dichlorobenzene. The mixture was precipitated by adding hexane and methanol, then centrifuged and redissolved in dichlorobenzene. The purification step was repeated and finally the precipitate was dissolved in a mixture of dichlorobenzene:chloroform 3:2 vol/vol to obtain a 1 mM arenethiolate-capped PbS QDs solution. Device fabrication. Patterned ITO-coated glass substrates (Visiontek) were cleaned using TL1 solution (H2O/NH4OH/H2O2

5:1:1)

and

covered

with

a

40

nm

thick

layer

of

poly(3,4-

ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS, Clevios P 4083). The active layer was obtained using a single deposition step, avoiding the tedious and wasteful layer-by-layer approach. The hybrid composite (HC) layers were deposited as follows: concentrated PbS QD solutions (90 mg/ml in dichlorobenzene/chloroform 3:2) were blended with P3HT (13 mg/ml in dichlorobenzene) at 9:1 (henceforth referred to as HC91), 2:1 (HC21), and 1:1 (HC11) weight ratios and spin-casted at 1500 rpm for 60 sec yielding ~ 100 nm thick films. Mere QD and P3HT layers were deposited by spin casting the active materials in the same conditions as for nanocomposites (1500 rpm for 60 sec; QD solution at 90 mg/ml in dichlorobenzene/chloroform 3:2 and P3HT solution at 13 mg/ml in dichlorobenzene); thinner and more rough QD films are obtained compared to P3HT and HC films. The samples marked with an x (QDx and HCx) were chemically then thermally treated: 1% vol/vol 1,3 benzenedithiol (BDT, chosen in the attempt to preserve conjugated ligands and the enhanced broadband optical absorption of QDs)6 solution in acetonitrile was dropped on the active layers and spin-coated at 1500 rpm for 30 sec, then rinsed with mere acetonitrile; an annealing step at 110 °C for 30 min was then performed. The thin films were kept under vacuum overnight. LiF (0.6 nm) and Al (130 nm) layers were deposited on top of the nanocomposites by thermal evaporation. The device area (typically ~ 4 mm2) was determined by the overlap of the ITO and Al electrodes and accurately measured using an optical microscope.



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Transmission electron microscopy. The TEM images were recorded with a Jeol Jem 1011 microscope operated at an accelerating voltage of 100 kV. HC samples for analysis were prepared by spin-coating the blend solutions glass/PEDOT:PSS substrates, then floated off the substrate onto the surface of a water bath, and transferred to carbon-coated Cu grids. Electrical device characterization. The current–voltage characteristics were determined using an Air Mass 1.5 global (AM 1.5 G) solar simulator (Spectra Physics Oriel150W) with an irradiation intensity of 100 mW cm-2 and recorded with a Keithley 2400 source meter. Optical measurements. Absorption spectra of thin-films were measured with a Jasko V-670 spectrophotometer. Fluorescence decay kinetics were detected with a streak camera Hamamatsu C5680; a femtosecond Yb:KGW oscillator (Light Conversion Ltd.) was used as excitation source. The oscillator produced 80 fs, 1030 nm light pulses at 76 MHz repetition rate, which were frequency doubled to 515 nm, attenuated, and focused into ~100 µm diameter spot on the sample, resulting in about 100 mW/cm2 average excitation power. The maximum time resolution of the whole system was about 3 ps. Photoelectrical measurements. Transient photocurrent measurements were performed by using an Agilent Technologies oscilloscope DSO5054A with 1 MΩ input resistance. A subnanosecond laser of EKSPLA Ltd generating about 500 ps duration pulses at 532 nm or 366 nm wavelength with 10 Hz repetition rate was used as excitation source. In order to minimise sample degradation, the voltage was applied by using functional generator AFG3000 with ~ 100 ms duration pulses. Positive voltage was applied to Al electrode in order to avoid carrier injection. The photocurrent, I(t) =n(t)m(t), was investigated by measuring the voltage drop on the sample capacitance ଵ

acting

as

an

integrating

capacitor.

The

time-dependent

voltage

drop

was

equal

to

௧

∆ܷሺ‫ݐ‬ሻ = ‫׬‬଴ ݊ሺ‫ݐ‬ሻߤሺ‫ݐ‬ሻ݀‫ݐ‬, where C is the sample capacitance, n(t) and µ(t) are time-dependent charge carrier ஼ number and corresponding mobility, respectively. The integrated photocurrent kinetics were corrected for the limited RC of the system by the procedure suggested in ref. 25. The Time Delayed Collection Field (TDCF) measurements were performed by using the same experimental setup, in which the electrical carrier extraction pulses were applied with variable delay after optical excitation pulses. Voltage dependence of the carrier generation efficiency was measured by keeping constant the carrier extraction pulse voltage and changing the voltage applied during the optical excitation pulse. Carrier extraction efficiency was investigated by keeping constant the generation voltage during the optical excitation, but changing the extraction


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voltage. Carrier recombination was investigated by keeping both the generation and the extraction voltages constant and changing the delay time between the optical excitation and extraction pulses. Contributions of the “dark” current and displacement current related to charging of the sample capacitance were eliminated by calculating difference between extracted charges with and without excitation pulse.

3. Experimental results and discussion. 3.1. Optical properties. Here we take advantage of our previous demonstration that the pendant moieties of ligands at the QD surface mediate non-covalent bonding interactions with the side chains of conjugated polymers, drastically impacting nanocomposite morphology during formation from blend solutions.17 Indeed, the solution-phase replacement of the electrically insulating (Pb-)oleate ligands coming from the synthetic procedure with short arenethiolate molecules permits to achieve polymer/QD phase segregation in interconnected nanometer size scale domains and comparable morphologies in several HCs.17,18 Such optimized and stable HC morphology permits the investigation of charge carrier generation and extraction processes in HC-based solar cells pursued in this study (TEM images of the HCs used in this work are shown in Supporting Figure S1). Absorption spectra of the investigated films are presented in Figure 1. Pure P3HT absorption spectrum is typical of this material with a strong absorption band in the 450 nm 650 nm region with vibronic features. Thin-films of arenethiolate-capped PbS QDs show broad featureless absorbance spanning the entire visible spectral region; the optical absorption onset and the first excitonic peak lye in the near infrared spectral region and are not shown here.17,18 Possible formation of cracks and voids upon chemical and thermal treatments of pure QD and HC films with high QD concentration casted in a single deposition step apparently induced appreciable scattering of the incident light that significantly distorted their absorption spectrum. We however assume that solid-phase ligand exchange of benzenethiol-based ligands with benzenedithiols did not markedly affect optical absorption of QD domains, thus we impose that absorption spectra of QDx and HCx91 samples may coincide to that of their non-treated analogues, QD and HC91, respectively. Absorption spectra of HC films roughly correspond to the sum of the isolated P3HT and QD spectra with contributions proportional to their concentrations.17 We therefore used excitation wavelengths that permit to almost selectively excite P3HT (at 532 nm) and the QDs (at 366 nm). Indeed, approximate estimation of fractions of photons absorbed by the two HC components suggests that QDs absorb 366 nm light by approximately 80-90% in HCx11 and HCx21 samples and



more than 95% in HC91 and HCx91 samples; P3HT instead absorbs 532 nm light by approximately 90% in HCx11 and HCx21 samples and 30-50% in HC91 and HCx91 samples (see Experimental Section for details on HC composition and nomenclature).

2.5 QDx QD HCx91 HC91 HCx21 HCx11 P3HT

2.0 Optical density

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.5 1.0

0.5

0.0

400 366 nm

500

600 532 nm

700 Wavelength, nm

Figure 1. Absorption spectra of the investigated samples. Dashed lines show estimated contribution of QDs to optical absorption in different samples. This estimation was made by decomposition of the HC film spectra into QD and polymer absorption assuming that it is not affected by weight ratio in corresponding HCs.

The steady-state and time-resolved photoluminescence (PL) spectra of neat P3HT and HCs are dominated by polymer emission. The sensitivity of the streak camera at around 1 µm, where QD luminescence is expected17 was not sufficient to detect it. Both PL spectra and kinetics reveal P3HT quenching by QDs proportional to QD volume fraction in HCs; such a quenching is further enhanced by post-deposition treatments in HCx samples. The hypsochromic shift of the PL spectrum in HCs with high QD content can also be related to the PL quenching; indeed, steady-state polymer PL is usually red-shifted due to exciton localization in low energy states within distributed density of states, while in HC samples with high QD concentration PL quenching is faster than exciton transfer to low energy states, therefore preventing PL bathochromic shift. At the same time, large QD concentrations may reduce the crystallinity of polymer domains leading to shorter exciton delocalization length. As commonly assumed, the observed P3HT PL quenching is ascribed to electron and/or excitation transfer to QDs, neglecting


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other non-radiative deactivation pathways. The PL quenching is slow and inefficient compared to P3HT composites with fullerene derivatives, in which it takes place on fs time scale. As an example, 5 wt% of PCBM quenches the P3HT PL more than 5 times and, depending on the morphology, 30 wt% quenches 50-100 times,26 whereas QDs in HCx21 sample (representing 66 wt%, which corresponds to about 30% by volume) quench the P3HT PL less than 2 times. Such a strong difference in the extent of PL quenching occurs regardless of P3HT/QD and P3HT/PCBM film morphologies and can thus be ascribed to much slower electron transfer from excited P3HT to QDs compared to PCBM. This is particularly evident for the as-casted samples, which show poorly efficient PL quenching and, as we will show below, weak photoconductivity. Post-deposition treatments enhance P3HT-to-QD electron transfer efficiency, whereas resonant energy transfer, which is less sensitive to the donor/acceptor distance, is expected to dominate in non-treated samples.

70

10 Normalized PL, arb.un.

Fluorescence intensity, arb.un.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30

1

20

40

20

60 80 Time, ps

100

P3HT HCx11 HCx21 HC91 HCx91

10 0 600

650

700

750

800

850

Wavelength, nm Figure 2. Fluorescence spectra of the investigated films. Inset shows decay kinetics of spectrally integrated fluorescence intensity measured under 532 nm excitation.

3.2. Transient photocurrent. 366 nm excitation. Since 366 nm excitation light is dominantly absorbed by the QDs, we can selectively study the processes initiated by photo-excitation of QDs. Moreover, the large optical density at 366 nm of HC91 and HCx91 samples ensures that excitation light is almost completely absorbed nearby (within the first ~ 10 nm) the positively



biased transparent electrode. As a result, holes are rapidly collected at the electrode poorly contributing to photocurrent dynamics, which therefore are dominated by transient electrons. This approximation is not valid for the HCx21, HCx11 and pure P3HT samples, in which incident light is absorbed in the entire film thickness and the fraction of photons absorbed by P3HT becomes relevant.

0.0

Extracted charge, nC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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QD

Vapp= 0 V

Vapp= 2 V

QDx

-0.5 HC91

HCx91

-1.0

HCx21 HCx11 P3HT

0.01 0.1

x5

x5

1

10

0.01 0.1

1

10

Time, µs Figure 3. Kinetics of the extracted charge from all investigated samples at 0 V and 2 V applied voltages (internal electric fields equal to 0.7·10-5 V/cm and 2.7·10-5 V/cm) measured under 366 nm excitation. The data were corrected for different absorbed light values by different samples.

Charge carrier extraction kinetics in the treated and untreated samples at 0 V and 2 V applied biases are shown in Figure 3. QDx and HCx91 samples exhibit about 5 times faster carrier extraction than non-treated analogs. Similar trend can be observed for the total extracted charge values, which are significantly larger for the treated samples (about 3-9 times, compared to the non-treated samples). Extracted charges at 2 V are similar for QD and HC91 films and analogously improved upon post-deposition treatments, suggesting that efficient charge carrier separation and subsequent extraction may occur also in the absence of P3HT, although it requires the application of strong external electric fields. Indeed, at 0 V, the presence of polymer in HC91 and HCx91 samples enhances the extracted charge


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values by 3-4 times compared to mere QD and QDx samples. Since at 366 nm, electron-hole pairs are mainly photogenerated in the QD domains, we can conclude that charge carrier separation at low electric field takes place mainly by QD-to-P3HT hole transfer, which is inefficient according to ref. 17. On the other hand, strong electric fields are required to enable separation and extraction of charge carriers photo-generated in the QD domains. This findings refine the conclusion made in ref. 17 of hindered QD-to-P3HT hole transfer, exploiting electrical measurements that are more sensitive to detect low carrier densities than the optical measurements used in ref. 17. In case of HCs with lower QD concentrations, the extracted charge values are lower and such extraction slower, if compared to HCs with higher QD content. This can be ascribed to the absence of continuous QD domains ensuring charge percolation pathways and to lower

probability of charge separation, as already demonstrated also for purely organic,

polymer/fullerene derivative, blends.27 In addition, we have estimated the average charge carrier transit time by the procedure illustrated in insert of Figure 5c and consequently evaluated the average carrier mobilities, which are listed in Table 1. As discussed above, we assumed that these values are mainly related to electron mobilities, although holes may slightly contribute to photocurrent, especially at early delays. Comparable mobility values for HCx91 and HCx11 samples were also obtained by Photo-CELIV method,28 despite HCs with lower QD content are affected by high dark current. The obtained electron mobility in HCx21 sample (~ 66 wt.% of QDs) is very close to that reported in ref. [16] for composites comprising P3HT and PbS QDs ligand-exchanged in solid-phase with 1,4-benzenedithiol at 75wt.% QD loading (larger mobility dependence on QD concentration was also reported).22

Table 1. Electron mobilities [cm2/Vs] in different samples evaluated from the integrated photocurrent kinetics and CELIV method (the latter values are reported in parenthesis).

Eintern, V/cm

QD

QDx

HC91

HCx91

HCx21

HCx11

2.7·10-5

0.6·10-4

3·10-4

1.0·10-4

4.5·10-4

3.3·10-4

2.5·10-4

1.7·10-5

0.4·10-4

2.2·10-4

0.7·10-4

4·10-4

3.1·10-4

2.3·10-4

0.7·10-5

0.25·10-4

1.8·10-4

0.6·10-4

3·10-4 (2.6 10-4)

2.1·10-4

2.1·10-4 (3.1·10-4)

Larger carrier mobilities are observed in all samples upon applying an external electric field, which is typical of disordered systems. The extraction kinetics, as well as the electron mobility values for QD and HC91 samples were found to be very similar and showed analogous improvements upon post-deposition treatments, as in QDx and



HCx91 samples. The high concentration of QDs in the HC91 and HCx91 samples ensures the presence of continuous QD domains that does not markedly alter electron mobility compared to corresponding mere QD and QDx samples. The presence of 10% wt of P3HT, instead, yielded much larger photocurrent values for the HCs compared to pure QD films, indicating that polymer plays a crucial role in the charge carrier generation processes.

532 nm excitation. Figure 4 shows the voltage dependence of the total extracted charges under 532 nm excitation, which is relevantly absorbed by P3HT. At an applied voltage of 2 V, rather comparable extracted charge values are obtained for HC and QD films. However, only the HCx91 sample shows substantial extracted charge values at negative biases, i.e. at operation conditions of solar cells, whereas other samples at negative biases produce very low current. This drawback is particularly clear for neat QD samples, which show quite large extracted charge values at high voltages that largely decreases at zero and negative biases.

1.2


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0.9

HCx91 HCx21 HCx11 HC91 QD QDx P3HT

0.6

0.3

0.0 -0.5

0.0

0.5

1.0

1.5

2.0

Vgen, V Figure 4. Voltage dependence of the charge extracted during 10 µs from different samples under their excitation of at 532 nm. The data were corrected for different absorbed light values by different samples.

Charge carrier generation, separation, and extraction processes are expected to be more complex under 532 nm excitation because both QDs and P3HT absorb incident light and optical densities in this spectral region are lower than at 366 nm. As a consequence, excitation light was absorbed in the entire film thickness, although its intensity gradually decreased by 2-3 times throughout the sample, hence photo-generated charge carriers were more


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concentrated close to the positively biased transparent electrode. Consequently, holes travelled in average shorter distances than electrons and their contribution to the photocurrent was substantial, though it can be estimated to be 2-3 times lower than that of electrons.

1.0

Vapp=2 V Vapp=0 V

0.0

b

Extracted charge, a.u.

a 0.5 QD QDx

0.5

-0.5

-1.0

HCx21 QDx HCx11 HCx91

0.0 0.00

HC91

0.05

0.10 Time, µs

0.15

HCx91 1

-1.5

-2.0

HCx21

HCx11 x20

P3HT

-2.5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x20

C

τtransit QD

0.1

0.01

0.1

10 Time, µs

0.1

10

1E-3

0.01

0.1 Time, µs

1

10

Figure 5. a) Kinetics of the integrated extracted charge from different samples measured at 0 V, and 2 V applied voltages (internal fields equal to 0.7·10-5 V/cm and 2.7·10-5 V/cm) under excitation at 532 nm. The data were corrected for different absorbed light values by different samples. b) Normalized initial parts of the carrier extraction kinetics from treated samples. c) Illustration of the carrier transit time determination procedure.

Figure 5a shows carrier extraction kinetics under 532 nm excitation. Carrier extraction kinetics from pure QD samples are identical, within measurement accuracy, to those obtained under 366 nm excitation because in both cases light is absorbed by QDs and hole mobility is expected to be similar to that of electrons. Charge carrier extraction from the untreated HC91 sample is also very similar under both excitation wavelengths, which is less



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expected because more than half of incident photons are absorbed by the polymer. However, P3HT excitation apparently negligibly contributes to photocurrent, because the total extracted charge from HC91 sample at 2 V is almost two-times lower than from the QD sample. This evidence supports the aforementioned conclusion that P3HT-to-QD electron transfer is not efficient in non-treated samples, thus accounting for P3HT-to-QD excitation energy transfer as most plausible cause of P3HT PL quenching. The carrier extraction kinetics at early delays (0.01 – 0.1 µs) is faster in the treated HCs than in QDx films, as shown in Figure 5b that compares normalised initial parts of the kinetics. Since charge carrier transport in HCs via QD domains is not expected to be faster than in mere QDx films, the observed fast charge carrier extraction component can be attributed to the holes transported in the P3HT domains. The fast hole extraction time well agrees with the charge carrier extraction kinetics from the pure P3HT film. Additionally, it shows that the charge carrier generation efficiency under P3HT excitation is substantially enhanced in treated blends. The role of polymer is particularly clear and important for HCx91 sample, in which the total extracted charge is much larger than in other samples and also relatively weakly dependent on the applied voltage (see Figure 5). Consequently, we attribute charge separation to the P3HT-to-QD electron transfer. Both these processes require no electric field assistance and ensure efficient extraction of charge carriers separated at the hybrid interface. In treated HC samples with lower QD concentration (namely, HCx11 and HCx21), 532 nm incident light is dominantly absorbed by P3HT. These samples showed low photocurrent values, in agreement with modest PL quenching, which indicate that only a small fraction of P3HT excitons split into separated charge carriers. Additionally, charge carriers photo-generated in QD domains may be inefficiently extracted, due to the absence of continuous percolation pathways. In summary, transient photocurrent investigations revealed different charge carrier generation mechanisms in treated and non-treated samples under almost selective excitation of QDs and P3HT. Generation, separation, and extraction of charge carriers inside excited QD domains requires electric field assistance, whereas excitation of P3HT promotes interfacial charge carrier separation and efficient charge extraction at low electric field, , although these processes are efficient only in treated HCs at high QD concentration.

3.3. Time Delayed Collection Field (TDCF) investigations. Internal quantum efficiency of solar cells is determined by effective generation of charge carriers and their extraction at the electrodes, which both may depend on the sample composition, treatment, and investigation


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conditions. In addition, the charge carrier generation can be regarded as a two-step process starting with the formation of Coulomb-bound charge pairs followed by separation into free charge carriers producing photocurrent. In order to discriminate these processes, we exploited the TDCF technique under 532 nm light excitation.

6

a

6

b QD

QD Normalized extracted charge

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QDx

QDx

4

4

HC91

HC91 HCx91

HCx91 2

2

HCx21

HCx21 HCx11

HCx11 0

0 1000 2000 Generation voltage, mV

0

0

1000

2000

Extraction voltage, mV

Figure 6. a) Dependence of the normalized extracted charge on the generation voltage at 2 V extraction voltage (Eintern.= 2.7·10-5 V/cm), b) dependence of the normalized extracted charge on the extraction voltage at 0 V (Eintern.= 0.7·10-5 V/cm) (empty squares) and –0,5 V (Eintern.= 0.2·10-5 V/cm) (full squares) generation voltages.

Figure 6a shows the dependence of the charge carrier generation efficiency on the applied electric field, which was obtained by changing the generation voltage applied during the excitation pulse, but keeping constant the carrier extraction voltage and the extraction pulse delay. Different charge carrier generation efficiencies in our samples required different suitable excitation intensities, therefore the measured curves were normalized. At 2 V extraction voltage, an extraction pulse was applied 90 ns after the optical excitation (this represents the minimal delay time of our function generator). All samples with the exception of QDx showed weak dependence of the extracted charge values on the generation voltage, even at negative biases where the internal electric field was very weak. In case of charge generation by P3HT-to-QD electron transfer, such a weak dependence is expected because electron transfer can hardly be significantly affected by relatively weak electric fields. On the other hand, charge generation mechanism inside pure QD films or QD domains in HCs remains less clear, also considering that carrier generation



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efficiency has been found field-dependent. Carrier generation in pure QD film was attributed to the band bending that establish an electric field near contacts.20,29 This mechanism implies the influence of the external applied voltage on charge generation, with an extent that depends on the band bending configuration and near-contact field strength. The efficiency of charge carrier generation and extraction as a function of the applied electric field was investigated by changing the extraction voltage, while keeping constant generation voltage and extraction pulse delay. Two generation voltages of 0 V and –500 mV were used (see Figure 6b). Extracted charge curves obtained at 0 V generation voltage were normalized to one, while curves obtained at –500 mV were multiplied by the same value, thus showing correct ratios of extracted charge values obtained at 0 and –500 mV generation voltages. The data obtained for all treated HCx films were independent of the generation voltage in agreement with the very weak electric field dependence of the charge pair generation efficiency. For QDx and, to lesser extent, HC91 samples the extracted charge at 0 V generation voltage was larger than at –500 mV, which also agrees with the data presented in Figure 6a. The signals for the as-casted QD sample were too low for measurements under –500 mV generation voltage. Strong voltage dependence of the extracted charge values shows that charge carrier extraction efficiency rather than generation is the major process determining voltage dependences of the photocurrent values presented in Figure 4. In operating solar cells, this limits the short circuit current and the fill factor. On the other hand, fielddependent charge pair generation may be also important in case of pure QD films and non-treated HCs. Dependences of the extracted charge on the delay time between optical excitation and electrical extraction pulses, presented in Figure 7, give information about the charge carrier recombination kinetics. The measurements were performed at 0 V generation voltage. These experimental conditions represent processes at short circuit operation of solar cells.


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a

QD QDx HC91 Normalized extracted charge

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HCx91 HCx21 HCx11 0.1

1

10

100

4

b

2

Iex=I0 Iex=8I0

0.1

1 10 Dealy time, µs

100

Figure 7. a) Dependence of the extracted charge on the delay time between excitation and extraction pulses. Generation and extraction voltages were 0 V (Eintern.= 0.7·10-5 V/cm) and 2 V (Eintern.= 2.7·10-5 V/cm), respectively. Blue and green lines show charge values extracted by generation and delayed extraction voltages respectively, whereas black lines show total extracted charge corresponding to sum of both parts. b) Delay time dependences of the extracted charge from HCx21 sample at commonly used and at 8 times larger excitation intensities.

Figure 7 shows charge values obtained upon applying generation and extraction voltages (blue and green lines, respectively), as well as the total extracted charge. It should be noted that the shape of the recombination kinetics during initial delays was independent of the excitation intensity (varied up to 8 times) and that the recombination rate increased by increasing the excitation intensity only after tens of µs (see Figure 7b), at which non-geminate recombination played a relevant role. This implies that geminate recombination dominated until several microseconds after excitation. Under the assumption that only a small fraction of charge carriers are extracted by the generation voltage, which is approximately valid for devices based on QD, HCx21, and HCx11 as photo-active layers, delay time dependence of



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the charge extracted by extracting voltage (green curves in Figure 7a) represents the charge carrier recombination kinetics. The recombination occurs on a microsecond time scale, but in QD films it is several times faster than in HCx21 and HCx11 films. In the other samples, charge extraction at 0 V applied voltage and charge carrier recombination are balanced. In these cases, charge recombination is approximately represented by the time dependence of the total extracted charge (black curves in Figure 7a). Although charge carrier recombination and extraction are non-exponential processes, the initial parts of corresponding kinetics may be reasonably approximated by exponential decay functions and this approximation gives the initial geminate recombination rates, τg (listed in Table 2).

Table 2. Initial geminate recombination rates in different samples evaluated from TDCF investigation.

τg / µs

QD

QDx

HC91

HCx91

HCx21

HCx11

1.7

1.3

2.5

8.1

4.1

3.9

The recombination rates in treated HCx samples are 2-3 times slower than in other thin films. This finding well agrees with the aforementioned conclusion that only in treated HCx films the majority of the charge carriers are separated between P3HT (holes) and QDs (electrons). Such separation is hindered in pure QD and untreated HC samples. Internal electric field of about 0.7·10-5 V/cm present at shot circuit conditions (at 0 V applied voltage) is not sufficient to completely separate charge carriers photo-generated in pure QD films or in QD domains of HCs and prevent recombination. Therefore, only in HCx91 sample, the efficiency of this process exceeds 50 %, indeed yielding rather efficient devices (see Supporting Figure S2). Figure 8 summarizes our results by representing the major photo-induced processes that take place at short circuit conditions in different samples.


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Figure 8. Major photo-induced processes in the different investigated films at short circuit conditions. Stars indicate exciton formation, whereas line thickness corresponds to the efficiency of the photo-induced processes.

4. Conclusion remarks. In summary, we discussed the major photo-induced processes determining hybrid P3HT/PbS QD solar cell efficiency. Charge carrier generation processes were found to be very different when incident photons are absorbed by QDs and P3HT constituting the HCs and strongly dependent on the post-deposition chemical and thermal treatments. Excitation of QDs efficiently creates charge carriers inside QD domains, however in absence of external electric fields, such as at short circuit conditions, charges undergo rapid geminate recombination. Hole transfer from photo-excited QDs to polymer is inefficient and slightly contributes to charge carrier generation. The efficiency of this process increases in treated nanocomposites, however still remaining two-three times lower than electron transfer from photo-excited polymer to the QDs. On the other hand, the fate of excitons created in polymer domains depends on the QD-to-P3HT stoichiometric ratio and on post-deposition treatments. In treated HCs, electron transfer from photo-excited polymer to QDs efficiently creates free charge carriers, whereas in non-treated HCs the electron transfer rate is comparable to the rate of other exciton relaxation pathways, therefore about half of P3HT excitons recombine without producing charges. Electron and hole separation between P3HT and QDs is crucial for preventing charge recombination and enabling efficient extraction at low electric field. The hole mobility over polymer domains is higher than that of electrons over QD domains even in HCs with 90 wt% of QD concentration, therefore the extraction efficiency is mainly determined by the electron mobility. Geminate carrier recombination was found to play a very important role: only in the HCx91 sample, in which the geminate recombination rate is reduced and the electron mobility is sufficiently high, more than 50 % of photo-generated charge carriers are extracted at short circuit conditions. In other HC samples, geminate recombination reduces the extracted charge efficiency to 10-20 %, apparently because of reduced



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percolation pathways. As a result, high QD concentration in HCs is necessary to ensure efficient charge carrier generation and extraction, although this increases the fraction of photons absorbed by QDs, which generate free charge carriers less efficiently at low electric field. Non-geminate recombination, which at our pulsed excitation conditions played only mild role, can be expected as relevant in real solar cell operation conditions, in which interfacial charge carrier accumulation could take place. In conclusion, three major factors limit the operation efficiency of P3HT/PbS QD-based solar cells: i) strongly fielddependent separation of generated charge pairs inside PbS QD domains that favor geminate recombination at buildin field, thus reducing short circuit current and fill factor; ii) slow and inefficient QD-to-P3HT hole transfer and relatively slow P3HT-to-QD electron transfer demanding for high QD concentration in HCs; and c) low electron mobility over QD domains most likely related to QD surface trap states, further exacerbated by solid-phase ligandexchange procedures. Consequently, in order to increase the efficiency of solar cells based on polymer/quantum dot composite nanomaterials, future research should deeply address charge transfer processes at the hybrid interfaces and pursue more effective nanocrystal surface chemical modifications.

Supporting Information Available TEM images of the polymer/quantum dot composite nanomaterials and current-voltage characteristics of devices thereof. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgments The work was partly financed by Research Council of Lithuania, project MIP-085/2015 and by Progetto di ricerca PON R&C 2007-2013 (Avviso n. 713/Ric. del 29 ottobre 2010) MAAT-Molecular NAnotechnology for HeAlth and EnvironmenT (Project Number: PON02_00563_3316357). C.G. thanks the ‘Future In Research’ program by Regione Puglia (code: ZCZP7C3).

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