Photoinduced Charge Transfer in Hybrid Systems of CuInS2

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Photoinduced Charge Transfer in Hybrid Systems of CuInS Nanocrystals and Conductive Polymer 2

Rany Miranti, Yuliar Firdaus, Eduard Fron, Mark Van der Auweraer, Prof. Dr. Jürgen Parisi, and Holger Borchert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06618 • Publication Date (Web): 08 Sep 2015 Downloaded from http://pubs.acs.org on September 14, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 55

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

The Journal of Physical Chemistry

Photoinduced Charge Transfer in Hybrid Systems of CuInS2 Nanocrystals and Conductive Polymer Rany Miranti*, Yuliar Firdaus‡, Eduard Fron‡, Mark Van der Auweraer‡, Jürgen Parisi†, Holger Borchert†* †

University of Oldenburg, Department of Physics, Energy and Semiconductor Research Laboratory, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany

‡

Laboratory of Photochemistry and Spectroscopy, Division of Molecular Imaging and

Photonics, Chemistry Department, K.U.Leuven, Celestijnenlaan 200F, B2404, 3001 Leuven, Belgium

KEYWORDS Charge transfer, CuInS2/polymers, nanocrystals, photoinduced absorption, photoluminescence

ACS Paragon Plus Environment

1


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

Page 2 of 55

ABSTRACT. Colloidal CuInS2 nanocrystals are a promising alternative to toxic cadmium or lead chalcogenide nanocrystals that are widely studied as absorbing material in hybrid solar cells. Photovoltaic devices with colloidal CuInS2 nanoparticles suffer, however, still from low performance. The present study focusses on a detailed investigation of charge transfer as an elemental process involved in the energy conversion process. Therefore, the excited state properties and the process of charge transfer in CuInS2 (CIS) nanocrystal/polymer composites were studied by applying quasi steady-state photo-induced absorption (PIA) and steady state photoluminescence (PL) as well as time-resolved photoluminescence (PL) spectroscopy. The excited state dynamics of our systems was studied using time-correlated single photon counting. We examined two different composites, namely CuInS2 nanocrystals combined with either poly(3-hexylthiophene) (P3HT) or poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). Optical absorption and emission spectra of these hybrid material systems exhibit luminescence quenching and polaronic photoinduced absorption indicating photo-induced charge transfer. By systematic variations of the composition of the films, the material ratios favoring efficient charge transfer were determined.


2

Page 3 of 55

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. Introduction Colloidal inorganic semiconductor nanocrystals (NCs) have gained much attention in the scientific community because of their stability, solution-processability and their ability to be combined with organic semiconductors, such as conjugated polymer. Because of these properties both the inorganic and organic materials can be processed from the same solvent during the fabrication of bulk heterojunction (BHJ) solar cells. Various organic/inorganic solar cells based on colloidal nanocrystals have been fabricated; here, among others, CdSe1–9, PbS10,11, PbSe12, and ZnO13–15 are of interest. In combination with conjugated polymers, currently the power conversion efficiency (PCE) of BHJ solar cells based on CdSe is reaching 4.7% 1. With alloys of PbS and PbSe, PCEs up to 5.5% were achieved 12. A drawback of the Cd- and Pb-based material systems holding currently the record efficiencies for hybrid solar cells is the toxicity of the compounds. This motivates research on more environmentally friendly (Cd- and Pb-free) materials, like copper indium disulfide (CuInS2, CIS) nanocrystals. Indeed, a recent study by Chen et al.16 points to a high chemical stability and low cytotoxicity of colloidal CIS NCs. We previously reported on BHJ solar cells based on colloidal wurtzite CIS NCs with different shapes and ligand exchange treatments. 17,18 Therein, the best result with CIS NCs was obtained after hexanethiol ligand exchange treatment and in combination with poly-(3-hexylthiophene) (P3HT), but the achieved PCE did not exceed 0.05%. Other studies of hybrid solar cells with CIS NCs synthesized using a colloidal approach reported quite similar performance data. Schottky solar cells based on CIS NCs were found to reach a PCE of about 0.1%

21

19,20

and solar

cells with a heterojunction between colloidal CIS and ZnO NCs yielded an efficiency of about 0.6%

22,23

. Higher device performance up to 2.8% PCE was achieved for CIS/polymer hybrid

solar cells, where the CIS phase was synthesized, as an alternative to classical colloidal


3


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

Page 4 of 55

chemistry, in situ in the polymer without using additional organic ligands.24 Despite the progress made, this approach implies, however, the difficulties that the synthesis of the inorganic NCs is dependent on the polymer, and that size and shape of the CIS NCs as well as the morphology of the blends are difficult to control. These issues limit the options to further improve the systems based on the in situ synthesis approach. Till date, it is still not clear why colloidal CIS NCs provide a low performance in hybrid solar cells. In general, the performance of an excitonic solar cell is among others critically influenced by the charge generation and transfer at donor/acceptor interfaces. Many studies have been conducted to investigate the fate of long-lived excited states in P3HT as well as charge transfer from P3HT to organic fullerene acceptors.25,26 Another polymer frequently used in photovoltaics is

the

low

bandgap

polymer

poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-

b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). This polymer is often used because of its smaller optical bandgap of Eg = ~1.4 eV, its broad absorption band, and its structure of alternating electron-rich and electron-deficient units. P3HT and PCPDTBT are well established bench mark materials for BHJ solar cells in combination with PCBM.27–29 Whereas the photoinduced charge carrier generation and transfer process are well studied in these blends 11,30–35

, corresponding studies on polymer/nanoparticle blends are more rare.36–38 In the case of

polymer/CIS systems, only a few studies exist. Krause et al.18 studied the charge transfer in hybrid systems with CIS NCs by electron spin resonance (ESR) and showed evidence for hole transfer from CIS NCs to P3HT. Kruszynska et al.39 investigated the charge transfer in CIS/P3HT blends by photoinduced absorption (PIA) spectroscopy which also provided evidence for charge transfer. However, combinations with conductive polymers other than P3HT were not


4

Page 5 of 55

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


examined, and even with P3HT, the existing works were restricted to quasi steady state measurements and did not take into account the influence of the material ratio in the blends. 18,39 Here, we present a comprehensive study of the charge generation and transfer mechanism in CIS NC/polymer blends, with the bench mark polymers P3HT and PCPDTBT. Thereby, systematic variations of the composition of the binary blends were considered in both cases. We combine photoluminescence (PL) and PIA spectroscopy to investigate long-lived excited species as well as photo-induced charge transfer in both P3HT/CIS NC and PCPDTBT/CIS NC blends. PL quenching is an indirect method to examine the charge transfer mechanism between donor and acceptor, but cannot generally distinguish between charge transfer and Förster40,41 or Dexter42 type resonance energy transfer (FRET). In contrast, PIA is a useful method to directly investigate the photo-induced charge transfer in a blend. Beyond quasi-steady state measurements, time-resolved photoluminescence (TRPL) spectroscopy was used to probe the kinetics of photo-induced charge transfer.

2. Experimental Section 2.1 Synthesis of CuInS2 NCs Pyramidal CuInS2 NCs were synthesized by a colloidal process reported in detail elsewhere18. Briefly, the synthesis started by dissolving 1 mmol copper (I) acetate, 1 mmol indium (III) acetate, and 1 mmol tri-octylphosphine oxide in 10 ml oleylamine. The mixture was stirred under vacuum for 20 min. Subsequently, the atmosphere was changed into nitrogen flow and the solution was heated. A mixture of 0.5 mmol of 1-dodecanethiol and 0.25 mmol of tertdodecanethiol was then injected into the solution, when the temperature of the solution reached 220°C. The temperature of the solution was then kept at 240°C for 1h. The reaction was stopped


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

Page 6 of 55

by cooling down to room temperature afterwards. The purification of as-synthesized CIS NCs was done by three cycles of precipitation with ethanol and dissolution with chlorobenzene. A typical TEM image and size distribution of as-synthesized CIS NCs are shown in Figure S1, Supporting Information. Accordingly, in a spherical approximation of the crystal shape, the NCs have an average diameter of 9.2 nm with standard deviation of 17 %. 2.2 Sample preparation The two kinds of polymers used in this study are poly(3-hexylthiophene) (P3HT) and poly[2,6(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-enzothiadiazole)] (PCPDTBT). P3HT was purchased from Rieke Metals, Inc (MW=50.000-70.000, RR=91-94%) and PCPDTBT from Sigma Aldrich. Two different substrates were used for different purposes. Sapphire substrates were used for steady state PL and PIA measurements, and quartz substrates for TRPL measurements. All the substrates were rinsed with distilled water, acetone, and isopropanol prior to their application. In the solutions used to prepare the samples, the concentration of the polymer was kept constant at 10 mg/mL. The required amount of CIS nanoparticles was dissolved in chlorobenzene before mixing with the polymer, so that the resulting CIS percentages in the polymer were 0%, 30%, 45%, 60%, 75%, and 90% by weight, respectively. All the films were spin-coated from chlorobenzene solution onto the substrates at 1000 rpm for 80 s. The CIS NCs/P3HT blend films were annealed at 150°C for 15 minutes prior the measurements, whereas the CIS NCs/PCPDTBT films could be used without a subsequent annealing procedure. The sample preparations were done inside a glove-box with the nitrogen flow environment. Except for the PIA and steady-state PL measurements, all the samples were encapsulated. In the case of the measurements in the PIA setup, encapsulation was omitted for preventing the additional absorption due to the encapsulation materials. However, care was taken to transfer the samples


6

Page 7 of 55

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


from the glove box into the vacuum-cryostat in a short period of time in order to reduce the risk of degradation of the samples. UV/Vis absorption spectroscopy was carried out with a Varian Cary 100 spectrophotometer. 2.3 Quasi steady state Photoinduced Absorption (PIA) & Steady state Photoluminescence (PL) The quasi-steady state PIA and PL measurements were performed in the same setup. The prepared samples were mounted into a nitrogen flow cryostat and kept under vacuum conditions. The temperature was varied between 80 K and room temperature. The samples were excited by a solid state laser, equipped with an optical chopper for photomodulation and, for the PIA experiments, probed with a white light halogen lamp. For both measurements, an incident laser beam with excitation wavelengths of 532 nm or 660 nm were used for P3HT and PCPDTBT, respectively. For the PIA experiments, both light sources were focused onto the same point of the sample with a diameter of 4 mm2. Laser induced changes in transmission, as probed with the white light source were measured by lock-in detection. The frequency of the modulation time was kept at 80 Hz. The signals were detected by two types of detectors depending on the wavelength (550-1100 nm by a silicon detector and 1100-5550 nm by an InSb detector). The resulting spectrum shows the relative changes of transmission as a function of energy probed.43,44 To enable a quantitative comparison of films with different nanocrystal content, the steady state PL and PIA spectra were normalized to the absorption by the polymer component in the films at the respective excitation wavelength (i.e., at 532 nm for P3HT and 660 nm for PCPDTBT). Therefore the absorption spectra of the blend films were measured with an UV-Vis absorption spectrometer. In detail, the UV-Vis spectrometer measures the transmission T of the films, from which the absorbance, or optical density,

OD10,blend is calculated neglecting


7


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

Page 8 of 55

reflection as , = − . The absorbance of the blend films was considered as the sum of the contributions from the polymer and NC phase, and the individual contributions were estimated from the spectra. With x denoting the fraction of the absorbance attributed to the polymer, the absorption by the polymer, Apolymer, can finally be calculated as follows: ≈ 1 − = 1 − 10∙

,!"#

%$A more detailed description of this procedure can be found in the Supporting Information (S5).

2.4 Time-resolved Photoluminescence (TRPL) Spectroscopy. The time-resolved properties and excited state dynamics of our systems were studied using the time-correlated single photon counting (TCSPC) technique45. The TCSPC setup46 consists of a Tsunami Ti:Sapphire laser (Spectra Physics) pumped by a Millenia XS CW-laser (Spectra Physics), resulting in pulsed laser light, which is tuneable between 760 and 1100 nm, with a repetition rate of 81 MHz. Using a pulse picker (GWU) the repetition rate could be decreased to 8.1 MHz. The frequency of the light is doubled with a flexible harmonic generator (GWU-FHG from GWU Lasertechnik). Emission from the sample was detected with a cooled R3809U-51 MCP-PMT detector from Hamamatsu after passing through with a polarizer at the magic angle (54.7°) and a subtractive monochromator. The signals were processed using an SPC 430 (Becker & Hickl GmbH) computer card. The instrument response function (IRF) was recorded using a LUDOX scattering solution, and its Full Width at Medium Height (FWHM) amounted to 20-30 ps. The photoluminescence decay curves were analysed by fitting the data to a convolution of the instrumental response function with a decay function for a δ-pulse. The fitting was done with the TRFA Global Analysis Program47 based on a Marquardt-Levenberg minimization algorithm and


8

Page 9 of 55

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


a Gaussian-weighted nonlinear least-square fitting. The quality of each fit was assessed by the random distribution of the residuals, their autocorrelation function and the value of the reduced chi-square parameter (χ2)48,49, which was around 1.1. All decays were fitted with the function: 1

&'( )*+ = ∑ ∙ -./ 0− 3, where and 5 are fitting parameters describing the amplitude and 2$

decay-time of the n-th decay component, respectively. The relative contribution 6 of the n-th 7 2$

component to the stationary PL can be calculated as 6 = ∑ $

8 78 28

, so that ∑9 69 = 1. The

amplitude-weighted average decay time constant is obtained as 〈5〉 =

∑8 78 ∙28 ∑8 78

. Concerning the

precision of the analysis, we estimated the error of the values derived for the lifetimes to be ±10 ps, and the relative error of the amplitudes to be below 10%. Furthermore, one should take into account that these standard deviations, estimated by the curve fitting program, assume that the parameters are uncorrelated. In reality the values of e.g. the decay times and the pre-exponential factors (amplitudes) are correlated. Hence the standard deviations as given here should be considered as a lower limit.

3. Results and discussion 3.1. Steady state absorption and photoluminescence quenching Figure 1 shows the absorption and photoluminescence spectra as well as energy levels of the film of pristine donor and acceptor materials used in this work. Figures 2 and 3 show in addition the absorption and photoluminescence spectra of the hybrid films. The pristine P3HT film shows an absorption in the visible region (black curve in Figures 2(a)) with two well-pronounced vibronic bands located at 525 nm (2.36 eV), 558 nm (2.22 eV) and a red shifted shoulder at 606 nm (2.05 eV) which can be attributed to the vibronic progression of the lowest < − < ∗ transition ACS Paragon Plus Environment

9


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

Page 10 of 55

of P3HT molecules arranged in lamellae.50,51 The PL spectrum of a pristine P3HT film at room temperature, as shown in Figure 1, displays a maximum at 724 nm (1.71 eV) and a shoulder at 670 nm (1.85 eV) and 800-820 nm, corresponding to the (0-1), (0-0) and (0-2) vibronic transitions, respectively. The Stokes shift between the (0-0) vibronic bands amounts to 0.20 eV while the optical band gap amounts to 1.95 eV. At 80 K (Figs. 1(a) and 3(a)) the emission maxima undergo a red shift to 682 nm (1.82 eV), 747 nm (1.66 eV), 833 nm (1.49 eV) and 954 nm (1.3 eV), for the (0-0), (0-1), (0-2), and (0-3) vibronic transitions, respectively and the intensity of the forbidden (0-0) transition is decreased significantly.50,52,53 In the films the pristine PCPDTBT has a broad absorption band (grey line in Fig. 1(a) and black line in Fig. 2(b)), with two main electronic transitions around ~ 415 nm and ~ 730 nm. In the latter transition two shoulders at 701 nm (1.77 eV) and 740 nm (1.68 eV) can be observed which can be attributed to the (0-1) and (0-0) vibronic transitions. At room temperature the emission of a pristine film of PCPDTBT (Fig. 1 (a)) has a maximum at 845 nm (1.49 eV) and a shoulder at 935 nm (1.33) eV, which can be attributed to the (0-0) and (0-1) vibronic transitions. The Stokes shift between the (0-0) vibronic bands amounts to 0.19 eV, which is similar to the value obtained for P3HT, while the optical band gap amounts to 1.60 eV. At 80 K (Figs. 1(a) and 3(b)) The PL spectrum of a pristine PCPDTBT thin film can be deconvoluted into two Gaussian peaks as shown in Figure S2(c). Compared to RT the emission maxima at 80 K undergo a red shift to 880 nm (1.41) eV and 992 nm (1.25 eV) for the (0-0) and (0-1) vibronic transitions, respectively. In analogy to P3HT the intensity of the (0-1) transition is increased significantly relative to that of the (0-0) transition. The absorption spectrum of the CIS NCs dissolved in chlorobenzene at RT (Fig. 1(a)) is characterized by a broad band extending to ~ 800 nm (1.55 eV) with a shoulder at ~ 715 nm


10

Page 11 of 55

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


(1.74 eV). Thin film of CIS NCs showed no emission at RT, on the other hand at 80 K, a narrow band with a maximum at 980 nm (1.27 eV) is observed (green curve in Fig. 1(a)). Values for the energy levels as determined by cyclic voltammetry scatter strongly when comparing various reports (see Fig. 1b). This concerns in particular the lowest unoccupied molecular orbital (LUMO) of P3HT as well as the valence and conduction band edge of CIS NCs. In the latter case, it is worth to point out that energy levels measured for CIS NCs with wurtzite crystal structure determined in a previous work in our group17 are substantially closer to the vacuum level than values measured by other groups for CIS NCs with chalcopyrite crystal structure54. Whether the crystal structure provides a physical reason for the different energy levels, or whether the measured values just scatter (like for the LUMO of P3HT), remains an unanswered questioned till date. The large uncertainties in the knowledge of the energy levels make it difficult to predict the nature of the heterojunctions. In the case of P3HT/CIS NC composites, there is furthermore a strong spectral overlap between the absorption of CIS NCs and the photoluminescence of P3HT, as shown in Figure 1(a). Therefore, energy transfer from P3HT to CIS can be expected. From the data available on the energy levels, it is not obvious, whether the P3HT/CIS NC system forms a type I or type II heterojunction. However, the observation of the hole transfer from CIS NCs to P3HT in ESR measurements18 points more to a type II heterojunction. Also for the whole range of LUMO values of P3HT and the conduction band of CIS (Figure 1(b)) electron transfer from excited P3HT to CIS NCs is in principle possible. In the case of PCPDTBT/CIS NC composites, the small overlap between the absorption of the CIS NCs and the PL of PCPDTBT or between the absorption of PCPDTBT and the PL of CIS NCs suggests only a weak probability for energy transfer. This is due to the large Stokes shift of


11


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

Page 12 of 55

both PCPDTBT and the CIS NCs. For the latter combination, the energy level alignment corresponds more clearly to a type II heterojunction suitable for charge separation at the interface if the band edges determined earlier for the CIS NCs17 are correct. With the levels measured for our wurtzite-type CIS NCs, the nanoparticles would play the role of the electron donor and the polymer the role of the acceptor. A similar situation was recently reported for blends of PCPDTBT with alloyed (ZnS)x(CuInS2)1-x nanocrystals.55 Furthermore, it is observed that the PL of both polymers is shifted to the red upon decreasing the temperature to 80K (Fig. 1(a)). As charge and energy transfer will quench the PL of the conjugated polymer, the occurrence of these processes was studied by photoluminescence quenching in the blends. To investigate the energy and charge transfer from the polymers to the CIS NCs, the concentration of CIS NCs in the blends was systematically increased. Figure 2 shows the influence of the CIS NCs on the UV-Vis absorption spectra for all conjugated polymer/CIS NCs thin films. Increasing the concentration of CIS NCs in the P3HT/CIS NCs blends does not result in significant changes or shifts of the bands observed for P3HT, but clearly yields an increased absorption in the spectral regions below 500 nm and above 650 nm, respectively (see Fig 2(a)). This indicates that the CIS NCs absorb light in a spectral region which is complementary to that of pristine P3HT. Figure 2(b) depicts the absorption spectra of blend films of PCPDTBT with different concentrations of CIS NCs. Upon increasing the fraction of CIS NCs in the PCPDTBT/CIS NCs blend, the feature at ~730 nm remains rather unaffected, but there is a pronounced increase of absorption in the range below 600 nm due to the absorption of the CIS NCs. The changes in the PL spectra of the polymers with increasing concentrations of CIS NCs in the blends at 80 K are presented in Figure 3. To enable a quantitative comparison, the intensity of the spectra is corrected in intensity by the absorption of the polymer component in the


12

Page 13 of 55

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


composite films at the excitation wavelength (see experimental section and Supporting Information (S5) for details). The PL spectra of P3HT/CIS NCs films, as shown in Figure 3(a), display similar features as those of pristine P3HT. When the spectra were deconvoluted using a combination of four Gaussian peaks (see Figure S2(a), Supporting Information) no significant changes of the wavelengths of the maxima or relative intensities of the different components were observed, as a function of the content of CIS NCs. However, the overall intensity of the PL signal in all emission peaks continuously decreases with increasing NC content. E.g. for the blend with 90% CIS NCs the PL intensity of P3HT is quenched by a factor of about 90%. The reduction of the PL intensity suggests a fast deactivation of the P3HT singlet exciton, caused by photo-induced electron or energy transfer from P3HT to the CIS NCs. When we performed the Stern-Volmer analysis (see Supporting Information, Figure S2(b)) it was observed that the plot shows a linear regime at low NCs loading, but a superlinear increase above 60 wt% CIS NCs. Fan et al. observed similar behavior in blends of conjugated polymer and Au nanocrystals and interpreted the superquenching as an indication for efficient energy or charge transfer between the polymer and the Au NCs62 . Thus, the analysis suggests that energy or charge transfer becomes efficient at loading above 60 wt% for the combination of P3HT and CIS NCs. Turning to the other polymer, PCPDTBT, upon addition of CIS NCs the PL intensity of the blends decreases compared to that of a pristine PCPDTBT film. A high CIS NCs concentration of 75 wt% resulted in a PL quenching by 40% in both emission peaks. Compared to the PL quenching observed in the P3HT/CIS NCs system, PCPDTBT/CIS NCs blends show less quenching, which indicates less photo-induced transfer in the system or substantial back electron transfer to the < − < ∗ excited state of the polymer. Furthermore, the Stern-Volmer plot of PCPDTBT/CIS NCs composites shows only a slight increase of the quenching for the highest


13


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

Page 14 of 55

concentration of CIS NCs in the blends (see Fig. S2(d)), which suggest a less pronounced charge transfer. One should note that due to the small overlap of the photoluminescence spectrum of PCPDTBT and the absorption spectrum of the CIS NCs (Fig. 1 (a)) the possibility of energy transfer is reduced strongly compared to the system P3HT/CIS NCs.

3.2. Time resolved photoluminescence To understand the carrier generation in the blends more in detail, we determined the photoluminescence decays of pristine polymer films and polymer films containing CIS NCs at room temperature. Figure 4 (a,b) shows the PL decays obtained for pristine P3HT along with those of blended P3HT/CIS NCs films, wherein the polymer/nanocrystal ratio was varied systematically. The films were excited at 500 nm and the corresponding decays recorded by monitoring the PL emission maxima at 675 nm (1.83 eV, Fig. 4a), and 725 nm (1.71 eV, Fig. 4b), which corresponds to the (0-0), and (0-1) transitions. Visual inspection of the decays shows that all decays are non-exponential and that the photoluminescence decays faster upon increasing the concentration of CIS NCs. A similar trend is observed for decays recorded at 675 nm (1.83 eV, Fig. 4a), and 725 nm (1.71 eV, Fig. 4b). At least qualitatively the photoluminescence quenching observed in the stationary measurements (at 80 K) (see Fig. 3) is confirmed by the TRPL measurements at room temperature. The photoluminescence decays could all be fitted to a sum of two exponentials (see experimental section) yielding the parameters shown in Table 1. &'( )*+ = exp A−

* * B + D exp A− B 5 5D

As more complex decays such as, e.g., a stretched exponential decay63 or a Gaussian distribution of decay rates64 can often also be analyzed as a sum of two exponentials and as there is no


14

Page 15 of 55

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


evidence for the presence of just two excited species especially the fast component of the decay, of which the decay time is close to the time-resolution of the set-up, can be an approximation for a more complex decay. The two decay times, however, allow one to calculate an amplitudeweighted average decay time 〈5〉 according to65,66 〈5〉 =

5 + D 5D , + D

A1 and A2 being the amplitudes of the decay components at t = 0 s. In order to estimate the efficiency of the quenching in the different blends compared to the pristine polymer, we defined a quenching efficiency ηQ (in %) as follows: 〈2!"#$% 〉

EF = A1 − 〈2

GH"IJ#K 〉

B ∙ 100%, where 〈5 〉 and 〈5 〉 are the amplitude-weighted

average photoluminescence decay-time of the blend and pristine polymer film, respectively. The TRPL analysis revealed in the framework of a bi-exponential fluorescence decay for the pure P3HT film a component with a short (5 ) decay time of about 70 ps and a long (5D ) decay time of about 950 ps which are within the experimental error independent of the emission wavelength (675 nm and 725 nm) (see Table 1). In a previous study of P3HT/PbS blends the PL decay of P3HT at 650 nm could also be analyzed as a sum of two exponentials67. Mikhnenko et al.68 reported, that a pronounced bi-exponential PL decay in thin films is usually observed in polymers with a large degree of energetic variation, such as P3HT. In the same report, they estimated the PL decay of P3HT films to occur on time-scales in the range of 600-900 ps68 while Banerji et al. found a shorter decay time of about 470 ps69. As discussed in our previous report67, and in agreement with reports by other groups69–71, the long decay time component in P3HT films can clearly be assigned to the monomolecular decay of the singlet exciton state.


15


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

Page 16 of 55

The fast decaying component of the emission of pristine P3HT has been attributed to several processes67,72 such as molecular relaxations, energy transfer to more stable exciton states or nonfluorescent states69,73–77 and, singlet-singlet, singlet-triplet and singlet-polaron annihilation.72,78–82 In contrast to earlier work67, using another P3HT sample, no decrease of the amplitude of the fast decaying component was observed upon increasing the detection wavelength. This excludes molecular relaxation processes and energy transfer to more stabilized exciton states with a red shifted emission, which anyhow occur to a large extent on shorter time scales.69,73,76 The laser intensity

used

in

the

TRPL

luminescence

experiments

(9.6

mW/cm2,

~2.8×109

photons/(pulse·cm2)) was much below the expected onset of singlet-singlet annihilation at (1011 1012 photons/(pulse·cm2)).72,78–80 which suggests that the fast component observed in the analysis of the fluorescence decays obtained by TCSPC is not due to singlet-singlet annihilation. Hence energy transfer to non-fluorescent traps is the most likely explanation for this fast decaying component. Finally one should mention that the average decay time 〈5〉 determined for the excited state in pristine P3HT at both detection wavelengths is about 600-700 ps, in agreement with values of the average decay time reported in previous literature.69,71 Upon increasing the loading by CIS NCs the PL decay of P3HT becomes progressively faster. The contribution of the fast decaying component increases while the long decay time and the average decay time decrease progressively. The short decay time at first increases up to a loading of 45 to 60% and then starts to decrease. This suggests that beside relaxation and quenching by non-fluorescent traps other processes start to contribute to the fast decay from loading of 45 wt % on. Both at 650 and 725 nm similar trends and decay parameters are observed. As can be seen from Table 1, the PL decay is quenched (with efficiency ηN ) by approximately by 30%, 70% and 90% at a loading of 60 wt%, 75 wt%, and 90 wt% CIS NCs, observed at both detection


16

Page 17 of 55

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


wavelengths. The broad range of decay times of the quenching (which probably reflects a nonexponential decay) is due to the fact that the charge or energy transfer, occurring on a time scale of a few tens of ps for P3HT chromophores adjacent to CIS NCs, has to be preceded by exciton hopping for P3HT chromophores at a larger distance from the QDs.83 The observation that even at a loading of 90%, where in the case of homogeneous mixing all polymer chains should be in contact with CIS NCs still a component with a decay time close to 200 ps is observed suggests the occurrence of a phase separation where a significant fraction of the P3HT chains is too far from the CIS NCs to be quenched without preceding exciton hopping. The quenching can be attributed to either charge or energy transfer from the excited state of P3HT to the CIS NCs. We also investigated the ratio between the average decay time in pristine P3HT and the blends (5'OPQ ⁄5 ) at both detection wavelength. The corresponding plot is shown in the supporting information (see Figure S3(a)). The ratio strongly increases at about 60 wt% at both detection wavelengths, meaning that decay time reduction becomes more pronounced above this threshold. This result is also supported by the PL quenching efficiency (ηN ), plotted in the Figure S3(b), supporting information. Figure 4c shows the photoluminescence decays of pristine PCPDTBT and blends of PCPDTBT and CIS NCs excited at 660 nm and recorded at S = 850 nm (1.45 eV), which is corresponding to the 0-0 transition of PCPDTBT. Qualitative inspection of the decays shows that the decay becomes faster upon increasing the loading by CIS NCs; however the increase of the decay rate is less pronounced than for the combination of P3HT and CIS NCs. For all samples the fluorescence decay could be analyzed as a sum of two exponentials. Although one should , as in the case of the blended films of P3HT and PbS NCs67 or P3HT and CIS NCs (cfr. supra), be careful with the interpretation of the individual decay parameters obtained in this way, these


17


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

Page 18 of 55

parameters allow to estimate an average decay rate and associated quantum efficiency ηN in the way described above. Similar to P3HT, the photoluminescence decay of pristine PCPDTBT also can be analyzed as a sum of two exponentials with a fast decaying component with a decay time, 5 , of 31 ps, which is at the limit of the time-resolution of the set-up, and a slower decaying component with a decay time, 5D , of 226 ps, resulting in an average decay time, 〈5〉, of 150 ps. The decay time of the slowly decaying component (5D ) of pristine PCPDTBT is consistent with the decay rate obtained in previous reports of excitonic decay in PCPDTBT, where it was attributed to the decay of the population of singlet excited species of PCPDTBT34,84. Furthermore picosecond pump-probe experiments by Hwang on pristine PCPDTBT showed a bi-exponential decay with decay times of 20 ps and 120 ps for the singlet excited state. The shorter decay time obtained for the slow decaying component could be due to the occurrence of singlet-singlet, singlet-triplet and singletpolaron annihilation due to the high intensity of the pump laser pulse at the high laser intensities used in pump probe experiments.

72,78–82

Jarzab et al.34 showed a broad range of values for the

photoluminescence decay time of PCPDTBT, which was probably related to the exposure of the sample to oxygen. Compared to samples exposed to oxygen, where a population decay time of ~78 ps34 was recovered, freshly prepared samples showed a longer population decay time (~220 ps) ), similar to the one we observed. Despite of the smaller energetic disorder compared to P3HT, which generally leads to a mono-exponential decay68, we observed that addition of a second fast decaying component to the analysis of the decays led to a significant improvement the quality of the fits to the decay curves. In analogy to the results obtained for the blends of P3HT and CIS NCs, the addition of CIS NCs in the blends induced a slight decrease of the average decay time as well as of the long


18

Page 19 of 55

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


decay time component and its relative weight while the decay time of the fast decaying component shows a slight increase upon increasing the loading of CIS NCs in the blends. Contrary to the results obtained for P3HT, the changes in the decay of the PL became pronounced in PCPDTBT/CIS NCs blends only for a loading by CIS NCs of at least 30% to 45%. E.g., at loading of 75% CIS NCs the efficiency of the quenching amounted to 21%. Plots of the ratio of the average decay time (5'V'QWQ ⁄5 ) and the PL quenching efficiency (ηN ) are shown in Figure S3(c,d) and indicate that the increase of this ratio upon increasing the loading of CIS NCs is much less pronounced compared to the P3HT/CIS system. Also the limiting value of the short decay time is at high loadings much larger than for the P3HT/CIS NCs system. This suggests that the charge or energy transfer between the excited polymer and adjacent CIS NCs is significantly slower.

3.3. Photo-induced absorption in polymer/nanocrystal blends PL quenching alone cannot distinguish between photo-induced charge transfer and other intermolecular interactions such as energy transfer. Quasi steady state PIA monitors the change in the transmission spectrum due to absorption by long-lived species such as polarons or triplets. Therefore, PIA spectroscopy is suitable to investigate the occurrence of charge transfer reactions at the donor-acceptor interface. Figure 5 shows the in-phase PIA signal for P3HT and P3HT/CIS NCs blends at (a) 80 K and (b) 295 K using an excitation wavelength of 532 nm and a modulation frequency of 80 Hz. At T = 80 K, all spectra show broad photo-induced absorption features below the bandgap of P3HT (Eg ~ 1.8 eV) indicating the generation of long-lived charge species under the laser light irradiation. All measured spectra consist of positive (PIA) and negative (photobleaching (PB))


19


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

Page 20 of 55

peaks. The PB feature around 2.0 eV is due to ground state depletion of the absorption of the conjugated polymer. When photoexcited, pristine P3HT shows a broad PIA signal between 1.05 eV and 1.85 eV, that originates from several contributions. A detailed discussion of existing literature and the assignment of the PIA features of pristine P3HT was given in a previous publication on P3HT/PbS hybrid blends67. Briefly, the peak at 1.05 eV is assigned to a long lived singlet species of which the nature is unclear and the peaks at 1.23 eV and 1.83 eV are characteristics of polarons generated in P3HT. The peak at 1.23 eV corresponds to the dipoleallowed transition from the semi-occupied molecular orbital (SOMO, a state associated with a localized polaron) to the LUMO of P3HT, whereas the peak at higher energy (1.83 eV) can be assigned to the corresponding transition in the case of delocalized polarons, as also described in refs 26,30,70,85. For comparison, we measured also PIA spectra of pure CIS NCs films (see Figure S4 Supporting Information), but only weak signals, one order magnitude lower in intensity, were found. The most prominent feature was a peak at ~1.6 eV, which matches roughly the energy of the first excitonic transition in the CIS NCs and can therefore be assigned to photobleaching of the ground state. Turning to the blends, the signal of the singlet exciton state in P3HT is decreased upon adding CIS NCs and completely vanished when the loading of CIS NCs reached 90 wt%, whereas the delocalized polaron signature at 1.83 eV is slightly increased. Simultaneously, a new peak appears at 0.38 eV, which can be attributed to a transition from the HOMO in P3HT to the SOMO associated with localized polarons, and which increases strongly after adding 75 wt% CIS NCs. We can draw the conclusion that, in agreement with the findings from the analysis of the PL decay (table 1), the efficiency of the electron transfer increased strongly when the loading of CIS NCs reached 75 wt% in the blend with P3HT. Further addition


20

Page 21 of 55

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


of CIS NCs to a loading of 90 wt% results in a broad peak in the range of 0.3-0.7 eV. Since the pure NC films exhibited a weak PIA signal in this region as well (see S4, Supporting Information), this feature observable at high NC loading only can be attributed to species such as e.g. polarons present in larger domains of CIS NCs. We also tried to measure PIA spectra at room temperature, which corresponds better to real conditions for solar cell operation (see Fig. 5b). At room temperature, the peaks around 1.0-1.8 eV get broadened. Nevertheless, upon adding 75 wt% CIS NCs to the blend, the PIA signal of the polaron transition at ~0.3 eV as well as a pronounced feature indicative for delocalized polarons (around 1.8 eV) can be recognized and provide evidence for charge transfer at room temperature. Contrary to what was observed at 80 K the amplitude of the PIA signal was smaller for a sample with 90% loading with CIS NCs compared to a sample with 75% loading. This might be attributed to larger carrier mobilities (electrons) leading to a faster recombination and hence smaller pseudo-stationary signal68. Figure 5(c) shows the photo-induced absorption of a pristine PCPDTBT film along with the spectra of PCPDTBT/CIS NCs blends at T = 80 K. The spectrum of pristine PCPDTBT exhibits a negative band centered at 1.55 eV in good agreement with previous reports

84,86

and can be

attributed to ground state depletion. The PIA signals consisted further of a strong peak at 1.0 eV and a broad peak around 0.3-0.7 eV. Transient absorption measurements by other groups indicated the presence of singlet excitons generated after photoexcitations with the decay time of about 60-78 ps, in the range of 0.62-1.03 eV.84,86 The TRPL measurement in this work shows that the decay time of the excited single state of a pristine PCPDTBT film is about 200 ps, which provides the possibility that the formation of the singlet excitons occurs immediately after photoexciton generation and that this species is converted at least partially by intersystem crossing to the lowest triplet energy state. It is quite unlikely that signals in the PIA spectra can be attributed


21


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

Page 22 of 55

to these singlet excited species as their very small decay time will lead to an extremely low concentration of those species. The peak at 1.0 eV has been assigned to a triplet-triplet absorption in pristine PCPDTBT films in a previous report by by Di Nuzzo et al.33. The broad peak around 0.3-0.7 eV can be assigned to the dipole allowed transition of the free polarons of PCPDTBT33, because a similar feature was observed in a study of PCBM/PCPDTBT blends33. In that study, increasing intensity of the free polaron signal was also accompanied by an increase of the peak at 1.0 eV as the result of population of the lowest triplet state via a transition from the charge transfer state in the blend33. In the case of PCPDTBT/CIS NCs, these features are more pronounced as a consequence of the population of a triplet excited state by back-electron transfer from the CB of the CIS NCs to the LUMO of PCPDTBT. Addition of CIS NCs by 60 wt% to the PCPDTBT/CIS NCs blend leads to slight enhancement of the peak at 1.0 eV. However, the peaks remain similar with further addition of CIS NCs in the blend. Moreover, a broad peak in the IR region shows increasing intensity upon addition of CIS NCs. This implies that the addition of CIS NCs to the blend has no further effect to the triplet state population in PCPDTBT. On the other hand it contributes to the population of polarons. Considering the PIA spectra of the pure CIS NCs film (see SI, Fig. S4), the broad peak at the NIR region could at least partly also originate from the CIS NCs phase. Furthermore, the PIA signals of the PCPDTBT/CIS blend with loading of 60 wt% CIS NCs at room temperature show persistent signals compared to pristine PCPDTBT as shown in Fig. 5(d). The loading of 75 wt% of CIS NCs in the blend increased slightly the PIA signal at the NIR region. In summary, these results indicated that the charge transfer process is less effective in PCPDTBT/blends compared to the P3HT/CIS blends.


22

Page 23 of 55

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


Summary and Conclusion We have comprehensively studied the fate of photo-generated electron-hole pairs in composite films of CIS NCs and the conductive polymers, P3HT and PCPDTBT, by combining PL quenching and PIA measurements. It was found that in both composites, CIS NCs quench the photoluminescence and the photoluminescence life time of the conjugated polymers. The extent of PL quenching increased with the loading of CIS NCs in the films. Since PL quenching alone cannot distinguish between charge transfer at the donor-acceptor interface and concurring processes like energy transfer, we applied additionally PIA spectroscopy. The measurements provided a strong indication for successful charge transfer via the detection of long-lived polarons generated in P3HT/CIS NCs blends. Systematic variations of the NC/polymer ratio revealed the charge transfer to be efficient at a CIS NC loading of 75 % by weight. In contrast, the PIA spectra of PCPDTBT/CIS NCs blends showed only a weak polaron signal. Furthermore, PL quenching and the reduction of the decay time of singlet excitons were less pronounced in the films with PCPDTBT. All this indicates that photo-induced charge transfer between CIS NCs and PCPDTBT is less effective than in case of the P3HT/CIS system. The increased triplet signal in the PCPDTBT/CIS NCs blends suggests that the initially formed geminate electron hole pair decays by back electron transfer leading to a triplet excited state of PCPDTBT. Moreover, the small overlap of absorption and PL spectra implies also limited probability of energy transfer in the PCPDTBT/CIS films.


23


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

Page 24 of 55

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge funding by the “EWE Nachwuchsgruppe Dünnschicht-Photovoltaik” by the EWE AG, Oldenburg, Germany. Rany Miranti is grateful for the scholarship by the German Academic Exchange Service (DAAD). This work was supported by the IMEC Leuven (PhD grant to Y.F.), and by the EU through FP7 People Herodot (grant 214954). We are indebted to the research council of KU Leuven through GOA 2006/2, 2011/3 and to Belspo through IAP VI/27 en IAP VII/05.

ASSOCIATED CONTENT Supporting Information TEM image and size distribution of CIS NCs, the deconvolution of the PL spectra of CIS NCs/polymer films and the Stern-Volmer plots, the lifetime ratio and the PL quenching efficiency EF of CIS NCs/polymer films, photoinduced absorption (PIA) spectrum of pure CIS NCs, and estimation of the absorption of light in blend films. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.


24

Page 25 of 55

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


REFERENCES

(1)

Zhou, R.; Stalder, R.; Xie, D.; Cao, W.; Zheng, Y.; Yang, Y.; Plaisant, M.; H. Holloway, P.; S. Schanze, K.; R. Reynolds, J.; et al. Enhancing the Efficiency of Solution-Processed Polymer : Colloidal Nanocrystal Hybrid Photovoltaic Cells Using Ethanedithiol Treatment. ACS Nano 2013, 7, 4846–4854.

(2)

Radychev, N.; Lokteva, I.; Witt, F.; Kolny-Olesiak, J.; Borchert, H.; Parisi, J. Physical Origin of the Impact of Different Nanocrystal Surface Modifications on the Performance of CdSe / P3HT Hybrid Solar Cells. J. Phys. Chem. C 2011, 115, 14111–14122.

(3)

Kumari, K.; Chand, S.; Vankar, V. D.; Kumar, V. Enhancement in Hole Current Density on Polarization in Poly ( 3-Hexylthiophene ): Cadmium Selenide Quantum Dot Nanocomposite Thin Films. Appl. Phys. Lett. 2009, 94, 213503.

(4)

Zhou, Y.; Eck, M.; Veit, C.; Zimmermann, B.; Rauscher, F.; Niyamakom, P.; Yilmaz, S.; Dumsch, I.; Allard, S.; Scherf, U.; et al. Enhancement for Bulk-Heterojunction Hybrid Solar Cells Based on Acid Treated CdSe Quantum Dots and Low Bandgap Polymer PCPDTBT. Sol. Energy Mater. Sol. Cells 2011, 95, 1232–1237.

(5)

Zhou, Y.; Riehle, F. S.; Yuan, Y.; Schleiermacher, H.; Niggemann, M.; Urban, G. A.; Krüger, M. Improved Efficiency of Hybrid Solar Cells Based on Non-Ligand-Exchanged CdSe Quantum Dots and Poly(3-hexylthiophene ). Appl. Phys. Lett. 2010, 96, 013304.

(6)

Freitas, J. N. De; Grova, I. R.; Akcelrud, L. C.; Arici, E.; Sariciftci, N. S.; Nogueira, A. F. The Effects of CdSe Incorporation into Bulk Heterojunction Solar Cells. J. Mater. Chem. 2010, 20, 4845–4853.

(7)

Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G. Photovoltaic Devices with a Low Band Gap Polymer and CdSe Nanostructures Exceeding 3 % Efficiency. Nano Lett. 2010, 10, 239–242.

(8)

Dayal, S.; Reese, M. O.; Ferguson, A. J.; Ginley, D. S.; Rumbles, G.; Kopidakis, N. The Effect of Nanoparticle Shape on the Photocarrier Dynamics and Photovoltaic Device Performance of Poly(3-hexylthiophene):CdSe Nanoparticle Bulk Heterojunction Solar Cells. Adv. Funct. Mater. 2010, 20, 2629–2635.

(9)

Eck, M.; Pham, C. Van; Zu, S.; Neukom, M.; Sessler, M.; Scheunemann, D.; Erdem, E.; Weber, S.; Borchert, H.; Ruhstalle, B.; et al. Improved Efficiency of Bulk Heterojunction Hybrid Solar Cells by Utilizing CdSe Quantum Dot-Graphene Nanocomposites. Phys. Chem. Chem. Phys. 2014, 16, 12251–12260.

(10)

Firdaus, Y.; Vandenplas, E.; Justo, Y.; Gehlhaar, R.; Cheyns, D.; Hens, Z.; Van der Auweraer, M. Enhancement of the Photovoltaic Performance in P3HT : PbS Hybrid Solar Cells Using Small Size PbS Quantum Dots. J. Appl. Phys. 2014, 116, 094305.


25


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

Page 26 of 55

(11)

Piliego, C.; Manca, M.; Kroon, R.; Yarema, M.; Szendrei, K.; Andersson, M. R.; Heiss, W.; Loi, M. A. Charge Separation Dynamics in A Narrow band gap polymer – PbS Nanocrystal Blend for Efficient Hybrid Solar Cells. J. Mater. Chem. 2012, 22, 24411– 24416.

(12)

Liu, Z.; Sun, Y.; Yuan, J.; Wei, H.; Huang, X.; Han, L.; Wang, W.; Wang, H.; Ma, W. High-Efficiency Hybrid Solar Cells Based on Polymer / PbSx Se1-x Nanocrystals Benefi ting from Vertical Phase Segregation. Adv. Mater. 2013, 25, 5772–5778.

(13)

Beek, W. J. E.; Wienk, M. M.; Kemerink, M.; Yang, X.; Janssen, R. A. J. Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells. J. Phys. Chem. B 2005, 105, 9505–9516.

(14)

Ferreira, S. R.; Davis, R. J.; Lee, Y.; Lu, P.; Hsu, J. W. P. Effect of Device Architecture on Hybrid Zinc Oxide Nanoparticle : Poly(3-hexylthiophene) Blend Solar Cell Performance and Stability. Org. Electron. 2011, 12, 1258–1263.

(15)

Bhat, S. V.; Govindaraj, A.; Rao, C. N. R. Hybrid Solar Cell Based on P3HT – ZnO Nanoparticle Blend in the Inverted Device Configuration. Sol. Energy Mater. Sol. Cells 2011, 95, 2318–2321.

(16)

Chen, C.; Wu, D.; Chan, Y.; Lin, C. C.; Chung, P.; Hsiao, M.; Liu, R. Evaluations of the Chemical Stability and Cytotoxicity of CuInS2 and CuInS2/ZnS Core/Shell Quantum Dots. J. Phys. Chem. C 2015, 119, 2852–2860.

(17)

Radychev, N.; Scheunemann, D.; Kruszynska, M.; Frevert, K.; Miranti, R.; KolnyOlesiak, J.; Borchert, H.; Parisi, J. Investigation of the Morphology and Electrical Characteristics of Hybrid Blends Based on Poly(3-hexylthiophene) and Colloidal CuInS2 Nanocrystals of Different Shapes. Org. Electron. 2012, 13, 3154–3164.

(18)

Krause, C.; Miranti, R.; Witt, F.; Neumann, J.; Fenske, D.; Parisi, J.; Borchert, H. Charge Transfer and Recombination in Organic/Inorganic Hybrid Composites with CuInS2 Nanocrystals Studied by Light-Induced Electron Spin Resonance. Sol. Energy Mater. Sol. Cells 2014, 124, 241–246.

(19)

Arici, E.; Sariciftci, N. S.; Meissner, D. Hybrid Solar Cells Based on Nanoparticles of CuInS2 in Organic Matrices. Adv. Funct. Mater. 2003, 13, 165–171.

(20)

Yue, W.; Han, S.; Peng, R.; Shen, W.; Geng, H.; Wu, F.; Tao, S.; Wang, M. CuInS2 Quantum Dots Synthesized by A Solvothermal Route and Their Application as Effective Electron Acceptors for Hybrid Solar Cells. J. Mater. Chem. 2010, 20, 7570–7578.

(21)

Borchert, H.; Scheunemann, D.; Frevert, K.; Witt, F.; Klein, A.; Parisi, J. Schottky Solar Cells with CuInS2 Nanocrystals as Absorber Material. Zeitschrift für Phys. Chemie 2015, 229, 191–203.


26

Page 27 of 55

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


(22)

Scheunemann, D.; Wilken, S.; Parisi, J.; Borchert, H. Towards Depleted Heterojunction Solar Cells with CuInS2 and ZnO Nanocrystals. Appl. Phys. Lett. 2013, 103, 133902.

(23)

Scheunemann, D.; Wilken, S.; Parisi, J.; Borchert, H. Investigation of the Spatially Dependent Charge Collection Probability in CuInS2/ZnO Colloidal Nanocrystal Solar Cells. ACS Photonics 2015, 2, 864–875.

(24)

Rath, T.; Edler, M.; Haas, W.; Fischereder, A.; Moscher, S.; Schenk, A.; Trattnig, R.; Sezen, M.; Mauthner, G.; Pein, A.; et al. A Direct Route Towards Polymer/Copper Indium Sulfide Nanocomposite Solar Cells. Adv. Energy Mater. 2011, 1, 1046–1050.

(25)

Cook, S.; Katoh, R.; Furube, A. Ultrafast Studies of Charge Generation in PCBM:P3HT Blend Films Following Excitation of the Fullerene PCBM. J. Phys. Chem. C 2009, 113, 2547–2552.

(26)

Österbacka, R.; An, C. P.; Jiang, X. M.; Vardeny, Z. V. Two-Dimensional Electronic Excitations in Self-Assembled Conjugated Polymer Nanocrystals. Science (80-. ). 2000, 287, 839–842.

(27)

Morana, M.; Wegscheider, M.; Bonanni, A.; Kopidakis, N.; Shaheen, S.; Scharber, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. Bipolar Charge Transport in PCPDTBTPCBM Bulk-Heterojunctions for Photovoltaic Applications. Adv. Funct. Mater. 2008, 18, 1757–1766.

(28)

Mühlbacher, D.; Scharber, M.; Morana, M.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. High Photovoltaic Performance of A Low-Bandgap Polymer. Adv. Mater. 2006, 18, 2884– 2889.

(29)

Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.; Heeger, A. J.; Bazan, G. C. Efficiency Enhancement in Low-Bandgap Polymer Solar Cells by Processing with Alkane Dithiols. Nat. Mater. 2007, 6, 497–500.

(30)

Hwang, I.; Moses, D.; Heeger, A. J. Photoinduced Carrier Generation in P3HT / PCBM Bulk Heterojunction Materials. J. Phys. Chem. C 2008, 112, 4350–4354.

(31)

Piris, J.; Dykstra, T. E.; Bakulin, A. A.; Loosdrecht, P. H. M. Van; Knulst, W.; Trinh, M. T.; Schins, J. M.; Siebbeles, L. D. A. Photogeneration and Ultrafast Dynamics of Excitons and Charges in P3HT/PCBM Blends. J. Phys. Chem. C 2009, 113, 14500–14506.

(32)

Soci, C.; Hwang, I.; Moses, D.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. J.; Heeger, A. J. Photoconductivity of A Low-Bandgap Conjugated Polymer. Adv. Funct. Mater. 2007, 17, 632–636.

(33)

Di Nuzzo, D.; Aguirre, A.; Shahid, M.; Gevaerts, V. S.; Meskers, S. C. J.; Janssen, R. A. J. Improved Film Morphology Reduces Charge Carrier Recombination into the Triplet


27


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

Page 28 of 55

Excited State in a Small Bandgap Polymer-Fullerene Photovoltaic Cell. Adv. Mater. 2010, 22, 4321–4324. (34)

Jarzab, D.; Cordella, F.; Gao, J.; Scharber, M.; Egelhaaf, H.; Loi, M. A. Low-Temperature Behaviour of Charge Transfer Excitons in Narrow-Bandgap Polymer-Based Bulk Heterojunctions. Adv. Energy Mater. 2011, 1, 604–609.

(35)

Piliego, C.; Loi, M. A. Charge Transfer State in Highly Efficient Polymer–Fullerene Bulk Heterojunction Solar Cells. J. Mater. Chem. 2012, 22, 4141–4150.

(36)

Noone, K. M.; Subramaniyan, S.; Zhang, Q.; Cao, G.; Jenekhe, S. A.; Ginger, D. S. Photoinduced Charge Transfer and Polaron Dynamics in Polymer and Hybrid Photovoltaic Thin Films : Organic vs Inorganic Acceptors. J. Phys. Chem. C 2011, 115, 24403–24410.

(37)

Noone, K. M.; Anderson, N. C.; Horwitz, N. E.; Munro, A. M.; Kulkarni, A. P.; Ginger, D. S. Absence of Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Conjugated Polymers. ACS Nano 2009, 3, 1345–1352.

(38)

Wang, P.; Abrusci, A.; Wong, H. M. P.; Svensson, M.; Andersson, M. R.; Greenham, N. C. Photoinduced Charge Transfer and Efficient Solar Energy Conversion in a Blend of a Red Polyfluorene Copolymer with CdSe Nanoparticles. nano Lett. 2006, 6, 1789–1793.

(39)

Kruszynska, M.; Knipper, M.; Kolny-olesiak, J.; Borchert, H.; Parisi, J. Charge Transfer in Blends of P3HT and Colloidally Prepared CuInS2 Nanocrystals. Thin Solid Films 2011, 519, 7374–7377.

(40)

Förster T. Transfer Mechanisms of Electronic Excitation. Disc. Far. Soc. 1959, 27, 7–17.

(41)

Agranovich, V.M.; Galanin, M. D. Electronic Excitation Energy Transfer in Condensed Matter, Modern Problems in Condensed Matter Science 3; North Holland Publishing Company: Amsterdam, 1982.

(42)

Dexter D. L. A Theory of Sensitized Luminescence in Solids. J. Chem. Phys. 1953, 21, 836–850.

(43)

Pientka, M.; Dyakonov, V.; Meissner, D.; Rogach, A.; Talapin, D.; Weller, H.; Lutsen, L.; Vanderzande, D. Photoinduced Charge Transfer in Composites of Conjugated Polymers and Semiconductor Nanocrystals. Nanotechnology 2004, 15, 163–170.

(44)

Heinemann, M. D.; von Maydell, K.; Zutz, F.; Kolny-Olesiak, J.; Borchert, H.; Riedel, I.; Parisi, J. Photo-induced Charge Transfer and Relaxation of Persistent Charge Carriers in Polymer/Nanocrystal Composites for Applications in Hybrid Solar Cells. Adv. Funct. Mater. 2009, 19, 3788–3795.


28

Page 29 of 55

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


(45)

O’Connor, D. V.; Phillips, D. Time-correlated single photon counting; Academic Press: London: Orlando, 1984.

(46)

Maus, M.; Rousseau, E.; Cotlet, M.; Schweitzer, G.; Hofkens, J.; Van der Auweraer, M.; De Schryver, F. C.; Krueger, A. New Picosecond Laser System for Easy Tunability Over the Whole Ultraviolet/Visible/Near Infrared Wavelength Range Based on Flexible Harmonic Generation and Optical Parametric Oscillation. Rev. Sci. Instrum. 2001, 72, 36– 40.

(47)

Program Developed in a Cooperation between the Management of Technology Institute (Belarusian State University) and The Division of Photochemistry and Spectroscopy (University of Leuven).

(48)

Boens, N.; Qin, W.; Basarić, N.; Hofkens, J.; Ameloot, M.; Pouget, J.; Lefèvre, J. P.; Valeur, B.; Gratton, E.; VandeVen, M.; et al. Fluorescence Lifetime Standards for Time and Frequency Domain Fluorescence Spectroscopy. Anal. Chem. 2007, 79, 2137–2149.

(49)

D. Caterall. Time-Resolved Spectroscopy in Biochemistry and Biology. In NATO ASI Series A; Plenum Press: New York, 1983.

(50)

Spano, F. C. Modeling Disorder in Polymer Aggregates: The Optical Spectroscopy of Regioregular Poly(3-hexylthiophene) Thin Films. J. Chem. Phys. 2005, 122, 234701.

(51)

Spano, F. C. Absorption in Regio-regular Poly (3-hexyl) Thiophene Thin Films: Fermi Resonances, Interband Coupling and Disorder. Chem. Phys 2006, 325, 22–35.

(52)

Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of Interchain Interactions on the Absorption and Emission of Poly(3-hexylthiophene). Phys. Rev. B 2003, 67, 064203.

(53)

Spano, F. C.; Clark, J.; Silva, C.; Friend, R. H. Determining Exciton Coherence from the Photoluminescence Spectral Line Shape in Poly(3-hexylthiophene) Thin Films. J. Chem. Phys. 2009, 130, 074904.

(54)

Zhong, H.; Lo, Ќ. S. S.; Mirkovic, T.; Li, Y.; Ding, Y.; Li, Y.; Scholes, G. D. Noninjection Gram-Scale Synthesis of Monodisperse Pyramidal CuInS2 Nanocrystals and Their Size-Dependent Properties. ACS Nano 2010, 4, 5253–5262.

(55)

Radychev, N.; Kempken, B.; Krause, C.; Li, J.; Kolny-olesiak, J.; Borchert, H. Photovoltaic Response of Hybrid Solar Cells with Alloyed ZnS – CuInS2 Nanorods. 2015, 21, 92–99.

(56)

Wilken, S.; Scheunemann, D.; Wilkens, V.; Parisi, J.; Borchert, H. Improvement of ITOFree Inverted Polymer-Based Solar Cells by Using Colloidal Zinc Oxide Nanocrystals as Electron-Selective Buffer Layer. Org. Electron. 2012, 13, 2386–2394.


29


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

Page 30 of 55

(57)

Hou, J.; Tan, Z.; Yan, Y.; He, Y.; Yang, C.; Li, Y. Synthesis and Photovoltaic Properties of Two-Dimensional Conjugated Polythiophenes with Bi(thienylenevinylene) Side Chains. J. Am. Chem. Soc. 2006, 128, 4911–4916.

(58)

Hou, J.; Chen, T. L.; Zhang, S.; Chen, H.; Yang, Y. PCPDTBSe, a New Low Band Gap Polymer in Polymer Solar Cells. J. Phys. Chem. C 2009, 113, 1601–1605.

(59)

Richter, T. V; Braun, C. H.; Scheuble, M.; Crossland, E. J. W.; Stelzl, F.; Wu, U.; Ludwigs, S. Regioregular Polythiophenes with Alkylthiophene Side Chains. Macromolecules 2012, 45, 5782–5788.

(60)

Shin, W. S.; Kim, S. C.; Lee, S.; Jeon, H.; Kim, M.; Naidu, B. V. K.; Jin, S.; Lee, J.; Lee, J. A. E. W.; Gal, Y. Synthesis and Photovoltaic Properties of a Low-Band-Gap Polymer Consisting of Alternating Thiophene and Benzothiadiazole Derivatives for BulkHeterojunction and Dye-Sensitized Solar Cells. J. Polym. Sci., Part A Polym. Chem. 2007, 45, 1394–1402.

(61)

Al-Ibrahim, M.; Roth, H.; Schroedner, M.; Konkin, A.; Zhokhavets, U.; Gobsch, G.; Scharff, P.; Sensfuss, S. The iInfluence of the Optoelectronic Properties of Poly (3alkylthiophenes ) on the Device Parameters in Flexible Polymer Solar Cells. Org. Electron. 2005, 6, 65–77.

(62)

Fan, C.; Wang, S.; Hong, J. W.; Bazan, G. C.; Plaxco, K. W.; Heeger, A. J. Beyond Superquenching: Hyper-Efficient Energy Transfer from Conjugated Polymers to Gold Nanoparticles. Proc. Natl. Acad. Sci. U. S. A. 2003, 100, 6297–6301.

(63)

Van der Auweraer, M.; Ballet, P.; De Schryver, F. C.; Kowalczyk, A. Parameter Recovery and Discrimination Between Different Types of Fluorescence Decays Obtained for Dipole-Dipole Energy Transfer in Low-Dimensional Systems. Chem. Phys. 1994, 187, 399–416.

(64)

Verbeek, G.; Vaes, A.; Van der Auweraer, M.; De Schryver, F. C.; Geelen, C.; Terrell, D.; De Meutter, S. Gaussian Distributions of the Decay Times of the Singlet Excited State of Aromatic Amines Dispersed in Polymer Films. Macromolecules 1993, 26, 472–478.

(65)

Lakowicz, J. R. Principles of Fluorescence Spectroscopy 3rd Ed.; 2006.

(66)

Berberan-Santos, M. N.; Bodunov, E. N.; Valeur, B. Mathematical Functions for the Analysis of Luminescence Decays with Underlying Distributions: 2. Becquerel (Compressed Hyperbola) and Related Decay Functions. Chem. Phys. 2005, 317, 57–62.

(67)

Firdaus, Y.; Miranti, R.; Fron, E.; Khetubol, A.; Vandenplas, E.; Cheyns, D.; Borchert, H.; Parisi, J.; Van der Auweraer, M. Charge Separation Dynamics at Bulk Heterojunctions Between Poly(3-hexylthiophene) and PbS Quantum Dots. J. Appl. Phys. 2015, 118, 055502.


30

Page 31 of 55

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


(68)

Mikhnenko, O. V; Kuik, M.; Lin, J.; Van Der Kaap, N.; Nguyen, T.; Blom, P. W. M. Trap-Limited Exciton Diffusion in Organic Semiconductors. Adv. Mater 2014, 26, 1912– 1917.

(69)

Banerji, N.; Cowan, S.; Vauthey, E.; Heeger, A. J. Ultrafast Relaxation of the Poly (3hexylthiophene) Emission Spectrum. Phys. Chem. C 2011, 115, 9726–9739.

(70)

Beek, W. J. E.; Wienk, M. M.; Janssen, R. A. J. Hybrid Solar Cells from Regioregular Polythiophene and ZnO Nanoparticles. Adv. Funct. Mater. 2006, 16, 1112–1116.

(71)

Cook, S.; Furube, A.; Katoh, R. Analysis of the Excited States of Regioregular Polythiophene P3HT. Energy Environ. Sci. 2008, 1, 294–299.

(72)

Scheblykin, I. G.; Yartsev, A.; Pullerits, T.; Gulbinas, V.; Sundström, V. Excited State and Charge Photogeneration Dynamics in Conjugated Polymers. J.Phys. Chem. B 2007, 111, 6303–6321.

(73)

Xie, Y.; Li, Y.; Xiao, L.; Qiao, Q.; Dhakal, R.; Zhang, Z.; Gong, Q.; Galipeau, D.; Yan, X. Femtosecond Time-Resolved Fluorescence Study of P3HT / PCBM Blend Films. J. Phys. Chem. 2010, 114, 14590–14600.

(74)

Nakamura, T.; Araki, Y.; Ito, O.; Takimiya, K.; Otsubo, T. Fluorescence Up-Conversion Study of Excitation Energy Transport Dynamics in Oligothiophene-Fullerene Linked Dyads. J. Phys. Chem. A 2008, 112, 1125–1132.

(75)

Westenhoff, S.; Daniel, C.; Friend, R. H.; Silva, C.; Sundström, V.; Yartsev, A. Exciton Migration in a Polythiophene: Probing the Spatial and Energy Domain by Line-Dipole Förster-Type Energy Transfer. J. Chem. Phys. 2005, 122, 094903.

(76)

T. Kobayashi; Yoshizawa, M.; Stamm, U.; Taiji, M.; Hasegawa, M. Relaxation Dynamics of Photoexcitations in Polydiacetylenes and Polythiophene. J. Opt. Soc. Am. B 1990, 7, 1558–1578.

(77)

Westenhoff, S.; Beenken, W. J. D.; Friend, R. H.; Greenham, N. C.; Yartsev, A.; Sundström, V. Anomalous Energy Transfer Dynamics due to Torsional Relaxation in a Conjugated Polymer. Phys. Rev. Lett. 2006, 97, 166804.

(78)

Martini, I.; Smith, A.; Schwartz, B. Exciton-exciton Annihilation and the Production of Interchain Species in Conjugated Polymer Films: Comparing the Ultrafast Stimulated Emission and Photoluminescence Dynamics of MEH-PPV. Phys. Rev. B 2004, 69, 035204.

(79)

Nguyen, T.-Q.; Martini, I. B.; Liu, J.; Schwartz, B. J. Controlling Interchain Interactions in Conjugated Polymers: The Effects of Chain Morphology on Exciton−Exciton Annihilation and Aggregation in MEH−PPV Films. J. Phys. Chem. B 2000, 104, 237–255.


31


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

Page 32 of 55

(80)

Cook, S.; Liyuan, H.; Furube, A.; Katoh, R. Singlet Annihilation in Films of Regioregular Poly(3-hexylthiophene): Estimates for Singlet Diffusion Lengths and the Correlation Between Singlet Annihilation Rates and Spectral Relaxation. J. Phys. Chem. C 2010, 114, 10962–10968.

(81)

Steiner, F.; Vogelsang, J.; Lupton, J. M. Singlet-Triplet Annihilation Limits Exciton Yield in Poly(3-hexylthiophene). Phys. Rev. Lett. 2014, 112, 137402.

(82)

Sahoo, D.; Tian, Y.; Sforazzini, G.; Anderson, L.; Scheblykin, I. G. Photo-induced Fluorescence Quenching in Conjugated Polymers Dispersed in Solid Matrices at Low Concentration. J. Mater. Chem. C 2014, 2, 6601–6608.

(83)

Pandey, L.; Van der Auweraer, M. Energy Transfer Dynamics in Organic Light Emitting Diode Emission Layers Doped with Triplet Emitters. J. Appl. Phys. 2011, 110, 053712.

(84)

Hwang, I.; Soci, C.; Moses, D.; Zhu, Z.; Waller, D.; Gaudiana, R.; Brabec, C. J.; Heeger, A. J. Ultrafast Electron Transfer and Decay Dynamics in a Small Band Gap Bulk Heterojunction Material. Adv. Mater 2007, 19, 2307–2312.

(85)

Hal, P. A. Van; Christiaans, M. P. T.; Wienk, M. M.; Kroon, J. M.; Janssen, A. J. Photoinduced Electron Transfer from Conjugated Polymers to TiO 2. J. Phys. Chem. C 1999, 103, 4352–4359.

(86)

Etzold, F.; Howard, I. A.; Forler, N.; Cho, D. M.; Meister, M.; Mangold, H.; Shu, J.; Hansen, M. R.; Müllen, K.; Laquai, F. The Effect of Solvent Additives on Morphology and Excited-State Dynamics in PCPDTBT : PCBM Photovoltaic Blends. J. Am. Chem. Soc. 2012, 134, 10569–10583.


32

Page 33 of 55

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


Figure Captions

Figure 1. (a) Normalized absorption and PL spectra showing the overlap of the PL spectrum of the donor with the absorption of the acceptor in the donor/acceptor systems consisting of P3HT/CIS NCs and CIS NCs/PCPDTBT (PL of P3HT: at RT (hollow red circles) and at 80K (red line); absorption of CIS NCs (black line); absorption of PCPDTBT (grey line); PL of CIS NCs (dotted green line); and PL of PCPDTBT at RT (hollow blue circles) and at 80K (blue line)) (b) Scheme of the energy levels of the involved materials and possible energy or charge transfer processes. Values for the energy levels were taken from the references indicated.17,28,54,56–61

Figure 2. UV Vis absorption spectra of CIS NCs in chlorobenzene solution at RT (grey line) along with UV Vis absorption spectra of (a) P3HT/CIS NCs blend films with different concentration of CIS NCs in the blend (0 wt% (black line), 15 wt% (red line), 30 wt% (green line), 45 wt % (blue line), 60 wt% (cyan line), 75 wt% (magenta line) and 90 wt% (purple line)), as well as (b) PCPDTBT/CIS NCs blends with different concentration of CIS NCs in the blend (0 wt% (black line), 60 wt% (red line), and 75 wt% (green line)). The spectra of thin films were normalized at 532 nm and 660 nm for respectively P3HT and PCPDTBT. These wavelengths correspond to the excitation wavelengths to be used later in the PL and PIA experiments (cfr. infra). Figure 3. (a) Photoluminescence quenching at 80 K of (a) P3HT/CIS NCs blends with different concentration of CIS NCs (0 wt% (black line), 15 wt% (red line), 30 wt% (blue line), 60 wt% (green line), 75 wt% (cyan line), and 90 wt% (magenta line), as well as (b) PCPDTBT/CIS NCs blends with different concentration of CIS NCs (0 wt% (black line), 60 wt% (red line), and 75


33


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

Page 34 of 55

wt% (green line)). The spectra are normalized to the absorption by the polymer component at the excitation wavelength as described in the experimental section. Figure 4. PL decay of P3HT films recorded at (a) λem= 675 nm, (b) λem= 725 nm and (c) PL decay of PCPDTBT films recorded at λem= 850 nm. The polymer films contained 0 wt% (black), 15 wt% (red), 30 wt% (green), 45 wt% (blue), 60 wt% (cyan), 75 wt% (magenta), 90 wt% (dark yellow) CIS NCs. The photoluminescence decay curves were analysed by fitting as described in the experimental section. The instrumental response function (grey line) is also shown. Figure 5. PIA spectra of P3HT/CIS NCs blends measured at (a) 80K, and (b) 295 K with excitation at 532 nm. The loading of the polymer films with CIS NCs was 0 wt% (black line), 15 wt% (red line), 30 wt% (blue line), 60 wt% (green line), 75 wt% (cyan line), and 90 wt% (magenta line). PIA spectra of PCPDTBT/CIS NCs blends measured at (c) 80K, and (d) 295 K with excitation at 660 nm. The loading of the polymer films with CIS NCs was 0 wt% (black line), 60 wt% (red line), and 75 wt% (green line). The spectra are normalized to the absorption by the polymer component at the excitation wavelength as described in the experimental section.

Table headings Table 1 Decay parameters obtained from bi-exponential fitting of the photoluminescence decay of P3HT and PCPDTBT samples with increasing concentration of CIS NCs. The photoluminescence decays were obtained at room temperature and excitation occurred at respectively 500 nm and 660 nm for respectively P3HT and PCPDTBT. From the fitting procedure, we estimate the error for the lifetimes reported to be ±10 ps, and the relative error of the amplitudes to be below 10 %.


34

Page 35 of 55

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


Figure 1


35


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

Page 36 of 55

Figure 2


36

Page 37 of 55

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


Figure 3


37


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

Page 38 of 55

Figure 4


38

Page 39 of 55

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


Figure 5


39


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

τ1 (ps)

XZ XY +XZ

τ2 (ps)

(ps)

Quenching efficiency [\ (%)

χ2

0.31

66

0.69

946

676

-

1.06

15

0.35

83

0.65

853

586

13

1.15

30

0.41

124

0.59

828

541.2

20

1.11

45

0.37

182

0.63

744

537

21

1.10

60

0.50

177

0.50

727

452

33

1.07

75

0.66

113

0.34

455

229

66

1.00

90

0.78

44

0.22

184

74.2

89

1.22

λem= 725nm. 0

0.29

72

0.71

951

699

-

1.10

15

0.36

140

0.64

869

603

14

1.09

30

0.39

148

0.61

832

563

19

1.15

45

0.52

133

0.48

694

405

42

1.06

60

0.49

182

0.51

729

459

34

1.03

75

0.65

122

0.35

461

243

65

1.04

90

0.81

38

0.19

173

64.2

91

0.93

Compound

XY XY +XZ

Page 40 of 55

P3HT + CIS NCs (wt%) λem= 675nm. 0

PCPDTBT+ CIS NCs (wt%) λem= 850nm. 0

0.39

31

0.61

226

150

-

1.04

15

0.24

67

0.76

199

167

0

1.10

30

0.38

42

0.62

224

155

0

1.13

45

0.32

57

0.68

186

144

4

1.07

60

0.37

68

0.63

176

136

10

1.02

75

0.46

75

0.54

161

122

20

1.02

Table 1


40

Page 41 of 55

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


TOC graphics


41


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

82x66mm (300 x 300 DPI)


Page 42 of 55

Page 43 of 55

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


101x86mm (300 x 300 DPI)



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

82x73mm (300 x 300 DPI)


Page 44 of 55

Page 45 of 55

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


82x73mm (300 x 300 DPI)



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

82x76mm (300 x 300 DPI)


Page 46 of 55

Page 47 of 55

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


82x73mm (300 x 300 DPI)



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

82x69mm (300 x 300 DPI)


Page 48 of 55

Page 49 of 55

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


82x68mm (300 x 300 DPI)



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

82x67mm (300 x 300 DPI)


Page 50 of 55

Page 51 of 55

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


82x75mm (300 x 300 DPI)



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

82x73mm (300 x 300 DPI)


Page 52 of 55

Page 53 of 55

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


82x72mm (300 x 300 DPI)



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

70x44mm (300 x 300 DPI)


Page 54 of 55

Page 55 of 55

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


82x75mm (300 x 300 DPI)


Photoinduced Charge Transfer in Hybrid Systems of CuInS2

Recommend Documents