Disparity in Optical Charge Generation and Recombination Processes

Subscriber access provided by UNIVERSITY OF MICHIGAN LIBRARY

Article

Disparity in Optical Charge Generation and Recombination Processes in Upright and Inverted PbS Quantum Dot Solar Cells Abay Dinku Gadisa, Yukihiro Hara, Yulan Fu, Kristina Vrouwenvelder, Jillian L Dempsey, Edward Thaddeus Samulski, and Rene Lopez J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512305x • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 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 28

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

Disparity in Optical Charge Generation and Recombination Processes in Upright and Inverted PbS Quantum Dot Solar Cells Abay Gadisa, † Yukihiro Hara, ‡ Yulan Fu, ‡ Kristina T. Vrouwenvelder, † Jillian L. Dempsey, † Edward T. Samulski, † Rene Lopez‡* †

Department of Chemistry, University of North Carolina at Chapel Hill, Caudill and Kenan Laboratories CB 3290 Chapel Hill, NC 27599, USA ‡ Department of Physics and Astronomy University of North Carolina at Chapel Hill Phillips Hall CB 3255, Chapel Hill, NC 27599, USA

KEYWORDS. quantum dot; PbS; solar cell; recombination; mobility; photocurrent Abstract The role of optical charge generation and nongeminate recombination on the photocurrent of upright and inverted colloidal PbS quantum dot solar cells is investigated. With a controlled active layer thickness, upright (PbS/fullerene) devices are found to present overall better photovoltaic performance relative to inverted devices, notwithstanding the better NIR photoconversion efficiency in the latter. Through detailed analysis and numerical optoelectronic simulations, we show that beyond incidental differences, these two device architectures have fundamentally dissimilar properties that stem from their particular optical generation characteristics and the nature of the recombination processes at play, with the inverted devices

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 28

affected only by trap-assisted losses and the upright ones suffering from enhanced bimolecular recombination. This study unveils the role of device geometry and inherent material properties on the carrier generation and collection efficiency of the light-generated photocurrent in colloidal quantum dot solar cells. Introduction Semiconducting colloidal quantum dots (CQDs) such as PbS and PbSe have been earmarked for application in optoelectronic devices such as solar cells,1–4 light emitting diodes,5–7 photodetectors,8 and other sensors.9 Facile solution processability due to their colloidal nature, and tunable light absorption characteristics owing to the effects of quantum confinement underlies the intense interest in CQDs.10,11 Several critical parameters such as surface passivation,12 particle size,13 and midgap states1 need to be understood and controlled in order to utilize CQDs in efficient optoelectronic devices. Despite these uncertainties PbS based CQD solar cells have reached power conversion efficiencies of up to 8.5%1,12 soon after inception, through several material and device engineering strategies: Devices exhibit remarkable short-circuit currents in excess of 20 mA/cm2.1–3,14 However, most of this current yield is generated from absorption of high energy photons (wavelength range from 400 to 800 nm), while the near-infrared (NIR) photon external quantum efficiency (EQE) is limited to less than 50%. As PbS CQDs possess propensity to absorb well inside the near IR region (800 nm to up to 1500 nm depending on dot size), it is important to investigate what the benefits and shortcomings of the different device architectures and the concomitant charge transport, collection efficiency, and recombination processes.15,16 Here, we have investigated the optical and electronic differences between two different solar cell architectures – upright (generates holes moving toward the conductive glass substrate) and


2

Page 3 of 28

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


inverted (generates holes moving away from the conductive glass substrate) – utilizing two different PbS thicknesses (thin 65 nm and thick 200 nm). Although photovoltaic performance is not very different across device types, we have found that the EQEs of these two device types show marked differences in the UV (thin devices) and NIR region (thick devices) with 30−50% more NIR photocurrent harvested in the inverted cell. Through detailed analysis and numerical simulations, we show that these differences stem primarily from the characteristic optical profiles of each device. However, this is not the source of the different device behaviors, as we have also identified differences in the nature of the recombination processes at play: Inverted devices are primarily affected by trap assisted losses while the upright ones suffer from additional bimolecular recombination. Our results explain why inverted devices are better NIR photo-converters and, more importantly, bring a clear insight into improved device engineering. The key physics engineering dichotomy, charge transport versus light absorption, needs to be attenuated to develop more efficient CQD-based photovoltaic technology. Experimental Section Materials Lead oxide (PbO (III), 99.999%), oleic acid (99%), 1-octadecene (ODE, 95.0%), hexamethyldisilathiane (TMS, synthesis grade), 3-mercaptopropionic acid (MPA, 99%), 1,3benzenedithiol (BDT, 99%), anhydrous octane (99%), and anhydrous acetonitrile were purchased from Sigma Aldrich and used without further purification. Acetone, methanol, and hexane were purchased from Fisher Scientific. TiO2 paste and [6,6] phenyl C61 butyric acid methyl ester (PCBM, 99.5%) were acquired from Dyesol and Nano-C, respectively. Synthesis of PbS Colloidal Quantum Dots


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 28

PbS colloidal quantum dots (CQDs) were synthesized following the procedures published in literatures.17,18 0.45 g (2 mmol) of PbO (III) and 1.13 g (4 mmol) of oleic acid were dissolved in 17.7 ml of ODE and stirred at 100 ºC under vacuum for over 1 h. The solution was then heated up to 150 ºC and N2 gas was flowed to the reaction flask. 0.21 ml (1 mmol) of TMS solution in 5.1 ml of ODE was prepared in another flask under N2. The TMS solution was injected to PbO/oleic acid solution and stirred for 150 s followed by quenching in an ice bath. The PbS colloidal solution was then transferred to centrifuge tubes and precipitated in acetone. After centrifugation (7,500 rpm, 10 min), the centrifuge-decant method was applied to clean the PbS. The PbS was collected with a small amount of hexane and then precipitated in acetone followed by centrifugation. This step was repeated more than 7 times and then switched to octane and methanol instead of hexane and acetone, respectively. After repeating the last step, the PbS was dispersed in octane. Fabrication of PbS Film PbS CQDs were deposited on substrates by spin-coating. In the present paper, two different substrates were used: indium tin oxide (ITO)/glass and TiO2 film spin-cast on the ITO/glass. 250 mg/ml of TiO2 suspension was prepared and deposited on an ITO glass by spin-coating followed by annealing at 400 ºC for 30 min. The following deposition protocol was applied to fabricate PbS films on these substrates. 25 mg/ml PbS solution was casted and spin coated at 4,500 rpm for 10 s. Then the ligand exchange solution was casted and reacted with the PbS layer for 10 s before spinning. A mix ligand solution of 0.1 % MPA and 0.1 % BDT in acetonitrile was used in the present study. Finally, the film was rinsed with acetonitrile and octane. The whole steps were repeated 6 or 16 times to obtain desired thickness of PbS film. Fabrication of PbS Solar Cells


4

Page 5 of 28

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


PCBM was deposited on PbS/ITO substrates by spin-coating from a 15 mg/ml chlorobenzene solution. 10 nm of Ca and then 100 nm of Al were subsequently deposited in a thermal evaporator at a base pressure of 10-6 mbar. Inverted devices were completed by thermal evaporation of a 10 nm of MoO3 and 100 nm on PbS layer giving a complete structure of ITO/TiO2/PbS/MoO3-Al. The spin-casting of PCBM and the photovoltaic characteristics measurements were made in a nitrogen-filled glove box (O2 < 0.1 ppm, H2O < 0.1 ppm). Characterization of PbS Solar Cells Absorption measurement: The absorption spectrum of PbS CQDs in octane was measured by UV-Vis and NIR spectroscopy (Cary 5000 UV-Vis and NIR spectrophotometer). Current-voltage measurements: Current-voltage measurements were performed using a Keithley 2400 source meter. The photovoltaic characteristics of the solar cells were recorded under a simulated A.M. 1.5G (1000 W/m2) solar illumination from a Newport solar simulator. Neutral density filters were used to vary the intensity of incident light. EQE measurement: To measure the EQE of the devices, light originating from a halogen lamp was passed through a monochromator to produce a monochromatic light that was directly incident onto the solar cells resulting into a photocurrent, which was measured by a lock-in amplifier. Results and Discussions In order to investigate the optical and electronic properties of the two types of solar cell designs, the following devices were constructed on an ITO (indium tin oxide) glass substrate: 1) an upright structure with ITO/PbS/PCBM/Ca/Al in which the PbS/PCBM forms a p-n junction (the PCBM also serves as electron transporting layer19,20 and adds some absorption at short


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 28

wavelengths21), and 2) an inverted structure-ITO/TiO2/PbS/MoO3/Al, where the p-n junction is formed at the TiO2/PbS interface (See Figure 1 (a)). The absorption peak corresponding to the first exciton absorption energy for PbS CQDs used in this investigation is observed at 1000 nm in octane (See Figure 1 (b)). Based on this absorption spectrum, the size of the PbS nanoparticles was estimated to be ca. 3.7 nm (conduction band 3.8 eV, valence band 5.1 eV, and the energy band gab 1.3 eV).22 It should be noted that the band edges of PbS nanoparticles depend on the capping ligands and the band gap energies with MPA and BDT are comparable.23

Figure 1. (a) The schematic diagram of the solar cells, (b) the absorption spectrum of PbS CQD in octane, and (c) the current-voltage characteristics of the inverted and upright solar cells comprising 65 nm and 200 nm PbS photoactive layers. The current-voltage (J-V) photovoltaic characteristics of all devices are shown in Figure 1 (c). Similar short circuit photocurrents (Jsc) and open circuit-voltages (Voc) were recorded in


6

Page 7 of 28

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


devices comprising the same thickness PbS films, with a slight difference favoring the upright devices in all cases. Inverted devices presented lower fill factors (FF) attributed to a higher series and lower shunt resistances (see Table 1) originating from a slight degradation of ITO as a result of a compulsory thermal annealing (at 400° C) of the TiO2 layer spin-cast on ITO. It is worth noting that the upright structure exhibiting the best photovoltaic characteristics was fabricated without any thermal treatments.24,25 Table 1. Photovoltaic parameters of the solar cells measured under A. M. 1.5 one sun illumination. Device type

PbS-thickness (nm)

Jsc (mA/cm )

a

Rseries

b

Rshunt

Voc

FF

Efficiency

(V)

(%)

(%)

(Ω cm2)

(Ω cm2)

2

Upright

65

6.89

0.55

58.55

2.22

5.09

1050.33

Inverted

65

6.44

0.51

44.91

1.47

14.20

345.53

Upright

200

11.19

0.57

52.37

3.34

7.66

352.44

Inverted

200

11.02

0.54

41.25

2.45

12.88

150.93

a

series resistance; b shunt resistance

The EQE of these solar cells is shown in Figure 2. In the 200 nm thick PbS layer devices, the EQE spectra are nearly the same regardless of device type for most of the spectral range. However, near the NIR absorption peak, EQE at λ = 1025 nm is 1.5 times larger for the inverted vs. the upright device. For the 65 nm thin PbS layer devices, the yield of red and NIR photocurrent is nearly the same, with a slight advantage for the inverted device. For the blue-UV wavelengths, the upright device clearly outperforms the inverted device by a significant margin.


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 28

Figure 2. The external quantum efficiency (EQE) and differences of the inverted and upright solar cells comprising either 65 or 200 nm thick PbS photoactive film. In order to explain both the major and the more subtle differences, we have calculated the absorption of the active layers with a complete numerical model of the solar cell’s optical electric fields utilizing the optical constants obtained from spectroscopic ellipsometry for each layer (Figure S1 supplemental information).26 The depth-dependent energy dissipation (Q(z)) per unit time and unit volume through each solar cell layer is given by

|| ,27,28 where , , , , and represent the speed of light in vacuum, the permittivity of free space, the absorption coefficient, the real index of refraction, and the electric field at the depth within the cell, respectively. This procedure allows us to exclude parasitic absorption in the contacts and charge barrier layers that do not contribute to the photocurrent. The results of these calculations are presented in Figure 3.


8

Page 9 of 28

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. Calculated absorption spectra in 65 nm and 200 nm PbS layer in the inverted and upright solar cells. The calculation mimics the EQE curves (Figure 2) well, pointing to large absorbed-photonto-electron conversion efficiencies. In particular, the calculation shows that for devices with a 200 nm thick PbS layer, the NIR-photons (near 1000 nm) are indeed slightly more effectively absorbed in the inverted devices, while photons in the red region (1100 nm) are nearly equally well absorbed in both device types. This is the result of different interference behavior created by the dissimilar layering within the two types of devices. However, the interference waves at the UV-blue region are presented less in the results of EQE curves (Figure 2 (a)) than the optical absorption model in Figure 3. This is typical of minor non-uniformities in film thickness. Nevertheless, the uptake in the photocurrent production below ∼550 nm for thin upright devices on integration can be attributed to a light absorption contribution from the PCBM layer. This uptake is minimal for the thicker PbS layers as few blue-green photons reach the PCBM. The differences in light interference patterns causes overall higher total absorption in upright relative to inverted devices leading to their higher Jsc as shown in Figure 1 (c). Building on the optical model, we have verified this is indeed the case by solving the drift-diffusion equations for a complete semiconductor model for each heterojunction with the parameters listed in Table 2.


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 28

The calculated J-V curves for all the devices are shown in Figure 4. This calculation shows that the optical profiles are consonant with the experimental photocurrents densities.

Figure 4. Calculated current-voltage curve by finite element method for upright and inverted devices. Devices parameters are listed in Table 2. Table 2. Parameters employed in simulations of PbS solar cells. 20, 24, 25, 27 PbS

TiO2

PCBM

Doping (1/cm3)

1017

5×1017

1015

Band-gap (eV)

1.3

3.2

2.4

Affinity (V)

3.8

4.1

4

Permittivity

17*

50

3.9

5.1×10-3

3×10-5

2×10-3

Hole Mobility (cm2/(Vs))

3×10-4

3×10-6

2×10-3

Electron Lifetime (s)

31×10-6

0.5

2×10-4

Hole Lifetime (s)

31×10-6

0.5

2×10-4

Electron Mobility (cm2/(Vs))

* The PbS permittivity is measured as shown in Figure S2.


10

Page 11 of 28

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


Comparing all the devices considered in this study (Figures 4 and 1 (c)), Jsc in model and experiment follow the same order: Jsc thick upright≥ Jsc thick inverted> Jsc thin upright≥ Jsc thin inverted; the Jsc ordering strictly follows optical generation characteristics. However, the complete optoelectronic model of the heterojunction predicts in general larger current densities than those measured. This discrepancy is more pronounced in the upright device. This could be due to differences in the electric fields in the depletion regions of the upright and inverted solar cells as they certainly cannot be exactly the same. Affinities, band-gaps, doping profiles, and layer thickness of PCBM and TiO2 are all different. However, the similarity between Voc across the board leads us to believe that such internal fields are not significantly different across device types. In order to uncover a more fundamental reason to explain dissimilarity in current densities, we have investigated in greater detail the charge recombination process in the upright and inverted solar cells having a 200 nm thick PbS film by employing light-intensity dependent photovoltaic attributes. Figure 5 shows the photovoltaic performance of the solar cells recorded under various light intensities ((a) upright and (b) inverted); the simulated counterparts, respectively are shown in (c) and (d). In this simulation, we can appreciate the limitations of the electrical model. A simple recombination term and the same formulation for both device types fails to capture the details of the high reverse bias behavior and the more subtle changes in slope in the forward bias region. Clearly, the particular interfaces of the upright or inverted devices have a critical effect on recombination, beyond just the internal recombination in the PbS layer.


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 28

Figure 5. Current-voltage characteristics of (a) the upright, and (b) inverted solar cells, as a function of light intensity. (c) and (d) simulated current-voltage characteristics of the same devices, respectively. Figure 6 shows a more detailed analysis of the light intensity dependence effects on Voc and photocurrent density thorough the power exponent, α, in the relationship, ∝ , where


12

Page 13 of 28

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


is the photo-generated current density (the difference of the total current measured under illumination, and the dark current), is the incident light intensity, and α quantifies the linearity of their relationship. α becomes unity when all charges are collected without loss, and sub-linear values may result from a carrier mobility imbalance, space charges or bimolecular recombination.29–31

Figure 6. Dependence of Voc on light intensity, and the proportionality factor α relating current density to light intensity ( ∝ _!"#"$%&) respectively. The solid lines in (a) are fits generated according to the equations embedded in the figure. As the observed α turns out to be sub-linear for both device types (Figure 6 (b)). It should be noted that the trend of curves was consistent although 2-4 % errors in α were observed for both devices. Close examination reveals that α is about 0.9 for the upright devices for measurements performed in the voltage range of -1 V to near Voc, while for the inverted devices α is 0.94 - 0.97 in reverse bias, and then follows the same trend as for the upright cell for


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 28

voltages ranging from short-circuit to Voc. Employing the ideal diode equation '()

*+ ,

012

ln / 0 + 16,32–34 (where , 7, , 8 and 9 represent the reverse saturation current density, 3

elementary charge, diode ideality factor, Boltzmann constant, and absolute temperature, respectively), Voc can be linked to both carrier generation and recombination processes through , and , respectively. If is close to 1, the carrier loss is considered to be dominated by bimolecular recombination while larger values of express trap-assisted recombination.34,35 The ideality factor , calculated to be nearly 1.45 (See Figure 6 (a)) for both inverted and upright solar cells, suggests that some trap-assisted recombination is involved near Voc. It follows from this analysis that the upright cell is more susceptible to carrier losses at Jsc, while both cells are characterized by similar type and magnitude of recombination losses at Voc. By analyzing the charge collection efficiency of the cells, the voltage range over which bimolecular and trap-assisted recombination occurs in each type of device is shown in Figure 7. The photocurrent of the cells comprising PbS film of thickness : is expressed as 7 ; 0

012 ?@,A

12 ?@,JA

>. Figures 7 (a) and (b) show the collection probability of

the upright and inverted cells, respectively. The regions where the collection probability is


14

Page 15 of 28

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


limited by particular loss mechanisms are highlighted.29 Namely bimolecular recombination where the Pc curves do not overlap with each other and trap-assisted losses when they do overlap, illustrating the light intensity independence of the process.

Figure 7. Charge collection probabilities of (a) the upright and (b) inverted solar cells, which were obtained by normalizing the photocurrent measured at the various light intensities with the saturated photocurrent measured at -1 V. The ovals show regions dominated by bimolecular (light intensity-dependent) and trap-assisted (light intensityindependent) recombination losses. We have thus shown that despite a better fill factor and Voc, the upright 200 nm thick PbS device is actually operating with higher bimolecular recombination than its inverted counterpart. The trap-assisted loss recorded near Voc is consistent with previous reports.36 On the other hand, the stronger bimolecular recombination recorded in the upright device is somewhat unexpected


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 28

given its overall better performance. Due to this stronger bimolecular recombination in upright device, the loss in the current density comparing the experimental Jsc (Figure 1 (c) and simulated Jsc (Figure 4) is larger in the upright device than in the inverted device. One possibility is electron backflow across the PbS/PCBM interface; since the conduction band of PbS22 and the LUMO of PCBM37 are nearly the same, this recombination path could dominate.24 Suitable interfacial modifications would be necessary to minimize this effect. Conclusions In summary, we have shown that the TiO2 optical spacer creates a positive interference effect that enhances NIR absorption in inverted devices relative to upright systems. Despite overall similar integrated photocurrent densities, the proportion of the different loss mechanisms between devices is not the same. While the signature of trap-induced recombination has been observed in both cells near Voc, the enhanced bimolecular recombination recorded in the upright devices is most probably caused by an electron backflow from PCBM to PbS. Nevertheless, we have shown that the upright device can potentially produce higher photocurrents than its inverted counterpart for a given PbS thickness, and it delivers increased Voc and fill factors. Given these findings and its advantageous all-room temperature processing, the upright architecture appears to be more attractive than the inverted cell in the long run, and the further improvement would be achievable by eliminating the bimolecular recombination.


16

Page 17 of 28

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


AUTHOR INFORMATION Corresponding author ∗

e-mail: [email protected] Tel: 919-962-7216

■ ACKNOWLEDGMENT This material is based upon work funded by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences under Award Number DE-SC0006416. We also acknowledge support by Research Corporation for the Science Advancement #22371. Authors would also like to thank Chapel Hill Analytical and Nanofabrication Laboratory (CHANL) for all their assistance.

■ Supporting Information Available. The optical constants of solar cell components; capacitance measurement of the devices. This material is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES

(1)

Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; et al. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577–582.

(2)

Tang, J.; Sargent, E. H. Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress. Adv. Mater. 2011, 23, 12–29.

(3)

Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Schottky Solar Cells Based on Colloidal Nanocrystal Films. Nano Lett. 2008, 8, 3488– 3492.

(4)

Chuang, C.-H. M.; Brown, P. R.; Bulović, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 796–801.


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 28

(5)

Steckel, J. S.; Coe-Sullivan, S.; Bulović, V.; Bawendi, M. G. 1.3µm to 1.55µm Tunable Electroluminescence from PbSe Quantum Dots Embedded within an Organic Device. Adv. Mater. 2003, 15, 1862–1866.

(6)

Konstantatos, G.; Huang, C.; Levina, L.; Lu, Z.; Sargent, E. H. Efficient Infrared Electroluminescent Devices Using Solution-Processed Colloidal Quantum Dots. Adv. Funct. Mater. 2005, 15, 1865–1869.

(7)

Sun, L.; Choi, J. J.; Stachnik, D.; Bartnik, A. C.; Hyun, B.-R.; Malliaras, G. G.; Hanrath, T.; Wise, F. W. Bright Infrared Quantum-Dot Light-Emitting Diodes through Inter-Dot Spacing Control. Nat. Nanotechnol. 2012, 7, 369–373.

(8)

Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H. Ultrasensitive Solution-Cast Quantum Dot Photodetectors. Nature 2006, 442, 180–183.

(9)

Zhang, C.-Y.; Yeh, H.-C.; Kuroki, M. T.; Wang, T.-H. Single-Quantum-Dot-Based DNA Nanosensor. Nat. Mater. 2005, 4, 826–831.

(10)

Semonin, O. E.; Luther, J. M.; Choi, S.; Chen, H.-Y.; Gao, J.; Nozik, A. J.; Beard, M. C. Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell. Science 2011, 334, 1530–1533.

(11)

Cunningham, P. D.; Boercker, J. E.; Foos, E. E.; Lumb, M. P.; Smith, A. R.; Tischler, J. G.; Melinger, J. S. Enhanced Multiple Exciton Generation in Quasi-One-Dimensional Semiconductors. Nano Lett. 2011, 11, 3476–3481.

(12)

Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; et al. Colloidal-Quantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765–771.

(13)

Yoon, W.; Boercker, J. E.; Lumb, M. P.; Placencia, D.; Foos, E. E.; Tischler, J. G. Enhanced Open-Circuit Voltage of PbS Nanocrystal Quantum Dot Solar Cells. Sci. Rep. 2013, 3, 2225.

(14)

Rath, A. K.; Bernechea, M.; Martinez, L.; Arquer, F. P. G. De; Osmond, J.; Konstantatos, G. Solution-Processed Inorganic Bulk Nano-Heterojunctions and Their Application to Solar Cells. 2012, 6, 529–534.

(15)

Liu, Y.; Gibbs, M.; Puthussery, J.; Gaik, S.; Ihly, R.; Hillhouse, H. W.; Law, M. Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids. Nano Lett. 2010, 10, 1960–1969.

(16)

Bisri, S. Z.; Piliego, C.; Yarema, M.; Heiss, W.; Loi, M. A. Low Driving Voltage and High Mobility Ambipolar Field-Effect Transistors with PbS Colloidal Nanocrystals. Adv. Mater. 2013, 25, 4309–4314.


18

Page 19 of 28

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


(17)

Hines, M. a.; Scholes, G. D. Colloidal PbS Nanocrystals with Size-Tunable Near-Infrared Emission: Observation of Post-Synthesis Self-Narrowing of the Particle Size Distribution. Adv. Mater. 2003, 15, 1844–1849.

(18)

Noone, K. M.; Strein, E.; Anderson, N. C.; Wu, P.-T.; Jenekhe, S. a; Ginger, D. S. Broadband Absorbing Bulk Heterojunction Photovoltaics Using Low-Bandgap SolutionProcessed Quantum Dots. Nano Lett. 2010, 10, 2635–2639.

(19) Mihailetchi, V. D.; van Duren, J. K. J.; Blom, P. W. M.; Hummelen, J. C.; Janssen, R. a. J.; Kroon, J. M.; Rispens, M. T.; Verhees, W. J. H.; Wienk, M. M. Electron Transport in a Methanofullerene. Adv. Funct. Mater. 2003, 13, 43–46. (20)

Jarzab, D.; Szendrei, K.; Yarema, M.; Pichler, S.; Heiss, W.; Loi, M. a. Charge-Separation Dynamics in Inorganic-Organic Ternary Blends for Efficient Infrared Photodiodes. Adv. Funct. Mater. 2011, 21, 1988–1992.

(21)

Song, Q. L.; Gong, C.; Yang, H. Bin; Li, C. M. Photocurrent Generation in Plain [6,6]Phenyl C61 Butyric Acid Methyl Ester. Sol. Energy Mater. Sol. Cells 2010, 94, 2422– 2425.

(22) Pattantyus-abraham, A. G.; Kramer, Ќ. I. J.; Barkhouse, Ќ. A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Sargent, E. H. DepletedHeterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374–3380. (23)

Brown, P. R.; Kim, D.; Lunt, R. R.; Zhao, N.; Bawendi, M. G.; Grossman, J. C.; Bulovi, V. Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange. ACS Nano 2014, 8, 5863–5872.

(24)

Zhao, N.; Osedach, T. P.; Chang, L.-Y.; Geyer, S. M.; Wanger, D.; Binda, M. T.; Arango, A. C.; Bawendi, M. G.; Bulovic, V. Colloidal PbS Quantum Dot Solar Cells with High Fill Factor. ACS Nano 2010, 4, 3743–3752.

(25)

Kim, G.; Kim, H.; Walker, B.; Choi, H.; Yang, C.; Park, J.; Kim, J. Y. Effects of Ionic Liquid Molecules in Hybrid PbS Quantum Dot − Organic Solar Cells. ACS Appl. Matter. Interfaces 2013, 13, 1757–1760.

(26)

Law, M.; Beard, M. C.; Choi, S.; Luther, J. M.; Hanna, M. C.; Nozik, A. J. Determining the Internal Quantum Efficiency of PbSe Annocrystal Solar Cells with the Aid of an Optical Model. Nano Lett. 2008, 8, 3904–3910.

(27)

Pettersson, L. A. A.; Roman, L. S.; Inganäs, O. Modeling Photocurrent Action Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86, 487.

(28)

Persson, N.-K.; Arwin, H.; Inganas, O. Optical Optimization of Polyfluorene-Fullerene Blend Photodiodes. J. Appl. Phys. 2005, 97, 034503.


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 28

(29)

Cowan, S. R.; Roy, A.; Heeger, A. J. Recombination in Polymer-Fullerene Bulk Heterojunction Solar Cells. Phys. Rev. B 2010, 82, 245207.

(30)

Proctor, C. M.; Kim, C.; Neher, D.; Nguyen, T.-Q. Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole-Based Solution-Processed Small Molecule Solar Cells. Adv. Funct. Mater. 2013, 23, 3584–3594.

(31)

Cowan, S. R.; Banerji, N.; Leong, W. L.; Heeger, A. J. Charge Formation, Recombination, and Sweep-Out Dynamics in Organic Solar Cells. Adv. Funct. Mater. 2012, 22, 1116– 1128.

(32)

Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganäs, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer-Fullerene Solar Cells. Nat. Mater. 2009, 8, 904–909.

(33)

Wetzelaer, G. a. H.; Kuik, M.; Lenes, M.; Blom, P. W. M. Origin of the Dark-Current Ideality Factor in Polymer:fullerene Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 99, 153506.

(34)

Koster, L. J. a.; Mihailetchi, V. D.; Ramaker, R.; Blom, P. W. M. Light Intensity Dependence of Open-Circuit Voltage of Polymer:fullerene Solar Cells. Appl. Phys. Lett. 2005, 86, 123509.

(35)

Szendrei, K.; Gomulya, W.; Yarema, M.; Heiss, W.; Loi, M. a. PbS Nanocrystal Solar Cells with High Efficiency and Fill Factor. Appl. Phys. Lett. 2010, 97, 203501/1–203501/3.

(36)

Ryan, J. W.; Marin-beloqui, J. M.; Albero, J.; Palomares, E. Nongeminate Recombination Dynamics − Device Voltage Relationship in Hybrid PbS Quantum Dot/C60 Solar Cells. J. Phys. Chem. C 2013, 117, 17470-17476.

(37)

Larson, B. W.; Whitaker, J. B.; Wang, X.; Popov, A. A.; Rumbles, G.; Kopidakis, N.; Strauss, S. H.; Boltalina, O. V. Electron Affinity of Phenyl − C61 − Butyric Acid Methyl Ester (PCBM). J. Phys. Chem. C 2013, 117, 14958-14964.


20

Page 21 of 28

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. (a) The schematic diagram of the solar cells, (b) the absorption spectrum of PbS CQD in octane, and (c) the current-voltage characteristics of the inverted and upright solar cells comprising 65 nm and 200 nm PbS photoactive layers. 102x129mm (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

Figure 2. The external quantum efficiency (EQE) and differences of the inverted and upright solar cells comprising either 65 or 200 nm thick PbS photoactive film. 98x120mm (300 x 300 DPI)


Page 22 of 28

Page 23 of 28

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. Calculated absorption spectra in 65 nm and 200 nm PbS layer in the inverted and upright solar cells. 56x39mm (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

Figure 4. Calculated current-voltage curve by finite element method for upright and inverted devices. Devices parameters are listed in Table 2. 57x40mm (300 x 300 DPI)


Page 24 of 28

Page 25 of 28

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. Current-voltage characteristics of (a) the upright, and (b) inverted solar cells, as a function of light intensity. (c) and (d) simulated current-voltage characteristics of the same devices, respectively. 167x169mm (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

Figure 6. Dependence of Voc on light intensity, and the proportionality factor α relating current density to light intensity (Jph ∝Light_Intensity હ) respectively. The solid lines in (a) are fits generated according to the equations embedded in the figure. 82x41mm (300 x 300 DPI)


Page 26 of 28

Page 27 of 28

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 7. Charge collection probabilities of (a) the upright and (b) inverted solar cells, which were obtained by normalizing the photocurrent measured at the various light intensities with the saturated photocurrent measured at -1 V. The ovals show regions dominated by bimolecular (light intensity-dependent) and trapassisted (light intensity-independent) recombination losses. 94x53mm (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

TOC 44x25mm (300 x 300 DPI)


Page 28 of 28

Disparity in Optical Charge Generation and Recombination Processes

Recommend Documents